Predicting the wind speeds of solar-like stars

Abstract

Some aspects of stellar magnetic activity such as X-ray emission are relatively easy to observe, while others, such as the geometry of the magnetic field are rather more difficult. Indeed, typically only the large-scale (or low-order) components of the field can be mapped. Most elusive of all is the hot, tenuous stellar wind, yet the wind speed is a crucial quantity governing the angular momentum loss of the star. We demonstrate, however, that wind speeds can be predicted reliably even from fairly low-resolution magnetograms. We use an empirically-derived model of the solar wind that predicts the distribution of wind speeds based on surface magnetograms. Using solar magnetic field measurements spanning several magnetic cycles, we demonstrate how changes in the surface magnetic field on various length-scales affect the coronal structure and hence the wind speed. We find that only low-order field components are needed to characterise the wind speed. We compare the variations in wind speed with variations in the X-ray emission over several cycles and show that while the small-scale field has a significant effect on Lx, it has little effect on the wind speed. This suggests that the large number of stellar magnetograms that are becoming available can be used to predict the stellar wind speeds.