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

Abstract

We comparatively studied the long-term variation 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 Fujiki et al.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 anticorrelation 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.

The role and contribution of magnetic fields, characterized via their magnetic flux, to the statistical structuring of the solar atmosphere

“…Polar brightening is a special phenomenon in the solar quiet atmosphere. It appears in the chromosphere at the minimum time of a solar cycle manifesting itself in observations in the Ca II K line 11 , 12 and the radio 17 GHz 13 , 15 – 18 band. Thus, the observed long-term variation of chromospheric brightness is in anti-phase with the solar cycle around the solar poles.…”

“…The full-disk solar chromosphere has also been observed respectively at radio 17 GHz (1.76 cm) of the Nobeyama Radioheliograph at the National Astronomical Observatory of Japan 13 and at radio 37 GHz (0.81 cm) of the 13.7-m Cassegrain radio telescope at the Aalto University Metsahovi Radio Observatory in Finland 14 . The temporal-latitudinal distribution of radio synoptic maps at each of these two frequencies presents a butterfly diagram, and they also reveal that "butterflies" are hot structures in the chromosphere [13][14][15][16][17][18] . Radio flux at 17 GHz measured from active regions may come from the chromosphere and the corona 13 .…”