TOWARDS AN MHD THEORY FOR THE STANDOFF DISTANCE OF EARTHS BOW SHOCK

An MHD theory is developed for the standoff distance a, of the bow sho ck and the thickness Delta(ms) of the magnetosheath, using the empiric al Spreiter et al. relation Delta(ms) = kX and the MHD density ratio X across the shock. The theory includes as special cases the well-known gasdynamic theory and associated phenomenological MHD-like models for Delta(ms) and a(s). In general, however, MHD effects produce major di fferences from previous models, especially at low Alfven (M(A)) and so nic (M(S)) Mach numbers. The magnetic field orientation, M(A), M(S), a nd the ratio of specific heats gamma are all important variables of th e theory. In contrast, the fast mode Mach number need play no direct r ole. Three principal conclusions are reached. First, the gasdynamic an d phenomenological models miss important dependances on field orientat ion and M(S) and generally provide poor approximations to the MHD resu lts. Second, changes in field orientation and M(S) are predicted to ca use factor of similar to 4 changes in Delta(ms) at low M(A). These eff ects should be important when predicting the shock's location or calcu lating gamma from observations. Third, using Spreiter et al.'s value f or k in the MHD theory leads to maximum a(s), values at low M(A) and n ominal M(S) that are much smaller than observations and MHD simulation s require. Resolving this problem requires either the modified Spreite r-like relation and larger K found in recent MHD simulations and/or a breakdown in the Spreiter-like relation at very low M(A).