This paper summarises some of the recent progress that has been made in understanding astrophysical plasma turbulence in the solar wind, from in situ spacecraft observations. At large scales, where the turbulence is predominantly Alfvenic, measurements of critical balance, residual energy and three-dimensional structure are discussed, along with comparison to recent models of strong Alfvenic turbulence. At these scales, a few per cent of the energy is also in compressive fluctuations, and their nature, anisotropy and relation to the Alfvenic component is described. In the small-scale kinetic range, below the ion gyroscale, the turbulence becomes predominantly kinetic Alfven in nature, and measurements of the spectra, anisotropy and intermittency of this turbulence are discussed with respect to recent cascade models. One of the major remaining questions is how the turbulent energy is dissipated, and some recent work on this question, in addition to future space missions which will help to answer it, are briefly discussed.

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Recent progress in astrophysical plasma turbulence from solar wind observations

The first two orbits of the Parker Solar Probe (PSP) spacecraft have enabled the first in situ measurements of the solar wind down to a heliocentric distance of 0.17 au (or 36 Rs). Here, we present an analysis of this data to study solar wind turbulence at 0.17 au and its evolution out to 1 au. While many features remain similar, key differences at 0.17 au include: increased turbulence energy levels by more than an order of magnitude, a magnetic field spectral index of -3/2 matching that of the velocity and both Elsasser fields, a lower magnetic compressibility consistent with a smaller slow-mode kinetic energy fraction, and a much smaller outer scale that has had time for substantial nonlinear processing. There is also an overall increase in the dominance of outward-propagating Alfvenic fluctuations compared to inward-propagating ones, and the radial variation of the inward component is consistent with its generation by reflection from the large-scale gradient in Alfven speed. The energy flux in this turbulence at 0.17 au was found to be ~10% of that in the bulk solar wind kinetic energy, becoming ~40% when extrapolated to the Alfven point, and both the fraction and rate of increase of this flux towards the Sun is consistent with turbulence-driven models in which the solar wind is powered by this flux.

https://iopscience.iop.org/article/10.3847/1538-4365/ab60a3/pdf

The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere

We investigate the anisotropy of Alfvenic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct com- parison is made by measuring the anisotropic second-order structure functions in both data sets. In the solar wind, the perpendicular spectral index of the magnetic field is close to −5/3. In the forced simulation, it is close to −5/3 for the velocity and −3/2 for the magnetic field. In the decaying simulation, it is −5/3 for both fields. The spectral index becomes steeper at small angles to the local magnetic field direction in all case s. We also show that when us- ing the global rather than local mean field, the anisotropic s caling of the simulations cannot always be properly measured.

/pdf/anisotropy-of-alfvenic-turbulence-in-the-solar-wind-and-1ivhwij1nn.pdf

Anisotropy of Alfvénic turbulence in the solar wind and numerical simulations

We investigate the anisotropy of Alfvenic turbulence in the inertial range of slow solar wind and in both driven and decaying reduced magnetohydrodynamic simulations. A direct comparison is made by measuring the anisotropic second-order structure functions in both data sets. In the solar wind, the perpendicular spectral index of the magnetic field is close to -5/3. In the forced simulation, it is close to -5/3 for the velocity and -3/2 for the magnetic field. In the decaying simulation, it is -5/3 for both fields. The spectral index becomes steeper at small angles to the local magnetic field direction in all cases. We also show that when using the global rather than local mean field, the anisotropic scaling of the simulations cannot always be properly measured.

Anisotropy of Alfv\'enic Turbulence in the Solar Wind and Numerical Simulations

The question is addressed as to what extent incompressible magnetohydrodynamics can describe random magnetic and velocity fluctuations measured in the solar wind. It is demonstrated that distributions of spectral indices for the velocity, magnetic field, and total energy obtained from high-resolution numerical simulations of magnetohydrodynamic turbulence are qualitatively and quantitatively similar to solar wind observations at 1?AU. Both simulations and observations show that in the inertial range the magnetic field spectrum Eb is steeper than the velocity spectrum Ev with Eb Ev and that the magnitude of the residual energy ER = Ev ? Eb decreases nearly following a k ?2 ? scaling.

https://iopscience.iop.org/article/10.1088/2041-8205/741/1/L19/pdf

Spectral Scaling Laws in Magnetohydrodynamic Turbulence Simulations and in the Solar Wind

Solar wind measurements over the period from 1995 through 2006 are used to study fluctuations in the plasma bulk velocity and magnetic field over the range of magnetohydrodynamic (MHD) scales commonly referred to as the inertial range. Power spectra of the solar wind velocity, magnetic field, proton density, total energy (kinetic plus magnetic), and cross-helicity are analyzed for 176 time intervals in which the interplanetary magnetic field is restricted to a single magnetic sector. The data yield measurements of the normalized cross-helicity σc, the ratio of the cross-helicity spectrum to the energy spectrum, that span the entire inertial range at 1 AU and extend previous measurements by more than one decade in wavenumber. The results show that σc is approximately constant throughout the inertial range, independent of wavenumber, consistent with existing theoretical ideas. At the highest frequencies measured, |σc| is observed to decrease toward zero, however, this decrease is caused by measurement noise...

J. E. Borovsky

Papers

Scale invariance of normalized cross-helicity throughout the inertial range of solar wind turbulence