Space weather

F10.7 cm Solar Flux Explained: Reading SFI for Band Conditions

F10.7 cm solar flux, often abbreviated SFI (Solar Flux Index), is a daily measurement of the sun’s radio emissions at 10.7 cm wavelength (2.8 GHz). It has been measured continuously since 1947 and serves as the standard proxy for solar activity, especially for ham radio propagation forecasting. Higher SFI generally lifts foF2, raises the Maximum Usable Frequency (MUF), and opens higher bands. Lower SFI does the opposite.

Why it matters for HF operating

SFI is the single most reliable indicator of “how high are the bands today?” During a low SFI period (under 80), 20 meters may be the highest reliably open band, and 15 meters and 10 meters may be closed for weeks at a time. During a high SFI period (over 150), 10 meters and even 6 meters can carry signals worldwide for hours each day. Operators use SFI to set expectations: look at the number, then pick a band that fits the activity level.

Key values to know

SFI Solar activity What to expect on HF
< 70 Solar minimum Lower bands dominant; 10m and 12m mostly closed
70 to 100 Quiet 20m, 17m reliable; 15m sporadic
100 to 150 Moderate 15m and 12m open daily; 10m starting to open
150 to 200 Active 10m worldwide; 6m possible on F2
> 200 High activity All HF bands strong; flare risk elevated
  • Update cadence. Published once per day from the Penticton observatory in Canada.
  • Smoothed value. A 90-day smoothed average is also published for solar-cycle tracking.
  • No direct flare information. SFI tracks background activity. Flares are reported separately as X-ray flux events.

Common misconceptions

A higher SFI does not guarantee good HF conditions today. The smoothed solar flux drives long-term ionospheric ionization, but day-to-day bands also depend on Kp (geomagnetic state), recent flare activity, and seasonal effects. An SFI of 180 with a Kp of 6 produces worse HF than an SFI of 120 with a Kp of 1. SFI sets the ceiling; geomagnetic conditions determine how close to that ceiling you actually get.

  • Sunspot number: the closely related solar activity counter
  • Solar cycle: the 11-year rhythm SFI tracks
  • foF2: the ionospheric measurement SFI predicts
  • MUF: the operational ceiling SFI helps lift
  • Solar flares: separate event-based flux that SFI does not capture

SkyWave shows current F10.7 cm solar flux on the Go screen with plain-language context for what it means for today’s bands. See it in the app →