Twofold effect of Alfvén waves on the transverse gravitational instability

¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Campus Morelia, A. P. 72-3 (Xangari), 58089, Morelia, Michoacán, México
² CNRS, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France e-mail: passot@obs-nice.fr

Corresponding author: A. Gazol, a.gazol@astrosmo.unam.mx

Received: 22 January 2003
Accepted: 25 March 2003

Abstract

This paper is devoted to the study of the gravitational instability of a medium permeated by a uniform magnetic field along which a circularly polarized Alfvén wave propagates. We concentrate on the case of perturbations purely transverse to the ambient field by means of direct numerical simulations of the MHD equations and of a linear stability analysis performed on a moderate amplitude asymptotic model. The Alfvén wave provides an extra stabilizing pressure when the scale of perturbations is sufficiently large or small compared to the Jeans length L_J. However, there is a band of scales around L_J for which the Alfvén wave is found to have a destabilizing effect. In particular, when the medium is stable in the absence of waves, the gravitational instability can develop when the wave amplitude lies in an appropriate range. This effect appears to be a consequence of the coupling between Alfvén and magnetosonic waves. The prediction based on a WKB approach that the Alfvén wave pressure tensor is isotropic and thus opposes gravity in all directions is only recovered for large amplitude waves for which the coupling between the different MHD modes is negligible.