# data/ — SIDC SILSO Monthly Mean Total Sunspot Number

## Source

- **Provider**: SIDC — Solar Influences Data Analysis Center, Royal Observatory of Belgium
- **Data URL**: <https://www.sidc.be/SILSO/INFO/snmtotcsv.php>
- **Version**: International Sunspot Number v2.0 (ISN v2, recalibrated; Clette et al. 2016, 2021)
- **Coverage**: Monthly values from January 1749 to present (~3300+ observations)
- **Definition**: Monthly mean of daily total international sunspot number (ISN),
  counting both sunspot groups and individual spots weighted by a standard formula.

## License

**CC-BY-NC-4.0** — Creative Commons Attribution–NonCommercial 4.0 International.
<https://creativecommons.org/licenses/by-nc/4.0/>

Attribution: SIDC, Royal Observatory of Belgium, "SILSO Monthly Mean Total Sunspot Number."
Non-commercial use only. If this benchmark is used commercially, a separate data agreement
with SIDC is required.

## How to rebuild `data.csv`

```bash
bash download.sh
```

The script:

1. `curl`s the SILSO semicolon-separated CSV (`snmtot_raw.csv`).
2. Parses columns: `year;month;decimal_year;ssn;ssn_std;n_obs;provisional`.
3. Drops rows where `ssn < 0` (SILSO missing-value sentinel is −1).
4. Computes `t_year = year + (month − 0.5) / 12` (mid-month decimal year).
5. Emits `data.csv` with col 0 = `ssn` (output).

`snmtot_raw.csv` and `data.csv` are both **gitignored**; only `download.sh` and this
`README.md` are committed. Rebuild from the script on a fresh clone.

## Column meanings

| index | name      | role   | units         | description |
|------:|-----------|--------|---------------|-------------|
| 0     | `ssn`     | output | dimensionless | Monthly mean total international sunspot number (ISN v2). Mean of daily totals within the calendar month. |
| 1     | `t_year`  | input  | decimal year  | Decimal year at the midpoint of the month, e.g. 1818.042 for January 1818. Continuous time coordinate. |
| 2     | `year`    | input  | integer year  | Integer calendar year. |
| 3     | `month`   | input  | integer 1–12  | Calendar month. Captures any intra-annual cycle in sunspot activity. |
| 4     | `ssn_std` | input  | dimensionless | Standard deviation of daily SSN values within the month. Higher values indicate more day-to-day variability. Not used by current ground_truth. |

## Shape

3327 rows × 5 columns (as of 2026-04-22; monthly observations from January 1749 to present,
after filtering missing values). Benchmark entries typically focus on the more
reliable instrumental record from 1818 onward.

## Notes

- **ISN v2.0**: The SILSO v2 recalibration (Clette & Lefevre 2016) rescaled the historical
  series by a factor of ~1.5 relative to the old v1 (Zürich) series. Always check which
  version is used when comparing to published solar-cycle analyses.
- **Solar cycles**: The ~11-year Schwabe cycle (sunspot number) and ~22-year Hale cycle
  (magnetic polarity) dominate the power spectrum. Cycle length varies between ~9 and ~14
  years. No accepted closed-form formula captures this variability.
- **Provisional values**: Recent months may be marked as provisional (`provisional` column);
  these are included but may be revised in future SILSO updates.