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Results for “AMBIPOLAR DIFFUSION”

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

The formation of sharp structures by ambipolar diffusion

The effect of ambipolar diffusion is investigated using simple numerical models. Examples are shown where sharp structures develop around magnetic nulls. In contrast to the case of ordinary diffusion, the magnetic field topology is conserved by ambipolar diffusion. This is demonstrated in an example where differential rotation winds up an initially uniform magnetic field and brings oppositely oriented field lines close together. It is argued that ambipolar diffusion produces structures of scales small enough for reconnection to occur.

Brandenburg, Axel↗
DOE OSTI2014

Formation of magnetized prestellar cores with ambipolar diffusion and turbulence

We investigate the roles of magnetic fields and ambipolar diffusion during prestellar core formation in turbulent giant molecular clouds, using three-dimensional numerical simulations. Our simulations focus on the shocked layer produced by a converging large-scale flow and survey varying ionization and the angle between the upstream flow and magnetic field. We also include ideal magnetohydrodynamic (MHD) and hydrodynamic models. From our simulations, we identify hundreds of self-gravitating cores that form within 1 Myr, with masses M ∼ 0.04-2.5 M {sub ☉} and sizes L ∼ 0.015-0.07 pc, consistent with observations of the peak of the core mass function. Median values are M = 0.47 M {sub ☉} and L = 0.03 pc. Core masses and sizes do not depend on either the ionization or upstream magnetic field direction. In contrast, the mass-to-flux ratio does increase with lower ionization, from twice to four times the critical value. The higher mass-to-flux ratio for low ionization is the result of enhanced transient ambipolar diffusion when the shocked layer first forms. However, ambipolar diffusion is not necessary to form low-mass supercritical cores. For ideal MHD, we find similar masses to other cases. These masses are one to two orders of magnitude lower than the value M {sub mag,} {sub sph} = 0.007B {sup 3}/(G {sup 3/2}ρ{sup 2}) that defines a magnetically supercritical sphere under post-shock ambient conditions. This discrepancy is the result of anisotropic contraction along field lines, which is clearly evident in both ideal MHD and diffusive simulations. We interpret our numerical findings using a simple scaling argument that suggests that gravitationally critical core masses will depend on the sound speed and mean turbulent pressure in a cloud, regardless of magnetic effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

GMC Collisions as Triggers of Star Formation. IV. The Role of Ambipolar Diffusion

We investigate the role of ambipolar diffusion (AD) in collisions between magnetized giant molecular clouds (GMCs), which may be an important mechanism for triggering star cluster formation. Three-dimensional simulations of GMC collisions are performed using a version of the Enzo magnetohydrodynamics code that has been extended to include AD. The resistivities are calculated using the 31-species chemical model of Wu et al. (2015). We find that in the weak-field, 10 μG case, AD has only a modest effect on the dynamical evolution during the collision. However, for the stronger-field, 30 μG case involving near-critical clouds, AD results in the formation of dense cores in regions where collapse is otherwise inhibited. The overall efficiency of formation of cores with n{sub H}⩾10{sup 6} cm{sup −3} in these simulations is increases from about 0.2% to 2% once AD is included, comparable to observed values in star-forming GMCs. The gas around these cores typically has relatively slow infall at speeds that are a modest fraction of the free-fall speed.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Ambipolar diffusion in the middle atmosphere

In the middle atmosphere above 60 km, the electron concentration increases with altitude, reaching values of 10 to the 10th per cu m in the daytime ionospheric E region near 100 km. The electrons are more mobile than the ions and diffuse more rapidly through the neutral atmosphere. The electron diffusion polarizes the medium, causing an electric field to develop that acts to retard the electron diffusion and enhance the conduction current of ions. A global zonally averaged numerical model of atmosheric electricity from the ground to 100 km is used to examine the effect of ambipolar diffusion and the earth's geomagnetic field on the currents and fields in the middle atmosphere. The results show that above about 65 km, ambipolar diffusion generates local electric fields and conduction currents that balance electron diffusion currents. The electric fields and conduction currents are a few orders of magnitude larger than the vertical fields and currents calculated from the downward mapping of the ionospheric potential without taking electron diffusion into account. Ambipolar diffusion does not alter the total current flowing in the global circuit. It is a local effect where enhanced conduction currens flow to balance the electron diffusion current.

