Wavelength dependence of solar irradiance enhancement during X-class flares and its influence on the upper atmosphere

The wavelength dependence of solar irradiance enhancement during flare events is one of the important factors in determining how the Thermosphere-Ionosphere (T-I) system responds to flares. To investigate the wavelength dependence of flare enhancement, the Flare Irradiance Spectral Model (FISM) was run for 61 X-class flares. The absolute and the percentage increases of solar irradiance at flare peaks, compared to pre-flare conditions, have clear wavelength dependences. The 0-14. nm irradiance increases much more (~680% on average) than that in the 14-25. nm waveband (~65% on average), except at 24. nm (~220%). The average percentage increases for the 25-105. nm and 122-190. nm wavebands are ~120% and ~35%, respectively. The influence of 6 different wavebands (0-14. nm, 14-25. nm, 25-105. nm, 105-120. nm, 121.56. nm, and 122-175. nm) on the thermosphere was examined for the October 28th, 2003 flare (X17-class) event by coupling FISM with the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) under geomagnetically quiet conditions (Kp=1). While the enhancement in the 0-14. nm waveband caused the largest enhancement of the globally integrated solar heating, the impact of solar irradiance enhancement on the thermosphere at 400. km is largest for the 25-105. nm waveband (EUV), which accounts for about 33. K of the total 45. K temperature enhancement, and ~7.4% of the total ~11.5% neutral density enhancement. The effect of 122-175. nm flare radiation on the thermosphere is rather small. The study also illustrates that the high-altitude thermospheric response to the flare radiation at 0-175. nm is almost a linear combination of the responses to the individual wavebands. The upper thermospheric temperature and density enhancements peaked 3-5. h after the maximum flare radiation.