Search NASA⌕ Search

SEARCH · Search NASA

Results for “(alpha)-variation”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

H-alpha redshifts as a diagnostic of solar flare heating

The dynamics of chomospheric condensations formed during chromospheric evaporation are studied using coordinated X-ray and H-alpha observations of five solar flares. It is shown that the peak downflow velocity of condensations predicted by simple hydrodynamic compression of the chromosphere is in good agreement with empirical downflow velocities implied by impulsive phase H-alpha red wing Doppler shifts. It is found that the H-alpha wing redshift provides a diagnostic of the pressure excess in the evaporating region and the coronal energy flux driving chromospheric evaporation. Because preflare coronal pressures derived for each of the flares are high, it is suggested that the chromosphere succeeds in radiating excess flare heat energy without undergoing explosive chromospheric evaporation.

Zarro, Dominic M.↗
NASA NTRS1989

Alpha particles in field-aligned beams upstream of the bow shock - Simulations

The possibility that field-aligned ion beams (FABs) formed by direct reflection could contain alpha particles is studied using self-consistent hybrid simulations of an oblique collisionless, supercritical shock. It is found that alpha particles can backstream from a shock in which the angle between the shock normal and the upstream magnetic field is 45 degrees. The deficit of alpha particles in the FABs, expressed as a relative density ratio to H(+) normalized to the solar wind He(2+) to H(+) density ratio, varies between about 0.1 and less than 0.01 and depends on the upstream H(+) and He(2+) temperatures.

Burgess, D.↗
NASA NTRS1989

Effects of various consolidation techniques on microstructure, strength, and reliability of alpha-SiC

The effects of slurry pressing, HIP, and sinter-HIP on the strength, microstructure, and critical processing flaws in alpha-SiC are examined. Also, the feasibility of glass encapsulation is determined. Baseline strength was improved by about 25 percent by using slurry pressing instead of dry pressing. HIP further improved average strength to as high as 655 MPa. Sinter-HIP did not noticeably improve average strength. Although process-related flaws such as shrinkage cracks and large voids were not seen in HIPed alpha-SiC, surface-related flaws were observed. It is suggested that surface finishing by lapping, polishing, and heat treating might reduce the effect of surface-related flaws, improving the Weibull modulus in HIPed alpha-SiC.

Dutta, Sunil↗
NASA NTRS1988

Coordinated soft X-ray and H-alpha observation of solar flares

Soft X-ray, Ca XIX, and H-alpha observations obtained for a set of four solar flares in the impulsive phase are analyzed. A blue asymmetry was observed in the coronal Ca XIX line during the soft-Xray rise phase in all of the events. A red asymmetry was observed simultaneously in chromospheric H-alpha at spatial locations associated with enhanced flare heating. It is shown that the impulsive phase momentum of upflowing soft X-ray plasma equalled that of the downflowing H-alpha plasma to within an order of magnitude. This supports the explosive chromospheric evaporation model of solar flares.

Zarro, D. M.↗
NASA NTRS1989

Doppler shifted H Ly alpha emission from Jupiter's aurora

High-spectral-resolution IUE observations of the aurora on Jupiter were obtained in order to search for Doppler shifted H-Ly-alpha emission produced through charge exchange by fast precipitating protons. Although no emission has been observed corresponding to proton energies greater than 200 eV, a large fraction of the H-Ly-alpha emission has appeared Doppler shifted, mainly toward the blue, by about 50 km/s. These results show that the acceleration of ionospheric plasma in an H2 atmosphere can lead to bright Ly-alpha emission, setting constraints on the production of the outer planet airglow emissions.

Clarke, John T.↗
NASA NTRS1989

A study of excess H-alpha emission in chromospherically active M dwarf

Spectroscopic observations from three observatories are combined to study the properties of the excess H-alpha emission which characterizes the most chromospherically active subset of the M dwarf stars, known as the dMe stars. It is demonstrated that the excess H-alpha luminosity from these stars is a monotonically decreasing function of their (R-I) color, and evidence is presented which suggests that the product of the mean surface brightness and the mean filling factor of the emissive regions is essentially constant with color. Another significant result of the study is a linear correlation between the excess luminosity in H-alpha and the coronal X-ray luminosity.

Young, Arthur↗
NASA NTRS1989

Rotational velocities of newly discovered, low-mass members of the Alpha Persei cluster

About 30 new, low-mass members of the young open cluster Alpha Persei are identified via a proper-motion study and subsequent photometric and spectroscopic observations. Membership in the cluster is confirmed for a number of the fainter proper-motion candidates from Heckman, Dieckvoss, and Kox (1956). Coordinates, finding charts, BVRI photometry, and rotational velocities are provided for most of the stars. At least two of the stars show peculiar H-alpha emission profiles, with weak but very broad emission wings, and relatively narrow absorption reversals. The rotational velocity distribution for low-mass stars in the Alpha Per cluster are compared with recently derived rotational velocity distributions for T Tauri stars, placing strong constraints on the mechanisms for angular momentum loss during pre-main-sequence evolution.

