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Singh, N. K., Kaepylae, M. J., Brandenburg, A., Kaepylae, P. J., Lagg, A. & Virtanen, I. (2018). Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution. Astrophysical Journal, 863(2), Article ID 182.
Open this publication in new window or tab >>Bihelical Spectrum of Solar Magnetic Helicity and Its Evolution
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2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 863, no 2, article id 182Article in journal (Refereed) Published
Abstract [en]

Using a recently developed two-scale formalism to determine the magnetic helicity spectrum, we analyze synoptic vector magnetograms built with data from the Vector Spectromagnetograph instrument on the Synoptic Optical Long-term Investigations of the Sun telescope during 2010 January-2016 July. In contrast to an earlier study using only three Carrington rotations (CRs), our analysis includes 74 synoptic CR maps. We recover here bihelical spectra at different phases of solar cycle. 24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than 20% of the analyzed maps, however, show violations of the expected sign rule.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
dynamo, magnetohydrodynamics (MHD), Sun: magnetic fields, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-234607 (URN)10.3847/1538-4357/aad0f2 (DOI)000442628400018 ()2-s2.0-85052381323 (Scopus ID)
Note

QC 20180914

Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2018-09-14Bibliographically approved
Brandenburg, A. & Giampapa, M. S. (2018). Enhanced Stellar Activity for Slow Antisolar Differential Rotation. Astrophysical Journal Letters, 855(2), Article ID L22.
Open this publication in new window or tab >>Enhanced Stellar Activity for Slow Antisolar Differential Rotation
2018 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 855, no 2, article id L22Article in journal (Refereed) Published
Abstract [en]

High-precision photometry of solar-like members of the open cluster M67 with Kepler/K2 data has recently revealed enhanced activity for stars with a large Rossby number, which is the ratio of rotation period to the convective turnover time. Contrary to the well established behavior for shorter rotation periods and smaller Rossby numbers, the chromospheric activity of the more slowly rotating stars of M67 was found to increase with increasing Rossby number. Such behavior has never been reported before, although it was theoretically predicted to emerge as a consequence of antisolar differential rotation (DR) for stars with Rossby numbers larger than that of the Sun, because in those models the absolute value of the DR was found to exceed that for solar-like DR. Using gyrochronological relations and an approximate age of 4 Gyr for the members of M67, we compare with computed rotation rates using just the B - V color. The resulting rotation-activity relation is found to be compatible with that obtained by employing the measured rotation rate. This provides additional support for the unconventional enhancement of activity at comparatively low rotation rates and the possible presence of antisolar differential rotation.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
stars: activity, stars: late-type, stars: magnetic field, starspots
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-225190 (URN)10.3847/2041-8213/aab20a (DOI)000427499600002 ()2-s2.0-85044159388 (Scopus ID)
Note

QC 20180405

Available from: 2018-04-05 Created: 2018-04-05 Last updated: 2018-04-05Bibliographically approved
Schober, J., Rogachevskii, I., Brandenburg, A., Boyarsky, A., Fröhlich, J., Ruchayskiy, O. & Kleeorin, N. (2018). Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. II. Simulations. Astrophysical Journal, 858(2), Article ID 124.
Open this publication in new window or tab >>Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. II. Simulations
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2018 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 2, article id 124Article in journal (Refereed) Published
Abstract [en]

Using direct numerical simulations (DNS), we study laminar and turbulent dynamos in chiral magnetohydrodynamics with an extended set of equations that accounts for an additional contribution to the electric current due to the chiral magnetic effect (CME). This quantum phenomenon originates from an asymmetry between left-and right-handed relativistic fermions in the presence of a magnetic field and gives rise to a chiral dynamo. We show that the magnetic field evolution proceeds in three stages: (1) a small-scale chiral dynamo instability, (2) production of chiral magnetically driven turbulence and excitation of a large-scale dynamo instability due to a new chiral effect (alpha(mu) effect), and (3) saturation of magnetic helicity and magnetic field growth controlled by a conservation law for the total chirality. The alpha(mu) effect becomes dominant at large fluid and magnetic Reynolds numbers and is not related to kinetic helicity. The growth rate of the large-scale magnetic field and its characteristic scale measured in the numerical simulations agree well with theoretical predictions based on mean-field theory. The previously discussed two-stage chiral magnetic scenario did not include stage (2), during which the characteristic scale of magnetic field variations can increase by many orders of magnitude. Based on the findings from numerical simulations, the relevance of the CME and the chiral effects revealed in the relativistic plasma of the early universe and of protoneutron stars are discussed.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2018
Keywords
early universe, magnetic fields, magnetohydrodynamics (MHD), relativistic processes, stars: neutron, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-230523 (URN)10.3847/1538-4357/aaba75 (DOI)000433065500011 ()2-s2.0-85047427531 (Scopus ID)
Funder
Swedish Research CouncilEU, Horizon 2020, 665667
Note

