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

Ken'ichi Fujiki   Submitted: 2019-02-28 23:15

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

Authors: Ken'ichi Fujiki, Kiyoto Shibasaki, Seiji Yashiro, Munetoshi Tokumaru, Kazumasa Iwai, Satoshi Masuda
Projects: Nobeyama Radioheliograph

Publication Status: Solar Physics 2019 in press (DOI: 10.1007/s11207-019-1418-6)
Last Modified: 2019-03-05 12:48
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Relationship Between Solar Wind Speed and Coronal Magnetic Field Properties  

Ken'ichi Fujiki   Submitted: 2015-07-13 18:37

We have studied the relationship between the solar-wind speed [V] and the coronal magnetic-field properties (a flux expansion factor [f] and photospheric magnetic-field strength [Bs]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that V is inversely proportional to f and found that V tends to increase with Bs if f is the same. As a consequence, we find that V has extremely good linear correlation with Bs/f. However, this linear relation of V and Bs/f cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between V and f has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.

Authors: Ken'ichi Fujiki, Munetoshi Tokumaru, Tomoya Iju, Kazuyuki Hakamada, Masayoshi Kojima
Projects: None

Publication Status: Solar Physics (accepted)
Last Modified: 2015-07-15 13:21
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Comparative Study of Microwave Polar Brightening, Coronal Holes, and Solar Wind Over the Solar Poles
Relationship Between Solar Wind Speed and Coronal Magnetic Field Properties

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