The Van Allen radiation belt is a zone of charged particles that are trapped by a planet's magnetosphere. Most of these particles come from the solar wind. Earth has two main radiation belts, and sometimes other belts can form temporarily. These belts are named after James Van Allen, who wrote about them in 1958.

Earth's two main belts stretch from about 640 to 58,000 km (400 to 36,040 mi) above Earth's surface. Radiation levels in this area change depending on location. The belts are located in the inner part of Earth's magnetic field. They trap high-energy electrons and protons. Other particles, such as alpha particles, are found in smaller amounts. Most of the particles in the belts come from the solar wind, while some arrive from space as cosmic rays. Earth's magnetic field traps these particles, which helps protect the atmosphere from damage.

The belts can harm satellites. These spacecraft must have special shielding to protect their sensitive parts if they travel near the belts for long periods. Astronauts who passed through the Van Allen belts during the Apollo missions received a very small and safe amount of radiation.

In 2013, the Van Allen Probes discovered a third radiation belt that lasted for about four weeks.

Discovery

Kristian Birkeland, Carl Størmer, Nicholas Christofilos, and Enrico Medi studied the possibility of charged particles being trapped in 1895, creating a theory about radiation belts. The second Soviet satellite, Sputnik 2, which had instruments created by Sergei Vernov, and later the US satellites Explorer 1 and Explorer 3, proved the existence of the belt in early 1958. This belt was later named after James Van Allen from the University of Iowa. The radiation belts were first mapped by Explorer 4, Pioneer 3, and Luna 1.

The term "Van Allen belts" refers to the radiation belts around Earth. Similar belts have been found around other planets. The Sun does not have long-term radiation belts because it lacks a strong, steady magnetic field. Earth's atmosphere keeps the belts' particles in areas above 200–1,000 km (120–620 mi), and the belts do not extend beyond 8 Earth radii (R E). The belts are limited to a space that spans about 65 degrees on either side of the celestial equator.

In 1958, the United States tested small nuclear bombs at an altitude of 300 miles, causing a temporary increase in the number of electrons in the radiation belts. These tests, called Project Argus, were meant to test the Christofilos effect, the idea that nuclear explosions in space could release enough electrons trapped by Earth's magnetic field to stop the warheads on intercontinental ballistic missiles. The project ended because of a treaty banning nuclear testing in the atmosphere and concerns that extra radiation might harm the Apollo moon mission.

Research

The NASA Van Allen Probes mission seeks to understand how fast-moving electrons and ions in space form or change when solar activity and the solar wind change. Studies supported by NASA’s Institute for Advanced Concepts have suggested using magnetic scoops to collect antimatter that naturally exists in Earth’s Van Allen belts, though only about 10 micrograms of antiprotons are believed to be present in the entire belt.

The Van Allen Probes mission launched successfully on August 30, 2012. The main mission was planned to last two years, with supplies expected to last four years. The probes were shut down in 2019 after running out of fuel, and the last probe is expected to leave Earth’s orbit during the 2030s. NASA’s Goddard Space Flight Center oversees the Living With a Star program, which includes the Van Allen Probes and the Solar Dynamics Observatory (SDO). The Applied Physics Laboratory was responsible for building and managing the instruments used in the Van Allen Probes.

Radiation belts exist around other planets and moons in the Solar System that have strong and stable magnetic fields. These belts have been observed at Jupiter, Saturn, Uranus, and Neptune through direct measurements by spacecraft such as Galileo and Juno at Jupiter, Cassini–Huygens at Saturn, and Voyager and Pioneer missions. Scientists have also used radio signals from energetic particles trapped in a planet’s magnetic field to detect radiation belts, including at Jupiter and the ultracool dwarf star LSR J1835+3259. Mercury may be able to trap charged particles in its magnetic field, but its rapidly changing magnetosphere may not support stable radiation belts. Venus and Mars do not have radiation belts because their magnetic fields do not trap charged particles in orbit.

Geomagnetic storms can cause the number of electrons in radiation belts to increase or decrease quickly, often within a day or less. Long-term processes shape the overall structure of the belts. After electrons are injected into the belts, their numbers often decrease over time. These time periods are called "lifetimes." Data from Van Allen Probe B’s Magnetic Electron Ion Spectrometer (MagEIS) show that electrons in the inner belt have lifetimes longer than 100 days, electrons in the space between the belts have lifetimes of about one or two days, and electrons in the outer belt have lifetimes of roughly five to 20 days, depending on their energy.

Inner belt

The inner Van Allen Belt usually spans from an altitude of 1,000 km (620 mi) to 12,000 km (7,500 mi) above Earth’s surface, or from 0.2 to 2 Earth radii (L values of 1.2 to 3). In some situations, such as during strong solar activity or near the South Atlantic Anomaly, the inner edge of the belt can get as close as 200 km above Earth. The inner belt holds large numbers of electrons with energies in the hundreds of keV and protons with energies over 100 MeV. These particles are trapped by the strong magnetic fields in this region, which are stronger than those in the outer belt.

