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2012-10-10 | Bodily tides near the 1:1 spin-orbit resonance. Correction to Goldreich's dynamical model | Spin-orbit coupling is often described in the "MacDonald torque" approach
which has become the textbook standard. Within this method, a concise
expression for the additional tidal potential, derived by MacDonald (1964; Rev.
Geophys. 2, 467), is combined with an assumption that the Q factor is
frequency-independent (i.e., that the geometric lag angle is constant in time).
This makes the approach unphysical because MacDonald's derivation of the said
formula was implicitly based on keeping the time lag frequency-independent,
which is equivalent to setting Q to scale as the inverse tidal frequency. The
contradiction requires the MacDonald treatment of both non-resonant and
resonant rotation to be rewritten.
The non-resonant case was reconsidered by Efroimsky & Williams (2009; CMDA
104, 257), in application to spin modes distant from the major
commensurabilities. We continue this work by introducing the necessary
alterations into the MacDonald-torque-based model of falling into a 1:1
resonance. (For the original version of the model, see Goldreich 1966; AJ 71,
1.)
We also study the effect of the triaxiality on both circulating and librating
rotation near the synchronous state. Circulating rotation may evolve toward the
libration region or toward a spin rate larger than synchronous
(pseudosynchronous spin). Which behaviour depends on the eccentricity, the
triaxiality of the primary, and the mass ratio of the secondary and primary
bodies. The spin evolution will always stall for the oblate case. For
small-amplitude librations, expressions are derived for the libration
frequency, damping rate, and average orientation.
However, the stability of pseudosynchronous spin hinges upon the dissipation
model. Makarov and Efroimsky (2012; arXiv:1209.1616) have found that a more
realistic dissipation model than the corrected MacDonald torque makes
pseudosynchronous spin unstable. | 1210.2923v3 |
2012-11-06 | Tidal resonance locks in inspiraling white dwarf binaries | We calculate the tidal response of helium and carbon/oxygen (C/O) white dwarf
(WD) binaries inspiraling due to gravitational wave emission. We show that
resonance locks, previously considered in binaries with an early-type star,
occur universally in WD binaries. In a resonance lock, the orbital and spin
frequencies evolve in lockstep, so that the tidal forcing frequency is
approximately constant and a particular normal mode remains resonant, producing
efficient tidal dissipation and nearly synchronous rotation. We show that
analogous locks between the spin and orbital frequencies can occur not only
with global standing modes, but even when damping is so efficient that the
resonant tidal response becomes a traveling wave. We derive simple analytic
formulas for the tidal quality factor Q and tidal heating rate during a g-mode
resonance lock, and verify our results numerically. We find that Q ~ 10^7 for
orbital periods ~ 1 - 2 hr in C/O WDs, and Q ~ 10^9 for P_orb ~ 3 - 10 hr in
helium WDs. Typically tidal heating occurs sufficiently close to the surface
that the energy should be observable as surface emission. Moreover, near an
orbital period of ~ 10 min, the tidal heating rate reaches ~ 10^{-2} L_\sun,
rivaling the luminosities of our fiducial WD models. Recent observations of the
13-minute double-WD binary J0651 are roughly consistent with our theoretical
predictions. Tides naturally tend to generate differential rotation; however,
we show that the fossil magnetic field strength of a typical WD can maintain
solid-body rotation down to at least P_orb ~ 10 min even in the presence of a
tidal torque concentrated near the WD surface. | 1211.1393v4 |
2017-04-18 | Outbursts of the intermediate-mass black hole HLX-1: a wind instability scenario | We model the intermediate-mass black hole HLX-1, using the Hubble Space
Telescope, XMM-Newton and Swift. We quantify the relative contributions of a
bluer component, function of X-ray irradiation, and a redder component,
constant and likely coming from an old stellar population. We estimate a black
hole mass of about (2^{+2}_{-1}) x 10^4 M_{sun}, a spin parameter a/M ~ 0.9 for
moderately face-on view, and a peak outburst luminosity of about 0.3 times the
Eddington luminosity. We discuss the discrepancy between the characteristic
sizes inferred from the short X-ray timescale (R ~ a few 10^{11} cm) and from
the optical emitter (R sqrt[cos theta] ~ 2.2 x 10^{13} cm). One possibility is
that the optical emitter is a circumbinary disk; however, we disfavour this
scenario because it would require a very small donor star. A more plausible
scenario is that the disk is large but only the inner annuli are involved in
the X-ray outburst. We propose that the recurrent outbursts are caused by an
accretion-rate oscillation driven by wind instability in the inner disk. We
argue that the system has a long-term-average accretion rate of a few percent
Eddington, just below the upper limit of the low/hard state; a wind-driven
oscillation can trigger transitions to the high/soft state, with a recurrence
period of ~1 year (much longer than the binary period, which we estimate as ~10
days). The oscillation that dominated the system in the last decade is now
damped such that the accretion rate no longer reaches the level required to
trigger a transition. Finally, we highlight similarities between disk winds in
HLX-1 and in the Galactic black hole V404 Cyg. | 1704.05468v1 |
2017-03-02 | Pinch dynamics in a low-$β$ plasma | The relaxation of a helical magnetic field ${\bf B}({\bf x}, t)$ in a
high-conductivity plasma contained in the annulus between two perfectly
conducting coaxial cylinders is considered. The plasma is of low density and
its pressure is negligible compared with the magnetic pressure; the flow of the
plasma is driven by the Lorentz force and and energy is dissipated primarily by
the viscosity of the medium. The axial and toroidal fluxes of magnetic field
are conserved in the perfect-conductivity limit, as is the mass per unit axial
length. The magnetic field relaxes during a rapid initial stage to a force-free
state, and then decays slowly, due to the effect of weak resistivity $\eta$,
while constrained to remain approximately force-free. Interest centres on
whether the relaxed field may attain a Taylor state; but under the assumed
conditions with axial and toroidal flux conserved inside every cylindrical
Lagrangian surface, this is not possible. The effect of an additional
$\alpha$-effect associated with instabilities and turbulence in the plasma is
therefore investigated in exploratory manner. An assumed pseudo-scalar form of
$\alpha$ proportional to $q\,\eta\, ({\bf j}\cdot {\bf B})$ is adopted, where $
{\bf j}=\nabla\times {\bf B}$ and $q$ is an $\mathcal{O}(1)$ dimensionless
parameter. It is shown that, when $q$ is less that a critical value $q_c$, the
evolution remains smooth and similar to that for $q=0$; but that if $q>q_c$,
negative-diffusivity effects act on the axial component of $\bf B$, generating
high-frequency rapidly damped oscillations and an associated transitory
appearance of reversed axial field. However, the scalar quantity $\gamma={\bf
j}\cdot {\bf B}/B^2$ remains highly non-uniform, so that again the field shows
no sign of relaxing to a Taylor state for which $\gamma$ would have to be
constant. | 1703.00708v1 |
2018-09-24 | Laser cooling and magneto-optical trapping of molecules analyzed using optical Bloch equations and the Fokker-Planck-Kramers equation | We study theoretically the behavior of laser-cooled calcium monofluoride
(CaF) molecules in an optical molasses and magneto-optical trap (MOT), and
compare our results to recent experiments. We use multi-level optical Bloch
equations to estimate the force and the diffusion constant, followed by a
Fokker-Planck-Kramers equation to calculate the time-evolution of the velocity
distribution. The calculations are done in three-dimensions, and we include all
the relevant energy levels of the molecule and all the relevant frequency
components of the light. Similar to simpler model systems, the
velocity-dependent force curve exhibits Doppler and polarization-gradient
forces of opposite signs. We show that the temperature of the MOT is governed
mainly by the balance of these two forces. Our calculated MOT temperatures and
photon scattering rates are in broad agreement with those measured
experimentally over a wide range of parameters. In a blue-detuned molasses, the
temperature is determined by the balance of polarization gradient cooling, and
heating due to momentum diffusion, with no significant contribution from
Doppler heating. In the molasses, we calculate a damping rate similar to the
measured one, and steady-state temperatures that have the same dependence on
laser intensity and applied magnetic field as measured experimentally, but are
consistently a few times smaller than measured. We attribute the higher
temperatures in the experiments to fluctuations of the dipole force which are
not captured by our model. We show that the photon scattering rate is strongly
influenced by the presence of dark states in the system, but that the
scattering rate does not go to zero even for stationary molecules because of
the transient nature of the dark states. | 1809.08833v4 |
2019-09-13 | Multi-Particle Collisions in Microgravity: Coefficient of Restitution and Sticking Threshold for Systems of Mm-Sized Particles | The current model of planet formation lacks a good understanding of the
growth of dust particles inside the protoplanetary disk beyond mm sizes. In
order to investigate the low-velocity collisions between this type of
particles, the NanoRocks experiment was flown on the International Space
Station (ISS) between September 2014 and March 2016. We present the results of
this experiment. We quantify the damping of energy in systems of multiple
particles in the 0.1 to 1 mm size range while they are in the bouncing regime,
and study the formation of clusters through sticking collisions between
particles. We developed statistical methods for the analysis of the large
quantity of collision data collected by the experiment. We measured the average
motion of particles, the moment of clustering, and the cluster size formed. In
addition, we ran simple numerical simulations in order to validate our
measurements. We computed the average coefficient of restitution (COR) of
collisions and find values ranging from 0.55 for systems including a population
of fine grains to 0.94 for systems of denser particles. We also measured the
sticking threshold velocities and find values around 1 cm/s, consistent with
the current dust collision models based on independently collected experimental
data. Our findings have the following implications that can be useful for the
simulation of particles in PPDs and planetary rings: (1) The average COR of
collisions between same-sized free-floating particles at low speeds (< 2 cm/s)
is not dependent on the collision velocity; (2) The simplified approach of
using a constant COR value will accurately reproduce the average behavior of a
particle system during collisional cooling; (3) At speeds below 5 mm/s, the
influence of particle rotation becomes apparent on the collision behavior; (4)
Current dust collision models predicting sticking thresholds are robust. | 1909.06417v1 |
2019-10-14 | A search for optical AGN variability in 35,000 low-mass galaxies with the Palomar Transient Factory | We present an analysis of the long-term optical variability for $\sim50,000$
nearby (z<0.055) galaxies from the NASA-Sloan Atlas, $35,000$ of which are
low-mass ($M_{\ast}<10^{10}~M_{\odot}$). We use difference imaging of Palomar
Transient Factory (PTF) R-band observations to construct light curves with
typical baselines of several years. We then search for subtle variations in the
nuclear light output. We determine whether detected variability is AGN-like by
assessing the fit quality to a damped random walk model. We identify 424
variability-selected AGN, including 244 with stellar masses between $10^{7}$
and $10^{10}~M_{\odot}$. 75% of low-mass galaxies with AGN-like variability
have narrow emission lines dominated by star formation. After controlling for
nucleus magnitude, the fraction of variable AGN is constant down to
$M_{\ast}=10^{9}~M_{\odot}$, suggesting no drastic decline in the BH occupation
fraction down to this stellar mass regime. Combining our NASA-Sloan Atlas
sample with samples of nearby galaxies with broad H$\alpha$ emission, we find
no dependence of variability properties with black hole mass. However, we
caution that the variable AGN fraction is strongly dependent on baseline. For
baselines less than two years, the variable fraction for the full sample is
0.25%, compared to 1.0% for baselines longer than two years. Finally, comparing
Stripe 82 light curves (Baldassare et al. 2018) to PTF light curves, we find
populations of changing-look AGN: 8 galaxies that are variable in Stripe 82,
but quiescent in PTF, and 15 galaxies where the reverse is true. Our PTF work
demonstrates the promise of long-term optical variability searches in low-mass
galaxies for finding AGNs missed by other selection techniques. | 1910.06342v1 |
2019-10-30 | A priori bounds for the $Φ^4$ equation in the full sub-critical regime | We derive a priori bounds for the $\Phi^4$ equation in the full sub-critical
regime using Hairer's theory of regularity structures. The equation is formally
given by \begin{equation} \label{e}(\partial_t-\Delta)\phi = -\phi^3 + \infty
\phi +\xi, \tag{$\star$} \end{equation} where the term $+\infty \phi$
represents infinite terms that have to be removed in a renormalisation
procedure. We emulate fractional dimensions $d<4$ by adjusting the regularity
of the noise term $\xi$, choosing $\xi \in C^{-3+\delta}$. Our main result
states that if $\phi$ satisfies this equation on a space-time cylinder $P=
(0,1) \times \{ |x| \leq 1 \}$, then away from the boundary $\partial P$ the
solution $\phi$ can be bounded in terms of a finite number of explicit
polynomial expressions in $\xi$, and this bound holds uniformly over all
possible choices of boundary data for $\phi$. The derivation of this bound
makes full use of the super-linear damping effect of the non-linear term
$-\phi^3$. A key part of our analysis consists of an appropriate re-formulation
of the theory of regularity structures in the specific context of \eqref{e},
which allows to couple the small scale control one obtains from this theory
with a suitable large scale argument. Along the way we make several new
observations and simplifications. Instead of a model $(\Pi_x)_x$ and the family
of translation operators $(\Gamma_{x,y})_{x,y}$ we work with just a single
object $(\mathbb{X}_{x, y})$ which acts on itself for translations, very much
in the spirit of Gubinelli's theory of branched rough paths. Furthermore, we
show that in the specific context of \eqref{e} the hierarchy of continuity
conditions which constitute Hairer's definition of a \emph{modelled
distribution} can be reduced to the single continuity condition on the
"coefficient on the constant level". | 1910.13854v2 |
2019-10-31 | One-point probability distribution function from spherical collapse: Early Dark Energy (EDE) vs. $Λ$CDM | We compute the one-point PDF of an initially Gaussian dark matter density
field using spherical collapse (SC). We compare the results to other forms
available in the literature and also compare the PDFs in the $\Lambda$CDM model
with an early dark energy (EDE) model. We find that the skewed log-normal
distribution provides the best fit to the non-linear PDF from SC for both
cosmologies, from $a=0.1$ to 1 and for scales characterized by the comoving
width of the Gaussian: $\sigma_G = 0.5, 1, 2$. To elucidate the effect of
cosmology, we examine the linear and non-linear growth rates through test
cases. For overdensities, when the two models have the same initial density
contrast, the differences due to cosmology are amplified in the non-linear
regime, whereas, if the two models have the same linear density contrast today,
then the differences in cosmology are damped in the non-linear regime. This
behaviour is in contrast with voids, where the non-linear growth becomes
`self-regulatory' and is less sensitive to cosmology and initial conditions. To
compare the PDFs, we examine the difference of the PDFs and evolution of the
width of the PDF. The trends with scale and redshift are as expected. A
tertiary aim of this paper was to check if the fitting form for the non-linear
density-velocity divergence relation, derived for constant equation of state
($w$) models by Nadkarni-Ghosh holds for the EDE model. We find that it does
with an accuracy of 4\%, thus increasing its range of validity. | 1910.14347v3 |
2013-10-31 | Spatially Resolved Emission of a High Redshift DLA Galaxy with the Keck/OSIRIS IFU | We present the first Keck/OSIRIS infrared IFU observations of a high redshift
damped Lyman-alpha (DLA) galaxy detected in the line of sight to a background
quasar. By utilizing the Laser Guide Star Adaptive Optics (LGSAO) to reduce the
quasar PSF to FWHM~0.15 arcsec, we were able to search for and map the
foreground DLA emission free from the quasar contamination. We present maps of
the H-alpha and [OIII] $\lambda \lambda$ 5007, 4959 emission of DLA 2222-0946
at a redshift of z ~ 2.35. From the composite spectrum over the H-alpha
emission region we measure a star formation rate of 9.5 $\pm$ 1.0 M$_{\odot}$
year$^{-1}$ and a dynamical mass, M$_{dyn}$ = 6.1 x 10$^9$ M$_{\odot}$. The
average star formation rate surface density is < \Sigma_{SFR} > = 0.55
M$_{\odot}$ yr$^{-1}$ kpc$^{-2}$, with a central peak of 1.7 M$_{\odot}$
yr$^{-1}$ kpc$^{-2}$. Using the standard Kennicutt-Schmidt relation, this
corresponds to a gas mass surface density of $\Sigma_{gas}$ = 243 M$_{\odot}$
pc$^{-2}$. Integrating over the size of the galaxy we find a total gas mass of
M$_{gas}$ = 4.2 x 10$^9$ M$_{\odot}$. We estimate the gas fraction of DLA
2222-0946 to be $f_{gas}$ ~ 40%. We detect [NII]$\lambda$6583 emission at 2.5
sigma significance with a flux corresponding to a metallicity of 75% solar.
Comparing this metallicity with that derived from the low-ion absorption gas ~6
kpc away, ~30% solar, indicates possible evidence for a metallicity gradient or
enriched in/outflow of gas. Kinematically, both H-alpha and [OIII] emission
show relatively constant velocity fields over the central galactic region.
