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

2008 ◽  
Author(s):  
R Klein ◽  
P Li ◽  
C McKee ◽  
R Fisher
Keyword(s):  
Ambipolar Diffusion ◽  
Mhd Turbulence ◽  
Turbulence Statistics ◽  
Ideal Mhd

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 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.

scholarly journals DUST TRANSPORT IN MRI TURBULENT DISKS: IDEAL AND NON-IDEAL MHD WITH AMBIPOLAR DIFFUSION

2015 ◽  
Vol 801 (2) ◽  
pp. 81 ◽  
Author(s):  
Zhaohuan Zhu ◽  
James M. Stone ◽  
Xue-Ning Bai
Keyword(s):  
Ambipolar Diffusion ◽  
Dust Transport ◽  
Ideal Mhd

scholarly journals Ideal MHD turbulence: the inertial range spectrum with collisionless dissipation

2015 ◽  
Vol 3 ◽  
Author(s):  
Rudolf A. Treumann ◽  
Wolfgang Baumjohann ◽  
Yasuhito Narita
Keyword(s):  
Inertial Range ◽  
Mhd Turbulence ◽  
Ideal Mhd ◽  
Collisionless Dissipation

scholarly journals Do we need non-ideal MHD to model protostellar discs?

2020 ◽  
Author(s):  
James Wurster
Keyword(s):  
Hall Effect ◽  
Ambipolar Diffusion ◽  
The Other ◽  
Dominant Term ◽  
Ideal Mhd ◽  
Field Geometry ◽  
Formation And Evolution ◽  
Ideal Magnetohydrodynamics ◽  
Protostellar Discs ◽  
The Magnetic Field

Abstract We investigate and discuss protostellar discs in terms of where the various non-ideal magnetohydrodynamics (MHD) processes are important. We find that the traditional picture of a magnetised disc (where Ohmic resistivity is dominant near the mid-plane, surrounded by a region dominated by the Hall effect, with the remainder of the disc dominated by ambipolar diffusion) is a great oversimplification. In simple parameterised discs, we find that the Hall effect is typically the dominant term throughout the majority of the disc. More importantly, we find that in much of our parameterised discs, at least two non-ideal processes have coefficients within a factor of 10 of one another, indicating that both are important and that naming a dominant term underplays the importance of the other terms. Discs that were self-consistently formed in our previous studies are also dominated by the Hall effect, and the ratio of ambipolar diffusion and Hall coefficients is typically less than 10, suggesting that both terms are equally important and listing a dominant term is misleading. These conclusions become more robust once the magnetic field geometry is taken into account. In agreement with the literature we review, we conclude that non-ideal MHD processes are important for the formation and evolution of protostellar discs. Ignoring any of the non-ideal processes, especially ambipolar diffusion and the Hall effect, yields an incorrect description of disc evolution.

scholarly journals Origin of CH+ in diffuse molecular clouds

2017 ◽  
Vol 600 ◽  
pp. A114 ◽  
Author(s):  
Valeska Valdivia ◽  
Benjamin Godard ◽  
Patrick Hennebelle ◽  
Maryvonne Gerin ◽  
Pierre Lesaffre ◽  
...  
Keyword(s):  
Velocity Distribution ◽  
Chemical Evolution ◽  
Molecular Clouds ◽  
Ambipolar Diffusion ◽  
The Other ◽  
Small Scale ◽  
Neutral Drift ◽  
Physical Conditions ◽  
Mhd Simulations ◽  
Ideal Mhd

