bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamo generated field emergence through recurrent plasmoid ejections
Authors: Warnecke Jörn , Brandenburg Axel
Published: August 2011

Magnetic buoyancy is believed to drive the transport of magnetic flux tubes from the convection zone to the surface of the Sun. The magnetic fields form twisted loop-like structures in the solar atmosphere. In this paper we use helical forcing to produce a large-scale dynamo-generated magnetic field, which rises even without magnetic buoyancy. A two layer system is used as computational domain where the upper part represents the solar atmosphere. Here, the evolution of the magnetic field is solved with the stress–and–relax method. Below this region a magnetic field is produced by a helical forcing function in the momentum equation, which leads to dynamo action. We find twisted magnetic fields emerging frequently to the outer layer, forming arch-like structures. In addition, recurrent plasmoid ejections can be found by looking at space–time diagrams of the magnetic field. Recent simulations in spherical coordinates show similar results.
Global magnetic cycles in rapidly rotating younger suns
Authors: Nelson Nicholas J. , Brown Benjamin P. , Browning Matthew K. , Brun Allan Sacha , Miesch Mark S. , Toomre Juri
Published: August 2011

Observations of sun-like stars rotating faster than our current sun tend to exhibit increased magnetic activity as well as magnetic cycles spanning multiple years. Using global simulations in spherical shells to study the coupling of large-scale convection, rotation, and magnetism in a younger sun, we have probed effects of rotation on stellar dynamos and the nature of magnetic cycles. Major 3-D MHD simulations carried out at three times the current solar rotation rate reveal hydromagnetic dynamo action that yields wreaths of strong toroidal magnetic field at low latitudes, often with opposite polarity in the two hemispheres. Our recent simulations have explored behavior in systems with considerably lower diffusivities, achieved with sub-grid scale models including a dynamic Smagorinsky treatment of unresolved turbulence. The lower diffusion promotes the generation of magnetic wreaths that undergo prominent temporal variations in field strength, exhibiting global magnetic cycles that involve polarity reversals. In our least diffusive simulation, we find that magnetic buoyancy coupled with advection by convective giant cells can lead to the rise of coherent loops of magnetic field toward the top of the simulated domain.
Magnetohydrostatic equilibrium in starspots: dependences on color (Teff) and surface gravity (g)
Authors: Rajaguru S. P. , Hasan S. S.
Published: August 2011

Temperature contrasts and magnetic field strengths of sunspot umbrae broadly follow the thermal-magnetic relationship obtained from magnetohydrostatic equilibrium. Using a compilation of recent observations, especially in molecular bands, of temperature contrasts of starspots in cool stars, and a grid of Kurucz stellar model atmospheres constructed to cover layers of sub-surface convection zone, we examine how the above relationship scales with effective temperature (Teff), surface gravity g and the associated changes in opacity of stellar photospheric gas. We calculate expected field strengths in starpots and find that a given relative reduction in temperatures (or the same darkness contrasts) yield increasing field strengths against decreasing Teff due to a combination of pressure and opacity variations against Teff.
Solar activity and differential rotation
Authors: Vats Hari Om , Chandra Satish
Published: August 2011

The coronal sidereal rotation rate as a function of latitude for each year, extending from 1992 to 2001 for soft X-ray images and from 1998 - 2005 for radio images are obtained. The present analysis reveals that the equatorial rotation rate of the corona is comparable to the photosphere and the chromosphere, However, at the higher latitudes, the corona rotation quite differently than the photosphere and chromosphere. The latitude differential obtained by both radio and X-ray images is quite variable throughout the period of the study. The equatorial rotation period seems to vary almost systematically with sunspot numbers which indicates its dependence on the phases of the solar activity cycle.
Numerical simulations of magnetic structures
Authors: Kitiashvili I. N. , Kosovichev A. G. , Wray A. A. , Mansour N. N.
Published: August 2011

We use 3D radiative MHD simulations of the upper turbulent convection layer for investigation of physical mechanisms of formation of magnetic structures on the Sun. The simulations include all essential physical processes, and are based of the LES (Large-Eddy Simulations) approach for describing the sub-grid scale turbulence. The simulation domain covers the top layer of the convection zone and the lower atmosphere. The results reveal a process of spontaneous formation of stable magnetic structures from an initially weak vertical magnetic field, uniformly distributed in the simulation domain. The process starts concentration of magnetic patches at the boundaries of granular cells, which are subsequently merged together into a stable large-scale structure by converging downdrafts below the surface. The resulting structure represents a compact concentration of strong magnetic field, reaching 6 kG in the interior. It has a cluster-like internal structurization, and is maintained by strong downdrafts extending into the deep layers.
Investigation of a sunspot complex by time-distance helioseismology
Authors: Kosovichev A. G. , Duvall T. L.
Published: August 2011

