Bifurcated Region 2 Field-Aligned Currents Associated With Substorms

We analyze the magnetosphere-ionosphere field-aligned currents associated with substorms using the Active Magnetosphere and Planetary Electrodynamics Response Experiment. We report a new phenomenon related to substorm onset: the development of a second pair of R2 field-aligned currents located equatorward of the preexisting R1/R2 currents, especially in the dusk sector. The appearance of such events favors the summer hemisphere, suggesting that ionospheric conductance modulates this phenomenon. During ongoing geomagnetic activity, the events have a quasiperiodicity of approximately 1 hr. We suggest that this phenomenon is related to subauroral polarization streams, which are strong westward flows in the dusk sector midlatitude ionosphere. This paper proposes a new mechanism that describes the formation of these current bifurcations: Consecutive particle injections into the inner magnetosphere during disturbed geomagnetic conditions cause separate partial ring currents to form, leading to the presence of distinct R2 current systems, all flowing into the R1 current.