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

## TL;DR

This study analyzes long-term solar polar data, revealing strong correlations between microwave brightening, solar wind velocity, and coronal hole size, and clarifies the physical relationships among these solar phenomena.

## Contribution

It establishes that polar microwave brightness temperature is a more direct indicator of solar wind speed than magnetic field strength or flux expansion rate.

## Key findings

- High correlation (CC=0.91) between polar brightness temperature and solar wind velocity.
- Polar brightening closely matches the probability distribution of polar coronal hole area.
- The V-T_b relationship reflects the Wang-Sheeley model linking solar wind speed and magnetic flux expansion.

## Abstract

We comparatively studied the long-term variation (1992-2017) in 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 Kitt Peak National Solar Observatory. 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.