Incoherent scatter radar measurements of ion-to-neutral collision frequencies and neutral temperatures in the D-region ionosphere

We have analyzed all the existing European incoherent scatter (EISCAT) radar VHF measurements carried out together with the neutral temperature measurements from LIDAR collocated at Tromsø, Norway. Incoherent scatter radar (ISR) spectral parameters are estimated from the backscattered signal autocorrelation function by fitting the D-region lag profiles (pulse-to-pulse fitting). This study focuses on the ISR spectral width which is a function of the ion-to-neutral collision frequency, neutral temperature, and ion mass. Using the neutral temperatures obtained from LIDAR, we have measured for the first time the ion-to-neutral collision frequency in the mesosphere lower thermosphere (MLT) altitude range (80-100 km) by fitting the ISR spectral width. The model ion-to-neutral collision frequencies estimated using MSIS neutral densities are found to be underestimated on average by 1.5 in comparison to the measurements. Also, the ISR collision frequencies showed large temporal variations caused by neutral density fluctuations, which are absent in the model values. These large-scale neutral density fluctuations which are thought to be caused by atmospheric waves are found to have amplitudes as large as 50% of the background density. This indicates that the ISR spectral width is largely influenced by the neutral density fluctuations. In light of these observations, the inherent limitations of inferring the neutral temperatures from ISR spectral width are studied. This study showcases the challenges of estimating neutral temperatures from ISR spectral width in the MLT altitudes, which is significant in the context of the upcoming EISCAT_3D.