417 research outputs found
Measurements of wind-wave growth and swell decay during the joint North Sea wave project (JONSWAP).
Wavo spectra were measured along a profile extending 160 km into the North Sea westward from Sylt for a period of ten weeks in 1969. Currents, tides, air-sea temperature differences and turbulence in the atmospheric boundary layer were also measured. the goal of the experiment (described in Part 1) was to determine the structure of the source function governing the energy balance of the wave spectrum, with particular emphasis on wave growth under stationary offshore wind conditions (Part 2) and the attention of swell in water of finito depth (Part 3). The source functions of wave spectra generated by offshore winds exhibit a characteristic plus-minus signature associated with the shift of the sharp spectral peak towards lower frequencies. The two-lobed distribution of the source function can be explained quantitively by the nonlinear transfer due to resonant wave-wave interactions (second order Bragg scattering). The evolution of a pronounced peak and its shift towards lower frequencies can also be understood as a self-stabilizing feature of this process. The decay rates determined for incoming swell varied considerably, but energy attenuation factors of two along the length of the profile were typical. This is in order of magnitude agreement with expected damping rates due to bottom friction. However, the strong tidal modulation predicted by theory for the case of a quadratic bottom friction law was not observed. Adverse winds did not affect the decay rate. Computations also rule out wave-wave interactions or dissipation due to turbulence outside the bottom boundary layer as effective mechanisms of swell attenuation. We conclude that either the generally accepted friction law needs to be significantly modified or that some other mechanism, such as scattering by bottom irregularities, is the cause of the attenuation. The dispersion characteristics of thw swells indicated rather nearby origins, for which the classical DELTA-event model was generally inapplicable. A strong Doppler modulation by tidal currents was also observed. (A
Dynamics of lattice pinned charge stripes
We study the transversal dynamics of a charged stripe (quantum string) and
show that zero temperature quantum fluctuations are able to depin it from the
lattice. If the hopping amplitude t is much smaller than the string tension J,
the string is pinned by the underlying lattice. At t>>J, the string is depinned
and allowed to move freely, if we neglect the effect of impurities. By mapping
the system onto a 1D array of Josephson junctions, we show that the quantum
depinning occurs at t/J = 2 / pi^2. Besides, we exploit the relation of the
stripe Hamiltonian to the sine-Gordon theory and calculate the infrared
excitation spectrum of the quantum string for arbitrary t/J values.Comment: 4 pages, 2 figure
Energy spectra of the ocean's internal wave field: theory and observations
The high-frequency limit of the Garrett and Munk spectrum of internal waves
in the ocean and the observed deviations from it are shown to form a pattern
consistent with the predictions of wave turbulence theory. In particular, the
high frequency limit of the Garrett and Munk spectrum constitutes an {\it
exact} steady state solution of the corresponding kinetic equation.Comment: 4 pages, one color figur
Linking surface morphology, composition, and activity on the nucleus of 67P/Churyumov-Gerasimenko
Beyond the Fokker-Planck equation: Pathwise control of noisy bistable systems
We introduce a new method, allowing to describe slowly time-dependent
Langevin equations through the behaviour of individual paths. This approach
yields considerably more information than the computation of the probability
density. The main idea is to show that for sufficiently small noise intensity
and slow time dependence, the vast majority of paths remain in small space-time
sets, typically in the neighbourhood of potential wells. The size of these sets
often has a power-law dependence on the small parameters, with universal
exponents. The overall probability of exceptional paths is exponentially small,
with an exponent also showing power-law behaviour. The results cover time spans
up to the maximal Kramers time of the system. We apply our method to three
phenomena characteristic for bistable systems: stochastic resonance, dynamical
hysteresis and bifurcation delay, where it yields precise bounds on transition
probabilities, and the distribution of hysteresis areas and first-exit times.
