Protostellar birth with ambipolar and ohmic diffusion

Context. The transport of angular momentum is fundamental during the formation of low-mass stars; too little removal and rotation ensures stellar densities are never reached, too much and the absence of rotation means no protoplanetary disks can form. Magnetic diffusion is seen as a pathway to resolving this long-standing problem. Aims. We aim to investigate the impact of including resistive magnetohydrodynamics (MHD) in simulations of the gravitational collapse of a 1 M⊙ gas sphere, from molecular cloud densities to the formation of the protostellar seed; the second Larson core. Methods. We used the adaptive mesh refinement code RAMSES to perform two 3D simulations of collapsing magnetised gas spheres, including self-gravity, radiative transfer in the form of flux-limited diffusion, and a non-ideal gas equation of state to describe H2 dissociation which leads to the second collapse. The first run was carried out under the ideal MHD approximation, while ambipolar and ohmic diffusion was incorporated in the second calculation using resistivities computed from an equilibrium chemical network. Results. In the ideal MHD simulation, the magnetic field dominates the energy budget everywhere inside and around the first hydrostatic core, fueling interchange instabilities and driving a low-velocity outflow above and below the equatorial plane of the system. High magnetic braking removes essentially all angular momentum from the second core. On the other hand, ambipolar and ohmic diffusion create a barrier which prevents amplification of the magnetic field beyond 0.1 G in the first Larson core which is now fully thermally supported. A significant amount of rotation is preserved and a small Keplerian-like disk forms around the second core. The ambipolar and ohmic diffusions are effective at radii below 10 AU, indicating that a least ~1 AU is necessary to investigate the angular momentum transfer and the formation of rotationally supported disks. Finally, when studying the radiative efficiency of the first and second core accretion shocks, we found that it can vary by several orders of magnitude over the 3D surface of the cores. Conclusions. This proves that magnetic diffusion is a prerequisite to star formation. Not only does it enable the formation of protoplanetary disks in which planets will eventually form, it also plays a determinant role in the formation of the protostar itself.

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Zooming in on the Formation of Protoplanetary Disks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313008107 ◽

2013 ◽

Vol 8 (S299) ◽

pp. 131-135 ◽

Cited By ~ 3

Author(s):

Åke Nordlund ◽

Troels Haugbølle ◽

Michael Küffmeier ◽

Paolo Padoan ◽

Aris Vasileiades

Keyword(s):

Magnetic Field ◽

Energy Efficient ◽

Adaptive Mesh Refinement ◽

Accretion Rate ◽

Magnetic Energy ◽

Protoplanetary Disks ◽

Adaptive Mesh ◽

Gravitational Energy ◽

Solar Mass ◽

The Magnetic Field

AbstractWe use the adaptive mesh refinement code RAMSES to model the formation of protoplanetary disks in realistic star formation environments. The resolution scales over up to 29 powers of two (~ 9 orders of magnitude) covering a range from outer scales of 40 pc to inner scales of 0.015 AU. The accretion rate from a 1.5 solar mass envelope peaks near 10−4 M⊙ about 6 kyr after sink particle formation and then decays approximately exponentially, reaching 10−6 M⊙ in 100 kyr. The models suggest universal scalings of physical properties with radius during the main accretion phase, with kinetic and / or magnetic energy in approximate balance with gravitational energy. Efficient accretion is made possible by the braking action of the magnetic field, which nevertheless allows a near-Keplerian disk to grow to a 100 AU size. The magnetic field strength ranges from more than 10 G at 0.1 AU to less than 1 mG at 100 AU, and drives a time dependent bipolar outflow, with a collimated jet and a broader disk wind.

