Eulerian and Lagrangian means in rotating, magnetohydrodynamic flows II. Braginsky’s nearly axisymmetric dynamo

The Hybrid Euler–Lagrange (HEL) approach has been usefully applied to weakly dissipative systems characterised by waves riding on mean flow. Soward (Phil. Trans. R. Soc. Lond. A 1972, 272, 431) showed how the HEL-formulation could elucidate remarkable features of the nearly axisymmetric large magnetic Reynolds number dynamo of Braginsky (JETP 1964, 47, 1084). Since Braginsky’s treatment of the nearly axisymmetric dynamo relies on azimuthal averages, those can only be taken when the azimuth is a coordinate direction. In that respect, the unified derivation and presentation of the HEL-equations governing rotating magnetohydrodynamic convective flows, as later reviewed and extended by Roberts and Soward (Geophys. Astrophys. Fluid Dyn. 2006, 100, 457), suffer the shortcoming that it was developed relative to rectangular Cartesian coordinates. Here we undertake those modifications needed to transform the rectangular Cartesian coordinate formulation into cylindrical polar coordinates. Being a Lagrangian description, application of the HEL-method means that the variables used, dependent on coordinates x, do not describe conditions at the position P: x but on conditions elsewhere at some displaced position PL: xL(x, t) = x + ξ (x, t), generally dependent on time t. To address this issue Soward and Roberts (J. Fluid Mech. 2010, 661, 45) invoked an idea pioneered by Moffatt (J. Fluid Mech. 1986, 166, 359), whereby the point x is dragged to xL(x, t) by a “fictitious steady flow” η(x, t) in a unit of “fictitious time”. This is the “Lie dragging” technique of general tensor calculus, which we apply here to the HEL-equations governing Braginsky’s nearly axisymmetric dynamo. We consider the “effective-variables” introduced by Braginsky, appropriate for small displacement ξ , and show that η, rather than ξ , is their natural expansion variable. As well as revisiting Braginsky’s kinematic dynamo, we reassess the hydromagnetic extensions of Tough and Roberts (Phys. Earth Planet. Inter. 1968, 1, 288)

SARS CoV subunit vaccine: Antibodymediated neutralisation and enhancement

1. A SARS vaccine was produced based on recombinant native full-length Spike-protein trimers (triSpike) and efficient establishment of a vaccination procedure in rodents. 2. Antibody-mediated enhancement of SARS-CoV infection with anti-SARS-CoV Spike immune-serum was observed in vitro. 3. Antibody-mediated infection of SARS-CoV triggers entry into human haematopoietic cells via an FcγR-dependent and ACE2-, pH-, cysteine-protease-independent pathways. 4. The antibody-mediated enhancement phenomenon is not a mandatory component of the humoral immune response elicited by SARS vaccines, as pure neutralising antibody only could be obtained. 5. Occurrence of immune-mediated enhancement of SARS-CoV infection raises safety concerns regarding the use of SARS-CoV vaccine in humans and enables new ways to investigate SARS pathogenesis (tropism and immune response deregulation)

Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium

We report the observation of dynamo action in the VKS experiment, i.e., the generation of magnetic field by a strongly turbulent swirling flow of liquid sodium. Both mean and fluctuating parts of the field are studied. The dynamo threshold corresponds to a magnetic Reynolds number Rm \sim 30. A mean magnetic field of order 40 G is observed 30% above threshold at the flow lateral boundary. The rms fluctuations are larger than the corresponding mean value for two of the components. The scaling of the mean square magnetic field is compared to a prediction previously made for high Reynolds number flows.Comment: 4 pages, 5 figure

Effect of anharmonicities in the critical number of trapped condensed atoms with attractive two-body interaction

We determine the quantitative effect, in the maximum number of particles and other static observables, due to small anharmonic terms added to the confining potential of an atomic condensed system with negative two-body interaction. As an example of how a cubic or quartic anharmonic term can affect the maximum number of particles, we consider the trap parameters and the results given by Roberts et al. [Phys. Rev. Lett. 86, 4211 (2001)]. However, this study can be easily transferred to other trap geometries to estimate anharmonic effects.Comment: Total of 5 pages, 3 figures and 1 table. To appear in Phys. Rev.