Context. Molecular clouds are known to be magnetised and to display a turbulent and complex structure where warm and cold phases are interwoven. The turbulent motions within molecular clouds transport molecules, and the presence of magnetic fields induces a relative velocity between neutrals and ions known as the ion-neutral drift (vd). These effects all together can influence the chemical evolution of the clouds. Aims. This paper assesses the roles of two physical phenomena which have previously been invoked to boost the production of CH+ under realistic physical conditions: the presence of warm H2 and the increased formation rate due to the ion-neutral drift. Methods. We performed ideal magnetohydrodynamical (MHD) simulations that include the heating and cooling of the multiphase interstellar medium (ISM), and where we treat dynamically the formation of the H2 molecule. In a post-processing step we compute the abundances of species at chemical equilibrium using a solver that we developed. The solver uses the physical conditions of the gas as input parameters, and can also prescribe the H2 fraction if needed. We validate our approach by showing that the H2 molecule generally has a much longer chemical evolution timescale compared to the other species. Results. We show that CH+ is efficiently formed at the edge of clumps, in regions where the H2 fraction is low (0.3−30%) but nevertheless higher than its equilibrium value, and where the gas temperature is high (≳ 300 K). We show that warm and out of equilibrium H2 increases the integrated column densities of CH+ by one order of magnitude up to values still ~ 3−10 times lower than those observed in the diffuse ISM. We balance the Lorentz force with the ion-neutral drag to estimate the ion-drift velocities from our ideal MHD simulations. We find that the ion-neutral drift velocity distribution peaks around ~ 0.04 km s-1, and that high drift velocities are too rare to have a significant statistical impact on the abundances of CH+. Compared to previous works, our multiphase simulations reduce the spread in vd, and our self-consistent treatment of the ionisation leads to much reduced vd. Nevertheless, our resolution study shows that this velocity distribution is not converged: the ion-neutral drift has a higher impact on CH+ at higher resolution. On the other hand, our ideal MHD simulations do not include ambipolar diffusion, which would yield lower drift velocities. Conclusions. Within these limitations, we conclude that warm H2 is a key ingredient in the efficient formation of CH+ and that the ambipolar diffusion has very little influence on the abundance of CH+, mainly due to the small drift velocities obtained. However, we point out that small-scale processes and other non-thermal processes not included in our MHD simulation may be of crucial importance, and higher resolution studies with better controlled dissipation processes are needed.

scholarly journals A Fast Explicit Scheme for Solving MHD Equations with Ambipolar Diffusion

2010 ◽  
Vol 6 (S270) ◽  
pp. 415-419
Author(s):  
Jongsoo Kim
Keyword(s):  
Small Time ◽  
Ambipolar Diffusion ◽  
Mhd Equations ◽  
Time Step ◽  
Good Test ◽  
Diffusion Term ◽  
Spatial Gradients ◽  
Ideal Mhd ◽  
Induction Equation ◽  
The Ideal

AbstractWe developed a fast numerical scheme for solving ambipolar diffusion MHD equations with the strong coupling approximation, which can be written as the ideal MHD equations with an additional ambipolar diffusion term in the induction equation. The mass, momentum, magnetic fluxes due to the ideal MHD equations can be easily calculated by any Godunov-type schemes. Additional magnetic fluxes due to the ambipolar diffusion term are added in the magnetic fluxes, because of two same spatial gradients operated on the advection fluxes and the ambipolar diffusion term. In this way, we easily kept divergence-free magnetic fields using the constraint transport scheme. In order to overcome a small time step imposed by ambipolar diffusion, we used the super time stepping method. The resultant scheme is fast and robust enough to do the long term evolution of star formation simulations. We also proposed that the decay of alfen by ambipolar diffusion be a good test problem for our codes.

Temperature and Entropy in Ideal Magnetohydrodynamic Turbulence

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
John V. Shebalin
Keyword(s):  
Dynamical System ◽  
Probability Density Function ◽  
Fourier Analysis ◽  
Numerical Experiments ◽  
Magnetic Energy ◽  
Fourier Mode ◽  
Mhd Turbulence ◽  
Magnetohydrodynamic Turbulence ◽  
Ideal Mhd ◽  
Ideal Magnetohydrodynamic

Fourier analysis of incompressible, homogeneous magnetohydrodynamic (MHD) turbulence produces a model dynamical system on which to perform numerical experiments. Statistical methods are used to understand the results of ideal (i.e., nondissipative) MHD turbulence simulations, with the goal of finding those aspects that survive the introduction of dissipation. This statistical mechanics is based on a Boltzmannlike probability density function containing three “inverse temperatures,” one associated with each of the three ideal invariants: energy, cross helicity, and magnetic helicity. However, these inverse temperatures are seen to be functions of a single parameter that may defined as the “temperature” in a statistical and thermodynamic sense: the average magnetic energy per Fourier mode. Here, we discuss temperature and entropy in ideal MHD turbulence and their use in understanding numerical experiments and physical observations.

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