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Axel Brandenburg - One of the best experts on this subject based on the ideXlab platform.

  • an azimuthal Dynamo wave in spherical shell convection
    The Astrophysical Journal, 2013
    Co-Authors: Elizabeth Cole, Axel Brandenburg, P J Kapyla, Maarit J Mantere
    Abstract:

    We report the discovery of an azimuthal Dynamo wave of a low-order (m = 1) mode in direct numerical simulations (DNS) of turbulent convection in spherical shells. Such waves are predicted by mean-field Dynamo Theory and have been obtained previously in mean-field models. An azimuthal Dynamo wave has been proposed as a possible explanation for the persistent drifts of spots observed on several rapidly rotating stars, as revealed through photometry and Doppler imaging. However, this has been judged unlikely because evidence for such waves from DNS has been lacking. Here we present DNS of large-scale magnetic fields showing a retrograde m = 1 mode. Its pattern speed is nearly independent of latitude and does not reflect the speed of the differential rotation at any depth. The extrema of magnetic m = 1 structures coincide reasonably well with the maxima of m = 2 structures of the temperature. These results provide direct support for the observed drifts being due to an azimuthal Dynamo wave.

  • azimuthal Dynamo wave in spherical shell convection
    arXiv: Solar and Stellar Astrophysics, 2013
    Co-Authors: Elizabeth Cole, P J Kapyla, Maarit J Mantere, Axel Brandenburg
    Abstract:

    We report the finding of an azimuthal Dynamo wave of a low-order (m=1) mode in direct numerical simulations (DNS) of turbulent convection in spherical shells. Such waves are predicted by mean field Dynamo Theory and have been obtained previously in mean-field models. Observational results both from photometry and Doppler imaging have revealed persistent drifts of spots for several rapidly rotating stars, but, although an azimuthal Dynamo wave has been proposed as a possible mechanism responsible for this behavior, it has been judged as unlikely, as practical evidence for such waves from DNS has been lacking. The large-scale magnetic field in our DNS, which is due to self-consistent Dynamo action, is dominated by a retrograde m=1 mode. Its pattern speed is nearly independent of latitude and does not reflect the speed of the differential rotation at any depth. The extrema of magnetic m=1 structures coincide reasonably with the maxima of m=2 structures of the temperature. These results provide direct support for the observed drifts being due to an azimuthal Dynamo wave.

  • astrophysical magnetic fields and nonlinear Dynamo Theory
    arXiv: Astrophysics, 2004
    Co-Authors: Axel Brandenburg, Kandaswamy Subramanian
    Abstract:

    The current understanding of astrophysical magnetic fields is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a self-excited Dynamo, which involves flows that can amplify a weak 'seed' magnetic field exponentially fast. Particular emphasis is placed on the nonlinear saturation of the Dynamo. Analytic and numerical results are discussed both for small scale Dynamos, which are completely isotropic, and for large scale Dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale Dynamos is the so-called alpha effect which explains the generation of a mean field if the turbulence lacks mirror symmetry, i.e. if the flow has kinetic helicity. Large scale Dynamos produce small scale helical fields as a waste product that quench the large scale Dynamo and hence the alpha effect. With this in mind, the microscopic Theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic fields are reviewed.

  • the inverse cascade and nonlinear alpha effect in simulations of isotropic helical hydromagnetic turbulence
    The Astrophysical Journal, 2001
    Co-Authors: Axel Brandenburg
    Abstract:

    A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which is essentially the prototype of the α2 Dynamo of mean field Dynamo Theory, produces strong Dynamo action with an additional large-scale field on the scale of the box (at wavenumber k = 1; forcing is at k = 5). This large-scale field is nearly force free and exceeds the equipartition value. As the magnetic Reynolds number Rm increases, the saturation field strength and the growth rate of the Dynamo increase. However, the time it takes to build up the large-scale field from equipartition to its final superequipartition value increases with magnetic Reynolds number. The large-scale field generation can be identified as being due to nonlocal interactions originating from the forcing scale, which is characteristic of the α-effect. Both α and turbulent magnetic diffusivity ηt are determined simultaneously using numerical experiments where the mean field is modified artificially. Both quantities are quenched in an Rm-dependent fashion. The evolution of the energy of the mean field matches that predicted by an α2 Dynamo model with similar α and ηt quenchings. For this model an analytic solution is given that matches the results of the simulations. The simulations are numerically robust in that the shape of the spectrum at large scales is unchanged when changing the resolution from 303 to 1203 mesh points, or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity) from 1 to 100. Increasing the forcing wavenumber to 30 (i.e., increasing the scale separation) makes the inverse cascade effect more pronounced, although it remains otherwise qualitatively unchanged.

