  One of the most important parameters in ionospheric plasma research also
having a wide practical application in wireless satellite telecommunications is
the total electron content (TEC) representing the columnal electron number
density. The F region with high electron density provides the biggest
contribution 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 cause
of satellite signal disturbances. However, sudden intensive ionization
processes like those induced by solar X ray flares can cause relative increases
of electron density that are significantly larger in the D-region than in
regions at higher altitudes. Therefore, one cannot exclude a priori the D
region from investigations of ionospheric influences on propagation of
electromagnetic signals emitted by satellites. We discuss here this problem
which has not been sufficiently treated in literature so far. The obtained
results are based on data collected from the D region monitoring by very low
frequency radio waves and on vertical TEC calculations from the Global
Navigation Satellite System (GNSS) signal analyses, and they show noticeable
variations in the D region electron content (TECD) during activity of a solar X
ray flare (it rises by a factor of 136 in the considered case) when TECD
contribution to TEC can reach several percent and which cannot be neglected in
practical applications like global positioning procedures by satellites.
