Change search
Link to record
Permanent link

Direct link
BETA
Alternative names
Publications (10 of 107) Show all publications
Brandenburg, A. (2019). Ambipolar diffusion in large Prandtl number turbulence. Monthly notices of the Royal Astronomical Society, 487(2), 2673-2684
Open this publication in new window or tab >>Ambipolar diffusion in large Prandtl number turbulence
2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 487, no 2, p. 2673-2684Article in journal (Refereed) Published
Abstract [en]

We study the effects of ambipolar diffusion (AD) on hydromagnetic turbulence. We consider the regime of large magnetic Prandtl number, relevant to the interstellar medium. In most of the cases, we use the single-fluid approximation where the drift velocity between charged and neutral particles is proportional to the Lorentz force. In two cases we also compare with the corresponding two-fluid model, where ionization and recombination are included in the continuity and momentum equations for the neutral and charged species. The magnetic field properties are found to be well represented by the single-fluid approximation. We quantify the effects of AD on total and spectral kinetic and magnetic energies, the ohmic and AD dissipation rates, the statistics of the magnetic field, the current density, and the linear polarization as measured by the rotationally invariant E and B mode polarizations. We show that the kurtosis of the magnetic field decreases with increasing AD. The E mode polarization changes its skewness from positive values for small AD to negative ones for large AD. Even when AD is weak, changes in AD have a marked effect on the skewness and kurtosis of E, and only a weak effect on those of B. These results open the possibility of employing E and B mode polarizations as diagnostic tools for characterizing turbulent properties of the interstellar medium.

Place, publisher, year, edition, pages
Oxford University Press, 2019
Keywords
dynamo, MHD, polarization, turbulence, ISM: magnetic fields
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-255366 (URN)10.1093/mnras/stz1509 (DOI)000474919700083 ()
Note

QC 20190731

Available from: 2019-07-31 Created: 2019-07-31 Last updated: 2019-07-31Bibliographically approved
Li, X.-Y., Svensson, G., Brandenburg, A. & Haugen, N. E. (2019). Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds. Atmospheric Chemistry And Physics, 19(1), 639-648
Open this publication in new window or tab >>Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds
2019 (English)In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, no 1, p. 639-648Article in journal (Refereed) Published
Abstract [en]

Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations (DNS) with droplets being modeled as Lagrangian particles. The supersaturation field is calculated directly by simulating the temperature and water vapor fields instead of being treated as a passive scalar. Thermodynamic feedbacks to the fields due to condensation are also included for completeness. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as t 1/2 . Also, for the first time, we explicitly demonstrate that the time evolution of the size distribution is sensitive to the Reynolds number, but insensitive to the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increase with increasing Reynolds number; therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the mean updraft velocity is almost zero.

Place, publisher, year, edition, pages
Copernicus GmbH, 2019
Keywords
air temperature, cloud droplet, computer simulation, hydrodynamics, Reynolds number, size distribution, stratiform cloud, supersaturation, surface area, thermodynamics, water vapor
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-248274 (URN)10.5194/acp-19-639-2019 (DOI)000456052700001 ()2-s2.0-85060017700 (Scopus ID)
Note

QC 20190408

Available from: 2019-04-08 Created: 2019-04-08 Last updated: 2019-04-08Bibliographically approved
Brandenburg, A., Kahniashvili, T., Mandal, S., Pol, A. R., Tevzadze, A. G. & Vachaspati, T. (2019). Dynamo effect in decaying helical turbulence. Physical Review Fluids, 4(2), Article ID 024608.
Open this publication in new window or tab >>Dynamo effect in decaying helical turbulence
Show others...
2019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 2, article id 024608Article in journal (Refereed) Published
Abstract [en]

We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity-regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t(-1/2). This is even slower than in the fully helical case, where it decays like t(-2/3). In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t(-2/3) decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-245908 (URN)10.1103/PhysRevFluids.4.024608 (DOI)000459585000001 ()2-s2.0-85062426577 (Scopus ID)
Note

QC 20190313

Available from: 2019-03-13 Created: 2019-03-13 Last updated: 2019-04-04Bibliographically approved
Brandenburg, A., Bracco, A., Kahniashvili, T., Mandal, S., Pol, A. R., Petrie, G. J. D. & Singh, N. K. (2019). E and B Polarizations from Inhomogeneous and Solar Surface Turbulence. Astrophysical Journal, 870(2), Article ID 87.
Open this publication in new window or tab >>E and B Polarizations from Inhomogeneous and Solar Surface Turbulence
Show others...
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, Vol. 870, no 2, article id 87Article in journal (Refereed) Published
Abstract [en]

