Interacting loops: final proton orbit positions (colour-coded by orbit energy) passing through either the upper or lower boundary of the MHD simulations, using different snapshots of the MHD loop experiment in which two loops merge. The background colour indicates the current structure (|j|) in the midplane of the domain, at the time when the orbits are initialised (with the key to the background colour seen in the second colour bar, in non-dimensional units).