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Publications (5 of 5) Show all publications
Zhou, H. & Blackman, E. G. (2024). Helical dynamo growth and saturation at modest versus extreme magnetic Reynolds numbers. Physical review. E, 109(1), Article ID 015206.
Open this publication in new window or tab >>Helical dynamo growth and saturation at modest versus extreme magnetic Reynolds numbers
2024 (English)In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 109, no 1, article id 015206Article in journal (Refereed) Published
Abstract [en]

Understanding magnetic field growth in astrophysical objects is a persistent challenge. In stars and galaxies, turbulent flows with net kinetic helicity are believed to be responsible for driving large-scale magnetic fields. However, numerical simulations have demonstrated that such helical dynamos in closed volumes saturate at lower magnetic field strengths when increasing the magnetic Reynolds number Rm. This would imply that helical large-scale dynamos cannot be efficient in astrophysical bodies without the help of helicity outflows such as stellar winds. But do these implications actually apply for very large Rm? Here we tackle the long-standing question of how much helical large-scale dynamo growth occurs independent of Rm in a closed volume. We analyze data from numerical simulations with a new method that tracks resistive versus nonresistive drivers of helical field growth. We identify a presaturation regime when the large-scale field grows at a rate independent of Rm, but to an Rm-dependent magnitude. The latter Rm dependence is due to a dominant resistive contribution, but whose fractional contribution to the large-scale magnetic energy decreases with increasing Rm. We argue that the resistive contribution would become negligible at large Rm and an Rm-independent dynamical contribution would dominate if the current helicity spectrum in the inertial range is steeper than k0. As such helicity spectra are plausible, this renews optimism for the relevance of closed dynamos. Our work pinpoints how modest Rm simulations can cause misapprehension of the Rm→∞ behavior.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-226914 (URN)10.1103/PhysRevE.109.015206 (DOI)001156853200003 ()38366478 (PubMedID)2-s2.0-85183455359 (Scopus ID)
Available from: 2024-03-04 Created: 2024-03-04 Last updated: 2024-03-04Bibliographically approved
Brandenburg, A., Zhou, H. & Sharma, R. (2023). Batchelor, Saffman, and Kazantsev spectra in galactic small-scale dynamos. Monthly notices of the Royal Astronomical Society, 518(3), 3312-3325
Open this publication in new window or tab >>Batchelor, Saffman, and Kazantsev spectra in galactic small-scale dynamos
2023 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 518, no 3, p. 3312-3325Article in journal (Refereed) Published
Abstract [en]

The magnetic fields in galaxy clusters and probably also in the interstellar medium are believed to be generated by a small-scale dynamo. Theoretically, during its kinematic stage, it is characterized by a Kazantsev spectrum, which peaks at the resistive scale. It is only slightly shallower than the Saffman spectrum that is expected for random and causally connected magnetic fields. Causally disconnected fields have the even steeper Batchelor spectrum. Here, we show that all three spectra are present in the small-scale dynamo. During the kinematic stage, the Batchelor spectrum occurs on scales larger than the energy-carrying scale of the turbulence, and the Kazantsev spectrum on smaller scales within the inertial range of the turbulence – even for a magnetic Prandtl number of unity. In the saturated state, the dynamo develops a Saffman spectrum on large scales, suggestive of the build-up of long-range correlations. At large magnetic Prandtl numbers, elongated structures are seen in synthetic synchrotron emission maps showing the parity-even E polarization. We also observe a significant excess in the E polarization over the parity-odd B polarization at subresistive scales, and a deficiency at larger scales. This finding is at odds with the observed excess in the Galactic microwave foreground emission, which is believed to be associated with larger scales. The E and B polarizations may be highly non-Gaussian and skewed in the kinematic regime of the dynamo. For dust emission, however, the polarized emission is always nearly Gaussian, and the excess in the E polarization is much weaker. 

Keywords
dynamo, MHD, polarization, turbulence, galaxies: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-215120 (URN)10.1093/mnras/stac3217 (DOI)000921145500009 ()
Available from: 2023-03-03 Created: 2023-03-03 Last updated: 2023-03-03Bibliographically approved
Zhou, H., Sharma, R. & Brandenburg, A. (2022). Scaling of the Hosking integral in decaying magnetically dominated turbulence. Journal of Plasma Physics, 88(6), Article ID 905880602.
Open this publication in new window or tab >>Scaling of the Hosking integral in decaying magnetically dominated turbulence
2022 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 88, no 6, article id 905880602Article in journal (Refereed) Published
Abstract [en]

The Saffman helicity invariant of Hosking & Schekochihin (Phys. Rev. X, vol. 11, issue 4, 2021, 041005), which we here call the Hosking integral, has emerged as an important quantity that may govern the decay properties of magnetically dominated non-helical turbulence. Using a range of different computational methods, we confirm that this quantity is indeed gauge invariant and nearly perfectly conserved in the limit of large Lundquist numbers. For direct numerical simulations with ordinary viscosity and magnetic diffusivity operators, we find that the solution develops in a nearly self-similar fashion. In a diagram quantifying the instantaneous decay coefficients of magnetic energy and integral scale, we find that the solution evolves along a line that is indeed suggestive of the governing role of the Hosking integral. The solution settles near a line in this diagram that is expected for a self-similar evolution of the magnetic energy spectrum. The solution will settle in a slightly different position when the magnetic diffusivity decreases with time, which would be compatible with the decay being governed by the reconnection time scale rather than the Alfven time.