Sunspot regions often form complexes of activity that may live for several solar rotations, and represent a major component of the Sun's magnetic activity. It had been suggested that the close appearance of active regions in space and time might be related to common subsurface roots, or “nests” of activity. EUV images show that the active regions are magnetically connected in the corona, but subsurface connections have not been established. We investigate the subsurface structure and dynamics of a large complex of activity, NOAA 10987-10989, observed during the SOHO/MDI Dynamics run in March-April 2008, which was a part of the Whole Heliospheric Interval (WHI) campaign. The active regions in this complex appeared in a narrow latitudinal range, probably representing a subsurface toroidal flux tube. We use the MDI full-disk Dopplergrams to measure perturbations of travel times of acoustic waves traveling to various depths by using time-distance helioseismology, and obtain sound-speed and flow maps by inversion of the travel times. The subsurface flow maps show an interesting dynamics of decaying active regions with persistent shearing flows, which may be important for driving the flaring and CME activity, observed during the WHI campaign. Our analyses, including the seismic sound-speed inversion results and the distribution of deep-focus travel-time anomalies, gave indications of diverging roots of the magnetic structures, as could be expected from Ω-loop structures. However, no clear connection in the depth range of 0-48 Mm among the three active regions in this complex of activity was detected.
Vector magnetic field and vector current density in and around the δ-spot NOAA 10808
Authors: Bommier Véronique , Degl'Innocenti Egidio Landi , Schmieder Brigitte , Gelly Bernard
Published: August 2011

The context is that of the so-called “fundamental ambiguity” (also azimuth ambiguity, or 180° ambiguity) in magnetic field vector measurements: two field vectors symmetrical with respect to the line-of-sight have the same polarimetric signature, so that they cannot be discriminated. We propose a method to solve this ambiguity by applying the “simulated annealing” algorithm to the minimization of the field divergence, added to the longitudinal current absolute value, the line-of-sight derivative of the magnetic field being inferred by the interpretation of the Zeeman effect observed by spectropolarimetry in two lines formed at different depths. We find that the line pair Fe I λ 6301.5 and Fe I λ 6302.5 is appropriate for this purpose. We treat the example case of the δ-spot of NOAA 10808 observed on 13 September 2005 between 14:25 and 15:25 UT with the THEMIS telescope. Besides the magnetic field resolved map, the electric current density vector map is also obtained. A strong horizontal current density flow is found surrounding each spot inside its penumbra, associated to a non-zero Lorentz force centripetal with respect to the spot center (i.e., oriented towards the spot center). The current wrapping direction is found to depend on the spot polarity: clockwise for the positive polarity, counterclockwise for the negative one. This analysis is made possible thanks to the UNNOFIT2 Milne-Eddington inversion code, where the usual theory is generalized to the case of a line (Fe I λ 6301.5) that is not a normal Zeeman triplet line (like Fe I λ 6302.5).
Substructure of quiet sun bright points
Authors: Andic Aleksandra , Chae Jongchul , Goode Phillip R.
Published: August 2011

Since photospheric bright points (BPs) were first observed, there has been a question as to how are they structured. Are they just single flux tubes or a bundle of the flux-tubes? Surface photometry of the quiet Sun (QS) has achieved resolution close to 0.1″ with the New Solar Telescope at Big Bear Solar Observatory. This resolution allowed us to detect a richer spectrum of BPs in the QS. The smallest BPs we observed with TiO 705.68 nm were 0.13″, and we were able to resolve individual components in some of the BPs clusters and ribbons observed in the QS, showing that they are composed of the individual BPs. Average size of observed BPs was 0.22″.
Two types of coronal bright points their characteristics, and evolution
Authors: Sattarov Isroil , Karachik Nina V. , Sherdanov Chori T. , Tillaboev Azlarxon M. , Pevtsov Alexei A.
Published: August 2011

Using maximum brightness of coronal bright point's (CBP) as a criterion, we separate them on two categories: dim CBPs, associated with areas of a quiet Sun, and bright CBPs, associated with an active Sun. This study reports on characteristics of two types of CBPs and their evolution.
Distribution of magnetic shear angle in an emerging flux region
Authors: Gosain Sanjay
Published: August 2011

We study the distribution of magnetic shear in an emerging flux region using the high-resolution Hinode/SOT SP observations. The distribution of mean magnetic shear angle across the active region shows large values near region of flux emergence i.e., in the middle of existing bipolar region and decreases while approaching the periphery of the active region.
Damping rates of p-modes by an ensemble of randomly distributed thin magnetic flux tubes
Authors: Gascoyne Andrew , Jain Rekha
Published: August 2011

The magnetohydrodynamic (MHD) sausage tube waves are excited in the magnetic flux tubes by p-mode forcing. These tube waves thus carry energy away from the p-mode cavity which results in the deficit of incident p-mode energy. We calculate the loss of incident p-mode energy as a damping rate of f- and p-modes. We calculate the damping rates of f- and p-modes by a model Sun consisting of an ensemble of many thin magnetic flux tubes with varying plasma properties and distributions. Each magnetic flux tube is modelled as axisymmetric, vertically oriented and untwisted. We find that the magnitude and the form of the damping rates are sensitive to the plasma-β of the tubes and the upper boundary condition used.
Studies of waves in sunspots using spectropolarimetric observations
Authors: MacDonald Gordon A. , Rajaguru S. P.
Published: August 2011