We also discuss the effect of coloured noise.Comment: 37 pages, 11 figure
Global climate models violate scaling of the observed atmospheric variability
We test the scaling performance of seven leading global climate models by
using detrended fluctuation analysis. We analyse temperature records of six
representative sites around the globe simulated by the models, for two
different scenarios: (i) with greenhouse gas forcing only and (ii) with
greenhouse gas plus aerosol forcing. We find that the simulated records for
both scenarios fail to reproduce the universal scaling behavior of the observed
records, and display wide performance differences. The deviations from the
scaling behavior are more pronounced in the first scenario, where also the
trends are clearly overestimated.Comment: Accepted for publishing in Physical Review Letter
Surface evolution of the Anhur region on comet 67P/Churyumov-Gerasimenko from high-resolution OSIRIS images
Context. The southern hemisphere of comet 67P/Churyumov-Gerasimenko (67P) became observable by the Rosetta mission in March 2015, a fe months before cometary southern vernal equinox. The Anhur region in th southern part of the comet's larger lobe was found to be highly eroded resolution images of the Anhur region pre- and post-perihelion acquire by the OSIRIS imaging system on board the Rosetta mission. The Narro Angle Camera is particularly useful for studying the evolution in Anhu in terms of morphological changes and color variations. Methods Radiance factor images processed by the OSIRIS pipeline wer coregistered, reprojected onto the 3D shape model of the comet, an corrected for the illumination conditions. Results: We find number of morphological changes in the Anhur region that are related t formation of new scarps; removal of dust coatings; localized resurfacin in some areas, including boulders displacements; and vanishin structures, which implies localized mass loss that we estimate to b higher than 50 million kg. The strongest changes took place in an nearby the Anhur canyon-like structure, where significant dust cover wa removed, an entire structure vanished, and many boulders wer rearranged. All such changes are potentially associated with one of th most intense outbursts registered by Rosetta during its observations which occurred one day before perihelion passage. Moreover, in the nich at the foot of a new observed scarp, we also see evidence of water ic exposure that persisted for at least six months. The abundance of wate ice, evaluated from a linear mixing model, is relatively high (>20%) Our results confirm that the Anhur region is volatile-rich and probabl is the area on 67P with the most pristine exposures near perihelion. Th movies associated to Figs. 2, 7, 8, and 10 are available at http://https://www. anda.or
Dynamics of Stripes in Doped Antiferromagnets
We study the dynamics of the striped phase, which has previously been
suggested to be the ground state of a doped antiferromagnet. Starting from the
t-J model, we derive the classical equation governing the motion of the charged
wall by using a ficticious spin model as an intermediate step. A wave-like
equation of motion is obtained and the wall elasticity and mass density
constants are derived in terms of the t and J parameters. The wall is then
regarded as an elastic string which will be trapped by the pinning potential
produced by randomly distributed impurities. We evaluate the pinning potential
and estimate the threshold electric field which has to be applied to the system
in order to release the walls. Besides, the dynamics of the stripe in the
presence of a bias field below the threshold is considered and the high- and
low-temperature relaxation rates are derived.Comment: 22 pages, RevTeX, 3 PS-figure
Magnetic neutron scattering in hole doped cuprate superconductors
A review is presented of the static and dynamic magnetic properties of
hole-doped cuprate superconductors measured with neutron scattering. A wide
variety of experiments are described with emphasis on the monolayer
La_{2-x}(Sr,Ba)_{x}CuO_{4} and bilayer YBa_{2}Cu_{3}O_{6+x} cuprates. At zero
hole doping, both classes of materials are antiferromagnetic insulators with
large superexchange constants of J > 100 meV. For increasing hole doping, the
cuprates become superconducting at a critical hole concentration of
x_{c}=0.055. The development of new instrumentation at neutron beam sources
coupled with the improvement in materials has lead to a better understanding of
these materials and the underlying spin dynamics over a broad range of hole
dopings. We will describe how the spin dispersion changes across the insulating
to superconducting boundary as well as the static magnetic properties which are
directly coupled with the superconductivity. Experiments directly probing the
competing magnetic and superconducting order parameters involving magnetic
fields, impurity doping, and structural order will be examined. Correlations
between superconductivity and magnetism will also be discussed.Comment: 14 pages, 18 figures. To be published in Journal of the Physical
Society of Japa
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