Ions in the F region ionosphere at 150-400 km altitude consist mainly of molecular NO+ and O2+, and atomic O+. Incoherent scatter (IS) radars are sensitive to the molecular-to-atomic ion density ratio, but its effect to the observed incoherent scatter spectra is almost identical with that of the ion temperature. It is thus very difficult to fit both the ion temperature and the fraction of O+ ions to the observed spectra. In this paper, we introduce a novel combination of Bayesian filtering, smoothness priors, and chemistry modeling to solve for F1 region O+ ion fraction from EISCAT Svalbard IS radar (75.43° corrected geomagnetic latitude) data during the international polar year (IPY) 2007-2008. We find that the fraction of O+ ions in the F1 region ionosphere is controlled by ion temperature and electron production. The median value of the molecular-to-atomic ion transition altitude during IPY varies from 187 km at 16-17 MLT to 208 km at 04-05 MLT. The ion temperature has maxima at 05-06 MLT and 15-16 MLT, but the transition altitude does not follow the ion temperature, because photoionization lowers the transition altitude. A daytime transition altitude maximum is observed in winter, when lack of photoionization leads to very low daytime electron densities. Both ion temperature and the molecular-to-atomic ion transition altitude correlate with the Polar Cap North geomagnetic index. The annual medians of the fitted transition altitudes are 14-32 km lower than those predicted by the International Reference Ionosphere.

We have carried out a statistical study of neutral atmospheric parameters in the mesosphere lower thermosphere (MLT) region, by utilizing simultaneous measurements from the EISCAT VHF radar and sodium LIDAR collocated at  Tromsø , Norway. This study focuses on the incoherent scatter (IS) spectral width, which is a function of the ion-neutral collision frequency, ion temperature, (equal to neutral temperatures in the D-region), and ion mass. Using the neutral temperatures obtained from LIDAR, and ion mass estimated using a chemistry model, we have measured the ion-neutral collision frequency in the 80-100 km altitudes by fitting the spectral width. The study shows that the current widely used formulae underestimate the ion-neutral collision frequency on average by 1.53\(\pm\)0.24 in comparison to the measurements. Also, the measured collision frequencies showed large temporal variations due to neutral density fluctuations, indicating the presence of atmospheric waves. The amplitudes of these waves are found to be as large as 50% of the background densities. This suggests that individual spectral width measurements are likely influenced by these random neutral density fluctuations, which can have a significant impact on the IS temperature fits. In addition, for altitudes below 85 km, the ion mass increases drastically indicating the presence of heavy cluster ions. The dominance of heavy ions makes it further challenging to extract the temperature values from the spectral width at these altitudes. In light of these observations, the inherent limitations of inferring temperatures from IS spectral width in the MLT altitudes are studied.