Characterization of Solar Flare Effects Observed by High Frequency Radar

Short-wave fadeout (SWF)  is a well-known anomaly in radio waves that occurs following solar flares directed towards Earth, causing significant disruptions in high-frequency systems that traverse the ionosphere. This disruption is a result of soft and hard X-rays emitted by the flares, which penetrate the D layer of the ionosphere, leading to enhanced ionization and substantial absorption of high-frequency signals over a large portion of the dayside for an extended period, usually lasting an hour or more. We explore the utilization of Super Dual Auroral Radar Network (SuperDARN) observations to analyze SWF events. By conducting a superposed epoch analysis on multiple observed features, we identify the typical characteristics of SWF. It is observed that the number of SuperDARN ground scatter echoes experiences a sudden and significant drop (occurring within approximately 100 seconds) immediately after a solar flare. The suppression of echoes reaches its maximum depth within a few tens of minutes and then gradually recovers to pre-SWF conditions over a period of about half an hour. The extent of echo suppression is influenced by the solar zenith angle, radio wave frequency, and flare intensity. Additionally, ground scatter echoes often exhibit a sudden phase change, resulting in a notable increase in apparent Doppler velocity known as the "Doppler flash," which statistically precedes the decrease in ground scatter echoes. The characterization of SWF effects based on SuperDARN ground scatter observations during several X-class solar flares. Furthermore, we investigate the relationship between the peak Doppler flash and solar zenith angle, frequency, and the peak intensity of solar flux.