| 1. |
- Viviani, M., et al.
(författare)
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Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616
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Tidskriftsartikel (refereegranskat)abstract
- Context. Both dynamo theory and observations of stellar large-scale magnetic fields suggest a change from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation. Aims. We seek to understand this transition using numerical simulations. Methods. We use three-dimensional simulations of turbulent magnetohydrodynamic convection in spherical shell wedges and considered rotation rates between 1 and 31 times the solar value. Results. We find a transition from axi- to nonaxisymmetric solutions at around 1.8 times the solar rotation rate. This transition coincides with a change in the rotation profile from antisolar- to solar-like differential rotation with a faster equator and slow poles. In the solar-like rotation regime, the field configuration consists of an axisymmetric oscillatory field accompanied by an m = 1 azimuthal mode (two active longitudes), which also shows temporal variability. At slow (rapid) rotation, the axisymmetric (nonaxisymmetric) mode dominates. The axisymmetric mode produces latitudinal dynamo waves with polarity reversals, while the nonaxisymmetric mode often exhibits a slow drift in the rotating reference frame and the strength of the active longitudes changes cyclically over time between the different hemispheres. In the majority of cases we find retrograde waves, while prograde waves are more often found from observations. Most of the obtained dynamo solutions exhibit cyclic variability either caused by latitudinal or azimuthal dynamo waves. In an activity-period diagram, the cycle lengths normalized by the rotation period form two different populations as a function of rotation rate or magnetic activity level. The slowly rotating axisymmetric population lies close to what in observations is called the inactive branch, where the stars are believed to have solar-like differential rotation, while the rapidly rotating models are close to the superactive branch with a declining cycle to rotation frequency ratio and an increasing rotation rate. Conclusions. We can successfully reproduce the transition from axi- to nonaxisymmetric dynamo solutions for high rotation rates, but high-resolution simulations are required to limit the effect of rotational quenching of convection at rotation rates above 20 times the solar value.
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| 2. |
- Hackman, T., et al.
(författare)
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From convective stellar dynamo simulations to Zeeman-Doppler images
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 682
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Tidskriftsartikel (refereegranskat)abstract
- Context. Zeeman-Doppler imaging (ZDI) is used to reconstruct the surface magnetic field of late-type stars from high-resolution spectropolarimetric observations. The results are usually described in terms of characteristics of the field topology, such as poloidality versus toroidality and axisymmetry versus non-axisymmetry, in addition to the field strength. Aims. In this study, we want to test how well these characteristics are preserved when applying the ZDI method to simulated data. We are particularly interested in how accurately the field topology is preserved and to what extent stellar parameters, such as projected rotation velocity and rotation axis inclination, influence the reconstruction. Methods. For these tests, we used published magnetic field vector data from direct numerical magnetohydrodynamic simulations taken near the surface of the simulation domain. These simulations have variable rotation rates and therefore represent different levels of activity of an otherwise Sun-like setup with a convective envelope of solar thickness. Our ZDI reconstruction is based on spherical harmonics expansion. By comparing the original values to those of the reconstructed images, we study the ability to reconstruct the surface magnetic field in terms of various characteristics of the field. Results. In general, the ZDI method works as expected. The main large-scale features are reasonably well recovered, but the strength of the recovered magnetic field is just a fraction of the original input. The quality of the reconstruction shows clear correlations with the data quality. Furthermore, there are some spurious dependencies between stellar parameters and the characteristics of the field. Conclusions. Our study uncovers some limits of ZDI. Firstly, the recovered field strength will generally be lower than the ‘real’ value, as smaller structures with opposite polarities will be blurred in the inversion. This is also seen in the relative distribution of magnetic energy in terms of the angular degree `. Secondly, the axisymmetry is overestimated. The poloidality versus toroidality is better recovered. The reconstruction works better for a stronger field and faster rotation velocity. Still, the ZDI method works surprisingly well even for a weaker field and slow rotation provided the data have a high signal-to-noise ratio and good rotation phase coverage.
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| 3. |
- Kapyla, M. J., et al.
