Presunrise ion temperature enhancement observed at 600 km low- and mid-latitude ionosphere

The quiet-time low- and mid-latitude topside ionospheric ion temperature measured with ROCSAT-1/IPEI instrument is studied for local time, longitudinal, latitudinal, and seasonal variations for the solar maximum year of 2000. The statistical result shows two significant observations at the presunrise sector. Namely, the earliest presunrise ion temperature increase at 600 km low- and mid-latitude ionosphere always starts in the winter hemisphere for both summer and winter seasons; and the strongest presunrise ion-heating region is located in the longitudinal region between 165° and 195° during June summer and between 285° and 345° during December winter. Our simple calculation indicates that the temperature increase at the satellite altitude results from the heating process of photoelectrons that are produced at the magnetic conjugate-point where sunrise is at an earlier time. However, the mechanism to enhance the photoelectron heating at the strongest presunrise ion-heating region is still not clear, because the observed ion density and the field flow data fail to lend a clear support to the proposed heating mechanism for the current observations.