Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics

TL;DR

This study uses high-resolution simulations to analyze how ambipolar diffusion affects turbulence statistics in weakly ionized, magnetized molecular clouds, revealing significant spectral steepening and changes in density and magnetic energy distributions.

Contribution

It introduces detailed turbulence statistics for sub-Alfvenic, non-ideal MHD simulations with ambipolar diffusion, expanding understanding beyond ideal MHD assumptions.

Findings

Ambipolar diffusion steepens velocity and magnetic power spectra.

Density PDFs and magnetic energies vary with AD Reynolds number.

Neutral gas spectra resemble Burgers turbulence at low AD Reynolds number.

Abstract

Most numerical investigations on the role of magnetic fields in turbulent molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However, MCs are weakly ionized, so that the time scale required for the magnetic field to diffuse through the neutral component of the plasma by ambipolar diffusion (AD) can be comparable to the dynamical time scale. We have performed a series of 256^3 and 512^3 simulations on supersonic but sub-Alfvenic turbulent systems with AD using the Heavy-Ion Approximation developed in Li, McKee, & Klein (2006). Our calculations are based on the assumption that the number of ions is conserved, but we show that these results approximately apply to the case of time-dependent ionization in molecular clouds as well. Convergence studies allow us to determine the optimal value of the ionization mass fraction when using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent systems. We find that ambipolar diffusion steepens the velocity and magnetic power spectra compared to the ideal MHD case. Changes in the density PDF, total magnetic energy, and ionization fraction are determined as a function of the AD Reynolds number. The power spectra for the neutral gas properties of a strongly magnetized medium with a low AD Reynolds number are similar to those for a weakly magnetized medium; in particular, the power spectrum of the neutral velocity is close to that for Burgers turbulence.