Nighttime increases in ionosphere electron content (a statistical and experimental study)

Variations of nighttime ionosphere total electron content (TEC) have been studied using the Faraday rotation angle data obtained at middle latitudes (Almaty, Kazakhstan) from the geostationary satellite ETS-II during the years 1985-1986. Critical frequency values of the ionosphere F2-layer shown in the vertical sounding ionograms measured at Alma-Ata station (43.25 degreesN, 76.92 degreesE) have been used throughout the study. Morphological and statistical analyses are used to investigate the anomalous nighttime increases (ANI) in TEC that occurred during the years. The ANI events are looked at in terms of their amplitude and probability of occurrence, and their duration and peak time. Special features in the events are found to interpret the ANI as a travelling ionosphere disturbance. By examining the hourly, nightly values foF2 and TEC over the two years it has been suggested that the ANI events exist on an ionosphere background which contains regular quasi-periodical oscillations in the period range of 4-30 days. (C) 2001 Elsevier Science Ltd. All rights reserved.