Influence of surface stressing on stellar coronae and winds

Abstract

The large-scale field of the Sun is well represented by its lowest energy (or potential) state. Recent observations, by comparison, reveal that many solar-type stars show large-scale surface magnetic fields that are highly non-potential - that is, they have been stressed above their lowest energy state. This non-potential component of the surface field is neglected by current stellar wind models. The aim of this paper is to determine its effect on the coronal structure and wind. We use Zeeman-Doppler surface magnetograms of two stars - one with an almost potential, one with a non-potential surface field - to extrapolate a static model of the coronal structure for each star. We find that the stresses are carried almost exclusively in a band of unidirectional azimuthal field that is confined to mid-latitudes. Using this static solution as an initial state for a magnetohydrodynamic (MHD) wind model, we then find that the final state is determined primarily by the potential component of the surface magnetic field. The band of azimuthal field must be confined close to the stellar surface, as it is not compatible with a steady-state wind. By artificially increasing the stellar rotation rate, we demonstrate that the observed azimuthal fields cannot be produced by the action of the wind but must be due to processes at or below the stellar surface. We conclude that the background winds of solar-like stars are largely unaffected by these highly stressed surface fields. Nonetheless, the increased flare activity and associated coronal mass ejections that may be expected to accompany such highly stressed fields may have a significant impact on any surrounding planets.