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NASA NTRS1982

Acoustic waves in supernova remnants

When the Mach number of a supernova shock wave, propagating through the hot interstellar gas, is less than the critical value 2.76, the interaction of the shock with an interstellar cloud produces a reflected pressure pulse that propagates away from the cloud as an acoustic wave. A preliminary analysis indicates that about 4% of the energy of an exploding supernova shell is converted into such waves. When the postshock gas temperature exceeds 2 x 10 to the 6th K, waves as short as 6 pc are damped in less than a wavelength, returning the energy to the hot gas. Longer waves, especially at late stages, are less strongly damped, and are superposed in the hot gas to create a fluctuating magnetoacoustic field, with typical periods of about 100,000 years. In the warm neutral clouds at temperatures of about 8000 K these waves are rapidly damped by plasma slip (ambipolar diffusion), providing a heat source that may account for the temperatures of these clouds.

Spitzer, L., Jr.↗

NASA NTRS1982

The effects of moisture on trapped mountain lee waves

The effects of latent heat release on the dynamics of mountain lee waves are examined with the aid of two-dimensional numerical simulations, for several situations in which the Scorer parameter has a nearly two-layer vertical structure. Changes in the moisture in the lowest layer are found to produce three fundamentally different behaviors: (1) resonant waves in an absolutely stable environment are distorted and untrapped by an increase in moisture; (2) resonant waves in a conditionally unstable layer are destroyed by an increase in moisture; and (3) resonant waves in a moist environment are detuned by a decrease in moisture. Changes in the humidity in the upper layer are found to amplify or damp the wave response, depending on the depth of the lower layer. In most situations, the wave response is significantly more complicated than that predicted by simply replacing the dry stability with an equivalent moist stability in the saturated layer.

Durran, D. R.↗

NASA NTRS1983

Stationary waves in the winter stratosphere - Seasonal and interannual variability

The zonal mean wind and stationary waves 1 and 2 in the January 1973 - February 1977 northern winter stratosphere are determined, on the basis of monthly average conventional and satellite data, by diagnosing wave propagation with the aid of the Eliassen-Palm (1961) flux and the refractive index-squared. Both the seasonal trend and the interannual variability of the amplitude of wave 1 reflect variations in zonal mean wind and the corresponding refractive index. During each year studied, a midwinter minimum in wave 1 amplitude occurs at and above 1 mb as a consequence of a midwinter curvature maximum. In three of the four sudden warmings of wave 1 type which occurred during the study period, the month prior to the warming was characterized by abnormally weak winds near the climatological mean jet, a detached secondary jet at low and high altitude, and abnormally large wave 1 amplitude throughout the upper and middle stratosphere.

Smith, A. K.↗

NASA NTRS1983

Upstream waves and particles /Tutorial Lecture/

The plasma waves, MHD waves, energetic electrons and ions associated with the proximity of the region upstream from terrestrial, planetary and interplanetary shocks are discussed in view of observations and current theories concerning their origin. These waves cannot be separated from the study of shock structure. Since the shocks are supersonic, they continually overtake any ULF waves created in the plasma in front of the shock. The upstream particles and waves are also of intrinsic interest because they provide a plasma laboratory for the study of wave-particle interactions in a plasma which, at least at the earth, is accessible to sophisticated probing. Insight may be gained into interstellar medium cosmic ray acceleration through the study of these phenomena.

Russell, C. T.↗

NASA NTRS1983

Apparent electrostatic ion cyclotron waves in the diffuse aurora

Emissions that have properties consistent with electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. Peaks were observed in the power spectrum of the electric field near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H(+) and O(+) EIC waves. The two analyses indicated that both emission bands were due to waves propagating eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting Landau damping require that the source be local. Magnetometer data indicated the presence of a downward parallel current density of 5 microamps/sq m. Sufficient free energy for the waves was available from this current, although the waves were observed frequently at altitudes where the ion-neutral collision frequency exceeded the oxygen cyclotron frequency.

Bering, E. A.↗

NASA NTRS1983

Saturn's magnetosphere - Observations of ion cyclotron waves near the Dione L shell

The mode of propagation and generational mechanism of quasi-periodic waves shown to be present near the Dione L shell by high time resolution measurements made with the Pioneer 11 vector helium magnetometer are investigated. The waves have a characteristic period of 18 s, well below the proton gyroperiod, and a typical amplitude of 5 nT. Ion cyclotron resonance of the waves with the dominant ions appears capable of generating the waves. Theoretical arguments based on the growth rates of the waves suggest that O(2+) is more likely to be responsible than O(+), but that a resonance involving H(+) ions with energies of a few keV cannot be excluded. Pitch angle scattering of heavy ions by the waves should cause precipitation with the possible production of an aurora near 67 deg latitude.

