A COMPARISON OF SPECULARLY REFLECTED GYRATING ION ORBITS WITH OBSERVED SHOCK FOOT THICKNESSES

ISEE 1 and 2 flux gate magnetometer data has been used to measure the length or thickness of the 'foot' of 60 quasi-perpendicular supercritical bow shocks along the shock normal. These bow shock crossings are contrasted with 10 quasi-perpendicular subcritical shocks which do not exhibit the precursor foot. For the supercritical shocks THETA //B//n, the angle between the shock normal and the upstream magnetic field, ranges from 43 degree to 88 degree . The range of THETA //B//n for the laminar shocks is 56 degree to 76 degree . The extent of the foot structure is compared to the orbit of an ion having solar wind velocity and undergoing simple specular reflection about the shock normal. Good quantitative agreement between the measured value of the foot and the predicted value of approximately 0. 7 ion gyroradii is evident.