What is the Radiation Belt?
Radiation belts are regions where high energy electrons and protons are drifting around the Earth.
1. Discovery of the radiation belt
There is an area in the space around the Earth where high-energy particles exist that we refer to as a “radiation belt”. In 1958, Dr. James Van Allen at the University of Iowa discovered this radiation belt. Van Allen’s group launched the Explorer 1 and 3 spacecraft, which were equipped with a Geiger counter to measure cosmic ray. After the launch, the Geiger counter picked up unexpected values that were sometimes too high and sometimes too low. After this, Explorer 4 and Pioneer 3 were equipped with improved instruments that revealed that the unexpected values indicated the existence of high-energy particles around the Earth.
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2. Distribution of the radiation belt
Radiation belts are circularly trapped around the Earth and are separated into two regions: inner and outer. The electron flux of the inner belt has a peak around 1.5–2 Earth radii distance (about 3,000 km) from the center of the Earth, while the outer belt has a maximum flux around 4–5 Earth radii (about 20,000 km). A part of the outer radiation belt reaches the geostationary orbit, which is located at about 6.6 Earth radii from the center of the Earth. The region between the inner and outer radiation belt is referred to as the slot region, where the electron flux intensity is weak compared with the other two regions.
The outer radiation belt is fairly variable. Solar wind conditions, which affect the magnetosphere, cause strong enhancement and/or loss of the electron flux, whose variation sometimes increases/decreases by a few orders of magnitude. The risk of spacecraft anomalies tends to increase if the spacecraft experiences a strong enhancement of the flux.
Above Brazil, the magnetic field intensity is relatively weak compared with other regions. This is referred to as the “South Atlantic Magnetic Anomaly”. In this region, the magnetic field conversing effect is relatively weak, which enables high-energy particles to penetrate more deeply into the atmosphere. This means that spacecraft and astronauts working within a few hundred altitudes of the South Atlantic area may have an increased risk of hazards associated with radiation. Moreover, there are some reports that instrument anomalies tend to increase in the South Atlantic area.
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3. Characteristics of radiation belt electrons
Electrons in radiation belts have a broad range of energy. Usually, the number of electrons decreases at higher energies. At higher energies, the position of the maximum flux tends to be located at the inner position, and the slot region is more clearly observed than at lower energies.
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4. Relationship between the radiation belt and the sun
The sun releases the solar wind, which is a combination of the flow of high-speed charged particles (plasma) and the sun’s own dragging magnetic field (or “interplanetary magnetic field” (IMF)). The speed of the solar wind is usually about 400-450 km/s near the Earth.
The solar wind is not steady: coronal mass ejection (CME) suddenly releases a large amount of solar corona mass, and the co-rotating interaction region (CIR) where the fast solar wind interacts with the slow wind creates the compressed plasma and IMF. These lead to a strongly turbulent state of the solar wind for several hours. When the “active” solar wind blows the earth’s magnetic field (magnetosphere), the high-energy electrons of the radiation belt are enhanced in and/or lost from the magnetosphere.
Process of radiation belt enhancement and loss
Electron acceleration and loss mechanisms in the magnetosphere control the electron flux variation of the radiation belts. The nonlinear nature of plasma in a non-uniform magnetic field makes it difficult to understand the physics associated with the acceleration and loss. Much research attention has been devoted to identifying the mechanisms associated with solar wind activity, with recent studies indicating that the CIR is more effective than the CME in terms of enhancing the electron radiation belt. Here, we briefly describe the processes associated with radiation belt acceleration (enhancement) and loss.
Electron acceleration
If low-energy electrons are accelerated and energized, the number of high-energy electrons and their electron flux in the high-energy range increases. There are two acceleration properties: adiabatic and non-adiabatic acceleration.
* Adiabatic acceleration
This process increases charged-particle energy by dragging a particle into a region in which the magnetic field intensity is stronger.
* Non-adiabatic acceleration
This process accelerates electrons at a local position through wave-particle interactions that break the first-adiabatic invariants.
One cause of adiabatic acceleration is ultra-low-frequency (ULF) waves in the magnetosphere. This oscillation transports radiation belt electrons in both the inward and outward directions. Electrons transported in the inward/outward direction experience stronger/weaker magnetic fields, and thus the adiabatic process increases/decreases the electron energy. The total electron energy and total amount of electron flux in the radiation belt increase if the number of electrons transported in the inward direction is higher than the number of electrons transported in the outward direction.
The most important element in the non-adiabatic acceleration of an electron is wave-particle interactions between electromagnetic waves and electrons. A whistler wave, which is one of the waves in plasma, can effectively scatter electrons in very short time scale (less than sec. order). Scattering associated with whistler waves both increases/decreases the electron energy and changes their trajectories.
Electron loss
Electron loss is identified with decrease of the number of electrons in a certain energy range. There are two reasons this can happen.
(1) Electrons decrease their energy.
(2) Electrons escape from the magnetosphere.
