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.