(författare)
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Multiple dynamo modes as a mechanism for long-term solar activity variations
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 589
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Tidskriftsartikel (refereegranskat)abstract
- Context. Solar magnetic activity shows both smooth secular changes, such as the modern Grand Maximum, and quite abrupt drops that are denoted as grand minima, such as the Maunder Minimum. Direct numerical simulations (DNS) of convection-driven dynamos off er one way of examining the mechanisms behind these events. Aims. In this work, we analyze a solution of a solar-like DNS that was evolved for roughly 80 magnetic cycles of 4.9 years and where epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. Methods. The DNS employed is a semi-global (wedge-shaped) magnetoconvection model. For the data analysis tasks we use Ensemble Empirical Mode Decomposition and phase dispersion methods, as they are well suited for analyzing cyclic (non-periodic) signals. Results. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like (equatorward migration at low latitudes and poleward at high latitudes). This mode is accompanied by a higher frequency mode near the surface and at low latitudes, showing poleward migration, and a low-frequency mode at the bottom of the convection zone. The low-frequency mode is almost purely antisymmetric with respect to the equator, while the dominant mode has strongly fluctuating mixed parity. The overall behavior of the dynamo solution is extremely complex, exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum; during this epoch the bottom toroidal magnetic field obtains a maximum, demonstrating that the interpretation of grand minima-type events is non-trivial. The hemispherical asymmetries are seen only in the magnetic field, while the velocity field exhibits considerably weaker asymmetry. Conclusions. We interpret the overall irregular behavior as being due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low-activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.
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| 4. |
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| 5. |
- Käpylä, Petri J., et al.
(författare)
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Convection-driven spherical shell dynamos at varying Prandtl numbers
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 599
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- van Daalen, Kim R., et al.
(författare)
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The 2024 Europe report of the lancet countdown on health and climate change : unprecedented warming demands unprecedented action
- 2024
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Ingår i: The Lancet Public Health. - : Elsevier. - 2468-2667. ; 9:7, s. e495-e522
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Forskningsöversikt (refereegranskat)abstract
- Record-breaking temperatures were recorded across the globe in 2023. Without climate action, adverse climate-related health impacts are expected to worsen worldwide, affecting billions of people. Temperatures in Europe are warming at twice the rate of the global average, threatening the health of populations across the continent and leading to unnecessary loss of life. The Lancet Countdown in Europe was established in 2021, to assess the health profile of climate change aiming to stimulate European social and political will to implement rapid health-responsive climate mitigation and adaptation actions. In 2022, the collaboration published its indicator report, tracking progress on health and climate change via 33 indicators and across five domains.This new report tracks 42 indicators highlighting the negative impacts of climate change on human health, the delayed climate action of European countries, and the missed opportunities to protect or improve health with health-responsive climate action. The methods behind indicators presented in the 2022 report have been improved, and nine new indicators have been added, covering leishmaniasis, ticks, food security, health-care emissions, production and consumption-based emissions, clean energy investment, and scientific, political, and media engagement with climate and health. Considering that negative climate-related health impacts and the responsibility for climate change are not equal at the regional and global levels, this report also endeavours to reflect on aspects of inequality and justice by highlighting at-risk groups within Europe and Europe's responsibility for the climate crisis.
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| 7. |
- Warnecke, J., et al.
(författare)
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Influence of a coronal envelope as a free boundary to global convective dynamo simulations
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 596
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We explore the effects of an outer stably stratified coronal envelope on rotating turbulent convection, differential rotation, and large-scale dynamo action in spherical wedge models of the Sun. Methods. We solve the compressible magnetohydrodynamic equations in a two-layer model with unstable stratification below the surface, representing the convection zone, and a stably stratified coronal envelope above. The interface represents a free surface. We compare our model to models that have no coronal envelope. Results. The presence of a coronal envelope is found to modify the Reynolds stress and the Lambda effect resulting in a weaker and non-cylindrical differential rotation. This is related to the reduced latitudinal temperature variations that are caused by and dependent on the angular velocity. Some simulations develop a near-surface shear layer that we can relate to a sign change in the meridional Reynolds stress term in the thermal wind balance equation. Furthermore, the presence of a free surface changes the magnetic field evolution since the toroidal field is concentrated closer to the surface. In all simulations, however, the migration direction of the mean magnetic field can be explained by the Parker-Yoshimura rule, which is consistent with earlier findings. Conclusions. A realistic treatment of the upper boundary in spherical dynamo simulations is crucial for the dynamics of the flow and magnetic field evolution.
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| 8. |
- Warnecke, J., et al.
