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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Nonlinear stability of solar type 3 radio bursts. 2: Application to observations near 1 AU

A set of rate equations including strong turbulence effects and anomalous resitivity are solved using parmeters which model several solar type 3 bursts. Exciter distributions observed at 1 AU are excitation of the linear bump-in-tail instability, amplifying Langmuir waves above the threshold for the oscillating two stream instability (OTSI). The OTSI, and the attendant anomalous resistivity produce a rapid spectral transfer of Langmuir waves to short wavelengths, out of resonance with the electron exciter. Further energy loss of the beam is thus precluded. The various parameters needed to model the bursts are extrapolated inside 1 AU with similar results. Again, the OTSI is excited and decouples the electron beam from the Langmuir radiation. Reabsorption of the Langmuir waves by the beam is shown to be unimportant in all cases, even at 0.1 AU. The theory provides a natural explanation for the observed realationship between radio flux, and the electron flux.

Goldstein, M. L.↗
NASA NTRS1986

Generation of lower-hybrid noise by superthermal cross-field ion currents

A kinetic model for generating lower-hybrid waves by a low-density, superthermal ion beam directed across a magnetic field is investigated. Growth rate calculations demonstrate the salient features of the process, in particular, the small threshold drift of several background ion thermal speeds required for instability. The mechanism is suitable for collisionless dissipation in systems which evince the characteristics of anomalous resistivity in the presence of relatively small electrical currents.

Barbosa, D. D.↗
NASA NTRS1946

Tank Tests of an Alternate Hull Form for the Consolidated Vultee PB2Y-3 Airplane

Tests have been made in Langley tank no. I of a dynamic model of the Consolidated Vultee PB2Y-3 airplane. These tests were made using an alternate hull form, the purpose of which was to reduce the bow spray and eliminate the landing instability which are objectionable features of the production design. The major differences from the PB2Y-3 hull included a deeper step to improve the landing stability , and a lengthened forebody and increased beam to reduce the sway in the propellers and on the flaps. The tests showed that the spray characteristics of the revised hull form were much better than that ot the production design. In addition the take-off and landing stability of the model with the alternate hull were satisfactory.

Land, Norman S.↗
NASA NTRS1995

Low-frequency waves associated with Langmuir waves in solar wind

The Ulysses spacecraft has detected several events of low-frequency electromagnetic waves in association with Langmuir waves in the solar wind. The high time resolution observations show that the Langmuir waves are very intense and occur as broad peaks superposed by collapsing millisecond spikes. The low-frequency waves are identified as electromagnetic lower hybrid waves. The observed energy densities of these waves often exceed the strong turbulence thresholds. It is shown that none of the parametric decay instabilities involving Langmuir and low-frequency waves are energetically favorable to explain the present observations. The low-frequency waves are proposed to arise from currents associated with gradients in the electron beam originating at sites where Langmuir waves scatter the beam electrons.

Thejappa, G.↗
NASA NTRS1962

ELASTIC STABILITY CONSIDERATIONS IN AIRCRAFT STRUCTURAL DESIGN

General instability in shell type structures is a relatively infrequent phenomenon. By general instability is meant that a complete section of a structure becomes elastically unstable, as compared with local crippling or buckling. The first step in design is to rough size the structure and determine its compatibility with air vehicle configuration and system instillations. After this step has been completed, it is possible to survey the layouts to determine whether general instability is likely to occur. The general parameters of length, diameter, frame spacing, element stiffness, etc., are than investigated to determine if a general failing stress exists which is less than any local column, beam column, or panel failing stress. In this way, a well-balanced design is determined and general stability failing modes eliminated.

