E-Print Archive

There are 4291 abstracts currently viewable.


Search:

Advanced Search
Options
Main Page Add New E-Print Submitter
Information
Feedback
News Help/FAQ About Preferences
Manage Key Phrase
Notification
* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Helicity proxies from linear polarisation of solar active regions  

Axel Brandenburg   Submitted: 2020-09-12 08:56

Abstract: The α effect is believed to play a key role in the generation of the solar magnetic field. A fundamental test for its significance in the solar dynamo is to look for magnetic helicity of opposite signs in the two hemispheres, and at small and large scales. However, measuring magnetic helicity is compromised by the inability to fully infer the magnetic field vector from observations of solar spectra, caused by what is known as the "pi ambiguity" of spectropolarimetric observations. We decompose linear polarisation into parity-even and parity-odd E and B polarisations, which are not affected by the "pi ambiguity". Furthermore, we study whether the correlations of spatial Fourier spectra of B and parity-even quantities such as E or temperature T are a robust proxy for magnetic helicity of solar magnetic fields. We analyse polarisation measurements of active regions observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics observatory. Theory predicts the magnetic helicity of active regions to have, statistically, opposite signs in the two hemispheres. We then compute the parity-odd E B and T B correlations, and test for systematic preference of their sign based on the hemisphere of the active regions. We find that: (i) E B and T B correlations are a reliable proxy for magnetic helicity, when computed from linear polarisation measurements away from spectral line cores, and (ii) E polarisation reverses its sign close to the line core. Our analysis reveals Faraday rotation to not have a significant influence on the computed parity-odd correlations. The EB decomposition of linear polarisation appears to be a good proxy for magnetic helicity independent of the "pi ambiguity". This allows us to routinely infer magnetic helicity directly from polarisation measurements.

Authors: A. Prabhu, A. Brandenburg, M. J. Käpylä, A. Lagg
Projects: None

Publication Status: Astron. Astrophys. 641, A46 (2020)
Last Modified: 2020-09-14 23:53
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection  

Axel Brandenburg   Submitted: 2020-08-04 12:33

We test the sensitivity of hydrodynamic and magnetohydrodynamic turbulent convection simulations with respect to Mach number, thermal and magnetic boundary conditions, and the centrifugal force. We find that varying the luminosity, which also controls the Mach number, has only a minor effect on the large-scale dynamics. A similar conclusion can also be drawn from the comparison of two formulations of the lower magnetic boundary condition with either vanishing electric field or current density. The centrifugal force has an effect on the solutions, but only if its magnitude with respect to acceleration due to gravity is by two orders of magnitude greater than in the Sun. Finally, we find that the parameterisation of the photospheric physics, either by an explicit cooling term or enhanced radiative diffusion, is more important than the thermal boundary condition. In particular, runs with cooling tend to lead to more anisotropic convection and stronger deviations from the Taylor-Proudman state. In summary, the fully compressible approach taken here with the Pencil Code is found to be valid, while still allowing the disparate timescales to be taken into account.

Authors: Käpylä, P. J., Gent, F. A., Olspert, N., Käpylä, M. J., & Brandenburg, A.
Projects: None

Publication Status: Geophys. Astrophys. Fluid Dyn. 114, 8-34 (2020)
Last Modified: 2020-08-05 14:12
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

f-mode strengthening from a localized bipolar subsurface magnetic field  

Axel Brandenburg   Submitted: 2020-05-23 01:40

Recent numerical work in helioseismology has shown that a periodically varying subsurface magnetic field leads to a fanning of the f-mode, which emerges from the density jump at the surface. In an attempt to model a more realistic situation, we now modulate this periodic variation with an envelope, giving thus more emphasis on localized bipolar magnetic structures in the middle of the domain. Some notable findings are: (i) compared to the purely hydrodynamic case, the strength of the f-mode is significantly larger at high horizontal wavenumbers k, but the fanning is weaker for the localized subsurface magnetic field concentrations investigated here than the periodic ones studied earlier; (ii) when the strength of the magnetic field is enhanced at a fixed depth below the surface, the fanning of the f-mode in the kω diagram increases proportionally in such a way that the normalized f-mode strengths remain nearly the same in different such cases; (iii) the unstable Bloch modes reported previously in case of harmonically varying magnetic fields are now completely absent when more realistic localized magnetic field concentrations are imposed beneath the surface, thus suggesting that the Bloch modes are unlikely to be supported during most phases of the solar cycle; (iv) the f-mode strength appears to depend also on the depth of magnetic field concentrations such that it shows a relative decrement when the maximum of the magnetic field is moved to a deeper layer. We argue that detections of f-mode perturbations such as those being explored here could be effective tracers of solar magnetic fields below the photosphere before these are directly detectable as visible manifestations in terms of active regions or sunspots.

