Relationship Between Solar-Wind Speed and Coronal Magnetic-Field Properties

Abstract

We have studied the relationship between the solar-wind speed $[V]$ and the coronal magnetic-field properties (a flux-expansion factor [$f$] and photospheric magnetic-field strength [$B_{\mathrm{S}}$]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that $V$ is inversely proportional to $f$ and found that $V$ tends to increase with $B_{\mathrm{S}}$ if $f$ is the same. As a consequence, we find that $V$ has an extremely good linear correlation with $B_{\mathrm{S}}/f$. However, this linear relation of $V$ and $B_{\mathrm{S}}/f$ cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between $V$ and $f$ has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.