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 occurrence ofa SAR arc which was observed continuously through an 8-hour 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 ionospheric response. The arc was colocated with a deep, narrow plasma trough and a region of enhanced westward plasma convection of similar width simate3 immediately equatorward of the low-latitude extent of plasma sheet particle precipitation. A region of low-energy ion precipitation was observed atthe equatorward e ge of the SAR arc during aperiod of spatial/temporal coincident satellite/radar observations near the Millstone Hill longitude. The width of the SAR arc and related phenomena was of the order of 2 ø, and the ~200-R emission was associated with an electron temperature of ~3500øK and a 10x reduction f plasma density atan altitude of450 kin. The best-fit model for the emission i tensities of both the SAR arc and the background airglow suggests that either the electron temperature atthe center of the SAR arc was somewhat higher than observed by the radar (~4000øK), or the neutral densities, [0 2] and [O], were increased by factors of 2 and 4, respectively, with respect tothe MSIS values. The ionospheric t ough and a colocated region of enhancext sunward convection (500 1700 m s -1) were observed in conjugate hemispheres throughout he local time range 18 02 MLT. The 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 longduration observations of a colocated SAID/SAR arc event. A narrow ionospheric trough developed during the interval when the SAID velocity was >1000 m s -• and was accompanied by a weak (100 R) 630.0-nm emission. As the velocity fell to -700 m s -1, the density in the trough recovered somewhat, and the arc intensity rose to -300 R above background. This brighter period of the SAR arc occurred within a fossil trough/SAID. We conclude that there is a close spatial and temporal association among these three types of subauroral low-altitude phenomena the SAR arc, the SAID even[ and the fossil (convection-related) troughand that this is indicative of the interrelationship of the magnetospheric processes and boundaries which are involved in their formation.