STRUCTURE OF THE DAYSIDE MAGNETOPAUSE FOR LOW MAGNETIC SHEAR

We have analyzed 22 AMPTE/IRM satellite passes through the low-latitude magnetopause region for which the magnetic shear, i.e., the field rotation angle on transit from the magnetosheath to the magnetosphere, was less than 30-degrees. We found that on all passes a key time could be identified where the proton temperature, and usually also in the electron temperature and the temperature anisotropies, show net and often discontinuous changes. A change in plasma flow direction also occurs at this time. The small field rotations that occur have no fixed relationship to the key time. Earthward of the key time, the plasma has the characteristics of the magnetopause boundary layer, i.e., reduced density and bulk velocity, and in particular an electron temperature anisotropy with T(e parallel-to) > T(e perpendicular-to), until further and usually more dramatic changes in plasma thermal properties mark the entry into the magnetosphere proper. Boundary layer durations varied widely, from 4s to 14 min. On the magnetosheath side we observed a layer of plasma density depletion (by a factor of two or more) and concurrent magnetic field pile-up in less than half the crossings. This depletion layer was 3 min wide on average and often characterized by a drop in T(p parallel-to) as predicted. From the observational evidence we conclude that the changes in plasma thermal and flow properties at the key time mark the crossing of the magnetopause under conditions of low magnetic shear. It is tempting to attribute these changes to the crossing of a topological boundary, e.g., a transition from open interplanetary to closed geomagnetic field lines. There is evidence, however, that in some cases the field lines immediately earthward of the key time are not closed. The consistent presence of a boundary layer inside the low-shear magnetopause confirms earlier inferences that solar wind plasma can enter the magnetosphere regardless of field orientation. Inspection of the velocity distribution functions indicates that the plasma is heated upon entry. Taking measured plasma velocities along the magnetopause normal direction, we have inferred an average magnetopause speed of 11 km/s, an indication that the low-shear magnetopause moves more slowly. The duration of the temperature transition translates into a thickness of often less than 50 km, comparable with the typical proton gyroradius. For the plasma depletion layer and the boundary layer we obtained average thicknesses of 0.4 and 0.3 R(E), respectively.