1,901 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
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