Dynamo effect in unstirred self-gravitating turbulence

TL;DR

This study uses simulations to demonstrate that self-gravity in unstirred, subsonic turbulence can generate magnetic fields through dynamo action once a critical magnetic Reynolds number is exceeded, with energy transfer influenced by gravitational collapse.

Contribution

It reveals the critical conditions for dynamo action driven by self-gravity in unstirred turbulence and details the energy transfer mechanisms during collapse and magnetic field amplification.

Findings

Dynamo action occurs above a magnetic Reynolds number of about 25.

Energy transfer shifts from kinetic to magnetic as collapse progresses.

Magnetic field amplification is driven by tangling of the initial field.

Abstract

In many astrophysical environments, self-gravity can generate kinetic energy, which, in principle, is available for driving dynamo action. Using direct numerical simulations, we show that in unstirred self-gravitating subsonic turbulence with helicity and a magnetic Prandtl number of unity, there is a critical magnetic Reynolds number of about 25 above which the work done against the Lorentz force exceeds the Ohmic dissipation. The collapse itself drives predominantly irrotational motions that cannot be responsible for dynamo action. We find that, with a weak magnetic field, one-third of the work done by the gravitational force goes into compressional heating and the remaining two-thirds go first into kinetic energy of the turbulence before a fraction of it is converted further into magnetic and finally thermal energies. Close to the collapse, however, these fractions change toward 1/4 and 3/4 for compressional heating and kinetic energy, respectively. When the magnetic field is strong, the compressional heating fraction is unchanged. Out of the remaining kinetic energy, one quarter goes directly into magnetic energy via work against the Lorentz force. The fraction of vortical motions diminishes in favor of compressive motions that are almost exclusively driven by the Jeans instability. For an initially uniform magnetic field, field amplification at scales larger than those of the initial turbulence are driven by tangling.