72,747 research outputs found

### Nonlinear magnetoacoustic waves in a cold plasma

The equations describing planar magnetoacoustic waves of permanent form in a cold plasma are rewritten so as to highlight the presence of a naturally small parameter equal to the ratio of the electron and ion masses. If the magnetic field is not nearly perpendicular to the direction of wave propagation, this allows us to use a multiple-scale expansion to demonstrate the existence and nature of nonlinear wave solutions. Such solutions are found to have a rapid oscillation of constant amplitude superimposed on the underlying large-scale variation. The approximate equations for the large-scale variation are obtained by making an adiabatic approximation and in one limit, new explicit solitary pulse solutions are found. In the case of a perpendicular magnetic field, conditions for the existence of solitary pulses are derived. Our results are consistent with earlier studies which were restricted to waves having a velocity close to that of long-wavelength linear magnetoacoustic waves

### A solitary-wave solution to a perturbed KdV equation

We derive the approximate form and speed of a solitary-wave solution to a perturbed KdV equation. Using a conventional perturbation expansion, one can derive a first-order correction to the solitary-wave speed, but at the next order, algebraically secular terms appear, which produce divergences that render the solution unphysical. These terms must be treated by a regrouping procedure developed by us previously. In this way, higher-order corrections to the speed are obtained, along with a form of solution that is bounded in space. For this particular perturbed KdV equation, it is found that there is only one possible solitary wave that has a form similar to the unperturbed soliton solution

### Understanding fragility in supercooled Lennard-Jones mixtures. I. Locally preferred structures

We reveal the existence of systematic variations of isobaric fragility in
different supercooled Lennard-Jones binary mixtures by performing molecular
dynamics simulations. The connection between fragility and local structures in
the bulk is analyzed by means of a Voronoi construction. We find that clusters
of particles belonging to locally preferred structures form slow, long-lived
domains, whose spatial extension increases by decreasing temperature. As a
general rule, a more rapid growth, upon supercooling, of such domains is
associated to a more pronounced super-Arrhenius behavior, hence to a larger
fragility.Comment: 14 pages, 14 figures, minor revisions, one figure adde

### Weakly nonlinear waves in magnetized plasma with a slightly non-Maxwellian electron distribution. Part 2, Stability of cnoidal waves

We determine the growth rate of linear instabilities resulting from long-wavelength transverse perturbations applied to periodic nonlinear wave solutions to the SchamelâKortewegâde VriesâZakharovâKuznetsov (SKdVZK) equation which governs weakly nonlinear waves in a strongly magnetized cold-ion plasma whose electron distribution is given by two Maxwellians at slightly different temperatures. To obtain the growth rate it is necessary to evaluate non-trivial integrals whose number is kept to a minimum by using recursion relations. It is shown that a key instance of one such relation cannot be used for classes of solution whose minimum value is zero, and an additional integral must be evaluated explicitly instead. The SKdVZK equation contains two nonlinear terms whose ratio b increases as the electron distribution becomes increasingly flat-topped. As b and hence the deviation from electron isothermality increases, it is found that for cnoidal wave solutions that travel faster than long-wavelength linear waves, there is a more pronounced variation of the growth rate with the angle Îž at which the perturbation is applied. Solutions whose minimum values are zero and which travel slower than long-wavelength linear waves are found, at first order, to be stable to perpendicular perturbations and have a relatively narrow range of Îž for which the first-order growth rate is not zero

### Vapor-liquid surface tension of strong short-range Yukawa fluid

The thermodynamic properties of strong short-range attractive Yukawa fluids,
k=10, 9, 8, and 7, are determined by combining the slab technique with the
standard and the replica exchange Monte Carlo (REMC) methods. A good agreement
was found among the coexistence curves of these systems calculated by REMC and
those previously reported in the literature. However, REMC allows exploring the
coexistence at lower temperatures, where dynamics turns glassy. To obtain the
surface tension we employed, for both methods, a procedure that yields the
pressure tensor components for discontinuous potentials. The surface tension
results obtained by the standard MC and REMC techniques are in good agreement.Comment: 6 pages, 4 figure

### The visual standards for the selection and retention of astronauts, part 2

In preparation for the various studies planned for assessing visual capabilities and tasks in order to set vision standards for astronauts, the following pieces of equipment have been assembled and tested: a spectacle obstruction measuring device, a biometric glare susceptibility tester, a variable vergence amplitude testing device, an eye movement recorder, a lunar illumination simulation chamber, a night myopia testing apparatus, and retinal adaption measuring devices

### Dynamics of uniaxial hard ellipsoids

We study the dynamics of monodisperse hard ellipsoids via a new event-driven
molecular dynamics algorithm as a function of volume fraction $\phi$ and aspect
ratio $X_0$. We evaluate the translational $D_{trans}$ and the rotational
$D_{rot}$ diffusion coefficient and the associated isodiffusivity lines in the
$\phi-X_0$ plane. We observe a decoupling of the translational and rotational
dynamics which generates an almost perpendicular crossing of the $D_{trans}$
and $D_{rot}$ isodiffusivity lines. While the self intermediate scattering
function exhibits stretched relaxation, i.e. glassy dynamics, only for large
$\phi$ and $X_0 \approx 1$, the second order orientational correlator $C_2(t)$
shows stretching only for large and small $X_0$ values. We discuss these
findings in the context of a possible pre-nematic order driven glass
transition.Comment: accepted by Phys. Rev. Let

### Sources of Financial Stress in Agricultural Cooperatives

Financial stress in agricultural cooperatives may be due to a combination of three factors: inadequate profitability, excessive debt, or high interest rates. This paper uses an analytical technique to determine the relative degree of financial stress in agricultural cooperatives attributable to each factor. Roughly 30 percent of agricultural cooperatives in our sample suffered financial stress from 1987 through 1992. The analysis indicates that the greatest portion of financial stress, 54 percent, originated from low earnings. High interest rates accounted for roughly 24 percent of the financial stress while leverage accounts for the remaining 22 percent. The results also indicate that smaller cooperatives are more than twice as likely to face financial stress than larger cooperatives. Small cooperatives are more likely to face profitability problems whereas large cooperatives are more likely to face debt and interest rate problems.Agribusiness, Agricultural Finance,

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