Showing papers on "Wavelength published in 1974"

The wavelengths in sunlight effective in producing skin cancer: a theoretical analysis.

Abstract: DNA is taken as the target for skin cancer induced by ultraviolet light, and the known data on the sensitivity of DNA as a function of wavelength are summarized. The sun's spectrum at the surface of the earth and the DNA action spectrum are used to calculate the carcinogenic effectiveness as a function of wavelength. The most effective wavelengths at 30°N latitude are <305 nm, and a 1% change in atmospheric ozone results in a 2% change in the effective dose of ultraviolet light. Since both the basic biological and physical data are reasonably precise, the major requirement for a quantitative evaluation of the dose response relation for ultraviolet-induced skin cancer in man is better epidemiological data to compare with data from animal models.

Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the solar system

Abstract: Spin-allowed Fe2+ absorption bands occur in the visible and near-infrared diffuse reflectance spectra of most pyroxenes. The wavelengths of the bands centered near 1 μm and 2 μm vary as functions of pyroxene composition, making possible mineralogical and chemical deductions based on spectral reflectance curves. Typically, pyroxene bands are well developed in relation to absorption features in the spectra of other rock-forming minerals and glass; thus pyroxene often dominates the spectral curves of pyroxene-bearing rocks. Telescopic spectra of the moon and some other solar system objects contain absorption features that can be interpreted in terms of pyroxenes mixed with other minerals and/or glass.

Spontaneous rupture of thin liquid films

Abstract: The rupture of a liquid film on a solid surface and of a free liquid film have been studied using hydrodynamic stability theory. The films are not thicker than several hundred Angstrom. A small perturbation applied to the free interface generates motions in the film, and the assumption is made that the Navier–Stokes equations can be used to describe them. The difference in forces acting upon an element of liquid in a thin film and in a bulk fluid is accounted for by introducing a body force in the Navier–Stokes equations. This force is calculated from the potential energy per unit volume in the liquid caused by the London–van der Waals interactions with the surrounding molecules of the liquid and with those of the solid. If the perturbation grows, it leads to the rupture of the film. The range of wavelengths of the perturbation for which instability occurs is established and the time of rupture is evaluated. The effect of insoluble and soluble surface active agents is analyzed. Available experimental data concerning condensation on a solid surface and coalescence of bubbles are explained on the basis of the obtained results.

Computer extension and analytic continuation of stokes expansion for gravity waves

Abstract: Stokes' infinitesimal-wave expansion for steady progressive free-surface waves has been extended to high order using a computer to perform the coefficient arithmetic. Stokes' expansion has been found to be incapable of yielding the highest wave for any value of the water depth since convergence is limited by a square-root branch-point some distance short of the maximum. By reformulating the problem using a different independent parameter, the highest waves are obtained correctly. Series summation and analytic continuation are facilitated by the use of Pade approximants. The method is valid in principle for any finite value of the wavelength and solutions of high accuracy can be obtained for most values of the wave height and water depth.

Characteristics of North African Easterly Waves During the Summers of 1968 and 1969

Abstract: This study uses both spectral and compositing analyses to determine the structure of North African easterly waves during the summers of 1968 and 1969. The average period and wavelength computed by the power-spectrum method are 4.5 days and 3800 km and for the compositing analysis 3.8 days and 3100 km. Although the period and wavelength of individual waves vary considerably, the spectral results are in qualitative agreement with the compositing results except in extreme northern Africa where eastward-moving disturbances of the same period affect the spectral computations. The maximum surface amplitudes of u, v, p and Td associated with the waves occur about 20N near the mean location of the east-west oriented region of minimum surface pressure. East of the Greenwich meridian the amplitude decreases and the wave motion is barely detectable with surface data near 25E. The compositing results show that the waves influence latitudes between 5 and 3ON and that wave features tilt from southwest to north...

Whistler waves observed upstream from collisionless shocks

Abstract: Waves in the frequency range 0.5 - 4 Hz were studied in the region upstream of the earth's bow shock using data from the fluxgate magnetic field experiment on IMP-6. Analysis of 150 examples of these waves during a three month interval indicates that amplitudes are generally less than 1 or 2 gammas and propagation directions generally make angles of between 20 and 40 degrees with the field direction. The waves as measured in the spacecraft frame of reference are either left or right hand polarized with respect to the average field direction. It is concluded that the observed waves are right handed waves in the plasma frame of reference with wavelengths of approximately 100 km propagating upstream in the whistler mode. Doppler shifting reduces the observed frequencies in the spacecraft frame and reverses the observed polarization for those waves propagating more directly upstream. Similar waves are seen ahead of most interplanetary shocks.

