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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Wave speeds in the corona and the dynamics of mass ejections

A disturbance or coronal mass ejection being advected by the solar wind will expand at the fastest local characteristic speed - typically approximately the fast-mode speed. To estimate this characteristic wave speed and the velocity field in the ambient corona, it is necessary to know the magnetic field, temperature, and density. Only the density is known from coronal observations. The temperature, magnetic field, and velocity are not yet directly measured in the outer corona and must be estimated from a model. In this study, it is estimated that the magnetic field, solar wind velocity, and characteristic speeds use the MHD model of coronal expansion between 1 and 5 solar radii (R solar radii) with a dipole magnetic field at the base. This model, for a field strength of about 2 gauss at the base, gives flow speeds at low latitudes (near the heliospheric current sheet) of 250 km/s at 5 R solar radii and, 50 km/s at 2 solar radii, and fast-mode speeds to 400 to 500 km/s everywhere between 2 and 5 solar radii. This suggests that the outer edge of a velocity of mass ejection reported by MacQueen and Fisher (1983) and implies that the acceleration mechanism for coronal mass ejections is other than simple entrainment in the solar wind.

Suess, S. T.↗
NASA NTRS1990

Enstrophy budget in decaying compressible turbulence

The interaction of vorticity and shock waves in decaying compressible turbulence is investigated by direct numerical simulation of the Navier-Stokes equation using a spectral method with 64-cubed collocation points. The Taylor microscale Reynolds number decreases from 36 to 17 and the rms Mach number from 0.8 to 0.4 during the decay. It is observed that vorticity is created inside curved shocks through baroclinic interaction and intensified by the compression of fluid elements. Vortex stretching is, on the other hand, dominant outside shocks and is responsible for enhancing the strength of vorticity. A dipole of vorticity is typically created at the vertex of curved shocks, the structure of which is explained in terms of jump relations of velocity derivatives across a curved shock.

Kida, Shigeo↗
NASA NTRS1993

A 250 GHz planar low noise Schottky receiver

A planar quasi-optical Schottky receiver based on the quasi-integrated horn antenna has been developed and tested over the 230-280 GHz bandwidth. The receiver consists of a planar GaAs Schottky diode placed at the feed of a dipole-probe suspended on a thin dielectric membrane in an etched-pyramidal horn cavity. The diode has a 1.21 micron anode diameter and a low parasitic capacitance due to the use of an etched surface channel. The antenna-mixer results in a measured DSB conversion loss and noise temperature at 258 GHz of 7.2 dB +/- 0.5 dB and 1310 K +/- 70 K, respectively, at room temperature. The design is compatible with SIS mixers, and the low cost of fabrication and simplicity makes it ideal for submillimeter-wave imaging arrays requiring a 10-20 percent bandwidth.

Ali-Ahmad, Walid Y.↗
NASA NTRS1993

Auroral arc thicknesses as predicted by various theories

Twelve electron-acceleration mechanisms and 10 generator mechanisms for auroral arcs are examined, and a characteristic auroral-arc thickness is worked out for each mechanism except one. The arc thicknesses are then mapped down to the ionosphere along the terrestrial magnetic-field lines; near the Earth, a dipole magnetic-field model is used, and farther from the Earth, the mapping includes the effects of magnetic-field-line draping. The 21 theoretical models all predict auroral-arc thicknesses that are at least an order of magnitude wider than the optically observed arcs. As an alternative explanation of the observed narrow auroral arcs, the acceleration of ionospheric electrons to produce airglow in electrical discharge mechanisms appears improbable. Also unsuccessfully explored is the possibility that the observed narrow auroral arcs are caused by interference effects when Alfven waves reflect off the ionosphere. Suggestions are made for future ground-based auroral-arc measurements.

