Publikationen
J. Lübke, P. Reichherzer, S. Aerdker, F. Effenberger, M. Wilbert, H. Fichtner, R. GrauerModeling Cosmic-Ray Transport: Magnetized versus Unmagnetized Motion in Astrophysical Magnetic TurbulenceCosmic-ray transport in turbulent astrophysical environments remains a multifaceted problem, and despite decades of study the impact of complex magnetic field geometry -- evident in simulations and observations -- has only recently received more focused attention. To understand how ensemble-averaged transport behavior emerges from the intricate interactions between cosmic rays and structured magnetic turbulence, we run test-particle experiments in snapshots of a strongly turbulent magnetohydrodynamics simulation. We characterize particle-turbulence interactions via the gyro radii of particles and their experienced field-line curvatures, which reveals two distinct transport modes: magnetized motion, where particles are tightly bound to strong coherent flux tubes and undergo large-scale mirroring; and unmagnetized motion characterized by chaotic scattering through weak and highly tangled regions of the magnetic field. We formulate an effective stochastic process for each mode: compound subdiffusion with long mean free paths for magnetized motion, and a Langevin process with short mean free paths for unmagnetized motion. A combined stochastic walker that alternates between these two modes accurately reproduces the mean squared displacements observed in the test-particle data. Our results emphasize the critical role of coherent magnetic structures in comprehensively understanding cosmic-ray transport and lay a foundation for developing a theory of geometry-mediated transport. accepted and to appear in J. Plasma Phys. (2025) 
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André Giesecke, Mike Wilbert, Ján Šimkanin, Rainer Grauer, Frank StefaniThe role of magnetic boundaries in kinematic and self-consistent magnetohydrodynamic simulations of precession-driven dynamo action in a closed cylinderWe numerically examine dynamo action generated by a flow of an electrically conducting fluid in a precessing cylindrical cavity. We compare a simplified kinematic approach based on the solution of the magnetic induction equation with a prescribed velocity field with the results from a self-consistent three-dimensional simulation of the complete set of magnetohydrodynamic equations. In all cases, we observe a minimum for the onset of dynamo action in a transitional regime, within which the hydrodynamic flow undergoes a change from a large-scale to a more small-scale, turbulent behaviour. However, significant differences in the absolute values for the critical magnetic Reynolds number occur depending on the physical properties of the external layers surrounding the flow active domain. The strong influence of the electromagnetic properties of outer layers with the large variation of the critical magnetic Reynolds number can be related to the existence of two different branches with dynamo action. In contrast to the kinematic models, the nonlinear MHD simulations reveal a small scale dynamo solution with the magnetic energy remaining significantly smaller than the kinetic energy of the flow. In irregular intervals, we observe dynamo bursts with a local concentration of the magnetic field, resulting in a global increase of the magnetic energy by a factor of 3 to 5. However, diffusion of the local patches caused by strong local shear is too rapid, causing these features to exist for only a short period so that their dynamical impact on the dynamo remains small. Physics of Fluids 37 (2025) 86607  
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J. Lübke, F. Effenberger, M. Wilbert, H. Fichtner, R. GrauerAnisotropic Cosmic Ray Transport resulting from Magnetic Mirroring and Resonant Curvature ScatteringThe transport of cosmic rays through turbulent astrophysical plasmas still constitutes an open problem. Building on recent progress, we study the combined effect of magnetic mirroring and resonant curvature scattering on parallel and perpendicular transport. We conduct test particle simulations in snapshots of an anisotropic magnetohydrodynamics simulation, and record magnetic moment variation and field line curvature around pitch angle reversals. We find for strongly magnetized particles that (i) pitch angle reversals may occur either in coherent regions of the field with small variation of the magnetic moment via magnetic mirroring, or in chaotic regions of the field with strong variation of the magnetic moment via resonant curvature scattering; (ii) parallel transport can be modeled as a Lévy walk with a truncated power-law distribution based on pitch angle reversal times; and (iii) perpendicular transport is enhanced by resonant curvature scattering in synergy with chaotic field line separation, and diminished by magnetic mirroring due to confinement in locally ordered field line bundles. While magnetic mirroring constitutes the bulk of reversal events, resonant curvature scattering additionally acts on trajectories which fall in the loss cones of typical mirroring structures and thus provides the cut-off for the reversal time distribution. Our results, which highlight the role of the magnetic field line geometry in cosmic ray transport processes, are consistent with energy-independent diffusion coefficients. We conclude by considering how energy-dependent observations could arise from an intermittently inhomogeneous interstellar medium. submitted (2025)
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Jan Friedrich, Mike Wilbert, Raffaele MarinoNonlocal contributions to the turbulent cascade in magnetohydrodynamic plasmasWe present evidence for nonlocal contributions to the turbulent energy cascade in magnetohydrodynamic (MHD) plasmas. Therefore, we revisit a well-known result derived directly from the MHD equations, i.e., the Politano and Pouquet law for the transfer of kinetic and magnetic energy in scale. We propose adding a term that accounts for nonlocal transfer and represents the influence of fluctuations from large scales due to the Alfvén effect. Supported by direct numerical simulations of homogeneous and isotropic MHD turbulence, we verify that in some plasma configurations, neglecting the additional nonlocal term might consistently overestimate energy dissipation rates and, thus, the contributions of turbulent energy dissipation potentially affecting solar wind heating, i.e., a central puzzle in space plasma physics that motivates the present work. Phys. Rev. E 109 (2024) 45208
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Jeremiah Lübke, Frederic Effenberger, Mike Wilbert, Horst Fichtner, Rainer GrauerTowards Synthetic Magnetic Turbulence with Coherent StructuresSynthetic turbulence is a relevant tool to study complex astrophysical and space plasma environments inaccessible by direct simulation. However, conventional models lack intermittent coherent structures, which are essential in realistic turbulence. We present a novel method, featuring coherent structures, conditional structure function scaling and fieldline curvature statistics comparable to magnetohydrodynamic turbulence. Enhanced transport of charged particles is investigated as well. This method presents significant progress towards physically faithful synthetic turbulence. EPL 145 (2024) 43001
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Mike Wilbert, André Giesecke, and Rainer GrauerNumerical Investigation of the Flow inside a Precession driven cylindrical Cavity with additional Baffles using an Immersed Boundary MethodIn this paper we present a numerical approach to solve the Navier-Stokes equations for arbitrary vessel geometries by combining a Fourier-Spectral method with a direct forcing Immersed Boundary method which allows to consider solid-fluid interactions. The approach is applied to a paradigmatic setup motivated by the precession dynamo experiment currently under construction at Helmholtz- Zentrum Dresden-Rossendorf (HZDR) are presented. The experiment consists of a fluid filled cylinder rotating about 2 axes which induces a precession driven flow inside the cavity. The cylinder is also equipped with baffles at the end caps with adjustable penetration depth to impact the flow. The numerical details as well as simulation results for the spin up and precession driven flow in a circular cylinder with additional baffles are presented. Phys. Fluids 34 (2022) 96607
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