894 research outputs found
Effect of Greenhouse Gases on Thermal Emissivity by Clouds
Greenhouse gases, most importantly water vapor, increase the emissivity and
decrease the albedo of clouds for thermal radiation. The modifications, which
can be of order 10% for optically thick clouds, depend on the attenuation
coefficient of the greenhouse gases, and also on the
attenuation coefficient, , the single-scattering albedo , and the scattering phase function of the
cloud particulates. Cold, high-altitude clouds with low partial pressures of
water vapor have smaller emissivities for thermal radiation and larger albedos
than otherwise identical but warmer low-altitude clouds with higher partial
pressures of water vapor. In -stream scattering theory, these phenomena can
be quantified with the intensity emissivities of the streams
, and with upward or downward flux emissivities,
and . The emissivities are the
ratios of the outgoing thermal intensities or fluxes to those of a reference
black cloud. Emission from optically-thick, isothermal clouds with scattering,
as well as absorption and emission, is limb darkened. Intensity emissivities
for streams that are nearly normal to the cloud surface are
larger than those of more nearly horizontal streams. The limb darkening
increases with increasing values of the single scattering albedo . For fixed values of , the onset of limb darkening with
increasing zenith angle is more abrupt for phase functions with more forward
scattering. Black clouds, which have only absorption and emission but no
scattering, have unit (Lambertian) emissivities, , for all
stream directions
Instantaneous Clear Sky Radiative Forcings of Halogenated Gases
The clear sky instantaneous radiative forcings of the 14 halogenated gases
previously shown to have the largest contribution to global warming, were
found. The calculation used the absorption cross sections for the halogenated
gases which are assumed to be independent of temperature as well as over 1/3
million line strengths for the 5 naturally occurring greenhouse gases: HO,
CO, O, CH and NO, from the Hitran database. The total radiative
forcing of the halogenated gases at their 2020 concentrations is 0.52 (0.67)
W/m at the tropopause (mesopause). Over half of this forcing is due to
CFC11 and CFC12 whose concentrations are declining as a result of the Montreal
Protocol. The rate of total forcing change for all 14 halogenated gases is 1.5
(2.2) mW/m/year at the tropopause (mesopause). The calculations assumed a
constant altitude concentration for all halogenated gases except CFC11, CFC12
and SF. Using the observed altitude dependence for those 3 molecules
reduced their radiative forcings by about 10%. The global warming potential
values were comparable to those given by the Intergovernmental Panel on Climate
Change. The contribution of a gas to global warming was estimated using the
forcing power per molecule defined as the derivative of its radiative forcing
with respect to its column density. For the present atmosphere, the
per-molecule forcing powers of halogenated gases are orders of magnitude larger
than those for the 5 naturally occuring greenhouse gases because the latter
have much higher concentrations and are strongly saturated. But, the rates of
concentration increase of the 5 main greenhouse gases are orders of magnitude
greater than that of any halogenated gas. Assuming the temperature increase
caused by each gas is proportional to its radiative forcing increase, the 14
halogenated gases are responsible for only 2% of the total global warming.Comment: arXiv admin note: text overlap with arXiv:2103.16465,
arXiv:2006.0309
Deuterium Lamb shift via quenching-radiation anisotropy measurements
The Lamb shift of a hydrogenic ion can be deduced from the anisotropy in the angular distribution of the 2s12-1s12 electric field quenching radiation. The accuracy of our previous anisotropy measurement for deuterium is improved to about ± 150 ppm. The derived Lamb shift is (1059.36±0.16) MHz. The sources of error are carefully analyzed and the prospects for further improvements in the accuracy are discussed. © 1978 The American Physical Society
Measurements of an e 1-M 1 interference effect in the electric-field quenching of spin-polarized He+2s12 ions
