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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Turbulence in deep radio occultations

The effects of turbulence in the focal region of spherical or weakly oblate refracting bodies on the evolute flashes associated with the crossing of the evolute of the planetary limb by an occulted spacecraft are investigated. The approximate theory of focal evolutes in oblate refractivity fields developed by Eshleman et al. (1979) is combined with a generalized weak scattering scintillation theory that includes the effect of focusing around a curved limb to obtain expressions for the scintillation index and power spectrum. Results are then applied to the radio observations of the crossing of the focal evolute of Jupiter by Voyager 1, in which no flash was detected, and to the effects of the solar coronal plasma on the gravitational lens of the sun.

Haugstad, B. S.↗
NASA NTRS1982

The periodic variation of spokes in Saturn's rings

The discovery of a periodic variation in spoke activity in Saturn's rings from the analysis of Voyager images is reported. A Fourier power spectrum was computed using a data set generated by quantifying spoke activity observed on the morning (western) half of the rings in Voyager images spanning 5.5 days (about 12 Saturn rotations). A peak approximately 14 sigma above the noise in the frequency domain was found at a period of 621 + or - 22 min. Within the formal error, this value is consistent with the 639.4-min rotation period of Saturn's magnetic field. Maximum spoke activity is most likely to be observed on the morning half of the rings when a particular magnetic field sector coincides with this area. This magnetic sector contains the region which is aligned with local noon at the time of the emission of the SKR (Saturn kilometric radiation). These results suggest that the fundamental period of spoke variation is that of Saturn's magnetic field, and that spoke activity is associated with the region of the field which gives rise to the SKR. Passage of this region through Saturn's shadow may play a significant role in the creation and/or rejuvenation of spokes.

Porco, C. C.↗
NASA NTRS1982

The noise of BL Lacertae

Results are presented from an intensive optical and IR monitoring program of the flux and polarization characteristics of BL Lac. It is found that the polarization variations increase in amplitude with increasing time interval, and that the path traced out by the polarization vector in the Q-U plane is a random walk. In view of earlier measurements of BL Lac, the polarization fluctuations can be represented at low frequencies by the flat power spectrum of white noise, up to a frequency of 0.05 cycles/day. Above this frequency, the spectrum steepens to that of a random walk. A model for BL Lac suggested by the polarimetric noise can be constructed from independent sources of light with randomly oriented, strong polarization. Small random differences in spectral index from source to source could also explain the variable wavelength dependence of polarization.

Moore, R. L.↗
NASA NTRS1982

Shape and texture

Methods used to measure the geometrical properties of regions in a segmented image are discussed including the use of centroids, moments, and principle axes. In addition, statistical picture properties, particularly those which describe visual texture, are discussed. Gray level statistics, local property statistics, and autocorrelation and power spectrum are addressed.

Rosenfeld, A.↗
NASA NTRS1983

Large amplitude MHD waves upstream of the Jovian bow shock

Observations of large amplitude magnetohydrodynamics (MHD) waves upstream of Jupiter's bow shock are analyzed. The waves are found to be right circularly polarized in the solar wind frame which suggests that they are propagating in the fast magnetosonic mode. A complete spectral and minimum variance eigenvalue analysis of the data was performed. The power spectrum of the magnetic fluctuations contains several peaks. The fluctuations at 2.3 mHz have a direction of minimum variance along the direction of the average magnetic field. The direction of minimum variance of these fluctuations lies at approximately 40 deg. to the magnetic field and is parallel to the radial direction. We argue that these fluctuations are waves excited by protons reflected off the Jovian bow shock. The inferred speed of the reflected protons is about two times the solar wind speed in the plasma rest frame. A linear instability analysis is presented which suggests an explanation for many of the observed features of the observations.

Goldstein, M. L.↗
NASA NTRS1983

MTF analysis of LANDSAT-4 Thematic Mapper

The spatial radiance distribution of a ground target must be known to a resolution at least four to five times greater than that of the system under test when measuring a satellite sensor's modulation transfer function. Calibration of the target requires either the use of man-made special purpose targets with known properties, e.g., a small reflective mirror or a dark-light linear pattern such as line or edge, or use of relatively high resolution underflight imagery to calibrate an arbitrary ground scene. Both approaches are to be used in addition a technique that utilizes an analytical model for the scene spatial frequency power spectrum is being investigated as an alternative to calibration of the scene.

