Heliophysics Science Division
Don Fairfield - Abstract

The Location of Low Mach Number Bow Shocks at Earth


D. H. Fairfield
NASA/GSFC

On April 26--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 spacecraft corresponded to shock subsolar distances of 45 and 42 RE respectively on these days which are the most distant locations that the shock has ever been seen. Shock observations by 3 other spacecraft on these days along with 34 previously reported distant shocks are used to compare with the predictions of different models. A recent MHD bow shock model of Cairns and Lyon [1995] predicts the observed locations quite well as does a modified gas dynamic model of Farris and Russell [1994] if a new Mach-number-dependent shape parameter is used. Bow shock predictions are limited by uncertainties in measurements of the very low densities and uncertainties in the position and shape of the magnetopause. Asymmetries in the shock shape caused by the interplanetary magnetic field direction and not accounted for by models are another likely source of uncertainty. These uncertainties make it impossible to clearly favor one theoretical model over another.