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

Earth's two main belts stretch from about 640 to 58,000 kilometers (400 to 36,040 miles) above the planet's surface. Radiation levels in this area vary. The belts are located in the inner part of Earth's magnetic field. They trap high-energy electrons and protons. Other particles, like alpha particles, are less common. Most of the particles in the belts are believed to come from the solar wind, while others arrive as cosmic rays. Earth's magnetic field captures these particles, which helps protect the atmosphere from damage.

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

In 2013, scientists using the Van Allen Probes discovered a third radiation belt that existed 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 that explained how radiation belts could form. The second Soviet satellite, Sputnik 2, which had detectors designed by Sergei Vernov, and later the US satellites Explorer 1 and Explorer 3, confirmed the existence of the belt in early 1958. This belt was later named the Van Allen belts after James Van Allen from the 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 around Earth. Similar radiation belts have been found around other planets. The Sun does not have long-term radiation belts because it lacks a stable magnetic field that covers the entire planet. Earth's atmosphere keeps the belts' particles limited to areas above 200–1,000 km (120–620 mi), and the belts do not extend beyond 8 Earth radii (Rₑ). The belts are contained within a space that spans about 65° on either side of the celestial equator.

In 1958, the United States tested low-yield 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, which suggested that nuclear explosions in space could release enough electrons trapped by 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 seeks to understand how high-energy electrons and ions in space form or change when solar activity and the solar wind change. Studies funded by the NASA Institute for Advanced Concepts have suggested using magnetic scoops to collect antimatter that naturally exists in Earth’s Van Allen belts. However, 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 primary mission was planned to last two years, with fuel expected to last four years. The probes were shut down in 2019 when fuel ran out, and the last probe is expected to leave Earth’s orbit during the 2030s. NASA’s Goddard Space Flight Center manages 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 on 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 around Jupiter, Saturn, Uranus, and Neptune using spacecraft such as Galileo and Juno at Jupiter, Cassini–Huygens at Saturn, and Voyager and Pioneer missions. Scientists have also used radio emissions from high-energy particles trapped in magnetic fields to detect radiation belts, including around Jupiter and the ultracool dwarf star LSR J1835+3259. Mercury may have the ability to trap charged particles in its magnetic field, but its rapidly changing magnetosphere might not support stable radiation belts. Venus and Mars do not have radiation belts because their magnetic fields do not trap charged particles effectively.

Geomagnetic storms can cause the number of electrons in radiation belts to increase or decrease quickly, often within one day. Over longer periods, other processes shape the overall structure of the belts. After an increase in electron numbers, scientists often observe a gradual decrease, called "lifetimes." Data from the Van Allen Probe B’s Magnetic Electron Ion Spectrometer (MagEIS) shows that electrons in the inner belt have long lifetimes (more than 100 days). Electrons in the region between the belts, called the "slot," have lifetimes of about one or two days. In the outer belt, lifetimes vary depending on energy, ranging from about five to 20 days.

Inner belt

The inner Van Allen Belt usually extends from a height of 0.2 to 2 Earth radii (L values of 1.2 to 3) or 1,000 km (620 mi) to 12,000 km (7,500 mi) above Earth. In some situations, such as during strong solar activity or in areas like 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 electrons with energies in the hundreds of keV and protons with energies 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 energies above 50 MeV in the lower belts are created when neutrons, formed by collisions between cosmic rays and atoms in the upper atmosphere, undergo beta decay. Lower-energy protons are thought to come from proton diffusion, which happens 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 a pattern resembling "zebra stripes" in the radiation belts. An early theory suggested that Earth’s rotation, combined with the tilt of its magnetic field, created a weak electric field that caused the stripes. However, a 2016 study found that the stripes were caused by the influence of ionospheric winds on 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 belt is shaped like a donut, starting at an altitude of 3 Earth radii (about 13,000 kilometers or 8,100 miles) and extending to 10 Earth radii (about 60,000 kilometers or 37,300 miles) above Earth’s surface. Its strongest energy is usually found between 4 to 5 Earth radii. The outer electron radiation belt is mainly created by electrons moving inward and gaining energy from waves in space. These electrons are also lost through collisions with Earth’s atmosphere, escaping into space, or moving outward. The paths of high-energy protons are large enough to reach Earth’s atmosphere. In the outer belt, electrons have very high numbers, but near the edge (close to where Earth’s magnetic field lines open into space), their numbers can drop to levels similar to space between planets within about 100 kilometers (62 miles)—a decrease of 1,000 times.

