Van Allen Belt: A Rampart Against Cosmic Particles

Van Allen belts: a journey into the heart of radiation

The discovery of a third radiation belt

NASA's Van Allen Twin Probes, launched on August 30, 2012, revealed a new radiation belt around Earth. The mission discovered a third radiation belt previously unknown within these dangerous regions of space. The Van Allen probes consist of two identical satellites, judiciously separated in space to better spot events, map this region, catalog high-energy particles and track the magnetic waves escaping from the belts.

The magnetosphere and the work of James Van Allen

The magnetosphere was described in 1958 by James Van Allen (1914-2006) through measurements made by Geiger counters aboard the first American satellite, Explorer 1. It was at that time that this region of space called the Van Allen belt was discovered. Data from Explorer 1, supplemented by those from Explorer 3, made it possible to demonstrate the existence of radiation belts surrounding Earth, a result published by Van Allen and his collaborators in the journal Jet Propulsion in 1958.

Structure and dynamics of the belts

The Van Allen belt, affected by solar storms, consists of two distinct parts of radiation with different characteristics. The first, closest to Earth, is located between 700 and 10,000 km altitude; it consists mainly of high-energy protons. The second, the larger outer part, is located between 13,000 and 65,000 km altitude; it consists of high-energy electrons. These two belts are separated by a region called the "slot", generally devoid of energetic particles.

This discovery shows the dynamic and variable nature of the radiation belts of our planet. The third belt, observed in 2012, persisted for four weeks and then disappeared, annihilated by an interplanetary shock wave — that of the solar eruption of August 31, 2012 — according to Daniel Baker, principal investigator of the REPT instrument at the University of Colorado. The results were published in the journal Science on February 28, 2013.

Origin of trapped particles: capture mechanisms

Magnetic trapping

The charged particles that populate the Van Allen belts are not there by chance. They are trapped by Earth's magnetic field, which acts like an immense dipolar magnet. A charged particle entering this field describes a helical motion around the magnetic field lines: it spirals along a line while drifting longitudinally around Earth. At the poles, where the field lines converge, the particle undergoes a magnetic mirror effect: it is reflected back toward the equator, oscillating between the two hemispheres. This confinement mechanism explains the persistence of the belts over periods that can reach years.

The CRAND mechanism and the inner belt

The origin of the high-energy protons of the inner belt has long puzzled physicists. As early as the 1960s, it was proposed that these protons come from the process of cosmic ray albedo neutron decay (CRAND). In this mechanism, galactic cosmic rays penetrate the upper atmosphere and collide with neutral atoms, producing neutrons. These neutrons, unstable, decay into protons and electrons. The protons, which retain most of the neutron's kinetic energy, are then trapped by Earth's magnetic field and feed the inner belt.

In 2017, a team led by Xinlin Li published in the journal Nature measurements confirming that CRAND is indeed the main source of relativistic electrons near the inner edge of the inner belt, and that it also contributes to electrons elsewhere in the inner belt. The neutron density measured in near-Earth space — about \(2 \times 10^{-9}\) per cubic centimeter — confirmed theoretical estimates. For the outer belt, electrons come mainly from the solar wind and plasma injections during geomagnetic storms.

South Atlantic Anomaly

Earth's magnetic field is not perfectly symmetric. Above the South Atlantic, a region known as the South Atlantic Anomaly (SAA) sees the geomagnetic field weaken, allowing the inner belt to dip to about 200 km altitude. Low Earth orbit satellites, including the International Space Station (ISS), regularly pass through this anomaly and experience higher than normal radiation levels. The ISS, with its orbital inclination of 51.6°, requires additional shielding to protect its electronic equipment and occupants. The SAA is thus a major factor in planning low Earth orbit missions.

