Van Allen Radiation Belt

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 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 200Š1,000 km, while the belts do not extend past 7 Earth radii RE. The belts are confined to a volume which extends about 65” from the celestial equator.

A radiation belt is a layer of energetic charged particles that is held in place around a magnetized planet, such as the Earth, by the 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 and as a result the Earth's belts bear his name. The main belts extend from an altitude of about 1,000 to 60,000 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. The belts are located in the inner region of the Earth's magnetosphere. The belts contain energetic electrons that form the outer belt and a combination of protons and electrons that form the inner belt. The radiation belts additionally contain lesser amounts of other nuclei, such as alpha particles. The belts endanger satellites, which must protect their sensitive components with adequate shielding if their orbit spends significant time in the radiation belts.

In 2013, NASA reported that the Van Allen Probes had discovered a transient, third radiation belt, which was observed for four weeks until destroyed by a powerful, interplanetary shock wave from the Sun.

The Van Allen radiation belt is a torus of energetic charged particles (i.e. a plasma) around Earth, trapped by Earth's magnetic field. The Van Allen belts are closely related to the polar aurora where particles strike the upper atmosphere and fluoresce. 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 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 Earth's atmosphere limits the belts' particles to regions above 200-1000 km, while the belts do not extend past 7 Earth radii RE. The belts are confined to an area which extends about 65” from the celestial equator.

The Outer Van Allen Belt

Enlarged View of this image

The large outer radiation belt is almost toroidal in shape, extending from an altitude of about three to ten Earth radii (RE) or 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 belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals, losses to magnetopause, and the outward radial diffusion.

The outer belt consists mainly of high energy (0.1Š10 MeV) electrons trapped by the Earth's 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 and at the outer edge (close to the 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 (62 mi), 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 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 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.

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 generated when a mass coronal ejection is created by 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 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 owing 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 which eventually destroyed it.

The Inner Van Allen Belt

While protons form one radiation belt, trapped electrons present two distinct structures, the inner and outer belt. The inner electron Van Allen Belt extends typically from an altitude of 0.2 to 2 Earth radii (L values of 1 to 3) or 600 miles (1,000 km) to 3,700 miles (6,000 km) above the Earth.

In certain cases when solar activity is stronger or in geographical areas such as the South Atlantic Anomaly (SAA), the inner boundary may go down 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 discovered in the radiation belts by NASA in their energetic particle experiment, RBSPICE. The reason reported was that due to the tilt in Earth's magnetic field axis, the planetÕs rotation generated an oscillating, weak electric field that permeates through the entire inner radiation belt. The field affects the electrons as if they behave like fluids.

The global oscillations slowly stretch and fold the fluid resulting in the striped pattern observed across the entire inner belt, extending from above EarthÕs atmosphere, about 800 km above the planetÕs surface up to roughly 13,000 km.

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 (the Starfish Prime test) 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. Electronics on satellites must be hardened against radiation to operate reliably.

The Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation. An object satellite shielded by 3 mm of aluminum will receive about 2500 rem (25 Sv) per year.

Proponents of the Apollo Moon Landing Hoax 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. Van Allen himself, still alive and living in Iowa City, has dismissed these ideas.

In practice, Apollo astronauts who travelled to the moon spent very little time in the belts and received a harmless dose. Nevertheless NASA deliberately timed Apollo launches, and used lunar transfer orbits that only skirted the edge of the belt over the equator to minimise the radiation. Astronauts who visited the moon probably have a slightly higher risk of cancer during their lifetimes, but still remain unlikely to become ill because of it.

The Van Allen Belt's And Why They Exist

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 and forth between the earth's poles, where the magnetic field increases.

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 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.

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. Thomas Gold has argued that the outer belt is left over from the aurora while Dr Alex Dessler has argued that the belt is a result of volcanic activity.

In another view, the belts could be considered a flow of electric current that is fed by the solar wind. With the protons being positive and the electrons being negative, the area between the belts is sometimes subjected to a current flow, which "drains" away. The belts are also thought to drive aurora, lightning and many other electrical effects.

Removing The Belts

The belts are a hazard for artificial satellites and moderately 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 highly electrically charged tethers in orbit. The idea is that the electrons would be deflected by the large electrostatic fields and intersect the atmosphere and harmlessly dissipate.

Some scientists, however, theorize that the Van Allen belts carry some additional protection against solar wind, which means that a weakening of the belts could harm electronics and organisms, and that they may influence the Earth's telluric current, dissipating the belts could influence the behavior of Earth's magnetic poles. Read more ...

In the News ...

Star Trek-like invisible shield found thousands of miles above Earth   PhysOrg - November 26, 2014

A team led by the University of Colorado Boulder has discovered an invisible shield some 7,200 miles above Earth that blocks so-called "killer electrons," which whip around the planet at near-light speed and have been known to threaten astronauts, fry satellites and degrade space systems during intense solar storms. The barrier to the particle motion was discovered in the Van Allen radiation belts, two doughnut-shaped rings above Earth that are filled with high-energy electrons and protons. The latest mystery revolves around an "extremely sharp" boundary at the inner edge of the outer belt at roughly 7,200 miles in altitude that appears to block the ultrafast electrons from breeching the shield and moving deeper towards Earth's atmosphere.

'Zebra Stripes' in Earth's Magnetic Field Have Surprising Source   Live Science - April 3, 2014

Strange stripe-like features in Earth's magnetic field are caused by the planet's spin, and not by the constant bombardment of solar particles as previously thought, scientists say. The so-called "zebra stripes" form when the electric field around Earth generated by the planet's rotation - previously thought to be too weak to impact the fast-moving particles - creates a striped pattern in the inner electron belt.

Earth's radiation belt: Van Allen probes reveal 'zebra stripes' in space   Science Daily - March 20, 2014

Scientists have discovered a new, persistent structure in one of two radiation belts surrounding Earth. NASA's twin Van Allen Probes spacecraft have shown that high-energy electrons in the inner radiation belt display a persistent pattern that resembles slanted zebra stripes. Surprisingly, this structure is produced by the slow rotation of Earth, previously considered incapable of affecting the motion of radiation belt particles, which have velocities approaching the speed of light.

NASA Discovers New Radiation Belt Around Earth   Live Science - February 28, 2013

A ring of radiation previously unknown to science fleetingly surrounded Earth last year before being virtually annihilated by a powerful interplanetary shock wave, scientists say. NASA's twin Van Allen space probes, which are studying the Earth's radiation belts, made the cosmic find. The surprising discovery - a new, albeit temporary, radiation belt around Earth - reveals how much remains unknown about outer space, even those regions closest to the planet, researchers added. After humanity began exploring space, the first major find made there were the Van Allen radiation belts, zones of magnetically trapped, highly energetic charged particles first discovered in 1958.