Field line resonance interference model for multiple auroral arc generation

Abstract

A simple model for the generation of poleward moving multiple auroral structures in the nightside ionosphere is described. The model is based upon the premise that field line resonance oscillations are responsible for auroral arc generation. The auroral structures are considered to be a result of interference between Alfven field line resonance oscillations on different L shells. Because of the dependence of the Alfven resonance frequency on L shell, any two neighboring L shells which oscillated initially with the same phase, after some time appear to be in opposite phase. That results in the appearance of counter‐directed electric fields at these L shells and a large field‐aligned current between them, which can produce the electron acceleration along the magnetic field and be responsible for auroral arc generation. The out of phasing between the oscillating L shells begins first at low latitudes and then propagates poleward producing poleward moving auroral arcs. The model requires that oscillations be maintained for many cycles of the waves and the source for energy is ultimately derived from magnetospheric convection in the plasma sheet. The field line resonance interference model explains periodic behavior in such auroral activity having frequencies in the Pc5 range. It is significant that the period of oscillation may be greater than that for the Alfven resonant oscillations, which implies that two separate frequencies may appear on the same field line. The poleward propagation of the auroral arcs in this model is a result of the poleward propagation of interference maxima rather than any real plasma or wave propagation.