Statistical relationships between high‐latitude ionospheric F region/topside upflows and their drivers: DE 2 observations

Abstract

A statistical analysis is conducted on the relationships among high‐latitude topside (850 – 950 km altitude) ionospheric plasma parameters and precipitating soft (≤1 keV) electron characteristics based on DE 2 satellite measurements from seven auroral zone passes. The parameters examined statistically for these relationships are 1137 independent samples of the field‐aligned ion flow velocities, fluxes, Mach numbers, densities, ion and electron temperatures, and soft electron energy fluxes and average or characteristic energies. We find that both ion upward velocities and upward fluxes are well correlated with electron and ion temperatures. Least squares fits to the data averaged in restricted bins show the following correlation coefficients: Ion upward velocity with Te, correlation coefficient r= 0.97; with Ti, r= 0.94; for ion upflux with Te, r= 0.97; with Ti, r= 0.91. The somewhat higher correlations with Te than Ti of both upflow velocities and upfluxes suggest the important role of enhanced ambipolar electric fields associated with enhanced Te, as heated by both direct collisions with the precipitating electrons as well as downward magnetospheric heat fluxes. The largest (≥1010 ions cm−2 s−1) ion upfluxes are associated with “ultrasoft” electron precipitation having average energies of ≤80 eV. Significant anticorrelations of electron (r= −0.90) and ion (r= −0.89) temperatures with the average energies of the precipitating soft electrons suggest that for the same precipitation energy flux, the lowest‐energy precipitating electrons are most effective in heating the topside thermal electrons. Finally, analysis of ion field‐aligned flow Mach numbers shows that these Mach numbers were almost always less than 0.4 and are typically less than 0.2. Such Mach number measurements suggest that low‐speed approximations in fluid transport models are usually valid for ≤1000 km altitude, even at high latitudes.