On the effects of solar wind dynamic pressure on the anisotropic terrestrial magnetosheath

We apply our recent three-dimensional anisotropic MHD model of magnetosheat h flow [Erkaev et al., 1999] to study quantitatively effects of solar wind dynamic pressure (P-d infinity) and Alfven Mach number (M-a infinity) on th e anisotropic magnetosheath and the plasma depletion layer (PDL) in the sub solar region. Given the wide range over which these two parameters vary, th eir influence on the magnetosheath structure may be significant. Our analys is is applicable to quasisteady changes in the interplanetary medium. Follo wing our earlier work; and in general agreement with the data, we define th e sunward edge of the PDL by beta(parallel to) = 1, where pll is the proton beta parallel to the magnetic field. We first discuss changes in P-d infin ity occurring under constant M-a infinity. In this case, a rescaling of the parameters yields the effects on the magnetosheath. We then study quantita tively a changing dynamic pressure through a varying Alfven Mach number. We obtain profiles of key magnetosheath parameters and the width of the PDL f or Alfven Mach numbers representative of the solar wind at Earth orbit. Gra dients in parameter profiles become steeper and shift toward the magnetopau se as M-a infinity increases. We find that PDL width varies as 1/M-a infini ty(2) even in the anisotropic magnetosheath. Using our model to study the m agnetosheath location where the electromagnetic ion cyclotron wave (EICW) i nstability dominates over the mirror instability, we find that this locatio n occurs well inside the PDL. In addition, we estimated the fraction of the PDL width occupied by the EICWs as a function of solar wind Alfven Mach nu mber. We conclude that the EICW regime is contained in, but is not co-exten sive with, the PDL. Examining critically this issue by changing the PDL ide ntification criterion to others based on a density decrease relative to the bow shock value and a systematic drop toward the magnetopause, we find tha t this result still holds, but the region where EICWs are destabilized occu pies a different fraction of the PDL thus defined. Some model results are c ompared with documented data from an inbound crossing of the magnetosheath made on December 24, 1994. Good agreement with model predictions are obtain ed.