On the Ionospheric Closure of Field-Aligned Currents and its M-I Coupling Implications

Abstract:

More than 50 years ago, Boström (1964) suggested two different configurations for the current system associated with an auroral arc: In ‘case 1’, field-aligned current filaments at the ends of the arc are connected by a Cowling electrojet along the arc, driven by a longitudinal (essentially E–W) electric field and a magnetospheric generator dominated by electric fields and currents in azimuthal direction. In ‘case 2’, field-aligned current sheets at the sides of the arc are connected by Pedersen current across the arc, driven by a meridional (essentially N–S) electric field and a magnetospheric generator dominated by electric fields and currents in radial direction; in this case a divergence free Hall electrojet flows along the arc. Since this early paper, written before the actual discovery of the field-aligned currents by in-situ magnetic field measurements, the ‘case 2’ configuration was found to be associated with quiet auroral arcs and, on a larger scale, with the Region1 / Region 2 current system of the quiet auroral oval. Dynamic magnetospheric phenomena, like the magnetosphere–ionosphere (M–I) coupling of bursty bulk flows or, on a larger scale, the substorm current wedge, appear to be associated rather with the ‘case 1’ configuration. In the real world, the two configurations can become entangled, either by preserving their basic features (e.g. embedded in each other, in particular during disturbed times), or by being mixed up in a new, hybrid configuration, with sheet-like field-aligned currents (‘case 2’) coupled to a divergent electrojet (‘case 1’). Observational evidence and theoretical results suggest that this hybrid configuration could be associated with the substorm growth phase, perhaps providing the transition between ‘case 2’ and ‘case 1’.