Project F1:
"First-principle synthetic turbulence for energetic particle propagation in astrophysical environments"

Project Description

An understanding of charged energetic particles in turbulent magnetic fields continues to be a focus of astrophysical research, reaching from cosmic-ray transport in the solar wind, via that in the interstellar medium to that in the galactic halos and jets of active galaxies. Given the complex structure of the various turbulent astrophysical environments, combined with the fact that strong turbulence exhibits pronounced non-Gaussian structure strongly influencing the particle propagation, it is highly desirable for simulations of such transport to have an as simple as possible but, at the same time, an as comprehensive as necessary method to generate turbulent electromagnetic fields that are characteristic for a given astrophysical system.
Specifically, spontaneous alignment between flow and magnetic field leads to the formation of coherent, nearly singular structures (e.g., current sheets), which are long-lived, exhibit intense amplitudes, and break the scale-invariance of the system. They lead to power-law tails and anomalous scaling of the distribution of fluctuations, i.e., intermittency, which is observed in various astrophysical environments and has recently been discussed to play a role for cosmic-ray diffusion.
In Phase 1, we generated synthetic intermittent MHD turbulence, determined running diffusion coefficients as input for cosmic-ray transport models, and studied (super)diffusive transport in different astrophysical environments together with the A projects. It also became clear that coherent structures, such as local magnetic-field curvature, play a significant role.
In Phase 2 we extend the approach by using the so-called instanton method, this way providing a geometrically accurate representation of coherent, nearly singular structures, to address the following key questions:

1. How can synthetic turbulent fields, that exhibit the intermittent properties and coherent structures of astrophysical systems, be designed efficiently with the instanton method?
2. What are the characteristics of the transport of charged energetic particles in instanton-generated intermittent astrophysical turbulence?
3. What is the impact of nearly singular structures in the turbulence on the particle transport and related observables in cosmic-ray propagation?