loading page

The Large Scale Dynamo in Stratified Rotating Objects
  • Ethan Vishniac,
  • Amir Jafari
Ethan Vishniac

Corresponding Author:[email protected]

Author Profile
Amir Jafari
Author Profile

Abstract

We revisit the foundations of mean-field dynamo theory, focussing on the possibility that magnetic field growth is driven by magnetic helicity. We discuss the generation of a large scale magnetic helicity flux in the absence of any large scale magnetic field. We calculate this flux to fourth order in the field variables, and to first order in the shear and rotation, in a differentially rotating system in a variety of interesting limits. We show that this contribution to the magnetic helicity flux typically dominates over all other contributions, including the diffusion term. In an inhomogeneous system, like a galaxy or a star, this leads to the large scale separation of magnetic helicity, which dominates the kinematic helicity after about one eddy turn over time, i.e. the kinematic dynamo limit is never realized. We apply this work to magnetic dynamos in galaxies, stars, and accretion disks. We give a qualitative account of the growth of a large scale magnetic field in a young disk galaxy with no initial large scale field. It begins with an incoherent dynamo and a magnetic helicity flux operating independently of any large scale field and ends with a transition from linear growth to saturation due to turbulent mixing. Applying this to the growth of large scale magnetic fields in galactic disks we find that the time required to reach saturation is a few rotational periods. A qualitative application to stellar dynamos in low mass stars reproduces the magnetic field amplitudes seen in slow and rapid rotators. For accretion disks we recover earlier results for the magnetic helicity flux driven dynamo in simulated and real disks, including the suppression of the dynamo in unstratified simulations with low magnetic Prandtl number.