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Cosmic magnetic fields with masclet: an application to galaxy clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa877 ◽

2020 ◽

Vol 494 (2) ◽

pp. 2706-2717

Author(s):

Vicent Quilis ◽

José-María Martí ◽

Susana Planelles

Keyword(s):

Magnetic Field ◽

Galaxy Clusters ◽

Adaptive Mesh Refinement ◽

Adaptive Mesh ◽

Pure Compression ◽

Ideal Magnetohydrodynamics ◽

Relative Errors ◽

Cluster Values ◽

The Magnetic Field ◽

The Ideal

ABSTRACT We describe and test a new version of the adaptive mesh refinement cosmological code masclet. The new version of the code includes all the ingredients of its previous version plus a description of the evolution of the magnetic field under the approximation of the ideal magnetohydrodynamics (MHD). To preserve the divergence-free condition of MHD, the original divergence cleaning algorithm of Dedner et al. (2002) is implemented. We present a set of well-known 1D and 2D tests, such as several shock tube problems, the fast rotor, and the Orszag–Tang vortex. The performance of the code in all the tests is excellent with estimated median relative errors of ∇ · B in the 2D tests smaller than 5 × 10−5 for the fast rotor test, and 5 × 10−3 for the Orszag–Tang vortex. As an astrophysical application of the code, we present a simulation of a cosmological box of 40 comoving Mpc side length in which a primordial uniform comoving magnetic field of strength 0.1 nG is seeded. The simulation shows how the magnetic field is channelled along the filaments of gas and is concentrated and amplified within galaxy clusters. Comparison with the values expected from pure compression reveals an additional amplification of the magnetic field caused by turbulence in the central region of the cluster. Values of the order of ∼1µG are obtained in clusters at z ∼ 0 with median relative errors of ∇ · B below 0.4 per cent. The implications of a proper description of the dynamics of the magnetic field and their possible observational counterparts in future facilities are discussed.

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Magnetized interstellar molecular clouds – II. The large-scale structure and dynamics of filamentary molecular clouds

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz653 ◽

2019 ◽

Vol 485 (4) ◽

pp. 4509-4528 ◽

Cited By ~ 7

Author(s):

Pak Shing Li ◽

Richard I Klein

Keyword(s):

Magnetic Field ◽

Strong Magnetic Field ◽

Adaptive Mesh Refinement ◽

Molecular Clouds ◽

Large Scale ◽

Velocity Structure ◽

Strong Support ◽

Adaptive Mesh ◽

Dark Cloud ◽

The Magnetic Field

Abstract We perform ideal magnetohydrodynamics high-resolution adaptive mesh refinement simulations with driven turbulence and self-gravity and find that long filamentary molecular clouds are formed at the converging locations of large-scale turbulence flows and the filaments are bounded by gravity. The magnetic field helps shape and reinforce the long filamentary structures. The main filamentary cloud has a length of ∼4.4 pc. Instead of a monolithic cylindrical structure, the main cloud is shown to be a collection of fibre/web-like substructures similar to filamentary clouds such as L1495. Unless the line-of-sight is close to the mean field direction, the large-scale magnetic field and striations in the simulation are found roughly perpendicular to the long axis of the main cloud, similar to L1495. This provides strong support for a large-scale moderately strong magnetic field surrounding L1495. We find that the projection effect from observations can lead to incorrect interpretations of the true three-dimensional physical shape, size, and velocity structure of the clouds. Helical magnetic field structures found around filamentary clouds that are interpreted from Zeeman observations can be explained by a simple bending of the magnetic field that pierces through the cloud. We demonstrate that two dark clouds form a T-shaped configuration that is strikingly similar to the infrared dark cloud SDC13, leading to the interpretation that SDC13 results from a collision of two long filamentary clouds. We show that a moderately strong magnetic field (${{\cal M}_{\rm A}}\sim 1$) is crucial for maintaining a long and slender filamentary cloud for a long period of time ∼0.5 Myr.

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Constrained transport and adaptive mesh refinement in the Black Hole Accretion Code

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935559 ◽

2019 ◽

Vol 629 ◽

pp. A61 ◽

Cited By ~ 7

Author(s):

Hector Olivares ◽

Oliver Porth ◽

Jordy Davelaar ◽

Elias R. Most ◽

Christian M. Fromm ◽

...

