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Brandenburg, A., Käpylä, P. J., Rogachevskii, I. & Yokoi, N. (2025). Helicity Effect on Turbulent Passive and Active Scalar Diffusivities. Astrophysical Journal, 984(1), Article ID 88.
Open this publication in new window or tab >>Helicity Effect on Turbulent Passive and Active Scalar Diffusivities
2025 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 984, no 1, article id 88Article in journal (Refereed) Published
Abstract [en]

Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various τ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate.

Place, publisher, year, edition, pages
American Astronomical Society, 2025
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-363455 (URN)10.3847/1538-4357/adc691 (DOI)001479281700001 ()2-s2.0-105004203942 (Scopus ID)
Note

QC 20250519

Available from: 2025-05-15 Created: 2025-05-15 Last updated: 2025-05-19Bibliographically approved
Neronov, A., Vazza, F., Brandenburg, A. & Caprini, C. (2025). Intergalactic magnetism in a γ -ray beam as a model of Porphyrion. Astronomy and Astrophysics, 696, Article ID L8.
Open this publication in new window or tab >>Intergalactic magnetism in a γ -ray beam as a model of Porphyrion
2025 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 696, article id L8Article in journal (Refereed) Published
Abstract [en]

We estimate the magnetic field in the jets of the recently discovered 7 Mpc long Porphyrion system. We used nondetection of the system in gamma-rays to derive a lower bound on the co-moving magnetic field strength at the level of 10 nG (comoving). This value is consistent with recent estimates of magnetic fields in the filaments of the large-scale structure.We discuss the possibility that instead of being the extreme case of a radio jet formation scenario, Porphyrion actually traces a very high-energy -ray beam emitted by an active galactic nucleus. In such a model, jets do not need to spread into the voids of the large-scale structure to appear straight on a very large distance range, and several anomalies of the standard radio jet scenarios can be solved at once.

Place, publisher, year, edition, pages
EDP Sciences, 2025
Keywords
Galaxies: active, Galaxies: jets, Gamma rays: galaxies, Large-scale structure of Universe, Magnetic fields, Radio lines: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-362528 (URN)10.1051/0004-6361/202453099 (DOI)2-s2.0-105002282104 (Scopus ID)
Note

QC 20250428

Available from: 2025-04-16 Created: 2025-04-16 Last updated: 2025-04-28Bibliographically approved
Sharma, R., Brandenburg, A., Subramanian, K. & Vikman, A. (2025). Lattice simulations of axion-U(1) inflation: gravitational waves, magnetic fields, and scalar statistics. Journal of Cosmology and Astroparticle Physics, 2025(5), Article ID 079.
Open this publication in new window or tab >>Lattice simulations of axion-U(1) inflation: gravitational waves, magnetic fields, and scalar statistics
2025 (English)In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, Vol. 2025, no 5, article id 079Article in journal (Refereed) Published
Abstract [en]

We numerically study axion-U(1) inflation, focusing on the regime where the coupling between axions and gauge fields results in significant backreaction from the amplified gauge fields during inflation. These amplified gauge fields not only generate high-frequency gravitational waves (GWs), but also enhance spatial inhomogeneities in the axion field. GWs serve as key probe for constraining the coupling strength between the axion and gauge fields. We find that, when backreaction is important during inflation, the constraints on the coupling strength due to GW overproduction are relaxed compared to previous studies, in which backreaction matters only after inflation. Moreover, our results suggest that the probability density function (PDF) of axion fluctuations tends toward a Gaussian distribution even in cases where gauge field backreaction is important only after inflation. This aligns with previous studies where the same effect was observed for cases with strong backreaction during inflation. This finding can be crucial for future studies of primordial black hole (PBH) formation, which can further constrain the coupling strength. We also calculate the spectrum of the produced magnetic fields in this model and find that their strength is compatible with the observed lower limits.

