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Using magnetohydrodynamic simulations of Fluctuation dynamos in turbulent flows with rms Mach numbers $\mathcal{M} \approx 0.2, 1.1$ and $3$, we show that magnetic pressure forces play a crucial role in dynamo saturation in supersonic flows. Firstly, as expected when pressure forces oppose compression, an increase in anti-correlation between density and magnetic field strengths obtains even in subsonic flows with the anti-correlation arising from the intense but rarer magnetic structures. In supersonic flows, due to stronger compressive motions density and magnetic field strength continue to maintain a positive correlation. However, the degree of positive correlation decreases as the dynamo saturates. Secondly, we find that the unit vectors of $\nabla\rho$ and $\nabla B^{2}$ are preferentially anti-parallel to each other in subsonic flows. This is indicative of magnetic pressure opposing compression. This anti-parallel alignment persists in transonic and supersonic flows at dynamo saturation. However, compressive motions also lead to the emergence of a parallel alignment in these flows. Finally, we consider the work done against the components of the Lorentz force and the different sources of magnetic energy growth and dissipation. We show that while in subsonic flows, suppression of field line stretching is dominant in saturating the dynamo, the picture is different in supersonic flows. Both field line stretching and compression amplifies the field initially. But the growing magnetic pressure opposes further compression of magnetic flux which then dominates the saturation of the dynamo., Comment: Submitted to MNRAS

Using magnetohydrodynamic simulations of fluctuation dynamos, we perform broad-bandwidth synthetic observations to investigate the properties of polarized synchrotron emission and the role that Faraday rotation plays in inferring the polarized structures in the intracluster medium (ICM) of galaxy clusters. In the saturated state of the dynamo, we find a Faraday depth (FD) dispersion $\sigma_{\rm FD} \approx 100$ rad m$^{-2}$, in agreement with observed values in the ICM. Remarkably, the FD power spectrum is qualitatively similar to $M(k)/k$, where $M(k)$ is the magnetic spectrum and $k$ the wavenumber. However, this similarity is broken at high $k$ when FD is obtained by applying RM synthesis to polarized emission from the ICM due to poor resolution and complexities of spectrum in FD space. Unlike the Gaussian probability distribution function (PDF) obtained for FD, the PDF of the synchrotron intensity is lognormal. A relatively large $\sigma_{\rm FD}$ in the ICM gives rise to strong frequency-dependent variations of the pixel-wise mean and peak polarized intensities at low frequencies ($\lesssim 1.5\,{\rm GHz}$). The mean fractional polarization $\langle p \rangle$ obtained at the resolution of the simulations increases from $, Comment: Published in MNRAS

The Square Kilometre Array (SKA) will answer fundamental questions about the origin, evolution, properties, and influence of magnetic fields throughout the Universe. Magnetic fields can illuminate and influence phenomena as diverse as star formation, galactic dynamics, fast radio bursts, active galactic nuclei, large-scale structure, and dark matter annihilation. Preparations for the SKA are swiftly continuing worldwide, and the community is making tremendous observational progress in the field of cosmic magnetism using data from a powerful international suite of SKA pathfinder and precursor telescopes. In this contribution, we revisit community plans for magnetism research using the SKA, in light of these recent rapid developments. We focus in particular on the impact that new radio telescope instrumentation is generating, thus advancing our understanding of key SKA magnetism science areas, as well as the new techniques that are required for processing and interpreting the data. We discuss these recent developments in the context of the ultimate scientific goals for the SKA era., The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. B.M.G. and J.L.W. acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through Grant RGPIN-2015-05948 and of the Canada Research Chairs program. I.A. acknowledges support by a Ramón y Cajal grant (RYC-2013-14511) of the “Ministerio de Ciencia, Innovación, y Universidades (MICIU)” of Spain, financial support from MCIU through Grant AYA2016-80889-P, and through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía-CSIC (SEV-2017-0709). A.B [asu]. acknowledges financial support by the German Federal Ministry of Education and Research (BMBF) under Grant 05A17PB1 (Verbundprojekt D-MeerKAT). A.B. (Andrea Bracco) acknowledges the support of the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 843008 (MUSICA). M.H. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 772663). M.P. acknowledges funding from the INAF PRIN-SKA 2017 program 1.05.01.88.04. F.P. acknowledges support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN) under Grant Numbers ESP2015-65597-C4-4-R and ESP2017-86852-C4-2-R. M.R. acknowledges the “Departments of Excellence 2018-2022” grant awarded by the Italian Ministry of Education, University and Research (MIUR) L. 232/2016; Research grant TAsP (Theoretical Astroparticle Physics) funded INFN; Research grant “Deciphering the high-energy sky via cross correlation” funded by the agreement ASI-INAFn. 2017-14-H.0; research grant “From Darklight to Dark Matter: understanding the galaxy/matter connection to measure the Universe” No. 20179P3PKJ funded by MIUR. C.J.R. acknowledges financial support from the ERC Starting Grant “DRANOEL”, Number 714245. Partial support for L.R. comes from U.S. National Science Foundation Grant AST17-14205 to the University of Minnesota. J.M.S. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC), 2019-04848. X.S. is supported by the National Key R&D Program of China (2018YFA0404602). S.S. acknowledges support from the Science and Engineering Research Board (SERB) of the Department of Science and Technology (DST), Govt. of India, through Research Grant ECR/2017/001535. F.V. acknowledges financial support of the H2020 initiative through the ERC Starting Grant MAGCOW(714196).

