Climatic changes in the upper ionosphere: long-term variations and trends in the critical frequency of the F2 layer in the Kazakhstan region

Abstract

The long-term variability and trends in the variations of the ionospheric F2 layer critical frequencies (foF2) obtained from ground-based vertical sounding data at the Alma-Ata station [43.25N, 77.92E] for the period 07.1958 – 04.2018 are studied. Near-midday (11–14 LT), near-midnight (23–01 LT) and hourly median values ​​of foF2 are analyzed. It is shown that both foF2 variations and variations in the solar activity index F10.7 experience a regular 11-year cyclicity, a stable negative trend over a 60-year observation interval, as well as some long-term (approximately four 11-year solar cycles) oscillation. The dependencies of the foF2 trend on the season and time of day are studied. It was found that a stable negative foF2 trend is observed for all the conditions under consideration, except for the night time of the winter season, when the foF2 trend is practically absent. The maximum negative values ​​of the foF2 trend are observed during the daytime, the nighttime values ​​are significantly lower.

Critical frequencies show a good relationship with solar activity for the entire observation period and for each season. It was found that the dfoF2 values ​​(the difference between the observed foF2 and the model (quadratic regression between foF2 and F10.7), smoothed with an 11-year (132 months) window, also generally reflect the corresponding course of F10.7. However, the position of the maximum in the time distribution of dfoF2132, the transition to the phase of decreasing values, occurs earlier (around 1980) relative to the position of the maximum of F10.7132 (around 1986), which may indicate the influence of an additional (to solar activity) source on the ionosphere, possibly an anthropogenic source.

Calculations of foF2 variability for 22-year periods before and after 1980 showed a noticeable decrease in critical frequencies in the later period (1980-2001).