Responses of the ionospheric D-region to periodic and transient variations of the ionizing solar Lyα radiation

Solar radiation has the most important role in periodical variation of terrestrial atmospheric properties. Under unperturbed ionospheric conditions, the solar Lyα line has a dominant influence on ionization processes in the lowest ionospheric layer, the so called D-region. In this paper, we present periodical and transient variations in influences of the Lyα radiation on this ionospheric layer. In the case of periodical lower ionospheric changes we consider diurnal, seasonal and solar cycle variations and show analysis of acoustic and gravity waves induced by solar terminator. Influences of solar flares and eclipses on this atmospheric layer are analyzed as examples of sudden ionospheric disturbances. For decades, Very Low Frequency radio signals (3 – 30 kHz) are successfully used as a tool for monitoring of changes in the lower ionosphere, based on radio wave propagation through Earth-ionosphere waveguide along given trajectories and registration of their physical parameters (amplitude and phase delay). For the analysis conducted in this paper, we used records of the VLF DHO signal, emitted on 23.4 kHz frequency from transmitter in Germany and received in Serbia.