Since the first particle measurements done half a century ago, space plasmas have been extensively investigated by satellite-borne in-situ observations. They have. revealed that charged particles have a broad energy range from <1 eV up to >10 MeV in/around the Earth's magnetosphere; these plasmas form various structures in multiple scales, and often shows dynamic processes over a variety of time scales. The energy of source plasma is rather lower. Thermal energy of ionospheric particles is <1 eV, while solar wind electrons and ions have typical energies of~10 eV. and ~1 keV/nuc, respectively. Therefore, it can be said that the terrestrial. magnetosphere acts as a charged particle accelerator. It can also be applied to other Magnetospheres, such as Jovian and Kronian ones. However, details of particle acceleration processes and large-scale transport of mass and energy are not fully understood. Studying energetic particles in/around magnetospheres is important in order to clarify the mechanisms of such huge particle acceleration systems in space. The main objectives of this study are to develop innovative energetic plasma instruments (5-200 keV/q) for space exploration missions of a new era, as well as analysing energetic particle data obtained by currently-operating satellites. During a storm recovery phase on 15 May 2005, the Geotail spacecraft repeatedly observed high-energy (>180 keV) oxygen ions in the dayside magnetosheath near the equatorial plane. The focused time period is when Geotail observed the oxygen ions and the interplanetary magnetic field (IMF) was constantly northward. The magnetic reconnection occurrence northward and duskward of Geotail is indicated by the Walen analysis and convective flows in the magnetopause boundary layer. Anisotropic pitch angle distributions of ions suggest that high-energy oxygen ions escaped from the northward of Geotail along the reconnected magnetic field lines. From the low-energy particle precipitation in the polar cap observed by DMSP that is consistent with magnetic reconnection occurring between the magnetosheath field lines and the magnetospheric closed field lines, it is concluded that these oxygen ions are of ring current origin. The results thus suggest a new escape route of oxygen ions during northward IMF. In the present event, this escape mechanism is more dominant than the leakage via the finite Larmor radius effect across the dayside equatorial magnetopause. Lessons from the past/present observations and future perspectives are discussed in the middle of this thesis. It is intensively argued that the energy coverage in plasma measurements, including medium-energy range (5-200 keV/q), is significant for future explorations of Earth and Planetary magnetospheres. In fact, detailed discussion on number/energy fluxes in the above event study on the oxygen ion escape was difficult due to the poor energy coverage (and no mass discrimination in the low-energy range). The instruments should cover full solid angle with higher and more reliable sensitivity than that of previous ones. It is also pointed out that noise attenuation is indispensable for inner magnetospheric explorations and observations during solar energetic particle events. The importance of the medium-energy particle measurements in the planetary magnetospheres is also discussed. In order to achieve the full energy coverage in future observations, a new electrostatic analyser (ESA) that enables medium-energy particle measurements is proposed. The design of a test model realises the uppermost measurement energy of~200 keV/q with applied high voltage of±5 kV (the maximum electric field in the curved plates of 2 kV/mm avoids the risk of the discharge). Full solid-angle coverage is achieved by its novel structure. Laboratory experiments with the test model analyser show that its performance agrees with numerical simulations. The design is well suited for combination with a mass analysis unit, while the novel design can also be applied to medium-energy electron measurements. Along with energy analysis, the mass discrimination and the charge state identification are important for ion measurements. Therefore, a combination of ESA, time-of-flight (TOF) mass analysis unit, and the solid-state detectors are utilised. ESA determines energy-per-charge (E/q) of each incoming ion, TOF provides the ion velocity (v), and the solid-state detector measure total energy of each ion (E); those independent determinations unambiguously give ion energy (E), mass (m), and charge state (q). A sophisticated ion mass spectrometer for medium-energy range (~10-200 keV/q) is also developed. The wide field-of-view (~360°fan) enables acquisition of 3-D distribution functions for all the major ions, by utilising spacecraft spin motions. The mass analysis unit that measures ion TOF is designed to detect secondary electrons by a single microchannel plate, which realises a lightweight and a simple structure. Laboratory, experiments with a test model show that the performance of mass spectroscopy agrees with numerical simulations. For charge state measurements of medium-energy ions, a single-sided silicon strip detector is newly applied. Based on laboratory experiments, it is shown that the detector has low noise levels which provide sufficient energy resolutions for charge state measurements of mediumenergy ions. It is also demonstrated that energy loss at a dead-layer is a critical factor for energy resolution and measurement energy threshold. It is found that a thickness of the dead-layer is ~370 nm, and concluded that the thinner dead-layer would provide the better performance for the heavy ion detection. Single-sided silicon strip detector with a thin dead-layer is a crucial technique for medium-energy ion measurements in the next generation satellite-borne missions. Design of a medium-energy electron instrument (~5-80 keV) is also presented. Serious problem in electron measurements in the radiation belt is the background noise. High-energy particles penetrating the sensor shielding generate spurious signals, and their count rate can be comparable to that of the true signals. Ion instruments with mass analysis units can reject the spurious signals by taking double coincidence of the secondary electron signals, while electron instruments cannot. In order to attenuate such background noise during medium-energy electron measurements, the double, energy analyses (DEA) method is proposed. DEA is conducted by a combination of an electrostatic analyser and avalanche photo-diodes (APDs); ESA and APD independently determine the energy of each incoming particle. By using the DEA method, therefore, the penetrating particles can be rejected when the two energy determinations are inconsistent; spurious noise are caused only when the deposited energy at an APD is by chance consistent with the measured energy by ESA. The noise count rate is formulated and the advantage of DEA method is shown quantitatively. Medium-energy ion/electron instruments presented in this thesis will surely be appreciated for upcoming space missions that will observe energetic plasma structures/phenomena of Earth and other planetary magnetospheres.

