VARIABILITY OF IONOSPHERIC IONIZATION OVER EURASIA ACCORDING TO DATA FROM A HIGH-LATITUDE IONOSONDE CHAIN DURING EXTREME MAGNETIC STORMS IN 2015

We have examined longitudinal-temporal variations in ionospheric parameters over Eurasia by analyzing data from a chain of high-latitude ionosondes along a latitude circle similar to 70 degrees N (geomagnetic latitudes 58 degrees< Glat <65 degrees) in the longitudinal sector 26-171 degrees E during severe magnetic storms of solar cycle 24 in March and June 2015. To analyze the response of ionospheric ionization to geomagnetic disturbances, we have used ionosonde data on hourly average critical frequency f(o)F2 of the ionospheric F2 layer. Strong differences were observed between common peculiarities of temporal variations in f(o)F2 for the analyzed periods of magnetic storms, which are likely associated with the characteristic features of the seasonal and diurnal variations in the background high-latitude ionosphere of the given geographic region. During the main and early recovery phases of magnetic storms there were periods of blackouts of ionosonde radio signals. Differences in the character of the ionospheric response to geomagnetic dis turbances have been noted. This is probably due to seasonal features of the probability of occurrence of the ionospheric storm positive or negative phase in different seasons of the year. The trends of increasing ionospheric ionization over the vast region of Eastern, Western Siberia and Europe after the end of the extreme magnetic storm in March 2015, according to measurements from the chain of high-latitude ionosondes, may be associated with the formation of an area of increased [O]/[N-2 ] ratio over this territory. Such an increase in ionospheric ionization exceeding the background level of f(o)F2 values can be considered as a clear manifestation of the after-effect of magnetic storms.