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At least 955 records · Page 53
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NASA NTRS1990

Three-dimensional potential flows from functions of a 3D complex variable

Potential, or ideal, flow velocities can be found from the gradient of an harmonic function. An ordinary complex valued analytic function can be written as the sum of two real valued functions, both of which are harmonic. Thus, 2D complex valued functions serve as a source of functions that describe two-dimensional potential flows. However, this use of complex variables has been limited to two-dimensions. Recently, a new system of three-dimensional complex variables has been developed at the NASA Ames Research Center. As a step toward application of this theory to the analysis of 3D potential flow, several functions of a three-dimensional complex variable have been investigated. The results for two such functions, the 3D exponential and 3D logarithm, are presented in this paper. Potential flows found from these functions are investigated. Important characteristics of these flows fields are noted.

Kelly, Patrick↗
NASA NTRS1990

Total solar irradiance variability - 5 years of ERBE data

Data obtained by the ERBS solar monitors measuring total solar irradiance variability are discussed. The ERBS and NOAA-99 monitors derived 1365 W/sq m as the magnitude of the solar irradiance, normalized to 1 astronomical unit. The NOAA-10 monitor yielded 1363 W/m as the magnitude of the irradiance. The long-term precision of the monitors was demonstrated by the detection of the decreasing and increasing trends in the irradiance at levels of the order of 0.03 to 0.05 percent per year. The ERBS and NOAA-9 measurements demonstrated that solar variability exists in a systematic mode which is directly correlated with the solar magnetic activity, indicated by sunspot activity. It is argued that during the decline of sunspot cycle 22, the solar irradiance variability may be entirely different from that which was observed during the decline of cycle 21.

Lee, Robert B., III↗
NASA NTRS1991

The connection between coronal bright points and the variability of the quiet-sun extreme-ultraviolet emission

An analysis of the EUV/Skylab data recorded at six wavelengths simultaneously and scanning the chromosphere to the corona is reported. The results show that, whether in a coronal hole or a quiet region, the significant variable emission in the quiet sun is preferentially localized in the enhanced emission produced by EUV bright points in the boundaries of network cells. The spatial density of the variable and enhanced emission varies with temperature, with a minimum at 3 x 10 exp 5 K which indicates two favored temperature distributions of small-scale loops in the quiet sun, one below 3 x 10 exp 5 K and one at coronal temperatures. The enhanced emission occupies between 10 and 25 percent of the solar surface, depending on the temperature, while the variable emission covers between 5 and 15 percent. For a given region, the spatial density at a given temperature does not vary in time by more than 10 percent about its mean value.

Habbal, Shadia R.↗
NASA NTRS1992

The optical variability of PKS 2155 - 304

Results of photometric monitoring of the BL Lacertae object PKS 2155 - 304 between 1979 and 1990 are presented. Optical variations were observed on time scales ranging from several minutes to several years. The variations detected with time scales on the order of minutes are among the fastest optical variations detected for a BL Lacertae object. These rapid variations provided constraints on the mass of the supermassive black hole, assumed to be present at the center of the object, of the range between 5.5 +/-0.4 x 10 exp 8 and 8.1 +/-1.0 x 10 exp 9 solar masses. The results indicate that the characteristic variability time scale for the optical variability is shorter than the characteristic variability time scale established for the UV of approximately 50 d reported by Treves et al. (1989).

Carini, Michael T.↗
NASA NTRS1992

Variables to be considered for further human exploration of space

Now embarked on its fourth decade of space exploration, the United States is turning from singular forays to long-term, evolutionary exploration and utilization of space. But while the goals of U.S. space exploration have changed, the many technical variables which exist for mission planners are essentially the same ones an earlier generation faced at the beginning of the Space Age. How these variables are defined and then answered will determine when and in what form future U.S. expeditions will depart for the moon and Mars. This paper discusses both the emerging rationales for a more robust space program, and the many variables such efforts will entail, with the goal of providing a starting point for the many choices space planners will encounter in the future.

Cohen, Aaron↗
NASA NTRS1992

Variability of near-infrared emission lines in NGC 4151 - Implications for nuclear star formation

Multiaperture infrared spectra of the circumnuclear region of the Seyfert galaxy NGC 4151 taken over a 10 year period show that the N = 7-4 B-gamma hydrogen recombination line is variable. The H2 quadrupole line emission shows no sign of variability. It is shown that there is a substantial contribution to the nuclear B-gamma flux from the active nucleus, making it impossible to determine star formation rates from infrared hydrogen recombination lines in active galaxies. The lack of variability in the H2 line fluxes indicates that ionizing photons from the active nucleus is not important in exciting the molecular line emission.