While we detect some red and blueshifted clumps of emission, they do not
correspond with rotational signatures that support an edge-on disk
interpretation. | 1311.0045v1 |
2017-07-04 | Lattice symmetries and the topological protected transport of colloidal particles | The topologically protected transport of colloidal particles on top of
magnetic patterns of all possible single lattice constant two dimensional
magnetic point group symmetries is studied experimentally, theoretically, and
with numerical simulations. We examine the transport of colloidal particles in
response to modulation loops of the external field. We classify the modulation
loops into topologically distinct classes causing different transport. We show
that the lattice symmetry has a profound influence on the transport modes, the
accessibility of transport networks, and the individual addressability of
paramagnetic versus diamagnetic colloidal particles. We show how the transport
of colloidal particles above a two fold symmetric stripe pattern changes from
universal adiabatic transport at large elevations via a topologically protected
ratchet motion at intermediate elevations toward a non-transport regime at low
elevations. Transport above four fold symmetric patterns is closely related to
the transport above two fold symmetric patterns. There exists a family of three
fold symmetric patterns that vary as a function of the phase of the pattern. We
show how this family can be divided into two topologically distinct classes
supporting different transport modes and being protected by proper and improper
six fold symmetries. Both classes support individual control over the transport
of paramagnetic and diamagnetic particles. We discuss the topological
transition when moving the phase from one class of pattern to the other class.
The similarities and the differences in the lattice symmetry protected
transport of classical over-damped colloidal particles versus the topologically
protected transport in quantum mechanical systems are emphasized | 1707.00861v1 |
2018-05-15 | Nonreciprocal charge transport in two-dimensional noncentrosymmetric superconductors | Nonreciprocal charge transport phenomena are studied theoretically for
two-dimensional noncentrosymmetric superconductors under an external magnetic
field $B$. Rashba superconductors, surface superconductivity on the surface of
three-dimensional topological insulators, and transition metal dichalcogenides
(TMD) are representative systems, and the current-voltage $I$-$V$
characteristics, i.e., $V=V(I)$, for each of them is analyzed. $V(I)$ can be
expanded with respect to the current $I$ as $V(I)= \sum_{j=1,\infty} a_j(B,T)
I^j$, and the $(B,T)$-dependence of $a_j$ depends on the mechanism of the
charge transport. Above the mean field transition temperature $T_0$, the
fluctuation of the superconducting order parameter gives the additional
conductivity, i.e., paraconductivity. Extending the analysis to the nonlinear
response, we obtain the nonreciprocal charge transport expressed by $a_2(B,T) =
a_1(T) \gamma(T) B$, where $\gamma$ converges to a finite value at $T=T_0$.
Below $T_0$, the vortex motion is relevant to the voltage drop, and the
dependence of $a_j$ on $B,T$ is different depending on the system and
mechanisms. For the superconductors under the in-plane magnetic field, the
Kosterlitz-Thouless (KT) transition occurs at $T_{\rm KT}$. In this case
$\gamma$ has the characteristic temperature dependences such as $\gamma \sim
(T-T_{\rm KT})^{-3/2}$ near $T_{\rm KT}$. On the other hand, for TMD with
out-plane magnetic field, the KT transition is gone, and there are two possible
mechanisms for the nonreciprocal response. One is the anisotropy of the damping
constant for the motion of the vortex. In this case, $a_1(B) \sim B$ and
$a_2(B) \sim B^2$. The other one is the ratchet potential acting on the vortex
motion, which gives $a_1(B) \sim B$ and $a_2(B) \sim B$. Based on these
results, we propose the experiments to identify the mechanism of the
nonreciprocal charge transport. | 1805.05735v1 |
2019-02-01 | The Neutrino Puzzle: Anomalies, Interactions, and Cosmological Tensions | New physics in the neutrino sector might be necessary to address anomalies
between different neutrino oscillation experiments. Intriguingly, it also
offers a possible solution to the discrepant cosmological measurements of $H_0$
and $\sigma_8$. We show here that delaying the onset of neutrino free-streaming
until close to the epoch of matter-radiation equality can naturally accommodate
a larger value for the Hubble constant $H_0=72.3 \pm 1.4$ km/s/Mpc and a lower
value of the matter fluctuations $\sigma_8=0.786\pm 0.020$, while not degrading
the fit to the cosmic microwave background (CMB) damping tail. We achieve this
by introducing neutrino self-interactions in the presence of a non-vanishing
sum of neutrino masses. This strongly interacting neutrino cosmology prefers
$N_{\rm eff} = 4.02 \pm 0.29$, which has interesting implications for particle
model-building and neutrino oscillation anomalies. We show that the absence of
the neutrino free-streaming phase shift on the CMB can be compensated by
shifting the value of other cosmological parameters, hence providing an
important caveat to the detections made in the literature. Due to their impact
on the evolution of the gravitational potential at early times,
self-interacting neutrinos and their subsequent decoupling leave a rich
structure on the matter power spectrum. In particular, we point out the
existence of a novel localized feature appearing on scales entering the horizon
at the onset of neutrino free-streaming. While the interacting neutrino
cosmology provides a better global fit to current cosmological data, we find
that traditional Bayesian analyses penalize the model as compared to the
standard cosmological. Our analysis shows that it is possible to find radically
different cosmological models that nonetheless provide excellent fits to the
data, hence providing an impetus to thoroughly explore alternate cosmological
scenarios. | 1902.00534v2 |
2019-01-31 | Analysis and active control of geometrically nonlinear responses of smart FG porous plates with graphene nanoplatelets reinforcement based on Bézier extraction of NURBS | In this paper, we propose an effective computational approach to analyze and
active control of geometrically nonlinear responses of functionally graded (FG)
porous plates with graphene nanoplatelets (GPLs) reinforcement integrated with
piezoelectric layers. The key concept behind this work is to utilize
isogeometric analysis (IGA) based on B\'ezier extraction technique and
$C^0$-type higher-order shear deformation theory ($C^0$-HSDT). By applying
B\'ezier extraction, the original Non-Uniform Rational B-Spline (NURBS) control
meshes can be transformed into B\'ezier elements which allow us to inherit the
standard numerical procedure like the standard finite element method (FEM). In
this scenario, the approximation of mechanical displacement field is calculated
via $C^0$-HSDT whilst the electric potential field is considered as a linear
function across the thickness of each piezoelectric sublayer. The FG plate
includes internal pores and GPLs dispersed into metal matrix either uniformly
or non-uniformly along plate's thickness. To control responses of structures,
the top and bottom surfaces of FG plate are firmly bonded with piezoelectric
layers which are considered as sensor and actuator layers. The geometrically
nonlinear equations are solved by Newton-Raphson iterative procedure and
Newmark's integration. The influence of porosity coefficient, weight fraction
of GPLs as well as external electrical voltage on geometrically nonlinear
behaviors of plate structures with various distributions of porosity and GPLs
are thoroughly investigated. A constant displacement and velocity feedback
control approaches are then adopted to actively control geometrically nonlinear
static and dynamic responses, where structural damping effect is taken into
account, based on a closed-loop control with sensor and actuator layers. | 1902.10806v2 |
2019-03-19 | Phase mixing of nonlinear Alfven waves | Aims: This paper presents 2.5D numerical experiments of Alfv\'en wave phase
mixing and aims to assess the effects of nonlinearities on wave behaviour and
dissipation. In addition, this paper aims to quantify how effective the model
presented in this work is at providing energy to the coronal volume.
Methods: The model is presented and explored through the use of several
numerical experiments which were carried out using the Lare2D code. The
experiments study footpoint driven Alfv\'en waves in the neighbourhood of a
two-dimensional x-type null point with initially uniform density and plasma
pressure. A continuous sinusoidal driver with a constant frequency is used.
Each experiment uses different driver amplitudes to compare weakly nonlinear
experiments with linear experiments.
Results: We find that the wave trains phase-mix owing to variations in the
length of each field line and variations in the field strength. The
nonlinearities reduce the amount of energy entering the domain, as they reduce
the effectiveness of the driver, but they have relatively little effect on the
damping rate (for the range of amplitudes studied). The nonlinearities produce
density structures which change the natural frequencies of the field lines and
hence cause the resonant locations to move. The shifting of the resonant
location causes the Poynting flux associated with the driver to decrease.
Reducing the magnetic diffusivity increases the energy build-up on the resonant
field lines, however, it has little effect on the total amount of energy
entering the system. From an order of magnitude estimate, we show that the
Poynting flux in our experiments is comparable to the energy requirements of
the quiet Sun corona. However a (possibly unphysically) large amount of
magnetic diffusion was used however and it remains unclear if the model is able
to provide enough energy under actual coronal conditions. | 1903.08093v1 |
2019-07-30 | Rapid Reionization by the Oligarchs: The Case for Massive, UV-Bright, Star-Forming Galaxies with High Escape Fractions | The protagonists of cosmic reionization remain elusive. Faint star-forming
galaxies are leading candidates because they are numerous and may have
significant ionizing photon escape fractions ($f_{esc}$). Here we update this
picture via an empirical model that successfully predicts latest observations
(e.g., the drop in star-formation density at z>8). We generate an ionizing
spectrum for each galaxy in our model and constrain $f_{esc}$ using latest
measurements of the reionization timeline (e.g., Ly$\alpha$ damping of quasars
and galaxies at z>7). Assuming a constant $f_{esc}$, we find $M_{UV}$<-13.5
galaxies need $f_{esc}=0.21^{+0.06}_{-0.04}$ to complete reionization. The
inferred IGM neutral fraction is [0.9, 0.5, 0.1] at z=[8.2, 6.8, 6.2]$\pm$0.2,
i.e., the bulk of reionization transpires in 300 Myrs. Inspired by the emergent
sample of Lyman Continuum (LyC) leakers that overwhelmingly displays
higher-than-average star-formation surface density ($\Sigma$), we propose a
model relating $f_{esc}$ to $\Sigma$ and find
$f_{esc}\propto\Sigma^{0.4\pm0.1}$. Since $\Sigma$ falls by ~2.5 dex between
z=8 and z=0, our model explains the humble upper limits on $f_{esc}$ at lower
redshifts and its required evolution to ~0.2 at z>6. Within this model,
strikingly, <5% of galaxies with $M_{UV}$<-18 (the `oligarchs') account for
>80% of the reionization budget. In fact, faint sources ($M_{UV}$>-16) must be
relegated to a limited role to ensure high neutral fractions at z=7-8. Shallow
faint-end slopes of the UV luminosity function ($\alpha$>-2) and/or $f_{esc}$
distributions skewed toward bright galaxies produce the required late and rapid
reionization. We predict LyC leakers like COLA1 (z=6.6, $f_{esc}$~30%,
$M_{UV}$=-21.5) become increasingly common towards z~6 and that the drivers of
reionization do not lie hidden across the faint-end of the luminosity function,
but are already known to us. (abridged) | 1907.13130v2 |
2019-11-04 | Planet-disk interaction in disks with cooling: basic theory | Gravitational coupling between young planets and their parent disks is often
explored using numerical simulations, which typically treat the disk
thermodynamics in a highly simplified manner. In particular, many studies adopt
the locally isothermal approximation, in which the disk temperature is a fixed
function of the stellocentric distance. We explore the dynamics of
planet-driven density waves in disks with more general thermodynamics, in which
the temperature is relaxed towards an equilibrium profile on a finite cooling
timescale $t_{\rm c}$. We use both linear perturbation theory and direct
numerical simulations to examine the global structure of density waves launched
by planets in such disks. A key diagnostic used in this study is the behavior
of the wave angular momentum flux (AMF), which directly determines the
evolution of the underlying disk. The AMF of free waves is constant for slowly
cooling (adiabatic) disks, but scales with the disk temperature for rapidly
cooling (and locally isothermal) disks. However, cooling must be extremely
fast, with $\beta = \Omega t_{\rm c} \lesssim 10^{-3}$ for the locally
isothermal approximation to provide a good description of density wave dynamics
in the linear regime (relaxing to $\beta \lesssim 10^{-2}$ when nonlinear
effects are important). For intermediate cooling timescales, density waves are
subject to a strong linear damping. This modifies the appearance of
planet-driven spiral arms and the characteristics of axisymmetric structures
produced by massive planets: in disks with $\beta \approx 0.1$ -- $1$, a
near-thermal mass planet opens only a single wide gap around its orbit, in
contrast to the several narrow gaps produced when cooling is either faster or
slower. | 1911.01428v2 |
2019-11-18 | Wave measurements from ship mounted sensors in the Arctic marginal ice zone | Increased research interest and economic activity in the Arctic raise the
need for new observations of sea ice dynamics. Remote sensing as well as
mathematical and numerical models of wave propagation in sea ice would benefit
from more in situ data for validation. This study presents wave measurements in
the marginal ice zone (MIZ) obtained from ship mounted sensors. The system
combines altimeter readings from the ship bow with ship motion correction data
to provide estimated single point ocean surface elevation. Significant wave
height and mean wave period, as well as one-dimensional wave spectra are
derived from the combined measurements. The results are compared with
integrated parameters from a spectral wave model over a period of eight days in
the open ocean, and with spectra and integrated parameters derived from motion
detecting instruments placed on ice floes inside the MIZ. Mean absolute errors
of the integrated parameters are in the range 15.0-18.9% when comparing with
the spectral wave model and 1.0-9.6% when comparing with valid motion detecting
instruments. The spatial wave damping coefficient is estimated by looking at
the change in spectral wave amplitude found at discrete frequency values as the
ship was moving along the longitudinal direction of the MIZ within time
intervals where the wave field is found to be approximately constant in time.
As expected from theory, high frequency waves are effectively dampened by the
presence of sea ice. The observed wave attenuation rates compare favourably
with a two-layer dissipation model. Our methodology can be regarded as a simple
and reliable way to collect more waves-in-ice data as it can be easily added to
any ship participating to ice expeditions, at little extra cost. | 1911.07612v2 |
2019-11-29 | Convection-dominated dissolution for single and multiple immersed sessile droplets | We numerically investigate both single and multiple droplet dissolution with
droplets consisting of lighter liquid dissolving in a denser host liquid. The
significance of buoyancy is quantified by the Rayleigh number Ra which is the
buoyancy force over the viscous damping force. In this study, Ra spans almost
four decades from 0.1 to 400. We focus on how the mass flux, characterized by
the Sherwood number Sh, and the flow morphologies depend on Ra. For single
droplet dissolution, we first show the transition of the Sh(Ra) scaling from a
constant value to $Sh\sim Ra^{1/4}$, which confirms the experimental results by
Dietrich et al. (J. Fluid Mech., vol. 794, 2016, pp. 45--67). The two distinct
regimes, namely the diffusively- and the convectively-dominated regime, exhibit
different flow morphologies: when Ra>=10, a buoyant plume is clearly visible
which contrasts sharply to the pure diffusion case at low Ra. For multiple
droplet dissolution, the well-known shielding effect comes into play at low Ra
so that the dissolution rate is slower as compared to the single droplet case.
However, at high Ra, convection becomes more and more dominant so that a
collective plume enhances the mass flux, and remarkably the multiple droplets
dissolve faster than a single droplet. This has also been found in the
experiments by Laghezza et al. (Soft Matter, vol. 12, 2016, pp. 5787--5796). We
explain this enhancement by the formation of a single, larger plume rather than
several individual plumes. Moreover, there is an optimal Ra at which the
enhancement is maximized, because the single plume is narrower at larger Ra,
which thus hinders the enhancement. Our findings demonstrate a new mechanism in
collective droplet dissolution, which is the merging of the plumes, that leads
to non-trivial phenomena, contrasting the shielding effect. | 1911.13040v1 |
2020-02-26 | The Magnetized Vlasov-Ampère system and the Bernstein-Landau paradox | We study the Bernstein-Landau paradox in the collisionless motion of an
electrostatic plasma in the presence of a constant external magnetic field. The
Bernstein-Landau paradox consists in that in the presence of the magnetic
field, the electric field and the charge density fluctuation have an
oscillatory behavior in time. This is radically different from Landau damping,
in the case without magnetic field, where the electric field tends to zero for
large times. We consider this problem from a new point of view. Instead of
analyzing the linear magnetized Vlasov-Poisson system, as it is usually done,
we study the linear magnetized Vlasov-Amp\`ere system. We formulate the
magnetized Vlasov-Amp\`ere system as a Schr\"odinger equation with a
selfadjoint magnetized Vlasov-Amp\`ere operator in the Hilbert space of states
with finite energy. The magnetized Vlasov-Amp\`ere operator has a complete set
of orthonormal eigenfunctions, that include the Bernstein modes. The expansion
of the solution of the magnetized Vlasov-Amp\`ere system in the eigenfunctions
shows the oscillatory behavior in time. We prove the convergence of the
expansion under optimal conditions, assuming only that the initial state has
finite energy. This solves a problem that was recently posed in the literature.
