AbstractFor 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.

Large scale transverse fluctuations are more equipartioned than expected from a “quasi-static” regime

Pvr / Pbr

Pvn / Pbn

Pvt / Pbt

Shear in V (Vr(q)) is

the dominant energy influctuations at largescales.

Solar rotation

NonAlfvénic Alfvénic

f –1

f –3/2

f –5/3f –1

Note the curious flattening—seen, at times in v, density, |B|, and z –, and more prevalently closer to the Sun. It isnot an artifact, but a real feature.

Helios 2, 0.3 AU

Velocity (black) and Magnetic (red) spectra1 AU (Helios 2; same streams)

NonAlfvénic Alfvénic

Note again the flattening of the V spectrum

Velocity (black/green) and Magnetic (red) spectra~4.5 AU (Ulysses)

NonAlfvénic Alfvénic

(N.B. change in f range)

Velocity spectral slope isclearly –5/3, not –3/2 Still a flattening in V

Velocity (black) and Magnetic (red) spectra4.7 AU (ecliptic, Ulysses pre-Jupiter)

NonAlfvénic

The low-f and high-fspectra can have a transition that may indicate alack of cascade from large to smallscales. The turbulence is decaying,not driven.

B is flat (f –1) while V is already in an “inertial range.”

f –5/3

f –5/3

Velocity Spectral Slope vs radial distance,speed, latitude, and Alfvénicity (Ulysses)

+ > 675 km/s

|σc| > 0.5

0.5 > |σc| > 0.33

Near ecliptic (<20°)

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.