Van Allen radiation belt - AbsoluteAstronomy.com

The Van Allen radiation belt is a torus

Torus

In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

of energetic

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

charged particle

Charged particle

In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...

s (plasma

Plasma (physics)

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

) around Earth, which is held in place by Earth's magnetic field

Earth's magnetic field

Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...

. It is believed that most of the particles that form the belts come from solar wind

Solar wind

The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

, and other particles by cosmic rays. It is named after its discoverer, James Van Allen

James Van Allen

James Alfred Van Allen was an American space scientist at the University of Iowa.The Van Allen radiation belts were named after him, following the 1958 satellite missions in which Van Allen had argued that a Geiger counter should be used to detect charged particles.- Life and career :* September...

, and is located in the inner region of the Earth's magnetosphere

Magnetosphere

A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

. 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 particle

Alpha particle

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...

s. 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.

Discovery

Prior to the Space Age

Space Age

The Space Age is a time period encompassing the activities related to the Space Race, space exploration, space technology, and the cultural developments influenced by these events. The Space Age is generally considered to have begun with Sputnik...

, the possibility of trapped charged particles had been investigated by Kristian Birkeland

Kristian Birkeland

Kristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...

, Carl Stormer, and Nicholas Christofilos

Nicholas Christofilos

Nicholas Constantine Christofilos was a Greek-American physicist.Christofilos was born in Boston, Massachusetts and raised in Greece...

. The existence of the belt was confirmed by the Explorer 1 and Explorer 3

Explorer 3

Explorer 3 was an artificial satellite of the Earth, nearly identical to the first United States artificial satellite Explorer 1 in its design and mission...

missions in early 1958, under Dr. James Van Allen

James Van Allen

at the University of Iowa

University of Iowa

The University of Iowa is a public state-supported research university located in Iowa City, Iowa, United States. It is the oldest public university in the state. The university is organized into eleven colleges granting undergraduate, graduate, and professional degrees...

. The trapped radiation was first mapped out by Explorer 4

Explorer 4

Explorer 4 was a US satellite launched on July 26, 1958. It was instrumented by Dr. James van Allen's group. The Department of Defense's Advanced Research Projects Agency had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of nuclear...

, Pioneer 3

Pioneer 3

Pioneer 3 was a spin stabilized spacecraft launched at 05:45:12 UTC on 6 December 1958 by the U.S. Army Ballistic Missile Agency in conjunction with the National Aeronautics and Space Administration...

and Luna 1

Luna 1

Luna 1 , first known as First Cosmic Ship, then known as Mechta was the first spacecraft to reach the vicinity of the Moon and the first of the Luna program of Soviet automatic interplanetary stations successfully launched in the direction of the Moon.While traveling through the outer Van Allen...

.

The term Van Allen belts refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s. The Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

itself 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, while the belts do not extend past 7 Earth radii

Earth radius

Because the Earth is not perfectly spherical, no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km...

R_E. The belts are confined to an area which extends about 65°

Degree (angle)

A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

from the celestial equator

Celestial sphere

In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

Research

An upcoming NASA mission, Radiation Belt Storm Probes

Radiation Belt Storm Probes

The Radiation Belt Storm Probes is a NASA mission under the Living With a Star program. The goal of the LWS program is to develop the scientific understanding necessary to effectively address those aspects of the connected Sun-Earth system that directly affect life and society...

(RBSP), will go further and gain scientific understanding (to the point of predictability) of how populations of relativistic electrons and ions in space form or change in response to changes in solar activity and the solar wind.
The RBSP mission is currently scheduled for 2012. The primary mission is scheduled to last 2 years, with expendables expected to last for 4 years. NASA's Goddard Space Flight Center

Goddard Space Flight Center

The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...

manages the overall Living With a Star

Living With a Star

Living With a Star is a NASA scientific program to study those aspects of the connected Sun-Earth system that directly affect life and society. LWS is a crosscutting initiative with goals and objectives relevant to NASA's Exploration Initiative, as well as to NASA's Strategic Enterprises...

program of which RBSP is a project, along with Solar Dynamics Observatory

Solar Dynamics Observatory

The Solar Dynamics Observatory is a NASA mission which will observe the Sun for over five years. Launched on February 11, 2010, the observatory is part of the Living With a Star program...

