Cascade and Damping of AlfvénCyclotron Fluctuations: Application
to Solar Wind Turbulence 

Siming Liu Submitted: 20090418 04:55
It is well recognized that the presence of magnetic fields will lead to anisotropic energy cascade and dissipation of astrophysical turbulence. With the diffusion approximation and linear dissipation rates, we study the cascade and damping of Alfvéncyclotron fluctuations in solar plasmas numerically for two diagonal diffusion tensors, one
(isotropic) with identical components for the parallel and perpendicular directions (with respect to the magnetic field) and one with different components (nonisotropic). It is found that for the isotropic case the steadystate turbulence spectra are nearly isotropic in the inertial
range and can be fitted by a single powerlaw function with
a spectral index of 3/2, similar to the IroshnikovKraichnan phenomenology, while for the nonisotropic case the spectra vary greatly with
the direction of propagation. The energy fluxes in both cases are much higher in the perpendicular direction than in the parallel direction due to the angular dependence (or inhomogeneity) of the components. In addition, beyond the MHD regime the kinetic effects make the spectrum softer at higher wavenumbers. In the dissipation range the turbulence
spectrum cuts off at the wavenumber, where the damping rate becomes comparable to the cascade rate, and the cutoff
wavenumber changes with the wave propagation direction. The angleaveraged turbulence spectrum of the isotropic model resembles a broken power law, which cuts off at the maximum of the cutoff wavenumbers or the 4He cyclotron frequency. Taking into account the Doppler effects, the model naturally
reproduces the broken powerlaw turbulence spectra observed in the solar wind and predicts that a higher break frequency
always comes along with a softer dissipation range spectrum that may be caused by the increase of the turbulence intensity, the reciprocal of the plasma beta, and/or the angle between the solar wind velocity and the mean magnetic field. These predictions can be tested by detailed comparisons with more accurate observations.
Authors: Yanwei Jiang, Siming Liu, and Vahé Petrosian
Projects: RHESSI

Publication Status: Accepted by ApJ
Last Modified: 20090418 09:44



Stochastic Acceleration of ^{3}He and ^{4}He by Parallel Propagating Plasma Waves 

Siming Liu Submitted: 20040813 14:06
Stochastic acceleration of ^{3}He and ^{4}He from a thermal background by parallel propagating turbulent plasma waves with an assumed powerlaw spectrum of the wavenumber is studied. In the model the acceleration rate is reduced considerably when the interaction is dominated by one of the several resonant wave modes. At low energies, this happens for ^{4}He, but not for ^{3}He. As a result, the acceleration of ^{4}He is suppressed by a barrier below sim 100 keV nucleon^{1}, and there is a prominent quasithermal component in the ^{4}He spectra, while almost all ^{3}He ions are accelerated
to high energies. The model accounts for the large enrichment of ^{3}He observed in impulsive solar energetic particle events and fits the spectra of both ions with reasonable plasma parameters. This study also indicates that the acceleration, Coulomb losses and diffusive escape of particles from the acceleration site {it all} play important roles in shaping the ion spectra, which may explain the varied spectral shapes observed recently by the {it Advanced Composition Explorer}. However, for further quantitative analysis and to apply the model to the acceleration of other ions, the generation and evolution of the turbulence need to be addressed.
Authors: Siming Liu, Vahé Petrosian, and Glenn M. Mason
Projects: None

Publication Status: In press in ApJL
Last Modified: 20040813 14:06




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