By analyzing the GNSS (Global Navigational Satellite Systems) signals recorded at the IGS (International GNSS Service) network, we compare the effects of ionization of the upper atmosphere by a series of intense X-class solar flares during the 23rd and 24th solar cycles. We develop the methods for esti mating the geoeffectiveness of solar flares from the GNSS data and suggest using the rate of change of the ionospheric total electron content averaged over all the receiving stations located on the sunlit side of the Earth reduced to the solar zenith angle during the flare as the characteristic of the flare’s geoeffectiveness.
$^1$Moscow State University, Moscow, 119991 Russia
$^2$Schmidt Institute of Physics of the Earth, Russian Academy of Sciences, ul. B. Gruzinskaya 10, Moscow, 123995 Russia