ELASTIC STABILITY↗
NASA NTRS1981

Electromagnetic-wave excitation in a large laboratory beam-plasma system

The mechanism by which unstable electrostatic waves of a beam-plasma system are converted into observed electromagnetic waves is of current interest in space physics and in tokamak fusion research. The process involved in the conversion of electrostatic to electromagnetic waves at the critical layer is well understood. However, the radiation from uniform plasmas cannot be explained on the basis of this process. In connection with certain difficulties, it has not yet been possible to establish the involved emission processes by means of experimental observations. In the considered investigation these difficulties are overcome by employing a large laboratory plasma in a parameter range suitable for detailed diagnostics. A finite-diameter electron beam is injected into a uniform quiescent afterglow plasma of dimensions large compared with electromagnetic wavelengths. The considered generation mechanism concerning the electromagnetic waves is conclusively confirmed by observing the temporal evolution of an instability

Whelan, D. A.↗
NASA NTRS1990

Test results of a diffraction grating beam combiner

A laser diode beam combiner employing diffraction gratings has been fabricated and tested. The Grating Laser Beam Combiner (GLBC) uses two holographic diffraction gratings to incoherently combine the first order diffraction components of four 35 mW AlGaAs lasers. The grating rhomb design minimizes the transmitter sensitivity to the inherent frequency instability of laser diodes. The overall throughput of the combiner is 74 percent. Each laser was temperature controlled to 0.1 C and modulated with 110 Mbps QPPM data. Two lasers under modulation were coaligned to within 76 microrad with a combined average power of 45 mW.

Rall, Jonathan A.↗
NASA NTRS2017

Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini Comparisons from 2012 to 2015

Submillimeter emission lines of carbon monoxide (CO) in Titan's atmosphere provide excellent probes of atmospheric temperature due to the molecule's long chemical lifetime and stable, well constrained volume mixing ratio. Here we present the analysis of 4 datasets obtained with the Atacama Large Millimeter/Submillimeter Array (ALMA) in 2012, 2013, 2014, and 2015 that contain strong CO rotational transitions. Utilizing ALMA's high spatial resolution in the 2012, 2014, and 2015 observations, we extract spectra from 3 separate regions on Titan's disk using datasets with beam sizes ranging from 0.35 ×0.28'' to 0.39 ×0.34''. Temperature profiles retrieved by the NEMESIS radiative transfer code are compared to Cassini Composite Infrared Spectrometer (CIRS) and radio occultation science results from similar latitude regions. Disk-averaged temperature profiles stay relatively constant from year to year, while small seasonal variations in atmospheric temperature are present from 2012 to 2015 in the stratosphere and mesosphere ( approx. 100-500 km) of spatially resolved regions. We measure the stratopause (320 km) to in- crease in temperature by 5 K in northern latitudes from 2012 to 2015, while temperatures rise throughout the stratosphere at lower latitudes. We observe generally cooler temperatures in the lower stratosphere ( approx. 100 km) than those obtained through Cassini radio occultation measurements, with the notable exception of warming in the northern latitudes and the absence of previous instabilities; both of these results are indicators that Titan's lower atmosphere responds to seasonal effects, particularly at higher latitudes. While retrieved temperature profiles cover a range of latitudes in these observations, deviations from CIRS nadir maps and radio occultation measurements convolved with the ALMA beam-footprint are not found to be statistically significant, and discrepancies are often found to be less than 5 K throughout the atmosphere. ALMA's excellent sensitivity in the lower stratosphere (60-300 km) provides a highly complementary dataset to contemporary CIRS and radio science observations, including altitude regions where both of those measurement sets contain large uncertainties. The demonstrated utility of CO emission lines in the submillimeter as a tracer of Titan's atmospheric temperature lays the groundwork for future studies of other molecular species -particularly those that exhibit strong polar abundance enhancements or are pressure-broadened in the lower atmosphere, as temperature profiles are found to consistently vary with latitude in all three years by up to 15 K.

