Background
Radiation Belts and Space Storms
High-energy particles (ions and electrons) are trapped in the Earth's magnetic field and form the radiation belts. MeV electrons in the radiation belts are the highest-energy particles in geospace (Figure 1). Recent satellite (CRRES, Akebono, THEMIS, etc.) and ground-based observations, and modeling studies have revealed the detailed structure and variations of the inner magnetosphere. As shown below, the cross-energy coupling, the cross-regional coupling between the plasmasphere, the plasma sheet, the ring current, and the radiation belts, and the magnetosphere-ionosphere coupling have become important concepts for the understanding of the radiation belts and the inner magnetosphere [Ebihara and Miyoshi, 2011].
As shown in Figure 2, in space storms, the outer belt electrons decrease significantly during the main phase and then recover to, or often increase over, the prestorm level during the recovery phase [e.g., Baker et al., 1986; Nagai, 1988; Reeves et al., 1998, 2003; Miyoshi and Kataoka, 2005]. During huge magnetic storms, the radiation belts are largely deformed, and large flux enhancements are observed in the slot region and the inner belt [Baker et al., 2004].
External Source and Internal Acceleration for Relativistic Electrons
Two possible mechanisms have been proposed for the generation of relativistic electrons (see reviews by, e.g., Friedel et al. [2002], Shprits et al. [2008a, 2008b], Hudson et al. [2008], and Ebihara and Miyoshi [2011]). One is the external source process via quasi-adiabatic acceleration [Schulz and Lanzerotti, 1974]. In this process, the energy of electrons increases with their first and second adiabatic invariants conserved when electrons are transported from the plasma sheet to the inner magnetosphere. This process has been modeled as the stochastic radial diffusion process, which is a fundamental transport mode of energetic electrons. ULF Pc5 pulsations with periods of a few min have been considered as a main driver of radial transport via drift resonance with electrons [e.g., Rostoker et al., 1998; Hudson et al., 2001; Elkington et al., 1999; Elkington, 2006; Mathie and Mann, 2000].
Another candidate is termed the internal acceleration process. It has been suggested that resonant interactions by whistler mode waves cause relativistic electron acceleration inside the radiation belts [e.g., Summers et al., 1998; Miyoshi et al., 2003; Horne et al., 2005]. The free energy for exciting whistler mode waves is the temperature anisotropy of electrons with tens of keV [e.g., Kennel and Petscheck, 1966; Jordanova et al., 2010]. Subsequent wave-particle interactions including nonlinear processes will generate chorus waves [e.g., Katoh and Omura, 2007; Omura et al., 2008] that accelerate relativistic electrons of the outer belt [e.g., Summers and Ma, 2000; Summers et al., 2007].
Wave generation and resonant conditions are affected by the cold plasma distribution in the inner magnetosphere. Satellite observations have shown that MeV electron fluxes of the outer belt enhance outside the plasmasphere where intense whistler mode chorus waves are generated, suggesting the importance of whistler mode waves for electron acceleration [e.g., Meredith et al., 2003; Miyoshi et al., 2003, 2007; Horne et al., 2005; Y. Kasahara et al., 2009]. Thus whistler mode waves work as a mediating agent that can deliver energy from a low-energy electron population to a higher-energy one, and it is important for plasma/particles in a wide energy range from eV to MeV to dynamically cooperate via wave-particle interactions. Magnetosonic mode waves are also plausible candidates for internal acceleration [Horne et al., 2007, Meredith et al., 2008].
Loss of Radiation Belt Electrons
The electron flux enhancement process is a result of a delicate balance between acceleration and loss of relativistic electrons [Reeves et al., 2003], so that loss processes are as important as acceleration processes. Several possibilities for loss processes have been proposed (see reviews by Millan and Thorne [2007] and Turner et al. [2012]). Although adiabatic deceleration always operates during ring current evolution [Kim and Chan, 1997], nonadiabatic loss processes also work during space storms. Magnetopause shadowing and subsequent outward radial diffusion may cause rapid loss of outer belt electrons [e.g., Brautigam and Albert, 2000; Miyoshi et al., 2003; Shprits et al., 2006; Matsumura et al., 2011; Turner et al., 2012]. Pitch angle scattering by electromagnetic ion cyclotron waves (EMIC) and whistler mode waves is important in that it causes relativistic electrons to precipitate to the atmosphere [e.g., Thorne and Kennel, 1971; Lyons et al., 1972; Abel and Thorne, 1998; Li et al., 2007; Miyoshi et al., 2008; Jordanova et al., 2008]. The loss processes associated with pitch angle scattering are expected to be localized, so that multipoint satellite and ground-based observations are important.