Prestwich, Andrea H.↗
NASA NTRS1992

On the origin of variable 511 keV line emission from the Galactic center region

Variable narrow-line emission at 511 keV, due to positron annihilation, has been observed from the region of the Galactic center for over a decade with high-resolution Ge spectrometers. The variable nature of this emission suggests that a significant fraction of the observed radiation is produced by a single source in the central region of the Galaxy. Recent observations with an imaging gamma-ray spectrometer of low energy resolution have revealed a daylong burst of annihilation radiation from the X-ray source 1E 1740.7-2042 located at an angular distance of 0.9 deg from the Galactic center and aligned with a dense molecular cloud. It is proposed that the variable narrow 511 keV line emission is due to positrons released impulsively (time scale of about 1 day) from 1E 1740.7-2942 into the molecular cloud where they slow down and annihilate on a longer time scale of up to a year.

Ramaty, R.↗
NASA NTRS1992

Long-term variability in low-mass X-ray binaries - A study using data from Vela 5B

A complete sample of low-mass X-ray binaries in uncrowded fields obtained from Vela 5B is studied here for periodic and aperiodic time variability on time scales of more than one day. The strong 176-day periodicity in X1820 - 303 is confirmed, and evidence is found for a significant 77-day period in Cyg X-2 and for variability on time scales of less than about 200 days in Cyg X-3. Limits are set on the possible amplitude of candidate periodicities in the literature for Ser X-1 and X0614 + 091. The evidence reported here suggests that while long-term cyclic variability is common among high-mass binaries, it is a rare occurrence in low-mass systems.

Smale, Alan P.↗
NASA NTRS1993

Elements Of Theory Of Multidimensional Complex Variables

Two reports describe elements of theory of multidimensional complex variables, with emphasis on three dimensions. First report introduces general theory. Second, presents further developments in theory of analytic functions of single three-dimensional variable and applies theory to representation of ideal flows. Results of preliminary studies suggest analytic functions of new three-dimensional complex variables useful in numerous applications, including representing of three-dimensional flows and potentials.

Martin, E. Dale↗
NASA NTRS1993

Kalman-Filter Estimation Of Unmeasured Output Variables

Kalman-filter "observer" algorithm used in estimating unmeasured output variables of turbofan engine. Unmeasured output variables needed along with measured output variables as feedback data to characterize state of aircraft-and-engine system for control purposes. Subsystem or processor implementing Kalman-filter algorithm incorporated into adaptive digital control system, called "performance-seeking" control system, developed to compute engine-control settings to optimize performance of aircraft under given pseudo-steady-state cruise conditions. Basic concept of algorithm not restricted to aircraft-engine application and can be extended to internal-combustion engines in land vehicles.

Gilyard, Glenn B.↗
NASA NTRS1992

The interannual variability of polar cap recessions as a measure of Martian climate and weather: Using Earth-based data to augment the time line for the Mars observer mapping mission

The recessions of the polar ice caps are the most visible and most studied indication of seasonal change on Mars. Circumstantial evidence links these recessions to the seasonal cycles of CO2, water, and dust. The possible advent of a planet encircling storm during the Mars Observer (MO) mission will provide a detailed correlation with a cap recession for that one Martian year. That cap recession will then be compared with other storm and nonstorm years. MO data will also provide a stronger link between cap recessions and the water and CO2 cycles. Cap recession variability might also be used to determine the variability of these cycles. After nearly a century of valiant attempts at measuring polar cap recessions, including Mariner 9 and Viking data, MO will provide the first comprehensive dataset. In contrast to MO, the older data are much less detailed and precise and could be forgotten, except that it will still be the only information on interannual variability. By obtaining simultaneous Earth-based observations (including those from Hubble) during the MO mission, direct comparisons can be made between the datasets.

Martin, L. J.↗
NASA NTRS1993

Real-time transmission of digital video using variable-length coding

Huffman coding is a variable-length lossless compression technique where data with a high probability of occurrence is represented with short codewords, while 'not-so-likely' data is assigned longer codewords. Compression is achieved when the high-probability levels occur so frequently that their benefit outweighs any penalty paid when a less likely input occurs. One instance where Huffman coding is extremely effective occurs when data is highly predictable and differential coding can be applied (as with a digital video signal). For that reason, it is desirable to apply this compression technique to digital video transmission; however, special care must be taken in order to implement a communication protocol utilizing Huffman coding. This paper addresses several of the issues relating to the real-time transmission of Huffman-coded digital video over a constant-rate serial channel. Topics discussed include data rate conversion (from variable to a fixed rate), efficient data buffering, channel coding, recovery from communication errors, decoder synchronization, and decoder architectures. A description of the hardware developed to execute Huffman coding and serial transmission is also included. Although this paper focuses on matters relating to Huffman-coded digital video, the techniques discussed can easily be generalized for a variety of applications which require transmission of variable-length data.

