2,566 research outputs found
The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes. I. General formalism and the linear approximation
In this paper we investigate the idea of Hanasz & Lesch 1993 that the
galactic dynamo effect is due to the Parker instability of magnetic flux tubes.
In addition to the former approach, we take into account more general physical
conditions in this paper, by incorporating cosmic rays and differential forces
due to the axisymmetric differential rotation and the density waves as well. We
present the theory of slender magnetic flux tube dynamics in the thin flux tube
approximation and the Lagrange description. This is the application of the
formalism obtained for solar magnetic flux tubes by Spruit (1981), to the
galactic conditions. We perform a linear stability analysis for the
Parker-shearing instability of magnetic flux tubes in galactic discs and then
calculate the dynamo coefficients. We present a number of new effects which are
very essential for cosmological and contemporary evolution of galactic magnetic
fields. First of all we demonstrate that a very strong dynamo -effect
is possible in the limit of weak magnetic fields in presence of cosmic rays.
Second, we show that the differential force resulting from axisymmetric
differential rotation and the linear density waves causes that the
-effect is essentially magnified in galactic arms and switched off in
the interarm regions. Moreover, we predict a non-uniform magnetic field in
spiral arms and well aligned one in interarm regions. These properties are well
confirmed by recent observational results by Beck & Hoernes (1996)Comment: LaTeX, 15 pages, 8 figures, uses l-aa.sty and epsf.sty, minor
corrections to match the published version, Published in Astronomy &
Astrophysics, 321, 100
A Bayesian method for pulsar template generation
Extracting Times of Arrival from pulsar radio signals depends on the
knowledge of the pulsars pulse profile and how this template is generated. We
examine pulsar template generation with Bayesian methods. We will contrast the
classical generation mechanism of averaging intensity profiles with a new
approach based on Bayesian inference. We introduce the Bayesian measurement
model imposed and derive the algorithm to reconstruct a "statistical template"
out of noisy data. The properties of these "statistical templates" are analysed
with simulated and real measurement data from PSR B1133+16. We explain how to
put this new form of template to use in analysing secondary parameters of
interest and give various examples: We implement a nonlinear filter for
determining ToAs of pulsars. Applying this method to data from PSR J1713+0747
we derive ToAs self consistently, meaning all epochs were timed and we used the
same epochs for template generation. While the average template contains
fluctuations and noise as unavoidable artifacts, we find that the "statistical
template" derived by Bayesian inference quantifies fluctuations and remaining
uncertainty. This is why the algorithm suggested turns out to reconstruct
templates of statistical significance from ten to fifty single pulses. A moving
data window of fifty pulses, taking out one single pulse at the beginning and
adding one at the end of the window unravels the characteristics of the methods
to be compared. It shows that the change induced in the classical
reconstruction is dominated by random fluctuations for the average template,
while statistically significant changes drive the dynamics of the proposed
method's reconstruction. The analysis of phase shifts with simulated data
reveals that the proposed nonlinear algorithm is able to reconstruct correct
phase information along with an acceptable estimation of the remaining
uncertainty.Comment: 21 pages, 16 figures, submitted to MNRA
Strong magnetic fields and large rotation measures in protogalaxies by supernova seeding
We present a model for the seeding and evolution of magnetic fields in
protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy
provide magnetic seed fields, which are subsequently amplified by compression,
shear flows and random motions. We implement the model into the MHD version of
the cosmological N-body / SPH simulation code GADGET and we couple the magnetic
seeding directly to the underlying multi-phase description of star formation.
We perform simulations of Milky Way-like galactic halo formation using a
standard LCDM cosmology and analyse the strength and distribution of the
subsequent evolving magnetic field. A dipole-shape divergence-free magnetic
field is injected at a rate of 10^{-9}G / Gyr within starforming regions, given
typical dimensions and magnetic field strengths in canonical SN remnants.
Subsequently, the magnetic field strength increases exponentially on timescales
of a few ten million years. At redshift z=0, the entire galactic halo is
magnetized and the field amplitude is of the order of a few G in the
center of the halo, and 10^{-9} G at the virial radius. Additionally, we
analyse the intrinsic rotation measure (RM) of the forming galactic halo over
redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and
reaches absolute values around 1000 rad m^{-2}. While the halo virializes
towards redshift z=0, the intrinsic RM values decline to a mean value below 10
rad m^{-2}. At high redshifts, the distribution of individual starforming, and
thus magnetized regions is widespread. In our model for the evolution of
galactic magnetic fields, the seed magnetic field amplitude and distribution is
no longer a free parameter, but determined self-consistently by the star
formation process occuring during the formation of cosmic structures.Comment: 13 pages, 14 figures, accepted to MNRAS after moderate revisio
Extended Hodge Theory for Fibred Cusp Manifolds
For a particular class of pseudo manifolds, we show that the intersection
cohomology groups for any perversity may be naturally represented by extended
weighted harmonic forms for a complete metric on the regular stratum with
respect to some weight determined by the perversity. Extended weighted
harmonic forms are harmonic forms that are almost in the given weighted
space for the metric in question, but not quite. This result is akin to the
representation of absolute and relative cohomology groups for a manifold with
boundary by extended harmonic forms on the associated manifold with cylindrical
ends. As in that setting, in the unweighted case, the boundary values of
the extended harmonic forms define a Lagrangian splitting of the boundary space
in the long exact sequence relating upper and lower middle perversity
intersection cohomology groups.Comment: 26 page
The Ford Foundation in Egypt
Egypt has been and continues to be one of the major recipients of development assistance in the third world, yet very little effort has been made to assess the overall impact of this aid on Egypt A symposium was held on December 10th and 11th, 1983 in Oriental Hall at the American University in Cairo, organized by Cairo Papers in Social Science with financial support from the Cairo Office of the Ford Foundation. Egyptian government ministers, officials from representative donors and scholars experienced in development were invited to make formal presentations to the symposium. The essays presented in this issue were all presented at the symposiumhttps://fount.aucegypt.edu/faculty_book_chapters/1893/thumbnail.jp
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