We observe the acoustic velocity oscillations in and near active region NOAA 10960 on 8 June, 2007 using observations from the IBIS instrument at the Dunn Solar Telescope at NSO/Sacramento Peak and simultaneous Hinode BFI/SP data. Inversions were performed on the spectropolarimetric datasets in order to get magnetic field information for the AR. A time series of Doppler maps from line bisectors and Stokes V zero-crossing was constructed and allowed us to construct power maps for the AR. Past works by various authors have shown that acoustic power in the solar atmosphere is strongly influenced by magnetic field strength and inclination. Our study also explores this, but in addition, we also discuss the role of oscillations due to purely magnetized gas in the photosphere. Our preliminary results for this study are presented.
Towards physics-based helioseismic inversions of subsurface sunspot structure
Authors: Braun D. C. , Birch A. C. , Crouch A. D. , Rempel M.
Published: August 2011

Numerical computations of wave propagation through sunspot-like magnetic field structures are critical to developing and testing methods to deduce the subsurface structure of sunspots and active regions. We show that helioseismic analysis applied to the MHD sunspot simulations of Rempel and collaborators, as well as to translation-invariant models of umbral-like fields, yield wave travel-time measurements in qualitative agreement with those obtained in real sunspots. However, standard inversion methods applied to these data fail to reproduce the true wave-speed structure beneath the surface of the model. Inversion methods which incorporate direct effects of the magnetic field, including mode conversion, may be required.
Correlations of magnetic features and the torsional pattern
Authors: Muraközy Judit , Ludmány András
Published: August 2011

The striking similarity between the cyclic equatorward migration of the torsional oscillation belts and the sunspot activity latitudes inspired several attempts to seek an explanation of the torsional phenomenon in terms of interactions between flux ropes and plasma motions. The aim of the present work is to examine the spatial and temporal coincidences of the torsional waves and the emergence of sunspot magnetic fields. The locations of the shearing latitudes have been compared with the distributions of certain sunspot parameters by using sunspot data of more than two cycles. The bulges of the sunspot number and area distributions tend to be located within the ‘forward’ belts close to their poleward shearing borders. A possible geometry of the magnetic and velocity field interacion is proposed.
Photospheric data programs at the Debrecen Observatory
Authors: Győri L. , Baranyi T. , Ludmány A.
Published: August 2011

The primary task of the Debrecen Observatory is the most detailed, reliable and precise documentation of the solar photospheric activity. This long-term effort started with the continuation of the Greenwich photoheliograph program, this is the Debrecen Photoheliographic Data (DPD) sunspot catalogue based on ground-based observations. The profile of the work has later been extended to space-borne observations (SOHO/MDI and SDO), to magnetic fields and faculae as well as to higher temporal resolution (one hour) and nearly real-time data supply. The database also includes historical observations. The web-presentation developed for the material is easy to search and browse. We describe the main characteristics of these catalogs, and their advantages. We summarize the recent advances in the procedure of their compilation, and the available sets of the data and images.
Observations of density fluctuations in a quiescent prominence
Authors: Nakatsukasa Ken
Published: August 2011

Density fluctuations in a torsional quiescent prominence were studied with Solar Optical Telescope on board of Hinode satellite. Continuous observations were made in Ca II H line from ~15:00 UT May 03, 2008; the observational duration was ~1 hr with a cadence time ~ 30s. The emission intensity along the prominence axis as functions of altitudes and time shows fluctuations in brightness with local peaks in Fourier power spectra, indicating the presence of periods and intervallic displacements to be 3 ~ 11 min and 2 ~ 7 Mm respectively and statistically significant peaks at 368 ± 63s & 620 ± 41 s, and 3.3 ± 0.6 Mm regime. The distance is the line of sight projection. These intensity disturbances may perhaps be caused by density fluctuations originated from compressional waves in which the possible origin could be the combinations of p-mode oscillations induced at the surface and twisting of the flux tube.
Using SONG to probe rapid variability and evolution of starspots
Authors: Neff James E. , Hakkila Jon , Hill Frank , Jackiewicz Jason , Metcalfe Travis S. , Christensen-Dalsgaard Jørgen , Frandsen Søren , Grundahl Frank , Kjeldsen Hans , Jørgensen Uffe Gråe , Rasmussen Per Kjærgaard , Gu Sheng-Hong
Published: August 2011

The Stellar Observations Network Group (SONG) is being developed as a network of 1-meter spectroscopic telescopes designed for and primarily dedicated to asteroseismology. It is patterned after the highly successful GONG project. The Danish prototype telescope will be installed in Tenerife in early 2011. Ultimately we hope to have as many as 8 identical nodes providing continuous high-resolution spectroscopic observations for targets anywhere in the sky. The primary scientific goals of SONG are asteroseismology and the search for Earth-mass exoplanets. The spectroscopic requirements for these programs push the limits of current technology, but the resulting spectrograph design will enable many secondary science programs with less stringent requirements. Doppler imaging of starspots can be accomplished using continuous observations over several stellar rotations using identical instrumentation at each node. It should be possible to observe the evolution of starspot morphology in real-time, for example. We discuss the design and status of the SONG project in general, and we describe how SONG could be used to probe short timescale changes in stellar surface structure.
Structure of sunspots observed with Hinode Solar Optical Telescope
Authors: Choudhary Debi Prasad , MacDonald Gordon A. , Deng Na , Toshifumi Shimizu
Published: August 2011

