VELOCITY OF AURORAL ARCS DRIFTING EQUATORWARD FROM THE POLAR-CAP

The drift velocity of an auroral are is compared with the component of F-region plasma velocity in the same direction for ten cases where th e are is seen to move steadily equatorward for several minutes without any major change in appearance or orientation. In most cases the two velocities are close, but on two occasions the drift velocity of the a re is much higher than the plasma velocity. From the cases studied it appears that during the growth and recovery phase of the substorm cycl e the are moves with a velocity close to the convection velocity, but during the expansion phase this is not the case.