Overview

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.

Above, Top: Image of Pioneer 3.
Source: nssdc.gsfc.nasa.gov/
database/MasterCatalog?sc=1959-013A

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.

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, 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 Radiation Belt Storm Probe mission will study 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?