Image of TIME magazine cover at top left and an image of James Van Allen in from of a satellite.

Above, Top Left: Van Allen on May 4, 1959 cover of TIME magazine (May 4, 1959 Vol. LXXIII No. 18). Source: Time magazine covers web site.

Above, Bottom Right: James Van Allen at National Air & Space Museum (NASM), 1981, Photo courtesy of NASM. Explorer I model and Pioneer H probe in background.

The radiation belts are two donut-shaped regions of high-energy particles, mainly protons and electrons, trapped by the magnetic field of the Earth. These belts are often referred to as "The Van Allen Belts" because they were discovered by James Van Allen and his team at the University of Iowa. This scientific discovery was a first for the space-age.

The first American satellite, Explorer 1, was launched into Earth's orbit on a Jupiter C missile from Cape Canaveral, Florida, on January 31, 1958. Aboard Explorer 1 were a micrometeorite detector and a cosmic ray experiment designed by Dr. Van Allen and his graduate students. Data from Explorer 1 and Explorer 3 (launched March 26, 1958) were used by the Iowa group to detect the existence of charged particle radiation trapped by Earth's magnetic field - the inner radiation belt. The particles in this region are mainly high-energy protons (10-100 MeV range) which are trapped within about 600-6000 km (400-4000 miles) of the Earth's surface. These protons readily penetrate spacecraft and can, on prolonged exposure, damage instruments and be a hazard to astronauts. Both manned and unmanned spaceflights tend to stay out of this region.

Photos of the Pioneer 3 and Explorer 1 spacecrafts.

Above, Top: Image of Pioneer 3.

Above, Bottom: Image of Explorer 1 spacecraft. Courtesy of NASA/JPL

Pioneer 3 (launched 6 December 1958) and Explorer IV (launched July 26, 1958) both carried instruments designed and built by Dr. Van Allen. These spacecraft provided Van Allen additional data that led to discovery of a second radiation belt. This was the larger, outer radiation belt which is typically located about 10,000-65,000 km (6250-40,000 miles) above the Earth's surface and encircles the inner belt. The region of greatest intensity lies between about 14,500-19,000 km (9000-12,000 miles). This outer belt is much more variable than the inner one and changes dramatically in size, location and intensity. The particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are alpha particles and O+ oxygen ions, similar to those in the ionosphere but much more energetic, with energies of about 10 keV to 10 MeV. This mixture of ions suggests that the particles probably come from more than one source.

The inner belt is marked by great stability, but the outer belt is constantly changing. Radiation belt particles are lost, e.g. by collision with the rarefied gas of the outermost atmosphere, and new ones are frequently injected from the comet-like tail of the magnetosphere (the magnetotail). The particle population of the outer belt fluctuates widely and is generally weaker in energy (less than 1 MeV), rising to energies of order 10 MeV when geomagnetic storms occur. Geomagnetic storms are temporary disturbances of the magnetosphere (the space environment around Earth) usually driven by effects which occur on the sun. These storms (usually driven by the solar wind) cause fresh particles to be injected into the radiation belts from the magnetotail. The energy of the radiation belts falls to more typical quiet time levels during the subsequent days - known as the storm recovery phase.

It is this constant variability of the radiation belts which is of most interest to scientists. There are known phenomena which give rise to these changes but the radiation belts do not always respond in the same way to the drivers. For example, there is a close, but by no means simple, relationship between storms at Earth and changes in the radiation belts. Each of these storms was preceded by similar solar conditions. Due to complex processes that can occur simultaneously during the storm period, the radiation belts can be enhanced (left), depressed (middle), or essentially unchanged (right) compared with conditions before the storm.

Graphics showing the enhanced, depressed and essentially unchanged radiation belt conditions.

Images courtesy of G. Reeves, Los Alamos National Laboratory

In addition, temporary new belts can be created during magnetic storms, sometimes within minutes of the storm's onset. Solar energetic protons, accelerated at shock waves that emanate from the sun, can provide the "seed" population for new proton belts. Although it was once thought that the behavior of the radiation belts was well-understood, observations over the last decade have given rise to new and fundamental questions about the physical processes involved in the enhancement and decay of the belts and in the formation of new ones.

Geomagnetic storms can "pump up" the radiation belts, producing increased intensities of energetic electrons that can damage satellite electronics and can also represent a potential health hazard to astronauts on the International Space Station. The majority of our communications satellites operate in regions where they can be exposed to intense amounts of extremely energetic radiation belt particles.

