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Solar flares can severely disturb the Earth's ionosphere, leading to changes of atmospheric chemistry and degradation of Global Navigation Satellite System (GNSS) signals. The key to better assess these effects is to more accurately quantify the electron density enhancements induced by solar flares, which is still uncertain. Thanks to the good coverage and quiet electromagnetic environment provided by the Great Wall Station (GWS, 62.22°S, 58.96°W) in Antarctica, our Very‐Low‐Frequency (VLF) receiver has recorded clean VLF signatures produced by three X‐class solar flares from March to May in 2022. These measurements motivate us to revisit the uncertain question related to the ionospheric effects induced by solar flares. Different from previous studies, we model solar flare events from satellite measurements of X‐ray fluxes to ground‐based measurements of VLF transmitter signals by combining a flare irradiance model, an X‐ray propagation model, a D‐region chemistry model, and a VLF propagation model. For eight transmitting frequencies and propagation paths, modeling results can satisfactorily explain our VLF measurements at GWS during the three X‐class flares, especially so for the X1.3 event occurring on 30 March 2022. Moreover, the electron density results that reconcile space‐ and ground‐based measurements suggest that the VLF reflection height can be lowered to ∼66 km by an X1.1 flare, in line with previous studies. The present set of numerical models can be thus utilized in future studies to improve the nowcasting capability of VLF sensing technique for monitoring solar flares, as well as the accuracy of GNSS navigation/positioning during solar flares. Plain Language Summary: The X‐ray fluxes emitted during solar flares can significantly influence the Earth's ionosphere, leading to delays of Global Navigation Satellite System signals and atmospheric chemistry changes. A Very‐Low‐Frequency (VLF, 3–30 kHz) receiver has been recently deployed at the Great Wall Station in Antarctica, a location with a good coverage of navy transmitters in the northern hemisphere and quiet electromagnetic environment. This receiver has recorded the VLF signatures of three X‐class solar flare events from late March to early May in 2022. In this study, we report these VLF measurements during X‐class solar flares in Antarctica, and model these events using four well‐validated models, including a flare irradiance model, an X‐ray propagation model, a D‐region chemistry model, and a VLF propagation model. For eight transmitting frequencies and VLF propagation paths, modeling results show good agreements with the VLF measurements during the three X‐class flare events, as well as previous studies. This set of numerical models, after being validated using Great Wall Station measurements, can be used in future studies to better quantify the influence of solar flare events on the Earth's ionosphere and radio wave propagation. Key Points: We report ground‐based measurements of Very‐Low‐Frequency (VLF) signals during X‐class solar flares at the Great Wall station in AntarcticaWe model the responses of VLF transmitter signals to solar flares based upon the combination of four well‐validated modelsModeling results are consistent with the VLF transmitter signal measurements for eight propagation paths during three X‐class flare events [ABSTRACT FROM AUTHOR]

Based on the results of mathematical modeling, we consider the possibility to estimate the plasma frequency F2 layer maximum of the polar ionosphere (critical frequency, foF2) using frequency-sweeping radiation from a highly elliptical spacecraft orbit in the Arctic zone. Our modeling concerning the energy problem of radio sensing consisted of analyzing wave field parameters, received field strength, and SNR on two radio paths with the distances 1900 and 2500 km along the earth's surface, with the satellite height varying from 10,000 to 30,000 km. Radio path orientations were selected to be close to the classical limit cases of radio wave propagation in the anisotropic ionospheric plasma: quasi-longitudinal approximation and, to a large extent, the quasi-transversal one for the quiet midday and midnight conditions. As a result of these simulations and following specific spacecraft conditions, working with an optimal probing signal was proposed for the appropriate emission power for the onboard transmitter. In the inverse problem of radio sounding of an ionized media, common mathematical inaccuracy in foF2 calculated from the transionogram, frequency dependence of the probing signals magneto-ionic group delay, was estimated. Considering and founding a possible realization of the method, physical prerequisites are discussed based on the experimental data of radio waves passing the 16,000 km long radio path for Moscow–Antarctica (UAS Vernadsky). [ABSTRACT FROM AUTHOR]

We address deviations in the source bearing of natural pulsed extremely low frequency (ELF) radio signals (Q-bursts) in the Schumann resonance band. The model of smooth day – night non-uniformity is used. The observer is located at the Ukrainian Antarctic station "Akademik Vernadsky" (65.25° S and 64.25° W), and the source is positioned at the same meridian at latitudes of 20° S; 25° N, or 85° N. The equinox period is considered when the solar terminator line is coincident with a meridian. Thus, the source – observer meridian and the terminator meridian are separated by an angular distance varying with the local time of observer. The propagation parameters of ELF radio waves were computed using the full wave solution in the form of Riccati equation for the ambient day and ambient night profiles of mesosphere conductivity. The field spectra were found from the 2D telegraph equations (2DTE). Spectra of the source bearing and of the polarization coefficient were computed for different local times. The source bearing and the polarization coefficient do not exceed ±15° and ±0.35 correspondingly. The Q-bursts in the time domain were computed at the output of an example Schumann resonance receiver. Phasors of the Poynting vector were used for estimating the source bearing in the time domain. Maximum deviations in the source azimuth may reach ±4° directed toward to the midnight point. The eccentricity of the "elliptic" magnetic field phasor may reach 0.1. Impact of the day – night non-uniformity is comparable with that of the continuous Schumann resonance background. Therefore, terminator effect is detectable in records of extra-powerful Q-bursts exceeding the background noise by the factor of 10–20. • Realistic model of the Earth–ionosphere cavity with the day–night non-uniformity is used. • Propagation parameters of ELF radio waves were computed using the full wave solution in the form of Riccati equation. • Spectra of vertical electric and two orthogonal horizontal magnetic fields were found using 2D telegraph equations. • Observer was at 65.25°S and 64.25°W, and the source occupied the same meridian at latitudes 20°S; 25°N, or 85°N. • The equinox period was considered when the solar terminator is coincident with a meridian. • Spectra were computed of the source bearing and of polarization coefficient for different local time of observatory. • Deviation of the source bearing may reach ±15°, and of the polarization coefficient may reach the ±0.35 interval. • The Q-bursts were computed with account for the gain of an example Schumann resonance receiver. • Time domain phasors of horizontal Poynting vector were used for estimating the source bearing. • The time domain deviations may reach ±4°, and the Poynting vector tends to point to the midnight position. • Detection of terminator effect in the source bearing is possible for Q-bursts exceeding the background by a 10–20 factor. [ABSTRACT FROM AUTHOR]