Mode Composition Shapes Magnetic Anisotropy in Solar Wind Turbulence

TL;DR

This study investigates how different wave modes influence magnetic anisotropy in solar wind turbulence, revealing mode-dependent anisotropy patterns and their implications for energy transfer and particle dynamics.

Contribution

It demonstrates that Alfvénic and compressible modes exhibit distinct anisotropy behaviors, advancing understanding of turbulence mode composition in the solar wind.

Findings

Alfvénic fluctuations are broadly distributed in propagation angles.

Compressible fluctuations are concentrated near the quasi-parallel direction.

Compressible modes dominate at small scales as plasma beta approaches zero.

Abstract

Turbulence is a ubiquitous process that transfers energy across many spatial and temporal scales, thereby influencing particle transport and heating. Recent progress has improved our understanding of the anisotropy of turbulence with respect to the mean magnetic field; however, its exact form and implications for magnetic topology and energy transfer remain unclear. In this study, we investigate the nature of magnetic anisotropy in compressible magnetohydrodynamic (MHD) turbulence within low-$\beta$ solar wind using measurements from the Cluster spacecraft. By decomposing small-amplitude fluctuations into Alfv\'en and compressible modes, we reveal that magnetic anisotropy is largely mode dependent: Alfvenic fluctuations are broadly distributed in propagation angle, whereas compressible fluctuations are concentrated near the quasi-parallel (slab) direction, a feature closely linked to collisionless damping of compressible modes. For $\beta\rightarrow0$, compressible modes become dominant within the slab component at smaller scales. These findings advance our understanding of magnetic anisotropy in solar wind turbulence and offer a new perspective on the three-dimensional turbulence cascade, with broad implications for particle transport, acceleration, and magnetic reconnection.