Large-scale turbulence, shocks, and charged-particle acceleration

We discuss the physics of particle acceleration by shocks moving in turbulent astrophysical plasmas. We suggest that the observed lack of agreement of energetic particles and shock properties in the heliosphere is likely to be a result of fluctuations and turbulence in the upstream fluid. Shock acceleration takes time, so the observations of particles at any given point and time at a shock reflects some combination of the history prior to the time of observation. Further, since the particles are mobile, there is spatial averaging as well. It follows that the shock properties observed by a spacecraft should not correlate well with the accelerated particles. Additionally, the upstream turbulence, and in particular, the fluctuations in plasma density have a large effect on the downstream magnetic field. These effects may also help to understand the magnetic field observed by Voyager 1 in the heliosheath.