Keyword(s):

Magnetic Field ◽

Black Hole ◽

Adaptive Mesh Refinement ◽

Mesh Refinement ◽

Adaptive Mesh ◽

Gravitational Fields ◽

Black Hole Accretion ◽

Constrained Transport ◽

The Magnetic Field ◽

Hole Accretion

Context. Worldwide very long baseline radio interferometry (VLBI) arrays are expected to obtain horizon-scale images of supermassive black hole candidates and of relativistic jets in several nearby active galactic nuclei. This, together with the expected detection of electromagnetic counterparts of gravitational-wave signals, motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. Aims. The Black Hole Accretion Code (BHAC) is a publicliy available code intended to aid with the modeling of such sources by performing general relativistic magnetohydrodynamical simulations in arbitrary stationary spacetimes. New additions to the code are required in order to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. Methods. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, which are essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field. We also present a general class of prolongation operators for face-allocated variables compatible with them. Results. After presenting several standard tests for the new implementation, we show that the choice of the divergence-control method can produce qualitative differences in the simulation results for scientifically relevant accretion problems. We demonstrate the ability of AMR to decrease the computational costs of black hole accretion simulations while sufficiently resolving turbulence arising from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up for which the new AMR implementation is particularly advantageous. Conclusions. The CT methods and AMR strategies discussed here are currently being used in the simulations performed with BHAC for the generation of theoretical models for the Event Horizon Telescope collaboration.

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A systematic study of radiative torque grain alignment in the diffuse interstellar medium

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937177 ◽

2020 ◽

Vol 640 ◽

pp. A118 ◽

Cited By ~ 3

Author(s):

Stefan Reissl ◽

Vincent Guillet ◽

Robert Brauer ◽

François Levrier ◽

François Boulanger ◽

...

Keyword(s):

Magnetic Field ◽

Interstellar Medium ◽

Adaptive Mesh Refinement ◽

Adaptive Mesh ◽

Specific Pattern ◽

Data Cube ◽

Polarization Angle ◽

Grain Alignment ◽

Polarization Fraction ◽

The Magnetic Field

Context. Analyses of Planck data have demonstrated that the grain alignment efficiency is almost constant in the diffuse and translucent interstellar medium (ISM). Aims. We aim to test whether the radiative torque (RAT) theory is compatible with these new observational constraints on grain alignment. Methods. We combine a numerical magnetohydrodynamical simulation with a state-of-the-art radiative transfer post-processing code POLARIS which incorporates a physical dust model and the detailed physics of grain alignment by RATs. A dust model based on two distinct power-law-sized distributions of spherical graphite grains and oblate silicate grains was designed to reproduce the mean spectral dependence of extinction and polarization observed in the diffuse ISM. From a simulation of interstellar turbulence obtained with the adaptive-mesh-refinement code RAMSES, we extracted a data cube with physical conditions representative of the diffuse ISM. We post-process the RAMSES cube with POLARIS to compute the grain temperature and alignment efficiency in each cell of the cube. Finally, we simulate synthetic dust emission and polarization observations. Results. In our simulation, the grain alignment efficiency is well-correlated with the gas pressure, but not with the radiative torque intensity. Because of the low dust extinction in our simulation, the magnitude of the radiative torque varies little, decreasing only for column densities larger than 1022 cm−2. In comparing our synthetic maps with those obtained assuming a uniform alignment efficiency, we find no systematic difference and very small random differences. The dependencies of the polarization fraction p with the column density NH or with the dispersion in polarization angle S are also similar in both cases. The drop of grain alignment produced by the RAT model in the denser cells of the data cube does not significantly affect the patterns of the synthetic polarization maps, the polarization signal being dominated by the line-of-sight and beam integration of the geometry of the magnetic field. If a star is artificially inserted at the center of the simulation, the polarization fraction is increased everywhere, with no specific pattern around the star. The angle-dependence of the RAT efficiency is not observed in simulated maps and where the magnetic field is artificially set to a uniform configuration in the plane of the sky, it is only seen to be very weak in the optimal configuration. Conclusions. The RAT alignment theory is found to be compatible with the Planck polarization data for the diffuse and translucent ISM in the sense that both uniform alignment and RAT alignment lead to very similar simulated maps. To further test the predictions of the RAT theory in an environment where an important drop of grain alignment is expected, high-resolution polarization observations of dense regions must be confronted with numerical simulations sampling high-column densities (NH > 1022 cm−2) through dense clouds, given a sufficient statistical basis.

