394 results on '"Brandenburg, Axel"'
Search Results
2. Introduction to Origins of Biological Homochirality
- Author
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Brandenburg, Axel and Hochberg, David
- Published
- 2022
- Full Text
- View/download PDF
3. Backreaction of axion-SU(2) dynamics during inflation.
- Author
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Iarygina, Oksana, Sfakianakis, Evangelos I., Sharma, Ramkishor, and Brandenburg, Axel
- Published
- 2024
- Full Text
- View/download PDF
4. Insulin, chemistry, structure, and function of insulin and related hormones: Proceedings of the Second International Insulin Symposium, Aachen, Germany, September 4–7, 1979
- Author
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Dietrich Brandenburg, Axel Wollmer, Dietrich Brandenburg, Axel Wollmer and Dietrich Brandenburg, Axel Wollmer, Dietrich Brandenburg, Axel Wollmer
- Published
- 2019
5. Relic Gravitational Waves from the Chiral Plasma Instability in the Standard Cosmological Model
- Author
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Brandenburg, Axel, Clarke, Emma, Kahniashvili, Tina, Long, Andrew J., and Sun, Guotong
- Subjects
High Energy Physics - Phenomenology ,Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,High Energy Physics - Phenomenology (hep-ph) ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
In the primordial plasma, at temperatures above the scale of electroweak symmetry breaking, the presence of chiral asymmetries is expected to induce the development of helical hypermagnetic fields through the phenomenon of chiral plasma instability. It results in magnetohydrodynamic turbulence due to the high conductivity and low viscosity and sources gravitational waves that survive in the universe today as a stochastic polarized gravitational wave background. In this article, we show that this scenario only relies on Standard Model physics, and therefore the observable signatures, namely the relic magnetic field and gravitational background, are linked to a single parameter controlling the initial chiral asymmetry. We estimate the magnetic field and gravitational wave spectra, and validate these estimates with 3D numerical simulations., 8 pages, 4 figures, 1 table
- Published
- 2023
6. Inverse cascading for initial MHD turbulence spectra between Saffman and Batchelor
- Author
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Brandenburg, Axel, Sharma, Ramkishor, and Vachaspati, Tanmay
- Subjects
Plasma Physics (physics.plasm-ph) ,FOS: Physical sciences ,Physics - Plasma Physics - Abstract
In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density increases with time at small wavenumbers $k$, provided the spectrum at low $k$ is sufficiently steep. This is 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 is known not to occur. 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., 15 pages, 6 figures, 3 tables, submitted to J. Plasma Physics
- Published
- 2023
7. The Limited Roles of Autocatalysis and Enantiomeric Cross-Inhibition in Achieving Homochirality in Dilute Systems
- Author
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Brandenburg, Axel
- Published
- 2019
- Full Text
- View/download PDF
8. Dynamo-Generated Turbulence and Outflows from Accretion Discs
- Author
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Brandenburg, Axel
- Published
- 2000
9. Cross-helicity effect on $\alpha$-type dynamo in non-equilibrium turbulence
- Author
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Mizerski, Krzysztof A., Yokoi, Nobumitsu, and Brandenburg, Axel
- Subjects
Physics - Fluid Dynamics ,Physics - Plasma Physics - Abstract
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 (TSDIA), which allows to explicitly relate the mean turbulent Reynolds and Maxwell stresses and the mean electromotive force (EMF) to the spectral characteristics of turbulence, such as e.g. 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 (MHD) 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 MHD turbulence is also discussed., Comment: 23 pages, 3 figures
- Published
- 2023
10. Modified propagation of gravitational waves from the early radiation era
- Author
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He, Yutong, Pol, Alberto Roper, Brandenburg, Axel, AstroParticule et Cosmologie (APC (UMR_7164)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), and HEP, INSPIRE
- Subjects
Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,gravitation: model ,energy spectrum ,FOS: Physical sciences ,General Relativity and Quantum Cosmology (gr-qc) ,General Relativity and Quantum Cosmology ,Einstein Telescope ,gravitational radiation: propagation ,general relativity ,gravitational radiation: spectrum ,energy: density ,WKB approximation ,LIGO ,numerical calculations ,pulsar ,LISA ,[PHYS.GRQC] Physics [physics]/General Relativity and Quantum Cosmology [gr-qc] ,gravitational radiation: background ,scale: Planck ,VIRGO ,frequency: low ,slope ,[PHYS.GRQC]Physics [physics]/General Relativity and Quantum Cosmology [gr-qc] ,spectral ,[PHYS.ASTR] Physics [physics]/Astrophysics [astro-ph] ,[PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph] ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
We study the propagation of cosmological gravitational wave (GW) backgrounds from the early radiation era until the present day in modified theories of gravity. Comparing to general relativity (GR), we study the effects that modified gravity parameters, such as the GW friction $\alpha_{\rm M}$ and the tensor speed excess $\alpha_{\rm T}$, have on the present-day GW spectrum. We use both the WKB estimate, which provides an analytical description but fails at superhorizon scales, and numerical simulations that allow us to go beyond the WKB approximation. We show that a constant $\alpha_{\rm T}$ makes relatively insignificant changes to the GR solution, especially taking into account the constraints on its value from GW observations by the LIGO--Virgo collaboration, while $\alpha_{\rm M}$ can introduce modifications to the spectral slopes of the GW energy spectrum in the low-frequency regime depending on the considered time evolution of $\alpha_{\rm M}$. The latter effect is additional to the damping or growth occurring equally at all scales that can be predicted by the WKB approximation. In light of the recent observations by pulsar timing array (PTA) collaborations, and the potential observations by future detectors such as SKA, LISA, DECIGO, BBO, or ET, we show that, in most of the cases, constraints cannot be placed on the effects of $\alpha_{\rm M}$ and the initial GW energy density $\mathcal{E}_{\rm GW}^*$ separately, but only on the combined effects of the two, unless the signal is observed at different frequency ranges. In particular, we provide some constraints on the combined effects from the reported PTA observations., Comment: 36 pages, 11 figures, 2 tables
