Radio Science

A survey of polar cap F region electron densities measured by the Sondrestrom radar


  • R. M. Robinson


The Sondrestrom incoherent scatter radar has been used to study F region electron densities in the polar cap. The data were obtained while the radar antenna was scanned in the plane of the magnetic meridian. In this mode, electron densities are measured as a function of altitude and latitude so that ionospheric boundaries can be easily identified. One boundary that is present in much of the data is a vertical wall of electron density between 80 and 200 km altitude that separates the region of aurorally produced electron density from the polar regions where auroral precipitation is absent. On the basis of this criterion we identified more than 500 radar scans during which the radar measured F region electron densities in the polar cap. The data were sorted according to magnetic activity, solar flux, invariant latitude, and magnetic local time. Besides calculating average electron density profiles, we also examined the altitude and latitude variations of the F region peak density. The peak density in the polar cap F region varies by a factor of 4 between solar zenith angles of 85° and 120°. The relative amplitude of large-scale irregularities is approximately twice as large when the F region is not sunlit. The results of model calculations agree well with the observed local time variations in F region peak density. The highest densities are found in the afternoon local time sector where enhanced plasma from the sunlit hemisphere is convected toward the nightside. The lowest densities are found in the dawn sector where the polar hole is created by a convection cell situated entirely on the nightside. These results demonstrate the importance of solar illumination and convection in determining the properties of the polar F region.