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Investigation of the localized co-interchange instabilities in a rotamak plasma

Journal of Plasma Physics ◽

10.1017/s0022377800014057 ◽

1989 ◽

Vol 41 (3) ◽

pp. 517-522

Author(s):

W. K. Bertram

Keyword(s):

Magnetic Field ◽

Stability Criterion ◽

Vortex Point ◽

Ohm’S Law ◽

Ideal Mhd ◽

Ohm's Law ◽

The Magnetic Field ◽

The Ideal

If the rotamak is regarded as a spherical field-reversed mirror then, according to conventional ideal MHD analysis, it should be unstable to co-interchange modes localized near the vortex point of the magnetic field. It is shown that to study these instabilities in a typical rotamak plasma, the Hall term in Ohm's law cannot be ignored. The effect of the Hall term on the ideal MHD analysis of co-interchange modes is investigated and a stability criterion is derived.

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MAGNETIC FIELD IMPACT UPON TRIBOTECHNICAL CHARACTERISTICS OF PERMANENT CONNECTIONS IN RELATION TO FRICTION SHOCK ABSORBERS

Bulletin of Bryansk state technical university ◽

10.30987/1999-8775-2020-10-4-11 ◽

2020 ◽

Vol 2020 (10) ◽

pp. 4-11

Author(s):

Victor Tikhomirov ◽

Aleksandr Gorlenko ◽

Stanislav Volohov ◽

Mikhail Izmerov

Keyword(s):

Magnetic Field ◽

Friction Factor ◽

Physical Contact ◽

Active Centers ◽

Press Fit ◽

Electro Magnetic Field ◽

Investigation Methods ◽

Electro Magnetic ◽

The Impact ◽

The Magnetic Field

The work purpose is the investigation of magnetic field impact upon properties of friction steel surfaces at fit stripping with tightness through manifested effects and their wear visually observed. On the spots of a real contact the magnetic field increases active centers, their amount and saturation with the time of dislocation outlet, and has an influence upon tribo-mating. The external electro-magnetic field promotes the increase of the number of active centers at the expense of dislocations outlet on the contact surface, and the increase of a physical contact area results in friction tie strengthening and growth of a friction factor. By the example of friction pairs of a spentonly unit in the suspension of coach cars there is given a substantiation of actuality and possibility for the creation of technical devices with the controlled factor of friction and the stability of effects achieved is also confirmed experimentally. Investigation methods: the fulfillment of laboratory physical experiments on the laboratory plant developed and patented on bush-rod samples inserted with the fit and tightness. The results of investigations and novelty: the impact of the magnetic field upon the value of a stripping force of a press fit with the guaranteed tightness is defined. Conclusion: there is a possibility to control a friction factor through the magnetic field impact upon a friction contact.

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Qualitative and quantitative assessment of magnetic vestibular stimulation in humans

Journal of Vestibular Research ◽

10.3233/ves-201538 ◽

2020 ◽

Vol 30 (6) ◽

pp. 353-361

Author(s):

Rebecca S. Dewey ◽

Rachel Gomez ◽

Chris Degg ◽

David M. Baguley ◽

Paul M. Glover

Keyword(s):

Magnetic Field ◽

Objective Measure ◽

Quantitative Measure ◽

Vestibular Stimulation ◽

Initial Study ◽

Clinical Test ◽

Healthy Control ◽

Student’S T ◽

The Impact ◽

The Magnetic Field

The sensation of phantom motion or exhibition of bodily sway is often reported in the proximity of an MR scanner. It is proposed that the magnetic field stimulates the vestibular system. There are a number of possible mechanisms responsible, and the relative contributions of susceptibility on the otolithic receptors and the Lorentz force on the cupulae have not yet been explored. This exploratory study aims to investigate the impact of being in the proximity of a 7.0 T MR scanner. The modified clinical test of sensory interaction on balance (mCTSIB) was used to qualitatively ascertain whether or not healthy control subjects who passed the mCTSIB in normal conditions 1) experienced subjective sensations of dizziness, vertigo or of leaning or shifting in gravity when in the magnetic field and 2) exhibited visibly increased bodily sway whilst in the magnetic field compared to outside the magnetic field. Condition IV of the mCTSIB was video recorded outside and inside the magnetic field, providing a semi-quantitative measure of sway. For condition IV of the mCTSIB (visual and proprioceptive cues compromised), all seven locations/orientations around the scanner yielded significantly more sway than at baseline (p < 0.01 FDR). A Student’s t-test comparing the RMS velocity of a motion marker on the upper arm during mCTSIB condition IV showed a significant increase in the amount of motion exhibited in the field (T = 2.59; d.f. = 9; p = 0.029) compared to outside the field. This initial study using qualitative measures of sway demonstrates that there is evidence for MR-naïve individuals exhibiting greater sway while performing the mCTSIB in the magnetic field compared to outside the field. Directional polarity of sway was not significant. Future studies of vestibular stimulation by magnetic fields would benefit from the development of a sensitive, objective measure of balance function, which can be performed inside a magnetic field.

