Enhanced Magnetic Compressibility and Isotropic Scale-Invariance at Sub-Ion Larmor Scales in Solar Wind Turbulence

TL;DR

This study reveals increased magnetic compressibility and isotropy at sub-ion Larmor scales in solar wind turbulence, highlighting the role of kinetic Alfvén waves and Hall effects in shaping small-scale turbulence.

Contribution

It provides the first detailed observational evidence of isotropic, scale-invariant turbulence at sub-ion scales in the solar wind, linking it to kinetic wave physics and Hall effects.

Findings

Isotropy achieved at electron Larmor radius

Magnetic fluctuations consistent with kinetic Alfvén waves

Full statistical signature indicates a scale-invariant process

Abstract

The anisotropic nature of solar wind magnetic turbulence fluctuations is investigated scale-by-scale using high cadence in-situ magnetic field measurements from the Cluster and ACE spacecraft missions. The data span five decades in scales from the inertial range to the electron Larmor radius. In contrast to the inertial range, there is a successive increase towards isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. In the context of wave-mediated theories of turbulence, we show that this enhancement in magnetic fluctuations parallel to the local mean background field is qualitatively consistent with the magnetic compressibility signature of kinetic Alfven wave solutions of the linearized Vlasov equation. More generally, we discuss how these results may arise naturally due to the prominent role of the Hall term at sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show that the full statistical signature of the fluctuations at scales below the ion Larmor radius is that of a single isotropic globally scale-invariant process distinct from the anisotropic statistics of the inertial range.