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.

Coordinated Ground‐Based and Space‐Based Observations of Equatorial Plasma Bubbles

“…The dark streaks that cut through the EIA band represent reduced emissions caused by low density within plasma bubbles. The airglow signature of EPBs and the tilted structure were also illustrated in simulations (Retterer, 2010) and are consistent with the previous ground-based and spaceborne observations (Jin et al, 2018;Kelley et al, 2003;Kil et al, 2009;Li et al, 2018;Tsunoda et al, 1982), which will be further discussed in the next section. These streaks elongate from northwest to southeast direction, with a meridional length of ∼1,500-2,000 km and interbubble distance of ∼500-800 km.…”

“…One of the top research priorities in the global space weather community is to better understand the generation mechanisms and the dynamic features of EPBs, because they can severely disrupt the amplitude and phase of transionospheric radio waves so as to cause adverse effects on relevant communication and navigation systems. In addition, the altitudinal information of irregularities embedded within EPBs can be examined through observations of plume-like structures from coherent backscatter radar or incoherent scatter radar measurements (Ajith et al, 2015;Jin et al, 2018;Li et al, 2013;Rodrigues et al, 2018;Tulasi Ram et al, 2017;Yokoyama & Fukao, 2006). During quiet times, the prereversal enhancement (PRE) of the zonal electric field is responsible for the enhanced upward E × B drift after sunset, which elevates the ionospheric height and subsequently amplifies the growth rate of R-T The morphological features and spatial/temporal variability of EPBs have been widely investigated via case studies and statistical analysis using different observational methods.…”

Merging of Storm Time Midlatitude Traveling Ionospheric Disturbances and Equatorial Plasma Bubbles

“…The solar wind and IMF conditions during 7–8 September 2017 have been described in several recent papers (e.g., Aa et al, ; Jin et al, ; Lei et al, ; Li et al, ; Shen et al, ), which are also shown here in Figures a–d. It was a storm with a double main phase.…”

“…During storm time, the occurrence of EPBs can be enhanced or suppressed due to two different perturbation electric fields: (1) the prompt penetration electric field (PPEF), which is created by solar wind‐magnetosphere coupling after the southward turning of interplanetary magnetic field (IMF) B z , can superpose upon the normal PRE to facilitate the development of EPBs on the duskside (Abdu et al, ; Basu et al, , ; Huang et al, ; Ram Tulasi et al, ); (2) ionospheric disturbance dynamo electric field, which is caused by changes in global thermosphere circulation due to Joule heating in the auroral zone, can inhibit the occurrence of EPBs on the duskside (Carter et al, ; Li, Ning, Liu, et al, ; Ramsingh et al, ; Scherliess & Fejer, ). In addition, the substorm‐related shielding electric field could also influence the zonal electric field (Ebihara & Tanaka, ; Jin et al, ). Moreover, several studies have found that under favorable storm time PPEF/PRE conditions, the EPBs can rise to higher altitude with plasma depletion extending along the magnetic field lines to midlatitude regions (e.g., Foster & Rich, ; Kelley et al, ; Ma & Maruyama, ; Mendillo et al, ), while in some extreme cases, the depletion signatures can even be measured around 40° magnetic latitude (MLAT; e.g., Aa et al, ; Cherniak & Zakharenkova, ; Katamzi‐Joseph et al, ; Martinis et al, ).…”

Topside Ionospheric Conditions During the 7–8 September 2017 Geomagnetic Storm

“…It was observed that the EPBs in the Asian sector had west-tilted irregularities pattern that drifted toward the west due to poleward increasing westward drifts. There are other interesting features associated with this storm relating to plasma irregularities as reported in other studies; for example, the persistence of irregularities into the postmidnight remarkably extended bubbles along the magnetic field lines in the form of depleted flux tubes and reaching up to middle latitudes (Aa et al, 2018(Aa et al, , 2019Jin et al, 2018). Most of the previous studies on this storm event were focused on ground-based observations and during the main phases, with very limited knowledge about the topside ionospheric response and irregularities during this event, especially for the recovery phase.…”