Behavior of Multiple Auroral Emissions during a Substorm Expansion Phase Based on Polar VIS and UVI Observations

Initial auroral brightening and poleward expansion characterize substorms. Auroral emissions in Earth's northern hemisphere were observed during 8:00-9:00 UT on 6 November 1998 by the Visible Imaging System (VIS) and the Ultraviolet Imager (UVI) onboard the Polar spacecraft. Auroral morphology, such as brightness distribution and latitudinal extent, was studied, based on images taken by several emissions: 557.7, 130.4, 391.4, and 630.0 nm for VIS and LBH long, LBH short, 130.4, and 135.6 nm for UVI. For 391.4, 557.7, and 630.0 nm auroral emissions associated with low-energy "soft" precipitating electrons, the most poleward latitude of poleward expansion reached approximately 72°-74°. For the rest of far ultraviolet (FUV) auroral emissions, the most poleward latitude of poleward expansion reached approximately 68°-70°, lower than the visible aurora. High-energy auroral particles (mostly precipitating electrons) associated with FUV aurora seem to cause the aurora to be localized, with small latitudinal expansion. On the other hand, low-energy auroral particles (mostly soft electrons) associated with visible aurora cause the aurora to be expansive and reach higher latitudes. The explanation could be that there are more high-energy electrons in the near-Earth region corresponding to lower latitudes of auroral expansion than in the tailward region corresponding to higher latitudes of auroral expansion.