Scientists believe that protons with energies above 50 MeV in the lower parts of the inner belt come from a process called beta decay. This happens when neutrons, created by collisions between cosmic rays and atoms in the upper atmosphere, change into protons and electrons. Lower-energy protons are thought to come from proton diffusion, which occurs when magnetic fields change during geomagnetic storms.

Because the Van Allen Belts are slightly shifted from Earth’s center, the inner belt comes closest to Earth’s surface near the South Atlantic Anomaly.

In March 2014, the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) on the Van Allen Probes observed "zebra stripe" patterns in the radiation belts. At first, scientists thought these stripes were caused by Earth’s rotation creating a weak electric field due to the tilt of the magnetic field. However, a 2016 study suggested the stripes were caused by winds in Earth’s ionosphere affecting the radiation belts.

Outer belt

The outer belt is made mostly of high-energy electrons (0.1–10 MeV) that are trapped by Earth's magnetosphere. It changes more than the inner belt because it is affected more by the Sun. The outer belt is shaped like a donut, starting at an altitude of 3 Earth radii and extending to 10 Earth radii (13,000 to 60,000 kilometers above Earth's surface). Its strongest energy is usually found around 4 to 5 Earth radii. The outer electron radiation belt is formed mainly by electrons moving inward and gaining energy from waves in space called whistler-mode plasma waves. Electrons in the belt are also lost through collisions with Earth's atmosphere, movement into the magnetopause, and outward movement. The paths of high-energy protons are large enough to reach Earth's atmosphere. In the outer belt, electrons have a high number of particles. Near the outer edge of the belt, where Earth's magnetic field lines extend into space, the number of energetic electrons can drop to levels similar to those in space between planets, decreasing by a factor of 1,000 over about 100 kilometers.

In 2014, scientists found that the inner edge of the outer belt has a sharp boundary where electrons with energy greater than 5 MeV cannot enter. The reason for this boundary is not yet understood.

The outer belt contains a mix of electrons and ions, mostly energetic protons, but also some alpha particles and oxygen ions. These ions are similar to those in Earth's ionosphere but have much more energy. This mix suggests that the particles in the outer belt may come from more than one source.

The outer belt is larger than the inner belt, and its particle numbers change a lot. The number of high-energy particles can increase or decrease quickly during geomagnetic storms, which are caused by disturbances in the Sun's magnetic field and plasma. These changes happen because particles from the magnetosphere's tail are injected and accelerated during storms. Another reason for changes in the outer belt is interactions between particles and plasma waves of many different frequencies.

On February 28, 2013, scientists discovered a third radiation belt made of very high-energy charged particles. NASA's Van Allen Probe team said this belt was created by a coronal mass ejection from the Sun. This third belt splits the outer belt on its outer side, like a knife, and acts as a temporary storage area for particles for about a month before merging back with the outer belt.

The stability of the third belt is explained by Earth's magnetic field trapping the very high-energy particles as they leave the outer belt. Unlike the outer zone, which forms and disappears quickly due to interactions with the atmosphere, the particles in the third belt are too energetic to interact with atmospheric waves at low latitudes. This lack of interaction and trapping allows them to stay for a long time. They are only destroyed by rare events, such as shock waves from the Sun.

Flux values

In the radiation belts, the number of particles changes depending on location, energy, and solar activity. Protons with enough energy (more than 20 MeV) to go through 0.25 mm of aluminum can have up to 100,000 particles per square centimeter per second. Electrons with energy over 1.5 MeV can also go through that thickness of aluminum, and their numbers can reach up to one million particles per square centimeter per second.

The proton belts include protons with kinetic energy ranging from about 100 keV, which can go through 0.6 micrometers of lead, to over 400 MeV, which can go through 143 mm of lead.

Radiation levels in the belts are dangerous to humans if they are exposed for a long time. The Apollo missions reduced dangers for astronauts by sending spacecraft quickly through the thinner parts of the upper belts and avoiding the inner belts completely, except for the Apollo 14 mission, which passed through the center of the trapped radiation belts.

• Flux values under normal solar conditions
• AP8 MIN omnidirectional proton flux ≥ 100 keV
• AP8 MIN omnidirectional proton flux ≥ 1 MeV
• AP8 MIN omnidirectional proton flux ≥ 400 MeV

Antimatter confinement

In 2011, a study proved a previous idea that the Van Allen belt can trap antiparticles. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) experiment found antiproton levels much higher than expected from normal particle decays while passing through the South Atlantic Anomaly. This shows the Van Allen belts trap a large amount of antiprotons created when cosmic rays hit Earth's upper atmosphere. Scientists measured the energy of these antiprotons to be between 60 and 750 MeV.

