Birth and Life of Auroral Arcs Embedded in the Evening Auroral Oval

We report on all-sky camera observations at Ft. Simpson during the crossing of the FAST spacecraft on 09 March 2008 at about 19:00 MLT. FAST registered eight auroral arcs with the associated upward currents and two periods of downward currents during the crossing time of five minutes. All arcs were moving equatorward with speeds near 300 m/s. Some of them exhibited local broadening and subsequent unfolding. Most remarkable was the appearance of new arcs at the poleward border of the auroral oval, clearly marked by an Alfvénic arc. The FAST data on energy and energy flux of the precipitating electrons and the jumps of the transverse magnetic perturbation field through the arcs were evaluated for five of the arcs following the formalism of Haerendel [2007]. This led to very consistent values for the integral wave impedance, field-parallel conductance, Alfvénic transit time, arc width, proper motion, and total energy release including the ionospheric dissipation. The most significant result is that all equatorward motions of the arcs were consistent with being proper motions in the rest frame of the ambient plasma. This is observational evidence for the arcs feeding on the magnetic energy liberated by the release of shear stresses in a region of dominantly upward field-aligned currents.