In the ionosphere, at a critical altitude, varying from 2000 to 3000 km, the frequency of collisions between charged particles becomes negligible. The motion of these particles is therefore primarily controlled by the terrestrial magnetic field.

The magnetic field limits the possibilities of motion for charged particles in the ionosphere by forcing them to roll up around its field lines. Because of that, these particles are able to fill all the space controlled by this field. This area, which surrounds our planet, corresponds to the terrestrial magnetosphere. (video 1) Its lower limit is the ionosphere and extends to the deepest zones controlled by the geomagnetic field. Because of its interaction with the solar wind (picture 1) the magnetosphere is compressed at the side of the Sun and very much lengthened in the opposite direction, where it is stretched on several million kilometres (picture 2). The magnetosphere sometimes reacts violently to the moods of the Sun: explosive phenomena on its surface reflect in the magnetosphere (picture 3) and produce effects on the ground, particularly above the magnetic poles. (picture 4 & video 2)