Formation and Evolution of Low-Latitude F Region Field-Aligned Irregularities During the 7–8 September 2017 Storm: Hainan Coherent Scatter Phased Array Radar and Digisonde Observations

Abstract

In this paper, we present a study of the low-latitude field-aligned irregularities formation and evolution during the 7–8 September 2017 geomagnetic storm by analyzing data of the very high frequency coherent radar installed at Fuke, Hainan Island of China (19.5°N, 109.1°E; magnetic latitude 9.58°N) and a colocated Digisonde Portable Sounder. The prompt penetration of eastward interplanetary electric field associated with sudden southward turning of the interplanetary magnetic field Bz resulted in large ascent of the F layer, making conducive conditions at the bottomside of the layer for the growth of Rayleigh-Taylor instability and the development of the plasma irregularities in the postsunset hours. The irregularities persisted into the postmidnight sector when the southward interplanetary magnetic field Bz gradually decreased to the quiet time values. In addition, the base height of F layer at Fuke also showed a large elevation after midnight during two consecutive substorm onsets, suggesting that the substorm-induced overshielding penetration electric field may take over and modify the ambient zonal electric field in low-latitude ionosphere and induce the irregularities in the postmidnight sector. Moreover, different from the quiet time eastward movement of the irregularities observed over Fuke, the storm time irregularities displayed no zonal drift at the initial period and subsequently began drifting westward. The reversal of background plasma zonal drift velocity observed by Hainan digisonde characterized the storm time zonal drift pattern of the irregularities.