Average and unusual locations of the Earth's magnetopause and bow shock

Abstract

A best fit ellipse and hyperbola have been calculated to represent several hundred, magnetopause and bow shock positions observed by six IMP spacecraft. Average geocentric distances to the magnetopause and bow shock near the ecliptic plane are 11.0 RE and 14.6 RE in the sunward direction, 15.1 R E and 22.8 RE in the dawn meridian and 15.8 and 27.6 RE in the dusk meridian. The bow shock hyperbola is oriented in a direction consistent with that expected considering aberration of a radial solar wind. Observed magnetopause crossings agree well with theoretical predictions in the noon meridian plane but fall outside the theoretical boundaries in the dawn-dusk meridian planes. IMP 4 plasma data are used to demonstrate that the solar wind momentum flux is the prime factor controlling the orbit-to-orbit changes in the boundary positions. Data suggest that the interplanetary fie]d. orientation also affects the distance to the magnetopause boundary with more earthward crossings i corresponding to southwardfields. Six unusual bow shock locations up to 22 RE beyond the average position are found to be due to an enhanced standoff distance associated with a low Alfven Mach nwnber. The possibility is raised that the solar wind may have become sub-Alfvenic on July 31, 1967