Seed magnetic fields in turbulent small-scale dynamos

ABSTRACT Magnetic fields in galaxies and galaxy clusters are amplified from a very weak seed value to the observed $\mu$G strengths by the turbulent dynamo. The seed magnetic field can be of primordial or astrophysical origin. The strength and structure of the seed field, on the galaxy or galaxy cluster scale, can be very different, depending on the seed-field generation mechanism. The seed field first encounters the small-scale dynamo, thus we investigate the effects of the strength and structure of the seed field on the small-scale dynamo action. Using numerical simulations of driven turbulence and considering three different seed-field configurations: (1) uniform field, (2) random field with a power-law spectrum, and (3) random field with a parabolic spectrum, we show that the strength and statistical properties of the dynamo-generated magnetic fields are independent of the details of the seed field. We demonstrate that, even when the small-scale dynamo is not active, small-scale magnetic fields can be generated and amplified linearly due to the tangling of the large-scale field. In the presence of the small-scale dynamo action, we find that any memory of the seed field for the non-linear small-scale dynamo generated magnetic fields is lost and thus, it is not possible to trace back seed-field information from the evolved magnetic fields in a turbulent medium.

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Cosmic-ray driven dynamo in galactic disks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309031147 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 479-484 ◽

Cited By ~ 3

Author(s):

Michał Hanasz ◽

K. Otmianowska-Mazur ◽

H. Lesch ◽

G. Kowal ◽

M. Soida ◽

...

Keyword(s):

Magnetic Fields ◽

Supernova Remnants ◽

Large Scale ◽

Cosmic Ray ◽

Turbulent Energy ◽

Field Energy ◽

Small Scale ◽

Galactic Rotation ◽

Dynamo Action ◽

Magnetic Field Energy

AbstractWe present new developments on the Cosmic–Ray driven, galactic dynamo, modeled by means of direct, resistive CR–MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large–scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small–scale turbulent magnetic fields. Our new results include demonstration of the global–galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.

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Faraday rotation measure dependence on galaxy cluster dynamics

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1450 ◽

2019 ◽

Vol 487 (4) ◽

pp. 4768-4774 ◽

Cited By ~ 1

Author(s):

F A Stasyszyn ◽

M de los Rios

Keyword(s):

Magnetic Fields ◽

Physical Process ◽

Faraday Rotation ◽

Large Scale ◽

Galaxy Cluster ◽

Small Scale ◽

Dominant Mechanism ◽

Rotation Measure ◽

The Difference ◽

Coherence Lengths

ABSTRACT We study the magnetic fields in galaxy clusters through Faraday rotation measurements crossing systems in different dynamical states. We confirm that magnetic fields are present in those systems and analyse the difference between relaxed and unrelaxed samples with respect to the dispersion between their inherent Faraday rotation measurements (RM). We found an increase of this RM dispersion and a higher RM overlapping frequency for unrelaxed clusters. This fact suggests that a large-scale physical process is involved in the nature of unrelaxed systems and possible depolarization effects are present in the relaxed ones. We show that dynamically unrelaxed systems can enhance magnetic fields to large coherence lengths. In contrast, the results for relaxed systems suggests that a small-scale dynamo can be a dominant mechanism for sustaining magnetic fields, leading to intrinsic depolarization.

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Magnetic field amplification in accretion discs around the first stars: implications for the primordial IMF

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab531 ◽

2021 ◽

Vol 503 (2) ◽

pp. 2014-2032

Author(s):

