Driving Influences of Solar Flare Effects on the Doppler Flash Observed by SuperDARN HF Radars

Sudden enhancement in HF absorption is a well-known impact of solar flare-driven Short-Wave Fadeout (SWF). Less understood, is a perturbation of the radiowave frequency as it traverses the lower ionosphere in the early stages of SWF, also known as the Doppler flash. Pervious investigations have suggested two possible sources that might contribute to the manifestation of Doppler flash: first, enhancements of plasma density in the D and lower E regions; second, the lowering of the reflection point in the F region. In our previous work, we investigated a solar flare event using first principals modeling and SuperDARN HF radar observations and found that change in the F-region refractive index is the primary driver of the Doppler flash. In this study, we analyze studies on multiple solar flare events observed across different SuperDARN HF radars to determine how flare characteristics and geophysical conditions impact the Doppler flash. In addition, we use ISR data and first principles modeling to investigate the lowering of the F-region reflection points.