Ion temperature crests and troughs in the morning sector of the low-latitude and midlatitude topside ionosphere

The low-latitude and midlatitude topside ionospheric ion temperature at 600 km altitude has been measured with the Ionospheric Plasma and Electrodynamics Instrument (IPEI) instrument on board ROCSAT-1 satellite in the solar maximum year of 2000. Global hemispheric asymmetrical distribution of this topside ion temperature in the morning hour is presented here for the first time and is explained with the associated observation of the field-aligned ion flow and ion concentration. Crests of ion temperature are found along the dip latitudes between 10° and 25° in the winter hemisphere during the solstice seasons. The maximum in the temperature crests is located in a longitudinal region of positive magnetic declination in the southern hemisphere during the June solstice and in the longitudinal region of negative magnetic declination in the northern hemisphere during the December solstice. Ion compression from a downward field-aligned ion flow driven by morning neutral wind is concluded to cause the ion heating in the winter hemisphere as modeled by Bailey et al. [1975]. On the other hand, in the 0700-0800 LT sector, there are low-temperature regions at higher latitudes in a longitude region of negative magnetic declination in the southern hemisphere during the June solstice and in the longitudinal region of positive magnetic declination in the northern hemisphere during the December solstice. A low photoionization rate together with an ion expansion cooling process from the upward field-aligned plasma flow has resulted in such low temperature in these regions. As for temperature troughs observed near the dip equator in the morning hour, it is attributed to the convective heat loss in the summer hemisphere initially in the early morning hour and then to the rapid increase of plasma concentration at the dip equator in the late morning hour for all seasons.