Borovsky, Joseph E.↗
NASA NTRS1995

An extension of the Lighthill theory of jet noise to encompass refraction and shielding

A formalism for jet noise prediction is derived that includes the refractive 'cone of silence' and other effects; outside the cone it approximates the simple Lighthill format. A key step is deferral of the simplifying assumption of uniform density in the dominant 'source' term. The result is conversion to a convected wave equation retaining the basic Lighthill source term. The main effect is to amend the Lighthill solution to allow for refraction by mean flow gradients, achieved via a frequency-dependent directional factor. A general formula for power spectral density emitted from unit volume is developed as the Lighthill-based value multiplied by a squared 'normalized' Green's function (the directional factor), referred to a stationary point source. The convective motion of the sources, with its powerful amplifying effect, also directional, is already accounted for in the Lighthill format: wave convection and source convection are decoupled. The normalized Green's function appears to be near unity outside the refraction dominated 'cone of silence', this validates our long term practice of using Lighthill-based approaches outside the cone, with extension inside via the Green's function. The function is obtained either experimentally (injected 'point' source) or numerically (computational aeroacoustics). Approximation by unity seems adequate except near the cone and except when there are shrouding jets: in that case the difference from unity quantifies the shielding effect. Further extension yields dipole and monopole source terms (cf. Morfey, Mani, and others) when the mean flow possesses density gradients (e.g., hot jets).

Ribner, Herbert S.↗
NASA NTRS1982

An electric noise component with density 1/f identified on ISEE 3

The properties of the 1/f noise detected at the terminals of ISEE 3 antennas are described and related to the solar wind parameters. The 1/f noise was observed with the radio receivers of the three-dimensional radio mapping experiment using the S and Z dipole antennas. The noise spectra contained a negative spectral index component at frequencies lower than 0.7 of the plasma frequency, and 5-10 times the predicted thermal noise for the Z antenna. S-antenna measurements of the 1/f component revealed it to be deeply spin modulated with a minimum electric field in the direction of the solar wind. Modulation increases with increasing frequency, becomes negligible when the 1/f intensity is negligible with respect to the thermal noise, and increases with solar wind velocity. The possibilities that the noise is due either to waves or currents are discussed.

Hoang, S.↗
NASA NTRS1976

Ion cyclotron instability of drifting plasma clouds

The paper is concerned with a quantitative study of the frequency dispersion characteristics of the ion cyclotron mode in a realistic dipole magnetosphere where the particles are allowed to drift azimuthally. The adopted model assumes that the particles are injected at a constant L shell in an extended region around local midnight. A drift-convoluted distribution function is used to study the spatial and temporal characteristics of the ion cyclotron instability. Two cases are examined: one in which the cold plasma density is constant and the other in which the cold particle density is allowed to vary. The resulting growth rates are presented in both the frequency versus coordinate space and the frequency versus time space. Possible inferences regarding wave emissions such as IPDP (intervals of pulsations of diminishing period) events are discussed. It is shown that the frequency dispersive effects can be produced by either the drift effects or changing the cold plasma density.

Lin, C. S.↗
NASA NTRS1993

An 86-106 GHz quasi-integrated low noise Schottky receiver

An integrated planar receiver was developed and tested over the 82-112 GHz bandwidth. The quasi-integrated antenna used in the receiver has a high gain, a high Gaussian coupling efficiency, and a wide bandwidth. The novel mixer design consists of a planar GaAs Schottky diode placed at the feed of a dipole-probe suspended inside an integrated horn antenna. The diode uses an etched surface channel and a planar air bridge for reduced parasitic capacitance. At 92 GHz, the room-temperature antenna-mixer exhibits a double sideband conversion loss and noise temperature of 5.5 +/- 0.5 dB and 770 K +/- 50 K, respectively. The measured DSB conversion loss and noise temperature over a 20 GHz bandwidth (86 GHz-106 GHz) remain less than 6.2 dB +/- 0.5 dB and 1000 K +/- 50 K, respectively. The low cost of fabrication and simplicity of the design makes it ideal for millimeter- and submillimeter-wave receivers.

