Electrodynamics of the equatorial evening ionosphere: 1. Importance of winds in different regions

Abstract

The importance of winds at different altitudes and latitudes for the electrodynamics of the low-latitude evening ionosphere is examined with a model of the global coupled ionosphere-thermosphere system. The model reproduces the main observed features of the evening equatorial plasma vortex and the prereversal enhancement (PRE) of the vertical drift. The electrodynamics is driven primarily by the zonal wind forced by the diurnally varying zonal pressure-gradient force. The zonal wind lags the zonal pressure-gradient force owing to inertia. When ion drag is important, the time lag of the wind behind the pressure gradient force is shortened, and the high-altitude evening wind turns eastward earlier than the wind at lower altitudes, where ion drag is less important. Therefore, a vertical shear of the zonal wind tends to develop at altitudes around the transition between small and large ion drag at the bottom of the F region. This wind shear is closely associated with the vertical shear in the zonal convection velocity that is part of the evening plasma vortex. Unlike previous studies, we find that the winds driving the PRE lie mainly on field lines with apexes above the peak of the equatorial F layer, field lines that extend in magnetic latitude out to nearly 30• and encompass the entire evening equatorial ionization anomaly region. Contrary to previous suggestions, the westward convection in the bottomside of the evening plasma vortex is found to weaken, rather than strengthen, the PRE. Daytime winds have relatively little influence on the low-latitude evening electrodynamics.

Modeling the ionospheric prereversal enhancement by using coupled thermosphere‐ionosphere data assimilation

“…They demonstrate that the E × B vertical drift could be improved from “climatology” to “weather” for strong PRE by assimilating GPS‐TEC data. In some recent studies, upward plasma drift is shown to be a part of the evening plasma convection vortex driven by the neutral wind and ion drag effects [e.g., Kudeki and Bhattacharyya , ; Rodrigues et al ., ; Heelis et al ., ; Richmond et al ., ; Richmond and Fang , ]. The evening plasma vortex results from the eastward plasma flow at F layer height, the westward plasma flow in the E layer, and the vertical plasma flow that plays a role in balancing the zonal plasma shear.…”

“…As described in Richmond et al . [], the stronger eastward F layer ion convection due to the stronger eastward wind at EIA region leads to PRE strengthening. On the other hand, eliminating the low‐altitude westward wind is also found to PRE strengthening.…”

“…Thus, the modified neutral winds in experiment A are in consistent with the theoretical results given by Richmond et al . [] and lead to the stronger modeled PRE (Figure b). In experiment B, without change of wind strength by the assimilation, although the conductivities are changed, the intensification of PRE is, however, small.…”

“…In experiment B, without change of wind strength by the assimilation, although the conductivities are changed, the intensification of PRE is, however, small. Another important feature shown in Figure b is that the intensification of neutral winds occurs for both equatorial and low‐latitude regions, since the eastward wind responsible to PRE intensification is mainly those at low‐latitude EIA regions [ Richmond et al ., ].…”

“…[] and Richmond et al . [] have suggested the important role of eastward neutral wind at the F layer that drives ions to move with the winds, promoting their upward motion. The plasma convection vortex, eastward in the F layer, westward in the E layer, and upward around dusk, is adopted to give new insight into PRE formation.…”