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J Martinezsykora - One of the best experts on this subject based on the ideXlab platform.
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nonequilibrium ionization and Ambipolar Diffusion in solar magnetic flux emergence processes
Astronomy and Astrophysics, 2020Co-Authors: Daniel Nobregasiverio, Mats Carlsson, J Martinezsykora, F Morenoinsertis, Mikolaj SzydlarskiAbstract:Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as Ambipolar Diffusion, on the dynamics and thermodynamics of the flux emergence process.Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the Ambipolar Diffusion term of the generalized Ohm’s law.Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the Ambipolar Diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the Ambipolar Diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of Ambipolar Diffusion in the atmosphere.
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ion neutral interactions and non equilibrium ionization in the solar chromosphere
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: J Martinezsykora, Daniel Nobregasiverio, V H Hansteen, Bart De Pontieu, Jorrit Leenaarts, M Carlsson, Mikolaj SzydlarskiAbstract:The thermal structure of the chromosphere is regulated through a complex interaction of various heating processes, radiative cooling, and the ionization degree of the plasma. Here we study the impact on the thermal properties of the chromosphere when including the combined action of non-equilibrium ionization (NEI) of hydrogen and helium and ion-neutral interaction effects. We have performed a 2.5D radiative magnetohydrodynamic simulation including ion-neutral interaction effects by solving the generalized Ohm's law (GOL) as well as NEI for hydrogen and helium using the Bifrost code. The GOL equation includes Ambipolar Diffusion and the Hall term. We compare this simulation with another simulation that computes the ionization in local thermodynamic equilibrium (LTE) including ion-neutral interaction effects. Our numerical models reveal substantial thermal differences in magneto-acoustic shocks, the wake behind the shocks, spicules, low-lying magnetic loops, and the transition region. In particular, we find that heating through Ambipolar Diffusion in shock wakes is substantially less efficient, while in the shock fronts themselves it is more efficient, under NEI conditions than when assuming LTE.
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nonequilibrium ionization and Ambipolar Diffusion in solar magnetic flux emergence processes
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Daniel Nobregasiverio, Mats Carlsson, J Martinezsykora, F Morenoinsertis, Mikolaj SzydlarskiAbstract:Magnetic flux emergence has been shown to be a key mechanism for unleashing a wide variety of solar phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as Ambipolar Diffusion, on the dynamics and thermodynamics of the flux emergence process. Using the Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the Ambipolar Diffusion term of the Generalized Ohm's Law. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the Ambipolar Diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the Ambipolar Diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of Ambipolar Diffusion in the atmosphere.
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two dimensional radiative magnetohydrodynamic simulations of partial ionization in the chromosphere ii dynamics and energetics of the low solar atmosphere
The Astrophysical Journal, 2017Co-Authors: J Martinezsykora, Daniel Nobregasiverio, V H Hansteen, Mats Carlsson, Bart De Pontieu, Boris Vilhelm GudiksenAbstract:We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion-neutral interaction effects using the Generalized Ohm's Law, i.e., we include the Hall term and the Ambipolar Diffusion (Pedersen dissipation) in the induction equation. Our models span from the upper convection zone to the corona, with the photosphere, chromosphere and transition region partially ionized. Our simulations reveal that the interactions between ionized particles and neutral particles have important consequences for the magneto-thermodynamics of these modeled layers: 1) Ambipolar Diffusion increases the temperature in the chromosphere; 2) sporadically the horizontal magnetic field in the photosphere is diffused into the chromosphere due to the large Ambipolar Diffusion; 3) Ambipolar Diffusion concentrates electrical currents leading to more violent jets and reconnection processes, resulting in 3a) the formation of longer and faster spicules, 3b) heating of plasma during the spicule evolution, and 3c) decoupling of the plasma and magnetic field in spicules. Our results indicate that Ambipolar Diffusion is a critical ingredient for understanding the magneto-thermo-dynamic properties in the chromosphere and transition region. The numerical simulations have been made publicly available, similar to previous Bifrost simulations. This will allow the community to study realistic numerical simulations with a wider range of magnetic field configurations and physics modules than previously possible.