QC 20180724

Available from: 2018-07-24 Created: 2018-07-24 Last updated: 2018-07-24Bibliographically approved
Bushby, P. J., Käpylä, P. J., Masada, Y., Brandenburg, A., Favier, B., Guervilly, C. & Kaepylae, M. J. (2018). Large-scale dynamos in rapidly rotating plane layer convection. Astronomy and Astrophysics, 612, Article ID A97.
Open this publication in new window or tab >>Large-scale dynamos in rapidly rotating plane layer convection
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A97Article in journal (Refereed) Published
Abstract [en]

Context. Convectively driven flows play a crucial role in the dynamo processes that are responsible for producing magnetic activity in stars and planets. It is still not fully understood why many astrophysical magnetic fields have a significant large-scale component. Aims. Our aim is to investigate the dynamo properties of compressible convection in a rapidly rotating Cartesian domain, focusing upon a parameter regime in which the underlying hydrodynamic flow is known to be unstable to a large-scale vortex instability. Methods. The governing equations of three-dimensional non-linear magnetohydrodynamics (MHD) are solved numerically. Different numerical schemes are compared and we propose a possible benchmark case for other similar codes. Results. In keeping with previous related studies, we find that convection in this parameter regime can drive a large-scale dynamo. The components of the mean horizontal magnetic field oscillate, leading to a continuous overall rotation of the mean field. Whilst the large-scale vortex instability dominates the early evolution of the system, the large-scale vortex is suppressed by the magnetic field and makes a negligible contribution to the mean electromotive force that is responsible for driving the large-scale dynamo. The cycle period of the dynamo is comparable to the ohmic decay time, with longer cycles for dynamos in convective systems that are closer to onset. In these particular simulations, large-scale dynamo action is found only when vertical magnetic field boundary conditions are adopted at the upper and lower boundaries. Strongly modulated large-scale dynamos are found at higher Rayleigh numbers, with periods of reduced activity (grand minima-like events) occurring during transient phases in which the large-scale vortex temporarily re-establishes itself, before being suppressed again by the magnetic field.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
convection, dynamo, instabilities, magnetic fields, magnetohydrodynamics (MHD), methods: numerical
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-228265 (URN)10.1051/0004-6361/201732066 (DOI)000431339800006 ()2-s2.0-85047226182 (Scopus ID)
Note

QC 20180523

Available from: 2018-05-23 Created: 2018-05-23 Last updated: 2018-10-16Bibliographically approved
Zhang, H. & Brandenburg, A. (2018). Solar Kinetic Energy and Cross Helicity Spectra. Astrophysical Journal Letters, 862(2), Article ID L17.
Open this publication in new window or tab >>Solar Kinetic Energy and Cross Helicity Spectra
2018 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 862, no 2, article id L17Article in journal (Refereed) Published
Abstract [en]

We develop a formalism that treats the calculation of solar kinetic energy and cross helicity spectra in an equal manner to that of magnetic energy and helicity spectra. The magnetic helicity spectrum is shown to be equal to the vertical part of the current helicity spectrum divided by the square of the wavenumber. For the cross helicity, we apply the recently developed two-scale approach globally over an entire active region to account for the sign change between the two polarities. Using vector magnetograms and Dopplergrams of NOAA. 11158 and 12266, we show that kinetic and magnetic energy spectra have similar slopes at intermediate wavenumbers, where the contribution from the granulation velocity has been removed. At wavenumbers around 0.3 Mm(-1), the magnetic helicity is found to be close to its maximal value. The cross helicity spectra are found to be within about 10% of the maximum possible value. Using the two-scale method for NOAA. 12266, the global cross helicity spectrum is found to be particularly steep, similarly to what has previously been found in theoretical models of spot generation. In the quiet Sun, by comparison, the cross helicity spectrum is found to be small.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
dynamo, Sun: activity, Sun: magnetic fields, Sun: photosphere
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-233286 (URN)10.3847/2041-8213/aad337 (DOI)000440583700003 ()2-s2.0-85051504860 (Scopus ID)
Note

