  By explicitly taking into account effects of Alfven waves, I derive from a
simple energetics argument a fundamental relation which predicts solar wind
(SW) speeds in the vicinity of the earth from physical properties on the sun.
Kojima et al. recently found from their observations that a ratio of surface
magnetic field strength to an 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 an evidence of the Alfven wave which accelerates SW in
expanding flux tubes. The observations further require that fluctuation
amplitudes of magnetic field lines at the surface should be almost universal in
different coronal holes, which needs to be tested by future observations.
