6,279 research outputs found
Resonant Interactions Between Protons and Oblique Alfv\'en/Ion-Cyclotron Waves
Resonant interactions between ions and Alfv\'en/ion-cyclotron (A/IC) waves
may play an important role in the heating and acceleration of the fast solar
wind. Although such interactions have been studied extensively for "parallel"
waves, whose wave vectors are aligned with the background magnetic
field , much less is known about interactions between ions and
oblique A/IC waves, for which the angle between and is nonzero. In this paper, we present new numerical results on resonant
cyclotron interactions between protons and oblique A/IC waves in collisionless
low-beta plasmas such as the solar corona. We find that if some mechanism
generates oblique high-frequency A/IC waves, then these waves initially modify
the proton distribution function in such a way that it becomes unstable to
parallel waves. Parallel waves are then amplified to the point that they
dominate the wave energy at the large parallel wave numbers at which the waves
resonate with the particles. Pitch-angle scattering by these waves then causes
the plasma to evolve towards a state in which the proton distribution is
constant along a particular set of nested "scattering surfaces" in velocity
space, whose shapes have been calculated previously. As the distribution
function approaches this state, the imaginary part of the frequency of parallel
A/IC waves drops continuously towards zero, but oblique waves continue to
undergo cyclotron damping while simultaneously causing protons to diffuse
across these kinetic shells to higher energies. We conclude that oblique A/IC
waves can be more effective at heating protons than parallel A/IC waves,
because for oblique waves the plasma does not relax towards a state in which
proton damping of oblique A/IC waves ceases
Source amplitudes for active exterior cloaking
The active cloak comprises a discrete set of multipole sources that
destructively interfere with an incident time harmonic scalar wave to produce
zero total field over a finite spatial region. For a given number of sources
and their positions in two dimensions it is shown that the multipole amplitudes
can be expressed as infinite sums of the coefficients of the incident wave
decomposed into regular Bessel functions. The field generated by the active
sources vanishes in the infinite region exterior to a set of circles defined by
the relative positions of the sources. The results provide a direct solution to
the inverse problem of determining the source amplitudes. They also define a
broad class of non-radiating discrete sources.Comment: 21 pages, 17 figure
Effects of lattice distortion and JahnâTeller coupling on the magnetoresistance of La0.7Ca0.3MnO3 and La0.5Ca0.5CoO3 epitaxial films
Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T â TC, TC being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T âȘ TC/, the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of JahnâTeller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of JahnâTeller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the JahnâTeller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites
Convective Fingering of an Autocatalytic Reaction Front
We report experimental observations of the convection-driven fingering
instability of an iodate-arsenous acid chemical reaction front. The front
propagated upward in a vertical slab; the thickness of the slab was varied to
control the degree of instability. We observed the onset and subsequent
nonlinear evolution of the fingers, which were made visible by a {\it p}H
indicator. We measured the spacing of the fingers during their initial stages
and compared this to the wavelength of the fastest growing linear mode
predicted by the stability analysis of Huang {\it et. al.} [{\it Phys. Rev. E},
{\bf 48}, 4378 (1993), and unpublished]. We find agreement with the thickness
dependence predicted by the theory.Comment: 11 pages, RevTex with 3 eps figures. To be published in Phys Rev E,
[email protected], [email protected], [email protected]
The Giraffe Inner Bulge Survey (GIBS) II. Metallicity distributions and alpha element abundances at fixed Galactic latitude
High resolution (R22,500) spectra for 400 red clump giants, in four
fields within and , were obtained within the GIRAFFE
Inner Bulge Survey (GIBS) project. To this sample we added another 400
stars in Baade's Window, observed with the identical instrumental
configuration. We constructed the metallicity distributions for the entire
sample, as well as for each field individually, in order to investigate the
presence of gradients or field-to-field variations in the shape of the
distributions. The metallicity distributions in the five fields are consistent
with being drawn from a single parent population, indicating the absence of a
gradient along the major axis of the Galactic bar. The global metallicity
distribution is well fitted by two Gaussians. The metal poor component is
rather broad, with a mean at dex and dex.
The metal-rich one is narrower, with mean and
dex. The [Mg/Fe] ratio follows a tight trend with [Fe/H], with enhancement with
respect to solar in the metal-poor regime, similar to the one observed for
giant stars in the local thick disc. [Ca/Fe] abundances follow a similar trend,
but with a considerably larger scatter than [Mg/Fe]. A decrease in [Mg/Fe] is
observed at dex. This \textit{knee} is in agreement with our
previous bulge study of K-giants along the minor axis, but is 0.1 dex lower in
metallicity than the one reported for the Bulge micro lensed dwarf and
sub-giant stars. We found no variation in -element abundance
distributions between different fields.Comment: Accepted for publication in A&
Nonlinear Front Evolution of Hydrodynamic Chemical Waves in Vertical Cylinders
The nonlinear stability of three-dimensional reaction-diffusion fronts in vertical cylinders is considered using the viscous hydrodynamic fluid equations in the limit of infinite thermal diffusivity. A nonlinear front evolution equation is presented and used to examine the transition from nonaxisymmetric to axisymmetric convection observed in experiments performed in cylinders. Comparisons with experiments show excellent agreement in both the shape and speed of the front
Hydrodynamic Instability of Chemical Waves
We present a theory for the transition to convection for flat chemical wave fronts propagating upward. The theory is based on the hydrodynamic equations and the oneâvariable reactionâdiffusion equation that describes the chemical front for the iodateâarsenous acid reaction. The reaction term involves the reaction rate constants and the chemical composition of the mixture. This allows the discussion of the effects of the different chemical variables on the transition to convection. We have studied perturbations of different wavelengths on an unbounded flat chemical front and found that for wavelengths larger than a critical wavelength (λâłÎ»c) the perturbations grow in time, while for smaller wavelengths the perturbations diminish. The critical wavelength depends not only on the density difference between the unreacted and reacted fluids, but also on the speed and thickness of the chemical front
Modification of the eikonal relation for chemical waves to include fluid flow
Propagating wave fronts resulting from autocatalytic chemical reactions have been the focus of much recent research. For the most part, the hydrodynamics resulting from such reactions has been neglected. In this work, a relation is derived for the normal speed of a propagating wave front as a function of the local curvature when fluid motion is allowed. This ââeikonalââ equation is a generalization of one which was derived in the absence of fluid flow. It is also shown that small variations in the fluid density due to the chemical reaction do not change the form of the relation
Finite Thermal Diffusivity at Onset of Convection in Autocatalytic Systems: Discontinuous Fluid Density
A linear convective stability analysis for propagating autocatalytic reaction fronts includes density differences due to both thermal and chemical gradients. Critical parameters for the onset of convection are calculated for an unbounded geometry, a vertical slab, and a vertical cylinder. Thermal effects are important at unstable wavelengths well above the critical wavelength for the onset of convection
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