DEPENDENCE OF SOLAR-WIND POWER SPECTRA ON THE DIRECTION OF THE LOCAL MEAN MAGNETIC FIELD

By J. Podesta. 2009

185 citations recorded by the source corpus.

Wavelet analysis can be used to measure the power spectrum of solar-wind fluctuations along a line in any direction (θ, ϕ) with respect to the local mean magnetic field B0. This technique is applied to study solar-wind turbulence in high-speed streams in the ecliptic plane near solar minimum using magnetic field measurements with a cadence of eight vectors per second. The analysis of nine high-speed streams shows that the reduced spectrum of magnetic field fluctuations (trace power) is approximately azimuthally symmetric about B0 in both the inertial range and dissipation range; in the inertial range the spectra are characterized by a power-law exponent that changes continuously from 1.6 ± 0.1 in the direction perpendicular to the mean field to 2.0 ± 0.1 in the direction parallel to the mean field. The large uncertainties suggest that the perpendicular power-law indices 3/2 and 5/3 are both consistent with the data. The results are similar to those found by Horbury et al. at high heliographic latitudes. Comparisons between solar-wind observations and the theories of strong incompressible MHD turbulence developed by Goldreich & Sridhar and Boldyrev are not rigorously justified because these theories only apply to turbulence with vanishing cross-helicity although the normalized cross-helicity of solar-wind turbulence is not negligible. Assuming these theories can be generalized in such a way that the three-dimensional wavevector spectra have similar functional forms when the cross-helicity is nonzero, then for the interval of Ulysses data analyzed by Horbury et al. the ratio of the spectra perpendicular and parallel to B0 is more consistent with the Goldreich & Sridhar scaling P⊥/P∥ ∝ ν1/3 than with the Boldyrev scaling ν1/2. The analysis of high-speed streams in the ecliptic plane does not yield a reliable measurement of this scaling law. The transition from a turbulent MHD-scale energy cascade to a kinetic Alfvén wave (KAW) cascade occurs when k⊥ρi ≃ 1, which coincides with the spectral break. At slightly higher wavenumbers, in the dissipation range, there is a peak in the power ratio with P⊥/P∥ ≫ 1. The decay of this peak may be caused by the damping of KAWs, which is predicted to occur near k⊥ρi ≃ 4.

Subfields: Astronomy and Astrophysics, Computational Mechanics

View Paper: https://www.semanticscholar.org/paper/05eccaff3b68f9a13d55ab89ac5f608987f5310e

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Citations in the corpus

The Solar Probe Plus Mission: Humanity’s First Visit to Our Star (2016, background): “There is strong evidence that the nonlinear cascade is anisotropic, but it is not known how the anisotropy is generated and evolves (Horbury et al. 2008; Podesta 2009).”
Power and spectral index anisotropy of the entire inertial range of turbulence in the fast solar wind (2010, methodology): “We use the method devised by Horbury et al. (2008) and explained in more detail by Podesta (2009).”
INCORPORATING KINETIC PHYSICS INTO A TWO-FLUID SOLAR-WIND MODEL WITH TEMPERATURE ANISOTROPY AND LOW-FREQUENCY ALFVÉN-WAVE TURBULENCE (2011, background): “There is some evidence that the magnetic fluctuations in the solar wind are consistent with turbulence theories based on critical balance (Horbury et al. 2008; Podesta 2009; Forman et al. 2011).”
A prescription for the turbulent heating of astrophysical plasmas (2010, background): “The resulting state of critical balance is supported by numerical simulations (Cho & Vishniac 2000) and observations in the solar wind (Horbury et al. 2008; Podesta 2009).”
Anisotropy of Alfvénic turbulence in the solar wind and numerical simulations (2010, methodology): “Values close to both −5/3 and −3/2 have been observed previously in the solar wind (Horbury et al. 2008; Podesta 2009; Luo & Wu 2010; Wicks et al. 2010, 2011).”