A long-term trend in the F2-layer critical frequency as observed at Alma-Ata ionosonde station

Abstract

In this study, we combine monthly median values for the F2-layer critical frequency (foF2), measured at Alma-Ata ionosonde station [43.25°N, 76.92°E] between 1000 and 1400 (local time), with historical data on the monthly mean values for solar radio flux at 10.7 cm (F10.7) and the geomagnetic activity index (Ap) (available at http://www.swpc.noaa.gov/), over the period from 1957 to 2012. These data are used to derive long-term trends in the upper ionosphere and to discuss their importance in the context of coupling between solar and geomagnetic activity in the ionosphere at middle latitudes.