- Published
- 2023
11. Electromagnetic Conversion into Kinetic and Thermal Energies.
- Author
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Brandenburg, Axel and Protiti, Nousaba Nasrin
- Subjects
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KINETIC energy , *ELECTRIC conductivity , *ENERGY conversion , *ELECTROMAGNETIC waves , *ENERGY dissipation , *INFLATIONARY universe - Abstract
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. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
12. Galactic Dynamos.
- Author
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Brandenburg, Axel and Ntormousi, Evangelia
- Abstract
Spiral galaxies, including the Milky Way, have large-scale magnetic fields with significant energy densities. The dominant theory attributes these magnetic fields to a large-scale dynamo. We review the current status of dynamo theory and discuss various numerical simulations designed either to explain particular aspects of the problem or to reproduce galactic magnetic fields globally. Our main conclusions can be summarized as follows: Idealized direct numerical simulations produce mean magnetic fields, whose saturation energy density tends to decline with increasing magnetic Reynolds number. This is still an unsolved problem. Large-scale galactic magnetic fields of microgauss strengths can probably be explained only if helical magnetic fields of small or moderate length scales can be rapidly ejected or destroyed. Small-scale dynamos are important throughout a galaxy's life and probably provide strong seed fields at early stages. The circumgalactic medium (CGM) may play an important role in driving dynamo action at small and large length scales. These interactions between the galactic disk and the CGM may provide important insights into our understanding of galactic dynamos. We expect future research in galactic dynamos to focus on the cosmological history of galaxies and the interaction with the CGM as means of replacing the idealized boundary conditions used in earlier work. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
13. Turbulence with Magnetic Helicity That Is Absent on Average.
- Author
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Brandenburg, Axel and Larsson, Gustav
- Subjects
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TURBULENCE , *HALL effect , *CONSERVED quantity , *MAGNETIC traps , *ENERGY density , *PLASMA turbulence , *ROTATIONAL motion - Abstract
Magnetic helicity plays a tremendously important role when it is different from zero on average. Most notably, it leads to the phenomenon of an inverse cascade. Here, we consider decaying magnetohydrodynamic (MHD) turbulence as well as some less common examples of magnetic evolution under the Hall effect and ambipolar diffusion, as well as cases in which the magnetic field evolution is constrained by the presence of an asymmetry in the number density of chiral fermions, whose spin is systematically either aligned or anti-aligned with its momentum. In all those cases, there is a new conserved quantity: the Hosking integral. We present quantitative scaling results for the magnetic integral scale as well as the magnetic energy density and its spectrum. We also compare with cases were a magnetic version of the Saffman integral is initially finite. Rotation in MHD turbulence tends to suppress nonlinearity and thereby also inverse cascading. Finally, the role of the Hosking and magnetic Saffman integrals in shell models of turbulence is examined. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
14. Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics
- Author
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Miesch, Mark, Matthaeus, William, Brandenburg, Axel, Petrosyan, Arakel, Pouquet, Annick, Cambon, Claude, Jenko, Frank, Uzdensky, Dmitri, Stone, James, Tobias, Steve, Toomre, Juri, and Velli, Marco
- Published
- 2015
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15. Dissipative magnetic structures and scales in small-scale dynamos.
- Author
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Brandenburg, Axel, Rogachevskii, Igor, and Schober, Jennifer
- Subjects
- *
MAGNETIC structure , *ELECTRIC generators , *GALACTIC magnetic fields - Abstract
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. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
16. Batchelor, Saffman, and Kazantsev spectra in galactic small-scale dynamos.
- Author
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Brandenburg, Axel, Zhou, Hongzhe, and Sharma, Ramkishor
- Subjects
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ELECTRIC generators , *COSMIC background radiation , *PRANDTL number , *GALACTIC magnetic fields , *SYNCHROTRON radiation , *INTERSTELLAR medium , *GALAXY clusters - Abstract
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. [ABSTRACT FROM AUTHOR]
- Published
- 2023
- Full Text
- View/download PDF
17. Application of a helicity proxy to edge-on galaxies
- Author
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Brandenburg, Axel and Furuya, Ray S.
