107 research outputs found
Corrigendum to: The formation of chlorophyll from chlorophyllide in leaves containing proplastids is a four-step process [FEBS Letters 486 (2000) 243–246]
Polarization experiments
Possible instrumental set--ups for the measurement of CMB polarization are
reviewed in this article. We discuss existing and planned instruments, putting
special emphasis on observational, instrumental, and data processing issues for
the detection of very low polarization signals of prime cosmological interest.
A short prospective summary is included
Boundary layer instability at the top of the Earth's outer core
AbstractWe investigate the instability of mixed Ekman–Hartmann boundary layers arising in rotating incompressible magnetohydrodynamics flows in a parameter regime relevant to the Earth liquid core. Relying on the small depth of the boundary layer, we perform a local study in a half-space at a given co-latitude θ≠π/2, and assume a mean dipolar axial magnetic field with internal sources (the geodynamo). Instabilities are driven, for high enough Reynolds number, by the quadratic term in the momentum equation.Nonlinear stability can be proven using energy methods in the neighborhood of the poles (Nonlinearity 12 (2) (1999) 181). Next, following the work of Lilly (J. Atmos. Sci. 23 (1966) 481), we restrict our analysis to the linear growth phase. We describe the dependence of the critical Reynolds number in terms of θ and Elsasser number (measuring the relative strength of Lorentz and Coriolis forces). It turns out that no matter how large the Elsasser number is, there exists a critical band centered on the equator in which instabilities can occur. For geophysically relevant values of parameters, this band could extend as far as 45° away from the equator. This establishes the possibility of boundary layer instabilities near the Earth core-mantle boundary (CMB).We finally present a first attempt of interaction with field maps at the CMB and core flows derived from the secular variation of the field, and discuss the sensitivity of the instability onset not only on the boundary layer Reynolds number, but also on the direction of the flow
Multi-component power spectra estimation method for multi-detector observations of the Cosmic Microwave Background
We present a new method for multi-component power spectra estimation in
multi-frequency observations of the CMB. Our method is based on matching a
model to the cross and auto power spectra of observed maps. All the component
power spectra are estimated, as well as their mixing matrix. Noise power
spectra are also estimated. The method has been applied to full-sky Planck
simulations containing five astrophysical components and white noise. The beam
smoothing effect is taken into account.Comment: 7 pages, 1 figure, proc. of the CMBnet workshop, 20-21 Feb. 2003
Oxford, UK. New Astronomy Reviews (eds. A. Melchiorri, J.I. Silk) in pres
Cosmological constraints in Lambda-CDM and Quintessence paradigms with Archeops
We review the cosmological constraints put by the current CMB experiment
including the recent ARCHEOPS data, in the framework of Lambda-CDM and
quintessence paradigm. We show that well chosen combinations of constraints
from different cosmological observations lead to precise measurements of
cosmological parameters. The Universe seems flat with a 70 percents
contribution of dark energy with an equation of state very close to those of
the vacuum.Comment: to appear in New Astronomy Reviews, Proceedings of the CMBNET
Meeting, 20-21 February 2003, Oxford, U
Characterization and modelling of the hollow beam produced by a real conical lens
The properties of the hollow beam produced by a conical lens are studied in
detail. In particular, the impact of a rounded vertex is examined. It is shown
that it could lead to drastic changes in the transverse distribution of the
hollow beam, determined by the ratio between the transverse size of the
incident beam and the size of the blunt area. An adequate choice for this ratio
allows us to either minimize the losses or optimize the distribution symmetry.Comment: 13 pages, 10 figures, submitted to Optics Communication
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