Auroral arcs are longitudinally elongated regions of enhanced energy flux into the upper atmosphere in the form of accelerated electron beams and Joule heating in the lower ionosphere. Arc models commonly involve a distant source that applies electric fields (voltage generator) or field-aligned currents (current generator) on magnetic field lines that thread arcs [e.g.,Lysak, 1985]. Significant challenges stand in the way of identifying a source that can sustain observed levels of energy dissipation within stable arcs, and that can also explain their basic morphology: long, thin, often comprising multiple parallel structures, and often lasting tens of minutes with little variation [e.g., Lessard et al., 2007]. Furthermore, arcs are found over a very wide range of magnetic latitudes corresponding to regions ranging from the inner magnetosphere to the plasma sheet boundary layer and even the polar cap, posing a serious problem for any theory that is tied to a specific region or boundary, such as a pressure gradient at the inner edge of the plasma sheet, or a mid-tail reconnection line, for example. The existence of an upward field-aligned electrical current alone is not sufficient to generate an auroral arc. In addition there must be a mechanism to accelerate current carriers, and to account for downward fluxes of electromagnetic energy at levels comparable to those carried by electrons [Evans et al., 1977; Mallinckrodt and Carlson, 1985].
 This paper explores a mechanism intended to address these challenges. With this mechanism, structured enhancements of auroral energy flux (auroral arcs) result from magnetospheric convection and consequent transport of magnetic energy across field-aligned current (FAC) systems such as the Region 1/2 current systems, which can be wider than the arcs embedded within them. The concept of magnetic energy as a source of power for auroral arcs has been discussed by previous authors [e.g.,Haerendel et al., 1993] in the context of shrinking and expanding current circuits. Our study shows how magnetic energy transport can persist indefinitely in a quasi-static steady state and contribute significantly to observed levels of dissipation.
 The superposition of field-aligned currents and plasma convection was studied initially byMaltsev et al.  and Mallinckrodt and Carlson . They showed that an assumed source of FAC fixed relative to plasma convection, added to field-aligned propagation of the resulting disturbance at the Alfvén speedVA, leads to electromagnetic structures that are stationary in the source frame. These disturbances, known as stationary Alfvén waves, comprise surfaces of constant field and field-aligned current that are tilted slightly in the direction of background convection with respect toB at an angle α = tan−1(Vd/VA), where α is typically of the order of 10−4 radians. Whereas those initial studies assumed thin current sheets, Knudsen applied a two-fluid model to the interior of finite-width, drifting FACs to show they are intrinsically susceptible to structuring in density and electron energy at a scale that does not depend on the structure of an assumed source and that is not imposed by the ionosphere. Specifically, an initially unstructured FAC can structure into sheets of accelerated electron beams and density depletions under the action of cross-FAC plasma drift. The present study addresses the energetics of such a system.
 There have been a few experimental attempts to measure plasma flow across arcs and, by extension, across the current sheets in which they are embedded. Haerendel et al.  and Frey et al. used the EISCAT radar and ground-based cameras to infer cross-arc flows of the order of 100–200 m/s.Robinson et al.  reported a constant electric field of 7 mV/m tangential to an auroral arc as measured by a sounding rocket that was approximately constant across the arc; de la Beaujardiere et al. reported a similar observation using satellite and radar data. Motivated by these observations, in the following section we derive an expression for the effect of cross-current sheet flows on the energy budget of auroral flux tubes.