The Van Allen radiation belt is a region of charged particles with energy, most of which come from the solar wind. These particles are trapped and held around a planet by the planet's magnetosphere. Earth has two main belts, and sometimes other belts may form temporarily. The belts are named after James Van Allen, who wrote a scientific paper about them in 1958.

Earth's two main belts stretch from about 640 to 58,000 km (400 to 36,040 mi) above the planet'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 types of particles, like alpha particles, are found in smaller amounts. Most of the particles that make up the belts come from the solar wind, while some come from cosmic rays. Earth's magnetic field traps these particles, helping to protect the atmosphere from damage.

The belts can harm satellites. To stay safe, satellites must have special shielding for their sensitive parts if they spend time near the belts. During space missions, astronauts passing through the Van Allen belts received a very small and safe amount of radiation.

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

Discovery

In 1895, Kristian Birkeland, Carl Størmer, Nicholas Christofilos, and Enrico Medi studied the possibility of charged particles being trapped in space, creating a scientific foundation for understanding radiation belts. The second Soviet satellite, Sputnik 2, which had detectors designed by Sergei Vernov, and later the US satellites Explorer 1 and Explorer 3, proved the existence of these belts in early 1958. These belts were later named the Van Allen belts after James Van Allen from the University of Iowa. The first maps of the radiation belts were created 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 stable, global magnetic field. Earth's atmosphere keeps the belts' particles limited to areas above 200–1,000 km (120–620 mi). These belts do not extend beyond 8 Earth radii (Rₐ). The belts are contained within a region that spans about 65 degrees on either side of the celestial equator.

In 1958, the United States tested low-yield nuclear bombs at an altitude of 300 miles. These tests, called Project Argus, aimed to test the Christofilos effect, which suggested that nuclear explosions in space could release enough electrons trapped in Earth's magnetic field to disable warheads on intercontinental ballistic missiles. The project was stopped because of a treaty banning atmospheric nuclear testing and concerns that extra radiation might harm the Apollo moon mission.

Research

The NASA Van Allen Probes mission studies how high-energy electrons and ions in space change when solar activity or the solar wind changes. Some studies, funded by NASA's Institute for Advanced Concepts, suggest using magnetic scoops to collect small amounts of antimatter found in Earth's Van Allen belts. Scientists estimate only about 10 micrograms of antiprotons exist in the entire belt.

The Van Allen Probes launched successfully on August 30, 2012. The mission was planned to last two years, with fuel expected to last four years. The probes stopped working in 2019 when they ran out of fuel. The last probe is expected to re-enter Earth's atmosphere 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 managed the mission's design and instruments.

Radiation belts form around planets and moons with strong, stable magnetic fields. These belts have been found around Jupiter, Saturn, Uranus, and Neptune using spacecraft like Galileo, Juno, Cassini–Huygens, and Voyager. Scientists also use radio signals from energetic particles trapped in magnetic fields to detect radiation belts, such as those around Jupiter and the star LSR J1835+3259. Mercury may trap charged particles in its magnetic field, but its changing magnetosphere might not support stable belts. Venus and Mars lack radiation belts because their magnetic fields do not trap charged particles.

Geomagnetic storms can cause the number of electrons in radiation belts to increase or decrease quickly, often within one day. Long-term processes shape the overall structure of the belts. After electrons are added to the belts, their numbers often decrease over time. This decrease is measured as "lifetimes." Data from Van Allen Probe B's Magnetic Electron Ion Spectrometer (MagEIS) shows that electrons in the inner belt have lifetimes longer than 100 days. In the area between the belts, called the "slot," electrons have lifetimes of about one or two days. In the outer belt, lifetimes depend on energy and range from five to 20 days.

Inner belt

The inner Van Allen Belt usually stretches 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 may move closer to Earth, reaching about 200 km above the surface. The inner belt holds large numbers of high-energy electrons (hundreds of keV) and protons with energy greater than 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 energy over 50 MeV in the lower parts of the inner belt come from neutrons created when cosmic rays collide with atoms in Earth’s upper atmosphere. These neutrons then decay into protons. Lower-energy protons are thought to form when protons move through the magnetic field 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 Van Allen Probes observed a pattern of "zebra stripes" in the radiation belts using the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE). At first, researchers thought these stripes were caused by Earth’s rotation creating a weak electric field due to the tilt of Earth’s 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 often than the inner belt because it is easily affected by activity from the Sun. The outer belt has a shape similar to a donut, starting at an altitude of 3 Earth radii (about 13,000 kilometers) and extending to 10 Earth radii (about 60,000 kilometers) above Earth's surface. Its strongest energy levels are usually found between 4 to 5 Earth radii. The outer belt is formed mainly by electrons moving inward and gaining energy from waves in space. These electrons are also removed by collisions with Earth's atmosphere, escaping through the magnetopause, or moving outward. The large paths of energetic protons would allow them to reach Earth's atmosphere. Inside the outer belt, electrons have very high numbers, but near the edge close to the magnetopause, where Earth's magnetic field lines stretch 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 within about 100 kilometers.

