406 research outputs found
Three Dimensional MHD Wave Propagation and Conversion to Alfven Waves near the Solar Surface. I. Direct Numerical Solution
The efficacy of fast/slow MHD mode conversion in the surface layers of
sunspots has been demonstrated over recent years using a number of modelling
techniques, including ray theory, perturbation theory, differential eigensystem
analysis, and direct numerical simulation. These show that significant energy
may be transferred between the fast and slow modes in the neighbourhood of the
equipartition layer where the Alfven and sound speeds coincide. However, most
of the models so far have been two dimensional. In three dimensions the Alfven
wave may couple to the magneto-acoustic waves with important implications for
energy loss from helioseismic modes and for oscillations in the atmosphere
above the spot. In this paper, we carry out a numerical ``scattering
experiment'', placing an acoustic driver 4 Mm below the solar surface and
monitoring the acoustic and Alfvenic wave energy flux high in an isothermal
atmosphere placed above it. These calculations indeed show that energy
conversion to upward travelling Alfven waves can be substantial, in many cases
exceeding loss to slow (acoustic) waves. Typically, at penumbral magnetic field
strengths, the strongest Alfven fluxes are produced when the field is inclined
30-40 degrees from the vertical, with the vertical plane of wave propagation
offset from the vertical plane containing field lines by some 60-80 degrees.Comment: Accepted for the HELAS II/ SOHO 19/ GONG 2007 Topical Issue of Solar
Physic
Towards the deconstruction of M-theory
We argue that there is an equivalence of M-theory on T^3 \times A_{N-1} with
a four-dimensional non-supersymmetric quiver gauge theory on the Higgs branch.
The quiver theory in question has gauge group SU(N)^{N_4N_6N_8} and is
considered in a strong coupling and large N_{4,6,8} limit. We provide field-
and string-theoretical evidence for the equivalence making use of the
deconstruction technique. In particular, we find wrapped M2-branes in the mass
spectrum of the quiver theory at low energies.Comment: LaTeX, 15 pages, 4 figures, added reference
Interactive Quadratic Gravity
A quadratic semiclassical theory, regarding the interaction of gravity with a
massive scalar quantum field, is considered in view of the renormalizable
energy-momentum tensor in a multi-dimensional curved spacetime. According to
it, a self-consistent coupling between the square curvature term R^{2} and the
quantum field \Phi should be introduced in order to yield the "correct"
renormalizable energy-momentum tensor in quadratic gravity theories. The
subsequent interaction discards any higher-order derivative terms from the
gravitational field equations, but, in the expence, it introduces a geometric
source term in the wave equation for the quantum field. Unlike the conformal
coupling case (R\Phi ^{2}), this term does not represent an additional "mass"
and, therefore, the quantum field interacts with gravity not only through its
mass (or energy) content (~\Phi ^{2}), but also, in a more generic way
(R^{2}\Phi). Within this context, we propose a general method to obtain
mode-solutions for the quantum field, by means of the associated Green's
function in an anisotropic six-dimensional background.Comment: 12 pages, LaTeX, to appear in Physics Letters
Does Reverse Causality Underlie the Temporal Relationship between Depression and Crohn's Disease?
Background: Studies suggest that there is a temporal relationship between depression and Crohn's disease (CD) activity. However, these studies assumed a unidirectional relationship and did not examine the possibility of reverse causality and the risk of a spurious association due to the overlap of symptoms underlying the depression-CD relationship. We evaluated the existence of reverse causality reflected in a possible bidirectional relationship between patient-reported CD activity and an affective-cognitive dimension of depression. Methods: We studied 3307 adult volunteers with a self-reported diagnosis of CD who completed a baseline survey that included demographics, CD activity, and an affective-cognitive index of depression. Crohn's disease status and the affective-cognitive index of depression were also measured 6 and 12 months after the baseline evaluation. We used structural equation models to evaluate whether the effect of depression on future CD activity is stronger than the effect of CD activity on future depression. We calculated the likelihood that each of these hypotheses is supported by the data and calculated the likelihood ratio to provide a relative measure of which hypothesis best accounts for the data. Results: The results of the informative hypothesis testing showed the most support for the hypothesis stating that an affective-cognitive dimension of depression is a stronger predictor of patient-reported CD activity than the converse. Conclusions: The hypothesis that an affective-cognitive dimension of depression predicts patient-reported exacerbation of CD is 218 times more likely to account for the data than the converse
Air pollution, neighborhood deprivation, and autism spectrum disorder in the Study to Explore Early Development
