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.
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.
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.
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:
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.
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:
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.
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.
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.
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.
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:
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.
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.
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:
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