Gradient- and curl-type or E- and B-type polarizations have been routinely analyzed to study the physics contributing to the cosmic microwave background polarization and galactic foregrounds. They characterize the parity-even and parity-odd properties of the underlying physical mechanisms, such as, for example, hydromagnetic turbulence in the case of dust polarization. Here, we study spectral correlation functions characterizing the parity-even and parity-odd parts of linear polarization for homogeneous and inhomogeneous turbulence to show that only the inhomogeneous helical case can give rise to a parity-odd polarization signal. We also study nonhelical turbulence and suggest that a strong non-vanishing (here negative) skewness of the E polarization is responsible for an enhanced ratio of the EE to the BB (quadratic) correlation in both the helical and nonhelical cases. This could explain the enhanced EE/BB ratio observed recently for dust polarization. We close with a preliminary assessment of using the linear polarization of the Sun to characterize its helical turbulence without being subjected to the pi ambiguity that magnetic inversion techniques have to address.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2019
Keywords
dynamo, magnetohydrodynamics (MHD), Sun: magnetic fields, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-242979 (URN)10.3847/1538-4357/aaf383 (DOI)000455820100033 ()2-s2.0-85060253289 (Scopus ID)
Note

QC 20190206

Available from: 2019-02-06 Created: 2019-02-06 Last updated: 2019-02-06Bibliographically approved
Kapyla, P. J., Viviani, M., Kapyla, M. J., Brandenburg, A. & Spada, F. (2019). Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations. Geophysical and Astrophysical Fluid Dynamics, 113(1-2), 149-183
Open this publication in new window or tab >>Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
Show others...
2019 (English)In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 113, no 1-2, p. 149-183Article in journal (Refereed) Published
Abstract [en]

We consider the effect of a subadiabatic layer at the base of the convection zone on convection itself and the associated large-scale dynamos in spherical wedge geometry. We use a heat conduction prescription based on the Kramers opacity law which allows the depth of the convection zone to dynamically adapt to changes in the physical characteristics such as rotation rate and magnetic fields. We find that the convective heat transport is strongly concentrated towards the equatorial and polar regions in the cases without a substantial radiative layer below the convection zone. The presence of a stable layer below the convection zone significantly reduces the anisotropy of radial enthalpy transport. Furthermore, the dynamo solutions are sensitive to subtle changes in the convection zone structure. We find that the kinetic helicity changes sign in the deeper parts of the convection zone at high latitudes in all runs. This region expands progressively towards the equator in runs with a thicker stably stratified layer.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:kth:diva-252996 (URN)10.1080/03091929.2019.1571584 (DOI)000468550900008 ()2-s2.0-85061240477 (Scopus ID)
Note

QC 20190619

Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2019-06-19Bibliographically approved
Schober, J., Brandenburg, A., Rogachevskii, l. & Kleeorin, N. (2019). Energetics of turbulence generated by chiral MHD dynamos. Geophysical and Astrophysical Fluid Dynamics, 113(1-2), 107-130
Open this publication in new window or tab >>Energetics of turbulence generated by chiral MHD dynamos
2019 (English)In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 113, no 1-2, p. 107-130Article in journal (Refereed) Published
Abstract [en]

An asymmetry in the number density of left- and right-handed fermions is known to give rise to a new term in the induction equation that can result in a dynamo instability. At high temperatures, when a chiral asymmetry can survive for long enough, this chiral dynamo instability can amplify magnetic fields efficiently, which in turn drive turbulence via the Lorentz force. While it has been demonstrated in numerical simulations that this chiral magnetically driven turbulence exists and strongly affects the dynamics of the magnetic field, the details of this process remain unclear. The goal of this paper is to analyse the energetics of chiral magnetically driven turbulence and its effect on the generation and dynamics of the magnetic field using direct numerical simulations. We study these effects for different initial conditions, including a variation of the initial chiral chemical potential and the magnetic Prandtl number, . In particular, we determine the ratio of kinetic to magnetic energy, , in chiral magnetically driven turbulence. Within the parameter space explored in this study, reaches a value of approximately 0.064-0.074-independently of the initial chiral asymmetry and for . Our simulations suggest, that decreases as a power law when increasing by decreasing the viscosity. While the exact scaling depends on the details of the fitting criteria and the Reynolds number regime, an approximate result of is reported. Using the findings from our numerical simulations, we analyse the energetics of chiral magnetically driven turbulence in the early Universe.

