An empirical model of the extreme ultraviolet solar spectrum as a function of F10.7

Abstract

AbstractThe F10.7 solar index is frequently used as a proxy for the solar extreme ultraviolet (EUV) irradiance. However, the relationship between F10.7 and the EUV spectrum has not been quantified in great detail for Solar Cycles 23 and 24. We use more than 2800 spectra (2003–2010) from the Solar EUV Experiment (SEE) to construct an empirical model of the EUV spectrum as a function of F10.7P = (F10.7+F10.7A) where F10.7A is the 81 day average. We compare our spectra to the HEUVAC model and find the HEUVAC soft X‐ray irradiance is ∼65% larger than SEE during solar minimum and HEUVAC H Lymann continuum fluxes are ∼30% smaller. We derive power law relationships between F10.7P and the ionizing irradiance—the integrated flux within some EUV wavelength band—in units of energy flux (FE) and photon flux (FP) for five neutral species and show that the relationship between F10.7P and FE is more linear than that of FP and that the magnitude of FP and its solar cycle variation is species dependent. In addition, we derive power law relationships between F10.7P and the ionization frequencies of these species and confirm that F10.7P is a better EUV proxy than F10.7. Finally, we investigate the hardening of the spectrum and its consequences for the Venus, Earth, and Mars ionospheres; the ratio of the soft X‐ray to EUV irradiance appropriate for Earth's E and F layers stayed constant during the prolonged solar minimum. Our results can elucidate ionospheric processes such as the saturation effect.