- Subjects
Astrophysics::Cosmology and Extragalactic Astrophysics ,individual ,NGC 891 – galaxies ,magnetic fields ,Astrophysics::Galaxy Astrophysics ,dynamo –MHD– polarization – turbulence – galaxies - Abstract
We study the prospects of detecting magnetic helicity in galaxies by observing the dust polarization of the edge-on galaxy NGC 891. Our numerical results of mean-field dynamo calculations show that there should be a large-scale component of the rotationally invariant parity-odd B polarization that we predict to be negative in the first and third quadrants, and positive in the second and fourth quadrants. The large-scale parity-even E polarization is predicted to be negative near the axis and positive further away in the outskirts. These properties are shown to be mostly a consequence of the magnetic field being azimuthal and the polarized intensity being maximum at the centre of the galaxy and are not a signature of magnetic helicity.
- Published
- 2020
18. Chirality in Astrophysics
- Author
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Brandenburg, Axel
- Subjects
Quantitative Biology::Biomolecules ,Astrophysics - Solar and Stellar Astrophysics ,Fluid Dynamics (physics.flu-dyn) ,FOS: Physical sciences ,Physics - Fluid Dynamics ,Solar and Stellar Astrophysics (astro-ph.SR) ,Computer Science::Databases - Abstract
Chirality, or handedness, enters astrophysics in three distinct ways. Magnetic field and vortex lines tend to be helical and have a systematic twist in the northern and southern hemispheres of a star or a galaxy. Helicity is here driven by external factors. Chirality can also enter at the microphysical level and can then be traced back to the parity-breaking weak force. Finally, chirality can arise spontaneously, but this requires not only the presence of an instability, but also the action of nonlinearity. Examples can be found both in magnetohydrodynamics and in astrobiology, where homochirality among biomolecules probably got established at the origin of life. In this review, all three types of chirality production will be explored and compared., 20 pages, 6 figures, 1 table, to appear in the proceedings to Nobel Symposium 167: Chiral Matter, eds. E. Babaev, D. Kharzeev, M. Larsson, A. Molochkov, & V. Zhaunerchyk, World Scientific, in press
- Published
- 2021
19. Scaling of the Hosking integral in decaying magnetically dominated turbulence.
- Author
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Zhou, Hongzhe, Sharma, Ramkishor, and Brandenburg, Axel
- Subjects
TURBULENCE ,INTEGRALS ,PLASMA astrophysics - Abstract
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 Alfvén time. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
20. Active Region Formation through the Negative Effective Magnetic Pressure Instability
- Author
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Kemel, Koen, Brandenburg, Axel, Kleeorin, Nathan, Mitra, Dhrubaditya, and Rogachevskii, Igor
- Published
- 2013
- Full Text
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21. Compressible test-field method and its application to shear dynamos
- Author
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Käpylä, Maarit J., Rheinhardt, Matthias, Brandenburg, Axel, Computer Science Professors, Professorship Korpi-Lagg Maarit, KTH Royal Institute of Technology, Department of Computer Science, Aalto-yliopisto, and Aalto University
- Subjects
Fluid Dynamics (physics.flu-dyn) ,FOS: Physical sciences ,Physics - Fluid Dynamics ,Astrophysics - Instrumentation and Methods for Astrophysics ,Instrumentation and Methods for Astrophysics (astro-ph.IM) - Abstract
In this study we present a compressible test-field method (CTFM) for computing $\alpha$ effect and turbulent magnetic diffusivity tensors, as well as those relevant for mean ponderomotive force and mass source, applied to the full MHD equations. We describe the theoretical background of the method, and compare it to the quasi-kinematic test-field method, and to the previously studied variant working in simplified MHD (SMHD). We present several test cases using velocity and magnetic fields of the Roberts geometry, and also compare with the imposed-field method. We show that, for moderate imposed field strengths, the nonlinear CTFM (nCTFM) gives results in agreement with the imposed-field method. Comparison of different flavors of the nCTFM in the shear dynamo case also agree up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study non-helically forced shear flows, which exhibit large-scale dynamo action, and present a re-analysis of low Reynolds number, moderate shear systems, where we previously neglected the pressure gradient in the momentum equation, and found no coherent shear-current effect. Another key difference is that in the earlier study we used magnetic forcing to mimic small-scale dynamo action, while here it is self-consistently driven by purely kinetic forcing. The kinematic CTFM with general validity forms the core of our analysis. We still find no coherent shear-current effect, but do recover strong large-scale dynamo action that, according to our analysis, is driven through the incoherent effects., Comment: 23 pages, 11 figures, submitted to the Astrophysical Journal
- Published
- 2021
22. Leading-order nonlinear gravitational waves from reheating magnetogeneses
- Author
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Yutong He, Roper Pol, Alberto, Brandenburg, Axel, HEP, INSPIRE, AstroParticule et Cosmologie (APC (UMR_7164)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut National de Physique Nucléaire et de Physique des Particules du CNRS (IN2P3)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité), and Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Centre National de la Recherche Scientifique (CNRS)-Université de Paris (UP)
