Van Allen radiation belt
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The Van Allen radiation belt is a torus of energetic charged particles around Earth, trapped by Earth's magnetic field. When the belts "overload", particles strike the upper atmosphere and fluoresce, causing the polar aurora. The presence of a radiation belt had been theorized prior to the Space Age and the belt's presence was confirmed by the Explorer I on January 31, 1958 and Explorer III missions, under Doctor James Van Allen. The trapped radiation was first mapped out by Explorer IV and Pioneer III.
Qualitatively, it is very useful to view this belt as consisting of two belts around Earth, the inner radiation belt and the outer radiation belt. The particles are distributed such that the inner belt consists mostly of protons while the outer belt consists mostly of electrons. Within these belts are particles capable of penetrating about 1 g/cm2 (2) of shielding (e.g., 1 millimetre of lead).
The term Van Allen Belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun does not support long-term radiation belts. The atmosphere limits the belts' particles to regions above 200-1000 km (1), while the belts do not extend past 7 Earth radii RE (1). The belts are confined to an area which extends about 65° (1) from the celestial equator.
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The Inner Van Allen Belt
The inner radiation belt extends over altitudes of 650–6,300 km (up to one Earth radius). This ring is most concentrated in the Earth's equatorial plane. It consists mostly of protons on the order of 10–50 MeV, a by-product of collisions between cosmic ray ions and atoms of the atmosphere. The belt also contains electrons, low-energy protons, and oxygen atoms with energies of 1–100 keV (3). When these electrons strike the atmosphere they cause the polar aurora.
The intensity of the belt fluctuates, partly due to the influence of the solar cycle, and is strongest between 2,000 and 5,000 km. The inner radiation belt comes nearest to Earth's surface at the South Atlantic Anomaly.
The number of cosmic ray ions is relatively small and the inner belt therefore accumulates slowly, but because the trapped protons are very stable in this belt (with particle lifetimes of up to ten years), high intensities are reached as they build up over many years.
The belt was discovered by a Geiger counter on board the Explorer 1 satellite built by James Van Allen and the University of Iowa and launched on January 31, 1958 as part of the IGY. The instrumentation on board Explorer 1 actually registered no radiation at the altitude of the radiation belts, an anomaly which was explained by Explorer III's more sophisticated data recording capabilities as being due to intense radiation having overwhelmed the earlier detector.
The Outer Van Allen Belt
The outer radiation belt extends from an altitude of about 10,000–65,000 km and has its greatest intensity between 14,500–19,000 km. The outer belt is thought to consist of plasma trapped by the Earth's magnetosphere. The USSR's Lunik I reported that there were very few particles of high energy within the outer belt. The electrons here have a high flux and along the outer edge and electrons with kinetic energy E > 40 keV can drop to normal interplanetary levels within about 100 km (a decrease by a factor of 1000). This drop-off is a result of the solar wind.
The particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic. This mixture of ions suggests that ring current particles probably come from more than one source.
The outer belt is larger and more diffuse than the inner, surrounded by a low-intensity region known as the ring current. Unlike the inner belt, the outer belt's particle population fluctuates widely and is generally weaker in intensity (less than 1 MeV), rising when magnetic storms inject fresh particles from the tail of the magnetosphere, and then falling off again.
There is debate as to whether the outer belt was discovered by the US Explorer IV or the USSR Sputnik II/III.
Radial Diffusion Induced by Magnetic Fluctuations
A sudden increase in solar wind pressure can cause the radiation belts to change shape. In such an instance, particles on the sunward side of the planet will be carried inward (toward the planet), while particles on the far side of the planet will be carried further from the planet. This can give the radiation belts something of a tear-drop shape. After such an incident, the belts tend to return to a more spherical shape.
Without this sort of "mirroring", ions and electrons would not be trapped in the Earth's magnetosphere, but would instead follow their guiding field lines into the atmosphere, where they would be absorbed and become lost. What happens instead is that every time a trapped particle approaches Earth, it is reflected back. It is thus confined to the more distant section of the field line.
The Van Allen Belt's Impact on Space Travel
Solar cells, integrated circuits, and sensors can be damaged by radiation. In 1962, the Van Allen belts were temporarily amplified by a high-altitude nuclear explosion and several satellites ceased operation. Magnetic storms occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics and logic circuits have made satellites more vulnerable to radiation, as incoming ions may be as large as the circuit's charge. The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation.
A object satellite shielded by 3 mm of aluminum will receive about 2500 rem (3) (25 Sv) per year.
Conspiracy theorists have argued that space travel to the moon is impossible because the Van Allen radiation would kill or incapacitate an astronaut who made the trip. In practice, even at the peak of the belts, one could live for several months without receiving a lethal dose.
Apollo nevertheless deliberately timed their launches, and used lunar transfer orbits that only skirted the edge of the belt over the equator to minimise the radiation. Astronauts that have travelled to the moon probably have an increased lifetime risk of cancer, but would be expected not to (and did not) have noticeable illness.
Belts of Other Planets
The gas giant planets Jupiter, Saturn, Uranus and Neptune, all have intense magnetic fields with radiation belts similar to the Earth's outer belt.
Jupiter's belt is the strongest, first detected via its radio emissions in 1955 though not understood at the time. Jupiter's belt is strongly affected by its large moon Io, which loads it with many ions of sulfur and sodium from the moon's volcanoes.
Saturn seems to have an "inner belt" similar to the Earth's, observed by Pioneer 11 during its 1979 fly-by and probably produced by cosmic rays which eject neutrons from Saturn's planetary rings.
The Van Allen Belts and Why They Exist
The Soviets once accused the U.S. of creating the inner belt as a result of nuclear testing in Nevada. The U.S. has, likewise, accused the USSR of creating the outer belt through nuclear testing. It is uncertain how particles from such testing could escape the atmosphere and reach the altitudes of the radiation belts. Likewise, it is unclear why, if this is the case, the belts have not weakened since atmospheric testing was banned by treaty. Tom Gold has argued that the outer belt is left over from the aurora while Alex Dessler has argued that the belt is a result of volcanic activity.
It is generally understood that the Van Allen belts are a result of the collision of Earth's magnetic field with the solar wind. Radiation from the solar wind then becomes trapped within the magnetosphere. The trapped particles are repelled from regions of stronger magnetic field, where field lines converge. This causes the particle to bounce back or "mirror".
The gap between the inner and outer Van Allen belts is caused by low-frequency radio waves that eject any particles that would otherwise accumulate there. 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're actually generated by lightning within Earth's atmosphere. The radio waves they generate only 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.
Removing the belts
The belts are a hazard for artificial satellites and highly dangerous for human beings and difficult and expensive to shield against.
There is a proposal by the late Robert L. Forward called HiVolt which may be a way to drain at least the inner belt to 1% of its natural level within a year. The proposal involves deploying a highly electrically charged tether in orbit. The idea is that the electrons would be deflected by the large field this produces so they would intersect the atmosphere and be harmlessly dissipated.
References
- 1 : Introduction to Geomagnetically Trapped Radiation by Martin Walt (1994)
- 2 : The Radiation Belt and Magnetosphere by Wilmot Hess (1968)
- 3 : NASA http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970228a.html
External links
- An explanation of the belts (http://www.phy6.org/Education/Iradbelt.html)de:Van-Allen-Gürtel
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