We have examined variations in electron density Ne at ionospheric F1-layer heights during geomagnetic storms at the Irkutsk ionospheric station (52° N, 104° E). We have selected geomagnetic storms of varying intensity as well as quiet days for each event for the winter and summer seasons of 2003. We have analyzed the electron density in the daytime during geomagnetic storms at 150–190 km heights. Different effects of geomagnetic storms on Ne in different seasons at these heights were found. There is a slight change in the electron density during summer geomagnetic storms. On the other hand, there is an interesting effect in winter indicating the summer-winter asymmetry of the Ne response to the geomagnetic storms at these heights in 2003: in winter there is a significant influence of disturbances on Ne at a height of 190 km and a smaller effect at lower heights.

We have detected an anomalous electron density Ne increase in winter months in Irkutsk in some years of the period 2003–2014. This effect was manifested when we compared the experimental values obtained by the Irkutsk ionosonde with model calculations at F1-layer heights (120–200 km). Two anomalous time zones have been found. The first was observed in the period 2003–2006 near solar minimum. In this zone, 2003 is the year of maximum manifestation of the winter Ne increase over the entire research period. The second anomalous zone — 2012, 2013, 2014 — was detected during solar maximum. We have explored possible causes of the Ne change in winter at the F1-layer heights in all the years under study. We have found that the main factor causing the winter increase in Ne is significant geomagnetic disturbances in the above time periods.

We report the results of approximation of electron density Ne array obtained with a digisonde at the high-latitude station Norilsk (69.40° N, 88.10° E) during years of declining solar activity (2003–2006). The calculations are made using the author's semi-empirical model with new coefficients calculated specifically for the station Norilsk. We obtain altitudinal changes of annual variations in daily Ne at heights of the ionospheric layer F1 (120–200 km). Approximation of experimental data describes Ne quite satisfactorily at these heights. Nevertheless, there are periods with quite pronounced deviations of model values from the experiment. The presence of significant geomagnetic disturbances during these periods is probably one of the reasons for such deviations.

We have obtained seasonal variations in relative values of the main thermospheric gas components [O]/[N₂] and [O₂]/[O] during solar maximum. We have used our method and measurements made with the Norilsk digisonde (69.4° N, 88.1° E) at heights of the ionospheric layer F1 (120–200 km) in quiet and disturbed geomagnetic conditions. We have compared [O]/[N₂] and [O₂]/[O] ratios during solar maximum with the corresponding values for the long period of solar minimum (2007–2009) in Norilsk. The relative content of atomic oxygen particles has been found to increase during solar maximum by more than 35 % in winter and autumn on quiet and disturbed days. In spring and summer, the atmosphere is enriched with molecular oxygen particles by 20 % both on quiet and disturbed days of solar maximum as compared to the conditions of solar minimum.

We have estimated seasonal variations in the main thermospheric gas components [O]/[N₂] and [O₂]/[O] for the period 2014–2017. We have used the well-known authoring technique and electron density measurements made with the Irkutsk digisonde (52° N, 104° E) at ionospheric F1-layer heights under different geomagnetic activity conditions. We have found that at these heights during geomagnetic disturbances in all seasons the molecular component of the neutral composition of the thermosphere increases and the atomic component decreases. In comparison with 2014, [O₂]/[O] values increased by 2017 under quiet and disturbed geomagnetic conditions: up to 30 % and 20 % in summer and spring respectively; up to 10 % in winter and autumn. The [O]/[N₂] ratio decreased by an average of 15 % by 2017. The assumption has been confirmed that in summer under quiet geomagnetic conditions the relative molecular oxygen content [O₂]/[O] increases with decreasing solar activity.

We report the results of the analysis of annual variations in daily electron density (N) for various solar activity conditions — minimum, rise, and maximum (2007–2014) — obtained from digisonde measurements at the ionospheric station Norilsk (69.4° N, 88.1° E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermosphere characteristics are calculated to identify regularities of these variations exactly at Norilsk station. The height changes of annual variations in the noon electron density N are obtained in the F1 region (120–200 km). The experimental data approximation describes N quite satisfactorily at these heights in the daytime of different seasons under different solar activity conditions. It is shown that in the years of solar minimum at all heights of the F1 layer the tendency remains for maximum N in summer and for minimum N in winter. In later years and in the year of maximum solar activity, a characteristic feature of the behavior of N is the change in the phase of the annual variation by 180° in the range of heights from 170 to180 km: maximum N is observed in winter; and minimum, in summer.

