Studying Interstellar Turbulence Driving Scales using the Bispectrum

TL;DR

This paper explores the use of the bispectrum and bicoherence to identify turbulence driving scales and phase coupling in interstellar MHD turbulence, highlighting the influence of magnetic fields.

Contribution

It introduces a new application of the bispectrum and bicoherence for studying turbulence driving scales and phase interactions in the interstellar medium.

Findings

Bicoherence identifies turbulence driving scales.

Ordered magnetic fields enhance phase coupling.

Bispectrum reveals non-linear scale interactions.

Abstract

We demonstrate the utility of the bispectrum, the Fourier three-point correlation function, for studying driving scales of magnetohydrodynamic (MHD) turbulence in the interstellar medium. We calculate the bispectrum by implementing a parallelized Monte Carlo direct measurement method, which we have made publicly available. In previous works, the bispectrum has been used to identify non-linear scaling correlations and break degeneracies in lower-order statistics like the power spectrum. We find that the bicoherence, a related statistic which measures phase coupling of Fourier modes, identifies turbulence driving scales using density and column density fields. In particular, it shows that the driving scale is phase-coupled to scales present in the turbulent cascade. We also find that the presence of an ordered magnetic field at large-scales enhances phase coupling as compared to a pure hydrodynamic case. We therefore suggest the bispectrum and bicoherence as tools for searching for non-locality for wave interactions in MHD turbulence.