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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Measurements of potential of a cylindrical monopole antenna on a rotating spacecraft

Measurements of antenna potential on the Helios 1 spacecraft are presented and discussed. The behavior of the potential indicates that Lyman-alpha and shorter wavelengths play an important role in determining antenna and spacecraft potential, and that conversely, longer wavelengths are unimportant. As expected, a voltage spike usually occurs when the antenna is shadowed. However, when the spike is not present, the potential variations on an antenna which is not shadowed can be estimated. For this case, the spectrum is about (Fourier coefficient) 2 V x (2fo/f)2 where fo is the spacecraft spin frequency, and f is an even harmonic of the spin frequency. Suggestions for measuring low-frequency waves are briefly discussed in relation to the experimental results, and slight tipping of the spacecraft is recommended to avoid shadowing.

Kellogg, P. J.↗
NASA NTRS1979

Implicit finite-difference simulations of steady and unsteady transonic flows

Implicit methods for several fluid dynamic formulations have been developed and applied to steady-state and low-frequency transonic flows. The basic steps involved in the construction of implicit schemes include: selection of linearly stable accurate implicit difference operators, time-linearization of nonlinear terms, and approximate factorization of the implicit operators into easily solved systems of equations. The proposed schemes are found very efficient for the simpler formulations.

Ballhaus, W. F.↗
NASA NTRS1980

The structure of moderately strong winds at a mid-Atlantic coastal site /below 75m/

Velocity and temperature measurements were obtained from an instrumented 76-meter meteorological tower located near the Atlantic Ocean at Wallops Island, Virginia. The instrumentation consists of a slow-response cup-vane and resistance temperature system and a hot-film, thermocouple system for turbulence measurements. Results are presented for moderately strong winds from westerly directions (category I) and for on-shore winds from southerly directions (category II). Results from category I winds indicate the presence of low-frequency velocity fluctuations affecting all turbulence parameters similar to the observations made in Kansas and Minnesota. Winds of category II experience a change in surface roughness and surface temperature as they cross the coast line, resulting in the development of an internal boundary layer. The stable ocean air above the IBL shows extremely low turbulence levels (less than 3%). Because of the lack of turbulent mixing with adjacent layers, Coriolis effects are important and profiles with a maximum velocity at heights between 60-200 m exist.

Tieleman, H. W.↗
NASA NTRS1981

Plasma waves near Saturn - Initial results from Voyager 1

The Voyager 1 plasma wave instrument detected many familiar types of plasma waves during the encounter with Saturn, including ion-acoustic waves and electron plasma oscillations upstream of the bow shock, an intense burst of electrostatic noise at the shock, and chorus, hiss, electrostatic electron cyclotron waves, and upper hybrid resonance emissions in the inner magnetosphere. A clocklike Saturn rotational control of low-frequency radio emissions was observed, and evidence was obtained of possible control by the moon Dione. Strong plasma wave emissions were detected at the Titan encounter indicating the presence of a turbulent sheath extending around Titan, and upper hybrid resonance measurements of the electron density show the existence of a dense plume of plasma being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn.

Gurnett, D. A.↗
NASA NTRS1981

The statistical character of Pc 4 magnetic pulsations at synchronous orbit

A statistical study of 215 magnetic pulsation events identified on the basis of waveform and period as Pc 4 (45- to 150-s period) observed at synchronous orbit by ATS 6 is reported. The study was designed to be similar to that done with Pc 3 (10- to 45-s period) by Arthur et al. (1977). These Pc 4 are found to occur most often near 1800 local time but are observed at all local times. The dominant period is approximately 100 s, but a secondary peak in the distribution occurred at approximately 53 s. They tend to be nearly linearly polarized, principally transverse to the ambient magnetic field. The Pc 4 are found to divide naturally into two classes (radial and azimuthal) on the basis of the azimuth of the major axis of polarization. These classes divide further into low-frequency (less than or equal to 0.015 Hz) and high-frequency (greater than or equal to 0.015 Hz) classes. These four classes organize the remaining wave characteristics quite well. Division of the Pc 4 events into these classes does not lead to identification of a single source mechanism as it did with Pc 3. However, some correspondence between the classes and various proposed mechanisms does exist.

Arthur, C. W.↗
NASA NTRS1980

A high frequency silicon pressure sensor

Theoretical and design considerations as well as fabrication and experimental work involved in the development of high-frequency silicon pressure sensors with an ultra-small diaphragm are discussed. A sensor is presented with a rectangular diaphragm of 0.0127 cm x 0.0254 cm x 1.06 micron; the sensor has a natural frequency of 625 kHz and a sensitivity of 0.82 mv/v-psi. High-frequency results from shock tube testing and low-frequency (less than 50 kHz) comparison with microphones are given.

