Modulation of galactic protons in the heliosphere during the unusual solar minimum of 2006 to 2009

The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of galactic protons were recorded at Earth in late 2009 in contrast to expectations. Proton spectra observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the Resurs-DK1 satellite are presented together with the solutions of a comprehensive numerical model for the solar modulation of cosmic rays. The model is used to determine what mechanisms were mainly responsible for the modulation of protons during this period, and why the observed spectrum for 2009 was the highest ever recorded. From mid-2006 until December 2009 we find that the spectra became significantly softer because increasingly more low energy protons had reached Earth. To simulate this effect, the rigidity dependence of the diffusion coefficients had to decrease significantly below ~3 GeV. The modulation minimum period of 2009 can thus be described as relatively more "diffusion dominated" than previous solar minima. However, we illustrate that drifts still had played a significant role but that the observable modulation effects were not as well correlated with the waviness of the heliospheric current sheet as before. Protons still experienced global gradient and curvature drifts as the heliospheric magnetic field had decreased significantly until the end of 2009, in contrast to the moderate decreases observed during previous minimum periods. We conclude that all modulation processes contributed to the observed increases in the proton spectra for this period, exhibiting an intriguing interplay of these major mechanisms

Hydrogen induced surface metallization of β\beta-SiC(100)-(3×23\times 2) revisited by DFT calculations

Recent experiments on the silicon terminated $3\times 2$ SiC(100) surface indicated an unexpected metallic character upon hydrogen adsorption. This effect was attributed to the bonding of hydrogen to a row of Si atoms and to the stabilization of a neighboring dangling bond row. Here, on the basis of Density-Functional calculations, we show that multiple-layer adsorption of H at the reconstructed surface is compatible with a different geometry: besides saturating the topmost Si dangling bonds, H atoms are adsorbed at rather unusual sites, \textit{i.e.} stable bridge positions above third-layer Si dimers. The results thus suggest an alternative interpretation for the electronic structure of the metallic surfaceComment: 10 pages, 3 figures. Higher resolution figures may be obtained from the authors ([email protected]). Submitted to Physical Review Letters in September 2004, resubmitted after revision in February 200

First principle theory of correlated transport through nano-junctions

We report the inclusion of electron-electron correlation in the calculation of transport properties within an ab initio scheme. A key step is the reformulation of Landauer's approach in terms of an effective transmittance for the interacting electron system. We apply this framework to analyze the effect of short range interactions on Pt atomic wires and discuss the coherent and incoherent correction to the mean-field approach.Comment: 5 pages, 3 figure

The determinants of Facebook social engagement for National Tourism Organisations’ Facebook pages: a quantitative approach

This work explores how the National Tourism Organizations (NTOs) of the top 10 most visited countries by international tourists strategically employ Facebook to promote and market their destinations. Based on big data retrieved from the NTOs’ Facebook pages, and leveraging advanced metrics for capturing user engagement, the study sheds light on the factors contributing to superior level of social activity. The findings indicate that the way Facebook is tactically employed varies significantly across sampled NTOs. The panel data regression analyses suggest that engagement is positively affected by posting visual content (namely photos), and posting during the weekends, and negatively affected by evening posting. Post frequency displays no statistically significant effect on social engagement. The study also shows that most of the NTOs (except for Italy, Spain, Turkey and the UK) deploy Facebook with a top-down approach, and spontaneous user generated content (UGC) is allowed to a very little extent

A TDDFT study of the excited states of DNA bases and their assemblies

We present a detailed study of the optical absorption spectra of DNA bases and base pairs, carried out by means of time dependent density functional theory. The spectra for the isolated bases are compared to available theoretical and experimental data and used to assess the accuracy of the method and the quality of the exchange-correlation functional: Our approach turns out to be a reliable tool to describe the response of the nucleobases. Furthermore, we analyze in detail the impact of hydrogen bonding and $\pi$-stacking in the calculated spectra for both Watson-Crick base pairs and Watson-Crick stacked assemblies. We show that the reduction of the UV absorption intensity (hypochromicity) for light polarized along the base-pair plane depends strongly on the type of interaction. For light polarized perpendicular to the basal plane, the hypochromicity effect is reduced, but another characteristic is found, namely a blue shift of the optical spectrum of the base-assembly compared to that of the isolated bases. The use of optical tools as fingerprints for the characterization of the structure (and type of interaction) is extensively discussed.Comment: 31 pages, 8 figure

