Characteristics of high-latitude auroral arcs

Abstract:

During quiet times, field-aligned currents appear frequently far poleward of the main auroral oval, causing high-latitude (polar) auroral arcs. It is well-established that they occur preferably during northward IMF conditions and their location and motion is strongly influenced by the sign of IMF B_y. The role of IMF B_x less clear: Many studies indicate a weak dependence of high-latitude currents and polar arcs on negative (positive) IMF B_x in the northern (southern) hemisphere, other studies do not show such dependence. Here, we reinvestigate three different polar arc datasets to clarify the role of the B_x component. The study shows that there exists indeed a weak correlation with negative (positive) IMF B_x for northern (southern) polar arcs and connected field-aligned currents, but the results depend strongly on the selection criteria for events that are included in the corresponding datasets.

Another unresolved issue is the direction of plasma flows along polar arcs. While most studies report of sunward plasma flow along polar arcs, there exist also observations where the plasma flow is in anti-sunward direction on at least part of the arc. With help of 73 isolated high-latitude auroral arcs, seen in DMSP measurements close to the dawn-dusk meridian, we examine the plasma flow along the arcs and in the surrounding polar cap. Our results show, a clear positive correlation exists between ionospheric plasma flow along the arc and IMF B_z but only in the sunlit hemisphere. The plasma flow along the arc is stronger and more likely to be sunward for larger IMF B_z and for a larger magnetic energy flux in the solar wind (v_swB²). On average, the flow along the arcs is more sunward than the average flow across the polar cap regions dawn and duskward of the arc. The polar cap flows are not dependent on the IMF strength or direction (except IMF B_y).