In this review we will focus on a topic of fundamental importance for both plasma physics and astrophysics, namely the occurrence of large-amplitude low-frequency fluctuations of the fields that describe the plasma state. This subject will be treated within the context of the expanding solar wind and the most meaningful advances in this research field will be reported emphasizing the results obtained in the past decade or so. As a matter of fact, Ulysses’ high latitude observations and new numerical approaches to the problem, based on the dynamics of complex systems, brought new important insights which helped to better understand how turbulent fluctuations behave in the solar wind. In particular, numerical simulations within the realm of magnetohydrodynamic (MHD) turbulence theory unraveled what kind of physical mechanisms are at the basis of turbulence generation and energy transfer across the spectral domain of the fluctuations. In other words, the advances reached in these past years in the investigation of solar wind turbulence now offer a rather complete picture of the phenomenological aspect of the problem to be tentatively presented in a rather organic way.

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The Solar Wind as a Turbulence Laboratory

A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.

MHD structures, waves and turbulence in the solar wind : observations and theories

We present a series of models for the plasma properties along open magnetic flux tubes rooted in solar coronal holes, streamers, and active regions. These models represent the first self-consistent solutions that combine (1) chromospheric heating driven by an empirically guided acoustic wave spectrum; (2) coronal heating from Alfven waves that have been partially reflected, then damped by anisotropic turbulent cascade; and (3) solar wind acceleration from gradients of gas pressure, acoustic wave pressure, and Alfven wave pressure. The only input parameters are the photospheric lower boundary conditions for the waves and the radial dependence of the background magnetic field along the flux tube. We have not included multifluid or collisionless effects (e.g., preferential ion heating), which are not yet fully understood. For a single choice for the photospheric wave properties, our models produce a realistic range of slow and fast solar wind conditions by varying only the coronal magnetic field. Specifically, a two-dimensional model of coronal holes and streamers at solar minimum reproduces the latitudinal bifurcation of slow and fast streams seen by Ulysses. The radial gradient of the Alfven speed affects where the waves are reflected and damped, and thus whether energy is deposited below or above the Parker critical point. As predicted by earlier studies, a larger coronal "expansion factor" gives rise to a slower and denser wind, higher temperature at the coronal base, less intense Alfven waves at 1 AU, and correlative trends for commonly measured ratios of ion charge states and FIP-sensitive abundances that are in general agreement with observations. These models offer supporting evidence for the idea that coronal heating and solar wind acceleration (in open magnetic flux tubes) can occur as a result of wave dissipation and turbulent cascade.

http://iopscience.iop.org/article/10.1086/518001/pdf

Self-consistent Coronal Heating and Solar Wind Acceleration from Anisotropic Magnetohydrodynamic Turbulence

▪ Abstract Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small-scale turbulence in the ionized medium, giving information abou...

http://www.ifa.hawaii.edu/~reipurth/reviews/scalo2_araa.pdf

Interstellar Turbulence II: Implications and Effects

[1] It is argued here that the inner heliosphere is filled with a network of entangled magnetic flux tubes and that the flux tubes are fossil structures that originate at the solar surface. 65,860 flux tubes are collected from seven years of measurements with the ACE spacecraft at 1 AU by spotting the flux tube walls with large changes in the magnetic field direction and the vector flow velocity. The tube walls are associated with large changes in the ion entropy density and the alpha-to-proton ratio. The median size of the flux tubes at 1 AU is 4.4 x 10 5 km. The tubes are larger in slow wind than in fast wind. The tubes are on-average aligned with the Parker spiral, with a large spread in orientations. This large spread may be caused by slight misalignments of tubes in the corona. The flux tubes map to granule and supergranule sizes on the Sun. The amounts of magnetic flux in the tubes at 1 AU correspond to the amounts of magnetic flux in field concentrations in the magnetic carpet. It is argued that the flux tubes do not reconnect during the ∼ 100-h advection to 1 AU owing to the expansion of the solar wind. The flux tube texture impacts the flow properties of the solar wind, turbulence in the solar wind, energetic-particle propagation in the inner heliosphere, and the driving of the Earth's magnetosphere. A method for using measurements of the flux tube walls for the remote sensing of magnetic field dynamics in the magnetic carpet is suggested.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007JA012684

Flux tube texture of the solar wind: Strands of the magnetic carpet at 1 AU?

Magnetic field and plasma data, obtained by the Helios 1 and 2 spacecraft in the solar wind near 0.3 AU during the years 1975 to 1976, have been analyzed by calculating 12 kinds of spectra related to the Elsasser variables, δZ+ = δV + δVA, and δZ− = δV − δVA, where δV and δVA are the bulk velocity and Alfven velocity fluctuations, respectively. For small amplitude Alfven waves the fluctuation variable δZ+ simply relates to outward propagation and δZ− to an inward sense of propagation, if the ambient magnetic field B0 is directed inward. The frequency range analysed in this paper is 6×10−6 Hz to 6×10−3 Hz. It is found that (1) the autocorrelation length for δZ− is much larger than for δZ+ in both the high-speed and low-speed wind. (2) The power spectra of δZ−, 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 δZ+ tend to be isotropic with respect to the three polarization directions, while the spectra of δZ− are dominated by the radial component. In the high-frequency domain, the spectra of both δZ+ and δZ− 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 δZ−. 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 Alfven waves.

K. M. Thieme

Papers

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