Climatology of plasma density depletions observed by DMSP in the dawn sector

Prior to the launch of the Communication/Navigation Outage Forecasting System (C/NOFS) satellite, equatorial plasma bubbles (EPBs) were regarded as postsunset phenomena. However, during this recent solar minimum the planar Langmuir probe (PLP) on the C/NOFS satellite has detected very few EPBs after sunset; most plasma density depletions have been observed between local midnight and dawn. We take advantage of the long history of plasma density measurements by a similar sensor on Defense Meteorological Satellite Program (DMSP) spacecraft to determine whether this change is typical of solar minima in general or unique to the present extended quiet time. In 2008 and 2009 the DMSP occurrence rates of topside plasma depletions in the dawn sector were unexpectedly high around the June and December solstices and extremely low near the March and September equinoxes. Dawn sector measurements from solar minimum years 1996–1997 exhibit similar seasonal and longitudinal distributions, but occurrence rates are significantly lower. While our analysis suggests that prevailing low levels of solar EUV flux and driving electric fields establish conditions favorable for the growth of postmidnight depletions, the primary causes of observed seasonal‐longitudinal distributions remain unresolved.