The magnetosheath region adjacent to the dayside magnetopause: AMPTE/IRM observations

Abstract

We have studied 38 low‐latitude, dayside (0800‐1600 LT) magnetopause crossings by the AMPTE/IRM satellite to investigate the variations of key plasma parameters and the magnetic field in the magnetosheath region 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 across the magnetopause, that is, on the angle between the magnetosheath magnetic field and the geomagnetic field. When the magnetic shear is low (<30°), a magnetosheath transition layer, also called the “plasma depletion layer,” of 10‐min average width exists where the magnetosheath magnetic field piles up against the magnetopause. In this region the plasma density and plasma β as well as the proton and electron temperatures are lower than in the magnetosheath proper. The condition for the onset of the mirror instability is generally not met in the magnetosheath transition layer, where the plasma β often falls below 1, while it is marginally satisfied in the magnetosheath proper, where usually β>1. When the magnetic shear across the magnetopause is high (>60°), the near‐magnetopause magnetosheath is more disturbed. The magnetic field in this case does not pile up in the immediate 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 transition layer. Also in contrast to the low‐shear case, the mirror instability 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 tangential to the magnetopause is enhanced and rotates to become more perpendicular to the local magnetic field as the low‐shear magnetopause is approached. This flow behavior may be consistent with the formation of a stagnation line instead of a stagnation point at the subsolar magnetopause. Enhancement and rotation of the magnetosheath flow on approach to the magnetopause are rarely observed when the magnetic shear across the magnetopause is high. In essence, our observations provide evidence for high (low) rate of transfer of magnetic flux and mass across the magnetopause when the magnetic shear is high (low). The relationships between the electron and proton temperature anisotropies and β in the near‐magnetopause magnetosheath region are also examined. It is found that Te⊥/Te∥ remains close to 1 for the entire range of βe, whereas Tp⊥/Tp∥ is generally anticorrelated with βp∥. However, no universal relationship seems to exist between Tp⊥/Tp∥ and βp∥.

Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

“…This shear is itself near the outer edge of the velocity gradient, which is more than 1 R E outside the closed field line boundary. Phan et al [1994] noted such an increase in velocity in the dayside equatorial region between 0800 and1600 LT but only for cases of low shear in B (i.e., generally northwards fields as in the present case). Such high velocities near the flank magnetopause have also been reported by Petrinec et al [1997]; and Phan et al [1997], again for cases when B is orthogonal to V. also performed a global simulation and also found enhanced velocities that were faster than the solar wind; however, their high-velocity regions are wider than the current simulation, probably because of their much lower 1 R E grid resolution.…”

Dynamics and local boundary properties of the dawn-side magnetopause under conditions observed by Equator-S

“…This correspondence, and any dependence of magnetopause character on the magnetic shear across the boundary, is much weaker for earlier local time. Thus, the characteristics of the boundary region on the¯anks is much less dependent on magnetic shear, than it is in the sub-solar region (as found by Phan et al, 1994). That there is a tendency for TD character to increase is perhaps not unexpected on thē anks, since reconnection is likely to be predominant in the sub-solar region.…”

“…Older studies of the boundary region, however, have tended to concentrate on the sub-solar region (e.g. Berchem and Russell, 1982;Russell, 1995;Paschmann et al, 1985Paschmann et al, , 1986Phan and Paschmann, 1995), and surveys of magnetopause crossings have tended to omit the dawnside (Paschmann et al, 1986;Phan et al, 1994). The apogee of the Equator-S orbit (at 11.5 i ) was much smaller than that of ISEE or AMPTE (at 20.5 i ) and consequently was well placed to sample the magnetopause and near magnetosheath for long periods as the spacecraft moved through apogee.…”

Effect of He++ ions on the propagation of low‐frequency magnetohydrodynamic waves in the magnetosheath

“…In the magnetosheath, plasma beta (b = p/ B 2 8p ) has been found to vary widely and values of the order $10 have been observed [Phan et al, 1994]. Keeping this variation in view, numerical study is further carried out (for brevity plots are not shown) with the above-mentioned parameters but with various values of s k 2 0 (= b k 2 0 /2).…”