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At least 631 records · Page 35
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NASA NTRS2000

An Analysis of the Torsion-Rotation-Vibration Rotational Spectrum of the Lowest In-Plane Bend and First Excited Torsional State of the C(3V) Internal Rotor C2H5CN

C2H5CN (Propionitrile or ethyl cyanide) is a well known interstellar species abundantly observed in hot cores during the onset of star formation. The onset of star formation generally results in elevated temperature, which thermally populates may low lying vibrational states such as the 206/cm in-plane bend and the 212/cm first excited torsional state in C2H5CN. Unfortunately, these two states are strongly coupled through a complex series of torsion-vibration-rotation interactions, which dominate the spectrum. In order to understand the details of these interactions and develop models capable of predicting unmeasured transitions for astronomical observations in C2H5CN and similar molecules, several thousand rotational transitions in the lowest excited in-plane bend and first excited torsional state have been recorded, assigned and analyzed. The analysis reveals very strong a- and b-type Coriolis interactions and a number of other smaller interactions and has a number of important implications for other C3V torsion-rotation-vibration systems. The relative importance and the physical origins of the coupling among the rotational, vibrational and torsional motions will be presented along with a full spectroscopic analysis and supporting astronomical observations.

Pearson, J. C.↗
NASA NTRS2001

The Transition from Hydrogen Bonding to Ionization in (HCI)n(NH3)n and (HCI)n(H2O)n Clusters: Consequences for Anharmonic Vibrational Spectroscopy

Anharmonic vibrational frequencies and intensities are calculated for 1:1 and 2:2 (HCl)(sub n)(NH3)(sub n) and (HCl)(sub n)(H2O)(sub n) complexes, employing the correlation-corrected vibrational self-consistent field method with ab initio potential surfaces at the MP2/TZP computational level. In this method, the anharmonic coupling between all vibrational modes is included, which is found to be important for the systems studied. For the 4:4 (HCl)(sub n)(H2O)(sub n) complex, the vibrational spectra are calculated at the harmonic level, and anharmonic effects are estimated. Just as the (HCl)(sub n)(NH3)(sub n) structure switches from hydrogen-bonded to ionic for n=2, the (HCl)(sub n)(H2O)(sub n) switches to ionic structure for n=4. For (HCl)2(H2O)2, the lowest energy structure corresponds to the hydrogen-bonded form. However, configurations of the ionic form are separated from this minimum by a barrier of less than an O-H stretching quantum. This suggests the possibility of experiments on ionization dynamics using infrared excitation of the hydrogen-bonded form. The strong cooperative effects on the hydrogen bonding, and concomitant transition to ionic bonding, makes an accurate estimate of the large anharmonicity crucial for understanding the infrared spectra of these systems. The anharmonicity is typically of the order of several hundred wave numbers for the proton stretching motions involved in hydrogen or ionic bonding, and can also be quite large for the intramolecular modes. In addition, the large cooperative effects in the 2:2 and higher order (HCl(sub n)(H2O)(sub n) complexes may have interesting implications for solvation of hydrogen halides at ice surfaces.

Chaban, Galina M.↗
NASA NTRS1984

High-resolution IUE observations of interstellar absorption lines in the Vela supernova remnant

Ultraviolet spectra of 45 stars in the vicinity of the Vela supernova remnant were recorded by the short-wavelength echelle spectrograph aboard the International Ultraviolet Explorer (IUE). Over one-third of the stars show interstellar absorption lines at large radial velocities (greater than 60 km/s). The mapping of these high-velocity components in the sky suggests the motions are chaotic, rather than from a coherent expansion of the remnant material. In accord with earlier conclusions from Copernicus data, the gas at high velocity exhibits higher than normal ionization and shows substantially less depletion of nonvolatile elements than normal interstellar material at low velocities. Relatively strong lines from neutral carbon in the two excited fine-structure states indicate that the neutral clouds within the remnant have had their pressures enhanced by the passage of the blast wave from the supernova. Also, the remnant seems to show a significant enhancement in the abundances of low-velocity Si IV, C IV, and N V over those found in the general interstellar medium.

