155 research outputs found
Collision and Diffusion in Microwave Breakdown of Nitrogen Gas in and around Microgaps
The microwave induced breakdown of N2 gas in microgaps was modeled using the
collision frequency between electrons and neutral molecules and the effective
electric field concept. Low pressure breakdown at the threshold electric field
occurs outside the gap, but at high pressures it is found to occur inside the
microgap with a large threshold breakdown electric field corresponding to a
very large electron oscillation amplitude. Three distinct pressure regimes are
apparent in the microgap breakdown: a low pressure multipactor branch, a
mid-pressure Paschen branch, both of which occur in the space outside the
microgap, and a high pressure diffusion-drift branch, which occurs inside the
microgap. The Paschen and diffusion-drift branches are divided by a sharp
transition and each separately fits the collision frequency model. There is
evidence that considerable electron loss to the microgap faces accompanies the
diffusion-drift branch in microgaps.Comment: 4 figure
Annual, seasonal, and diel surface energy partitioning in the semiarid Sand Hills of Nebraska, USA
Study Region: The Nebraska Sand Hills consisting of four major land cover types: (1) lakes and wetlands (ā¼5% for both), (2) subirrigated meadows (ā¼10%), (3) dry valleys (ā¼20%), and (4) upland dunes (ā¼65%).
Study Focus: Examination of surface energy and water balances on multiple temporal scales with primary focus on latent heat flux (Ī»E), and evapotranspiration (ET), to gain a better understanding of the annual, seasonal, and diel properties of surface energy partitioning among different Sand Hills ecosystems to improve regional water resource management. New Hydrological Insights for the Region: Based on surface energy and water balance measurements using Bowen ratio energy balance systems at three locations during 2004, we find a strong spatial gradient between sites in latent (Ī»E) and sensible (H) heat flux due to differences in topography, soils, and plant community composition on all timescales. Seasonally, all land covers show the greatest Ī»E in summer. Our results show that subirrigated meadows, dry valleys, and upland dunes allocate roughly 81%, 50%, and 41% of available energy to Ī»E, respectively, during the growing season. The subirrigated meadow was the only cover type where cumulative annual ET surpassed cumulative annual precipitation (i.e. net loss of water to the atmosphere). Therefore, the dry valleys and upland dunes are where net groundwater recharge to the High Plains Aquifer is occurring
Rapid and highly variable warming of lake surface waters around the globe
In this first worldwide synthesis of in situ and satelliteāderived lake data, we find that lake summer surface water temperatures rose rapidly (global meanā=ā0.34Ā°C decadeā1) between 1985 and 2009. Our analyses show that surface water warming rates are dependent on combinations of climate and local characteristics, rather than just lake location, leading to the counterintuitive result that regional consistency in lake warming is the exception, rather than the rule. The most rapidly warming lakes are widely geographically distributed, and their warming is associated with interactions among different climatic factorsāfrom seasonally iceācovered lakes in areas where temperature and solar radiation are increasing while cloud cover is diminishing (0.72Ā°C decadeā1) to iceāfree lakes experiencing increases in air temperature and solar radiation (0.53Ā°C decadeā1). The pervasive and rapid warming observed here signals the urgent need to incorporate climate impacts into vulnerability assessments and adaptation efforts for lakes
The relationship between parenting, family interaction and childhood dental caries: A case-control study
a b s t r a c t The aim of this case-control study was to explore the relationship between parenting practices, parent echild interaction and childhood dental caries, using a sample of 5e8-year old children from the Netherlands. Cases were defined as children with four or more decayed, missing or filled teeth and controls were caries free. Cases (n Ā¼ 28) and controls (n Ā¼ 26) were recruited from a referral centre for paediatric dental care and a general dental practice, respectively. Parenting practices and parentechild interactions of the child's primary caregiver were observed using Structured Interaction Tasks and subsequently rated on seven dimensions: positive involvement, encouragement, problem-solving, discipline, monitoring, coercion and interpersonal atmosphere. All Structured Interaction Tasks were videotaped, and coded by trained and calibrated observers blind to the dental condition. Differences in parenting dimensions between cases and controls were analysed using multivariate analysis of variance, independent samples T-tests, c 2 -tests and multiple logistic regression analyses. Controls had significantly higher scores on the dimensions positive involvement, encouragement, problem-solving and interpersonal atmosphere, compared to cases. Parents of controls were also less likely to show coercive behaviours. These associations remained statistically significant after adjustment for the mother's education level, tooth brushing frequency and the frequency of consuming sugary foods and drinks, except for coercion. There was no significant difference in discipline between cases and controls. In conclusion, this case-control study found a significant relationship between parenting practices, parent echild interaction quality and childhood dental caries. Our findings suggest that parenting practices may be an important factor to consider in caries preventive programs
Modeling lakes and reservoirs in the climate system
Modeling studies examining the effect of lakes on regional and global climate, as well as studies on the influence of climate variability and change on aquatic ecosystems, are surveyed. Fully coupled atmosphereāland surfaceālake climate models that could be used for both of these types of study simultaneously do not presently exist, though there are many applications that would benefit from such models. It is argued here that current understanding of physical and biogeochemical processes in freshwater systems is sufficient to begin to construct such models, and a path forward is proposed. The largest impediment to fully representing lakes in the climate system lies in the handling of lakes that are too small to be explicitly resolved by the climate model, and that make up the majority of the lake-covered area at the resolutions currently used by global and regional climate models. Ongoing development within the hydrological sciences community and continual improvements in model resolution should help ameliorate this issue
Lake-size dependency of wind shear and convection as controls on gas exchange
High-frequency physical observations from 40 temperate lakes were used to examine the relative contributions of wind shear (u*) and convection (w*) to turbulence in the surface mixed layer. Seasonal patterns of u* and w* were dissimilar; u* was often highest in the spring, while w * increased throughout the summer to a maximum in early fall. Convection was a larger mixed-layer turbulence source than wind shear (u */w*-1 for lakes* and w* differ in temporal pattern and magnitude across lakes, both convection and wind shear should be considered in future formulations of lake-air gas exchange, especially for small lakes. Ā© 2012 by the American Geophysical Union.Jordan S. Read, David P. Hamilton, Ankur R. Desai, Kevin C. Rose, Sally MacIntyre, John D. Lenters, Robyn L. Smyth, Paul C. Hanson, Jonathan J. Cole, Peter A. Staehr, James A. Rusak, Donald C. Pierson, Justin D. Brookes, Alo Laas, and Chin H. W
Unveiling Soft Gamma-Ray Repeaters with INTEGRAL
Thanks to INTEGRAL's long exposures of the Galactic Plane, the two brightest
Soft Gamma-Ray Repeaters, SGR 1806-20 and SGR 1900+14, have been monitored and
studied in detail for the first time at hard-X/soft gamma rays.
