論文内容の要旨

論文題目 Observational Study of Non-stationary Shock Structure(非定常衝撃波に関する観測的研究)
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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 thecourse of the dissipation processes non-thermal particle are generated in association withthe bulk heating of the main body populations. Previous observations reveal that thesesupra-thermal electrons are observed to be accelerated up to 20 keV at thequasi-perpendicular region of the Earth’s bow shock. The electron acceleration processitself, however, still remains unclear. Recent numerical simulation results imply that
non-stationary behavior of a quasi-perpendicular shock front has a strong impact on thedissipative 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 itis difficult for single spacecraft observations, which is notorious for its inability to distinguisha spatial variation from a temporal one, to unambiguously identify the non-stationarity of ashock front.
The cyclic variation of the shock front structure seen in numerous simulations ofquasi-perpendicular shocks is widely known as the shock reformation process. Theinteraction 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 tothe 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 thereformation 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 fromseveral 10 s of keV to several MeV. When the shock angle (ΘBn, the angle between theupstream 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 astationary 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 theinfluence 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 andhas 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 theformation 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 theenergy of the solar wind ions is a few keV at most, the energy of the backstreaming ionsranges 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, wefound that the energetic ions appeared only when the upstream magnetic field wasconnected 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 (30keV-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 theproduction 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 inunderstanding 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 understandobservationally 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 tosearch 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 andripples, (B) Reformation event, and (C) Uncertain event. Examples are shown of a numberof bow shock phenomena observed by Cluster, including stationary shock, periodicstructure, reformation event, and candidate events for possibly new phenomena atquasi-perpendicular shocks that have not been discussed to date in the theoreticalframework.
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 eventis reported in detail. Previous computer simulation studies have suggested the importanceof 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 theself-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 ofgyro frequency time scale, namely the shock reformation. The two types of profiles are seenby the four spacecraft: (1) A steepened magnetic field profile and a large spiky electric fieldat the shock ramp, which we consider to be taken during the ”steepened” phase ofreformation. (2) A profile of magnetic field broadened toward shock upstream andfluctuating electric field in the foot region, which we consider to be taken duringthe ”broadened” phase. In order to better interpret the data, we carried out simulationsdedicated to comparison with the observations. The comparison of electromagnetic fieldstructure 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 inthe ”broadened” phase is tested positively in the data.
Many simulation studies have indicated that the shock reformation appears at high-Machnumber 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, totake into account the effect of shock angle on the shock structure, the Cluster data withseparation distance ~1300 km in 2005 are used to study the statistics of type of thequasi-perpendicular shocks. The number of selected high- Mach number andquasi-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 andare 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 obliqueshock. 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 aprecursor magnetic pulsation, detached by ~450 km from the main shock ramp, was seen togrow 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 themain 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 withthe properties of the quasi-perpendicular shock transition. In addition to these electrondynamics, 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 upstreamwave 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°), thatwere 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 theupstream beta.
Chapter 8 summarizes the novel points of these issues and the suggestions for the future
works.