Feature of Diurnal Double Maxima in the Topside Ionosphere Observed by ICON

This study investigates the diurnal variation of topside ionospheric plasma density using the Ionospheric Connection Explorer (ICON) observations from May to July in 2020 and 2021. The total ion density exhibits daytime double-maxima (DDM) patterns, also known as "bite-out" at magnetic latitudes from 10 degrees S to 20 degrees N and longitudes of 180 degrees-276 degrees E, but a single peak in other longitudes. The total ion density between 180 degrees and 276 degrees E reaches its first peak around 12 LT (Local Time) in both hemispheres and the second peak at 14-15 LT in the Northern Hemisphere, gradually shifting to around 16-17 LT in the Southern Hemisphere. The formation of the first peak is mainly influenced by vertical plasma drift, while the second peak is associated with both neutral wind and vertical plasma drift. Furthermore, this study provides direct observational evidence for the influence of neutral winds on the longitude and latitude differences of the topside DDM. Stronger southward magnetic meridional winds and vertical plasma drifts are observed at approximately 11-15 LT in the longitude sector with DDM compared to other longitudes, leading to a valley of ion density in the Southern Hemisphere around 15 LT. In the Northern Hemisphere, ion density continues to accumulate to form a second peak at 15 LT. Diurnal double maxima (DDM) occur in the daytime topside ionosphere around 180 degrees-276 degrees E in June Solstice This study presents probably evidence that neutral trans-equatorial winds cause the formation of the topside ionospheric DDM Simulations analyzed the respective contributions of neutral winds and E x B drift to the topside ionospheric DDM