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 barrier is still unknown.

The outer belt contains a mix of electrons and ions, mostly high-energy 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. 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 increases happen when particles are injected and sped up from the tail of Earth’s magnetosphere. Another reason for changes in the outer belt is interactions between particles and different types of waves in space.

On February 28, 2013, scientists discovered a third radiation belt made of extremely 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 on its outer side and existing separately for about a month before merging back with the outer belt.

The third belt stays stable for a long time because Earth’s magnetic field traps the extremely high-energy particles as they leave the outer belt. Unlike the outer belt, which changes quickly 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 stability means they remain for a long time until a rare event, like a shock wave from the Sun, destroys them.

Flux values

In the radiation belts, the number of particles changes greatly depending on location, energy, and solar activity. Protons with enough energy (more than 20 MeV) to pass through 0.25 mm of aluminum can reach up to 100,000 particles per square centimeter per second. Electrons with energy over 1.5 MeV can also pass through that thickness of aluminum, and their numbers can reach up to 1 million particles per square centimeter per second.

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

Radiation levels in the belts would be harmful to humans if they were exposed for a long time. The Apollo missions reduced risks for astronauts by sending spacecraft at high speeds through the less dense parts of the upper belts and avoiding the inner belts entirely, except for the Apollo 14 mission, where the spacecraft passed through the center of the trapped radiation belts.

Antimatter confinement

In 2011, a study showed that earlier ideas about the Van Allen belt trapping antiparticles were correct. The PAMELA experiment found much more antiprotons than expected from normal particle decays when passing through the South Atlantic Anomaly. This suggests the Van Allen belts trap a large number of antiprotons created when cosmic rays hit Earth's upper atmosphere. Scientists have measured the energy of these antiprotons as ranging from 60 to 750 MeV.

The very high energy from antimatter annihilation has led to suggestions that antiprotons could be used to power spacecraft. This idea depends on creating special tools to collect and store antimatter.

Implications for space travel

Spacecraft traveling beyond low Earth orbit enter a radiation zone called the Van Allen belts. Beyond these belts, they face more 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 from Earth’s center. This area is sometimes called the "safe zone."

Solar cells, circuits, and sensors on spacecraft can be damaged by radiation. Geomagnetic storms sometimes harm electronic parts on spacecraft. Smaller and more digital electronics are more vulnerable to radiation because their tiny electric charge can be affected by the charge of incoming ions. To work properly, satellite electronics must be made strong against 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 dangers. The astronauts had low radiation exposure because they spent only a short time passing through the belts.

Causes

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

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 move toward the poles, the magnetic field becomes stronger, slowing their side-to-side movement. This can reverse their direction, sending them back toward the equator. This causes the particles to bounce between the poles. In addition to spiraling and moving side to side, electrons move slowly eastward, while protons move slowly westward.

The area between the inner and outer Van Allen belts is sometimes called the "safe zone" or "safe slot." This is where medium Earth orbits are located. The gap forms because very low frequency (VLF) radio waves scatter particles, which adds ions to the atmosphere. Solar outbursts can also send particles into the gap, but these particles leave within days. Scientists once believed 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. The findings suggest that VLF radio waves are actually created by lightning in Earth’s atmosphere. These radio waves hit the ionosphere at an angle that allows them to pass through 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 radiation risks for astronauts. Similar ideas have been suggested for other planets, such as before exploring Europa, which is located inside Jupiter's radiation belt. Because radiation belts are part of a larger system, scientists do not yet know if removing them might cause unexpected problems.

One idea to remove the radiation 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 by Robert P. Hoyt and Robert L. Forward. Another method involves sending very-low-frequency (VLF) radio waves from Earth's surface into the Van Allen belts.