The Bernstein modes are not complete. To have a complete system it is necessary
to add eigenfunctions that are associated with eigenvalues at all the integer
multiples of the cyclotron frequency. These special plasma oscillations
actually exist on their own, without the excitation of the other modes. In the
limit when the magnetic fields goes to zero the spectrum of the magnetized
Vlasov-Amp\`ere operator changes drastically from pure point to absolutely
continuous in the orthogonal complement to its kernel, due to a sharp change on
its domain. This explains the Bernstein-Landau paradox. | 2002.11380v3 |
2020-07-24 | Convective turbulent viscosity acting on equilibrium tidal flows: new frequency scaling of the effective viscosity | Turbulent convection is thought to act as an effective viscosity ($\nu_E$) in
damping tidal flows in stars and giant planets. However, the efficiency of this
mechanism has long been debated, particularly in the regime of fast tides, when
the tidal frequency ($\omega$) exceeds the turnover frequency of the dominant
convective eddies ($\omega_c$). We present the results of hydrodynamical
simulations to study the interaction between tidal flows and convection in a
small patch of a convection zone. These simulations build upon our prior work
by simulating more turbulent convection in larger horizontal boxes, and here we
explore a wider range of parameters. We obtain several new results: 1) $\nu_E$
is frequency-dependent, scaling as $\omega^{-0.5}$ when $\omega/\omega_c
\lesssim 1$, and appears to attain its maximum constant value only for very
small frequencies ($\omega/\omega_c \lesssim 10^{-2}$). This
frequency-reduction for low frequency tidal forcing has never been observed
previously. 2) The frequency-dependence of $\nu_E$ appears to follow the same
scaling as the frequency spectrum of the energy (or Reynolds stress) for low
and intermediate frequencies. 3) For high frequencies ($\omega/\omega_c\gtrsim
1-5$), $\nu_E\propto \omega^{-2}$. 4) The energetically-dominant convective
modes always appear to contribute the most to $\nu_E$, rather than the resonant
eddies in a Kolmogorov cascade. These results have important implications for
tidal dissipation in convection zones of stars and planets, and indicate that
the classical tidal theory of the equilibrium tide in stars and giant planets
should be revisited. We briefly touch upon the implications for planetary
orbital decay around evolving stars. | 2007.12624v1 |
2020-10-05 | Cryogenic suspension design for a kilometer-scale gravitational-wave detector | We report the mirror suspension design for Large-scale Cryogenic
Gravitational wave Telescope, KAGRA, during bKAGRA Phase 1. Mirror thermal
noise is one of the fundamental noises for room-temperature gravitational-wave
detectors such as Advanced LIGO and Advanced Virgo. Thus, reduction of thermal
noise is required for further improvement of their sensitivity. One effective
approach for reducing thermal noise is to cool the mirrors. There are many
technical challenges that must be overcome to cool the mirrors, such as
cryocooler induced vibrations, thermal drift in suspensions, and reduction in
duty cycling due to the increased number of potential failure mechanisms. Our
mirror suspension has a black coating that makes radiative cooling more
efficient. For conduction cooling, we developed ultra high purity aluminum heat
links, which yield high thermal conductivity while keeping the spring constant
sufficiently small. A unique inclination adjustment system, called moving mass,
is used for aligning the mirror orientation in pitch. Photo-reflective
displacement sensors, which have a large range, are installed for damping
control on marionette recoil mass and intermediate recoil mass. Samarium cobalt
magnets are used for coil-magnet actuators to prevent significant change of
magnetism between room temperature and cryogenic temperature. In this paper,
the design of our first cryogenic payload and its performance during bKAGRA
Phase 1 are discussed. | 2010.01889v2 |
2021-01-21 | A Gauss-Seidel projection method with the minimal number of updates for stray field in micromagnetic simulations | Magnetization dynamics in magnetic materials is often modeled by the
Landau-Lifshitz equation, which is solved numerically in general. In
micromagnetic simulations, the computational cost relies heavily on the
time-marching scheme and the evaluation of stray field. Explicit marching
schemes are efficient but suffer from severe stability constraints, while
nonlinear systems of equations have to be solved in implicit schemes though
they are unconditionally stable. A better compromise between stability and
efficiency is the semi-implicit scheme, such as the Gauss-Seidel projection
method (GSPM) and the second-order backward differentiation formula scheme
(BDF2). At each marching step, GSPM solves several linear systems of equations
with constant coefficients and updates the stray field several times, while
BDF2 updates the stray field only once but solves a larger linear system of
equations with variable coefficients and a nonsymmetric structure. In this
work, we propose a new method, dubbed as GSPM-BDF2, by combing the advantages
of both GSPM and BDF2. Like GSPM, this method is first-order accurate in time
and second-order accurate in space, and is unconditionally stable with respect
to the damping parameter. However, GSPM-BDF2 updates the stray field only once
per time step, leading to an efficiency improvement of about $60\%$ than the
state-of-the-art GSPM for micromagnetic simulations. For Standard Problem \#4
and \#5 from National Institute of Standards and Technology, GSPM-BDF2 reduces
the computational time over the popular software OOMMF by $82\%$ and $96\%$,
respectively. Thus, the proposed method provides a more efficient choice for
micromagnetic simulations. | 2101.08574v1 |
2021-01-29 | Radiative Poincare type eon and its follower | We consider two consecutive eons $\hat{M}$ and $\check{M}$ from Penrose's
Conformal Cyclic Cosmology and study how the matter content of the past eon
($\hat{M}$) determines the matter content of the present eon ($\check{M}$) by
means of the reciprocity hypothesis. We assume that the only matter content in
the final stages of the past eon is a spherical wave described by Einstein's
equations with the pure radiation energy momentum tensor $$\hat{T}^{ij} =
\hat{\Phi}K^iK^j, \quad \hat{g}_{ij} K^iK^j = 0,$$ and with cosmological
constant $\hat{\Lambda}$ . We solve these Einstein's equations associating to
$\hat{M}$ the metric $\hat{g}=t^{-2}\big(-d t^2+h_t\big)$, which is a
Lorentzian analog of the Poincar\'e-Einstein metric known from the theory of
conformal invariants. The solution is obtained under the assumption that the
3-dimensional conformal structure $[h]$ on the $\mathscr{I}^+$ of $\hat{M}$ is
flat, that the metric $\hat{g}$ admits a power series expansion in the time
variable $t$, and that $h_0\in [h]$. Such solution depends on one real
arbitrary function of the radial variable $r$. Applying the reciprocal
hypothesis, $\hat{g}\to \check{g}=t^4\hat{g}$, we show that the new eon
$(\check{M},\check{g})$ created from the one containing a single spherical
wave, is filled at its initial state with three types of radiation: (i) the
damped spherical wave which continues its life from the previous eon, (ii) the
in-going spherical wave obtained as a result of a collision of the wave from
the past eon with the Bang hypersurface and (3) randomly scattered waves that
could be interpreted as perfect fluid with the energy density $\check{\rho}$
and the isotropic pressure $\check{p}$ such that
$\check{p}=\tfrac13\check{\rho}$. | 2101.12670v2 |
2021-03-24 | Magnetism and Spin Dynamics in Room-Temperature van der Waals Magnet Fe$_5$GeTe$_2$ | Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of
attention recently. However, the majority of these materials have Curie
temperatures that are well below room temperature, making it challenging to
incorporate them into device applications. In this work, we synthesized a
room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature
T$_c = 332$ K, and studied its magnetic properties by vibrating sample
magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy.
The experiments were performed with external magnetic fields applied along the
c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures
ranging from 300 K to 10 K. We have found a sizable Land\'e g-factor difference
between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Land\'e
g-factor values deviated from g = 2. This indicates contribution of orbital
angular momentum to the magnetic moment. The FMR measurements reveal that
Fe$_5$GeTe$_2$ has a damping constant comparable to Permalloy. With reducing
temperature, the linewidth was broadened. Together with the VSM data, our
measurements indicate that Fe$_5$GeTe$_2$ transitions from ferromagnetic to
ferrimagnetic at lower temperatures. Our experiments highlight key information
regarding the magnetic state and spin scattering processes in Fe$_5$GeTe$_2$,
which promote the understanding of magnetism in Fe$_5$GeTe$_2$, leading to
implementations of Fe$_5$GeTe$_2$ based room-temperature spintronic devices. | 2103.13433v2 |
2021-07-25 | High-overtone fits to numerical relativity ringdowns: beyond the dismissed n=8 special tone | In general relativity, the remnant object originating from an uncharged black
hole merger is a Kerr black hole. The approach to this final state is reached
through the emission of a late train of radiation known as the black hole
ringdown. The ringdown morphology is described by a countably infinite set of
damped sinusoids, whose complex frequencies are solely determined by the final
black hole's mass and spin. Recent results advocate that ringdown waveforms
from numerical relativity can be fully described from the peak of the strain
onwards if quasi-normal mode models with $N_{max}=7$ overtones are used. In
this work we extend this analysis to models with $N_{max}\geq 7$ up to
$N_{max}=16$ overtones by exploring the parameter bias on the final mass and
final spin obtained by fitting the nonprecessing binary black hole simulations
from the SXS catalogue. To this aim, we have computed the spin weight $-2$
quasi-normal mode frequencies and angular separation constants for the special
$(l=m=2, n=8,9)$ overtones for the Kerr spacetime. We find that a total of
$N_{max}\sim 6$ overtones are on average sufficient to model the ringdown
starting at the peak of the strain, although about $21\%$ of the cases studied
require at least $N_{max}\sim 12$ overtones to reach a comparable accuracy on
the final state parameters. Considering the waveforms from an earlier or later
point in time, we find that a very similar maximum accuracy can be reached in
each case, occurring at a different number of overtones $N_{max}$. We provide
new error estimates for the SXS waveforms based on the extrapolation and the
resolution uncertainties of the gravitational wave strain. Finally, we observe
substantial instabilities on the values of the best-fit amplitudes of the tones
beyond the fundamental mode and the first overtone, that, nevertheless, do not
impact significantly the mass and spin estimates. | 2107.11829v2 |
2021-12-23 | Real-time methods for spectral functions | In this paper we develop and compare different real-time methods to calculate
spectral functions. These are classical-statistical simulations, the Gaussian
state approximation (GSA), and the functional renormalization group (FRG)
formulated on the Keldysh closed-time path. Our test-bed system is the quartic
anharmonic oscillator, a single self-interacting bosonic degree of freedom,
coupled to an external heat bath providing dissipation analogous to the
Caldeira-Leggett model. As our benchmark we use the spectral function from
exact diagonalization with constant Ohmic damping. To extend the GSA for the
open system, we solve the corresponding Heisenberg-Langevin equations in the
Gaussian approximation. For the real-time FRG, we introduce a novel general
prescription to construct causal regulators based on introducing
scale-dependent fictitious heat baths. Our results explicitly demonstrate how
the discrete transition lines of the quantum system gradually build up the
broad continuous structures in the classical spectral function as temperature
increases. At sufficiently high temperatures, classical, GSA and
exact-diagonalization results all coincide. The real-time FRG is able to
reproduce the effective thermal mass, but overestimates broadening and only
qualitatively describes higher excitations, at the present order of our
combined vertex and loop expansion. As temperature is lowered, the GSA follows
the ensemble average of the exact solution better than the classical spectral
function. In the low-temperature strong-coupling regime, the qualitative
features of the exact result are best captured by our real-time FRG
calculation, with quantitative improvements to be expected at higher truncation
orders. | 2112.12568v3 |
2022-01-03 | Exact scalar (quasi-)normal modes of black holes and solitons in gauged SUGRA | In this paper we identify a new family of black holes and solitons that lead
to the exact integration of scalar probes, even in the presence of a
non-minimal coupling with the Ricci scalar which has a non-trivial profile. The
backgrounds are planar and spherical black holes as well as solitons of
$SU\left( 2\right) \times SU\left( 2\right) $ $\mathcal{N}=4$ gauged
supergravity in four dimensions. On these geometries, we compute the spectrum
of (quasi-)normal modes for the non-minimally coupled scalar field. We find
that the equation for the radial dependence can be integrated in terms of
hypergeometric functions leading to an exact expression for the frequencies.
The solutions do not asymptote to a constant curvature spacetime, nevertheless
the asymptotic region acquires an extra conformal Killing vector. For the black
hole, the scalar probe is purely ingoing at the horizon, and requiring that the
solutions lead to an extremum of the action principle we impose a Dirichlet
boundary condition at infinity. Surprisingly, the quasinormal modes do not
depend on the radius of the black hole, therefore this family of geometries can
be interpreted as isospectral in what regards to the wave operator
non-minimally coupled to the Ricci scalar. We find both purely damped modes, as
well as exponentially growing unstable modes depending on the values of the
non-minimal coupling parameter. For the solitons we show that the same
integrability property is achieved separately in a non-supersymmetric solutions
as well as for the supersymmetric one. Imposing regularity at the origin and a
well defined extremum for the action principle we obtain the spectra that can
also lead to purely oscillatory modes as well as to unstable scalar probes,
depending on the values of the non-minimal coupling. | 2201.00438v1 |
2022-06-02 | Axion dark matter from frictional misalignment | We study the impact of sphaleron-induced thermal friction on the axion
dark-matter abundance due to the interaction of an axion-like particle (ALP)
with a dark non-abelian gauge sector in a secluded thermal bath. Thermal
friction can either enhance the axion relic density by delaying the onset of
oscillations or suppress it by damping them. We derive an analytical formula
for the \emph{frictional adiabatic invariant}, which remains constant along the
axion evolution and which allows us to compute the axion relic density in a
general set-up. Even in the most minimal scenario, in which a single gauge
group is responsible for both the generation of the ALP mass and the friction
force, we find that the resulting dark-matter abundance from the misalignment
mechanism deviates from the standard scenario for axion masses $m_a\gtrsim 100
\; {\rm eV}$. We also generalize our analysis to the case where the gauge field
that induces friction and the gauge sector responsible for the ALP mass are
distinct and their couplings to the axion have a large hierarchy as can be
justified by means of alignment or clockwork scenarios. We find that it is easy
to open up the ALP parameter space where the resulting axion abundance matches
the observed dark-matter relic density both in the traditionally over- and
underabundant regimes. This conclusion also holds for the QCD axion. | 2206.01129v3 |
2022-06-11 | Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet Na2Co2TeO6: 23Na NMR study | We report on the results of 23Na NMR in the honeycomb lattice magnet
Na2Co2TeO6 which has been nominated as a Kitaev material. Measurements of
magnetic shift and width of the NMR line as functions of temperature and
magnetic field show that a spin-disordered phase does not appear up to a field
of 9 T. In the antiferromagnetic phase just below the Neel temperature TN, we
find a temperature region extending down to ~TN/2 where the nuclear
spin-lattice relaxation rate 1/T1 remains enhanced and is further increased by
a magnetic field. This region crosses over to a low temperature region
characterized by the rapidly decreasing 1/T1 which is less field-sensitive.
These observations suggest incoherent spin excitations with a large spectral
weight at low energies in the intermediate temperature region transforming to
more conventional spin-wave excitations at low temperatures. The drastic change
of the low-energy spin dynamics is likely caused by strong damping of spin
waves activated only in the intermediate temperature region, which may be
realized for triple-q magnetic order possessing partially-disordered moments as
scattering centers of spin waves. In the paramagnetic phase near TN, dramatic
field suppression of 1/T1 is observed. From analysis of the temperature
dependence of 1/T1 based on the renormalized-classical description of a
two-dimensional quantum antiferromagnet, we find the field-dependent spin
stiffness constant that scales with TN as a function of magnetic field. This
implies field suppression of the energy scale characterizing both
two-dimensional spin correlations and three-dimensional long-range order, which
may be associated with an increasing effect of frustration in magnetic fields. | 2206.05409v3 |
2022-07-05 | Emergence of a new HI 21-cm absorption component at z~1.1726 towards the gamma-ray blazar PKS~2355-106 | We report the emergence of a new HI 21-cm absorption at z_abs = 1.172635 in
the damped Lyman-alpha absorber (DLA) towards the gamma-ray blazar PKS 2355-106
(z_em~1.639) using science verification observations (June 2020) from the
MeerKAT Absorption Line Survey (MALS). Since 2006, this DLA is known to show a
narrow HI 21-cm absorption at z_abs = 1.173019 coinciding with a distinct metal
absorption line component. We do not detect significant HI 21-cm optical depth
variations from this known HI component. A high resolution optical spectrum
(August 2010) shows a distinct Mg I absorption at the redshift of the new HI
21-cm absorber. However, this component is not evident in the profiles of
singly ionized species. We measure the metallicity ([Zn/H] = -(0.77\pm0.11) and
[Si/H]= -(0.96\pm0.11)) and depletion ([Fe/Zn] = -(0.63\pm0.16)) for the full
system. Using the apparent column density profiles of Si II, Fe II and Mg I we
show that the depletion and the N(Mg I)/N(Si II) column density ratio
systematically vary across the velocity range. The region with high depletion
tends to have slightly larger N(Mg I)/N(Si II) ratio. The two HI 21-cm
absorbers belong to this velocity range. The emergence of z_abs = 1.172635 can
be understood if there is a large optical depth gradient over a length scale of
~0.35 pc. However, the gas producing the z_abs = 1.173019 component must be
nearly uniform over the same scale. Systematic uncertainties introduced by the
absorption line variability has to be accounted for in experiments measuring
the variations of fundamental constants and cosmic acceleration even when the
radio emission is apparently compact as in PKS 2355-106. | 2207.01807v1 |
2022-07-29 | Global spherically symmetric solutions to degenerate compressible Navier-Stokes equations with large data and far field vacuum | We consider the initial-boundary value problem (IBVP) for the isentropic
compressible Navier-Stokes equations (\textbf{CNS}) in the domain exterior to a
ball in $\mathbb R^d$ $(d=2\ \text{or} \ 3)$. When viscosity coefficients are
given as a constant multiple of the mass density $\rho$, based on some analysis
of the nonlinear structure of this system, we prove the global existence of the
unique spherically symmetric classical solution for (large) initial data with
spherical symmetry and far field vacuum in some inhomogeneous Sobolev spaces.
