Inner edge of the electron plasma sheet: Empirical models of boundary location

This study uses geosynchronous plasma observations and the inferred midnight equatorward edge of the diffuse aurora (based on DMSP data) to obtain hundreds of two-point locations of the inner edge of the electron plasma sheet. We reconfirm that the local time at L = 6.6 of the boundary separating the plasmasphere/electron trough (where cold ion densities can be high and the density of hot electrons is low) and the electron plasma sheet (where hot electron densities increase) varies with activity level. We also demonstrate a good correlation between the midnight boundary index (MBI) from DMSP and the local time of the geosynchronous electron plasma sheet inner edge. Models of the size and shape of the inner edge of the electron plasma sheet may be constructed using the equatorial mapping of the MBI values and the geosynchronous boundary crossing locations in local time. The model form used is an improved version of the zero-energy drift boundary approximation of Southwood and Kaye [1979]; the improved form is an analytic approximation to the exact solution for a shielded cross-tail electric field. Model parameters such as radial distance of the stagnation point range from 5 to well over 7 R_E depending on activity; the degree of shielding also appears to be greater for low activity than for high, with shielding parameter values of greater than 5 for low activity. When the boundary model is allowed to rotate freely, the inferred stagnation point appears to move tailward, not sunward, with increasing activity, but this result is poorly constrained.