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NASA NTRS1983

Modes of inter-hemispheric wave propagation

Attention is given to observations made in connection with the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE), taking into account the average 200 mb zonal wind for two periods during the First Special Observing Period of FGGE, and the 200 mb velocity potential. The meridional linkages implied in the observations involve sources south of 15 deg N, and the most active extratropical patterns are collocated with regions of tropical easterlies. The possibility is investigated that the theory of reduced Rossby wave propagation through easterlies is not sufficiently general to cover the considered observations, taking into account a shallow water model. It is found that the meridional interactions cannot be associated with Rossby waves because these waves connot be generated in the regions of zero absolute vorticity gradient within which the forcing is embedded.

Paegle, J.↗

NASA NTRS1986

Dynamic delamination crack propagation in a graphite/epoxy laminate

The dynamic delamination crack propagation behavior during ballistic tests of (90/0)5s T-300/934 graphite/epoxy laminates with embedded interfacial cracks was investigated using high speed photography. The impact on the beam-like specimen was produced with a silicon rubber ball, and the crack propagation speeds and the threshold impact velocities required to initiate dynamic crack propagation were determined for several crack positions. The results suggest that the mode of crack propagation depends on the specimen geometry as well as the loading condition. A simplified finite element analysis of the experimental data obtained from one of the midplane-cracked specimens was used to estimate the critical strain energy release rate, which may determine the onset of unstable crack propagation.

Sun, C. T.↗

NASA NTRS1991

Supercritical baroclinic disturbances under the influence of topography

An Eady model modified to include Ekman dissipation and sloping horizontal boundaries is used to investigate the nonlinear evolution of baroclinic waves in the presence of surface topography. The effectiveness of the topography to phase lock and equilibrate a given mode vs that of baroclinity to propagate and vacillate that mode is found to depend on the topographic height, its zonal structure, and the level of supercriticality. When topography is sinusoidal and of the same wavelength as the baroclinically most unstable mode, it induces a short-period amplitude modulation whose envelope represents the baroclinic evolution. The dynamics of this modulation is explored analytically. When the sinusoidal topography is of a longer zonal wavelength, a 'mixed' wave state persists with amplitude dominance shifting between the topographic mode and the baroclinic mode, depending on the relative strength of form drag and baroclinicity. Model results are also compared, in appropriate parameter limits, with near-resonance asymptotic results and annulus experiments.

Chou, Shih-Hung↗

NASA NTRS1995

Radial energy transport by magnetospheric ULF waves: Effects of magnetic curvature and plasma pressure

The 'radial' transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of one-fluid magnetohydrodynamics. (the term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plasma density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD waves is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a Wentzel-Kramer-Brillouin (WKB) approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boudnary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or box-model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2-4; and (3) broaden the spectral width of the resonance by a factor of 2-3. The effects are attributed to the existence of an 'Alfven buoyancy oscillation,' which approaches the usual shear mode Alfven wave at resonance, but unlike the shear Alfven mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfven buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distances.

Kouznetsov, Igor↗

NASA NTRS1989

Filamentation instability of magnetosonic waves in the solar wind environment

Intense magnetosonic waves, originally propagating at the right angle with the interplanetary magnetic field, can excite a purely growing mode along the interplanetary magnetic field together with two symmetric magnetosonic sidebands propagating obliquely across the magnetic field. This instability process leads to the filamentation of the magnetosonic pump waves. These two excited magnetosonic sideband modes propagate together perpendicularly across the magnetic field and, meanwhile, form a standing wave pattern along the magnetic field. The thresholds of this filamentation instability can be exceeded in the solar wind environment. It is predicted that the density fluctuations produced by the filamentation instability along the interplanetary magnetic field have wavelengths greater than, at least, a few earth radii. The polarization of the obliquely propagating magnetosonic waves excited by the filamentation instability is determined by the characteristics of the magnetosonic pump waves and the environmental plasmas.

Kuo, S. P.↗

NASA NTRS2015

Effect of Buckling Modes on the Fatigue Life and Damage Tolerance of Stiffened Structures

The postbuckling response and the collapse of composite specimens with a co-cured hat stringer are investigated experimentally and numerically. These specimens are designed to evaluate the postbuckling response and the effect of an embedded defect on the collapse load and the mode of failure. Tests performed using controlled conditions and detailed instrumentation demonstrate that the damage tolerance, fatigue life, and collapse loads are closely tied with the mode of the postbuckling deformation, which can be different between two nominally identical specimens. Modes that tend to open skin/stringer defects are the most damaging to the structure. However, skin/stringer bond defects can also propagate under shearing modes. In the proposed paper, the effects of initial shape imperfections on the postbuckling modes and the interaction between different postbuckling deformations and the propagation of skin/stringer bond defects under quasi-static or fatigue loads will be examined.