Moreover, the solutions we obtained have the conserved total mass and finite
total energy. $\rho$ keeps positive in the domain considered but decays to zero
in the far field, which is consistent with the facts that the total mass is
conserved, and \textbf{CNS} is a model of non-dilute fluids where $\rho$ is
bounded away from the vacuum. To prove the existence, on the one hand, we
consider a well-designed reformulated structure by introducing some new
variables, which, actually, can transfer the degeneracies of the time evolution
and the viscosity to the possible singularity of some special source terms. On
the other hand, it is observed that, for the spherically symmetric flow, the
radial projection of the so-called effective velocity $\boldsymbol{v} =U+\nabla
\varphi(\rho)$ ($U$ is the velocity of the fluid, and $\varphi(\rho)$ is a
function of $\rho$ defined via the shear viscosity coefficient $\mu(\rho)$:
$\varphi'(\rho)=2\mu(\rho)/\rho^2$), verifies a damped transport equation which
provides the possibility to obtain its upper bound. Then combined with the BD
entropy estimates, one can obtain the required uniform a priori estimates of
the solution. It is worth pointing out that the frame work on the
well-posedness theory established here can be applied to the shallow water
equations. | 2207.14494v1 |
2022-10-13 | The Planck clusters in the LOFAR sky. III. LoTSS-DR2: Dynamic states and density fluctuations of the intracluster medium | The footprint of LoTSS-DR2 covers 309 PSZ2 galaxy clusters, 83 of which host
a radio halo and 26 host a radio relic(s). It provides us an excellent
opportunity to statistically study the properties of extended cluster radio
sources, especially their connection with merging activities. We aim to
quantify cluster dynamic states to investigate their relation with the
occurrence of extended radio sources. We also search for connections between
intracluster medium (ICM) turbulence and nonthermal characteristics of radio
halos in the LoTSS-DR2. We analyzed XMM-Newton and Chandra archival X-ray data
and computed concentration parameters and centroid shifts that indicate the
dynamic states of the clusters. We also performed a power spectral analysis of
the X-ray surface brightness (SB) fluctuations to investigate large-scale
density perturbations and estimate the turbulent velocity dispersion. The power
spectral analysis results in a large scatter density fluctuation amplitude. We
therefore only found a marginal anticorrelation between density fluctuations
and cluster relaxation state, and we did not find a correlation between density
fluctuations and radio halo power. Nevertheless, the injected power for
particle acceleration calculated from turbulent dissipation is correlated with
the radio halo power, where the best-fit unity slope supports the turbulent
(re)acceleration scenario. Two different acceleration models, transit-time
damping and adiabatic stochastic acceleration, cannot be distinguished due to
the large scatter of the estimated turbulent Mach number. We introduced a new
quantity $[kT\cdot Y_X]_{r_\mathrm{RH}}$, which is proportional to the
turbulent acceleration power assuming a constant Mach number. This quantity is
strongly correlated with radio halo power, where the slope is also unity. | 2210.07284v1 |
2022-12-12 | Modified propagation of gravitational waves from the early radiation era | We study the propagation of cosmological gravitational wave (GW) backgrounds
from the early radiation era until the present day in modified theories of
gravity. Comparing to general relativity (GR), we study the effects that
modified gravity parameters, such as the GW friction $\alpha_{\rm M}$ and the
tensor speed excess $\alpha_{\rm T}$, have on the present-day GW spectrum. We
use both the WKB estimate, which provides an analytical description but fails
at superhorizon scales, and numerical simulations that allow us to go beyond
the WKB approximation. We show that a constant $\alpha_{\rm T}$ makes
relatively insignificant changes to the GR solution, especially taking into
account the constraints on its value from GW observations by the LIGO--Virgo
collaboration, while $\alpha_{\rm M}$ can introduce modifications to the
spectral slopes of the GW energy spectrum in the low-frequency regime depending
on the considered time evolution of $\alpha_{\rm M}$. The latter effect is
additional to the damping or growth occurring equally at all scales that can be
predicted by the WKB approximation. In light of the recent observations by
pulsar timing array (PTA) collaborations, and the potential observations by
future detectors such as SKA, LISA, DECIGO, BBO, or ET, we show that, in most
of the cases, constraints cannot be placed on the effects of $\alpha_{\rm M}$
and the initial GW energy density $\mathcal{E}_{\rm GW}^*$ separately, but only
on the combined effects of the two, unless the signal is observed at different
frequency ranges. In particular, we provide some constraints on the combined
effects from the reported PTA observations. | 2212.06082v3 |
2023-02-01 | Post-dynamical inspiral phase of common envelope evolution: Binary orbit evolution and angular momentum transport | After the companion dynamically plunges through the primary's envelope, the
two cores remain surrounded by a common envelope and the decrease of the
orbital period $P_\text{orb}$ stalls. The subsequent evolution has never been
systematically explored with multidimensional simulations. For this study, we
performed 3D hydrodynamical simulations of an envelope evolving under the
influence of a central binary star using an adaptively refined spherical grid.
We followed the evolution over hundreds of orbits of the central binary to
characterize the transport of angular momentum by advection, gravitational
torques, turbulence, and viscosity. We find that local advective torques from
the mean flow and Reynolds stresses associated with the turbulent flow dominate
the angular momentum transport, which occurs outward in a disk-like structure
about the orbital plane and inward along the polar axis. Turbulent transport is
less efficient, but can locally significantly damp or enhance the net angular
momentum radial transport and may even reverse its direction. Short-term
variability in the envelope is remarkably similar to circumbinary disks,
including the formation and destruction of lump-like overdensities, which
enhance mass accretion and contribute to the outward transport of eccentricity
generated in the vicinity of the binary. If the accretion onto the binary is
allowed, the orbital decay timescale settles to a nearly constant value
$\tau_\text{b} \sim 10^3$ to $10^4\,P_\text{orb}$, while preventing accretion
leads to a slowly increasing $\tau_\text{b} \sim 10^5\,P_\text{orb}$ at the end
of our simulations. Our results suggest that the post-dynamical orbital
contraction and envelope ejection will slowly continue while the binary is
surrounded by gas and that $\tau_\text{b}$ is often much shorter than the
thermal timescale of the envelope. | 2302.00691v2 |
2023-04-24 | Magnetic levitation by rotation | A permanent magnet can be levitated simply by placing it in the vicinity of
another permanent magnet that rotates in the order of 200 Hz. This surprising
effect can be easily reproduced in the lab with off-the-shelf components. Here
we investigate this novel type of magnetic levitation experimentally and
clarify the underlying physics. Using a 19 mm diameter spherical NdFeB magnet
as rotor magnet, we capture the detailed motion of levitating, spherical NdFeB
magnets, denoted floater magnets. We find that as levitation occurs, the
floater magnet frequency-locks with the rotor magnet, and, noticeably, that the
magnetization of the floater is oriented close to the axis of rotation and
towards the like pole of the rotor magnet. This is in contrast to what might be
expected by the laws of magnetostatics as the floater is observed to align its
magnetization essentially perpendicular to the magnetic field of the rotor.
Moreover, we find that the size of the floater has a clear influence on the
levitation: the smaller the floater, the higher the rotor speed necessary to
achieve levitation, and the further away the levitation point shifts. We verify
that magnetostatic interactions between the rotating magnets are responsible
for creating the equilibrium position of the floater. Hence, this type of
magnetic levitation does not rely on gravity as a balancing force to achieve an
equilibrium position. Based on theoretical arguments and a numerical model, we
show that a constant, vertical field and eddy-current enhanced damping is
sufficient to produce levitation from rest. This enables a gyroscopically
stabilised counter-intuitive steady-state moment orientation, and the resulting
magnetostatically stable, mid-air equilibrium point. The numerical model
display the same trends with respect to rotation speed and the floater magnet
size as seen in the experiments. | 2305.00812v3 |
2023-07-26 | Formulation and Implementation of Frequency-Dependent Linear Response Properties with Relativistic Coupled Cluster Theory for GPU-accelerated Computer Architectures | We present the development and implementation of the relativistic coupled
cluster linear response theory (CC-LR) which allows the determination of
molecular properties arising from time-dependent or time-independent electric,
magnetic, or mixed electric-magnetic perturbations (within a common gauge
origin), and take into account the finite lifetime of excited states via damped
response theory. We showcase our implementation, which is capable to offload
intensive tensor contractions onto graphical processing units (GPUs), in the
calculation of: \textit{(a)} frequency-(in)dependent dipole-dipole
polarizabilities of IIB atoms and selected diatomic molecules, with a emphasis
on the calculation of valence absorption cross-sections for the I$_2$
molecule;\textit{(b)} indirect spin-spin coupling constants for benchmark
systems such as the hydrogen halides (HX, X = F-I) as well the H$_2$Se-H$_2$O
dimer as a prototypical system containing hydrogen bonds; and \textit{(c)}
optical rotations at the sodium D line for hydrogen peroxide analogues
(H$_{2}$Y$_{2}$, Y=O, S, Se, Te). Thanks to this implementation, we are able
show the similarities in performance--but often the significant
discrepancies--between CC-LR and approximate methods such as density functional
theory (DFT). Comparing standard CC response theory with the equation of motion
formalism, we find that, for valence properties such as polarizabilities, the
two frameworks yield very similar results across the periodic table as found
elsewhere in the literature; for properties that probe the core region such as
spin-spin couplings, we show a progressive differentiation between the two as
relativistic effects become more important. Our results also suggest that as
one goes down the periodic table it may become increasingly difficult to
measure pure optical rotation at the sodium D line, due to the appearance of
absorbing states. | 2307.14296v2 |
2023-09-22 | Challenges in Quasinormal Mode Extraction: Perspectives from Numerical solutions to the Teukolsky Equation | The intricacies of black hole ringdown analysis are amplified by the absence
of a complete set of orthogonal basis functions for quasinormal modes. Although
damped sinusoids effectively fit the ringdown signals from binary black hole
mergers, the risk of overfitting remains, due to initial transients and
nonlinear effects. In light of this challenge, we introduce two methods for
extracting quasinormal modes in numerical simulations and qualitatively study
how the transient might affect quasinormal mode fitting. In one method, we
accurately fit quasinormal modes by using their spatial functional form at
constant time hypersurfaces, while in the other method, we exploit both spatial
and temporal aspects of the quasinormal modes. Both fitting methods leverage
the spatial behavior of quasinormal eigenfunctions to enhance accuracy,
outperforming conventional time-only fitting techniques at null infinity. We
also show that we can construct an inner product for which the quasinormal
eigenfunctions form an orthonormal (but not complete) set. We then conduct
numerical experiments involving linearly perturbed Kerr black holes in horizon
penetrating, hyperboloidally compactified coordinates, as this setup enables a
more precise isolation and examination of the ringdown phenomenon. From
solutions to the Teukolsky equation, describing scattering of an ingoing
gravitational wave pulse, we find that the contributions from early-time
transients can lead to large uncertainties in the fit to the amplitudes of
higher overtones ($n\geq 3$). While the methods we discuss here cannot be
applied directly to data from merger observations, our findings underscore the
persistence of ambiguities in interpreting ringdown signals, even with access
to both temporal and spatial information. | 2309.13204v3 |
2023-09-25 | Influence of density and viscosity on deformation, breakage, and coalescence of bubbles in turbulence | We investigate the effect of density and viscosity differences on a swarm of
large and deformable bubbles dispersed in a turbulent channel flow. For a given
shear Reynolds number, Re=300, and a constant bubble volume fraction, Phi=5.4%,
we perform a campaign of direct numerical simulations of turbulence coupled
with a phase-field method accounting for interfacial phenomena. For each
simulation, we vary the Weber number (We, ratio of inertial to surface tension
forces), the density ratio (r, ratio of bubble density to carrier flow density)
and the viscosity ratio (e, ratio of bubble viscosity to carrier flow
viscosity). Specifically, we consider two Weber numbers, We=1.50 and We=3.00,
four density ratios, from r=1 down to r=0.001, and five viscosity ratios, from
e=0.01 up to e=100. Our results show that density differences have a negligible
effect on breakage and coalescence phenomena, while a much stronger effect is
observed when changing the viscosity of the two phases. Increasing the bubble
viscosity with respect to the carrier fluid viscosity damps turbulence
fluctuations, makes the bubble more rigid, and strongly prevents large
deformations, thus reducing the number of breakage events. Local deformations
of the interface, on the contrary, depend on both density and viscosity ratios.
The opposite effect is observed for increasing bubble viscosities. We report
that these effects are mostly visible for larger Weber numbers, where surface
forces are weaker. Finally, we characterize the flow inside the bubbles; as the
bubble density is increased, we observe, as expected, an increase in the
turbulent kinetic energy (TKE) inside the bubble, while as the bubble viscosity
is increased, we observe a mild reduction of the TKE inside the bubble and a
strong suppression of turbulence. | 2309.13995v1 |
2023-11-24 | Black hole spectroscopy beyond Kerr: agnostic and theory-based tests with next-generation interferometers | Black hole spectroscopy is a clean and powerful tool to test gravity in the
strong-field regime and to probe the nature of compact objects. Next-generation
ground-based detectors, such as the Einstein Telescope and Cosmic Explorer,
will observe thousands of binary black hole mergers with large signal-to-noise
ratios, allowing for accurate measurements of the remnant black hole
quasinormal mode frequencies and damping times. In previous work we developed
an observable-based parametrization of the quasinormal mode spectrum of
spinning black holes beyond general relativity (ParSpec). In this paper we use
this parametrization to ask: can next-generation detectors detect or constrain
deviations from the Kerr spectrum by stacking multiple observations of binary
mergers from astrophysically motivated populations? We focus on two families of
tests: (i) agnostic (null) tests, and (ii) theory-based tests, which make use
of quasinormal frequency calculations in specific modified theories of gravity.
We consider in particular two quadratic gravity theories
(Einstein-scalar-Gauss-Bonnet and dynamical Chern-Simons gravity) and various
effective field theory-based extensions of general relativity. We find that
robust inference of hypothetical corrections to general relativity requires
pushing the slow-rotation expansion to high orders. Even when high-order
expansions are available, ringdown observations alone may not be sufficient to
measure deviations from the Kerr spectrum for theories with dimensionful
coupling constants. This is because the constraints are dominated by "light"
black hole remnants, and only few of them have sufficiently high
signal-to-noise ratio in the ringdown. Black hole spectroscopy with
next-generation detectors may be able to set tight constraints on theories with
dimensionless coupling, as long as we assume prior knowledge of the mass and
spin of the remnant black hole. | 2311.14803v3 |
2024-03-16 | Elasto-visco-plastic flows in benchmark geometries: I. 4 to 1 Planar Contraction | We present predictions for the flow of elastoviscoplastic (EVP) fluids in the
4 to 1 planar contraction geometry. The Saramito-Herschel-Bulkley fluid model
is solved via the finite-volume method with the OpenFOAM software. Both the
constitutive model and the solution method require using transient simulations.
