We study dynamics of relativistic coronal mass ejections (CMEs), from launching by shearing of foot-points (either slowly – the ‘Solar flare’ paradigm, or suddenly – the ‘star quake’ paradigm), to propagation in the preceding magnetar wind. For slow shear, most of the energy injected into the CME is first spent on the work done on breaking through the overlaying magnetic field. At later stages, sufficiently powerful CMEs may lead to the ‘detonation’ of a CME and opening of the magnetosphere beyond some equipartition radius req, where the decreasing energy of the CME becomes larger than the decreasing external magnetospheric energy. Post-CME magnetosphere relaxes via the formation of a plasmoid-mediated current sheet, initially at ∼req, and slowly reaching the light cylinder. Both the location of the foot-point shear and the global magnetospheric configuration affect the frequent/weak versus rare/powerful CME dichotomy – to produce powerful flares, the slow shear should be limited to field lines that close in near the star. After the creation of a topologically disconnected flux tube, the tube quickly (at ∼ the light cylinder) comes into force-balance with the preceding wind and is passively advected/frozen in the wind afterward. For fast shear (a local rotational glitch), the resulting large amplitude Alfvén waves lead to the opening of the magnetosphere (which later recovers similarly to the slow shear case). At distances much larger than the light cylinder, the resulting shear Alfvén waves propagate through the wind non-dissipatively.
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.
more » « less- NSF-PAR ID:
- 10379570
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 509
- Issue:
- 4
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 5075-5085
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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