Diurnal variation of the ion temperature

Abstract

Results on the diurnal variation of the ion temperature, obtained from the Thomson scatter spectra at Arecibo, in the height range 250-475 km are presented. At 250 km where the ion temperature (T_i) is expected to be nearly equal to the neutral temperature (T_n, T_i is generally observed to show diurnal maximum at 16 hr L.T. and diurnal minimum at 4 hr L.T., with no appreciable change in these times with solar activity. The average day to night T_i ratio at 250 km is found to be 1.45. These results are in apparent conflict with the satellite drag analysis in which Tn is found to have a diurnal maximum at 14 hr L.T. and day to night temperature ratio around 1.30. At 475 km where T_i, is significantly above T_n, the diurnal behavior of T_i is observed to be different on different days. The day-to-day and hour-to-hour variations in T_i at this height are found to be due to changes in the electron density and electron temperature and seem consistent in terms of an energy budget that includes heating of the ion gas by the hotter thermal electrons and cooling by the neutral atmosphere.