Magnetic Fields Around Galactic Discs

Magnetic fields in the discs of spiral galaxies are quite well understood, although, of course, many details still require investigation and future observations with new generations of radio telescopes will be valuable here. Magnetic configurations around galactic discs and, in particular, the magnetic field components perpendicular to galactic discs seem to be much more poorly understood and deserve further investigation both observationally and by modelling. Another problem to be addressed in future investigations is the magnetic configuration in galactic halos and, in particular, interactions with the intergalactic medium and various winds. Finally, the importance of the observational determination of such drivers of galactic dynamo action as mirror asymmetry of the turbulent galactic flows are briefly discussed.

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Magnetic Fields and Halos in Spiral Galaxies

Galaxies ◽

10.3390/galaxies7020054 ◽

2019 ◽

Vol 7 (2) ◽

pp. 54 ◽

Cited By ~ 4

Author(s):

Marita Krause

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Spiral Galaxies ◽

Cosmic Ray ◽

Radio Continuum ◽

Field Pattern ◽

Dynamo Action ◽

Disk Plane ◽

The Magnetic Field

Radio continuum and polarization observations reveal best the magnetic field structure and strength in nearby spiral galaxies. They show a similar magnetic field pattern, which is of spiral shape along the disk plane and X-shaped in the halo, sometimes accompanied by strong vertical fields above and below the central region of the disk. The strength of the total halo field is comparable to that of the disk. The small- and large-scale dynamo action is discussed to explain the observations with special emphasis on the rôle of star formation on the α − Ω dynamo and the magnetic field strength and structure in the disk and halo. Recently, with RM-synthesis of the CHANG-ES observations, we obtained the first observational evidence for the existence of regular magnetic fields in the halo. The analysis of the radio scale heights indicate escape-dominated radio halos with convective cosmic ray propagation for many galaxies. These galactic winds may be essential for an effective dynamo action and may transport large-scale magnetic field from the disk into the halo.

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Magnetic Field Strengths of Spiral Galaxies and the Far-Infrared/Radio Correlation

Symposium - International Astronomical Union ◽

10.1017/s0074180900190114 ◽

1990 ◽

Vol 140 ◽

pp. 241-241

Author(s):

A. J. Fitt ◽

P. Alexander

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Distribution ◽

Detection Rate ◽

Spiral Galaxies ◽

Far Infrared ◽

Magnetic Field Distribution ◽

Selection Effects ◽

The Magnetic Field ◽

Field Strengths

We have calculated equipartition magnetic fields for a complete, optically-selected sample of 165 spiral galaxies. The magnetic field distribution (fig. 1) is type independent, and shows remarkably little spread in values, around 1 decade in B. This is not due to selection effects because of the nature of the sample and the 95 percent detection rate.

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Decaying turbulence and magnetic fields in galaxy clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1918 ◽

2019 ◽

Vol 488 (3) ◽

pp. 3439-3445 ◽

Cited By ~ 1

Author(s):

Sharanya Sur

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Galaxy Clusters ◽

Power Law ◽

Dynamo Action ◽

Wide Range ◽

Major Merger ◽

Decaying Turbulence ◽

Field Decay ◽

The Magnetic Field

Abstract We explore the decay of turbulence and magnetic fields generated by fluctuation dynamo action in the context of galaxy clusters where such a decaying phase can occur in the aftermath of a major merger event. Using idealized numerical simulations that start from a kinetically dominated regime we focus on the decay of the steady state rms velocity and the magnetic field for a wide range of conditions that include varying the compressibility of the flow, the forcing wavenumber, and the magnetic Prandtl number. Irrespective of the compressibility of the flow, both the rms velocity and the rms magnetic field decay as a power law in time. In the subsonic case we find that the exponent of the power law is consistent with the −3/5 scaling reported in previous studies. However, in the transonic regime both the rms velocity and the magnetic field initially undergo rapid decay with an ≈t−1.1 scaling with time. This is followed by a phase of slow decay where the decay of the rms velocity exhibits an ≈−3/5 scaling in time, while the rms magnetic field scales as ≈−5/7. Furthermore, analysis of the Faraday rotation measure (RM) reveals that the Faraday RM also decays as a power law in time ≈t−5/7; steeper than the ∼t−2/5 scaling obtained in previous simulations of magnetic field decay in subsonic turbulence. Apart from galaxy clusters, our work can have potential implications in the study of magnetic fields in elliptical galaxies.

