Superbubbles, Galactic Dynamos and the Spike Instability

TL;DR

This paper proposes that a spike instability from superbubbles can fragment magnetic flux lines, enabling the alpha-Omega dynamo to generate galactic magnetic fields without violating flux freezing or expelling entire field lines.

Contribution

It introduces a novel mechanism involving spike instabilities to resolve flux expulsion issues in galactic dynamo theory.

Findings

Spike instability can cut magnetic flux lines into short segments.

Enough spikes exist to fragment all undesirable flux during a billion-year dynamo cycle.

Fragmented flux segments can be rotated to eliminate undesirable flux without expelling entire fields.

Abstract

We draw attention to a problem with the alpha-Omega dynamo when it is applied to the origin of the galactic magnetic field under the assumption of perfect flux freezing. The standard theory involves the expulsion of undesirable flux and, because of flux freezing, the mass anchored on this flux also must be expelled. The strong galactic gravitational field makes this impossible on energetic grounds. It is shown that if only short pieces of the undesirable field lines are expelled, then mass can flow down along these field lines without requiring much energy. This expulsion of only short lines of force can be accomplished by a spike instability associated with gigantic astrophysical superbubbles. The physics of this instability is discussed and the results enable an estimate to be made of the number of spikes in the galaxy. It appears that there are probably enough spikes to cut all the undesirable lines into pieces as short as a couple of kiloparsecs during a dynamo time of a billion years. These cut pieces then may be randomly rotated in a dynamo time by alpha-Omega diffusion and there is enough rotation to get rid of the undesirable flux without expelling the fields themselves. The spike process seems strong enough to allows the alpha-Omega dynamo to create the galactic field without any trouble from the boundary condition problem.