juliensimon/silso-sunspot-number

SILSO Daily Sunspot Number

Part of the Space Weather Datasets collection on Hugging Face.

Daily total sunspot numbers from the World Data Center SILSO at the Royal Observatory of Belgium. This is the longest continuous scientific observation in history, with systematic daily records since 1818 and international coordination since 1981. Currently 76,061 daily records.

Dataset description

The International Sunspot Number is the primary index of solar activity, tracking the number of sunspots visible on the solar disk each day. Sunspots are temporary phenomena on the Sun's photosphere caused by magnetic flux concentrations. Their number follows an approximately 11-year cycle (the Schwabe cycle) that profoundly affects space weather, satellite operations, radio communications, and Earth's upper atmosphere.

SILSO (Sunspot Index and Long-term Solar Observations) at the Royal Observatory of Belgium serves as the World Data Center for sunspot number computation, collecting observations from a worldwide network of stations.

Sunspots are regions where intense magnetic flux tubes (typically 0.1-0.3 T) emerge through the photosphere, inhibiting convective energy transport and creating dark spots roughly 1000-1500 K cooler than the surrounding ~5800 K surface. They range in size from small pores barely resolvable in modest telescopes to complex active regions spanning over 100,000 km. The daily sunspot number is computed using the Wolf formula (R = k(10g + s), where g is the number of sunspot groups, s is the total number of individual spots, and k is a station-dependent scaling factor), then combined across the observer network into a single international index.

The approximately 11-year Schwabe cycle in sunspot number is the most visible manifestation of the solar magnetic dynamo operating in the Sun's convective zone. At solar minimum, the disk may be spotless for weeks; at maximum, daily counts can exceed 200-300. This cycle drives variations in the total solar irradiance (order 0.1%), extreme ultraviolet flux (factor of 10 or more), solar flare and CME rates, and the overall heliospheric magnetic field strength. These variations have direct consequences for satellite drag (through thermospheric heating), HF radio propagation, radiation exposure for astronauts and polar-route aviation, and the modulation of galactic cosmic ray flux at Earth.

The SILSO Version 2.0 series, released in 2015, recalibrated the entire historical record back to 1818 to correct for discontinuities introduced by changes in the reference observer. This makes it the most homogeneous long-baseline solar activity record available, spanning over 200 years of daily observations and encompassing roughly 19 complete solar cycles -- an invaluable resource for studying long-term solar variability, cycle prediction, and Sun-climate relationships.

Schema

Column	Type	Description
date	date	Observation date
decimal_date	float64	Fractional year (e.g. 2024.5)
sunspot_number	Int64	Daily total sunspot number (null if missing)
std_dev	float64	Standard deviation from multiple stations
n_observations	Int64	Number of observing stations
is_provisional	bool	True if value is provisional (not yet definitive)

Quick stats

• 76,061 daily records (1818-01-01 to 2026-03-31)
• All-time maximum: 528 on 1870-08-26
• 75,879 provisional values
• Solar Cycle 25 (current): peak so far 290, mean 86.5

Usage

from datasets import load_dataset

ds = load_dataset("juliensimon/silso-sunspot-number", split="train")
df = ds.to_pandas()

# Plot solar cycles
import matplotlib.pyplot as plt
df["date"] = pd.to_datetime(df["date"])
monthly = df.set_index("date").resample("MS")["sunspot_number"].mean()
monthly.plot(figsize=(14, 4), title="Solar Cycles - Monthly Mean Sunspot Number")
plt.ylabel("Sunspot Number")
plt.show()

# Current solar cycle 25
sc25 = df[df["date"] >= "2019-12-01"]
print(f"Cycle 25 max so far: {sc25['sunspot_number'].max()}")

# Compare cycle amplitudes
df["year"] = pd.to_datetime(df["date"]).dt.year
yearly = df.groupby("year")["sunspot_number"].mean()

Data source

SILSO, World Data Center for the Sunspot Index, Royal Observatory of Belgium, Brussels.

Update schedule

Monthly (1st at 09:00 UTC) via GitHub Actions.

Related datasets

• solar-flare-index -- Solar flare observations
• kp-index -- Geomagnetic Kp index
• dst-index -- Geomagnetic Dst index

Pipeline

Source code: juliensimon/space-datasets

Support

If you find this dataset useful, please give it a ❤️ on the dataset page and share feedback in the Community tab! Also consider giving a ⭐️ to the space-datasets repo.

Citation

@dataset{silso_sunspot_number,
  author = {Simon, Julien},
  title = {SILSO Daily Sunspot Number},
  year = {2026},
  publisher = {Hugging Face},
  url = {https://huggingface.co/datasets/juliensimon/silso-sunspot-number},
  note = {Based on SILSO data, World Data Center for the Sunspot Index, Royal Observatory of Belgium}
}

License

CC-BY-4.0