Space weather and technological risks

Impact on satellites and electronic systems

The energetic particles of the Van Allen belts constitute a major risk for satellites and space systems. Relativistic electrons of the outer belt can penetrate shielding and cause electronic failures, disturbances on solar panels and internal electrical discharges. Telecommunications satellites in geostationary orbit, which evolve near the outer edge of the belt, are particularly exposed. GPS satellites, space observatories like Hubble, and probes in transit to higher orbits must cross the belts multiple times, accumulating radiation doses that can degrade their components over the years.

The May 2024 solar storm

In May 2024, the largest solar storm in two decades struck Earth. For several days, successive waves of high-energy charged particles from the Sun bombarded Earth's magnetosphere, producing exceptional auroras and temporarily disrupting some GPS communications. Thanks to NASA's CIRBE CubeSat (Colorado Inner Radiation Belt Experiment), scientists discovered that this storm had created two new temporary radiation belts, wedged between the two permanent Van Allen belts.

One of these new belts was composed mainly of electrons, while the other, more internal, contained a population of energetic protons — a composition never observed before. The proton belt, located in a magnetically more stable region, persisted much longer than previous temporary electron belts. The results, published on February 6, 2025 in the Journal of Geophysical Research: Space Physics, underline the importance of monitoring the belts to protect satellites launched toward geostationary orbit, which must cross them multiple times before reaching their final position.

Van Allen belts: a challenge for space exploration

Probe orbits and the REPT instrument

The two Van Allen Probes had roughly the same eccentric orbit, covering the entire radiation zone of the belts. The REPT instrument (Relativistic Electron Proton Telescope) aboard the probes revealed a third structure distinct from the Van Allen belt with a second empty zone of space, between the two outer belts. The data collected by the first twin spacecraft to analyze the Van Allen belt were published on February 28, 2013 in the journal Science.

The scientific institutions involved

Scientists observed the third belt for four weeks as a powerful shock wave struck the magnetosphere. The observations were made by researchers from LASP (Laboratory for Atmospheric and Space Physics), NASA's Goddard Space Flight Center, Los Alamos National Laboratory and the Institute for the Study of Earth at the University of New Hampshire. The Van Allen Probes mission operated for nearly seven years, well beyond its initial planned duration of two years, before being decommissioned in 2019.

The Van Allen belts: between protection and threat

The solar wind and the magnetosphere

Most solar energy is released into space as electromagnetic radiation, mainly as visible light, but the Sun also emits a flux of charged particles called the solar wind. This solar wind interacts strongly with the magnetosphere of planets and contributes to cleaning interplanetary space by ejecting gases and dust outside the solar system.

Earth's magnetic shield

Solar eruptions continuously send high-energy particles into space and, from time to time, a bubble of radioactive and superheated plasma reaches Earth. All the energetic particles emitted by the Sun toward Earth travel at high speed and are continuously brought back toward the poles of the magnetosphere. Thus the solar wind never strikes Earth's surface directly.

This magnetic bubble shelters us from solar radiation. It is the magnetosphere, and not the Van Allen belts themselves, that ensures Earth's protection: the belts are its manifestation, the reservoir of trapped particles. The magnetic field acts as a screen deflecting the electric current of the solar wind, which then flows outside the magnetosphere. Prolonged exposure to the densest fluxes of the Van Allen radiation belt can exceed the permissible dose limits for humans.

Apollo astronauts facing radiation

Few men have crossed the magnetosphere to find themselves facing solar radiation: only the Apollo astronauts who traveled to the Moon had to cross the Van Allen belt. The Apollo missions followed trajectories specifically calculated to minimize transit time in the most intense zones, avoiding the inner belt as much as possible. The astronauts spent about one hour in total in the most exposed regions.

According to onboard dosimeters, the total doses recorded for all Apollo missions ranged between 0.16 rad (Apollo 8) and 1.14 rad (Apollo 14) — approximately 1.6 to 11.4 millisieverts for the complete mission, not only the belt crossing. For comparison, an abdominal CT scan delivers about 10 millisieverts, and the annual limit for nuclear industry workers in France is set at 20 millisieverts. The Apollo command module, with its aluminum and stainless steel hull, offered sufficient shielding to keep these doses well below immediate danger thresholds.