The dissipation processes at collision-less plasma shocks are one of the most interesting issues in space plasma physics. Due to the collision-less nature of the plasmas, in the course of the dissipation processes non-thermal particle are generated in association with the bulk heating of the main body populations. Previous observations reveal that these supra-thermal electrons are observed to be accelerated up to 20 keV at the quasi-perpendicular region of the Earth's bow shock. The electron acceleration process itself, however, still remains unclear. Recent numerical simulation results imply that non-stationary behavior of a quasi-perpendicular shock front has a strong impact on the dissipative processes and thus on the electron acceleration mechanism. Observational support for this issue, however, has been rather scarce. One of the obvious reasons is that it is difficult for single spacecraft observations, which is notorious for its inability to distinguish a spatial variation from a temporal one, to unambiguously identify the non-stationarity of a shock front.

The cyclic variation of the shock front structure seen in numerous simulations of quasi-perpendicular shocks is widely known as the shock reformation process. The interaction between the reflected ions and the incoming ions/electrons excites micro-instabilities in the shock transition region, which make contribution to the dissipation required to attain the transition from the upstream to the downstream state, and possibly to the electron acceleration processes. The multi point measurements by Cluster-II enable us to investigate the non-stationary behavior with temporal and spatial variations being discriminated. In this thesis, we show robust evidence of the reformation process obtained from formation flying observations by Cluster and discuss the electron dynamics under the reformation process.

Turning our eyes to ions, it is well known that ions are also accelerated at the Earth's bow shock and energetic ions are at times observed in its upstream region. While the energy of the solar wind ions is at most a few keV, the energy of the backstreaming ions ranges from several 10 s of keV to several MeV. When the shock angle (ΘBn, the angle between the upstream magnetic field and the shock normal) is in the oblique range (~50°<ΘBn<~75°) the upstream ions put on the form of field aligned beam (FAB) whose energies are 10-18 keV, and its generation mechanism is nicely explained by an adiabatic theory for a stationary shock structure. Production mechanism of more energetic upstream ions, however, is an open question. Here we show discovery of a new member of the energetic upstream ions, generated only in the quasi-perpendicular regime, possibly under the influence of the reformation process.

In the parallel regime (ΘBn<45°), the upstream FAB is well-known to excite ULF waves. The waves are convected back to the shock front and this wave-shock front interaction leads to another type of shock front non-stationarity. While this concept is well known and has been studied extensively for the parallel regime, its application to the oblique regime (45°<ΘBn<60°) has not been performed. The third topic dealt with in this thesis is the formation flying observations of such a case.

The main body of this thesis is organized as follows.

We start from the discovery of a new member of the upstream energetic ions. In Chapter 3, we show that energetic ions are at times observed in the upstream of the Earth's bow shock and their origin is considered to be in the interaction with the shock front. While the energy of the solar wind ions is a few keV at most, the energy of the backstreaming ions ranges from ~5 keV to several MeV. In the present study we investigate backstreaming energetic ions in the upstream of the Earth's bow shock observed by Geotail during two coronal mass ejection (CME) events. The observed local magnetic field rotated significantly during the events. Using the bow shock model and the observed magnetic field data, we found that the energetic ions appeared only when the upstream magnetic field was connected to the bow shock. The energetic ions showed two distinct distribution function characteristics, namely, the field-aligned beam (FAB) and the loss-cone distribution, respectively. While the former is occasionally detected, the latter having higher energies (30 keV-several hundred keV, compared to <18 keV for FAB) has not been reported before. Using the bow shock model we can also estimate the shock angle at the point on the shock surface that the upstream field line is connected to, and find that the distribution function shape transits from FAB to the loss-cone distribution as the shock angle becomes larger (transition at ΘBn=70-80°). We discuss the possible mechanisms responsible for the production of the newly found member of the energetic upstream ion family. Many simulation studies show that the nonstationary structures (reformation and ripples) appear at high Mach number quasi-perpendicular shock regime. It is a possible that the energetic ions with energy up to 800 keV are accelerated at the quasi-perpendicular shock region under the non-stationary structure. These non-stationary structures may be important in understanding the particle acceleration mechanisms at the bow shock. These nonstationary structures have not been clearly identified by the experimental data.