Degradation of a quantum directional reference frame as a random walk

We investigate if the degradation of a quantum directional reference frame through repeated use can be modeled as a classical direction undergoing a random walk on a sphere. We demonstrate that the behaviour of the fidelity for a degrading quantum directional reference frame, defined as the average probability of correctly determining the orientation of a test system, can be fit precisely using such a model. Physically, the mechanism for the random walk is the uncontrollable back-action on the reference frame due to its use in a measurement of the direction of another system. However, we find that the magnitude of the step size of this random walk is not given by our classical model and must be determined from the full quantum description.Comment: 5 pages, no figures. Comments are welcome. v2: several changes to clarify the key results. v3: journal reference added, acknowledgements and references update

Transport of magnetic field by a turbulent flow of liquid sodium

We study the effect of a turbulent flow of liquid sodium generated in the von K\'arm\'an geometry, on the localized field of a magnet placed close to the frontier of the flow. We observe that the field can be transported by the flow on distances larger than its integral length scale. In the most turbulent configurations, the mean value of the field advected at large distance vanishes. However, the rms value of the fluctuations increases linearly with the magnetic Reynolds number. The advected field is strongly intermittent.Comment: 4 pages, 6 figure

A new interferometric study of four exoplanet host stars : {\theta} Cygni, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis

Studying exoplanet host stars is of the utmost importance to establish the link between the presence of exoplanets around various types of stars and to understand the respective evolution of stars and exoplanets. Using the limb-darkened diameter (LDD) obtained from interferometric data, we determine the fundamental parameters of four exoplanet host stars. We are particularly interested in the F4 main-sequence star, {\theta} Cyg, for which Kepler has recently revealed solar-like oscillations that are unexpected for this type of star. Furthermore, recent photometric and spectroscopic measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic radial velocity of \sim150 days. Models of this periodic change in radial velocity predict either a complex planetary system orbiting the star, or a new and unidentified stellar pulsation mode. We performed interferometric observations of {\theta} Cyg, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount Wilson, California) in several three-telescope configurations. We measured accurate limb darkened diameters and derived their radius, mass and temperature using empirical laws. We obtain new accurate fundamental parameters for stars 14 And, {\upsilon} And and 42 Dra. We also obtained limb darkened diameters with a minimum precision of \sim 1.3%, leading to minimum planet masses of Msini=5.33\pm 0.57, 0.62 \pm 0.09 and 3.79\pm0.29 MJup for 14 And b, {\upsilon} And b and 42 Dra b, respectively. The interferometric measurements of {\theta} Cyg show a significant diameter variability that remains unexplained up to now. We propose that the presence of these discrepancies in the interferometric data is caused by either an intrinsic variation of the star or an unknown close companion orbiting around it.Comment: 10 pages + 2 pages appendix, 16 figures, accepted for publication in A&

On the flexibility of the design of Multiple Try Metropolis schemes

The Multiple Try Metropolis (MTM) method is a generalization of the classical Metropolis-Hastings algorithm in which the next state of the chain is chosen among a set of samples, according to normalized weights. In the literature, several extensions have been proposed. In this work, we show and remark upon the flexibility of the design of MTM-type methods, fulfilling the detailed balance condition. We discuss several possibilities and show different numerical results

Resonant control of elastic collisions in an optically trapped Fermi gas of atoms

We have loaded an ultracold gas of fermionic atoms into a far off resonance optical dipole trap and precisely controlled the spin composition of the trapped gas. We have measured a magnetic-field Feshbach resonance between atoms in the two lowest energy spin-states, |9/2, -9/2> and |9/2, -7/2>. The resonance peaks at a magnetic field of 201.5 plus or minus 1.4 G and has a width of 8.0 plus or minus 1.1 G. Using this resonance we have changed the elastic collision cross section in the gas by nearly 3 orders of magnitude.Comment: 4 pages, 3 figure

Pauli Blocking of Collisions in a Quantum Degenerate Atomic Fermi Gas

We have produced an interacting quantum degenerate Fermi gas of atoms composed of two spin-states of magnetically trapped $^{40}$K. The relative Fermi energies are adjusted by controlling the population in each spin-state. Measurements of the thermodynamics reveal the resulting imbalance in the mean energy per particle between the two species, which is as large as a factor of 1.4 at our lowest temperature. This imbalance of energy comes from a suppression of collisions between atoms in the gas due to the Pauli exclusion principle. Through measurements of the thermal relaxation rate we have directly observed this Pauli blocking as a factor of two reduction in the effective collision cross-section in the quantum degenerate regime.Comment: 11 pages, 4 figure