Thelen, Alexander E.↗
NASA NTRS1979

Artificial stimulation of auroral electron acceleration by intense field aligned currents

A cesium-doped high explosion was detonated at 165 km altitude in the auroral ionosphere during quiet conditions. An Alfven wave pulse with a 200-mV/m electric field was observed, with the peak occurring 135 ms after the explosion at a distance of about 1 km. The count rate of fixed energy 2-keV electron detectors abruptly increased at 140 ms, peaked at 415 ms, and indicated a downward field-aligned beam of accelerated electrons. An anomalously high-field aligned beam of backscattered electrons was also detected. The acceleration is interpreted as due to production of an electrostatic shock or double layer between 300 and 800 km altitude. The structure was probably formed by an instability of the intense field-aligned currents in the Alfven wave launched by the charge-separation electric field due to the explosion.

Holmgren, G.↗
NASA NTRS2011

Observations and Effects of Dipolarization Fronts Observed in Earth's Magnetotail

Dipolarization fronts in Earth's magnetotail are characterized by sharp jumps in magnetic field, a drop in density, and often follow earthward fast plasma flow. They are commonly detected near the equatorial plane of Earth s tail plasma sheet. Sometimes, but not always, dipolarization fronts are associated with global substorms and auroral brightenings. Both Cluster, THEMIS, and other spacecraft have detected dipolarization fronts in a variety of locations in the magnetotail. Using multi-spacecraft analyses together with simulations, we have investigated the propagation and evolution of some dipolarization events. We have also investigated the acceleration of electrons and ions that results from such magnetic-field changes. In some situations, the velocities of fast earthward flows are comparable to the Alfven speed, indicating that the flow bursts might have been generated by bursty reconnection that occurred tailward of the spacecraft. Based on multi-spacecraft timing analysis, dipolarization fronts are found to propagate mainly earthward at 160-335 km/s and have thicknesses of ~900-1500 km, which corresponds to the ion inertial length or gyroradius scale. Following the passage of dipolarization fronts, significant fluctuations are observed in the x and y components of the magnetic field. These peaks in the magnetic field come approximately 1-2 minutes after passage of the dipolarization front. These Bx and By fluctuations propagate primarily dawnward and earthward. Field-aligned electron beams are observed coincident with those magnetic field fluctuations. Non-Maxwellian electron and ion distributions are observed that are associated with the dipolarization that may be unstable to a range of electrostatic and/or whistler instabilities. Enhanced electrostatic broadband noise at frequencies below and near the lower-hybrid frequency is also observed at or very close to these fronts. This broadband noise is thought to play a role in further energizing the particles. Such studies provide insights into the particle acceleration mechanisms associated with substorm dipolarization, and, in turn, the effects of those acceleration mechanisms on the structure and evolution of dipolarization fronts.

Goldstein, Melvyn L.↗
NASA NTRS1991

ISEE 1 observations of electrostatic ion cyclotron waves in association with ion beams on auroral field lines from about 2.5 to 4.5 R(E)

Quasi-monochromatic waves at about the hydrogen cyclotron frequency were observed as the ISEE 1 satellite traversed auroral field lines at radial distances of about 2.5-4.5 R(E) near midnight on June 19, 1981. Waves and both lower and higher frequencies were observed at higher altitudes, and possible electrostatic helium cyclotron and oxygen cyclotron waves occurred at lower altitudes. Upflowing hydrogen and oxygen beams and field-aligned currents occurred simultaneously. The features of the waves are most consistent with the current-driven mode. In addition, numerical studies of the linear dispersion relation, using parameters based on the observations, show that both the parallel and oblique two-stream modes and the ion-beam-driven modes were stable while oblique current-driven modes were unstable. The O(+) and H(+) distributions provide evidence for interactions with local electrostatic ion cyclotron waves and for the H(+)-O(+) two-stream instability at altitudes below the satellite.

