Hot O and nighttime ionospheric temperatures

By analysing a large set of St. Santin F region Incoherent Scatter (IS) dat a for low solar activity we find nighttime ion temperature (T-i) on average to be apparently slightly higher than electron temperature (T-e). We show that this difference is a bias incurred by the IS spectral analysis procedu re owing to the neglect of H+ ions in the 300-500 km altitude region. Furth er, we show T-e and T-i to depart from the neutral temperature T-n with inc reasing altitude. We perform a simulation of ion and electron temperatures and show that the difference between T-i, T-e and T-n can be caused by the presence of a small amount of hat oxygen. Using our simulation we estimate the ratio of hot oxygen to cold oxygen density to be 0.13% on average at 40 0-km altitude for low solar activity.