scholarly journals Sub‐Alfvénic Nonideal MHD Turbulence Simulations with Ambipolar Diffusion. I. Turbulence Statistics

2008 ◽  
Vol 684 (1) ◽  
pp. 380-394 ◽  
Author(s):  
Pak Shing Li ◽  
Christopher F. McKee ◽  
Richard I. Klein ◽  
Robert T. Fisher
Keyword(s):  
Ambipolar Diffusion ◽  
Mhd Turbulence ◽  
Turbulence Statistics

scholarly journals Dust settling and rings in the outer regions of protoplanetary discs subject to ambipolar diffusion

2018 ◽  
Vol 617 ◽  
pp. A117 ◽  
Author(s):  
A. Riols ◽  
G. Lesur
Keyword(s):  
Large Scale ◽  
Diffusion Theory ◽  
Ambipolar Diffusion ◽  
Mhd Turbulence ◽  
Low Velocity ◽  
Zonal Flows ◽  
T Tauri ◽  
Ideal Mhd ◽  
Weakly Ionized ◽  
Protoplanetary Discs

Context. Magnetohydrodynamic (MHD) turbulence plays a crucial role in the dust dynamics of protoplanetary discs. It affects planet formation, vertical settling, and is one possible origin of the large scale axisymmetric structures, such as rings, recently imaged by ALMA and SPHERE. Among the variety of MHD processes in discs, the magnetorotational instability (MRI) has raised particular interest since it provides a source of turbulence and potentially organizes the flow into large scale structures. However, the weak ionization of discs prevents the MRI from being excited beyond 1 AU. Moreover, the low velocity dispersion observed in CO and strong sedimentation of millimetre dust measured in T-Tauri discs are in contradiction with predictions based on ideal MRI turbulence. Aims. In this paper, we study the effects of non-ideal MHD and magnetized winds on the dynamics and sedimentation of dust grains. We consider a weakly ionized plasma subject to ambipolar diffusion characterizing the disc outer regions (≫1 AU). Methods. To compute the dust and gas motions, we performed numerical MHD simulations in the stratified shearing box, using a modified version of the PLUTO code. We explored different grain sizes from micrometre to few centimetres and different disc vertical magnetizations with plasma beta ranging from 103 to 105. Results. Our simulations show that the mm-cm dust is contained vertically in a very thin layer, with typical heightscale ≲0.4 AU at R = 30 AU, compatible with recent ALMA observations. Horizontally, the grains are trapped within the pressure maxima (or zonal flows) induced by ambipolar diffusion, leading to the formation of dust rings. For micrometre grains and strong magnetization, we find that the dust layer has a size comparable to the disc heightscale H. In this regime, dust settling cannot be explained by a simple 1D diffusion theory but results from a large scale 2D circulation induced by both MHD winds and zonal flows. Conclusions. Our results suggest that non-ideal MHD effects and MHD winds associated with zonal flows play a major role in shaping the radial and vertical distribution of dust in protoplanetary discs. Leading to effective accretion efficiency α ≃ 10−3–10−1, non-ideal MHD models are also a promising avenue to reconcile the low turbulent activity measured in discs with their relatively high accretion rates.

scholarly journals Turbulent dissipation, CH+ abundance, H2 line luminosities, and polarization in the cold neutral medium

2020 ◽  
Vol 500 (3) ◽  
pp. 3290-3308
Author(s):  
Eric R Moseley ◽  
B T Draine ◽  
Kengo Tomida ◽  
James M Stone
Keyword(s):  
Magnetic Field ◽  
Equations Of State ◽  
Mean Field ◽  
Line Emission ◽  
Ambipolar Diffusion ◽  
Neutral Medium ◽  
Mhd Turbulence ◽  
Turbulent Dissipation ◽  
Neutral Drift ◽  
Field Strengths