  • the inverse cascade and nonlinear alpha effect in simulations of isotropic helical hydromagnetic turbulence
    arXiv: Astrophysics, 2000
    Co-Authors: Axel Brandenburg
    Abstract:

    A numerical model of isotropic homogeneous turbulence with helical forcing is investigated. The resulting flow, which is essentially the prototype of the alpha^2 Dynamo of mean-field Dynamo Theory, produces strong Dynamo action with an additional large scale field on the scale of the box (at wavenumber k=1; forcing is at k=5). This large scale field is nearly force-free and exceeds the equipartition value. As the magnetic Reynolds number R_m increases, the saturation field strength and the growth rate of the Dynamo increase. However, the time it takes to built up the large scale field from equipartition to its final super-equipartition value increases with magnetic Reynolds number. The large scale field generation can be identified as being due to nonlocal interactions originating from the forcing scale, which is characteristic of the alpha-effect. Both alpha and turbulent magnetic diffusivity eta_t are determined simultaneously using numerical experiments where the mean-field is modified artificially. Both quantities are quenched in a R_m-dependent fashion. The evolution of the energy of the mean field matches that predicted by an alpha^2 Dynamo model with similar alpha and eta_t quenchings. For this model an analytic solution is given which matches the results of the simulations. The simulations are numerically robust in that the shape of the spectrum at large scales is unchanged when changing the resolution from 30^3 to 120^3 meshpoints, or when increasing the magnetic Prandtl number (viscosity/magnetic diffusivity) from 1 to 100. Increasing the forcing wavenumber to 30 (i.e. increasing the scale separation) makes the inverse cascade effect more pronounced, although it remains otherwise qualitatively unchanged.

K Mursula - One of the best experts on this subject based on the ideXlab platform.

  • occurrence of high speed solar wind streams over the grand modern maximum
    The Astrophysical Journal, 2015
    Co-Authors: K Mursula, Renata Lukianova, Lauri Holappa
    Abstract:

    In the declining phase of the solar cycle (SC), when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind (SW) streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity (GA) in the near-Earth space. Here, using a novel definition of GA at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged SW speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onward. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each of SCs 16-23. For most cycles the HSS activity clearly reaches a maximum in one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of SC 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar Dynamo Theory during this dramatic period of solar magnetism.

  • occurrence of high speed solar wind streams over the grand modern maximum
    arXiv: Solar and Stellar Astrophysics, 2015
    Co-Authors: K Mursula, Renata Lukianova, Lauri Holappa
    Abstract:

    In the declining phase of the solar cycle, when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity in the near-Earth space. Here, using a novel definition of geomagnetic activity at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged solar wind speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onwards. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each solar cycle 16-23. For most cycles the HSS activity clearly maximizes during one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of solar cycle 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar Dynamo Theory during this dramatic period of solar magnetism.

  • millennium scale sunspot number reconstruction evidence for an unusually active sun since the 1940s
    Physical Review Letters, 2003
    Co-Authors: I G Usoskin, K Mursula, S K Solanki, M Schussler, Katja Alanko
    Abstract:

    The extension of the sunspot number series backward in time is of considerable interest for Dynamo Theory, solar, stellar, and climate research. We have used records of the 10 Be concentration in polar ice to reconstruct the average sunspot activity level for the period between the year 850 to the present. Our method uses physical models for processes connecting the 10 Be concentration with the sunspot number. The reconstruction shows reliably that the period of high solar activity during the last 60 years is unique throughout the past 1150 years. This nearly triples the time interval for which such a statement could be made previously.

Lauri Holappa - One of the best experts on this subject based on the ideXlab platform.

