Characterization of Short-Wave Fadeout Seen in Daytime SuperDARN Ground Scatter Observations

Short-wave fadeout (SWF) is a well-known radio wave anomaly which follows Earth-directed solar flares and leads to severe disruption of transionospheric high-frequency systems. The disruption is produced by flare-enhanced soft and hard X-rays that penetrate to the Dlayer where they dramatically enhance ionization leading to heavy high-frequency absorption over much of the dayside for an hour or more. In this paper, we describe how Super Dual Auroral Radar Network (SuperDARN) observations can be exploited to analyze SWF events. Superposed epoch analysis of multiple signatures reveals the typical characteristics of SWF. The number of SuperDARN ground scatter echoes drops suddenly (approximate to 100s) and sharply after a solar flare, reaching a maximum depth of suppression within a few tens of minutes, and then recovering to pre-SWF conditions over half an hour or so. The depth of echo suppression depends on the solar zenith angle, radio wave frequency, and intensity of the flare. Furthermore, ground scatter echoes typically exhibit a sudden phase change leading to a dramatic increase in apparent Doppler velocity (the so-called Doppler flash), which statistically precedes the dropout in ground scatter echoes. We report here on the characterization of SWF effects in SuperDARN ground scatter observations produced by several X class solar flares. We also describe the functional dependence of peak Doppler flash on solar zenith angle, frequency, and peak intensity of solar flux. Plain Language Summary This study is about how energetic electromagnetic eruptions (solar flares) coming from the Sun alter the physical properties of the ionosphere which affects the over the horizon high-frequency (HF) radio communication channels, commonly known as short-wave fadeout (SWF). The study characterizes different facts and features of SWF seen in daytime observations of the SuperDARN HF radar (a monostatic HF transceiver system, which simulates a two-way HF communication link) during different solar flare events. The article also demonstrates how the physical location of HF transmitter/receiver, HF transmitter frequency, and intensity of solar flare impacts SWF.