Astrophysical significance of the anisotropic kinetic alpha effect

Department of Mathematics, University of Newcastle upon Tyne, NE1 7RU, UK

Corresponding author: A. Brandenburg, brandenb@nordita.dk

Received: 15 June 2001
Accepted: 4 October 2001

Abstract

The generation of large scale flows by the anisotropic kinetic alpha (AKA) effect is investigated in simulations with a suitable time-dependent space- and time-periodic anisotropic forcing lacking parity invariance. The forcing pattern moves relative to the fluid, which leads to a breaking of the Galilean invariance as required for the AKA effect to exist. The AKA effect is found to produce a clear large scale flow pattern when the Reynolds number, , is small as only a few modes are excited in linear theory. In this case the non-vanishing components of the AKA tensor are dynamically independent of the Reynolds number. For larger values of , many more modes are excited and the components of the AKA tensor are found to decrease rapidly with increasing value of . However, once there is a magnetic field (imposed and of sufficient strength, or dynamo-generated and saturated) the field begins to suppress the AKA effect, regardless of the value of . It is argued that the AKA effect is unlikely to be astrophysically significant unless the magnetic field is weak and is small.