This has produced a wealth of new scientific results, which we will review
here. Since SGR 1806-20 was particularly active during the last two years, more
than 300 short bursts have been observed with INTEGRAL. and their
characteristics have been studied with unprecedented sensitivity in the 15-200
keV range. A hardness-intensity anticorrelation within the bursts has been
discovered and the overall Number-Intensity distribution of the bursts has been
determined. In addition, a particularly active state, during which ~100 bursts
were emitted in ~10 minutes, has been observed on October 5 2004, indicating
that the source activity was rapidly increasing. This eventually led to the
Giant Flare of December 27th 2004, for which a possible soft gamma-ray (>80
keV) early afterglow has been detected.
The deep observations allowed us to discover the persistent emission in hard
X-rays (20-150 keV) from 1806-20 and 1900+14, the latter being in a quiescent
state, and to directly compare the spectral characteristics of all Magnetars
(two SGRs and three Anomalous X-ray Pulsars) detected with INTEGRAL.Comment: 8 pages, 7 figures, Presented at the conference "Isolated Neutron
Stars: from the Surface to the Interior", London, UK, 24-28 April 200
An exceptionally bright flare from SGR1806-20 and the origins of short-duration gamma-ray bursts
Soft-gamma-ray repeaters (SGRs) are galactic X-ray stars that emit numerous
short-duration (about 0.1 s) bursts of hard X-rays during sporadic active
periods. They are thought to be magnetars: strongly magnetized neutron stars
with emissions powered by the dissipation of magnetic energy. Here we report
the detection of a long (380 s) giant flare from SGR 1806-20, which was much
more luminous than any previous transient event observed in our Galaxy. (In the
first 0.2 s, the flare released as much energy as the Sun radiates in a quarter
of a million years.) Its power can be explained by a catastrophic instability
involving global crust failure and magnetic reconnection on a magnetar, with
possible large-scale untwisting of magnetic field lines outside the star. From
a great distance this event would appear to be a short-duration, hard-spectrum
cosmic gamma-ray burst. At least a significant fraction of the mysterious
short-duration gamma-ray bursts therefore may come from extragalactic
magnetars.Comment: 21 pages, 5 figures. Published in Natur
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Evaluating and improving modeled turbulent heat fluxes across the North American Great Lakes
Turbulent fluxes of latent and sensible heat are important physical processes
that influence the energy and water budgets of the North American Great
Lakes. These fluxes can be measured inĀ situ using eddy covariance techniques
and are regularly included as a component of lakeāatmosphere models. To help
ensure accurate projections of lake temperature, circulation, and regional
meteorology, we validated the output of five algorithms used in three popular
models to calculate surface heat fluxes: the Finite Volume Community Ocean
Model (FVCOM, with three different options for heat flux algorithm), the
Weather Research and ForecastingĀ (WRF) model, and the Large Lake
Thermodynamic Model. These models are used in research and operational
environments and concentrate on different aspects of the Great Lakes'
physical system. We isolated only the code for the heat flux algorithms from
each model and drove them using meteorological data from four over-lake
stations within the Great Lakes Evaporation Network (GLEN), where eddy
covariance measurements were also made, enabling co-located comparison. All
algorithms reasonably reproduced the seasonal cycle of the turbulent heat
fluxes, but all of the algorithms except for the Coupled OceanāAtmosphere
Response ExperimentĀ (COARE) algorithm showed notable overestimation of the
fluxes in fall and winter. Overall, COARE had the best agreement with eddy
covariance measurements. The four algorithms other than COARE were altered by
updating the parameterization of roughness length scales for air temperature
and humidity to match those used in COARE, yielding improved agreement
between modeled and observed sensible and latent heat fluxes.</p
A global database of lake surface temperatures collected by in situ and satellite methods from 1985-2009
Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985-2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.Sapna Sharma ā¦ Justin Brookes ā¦ Anna Rigosi et. al
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