Chakraborty S, Qian L, Baker J B H, Ruohoniemi J M, Kuyeng K, Mclnerney J M
Bradley Department of Electrical and Computer Engineering Virginia Tech Blacksburg VA USA.
National Center for Atmospheric Research Boulder CO USA.
J Geophys Res Space Phys. 2022 Jun;127(6):e2022JA030342. doi: 10.1029/2022JA030342. Epub 2022 Jun 3.
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.
高频吸收的突然增强是太阳耀斑驱动的短波衰落（SWF）的一个众所周知的影响。鲜为人知的是，在SWF早期，无线电波在穿越电离层时会出现频率扰动，即所谓的多普勒闪变。研究表明，可能导致其出现的有两个潜在原因：一是D区和较低E区等离子体密度的增强；二是F区反射点的降低。我们最近的工作利用第一性原理建模和超级双极光雷达网络（SuperDARN）高频雷达观测研究了一次太阳耀斑事件，发现F区折射率的变化是多普勒闪变的主要驱动因素。本研究分析了不同SuperDARN高频雷达观测到的多个太阳耀斑事件，以确定耀斑特征、传播无线电波的特性和地球物理条件如何影响多普勒闪变。此外，我们使用非相干散射雷达数据和第一性原理建模来研究导致F区反射点降低的物理机制。我们发现，（a）平均而言，E区和F区折射率的变化是多普勒闪变的主要驱动因素；（b）太阳天顶角、射线仰角、工作频率以及太阳耀斑在太阳圆盘上的位置会改变对多普勒闪变贡献最大的电离层区域；（c）电离层霍尔电导率和佩德森电导率的增加会导致白天向东电场的减小，从而降低低纬度和中纬度电离层中的垂直离子漂移，进而导致F区射线反射点降低。