- Subjects
polarization: spectrum ,spectrum: linear ,Cosmology and Nongalactic Astrophysics (astro-ph.CO) ,higher-order ,interferometer ,FOS: Physical sciences ,magnetic field ,General Relativity and Quantum Cosmology (gr-qc) ,GeV ,General Relativity and Quantum Cosmology ,TeV ,energy: density ,gravitational radiation: spectrum ,numerical calculations ,scale: quantum chromodynamics ,pulsar ,[PHYS.GRQC] Physics [physics]/General Relativity and Quantum Cosmology [gr-qc] ,temperature: reheating ,scale: electroweak interaction ,parametrization ,suppression ,helicity ,electromagnetic ,fractional ,[PHYS.GRQC]Physics [physics]/General Relativity and Quantum Cosmology [gr-qc] ,nonlinear ,[PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph] ,[PHYS.ASTR] Physics [physics]/Astrophysics [astro-ph] ,Astrophysics - Cosmology and Nongalactic Astrophysics - Abstract
We study the leading-order nonlinear gravitational waves (GWs) produced by an electromagnetic (EM) stress in reheating magnetogenesis scenarios. Both nonhelical and helical magnetic fields are considered. By numerically solving the linear and leading-order nonlinear GW equations, we find that the GW energy from the latter is usually larger. We compare their differences in terms of the GW spectrum and parameterize the GW energy difference due to the nonlinear term, $\Delta\mathcal{E}_{\rm GW}$, in terms of EM energy $\mathcal{E}_{\rm EM}$ as $\Delta\mathcal{E}_{\rm GW}=(\tilde p\mathcal{E}_{\rm EM}/k_*)^3$, where $k_*$ is the characteristic wave number, $\tilde p=0.84$ and $0.88$ are found in the nonhelical and helical cases, respectively, with reheating around the QCD energy scale, while $\tilde p=0.45$ is found at the electroweak energy scale. We also compare the polarization spectrum of the linear and nonlinear cases and find that adding the nonlinear term usually yields a decrease in the polarization that is proportional to the EM energy density. We parameterize the fractional polarization suppression as $|\Delta \mathcal{P}_{\rm GW}/\mathcal{P}_{\rm GW}|=\tilde r \mathcal{E}_{\rm EM}/k_*$ and find $\tilde r = 1.2 \times 10^{-1}$, $7.2 \times 10^{-4}$, and $3.2 \times 10^{-2}$ for the helical cases with reheating temperatures $T_{\rm r} = 300 {\rm TeV}$, $8 {\rm GeV}$, and $120 {\rm MeV}$, respectively. Prospects of observation by pulsar timing arrays, space-based interferometers, and other novel detection proposals are also discussed., Comment: 17 pages, 7 figures, 3 tables, to be submitted to PRD
- Published
- 2021
23. Collision Fluctuations of Lucky Droplets with Superdroplets.
- Author
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XIANG-YU LI, MEHLIG, BERNHARD, SVENSSON, GUNILLA, BRANDENBURG, AXEL, and HAUGEN, NILS E. L.
- Subjects
DROPLETS ,CLOUD droplets ,OPEN-ended questions ,COAGULATION - Abstract
It was previously shown that the superdroplet algorithm for modeling the collision–coalescence process can faithfully represent mean droplet growth in turbulent clouds. An open question is how accurately the superdroplet algorithm accounts for fluctuations in the collisional aggregation process. Such fluctuations are particularly important in dilute suspensions. Even in the absence of turbulence, Poisson fluctuations of collision times in dilute suspensions may result in substantial variations in the growth process, resulting in a broad distribution of growth times to reach a certain droplet size. We quantify the accuracy of the superdroplet algorithm in describing the fluctuating growth history of a larger droplet that settles under the effect of gravity in a quiescent fluid and collides with a dilute suspension of smaller droplets that were initially randomly distributed in space (“lucky droplet model”). We assess the effect of fluctuations upon the growth history of the lucky droplet and compute the distribution of cumulative collision times. The latter is shown to be sensitive enough to detect the subtle increase of fluctuations associated with collisions between multiple lucky droplets. The superdroplet algorithm incorporates fluctuations in two distinct ways: through the random spatial distribution of superdroplets and through the Monte Carlo collision algorithm involved. Using specifically designed numerical experiments, we show that both on their own give an accurate representation of fluctuations. We conclude that the superdroplet algorithm can faithfully represent fluctuations in the coagulation of droplets driven by gravity. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
24. Spontaneous Formation of Magnetic Flux Concentrations in Stratified Turbulence
- Author
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Kemel, Koen, Brandenburg, Axel, Kleeorin, Nathan, Mitra, Dhrubaditya, and Rogachevskii, Igor
- Published
- 2012
- Full Text
- View/download PDF
25. Ejections of Magnetic Structures Above a Spherical Wedge Driven by a Convective Dynamo with Differential Rotation
- Author
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Warnecke, Jörn, Käpylä, Petri J., Mantere, Maarit J., and Brandenburg, Axel
- Published
- 2012
- Full Text
- View/download PDF
26. Current Status of Turbulent Dynamo Theory: From Large-Scale to Small-Scale Dynamos
- Author
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Brandenburg, Axel, Sokoloff, Dmitry, and Subramanian, Kandaswamy
- Published
- 2012
- Full Text
- View/download PDF
27. Chandrasekhar–Kendall functions in astrophysical dynamos
- Author
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BRANDENBURG, AXEL
- Published
- 2011
- Full Text
- View/download PDF
28. Homochirality and the Need for Energy
- Author
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Plasson, Raphaël and Brandenburg, Axel
