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_b] in the polar region, we found good correlation coefficients (CCs) between V and T_b in the polar regions, CC = 0.91 (0.83) for the northern (southern) polar region, and we obtained the V -T_b relationship as V =12.6 (T^{_b−10 ,667 ) 1 /2}+432 . We also confirmed that the CC of V -T_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_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_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_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_b relationship is another expression of the Wang-Sheeley relationship (V - 1 /f ) in the polar regions.