2. Propagation through the ionosphere

Above an altitude of 70 km, we encounter the ionosphere. This is a medium partially ionized by solar X and UV radiation. The ionization produced during the day is maintained at night due to the low rate of recombination between electrons and ions at these altitudes. Ionization is measured by the electron concentration N _e (number of free electrons per unit volume), which exhibits a maximum at an altitude between around 250 km during the day and 400 km at night. N _e electron concentration therefore depends on altitude and local time. It also depends on geographical location and shows strong daily variations. We can see that the average ionization of the ionosphere follows fairly closely the variations in the solar activity cycle.

Free electrons in the ionosphere interact with...