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At least 1,153 records · Page 64
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NASA NTRS1984

Asteroid Lightcurve Inversion

The application of convex profile inversion (CPI) to the interpretation of asteroid lightcurves is discussed. This technique investigates the problem of extracting information about an asteroid's shape from its lightcurve. Whenever four ideal conditions are met, P is an estimator for the asteroids mean cross section C, a convex set defined as the average of all cross sections C(Z) cut by planes a distance z above the asteroids equatorial plane. C is therefore a 2-D average of the asteroids 3-D shape. The method is tested by inverting lightcurves generated analytically for geometrically scattered ellipsoids (GSE's) with semiaxes a or = B or = C. Using a defined 'distance measure' to quantify the difference between any two profiles, the deviation of P from C for GSE's as a function of lightcurve noise level, rotation phase sampling interval delta theta, and departure from ideal conditions is calibrated. The distance between P and a circle provides a gauge of the asteroid's nonsphericity and incorporates all the information contained in the lightcurve.

Ostrow, S. J.↗
NASA NTRS1983

Long-time states of inverse cascades in the presence of a maximum length scale

It is shown numerically, both for the two-dimensional Navier-Stokes (guiding-center plasma) equations and for two-dimensional magnetohydrodynamics, that the long-time asymptotic state in a forced inverse-cascade situation is one in which the spectrum is completely dominated by its own fundamental. The growth continues until the fundamental is dissipatively limited by its own dissipation rate. Previously announced in STAR as N83-23546

Hossain, M.↗
NASA NTRS1984

Measurements of mantle wave velocities and inversion for lateral heterogeneity and anisotropy. II - Analysis by the single-station method

In the present investigation, the single-station method reported by Brune et al. (1960) is utilized for an analysis of long-period Love(G) and Rayleigh(R) waves recorded on digital seismic networks. The analysis was conducted to study the lateral heterogeneity of surface wave velocities. The data set is examined, and a description is presented of the single-station method. Attention is given to an error analysis for velocity measurements, the estimation of geographical distribution of surface wave velocities, the global distribution of surface wave velocities, and the correlation between the surface wave velocities and the heat flow on the geoid. The conducted measurements and inversions of surface wave velocities are used as a basis to derive certain conclusions. It is found that the application of the single-station method to long-period surface waves recorded on digital networks makes it possible to reach an accuracy level comparable to great circle velocity measurements.

Nakanishi, I.↗
NASA NTRS1984

Waveform inversion of mantle Love waves - The Born seismogram approach

Normal mode theory, extended to the slightly laterally heterogeneous earth by the first-order Born approximation, is applied to the waveform inversion of mantle Love waves (200-500 sec) for the earth's lateral heterogeneity at l = 2 and a spherically symmetric anelasticity (Q sub mu) structure. The data are from the Global Digital Seismograph Network (GDSN). The l = 2 pattern is very similar to the results of other studies that used either different methods, such as phase velocity measurements and multiplet location measurements, or a different data set, such as mantle Rayleigh waves from different instruments. The results are carefully analyzed for variance reduction and are most naturally explained by heterogeneity in the upper 420 km. Because of the poor resolution of the data set for the deep interior, however, a fairly large heterogeneity in the transition zones, of the order of up to 3.5 percent in shear wave velocity, is allowed. It is noteworthy that Love waves of this period range can not constrain the structure below 420 km and thus any model presented by similar studies below this depth are likely to be constrained by Rayleigh waves (spheroidal modes) only.

Tanimoto, T.↗
NASA NTRS1984

Statistical tests of additional plate boundaries from plate motion inversions

The application of the F-ratio test, a standard statistical technique, to the results of relative plate motion inversions has been investigated. The method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance. This approach appears to be useful in determining whether additional plate boundaries are justified. Previous results have been confirmed favoring separate North American and South American plates with a boundary located beween 30 N and the equator. Using Chase's global relative motion data, it is shown that in addition to separate West African and Somalian plates, separate West Indian and Australian plates, with a best-fitting boundary between 70 E and 90 E, can be resolved. These results are generally consistent with the observation that the Indian plate's internal deformation extends somewhat westward of the Ninetyeast Ridge. The relative motion pole is similar to Minster and Jordan's and predicts the NW-SE compression observed in earthquake mechanisms near the Ninetyeast Ridge.

