What the SFI measures
The SFI is the strength of the Sun’s radio emission at a wavelength of 10.7 cm (2800 MHz), also written F10.7. It is measured in solar flux units (sfu), where 1 sfu equals 10-22 W per square metre per hertz. The value is recorded daily by the Penticton observatory in Canada, and NOAA publishes it for the whole world.
This radio emission tracks the same solar activity that produces ionising ultraviolet and X-ray radiation, which cannot be measured easily from the ground. So the SFI is a convenient stand-in for how much the Sun is ionising the upper atmosphere.
Typical values and what they mean
The SFI never drops far below about 65-70 at solar minimum and can exceed 250-300 at a strong solar maximum. As a practical guide:
- Below 80: only the lower HF bands are reliable; 15, 12 and 10 metres are mostly closed.
- 80 to 100: 20 and 17 metres are good in daylight, 15 metres opens on good paths.
- 100 to 150: 15 and 12 metres open regularly, 10 metres on the best days.
- Above 150: 10 metres and even 6 metres can offer worldwide openings.
SFI is not the whole story
A high SFI does not guarantee good conditions: a geomagnetic storm (high K or A index) or a flare-driven radio blackout can spoil a day with excellent flux. In the other direction, a modest SFI with quiet geomagnetic conditions often gives surprisingly good low-band DX at night.
Use the SFI to sunspot number converter to relate the flux to the sunspot count, and check the current value on the live solar data page.
