Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

談, 沁怡

English

A Study of Clausal Ellipsis in Mandarin Chinese and Two Chinese Dialects

博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

椹木, 幹人

Japanese

日本語を中心とした時空間メタファーの統合的研究 ―認知・機能言語学的アプローチ―

Matsunaga, Lucas Heiki

Doctoral Dissertation Abstract 
The Social and Cognitive Mechanisms for Cooperation in International Crises 
(国際的危機における協力行動の社会的および認知的メカニズム) 
 
 
Tohoku University  
Graduate School of International Cultural Studies 
Department of International Environment and Resources Policies 
Lucas Heiki Matsunaga 
 
 
Supervisor: Masahiro Sato 
Subsupervisor: Tatako Izumi 
 
 
 
 
 
 
 
 
ABSTRACT 
Over the past decade, global crises, such as climate change and the COVID-19 
pandemic, have posed significant challenges to global governance and resilient societies 
(Magnan et al., 2021). Cooperative public behavior is essential for addressing these 
challenges. However, policies that require cooperation might face barriers in fostering 
support from their population, particularly in a conflicted society. Common findings in the 
literature on behavioral economics, behavioral policymaking, and social psychology imply 
that lack of cooperation is causally rooted in people’s beliefs, perceptions, and social 
environment. Zero-sum thinking (i.e., the belief that one party’s gain is another party’s 
loss), egocentric bias (i.e., viewing one’s gains as unrelated to another’s negative 
outcome), social norms (i.e., expectations for certain conducts), risk perception, political 
ideologies, and cultural identities are examples of these causes (Fujii, 2006; Johnson et al., 
2021). Therefore, an in-depth examination of the social and psychological mechanisms that 
encourage cooperative behavior and beliefs in the contemporary world is required. Because 
of that, this dissertation enhances our understanding of these mechanisms through an 
international, multimethod, and interdisciplinary approach. After conducting rigorous 
studies, this dissertation provided insights into the pivotal role of social polarization and 
group conflict in shaping cooperative behavior in an increasingly uncertain global 
landscape. Furthermore, this study investigated the major impact of a disrupted global 
society on people’s willingness to cooperate and its significant contribution to maintaining 
social well-being and safety. Consequently, this dissertation is divided into eight chapters 
that present these impacts and contributions in detail. 
Chapter 1 provides the background that unifies the subsequent studies of this thesis 
and the theoretical approach. Accordingly, the science of behavioral public policy (BPP) is 
presented, and how it is related to the context in which I aim to advance contributions to 
global policymaking in the following chapters especially tracking solutions for the 
interconnected modern global challenges (i.e., climate change, pandemics, conflicts… see 
Figure 1). In addition, this chapter introduces new researchers and stakeholders to the field 
of BPP applied to global governance. Besides being concerned with ethical issues, such as 
freedom, well-being, and justice of our communities in a world surrounded by threatening 
uncertainties, this study provides original evidence-based solutions on how this field can 
improve security and resilience.  
Figure 1  
A Gordian Knot of Modern Global Crises. 
 
Note. The image is a visual representation of the intricate relationships between various 
global challenges. These challenges, including environmental issues (e.g., climate change 
and biodiversity loss), social and economic problems (e.g., discrimination and 
inequalities), and geopolitical problems (e.g., international conflicts), are interconnected 
and mutually influence one another. The diagram emphasizes the need for a holistic 
approach to address these issues, considering their interdependence and the importance of 
addressing the underlying social and psychological mechanisms.  
 
Chapter 2 describes how researchers can utilize structural equation modeling 
(SEM) to build tailored models for different stakeholders, fostering a logical and socially 
grounded understanding of their challenges. An eight-step framework is proposed for 
employing SEM in policy prescription to guide researchers in (1) moving beyond simple 
correlations, (2) complementing piecemeal regression analysis, (3) effectively assessing 
effect sizes and variable presentations, (4) examining how factors are presented 
descriptively along with paths and covariates, (5) addressing the need for multigroup 
analysis and mean differences, (6) selecting appropriate models, (7) exploring unexplained 
variance in SEM to identify avenues for future research, and (8) communicating findings 
with a diverse team. This chapter provides a key foundation for the analysis presented in 
Chapter 3 based on the framework. 
Chapter 3 presents the first empirical investigation conducted in this thesis, 
exploring the determinants of cooperative behavior during the COVID-19 pandemic, with 
a focus on the role of political views in an international sample from Brazil, Japan, Taiwan, 
and the United States. Accordingly, this study investigates why people cooperate with 
international public health regulations and the social, political, and psychological 
mechanisms underpinning them. The behavior of avoiding crowded areas as one of the 
major protective measures requested by authorities during the pandemic to limit the spread 
of clusters of infection and risks to citizens with vulnerable conditions is a key focus for 
understanding. This understanding is crucial in the metropolitan areas studied (i.e., 
Brasilia, New York, Tokyo, and Taipei) because of the crowdedness and difficulty 
avoiding social contact in the social and economic life of the cities. However, as 
highlighted in the literature, the cultural and political norms of countries are essential to 
understanding the reaction of citizens and governments to the COVID-19 pandemic. No 
study has addressed the impact of these factors on the psychological determinants that 
shape people’s motivation and efforts. Based on the research background and using a 
diverse, cross-cultural political sample, this study ascertains that people’s beliefs, 
subjective norms, perception of control of behavior and risk, and feelings of moral 
obligation predict efforts to avoid crowded areas (see Figure 2). These factors predict the 
protective behavior of avoiding crowded places differently according to cultural settings. 
In particular, this study highlights that political identity not only predicts the extent to 
which people avoid crowded areas in polarized countries but also their beliefs, risk 
perceptions, and feelings of moral obligation. Accordingly, policymakers, governmental 
actors, and citizens can improve their strategies to encourage cooperation, maximize social 
cohesion and well-being, and preserve people’s freedom.  
Figure 2 
Results of the structural equation model with regression paths and total variance of 
endogenous variables. 
 
