DAYSIDE HIGH-LATITUDE MAGNETIC IMPULSIVE EVENTS - THEIR CHARACTERISTICS AND RELATIONSHIP TO SUDDEN IMPULSES

On 17 December 1990 a series magnetic impulsive events (MIEs) were obs erved at high latitudes near local noon. EISCAT, situated some 5 hours of MLT away from the noon sector, detected simultaneous impulsive ele ctron density enhancements at heights between 90 and 120 km. The MIEs at noon were also associated with riometer absorption spikes. The corr elated EISCAT and riometer observations indicate that there was an elo ngated electron precipitation region some 3000 km wide stretching from local noon to morning. In close association with the impulsive electr on precipitation, VLF emissions were observed by groundbased stations in the morning side. We interpret the large scale electron precipitati on and VLF emissions as signatures of a global compression of the Eart h's magnetosphere. This is confirmed by the specific type of magnetic variations simultaneously recorded at the worldwide network of magneto meters. We conclude that the small scale MIEs with their drifting iono spheric current vortex structures can (but do not necessarily have to) occur in conjunction with large scale SIs. Moreover, MIEs and SIs hav e a common origin: the interaction of solar wind inhomogeneities with the Earth's magnetosphere. They do, however, represent different effec ts of the same primary agent.