Ionospheric D-region temperature relaxation and its influences on radio signal propagation after solar X-flares occurrence

In this paper our attention is focused on relations between radio signal propagation characteristics and temperature changes in D-region af- ter solar X-flare occurrence. We present temperature dependencies of electron plasma frequency, the parameter that describes medium conditions for prop- agation of an electromagnetic wave, and the refractive index which describes how this wave propagates. As an example for quantitative calculations based on obtained theoretical equations we choose the reaction of the D-region to the solar X-flare occurred on May 5 th, 2010. The ionospheric modelling is based on the experimental data obtained by low ionosphere observations using very low frequency radio signal.