Process (1) occurs when a magnetic storm develops and the magnetic field intensity in the inner magnetosphere decreases. This decrease prompts the adiabatic effect to reduce electron energy, which is referred to as “adiabatic loss”. If adiabatic loss is dominant during a magnetic storm, the electron energy recovers its pre-storm time energy after the storm declines. However, in many cases, the electron energy of the radiation belt increases or decreases compared with the energy in the pre-storm time, suggesting that the acceleration and/or loss processes associated with the non-adiabatic effect during the magnetic storm time vary the electron energy of the radiation belt. Process (2) is divided into two processes: electron precipitation into the Earth’s atmosphere, and magnetopause shadowing.
* Precipitation
Charged particles with gyromotion move parallel to a magnetic field line. The Earth’s magnetic field intensity increases towards the North and South poles. Because of the magnetic field convergence, some charged particles are reflected before reaching the atmosphere by the magnetic mirror force. Note that the mirror force reflects the particles more effectively when the gyrating particle trajectory is more perpendicular to the magnetic field line. These particles are permanently trapped between the Northern and Southern hemispheres if there is no scattering that changes their trajectories. When a geomagnetic storm occurs, plasma waves (e.g., whistler waves, ion cyclotron waves) are generated in the magnetosphere and scatter the trapped electrons, changing their trajectories. Some electrons precipitate into the Earth’s atmosphere without enough magnetic mirror force. Some of these precipitated electrons cannot return to the magnetosphere, so the trapped electron population decreases non-adiabatically.
* Magnetopause shadowing
Electrons move not only in the parallel direction between the Northern and Southern hemispheres but also in the perpendicular direction of both the magnetic field and its gradient. If the Earth’s magnetic field is symmetric at the polar axis, the perpendicular drift motion is entirely in the longitudinal direction. This drift motion is eastward for electrons (negatively charged particles) and westward for protons (positively charged particles).
When the solar wind dynamic pressure becomes strong, the magnetosphere is compressed. Then, the boundary of the magnetosphere interacts with the inner drift paths of electrons. If the electrons drifting around the Earth interact with the magnetopause boundary, they escape into interplanetary space. This is one of processes that decreases the number of electrons trapped in a radiation belt.
Many scientists who specialize in observation, theory, and computer simulation have been attempting to improve our understanding of the physics of radiation belts for over 50 years. We also have been studying radiation belts using GEMSIS-RB code [e.g. Saito et al. (2010) and (2012)] .
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5. Influences of the radiation belt on spacecraft
A spacecraft can become electrically charged if it is in an environment where it interacts with charged particles. If the charging exceeds a breakdown voltage between materials, a discharge can happen suddenly and cause damage to instruments, intermittent anomalous behavior, and catastrophic satellite failure. Spacecraft charging includes “surface charging” and “internal dielectric charging”. Surface charging is due to relatively low energy electrons around keV energy, while internal dielectric charging is due to electrons with relativistic energy.
Surface charging
When electrons with energy less than 100 keV collide with the surface of a spacecraft, a negative electric charge occurs on the surface (surface charging). After the charging, electrons tend to be reflected due to the negative potential of the spacecraft. Several release effects of the negative potential simultaneously occur, and the charging and release effects are thus balanced in a relatively quiet geospace environments and the surface charging is not so serious for the spacecraft. However, as the number of keV-electrons suddenly increases, the electric potential between materials (i.e., cables and circuits) suddenly increases. This may cause a breakdown, which is a source of the electric current associated with the discharge. This is what damages the spacecraft.
Internal dielectric charging
Electrons with energy larger than 100 keV can actually penetrate into the spacecraft. These high-energy electrons are what cause the charging on electric circuits and cables in the spacecraft (internal dielectric charging). When the number of high-energy electrons increases after a geomagnetic storm, a spacecraft has an increased risk related to the internal charging.
Examples that may be associated with surface charging
Oct. 23, 2003
Midori 2 suffered damage on its solar panel due to charging and electric discharge.
Environment
High-energy electron flux in the outer radiation belt suddenly increased. Strong solar wind dynamic pressure and interplanetary magnetic field are considered as causes of the flux enhancement in the radiation belt. The high flux level remained until Oct. 24, 2003.
Apr. 5, 2010
Galaxy 15 stopped responding to commands due to electrostatic discharge.
Environment
The direction of the interplanetary magnetic field (IMF) was south on Apr. 5, 2010. A strong injection of plasma into the magnetosphere was observed at the geostationary orbit in the midnight sector.
Examples that may be associated with internal dielectric charging
Jan. 20, 1994
BS-3a, a communications satellite in Japan, experienced a problem and stopped broadcasting. Anik E-1 and E-2, two communications satellites in Canada, also experienced a problem.
Environment
The high-energy electron flux increased up to 100 times and stayed that high until Jan. 22.
Jan. 11, 1997
A communication satellite, Telstar401, was disabled.
Environment
High-energy electron flux sharply increased up to 1000 times for a few hours. During this event, the bulk speed, density, and magnetic field of the solar wind stayed fairly strong for about 24 hours.