Global Dynamics of Subsurface Solar Active Regions

We present three-dimensional numerical simulations of a magnetic loop evolving in either a convectively stable or unstable rotating shell. The magnetic loop is introduced in the shell in such a way that it is buoyant only in a certain portion in longitude, thus creating an \Omega-loop. Due to the action of magnetic buoyancy, the loop rises and develops asymmetries between its leading and following legs, creating emerging bipolar regions whose characteristics are similar to the ones of observed spots at the solar surface. In particular, we self-consistently reproduce the creation of tongues around the spot polarities, which can be strongly affected by convection. We moreover emphasize the presence of ring-shaped magnetic structures around our simulated emerging regions, which we call "magnetic necklace" and which were seen in a number of observations without being reported as of today. We show that those necklaces are markers of vorticity generation at the periphery and below the rising magnetic loop. We also find that the asymmetry between the two legs of the loop is crucially dependent on the initial magnetic field strength. The tilt angle of the emerging regions is also studied in the stable and unstable cases and seems to be affected both by the convective motions and the presence of a differential rotation in the convective cases.Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication in Ap

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces.Comment: 13 pages, 10 figures, accepted for publication in A&

Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo

An extremely weak circularly polarized signature was recently discovered in spectral lines of the chemically peculiar Am star Sirius A. A weak surface magnetic field was proposed to account for the observed polarized signal, but the shape of the phase-averaged signature, dominated by a prominent positive lobe, is not expected in the standard theory of the Zeeman effect. We aim at verifying the presence of weak circularly polarized signatures in two other bright Am stars, beta UMa and theta Leo, and investigating the physical origin of Sirius-like polarized signals further. We present here a set of deep spectropolarimetric observations of beta UMa and theta Leo, observed with the NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares Deconvolution multiline procedure. To improve the signal-to-noise ratio and detect extremely weak signatures in Stokes V profiles, we co-added all available spectra of each star (around 150 observations each time). Finally, we ran several tests to evaluate whether the detected signatures are consistent with the behavior expected from the Zeeman effect. The line profiles of the two stars display circularly polarized signatures similar in shape and amplitude to the observations previously gathered for Sirius A. Our series of tests brings further evidence of a magnetic origin of the recorded signal. These new detections suggest that very weak magnetic fields may well be present in the photospheres of a significant fraction of intermediate-mass stars. The strongly asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant single-sign lobe) are not expected in the standard theory of the Zeeman effect and may be linked to sharp vertical gradients in photospheric velocities and magnetic field strengths

Allelopathic interactions between the brown algal genus Lobophora (Dictyotales, Phaeophyceae) and scleractinian corals

Allelopathy has been recently suggested as a mechanism by which macroalgae may outcompete corals in damaged reefs. Members of the brown algal genus Lobophora are commonly observed in close contact with scleractinian corals and have been considered responsible for negative effects of macroalgae to scleractinian corals. Recent field assays have suggested the potential role of chemical mediators in this interaction. We performed in situ bioassays testing the allelopathy of crude extracts and isolated compounds of several Lobophora species, naturally associated or not with corals, against four corals in New Caledonia. Our results showed that, regardless of their natural association with corals, organic extracts from species of the genus Lobophora are intrinsically capable of bleaching some coral species upon direct contact. Additionally, three new C-21 polyunsaturated alcohols named lobophorenols A-C (1-3) were isolated and identified. Significant all elopathic effects against Acropora muricata were identified for these compounds. In situ observations in New Caledonia, however, indicated that while allelopathic interactions are likely to occur at the macroalgal-coral interface, Lobophora spp. rarely bleached their coral hosts. These findings are important toward our understanding of the importance of allelopathy versus other processes such as herbivory in the interaction between macroalgae and corals in reef ecosystems

Gold-Silver Catalysts: Ruling Factors for Establishing Synergism

DPU and SOL immobilisation have been used to prepare 1 %AuAg/TiO2 with internal ratio 1 : 1 and 4 : 1 which have been studied as fresh, calcined in air at 300 \ub0C and reduced at 550 \ub0C in H2. TEM-EDS, XPS, UV-Vis and CO-DRIFT allowed to characterize the samples in terms of particle size, particle composition, exposure and oxidation state of metals. Correlating these characteristics to the catalytic behaviour we concluded that only Au-rich catalysts show synergistic effect, silver in bimetallic systems appears more resistant to oxidation than in monometallic one, thermal treatment enhances the SMSI thus producing (regardless to the post-treatment) almost the same amount of Au\u3b4+ and also Ag\u3b4+. Catalysts prepared by DPU (calcined in air or reduced in H2) are more active than SOL (fresh or calcined) probably due to the higher presence of gold at the surface

The SNARE VAMP7 Regulates Exocytic Trafficking of Interleukin-12 in Dendritic Cells

Interleukin-12 (IL-12), produced by dendritic cells in response to activation, is central to pathogen eradication and tumor rejection. The trafficking pathways controlling spatial distribution and intracellular transport of IL-12 vesicles to the cell surface are still unknown. Here, we show that intracellular IL-12 localizes in late endocytic vesicles marked by the SNARE VAMP7. Dendritic cells (DCs) from VAMP7-deficient mice are partially impaired in the multidirectional release of IL-12. Upon encounter with antigen-specific T cells, IL-12-containing vesicles rapidly redistribute at the immune synapse and release IL-12 in a process entirely dependent on VAMP7 expression. Consistently, acquisition of effector functions is reduced in T cells stimulated by VAMP7-null DCs. These results provide insights into IL-12 intracellular trafficking pathways and show that VAMP7-mediated release of IL-12 at the immune synapse is a mechanism to transmit innate signals to T cells

The analysis of meiosis of the B genome of common wheat.

6 p.Two intervarietal hybrids of common wheat, Triticum aestivum L., are meiotically analyzed using the C-banding staining method. The C-banding pattern of nine meiotic chromosomes (4A, 7A, and the seven of the B genome) permitted their unequivocal recognition at first metaphase plates. The pairing frequency of each B-genome chromosome arm was scored. Data on the pairing frequency of the arms, separately considered, are applied to calculate expected pairing of whole chromosomes and whole genomes. The application of mathematical models to predict the genome pairing using either equal or different frequencies per chromosome arm is discussed.Key words: meiotic analysis, Triticum aestivum L., C-banding

Magnetic helicity fluxes in interface and flux transport dynamos

Dynamos in the Sun and other bodies tend to produce magnetic fields that possess magnetic helicity of opposite sign at large and small scales, respectively. The build-up of magnetic helicity at small scales provides an important saturation mechanism. In order to understand the nature of the solar dynamo we need to understand the details of the saturation mechanism in spherical geometry. In particular, we want to understand the effects of magnetic helicity fluxes from turbulence and meridional circulation. We consider a model with just radial shear confined to a thin layer (tachocline) at the bottom of the convection zone. The kinetic alpha owing to helical turbulence is assumed to be localized in a region above the convection zone. The dynamical quenching formalism is used to describe the build-up of mean magnetic helicity in the model, which results in a magnetic alpha effect that feeds back on the kinetic alpha effect. In some cases we compare with results obtained using a simple algebraic alpha quenching formula. In agreement with earlier findings, the magnetic alpha effect in the dynamical alpha quenching formalism has the opposite sign compared with the kinetic alpha effect and leads to a catastrophic decrease of the saturation field strength with increasing magnetic Reynolds numbers. However, at high latitudes this quenching effect can lead to secondary dynamo waves that propagate poleward due to the opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and from meridional circulation alleviate catastrophic quenching.Comment: 9 pages, 14 figures, submitted to A &