The magnetopause for large magnetic shear: AMPTE/IRM observations

Abstract

A total of 21 passes of AMPTE/IRM through the dayside (0800–1600 hours local time) low‐latitude magnetospheric boundary region have been examined, all of which were characterized by large magnetic shear across the magnetopause. The purpose of the study was to use the improved accuracy and time resolution of the IRM plasma measurements to reexamine the occurrence of high‐speed flows as signatures of the reconnection process at the magnetopause. A total of 12 of the 21 passes showed magnetopause crossings with high‐speed flows. The duration of these flows was sometimes as short as 10 s and rarely more than 30 s. The occurrence of high‐speed flows is inversely correlated with β, the ratio of plasma to magnetic pressure. For β < 2 the ratio, ΔV*, of observed to theoretically predicted flow velocity changes is high (ΔV* = 0.74 on average), while for β > 2 the average ratio is low (ΔV* = 0.18). We tentatively interpret this to indicate that reconnection may occur preferentially for low β values. A total of nine high‐speed flows were subjected to quantitative momentum and energy balance test. The agreement between observations and theoretical predictions was generally quite good. The signs of the normal magnetic field, Bn, deduced from the momentum balance agreed with the sense of magnetic connection inferred independently from the measured proton heat flux direction. In one case the observations suggest that the satellite may have traversed the diffusion region of the reconnection process. The momentum balance results indicate that at times the effective mass of the ions in the magnetopause and boundary layer was of the order of twice the proton mass.