PPARCセミナー (2025/10/10)
PPARCセミナー (2025/10/10)
(1)
[Name]
Ayuto Kawakami
[Title]
(Review) Feasibility of the exploration of the subsurface structures of Jupiter’s icy moons by interference of Jovian hectometric and decametric radiation
[Abstract]
A new passive subsurface radar technique using interference patterns in the spectrum of the Jovian hectometric and decametric radiation (HOM/DAM) has been proposed, and investigated for implementation on JUICE (Jupiter Icy Moons Explorer)/ RPWI (Radio and Plasma Wave Instrument). When there occurs interference among Jovian radio waves directly from Jupiter (W1), those reflected at the ice crust surface (W2), and those reflected at the subsurface reflectors in the ice crust (W3), fine and wide interference patters can be found in the spectrum. Fine patterns are caused by interference between W1 and W2, and between W1 and W3. Wide patterns are caused by interference between W2 and W3. In order to observe these interference patterns, the receiver of JUICE/RPWI is required to resolve 100 Hz, and possess a downlink spectra with a frequency range of 2 MHz and resolution of 1 kHz. Based on the calculation of the attenuation rate of the radio waves in the ice from 80 K (surface) to 250 K (just above the subsurface ocean), the intensity of the subsurface echo was estimated. The radar waves are expected to reach just above the ice crust/liquid ocean boundary. However, due to extremely high attenuation, it is difficult to detect the echoes from ice crust/liquid ocean boundary. In order to apply the new passive subsurface radar methods, the duration of the coherence of the Jovian radio wave should be long enough (>3.3 ms if spacecraft’s altitude is 500 km).
(2)
[Name]
Natsuko Matsushita
[Title]
ひさき衛星の極端紫外分光データを用いたイオ軌道からエウロパ軌道にわたるプラズマ特性の導出
Plasma properties from Io’s to Europa’s orbit estimated from the Hisaki observation
[Abstract]
木星内部磁気圏では、イオに由来する硫黄や酸素のイオンがイオプラズマトーラスを形成し、エウロパ軌道まで広がっている。木星磁気圏の物質やエネルギー輸送のメカニズムを理解するためには、プラズマ特性の空間分布・時間変動の導出が不可欠であるが、これまでの観測が限定的なため、イオ周辺とエウロパ周辺の同時期の解析は行われてこなかった。そこで本研究では、視野が広く長期間の観測が可能なひさき衛星/EXCEEDから得られた極端紫外分光データを用いて、イオ軌道からエウロパ軌道にかけての電子密度、電子温度、イオン組成の動径方向分布を導出することを目的としている。
これまでに、プラズマ特性が動径方向にべき乗分布で表せると仮定した、新たなプラズマ診断手法の開発に取り組んできた。2015年1月のイオ火山噴火イベント後の期間(2015年3月~5月, 活発期)では、推定された電子密度と電子温度は、Bagenal et al. (2015) によるエウロパ軌道の結果と Yoshioka et al. (2018) によるイオ軌道の結果の範囲内にそれぞれ一致した。電子全体に対する高温電子の割合は、イオ軌道でもエウロパ軌道でも先行研究の約4倍の大きさとなった。イオン組成に関しては、高温電子の割合が動径方向外側ほど増加するにつれて、S3+の割合は増加しS+の割合は減少した。これは、トーラスのイオンが動径方向外側へ輸送されながら、電子衝突電離によるイオン化が進行している可能性を支持している。一方、2014年2月~4月の静穏期の結果からは、パラメータ空間上にχ2の極小値が数カ所にわたって存在している可能性が出てきた。
本セミナーでは、主にHot電子に関する先行研究との差分が生じた原因を紹介する。
Jupiter’s inner magnetosphere contains the Io plasma torus with dense sulfur and oxygen ions originating from SO₂ gases emitted through volcanic activities in Io. Understanding the mechanism of mass and energy requires knowledge of plasma parameters around the satellite. However, due to the limitation of past observations, simultaneous analysis of the regions around Io and Europa has not been carried out. This study aims to estimate the electron density, electron temperature, and ion composition from Io’s to Europa’s orbit using Hisaki/EXCEED data.
We have been developing a new plasma diagnosis method, assuming exponential radial distributions of plasma parameters. During the active period between March and May 2015, the estimated electron densities and core temperatures are consistent with Bagenal et al. (2015) and Yoshioka et al. (2018), but the hot electron fraction is approximately 4 times larger in both Io’s and Europa’s orbits. Both the hot electron fraction and the S³⁺ fraction increase with the radial distance, while the S⁺ fraction decreases. This indicates that electron impact ionization of sulfur ions proceeds as the ions are transported outward. Meanwhile, results from the quiet period between February and April 2014 suggest the possibility of multiple local minima for χ2 within the parameter space.
In this seminar, we will primarily introduce the causes of discrepancies with previous studies concerning hot electrons.