Ali-Ahmad, Walid Y.↗
NASA NTRS1992

Plasma observations near Neptune - Results from Voyager 2

Results from observations made by the Plasma Science experiment on Voyager 2 at Neptune are reviewed. The magnetosphere of Neptune is filled with a tenuous plasma, which consists of at least two components: a light ion, probably H(+) and a heavy ion, probably N(+). Triton's atmosphere or ionosphere is thought to be the source of both heavy and light ions. Much of the low energy plasma in the inner magnetosphere is concentrated near the magnetic equator and near closest approach to the planet. The large tilt of the magnetic dipole axis from the rotation axis produces a dynamic magnetosphere which goes from an earthlike configuration to a pole-on configuration and back every 16 hours. The polar cusp regions change location and size as the planet rotates; at the time of the inbound magnetopause crossing, the phase of Neptune's rotation was such that the spacecraft entered the magnetosphere through the southern polar cusp region. Outbound from Neptune observations made in the magnetosheath show a possible signature of diurnal oscillation of the plasma mantle that grows, shrinks, and rocks, in a diurnal cycle. After the encounter with Neptune's magnetosphere, an upstream wave event was observed when the interplanetary magnetic field connected to the bow shock. The low frequency waves observed appear to be a mixture of Alfvenic and/or fast mode waves propagating away from the planet.

Zhang, Ming↗
NASA NTRS1992

Submillimeter heterodyne spectroscopy and remote sensing of the upper atmosphere

Submillimeter-wavelength heterodyne spectroscopy by microwave limb sounding (MLS) from orbiting satellite is a measurement technique for studying Earth's stratosphere, mesosphere and lowerr thermosphere on a global scale. Development and deployment of such tools are timely, as stratospheric ozone shileds life from solar ultraviolet radiation but is depleted by pollution from industrial activities. MLS experiments on NASA's Upper Atmosphere Research Satellite (UARS) and Earth Observing System (EOS) are now being developed and implementend for global monitoring. Atmospheric thermal-emission spectra at millimeter and submillimeter wave-lengths are measured as the instrument field of view (FOV) is scanned through the limb from above. Atmospheric profiles of molecular abundances, temperature, pressure, wind and magnetic field can be determined from the measured emission spectra. Intensity of the emission can provide abundance and temperature. Measured linewidths, and emission from temperature-insensitive O2 lines, can provide pressure. Differentitaion of measured pressure with repsect ot measured height differential (obtaiend from the instrument FOV scan encoder) can also provide temperature through atmospheric hydrostatic equilibrium (which relates temperature to pressure and height differential). Doppler shifts of spectral lines can provide wind, and Zeeman splitting of the magnetic-dipole lines of O2 can provide magnetic field.

