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Publications (10 of 84) Show all publications
Brandenburg, A., Ashurova, M. B. & Jabbari, S. (2017). Compensating Faraday Depolarization by Magnetic Helicity in the Solar Corona. Astrophysical Journal Letters, 845(2), Article ID L15.
Open this publication in new window or tab >>Compensating Faraday Depolarization by Magnetic Helicity in the Solar Corona
2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 845, no 2, L15Article in journal (Refereed) Published
Abstract [en]

A turbulent dynamo in spherical geometry with an outer corona is simulated to study the sign of magnetic helicity in the outer parts. In agreement with earlier studies, the sign in the outer corona is found to be opposite to that inside the dynamo. Line-of-sight observations of polarized emission are synthesized to explore the feasibility of using the local reduction of Faraday depolarization to infer the sign of helicity of magnetic fields in the solar corona. This approach was previously identified as an observational diagnostic in the context of galactic magnetic fields. Based on our simulations, we show that this method can be successful in the solar context if sufficient statistics are gathered by using averages over ring segments in the corona separately for the regions north and south of the solar equator.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keyword
dynamo, magnetohydrodynamics (MHD), Sun: magnetic fields, turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-214332 (URN)10.3847/2041-8213/aa844b (DOI)000407971200003 ()2-s2.0-85028457905 (Scopus ID)
Note

QC 20170912

Available from: 2017-09-12 Created: 2017-09-12 Last updated: 2017-09-12Bibliographically approved
Käpylä, P. J., Käpylä, M. J., Olspert, N., Warnecke, J. & Brandenburg, A. (2017). Convection-driven spherical shell dynamos at varying Prandtl numbers. Astronomy and Astrophysics, 599, Article ID A4.
Open this publication in new window or tab >>Convection-driven spherical shell dynamos at varying Prandtl numbers
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, A4Article in journal (Refereed) Published
Abstract [en]

Context. Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims. We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods. We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and from 0.25 to 5, respectively). Differential rotation and large-scale magnetic fields are produced self-consistently. Results. We find that for high thermal diffusivity, the rotation profiles show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased, corresponding to an increase of the Prandtl number. This coincides with and results in a change of the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the clearly cyclic solutions disappear at the highest magnetic Reynolds numbers and give way to irregular sign changes or quasi-stationary states. The total (mean and fluctuating) magnetic energy increases as a function of the magnetic Reynolds number in the range studied here (5-151), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases, however, we find that two regimes are possible, where either differential rotation is strong and mean magnetic fields are relatively weak, or vice versa. Conclusions. Our simulations indicate a strong nonlinear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients in the parameter range studied here.

Place, publisher, year, edition, pages
EDP Sciences, 2017
Keyword
convection, turbulence, dynamo, magnetohydrodynamics (MHD), Sun: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-205494 (URN)10.1051/0004-6361/201628973 (DOI)000395821900028 ()2-s2.0-85013480640 (Scopus ID)
Note

QC 20170524

Available from: 2017-05-24 Created: 2017-05-24 Last updated: 2017-05-24Bibliographically approved
Li, X.-Y., Brandenburg, A., Haugen, N. E. & Svensson, G. (2017). Eulerian and Lagrangian approaches to multidimensional condensation and collection. Journal of Advances in Modeling Earth Systems, 9(2), 1116-1137.
Open this publication in new window or tab >>Eulerian and Lagrangian approaches to multidimensional condensation and collection
2017 (English)In: Journal of Advances in Modeling Earth Systems, ISSN 1942-2466, Vol. 9, no 2, 1116-1137 p.Article in journal (Refereed) Published
Abstract [en]

