Sudden phase anomalies of VLF radio transmitters signals (11.9 kHz) of RSDN-20 system registered in Yakutsk during 2009-2017

Sudden phase anomalies of VLF signals (11.9 kHz) of RSDN-20 system (Novosibirsk, Krasnodar and Khabarovsk transmitters) registered in Yakutsk during 2009-2017 are considered. A catalog of X-ray solar flares from C3.0 class and higher is compiled for the period 2009 - 2017. Each flare is associated with the daily average value of the F10.7 index in solar flux units to characterize the background solar X-ray radiation. The array is divided into intervals corresponding to the minimum solar activity (SA): the F10.7 index is less than 122 sfu (quartile 25%), maximum SA: index F10.7 is greater than 160 sfu (75% quartile) and medium SA: 122 the X-ray flux of solar flares is decreased from the solar activity (SA) minimum to the maximum one. There is an uneven distribution of the intensity of the background X-ray radiation from season to season. For the 24th solar cycle activity the main maxima of the F10.7 index and the maxima of the intensity of the solar background X-ray radiation fell on the winter seasons. By ground based methods (based on data on the SPA of VLF radio signals and average daily values of F10.7) it is obtained satisfactory estimates of the intensity of the X-ray radiation flux during solar flares.