Catell, C. A.↗
NASA NTRS1995

Radar investigation of barium releases over Arecibo Observatory, Puerto Rico

The NASA Combined Release and Radiation Effects Satellite (CRRES) El Coqui rocket campaign was successfully carried out in Puerto Rico during the period 18 May through 12 July 1992. This report describes five chemical release experiments in the upper ionosphere supported by Geospace Research, Inc. during the El Coqui campaign. Additional spin-off science is also discussed. The El Coqui releases are designated AA-1 (rocket 36-082), AA-2 (rocket 36-081), AA-3b (rocket 36-064), AA-4 (rocket 36-065), and AA-7 (rocket 36-083). Particular attention is paid to releases AA-2 and AA-4. These two experiments involved the illumination of ionospheric release regions with powerful high-frequency (HF) radio waves transmitted from the Arecibo HF facility. In the AA-2 experiment, microinstabilities excited by the HF wave in a Ba(+) plasma were examined. This release yielded a smooth plasma cloud that helped clarify several fundamental issues regarding the physics of wave plasma instabilities. During AA-2 extremely strong HF-induced Langmuir turbulence was detected with the Arecibo 430 MHz radar. CF3Br was released in the AA-4 study to create an ionospheric hole that focused the HF beam. This experiment successfully explored wave-plasma coupling in an O(+) ionosphere under conditions of very high HF electric field strengths.

Djuth, Frank T.↗
NASA NTRS1992

Intense laser beams; Proceedings of the Meeting, Los Angeles, CA, Jan. 23, 24, 1992

Various papers on intense laser beams are presented. Individual topics addressed include: novel methods of copper vapor laser excitation, UCLA IR FEL, lasing characteristics of a large-bore copper vapor laser (CVL), copper density measurement of a large-bore CVL, high-power XeCl excimer laser, solid state direct-drive circuit for pumping gas lasers, united energy model for FELs, intensity and frequency instabilities in double-mode CO2 lasers, comparison of output power stabilities of CO and CO2 lasers, increasing efficiency of sealed-off CO lasers, thermal effects in singlet delta oxygen generation, optical extraction from the chemical oxygen-iodine laser medium, generation and laser diagnostic analysis of bismuth fluoride. Also discussed are: high-Q resonator design for an HF overtone chemical lasers, improved coatings for HF overtone lasers, scaled atmospheric blooming experiment, simulation on producing conjugate field using deformable mirrors, paraxial theory of amplitude correction, potential capabilities of adaptive optical systems in the atmosphere, power beaming research at NASA, system evaluations of laser power beaming options, performance projections for laser beam power to space, independent assessment of laser power beaming options, removal of atmospheric CFCs by lasers, efficiency of vaporization cutting by CVL.

Wade, Richard C.↗
NASA NTRS1994

Fine structure in plasma waves and radiation near the plasma frequency in Earth's foreshock

Novel observations are presented of intrunsic fine structure in the frequency spectrum of electomagnetic (EM) radiation and plasma waves near the electron plasma frequency f(sub p) during a period of unusually high interplanetary magnetic field strength. Measured using the wideband receiver on the International Sun-Earth Explorer (ISEE) 1 spacecraft, fine-structured emissions are observed both in the solar wind and the foreshock, The fine structure is shown to correspond to emissions spaced above f(sub p) near half harmonies of the electon cyclotron frequency f(sub ce), i.e., near f(sub p) + nf(sub ce)/2. These appear to be the first space physics observations of emissions spaced by f(sub ce)/2. Indirect but strong arguments are used to discriminate between EM and electrostatic (ES) signals, to identify whether ISEE 1 is in the solar wind or the foreshock, and to determine the relative frequencies of the emissions and the local f(sub p). The data are consistent with generation of the ES and EM emissions in the foreshock, with subsequent propagation of the EM emissions into the solar wind. It remains possible that some emissions currently identified as ES have significant EM character. The ES and EM emisions often merge into one another with minimal changes in frequency, arguing that their source regions and generation mechanisms are related and imposing significant constraints on theories. The f(sub ce)/2 ES and EM fine structures observed may be intrinsic to the emission mechanisms or to superposition of two series of signals with f(sub ce) spacing that differ in starting frequency by f(sub ce)/2. Present theories for nonlinear wave coupling processes, cyclotron maser emission, and other linear instability processes are all unable to explain multiple EM and/or ES components spaced by approximately f(sub ce)/2 above f(sub p) for f(sub p)/f(sub ce) much greater than 1 and typical for shock beams parameters. Suitable avenues for further theoretical research are identified. Empirically, the observed fine structures appear very similar to those in split bnad and multiple-lane type II solar radio bursts; interpretation of both these type II fine structures in terms of f(sub ce)/2 splitting is suggested, thereby supporting and generalizing a suggestion by Wild (1950). A possible application to continuum radiation is mentioned. The ubiquity of these fine structures in the Earth's f(sub p) radiation and foreshock waves remains unknown. Only the ISEE 1 wideband receiver has sufficient frequency resolution (approximately less than or equal to 100 Hz) to perform a dedicated search. Further study of the ubiquity of these fine structures, of how reliably the splitting corresponds to f(sub ce)/2, and of the other interpretations above is necessary.

