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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Atmospheric structure and variability in areas of convective storms determined from 3-h rawinsonde data

The structure and variability of the atmosphere in areas of radar-observed convection were established by using 3-h rawinsonde and surface data from NASA's second Atmospheric Variability Experiment. Convective activity was shown to exist in areas where the low and middle troposphere is moist and the air is potentially and convectively unstable and has upward motion, in combination with positive moisture advection, at either the surface or within the boundary layer. The large variability of the parameters associated with convective storms over time intervals less than 12 h was also demonstrated so as to possibly produce a change in the probability of convective activity by a factor of 8 or more in 3 h. Between 30 and 60 percent of the total changes in parameters associated with convective activity over a 12-h period were shown to take place during a 3-h period. These large changes in parameters are related to subsynoptic-scale systems that often produce convective storms.

Wilson, G. S.↗
NASA NTRS1975

Stellar model chromospheres. IV - The formation of the H-epsilon feature in the sun /G2 V/ and Arcturus /K2 III/

The formation of the Balmer-series member H-epsilon in the near-red wing of the Ca II H line is discussed for two cases: the sun (H-epsilon absorption profile) and Arcturus (H-epsilon emission profile). It is shown that although the H-epsilon source functions in both stars are dominated by the Balmer-continuum radiation field through photoionizations, the line-formation problems in the two stars are quantitatively different, owing to a substantial difference in the relative importance of the stellar chromosphere temperature inversion as compared with the stellar photosphere.

Ayres, T. R.↗
NASA NTRS1981

H I and dust in Kutner's cloud

A map in the 1.420 GHz line of H I, with an angular resolution of 9.1 arcmin, is made toward Kutner's Cloud and the region between Kutner's Cloud and Heiles' Cloud. It is assumed that the cold H I in the clouds is located only in front of, and absorbs the H I background; the column density of cold H I, N(H I), is obtained from this absorption. The visual extinction for this region is determined from star counts on red and blue POSS prints. It is noted that most of the gas in the cloud is in the form of H2. The column density of H2 is obtained from the visual extinction and the extinction-to-gas ratio given by Bohlin et al. (1978).

Batrla, W.↗
NASA NTRS1982

A surge observed in H alpha and C IV

Results are presented of simultaneous measurements of H-alpha (MSDP at Meudon) and C IV (UVSP onboard SMM) of Active Region 2701 made on October 2, 1980. Isodensity and velocity maps were obtained for both lines and these maps were superimposed. Results show a good correlation between the H-alpha and C IV velocities with a surge being observed for 10 minutes. The base of the surge was determined to be located in a bright point in C IV and H-alpha, while the escaping matter followed the same channel ('absorbing' in H-alpha, 'emitting' in C IV). It was found that the velocity along the surge was about 80 km/s in H-alpha and 100 km/s in C IV. In addition, a loop appeared in C IV during the surge. It is concluded that the vertical pressure gradient was capable of driving the surge.

Schmieder, B.↗
NASA NTRS1986

The sensitivity of H alpha profiles to rapid electron beam fluctuations

In order to understand the temporal relationship between H alpha and hard X-ray emission predicted by the nonthermal electron thick target model of impulsive-phase energy transport the time-dependent theoretical H alpha profiles were computed for the dynamic model atmospheres of Fisher, Canfield, and McClymont, which simulate the effects of an impulsively initiated power-law beam of electrons. On the basis of the physical analysis it was expected that a very rapid H alpha response to an instantaneous increase in the flux of a nonthermal deka-keV electron beam, as compared to the timescale associated with the propagation of these electrons over characteristic flare coronal loop spatial scales. It was concluded that observational efforts to test the thick target nonthermal electron model through detection of impulsive H alpha brightenings associated with impulsive hard X-ray or microwave bursts should initially focus attention on the H alpha line center. Additional simultaneous blue-wing measurements will have substantial diagnostic potential.

