Abnormal Deep Plasma Depletions at Geomagnetic Equator Observed by CHAMP in the Pre-midnight Sector

Based on CHAMP observations, we notice some abnormal deep plasma depletions are present at geomagnetic equator. Their electron densities are much lower than the ordinary densities of the equatorial ionization anomaly trough. These depletions are also not the result of plasma bubbles because of their smooth profiles. We count the deep depletions and give their local time evolutions and climatology. The results show almost all the deep depletions occur in the pre-midnight sector and in the seasons/longitudes where the peak height of F2 layer (hmf2) is high. The depletions are present at a broad range of longitudes. The large plasma depletions with broad longitudinal width are named as the Broad Plasma Depletions (BPDs). By some investigations, the formation of BPDs is explained as the satellite below hmf2. We suggest the depletions observed by CHAMP are also below hmf2 but measured along latitude. Furthermore, most deep depletions are present at the seasons/longitudes where the upward plasma drift velocity is large. The upward plasma drift velocity has same climatology with plasma bubbles occurrence probability. It explains why the BPDs are often detected with plasma bubbles. Some deep depletions correspond to small upward plasma drift velocity. The hmf2 might change by other process except the upward plasma drift.