228 research outputs found
Mean-field effects in the Galloway-Proctor flow
The coefficients defining the mean electromotive force in a Galloway-Proctor
flow are determined. This flow shows a two-dimensional pattern and is helical.
The pattern wobbles in its plane. Apart from one exception a circular motion of
the flow pattern is assumed. This corresponds to one of the cases considered
recently by Courvoisier, Hughes and Tobias (2006, Phys. Rev. Lett., 96,
034503). An analytic theory of the alpha effect and related effects in this
flow is developed within the second-order correlation approximation and a
corresponding fourth-order approximation. In the validity range of these
approximations there is an alpha effect but no gamma effect, or pumping effect.
Numerical results obtained with the test-field method, which are independent of
these approximations, confirm the results for alpha and show that gamma is in
general nonzero. Both alpha and gamma show a complex dependency on the magnetic
Reynolds number and other parameters that define the flow, that is, amplitude
and frequency of the wobbling motion. Some results for the magnetic diffusivity
eta_t and a related quantity are given, too. Finally a result for alpha in the
case of a randomly varying flow without the aforementioned circular motion is
presented. This flow may be a more appropriate model for studying the alpha
effect and related effects in flows that are statistical isotropic in a plane.Comment: 12 pages, 14 figures, submitted to MNRA
The alpha-effect in rotating convection: a comparison of numerical simulations
Numerical simulations are an important tool in furthering our understanding
of turbulent dynamo action, a process that occurs in a vast range of
astrophysical bodies. It is important in all computational work that
comparisons are made between different codes and, if non-trivial differences
arise, that these are explained. Kapyla et al (2010: MNRAS 402, 1458) describe
an attempt to reproduce the results of Hughes & Proctor (2009: PRL 102, 044501)
and, by employing a different methodology, they arrive at very different
conclusions concerning the mean electromotive force and the generation of
large-scale fields. Here we describe why the simulations of Kapyla et al (2010)
are simply not suitable for a meaningful comparison, since they solve different
equations, at different parameter values and with different boundary
conditions. Furthermore we describe why the interpretation of Kapyla et al
(2010) of the calculation of the alpha-effect is inappropriate and argue that
the generation of large-scale magnetic fields by turbulent convection remains a
problematic issue.Comment: Submitted to MNRAS. 5 pages, 3 figure
Real time localization of Gamma Ray Bursts with INTEGRAL
The INTEGRAL satellite has been successfully launched in October 2002 and has
recently started its operational phase. The INTEGRAL Burst Alert System (IBAS)
will distribute in real time the coordinates of the GRBs detected with
INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe
the main IBAS characteristics and report on the initial results. During the
initial performance and verification phase of the INTEGRAL mission, which
lasted about two months, two GRBs have been localized with accuracy of about
2-4 arcmin. These observations have allowed us to validate the IBAS software,
which is now expected to provide quick (few seconds delay) and precise (few
arcmin) localization for about 10-15 GRBs per year.Comment: 6 pages, latex, 3 figures, submitted to Adv. Sp. Res., Proceedings of
the 34th COSPAR Scientific Assembly, Houston, 10-19 October 200
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