THE CRITICAL FREQUENCY OF THE IONOSPHERIC F2-LAYER AS OBTAINED FROM IONOSONDE DATA AND OBSERVATIONS OF SOLAR RADIO BURSTS

Subject and Purpose. Studying the time variations shown by the critical frequencies of the ionospheric F2 layer through comparative analysis of ionosonde data and observations of type III solar radio bursts. Methods and Methodology. In this work, two independent methods have been used for identifying critical frequencies in the ionosphere, namely that of vertical sounding and observations of type III solar radio bursts near their cut-off frequency in the ionosphere. One of the ionosondes used for vertical sounding was located near Zmiiv (Kharkiv Region), rather close to the UTR-2 radio astronomy observatory where the solar bursts were observed. The radiation from such bursts represented probe signals for transmissive sounding. The solar radiation was received with an element of a low-frequency (1 to 40 MHz) antenna array. Results. On May 22, 2021 variations in the critical frequency f0F2 of the ionospheric F2-layer were followed between 07:00 and 17:00 UT. The value reached a maximum of 5.9 MHz at 07:45 to 08:00 UT and then decreased smoothlyto 4.9 MHz, stayіng there from 15:30 till 16:45 UT. At that time, a storm of type III solar bursts was recorded with the antenna for radio observations at 1…40 MHz, revealing a cut-off effect for the bursts. As has been found,their cut-off frequency can be used for estimating the critical frequency f0F2 in the ionosphere. Conclusions. The comparative analysis of solar burst observations and frequency-and-time measurements with an ionosonde has shown possibilities for evaluating the critical frequency f0F2 in the ionosphere from the data on the cut-off frequency for solar radio-frequ