We studied 16 sunspots with different sizes and shapes using the observations with the Hinode Solar Optical Telescope. The ratio of G-band and CaII H images reveal rich structures both within the umbra and penumbra of most spots. The striking features are the compact blob at the foot point of the umbra side of the penumbral fibrils with disk center-limb side asymmetry. In this paper, we present properties of these features using the spectropolarimetry and images in G-band, CaII and blue filters. We discuss the results using the contemporary models of the sunspots.
Microwave Depolarization above Sunspots
Authors: Lee Jeongwoo , White Stephen M.
Published: August 2011

Microwave emissions from sunspots are circularly polarized in the sense of rotation (right or left) determined by the polarity (north or south) of coronal magnetic fields. However, they may convert into unpolarized emissions under certain conditions of magnetic field and electron density in the corona, and this phenomenon of depolarization could be used to derive those parameters. We propose another diagnostic use of microwave depolarization based on the fact that an observed depolarization strip actually represents the coronal magnetic polarity inversion line (PIL) at the heights of effective mode coupling, and its location itself carries information on the distribution of magnetic polarity in the corona. To demonstrate this diagnostic utility we generate a set of magnetic field models for a complex active region with the observed line-of-sight magnetic fields but varying current density distribution and compare them with the 4.9 GHz polarization map obtained with the Very Large Array (VLA). The field extrapolation predicts very different locations of the depolarization strip in the corona depending on the amount of electric currents assumed to exist in the photosphere. Such high sensitivity of microwave depolarization to the coronal magnetic field can therefore be useful for validating electric current density maps inferred from vector magnetic fields observed in the photosphere.
Damping and the period ratio P1/2P2 of non-adiabatic slow mode
Authors: Kumar N. , Kumar A.
Published: August 2011

We investigate the combined effects of thermal conduction, compressive viscosity and optically thin radiative losses on the period ratio, P1/2P2, (P1 is the period of the fundamental mode and P2 is the period of its first harmonic) of a slow mode propagating one dimensionally. We obtain the dispersion relation and solve it to study the influence of non-ideal effects on the period ratio. The dependence of period ratio on thermal conductivity, compressive viscosity and radiative losses has been shown graphically. It is found that the effect of thermal conduction on the period ratio is negligible while compressive viscosity and radiation have sufficient effects for small loops and large loops respectively.
Pre-Eruption Magnetic Configurations in the Active-Region Solar Photosphere
Authors: Georgoulis Manolis K.
Published: August 2011

Most solar eruptions occur above strong photospheric magnetic polarity inversion lines (PILs). What overlays a PIL is unknown, however, and this has led to a debate over the existence of sheared magnetic arcades vs. helical magnetic flux ropes. We argue that this debate may be of little meaning: numerous small-scale magnetic reconnections, constantly triggered in the PIL area, can lead to effective transformation of mutual to self magnetic helicity (i.e. twist and writhe) that, ultimately, may force the magnetic structure above PILs to erupt to be relieved from its excess helicity. This is preliminary report of work currently in progress.
Empirically evaluating the impact of adjudicative tribunals in the health sector: context, challenges and opportunities
Authors: Hoffman Steven J. , Sossin Lorne
Published: August 2011

Adjudicative tribunals are an integral part of health system governance, yet their real-world impact remains largely unknown. Most assessments focus on internal accountability and use anecdotal methodologies; few, studies if any, empirically evaluate their external impact and use these data to test effectiveness, track performance, inform service improvements and ultimately strengthen health systems. Given that such assessments would yield important benefits and have been conducted successfully in similar settings (e.g. specialist courts), their absence is likely attributable to complexity in the health system, methodological difficulties and the legal environment within which tribunals operate. We suggest practical steps for potential evaluators to conduct empirical impact evaluations along with an evaluation matrix template featuring possible target outcomes and corresponding surrogate endpoints, performance indicators and empirical methodologies. Several system-level strategies for supporting such assessments have also been suggested for academics, health system institutions, health planners and research funders. Action is necessary to ensure that policymakers do not continue operating without evidence but can rather pursue data-driven strategies that are more likely to achieve their health system goals in a cost-effective way.
Modeling surf zone tracer plumes: 1. Waves, mean currents, and low‐frequency eddies
Authors: Feddersen Falk , Clark David B. , Guza R. T.
Published: August 2011

A model that accurately simulates surf zone waves, mean currents, and low‐frequency eddies is required to diagnose the mechanisms of surf zone tracer transport and dispersion. In this paper, a wave‐resolving time‐dependent Boussinesq model is compared with waves and currents observed during five surf zone dye release experiments. In a companion paper, Clark et al. (2011) compare a coupled tracer model to the dye plume observations. The Boussinesq model uses observed bathymetry and incident random, directionally spread waves. For all five releases, the model generally reproduces the observed cross‐shore evolution of significant wave height, mean wave angle, bulk directional spread, mean alongshore current, and the frequency‐dependent sea surface elevation spectra and directional moments. The largest errors are near the shoreline where the bathymetry is most uncertain. The model also reproduces the observed cross‐shore structure of rotational velocities in the infragravity (0.004 < f < 0.03 Hz) and very low frequency (VLF) (0.001 < f < 0.004 Hz) bands, although the modeled VLF energy is 2–3 times too large. Similar to the observations, the dominant contributions to the modeled eddy‐induced momentum flux are in the VLF band. These eddies are elliptical near the shoreline and circular in the mid surf zone. The model‐data agreement for sea swell waves, low‐frequency eddies, and mean currents suggests that the model is appropriate for simulating surf zone tracer transport and dispersion.
Absence of ABCG2-mediated mucosal detoxification in patients with active inflammatory bowel disease is due to impeded protein folding
Authors: Deuring J. Jasper , de Haar Colin , Koelewijn Chantal L. , Kuipers Ernst J. , Peppelenbosch Maikel P. , van der Woude C. Janneke
Published: August 2011

