Small scale anisotropy of magnetic turbulence in the solar wind (original) (raw)

Persistence of small-scale anisotropy of magnetic turbulence as observed in the solar wind

Europhysics Letters (epl), 2006

The anisotropy of magnetophydrodynamic turbulence is investigated by using solar wind data from the Helios 2 spacecraft. We investigate the behaviour of the complete high-order moment tensors of magnetic field increments and we compare the usual longitudinal structure functions which have both isotropic and anisotropic contributions, to the fully anisotropic contribution. Scaling exponents have been extracted by an interpolation scaling function. Unlike the usual turbulence in fluid flows, small-scale magnetic fluctuations remain anisotropic. We discuss the radial dependence of both anisotropy and intermittency and their relationship.

Nonaxisymmetric Anisotropy of Solar Wind Turbulence

Physical Review Letters, 2011

A key prediction of turbulence theories is frame-invariance, and in magnetohydrodynamic (MHD) turbulence, axisymmetry of fluctuations with respect to the background magnetic field. Paradoxically the power in fluctuations in the turbulent solar wind are observed to be ordered with respect to the bulk macroscopic flow as well as the background magnetic field. Here, non-axisymmetry across the inertial and dissipation ranges is quantified using in-situ observations from Cluster. The observed inertial range non-axisymmetry is reproduced by a 'fly through' sampling of a Direct Numerical Simulation of MHD turbulence. Furthermore, 'fly through' sampling of a linear superposition of transverse waves with axisymmetric fluctuations generates the trend in non-axisymmetry with power spectral exponent. The observed non-axisymmetric anisotropy may thus simply arise as a sampling effect related to Taylor's hypothesis and is not related to the plasma dynamics itself.

3D Anisotropy of Solar Wind Turbulence, Tubes, or Ribbons?

The Astrophysical Journal, 2018

We study the anisotropy with respect to the local magnetic field of turbulent magnetic fluctuations at magnetofluid scales in the solar wind. Previous measurements in the fast solar wind obtained axisymmetric anisotropy, despite that the analysis method allows nonaxisymmetric structures. These results are probably contaminated by the wind expansion that introduces another symmetry axis, namely, the radial direction, as indicated by recent numerical simulations. These simulations also show that while the expansion is strong, the principal fluctuations are in the plane perpendicular to the radial direction. Using this property, we separate 11 yr of Wind spacecraft data into two subsets characterized by strong and weak expansion and determine the corresponding turbulence anisotropy. Under strong expansion, the small-scale anisotropy is consistent with the Goldreich & Sridhar critical balance. As in previous works, when the radial symmetry axis is not eliminated, the turbulent structures are field-aligned tubes. Under weak expansion, we find 3D anisotropy predicted by the Boldyrev model, that is, turbulent structures are ribbons and not tubes. However, the very basis of the Boldyrev phenomenology, namely, a cross-helicity increasing at small scales, is not observed in the solar wind: the origin of the ribbon formation is unknown.

Magnetically dominated structures as an important component of the solar wind turbulence

Annales Geophysicae, 2007

This study focuses on the role that magnetically dominated fluctuations have within the solar wind MHD turbulence. It is well known that, as the wind expands, magnetic energy starts to dominate over kinetic energy but we lack of a statistical study apt to estimate the relevance of these fluctuations depending on wind speed, radial distance from the sun and heliographic latitude. Our results suggest that this kind of fluctuations can be interpreted as non-propagating structures, advected by the wind during its expansion. In particular, observations performed in the ecliptic revealed a clear radial dependence of these magnetic structures within fast wind, but not within slow wind. At short heliocentric distances (~0.3 AU) the turbulent population is largely dominated by Alfvénic fluctuations characterized by high values of normalized cross-helicity and a remarkable level of energy equipartition. However, as the wind expands, a new-born population, characterized by lower values of Alfvénicity and a clear imbalance in favor of magnetic energy becomes visible and clearly distinguishable from the Alfvénic population largely characterized by an outward sense of propagation. We estimate that more than 20% of all the analyzed intervals of hourly scale within fast wind are characterized by normalized cross-helicity close to zero and magnetic energy largely dominating over kinetic energy. Most of these advected magnetic structures result to be non-compressive and might represent the crossing of the border between adjacent flux tubes forming, as suggested in literature, the advected background structure of the interplanetary magnetic field. On the other hand, their features are also well fitted by the Magnetic Field Directional Turnings paradigm as proposed in literature.

