Magnetocentrifugal Winds in Three Dimensions: A Nonaxisymmetric Steady State

Abstract

Outflows can be loaded and accelerated to high speeds along rapidlyrotating, open magnetic field lines by centrifugal forces. Whether suchmagnetocentrifugally driven winds are stable is a long-standingtheoretical problem. As a step toward addressing this problem, weperform the first large-scale 3D MHD simulations that extend to adistance ~102 times beyond the launching region, startingfrom steady 2D (axisymmetric) solutions. In an attempt to drive the windunstable, we increase the mass loading on one half of the launchingsurface by a factor of sqrt(10) and reduce it by the same factor on theother half. The evolution of the perturbed wind is followed numerically.We find no evidence for any rapidly growing instability that coulddisrupt the wind during the launching and initial phase of propagation,even when the magnetic field of the magnetocentrifugal wind istoroidally dominated all the way to the launching surface. The stronglyperturbed wind settles into a new steady state, with a highly asymmetricmass distribution. The distribution of magnetic field strength is, incontrast, much more symmetric. We discuss possible reasons for theapparent stability, including stabilization by an axial poloidalmagnetic field, which is required to bend field lines away from thevertical direction and produce a magnetocentrifugal wind in the firstplace.