(författare)
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Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 609
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We investigate dynamo action in global compressible solar-like convective dynamos in the framework of mean-field theory. Methods. We simulate a solar-type star in a wedge-shaped spherical shell, where the interplay between convection and rotation self-consistently drives a large-scale dynamo. To analyze the dynamo mechanism we apply the test-field method for azimuthally (φ) averaged fields to determine the 27 turbulent transport coefficients of the electromotive force, of which six are related to the α tensor. This method has previously been used either in simulations in Cartesian coordinates or in the geodynamo context and is applied here for the first time to fully compressible simulations of solar-like dynamos. Results. We find that the φφ-component of the α tensor does not follow the profile expected from that of kinetic helicity. The turbulent pumping velocities significantly alter the effective mean flows acting on the magnetic field and therefore challenge the flux transport dynamo concept. All coefficients are significantly affected by dynamically important magnetic fields. Quenching as well as enhancement are being observed. This leads to a modulation of the coefficients with the activity cycle. The temporal variations are found to be comparable to the time-averaged values and seem to be responsible for a nonlinear feedback on the magnetic field generation. Furthermore, we quantify the validity of the Parker-Yoshimura rule for the equatorward propagation of the mean magnetic field in the present case.
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| 9. |
- Kaepylae, Petri J., et al.
(författare)
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Extended Subadiabatic Layer in Simulations of Overshooting Convection
- 2017
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Ingår i: Astrophysical Journal Letters. - : IOP PUBLISHING LTD. - 2041-8205 .- 2041-8213. ; 845:2
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Käpylä, Petri J., et al.
(författare)
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Effects of enhanced stratification on equatorward dynamo wave propagation
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 778:1, s. 41-
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Tidskriftsartikel (refereegranskat)abstract
- We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification, we find quasi-steady dynamo solutions for moderate rotation and oscillatory ones with poleward migration of activity belts for more rapid rotation. For stronger stratification, the growth rate tends to become smaller. Furthermore, a transition from quasi-steady to oscillatory dynamos is found as the Coriolis number is increased, but now there is an equatorward migrating branch near the equator. The breakpoint where this happens corresponds to a rotation rate that is about three to seven times the solar value. The phase relation of the magnetic field is such that the toroidal field lags behind the radial field by about pi/2, which can be explained by an oscillatory alpha(2) dynamo caused by the sign change of the alpha-effect about the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an m = 1 mode is excited for a full 2 pi azimuthal extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.
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| 11. |
- Käpylä, Petri J., et al.
(författare)
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Effects of strong stratification on equatorward dynamo wave propagation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification we find quasi-steady solutions for moderate rotation and oscillatory dynamos with poleward migration of activity belts for more rapid rotation. For stronger stratification a similar transition as a function of the Coriolis number is found, but with an equatorward migrating branch near the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an m=1 mode is excited for a full 2π φ-extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.
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| 12. |
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| 13. |
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| 14. |
- Warnecke, Jörn, et al.
(författare)
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Ejections of Magnetic Structures Above a Spherical Wedge Driven by a Convective Dynamo with Differential Rotation
- 2012
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Ingår i: Solar Physics. - : Springer Science and Business Media LLC. - 0038-0938 .- 1573-093X. ; 280:2, s. 299-319
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Tidskriftsartikel (refereegranskat)abstract
- We combine a convectively driven dynamo in a spherical shell with a nearly isothermal density-stratified cooling layer that mimics some aspects of a stellar corona to study the emergence and ejections of magnetic field structures. This approach is an extension of earlier models, where forced turbulence simulations were employed to generate magnetic fields. A spherical wedge is used which consists of a convection zone and an extended coronal region to a parts per thousand aEuro parts per thousand 1.5 times the radius of the sphere. The wedge contains a quarter of the azimuthal extent of the sphere and 150(a similar to) in latitude. The magnetic field is self-consistently generated by the turbulent motions due to convection beneath the surface. Magnetic fields are found to emerge at the surface and are ejected to the coronal part of the domain. These ejections occur at irregular intervals and are weaker than in earlier work. We tentatively associate these events with coronal mass ejections on the Sun, even though our model of the solar atmosphere is rather simplistic.
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| 15. |
- Warnecke, J., et al.
(författare)
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Investigating Global Convective Dynamos with Mean-field Models : Full Spectrum of Turbulent Effects Required
- 2021
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 919:2
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Tidskriftsartikel (refereegranskat)abstract
- The role of turbulent effects for dynamos in the Sun and stars continues to be debated. Mean-field (MF) theory provides a broadly used framework to connect these effects to fundamental magnetohydrodynamics. While inaccessible observationally, turbulent effects can be directly studied using global convective dynamo (GCD) simulations. We measure the turbulent effects in terms of turbulent transport coefficients, based on the MF framework, from an exemplary GCD simulation using the test-field method. These coefficients are then used as an input into an MF model. We find a good agreement between the MF and GCD solutions, which validates our theoretical approach. This agreement requires all turbulent effects to be included, even those which have been regarded as unimportant so far. Our results suggest that simple dynamo models, as are commonly used in the solar and stellar community, relying on very few, precisely fine-tuned turbulent effects, may not be representative of the full dynamics of dynamos in global convective simulations and astronomical objects.