QC 20180821

Available from: 2018-08-21 Created: 2018-08-21 Last updated: 2018-10-16Bibliographically approved
Perri, B. & Brandenburg, A. (2018). Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface. Astronomy and Astrophysics, 609, Article ID A99.
Open this publication in new window or tab >>Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, article id A99Article in journal (Refereed) Published
Abstract [en]

Context. The formation of sunspots requires the concentration of magnetic flux near the surface. The negative effective magnetic pressure instability (NEMPI) might be a possible mechanism for accomplishing this, but it has mainly been studied in simple systems using an isothermal equation of state without a natural free surface. Aims. We study NEMPI in a stratified Cartesian mean-field model where turbulence effects are parameterized. We use an ideal equation of state and include radiation transport, which establishes selfconsistently a free surface. Methods. We use a Kramers-type opacity with adjustable exponents chosen such that the deeper layers are approximately isentropic. No convection is therefore possible in this model, allowing us to study NEMPI with radiation in isolation. We restrict ourselves to two-dimensional models. We use artificially enhanced mean-field coefficients to allow NEMPI to develop, thereby making it possible to study the reason why it is much harder to excite in the presence of radiation. Results. NEMPI yields moderately strong magnetic flux concentrations a certain distance beneath the surface where the optical depth is unity. The instability is oscillatory and in the form of upward traveling waves. This seems to be a new effect that has not been found in earlier models without radiative transport. The horizontal wavelength is about ten times smaller than what has previously been found in more idealized isothermal models. Conclusions. In our models, NEMPI saturates at field strengths too low to explain sunspots. Furthermore, the structures appear too narrow and too far beneath the surface to cause significant brightness variations at the radiative surface. We speculate that the failure to reproduce effects resembling sunspots may be related to the neglect of convection.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
Hydrodynamics, Radiative transfer, Sun: atmosphere, Sunspots
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-222504 (URN)10.1051/0004-6361/201730421 (DOI)000423437600004 ()2-s2.0-85041195183 (Scopus ID)
Note

QC 20180212

Available from: 2018-02-12 Created: 2018-02-12 Last updated: 2018-02-12Bibliographically approved
Brandenburg, A., Durrer, R., Kahniashvili, T., Mandalc, S. & Yin, W. W. (2018). Statistical properties of scale-invariant helical magnetic fields and applications to cosmology. Journal of Cosmology and Astroparticle Physics (8), Article ID 034.
Open this publication in new window or tab >>Statistical properties of scale-invariant helical magnetic fields and applications to cosmology
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2018 (English)In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 8, article id 034Article in journal (Refereed) Published
Abstract [en]

We investigate the statistical properties of isotropic, stochastic, Gaussian distributed, helical magnetic fields characterized by different shapes of the energy spectra at large length scales and study the associated realizability condition. We discuss smoothed magnetic fields that are commonly used when the primordial magnetic field is constrained by observational data. We are particularly interested in scale-invariant magnetic fields that can be generated during the inflationary stage by quantum fluctuations. We determine the correlation length of such magnetic fields and relate it to the infrared cutoff of perturbations produced during inflation. We show that this scale determines the observational signatures of the inflationary magnetic fields on the cosmic microwave background. At smaller scales, the scale-invariant spectrum changes with time. It becomes a steeper weak-turbulence spectrum at progressively larger scales. We show numerically that the critical length scale where this happens is the turbulent-diffusive scale, which increases with the square root of time.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2018
Keywords
Magnetohydrodynamics, primordial magnetic fields, CMBR polarisation, physics of the early universe
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:kth:diva-234622 (URN)10.1088/1475-7516/2018/08/034 (DOI)000442637500001 ()2-s2.0-85053048248 (Scopus ID)
Note

QC 20180913

Available from: 2018-09-13 Created: 2018-09-13 Last updated: 2018-10-16Bibliographically approved
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, article id 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
Keywords
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, article id 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
Keywords
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, p. 1116-1137Article 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
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ORCID iD: ORCID iD iconorcid.org/0000-0002-7304-021X

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