2025-02-06
セミナー

PPARC セミナー (2025/02/07)

 

PPARC セミナー (2025/02/07)

(1) Chizuru Nose

[Title]  
Influence of the solar wind on the hydrogen airglow in the Venusian upper atmosphere observed by Hisaki and VEX

[Abstract]
One of the big questions for Venus is whether or not Venus had an Ocean in the past. While Venusian current atmosphere is extremely dry, isotopic measurements of the D/H ratio suggest that Venus likely retained significant amounts of water in its past. Observations of hydrogen and oxygen in Venusian upper atmosphere can provide constraints on these processes. Results from Venus Express have shown the presence of hot and cold components in the Venusian hydrogen corona at different scale heights. It was suggested that charge exchange between the cold component and ionospheric or solar wind protons play a significant role in producing the hot component (Chaufray et al. 2012).Observations from Hisaki have shown that, when the high-speed solar wind originating from a corotating interaction region (CIR), arrived, the hydrogen column density derived from Ly-α increased by approximately 10% over a few days and then remained almost constant for weeks. The column density derived from Ly-β almost constant during this period. One possible explanation for the ~10% variations in Ly-α and the relatively stable Ly- is an increased high-altitude hot hydrogen abundance due to interaction between the Venusian upper atmosphere and the high speed solar wind. However, since the Hisaki observations measured the globally averaged hydrogen column density, they could not resolve altitude-dependent variations.
In this seminar, we introduce the method used to derive hydrogen density and temperature from the Venusian hydrogen corona model by Chaufray et al. (2012, 2015) and the observed Ly-α emission intensity profile. We also discuss perspectives on understanding the interaction between the Venusian hydrogen atmosphere and the solar wind.

(2) Natsuko Matsushita

[Title]
Plasma parameters at Europa’s orbit estimated from the Hisaki observation
~ Improvement and evaluation of the method ~

 
ひさき衛星の極端紫外分光データを用いたエウロパ軌道におけるプラズマ診断
～手法の改良・評価～

[Abstract]
Europa (9.4 RJ from Jupiter) has a tenuous molecular oxygen atmosphere produced by magnetospheric plasma sputtering on its surface. To improve our understanding of the production and loss of the atmosphere, the density and temperature of the magnetospheric plasma around the satellite must be known. This study analyzed JAXA’s Hisaki data observed from March 1st to May 14th, 2015, to estimate the electron density, electron temperature, and ion composition at Europa’s orbit.
     An ultraviolet spectrograph (EXCEED) aboard Hisaki measured the sulfur and oxygen ion emission lines in the extreme ultraviolet (EUV) wavelength range (55-145 nm). The Jovian magnetosphere is filled with plasmas originating from satellite Io (5.9 RJ). The torus emission intensity peaks around Io’s orbit and decays with increasing radial distance from the planet. At Europa’s orbit, the brightness was so weak that contaminations from the terrestrial radiation belt and foreground geocoronal emissions were carefully removed, and the spectrograph data were integrated between LT20 and LT4 for 2.5 months (about 15,500 min).
     The emission intensity is a product of the ion density and natural transition probability along the line of sight. The torus ions are excited by electron impact, so that the ion density of a certain energy level depends on the density and temperature of the electrons. We used the CHIANTI atomic database to find the best-fit plasma parameters of the observed spectrum by minimizing the chi-square, a method known as plasma diagnosis. From plasma diagnosis, the electron density and electron temperature at Europa’s orbit were determined to be 246 ± 30 cm^-3 and 6.1 ± 1.5 eV, respectively, consistent with in situ observations (Bagenal et al., 2015). On the other hand, the high fraction of hot electrons (39%) has led to the need to verify whether the current plasma diagnosis method is objectively correct.
     The current plasma diagnosis assumes for convenience that the plasma parameters are uniform in the line-of-sight direction, but in reality density and temperature vary in the radial direction as well. In this seminar, we will evaluate the current plasma diagnostic method by creating a forward model in which the density varies exponentially in the radial direction.　

 
衛星エウロパは木星から9.4RJ 離れており、木星磁気圏のプラズマが衛星表面にスパッタリングすることによって生成される酸素大気に覆われている。エウロパ大気の生成散逸についての理解を深めるためには、衛星周辺のプラズマ状態を知ることが不可欠である。そこで本研究では、JAXAのひさき望遠鏡を用いて、エウロパ周辺の電子密度や電子温度、イオン組成といったプラズマパラメータを推定することを目的とする。
　木星磁気圏のプラズマの大部分は、衛星イオ (5.9RJ) の火山活動によって生じている。イオプラズマトーラスの密度は、イオから動径方向外側に向かうほど小さくなっている。硫黄や酸素のイオンは、高温の電子との衝突によって紫外線を発光するため、イオから離れたエウロパ軌道では発光が非常に弱い。そこで、S/N比を改善するために2015年3月1日から5月14日までの約2.5ヶ月(約15,500分)のひさき望遠鏡のデータを積分した。検出された硫黄と酸素のスペクトルから、プラズマ診断と呼ばれる手法によって、電子密度・高温電子の割合・硫黄と酸素イオンの割合を導出した。その結果、エウロパ軌道における電子密度は246±30 cm^-3、電子温度は6.1±1.5 eVと求められ、その場観測の結果(Bagenal et al., 2015)と整合的になった。一方、高温電子の割合が39%と高くなり、現在のプラズマ診断の方法が客観的に正しいかを検証する必要が生じてきた。
　プラズマ診断では、CHIANTI原子データベースをもとに計算された各種イオンの体積放射率を視線方向に積分することで、モデルのスペクトルを作成する。現在のプラズマ診断では、便宜上視線方向にプラズマパラメータが一様であるという仮定を置いているが、実際は動径方向にも密度や温度変化がある。そこで、今回のセミナーでは、密度が動径方向に指数関数的に変化するようなフォワードモデルを作成し、現在のプラズマ診断手法の評価を行う。