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Effects from the June 10, 2021 solar eclipse in the ionosphere over Kharkiv: results from ionosonde measurements

Background. Ionospheric effects from solar eclipses (SEs) have been studied for over a century. The studies intensified in the 1960–1970, when rockets, satellites, ionosonde networks, and incoherent scatter radars became actively involved in the investigations. The following basic features of SEs displayed in the ionosphere have been established: a decrease in the electron density and the electron, ion, and neutral temperatures, changes in the dynamics of the ionosphere, and the generation of acoustic and atmospheric gravity waves. The recurring regular features are observed together with the features pertaining to each individual SE. Therefore, the study of the effects of each new solar eclipse is an urgent task.

The purpose of this paper is to present the results of analysis of the ionogram measurements acquired by the digisonde located at the Radiophysical Observatory, V. N. Karazin Kharkiv National University.

Techniques and Methodology. The observations of the state of the ionosphere were made using the digisonde specifically developed by the V. N. Karazin Kharkiv National University and located at the V. N. Karazin Kharkiv National University Radiophysical Observatory (49°38' N, 36°20' E). The data processing included the following. First, the time series of critical F2-layer frequencies, foF2, of the minimum frequency, fmin, observed on an ionogram, and of the virtual height were plotted. Second, the trends and the differences between the initial series and the trends were calculated. Third, given the foF2(t) values, the electron densities N(t) and their increments ΔN(t) were calculated, and fourth, the systems spectral analysis of ΔN(t) dependences was conducted to determine the spectral content.

Results. A maximum decrease of 3.7–3.8% in the F2 peak electron density has been detected. This value agrees very well with the calculated value of 3.5–3.8%. The time delay between the minimum in the electron density observed and the maximum value of the phase of the solar eclipse was determined to be 12.5 min, which is in good agreement with the calculated value of 12.8 min. The time delay suggests that the linear loss coefficient was 1.3´10–3 s–1 and the production rate was (3.8–3.9)×108 m–3∙s–1. The SE was associated with the generation of quasi-periodic oscillations in the electron density and the F2 layer virtual height, within the period range of 10–15 min, and with amplitudes of 1.7–4% and 9.1–11.4%, respectively. The fluctuations in the minimum frequency observed on the ionograms showed a factor of 3–4 times decrease in the course of the SE. In addition, a decrease in the averaged values was observed to occur from 3 to 2.85 MHz (by 5%), which suggested that the radio wave attenuation by absorption decreased due to a decrease in the lower-ionosphere electron density below 100 km altitude. This decrease was estimated to be about 2%, while the decrease determined from the observations gave 2.2%.

Conclusions. The partial solar eclipse with a very small value of the maximum magnitude (0.112) and obscuration of 4.4% acted to produce a few effects observed in the ionosphere with the ionosonde.