Authors: Authors:Nishant K. Singh, Harsha Raichur, Maarit J. Käpylä, Matthias Rheinhardt, Axel Brandenburg, Petri J. Käpylä
Projects: None

Publication Status: Geophys. Astro. Fluid 2020, 114, 196-212 (2020)
Last Modified: 2020-05-27 13:13
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

The nature of mean-field generation in three classes of optimal dynamos  

Axel Brandenburg   Submitted: 2020-04-17 13:13

Abstract: In recent years, several optimal dynamos have been discovered. They minimize the magnetic energy dissipation or, equivalently, maximize the growth rate at a fixed magnetic Reynolds number. In the optimal dynamo of Willis (2012, Phys. Rev. Lett. 109, 251101), we find mean-field dynamo action for planar averages. One component of the magnetic field grows exponentially while the other decays in an oscillatory fashion near onset. This behavior is different from that of an α 2 dynamo, where the two non-vanishing components of the planar averages are coupled and have the same growth rate. For the Willis dynamo, we find that the mean field is excited by a negative turbulent magnetic diffusivity, which has a non-uniform spatial profile near onset. The temporal oscillations in the decaying component are caused by the corresponding component of the diffusivity tensor being complex when the mean field is decaying and, in this way, time-dependent. The growing mean field can be modeled by a negative magnetic diffusivity combined with a positive magnetic hyperdiffusivity. In two other classes of optimal dynamos of Chen et al. (2015, J. Fluid Mech. 783, 23), we find, to some extent, similar mean-field dynamo actions. When the magnetic boundary conditions are mixed, the two components of the planar averaged field grow at different rates when the dynamo is 15% supercritical. When the mean magnetic field satisfies homogeneous boundary conditions (where the magnetic field is tangential to the boundary), mean-field dynamo action is found for one-dimensional averages, but not for planar averages. Despite having different spatial profiles, both dynamos show negative turbulent magnetic diffusivities. Our finding suggests negative turbulent magnetic diffusivities may support a broader class of dynamos than previously thought, including these three optimal dynamos.

Authors: Axel Brandenburg, Long Chen
Projects: None

Publication Status: J. Plasma Phys. 86 (2020) 905860110
Last Modified: 2020-04-18 12:48
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Magnetic helicity dissipation and production in an ideal MHD code  

Axel Brandenburg   Submitted: 2020-03-17 21:01

Abstract: We study a turbulent helical dynamo in a periodic domain by solving the ideal magnetohydrodynamic (MHD) equations with the FLASH code using the divergence-cleaning eight-wave method and compare our results with direct numerical simulations (DNS) using the Pencil Code. At low resolution, FLASH reproduces the DNS results qualitatively by developing the large-scale magnetic field expected from DNS, but at higher resolution, no large-scale magnetic field is obtained. In all those cases in which a large-scale magnetic field is generated, the ideal MHD results yield too little power at small scales. As a consequence, the small-scale current helicity is too small compared with that of the DNS. The resulting net current helicity has then always the wrong sign, and its statistical average also does not approach zero at late times, as expected from the DNS. Our results have implications for astrophysical dynamo simulations of stellar and galactic magnetism using ideal MHD codes.

Authors: Axel Brandenburg, Evan Scannapieco
Projects: None

Publication Status: Astrophys. J. 889, 55 (2020)
Last Modified: 2020-03-18 13:05
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

The time step constraint in radiation hydrodynamics  

Axel Brandenburg   Submitted: 2020-01-18 04:38

Explicit radiation hydrodynamic simulations of the atmospheres of massive stars and of convection in accretion discs around white dwarfs suffer from prohibitively short time steps due to radiation. This constraint is related to the cooling time rather than the radiative pressure, which also becomes important in hot stars and discs. We show that the radiative time step constraint is governed by the minimum of the sum of the optically thick and thin contributions rather than the smaller one of the two. In simulations with the Pencil Code, their weighting fractions are found empirically. In three-dimensional convective accretion disc simulations, the Deardorff term is found to be the main contributor to the enthalpy flux rather than the superadiabatic gradient. We conclude with a discussion of how the radiative time step problem could be mitigated in certain types of investigations.

