Evolution of plasma turbulence excited with particle beams

Particles ejected from the Sun that stream through the surrounding plasma of the solar wind are causing instabilities. These generate wavemodes in a certain frequency range especially within shock regions, where particles are accelerated. The aim of this paper is to investigate of amplified Alfvenic wavemodes in driven incompressible magnetohydrodynamic turbulence. Results of different heliospheric scenarios from isotropic and anisotropic plasmas, as well as turbulence near the critical balance are shown. The energy transport of the amplified wavemode is governed by the mechanisms of diffusion, convection and dissipation of energy in wavenumber space. The strength of these effects varies with energy and wavenumber of the mode in question. Two-dimensional energy spectra of spherical k-space integration that permit detailed insight into the parallel and perpendicular development are presented. The evolution of energy injected through driving shows a strong energy transfer to perpendicular wavemodes. The main process at parallel wavemodes is the dissipation of energy in wavenumber space. The generation of higher harmonics along the parallel wavenumber axis is observed. We find evidence for a critical balance in our simulations.Comment: Accepted for publication in A&

Particle scattering in turbulent plasmas with amplified wave modes

Our calculations show a good agreement of particle simulations and the QLT for broad-band turbulent spectra; for higher turbulence levels and particle beam driven plasmas, the QLT approximation gets worse. Especially the resonance gap at mu = 0 poses a well-known problem for QLT for steep turbulence spectra, whereas test-particle computations show no problems for the particles to scatter across this region. The reason is that the sharp resonant wave-particle interactions in QLT are an oversimplification of the broader resonances in test-particle calculations, which result from nonlinear effects not included in the QLT. We emphasise the importance of these results for both numerical simulations and analytical particle transport approaches, especially the validity of the QLT

Deciphering the physical basis of the intermediate-scale instability

We study the underlying physics of cosmic-ray (CR) driven instabilities that play a crucial role for CR transport across a wide range of scales, from interstellar to galaxy cluster environments. By examining the linear dispersion relation of CR-driven instabilities in a magnetised electron-ion background plasma, we establish that both, the intermediate and gyroscale instabilities have a resonant origin and show that these resonances can be understood via a simple graphical interpretation. These instabilities destabilise wave modes parallel to the large-scale background magnetic field at significantly distinct scales and with very different phase speeds. Furthermore, we show that approximating the electron-ion background plasma with either magnetohydrodynamics (MHD) or Hall-MHD fails to capture the fastest growing instability in the linear regime, namely the intermediate-scale instability. This finding highlights the importance of accurately characterising the background plasma for resolving the most unstable wave modes. Finally, we discuss the implications of the different phase speeds of unstable modes on particle-wave scattering. Further work is needed to investigate the relative importance of these two instabilities in the non-linear, saturated regime and to develop a physical understanding of the effective CR transport coefficients in large-scale CR hydrodynamics theories.Comment: 14 pages, 3 figures, submitted to JPP Letters, comments welcom

