Space weather research and the radiation belts
Space infrastructure such as GPS and meteorological satellites are indispensable to our lives in modern society. These satellites operate in the radiation belts. Manned space activities such as the Space Shuttle also take place at the bottom of the inner radiation belt. The high-energy particles can cause operational anomalies with satellites and exert a dangerous impact on the mankind’s long-term stay in space. In fact, it was reported that radiation-belt particles have wrecked satellites and disrupted live TV broadcasts. Solar-terrestrial science closely associated with human activities is called “space-weather research.” For humankind to act safely and comfortably in the outer space, the study of the radiation belts in space-weather research is especially important.
Reforming mechanism of the outer belt
How does it increase?
What is happening to the radiation belts during magnetic storms? As both the “disappearance” and “reforming” of the outer belt are important themes, I would like to focus now on the “reforming,” the process whereby the outer belt fills again with high-energy particles after temporarily vanishing. Key to understanding the “reforming” is the acceleration of particles, which is an important research theme for space physics.
Charged particles in the magnetic field are characterized by maintaining a constant ratio of the magnetic field’s intensity and the particles’ energy. Therefore, when solar-wind plasma enters the magnetosphere and is conveyed close to the Earth where the magnetic field is strong, their energy increases. This acceleration is called adiabatic acceleration. Particles in the radiation belts, however, have far higher energy compared to the energy level of adiabatic accelerated plasma originating from the solar wind. To explain this discrepancy, it is guessed that non-adiabatic acceleration mechanisms exist somewhere in the magnetosphere. The question of where and how the non-adiabatic acceleration occurs remains unsolved and is an important issue. In the past decade, there has been ardent theoretical and observational research conducted on two mechanisms, “external supply process” and “internal acceleration process.”
External supply process
The external supply process assumes that non-adiabatic acceleration takes place in the magnetotail and then adiabatic acceleration takes place as particles are transported where the magnetic field is strong. The classic 1970s theory explained the formation of the radiation belts by applying this process. In addition, theoretical research in the late 1990s pointed out that magnetic pulsations in the magnetosphere called the magnetohydrodynamic (MHD) wave could take on efficient transportation of high-energy electrons. This supposition is being examined by observations.
Internal acceleration process
The internal acceleration process assumes that non-adiabatic acceleration takes place inside the radiation belts. In this theory, “seed particles” that have lower energy than the radiation belt’s particles are accelerated to higher energy by their microscopic interaction with plasma waves in the magnetosphere to form a macroscopic radiation belt structure. Plasma and particles, which have a wide range of energy extending to more than six orders of magnitude including cold plasma (several eV) affecting characteristics of plasma waves, are involved in this acceleration process.
In the past, it was thought that, considering the actual plasma environment in the inner magnetosphere and acceleration efficiency, even if it were theoretically possible, “internal acceleration” was insufficient to form the outer belt. In the 1990s, however, AKEBONO, etc., observed that the plasma waves and seed particles causing this process existed, and the background plasma environment is also in a state to facilitate this acceleration process. Thus, we now know, though fragmentarily, that internal acceleration plays an important role in the formation of the outer belt.
Significance of direct observation at the equatorial plane
In order to examine which process (external supply process or internal acceleration process) occurs more efficiently, we need to measure an amount called the phase-space density (velocity distribution function) of particles. When the relativistic electrons of the outer belt increase, it is expected that, with the external supply process, the phase-space density increase monotonically with radius. Meanwhile, with the internal acceleration process, the phase-space density must have a peak inside the outer belt. In order to measure the phase-space density, we need to observe the radiation belt’s particles in a wide range of energy and background magnetic fields.
Since the CRRES satellite in the early 1990s, however, no scientific satellite capable of conducting such observations has been launched into the equatorial plane in the magnetosphere. Instead, efforts to estimate the phase-space density have been made using data obtained by other polar-orbit satellites. As a result, data suggesting the external supply were obtained. Meanwhile, much data suggesting internal acceleration were reported as well. Thus, the two theories are still contentious with no conclusion. In addition, the data indicated that the occurrence of the two processes varies depending on distance from the Earth, local time, and type of magnetic storms. Without measurement of the phase-space density at the equatorial plane of the magnetosphere, uncertainty is inevitable. Thus, a decisive understanding of the two processes has not yet been obtained.