Van Allen Discovers the Earth's Radiation Belts

Date July 26, 1958

James Van Allen pioneered the use of artificial satellites for Earth studies, which led to the discovery of electrically charged particles trapped within the Earth’s magnetic field. These fields were later named the Van Allen radiation belts, after their discoverer.

Also known as Van Allen radiation belts; Van Allen belts

Locale Iowa City, Iowa

Key Figures

• James Van Allen (1914-2006), American physicist

Summary of Event

William Gilbert , English physician and physicist, wrote in 1600 that Earth has a magnetic field similar to a bar magnet. He reported that a compass points roughly north and south and that it is the magnetic poles of the earth’s magnet that attract the compass needle. Because the magnetic field has two poles, it is a dipole field, as opposed to a monopole field, of an electrically charged particle.

Other research over the centuries has increased an understanding of the magnetic field. Roughly 95 percent of the field is the result of sources deep within the interior of the earth, while the remainder is attributable to electrical currents in the upper atmosphere and other sources in space. The poles are not stationary, but wander over the face of the earth. The poles are currently located in the islands north of Canada. It can be inferred, therefore, that the interior source is not a permanent bar magnet but rather a dynamo effect generated in the liquid, metallic outer core of the earth.

Until the late 1950’s, the field and its changes could be charted only on the earth’s surface. This hindered the understanding of the earth’s magnetic field and inhibited the development of better field-generation theories and determination of the field’s shape, strength, and volume of space that it occupied. This changed, however, with the development of artificial satellites. The idea of launching a payload into Earth orbit had fascinated scientists such as Robert H. Goddard for several decades.

As a result of the rocket and jet experimentation conducted during World War II, the dream of launching artificial satellites came closer to reality. The V-2 rockets designed by Wernher von Braun and others for the destruction of English cities in World War II could reach an altitude of 100 kilometers. Although they did not reach the speeds necessary to place a satellite in orbit, they were a step in the right direction. After the war, captured German scientists and rockets formed the basis for the United States’ space efforts, such as Projects Mercury, Gemini, and Apollo.

James Van Allen started studying cosmic rays as an undergraduate. He received his Ph.D. in 1939. During the war, he served as a naval officer and worked on a proximity fuse for artillery shells. This device used a radar signal emanating from the shell that reflected back from the target to trigger the fuse that caused the shell to explode. Van Allen worked to miniaturize the electronic components needed for the small confines of the shell. After the war, Van Allen worked to reduce the instrument packages being sent aloft in the captured V-2 rockets. These rockets could go higher because they were now lifting smaller payloads, but they were still not capable of orbiting these payloads into permanent Earth orbit. By 1954, however, Van Allen and his colleagues began talking about the possibility of using larger, more powerful rockets which were then under development. In 1955, President Dwight D. Eisenhower announced that the United States would launch an artificial satellite within two years.

Scientists designated the time period from July 1, 1957, to December 31, 1958, as the International Geophysical Year . During this time period, the earth and its surrounding area were to be studied intensely by scientists around the world to learn more about the planet. The Soviets announced their intention to launch an artificial satellite as part of this study. They launched Sputnik 1 into Earth orbit on October 4, 1957.

In order to demilitarize the space effort, the United States had designed the rockets of the Vanguard program from the ground up. The Soviets, on the other hand, used a rocket designed for the delivery of nuclear weapons. The Vanguard rockets proved unreliable. The United States finally launched Explorer 1 on January 31, 1958, using a military missile.

As part of the design effort, Van Allen utilized his experience by reducing payload experiments into a package of smaller mass for launch by America’s less powerful rockets. Although the payloads were smaller, they were more sophisticated because of the efforts of Van Allen and others.

Van Allen’s interest in cosmic rays was also apparent when, on July 26, 1958, the United States included a Geiger counter in its launch of Explorer 4 to detect space radiation. When the counter’s radio signal was transmitted to Earth for analysis, it did something strange: It increased to a maximum, decreased to zero, and then increased again to a maximum. Van Allen correctly interpreted this not as a result of an actual decrease in radiation but as a result of the instrument’s inability to handle high levels of radiation. This is analogous to turning the volume of a radio too high when the sound becomes distorted as the electronics are driven beyond their design limits.

Further study revealed the nature of the radiation: The Earth’s magnetic field temporarily traps electrons and other electrically charged particles emitted from the sun in the solar wind . Some of the particles also may come from Earth’s upper atmosphere as its gases interact with the solar wind particles. When an electrically charged particle encounters a magnetic field, the particle may engage in one of three motions: One, if the particle moves parallel to the field, the field does not affect the particle’s motion. Two, if the particle is moving perpendicular to the field, the particle will assume a circular motion perpendicular to the field. Finally, if the particle encounters the field at any other direction, the charged particle will move in a spiral motion around the magnetic field.

The earth’s magnetic field fans out at the magnetic pole in the Southern Hemisphere, arcs over Earth’s equator, and converges on the magnetic pole in the Northern Hemisphere. The field is strongest at the poles and weakest halfway between. The field reaches from Earth’s surface into space, decreasing in strength the farther it is from Earth. Particles such as electrons enter the field and spiral along the field lines. As the field strength increases near the poles, the particles bounce off this area and spiral toward the opposite poles. The particles may perform this spiral-bounce motion many times before they finally escape from the field into outer space.

