Cosmic particles, once a topic of mystery and science fiction, are found in abundance in Earth's vicinity. In the late 1950's, James Arthur Van Allen launched a probe to learn more about them. What exactly are the Van Allen Belts, and what are their secrets?
Public Interest and Early Exploration
As children, many “baby boomers" watched the popular Bell Labs Series of science films. Among its eight popular entries, one film was entitled, “The Strange Case of the Cosmic Rays" (1957). Pursuing the nature of cosmic rays was likened to solving an Edgar Allen Poe mystery. As circumstance would have it, the release of this movie occurred just before a "quantum leap" in understanding cosmic rays occurred.
Arguably the most famous scientist/detective in pursuit of cosmic rays was James Van Allen (1914-2006). On January 31, 1958, Van Allen and his University of Iowa team of scientists launched Explorer 1 on a Jupiter C rocket, complete with on-board particle counter to detect cosmic rays. Allen wanted to know how the particle count varies in number with distance from the magnetic equator. Unfortunately, the data collection of Explorer 1 was limited because of intermittent sampling, since there was no on-board recorder. Explorer 3, however, was equipped with such a recorder. The superior data set revealed a multitude of cosmic particles at higher altitude.
Earth's magnetic field interacts with highly energized particles from space, and with some particles actually formed within the magnetic field. Particles from the solar wind, on the other hand, are deflected by Earth's magnetic field. The solar wind compacts the part of the torus-shaped magnetic field facing the Sun. The part opposite the Sun is elongated. An inner belt, elevation about 2,000 to 5,000 km, contains protons and atoms stripped of their electrons. The outer radiation belt (about 13,000 to 19,000 km) is rich in electrons coming from the magnetic tail of Earth’s magnetosphere. The particle-rich portion of the magnetic field is from about 20 degrees below to 20 degrees above the geomagnetic equator.
Differences Between Inner and Outer Belts
Inner belt particles have energies on the order of millions of electron volts (MeVs) and are restrained to a compact region. The inner belt, or radiation belt is generally associated with cosmic phenomena, whereas the outer belt involves lower energy particles - typically less than 1 MeV—and is associated with the ring current, magnetic storms, and auroras. Interestingly, other planets may possess such belt structures.
Highly energized particles spiral around lines of magnetic force, while maintaining their forward motion. As they approach the Earth's surface and the magnetic field grows stronger, the particles are reflected back in the opposite direction. This can happen repeatedly. In addition to this, there is a charged particle “drift"—electrons and protons revolving in opposite directions. This drift of charges around the Earth is equivalent to what is termed the ring current. Particles that succeed in precipitating toward Earth from this ring current are responsible for certain auroral effects termed Ring Current Auroras.
There has been controversy in connection with radiation and the Van Allen Belts. Have astronauts been exposed to dangerous levels of radiation during space flights? The feeling is they have been exposed to a higher level than is considered acceptable for the general population, but not to anything near a life-threatening level. Low Earth orbit exposes astronauts to only a small radiation bombardment, however the Moon-bound Apollo astronauts received higher exposure, since they passed through the regions containing high-energy particles. Future astronauts to deep space will have to deal with hazardous levels of radiation but can expect better radiation exposure management. A much larger problem caused by interaction of the Van Allen belt with the electronics of satellites and space stations.
Satellites and Space Stations
Electronics of satellites have become, if anything, more susceptible to problems from the charged particles present in the Van Allen radiation belt. This is because of their increasing miniaturization and sensitivity. Part of the current solution has been the “hardening" of electronic packages and sometimes turning them off when passing through regions of intense radiation. Hardening requires the use of less-sensitive components, more tolerant circuit designs, and radiation shielding.
Other Planetary Magnetospheres
Even as earth's magnetosphere plays an important part with how charged particles affect astronauts, other planets (Mercury, Jupiter, Saturn, Uranus, and Neptune) have magnetospheres, each of them different in nature. What role they play in planetary astronomy will undoubtedly be discussed in other articles written by Bright Hub contributors.