PlasmaAstro-TOK-Seminar 2026
 

Abstract: Gamma-ray bursts (GRBs) and active galactic nuclei (AGNs) launch plasma outflows moving at nearly the speed of light from their central engines, and they create relativistic shock waves. Due to their extreme energies, these shocks are expected to accelerate particles via first-order Fermi acceleration, in which particles gain energy by repeatedly crossing the shock front, potentially explaining the origin of cosmic rays, especially ultra-high-energy cosmic rays.However, whether charged particles can indeed be efficiently accelerated at relativistic shocks remains under debate. A key condition for such acceleration is the presence of strong magnetic turbulence around the shock, which allows particles to be scattered and cross the shock multiple times. Yet, the physical mechanism that generates such intense turbulence is still poorly understood.In this study, we investigate a scenario where turbulence is driven downstream of the shock through the interaction between the shock front and density fluctuations pre-existing in the upstream medium. We perform 3D relativistic magnetohydrodynamic (MHD) simulations to model the turbulent structure, followed by test-particle simulations to trace particle trajectories in the resulting fields.Our results show that strong magnetic turbulence develops in the downstream region, where the magnetic field is amplified by the turbulence through the small-scale dynamo, leading to efficient shock acceleration by particle scattering.In my talk, we will discuss the conditions under which particle acceleration occurs, the nature of acceleration within magnetic turbulence, and the properties of the generated turbulence in comparison with GRB observations. [mehr]

Abstract: Understanding the complex transport of particles in turbulent plasmas is of great relevance in various fields. In astrophysics, the diffusive transport of high-energy particles is often described in an ensemble-averaged way, employing a transport equation that describes the time evolution of the particles distribution function in space and momentum. The standard transport equation can also be re-written into a set of stochastic differential equation. This allows for a Monte-Carlo approach of solving for the particles distribution function, which has been successfully applied to particle transport in the heliosphere, diffusive shock acceleration and recently to shear acceleration. Furthermore, the stochastic model allows for an extension to Levy flights, modeling non-Gaussian diffusion. Particle transport with such underlying power-law jump length distributions cannot be represented by a normal transport equation, but by those including fractional derivatives. While solving fractional differential equations is difficult, stochastic models provide an elegant way to solve for the particles distribution function.In this talk, I will discuss the connection between Fokker-Planck equations and stochastic differential equations, show applications to particle transport and acceleration and give a brief outlook to Levy flights. [mehr]

Abstract: The Rayleigh–Taylor instability (RTI) is a key mechanism driving mixing and structure formation in magnetised astrophysical plasmas. In many environments of the interstellar medium the gas is only partially ionised, so that ions and neutrals can drift relative to each other and exchange momentum through collisions. The linear behaviour of the magnetised RTI in this regime has remained poorly characterised.In this talk I present a two-fluid study of the magnetised RTI in partially ionised plasmas. I first derive the linear dispersion relation including ion–neutral collisional coupling. The analysis shows how the growth rate depends on the collision frequency, ionisation fraction and magnetic field orientation, and recovers the ideal MHD and hydrodynamic limits.I then compare these predictions with high-resolution two-fluid simulations performed with the MPI-AMRVAC code. The numerical results reproduce the theoretical growth rates over a wide range of coupling regimes.Finally, I discuss the nonlinear evolution of the instability. The simulations show that ambipolar diffusion redistributes part of the kinetic energy into ion–neutral drift and modifies the morphology of the mixing layer, producing smoother interfaces and reduced small-scale structure compared to ideal MHD. [mehr]