Understanding F10.7 Index
From radio flux to orbital decay
The chain from Sun to satellite is indirect but well understood. F10.7 tracks the Sun's extreme-ultraviolet radiation, which is absorbed high in the thermosphere and heats it. A hotter thermosphere expands upward, so a spacecraft at a fixed altitude finds itself in denser air, which increases aerodynamic drag and drains orbital energy. Because density falls off exponentially with height, the effect is dramatic in the lower part of low Earth orbit and negligible above roughly 800 km. Density models such as NRLMSISE-00 and JB2008 take both the daily F10.7 and its 81-day average as inputs, alongside geomagnetic indices, and high-precision numerical propagators call these models at each step to compute a satellite's drag. Simpler analytic propagators such as SGP4 do not read F10.7 directly; they instead carry the drag effect in a fixed B* term fitted into each TLE.
| Solar-cycle phase | Typical F10.7 (SFU) | Effect on LEO drag |
|---|---|---|
| Deep minimum | 64–70 | Cool, contracted thermosphere; slow decay |
| Rising / moderate | 100–150 | Atmosphere expands; decay noticeably faster |
| Solar maximum | 150–250+ | Hot, puffed-up thermosphere; rapid decay, earlier re-entries |
Why measure radio flux instead of EUV?
The radiation that actually heats the atmosphere is extreme ultraviolet, so why track a radio signal? Because EUV is absorbed before it reaches the ground and can only be measured from space, using instruments that are expensive and degrade over time. The 10.7 cm radio flux, by contrast, passes through cloud and atmosphere and can be recorded cheaply and consistently from the ground every day. It correlates closely with EUV output and with the sunspot number, so it captures the same activity at a fraction of the cost. The Canadian record has run without a break since February 1947 — measured near Ottawa, then at the Algonquin Radio Observatory, and at the Dominion Radio Astrophysical Observatory near Penticton, British Columbia, since 1991 — giving one of the longest, most uniform datasets in all of solar physics. Values are reported both as 'observed' flux and as 'adjusted' flux, scaled to a standard Sun–Earth distance of one astronomical unit.
F10.7 in the space-weather toolkit
F10.7 captures the slow, solar-cycle side of space weather — the gradual EUV heating that sets the atmosphere's baseline density over weeks to years. It works hand in hand with the Kp index, which measures fast geomagnetic disturbances driven by coronal mass ejections and geomagnetic storms. Those storms can spike thermospheric density within hours: in February 2022 a moderate storm raised drag enough to destroy dozens of newly launched Starlink satellites before they could raise their orbits. Density models therefore use F10.7 for the baseline and Kp for the sudden surges, and the two together underpin position prediction, conjunction screening and the forecasts behind the live satellite re-entry tracker.