Jenkins, E. B.↗
NASA NTRS1994

Morphological changes in the NGC 6251 jet

The giant radio galaxy NGC 6251 is a particularly good object for observational tests of relativistic jet models. Due to its high declination and approximately 0.5 Jy radio nucleus, high-quality Very Long Baseline Interferometry (VLBI) images of the central regions of the source can be made with northern hemisphere arrays. In addition, the large-scale radio morphology strongly suggests that the radio axis lies close to the plane of the sky, so Doppler boosting should be less extreme than in the core-dominated superluminal sources. Earlier 18 cm VLBI observations of NGC 6251 revealed an unexpectedly large jet/counterjet brightness ratio and small transverse motion of a feature in the parsec-scale jet. These early results are difficult to reconcile with the simplest symmetric relativistic jet models. In this paper we present a third-epoch 18 cm VLBI image of the parsec-scale radio jet in NGC 6251, and compare jet morphology over a 5 year time span. The jet shows a minor brightness peak at nearly the same distance from the core as the '25 mas knot' seen in the first- and second-epoch VLBI images. This feature is much less pronounced in the third epoch, and a relatively bright, new knot has appeared approximately 12 mas from the core. If this new component had a constant brightness during the 5 years separating the first and third observing epochs, then it must have moved away from the core with an apparent speed of at least 1.2c (compared with an upper limit of 0.23c for motion of the 25 mas knot). However, we cannot yet rule out a local brightening of the inner jet in favor of a new moving component. We determine a lower limit for the jet/couterjet brightness ratio of 100:1 within 6 mas of the core. We also present a new Very Large Array (VLA) image of the kpc-scale jet with 3 sec resolution, made from data obtained during the VLBI observations. The rate of decrease in jet surface brightness from parsec to kiloparsec scales is similar to jets in known superluminal radio sources.

Jones, Dayton L.↗
NASA NTRS1995

Navier-Stokes Computations on Full Wing-Body Configuration with Oscillating Control Surfaces

Unsteady Navier-Stokes simulations have been performed for vortical flows over an "arrow-wing" configuration of a supersonic transport in the transonic regime. Computed steady pressures and integrated force coefficients with and without control surface deflection at a moderate angle of attack are compared with experiment. For unsteady cases, oscillating trailing-edge control surfaces are modeled by using moving grids. Response characteristics between symmetric and antisymmetric oscillatory motions of the control surfaces on the left and right wings are studied. The antisymmetric case produces higher lift than the steady case with no deflection and the unsteady symmetric case produces higher lift than the antisymmetric case. The detailed analysis of the wake structure revealed a strong interaction between the primary vortex and the wake vortex sheet from the flap region when the flap is deflected up.

Obayashi, Shigeru↗
NASA NTRS1999

Role of Ocean Currents and Bottom Pressure Variability on Seasonal Polar Motion

Changes in the ocean angular momentum (OAM) components about the equatorial axes, either due to fluctuations in currents or bottom pressure (mass redistribution), can induce movements of the Earth's pole of rotation, commonly referred to as polar motion or wobble. Output from a 1 deg resolution ocean model is used to calculate the effective equatorial OAM functions chi(sub 1, sup O) and chi(sub 2, sup O), corresponding to polar motion excitation about the equatorial axis pointing to the Greenwich and 90 deg E meridians, respectively. Time series of chi(sup O) are combined with similar atmospheric series chi(sup A), computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses, to interpret the observed low-frequency polar motion excitation for the period 1985-1996. Results indicate that the oceans are a very important excitation source for the Chandler (approximately 433 days), annual, and semiannual wobbles, providing for much better amplitude and phase agreement with the observed excitation at these periods, in comparison with what is obtained when only the atmosphere is considered. Both oceanic mass and motion terms are found to be important but with mass signals having somewhat larger amplitudes. The role of regional variability in ocean currents and bottom pressure in contributing to chi(sup O) signals is quantified. Midlatitude regions (approximately 30 deg - 70 deg) figure prominently as places of strong local oceanic excitation signals. The North Pacific basin is found to be generally important for chi(sup O) excitation, while the Southern Ocean is important for both chi(sub 1, sup O) and chi(sub 2, sup O). The largest positive covariances of local with global chi(sup O) signals occur in the Kuroshio region near the western boundary of the North Pacific for chi(sub 1, sup O) and southwest of Australia for chi(sub 2, sup O).

