D‐Region Ion‐Neutral Collision Frequency Observed by Incoherent Scatter Spectral Width Combined With LIDAR Measurements

Abstract 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 near Tromsø, Norway. This study focuses on the spectral width of the incoherent scatter (IS) signal, which is a function of 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 obtained using a chemistry model, we have estimated the ion‐neutral collision frequency in 80–100 km altitudes by fitting a theoretical IS spectrum to the EISCAT‐VHF measurements. These fitted ion‐neutral collision frequencies are then compared with the standard model values obtained using MSIS neutral densities. The study shows that the current widely used model underestimates the ion‐neutral collision frequency on average by . Also, the fitted ion‐neutral collision frequencies showed large temporal variations due to neutral density fluctuations, which are absent in the MSIS model. The study demonstrates that these random neutral density fluctuations caused by atmospheric waves can heavily influence the IS spectral width measurements and, therefore, can have a significant impact on ISR analysis when fitting neutral temperatures. The study also demonstrates the presence of heavy cluster ions below 85 km. Large uncertainties in the ion mass make it further challenging to extract the neutral temperature from spectral width below 85 km. In light of these observations, the inherent limitations of inferring temperatures from IS spectral width in the MLT altitudes are discussed.