Abstract
The magnetosphere-ionosphere system associated with the nighttime auroral region is considered for quiet conditions, and it is shown that in the presence of an electric field an ionospheric electron density perturbation that is narrow in latitude and broad in local time will grow exponentially in time, driving an ac current through the magnetosphere-ionosphere system. Under favorable conditions a field-aligned current-driven instability is excited in the lower magnetosphere, and associated wave-particle interactions give rise to pitch angle scattering of energetic magnetospheric particles and an anomalous resistivity along field lines. In a region of upward field-aligned current the pitch angle scattering and the field-aligned electric field (arising from anomalous resistivity) lead to an enhanced flux of precipitating magnetospheric electrons and an energization of these electrons that in turn give rise to an enhanced auroral electrojet current and a nearly coincident visible auroral arc.
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