Nonlinear Interactions in Spherically Polarized Alfv\'{e}nic Turbulence

TL;DR

This paper investigates large-amplitude Alfvénic turbulence in the solar wind, revealing nonlinear interactions and alignment effects that influence energy transfer mechanisms beyond the small-amplitude approximation.

Contribution

It introduces a new method to analyze finite-amplitude turbulence through constant magnitude rotations, uncovering nonlinear interactions and alignment effects in spherically polarized Alfvén waves.

Findings

Signatures of finite-amplitude effects exist deep into the MHD range.

Subdominant modes are relatively compressible.

Alignment may reduce nonlinearity or maintain coherence for energy transfer.

Abstract

Turbulent magnetic field fluctuations observed in the solar wind often maintain a constant magnitude condition accompanied by spherically polarized velocity fluctuations; these signatures are characteristic of large-amplitude Alfv\'{e}n waves. Nonlinear energy transfer in Alfv\'{e}nic turbulence is typically considered in the small-amplitude limit where the constant magnitude condition may be neglected; in contrast, nonlinear energy transfer in the large-amplitude limit remains relatively unstudied. We develop a method to analyze finite-amplitude turbulence through studying fluctuations as constant magnitude rotations in the stationary wave (de Hoffmann-Teller) frame, which reveals that signatures of finite-amplitude effects exist deep into the MHD range. While the dominant fluctuations are consistent with spherically-polarized large-amplitude Alfv\'{e}n waves, the subdominant mode is relatively compressible. Signatures of nonlinear interaction between the finite-amplitude spherically polarized mode with the subdominant population reveal highly aligned transverse components. In theoretical models of Alfv\'{e}nic turbulence, alignment is thought to reduce nonlinearity; our observations require that alignment is sufficient to either reduce shear nonlinearity such that non-Alfv\'{e}nic interactions may be responsible for energy transfer in spherically polarized states, or that counter-propagating fluctuations maintain anomalous coherence, which is a predicted signature of reflection-driven turbulence.