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Connecting magnetic fields from sub-galactic scale to clusters of galaxies and beyond with cosmological MHD simulations

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316006396 ◽

2015 ◽

Vol 11 (A29B) ◽

pp. 699-699

Author(s):

Klaus Dolag ◽

Alexander M. Beck ◽

Alexander Arth

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Heat Transport ◽

Galaxy Clusters ◽

Large Scale ◽

High Redshift ◽

Galactic Halos ◽

Rotation Measure ◽

The Magnetic Field ◽

Good Agreement

AbstractUsing the MHD version of Gadget3 (Stasyszyn, Dolag & Beck 2013) and a model for the seeding of magnetic fields by supernovae (SN), we performed simulations of the evolution of the magnetic fields in galaxy clusters and study their effects on the heat transport within the intra cluster medium (ICM). This mechanism – where SN explosions during the assembly of galaxies provide magnetic seed fields – has been shown to reproduce the magnetic field in Milky Way-like galactic halos (Beck et al. 2013). The build up of the magnetic field at redshifts before z = 5 and the accordingly predicted rotation measure evolution are also in good agreement with current observations. Such magnetic fields present at high redshift are then transported out of the forming protogalaxies into the large-scale structure and pollute the ICM (in a similar fashion to metals transport). Here, complex velocity patterns, driven by the formation process of cosmic structures are further amplifying and distributing the magnetic fields. In galaxy clusters, the magnetic fields therefore get amplified to the observed μG level and produce the observed amplitude of rotation measures of several hundreds of rad/m2. We also demonstrate that heat conduction in such turbulent fields on average is equivalent to a suppression factor around 1/20th of the classical Spitzer value and in contrast to classical, isotropic heat transport leads to temperature structures within the ICM compatible with observations (Arth et al. 2014).

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Enhancement of Galactic Dynamos by Density Waves

Symposium - International Astronomical Union ◽

10.1017/s0074180900189739 ◽

1990 ◽

Vol 140 ◽

pp. 127-130 ◽

Cited By ~ 1

Author(s):

Makoto Tosa ◽

Masashi Chiba

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Density Wave ◽

Wave Perturbation ◽

Density Waves ◽

Dynamo Action ◽

Galactic Dynamos ◽

The Magnetic Field ◽

Effects Of Density

We examine effects of density waves on the local galactic αω-dynamo. Oscillations of the magnetic field and the dynamo parameters due to the density wave perturbation irreversibly couple with the dynamo action to enhance the growth of the magnetic fields.

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MRE Damping Characteristics Evaluation

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.699.31 ◽

2016 ◽

Vol 699 ◽

pp. 31-36 ◽

Cited By ~ 1

Author(s):

Eduard Chirila ◽

Ionel Chirica ◽

Doina Boazu ◽

Elena Felicia Beznea

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Smart Materials ◽

Magnetorheological Elastomers ◽

Damping Characteristics ◽

The Subject ◽

Energy Absorbing ◽

The Magnetic Field ◽

Material Form

The paper addresses the study of the damping characteristics estimation and behaviour of the magnetorheological elastomers (MREs) in the absence of magnetic field. This type of material actively changes the size, internal structure and viscoelastic characteristics under the external influences. These particular composite materials whose characteristics can vary in the presence of a magnetic fields are known as smart materials. The feature which causes the variation of properties in magnetic fields is explained by the existence of polarized particles which change the material form by energy absorbing. Damping is a special characteristic that influences the vibratory of the mechanical system. As an effect of this property is the reducing of the vibration amplitudes by dissipating the energy stored during the vibratory moving. The main characteristic that is based on the determination of the damping coefficient is the energy loss, which is the subject of the present paper. Before to start the characteristics determination in the presence of the magnetic field, it is necessary to study these characteristics in the absence of magnetic field. The MRE specimens have been manufactured and tested under the light conditions (non magnetic field). A special experimental test rig was built to investigate the response of the MRE specimens under the charging force. The experimental results show that the loss energy of the MRE specimen can be determined from the charging-discharging curves versus displacement. The results of the MRE specimen are presented in this paper: MRE with feromagnetic particles not exposed in magnetic field during fabrication.