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two dimensional radiative magnetohydrodynamic simulations of the importance of partial ionization in the chromosphere
The Astrophysical Journal, 2012Co-Authors: J Martinezsykora, Bart De Pontieu, V H HansteenAbstract:The bulk of the solar chromosphere is weakly ionized and interactions between ionized particles and neutral particles likely have significant consequences for the thermodynamics of the chromospheric plasma. We investigate the importance of introducing neutral particles into the MHD equations using numerical 2.5D radiative MHD simulations obtained with the Bifrost code. The models span the solar atmosphere from the upper layers of the convection zone to the low corona, and solve the full MHD equations with non-gray and non-LTE radiative transfer, and thermal conduction along the magnetic field. The effects of partial ionization are implemented using the generalized Ohm's law, i.e., we consider the effects of the Hall term and Ambipolar Diffusion in the induction equation. The approximations required in going from three fluids to the generalized Ohm's law are tested in our simulations. The Ohmic Diffusion, Hall term, and Ambipolar Diffusion show strong variations in the chromosphere. These strong variations of the various magnetic diffusivities are absent or significantly underestimated when, as has been common for these types of studies, using the semi-empirical VAL-C model as a basis for estimates. In addition, we find that differences in estimating the magnitude of Ambipolar Diffusion arise depending on which method is used to calculate the ion-neutral collision frequency. These differences cause uncertainties in the different magnetic diffusivity terms. In the chromosphere, we find that the Ambipolar Diffusion is of the same order of magnitude or even larger than the numerical Diffusion used to stabilize our code. As a consequence, Ambipolar Diffusion produces a strong impact on the modeled atmosphere. Perhaps more importantly, it suggests that at least in the chromospheric domain, self-consistent simulations of the solar atmosphere driven by magnetoconvection can accurately describe the impact of the dominant form of resistivity, i.e., Ambipolar Diffusion. This suggests that such simulations may be more realistic in their approach to the lower solar atmosphere (which directly drives the coronal volume) than previously assumed.
Daniel Nobregasiverio - One of the best experts on this subject based on the ideXlab platform.
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nonequilibrium ionization and Ambipolar Diffusion in solar magnetic flux emergence processes
Astronomy and Astrophysics, 2020Co-Authors: Daniel Nobregasiverio, Mats Carlsson, J Martinezsykora, F Morenoinsertis, Mikolaj SzydlarskiAbstract:Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as Ambipolar Diffusion, on the dynamics and thermodynamics of the flux emergence process.Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the Ambipolar Diffusion term of the generalized Ohm’s law.Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the Ambipolar Diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the Ambipolar Diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of Ambipolar Diffusion in the atmosphere.
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ion neutral interactions and non equilibrium ionization in the solar chromosphere
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: J Martinezsykora, Daniel Nobregasiverio, V H Hansteen, Bart De Pontieu, Jorrit Leenaarts, M Carlsson, Mikolaj SzydlarskiAbstract:The thermal structure of the chromosphere is regulated through a complex interaction of various heating processes, radiative cooling, and the ionization degree of the plasma. Here we study the impact on the thermal properties of the chromosphere when including the combined action of non-equilibrium ionization (NEI) of hydrogen and helium and ion-neutral interaction effects. We have performed a 2.5D radiative magnetohydrodynamic simulation including ion-neutral interaction effects by solving the generalized Ohm's law (GOL) as well as NEI for hydrogen and helium using the Bifrost code. The GOL equation includes Ambipolar Diffusion and the Hall term. We compare this simulation with another simulation that computes the ionization in local thermodynamic equilibrium (LTE) including ion-neutral interaction effects. Our numerical models reveal substantial thermal differences in magneto-acoustic shocks, the wake behind the shocks, spicules, low-lying magnetic loops, and the transition region. In particular, we find that heating through Ambipolar Diffusion in shock wakes is substantially less efficient, while in the shock fronts themselves it is more efficient, under NEI conditions than when assuming LTE.