- Published
- 2010
- Full Text
- View/download PDF
29. Advances in Theory and Simulations of Large-Scale Dynamos
- Author
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Brandenburg, Axel
- Published
- 2009
- Full Text
- View/download PDF
30. Dynamo effect in unstirred self-gravitating turbulence.
- Author
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Brandenburg, Axel and Ntormousi, Evangelia
- Subjects
- *
ELECTRIC generators , *TURBULENCE , *LORENTZ force , *KINETIC energy , *REYNOLDS number - Abstract
In many astrophysical environments, self-gravity can generate kinetic energy, which, in principle, is available for driving dynamo action. Using direct numerical simulations, we show that in unstirred self-gravitating subsonic turbulence with helicity and a magnetic Prandtl number of unity, there is a critical magnetic Reynolds number of about 25 above which the work done against the Lorentz force exceeds the Ohmic dissipation. The collapse itself drives predominantly irrotational motions that cannot be responsible for dynamo action. We find that, with a weak magnetic field, one-third of the work done by the gravitational force goes into compressional heating and the remaining two-thirds go first into kinetic energy of the turbulence before a fraction of it is converted further into magnetic and finally thermal energies. Close to the collapse, however, these fractions change toward 1/4 and 3/4 for compressional heating and kinetic energy, respectively. When the magnetic field is strong, the compressional heating fraction is unchanged. Out of the remaining kinetic energy, one quarter goes directly into magnetic energy via work against the Lorentz force. The fraction of vortical motions diminishes in favour of compressive motions that are almost exclusively driven by the Jeans instability. For an initially uniform magnetic field, field amplification at scales larger than those of the initial turbulence are driven by tangling. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
31. Compressible Test-field Method and Its Application to Shear Dynamos.
- Author
-
Käpylä, Maarit J., Rheinhardt, Matthias, and Brandenburg, Axel
- Subjects
ELECTRIC generators ,COSMIC magnetic fields ,PONDEROMOTIVE force ,CROWDSOURCING ,MAGNETISM ,SHEAR flow ,COMPRESSIBLE flow - Abstract
In this study, we present a compressible test-field method (CTFM) for computing α- effect and turbulent magnetic diffusivity tensors, as well as those relevant for the mean ponderomotive force and mass source, applied to the full MHD equations. We describe the theoretical background of the method and compare it to the quasi-kinematic test-field method and to the previously studied variant working in simplified MHD (SMHD). We present several test cases using velocity and magnetic fields of the Roberts geometry and also compare with the imposed-field method. We show that, for moderate imposed-field strengths, the nonlinear CTFM (nCTFM) gives results in agreement with the imposed-field method. A comparison of different flavors of the nCTFM in the shear dynamo case also yields agreement up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study nonhelically forced shear flows, which exhibit large-scale dynamo action, and present a reanalysis of low-Reynolds-number, moderate shear systems, where we previously ignored the pressure gradient in the momentum equation and found no coherent shear-current effect. Another key difference is that in the earlier study we used magnetic forcing to mimic small-scale dynamo action, while here it is self-consistently driven by purely kinetic forcing. The kinematic CTFM with general validity forms the core of our analysis. We still find no coherent shear-current effect, but do recover strong large-scale dynamo action that, according to our analysis, is driven by incoherent effects. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
