ASTROPHYSICS

The acceleration of charged particles is still one of the most important problems in astrophysics. Cosmic rays, which mainly consist of protons, show a broad spectrum with energies up to 10²¹ eV, which can be produced in collisionless shocks. However, many questions are still open regarding the acceleration process and the process of shock formation. To study this complex process with non-linear methods, researchers used JUQUEEN to investigate different aspects of the shock formation process and further applications.

The acceleration of charged particles is still one of the most important problems in astrophysics. Cosmic rays, which mainly consist of protons, show a broad spectrum with energies up to 10²¹ eV, which can be produced in collisionless shocks. However, many questions are still open regarding the acceleration process and the process of shock formation. The universe is widely made of low-density plasma, in which a shock cannot form due to the interaction of individual particles because particle collisions are just too rare. It is rather the interaction of particles with surrounding fields that triggers the shock formation. Small deviations from an equilibrium state can generate plasma instabilities that form electromagnetic fields, which then interact back with the particles and lead to shock formation.

It is necessary to study this complex process with non-linear methods. In a project with principal investigator Anne Stockem Novo from Ruhr-University Bochum, high performance computing on JUQUEEN is used to investigate different aspects of the shock formation process and further applications.

A strong, large-scale magnetic field is required for efficient shock formation and acceleration of particles. Electromagnetic fluctuations seed a small-scale field, which is amplified by the Weibel instability [1], undergoing a non-linear merging phase which can be well studied with large-scale kinetic simulations [2,3] shown in Fig. 1.

While most works deal with an idealised symmetric system, it is important to take into account less ideal conditions for understanding the realistic, non-ideal scenario. Fig. 2 shows a 3D simulation of shear flows where the Mushroom instability (MI) occurs [4].

Collisionless shocks can also be generated in the laboratory. The power of current laser systems is high enough to reproduce collisionless processes. Numerical simulations are essential to guide experiments. Fig. 3 shows the interaction of a laser pulse with a hydrogen target (a) and the trajectories of accelerated ions to several MeV of energy (b).

References: 
[1] E. S. Weibel, Phys. Rev., 114 , 18, 1959.
[2] R. A. Fonseca, L. O. Silva, F. S. Tsung et al., Lecture Notes in Comput. Sci. 2331, 342, 2002.
[3] R. A. Fonseca, S. F. Martins, L. O. Silva et al., Plasma Phys. Controlled Fusion, 50, 124034, 2008.
[4] E. P. Alves, T. Grismayer, R. A. Fonseca and L. O. Silva, Physical Review E, 92, 021101(R), 2015.

Scientific contact:
Dr. Anne Stockem Novo
Institute for theoretical physics IV
Ruhr-Universität Bochum
Universitätsstraße 150, D-44801 Bochum (Germany)
e-mail: anne @ tp4.rub.de