Cross-Regional Coupling and Cross-Energy Coupling
Figure 3 summarizes the transport and acceleration mechanisms in the inner magnetosphere in the form of an L-energy diagram. For radial diffusion (the blue arrow), electrons move earthward with increasing energy due to the conservation of the first and second adiabatic invariants. ULF Pc5 waves can be the main driver of the radial diffusion. On the other hand, for in situ acceleration by waves (the red arrow), subrelativistic electrons are accelerated to MeV energies by whistler mode/magnetosonic waves that are generated by a plasma instability of ring current electrons and ions. Plasma waves, such as whistler waves and magnetosonic waves, deliver energies from the population of ring current electrons and ions to the population of subrelativistic electrons via wave-particle interactions, and the relativistic electron flux increases inside the outer belt. In this process, thermal plasma also plays an important role as the ambient medium. Because the transport of ring current electrons and thermal plasma is predominantly controlled by convective electric fields, the process of convection may affect the relativistic electron dynamics in the internal acceleration process.
The cross-energy coupling among plasma/particle populations in the inner magnetosphere with energies widely differing by more than 6 orders of magnitude (eV to MeV) plays a role in generating MeV electrons of the outer belt via wave-particle interactions. Moreover, the cross-regional coupling between the magnetosphere and the ionosphere drives the dynamical evolution of convective electric fields in the inner magnetosphere [e.g., Ebihara et al., 2004]. Therefore, the formation of the radiation belt is a manifestation of the cross-energy and cross-regional couplings in geospace [Mann et al., 2006; Mann, 2008, Ebihara and Miyoshi, 2011], which are the key concept of the ERG project in understanding how relativistic electrons are generated in geospace during space storms.
Radiation Belts and Space Weather
The study of relativistic electrons in the radiation belts is also important for understanding space weather [Baker, 2002]. Space infrastructure, such as GPS, meteorological satellites, and telecommunications satellites that operate in the radiation belts, is indispensable in our modern society. High-energy particles can cause operational anomalies to satellites. In fact, it has been suggested that satellite anomalies are closely related with large enhancement of relativistic electron fluxes of the outer belt [Pilipenko et al., 2006]. Deep dielectric charging in the satellite by the enhancement of relativistic electrons is one of the major causes of satellite anomalies. Furthermore, the International Space Station (ISS) also operates at the bottom of the inner and outer radiation belts. An astronaut during extravehicular activity at ISS is exposed to radiation not only by high-energy protons but also by relativistic electrons, because MeV electrons can penetrate into his/her spacesuit [National Research Council, 2000]. To control the exposed dose of an astronaut during extravehicular activity at ISS, information about current and future conditions of relativistic electron environment is important.The study of relativistic electrons in the radiation belts is also important for understanding space weather [Baker, 2002]. Space infrastructure, such as GPS, meteorological satellites, and telecommunications satellites that operate in the radiation belts, is indispensable in our modern society. High-energy particles can cause operational anomalies to satellites. In fact, it has been suggested that satellite anomalies are closely related with large enhancement of relativistic electron fluxes of the outer belt [Pilipenko et al., 2006]. Deep dielectric charging in the satellite by the enhancement of relativistic electrons is one of the major causes of satellite anomalies. Furthermore, the International Space Station (ISS) also operates at the bottom of the inner and outer radiation belts. An astronaut during extravehicular activity at ISS is exposed to radiation not only by high-energy protons but also by relativistic electrons, because MeV electrons can penetrate into his/her spacesuit [National Research Council, 2000]. To control the exposed dose of an astronaut during extravehicular activity at ISS, information about current and future conditions of relativistic electron environment is important.
To examine what mechanisms mainly contribute to the evolution of the outer belt electrons, it is important to measure plasma, fields, and waves near the magnetic equator. The CRRES and Akebono observations suggested the importance of observations at the equatorial plane [Seki et al., 2005]. Moreover, the phase space density profile is a key to discriminating between the external supply process and the internal acceleration process [Green and Kivelson, 2004; Chen et al., 2007]. Comprehensive observations in the inner magnetosphere, however, have never been realized, because the strong radiation environment causes serious contamination of particle measurements. Thus the acceleration mechanism has not been clearly understood yet.
Future Perspectives
The Earth's radiation belts are a unique natural laboratory for developing our understanding of the generation processes of relativistic particles which operate all over the universe. All magnetized planets in our solar system except for Mercury have radiation belts [Mauk and Fox, 2010]. The latest knowledge on the Earth's radiation belts will contribute to the understanding of the planetary radiation belts. For example, in the Jovian radiation belts, where ultrarelativistic electrons are generated [Bolton et al., 2002; Ezoe et al., 2010], radial diffusion would be important for the electron acceleration process in the Jovian magnetosphere [e.g., Goertz et al., 1979]. On the other hand, the nonadiabatic acceleration process via wave-particle interactions has been proposed, based on recent studies of the Earth's radiation belts [Horne et al., 2008]. Detailed understanding of the relativistic electron acceleration process in the Earth's radiation belts based on the results of the ERG project will shed light on how high-energy electrons are generated in the planetary magnetospheres and the universe.