Schowengerdt, R.↗
NASA NTRS1983

Gain Stability Measurements at S-band and X-band

The Search for Extraterrestrial Intelligence (SETI) requires low noise broadband total power radiometers of high gain stability. Preliminary results of SETI field tests for a variety of receiver configurations at Deep Space Station 13 are discussed. The power spectra of Deep Space Network low noise receiving systems exhibit I/F noise whose spectral index is significantly smaller than unity. The gain stability of the S-band systems tested is almost 0.0001. An appendix presents a derivation of the power spectrum of the output of a square law detector.

Gulkis, S.↗
NASA NTRS1982

Temporal variability of ultraviolet cloud features in the Venus stratosphere

The observed temporal variability of ultraviolet cloud brightness in over two months of Pioneer Venus Orbiter Cloud Photopolarimeter imaging is considered. It is found that the longitudinal mean brightness record shows evidence of two planetary-scale waves. One appears to be centered on the equator but has significant amplitude at all latitudes. The other is centered at midlatitudes and is more confined latitudinally. Visually, the two waves correspond to the longitudinal motion of the dark equatorial band and the oscillation in latitude of the bright polar bands, respectively. The slope of the longitudinal cloud brightness power spectrum at intermediate wavenumbers fluctuates with a periodicity of approximately 2-3 weeks in southern midlatitudes. The polar regions in both hemispheres vary in brightness on a time scale of approximately two months. The disk-integrated brightness increased with time throughout the nominal mission.

Del Genio, A. D.↗
NASA NTRS1982

Observations of tropospheric phase scintillations at 5 GHz on vertical paths

The article presents observations of turbulence-induced tropospheric phase fluctuations measured at 5 GHz on the near-vertical paths relevant to many astronomical and geophysical measurements. The data are summarized as phase power spectra, structure functions, and Allan variances. Comparisons to other microwave observations indicate relatively good agreement in both the level and shape of the power spectrum of these tropospheric phase fluctuations. Implications for precision Doppler tracking of spacecraft and geodesy/radio interferometry are discussed.

Armstrong, J. W.↗
NASA NTRS1982

A dispersion law for solar oscillations

The simple model of p-mode solar oscillations of Liebacher and Stein (1981), in which the acoustic vibrations are trapped in a resonant cavity taking the form of a spherical shell below the solar surface, is compared with Doppler shift observations of vertical velocities. The model is shown to predict a modified dispersion law in which the sound travel time across the cavity is a function of the ratio of the temporal frequency to the horizontal wavenumber, resulting in a single curve when the temporal frequency is plotted against the wavenumber. Frequencies derived from a two-dimensional power spectrum of velocity observations are found to conform to a modified version of the dispersion relation, and that only when the fundamental mode is excluded. Results thus suggest that all modes with the same frequency/wavenumber ratio are trapped in an identical cavity, or, more plausibly, that the difference in upper boundary conditions for different modes has minimal effect on the resulting frequencies.

Duvall, T. L., Jr.↗
NASA NTRS1982

Band-pass filtering of one year of daily mean pressures on Mars

The sol-mean atmospheric pressures measured at the two Viking landers exhibit fluctuations ranging in period from seasonal to a few days. The time series are highly nonstationary, which severely limits standard methods such as power-spectrum analysis. Advantage is taken of the rather precise annual periodicity of these data to design sharp numerical filters and use them to split the series at each lander into six time series, each containing oscillations associated with a discrete frequency band. This analysis reveals 2- to 4-sol waves during the cold seasons at both landers, and 8-sol waves during the spring at Lander 2, confirming previous results from spectral analysis. As previously predicted, high-frequency waves appear suppressed during the second dust storm. Certain low-frequency signals, probably associated with the global dust storms, are detected and demonstrated to be significant for the first time.

Niver, D. S.↗
NASA NTRS1981

Observations of dynamical phenomena in sunspots

A preliminary report of the results of one observing run based on data from one spectral line, the photospheric magnetic line Fe 6303, is presented as part of a series of observations of dynamical phenomena in sunspots using photographic spectra with the SPO vacuum tower telescope and echelle spectrograph. The ejection of a magnetic feature from the outer edge of the penumbra was observed. The initial total field strength of the feature was about 1000 gauss, which appeared to decrease as the feature moved away from the sunspot. The proper motion was about 2 km/s, and the velocity field measured in the V profile showed a downflow of 400 m/s on the spotward side of the moving magnetic feature. Umbral oscillations at the photospheric level with a herringbone structure characteristic of horizontally propagating waves, suggesting some overtone mode of membrane oscillation in the umbra, were seen. The peak amplitude of the oscillation was about 200 m/s, and the mean power spectrum had several clear peaks.

