Acceleration of charged particles by reconnection by small solar flares in twisted loops

Solar flares produce large numbers of high energy ions and electrons. The primary energy release in solar flares is almost certainly magnetic reconnection, and the electric fields associated with reconnection are a strong candidate as a mechanism for particle acceleration. Test particle studies are a very useful tool to understanding this process, and particle acceleration in idealized steady 2D geometries has been widely studied with this approach. We extend this to consider both time-dependent and 3D fields, coupling a test particle approach with 3D MHD simulations of reconnecting fields. Time-dependent fields are used, so that the time evolution of the energy spectra and other properties can be explored. Results are presented for particle acceleration in fields arising in reconnecting current sheets which arise in the nonlinear phase of kink instability of a twisted coronal loop (Hood et al; Astron Astrophys. 506, 913 , 2009). This models small solar flares occurring in single loops. We compare behaviour in the early phase, which has a single monolithic helical current sheet, with the later phase, in which the current sheet structure is turbulent and fragmented, which allows particles to undergo multiple accelerations. In the turbulent phase, particles are accelerated throughout the loop volume, which mitigates some of the problems associated with the highly localised acceleration region postulated in the "standard flare model". We present results for the energy spectra, spatial distribution and pitch angles of the accelerated particles, and explore how these depend on the properties of the twisted coronal loop.