Place, publisher, year, edition, pages
Taylor & Francis Group, 2019
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-252995 (URN)10.1080/03091929.2018.1515313 (DOI)000468550900006 ()2-s2.0-85053381281 (Scopus ID)
Note

QC 20190619

Available from: 2019-06-19 Created: 2019-06-19 Last updated: 2019-06-19Bibliographically approved
Bracco, A., Candelaresi, S., Del Sordo, F. & Brandenburg, A. (2019). Is there a left-handed magnetic field in the solar neighborhood?: Exploring helical magnetic fields in the interstellar medium through dust polarization power spectra. Astronomy and Astrophysics, 621, Article ID A97.
Open this publication in new window or tab >>Is there a left-handed magnetic field in the solar neighborhood?: Exploring helical magnetic fields in the interstellar medium through dust polarization power spectra
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A97Article in journal (Refereed) Published
Abstract [en]

Context. The analysis of the full-sky Planck polarization data at 850 mu m revealed unexpected properties of the E- and B-mode power spectra of dust emission in the interstellar medium (ISM). The positive cross-correlations over a wide range of angular scales between the total dust intensity, T, and both E and (most of all) B modes has raised new questions about the physical mechanisms that affect dust polarization, such as the Galactic magnetic field structure. This is key both to better understanding ISM dynamics and to accurately describing Galactic foregrounds to the polarization of the cosmic microwave background (CMB). In particular, in the quest to find primordial B modes of the CMB, the observed positive cross-correlation between T and B for interstellar dust requires further investigation towards parity-violating processes in the ISM. Aims. In this theoretical paper we investigate the possibility that the observed cross-correlations in the dust polarization power spectra, and specifically the one between T and B, can be related to a parity-odd quantity in the ISM such as the magnetic helicity. Methods. We produce synthetic dust polarization data, derived from 3D analytical toy models of density structures and helical magnetic fields, to compare with the E and B modes of observations. We present several models. The first is an ideal fully helical isotropic case, such as the Arnold-Beltrami-Childress field. Second, following the nowadays favored interpretation of the T-E signal in terms of the observed alignment between the magnetic field morphology and the filamentary density structure of the diffuse ISM, we design models for helical magnetic fields wrapped around cylindrical interstellar filaments. Lastly, focusing on the observed T-B correlation, we propose a new line of interpretation of the Planck observations advocating the presence of a large-scale helical component of the Galactic magnetic field in the solar neighborhood. Results. Our analysis shows that: I) the sign of magnetic helicity does not affect E and B modes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce the Planck results; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain the T-B correlation seen in the Planck data. Such a magnetic-field configuration would also account for the observed large-scale T-E correlation. Conclusions. This work suggests a new perspective for the interpretation of the dust polarization power spectra that supports the imprint of a large-scale structure of the Galactic magnetic field in the solar neighborhood.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
ISM: magnetic fields, dust, extinction, local insterstellar matter, ISM: structure, cosmic background radiation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-242972 (URN)10.1051/0004-6361/201833961 (DOI)000455571900001 ()2-s2.0-85060370716 (Scopus ID)
Note

QC 20190201

Available from: 2019-02-01 Created: 2019-02-01 Last updated: 2019-02-01Bibliographically approved
Losada, I., Warnecke, J., Brandenburg, A., Kleeorin, N. & Rogachevskii, l. (2019). Magnetic bipoles in rotating turbulence with coronal envelope. Astronomy and Astrophysics, 621, Article ID A61.
Open this publication in new window or tab >>Magnetic bipoles in rotating turbulence with coronal envelope
Show others...
2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A61Article in journal (Refereed) Published
Abstract [en]

Context. The formation mechanism of sunspots and starspots is not yet fully understood. It is a major open problem in astrophysics. Aims. Magnetic flux concentrations can be produced by the negative effective magnetic pressure instability (NEMPI). This instability is strongly suppressed by rotation. However, the presence of an outer coronal envelope was previously found to strengthen the flux concentrations and make them more prominent. It also allows for the formation of bipolar regions (BRs). We aim to understand the important issue of whether the presence of an outer coronal envelope also changes the excitation conditions and the rotational dependence of NEMPI. Methods. We have used direct numerical simulations and mean-field simulations. We adopted a simple two-layer model of turbulence that mimics the jump between the convective turbulent and coronal layers below and above the surface of a star, respectively. The computational domain is Cartesian and located at a certain latitude of a rotating sphere. We investigated the effects of rotation on NEMPI by changing the Coriolis number, the latitude, the strengths of the imposed magnetic field, and the box resolution. Results. Rotation has a strong impact on the process of BR formation. Even rather slow rotation is found to suppress BR formation. However, increasing the imposed magnetic field strength also makes the structures stronger and alleviates the rotational suppression somewhat. The presence of a coronal layer itself does not significantly reduce the effects of rotational suppression.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2019
Keywords
magnetohydrodynamics (MHD), turbulence, dynamo, Sun: magnetic fields, Sun: rotation, Sun: activity
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-241309 (URN)10.1051/0004-6361/201833018 (DOI)000455172300001 ()2-s2.0-85059893132 (Scopus ID)
Note

