The radiation belts are the highest energy component of the inner magnetosphere, where the kinetic energy range is well above \(3 \: KeV\). The shape of the belts encircling the planet is toroidal-like around the magnetic axis. Approximatively, the motion of each particle is at constant energy, subject only to the Lorentz force.
On typical geomagnetic field lines, attached to the Earth at both ends, the motion of charged particles would soon lead them into the atmosphere, where they would collide and lose their energy. However, the additional feature of trapped motion usually prevents this from happening. The magnetic force is much stronger near the Earth than far away, and on any field line it is greatest at the ends, where the line enters the atmosphere. Thus electrons and ions can remain trapped for a long time, bouncing back and forth from one hemisphere to the other. In this way the radiation belts of the Earth are formed.
The additional drift motion of trapped particles around Earth (ions drift clockwise, electrons the other) is equivalent to an electric current circling clockwise. That is the so-called ring current, whose magnetic field slightly weakens the field observed over most of the Earth's surface. During magnetic storms the ring current receives many additional particles from the nightside ``tail'' of the magnetosphere and its effect increases, though at the Earth's surface it is always very small, only rarely exceeding 1\% of the total magnetic field intensity. The years 1957, 1958 were designated as the ``International Geophysical Year'' (IGY), and both the USA and the Soviet Union (Russia) prepared to launch at that time artificial satellites. Russia successfully orbited its first Sputnik (``satellite'') on October 4, 1957, but after the failure of the US Vanguard, then quickly assembled an alternative rocket carrying a different satellite, the small Explorer 1 built by James Van Allen and his team at the University of Iowa. It was launched on 31 January, 1958. Explorer 1 carried only one instrument, a Geiger counter designed to observe cosmic rays. The experiment worked quite well at low altitudes, but at the top of the orbit no particles at all were counted. Explorer 3, 3 months later revealed that the zero counts actually represented a very high level of radiation. The Earth has two regions of trapped fast particles. The inner radiation belt discovered by Van Allen is relatively compact, extending perhaps one or two Earth radii above the equator (\(1~ R_{e} = 6371 ~km\) ). It consists of very energetic protons (in the energy range 0.1 - 500 MeV), a product of collisions by cosmic ray ions with atoms of the atmosphere, and decay of neutrons produced by this interaction (\(n\rightarrow p+e^{-}+\bar{\nu_{e}}\)). The number of such ions is relatively small, and the inner belt therefore accumulates slowly, but because trapping near Earth is very stable, rather high intensities are reached.
Further out is the large region of the ring current, containing ions and electrons of much lower energy (the most energetic among them also known as the outer radiation belt). Unlike the inner belt, this population fluctuates widely, rising when magnetic storms inject fresh particles from the tail of the magnetosphere, then gradually falling off again. The ring current energy is mainly carried by the ions, most of which are protons. However, there are also helium nuclei. In addition, a certain percentage are \(O^{+}\) oxygen ions, similar to those in the ionosphere of the Earth, though much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source. Auroral electrons have energy in the range \(1000-15.000 ~\)eV, protons in the inner belt perhaps \(50 ~\)MeV, while the energy of cosmic ray ions may reach the GeV. In contrast, air molecules in the atmosphere only have about \(0.03 ~\)eV. The main source of energy seems to be the solar wind, but the ways this energy is transported and distributed in the magnetosphere are not yet completely clear. Anyway it is clear that the actual population is very dynamic over different time scales. Slow variations arise from the 11 year solar cycle, inducing a periodicity of the low altitude trapped population, and from the secular changes in the geomagnetic field which drifts the dipole center away from the center of the Earth at a rate of approx \(2.5\) \(km/year\). The main asymmetry in the geomagnetic field. due both to the Internal Field dipole shift and tilt, together with the effect of the supersonic solar wind that, propagating in the upstream direction, impinges on the Earth magnetosphere, compressing the field on the sun ward side and distends the field into a tail extending several milion kilometers down wind from the Earth. The main effect on the radiation belts is the creation of the so-called South Altantic Anomaly.(SAA).