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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
Article

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.

Keywords

Solar wind Interplanetary scintillation Radioheliograph Coronal holes Magnetic fields Solar Cycle 

Notes

Acknowledgements

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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