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Research - Particle Acceleration
The acceleration of cosmic rays is a complex topic: A large number of objects in the universe accelerate particles to non-thermal energies. This ranges from acceleration at shocks in the interstellar medium, to acceleration at compact objects such as pulsars, to extended objects such as jets from active galactic nuclei. Therefore, there is also a wide range of acceleration mechanisms
One of the most important mechanisms discussed in astrophysics is Fermi acceleration at shock fronts. Even though the mechanism has been known in principle for years, many details are still being are still being researched. For example, the question of how particles are pre-accelerated, or how the shock front is affected by the the shock front is changed by the high-energy particles that are accelerated there.
With particle-in-cell simulations, we try to find out, for example, how particles arrive at shock fronts in the solar wind (Kinetic Simulations of Electron Acceleration at Mercury (Büchner, Kilian \emph{et al.} , 2018, Magnetic Fields in the Solar System )) are accelerated. Or how overtaking shock fronts in active galactic nuclei affect particle acceleration. (Simulating the injection of magnetized plasma without electromagnetic precursor wave (Kilian and Spanier 2018 Journal of Computational Physics )).
Another approach is the parameterised consideration of Fermi acceleration in AGN radiation models (A Numerical Model of Parsec-scale SSC Morphologies and Their Radio Emission (Richter and Spanier 2016 The Astrophysical Journal )). While this does not clarify how acceleration mechanisms work, it does allow us to find out what the radiation signatures of different mechanisms look like.
Completely different acceleration mechanisms, such as those caused by filamentation instability (The Influence of the Mass Ratio on the Acceleration of Particles by Filamentation Instabilities (Burkart, Elbracht, Ganse, and Spanier 2010 The Astrophysical Journal )) can now also be readily inferred by numerical methods.
