Energy transfer between various groups of electrons in the process of resonant wave-particle interaction
D.R. Shklyar
Space Research Institute of RAS, Moscow, Russia
Search for effective mechanisms of electron energization remains the most discussed problem in the physicsof Earth's radiation belts. Successful implementation of RBSP (now Van Allen Probes) mission, which includes wave and particle measurements in the radiation belts in wide ranges of frequencies and energies, has provided a unique basis for experimental and theoretical study of the problem stated above. Of various mechanismsfor electron energization in the radiation belts those related to wave-particle interactions with quasi-monochromatic or wide-spectrum waves are discussed most often. While resonant interaction with quasi-monochromatic wave may lead to a significant particle acceleration during a fraction of its bounce period, the interaction with a wide spectrum of waves leads to particle diffusion in the phase space, and the characteristic heating time is usually much larger than the particle bounce period. One of the consequences of resonant wave-particle interactions consists in energy exchange between waves and resonantparticles, and in particular, in the possibility of particle energization at the expense of wave energy. Such energization, however, has essential limitation connected with that the wave energy density is usually much smaller than the resonant particle energy density, as has been pointed out by Olsen et al (1987). Thus, a direct energy exchange between waves and resonant particles cannot lead to significant particle energization during the time comparable with the particle bounce period, but would rather result in fast wave damping. In this respect, energy exchange between two groupsof particles mediated by the wave, which is the constituent part of resonant wave-particle interactions, seems morepromising. Obviously, in order for this process to be of importance for particle energization, the energy should be transferred from lower energy particles to higher energy particles. The question arises whether such processis permitted by thermodynamics. The answer to this question is positive for the case of unstable plasma.Indeed, in this case, the free energy contained in an unstable particle distribution is transferred to wavesthat are excited due to plasma instability. On the contrary, in the case of stable plasma distribution, the waves are damped giving their energy to resonant particles, which therefore are heated. In the process of energy transferbetween two groups of particles both processes operate simultaneously, and if the lower energy part of plasma distribution is unstable while the higher energy part is stable, then the wave-mediated energy transferfrom lower energy particles to higher energy particles takes place. It is necessary to keep in mind that only a partof free energy of unstable distribution can go to higher energy particles, since a part of free energy is spenton wave excitation. In this report we discuss the process described above by the example of resonant wave-particle interactionsbetween lightning-induced whistler-mode wave packets and energetic electrons in the magnetosphere.
Olsen, R. C., S. D. Shawhan, D. L. Gallagher, J. L. Green, C. R. Chappell, and R. A. Anderson (1987), Plasma observations at the Earth's magnetic equator, J. Geophys. Res., 92(A3),
2385-2407.