Journal Article

MRI channel flows in vertically stratified models of accretion discs

Henrik N. Latter, Sebastien Fromang and Oliver Gressel

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 406, issue 2, pages 848-862
Published in print August 2010 | ISSN: 0035-8711
Published online July 2010 | e-ISSN: 1365-2966 | DOI:
MRI channel flows in vertically stratified models of accretion discs

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Simulations of the magnetorotational instability (MRI) in ‘unstratified’ shearing boxes exhibit powerful coherent flows, whereby the fluid vertically splits into countermoving planar jets or ‘channels’. Channel flows correspond to certain axisymmetric linear MRI modes, and their preponderance follows from the remarkable fact that they are approximate non-linear solutions of the MHD equations in the limit of weak magnetic fields. We show in this paper, analytically and with one-dimensional numerical simulations, that this property is also shared by certain axisymmetric MRI modes in vertically stratified shearing boxes. These channel flows rapidly capture significant amounts of magnetic and kinetic energy, and thus are vulnerable to secondary shear instabilities. We examine these parasites in the vertically stratified context, and estimate the maximum amplitudes that channels attain before they are destroyed. These estimates suggest that a dominant channel flow will usually drive the disc's magnetic field to thermal strengths. The prominence of these flows and their destruction place enormous demands on simulations, but channels in their initial stages also offer a useful check on numerical codes. These benchmarks are especially valuable given the increasing interest in the saturation of the stratified MRI. Lastly, we speculate on the potential connection between ‘run-away’ channel flows and outburst behaviour in protostellar and dwarf nova discs.

Keywords: accretion, accretion discs; instabilities; MHD; turbulence

Journal Article.  12614 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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