Orientation and shape of the Earth's bow shock in three dimensions

Abstract

Nearly 2500 shock crossings from HEOS-1, HEOS-2 and 5 IMP spacecraft, covering most of the northern and part of the southern bow shock surface for X values X > − 20 R E , have been used to carry out a detailed study of the three-dimensional shape and location of the bow shock. The influence of the different solar wind conditions has been reduced by normalising the observed crossings to an average solar wind dynamical pressure ( N 0 = 9.4 cm −3, V 0 = 450 km s −1). It has been shown that the shock surface is symmetric with respect to the ecliptic plane and intersects the coordinate axes at 11.9 R E ( X), + 27.0 and − 22.9 R E ( Y), + 23.9 and − 24.5 R E ( Z) for the average dynamical pressure ( N 0 = 9.4 cm −3, V 0 = 450 km s −1, with M A = 9.3 , M MS = 6.1) . The observed aberration of the shock surface is 8.9° ± 1°, i.e. 5.1° larger than the aberration predicted from the Earth's motion. This asymmetry around the solar wind apparent direction is described by equation (6) for different Mach numbers M A and confirms the predictions of Walters [ J. geophys. Res. 71, 1319 (1964)] and Michel [ J. geophys. Res. 70, 1 (1965)]. The magnetosheath thickness is 3.3 R E along the X-axis, 11.4 R E (+ Y), 8.7 R E (− Y), 9.9 R E (+ Z) and 10.9 R E along the negative Z axis.