Session X6: Invited Session: The Dynamics of Waves and Energetic Particles: Theoretical Models

Alfven waves and their excitations by energetic particles in fusion and space plasmas

Hydromagnetic Alfven waves are fundamental electromagnetic oscillations in magnetically confined plasmas both in laboratories and in space. The anisotropic shear Alfven wave is particularly interesting due to its near incompressibility, and its group velocity being at the Alfven speed and parallel to the confining magnetic field. In realistic plasmas, shear Alfven waves consist of both the continuous spectrum due to the radial nonuniformities, and discrete eigenmodes due to the near periodic properties along the magnetic field. Energetic particles, meanwhile, are prevalent in laboratory fusion as well as solar-terrestial space plasmas. As the typical energetic particle velocities are comparable to the Alfven velocity, they can readily resonate with the waves and excite shear Alfven waves within the continuous and/or the discrete spectra. We shall discuss how excitations can be analyzed within the theoretical framework of a generalized linear fishbone-like dispersion relation. We shall also discuss issues of nonlinear physics pertinent to both the waves and the energetic particles; emphasizing the crucial roles of equilibrium geometries and nonuniformities.   

Understanding the global excitation of whistler-mode chorus and its effects on Earth's radiation belt dynamics

Whistler mode chorus emissions are excited in the inner magnetosphere following the convective injection of energetic electrons from a source region in the plasma sheet. On the night-side pronounced increase in the flux of anisotropic electrons leads to large linear wave growth and the onset of non-linear processes, which are responsible for the formation of discrete chorus elements in two distinct band above and below one half the electron gyrofrequency. Rapid pitch-angle scattering and energy diffusion causes a severe depletion in electron flux (leading to diffuse auroral precipitation) and the development of highly anisotropic pitch-angle distributions during convective transport to the dayside. The modified electron distributions approach a state of marginal stability, but whistler-mode instability can be triggered by macroscopic changes in plasma density and magnetic field. The global distribution of excited chorus emissions is responsible for microburst of energetic electron precipitation and the stochastic acceleration of electrons to relativistic energies in the recovery phase of magnetic storms. Chorus is also the responsible for the origin of hiss within the plasmasphere, which provides the major scattering loss responsible for the two-zone structure of the energetic electron radiation belts.   

Solar Energetic Particles: Wave Generation and Particle Acceleration at Shocks

Ion and electron acceleration at shock waves accounts for many if not most of the energetic particle enhancements observed throughout the heliosphere. In most cases the association between the energetic particles and shocks is clear. In other cases, including the so-called anomalous cosmic rays and the high-energy solar energetic particles observed following the onset of well-connected solar flare events, the association has been less clear and more controversial. The theory of shock acceleration is first reviewed including the shock drift mechanism, the theory of diffusive shock acceleration (DSA), and some of the complications that occur when applying simple versions of these processes to observed shocks and particle enhancements. A critical feature of DSA is that the upstream accelerating protons excite hydromagnetic waves, which dramatically decrease the timescale for acceleration. The waves are transmitted at the shock to enhance the downstream wave intensity, they affect particle injection out of the solar wind at the shock, and they can grow to sufficient amplitudes upstream that nonlinear processes become important. Several examples of wave and energetic particle enhancements will be shown.