F.2.3 S10.7

The NASA/ESA Solar and Heliospheric Observatory (SOHO) research satellite operates in a halo orbit at the Lagrange Point 1 (L1) on the Earth-Sun line and has an uninterrupted view of the Sun. One of the instruments on SOHO is the Solar Extreme-ultraviolet Monitor (SEM) that measures the 26–34 nm solar EUV emission with 15-second time resolution in its first order broadband wavelength range. The integrated 26–34 nm emission is normalized. The normalized value is converted to sfu through linear regression with F10.7 (for historical data) over the common time frame and the resulting index is called S10.7.

The broadband (wavelength integrated) SEM 26-34 nm irradiances, represented by the S10.7 index, are EUV line emissions dominated by the chromospheric He II line at 30,4 nm with contributions from other chromospheric and coronal lines. This energy principally comes from solar active regions, plage, and network. Once the photons reach the Earth, they are deposited (absorbed) in the terrestrial thermosphere mostly by atomic oxygen above 200 km.