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nonequilibrium ionization and Ambipolar Diffusion in solar magnetic flux emergence processes
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Daniel Nobregasiverio, Mats Carlsson, J Martinezsykora, F Morenoinsertis, Mikolaj SzydlarskiAbstract:Magnetic flux emergence has been shown to be a key mechanism for unleashing a wide variety of solar phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as Ambipolar Diffusion, on the dynamics and thermodynamics of the flux emergence process. Using the Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H2 molecules, and the Ambipolar Diffusion term of the Generalized Ohm's Law. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the Ambipolar Diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H2 molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H2 molecule formation to the thermal energy during the flux emergence process. We find that the Ambipolar Diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of Ambipolar Diffusion in the atmosphere.
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two dimensional radiative magnetohydrodynamic simulations of partial ionization in the chromosphere ii dynamics and energetics of the low solar atmosphere
The Astrophysical Journal, 2017Co-Authors: J Martinezsykora, Daniel Nobregasiverio, V H Hansteen, Mats Carlsson, Bart De Pontieu, Boris Vilhelm GudiksenAbstract:We investigate the effects of interactions between ions and neutrals on the chromosphere and overlying corona using 2.5D radiative MHD simulations with the Bifrost code. We have extended the code capabilities implementing ion-neutral interaction effects using the Generalized Ohm's Law, i.e., we include the Hall term and the Ambipolar Diffusion (Pedersen dissipation) in the induction equation. Our models span from the upper convection zone to the corona, with the photosphere, chromosphere and transition region partially ionized. Our simulations reveal that the interactions between ionized particles and neutral particles have important consequences for the magneto-thermodynamics of these modeled layers: 1) Ambipolar Diffusion increases the temperature in the chromosphere; 2) sporadically the horizontal magnetic field in the photosphere is diffused into the chromosphere due to the large Ambipolar Diffusion; 3) Ambipolar Diffusion concentrates electrical currents leading to more violent jets and reconnection processes, resulting in 3a) the formation of longer and faster spicules, 3b) heating of plasma during the spicule evolution, and 3c) decoupling of the plasma and magnetic field in spicules. Our results indicate that Ambipolar Diffusion is a critical ingredient for understanding the magneto-thermo-dynamic properties in the chromosphere and transition region. The numerical simulations have been made publicly available, similar to previous Bifrost simulations. This will allow the community to study realistic numerical simulations with a wider range of magnetic field configurations and physics modules than previously possible.
Telemachos Ch Mouschovias - One of the best experts on this subject based on the ideXlab platform.
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protostar formation in magnetic molecular clouds beyond ion detachment ii typical axisymmetric solution
The Astrophysical Journal, 2007Co-Authors: Konstantinos Tassis, Telemachos Ch MouschoviasAbstract:We follow the Ambipolar-Diffusion-driven formation and evolution of a fragment in a magnetically supported molecular cloud, until a hydrostatic protostellar core forms at its center. This problem was formulated in Paper I. We determine the density, velocity, and magnetic field as functions of space and time, and the contribution of Ambipolar Diffusion and Ohmic dissipation to the resolution of the magnetic flux problem of star formation. The issue of whether the magnetic field ever decouples from the (neutral) matter is also addressed. We also find that the electrons do not decouple from the field lines before thermal ionization becomes important and recouples the magnetic field to the neutral matter. Ohmic dissipation becomes more effective than Ambipolar Diffusion as a flux reduction mechanism only at the highest densities (a few × 1012 cm-3). In the high-density central parts of the core, the magnetic field acquires an almost spatially uniform structure, with a value that, at the end of the calculation (nn ≈ 5 × 1014 cm-3), is found to be in excellent agreement with meteoritic measurements of magnetic fields in the protosolar nebula. Outside the hydrostatic protostellar core, a concentration of magnetic flux (a "magnetic wall") forms, which gives rise to a magnetic shock. This magnetic shock is the precursor of the repeated shocks previously found by Tassis & Mouschovias, which cause spasmodic accretion onto the hydrostatic core at later times.
