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Abstract

Abstract. For a while it seemed like a simple fluid-like, self-similar, Kolmogoroff cascade was the easy explanation for the nature and evolution of the majority of solar wind fluctuations. More recently we have found that:

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Abstract

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  1. Abstract For a while it seemed like a simple fluid-like, self-similar, Kolmogoroff cascade was the easy explanation for the nature and evolution of the majority of solar wind fluctuations. More recently we have found that: • the cascade is not stirred at large scales, and the large scales present a number of puzzles; • the velocity and magnetic spectra evolve differently with different “inertial ranges” in both slope (until far from the Sun) and wavenumber range (everywhere); • anisotropy in both variances and spectral characteristics are the order of the day and are strongly scale dependent; and • it is not clear what fraction of the fluctuations should be considered to be “turbulent” as opposed to, for example, convected structures or discontinuities. This talk will review some recent results in these areas, and attempt to characterize where we are in our understanding.

  2. Large scale transverse fluctuations are more equipartioned than expected from a “quasi-static” regime Pvr/ Pbr Shear in V (Vr(q)) is the dominant energy in fluctuations at large scales. Solar rotation Pvn/ Pbn Pvt/ Pbt

  3. Helios 2, 0.3 AU NonAlfvénic Alfvénic f –1 f –5/3 f –1 f –3/2 Note the curious flattening—seen, at times in v, density, |B|, and z –, and more prevalently closer to the Sun. It is not an artifact, but a real feature.

  4. Velocity (black) and Magnetic (red) spectra1 AU (Helios 2; same streams) NonAlfvénic Alfvénic Note again the flattening of the V spectrum

  5. Velocity (black/green) and Magnetic (red) spectra~4.5 AU (Ulysses) NonAlfvénic Alfvénic Velocity spectral slope is clearly –5/3, not –3/2 Still a flattening in V (N.B. change in f range)

  6. Velocity (black) and Magnetic (red) spectra4.7 AU (ecliptic, Ulysses pre-Jupiter) NonAlfvénic B is flat (f –1)while V is already in an “inertial range.” The low-f and high-f spectra can have a transition that may indicate a lack of cascade from large to small scales. The turbulence is decaying, not driven. f –5/3 f –5/3

  7. Velocity Spectral Slope vs radial distance,speed, latitude, and Alfvénicity (Ulysses) Near ecliptic (<20°) 0.5 > |σc| > 0.33 + > 675 km/s |σc| > 0.5

  8. Conclusions • Don’t trust 1 AU – spectra are rapidly evolving, and eventually reach a “Kolmogoroff” spectrum for both velocity and magnetic field, which take different paths to get there. • The only really Alfvénic fluctuations (both in correlation and in amplitude) are in “smooth” streams nearer the Sun. • The large-scale fluctuations are not well understood, and there does not seem to be a simple “non-WKB” or “quasi-static” regime. • We need to rethink the idea of “the inertial range” for the solar wind. • Cross-helicity strongly determines the rate of evolution (as expected). • In the inner heliosphere, the evolution of velocity fluctuations is often energetically more important that of the magnetic field. • Turbulence in the solar wind is decaying rather than driven. • We still don’t know whether low cross-helicity regions near the Sun represent “older turbulence” or a different origin—they do have more “evolved” spectra, but we await new missions to determine the origin of the spectrum nearer the Sun.

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