Magnetic field orientation effects on the standoff distance of Earth's bow shock

Abstract

Three‐dimensional, global MHD simulations of solar wind flow onto a prescribed magnetopause obstacle are used to show that a bow shock's nose location as and the relative subsolar magnetosheath thickness Δms/amp are strong functions of the IMF cone angle θ (between vsw and Bsw) and the Alfven Mach number MA For a given MA the shock is more distant for higher θ (restricted to the interval 0–90° by symmetries), while as/amp and Δms/amp increase with decreasing MA for θ ≳ 20° but decrease with decreasing MA for θ ∼ 0°. Large differences in Δms/amp are predicted between θ = 0° and 90° at low MA, with smaller differences remaining even at MA ∼ 10. The θ = 0° results confirm and extend the previous work of Spreiter and Rizzi [1974]. The simulations show that successful models for the subsolar shock location cannot subsume the dependences on MA and θ into a sole dependence on Mms. Instead, they confirm a recent prediction [Cairns and Grabbe, 1994] that as/amp and Δms/amp should depend strongly on θ and MA for MA ≲ 10 (as well as on other MHD variables). Detailed comparisons between theory and data remain to be done. However, preliminary comparisons show good agreement, with distant shock locations found for low MA and large θ ≳ 45° and closer locations found for θ ≲ 20° even at MA ∼ 8.