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Publications (10 of 252) Show all publications
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
Brandenburg, A., Kamada, K., Mukaida, K., Schmitz, K. & Schober, J. (2023). Chiral magnetohydrodynamics with zero total chirality. Physical Review D: covering particles, fields, gravitation, and cosmology, 108(6), Article ID 063529.
Open this publication in new window or tab >>Chiral magnetohydrodynamics with zero total chirality
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2023 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 108, no 6, article id 063529Article in journal (Refereed) Published
Abstract [en]

We study the evolution of magnetic fields coupled with chiral fermion asymmetry in the framework of chiral magnetohydrodynamics with zero initial total chirality. The initial magnetic field has a turbulent spectrum peaking at a certain characteristic scale and is fully helical with positive helicity. The initial chiral chemical potential is spatially uniform and negative. We consider two opposite cases where the ratio of the length scale of the chiral plasma instability (CPI) to the characteristic scale of the turbulence is smaller and larger than unity. These initial conditions might be realized in cosmological models, including certain types of axion inflation. The magnetic field and chiral chemical potential evolve with inverse cascading in such a way that the magnetic helicity and chirality cancel each other at all times, provided there is no spin flipping. The CPI timescale is found to determine mainly the time when the magnetic helicity spectrum attains negative values at high wave numbers. The turnover time of the energy-carrying eddies, on the other hand, determines the time when the peak of the spectrum starts to shift to smaller wave numbers via an inverse cascade. The onset of helicity decay is determined by the time when the chiral magnetic effect becomes efficient at the peak of the initial magnetic energy spectrum, provided the CPI does not grow much. When spin flipping is important, the chiral chemical potential vanishes at late times and the magnetic helicity becomes constant, which leads to a faster increase of the correlation length. This is in agreement with what is expected from magnetic helicity conservation and also happens when the initial total chirality is imbalanced. Our findings have important implications for baryogenesis after axion inflation.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-224276 (URN)10.1103/PhysRevD.108.063529 (DOI)001093991600003 ()2-s2.0-85175076493 (Scopus ID)
Available from: 2023-12-18 Created: 2023-12-18 Last updated: 2023-12-18Bibliographically approved
Sarin, N., Brandenburg, A. & Haskell, B. (2023). Confronting the Neutron Star Population with Inverse Cascades. Astrophysical Journal Letters, 952(1), Article ID L21.
Open this publication in new window or tab >>Confronting the Neutron Star Population with Inverse Cascades
2023 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 952, no 1, article id L21Article in journal (Refereed) Published
Abstract [en]

The origin and evolution of magnetic fields of neutron stars from birth have long been a source of debate. Here, motivated by recent simulations of the Hall cascade with magnetic helicity, we invoke a model where the large-scale magnetic field of neutron stars grows as a product of small-scale turbulence through an inverse cascade. We apply this model to a simulated population of neutron stars at birth and show how this model can account for the evolution of such objects across the  diagram, explaining both pulsar and magnetar observations. Under the assumption that small-scale turbulence is responsible for large-scale magnetic fields, we place a lower limit on the spherical harmonic degree of the energy-carrying magnetic eddies of ≈40. Our results favor the presence of a highly resistive pasta layer at the base of the neutron star crust. We further discuss the implications of this paradigm on direct observables, such as the nominal age and braking index of pulsars.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-221063 (URN)10.3847/2041-8213/ace363 (DOI)001036249100001 ()2-s2.0-85166239231 (Scopus ID)
Available from: 2023-09-26 Created: 2023-09-26 Last updated: 2023-09-26Bibliographically approved
Mizerski, K. A., Yokoi, N. & Brandenburg, A. (2023). Cross-helicity effect on α-type dynamo in non-equilibrium turbulence. Journal of Plasma Physics, 89(4), Article ID 905890412.
Open this publication in new window or tab >>Cross-helicity effect on α-type dynamo in non-equilibrium turbulence
2023 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 89, no 4, article id 905890412Article in journal (Refereed) Published
Abstract [en]

Turbulence is typically not in equilibrium, i.e. mean quantities such as the mean energy and helicity are typically time-dependent. The effect of non-stationarity on the turbulent hydromagnetic dynamo process is studied here with the use of the two-scale direct-interaction approximation, which allows one to explicitly relate the mean turbulent Reynolds and Maxwell stresses and the mean electromotive force to the spectral characteristics of turbulence, such as the mean energy, as well as kinetic and cross-helicity. It is demonstrated that the non-equilibrium effects can enhance the dynamo process when the magnetohydrodynamic turbulence is both helical and cross-helical. This effect is based on the turbulent infinitesimal-impulse cross-response functions, which do not affect turbulent flows in equilibrium. The evolution and sources of the cross-helicity in magnetohydrodynamic turbulence are also discussed.

