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dc.contributor.advisorSpanier, F
dc.contributor.authorSchreiner, Cedric
dc.descriptionPhD (Space Physics), North-West University, Potchefstroom Campus, 2017en_US
dc.description.abstractThe acceleration and transport of solar energetic particles have been intensively studied ever since the discovery of relativistic particles originating from the Sun. Both processes are tightly connected to the dynamics of the solar wind and the turbulent interactions of plasma waves. While advances in both theoretical modeling and observations have been made over the years, there are still many details which are not understood yet. Solar wind turbulence on its own is a complicated matter, and especially the regime of kinetic turbulence poses many open questions. Kinetic turbulence involves plasma waves at high frequencies and small wave lengths, where their interactions with the charged particles in the plasma become important. Compared to the well-understood energy spectrum in the inertial range, a steepening of the spectral slope is expected in the kinetic regime, which is generally attributed to the effects of dispersion and energy dissipation by resonant interactions with the particles. However, no complete model for the composition and behavior of the turbulent waves in this so-called dissipation range is available, yet. Observations suggest that kinetic Alfvén waves are responsible for turbulence in the dissipation range. However, whistler waves, which are also detected in various regions of the solar wind, may also contribute. This latter case is especially interesting, because whistler waves allow for the transport of energy to frequencies above the proton cyclotron frequency and may, therefore, interact with both thermal and high energy electrons in the solar wind plasma. A particle-in-cell code is employed to simulate dispersive waves and their interaction with charged particles in the plasma. As a preliminary study and a first step towards simulations of dissipation range turbulence, the cyclotron resonance of thermal protons and dispersive Alfvén waves and their strongly damped analog at higher frequencies, the proton cyclotron waves, is modeled. To quantitatively analyze the dissipation of these waves, a method is developed which allows to extract the waves' damping rates from simulation data. Extensive tests show that cyclotron damping is recovered correctly in the simulation, which is a crucial prerequisite for a correct model of dissipation range turbulence. Similar to the case of turbulence, sophisticated models for the transport of solar energetic particles in an environment that is dominated by non-dispersive waves are available. However, the effect of dispersive waves on particle transport is less well-understood, which is partly due to the more difficult treatment of dispersive waves in theoretical models. The theoretical approach to describe particle transport is usually based on the quasi-linear approximation, which assumes that resonant scattering processes can be described by diffusion in phase space. Using particle-in-cell simulations again, the resonant interaction of energetic electrons and dispersive waves is studied. The particles are scattered off of the waves' electromagnetic fields, creating a specific resonance pattern in phase space. The simulation data is compared to analytical predictions, which can be obtained from a model originally based on magnetostatic quasi-linear theory and which has recently been enhanced in order to allow for the description of dispersive waves. While the model predictions and the simulation results gen-erally agree, it can be seen that the resonant interaction of energetic particles and a single wave does not lead to diffusion, but rather to trapping of the particles in the electromagnetic fields of the wave. Diffusion can only occur when several waves with different frequencies, wave lengths, or directions of propagation are present. Even though these simulations do not model particle transport in turbulence, they contribute to a better understanding of the micro-physical properties of the scattering processes which are responsible for the transport and acceleration of solar energetic particles. Finally, kinetic turbulence is directly studied in simulations. A set of initially excited whistler waves is used as a seed population for the development of a turbulent cascade. Whistler waves are chosen because they allow for a continuation of the spectrum above the proton cyclotron frequency into a regime where the interaction of the waves with electrons becomes dominant. The simulations are analyzed especially with regard to the shape of the energy spectrum, since very little is known about the typical spectral index. However, no consistent picture of the dependence of the spectral shape on the physical parameters is obtained, yet. Extended parameter studies, which might yield more conclusive results, are hindered by the limited amount of computational resources available for this work. They remain as an eligible task for future projects. Despite the absence of a detailed picture of kinetic turbulence, the simulations support the idea that the magnetic energy spectrum in the kinetic regime is steeper than in Alfvénic turbulence. It can also be assumed that a spectral break is produced at the transition into the dissipation range. The spectrum forms an even steeper power law after the break. Choosing two similar setups for simulations of whistler turbulence as a basis, the transport of energetic electrons in kinetic turbulence is investigated. The analysis shows that the steep energy spectra in the kinetic regime lead to a particular dominance of waves at low wave numbers. These waves carry most of the energy and, thus, are most important for the interactions with the energetic particles. Although particles may resonate with waves at higher wave numbers (in the dispersive or dissipative regime), these interactions do not seem to contribute significantly to the transport mechanism. Comparison with a theoretical model suggests that the turbulent spectrum can be approximated by the relatively at regime at low wave numbers, before the spectral break is encountered. Although the model predictions are not very accurate, the basic features of the pitch angle diffusion coefficient derived from the particle data can be recovered. This is especially interesting, since the model is derived for Alfvénic turbulenceen_US
dc.publisherNorth-West University (South Africa) , Potchefstroom Campusen_US
dc.subjectSolar winden_US
dc.subjectKinetic plasmaen_US
dc.subjectPlasma wavesen_US
dc.subjectWave dampingen_US
dc.subjectParticle transporten_US
dc.subjectNumerical simulationen_US
dc.titleNumerical modelling of the microphysical foundation of astrophysical particle accelerationen_US

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    This collection contains the original digitized versions of research conducted at the North-West University (Potchefstroom Campus)

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