MHD simulations of Earth's bow shock : interplanetary magnetic field orientation effects on shape and position

The location and geometry of Earth's bow shock vary considerably with the solar wind conditions. More specifically, Earth's bow shock is formed by the steepening of fast mode waves, whose speed v ms depends upon the angle θ bn between the local shock normal n and the magnetic field vector B IMF, as well as the Alfvén and sound speeds (v A and c S ). Since v ms is a minimum for θ bn = 0° and low Alfvén Mach number M A , and maximum for θ bn = 90° and high M A , this implies that as θ IMF (the angle between B IMF and v sw ) varies, the magnitude of v ms should vary also across the shock, leading to changes in shape. This paper presents 3-D MHD simulation data which illustrate the changes in shock location and geometry in response to changes in θ IMF and M A , for 1.4 ≤ M A ≤ 9.7 and 0° ≤ θ IMF ≤ 90°. Specifically, for oblique IMF the shock's geometry is shown to become skewed in planes containing B IMF (e.g., the x − z plane). This is also emphasized in the terminator plane data, where the shock is best represented by ellipses, with centers translated along the z axis. For the θ IMF = 90° simulations the shock is symmetric about the x axis in both the x − y and x − z planes. Simulations for field-aligned flow (θ IMF = 0°) show a dimpling of the nose of the shock as M A → 1. The simulations also illustrate the general movement of the shock in response to changes in M A ; high M A shocks are found closer to Earth than low M A shocks. Farris et al.'s [1991] magnetopause model is used in the simulations, and we discuss the limitations of this, as well as the expected results using a self-consistent model.12 page(s