ANALYTIC MHD THEORY FOR EARTHS BOW SHOCK AT LOW MACH NUMBERS

A previous MHD theory for the density jump at the Earth's bow shock, w hich assumed the Alfven (M(A)) and sonic (M(s)) Mach numbers are both >> 1, is reanalyzed and generalized. It is shown that the MHD jump equ ation can be analytically solved much more directly using perturbation theory, with the ordering determined by M(A) and M(s), and that the f irst order perturbation solution is identical to the solution found in the earlier theory. The second-order Perturbation solution is calcula ted, whereas the earlier approach cannot be used to obtain it. The sec ond-order terms generally are important over most of the range of M(A) and M(s) in the solar wind when the angle a between the normal to the bow shock and magnetic field is not close to 0 degrees or 180 degrees (the solutions are symmetric about 90 degrees). This new perturbation solution is generally accurate under most solar wind conditions at 1 AU, with the exception of low Mach numbers when theta is close to 90 d egrees. In this exceptional case the new solution does not improve on. the first-order solutions obtained earlier, and the predicted density ratio can vary by 10-20% from the exact numerical MHD solutions. For theta similar to 90 degrees another perturbation solution is derived t hat predicts the density ratio much more accurately. This second solut ion is typically accurate for quasi-perpendicular conditions. Taken to gether, these two analytical solutions are generally accurate for the Earth's bow shock, except in the rare circumstance that M(A) less than or equal to 2. MHD and gasdynamic simulations have produced empirical models ih which the shock's standoff distance alpha(s) is linearly re lated to the density jump ratio X at the subsolar point. Using an empi rical relationship between alpha(s) and X obtained from MHD simulation s, alpha(s) values predicted using the MHD solutions for X are compare d with the predictions of phenomenological models commonly used for mo deling observational data, and with the predictions of a modified phen omenological model proposed recently. The similarities and differences between these results are illustrated using plots bf X and alpha(s) p redicted for the Earth's bow shock. The plots. Show that the new analy tic solutions agree very well with the exact numerical MHD solutions a nd that these MHD solutions should replace the the corresponding pheno menological relations in comparisons with data. Furthermore; significa nt differences exist between the standoff distances Predicted at low M (A) using the MHD models versus those predicted by the new modified ph enomenological model. These differences should be amenable to observat ional testing.