Stauffer, John R.↗
NASA NTRS1990

Alpha particle heating in hot diamagnetic cavities

Observational data from ISEE 1 are analyzed and a one-dimensional electromagnetic hybrid computer simulation is conducted for the heating of solar wind alpha particles in hot diamagnetic cavities (HDCs). In the simulation, which envisions alpha heating by ion-ion instabilities, low beam densities excite the proton/proton right-hand resonant instability that then pitch-angle scatters the beam without significantly heating the alphas. At greater beam densities, the proton/proton nonresonant instability undergoes saturation through a trapping of all three ion components. These results support the Thomsen et al. (1988) hypothesis that the nonresonant instability is the primary source of ion heating in hot diamagnetic cavities.

Galvez, Miguel↗
NASA NTRS1989

In-flight flow visualization characteristics of the NASA F-18 high alpha research vehicle at high angles of attack

Surface and off-surface flow visualization techniques have been used to visualize the three-dimensional separated flows on the NASA F-18 high alpha research vehicle at high angles of attack. Results near alpha = 25 deg to 26 deg and alpha = 45 deg to 49 deg are presented. Both the forebody and leading-edge extension (LEX) vortex cores and breakdown locations were visualized using smoke. Forebody and LEX vortex separation lines on the surface were defined using an emitted-fluid technique. A laminar separation bubble was also detected on the nose cone using the emitted fluid technique and was similar to that observed in the wind-tunnel test, but not as extensive. Regions of attached, separated, and vortical flow were noted on the wing and the leading-edge flap using tufts and flow cones, and compared well with limited wind-tunnel results.

Fisher, David F.↗
NASA NTRS1990

F-18 high alpha research vehicle surface pressures - Initial in-flight results and correlation with flow visualization and wind-tunnel data

Flight tests with the NASA F-18 high-alpha research vehicle (HARV) have yielded pressure distributions at angles of attack from 10 to 50 deg, at Mach 0.23 to 0.6, at five fuselage forebody stations and three on the leading-edge extensions (LEXs). Correlations are made between these data and both previously obtained HARV flow visualizations and wind tunnel model test results. The general trend is one in which the forebody's maximum suction pressure peaks increase in magnitude, after their first appearance at alpha of about 19 deg, with increasing alpha. LEX pressure-distribution trends involve the inward progression of the maximum suction peaks, an increase in the magnitude of the maximum pressure peaks up to pressure core breakdown, and the decrease and general flattening of the pressure distribution beyond the LEX primary vortex breakdown.

Fisher, David F.↗
NASA NTRS1990

A comparison of H-alpha intensity and radio wave scattering on eight low-latitude lines of sight

Radio scattering and H-alpha measurements for eight sources in Cygnus are compared. The diameter of the radio scattering disk at 1 GHz is found to be correlated with emission measure as determined from the H-alpha measurements, and that strong radio scattering characterized by a diameter of about 30 milliarcsec is produced in regions with emission measures of about 100/cm exp 6 pc or less. The slope of diameter-emission measure correlation is steeper than would be the case if all lines of sight were characterized by the same turbulent outer scale and modulation index. It is suggested that the lines of sight to unscattered or lightly scattered sources traverse only a diffuse medium which is revealed by H-alpha observations at high latitudes. Heavily scattered sources are viewed through an additional component of interstellar H II which has turbulence with different properties.

Spangler, Steven R.↗
NASA NTRS1990

Analytical aerodynamic model of a high alpha research vehicle wind-tunnel model

A 6 DOF analytical aerodynamic model of a high alpha research vehicle is derived. The derivation is based on wind-tunnel model data valid in the altitude-Mach flight envelope centered at 15,000 ft altitude and 0.6 Mach number with Mach range between 0.3 and 0.9. The analytical models of the aerodynamics coefficients are nonlinear functions of alpha with all control variable and other states fixed. Interpolation is required between the parameterized nonlinear functions. The lift and pitching moment coefficients have unsteady flow parts due to the time range of change of angle-of-attack (alpha dot). The analytical models are plotted and compared with their corresponding wind-tunnel data. Piloted simulated maneuvers of the wind-tunnel model are used to evaluate the analytical model. The maneuvers considered are pitch-ups, 360 degree loaded and unloaded rolls, turn reversals, split S's, and level turns. The evaluation finds that (1) the analytical model is a good representation at Mach 0.6, (2) the longitudinal part is good for the Mach range 0.3 to 0.9, and (3) the lateral part is good for Mach numbers between 0.6 and 0.9. The computer simulations show that the storage requirement of the analytical model is about one tenth that of the wind-tunnel model and it runs twice as fast.

