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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Lower hybrid waves excited through linear mode coupling and the heating of ions in the auroral and subauroral magnetosphere

Two related aspects of the linear mode conversion mechanism are considered: (1) the controlled heating of suprathermal ions in the ionosphere and magnetosphere over a powerful VLF/ELF transmitter using lower hybrid (LH) waves excited by the electromagnetic (EM) transmitter signals through linear mode conversion; (2) the excitation of intense LH waves in the auroral regions through the linear conversion of VLF/ELF EM auroral hiss, and the subsequent heating of ions by the excited LH waves. Experimental results are used to demonstrate that strong LH waves can be excited by EM waves down to frequencies as low as 2 kHz in the subauroral low-altitude magnetosphere. It is also shown that an approximately 250 kW VLF/ELF transmitter operating in the subauroral region at 2 kHz could excite sufficiently intense LH waves to heat suprathermal 1 eV H(+) ions and 16 eV O(+) ions to roughly 50 eV in a region extending in altitude from 1000 to 5000 km above the transmitter with a horizontal scale of about 500 km. If coherent wave stochastic heating occurs, the O(+) energy gain could be as large as 200 eV.

Bell, T. F.↗
NASA NTRS1992

Observations of a new class of upstream waves with periods near 3 seconds

A new class of ULF waves with periods near 3 s in the earth's upstream region is found by examining the high time resolution magnetic field data from the ISEE spacecraft. These waves are observed in the part of the upstream region which is magnetically connected to the bow shock, but only when the solar wind plasma beta is high (greater than 1). The waves are always right-handed, nearly circularly polarized in the spacecraft frame. The directions of the wave vectors are in the general direction of the average magnetic field, and the waves are convected downstream in the spacecraft frame. This study of these waves has shown that they appear to be intrinsically left-handed ion cyclotron waves in the plasma rest frame.

Le, G.↗
NASA NTRS1992

Electron acceleration by Alfven waves in the magnetosphere

The self-consistent electron kinetics of Alfven waves on the electron inertial scale is studied using a 2D hybrid-kinetic description. The ions follow a fluid description for Alfven waves at frequencies below the ion cyclotron frequency. The parallel electron dynamics is treated kinetically using particle-in-cell techniques. In this model, the electron plasma mode is eliminated, and only the physics of the Alfven waves is retained. At sufficiently large amplitudes, it is found that oblique Alfven waves break due to finite electron inertia in a cold plasma. The consequence of wave breaking is the formation of an electron beam which can be unstable to the beam-plasma instability. The electrons supporting the parallel current thermalize into a non-Maxwellian distribution with an energetic tail up to several keV, assuming a reasonable magnetospheric Alfven speed. In hot plasma simulations, electron trapping is the principal mechanism of electron acceleration. It is proposed that wave breaking or electron trapping of oblique Alfven waves at 1 R(E) can result in electron acceleration and may explain some observed auroral phenomena.

Hui, C.-H.↗
NASA NTRS1992

An assessment of lightning or in situ instabilities as a source for whistler mode waves in the night ionosphere of Venus

Low-altitude data acquired on orbits 484-526 of the Pioneer Venus Orbiter are used to assess the most probable source of whistler mode waves in the nightside ionosphere of Venus. The effect of electron temperature on whistler mode propagation is discussed, with particular emphasis on the growth rate of gyroresonant instabilities. The wave intensity as observed with the Pioneer Venus Orbiter electric field detector is compared with various plasma parameters to determine if it is likely that the waves are generated by an in situ instability. It is shown that gyroresonant whistler mode instabilities do not explain the Pioneer Venus wave data. It is demonstrated that the magnetic field strength controls the wave intensity, rather than density, and it is concluded that the electron Beta(e) is the primary factor in the occurrence of the 100-Hz waves detected in the nightside ionosphere of Venus. The present analysis supports the interpretation that these waves are due to lightning in the Venus atmosphere.