ABSTRACT In the cold neutral medium, high out-of-equilibrium temperatures are created by intermittent dissipation processes, including shocks, viscous heating, and ambipolar diffusion. The high-temperature excursions are thought to explain the enhanced abundance of CH+ observed along diffuse molecular sightlines. Intermittent high temperatures should also have an impact on H2 line luminosities. We carry out simulations of magnetohydrodynamic (MHD) turbulence in molecular clouds including heating and cooling, and post-process them to study H2 line emission and hot-gas chemistry, particularly the formation of CH+. We explore multiple magnetic field strengths and equations of state. We use a new H2 cooling function for $n_{\text{H}}\le 10^5\, {\text{cm}}^{-3}$, $T\le 5000\, {\text{K}}$, and variable H2 fraction. We make two important simplifying assumptions: (i) the H2/H fraction is fixed everywhere and (ii) we exclude from our analysis regions where the ion–neutral drift velocity is calculated to be greater than 5 km s−1. Our models produce H2 emission lines in accord with many observations, although extra excitation mechanisms are required in some clouds. For realistic root-mean-square (rms) magnetic field strengths (≈10 μG) and velocity dispersions, we reproduce observed CH+ abundances. These findings contrast with those of Valdivia et al. (2017) Comparison of predicted dust polarization with observations by Planck suggests that the mean field is ≳5 µG, so that the turbulence is sub-Alfvénic. We recommend future work treating ions and neutrals as separate fluids to more accurately capture the effects of ambipolar diffusion on CH+ abundance.

scholarly journals Ambipolar Diffusion Effects on Weakly Ionized Turbulence Molecular Clouds

2010 ◽  
Vol 6 (S270) ◽  
pp. 421-424
Author(s):  
Pak Shing Li ◽  
Christopher F. McKee ◽  
Richard I. Klein
Keyword(s):  
Molecular Clouds ◽  
Heavy Ion ◽  
Ambipolar Diffusion ◽  
Mhd Turbulence ◽  
Neutral Gas ◽  
Diffusion Effects ◽  
Ideal Mhd ◽  
Weakly Ionized ◽  
Two Fluid ◽  
Gas Component

AbstractAmbipolar diffusion (AD) is a key process in molecular clouds (MCs). Non-ideal MHD turbulence simulations are technically very challenging because of the large Alfvén speed of ions in weakly ionized clouds. Using the Heavy-Ion Approximation method (Li, McKee & Klein 2006), we have carried out two-fluid simulations of AD in isothermal, turbulent boxes at a resolution of 5123, to investigate the effect of AD on the weakly ionized turbulence in MCs. Our simulation results show that the neutral gas component of the two-fluid system gradually transforms from an ideal MHD turbulence system to near a pure hydrodynamic turbulence system within the standard AD regime, in which the neutrals and ions are coupled over a flow time. The change of the turbulent state has a profound effect on the weakly ionized MCs.

Effects of Ambipolar Diffusion on Prominence Thread Models

1994 ◽  
Vol 144 ◽  
pp. 315-321 ◽  
Author(s):  
M. G. Rovira ◽  
J. M. Fontenla ◽  
J.-C. Vial ◽  
P. Gouttebroze
Keyword(s):  
Energy Balance ◽  
Transition Region ◽  
Ambipolar Diffusion ◽  
Line Profiles ◽  
Balance Equations ◽  
Model Calculations ◽  
Partial Frequency ◽  
Frequency Redistribution ◽  
Partial Frequency Redistribution ◽  
Theoretical Structure

AbstractWe have improved previous model calculations of the prominence-corona transition region including the effect of the ambipolar diffusion in the statistical equilibrium and energy balance equations. We show its influence on the different parameters that characterize the resulting prominence theoretical structure. We take into account the effect of the partial frequency redistribution (PRD) in the line profiles and total intensities calculations.

Inverse cascade in simulated MHD turbulence driven by small scale source of magnetic helicity

2016 ◽  
Vol 52 (1) ◽  
pp. 261-268
Author(s):  
R. Stepanov ◽  
◽  
V. Titov ◽  
◽  
Keyword(s):  
Magnetic Helicity ◽  
Small Scale ◽  
Mhd Turbulence ◽  
Inverse Cascade
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