AURORAL SIGNATURE OF LOBE RECONNECTION

We report specific changes in the dayside auroral morphology in the wi nter hemisphere which occur in response to sharp transitions between n orthward 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 sw itch in the location of the 630.0 nm aurora: the cusp aurora situated at approximate to 74 degrees MLAT disappeared and another form this ti me situated at approximate to 77 - 78 degrees MLAT appeared simultaneo usly (within 1 min.). We suggest that the lower- and higher-latitude a uroras correspond to injections of magnetosheath plasma associated wit h, respectively, magnetic reconnection at low and high magnetopause la titudes. They may be called cusp/LLBL and cusp/mantle auroras, respect ively. According to this interpretation the cusp/mantle aurora thus co rresponds to reconnection tailward of the cusp, the so-called lobe rec onnection. The auroral signature is observed to last for a few tens of minutes, indicating that lobe reconnection can occur in a quasi-stead y mode. During the 17 December 1992 case event sunward plasma convecti on in the polar cap was inferred from magnetometer records obtained du ring the period when the high-latitude aurora occurred.