Basic properties of solar wind MHD turbulence near 0.3 AU analyzed by means of Elsässer variables

Abstract

and Alfv6n velocity fluctuations, respectively. For small amplitude Alfv6n waves the fluctuation variable •Z + simply relates to outward propagation and •Z-to an inward sense of propagation, if the ambient magnetic field Bo is directed inward. The frequency range analysed in this paper is 6 x 10 -6 Hz to 6 x 10 -3 Hz. It is found that (1) the autocorrelation length for $Z-is much larger than for/SZ + in both the high-speed and low-speed wind. (2) The power spectra of •Z-, especially in high-wind speed, are steeper in the low-frequency range and flatten in the high-frequency range. (3) In the low-frequency range, the power spectra for the components of •Z + tend to be isotropic with respect to the three polarization directions, while the spectra of •Z-are dominated by the radial component. In the high-frequency domain, the spectra of both •Z + and •Z-are dominated by the transverse component in high-speed wind and are more isotropic in low-speed wind. (4) The spectra related to the residual energy or the cross-correlation in low-speed flows have a power law with the slope near to -5/3. However, in high-speed flows the corresponding data are widely distributed in a cloud of points with an upper envelope near to the spectrum of •Z-. The origin of all these spectra and their importance for the solar wind physics have also been discussed. Several generation mechanisms are suggested as candidates. In the flat part of e-spectrum, the fluctuations may be generated by non-local (in wave number space) interactions with the low-frequency part of the e + spectrum, or just by parametric decay of the high-frequency part of the e + spectrum. The steep part of e-(f) may be related to small-scale stream tubes, or be influenced by pressure waves, nonlinear cascading, and the interaction with the outgoing Alfv6n waves. wave description are entirely in contradiction. To get out of this dilemma,'Dobrowolny et al. [1980a, b] proposed a model known as "dynamic alignment," in which the fluctuations are conceived as the final state of a turbulent relaxation of cross-helicity, resulting in a disappearance of nonlinear interactions. Instead of considering only local interactions in k space, Matthaeus et al. [1983] suggested that outward propagating fluctuations might even be generated outside the Alfv•nic critical point by nonlocal interactions in wave number space. However, the recent observations did not support these conjectures [Roberts et al., 1987a, b; Luttrell and Richter, 1988], and the problem how to describe the solar wind fluctuations remained unsolved. The observational findings of Denskat and Neubauer [1982] and Bavassano et al. [1982a, b] that the slope of the spectrum in the low-frequency range of Alfv•nic fluctuations becomes steeper with increasing heliospheric distance cannot be explained by the WKB theory of pure Alfv•n waves. This evidence strongly suggests that nonlinear interactions in fact occur between the fluctuations that still preserve some properties of Alfv•n waves. The nonlinear interactions couple the primord...

Intermittent turbulence in the heliosheath and the magnetosheath plasmas based on Voyager and THEMIS data

“…15, cases (c) and (d) in Table 2. We can generally notice only small differences between z + and z − , and therefore the outgoing and ingoing fluctuations seem to be similar, which is roughly consistent with equipartition suggested by Tu et al (1989). On the other hand, behind the bow shock for small plasma β ∼ 1 (when the thermal pressure and the magnetic pressure are similar in the magnetized plasma), but with a moderate Alfvénic Mach number M A ≈ 9, Fig.…”

Evolution of plasma turbulence excited with particle beams

“…High Reynolds numbers in combination with massive energy injection, as seen in, e.g., the solar wind, are strong indicators of a highly turbulent state. In situ measurements of the energy spectrum (Tu et al 1989) agree with this fact. To simulate conditions within the turbulent heliospheric plasma, the research group at the University of Würzburg has developed a simulation code, Gismo.…”

Evidence for Nonlinear Development of Magnetohydrodynamic Scale Intermittency in the Inner Heliosphere

“…Here we employ the Karman-Taylor approach for modeling the evolution of λ C and Z (Matthaeus et al 1998), obtaining the scalings λ C (r) ∼ √ (r) and Z 2 (r) ∼ 1/r (Matthaeus et al 1998). Then, the corresponding MHD timescale, in case of zero cross-helicity, is t NL (r) = λ C (r)/Z(r) ∼ r. However, this simplification is well satisfied beyond a few astronomical units (AU), but it is not a reasonable simplification at distances less than 1 AU from the Sun (Tu et al 1989). Matthaeus et al (2004) have shown that for nonzero cross-helicity σ c , along with the equations for the evolution of λ C and Z, a third simplified equation for σ c can be written, which causes a reduction in the effective decay of the above quantities by a factor f. In turn, the nonlinear time becomes t NL (r) = f λ C (r)/Z(r), where f is a function of σ c and it is defined as (1…”

“…Therefore, the fact that plasma data (81 s) are not available at the cadence of the magnetic field data (6 s) does not present a problem for the computation of the global cross-helicity. This is however the reason we did not consider analysis in terms of Elsasser fluctuation variables (Tu et al 1989), staying instead with magnetic field fluctuations.…”