The spatial relationship of field-aligned currents and auroral electrojets to the distribution of nightside auroras

In this paper, nearly simultaneous sets of ground-based magnetometer data, magnetic perturbations recorded at 800-km altitude by the polar-orbiting Triad satellite and auroral imagery and information on precipitating electrons in the energy range 200 eV < E < 20 keV obtained from the DMSP satellites are analyzed for intervals of moderate magnetospheric activity during which substorms were in progress. It is found that the boundaries of intense (>2 kR at 5577 Å) auroral features coincide with the boundaries of field-aligned current regions defined by the Triad data. In the morning sector the downward field-aligned current flows in the poleward portion of the auroral oval, and the intensity of this current appears not to be a function of the intensity of the related auroral features. These results suggest that the current is carried by upgoing thermal electrons from the ionosphere. On the equatorward side of the oval the region of the upward field-aligned current coincides with the bright aurora regime and is probably carried by precipitating energetic electrons. In the evening sector, regions of intense upward field-aligned currents are found to coincide with discrete auroral arcs and fluxes of precipitating electrons with energies peaked at several keV in the region immediately poleward of the eastward electrojet. This region is thought to coincide with the extension of the westward electrojet from the morning sector, and precipitating electrons in the bright auroral forms are thought to carry the upward field-aligned current in this region. In a narrow latitudinal area poleward of the upward current where no auroral luminosity is detected, downward field-aligned current is often observed, indicating the importance of ionospheric thermal electrons carrying the downward current. This current is thought to be connected to the equatorward Pedersen current of the equatorward electric field that is dominant in the westward electrojet region. In the equatorward half of the evening auroral oval the diffuse aurora is located in the region of the downward field-aligned current and, in general, collocated with the eastward electrojet.