A modelling study of the latitudinal variations in the nighttime plasma temperatures of the equatorial topside ionosphere during northern winter at solar maximum

Latitudinal variations in the nighttime plasma temperatures of the equatori al topside ionosphere during northern winter at solar maximum have been exa mined by using values modelled by SUPIM (Sheffield University Plasmasphere Ionosphere Model) and observations made by the DMSP F10 satellite at 21.00 LT near 800 km altitude. The modelled values confirm that the crests observ ed near 15 degrees latitude in the winter hemisphere are due to adiabatic h eating and the troughs observed near the magnetic equator are due to adiaba tic cooling as plasma is transported along the magnetic field lines from th e summer hemisphere to the winter hemisphere. The modelled values also conf irm that the interhemispheric plasma transport needed to produce the requir ed adiabatic heating/cooling can be induced by F-region neutral winds. It i s shown that the longitudinal variations in the observed troughs and crests arise mainly from the longitudinal variations in the magnetic meridional w ind. At longitudes where the magnetic declination angle is positive the eas tward geographic zonal wind combines with the northward (summer hemisphere to winter hemisphere) geographic meridional wind to enhance the northward m agnetic meridional wind. This leads to deeper troughs and enhanced crests. At longitudes where the magnetic declination angle is negative the eastward geographic zonal wind opposes the northward geographic meridional wind and the trough depth and crest values are reduced. The characteristic features of the troughs and crests depend, in a complicated manner, on the field-al igned flow of plasma, thermal conduction, and inter-gas heat transfer. At t he latitudes of the troughs/crests, the low/high plasma temperatures lead t o increased/decreased plasma concentrations.