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At least 415 records · Page 23

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NASA NTRS1978

An instability of finite amplitude circularly polarized Alfven waves

A demonstration is presented that a finite-amplitude circularly-polarized Alfven wave is generally unstable in a MHD fluid. The wave decays by a four-wave coupling process in which the daughter waves are forward propagating random density and magnetic fluctuations and a backward-propagating magnetic wave. For parameters typical of the solar corona and the solar wind (thermal to magnetic energy density ratios between 0.1 and 1, and values between 0.1 and 0.9 for the ratio of magnetic energy density of the initial Alfven wave to that of the background magnetic field), large decay rates are found.

Goldstein, M. L.↗

NASA NTRS1977

Amplitude scintillation at 2 and 30 GHz on earth space paths

Extensive amplitude scintillation measurements have been made simultaneously at 2.075 and 30 GHz on earth-space propagation paths. These measurements were performed as the Applications Technology Satellite (ATS-6) was moved slowly from a synchronous position over India to a new synchronous position over the United States. The variance, path loss, covariance, and spectra are discussed as functions of the path elevation angle. These results are also compared with earlier simultaneous scintillation measurements at 20 and 30 GHz during the movement of ATS-6 to its position over India.

Hodge, D. B.↗

NASA NTRS1978

Collinear interaction of noise with a finite-amplitude tone

The interaction between a low-level noise band and a finite-amplitude tone is studied. Experiments are performed in the air and in a plane-progressive tube. Three cases are considered: where the tone is located at the low end of a three-octave noise band, where the tone is at the upper end of a narrower noise band, and where the tone is over an octave above a one-octave noise band. It is found that the noise portion of the spectrum becomes broadband and flat at short distances. Both these characteristics become extensive before the tone reaches the shock-formation stage. The source of the high-frequency noise energy is identified as the tone and its harmonics rather than the original noise band. Good agreement is found between these results and calculations based on the Earnshaw solution. It is noted that the study is directly applicable to jet aircraft noise.

Webster, D. A.↗

NASA NTRS1978

A bandwidth compressive modulation system using multi-amplitude minimum shift keying /MAMSK/

A bandwidth compressive modem making use of multi-amplitude minimum shift keying (MAMSK) has been designed and implemented in a laboratory environment at microwave frequencies. This system achieves a substantial bandwidth reduction over binary PSK and operates within 0.5 dB of theoretical performance. A number of easily implemented microwave transmitters have been designed to generate the required set of 16 signals. The receiver has been designed to work at 1 Mbit/s and contains the necessary phase tracking, AGC, and symbol synchronization loops as well as a lock detector, SNR estimator and provisions for differential decoding. This paper describes this entire system and presents the experimental results.

Weber, W. J., III↗

NASA NTRS1978

A white-light amplitude interferometer with 180-degree rotational shear

The paper reports on the fabrication, alignment, and test techniques of an equal-path, wave-front, folding amplitude interferometer with 180-degree rotational shear. The device may be used for measuring atmospheric phase-perturbing properties and designing optical filters used in processing spatial information. The instrument has been used to photograph the Michelson Stellar interferometer fringes.

Breckinridge, J. B.↗

NASA NTRS1978

Small amplitude waves in a hot relativistic two-fluid plasma

The dispersion relation for small amplitude waves in a hot relativistic plasma embedded in a uniform magnetic field is derived. Four plasma modes exist for propagation oblique to the magnetic field, and the mode properties are summarized in a hot relativistic generalization of the cold plasma pond diagram of Stix (1962). The two-fluid approximation is considered, and two-fluid equations are applied in this procedure which summarizes properties of linear waves in an unbounded magnetized relativistic plasma by means of a parameter-space diagram of wave phase velocity.

Hyun, S.↗

NASA NTRS1979

An unusually large westerly amplitude of the quasi-biennial oscillation

A time-height section of the quasi-biennial oscillation is presented for the period 1950-1978. The data are from the Canal Zone station through June 1970 and from Kwajalein from July 1970 through April 1978 (both stations are near 9 deg N). The most striking feature in the new data is the unusually strong westerly phase which occurred in the winter of 1977-1978. The magnitude of the westerly amplitude in December 1978 was over 60 m/sec at and above 10 mb. The easterly phase below the westerly phase of the 1977-1978 winter was also unusually large. The other features in the new data are consistent with past observations.

Coy, L.↗

NASA NTRS1979

Large amplitude substorm motion of the magnetotail boundary

During a period of several days when the solar wind was relatively quiet, the magnetotail boundary near lunar distance swept back and forth past Explorer 33 as the spacecraft traveled through a distance perpendicular to the earth-sun line of about 17 earth radii. The boundary crossings are remarkably well-correlated with peaks in the AE index, even though the magnitudes of the peaks are only on the order of 200 nT. Examination of the regions of multiple crossings on other Explorer 33 and 35 orbits reveals that they commonly cover large distances perpendicular to the earth-sun line, although correlation with AE is often obscured, probably by external solar wind conditions. It is concluded that a substantial fraction of the scatter in published statistical plots of boundary crossing positions can be accounted for in terms of an internal substorm-related motion with an amplitude of 5-8 earth radii. The data suggest that the motion takes the form of a compressional deformation wave convecting down the tail.

Crooker, N. U.↗

NASA NTRS1979

A two-dimensional white-light amplitude interferometer

A review of the applications of a point-symmetric, 180-degree, rotational shearing interferometer is given. Material includes, photographs of the Michelson stellar interferometer fringes from Alpha-Lyr, measures of amplitude of phase excursions in the atmosphere, and a measure of the order of interference of speckle patterns. New material includes unpublished double star speckle patterns to illustrate the isoplanatic patch, a measure of the time fluctuations in the earth's atmosphere, and measurements of the properties of a 256 channel linear reticon.