Stein, S.↗
NASA NTRS1984

Simulation study of a new inverse-pinch high Coulomb transfer switch

A simulation study of a simplified model of a high coulomb transfer switch is performed. The switch operates in an inverse pinch geometry formed by an all metal chamber, which greatly reduces hot spot formations on the electrode surfaces. Advantages of the switch over the conventional switches are longer useful life, higher current capability and lower inductance, which improves the characteristics required for a high repetition rate switch. The simulation determines the design parameters by analytical computations and comparison with the experimentally measured risetime, current handling capability, electrode damage, and hold-off voltages. The parameters of initial switch design can be determined for the anticipated switch performance. Results are in agreement with the experiment results. Although the model is simplified, the switch characteristics such as risetime, current handling capability, electrode damages, and hold-off voltages are accurately determined.

Choi, S. H.↗
NASA NTRS1984

Inverse Modelling to Obtain Head Movement Controller Signal

Experimentally obtained dynamics of time-optimal, horizontal head rotations have previously been simulated by a sixth order, nonlinear model driven by rectangular control signals. Electromyography (EMG) recordings have spects which differ in detail from the theoretical rectangular pulsed control signal. Control signals for time-optimal as well as sub-optimal horizontal head rotations were obtained by means of an inverse modelling procedures. With experimentally measured dynamical data serving as the input, this procedure inverts the model to produce the neurological control signals driving muscles and plant. The relationships between these controller signals, and EMG records should contribute to the understanding of the neurological control of movements.

Kim, W. S.↗
NASA NTRS1983

The NYU inverse swept wing code

An inverse swept wing code is described that is based on the widely used transonic flow program FLO22. The new code incorporates a free boundary algorithm permitting the pressure distribution to be prescribed over a portion of the wing surface. A special routine is included to calculate the wave drag, which can be minimized in its dependence on the pressure distribution. An alternate formulation of the boundary condition at infinity was introduced to enhance the speed and accuracy of the code. A FORTRAN listing of the code and a listing of a sample run are presented. There is also a user's manual as well as glossaries of input and output parameters.

Bauer, F.↗
NASA NTRS1985

Inverse-Compton gamma rays in the galaxy

Compton gamma rays with energies 1 MeV largely results from scattering between electrons, with energies 100 MeV, and photons in the optical and infrared range and the 2.7 K universal blackbody radiation. An empirical model of the inverse Compton (IC) gamma ray production in the Galaxy is presented, using the most recent estimate of the interstellar electron spectrum given by Webber and a combination of optical and infrared observations to determine the galactic distribution of the various components of the interstellar photon field. The present analysis has an improved precision since the spectral distribution of the IC source function as well as that of the interstellar photon field are more accurately taken into account. The exact evaluation of the IC process is applied and different electron distribution models are considered.

Bloemen, J. B. G. M.↗
NASA NTRS1985

Study of non-thermal photon production under different scenarios in solar flares. 2: The Compton inverse and Bremsstrahlung models and fittings

Energy spectra of photons emitted from Bremsstrahlung (BR) of energetic electrons with matter, is obtained from the deconvolution of the electron energy spectra. It can be inferred that the scenario for the production of X-rays and gamma rays in solar flares may vary from event to event. However, it is possible in many cases to associated low energy events to impulsive acceleration, and the high energy phase of some events to stochastic acceleration. In both cases, flare particles seem to be strongly modulated by local energy losses. Electric field acceleration, associated to neutral current sheets is a suitable candidate for impulsive acceleration. Finally, that the predominant radiation process of this radiation is the inverse Compton effect due to the local flare photon field.

Perez-Peraza, J.↗
NASA NTRS1984

Source mechanisms of earthquakes near mid-ocean ridges from body waveform inversion - Implications for the early evolution of oceanic lithosphere

An evaluation is presented of the tectonics of the near-ridge environment based on the source mechanisms of earthquakes in young oceanic lithosphere. A catalog of near-ridge earthquakes is presented, and source parameters are determined from body waveform inversions. Source parameter-age relations are examined, and the near-ridge seismicity of the Indian Ocean is discussed. Deep normal faulting in young oceanic lithosphere is addressed, as is the relationship between thermoelastic stress and near-ridge earthquakes. The possibility of secondary convection beneath young oceanic lithosphere is considered. Finally, the broader tectonic implications of these results for the evolution of young oceanic lithosphere are discussed.

