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°N, 136.10°E, magnetic latitude 25°N) in Japan are studied using the electron and ion temperature ( T e and T i , respectively) data measured by the radar during nearly a full solar cycle (1986–1995). A comprehensive picture 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, while 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, while daytime T e decreases, the corresponding T i increases. The diurnal variation of T e is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in T e are illustrated through model calculations using the Sheffield University plasmasphere‐ionosphere model.