Ion Acceleration at Quasi-Parallel Bow Shocks: Linking Observations, Theory and Modelling

Abstract:

Shock fronts are known to be efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are frequently seen upstream of the bow shock. This energization is generally attributed to diffusive shock acceleration, however, for this process to become effective it is first required that the ions undergo an initial acceleration. How and where this acceleration takes place has not yet been determined, and it is one of the key unsolved issues in shock acceleration theory. There are a few proposed models, based for example on resonant wave-particle interaction or particle scattering from the quasi-perpendicular shock section, but there is yet no consensus and there is a lack of observational validation for the models. We here investigate the nature of these acceleration processes for a series of Cluster spacecraft crossings of the quasi-parallel bow shock. Focus is put on detailed observations of coherent beams within the turbulent transition region and their coupling to magnetic field pulsations and structures. By comparing the observations against predictions from theoretical models as well as with 3-D hybrid simulations, we can evaluate the potential source processes involved in the acceleration, and begin to understand the origin of the variety of magnetic and particle signatures seen in the data.