Place, publisher, year, edition, pages
IOP Publishing, 2025
Keywords
inflation, physics of the early universe, primordial black holes, primordial magnetic fields
National Category
Astronomy, Astrophysics and Cosmology Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-364147 (URN)10.1088/1475-7516/2025/05/079 (DOI)2-s2.0-105005857873 (Scopus ID)
Note

QC 20250604

Available from: 2025-06-04 Created: 2025-06-04 Last updated: 2025-06-04Bibliographically approved
Brandenburg, A. & Scannapieco, E. (2025). Magnetically Assisted Vorticity Production in Decaying Acoustic Turbulence. Astrophysical Journal, 983(2), Article ID 105.
Open this publication in new window or tab >>Magnetically Assisted Vorticity Production in Decaying Acoustic Turbulence
2025 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 983, no 2, article id 105Article in journal (Refereed) Published
Abstract [en]

We study vorticity production in isothermal, subsonic, acoustic (nonvortical), and decaying turbulence due to the presence of magnetic fields. Using three-dimensional numerical simulations, we find that the resulting kinetic energy cascade follows the ordinary Kolmogorov phenomenology involving a constant spectral energy flux. The nondimensional prefactor for acoustic turbulence is larger than the standard Kolmogorov constant due to the inefficient dissipation of kinetic energy. We also find that the Lorentz force can drive vortical motions even when the initial field is uniform by converting a fraction of the acoustic energy into vortical energy. This conversion is shown to be quadratic in the magnetic field strength and linear in the acoustic flow speed. By contrast, the direct production of vortical motions by a non-force-free magnetic field is linear in the field strength. Our results suggest that magnetic fields play a crucial role in vorticity production in cosmological flows, particularly in scenarios where significant acoustic turbulence is prevalent. We also discuss the implications of our findings for the early Universe, where magnetic fields may convert acoustic turbulence generated during cosmological phase transitions into vortical turbulence.

Place, publisher, year, edition, pages
American Astronomical Society, 2025
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-363867 (URN)10.3847/1538-4357/adbe38 (DOI)001464713700001 ()2-s2.0-105002797365 (Scopus ID)
Note

QC 20250526

Available from: 2025-05-26 Created: 2025-05-26 Last updated: 2025-05-26Bibliographically approved
Dehman, C. & Brandenburg, A. (2025). Reality of inverse cascading in neutron star crusts. Astronomy and Astrophysics, 694, Article ID A39.
Open this publication in new window or tab >>Reality of inverse cascading in neutron star crusts
2025 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 694, article id A39Article in journal (Refereed) Published
Abstract [en]

The braking torque that dictates the timing properties of magnetars is closely tied to the large-scale dipolar magnetic field on their surface. The formation of this field has been a topic of ongoing debate. One proposed mechanism, based on macroscopic principles, involves an inverse cascade within the neutron star's crust. However, this phenomenon has not been observed in realistic simulations. In this study, we provide compelling evidence supporting the feasibility of the inverse cascading process in the presence of an initial helical magnetic field within realistic neutron star crusts and discuss its contribution to the amplification of the large-scale magnetic field. Our findings, derived from a systematic investigation that considers various coordinate systems, peak wavenumber positions, crustal thicknesses, magnetic boundary conditions, and magnetic Lundquist numbers, reveal that the specific geometry of the crustal domain-with its extreme aspect ratio-requires an initial peak wavenumber from small-scale structures for the inverse cascade to occur. However, this same aspect ratio confines the cascade to structures on the scale of the crust, making the formation of a large-scale dipolar surface field unlikely. Despite these limitations, the inverse cascade remains a significant factor in the magnetic field evolution within the crust and may help explain highly magnetized objects with weak surface dipolar fields, such as low-field magnetars and central compact objects.