Correction to the velocity‐per‐fringe relationship for the VISAR interferometer

Abstract: A Doppler shift in wavelength can result in a very slight change in the index of refraction of a transparent material due to disperison. This effect, if not accounted for, produces an error of a few percent in the velocities determined using the recently developed velocity interferometer for diffusely reflecting surfaces.

A theory of medium-scale traveling ionospheric disturbances

Abstract: Analysis of the excitation of atmospheric gravity waves leads to a hypothesis concerning the nature of medium-scale traveling ionospheric disturbances (TID's) and their relation to large-scale TID's Stationary phase techniques are used to derive the asymptotic fluctuations of velocity and electron density generated by a line source (eg, the auroral electrojet) Dissipation by viscosity and thermal conductivity is included in an approximate manner The gravity wave response includes a discrete spectrum of upper-atmospheric guided modes and a continuous spectrum of freely propagating waves; these two classes of waves can be associated with large-scale and medium-scale TID's, respectively The medium-scale TID's, in turn, are primarily composed of two families of waves: those that propagate obliquely upward from the source and reach the F region directly and those that propagate initially downward and reach the F region only after reflection from the earth For an E region source the earth-reflected waves appear as nearly monochromatic wave packets, whereas the direct waves appear for the most part as single pulses; this qualitative difference corresponds to J E Titheridge's observational distinction between periodic and nonperiodic TID's It is suggested that this dichotomy may imply the dominance of upper-atmospheric (as opposed to tropospheric) sources In this connection the average fluctuations of the auroral electrojet are shown to be sufficient to generate sizable medium-scale TID's that propagate to large horizontal distances with no loss of amplitude despite the absence of ducting mechanisms Such propagation is possible because amplitude attenuation due to cylindrical spreading is counteracted by the fact that the lower-frequency waves, which reach the F region at the greater horizontal distances, are generated with larger amplitudes Several properties of the calculated response, including the linear increase of period and wavelength with distance from the source, might be useful in future observational studies designed to identify the sources of medium-scale TID's

The stability of planetary waves on an infinite beta‐plane

Abstract: The stability of a planetary wave on an infinite s‐plane is examined. There are two parameters in the problem, (a) ? = Uκ2/β where U is the velocity amplitude of the planetary wave and κ its wavenumber, and (b) the direction of the wavenumber of the planetary wave. For large M, the problem reduces to the Rayleigh problem for a sinusoidal velocity distribution. The growth rate of the most unstable wave is 0.27 Uκ. 38 % of the energy lost by this wave is transferred to wavenumber 0.59 κ and 61 % is transferred to wavenumber 1.16 κ. As ? decreases and β effects become more important, there are fewer unstable waves, but unstable waves can always be found. For small M, the disturbance comprises two waves which form a resonantly interacting triad with the primary wave. Some geophysical applications are discussed. It is shown, for instance, that for strong currents like the Gulf Stream, this type of instability is much more important than inertial instability. Also, observations indicate that the dominant eddies...

Amplification and absorption of electromagnetic waves in overdense plasmas

Abstract: The spatial variation of the amplitude of electromagnetic radiation propagating into an inhomogeneous plasma is discussed in reference to nonlinear interaction of HCN laser radiation with plasmas and to experiments on r.f. heating of the ionosphere. Previous results on the ordinary wave and on the extraordinary wave at normal incidence are reviewed with emphasis on the physical processes affecting the amplitude behaviour. New numerical results are obtained starting from an integral representation of the solution of the wave equation for waves in a cold, inhomogeneous, magnetized plasma slab. Resonance absorption is discussed for the cases of normal incidence in the presence of a magnetic field (the Budden problem) and oblique incidence in the absence of a magnetic field.

Standing waves on beaches

Abstract: Simultaneous measurements of run-up spectra and nearshore wave spectra associated with standing waves have been made on a natural beach. Digital wave staffs and bottom-mounted pressure sensors were placed in a line at various distances offshore from a digital run-up meter. The run-up meter, which measured the position of the edge of the water, consisted of a series of electrical contacts suspended above the beach face and spaced 30 cm apart. Because of the possible influence of nonlinear interactions on the spectra, bicoherence was estimated, and the frequency range appropriate to a linearized model was defined. In this low-frequency region of the spectrum the cross-spectra phase angles between data records had values near 0 or π at frequencies of high coherence. The energy spectrum levels and the observed phase differences between records were in good agreement with the linear shallow water wave theory (Lamb, 1932) for standing waves on a plane beach. The general reflection coefficient for the data, estimated from the disagreement between data and standing wave theory, was found to be approximately 0.7. This result indicates that the theory of Miche (1944) for reflection from a plane beach predicts a reflection coefficient that is too high by a factor of approximately 1.5.