Authors: Axel Brandenburg, Upasana Das
Projects: None

Publication Status: Geophys. Astrophys. Fluid Dyn. 114, 162-195 (2020)
Last Modified: 2020-01-19 14:06
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Spectral magnetic helicity of solar active regions between 2006 and 2017  

Axel Brandenburg   Submitted: 2019-12-07 00:53

We compute magnetic helicity and energy spectra from about 2485 patches of about 100 megameters (Mm) side length on the solar surface using data from Hinode during 2006-2017. An extensive database is assembled where we list magnetic energy and helicity, large- and small-scale magnetic helicity, mean current helicity density, fractional magnetic helicity, and correlation length along with the Hinode map identification number (MapID), as well as Carrington latitude and longitude for each MapID. While there are departures from the hemispheric sign rule for magnetic and current helicities, the weak trend reported here is in agreement with the previous results. This is argued to be a physical effect associated with the dominance of individual active regions that contribute more strongly in the better resolved Hinode maps. In comparison with earlier work, the typical correlation length is found to be 6-8 Mm, while the length scale relating magnetic and current helicity to each other is found to be around 1.4 Mm.

Authors: Gosain, S., Brandenburg, A.
Projects: None

Publication Status: Astrophys. J. 882, 80 (2019)
Last Modified: 2019-12-07 15:34
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

A global two-scale helicity proxy from pi-ambiguous solar magnetic fields  

Axel Brandenburg   Submitted: 2019-10-15 22:42

If the α effect plays a role in the generation of the Sun's magnetic field, the field should show evidence of magnetic helicity of opposite signs at large and small length scales. Measuring this faces two challenges: (i) in weak-field regions, horizontal field measurements are unreliable because of the pi ambiguity, and (ii) one needs a truly global approach to computing helicity spectra in the case where one expects a sign reversal across the equator at all wavenumbers. Here we develop such a method using spin-2 spherical harmonics to decompose the linear polarization in terms of the parity-even and parity-odd E and B polarizations, respectively. Using simple one- and two-dimensional models, we show that the product of the spectral decompositions of E and B, taken at spherical harmonic degrees that are shifted by one, can act as a proxy of the global magnetic helicity with a sign that represents that in the northern hemisphere. We then apply this method to the analysis of solar synoptic vector magnetograms, from which we extract a pseudo-polarization corresponding to a "pi-ambiguated" magnetic field, i.e., a magnetic field vector that has no arrow. We find a negative sign of the global EB helicity proxy at spherical harmonic degrees of around 6. This could indicate a positive magnetic helicity at large length scales, but the spectrum fails to capture clear evidence of the well-known negative magnetic helicity at smaller scales. This method might also be applicable to stellar and Galactic polarization data.

Authors: Axel Brandenburg
Projects: CORONAS-F/RESIK

Publication Status: Astrophys. J. 883, 119 (2019)
Last Modified: 2019-10-16 12:42
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Reversed dynamo at small scales and large magnetic Prandtl number  

Axel Brandenburg   Submitted: 2019-08-24 21:21

Abstract: 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 opposite to 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 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 large eddy simulations 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.

Authors: Axel Brandenburg, Matthias Rempel
Projects: None

Publication Status: Astrophys. J. 879, 57 (2019)
Last Modified: 2019-08-25 16:48
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Magnetic Helicity from Multipolar Regions on the Solar Surface  

Axel Brandenburg   Submitted: 2019-06-13 13:29

The emergence of dipolar magnetic features on the solar surface is an idealization. Most of the magnetic flux emergence occurs in complex multipolar regions. Here, we show that the surface pattern of magnetic structures alone can reveal the sign of the underlying magnetic helicity in the nearly force-free coronal regions above. The sign of the magnetic helicity can be predicted to good accuracy by considering the three-dimensional position vectors of three spots on the sphere ordered by their relative strengths at the surface and compute from them the skew product. This product, which is a pseudoscalar, is shown to be a good proxy for the sign of the coronal magnetic helicity.