The very high energy from antimatter annihilation has led to ideas for using antiprotons to power spacecraft. This idea depends on creating tools to collect and store antimatter.

Implications for space travel

Spacecraft that travel beyond low Earth orbit enter the radiation zone of the Van Allen belts. Beyond these belts, spacecraft face more dangers from cosmic rays and solar particle events. A region between the inner and outer Van Allen belts lies between 2 to 4 Earth radii and is sometimes called the "safe zone."

Solar cells, circuits, and sensors can be harmed by radiation. Geomagnetic storms sometimes damage electronic parts on spacecraft. Making electronics smaller and more digital has made satellites more likely to be affected by radiation, because the charge in these circuits is now similar to the charge from incoming ions. Electronics on satellites must be protected from radiation to work properly. The Chandra Space Telescope turns off its sensors when passing through the Van Allen belts. The INTEGRAL space telescope was placed in an orbit designed to avoid spending time inside the belts.

The Apollo missions were the first time humans traveled through the Van Allen belts, which was one of several radiation dangers known to mission planners. The astronauts had low radiation exposure in the Van Allen belts because they spent only a short time traveling through them.

Causes

The inner and outer Van Allen belts form due to different processes. The inner belt is mainly made of high-energy protons created when neutrons break down. These neutrons form when cosmic rays collide with Earth's upper atmosphere. The outer Van Allen belt is mostly made of electrons. These electrons are sent from Earth's magnetic tail during geomagnetic storms and gain energy through interactions with waves in space.

In the inner belt, particles from the Sun are trapped by Earth's magnetic field. These particles spiral around magnetic field lines while moving side to side along them. As they travel toward the poles, the magnetic lines get closer together, slowing their side-to-side movement and sometimes reversing it. This causes the particles to bounce back and forth between Earth's poles. Electrons also drift slowly eastward, while protons drift westward.

The space between the inner and outer Van Allen belts is called the "safe zone" or "safe slot," where medium Earth orbits are located. This gap forms because very low frequency (VLF) radio waves scatter particles, adding ions to the atmosphere. Solar outbursts can send particles into the gap, but these particles leave within days. Earlier, scientists thought VLF radio waves came from turbulence in the radiation belts. However, recent research by J.L. Green from the Goddard Space Flight Center compared lightning activity data from the Microlab 1 spacecraft with radio wave data from the IMAGE spacecraft. This showed that VLF radio waves are actually created by lightning in Earth's atmosphere. These waves reach the ionosphere at high latitudes, where the gap nears the upper atmosphere. Scientists are still discussing these findings.

Proposed removal

Removing electrically charged particles from the Van Allen belts could create new paths for satellites and improve safety for astronauts traveling in space. Removing radiation belts around other planets has also been suggested. For example, scientists might consider this before exploring Europa, a moon that orbits inside Jupiter's radiation belt. Because the radiation belts are part of a complicated system, it is unclear if removing them might cause unexpected problems. One idea to remove the radiation fields from Earth's Van Allen belts is called High Voltage Orbiting Long Tether, or HiVOLT. This concept was first proposed by Russian physicist V. V. Danilov and later improved upon by Robert P. Hoyt and Robert L. Forward. Another plan to reduce the Van Allen belts includes sending very-low-frequency (VLF) radio waves from Earth's surface into the belts.

Additional sources

• Adams, L.; Daly, E. J.; Harboe-Sorensen, R.; Holmes-Siedle, A. G.; Ward, A. K.; Bull, R. A. (December 1991). "Studying SEU and total dose in geostationary orbit during normal and solar flare conditions." IEEE Transactions on Nuclear Science. Volume 38, issue 6, pages 1686–1692. Bibcode: 1991ITNS…38.1686A. DOI: 10.1109/23.124163. OCLC 4632198117.
• Holmes-Siedle, Andrew; Adams, Len (2002). Handbook of Radiation Effects (2nd edition). Published by Oxford University Press in Oxford and New York. ISBN 978-0-19-850733-8. LCCN 2001053096. OCLC 47930537.
• Shprits, Yuri Y.; Elkington, Scott R.; Meredith, Nigel P.; Subbotin, Dmitriy A. (November 2008). "Review of modeling of losses and sources of relativistic electrons in the outer radiation belt." Journal of Atmospheric and Solar-Terrestrial Physics. Volume 70, issue 14. Part I: Radial transport, pages 1679–1693, DOI: 10.1016/j.jastp.2008.06.008; Part II: Local acceleration and loss, pages 1694–1713, DOI: 10.1016/j.jastp.2008.06.014.