Relation Between Coronal Hole Areas and Solar Wind Speeds Derived from Interplanetary Scintillation Measurements

Abstract

We investigate the relation between coronal hole (CH) areas and solar wind speeds during 1995 – 2011 using the potential field (PF) model analysis of magnetograph observations and interplanetary scintillation (IPS) observations by the Institute for Space-Earth Environmental Research (formerly Solar-Terrestrial Environment Laboratory) of Nagoya University. We obtained a significant positive correlation between the CH areas ($A$) derived from the PF model calculations and solar wind speeds ($V$) derived from the IPS observations. The correlation coefficients between them are usually high, but they drop significantly in solar maxima. The slopes of the $A$ – $V$ relation are roughly constant except for the period around solar maximum, when flatter or steeper slopes are observed. The excursion of the correlation coefficients and slopes at solar maxima is ascribed partly to the effect of rapid structural changes in the coronal magnetic field and solar wind, and partly to the predominance of small CHs. It is also demonstrated that $V$ is inversely related to the flux expansion factor ($f$) and that $f$ is closely related to $A^{-1/2}$; hence, $V \propto A^{1/2}$. A better correlation coefficient is obtained from the $A^{1/2}$ – $V$ relation, and this fact is useful for improving space weather predictions. We compare the CH areas derived from the PF model calculations with He i 1083 nm observations and show that the PF model calculations provide reliable estimates of the CH area, particularly for large $A$.