On magnetic fields, stellar coronae and dynamo action in late-type dwarfs

Abstract

The chromospheric and coronal emission from late-type dwarf stars is thought to depend on the stellar magnetic fields. The observed relations between the average surface magnetic fields Bsfs, the filling factors fs and the Rossby numbers RO are discussed and compared with the predictions of simple dynamo theory. The Ca II excess flux density ΔFHK as a fraction of the bolometric flux is found to correlate most closely with RO, a conclusion similar to that drawn by Noyes et al. Coronal scaling laws, based on the minimum energy loss hypothesis of Hearn, are compared with observed coronal parameters, and on average can account for the observed trends. The total coronal heating flux required is found to scale approximately as RO-1. If magnetic reconnection provides this flux, a combination of the plasma parameters required to match the empirical heating flux can be found. The scalings of the implied coronal field and surface magnetic flux with RO cannot be reconciled with magnetic flux conservation, suggesting that not all the surface flux extends to the corona. The dependence of chromospheric and coronal parameters on RO is not yet understood. However, the correlation between the Ca II excess flux density ΔFHK and the X-ray flux (more strictly, the coronal emission measure) can be accounted for by relating these quantities through the electron pressure.