THE MAGNETOSHEATH REGION ADJACENT TO THE DAYSIDE MAGNETOPAUSE - AMPTEIRM OBSERVATIONS/

We have studied 38 low-latitude, dayside (0800-1600 LT) magnetopause c rossings by the AMPTE/IRM satellite to investigate the variations of k ey plasma parameters and the magnetic field in the magnetosheath regio n adjacent to the dayside magnetopause. We find that the structures of the key plasma parameters and the magnetic field and the dynamics of plasma flows in this region depend strongly on the magnetic shear acro ss the magnetopause, that is, on the angle between the magnetosheath m agnetic field and the geomagnetic field. When the magnetic shear is lo w (<30 degrees), a magnetosheath transition layer, also called the ''p lasma depletion layer,'' of 10-min average width exists where the magn etosheath magnetic field piles up against the magnetopause. In this re gion the plasma density and plasma beta as well as the proton and elec tron temperatures are lower than in the magnetosheath proper. The cond ition for the onset of the mirror instability is generally not met in the magnetosheath transition layer, where the plasma beta often falls below 1, while it is marginally satisfied in the magnetosheath proper, where usually beta>1. When the magnetic shear across the magnetopause is high (>60 degrees), the near-magnetopause magnetosheath is more di sturbed. The magnetic field in this case does not pile up in the immed iate vicinity of the magnetopause, and no systematic variations in the plasma parameters are observed in this region until the encounter of the magnetopause current layer; that is, there is no magnetosheath tra nsition layer. Also in contrast to the low-shear case, the mirror inst ability threshold is marginally satisfied throughout the magnetosheath . The plasma flow pattern in the magnetosheath region adjacent to the dayside magnetopause is also found to depend strongly on the magnetic shear across the magnetopause: the magnetosheath flow component tangen tial to the magnetopause is enhanced and rotates to become more perpen dicular to the local magnetic field as the low-shear magnetopause is a pproached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnet opause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear acros s the magnetopause is high. In essence, our observations provide evide nce for high (low) rate of transfer of magnetic flux and mass across t he magnetopause when the magnetic shear is high (low). The relationshi ps between the electron and proton temperature anisotropies and beta i n the near-magnetopause magnetosheath region are also examined. It is found that T-c perpendicular to /T-c parallel to remains close to 1 fo r the entire range of beta(c), whereas T-p perpendicular to/T(p parall el to)is generally anticorrelated with beta(p parallel to). However, n o universal relationship seems to exist between T-p perpendicular to/T -p parallel to and beta(p parallel to).