Illustration of initial particle grid and electromagnetic environment in our study. a) shows the grid of initial particle positions (shown as black orbs) located in the coronal region of MHD simulations of a stable AR (Bourdin et al. 2013). Both a) and b) show interpolated magnetic field lines (grey lines) and observed magnetogram at the base of the simulations, and isosurfaces of parallel electric field E_||; a) shows opaque pink and purple surfaces of positive and negative E_|| respectively, encompassing regions of current at 25% of the peak value of E_|| in the corona, while b) also includes transparent surfaces of E_|| at 1% of the coronal peak value (~3.5 V m^-1). For reference, three green orbs labelled “A”, “B” and “C” indicate the initial positions of orbits studied in detail in Sect. 4.