Winter Nighttime Enhancement of the Midlatitude Ionosphere: Contribution From the Diffusive and Wind‐Driven Plasma Transport

Abstract

AbstractThe characteristics of winter nighttime enhancement (WNE) of the midlatitude ionosphere at 300, 550, and 750 km altitude are presented, based on the radio occultation data from the Constellation Observing System for Meteorology, Ionosphere, and Climate. WNE at 750 km altitude has earlier start time and larger increasing rate than that at 550 km, possibly due to the downward plasma diffusion from above that dominates the most upper part of the ionosphere. Theoretical calculation of the plasma transport at 300 km altitude reveals the largest effect of downward diffusion at 45° geomagnetic latitude, consistent with the latitudinal distribution of WNE. Meanwhile, the wind‐driven plasma transport also contributes to WNE, at 30°W–180°W geographic longitude in the Southern Hemisphere. The strength and end time of WNE vary with longitude, and are correlated with the longitudinal variations of the enhancement in the conjugate (summer) hemisphere. This may be attributed to the transport processes in the interhemispheric geomagnetic flux tubes, which redistribute and balance the plasma between the conjugate hemispheres at night. The plasma flux from above is hence an important plasma source for further understanding of both the winter and summer nighttime enhancements.