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One of the most important parameters in ionospheric plasma research alsohaving a wide practical application in wireless satellite telecommunications isthe total electron content (TEC) representing the columnal electron numberdensity. The F region with high electron density provides the biggestcontribution to TEC while the relatively weakly ionized plasma of the D region(60 km - 90 km above Earths surface) is often considered as a negligible causeof satellite signal disturbances. However, sudden intensive ionizationprocesses like those induced by solar X ray flares can cause relative increasesof electron density that are significantly larger in the D-region than inregions at higher altitudes. Therefore, one cannot exclude a priori the Dregion from investigations of ionospheric influences on propagation ofelectromagnetic signals emitted by satellites. We discuss here this problemwhich has not been sufficiently treated in literature so far. The obtainedresults are based on data collected from the D region monitoring by very lowfrequency radio waves and on vertical TEC calculations from the GlobalNavigation Satellite System (GNSS) signal analyses, and they show noticeablevariations in the D region electron content (TECD) during activity of a solar Xray flare (it rises by a factor of 136 in the considered case) when TECDcontribution to TEC can reach several percent and which cannot be neglected inpractical applications like global positioning procedures by satellites.

Behaviour of electron content in the ionospheric D-region during solar X-ray flares