Low‐latitude dayside magnetopause and boundary layer for high magnetic shear: 1. Structure and motion

Abstract

A large number (128) of Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) crossings of the low‐latitude (<30°) dayside (0800–1600 LT) high‐shear (>45°) magnetopause (MP) and the low‐latitude boundary layer (LLBL) have been analyzed in detail in order to study the structure and motion of the MP and LLBL regions. From a superposed epoch analysis of 59 of these events, we found the average high‐shear MP to be characterized by large changes in the magnetic field magnitude, together with small increases of the parallel ion and electron temperatures as one crosses the boundary from the magnetosheath. Accelerated plasma flows, indicative of magnetic reconnection, are often seen in the MP/LLBL region, and in conjunction with these flows, one often observes an enhancement of the parallel ion temperature. Large changes in the plasma thermal properties occur at the inner edge of the LLBL, where the bulk of the magnetospheric particles is depleted. Using measured plasma flow velocity normal to the MP, we have estimated the MP speed. Comparing the results of the analysis of the 128 high‐shear cases with results on 22 low‐shear crossings reported in an earlier paper, we conclude that the high‐shear MP moves on average twice as fast as the low‐shear one, while the duration of the crossings of the high‐shear MP/LLBL is half that of the low‐shear crossings, so that the average thickness of the MP/LLBL is nearly independent of the local magnetic shear. In the range covered, the average MP/LLBL width is also found to be independent of the distance from the subsolar point. The normal speed (thickness) of the MP/LLBL is correlated (anticorrelated) with the magnetosheath plasma β. Furthermore, the normal speed of the MP tends to be anticorrelated with the crossing duration of the LLBL, implying that the width of the region does not vary as much as crossing durations imply. The (geometric) average widths of the high‐shear and low‐shear MP/LLBL are 1380 km (31 ri,sheath 15 ri,sphere) and 1100 km (27 ri,sheath, 12 ri,sphere), respectively, where ri,sheath and ri,sphere are the average gyroradii of magnetosheath and magnetospheric protons. The average width of the entire region of current flow at the magnetopause is 630 km (14 ri,sheath, 7 ri,sphere). The width of the current flow region is also found to be anticorrelated with the magnetosheath β, while it is independent of ri,sheath and ri,sphere.