Background: To examine whether neighborhood deprivation modifies the association between early life air pollution exposure and autism spectrum disorder (ASD), we used resources from a multisite case-control study, the Study to Explore Early Development. Methods: Cases were 674 children with confirmed ASD born in 2003-2006; controls were 855 randomly sampled children born during the same time period and residents of the same geographic areas as cases. Air pollution was assessed by roadway proximity and particulate matter <2.5 µm (PM2.5) exposure during pregnancy and first year of life. To characterize neighborhood deprivation, an index was created based on eight census tract-level socioeconomic status-related parameters. The continuous index was categorized into tertiles, representing low, moderate, and high deprivation. Logistic regression was used to estimate odds ratios (ORs) and corresponding 95% confidence intervals (CIs). Results: Neighborhood deprivation modified (Pfor interaction = 0.08) the association between PM2.5 exposure during the first year of life and ASD, with a stronger association for those living in high (OR = 2.42, 95% CI = 1.20, 4.86) rather than moderate (OR=1.21, 95% CI = 0.67, 2.17) or low (OR=1.46, 95% CI = 0.80, 2.65) deprivation neighborhoods. Departure from additivity or multiplicativity was not observed for roadway proximity or exposures during pregnancy. Conclusion: These results provide suggestive evidence of interaction between neighborhood deprivation and PM2.5 exposure during the first year of life in association with ASD
A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints
Using RHESSI data, we have analyzed some 172 hard X-ray peaks during 53 solar
flares which exhibited a double-footpoint structure. Fitting both footpoints
with power-laws, we find that spectral index differences range mostly between 0
to 0.6, and only rarely go beyond. Asymmetries between footpoints were not
observed to be significantly dependent on their mean heliographic position,
their relative position with respect to each other, nor their orientation with
respect to the solar equator. Assuming a symmetric acceleration process, it is
also clear that differences in footpoint spectral indices and footpoint flux
ratios can seldom be attributed to a difference in column densities between the
two legs of a coronal loop. Our results corroborate better the magnetic mirror
trap scenario. Moreover, footpoint asymmetries are more marked during times of
peak HXR flux than when averaging over the whole HXR burst, suggesting that the
magnetic configuration evolves during individual HXR bursts. We observed also a
linear correlation between the peak 50-keV flux and the peak GOES 1-8A channel
flux, and that HXR burst duration seem correlated with loop length.Comment: 20 pages, 13 figures. Published in Solar Physic
How Coupling Determines the Entrainment of Circadian Clocks
Autonomous circadian clocks drive daily rhythms in physiology and behaviour.
A network of coupled neurons, the suprachiasmatic nucleus (SCN), serves as a
robust self-sustained circadian pacemaker. Synchronization of this timer to the
environmental light-dark cycle is crucial for an organism's fitness. In a
recent theoretical and experimental study it was shown that coupling governs
the entrainment range of circadian clocks. We apply the theory of coupled
oscillators to analyse how diffusive and mean-field coupling affects the
entrainment range of interacting cells. Mean-field coupling leads to amplitude
expansion of weak oscillators and, as a result, reduces the entrainment range.
We also show that coupling determines the rigidity of the synchronized SCN
network, i.e. the relaxation rates upon perturbation. %(Floquet exponents). Our
simulations and analytical calculations using generic oscillator models help to
elucidate how coupling determines the entrainment of the SCN. Our theoretical
framework helps to interpret experimental data
Early Life Exposure to Air Pollution and Autism Spectrum Disorder: Findings from a Multisite Case-Control Study
BACKGROUND: Epidemiologic studies have reported associations between prenatal and early postnatal air pollution exposure and autism spectrum disorder (ASD); however, findings differ by pollutant and developmental window. OBJECTIVES: We examined associations between early life exposure to particulate matter ≤2.5 µm in diameter (PM2.5) and ozone in association with ASD across multiple US regions. METHODS: Our study participants included 674 children with confirmed ASD and 855 population controls from the Study to Explore Early Development, a multi-site case-control study of children born from 2003 to 2006 in the United States. We used a satellite-based model to assign air pollutant exposure averages during several critical periods of neurodevelopment: 3 months before pregnancy; each trimester of pregnancy; the entire pregnancy; and the first year of life. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs), adjusting for study site, maternal age, maternal education, maternal race/ethnicity, maternal smoking, and month and year of birth. RESULTS: The air pollution-ASD associations appeared to vary by exposure time period. Ozone exposure during the third trimester was associated with ASD, with an OR of 1.2 (95% CI: 1.1, 1.4) per 6.6 ppb increase in ozone. We additionally observed a positive association with PM2.5 exposure during the first year of life (OR = 1.3 [95% CI: 1.0, 1.6] per 1.6 µg/m increase in PM2.5). CONCLUSIONS: Our study corroborates previous findings of a positive association between early life air pollution exposure and ASD, and identifies a potential critical window of exposure during the late prenatal and early postnatal periods
Project of VEPP-2000 electron-positron collider
The status of VEPP-2M collider is presented. Implementation of Round Colliding Beams (RCB) concept in the new collider VEPP-2M is outlined, potential advantages of RCB over the flat colliding beams are discussed. The main desing parameters and features of this VEPP-2000 collider are reported
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