Xenotoxic damage in inflammatory diseases, including IBD (inflammatory bowel disease), is compounded by reduced activity of the xenobiotic transporter ABCG2 (ATP-binding-cassette G2) during the inflammatory state. An association between the activation of the unfolded protein response pathway and inflammation prompted us to investigate the possibility that reduced ABCG2 activity is causally linked to this response. To this end, we correlated expression of ABCG2 and the unfolded protein response marker GRP78 (glucose-regulated protein of 78 kDa) in colon biopsies from healthy individuals (n=9) and patients with inactive (n=67) or active (n=55) IBD, ischaemic colitis (n=10) or infectious colitis (n=14). In addition, tissue specimens throughout the small bowel from healthy individuals (n=27) and from patients with inactive (n=9) or active (n=25) Crohn's disease were co-stained for ABCG2 and GRP78. In all biopsies from patients with active inflammation, irrespective of the underlying disease, an absolute negative correlation was observed between epithelial ABCG2 expression and GRP78 expression, suggesting that inflammation-dependent activation of the unfolded protein response is responsible for suppression of ABCG2 function. The link between the unfolded protein response and functional ABCG2 expression was further corroborated by live imaging of ABCG2-expressing cells, which showed that various inflammatory mediators, including nitric oxide, activate the unfolded protein response and concomitantly reduce plasma membrane localization as well as transport function of ABCG2. Thus a novel mechanism for explaining xenobiotic stress during inflammation emerges in which intestinal inflammation activates the unfolded protein response, in turn abrogating defences against xenobiotic challenge by impairing ABCG2 expression and function.
Composition dependence of local piezoelectric nonlinearity in (0.3)Pb(Ni0.33Nb0.67)O3-(0.7)Pb(ZrxTi1−x)O3 films
Authors: Griggio F. , Jesse S. , Qu W. , Kumar A. , Ovchinnikov O. , Tinberg D. S. , Kalinin S. V. , Trolier-McKinstry S.
Published: August 2011

The composition dependence of dielectric and piezoelectric nonlinearities were characterized in epitaxially grown (0.3)Pb(Ni0.33Nb0.67)O3-(0.7)Pb(ZrxTi1−x)O3 thin films deposited on SrTiO3. Tetragonal, morphotropic phase boundary (MPB) and rhombohedral films were prepared by changing the Zr/Ti ratio. The relative dielectric permittivity ɛr and the converse piezoelectric coefficient d33,f were found to follow the Rayleigh law. The local piezoelectric nonlinearity map showed the formation of micron-sized clusters of higher nonlinear activity for the MPB and rhombohedral compositions. The ratios of the irreversible to the reversible Rayleigh constants αɛ/ɛinit and the spatially averaged αd/d33,init ratio were larger for the rhombohedral and MPB compositions compared to the tetragonal composition. The larger dielectric and piezoelectric nonlinearities observed for the rhombohedral sample are interpreted in terms of a higher domain wall mobility due to a smaller ferroelectric distortion and superior crystal quality.
Structural transitions in hypersphere fluids: Predictions of Kirkwood's approximation
Authors: Piasecki Jarosław , Szymczak Piotr , Kozak John J.
Published: August 2011

We use an analytic criterion for vanishing of exponential damping of correlations developed previously [J. Piasecki et al., J. Chem. Phys. 133, 164507 (2010)]10.1063/1.3491039 to determine the threshold volume fractions for structural transitions in hard sphere systems in dimensions D = 3, 4, 5, and 6, proceeding from the Yvon-Born-Green hierarchy and using the Kirkwood superposition approximation. We conclude that the theory does predict phase transitions in qualitative agreement with numerical studies. We also derive, within the superposition approximation, the asymptotic form of the analytic condition for occurrence of a structural transition in the D → ∞ limit.
Transient chaos in optical metamaterials
Authors: Ni Xuan , Lai Ying-Cheng
Published: August 2011

We investigate the dynamics of light rays in two classes of optical metamaterial systems: (1) time-dependent system with a volcano-shaped, inhomogeneous and isotropic refractive-index distribution, subject to external electromagnetic perturbations and (2) time-independent system consisting of three overlapping or non-overlapping refractive-index distributions. Utilizing a mechanical-optical analogy and coordinate transformation, the wave-propagation problem governed by the Maxwell’s equations can be modeled by a set of ordinary differential equations for light rays. We find that transient chaotic dynamics, hyperbolic or nonhyperbolic, are common in optical metamaterial systems. Due to the analogy between light-ray dynamics in metamaterials and the motion of light in matter as described by general relativity, our results reinforce the recent idea that chaos in gravitational systems can be observed and studied in laboratory experiments.
Shear driven electromagnetic drift-waves in a nonuniform dense magnetoplasma
Authors: Tariq Sabeen , Mirza Arshad M. , Masood Waqas
Published: August 2011

