Expanding Auroral Loops

AbstractA new scenario is presented for the energy supply to the auroral acceleration process. It applies to auroral arcs, which are propagating into regions of magnetic fields with shears with lower than those existing behind the arc. This pertains in particular to expanding U‐loops or other active protrusions. A Poynting flux, emerging out of the interior of the associated current system with strongly sheared field, flows into the auroral acceleration region or fracture zone. One half of the energy is consumed by the acceleration process. The other half flows (mainly upward) into the current sheet and is expended by shearing the newly incorporated field into the direction of the internal field. This is enabled by the magnetic connectivity being broken inside the region of parallel electric potential drops. The latter are formally attributed to the presence of an anomalous resistivity in the auroral current sheet. Simple relations describe the energy transport and consumption. An important quantity is the width of the arc. It follows from the balance of the energy transport inside and out of the acceleration region. Since the process involves first breaking of the field lines, to be followed by building up shear stresses, the name “constructive magnetic fractures” has been chosen for distinguishing it from “destructive fractures,” which applies to embedded arcs. Which of these two processes is acting can be easily recognized by the direction of motion of the auroral rays or folds, whether they are opposed to or in parallel with the convective flow behind the arc.