The discovery described in Chapter 3 motivated us even more to understand observationally the non-stationarity of shocks. In Chapter 4, we search for nonstationary shock structures at the quasi-perpendicular bow shock by using the Cluster data. In order to search for the non-stationary shock structures, we performed event survey of critical shock events observed at quasi-perpendicular bow shock with parameters Alfven Mach number (Ma) >3 and ΘBn>60° in 2002. The number of selected events is 41. Correlation analysis between spacecraft pairs is used to examine shock structure. The shock structures observed by each spacecraft pair can be classified into 3 types of shock structures based on the observational time delay and the cross-correlation coefficient; (A) Stationary event and ripples, (B) Reformation event, and (C) Uncertain event. Examples are shown of a number of bow shock phenomena observed by Cluster, including stationary shock, periodic structure, reformation event, and candidate events for possibly new phenomena at quasi-perpendicular shocks that have not been discussed to date in the theoretical framework.

The survey through the dataset made us detect one event that seems to be a robust evidence of the reformation process of a quasi-perpendicular shock. In Chapter 5, this event is reported in detail. Previous computer simulation studies have suggested the importance of the non-stationary behavior of the collisionless shock structure in the context of energy dissipation. This issue, however, has been difficult to deal with by single spacecraft observations. A cyclic temporal variation of a shock front is widely known as the self-reformation process. On April 20, 2002, Cluster four probes, with the maximum inter-spacecraft distance of ~150 km, crossed a quasi-perpendicular shock front within ~1.3 s. The magnetic and electric field data taken by the four spacecraft with the time differences showed different characteristics, which we attribute to the shock front non-stationarity of gyro frequency time scale, namely the shock reformation. The two types of profiles are seen by the four spacecraft: (1) A steepened magnetic field profile and a large spiky electric field at the shock ramp, which we consider to be taken during the "steepened" phase of reformation. (2) A profile of magnetic field broadened toward shock upstream and fluctuating electric field in the foot region, which we consider to be taken during the "broadened" phase. In order to better interpret the data, we carried out simulations dedicated to comparison with the observations. The comparison of electromagnetic field structure is found to support the above interpretation. Moreover, a prediction from the simulations that a class of electrostatic wave mode would be observed at the shock front in the "broadened" phase is tested positively in the data.

Many simulation studies have indicated that the shock reformation appears at high-Mach number and quasi-perpendicular shocks, while the shock structure at less oblique shock has not been discussed so much using experimental and simulation data. In Chapter 6, to take into account the effect of shock angle on the shock structure, the Cluster data with separation distance ~1300 km in 2005 are used to study the statistics of type of the quasi-perpendicular shocks. The number of selected high- Mach number and quasi-perpendicular shock events (Ma>3 and ΘBn>45°) is 76 in 2005. The experimental data show that the oblique shock fronts put on nonstationary behaviors due to interaction with the upstream waves that are generated by the backstreaming ions in the upstream and are convected toward the shock front. While the process of the non-stationary structure shares many characteristics with previous self-reforming quasi-parallel shock simulations, there are differences, which are the target of the study described in Chapter 7.

In Chapter 7, we found a non-stationary and reforming shock structure at the oblique shock. The oblique shock, observed by Cluster at a separation distance of ~1300 km, had Alfven Mach number (Ma) of ~6.4 and the shock angle of ~55°. All the four Cluster spacecraft saw essentially the main shock ramp structure and ultra-low frequency (ULF) waves (0.03 Hz in the spacecraft frame) in the upstream of the shock front presumably excited by field-aligned beam (FAB) ions. The non-stationarity behavior was captured as a precursor magnetic pulsation, detached by ~450 km from the main shock ramp, was seen to grow in time (over the 2 minutes interval during the observations by the Cluster formation). The amplitude of the largest pulsation was so high that it was starting to play the role of the main shock ramp. Furthermore, the local magnetic field was so modulated by upstream ULF waves that the new shock front behaved as a perpendicular shock. Indeed flat-topped electrons and spiky electric field were seen at the new shock front, which is consistent with the properties of the quasi-perpendicular shock transition. In addition to these electron dynamics, slowing-down and moderate heating of the solar wind ions were also seen. The observations suggest that a cyclic shock front reformation is induced in a class of oblique shocks by the upstream ULF wave convected downstream-wise by the solar wind flow. In striking contrast to the well-known quasi-parallel shock cases, which are also upstream wave driven, the reformation at the oblique shock is found to occur in a coherent manner, with the shock front itself having quasi-perpendicular properties. Two more oblique shock events (1st: Ma=6.1, βi=1.2, and ΘBn=61°; 3rd: Ma=6.2, βi=0.26, and ΘBn=49°), that were obtained within 1.5 hour from the one described above and had similar shock parameters but different temporal behavior are also described in detail to indicate the sensitivity of the oblique shock behavior to the parameters such as the shock angle and the upstream beta.

Chapter 8 summarizes the novel points of these issues and the suggestions for the future works.

