Van Allen radiation belts

The Van Allen radiation belts are concentrated rings of ionized particles in Earth’s magnetosphere. Detailed study of the radiation belts led to an understanding of certain phenomena occurring in the ionosphere and the determination of the physical properties of the exosphere.

Overview

The Van Allen radiation belts are concentrated, torus-shaped regions of charged particles within Earth’s magnetosphere. These particles, made up of protons, electrons, and other ions, spiral about in great numbers between Earth’s magnetic poles. The magnetic and charged particles within the Van Allen belts can be divided into four regions: the Van Allen geomagnetically trapped radiation region, the auroral region, the magnetosheath, and interplanetary space. The inner and outer belts are part of the Van Allen geomagnetically trapped radiation region. In discussions of the Van Allen belts, the magnetic storm is often referred to as a third radiation belt.

The inner zone stretches from about 1,000 to more than 5,000 kilometers above Earth. It is mainly independent of time. Its composition is nearly consistent with that expected for the decay products of cosmic-ray-produced neutrons in the atmosphere (a neutron is an elementary, neutral particle of mass); this zone is of cosmic-ray origin. The radiation in the middle of the inner zone is composed of electrons with energies exceeding forty kilo-electron volts (keV) and protons with energies greater than forty million electron volts (MeV). (Electrons are elementary particles with a negative charge; protons are positively charged elementary particles.) In the inner belt, many of the high-energy protons are capable of penetrating several inches of lead. At the edge of the inner zone, in the region of geomagnetic latitudes 35° to 40°, low-energy electrons are found. The decay of light-scattering neutrons gives rise to high-energy protons. Beyond Earth’s magnetic field, the mean ionizing capacity is 2.5 times higher than the minimum ionizing capacity. Particles in the inner zone are stable and exist for a long period of time.

The outer zone stretches about 15,000 to 25,000 kilometers above Earth. This zone undergoes very large temporal fluctuations, appearing to be caused by solar activity and auroras, atmospheric heating, and magnetic storms. The outer belt contains soft particles; it is of solar origin. The outer zone contains electrons with more than forty keV in energy and protons with more than sixty MeV in energy. The outer zone has greater geophysical significance than the inner zone. According to the comparison of E. V. Gorchakov, the boundaries of the outer zone coincide with isochasms (lines of equal probability of auroras). Trapped particles introduce magnetic effects in the outer radiation belt. This effect was measured by Luna 1. The increase in Ionization of the outer zone is unstable. Particles exist for a short period of time compared with those of the inner belt.

A third radiation belt is produced by magnetic storms. Protons are transported from the Sun in a corpuscular stream and injected by magnetic field perturbations into Earth’s field. The charge exchange with neutral hydrogen in Earth’s exosphere is the fastest mechanism of removal. This is about a hundred times faster than scattering from ions in the exosphere. With the exception of trapped radiation, the entire region in the magnetic cavity is known as the auroral region. Auroral particles, the islands or pulses in the long tail and spikes at high latitudes of 1,000 kilometers are phenomena that occur in the auroral region. Electrons of uniform angular distribution have a roughly constant intensity between 100 and 180 kilometers in altitude.

The magnetosheath lies between the shock front formed by the solar wind and the magnetic cavity. Islands of electrons have been observed in the magnetosheath. At its widest, the magnetosheath is about four times the radius of Earth. It contains a compressed, seemingly chaotic interplanetary magnetic field. The interplanetary field connected to the Sun is predominantly in the ecliptic plane. The field terminates when the solar wind undergoes a shock transition to subsonic flow.

The lifetime of trapped particles decreases with distance from Earth. The lifetime of electrons with energies greater than one MeV at a distance of 1.2 to 1.5 times Earth’s radius is about a year. The lifetime of the same electrons is reduced to days and months at a distance of 1.5 to 2.5 times Earth’s radius. At even greater distances, the lifetime of the particles is measured in minutes. Because Earth is strongly influenced by the Sun’s magnetic field, Earth’s geomagnetic field does not decrease indefinitely with increasing distance. The solar wind pushes Earth’s magnetic field and is deflected by it. At about ten Earth radii, the radiation belt ends abruptly.

Particles of trapped radiation may be lost in two ways. During a magnetic storm, the magnetosphere may lose or gain particles. This occurs at distances of 1.0 to 1.5 times Earth’s radius. The other mechanism occurs at distances greater than eight times Earth’s radius. Small, rapid variation in the magnetic field at such distances scatters trapped particles, dumping them into the atmosphere. In a similar fashion, it is seen that charged particles in Uranus’s magnetosphere are swept down into the planet’s upper atmosphere by collisions with particles in its ring system.

