The detailed spatial structure of field-aligned currents comprising the substorm current wedge

We present a comprehensive two‐dimensional view of the field‐aligned currents (FACs) during the late growth and expansion phases for three isolated substorms utilizing in situ observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and from ground‐based magnetometer and optical instrumentation from the Canadian Array for Realtime Investigations of Magnetic Activity and Time History of Events and Macroscale Interactions during Substorms ground‐based arrays. We demonstrate that the structure of FACs formed during the expansion phase and associated with the substorm current wedge is significantly more complex than a simple equivalent line current model comprising a downward FAC in the east and upward FAC in the west. This two‐dimensional view demonstrates that azimuthal bands of upward and downward FACs with periodic structuring in latitude form across midnight and can span up to 8 h of magnetic local time. However, when averaged over latitude, the overall longitudinal structure of the net FACs resembles the simpler equivalent line current description of the substorm current wedge (SCW). In addition, we demonstrate that the upward FAC elements of the structured SCW are spatially very well correlated with discrete aurora during the substorm expansion phase and that discrete changes in the FAC topology are observed in the late growth phase prior to auroral substorm expansion phase onset. These observations have important implications for determining how the magnetosphere and ionosphere couple during the late growth phase and expansion phase, as well as providing important constraints on the magnetospheric generator of the FACs comprising the SCW. Key Points There is evidence of a change/reduction in FAC strength preceding substorm onset First two‐dimensional observation of the relation of discrete aurora and FACs Highly structured azimuthal bands of enhanced upward and downward FAC form the SCW