In this benchmark geometry, whereas viscoelastic fluids may exhibit two
vortices, referred to as lip and corner vortices, we find that EVP materials
are unyielded in the concave corners. They are also unyielded along the
mid-plane of both channels, but not around the contraction area where all
stress components are larger. When the Bingham or the Weissenberg numbers are
lower than critical values, and then, a steady state is reached. When these two
dimensionless numbers increase while they remain below the respective critical
values, which are interdependent, (a) the unyielded regions expand and shift in
the flow direction, and (b) the maximum velocity increases at the entrance of
the contraction. Increasing material elasticity collaborates with increasing
the yield stress, which expands the unyielded areas, because it deforms the
material more prior to yielding compared to stiffer materials. Above the
critical Weissenberg number, transient variations appear for longer times in
all variables, including the yield surface, instead of a monotonic approach to
the steady state. They may lead to oscillations which are damped or of constant
amplitude or approach a flow with rather smooth path lines but complex stress
field without a plane of symmetry, under creeping conditions. These patterns
arise near the entrance of the narrow channel, where the curvature of the path
lines is highest and its coupling with the increased elasticity triggers a
purely elastic instability. Similarly, a critical value of the yield stress
exists above which such phenomena are predicted. | 2403.10890v1 |
1994-05-02 | Damped Lyman Alpha Systems vs. Cold + Hot Dark Matter | Although the Cold + Hot Dark Matter (CHDM) cosmology provides perhaps the
best fit of any model to all the available data at the current epoch, CHDM
produces structure at relatively low redshifts and thus could be ruled out if
there were evidence for formation of massive objects at high redshifts. Damped
Ly$\alpha$ systems are abundant in quasar absorption spectra and thus provide
possibly the most significant evidence for early structure formation, and thus
perhaps the most stringent constraint on CHDM. Using the numbers of halos in
N-body simulations to normalize Press-Schechter estimates of the number
densities of protogalaxies as a function of redshift, we find that CHDM with
$\Omega_c/\Omega_\nu/\Omega_b = 0.6/0.3/0.1$ is compatible with the damped
Ly$\alpha$ data at $\le 2.5$, but that it is probably incompatible with the
limited $z>3$ damped Ly$\alpha$ data. The situation is uncertain because there
is very little data for $z>3$, and also it is unclear whether all damped
Ly$\alpha$ systems are associated with collapsed protogalaxies. The predictions
of CHDM are quite sensitive to the hot (neutrino) fraction, and we find that
$\Omega_c/\Omega_\nu/\Omega_b = 0.675/0.25/0.075$ is compatible even with the
$z>3$ data. This corresponds to lowering the neutrino mass from 6.8 to 5.7 eV,
for $H_0=50\kmsMpc$. In CHDM, the higher redshift damped Ly$\alpha$ systems are
predicted to have lower masses, which can be checked by measuring the velocity
widths of the associated metal line systems. | 9405003v1 |
1995-03-24 | High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers | We have obtained high signal:to:noise optical spectroscopy at 5\AA\
resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have
been analyzed to create new samples of high redshift Lyman-limit and damped
Lyman-$\alpha$ absorbers. These data have been combined with published data
sets in a study of the redshift evolution and the column density distribution
function for absorbers with $\log$N(HI)$\ge17.5$, over the redshift range 0.01
$<$ z $<$ 5. The main results are: \begin{itemize} \item Lyman limit systems:
The data are well fit by a power law $N(z) = N_0(1 + z)^{\gamma}$ for the
number density per unit redshift. For the first time intrinsic evolution is
detected in the product of the absorption cross-section and comoving spatial
number density for an $\Omega = 1$ Universe. We find $\gamma = 1.55$ ($\gamma =
0.5$ for no evolution) and $N_0 = 0.27$ with $>$99.7\% confidence limits for
$\gamma$ of 0.82 \& 2.37. \item Damped \lya systems: The APM QSOs provide a
substantial increase in the redshift path available for damped surveys for
$z>3$. Eleven candidate and three confirmed damped Ly$\alpha$ absorption
systems, have been identified in the APM QSO spectra covering the redshift
range $2.8\le z \le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed
and candidate damped \lya absorbers with previous surveys, we find evidence for
a turnover at z$\sim$3 or a flattening at z$\sim$2 in the cosmological mass
density of neutral gas, $\Omega_g$. \end{itemize} The Lyman limit survey
results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we
describe the results for the DLA population of absorbers. | 9503089v1 |
1997-05-15 | Cosmological Constraints from High-Redshift Damped Lyman-Alpha Systems | Any viable cosmological model must produce enough structure at early epochs
to explain the amount of gas associated with high-redshift damped Ly$\alpha$
systems. We study the evolution of damped Ly$\alpha$ systems at redshifts $z\ge
2$ in cold dark matter (CDM) and cold+hot dark matter (CDM+HDM) models using
both N-body and hydrodynamic simulations. Our approach incorporates the effects
of gas dynamics, and we find that all earlier estimates which assumed that all
the baryons in dark matter halos would contribute to damped Ly$\alpha$
absorption have overestimated the column density distribution $f(N)$ and the
fraction of neutral dense gas $\Omega_g$ in damped Ly$\alpha$ systems. The
differences are driven by ionization of hydrogen in the outskirts of galactic
halos and by gaseous dissipation near the halo centers, and they tend to
exacerbate the problem of late galaxy formation in CDM+HDM models. We only
include systems up to the highest observed column density $N\sim 10^{21.8}$
cm$^{-2}$ in the estimation of $\Omega_g$ for a fair comparison with data. If
the observed $f(N)$ and $\Omega_g$ inferred from a small number of confirmed
and candidate absorbers are robust, the amount of gas in damped Ly$\alpha$
systems at high redshifts in the $\Omega_\nu=0.2$ CDM+HDM model falls well
below the observations. | 9705113v1 |
2001-01-03 | Galactic Chemical Abundances at z>3 I: First Results from the Echellette Spectrograph and Imager | We present the first results from an ongoing survey to discover and measure
the metallicity of z>3 damped Lya systems with the Echellette Spectrograph and
Imager (ESI) on the Keck II telescope. Our motivation arises from a recent
study on the damped Lya systems suggesting only mild evolution in the cosmic
metallicity from z~2 to 4. The Echellette Spectrograph and Imager, which
provides two complementary spectroscopic modes, is the ideal instrument for a
z>3 damped Lya survey. We describe our observing strategy and report on the
discovery and analysis of 5 new z>3 damped Lya systems acquired in a single
night of observing. These observations further support the principal
conclusions of the previous study: (1) the cosmic metallicity in neutral gas
inferred from the damped Lya systems does not evolve significantly from z~2 to
4; (2) the unweighted metallicity exhibits a statistically significant decrease
with increasing redshift; and (3) not a single damped Lya system has a
metallicity below [Fe/H]=-3. We discuss the implications of these results and
comment on recent theoretical studies which attempt to explain the
observations. | 0101029v1 |
2002-01-17 | Self-shielding Effects on the Column Density Distribution of Damped Lyman Alpha Systems | We calculate the column density distribution of damped Lyman alpha systems,
modeled as spherical isothermal gaseous halos ionized by the external cosmic
background. The effects of self-shielding introduce a hump in this
distribution, at a column density N_{HI} \sim 1.6x10^{17} X^{-1} cm^{-2}, where
X is the neutral fraction at the radius where self-shielding starts being
important. The most recent compilation of the column density distribution by
Storrie-Lombardi & Wolfe shows marginal evidence for the detection of this
feature due to self-shielding, suggesting a value X \sim 10^{-3}. Assuming a
photoionization rate \Gamma \sim 10^{-12} s^{-1} from the external ionizing
background, the radius where self-shielding occurs is inferred to be about
3.8kpc. If damped Lyman alpha systems consist of a clumpy medium, this should
be interpreted as the typical size of the gas clumps in the region where they
become self-shielding. Clumps of this size with typical column densities N_H
\sim 3x10^{20} cm^{-2} would be in hydrostatic equilibrium at the
characteristic photoionization temperature \sim 10^4 K if they do not contain
dark matter. Since this size is similar to the overall radius of damped \lya
systems in Cold Dark Matter models, where all halos are assumed to contain
similar gas clouds producing damped absorbers, this suggests that the gas in
damped absorbers is in fact not highly clumped. | 0201275v2 |
2002-04-30 | Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems | Molecular hydrogen is quite underabundant in damped Lyman-alpha systems at
high redshift, when compared to the interstellar medium near the Sun. This has
been interpreted as implying that the gas in damped Lyman-alpha systems is
warm. like the nearby neutral intercloud medium, rather than cool, as in the
clouds which give rise to most H I absorption in the Milky Way. Other lines of
evidence suggest that the gas in damped Lyman-alpha systems -- in whole or part
-- is actually cool; spectroscopy of neutral and ionized carbon, discussed
here, shows that the damped Lyman-alpha systems observed at lower redshift z
$<$ 2.3 are largely cool, while those seen at z $>$ 2.8 are warm (though not
devoid of H2). To interpret the observations of carbon and hydrogen we
constructed detailed numerical models of H2 formation under the conditions of
two-phase thermal equilibrium, like those which account for conditions near the
Sun, but with varying metallicity, dust-gas ratio, $etc$. We find that the low
metallicity of damped Lyman-alpha systems is enough to suppress H2 formation by
many orders of magnitude even in cool diffuse clouds, as long as the ambient
optical/uv radiation field is not too small. For very low metallicity and under
the most diffuse conditions, H2 formation will be dominated by slow gas-phase
processes not involving grains, and a minimum molecular fraction in the range
$10^{-8}-10^{-7}$ is expected. | 0204515v1 |
2003-05-12 | Ordinary and Viscosity-Damped MHD Turbulence | We compare the properties of ordinary strong magnetohydrodynamic (MHD)
turbulence in a strongly magnetized medium with the recently discovered
viscosity-damped regime. We focus on energy spectra, anisotropy, and
intermittency. Our most surprising conclusion is that in ordinary strong MHD
turbulence the velocity and magnetic fields show different high-order structure
function scalings. Moreover this scaling depends on whether the intermittency
is viewed in a global or local system of reference. This reconciles seemingly
contradictory earlier results. On the other hand, the intermittency scaling for
viscosity-damped turbulence is very different, and difficult to understand in
terms of the usual phenomenological models for intermittency in turbulence. Our
remaining results are in reasonable agreement with expectations. First, we find
that our high resolution simulations for ordinary MHD turbulence show that the
energy spectra are {\it compatible} with a Kolmogorov spectrum, while
viscosity-damped turbulence shows a shallow $k^{-1}$ spectrum for the magnetic
fluctuations. Second, a new numerical technique confirms that ordinary MHD
turbulence exhibits Goldreich-Sridhar type anisotropy, while viscosity-damped
MHD turbulence shows extremely anisotropic eddy structures. Finally, we show
that many properties of incompressible turbulence for both the ordinary and
viscosity-damped regimes carry over to the case of compressible turbulence. | 0305212v2 |
2003-09-17 | Observational Tests of Damping by Resonant Absorption in Coronal Loop Oscillations | One of the proposed damping mechanisms of coronal (transverse) loop
oscillations in the kink-mode is resonant absorption as a result of the Alfven
speed variation at the outer boundary of coronal loops. Analytical expressions
for the period and damping time exist for loop models with thin non-uniform
boundaries. Here we measure the thickness of the non-uniform layer in
oscillating loops for 11 events, by forward-fitting of the cross-sectional
density profile and line-of-sight integration to the cross-sectional fluxes
observed with TRACE 171 A. This way we model the internal and external electron
density of the coronal plasma in oscillating loops. This allows us to test the
theoretically predicted damping rates for thin boundaries as function of the
density ratio. We find that the density ratio predicted by the damping time is
higher than the density ratio estimated from the background fluxes. The lower
densities modeled from the background fluxes are likely to be a consequence of
the neglected hotter plasma that is not detected with the TRACE 171 A filter.
Taking these correction into account, resonant absorption predicts damping
times of kink-mode oscillations that are commensurable with the observed ones
and provides a new diagnostic of the density contrast of oscillating loops. | 0309470v1 |
2005-03-01 | Metal Abundances in a Damped Lyman-alpha System Along Two Lines of Sight at z=0.93 | We study metal abundances in the z=0.9313 damped Lya system observed in the
two lines-of-sight, A and B, toward the gravitationally-lensed double QSO
HE0512-3329. Spatially resolved STIS spectra constrain the neutral-gas column
density to be LogN(HI)=20.5 in both Aand B. UVES spectra (spectral resolution
FWHM=9.8 km/s) show, in contrast, significant line-of-sight differences in the
column densities of MnII and FeII; these are not due to observational
systematics. We find that [Mn/H]=-1.44 and [Fe/H]=-1.52 in damped Lya system A,
while [Mn/H]=-0.98 and [Fe/H]>-1.32, and possibly as high as [Fe/H] approx. -1
in damped Lya system B. A careful assessment of possible systematic errors
leads us to conclude that these transverse differences are significant at a 5
sigma level or greater. Although nucleosynthesis effects may also be at play,
we favor differential dust-depletion as the main mechanism producing the
observed abundance gradient. The transverse separation is 5 kpc at the redshift
of the absorber, which is also likely to be the lensing galaxy. The derived
abundances therefore probe two opposite sides of a single galaxy hosting both
damped Lya systems. This is the first time firm abundance constraints have been
obtained for a single damped system probed by two lines-of-sight. The
significance of this finding for the cosmic evolution of metals is discussed. | 0503026v1 |
2000-08-26 | Adsorbate aggregation and relaxation of low-frequency vibrations | We present a study of resonant vibrational coupling between adsorbates and an
elastic substrate at low macroscopic coverages. In the first part of the paper
we consider the situation when adsorbates form aggregates with high local
coverage. Based upon our previously published theory, we derive formulas
describing the damping rate of adsorbate vibrations for two cases of such
aggregation: (i) adsorbates attached to step edges and (ii) adsorbates forming
two-dimensional islands. We have shown that damping is governed by local
coverage. Particularly, for a wide range of resonant frequencies, the damping
rate of adsorbates forming well separated islands is described by the damping
rate formula for a periodic overlayer with the coverage equal to the local
coverage in the island. The second part of the paper is devoted to facilitating
the evaluation of damping rates for a disordered overlayer. The formula
describing the damping rate involves the parameter $\beta$ which is related to
the local density of phonon states at the substrate surface and does not allow
a closed-form representation. For substrates of isotropic and cubic symmetries,
we have developed a good analytical approximation to this parameter. For a vast
majority of cubic substrates the difference between the analytical
approximation and numerical calculation does not exceed 4%. | 0008389v1 |
2004-10-26 | Mean-field treatment of the damping of the oscillations of a 1D Bose gas in an optical lattice | We present a theoretical treatment of the surprisingly large damping observed
recently in one-dimensional Bose-Einstein atomic condensates in optical
lattices. We show that time-dependent Hartree-Fock-Bogoliubov (HFB)
calculations can describe qualitatively the main features of the damping
observed over a range of lattice depths. We also derive a formula of the
fluctuation-dissipation type for the damping, based on a picture in which the
coherent motion of the condensate atoms is disrupted as they try to flow
through the random local potential created by the irregular motion of
noncondensate atoms. We expect this irregular motion to result from the
well-known dynamical instability exhibited by the mean-field theory for these
systems. When parameters for the characteristic strength and correlation times
of the fluctuations, obtained from the HFB calculations, are substituted in the
damping formula, we find very good agreement with the experimentally-observed
damping, as long as the lattice is shallow enough for the fraction of atoms in
the Mott insulator phase to be negligible. We also include, for completeness,
the results of other calculations based on the Gutzwiller ansatz, which appear
to work better for the deeper lattices. | 0410677v4 |
1998-10-16 | Fermion Damping in a Fermion-Scalar Plasma | In this article we study the dynamics of fermions in a fermion-scalar plasma.
We begin by obtaining the effective in-medium Dirac equation in real time which
is fully renormalized and causal and leads to the initial value problem. For a
heavy scalar we find the novel result that the decay of the scalar into fermion
pairs in the medium leads to damping of the fermionic excitations and their
in-medium propagation as quasiparticles. That is, the fermions acquire a width
due to the decay of the heavier scalar in the medium. We find the damping rate
to lowest order in the Yukawa coupling for arbitrary values of scalar and
fermion masses, temperature and fermion momentum. An all-order expression for
the damping rate in terms of the exact quasiparticle wave functions is
established. A kinetic Boltzmann approach to the relaxation of the fermionic
distribution function confirms the damping of fermionic excitations as a
consequence of the induced decay of heavy scalars in the medium. A
linearization of the Boltzmann equation near equilibrium clearly displays the
relationship between the damping rate of fermionic mean fields and the fermion
interaction rate to lowest order in the Yukawa coupling directly in real time. | 9810393v2 |
2006-01-06 | Wave energy localization by self-focusing in large molecular structures: a damped stochastic discrete nonlinear Schroedinger equation model | Wave self-focusing in molecular systems subject to thermal effects, such as
thin molecular films and long biomolecules, can be modeled by stochastic
versions of the Discrete Self-Trapping equation of Eilbeck, Lomdahl and Scott,
and this can be approximated by continuum limits in the form of stochastic
nonlinear Schroedinger equations.
Previous studies directed at the SNLS approximations have indicated that the
self-focusing of wave energy to highly localized states can be inhibited by
phase noise (modeling thermal effects) and can be restored by phase damping
(modeling heat radiation).
We show that the continuum limit is probably ill-posed in the presence of
spatially uncorrelated noise, at least with little or no damping, so that
discrete models need to be addressed directly. Also, as has been noted by other
authors, omission of damping produces highly unphysical results.
Numerical results are presented for the first time for the discrete models
including the highly nonlinear damping term, and new numerical methods are
introduced for this purpose. Previous conjectures are in general confirmed, and
the damping is shown to strongly stabilize the highly localized states of the
discrete models. It appears that the previously noted inhibition of nonlinear
wave phenomena by noise is an artifact of modeling that includes the effects of
heat, but not of heat loss. | 0601017v1 |
2007-11-15 | Effect of the steady flow on spatial damping of small-amplitude prominence oscillations | Aims. Taking account of steady flow in solar prominences, we study its
effects on spatial damping of small-amplitude non-adiabatic magnetoacoustic
waves in a homogeneous, isothermal, and unbounded prominence plasma. Methods.
We model the typical feature of observed damped oscillatory motion in
prominences, removing the adiabaticity assumption through thermal conduction,
radiation and heating. Invoking steady flow in MHD equations, we linearise them
under small-amplitude approximation and obtain a new general dispersion
relation for linear non-adiabatic magnetoacoustic waves in prominences Results.
