Improved bow shock model with dependence on the IMF strength

Abstract

The presence of the bow shock in front of an obstacle immersed into a supersonic flow of collisionless plasma has been theoretically predicted and experimentally confirmed more than 40 years ago. However, in spite of a great effort of theoreticians and experimenters, we are still not able to predict the bow shock location under varying upstream conditions with a sufficient accuracy. Gasdynamic and MHD models cannot account for kinetics effects, whereas the kinetic models can be used only to study particular phenomena due to limited computer capacity. Models based on fits of experimental data usually expect that the bow shock location is a simple function of several upstream parameters as the solar wind dynamic pressure, upstream Mach number, and direction or magnitude of the interplanetary magnetic field. Since the bow shock location would be determined by the downstream parameters, these models implicitly assume that these parameters are unambiguously determined by the upstream conditions. The uncertainty of the bow shock prediction is usually attributed to the uncertainty in determination of the upstream state or to an irregular bow shock motion. We have accumulated a large set of bow shock crossings observed by INTERBALL-1, MAGION-4, GEOTAIL, IMP 8, and CLUSTER-2 spacecraft and complemented this set with upstream measurements of WIND with motivation to improve the accuracy of a prediction of the bow shock location provided by the Němeček and Šafránková (J. Atmos. Terr. Phys. 53 (1991) 1049) model. Although proposed corrections conserve a simplicity of the original model, they decrease significantly the most probable error of the prediction.