Auroral signature of lobe reconnection

Abstract

We report specific changes in the dayside auroral morphology in the winter hemisphere which occur in response to sharp transitions between northward and southward‐directed interplanetary magnetic fields (IMF). In two case examples we show how a switch between large negative and large positive IMF BZ component was accompanied by a corresponding switch in the location of the 630.0 nm aurora: the cusp aurora situated at ≈ 74° MLAT disappeared and another form this time situated at ≈ 77–78° MLAT appeared simultaneously (within 1 min.). We suggest that the lower‐ and higher‐latitude auroras correspond to injections of magnetosheath plasma associated with, respectively, magnetic reconnection at low and high magnetopause latitudes. They may be called cusp/LLBL and cusp/mantle auroras, respectively. According to this interpretation the cusp/mantle aurora thus corresponds to reconnection tailward of the cusp, the so‐called lobe reconnection. The auroral signature is observed to last for a few tens of minutes, indicating that lobe reconnection can occur in a quasi‐steady mode. During the 17 December 1992 case event sunward plasma convection in the polar cap was inferred from magnetometer records obtained during the period when the high‐latitude aurora occurred.