A Transient Auroral Event on the Dayside

On 5 December, 1986, a strong perturbation lasting for about 10 minutes was observed by high-latitude magnetometers, the incoherent scatter radar at Sondrestrom, auroral photometers, and satellites. The disturbance occurred on closed field lines. A pair of field aligned currents separated in the east-west sense and moving westward (tailward) at 4-5 km/s is consistent with the data, producing a twin vortex pattern of Hall currents. We interpret this event as an impulsive penetration of solar wind plasma through the magnetopause; it was not a flux transfer event. Other events reported in the literature may have a similar explanation.