Stability of fast solitary structures on auroral field lines

Abstract

The stability of the fast solitary structures which were observed onboard several auroral crossing satellites is analyzed as a dynamical system and investigated numerically. These large-amplitude potential spikes are supported by trapped electron populations. For parameters of low and mid-altitude auroral passes with gyro-to-bounce frequency ratios significantly larger than unity, the potential spikes are very resilient, while for lower magnetic fields, at ratios below unity, they develop unstable undulations in the transverse direction. The evolution of the solitary structures is related to changes in the trajectories of the trapped electrons. It is shown here that the coupling of the parallel and perpendicular dynamics is stronger when the above ratio decreases, resulting in a bifurcation of trajectories. The addition of a small perturbation to the large amplitude structure leads to a very different response of the trapped electrons in the two configurations. The electron behavior reflects the lack of spike stability at small gyro-to-bounce frequency ratios.