Note. The numbers above the paths indicate standardized regression weights, and the 
numbers above the endogenous factors are the total effects. 
Following our main findings presented in Chapter 3, which state the substantial 
effect of social polarization on cooperation toward global policies, Chapter 4 verifies the 
potential effect of social cohesion on maintaining well-being and mental health during the 
pandemic. This study included data from 896 participants from the United States, Brazil, 
and Japan between December 31, 2020, and March 3, 2021. These data were employed to 
conduct a multiple linear regression using depression symptoms as the dependent variable. 
In contrast, social cohesion and demographic factors, such as age and gender, were the 
independent variables. The results indicated that social cohesion substantially correlated 
with depression across the three cultures, whereas other factors varied according to the 
location (see Figure 3). Hence, policymakers should encourage strong social ties within 
communities and reduce social polarization to manage the high prevalence of depression. 
Figure 3 
The relationship between social cohesion and scores of reported symptoms of depression 
by city. 
 
Note. Lines are predicted lines and circles are mean scores of observed individuals. The 
observed individuals might overlap between themselves when they are present in the same 
coordination. 
After verifying the significance of social division as the primary source of 
noncooperation and lack of well-being in society, Chapter 5 explores a vital belief 
influencing noncooperative behavior, especially within social and group conflicts. The 
rising academic and social interest in zero-sum thinking (i.e., the perception that one’s gain 
is another’s loss) was addressed as an essential mechanism for mediating conflict and 
consensus-building. Therefore, a scoping review of 67 empirical papers containing 192 
global studies published in peer-reviewed journals on zero-sum thinking in the past decade 
was conducted. This review provided state-of-the-art research on the psychology of zero-
sum thinking and its relevance for addressing contemporary social problems of intergroup 
conflict, such as wars, climate change mitigation, and prejudice. Furthermore, a 
comprehensive view of how it is measured, conceptualized, and researched was provided, 
leading us to build a new research agenda to address the gaps in the literature. 
Accordingly, our findings support academic advances and policy insights that promote 
consensus-building in our global society. 
Chapter 6 presents a study conducted in Japan and Wales, exploring the enduring 
influence of egocentric bias in negotiation contexts despite fair settings. This 
chapter explores how individuals navigate the interplay between egocentric bias and justice 
mechanisms (i.e., procedural and retributive justice) and their impact on zero-sum 
thinking. These dynamics are explored across four studies involving 947 participants 
through simulated negotiation scenarios such as car trade deals and international 
environmental negotiations. Our findings reveal a robust egocentric bias that dominates 
judgments even when justice mechanisms, including procedural or retributive fairness, are 
introduced. Consequently, our findings indicate that negotiators view personal gains as 
independent of losses incurred by others, which hinders collaborative outcomes. In 
particular, procedural fairness has a moderating effect, fostering perceptions of fairness 
and reducing zero-sum thinking in specific scenarios (car trade). However, its influence 
diminishes in complex international contexts. Retributive justice that aims to discourage 
noncooperative behavior backfires by intensifying punitive sentiments, which may have 
undesirable effects in some situations. By highlighting these dynamics, this study paves the 
way for developing more effective negotiation strategies that promote cooperation and 
foster successful conflict resolution in real-world settings. 
A general discussion is presented in Chapter 7. The interconnectedness of the 
research is highlighted through an overview of the findings from previous chapters, 
emphasizing emerging themes and insights. Subsequently, the academic and social 
contributions of this dissertation are identified by discussing how the research advances the 
understanding of cooperative behavior, social cohesion, zero-sum thinking, and justice 
mechanisms in the context of global crises. In addition, this chapter explores the potential 
applications of these findings in shaping policies, interventions, and strategies to promote 
cooperation, resilience, and well-being in the face of global challenges by different 
approaches to behavioral interventional plans, especially helping motivate cooperative 
action or designing better decision frameworks (see Figure 4). Finally, potential avenues for 
future research are outlined based on the gaps, limitations, and unanswered questions 
identified in the previous chapters by proposing research directions that could explore the 
complex interplay of psychological, social, and political factors influencing cooperation and 
conflict in diverse cultural and international contexts. Chapter 8 presents a general 
conclusion addressing and tying the major results from all studies, acknowledgments, and 
the list of publications derived from this dissertation.  
Figure 4  
Different approaches to behavioral intervention plans. 
 




国際的危機における協力行動の社会的および認知的メカニズム

Akiyama, Keito

ツァリスエントロピーによるニューラルネットワークの最適化問題

Tohoku University博士（理学）要約のみthesi

Koyama, Shungo

  
 
博士論文 
 
Photochemical production of formaldehyde and  
its stable carbon isotope evolution on early Mars 
 
初期火星におけるホルムアルデヒドの光化学生成と安定炭素同位体進化 
 
 
 
 
 
 
小山 俊吾 
 
2024年  

Doctoral Thesis 
 
 
Photochemical production of formaldehyde and  
its stable carbon isotope evolution on early Mars 
 
初期火星におけるホルムアルデヒドの光化学生成と安定炭素同位体進化 
 
 
Shungo Koyama 
(小山 俊吾) 
 
Department of Geophysics 
Graduate School of Science 
Tohoku University 
 
Thesis Committee Members 
 
Professor Naoki Terada (Chair, co-supervisor) 
Visiting Professor Ann Carine Vandaele (co-supervisor) 
Professor Véronique Dehant (UCLouvain, ROB, co-supervisor) 
Associate Professor Yoshihiro Furukawa (Department of Earth Science) 
Professor Yuto Katoh 
Professor Yasumasa Kasaba 
Professor Fuminori Tsuchiya 
Assistant Professor Takeshi Kuroda 
 
 
2024  
 
 
 
  
  