Bergman, E. A.↗
NASA NTRS1985

Finite field of view effects on inversion of limb thermal emission observations

It is pointed out that the technique of thermal emission spectroscopy provides an effective means for remote sounding of stratospheric temperature structure and constituent distributions. One procedure for measuring the stratospheric infrared spectrum involves the conduction of observations along ray paths tangent to the stratospheric limb. Thermal emission limb tangent observations have certain advantages compared to other types of observations. The techniques for determining temperature and trace gas distributions from limb thermal emission radiances are based on the assumption that the bulk of opacity lies near the tangent point. Ideally, the field of view (FOV) of the observing instrument should be very small. The effect of a finite FOV is to reduce the spatial resolution of the retrieved temperature and constituent profiles. The present investigation is concerned with the effects of the FOV on the inversion of infrared thermal emission measurements for balloon platforms. Attention is given to a convenient method for determining the weighting functions.

Abbas, M. M.↗
NASA NTRS1983

Three-dimensional inversion of satellite observed radiances A proposal

Fully three-dimensional temperature fields are obtained from observed satellite radiances through the use of variational methods which generalize the method of Wahba and Wendelberger (1980). This variational formalism allows a variety of different information types to contribute to the inversion process. Full use can be made of the horizontal smoothness of the temperature field in the calculation of three-dimensional retrievals. The present method facilitates the acquisition of maximum vertical resolution, accomplishes cloud effect filtering, and minimizes the effects of instrument noise, to yield horizontally coherent retrievals.

Hoffman, R. N.↗
NASA NTRS1985

Microwave inversion of leaf area and inclination angle distributions from backscattered data

The backscattering coefficient from a slab of thin randomly oriented dielectric disks over a flat lossy ground is used to reconstruct the inclination angle and area distributions of the disks. The disks are employed to model a leafy agricultural crop, such as soybeans, in the L-band microwave region of the spectrum. The distorted Born approximation, along with a thin disk approximation, is used to obtain a relationship between the horizontal-like polarized backscattering coefficient and the joint probability density of disk inclination angle and disk radius. Assuming large skin depth reduces the relationship to a linear Fredholm integral equation of the first kind. Due to the ill-posed nature of this equation, a Phillips-Twomey regularization method with a second difference smoothing condition is used to find the inversion. Results are obtained in the presence of 1 and 10 percent noise for both leaf inclination angle and leaf radius densities.

Lang, R. H.↗
NASA NTRS1984

X-ray inverse Compton emission from the radio halo of M87

A significant fraction of known galaxies contain an active galactic nucleus (AGN) at their cores, the site of violent activity and non-stellar radiation seen across the entire electromagnetic spectrum. This activity is thought to be due to the accretion of gas onto a massive black hole. A fraction of AGNs also eject collimated beams of energetic material, usually seen by virtue of its synchrotron emission in the radio band. Efforts to study these jets from AGNs in the X-ray band with the Einstein Observatory has led to several detections, most notably the jets in the nearby radio galaxies Centaurus A and Virgo A = M87. In their study of M87, Schreier, Gorenstein and Feigelson (1982) noted that, in addition to the synchrotron jet 10"-20" from the nucleus, X-rays appear to be generated in the diffuse radio halo 2'-5' from the nucleus. This finding may be particularly important as it may constitute the first known case of X-ray inverse Compton emission from AGN ejecta, allowing for the first time direct determination of the magnetic field strengths.

Feigelson, E. D.↗
NASA NTRS1985

X-ray inverse Compton emission from the radio halo of M87

M87 has been observed in the 0.2-4 KeV X-ray band using the High Resolution Imager on the Einstein Observatory, and at 1.452 GHz using the Very Large Array. The radio map showed that the halo contained prominent asymmetries to the east and southwest. The X-ray map indicated similar asymmetries, but they were imbedded in the diffuse hot gas that surrounds the core out to a radius of several arcminutes. The hot X-ray emitting gas was assumed to be spherically symmetric and could, therefore, be subtracted from the image. The resultant image was asymmetric with major lobes to the east and southwest that coincide approximately with the asymmetries in the radio halo. The data indicates that inverse Compton emission is a plausible model for the X-rays coming from the asymmetric component.

Wood, P. A. D.↗
NASA NTRS1985

Earthquake source mechanisms from body-waveform inversion and intraplate tectonics in the northern Indian Ocean

Double-couple point-source parameters for 11 of the largest intraplate earthquakes in the northern Indian Ocean during the last 20 years were determined from a formal inversion of the long-period P and SH waveforms. Two major intraplate tectonic provinces are distinguished in the northern Indian Ocean. The plate-wide stress pattern found and the high level of intraplate seismicity are probably the results of substantial resistance, along the Himalayan continental collision zone, to the continued northward motion of the western portion of the Indian plate.

Bergman, E. A.↗