Canfield, Richard C.↗
NASA NTRS1986

Continued observations of the H Ly alpha emission from Uranus

Observations of Uranus obtained over four years with the IUE Observatory supports the initial identification of a bright H Ly alpha flux which varies independently of the solar H Ly alpha flux, implying a largely self-excited emission. An average brightness of 1400 Rayleighs is derived, and limits for the possible contribution by reflected solar H Ly alpha emission, estimated to be about 200 Rayleighs, suggest that the remaining self-excited emission is produced by an aurora. Based on comparison with solar wind measurements obtained in the vicinity of Uranus by Voyager 2 and Pioneer 11, no evidence for correlation between the solar wind density and the H Ly alpha brightness is found. The upper limit to H2 emission gives a lower limit to the ratio of H Ly alpha/H2 emissions of about 2.4, suggesting that the precipitating particles may be significantly less energetic on Uranus than those responsible for the aurora on Jupiter. The average power in precipitating particles is estimated to be of the order of 10 to the 12th W.

Clarke, J.↗
NASA NTRS1987

The H I distribution in clouds within galaxies

An interpretation is given for the relatively flat distribution of atomic hydrogen as a function of radius within the visible disks of spiral galaxies and the existence of a correlation between H I surface density and morphological class of spiral galaxy. The H I is mostly contained in constant column density envelopes of molecular clouds. The clouds have a two-dimensional filling factor of unity when a galaxy is viewed face on. The column density of the cloud envelopes depends on the ratio of flux of H2 dissociating radiation in the ambient medium to envelope gas density. The flat H I profiles imply the gas density and pressure follow the ultraviolet ambient flux on kiloparsec scales throughout a given galaxy. Later-type galaxies have larger ultraviolet surface brightnesses, as deduced from the H-alpha surface brightnesses. Therefore, thicker protective H I envelopes develop in late-type spirals.

Shaya, E. J.↗
NASA NTRS1988

Molecules, grains, and shocks - A comparison of CO, H I, and IRAS data

The IR and H I properties and CO content of a set of 26 isolated, roughly degree-sized interstellar clouds are compared. It is argued that G230.1-28.4, the cloud with the narrowest H I line, has recently been shocked by the same collision that a neighboring cloud, G228.0-28.6, is currently undergoing. The ratio S(100)/N(H I) is found to range up to much larger values than predicted theoretically, where N(H) represents the column density of H nuclei and S(100) is the 100 micron surface brightness. Clouds with the highest such values are observed to contain CO, and it is inferred that they contain H2. The (H2/H I) ratio of a cloud increases as the measure of its internal velocity differences decreases. The dependence of S(60)/S(100) on V(LSR) implies that fast shocks preferentially destroy large grains or produce small grains, or both.

Heiles, Carl↗
NASA NTRS1988

The stellar radiation field and the ionization of H and He in the local interstellar medium

Detailed photoionization calculations for the local interstellar medium (LISM) are presented based on constraints imposed by H I column density estimates derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. It is found that the nearby hot white dwarfs dominate the stellar contributions to the EUV radiation field. Considering stellar contribution to the EUV background alone, the resulting lower limits to the hydrogen and helium fractional ionization in the local diffuse cloud, at the sun, are 0.091 and 0.089, respectively. This result is insensitive to the absorbing geometry of the local cloud. The comparable ionizations of H and He near the sun are due to the lack of absorption from photospheric helium in the hot DA white dwarfs, to the difference in optical depths at wavelengths shortward of the H I and He I ionization edges, and to the fact that the hydrogen ionization controls the electron number density. The derived H and He ionization limits have important implications for the interpretation of the He I and H I backscattering results, for which uncertainties in charge-exchange interactions occurring in or near the heliopause reflect additional significant uncertainties in determining the ionization in the LISM.

Bruhweiler, Frederick C.↗
NASA NTRS1990

H I absorption toward cooling flows in clusters of galaxies

An H I survey of 14 cooling flow clusters and two noncooling flow clusters was conducted, and H I absorption features were detected against the nuclear radio continuum sources of two cooling flow dominant (CFD) galaxies, 2A 0335 + 096 and MKW3s. The absorption features are broad and redshifted with respect to the stellar absorption-line velocity of the CFDs by 90-225 km/s. This indicates that the H I is falling onto, and is probably gravitationally bound to, the CFDs. The kinematics of the H I clouds suggest a possible kinematic link between the warm and cold phases of the intracluster medium. The clouds are orders of magnitude smaller in radius and mass and larger in density than Galactic H I clouds. The detected CFDs have mass-accretion rates that are about 2.5 times larger than the CFDs that were not detected.

