On the proper motion of auroral arcs

The ability of the tristatic UHF radar of EISCAT to measure plasma velocities in the F region has been employed for the assessment of such motions in the proximity of and relative to auroral arcs. This required real-time tracking of the arcs with the radar upon command from the ground station monitoring the aurora. Slow relative motions of arc and plasma of the order of 30-90 m s−1 were deduced from the observations, but the error sources are considerable. Three cases are presented, representing different situations. The arc may move into an auroral current system or out of it. The first case corresponds to an energy reduction, the second to a buildup of the energy content of the current system. Oscillatory motions, alternating between these situations, exist. The oscillations lie in the Pc 5 range and testify for an interaction of the energy release region (auroral acceleration region) and the generator via shear Alfven waves. The events are interpreted in the light of the “fracture model” of auroral arcs developed by the first author. Although relative motions of the observed magnitude are predicted by this theory, other explanations are conceivable.