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| 16. |
- Warnecke, Jörn, et al.
(författare)
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Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers
- 2023
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Ingår i: Nature Astronomy. - : Springer Nature. - 2397-3366. ; 7:6, s. 662-668
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic fields on small scales are ubiquitous in the Universe. Although they can often be observed in detail, their generation mechanisms are not fully understood. One possibility is the so-called small-scale dynamo (SSD). Prevailing numerical evidence, however, appears to indicate that an SSD is unlikely to exist at very low magnetic Prandtl numbers (PrM) such as those that are present in the Sun and other cool stars. Here we have performed high-resolution simulations of isothermal forced turbulence using the lowest PrM values achieved so far. Contrary to earlier findings, the SSD not only turns out to be possible for PrM down to 0.0031 but also becomes increasingly easier to excite for PrM below about 0.05. We relate this behaviour to the known hydrodynamic phenomenon referred to as the bottleneck effect. Extrapolating our results to solar values of PrM indicates that an SSD would be possible under such conditions.
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| 17. |
- Warnecke, Jörn, et al.
(författare)
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ON THE CAUSE OF SOLAR-LIKE EQUATORWARD MIGRATION IN GLOBAL CONVECTIVE DYNAMO SIMULATIONS
- 2014
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 796:1
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Tidskriftsartikel (refereegranskat)abstract
- We present results from four convectively driven stellar dynamo simulations in spherical wedge geometry. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons, we show that the migration direction can be explained by an alpha Omega dynamo wave following the Parker-Yoshimura rule. We conclude that the equatorward migration in this and previous work is due to a positive (negative) alpha effect in the northern (southern) hemisphere and a negative radial gradient of Omega outside the inner tangent cylinder of these models. This idea is supported by a strong correlation between negative radial shear and toroidal field strength in the region of equatorward propagation.
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| 18. |
- Warnecke, Jörn, 1982-, et al.
(författare)
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Solar-like differential rotation in a convective dynamo with a coronal envelope
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We report on the results of four convective dynamo simulations with an souter coronal layer. The magnetic field is self-consistently generated by the convectivemotions beneath the surface. Above the convection zone we include a polytropic layerthat extends to 1.6 solar radii. The temperature increases in this regionto ≈8 times the value at the surface, corresponding to ≈1.2 times the value at the bottom of the spherical shell. We associate this region with the solar corona. We find a solar-like differential rotation with radial contours of constant rotation rate, together with a solar-like meridionalcirculation and a near-surface shear layer. This spoke-like rotation profile is caused by a non-zero latitudinalentropy gradient which violates the Taylor-Proudman balance via thebaroclinic term. The lower density stratification compared with the Sun leads to anequatorward return flow above the surface. The mean magnetic field is in most of the casesoscillatory with equatorward migration in one case. In other cases the equatorward migration is overlaid by stationary oreven poleward migrating mean fields.
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| 19. |
- Warnecke, Jörn, et al.
(författare)
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SPOKE-LIKE DIFFERENTIAL ROTATION IN A CONVECTIVE DYNAMO WITH A CORONAL ENVELOPE
- 2013
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 778:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on the results of four convective dynamo simulations with an outer coronal layer. The magnetic field is self-consistently generated by the convective motions beneath the surface. Above the convection zone, we include a polytropic layer that extends to 1.6 solar radii. The temperature increases in this region to approximate to 8 times the value at the surface, corresponding to approximate to 1.2 times the value at the bottom of the spherical shell. We associate this region with the solar corona. We find solar-like differential rotation with radial contours of constant rotation rate, together with a near-surface shear layer. This non-cylindrical rotation profile is caused by a non-zero latitudinal entropy gradient that offsets the Taylor-Proudman balance through the baroclinic term. The meridional circulation is multi-cellular with a solar-like poleward flow near the surface at low latitudes. In most of the cases, the mean magnetic field is oscillatory with equatorward migration in two cases. In other cases, the equatorward migration is overlaid by stationary or even poleward migrating mean fields.
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| 20. |
- Drinnenberg, IA, et al.
