Space weather

Sunspot Number (SSN): The Solar Cycle’s Heartbeat

Sunspot number (SSN) is a daily count of active regions on the sun, weighted by group size and individual spot count. It has been observed continuously since the 17th century and is the longest-running solar activity record. For HF radio, SSN correlates closely with the Solar Flux Index (SFI) and the state of the F2 layer. When sunspots are abundant, the higher bands open; when sunspots are sparse, HF activity concentrates on the lower bands.

Why it matters for HF operating

SSN tells you where the solar cycle is and what HF bands are likely to support activity. During solar maximum, when SSN regularly exceeds 150, the upper HF bands (15 meters, 12 meters, 10 meters) carry signals worldwide. During solar minimum, when SSN can sit near zero for months, the upper bands often go quiet for extended periods and 40 meters and 80 meters become the workhorse bands. Operators planning equipment investments or antenna projects use SSN to anticipate which bands pay off.

Key values to know

SSN Cycle phase What to expect on HF
0 to 10 Solar minimum 10m closed; 15m and 17m sporadic; 20m and below reliable
30 to 70 Rising / falling 15m and 12m opening; 10m starting
70 to 130 Active All bands open daily
130+ Solar maximum 10m worldwide; 6m via F2 layer possible
  • Two flavors. “International Sunspot Number” (SILSO) and “American Sunspot Number” (NOAA) use slightly different formulas; both are published.
  • Smoothed series. A 13-month smoothed SSN is the standard for solar-cycle tracking.
  • Lag with HF. Short-term spikes in SSN can take a few days to translate into ionospheric improvement.

Common misconceptions

SSN is not a perfect HF predictor. Two days with the same SSN can produce very different conditions if one has elevated Kp and the other does not. Geomagnetic state and recent flare activity often dominate the day-to-day picture, even at high SSN. The right way to use SSN is as the long-term context, with SFI and Kp as the daily tactical signals.

SkyWave’s data sources include sunspot context from the NOAA Space Weather Prediction Center (SWPC) so you can read SSN alongside today’s bands. See it in the app →