Observations of the Relationship between Magnetic Anisotropy and Mode Composition in Low-$β$ Solar Wind Turbulence
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 Letter, we investigate the nature of magnetic anisotropy in compressible magnetohydrodynamic (MHD) turbulence within low-$\beta$ solar wind using Cluster spacecraft measurements. By decomposing small-amplitude fluctuations into Alfv\'en and compressible modes, we reveal that the anisotropy is strongly mode dependent: quasi-parallel (`slab') energy contains both Alfv\'en and compressible modes, whereas quasi-perpendicular (`two-dimensional'; 2D) energy is almost purely Alfv\'enic, a feature closely linked to collisionless damping of compressible modes. These findings elucidate the physical origin of the long-standing `slab+2D' empirical model and offer a new perspective on the turbulence cascade across the full three-dimensional wavevector space.