Pre-onset auroral signatures and subsequent development of substorm auroras: a development of ionospheric loop currents at the onset latitudes

摘要

Substorm auroras observed on 17 January 1994 were localized within the fieldof view of an all-sky imager installed at Dawson City (DWS, 65.7°ILAT). In association with the enhancement of the anti-sunward convection inthe polar cap and the ion flux enhancement in 1–6 keV at geosynchronousaltitudes, a wave-like structure propagating equatorward to the onsetlatitudes with a high wave number in azimuth ( ~ 76, ~ 120 s) was observed 30 min prior to the activation in theequatorward latitudes. The activation of the auroras in the equatorwardlatitudes and the subsequent poleward expansion lasted for approximately6 min until a diffuse aurora formed. The auroras in the last 6 min wereisolated and localized within the field of view of DWS, from 400 km west to400 km east, and accompanied the magnetic pulse at the optical station. Themagnetic pulse is interpreted by the propagating ionospheric current loopwith a size comparable to the isolated auroras (~ 1000 km). Weconclude that the wave-like structures in the pre-onset interval relate tothe intrusion of the plasma-sheet plasmas from the tail by the convection.The plasmas from the tail eventually developed the ionospheric loop currentsat the onset latitudes, in association with the triggering of the bead-likerippling of auroras and subsequent breaking out from the onsetlatitudes.