Linear characteristic properties of high- and low-frequency (in comparison with the cyclotron frequency) electromagnetic drift-waves are studied in a nonuniform, dense magnetoplasma (composed of electrons and ions), in the presence of parallel (magnetic field-aligned) velocity shear, by using quantum magnetohydrodynamic model. By applying the drift-approximation (viz., |∂ t|≪ωci≪ωce) to the quantum momentum equations, together with the continuity equations and the Poisson equation, we derive the governing equations for electromagnetic drift-waves with the shear flow. These linear equations are then Fourier transformed to obtain the dispersion relation in both high-frequency and low-frequency regimes. The dispersion relations are then discussed under various limiting cases.
Kinetics of polar mesospheric plasma layers: Comparison of theoretical results with observations
Authors: Sodha M. S. , Misra Shikha , Mishra S. K. , Dixit Amrit
Published: August 2011

This paper presents an analytical model for the physical understanding of the charge distribution on ice dust particles in plasma layers of polar mesospheric clouds PMCs (Noctilucent clouds and polar mesospheric summer echoes). For the case of pure ice dust (with high work function), the charging of the particles occurs only because of the accretion of electronic and ionic species on the surface of ice grains. The analysis is based on the number and energy balance of constituents and allows the charge to be only an integral multiple (positive or negative) of the electronic charge. Amongst other interesting results, the theory explains the observed charge distribution on pure ice particles and corresponding reduction of electron density (viz., Bite out) in the PMCs.
Laser plasma interaction studies in the context of shock ignition—Transition from collisional to collisionless absorption
Authors: Klimo O. , Tikhonchuk V. T. , Ribeyre X. , Schurtz G. , Riconda C. , Weber S. , Limpouch J.
Published: August 2011

The shock ignition concept implies laser pulse intensities higher than 1015 W/cm2 (at the wavelength of 351 nm), which is the commonly accepted limit where the inverse bremsstruhlung absorption dominates. The transition from collisional to collisionless absorption in laser plasma interactions at higher intensities is studied in the present paper with the help of large scale one-dimensional particle-in-cell simulations. The initial parameters are defined by the hydrodynamic simulations corresponding to recent experiments. The simulations predict that a quasi-steady regime of laser plasma interaction is attained where the total laser energy absorption stays on the level of ∼65% in the laser intensity range 1015–1016 W/cm2. However, the relation between the collisional and collisionless processes changes significantly. This is manifested in the energy spectrum of electrons transporting the absorbed laser energy and in the spectrum of the reflected laser light.
Exact momentum conservation laws for the gyrokinetic Vlasov-Poisson equations
Authors: Brizard Alain J. , Tronko Natalia
Published: August 2011

The exact momentum conservation laws for the nonlinear gyrokinetic Vlasov-Poisson equations are derived by applying the Noether method on the gyrokinetic variational principle [A. J. Brizard, Phys. Plasmas 7, 4816 (2000)]. From the gyrokinetic Noether canonical-momentum equation derived by the Noether method, the gyrokinetic parallel momentum equation and other gyrokinetic Vlasov-moment equations are obtained. In addition, an exact gyrokinetic toroidal angular-momentum conservation law is derived in axisymmetric tokamak geometry, where the transport of parallel-toroidal momentum is related to the radial gyrocenter polarization, which includes contributions from the guiding-center and gyrocenter transformations.
Thermodynamic stability of a bi-layer of copper nitride on Cu(100) surface
Authors: Pushpa Raghani , Williamson Izaak , Jones Barbara
Published: August 2011

Ultrathin insulating films composed of a few atomic layers are being extensively used for controlling the electronic coupling of nanostructures deposited on a substrate. Ultrathin film, for example, a single layer of Cu2N deposited on a Cu(100) surface (known as Cu2N/Cu(100) surface) has been used to determine the spectral properties of nanomagnets using scanning tunneling spectroscopy. However, recent experiments that measure spin relaxation times in a single atom suggest that the single layer of Cu2N does not provide efficient electronic decoupling. In this work, we study the thermodynamic stability of a bi-layer of copper nitride on the Cu(100) surface. We calculate adsorption and co-adsorption energies of Cu and N as a function of their concentration on the Cu2N/Cu(100) surface using density functional theory. We find that the adsorption and co-adsorption energies of Cu and N on the Cu2N/Cu(100) surface are of the order of a few eV. This suggests that the bi-layer of copper nitride is thermodynamically stable on the Cu(100) surface. We also find that the work function of N-adsorbed Cu2N/Cu(100) increases with the N concentration, suggesting a better insulating character of the bi-layer of copper nitride on the Cu(100) surface.
Electronic and vibrational spectroscopy of intermediates in methane-to-methanol conversion by CoO+
Authors: Altinay Gokhan , Kocak Abdulkadir , Silva Daluz Jennifer , Metz Ricardo B.
Published: August 2011

