Temporal variations of the ion‐neutral collision frequency from EISCAT observations in the polar lower ionosphere during periods of geomagnetic disturbances

The ion‐neutral collision frequency in the lower ionosphere (106–135 km) was estimated using data from the European Incoherent Scatter (EISCAT) radar at Tromsø, Norway during the Dynamics and Energetics of the Lower Thermosphere in Aurora 2 (DELTA‐2) campaign in 2009. Vertical component of the ion velocity was used with the ion momentum equation for the calculations. The ion‐neutral collision frequency was found to be approximately equivalent to that predicted using modeled density data. However, notable increases were found above 126.8 km during natural ionospheric heating events. A depression in calculated values was also found between 114.6 km and 126.8 km just after cessation of a heating event. This paper discusses contributions of the vertical thermospheric motion to variations of the ion‐neutral collision frequency.