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protostar formation in magnetic molecular clouds beyond ion detachment ii typical axisymmetric solution
arXiv: Astrophysics, 2007Co-Authors: Konstantinos Tassis, Telemachos Ch MouschoviasAbstract:We follow the Ambipolar-Diffusion--driven formation and evolution of a fragment in a magnetically supported molecular cloud, until a hydrostatic protostellar core forms at its center. This problem was formulated in Paper I. We determine the density, velocity and magnetic field as functions of space and time, and the contribution of Ambipolar Diffusion and Ohmic dissipation to the resolution of the magnetic flux problem of star formation. The issue of whether the magnetic field ever decouples from the (neutral) matter is also addressed. We also find that the electrons do not decouple from the field lines before thermal ionization becomes important and recouples the magnetic field to the neutral matter. Ohmic dissipation becomes more effective than Ambipolar Diffusion as a flux reduction mechanism only at the highest densities (a few times 10^12 particles per cubic cm). In the high-density central parts of the core, the magnetic field acquires an almost spatially uniform structure, with a value that, at the end of the calculation (number density ~ 5 times 10^14 particles per cubic cm), is found to be in excellent agreement with meteoritic measurements of magnetic fields in the protosolar nebula. Outside the hydrostatic protostellar core, a concentration of magnetic flux (a "magnetic wall") forms, which gives rise to a magnetic shock. This magnetic shock is the precursor of the repeated shocks found by Tassis & Mouschovias (2005) which cause spasmodic accretion onto the hydrostatic core at later times.
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Ambipolar Diffusion and star formation formation and contraction of axisymmetric cloud cores i formulation of the problem and method of solution
The Astrophysical Journal, 1992Co-Authors: Robert Fiedler, Telemachos Ch MouschoviasAbstract:We formulate the problem of the formation and contraction of axially symmetric, isothermal, self-gravitating, molecular cloud fragments (or cores) due to Ambipolar Diffusion in magnetically supported parent clouds. The initial reference states are taken to be static and uniform with the magnetic field parallel to the axis of symmetry. The two-fluid MHD equations describing the evolution contain three dimensionless free parameters: α 0 , the ratio of magnetic and thermal pressures in the initial state, ν ff,0 , essentially the initial ratio of the free-fall and neutral-ion collision times, and the exponent k in the relation between the ion and neutral densities n i ∞n n k
James M Stone - One of the best experts on this subject based on the ideXlab platform.
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dust transport in mri turbulent disks ideal and non ideal mhd with Ambipolar Diffusion
The Astrophysical Journal, 2015Co-Authors: James M StoneAbstract: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 and , but in the AD dominated regime both Scr and Scz are . 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.
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turbulence in the outer regions of protoplanetary disks i weak accretion with no vertical magnetic flux
The Astrophysical Journal, 2013Co-Authors: Jacob B Simon, James M Stone, Philip J Armitage, Kris BeckwithAbstract:We use local numerical simulations to investigate the strength and nature of magnetohydrodynamic (MHD) turbulence in the outer regions of protoplanetary disks, where Ambipolar Diffusion is the dominant non-ideal MHD effect. The simulations include vertical stratification and assume zero net vertical magnetic flux. We employ a super time-stepping technique to ameliorate the Courant restriction on the diffusive time step. We find that in idealized stratified simulations, with a spatially constant Ambipolar Elsasser number Am, turbulence driven by the magnetorotational instability (MRI) behaves in a similar manner as in prior unstratified calculations. Turbulence dies away for Am ? 1, and becomes progressively more vigorous as Ambipolar Diffusion is decreased. Near-ideal MHD behavior is recovered for Am ? 103. In the intermediate regime (10 ? Am ? 103) Ambipolar Diffusion leads to substantial increases in both the period of the MRI dynamo cycle and the characteristic scales of magnetic field structures. To quantify the impact of Ambipolar physics on disk accretion, we run simulations at 30 AU and 100 AU that include a vertical Am profile based upon far-ultraviolet (FUV) ionized disk models. These models develop a vertically layered structure analogous to the Ohmic dead zone that is present at smaller radii. We find that, although the levels of surface turbulence can be strong (and consistent with constraints on turbulent line widths at these radii), the inferred accretion rates are at least an order of magnitude smaller than those observed in T Tauri stars. This discrepancy is very likely due to the assumption of zero vertical magnetic field in our simulations and suggests that vertical magnetic fields are essential for MRI-driven accretion in the outer regions of protoplanetary disks.
