Critical kinetic plasma processes in relativistic astrophysics [Elektronische Ressource] / vorgelegt von Claus H. Jaroschek

Claus H. Jaroschek

Sujets

• Astronomie

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2005
• Nombre de lectures: 26
• Langue: English
• Poids de l'ouvrage: 18 Mo

Extrait

Ludwig-Maximilians-Universit¨at Munc¨ hen
Critical Kinetic Plasma Processes
in
Relativistic Astrophysics
Dissertation
der Fakult¨ at fur¨ Physik
der Ludwig-Maximilians-Universit¨ at
M¨ unchen
vorgelegt von
Claus H. Jaroschek
aus Wurzburg¨
M¨ unchen, M¨ arz 20051. Gutachter: Prof.Dr. Harald Lesch
2. Gutachter: Prof.Dr. Rudolf A. Treumann
Tag der mundl¨ ichen Pru¨fung: 20. Juli 2005In the universe, there are things that are known,
and things that are unknown,
and in between, there are doors ...
If the doors of perception were cleansed,
everything would appear to man as it is, infinite.
William Blake, 1790
Ich widme diese Arbeit
all’ diesen Menschen, die mir nahestehen
und immer noch zu mir halten
obschon ich Ihnen bis jetzt nie die Zeit schenken konnte, die Ihnen zusteht ...Abstract
Plasma astrophysics deals with collective plasma processes in astrophysical scenarios. As observational
astronomy pushes towards unprecedented resolutions in space and time, the focus of theoretical research
necessarily ventures towards a description of the plasma microphysics. On microphysical scales the
plasma is pervasively collisionless and the magnetohydrodynamic approximation breaks down. Conse-
quently theoretical concepts rely on a kinetic plasma description as the most sophisticated plasma model.
The present work discusses some fundamental kinetic plasma processes in relativistic astrophysics: Fast
Magnetic Reconnection (FMR) associated with discontinuities in the magnetic field topology, and the
Coupled Two-Stream-Weibel instability (CTW) in the wake of collisionless shocks. Both processes are
ubiquitous in astrophysical sites, prevail over competing plasma modes because of dominant growth
rates, experience significant relativistic modifications, and develop essential features solely in the highly
non-linear regime. The computational representation invokes the entire 6D phase space. These charac-
teristics distinguish FMR and the CTW as distinctively critical processes.
FMR and the CTW are studied here in the framework of self-consistent, relativistic and fully electro-
9magnetic Particle-In-Cell (PIC) simulations. Typical scenarios comprise ensembles of 10 particles and
4endure for several 10 time steps. The computational task is challenging and completely in the realm of
the massively parallelized architectures of state-of-the-art supercomputers.
We present the first self-consistent 3D simulations of FMR in relativistic pair plasma. Focusing on the
mechanism of particle acceleration we show that the highly dynamic evolution of the current sheet in
the non-linear regime is the essential stage. Therein non-stationary acceleration zones arise in the su-
perposition of the relativistic tearing and the relativistic drift kink mode as competing current sheet
instabilities. Though the topology of electromagnetic fields is highly turbulent, the FMR process shows
sthe remarkable quality to generate smooth and stable power-laws f(γ)dγ∝ γ dγ in the particle distri-
bution function (PDF) out of an initial Maxwellian. The upper PDF cut-off in relativistic energy γ is
determined by the ratio of light to Alfv´en velocity c/v . The power-law index assumes s? 1within
A
the reconnection X-zone irrespective of parameter variations. Intriguingly the power-law index appears
as the universal characteristic of the source process. The associated synchrotron spectra provide a valid
description of the extremely hard spectra and rapid variabilities of ‘Flat Spectrum Radio Quasars’.
Conceptual γ-ray burst (GRB) synchrotron emission models depend on a plasma process which ensures
efficient magnetic field generation. The CTW converts bulk-kinetic energy of counter-streaming plasma
shells into Weibel magnetic fields. Pivoted by the linear analysis of the CTW, the PIC simulations
confirm the correspondence between saturation magnetic fields and bulk-kinetic energy. Plasma shell
collisions in GRBs are either associated with internal or external shocks. As direct consequence of the
energy dependence the CTW evolves from a complex 3D topology in internal collisions towards quasi-
2D, Weibel-dominated conformalizations at the higher external shock energies. The PIC results prove
that the Weibel fields are sufficiently strong to sustain synchrotron emission scenarios, particularly in
external shocks. By determining the first lifetime limits we show that Weibel fields are also sufficiently
long-lived with respect to typical synchrotron cooling times. We further identify the stability-limiting