Place, publisher, year, edition, pages
EDP Sciences, 2025
Keywords
magnetic fields, stars: evolution, stars: interiors, stars: magnetars, stars: magnetic field, stars: neutron
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-360051 (URN)10.1051/0004-6361/202451904 (DOI)001411854100006 ()2-s2.0-85217023483 (Scopus ID)
Note

QC 20250226

Available from: 2025-02-17 Created: 2025-02-17 Last updated: 2025-02-26Bibliographically approved
Vachaspati, T. & Brandenburg, A. (2025). Spectra of magnetic fields from electroweak symmetry breaking. Physical Review D: covering particles, fields, gravitation, and cosmology, 111(4), Article ID 043541.
Open this publication in new window or tab >>Spectra of magnetic fields from electroweak symmetry breaking
2025 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 111, no 4, article id 043541Article in journal (Refereed) Published
Abstract [en]

We characterize magnetic fields produced during electroweak symmetry breaking by nondynamical numerical simulations based on the Kibble mechanism. The generated magnetic fields were thought to have an energy spectrum k3 for small wave numbers k, but here we show that it is actually a spectrum k4 along with characteristic fluctuations in the magnetic helicity. Using scaling results from magnetohydrodynamics simulations for the evolution and assuming that the initial magnetic field is coherent on the electroweak Hubble scale, we estimate the magnetic field strength to be ∼10-13 G on kpc scales at the present epoch for nonhelical fields. For maximally helical fields we obtain ∼10-10 G on Mpc scales. We also give scalings of these estimates for partially helical fields.

Place, publisher, year, edition, pages
American Physical Society (APS), 2025
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-361170 (URN)10.1103/PhysRevD.111.043541 (DOI)001435471700017 ()2-s2.0-85219029103 (Scopus ID)
Note

QC 20250317

Available from: 2025-03-12 Created: 2025-03-12 Last updated: 2025-03-17Bibliographically approved
Rogachevskii, I., Kleeorin, N. & Brandenburg, A. (2025). Theory of the Kinetic Helicity Effect on Turbulent Diffusion of Magnetic and Scalar Fields. Astrophysical Journal, 985(1), Article ID 18.
Open this publication in new window or tab >>Theory of the Kinetic Helicity Effect on Turbulent Diffusion of Magnetic and Scalar Fields
2025 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 985, no 1, article id 18Article in journal (Refereed) Published
Abstract [en]

Kinetic helicity is a fundamental characteristic of astrophysical turbulent flows. It is not only responsible for the generation of large-scale magnetic fields in the Sun, stars, and spiral galaxies, but it also affects turbulent diffusion, resulting in the dissipation of large-scale magnetic fields. Using the path integral approach for random helical velocity fields with a finite correlation time and large Reynolds numbers, we show that turbulent magnetic diffusion is reduced by the kinetic helicity, while the turbulent diffusivity of a passive scalar is enhanced by the helicity. The latter can explain the results of recent numerical simulations for forced helical turbulence. One of the crucial reasons for the difference between the kinetic helicity effect on magnetic and scalar fields is related to the helicity dependence of the correlation time of a turbulent velocity field.

Place, publisher, year, edition, pages
American Astronomical Society, 2025
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-364001 (URN)10.3847/1538-4357/adcec0 (DOI)001487316400001 ()2-s2.0-105005166820 (Scopus ID)
Note

QC 20250603

Available from: 2025-06-02 Created: 2025-06-02 Last updated: 2025-06-03Bibliographically approved
Brandenburg, A. & Banerjee, A. (2025). Turbulent magnetic decay controlled by two conserved quantities. Journal of Plasma Physics, 91(1), Article ID E5.
Open this publication in new window or tab >>Turbulent magnetic decay controlled by two conserved quantities
2025 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 91, no 1, article id E5Article in journal (Refereed) Published
Abstract [en]