Mcnamara, Brian R.↗
NASA NTRS1989

High resolution observations of compact H II regions at 230 GHz

Based on the idea that star formation goes on progressively in molecular clouds, a search was conducted for protostars by mapping compact H II regions at a frequency of 250 GHz. The IRAM 30 m radio telescope was used with a (3)He cooled bolometer. Twenty compact H II regions usually obtaining twice the expected free-free flux density, positionally coincident with the H II region, were observed. Even fine structure within the H II regions can be traced in the maps as in the case of G75.84+0.40 near ON-2. The high degree of coincidence between the 250 and 5 GHz map of Harris shows that the excess flux density observed must come from dust mixed with the ionized gas. Part of the dust must however be accumulated in the outer part of the H II region, since in some cases the contours are shifted outwards relative to the radio maps. This is consistent with the fact that in those cases where enough information is available to make a model fit, temperatures were derived of 80 + or - 30 K.

Wink, JOERN-E.↗
NASA NTRS1990

Galaxy interactions and star formation: Results of a survey of global H-alpha emission in spiral galaxies in 8 clusters

Kennicutt and Kent (1983) have shown that the global H alpha emission from a spiral galaxy is an indicator of the formation rate of massive stars. Moss, Whittle and Irwin (1988) have surveyed two clusters (Abell 347 and 1367) for galaxies with H alpha emission using a high dispersion objective prism technique. The purpose of the survey is to investigate environmental effects on star formation in spiral galaxies, and in particular to ascertain whether star formation is enhanced in cluster spirals. Approximately 20 percent of CGCG galaxies were detected in emission. Two plates of excellent quality were obtained for each of the two clusters, and galaxies were only identified to have emission if this was detected on both plates of a plate pair. In this way, plate flaws and other spurious identifications of emission could be rejected, and weak emission confirmed. The results of this survey have been discussed by Moss (1987). The detected galaxies are of types SO-a and later. The frequency with which galaxies are detected in emission increases towards later morphological type as expected (cf. Kennicutt and Kent 1983). There is no evidence of any dependence of the frequency of detected emission on the absolute magnitude of the galaxy (cf. Moss and Whittle 1990), but there is a strong correlation between a disturbed morphological appearance of the galaxy and the detection of emission. Furthermore it is found that the emission is more centrally concentrated in those galaxies which show a disturbed morphology. It may be noted that the objective prism plate gives a spectrum of a 400 A region around rest wavelength H alpha, but superposed on this is the H alpha emission from the galaxy which, because the light is essentially monochromatic, results in a true two-dimensional image of the H alpha distribution. The visual appearance of the emission on the prism plates was classified according to its diffuseness on a 5 point scale (very diffuse, diffuse, intermediate, compact, and very compact). In tabular form, the relation is shown between this classification and a morphologically disturbed appearance for the galaxy.

Moss, C.↗
NASA NTRS1991

Integrated flight/propulsion control design for a STOVL aircraft using H-infinity control design techniques

Results are presented from an application of H-infinity control design methodology to a centralized integrated flight propulsion control (IFPC) system design for a supersonic Short Takeoff and Vertical Landing (STOVL) fighter aircraft in transition flight. The emphasis is on formulating the H-infinity control design problem such that the resulting controller provides robustness to modeling uncertainties and model parameter variations with flight condition. Experience gained from a preliminary H-infinity based IFPC design study performed earlier is used as the basis to formulate the robust H-infinity control design problem and improve upon the previous design. Detailed evaluation results are presented for a reduced order controller obtained from the improved H-infinity control design showing that the control design meets the specified nominal performance objectives as well as provides stability robustness for variations in plant system dynamics with changes in aircraft trim speed within the transition flight envelope. A controller scheduling technique which accounts for changes in plant control effectiveness with variation in trim conditions is developed and off design model performance results are presented.

Garg, Sanjay↗
NASA NTRS1991

Echo mapping of broad H-beta emission in NGC 5548

A one-dimensional echo map of the broad H-beta emission-line region in the Seyfert 1 galaxy NGC 5548 is reconstructed by using a maximum entropy technique to model the variable optical continuum and integrated H-beta emission-line fluxes observed during the 1989-1990 monitoring campaign as reported by Peterson et al. (1991). The echo map has a strong peak at a time delay of 20 lt-days which has an unresolved full width of 20 days at half-maximum. The H-beta response at time delay zero is less than one-third of that at 20 days, implying a deficit of H-beta-emitting gas near the line of sight to the continuum source. This rules out spherically symmetric and edge-on disk geometries for isotropically emitting H-beta clouds.

