Ambipolar diffusion in large Prandtl number turbulence

Brandenburg, Axel

doi:10.1093/mnras/stz1509

Cited by 11 publications

(13 citation statements)

References 32 publications

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“…This is different from a full two-fluid model, where there are separate equations which describe the time evolution of neutrals and charges, so that there is no assumption about their collisional timescale. It has been shown that in many situations, the single fluid MHD (extended with ambipolar terms) and the full two-fluid treatment give very similar results (Brandenburg 2019;Popescu Braileanu 2020).…”

Section: Introductionmentioning

confidence: 94%

“…It is the most important process of magnetic field decay in neutron stars (Jones 1987;Hoyos et al 2010;Sinha 2020). Ambipolar diffusion has significant effects on turbulence present in the interstellar medium (Brandenburg 2019) or as produced by the magnetorotational instability in weakly ionized disks (Bai & Stone 2011). In solar context, ambipolar diffusion has an important contribution in the chromosphere.…”

Section: Introductionmentioning

confidence: 99%

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Effects of ambipolar diffusion on waves in the solar chromosphere

Braileanu

Keppens

2021

A&A

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Context. The chromosphere is a partially ionized layer of the solar atmosphere that mediates the transition between the photosphere where the gas motion is determined by the gas pressure and the corona dominated by the magnetic field. Aims. We study the effect of partial ionization for 2D wave propagation in a gravitationally stratified, magnetized atmosphere characterized by properties that are similar to those of the solar chromosphere. Methods. We adopted an oblique uniform magnetic field in the plane of propagation with a strength that is suitable for a quiet sun region. The theoretical model we used is a single fluid magnetohydrodynamic approximation, where ion-neutral interaction is modeled by the ambipolar diffusion term. Magnetic energy can be converted into internal energy through the dissipation of the electric current produced by the drift between ions and neutrals. We used numerical simulations in which we continuously drove fast waves at the bottom of the atmosphere. The collisional coupling between ions and neutrals decreases with the decrease in the density and the ambipolar effect thus becomes important. Results. Fast waves excited at the base of the atmosphere reach the equipartition layer and are reflected or transmitted as slow waves. While the waves propagate through the atmosphere and the density drops, the waves steepen into shocks. Conclusions. The main effect of ambipolar diffusion is damping of the waves. We find that for the parameters chosen in this work, the ambipolar diffusion affects the fast wave before it is reflected, with damping being more pronounced for waves which are launched in a direction perpendicular to the magnetic field. Slow waves are less affected by ambipolar effects. The damping increases for shorter periods and greater magnetic field strengths. Small scales produced by the nonlinear effects and the superposition of different types of waves created at the equipartition height are efficiently damped by ambipolar diffusion.

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Section: Introductionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Effects of ambipolar diffusion on waves in the solar chromosphere

Braileanu

Keppens

2021

A&A

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“…Hence, the diffuse warm gas can be treated as a single entity in dynamo models, and we do not consider the effects of ambipolar diffusion on the magnetic field [e.g. 98].…”

Section: Boundary Conditions In Mean-field Modelsmentioning

confidence: 99%

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

et al. 2019

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Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Further, we measure the velocity dispersion of H I gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges of dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory.

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“…Besides the nonlinear regime, XL16 also studied the kinematic dynamo at a large and small magnetic Prandtl number, and at different ionization fractions. They found new regimes of kinematic dynamo, including the transitional stage in a high-Prandtl number medium with the shift of the correlation length of magnetic fields from the resistive scale to the viscous scale, and the damping stage of dynamo in a weakly ionized medium (see also [22]). XL16 predictions on the dynamo behavior resulting from the weak coupling between neutrals and ions at a low ionization fraction have been numerical confirmed by the two-fluid dynamo simulation [32].…”

Section: Introductionmentioning

confidence: 97%

“…Most earlier theoretical studies on turbulent dynamo focused on its kinematic regime with negligible magnetic back reaction [15,16,17,18,19]. For the kinematic regime of dynamo, the dynamo growth of magnetic fields and the magnetic field structure are sensitive to the plasma parameters, including both the ionization fraction [17,20] and the magnetic Prandtl number [21,22]. For astrophysical settings involving large length scales and long timescales, the kinematic dynamo alone is insufficient to explain the dynamo evolution of magnetic fields and their dynamical roles in the multi-scale astrophysical processes.…”

Section: Introductionmentioning

confidence: 99%

Small-scale turbulent dynamo in astrophysical environments: nonlinear dynamo and dynamo in a partially ionized plasma

Lazarían

2021

Preprint

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Small-scale turbulent dynamo is responsible for the amplification of magnetic fields on scales smaller than the driving scale of turbulence in diverse astrophysical media. Most earlier dynamo theories concern the kinematic regime and smallscale magnetic field amplification. Here we review our recent progress in developing the theories for the nonlinear dynamo and the dynamo regime in a partially ionized plasma. The importance of reconnection diffusion of magnetic fields is identified for both the nonlinear dynamo and magnetic field amplification during gravitational contraction. For the dynamo in a partially ionized plasma, the coupling state between neutrals and ions and the ion-neutral collisional damping can significantly affect the dynamo behavior and the resulting magnetic field structure. We present both our analytical predictions and numerical tests with a two-fluid dynamo simulation on the dynamo features in this regime. In addition, to illustrate the astrophysical implications, we discuss several examples for the applications of the dynamo theory to studying magnetic field evolution in both preshock and postshock regions of supernova remnants, in weakly magnetized molecular clouds, during the (primordial) star formation, and during the first galaxy formation.

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Ambipolar diffusion in large Prandtl number turbulence

Cited by 11 publications

References 32 publications

Effects of ambipolar diffusion on waves in the solar chromosphere

Effects of ambipolar diffusion on waves in the solar chromosphere

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

Small-scale turbulent dynamo in astrophysical environments: nonlinear dynamo and dynamo in a partially ionized plasma

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