Strangeway, R. J.↗
NASA NTRS1992

Wave diffraction in weak cosmic-ray-modified shocks

Weakly multidirectional, long-wavelength cosmic-ray-modified shocks are studied via multiple scale perturbation techniques. The effects of diffraction are discussed in terms of Green's function solutions of the linearized 1 + 3D Burgers and 1 + 3D KdVB equations, and also in terms of solutions with singular Dirac delta initial distributions. The solutions show a monotonic decrease of the wave-front curvature with increasing time owing to the effects of wave diffraction. The shape of the wave surface is discussed in terms of solutions S to the wave eikonal equation corresponding to singular initial conditions. For the fast magnetosonic wave propagating in the positive x-direction, the wave phase surface S = 0 has elliptic cross sections with the planes x = constant and has a convex paraboloidal shape. Plane-wave solutions of the 1 + 3D KdVB equation are discussed.

Webb, G. M.↗
NASA NTRS1992

Plasma wave phenomena observed at interplanetary shocks by the Ulysses URAP experiment

Results of a study of 24 interplanetary shocks observed by the Unified Radio and Plasma Wave Experiment (URAP) on the Ulysses spacecraft are presented. These shocks, observed between approximately 1 and 4 AU, display a variety of wave phenomena similar to those detected in earlier studies of shocks near 1 AU. The correspondence of the observed low frequency magnetic and electric field waves with the parallel index of refraction for whistler waves was investigated. Observed B/E ratios are found to be typically about a factor of 0.7 times the computed index of refraction, supporting the whistler interpretation of these waves, but also implying a prevalent electrostatic wave component which may be due to whistlers propagating at an angle to the interplanetary magnetic field. A statistical correlation of the amplitudes of the various types of waves with shock and solar wind properties is presented.

Lengyel-Frey, D.↗
NASA NTRS1992

Lower stratosphere waves during 1986-1989 southern springs

Global NMC height analyses and Nimbus 7 TOMS Grid-T Version 6 data are used to investigate the horizontal structure of ozone and 50-mb height waves for the 1986-1989 springs of the Southern Hemisphere. Empirical orthogonal functions (EOFs) are obtained for the ozone and height data independently to identify the meridional structure of observed long waves as well as their propagation characteristics. The leading EOFs for both the ozone and 50-mb height waves exhibit wavenumber 1 and 2 patterns. The correlation coefficients between ozone and 50-mb height principal components are high. They indicate high/low ozone values associated with high/low values of 50-mb heights. This distribution is consistent with the advection of ozone by planetary waves south of the ozone maximum, as shown by a simple linear wave model. The observed total ozone amplitudes for waves 1 and 2 for most years are consistent with the total ozone predictions by the wave model.

Nogues-Paegle, Julia↗
NASA NTRS1993

Nonlinear waves and solitons in molecular clouds

We begin a study of nonlinear wave phenomena in molecular clouds. These clouds exhibit highly nonlinear structure that is often described in terms of 'clumps' and 'filaments' which are bouncing around, twisting, and colliding within the cloud. These clouds are important because they ultimately produce the initial conditions for the star formation process. Our motivation is to explore the possibility that solitons (i.e., spatially localized, single-hump wave entities which often exhibit remarkable stability) can live in these molecular clouds and produce their observed structure. In this paper we focus on the case of one spatial dimension, and we show that a rich variety of nonlinear waves can exist in molecular cloud fluid systems (where self-gravity is included). We show that in the absence of magnetic fields no true soliton solutions are allowed, although highly nonlinear waves (whose crests become widely spaced and thus soliton-like) do exist. For clouds with embedded magnetic fields, we derive a model equation which describes the behavior of wave phenomena; this model equation allows solutions which correspond to nonlinear waves, solitons, and topological solitons. We briefly consider the stability of these wave entities and discuss the possible role they play in molecular cloud dynamics.