Aspects of geodesical motion with Fisher-Rao metric: classical and quantum

The purpose of this article is to exploit the geometric structure of Quantum Mechanics and of statistical manifolds to study the qualitative effect that the quantum properties have in the statistical description of a system. We show that the end points of geodesics in the classical setting coincide with the probability distributions that minimise Shannon's Entropy, i.e. with distributions of zero dispersion. In the quantum setting this happens only for particular initial conditions, which in turn correspond to classical submanifolds. This result can be interpreted as a geometric manifestation of the uncertainty principle.Comment: 15 pages, 5 figure

Hamilton-Jacobi approach to Potential Functions in Information Geometry

The search for a potential function $S$ allowing to reconstruct a given metric tensor $g$ and a given symmetric covariant tensor $T$ on a manifold $\mathcal{M}$ is formulated as the Hamilton-Jacobi problem associated with a canonically defined Lagrangian on $T\mathcal{M}$. The connection between this problem, the geometric structure of the space of pure states of quantum mechanics, and the theory of contrast functions of classical information geometry is outlined.Comment: 16 pages. A discussion on the Kullback-Leibler divergence has been added. To appear in Journal of Mathematical Physic

Molecular Mechanism of DNA Topoisomerase I-Dependent rDNA Silencing: Sir2p Recruitment at Ribosomal Genes

Saccharomyces cerevisiae sir2Δ or top1Δ mutants exhibit similar phenotypes involving ribosomal DNA, including (i) loss of transcriptional silencing, resulting in non-coding RNA hyperproduction from cryptic RNA polymerase II promoters; (ii) alterations in recombination; and (iii) a general increase in histone acetylation. Given the distinct enzymatic activities of Sir2 and Top1 proteins, a histone deacetylase and a DNA topoisomerase, respectively, we investigated whether genetic and/or physical interactions between the two proteins could explain the shared ribosomal RNA genes (rDNA) phenotypes. We employed an approach of complementing top1Δ cells with yeast, human, truncated, and chimeric yeast/human TOP1 constructs and of assessing the extent of non-coding RNA silencing and histone H4K16 deacetylation. Our findings demonstrate that residues 115–125 within the yeast Top1p N-terminal domain are required for the complementation of the top1Δ rDNA phenotypes. In chromatin immunoprecipitation and co-immunoprecipitation experiments, we further demonstrate the physical interaction between Top1p and Sir2p. Our genetic and biochemical studies support a model whereby Top1p recruits Sir2p to the rDNA and clarifies a structural role of DNA topoisomerase I in the epigenetic regulation of rDNA, independent of its known catalytic activity

The drag force in two-fluid models of gas-solid flows

Currently, the two most widespread methods for modelling the particulate phase in numerical simulations of gas-solid flows are discrete particle simulation (see, e.g., Mikami, Kamiya, & Horio, 1998), and the two-fluid approach, e.g., kinetic theory models (see, e.g., Louge, Mastorakos, & Jenkins, 1991). In both approaches the gas phase is described by a locally averaged Navier-Stokes equation and the two phases are usually coupled by a drag force. Due to the large density difference between the particles and the gas, inter-phase forces other than the drag force are usually neglected, so it plays a significant role in characterising the gas-solid flow. Yasuna, Moyer, Elliott, and Sinclair (1995) have shown that the solution of their model is sensitive to the drag coefficient. In general, the performance of most current models depends critically on the accuracy of the drag force formulation