Davila, Carlos G.↗

NASA NTRS1986

Ray tracing survey of Z mode emissions from source regions in the high-altitude auroral zone

The ray tracing computer program described by Menietti and Lin (1985) was used to investigate the propagation of Z mode emissions from sources in the auroral zone. The calculation results indicate that the propagation is primarily perpendicular to the magnetic field line (horizontal propagation) for almost all sources considered, except when the wave source is assumed to be unrestricted by the cyclotron maser mechanism. While the propagation is initially downward, refraction rapidly increases the wave normal angle, and only propagation at near-constant altitude occurs. The frequency bandwidth estimated from the ray paths was found to be less than that observed by the DE 1 satellite. Large horizontal magnetic field gradients associated with field-aligned currents produce downward propagation but are still not sufficient to produce the observed bandwidths. The results suggest that wave growth may be due to mechanisms in addition to cyclotron maser resonance.

Menietti, J. D.↗

NASA NTRS1986

On the applicability of relativistic dispersion to auroral zone electron distributions

An examination is made to identify appropriate times to include weakly relativistic wave dispersion in calculations of auroral zone electron distributions. Two dispersion relations are examined. First, solutions are found for a ring distribution of particles around a cold ambient plasma. The solutions are found only for modes near the electron gyrofrequency. A trapped mode which is decoupled from the freely propagating R-X mode of cold plasmas is found to be the most unstable mode. Wave dispersion is examined with a spherically symmetric Dory-Guest-Harris (DGH) distribution, which describes a shell distribution that has a peak momentum larger than the thermal spread. The trapped mode is again found to be the most unstable. The analysis indicates it is beneficial to include relativistic effects whenever calculating auroral plasma distributions, and that a simple model will have sufficient accuracy.

Strangeway, R. J.↗

NASA NTRS1993

Global normal-mode Rossby waves observed in stratospheric ozone data

Westward-propagating Rossby normal-mode planetary waves are documented in stratospheric ozone data using Solar Backscatter Ultraviolet (SBUV) satellite measurements. These modes are evidenced by enhanced spectral power and near-global coherence for westward-traveling zonal wave 1 oscillations with periods of 5-10 days. The ozone waves have maxima in high latitudes of the middle stratosphere (due to transport) and over midlatitudes in the upper stratosphere (due to photochemistry). These modes are nearly continuous throughout the eight years of SBUV observations, with maximum global coherence during the equinoxes. The upper-stratospheric waves are symmetric (in phase) between hemispheres, even for modes previously identified as antisymmetric in geopotential height. This behavior is due to differing wave vertical structure in each hemisphere; the planetary temperature waves are nearly in phase in the upper stratosphere, even though the height waves are out of phase. The observed ozone waves are furthermore compared to calculations based on linear wave transport and photochemistry, incorporating derived wind and temperature fields. Good agreement is found, showing that normal modes provide an idealized context to study the linear wave behavior of trace constituents in the real atmosphere.

Randel, William J.↗

NASA NTRS1992

Loss-cone-driven ion cyclotron waves in the magnetosphere

The study examines the theoretical properties of linear ion cyclotron waves propagating in the magnetosphere at arbitrary angles to the background magnetic field. It is found that in some cases the linear wave growth of modes with oblique propagation can dominate that of the parallel propagating electromagnetic ion cyclotron (EMIC) wave. The growth rate of the loss-cone-driven mode depends strongly on the depth of the loss cone. A simple analytical theory which explains the scaling of the growth rate of the oblique mode with respect to various parameters is presented. The loss-cone-driven mode is an electromagnetic mode which is preferentially nearly linearly polarized. The wave field which results from the oblique mode in its perferentially nearly linearly polarized form are nearly perpendicular to B0 and are such that they may be difficult to distinguish from those of a linearly polarized parallel propgating EMIC wave.

Denton, Richard E.↗

NASA NTRS2003

Structure and Stability of One-Dimensional Detonations in Ethylene-Air Mixtures

The propagation of one-dimensional detonations in ethylene-air mixtures is investigated numerically by solving the one-dimensional Euler equations with detailed finite-rate chemistry. The numerical method is based on a second-order spatially accurate total-variation-diminishing scheme and a point implicit, first-order-accurate, time marching algorithm. The ethylene-air combustion is modeled with a 20-species, 36-step reaction mechanism. A multi-level, dynamically adaptive grid is utilized, in order to resolve the structure of the detonation. Parametric studies over an equivalence ratio range of 0.5 less than phi less than 3 for different initial pressures and degrees of detonation overdrive demonstrate that the detonation is unstable for low degrees of overdrive, but the dynamics of wave propagation varies with fuel-air equivalence ratio. For equivalence ratios less than approximately 1.2 the detonation exhibits a short-period oscillatory mode, characterized by high-frequency, low-amplitude waves. Richer mixtures (phi greater than 1.2) exhibit a low-frequency mode that includes large fluctuations in the detonation wave speed; that is, a galloping propagation mode is established. At high degrees of overdrive, stable detonation wave propagation is obtained. A modified McVey-Toong short-period wave-interaction theory is in excellent agreement with the numerical simulations.

