Scale-Dependent Dynamic Alignment in MHD Turbulence: Insights into Intermittency, Compressibility, and Imbalance Effects

TL;DR

This study investigates how scale-dependent dynamic alignment in MHD turbulence varies with factors like imbalance and compressibility, using WIND data, revealing that alignment behaviors depend on scale, imbalance, and wave interactions.

Contribution

It provides empirical evidence for scale-dependent dynamic alignment in solar wind turbulence and links it to imbalanced Els"asser field interactions, expanding understanding of turbulence dynamics.

Findings

SDDA appears at energy-containing scales and misaligns at inertial scales.

Alignment angles inversely correlate with field gradient intensity.

Alignment behavior varies with Els"asser imbalance and scale.

Abstract

Scale-Dependent Dynamic Alignment (SDDA) in Els\"asser field fluctuations is theorized to suppress nonlinearities and modulate the energy spectrum. Limited empirical evidence exists for SDDA within the solar wind turbulence's inertial range. We analyzed data from the WIND mission to assess the effects of compressibility, intermittency, and imbalance on SDDA. SDDA consistently appears at energy-containing scales, with a trend toward misalignment at inertial scales. Compressible fluctuations show no increased alignment; however, their impact on SDDA's overall behavior is minimal. The alignment angles inversely correlate with field gradient intensity, likely due to "anomalous" or "counterpropagating" wave packet interactions. This suggests that SDDA originates from mutual shearing of Els\"asser fields during imbalanced ($\delta \boldsymbol{z}^{\pm} \gg \delta \boldsymbol{z}^{\mp}$) interactions. Rigorous thresholding on field gradient intensity reveals SDDA signatures across much of the inertial range. The scaling of Els\"asser increments' alignment angle, $\Theta^{z}$, steepens with increasing global Alfv\'enic imbalance, while the angle between magnetic and velocity field increments, $\Theta^{ub}$, becomes shallower. $\Theta^{ub}$ only correlates with global Els\"asser imbalance, steepening as the imbalance increases. Furthermore, increasing alignment in $\Theta^{ub}$ persists deep into the inertial range of balanced intervals but collapses at large scales for imbalanced ones. Simplified theoretical analysis and modeling of high-frequency, low-amplitude noise in the velocity field indicate significant impacts on alignment angle measurements even at very low frequencies, with effects growing as global imbalance increases.