401 research outputs found
Examining Periodic Solar Wind Density Structures Observed in the SECCHI Heliospheric Imagers
We present an analysis of small-scale, periodic, solar-wind density
enhancements (length-scales as small as \approx 1000 Mm) observed in images
from the Heliospheric Imager (HI) aboard STEREO A. We discuss their possible
relationship to periodic fluctuations of the proton density that have been
identified at 1 AU using in-situ plasma measurements. Specifically, Viall,
Kepko, and Spence (2008) examined 11 years of in-situ solar-wind density
measurements at 1 AU and demonstrated that not only turbulent structures, but
also non-turbulent periodic density structures exist in the solar wind with
scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall,
Spence, and Kasper (2009) analyzed the {\alpha} to proton solar-wind abundance
ratio measured during one such event of periodic density structures,
demonstrating that the plasma behavior was highly suggestive that either
temporally or spatially varying coronal source plasma created those density
structures. Large periodic density structures observed at 1 AU, which were
generated in the corona, can be observable in coronal and heliospheric
white-light images if they possess sufficiently high density contrast. Indeed,
we identify such periodic density structures as they enter the HI field of view
and follow them as they advect with the solar wind through the images. The
smaller periodic density structures that we identify in the images are
comparable in size to the larger structures analyzed in-situ at 1 AU, yielding
further evidence that periodic density enhancements are a consequence of
coronal activity as the solar wind is formed.Comment: 15 pages, 12 figures. The final publication is available at
http://www.springerlink.co
A Coupled-Cluster Formulation of Hamiltonian Lattice Field Theory: The Non-Linear Sigma Model
We apply the coupled cluster method (CCM) to the Hamiltonian version of the
latticised O(4) non-linear sigma model. The method, which was initially
developed for the accurate description of quantum many-body systems, gives rise
to two distinct approximation schemes. These approaches are compared with each
other as well as with some other Hamiltonian approaches. Our study of both the
ground state and collective excitations leads to indications of a possible
chiral phase transition as the lattice spacing is varied.Comment: 44 Pages, 14 figures. Uses Latex2e, graphicx, amstex and geometry
package
Thermal diffusion of supersonic solitons in an anharmonic chain of atoms
We study the non-equilibrium diffusion dynamics of supersonic lattice
solitons in a classical chain of atoms with nearest-neighbor interactions
coupled to a heat bath. As a specific example we choose an interaction with
cubic anharmonicity. The coupling between the system and a thermal bath with a
given temperature is made by adding noise, delta-correlated in time and space,
and damping to the set of discrete equations of motion. Working in the
continuum limit and changing to the sound velocity frame we derive a
Korteweg-de Vries equation with noise and damping. We apply a collective
coordinate approach which yields two stochastic ODEs which are solved
approximately by a perturbation analysis. This finally yields analytical
expressions for the variances of the soliton position and velocity. We perform
Langevin dynamics simulations for the original discrete system which fully
confirm the predictions of our analytical calculations, namely noise-induced
superdiffusive behavior which scales with the temperature and depends strongly
on the initial soliton velocity. A normal diffusion behavior is observed for
very low-energy solitons where the noise-induced phonons also make a
significant contribution to the soliton diffusion.Comment: Submitted to PRE. Changes made: New simulations with a different
method of soliton detection. The results and conclusions are not different
from previous version. New appendixes containing information about the system
energy and soliton profile
Iron storage capacities and associated ferritin gene expression among marine diatoms
In large regions of the ocean, low iron availability regulates diatom growth and species composition. Diatom species often vary in their physiological response to iron enrichment, with natural and artificial iron additions in iron-limited regions of the ocean resulting in large blooms of primarily pennate diatoms. The ability of pennate diatoms to proliferate following pulse iron additions has been partly attributed to their ability to acquire and store excess intracellular iron in the iron storage protein ferritin. Recent transcriptome sequencing of diatoms indicate that some centric diatoms also possess ferritin. Using a combination of physiological and molecular techniques, we examined the iron storage capacities and associated ferritin gene expression in phylogenetically diverse centric and pennate diatoms grown under high and low iron concentrations. There were no systematic differences among ferritin-containing and non-containing diatom lineages in their ability to store iron in excess of that needed to support maximum growth rates. An exception, however, was the ferritin-containing pennate diatom Pseudo-nitzschia granii, native to iron-limited waters of the Northeast Pacific Ocean. This species exhibited an exceptionally large luxury iron storage capacity and increased ferritin gene expression at high iron concentrations, supporting a role in long-term iron storage. By contrast, two other diatoms species that exhibited minimal iron storage capacities contained two distinct ferritin genes where one ferritin gene increased in expression under iron limitation while the second showed no variation with cellular iron status. We conclude that ferritin may serve multiple functional roles that are independent of diatom phylogeny
Integrable Open Spin Chains in Defect Conformal Field Theory