The presence of steady flow breaks the symmetry of forward and backward
propagating MHD wave modes in prominences. The steady flow has dramatic
influence on the propagation and damping of magnetoacoustic and thermal waves.
Depending upon the direction and strength of flow the magnetoacoustic and
thermal modes can show both the features of wave amplification and damping. At
the wave period of 5 min where the photospheric power is maximum, the slow mode
shows wave amplification. However, in the absence of steady flow the slow mode
wave shows damping. Conclusions. For the wave period between 5 min and 15 min,
the amplification length for slow mode, in the case of prominence regime 1.1,
varies between 3.4*10^11 m to 2*10^12 m. Dramatic influence of steady flow on
small-amplitude prominence oscillations is likely to play an important role in
both wave detection and prominence seismology. | 0711.2353v1 |
2008-02-07 | Cascade and Damping of Alfvén-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum | With the diffusion approximation, we study the cascade and damping of
Alfv\'{e}n-cyclotron fluctuations in solar plasmas numerically. Motivated by
wave-wave couplings and nonlinear effects, we test several forms of the
diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion
tensor in the wave vector space, the turbulence spectrum in the inertial range
can be fitted with power-laws with the power-law index varying with the wave
propagation direction. For several locally isotropic but inhomogeneous
diffusion coefficients, the steady-state turbulence spectra are nearly
isotropic in the absence of damping and can be fitted by a single power-law
function. However, the energy flux is strongly polarized due to the
inhomogeneity that leads to an anisotropic cascade. Including the anisotropic
thermal damping, the turbulence spectrum cuts off at the wave numbers, where
the damping rates become comparable to the cascade rates. The combined
anisotropic effects of cascade and damping make this cutoff wave number
dependent on the wave propagation direction, and the propagation direction
integrated turbulence spectrum resembles a broken power-law, which cuts off at
the maximum of the cutoff wave numbers or the $^4$He cyclotron frequency.
Taking into account the Doppler effects, the model can naturally reproduce the
broken power-law wave spectra observed in the solar wind and predicts that a
higher break frequency is aways accompanied with a greater spectral index
change that may be caused by the increase of the Alfv\'{e}n Mach number, the
reciprocal of the plasma beta, and/or the angle between the solar wind velocity
and the mean magnetic field. These predictions can be tested by future
observations. | 0802.0910v1 |
2011-04-13 | Evolution of inclined planets in three-dimensional radiative discs | While planets in the solar system only have a low inclination with respect to
the ecliptic there is mounting evidence that in extrasolar systems the
inclination can be very high, at least for close-in planets. One process to
alter the inclination of a planet is through planet-disc interactions. Recent
simulations considering radiative transport have shown that the evolution of
migration and eccentricity can strongly depend on the thermodynamic state of
the disc. We extend previous studies to investigate the planet-disc
interactions of fixed and moving planets on inclined and eccentric orbits. We
also analyse the effect of the disc's thermodynamic properties on the orbital
evolution of embedded planets in detail. The protoplanetary disc is modelled as
a viscous gas where the internally produced dissipation is transported by
radiation. For locally isothermal discs, we confirm previous results and find
inclination damping and inward migration for planetary cores. For low
inclinations i < 2 H/r, the damping is exponential, while di/dt is proportional
to i^-2 for larger i. For radiative discs, the planetary migration is very
limited, as long as their inclination exceeds a certain threshold. If the
inclination is damped below this threshold, planetary cores with a mass up to
approximately 33 Earth masses start to migrate outwards, while larger cores
migrate inwards right from the start. The inclination is damped for all
analysed planet masses. In a viscous disc an initial inclination of embedded
planets will be damped for all planet masses. This damping occurs on timescales
that are shorter than the migration time. If the inclination lies beneath a
certain threshold, the outward migration in radiative discs is not handicapped.
Outward migration is strongest for circular and non-inclined orbits. | 1104.2408v1 |
2011-07-12 | Mode conversion of radiatively damped magnetogravity waves in the solar chromosphere | Modelling of adiabatic gravity wave propagation in the solar atmosphere
showed that mode conversion to field guided acoustic waves or Alfv\'en waves
was possible in the presence of highly inclined magnetic fields. This work aims
to extend the previous adiabatic study, exploring the consequences of radiative
damping on the propagation and mode conversion of gravity waves in the solar
atmosphere. We model gravity waves in a VAL-C atmosphere, subject to a uniform,
and arbitrarily orientated magnetic field, using the Newton cooling
approximation for radiatively damped propagation. The results indicate that the
mode conversion pathways identified in the adiabatic study are maintained in
the presence of damping. The wave energy fluxes are highly sensitive to the
form of the height dependence of the radiative damping time. While simulations
starting from 0.2 Mm result in modest flux attenuation compared to the
adiabatic results, short damping times expected in the low photosphere
effectively suppress gravity waves in simulations starting at the base of the
photosphere. It is difficult to reconcile our results and observations of
propagating gravity waves with significant energy flux at photospheric heights
unless they are generated in situ, and even then, why they are observed to be
propagating as low as 70 km where gravity waves should be radiatively
overdamped. | 1107.2208v1 |
2013-09-23 | Phonon-mediated damping of mechanical vibrations in a finite atomic chain coupled to an outer environment | We study phonon-mediated damping of mechanical vibrations in a finite
quantum-mechanical atomic-chain model. Our study is motivated by the quest to
understand the quality factors (Q) of nanomechanical resonators and
nanoelectromechanical systems (NEMS), as well as actual experiments with
suspended atomic chains and molecular junctions. We consider a finite atomic
chain which is coupled to a zero-temperature outer environment, modeled as two
additional semi-infinite chains, thus inducing "clamping-losses". Weak coupling
to the outer environment ensures that the clamping losses are small, and that
the initially discrete nature of the phonon spectrum is approximately
maintained. We then consider a phonon damping process known as "Landau-Rumer
damping", where phonons in the excited mode of vibration decay into other modes
through anharmonic phonon-phonon interaction. The approximately discrete nature
of the phonon spectrum leads to sharp nonmonotonic changes in Q as parameters
are varied, and to the appearance of resonances in the damping. The latter
correspond to the existence of decay processes where the participating phonons
approximately conserve energy. We explore means to control the damping by
changing either the number of atoms in the chains or the ratio between the
longitudinal and transverse speeds of sound, thereby suggesting future
experiments to observe this resonance-like behavior. | 1309.5772v1 |
2015-03-31 | Damping of Confined Excitations Modes of 1D Condensates in an Optical Lattice | We study the damping of the collective excitations of Bose-Einstein
condensates in a harmonic trap potential loaded in an optical lattice. In the
presence of a confining potential the system is non-homogeneous and the
collective excitations are characterized by a set of discrete confined
phonon-like excitations. We derive a general convenient analytical description
for the damping rate, which takes into account, the trapping potential and the
optical lattice, for the Landau and Beliaev processes at any temperature, $T$.
At high temperature or weak spatial confinement, we show that both mechanisms
display linear dependence on $T$. In the quantum limit, we found that the
Landau damping is exponentially suppressed at low temperatures and the total
damping is independent of $T$. Our theoretical predictions for the damping rate
under thermal regime is in completely correspondence with the experimental
values reported for 1D condensate of sodium atoms. We show that the laser
intensity can tune the collision process, allowing a \textit{resonant effect}
for the condensate lifetime. Also, we study the influence of the attractive or
repulsive non-linear terms on the decay rate of the collective excitations. A
general expression of the renormalized Goldstone frequency has been obtained as
a function of the 1D non-linear self-interaction parameter, laser intensity and
temperature. | 1503.08884v2 |
2015-08-06 | On the spatial scales of wave heating in the solar chromosphere | Dissipation of magnetohydrodynamic (MHD) wave energy has been proposed as a
viable heating mechanism in the solar chromospheric plasma. Here, we use a
simplified one-dimensional model of the chromosphere to theoretically
investigate the physical processes and the spatial scales that are required for
the efficient dissipation of Alfv\'en waves and slow magnetoacoustic waves. We
consider the governing equations for a partially ionized hydrogen-helium plasma
in the single-fluid MHD approximation and include realistic wave damping
mechanisms that may operate in the chromosphere, namely Ohmic and ambipolar
magnetic diffusion, viscosity, thermal conduction, and radiative losses. We
perform an analytic local study in the limit of small amplitudes to
approximately derive the lengthscales for critical damping and efficient
dissipation of MHD wave energy. We find that the critical dissipation
lengthscale for Alfv\'en waves depends strongly on the magnetic field strength
and ranges from 10~m to 1~km for realistic field strengths. The damping of
Alfv\'en waves is dominated by Ohmic diffusion for weak magnetic field and low
heights in the chromosphere, and by ambipolar diffusion for strong magnetic
field and medium/large heights in the chromosphere. Conversely, the damping of
slow magnetoacoustic waves is less efficient, and spatial scales shorter than
10~m are required for critical damping. Thermal conduction and viscosity govern
the damping of slow magnetoacoustic waves and play an equally important role at
all heights. These results indicate that the spatial scales at which strong
wave heating may work in the chromosphere are currently unresolved by
observations. | 1508.01497v1 |
2015-11-11 | A statistical study of decaying kink oscillations detected using SDO/AIA | Despite intensive studies of kink oscillations of coronal loops in the last
decade, a large scale statistically significant investigation of the
oscillation parameters has not been made using data from the Solar Dynamics
Observatory (SDO).
We carry out a statistical study of kink oscillations using Extreme
Ultra-Violet (EUV) imaging data from a previously compiled catalogue.
We analysed 58 kink oscillation events observed by the Atmospheric Imaging
Assembly (AIA) onboard SDO during its first four years of operation
(2010-2014). Parameters of the oscillations, including the initial apparent
amplitude, period, length of the oscillating loop, and damping are studied for
120 individual loop oscillations.
Analysis of the initial loop displacement and oscillation amplitude leads to
the conclusion that the initial loop displacement prescribes the initial
amplitude of oscillation in general. The period is found to scale with the loop
length, and a linear fit of the data cloud gives a kink speed of Ck
=(1330+/-50) km s-1 . The main body of the data corresponds to kink speeds in
the range Ck =(800-3300) km s-1. Measurements of 52 exponential damping times
were made, and it was noted that at least 22 of the damping profiles may be
better approximated by a combination of non-exponential and exponential
profiles, rather than a purely exponential damping envelope. There are an
additional 10 cases where the profile appears to be purely non-exponential, and
no damping time was measured. A scaling of the exponential damping time with
the period is found, following the previously established linear scaling
between these two parameters. | 1511.03558v1 |
2016-11-17 | A stable partitioned FSI algorithm for rigid bodies and incompressible flow. Part I: Model problem analysis | A stable partitioned algorithm is developed for fluid-structure interaction
(FSI) problems involving viscous incompressible flow and rigid bodies. This
{\em added-mass partitioned} (AMP) algorithm remains stable, without
sub-iterations, for light and even zero mass rigid bodies when added-mass and
viscous added-damping effects are large. The scheme is based on a generalized
Robin interface condition for the fluid pressure that includes terms involving
the linear acceleration and angular acceleration of the rigid body. Added-mass
effects are handled in the Robin condition by inclusion of a boundary integral
term that depends on the pressure. Added-damping effects due to the viscous
shear forces on the body are treated by inclusion of added-damping tensors that
are derived through a linearization of the integrals defining the force and
torque. Added-damping effects may be important at low Reynolds number, or, for
example, in the case of a rotating cylinder or rotating sphere when the
rotational moments of inertia are small. In this first part of a two-part
series, the properties of the AMP scheme are motivated and evaluated through
the development and analysis of some model problems. The analysis shows when
and why the traditional partitioned scheme becomes unstable due to either
added-mass or added-damping effects. The analysis also identifies the proper
form of the added-damping which depends on the discrete time-step and the
grid-spacing normal to the rigid body. The results of the analysis are
confirmed with numerical simulations that also demonstrate a second-order
accurate implementation of the AMP scheme. | 1611.05711v1 |
2017-01-30 | Torsional Alfvén resonances as an efficient damping mechanism for non-radial oscillations in red giant stars | Stars are self-gravitating fluids in which pressure, buoyancy, rotation and
magnetic fields provide the restoring forces for global modes of oscillation.
Pressure and buoyancy energetically dominate, while rotation and magnetism are
generally assumed to be weak perturbations and often ignored. However,
observations of anomalously weak dipole mode amplitudes in red giant stars
suggest that a substantial fraction of these are subject to an additional
source of damping localised to their core region, with indirect evidence
pointing to the role of a deeply buried magnetic field. It is also known that
in many instances the gravity-mode character of affected modes is preserved,
but so far no effective damping mechanism has been proposed that accommodates
this aspect. Here we present such a mechanism, which damps the oscillations of
stars harbouring magnetised cores via resonant interactions with standing
Alfv\'en modes of high harmonic index. The damping rates produced by this
mechanism are quantitatively on par with those associated with turbulent
convection, and in the range required to explain observations, for realistic
stellar models and magnetic field strengths. Our results suggest that magnetic
fields can provide an efficient means of damping stellar oscillations without
needing to disrupt the internal structure of the modes, and lay the groundwork
for an extension of the theory of global stellar oscillations that incorporates
these effects. | 1701.08771v1 |
2018-03-30 | Damping of gravitational waves in a viscous Universe and its implication for dark matter self-interactions | It is well known that a gravitational wave (GW) experiences the damping
effect when it propagates in a fluid with nonzero shear viscosity. In this
paper, we propose a new method to constrain the GW damping rate and thus the
fluid shear viscosity. By defining the effective distance which incorporates
damping effects, we can transform the GW strain expression in a viscous
Universe into the same form as that in a perfect fluid. Therefore, the
constraints of the luminosity distances from the observed GW events by LIGO and
Virgo can be directly applied to the effective distances in our formalism. We
exploit the lognormal likelihoods for the available GW effective distances and
a Gaussian likelihood for the luminosity distance inferred from the
electromagnetic radiation observation of the binary neutron star merger event
GW170817. Our fittings show no obvious damping effects in the current GW data,
and the upper limit on the damping rate with the combined data is $6.75 \times
10^{-4}\,{\rm Mpc}^{-1}$ at 95\% confidence level. By assuming that the dark
matter self-scatterings are efficient enough for the hydrodynamic description
to be valid, we find that a GW event from its source at a luminosity distance
$D\gtrsim 10^4\;\rm Mpc$ can be used to put a constraint on the dark matter
self-interactions. | 1803.11397v1 |
2018-08-22 | Constructing a boosted, spinning black hole in the damped harmonic gauge | The damped harmonic gauge is important for numerical relativity computations
based on the generalized harmonic formulation of Einstein's equations, and is
used to reduce coordinate distortions near binary black hole mergers. However,
currently there is no prescription to construct quasiequilibrium binary black
hole initial data in this gauge. Instead, initial data are typically
constructed using a superposition of two boosted analytic single black hole
solutions as free data in the solution of the constraint equations. Then, a
smooth time-dependent gauge transformation is done early in the evolution to
move into the damped harmonic gauge. Using this strategy to produce initial
data in damped harmonic gauge would require the solution of a single black hole
in this gauge, which is not known analytically. In this work we construct a
single boosted, spinning, equilibrium BH in damped harmonic coordinates as a
regular time-independent coordinate transformation from Kerr-Schild
coordinates. To do this, we derive and solve a set of 4 coupled, nonlinear,
elliptic equations for this transformation, with appropriate boundary
conditions. This solution can now be used in the construction of damped
harmonic initial data for binary black holes. | 1808.07490v3 |
2018-12-13 | Neutrino damping in a fermion and scalar background | We consider the propagation of a neutrino in a background composed of a
scalar particle and a fermion using a simple model for the coupling of the form
$\lambda\bar f_R\nu_L\phi$. In the presence of these interactions there can be
damping terms in the neutrino effective potential and index of refraction. We
calculate the imaginary part of the neutrino self-energy in this case, from
which the damping terms are determined. The results are useful in the context
of Dark Matter-neutrino interaction models in which the scalar and/or fermion
constitute the dark-matter. The corresponding formulas for models in which the
scalar particle couples to two neutrinos via a coupling of the form
$\lambda^{(\nu\nu\phi)}\bar\nu^c_R\nu_L\phi$ are then obtained as a special
case, which can be important also in the context of neutrino collective
oscillations in a supernova and in the Early Universe hot plasma before
neutrino decoupling. A particular feature of our results is that the damping
term in a $\nu\phi$ background is independent of the antineutrino-neutrino
asymmetry in the background. Therefore, the relative importance of the damping
term may be more significant if the neutrino-antineutrino asymmetry in the
background is small, because the leading $Z$-exchange and $\phi$-exchange
contributions to the effective potential, which are proportional to the
neutrino-antineutrino asymmetry, are suppressed in that case, while the damping
term is not. | 1812.05672v2 |
2016-03-01 | A comparative study of protocols for secure quantum communication under noisy environment: single-qubit-based protocols versus entangled-state-based protocols | The effect of noise on various protocols of secure quantum communication has
been studied. Specifically, we have investigated the effect of amplitude
damping, phase damping, squeezed generalized amplitude damping, Pauli type as
well as various collective noise models on the protocols of quantum key
distribution, quantum key agreement,quantum secure direct quantum communication
and quantum dialogue. From each type of protocol of secure quantum
communication, we have chosen two protocols for our comparative study; one
based on single qubit states and the other one on entangled states. The
comparative study reported here has revealed that single-qubit-based schemes
are generally found to perform better in the presence of amplitude damping,
phase damping, squeezed generalized amplitude damping noises, while
entanglement-based protocols turn out to be preferable in the presence of
collective noises. It is also observed that the effect of noise entirely
depends upon the number of rounds of quantum communication involved in a scheme
of quantum communication. Further, it is observed that squeezing, a completely
quantum mechanical resource present in the squeezed generalized amplitude
channel, can be used in a beneficial way as it may yield higher fidelity
compared to the corresponding zero squeezing case. | 1603.00178v1 |
2014-04-01 | Stellar dynamics in gas: The role of gas damping | In this paper, we consider how gas damping affects the dynamical evolution of
gas-embedded star clusters. Using a simple three-component (i.e. one gas and
two stellar components) model, we compare the rates of mass segregation due to
two-body relaxation, accretion from the interstellar medium, and gas dynamical
friction in both the supersonic and subsonic regimes. Using observational data
in the literature, we apply our analytic predictions to two different
astrophysical environments, namely galactic nuclei and young open star
clusters. Our analytic results are then tested using numerical simulations
performed with the NBSymple code, modified by an additional deceleration term
to model the damping effects of the gas.