Authors: Philippe-A. Bourdin, Axel Brandenburg
Projects: None

Publication Status: ApJ, 869:3 (7pp), 2018
Last Modified: 2019-06-15 17:36
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

E and B polarizations from inhomogeneous and solar surface turbulence  

Axel Brandenburg   Submitted: 2019-04-25 21:32

Abstract: 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, 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 nonvanishing (here negative) skewness of the E polarization is responsible for an enhanced ratio of the EE to the BB (quadratic) correlation in both 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 linear polarization of the Sun to characterize its helical turbulence without being subjected to the pi ambiguity that magnetic inversion techniques have to address.

Authors: Axel Brandenburg, Andrea Bracco, Tina Kahniashvili, Sayan Mandal, Alberto Roper Pol, Gordon J. D. Petrie, Nishant K. Singh
Projects: None

Publication Status: Astrophys. J. 870, 87 (2019)
Last Modified: 2019-05-01 12:05
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Magnetic helicity and fluxes in an inhomogeneous α squared dynamo  

Axel Brandenburg   Submitted: 2019-03-02 12:48

Much work on turbulent three-dimensional dynamos has been done using triply periodic domains, in which there are no magnetic helicity fluxes. Here we present simulations where the turbulent intensity is still nearly homogeneous, but now there is a perfect conductor boundary condition on one end and a vertical field or pseudo-vacuum condition on the other. This leads to migratory dynamo waves. Good agreement with a corresponding analytically solvable α 2 dynamo is found. Magnetic helicity fluxes are studied in both types of models. It is found that at moderate magnetic Reynolds numbers, most of the magnetic helicity losses occur at large scales. Whether this changes at even larger magnetic Reynolds numbers, as required for alleviating the catastrophic dynamo quenching problem, remains still unclear.

Authors: A. Brandenburg
Projects: None

Publication Status: Astron. Nachr. 339, 631-640 (2018)
Last Modified: 2019-03-05 12:47
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars  

Axel Brandenburg   Submitted: 2018-12-26 02:55

We seek to understand the transition from nearly axisymmetric configurations at solar rotation rates to nonaxisymmetric configurations for rapid rotation using 3D numerical simulations of turbulent convection and considering rotation rates between 1 and 30 times the solar value. 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 drift in the rotating reference frame and the strength of the active longitudes changes cyclically over time between the different hemispheres. 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 is called the inactive branch in observations, while the rapidly rotating models are close to the superactive branch with a declining cycle to rotation frequency ratio with increasing rotation rate. 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.

Authors: Viviani, M., Warnecke, J., Käpylä, M. J., Käpylä, P. J., Olspert, N., Cole-Kodikara, E. M., Lehtinen, J. J., & Brandenburg, A.
Projects: None

Publication Status: A&A 616, A160 (2018)
Last Modified: 2018-12-26 12:06
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Advances in mean-field dynamo theory and applications to astrophysical turbulence  

Axel Brandenburg   Submitted: 2018-09-05 00:47

Recent advances in mean-field theory are reviewed and applications to the Sun, late-type stars, accretion disks, galaxies, and the early Universe are discussed. We focus particularly on aspects of spatio-temporal nonlocality, which is one of the main insights that emerged from applying the test-field method to magnetic fields of different length and timescales. We also review the status of nonlinear quenching and the relation to magnetic helicity, which is an important observational diagnostic of modern solar dynamo theory. Both solar and some stellar dynamos seem to operate in an intermediate regime that has not yet been possible to model successfully. This regime is bracketed by antisolar-like differential rotation on one end and stellar activity cycles belonging to the superactive stars on the other. The difficulty in modeling this regime may be related to shortcomings in modelling solar/stellar convection. On galactic and extragalactic length scales, the observational constraints are still less stringent and uncertain, but recent advances both in theory and in observations suggest that more conclusive comparisons may soon be possible. The possibility of inversely cascading magnetic helicity throughout all of the early Universe is particularly exciting in explaining the lower limits of magnetic fields on cosmological length scales and the possibility of parity breaking and finite helicity of such a field.

