Abstract
AbstractThe 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 field‐aligned currents and wavefield in the rear of the arc.
Highlights
Ackerson and Frank (1972) discovered that the energy-time spectrogram of precipitating electrons in the evening auroral zone had the shape of an “inverted V,” that is, the energy increased to a maximum and subsequently decreased as the spacecraft passed through the precipitation band
A later survey of the electric fields measured during the same mission (S3-3) revealed that the inverted V structures were less common than single S-shaped potential structures (Mozer et al, 1980)
Far extending structured auroral arcs appearing in the evening auroral oval during and after substorms belong to the most striking auroral phenomena, in particular because of their longevity
Summary
The observation that auroral arcs were related with the precipitation of nearly mono-energetic electrons (Evans, 1968; McIlwain, 1960) led to the conclusion that acceleration by field-parallel electric fields were at work. Ackerson and Frank (1972) discovered that the energy-time spectrogram of precipitating electrons in the evening auroral zone had the shape of an “inverted V,” that is, the energy increased to a maximum and subsequently decreased as the spacecraft passed through the precipitation band. Lysak and Dum (1983) used a two-dimensional MHD model to calculate multiply reflected Alfvén waves between generator and ionosphere including anomalous resistivity where the parallel current exceeded a critical threshold They showed that the energy derived from twisting the magnetic field was mostly absorbed in the region of anomalous resistivity and dominantly converted into the energy of runaway electrons. Most important for supporting the validity of the fracture model was the observational proof that auroral arcs are not frozen into the plasma frame but have a proper motion This was achieved by simultaneous measurements of the electric field in the F region by means of an incoherent radar and optical tracing of the arc motion by Haerendel et al (1993) and Frey et al (1996). In the Conclusions Section, all modifications will be summarized with respect to their physical meaning
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