Piyush Sharda ◽

Christoph Federrath ◽

Mark R Krumholz ◽

Dominik R G Schleicher

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Strength ◽

Large Scale ◽

Initial Mass Function ◽

Accretion Discs ◽

Small Scale ◽

Dynamo Action ◽

Initial Field ◽

First Stars

ABSTRACT Magnetic fields play an important role in the dynamics of present-day molecular clouds. Recent work has shown that magnetic fields are equally important for primordial clouds, which form the first stars in the Universe. While the primordial magnetic field strength on cosmic scales is largely unconstrained, theoretical models strongly suggest that a weak seed field existed in the early Universe. We study how the amplification of such a weak field can influence the evolution of accretion discs around first stars, and thus affect the primordial initial mass function (IMF). We perform a suite of 3D ideal magneto-hydrodynamic simulations with different initial field strengths and numerical resolutions. We find that, in simulations with sufficient spatial resolution to resolve the Jeans scale during the collapse, even initially weak magnetic fields grow exponentially to become dynamically important due to both the so-called small-scale turbulent dynamo and the large-scale mean-field dynamo. Capturing the small-scale dynamo action depends primarily on how well we resolve the Jeans length, while capturing the large-scale dynamo depends on the Jeans resolution as well as the maximum absolute resolution. Provided enough resolution, we find that fragmentation does not depend strongly on the initial field strength, because even weak fields grow to become strong. However, fragmentation in runs with magnetic fields differs significantly from those without magnetic fields. We conclude that the development of dynamically strong magnetic fields during the formation of the first stars is likely inevitable, and that these fields had a significant impact on the primordial IMF.

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Global MHD phenomena and their importance for stellar surfaces

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311015146 ◽

2010 ◽

Vol 6 (S273) ◽

pp. 141-147

Author(s):

Rainer Arlt

Keyword(s):

Magnetic Fields ◽

Mean Field ◽

Momentum Equation ◽

Small Scale ◽

Dynamo Action ◽

Complex Activity ◽

Mhd Instabilities ◽

Stellar Magnetic Fields ◽

Mean Field Dynamo

AbstractThis review is an attempt to elucidate MHD phenomena relevant for stellar magnetic fields. The full MHD treatment of a star is a problem which is numerically too demanding. Mean-field dynamo models use an approximation of the dynamo action from the small-scale motions and deliver global magnetic modes which can be cyclic, stationary, axisymmetric, and non-axisymmetric. Due to the lack of a momentum equation, MHD instabilities are not visible in this picture. However, magnetic instabilities must set in as a result of growing magnetic fields and/or buoyancy. Instabilities deliver new timescales, saturation limits and topologies to the system probably providing a key to the complex activity features observed on stars.

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The macrodynamics of α-effect dynamos in rotating fluids

Journal of Fluid Mechanics ◽

10.1017/s0022112075000390 ◽

1975 ◽

Vol 67 (3) ◽

pp. 417-443 ◽

Cited By ~ 140

Author(s):

W. V. R. Maekus ◽

M. R. E. Proctor

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Magnetic Energy ◽

Finite Amplitude ◽

Small Scale ◽

Past Study ◽

Ohmic Loss ◽

General Will ◽

Rotating Systems ◽

Stellar Magnetic Fields

Past study of the large-scale consequences of forced small-scale motions in electrically conducting fluids has led to the ‘α-effect’ dynamos. Various linear kinematic aspects of these dynamos have been explored, suggesting their value in the interpretation of observed planetary and stellar magnetic fields. However, large-scale magnetic fields with global boundary conditions can not be force free and in general will cause large-scale motions as they grow. I n this paper the finite amplitude behaviour of global magnetic fields and the large-scale flows induced by them in rotating systems is investigated. In general, viscous and ohmic dissipative mechanisms both play a role in determining the amplitude and structure of the flows and magnetic fields which evolve. In circumstances where ohmic loss is the principal dissipation, it is found that determination of a geo- strophic flow is an essential part of the solution of the basic stability problem. Nonlinear aspects of the theory include flow amplitudes which are independent of the rotation and a total magnetic energy which is directly proportional to the rotation. Constant a is the simplest example exhibiting the various dynamic balances of this stabilizing mechanism for planetary dynamos. A detailed analysis is made for this case to determine the initial equilibrium of fields and flows in a rotating sphere.

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Magnetosheath-cusp interface

Annales Geophysicae ◽

10.5194/angeo-22-183-2004 ◽

2004 ◽

Vol 22 (1) ◽

pp. 183-212 ◽

Cited By ~ 20

Author(s):

S. Savin ◽

L. Zelenyi ◽

S. Romanov ◽

I. Sandahl ◽

J. Pickett ◽

...

