Coexistence of two substorms in the midnight sector

This report presents clear evidence that the expansive phase of a substorm and the recovery phase of the previous substorm can coexist in the same local time sector. This evidence is demonstrated by the fact that a new auroral bulge develops rapidly within the bulge of the previous substorm. It is also found that during the expansive phase of the first substorm (namely, when the usual poleward expansion of auroras was taking place in the equatorward half of the auroral oval) an equatorward shift of auroras was observed in the poleward half of the oval. This phenomenon occurred when the interplanetary magnetic field had a large southward component. Characteristics of the development of the auroral electrojets and of the corresponding mid-latitude magnetic signatures are also examined