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NASA NTRS1992

Model atmospheres and radiation of magnetic neutron stars. I - The fully ionized case

Model neutron star atmospheres are calculated for typical cooling stars with a strong magnetic field and effective temperatures of 10 exp 5 to 10 exp 6 K. The effect of anisotropic photon diffusion in two normal modes are examined under the assumption that the opacity is due solely to the bremsstrahlung and Thomson scattering processes under conditions of LTE that are expected to prevail at the temperatures and densities obtained. The main aspects of anisotropic photon diffusion, and an original procedure for calculating model atmospheres and emitted spectra are discussed. Representative calculated spectra are given, and it is found that the hard spectral excess characterizing the nonmagnetic case, while still present, becomes less prominent in the presence of magnetic fields in the range of 10 exp 11 to 10 exp 13 G.

Shibanov, Iu. A.↗
NASA NTRS1991

Preliminary DMR measurements of the CMB isotropy

The COBE Differential Microwave Radiometers (DMR) instrument has produced preliminary full-sky maps at frequencies 31.5, 53, and 90 GHz. The redundant channels and matched beams at three frequencies distinguish the DMR from previous large-scale surveys. Galactic emission is seen unambiguously at all three frequencies. The only large-scale anisotropy detected in the cosmic microwave background is the dipole anisotropy. There is no clear evidence for any other large-angular-scale feature in the maps. Without correcting for any systematic effects, we are able to place limits DeltaT/T sub 0 less than 3 x 10 exp -5 for the rms quadrupole amplitude, DeltaT/T sub 0 less than 4 x 10 exp -5 for monochromatic fluctuations, and DeltaT/T sub 0 less than 4 x 10 exp -5 for Gaussian fluctuations (all limits are 95 percent C.L. with TO = 2.735 K). The data limit DeltaT/T sub 0 less than 10 exp -4 for any feature larger than 7 deg. We briefly review the DMR and discuss some implications of these results in cosmology.

Smoot, G. F.↗
NASA NTRS1992

Microgravity monitoring instrument development and application to vernier guidance, navigation, and vehicle control

The fact that a spacecraft traveling through the 'vacuum' of space conforms to the classical Keplerian ellipse has recently been disproven. It is now well known that such a vehicle is acted on by many external forces such as drag in the rarefied particle atmosphere, solar wind and particle impact. This paper discusses the development of sensors and sensor systems to measure these minute forces of acceleration/deceleration. Four systems will be discussed: a 10 exp -4 g system, a 10 exp -6 g system, a 10 exp -(6-8) g system and a 10 exp -9 g system. The design of each system will be explained along with the advantages/disadvantages of each. Various applications unique to each system will be discussed. Configurations, design schemes, test plans and calibration procedures, both in the ground laboratory and inflight, will be presented. The current design/development/operational status of each system will be examined and future plans discussed. Application to aerodynamic studies and vernier guidance, navigation, and vehicle control will also be examined.

Howell, Joseph J.↗
NASA NTRS1993

Two-dimensional velocity, optical risetime, and peak current estimates for natural positive lightning return strokes

Velocities, optical risetimes, and transmission line model peak currents for seven natural positive return strokes are reported. The average 2D positive return stroke velocity for channel segments of less than 500 m in length starting near the base of the channel is 0.8 +/- 0.3 x 10 exp 8 m/s, which is slower than the present corresponding average velocity for natural negative first return strokes of 1.7 +/- 0.7 x 10 exp 8/s. It is inferred that positive stroke peak currents in the literature, which assume the same velocity as negative strokes, are low by a factor of 2. The average 2D positive return stroke velocity for channel segments of greater than 500 m starting near the base of the channel is 0.9 +/- 0.4 x 10 exp 8 m/s. The corresponding average velocity for the present natural negative first strokes is 1.2 +/- 0.6 x 10 exp 8 m/s. No significant velocity change with height is found for positive return strokes.

Mach, Douglas M.↗
NASA NTRS1993

Constraints on decaying neutrinos from the far-ultraviolet extragalactic background light

We consider light neutrinos as dark matter candidates, concentrating on those proposed by Sciama, with rest energies near 30 eV and decay lifetimes of (2 +/- 1) x 10 exp 23 s. Using equations developed by Wesson et al. (1987) for the visible extragalactic background light, and assuming that the neutrinos are clustered in galactic halos similar to that around our own Galaxy, we show that their decay would be capable of significant contributions to the far-ultraviolet background. Observations in this region by Martin and Bowyer (1989) limit the possible mass of the neutrino halos to 2 x 10 exp 11 M(solar), which is less than the required galactic 'missing mass'. In order to form galactic halos of mass 5 x 10 exp 11 M(solar), such neutrinos would have to decay with lifetimes of at least 3 x 10 exp 23 s to be consistent with the observations. Sciama's proposal remains marginally viable, but more observational data will test it and similar neutrino models critically.