The results of our simulations are in reasonable agreement with our analytic
predictions, and demonstrate that gas damping can significantly accelerate the
rate of mass segregation. A stable state of approximate energy equilibrium
cannot be achieved in our model if gas damping is present, even if Spitzer's
Criterion is satisfied. This instability drives the continued dynamical
decoupling and subsequent ejection (and/or collisions) of the more massive
population. Unlike two-body relaxation, gas damping causes overall cluster
contraction, reducing both the core and half-mass radii. If the cluster is mass
segregated (and/or the gas density is highest at the cluster centre), the
latter contracts faster than the former, accelerating the rate of core
collapse. | 1404.0379v1 |
2014-04-26 | Landau damping effects on dust-acoustic solitary waves in a dusty negative-ion plasma | The nonlinear theory of dust-acoustic waves (DAWs) with Landau damping is
studied in an unmagnetized dusty negative-ion plasma in the extreme conditions
when the free electrons are absent. The cold massive charged dusts are
described by fluid equations, whereas the two-species of ions (positive and
negative) are described by the kinetic Vlasov equations. A Korteweg de-Vries
(KdV) equation with Landau damping, governing the dynamics of weakly nonlinear
and weakly dispersive DAWs, is derived following Ott and Sudan [Phys. Fluids
{\bf 12}, 2388 (1969)]. It is shown that for some typical laboratory and space
plasmas, the Landau damping (and the nonlinear) effects are more pronounced
than the finite Debye length (dispersive) effects for which the KdV soliton
theory is not applicable to DAWs in dusty pair-ion plasmas. The properties of
the linear phase velocity, solitary wave amplitudes (in presence and absence of
the Landau damping) as well as the Landau damping rate are studied with the
effects of the positive ion to dust density ratio $(\mu_{pd})$ as well as the
ratios of positive to negative ion temperatures $(\sigma)$ and masses $(m)$. | 1404.6623v3 |
2018-05-29 | Basic microscopic plasma physics from N-body mechanics | Computing is not understanding. This is exemplified by the multiple and
discordant interpretations of Landau damping still present after seventy years.
For long deemed impossible, the mechanical N-body description of this damping,
not only enables its rigorous and simple calculation, but makes unequivocal and
intuitive its interpretation as the synchronization of almost resonant passing
particles. This synchronization justifies mechanically why a single formula
applies to both Landau growth and damping. As to the electrostatic potential,
the phase mixing of many beam modes produces Landau damping, but it is
unexpectedly essential for Landau growth too. Moreover, collisions play an
essential role in collisionless plasmas. In particular, Debye shielding results
from a cooperative dynamical self-organization process, where "collisional"
deflections due to a given electron diminish the apparent number of charges
about it. The finite value of exponentiation rates due to collisions is crucial
for the equivalent of the van Kampen phase mixing to occur in the N-body
system. The N-body approach incorporates spontaneous emission naturally, whose
compound effect with Landau damping drives a thermalization of Langmuir waves.
O'Neil's damping with trapping typical of initially large enough Langmuir waves
results from a phase transition. As to collisional transport, there is a smooth
connection between impact parameters where the two-body Rutherford picture is
correct, and those where a collective description is mandatory. The N-body
approach reveals two important features of the Vlasovian limit: it is singular
and it corresponds to a renormalized description of the actual N-body dynamics. | 1805.11408v2 |
2019-04-25 | High Spin-Wave Propagation Length Consistent with Low Damping in a Metallic Ferromagnet | We report ultra-low intrinsic magnetic damping in
Co$_{\text{25}}$Fe$_{\text{75}}$ heterostructures, reaching the low $10^{-4}$
regime at room temperature. By using a broadband ferromagnetic resonance
technique, we extracted the dynamic magnetic properties of several
Co$_{\text{25}}$Fe$_{\text{75}}$-based heterostructures with varying
ferromagnetic layer thickness. By estimating the eddy current contribution to
damping, measuring radiative damping and spin pumping effects, we found the
intrinsic damping of a 26\,nm thick sample to be $$\alpha_{\mathrm{0}} \lesssim
3.18\times10^{-4}$. Furthermore, using Brillouin light scattering microscopy we
measured spin-wave propagation lengths of up to $(21\pm1)\,\mathrm{\mu m}$ in a
26 nm thick Co$_{\text{25}}$Fe$_{\text{75}}$ heterostructure at room
temperature, which is in excellent agreement with the measured damping. | 1904.11321v3 |
2020-09-25 | Temperature dependence of the damping parameter in the ferrimagnet Gd$_3$Fe$_5$O$_{12}$ | The damping parameter ${\alpha}_{\text{FM}}$ in ferrimagnets defined
according to the conventional practice for ferromagnets is known to be strongly
temperature dependent and diverge at the angular momentum compensation
temperature, where the net angular momentum vanishes. However, recent
theoretical and experimental developments on ferrimagnetic metals suggest that
the damping parameter can be defined in such a way, which we denote by
${\alpha}_{\text{FiM}}$, that it is free of the diverging anomaly at the
angular momentum compensation point and is little dependent on temperature. To
further understand the temperature dependence of the damping parameter in
ferrimagnets, we analyze several data sets from literature for a ferrimagnetic
insulator, gadolinium iron garnet, by using the two different definitions of
the damping parameter. Using two methods to estimate the individual sublattice
magnetizations, which yield results consistent with each other, we found that
in all the used data sets, the damping parameter ${\alpha}_{\text{FiM}}$ does
not increase at the angular compensation temperature and shows no anomaly
whereas the conventionally defined ${\alpha}_{\text{FM}}$ is strongly dependent
on the temperature. | 2009.12073v2 |
2020-09-25 | A Complex Stiffness Human Impedance Model with Customizable Exoskeleton Control | The natural impedance, or dynamic relationship between force and motion, of a
human operator can determine the stability of exoskeletons that use
interaction-torque feedback to amplify human strength. While human impedance is
typically modelled as a linear system, our experiments on a single-joint
exoskeleton testbed involving 10 human subjects show evidence of nonlinear
behavior: a low-frequency asymptotic phase for the dynamic stiffness of the
human that is different than the expected zero, and an unexpectedly consistent
damping ratio as the stiffness and inertia vary. To explain these observations,
this paper considers a new frequency-domain model of the human joint dynamics
featuring complex value stiffness comprising a real stiffness term and a
hysteretic damping term. Using a statistical F-test we show that the hysteretic
damping term is not only significant but is even more significant than the
linear damping term. Further analysis reveals a linear trend linking hysteretic
damping and the real part of the stiffness, which allows us to simplify the
complex stiffness model down to a 1-parameter system. Then, we introduce and
demonstrate a customizable fractional-order controller that exploits this
hysteretic damping behavior to improve strength amplification bandwidth while
maintaining stability, and explore a tuning approach which ensures that this
stability property is robust to muscle co-contraction for each individual. | 2009.12446v1 |
2020-11-26 | On the stabilization of breather-type solutions of the damped higher order nonlinear Schrödinger equation | Spatially periodic breather solutions (SPBs) of the nonlinear Schr\"o\-dinger
(NLS) equation are frequently used to model rogue waves and are typically
unstable. In this paper we study the effects of dissipation and higher order
nonlinearities on the stabilization of both single and multi-mode SPBs in the
framework of a damped higher order NLS (HONLS) equation. We observe the onset
of novel instabilities associated with the development of critical states which
result from symmetry breaking in the damped HONLS system. We broaden the
Floquet characterization of instabilities of solutions of the NLS equation,
using an even 3-phase solution of the NLS as an example, to show instabilities
are associated with degenerate complex elements of both the periodic and
continuous Floquet spectrum. As a result the Floquet criteria for the
stabilization of a solution of the damped HONLS centers around the elimination
of all complex degenerate elements of the spectrum. For an initial SPB with a
given mode structure, a perturbation analysis shows that for short time only
the complex double points associated with resonant modes split under the damped
HONLS while those associated with nonresonant modes remain effectively closed.
The corresponding damped HONLS numerical experiments corroborate that
instabilities associated with nonresonant modes persist on a longer time scale
than the instabilities associated with resonant modes. | 2011.13334v1 |
2020-12-22 | Comparison of local and global gyrokinetic calculations of collisionless zonal flow damping in quasi-symmetric stellarators | The linear collisionless damping of zonal flows is calculated for
quasi-symmetric stellarator equilibria in flux-tube, flux-surface, and
full-volume geometry. Equilibria are studied from the quasi-helical symmetry
configuration of the Helically Symmetric eXperiment (HSX), a broken symmetry
configuration of HSX, and the quasi-axial symmetry geometry of the National
Compact Stellarator eXperiment (NCSX). Zonal flow oscillations and long-time
damping affect the zonal flow evolution, and the zonal flow residual goes to
zero for small radial wavenumber. The oscillation frequency and damping rate
depend on the bounce-averaged radial particle drift in accordance with theory.
While each flux tube on a flux surface is unique, several different flux tubes
in HSX or NCSX can reproduce the zonal flow damping from a flux-surface
calculation given an adequate parallel extent. The flux-surface or flux-tube
calculations can accurately reproduce the full-volume long-time residual for
moderate $k_x$, but the oscillation and damping time scales are longer in local
representations, particularly for small $k_x$ approaching the system size. | 2012.12213v2 |
2020-12-31 | Damping of slow surface kink modes in solar photospheric waveguides modeled by one-dimensional inhomogeneities | Given the recent interest in magnetohydrodynamic (MHD) waves in pores and
sunspot umbrae, we examine the damping of slow surface kink modes (SSKMs) by
modeling solar photospheric waveguides with a cylindrical inhomogeneity
comprising a uniform interior, a uniform exterior, and a continuous transition
layer (TL) in between. Performing an eigen-mode analysis in linear, resistive,
gravity-free MHD, our approach is idealized in that, among other things, our
equilibrium is structured only in the radial direction. We can nonetheless
address two damping mechanisms simultaneously, one being the Ohmic resistivity,
and the other being the resonant absorption of SSKMs in the cusp and
Alfv$\acute{\rm e}$n continua. We find that the relative importance of the two
mechanisms depends sensitively on the magnetic Reynolds number ($R_{\rm m}$).
Resonant absorption is the sole damping mechanism for realistically large
values of $R_{\rm m}$, and the cusp resonance in general dominates the
Alfv$\acute{\rm e}$n one unless the axial wavenumbers are at the lower end of
the observationally relevant range. We also find that the thin-boundary
approximation holds only when the TL-width-to-radius ratios are much smaller
than nominally expected. The Ohmic resistivity is far more important for
realistically small $R_{\rm m}$. Even in this case, SSKMs are only marginally
damped, with damping-time-to-period-ratios reaching $\sim 10$ in the parameter
range we examine. | 2012.15426v1 |
2021-02-24 | Finding the mechanism of wave energy flux damping in solar pores using numerical simulations | Context. Solar magnetic pores are, due to their concentrated magnetic fields,
suitable guides for magnetoacoustic waves. Recent observations have shown that
propagating energy flux in pores is subject to strong damping with height;
however, the reason is still unclear. Aims. We investigate possible damping
mechanisms numerically to explain the observations. Methods. We performed 2D
numerical magnetohydrodynamic (MHD) simulations, starting from an equilibrium
model of a single pore inspired by the observed properties. Energy was inserted
into the bottom of the domain via different vertical drivers with a period of
30s. Simulations were performed with both ideal MHD and non-ideal effects.
Results. While the analysis of the energy flux for ideal and non-ideal MHD
simulations with a plane driver cannot reproduce the observed damping, the
numerically predicted damping for a localized driver closely corresponds with
the observations. The strong damping in simulations with localized driver was
caused by two geometric effects, geometric spreading due to diverging field
lines and lateral wave leakage. | 2102.12420v1 |
2022-04-08 | Damped Strichartz estimates and the incompressible Euler--Maxwell system | Euler--Maxwell systems describe the dynamics of inviscid plasmas. In this
work, we consider an incompressible two-dimensional version of such systems and
prove the existence and uniqueness of global weak solutions, uniformly with
respect to the speed of light $c\in (c_0,\infty)$, for some threshold value
$c_0>0$ depending only on the initial data. In particular, the condition
$c>c_0$ ensures that the velocity of the plasma nowhere exceeds the speed of
light and allows us to analyze the singular regime $c\to\infty$.
The functional setting for the fluid velocity lies in the framework of
Yudovich's solutions of the two-dimensional Euler equations, whereas the
analysis of the electromagnetic field hinges upon the refined interactions
between the damping and dispersive phenomena in Maxwell's equations in the
whole space. This analysis is enabled by the new development of a robust
abstract method allowing us to incorporate the damping effect into a variety of
existing estimates. The use of this method is illustrated by the derivation of
damped Strichartz estimates (including endpoint cases) for several dispersive
systems (including the wave and Schr\"odinger equations), as well as damped
maximal regularity estimates for the heat equation. The ensuing damped
Strichartz estimates supersede previously existing results on the same systems. | 2204.04277v3 |
2022-05-11 | A new look at the frequency-dependent damping of slow-mode waves in the solar corona | Being directly observed in the Doppler shift and imaging data and indirectly
as quasi-periodic pulsations in solar and stellar flares, slow magnetoacoustic
waves offer an important seismological tool for probing many vital parameters
of the coronal plasma. A recently understood active nature of the solar corona
for magnetoacoustic waves, manifested through the phenomenon of wave-induced
thermal misbalance, led to the identification of new natural mechanisms for the
interpretation of observed properties of waves. A frequency-dependent damping
of slow waves in various coronal plasma structures remains an open question, as
traditional wave damping theories fail to match observations. We demonstrate
that accounting for the back-reaction caused by thermal misbalance on the wave
dynamics leads to a modification of the relationship between the damping time
and oscillation period of standing slow waves, prescribed by the linear theory.
The modified relationship is not of a power-law form and has the equilibrium
plasma conditions and properties of the coronal heating/cooling processes as
free parameters. It is shown to readily explain the observed scaling of the
damping time with period of standing slow waves in hot coronal loops.
Functional forms of the unknown coronal heating process, consistent with the
observed frequency-dependent damping, are seismologically revealed. | 2205.05346v1 |
2022-12-13 | The Effect of Internal Damping on Locomotion in Frictional Environments | The gaits of undulating animals arise from a complex interaction of their
central nervous system, muscle, connective tissue, bone, and environment. As a
simplifying assumption, many previous studies have often assumed that
sufficient internal force is available to produce observed kinematics, thus not
focusing on quantifying the interconnection between muscle effort, body shape,
and external reaction forces. This interplay, however, is critical to
locomotion performance in crawling animals, especially when accompanied by body
viscoelasticity. Moreover, in bio-inspired robotic applications, the body's
internal damping is indeed a parameter that the designer can tune. Still, the
effect of internal damping is not well understood. This study explores how
internal damping affects the locomotion performance of a crawler with a
continuous, visco-elastic, nonlinear beam model. Crawler muscle actuation is
modeled as a traveling wave of bending moment propagating posteriorly along the
body. Consistent with the friction properties of the scales of snakes and
limbless lizards, environmental forces are modeled using anisotropic Coulomb
friction. It is found that by varying the crawler body's internal damping, the
crawler's performance can be altered, and distinct gaits could be achieved,
including changing the net locomotion direction from forward to back. We will
discuss this forward and backward control and identify the optimal internal
damping for peak crawling speed. | 2212.06290v1 |
2023-01-19 | Inverse Problems of Identifying the Unknown Transverse Shear Force in the Euler-Bernoulli Beam with Kelvin-Voigt Damping | In this paper, we study the inverse problems of determining the unknown
transverse shear force $g(t)$ in a system governed by the damped
Euler-Bernoulli equation $\rho(x)u_{tt}+\mu(x)u_t+ (r(x)u_{xx})_{xx}+
(\kappa(x)u_{xxt})_{xx}=0, ~(x,t)\in (0,\ell)\times(0,T],$ subject to the
boundary conditions $u(0,t) =0$, $u_{x}(0,t)=0$,
$\left[r(x)u_{xx}+\kappa(x)u_{xxt}\right]_{x=\ell} =0$,
$-\left[\big(r(x)u_{xx}+\kappa(x)u_{xxt}\big)_{x}\right]_{x=\ell}=g(t)$, $t\in
[0,T]$, from the measured deflection $\nu(t):=u(\ell,t)$, $t \in [0,T]$, and
from the bending moment $\omega(t):=-\left(
r(0)u_{xx}(0,t)+\kappa(0)u_{xxt}(0,t) \right)$, $t \in [0,T]$, where the terms
$(\kappa(x)u_{xxt})_{xx}$ and $\mu(x)u_t$ account for the Kelvin-Voigt damping
and external damping, respectively.
