Estimation of the Ionospheric D-Region Ionization Caused by X-Class Solar Flares Based on VLF Observations

In this paper, we study the ionization-recombination processes in the lower ionosphere during solar flares of various classes in June 2014 and September 2017. For the first time, ionization and recombination rates of the ionospheric D-region were estimated using the experimental data on variations in the amplitude and phase characteristics of very low frequencies (VLF) signals and two-channel data from the GOES satellite (0.05-0.4 nm and 0.1-0.8 nm). The empirical two-parameter Wait-Ferguson model was used to calculate temporal changes in the electron concentration Ne during solar flares of different classes. GOES satellite data and black body model were used to estimate the X-ray fluxes in the wavelength spectral range lambda <= 0.3 nm. Joint analysis of the temporal evolution of the Ne vertical profile in the lower ionosphere and X-ray fluxes in different wavelength ranges was carried out. As a result, the values of the ionization and recombination rate coefficients were obtained, and the spectral ranges of radiation that have the greatest impact on Ne at given heights were determined. Calculated ionization and recombination rate coefficients and ranges were successfully verified using VLF data during different solar flares. The results obtained in this work can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes. Solar flares have a significant impact on the state and dynamics of the lower ionosphere and, as a consequence, on the conditions for the radio waves propagation in a wide frequency range. Changes occurring in the ionospheric D-region are mainly due to variations in the amplitude and phase of VLF-signals. Analysis of the variation of these parameters together with the X-ray fluxes recorded by the GOES satellite during solar flares of June 2014 and September 2017 allowed us to study the processes occurring in the ionosphere. In this paper, we empirically estimated the ionization and recombination rates and the spectral ranges of radiation that have the greatest impact on the ionization processes of the lower ionosphere during solar flares. The results obtained can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes. VLF signals data and GOES flux data were used to calculate the ionization and recombination rates in the ionospheric D-region The spectral ranges of radiation with the greatest influence on the ionization processes were estimated Obtained values of ionization and recombination rates can be used to estimate time variations of electron concentration during flares