2,688 research outputs found
On the theory of Gamma Ray Amplification through Stimulated Annihilation Radiation (GRASAR)
The theory of photon emission, absorption, and scattering in a relativistic plasma of positrons, electrons, and photon was studied. Expressions for the emissivities and absorption coefficients of pair annihilation, pair production, and Compton scattering are given and evaluated numerically. The conditions for negative absorption were investigated. In a system of photons and e(+) - e(-) pairs, an emission line at at approximately 0.43 MeV can be produced by grasar action provided that the pair chemical potential exceeds approximately 1 MeV. At a temperature of approximately 10 to the 9th power. This requires a pair density approximately 10 to the 30th power cm to the (-3) power a value much larger than the thermodynamic equilbrium pair density at this temperature. This emission line could account without a gravitational redshift for the observed lines at this energy from gamma ray bursts
Criterion for purely elastic Taylor-Couette instability in the flows of shear-banding fluids
In the past twenty years, shear-banding flows have been probed by various
techniques, such as rheometry, velocimetry and flow birefringence. In micellar
solutions, many of the data collected exhibit unexplained spatio-temporal
fluctuations. Recently, it has been suggested that those fluctuations originate
from a purely elastic instability of the flow. In cylindrical Couette geometry,
the instability is reminiscent of the Taylor-like instability observed in
viscoelastic polymer solutions. In this letter, we describe how the criterion
for purely elastic Taylor-Couette instability should be adapted to
shear-banding flows. We derive three categories of shear-banding flows with
curved streamlines, depending on their stability.Comment: 6 pages, 3 figure
Potential "ways of thinking" about the shear-banding phenomenon
Shear-banding is a curious but ubiquitous phenomenon occurring in soft
matter. The phenomenological similarities between the shear-banding transition
and phase transitions has pushed some researchers to adopt a 'thermodynamical'
approach, in opposition to the more classical 'mechanical' approach to fluid
flows. In this heuristic review, we describe why the apparent dichotomy between
those approaches has slowly faded away over the years. To support our
discussion, we give an overview of different interpretations of a single
equation, the diffusive Johnson-Segalman (dJS) equation, in the context of
shear-banding. We restrict ourselves to dJS, but we show that the equation can
be written in various equivalent forms usually associated with opposite
approaches. We first review briefly the origin of the dJS model and its initial
rheological interpretation in the context of shear-banding. Then we describe
the analogy between dJS and reaction-diffusion equations. In the case of
anisotropic diffusion, we show how the dJS governing equations for steady shear
flow are analogous to the equations of the dynamics of a particle in a quartic
potential. Going beyond the existing literature, we then draw on the Lagrangian
formalism to describe how the boundary conditions can have a key impact on the
banding state. Finally, we reinterpret the dJS equation again and we show that
a rigorous effective free energy can be constructed, in the spirit of early
thermodynamic interpretations or in terms of more recent approaches exploiting
the language of irreversible thermodynamics.Comment: 14 pages, 6 figures, tutorial revie
The Influence of Statistical versus Exemplar Appeals on Indian Adults’ Health Intentions: An Investigation of Direct Effects and Intervening Persuasion Processes
In two separate investigations, we examined the persuasive effectiveness of statistical versus exemplar appeals on Indian adults’ smoking cessation and mammography screening intentions. To more comprehensively address persuasion processes, we explored whether message response and perceived message effectiveness functioned as antecedents to persuasive effects. Results showed that statistical appeals led to higher levels of health intentions than exemplar appeals. In addition, findings from both studies indicated that statistical appeals stimulated more attention and were perceived as more effective than anecdotal accounts. Among male smokers, statistical appeals also generated greater cognitive processing than exemplar appeals. Subsequent mediation analyses revealed that message response and perceived message effectiveness fully carried the influence of appeal format on health intentions. Given these findings, future public health initiatives conducted among similar populations should design messages that include substantive factual information while ensuring that this content is perceived as credible and valuable. [ABSTRACT FROM PUBLISHER
Interannual variability of the air-sea flux of oxygen in the North Atlantic
In studies using timeseries observations of atmospheric O[subscript 2]/N[subscript 2] to infer the fate of fossil fuel CO[subscript 2], it has been assumed that multi-year trends in observed O[subscript 2]/N[subscript 2] are insensitive to interannual variability in air-sea fluxes of oxygen. We begin to address the validity of this assumption by investigating the magnitude and mechanisms of interannual variability in the flux of oxygen across the sea surface using a North Atlantic biogeochemical model. The model, based on the MIT ocean general circulation model, captures the gross patterns and seasonal cycle of nutrients and oxygen within the basin. The air-sea oxygen flux exhibits significant interannual variability in the North Atlantic, with a standard deviation (0.36 mol m[superscript −2] y[superscript −1]) that is a large fraction of the mean (0.85 mol m[superscript −2] y[subscript −1]). This is primarily a consequence of variability in winter convection in the subpolar gyre.Goddard Space Flight Center (Grants NGTS-30189 and NCC5-244
Development of the drag coefficient of a sphere translating through a viscoelastic fluid
Presentation at 15th OpenFOAM Workshop, June 22-25, 2020, Arlington, VA, USA
Climate-Smart Agriculture (CSA) Technologies in Asia CCAFS Workshop Report
A regional workshop on Climate-Smart Agriculture (CSA) technologies was
organized by the International Rice Research Institute (IRRI) in collaboration with the
Consultative Group for International Agricultural Research (CGIAR) research
program for Climate Change, Agriculture, and Food Security (CCAFS), with financial
support from the United Nations Environment Programme (UNEP). Participants from
thirteen (13) Asian countries attended the workshop, which consisted of two days of
presentations on technical, organizational, and financial aspects of CSA technologies
in Metro Manila, Philippines and a field visit to the IRRI campus in Los Baños,
Philippines to view the development and testing of CSA technologies
Thermal Comptonization in Mildly Relativistic Pair Plasmas
We use a Monte Carlo simulation to calculate the spectra of mildly
relativistic thermal plasmas in pair balance. We use the exact integral
expression for the electron-positron thermal annihilation spectrum, and provide
accurate expressions for the Gaunt factors of electron-ion, electron-electron,
and electron-positron thermal bremsstrahlung in the transrelativistic
temperature regime. The particles are assumed to be uniformly distributed
throughout a sphere, and the pair opacity is self-consistently calculated from
the energy and angular distribution of scattered photons. The resultant photon
spectra are compared with the nonrelativistic diffusion treatment of Sunyaev
and Titarchuk, the bridging formulas of Zdziarski, and the relativistic
corrections proposed by Titarchuk. We calculate allowed pair-balanced states of
thermal plasmas with no pair escape which include bremsstrahlung and internal
soft photons. The results are presented in the spectral index/compactness
plane, and can be directly compared with observations of spectra from AGNs and
Galactic black hole candidates. By comparing with X-ray spectral indices of
Seyfert AGNs and compactnesses inferred from X-ray variability data, we find
that the allowed solutions for pair equilibrium plasma imply that the
temperatures of Seyfert galaxies are keV. This prediction can be
tested with more sensitive gamma-ray observations of Seyfert galaxies. We find
that if the X-ray variability time scale gives an accurate measure of the
compactness, pair-dominated solutions are inconsistent with the data.Comment: 32 pages with 9 figures, compressed and uuencoded postscrip
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