OBSERVATION OF TWEEK-ATMOSPHERICS AFTER SOLAR FLARES AT THE "AKADEMIK VERNADSKY" UKRAINIAN ANTARCTIC STATION

Tweek-atmospherics (tweeks), along with radio transmission by VLF radio stations, are used to study the lower ionosphere. Electromagnetic pulse radiation, which has been excited by the lightning discharges, has a maximum spectral density at extra low frequencies range (ELF, 300...3000 Hz) and very low frequencies (VLF, 3...30 kHz). The Earth-ionosphere cavity serves as a waveguide for electromagnetic waves in these frequency ranges. On the spectrogram of the tweek, the initial part is a linearly polarized broadband signal, and then a number of individual harmonics are observed. Their instantaneous frequencies decrease, asymptotically approaching approximately multiples of the cutoff frequencies of the waveguide. The single position method for lightning location and estimation of the ELF wave's reflection heights in the lower ionosphere by tweeks has been implemented into the computational algorithm. The clusters with approximately the same azimuths and distances to sources which have been obtained during the same night have been identified upon the ensemble of tweek-atmospheric records. The data were accumulated at the Akademik Vernadsky Ukrainian Antarctic Station in 2021. These data were selected for 6 nights of summer, autumn and winter of the southern hemisphere, during periods of disturbed magnetosphere after solar flares. The location of the receiving complex in the near-polar region makes it possible to register tweek sources in two world thunderstorm centers with geographic azimuths from –60° to 130°. By experimental results it has been shown that, under conditions the three-hour planetary index Kp = 4...6, the measuring complex at the Akademik Vernadsky Ukrainian Antarctic Station stably detects ELF – VLF atmospherics propagating at night in the near-Earth waveguide. Processing records of such signals using the same algorithm as for tweek-atmospherics under normal conditions of an undisturbed magnetosphere demonstrates the coordinates of the sources of atmospherics. This also applies to the area of the Southern Magnetic Anomaly in the Brazilian region, where satellite observation of lightning discharges is nearly impossible after solar flares. The vast majority of thunderstorm centers for period studied were confined to continental regions, also tweek sources have been observed with coordinates associated with a latitude band around 40 S of the Atlantic and Indian Oceans. Data on the location of tweek sources (lightning discharges) coincide quite closely with predictions according to climate patterns. By studying the spectra of tweeks, there has also been detected an effect of non-reciprocity of the propagation in the East – West and West – East directions. The mechanism of this phenomenon should probably be close to the same mechanism during the reflection of ELF – VLF radiation in the undisturbed ionosphere. A sharp maximum in the average number of harmonics (that is, a minimum of attenuation coefficients) was observed when tweek atmospherics arrived from a geographic azimuth of 110°. This feature can be explained by the joint manifestation of the non-reciprocity effect and the specific layout of the Southern Magnetic Anomaly relative to the receiving point at the Akademik Vernadsky Ukrainian Antarctic Station.