Beautiful auroral displays occur when the charged particles are dumped into Earth’s upper atmosphere. Solar flares eject into space streams of high-energy protons and electrons. When these beams of high-energy particles are directed toward Earth, Earth’s magnetic field is partially disrupted. Particles trapped within the field lines can escape downward toward Earth at the lower ends of the radiation belts. High-energy particles, reinforced with particles from the Sun, energize the upper atmosphere, causing luminous and often colorful auroras.

Knowledge Gained

The years 1957 and 1958 were designated the International Geophysical Year (IGY), an international scientific tour de force to advance understanding of Earth sciences. As contributions to IGY, the Soviet Union and the United States both pledged to place a satellite in orbit about the Earth. Russia’s Sputnik 1 beat the American effort to orbit. However, the American effort was the first to gather useful scientific information. With the data returned by Explorer 1, America’s first artificial satellite, a high-energy radiation belt was detected by James A. Van Allen and his assistants, George H. Ludwig, Carl E. McIlwain, and Ernest C. Ray. The same observations were made by Explorer 3, launched by the US Army on March 26, 1958, and Sputnik 3, launched by the Soviet Union on May 15, 1958.

Later, a satellite was launched as part of Project Argus, which studied the location, height, and yield of electron blasts. This project was carried out by the Advanced Research Projects Agency. The belt of electrons produced by the Argus nuclear explosions developed at a distance of twice Earth’s radius. Explorer 4, launched on July 26, 1958, carried four Geiger counters to handle high levels of radiation. One of these Geiger counters was shielded with a thin layer of lead to keep out most of the radiation. The satellite reached a height of 2,200 kilometers and registered an intensity of high-energy radiation. From the data returned, scientists concluded that Earth is surrounded by belts of high-energy radiation consisting of particles originating from the Sun and trapped in the lines of force of Earth’s magnetic field. These were named the “Van Allen radiation belts.”

Explorer 4 obtained a kidney-shaped intensity contour of Earth’s inner belt. Data from early Pioneer spacecraft suggested a solar origin of soft particles populating the outer zone. Three Pioneer probes and Luna 1 discovered the crescent-shaped intensity contours of the outer belt. Sputnik 3 data helped identify the bulk of the outer belt particles as low-energy electrons (ten to fifty keV).

Several more human-made belts were produced in 1962. The Starfish Project, an American venture, created a belt much wider than the Argus Belt. The decay of some of the particles took several years in low altitudes. In the same year, the Soviets created at least three similar belts. More sophisticated versions of the instrumentation used in these early probes of Earth were incorporated into spacecraft sent to other planets in the solar system. Probes to Mercury, Jupiter, Saturn, Uranus, and Neptune have discovered radiation belts similar to Earth’s Van Allen belts. A planet needs a magnetic field to trap charged particles in a radiation belt or system of radiation belts. For that very reason, there are no significant belts of trapped charged particles at Venus or Mars. Exploration of the Van Allen belts continued into the twenty-first century. In August 2012, NASA launched the Van Allen Probes mission which lasted until the probe was deactivated in 2019. The goal of this mission was to understand how populations of ions and electrons responded to solar activity and solar winds.

Context

In the earliest days of space exploration, gauging the intensity of Earth’s radiation belts with uncrewed spacecraft was crucial as a first step toward sending humans into space. Both the United States and the Soviet Union had a vested interest in the results of early investigations of the magnetosphere.

The Van Allen belts, while lifesaving in that they keep dangerous radiation from reaching the surface of the planet, are potentially hazardous to Earth-orbiting spacecraft. They threaten electronic systems and instrumentation and can interfere with radio transmissions. In the late 1950s, it was not known just how hazardous the radiation surrounding Earth would prove to humans. While it would not be wise to base a space station within the radiation belts, the belts themselves pose little threat to humans, who quickly punch through on voyages of exploration beyond the Earth; that was the case back in the Apollo program and was also true of the National Aeronautics and Space Administration’s(NASA’s) planned Constellation program flights to the International Space Station, the Moon, and later to Mars, though this program was ultimately shelved due to a lack of funding. However, leaving the safety of orbit beneath the Van Allen belts does expose astronauts to the potential hazards of ionizing radiation streaming outward into the solar system from the Sun. Therefore, special protection must be provided to humans on expeditions beyond low-Earth orbit.

The relationship between auroras and the Van Allen belts has been studied for decades, but although the overall phenomenon has been well characterized, all is not completely understood. Scientists do know that most bright auroras are produced by electrons dumped into Earth’s atmosphere by solar flares. The auroral particles are the electrons escaping from the outer Van Allen radiation belt. The average kinetic energy of the electrons is thirty-two keV. The leakage of corpuscular radiation into the auroral zones is the most important loss of corpuscular radiation from the outer Van Allen belt.

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