There are two regions in the field that have high radiation levels. Aligned with the center of Earth, they are both donut-shaped, with crescent-shaped cross sections. The inner surface begins at 3,000 kilometers above the earth’s surface and is at its thickest portion at 5,000 kilometers. The outer region is 16,000 kilometers from the earth’s surface and is 6,500 kilometers thick. Although the particles consist mostly of electrons, the inner belt does contain some protons and other particles.

In honor of Van Allen’s discovery, these regions of high-level radiation were named the Van Allen radiation belts, or simply the Van Allen belts. The fact that the earth has these belts of trapped particles has far-reaching implications that will affect how space is utilized now and in the future.

Significance

Before exploring near-Earth space with artificial satellites, scientists had a simplistic view of the earth’s magnetic field. It was considered a simple dipole field having no spectacular features. Now, it is known that the solar wind distorts the shape of the magnetic field. The interactions between the wind and field push the field into the side closest to the sun and pull it into a tail on the opposite side.

The sun goes through an eleven-year cycle in which it is quiet, then becomes more active, and then returns to the quiescent stage. During its active phase, the sun’s surface sends streamers of hot, ionized gases into space. Because these gases are electrically charged, they encounter the earth’s magnetic field. This plays havoc with radio communication on Earth because the ionized layers within Earth’s atmosphere that reflect radio waves interact with the solar wind. As the particles move along the magnetic-field lines, some interact with air molecules high in the atmosphere, thereby producing the northern lights, or aurora borealis.

The interaction between the magnetic field and the upper atmosphere also may relate to the ozone “holes” found in upper atmosphere. The ozone atmospheric layer provides protection against ultraviolet radiation in sunlight. Anything that affects the amount of ozone is of interest because its depletion will result in an increase of skin cancers.

It has been learned that orbiting satellites are a very convenient method for communication, navigation, and Earth monitoring. As more satellites are placed in orbit, however, convenient orbital paths are filling up, creating the need to expand space usage farther from Earth. The Van Allen radiation belts will complicate this goal because their high levels of radiation require the use of extra shielding for the satellite’s instruments, thereby increasing the mass of the payload.

The Van Allen radiation belts also explain a mystery about Jupiter: For decades, scientists have detected radio signals that originate from that planet. It is now known that Jupiter has a strong magnetic field with associated Van Allen radiation belts. The trapped particles are producing the radio waves as they spiral along the magnetic field lines.

The spiraling and bouncing behavior of the particles in the field leads to the energy crisis. Because convenient energy sources are becoming depleted, other methods must be found for generating the needed energy. One method is the fusion of hydrogen nuclei into helium with the release of energy. For fusion to occur, however, high temperatures of millions of degrees are needed, but no material can contain anything that hot. It is possible to produce a magnetic “bottle,” where the high temperature hydrogen plasma spirals along the field and bounces at the ends of the “bottle” where the field increases in strength.

Bibliography

Frank, L. A., and James Van Allen. “A Survey of Magnetospheric Boundary Phenomena.” In Research in Geophysics. Vol. 1, Sun, Upper Atmosphere, and Space, edited by Hugh Odishaw. Cambridge, Mass.: MIT Press, 1964. This chapter includes many figures of the earth’s magnetic field and of the trapped particles moving along the field. Although it is somewhat dated, when studied with other articles in this bibliography, a historical perspective of the belts may be gained.

Lemaire, J. F., D. Heynderickx, and D. N. Baker, eds. Radiation Belts: Models and Standards. Washington, D.C.: American Geophysical Union, 1996. Discussion of methods for studying the Van Allen radiation belts and their place in the study of earth science generally. Bibliographic references.

Miroshnichenko, Leonty I. Radiation Hazard in Space. Boston: Kluwer Academic, 2003. A study of the Van Allen radiation belts and other radiation sources in space, discussing the dangers they pose to astronauts. Bibliographic references.

O’Brien, Brian. “Radiation Belts.” Scientific American 208 (May, 1963): 84-96. Although written five years after the discovery of the Van Allen radiation belts, this article provides a wealth of information about them, including maps of the belts around the earth, and the shape of the earth’s magnetic field.

Roederer, J. G. Dynamics of Geomagnetically Trapped Radiation. New York: Springer-Verlag, 1970. This is a highly technical text that contains calculus-level mathematics.

Van Allen, James. “Interplanetary Particles and Fields.” Scientific American 233 (September, 1975): 160-173. This is a good update article about the Van Allen radiation belts and contains many figures.

Williams, Donald. “Charged Particles Trapped in the Earth’s Magnetic Field.” In Advances in Geophysics. Vol. 15, edited by H. E. Landsberg and J. Van Mieghen. New York: Academic Press, 1971. This technical article surveys the types of trapped particles, what their sources are, how they escape from the magnetic field, and how they are transported. Presents future directions for research. Contains 210 references.