Bizon, Thomas P.↗
NASA NTRS1992

The global structure of the annual and semiannual sea surface height variability from Geosat altimeter data

Two years of data from the Geosat Exact Repeat Mission are employed to study the annual and semiannual variability of sea-surface height on a global basis. The data are treated with atmospheric corrections and interpolations, and the orbit error of about 40 cm RMS is purged. The spatial variability of the Geosat data is constructed for particular frequencies, and the estimate of the M2 tidal error indicates that the error is aliased to 1.15 cycles/yr and has an unusual spatial pattern. Sine and cosine coefficients are derived for the annual and semiannual frequencies by means of a least squares fit yielding the amplitude and phase of the changes in sea-surface height. A 180-deg phase difference is noted between the Northern and Southern Hemispheres for the annual variability, and large-scale westward propagating waves are identified. The Geosat data also indicate the phase relationships between major current systems and the systems of variations at work in the Intertropical Convergence Zone.

Jacobs, Gregg A.↗
NASA NTRS1992

Soil moisture variability within remote sensing pixels

The effects of topography and the level of soil moisture on the variability of soil moisture within remote sensing pixels were assessed during the First ISLSCP Field Experiment (FIFE) during 1987 and 1989. Soil moisture data from flat, sloped, and valley-shaped pixels were obtained over a wide range of moisture conditions. Relative elevation data were obtained for each study area to create digital elevation models with which to quantify topographic variability. Within-pixel soil moisture variability was shown to increase with increased topographic heterogeneity. The flat pixel had significantly lower standard deviations and fewer outlier points than the slope and valley pixels. Most pixel means had a positive skewness, indicating that most pixels will have areas of markedly higher than average soil moisture.

Charpentier, Michael A.↗
NASA NTRS1993

The variable stars of the young LMC cluster NGC 2164

The present search of the LMC cluster NGC 2164 for variable stars has uncovered one new member classical Cepheid variable with 3.772-day period; attention is also given to photometry for a previously unknown field overtone Cepheid variable with 3.4626-day period, and the 10.6878-day period HV 12078, which may be a member of the young NGC 2156 cluster. The clear separation of fundamental and overtone pulsators in the period-luminosity-color relation of known LMC cluster Cepheids establishes that the NGC 2164 member is a true overtone.

Welch, Douglas L.↗
NASA NTRS1992

Variable EUV emission in the quiet sun and coronal heating

We review the characteristics of the variable emission from the small scale structure in the quiet sun, in view of the recent theoretical proposals that microflares are responsible for the heating of the corona and the solar wind. We consider the observational properties of the variable emission in quiet regions and coronal holes. Our results are based primarily on simultaneous multiwavelength EUV observations, supplemented by combinations of simultaneous cm radio, He I 10830 A, X-ray and line of sight photospheric magnetic field measurements. We show that the variable emission from the small-scale structure has surprisingly well-defined properties. Yet, within the limit of the temporal and spatial resolution of data currently available, the radiative losses, from this component of the emission, are a factor of ten smaller than that in the quiet sun. Hence, the theory of microflare heating cannot rely on these observations for support.

Habbal, S. R.↗
NASA NTRS1993

Evidence for two distinct morphological classes of gamma-ray bursts from their short time scale variability

We have analyzed the 241 bursts for which peak counts (C)max exist in the publicly available Burst and Transient Source Experiment (BATSE) catalog. Introducing peak counts in 1024 ms as a measure of burst brightness B and the ratio of peak counts in 64 and 1024 ms as a measure of short timescale variability V, we find a statistically significant correlation between the brightness and the short time scale variability of gamma-ray bursts. The bursts which are smoother on short time scales are both faint and bright, while the bursts which are variable on short time scales are faint only, suggesting the existence of two distinct morphological classes of bursts.

Lamb, D. Q.↗
NASA NTRS1993

Observations of entrainment and time variability in the HH 47 jet

We present new Fabry-Perot images of the HH 47 jet that show the first clear evidence for entrainment in a jet from a young star. The material in the jet moves faster down the axis of the flow and slower at the edges, similar to viscous flow in a pipe. The higher excitation lines occur along the edges of the jet, as expected if entrainment accelerates and heats the ambient material. We confirm previous observations of multiple bow shocks in this system. Together, time variability and entrainment produce much of the observed shock-excited gas in this object. Our data show that the 'wiggles' along the jet are not caused by jet material tied to a spiraling magnetic field, but instead result from time variability, variable ejection angles, or inhomogeneities in the flow. The gas entrained in the HH 47 jet may be atomic; our results do not provide direct evidence that stellar jets drive molecular outflows.

Hartigan, Patrick↗