Turbulence is argued to play a crucial role in cloud droplet growth. The combined problem of turbulence and cloud droplet growth is numerically challenging. Here an Eulerian scheme based on the Smoluchowski equation is compared with two Lagrangian superparticle (or superdroplet) schemes in the presence of condensation and collection. The growth processes are studied either separately or in combination using either two-dimensional turbulence, a steady flow or just gravitational acceleration without gas flow. Good agreement between the different schemes for the time evolution of the size spectra is observed in the presence of gravity or turbulence. The Lagrangian superparticle schemes are found to be superior over the Eulerian one in terms of computational performance. However, it is shown that the use of interpolation schemes such as the cloud-in-cell algorithm is detrimental in connection with superparticle or superdroplet approaches. Furthermore, the use of symmetric over asymmetric collection schemes is shown to reduce the amount of scatter in the results. For the Eulerian scheme, gravitational collection is rather sensitive to the mass bin resolution, but not so in the case with turbulence. Plain Language Summary The bottleneck problem of cloud droplet growth is one of the most challenging problems in cloud physics. Cloud droplet growth is neither dominated by condensation nor gravitational collision in the size range of 15 mu m similar to 40 mu m [1]. Turbulence-generated collection has been thought to be the mechanism to bridge the size gap, i.e., the bottleneck problem. This study compares the Lagrangian and Eulerian schemes in detail to tackle with the turbulence-generated collection.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2017
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:kth:diva-211615 (URN)10.1002/2017MS000930 (DOI)000406239300020 ()2-s2.0-85018911257 (Scopus ID)
Funder
Swedish Research Council, 20125797Knut and Alice Wallenberg Foundation, KAW 2014.0048Wenner-Gren Foundations
Note

QC 20170811

Available from: 2017-08-11 Created: 2017-08-11 Last updated: 2017-08-11Bibliographically approved
Brandenburg, A., Mathur, S. & Metcalfe, T. S. (2017). Evolution of Co-existing Long and Short Period Stellar Activity Cycles. Astrophysical Journal, 845(1), Article ID 79.
Open this publication in new window or tab >>Evolution of Co-existing Long and Short Period Stellar Activity Cycles
2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 845, no 1, 79Article in journal (Refereed) Published
Abstract [en]

The magnetic activity of the Sun becomes stronger and weaker over roughly an 11 year cycle, modulating the radiation and charged particle environment experienced by the Earth as "space weather." Decades of observations from the Mount Wilson Observatory have revealed that other stars also show regular activity cycles in their Ca II H+K line emission, and identified two different relationships between the length of the cycle and the rotation rate of the star. Recent observations at higher cadence have allowed the discovery of shorter cycles with periods between 1-3 years. Some of these shorter cycles coexist with longer cycle periods, suggesting that two underlying dynamos can operate simultaneously. We combine these new observations with previous data, and show that the longer and shorter cycle periods agree remarkably well with those expected from an earlier analysis based on the mean activity level and the rotation period. The relative turbulent length scales associated with the two branches of cyclic behavior suggest that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, whereas a dynamo operating in deeper layers may dominate in less active stars. However, several examples of equally prominent long and short cycles have been found at all levels of activity of stars younger than 2.3 Gyr. Deviations from the expected cycle periods show no dependence on the depth of the convection zone or on the metallicity. For some stars that exhibit longer cycles, we compute the periods of shorter cycles that might be detected with future high-cadence observations.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-214511 (URN)10.3847/1538-4357/aa7cfa (DOI)000407627500004 ()2-s2.0-85028007826 (Scopus ID)
Note

QC 20170929

Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2017-09-29Bibliographically approved
Brandenburg, A., Kahniashvili, T., Mandal, S., Pol, A. R., Tevzadze, A. G. & Vachaspati, T. (2017). Evolution of hydromagnetic turbulence from the electroweak phase transition. Physical Review D: covering particles, fields, gravitation, and cosmology, 96(12), Article ID 123528.
Open this publication in new window or tab >>Evolution of hydromagnetic turbulence from the electroweak phase transition
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2017 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 96, no 12, 123528Article in journal (Refereed) Published
Abstract [en]