32. Polarization of gravitational waves from helical MHD turbulent sources.
- Author
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Roper Pol, Alberto, Mandal, Sayan, Brandenburg, Axel, and Kahniashvili, Tina
- Published
- 2022
- Full Text
- View/download PDF
33. Prospective Prediction of Future SARS-CoV-2 Infections Using Empirical Data on a National Level to Gauge Response Effectiveness
- Author
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Blanco, Natalia, Stafford, Kristen, Lavoie, Marie-Claude, Brandenburg, Axel, Gorna, Maria W., and Merski, Matthew
- Subjects
FOS: Biological sciences ,Populations and Evolution (q-bio.PE) ,Quantitative Biology - Populations and Evolution - Abstract
Predicting an accurate expected number of future COVID-19 cases is essential to properly evaluate the effectiveness of any treatment or preventive measure. This study aimed to identify the most appropriate mathematical model to prospectively predict the expected number of cases without any intervention. The total number of cases for the COVID-19 epidemic in 28 countries was analyzed and fitted to several simple rate models including the logistic, Gompertz, quadratic, simple square, and simple exponential growth models. The resulting model parameters were used to extrapolate predictions for more recent data. While the Gompertz growth models (mean R2 = 0.998) best fitted the current data, uncertainties in the eventual case limit made future predictions with logistic models prone to errors. Of the other models, the quadratic rate model (mean R2 = 0.992) fitted the current data best for 25 (89 %) countries as determined by R2 values. The simple square and quadratic models accurately predicted the number of future total cases 37 and 36 days in advance respectively, compared to only 15 days for the simple exponential model. The simple exponential model significantly overpredicted the total number of future cases while the quadratic and simple square models did not. These results demonstrated that accurate future predictions of the case load in a given country can be made significantly in advance without the need for complicated models of population behavior and generate a reliable assessment of the efficacy of current prescriptive measures against disease spread., 20 pages, 2 tables, 3 figures followed by 12 pages of supporting information
- Published
- 2020
34. Magnetic twist: a source and property of space weather
- Author
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Mitra Dhrubaditya, Brandenburg Axel, and Warnecke Jörn
- Subjects
MHD ,turbulence ,solar activity ,coronal mass ejection (CME) ,solar wind ,Meteorology. Climatology ,QC851-999 - Abstract
Aim: We present evidence for finite magnetic helicity density in the heliosphere and numerical models thereof, and relate it to the magnetic field properties of the dynamo in the solar convection zone. Methods: We use simulations and solar wind data to compute magnetic helicity either directly from the simulations or indirectly using time series of the skew-symmetric components of the magnetic correlation tensor. Results: We find that the solar dynamo produces negative magnetic helicity at small scales and positive at large scales. However, in the heliosphere these properties are reversed and the magnetic helicity is now positive at small scales and negative at large scales. We explain this by the fact that a negative diffusive magnetic helicity flux corresponds to a positive gradient of magnetic helicity, which leads to a change of sign from negative to positive values at some radius in the northern hemisphere.
- Published
- 2012
- Full Text
- View/download PDF
35. Self-Regulating Supernova Heating in Interstellar Medium Simulations
- Author
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Sarson, Graeme R., Shukurov, Anvar, Nordlund, Åke, Gudiksen, Boris, and Brandenburg, Axel
- Published
- 2004
- Full Text
- View/download PDF
36. Outflows from Dynamo-Active Protostellar Accretion Discs
- Author
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von Rekowski, Brigitta, Brandenburg, Axel, Dobler, Wolfgang, and Shukurov, Anvar
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- 2004
- Full Text
- View/download PDF
37. The effects of spiral arms on the multi-phase ISM
- Author
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Shukurov, Anvar, Sarson, Graeme R., Nordlund, Åke, Gudiksen, Boris, and Brandenburg, Axel
- Published
- 2004
- Full Text
- View/download PDF
38. Stochastic Excitation of Gravity Waves by Overshooting Convection in Solar-Type Stars
- Author
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Dintrans, Boris, Brandenburg, Axel, Nordlund, Åke, and Stein, Robert F.
- Published
- 2003
- Full Text
- View/download PDF
39. Spectral characterisation of inertial particle clustering in turbulence.
- Author
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Haugen, Nils E. L., Brandenburg, Axel, Sandin, Christer, and Mattsson, Lars
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CLUSTERING of particles ,TURBULENCE ,COMPRESSIBLE flow ,INCOMPRESSIBLE flow ,POWER spectra ,RADIAL distribution function - Abstract
Clustering of inertial particles is important for many types of astrophysical and geophysical turbulence, but it has been studied predominately for incompressible flows. Here, we study compressible flows and compare clustering in both compressively (irrotationally) and vortically (solenoidally) forced turbulence. Vortically and compressively forced flows are driven stochastically either by solenoidal waves or by circular expansion waves, respectively. For compressively forced flows, the power spectrum of the density of inertial particles is a useful tool for displaying particle clustering relative to the fluid density enhancement. Power spectra are shown to be particularly sensitive for studying large-scale particle clustering, while conventional tools such as radial distribution functions are more suitable for studying small-scale clustering. Our primary finding is that particle clustering through shock interaction is particularly prominent in turbulence driven by spherical expansion waves. It manifests itself through a double-peaked distribution of spectral power as a function of Stokes number. The two peaks are associated with two distinct clustering mechanisms; shock interaction for smaller Stokes numbers and the centrifugal sling effect for larger values. The clustering of inertial particles is associated with the formation of caustics. Such caustics can only be captured in the Lagrangian description, which allows us to assess the relative importance of caustics in vortically and compressively forced turbulence. We show that the statistical noise resulting fromthe limited number of particles in the Lagrangian description can be removed from the particle power spectra, allowing us a more detailed comparison of the residual spectra. We focus on the Epstein drag law relevant for rarefied gases, but show that our findings apply also to the usual Stokes drag. [ABSTRACT FROM AUTHOR]