At room temperature, cobalt oxide cations directly convert methane to methanol with high selectivity but very low efficiency. Two potential intermediates of this reaction, the [HO–Co–CH3]+ insertion intermediate and [H2O–Co=CH2]+ aquo-carbene complex are produced in a laser ablation source and characterized by electronic and vibrational spectroscopy. Reaction of laser-ablated cobalt cations with different organic precursors seeded in a carrier gas produces the intermediates, which subsequently expand into vacuum and cool. Ions are extracted into a time-of-flight mass spectrometer and spectra are measured via photofragment spectroscopy. Photodissociation of [HO–Co–CH3]+ in the visible and via infrared multiple photon dissociation (IRMPD) makes only Co+ + CH3OH, while photodissociation of [H2O–Co=CH2]+ produces CoCH2+ + H2O. The electronic spectrum of [HO–Co–CH3]+ shows progressions in the excited state Co–C stretch (335 cm−1) and O–Co–C bend (90 cm−1); the IRMPD spectrum gives νOH = 3630 cm−1. The [HO–Co–CH3]+(Ar) complex has been synthesized and its vibrational spectrum measured in the O–H stretching region. The resulting spectrum is sharper than that obtained via IRMPD and gives νOH = 3642 cm−1. Also, an improved potential energy surface for the reaction of CoO+ with methane has been developed using single point energies calculated by the CBS-QB3 method for reactants, intermediates, transition states and products.
A systematic formulation of the virial expansion for nonadditive interaction potentials
Authors: Hellmann Robert , Bich Eckard
Published: August 2011

A new formulation of the virial expansion for a classical gas is derived without the restriction to pairwise-additive interaction potentials. Explicit expressions up to the eighth virial coefficient, suitable for numerical evaluation, are given in the form of integrals over sums of cluster diagrams. Compared with previous approaches, the number of cluster diagrams increases more moderately with increasing order of the virial coefficient. Thus, the new formulation should be particularly useful for the computation of high-order virial coefficients.
Strongly coupled bio-plasmonic system: Application to oxygen sensing
Authors: Dutta-Gupta Shourya , Martin Olivier J. F.
Published: August 2011

We investigate theoretically the strong coupling between surface plasmon resonances (SPRs) and absorption bands of hemoglobin. When the surface plasmon resonance spectrally overlaps the absorption bands of hemoglobin, the system is strongly coupled and its dispersion diagram exhibits an anti-crossing. Working in the conditions of strong coupling enhances the sensitivity of a SPR sensor up to a factor of 10. A model for the permittivity of hemoglobin, both in oxygenated and deoxygenated states, is presented and the study is carried out for both angle and wavelength modulated SPR sensors. Finally, a differential measurement is shown to increase the sensitivity further.
Design of resonant-cavity-enhanced multi-band photodetectors
Authors: Lao Yan-Feng , Ariyawansa Gamini , Unil Perera A. G.
Published: August 2011

A theoretical analysis to improve the quantum efficiency of detectors sensing in multiple spectral bands is presented. The effective coupling between the incoming light and multiple absorbing regions for simultaneously improving the multi-band absorption efficiency is obtained by using resonant-cavity structures. An optimized cavity with only a Au bottom reflector gives rise to an enhancement factor of 11 in absorption compared to the conventional detector without the cavity. Further improvement, by a factor of 26, can be attained with the aid of a dual-band Bragg reflector placed at the top. The resulting multi-band resonant-cavity detector increases the response in three out of four detection bands contributing to the spectral range from visible to long-wave infrared (IR). The optimized detector is capable of serving multiple purposes, such as regular IR detection for atmospheric windows, gas sensing, and for optical communications.
Thermal and mechanical denaturation properties of a DNA model with three sites per nucleotide
Authors: Florescu Ana-Maria , Joyeux Marc
Published: August 2011

In this paper, we show that the coarse grain model for DNA, which has been proposed recently by Knotts et al. [J. Chem. Phys. 126, 084901 (2007)]10.1063/1.2431804, can be adapted to describe the thermal and mechanical denaturation of long DNA sequences by adjusting slightly the base pairing contribution. The adjusted model leads to (i) critical temperatures for long homogeneous sequences that are in good agreement with both experimental ones and those obtained from statistical models, (ii) a realistic step-like denaturation behaviour for long inhomogeneous sequences, and (iii) critical forces at ambient temperature of the order of 10 pN, close to measured values. The adjusted model furthermore supports the conclusion that the thermal denaturation of long homogeneous sequences corresponds to a first-order phase transition and yields a critical exponent for the critical force equal to σ = 0.70. This model is both geometrically and energetically realistic, in the sense that the helical structure and the grooves, where most proteins bind, are satisfactorily reproduced, while the energy and the force required to break a base pair lie in the expected range. It therefore represents a promising tool for studying the dynamics of DNA-protein specific interactions at an unprecedented detail level.
Statistical mechanics of hydrated electron recombination in liquid and supercritical water
Authors: Schiller Robert , Horváth Ákos
Published: August 2011