Keyword(s):

Boundary Layer ◽

Solar Wind ◽

Magnetic Fields ◽

Large Scale ◽

Traditional Approach ◽

Qualitative Difference ◽

Small Scale ◽

Nonlinear Phenomena ◽

Solar Wind Interaction ◽

Linear Superposition

Abstract. We advance the achievements of Interball-1 and other contemporary missions in exploration of the magnetosheath-cusp interface. Extensive discussion of published results is accompanied by presentation of new data from a case study and a comparison of those data within the broader context of three-year magnetopause (MP) crossings by Interball-1. Multi-spacecraft boundary layer studies reveal that in ∼80% of the cases the interaction of the magnetosheath (MSH) flow with the high latitude MP produces a layer containing strong nonlinear turbulence, called the turbulent boundary layer (TBL). The TBL contains wave trains with flows at approximately the Alfvén speed along field lines and "diamagnetic bubbles" with small magnetic fields inside. A comparison of the multi-point measurements obtained on 29 May 1996 with a global MHD model indicates that three types of populating processes should be operative: large-scale (∼few RE) anti-parallel merging at sites remote from the cusp; medium-scale (few thousandkm) local TBL-merging of fields that are anti-parallel on average; small-scale (few hundredkm) bursty reconnection of fluctuating magnetic fields, representing a continuous mechanism for MSH plasma inflow into the magnetosphere, which could dominate in quasi-steady cases. The lowest frequency (∼1–2mHz) TBL fluctuations are traced throughout the magnetosheath from the post-bow shock region up to the inner magnetopause border. The resonance of these fluctuations with dayside flux tubes might provide an effective correlative link for the entire dayside region of the solar wind interaction with the magnetopause and cusp ionosphere. The TBL disturbances are characterized by kinked, double-sloped wave power spectra and, most probably, three-wave cascading. Both elliptical polarization and nearly Alfvénic phase velocities with characteristic dispersion indicate the kinetic Alfvénic nature of the TBL waves. The three-wave phase coupling could effectively support the self-organization of the TBL plasma by means of coherent resonant-like structures. The estimated characteristic scale of the "resonator" is of the order of the TBL dimension over the cusps. Inverse cascades of kinetic Alfvén waves are proposed for forming the larger scale "organizing" structures, which in turn synchronize all nonlinear cascades within the TBL in a self-consistent manner. This infers a qualitative difference from the traditional approach, wherein the MSH/cusp interaction is regarded as a linear superposition of magnetospheric responses on the solar wind or MSH disturbances. Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (turbulence; nonlinear phenomena)

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Viscosity and Large-Scale Magnetic Fields from Accretion Disc Dynamos

International Astronomical Union Colloquium ◽

10.1017/s0252921100042639 ◽

1997 ◽

Vol 163 ◽

pp. 190-200

Author(s):

Christopher A. Tout

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Small Scale ◽

Cascade Process ◽

Jet Formation ◽

Magnetohydrodynamic Turbulence ◽

Discs Large ◽

Scale Field ◽

Large Scale Field ◽

Nova Outbursts

AbstractWe review those processes associated with accretion discs that are probably influenced by magnetic fields, specifically, accretiondisc viscosity, energy dissipation and jet formation. We consider how magnetic instabilities in the disc can lead to a self-sustaining dynamical dynamo and how this is manifested as magnetohydrodynamic turbulence in numerical simulations. We show that currently these models do not fit with observational constraints imposed by dwarf-nova outbursts. We also show that the drop in ionisation fraction does not lead to the apparently necessary drop in viscosity in quiescent cataclysmic variable discs. Large-scale magnetic fields are required to launch and collimate jets form discs. We describe an inverse cascade process that can construct sufficient large-scale field from small-scale field generated by a dynamo.

