24 research outputs found
Magnetic field reversals and galactic dynamos
We argue that global magnetic field reversals similar to those observed in
the Milky Way occur quite frequently in mean-field galactic dynamo models that
have relatively strong, random, seed magnetic fields that are localized in
discrete regions. The number of reversals decreases to zero with reduction of
the seed strength, efficiency of the galactic dynamo and size of the spots of
the seed field. A systematic observational search for magnetic field reversals
in a representative sample of spiral galaxies promises to give valuable
information concerning seed magnetic fields and, in this way, to clarify the
initial stages of galactic magnetic field evolution
Biermann Mechanism in Primordial Supernova Remnant and Seed Magnetic Fields
We study generation of magnetic fields by the Biermann mechanism in the
pair-instability supernovae explosions of first stars. The Biermann mechanism
produces magnetic fields in the shocked region between the bubble and
interstellar medium (ISM), even if magnetic fields are absent initially. We
perform a series of two-dimensional magnetohydrodynamic simulations with the
Biermann term and estimate the amplitude and total energy of the produced
magnetic fields. We find that magnetic fields with amplitude
G are generated inside the bubble, though the amount of
magnetic fields generated depend on specific values of initial conditions. This
corresponds to magnetic fields of erg per each supernova
remnant, which is strong enough to be the seed magnetic field for galactic
and/or interstellar dynamo.Comment: 12 pages, 3 figure
The effects of vertical outflows on disk dynamos
We consider the effect of vertical outflows on the mean-field dynamo in a
thin disk. These outflows could be due to winds or magnetic buoyancy. We
analyse both two-dimensional finite-difference numerical solutions of the
axisymmetric dynamo equations and a free-decay mode expansion using the
thin-disk approximation. Contrary to expectations, a vertical velocity can
enhance dynamo action, provided it is not too strong. In the nonlinear regime
this can lead to super-exponential growth of the magnetic field.Comment: 14 pages, final version after referee comments, accepted in A&
Galactic dynamos with captured magnetic flux and an accretion flow
We examine the behaviour of an axisymmetric galactic dynamo model with a
radial accretion flow in the disc. We also introduce a vertical magnetic flux
through the galactic midplane, to simulate the presence of a large scale
magnetic field trapped by the galaxy when forming. The trapped vertical flux is
conserved and advected towards the disc centre by the radial flow. We confirm
that accretion flows of magnitude several km/s through a significant part of
the galactic disc can markedly inhibit dynamo action. Moreover, advection of
the vertical flux in general results in mixed parity galactic fields. However,
the effect is nonlinear and non-additive -- global magnetic field energies are
usually significantly smaller that the sum of purely dynamo generated and
purely advected field energies. For large inflow speeds, a form of
`semi-dynamo' action may occur.
We apply our results to the accumulation and redistribution, by a radial
inflow, of a vertical magnetic flux captured by the Galactic disc. Taking
representative values, it appears difficult to obtain mean vertical fields near
the centre of the Milky Way that are much in excess of 10 microgauss, largely
because the galactic dynamo and turbulent magnetic diffusion modify the
external magnetic field before it can reach the disc centre.Comment: 18 pages, 12 figures, LaTE
Constraining the Magnetic Effects on HI Rotation Curves and the Need for Dark Halos
The density profiles of dark halos are usually inferred from the rotation
curves of disk galaxies based on the assumption that the gas is a good tracer
of the gravitational potential of the galaxies. Some authors have suggested
that magnetic pinching forces could alter significantly the rotation curves of
spiral galaxies. In contrast to other studies which have concentrated in the
vertical structure of the disk, here we focus on the problem of magnetic
confinement in the radial direction to bound the magnetic effects on the HI
rotation curves. It is shown that azimuthal magnetic fields can hardly speed up
the HI disk of galaxies as a whole. In fact, based on virial constraints we
show that the contribution of galactic magnetic fields to the rotation curves
cannot be larger than ~10 km/s at the outermost point of HI detection, if the
galaxies did not contain dark matter at all, and up to 20 km/s in the
conventional dark halo scenario. The procedure to estimate the maximum effect
of magnetic fields is general and applicable to any particular galaxy disk. The
inclusion of the surface terms, namely the intergalactic (thermal, magnetic or
ram) pressure, does not change our conclusions. Other problems related with the
magnetic alternative to dark halos are highlighted. The relevance of magnetic
fields in the cuspy problem of dark halos is also discussed.Comment: 12 pages, 1 figure, accepted for publication in The Astrophysical
Journa
Constraining the regular Galactic Magnetic Field with the 5-year WMAP polarization measurements at 22 GHz
[ABRIDGED] The knowledge of the regular component of the Galactic magnetic
field gives important information about the structure and dynamics of the Milky
Way, as well as constitutes a basic tool to determine cosmic rays trajectories.
