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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 ~1g/cm2 (2) of shielding (1 millimeter
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 RE (1). The belts are confined to an area which extends about
65° (1) from the celestial equator.
The Inner Van Allen Belt
The inner radiation belt extends over altitudes of 650-6,300 km (up to one
RE). 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 2-5,000 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 E > 40 Kev electrons can drop to normal interplanetary levels within about 100km (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 somewhat 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.
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 US of creating the inner belt as a result of nuclear testing in Nevada. The US 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. 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."
See also: Sherwood
machine
The Van Allen Belt's Impact on the space elevator
When the Apollo astronauts travelled to the moon the astronauts received about 1% of a fatal dose in the few hours they were
crossing these regions of space. By way of contrast a space elevator will spend anywhere from hours to weeks in these regions,
and if the final destination is geosynchronous orbit, the
length of stay could be indefinite. Without shielding, this could pose a serious risk to passengers.
As with nuclear power, the problem is that the necessary radiation shielding is very heavy - much heavier than the people it
protects; having to lift the passengers as well as the shielding may increase the ticket price many times over the equivalent
quantity of freight (since freight largely doesn't care about radiation issues and doesn't require shielding).
The radiation belts are based on Earth's magnetic field, which is tilted at about 11 degrees from its rotational axis. They
are further distorted by the solar wind, giving them a teardrop shape. Due to this, the elevator will encounter varying
intensities of radiation; especially concerning is the inner belt.
One proposal for two way elevator systems to deal with the outer belt is to have extra shielding "in-place" along the cable
that is carried up by a climbing elevator, and carried back down by a descending elevator. While this adds constant weight to the
elevator (as if a "permanent payload"), it adds the weight to the elevator where the cable is thickest and most able to tolerate
extra payload. The "weak point" of the elevator is where it meets the Earth.
Another type of shielding is so-called "active" shielding. One such type involves electromagnetic fields to deflect low-energy
radiation. Another type of active shielding is the Multilayer High Temperature Superconductor Protection System, which involves
using high-temperature superconducting materials to produce strong magnetic fields for deflection. [1]
. In theory, anything that produces a
strong magnetic field could be used to deflect the radiation, but the strength of the magnetic field produced given the weight of
the materials required can be a limiting factor. Active shielding, in its current designs, is very effective at shielding from
protons of energies up to 200MeV, but is largely ineffective against galactic cosmic radiation (GCR) [2] . As the dangerous inner Van Allen belt
consists mostly of protons from energies between 10 and 100 MeV , and particles in the outer van allen belts are lower energy
(around 1 MeV) [[3] ], active shielding is a realistic option for
the transit up to GEO. However, since it is ineffective against GCR, long-term human stays at GEO would require physical
shielding in the structure they are to stay at.
There is also a proposal by the late Bob Forward called HiVolt which may be a way to drain at least parts of
the Van Allen belts to 1% of their natural level within a year.
External Links
References:
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