Simulations of a Conducting Sphere Moving through Magnetized Plasma: Alfv\'en Wings, Slow Magnetosonic Wings, and Drag Force

Abstract

Plasma-mediated interaction between astrophysical objects can play an important role and produce electromagnetic radiation in various binary systems, ranging from planet-moon and star-planet systems to binary compact objects. We perform 3D magnetohydrodynamic numerical simulations to study an ideal magnetized plasma flowing past an unmagnetized conducting sphere. Such flow generates magnetic disturbances and produces a drag force on the sphere, and we explore the corresponding drag coefficient as a function of the flow speed relative to Alfv\'en speed and the $\beta$ parameter of the background plasma. We find that the drag is generally well-described by the Alfv\'en wing model, but we also show that slow magnetosonic waves provide a correction through their own wing-like features. These give rise to a nontrivial dependence of the drag coefficient on the plasma $\beta$, as well as enhanced drag as the flow speed approaches the Alfv\'en speed.