Observations of Diffuse Aurora at the Geomagnetic Pole D. P. Steele, et al. (Cogger, MacDougall, McEwen, Oznovich, Weber, etc.) 1. Introduction Galperin and Feldstein (1985) proposed a classification of auroral morphology that included a faint diffuse emission region poleward of the zone of discrete arcs. They termed the emission in this poleward region "polar diffuse aurora". The observational support for the existence of this region was limited, perhaps because of the weakness of the emissions, and their location poleward of the main auroral oval. A sensitive CCD-based all-sky imaging system fielded near the geomagneic pole over the last two winters has frequently observed very faint (30 - 250 R) diffuse emission by [OI] at 630.0 nm. This report presents a survey of several cases of such emission, using multiwavelength optical observations from a ground site at Eureka, Canada (80.05 N, -86.41 E, 88.6 PACE geo- magnetic), digital ionosonde observations from the same site, near- simultaneous precipitating particle data from the DMSP F10 and F11 satellites in polar orbits over Eureka, and supporting interplanetary magnetic field (IMF) and solar wind observations from the International Solar Terrestrial Physics (ISTP) satellite WIND, located in the solar wind at the L1 Lagrangian point upstream from Earth. The characteristic features of the diffuse emissions include a distinct, roughly Sun-aligned emission boundary, a drift of the boundary in the direction of the Y component of the IMF (in the northern hemisphere) and, frequently, an association with discrete polar arcs. The cases presented here occurred during predominantly northward IMF, when the polar cap was sharply contracted along the dawn and dusk flanks (as shown by DMSP F10 crossings along the dusk - dawn meridian). 2. Observations 2.1 December 21, 1994, 2100 - 2200 UT The 630 nm images in Figure 1 are 60-s exposures taken at intervals of 2 to 3 minutes. Geographic north is to the top, geographic east is to the right, and the sun is to the lower left in these images. They show two faint arc-like features drifting dawnward (<100 R brightness overhead), followed by a broad diffuse band of emission of peak brightness 60 R behind a sun-aligned boundary. The width of the diffuse emission feature is much greater than that of the arcs preceding it, exceeding 300 km at 2148 UT. The diffuse emission drifts dawnward at about 150 m/s. Corresponding emission features are barely detectable in simultaneous images at 557.7 nm. A Canadian Advanced Digital Ionosonde located at Eureka measured a virtual reflection height at 3 MHz of 270 - 300 km during this time. The ionospheric drift was variable but typically antisunward and dawnward, consistent with the observed drift of the optical emission boundary. The interplanetary field pointed northward (Bz > 0) and dawnward (By < 0) throughout the interval, as measured by the Magnetic Fields Investigation (MFI) on the International Solar Terrestrial Physics (ISTP) spacecraft WIND. The Solar Wind Experiment (SWE) onboard WIND measured typical solar wind speeds of 450 km/s, and moderately low densities (5 cm^-3). 2.2 December 22, 1994, 0700 - 0830 UT As in the event on the preceding day, an extended diffuse region of 630.0 nm emission drifted from the dusk hemisphere toward the geomagnetic pole (see Figure 2). Faint arcs (< 200 R of 630.0 nm) were observed intermittently, both dawnward of the drifting emission region and, briefly, at its leading edge (see the image at 0742:51 UT). Between about 0729 and 0750, the leading edge of the diffuse region dimmed while continuing its slow dawnward motion. Thereafter a second "front" of emission moved dawnward. CADI data showed ill-defined drift before 0755 UT, and drift sunward and dawnward after 0755. The 3 MHz virtual height was greater than 290 km throughout the period. As in the first event, the IMF pointed northward and dawnward during this period. (Speed, density?) 2.3 December 22, 1994, 1700 - 1800 UT This event began ten hours after the previous one, occurring near local noon. A region of diffuse 630.0 nm emission drifted into the field of view, this time from the dawn hemisphere, and stopped at about 1730 UT before reaching the zenith (Figure 3). Thereafter the diffuse emission retreated slightly toward the dawn sector. A faint arc appeared beyond the leading edge of the diffuse emission from 1724 UT to at least 1749 UT. Observations were interrupted for 30 min at 1800 UT by the daily data backup. CADI data indicated a 3 MHz virtual height of 275 - 290 km, and approximately antisunward drift. The IMF pointed northward and dawnward as in the first two events. (Speed, density?) The DMSP F11 satellite flew through the Polar Camera field of view at 1658 - 1701 UT, just before the appearance of the diffuse emission at Eureka. Figure 4 shows a 630.0 nm image taken starting at 1657:53 UT, that has been mapped to geographic coordinates assuming an emission height of 250 km. Coastlines are shown, and the image has been rotated to place the solar azimuth at the top of the image. The horizontal white line shows the DMSP F11 track at 250 km altitude, with diamonds at 1-min intervals, and a thicker line showing the path of the satellite during the exposure time of the image. Between 1658 and 1659 UT the F11 satellite overflew the diffuse emission, just before it drifted into the Polar Camera field of view. (Check MSP data??) The energy-time spectrogram in Figure 5 covers the time period from 1658:00 UT to 1701:00 UT, while F11 was above the Polar Camera field of view. Weak fluxes of precipitating electrons are seen, along with essentially zero ion precipitation, similar to polar rain. The electron fluxes observed between 1658:00 and 1659:00 UT, during which time F11 was on field lines threading the diffuse emission, appear slightly higher than those observed after 1659:00. Figure 6 shows 20-s averages of the precipitating electron spectra during this 3-min period.