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Hydromagnetic dynamos in rotating spherical fluid shells in dependence on the Prandtl number, density stratification and electromagnetic boundary conditions

Contributions to Geophysics and Geodesy ◽

10.1515/congeo-2015-0007 ◽

2014 ◽

Vol 44 (4) ◽

pp. 293-312 ◽

Cited By ~ 3

Author(s):

Tomáš Šoltis ◽

Ján Šimkanin

Keyword(s):

Magnetic Field ◽

Electrical Conductivity ◽

Magnetic Fields ◽

Prandtl Number ◽

Inner Core ◽

Polar Regions ◽

Number Density ◽

Magnetic Diffusion ◽

Spherical Fluid ◽

The Magnetic Field

Abstract We present an investigation of dynamo in a simultaneous dependence on the non-uniform stratification, electrical conductivity of the inner core and the Prandtl number. Computations are performed using the MAG dynamo code. In all the investigated cases, the generated magnetic fields are dipolar. Our results show that the dynamos, especially magnetic field structures, are independent in our investigated cases on the electrical conductivity of the inner core. This is in agreement with results obtained in previous analyses. The influence of non-uniform stratification is for our parameters weak, which is understandable because most of the shell is unstably stratified, and the stably stratified region is only a thin layer near the CMB. The teleconvection is not observed in our study. However, the influence of the Prandtl number is strong. The generated magnetic fields do not become weak in the polar regions because the magnetic field inside the tangent cylinder is always regenerated due to the weak magnetic diffusion.

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Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834296 ◽

2018 ◽

Vol 620 ◽

pp. A191 ◽

Cited By ~ 5

Author(s):

M. Benko ◽

S. J. González Manrique ◽

H. Balthasar ◽

P. Gömöry ◽

C. Kuckein ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Temporal Evolution ◽

Vertical Component ◽

Field Vector ◽

Linear Increase ◽

Vertical Magnetic Field ◽

Magnetic Diffusion ◽

The Magnetic Field ◽

Linear Decay

Context. It has been empirically determined that the umbra-penumbra boundaries of stable sunspots are characterized by a constant value of the vertical magnetic field. Aims. We analyzed the evolution of the photospheric magnetic field properties of a decaying sunspot belonging to NOAA 11277 between August 28–September 3, 2011. The observations were acquired with the spectropolarimeter on-board of the Hinode satellite. We aim to prove the validity of the constant vertical magnetic-field boundary between the umbra and penumbra in decaying sunspots. Methods. A spectral-line inversion technique was used to infer the magnetic field vector from the full-Stokes profiles. In total, eight maps were inverted and the variation of the magnetic properties in time were quantified using linear or quadratic fits. Results. We find a linear decay of the umbral vertical magnetic field, magnetic flux, and area. The penumbra showed a linear increase of the vertical magnetic field and a sharp decay of the magnetic flux. In addition, the penumbral area quadratically decayed. The vertical component of the magnetic field is weaker on the umbra-penumbra boundary of the studied decaying sunspot compared to stable sunspots. Its value seem to be steadily decreasing during the decay phase. Moreover, at any time of the sunspot decay shown, the inner penumbra boundary does not match with a constant value of the vertical magnetic field, contrary to what is seen in stable sunspots. Conclusions. During the decaying phase of the studied sunspot, the umbra does not have a sufficiently strong vertical component of the magnetic field and is thus unstable and prone to be disintegrated by convection or magnetic diffusion. No constant value of the vertical magnetic field is found for the inner penumbral boundary.

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