Smith, E. J.↗

NASA NTRS1983

VLF wave injections from the ground

Experiments on wave-particle interactions using VLF whistler-mode waves injected into the magnetosphere from Antartica are described. The injected signals are single-frequency coherent waves whose amplitudes and frequencies may be changed slowly with time, or else two or more coherent wave trains transmitted simultaneously to determine the nature of the response to multifrequency excitation. The waves may be amplified 30 dB or more and may trigger intense emissions having bandwidths that vary from a few to several hundred Hertz. In most cases significant growth and triggering occur only when the driving signal is essentially monochromatic (bandwidth 10 Hz). If two frequencies are transmitted simultaneously the signal at the lower frequency tends to be suppressed by 20 dB or more. These results are interpreted in terms of a feedback interaction between the waves and counter-streaming cyclotron resonant electrons in a region several hundred wavelengths long, centered on the magnetic equator.

Helliwell, R. A.↗

NASA NTRS1983

900 km of digital ocean wave spectra from the SEASAT SAR

One of the primary reasons for including the SAR in the complement of ocean microwave instruments on SEASAT was the global ocean wave spectra. An accurate, timely, and global knowledge of the full two dimensional or directional surface wave spectra has great value in both operational wave forecasting and in wave climatology, especially in the Southern Oceans. The sparse sampling obtainable from a single orbiting SAR would by itself be inadequate to reconstruct a global wave field, but when supplemented with auxiliary knowledge of the winds, the SAR might provide essential updates to a global wave model.

Beal, R. C.↗

NASA NTRS1984

SAR imagery of ocean-wave swell traveling in an arbitrary direction

The intensity wave like patterns observed in Synthetic Aperture Radar (SAR) are known to be caused by two mechanisms: the microwave radar cross sectional amplitude modulation due to tilt and hydrodynamic interaction of the long ocean waves, and intensity modulation due to the motion of the long ocean waves. Two dimensional closed form expressions of intensity wave patterns based on ocean wave swell are developed. They illustrate the relative importance of the amplitude and motion modulations; they also show that velocity bunching and a distortion due to the phase velocity of the ocean wave field are independent of the focus adjustment, provided that the second order temporal effects are neglected. Second order effects are small only over a limited range of ocean/radar parameters.

Rufenach, C. L.↗

NASA NTRS1983

Strong turbulence and atmospheric waves in stellar occultations

General techniques for producing model lightcurves for a variety of realistic atmospheric irregularities, including turbulence and inertia-gravity waves, are presented and applied. The restrictions of weak scintillation theory are relaxed and model lightcurves are constructed using wave optics for atmospheres with strong, anisotropic turbulence. This is accomplished by numerical simulations which model the propagation of a wave through a phase-changing screen while maintaining complete amplitude and phase information from the wave. The results are compared with available weak scintillation theory and with recent occultation data. The effects of large scale atmospheric waves with realistic horizontal structure are examined, and the reliability of the numerical inversion method in retrieving the true atmospheric vertical structure under conditions of strong ray crossing and horizontal inhomogeneities is assessed. The nature of model lightcurve spikes generated using geometric optics and wave optics are compared.

French, R. G.↗

NASA NTRS1983

L band SAR ocean wave observations during Marsen

The five-sided flight pattern made it possible to view the same ocean surface patch from five different directions. The results obtained from analyzing radar and in situ measurements indicate that the focus dependence for optimum imaging is that for a surface moving with a speed that is approximately equal to the wave phase velocity. The finding also suggest that azimuthally traveling waves can be visible as range traveling waves when the necessary focus adjustments are made in the SAR processor and that the visibility of azimuthally travelling waves does not improve with decreasing integration time. What is more, the spectra and images of azimuthally traveling waves do not show observable distortions compared with those for range traveling waves.

Jain, A.↗

NASA NTRS1983

Macroscopic Lagrangian description of warm plasmas. II Nonlinear wave interactions

A macroscopic Lagrangian is simplified to the adiabatic limit and expanded about equilibrium, to third order in perturbation, for three illustrative cases: one-dimensional compression parallel to the static magnetic field, two-dimensional compression perpendicular to the static magnetic field, and three-dimensional compression. As examples of the averaged-Lagrangian method applied to nonlinear wave interactions, coupling coefficients are derived for interactions between two electron plasma waves and an ion acoustic wave, and between an ordinary wave, an electron plasma wave, and an ion acoustic wave.