 
Abstract 
The potential habitability of Mars has long been a driving force behind both the previous and 
ongoing Martian explorations. Present-day Mars is extremely cold and dry, but 
geomorphological and geochemical evidence indicates a warm environment compatible with 
surface liquid water on early Mars. In addition to water, the existence of genomic and catalytic 
bio-molecules that support the fundamentals of life is critical for habitability. The Sample 
Analysis at Mars instrument onboard the Curiosity rover discovered organic matter in the early 
Martian sediment. Moreover, a recent analysis discovered highly variable and 13C-depleted 
carbon isotopic values in its organic. However, its origin and the availability of biologically 
important molecules on early Mars remain uncertain. 
Formaldehyde (H2CO), a highly soluble and reactive molecule, can be formed through 
photochemical reactions in planetary atmospheres. Dissolved H2CO in water would 
subsequently form diverse organic matter, including bio-important molecules, such as sugars 
and amino acids, by the formose-type reaction. Moreover, H2CO can be generated from 13C-
depleted CO derived from CO2 photolysis in the atmosphere. The deposition of this 
photochemically produced H2CO may explain the origin of the strong isotopic depletion 
observed in Martian organic matter. Therefore, investigating the atmospheric synthesis of 
H2CO on early Mars is crucial for understanding prebiotic chemistry toward the potential of 
ancient Martian life. The goal of this thesis is to elucidate the prebiotic chemistry involved in 
the formation of organic matter from H2CO, and to advance the understanding of the early 
Martian habitability. This paper first investigated the global average production and deposition 
of H2CO in Chapter 2, followed by its global distribution in Chapter 3, and finally its stable 
carbon isotope evolution in Chapter 4. 
First, we investigated the global average atmospheric production and deposition of H2CO in a 
2-bar CO2-dominated atmosphere with H2 and CO on early Mars at ~4–3 billion years ago. We 
adapted a 1D photochemistry model, PROTEUS (Photochemical and RaiatiOn Transport 
model for Extensive USe), for early Martian conditions. We utilized the temperature and H2O 
vapor density profiles from the global mean results computed by a 3D Paleo-Mars Global 
Climate Model (PMGCM) for a 2-bar CO2-dominated atmosphere with 0, 3, and 6 % H2 and 
an obliquity of 40°. Our results show that a larger amount of H2 leads to a more significant 
production of H2CO owing to the reduction in O2 abundance in the lower atmosphere. Two 
mechanisms cause this O2 reduction: (1) the chemical reaction with H atoms at high altitudes 
above 100 km, resulting in a decrease in O2 downward flux, and (2) the HCO catalytic cycle at 
~ 50–60 km reducing O2 in the lower atmosphere below 60 km. In a warm climate under a 6% 
H2 condition, the number density of H2CO is ~ 5 × 109 cm−3 near the surface, and its deposition 
flux into the ocean is 3 × 109 cm−2 s−1 assuming that early Mars had a hydrological cycle similar 
to Earth’s. The sensitivity analysis of water vapor implies that H2CO production could have 
varied locally in correlation with the abundance of water vapor. Given the previously reported 
conversion rate from H2CO to ribose, the calculated H2CO deposition flux suggests a 
continuous supply of bio-important sugars on Noachian and early Hesperian Mars. 
Second, we calculated the global distribution of H2CO deposition across the globe in a 
PMGCM by creating a lookup table of H2CO production for key atmospheric parameters: 
surface temperature, surface pressure, surface H2O vapor, and UV flux. We assumed a warm 
climate scenario with a 2-bar CO2 atmosphere with 6% H2 and 1% CO. The rainout scheme of 
H2CO is newly implemented in the PMGCM. As a result, the H2CO deposition flux is higher 
in regions with high concentrations of H2O vapor and efficient liquid water precipitation. Due 
to this trend, the deposition of H2CO concentrates on lower latitudes. Considering the carbon 
isotope fractionation induced by CO2 photolysis, organic matter derived from photochemically 
produced H2CO at low latitudes would be depleted in 13C more than that at high latitudes, 
which could be validated by future missions. At Gale crater, the deposition flux was estimated 
to be approximately 108 cm−2 s-1. The amount of organic matter estimated from this H2CO 
deposition is compatible with the value measured by the Curiosity rover, suggesting that 
photochemically produced H2CO could have contributed to the synthesis of the observed 
organic matter. Furthermore, the presence of calcium and boron minerals at Gale crater 
supports the potential synthesis of ribose in this region. 
Third, we focused on the evolution of the carbon isotopic ratio in H2CO. We developed a 
coupled 1D photochemistry-climate evolution model incorporating carbon isotope 
fractionation processes induced by CO2 photolysis, carbon escape to space, and volcanic 
outgassing in an early Martian atmosphere of 0.5–2 bar, composed mainly of CO2, CO, and H2 
to track the evolution of the carbon isotopic composition of C-bearing species. The calculated 
carbon isotopic ratio in H2CO shows a significant depletion in 13C with a minimum ¹³C value 
of approximately −200‰, resulting from the isotopic fractionation induced by CO2 photolysis. 
This ratio varies with changes in the background atmospheric conditions, such as CO/CO2 ratio, 
surface pressure, and H2 outgassing rate. Conversely, CO2 becomes enriched in 13C, consistent 
with an estimate from the carbonates preserved in the Martian meteorite Allan Hills 84001. 
These findings imply that certain amounts of the organic matter containing strongly depleted 
13C in the early Martian sediment measured by the Curiosity rover could have originated from 
the photochemically produced H2CO, undergoing subsequent condensation processes in water, 
such as formose-type reactions, during transient melting events in the late Noachian to 
Hesperian periods. Mixing with other organic sources, such as meteoritic material with higher 
δ13C values, could explain the observed variable δ13C values in Martian organic matter. 
This thesis explored the photochemical processes that produce H2CO and its carbon isotopic 
signatures, as well as the subsequent formation of organic matter on early Mars. Our findings 
suggest that a weakly reducing early Martian atmosphere was suitable for H2CO photochemical 
production. Moreover, we showed that the amount and carbon isotopic ratio of organic matter 
originating from photochemically produced H2CO are consistent with the measurement by the 
Curiosity rover. These results indicate that early Mars not only retained surface liquid water 
but also likely hosted organic compounds such as ribose, a prebiotic precursor originating from 
H2CO. 
Future work will involve estimating the global concentration of H2CO on early Mars using a 
global climate model that includes the transport of H2CO through water flow and dry-wet cycle. 
Additionally, modeling chemical reactions in water is required to estimate the degradation rate 
of H2CO after its deposition. Investigating the production of glycolaldehyde on early Mars is 
also necessary to fully understand the organic synthesis from H2CO by formose-type reaction. 
Expanding the photochemistry-climate evolution model developed in this thesis to track the 
oxygen isotopic ratio in CO2 would be helpful to constrain the early Martian atmospheric 
evolution. Additional isotopic constraints across different Martian epochs, brought by the 
upcoming sample return missions, will enhance our understanding of the origins of organic 
matter and the evolution of Mars’ atmosphere. 


