Forecasting Solar Wind Speeds

By explicitly taking into account the effects of Alfvén waves, I derive from a simple energetics argument a fundamental relation that predicts solar wind (SW) speeds in the vicinity of Earth from physical properties on the Sun. Kojima et al. recently found from observations that the ratio of surface magnetic field strength to the expansion factor of open magnetic flux tubes is a good indicator of the SW speed. I show by using the derived relation that this nice correlation is evidence of Alfvén wave acceleration of the SW in expanding flux tubes. The observations further require that the fluctuation amplitudes of magnetic field lines at the surface be almost universal in different coronal holes, which needs to be tested with future observations.