We present the results of the analysis of the measurements data of amplitudes and phases of signals from GQD (19.58 kHz) and GBZ (22.1 kHz) VLF transmitters in the geophysical observatory of IDG RAS “Mikhnevo” (GO MIK) to assess the impact of solar flares of different classes that occurred in June 2014 on the height profile of the electronic concentration $Ne$ in the lower ionosphere. The use of two-channel data from the GOES satellite (0.05-0.4 nm and 0.1-0.8 nm) allowed us to estimate the fluxes of X-rays in harder spectral ranges. The analysis of the dynamics of the $Ne$ height profile and X-ray fluxes in different wavelength ranges allowed us to estimate the ionization and recombination rates and determine the spectral ranges of radiation that have the greatest influence on the dynamics of the electron concentration at considered heights.

The Schumann resonance (SR), the source of which is the global thunderstorm activity is constantly observed in the Earth-ionosphere waveguide. Changes in the parameters of SR signals caused by geophysical disturbances make it possible to study the state and dynamics of the lower ionosphere. When calculating the SR parameters, there are problems associated with the impact of electromagnetic interference of natural and anthropogenic origin. The main natural sources of interference are signals associated with the radiation of nearby lightning discharges, as well as the influence of the Alfvén ionospheric resonator. The paper presents a new method for calculating the SR parameters,which significantly reduces the impact of these interferences. The developed technique significantly increased the temporal resolution of the obtained data on the frequency and amplitude of the SR. Due to this, it became possible to study the influence of fast heliogeophysical disturbances (such as solar X-ray flares) on the lower ionosphere and, as a consequence, on the parameters of the SR. An analysis of the experimental data made it possible to establish a linear dependence of the SR frequency on the logarithm of the X-ray flux in the range up to 0.2 nm during a class X solar flare.

This paper presents the results of modeling the lower ionosphere response to solar X-ray flares of C-, M- and X-classes. The model is based on a 5-component scheme of the ionization-recombination cycle of the ionospheric D-region. Input parameters of the plasma-chemical model under different heliogeophysical conditions corresponding to selected X-ray flares were determined by using data received from AURA, SDO and GOES satellites. Verification of the obtained results was carried out with use of ground-based radiophysical measurements taken at the geophysical observatory Mikhnevo. Results of comparing the calculated and experimental the radio wave amplitude variations along six European very low frequency (VLF) paths show that the root mean square error (RMSE) does not exceed 1.5 dB for ~70% of cases including X-class flares during which the amplitude jump on some paths reaches 8 dB. Qualitative and quantitative analysis of the verification results of the VLF signal amplitude has showed the good predictive capability of the built model for describing weak and moderate ionospheric disturbances.