The effect of rotation on the buoyant rise of magnetic flux tubes in accretion disks

Abstract

The dynamics of buoyant magnetic flux tubes in thin accretion disks is studied under isothermal conditions by means of numerical simulations. The influence of rotation on the rising behavior of the flux tube is examined and the role of a weak magnetic field line twist within the tube is investigated. By employing the adaptive mesh code NIRVANA the 3D simulations have effective resolution higher than in any previous numerical work on that topic. The fate of the flux tube strongly depends on the presence or absence of rotation respective differential rotation. Rotation effectively slows down the vertical ascend of the flux tube largely as a consequence of the Coriolis force acting on the surrounding flow which, in turn, reacts upon the tube. The detailed behavior also depends on the amount of twist. In accretion disks, a weakly twisted flux tube is disrupted and its rise is halted due to the impact of the magnetic shear instability which is driven by the interaction between the background rotational shear flow and (poloidal) twist field. As a consequence, the magnetic structure is captured in the inner disk region (z < H0, H0: disk scale height) a much longer time than suggested by the buoyant time scale in a non-rotating atmosphere. Untwisted accretion disk flux tubes do not break up quickly into pieces, as was found for corresponding tubes embedded in a non-rotating environment, but retain some degree of coherence albeit the stabilizing effect of twist is missing. In general, the numerical results are in gross contradiction to what postulates a highly simplified 1D picture based on the thin flux tube approximation.