1,244 research outputs found
Band-edge-induced Bragg diffraction in two-dimensional photonic crystals
Two-dimensional photonic crystals composed of two orthogonal volume diffraction gratings have been photogenerated in photopolymers. When the read beam is set at the Bragg angle, the diffraction efficiency of the transmission grating is strongly enhanced at the band edge of the reflection grating recorded in the material. Such a device provides Bragg operation and enhancement of the diffraction efficiency of the thin diffraction grating together with good wavelength selectivity. Such advantages could be interesting for optical
signal processing
Prospects for Neutron Star Parameter Estimation using Gravitational Waves from f-modes Associated with Magnetar Flares
Magnetar vibrational modes are theorized to be associated with energetic
X-ray flares. Regular searches for gravitational waves from these modes have
been performed by Advanced LIGO and Advanced Virgo, with no detections so far.
Presently, search results are given in limits on the root-sum-square of the
integrated gravitational-wave strain. However, the increased sensitivity of
current detectors and the promise of future detectors invite the consideration
of more astrophysically motivated methods. We present a framework for
augmenting gravitational wave searches to measure or place direct limits on
magnetar astrophysical properties in various search scenarios using a set of
phenomenological and analytic models.Comment: 9 pages, 5 figure
Multivariate Regression Analysis of Gravitational Waves from Rotating Core Collapse
We present a new multivariate regression model for analysis and parameter
estimation of gravitational waves observed from well but not perfectly modeled
sources such as core-collapse supernovae. Our approach is based on a principal
component decomposition of simulated waveform catalogs. Instead of
reconstructing waveforms by direct linear combination of physically meaningless
principal components, we solve via least squares for the relationship that
encodes the connection between chosen physical parameters and the principal
component basis. Although our approach is linear, the waveforms' parameter
dependence may be non-linear. For the case of gravitational waves from rotating
core collapse, we show, using statistical hypothesis testing, that our method
is capable of identifying the most important physical parameters that govern
waveform morphology in the presence of simulated detector noise. We also
demonstrate our method's ability to predict waveforms from a principal
component basis given a set of physical progenitor parameters
A gross and microscopic study of the bone marrow of clinicallly accessible bones of the immature, the mature, and the aged cat
Call number: LD2668 .T4 1961 F7
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The Influence of Southern Ocean Shortwave Cloud Feedbacks on the Coupled Climate System
The Southern Ocean, a unique region where clouds, ocean dynamics and sea ice interact to influence climate, has historically been poorly modeled and observed. Here, we improve a global climate model and use newly-available surface-independent cloud observations to establish how Southern Ocean cloud feedbacks impact global climate change. We modify the Community Earth System Model (CESM) by increasing supercooled cloud liquid to better match observations over the Southern Ocean. In the modified model, two extratropical cloud feedbacks cause equilibrium climate sensitivity (ECS, the equilibrium warming in response to doubled CO2) to increase from 4.1 K in the control to 5.6 K. First, reduced conversion of cloud ice to liquid at high southern latitudes decreases the magnitude of a negative cloud phase feedback. Second, warming is amplified in the mid-latitudes by a larger positive shortwave cloud cover feedback. Despite the 1.5 K ECS increase, transient 21st century warming hardly increases in the modified model over the control because ocean heat uptake moves heat input by extratropical cloud feedbacks to depth. Persistent extratropical ocean heat uptake implies that extratropical cloud biases may not be as important to 21st century warming as biases in other regions. Next, we determine how interactions with sea ice impact Southern Ocean cloud feedbacks. We use surface-independent cloud observations to diagnose how present-day sea ice–cloud interactions during spring and summer impact top-of-atmosphere albedo. Observed low cloud cover and opacity are larger over open water compared to over sea ice. The cloud opacity increase is due to an ice-toward-liquid cloud phase shift with no change in air-sea coupling. Even with the cloud response, top-of-atmosphere albedo decreases as sea ice retreats. In CESM, the cloud and albedo responses to sea ice variability are of the same sign but larger in magnitude than the observed responses. The modeled cloud opacity increase is linked to strengthened air-sea coupling rather than a cloud phase shift. Strengthened air-sea coupling with decreasing sea ice could impact model-predicted cloud feedbacks in a way inconsistent with observations. Our results highlight how Southern Ocean shortwave cloud feedbacks influence climate change in the coupled climate system.</p
Exploring a search for long-duration transient gravitational waves associated with magnetar bursts
Soft gamma repeaters and anomalous X-ray pulsars are thought to be magnetars,
neutron stars with strong magnetic fields of order --. These objects emit intermittent bursts
of hard X-rays and soft gamma rays. Quasiperiodic oscillations in the X-ray
tails of giant flares imply the existence of neutron star oscillation modes
which could emit gravitational waves powered by the magnetar's magnetic energy
reservoir. We describe a method to search for transient gravitational-wave
signals associated with magnetar bursts with durations of 10s to 1000s of
seconds. The sensitivity of this method is estimated by adding simulated
waveforms to data from the sixth science run of Laser Interferometer
Gravitational-wave Observatory (LIGO). We find a search sensitivity in terms of
the root sum square strain amplitude of for a half sine-Gaussian waveform with a central
frequency and a characteristic time . This corresponds to a gravitational wave energy of
, the same order of
magnitude as the 2004 giant flare which had an estimated electromagnetic energy
of , where is the distance to SGR 1806-20. We
present an extrapolation of these results to Advanced LIGO, estimating a
sensitivity to a gravitational wave energy of for a magnetar at a distance of .