Wave-Vector Dependence of Magnetic-Turbulence Spectra in the Solar Wind

Physical Review Letters, 2010

Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range jkj 1:5 Â 10 À3 rad=km. The energy distribution exhibits anisotropic features characterized by a prominently extended structure perpendicular to the mean field preferring the ecliptic north direction and also by a moderately extended structure parallel to the mean field. From the three-dimensional energy distribution wave vector anisotropy is estimated with respect to directions parallel and perpendicular to the mean magnetic field, and the result suggests the dominance of quasi-two-dimensional turbulence toward smaller spatial scales.

On the scaling properties of anisotropy of interplanetary magnetic turbulent fluctuations

Europhysics Letters (epl), 2010

The anisotropic character of interplanetary magnetic-field turbulence has been studied through the analysis of Cluster data. The full tensor of the mixed second-order structure functions has been used to quatitatively measure the degree of anisotropy and its effect on small-scale turbulence. Three different regions of the near-Earth space have been studied, namely the solar wind, the Earth's foreshock and magnetosheath. While in the undisturbed solar wind the observed strong anisotropy is mainly due to the large-scale magnetic field, near the magnetosphere other sources of anisotropy influence the magnetic-field properties.

Anisotropic Intermittency of Magnetohydrodynamic Turbulence

The Astrophysical Journal, 2014

A higher-order multiscale analysis of spatial anisotropy in inertial range magnetohydrodynamic turbulence is presented using measurements from the STEREO spacecraft in fast ambient solar wind. We show for the first time that, when measuring parallel to the local magnetic field direction, the full statistical signature of the magnetic and Elsässer field fluctuations is that of a non-Gaussian globally scale-invariant process. This is distinct from the classic multi-exponent statistics observed when the local magnetic field is perpendicular to the flow direction. These observations are interpreted as evidence for the weakness, or absence, of a parallel magnetofluid turbulence energy cascade. As such, these results present strong observational constraints on the statistical nature of intermittency in turbulent plasmas.

Radial Evolution of the Wavevector Anisotropy of Solar Wind Turbulence Between 0.3 and 1 Au

The Astrophysical Journal, 2013

We present observations of the power spectral anisotropy in wave-vector space of solar wind turbulence, and study how it evolves in interplanetary space with increasing heliocentric distance. For this purpose we use magnetic field measurements made by the Helios-2 spacecraft at three positions between 0.29 and 0.9 AU. To derive the power spectral density (PSD) in (k , k ⊥)-space based on single-satellite measurements is a challenging task not yet accomplished previously. Here we derive the spectrum PSD 2D (k , k ⊥) from the spatial correlation function CF 2D (r , r ⊥) by a transformation according to the projection-slice theorem. We find the so constructed PSDs to be distributed in k-space mainly along a ridge that is more inclined toward the k ⊥ than k axis, a new result which probably indicates preferential cascading of turbulent energy along the k ⊥ direction. Furthermore, this ridge of the distribution is found to gradually get closer to the k ⊥ axis, as the outer scale length of the turbulence becomes larger while the solar wind flows further away from the Sun. In the vicinity of the k axis, there appears a minor spectral component that probably corresponds to quasi-parallel Alfvénic fluctuations. Their relative contribution to the total spectral density tends to decrease with radial distance. These findings suggest that solar wind turbulence undergoes an anisotropic cascade transporting most of its magnetic energy towards larger k ⊥ , and that the anisotropy in the inertial range is radially developing further at scales that are relatively far from the ever increasing outer scale.

Magnetohydrodynamic Turbulence in the Solar Wind

Annual Review of Astronomy and Astrophysics, 1995

Recent work in describing the solar wind as an MHD turbulent fluid has shown that the magnetic fluctuations are adequately described as time stationary and to some extent as spatially homogeneous. Spectra of the three rugged invariants of incompressible MHD are the principal quantities used to characterize the velocity and magnetic field fluctuations. Unresolved issues concerning the existence of actively developing turbulence are discussed. INTRODUCTION To describe a fluid system as turbulent is to say that the dynamical fluid variables exhibit complex and essentially non-reproducible behavior as a function of time. This is generally due to the presence of nonlinearities in the fluid equations which strongly couple a large number of degrees of freedom. Turbulent systems are usually very far from equilibrium states for which detailed analytically tractable theories might exist. By all appearances, the solar wind plasma flow and the interplanetary magnetic field carried along with it are such a turbulent system. In the zero momentum frame, the magnetic and velocity field fluctuations are energetically comparable to the mean magnetic field over length scales of order I AU and display the type of complicated behavior expected of turbulence. velocity and magnetic fields in magnetohydrodynamie turbulence, EOS,