Energy Deposition into the Ionosphere during a Solar Flare with Extreme-ultraviolet Late Phase

Abstract Solar extreme-ultraviolet (EUV) irradiance is the dominant energy source for ionizing and heating the Earth’s upper atmosphere. It is common to assume that the spectra of different EUV lines have the same trend to fill the solar EUV irradiance gap for modeling purposes due to inadequate EUV irradiance measurements. However, the spectra across the EUV bands may not vary in the same trend. The flare radiation energy release in the EUV (10–120 nm) is about twice as much as X-rays (0.1–10 nm) during flare interval ∼03–06 UT on 2012 October 23. By assimilating the observed nonuniform varying, time-dependent, and high-resolution solar spectrum from the Solar Dynamics Observatory mission into the modeling framework, we provide the first direct evidence of nonuniform varying solar EUV fluxes during the solar flare EUV late phase (ELP) having appreciable effects on the dayside ionosphere. The total EUV radiation energy release (5.838 × 10 28 erg) during the flare ELP is larger than that (5.698 × 10 28 erg) during the flare main phase. The ELP of an X1.8-class solar flare on 2012 October 23 can increase the dayside ionospheric density at the subsolar point by ∼5 TECU and the ionospheric density enhancements extend from the bottom to the peak of the F2 region at low latitudes with relative changes ranging from ∼20% to ∼100%. Our results highlight the importance of incorporating a realistic, high spectral and temporal resolution solar irradiance spectrum into numerical models to capture the observed time-varying ionospheric response to solar flares.