Particle acceleration and transport at shocks propagating through turbulent media

The presence of energetic particles is found in many astrophysical environments, and the mechanisms by which such particles are accelerated to high energies are still not completely clear. Collisionless shocks and plasma turbulence, spectacular phenomena, are prime candidates to explain particle acceleration. I will begin by presenting some aspects of electron acceleration, considering the interaction of suprathermal electrons with supercritical, quasi-perpendicular shocks. The role of shock ripples in enhancing electron energisation will be addressed. I will briefly touch on the role of plasma turbulence in providing efficient electron acceleration.

Then I will discuss the interaction between an oblique, supercritical shock and fully developed plasma turbulence. I will present recent hybrid kinetic simulations, where a shock propagates in pre-existing, coherent turbulence. In particular, in these simulations we addressed the phase space diffusion of coherent Field Aligned Beams (FABs), typical of oblique shocks in the upstream turbulent medium, using a novel technique, relying on the coarse-graining of the Vlasov equation.

These simulations yield interesting, novel results: (i) increasing the initial turbulence level and the presence of coherent structures ahead of a shock result in an enhanced particles transport in both ordinary and velocity space, and (ii) the turbulent, parallel electric field plays a key role in particle velocity diffusion.
These results are relevant for a variety of systems, ranging from the Earth's bow shock interacting with solar wind turbulence to larger astrophysical systems, and might be crucial for the understanding of acceleration and heating processes in space plasmas.