Statistical analysis of F region and topside ionospheric ion field‐aligned flows at high latitudes

Abstract

A statistical examination is made of ionospheric ion field‐aligned flow velocities, ion densities, and electron and ion temperatures as measured at high latitudes in the Northern Hemisphere with the Dynamics Explorer (DE) 2 spacecraft in the 300–1000 km altitude range. We explore the characteristic trends and morphology of these field‐aligned ion flows in terms of their variations with invariant latitude, altitude, solar zenith angle, plasma temperatures, and geomagnetic activity. Enhancements in the electron and ion temperatures, field‐aligned ion upflows, and ion densities are frequently observed above 600 km over the statistical cleft and auroral oval regions and over wide latitudes on the nightside. Intermittent, subsonic auroral upflows often reach bulk speeds of up to several hundreds meters per second. Upfluxes over the statistical auroral oval are generally in the range 108–109 ions cm−2 s−1, and occasionally up to 1010 ions cm−2 s−1. The nocturnal upflows have lower densities than those on the dayside. Statistical altitude profiles indicate that upflows accelerate above 450 km on the dayside, while detectable nightside upflows start in the lower altitudes. Upfluxes are more closely correlated to the electron temperatures than to the ion temperatures. The topside ion upflow fluxes generally are uncorrelated with Kp index. Downflows are more frequent over the statistical polar cap, especially above 600 km. The electron temperatures are usually 1000 K higher than the ion temperatures on the dayside; on the nightside the mean electron temperatures are comparable or occasionally even lower than those of the ions. The mean day‐night temperature asymmetry is more than 1000 K for electrons and is smaller for the ions. Plasma temperatures generally increase with Kp index. Field‐aligned ion flows of 0–100 m s−1 dominate the high‐latitude F region (300–500 km altitude), and are mostly down‐ward on the dayside but more consistently upward on the nightside. Density enhancements are frequently observed in the dayside statistical auroral region. Density depletions of about 30% are present in the nightside statistical auroral F region, and the depletions deepen with increasing magnetic activity.