Comparison of two types of polar cap aurora: Simultaneous obserbations with ASI and ISR at Resolute Bay, Canada

Polar cap auroras often appear in the polar cap region during the northward IMF conditions.In general, the polar cap is defined as a region of open magnetic field lines; thus, the polar cap aurora should be a phenomenon which originates from the magnetospheric lobe or solar wind.In this study, however, the term "polar cap aurora" is simply used as auroral features which appear at the polar cap latitudes.In the past studies, polar cap auroras have been roughly classified into the following two types.The one type is a single isolated arc showing relatively small time variation.This type of arc has a structure extending towards the sun and then is sometimes called as sun-aligned arc.Such quiet polar cap arcs are known to move in the dawn-dusk direction depending on the sign of the IMF By.Another type of polar cap aurora is composed of multiple arcs propagating poleward intermittently.This type of polar cap aurora is mostly observed in the dawnside polar cap.The source regions and generation mechanisms of these two types of polar cap aurora have not been revealed so far.In particular, for the latter type, its generation mechamism has not been understood at all mainly due to limitations in the time resolution of the radio and optical observations.To answer these questions, we need to directly compare the electromagnetic structure in the vicinity of these two types of arcs.In this study, we have carried out simultaneous observations of polar cap aurora by combining data from an allsky airglow imager (Optical Mesosphere Theremosphere Imagers: OMTIs) with those from incoherent scatter radar (Resolute Bay Incoherent Scatter Radar: RISR) at Resolute bay, Canada.Then, we investigated temporal evolution of the parameters of plasma in the vicinity of the arcs.Especially, we clarified the differences between an isolated single arc on the dusk side and multiple arcs propagating poleward on the dawn side.On January 6, 2013, polar cap auroras were observed at Resolute Bay during two intervals: 0820-1120 UT and 1300-1500 UT.The arc observed during 0820-1120 UT corresponds to the isolated arc on the dusk side.The optical intensity of the 630.0 nm emission from the arcs was about 300-500 R. The line-of-sight (LOS) ion velocity changed rapidly from 70 m/s to -540 m/s when the arc passed through the field-of-view (FOV) of RISR.This variation in the LOS velocity corresponds to a shear structure in the background ionospheric convection surrounding the arc.During the passage of the arc, the electron density at an altitude of 200 km increased from 10 9.5 to 10 11.1 m -3 .The IMF By was stable at about 4 nT.The arcs observed during 1300-1500 UT correspond to the poleward-propagating multiple arcs on the dawn side.The 630.0 nm emission intensity was about 500-700 R, which was relatively higher than that of the arc on the duskside.The LOS ion velocity changed from 760 m/s to -100 m/s when the arc passed through the FOV of RISR.The electron density at an altitude of 200 km increased from 10 9.4 to 10 10.8 m -3 in response to the passage of the arcs.The electron and ion temperatures increased up to 1800 K and 2500 K, respectively.For both the case of the polar cap arcs, an abrupt change was identified in the LOS ion velocity when the arcs passed through the FOV of RISR.This corresponds to the shear in the background convection closely associated with the arcs.Now we are trying to compute the 2D distribution of the convection by using the data from beams pointing toward 11 different directions.By examining the flow pattern in the vicinity of the arcs, we will discuss the differences in the electromagnetic structure between the two types of polar cap aurora.

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