日本地球惑星科学連合2023年大会

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[J] 口頭発表

セッション記号 P (宇宙惑星科学) » P-EM 太陽地球系科学・宇宙電磁気学・宇宙環境

[P-EM16] 太陽圏・惑星間空間

2023年5月22日(月) 15:30 〜 16:45 101 (幕張メッセ国際会議場)

コンビーナ:岩井 一正(名古屋大学 宇宙地球環境研究所)、成行 泰裕(富山大学学術研究部教育学系)、西野 真木(宇宙航空研究開発機構宇宙科学研究所)、坪内 健(電気通信大学)、座長:岩井 一正(名古屋大学 宇宙地球環境研究所)、成行 泰裕(富山大学学術研究部教育学系)、西野 真木(東京大学大学院理学系研究科地球惑星科学専攻)、小財 正義(情報・システム研究機構 データサイエンス共同利用基盤施設 極域環境データサイエンスセンター)

15:45 〜 16:00

[PEM16-02] Density oscillations in the solar corona seen in radio occultation measurements and a MHD simulation

*千葉 翔太1庄田 宗人2今村 剛1 (1.東京大学大学院 新領域創成科学研究科、2.東京大学大学院 理学系研究科)


キーワード:太陽風、電波掩蔽観測、太陽圏

The solar wind is a supersonic plasma flow streamed from the solar corona. The solar wind acceleration mainly occurs in the outer corona at heliocentric distances of about 2–10 RS (=solar radii), where the coronal heating by magnetohydrodynamic waves and the wave-induced magnetic pressure are thought to play major roles in the acceleration. The mechanisms have not been fully confirmed because the acceleration region is where no spacecraft has ever reached to date. Recently, however, an inner heliosphere observation network is getting ready, by such as NASA's Parker Solar Probe and ESA's Solar orbiter and BepiColombo.
Radio occultation observations cover the acceleration region fully and can obtain information complementary to in-situ observations. The radio occultation observations are conducted during the passage of a spacecraft on the opposite side of the sun as seen from the Earth. Inhomogeneity of coronal plasma density structure traversing the ray path disturbs radio waves' frequency so that we can interpret the received frequency fluctuations as density fluctuations in the coronal plasma. Previous observations detected quasi-periodic components thought to represent magnetoacoustic waves (e.g., Efimov et al., 2012; Miyamoto et al., 2014). The details of the detected waves still have not been investigated.
A recent MHD simulation has reproduced the formation of the solar wind based on the wave/turbulence-driven scenario, in which ubiquitous presence of density fluctuation is found. We applied the spectral analysis to the density fluctuations to compare them with the wave components observed by radio occultation observations conducted by JAXA’s Akatsuki spacecraft in 2016. The time-spatial spectrum of density fluctuations has two components whose phase speeds correspond to the Alfven speed and sound speed, and these two components are considered to be fast and slow modes. The dominant periods of the slow modes in the model are longer than 100 s, which is consistent with the density fluctuations observed by the radio occultation. The periods of the fast modes in the model are about 20–100 s; such short-period components are also seen in the radio occultation observations. Histograms of the wave’s period detected in the model and observations have a similar distribution, which has two peaks around less than 100 seconds and more than 100 seconds, so these different modes might have been observed by radio occultation simultaneously.