On a variety of wave phenomena in chemical reactions

Abstract: A variety of wave phenomena are analyzed and discussed for systems in which chemical reactions and transport take place. Certain families of wave solutions of reaction‐transport equations arise owing to the weak stability of a reference state to a class of perturbations. We consider both wave induction by heterogeneities and autonomous waves and seek perturbation solutions which provide the dispersion relation and the wave vector dependence of the amplitude for one‐parameter families of waves characterized by the wave vector. For the case of an arbitrary reaction mechanism possessing a homogeneous steady state we derive, by use of bifurcation theory and frequency renormalization, small amplitude autonomous plane waves and standing and rotating waves. We find solutions corresponding to long wavelength waves, static structures, and phenomena existing only at intermediate frequencies and wavelengths. The theory is found to have a nonuniformity in convergence in the core region of pacemaker and spiral‐like so...

Rain-induced cross-polarization at centimeter and millimeter wavelengths

Abstract: Rain-induced cross-polarization is an important factor in design of dual-polarization microwave radio communication systems. We present current estimates of this effect based upon calculated differential characteristics of canted oblate raindrops and their relationship to experiments. Measured differential attenuation and cross-polarization, mainly at 18 GHz, are used to determine two empirical parameters: an effective average of the absolute value of the canting angle and a measure of the imbalance between positive and negative canting angles. We can then provide estimates for median values of cross-polarization discriminations at other frequencies; these are found to agree fairly well with available measured data. Differential phase shift is the dominant factor in the rain-induced cross-polarization at frequencies below about 10 GHz, and differential attenuation becomes increasingly important at higher frequencies. For a given rain fading, the cross-polarization decreases with increase in frequency and is relatively insensitive to the rain rate, whereas for a given amount of rain the cross-polarization increases with frequency up to about 35 GHz. The cross-polarization discrimination of circularly polarized waves is much poorer than that of linearly polarized waves. When the angle α between the direction of propagation and the axis of symmetry of oblate raindrops is not equal to π/2, as on earth-space paths in satellite communication systems, the differential attenuation and differential phase shift can be approximated by sin2 α times those for α = π/2, which is the condition for terrestrial paths.

Propagation and mode conversion of lower-hybrid waves generated by a finite source

Abstract: The propagation of electrostatic plasma waves, and their subsequent conversion into hot plasma waves at the lower hybrid frequency is calculated for realistic density profiles and finite rf sources in a slab geometry. A finite length slow wave source having a potential distribution phi ~ cosk0z is found to generate spatial oscillations having a well-defined wavelength. These oscillations are confined to regions bounded by conical curves originating at the ends of the source. The axial distance of rf energy propagation to the lower hybrid layer is found to be greater than the radial distance of propagation by a factor of the order (mi/me)1/2. The conversion at the lower hybrid layer of the electrostatic cold plasma waves excited by a finite source into propagating hot plasma waves is calculated. It is shown that collisional damping at the lower hybrid layer may predominate over mode conversion even for relatively low collision frequencies.

Space and wavelength dependence of speckle intensity

Abstract: A unified, analysis is presented for the spatial and the spectral sensitivity of speckle (the rapid spatial variations which occur in an image when illumination of narrow spectral width is used) in a space-invariant linear system. In prior work considering speckle size, others have shown that its spatial variation is functionally dependent primarily on the autocorrelation function of the system's impulse response, but effects of varying the wavelength were largely ignored. In the present paper we treat the general problem in which a diffuse object, illuminated by a collimated, monochromatic beam, is imaged by a system whose amplitude impulse response isz(x, η), wherex and η are space and normalized (temporal) frequency coordinates, respectively. An expression is derived for the multidimensional autocorrelation functionRu(Δx,η1,η2) of the intensityu(x,η) in the image plane. Functionally, it depends upon a convolution of the system autocorrelation functionRu(Δx,η1,η2) with the characteristic function of the distribution function for heights, which is used to model the input object's surface. Examples are presented; and, it is shown that one can infer valuable information about the variation of heights for points on the surface of the input diffuse object, which are separated by much less than the classical resolution limit.

