Magnetic Helicity Reversal in the Corona at Small Plasma Beta

TL;DR

This paper demonstrates that magnetic helicity reverses sign within the solar corona when plasma beta drops below one, revealing a key process in magnetic field evolution that is not directly observable by current spacecraft.

Contribution

It identifies the conditions under which magnetic helicity reverses sign in the solar corona using realistic simulations, linking plasma beta to helicity behavior.

Findings

Helical sign reversal occurs when plasma beta drops below unity.

Reversal happens within the nearly force-free coronal magnetic field.

Changes in magnetic structure dominance accompany the helicity reversal.

Abstract

Solar and stellar dynamos shed small-scale and large-scale magnetic helicity of opposite signs. However, solar wind observations and simulations have shown that some distance above the dynamo both the small-scale and large-scale magnetic helicities have reversed signs. With realistic simulations of the solar corona above an active region now being available, we have access to the magnetic field and current density along coronal loops. We show that a sign reversal in the horizontal averages of the magnetic helicity occurs when the local maximum of the plasma beta drops below unity and the field becomes nearly fully force free. Hence, this reversal is expected to occur well within the solar corona and would not directly be accessible to in-situ measurements with the Parker Solar Probe or SolarOrbiter. We also show that the reversal is associated with subtle changes in the relative dominance of structures with positive and negative magnetic helicity.