(SDO). The Applied Physics Laboratory

Applied Physics Laboratory

The Johns Hopkins University Applied Physics Laboratory , located in Howard County, Maryland near Laurel and Columbia, is a not-for-profit, university-affiliated research center employing 4,500 people. APL is primarily a defense contractor. It serves as a technical resource for the Department of...

is responsible for the overall implementation and instrument management for RBSP.

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

Voyager 2

The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

) only nominally confirmed the existence of similar belts on Uranus

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

and Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

Outer belt

The large outer radiation belt extends from an altitude of about three to ten Earth radii (R_E) or 13,000 to 60,000 kilometres above the Earth's surface. Its greatest intensity is usually around 4–5 R_E. 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 belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals, losses to magnetopause

Magnetopause

The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet’s magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the...

, and the outward radial diffusion. The outer belt consists mainly of high energy (0.1–10 MeV) electrons trapped by the Earth's magnetosphere

Magnetosphere

. The gyroradii for energetic protons would be large enough to bring them into contact with the Earth's atmosphere. The electrons here have a high flux

Flux

In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...

and at the outer edge (close to the magnetopause

Magnetopause

), where geomagnetic field lines open into the geomagnetic "tail", fluxes of energetic electrons can drop to the low interplanetary levels within about 100 km (a decrease by a factor of 1,000).

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

Ionosphere

The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

but much more energetic. This mixture of ions suggests that ring current

Ring current

A ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic particles.-Earth's 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 as a consequence of 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.

There is debate as to whether the outer belt was discovered by the United States Explorer 4

Explorer 4

or the USSR

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

Sputnik 2

Sputnik 2 , or Prosteyshiy Sputnik 2 ), was the second spacecraft launched into Earth orbit, on November 3, 1957, and the first to carry a living animal, a dog named Laika. Sputnik 2 was a 4-meter high cone-shaped capsule with a base diameter of 2 meters...

Sputnik 3

Sputnik 3 was a Soviet satellite launched on May 15, 1958 from Baikonur cosmodrome by a modified R-7/SS-6 ICBM. It was a research satellite to explore the upper atmosphere and the near space, and carried a large array of instruments for geophysical research....

Inner belt

The inner Van Allen Belt extends from an altitude of 100–10,000 km (0.01 to 1.5 Earth radii) above the Earth's surface, and contains high concentrations of energetic protons with energies exceeding 100 MeV and electrons in the range of hundreds of keV, trapped by the strong (relative to the outer belts) magnetic fields in the region.

It is believed that protons of energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the beta decay

Beta decay

In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...

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

South Atlantic Anomaly

The South Atlantic Anomaly is an area where the Earth's inner Van Allen radiation belt comes closest to the Earth's surface. This leads to an increased flux of energetic particles in this region and exposes orbiting satellites to higher than usual levels of radiation...

(SAA).

Flux values

In the belts, at a given point, the flux

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 microns 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 the event.

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.

Antimatter confinement

In 2011, a study has confirmed earlier speculation that the Van Allen belt could confine antiparticles. The PAMELA experiment detected orders of magnitude higher levels of antiproton

Antiproton

The antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy....

s than are expected from normal particle decay

Particle decay

Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...

s while passing through the SAA. 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

Missions beyond low earth orbit leave the protection of the geomagnetic field, and transit the Van Allen belts. Thus they may need to be shielded against exposure to cosmic rays, Van Allen radiation, or solar flare

Solar flare

A solar flare is a sudden brightening observed over the Sun surface or the solar limb, which is interpreted as a large energy release of up to 6 × 1025 joules of energy . The flare ejects clouds of electrons, ions, and atoms through the corona into space. These clouds typically reach Earth a day...

s. The region between two to four earth radii lies between the two radiation belts and is sometimes referred to as the "safe zone".