We study the influence of initial conditions on the magnetic field amplification during the collapse of a magnetised gas cloud. We focus on the dependence of the growth and saturation level of the dynamo generated field on the turbulent properties of the collapsing cloud. In particular, we explore the effect of varying the initial strength and injection scale of turbulence and the initial uniform rotation of the collapsing magnetised cloud. In order to follow the evolution of the magnetic field in both the kinematic and the nonlinear regime, we choose an initial field strength of $\simeq 1\,\mkG$ with the magnetic to kinetic energy ratio, $E_{\rm m}/E_{\rm k} \sim 10^{-4}$. Both gravitational compression and the small-scale dynamo initially amplify the magnetic field. Further into the evolution, the dynamo-generated magnetic field saturates but the total magnetic field continues to grow because of compression. The saturation of the small-scale dynamo is marked by a change in the slope of $B/\rho^{2/3}$ and by a shift in the peak of the magnetic energy spectrum from small scales to larger scales. For the range of initial Mach numbers explored in this study, the dynamo growth rate increases as the Mach number increases from $v_{\rm rms}/c_{\rm s}\sim 0.2$ to 0.4 and then starts decreasing from $v_{\rm rms}/c_{\rm s}\sim 1.0$. We obtain saturation values of $E_{\rm m}/E_{\rm k} = 0.2 - 0.3$ for these runs. Simulations with different initial injection scales of turbulence also show saturation at similar levels. For runs with different initial rotation of the cloud, the magnetic energy saturates at $E_{\rm m}/E_{\rm k}\sim 0.2 - 0.4$ of the equipartition value. (Abridged), Comment: submitted to MNRAS, reviewer's comments addressed

Recent FERMI observations provide a lower limit of 10^{-15} G for the magnetic field strength in the intergalactic medium (IGM). This is consistent with theoretical expectations based on the Biermann battery effect, which predicts such IGM fields already at redshifts z~10. During gravitational collapse, such magnetic fields can be amplified by compression and by turbulence, giving rise to the small-scale dynamo. On scales below the Jeans length, the eddy turnover timescale is much shorter than the free-fall timescale, so that saturation can be reached during collapse. This scenario has been tested and confirmed with magneto-hydrodynamical simulations following the collapse of a turbulent, weakly magnetized cloud. Based on a spectral analysis, we confirm that turbulence is injected on the Jeans scale. For the power spectrum of the magnetic field, we obtain the Kazantsev slope which is characteristic for the small-scale dynamo. A calculation of the critical length scales for ambipolar diffusion and Ohmic dissipation shows that these scales are always small enough to allow significant amplification of the magnetic field by small-scale eddies. We discuss potential implications for the protostellar accretion disk, with particular focus on the magneto-rotational instability, which may change the morphology of the disk and reduce the accretion rate by a factor of a few., 4 pages, 2 figures, conference proceedings from "Cosmic Radiation Fields: Sources in the early Universe", 2010, Hamburg, Germany