The supernova-regulated ISM – II. The mean magnetic field

Gent, Frederick A.; Shukurov, Anvar; Sarson, Graeme R.; Fletcher, Andrew; Mantere, M. J.

doi:10.1093/mnrasl/sls042

Cited by 92 publications

(131 citation statements)

References 8 publications

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“…However, this leads to physically unacceptable values of the correlation length of the density fluctuations in excess of 200 pc, whereas physically justifiable values (and those observed in the ISM) are about 100 pc or less. It is also worth noting that there is no need for the large-scale density to be perfectly uniform in x and y and only depend on z: the horizontal averaging disregards this fact (for details, see Gent et al 2013a). …”

Section: Gas Density Fluctuationsmentioning

confidence: 99%

Topological signatures of interstellar magnetic fields – I. Betti numbers and persistence diagrams

Makarenko

Shukurov

Henderson

et al. 2018

Monthly Notices of the Royal Astronomical Society

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The interstellar medium (ISM) is a magnetised system in which transonic or supersonic turbulence is driven by supernova explosions. This leads to the production of intermittent, filamentary structures in the ISM gas density, whilst the associated dynamo action also produces intermittent magnetic fields. The traditional theory of random functions, restricted to second-order statistical moments (or power spectra), does not adequately describe such systems. We apply topological data analysis (TDA), sensitive to all statistical moments and independent of the assumption of Gaussian statistics, to the gas density fluctuations in a magnetohydrodynamic (MHD) simulation of the multi-phase ISM. This simulation admits dynamo action, so produces physically realistic magnetic fields. The topology of the gas distribution, with and without magnetic fields, is quantified in terms of Betti numbers and persistence diagrams. Like the more standard correlation analysis, TDA shows that the ISM gas density is sensitive to the presence of magnetic fields. However, TDA gives us important additional information that cannot be obtained from correlation functions. In particular, the Betti numbers per correlation cell are shown to be physically informative. Magnetic fields make the ISM more homogeneous, reducing the abundance of both isolated gas clouds and cavities, with a stronger effect on the cavities. Remarkably, the modification of the gas distribution by magnetic fields is captured by the Betti numbers even in regions more than 300 pc from the midplane, where the magnetic field is weaker and correlation analysis fails to detect any signatures of magnetic effects.

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Section: Gas Density Fluctuationsmentioning

confidence: 99%

Topological signatures of interstellar magnetic fields – I. Betti numbers and persistence diagrams

Makarenko

Shukurov

Henderson

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…In IC 342 (like in NGC 6946 and M 33) the magnetic energy density decreases more slowly than the turbulent energy density and dominates beyond about 5 radius, while models of field amplification by small-scale and mean-field dynamos predict a close similarity between these energy components Gent et al 2013). This may point towards an additional source of turbulence supporting mean-field dynamo action in the outer disk.…”

