Patrick Kilian

Emission of Type II Radio Bursts - Single-Beam versus Two-Beam Scenario

The foreshock region of a CME shock front, where shock accelerated electrons form a beam population in the otherwise quiescent plasma is generally assumed to be the source region of type II radio bursts. Nonlinear wave interaction of electrostatic waves excited by the beamed electrons are the prime candidates for the radio waves’ emission.To address the question whether a single, or two counterpropagating beam populations are a requirement for this process, we have conducted 2.5D particle-in-cell simulations using the fully relativistic ACRONYM code.Results show indications of three-wave interaction leading to electromagnetic emission at the fundamental and harmonic frequency for the two-beam case. For the single-beam case, no such signatures were detectable.

Gyrokinetic and kinetic particle-in-cell simulations of guide-field reconnection. I. Macroscopic effects of the electron flows

In this work, we compare gyrokinetic (GK) with fully kinetic Particle-in-Cell (PIC) simulations of magnetic reconnection in the limit of strong guide field. In particular, we analyze the limits of applicability of the GK plasma model compared to a fully kinetic description of force free current sheets for finite guide fields (bg). Here, we report the first part of an extended comparison, focusing on the macroscopic effects of the electron flows. For a low beta plasma (βi = 0.01), it is shown that both plasma models develop magnetic reconnection with similar features in the secondary magnetic islands if a sufficiently high guide field (bg ≳ 30) is imposed in the kinetic PIC simulations. Outside of these regions, in the separatrices close to the X points, the convergence between both plasma descriptions is less restrictive (bg ≳ 5). Kinetic PIC simulations using guide fields bg ≲ 30 reveal secondary magnetic islands with a core magnetic field and less energetic flows inside of them in comparison to the GK o...

Nonlinear Wave Interactions as Emission Process of Type II Radio Bursts

The emission of fundamental and harmonic frequency radio waves of type II radio bursts are assumed to be products of three-wave interaction processes of beam-excited Langmuir waves. Using a particle-in-cell code, we have performed simulations of the assumed emission region, a coronal mass ejection foreshock with two counterstreaming electron beams. Analysis of wavemodes within the simulation shows self-consistent excitation of beam-driven modes, which yield interaction products at both fundamental and harmonic emission frequencies. Through variation of the beam strength, we have investigated the dependence of energy transfer into electrostatic and electromagnetic modes, confirming the quadratic dependence of electromagnetic emission on electron beam strength.

The Influence of the Mass Ratio on Particle Acceleration by the Filamentation Instability

Observations indicate that several types of astrophysical sources produce relativistic jets that interact with the intergalactic medium, creating regions of counterstreaming plasma. Under these conditions the plasma is susceptible to filamentation instabilities. Analytical analysis of this environment is highly non-trivial, which leads to the extensive use of computer simulations to study these conditions and the connection to the energetic photons and particles emanating from these sources. To make simulations feasible one has to make a couple of simplifications to reduce the computational complexity to a level that is reachable with todays computers. One such simplification is the reduction of the proton mass compared to the electron mass. This project tries to assess what the lower limit of this quantity is that still allows a realistic representation of the situation in nature.

Instabilities of collisionless current sheets revisited: The role of anisotropic heating

In this work, we investigate the influence of the anisotropic heating on the spontaneous instability and evolution of thin Harris-type collisionless current sheets, embedded in antiparallel magnetic fields. In particular, we explore the influence of the macroparticle shape-function using a 2D version of the PIC code ACRONYM. We also investigate the role of the numerical collisionality due to the finite number of macroparticles in PIC codes. It is shown that it is appropriate to choose higher order shape functions of the macroparticles compared to a larger number of macroparticles per cell. This allows to estimate better the anisotropic electron heating due to the collisions of macroparticles in a PIC code. Temperature anisotropies can stabilize the tearing mode instability and trigger additional current sheet instabilities. We found a good agreement between the analytically derived threshold for the stabilization of the anisotropic tearing mode and other instabilities, either spontaneously developing or i...