Adams, Fred C.↗
NASA NTRS1993

Wave generation in a sunspot

In this paper we analyze the generation of waves in a sunspot by extending Stein's hydrodynamic approach to the turbulent medium permeated by a strong uniform magnetic field oriented parallel to the gravity. For wave sources appropriate to the sunspot, we consider magnetic perturbations and entropy changes as well as turbulent convection. To describe the anisotropy imposed by the sunspot, we use a one-dimensional correlation function relating the turbulent eddies separated along the symmetry axis of the spot. This treatment yields several interesting possibilities for wave generation in a sunspot. First, it is demonstrated that the entropy change and magnetic perturbation can lead to a relative enhancement of acoustic wave emission. Second, the energy flux of Alfven waves may be comparable to that of acoustic waves in sunspots. Third, the anisotropy of the sunspot dynamics can lead to wave energy spectrum in a form which may explain the origin of umbral atmospheric oscillations.

Lee, Jeongwoo W.↗
NASA NTRS1992

Alfven wave reflection and heating in coronal holes - Theory and observation

We present evidence for significant reflection of Alfven waves in an isothermal, hydrostatic model corona and that heating in coronal holes is provided by Alfven waves. For Alfven waves with periods of 5 min, upward propagating waves are reflected if the temperature is less than 10 exp 6 K, but escape into the solar wind if the temperature is greater than 10 exp 6 K. This sensitive temperature dependence may provide the self-limiting mechanism that has been suspected to exist because the reflected waves result in heating which raises the temperature which, in turn, decreases the reflection. The reflection occurs mostly inside of about 6 solar radii, depending on temperature, wave period, and magnetic field strength and geometry. The importance of this process has often been overlooked due to a poor choice of coronal Alfven speed and temperature. SOHO is well-suited to measure whether the required properties for reflection exist. Solar Probe, however, is the only definitive experiment to show if the waves actually exist to the degree necessary.

Suess, S. T.↗
NASA NTRS1993

The cutoff frequency for fast-mode magnetohydrodynamic waves in an isothermal atmosphere with a uniform horizontal magnetic field

This study analytically examines conditions for reflection of MHD fast-mode waves propagating upward in an isothermal atmosphere. A new method of transforming the linearized wave equation into Klein-Gordon form is utilized to calculate a local cutoff (critical) frequency for these waves. This critical frequency determines the height in the atmosphere at which reflection dominates and above which wave propagation is effectively cut off. Comparison of our results to those previously obtained shows that earlier calculations of the critical frequency for MHD fast mode waves were done incorrectly. The results may be helpful in explaining the short-period end of the spectrum of the solar global p-mode oscillations. They may also be important in studies of wave propagation and wave trapping in highly magnetized stellar atmospheres.

Stark, B. A.↗
NASA NTRS1993

On the generation of plasma waves in Saturn's inner magnetosphere

Voyager 1 plasma wave measurements of Saturn's inner magnetosphere are reviewed with regard to interpretative aspects of the wave spectrum. A comparison of the wave emission profile with the electron plasma frequency obtained from in situ measurements of the thermal ion density shows good agreement with various features in the wave data identified as electrostatic modes and electromagnetic radio waves. Theoretical calculations of the critical flux of superthermal electrons able to generate whistler-mode waves and electrostatic electron cyclotron harmonic waves through a loss-cone instability are presented. The comparison of model results with electron measurements shows excellent agreement, thereby lending support to the conclusion that a moderate perpendicular anisotropy in the hot electron distribution is present in the equatorial region of L = 5-8.

Barbosa, D. D.↗
NASA NTRS1993

Wave-turbulence dynamics in the stably stratified boundary layer

New data obtained at the Boulder Atmospheric Observatory are analyzed to obtain separation of wave, turbulence, and mean field necessary for a complete treatment of wave-turbulence interaction. The data were compared with a linear stability analysis of the background atmospheric state, showing good agreement between measured wave parameters (such as wavelength, period, and vector phase velocity) and the eigenvalues of the linear solution. The analysis of the budgets of wave heat flux and temperature variance revealed the essential role of wave-turbulence interaction in maintaining a large amplitude temperature wave and countergradient heat flux. A mechanism for the maintenance of turbulence by waves in strongly stratified boundary layers is described, which emphasizes that the time-mean Richardson number is an irrelevant parameter at such times.