Tzur, I.↗
NASA NTRS1973

Ambipolar diffusion in the F1-region of the ionosphere.

Ambipolar diffusion is discussed for the case of a weakly-ionized, multicomponent plasma. Shortcomings of some of the previous formulations of this problem are pointed out. In particular, it is shown that the polarization electrostatic field acts to couple the motion of the various ions with the result that the diffusion velocity of each ion depends on the density gradients of all the ions.

Schunk, R. W.↗
DOE OSTI2015

Global simulations of protoplanetary disks with OHMIC resistivity and ambipolar diffusion

Protoplanetary disks (PPDs) are believed to accrete onto their central T Tauri star because of magnetic stresses. Recently published shearing box simulations indicate that Ohmic resistivity, ambipolar diffusion (AD) and the Hall effect all play important roles in disk evolution. In the presence of a vertical magnetic field, the disk remains laminar between 1–5 AU, and a magnetocentrifugal disk wind forms that provides an important mechanism for removing angular momentum. Questions remain, however, about the establishment of a true physical wind solution in the shearing box simulations because of the symmetries inherent in the local approximation. We present global MHD simulations of PPDs that include Ohmic resistivity and AD, where the time-dependent gas-phase electron and ion fractions are computed under FUV and X-ray ionization with a simplified recombination chemistry. Our results show that the disk remains laminar, and that a physical wind solution arises naturally in global disk models. The wind is sufficiently efficient to explain the observed accretion rates. Furthermore, the ionization fraction at intermediate disk heights is large enough for magneto-rotational channel modes to grow and subsequently develop into belts of horizontal field. Depending on the ionization fraction, these can remain quasi-global, or break-up into discrete islands of coherent field polarity. The disk models we present here show a dramatic departure from our earlier models including Ohmic resistivity only. It will be important to examine how the Hall effect modifies the evolution, and to explore the influence this has on the observational appearance of such systems, and on planet formation and migration.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Dust transport in MRI turbulent disks: Ideal and non-ideal MHD with ambipolar diffusion

We study dust transport in turbulent protoplanetary disks using three-dimensional global unstratified MHD simulations including Lagrangian dust particles. The turbulence is driven by the magnetorotational instability (MRI) with either ideal or non-ideal MHD that includes ambipolar diffusion (AD). In ideal MHD simulations, the surface density evolution (except for dust that drifts fastest), turbulent diffusion, and vertical scale height of dust can all be reproduced by simple one-dimensoinal and/or analytical models. However, in AD dominated simulations which simulate protoplanetary disks beyond 10s of AU, the vertical scale height of dust is larger than previously predicted. To understand this anomaly in more detail, we carry out both unstratified and stratified local shearing box simulations with Lagrangian particles, and find that turbulence in AD dominated disks has very different properties (e.g., temporal autocorrelation functions and power spectra) than turbulence in ideal MHD disks, which leads to quite different particle diffusion efficiency. For example, MRI turbulence with AD has a longer correlation time for the vertical velocity, which causes significant vertical particle diffusion and large dust scale height. In ideal MHD the Schmidt numbers (Sc) for radial and vertical turbulent diffusion are Sc{sub r}∼1 and Sc{sub z}≳3, but in the AD dominated regime both Sc{sub r} and Sc{sub z} are ≲1. Particle concentration in pressure bumps induced by MRI turbulence has also been studied. Since non-ideal MHD effects dominate most regions in protoplanetary disks, our study suggests that modeling dust transport in turbulence driven by MRI with non-ideal MHD effects is important for understanding dust transport in realistic protoplanetary disks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Alfvénic turbulence beyond the ambipolar diffusion scale

We investigate the nature of the Alfvénic turbulence cascade in two-fluid magnetohydrodynamic (MHD) simulations in order to determine if turbulence is damped once the ion and neutral species become decoupled at a critical scale called the ambipolar diffusion scale (L{sub AD}). Using mode decomposition to separate the three classical MHD modes, we study the second-order structure functions of the Alfvén mode velocity field of both neutrals and ions in the reference frame of the local magnetic field. On scales greater than L{sub AD} we confirm that two-fluid turbulence strongly resembles single-fluid MHD turbulence. Our simulations show that the behavior of two-fluid turbulence becomes more complex on scales less than L{sub AD}. We find that Alfvénic turbulence can exist past L{sub AD} when the turbulence is globally super-Alfvénic, with the ions and neutrals forming separate cascades once decoupling has taken place. When turbulence is globally sub-Alfvénic and hence strongly anisotropic, with a large separation between the parallel and perpendicular decoupling scales, turbulence is damped at L{sub AD}. We also find that the power spectrum of the kinetic energy in the damped regime is consistent with a k{sup −4} scaling (in agreement with the predictions of Lazarian et al.).

