Erosion and recovery of the plasmasphere in the plasmapause region

Understanding the basic plasmasphere erosion/recovery cycle remains a major, as yet largely unmet, challenge to the space science community. We do not yet have a description of the formation of a new plasmapause boundary, nor have we been able to map the evidently complex electric fields that develop at subauroral latitudes during the process of plasmasphere erosion. Density structure regularly observed in the plasmapause region suggests that instabilities play an as yet unassessed role in the erosion/recovery cycle. Electron density interior to a newly formed plasmapause boundary tends to be reduced by factors of up to 3 in association with the erosion process, so that refilling during recovery occurs there as well as in the more deeply depleted plasmatrough region beyond. The number of electrons lost from this interior region, apparently through interchange with the ionosphere, can be of order 50% of the number lost from beyond the new boundary through flow perpendicular to B. Evidence has been found that of order 20% of the plasma removed from the main plasmasphere during an erosion event remains in the outer afternoon-dusk magnetosphere for extended periods. It is not yet known whether eroded plasmas entering the Earth's boundary layers make a geophysically important contribution to the plasma sheet. New insights into these and other important questions await both future photon and radio imaging of the plasmasphere from high altitude as well as continued work with certain excellent, as yet only partially exploited, satellite data sets.