初期火星におけるホルムアルデヒドの光化学生成と安定炭素同位体進化

及川, 一真

 
博士論文 
 
 
 
 
 
Brachiopod Isotope Thermometers: Optical Shell Portions 
and Appropriate Analytical Methods 
（腕足動物殻の地質温度計利用に向けた最適な部位の
特定と解析手法の検討） 
 
 
 
 
 
 
 
Kazuma OIKAWA 
 
 
 
 
令和 6年 
はじめに 
 
顕生代（過去約 5 億 3880 万年間）を通じて保存状態の良い化石が豊富に産出する腕足動物
は，超長期スケールの古海洋環境変動を復元する上で重要な古環境アーカイブである．特に，
その酸素同位体組成は海水温の復元に広く用いられてきたが，必ずしも信頼性の高い古水温復
元には至っておらず，以下のような課題が存在する． 
第一に，特定の種や殻の部位では生物学的同位体効果の影響を受け，初生的に海水と同位体
平衡で形成されていない例が報告されている．したがって，生物学的同位体効果の影響を受け
ない種および部位を特定することが重要である．第二に，化石試料の化学組成が続成作用によ
る改変を受けていないかを判定する手法は，定性的な判断に依存する場合が多く，客観性に欠
けるとともに，解析に多大な手間と時間を要する．そのため，汎用性が高く，簡便かつ高精度
に試料の保存状態を評価できる手法の開発が求められる．第三に，酸素同位体温度計を用いた
温度推定には，炭酸塩析出時の周囲の水の同位体比を仮定する必要がある．しかし，対象とす
る時代が古くなるほどこの仮定の不確実性が増大するため，超長期的な温度復元には，前提条
件を必要としない温度計の利用が望ましい． 
これらの課題を踏まえ，分析技術の発展を見据えつつ，生物学的同位体効果や続成作用の影
響を事前に評価し，適切な殻部位を特定する手法の開発が求められる．よって本研究では，以
下の 3点を目的とする． 
1. 現生腕足動物殻の石灰化メカニズムを地球化学的視点から評価し，生物学的同位体効果の
影響を詳細に検討する． 
2. 炭酸塩クランプトアイソトープ分析の技術的発展をレビューし，腕足動物殻への応用可能
性を評価する． 
3. 生物学的同位体効果や続成作用の影響を事前に評価する指標として，ラマン分光法が有用
であるかを検討し，最適な殻部位について考察する． 
以下に述べる「現生腕足動物殻における生物学的同位体効果の検討」は博士論文第 1 章に，
「近年の炭酸塩クランプトアイソトープ分析の精度および確度向上に向けた取り組み」は第 2
章に，「一次スクリーニング指標としてのラマン分光法の評価」は第 3 章に対応する．また，
博士論文第 2章については，国際誌に公表済である（Oikawa et al., 2024）． 
 
 
現生腕足動物殻への生物学的同位体効果の検討 
 
【研究手法】 
島根県隠岐島沖水深 61 m より採取された現生腕足動物 Pictothyris picta について，一次層
（PL-Ont），二次層上部（SL-Ont）の炭素・酸素同位体比および微量金属元素濃度を測定した． 
サンプリングはマイクロドリルを用いて殻の成長方向に 0.1–0.2mm間隔で行い，炭素・酸素同
位体分析用と微量金属元素濃度分析用とで交互に用いた．  
 
【結果・考察】 
 P. picta 殻の炭素同位体比は，PL-Ont および SL-Ont のいずれにおいても，周囲の海水と同
位体平衡下で沈積した場合の推定値より低い値を示した．特に，PL-Ont では顕著な低下が認
められた（図 1）．また，酸素同位体比に関しては，PL-Ont では平衡値より低いピークが確認
されたのに対し，SL-Ont では殻全体が同位体平衡の範囲内であった． 
炭素・酸素同位体比の低下を引き起こす要因として，まず代謝効果（metabolic effect）が考え
られる．これは，呼吸由来の CO2 が石灰化反応に利用されることで生じるものであり，主に
炭素同位体比の低下を引き起こす．また，殻の成長速度が速い時期には，反応速度論的同位体
効果（kinetic isotope effect）の影響も考慮する必要がある．この効果は炭素・酸素同位体比の両
方に影響を及ぼすことが知られている．本研究の結果をこれらの要因と照らし合わせると，PL-
Ont では代謝効果と反応速度論的同位体効果の双方が作用していると推測される．これに対し
て，SL-Ont では代謝効果のみが影響を与えており，反応速度論的同位体効果の影響はほとん
ど見られなかった． 
PL-Ont および SL-Ont における Mg/Ca 比と酸素同位体比の相関係数はそれぞれ r = 0.19, -
0.15 であり，統計的な有意な相関は認められなかった．しかし，成長段階ごとに特徴的な傾向
が確認された．特に，SL-Ont においては，成長速度が速い時期ほど Mg/Ca 比が低下する傾向
が認められ，これは有孔虫において報告されている石灰化促進のための Mg 除去機構が，腕足
動物においても機能している可能性を示唆する． 一次層および二次層の Mg/Ca 比の違いに基
づき，殻を 3 つのセクター（Sector 1，Sector 2，Sector 3）に分類した結果，SL-Ont の Sector 
1 では Mg の顕著な枯渇が観察され，Mg 除去が強く作用していることが示唆された．これに
対し，成長速度の遅い Sector 3 では Mg/Ca 比が高く，Mg 除去が弱い，あるいは完全に停止
している可能性が高い．これらの結果は，Mg 除去の強度が成長段階に応じて変化し，石灰化
サイトの化学組成を調節していることを示している．  
SL-Ont では，殻全体が酸素同位体平衡下で形成されており，成長速度が速い時期において
も反応速度論的同位体効果の影響を受けなかった．この要因として，成長速度が速い時期に 
Mg 除去を含む石灰化促進のための生理学的機構が卓越することが関与していると考えられる．
さらに，微量金属元素濃度の解析を通じて，これらの生物学的同位体効果の影響を定量的に評
価できる可能性が示唆された．  
 
