Distribution of auroral arcs during quiet geomagnetic conditions

Auroral arcs observed from the Greenland all-sky camera network during quiet intervals (AE less than 50 nT) were ordered in corrected geomagnetic latitude/time mass plots for different values of AE. With decreasing AE the pattern of oval-aligned arcs dissolves, so that for AE less than 20 nT practically all arcs belong to the so-called polar cap pattern of discrete arcs. It is demonstrated that this pattern is limited toward equator by the trapping boundary of energetic electrons which is regarded as the boundary between open and closed field lines. Toward the pole the pattern is limited by the polar precipitation boundary of electrons of average energy lower that 500 eV, although cross-polar cap arcs do occur isolated from the electron precipitation belt, especially during very low activity. Thus, the polar cap arc pattern appears to consist essentially of discrete arcs embedded in the inner low-energy electron precipitation belt. The oval arc pattern, which is more prominent for AE greater than 30 nT, is situated on the equatorial side of the trapping boundary. It forms part of the classical auroral oval, and the arcs are embedded in the outer higher-energy part of the auroral electron precipitation belt.