Authors: Axel Brandenburg
Projects: None

Publication Status: J. Plasma Phys. 84, 735840404
Last Modified: 2018-09-05 12:43
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Solar Kinetic Energy and Cross Helicity Spectra  

Axel Brandenburg   Submitted: 2018-08-04 08:48

The relation between magnetic and velocity fields in the solar atmosphere is a topic that can now be addressed quantitatively at the statistical level. Here we analyze kinetic energy and cross helicity spectra along with magnetic energy and helicity spectra in two solar active regions using vector magnetograms and Dopplergrams of NOAA~11158 and 12266. Within these active regions, we find similar slopes for kinetic and magnetic energy spectra 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. We also develop a two-scale method for cross helicity spectra, which allows us to take the cancellation from the bipolarity of active regions into account. In the quiet Sun, by comparison, the cross helicity spectrum is found to be small.

Authors: Hongqi Zhang, Axel Brandenburg
Projects: None

Publication Status: Astrophys. J. Lett. 862, L17 (2018)
Last Modified: 2018-08-07 11:10
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Enhanced stellar activity for slow antisolar differential rotation?  

Axel Brandenburg   Submitted: 2018-04-14 17:44

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.

Authors: Axel Brandenburg, Mark S. Giampapa
Projects: None

Publication Status: Astrophys. J. Lett. 855, L22 (2018)
Last Modified: 2018-04-18 04:34
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars  

Axel Brandenburg   Submitted: 2018-03-11 23:22

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.

Authors: Warnecke, J., Rheinhardt, M., Käpylä, P. J., Käpylä, M. J., & Brandenburg, A.
Projects: None

Publication Status: Astron. Astrophys. 609, A51 (2018)
Last Modified: 2018-03-12 09:45
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface  

Axel Brandenburg   Submitted: 2018-02-03 05:30

Abstract: The formation of sunspots requires the concentration of magnetic flux near the surface. The negative 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. 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. 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, making it therefore possible to study the reason why it is much harder to excite in the presence of radiation. NEMPI yields moderately strong magnetic flux concentrations a certain distance beneath the surface where optical depth is unity. The instability is oscillatory and in the form of upward travelling 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 been found previously in more idealized isothermal models. 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.

Authors: Perri, B., & Brandenburg, A.
Projects: None

Publication Status: A&A 609, A99 (2018)
Last Modified: 2018-02-05 22:07
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

Two-scale analysis of solar magnetic helicity  

Axel Brandenburg   Submitted: 2017-12-12 04:33

We develop a two-scale formalism to determine global magnetic helicity spectra in systems where the local magnetic helicity has opposite signs on both sides of the equator, giving rise to cancelation with conventional methods. We verify this approach using first a synthetic one-dimensional magnetic field and then two-dimensional slices from a three-dimensional α effect-type dynamo-generated magnetic field with forced turbulence of opposite helicity above and below the midplane of the domain. We then apply this formalism to global solar synoptic vector magnetograms. To improve the statistics, data from three consecutive Carrington rotations (2161-2163) are combined into a single map. We find that the spectral magnetic helicity representative of the northern hemisphere is negative at all wavenumbers and peaks at ~ 0.06 Mm-1 (scales around 100 Mm). There is no evidence of bihelical magnetic fields that are found in three-dimensional turbulence simulations of helicity-driven α effect-type dynamos.

Authors: Brandenburg, A., Petrie, G. J. D., & Singh, N. K.
Projects: None

Publication Status: Astrophys. J. 836, 21 (2017)
Last Modified: 2017-12-13 07:55
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 

High-wavenumber solar f-mode strengthening prior to active region formation  

Axel Brandenburg   Submitted: 2017-10-21 07:56

We report a systematic strengthening of the local solar surface or fundamental f-mode 1-2 days prior to the emergence of an active region (AR) in the same (corotating) location. Except for a possibly related increase in the kurtosis of the magnetic field, no indication can be seen in the magnetograms at that time. Our study is motivated by earlier numerical findings of Singh et al. (2014) which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f-mode fans out in the diagnostic kω diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and show for six isolated ARs, 11130, 11158, 11242, 11105, 11072, and 11768, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f-mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. Furthermore, we study two ARs, 12051 and 11678, apart from a magnetically quiet patch lying next to AR~12529, to demonstrate the challenges in extracting such a precursor signal when a newly forming AR emerges in a patch that lies in close proximity of one or several already existing ARs which are expected to pollute neighboring patches. We then discuss plausible procedures for extracting precursor signals from regions with crowded environments. The idea that the f-mode is perturbed days before any visible magnetic activity occurs at the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.

