Substorm Overshielding Electric Field at Low Latitude on the Nightside as Observed by the HF Doppler Sounder and Magnetometers

Abstract

The convection electric field increases during the growth phase of substorms, driving the DP 2 ionospheric currents at high‐to‐equatorial latitudes, intensifying the eastward equatorial electrojet (EEJ) on the dayside. During the expansion phase, the electric field is often reversed; i.e., overshielding occurs at subauroral‐to‐equatorial latitudes where the EEJ turns to the westward counterelectrojet (CEJ). In this paper, we show that the HF Doppler sounders detected the eastward overshielding electric field at low latitudes on the nightside simultaneously with the CEJ on the dayside. We also show that the overshielding often occurs during the substorm recovery due to the convection reduction, resulting in a two‐step form in both the dayside CEJ and nightside electric field. The opposite direction of the electric field on the dayside and nightside is consistent with the dusk‐to‐dawn potential electric field associated with the region 2 field‐aligned currents intensified by the substorm. The overshielding electric field was found to drive an eastward electrojet with appreciable magnitude in the nighttime equatorial ionosphere, which in turn causes an equatorial enhancement of the midnight positive bay.

Response of the Ionosphere‐Plasmasphere Coupling to the September 2017 Storm: What Erodes the Plasmasphere so Severely?

“…From Figure it can be said that a westward (convection) electric field has penetrated for about 6 hr at least in the nightside ionosphere. The nightside EEJ was an order of magnitude smaller than the dayside EEJ as shown for geomagnetic storms (Tsuji et al, ; Wei et al, ) and substorms (Hashimoto et al, ), which indicates penetration of the electric field to the nightside equatorial ionosphere.…”

“…The cause of the equatorial EEJ variations during storms is attributed to DP2 magnetic field variations (convection) due to IMF (e.g., Nishida, 1968;Kikuchi et al, 1996Kikuchi et al, , 2010, dynamic pressure (Sibeck et al, 1998), and Region 2 field-aligned current enhancement associated with the substorm onset (Ebihara et al, 2014;Fejer et al, 1979;Hashimoto et al, 2011Hashimoto et al, , 2017Kikuchi et al, 2000). However, it is challenging to quantify the relative contributions of individual effects.…”

Interhemispheric Asymmetry in Response of Low‐Latitude Ionosphere to Perturbation Electric Fields in the Main Phase of Geomagnetic Storms

“…Coming to the coupling that further transfers the energy accumulated at polar latitudes during storm time to middle and lower latitudes, it is manifested by means of transfer of electric fields, equatorward TADs and disturbed thermospheric meridional winds. Reduction in transmission efficiency can be different for different longitude sectors due to geometrical differences (Hashimoto et al, 2017;Kikuchi, 2014). Moreover, the processes such as electric field and TADs could transmit the part of energy across the dip equator (Bruinsma & Forbes, 2007;Lu et al, 2012;Pradipta et al, 2016).…”

“…The equatorward winds launched during the MP of the storm later on reach the middle and middle-to-low latitudes forming the disturbance dynamo wind system that produces opposing electric fields to the prevailing fields (Blanc & Richmond, 1980;Fuller-Rowell et al, 2002;Fejer et al, 2016), whereas the convection electric field associated with interplanetary electric field (IEF Ey), derived as IEF Ey = −Vsw × Bz, is transmitted instantaneously from high latitudes to the dip equator (Senior & Blanc, 1984), known as prompt penetration electric field (PPEF; Kikuchi & Araki, 1979;Kelley et al, 1979;Kikuchi et al, 1996Kikuchi et al, , 2008. As far as PPEF is concerned, its magnitude and efficiency during each case vary for daytime and nighttime conditions with anomalous dawn and dusk behaviors (Hashimoto et al, 2017;Kikuchi et al, 2008;Milan et al, 2017;Peymirat et al, 2000). PPEF can immediately affect the equatorial ionospheric TEC (total electron content) when compared to TADs during the MP (Dashora et al, 2009;Maruyama et al, 2005).…”

“…Such transmission can continue for more than a few hours in some cases sweeping over from midlatitudes to the dip equator and across it Huang et al, 2007;Huba et al, 2005;Maruyama et al, 2005). Further, the instantaneous PPEF that originates in respective northern and southern polar regions traversing toward dip equator suffers geometric attenuation (Hashimoto et al, 2017;Kikuchi & Araki, 1979). Due to lack of observations and large variability from case to case in space and time, almost all the mechanisms of magnetosphere-ionosphere coupling mentioned above including the ones coupling high and low latitudes have not been clearly understood.…”