Torus-Stable Zone Above Starspots

TL;DR

This paper investigates the magnetic conditions above starspots that can suppress stellar coronal mass ejections, explaining the low observed CME rates on cool stars through a model of the torus-stable zone.

Contribution

It introduces a potential field model to estimate the extent of the torus-stable zone above starspots, linking magnetic field configurations to CME suppression.

Findings

The upper bound of the TSZ increases with bipole size, dipole strength, and source surface radius.

A secondary TSZ can form at higher altitudes, possibly leading to failed eruptions.

Extended TSZs may explain the low CME detection rate on cool stars.

Abstract

Whilst intense solar flares are almost always accompanied by a coronal mass ejection (CME), reports on stellar CMEs are rare, despite the frequent detection of stellar 'super flares'. The torus instability of magnetic flux ropes is believed to be one of the main driving mechanisms of solar CMEs. Suppression of the torus instability, due to a confining background coronal magnetic field that decreases sufficiently slowly with height, may contribute to the lack of stellar CME detection. Here we use the solar magnetic field as a template to estimate the vertical extent of this 'torus-stable zone' (TSZ) above a stellar active region. For an idealised potential field model comprising the fields of a local bipole (mimicking a pair of starspots) and a global dipole, we show that the upper bound of the TSZ increases with the bipole size, the dipole strength, and the source surface radius where the coronal field becomes radial. The boundaries of the TSZ depend on the interplay between the spots' and the dipole's magnetic fields, which provide the local- and global-scale confinement, respectively. They range from about half the bipole size to a significant fraction of the stellar radius. For smaller spots and an intermediate dipole field, a secondary TSZ arises at a higher altitude, which may increase the likelihood of 'failed eruptions'. Our results suggest that the low apparent CME occurrence rate on cool stars is, at least partially, due to the presence of extended TSZs.