本論文は、八章からなる。

第一章は、本論文が主眼とする、衝撃波面の非定常構造に関するこれまでの研究を紹介している。非定常衝撃波とは、衝撃波面が時間的・空間的に変動している構造をさす。これまで、理論・数値シミュレーションを中心に、衝撃波面が時間的・空間的に非定常な構造(衝撃波再形成やリップル)をしていることが示唆されてきた。非定常構造は、衝撃波遷移層で起こる粒子加速機構に大きく影響するため、磁気圏物理だけでなく、天文分野においても非常に重要な問題となっている。しかし、これまでの単一衛星観測では、原理的に時空間構造を分離することが難しいという問題があり、非定常構造が観測的に示されたことはない。しかしながら、編隊飛行衛星を用いることにより、衝撃波面の時間・空間変動を分離することができる。編隊飛行衛星を使用し、特に衝撃波再形成を同定し、無衝突衝撃波でおこる粒子加速機構を理解する、というのが本論文の独特の着眼点である。

第二章は、本研究で使用した磁気圏探査機(Cluster、Geotail)及び観測機器に関しての説明である。1994年に日米共同で打ち上げられたGeotail衛星に関しての説明と、2000年に欧州宇宙機関によって打ち上げられた、初の編隊観測衛星であるClusterに関しての説明をしている。

第三章は、衝撃波上流で観測された高エネルギー粒子の加速機構に関して議論している。コロナ質量噴出イベント中に~20keVまで加速された沿磁力線ビーム(FABs)と~800keVまで加速されたロスコーン型分布の二つの高エネルギー粒子分布が観測された。後者は新発見である。二つの分布は、衝撃波角に依存して変化しており、FABsが断熱加速理論から予想されるエネルギーとよい一致をしていたのに対してロスコーン型分布を生成することは難しく、衝撃波角により加速機構が異なること、かつ、ロスコーン型生成には衝撃波遷移層の非定常性が効いている可能性という新しい知見をもたらした。

第四章は、Clusterによる観測データを用いて、準垂直衝撃波における非定常構造の特徴を示している。結果から、これまでの数値シミュレーションから示唆されていた衝撃波再形成やリップルとは異なる非定常構造も引き起こされている、という新しい知見をもたらした。

第五章は、四章で発見された衝撃波再形成イベントに関して、さらに詳細に調べている。本研究では、観測をもとにした数値シミュレーション結果と衛星観測データを詳細に比較することにより、衝撃波再形成が実際に起こっていることを観測的に証明した。さらに、衝撃波再形成に伴い励起されていた波動を解析した結果、理論・シミュレーション研究から報告されていた変形二流体不安定性の特徴と矛盾しない、などの新しい知見をもたらしている。

第六章は、衝撃波角50°付近での斜め衝撃波面の構造に関して統計的に調べた。これまでの研究では、衝撃波角50°付近の構造に関しては、理論的にも観測的にもあまり調べられてこなかった。そのため、衝撃波角50°付近における衝撃波構造がどのような特徴を持っているかは、よくわかっていない。本研究では、編隊飛行衛星を用いることにより、衝撃波角50°付近の斜め衝撃波は、準垂直衝撃波とは異なるメカニズムによって衝撃波面が非定常構造になっている、などの新しい知見をもたらしている。

第七章は、衝撃波角50°付近の3イベントに注目し、衝撃波上流のパラメータの微妙な違いが、衝撃波面の構造を大きく変えることを議論し示した。さらに、衝撃波上流で励起された波の急峻化によって新しい衝撃波面が形成され、そして衝撃波面の再形成が起こる、という新しい知見をもたらした。このメカニズムは、平行衝撃波での再形成メカニズムと非常によく似ており、衝撃波角50°では初めて発見された。

第八章は、結論である。本論文は、編隊観測衛星を使用して衝撃波面の非定常構造を同定することにより、これまでの単一衛星観測では成し得なかった衝撃波面の時空間変動を観測的に調べることができることを示した。そして、衝撃波角の違いにより非定常構造を引き起こす原因が異なるという、新しい知見をもたらした。また、非定常衝撃波構造とそこで起こっている粒子加速機構に関しても新しい知見をもたらした。本論文の主要な成果は、これまで大規模数値実験によって示唆されていた垂直衝撃波における衝撃波再形成が実際に起こっているという証拠を観測的に示し、さらには、衝撃波角50°においても、衝撃波面が再形成していることを観測的に発見したことである。

なお、第3章、第5章、第7章は、藤本・篠原・長谷川(ISAS/JAXA)、S.J.Schwartz(Imperial College)、松清(九州大)らとの共同研究であるが、論文提出者が主体となって解析を行い、その寄与は十分である。

したがって、博士(理学)の学位を授与できると認める。