Examining the role of flare-driven D-region electron density enhancement
on Doppler Flash
Abstract
Trans–ionospheric high frequency (HF) signals experience a strong
attenuation following a solar flare, commonly referred to as Short–Wave
Fadeout (SWF). Although solar flare-driven HF absorption is a well-known
impact of SWF, the occurrence of a frequency shift on radio wave signal
traversing the lower ionosphere in the early stages of SWF, also known
as “Doppler Flash”, is newly reported and not well understood. Some
prior 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. Observations and modeling evidence
regarding the manifestation and evolution of Doppler Flash in the
ionosphere are limited. This study seeks to advance our understanding of
the initial impacts of solar flare-driven SWF. We use WACCM-X to
estimate flare-driven enhanced ionization in D, E, and F-regions and a
ray-tracing code (Pharlap) to simulate a 1-hop HF communication through
the modified ionosphere. Once the ray traveling path has been
identified, the model estimates the Doppler frequency shift along the
ray path. Finally, the outputs are validated against observations of SWF
made with SuperDARN HF radars. We find that changes in refractive index
in the D and lower E regions due to plasma density enhancement are the
primary cause of Doppler Flash.