33,485 research outputs found
On the variational homotopy perturbation method for nonlinear oscillators
In this paper we discuss a recent application of a variational homotopy
perturbation method to rather simple nonlinear oscillators . We show that the
main equations are inconsistent and for that reason the results may be of
scarce utility
Bi-Directional Energy Cascades and the Origin of Kinetic Alfv\'enic and Whistler Turbulence in the Solar Wind
The observed sub-proton scale turbulence spectrum in the solar wind raises
the question of how that turbulence originates. Observations of keV energetic
electrons during solar quite-time suggest them as possible source of free
energy to drive the turbulence. Using particle-in-cell simulations, we explore
how free energy in energetic electrons, released by an electron two-stream
instability drives Weibel-like electromagnetic waves that excite wave-wave
interactions. Consequently, both kinetic Alfv\'enic and whistler waves are
excited that evolve through inverse and forward magnetic energy cascades.Comment: 12 pages, 5 figures, Submitted to Physical Review Letter
Solving the Hamilton-Jacobi equation for gravitationally interacting electromagnetic and scalar fields
The spatial gradient expansion of the generating functional was recently
developed by Parry, Salopek, and Stewart to solve the Hamiltonian constraint in
Einstein-Hamilton-Jacobi theory for gravitationally interacting dust and scalar
fields. This expansion is used here to derive an order-by-order solution of the
Hamiltonian constraint for gravitationally interacting electromagnetic and
scalar fields. A conformal transformation and functional integral are used to
derive the generating functional up to the terms fourth order in spatial
gradients. The perturbations of a flat Friedmann-Robertson-Walker cosmology
with a scalar field, up to second order in spatial gradients, are given. The
application of this formalism is demonstrated in the specific example of an
exponential potential.Comment: 14 pages, uses amsmath,amssymb, referees' suggestions implemented, to
appear in Classical and Quantum Gravit
Densities and abundances of hot cometary ions in the coma of P/Halley
On its flight by P/Halley, the Giotto spacecraft carried a High Energy Range Spectrometer (HERS) for measuring the properties of cometary ions picked up by the solar wind in the nearly collisionless regions of the coma. Preliminary estimates of the ion densities observed by HERS were reevaluated and extended; density profiles along the Giotto trajectory are presented for 13 values of ion mass/charge. Comparison with the physical-chemical model of the interaction of sunlight and the solar wind with the comet by other researchers reveals that, with the exception of protons and H2(+), all ion densities were at least an order of magnitude higher than predicted. The high ion densities cannot be explained on the basis of compression of the plasma, but require additional or stronger ionization mechanisms. Ratios of the densities of different ion species reveal an overabundance of carbonaceous material and an underabundance of H2(+) compared to the predictions of the Schmidt. While the densities of solar wind ions (H(+) and He(++)) changed sharply across a magnetic discontinuity located 1.35(10)(exp 5) km from the comet, this feature, which has been called both the 'cometopause' and the 'magnetic pileup boundary' was barely distinguishable in the density profiles of hot cometary ions. This result is consistent with the interpretation that the magnetic pileup boundary detected by Giotto was caused by a discontinuity in the solar wind and is not an intrinsic feature of the interaction of the solar wind with an active comet
High temperature glass thermal control structure and coating
A high temperature stable and solar radiation stable thermal control coating is described which is useful either as such, applied directly to a member to be protected, or applied as a coating on a re-usable surface insulation (RSI). It has a base coat layer and an overlay glass layer. The base coat layer has a high emittance, and the overlay layer is formed from discrete, but sintered together glass particles to give the overlay layer a high scattering coefficient. The resulting two-layer space and thermal control coating has an absorptivity-to-emissivity ratio of less than or equal to 0.4 at room temperature, with an emittance of 0.8 at 1200 F. It is capable of exposure to either solar radiation or temperatures as high as 2000 F without significant degradation. When used as a coating on a silica substrate to give an RSI structure, the coatings of this invention show significantly less reduction in emittance after long term convective heating and less residual strain than prior art coatings for RSI structures
Triangular buckling patterns of twisted inextensible strips
When twisting a strip of paper or acetate under high longitudinal tension,
one observes, at some critical load, a buckling of the strip into a regular
triangular pattern. Very similar triangular facets have recently been observed
in solutions to a new set of geometrically-exact equations describing the
equilibrium shape of thin inextensible elastic strips. Here we formulate a
modified boundary-value problem for these equations and construct post-buckling
solutions in good agreement with the observed pattern in twisted strips. We
also study the force-extension and moment-twist behaviour of these strips by
varying the mode number n of triangular facets
An effective Hamiltonian for 2D black hole Physics
In another application of the methods of Henneaux, Teitelboim, and Vergara
developed for diffeomorphisms invariant models, the CGHS theory of 2D black
holes is focused in order to obtain the true degrees of freedom, the simplectic
structure and the {\it effective} Hamiltonian that rules the dynamics in
reduced phase-space.Comment: To appear in Europhysics Letter
Elastohydrodynamic study of actin filaments using fluorescence microscopy
We probed the bending of actin subject to external forcing and viscous drag.
Single actin filaments were moved perpendicular to their long axis in an
oscillatory way by means of an optically tweezed latex bead attached to one end
of the filaments. Shapes of these polymers were observed by epifluorescence
microscopy. They were found to be in agreement with predictions of semiflexible
polymer theory and slender-body hydrodynamics. A persistence length of m could be extracted.Comment: RevTex, 4 pages, 5 eps figs, submitted to PR
Covariant quantum measurements may not be optimal
Quantum particles, such as spins, can be used for communicating spatial
directions to observers who share no common coordinate frame. We show that if
the emitter's signals are the orbit of a group, then the optimal detection
method may not be a covariant measurement (contrary to widespread belief). It
may be advantageous for the receiver to use a different group and an indirect
estimation method: first, an ordinary measurement supplies redundant numerical
parameters; the latter are then used for a nonlinear optimal identification of
the signal.Comment: minor corrections, to appear in J. Mod. Opt. (proc. of Gdansk conf.
Observations of solar wind ion charge exchange in the comet Halley coma
Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) observations of solar wind ions show charge exchange effects and solar wind compositional changes in the coma of comet Halley. As the comet was approached, the He(++) to proton density ratio increased until about 1 hour before closest approach after which time it decreased. Abrupt increases in this ratio were also observed in the beginning and near the end of the so-called Mystery Region (8.6 - 5.5(10)(exp 5) km from the comet along the spacecraft trajectory). These abrupt increases in the density ratio were well correlated with enhanced fluxes of keV electrons as measured by the Giotto plasma electron spectrometer. The general increase and then decrease of the He(++) to proton density ratio is quantitatively consistent with a combination of the addition of protons of cometary origin to the plasma and loss of plasma through charge exchange of protons and He(++). In general agreement with the solar wind proton and He(++) observations, solar wind oxygen and carbon ions were observed to charge exchange from higher to lower charge states with decreasing distance to the comet. The more abrupt increases in the He(++) to proton and the He(++) to O(6+) density ratios in the mystery region require a change in the solar wind ion composition in this region while the correlation with energetic electrons indicates processes associated with the comet
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