Distribution and origin of broad plasma depletions in the equatorial F region observed by the C/NOFS satellite

Broad plasma depletions (BPDs) represent bubble-like plasma depletions in the equatorial F region whose longitudinal widths (> 5 degree) are greater than those of regular bubbles. Their occurrence in satellite observations is understood in terms of the uplift of the ionosphere; BPDs occur when satellites pass through the bottomside of bubbles. However, a merger of bubbles is also suggested as the cause of BPDs. We investigate the origin of BPDs by examining the occurrence climatology of BPDs and its association with vertical plasma motion. Our preliminary results derived from the C/NOFS observations in 2008-2012 show that BPDs occur more frequently during lower solar activity, during higher magnetic activity, and at lower altitudes. BPDs during solar maximum and minimum show different behavior. BPDs occur frequently at premidnight and during the equinoxes and December solstices during solar maximum, whereas BPDs occur predominantly at postmidnight and during the June solstices. Our study investigates how well the observed characteristics of BPD can be explained by the vertical plasma motion and assess the origin of BPDs based on this investigation result.