(författare)
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EvoChromo: towards a synthesis of chromatin biology and evolution
- 2019
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Ingår i: Development (Cambridge, England). - : The Company of Biologists. - 1477-9129 .- 0950-1991. ; 146:19
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Tidskriftsartikel (refereegranskat)abstract
- Over the past few years, interest in chromatin and its evolution has grown. To further advance these interests, we organized a workshop with the support of The Company of Biologists to debate the current state of knowledge regarding the origin and evolution of chromatin. This workshop led to prospective views on the development of a new field of research that we term ‘EvoChromo’. In this short Spotlight article, we define the breadth and expected impact of this new area of scientific inquiry on our understanding of both chromatin and evolution.
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| 21. |
- Losada, Illa Rivero, et al.
(författare)
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A new look at sunspot formation using theory and observations
- 2016
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Ingår i: Proceedings of the International Astronomical Union. - : Cambridge University Press. ; , s. 46-59
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Konferensbidrag (refereegranskat)abstract
- Sunspots are of basic interest in the study of the Sun. Their relevance ranges from them being an activity indicator of magnetic fields to being the place where coronal mass ejections and flares erupt. They are therefore also an important ingredient of space weather. Their formation, however, is still an unresolved problem in solar physics. Observations utilize just 2D surface information near the spot, but it is debatable how to infer deep structures and properties from local helioseismology. For a long time, it was believed that flux tubes rising from the bottom of the convection zone are the origin of the bipolar sunspot structure seen on the solar surface. However, this theory has been challenged, in particular recently by new surface observation, helioseismic inversions, and numerical models of convective dynamos. In this article we discuss another theoretical approach to the formation of sunspots: the negative effective magnetic pressure instability. This is a large-scale instability, in which the total (kinetic plus magnetic) turbulent pressure can be suppressed in the presence of a weak large-scale magnetic field, leading to a converging downflow, which eventually concentrates the magnetic field within it. Numerical simulations of forced stratified turbulence have been able to produce strong superequipartition flux concentrations, similar to sunspots at the solar surface. In this framework, sunspots would only form close to the surface due to the instability constraints on stratification and rotation. Additionally, we present some ideas from local helioseismology, where we plan to use the Hankel analysis to study the pre-emergence phase of a sunspot and to constrain its deep structure and formation mechanism.
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| 22. |
- Lühr, H, et al.
(författare)
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Fine-structure of field-aligned currente sheets deduced from spacecraft and ground-based observations - initial Freja results
- 1994
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 21, s. 1883-1886
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Tidskriftsartikel (refereegranskat)abstract
- Due to its special orbit-over large segments tangential to the auroral oval and at altitudes around 1600 km-FREJA provides measurements from a sparsely sampled region. Here we will introduce initial results from the magnetic field experiment. For one event occurring about an hour after midnight we investigate the fine structure of the field-aligned currents associated with a westward electrojet. From simultaneous ground-based observations we deduce the orientation and the spatial and temporal variations of the electrojet. Space-borne magnetic field measurements provide estimates of the field-aligned current density. We deduce a lower limit of 1.75 km for the thickness of field-aligned current filaments. The comparison between electric and magnetic field recordings on FREJA suggests a poor correlation of these quantities for this event.
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| 23. |
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| 24. |
- Warnecke, H., et al.
(författare)
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New metrological capabilities for measurements of dynamic liquid flows
- 2022
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Ingår i: Metrologia. - : IOP Publishing Ltd. - 0026-1394 .- 1681-7575. ; 59:2
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Tidskriftsartikel (refereegranskat)abstract
- The capability to calibrate flow and volume devices dynamically has gained increasing interest over the years. Within the scope of the EMPIR project 17IND13 'Metrology for real-world domestic water metering', several test rigs were developed with which dynamic flow profiles can be generated and measured that reflect characteristics of real-world drinking water consumption. The dynamic component of the test rigs is realized based on different technologies such as valves, cavitation nozzles or piston provers. For validation purposes, an intercomparison of the test rigs was carried out in the scope of an EURAMET pilot study no. 1506. Between September 2020 and February 2021, a transfer standard specially developed for the intercomparison was calibrated at eight laboratories. The measurement error was determined for three dynamic flow profiles representative of drinking water consumption in Europe. In addition to determining the measurement errors and the degree of equivalence, five additional key parameters were derived to characterize the test rig properties: (1) repeatability of the profile measurements, (2) mean value of the residuals, (3) deviation between measured total mass and total mass resulting from the given profile and (4) duration of the flow change for an increasing change (5) and duration of the flow change for a decreasing change. These key parameters comprehensively describe the quality with which the dynamic flow profiles were generated and measured on the test rigs and can be used for evaluations in future intercomparisons of this kind. A main outcome of the intercomparison is that there is no technology to be preferred in terms of technical implementation. All test rigs agree well with each other, taking into account their expanded measurement uncertainties.
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