Acceleration of charged particles in solar flares by magnetic reconnection in twisted coronal loops

A coronal loop twisted by photospheric footpoint motions may become unstable to the ideal kink mode. Numerical simulations show that, in the nonlinear phase of this instability, current sheets develop, leading to magnetic reconnection and energy dissipation - this is manifest as a confined flare. The electric fields associated with these fragmented current sheets are an efficient accelerator of charged particles. Test particle simulations, coupled to 3D MHD simulations, are used to determine the time-evolution of particle populations, and show that the loop quickly fills with high-energy ions and electrons. Firstly, we consider the evolution of loops whose initial magnetic field configuration is kink-unstable, consisting of a one-dimensional twisted flux tube. Then, we model loops with initially purely-axial field, which become twisted as a result of slow footpoint motions and thus become unstable; this model also includes the effects of flux tube expansion from the footpoints to the loop apex. Results are also presented showing the effects of collisions in the denser chromospheric plasma near the loop footpoints. Thus, transport is considered along with acceleration. Properties such as energy spectra and pitch-angle distributions are calculated, as well as spatial and temporal variations of particle properties, which may be compared with data.