ISEE plasma observations near the subsolar magnetopause

The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point. Measurements of complete two-dimensional ion and electron distributions every 3 or 12 s, and of three-dimensional distributions every 12 or 48 s by the LASL/MPI instrumentation on both spacecraft allow a detailed study of the plasma properties with unprecedented temporal resolution. This paper presents observations obtained during four successive inbound orbits in November 1977, containing a total of 9 magnetopause crossings, which occurred under widely differing orientations of the external magnetic field. The main findings are: (1) The magnetosheath flow near the magnetopause is characterized by large fluctuations, which often appear to be temporal in nature. (2) Between 0.1 and 0.3R _E outside the magnetopause, the plasma density and pressure often start to gradually decrease as the magnetopause is approached, in conjunction with an increase in magnetic field strength. These observations are in accordance with the formation of a depletion layer due to the compression of magnetic flux tubes. (3) In cases where the magnetopause can be well resolved, it exhibits fluctuations in density, and especially pressure and bulk velocity around average magnetosheath values. The pressure fluctuations are anticorrelated with simultaneous magnetic field pressure changes. (4) In ope case the magnetopause is characterized by substantially displaced electron and proton boundaries and a proton flow direction change from upwards along the magnetopause to a direction tranverse to the geomagnetic field. These features are in agreement with a model of the magnetopause described by Parker. (5) The character of the magnetopause sometimes varies strongly between ISEE-1 and -2 crossings which occur 1 min apart. At times this is clearly the result of highly non-uniform motions. There are also cases where there is very good agreement between the structures observed by the two satellites. (6) In three of the nine crossings no boundary layer was present adjacent to the magnetopause. More remarkably, two of the three occurred while the external magnetic field had a substantial southward component, in clear contradiction to expectations from current reconnection models. (7) The only thick (low-latitude) boundary layer (LLBL) observed was characterized by sharp changes at its inner and outer edges. This profile is difficult to reconcile with local plasma entry by either direct influx or diffusion. (8) During the crossings which showed no boundary layer adjacent to the magnetopause, magnetosheath-like plasma was encountered sometime later. Possible explanations include the sudden formation of a boundary layer at this location right at the time of the encounter, and a crossing of an inclusion of magnetosheath plasma within the magnetosphere. (9) The flow in the LLBL is highly variable, observed directions include flow towards and away from the subsolar point, along the geomagnetic field and across it, tangential and normal to the magnetopause. Some of these features clearly are nonstationary. The scale size over which the flow directions change exceeds the separation distance (several hundred km) of the two spacecraft.