Dynamics of the region 1 Birkeland current oval derived from the active magnetosphere and planetary electrodynamics response experiment (AMPERE)

[1] The region 1 (R1) and region 2 current systems typically form concentric rings of field-aligned currents in the polar ionospheres; we term the inner ring the R1 oval. We apply an automated fitting scheme to field-aligned current densities provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) and identify the latitude of maximum R1 current at all magnetic local times to yield the size of the R1 oval. We investigate the dynamics of the R1 oval size in response to geomagnetic activity for two cases corresponding to: repeated substorm activations with a minimally enhanced ring current; a significant ring current enhancement with multiple substorms. During the first event the dynamics of the R1 oval size reflected an expanding-contracting polar cap: during substorm growth phase dayside reconnection added open magnetic flux to the polar cap, expanding the R1 oval equatorward. Tail reconnection during the substorm expansion phase converted open into closed magnetic flux and the polar cap contracts as reflected by the poleward retreat of the R1 oval. During the period of enhanced ring current intensity the R1 oval grew to larger sizes during each substorm growth phase than it did during the other event, consistent with the suggestion that a stronger ring current stabilizes the magnetospheric tail to the onset of magnetic reconnection. The presented methodology allows AMPERE data to be condensed into a single parameter, the R1 oval size, which reflects magnetospheric dynamics and provides a convenient measure of the instantaneous magnetospheric system state in both hemispheres.