COORDINATED STABLE AURORAL RED ARC OBSERVATIONS - RELATIONSHIP TO PLASMA CONVECTION

During the March 20-22, 1990, magnetic storm, Millstone Hill radar and DMSP satellite observations detailed the conditions surrounding the o ccurrence of a SAR arc which was observed continuously through an 8-ho ur interval from dusk till past midnight in the North American sector. All-sky imaging with a 630.0-nm imager continually monitored the two- dimensional position and magnitude of the SAR arc emission while radar scans and satellite overflights measured magnetospheric inputs and io nospheric response. The arc was colocated with a deep, narrow plasma t rough and a region of enhanced westward plasma convection of similar w idth situated immediately equatorward of the low-latitude extent of pl asma sheet particle precipitation. A region of low-energy ion precipit ation was observed at die equatorward edge of the SAR arc during a per iod of spatial/temporal coincident satellite/radar observations near t he Millstone Hill longitude. The width of the SAR arc and related phen omena was of the order of 2-degrees, and the approximately 200-R emiss ion was associated with an electron temperature of approximately 3500- degrees-K and a 10x reduction of plasma density at an altitude of 450 km. The best-fit model for the emission intensities of both the SAR ar c and the background airglow suggests that either the electron tempera ture at the center of the SAR arc was somewhat higher than observed by the radar (approximately 4000-degrees-K), or the neutral densities, [ O2] and [O], were increased by factors of 2 and 4, respectively, with respect to the MSIS values. The ionospheric trough and a colocated reg ion of enhanced sunward convection (500 - 1700 m s-1) were observed in conjugate hemispheres throughout the local time range 18 - 02 MLT. Th e convection feature seen in association with the SAR arc had many of the characteristics of a subauroral ion drifts (SAID) event; we report here the first long-duration observations of a colocated SAID/SAR arc event. A narrow ionospheric trough developed during the interval when the SAID velocity was >1000 m s-1 and was accompanied by a weak (100 R) 630.0-nm emission. As the velocity fell to approximately 700 m s-1, the density in the trough recovered some-what, and the arc intensity rose to approximately 300 R above background. This brighter period of the SAR arc occurred within a fossil trough/SAID. We conclude that the re is a close spatial and temporal association among these three types of subauroral low-altitude phenomena - the SAR arc, die SAID event, a nd the fossil (convection-related) trough - and that this is indicativ e of the interrelationship of the magnetospheric processes and boundar ies which are involved in their formation.