PLASMA TEMPERATURE-VARIATIONS IN THE IONOSPHERE OVER THE MIDDLE AND UPPER-ATMOSPHERE RADAR

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85 degrees N, 136.10 degrees E, mag netic latitude 25 degrees N) in Japan are studied using the electron a nd ion temperature (T-e and T-i, respectively) data measured by the ra dar during nearly a full solar cycle (1986-1995). A comprehensive pict ure of the diurnal, seasonal, and solar activity variations of T-e and T-i is presented for the altitude range 200-550 km. The temperatures T-e and T-i are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season; whi le daytime T-e is highest in summer and lowest in equinox, daytime T-i is highest in equinox and lowest in summer. With solar activity, whil e daytime T-e decreases, the corresponding T-i increases. The diurnal variation of T-e is characterized by morning and evening peaks. The oc currence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoel ectron heating of the morning and evening electron gas. However, neutr al winds play a dominant role in the appearance of the peaks. A polewa rd wind, which reduces the electron density to a low value before suns et, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in T-e are illustrate d through model calculations using the Sheffield University plasmasphe re-ionosphere model.