Beyond the plasmapause: the "plasma trough"
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
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.