Driving Influences of the Doppler Flash Observed by SuperDARN HF Radars in Response to Solar Flares

Abstract

Sudden enhancement in high‐frequency absorption is a well‐known impact of solar flare‐driven Short‐Wave Fadeout (SWF). Less understood, is a perturbation of the radio wave frequency as it traverses the ionosphere in the early stages of SWF, also known as the Doppler flash. Investigations have suggested two possible sources that might contribute to it’s manifestation: first, enhancements of plasma density in the D‐and lower E‐regions; second, the lowering of the F‐region reflection point. Our recent work investigated a solar flare event using first principles modeling and Super Dual Auroral Radar Network (SuperDARN) HF radar observations and found that change in the F‐region refractive index is the primary driver of the Doppler flash. This study analyzes multiple solar flare events observed across different SuperDARN HF radars to determine how flare characteristics, properties of the traveling radio wave, and geophysical conditions impact the Doppler flash. In addition, we use incoherent scatter radar data and first‐principles modeling to investigate physical mechanisms that drive the lowering of the F‐region reflection points. We found, (a) on average, the change in E‐ and F‐region refractive index is the primary driver of the Doppler flash, (b) solar zenith angle, ray’s elevation angle, operating frequency, and location of the solar flare on the solar disk can alter the ionospheric regions of maximum contribution to the Doppler flash, (c) increased ionospheric Hall and Pedersen conductance causes a reduction of the daytime eastward electric field, and consequently reduces the vertical ion‐drift in the lower and middle latitude ionosphere, which results in lowering of the F‐region ray reflection point.

Heliophysics and amateur radio: citizen science collaborations for atmospheric, ionospheric, and space physics research and operations

“…Figure 14 (from Collins et al, 2023) shows the response of a network of Grape Personal Space Weather Stations to X-ray solar flares on 28 October 2021. The response is a Doppler "flash", similar to the signature observed by SuperDARN radars (Chakraborty et al, 2018;Chakraborty et al, 2021;Chakraborty et al, 2022). Figure 14A presents NOAA GOES-17 0.1-0.8 nm band X-ray flux measurements showing an X1 class flare at ∼1535 UTC and a C4.9 class flare at ∼1738 UTC.…”

“…Collisional absorption due to this D-region enhancement can cause complete fading out of dayside HF radio communications for periods ranging from a few minutes to an hour or more. Because solar EUV and Xray energy propagate at the speed of light, it takes ∼8 min for flares to travel from the Sun to the Earth and no advanced warning of these impacts is possible (Dellinger, 1937;Benson, 1964;McNamara, 1979;Chakraborty et al, 2018;Chakraborty et al, 2019;Chakraborty et al, 2021;Chakraborty et al, 2022).…”

Statistical analysis for EUV dynamic spectra and their impact on the ionosphere during solar flares

“…The phenomena caused by the sudden ionization of the Earth's upper atmosphere by X-rays and EUV emitted from solar flares are widely known as sudden ionospheric disturbances (SIDs) (Davies 1990;Donnely 1969;Mitra 1974). SIDs are sudden disturbances of the ionosphere and are a superset of events, which consists of Dellinger effect (Dellinger 1937), short-wave fadeout (SWF) (e.g., Chakraborty et al 2018Chakraborty et al , 2019Chakraborty et al , 2022Fiori et al 2018Fiori et al , 2022, sudden frequency and phase deviation (SFD/SPD) (Chakraborty et al 2018;Watanabe and Nishitani 2013), solar flare effects (SFEs) (e.g., Curto et al 2018), and sudden increase in total electron content (SITEC) (e.g., Tsugawa et al 2006;Zhang et al 2011). In 1957, the International Geophysical Year, much was offered from around the world on SIDs, and research in this field began to advance dramatically (Curto 2020).…”