21,092 research outputs found
What can we say about seed fields for galactic dynamos?
We demonstrate that a quasi-uniform cosmological seed field is a much less
suitable seed for a galactic dynamo than has often been believed. The age of
the Universe is insufficient for a conventional galactic dynamo to generate a
contemporary galactic magnetic field starting from such a seed, accepting
conventional estimates for physical quantities. We discuss modifications to the
scenario for the evolution of galactic magnetic fields implied by this result.
We also consider briefly the implications of a dynamo number that is
significantly larger than that given by conventional estimates
Parity fluctuations in stellar dynamos
Observations of the solar butterfly diagram from sunspot records suggest
persistent fluctuation in parity, away from the overall, approximately dipolar
structure. We use a simple mean-field dynamo model with a solar-like rotation
law, and perturb the -effect. We find that the parity of the magnetic
field with respect to the rotational equator can demonstrate what we describe
as resonant behaviour, while the magnetic energy behaves in a more or less
expected way. We discuss possible applications of the phenomena in the context
of various deviations of the solar magnetic field from dipolar symmetry, as
reported from analysis of archival sunspot data. We deduce that our model
produces fluctuations in field parity, and hence in the butterfly diagram, that
are consistent with observed fluctaions in solar behaviour
Changing times of feminism and higher education: From community to employability
This article discusses the creation of space and time for feminist approaches in higher education in the context of shifting community and employment relations and the restructuring of higher education space-time. It draws on the reflections of three feminist academics concerning aspects of their work biographies in two very different higher education settings. It explores the shift from working in an academic department concerned with community studies to one concerned with education and related employment. The article focuses on the attempt to sustain feminist practices through these changing times and settings and is informed by the work on time and space by Barbara Adam, Henri Lefebvre and Doreen Massey. © 2011 Taylor & Francis
Magnetic fields in barred galaxies. II. Dynamo models
We study the generation and maintenance of large-scale magnetic fields in
barred galaxies. We take a velocity field (with strong noncircular components)
from a published gas dynamical simulation of Athanassoula (1992), and use this
as input to a galactic dynamo calculation. Our work is largely motivated by
recent high quality VLA radio observations of the barred galaxy NGC 1097, and
we compare our results in detail with the regular magnetic fields deduced from
these observations. We are able to reproduce most of the conspicuous
large-scale features of the observed regular field, including the field
structure in the central regions, by using a simple mean-field dynamo model in
which the intensity of interstellar turbulence (more precisely, the turbulent
diffusivity) is enhanced by a factor of 2-6 in the dust lanes and near the
circumnuclear ring. We argue that magnetic fields can be dynamically important,
and therefore should be included in models of gas flow in barred galaxies.Comment: 16 pages, 13 figures, submitted to Astronomy and Astrophysics.
Revised version (changes shown in bold face
Anisotropic dark energy and CMB anomalies
We investigate the breaking of global statistical isotropy caused by a dark
energy component with an energy-momentum tensor which has point symmetry, that
could represent a cubic or hexagonal crystalline lattice. In such models
Gaussian, adiabatic initial conditions created during inflation can lead to
anisotropies in the cosmic microwave background whose spherical harmonic
coefficients are correlated, contrary to the standard assumption. We develop an
adaptation of the line of sight integration method that can be applied to
models where the background energy-momentum tensor is isotropic, but whose
linearized perturbations are anisotropic. We then show how this can be applied
to the cases of cubic and hexagonal symmetry. We compute quantities which show
that such models are indistinguishable from isotropic models even in the most
extreme parameter choices, in stark contrast to models with anisotropic initial
conditions based on inflation. The reason for this is that the dark energy
based models contribute to the CMB anistropy via the inegrated Sachs-Wolfe
effect, which is only relevent when the dark energy is dominant, that is, on
the very largest scales. For inflationary models, however, the anisotropy is
present on all scales.Comment: 18 pages, 9 figure
Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models
Observations show that the photospheric solar magnetic dipole usually does
not vanish during the reversal of the solar magnetic field, which occurs in
each solar cycle. In contrast, mean-field solar dynamo models predict that the
dipole field does become zero. In a recent paper Moss et al. (2013) suggested
that this contradiction can be explained as a large-scale manifestation of
small-scale magnetic fluctuations of the surface poloidal field. Here we
compare this interpretation with WSO (Wilcox Solar Observatory) photospheric
magnetic field data in order to determine the amplitude of magnetic
fluctuations required to explain the phenomenon and to compare the results with
predictions from a simple dynamo model which takes these fluctuations into
account. We demonstrate that the WSO data concerning the magnetic dipole
reversals are very similar to the predictions of our very simple solar dynamo
model, which includes both mean magnetic field and fluctuations. The ratio
between the rms value of the magnetic fluctuations and the mean field is
estimated to be about 2, in reasonable agreement with estimates from sunspot
data. The reversal epoch, during which the fluctuating contribution to the
dipole is larger than that from the mean field, is about 4 months. The memory
time of the fluctuations is about 2 months. Observations demonstrate that the
rms of the magnetic fluctuations is strongly modulated by the phase of the
solar cycle. This gives additional support to the concept that the solar
magnetic field is generated by a single dynamo mechanism rather than also by
independent small-scale dynamo action. A suggestion of a weak nonaxsymmetric
magnetic field of a fluctuating nature arises from the analysis, with a
lifetime of about 1 year.Comment: 9 pages, 10 figures, accepted versio
Dynamo models and differential rotation in late-type rapidly rotating stars
Increasing evidence is becoming available about not only the surface
differential rotation of rapidly rotating cool stars but, in a small number of
cases, also about temporal variations, which possibly are analogous to the
solar torsional oscillations. Given the present difficulties in resolving the
precise nature of such variations, due to both the short length and poor
resolution of the available data, theoretical input is vital to help assess the
modes of behaviour that might be expected, and will facilitate interpretation
of the observations. Here we take a first step in this direction by studying
the variations in the convection zones of such stars, using a two dimensional
axisymmetric mean field dynamo model operating in a spherical shell in which
the only nonlinearity is the action of the azimuthal component of the Lorentz
force of the dynamo generated magnetic field on the stellar angular velocity.
We consider three families of models with different depths of dynamo-active
regions. For moderately supercritical dynamo numbers we find torsional
oscillations that penetrate all the way down to the bottom of the convection
zones, similar to the case of the Sun. For larger dynamo numbers we find
fragmentation in some cases and sometimes there are other dynamical modes of
behaviour, including quasi-periodicity and chaos. We find that the largest
deviations in the angular velocity distribution caused by the Lorentz force are
of the order of few percent, implying that the original assumed `background'
rotation field is not strongly distorted.Comment: Astronomy and Astrophysics, in pres
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