It can also provide clear windows where primordial magnetic fields could be
detected. We want to obtain the regular (large scale) pattern of the magnetic
field distribution of the Milky Way that better fits the polarized synchrotron
emission as seen by the 5-year WMAP data at 22 GHz. We have done a systematic
study of a number of Galactic magnetic field models: axisymmetric, bisymmetric,
logarithmic spiral arms, concentric circular rings with reversals and
bi-toroidal. We have explored the parameter space defining each of these models
using a grid-based approach. In total, more than one million models are
computed. The model selection is done using a Bayesian approach. For each
model, the posterior distributions are obtained and marginalised over the
unwanted parameters to obtain the marginal 1-D probability distribution
functions. In general, axisymmetric models provide a better description of the
halo component, although attending to their goodness-of-fit, the rest of the
models cannot be rejected. In the case of disk component, the analysis is not
very sensitive for obtaining the disk large scale structure, because of the
effective available area (less than 8% of the whole map and less than 40% of
the disk). Nevertheless, within a given family of models, the best-fit
parameters are compatible with those found in the literature. The family of
models that better describes the polarized synchrotron halo emission is the
axisymmetric one, with magnetic spiral arms with a pitch angle of ~24 degrees,
and a strong vertical field of 1 microG at z ~ 1 kpc. When a radial variation
is fitted, models require fast variations.Comment: 14 pages, 9 figures. Accepted for publication in A&
Non-local effects in the mean-field disc dynamo. II. Numerical and asymptotic solutions
The thin-disc global asymptotics are discussed for axisymmetric mean-field
dynamos with vacuum boundary conditions allowing for non-local terms arising
from a finite radial component of the mean magnetic field at the disc surface.
This leads to an integro-differential operator in the equation for the radial
distribution of the mean magnetic field strength, in the disc plane at a
distance from its centre; an asymptotic form of its solution at large
distances from the dynamo active region is obtained. Numerical solutions of the
integro-differential equation confirm that the non-local effects act similarly
to an enhanced magnetic diffusion. This leads to a wider radial distribution of
the eigensolution and faster propagation of magnetic fronts, compared to
solutions with the radial surface field neglected. Another result of non-local
effects is a slowly decaying algebraic tail of the eigenfunctions outside the
dynamo active region, , which is shown to persist in nonlinear
solutions where -quenching is included. The non-local nature of the
solutions can affect the radial profile of the regular magnetic field in spiral
galaxies and accretion discs at large distances from the centre.Comment: Revised version, as accepted; Geophys. Astrophys. Fluid Dyna
On the Origin of Cosmic Magnetic Fields
We review the literature concerning how the cosmic magnetic fields pervading
nearly all galaxies actually got started. some observational evidence involves
the chemical abundance of the light elements Be and B, while another one is
based on strong magnetic fields seen in high red shift galaxies. Seed fields,
whose strength is of order 10^{-20} gauss, easily sprung up in the era
preceding galaxy formation. Several mechanisms are proposed to amplify these
seed fields to microgauss strengths. The standard mechanism is the Alpha-Omega
dynamo theory. It has a major difficulty that makes unlikely to provide the
sole origin. The difficulty is rooted in the fact that the total flux is
constant. This implies that flux must be removed from the galactic discs. This
requires that the field and flux be separated, for otherwise interstellar mass
must be removed from the deep galactic gravitational and then their strength
increased by the alpha omega theory.Comment: 90 pages and 6 figures; accepted for publication in Reports of
Progress in Physics as an invited revie