近年の炭酸塩クランプトアイソトープ分析の精度および確度向上
に向けた取り組み 
 
【趣旨】 
炭酸塩クランプトアイソトープ温度計は，炭酸塩中の 13C-18O 結合の存在度が形成温度と強
図 1. Pictothyris picta殻の成長方向に沿った炭素同位体比（δ13C），酸素同位体比（δ18O），Mg/Ca
比および Sr/Ca比の変動． 
く相関することに基づく，比較的新しい温度計である．本温度計は，炭酸塩から生成された CO2
の同位体組成のみを用いて温度を算出できるため，地質学的時間スケールにおける地球表層水
温の復元に有用なツールとして注目されている．しかし，開発当初は，膨大な試料量（10 mg）
の必要性，有機物コンタミネーション除去に伴う煩雑な前処理，さらには国際的に認められた
標準試料の欠如といった課題があり，古環境復元への応用に際して多くの制約が存在していた．
これらの課題に対処するため，温度計の開発以降，炭酸塩クランプトアイソトープ分析の高精
度化および高確度化に向けた改良が進められている． 例えば，有機物除去に関しては，初期の
ガスクロマトグラフィーによる分離に加え，真空系内で吸着剤を通過させることにより有機物
を除去する簡便な手法が開発された．試料量の削減については，炭酸塩自動前処理システムの
導入および改良されたデュアルインレットプロトコル（LIDI）の併用により，従来の測定精度
を維持しつつ，必要な試料量が 10 mg から 1.5 mg へと大幅に削減された．さらに，近年，赤
外レーザー吸収分光法を用いた炭酸塩クランプトアイソトープ分析技術の開発が進められて
おり，既に従来の同位体質量分析計と同等の精度および試料量での測定が可能となっている．
この手法のさらなる発展により，より微量な試料の測定が可能となることが期待される． 
 
一次スクリーニング指標としてのラマン分光法の評価 
 
【研究手法】 
本研究では，腕足動物殻のラマ
ン分光データを用いて，生物学的
同位体効果および続成作用の影
響を評価できるかを検討した．試
料として，岩手県大槌湾で採取さ
れた現生腕足動物  Terebratalia 
coreanica の 3 個体に加え，先行
研究（Fujioka et al., 2019）で用い
られた秋田県八峰町峰浜産の腕
足 動 物 化 石 （ Terebratalia 
coreanica，3.85–2.75 Ma），および
藤岡（2022）による千葉県市原市
図 2. 腕足動物殻の測定位置および使用したラマン分光デ
ータの概略図． 
瀬又産の腕足動物化石（Terebratalia coreanica，0.35–0.25 Ma）を 3 個体使用した．さらに，殻
の微量元素濃度が低く，純粋な方解石に近い結晶性を持つ Basiliola lucida の現生試料 2 個体
を比較対象とした．各個体の腹殻を最大成長軸に沿って半割し，断面が観察できるように
Struers社の Caldfix-2を用いて樹脂に包埋した．試料表面は#400，#800，#2000，#3000，#4000，
#6000，3μm，1μmのダイヤモンドペーストを，この順で用いて研磨を施した． 
ラマン分光分析は殻断面に対して深さ方向のライン測定を実施した．特に，1088 cm-1 付近
に認められる ν1 ピークに着目し，ピーク中心位置および半値幅を解析した．ピーク中心位置
の精度を向上させるため，Ne ランプの輝線（1705 cm-1 付近に出現）を基準とした波数補正を
実施した． 
 
【結果・考察】 
図 3 に，ν1 ピークの中心位置および半値幅を横軸，殻の外側表面からの相対深度を縦軸と
したプロットを示す．現生 T. coreanica においては，一次層および二次層上部ではピーク中心
位置が低波数側に分布し，半値幅が大きい傾向が確認された．この特徴は，これらの部位が相
対的に成長速度が速く，Sr/Mg 比が高いことを反映していると考えられる．二次層中央部では，
相対深度が増すにつれてピーク中心位置が徐々に高波数側へシフトし，半値幅は減少する傾向
を示した．この変化は，結晶性が純粋な方解石に近づいていることを示唆する．さらに，二次
層下部ではピーク中心位置がより高波数側へ移行し，半値幅も再び増加する傾向が認められた． 
化石殻の半値幅は，全体的に現生試料よりも小さく，特に堆積年代の古い峰浜産試料におい
て顕著な縮小が確認された．この変化は，続成作用の初期段階における不純物（微量元素や有
機物）の溶脱を反映している可能性が高く，一次層や二次層上部ではその影響がより顕著に現
れていると考えられる．また，化石殻のピーク中心位置も，現生試料と同様に，上部から下部
にかけて低波数側から高波数側へ移行する傾向を示した．しかし，深度ごとのばらつきは現生
試料よりも大きく，特に相対深度が深い部位ほどその変動が顕著であった．このばらつきの増
大は，化石殻が陸水性続成作用の影響を受けた結果，微量元素濃度が改変され，結晶格子の結
合距離が変化したことに起因すると考えられる． 
以上の結果から，ラマン分光データを用いることで，成長速度の速い殻の部位や続成変質の
影響を評価できることが示された．また，相対深度 60–90% の領域では，純粋な方解石に最も
近い結晶性を示し，生物学的同位体効果の影響が最小限であるとともに，続成作用の影響も受
けにくいことが示唆された．これらの知見を総合すると，相対深度 60–90% の領域は，古水温
復元に適した部位であると考えられる． 
 
まとめ 
 
Pictothyris picta 殻の炭素・酸素同位体比は，PL-Ont において顕著な低下を示し，代謝効果
および反応速度論的同位体効果の影響を受けていると考えられる．これに対し，SL-Ont では
代謝効果のみが作用し，酸素同位体比は殻全体が同位体平衡の範囲内に維持されていた．さら
に，SL-Ont では成長速度が速い時期に Mg 除去が促進され，石灰化サイトの化学組成が動的
図 3. 現生および化石腕足動物殻における ν1ピーク中心位置および半値幅（FWHM）の深度
プロファイル． 
に調節されている可能性が示唆された． 
炭酸塩クランプトアイソトープ分析は，近年の技術開発により，試料量の大幅な削減が可能
となっただけでなく，前処理の簡素化や赤外レーザー吸収分光法を用いた分析技術の発展も著
しい．今後，さらなる技術革新により，より微量な試料を用いて高精度な測定が可能となるこ
とが期待される． 
ラマン分光データの解析により，成長速度の速い時期に形成された殻では，ν1 ピークの中
心位置が低波数側へシフトし，半値幅が増加する傾向が認められた．また，化石殻においては，
不純物の溶脱に伴う半値幅の減少や，微量金属元素濃度の改変に起因するピーク中心位置の変
動が確認された．今後，微量元素濃度とラマン分光データの関係を定量的に解析し，生物学的
同位体効果および続成作用の影響をより精緻に評価する必要がある．さらに，相対深度 60–90% 
の領域は，純粋な方解石に最も近い結晶性を示し，生物学的同位体効果および続成作用の影響
が最小限であることから，古水温復元に最適な部位であると考えられる． 
 
