The Van Allen radiation belt is a torus of energetic charged particles (plasma) around Earth, which is held in place by Earth’s magnetic field. It is believed that most of the particles that form the belts come from solar wind, and other particles by cosmic rays. It is named after its discoverer, James Van Allen, and is located in the inner region of the Earth’s magnetosphere. It is split into two distinct belts, with energetic electrons forming the outer belt and a combination of protons and electrons forming the inner belts. In addition, the radiation belts contain lesser amounts of other nuclei, such as alpha particles. The belts pose a hazard to satellites, which must protect their sensitive components with adequate shielding if their orbit spends significant time in the radiation belts.
Van Allen radiation belts do exist on other planets in the solar system, whenever a planet or moon has a magnetic field that is powerful enough to sustain a radiation belt. However, many of these radiation belts have been poorly mapped. The Voyager Program (namely Voyager 2) only nominally confirmed the existence of similar belts on Uranus and Neptune.
Causes
Simulated Van Allen Belts generated by a plasma thruster in tank #5 Electric Propulsion Laboratory at the then-called Lewis Research Center, Cleveland, Ohio.
It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of so-called “albedo” neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized though wave-particle interactions.
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In the inner belt, particles are trapped in the Earth’s nonlinear magnetic field. Particles gyrate and move along field lines radiating energy in the form of gamma rays. As particles encounter regions of larger density of magnetic field lines, their “longitudinal” velocity is slowed and can be reversed, reflecting the particle, radiating energy and producing aurora borialis. This causes the particle to bounce back and forth between the Earth’s poles, losing all its energy.[23] Globally, the motion of this trapped particles is chaotic [24]. The effect of these planetary magnetic fields is to protect planets from the outer space radiation.
A gap between the inner and outer Van Allen belts, sometimes called safe zone or safe slot, is caused by the Very Low Frequency (VLF) waves which scatter particles in pitch angle which results in the gain of particles to the atmosphere. Solar outbursts can pump particles into the gap but they drain again in a matter of days. The radio waves were originally thought to be generated by turbulence in the radiation belts, but recent work by James Green of the NASA Goddard Space Flight Center comparing maps of lightning activity collected by the Micro Lab 1 spacecraft with data on radio waves in the radiation-belt gap from the IMAGE spacecraft suggests that they are actually generated by lightning within Earth’s atmosphere. The radio waves they generate strike the ionosphere at the right angle to pass through it only at high latitudes, where the lower ends of the gap approach the upper atmosphere. These results are still under scientific debate.
There have been nuclear tests in space that have caused artificial radiation belts. Starfish Prime, a high altitude nuclear test, created an artificial radiation belt that damaged or destroyed as many as one third of the satellites in low earth orbit at the time.
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The Earth actually has two radiation belts of different origins. The inner ring is called the Van Allen belt after its discoverer James Van Allen of the University of Iowa. This atmospheric radiation was first detected by Geiger counters onboard the Explorer 1 satellite during the International Geophysical Year and was substantiated by data from later Explorer satellites.
The Van Allen belt extends above the equator at an altitude of about 4,000 miles (6437 kilometers).
This belt is populated by very energetic protons in the 10-100 MeV range (a byproduct of collisions by cosmic rays with atoms of the atmosphere).
The cosmic radiation has a rather low intensity (comparable to starlight) and only by accumulating particles over the span of years does the inner belt reach its high intensity. These particles can readily penetrate spacecraft and prolonged exposure can damage instruments and be a hazard to astronauts.
The space probes Pioneer 3 and 4 detected the outer radiation belt. It is nowadays seen as part of the plasma trapped in the magnetosphere. The name radiation belt is usually applied to the more energetic part of the plasma population (e.g., ions of about 1 MeV of energy).
The more numerous lower-energy particles are known as the “ring current,” since they carry the current responsible for magnetic storms. Most of the ring current energy resides in the ions (typically with 0.05 MeV), but energetic electrons can also be found.
In 1990, the satellite Combined Release and Radiation Effects Satellite (CRRES) discovered a third radiation belt located between the inner and outer Van Allen belts. Also, around May 8, 1998, there were a series of large, solar disturbances that caused a new radiation belt to form in the so-called “slot region” between the inner and outer van Allen belts. The new belt eventually disappeared once the solar activity subsided.
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