Comparative Study of Microwave Polar Brightening, Coronal Holes, and Solar Wind over the Solar Poles

Abstract

We compared the long-term variation (1992 – 2017) in solar polar brightening observed with the Nobeyama Radioheliograph, the polar solar-wind velocity with interplanetary scintillation observations at the Institute for Space-Earth Environmental Research, and the coronal-hole distribution computed by potential-field calculations of the solar corona using synoptic magnetogram data obtained at the National Solar Observatory/Kitt Peak. First, by comparing the solar-wind velocity [$V$] and the brightness temperature [$T_{\mathrm{b}}$] in the polar region, we found good correlation coefficients (CCs) between $V$ and $T_{\mathrm{b}}$ in the polar regions, CC = 0.91 (0.83) for the northern (southern) polar region, and we obtained the $V$–$T_{ \mathrm{b}}$ relationship as $V = 12.6$ $(T_{\mathrm{b}}-10{,}667)^{1/2}+432$. We also confirmed that the CC of $V$–$T_{\mathrm{b}}$ is higher than those of $V$–$B$ and $V$–$B/f$, where $B$ and $f$ are the polar magnetic-field strength and magnetic-flux expansion rate, respectively. These results indicate that $T_{\mathrm{b}}$ is a more direct parameter than $B$ or $B/f$ for expressing solar-wind velocity. Next, we analyzed the long-term variation of the polar brightening and its relation to the area of the polar coronal hole [$A$]. As a result, we found that the polar brightening matches the probability distribution of the predicted coronal hole and that the CC between $T_{\mathrm{b}}$ and $A$ is remarkably high, CC = 0.97. This result indicates that the polar brightening is strongly coupled to the size of the polar coronal hole. Therefore, the reasonable correlation of $V$ – $T_{\mathrm{b}}$ is explained by $V$ – $A$. In addition, by considering the anti-correlation between $A$ and $f$ found in a previous study, we suggest that the $V$ – $T_{\mathrm{b}}$ relationship is another expression of the Wang–Sheeley relationship ($V$ – $1/f$) in the polar regions.