  • occurrence of high speed solar wind streams over the grand modern maximum
    The Astrophysical Journal, 2015
    Co-Authors: K Mursula, Renata Lukianova, Lauri Holappa
    Abstract:

    In the declining phase of the solar cycle (SC), when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind (SW) streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity (GA) in the near-Earth space. Here, using a novel definition of GA at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged SW speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onward. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each of SCs 16-23. For most cycles the HSS activity clearly reaches a maximum in one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of SC 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar Dynamo Theory during this dramatic period of solar magnetism.

  • occurrence of high speed solar wind streams over the grand modern maximum
    arXiv: Solar and Stellar Astrophysics, 2015
    Co-Authors: K Mursula, Renata Lukianova, Lauri Holappa
    Abstract:

    In the declining phase of the solar cycle, when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity in the near-Earth space. Here, using a novel definition of geomagnetic activity at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged solar wind speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onwards. We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each solar cycle 16-23. For most cycles the HSS activity clearly maximizes during one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of solar cycle 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar Dynamo Theory during this dramatic period of solar magnetism.

G. A. Kovaltsov - One of the best experts on this subject based on the ideXlab platform.

  • revisited sunspot data a new scenario for the onset of the maunder minimum
    The Astrophysical Journal, 2011
    Co-Authors: J M Vaquero, M C Gallego, I G Usoskin, G. A. Kovaltsov
    Abstract:

    The Maunder minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar Dynamo Theory dealing with long-term solar activity evolution. Here, we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636-1642, i.e., one solar cycle before the beginning of the Maunder minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder minimum, from 60-70 down to about 20, implying a possibly gradual onset of the minimum with reduced activity started two cycles before it. This revised scenario of the Maunder minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar Dynamo theories focused on long-term studies and occurrence of Grand minima.

  • Revisited sunspot data: A new scenario for the onset of the Maunder minimum
    The Astrophysical Journal, 2011
    Co-Authors: Ilya Usoskin, G. A. Kovaltsov
    Abstract:

    Maunder Minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar Dynamo Theory dealing with long-term solar activity evolution. Here we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636--1642, i.e., one solar cycle before the beginning of the Maunder Minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder Minimum, from 60--70 down to about 20, implying a possibly gradual onset of the Minimum with reduced activity started two cycles before it. This revised scenario of the Maunder Minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar Dynamo theories focused on long-term studies and occurrence of Grand minima.

R. Beck - One of the best experts on this subject based on the ideXlab platform.

  • magnetic field structure of the large magellanic cloud from faraday rotation measures of diffuse polarized emission
    The Astrophysical Journal, 2012
    Co-Authors: S A Mao, R. Beck, N M Mccluregriffiths, B M Gaensler, M Haverkorn, David Mcconnell, M Wolleben, S Stanimirovic, J M Dickey
    Abstract:

    We present a study of the magnetic field of the Large Magellanic Cloud (LMC), carried out using diffuse polarized synchrotron emission data at 1.4 GHz acquired at the Parkes Radio Telescope and the Australia Telescope Compact Array. The observed diffuse polarized emission is likely to originate above the LMC disk on the near side of the galaxy. Consistent negative rotation measures (RMs) derived from the diffuse emission indicate that the line-of-sight magnetic field in the LMC's near-side halo is directed coherently away from us. In combination with RMs of extragalactic sources that lie behind the galaxy, we show that the LMC's large-scale magnetic field is likely to be of quadrupolar geometry, consistent with the prediction of Dynamo Theory. On smaller scales, we identify two brightly polarized filaments southeast of the LMC, associated with neutral hydrogen arms. The filaments' magnetic field potentially aligns with the direction toward the Small Magellanic Cloud (SMC). We suggest that tidal interactions between the SMC and the LMC in the past 109 years are likely to have shaped the magnetic field in these filaments.