Keywords
astrophysical plasmas, plasma dynamics, plasma nonlinear phenomena
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-221074 (URN)10.1017/S0022377823000545 (DOI)001043287300001 ()2-s2.0-85168806727 (Scopus ID)
Available from: 2023-09-25 Created: 2023-09-25 Last updated: 2023-09-25Bibliographically approved
Brandenburg, A., Rogachevskii, I. & Schober, J. (2023). Dissipative magnetic structures and scales in small-scale dynamos. Monthly notices of the Royal Astronomical Society, 518(4), 6367-6375
Open this publication in new window or tab >>Dissipative magnetic structures and scales in small-scale dynamos
2023 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 518, no 4, p. 6367-6375Article in journal (Refereed) Published
Abstract [en]

Small-scale dynamos play important roles in modern astrophysics, especially on galactic and extragalactic scales. Owing to dynamo action, purely hydrodynamic Kolmogorov turbulence hardly exists and is often replaced by hydromagnetic turbulence. Understanding the size of dissipative magnetic structures is important in estimating the time-scale of galactic scintillation and other observational and theoretical aspects of interstellar and intergalactic small-scale dynamos. Here we show that, during the kinematic phase of the small-scale dynamo, the cutoff wavenumber of the magnetic energy spectra scales as expected for large magnetic Prandtl numbers, but continues in the same way also for moderately small values – contrary to what is expected. For a critical magnetic Prandtl number of about 0.3, the dissipative and resistive cutoffs are found to occur at the same wavenumber. In the non-linearly saturated regime, the critical magnetic Prandtl number becomes unity. The cutoff scale now has a shallower scaling with magnetic Prandtl number below a value of about three, and a steeper one otherwise compared to the kinematic regime.

Keywords
dynamo, MHD, turbulence, galaxies: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-214545 (URN)10.1093/mnras/stac3555 (DOI)000904583300009 ()
Available from: 2023-02-10 Created: 2023-02-10 Last updated: 2023-02-10Bibliographically approved
Brandenburg, A. & Protiti, N. N. (2023). Electromagnetic Conversion into Kinetic and Thermal Energies. Entropy, 25(9), Article ID 1270.
Open this publication in new window or tab >>Electromagnetic Conversion into Kinetic and Thermal Energies
2023 (English)In: Entropy, E-ISSN 1099-4300, Vol. 25, no 9, article id 1270Article in journal (Refereed) Published
Abstract [en]

The conversion of electromagnetic energy into magnetohydrodynamic energy occurs when the electric conductivity changes from negligible to finite values. This process is relevant during the epoch of reheating in the early universe at the end of inflation and before the emergence of the radiation-dominated era. We find that the conversion into kinetic and thermal energies is primarily the result of electric energy dissipation, while magnetic energy only plays a secondary role in this process. This means that since electric energy dominates over magnetic energy during inflation and reheating, significant amounts of electric energy can be converted into magnetohydrodynamic energy when conductivity emerges before the relevant length scales become stable.

Keywords
electric energy, cosmological inflation, emergence of conductivity
National Category
Energy Engineering
Identifiers
urn:nbn:se:su:diva-223211 (URN)10.3390/e25091270 (DOI)001078629400001 ()37761569 (PubMedID)2-s2.0-85172256374 (Scopus ID)
Available from: 2023-11-06 Created: 2023-11-06 Last updated: 2023-11-06Bibliographically approved
Mtchedlidze, S., Domínguez-Fernández, P., Du, X., Schmidt, W., Brandenburg, A., Niemeyer, J. & Kahniashvili, T. (2023). Inflationary and Phase-transitional Primordial Magnetic Fields in Galaxy Clusters. Astrophysical Journal, 944(1), Article ID 100.
Open this publication in new window or tab >>Inflationary and Phase-transitional Primordial Magnetic Fields in Galaxy Clusters
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2023 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 944, no 1, article id 100Article in journal (Refereed) Published
Abstract [en]

Primordial magnetic fields (PMFs) are possible candidates for explaining the observed magnetic fields in galaxy clusters. Two competing scenarios of primordial magnetogenesis have been discussed in the literature: inflationary and phase-transitional. We study the amplification of both large- and small-scale correlated magnetic fields, corresponding to inflation- and phase transition–generated PMFs, in a massive galaxy cluster. We employ high-resolution magnetohydrodynamic cosmological zoom-in simulations to resolve the turbulent motions in the intracluster medium. We find that the turbulent amplification is more efficient for the large-scale inflationary models, while the phase transition–generated seed fields show moderate growth. The differences between the models are imprinted on the spectral characteristics of the field (such as the amplitude and the shape of the magnetic power spectrum) and therefore also on the final correlation length. We find a one order of magnitude difference between the final strengths of the inflation- and phase transition–generated magnetic fields, and a factor of 1.5 difference between their final coherence scales. Thus, the final configuration of the magnetic field retains information about the PMF generation scenarios. Our findings have implications for future extragalactic Faraday rotation surveys with the possibility of distinguishing between different magnetogenesis scenarios.

