Higher-order Analysis of Three-dimensional Anisotropy in Imbalanced Alfvénic Turbulence
Year: 2025
Type: article
Abstract: We examine the applicability of homogeneous, negligible cross-helicity magnetohydrodynamic (MHD) turbulence models incorporating “critical balance” (CB) and “scale-dependent dynamic alignment” (SDDA) to Parker Solar Probe observations of a highly Alfvénic, high cross-helicity solar wind stream. At energy-injection scales, both Alfvénic modes satisfy χ λ ± ≡ τ A ± / τ n l ± ≪ 1 , where τ A ± and τ n l ± denote the linear and nonlinear timescales. The outward cascade remains weak, χ λ + ≈ 0.2 , across all scales, while the inward cascade reaches CB at the onset of the inertial range ( χ λ − ∼ 1 ), yet with spectral scalings that depart from the canonical weak-to-strong transition. Within the domain conventionally designated as the inertial range, we identify two statistically distinct subranges. At larger scales ( R 2 ; 200–6000, d i ), the average eddy displays a field-aligned, tube-like morphology. An inverse correlation is observed between alignment angle and gradient intensity, and the conditional structure-function exponents ζ n —measured perpendicular to both the local mean field and fluctuation direction—agree with the predictions of B. D. G. Chandran et al. and A. Mallet & A. A. Schekochihin, although the parallel and displacement components show more concave scaling than anticipated. At smaller scales ( R 1 ; 10–100, d i ), spectra steepen, eddies become ribbon-like, and intermittency weakens. Within a narrow interval near the ion characteristic scales, eddies approach isotropy before the trend of increasing anisotropy resumes at smaller scales. Analysis using five-point increments further demonstrates a stronger multifractal character at kinetic scales than is resolved with conventional two-point methods. Finally, we discuss the influence of solar wind expansion, finite cross helicity, “anomalous coherence,” and the emergence of a “helicity barrier” in modifying CB/SDDA phenomenology and shaping the statistical properties of solar wind turbulence.
Source: The Astrophysical Journal
Institutions: Space Sciences Laboratory, University of California, Berkeley, Aristotle University of Thessaloniki, University of California, Los Angeles, University of New Hampshire, Institute for the Science and Technology of Plasmas, National Research Council, University of Arizona, Centre National de la Recherche Scientifique, Université Côte d'Azur, Lagrange Laboratory, Observatoire de la Côte d’Azur, University of Florence
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W4415685504