Critically Balanced Turbulence In Magnetized Plasmas: Implications For Dissipation-scale Turbulence In The Solar Wind

Details Critically Balanced Turbulence In Magnetized Plasmas: Implications For Dissipation-scale Turbulence In The Solar Wind Critically Balanced Turbulence In Magnetized Plasmas: Implications For Dissipation-scale Turbulence In The Solar Wind

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...210.5501h&link_type=abstract

American Astronomical Society Meeting 210, #55.01; Bulletin of the American Astronomical Society, Vol. 39, p.165

Astronomy and Astrophysics

Astronomy

Scientific paper

We present an analytical cascade model for turbulence in weakly collisional plasmas. Key assumptions are spectrally local nonlinear energy transport, critical balance, and the applicability of linear dissipation rates. The model follows the nonlinear cascade of energy from the MHD regime, through the transition at the ion Larmor radius, and into the kinetic Alfven wave regime. For a large inertial range, turbulent fluctuations remain at low frequency due to the strong anisotropy of the MHD cascade; such turbulence is optimally described by gyrokinetics. Observational estimates show that the gyrokinetic approximation is well satisfied for typical solar wind parameters. Steady-state solutions of the gyrokinetic model for the slow solar wind yield three conclusions: (1) beyond the observed break in the magnetic energy spectrum, one expects an exponential cut-off; (2) the widely held interpretation that this dissipation range obeys power-law behavior is an artifact of instrumental sensitivity limitations; and, (3) over the range of parameters relevant to the solar wind, the observed variation of dissipation range spectral indices from -2 to -4 is naturally explained by the varying effectiveness of Landau damping, from an undamped prediction of -7/3 to a strongly damped index around -4. A cascade model based on the hot plasma dispersion relation enables evaluation of the relative importance of the Landau and ion cyclotron resonances in the dissipation of the slow solar wind. The model predicts that, for plasmas with beta < 0.1, the ion cyclotron resonance controls the breakpoint frequency and the steepened spectrum; for beta > 0.1, the breakpoint occurs at the ion Larmor radius, and the damping may be a combination of Landau damping and ion cyclotron damping. This simple cascade model can connect observational data to detailed nonlinear gyrokinetic simulations aimed at unraveling the mechanisms at work in the turbulent solar wind.

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