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

摘要

We document the activationof transient polar arcs emanating from the cusp within a 15 min longintermediate phase during the transition from a standard two-cell convectionpattern, representative of a strongly southward interplanetary magnetic field (IMF),to a "reverse" two-cell pattern, representative of strongly northwardIMF conditions. During the 2–3 min lifetime of the arc, its base in the cusp,appearing as a bright spot, moved eastward toward noon by ~ 300 km. As the arcmoved, it left in its "wake" enhanced cusp precipitation. The polararc is a tracer of the activation of a lobe convection cell with clockwisevorticity, intruding into the previously established large-scale distortedtwo-cell pattern, due to an episode of localized lobe reconnection. The lobecell gives rise to strong flow shear (converging electric field) and anassociated sheet of outflowing field-aligned current, which is manifested by thepolar arc. The enhanced cusp precipitation represents, in our view, theionospheric footprint of the lobe reconnection process.Magnetospheric physics (auroralphenomena; magnetopause, cusp, and boundary layers; plasma convection)