2025-07-11

PPARCセミナー (2025/07/14)

(1)
[Name]
Ayuto Kawakami

[Title]
(Review) Feasibility of Passive Sounding of Uranian Moons Using Uranian Kilometric Radiation -A. Romero-Wolf et al.(2024)

[Abstract]
氷衛星の内部構造を特定する手法で、以前から用いられたものとしてMagnetic sounding(=電磁探査)があり、エウロパなどの内部海の特定に使用された。この論文では、天王星の5つの主な氷衛星であるミランダ、アリエル、アンブリエル、ティタニア、オベロンの内部海の有無やその特性を探査するために、新たな手法として、100〜900 kHz帯域のUKR(Uranian Kilometric Radiation)の衛星表面での反射を用いたPassive radar soundingを提案する。Passive radar soundingは、先立って木星におけるJovian Kilometric Radiationを用いたガリレオ衛星に対してなされ、地球における太陽電波放射の反射で実証されている。ここでは、CassiniのRPWSが時間分解能などの面で改善された機器を想定し、UKRに対しても同様の手法が適用可能であるかを考える。そのために、電波源の強度、放射パターンによる影響、周囲の電子密度によって発生するプラズマノイズなどの受信機の特性、氷の組成による電波の減衰といった氷衛星の表面の特徴について評価する。また、これらの影響を統合して感度の見積もりと観測データの見積もり、およびPassive soundingをより精度良く行うためのUKRの掩蔽観測について議論する。その結果、この手法は、電磁探査の補完になりえることが示され、衛星内の冷たく薄い海洋を直接的かつ明確に検出し、温かく厚い海洋または海洋がない場合でも内部構造に強い制約をもたらす可能性があることが導かれる。また、将来の天王星への探査ミッションにおいて、フライバイ高度と接近するタイミングに関する制約が示される。

A method for identifying the internal structure of icy moons, known as Magnetic sounding, has been used previously to identify subsurface oceans on bodies of Europa etc. This paper proposes a new method for exploring the presence and characteristics of subsurface oceans on Uranus’s five major icy moons —Miranda, Ariel, Umbriel, Titania, and Oberon— using passive radar sounding based on reflections of Uranian Kilometric Radiation (UKR) in the 100–900 kHz band from the moon’s surface. Passive radar sounding has been previously applied to Jupiter’s Galilean moons using Jovian Kilometric Radiation and demonstrated with solar radio emissions reflected on Earth. Here, we consider whether a similar approach can be applied to UKR, assuming an instrument analogous to Cassini’s Radio Plasma Wave Science (RPWS) with improved time resolution. We discuss the characteristics of the radio source intensity, radiation pattern effects, plasma noise generated by surrounding electron density, and the attenuation of radio waves due to ice composition. We integrate these factors to estimate sensitivity and observational data for passive sounding and discuss UKR occultation observations for more accurate measurements. The results indicate that this method could complement Magnetic sounding by directly and unambiguously detecting cold, thin oceans within the moons and providing strong constraints on interior structure in cases of warm, thick oceans or no oceans at all. Additionally, it highlights constraints related to flyby altitudes and encounter timing for future Uranus exploration missions.

(2)
[Name]
Hiroshige Yamaguchi

[Title]
Determining the acceleration regions of in situ electrons using remote radio and X-ray observations

[Abstract]
Solar energetic particles (SEP) in the heliosphere are produced by flaring processes on the Sun or by shocks driven by coronal mass ejections. These particles are regularly detected remotely as electromagnetic radiation (X-rays or radio emission) or in situ by spacecraft monitoring the Sun and the heliosphere.
This study aims to figure out the origin and acceleration mechanisms of SEP in the heliosphere by integrating remote observations with in situ electron measurements.

This research focuses on events observed on October 3, 2023, involving a type II radio burst, type III radio bursts, hard X-ray (HXR) emission detected from Earth, and in situ electrons recorded by the Solar Orbiter spacecraft. These observations are combined with a three-dimensional (3D) representation of the electron acceleration locations based on a magnetohydrodynamic (MHD) model of the solar corona in order to investigate the origin and connectivity of electrons observed remotely at the Sun to in situ electrons. They found a strong magnetic link between a Type II radio burst (caused by a shock wave) and electrons detected by Solar Orbiter. While Type III radio bursts and X-rays were also present, the Type II burst’s location was most directly connected to the spacecraft. The electron data suggested shock acceleration.

This study concluded that Solar Orbiter’s electrons likely came from two distinct sources: a minor contribution from a far-side solar flare and a major one from a shock wave near the Sun’s eastern limb. These two sources were physically separated and connected to Solar Orbiter by different magnetic field lines.