A RADIATION BELT is a zone of energetic charged particles , most of
which originate from the solar wind that is captured by and held
around a planet by that planet's magnetic field . The
Earth has two
such belts and sometimes others may be temporarily created. The
discovery of the belts is credited to
James Van Allen
James Van Allen , and as a
result the Earth's belts are known as the VAN ALLEN BELTS. Earth's two
main belts extend from an altitude of about 500 to 58,000 kilometers
above the surface in which region radiation levels vary. Most of the
particles that form the belts are thought to come from solar wind and
other particles by cosmic rays . By trapping the solar wind, the
magnetic field deflects those energetic particles and protects the
Earth's atmosphere from destruction.
The belts are located in the inner region of the Earth's
magnetosphere . The belts trap energetic electrons and protons . Other
nuclei, such as alpha particles , are less prevalent. The belts
endanger satellites , which must have their sensitive components
protected with adequate shielding if they spend significant time in
that zone. In 2013,
NASA reported that the
Van Allen Probes had
discovered a transient, third radiation belt, which was observed for
four weeks until it was destroyed by a powerful, interplanetary shock
wave from the
* 1 Discovery
* 2 Research
* 3 Inner belt
* 4 Outer belt
* 7 Implications for space travel
* 8 Causes
* 9 Proposed removal
* 10 See also
* 11 Notes
* 12 References
* 13 Additional sources
* 14 External links
Kristian Birkeland ,
Carl Størmer , and
Nicholas Christofilos had
investigated the possibility of trapped charged particles before the
Space Age .
Explorer 1 and
Explorer 3 confirmed the existence of the
belt in early 1958 under
James Van Allen
James Van Allen at the
University of Iowa
University of Iowa .
The trapped radiation was first mapped by
Explorer 4 ,
Pioneer 3 and
Luna 1 .
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, as it lacks a stable, global, dipole field. The
Earth's atmosphere limits the belts' particles to regions above
200–1,000 km, (124–620 miles) while the belts do not extend past
Earth radii RE. The belts are confined to a volume which extends
about 65° on either side of the celestial equator .
Jupiter's variable radiation belts
Van Allen Probes mission aims at understanding (to the point
of predictability) how populations of relativistic electrons and ions
in space form or change in response to changes in solar activity and
the solar wind.
NASA Institute for Advanced Concepts –funded studies
have proposed magnetic scoops to collect antimatter that naturally
occurs in the Van Allen belts of Earth, although only about 10
micrograms of antiprotons are estimated to exist in the entire belt.
Van Allen Probes mission successfully launched on August 30,
2012. The primary mission is scheduled to last two years with
expendables expected to last four. NASA's Goddard Space Flight Center
Living With a Star program of which the Van Allen Probes
is a project, along with
Solar Dynamics Observatory (SDO). The Applied
Physics Laboratory is responsible for the implementation and
instrument management for the Van Allen Probes.
Radiation belts exist around other planets and moons in the solar
system that have magnetic fields powerful enough to sustain them. To
date, most of these radiation belts have been poorly mapped. The
Voyager Program (namely
Voyager 2 ) only nominally confirmed the
existence of similar belts around
Cutaway drawing of two radiation belts around Earth: the inner
belt (red) dominated by protons and the outer one (blue) by electrons.
The inner Van Allen Belt extends typically from an altitude of 0.2 to
Earth radii (L values of 1 to 3) or 1,000 km (620 mi) to 6,000 km
(3,700 mi) above the Earth. In certain cases when solar activity is
stronger or in geographical areas such as the
South Atlantic Anomaly
South Atlantic Anomaly ,
the inner boundary may decline to roughly 200 kilometers above the
Earth's surface. The inner belt contains high concentrations of
electrons in the range of hundreds of keV and energetic protons with
energies exceeding 100 MeV, trapped by the strong (relative to the
outer belts) magnetic fields in the region.
It is believed that proton energies exceeding 50 MeV in the lower
belts at lower altitudes are the result of the beta decay of neutrons
created by cosmic ray collisions with nuclei of the upper atmosphere.
The source of lower energy protons is believed to be proton diffusion
due to changes in the magnetic field during geomagnetic storms.
Due to the slight offset of the belts from Earth's geometric center,
the inner Van Allen belt makes its closest approach to the surface at
the South Atlantic Anomaly.
On March 2014, a pattern resembling 'zebra stripes' was observed in
the radiation belts by the
Radiation Belt Storm Probes Ion Composition
Experiment (RBSPICE) onboard
Van Allen Probes . The reason reported
was that due to the tilt in
Earth's magnetic field axis, the
planet’s rotation generated an oscillating, weak electric field that
permeates through the entire inner radiation belt. It was later
demonstrated that the zebra stripes were in fact an imprint of
ionospheric winds on radiation belts.