We present new simulations of decaying hydromagnetic turbulence for a relativistic equation of state relevant to the early Universe. We compare helical and nonhelical cases either with kinetically or magnetically dominated initial fields. Both kinetic and magnetic initial helicities lead to maximally helical magnetic fields after some time, but with different temporal decay laws. Both are relevant to the early Universe, although no mechanisms have yet been identified that produce magnetic helicity with strengths comparable to the big bang nucleosynthesis limit at scales comparable to the Hubble horizon at the electroweak phase transition. Nonhelical magnetically dominated fields could still produce picoGauss magnetic fields under most optimistic conditions. Only helical magnetic fields can potentially have nanoGauss strengths at scales up to 30 kpc today.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-221006 (URN)10.1103/PhysRevD.96.123528 (DOI)000418658400005 ()2-s2.0-85040166331 (Scopus ID)
Note

QC 20180115

Available from: 2018-01-15 Created: 2018-01-15 Last updated: 2018-01-15Bibliographically approved
Kaepylae, P. J., Rheinhardt, M., Brandenburg, A., Arlt, R., Kaepylae, M. J., Lagg, A., . . . Warnecke, J. (2017). Extended Subadiabatic Layer in Simulations of Overshooting Convection. Astrophysical Journal Letters, 845(2), Article ID L23.
Open this publication in new window or tab >>Extended Subadiabatic Layer in Simulations of Overshooting Convection
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2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 845, no 2, L23Article in journal (Refereed) Published
Abstract [en]

We present numerical simulations of hydrodynamic overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper parts of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keyword
convection, hydrodynamics, turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-214331 (URN)10.3847/2041-8213/aa83ab (DOI)000408251700003 ()2-s2.0-85028452048 (Scopus ID)
Note

QC 20170913

Available from: 2017-09-13 Created: 2017-09-13 Last updated: 2017-09-13Bibliographically approved
Kahniashvili, T., Brandenburg, A., Durrer, R., Tevzadze, A. G. & Yin, W. (2017). Scale-invariant helical magnetic field evolution and the duration of inflation. Journal of Cosmology and Astroparticle Physics (11), Article ID 002.
Open this publication in new window or tab >>Scale-invariant helical magnetic field evolution and the duration of inflation
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2017 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 11, 002Article in journal (Refereed) Published
Abstract [en]

We consider a scale-invariant helical magnetic field generated during inflation. We show that, if the mean magnetic helicity density of such a field is measured, it can be used to determine a lower bound on the duration of inflation. Even if we just have upper bounds on the helicity, these can be used to derive constraints on the minimal duration if one assumes that the magnetic field generated during inflation is helical. Using three-dimensional simulations, we show that an initially scale-invariant field develops, which is similar both with and without magnetic helicity. In the fully helical case, however, the magnetic field appears to have a more pronounced folded structure.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-220621 (URN)10.1088/1475-7516/2017/12/002 (DOI)000417562300002 ()2-s2.0-85039840529 (Scopus ID)
Note

QC 20180112

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-01-12Bibliographically approved
Jabbari, S., Brandenburg, A., Kleeorin, N. & Rogachevskii, I. (2017). Sharp magnetic structures from dynamos with density stratification. Monthly notices of the Royal Astronomical Society, 467(3), 2753-2765.
Open this publication in new window or tab >>Sharp magnetic structures from dynamos with density stratification
2017 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 467, no 3, 2753-2765 p.Article in journal (Refereed) Published
Abstract [en]