参考文献 
 
Fujioka, H., Takayanagi, H., Yamamoto, K., and Iryu, Y., 2019, The effects of meteoric diagenesis on the 
geochemical composition and microstructure of Pliocene fossil Terebratalia coreanica and Laqueus 
rubellus brachiopod shells from northeastern Japan. Prog. Earth Planet. Sci., 6, 45, 
doi:10.1186/s40645-019-0289-7. 
藤岡 大., 2022MS. 腕足動物殻の構造および化学組成は 続成作用によってどのように改変さ
れるのか. 東北大学大学院理学研究科地学専攻博士論文． 
Oikawa, K., Abe, O. and Iryu, Y., 2024, Recent efforts to improve the accuracy and precision of 
carbonate clumped-isotope analysis. Isl. Arc., 33, e12537, doi: 10.1111/iar.12537. 
 


Brachiopod Isotope Thermometers: Optical Shell Portions and Appropriate Analytical Methods

Tohoku University博士（薬科学）thesi

井手, 皓太

光酸化還元触媒を用いたワンポット二環性シクロプロパン合成法の開発および海産性ピロロイミノキノンアルカロイドの効率的全合成

Kinoshita, Yutaka

（別紙様式５）                                       
論 文 内 容 要 旨 
(NO．1) 
氏  名 木下 豊 提出年 ２０２４年 
学位論文の 
題   目 
Spatiotemporal patterns and correlations in active matter featuring alignment interactions 
(配向相互作用を持つアクティブマターにおける時空パターンと相関) 
 
 
論 文 目 次 
List of Conventions v 
1 Introduction 1 
1.1 Active matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 
1.1.1 Basic concept of active matter . . . . . . . . . . . . . . . . . . . . . . . . . 1 
1.1.2 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 
1.1.3 Dry active matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 
1.2 Passive nematic liquid crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 
1.2.1 Alignment tensor (Q-tensor) . . . . . . . . . . . . . . . . . . . . . . . . . . 5 
1.2.2 Phenomenological free energy . . . . . . . . . . . . . . . . . . . . . . . . . 5 
1.2.3 Time-evolution equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 
1.2.4 Topological defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 
1.3 Active nematic liquid crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 
1.3.1 Examples of experimental systems . . . . . . . . . . . . . . . . . . . . . . . 9 
1.3.2 Theoretical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 
1.3.3 Hydrodynamic instabilities and topological defects . . . . . . . . . . . . . . 12 
1.3.4 Active turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 
1.3.5 Effect of boundary walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 
Confinement in channel geometry . . . . . . . . . . . . . . . . . . . . . . . 13 
Friction with substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 
1.4 Active Solids featuring alignment interaction . . . . . . . . . . . . . . . . . . . . . 14 
1.4.1 An example of experimental system . . . . . . . . . . . . . . . . . . . . . . 14 
1.4.2 Order-disorder transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 
1.4.3 Long-range order in active solids . . . . . . . . . . . . . . . . . . . . . . . . 17 
1.5 Scope of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 
1.5.1 Effect of a uniform field on active nematics (Chap. 2) . . . . . . . . . . . . . 19 
1.5.2 Effect of a random field on active nematics (Chap. 3) . . . . . . . . . . . . . 19 
1.5.3 Fluctuation in active solid featuring alignment interaction (Chap. 4) . . . . . 20 
1.6 Structure of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 
2 Active nematics under an electric field 22 
2.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 
（別紙様式５）                                       
2.1.1 Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 
2.1.2 Linear stability analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 
2.2 Numerical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 
2.2.1 Method and parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 
2.2.2 Spatiotemporal patterns and flow anisotropy . . . . . . . . . . . . . . . . . . 27 
2.2.3 Distributions and correlation functions . . . . . . . . . . . . . . . . . . . . 30 
2.2.4 Defect dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 
2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 
3 Active nematics with quenched random field 37 
3.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 
3.2 Disorder-dominated regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 
3.3 Numerical simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 
3.4 Spatial patterns and orientational freezing . . . . . . . . . . . . . . . . . . . . . . . 44 
3.5 Spatial correlation functions and correlation lengths . . . . . . . . . . . . . . . . . 45 
3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 
4 Active solids with alignment interaction 48 
4.1 Theoretical description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 
4.2 Fluctuation spectra in the collectively migrating state . . . . . . . . . . . . . . . . . 50 
4.3 Rate of entropy production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 
4.4 Numerical evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 
5 Conclusion 68 
A Details of the linear stability analysis 70 
B Angle distribution for sinusoidal director undulation 71 
C Fourier transformation 72 
C.1 Fourier transformation in time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 
C.2 Fourier series in space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 
D Definition and derivation of the elasticity matrix in Fourier space 75 
E Wiener–Khinchin theorem 77 
References 78 
Work List 83 
Acknowledgements 84 
 
 
 
 
 
 
 
(NO．2) 
（別紙様式５）                                       
Introduction 
The collective motion of active matter, in which individual constituent elements 
continuously inject energy into the system, has garnered significant attention over the past few 
decades. Active matter constitutes a class of nonequilibrium systems exhibiting unique dynamical 
behaviors that are absent in equilibrium systems. A unified understanding of the mechanisms 
governing these dynamics is critical for unraveling pattern formations and fluctuation properties in 
active matter. This thesis focuses on active matter systems where constituents align their directions 
of motion via mutual interactions, analyzing two representative cases: the pattern formation in 
active nematic liquid crystals under external fields and the fluctuation properties of active solids 
featuring alignment interactions. The key studies are summarized as follows: 
 
