Using AMPERE to Identify and Characterize Onsets of Birkeland Currents

Abstract:

Data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) was used to study the development of the global Birkeland currents. The integrated total current was used to identify onsets of Birkeland currents preceded by periods of quiescence. Onset events were identified automatically by requiring quiescence for three hours preceding onset, an increase of a specified total current within 40 minutes, and sustained enhanced total current for 30 minutes. Three ranges of total current increase were chosen representing equal numbers of events: 1 to 1.2 MA; 1.2 to 1.5 MA, and greater than 1.5 MA. A Fermi function fit to the total current was used to identify an objective onset time used in superposed epoch analysis. The current onsets are closely correlated with enhancements of magnetospheric forcing as indicated by the interplanetary magnetic field (IMF) BZ and the solar wind electric field. The dayside current quiescent baseline is seasonally dependent but the time profile preceding and through onset is consistent for all seasons. The average nightside total current baseline and onset profile are independent of season. The nightside onset is delayed relative to the increase of the dayside current and is 2.8 times more rapid than the dayside current increase. The fit AMPERE magnetic perturbations and radial currents were averaged to determine the average spatial development of the currents. While the dayside patterns of Birkeland currents depend on the IMF as expected for convection dependence on IMF BY, the nightside current distributions develop consistently regardless of the sign of the IMF BY and display a persistent Harang discontinuity signature. The analyses reveal that the development and intensification of nightside currents corresponds to strengthening of the currents at all local times. Unexpectedly, the strongest nightside onset occurs first in the post-midnight sector and not near the average sub-storm onset local time.