SPACE-TIME STRUCTURE OF THE MORNING AURORA INFERRED FROM COINCIDENT DMSP-F6, DMSP-F8, AND SONDRESTROM INCOHERENT-SCATTER RADAR OBSERVATIONS

On rare occasions, observations from the DMSP-F6 and -F8 spacecraft an d the Sondrestrom incoherent scatter radar coincide in space. Such coi ncidence offers a unique opportunity to study temporal vs spatial vari ations on a small scale. We discuss data from one of those occasions, with observations made in the dawn sector in the presence of moderate auroral precipitation during a magnetically quiet period. The DMSP sat ellites measured vertical electron and ion flux and cross-track plasma drift while the radar measured the ionospheric electron density distr ibution and line-of-sight plasma velocities. We combine these data set s to construct a two-dimensional map of a possible auroral pattern abo ve Sondrestrom. It is characterized by the following properties. No di fference is seen between the gross precipitation patterns measured alo ng the DMSP-F6 and -F8 trajectories (separated by 32 km in magnetic ea st-west direction and some 4 s in travel time in magnetic north-south direction), except that they are not exactly aligned with the L shells . However, F6 and F8 observed minor differences in the small-scale str uctures. More significant differences are found between small-scale fe atures in the DMSP precipitation measurements and in radar observation s of the E-region plasma density distribution. These measurements are separated by 74 km, equivalent to 2.4-degrees, in magnetic longitude, and 0-40 s in time along the spacecraft trajectories (varying with mag netic latitude). Large-scale magnetospheric-ionospheric surfaces such as plasma flow reversal, poleward boundary of the keV ion and electron precipitation, and poleward boundary of E-region ionization, coincide . The combined data suggest that the plasma flow reversal delineates t he polar cap boundary, that is, the boundary between precipitation cha racteristic for the plasma mantle and for the plasma sheet boundary la yer.