Recent direct numerical simulations (DNS) of large-scale turbulent dynamos in strongly stratified layers have resulted in surprisingly sharp bipolar structures at the surface. Here, we present new DNS of helically and non-helically forced turbulence with and without rotation and compare with corresponding mean-field simulations (MFS) to show that these structures are a generic outcome of a broader class of dynamos in density-stratified layers. The MFS agree qualitatively with the DNS, but the period of oscillations tends to be longer in the DNS. In both DNS and MFS, the sharp structures are produced by converging flows at the surface and might be driven in non-linear stage of evolution by the Lorentz force associated with the large-scale dynamo-driven magnetic field if the dynamo number is at least 2.5 times supercritical.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2017
Keyword
dynamo, turbulence, sunspots
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-206233 (URN)10.1093/mnras/stx148 (DOI)000398419400020 ()
Note

QC 20170517

Available from: 2017-05-17 Created: 2017-05-17 Last updated: 2017-05-17Bibliographically approved
Brandenburg, A., Schober, J. & Rogachevskii, I. (2017). The contribution of kinetic helicity to turbulent magnetic diffusivity. Astronomical Notes - Astronomische Nachrichten, 338(7), 790-793.
Open this publication in new window or tab >>The contribution of kinetic helicity to turbulent magnetic diffusivity
2017 (English)In: Astronomical Notes - Astronomische Nachrichten, ISSN 0004-6337, E-ISSN 1521-3994, Vol. 338, no 7, 790-793 p.Article in journal (Refereed) Published
Abstract [en]

Using numerical simulations of forced turbulence, we show that for magnetic Reynolds numbers larger than unity, that is, beyond the regime of quasilinear theory, the turbulent magnetic diffusivity attains an additional negative contribution that is quadratic in the kinetic helicity. In particular, for large magnetic Reynolds numbers, the turbulent magnetic diffusivity without helicity is about twice the value with helicity. Such a contribution was not previously anticipated, but, as we discuss, it turns out to be important when accurate estimates of the turbulent magnetic diffusivity are needed.

Place, publisher, year, edition, pages
WILEY-V C H VERLAG GMBH, 2017
Keyword
magnetic fields, magnetohydrodynamics (MHD), turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-215839 (URN)10.1002/asna.201713384 (DOI)000412073100004 ()2-s2.0-8503024580 (Scopus ID)
Note

QC 20171017. 

NSF Astrophysics and Astronomy Grant Program, 1615100. Research Council of Norway, FRINATEK 231444. National Science Foundation, CNS-0821794. University of Colorado Boulder. University of Colorado Denver. National Center for Atmospheric Research.

Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2017-10-17Bibliographically approved
Brandenburg, A., Schober, J., Rogachevskii, I., Kahniashvili, T., Boyarsky, A., Frohlich, J., . . . Kleeorin, N. (2017). The Turbulent Chiral Magnetic Cascade in the Early Universe. Astrophysical Journal Letters, 845(2), Article ID L21.
Open this publication in new window or tab >>The Turbulent Chiral Magnetic Cascade in the Early Universe
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2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 845, no 2, L21Article in journal (Refereed) Published
Abstract [en]

The presence of asymmetry between fermions of opposite handedness in plasmas of relativistic particles can lead to exponential growth of a helical magnetic field via a small-scale chiral dynamo instability known as the chiral magnetic effect. Here, we show, using dimensional arguments and numerical simulations, that this process produces through the Lorentz force chiral magnetically driven turbulence. A k(-2) magnetic energy spectrum emerges via inverse transfer over a certain range of wavenumbers k. The total chirality (magnetic helicity plus normalized chiral chemical potential) is conserved in this system. Therefore, as the helical magnetic field grows, most of the total chirality gets transferred into magnetic helicity until the chiral magnetic effect terminates. Quantitative results for height, slope, and extent of the spectrum are obtained. Consequences of this effect for cosmic magnetic fields are discussed.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keyword
dynamo, early universe, magnetic fields, magnetohydrodynamics (MHD), turbulence
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-214330 (URN)10.3847/2041-8213/aa855d (DOI)000408251700001 ()2-s2.0-85028448126 (Scopus ID)
Note

QC 20170913

Available from: 2017-09-13 Created: 2017-09-13 Last updated: 2017-09-13Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0002-7304-021X

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