Trends in the F2-layer critical frequency from data obtained at the ionospheric station Yakutsk during the period from 1956 to 2017

A statistical analysis of vertical ionospheric sounding data from the Yakutsk station (62.01° N, 129.43° E, 57.12° MLAT) for the period from 1956 to 2017 encompassing six solar cycles has been carried out to identify long-term changes in the F2 layer of the subauroral ionosphere and their relationship with solar and geomagnetic activity. We examined variations in one of the main parameters of the ionospheric F2 layer, the critical frequency. A high correlation was found between the F2-layer critical frequency and the solar activity index F10.7. It is shown that during six solar cycles (cycles 19–24) there were negative trends in annual average F2-layer critical frequencies both at midday and at midnight. It has been revealed that foF2 trends depend on the season and time of day. Absolute values of the trends are higher in equinoctial and summer seasons. Peak negative trends are observed at midday during equinoctial months, reaching approximately –11 kHz/year.