Waters, J. W.↗
NASA NTRS1961

A Study of the Free Oscillations of the Earth

Published observations on the toroidal oscillations of the earth are critically reviewed. A supplementary analysis of the record obtained by the Lamont strain seismometer is presented. Eleven toroidal modes are identified, and it is concluded that the periods are known to within 1 per cent. A perturbation scheme involving the ratio of the angular velocity of the earth to the resonant frequency is used in calculating the effects due to the rotation of the earth on the resonant frequency. The free oscillations are viewed as a superposition of traveling waves. In a nonrotating system two traveling waves combine to produce a stationary standing wave. In a rotating system, the rotation distinguishes between waves that travel in the direction of rotation and those that travel in the opposite direction. Rotation removes a degeneracy and results in a splitting of a spectral peak of order l into 2 times l plus 1 peaks. The fractional displacement in frequency for the lowest-order toroidal oscillations is 1/206 and of the same order as the Q of the peak, so that splitting will probably not be observed in the toroidal oscillations. Viewed locally, rotation causes a particle to precess about a direction parallel to the axis of rotation. This precession will cause a variation of amplitude with time if the motion is recorded by an instrument with an anisotropic response function. Care is therefore needed in studying the time decay of a given spectral peak. Rotation also couples the normal coordinates so that a motion that is initially purely horizontal will develop a vertical component. It is expected that vertical seismometers should record particle motion with the toroidal frequencies. The perturbations of the toroidal oscillations due to core-mantle interaction are treated in detail. An exact expression is obtained for the rate of energy dissipated by a finitely conducting plate oscillating across a magnetic field. The energy dissipated at the core-mantle boundary due to viscous and hydromagnetic coupling is shown to be insignificant as compared with the energy dissipated within the mantle. The toroidal magnetic field leaking into the lower mantle combines with the dipole field, resulting in a stress on the mantle, tending to stiffen the lower boundary. The stress is of sufficient magnitude to produce a displacement toward higher frequency in the lower-order toroidal oscillations. Observations on the (sub 0) T (sub 2) oscillations lead to an estimate of the toroidal magnetic field in the lower mantle. A calculation of elastic energy in the low-order oscillations suggests a value of 10 (sup 18) ergs per cycles per hour for the energy density at low frequencies in the Chilean earthquake. Each mode of oscillation has a characteristic radial distribution of elastic energy associated with it. This distribution determines which parts of the earth contribute most heavily in determining a particular resonant frequency. The distribution of energy for the lower 17 modes for a homogeneous and a Gutenberg model earth is calculated. The resonant frequencies for models of the earth based on the Gutenberg and Lehmann distribution of elastic properties are presented. It is shown that the Gutenberg model earth fits the observations more closely than the Lehmann model and that a slight alteration of the Gutenberg model gives a significantly better fit to the observations. The alteration involves a lower shear-wave velocity in the lower mantle while the Gutenberg velocity distribution is maintained in the upper mantle. Various studies of the earth's oscillations coupled with surface-wave investigations substantiate Gutenberg's hypothesis of a layer of low velocity in the upper mantle. The physical conditions required for the formation of a region of low velocity are examined in detail. The results confirm Birch's earlier statement that a temperature gradient in excess of 6 degrees to 7 degrees per kilometer is needed to produce a decrease in velocity. The low-velocity layer does not require that the temperature approach or exceed the melting temperature. If tile upper mantle is homogeneous, the region of lower velocity should commence at the base of the crust and extend to 150 kilometers under the oceans and about 100 kilometers under continental regions. The distribution of thermal conductivity and radioactivity consistent with the low-velocity layer is also considered.

MacDonald, Gordon J. F.↗
NASA NTRS1997

Physics of Rotation Powered Pulsars and Their Nebulae

This letter is my progress report for the Astrophysics Theory grant. The first goal of the research supported by this grant is to produce a time dependent theory of the unsteady relativistic collisionless shock wave terminating the relativistic wind from a pulsar, and compare the predicted surface brightness fluctuations to Hubble Space Telescope observations of the wisps in the Crab Nebula. The second goal is to model the production of electron-positron pairs over the polar caps of rotation powered pulsars, and use the results to predict the heating of the surface due to particle trapping and bombardment of the atmosphere at the base of the polar field lines. We have succeeded in creating a one dimensional hybrid code, in which the electron-positron pairs incident on the shock structure are modeled as a relativistic, adiabatic, ideal MHD fluid, while the heavy ions are treated as particles using a particle-in-cell algorithm. The electromagnetic fields are evaluated from the currents and charge densities in the pairs and the ions, while the particles and the fluid accelerate in response to the computed self-consistent electromagnetic fields. The results are promising, in that the underlying ion cyclotron instability generates finite amplitude, propagating magnetosonic waves in the pairs, whose wavelengths and frequencies, when translated into physical units, are comparable to the observed running waves of brightness observed by HST near the Crab pulsar. The code is undergoing a number of tests, to assure us that this preliminary correspondence is not an artifact. In the coming year, the observational appearance of the models will be computed and compared to the HST observations of the Crab now in hand, and used to predict the HST results which will be obtained the year after next. WE also developed a one dimensional cascade theory for pair creation over pulsars' polar caps. A linear integral equation describing the synchrotron cascade has been derived and solved by iterative techniques, in the case when a high energy electron moving parallel to a star centered dipole magnetic field initiates the cascade through curvature gamma ray emission.