When a beam of spin-polarized metastable He+2s12 ions is quenched by an electric field E, the emitted radiation intensity contains an asymmetry term which is proportional to kE ×P, where P is the spin-polarization vector and k is the direction of observation. The effect is due to interference between spontaneous magnetic-dipole (M 1) and induced electric-dipole (E 1) decay modes to the ground state. At |E|=38.14 V/cm, the measured asymmetry is (0.3230.085) × 10-3 in agreement with the theoretical value 0.3443 × 10-3. The experiment provides the first measurement of the relativistic M 1 matrix element for the 2s12-1s12 transition in a hydrogenic ion. The paper contains an extensive discussion of all possible asymmetry effects, including higher-order relativistic and electric-field perturbation corrections. © 1982 The American Physical Society
Measurement of the n=2 Lamb shift in He+ by the anisotropy method
A high-precision measurement of the 2s 2S1/22p 2P1/2 Lamb shift in He+ by the quenching-anisotropy method is reported. The theory and experimental method are described in detail. The measured value of 14042.520.16 MHz (11 parts per million) rivals the accuracy of Lamb-shift measurements in hydrogen by microwave resonance. By subtracting the known low-order terms in the Lamb shift, we interpret the results as a measurement of the order (Z)6mc2 and higher contributions to the electron self-energy GSE(Z). The various contributions to the Lamb shift are discussed, and a revised value for GSE(Z) at low Z is extracted from high-Z calculations. The theoretical value for the Lamb shift is 14042.510.2 MHz, in excellent agreement with experiment. The results provide the most sensitive available determination of GSE(Z) for low Z. Measurements and calculations for hydrogen and other members of the isoelectronic sequence are discussed. © 1991 The American Physical Society
Asymmetry measurement of the 2s1/2(hy1s1/2 relativistic magnetic-dipole matrix element in He+
When a beam of spin-polarized He+(2s1/2) ions is quenched by an electric field E, the emitted radiation intensity contains a left-right-asymmetry term proportional to Pk×E, where P is the spin-polarization vector and k is the observation direction. The resulting asymmetry is proportional to the relativistic magnetic dipole matrix element 1s1/2,1/2M1,02s 1/2,1/2. The measured asymmetry (2.935±0.337)×10-4 corresponds to the matrix element 1s1/2,1/2M1,02s 1/2,1/2 =-(0.2725±0.0313)α2eLatin small letter h with stroke/mc, in agreement with the theoretical value -0.2794α2eLatin small letter h with stroke/mc. The measurement provides a direct test of the relativistic corrections to the magnetic dipole transition operator. © 1986 The American Physical Society
He+ 2p state lifetime by a quenching-asymmetry measurement
An interference asymmetry in the angular distribution of the Ly quenching radiation emitted by He+ ions in the metastable 2s1/2 state is measured to high precision to obtain the lifetime of the 2p1/2 state. The derived lifetime of (0.997 170.000 75)×10-10 s is the most accurate available for a fundamental atomic system. A detailed discussion of systematic corrections is included. The result is in good agreement with theory, thereby confirming the theory of radiative transition probabilities to 0.075%, and indicating that differences between theory and experiment for the alkali metals must be due to either inadequate wave functions or experimental errors. © 1992 The American Physical Society
Elephants, trees, grass, grazers : relationships between climate soils, vegetation and large herbivores in a semi-arid savanna ecosysteem (Tsavo, Kenya)
Based on descriptions of the climate, soils, vegetation and large herbivore populations, relationships are described between the different components of the Tsavo ecosystem. It appears that there is a strong correlation between the floristic composition of the vegetation and the physical environment (climate and soil). The structure of the vegetation, however, is related more to the density of elephants and the activity of man (fire). The production of the vegetation is strongly related to rainfall and the percentage cover of the relevant vegetation component. Large herbivore distribution and density is related to human activity and the amount and type of forage that is available in a dry year. Static and dynamic models describing these relationships quantitatively are developed and, on the basis of simulations, some conclusions are drawn on the future management options for the national park and the nearby ranches.</p
Lamb shift in He+: Resolution of a discrepancy between theory and experiment
The Lamb shift in helium ions were investigated to verify or remove a discrepancy between theory and experiment by the anisotropy method. The method was used for the elimination of residual polarization sensitivity of the photon detectors. The results showed that the experimental value of a Lamb shift were in agreement with the theoretical values
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