Computer simulation of Alfvén waves and double layers along auroral magnetic field lines

A plasma simulation has been developed to model interactions between inertial Alfvén waves and double layers and to investigate their relative contributions to auroral particle acceleration. We use a novel one-dimensional particle-in-cell code, with periodic boundary conditions, to model the nonlinear excitation of current-driven weak double layers via the free energy supplied by an inertial Alfvén wave. Analysis of the simulation output shows that double layers are not the agent primarily responsible for electron acceleration. Rather, the inertial Alfvén wave accelerates groups of electrons into a steepening beam as it encounters them. As the beam electrons reenter the main distribution, decelerated by anomalous resistive effects, they are replaced by electrons farther downstream. Hence, the particles do not free-stream over the length of the channel. Furthermore, this wave action persists even when the system is linearly stable to ion-acoustic modes, precluding the possibility that this behavior is brought about by the formation of ion-acoustic double layers.