The photochemical yield of hydrated electrons as a function of temperature in liquid and supercritical water is treated in terms of energy fluctuations of the medium. The geminate pair, consisting of a positive ion and a hydrated electron, is regarded as a H-like atom embedded in a completely relaxed dielectric continuum. If the local medium energy is larger than the ionization energy of this atom, the electron escapes its geminate partner. By making use of the classical theory of energy fluctuations, escape probability is described by a simple explicit function, the variable of which is a combination of temperature, relative permittivity, and specific heat. First our earlier calculations on the recombination of solvated electrons, produced by ionizing radiation in a number of polar liquids, are improved and then the theory is compared with the experimental results on temperature dependent electron survival by Kratz et al. [S. Kratz, J. Torres-Alcan, J. Urbanek, J. Lindner, and P. Vöhringer, Phys. Chem. Chem. Phys. 12, 12169 (2010)10.1039/c0cp00762e]. Two adjustable parameters are needed to achieve reasonable quantitative agreement.
Role of spinel substrate in the morphology of BiFeO3-CoFe2O4 epitaxial nanocomposite films
Authors: Stern Ilan , He Jibao , Zhou Xiaolan , Silwal Punam , Miao Ludi , Vargas Jose Marcelo , Spinu Leonard , Kim Dae Ho
Published: August 2011

Epitaxial thin films of BiFeO3-CoFe2O4 composite were grown on spinel MgAl2O4 substrates with a (001) orientation. The composite formed an intriguing nanogrid structure where perovskite BiFeO3 forms linear mounds embedded in a matrix of spinel CoFe2O4. The structure makes a stark distinction with similar composite films on (001) perovskite SrTiO3 substrates where CoFe2O4 forms nanopillars surrounded by a BiFeO3 matrix. This result shows the interface between the film and the substrate that plays a determining role during the formation of the composite.
Communication: Imaging wavefunctions in dissociative photoionization
Authors: Scott Hopkins W. , Mackenzie Stuart R.
Published: August 2011

The dissociative ionization dynamics of excited electronic states of the xenon dimer, Xe2, have been studied using velocity map ion imaging (VMI). A one-colour, (2+1) resonant excitation scheme was employed to first excite and then ionize selected vibrational levels of the Xe2 6p 2[1/2]0 $0_g^ +$0g+ Rydberg state. Cationic fragments were then detected by the VMI. The data provide an outstanding example of the reflection principle in photodissociation with the full nodal structure of the Rydberg state wavefunctions clearly observed in the final Xe+ kinetic energy distributions without the need for scanning the excitation energy. Fitting of the observed distributions provides detailed and precise information on the form of the Xe2+ I(1/2g) potential energy curve involved which is in excellent agreement with the results of photoelectron imaging studies [Shubert and Pratt, J. Chem. Phys. 134, 044315 (2011) 10.1063/1.3533361]. Furthermore, the anisotropy of the product angular distributions yields information on the evolution of the electronic character of the ionic state with internuclear separation, R. The combination of the nature of dissociative ionization and the extent of the bound state wavefunctions provide information over an unusually wide range of internuclear separation R (ΔR > 0.75 Å). This would normally require scanning over a considerable energy region but is obtained in these studies at a fixed excitation energy.
Weak localization and percolation effects in annealed In2O3-ZnO thin films
Authors: Shinozaki B. , Ezaki S. , Hidaka K. , Makise K. , Asano T. , Yano K. , Nakamura H.
Published: August 2011

We have investigated the temperature T and magnetic field H dependences of the sheet resistance R□ of thin (In2O3)0.975-(ZnO)0.025 films with different resistivities and carrier densities prepared by postannealing in air at various annealing temperatures Ta. Regarding the magnetoconductance Δσ(H) ≡ 1/R□(H) − 1/R□(0) of films with large values of sheet resistance R□, agreement between weak localization theory and the data cannot be obtained for any value of the localization length $L_{{\rm in}} (T) = \sqrt {D\tau _{{\rm in}} (T)}$L in (T)=Dτ in (T), where D and τin are the diffusion constant and inelastic scattering time, respectively. Taking account of the inhomogeneous morphology confirmed by Scanning Electron Microscopy (SEM) observation, we introduced the effective sheet resistance R□eff given by R□eff = α × R□meas., where the strength of reduction factor α is less than unit, α ⩽ 1. Using a suitable value of α(Ta), we successfully fitted the theory to data for Δσeff(H, T), regarding Lin2(T) as a fitting parameter in the region 2.0 K⩽T ⩽ 50 K. It was confirmed that the rate 1/τin(T) is given by the sum of the electron-electron and electron-phonon inelastic scattering rates.
SIRT6 in mouse spermatogenesis is modulated by diet-induced obesity
Authors: Palmer Nicole O. , Fullston Tod , Mitchell Megan , Setchell Brian P. , Lane Michelle
Published: August 2011

Male obesity is associated with reduced sperm function and increased incidence of sperm DNA damage; however, the underlying molecular mechanisms have not yet been identified. Mammalian SIRT6 protein is involved in caloric-dependant DNA damage repair in other tissue types, yet a possible role for SIRT6 in male obesity and subfertility has not been investigated previously. To assess SIRT6 levels and activity in the testes, male mice (n = 12 per diet) were fed either a control diet (CD; 6% fat) or a high-fat diet (HFD; 21% fat) for 16 weeks before the collection of testes and spermatozoa. SIRT6 protein was localised to the nucleus of transitional spermatids and the acrosome of mature spermatozoa, with levels significantly decreased in HFD-fed male mice (P < 0.05). This decrease in SIRT6 protein was associated with transitional spermatids having increased levels of acetylated H3K9 in the nucleus (P < 0.01) and increased DNA damage (P < 0.001). We propose a role for SIRT6 in spermiogenesis and potentially protamination processes, which are known to be compromised by male obesity.