The decay of a turbulent magnetic field is slower with helicity than without. Furthermore, the magnetic correlation length grows faster for a helical than a non-helical field. Both helical and non-helical decay laws involve conserved quantities: the mean magnetic helicity density and the Hosking integral. Using direct numerical simulations in a triply periodic domain, we show quantitatively that in the fractionally helical case the mean magnetic energy density and correlation length are approximately given by the maximum of the values for the purely helical and purely non-helical cases. The time of switchover from one to the other decay law can be obtained on dimensional grounds and is approximately given by $I_{H}<^>{1/2}I_{M}<^>{-3/2}$, where $I_{H}$ is the Hosking integral and $I_{M}$ is the mean magnetic helicity density. An earlier approach based on the decay time is found to agree with our new result and suggests that the Hosking integral exceeds naive estimates by the square of the same resistivity-dependent factor by which also the turbulent decay time exceeds the Alfv & eacute;n time. In the presence of an applied magnetic field, the mean magnetic helicity density is known to be not conserved, and we show that then also the Hosking integral is not conserved.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2025
Keywords
astrophysical plasmas, plasma simulation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-358760 (URN)10.1017/S0022377824001508 (DOI)001390487900001 ()2-s2.0-85216378771 (Scopus ID)
Note

QC 20250121

Available from: 2025-01-21 Created: 2025-01-21 Last updated: 2025-02-06Bibliographically approved
Iarygina, O., Sfakianakis, E. I., Sharma, R. & Brandenburg, A. (2024). Backreaction of axion-SU(2) dynamics during inflation. Journal of Cosmology and Astroparticle Physics, 2024(4), Article ID 018.
Open this publication in new window or tab >>Backreaction of axion-SU(2) dynamics during inflation
2024 (English)In: Journal of Cosmology and Astroparticle Physics, E-ISSN 1475-7516, Vol. 2024, no 4, article id 018Article in journal (Refereed) Published
Abstract [en]

We consider the effects of backreaction on axion-SU(2) dynamics during inflation. We use the linear evolution equations for the gauge field modes and compute their backreaction on the background quantities numerically using the Hartree approximation. We show that the spectator chromo-natural inflation attractor is unstable when back-reaction becomes important. Working within the constraints of the linear mode equations, we find a new dynamical attractor solution for the axion field and the vacuum expectation value of the gauge field, where the latter has an opposite sign with respect to the chromo-natural inflation solution. Our findings are of particular interest to the phenomenology of axion-SU(2) inflation, as they demonstrate the instability of the usual trajectory due to large backreaction effects. The viable parameter space of the model becomes significantly altered, provided future non-Abelian lattice simulations confirm the existence of the new dynamical attractor. In addition, the backreaction effects lead to characteristic oscillatory features in the primordial gravitational wave background that are potentially detectable with upcoming gravitational wave detectors.

Place, publisher, year, edition, pages
IOP Publishing, 2024
Keywords
axions, inflation, physics of the early universe, primordial gravitational waves (theory)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-345703 (URN)10.1088/1475-7516/2024/04/018 (DOI)001199520600004 ()2-s2.0-85189880274 (Scopus ID)
Note

QC 20240418

Available from: 2024-04-18 Created: 2024-04-18 Last updated: 2024-05-13Bibliographically approved
Schober, J., Rogachevskii, I. & Brandenburg, A. (2024). Chiral Anomaly and Dynamos from Inhomogeneous Chemical Potential Fluctuations. Physical Review Letters, 132(6), Article ID 065101.
Open this publication in new window or tab >>Chiral Anomaly and Dynamos from Inhomogeneous Chemical Potential Fluctuations
2024 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 132, no 6, article id 065101Article in journal (Refereed) Published
Abstract [en]

In the standard model of particle physics, the chiral anomaly can occur in relativistic plasmas and plays a role in the early Universe, protoneutron stars, heavy-ion collisions, and quantum materials. It gives rise to a magnetic instability if the number densities of left- and right-handed electrically charged fermions are unequal. Using direct numerical simulations, we show this can result just from spatial fluctuations of the chemical potential, causing a chiral dynamo instability, magnetically driven turbulence, and ultimately a large-scale magnetic field through the magnetic α effect.

Place, publisher, year, edition, pages
American Physical Society (APS), 2024
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-343467 (URN)10.1103/PhysRevLett.132.065101 (DOI)38394574 (PubMedID)2-s2.0-85184149712 (Scopus ID)
Note

QC 20240215

Available from: 2024-02-15 Created: 2024-02-15 Last updated: 2024-08-28Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-7304-021X

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