Unveiling the true plasma behaviour around comets by spacecraft charging simulations

Research Project

Project/Area Number	22KF0123
Project/Area Number (Other)	22F22723 (2022)
Research Category	Grant-in-Aid for JSPS Fellows
Allocation Type	Multi-year Fund (2023) Single-year Grants (2022)
Section	外国
Review Section	Basic Section 17010:Space and planetary sciences-related
Research Institution	The University of Tokyo
Principal Investigator	笠原慧東京大学, 大学院理学系研究科(理学部), 准教授 (00550500)
Co-Investigator(Kenkyū-buntansha)	BERGMAN SOFIA 東京大学, 大学院理学系研究科(理学部), 外国人特別研究員
Project Period (FY)	2023-03-08 – 2024-03-31
Project Status	Granted (Fiscal Year 2023)
Budget Amount *help	¥1,200,000 (Direct Cost: ¥1,200,000) Fiscal Year 2023: ¥400,000 (Direct Cost: ¥400,000) Fiscal Year 2022: ¥800,000 (Direct Cost: ¥800,000)
Keywords	Cometary plasma / spacecraft charging
Outline of Research at the Start	欧日共同の彗星探査ミッションComet Interceptor (2029年頃打ち上げ予定)では，彗星プラズマと太陽風の相互作用を観測する．この際，探査機の帯電状態によっては，計測されるプラズマの軌道が乱され，観測の妨げとなる可能性がある．本研究では，彗星環境における探査機の帯電状態を事前にシミュレートすることにより，探査機の設計及び打ち上げ後のデータ解析の指針となる情報を提供する．
Outline of Annual Research Achievements	In 2019, Comet Interceptor was selected by ESA as the first F-class mission. This mission will make a flyby of a long-period comet and for the first time make multi point measurements in the cometary environment using three spacecraft: Spacecraft A (ESA), Probe B1 (JAXA) and Probe B2 (ESA). All spacecraft will carry plasma instruments, allowing, for the first time, a threedimensional study of the cometary plasma environment. The plasma measurements are, however, expected to be affected by the spacecraft potential, as already observed by other cometary missions (e.g. the Rosetta mission, Bergman et al., 2020). Throughout the flyby, the spacecraft will pass by several plasma regions with different characteristics, and hence the spacecraft potential is expected to vary. In this study, we estimated the spacecraft potential of Probe B1 of Comet Interceptor throughout the cometary flyby using Particle-In-Cell (PIC) simulations and study the influence on the plasma measurements. The main results are as follows: (1) At a low relative flyby velocity, a more effective cooling of the electrons in the inner coma results in less negative spacecraft potentials for a Halley-type comet. (2) Secondary electron emissions from neutral impacts charge the spacecraft positive in most environments at a high relative flyby velocity.
Current Status of Research Progress	Current Status of Research Progress 2: Research has progressed on the whole more than it was originally planned. Reason The original plan is as follows: November-December: Development of a spacecraft model to be used in the spacecraft Plasma Interaction Software (SPIS). A model of the ion sensor will be implemented in the model which will subsequently be used to trace low-energy ions in the simulations. The information about spacecraft B1 and the sensor needed to develop the model is available at the host institute in Japan, and an extended stay is hence necessary to carry out the proposed research. January-March: Simulation running. The spacecraft model will be implemented in SPIS. A suitable model for the plasma environment around the spacecraft will be chosen based on available data and publications from previous comet missions (e.g., the Rosetta mission, the Giotto mission, and the Japanese Suisei/Sakigake missions). We have successfully completed these items and have already got preliminary results.
Strategy for Future Research Activity	2022年度までに，複数の彗星環境中で探査機の帯電シミュレーションを実施し，結果を議論している．2023年度は，以下のような研究を実施予定である．・これまで太陽電池パドルの表面を導体としていたが，探査機の設計進捗に伴い，コスト・スケジュールの制約から，表面に高抵抗部材・絶縁物が残る可能性が出てきた．このため，パドル表面を絶縁物とした場合のシミュレーションを実施し，帯電量を評価する．・得られた電位分布に対して観測対象であるイオンの粒子軌道トレースを実施し，観測への影響を評価する．