A statistical study of evening sector arcs and electrojets Original Research Article

We present results of a statistical study of evening sector auroral arcs associated with electrojets. The study (including ∼ 1000 all-sky camera (ASC) frames) is based on data of the MIRACLE instrument network. An automatic search engine is used to define the arc periods from ASC data and the recordings of the IMAGE magnetometer network are used to estimate the strength and location of the equivalent electrojet currents. The maximum current densities and intensities of the equivalent electrojets vary in the ranges 200–600 A/km and 200–400 kA, respectively. In 85 % of cases with single arcs the current system is of a convection reversal type (an eastward electrojet equatorward of a westward electrojet). Usually the arc is within 1° distance from the latitude of maximum eastward current density which is ≤4° south from the transition region between the westward and eastward electrojets. Most of such events were observed in the dusk sector, not in the premidnight-sector, which is generally considered as the typical Harang discontinuity (HD) region of convection reversal type currents. Furthermore, the poleward boundary of the westward current is very often poleward of the northernmost IMAGE station (CGM lat. ∼76), which may mean that the westward currents are not real horizontal currents but equivalent currents due to field aligned R1 currents. This implies that a stable arc often resides a few degrees equatorward to the evening shell potential minimum. We discuss the implications of these findings from the viewpoint of previous HD-studies.