Midlatitude nighttime enhancement in F region electron density from global COSMIC measurements under solar minimum winter condition

Ionospheric electron density profiles retrieved from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites measurements from 6 November 2006 to 5 February 2007 are used to study the ionospheric nighttime electron density enhancements under winter, solar minimum, and geomagnetically quiet conditions. In this work, the peak electron densities of F₂ layer (N_nF₂) derived from COSMIC measurements are found to be in reasonably good agreement with ionosonde observations during the night. Therefore the morphology of nighttime enhancements is investigated at geomagnetic midlatitudes (MLAT 20-60°) in the northern hemisphere using COSMIC observations. The enhancements of electron density are evident near the F₂ layer peak at most latitudes and longitudes; however, significant variations in the latitudinal dependence of the occurrence time and net magnitudes of the enhancements are found in different regions. The characteristics of enhancements in the North Atlantic Ocean sector are distinctly different from those in the eastern part of the North American sector and also from those at longitudes from Europe to Asia, in the Pacific Ocean, and in the western part of the North American sectors. The longitudinal variations of the morphology of the electron density enhancements during nighttime are possibly caused by different downward plasma flux, meridional winds, and electric field drifts under the effect of the geomagnetic field.