The incompressible energy cascade rate in anisotropic solar wind turbulence

TL;DR

This study analyzes how magnetic guide fields induce anisotropy in solar wind turbulence, revealing that the perpendicular energy cascade dominates near the Sun and correlates with plasma temperature, using Parker Solar Probe data.

Contribution

It provides the first detailed quantification of anisotropic energy cascade rates in the solar wind across different distances and conditions, using exact MHD relations and observational data.

Findings

Perpendicular cascade dominates over parallel near the Sun.

Energy cascade rates correlate with plasma temperature.

Variance anisotropy ratios are independent of heliocentric distance.

Abstract

Context. The presence of a magnetic guide field induces several types of anisotropy in solar wind turbulence. The energy cascade rate between scales in the inertial range depends strongly on the direction of this magnetic guide field, splitting the energy cascade according to the parallel and perpendicular directions with respect to magnetic guide field. Aims. Using more than two years of Parker Solar Probe (PSP) observations, the isotropy and anisotropy energy cascade rates are investigated. The variance and normalized fluctuation ratios, the kinetic and magnetic energies, and the normalized cross-helicity and residual energy are studied. The connection between the heliocentric distance, the local temperature of the plasma, and the energy cascade components is made. Methods. Using exact relations for fully developed incompressible magnetohydrodynamic (MHD) turbulence, the incompressible energy cascade rate is computed. In particular, using the isotropy and 2D and slab assumptions, the isotropic, perpendicular, and parallel energy cascade rate components are estimated. Results. The variance anisotropy ratios, for both velocity and magnetic fields, do not exhibit a dependence with respect to the heliocentric distance $r$ between 0.2 and 0.8 au. While the velocity normalized fluctuation ratio shows a dependence with $r$, the magnetic normalized fluctuation ratio does not. A strong correlation between the isotropic and anisotropic energy cascade rates and the temperature is found. A clear dominance of the perpendicular cascades over the parallel cascades as PSP approaches the Sun is observed. A dominant 2D cascade and/or geometry over the slab component in slow solar wind turbulence in the largest MHD scales is observed.