Large jets from small-scale magnetic fields^*

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany e-mail: rmo@mpa-garching.mpg.de

Received: 25 August 2009
Accepted: 28 November 2009

Abstract

We consider the conditions under which a rotating magnetic object can produce a magnetically powered outflow in an initially unmagnetized medium stratified under gravity. 3D MHD simulations are presented in which the footpoints of localized, arcade-shaped magnetic fields are placed in rotation. We show how the effectiveness of producing a collimated magnetically powered outflow depends on the rotation rate, and both the strength and the geometry of the field. The flows produced by uniformly rotating, non-axisymmetric fields are found to consist mainly of buoyant plumes heated by the dissipation of rotational energy. Collimated magnetically powered flows are formed if the field and the rotating surface are arranged such that a toroidal magnetic field is produced. This requires that the arcades' footpoints are differentially rotating. These jets are well-collimated and we follow their propagation through the stratified atmosphere over a distance 100 times the source size. The magnetic field is tightly wound and its propagation is dominated by the development of non-axisymmetric instabilities. We observe a Poynting flux conversion efficiency of over 75% in the longest simulations. Applications to the collapsar model and protostellar jets are discussed.