  The hydrogen Ly-{\alpha} line has a dominant influence in photo-ionization
processes in the unperturbed terrestrial ionospheric D region. In this paper,
we present a procedure of calculating the rate of photo-ionization induced by
Ly-{\alpha} photons based on relaxation of electron density after intensive
perturbations like those caused by solar X flares. This theory is applied to
the ends of relaxation periods following three cases of solar X flares from May
5, 2010, February 18, 2011 and March 24, 2011. The necessary data on low
ionospheric plasma parameters were collected by the very low frequency (VLF)
radio-wave technics. The electron concentration is calculated from the
amplitude and phase of the VLF signal emitted by the DHO transmitter in Germany
and recorded by a receiver located in Serbia.