- Published
- 2022
- Full Text
- View/download PDF
40. Scale Dependence of Magnetic Helicity in the Solar Wind
- Author
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Brandenburg, Axel, Subramanian, Kandaswamy, Balogh, Andre, and Goldstein, Melvyn L
- Subjects
Astrophysics - Abstract
We determine the magnetic helicity, along with the magnetic energy, at high latitudes using data from the Ulysses mission. The data set spans the time period from 1993 to 1996. The basic assumption of the analysis is that the solar wind is homogeneous. Because the solar wind speed is high, we follow the approach first pioneered by Matthaeus et al. by which, under the assumption of spatial homogeneity, one can use Fourier transforms of the magnetic field time series to construct one-dimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozen-in-flow hypothesis is valid. That is a well-satisfied assumption for the data used in this study. The magnetic helicity derives from the skew-symmetric terms of the three-dimensional magnetic correlation tensor, while the symmetric terms of the tensor are used to determine the magnetic energy spectrum. Our results show a sign change of magnetic helicity at wavenumber k approximately equal to 2AU(sup -1) (or frequency nu approximately equal to 2 microHz) at distances below 2.8AU and at k approximately equal to 30AU(sup -1) (or nu approximately equal to 25 microHz) at larger distances. At small scales the magnetic helicity is positive at northern heliographic latitudes and negative at southern latitudes. The positive magnetic helicity at small scales is argued to be the result of turbulent diffusion reversing the sign relative to what is seen at small scales at the solar surface. Furthermore, the magnetic helicity declines toward solar minimum in 1996. The magnetic helicity flux integrated separately over one hemisphere amounts to about 10(sup 45) Mx(sup 2) cycle(sup -1) at large scales and to a three times lower value at smaller scales.
- Published
- 2011
- Full Text
- View/download PDF
41. Astrophysical magnetic fields and nonlinear dynamo theory
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Brandenburg, Axel and Subramanian, Kandaswamy
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- 2005
- Full Text
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42. Robustness of truncated α Ω dynamos with a dynamic α
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Covas, Eurico, Tworkowski, Andrew, Tavakol, Reza, and Brandenburg, Axel
- Published
- 1997
- Full Text
- View/download PDF
43. Magnetic Mysteries
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Brandenburg, Axel
- Published
- 2001
44. Turbulent radiative diffusion and turbulent Newtonian cooling.
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Brandenburg, Axel and Das, Upasana
- Subjects
- *
STELLAR atmospheres , *COOLING , *SQUARE root , *TURBULENCE , *RADIATION , *STELLAR radiation - Abstract
Radiation transport plays an important role in stellar atmospheres, but the effects of turbulence are being obscured by other effects such as stratification. Using radiative hydrodynamic simulations of forced turbulence, we determine the decay rates of sinusoidal large-scale temperature perturbations of different wavenumbers in the optically thick and thin regimes. Increasing the wavenumber increases the rate of decay in both regimes, but this effect is much weaker than for the usual turbulent diffusion of passive scalars, where the increase is quadratic for small wavenumbers. The turbulent decay is well described by an enhanced Newtonian cooling process in the optically thin limit, which is found to show a weak increase proportional to the square root of the wavenumber. In the optically thick limit, the increase in turbulent decay is somewhat steeper for wavenumbers below the energy-carrying wavenumber of the turbulence, but levels off toward larger wavenumbers. In the presence of turbulence, the typical cooling time is comparable to the turbulent turnover time. We observe that the temperature takes a long time to reach equilibrium in both the optically thin and thick cases, but in the former, the temperature retains smaller scale structures for longer. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
45. The scalar, vector, and tensor modes in gravitational wave turbulence simulations.
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Brandenburg, Axel, Gogoberidze, Grigol, Kahniashvili, Tina, Mandal, Sayan, Pol, Alberto Roper, and Shenoy, Nakul
- Subjects
- *
GRAVITATIONAL waves , *TURBULENCE , *QUANTUM phase transitions , *WAVENUMBER , *MAGNETOHYDRODYNAMICS , *STRAIN energy , *STELLAR oscillations - Abstract
We study the gravitational wave (GW) signal sourced by primordial turbulence that is assumed to be present at cosmological phase transitions like the electroweak and quantum chromodynamics phase transitions. We consider various models of primordial turbulence, such as those with and without helicity, purely hydrodynamical turbulence induced by fluid motions, and magnetohydrodynamic turbulence whose energy can be dominated either by kinetic or magnetic energy, depending on the nature of the turbulence. We also study circularly polarized GWs generated by parity violating sources such as helical turbulence. Our ultimate goal is to determine the efficiency of GW production through different classes of turbulence. We find that the GW energy and strain tend to be large for acoustic or irrotational turbulence, even though its tensor mode amplitude is relatively small at most wave numbers. Only at very small wave numbers is the spectral tensor mode significant, which might explain the efficient GW production in that case. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