Yungster, S.↗

NASA NTRS1992

Meridionally propagating interannual-to-interdecadal variability in a linear ocean-atmosphere model

Meridional oscillation modes in a global, primitive-equation coupled ocean-atmosphere model have been analyzed in order to determine whether they contain such meridionally propagating modes as surface-pressure perturbations with years-to-decades oscillation periods. A two-layer global ocean model and a two-level global atmosphere model were then formulated. For realistic parameter values and basic states, meridional modes oscillating at periods of several years to several decades are noted to be present in the coupled ocean-atmosphere model; the oscillation periods, travel times, and meridional structures of surface pressure perturbations in one of the modes are found to be comparable to the corresponding characteristics of observed sea-level pressure perturbations.

Mehta, Vikram M.↗

NASA NTRS1995

Low-frequency fluctuations in the magnetosheath near the magnetopause

There are four low-frequency modes which may propagate in a high-beta nearly bi-Maxwellian plasma. These are the magnetosonic, Alfven, ion acoustic, and mirror modes. This manuscript defines a procedure based on linear Vlasov theory for the unique identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode deviation is calculated using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) spacecraft to identify the modes observed in the terrestial magnetosheath near the magnetopause. As well as determining the mode which best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique. Using 17 time periods temporally close to a magnetopause crossing, and confining our study to the frequency range from 0.01 to 0.04 Hz, we find that the only clearly identified mode in this frequency range is the mirror mode. Most commonly, the quasi-perpendicular mirror mode (with wave vector k roughly perpendicular to the background magnetic field B(sub zero) is observed. In two events the quasi-parallel mirror mode k parallel B(sub zero) was identified.

Denton, Richard E.↗

NASA NTRS1972

Time-spectral characteristics of large-scale cloud systems in the tropical Pacific.

Study of time series of areal cloud coverage over the central Pacific to determine the large-scale variations of tropical disturbance activity for a four-month period in 1967. The persistent and fluctuating components are found to be more prevalent in regions of high and low mean cloudiness, respectively, with major longitudinal variations in activity most evident in the Southern Hemisphere. Normalized power spectra show that the transient activity occurring in the equatorial zone is dominated by shorter periods of about four days, while longer periods are dominant away from the equator. Coherency magnitudes and phases between different locations indicate that propagating cloud systems are most identifiable at lower frequencies in the Southern Hemisphere, and at somewhat higher frequencies in the Northern Hemisphere. Activity with periods in excess of five days consists largely of both westward and eastward propagation on the planetary wave scale. The shorter periods appear to consist of both synoptic-scale and planetary-scale modes, with westward propagation dominant near the equator.

Sikdar, D. N.↗

NASA NTRS1994

Identification of low-frequency fluctuations in the terrestrial magnetosheath

On the basis of magnetohydrodynamic (MHD) theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-beta anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.

Song, P.↗

NASA NTRS1987

An analysis of electromagnetic coupling and eigenfrequencies for microwave electrothermal thruster discharges

This paper discusses the basic approach to developing a comprehensive electromagnetic model for a microwave electrothermal discharge. The discharge and surrounding region are modeled as a section of waveguide loaded with a uniform, cold, lossy plasma. A characteristic equation has been derived describing all possible system modes. In particular the TM(01) mode is numerically solved. Propagation and attenuation constants for the TM(01) mode are found numerically for a given range of electron-neutral collision frequencies and plasma densities. Results are used to interpret a number of experimental observations in helium. Specifically, they are used to explain high coupling efficiencies and ease of cavity tuning at high pressures.

Frasch, L. L.↗

NASA NTRS1990

Low-frequency radio emissions at Neptune

The Voyager 2 plasma wave receiver detected weak radio emissions from Neptune's magnetosphere in the frequency range of 3 - 60 kHz. The emissions occurred in bursts lasting for typically 1.5 hours, often occurring twice per planetary rotation. Most of these radio bursts were detected within several degrees of the magnetic equatorial plane. During the passage through the magnetosphere, electrostatic upper hybrid resonance bands were observed close to the magnetic equator in conjunction with intensifications of the radio emissions at frequencies close to and above the upper hybrid bands. Further, near closest approach, the radio emissions were observed to cross the right-hand cutoff frequency with no apparent attenuation. It is concluded that the Neptunian radio emissions below about 60 kHz are produced by mode conversion from the upper hybrid waves and propagate in the ordinary mode into beams within about 12 deg of the magnetic equator. There is also evidence of an extraordinary mode emission at about 60 kHz which is apparently generated by an entirely different source from the escaping continuum radiation.

Kurth, W. S.↗

NASA NTRS1980

Examination of group-velocity criterion for breakdown of vortex flow in a divergent duct

A group-velocity criterion for vortex breakdown implied by wave trapping theory is applied to vortex flows in a slightly divergent duct that exhibits breakdown. The group velocities for both symmetric (n = 0) and nonsymmetric (n = plus or - 1) modes of wave propagation are calculated for the experimental data. It is found that the flow ahead of the breakdown region is always supercritical and stable to these modes of disturbances. However, the flow field behind the breakdown region may be either supercritical or subcritical to the modes n = 0 and n = 1, and always supercritical to mode n = -1. The flow field behind this breakdown region is unstable to the asymmetric mode disturbance (n = 1) for a finite range of wavenumbers. The calculated frequencies of the unstable disturbances are in good agreement with the frequencies obtained from the experimental measurements.

Tsai, C.-Y.↗