Dangerous Radiation belts around the Earth in a doughnut shape
The radiation belts (also known as the van Allen belts) are toroidal regions encircling the Earth, in which very energetic (that is, very fast) particles are found. These particles are essentially trapped in the geomagnetic field. One can consider these particles to be a form of ionising radiation. As such, these particles constitute a real danger for humans and for spacecraft in orbit around the Earth.
The energetic particles in the radiation belts move along the Earth’s magnetic field lines. In the magnetosphere, which largely consists of low-energy ionized gas originating from Earth, the magnetic field of our planet traps high-energy charged particles emitted from the Sun.
Two Van Allen belts and their distance from earth (heigth)
These particles are located in two particular “doughnut-shaped” regions of the magnetosphere, better known as the Van Allen belts. The belts are named after the American physicist James Alfred Van Allen who discovered them with a Geiger counter onboard Explorer 1, the first American satellite, in 1958.
- The inner belt is centered at a height between 300 and 1,000 kilometers above the Earth and reaches up to about 10,000 kilometers, consisting mainly of energetic protons with energies exceeding 100 MeV and, to a lesser degree, of ions and electrons.
- The outer belt extends from an altitude of about 10,000-40,000 kilometers, comprising mainly high energy (0.1–10 MeV) electrons.
Between the inner and outer belt there is a slot" region where the electron flux decreases from its value in the inner zone, before rising in the outer zone. The location and extent of the inner belt, the slot and the outer zone depend on the energy of the electrons.
At high latitudes, the outer electron belt reaches very low altitudes.
3 superposed motions of radiation belt particles
Generally speaking, electrically charged particles remain trapped in the Earth's magnetic field. Their trajectories can in a first approximation be decomposed into three superposed motions, at least for particles whose energy is not too high :
- A gyratory motion perpendicular to the magnetic field lines
- An oscillating movement along the magnetic flield lines between two mirror points located in each hemisphere
- A slower movement of drift in longitude, eastward for electrons and westward for energetic protons.
The azimuthal movement of ions and electrons in opposite directions produces a gigantic ring current which is responsible for the decrease in the intensity of the horizontal component of the magnetic field during magnetic storms.
This global movement explains the toroidal shape of the belts. The axis of symmetry of the belts coincides with the axis of the Earth's magnetic dipole, which is inclined 11 degrees to the Earth's axis of rotation.
In addition, the centre of the dipole is more than 500 km offset, in the direction of the North Pacific, from the planet's centre of gravity. Because of this offset, energetic protons penetrate deeper into the Earth's atmosphere over the South Atlantic. This region is called the South Atlantic Anomaly (SAA).
Sources of the high-energy particles
When these high-energy particles collide with nitrogen or oxygen atoms in the atmosphere, a nuclear reaction is produced in which a neutron is created. About 10% of these neutrons escape to space and rapidly decay into protons and electrons. These are captured by the magnetic field.
The electron population in the outer zone varies much more rapidly over time than in the inner belt, especially during magnetic storms. The main sources of particles in the outer belt are the solar wind and the magnetosphere. The presence of helium ions and oxygen show that the ionosphere also contributes.
Intensity depends on the altitude
Trapped particles can be lost through collisions with the Earth's atmosphere. In particular, protons that descend too low in the atmosphere tend to lose their charge. Below about 400 km, the intensity of the radiation belts is low because the particles undergo many collisions.
However, at a radial distance of 8000 km, particles can remain trapped in the magnetic field for several years. The intensity of the fluxes depends considerably on the altitude. The very high energy proton fluxes encountered in the inner zone are the most damaging for satellites, especially for the photoelectric cells of solar panels.