Discrete auroral arcs generated by ionospheric feedback instability

Abstract

Currently, the hypothesis that discrete auroral arcs are produced by fluxes of magnetospheric electrons accelerated along the ambient magnetic field into the ionosphere by linear/non-linear mechanisms associated with magnetic field-aligned currents (FACs) carried by ultra-low-frequency (ULF) Alfven waves is confirmed by a large number of experimental and theoretical works. This correlation arises from the fact that dispersive Alfven waves have a component of the electric field parallel to the ambient magnetic field. This field can accelerate electrons into the ionosphere and produce some types of the discrete aurora. Two main questions studies of ULF waves and the discrete aurora seek to answer are: 1) What physical mechanism(s) generates the ULF waves? 2) What parameters of the coupled magnetosphere-ionosphere system define frequency, dynamics, and spatial structure of these waves? Ionospheric Feedback instability (IFI) is one of the most self-consistent physic models that aims to answer these two questions. The basic idea of IFI is that ULF waves can be generated due to the active interaction between FAC and ionospheric plasma under some favorable conditions, for example, large electric field and low plasma density in the ionosphere.