Cairns, Iver H.↗
NASA NTRS2002

Whirl Flutter Stability of Two-Bladed Proprotor/Pylon Systems In High Speed Flight

The lack of polar symmetry in two-bladed rotors leads to equations of motion with periodic coefficients in axial flight, which is contrary to three or more bladed rotors that result in constant coefficient equations. With periodic coefficients, the analysis becomes involved, as a result very few studies have been directed towards the analysis of two-bladed rotors. In this paper, the aeroelastic stability of two-bladed proprotor/pylon/wing combinations is examined in high speed axial flight. Several parametric studies are carried out to illustrate the special nature of two-bladed proprotors and to better understand the mechanism of whirl-flutter in such rotors. The wing beam bending mode for two-bladed rotors is found to be stable over the range of parameters examined, a behaviour very different from three-bladed rotors. Also, the wing torsion mode exhibits a new type of instability similar to a wing torsional divergence scouring at I/rev frequency. This type of behaviour is not seen in three and more bladed rotors. The interaction between wing chordwise bending and torsion modes is found to be much greater in the case of two-bladed rotors and, over the range of parameters considered, these two modes govern the stability of the system.

Singh, Beerinder↗
NASA NTRS1986

A double layer model for solar X-ray and microwave pulsations

The wide range of wavelengths over which quasi-periodic pulsations have been observed suggests that the mechanism causing them acts upon the supply of high energy electrons driving the emission processes. A model is described which is based upon the radial shrinkage of a magnetic flux tube. The concentration of the current, along with the reduction in the number of available charge carriers, can rise to a condition where the current demand exceeds the capacity of the thermal electrons. Driven by the large inductance of the external current circuit, an instability takes place in the tube throat, resulting in the formation of a potential double layer, which then accelerates electrons and ions to MeV energies. The double layer can be unstable, collapsing and reforming repeatedly. The resulting pulsed particle beams give rise to pulsating emission which are observed at radio and X-ray wavelengths.

Tapping, K. F.↗
NASA NTRS1987

Structural modeling for multicell composite rotor blades

Composite material systems are currently good candidates for aerospace structures, primarily for the design flexibility they offer, i.e., it is possible to tailor the material and manufacturing approach to the application. A working definition of elastic or structural tailoring is the use of structural concept, fiber orientation, ply stacking sequence, and a blend of materials to achieve specific performance goals. In the design process, choices of materials and dimensions are made which produce specific response characteristics, and which permit the selected goals to be achieved. Common choices for tailoring goals are preventing instabilities or vibration resonances or enhancing damage tolerance. An essential, enabling factor in the design of tailored composite structures is structural modeling that accurately, but simply, characterizes response. The objective of this paper is to present a new multicell beam model for composite rotor blades and to validate predictions based on the new model by comparison with a finite element simulation in three benchmark static load cases.

Rehfield, Lawrence W.↗