In 2014, scientists found that the inner edge of the outer belt has a sharp boundary where electrons with very high energy (>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 much more energetic. 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. High-energy particle levels can rise or fall 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. Another reason for changes is interactions between particles and different types of plasma waves.

On February 28, 2013, scientists discovered a third radiation belt made of high-energy charged particles. NASA's Van Allen Probe team said this belt was created by a coronal mass ejection from the Sun. It is described as splitting the outer belt like a knife and existing separately for about a month before merging back with the outer belt.

The third belt stays stable for a long time because its extremely fast-moving particles are trapped by Earth's magnetic field after leaving the outer belt. Unlike the outer belt, which changes daily due to interactions with the atmosphere, the particles in the third belt are too energetic to be scattered by atmospheric waves at low latitudes. This lack of scattering and trapping allows them to remain for a long time, until an unusual event, like a shock wave from the Sun, destroys them.

Flux values

The number of particles in the belts changes a lot depending on where you are, their energy, and how active the Sun is. 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 each second. Electrons with more than 1.5 MeV can go through that same aluminum thickness, and their numbers can reach up to a million per square centimeter each second.

Proton belts have protons with energy 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 in the belts is very dangerous to humans if they stay there for a long time. The Apollo missions reduced risks by sending spacecraft quickly through the thinner parts of the upper belts, avoiding the inner belts entirely, except for Apollo 14, which passed through the center of the trapped radiation belts.

Antimatter confinement

In 2011, a study proved a previous idea that the Van Allen belt can trap antiparticles. The PAMELA experiment found much higher levels of antiprotons than expected from normal particle decays while passing through the South Atlantic Anomaly. This shows the Van Allen belts trap a large number of antiprotons created when cosmic rays hit Earth's upper atmosphere. The energy levels of these antiprotons range from 60 to 750 MeV.

The huge amounts of energy from antimatter annihilation have led to ideas for using antiprotons to power spacecraft. This concept depends on creating special 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. These belts are regions around Earth that trap charged particles. Beyond the belts, spacecraft face additional dangers from cosmic rays and solar particle events. Between the inner and outer Van Allen belts lies a region located 2 to 4 Earth radii away from Earth’s center. This area is sometimes called the "safe zone" because it has lower radiation levels.

Radiation can damage solar cells, integrated circuits, and sensors on spacecraft. Geomagnetic storms sometimes harm electronic parts on spacecraft. As electronics have become smaller and more advanced, they are now more sensitive to radiation. The total electric charge in these circuits is now similar to the charge of incoming ions. To function properly, satellite electronics must be protected from radiation. The Chandra Space Telescope turns off its sensors when passing through the Van Allen belts. The INTEGRAL space telescope was placed in an orbit that avoids spending time inside the belts.

The Apollo missions were the first time humans traveled through the Van Allen belts. Mission planners knew about the radiation risks. The astronauts had low exposure to radiation in the belts because they spent only a short time passing through them.

Causes

The inner and outer Van Allen belts are created by different processes. The inner belt is mostly made up of high-energy protons. These protons come from neutrons, which are formed when cosmic rays hit the upper atmosphere. The outer Van Allen belt is mainly made of electrons. These electrons are sent from the geomagnetic tail during geomagnetic storms and gain energy through interactions between waves and particles.

In the inner belt, particles from the Sun are trapped by Earth’s magnetic field. These particles move in a spiral around the magnetic field lines while also moving side to side along those lines. As particles travel toward the poles, the magnetic field becomes stronger, slowing their side-to-side movement and sometimes reversing it. This causes the particles to bounce back and forth between Earth’s poles. Electrons in the inner belt also slowly drift eastward, while protons drift westward.

There is a space between the inner and outer Van Allen belts. This area is sometimes called the "safe zone" or "safe slot," and it is where medium Earth orbits are located. This gap is caused by very low frequency (VLF) radio waves, which scatter particles and send them into the atmosphere. Solar outbursts can also send particles into the gap, but these particles usually leave within a few 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 maps from the Microlab 1 spacecraft with VLF radio wave data from the IMAGE spacecraft. The findings suggest that VLF radio waves are actually created by lightning in Earth’s atmosphere. These radio waves reach the ionosphere at the correct angle only at high latitudes, where the gap nears the upper atmosphere. Scientists are still discussing these results.

Proposed removal

Removing charged particles from the Van Allen belts could create new paths for satellites and reduce risks for astronauts traveling in space. Scientists have also considered removing radiation belts around other planets, such as before exploring Europa, a moon that orbits inside Jupiter's radiation belt. Because radiation belts are part of a complicated system, it is unclear if removing them might cause unexpected problems.

One idea for removing the radiation from Earth's Van Allen belts is called High Voltage Orbiting Long Tether, or HiVOLT. This concept was first suggested by Russian physicist V. V. Danilov and later improved by Robert P. Hoyt and Robert L. Forward. Another idea involves sending very-low-frequency (VLF) radio waves from Earth's surface into the Van Allen belts.