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