The sun’s ultraviolet light ionizes some of the neutral gases in Earth’s atmosphere. Above 80 km above the Earth’s surface, ionized atoms have little chance to become neutral again due to the low abundance of particles. Because of this, a region of permanent plasma is formed at these altitudes called the ionosphere. In the middle of this region, the electron number density is about 100,000 per cc, the temperature is about 1000 Kelvin, and the magnetic field strength is about 10 microtesla.

The ionosphere extends from about 80 km upwards. At some point above it merges with the plasmasphere. The plasmasphere is a torus-shaped region within the Earth’s radiation belt that rotates with the Earth once every twenty-four hours. Here the electron temperature (5000 Kelvin) is low but the density (500 per cc) is high. At the equator this sphere extends to about 4 Earth radii. Beyond this the density suddenly drops to only 1 per cc, and this boundary is called the plasmapause.

Far above the surface, Earth’s plasma sheet rains down particles. Neutral particles in the Earth’s atmosphere are further ionized due to this precipitation. And the light produced by this process is what we call aurora. The aurora is seen in an oval shell surrounding the Earth’s magnetic poles. The shape of this shell is determined by the distribution of the magnetic field lines of the plasma sheet. At the center of the auroral oval is the polar cap from which field lines extend to the magnetotail lobes.