Neutral density depletions associated with equatorial plasma bubbles as observed by the CHAMP satellite

Using CHAMP observations from 2002 to 2005 we investigate neutral density depletions (NDDs) associated with equatorial plasma bubbles (EPBs). The seasonal–longitudinal distribution of NDDs generally follows that of EPBs. However, there are several important differences between them. The maximum NDD occurrence rate is much smaller than the maximum EPB occurrence. NDDs occur at latitudes north and south of the dip equator with an offset of about 15∘15∘, which is collocated with the Appleton anomaly peaks and slightly poleward of EPB occurrence maxima. The NDD occurrence maximizes around 21 LT, and has nearly died out after 23 LT. Meanwhile, the EPB occurrence shows a broad maximum between 20 and 24 LT. NDD distribution deviates slightly from that of EPBs shifted toward the region of high ion–neutral interaction. Based on our statistical results, as well as on some physics-based calculations, we suggest that an enhanced friction between ions and neutrals is needed for the NDD generation.