Temporal and spatial dynamics of the regions 1 and 2 Birkeland currents during substorms

We use current density data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to identify the location of maximum region 1 current at all magnetic local times (MLTs). We term this location the R1 oval. Comparing the R1 oval location with particle precipitation boundaries identified in DMSP data, we find that the R1 oval is located on average within 1 degrees of particle signatures associated with the open/closed field line boundary (OCB) across dayside and nightside MLTs. We hence conclude that the R1 oval can be used as a proxy for the location of the OCB. Studying the amount of magnetic flux enclosed by the R1 oval during the substorm cycle, we find that the R1 oval flux is well organized by it: during the growth phase the R1 oval location moves equatorward as the amount of magnetic flux increases whereas after substorm expansion phase onset significant flux closure occurs as the R1 current location retreats to higher latitudes. For about 15 min after expansion phase onset, the amount of open magnetic flux continues to increase indicating that dayside reconnection dominates over nightside reconnection. In the current density data, we find evidence of the substorm current wedge and also show that the dayside R1 currents are stronger than their nightside counterpart during the substorm growth phase, whereas after expansion phase onset, the nightside R1 currents dominate. Our observations of the current distribution and OCB movement during the substorm cycle are in excellent agreement with the expanding/contracting polar cap paradigm.