The main purpose of this study is to analyze the Kelvin-Voigt damping effect
on determining the unknown transverse shear force (boundary input) through the
given boundary measurements. The inverse problems are transformed into
minimization problems for Tikhonov functionals, and it is shown that the
regularized functionals admit unique solutions for the inverse problems. By
suitable regularity on the admissible class of shear force $g(t),$ we prove
that these functionals are Fr\'echet differentiable, and the derivatives are
expressed through the solutions of corresponding adjoint problems posed with
measured data as boundary data associated with the direct problem. The
solvability of these adjoint problems is obtained under the minimal regularity
of the boundary data $g(t)$, which turns out to be the regularizing effect of
the Kelvin-Voigt damping in the direct problem. | 2301.07931v1 |
2023-03-28 | Escape Kinetics of an Underdamped Colloidal Particle from a Cavity through Narrow Pores | It is often desirable to know the controlling mechanism of survival
probability of nano - or microscale particles in small cavities such as, e.g.,
confined submicron particles in fiber beds of high-efficiency filter media or
ions/small molecules in confined cellular structures. Here we address this
issue based on numerical study of the escape kinetics of inertial Brownian
colloidal particles from various types of cavities with single and multiple
pores. We consider both the situations of strong and weak viscous damping. Our
simulation results show that as long as the thermal length is larger than the
cavity size the mean exit time remains insensitive to the medium viscous
damping. On further increasing damping strength, a linear relation between
escape rate and damping strength emerges gradually. This result is in sharp
contrast to the energy barrier crossing dynamics where the escape rate exhibits
a turnover behavior as a function of the damping strength. Moreover, in the
ballistic regime, the exit rate is directly proportional to the pore width and
the thermal velocity. All these attributes are insensitive to the cavity as
well as the pore structures. Further, we show that the effects of pore
structure variation on the escape kinetics are conspicuously different in the
low damping regimes compared to the overdamped situation. Apart from direct
applications in biology and nanotechnology, our simulation results can
potentially be used to understand diffusion of living or artificial micro/nano
objects, such as bacteria, virus, Janus Particle etc. where memory effects play
dictating roles. | 2303.16092v1 |
2023-08-22 | Investigating the characteristic shape and scatter of intergalactic damping wings during reionization | Ly$\alpha$ damping wings in the spectra of bright objects at high redshift
are a useful probe of the ionization state of the intergalactic medium during
the reionization epoch. It has recently been noted that, despite the
inhomogeneous nature of reionization, these damping wings have a characteristic
shape which is a strong function of the volume-weighted average neutral
hydrogen fraction of the intergalactic medium. We present here a closer
examination of this finding using a simulation of patchy reionization from the
Sherwood-Relics simulation suite. We show that the characteristic shape and
scatter of the damping wings are determined by the average neutral hydrogen
density along the line of sight, weighted by its contribution to the optical
depth producing the damping wing. We find that there is a redshift dependence
in the characteristic shape due to the expansion of the Universe. Finally, we
show that it is possible to differentiate between the shapes of damping wings
in galaxies and young (or faint) quasars at different points in the
reionization history at large velocity offsets from the point where the
transmission first reaches zero. | 2308.11709v1 |
2023-10-02 | Characterizing the Velocity-Space Signature of Electron Landau Damping | Plasma turbulence plays a critical role in the transport of energy from
large-scale magnetic fields and plasma flows to small scales, where the
dissipated turbulent energy ultimately leads to heating of the plasma species.
A major goal of the broader heliophysics community is to identify the physical
mechanisms responsible for the dissipation of the turbulence and to quantify
the consequent rate of plasma heating. One of the mechanisms proposed to damp
turbulent fluctuations in weakly collisional space and astrophysical plasmas is
electron Landau damping. The velocity-space signature of electron energization
by Landau damping can be identified using the recently developed field-particle
correlation technique. Here, we perform a suite of gyrokinetic turbulence
simulations with ion plasma beta values of 0.01, 0.1, 1, and 10 and use the
field-particle correlation technique to characterize the features of the
velocity-space signatures of electron Landau damping in turbulent plasma
conditions consistent with those observed in the solar wind and planetary
magnetospheres. We identify the key features of the velocity-space signatures
of electron Landau damping as a function of varying plasma \beta_i to provide a
critical framework for interpreting the results of field-particle correlation
analysis of in situ spacecraft observations of plasma turbulence. | 2310.01242v2 |
2023-10-07 | OEDG: Oscillation-eliminating discontinuous Galerkin method for hyperbolic conservation laws | Controlling spurious oscillations is crucial for designing reliable numerical
schemes for hyperbolic conservation laws. This paper proposes a novel, robust,
and efficient oscillation-eliminating discontinuous Galerkin (OEDG) method on
general meshes, motivated by the damping technique in [Lu, Liu, and Shu, SIAM
J. Numer. Anal., 59:1299-1324, 2021]. The OEDG method incorporates an OE
procedure after each Runge-Kutta stage, devised by alternately evolving
conventional semidiscrete DG scheme and a damping equation. A novel damping
operator is carefully designed to possess scale-invariant and
evolution-invariant properties. We rigorously prove optimal error estimates of
the fully discrete OEDG method for linear scalar conservation laws. This might
be the first generic fully-discrete error estimates for nonlinear DG schemes
with automatic oscillation control mechanism. The OEDG method exhibits many
notable advantages. It effectively eliminates spurious oscillations for
challenging problems across various scales and wave speeds, without
problem-specific parameters. It obviates the need for characteristic
decomposition in hyperbolic systems. It retains key properties of conventional
DG method, such as conservation, optimal convergence rates, and
superconvergence. Moreover, it remains stable under normal CFL condition. The
OE procedure is non-intrusive, facilitating integration into existing DG codes
as an independent module. Its implementation is easy and efficient, involving
only simple multiplications of modal coefficients by scalars. The OEDG approach
provides new insights into the damping mechanism for oscillation control. It
reveals the role of damping operator as a modal filter and establishes close
relations between the damping and spectral viscosity techniques. Extensive
numerical results confirm the theoretical analysis and validate the
effectiveness and advantages of the OEDG method. | 2310.04807v1 |
2023-12-07 | Probing levitodynamics with multi-stochastic forces and the simple applications on the dark matter detection in optical levitation experiment | If the terrestrial environment is permeated by dark matter, the levitation
experiences damping forces and fluctuations attributed to dark matter. This
paper investigates levitodynamics with multiple stochastic forces, including
thermal drag, photon recoil, feedback, etc., assuming that all of these forces
adhere to the fluctuation-dissipation theorem. The ratio of total damping to
the stochastic damping coefficient distinguishes the levitodynamics from cases
involving only one single stochastic force. The heating and cooling processes
are formulated to determine the limits of temperature change. All sources of
stochastic forces are comprehensively examined, revealing that dark matter
collisions cannot be treated analogously to fluid dynamics. Additionally, a
meticulous analysis is presented, elucidating the intricate relationship
between the fundamental transfer cross-section and the macroscopic transfer
cross-section. While the dark damping coefficient is suppressed by the mass of
the levitated particle, scattering can be coherently enhanced based on the
scale of the component microscopic particle, the atomic form factor, and the
static structure factor. Hence, dark damping holds the potential to provide
valuable insights into the detection of the macroscopic strength of fundamental
particles. We propose experimental procedures for levitation and employ linear
estimation to extract the dark damping coefficient. Utilizing current
levitation results, we demonstrate that the fundamental transfer cross section
of dark matter can be of the order $\sigma^{\rm D}_{T}\lsim {\cal
O}(10^{-26})\rm cm^2$. | 2312.04202v2 |
2024-01-23 | Damped kink motions in a system of two solar coronal tubes with elliptic cross-sections | This study is motivated by observations of coordinated transverse
displacements in neighboring solar active region loops, addressing specifically
how the behavior of kink motions in straight two-tube equilibria is impacted by
tube interactions and tube cross-sectional shapes.We work with linear, ideal,
pressureless magnetohydrodynamics. Axially standing kink motions are examined
as an initial value problem for transversely structured equilibria involving
two identical, field-aligned, density-enhanced tubes with elliptic
cross-sections (elliptic tubes). Continuously nonuniform layers are implemented
around both tube boundaries. We numerically follow the system response to
external velocity drivers, largely focusing on the quasi-mode stage of internal
flows to derive the pertinent periods and damping times. The periods and
damping times we derive for two-circular-tube setups justify available modal
results found with the T-matrix approach. Regardless of cross-sectional shapes,
our nonuniform layers feature the development of small-scale shears and energy
accumulation around Alf\'ven resonances, indicative of resonant absorption and
phase-mixing. As with two-circular-tube systems, our configurational symmetries
make it still possible to classify lower-order kink motions by the polarization
and symmetric properties of the internal flows; hence such mode labels as $S_x$
and $A_x$. However, the periods and damping times for two-elliptic-tube setups
further depend on cross-sectional aspect ratios, with $A_x$ motions
occasionally damped less rapidly than $S_x$ motions. We find uncertainties up
to $\sim 20\%$ ($\sim 50\%$) for the axial Alfven time (the inhomogeneity
lengthscale) if the periods (damping times) computed for two-elliptic-tube
setups are seismologically inverted with canonical theories for isolated
circular tubes. | 2401.12885v2 |
1995-02-08 | The Chemical Evolution of Damped Lyman Alpha Galaxies | Measurements of element abundances in damped Lyman alpha systems are
providing new means to investigate the chemical evolution of galaxies,
particularly at early times. We review progress in this area, concentrating on
recent efforts to extend the range of existing surveys to both higher and lower
redshifts. | 9502047v1 |
1996-01-19 | The Chemical Enrichment History of Damped Lyman-alpha Galaxies | Studies of damped Lyman-alpha absorption systems in quasar spectra are
yielding very interesting results regarding the chemical evolution of these
galaxies. We present some preliminary results from such a program. | 9601098v1 |
1997-01-30 | Initial Chemical Enrichment in Galaxies | We present evidence that damped Lyman-alpha galaxies detected in spectra of
quasars may not have started forming stars until the redshift z~3. If damped
Lyman-alpha absorbers are the progenitors of disk galaxies, then the above
result may indicate that star formation in galactic disks first began at z~3. | 9701241v1 |
1997-10-24 | The N/Si Abundance Ratio in Fifteen Damped Lyman-alpha Galaxies: Implications for the Origin of Nitrogen | Galactic chemical evolution model calculations indicate that there should be
considerable scatter in the observed N/O ratios at a fixed metallicity (O/H)
for galaxies with very low metallicities due to the delayed release of primary
N from intermediate mass stars relative to that of O from short-lived massive
stars. Moreover, the scatter should increase progressively toward decreasing
metallicity. Such effects have not been convincingly demonstrated by
observations of H II regions in nearby metal-poor galaxies, raising doubts
about the time-delay model of primary N production. Pettini et al and Lipman et
al realized the utility of high-redshift damped Lyman-alpha galaxies for
gaining further insights into the origin of N and discussed abundances in three
damped Lyman-alpha galaxies. Since abundance measurements for O are generally
unavailable for damped Lyman-alpha galaxies, they used N/Si or N/S in place of
N/O under the reasonable assumption that the abundance ratios O/Si and O/S are
the same as solar in damped Lyman-alpha galaxies. We discuss observations of
heavy element abundances in 15 high-redshift (z>2) damped Lyman-alpha galaxies,
many of which have metallicities comparable to or lower than the lowest
metallicity galaxy known locally (I Zw 18). We find that the N/Si ratios in
damped Lyman-alpha galaxies exhibit a very large scatter (about 1 dex) at
[Si/H]~-2 and there is some indication that the scatter increases toward
decreasing metallicity. Considerations of various sources of uncertainties
suggest that they are not likely the main causes of the large scatter. These
results thus provide strong support for the time-delay model of primary N
production in intermediate mass stars if, indeed, O/Si=solar in damped
Lyman-alpha galaxies. | 9710266v2 |
2001-06-05 | On Nonlinear Alfvén Waves Generated by Cosmic Ray Streaming Instability | Nonlinear damping of parallel propagating Alfv\'en waves in high-$\beta$
plasma is considered. Trapping of thermal ions and Coulomb collisions are taken
into account. Saturated damping rate is calculated. Applications are made for
cosmic ray propagation in the Galaxy. | 0106078v1 |
2001-10-15 | The UCSD HIRES/KeckI Damped Lya Abundance Database: II. The Implications | We present a comprehensive analysis of the damped Lya abundance database
presented in the first paper of this series. This database provides a
homogeneous set of abundance measurements for many elements including Si, Cr,
Ni, Zn, Fe, Al, S, Co, O, and Ar from 38 damped Lya systems with z > 1.5. With
little exception, these damped Llya systems exhibit very similar relative
abundances. There is no significant correlation in X/Fe with [Fe/H] metallicity
and the dispersion in X/Fe is small at all metallicity.
We search the database for trends indicative of dust depletion and in a few
cases find strong evidence. Specifically, we identify a correlation between
[Si/Ti] and [Zn/Fe] which is unambiguous evidence for depletion.
We present a discussion on the nucleosynthetic history of the damped Lya
systems by focusing on abundance patterns which are minimally affected by dust
depletion. We find [Si/Fe] -> +0.25 dex as [Zn/Fe] -> 0 and that the [Si/Fe]
values exhibit a plateau of ~+0.3 dex at [Si/H] < -1.5 dex. Together these
trends indicate significant alpha-enrichment in the damped Lya systems at low
metallicity, an interpretation further supported by the observed O/Fe, S/Fe and
Ar/Fe ratios. We also discuss Fe-peak nucleosynthesis and the odd-even effect.
To assess the impact of dust obscuration, we present estimates of the
dust-to-gas ratios for the damped Lya sightlines and crudely calculate dust
extinction corrections. The distribution of extinction corrections suggests the
effects of dust obscuration are minimal and that the population of 'missing'
damped systems has physical characteristics similar to the observed sample.
We update our investigation on the chemical evolution of the early universe
in neutral gas. [significantly abridged] | 0110351v1 |
2005-09-05 | Comment on "Damping of Tensor Modes in Cosmology" | We provide an analytic solution to the short wave length limit of the
integro-differential equation describing the damping of the tensor modes of
gravitational waves. | 0509096v2 |
1997-02-12 | Crossover from coherent to incoherent dynamics in damped quantum systems | The destruction of quantum coherence by environmental influences is
investigated taking the damped harmonic oscillator and the dissipative
two-state system as prototypical examples. It is shown that the location of the
coherent-incoherent transition depends to a large degree on the dynamical
quantity under consideration. | 9702115v1 |
1998-06-05 | Dielectric formalism and damping of collective modes in trapped Bose-Einstein condensed gases | We present the general dielectric formalism for Bose-Einstein condensed
systems in external potential at finite temperatures. On the basis of a model
arising within this framework as a first approximation in an intermediate
temperature region for large condensate we calculate the damping of low-energy
excitations in the collisionless regime. | 9806079v1 |
1999-05-27 | Do correlations create an energy gap in electronic bilayers? Critical analysis of different approaches | This paper investigates the effect of correlations in electronic bilayers on
the longitudinal collective mode structure. We employ the dielectric
permeability constructed by means of the classical theory of moments. It is
shown that the neglection of damping processes overestimates the role of
correlations. We conclude that the correct account of damping processes leads
to an absence of an energy gap. | 9905405v1 |
1999-11-16 | Damping of low-energy excitations of a Bose-condensed gas in the hydrodynamic regime | We develop a theory to describe the damping of elementary excitations of a
Bose-condensed gas in the hydrodynamic regime for the thermal cloud. We discuss
second sound in a spatially homogeneous gas and the lowest excitations of a
trapped condensate. | 9911238v2 |
2002-04-18 | Faraday patterns in Bose-Einstein condensates. Amplitude equation for rolls in the parametrically driven, damped Gross-Pitaevskii equation | The parametrically driven, damped Gross-Pitaevskii equation, which models
Bose-Einstein condensates in which the interatomic s-wave scattering length is
modulated in time, is shown to support spatially modulated states in the form
of rolls. A Landau equation with broken phase symmetry is derived, which
governs the dynamics of the roll amplitude. | 0204406v1 |
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