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Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935695 ◽

2019 ◽

Vol 630 ◽

pp. A99 ◽

Cited By ~ 5

Author(s):

A. Lavail ◽

O. Kochukhov ◽

G. A. J. Hussain

Keyword(s):

Magnetic Field ◽

High Resolution ◽

Magnetic Fields ◽

Large Scale ◽

Near Infrared ◽

Field Measurements ◽

T Tauri Stars ◽

Small Scale ◽

Surface Magnetic Field ◽

T Tauri

Aims. In this paper, we aim to characterise the surface magnetic fields of a sample of eight T Tauri stars from high-resolution near-infrared spectroscopy. Some stars in our sample are known to be magnetic from previous spectroscopic or spectropolarimetric studies. Our goals are firstly to apply Zeeman broadening modelling to T Tauri stars with high-resolution data, secondly to expand the sample of stars with measured surface magnetic field strengths, thirdly to investigate possible rotational or long-term magnetic variability by comparing spectral time series of given targets, and fourthly to compare the magnetic field modulus ⟨B⟩ tracing small-scale magnetic fields to those of large-scale magnetic fields derived by Stokes V Zeeman Doppler Imaging (ZDI) studies. Methods. We modelled the Zeeman broadening of magnetically sensitive spectral lines in the near-infrared K-band from high-resolution spectra by using magnetic spectrum synthesis based on realistic model atmospheres and by using different descriptions of the surface magnetic field. We developped a Bayesian framework that selects the complexity of the magnetic field prescription based on the information contained in the data. Results. We obtain individual magnetic field measurements for each star in our sample using four different models. We find that the Bayesian Model 4 performs best in the range of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not detect a strong rotational variation of ⟨B⟩ with a mean peak-to-peak variation of 0.3 kG. Our confidence intervals are of the same order of magnitude, which suggests that the Zeeman broadening is produced by a small-scale magnetic field homogeneously distributed over stellar surfaces. A comparison of our results with mean large-scale magnetic field measurements from Stokes V ZDI show different fractions of mean field strength being recovered, from 25–42% for relatively simple poloidal axisymmetric field topologies to 2–11% for more complex fields.

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Magnetic coupling of waves and oscillations in the lower solar atmosphere: can the tail wag the dog?

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2005.1703 ◽

2005 ◽

Vol 364 (1839) ◽

pp. 351-381 ◽

Cited By ~ 59

Author(s):

Robert Erdélyi

Keyword(s):

Magnetic Fields ◽

Solar Atmosphere ◽

Acoustic Waves ◽

Large Scale ◽

Magnetic Coupling ◽

Small Scale ◽

Coupling Mechanism ◽

Frequency Shifts ◽

Characteristic Features ◽

Lower Solar Atmosphere

Can the ubiquitously magnetic solar atmosphere have any effect on solar global oscillations? Traditionally, solar atmospheric magnetic fields are considered to be somewhat less important for the existence and characteristic features of solar global oscillations ( p , f and the not-yet-observed g -modes). In this paper, I demonstrate the importance of the presence of magnetism and plasma dynamics for global resonant oscillations in the solar atmosphere. In particular, in the lower part of the solar atmosphere there are both coherent and random components of magnetic fields and velocity fields, each of which contribute on its own to the line widths and frequency variations of solar global acoustic waves. Changes in the coherent large-scale atmospheric magnetic fields cause frequency shifts of global oscillations over a solar cycle. The random character of the continuously emerging, more localized, magnetic carpet (i.e. small-scale, possibly even sub-resolution, loops) gives rise to additional frequency shifts. On the other hand, random and organized surface and sub-surface flows, like surface granulation, meridional flows or differential rotation, also affect the coupling mechanism of global oscillations to the lower magnetic atmosphere. The competition between magnetic fields and flows is inevitable. Finally, I shall discuss how solar global oscillations can resonantly interact with the overlaying inhomogeneous lower solar atmosphere embedded in a magnetic carpet. Line width broadening and distorsion of global acoustic modes will be discussed. The latter is suggested to be tested and measured by using ring-analysis techniques.

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