Cosmic magnetism: The plasma physics of the recombining universe

This thesis presents an analytical and computational approach to modelling partially ionised, spatially-inhomogeneous and recombining plasmas. The specific context for this study is astrophysical plasmas, the early Universe in particular. Two models are investigated in detail: a magnetohydrodynamic (MHD) plasma model to simulate partially ionised plasmas; and a fully electromagnetic/kinetic model, used to study recombining plasmas. The first section further develops an existing computational model of a partially ionised plasma as a mixture of two cospatial fluids: an MHD plasma and a neutral gas. In order to model the interaction between the plasma and neutral gas populations ab initio, a collisional momentum exchange term was added to the momentum equation of each fluid. The model was used to investigate the combined response to different wave modes driven in the plasma or the neutral gas. The momentum coupling between the plasma and the neutral gas leads to complex interactions between the two populations. In particular, the propagation of plasma waves induces waves in the neutral gas by virtue of the collisional momentum exchange between the velocity fields of each fluid. This means that the normal wave modes of each independent fluid are modified to produce a combined, hybrid response, with the intriguing possibility that neutral gas can not only respond indirectly to magnetic fluctuations but also generate them via sound waves. This model is used to examine an existing observational method known as the ‘Chandrasekhar-Fermi method’ (CF53) for the diagnosis of magnetic fields in astrophysical plasmas. CF53 is commonly applied to objects such as nebulae and molecular clouds which are partially-ionised plasmas. It assumes that the gas motion can be used to infer the magnetic field strength, given coupling between Alfv´en waves in the plasma and the thermal motion of the neutral gas. Computational results show that this method may need to be refined, and that certain assumptions made should be re-evaluated. This is consistent with reports in the literature of CF53 under- or over-estimating the magnetic fields in objects such as molecular clouds. The second part of this thesis concentrates on the non-equilibrium evolution of magnetic field structures at the onset of the large-scale recombination of an inhomogeneouslyionised plasma, such as the Universe was during the epoch of recombination. The conduction currents sustaining the magnetic structure will be removed as the charges comprising them combine into neutrals. The effect that a decaying magnetic flux has on the acceleration of remaining charged particles via the transient induced electric field is considered. Since the residual charged-particle number density is small as a result of decoupling, the magnetic and electric fields can be considered essentially to be imposed, neglecting for now the feedback from any minority accelerated population. The electromagnetic treatment of this phase transition can produce energetic electrons scattered throughout the Universe. Such particles could have a significant effect on cosmic evolution in several ways: (i) their presence could influence the overall physics of the recombination era; and (ii) a population of energetic particles might lend a Coulomb contribution to localized gravitational collapse. This is confirmed by a numerical simulation in which a magnetic domain is modelled as a uniform field region produced by a thin surrounding current sheet. The imposed decay of the current sheet simulates the formation of neutrals characteristic of the decoupling era, and the induced electric field accompanying the magnetic collapse is able to accelerate ambient stationary electrons (that is, electrons not participating in the current sheet) to energies of up to order 10keV. This is consistent with theoretical predictions

Compressibility Effects on the Kelvin-Helmholtz Instability and Mixing Layer Flows

The objective of the thesis is to analyze, understand and explicate the various physical mechanisms underlying the suppression of instability and mixing in compressible mixing layers. The investigation comprises of three studies which employ linear analysis and Direct Numerical Simulation (DNS). The first study examines the effect of compressibility on the underlying planar Kelvin-Helmholtz (KH) instability. The transformative influence of compressibility on the ubiquitous free shear-flow instability is investigated. This study focuses on the change in the character of pressure from a Lagrange-multiplier in incompressible flows to a thermodynamic variable in compressible flows. Linear analysis reveals that compressibility engenders the formation of a dilatational-interface-layer (DIL) within which the velocity perturbation is wave-like rather than vortical. Inherently unsteady dilatational action is shown to disrupt vortex merging and roll-up leading to suppression of KH instability. The second study examines the effect of perturbation alignment and non-linear interaction on the stability of compressible mixing layers. Linear analysis clearly shows that compressibility effects diminish with increasing obliqueness of the perturbation with respect to the shear plane. Notably, spanwise perturbations are impervious to Mach number effects. The non-linear effects are examined using DNS. It is shown that triadic interactions among the perturbation wavemodes lead to new perturbation wavemodes that are aligned closed to the spanwise directions and hence unstable. The third study examines mixing layer flow structure at various Mach numbers. At low speeds, the mixing layers exhibit strong spanwise rollers and short streamwise ribs. The effect of Mach number on the evolution of structures and the interaction between them are investigated in detail. With increasing Mach numbers, the spanwise rollers are suppressed. In the absence of spanwise rollers, the streamwise ribs align to form streamwise structures

