Department of Physics, Massachusetts Institute of Technology, Cambridge

Nine bow shock crossings from late 1994 are examined using data from the IMP 8 spacecraft as input to the Rankine-Hugoniot (RH) conservation equations. In addition, two crossings from the same time period are examined using WIND spacecraft data. A nonlinear chi-square minimization method is used to obtain the best fit bow shock (BS) normal and speed for each crossing. The shock normals and speeds are also calculated neglecting temperature, as well as using only the magnetic coplanarity and velocity coplanarity techniques. All of the methods generally give mutually consistent results. The orientations of the shock normals are consistent with the nominal bow shock shape. The shocks were expected to move according to a simple "breathing" model in which the bow shock moves inward and outward from Earth according to changes in external solar wind conditions. However, the results show outward motion of the bow shock for all but one of all the crossings considered. Moreover, the direction of motion of the BS depends on the exact choice of the shock normal so that a slightly different shock normal can give a different sense of motion of the BS. We conclude that the RH equations that we may use may be missing some terms which would provide an accurate description of the balance on both sides of the BS. In particular, the BS acceleration at its flanks is not taken into account by these equations.

J. Geophys. Res., 102, No. A9, 19761-19772, Sept. 1997