MHD SIMULATIONS OF EARTHS BOW SHOCK AT LOW MACH NUMBERS - STANDOFF DISTANCES

Global, three-dimensional, ideal MHD simulations of Earth's bow shock are reported for low Alfven Mach numbers M(A) and quasi-perpendicular magnetic field orientations. The simulations use a hard, infinitely co nducting magnetopause obstacle, with axisymmetric three-dimensional lo cation given by a scaled standard model, to directly address previous gasdynamic (GD) and field-aligned MHD (FA-MHD) work. Tests of the simu lated shocks' density jumps X for 1.4 less than or similar to M(A) les s than or similar to 10 and the high M(A) shock location, and reproduc tion of the GD relation between magnetosheath thickness and X for quas i-gasdynamic MHD runs with M(A) his, confirm that the MHD code is work ing correctly. The MHD simulations show the standoff distance a(s) inc reasing monotonically with decreasing M(A). Significantly larger a, ar e found at low MA than predicted by GD and phenomenological MHD models and FA-MHD simulations, as required qualitatively by observations. Th e GD and FA-MHD predictions err qualitatively, predicting either const ant or decreasing a(s) with decreasing M(A). This qualitative differen ce between quasiperpendicular MHD and FA-MHD simulations is direct evi dence for a(s) depending on the magnetic field orientation theta. The enhancement factor over the phenomenological MHD predictions at M(A) s imilar to 2.4 agrees quantitatively with one observational estimate. A linear relationship is found between the magnetosheath thickness and X, modified both quantitatively and intrinsically by MHD effects from the GD result. The MHD and GD results agree in the high M(A) limit. An MHD theory is developed for a(s), restricted to sufficiently perpendi cular theta and high sonic Mach numbers M(S). It explains the simulati on results with excellent accuracy. Observational and further simulati on testing of this MHD theory, and of its predicted M(A), theta, and M (S) effects, is desirable.