Kim, H.↗

NASA NTRS1983

Wave structure in the Venus ionosphere downstream of the terminator

In the lower ionosphere of Venus, just nightward of the terminator, instruments on the Pioneer Venus Orbiter have revealed nearly coherent wave trains in the electron density, N(e), temperature, T(e), and in the east-west component of the magnetic field, B(E). These waves exist primarily below 200 km. They have north-south wavelengths of the order of 150 km and amplitudes in N(e) and T(e) of about a factor of 2 or 3. B(E) has an amplitude of about 30 nT but no net value averaged over the waves. A unique phase relationship exists between these three parameters. N(e) and T(e) vary approximately inversely, suggesting that the waves represent vertical plasma motions. N(e) maxima and minima tend to occur at zero crossings of B(E), i.e., within regions of vertical current. The wave energy is believed to be derived from the steep plasma pressure gradient at the terminator which accelerates ionospheric plasma nightward. The generation process is unknown, but it may involve gradient driven interchange instabilities, or shear instabilities produced by ion-neutral drag at lower altitudes. Whatever their origin, the waves are important because they represent an energy sink for the transterminator flow that is largely responsible for the maintenance of the nightside ionosphere.

Brace, L. H.↗

NASA NTRS1984

Magnetosonic waves adjacent to the plasma sheet in the distant magnetotail - ISEE-3

The presence of large-amplitude magnetosonic waves in the distant plasma sheet boundary layers is demonstrated, and their properties are discussed. The waves for almost all the events examined are right-hand circularly polarized, with the degree of polarization varying from 1.04 to 1.87, i.e., from circular to slightly elliptical. The waves are all propagating nearly along the magnetic field. Their periods vary from 3.7 seconds to as long as 35 seconds, with a median value of about 10 seconds, and are slightly larger than the proton gyroperiod in all but two cases. Most waves have a relatively large compressional component varying from 30 to 90 percent of the wave amplitude. Several possible generation mechanisms are discussed, along with the role that the waves may play in the heating and expansion of the plasma sheet.

Tsurutani, B. T.↗

NASA NTRS1984

Mechanics of a strong subsynoptic gravity wave deduced from satellite and surface observations

The three-dimensional structure and dynamics of a strong, tropospheric gravity wave event continuously observed from both satellite and surface facilities reveal the subsynoptic character of this 'wave of depression', suggesting that the gravity wave originated in the upper troposphere near a jet streak, and, being quasi-hydrostatic, was relatively nondispersive and long lived. Theory is applied in order both to explain and estimate gravity wave properties throughout the troposphere. Further improvements in the descriptions presented may require the development of solitary wave theory for deep depression waves in shear flow.

Pecnick, M. J.↗

NASA NTRS1984

Kelvin wave packets and flow acceleration - A comparison of modeling and observations

Atmospheric Kelvin waves, as revealed by temperatures obtained from the recent Limb Infrared Monitor of the Stratosphere (LIMS) experiment, commonly occur in packets. A simple two-dimensional gravity-wave model is used to study the upward propagation of these packets through different zonal mean wind profiles derived from the LIMS data. The observed prevalence of high frequency waves in the lower mesosphere and low frequency waves in the lower stratosphere can be exlained by dispersion of energy associated with the range of frequencies comprising a packet. Dominant wave frequencies at upper and lower levels are more distinctly separated if the packet propagates through a layer of westerly winds. Due to dispersion and shear effects, a packet of short temporal length at low levels will have a considerably extended impact on a layer of westerly winds at higher levels. Observed and modeled westerly accelerations resulting from packet absorption occur in the same layer, and are similar in magnitude and duration. These results support the theory that Kelvin waves are responsible for the westerly phase of the semiannual oscillation.

Coy, L.↗

NASA NTRS1984

SAR internal wave signature experiment

Internal wave and surface wave observations by research vessels will be conducted in conjunction with aircraft SAR images. Synthetic Aperture Radar Internal Wave Signature Experiment (SARSEX) provides an opportunity to acquire aircraft and spacecraft SAR imagery of internal waves with appropriate sea truth data. Objectives are to investigate basic mechanisms (both hydrodynamic and electromagnetic) responsible for internal wave signatures and quantitatively test theories and models for predicting internal wave signatures from given ocean and radar parameters.

Winokur, R. S.↗

NASA NTRS1984

Nonlinear evolution of astrophysical Alfven waves

Nonlinear Alfven waves were studied using the derivative nonlinear Schrodinger equation as a model. The evolution of initial conditions, such as envelope solitons, amplitude-modulated waves, and band-limited noise was investigated. The last two furnish models for naturally occurring Alfven waves in an astrophysical plasma. A collapse instability in which a wave packet becomes more intense and of smaller spatial extent was analyzed. It is argued that this instability leads to enhanced plasma heating. In studies in which the waves are amplified by an electron beam, the instability tends to modestly inhibit wave growth.

Spangler, S. R.↗