MHD Turbulence and Particle Acceleration
When
Where
Dr. Andrey Beresnyak
National Research Council Fellow
Naval Research Laboratory
Outer space is filled with collisionless plasmas and energetic particles. Non-thermal emission is prominently present on the Sun. Fast particles, brought by the solar wind, penetrate the Earth's magnetosphere and ionize the upper layers of the atmosphere. The particle acceleration is often due to release of magnetic energy, as in solar flares, or due to shocks, e.g. CME shocks, or due to turbulence. In all these cases interaction with turbulence is crucial to understand acceleration. The perturbations created by the particles themselves often play a role in their scattering.
Solar flares are X-ray flashes up to 10^32 erg of X-rays emitted from solar corona, which apparently convert magnetic energy into kinetic energy of fast particles, but how this conversion happens is still a matter of debate. Recently it has been realized that quickly reconnecting current layers, which are the key culprits in the solar flares and coronal mass ejections, are likely to be turbulent, which challenged the old view that the current layer is a special location where high electric field, directed along magnetic field, accelerates particles. The common wisdom, however, was that turbulence is only able to accelerate by relatively slow second-order process. Recently we found an analytical connection between first order acceleration rate and a certain statistical property of turbulence which is non-zero in the case of turbulence driven by reconnection. If this generic mechanism is at play, it would be not surprising that non-thermal particles are ubiquitous in magnetically-dominated environments. I studied MHD turbulence driven by outside force and by reconnecting magnetic field by means of large-scale simulations. Also, I developed particle-MHD coupled code Hephaestus that addresses the issue of particles backreacting on the fluid. I will report on a recent progress in theory regarding almost all basic aspects of MHD turbulence, motivated to the great part by high-resolution numerics. This progress allowed us to proceed with observationally motivated studies.
Host: Dr. Joe Giacalone