Observations of polar ion outflows

The characteristics of the polar ion outflows as observed in the topside polar ionosphere by the Dynamics Explorer Retarding Ion Mass Spectrometer are reported in this study. The study is restricted to altitudes between 1000 and 4000 km in order to focus on the phenomenon of the “classical polar wind.” Except insofar as they convect into the polar cap at these altitudes, auroral and cusp/cleft outflows are specifically excluded from this study. Using a method based on the “relative wind” of ions as seen from the moving spacecraft, averages and variances of the magnetic field‐aligned ion flux and velocity, and the species densities, are derived and binned for examination of their altitudinal, seasonal, and magnetic activity dependencies. The data set used encompasses the period from autumn 1981 through 1983, the altitude range from 1000 to 4000 km, and invariant latitudes greater than 70°. Observations of H + , He + , and O + with energies down to spacecraft potential are reported. It is found that the polar outflows are not well correlated with common indicators of solar or geomagnetic activity. The average parameters of the outflows exhibit a winter enhancement of He + flux, a winter decrease in H + flux, and a summer enhancement of the asymptotic outflow speeds of all species. The solar F 10 .7 (and hence EUV) flux is correlated with the acceleration profile of the flows, with slower flow velocities at 2500 km altitude for active solar conditions. Evidence is found that the H + flows are, at times, subsonic in the altitude range studied, in contrast to cold polar wind models.