Kahng, S. K.↗
NASA NTRS1981

Influence of combined visual and vestibular cues on human perception and control of horizontal rotation

Measurements are made of manual control performance in the closed-loop task of nulling perceived self-rotation velocity about an earth-vertical axis. Self-velocity estimation is modeled as a function of the simultaneous presentation of vestibular and peripheral visual field motion cues. Based on measured low-frequency operator behavior in three visual field environments, a parallel channel linear model is proposed which has separate visual and vestibular pathways summing in a complementary manner. A dual-input describing function analysis supports the complementary model; vestibular cues dominate sensation at higher frequencies. The describing function model is extended by the proposal of a nonlinear cue conflict model, in which cue weighting depends on the level of agreement between visual and vestibular cues.

Zacharias, G. L.↗
NASA NTRS1980

Air transport flight parameter measurements program - Concepts and benefits

A program is described in which statistical flight loads and operating practice data for both narrow- and wide-body airline transport aircraft, intended primarily for use by manufacturers in updating design criteria, are obtained from existing, on-board digital flight data recorders. Procedures for editing and processing the data are explained, and differences between these and past NACA/NASA analog data are discussed. One major such difference is the automatic bandpass filtering of normal acceleration data to separate high-frequency gust response from low-frequency maneuver response. Plans and preliminary efforts for the development of an on-board data processing system, able to derive statistical aircraft operating parameters directly from real-time data, are also reviewed.

Morris, G. J.↗
NASA NTRS1981

Decameter studies of the 5 September 1973 flare

The spectra and positions of the meter-decameter wavelength radio sources associated with the flare of September 5, 1973 are discussed. The evolution of the size of the type II burst source is discussed and it is shown that it fluctuates by a factor of 10, or larger. Consequently, the potential and kinetic energies associated with the shock are uncertain by the same factor. By a comparison of the positions of the type II and type III sources, it is concluded that while the shock wave associated with the type II was guided along high loops, the type III electrons were injected along open field lines which diverged within a short height in the corona. The characteristics of a particularly interesting type III burst with a low-frequency cut-off are discussed. It is argued that nearby loops were not disrupted by the shock and that the energetic electrons produced during the event must have been injected at several sites and guided along open field lines at large distances from the flare to produce type III bursts.

Gergely, T. E.↗
NASA NTRS1981

Frequency band broadening of magnetospheric VLF emissions near the equator

The broadening of the whistler mode VLF emission band has frequently been observed by the equatorially orbiting S3-A (Explorer 45) satellite outside the midnight sector of the plasmasphere, during periods of geomagnetic disturbance. Prior to the broadening, the band of this emission is narrow with a sharp gap at the half electron gyrofrequency. The gradual broadening of the emission band on the low-frequency side is associated with the simultaneously observed spreading of the anisotropy of the ring current electrons to higher and wider energy ranges. Using the modeled distribution function, the linear growth rates of the cyclotron instability are calculated numerically. The results suggest that broadening of the VLF emission band near the plasmasphere can be caused by spreading of the ring current electron anisotropy toward higher energies.

Maeda, K.↗
NASA NTRS1981

An upper bound to the lightning flash rate in Jupiter's atmosphere

Lewis (1980) examined Voyager optical measurements and low-frequency radio-wave observations related to lightning discharges in Jupiter's atmosphere using whistler measurements from the plasma-wave system and a specific set of assumptions. Estimates of the average planetary lightning stroke rate were found to be from 0.04 to 0.0001 flashes per square kilometer per year. In the present paper, the planetary lightning rate is demonstrated to be as high as several tens of flashes per square kilometer per year. The same whistler data are used; however, different physical assumptions about the source area, including whistler paths and whistler amplitude distributions are incorporated.

Scarf, F. L.↗
NASA NTRS1981

Observations of rich clusters of galaxies at metre wavelengths

Observations have been made at 10 frequencies between 50 and 120 MHz of 17 rich, X-ray emitting clusters of galaxies with the 78 x 156 m dipole array al Llanherne. The observed flux densities were compared to the flux densities expected on the basis of the known discrete sources in the fields. In no case was a significant flux excess found that might have indicated the presence of a diffuse halo component of radio emission in the cluster. For those clusters in which spectral indices could be determined, the spectra all tend to be much steeper than is normal for extragalactic radio sources, although a strict correlation between the X-ray luminosity and the low-frequency radio luminosity or spectral index is not found. The occurrence of large halo sources such as that which is present in the Coma cluster seems to be quite unusual.

