  The solar Ly{\alpha} line emission can be considered as the dominant source
of ionization processes in the ionospheric D-region at altitudes above 70 km
during unperturbed conditions. However, large sudden impacts of radiation in
some other energy domains can also significantly influence the ionization rate
and, in this paper, we present a study on the contribution of Ly {\alpha}
radiation to the ionization rate when the ionosphere is disturbed by solar
X-flares. We give relevant analytical expressions and make calculations and
numerical simulations for the low ionosphere using data collected by the VLF
receiver located in Serbia for the VLF radio signal emitted by the DHO
transmitter in Germany.
