Reversal of Ionospheric Density Perturbations at Daybreak due to Geomagnetic Activities

A statistical study is performed on the vertical electron density profiles observed by the GPS Occultation Experiment aboard satellites of the FORMOSAT-3/COSMIC mission. The study examines the influence of geomagnetic disturbances on the mid-latitude density in the dawn/pre-dawn sector of the ionosphere under different ranges of solar zenith angles, covering locations before and after daybreak. The statistical observations are governed by multiple effects, corresponding to different mechanisms induced by the occurrence of auroras. One effect is a reduction of the density in the F2 region, whose importance becomes secondary when the auroras are associated with strong geomagnetic disturbances. The primary effect features an increase in the density at locations before daybreak, but an opposite change when the local ionosphere is already sunlit. This observed effect of aurora-induced reversal of daybreak density perturbation (ARDDP) can be explained by the dynamo mechanism associated with equatorward neutral wind at subauroral latitudes during auroral particle precipitation.