Seed fields for galactic dynamos by the magnetorotational instabilities

Abstract

A linear but global numerical model for the magnetorotational instability (MRI) in 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 approached by 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, Bmin 10-25 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 the magnetic field modes of quadrupolar symmetry.