Dynamics of the aurora and associated convection currents during a cusp bifurcation event

We report the detailed two‐dimensional evolution of a transient auroral bifurcation in the cusp region which occurred during an interval of eastward (By>0) oriented interplanetary magnetic field (IMF). The actual auroral bifurcation is characterized by the near‐simultaneous activation and intensification of two latitudinally separated auroral forms which both expanded westward across the 1200 MLT meridian, after their initial appearance in the postnoon sector. The northernmost form (called type 2) is typical of periods of northward IMF orientation while the southernmost form (called type 1) represents a continuation and intensification of a type of auroral activity which is characteristic of intervals of southward IMF orientation. The auroral bifurcation is commonly observed when the IMF is in an intermediate regime of clock angles (∼45°–90°), i.e., when By is the dominating IMF component. We explain the transient cusp bifurcation in terms of a short‐lived activation of high‐latitude reconnection and an associated lobe cell convection/current system and type 2 aurora embedded in a long interval of active low‐latitude reconnection/type 1 aurora.