Fundamental and harmonic plasma emission in different plasma environments (Research Note)

Aims: Emission of radio waves from plasmas through plasma emission with fundamental and harmonic frequencies is a familiar process known from solar type II radio bursts. Current models assume the existence of counterstreaming electron beam populations excited at shocks as sources for these emission features, which limits the plasma parameters to reasonable heliospheric shock conditions. However, situations in which counterstreaming electron beams are present can also occur with different plasma parameters, such as higher magnetisation, including but not limited to our Sun. Similar radio emissions might also occur from these situations. Methods: We used particle-in-cell simulations, to compare plasma microphysics of radio emission processes from counterstreaming beams in different plasma environments that differed in density and magnetization. Results: Although large differences in wave populations are evident, the emission process of type II bursts appears to be qualitatively unaffected and shows the same behaviour in all environments.

Plasma waves as a benchmark problem

A large number of wave modes exist in a magnetized plasma. Their properties are determined by the interaction of particles and waves. In a simulation code, the correct treatment of field quantities and particle behavior is essential to correctly reproduce the wave properties. Consequently, plasma waves provide test problems that cover a large fraction of the simulation code. The large number of possible wave modes and the freedom to choose parameters make the selection of test problems time consuming and comparison between different codes difficult. This paper therefore aims to provide a selection of test problems, based on different wave modes and with well defined parameter values, that is accessible to a large number of simulation codes to allow for easy benchmarking and cross validation. Example results are provided for a number of plasma models. For all plasma models and wave modes that are used in the test problems, a mathematical description is provided to clarify notation and avoid possible misunderstanding in naming.

Recovering the Damping Rates of Cyclotron Damped Plasma Waves from Simulation Data

Plasma waves with frequencies close to the particular gyrofrequencies of the charged particles in the plasma lose energy due to cyclotron damping. We briefly discuss the gyro-resonance of low frequency plasma waves and ions particularly with regard to particle-in-cell (PiC) simulations. A setup is outlined which uses artificially excited waves in the damped regime of the wave modes dispersion relation to track the damping of the waves electromagnetic fields. Extracting the damping rate directly from the field data in real or Fourier space is an intricate and non-trivial task. We therefore present a simple method of obtaining the damping rate {\Gamma} from the simulation data. This method is described in detail, focusing on a step-by-step explanation of the course of actions. In a first application to a test simulation we find that the damping rates obtained from this simulation generally are in good agreement with theoretical predictions. We then compare the results of one-, two- and three-dimensional simulation setups and simulations with different physical parameter sets.

Kinetic Simulations of Electron Acceleration at Mercury

In preparation of the ESA-JAXA mission Bepi Colombo we reconsidered the electron acceleration near Mercury. We first reviewed the existing observations starting from NASA’s Mariner-10 (1974–1975). Some of them later were shown to be inaccurate. Recently NASA’s Messenger mission newly observed energetic electrons including bursts of energies up to 100–200 keV. This by far exceeds the electron energies in the upstream solar wind. The acceleration mechanisms are, however, still not well understood. We derive models of electron acceleration near Mercury by passing strong interplanetary shocks, by reconnection at the magnetopause and in the Hermean magnetotail. We obtained the resulting electron energies and spectra in the near-Mercury MHD- and kinetic plasma turbulence as well as due to electric field structures by means of test particle calculations and also by fully self-consistent kinetic two- and three-dimensional PIC-code simulations whose results and, therefore, the acceleration mechanisms should be verified by the coming ESA-JAXA Bepi-Colombo mission to Mercury.

Short note: Note on the use of Yee-lattices in (semi-) implicit particle-in-cell codes

A modification of the implicit algorithm for particle-in-cell simulations proposed by Petrov and Davis (2011) [1] is presented. The original lattice arrangement is not inherently divergence-free, possibly leading to unphysical results. This arrangement is replaced by a staggered mesh resulting in a reduction of the divergence of the magnetic field by several orders of magnitude.