Project Description
Diffusive shock acceleration (DSA) is one of the most promising mechanisms for accelerating cosmic rays. It is expected to operate in many astrophysical environments, such as supernova remnants, galaxy clusters and superbubbles, and it naturally produces spectra similar to the observed ones. Several aspects of DSA are, however, still not understood: how the global drivers are linked to wave-particle interactions, what role turbulence plays, and which spectral slopes and maximum energies can be reached. In this project, we combine kinetic-scale simulations with macroscopic transport models in order to follow cosmic-ray acceleration from the kinetic to the global scales. Using supernova remnants as laboratories for DSA, we will address the following open questions:
- Injection: to take part in DSA, particles first have to be pre-accelerated out of the thermal pool. This is particularly challenging for electrons, whose gyroradii are much smaller than the scale of the turbulent fluctuations. We will determine the ratio of electrons to protons injected into DSA at supernova-remnant shocks;
- Magnetic-field orientation and amplification: DSA is most efficient at parallel shocks, while drifts become important at oblique and perpendicular shocks. Moreover, observations require magnetic fields at the shock that are much stronger than in the surrounding interstellar medium. We will investigate how instabilities, such as the non-resonant Bell instability, and turbulence amplify and structure the magnetic field;
- Particle diffusion: the resulting energy spectra depend on how long particles remain close to the shock front, which is determined by the structure, the spatial profile and the intermittency of the turbulence near the shock.
Simulations of these processes are computationally very expensive. To reduce the cost, we will use, to our knowledge for the first time for these problems, semi-implicit particle-in-cell (PIC) simulations to study electron injection and two-fluid 10-moment simulations to study the dynamics of the shock front. From these local simulations, we will derive diffusion coefficients in space and momentum as well as the magnetic-field topology. These enter macroscopic transport models based on stochastic differential equations, implemented in the open-source propagation framework CRPropa and building on developments of Phase 1 in projects F1 and A3. The electron-to-proton ratios obtained for DSA will be provided to other CIM projects. In this way, the project covers the full multiscale dynamics of cosmic-ray acceleration, from kinetic to global scales.