Horne, Keith↗
NASA NTRS1991

A study of solar flare energy transport based on coordinated H-alpha and X-ray observations

The temporal evolution of the ratio between H-alpha to nonthermal hard X-ray emission was investigated using coordinated H-alpha and hard- and soft-X-ray observations of five solar flares (on May 7, June 23, June 24, and June 25, 1980 and on April 30, 1985). These observations were used to estimate the emitted flare energy flux F(H-alpha) in H-alpha, the flux of F(2O) energy deposited by nonthermal electrons with energies above 20 keV, and the pressure p(c) of soft X-ray-emitting plasma as functions of time during the impulsive phase of each flare. It was found that the F(H-alpha)/F(2O) ratio shows a power-law dependence on F(2O), with a slope that differs slightly from that predicted by the static thick-target model of solar transport. Results also indicate that the power-law dependence is modified by hydrostatic pressure effects.

Canfield, Richard C.↗
NASA NTRS1991

Theoretical characterization of the potential energy surface for the reversible reaction H + O2 yields HO2(asterisk) yields OH + O. III - Computed points to define a global potential energy surface

Computed energies and geometries are reported which, combined with previously published calculations, permit a global representation of the potential energy surface for the reaction H + O2 yields HO2(asterisk) yields OH + O. These new calculations characterize the potential energy surface (PES) for all H atom angles of approach to O2 and for the region of the inner repulsive wall. The region of the T-shaped H-O2 exchange saddle point is connected with the constrained energy minimum (CEM) path, and a new collinear H-O2 exchange saddle point is characterized which lies only 9 kcal/mol above the H + O2 asymptote. A vibrational analysis which utilizes local cubic and quartic polynomial representations of the PES along the CEM path has been carried out. Optimal geometries, energies, and harmonic frequencies are reported along with anharmonic analyses for the O2 and OH asymptotes and for the HO2 minimum region of the PES.

Walch, Stephen P.↗
NASA NTRS1992

M-H characteristics and demagnetization resistance of samarium-cobalt permanent magnets to 300 C

The influence of temperature on the M-H demagnetization characteristics of permanent magnets is important information for the full utilization of the capabilities of samarium-cobalt magnets at high temperatures in demagnetization-resistant permanent magnet devices. In high temperature space power converters, such as free-piston Stirling engine driven linear alternators, magnet demagnetization can occur as a long-term consequence of thermal agitation of domains and of metallurgical change, and also as an immediate consequence of too large an applied field. Investigated here is the short-term demagnetization resistance to applied fields derived from basic M-H data. This quasistatic demagnetization data was obtained for commercial, high-intrinsic-coercivity, Sm2Co17-type magnets from 5 sources, in the temperature range 23 to 300 C. An electromagnet driven, electronic hysteresigraph was used to test the 1-cm cubic samples. The observed variation of the 2nd quadrant M-H characteristics was a typical rapid loss of M-coercivity and a relatively lesser loss of remanence with increasing temperature. The 2nd quadrant M-H curve knee point is used to define the limits of operation safe against irreversible demagnetization due to an excessive bucking field for a given flux density swing at temperature. Such safe operating area plots are shown to differentiate the high temperature capabilities of the samples from different sources. For most of the samples, their 2nd quadrant M-H loop squareness increased with temperature, reaching a peak or a plateau above 250 C.

Niedra, Janis M.↗
NASA NTRS1991

Integrated flight/propulsion control design for a STOVL aircraft using H-infinity control design techniques

Results are presented from an application of H(infinity) control design methodology to a centralized integrated flight/propulsion control (IFPC) system design for a supersonic short take-off and vertical landing (STOVL) fighter aircraft in transition flight. The emphasis is on formulating the H(infinity) control design problem such that the resulting controller provides robustness to modeling uncertainties and model parameter variations with flight condition. Experience gained from a preliminary H(infinity)=based IFPC design study performed earlier is used as the base to formulate the robust H(infinity) control design problem and improve the previous design. Detailed evaluation results are presented for a reduced-order controller obtained from the improved H(infinity) control design showing that the control design meets the specified nominal performance objectives as well as provides stability robustness for variations in plant system dynamics with chnages in aircraft trim speed within the transition flight envelope.

Garg, Sanjay↗