Model-generated image showing the two main radiation belts

Model-generated image showing the two main radiation belts, the outer belt and the inner belt. The model was developed at the Air Force Research Laboratory. Shown here are representative orbits for three GPS and one geosynchronous spacecraft.

Figure courtesy R. V. Hilmer, Air Force Research Laboratory

Understanding the radiation belt environment and its variability has extremely important practical applications in the areas of spacecraft operations, spacecraft and spacecraft system design, and mission planning and astronaut safety.

NASA's Van Allen Probes mission is studying this radiation belt environment with emphasis on the variability of the outer radiation belt because this region is the most dynamic part of the radiation belts and has high practical relevance.

Specifically, the goal of the mission is to understand the acceleration, global distribution and variability of energetic electrons and ions in the radiation belts.

It is anticipated that the following questions will be answered using data from this mission:

  • Which physical processes produce radiation belt enhancements?
  • What are the dominant mechanisms for high-energy electron loss?
  • What factors affect radiation belt dynamics?

ScienceSpace Weather

Artist depiction of sun and coronal mass ejection.

The source of space weather, our dynamic sun, shown with a coronal mass ejection that will interact with the terrestrial magnetosphere producing geospace storms.
Credit: NASA

Everyone is familiar with changes in the weather on Earth. But "weather" also occurs in space. Just as it drives weather on Earth, the sun is responsible for disturbances in our space environment.

Besides emitting a continuous stream of plasma called the solar wind, the sun periodically releases billions of tons of matter in what are called coronal mass ejections. These immense clouds of material, when directed towards Earth, can cause large magnetic storms in the space environment around Earth, the magnetosphere and the upper atmosphere.

Image of man in spacesuit floating in space with Earth in background.

In Earth orbit and in interplanetary space, humans are directly exposed to space weather and its potentially dangerous impact.
Credit: NASA

The term space weather generally refers to conditions on the sun, in the solar wind, and within Earth's magnetosphere, ionosphere and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health.

Magnetic storms produce many noticeable effects on and near Earth:

  • Aurora borealis, the northern lights, and aurora australis, the southern lights
  • Communication disruptions
  • Radiation hazards to orbiting astronauts and spacecraft
  • Current surges in power lines
  • Orbital degradation
  • Corrosion in oil pipelines

Image of Solar Flare

Forecasting Space Weather Is a Complex Business

  • Space weather forecasts are important because radiation from particles from the sun associated with large solar flares can be hazardous to unprotected astronauts, airplane occupants and satellites.
  • Scientists recently reported improved methods of forecasting periods of low flare probability.
  • Flares are often cause for concern as they can foretell of intense effects at Earth.

Imaging the Corona is a Major Step in Space Weather Forecasting

Image of solar activity from October 26, 2003.
  • Detecting phenomena occurring on the surface of the sun allows us to monitor solar activity, but knowing that something is heading towards Earth is a key measurement.
  • Images of eruptions in the solar corona from NASA's SOHO spacecraft have provided invaluable monitoring capabilities of approaching coronal mass ejections - energetic eruptions on the sun and primary cause of major geomagnetic storms.

Artist depiction of sun with the two STEREO spacecraft.

Artist's concept of the twin STEREO observatory studying the sun.
Credit: NASA/JHU Applied Physics Laboratory
Click on image above to enlarge.

Stereoscopic Imaging Will Provide an Extra Dimension for Forecasting

A major advance in space weather forecasting will come from our ability to determine the speed at which the phenomenon is moving. This will be achieved with stereoscopic views of the sun from NASA's Solar TErrestrial RElations Observatory (STEREO) spacecraft. This pair of spacecraft will use 3-D vision to construct a global picture of the sun and its influences.

A diagram of the STEREO spacecrafts in orbit around Earth.

Solar Signatures Are Just One Way to Monitor Space Weather

Sometimes dramatic events on the sun can be the precursor of huge storms in geospace - Earth's near space environment - but at other times they can have little or no effect. Understanding how this geospace region responds to a variety of solar drivers is the key to predicting space weather.

Similar solar events can cause the Earth's Northern Lights to glow or remain dim.

Image of Earth and appearance of Northern Lights viewed from space in June 2000.

June 2000

Image of Earth and appearance of Northern Lights viewed from space in July 2000.

July 2000

Northern Light viewed from space.

The solar signatures preceding these events told of great things to come. June 2000 merely fizzled out, but July 2000 saw one of the most spectacular auroral displays of the solar cycle.