Overduin, J. M.↗
NASA NTRS1993

First observations of CO and HCN on Neptune and Uranus at millimeter wavelengths and the implications for atmospheric chemistry

Observations are presented which show that CO is present in both the troposphere and stratosphere of Neptune, whereas is confined to the Neptune stratosphere with a mean mole fraction in the 0.003-30 mbar pressure level range of 1.0 x 10 exp -9. CO is present in both the stratosphere and in the troposphere with a uniformly mixed model fraction of 1.2 x 10 exp -6. Upper limits of 1.0 x 10 exp -10 and 3.0 x 10 exp -8 mole fractions are derived for HCN and CO respectively on Uranus. The origin of these species in the atmosphere of Neptune and their nondetection in that of Uranus are discussed in detail. It is concluded that the most plausible scenario involves upward convection of CO and N2 from Neptune's deep interior and a failure of chemical equilibrium at deep atmospheric levels, allowing excess CO and presumably N2 to reach the upper atmosphere. Nondetection in Uranus may be explained by the lack of a significant internal heat source in the planet and consequent suppression of vertical convection.

Marten, A.↗
NASA NTRS1993

The mass function of clusters of galaxies

The mass function of clusters of galaxies is derived separately from optical and X-ray observations of their respective properties. The mass function was extended to the faint end, to lower masses than the rich clusters, using small groups of galaxies. The derived mass function is within the mass range from approximately 10 exp 12 to not less than 10 exp 15/h solar masses. The deduced density range n is approximately 10 exp -2 to 10 exp -8 cubic h/cubic Mpc. The observed shape of the mass function exhibits a shallow power-law decrease with mass at the faint end, and a steep exponential decrease with mass at the massive end. A summary of some constraints placed by the observed mass function on current cosmological models is presented.

Bahcall, Neta A.↗
NASA NTRS1993

Physical properties and evolutionary time scales of disks around solar-type and intermediate mass stars

Recent observations of circumstellar disks and their evolutionary timescales are reviewed. It is concluded that disks appear to be a natural outcome of the star-formation process. The disks surrounding young stars initially are massive, with optically thick structures comprised of gas and micron-sized grains. Disk masses are found to range from 0.01 to 0.2 solar masses for solar-type PMS stars, and from 0.01 to 6 solar masses for young, intermediate mass stars. Massive, optically thick accretion disks have accretion rates between 10 exp -8 and 10 exp -6 solar masses/yr for solar type PMS stars and between 10 exp -6 and 10 exp -4 solar masses/yr for intermediate stars. The results suggest that a significant fraction of the mass comprising the star may have passed through a circumstellar accretion disk.

Strom, Stephen E.↗
NASA NTRS1992

Extragalactic scenarios for gamma-ray bursts

The cosmological distance scale to gamma-ray bursts is indicated by the observed distribution of the bursts in angle and intensity. The redshifts of the weakest and the strongest bursts detected by the BATSE experiment are estimated to be z = 1.5 and z = 0.2, respectively, while the strongest bursts detected by the PVO in its 11 years of operation were at z = 0.05. The bursts' rate is about 10 exp -6 per year in a galaxy like ours, and a typical energy release is about 10 exp 51 ergs, assuming isotropic emission. The rate of the neutron star and black hole collisions in close binaries is somewhat higher, and those collisions release 10 exp 53 - 10 exp 54 ergs in a burstlike form, making them the primary candidates for the observed gamma-ray events. Contrary to the common belief, no observations available to date are in conflict with the cosmological distance to the bursters. A detection of gravitational lensing of some bursts, the redshift of the weakest events, and the absorption of soft X-rays by the interstellar matter in the plane of our galaxy, are the specific predictions of the cosmological scenario.

Paczynski, Bohdan↗
NASA NTRS1993

Observational evidence for hotter transition region loops within the supergranular network

We correlate EUV spectroheliograms of the quiet sun taken by the Harvard EUV Spectroheliometer on Skylab with a magnetogram taken within 15-20 minutes by the 40 channel magnetograph at Kitt Peak. Focus is on the O VI and Ne VII ions which are formed in the hotter transition region and the Mg x ion which is formed in the corona. Since no imaged observations in lines from T = 2 x 10 exp 5 - 10 exp 6 K temperature region have been made since Skylab, these data are unique in providing opportunities to investigate the hotter transition region and its relationship to the corona. The signature of an EUV intranetwork loop is a small EUV brightening located on the neutral line of a small magnetic bipole within the supergranular network. The data clearly show such features. Some of these features are visible at coronal temperatures, indicating that network loops can reach 10 exp 6 K. Others are bright in the lines formed below 10 exp 6 K but occur in regions of depleted coronal emission, which cannot be explained by models based on back-heating from a large-scale corona. We conclude that some fraction of the quiet solar EUV output is generated within network loops that are effectively insulated from the corona.

