Lagrangian statistics of a shock-driven turbulent dynamo in decaying turbulence

TL;DR

This study uses advanced 3D MHD simulations to analyze how turbulent dynamo processes amplify magnetic fields behind shocks in a stratified medium, revealing unique growth characteristics during turbulence decay.

Contribution

It presents novel Lagrangian-based analysis of magnetic field amplification in shock-driven turbulence, highlighting differences from periodic box simulations and providing new phenomenological insights.

Findings

Magnetic field growth occurs during turbulence decay with no distinct phases.

Growth rate aligns with compressive turbulence expectations.

Lagrangian spectra match inertial range scalings in turbulence.

Abstract

Small-scale fluctuating magnetic fields of order $n$G are observed in supernova shocks and galaxy clusters, where its amplification is likely caused by the Biermann battery mechanism. However, these fields cannot be amplified further without the turbulent dynamo, which generates magnetic energy through the stretch-twist-fold (STF) mechanism. Thus, we present here novel three-dimensional magnetohydrodynamic (MHD) simulations of a laser-driven shock propagating into a stratified, multiphase medium, to investigate the post-shock turbulent magnetic field amplification via the turbulent dynamo. The configuration used here is currently being tested in the shock tunnel at the National Ignition Facility (NIF). In order to probe the statistical properties of the post-shock turbulent region, we use $384 \times 512 \times 384$ tracers to track its evolution through the Lagrangian framework, thus providing a high-fidelity analysis of the shocked medium. Our simulations indicate that the growth of the magnetic field, which accompanies the near-Saffman kinetic energy decay ($E_{\textrm{kin}} \propto t^{-1.15})$ without turbulence driving, exhibits slightly different characteristics as compared to periodic box simulations. Seemingly no distinct phases exist in its evolution, because the shock passage and time to observe the magnetic field amplification during the turbulence decay are very short ($\sim\!0.3$ of a turbulent turnover time). Yet, the growth rate is still consistent with those expected for compressive (curl-free) turbulence driving in subsonic, compressible turbulence. Phenomenological understanding of the dynamics of the magnetic and velocity fields are also elucidated via Lagrangian frequency spectra, which are consistent with the expected inertial range scalings in the Eulerian-Lagrangian bridge.