Multiple-Satellite Studies of Plasmasphere Erosion and its Effects on the 
Low-Energy Ring Current 

Thomsen, M.F., Gary, P., Borovsky, J.E. 


During episodes of enhanced magnetospheric convection, the outer layer of the 
plasmasphere finds itself on drift orbits that no longer close around the earth 
but rather extend sunward to the dayside magnetopause. This outer layer of 
dense, cold material thus erodes from the plasmasphere and forms a drainage 
plume (or "tail") extending sunward toward the magnetopause. This drainage plume 
is often seen in geosynchronous plasma measurements and has now been imaged 
directly by the EUV instrument on the IMAGE spacecraft. In the same region, ring 
current ions, with energies above 10 keV, follow drift orbits that are dominated 
by the gradient and curvature of the magnetic field and hence are largely 
orthogonal to the drainage plume. Thus, ring current ions will drift across such 
a plume, temporarily traversing a region of appreciably elevated cold plasma 
density, with an associated lowering of the resonant energy for interaction with 
ion cyclotron waves. Using measurements from the LANL MPA instruments on several 
geosynchronous satellites, we examine the intensities of the hot ions before and 
after they encounter a plume, seeking evidence for enhanced pitch-angle 
scattering losses during the plume transit. Such measurements would enable a 
quantitative assessment of the importance of the more general precipitation 
losses in the overlap region of the ring current and the plasmasphere. 

_______________
Presented at the 2002 Spring A.G.U. Meeting, 
Washington, D.C., U.S.A., 28-31 May 2002