The terrestrial upper atmosphere includes a region rich of free electrons and ions, known as ionosphere, which extends from 50 km of altitude upwards. The high concentration of free electrons makes the ionosphere an electromagnetic medium, sensitive to the passage of electromagnetic waves and to the presence of charged particles coming from the outer space. The electron concentration depends on the height above sea level, being larger at around 300-400 km, in the so-called “F region”. Below the F region, the “E” and “D” regions present a weaker ionization, but show a high sensitivity to the charged particles injection occurring when an ionospheric storm onsets.
The occurrence of an ionospheric storm is associated with an anomalous distribution of the electrons concentration, forming regions into which the density is significantly lower or higher with respect to the ambient medium. When these conditions happen, the ionosphere behaves unevenly and the regions of anomalous electrons density distribution identify the ionospheric irregularities. At the edges of such irregularities, the ionosphere is strongly unstable and gives rise to a variety of effects on the ionospheric weather.
The cause of the storms is of external origin: the solar eruptions trigger the enhancement of solar wind (supersonic flux of charged particles moving from the Sun to the outer space) density and speed, exciting, on turn, the geomagnetic field variations on the Earth. The prediction and the mitigation of the stormy ionospheric weather is part of the space weather.