High altitude observations of electron temperature and a possible north-south asymmetry

Measurements of electron temperature made by the thermal electron energy di stribution (TED) instrument on board the EXOS-D (Akebono) satellite have be en analysed. From the data taken between 1989 and 1995, averaged daytime an d nighttime temperature profiles for different geophysical conditions have been produced. These profiles represent the averaged thermal electron tempe rature between 1000 and 8000 km altitude for conditions of high (F10.7 > 15 0) and low (F10.7 < 120) solar activity. Results indicate that increased so lar activity has a marked effect on the electron temperature. At 8000 km al titude, the typical low-latitude daytime electron temperature is around 800 0 K. The nighttime electron temperature at 8000 km is around 4000 K. The av eraged daytime difference between high and low solar activity conditions is around 1000 K at altitudes above 2500 km. Between 1000 and 2000 km altitud e this situation is reversed, and the electron temperature is comparatively higher during periods of low solar activity during both day and night. Com position changes in the region are proposed as a mechanism for this reversa l. In addition, there is evidence of an asymmetry in thermal electron tempe rature between the northern and southern hemispheres. The averaged electron temperature is found to be comparatively higher in the northern hemisphere during the daytime and comparatively higher in the southern hemisphere dur ing the nighttime. This difference between hemispheres is particularly evid ent during the nighttime, and during the rapid heating and cooling periods around sunrise and sunset. Possible reasons for the asymmetry are discussed . Profiles are also presented for conditio:ns of high (Ap>30) and low (Ap < 20) magnetic activity. Analysis has confirmed that geomagnetic activity ha s little effect on electron temperature below L = 2.2. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.