Wavelike Properties of the Alpha Rhythm

Abstract: Apparent similarities between brain waves and physical waves are considered. The distribution of power in the alpha rhythm over frequency and spatial wavelength is measured in several subjects. This is accomplished by means of spatial-temporal Fourier analyses of scalp surface potentials using multichannel recordings from electrodes placed along the midline. It is found that the spectrum of midline alpha oscillations peaks for long wavelengths (?20 cm). A dispersion relation relating frequency to spatial wavelength apparently exists for frequencies near the peak alpha frequency. That is, in all 13 records, frequency components just above the peak alpha frequency have shorter midline wavelengths than low alpha frequency components. The relationship of these results to theoretical descriptions of the EEG and to other experiments is discussed.

Interferometric measurement of electron and vapor densities in a high‐current vacuum arc

Abstract: Electron and vapor densities in a 0.4–4.0‐kA copper vapor (vacuum) arc were investigated by means of optical interferometry. Nonsimultaneous measurements were made with a visible wavelength (0.63 μm) and an infrared wavelength (10.6 μm) to separate the electron and vapor contributions. The electron contribution to the index of refraction was found to dominate at both wavelengths. An upper limit to the ratio of copper vapor density to electron density of 2.8 could be set as a result of the 0.63‐μm measurement. The integral of the electron density over the optical pathlength was measured at various lateral positions of the arc by means of the 10.6‐μm interferometer; the Abel transformation was applied to these results to obtain the radial profile of the electron density. A diffuse electron density profile is observed, with the axial densities ranging from 1 × 1014/cm3 at 0.4 kA to 2 × 1015/cm3 at 4 kA. At the transition from the diffuse or quiescent mode to the high‐voltage or constricted mode, which occurred at about 3 kA with the electrode geometry used in this study, a sudden decrease in electron density from 1.7 × 1015/cm3 to 0.7 × 1015/cm3 was observed in the center of the discharge. The measured densities were compared with existing theoretical predictions.

HO2 detected by laser magnetic resonance

Abstract: Far‐infrared absorption spectra of HO2 in the gas phase have been detected at six wavelengths of a water vapor laser magnetic resonance spectrometer. The identification of HO2 as the absorbing molecule is based on a partial analysis of the spectra and on a variety of different chemical methods used to produce the radical. Approximate values of rotational constants and spin doublet separations are derived from the spectra.

Thermoelastic waves in anisotropic solids

Abstract: As motivated by the recent discovery of heat pulses propagating in dielectric crystals at low temperature, a continuum theory of thermoelasticity, which is modified to include the effect of thermal phonon relaxation, is applied to investigate the propagation of plane harmonic waves in unbounded anisotropic solids Four characteristic wave speeds are found, three being analogous to those of isothermal or adiabatic elastic waves The fourth wave, which is predominantly a temperature disturbance, corresponds to the heat pulses known also as the second sound Velocity, slowness, and wave surfaces of the thermoelastic waves are analyzed and are illustrated with numerical and graphical results for NaF and solid helium crystals A new definition of the group velocity for waves in a dissipative and dispersive anisotropic medium is proposed and is calculated and compared with the energy transport speed of thermoelastic waves

Holography--a new tool for soil dynamics

Abstract: Holographic interferometry was used to find dynamic displacement of the surface of a sand half-space model excited by a harmonically vibrating circular footing. The screening effectiveness of wave barriers composed of rows of vertical cylindrical holes was studied using this technique. The general results indicate that: (1) Holographic interferometry is a powerful tool for measurement of static as well as dynamic displacement of the surface of sand half-space model; and (2) stroboscopic, double exposure, holographic interferometry can be used to get "stopped motion" records of traveling waves. Specific results for barriers indicate that the scaled hole diameter (diameter/wave-length of Rayleigh wave) should be at least 1/6, the net scaled spacing ((c-c spacing - diameter)/wavelength) should be less than 1/4, and the solid-filled hole barriers behave fundamentally differently than fluid-filled or void holes. /ASCE/

Optical characteristics of twisted nematic liquid-crystal films

Abstract: A twisted nematic liquid crystal is shown by theory and experiment to rotate incident plane-polarised light by π/2 only at specified wavelengths. At other wavelengths, elliptically polarised light is produced. This action determines the contrast ratio of some twisted nematic devices.

