Anomalous F region response to moderate solar flares

Abstract

Ionograms recorded with a dynasonde at Bear Lake Observatory, Utah, during moderate solar x‐ray flares exhibit characteristic enhancements to the E and F1 region ionosphere. However, during these same flares, the peak electron density of the ionosphere (NmF2) unexpectedly decreases, recovering after the flare ends. In order to reconcile this anomalous behavior with expected increases to the total electron content (TEC), we undertake a modeling effort using the Time‐Dependent Ionospheric Model (TDIM) developed at Utah State University. For solar input, a simple flare time irradiance model is created, using measurements from the Solar EUV Experiment instrument on the TIMED spacecraft. TDIM simulations show that the anomalous NmF2 response can be explained by assuming a rapid electron temperature increase, which increases the O+ scale height, moving plasma to higher altitudes. The model results are able to reproduce both the decreasing NmF2 as well as the expected TEC enhancement.