The Role of Flare-Driven Ionospheric Electron Density Changes on the Doppler Flash Observed by SuperDARN HF Radars

Trans-ionospheric high frequency (HF: 3–30 MHz) signals experience strong attenuation following a solar flare-driven sudden ionospheric disturbance (SID). Solar flare-driven HF absorption, referred to as short-wave fadeout, is a well-known impact of SIDs, but the initial Doppler frequency shift phenomena, also known as “Doppler flash” in the traveling radio wave is not well understood. This study seeks to advance our understanding of the initial impacts of solar flare-driven SID using a physics-based whole atmosphere model for a specific solar flare event. First, we demonstrate that the Doppler flash phenomenon observed by Super Dual Auroral Radar Network (SuperDARN) radars can be successfully reproduced using first-principles based modeling. The output from the simulation is validated against SuperDARN line-of-sight Doppler velocity measurements. We then examine which region of the ionosphere, D, E, or F, makes the largest contribution to the Doppler flash. We also consider the relative contribution of change in refractive index through the ionospheric layers versus lowered reflection height. We find: (a) the model is able to reproduce radar observations with an root-median-squared-error and a mean percentage error (δ) of 3.72 m/s and 0.67%, respectively; (b) the F-region is the most significant contributor to the total Doppler flash (∼48%), 30% of which is contributed by the change in F-region's refractive index, while the other ∼18% is due to change in ray reflection height. Our analysis shows lowering of the F-region's ray reflection point is a secondary driver compared to the change in refractive index.