Keywords
Magnetohydrodynamical simulations, Galaxy clusters, Primordial magnetic fields, Intracluster medium
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-215791 (URN)10.3847/1538-4357/acb04d (DOI)000936676700001 ()2-s2.0-85148454827 (Scopus ID)
Available from: 2023-03-30 Created: 2023-03-30 Last updated: 2023-03-30Bibliographically approved
Brandenburg, A., Sharma, R. & Vachaspati, T. (2023). Inverse cascading for initial magnetohydrodynamic turbulence spectra between Saffman and Batchelor. Journal of Plasma Physics, 89(6), Article ID 905890606.
Open this publication in new window or tab >>Inverse cascading for initial magnetohydrodynamic turbulence spectra between Saffman and Batchelor
2023 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 89, no 6, article id 905890606Article in journal (Refereed) Published
Abstract [en]

In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density is known to increase with time at small wavenumbers k, provided the spectrum at low k is sufficiently steep. This process is called inverse cascading and occurs for an initial Batchelor spectrum, where the magnetic energy per linear wavenumber interval increases like k(4). For an initial Saffman spectrum that is proportional to k(2), however, inverse cascading has not been found in the past. We study here the case of an intermediate k(3) spectrum, which may be relevant for magnetogenesis in the early Universe during the electroweak epoch. This case is not well understood in view of the standard Taylor expansion of the magnetic energy spectrum for small k. Using high resolution MHD simulations, we show that, also in this case, there is inverse cascading with a strength just as expected from the conservation of the Hosking integral, which governs the decay of an initial Batchelor spectrum. Even for shallower k(alpha) spectra with spectral index alpha > 3/2, our simulations suggest a spectral increase at small k with time t proportional to t(4 alpha/9-2/3). The critical spectral index of alpha = 3/2 is related to the slope of the spectral envelope in the Hosking phenomenology. Our simulations with 2048(3) mesh points now suggest inverse cascading even for an initial Saffman spectrum.

Keywords
astrophysical plasmas
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-225064 (URN)10.1017/S0022377823001253 (DOI)001113341700001 ()
Available from: 2024-01-09 Created: 2024-01-09 Last updated: 2024-01-09Bibliographically approved
Carenza, P., Sharma, R., Marsh, M. C., Brandenburg, A. & Ravensburg, E. (2023). Magnetohydrodynamics predicts heavy-tailed distributions of axion-photon conversion. Physical Review D: covering particles, fields, gravitation, and cosmology, 108(10), Article ID 103029.
Open this publication in new window or tab >>Magnetohydrodynamics predicts heavy-tailed distributions of axion-photon conversion
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2023 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 108, no 10, article id 103029Article in journal (Refereed) Published
Abstract [en]

The interconversion of axionlike particles (ALPs) and photons in magnetized astrophysical environments provides a promising route to search for ALPs. The strongest limits to date on light ALPs use galaxy clusters as ALP-photon converters. However, such studies traditionally rely on simple models of the cluster magnetic fields, with the state-of-the-art being Gaussian random fields (GRFs). We present the first systematic study of ALP-photon conversion in more realistic, turbulent fields from dedicated magnetohydrodynamic (MHD) simulations, which we compare with GRF models. For GRFs, we analytically derive the distribution of conversion ratios at fixed energy and find that it follows an exponential law. We find that the MHD models agree with the exponential law for typical, small-amplitude mixings but exhibit distinctly heavy tails for rare and large mixings. We explain how non-Gaussian features, e.g., coherent structures and local spikes in the MHD magnetic field, are responsible for the heavy tail. Our results suggest that limits placed on ALPs using GRFs are robust.

National Category
Subatomic Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-225379 (URN)10.1103/PhysRevD.108.103029 (DOI)001121863200008 ()2-s2.0-85178414712 (Scopus ID)
Available from: 2024-01-19 Created: 2024-01-19 Last updated: 2024-01-19Bibliographically approved
Brandenburg, A. (2023). Quadratic growth during the COVID-19 pandemic: merging hotspots and reinfections. Journal of Physics A: Mathematical and Theoretical, 56(4), Article ID 044002.
Open this publication in new window or tab >>Quadratic growth during the COVID-19 pandemic: merging hotspots and reinfections
2023 (English)In: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 56, no 4, article id 044002Article in journal (Refereed) Published
Abstract [en]

The existence of an exponential growth phase during early stages of a pandemic is often taken for granted. However, for the 2019 novel coronavirus epidemic, the early exponential phase lasted only for about six days, while the quadratic growth prevailed for forty days until it spread to other countries and continued, again quadratically, but with a shorter time constant. Here we show that this rapid phase is followed by a subsequent slow-down where the coefficient is reduced to almost the original value at the outbreak. This can be explained by the merging of previously disconnected sites that occurred after the disease jumped (nonlocally) to a relatively small number of separated sites. Subsequent variations in the slope with continued growth can qualitatively be explained as a result of reinfections and variations in their rate. We demonstrate that the observed behavior can be described by a standard epidemiological model with spatial extent and reinfections included. Time-dependent changes in the spatial diffusion coefficient can also model corresponding variations in the slope.

Keywords
quadratic growth, SIR model, front propagation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-215879 (URN)10.1088/1751-8121/acb743 (DOI)000931784500001 ()2-s2.0-85148067420 (Scopus ID)
Available from: 2023-03-30 Created: 2023-03-30 Last updated: 2023-03-30Bibliographically approved
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