Measurements and Modeling of the Responses of VLF Transmitter Signals to X-Class Solar Flares at the Great Wall Station in Antarctica

Solar flares can severely disturb the Earth's ionosphere, leading to changes of atmospheric chemistry and degradation of Global Navigation Satellite System (GNSS) signals. The key to better assess these effects is to more accurately quantify the electron density enhancements induced by solar flares, which is still uncertain. Thanks to the good coverage and quiet electromagnetic environment provided by the Great Wall Station (GWS, 62.22 degrees S, 58.96 degrees W) in Antarctica, our Very-Low-Frequency (VLF) receiver has recorded clean VLF signatures produced by three X-class solar flares from March to May in 2022. These measurements motivate us to revisit the uncertain question related to the ionospheric effects induced by solar flares. Different from previous studies, we model solar flare events from satellite measurements of X-ray fluxes to ground-based measurements of VLF transmitter signals by combining a flare irradiance model, an X-ray propagation model, a D-region chemistry model, and a VLF propagation model. For eight transmitting frequencies and propagation paths, modeling results can satisfactorily explain our VLF measurements at GWS during the three X-class flares, especially so for the X1.3 event occurring on 30 March 2022. Moreover, the electron density results that reconcile space- and ground-based measurements suggest that the VLF reflection height can be lowered to similar to 66 km by an X1.1 flare, in line with previous studies. The present set of numerical models can be thus utilized in future studies to improve the nowcasting capability of VLF sensing technique for monitoring solar flares, as well as the accuracy of GNSS navigation/positioning during solar flares.