Magnetic field orientation effects on the standoff distance of Earth's bow shock

Three-dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose 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 shock is more distant for higher theta (restricted to the interval 0 - 90 degrees by symmetries), while a(s)/a(mp) and Delta(ms)/a(mp) increase 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. Large differences in Delta(ms)/a(mp) are predicted between theta = 0 degrees and 90 degrees at low M(A), with smaller differences remaining even at M(A) similar to 10. The theta = 0 degrees results confirm and extend the previous work of Spreiter and Rizzi [1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences on M(A) and theta into a sole dependence on M(ms). Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] that 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 variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for low M(A) and large theta greater than or similar to 45 degrees and closer locations found for theta less than or similar to 20 degrees even at M(A) similar to 8.