Ponte, Rui M.↗
NASA NTRS1993

Morphological Changes in the NGC 6251 Jet

The giant radio galaxy NGC 6251 is a particularly good object for observational tests of relativistic jet models. Due to its high declination and similar 0.5 Jy radio nucleus, high quality VLBI images of the central regions of the source can be made with northern hemisphere arrays. In addition, the large-scale radio morphology strongly suggests that the radio axis lies close to the plane of the sky, so Doppler boosting should be less extreme than in the core-dominated superluminal sources. Earlier 18-cm VLBI observations of NGC 6251 revealed an unexpectedly large jet/counterjet brightness ratio and small transverse motion of a feature in the parsec-scale jet. These early results are difficult to reconcile with the simplest symmetric relativistic jet models.

Wehrle, A. E.↗
NASA NTRS1973

Brownian motion of electrons in time-dependent magnetic fields.

The behavior of a weakly ionized plasma in slowly varying time-dependent magnetic fields is studied through an extension of Williamson's stochastic theory. In particular, attention is focused on the properties of electron diffusion in the plane perpendicular to the direction of the magnetic field, when the field strength is large. It is shown that, in the strong field limit, the classical 1/B-squared dependence of the perpendicular diffusion coefficient is obtained for two models in which the field B(t) is monotonic in t and for two models in which B(t) possesses at least one turning point.

Iverson, G. J.↗
NASA NTRS1992

Search for evidence of a clock related to the solar 154 day complex of periodicities

Evidence that has recently been compiled (Bai and Sturrock 1991) indicates that the enigmatic 154-day periodicity in solar activity may be viewed as part of a complex of periodicities that are approximate multiples of 25.8 days, suggesting that the Sun contains a 'clock' with frequency in the range 440 to 463 nano Hz. The clock may comprise either an oscillator or a rotator, each of which may be either real or virtual. We have reconsidered a previous spectrum analysis of the Zurich sunspot-number sequence by Knight, Schatten, and Sturrock (1979) which revealed a sharp, persistent and significant periodicity with a period of 12.072 days, corresponding to a frequency of about 958.8 nano Hz. This periodicity may be regarded as the (second) upper sideband of the second harmonic (2nu(sub R) + 2nu(sub E)) of a fundamental frequency of 447.7 nano Hz that is clearly within the search band. In this expression, nu(sub R) is the sidereal frequency of the hypothetical rotator and nu(sub E) is the frequency (31.69 nano Hz) of the Earth in its orbital motion around the Sun. In analyzing sunspot area data derived from the Greenwich data set, and on noting that any frequency is defined only to within the Nyquist frequency, we find clear evidence not only for the upper sideband of the second harmonic, but also for the second harmonic (2nu(sub R)) and the lower sideband of the second harmonic (2nu(sub R) - 2nu(sub E)). There is no strong peak at the fundamental frequency in the Greenwich data, but there is in the Zurich sunspot data. The effect of a linear oscillator is, to the lowest order in the amplitude, the same as the combined effect of two rotators of opposite polarities. A rotator that has arbitrary orientation with respect to the ecliptic may influence the outer layers of the Sun and thereby modulate the occurrence of solar activity such as sunspots. By analyzing a simple model, we find that such a rotator would influence surface activity in such a way that the spectrum of a 'signal' (such as the record of sunspots), as seen from the Earth, would contain components with frequencies that are certain integral combinations of nu(sub R) and nu(sub E). The amplitudes of the various components depend sensitively on theta, the angle between the axis of the rotator and the axis of the Earth's orbital motion. This simple model therefore offers a kinematical (but not dynamical) interpretation of the sunspot spectrum. The present analysis, while offering support of our conjectures that the Sun contains a clock that regulates the 154-day complex of periodicities, cannot distinguish between an osillator or a rotator (that might be a traveling wave), nor between a real rotator or a virtual rotator (that might be an apparent traveling wave due to the aliasing effect of an oscillator in a rotating system). Further analysis of sunspot and other data sets will be required to confirm the existence of such clock and (if it is real) to determine its physical nature.

Sturrock, P. A.↗
NASA NTRS1976

On the response of superpressure balloons to displacements from equilibrium density level

The response of a superpressure balloon to an initial displacement from its constant-density floating level is examined. An approximate solution is obtained to the governing vertical equation of motion for constant-density superpressure balloons. This solution is used to filter out neutrally buoyant oscillations in balloon records despite the nonlinear behavior of the balloon. The graph depicting the pressure data after deconvolution between the raw pressure data and the normalized balloon wavelet shows clearly the strong filtering-out of the neutral buoyancy oscillations.

