Coordinated stable auroral red arc observations: Relationship to plasma convection

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Abstract
During the March 20‐22, 1990, magnetic storm, Millstone Hill radar and DMSP satellite observations detailed the conditions surrounding the occurrence of a SAR are 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 situated immediately equatorward of the low‐latitude extent of plasma sheet particle precipitation. A region of low‐energy ion precipitation was observed at the equatorward edge of the SAR arc during a period 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°, and the ∼200‐R emission was associated with an electron temperature of ∼3500°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 arc and the background airglow suggests that either the electron temperature at the center of the SAR arc was somewhat higher than observed by the radar (∼4000°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 region of enhanced sunward convection (500 ‐ 1700 m s−1) were observed in conjugate hemispheres throughout the 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 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 ∼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 event, and the fossil (convection‐related) trough ‐ and that this is indicative of the interrelationship of the magnetospheric processes and boundaries which are involved in their formation.