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Statistics of Radio-X-Ray Emission and Magnetic Fields in the Intergalactic Medium

Symposium - International Astronomical Union ◽

10.1017/s0074180900241168 ◽

1990 ◽

Vol 139 ◽

pp. 414-415

Author(s):

Hitoshi Hanami

Keyword(s):

Magnetic Field ◽

Cosmic Rays ◽

Magnetic Fields ◽

Intergalactic Medium ◽

Relativistic Electrons ◽

X Ray ◽

Cooling Flows ◽

Hot Plasma ◽

Inverse Compton ◽

The Magnetic Field

X-ray observations have demonstrated that the intergalactic medium in many clusters (cf. Coma, Perseus) contains a thin, hot plasma that may be produced by the accretion process in the gravitational potential of clusters with radiative cooling; this is usually called “cooling flows” (Fabian, Nulsen, and Canizares 1984; Sarazin 1986). On the other hand, the existence of radio halos in some clusters has been reported (Coma: Jaffe, Perola, and Valentijn 1976; A401: Roland et al. 1981). In addition, many elliptical galaxies in the center of clusters are also strong synchrotron radio sources. These radio emissions provide evidence for large amounts of relativistic electrons associated with the active phenomena in or around these galaxies and clusters. We can estimate the values or limits on the magnetic field in the cluster from the limits on the inverse Compton X-ray emission with the synchrotron radio emission (cf. Jaffe 1980). The intracluster field strength Bo is roughly 1 μG. It has been suggested that the influence of cosmic rays and magnetic fields is important for the properties and dynamics of the intercluster medium (Böhringer and Morfill 1988; Soker and Sarazin 1989). If cooling flows are real, this inward flow can impede the escape of the cosmic rays from the central galaxies in clusters and enhance the magnetic field. The confinement of the cosmic rays and the magnetic field in the center of clusters affects the gas of the intracluster medium.

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3D model of magnetic fields evolution in dwarf irregular galaxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311007344 ◽

2010 ◽

Vol 6 (S274) ◽

pp. 389-392

Author(s):

Hubert Siejkowski ◽

Marian Soida ◽

Katarzyna Otmianowska-Mazur ◽

Michał Hanasz ◽

Dominik J. Bomans

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Gravitational Potential ◽

Spiral Galaxies ◽

Cosmic Ray ◽

Dynamo Model ◽

Slow Rotation ◽

Irregular Galaxies ◽

Dwarf Irregular Galaxies ◽

The Magnetic Field

AbstractRadio observations show that magnetic fields are present in dwarf irregular galaxies (dIrr) and its strength is comparable to that found in spiral galaxies. Slow rotation, weak shear and shallow gravitational potential are the main features of a typical dIrr galaxy. These conditions of the interstellar medium in a dIrr galaxy seem to unfavourable for amplification of the magnetic field through the dynamo process. Cosmic-ray driven dynamo is one of the galactic dynamo model, which has been successfully tested in case of the spiral galaxies. We investigate this dynamo model in the ISM of a dIrr galaxy. We study its efficiency under the influence of slow rotation, weak shear and shallow gravitational potential. Additionally, the exploding supernovae are parametrised by the frequency of star formation and its modulation, to reproduce bursts and quiescent phases. We found that even slow galactic rotation with a low shearing rate amplifies the magnetic field, and that rapid rotation with a low value of the shear enhances the efficiency of the dynamo. Our simulations have shown that a high amount of magnetic energy leaves the simulation box becoming an efficient source of intergalactic magnetic fields.