Levanon, N.↗
NASA NTRS1984

Inflow of halo gas from the direction of the Galactic north pole

A far-UV echelle spectrum has been obtained of the UV-bright star vZ1128 in the globular cluster M3. This cluster lies in a direction 11 deg off the Galactic north pole at a distance of about 10 kpc. Only the usually strong interstellar lines are recognizable in the faint spectrum, and they show absorption at velocities from near +40 km/s to -100 km/s. Since in the direction observed the line-of-sight component of the Milky Way rotation is negligible, the detected velocities demonstrate for this direction the motion of gas from the Milky Way halo toward the disk. Gas returning to the disk in a galactic fountainlike flow may explain the observations.

De Boer, K. S.↗
NASA NTRS1988

Studies of the dynamics of the twin-lift system

A full set of equations of motion for the twin lift system, linearized about a hover trim condition were derived and presented. It is shown that this full set of equations of motion decouples into simpler sets of equations of motion if the aerodynamic coupling derivatives of the helicopters are neglected. One of these decoupled sets of equations of motion (referred to as the planar set) was studied at length. The other decoupled set (referred to as the nonplanar set) is examined here. It is shown that when the geometric configuration of the twin-lift is symmetric that a further decoupling is possible into antisymmetric and symmetric sets of equations. One set of these reduced equations of motion referred to as the antisymmetric set is directly equivalent to the longitudinal motion of a single helicopeter with a sling load. The second set, referred to as the symmetric set corresponds to the rotation of the entire system without load motion. As in the case of the planar symmetric motion, the location of the tether attachment point influences the stability of the nonplanar symmetric mode. The trend is opposite however in the nonplanar case in that an attachment point below the helicopter center of gravity gives a favorable effect on the stability. The effect is not as strong however as the unfavorable effect on the planar symmetric mode.

Curtiss, H. C., Jr.↗
NASA NTRS1989

Magnetospheric ion bombardment profiles of satellites - Europa and Dione

Bombardment profiles generated by tracking ions in magnetospheric plasmas onto the surface of a satellite with a suitable description of the ion motion are used to calculate the spatial dependence across a satellite surface of the ion bombardment/implantation rate for satellites embedded in planetary magnetospheric plasmas. Attention is given to the results of a parameter study; a general dependency on ion gyroradius and pitch angle is noted, together with a strong dependence of access to the leading hemisphere on pitch-angle distribution. Gyromotion is found to cause differences in the bombardment of the inner and outer hemisphere. Reasonable speed and pitch-angle distributions are used to calculate profiles for sulfur ions incident on Europa and oxygen ones incident on Dione.

Pospieszalska, M. K.↗
NASA NTRS1994

Radio and X-ray observations of the gamma-ray bright quasar PKS 0528+134

We present a study of the z = 2.07 quasar PKS 0528+134, which has been detected as an extraordinarily luminous gamma-ray source. Its radio properties are highly variable in both total and polarized flux density. Milliarcsecond-scale maps from global very long base interferometry (VLBI) experiments, an X-ray spectrum from ROSAT Position Sensitive Proportional Counter (PSPC) observations, and light curves in total flux density and polarization are used to investigate the geometry, radiation mechanism, and physical environment of the emission region in the source. The VLBI images reveal a bent jet extending toward the northeast on parsec scales, with less intense knots of emission appearing on the opposite side of the brightest spot. The position of the core usually found in such sources is unclear. The polarization angle is stable despite strong variability in polarized flux density and indicates that the magnetic field is aligned with the jet axis as defined by our 8.4 GHz image. The ROSAT X-ray flux density of PKS 0528+134 in 1991 March is measured to be 1.6 micro Jy at 1 keV, with a very steep spectral (`energy') index sigma(sub x) approximately equal to 2.2. The X-ray observations reveal the presence of cold gas along the line of sight significantly in excess of that present in the Galaxy. A strong radio flare began within two months of the first observation of a high flux of gamma-rays from PKS 0528+134 by Hunter et al. Using the geometry and spectral chacateristics determined by our VLBI observations, a synchrotron self-Compton calculation indicates that relativistic bulk motion is required in PKS 0528+134, with an estimated Doppler beaming factor delta approximately greater than 4.3, similar to the value delta approximately greater than 7 required to explain the low optical depth of the gamma-rays to photon-photon pair production. We suggest that the core activity of PKS 0528+134 is sporadic in nature, with the nonthermal outburst starting in 1991 representing a violent period involving ejection of relativistic plasma along a compact jet pointed almost directly toward us.

