Near-simultaneous Polar and DMSP measurements of topside ionospheric field-aligned flows at high latitudes

Near-simultaneous observations of topside O+ parallel flows are presented for four periods of measurement by the Polar and DMSP satellites during April 1996. The Polar measurements were from southern perigee measurements near 5000 km altitude, while the DMSP measurements were from 840 km altitude. In general, the velocities were upward at expected cleft and auroral latitudes, typically about 2-10 km s(-1) at 5000 km altitude, and 0-2 km s(-1) at 840 km altitude. At the highest, polar cap latitudes, downward velocities were more frequent at both altitudes, but especially at the lower 840 km altitude. The downward velocities were typically a few hundred meters per second at 840 km altitude, and 0-1 km s(-1) at 5000 km altitude. In some instances, downward velocities were observed at 840 km altitude while upward O+ flows were observed at 5000 km altitude, possibly on the same flux tube. The O+ densities were characteristically 1-10 O+ cm(-3) at 5000 km altitude and 10(3) -10(4) O+ cm(-3) at 840 km altitude, while the O+ fluxes were characteristically 10(5) - 10(7) O+ cm(-1) s(-1) at 5000 km altitude and characteristically 10(7) - 10(9) O+ cm(-2) s(-1) at 840 km altitude. We have also examined the dual-altitude parameter measurements for a polar cap field line, the Polar and DMSP measurements approximately 30 min apart, and compared them with results from a transport simulation. The simulated high-altitude velocity altitude profiles for the period during and after the initiation of the auroral processes generally bracketed the observations, but the observed downward velocities (500 - 600 m s(-1)) at 840 km altitude were much larger in magnitude than those in the transport simulation, and the simulated densities were several times higher than those observed at both altitudes.