Laboratory simulation of the Van Allen belt's influence on the
Solar Wind; these aurora-like Birkeland currents were created by the
Kristian Birkeland in his terrella , a magnetized anode
globe in an evacuated chamber
The outer belt consists mainly of high energy (0.1–10 MeV )
electrons trapped by the Earth's magnetosphere. It is more variable
than the inner belt as it is more easily influenced by solar activity.
It is almost toroidal in shape, beginning at an altitude of three and
extending to ten
Earth radii (RE) 13,000 to 60,000 kilometres (8,100
to 37,300 mi) above the Earth's surface. Its greatest intensity is
usually around 4–5 RE. The outer electron radiation belt is mostly
produced by the inward radial diffusion and local acceleration due
to transfer of energy from whistler-mode plasma waves to radiation
Radiation belt electrons are also constantly removed
by collisions with Earth's atmosphere, losses to the magnetopause ,
and their outward radial diffusion. The gyroradii of energetic protons
would be large enough to bring them into contact with the Earth's
atmosphere. Within this belt, the electrons have a high flux and at
the outer edge (close to the magnetopause), where geomagnetic field
lines open into the geomagnetic "tail" , the flux of energetic
electrons can drop to the low interplanetary levels within about 100
km (62 mi), a decrease by a factor of 1,000.
In 2014 it was discovered that the inner edge of the outer belt is
characterized by a very sharp transition, below which highly
relativistic electrons (> 5MeV) cannot penetrate. The reason for this
shield-like behavior is not well understood.
The trapped 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
are much more energetic. This mixture of ions suggests that ring
current particles probably come from more than one source.
The outer belt is larger than the inner belt and its particle
population fluctuates widely. Energetic (radiation) particle fluxes
can increase and decrease dramatically in response to geomagnetic
storms , which are themselves triggered by magnetic field and plasma
disturbances produced by the Sun. The increases are due to
storm-related injections and acceleration of particles from the tail
of the magnetosphere.
On February 28, 2013, a third radiation belt, consisting of
high-energy ultrarelativistic charged particles, was reported to be
discovered. In a news conference by NASA's Van Allen Probe team, it
was stated that this third belt is a product of mass coronal ejection
from the Sun. It has been represented as a separate creation which
splits the Outer Belt, like a knife, on its outer side, and exists
separately as a storage container of particles for a month's time,
before merging once again with the Outer Belt.
The unusual stability of this third, transient belt has been
explained as due to a 'trapping' by the
Earth's magnetic field of
ultrarelativistic particles as they are lost from the second,
traditional outer belt. While the outer zone, which forms and
disappears over a day, is highly variable due to interactions with the
atmosphere, the ultrarelativistic particles of the third belt are
thought to not scatter into the atmosphere, as they are too energetic
to interact with atmospheric waves at low latitudes. This absence of
scattering and the trapping allows them to persist for a long time,
finally only being destroyed by an unusual event, such as the shock
wave from the Sun.
In the belts, at a given point, the flux of particles of a given
energy decreases sharply with energy.
At the magnetic equator , electrons of energies exceeding 500 keV
(resp. 5 MeV) have omnidirectional fluxes ranging from 1.2×106 (resp.
3.7×104) up to 9.4×109 (resp. 2×107) particles per square
centimeter per second.
The proton belts contain protons with kinetic energies ranging from
about 100 keV (which can penetrate 0.6 µm of lead ) to over 400 MeV
(which can penetrate 143 mm of lead).
Most published flux values for the inner and outer belts may not show
the maximum probable flux densities that are possible in the belts.
There is a reason for this discrepancy: the flux density and the
location of the peak flux is variable (depending primarily on solar
activity), and the number of spacecraft with instruments observing the
belt in real time has been limited. The
Earth has not experienced a
solar storm of
Carrington event intensity and duration while
spacecraft with the proper instruments have been available to observe
Regardless of the differences of the flux levels in the Inner and
Outer Van Allen belts, the beta radiation levels would be dangerous to
humans if they were exposed for an extended period of time. The Apollo
missions minimised hazards for astronauts by sending spacecraft at
high speeds through the thinner areas of the upper belts, bypassing
inner belts completely.