Arons, Jonathan↗
NASA NTRS1997

Effective Collision Strengths for Electron Impact Excitation of Inelastic Transitions in S III

We have calculated electron collisional excitation strengths for all electric dipole forbidden, semi-forbidden, and allowed transitions among the lowest 17 LS states 3s(exp 2)3p(exp 2) P-3, D-1, S-1, 3s3p(exp 3)S-5(exp 0), D-3(exp 0), P-3(exp 0), P-1(exp 0), S-3(exp 0), D-1(exp 0), 3S(exp 2)3p3d D-1(exp 0), F-3(exp 0), P-3(exp 0), D-3(exp 0), F-3(exp 0), P-1(exp 0), and 3S(exp 2)3p4S P-3(exp 0), P-l(exp 0) of S III using the R-matrix method. These S m states are represented by fairly extensive configuration-interaction wave functions that yield excited state energies in close agreement with recent laboratory measurements. Rydberg series of resonances converging to the excited state thresholds are explicitly included in the scattering calculation. The effective collision strengths are determined assuming Maxwellian distribution of electron energies. These are listed over a wide temperature range ([0.5-10] x 10(exp 4) K) and compared, where possible, with other available calculations. Subject headings: atomic data - atomic processes

Tayal, S. S.↗
NASA NTRS2017

Electric Potential and Electric Field Imaging with Dynamic Applications: 2017 Research Award Innovation

The technology and methods for remote quantitative imaging of electrostatic potentials and electrostatic fields in and around objects and in free space is presented. Electric field imaging (EFI) technology may be applied to characterize intrinsic or existing electric potentials and electric fields, or an externally generated electrostatic field may be used for illuminating volumes to be inspected with EFI. The baseline sensor technology (e-Sensor) and its construction, optional electric field generation (quasi-static generator), and current e- Sensor enhancements (ephemeral e-Sensor) are discussed. Critical design elements of current linear and real-time two-dimensional (2D) measurement systems are highlighted, and the development of a three dimensional (3D) EFI system is presented. Demonstrations for structural, electronic, human, and memory applications are shown. Recent work demonstrates that phonons may be used to create and annihilate electric dipoles within structures. Phonon induced dipoles are ephemeral and their polarization, strength, and location may be quantitatively characterized by EFI providing a new subsurface Phonon-EFI imaging technology. Initial results from real-time imaging of combustion and ion flow, and their measurement complications, will be discussed. These new EFI capabilities are demonstrated to characterize electric charge distribution creating a new field of study embracing areas of interest including electrostatic discharge (ESD) mitigation, crime scene forensics, design and materials selection for advanced sensors, combustion science, on-orbit space potential, container inspection, remote characterization of electronic circuits and level of activation, dielectric morphology of structures, tether integrity, organic molecular memory, atmospheric science, and medical diagnostic and treatment efficacy applications such as cardiac polarization wave propagation and electromyography imaging.

Generazio, Ed↗
NASA NTRS1993

Source characteristics of Jovian hectometric radio emissions

Direct confirmation that low-frequency Jovian hectometric (HOM) radio emissions centered near 0 deg central meridian longitude consist of distinct, oppositely polarized northern and southern beams has been achieved using data from the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. Distinct northern and southern beams were observed in the frequency range from approximately 300 kHz to 1 MHz for at least eight Jovian rotations during the Ulysses inbound pass at distances from 100 to 40 R(sub j). The radiation from the two magnetic hemispheres was measured from different Jovigraphic longitudes and magnetic (or centrifugal) latitudes. Observed temporal variations in the radio intensities, with time scales on the order of 30 min, may result either from longitudinal variations of the HOM sources or from longitudinal density variations in the Io plasma torus. Using the URAP direction-finding capabilities and assuming a tilted dipole planetary magnetic field model, the three-dimensional HOM source locations, the L shell through these source locations, and the beam opening angles were independently deduced. The HOM sources were found to originate at approximately 3 R(sub j) and on low L shells (L approximately 4 to 6), with beam opening angles ranging from 10 to 50 deg.