Active nematics 
Active nematics are nematic liquid crystals where each molecule creates hydrodynamic stress 
along its alignment axis. We begin with an overview of active nematic liquid crystals, including 
experimental systems such as microtubule-kinesin suspensions, bacterial colonies, and cellular 
tissues. We introduce the theoretical framework based on extensions of equations for passive 
nematic liquid crystals, incorporating an activity term. We review key phenomena such as chaotic 
active turbulence, the characteristic dynamics of topological defects, and the influence of external 
controls on these behaviors. We numerically investigate the influence of an electric field on the flow 
patterns and defect dynamics of two-dimensional active nematic liquid crystals (Chap. 2). Our 
findings show that field-induced reorientation of the director leads to three distinct regimes: (1) 
active turbulence with chaotic defect motion, (2) a laning state, stripe-like alternating laminar flows, 
and (3) a uniformly aligned state without flow. The boundary between (2) and (3) is estimated with 
linear stability analysis of the uniform state. The average flow speed and anisotropy reach their 
peaks at the laning state that appears in intermediate field strength. Localized vortices are created 
and disappear leading to periodic oscillation of the states between the active turbulence and laning 
state in the intermediate regime. In this regime, an undulation instability of the lanes forms 
localized vortices inside them. They collapse to create two pairs of topological defects, which 
recombine and annihilate to restore the laning state. Next, we explore the influence of random 
fields on two-dimensional active nematic liquid crystals (Chap. 3). In biological systems, internal or 
external cellular environments often contain structural heterogeneities that disrupt the alignment 
of liquid crystals. We model these effects by a quenched random field imposed on the nematic liquid 
crystals. Our simulations show that the system undergoes a transition from the active turbulence to 
a frozen state in which the alignment pattern is arrested with a strong random field. In this frozen 
state, the nematic director locally aligns with the random field. We found short-range orientational 
correlations similar to random passive nematics, while the flow velocity retains long-range 
correlations that decay logarithmically reflecting the form of the Green function of the Stokes 
equation. This phenomenon is independent of the type of active stress (extensile or contractile). The 
coexistence of local order and global flow is a novel feature that characterizes active nematics with 
(NO．3) 
（別紙様式５）                                       
quenched randomness. 
 
Active solids 
Active matter at high density often exhibits solid states that have attracted attention in recent 
years. We focus on active solids, elastically deformable materials composed of self-propelled 
particles. The self-driving of each component causes non-equilibrium fluctuations. A coupling 
between the orientation of each component and its velocity induces drastic changes in the nature of 
collective motion. An experimental realization of such systems is a network of HexBug® robots 
connected by springs. In this system, the alignment of the robots causes the migration of the solid. 
On the other hand, simulations show that the direction of self-propulsion becomes random when the 
rotational noise is large. The static properties of this system have been investigated in both the 
random and migrating states, and it is shown that the direction of self-propulsion exhibits a 
quasi-long-range order in the migrating state. However, the dynamic properties of this state have 
not been investigated. We then examine fluctuation spectra in active solids featuring polar 
alignment interactions (Chap. 4). The study reveals unique excitation modes arising from the 
coupling between activity and alignment interactions. They can be obtained by solving the 
linearized equations for fluctuations from a noise-free state. The fluctuations reflect the anisotropy 
of the elastic interaction and the direction of migration. The peaks shift to higher wavenumber and 
frequency as the coupling or the self-propulsion speed increases. The results of the numerical 
simulation and the analytical solution are in good agreement. These nonequilibrium fluctuations 
differ fundamentally from equilibrium fluctuations, as evidenced by the rate of entropy production 
that quantifies the rate of energy dissipation or irreversibility associated with the system's 
deviation from equilibrium, based on the probabilities of forward and reverse transitions. It is 
known that the rate of entropy production in a steady state is proportional to the off-diagonal 
components of the fluctuation spectrum matrix. In this system, only the coupling between 
fluctuations perpendicular to the migration direction and the self-propelling angle contributes to it. 
 
Conclusion 
By investigating systems where activity and alignment interactions coexist, this work uncovers 
qualitatively distinct dynamics, including novel pattern formations and characteristics of 
non-equilibrium fluctuations. These findings lay the groundwork for future studies on active matter 
systems with alignment interactions, offering potential applications in understanding biological 
collectives and designing advanced biomaterials. 
(NO．4) 
別 紙 
 
論文審査の結果の要旨 
 
アクティブマターと呼ばれる物質群は自己駆動するという特性を持ち、従来の物理学の研究の
メインの対象であった平衡系にはない強い非平衡性に起因する振る舞いを示す物質群である。こ
のような物質においては、構成要素間で自己駆動の方向を揃えようとする相互作用により誘起さ
れる集団運動等の非平衡現象が多くの関心を集めている。 
木下氏提出の博士論文は、アクティブマターの例として、軸性の異方性を持つアクティブネマ
チック液晶に電場を作用させた系（第２，３章）および、極性の異方性を持つアクティブ粒子を
弾性体中に埋め込んだアクティブ固体（第４章）の２つの系についての研究成果を３部構成で報
告したものである。木下氏の得た目立った成果としては、一様な静電場下やランダム電場下での
アクティブネマチック液晶系における構造相転移や新規な運動と構想を有する相の出現、アクテ
ィブ固体系におけるフォノンに似た集団励起モードの出現などがあげられる。 
まず第１部では、方向のそろった一様状態にある２次元アクティブネマチック液晶の面内に静
電場が印加されたときに、一様状態が不安定化する条件を運動方程式の線形解析から求め、さら
に電場の強度をあげてゆくときに、系の状態が一様相、層状構造を持つ相、アクティブ乱流相の
ように相変化することと、構造欠陥の動力学挙動をシミュレーションで明らかにした。 
続く第２部では、２次元アクティブネマチック液晶に空間的に固定されたランダムな電場を印
加するシミュレーションを実行し、ランダム電場の強度がある臨界値以上になったときに速度場
の相関関数に対数的な挙動が現れることを見出した。 
最後の第３部では、一方向に自走する自己駆動粒子をバネで結合させたアクティブ固体のモデ
ルを構成し、一様に運動する基準状態の周りでの線形解析を行うことで、固体の変位場の方向と
自己駆動の方向との間のアクティブ性由来の相互作用によって、フォノンに類似の集団励起が生
じることを見出した。 
これらの成果は、自己駆動系における運動の方向と外部環境とのカップリングの効果を明らか
にしたものであり、統計物理学における基礎研究として有用かつ価値のあるものであると認めら
れる。これは申請者が自立して研究活動を行うために必要な高度の研究能力と学識を有している
ことを示しており、木下氏提出の博士論文は「博士（理学）」の学位論文として合格であると認
める。 


