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Dynamo Mechanism (dynamo + mechanism)

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Physics and Astronomy100%

Selected Abstracts

Origin and evolution of magnetars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Lilia Ferrario
ABSTRACT We present a population synthesis study of the observed properties of the magnetars investigating the hypothesis that they are drawn from a population of progenitors that are more massive than those of the normal radio pulsars. We assume that the anomalous X-ray emission is caused by the decay of a toroidal or tangled up field that does not take part in the spin-down of the star. Our model assumes that the magnetic flux of the neutron star is distributed as a Gaussian in the logarithm about a mean value that is described by a power law , where Mp is the mass of the progenitor. We find that we can explain the observed properties of the magnetars for a model with ,0= 2 × 1025 G cm2 and ,= 5 if we suitably parametrize the time evolution of the anomalous X-ray luminosity as an exponentially decaying function of time. Our modelling suggests that magnetars arise from stars in the high-mass end (20 M,,Mp, 45 M,) of this distribution. The lower mass progenitors are assumed to give rise to the radio pulsars. The high value of , can be interpreted in one of two ways. It may indicate that the magnetic flux distribution on the main sequence is a strong function of mass and that this is reflected in the magnetic fluxes of the neutron stars that form from this mass range (the fossil field hypothesis). The recent evidence for magnetic fluxes similar to those of the magnetars in a high fraction (,25 per cent) of massive O-type stars lends support to such a hypothesis. Another possibility is that the spin of the neutron star is a strong function of the progenitor mass, and it is only for stars that are more massive than ,20 M, that magnetar-type fields can be generated by the ,,, dynamo mechanism (the convective dynamo hypothesis). In either interpretation, it has to be assumed that all or a subset of stars in the mass range ,20,45 M,, which on standard stellar evolution models lead to black holes via the formation of a fall-back disc, must give rise to magnetars. Unlike with the radio pulsars, the magnetars only weakly constrain the birth spin period, due to their rapid spin-down. Our model predicts a birthrate of ,1.5,3 × 10,3 yr,1 for the magnetars. [source]

Numerical experiments on dynamo action in sheared and rotating turbulence

ASTRONOMISCHE NACHRICHTEN, Issue 7 2008
T. A. Yousef
Abstract Numerical simulations of forced turbulence in elongated shearing boxes are carried out to demonstrate that a nonhelical turbulence in conjunction with a linear shear can give rise to a mean-field dynamo. Exponential growth of magnetic field at scales larger than the outer (forcing) scale of the turbulence is found. Over a range of values of the shearing rate S spanning approximately two orders of magnitude, the growth rate of the magnetic field is proportional to the imposed shear, , , S, while the characteristic spatial scale of the field is lB , S,1/2. The effect is quite general: earlier results for the nonrotating case by Yousef et al. (2008) are extended to shearing boxes with Keplerian rotation; it is also shown that the shear dynamo mechanism operates both below and above the threshold for the fluctuation dynamo. The apparently generic nature of the shear dynamo effect makes it an attractive object of study for the purpose of understanding the generation of magnetic fields in astrophysical systems. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Are solar cycles predictable?

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
M. Schüssler
Abstract Various methods (or recipes) have been proposed to predict future solar activity levels , with mixed success. Among these, some precursor methods based upon quantities determined around or a few years before solar minimum have provided rather high correlations with the strength of the following cycles. Recently, data assimilation with an advection-dominated (flux-transport) dynamo model has been proposed as a predictive tool, yielding remarkably high correlation coefficients. After discussing the potential implications of these results and the criticism that has been raised, we study the possible physical origin(s) of the predictive skill provided by precursor and other methods. It is found that the combination of the overlap of solar cycles and their amplitude-dependent rise time (Waldmeier's rule) introduces correlations in the sunspot number (or area) record, which account for the predictive skill of many precursor methods. This explanation requires no direct physical relation between the precursor quantity and the dynamo mechanism (in the sense of the Babcock-Leighton scheme or otherwise). (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Nonlinear simulations of magnetic instabilities in stellar radiation zones: The role of rotation and shear

ASTRONOMISCHE NACHRICHTEN, Issue 10 2007
A.S. Brun
Abstract Using the 3-dimensional ASH code, we have studied numerically the instabilities that occur in stellar radiation zones in presence of large-scale magnetic fields, rotation and large-scale shear. We confirm that some configurations are linearly unstable, as predicted by Tayler and collaborators, and we determine the saturation level of the instability. We find that rotation modifies the peak of the most unstable wave number of the poloidal instability but not its growth rate as much as in the case of the m = 1 toroidal instability for which it is changed to , = /,. Further in the case with rotation and shear, we found no sign of the dynamo mechanism suggested recently by Spruit even though we possess the essential ingredients (Tayler's m = 1 instability and a large scale shear) supposedly at work. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]