The location of low Mach number bow shocks at Earth

On April 26 and 27 and May 10-12, 1999, unusually low solar wind densities produced unusually low Alfven Mach numbers that moved the Earth's bow shock far out past its normal location. The shocks observed by the Wind spacecra ft corresponded to shock subsolar distances of 45 and 42 R-E, respectively, on these days which are the most distant locations at which the shock has ever been seen. Shock observations by three other spacecraft on these days along with 34 previously reported distant shocks are used to compare with t he predictions of different models. A recent MHD bow shock model of Cairns and Lyon [1995] predicts the observed locations quite well as does a modifi ed gasdynamic model of Farris and Russell [1994] if a new Mach-number-depen dent shape parameter is used. A third model of Verigin et al. [1997] also p redicts a shock shape and is also quite good. Bow shock predictions are lim ited by uncertainties in measurements of the very low densities and uncerta inties in the position and shape of the magnetopause. Asymmetries in the sh ock shape caused by the interplanetary magnetic field direction and not acc ounted for by models are another likely source of uncertainty. These uncert ainties make it impossible to clearly favor one theoretical model over anot her.