Solving the Chain - One Link at a Time

  • Predicting space weather is a complex problem, because of the intricate interactions between many systems - the sun, solar wind, Earth's magnetic field, and Earth's atmosphere. Periods of intense activity at Earth can occur during so called "quiet times" on the sun.
  • Space weather is a complicated series of events. This improved ability to predict solar quiet times is the first link.
  • Just as no one would expect to be able to predict weather in the lower atmosphere solely by observing the sun, understanding the upper atmosphere of the Earth requires an understanding of how Earth 's atmosphere responds to changes in the space environment.
  • Programs, such as NASA's Living With a Star-Geospace mission will provide us with the vital information needed to allow full understanding of the space weather chain.

ScienceImpact on Earth

Many phenomena associated with space weather occur within the Earth's magnetosphere and upper atmosphere. Geomagnetic storms can be accompanied by enhancements in the radiation belts and complex changes in the ionosphere and thermosphere.


Image of astronaut in space

When high-energy particles – those moving with enough energy to knock electrons out of atoms – collide with human tissue, they alter the chemical bonds between the molecules that make up the tissue''s cells. Sometimes the damage is too great for a cell to repair and it no longer functions properly. Damage to DNA within cells may even lead to cancer – causing mutations.

During geomagnetic storms, the increased density and energy of particles trapped in the radiation belts means a greater chance that an astronaut will be hit by a damaging particle. That''s why the International Space Station has increased shielding around crew quarters, and why NASA carefully monitors each astronaut''s radiation exposure throughout his or her career.

The magnetosphere and atmosphere keep most harmful radiation from reaching the surface of the Earth, but damaging radiation does penetrate the upper levels of the atmosphere. High-flying airplanes, and those flying over the North Pole, are exposed to more radiation than when at sea level. Geomagnetic storms can also alter the shape of the Earth''s protective magnetosphere, sometimes allowing more high-energy particles into the upper levels of our atmosphere. During these times, people in airplanes face increased exposure to damaging radiation and flights are sometimes rerouted to protect them.

RBSP will help develop better predictive models so that astronauts will have increased warning of storms.

Satellite Operations

Image of satellite passing over earth

Most spacecraft in Earth orbit operate partly or entirely within the radiation belts. During periods of intense space weather, the density of particles within the belts increases, making it more likely that sensitive electronics will be hit by a charged particle.

Ions striking satellites can overwhelm sensors, damage solar cells, and degrade wiring and other equipment. When conditions get especially rough in the radiation belts, satellites often switch to a safe mode to protect their systems.

Geomagnetic storms can also:

  • Disrupt radio frequency signals as they travel between satellites and ground stations—including Global Positioning Systems and your satellite televisions and car radios. Plasma bubbles (regions of dense ionized gas) form in the atmosphere, and can disrupt a signal passing through them.
  • Change the shape of the Earth''s atmosphere so that it moves into the path of satellites that normally fly above it. The increased drag on the satellite slows the spacecraft and changes its orbit, which will need to be corrected.
  • Cause electric charges to build up inside spacecraft that overwhelm systems when they discharge.

RBSP will help identify the conditions that can disrupt satellite operations, and lead to the development of better technologies that can withstand, or protect satellites during, geomagnetic storms.

Power and Communications

Image of power lines on hillside

Large changes in the magnetic field near the Earth''s surface that are associated with geomagnetic storms can induce currents that flow through man-made structures such as railroad systems, power transmission lines, and pipelines. These currents can cause minor disruptions in service, or major problems such as blackouts affecting thousands of people. On Oct. 30, 2003, a geomagnetic storm caused a power failure in Sweden, and on March 13, 1989, six million people lost power when a geomagnetic storm caused a power grid failure in Quebec, Ontario.

RBSP will help develop better predictive models that could give technology operators advance warning of when their systems might be in danger from powerful electric currents induced by space weather phenomena.

ScienceScience Objectives

The primary science objective of the RBSP mission is to provide understanding, ideally to the point of predictability, of how populations of relativistic electrons and penetrating ions in space form or change in response to variable inputs of energy from the sun.

To understand origin and variability of high energy electrons and protons in Earth''s radiation belt the RBSP mission will identify and quantify the processes that cause acceleration, redistribution, and loss of energetic particles across the inner magnetosphere. Radiation belt electrons of energies greater than several hundred keV and ions with energies greater than several MeV create hazardous conditions for satellite operation and human exploration of space. Dynamic variability of these particle populations in response to varying geomagnetic conditions has been a mystery for more then five decades since the discovery of the belts.

The overarching science questions addressed by the RBSP mission are:

• Which physical processes produce radiation belt enhancement events?

Acceleration Mechanisms

Radial Transport

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Newkirk, L., and M. Walt (1968), Radial Diffusion Coefficient for Electrons at Low L Values, J. Geophys. Res., 73(3), 1013-1017.

