Dayside optical and magnetic correlation events

Abstract

Sudden, short‐lived dayside auroral activity is often observed equatorward of the quiescent auroral oval. Several cases from the 1996 South Pole and United States Automatic Geophysical Observatory data were examined. In all cases the optical events were accompanied by well‐correlated magnetic impulsive events. Most optical auroral activity was primarily in 630‐nm emission (soft electron precipitation presumably of plasma sheet or magnetosheath origin). Most optical events also show the presence of much shorter lived 427.8‐nm emission (harder electron precipitation with associated electron acceleration). In almost all events the keograms showed repeated poleward propagation, indicating that the event started at lower latitudes and propagated to higher latitudes. The optical emissions showed distinct periodicities, which usually correlated well with the magnetic signature. All the events began equatorward of the preexisting quiescent aurora, indicating that they initiated in the region of closed field lines. The interplanetary magnetic field Bz component prior to the events was either small or positive in most cases. The majority of the observed events were consistent with being triggered by interplanetary Bz or solar wind pressure change. For some events, no specific trigger was found.