Breckinridge, J. B.↗

NASA NTRS1979

Theory of the large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma

An exact solution of the kinetic and electromagnetic equations for a large-amplitude plane magnetoacoustic wave propagating transverse to the magnetic field in a hot collisionless plasma is presented. The solution gives simple relations among the magnetic-field strength, density, stress tensor, and plasma velocity, all of which are measurable in the interplanetary plasma. These relations are independent of the electron and ion velocity distributions, subject to certain restrictions on 'high-velocity tails.' The magnetic field of the wave is linearly polarized. The wave steepens to form a shock much as the analogous waves of MHD theory do.

Barnes, A.↗

NASA NTRS1981

Amplitude and angle of arrival measurements on a 28.56 GHz Earth-space path

The amplitude and angle of arrival measurements on an Earth-space path using the 28.56 GHz COMSTAR D3 satellite beacon are described. These measurements were made by the Ohio State University ElectroScience Laboratory during the period September 1978 to September 1979. Monthly, quarterly, and annual distributions of attenuation, angle of arrival, and variance of both these parameters are reported. During this period, fades exceeding 29 dB for .00% of the time and angle of arrival fluctuations exceeding .12 degrees for .01% of the time were observed.

Devasirvatham, D. M. J.↗

NASA NTRS1980

Radiation damping of long, finite-amplitude internal waves

Numerical solutions of a damped, nonlinear wave equation are presented. The equation describes the propagation of waves in a narrow thermocline or inversion which lose energy by exciting internal waves in the weakly stratified ambient environment. The results provide estimates for the persistence of finite-amplitude internal waves propagating in a thermoclinic waveguide.

Pereira, N. R.↗

NASA NTRS1980

Lunar near-surface shear wave velocities at the Apollo landing sites as inferred from spectral amplitude ratios

The horizontal-to-vertical amplitude ratios of the long-period seismograms are reexamined to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity-depth functions are well represented by two piecewise linear segments, although the presence of first-order discontinuities cannot be ruled out.

Horvath, P.↗

NASA NTRS1981

Solar cycle signal in air temperature in North America - Amplitude, gradient, phase and distribution

The considered investigation was motivated by three factors. One is related to an extension of single-channel MESA to multi-channel by Strand (1977), Morf et al. (1978), and Jones (1978). MESA is a high-resolution signal processing and spectrum analysis technique due to Burg (1975). The considered developments resulted in the discovery of the 11-year solar cycle signal in the change of the length of day by Currie (1980, 1981). They also led Currie (1981) to study the phase spectrum of the 11-year term in height H of sea level. The investigation tries to clarify the phase relations among the involved parameters. The second factor is connected with an application of the linear time domain technique used by Currie (1981) to temperature records to obtain more accurate information regarding the signal amplitude. The third factor of motivation is related to increases in the number of stations available for an analysis, the greater average length of the records, and the more accurate data set.

Currie, R. G.↗

NASA NTRS1981

Lightning amplitude spectra in the interval from 100 kHz to 20 MHz

The electric radiation fields produced by lightning return strokes, stepped leaders, and intracloud discharge processes have been Fourier-analyzed to determine amplitude spectra for these processes from about 100 kHz to 20 MHz. The fields were recorded under conditions where the lightning locations were known and where the field propagation from the lightning sources to the recording site was entirely over salt water. The spectra for return strokes show an f exp -1 frequency dependence from 100 kHz to 2 MHz, an f exp -2 dependence between 2 and 10 MHz, and an f exp -5 decrease above 10 MHz. In the 1 to 20 MHz range, the spectra of the initial fast transition in return strokes, the initial fast-rising portion of leader steps and the fast transitions in positive intracloud pulses are surprisingly similar.

Weidman, C. D.↗

NASA NTRS1981

Large amplitude stationary Rossby waves in the Southern Hemisphere

Observational evidence of the characteristics of the large amplitude, short wavelength stationary waves found in the lee of South America and Australia during the complete first special observing period of the first GARP global experiment is presented. The structure and evolution of the waves are described and their possible origin discussed.

Kalnay, E.↗

NASA NTRS1982

An experimental study of small-amplitude drop oscillations in immiscible liquid systems

Measurements of the characteristics of small-amplitude shape oscillations of drops immersed in a host liquid have been carried out by acoustical means. The resonance frequencies of the first few modes have been measured, as well as the damping constant for the fundamental mode, as functions of the drop radius and viscosities of the two liquids. A qualitative photographic study during steady oscillations has revealed a simple internal fluid-particle flow field with no circulation. The theory available at the present time has been found to provide results which are in general agreement with experimental findings for low-viscosity liquids.

Trinh, E.↗

NASA NTRS1982

A note on the limits of Rossby wave amplitudes

Quasi-geostrophic dynamics, as usually formulated, centers on the conservation of pseudopotential vorticity (PV) on isobaric surfaces. The present study has the objective to investigate some simple constraints which conservation of PV imposes on internal Rossby waves. Attention is given to some of the limitations that conservation of potential vorticity imposes on the dynamics of the stratosphere. Another possible constraint imposed by hydrodynamic stability is also discussed. The quantitative limitations these constraints put on the amplitudes of internal Rossby waves are assessed and compared with observations of the lower stratosphere.

Lindzen, R. S.↗