Electron production by solar Ly-αα line radiation in the ionospheric D-region

The hydrogen Ly-αα 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-αα 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 techniques. 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.