Section: Dynamo Action In the Outer Diskmentioning

confidence: 99%

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Beck

2015

A&A

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Context. Magnetic fields play an important role in the formation and stabilization of spiral structures in galaxies, but the interaction between interstellar gas and magnetic fields has not yet been understood. In particular, the phenomenon of "magnetic arms" located between material arms is a mystery. Aims. The strength and structure of interstellar magnetic fields and their relation to spiral arms in gas and dust are investigated in the nearby and almost face-on spiral galaxy IC 342. Methods. The total and polarized radio continuum emission of IC 342 was observed with high spatial resolution in four wavelength bands with the Effelsberg and VLA telescopes. At λ6.2 cm the data from both telescopes were combined. I separated thermal and nonthermal (synchrotron) emission components with the help of the spectral index distribution and derived maps of the magnetic field strength, degree of magnetic field order, magnetic pitch angle, Faraday rotation measure, and Faraday depolarization. Results. IC 342 hosts a diffuse radio disk with an intensity that decreases exponentially with increasing radius. The frequency dependence of the scalelength of synchrotron emission indicates energy-dependent propagation of the cosmic-ray electrons, probably via the streaming instability. The equipartition strength of the total field in the main spiral arms is typically 15 μG, that of the ordered field about 5 μG. The total radio emission, observed with the VLA's high resolution, closely follows the dust emission in the infrared at 8 μm (Spitzer telescope) and 22 μm (WISE telescope). The polarized emission is not diffuse, but concentrated in spiral arms of various types: (1) a narrow arm of about 300 pc width, displaced inwards with respect to the eastern arm by about 200 pc, indicating magnetic fields compressed by a density wave; (2) a broad arm of 300-500 pc width around the northern arm with systematic variations in polarized emission, polarization angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of a helically twisted flux tube generated by the Parker instability; (3) a rudimentary magnetic arm in an interarm region in the north-west; (4) several broad spiral arms in the outer galaxy, related to spiral arms in the total neutral gas; (5) short features in the outer south-western galaxy, probably distorted by tidal interaction. Faraday rotation of the polarization angles reveals an underlying regular field of only 0.5 μG strength with a large-scale axisymmetric spiral pattern, probably a signature of a mean-field α − Ω dynamo, and an about 10× stronger field that fluctuates on scales of a few 100 pc. The magnetic field around the bar in the central region of IC 342 resembles that of large barred galaxies; it has a regular spiral pattern with a large pitch angle, is directed outwards, and is opposite to the large-scale regular field in the disk. Polarized emission at λ20.1 cm is strongly affected by Faraday depolarization in the western and northern parts of the galaxy. Helical fields extending fro...

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“…It is then natural to use sufficiently realistic numerical models to address these questions before the more difficult observational exploration. We use such simulations, as detailed in Gent (2012) and Gent et al (2013aGent et al ( , 2013b, which have nontrivial magnetic fields generated by dynamo action, to clarify the correlation (and other statistical) properties of the multiphase ISM. In particular, we compare the autocorrelation and crosscorrelation functions of the random (i.e., small-scale; see Section 2.3) velocity and magnetic fields and density fluctuations, as well as the Faraday depth and synchrotron intensity.…”

Section: Introductionmentioning

confidence: 99%

“…Open boundary conditions, permitting outflow and inflow, are used at the vertical (z) boundaries. See Gent (2012) and Gent et al (2013aGent et al ( , 2013b for further details on the boundary conditions used and on the other implementations described above.…”

mentioning

confidence: 99%

Supernova-regulated ISM. V. Space and Time Correlations

Hollins

Sarson

Shukurov

et al. 2017

ApJ

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We apply correlation analysis to random fields in numerical simulations of the supernova-driven interstellar medium (ISM) with the magnetic field produced by dynamo action. We solve the magnetohydrodynamic (MHD) equations in a shearing Cartesian box representing a local region of the ISM, subject to thermal and kinetic energy injection by supernova explosions, and parameterized, optically thin radiative cooling. We consider the cold, warm, and hot phases of the ISM separately; the analysis mostly considers the warm gas, which occupies the bulk of the domain. Various physical variables have different correlation lengths in the warm phase: 40, 50, and 60 pc for the random magnetic field, density, and velocity, respectively, in the midplane. The correlation time of the random velocity is comparable to the eddy turnover time, about 10 year 7 , although it may be shorter in regions with a higher star formation rate. The random magnetic field is anisotropic, with the standard deviations of its components b b b x y z having approximate ratios 0.5 0.6 0.6 in the midplane. The anisotropy is attributed to the global velocity shear from galactic differential rotation and locally inhomogeneous outflow to the galactic halo. The correlation length of Faraday depth along the z axis, 120 pc, is greater than for electron density, 60 90 pc -, and the vertical magnetic field, 60 pc. Such comparisons may be sensitive to the orientation of the line of sight. Uncertainties of the structure functions of synchrotron intensity rapidly increase with the scale. This feature is hidden in a power spectrum analysis, which can undermine the usefulness of power spectra for detailed studies of interstellar turbulence.

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The supernova-regulated ISM – II. The mean magnetic field

Cited by 92 publications

References 8 publications

Topological signatures of interstellar magnetic fields – I. Betti numbers and persistence diagrams

Topological signatures of interstellar magnetic fields – I. Betti numbers and persistence diagrams

Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency radio polarization study

Supernova-regulated ISM. V. Space and Time Correlations

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