Seed fields for galactic dynamos by the magnetorotational instability

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482, Potsdam, Germany e-mail: [gruediger;lkitchatinov]@aip.de
² Institute for Solar-Terrestrial Physics, PO Box 4026, Irkutsk 664033, Russian Federation e-mail: kit@iszf.irk.ru

Received: 12 March 2004
Accepted: 13 May 2004

Abstract

A linear but global numerical model for the magnetorotational instability (MRI) in a disk geometry is considered to estimate the instability parameters for galaxies. Similarity rules suggested by a local analysis are applied to reveal a universal behavior of the results of global calculations for large magnetic Prandtl numbers. The findings are used to estimate the MRI characteristics for galaxies with their very large magnetic Prandtl numbers which cannot be attained in any numerical simulations. The resulting minimum field for the instability,  G, is small compared to any seed fields currently discussed. The growth times of MRI are estimated to be on the order of the rotation period of the inner rigidly-rotating core, i.e. ~100 Myr. Global MRI excites preferentially magnetic field modes of quadrupolar symmetry.