MAGNETIC-FIELD ORIENTATION EFFECTS ON THE STANDOFF DISTANCE OF EARTHSBOW SHOCK

Three-dimensional, global MHD simulations of solar wind flow onto a pr escribed magnetopause obstacle are used to show that a bow shock's nos e location a, and the relative subsolar magnetosheath thickness Delta( ms)/a(mp) are strong functions of the IMF cone angle theta (between v( sw) and B-sw) and the Alfven Mach number M(A). For a given M(A) the sh ock is more distant for higher theta (restricted to the interval 0 - 9 0 degrees by symmetries), while a(s)/a(mp) and Delta(ms)/a(mp) increas e with decreasing M(A) for theta greater than or similar to 20 degrees but decrease with decreasing M(A) for theta similar to 0 degrees. Lar ge differences in Delta(ms)/a(mp) are predicted between theta = 0 degr ees and 90 degrees at low M(A), with smaller differences remaining eve n at M(A) similar to 10. The theta = 0 degrees results confirm and ext end the previous work of Spreiter and Rizzi [1974]. The simulations sh ow that successful models for the subsolar shock location cannot subsu me the dependences on M(A) and theta into a sole dependence on M(ms). Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] th at a(s)/a(mp) and Delta(ms)/a(mp) should depend strongly on theta and M(A) for M(A) less than or similar to 10 (as well as on other MHD vari ables). Detailed comparisons between theory and data remain to be done . However, preliminary comparisons show good agreement, with distant s hock locations found for low M(A) and large theta greater than or simi lar to 45 degrees and closer locations found for theta less than or si milar to 20 degrees even at M(A) similar to 8.