Low‐frequency hiss is known to play an important role in the precipitation of radiation belt electrons by cyclotron, Landau, and bounce resonances. To investigate the potential combined scattering effect caused by these resonant processes, we analyze the resonant conditions and develop a full relativistic test particle code to quantify the net pitch angle scattering efficiency. It is indicated that the three resonance processes can coexist to scatter electrons at different energies and pitch angles, with the net pitch angle scattering rates up to ~10⁻³ s⁻¹ for low‐frequency hiss ~175 pT at L = 4.5. Comparisons with the quasi‐linear theory results demonstrate that the cyclotron resonance is mainly responsible for the pitch angle scattering of electrons < ~ 80°, while both Landau and bounce resonances can affect the scattering of near‐equatorially mirroring electrons and their combined diffusion produces smaller scattering coefficients compared to quasi‐linear theory calculations.

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.

Solar flares have a severe impact on the near-earth space environment, during which the VLF signals observed by the ground-based instrument exhibit abnormal changes. However, the similarity and differences of VLF signal responses to solar flares over daytime and nighttime propagation paths are still unclear. Previous magnetograph measurements suggest that solar flares can also influence the dark hemisphere by the induction currents caused by the change in the ionospheric electrical conductivity of the sunlit hemisphere. To examine these effects, we have analyzed the solar flare effects on VLF propagation along two paths that are suited along the north–south direction, but with a time difference of 12 h. From late March to late May in the year of 2022, a total of 32 flare events with clear VLF responses are selected in order to analyze the similarity and differences between daytime and nighttime propagation paths. Different from the previous magnetograph measurements, it is found that the solar flare effects can only be observed from daytime VLF propagation paths. Moreover, present results show that the amplitude and phase variation of the VLF signal increases almost linearly with the magnitude of solar flares, and the stronger the solar flare, the more obvious the influence on VLF signals. However, the two paths exhibit notably different sensitivity in terms of amplitude and phase variation to solar flare class. Future studies that aim at nowcasting solar flare events using ground-based VLF receivers need to take these effects into account. The goal is to better understand the effects of solar flares on the lower ionosphere, with a view toward improving the nowcasting capability of the VLF technique for solar flares.

A large data set of auroral poleward/equatorward boundary, which are automatically identified from more than 60,000 images acquired by Polar ultraviolet imager, is used to study the variation of auroral boundaries associated with the interplanetary/geomagnetic environments and dynamic parameters of the magnetosphere. A total number of 3,805,000/1,215,000 points for poleward/equatorward boundary were identified from the data set. It is found that the poleward/equatorward boundaries show an equatorward movement with the increase of Kan‐Lee electric field. The poleward/equatorward boundaries in the nightside sector (1800–0600 magnetic local time (MLT)) show an equatorward motion with the increase of the value of IMF B_y and the north‐south electric field. The equatorward boundary (1800–0300 MLT) shows a quasi‐linear equatorward displacement with the increase of solar wind dynamic pressure and speed. The poleward boundary (excluding midday sector) and equatorward boundary expand equatorward with the increase of AE index. The auroral oval boundary model with input parameters of the three components of IMF, solar wind speed and density, and AE is developed by using multivariate regression method. Evaluation of the model shows that the mean absolute deviation of the model in every MLT sector is 1.3–2.1° magnetic latitude (MLAT) for the poleward boundary and 1.3–2.5° MLAT for the equatorward boundary, respectively, when input parameters are within valid value coverage (three components of IMF are each from −8 to +8 nT, and the values of solar wind speed and density, and AE are not more than 550 km/s, 20/cm⁻³, and 520 nT, respectively).