Because the distances in planetary space environments are so vast, space scientists typically measure them in terms of planetary radii. One Earth radius (typically abbreviated as "R" with a subscript "E") equals 6378 kilometers or 3906 miles.
Electron Volt (eV)
The electron volt (eV) is the standard unit in terms of which charged particle energy is expressed. An electron volt is the energy acquired by one electron as it is accelerated in an electric field through a potential difference of one volt. In the space environment of the Earth, charged particle energies range from 1 eV for the "cold" plasma of the plasmasphere to tens and even hundreds of MeV (MeV = mega-electron volts = 1,000,000 eV) for the trapped energetic ions and electrons of the radiation belts.
The charged particles that make up the ring current and radiation belts are trapped in the Earth's magnetic field, bouncing back and forth along the magnetic field lines between "mirror points" in the northern and southern hemispheres. These are the points in a non-uniform magnetic field toward either end of a field line where the magnetic field becomes strong enough to cause a particle traveling along that field line to reverse direction. (For more information, see the entry on pitch angle.) In addition to their bounce motion, the trapped particles also drift azimuthally (in longitude): ions to the west, electrons to the east. It is this drift motion that creates the ring current. The two-dimensional surface that these particles define through their combined bounce and drift motions is known as a "drift shell" or "L shell." Locations within the ring current/radiation belt region of the inner magnetosphere are typically given in terms of the distance, in Earth radii, from the center of the Earth to the point where a particular drift or L shell intersects the plane of the geomagnetic equator.