DPYcurrents in the cusp/cleft region: A crucial role of southward interplanetary magnetic field

Geomagnetic variations observed at the meridional chain of stations in Greenland during 1982–1992 are utilized for the study of relationships between occurrence of azimuthal currents in the dayside cusp/cleft region and changes in the interplanetary magnetic field (IMF) By and Bz components. Events were selected for analysis if (1) the Greenland West Coast stations were located near magnetic noon ∼ 1400 UT and (2) the absolute values of the IMF Bz were less than By. Sixteen events were selected under these conditions. The analysis shows that intensive azimuthal currents develop in the cusp/cleft region in association with the southward IMF despite the fact that the IMF By component had the same orientation during the few previous hours. The direction of these currents (westward or eastward) is specified by the sign of the By component (negative or positive, respectively). If the IMF By is not zero but Bz is mainly northward, no significant currents are observed in the cusp/cleft region at all. As a result, because magnetic substorms usually follow extended periods of southward Bz, the dayside cusp/cleft magnetic perturbations often precede substorm activity in the midnight sector of the auroral zone. This can simply be a connection between the dayside cusp/cleft and nighttime auroral electrojets and therefore could be used as the dayside substorm precursors.