Einaudi, F.↗
NASA NTRS1992

Simulations of the seasonal evolution of planetary waves in the stratosphere

Results are presented of experiments performed with the 3D primitive equation model of the middle atmosphere to elucidate the connections between the behavior of planetary waves in the stratosphere and in the upper troposphere during the seasonal cycle. In these experiments the geopotential height fields at the model's lower boundary consisted of a time-varying zonally symmetric component plus a time-independent wave 1. Experiments were also performed with the same time-varying zonally symmetric component plus a time-independent combination of wave 1 and 2. In each of the wave 1-only experiments, the wave 1 amplitude in the middle stratosphere has two maxima, one in fall and the other in spring, separated by a midwinter minimum. It is suggested that some important transient features in the seasonal evolution of stratospheric planetary waves do not depend on time variations in the amplitude or phase of the corresponding tropospheric waves.

Farrara, John D.↗
NASA NTRS1993

Solitary waves in the atmosphere

The weakly nonlinear theory for internal solitary waves is reviewed, and theoretical results of the vertical and horizontal structure of temperature, vertical displacements, and vertical and horizontal perturbations to the wind field associated with steadily propagating solitary waves are presented in two idealized atmospheric configurations. One configuration is representative of solitary waves observed in the lower troposphere and the other of solitary waves that occupy the entire troposphere. The important results of the theory are presented in a form that can be readily used by observationalists. The results obtained are then analyzed using actual rawinsonde data for two well-documented observations of atmospheric solitary waves, which are analogous to the two idealized configurations. The importance and difficulties of properly identifying the waveguide within which the solitary wave is confined are discussed. The fundamental role of a critical level in ducting the disturbances and thus in defining the thickness of the waveguide is illustrated in the example dealing with the solitary wave occupying the entire troposphere. Together, these two examples illustrate the decisions and compromises that must be made in applying the theory to the real atmosphere.

Rottman, James W.↗
NASA NTRS1993

High order hybrid numerical simulations of two dimensional detonation waves

In order to study multi-dimensional unstable detonation waves, a high order numerical scheme suitable for calculating the detailed transverse wave structures of multidimensional detonation waves was developed. The numerical algorithm uses a multi-domain approach so different numerical techniques can be applied for different components of detonation waves. The detonation waves are assumed to undergo an irreversible, unimolecular reaction A yields B. Several cases of unstable two dimensional detonation waves are simulated and detailed transverse wave interactions are documented. The numerical results show the importance of resolving the detonation front without excessive numerical viscosity in order to obtain the correct cellular patterns.

Cai, Wei↗
NASA NTRS1993

Planetary plasma waves

The primary types of plasma waves observed in the vicinity of the planets Venus, Mars, Earth, Jupiter, Saturn, Uranus, and Neptune are described. The observations are organized according to the various types of plasma waves observed, ordered according to decreasing distance from the planet, starting from the sunward side of the planet, and ending in the region near the closest approach. The plasma waves observed include: electron plasma oscillations and ion acoustic waves; trapped continuum radiation; electron cyclotron and upper hybrid waves; whistler-mode emissions; electrostatic ion cyclotron waves; and electromagnetic ion cyclotron waves.

Gurnett, Donald A.↗
NASA NTRS1993

Detection of traveling ionospheric disturbances induced by atmospheric gravity waves using the global positioning system

Natural and man-made events like earthquakes and nuclear explosions launch atmospheric gravity waves (AGW) into the atmosphere. Since the particle density decreases exponentially with height, the gravity waves increase exponentially in amplitude as they propagate toward the upper atmosphere and ionosphere. As atmospheric gravity waves approach the ionospheric heights, the neutral particles carried by gravity waves collide with electrons and ions, setting these particles in motion. This motion of charged particles manifests itself by wave-like fluctuations and disturbances that are known as traveling ionospheric disturbances (TID). The perturbation in the total electron content due to TID's is derived analytically from first principles. Using the tilted dipole magnetic field approximation and a Chapman layer distribution for the electron density, the variations of the total electron content versus the line-of-sight direction are numerically analyzed. The temporal variation associated with the total electron content measurements due to AGW's can be used as a means of detecting characteristics of the gravity waves. As an example, detection of tsunami generated earthquakes from their associated atmospheric gravity waves using the Global Positioning System is simulated.

Bassiri, Sassan↗