SIMULTANEOUS MEASUREMENTS OF O+ AND H+ TEMPERATURES IN THE TOPSIDE IONOSPHERE OVER ARECIBO

We have measured both proton (T-H+) and oxygen ion (T-O+) temperatures simultaneously from spectra obtained in the lower topside ionosphere using the Arecibo incoherent scatter radar (ISR). This is the first ti me that two ion temperatures and three ion compositions have been meas ured from the same IS spectrum, and, as far as we know, one of the fir st measurements of both ion temperatures in the ionosphere. First we d escribe our spectral-domain analysis technique, emphasizing the abilit y to determine goodness of fit, which allows of the need for and effec t of including a parameter to be numerically evaluated. We then show t hat common T-H+ and T-O+ allow insufficient freedom, and that separate temperatures are sufficient. A small negative bias in [He+] parameter is removed when the second temperature is allowed to be free while th e H+ fraction is not significantly affected. We also show that the mea surement cannot be made at altitudes where little O+ is present. The d ifference between T-H+ and T-O+ is zero (or at least very small) durin g the night, and rises to several hundred degrees at about 500 km shor tly after sunrise. During the middle of the day the temperature differ ence declines (presumably due to higher densities) and then rises agai n before sunset.