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Numerical determination of the cutoff frequency in solar models

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038387 ◽

2020 ◽

Vol 640 ◽

pp. A4 ◽

Cited By ~ 1

Author(s):

T. Felipe ◽

C. R. Sangeetha

Keyword(s):

Magnetic Field ◽

Wave Propagation ◽

Magnetic Fields ◽

Numerical Simulations ◽

Acoustic Waves ◽

Cutoff Frequency ◽

Quiet Sun ◽

Radiative Losses ◽

The Magnetic Field

Context. In stratified atmospheres, acoustic waves can only propagate if their frequency is higher than the cutoff value. The determination of the cutoff frequency is fundamental for several topics in solar physics, such as evaluating the contribution of the acoustic waves to the chromospheric heating or the application of seismic techniques. However, different theories provide different cutoff values. Aims. We developed an alternative method to derive the cutoff frequency in several standard solar models, including various quiet-Sun and umbral atmospheres. The effects of magnetic field and radiative losses on the cutoff are examined. Methods. We performed numerical simulations of wave propagation in the solar atmosphere using the code MANCHA. The cutoff frequency is determined from the inspection of phase-difference spectra computed between the velocity signal at two atmospheric heights. The process is performed by choosing pairs of heights across all the layers between the photosphere and the chromosphere to derive the vertical stratification of the cutoff in the solar models. Result. The cutoff frequency predicted by the theoretical calculations departs significantly from the measurements obtained from the numerical simulations. In quiet-Sun atmospheres, the cutoff shows a strong dependence on the magnetic field for adiabatic wave propagation. When radiative losses are taken into account, the cutoff frequency is greatly reduced and the variation of the cutoff with the strength of the magnetic field is lower. The effect of the radiative losses in the cutoff is necessary to understand recent quiet-Sun and sunspot observations. In the presence of inclined magnetic fields, our numerical calculations confirm that the cutoff frequency is reduced as a result of the reduced gravity experienced by waves that propagate along field lines. An additional reduction is also found in regions with significant changes in the temperature, which is due to the lower temperature gradient along the path of field-guided waves. Conclusions. Our results show solid evidence that the cutoff frequency in the solar atmosphere is stratified. The cutoff values are not correctly captured by theoretical estimates. In addition, most of the widely used analytical cutoff formulae neglect the effect of magnetic fields and radiative losses, whose role is critical for determining the evanescent or propagating nature of the waves.

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A dynamo amplifying the magnetic field of a Milky-Way-like galaxy

Astronomy and Astrophysics ◽

10.1051/0004-6361/202037835 ◽

2020 ◽

Vol 641 ◽

pp. A165

Author(s):

Evangelia Ntormousi ◽

Konstantinos Tassis ◽

Fabio Del Sordo ◽

Francesca Fragkoudi ◽

Rüdiger Pakmor

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Spiral Galaxies ◽

Mean Field ◽

Dark Matter Halo ◽

Random Component ◽

Galactic Magnetic Field ◽

The Mean ◽

The Magnetic Field ◽

Mean Field Dynamo

Context. The magnetic fields of spiral galaxies are so strong that they cannot qualify as primordial. Their typical values are over one billion times higher than any value predicted for the early Universe. Explaining this immense growth and incorporating it in galaxy evolution theories is one of the long-standing challenges in astrophysics. Aims. So far, the most successful theory for the sustained growth of the galactic magnetic field is the alpha-omega dynamo. This theory predicts a characteristic dipolar or quadrupolar morphology for the galactic magnetic field, which has been observed in external galaxies. So far, however, there has been no direct demonstration of a mean-field dynamo operating in direct, multi-physics simulations of spiral galaxies. We carry out such a demonstration in this work. Methods. We employed numerical models of isolated, star-forming spiral galaxies that include a magnetized gaseous disk, a dark matter halo, stars, and stellar feedback. Naturally, the resulting magnetic field has a complex morphology that includes a strong random component. Using a smoothing of the magnetic field on small scales, we were able to separate the mean from the turbulent component and analyze them individually. Results. We find that a mean-field dynamo naturally occurs as a result of the dynamical evolution of the galaxy and amplifies the magnetic field by an order of magnitude over half a Gyr. Despite the highly dynamical nature of these models, the morphology of the mean component of the field is identical to analytical predictions. Conclusions. This result underlines the importance of the mean-field dynamo in galactic evolution. Moreover, by demonstrating the natural growth of the magnetic field in a complex galactic environment, it brings us a step closer to understanding the cosmic origin of magnetic fields.

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