QC 20190125

Available from: 2019-01-25 Created: 2019-01-25 Last updated: 2019-01-25Bibliographically approved
Brandenburg, A. & Rempel, M. (2019). Reversed Dynamo at Small Scales and Large Magnetic Prandtl Number. Astrophysical Journal, 879(1), Article ID 57.
Open this publication in new window or tab >>Reversed Dynamo at Small Scales and Large Magnetic Prandtl Number
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 879, no 1, article id 57Article in journal (Refereed) Published
Abstract [en]

We show that at large magnetic Prandtl numbers, the Lorentz force does work on the flow at small scales and drives fluid motions, whose energy is dissipated viscously. This situation is the opposite of that in a normal dynamo, where the flow does work against the Lorentz force. We compute the spectral conversion rates between kinetic and magnetic energies for several magnetic Prandtl numbers and show that normal (forward) dynamo action occurs on large scales over a progressively narrower range of wavenumbers as the magnetic Prandtl number is increased. At higher wavenumbers, reversed dynamo action occurs, i.e., magnetic energy is converted back into kinetic energy at small scales. We demonstrate this in both direct numerical simulations forced by volume stirring and in large eddy simulations (LESs) of solar convectively driven small-scale dynamos. Low-density plasmas such as stellar coronae tend to have large magnetic Prandtl numbers, i.e., the viscosity is large compared with the magnetic diffusivity. The regime in which viscous dissipation dominates over resistive dissipation for large magnetic Prandtl numbers was also previously found in LESs of the solar corona, i.e., our findings are a more fundamental property of MHD that is not just restricted to dynamos. Viscous energy dissipation is a consequence of positive Lorentz force work, which may partly correspond to particle acceleration in close-to-collisionless plasmas. This is, however, not modeled in the MHD approximation employed. By contrast, resistive energy dissipation on current sheets is expected to be unimportant in stellar coronae.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
dynamo, hydrodynamics, magnetohydrodynamics (MHD), Sun: corona, turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-255394 (URN)10.3847/1538-4357/ab24bd (DOI)000474384700002 ()
Note

QC 20190729

Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-07-29Bibliographically approved
Brandenburg, A. (2019). The Limited Roles of Autocatalysis and Enantiomeric Cross-Inhibition in Achieving Homochirality in Dilute Systems. Origins of life and evolution of the biosphere, 49(1-2), 49-60
Open this publication in new window or tab >>The Limited Roles of Autocatalysis and Enantiomeric Cross-Inhibition in Achieving Homochirality in Dilute Systems
2019 (English)In: Origins of life and evolution of the biosphere, ISSN 0169-6149, E-ISSN 1573-0875, Vol. 49, no 1-2, p. 49-60Article in journal (Refereed) Published
Abstract [en]

To understand the effects of fluctuations on achieving homochirality, we employ a Monte-Carlo method where autocatalysis and enantiomeric cross-inhibition, as well as racemization and deracemization reactions are included. The results of earlier work either without autocatalysis or without cross-inhibition are reproduced. Bifurcation diagrams and the dependencies of the number of reaction steps on parameters are studied. In systems with 30,000 molecules, for example, up to a billion reaction steps may be needed to achieve homochirality without autocatalysis.

Place, publisher, year, edition, pages
SPRINGER, 2019
Keywords
DNA polymerization, Enantiomeric cross-inhibition, Origin of homochirality, Revision, 1.46
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-257827 (URN)10.1007/s11084-019-09579-4 (DOI)000481414300003 ()31286343 (PubMedID)2-s2.0-85068851650 (Scopus ID)
Note

QC 20190906

Available from: 2019-09-06 Created: 2019-09-06 Last updated: 2019-09-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-7304-021X

Search in DiVA

Show all publications