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 auror al UV emissions during an isolated substorm on January 9, 1997. The large-s cale upward field-aligned currents derived from the assimilative mapping of ionospheric electrodynamics (AMIE) procedure are found to generally coinci de with the relatively intense auroral emissions in the central auroral ova l, 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 brightes t, yet localized, discrete auroras shown in the UV images often lie at the boundary between upward and downward field-aligned currents, indicating tha t the current AMIE spatial resolution is unable to resolve the fine-scale f ilamentary field-aligned currents that are expected to be associated with t he localized auroral features. The initial auroral brightening at the subst orm onset occurs near the midnight region; intense auroral precipitation th en appears to shift toward dusk and to higher latitudes in the ionosphere a s the substorm proceeds. Using an improved time-dependent magnetic field mo del specified for this event, we find that the initial auroral intensificat ion 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 plasmapaus e. During the substorm expansion phase, the corresponding magnetospheric so urce 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 c onvection zone. The mapping of substorm-related field-aligned currents, on the other hand, is more confined close to the Earth, with the current densi ty peaks around geosynchronous altitude. The apparently different magnetosp heric source regions imply that the processes that produce! energetic preci pitating particles are not the same ones that generate the strongest magnet ic perturbations in the magnetosphere. The relative importance of electrost atic 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 field s at the early expansion phase, but they become less important at the late stage of the substorm expansion phase.