Equatorial broad plasma depletions associated with the evening prereversal enhancement and plasma bubbles during the 17 March 2015 storm

Abstract

Broad plasma depletions (BPDs) in the equatorial F region represent plasma depletions whose longitudinal and latitudinal scales are much greater than those of normal plasma bubbles. This study investigates the characteristics and origin of BPDs using the coincident ionospheric observations by the Communication/Navigation Outage Forecasting System, Defense Meteorological Satellite Program, and Swarm satellites during the 2015 St. Patrick's Day (17 March) storm. Two types of BPDs were detected before midnight during the main phase of the storm. One type of BPDs showed a gradual plasma density variation (Type 1) and the other type of BPDs showed a steep density gradient (Type 2) at the walls of BPDs. The Type 1 BPDs were detected with no signature of plasma bubbles nearby, whereas the Type 2 BPDs were accompanied by bubbles. The formation of the Type 1 BPDs is attributed to the uplift of the bottomside of the F region above the satellite altitude by the action of storm-induced electric fields. The steep walls of Type 2 BPDs are associated with the ionospheric uplift and the spatial discontinuity of the ionosphere produced by bubbles. The detection of BPDs that are more than 15° wide in latitude by the polar-orbit Swarm satellites arises from the elongation of bubbles along the magnetic field lines and the alignment of the elongation with the plane of the orbit.