Magnetic Noise and the Galactic Dynamo

Summary

Galactic magnetic fields are widely thought to be the product of a turbulent mean field dynamo. We find, however, that kinematic mean field theory is inapplicable for galactic parameters because there is no effective way to destroy the small-scale fluctuating magnetic fields. We find that this ‘magnetic noise’ grows exponentially with a time constant of 10⁴ years, while the dynamo grows with a 2 × 10⁸ year time scale. The dynamo field quickly becomes unobservable under such conditions and the kinematic approximation fails before the mean field grows significantly.

INTRODUCTION

Our galaxy and others are permeated by magnetic fields. They play an important role in star formation, in the support of molecular clouds against collapse, and in cosmic ray confinement. With a field strength of a few microgauss, they are comparable in in energy density to thermal energy, radiation, and cosmic rays. These fields are widely assumed to be the result of a dynamo operating on an initial seed field.

Dynamos work by folding magnetic field lines back on themselves constructively more often than destructively. Mean field theory assumes that the many folds in the field with no net contribution are destroyed, usually by resistivity. What would happen if these small disordered fields were not destroyed? They would obscure the growing large-scale field and might dominate the total magnetic energy. This is indeed a concern for galactic dynamo theory as magnetic loops 0.1 pc across need 10²² years to decay ohmically.