Temporal and Spatial Development of Birkeland and Electrojet Currents

Data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) and SuperMAG were used to determine relationships between Birkeland and ionospheric current systems. A superposed epoch analysis of AMPERE current density and SuperMAG ground magnetic field measurements was performed using 325 events featuring onsets in the northern-hemisphere Birkeland currents. AMPERE current density and SuperMAG ground magnetic perturbation averages confirmed independently that the temporal and spatial development of the onsets are coincident and lead to the fully formed Birkeland and electrojet currents. Dayside Birkeland currents were clearly present prior to onset but do not correspond to significant electrojet signatures. Differences in ground magnetic signatures relative to the Birkeland currents reflect key features of ionospheric electrodynamics. While the Birkeland currents are nearly symmetric relative to midnight, the electrojet signatures indicate a westward electrojet in the morning that is twice as strong as the eastward electrojet in the evening. In addition, the eastward electrojet occurs in the downward Region 2 while the westward electrojet occurs in the upward Region 2 and extends into the downward Region 1. The difference in the pre and post midnight electrojets is consistent with a Pedersen to Hall conductance ratio that is higher in the evening Region 1 upward currents than in the morning Region 2 upward current. This indicates a difference in the precipitation energies of electrons in the upward currents. The results illustrate how systematic comparisons of ionospheric electrodynamic signatures can be used to diagnose the structure of ionospheric conductivities.