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

A numerical study of the effects of ambipolar diffusion on the collapse of magnetic gas clouds

The gravitational collapse of isothermal, nonrotating magnetic gas clouds have been calculated numerically, including the effects of ambipolar diffusion. The fractional ionization in the clouds is approximated by a power-law function of the gas density, f = K/n to the q-power, where K and q are adjustable parameters. Eleven numerical experiments were run, and the results indicate that the asymptotic character of collapse is determined mainly by the value of q and is largely independent of the other parameters characterizing a cloud (e.g., K, cloud mass). In particular, there is nearly a one-to-one correspondence between q and the slope, x, of the central magnetic field strength-gas density relationship. If q is no more than 0.8, a cloud collapses asymptotically, as though the magnetic field were 'frozen' to the neutral matter. The magnetic field strength at the center of a collapsing cloud is strongly amplified during collapse even for values of q of about 1, despite extremely low values of fractional ionization. A discussion of the theoretical basis for this unexpected behavior is given. Possible implications of our results for the problems of magnetic braking of rotating protostars and star formation in general are also presented.

Black, D. C.↗
NASA NTRS1990

Energy balance in the solar transition region. I - Hydrostatic thermal models with ambipolar diffusion

The energy balance in the lower transition region is analyzed by constructing theoretical models which satisfy the energy balance constraint. The energy balance is achieved by balancing the radiative losses and the energy flowing downward from the corona. This energy flow is mainly in two forms: conductive heat flow and hydrogen ionization energy flow due to ambipolar diffusion. Hydrostatic equilibrium is assumed, and, in a first calculation, local mechanical heating and Joule heating are ignored. In a second model, some mechanical heating compatible with chromospheric energy-balance calculations is introduced. The models are computed for a partial non-LTE approach in which radiation departs strongly from LTE but particles depart from Maxwellian distributions only to first order. The results, which apply to cases where the magnetic field is either absent, or uniform and vertical, are compared with the observed Lyman lines and continuum from the average quiet sun. The approximate agreement suggests that this type of model can roughly explain the observed intensities in a physically meaningful way, assuming only a few free parameters specified as chromospheric boundary conditions.

Fontenla, J. M.↗
NASA NTRS1988

Ambipolar diffusion drifts and dynamos in turbulent gases

Ambipolar drift in turbulent fluids are considered. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, it is shown that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvenic (helical) turbulence. A simple dynamo theory is made and is compared with standard theories in which dissipation is caused by turbulent diffusion. The redistribution of magnetic flux in interstellar clouds is also discussed.

Zweibel, Ellen G.↗
DOE OSTI2014

Ambipolar and non-ambipolar diffusion in an rf plasma source containing a magnetic filter

By placing a magnetic filter across a rectangular plasma source (closed at one end with a ceramic plate and an rf antenna, and terminated at the opposite end by a grounded grid), we experimentally investigate the effect of conducting and insulating source walls on the nature of the plasma diffusion phenomena. The use of a magnetic filter creates a unique plasma, characterized by a high upstream electron temperature (T{sub e{sub u}}∼5 eV) near the rf antenna and a low downstream electron temperature (T{sub e{sub d}}∼1 eV) near the grid, which more clearly demonstrates the role of the source wall materials. For conducting walls a net ion current to ground is measured on the grid, and the plasma potential is determined by a mean electron temperature within the source. For insulating walls the plasma potential is determined by the downstream electron temperature (i.e., V{sub p}∼5.2T{sub e{sub d}} in argon), and the net current to the grid is exactly zero. Furthermore, by inserting a small additional upstream conductor (that can be made floating or grounded through an external circuit switch), we demonstrate that the plasma potential can be controlled and set to a low (V{sub p}∼5.2T{sub e{sub d}}), or high (V{sub p}∼5.2T{sub e{sub u}}) value.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