The high-energy, charged particles that are trapped into orbits by the geomagnetic field, forming radiation belts that surround the Earth. The belts consist primarily of electrons and protons and extend from a few hundred kilometers above the Earth to a distance of about 8 Re (Re = radius of Earth = 6371 km = 3959 mi). James Van Allen and coworkers discovered them in 1958 using radiation detectors carried on satellites Explorer 1 and 3, and they are often referred to as the Van Allen belts.
A charged particle under the influence of the geomagnetic field follows a trajectory that can be conveniently described as a superposition of three separate motions. The first motion, produced by the magnetic force acting at right angles to both the particle velocity and the magnetic field, is a rapid spiral about magnetic field lines. As the spiraling particle moves along the field line toward either the North Pole or South Pole, the increase in magnetic field strength causes the particle to be reflected so that it bounces between the Earth's two hemispheres. Superimposed on the spiral and bounce motions is a slow east-west drift; electrons drift eastward and protons or heavier ions drift westward. (The resulting current, called the ring current, acts to decrease the strength of the Earth's (surface) northward magnetic field at low latitudes.) Thus, individual trapped particles move completely around the Earth in a complicated pattern, their motion being constrained to lie on magnetic shells.
The spatial structure of trapped radiation shows two maxima: an inner radiation belt centered at about 1.5Re and an outer belt centered at 4–5Re. In the inner radiation belt, the most penetrating particles are protons with energies extending to several hundred megaelectronvolts (MeV). However, the flux of high-energy protons decreases rapidly with increasing distance from the Earth and becomes insignificant beyond 4Re. Electrons and low-energy protons with energies up to a few MeV occur throughout the stable trapping region. The electron energies extend to several MeV, and in the outer radiation belt electrons are the most penetrating component.
The intensity, energy spectrum, and spatial distribution of particles within the radiation belts vary with time. The most dramatic variations are associated with magnetic storms. The changes are most pronounced for particles in the outer belt where the magnetic variations are the largest. During a magnetic storm, the electron flux in the outer belt may increase by an order of magnitude or more.
It is believed that most of the very high energy protons (>50 MeV) in the inner belt result from the spontaneous decay of high-energy neutrons, produced by collisions of cosmic rays with atmospheric atoms. However, the vast majority of the radiation belt populations are ions and electrons that originate from either the atmosphere or the solar wind and are accelerated by processes only partly understood.
The radiation belts are just one feature of the space plasma environment. For example, plasma from the Sun is continually impinging on the Earth's magnetic field, resulting in a “cavity” known as the magnetosphere.
Because the conditions that lead to the formation of Earth's radiation belts are so general, it is believed that any planet or moon that has a large enough magnetic field will also have radiation belts. Jupiter, Saturn, Uranus, and Neptune have strong magnetic fields and very large, intense radiation belts analogous to those of the Earth.
The term “space weather” describes the conditions in space that affect Earth and its technological systems. It is a consequence of the behavior of the Sun, and the interaction of the solar wind with the Earth's magnetic field. The fluxes of electrons and protons trapped in the radiation belts can injure both personnel and equipment on board spacecraft if the vehicle is exposed to these energetic particles for a sufficiently long time. Because of the spatial structure of the belts, the degree of damage will be strongly dependent on the position in space and hence on the orbit of the vehicle. In the inner belt region, high-energy protons can pass through several centimeters of aluminum structure and injure components in the interior of the spacecraft. In most other regions of the radiation belts, the trapped particles are less penetrating, and damage is confined to exposed equipment such as solar cells.