Keywords
astrophysical plasmas, plasma simulation, plasma nonlinear phenomena
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-212436 (URN)10.1017/S002237782200109X (DOI)000880189000001 ()2-s2.0-85141916273 (Scopus ID)
Available from: 2022-12-13 Created: 2022-12-13 Last updated: 2022-12-13Bibliographically approved
Zhou, H. & Blackman, E. G. (2021). Influence of inhomogeneous stochasticity on the falsifiability of mean-field theories and examples from accretion disc modelling. Monthly notices of the Royal Astronomical Society, 507(2), 2735-2743
Open this publication in new window or tab >>Influence of inhomogeneous stochasticity on the falsifiability of mean-field theories and examples from accretion disc modelling
2021 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 507, no 2, p. 2735-2743Article in journal (Refereed) Published
Abstract [en]

Despite spatial and temporal fluctuations in turbulent astrophysical systems, mean-field theories can be used to describe their secular evolution. However, observations taken over time scales much shorter than dynamical time scales capture a system in a single state of its turbulence ensemble. Comparing with mean-field theory can falsify the latter only if the theory is additionally supplied with a quantified precision. The central limit theorem provides appropriate estimates to the precision only when fluctuations contribute linearly to an observable and with constant coherent scales. Here, we introduce an error propagation formula that relaxes both limitations, allowing for non-linear functional forms of observables and inhomogeneous coherent scales and amplitudes of fluctuations. The method is exemplified in the context of accretion disc theories, where inhomogeneous fluctuations in the surface temperature are propagated to the disc emission spectrum - the latter being a non-linear and non-local function of the former. The derived precision depends non-monotonically on emission frequency. Using the same method, we investigate how binned spectral fluctuations in telescope data change with the spectral resolving power. We discuss the broader implications for falsifiability of a mean-field theory.

Keywords
accretion, accretion discs, turbulence, methods: analytical, protoplanetary discs
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-198572 (URN)10.1093/mnras/stab2403 (DOI)000697380800077 ()
Available from: 2021-11-11 Created: 2021-11-11 Last updated: 2022-02-25Bibliographically approved
Zhou, H. & Blackman, E. G. (2021). On the shear-current effect: toward understanding why theories and simulations have mutually and separately conflicted. Monthly notices of the Royal Astronomical Society, 507(4), 5732-5746
Open this publication in new window or tab >>On the shear-current effect: toward understanding why theories and simulations have mutually and separately conflicted
2021 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 507, no 4, p. 5732-5746Article in journal (Refereed) Published
Abstract [en]

The shear-current effect (SCE) of mean-field dynamo theory refers to the combination of a shear flow and a turbulent coefficient β21 with a favourable negative sign for exponential mean-field growth, rather than positive for diffusion. There have been long-standing disagreements among theoretical calculations and comparisons of theory with numerical experiments as to the sign of kinetic (⁠βu21⁠) and magnetic (⁠βb21⁠) contributions. To resolve these discrepancies, we combine an analytical approach with simulations, and show that unlike βb21⁠, the kinetic SCE βu21 has a strong dependence on the kinetic energy spectral index and can transit from positive to negative values at O(10) Reynolds numbers if the spectrum is not too steep. Conversely, βb21 is always negative regardless of the spectral index and Reynolds numbers. For very steep energy spectra, the positive βu21 can dominate even at energy equipartition urms ≃ brms, resulting in a positive total β21 even though βb21<0⁠. Our findings bridge the gap between the seemingly contradictory results from the second-order-correlation approximation versus the spectral-τ closure, for which opposite signs for βu21 have been reported, with the same sign for βb21<0⁠. The results also offer an explanation for the simulations that find βu21>0 and an inconclusive overall sign of β21 for O(10) Reynolds numbers. The transient behaviour of βu21 is demonstrated using the kinematic test-field method. We compute dynamo growth rates for cases with or without rotation, and discuss opportunities for further work.

Keywords
dynamo, magnetic fields, MHD, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-198645 (URN)10.1093/mnras/stab2469 (DOI)000702151300074 ()2-s2.0-85116972587 (Scopus ID)
Available from: 2021-11-13 Created: 2021-11-13 Last updated: 2022-05-03Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-2991-5306

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