Flux values, normal solar conditions
AP8 MIN omnidirectional proton flux ≥ 100 keV
AP8 MIN omnidirectional proton flux ≥ 1 MeV
AP8 MIN omnidirectional proton flux ≥ 400 MeV
In 2011, a study confirmed earlier speculation that the Van Allen
belt could confine antiparticles. The PAMELA experiment detected
orders of magnitude higher levels of antiprotons than are expected
from normal particle decays while passing through the South Atlantic
Anomaly . This suggests the Van Allen belts confine a significant flux
of antiprotons produced by the interaction of the Earth's upper
atmosphere with cosmic rays. The energy of the antiprotons has been
measured in the range from 60–750 MeV.
IMPLICATIONS FOR SPACE TRAVEL
Comparison of geostationary ,
GLONASS , GALILEO , COMPASS
International Space Station
International Space Station ,
Hubble Space Telescope
Hubble Space Telescope and
Iridium constellation orbits, with the Van Allen radiation belts and
Earth to scale. The
Moon 's orbit is around 9 times larger than
geostationary orbit. (In the SVG file, hover over an orbit or its
label to highlight it; click to load its article.)
Spacecraft travelling beyond low
Earth orbit enter the zone of
radiation of the Van Allen belts. Beyond the belts, they face
additional hazards from cosmic rays and solar particle events . A
region between the inner and outer Van Allen belts lies at two to four
Earth radii and is sometimes referred to as the "safe zone".
Solar cells , integrated circuits , and sensors can be damaged by
radiation. Geomagnetic storms occasionally damage electronic
components on spacecraft. Miniaturization and digitization of
electronics and logic circuits have made satellites more vulnerable to
radiation, as the total electric charge in these circuits is now small
enough so as to be comparable with the charge of incoming ions.
Electronics on satellites must be hardened against radiation to
operate reliably. The
Hubble Space Telescope
Hubble Space Telescope , among other satellites,
often has its sensors turned off when passing through regions of
intense radiation. A satellite shielded by 3 mm of aluminium in an
elliptic orbit (200 by 20,000 miles (320 by 32,190 km)) passing the
radiation belts will receive about 2,500 rem (25 Sv ) per year (for
comparison, a full-body dose of 5 Sv is deadly). Almost all radiation
will be received while passing the inner belt.
The Apollo missions marked the first event where humans traveled
through the Van Allen belts, which was one of several radiation
hazards known by mission planners. The astronauts had low exposure in
the Van Allen belts due to the short period of time spent flying
through them. Apollo flight trajectories bypassed the inner belts
completely, and only passed through the thinner areas of the outer
Astronauts' overall exposure was actually dominated by solar
particles once outside Earth's magnetic field. The total radiation
received by the astronauts varied from mission to mission but was
measured to be between 0.16 and 1.14 rads (1.6 and 11.4 mGy ), much
less than the standard of 5 rem (50 mSv) per year set by the United
Energy Commission for people who work with
Simulated Van Allen Belts generated by a plasma thruster in tank
#5 at the Electric Propulsion Laboratory located 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 through wave-particle
In the inner belt, particles that originate from the
Sun are trapped
in the Earth's magnetic field. Particles spiral along the magnetic
lines of flux as they move "longitudinally" along those lines. As
particles move toward the poles, the magnetic field line density
increases and their "longitudinal" velocity is slowed and can be
reversed, reflecting the particle and causing them to bounce back and
forth between the Earth's poles. In addition to the spiral about and
motion along the flux lines, the electrons move slowly in an eastward
direction, while the ions move westward.
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 L. Green of the Goddard Space Flight
Center comparing maps of lightning activity collected by the Microlab
1 spacecraft with data on radio waves in the radiation-belt gap from
IMAGE spacecraft suggests that they are actually generated by
lightning within Earth's atmosphere. The radio waves they generate
strike the ionosphere at the correct angle to pass through only at
high latitudes, where the lower ends of the gap approach the upper
atmosphere. These results are still under scientific debate.
HIGH VOLTAGE ORBITING LONG TETHER, or HiVOLT, is a concept proposed
by Russian physicist V. V. Danilov and further refined by Robert P.
Robert L. Forward for draining and removing the radiation
fields of the Van Allen radiation belts that surround the Earth. A
proposed configuration consists of a system of five 100 km long
conducting tethers deployed from satellites, and charged to a large
voltage. This would cause charged particles that encounter the tethers
to have their pitch angle changed, thus over time dissolving the inner
belts. Hoyt and Forward's company, Tethers Unlimited, performed a
preliminary analysis simulation in 2011, and produced a chart
depicting a theoretical radiation flux reduction, to less than 1% of
current levels within two months for the inner belts that threaten LEO
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List of artificial radiation belts
List of plasma (physics) articles
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≈ 5.98×1024 kg.
* ^ Approximately 8.6 times (in radius and length) when the moon is
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