Using vertical sounding data obtained by the Irkutsk digisonde DPS-4 from 2003 to 2016, we have studied the frequency of occurrence of the F1 layer in winter conditions. The frequency of occurrence of the F1 layer in December–January is shown to be more than twice lower than that in February at any level of magnetic activity. At moderate and low solar activity under quiet geomagnetic conditions, the appearance of F1 layer in midlatitudes of the Northern Hemisphere may be caused by active thermodynamic processes, which lead to transformation or destruction of the circumpolar vortex at heights of the middle atmosphere. Such global dynamic changes occurring in the winter strato-mesosphere are often associated with sudden stratospheric warming events, which are accompanied by increased generation of atmospheric waves of various scales. These wave disturbances can propagate upward to the heights of the lower thermosphere and ionosphere, carrying a significant vertical flow of energy and causing variations in the composition, thermodynamic parameters of the neutral atmosphere and ionosphere.

We analyze the influence of geomagnetic disturbances on the electron density Ne at Norilsk ionospheric station (69° N; 88° E) at F1-layer heights (120–200 km). For the analysis, we have selected 25 moderate and weak geomagnetic disturbances for two seasons — spring and fall — of 2003–2014. Using the Ne values obtained from measurements made with the Norilsk digisonde during this period, we analyze Ne variations during geomagnetic disturbances in spring and fall for a long period of time. We determine the effect of spring-fall asymmetry occurring in all solar activity phases and manifesting itself in a significant decrease in the electron density during the main phase of fall storms at all heights in comparison with quiet days: up to 2.6 times at a height of 200 km and slightly less at lower heights. This phenomenon is not observed during spring disturbances: Ne variations are much weaker.

We present the results of the analysis of annual variations in daily electron density (N) at heights 140–160 km for the last solar minimum (2007–2009) obtained from digisonde measurements at the ionospheric station Irkutsk (52 °N, 104 °E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermospheric characteristics are calculated to identify regularities of these variations. We have revealed that a characteristic feature of the annual N variations during the solar minimum is a change in their phase by 180° in a relatively narrow altitude interval (170–180 km). These results and the new SEM coefficients are original and important for atmospheric and ionospheric physics.

The influence of geomagnetic disturbances on electron density Ne at F1 layer altitudes in different conditions of solar activity during the autumnal and vernal seasons of 2003–2015, according to the data from the Irkutsk digital ionospheric station (52° N, 104° Е) is examined. Variations of Ne at heights of 150–190 km during the periods of twenty medium-scale and strong geomagnetic storms have been analyzed. At these specified heights, a vernal–autumn asymmetry of geomagnetic storm effects is discovered in all periods of solar activity of 2003–2015: a considerable Ne decrease at a height of 190 km and a weaker effect at lower levels during the autumnal storms. During vernal storms, no significant Ne decrease as compared with quiet conditions was registered over the entire analyzed interval of 150−190 km.

Seasonal variations in relative values of the main gas components of the thermosphere [O]/[N2] and [O2]/[O] were obtained at the station Norilsk during the period 2003–2013. Estimates were made using a technique we devel-oped and data from the Norilsk digisonde (69° N, 88° E) for heights below the ionospheric F1-layer peak height under quiet and disturbed geomagnetic conditions in different seasons. We established that the [O2]/[O] ratio under quiet and disturbed geomagnetic conditions in all seasons by the end of 2013 decreased from 10 to 20 % relative to the values of 2003. The [O]/[N2] ratio for the same conditions increased to 17 %.

Long-term variations of [O]/[N2] and [O2]/[O] ratios of dominant gas components of the thermosphere are analyzed for 2003–2013. The variations are estimated using the technique developed by the authors and Irkutsk (52° N, 104° E) digisonde data for heights lower than the ionospheric layer F1 maximum, quiet and disturbed geomagnetic conditions, and different seasons. For summer quiet and disturbed conditions, the [O2]/[O] ratio is found to be maximum during long solar activity minimum.