VARIATIONS OF THE Es LAYER OVER NOVOSIBIRSK IN 23 AND 24 SOLAR CYCLES

Сontinuous observations have been carried out by vertical sounding at the ionospheric station in Novosibirsk (54.84°N, 83.23°E) since the late 1960s of the XX century. The digital ionosonde “PARUS” has been installed since 1995, thanks to which an archive of continuous series of ionospheric parameters obtained by one instrument in one place was accumulated in digital form. To study the morphological features of the behavior of the ionosphere over Novosibirsk, we chosen the critical frequency and virtual layer height Es (foEs and h’es, respectively) as parameters characterizing the state of the lower ionosphere. The accumulated material made it possible to analyze the observation probabilities (PEs) of this layer in each month depending on the time of day and height of appearance for the period from 1996 to 2019, which covers the 23rd and 24th eleven-year solar cycles. There was no direct dependence on the level of solar activity, which was represented by Wolf numbers (the number of sunspots). To study the conditions for the formation of the sporadic Es formation, the statistical relationship between solar and magnetic activities in the period from 1996 to 2019 was considered. It was shown that with an unstable relationship between solar and geomagnetic activities, the appearance of a sporadic layer is more likely. The probability of the appearance of Es shows a clear dependence on the season and hour of the day. In January-February 2002, the Es layer was rarely observed (PEs about 15%), while, as in other years, this value was on average 40-45%, and in 2011 it reached 80%. In July and August, the appearance of the Es layer is almost 100%. The most probable time of occurrence of the sporadic layer falls on the period from sunrise to sunset, regardless of the month of observation, and in summer there are two peaks, when it is observed 100% - these are local daytime and pre-sunset hours, the minimum PEs occur at night hours in winter. The number of occurrences of Es in specific altitude ranges is analyzed, the time series of this value repeats the solar cycle: low Es (100-110 km) are more often observed at solar minimum, and higher ones (115-130 km) - at solar maximum. A seasonal dependence was revealed: lower sporadic layers (100–130 km) appear in greater numbers in summer, and higher Es (> 135 km) in spring and winter. The formation of the Es layer during the preparation and realization of earthquakes in the region of 200–300 km from the ionospheric station is discussed. An unambiguous response of the lower ionosphere to these events has not been revealed.