Rotational Asymmetry of Earth's Bow Shock

Abstract

AbstractIn terms of global magnetohydrodynamic (MHD) simulations of the solar wind‐magnetosphere‐ionosphere system, this paper investigates the rotational asymmetry of the Earth's bow shock with respect to the Sun‐Earth line. We are limited to simple cases in which the solar wind is along the Sun‐Earth Line; and both the Earth's magnetic dipole moment and the interplanetary magnetic field (IMF) are perpendicular to the Sun‐Earth line. It is shown that even for the case of vanishing IMF strength the bow shock is not rotationally symmetric with respect to the Sun‐Earth line: the east‐west width of the cross section of the bow shock exceeds the north‐south width by about 9%~11% on the terminator plane (dawn‐dusk meridian plane) and its sunward side, and becomes smaller than the north‐south width by about 8% on the tailward side of the terminator plane. In the presence of the IMF, the configuration of the bow shock is affected by both the shape of the magnetopause and the anisotropy of fast magnetosonic wave speed. The magnetopause expands outward, being stretched along the IMF, and the extent of its expansion and stretch increases when the IMF rotates from north to south. In the magnetosheath, the fast magnetosonic wave speed is higher in the direction perpendicular to the magnetic field than that in the parallel direction. Therefore, the stretch direction of the magnetopause is perpendicular to the maximum direction of the fast magnetosonic wave speed, and their effects on the bow shock position are exactly opposite. The eventual shape of the bow shock depends on which effect dominates. On the tailward side of the terminator plane, the anisotropy of fast magnetosonic wave speed dominates, so the cross section of the bow shock is wider in the direction perpendicular to the IMF. On the terminator plane and its sunward side, the shape of the bow shock cross section depends on the orientation of the IMF: the bow shock cross section is still wider in the direction perpendicular to the IMF under generic northward or dawn‐dusk IMF cases, but it becomes narrower in the direction perpendicular to the IMF instead under generic southward IMF cases. In light of the intimate relationship between the shape of the bow shock and the orientation of the IMF, it is proposed to take the IMF as the datum direction so as to extract the parallel half width Rb// and the perpendicular half width Rb┴ as the scale parameters. In comparison with the commonly used east‐west half width yb and the north‐west half width zb, these parameters provide a more reasonable description of the geometry of the bow shock. Simulation data show that under the assumption of isotropic orientation of the IMF, the statistical averages of yb/zb and Rb‌‌/Rb┴ are both smaller than 1 on the terminator plane, which agrees with relevant observational conclusions.