What results from it, is named ionisation, i.e a production of ions and electrons, which are particles with opposite electric charges coming from a variety of atoms and atmospheric molecules. (picture 2)

It is only from an altitude of 60 km that the existence of a real ionised area is recognised. The so-called ionosphere, which extends from the mesosphere to the boundaries of the atmosphere (picture 3). As long as the pressure is sufficiently high, the neutral atmosphere inflicts its own conditions on the ionosphere. But when the ionisation level, i.e. the ratio of the charged particles to neutral particles is no more negligible, the ionosphere has its own conditions where the electric field binding the positively charged (ions) and negatively charged particles (electrons) needs to be taken into account.

Although the number of ions and electrons is insignificant compared to the number of neutral particles in the ionosphere, the presence of free electrons is very important. Indeed, the latter have a big influence on the propagation of radio waves with high frequency (HF: between 3 and 30 MHz) used, in particular, by radio amateurs. These waves are sent in space from a transmitter, are reflected by the ionosphere, and finally deviated towards a receiving station on the ground.

Ionosphere’s character varies according to the hour of day, the season, the geographical place and the solar and auroral activities (picture 4). This is why, the radio amateurs must have a lot of ingeniousness to calculate the frequency of emission according to the changing parameters in the ionosphere (picture 5 et 6), in order to maintain stability with communications. Nevertheless, it occurs that radio-communications are impossible. That happens at times of violent solar eruptions because they strongly disturb the ionosphere. (video 1)