Stabilizing effect of magnetic helicity on magnetic cavities in the intergalactic medium

TL;DR

This paper demonstrates that magnetic helicity within cavities in the intergalactic medium enhances their stability against disruptive instabilities, potentially explaining their longevity over hundreds of millions of years.

Contribution

It shows that internal helical magnetic fields can stabilize intergalactic cavities more effectively than external magnetic fields at comparable energy levels.

Findings

High magnetic helicity increases cavity stability.

Helical fields outperform external fields in stabilizing cavities.

Cavities with helical fields last over 100 million years.

Abstract

We investigate the effect of magnetic helicity on the stability of buoyant magnetic cavities as found in the intergalactic medium. In these cavities, we insert helical magnetic fields and test whether or not helicity can increase their stability to shredding through the Kelvin-Helmholtz instability and, with that, their lifetime. This is compared to the case of an external vertical magnetic field which is known to reduce the growth rate of the Kelvin-Helmholtz instability. By comparing a low-helicity configuration with a high helicity one with the same magnetic energy we find that an internal helical magnetic field stabilizes the cavity. This effect increases as we increase the helicity content. Stabilizing the cavity with an external magnetic field requires instead a significantly stronger field at higher magnetic energy. We conclude that the presence of helical magnetic fields is a viable mechanism to explain the stability of intergalactic cavities on time scales longer than 100 Myr.