Keywords:Geomagnetic storm, Ionosphere, Storm enhanced density, Equatorial ionization anomaly, Midlatitude, Convection electric field
The global electron density distribution in the ionosphere depends strongly on geographical latitude, longitude, and local time. The structure changes severely from the high-latitude to the equatorial regions associated with geomagnetic storms. The storm-time prominent ionospheric phenomena are tongue of ionization (TOI), storm-enhanced density (SED), and equatorial ionization anomaly (EIA). These phenomena are very dynamic because ionospheric electric fields and particle precipitation from the magnetosphere vary significantly during the development and decay of the geomagnetic storms. The generation mechanism of SED has been thought as local upward ExB drifts [e.g., Huang et al., 2005, Liu et al., 2016], westward plasma transportation from the nightside to the dayside by sub-auroral polarization stream (SAPS) [Foster et al., 2007], equatorward neutral winds [Anderson, 1976], and latitudinal expansion of the EIA [e.g., Kelley et al., 2004]. However, since such different mechanisms of the formation of SED have been proposed by many researchers, a comprehensive understanding of the cause of SED formation has not yet been done. In this study, we investigate the temporal and spatial evolutions of SED during the development and decay of geomagnetic storms using Global Navigation Satellite System (GNSS) Total Electron Content (TEC) data with high time and spatial resolutions, to identify the main mechanism responsible for SED. We also use solar wind data and geomagnetic indices to see the interplanetary condition and geomagnetic activity during geomagnetic storms. In the present analysis, we first calculate the average absolute TEC (ATEC) of 10 geomagnetically quiet days every month, referring to the list of quiet and disturbed days provided by GFZ. Next, we subtract the storm-time ATEC from the average ATEC and create global maps of the difference ATEC in geographical and geomagnetic coordinates. Several hours after the onset of the main phase of the geomagnetic storms, the enhanced TEC region with a narrow latitudinal width (5-10 degrees) appears at high latitudes (60-70 degrees in geomagnetic latitude (GMLAT)) of the afternoon sector (12-16 h, MLT: magnetic local time). The enhanced TEC region is located at a lower latitude of the equatorward wall of the midlatitude trough. As the geomagnetic storms grow, the enhanced TEC region expands to the nightside (~20 h, MLT) within a time scale of 20-30 minutes, and the region moves equatorward within several hours. After that, apart from the midlatitude enhanced TEC phenomena, another TEC enhancement takes place in the low-latitude region (15-30 degrees, GMLAT) on both sides of the dip equator. This observational fact cannot be explained by the latitudinal expansion of the EIA and westward transportation of a part of the higher latitude EIA by SAPS [Kelley et al., 2004; Foster et al., 2007]. As already proposed by Liu et al. , the midlatitude enhanced TEC signature may be caused by upward ExB drifts that lift the ionosphere in the sunlit region to higher altitudes. In future study, we verify the effect of the localized ExB drifts on the formation of SED from a comparison between the global TEC and ionospheric plasma flow observed by the SuperDARN radars.