本論文は8章からなる。第1章はイントロダクションであり、第2章では論文提出者が行った磁気圏観測について述べられている。第3章は過去の中間エネルギー粒子観測装置のレビューである。第4章では論文提出者が開発したカスプ型静電分析装置について述べられており、第5章では論文提出者が開発したイオン質量分析器について、第6章ではシリコン検出器を用いた中間エネルギーイオンのエネルギーおよび電荷(荷電状態)の同定について、第7章では中間エネルギー電子の測定に於けるノイズを除去する方法について述べられている。第8章は結論である。

本論文では、衛星データを用いた高エネルギー粒子ダイナミクスの解析を行うとともに、論文提出者は新時代の磁気圏探査に向け、5-200keV/qという中間エネルギー帯を対象とする新たなアイデアに基づく観測器を開発した。

2005年5月15日に発生した巨大磁気嵐の回復相において、ジオテイル衛星は昼側の低緯度マグネトシースを航行しており、高エネルギーの酸素イオンを繰り返し検出した。惑星間空間磁場が安定して北向きであった時間帯を詳細に解析したところ、ジオテイル衛星の北側かつ夕側で磁気リコネクションが起こっていたことが示された。観測された酸素イオンのピッチ角分布は、それらが磁気リコネクション領域から磁力線に沿って流出してきた事を示唆していた。また、DMSP衛星による低高度降込み粒子の観測の傍証を得てジオテイル衛星の観測した酸素イオンは、磁気圏内部のリングカレントが起源であると結論づけられ、北向きIMFの際の新たな酸素イオン流出経路が提唱された。

次に本論文では中間エネルギープラズマ粒子計測を可能にする新たな静電分析器(カスプ型静電分析器)が提案された。テストモデルの設計では、±5kVの高電位印加で約200keV/qの上限エネルギーを実現する。数値計算に基づいて製作したテストモデルを実験室において試験し、設計通りの性能が確認された。

続いて本論文では静電分析器、飛行時間(TOF)測定型質量分析器、および半導体検出器の組合せを利用して質量分析および電荷(荷電状態)の同定する手法の開発について述べた。中間エネルギーイオン(10-200keV/q)のための質量分析器は、数値計算を通じて設計され、テストモデルが実際に製作・試験された。ほぼ360度を網羅する視野は、3次元速度分布関数の取得を可能にする。丁OF計測に用いる2次電子を単体のマイクロチャンネルプレート(MCP)で検出する設計によって、構造の単純化・計量化が達成される。実験室におけるテストモデルの試験結果は、数値計算の結果とよく一致した。

さらに中間エネルギーイオンの荷電状態測定において低電気雑音を実現するため,片面シリコンストリップ検出器を新たに応用した。室内の中間エネルギーイオン計測実験において、十分な低雑音を確認すると同時に,静電分析器,質量分析器のテストモデルと組み合わせて荷電状態を同定する事にも成功した。

最後に本論文では、静電分析器とアバランシヱフォトダイオード(APD)により独立にエネルギー計測を行い、それらが一致する場合のみ信号と認めることにより、大幅に雑音を低減することができる2重エネルギー分析という新たな手法を提案し、このアイデアに基づき信号と雑音の計数率を定量的に議論した。

本論文で提示された中間エネルギーイオン/電子分析器は,ERG(Energization and Radiation in Geospace)やSCOPE(cross Scale-COupling in the Plasma universE)といった地球磁気圏の将来探査で磁気圏プラズマダイナミクスの詳細及び全体像を把握するのに必須であり、また他の惑星探査への応用も期待されるものである。新時代の太陽系プラズマ観測に於ける必須の新手法を論文提出者は網羅的に開発しており、この功績は審査員一同、極めて高く評価する。

なお、本論文第2章は長谷川洋、桂華邦裕、宮下幸長、西野真木、Thomas Soirelis、斎藤義文、向井利典との共同研究、本論文第4章は浅村和史、斎藤義文、高島健、平原聖文、向井利典との共同研究、本論文第5章は浅村和史、小笠原桂一、三谷烈史、斎藤義文、高島健、平原聖文、向井利典との共同研究、本論文第6章は三谷烈史、小笠原桂一、高島健、平原聖文、浅村和史との共同研究、本論文第7章は浅村和史、小笠原桂一、風間洋一、斎藤義文、高島健、平原聖文との共同研究であるが、論文提出者が主体となってデータの分析、機器の設計、試験、これらの検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。