Authors: Singh, N. K., Raichur, H., & Brandenburg, A.
Projects: None

Publication Status: Astrophys. J. 832, 120 (2017)
Last Modified: 2017-10-21 18:40
Go to main E-Print page  Edit Entry  Download Preprint  Submitter's Homepage Delete Entry 


[Older Entries]
Key
Go to main E-Print pageGo to main E-Print page.
Download PreprintDownload Preprint.
Submitter's HomepageSubmitters Homepage.
Edit EntryEdit Entry.
Delete AbstractDelete abstract.

Abstracts by Author
Helicity proxies from linear polarisation of solar active regions
Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection
f-mode strengthening from a localized bipolar subsurface magnetic field
The nature of mean-field generation in three classes of optimal dynamos
Magnetic helicity dissipation and production in an ideal MHD code
The time step constraint in radiation hydrodynamics
Spectral magnetic helicity of solar active regions between 2006 and 2017
A global two-scale helicity proxy from pi-ambiguous solar magnetic fields
Reversed dynamo at small scales and large magnetic Prandtl number
Magnetic Helicity from Multipolar Regions on the Solar Surface
E and B polarizations from inhomogeneous and solar surface turbulence
Magnetic helicity and fluxes in an inhomogeneous alpha squared dynamo
Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars
Advances in mean-field dynamo theory and applications to astrophysical turbulence
Solar Kinetic Energy and Cross Helicity Spectra
Enhanced stellar activity for slow antisolar differential rotation?
Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
Spontaneous flux concentrations from the negative effective magnetic pressure instability beneath a radiative stellar surface
Two-scale analysis of solar magnetic helicity
High-wavenumber solar f-mode strengthening prior to active region formation
Stellar mixing length theory with entropy rain
Bipolar region formation in stratified two-layer turbulence
Magnetic flux concentrations from turbulent stratified convection
Hydraulic effects in a radiative atmosphere with ionization
Magnetic helicity and energy spectra of a solar active region
A New Twist in Simulating Solar Flares
Intense bipolar structures from stratified helical dynamos
Magnetic flux concentrations from dynamo-generated fields
Near-polytropic stellar simulations with a radiative surface
Superflare occurrence and energies on G-, K- and M-type dwarfs
Magnetic Prandtl number dependence of the kinetic-to-magnetic dissipation ratio
Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
Mean-field dynamo action from delayed transport
Magnetic flux concentrations in a polytropic atmosphere
Magnetic helicity and energy spectra of a solar active region
Effects of enhanced stratification on equatorward dynamo wave propagation
Spoke-like differential rotation in a convective dynamo with a coronal envelope
Bipolar magnetic structures driven by stratified turbulence with a coronal envelope
Self-assembly of shallow magnetic spots through strongly stratified turbulence
Active region formation through the negative effective magnetic pressure instability
Magnetic twist: a source and property of space weather
Non-linear and chaotic dynamo regimes
Rotational effects on the negative magnetic pressure instability
Ejections of magnetic structures above a spherical wedge driven by a convective dynamo with differential rotation
Detection of negative effective magnetic pressure instability in turbulence simulations
Cyclic magnetic activity due to turbulent convection in spherical wedge geometry
Nonlinear small-scale dynamos at low magnetic Prandtl numbers
Dynamo-driven plasmoid ejections above a spherical surface
Scale-dependence of magnetic helicity in the solar wind
Magnetic helicity density and its flux in weakly inhomogeneous turbulence
Radiative transfer in decomposed domains
Astrophysical magnetic fields and nonlinear dynamo theory
Effect of the radiative background flux in convection
Strong mean field dynamos require supercritical helicity fluxes
The case for a distributed solar dynamo shaped by near-surface shear
Magnetic helicity evolution in a periodic domain with imposed field
Catastrophic alpha quenching alleviated by helicity flux and shear
Relaxation of writhe and twist of a bi-helical magnetic field
Doubly Helical Coronal Ejections from Dynamos and their Role in Sustaining the Solar Cycle
How magnetic helicity ejection helps large scale dynamos

Related Pages
MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

Archive Maintainer
Alisdair Davey



© 2000-2020 Solar Physics Group - Montana State University