Stability of fast solitary structures on auroral field lines

The stability of the fast solitary structures which were observed onboard s everal auroral crossing satellites is analyzed as a dynamical system and in vestigated numerically. These large-amplitude potential spikes are supporte d by trapped electron populations. For parameters of low and mid-altitude a uroral passes with gyro-to-bounce frequency ratios significantly larger tha n unity, the potential spikes are very resilient, while for lower magnetic fields, at ratios below unity, they develop unstable undulations in the tra nsverse direction. The evolution of the solitary structures is related to c hanges in the trajectories of the trapped electrons. It is shown here that the coupling of the parallel and perpendicular dynamics is stronger when th e above ratio decreases, resulting in a bifurcation of trajectories. The ad dition 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-t o-bounce frequency ratios. (C) 2001 Elsevier Science Ltd. All rights reserv ed.