Convection-driven erosion of the outer plasmasphere causes the plasmasphere to contract and creates an abrupt gradient or drop in the number density of the plasmaspheric plasma. This steep drop in density--by as much as two orders of magnitude over a distance of half an Earth radius--is known as the "plasmapause" (see figure). It marks the boundary of the main body of the plasmasphere, which becomes roughly circular in shape (with a slightly greater radius at dusk) as the magnetosphere quiets. The location of the plasmapause varies depending upon the degree of magnetospheric disturbance. At strongly active times it can form as close to the Earth as L = 2.5; typical locations for plasmapause formation lie between L ~ 3 and 5. In addition, smaller-scale variations in the location of the plasmapause--a few tenths of an Earth radius over a longitude range of about 10-30 degrees--give the plasmasphere a wavy structure. These wave-like irregularities are believed to result from transient, localized processes associated with substorms.

The depleted region beyond the plasmapause is referred to as the "plasma trough." Here typical densities are on the order of 1-10 particles per cubic centimeter, compared with a few thousand particles per cubic centimeter in the main plasmasphere. Within this generally depleted region, however, significant structuring is observed, particularly in the afternoon-dusk sector, indicating the presence of patches or plumes of dense plasmaspheric plasma. These outlying plasma structures consist of material that has been eroded from the main body of the plasmasphere and that is being convected toward the magnetopause or has become trapped in the plasma trough after convection has weakened. In the outer magnetosphere, at geosynchronous orbit (L ~ 6.6 ) and beyond, the outlying regions of dense plasma are relatively large, ~2-3 Earth radii across, persist for several days, and appear to be detached from the main body of the plasmasphere; those observed closer in, in the region between geosynchronous orbit and the plasmapause, are narrower, ~0.5-1 Earth radius across, and are probably connected to the plasmasphere. Smaller-scale density features--ranging in size from 1000 km to 50 km--have also been observed. (See figure.)

In addition to loss by convection-driven sweeping toward the dayside magnetopause, some plasmaspheric plasma appears to be lost, both from within the main plasmasphere and from the plasma trough region, by "dumping" into the ionosphere. This loss process can account for the removal from inside the plasmasphere of a significant amount of material--up to 50% that lost by erosion from the outer plasmasphere.

Re-filling the plasmasphere from below

Following erosion, which can last hours to tens of hours, plasma flowing upward along magnetic field lines from conjugate ionospheres begins to "refill" the depleted plasma trough and plasmasphere. Refilling of the plasmasphere typically requires several days. In order for refilling to occur, the counterstreaming plasmas must be thermalized (i. e., their kinetic energy must be converted to random thermal energy) and trapped. The plasma processes responsible for thermalizing and trapping the upflowing plasma-- electrostatic shocks, pitch-angle scattering, Coulomb collisions--have not been conclusively determined and are the subject of ongoing research.