Electron precipitation

Electron precipitation (also called energetic electron precipitation or EEP) is an atmospheric phenomenon that occurs when previously trapped electrons enter the Earth's atmosphere, thus creating communications interferences and other disturbances. Electrons are trapped in the Van Allen radiation belt by Earth's magnetic fields and begin to spiral around field lines in the radiation belt. They may remain there for an indefinite period of time (in some cases years). When broadband very low frequency (VLF) waves propagate the radiation belts, the electrons exit the radiation belt and "precipitate" (or travel) into the ionosphere (a region of Earth's atmosphere) where the electrons will collide with ions. Electron precipitation is regularly linked to ozone depletion. It is often caused by lightning strikes.

An electron's gyrofrequency is the number of times it revolves around a field line. VLF waves traveling through the magnetosphere, caused by lightning or powerful transmitters, propagate through the radiation belt. When those VLF waves hit the electrons with the same frequency as an electron's gyrofrequency, the electron exits the radiation belt and "precipitates" (because it will not be able to re-enter the radiation belt) throughout the Earth's atmosphere and ionosphere.

Often, as an electron precipitates, it is directed into the upper atmosphere where it may collide with neutral particles, thus depleting the electron's energy. If an electron makes it through the upper atmosphere, it will continue into the ionosphere. Groups of precipitated electrons can change the shape and conductivity of the ionosphere by colliding with atoms or molecules (usually oxygen or nitrogen based particles) in the region. When colliding with an atom, the electron strips the atom of its other electrons creating an ion. Collisions with the air molecules also release photons which provide a dim "aurora" effect. Because this occurs at such a high altitude, humans in aircraft are not affected by the radiation.

The ionization process, caused by electron precipitation in the ionosphere, increases its electrical conductivity which in turn brings the bottom of the ionosphere to a lower altitude. When this happens, ozone depletion occurs and certain communications may be disrupted. The lowered altitude of the ionosphere is temporary (unless electron precipitation is steady) while the ions and electrons rapidly react to form neutral particles.

...
Wikipedia