On the magnetospheric source regions of substorm‐related field‐aligned currents and auroral precipitation

This paper presents a detailed analysis of field‐aligned currents and auroral UV emissions during an isolated substorm on January 9, 1997. The large‐scale upward field‐aligned currents derived from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure are found to generally coincide with the relatively intense auroral emissions in the central auroral oval, and downward field‐aligned currents are mostly in the poleward edge (and with much weaker downward currents at the equatorward edge) of the auroral oval where auroral luminosity is considerably lower. However, the brightest, yet localized, discrete auroras shown in the UV images often lie at the boundary between upward and downward field‐aligned currents, indicating that the current AMIE spatial resolution is unable to resolve the fine‐scale filamentary field‐aligned currents that are expected to be associated with the localized auroral features. The initial auroral brightening at the substorm onset occurs near the midnight region; intense auroral precipitation then appears to shift toward dusk and to higher latitudes in the ionosphere as the substorm proceeds. Using an improved time‐dependent magnetic field model specified for this event, we find that the initial auroral intensification at the substorm onset maps to the inner central plasma sheet between −5 and −7 R E , with its earthward edge located just outside of the plasmapause. During the substorm expansion phase, the corresponding magnetospheric source region of auroral precipitation moves slightly tailward and toward the dusk flank near the low‐latitude boundary layer. At the late stage of the expansion phase, bright discrete auroras map to a narrow region of tailward plasma convection that is embedded in a wide and predominantly earthward convection zone. The mapping of substorm‐related field‐aligned currents, on the other hand, is more confined close to the Earth, with the current density peaks around geosynchronous altitude. The apparently different magnetospheric source regions imply that the processes that produce energetic precipitating particles are not the same ones that generate the strongest magnetic perturbations in the magnetosphere. The relative importance of electrostatic fields and the inductive electric fields during the substorm expansion phase is also investigated. It is shown that the inductive electric fields are comparable to or even larger in magnitude than the electrostatic fields at the early expansion phase, but they become less important at the late stage of the substorm expansion phase.