On the evolution of the Anisotropic Scaling of Magnetohydrodynamic Turbulence in the Inner Heliosphere

TL;DR

This study investigates how the anisotropic properties of magnetohydrodynamic turbulence in the solar wind evolve with distance from the Sun, revealing different behaviors in fast and slow wind streams and identifying a transition within the inertial range.

Contribution

It provides new observational insights into the radial evolution of turbulence anisotropy and identifies a transition in turbulence regimes near the first Parker Solar Probe perihelion.

Findings

Anisotropic signatures decrease with heliocentric distance in slow wind.

Fast wind retains near-Sun anisotropic properties, consistent with a dynamically aligned cascade.

A transition from weak to strong turbulence occurs at a specific scale, changing spectral indices.

Abstract

We analyze a merged Parker Solar Probe ($PSP$) and Solar Orbiter ($SO$) dataset covering heliocentric distances $13 \ R_{\odot} \lesssim R \lesssim 220$ $R_{\odot}$ to investigate the radial evolution of power and spectral-index anisotropy in the wavevector space of solar wind turbulence. Our results show that anisotropic signatures of turbulence display a distinct radial evolution when fast, $V_{sw} \geq ~ 400 ~km ~s^{-1}$, and slow, $V_{sw} \leq ~ 400 ~km ~s^{-1}$, wind streams are considered. The anisotropic properties of slow wind in Earth orbit are consistent with a ``critically balanced'' cascade, but both spectral-index anisotropy and power anisotropy diminish with decreasing heliographic distance. Fast streams are observed to roughly retain their near-Sun anisotropic properties, with the observed spectral index and power anisotropies being more consistent with a ``dynamically aligned'' type of cascade, though the lack of extended fast-wind intervals makes it difficult to accurately measure the anisotropic scaling. A high-resolution analysis during the first perihelion of PSP confirms the presence of two sub-ranges within the inertial range, which may be associated with the transition from weak to strong turbulence. The transition occurs at $\kappa d_{i} \approx 6 \times 10^{-2}$, and signifies a shift from -5/3 to -2 and -3/2 to -1.57 scaling in parallel and perpendicular spectra, respectively. Our results provide strong observational constraints for anisotropic theories of MHD turbulence in the solar wind.