Abstract
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 idealized 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.
Highlights
1.1 Torus instability in solar eruptionsSolar coronal mass ejections (CMEs) are rapid expulsions of magnetized plasma with velocity up to 3000 km s−1 and mass up to a few 1016 g (Webb & Howard 2012)
Using an idealised coronal magnetic field model, we estimate the extent of the torus-stable zone’ (TSZ) above a bipolar stellar active region embedded in a global dipole field
We find that the upper bound of the TSZ, defined by the torus instability critical height, hc, increases with the spot size ρ, the dipole strength g10, and the source surface radius Rs
Summary
Solar coronal mass ejections (CMEs) are rapid expulsions of magnetized plasma with velocity up to 3000 km s−1 and mass up to a few 1016 g (Webb & Howard 2012). The fastest CMEs are often associated with intense flaring (Vršnak et al 2005). In these events, the CME kinetic energy and the flare radiative energy are generally both of order 1032–1033 erg (Emslie et al 2012). Many solar CMEs originate from active regions that harbour kilogauss magnetic fields and sunspots. Prior to eruption, their coronal fields are thought to often evolve towards a ‘magnetic flux rope’ configuration, i.e. a current-carrying magnetic flux tube with coherent twist (Patsourakos et al 2020). The twisted structure is clearly visible in a significant fraction of CMEs (Vourlidas et al 2013)
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