  • magnetic field structure of the large magellanic cloud from faraday rotation measures of diffuse polarized emission
    arXiv: Astrophysics of Galaxies, 2012
    Co-Authors: S A Mao, R. Beck, N M Mccluregriffiths, B M Gaensler, M Haverkorn, David Mcconnell, M Wolleben, S Stanimirovic, J M Dickey
    Abstract:

    We present a study of the magnetic field of the Large Magellanic Cloud (LMC), carried out using diffuse polarized synchrotron emission data at 1.4 GHz acquired at the Parkes Radio Telescope and the Australia Telescope Compact Array. The observed diffuse polarized emission is likely to originate above the LMC disk on the near side of the galaxy. Consistent negative rotation measures (RMs) derived from the diffuse emission indicate that the line-of-sight magnetic field in the LMC's near-side halo is directed coherently away from us. In combination with RMs of extragalactic sources that lie behind the galaxy, we show that the LMC's large scale magnetic field is likely to be of quadrupolar geometry, consistent with the prediction of Dynamo Theory. On smaller scales, we identify two brightly polarized filaments southeast of the LMC, associated with neutral hydrogen arms. The filaments' magnetic field potentially aligns with the direction towards the Small Magellanic Cloud. We suggest that tidal interactions between the Small and the Large Magellanic Clouds in the past 10^9 years is likely to have shaped the magnetic field in these filaments.

  • Interstellar Magnetic Fields
    2011
    Co-Authors: R. Beck
    Abstract:

    Methods of investigating interstellar magnetic fields in spiral galaxies are reviewed. Linearly polarized radio continuum emission yields information about strength and structure of the field. Data are available for our Galaxy and several nearby galaxies. The field strengths vary between 3 and 13 , ?G; a correlation with the average surface mass density of the galaxies is indicated. The field of M31 is probably concentrated in a ring-like torus, as predicted by the Dynamo Theory. The field structure of M33, however, appears to be bisymmetric as expected in the primordial Theory of field origin. Other candidates for a bisymmetric field structure are M51, M81, M83, NGC 2903, and NGC 6946, but the observations are not yet conclusive. The magnetic field in IC 342 is too complicated to be described in terms of a simple model. The interaction processes between the field and the various components of interstellar gas should be investigated in more detail.

  • evolution of magnetic fields in galaxies in the frame of hierarchical structure formation cosmology future tests with the ska
    arXiv: Cosmology and Nongalactic Astrophysics, 2009
    Co-Authors: T G Arshakian, R. Beck, Marita Krause, Dmitry Sokolff, Rodion Stepanov
    Abstract:

    Results from simulations of hierarchical structure formation cosmology provide a tool to develop an evolutionary model of regular magnetic fields coupled to galaxy formation and evolution. We use the Dynamo Theory to derive the timescales of amplification and ordering of magnetic fields in disk and puffy galaxies. Galaxies similar to the Milky Way formed their disks at $z\approx10$ and regular fields of $\mu$G strength and a few kpc coherence length were generated within 2 Gyr (at $z\approx3$), but field ordering up to the coherence scale of the galaxy size took another 6 Gyr (at $z\approx0.5$). Giant galaxies formed their disk already at $z\approx10$, allowing more efficient Dynamo generation of strong regular fields (with kpc coherence length) already at $z\approx4$. Dwarf galaxies should have hosted fully coherent fields at $z\approx1$. This evolutionary scenario and number of predictions of the model can be tested by measurements of polarized synchrotron emission and Faraday rotation with the planned Square Kilometre Array. This model is used to simulate the evolution of regular fields in disk galaxies and the polarized radio sky as part of the Square Kilometer Array Design Studies (SKADS).

  • symmetry and direction of seed magnetic fields in galaxies
    Astronomy and Astrophysics, 1998
    Co-Authors: F Krause, R. Beck
    Abstract:

    Radio observations of large-scale magnetic fields in spiral galaxies suggest that these fields show in many cases a dominating S0 symmetry, i.e. even symmetry to the central plane and axisymmetry with respect to the rotational axis. In addition, detailed evaluation of Faraday rotation measures in five nearby galaxies reveals that in four cases the magnetic field lines are directed toward the centre of these galaxies. Dynamo Theory, successful in explaining the existence of such fields, does not contain any statement concerning the sign of the field which depends on the initial conditions. Hence we investigate whether there is a mechanism which produces seed fields with a definitely determined field direction. Neither pre-galactic seed fields, nor galactic battery seed fields, nor ionization fronts are able to imprint such a dominating direction.