Solar Physics

, 294:30 | Cite as

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

  • Ken’ichi FujikiEmail author
  • Kiyoto Shibasaki
  • Seiji Yashiro
  • Munetoshi Tokumaru
  • Kazumasa Iwai
  • Satoshi Masuda


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.


Solar wind Interplanetary scintillation Radioheliograph Coronal holes Magnetic fields Solar Cycle 



NoRH was operated by Nobeyama Solar Radio Observatory (NSRO), National Astronomical Observatory Japan (NAOJ) in 1992 – 2015. Since then, it has been operated by the International Consortium for the Continued Operation of Nobeyama Radioheliograph (ICCON). ICCON consists of ISEE/Nagoya University, NAOC, KASI, NICT, and GSFC/NASA. The IPS observations are carried out under the solar-wind program of ISEE/Nagoya University. We are grateful to NSO/KP for the use of their synoptic magnetograms. We also thank COHOWeb/NASA for the use of Ulysses/SWOOPS data. This work is partially supported by JSPS KAKENHI, Grant Number 18H01253.

Disclosure of Potential Conflicts of Interest

The authors declare that they have no conflicts of interest.


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Copyright information

© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Institute for Space-Earth Environmental ResearchNagoya UniversityNagoyaJapan
  2. 2.Solar Physics Research Inc.KasugaiJapan
  3. 3.Department of PhysicsThe Catholic University of AmericaWashingtonUSA
  4. 4.Code 671NASA Goddard Space Flight CenterGreenbeltUSA

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