PPARCセミナー (2025/07/07)

PPARCセミナー (2025/07/07)

(1)

[Name]

Haruki Okuda

[Title]

On the Factors Controlling the Relationship Between Type of Pulsating Aurora and Energy of Pulsating Auroral Electrons: Simultaneous Observations by Arase Satellite, Ground‐Based All‐Sky Imagers and EISCAT Radar

[Abstract]

Pulsating aurora (PsA) is a key form of diffuse aurora associated with the precipitation of electrons from the magnetosphere. Previous studies have suggested that PsA electrons may become more energetic—sometimes reaching sub-relativistic or relativistic energies—when chorus waves propagate to higher latitudes. However, direct evidence connecting PsA morphology, electron energy, and high-latitude chorus wave propagation has been limited. To address this, the authors analyzed an extended PsA event observed on 12 January 2021 using the Arase satellite, ground-based all-sky imagers, and EISCAT radar. They found that when PsA appeared more patchy, the electron energy increased and chorus waves were observed at magnetic latitudes above 20°, suggesting ducted propagation. Electron density variations observed by Arase indicate the presence of magnetospheric density ducts. These ducts likely allowed chorus waves to reach higher latitudes, resulting in enhanced resonant scattering and the precipitation of more energetic electrons. These results imply that both PsA morphology and electron energy are controlled by the presence or absence of such density structures in the magnetosphere.

(2)

[Name]

Takeru Kato

[Title]

On the ionospheric structure of Titan（review)

[Abstract]

本論文の発表以前、Titanの電離圏に関する直接的な知見は、主に1980年のvoyger1号による一度のflybyに限られていた。この探査により、Titanには大気由来の誘導磁気圏が存在すること、そして電離層が存在することは示唆されていたが、その詳細な構造や生成メカニズムは未解明な点が多かった。本研究では、2004年より土星系で観測を開始したcassiniによる17回のTitan flybyで得られたデータを解析し、電離層の構造を太陽天頂角（SZA）の観点から包括的に明らかにすることを目的とした。観測には主にLangmuir Probe（LP）が用いられ、合計34個の鉛直電子密度プロファイルが取得された。各プロファイルから密度ピーク、ピーク高度、電子温度を導出し、SZAに対する依存性を詳細に解析した。その結果、まず昼側の電離層を生成する主要なエネルギー源は太陽光子であることが明確に示された。これを裏付けるように、電子密度は昼側（約2500～3500 cm⁻³）が夜側（約400～1000 cm⁻³）に比べて平均で4倍高く、大きな違いが見られた。そして、昼側と夜側の間にはSZA 50°から100°にかけて広範な「遷移領域」が存在し、Titanの広がった大気の影響によって太陽光による電離作用がSZA 90°を超えても続いていることが特定された。電離層の構造もSZAに依存し、ピーク高度はSZA 50°未満の昼側ではSZAの増加と共に上昇する明確な傾向を示したが、SZA 50°以上の遷移領域と夜側では、その高度は大きくばらついていた。一方で、電子温度は昼夜を問わず約350～700 K（0.03～0.06 eV）の範囲で比較的安定しており、SZAへの強い依存は見られないという異なる傾向が明らかになった。Titanの昼側の電離層は主に太陽光によって駆動されていることが明確になった一方で、特に夜側ではその構造が大きく変動しうることが示された。この変動を完全に理解するためには、各flybyにおける磁気圏の状態やプラズマの流入方向を詳細に研究することが今後の課題である。

Prior to the publication of this paper, direct knowledge of Titan’s ionosphere was mainly limited to a single flyby by the voyager 1 in 1980. Although this exploration suggested the existence of an atmosphere-derived induced magnetosphere and an ionosphere on Titan, its detailed structure and formation mechanism remained largely unresolved. The purpose of this study is to comprehensively clarify the structure of the ionosphere in terms of the solar zenith angle (SZA) by analyzing data obtained during 17 Titan flybys by cassini, which started observations in the Saturn system in 2004. The Langmuir Probe (LP) was mainly used for the observations, and a total of 34 vertical electron density profiles were obtained. From each profile, the density peak, peak height, and electron temperature were derived, and their dependence on SZA was analyzed in detail. The results clearly indicate that the primary energy source producing the daytime ionosphere is solar photons. In support of this, the electron density on the day side (about 2500-3500 cm-³) was on average four times higher than that on the night side (about 400-1000 cm-³), a significant difference. A broad “transition zone” between the day and night sides was identified between SZA 50° and 100°, where the ionizing effects of sunlight continue beyond SZA 90° due to Titan’s extended atmosphere. The structure of the ionosphere also depended on SZA, and peak heights showed a clear trend of increasing with increasing SZA on the day side below 50° SZA, while the heights were highly scattered in the transition region above 50° SZA and on the night side. On the other hand, the electron temperature is relatively stable in the range of about 350-700 K (0.03-0.06 eV) both day and night, revealing a different trend, with no strong dependence on SZA. the structure can be highly variable, especially on the night side. In order to fully understand this variability, detailed studies of the magnetospheric conditions and plasma inflow directions at each flyby will be the subject of future work.