Quiet-Time Equatorial Plasma Fountain Observed by ICON

It is still poorly understood how Ionospheric Connection Explorer (ICON) can observe at ~600 km altitude the quiet-time equatorial plasma fountain and vertical E×B drift variation over the dip equator including its before-reversal evening increase known as the pre-reversal enhancement (PRE). To fill this knowledge gap, this study uses multi-instrument and multi-point observations to demonstrate the Equatorial Ionization Anomaly (EIA) and the EIA’s breaking down along with their respective underlying forward and revers fountains, and their equatorial vertical upward and downward E×B drift drivers. Jicamarca radar data validated the vertical E×B drift’s daily variation and evening PRE. Total electron content maps verified the EIA’s spatial variation at ~350 km altitude. Before and during the PRE, ICON observed the EIA as a two-peak/single-peak structure at higher/lower sunspot numbers. Underlying the EIA, the forward fountain drift pattern appeared as a latitudinal narrow (~2.5^o) equatorial vertical upward E×B drift enhancement and broader off-equatorial downward drifts related to field-aligned downward plasma diffusions. Underlying the breaking-down EIA, the reverse fountain showed an opposite drift pattern. As a conclusion, a ±1.25^oN (dip) latitude restriction should be applied to the ICON data to correctly specify the PRE by excluding off-equatorial drifts.