Field linkage and magnetic helicity density

TL;DR

This paper introduces a new visualization method for magnetic helicity density in stars, revealing how field linkages vary with stellar mass and rotation, and showing lower-mass stars are less efficient at generating large-scale helicity.

Contribution

A novel visualization approach for magnetic helicity density that classifies field linkages across stellar surfaces, enhancing understanding of magnetic properties in stars of different masses.

Findings

Lower-mass stars tend to have non-axisymmetric toroidal fields.

Stars on the lower-mass branch link through regions of mixed polarity.

Lower-mass stars are less efficient at generating large-scale helicity.

Abstract

The helicity of a magnetic field is a fundamental property that is conserved in ideal MHD. It can be explored in the stellar context by mapping large-scale magnetic fields across stellar surfaces using Zeeman-Doppler imaging. A recent study of 51 stars in the mass range 0.1-1.34 M$_\odot$ showed that the photospheric magnetic helicity density follows a single power law when plotted against the toroidal field energy, but splits into two branches when plotted against the poloidal field energy. These two branches divide stars above and below $\sim$ 0.5 M$_\odot$. We present here a novel method of visualising the helicity density in terms of the linkage of the toroidal and poloidal fields that are mapped across the stellar surface. This approach allows us to classify the field linkages that provide the helicity density for stars of different masses and rotation rates. We find that stars on the lower-mass branch tend to have toroidal fields that are non-axisymmetric and so link through regions of positive and negative poloidal field. A lower-mass star may have the same helicity density as a higher-mass star, despite having a stronger poloidal field. Lower-mass stars are therefore less efficient at generating large-scale helicity.