Cao, Jichang↗
NASA NTRS1991

In-flight flow visualization characteristics of the NASA F-18 high alpha research vehicle at high angles of attack

Surface and off-surface flow visualization techniques were used to visualize the 3-D separated flows on the NASA F-18 high alpha research vehicle at high angles of attack. Results near the alpha = 25 to 26 deg and alpha = 45 to 49 deg are presented. Both the forebody and leading edge extension (LEX) vortex cores and breakdown locations were visualized using smoke. Forebody and LEX vortex separation lines on the surface were defined using an emitted fluid technique. A laminar separation bubble was also detected on the nose cone using the emitted fluid technique and was similar to that observed in the wind tunnel test, but not as extensive. Regions of attached, separated, and vortical flow were noted on the wing and the leading edge flap using tufts and flow cones, and compared well with limited wind tunnel results.

Fisher, David F.↗
NASA NTRS1990

The circumstellar environment of the emission-line star LkH-alpha 101

The environs of the premain-sequence, emission-line star, LkH-alpha 101, have been observed by broad-band CCD imaging, high-resolution optical spectroscopy, IRAS imaging, and single-dish, as well as interferometric, millimeter-line mapping. LkH-alpha 101 exhibits a high mass-loss rate (0.000011) low-velocity (350 km/s) ionized wind typical of early-type premain-sequence stars (Barsony, 1989). The millimeter interferometer maps show that this wind has cleared out a cavity in the molecular cloud surrounding LkH-alpha 101, allowing the rapid expansion of the previously observed VLA H II region (Becker and White, 1988).

Barsony, M.↗
NASA NTRS1991

The carbon abundance of the Lyman-alpha clouds

A search for C IV 1548, 1550A absorption associated with Ly-alpha forest clouds is attempted, using a method similar to the composite spectrum method developed by Norris, Hartwick, and Peterson (1983). A detection of C IV 1548 absorption feature at not less than 99.99-percent significance level is tentatively reported. If confirmed, not all Ly-alpha clouds have primordial composition. Assuming that the absorption feature is real, the relative abundance of carbon of the Ly-alpha forest clouds is estimated to be roughly C/H = -3.2 from photoionization models. Some other interesting results are also discussed.

Lu, Limin↗
NASA NTRS1990

Lyman-alpha emission from young galaxy disks

An imaging survey of Ly-alpha disk absorption systems for Ly-alpha emission from the absorbing objects, which are candidate young disk galaxies, has been conducted. Limits are set to the flux in Ly-alpha from these objects of approximately 10 exp -16 ergs/sq cm s (3 sigma), which corresponds to a limit on the star formation rate of 2-7 solar masses/yr, in the absence of dust. This suggests that although disk structures form early, rapid star formation does not occur immediately, or else occurs in episodic fashion.

Cohen, Ross D.↗
NASA NTRS1992

Subsonic investigations of vortex interaction control for enhanced high-alpha aerodynamics of a chine forebody/Delta wing configuration

A proposed concept to alleviate high alpha asymmetry and lateral/directional instability by decoupling of forebody and wing vortices was studied on a generic chine forebody/ 60 deg. delta configuration in the NASA Langley 7 by 10 foot High Speed Tunnel. The decoupling technique involved inboard leading edge flaps of varying span and deflection angle. Six component force/moment characteristics, surface pressure distributions and vapor-screen flow visualizations were acquired, on the basic wing-body configuration and with both single and twin vertical tails at M sub infinity = 0.1 and 0.4, and in the range alpha = 0 to 50 deg and beta = -10 to +10 degs. Results are presented which highlight the potential of vortex decoupling via leading edge flaps for enhanced high alpha lateral/directional characteristics.

Rao, Dhanvada M.↗
NASA NTRS1991

A study of the H-alpha line in X1735 - 444

Forty six optical spectra containing the H-alpha line of X1735 - 444, which were obtained over three nights in May 1987, were analyzed. Using a convolution method, a radial velocity curve was derived for the whole H-alpha line. Combining the spectroscopic results with an updated photometric ephemeris, the probable sites for H-alpha emission in X1735 - 444 were determined. Results of the analyses suggest that two distinct regions contribute to the observed profile in X-1735 - 444: (1) the peak emission, which is associated with the accretion stream of material passing between the companion and the compact object; and (2) the base emission, which is produced symmetrically in the outer accretion disk.

Smale, Alan P.↗