SOLAR-WIND SPEED AND CORONAL FLUX-TUBE EXPANSION

The hypothesis that the solar wind speed at 1 AU and the rate of magnetic flux-tube expansion in the corona are inversely correlated is shown to be consistent with observations extending over the last 22 years. This empirical relationship allows the daily wind speeds at Earth to be predicted from a current-free extrapolation of the observed photospheric field into the corona. We attribute the narrow boundaries of high-speed wind streams to steep gradients in the flux-tube expansion rates at the edges of coronal holes. When a heliospheric current sheet is included in the model, we find that the flux tubes near the hole axis, although diverging more slowly than those near the hole boundary in the corona, have undergone the greatest net expansion at 1 AU, an effect consistent with the low densities within high-speed streams.