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effect of Ambipolar Diffusion on the nonlinear evolution of magnetorotational instability in weakly ionized disks
The Astrophysical Journal, 2011Co-Authors: Xuening Bai, James M StoneAbstract:We study the role of Ambipolar Diffusion (AD) on the nonlinear evolution of the magnetorotational instability (MRI) in protoplanetary disks using the strong coupling limit, which applies in very weakly ionized gas with electron recombination time much shorter than the orbital time so that a single-fluid treatment is sufficient. The effect of AD in this limit is characterized by the dimensionless number Am, the frequency at which neutral particles collides with ions normalized to the orbital frequency. We perform three-dimensional unstratified shearing-box simulations of the MRI over a wide range of Am as well as different magnetic field strengths and geometries. The saturation level of the MRI turbulence depends on the magnetic geometry and increases with the net magnetic flux. There is an upper limit to the net flux for sustained turbulence, corresponding to the requirement that the most unstable vertical wavelength be less than the disk scale height. Correspondingly, at a given Am, there exists a maximum value of the turbulent stress αmax. For Am 1, the largest stress is associated with a field geometry that has both net vertical and toroidal flux. In this case, we confirm the results of linear analyses that show the fastest growing mode has a non-zero radial wavenumber with a growth rate exceeding that of the pure vertical field case. We find there is a very tight correlation between the turbulent stress α and the plasma β ≡ P gas/P mag ≈ 1/2α at the saturated state of the MRI turbulence regardless of field geometry, and αmax rapidly decreases with decreasing Am. In particular, we find αmax ≈ 7 × 10–3 for Am = 1 and αmax ≈ 6 × 10–4 for Am = 0.1.
Christopher F Mckee - One of the best experts on this subject based on the ideXlab platform.
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Ambipolar Diffusion heating in turbulent systems
The Astrophysical Journal, 2012Co-Authors: Pak Shing Li, Andrew T Myers, Christopher F MckeeAbstract:The temperature of the gas in molecular clouds is a key determinant of the characteristic mass of star formation. Ambipolar Diffusion (AD) is considered one of the most important heating mechanisms in weakly ionized molecular clouds. In this work, we study the AD heating rate using two-fluid turbulence simulations and compare it with the overall heating rate due to turbulent dissipation. We find that for observed molecular clouds, which typically have Alfven Mach numbers of ~1 and AD Reynolds numbers of ~20, about 70% of the total turbulent dissipation is in the form of AD heating. AD has an important effect on the length scale where energy is dissipated: when AD heating is strong, most of the energy in the cascade is removed by ion-neutral drift, with a comparatively small amount of energy making it down to small scales. We derive a relation for the AD heating rate that describes the results of our simulations to within a factor of two. Turbulent dissipation, including AD heating, is generally less important than cosmic-ray heating in molecular clouds, although there is substantial scatter in both.
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sub alfvenic non ideal magnetohydrodynamic turbulence simulations with Ambipolar Diffusion iii implications for observations and turbulent enhancement
The Astrophysical Journal, 2012Co-Authors: Pak Shing Li, Christopher F Mckee, Richard I KleinAbstract:Ambipolar Diffusion (AD) is believed to be a crucial process for redistributing magnetic flux in the dense molecular gas that occurs in regions of star formation. We carry out numerical simulations of this process in regions of low ionization using the heavy-ion approximation. The simulations are for regions of strong field (plasma β = 0.1) and mildly supersonic turbulence (, corresponding to an Alfven Mach number of 0.67). The velocity power spectrum of the neutral gas changes from an Iroshnikov-Kraichnan spectrum in the case of ideal MHD to a Burgers spectrum in the case of a shock-dominated hydrodynamic system. The magnetic power spectrum shows a similar behavior. We use a one-dimensional radiative transfer code to post-process our simulation results; the simulated emission from the CS J = 2-1 and H13CO+ J = 1-0 lines shows that the effects of AD are observable in principle. Linewidths of ions are observed to be less than those of neutrals, and we confirm previous suggestions that this is due to AD. We show that AD is unlikely to affect the Chandrasekhar-Fermi method for inferring field strengths unless the AD is stronger than generally observed. Finally, we present a study of the enhancement of AD by turbulence, finding that AD is accelerated by factor 2-4.5 for non-self-gravitating systems with the level of turbulence we consider.