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Jaroschek, Claus (2005): Critical Kinetic Plasma Processes In Relativistic Astrophysics. Dissertation, LMU München: Faculty of Physics
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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. Consequently 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 characteristics 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 electromagnetic Particle-In-Cell (PIC) simulations. Typical scenarios comprise ensembles of 10^9 particles and endure for several 10^4 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 superposition 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 the remarkable quality to generate smooth and stable power-laws in the particle distribution function (PDF) out of an initial Maxwellian. The upper PDF cut-off in relativistic energy is determined by the ratio of light to Alfven velocity c/v_A. The power-law index assumes s~-1 within 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 Gamma-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 scattering. 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 CTW in favour of the electrostatic Two-Stream instability (TSI). The pulsar magnetosphere is the paradigmatic 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.