Zhang, Yun Fei↗
NASA NTRS2011

Flight Dynamics Aspects of a Large Civil Tiltrotor Simulation Using Translational Rate Command

An in-depth analysis of a Large Civil Tiltrotor simulation with a Translational Rate Command control law that uses automatic nacelle deflections for longitudinal velocity control and lateral cyclic for lateral velocity control is presented. Results from piloted real-time simulation experiments and offline time and frequency domain analyses are used to investigate the fundamental flight dynamic and control mechanisms of the control law. The baseline Translational Rate Command conferred handling qualities improvements over an attitude command attitude hold control law but in some scenarios there was a tendency to enter PIO. Nacelle actuator rate limiting strongly influenced the PIO tendency and reducing the rate limits degraded the handling qualities further. Counterintuitively, increasing rate limits also led to a worsening of the handling qualities ratings. This led to the identification of a nacelle rate to rotor longitudinal flapping coupling effect that induced undesired pitching motions proportional to the allowable amount of nacelle rate. A modification that applied a counteracting amount of longitudinal cyclic proportional to the nacelle rate significantly improved the handling qualities. The lateral axis of the Translational Rate Command conferred Level 1 handling qualities in a Lateral Reposition maneuver. Analysis of the influence of the modeling fidelity on the lateral flapping angles is presented. It is showed that the linear modeling approximation is likely to have under-predicted the side-force and therefore under-predicted the lateral flapping at velocities above 15 ft/s. However, at lower velocities, and therefore more weakly influenced by the side force modeling, the accelerations that the control law commands also significantly influenced the peak levels of lateral flapping achieved.

Lawrence, Ben↗
NASA NTRS1974

Mariner 9 observations of the Mars north polar hood

Mariner 9 photographs showing variations in the behavior of the Mars atmosphere over 21 consecutive days during northern winter are displayed. The photographs show the north polar cloud hood to be highly variable from day to day, with variations suggestive of development, motion, and decay of baroclinic waves. Lee wave clouds apparently composed primarily of water ice, convective cloud lines inferred to consist of CO2 ice, and dust clouds all occur frequently in the active region between latitudes 40 and 60 N. The wave clouds show that persistent surface westerlies and strong westerly shear occur in this region.

Briggs, G. A.↗
NASA NTRS1976

Ionospheric disturbances and gravity waves

The response of ionization to a gravity wave moving through the ionosphere is studied. Hydrodynamic equations are used, and local thermodynamic equilibrium is imposed for simplicity. The treatment involves a perturbation analysis, and the background medium is assumed to be time stationary, horizontally stratified, and known. It is shown that ionization may be locally resonant at each level for certain frequencies and directions, for which condition neutral and ionized particles are considered closely or critically coupled. The phase direction for this critical coupling is always downward in the absence of a magnetic field. A magnetic field results in two resonant directions for the same frequency, and these directions are mostly downward. Observed TID's associated with gravity waves may be indicative of such resonances. It is also noted that strong coupling may occur to neutral acoustic waves at high altitudes. Previous investigations restrict their use of momentum equations to the diffusion equation. The analysis also shows that such restrictions result in the neglect of terms arising from momentum transport due to any background ambipolar diffusion velocity and wave motion. These terms are mostly relevant at higher altitudes.

Eun, H.↗
NASA NTRS1989

Particle-in-cell simulations of stochastic electron acceleration

The results of a series of particle-in-cell simulations of stochastic wave-particle interaction are presented. The threshold for stochasticity was confirmed. The simulations demonstrate that in a strong magnetic field plasma waves with quiver velocities much less than the speed of light but above a certain threshold can stochastically accelerate electrons to energies far greater than 1 MeV. Moreover, self-consistency effects drive return currents and produce energetic runaway electrons that violate an invariant of motion.

Akimoto, K.↗