Cane, H. V.↗
NASA NTRS1981

DC-magnetic field vector measurement

A magnetometer experiment was designed to determine the local magnetic field by measuring the total of the Earth's magnetic field and that of an unknown spacecraft. The measured field vector components are available to all onboard experiments via the Spacelab command and data management system. The experiment consists of two parts, an electronic box and the magnetic field sensor. The sensor includes three independent measuring flux-gate magnetometers, each measuring one component. The physical background is the nonlinearity of the B-H curve of a ferrite material. Two coils wound around a ferrite rod are necessary. One of them, a tank coil, pumps the ferrite rod at approximately 20 kilohertz. As a consequence of the nonlinearity, many harmonics can be produced. The second coil (i.e., the detection coil) resonates to the first harmonic. If an unknown dc or low-frequency magnetic field exists, the amplitude of the first harmonic is a measure for the unknown magnetic field. The voltages detected by the sensors are to be digitized and transferred to the command and data management system.

Schmidt, R.↗
NASA NTRS1981

Radio Jupiter after Voyager - An overview of the planetary radio astronomy observations

An overview of Jupiter's low-frequency radio emission morphology as observed by the planetary radio astronomy (PRA) instrument onboard the Voyager spacecraft is presented. The PRA measurement capabilities and limitations are summarized, based on over two years of experience with the instrument. As a direct consequence of the PRA spacecraft observations, unprecedented in terms of their sensitivity and frequency coverage, at least three previously-unrecognized emission components have been discovered: broadband and narrow-band kilometric emission, and the lesser-arc decametric emission. Their properties are reviewed. In addition, the fundamental structure of the decameter wavelength and hectometer wavelength emission, now believed to be almost exclusively in the form of complex but repeating arc structures in the frequencytime domain, is described. Dramatic changes in the emission morphology of some components as a function of the sun-Jupiter-spacecraft angle (local time) are described. Finally, the PRA in situ measurements of the Io plasma torus hot-to-cold electron density and temperature ratios are summarized.

Boischot, A.↗
NASA NTRS1981

Auroral kilometric radiation - A theoretical review

Auroral kilometric radiation (AKR) is a high-density radio wave radiation in the frequency band from 50 to 750 kHz, with a peak around 250 kHz, that has been observed emanating from the auroral zone. In connection with its low frequency, the radiation can not penetrate through the ionosphere to earth, so all observations have been made by satellite. The AKR is closely correlated with the occurrence of discrete auroral arcs, which are believed to be generated by intense inverted V electron precipitation bands. A review is presented of several theories which have been proposed to explain the observed AKR. Attention is given to the conversion of electron cyclotron wave to O mode, the coherent amplification of gyroemission by velocity space instabilities, beam-driven electromagnetic instability via low-frequency turbulence, soliton radiation, loss cone instability, nonlinear beating of electrostatic waves, and the beam amplification of electromagnetic wave via coherent EIC density fluctuations.

Grabbe, C. L.↗
NASA NTRS1981

The AKR emission cone at low frequencies

It is noted that certain of the ISEE-1 observations between the plasmasphere and the auroral zone have revealed the emission cone of auroral kilometric radiation (AKR) unaffected by plasmaspheric refraction. At some distance from the source, the cone produced a sharp low-frequency boundary in the AKR signals, which was displaced above the cyclotron frequency. The variation of this boundary, together with other aspects of the AKR signals, suggested that the AKR emission cone closed toward a hollow, roughly 45 deg limit cone with decreasing frequency, duplicating the behavior previously found with ISIS-1 at the opposite end of the AKR spectrum. It is pointed out that the hollow limit cone at low frequencies is a new feature, not previously reported.

Calvert, W.↗
NASA NTRS1981

Dynamical predictability of monthly means

The concept of predictability which is conditioned by synoptic-scale disturbance instabilities is extended to that of time averages, which are determined by low-frequency planetary wave predictability, in an attempt to determine the theoretical upper limit of dynamical predictability of monthly means for prescribed, nonfluctuating external forcings. Sixty-day integrations of a global general circulation model with nine different initial conditions but identical boundary conditions of sea surface temperatures, snow, sea ice and soil moisture are carried out, where the rms vector wind error between the observed initial conditions is greater than 15 m/sec. It is found that while the variances among the first 30-day means, predicted from mostly different initial conditions, are significantly different from the variances due to random perturbations in the initial conditions, variances for days 31-60 are not so distinguishable. These results suggest that the evolution of long waves remains predictable for between one month and 45 days.

Shukla, J.↗
NASA NTRS1948

Transonic-flutter Investigation of Wings Attached to Two Low-acceleration Rocket-propelled Vehicles

Two low-acceleration transonic-flutter vehicles were launched and flown. The first carried two test wings, one of which fluttered at M = 0.92 at a frequency of 61.4 cycles per second. The reference flutter speed determined from two-dimensional theory for an unswept wing in incompressible flow is conservative when compared to the experimental flutter speed. The second vehicle carried two test wings, one of which failed at M = 0.71 because of low-frequency divergent oscillation. Since this failure was not caused by conventional flexure-torsion flutter, no comparison with a reference flutter speed can be made.

MISSILES, SPECIFIC TYPES↗