Optimal swimming of flagellated micro-organisms

Abstract: This paper studies flagellar undulations that propel a micro-organism at a given speed while minimizing its expenditure of hydrodynamical energy. The study is in two basic parts. The first part is a qualitative inquiry into the general nature of undulations that are hydrodynamically optimal in the instantaneous sense. The results indicate that an apparent sliding of the entire flagellum along its instantaneous axis is fundamental to such motions, although an additional deformation is necessary to compensate for the presence of the organism's head. Periodic or semi-periodic undulations are clearly suggested, and must consist of travelling waves propagated in the direction opposite to propulsion.The second part of the paper is a quantitative inquiry as to the values of parameters that optimize given periodic wave shapes in the time-average sense. The trade-off between wave amplitude and the number of wavelengths is of particular interest. Results are obtained for small amplitude sinusoidal waves and finite amplitude sawtooth waves. For the latter, a single wavelength with amplitude roughly one-sixth of the wavelength is optimal. The significance of the twitching movements of the head is investigated. The results are consistent with the qualitative study and emphasize the need to inhibit such motions. The implications of the dependence of resistive-force coefficients upon wave shape are considered, and the physical significance of rotational pitching motions is assessed.

On the backscatter instability of solar wind Alfvén waves

Abstract: A coherent Alfv6n wave propagating along the equilibrium magnetic field is unstable against decay into an Alfv6n wave traveling in the opposite direction and a sound wave (Sagdeer and Galeev, 1969). The stability of incoherent Alfv6n waves against this mode decay is considered, assuming that the sound waves are damped. The criterion for stabilizing waves with power spectral density In ok -N is N > 1. The results indicate that solar wind Alfv6n waves are stable in the wavelength range considered in Mariner 5 ob- servations, although a mild instability is predicted for longer wavelengths. According to the interpretation of Mariner 5 data by Belcher and Davis (1971) the high-velocity solar wind streams and their trailing edges are often characterized at 1 A U by large-amplitude hydromagnetic fluctuations that are almost purely Alfv6nic and almost completely outward propagating with respect to the wind. The term Alfv6nic is used here to de- scribe fluctuations in which the magnitude of the magnetic field is strictly constant, such as circularly polarized waves propagating along the unperturbed field direction. Among the processes that have been proposed for damping of hydromagnetic waves in the solar wind are linear collision- less damping (Barnes, 1966), nonlinear Landau damping (Hollweg, 1971; Lee and Volk, 1973), and steepening followed by dissipation in shocks (Cohen and Kulsrud, 1974). These mechanisms are inoperative for the Alfv6n mode; hence under their influence a random mixture of hydromagnetic waves damps into Alfv6nic fluctuations, such as those observed at 1 A U. On the other hand, the decay instability of Galeev and Oraevskii (1963) results from a mode coupling that is basically insensitive to polarization, and hence in particular can lead to decay of purely Alfv6nic fluctuations. We consider here the stability of solar wind Alfv6n waves against this decay mode. Galeev and Oraevskii noted that an Alfv6n wave is unstable against decay into another Alfv6n wave and a magneto- acoustic wave. For the simple case of propagation along the magnetic field (Sagdeer and Galeev, 1969) the magneto- acoustic wave is a sound wave and the scattered Alfv6n wave travels in the opposite direction from the parent wave. Sagdeev and Galeev consider a monochromatic circularly polarized parent wave; they introduce the decay products as small per- turbations and calculate the growth rate 'c, assuming no linear damping and small/5 -- (Cs/VA) 2, where Cs is the sound speed and vA is the Alfv6n speed. Their result may be written in the form

A theoretical model for short‐scale field‐aligned plasma density striations

Abstract: The short-scale (about 3 m), field-aligned plasma density striations found in ionospheric modification experiments are generated by the plasma turbulence resulting from ion acoustic-plasma wave parametric decay instabilities. The field-aligned geometry results from the fact that nonlinear wave coupling arises from thermal, ohmic heating terms rather than ponderomotive force (radiation pressure). The theory shows that field-aligned striations increase rapidly in intensity as the wavelength increases—in agreement with experiment.

Acoustic wave propagation along a fluid‐filled cylinder

Abstract: Phenomena of elastic wave propagation from a point source located in a fluid‐filled cylinder are investigated. Detailed consideration is given to the natural mode wave propagation for either fluid or elastic solid surroundings of infinite extent. Phase and group velocity dispersion curves are presented and the relative modal sensitivity, interference pattern, and decay rates are determined for typical sources having various dominant wavelengths. Curves showing actual waveforms including both axial and radial dependence are also presented for this coupled system.

Particle detecting system

Abstract: Submicron-sized particles are detected by a system which observes emission, as by scattering or by fluorescence from particles undergoing Brownian motion in a region of a fluid medium excited by an evanescent wave created adjacent an interface between the fluid medium and a multiple internal totally reflecting cell or light guide. The excited region can range in depth between several wavelengths and a fraction of the wavelength of the exciting beam, hence serves as an "aperture" having a dimension of about the same order of magnitude as the particles being detected. The particles can be classified according to size by examining the amplitude modulation arising out of the motion of the particles through the aperture.