46. Tensor spectrum of turbulence-sourced gravitational waves as a constraint on graviton mass.
- Author
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He, Yutong, Brandenburg, Axel, and Sinha, Aditya
- Published
- 2021
- Full Text
- View/download PDF
47. Magnetic field evolution in solar-type stars.
- Author
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Brandenburg, Axel, Kosovichev, Alexander, Strassmeier, Klaus, and Jardine, Moira
- Abstract
We discuss selected aspects regarding the magnetic field evolution of solar-type stars. Most of the stars with activity cycles are in the range where the normalized chromospheric Calcium emission increases linearly with the inverse Rossby number. For Rossby numbers below about a quarter of the solar value, the activity saturates and no cycles have been found. For Rossby numbers above the solar value, again no activity cycles have been found, but now the activity goes up again for a major fraction of the stars. Rapidly rotating stars show nonaxisymmetric large-scale magnetic fields, but there is disagreement between models and observations regarding the actual value of the Rossby number where this happens. We also discuss the prospects of detecting the sign of magnetic helicity using various linear polarization techniques both at the stellar surface using the parity-odd contribution to linear polarization and above the surface using Faraday rotation. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
48. Relic Gravitational Waves from the Chiral Magnetic Effect.
- Author
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Brandenburg, Axel, He, Yutong, Kahniashvili, Tina, Rheinhardt, Matthias, and Schober, Jennifer
- Subjects
- *
CORONAL mass ejections , *MAGNETIC flux density , *WHITE noise , *MAGNETIC fields , *GRAVITATIONAL waves , *RELICS , *MONOCHROMATIC light , *WAVENUMBER - Abstract
Relic gravitational waves (GWs) can be produced by primordial magnetic fields. However, not much is known about the resulting GW amplitudes and their dependence on the details of the generation mechanism. Here we treat magnetic field generation through the chiral magnetic effect (CME) as a generic mechanism and explore its dependence on the speed of generation (the product of magnetic diffusivity and characteristic wavenumber) and the speed characterizing the maximum magnetic field strength expected from the CME. When the latter exceeds the former (regime I), which is the regime applicable to the early universe, we obtain an inverse cascade with moderate GW energy that scales with the third power of the magnetic energy. When the generation speed exceeds the CME limit (regime II), the GW energy continues to increase without a corresponding increase of magnetic energy. In the early kinematic phase, the GW energy spectrum (per linear wavenumber interval) has opposite slopes in both regimes and is characterized by an inertial range spectrum in regime I and a white noise spectrum in regime II. The occurrence of these two slopes is shown to be a generic consequence of a nearly monochromatic exponential growth of the magnetic field. The resulting GW energy is found to be proportional to the fifth power of the limiting CME speed and the first power of the generation speed. [ABSTRACT FROM AUTHOR]
- Published
- 2021
- Full Text
- View/download PDF
49. ROBUSTNESS OF TRUNCATED αΩ dynamos with a dynamic α
- Author
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Covas, Eurico, Tworkowski, Andrew, Tavakol, Reza, and Brandenburg, Axel
- Published
- 1997
50. On the Existence of Shear-current Effects in Magnetized Burgulence.
- Author
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Käpylä, Maarit J., Vizoso, Javier Álvarez, Rheinhardt, Matthias, Brandenburg, Axel, and Singh, Nishant K.
- Subjects
BURGERS' equation ,SHEAR flow ,EXPONENTIAL functions ,ELECTRIC generators ,MAGNETISM ,NONLINEAR equations - Abstract
The possibility of explaining shear flow dynamos by a magnetic shear-current (MSC) effect is examined via numerical simulations. Our primary diagnostics is the determination of the turbulent magnetic diffusivity tensor η. In our setup, a negative sign of its component η
yx is necessary for coherent dynamo action by the SC effect. To be able to measure turbulent transport coefficients from systems with magnetic background turbulence, we present an extension of the test-field method (TFM) applicable to our setup where the pressure gradient is dropped from the momentum equation: the nonlinear TFM (NLTFM). Our momentum equation is related to Burgers' equation and the resulting flows are referred to as magnetized burgulence. We use both stochastic kinetic and magnetic forcings to mimic cases without and with simultaneous small-scale dynamo action. When we force only kinetically, negative ηyx are obtained with exponential growth in both the radial and azimuthal mean magnetic field components. Using magnetokinetic forcing, the field growth is no longer exponential, while NLTFM yields positive ηyx . By employing an alternative forcing from which wavevectors whose components correspond to the largest scales are removed, the exponential growth is recovered, but the NLTFM results do not change significantly. Analyzing the dynamo excitation conditions for the coherent SC and incoherent α and SC effects shows that the incoherent effects are the main drivers of the dynamo in the majority of cases. We find no evidence for MSC-effect-driven dynamos in our simulations. [ABSTRACT FROM AUTHOR]- Published
- 2020
- Full Text
- View/download PDF
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