Magnetic loop models: from Sun to stars

I review recent progress in determining the nature of the loop structures that form the coronae of solar-like stars. This progress has been driven by observational advances, in particular the new results from Xray satellites (Chandra and XMM-Newton) and the availability of surface magnetograms from Zeeman-Doppler imaging. It is now clear that stars that are similar to the Sun in mass, but which rotate more rapidly, have a very different magnetic field structure. Their surfaces are more heavily spotted, with spots appearing at all latitudes, extending all the way up to the rotation pole. Their coronae are correspondingly much brighter in X-rays, containing plasma that is hotter and denser than on the Sun. In addition, stellar coronae can support massive co-rotating prominences out to many stellar radii. Recent efforts in modelling these magnetic structures are now bringing together both the surface magnetograms and also the coronal X-ray emission. The resulting coronal loop models show complex loop structures on all scales, with much of the X-ray emission coming from high latitudes where is does not suffer rotational self-eclipse. The observed high densities and X-ray emission measures are a natural consequence of the high magnetic flux density at the surface. The stripping of the corona due to centrifugal effects at high rotation rates can also explain the saturation and supersaturation of X-ray emission with increasing rotation rates, and the recent observation of a high rotational modulation in a supersaturated star.