Solar cell

Solar cell

A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....

s, integrated circuit

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s, and sensor

Sensor

A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

s can be damaged by radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

. Geomagnetic storm

Geomagnetic storm

A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is a major component of space weather and provides the input for many other components of space weather...

s occasionally damage electronic components on spacecraft. Miniaturization and digitization of electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

and logic circuit

Logic gate

A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...

s have made satellites more vulnerable to radiation, as the total charge

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

in these circuits is now small enough so as to be comparable with the charge of incoming ions. Electronics on satellites must be hardened

Radiation hardening

Radiation hardening is a method of designing and testing electronic components and systems to make them resistant to damage or malfunctions caused by ionizing radiation , such as would be encountered in outer space, high-altitude flight, around nuclear reactors, particle accelerators, or during...

against radiation to operate reliably. The Hubble Space Telescope

Hubble Space Telescope

The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

, among other satellites, often has its sensors turned off when passing through regions of intense radiation. A satellite shielded by 3 mm of aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

in an elliptic orbit (200 by 20,000 miles) passing through the radiation belts will receive about 2,500 rem

Röntgen equivalent man

Named after Wilhelm Röntgen , the roentgen equivalent in man or rem is a unit of radiation dose equivalent...

(25 Sv

Sievert

The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

) per year. Almost all radiation will be received while passing the inner belt.

The Apollo astronauts traveled through the Van Allen radiation belts on the way to the moon; however, exposure was minimized by following a trajectory along the edge of the belts that avoided the strongest areas of radiation. The total radiation exposure to astronauts was estimated to be much less than the five (5) rem set by the U.S. Atomic Energy Commission for people who work with radioactivity.

Causes

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

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

" 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. Particles are trapped in the Earth's magnetic field because it is basically a magnetic mirror

Magnetic mirror

A magnetic mirror is a magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region....

. Particles gyrate around field lines and also move along field lines. As particles encounter regions of stronger magnetic field where field lines converge, their "longitudinal" velocity is slowed and can be reversed, reflecting the particle. This causes the particle to bounce back and forth between the Earth's poles, where the magnetic field increases.

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

Pitch angle

The pitch angle of a charged particle is the angle between the particle's velocity vector and the local magnetic field. This is a common measurement and topic when studying the magnetosphere. See Aurora and Ring current-Usage: Particle motion:...

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

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

IMAGE

IMAGE , or Explorer 78, was a NASA MIDEX mission that studied the global response of the Earth's magnetosphere to changes in the solar wind...

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

Starfish Prime

Starfish Prime was a high-altitude nuclear test conducted by the United States of America on July 9, 1962, a joint effort of the Atomic Energy Commission and the Defense Atomic Support Agency ....

, 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.

Removal

The belts are a hazard for artificial satellites and are moderately dangerous for human beings, but are difficult and expensive to shield against.

The Russian physicist V.V. Danilov proposed the use of a High Voltage Orbiting Long Tether

Tether

A tether is a cord, fixture, or signal that anchors something movable to a reference point which may be fixed or moving. There are a number of applications for tethers: balloons, kites, tethered wind-energy conversion systems, anchors, tethered water-flow energy conversion systems, towing, animal...

as a potential means to drain the radiation belt of high energy particles. The proposal involves deploying highly electrically charged tethers from satellites in orbit. Charged particles within the radiation belt encountering these tethers would be deflected by their large electrostatic fields onto paths that intersect with the atmosphere, where they would be harmlessly dissipated. Simulations have suggested that the inner belt could be drained to 1% of its natural electron flux within two months of HiVOLT operation.

External links

An explanation of the belts
Trapped particle radiation models—Introduction to the Trapped Radiation Belts.
SPENVIS - Space Environment, Effects, and Education System—Gateway to the SPENVIS orbital dose calculation software.
NASA Radiation Belt Storm Probe Mission

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