STRUCTURE OF THE DAYSIDE MAGNETOPAUSE FOR LOW MAGNETIC SHEAR

We have analyzed 22 AMPTE/IRM satellite passes through the low-latitud e magnetopause region for which the magnetic shear, i.e., the field ro tation angle on transit from the magnetosheath to the magnetosphere, w as 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 el ectron temperature and the temperature anisotropies, show net and ofte n discontinuous changes. A change in plasma flow direction also occurs at this time. The small field rotations that occur have no fixed rela tionship to the key time. Earthward of the key time, the plasma has th e characteristics of the magnetopause boundary layer, i.e., reduced de nsity and bulk velocity, and in particular an electron temperature ani sotropy with T(e parallel-to) > T(e perpendicular-to), until further a nd 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 lay er of plasma density depletion (by a factor of two or more) and concur rent magnetic field pile-up in less than half the crossings. This depl etion layer was 3 min wide on average and often characterized by a dro p in T(p parallel-to) as predicted. From the observational evidence we conclude that the changes in plasma thermal and flow properties at th e key time mark the crossing of the magnetopause under conditions of l ow magnetic shear. It is tempting to attribute these changes to the cr ossing of a topological boundary, e.g., a transition from open interpl anetary to closed geomagnetic field lines. There is evidence, however, that in some cases the field lines immediately earthward of the key t ime are not closed. The consistent presence of a boundary layer inside the low-shear magnetopause confirms earlier inferences that solar win d plasma can enter the magnetosphere regardless of field orientation. Inspection of the velocity distribution functions indicates that the p lasma is heated upon entry. Taking measured plasma velocities along th e magnetopause normal direction, we have inferred an average magnetopa use speed of 11 km/s, an indication that the low-shear magnetopause mo ves more slowly. The duration of the temperature transition translates into a thickness of often less than 50 km, comparable with the typica l proton gyroradius. For the plasma depletion layer and the boundary l ayer we obtained average thicknesses of 0.4 and 0.3 R(E), respectively .