Tests of a New Solar Flare Model Against D and E Region Ionosphere Data

We present results from a suite of models designed to simulate solar flare effects on the D and E region of the ionosphere. This suite includes models of the solar spectrum, the ionosphere and of HF radiowave propagation. A central component of this system is the development of photoelectron ionization enhancement factors with higher energy resolution in the soft X-ray spectral region that can be used to supplement existing ionization schemes currently implemented in upper atmospheric general circulation models. We tested this photoelectron model in the NCAR Thermosphere-Ionosphere-Mesosphere- Electrodynamics General Circulation Model (TIME-GCM) and in a photochemical model of the D region. In both cases, we compared predicted flare response using two different input solar flare spectra. One is the Flare Irradiance Spectral Model (FISM) and the other is a physics based model called NRLFLARE. Our predictions for the E region were compared with incoherent scatter radar data and suggest that enhanced flux in the 1-2 nm spectral region, as indicated by NRLFLARE, is important for reproducing the observations. For the D region, we combined our theoretical results for the X1.3 flare of 7 September 2017 with ray tracing calculations that suggest 20-40 db of 6.4 MHz absorption. This agrees with previously published observations and model estimates, all of which suggest greater HF absorption than the operational D region absorption prediction model (swpc.noaa.gov/products/d-region-absorption-predictions-d-rap). Finally, our theoretical comparison with previously published empirical models derived from very low frequency data was less clear due, in part, to large differences between the different empirical models.