  Variations in X-ray and EUV irradiance during solar flares lead to a
noticeable increase in the electron concentration in the illuminated part of
the Earth's ionosphere. Due to the large amount of experimental data
accumulated by Global Navigation Satellite Systems (GNSS), the total electron
content (TEC) response to the impulsive phase of a solar flare has been studied
quite well. However, recent studies have shown that large fraction of X-class
flares have second strong peak of warm coronal emission (which is called 'EUV
late phase'), whose influence on the ionization of ionospheric layers is not
yet clear. A combined analysis of successive solar emissions and the caused TEC
changes made it possible to numerically estimate the ionospheric response to
the impulsive, gradual, and late phases of the X2.9 solar flare occurred on
2011 November 3 and demonstrate the high geoeffectiveness of the rather weak Fe
XV 28.4 nm solar emission during the EUV late phase. It was found that the
ionospheric response to the relatively weak emissions of the EUV late phase of
the X2.9 solar flare amounted to almost a third of the TEC increase during the
impulsive phase.
