A Statistical Study on the Climatology of the Equatorial Plasma Depletions Occurrence at Topside Ionosphere During Geomagnetic Disturbed Periods

Using the in situ measurements of ROCSAT-1 satellite during 181 geomagnetic storms happened from July 1999 to June 2004, a superposed epoch analysis of the equatorial plasma depletions (EPDs) occurrence is conducted. At postsunset hours (1800-2200 LT), the EPDs occurrence is enhanced shortly at the storm onset, but afterward, a long-last suppression dominates. The EPDs occurrence at Midnight (2200-0200 LT) generally shares a similar pattern to that at postsunset hours. The occurrence at predawn (0200-0400) gradual increases near storm onset and reach its maximum at 6-9 hr and decays until 18 hr. For a given longitude at postsunset/midnight, the EPDs occurrence tends to be suppressed or promoted when the EPDs do or do not prevail. The disturbed vertical plasma drift generally determines the inhibition/promotion of the EPDs occurrence at postsunset/predawn. However, for predawn EPDs occurrence, the plasma vertical drift cannot well explain the longitudinal variation. The continuous observations from consecutive orbits of ROCSAT-1 are carefully compared and the result suggests that the geomagnetic storm-induced additional predawn EPDs are preferred to be the longer-lived developed EPDs rather than fresh EPDs. In addition, a possible mechanism concerning the background plasma density enhancement which might be related with the energetic electrons induced nighttime ionization is proposed.