Nye, A. H.↗
NASA NTRS1982

Study of rotor gust response by means of the local momentum theory

The vertical gust response of the helicopter rotor in cruising flight is studied analytically by means of the local momentum theory (LMT) and experimentally in a wind tunnel utilizing a gust generator. By introducing the unsteady aerodynamic effects into the LMT and by considering the elastic deformation of the rotor blade, the vibratory characteristics of flapping blades and of the rotor forces can be obtained. Since the LMT makes it possible to calculate the instantaneous load distribution along the rotor blade in desired azimuthal or timewise intervals, the effects of gradual penetration of the rotor into the gust can be studied and the Fourier analysis or power spectrum analysis of the rotor response applied to any kind of gust input. Some results obtained analytically are verified by the experimental tests performed in the wind tunnel which generates step, sinusoidal and random vertical gusts by the motion of a series of cascaded vanes upstream of the rotor.

Azuma, A.↗
NASA NTRS1983

Intensity dependence of the pulse profile and polarization of the Vela pulsar

Polarization measurements of the Vela pulsar PSR 0833-45 at 2295 MHz revealed that the radio pulse profile is composed of four components. The four component configuration was found to fit the observed dependence of the pulse profile on the intensity of the pulsar, and the dependence of the modulation index and power spectrum of the intensity fluctuations on the pulse-longitude. Deviations of the observed polarization position angles from those predicted by a rotating vector model were explained by the emission regions for each component being located at different distances from the star. The components are affected by retardation, aberration, and magnetic field curvature effects, all of which contribute to arrival of the components at different longitudes even though the components actually carry the position angle signature predicted by the rotating vector model.

Krishnamohan, S.↗
NASA NTRS1983

Evidence for quasi-periodic components in Dobson network total ozone records

Power spectrum analysis has been applied to the total ozone time series at each of a sample of Dobson stations with records of more than 15 years in the interval 1957 to 1981. The distributions of strongly periodic or quasiperiodic signals in these spectral estimates, for three spectral estimators and two levels of confidence, show distinct features. These appear at the previously known or suspected periods of 3.5-4.0 years, 27 months, and 21 months as well as the previously unreported period of 11.5 months. Possible geophysical implications are discussed.

Guthrie, P. D.↗
NASA NTRS1983

First VHF auroral radar interferometer observations

Auroral plasma instabilities were investigated using the radar interferometer technique based on observations obtained with a 49.92 MHz, 20-25 KW peak power pulsed radar located in Ithaca, NY (42.5 degrees N, 76.4 degrees W). Strong auroral echoes obtained during several highly active periods were analyzed. Phase differences between the signals received on the two antennas were utilized to accurately determine the E-W position, within the scattering volume, of localized scattering centers, and changes in this phase were used to determine the corresponding velocity. The radial (essentially N-S) motion was described by the signal Doppler shift. It is found that these data provide detailed information on the turbulent structure of the echoing region and show clearly that different features in the Doppler power spectrum often represent signals coming from different locations. It is concluded that these data can be utilized to determine full horizontal velocity vectors and hence the horizontal electric field, usually with a time resolution of the order of 15-30 s.

Providakes, J. F.↗
NASA NTRS1983

An analysis of periodicities in the 1470 to 1974 Beijing precipitation record

An analyis of a time series consisting of an annual index of dryness/wetness for the years 1470 to 1974 in Beijing, China is presented. Its power spectrum shows that dominant cycles occur with long periods of the order of 80 years. Cycles with periods of 11 and 22 years are weak or non-existent, but a significant signal at 18.7 years (which is also the period of a component of the lunar tide generating force) is detected. The long term variations in Beijing precipitation appear to lag long term (Gleissberg) variations in solar activity by nearly 75 years. A pattern which spans nearly 150 years in the Beijing record is found to be repeated with notable similarity.

Hameed, S.↗
NASA NTRS1983

Apparent electrostatic ion cyclotron waves in the diffuse aurora

Emissions that have properties consistent with electrostatic ion cyclotron (EIC) waves have been observed at low altitude in the diffuse aurora by a sounding rocket payload. Peaks were observed in the power spectrum of the electric field near the hydrogen and oxygen ion cyclotron frequencies. Doppler shift and polarization analyses have been performed using EIC wave parameters derived from linear theory. Both analyses indicated that these emissions had properties consistent with those expected for H(+) and O(+) EIC waves. The two analyses indicated that both emission bands were due to waves propagating eastward parallel to the poleward boundary of the diffuse aurora. The large local cold plasma density and resulting Landau damping require that the source be local. Magnetometer data indicated the presence of a downward parallel current density of 5 microamps/sq m. Sufficient free energy for the waves was available from this current, although the waves were observed frequently at altitudes where the ion-neutral collision frequency exceeded the oxygen cyclotron frequency.

Bering, E. A.↗