Effects of partial ionisation in the solar atmosphere

In this thesis techniques are developed for the simulation of partially ionised plasmas in the fluid approximation. These techniques are used to model the evolution of magnetic fields in the partially ionised regions of the solar atmosphere. Single fluid equations for a partially ionised plasma are derived based on the individual equations for each species. A Lagrangian Remap MHD code is then adapted to simulate a plasma of arbitrary degree of ionisation. The effects of the presence of neutrals on the propagation and damping of Alfv´en waves in the solar atmosphere are investigated. Ion-neutral collisions are shown to be an efficient damping mechanism for outwardly propagating Alfv´en waves of frequencies greater than 0.1 Hz, showing that high frequency waves in the outer solar atmosphere cannot originate at the surface of the Sun. Next simulations to show the effects of neutrals on the emergence of magnetic flux from beneath the solar surface into the outer atmosphere are performed. Results from 2D and 3D numerical experiments show that the presence of neutrals increases the amount of magnetic flux that can emerge into the corona. Furthermore, ion-neutral collisions are strong enough to dissipate currents perpendicular to the magnetic field as it emerges. This shows that ion-neutral collisions are a viable mechanism for the formation of force-free (j ∧B = 0) coronal magnetic field from sub-surface field, which is not the case when the plasma is assumed to be fully ionised.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Vortex Motions in the Solar Atmosphere: Definitions, Theory, Observations, and Modelling

Vortex flows, related to solar convective turbulent dynamics at granular scales and their interplay with magnetic fields within intergranular lanes, occur abundantly on the solar surface and in the atmosphere above. Their presence is revealed in high-resolution and high-cadence solar observations from the ground and from space and with state-of-the-art magnetoconvection simulations. Vortical flows exhibit complex characteristics and dynamics, excite a wide range of different waves, and couple different layers of the solar atmosphere, which facilitates the channeling and transfer of mass, momentum and energy from the solar surface up to the low corona. Here we provide a comprehensive review of documented research and new developments in theory, observations, and modelling of vortices over the past couple of decades after their observational discovery, including recent observations in Hα, innovative detection techniques, diverse hydrostatic modelling of waves and forefront magnetohydrodynamic simulations incorporating effects of a non-ideal plasma. It is the first systematic overview of solar vortex flows at granular scales, a field with a plethora of names for phenomena that exhibit similarities and differences and often interconnect and rely on the same physics. With the advent of the 4-m Daniel K. Inouye Solar Telescope and the forthcoming European Solar Telescope, the ongoing Solar Orbiter mission, and the development of cutting-edge simulations, this review timely addresses the state-of-the-art on vortex flows and outlines both theoretical and observational future research directions

Effects of partial ionisation in the solar atmosphere

In this thesis techniques are developed for the simulation of partially ionised plasmas in the fluid approximation. These techniques are used to model the evolution of magnetic fields in the partially ionised regions of the solar atmosphere. Single fluid equations for a partially ionised plasma are derived based on the individual equations for each species. A Lagrangian Remap MHD code is then adapted to simulate a plasma of arbitrary degree of ionisation. The effects of the presence of neutrals on the propagation and damping of Alfv´en waves in the solar atmosphere are investigated. Ion-neutral collisions are shown to be an efficient damping mechanism for outwardly propagating Alfv´en waves of frequencies greater than 0.1 Hz, showing that high frequency waves in the outer solar atmosphere cannot originate at the surface of the Sun. Next simulations to show the effects of neutrals on the emergence of magnetic flux from beneath the solar surface into the outer atmosphere are performed. Results from 2D and 3D numerical experiments show that the presence of neutrals increases the amount of magnetic flux that can emerge into the corona. Furthermore, ion-neutral collisions are strong enough to dissipate currents perpendicular to the magnetic field as it emerges. This shows that ion-neutral collisions are a viable mechanism for the formation of force-free (j ∧B = 0) coronal magnetic field from sub-surface field, which is not the case when the plasma is assumed to be fully ionised