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

Abstract

We apply our recent three‐dimensional anisotropic MHD model of magnetosheath flow [Erkaev et al., 1999] to study quantitatively effects of solar wind dynamic pressure (Pd∞ ) and Alfvén Mach number (Ma∞) on the anisotropic magnetosheath and the plasma depletion layer (PDL) in the subsolar region. Given the wide range over which these two parameters vary, their influence on the magnetosheath structure may be significant. Our analysis is applicable to quasi‐steady changes in the interplanetary medium. Following our earlier work, and in general agreement with the data, we define the sunward edge of the PDL by β‖ = 1, where β‖ is the proton beta parallel to the magnetic field. We first discuss changes in Pd∞ occurring under constant Ma∞. In this case, a rescaling of the parameters yields the effects on the magnetosheath. We then study quantitatively a changing dynamic pressure through a varying Alfvén Mach number. We obtain profiles of key magnetosheath parameters and the width of the PDL for Alfvén Mach numbers representative of the solar wind at Earth orbit. Gradients in parameter profiles become steeper and shift toward the magnetopause as Ma∞ increases. We find that PDL width varies as even in the anisotropic magnetosheath. Using our model to study the magnetosheath location where the electromagnetic ion cyclotron wave (EICW) instability dominates over the mirror instability, we find that this location 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 Alfvén Mach number. We conclude that the EICW regime is contained in, but is not co‐extensive with, the PDL. Examining critically this issue by changing the PDL identification criterion to others based on a density decrease relative to the bow shock value and a systematic drop toward the magnetopause, we find that this result still holds, but the region where EICWs are destabilized occupies a different fraction of the PDL thus defined. Some model results are compared with documented data from an inbound crossing of the magnetosheath made on December 24, 1994. Good agreement with model predictions are obtained.