MHD MODEL OF MAGNETOSHEATH FLOW - COMPARISON WITH AMPTE IRM OBSERVATIONS ON 24 OCTOBER, 1985/

We compare numerical results obtained from a steady-state MHD model of solar wind flow past the terrestrial magnetosphere with documented ob servations made by the AMPTE!IRM spacecraft on 24 October, 1985, durin g an inbound crossing of the magnetosheath. Observations indicate that steady conditions prevailed during this about 4 hour-long crossing. T he magnetic shear at spacecraft entry into the magnetosphere was 15 de grees. A steady density decrease and a concomitant magnetic field pile -up were observed during the 40 min interval just preceding the magnet opause crossing. In this plasma depletion layer (1) the plasma beta dr opped to values below unity; (2) the flow speed tangential to the magn etopause was enhanced; and (3) the local magnetic field and velocity v ectors became increasingly more orthogonal to each other as the magnet opause was approached (Phan er al., 1994). We model parameter variatio ns along a spacecraft orbit approximating that of AMPTE/IRM, which was at slightly southern GSE latitudes and about 1.5 h post-noon Local Ti me. We model the magnetopause as a tangential discontinuity, as sugges ted by the observations, and take as input solar wind parameters those measured by AMPTE/IRM just prior to its bow shock crossing. We find t hat computed field and plasma profiles across the magnetosheath and pl asma depletion layer match all observations closely. Theoretical predi ctions on stagnation line flow near this low-shear magnetopause are co nfirmed by the experimental findings. Our theory does not give, and th e data on this pass do not show, any localized density enhancements in the inner magnetosheath region just outside the plasma depletion laye r.