Particle acceleration in turbulent plasmas: colourizing the Fermi picture

Particle acceleration in magnetohydrodynamic (MHD) turbulence represents a fundamental dissipation process in a wide variety of astrophysical sources. This seminar discusses a generalization of the original Fermi scenario of point-like scatterings off discrete, moving structures to a turbulent MHD flow. The energization rate of a particle is first connected to the spatio-temporal gradients of the velocity of magnetic field lines; this notably implies that the intermittent nature of those gradients shapes the acceleration physics. This non-resonant Fermi process is then shown to account for the bulk of particle energization observed in numerical simulations (particle-in-cell and MHD) of relativistic, strong turbulence. Finally, I introduce a transport equation to describe the evolution of the distribution function in momentum space, paying proper account to the intermittent statistics of the accelerating structures. This transport model can reproduce the time evolution of particle energy distributions that are collected by particle tracking in the forced MHD turbulence of the Johns Hopkins University database.