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

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 ( 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∥.

[1]  B. Anderson,et al.  Electromagnetic ion cyclotron waves observed in the plasma depletion layer , 1991 .

[2]  B. Anderson,et al.  Magnetic pulsations from 0.1 to 4.0 Hz and associated plasma properties in the Earth's subsolar magnetosheath and plasma depletion layer , 1993 .

[3]  D. Curtis,et al.  The Plasma Instrument for AMPTE IRM , 1985, IEEE Transactions on Geoscience and Remote Sensing.

[4]  D. Fairfield.  The ordered magnetic field of the magnetosheath , 1967 .

[5]  G. Paschmann,et al.  ISEE plasma observations near the subsolar magnetopause , 1978 .

[6]  T. Eastman,et al.  Large-scale flow in the dayside magnetosheath , 1984 .

[7]  Wolfgang Baumjohann,et al.  The magnetopause for large magnetic shear: AMPTE/IRM observations , 1986 .

[8]  B. Anderson,et al.  He2+ and H+ dynamics in the subsolar magnetosheath and plasma depletion layer , 1991 .

[9]  Brian J. Anderson,et al.  Magnetic spectral signatures in the Earth's magnetosheath and plasma depletion layer , 1994 .

[10]  C. Russell,et al.  Structure and properties of the subsolar magnetopause for northward IMF: ISEE observations , 1990 .

[11]  H. Lühr,et al.  Magnetic field maxima in the low latitude boundary layer , 1992 .

[12]  C. Wu.  MHD FLOW PAST AN OBSTACLE : LARGE-SCALE FLOW IN THE MAGNETOSHEATH , 1992 .

[13]  G. Paschmann,et al.  Structure of the Dayside Magnetopause for Low Magnetic Shear , 1993 .

[14]  M. Kivelson,et al.  Anomalous aspects of magnetosheath flow and of the shape and oscillations of the magnetopause during an interval of strongly northward interplanetary magnetic field , 1993 .

[15]  Wolfgang Baumjohann,et al.  The magnetopause and boundary layer for small magnetic shear: Convection electric fields and reconnection , 1990 .

[16]  I. Papamastorakis,et al.  Evidence for magnetic field reconnection at the Earth's magnetopause , 1981 .

[17]  C. Russell,et al.  Spatial distributions of magnetic field fluctuations in the dayside magnetosheath , 1986 .

[18]  B. Anderson,et al.  Ion anisotropies in the magnetosheath , 1993 .

[19]  D. A. Bryant,et al.  Electrons in the boundary layers near the dayside magnetopause , 1991 .

[20]  L. Lees.  Interaction between the solar plasma wind and the geomagnetic cavity , 1964 .

[21]  B. Anderson,et al.  Low-frequency magnetic fluctuation spectra in the magnetosheath and plasma depletion layer , 1994 .

[22]  D. Baker,et al.  Lion roars and nonoscillatory drift mirror waves in the magnetosheath , 1982 .

[23]  D. H. Fairfield,et al.  Average and unusual locations of the Earth's magnetopause and bow shock , 1971 .

[24]  C. Russell,et al.  Energetic magnetosheath ions and the interplanetary magnetic field orientation , 1981 .

[25]  G. Paschmann,et al.  ISEE plasma observations near the subsolar magnetopause , 1978 .

[26]  C. Russell,et al.  The thickness of the magnetopause current layer: ISEE 1 and 2 observations , 1982 .

[27]  A. Summers,et al.  External aerodynamics of the magnetosphere , 1968 .

[28]  M. Heyn,et al.  Magnetosheath's parameters and their dependence on intensity and direction of the solar wind magnetic field , 1982 .

[29]  I. Papamastorakis,et al.  Plasma acceleration at the Earth's magnetopause: evidence for reconnection , 1979, Nature.

[30]  D. Hunten,et al.  Depletion of solar wind plasma near a planetary boundary , 1976 .

[31]  G. S. Stiles,et al.  Observations of plasma depletion in the magnetosheath at the dayside magnetopause , 1979 .

[32]  N. Sckopke,et al.  The magnetopause as a tangential discontinuity for large field rotation angles , 1984 .

[33]  C. Russell,et al.  ISEE 1 & 2 observations of the oscillating magnetopause , 1988 .

[34]  J. Midgley,et al.  Calculation by a moment technique of the perturbation of the geomagnetic field by the solar wind , 1963 .

[35]  Christopher T. Russell,et al.  ISEE observations of flux transfer events at the dayside magnetopause , 1979 .

[36]  C. Russell,et al.  Accelerated plasma flows at the near‐tail magnetopause , 1986 .

[37]  M. Acuna,et al.  The IRM Fluxgate Magnetometer , 1985, IEEE Transactions on Geoscience and Remote Sensing.

[38]  C. Russell,et al.  Structure and properties of the subsolar magnetopause for northward interplanetary magnetic field - Multiple-instrument particle observations , 1993 .

[39]  David Klumpar,et al.  Mass density and pressure changes across the dayside magnetopause , 1993 .