diffusion process as of ‘Bohm’-type, i.e. the diffusion coefficient exhibits the ∝ T/B-dependence and
herewith represents a conservative stability criterion. The CTW generates stable power-law spectra in
the magnetic fields implying power-law shaped PDFs as self-similar solutions for diffusive particle scat-
tering. This suggests a universal power-law index as the characteristic of the CTW process.
Imposing a magnetic guide field of well-defined strength suppresses the Weibel contributions of the CTWin favour of the electrostatic Two-Stream instability (TSI). The pulsar magnetosphere is the paradig-
matic scenario in which we discuss the mechanism of Coherent Collisionless Bremsstrahlung (CCB)
triggered by the TSI. The PIC simulations show that the CCB mechanism provides a valid description
of the phenomenon of ‘Giant Radio Pulses’ as recently observed from the Crab pulsar.Contents
1 Introduction 1
1.1 Motivation - Self-Consistent Modelling of
CriticalPlasmaProcesesinAstrophysicalScenarios............ 1
1.2 Magnetic Reconnection -
TopicalProblemsofaLong-DiscusedProces................ 4
1.3 The Weibel Mechanism -
MicrophysicalOriginofAstrophysicalMagneticFields........... 15
1.4 The Weibel-Two-Stream-Connection -
ThePlasmaMicrophysicsofJetsandPulsars................ 20
2 The Computational Method of Choice: Particle-In-Cell 23
3 Collisionless Magnetic Reconnection 33
3.1 The Relativistic Harris Equilibrium . . . ................... 3
3.2RelativisticParticlesinElectromagneticFields ........... 38
3.3 The Mechanism of Relativistic Particle Acceleration . ............ 40
3.3.1Abstract.............................. 40
3.3.2Introduction........................ 40
3.3.3SimulationDescription...................... 42
3.3.4SimulationResults..................... 4
General Time Evolution of Reconnection in a Pair Plasma . . . . . . 44
Initial Plasma Parameter / Acceleration Efficiency Correspondence . 50
Characteristicsofthe3DConfiguration................. 53
Detailed Analysis of the Acceleration Mechanism . . . . . . . . . . . 54
3.3.5Discussion................................. 59
Particle Acceleration in Electron-Proton Plasma Reconnection . . . . 59
Comparison with Previous Results on 2D Scenarios . . . . . . . . . . 60
Concluding Remarks on Particle Acceleration in FMR . . . . . . . . . 60
3.4SynchrotronSignaturesoftheSelf-ConsistentModel............ 62
3.4.1Abstract.............................. 62
3.4.2Introduction........................ 62
3.4.3SimulationModel......................... 63
3.4.4NumericalResultsandPhysicalPicture................. 64
Non-thermal Particle Generation in the Late-time Evolution . . . . . 64SynchrotronEmision........................... 65
3.4.5SummaryandConclusions............ 69
4 Magnetic Fields in 3D Weibel Scenarios 71
4.1 Magnetic Fields in γ-Ray Burst Models - Generation,
TopologyandLifetime............................. 71
4.2 Ultra-Relativistic Plasma Shell Collisions -
TheMagneticEquipartitionRatio....................... 84
4.2.1Abstract...................... 84
4.2.2Introduction............................ 84
4.2.3SimulationDescription.......................... 86
4.2.4 Dimensional Effects in Relativistic Plasma Shell Collisions .. 87
Linear Progenitors of the Preeminent Plasma Instability Modes . . . 87
EffectsofaFiniteThermalSpread:KineticModifications....... 91
4.2.5TheSaturatedandSteady-StateFinalMagneticFields.... 96
4.2.6SummaryandPerspectives........................ 104
4.3 Topology and Lifetime - A Critical Test for Synchrotron Emission Models 106
4.3.1Abstract.................................. 106
4.3.2Introduction................ 106
4.3.3SimulationDescription.......................... 108
4.3.4DiffusionLimitedLifetimeofMagneticFields..... 10
DiffusionCoefficientsObtainedbythePICSimulation ........ 110
DiffusionisIdentifiedas‘Bohm’-type.................. 115
4.3.5ConsequencesforSynchrotronEmision......... 16
4.4Self-ConsistentSynchrotronCascades..................... 19
5 The Weibel-Two-Stream Connection 125
5.1 The Electromagnetic Counterstreaming Instability
withMagneticGuideField........................... 125
5.2 The Connection to Pulsar Physics -
Stimulation of Collisionless Bremsstrahlung . . . . . ............ 135
6 Final Remarks 151
6.1Achievements.................................. 151
6.2Perspectives................... 157
A Dimensionless Representation of Physical Quantities 159
B Linear Theory of the 2D EM Counterstreaming Instability 163
List of Publications 177
Acknowledgements 179
Curriculum Vitae 181Chapter 1
Introduction
1.1 Motivation - Self-consistent Modelling of
Critical Plasma Processes in Astrophysical Scenarios
During the recent decades the observational techniques of modern astronomy have
reach