Longitudinal variation of critical frequencies in polar F-region Original Research Article

Based on data from a network of ionospheric stations located in the range of geographic longitudes 19°–285° and invariant latitudes 53°–70°N we have investigated diurnal behaviour variations in F2-layer critical frequencies for different seasons and different levels of solar activity. The study revealed that the longitudinal effect in diurnal foF2 variations is most conspicuous in the summertime at the invariant latitude about 55–57°N and manifests itself in the shift of the foF2 maximum into the evening and night-time hours on the Yakutsk (129.6°) and Ottawa (284°) meridians in the region of westward declination. It is likely that such a behaviour of foF2 is conditioned by a change in the dynamic regime of the high-latitude ionosphere associated with the magnetic anomaly.