配向相互作用を持つアクティブマターにおける時空パターンと相関

Kuwata, Asuka

（別紙様式５）                                       
論 文 内 容 要 旨 
(NO．1) 
氏  名 桑田 明日香 提出年 2024 年 
学位論文の 
題   目 
 
Large-scale Magnetic Field Model of Gamma-ray Burst Afterglow Polarization 
（ガンマ線バースト残光の偏光の大スケール乱流磁場モデル） 
 
 
論 文 目 次 
1 Introduction 
1.1 Background 
1.2 Afterglow polarimetric observation 
2 Afterglow theory 
2.1 Synchrotron spectrum and light curve of the afterglow 
2.2 Synchrotron emission integrated over the equal arrival time surface 
2.3 Energy dissipation process of the relativistic collisionless shocks 
2.4 Microscopic-scale field model 
3 Large-scale field model 
3.1 Synchrotron polarization with the large-scale turbulent magnetic field 
3.2 Zero-viewing angle observer 
3.3 Dependence on the field anisotropy 
3.4 Off-axis viewing angle 
3.5 Contribution of ordered magnetic field 
3.6 PD and PA spectra 
4 Discussion: comparison to observational data 
4.1 Late afterglows 
4.2 Early afterglows 
4.3 GW-associated GRB (off-axis) 
4.4 Radio afterglows 
5 Summary and future work 
 
論 文 要 旨 
The afterglows of gamma-ray bursts (GRB) are synchrotron emissions from electrons accelerated by relativistic 
shocks. Comparison of the light curves and the theoretical model suggests that the magnetic field downstream of 
the forward shocks is amplified at least two orders of magnitude stronger than the shock-compressed magnetic 
field of the interstellar medium. The amplification mechanism of such a strong magnetic field is a major mystery 
in the radiation mechanism of the afterglow, and proving it is important for constraining the physical quantities 
and radiation mechanism of GRB jets and the acceleration mechanism of high-energy cosmic rays. 
（別紙様式５）                                       
    (NO．2) 
    Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, 
have been discussed so far. However, it is difficult to verify either mechanism by simulations in a realistic setting, 
and the origin of the strong magnetic field suggested by observations is still unknown. In this work, we focus on 
the fact that the coherence length of the turbulent magnetic field amplified by the two amplification mechanisms 
differs by about 10 orders of magnitude, and attempt to settle this problem using synchrotron polarization, which 
is the only way to verify the turbulence scale observationally. Theoretical works of polarization have been 
extensively studied and compared with observations in the microscopic-scale turbulent magnetic field amplified 
by plasma instabilities, but they are inconsistent with some observations. In this work, we conduct the first 
quantitative study of the large-scale turbulent magnetic field amplified by magnetohydrodynamic instabilities, and 
construct a semi-analytic model of the polarization. In the constructed model, we examine the effects of the 
magnetic field anisotropy, the observer viewing angle, and the ordered magnetic field component. The results 
indicate that our model could explain all the polarimetric observational data to date that seem to be forward shock 
emission. We also discuss the observational predictions of our model and how to verify the turbulence models in 
future observations. 
別 紙 
 
論文審査の結果の要旨 
 
 桑田氏の研究目的は、ガンマ線バーストの残光を放射する相対論的速度の無衝突衝撃波（相対
論的衝撃波と呼ぶ）について、これまで手を付けられていなかった磁気乱流の理論モデルを構築
することである。そのモデルによるシンクロトロン放射の偏光の数値計算結果とこれまでの可視
と電波の偏光観測データとを比較することで、相対論的衝撃波の磁場増幅機構を解明することを
目指している。 
 ガンマ線バーストは宇宙一明るい天体現象であり、特異な大質量星の崩壊か高密度星の連星合
体を起源とする相対論的ジェットのガンマ線放射現象と考えられている。起源天体、相対論的ジ
ェットの生成と加速、ジェットが周辺媒質に作る相対論的衝撃波などの物理に謎が多く、国際的
に多くの研究者が注目している現象である。ガンマ線バーストの残光は相対論衝撃波からのシン
クロトロン放射であり、衝撃波における磁場は流体圧縮で予想される強さよりも 100 倍以上増幅
されていることがわかっている。しかしその増幅機構は不明であり、相対論的衝撃波の物理や相
対論的ジェットの物理量の確定、さらには高エネルギー宇宙線の加速機構の理解に直結した課題
となっている。これまで磁場増幅機構として、プラズマ不安定と磁気流体不安定が考えられてき
た。磁気流体不安定で増幅される乱流磁場のスケールの典型値は、プラズマ不安定で増幅される
小スケールの乱流より 10 桁程度も大きく、そのスケールの違いがシンクロトロン放射の偏光に現
れる。ところが偏光の理論はこれまで小スケール磁気乱流に基づくものしかなく、大スケール磁
気乱流に基づいた理論はオーダー評価のみであった。桑田氏は大スケール磁気乱流について、観
測との比較に耐えうる定量的な理論の構築に挑戦した。大スケールの磁気乱流をランダムな方向
と位相の３次元のアルフベン波の重ね合わせでモデル化し、相対論的速度の放射体に特有な等観
測時刻面を考慮した放射計算コード開発を行い、偏光の準解析的モデルの構築に成功した。 
 この研究による新しい知見は以下の３点である：(1)シンクロトロン放射強度を決める磁場の最
大スケールが放射領域サイズ程度であるという自然な仮定をすれば、可視域の偏光度の典型的な
観測値 1-3%が導かれることを見つけた。(2)乱流磁場が等方な場合に偏光の時間変化と周波数依存
性が小スケール磁場モデルと異なることを明確にし、観測的にモデルを区別しうることを示した。
(3)磁場の非等方性や観測者のジェット見込み角、一様磁場の影響を網羅的に調査した結果、小ス
ケール乱流磁場モデルが一部の偏光観測結果と矛盾するのに対し、大スケール乱流磁場モデルが
既存の全ての偏光観測と整合的であるという示唆を得た。 
これらの新しい知見を含む桑田氏の研究論文は、博士論文として適当である。そして論文は全
体として、自立して研究活動を行うに必要な高度の研究能力と学識を有することを示している。
以上の理由により、桑田氏が提出した博士論文を博士（理学）の学位論文として合格と認める。 


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