Auroral Arcs: The Fracture Theory Revisited

摘要

The fracture theory for auroral arcs, developed by the author since 1980, compares the decoupling of the magnetic field from the ionosphere by the auroral acceleration region (AAR) with the breaking of a solid rod. In the latter elastic energy stored by the bending is converted into kinetic energy of the stress release motion. Similarly, magnetic energy stored in sheared magnetic fields is temporarily converted into stress release motions and finally transported as Poynting flux into the AAR. The fracture theory has been especially applied to arcs embedded in the convection of the evening auroral oval. The present study subjects the different steps in the fracture process to a critical analysis in the light of new physical insights. This boils down to a revision of the illustrating cartoon used in the earlier publications, without having affecting the quantitative evaluations. The first revision concerns the height extent of the AAR. It must be largely increased. The second revision introduces a nearly 2-D magnetohydrodynamics (MHD) turbulence into the state of the AAR. This is supported by high-altitude electric field data and leads to new view of auroral rays. The third revision describes the transition from the AAR to the ionosphere as structured by so-called potential fingers, which contain substantial fractions of the total field-parallel potential drop. The most important modification pertains to the average U-shaped potential of a spontaneously propagating AAR. While the leading edge of the auroral current sheet is structured by stress release motions, the reverse flow in the rear section escapes simple interpretation. It is proposed that this flow is driven by a turbulent transport of reversed momentum from front to rear in response to the incompressibility of the magnetic field in the acceleration region. This leads to a revision of the fieldaligned currents and wavefield in the rear of the arc.