ASTROPHYSICS

The Sun not only emits light but also a constant stream of charged particles, which travel outward towards Earth and the other planets. On their way through the solar system, those particles interact with the Sun's magnetic field and also with each other, thus creating additional electric and magnetic fields. This mixture of mainly protons and electrons, together with their electric and magnetic fields, is known as the solar wind – a magnetized plasma emanating from the Sun.

Although rather close to Earth – at least on astrophysical scales – it is still cumbersome to take direct measurements in the solar wind since satellites can only cover a small fraction of the vastness of space with their studies. Therefore, many details of the complex interplay of particles and electromagnetic fields are still unknown and in the focus of current research. One specifically challenging topic is the turbulent behaviour of plasma waves, which are created as large-scale fluctuations in the electromagnetic fields of the plasma close to the Sun. By interaction of several waves these large-scale structures decay into smaller and smaller substructures via a cascade of plasma waves with decreasing wave lengths. At the smallest scales the energy of the waves is dissipated and transferred to the charged particles, which eventually leads to heating of the whole plasma.

While analytic models only describing low-frequency turbulence exist, these models fail for other kinds of plasma waves. This is where numerical simulations come into play, since they are capable of modelling the complex system of different waves, particles and electromagnetic fields self-consistently. The particle-in-cell (PiC) approach is one capable method to study turbulence in the solar wind. However, massive computing power is required and a supercomputer is crucial in order to be able to perform detailed simulations with high resolution.

The project 'Numerical Modeling of the Microphysical Foundation of Astrophysical Particle Acceleration' running on HPC system SuperMUC of LRZ deals with turbulence and particle acceleration in the solar wind. The plasma of the solar wind has different properties with regard to the turbulent cascade and this in turn changes the acceleration and transport of particles in the plasma.

The fundamental physics behind this project is the evolution of turbulence of dispersive plasma waves. In previous projects the researchers have investigated the wave-particle interaction for dispersive waves, which lead to the conclusion that PiC-codes are capable of modelling the actual physics.

The scientists have started to perform small-scale simulations that help to analyse the actual transport of energy between wave modes through a turbulent cascade. Here, the structure and the development of the electromagnetic fields is important. While the time evolution of the magnetic field, if viewed in physical space, seems like a random conglomeration of patches with varying field strength (Fig. 1), a clearer pattern can be observed in so-called Fourier space (Fig. 2), where small structures represent large length scales in physical space and the other way round. Figure 2 shows the development of a turbulent cascade from an initial distribution of only a few waves (top panel) to a wide range of waves with different amplitudes and wave lengths (bottom panel).

To understand the full picture, the researchers have to perform simulations spanning a wider range of frequencies, which makes the use of HPC systems like SuperMUC inevitable.