Optical and radar observations of the motion of auroral arcs

During a nine day observational campaign in February 1992 we used an image-intensified CCD TV camera along with the EISCAT radar to observe auroral arcs in the magnetic zenith above Kiruna. We determine the normal motions of auroral arcs near magnetic zenith and compare them with the plasma velocities measured by EISCAT. Our interest focuses on the relative motions of arcs and ionospheric plasma and the changes of tangential velocity as the plasma traverses the arcs. In all six cases, relative motions of are and plasma of the order of 200 m s(-1) are found. They are interpreted in relation to the current system to which the are belongs as a trace of an upward-directed field-aligned sheet current. In most cases the are moves so as to reduce the size and energy content of the current system, but the opposite case is also observed. There is a striking correlation between the N-S motion of arcs and plasma, but the arcs tend to lag behind.