These results suggest this search method can probe significantly below the
energy budgets for magnetar burst emission mechanisms such as crust cracking
and hydrodynamic deformation
Problems in Interpreting Unusually Large Burrows
Although marine burrows of unusually large dimensions have long been known in certain areas, they are probably much more widespread in the rock record than is generally recognized. Such burrows constitute a heterogeneous group, having little in common other than exceptional size. Yet their size alone unites them in difficulty of interpretation: e.g., densely spaced-dwelling burrows of combined dwelling-escape burrows as much as 12 cm in diameter and 5 m long; vertical dwelling burrows only 0.5 cm in diameter but up to 9 m long; possible escape structures as much as 24 cm in diameter and 3 m long, subsequently penetrated in some cases by secondary burrow-like structures.
Numerous special problems are encountered in the study and interpretation of burrows of these extreme dimensions: (1) field exposure and accessibility, so that the full extent, or a large part, of the structures can be studied; (2) preservation of the burrows in continuity, not merely in places where they pass through certain beds or within concretion horizons; (3) the fossilization barrier ; our knowledge of comparable modern structures of similar dimensions or of the animals responsible for them is negligible; and (4) the possibility that certain of these unusual structures were formed by physical rather than organic processes; again, our criteria for comparisons are limited.
The examples selected by us—from the Permian of Montana, Idaho, and Wyoming, the Cretaceous and Paleocene of northwestern Europe, and the Pleistocene of North Carolina—are intended primarily (1) to call additional attention to such intriguing structures, and (2) to illustrate some of the problems involved in interpreting their origin and function. Hopefully, future work will solve many of these problems
Optical Testing of Retroreflectors for Cryogenic Applications
A laser tracker (LT) is an important coordinate metrology tool that uses laser interferometry to determine precise distances to objects, points, or surfaces defined by an optical reference, such as a retroreflector. A retroreflector is a precision optic consisting of three orthogonal faces that returns an incident laser beam nearly exactly parallel to the incident beam. Commercial retroreflectors are designed for operation at room temperature and are specified by the divergence, or beam deviation, of the returning laser beam, usually a few arcseconds or less. When a retroreflector goes to extreme cold (.35 K), however, it could be anticipated that the precision alignment between the three faces and the surface figure of each face would be compromised, resulting in wavefront errors and beam divergence, degrading the accuracy of the LT position determination. Controlled tests must be done beforehand to determine survivability and these LT coordinate errors. Since conventional interferometer systems and laser trackers do not operate in vacuum or at cold temperatures, measurements must be done through a vacuum window, and care must be taken to ensure window-induced errors are negligible, or can be subtracted out. Retroreflector holders must be carefully designed to minimize thermally induced stresses. Changes in the path length and refractive index of the retroreflector have to be considered. Cryogenic vacuum testing was done on commercial solid glass retroreflectors for use on cryogenic metrology tasks. The capabilities to measure wavefront errors, measure beam deviations, and acquire laser tracker coordinate data were demonstrated. Measurable but relatively small increases in beam deviation were shown, and further tests are planned to make an accurate determination of coordinate errors
Landscape-level controls on dissolved carbon flux from diverse catchments of the circumboreal
Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 26 (2012): GB0E02, doi:10.1029/2012GB004299.While much of the dissolved organic carbon (DOC) within rivers is destined for mineralization to CO2, a substantial fraction of riverine bicarbonate (HCO3−) flux represents a CO2 sink, as a result of weathering processes that sequester CO2 as HCO3−. We explored landscape-level controls on DOC and HCO3− flux in subcatchments of the boreal, with a specific focus on the effect of permafrost on riverine dissolved C flux. To do this, we undertook a multivariate analysis that partitioned the variance attributable to known, key regulators of dissolved C flux (runoff, lithology, and vegetation) prior to examining the effect of permafrost, using riverine biogeochemistry data from a suite of subcatchments drawn from the Mackenzie, Yukon, East, and West Siberian regions of the circumboreal. Across the diverse catchments that we study, controls on HCO3− flux were near-universal: runoff and an increased carbonate rock contribution to weathering (assessed as riverwater Ca:Na) increased HCO3− yields, while increasing permafrost extent was associated with decreases in HCO3−. In contrast, permafrost had contrasting and region-specific effects on DOC yield, even after the variation caused by other key drivers of its flux had been accounted for. We used ionic ratios and SO4 yields to calculate the potential range of CO2 sequestered via weathering across these boreal subcatchments, and show that decreasing permafrost extent is associated with increases in weathering-mediated CO2 fixation across broad spatial scales, an effect that could counterbalance some of the organic C mineralization that is predicted with declining permafrost.Funding for this work was provided through
NSF-OPP-0229302 and NSF-OPP-0732985. Additional support to S.E.T.
was provided by an NSERC Postdoctoral Fellowship.2013-02-2
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