Reiner, M. J.↗
NASA NTRS2007

Antenna for Measuring Electric Fields Within the Inner Heliosphere

A document discusses concepts for the design of an antenna to be deployed from a spacecraft for measuring the ambient electric field associated with plasma waves at a location within 3 solar radii from the solar photosphere. The antenna must be long enough to extend beyond the photoelectron and plasma sheaths of the spacecraft (expected to be of the order of meters thick) and to enable measurements at frequencies from 20 Hz to 10 MHz without contamination by spacecraft electric-field noise. The antenna must, therefore, extend beyond the thermal protection system (TPS) of the main body of the spacecraft and must withstand solar heating to a temperature as high as 2,000 C while not conducting excessive heat to the interior of the spacecraft. The TPS would be conical and its axis would be pointed toward the Sun. The antenna would include monopole halves of dipoles that would be deployed from within the shadow of the TPS. The outer potion of each monopole would be composed of a carbon-carbon (C-C) composite surface exposed to direct sunlight (hot side) and a C-C side in shadow (cold side) with yttria-stabilized zirconia spacers in-between. The hot side cannot view the spacecraft bus, while the cold side can. The booms also can be tilted to minimize heat input to spacecraft bus. This design allows one to reduce heat input to the spacecraft bus to acceptable levels.

Sittler, Edward Charles↗
NASA NTRS1972

Ionization of multielectron atoms by fast charged particles.

Using plane waves to describe the incident and scattered particles, and screened hydrogenic and Coulomb functions to describe the atomic electrons before and after ejections, we have calculated the differential and total ionization cross sections of 11 atoms and one ion by electron impact, and ionization of helium by proton impact. The effective charges of the screened hydrogenic functions are fixed by the Hartree-Fock calculations. Calculations have been carried out for the atomic s, p, and d electrons. For low atomic numbers, we find reasonable agreement with the experimental data. For intermediate atomic numbers, we expect our results to overestimate the actual cross sections, since our choice of a unit charge for the Coulomb function of the ejected electrons will overestimate the atomic dipole potential strength, and in turn the high-energy cross sections. The advantage of the method presented here is that the ionization amplitude is given in analytic form. This may allow further analysis on this amplitude, and facilitates extension of the numerical integration for the cross section to high impact energies.

Omidvar, K.↗
NASA NTRSDate not supplied

Phased Array Characterization of Slat Noise Radiation from a High-Lift Common Research Model

A test campaign was conducted in the open-jet test section of the NASA Langley 14- by 22-Foot Subsonic Tunnel on a 10%-scale, semispan version of a High-Lift Common Research Model (CRM-HL) incorporating a leading-edge slat, trailing-edge flap and removable high-fidelity main landing gear. Far field acoustic measurements were obtained on baseline and acoustically treated model configurations using a traversing 97-microphone phased array that viewed the pressure side of the airframe. A modified version of the DAMAS deconvolution method incorporating corrections for shear layer acoustic wave decorrelation was employed to determine the locations and strengths of relevant noise sources along the span of the slat and in the vicinity of the flap edges and landing gear. The main goal of the test campaign was to use the array data to evaluate the effectiveness of various slat noise reduction concepts. It was found that slat-gap fillers produced significant broadband noise reduction for all Mach numbers and airfoil angles of attack that were investigated. Mach number scaling revealed an approximate velocity to the sixth power relationship for the slat noise at higher frequencies. Directivity measurements obtained from integration of DAMAS pressure-squared values over defined geometric zones around the model showed that baseline slat noise radiation approximated a dipole as a function of polar emission angle, becoming approximately omnidirectional with the introduction of the slat-gap filler. The test campaign clearly demonstrated the noise reduction benefits that can be obtained by applying appropriate treatments to leading edge slats on commercial transport-class aircraft.

William M Humphreys, Jr.↗