Excitation of transient lobe cell convection and auroral arc at the cusp poleward boundary during a transition of the interplanetary magnetic field from south to north

We document the activation of transient polar arcs emanating from the cusp within a 15 min long intermediate phase during the transition from a standa rd two-cell convection pattern, representative of a strongly southward inte rplanetary magnetic field (IMF), to a "reverse" two-cell pattern, represent ative of strongly northward IMF conditions. During the 2-3 min lifetime of the are, its base in the cusp, appearing as a bright spot, moved eastward t oward noon by similar to 300 km. As the are moved, it left in its "wake" en hanced cusp precipitation. The polar are is a tracer of the activation of a lobe convection cell with clockwise vorticity, intruding into the previous ly established large-scale distorted two-cell pattern, due to an episode of localized lobe reconnection. The lobe cell gives rise to strong flow shear (converging electric field) and an associated sheet of outflowing field-al igned current, which is manifested by the polar are. The enhanced cusp pre cipitation represents, in our view, the ionospheric footprint of the lobe r econnection process.