We demonstrate that the one-loop dilatation generator for the scalar sector
of a certain perturbation of N=4 Super Yang-Mills with fundamentals is the
Hamiltonian of an integrable spin chain with open boundary conditions. The
theory is a supersymmetric defect conformal field theory (dCFT) with the
fundamentals in hypermultiplets confined to a codimension one defect. We obtain
a K-matrix satisfying a suitably generalized form of the boundary Yang-Baxter
equation, study the Bethe ansatz equations and demonstrate how Dirichlet and
Neumann boundary conditions arise in field theory, and match to existing
results in the plane wave limit.Comment: 26 pages, LaTeX. v2: references added, typos correcte
Taxonomic and nutrient controls on phytoplankton iron quotas in the ocean
Phytoplankton iron contents (i.e., quotas) directly link biogeochemical cycles of iron and carbon and drive patterns of nutrient limitation, recycling, and export. Ocean biogeochemical models typically assume that iron quotas are either static or controlled by dissolved iron availability. We measured iron quotas in phytoplankton communities across nutrient gradients in the Pacific Ocean and found that quotas diverged significantly in taxon-specific ways from laboratory-derived predictions. Iron quotas varied 40-fold across nutrient gradients, and nitrogen-limitation allowed diatoms to accumulate fivefold more iron than co-occurring flagellates even under low iron availability. Modeling indicates such âluxuryâ uptake is common in large regions of the low-iron Pacific Ocean. Among diatoms, both pennate and centric genera accumulated luxury iron, but the cosmopolitan pennate genus Pseudo-nitzschia maintained iron quotas 10-fold higher than co-occurring centric diatoms, likely due to enhanced iron storage. Biogeochemical models should account for taxonomic and macronutrient controls on phytoplankton iron quotas
Origin of strange metallic phase in cuprate superconductors
The origin of strange metallic phase is shown to exist due to these two
conditions---(i) the electrons are strongly interacting such that there are no
band and Mott-Hubbard gaps, and (ii) the electronic energy levels are crossed
in such a way that there is an electronic energy gap between two energy levels
associated to two different wave functions. The theory is also exploited to
explain (i) the upward- and downward-shifts in the -linear resistivity
curves, and (ii) the spectral weight transfer observed in the soft X-ray
absorption spectroscopic measurements of the La-Sr-Cu-O Mott insulator.Comment: To be published in J. Supercond. Nov. Mag
Giant Gravitons - with Strings Attached (III)
We develop techniques to compute the one-loop anomalous dimensions of
operators in the super Yang-Mills theory that are dual to open
strings ending on boundstates of sphere giant gravitons. Our results, which are
applicable to excitations involving an arbitrary number of open strings,
generalize the single string results of hep-th/0701067. The open strings we
consider carry angular momentum on an S embedded in the S of the
AdSS background. The problem of computing the one loop anomalous
dimensions is replaced with the problem of diagonalizing an interacting Cuntz
oscillator Hamiltonian. Our Cuntz oscillator dynamics illustrates how the
Chan-Paton factors for open strings propagating on multiple branes can arise
dynamically.Comment: 66 pages; v2: improved presentatio
Effective theory of the Delta(1232) in Compton scattering off the nucleon
We formulate a new power-counting scheme for a chiral effective field theory
of nucleons, pions, and Deltas. This extends chiral perturbation theory into
the Delta-resonance region. We calculate nucleon Compton scattering up to
next-to-leading order in this theory. The resultant description of existing
p cross section data is very good for photon energies up to about 300
MeV. We also find reasonable numbers for the spin-independent polarizabilities
and .Comment: 29 pp, 9 figs. Minor revisions. To be published in PR
Cardiovascular risk reduction with liraglutide: An exploratory mediation analysis of the leader trial
OBJECTIVE The Liraglutide Effect and Action in Diabetes: Evaluation of Cardiovascular Outcome Results (LEADER) trial (ClinicalTrials.gov reg. no. NCT01179048) demonstrated a reduced risk of cardiovascular (CV) events for patients with type 2 diabetes who received the glucagon-like peptide 1 receptor agonist liraglutide versus placebo. The mechanisms behind this CV benefit remain unclear. We aimed to identify potential mediators for the CV benefit observed with liraglutide in the LEADER trial. RESEARCH DESIGN AND METHODS We performed exploratory analyses to identify potential mediators of the effect of liraglutide on major adverse CV events (MACE; composite of CV death, nonfatal myocardial infarction, or nonfatal stroke) from the following candidates: Glycated hemoglobin (HbA1c), body weight, urinary albumin-to-creatinine ratio (UACR), confirmed hypoglycemia, sulfonylurea use, insulin use, systolic blood pressure, and LDL cholesterol. These candidates were selected as CV risk factors on which liraglutide had an effect in LEADER such that a reduction in CV risk might result. We used two methods based on a Cox proportional hazards model and the new Vansteelandt method designed to use all available information from the mediator and to control for confounding factors. RESULTS Analyses using the Cox methods and Vansteelandt method indicated potential mediation by HbA1c (up to 41% and 83% mediation, respectively) and UACR (up to 29% and 33% mediation, respectively) on the effect of liraglutide on MACE. Mediation effects were small for other candidates. CONCLUSIONS These analyses identify HbA1c and, to a lesser extent, UACR as potential mediators of the CV effects of liraglutide. Whether either is a marker of an unmeasured factor or a true mediator remains a key question that invites further investigation
- âŠ