Changes in the state of the D and Eionospheric regions lead to variations in the amplitude-phase characteristics of VLF radio signals. The existing theoretical and empirical models of the...

The paper presents the results of the reconstruction of the effective height h' and the slope of the profile β of the electron concentration in the D layer of the ionosphere during X-ray flares of M- and X-classes at the midlatitudes. The reconstruction was carried out with data from measurements at the observatory of the Institute of Geospheres Dynamics of the Russian Academy of Sciences at the Mikhnevo observatory of amplitudes and the phases of signals from VLF transmitters GQD (19.6 kHz) and GBZ (22.1 kHz) that propagate along one mid-latitude path. Based on the calculations, the empirical dependencies of the parameters h' and β on the flare energy according to the X-ray flux measurements on the GOES satellite are in the range 0.05–0.4 nm. Criteria are proposed for the selection of solar flares for the verification of theoretical computational models.

An experimental study was made of the dependence of the electron-density perturbations in the D‑regions of the midlatitude ionosphere during solar flares on the spectral composition of X-ray radiation in the wavelength range from 0.01 to 0.4 nm, i.e., outside the measurement range of the GOES satellite. For this, the variation in the brightness temperature of the emission of six flares in 2014–2017 were calculated, and the composition of their radiation was determined. It is shown that, although only a small percent of the total flash radiation energy is in the 0.01–0.2 nm wavelength range, it is the hard components that are the main factor leading to a change in the reflection height of very low frequency (VLF) radio waves. It is shown that the efficiency of solar flares significantly depends on the previous solar activity, not only for class-C flares but also for powerful class-M and -X flares.