2,986 research outputs found
Comment on ``Validity of certain soft-photon amplitudes''
The criteria suggested by Welsh and Fearing (nucl-th/9606040) to judge the
validity of certain soft-photon amplitudes are examined. We comment on aspects
of their analysis which lead to incorrect conclusions about published
amplitudes and point out important criteria which were omitted from their
analysis.Comment: 6 pages plus 1 postscript figure, Revte
Remote sensing of tropical tropopause layer radiation balance using A-train measurements
Determining the level of zero net radiative heating (LZH) is critical to understanding parcel trajectory in the Tropical Tropopause Layer (TTL) and associated stratospheric hydration processes. Previous studies of the TTL radiative balance have focused on using radiosonde data, but remote sensing measurements from polar-orbiting satellites may provide the relevant horizontal and vertical information for assessing TTL solar heating and infrared cooling rates, especially across the Pacific Ocean. CloudSat provides a considerable amount of vertical information about the distribution of cloud properties relevant to heating rate analysis. The ability of CloudSat measurements and ancillary information to constrain LZH is explored. We employ formal error propagation analysis for derived heating rate uncertainty given the CloudSat cloud property retrieval algorithms. Estimation of the LZH to within approximately 0.5 to 1 km is achievable with CloudSat, but it has a low-altitude bias because the radar is unable to detect thin cirrus. This can be remedied with the proper utilization of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar backscatter information. By utilizing an orbital simulation with the GISS data set, we explore the representativeness of non-cross-track scanning active sounders in terms of describing the LZH distribution. In order to supplement CloudSat, we explore the ability of Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Scanning Radiometer-EOS (AMSR-E) to constrain LZH and find that these passive sounders are useful where the cloud top height does not exceed 7 km. The spatiotemporal distributions of LZH derived from CloudSat and CALIPSO measurements are presented which suggest that thin cirrus have a limited effect on LZH mean values but affect LZH variability
Modes of Growth in Dynamic Systems
Regardless of a system's complexity or scale, its growth can be considered to
be a spontaneous thermodynamic response to a local convergence of down-gradient
material flows. Here it is shown how growth can be constrained to a few
distinct modes that depend on the availability of material and energetic
resources. These modes include a law of diminishing returns, logistic behavior
and, if resources are expanding very rapidly, super-exponential growth. For a
case where a system has a resolved sink as well as a source, growth and decay
can be characterized in terms of a slightly modified form of the predator-prey
equations commonly employed in ecology, where the perturbation formulation of
these equations is equivalent to a damped simple harmonic oscillator. Thus, the
framework presented here suggests a common theoretical under-pinning for
emergent behaviors in the physical and life sciences. Specific examples are
described for phenomena as seemingly dissimilar as the development of rain and
the evolution of fish stocks.Comment: 16 pages, 6 figures, including appendi
Perturbation of magnetostatic modes observed by ferromagnetic resonance force microscopy
Magnetostatic modes of yttrium iron garnet (YIG) films are investigated by ferromagnetic resonance force microscopy. A thin-film "probe" magnet at the tip of a compliant cantilever introduces a local inhomogeneity in the internal field of the YIG sample. This influences the shape of the sample's magnetostatic modes, thereby measurably perturbing the strength of the force coupled to the cantilever. We present a theoretical model that explains these observations; it shows that the tip-induced variation of the internal field creates either a local "potential barrier" or "potential well" for the magnetostatic waves. The data and model together indicate that local magnetic imaging of ferromagnets is possible, even in the presence of long-range spin coupling, through the introduction of localized magnetostatic modes predicted to arise from sufficiently strong tip fields
Anatomy of the Soft-Photon Approximation in Hadron-Hadron Bremsstrahlung
A modified Low procedure for constructing soft-photon amplitudes has been
used to derive two general soft-photon amplitudes, a two-s-two-t special
amplitude and a two-u-two-t special amplitude
, where s, t and u are the Mandelstam variables.
depends only on the elastic T-matrix evaluated at four sets
of (s,t) fixed by the requirement that the amplitude be free of derivatives
(T/s and /or T/). Likewise
depends only on the elastic T-matrix evaluated at four sets
of (u,t). In deriving these amplitudes, we impose the condition that
and reduce to and
, respectively, their tree level approximations. The
amplitude represents photon emission from a sum of
one-particle t-channel exchange diagrams and one-particle s-channel exchange
diagrams, while the amplitude represents photon
emission from a sum of one-particle t-channel exchange diagrams and
one-particle u-channel exchange diagrams. The precise expressions for
and are determined by using the
radiation decomposition identities of Brodsky and Brown. We point out that it
is theoretically impossible to describe all bremsstrahlung processes by using
only a single class of soft-photon amplitudes. At least two different classes
are required: the amplitudes which depend on s and t or the amplitudes which
depend on u and t. When resonance effects are important, the amplitude
, not , should be used. For processes with
strong u-channel exchange effects, the amplitude should be
the first choice.Comment: 49 pages report # LA-UR-92-270
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