The Morphology of the Birkeland Current System During Substorms

Abstract:

Birkeland (field-aligned) currents link the magnetopause to the ionosphere, communicating stresses through the system. In the case of substorms, field-aligned currents are enhanced as a result of cross-tail current disruption in the tail. Global-scale current maps can be obtained from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). To identify the morphology of the field-aligned currents close to substorm onset it is necessary to correctly identify the time, latitude and longitude of onset. We perform a superposed epoch analysis of the morphology of the Birkeland currents during substorms accounting for all relevant temporal and spatial boundaries. We use substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE) technique and calculate average current maps of the current systems for two hours prior, and subsequent, to onset. We bin the onsets by the latitude of the current oval at onset, and we rotate each current map by the MLT at which the substorm onset was recorded.

In this way, we investigate the spatial and temporal characteristics of the currents flowing during a substorm. We observe a large-scale structuring in the currents in the same sense as the substorm current wedge, in contrast to previously reported studies. Furthermore, we do not see any statistical signature in these current systems that is consistent with a drop in the current magnitude prior to auroral dimming, as reported in the literature. We discuss this result in the context of other recent studies of the spatial studies of Birkeland currents during substorms with which we differ and discuss reasons as to why this may be the case.