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Local Acceleration due to Wave-Particle Interaction

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Horne, R.B., R. M. Thorne, Y. Y. Shprits, N. P. Meredith, S. A. Glauert, A. J. Smith, S. G. Kanekal, D. N. Baker, M. J. Engebretson, J. L. Posch, M. Spasojevic, U. S. Inan, J. S. Pickett, P. M. E. Decreau (2005), Wave acceleration of electrons in the Van Allen radiation belts, Nature, 437 (7056), 227–230.

Iles R. H. A., N. P. Meredith, A. N. Fazakerley, R. B. Horne (2006), Phase space density analysis of the outer radiation belt energetic electron dynamics, J. Geophys. Res., 111, A03204, doi:10.1029/2005JA011206.

Meredith, N. P., M. Cain, R. B. Horne, R. M. Thorne, D. Summers, and R. R. Anderson (2003), Evidence for chorus-driven electron acceleration to relativistic energies from a survey of geomagnetically disturbed periods, J. Geophys. Res., 108(A6), 1248, doi:10.1029/2002JA009764.

O'Brien, T. P., K. R. Lorentzen, I. R. Mann, N. P. Meredith, J. B. Blake, J. F. Fennell, M. D. Looper, D. K. Milling, and R. R. Anderson (2003), Energization of relativistic electrons in the presence of ULF power and MeV microbursts: Evidence for dual ULF and VLF acceleration, J. Geophys. Res., 108(A8), 1329, doi:10.1029/2002JA009784.

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Summers, D., R. Thorne, and F. Xiao (1998), Relativistic theory of wave-particle resonant diffusion with application to electron acceleration in the magnetosphere, J. Geophys. Res., 103(A9), 20487-20500.

Shock-Induced Transport. Slot Refilling and Formation of New Belts.

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Substorm Injections

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• What are the dominant mechanisms for relativistic electron loss?

Loss Mechanisms

Local Loss due to VLF/ELF/EMIC Waves

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Meredith N. P., R. B. Horne, S. A. Glauert, R. R. Anderson (2007), Slot region electron loss timescales due to plasmaspheric hiss and lightning-generated whistlers, J. Geophys. Res., 112, A08214, doi:10.1029/2007JA012413.

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Millan R. M., R. P. Lin, D. M. Smith, K. R. Lorentzen, and M. P. McCarthy, X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer, Geophys. Res. Lett., 29 (24), 2194, doi:10.1029/2002GL015922, 2002.

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Summers, D., and R. M. Thorne (2003), Relativistic electron pitch-angle scattering by electromagnetic ion cyclotron waves during geomagnetic storms, J. Geophys. Res., 108(A4), 1143, doi:10.1029/2002JA009489.

Thorne R. M., T. P. O'Brien, Y. Y. Shprits, D. Summers, R. B. Horne (2005), Timescale for MeV electron microburst loss during geomagnetic storms, J. Geophys. Res., 110, A09202, doi:10.1029/2004JA010882.

Magnetopause Losses

Desorgher, L., P. Bühler, A. Zehnder, and E. Flückiger (2000), Simulation of the outer radiation belt electron flux decrease during the March 26, 1995, magnetic storm, J. Geophys. Res., 105(A9), 21211-21233.

Li, X., D. Baker, M. Temerin, T. Cayton, E. Reeves, R. Christensen, J. Blake, M. Looper, R. Nakamura, and S. Kanekal (1997), Multisatellite observations of the outer zone electron variation during the November 3–4, 1993, magnetic storm, J. Geophys. Res., 102(A7), 14123-14140.

Ohtani S., Y. Miyoshi, H. J. Singer, J. M. Weygand (2009), On the loss of relativistic electrons at geosynchronous altitude: Its dependence on magnetic configurations and external conditions, J. Geophys. Res., 114, A01202, doi:10.1029/2008JA013391.

Shprits Y. Y., R. M. Thorne, R. Friedel, G. D. Reeves, J. Fennell, D. N. Baker, S. G. Kanekal (2006), Outward radial diffusion driven by losses at magnetopause, J. Geophys. Res., 111, A11214, doi:10.1029/2006JA011657.

Ukhorskiy, A. Y., B. J. Anderson, P. C. Brandt, N. A. Tsyganenko (2006), Storm-time evolution of the outer radiation belt: Transport and losses, J. Geophys. Res., 111, A11S03, doi:10.1029/2006JA011690.

• How do ring current and other geomagnetic processes affect radiation belt behavior?

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