Dowdy, James F., Jr.↗
NASA NTRS1993

Simultaneous observations of solar plage with the solar extreme ultraviolet rocket telescope and spectrograph (SERTS), the VLA, and the Kitt Peak magnetograph

We obtained simultaneous images of solar plage on 1991, May 7 with SERTS, the VLA,4 and the NASA/National Solar Observatory spectromagnetograph at the NSO/Kitt Peak Vacuum Telescope. Using intensity ratios of Fe XVI to Fe XV emission lines, we find that the coronal plasma temperature is (2.3-2.9) x 10 exp 6 K throughout the region. The column emission measure ranges from 2.5 x 10 exp 27 to l.3 x 10 exp 28 cm exp -5. The calculated structure and intensity of the 20 cm wavelength thermal bremsstrahlung emission from the hot plasma observed by SERTS is quite similar to the observed structure and intensity of the 20 cm microwave emission observed by the VLA. Using the Meyer (1991, 1992) revised coronal iron abundance, we find no evidence either for cool absorbing plasma or for contributions from thermal gyroemission. Using the observed microwave polarization and the SERTS plasma parameters, we calculate a map of the coronal longitudinal magnetic field. The resulting values, about 30-60 G, are comparable to extrapolated values of the potential field at heights of 5000 and 10,000 km.

Brosius, Jeffrey W.↗
NASA NTRS1993

Fourier transform infrared studies of the interaction of HCl with model polar stratospheric cloud films

Heterogeneous reactions involving hydrochloric acid adsorbed on the surfaces of polar stratospheric clouds (PSCs) are postulated to contribute to polar ozone loss. Using FTIR spectroscopy to probe the condensed phase, we have examined the interaction of HCl with ice and nitric acid trihydrate (NAT) films representative of types II and I PSCs, respectively. For HCl pressures in the range of 10 exp -7 to 10 exp -5 Torr, our FTIR studies show that a small amount of crystalline HCl-6H2O formed on or in ice at 155 K. However, for higher HCl pressures, we observed that the entire film of ice rapidly converted into an amorphous 4:1 H2O:HCl mixture. From HCl-uptake experiments with P(HCl) = 8 x 10 exp -7 Torr, we estimate roughly that the diffusion coefficient of HCl in ice is around 2 x 10 exp -12 sq cm/s at 158 K. For higher temperatures more closely approximating those found in the stratosphere, we were unable to detect bulk HCl uptake by ice. Indirect evidence suggests that HCl adsorption onto the surface of model PSC films inhibited the evaporation of both ice and NAT by 3-5 K.

Koehler, Birgit G.↗
NASA NTRS1992

Observed temporal variations in the Earth's gravity field from 16-year Starlette orbit analysis

Satellite laser ranging data to Starlette, collected during the period from 1975 to 1990, are analyzed to determine yearly values of the second degree annual (Sa) and semiannual (Ssa) tides, simultaneously with average values of other low degree and order tide parameters. The yearly fluctuations in the values for Sa and Ssa are associated with changes in the Earth's second degree zonal harmonic caused by meteorological excitation. The Starlette-determined mean values for the amplitude of the annual and semiannual variations in J2 are 32.3 x 10 exp -11 and 19.5 x 10 exp -11, respectively; while the rms about the mean values are 4.1 x 10 exp -11 and 6.3 x 10 exp -11, respectively. The annual delta-J2 is in good agreement with the value obtained from the combined effects of air mass redistribution without the oceanic inverted-barometer effects (non-IB) and hydrological change. Approximately 90 percent of the observed annual variation from Starlette is attributed to the meteorological mass redistribution occurring on the Earth's surface.

Cheng, M. K.↗
NASA NTRS1990

On a concurrent element-by-element preconditioned conjugate gradient algorithm for multiple load cases

Element-by-element preconditioned conjugate gradient (EBE-PCG) algorithms have been advocated for use in parallel/vector processing environments as being superior to the conventional LDL(exp T) decomposition algorithm for single load cases. Although there may be some advantages in using such algorithms for a single load case, when it comes to situations involving multiple load cases, the LDL(exp T) decomposition algorithm would appear to be decidedly more cost-effective. The authors have outlined an EBE-PCG algorithm suitable for multiple load cases and compared its effectiveness to the highly efficient LDL(exp T) decomposition scheme. The proposed algorithm offers almost no advantages over the LDL(exp T) algorithm for the linear problems investigated on the Alliant FX/8. However, there may be some merit in the algorithm in solving nonlinear problems with load incrementation, but that remains to be investigated.

Watson, Brian↗
NASA NTRS1993

Organometallic chemical vapor deposition and characterization of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates

The epitaxial growth of ZnGe(1-x)Si(x)P2-Ge alloys on GaP substrates by open tube organometallic chemical vapor deposition (OMCVD) is reported. The chemical composition of the alloys characterized by energy dispersive X-ray spectroscopy shows that alloys with x up to 0.13 can be deposited on (001) GaP. Epitaxial growth with mirror smooth surface morphology was achieved for x less than or equal to 0.05. Transmission electron microscopy (TEM) micrographs of these alloys show specular epitaxy and the absence of microstructural defects indicating a defect density of less than 10(exp 7) cm(sup -2). Selected area electron diffraction pattern of the alloy shows that the epitaxial layer crystallizes in the chalcopyrite structure with relatively weak superlattice reflections indicating certain degree of randomness in the cation sublattice. Hall measurements show that the alloys are p-type, like the unalloyed films; the carrier concentration, however, dropped about 10 times from 2 x 10(exp 18) to 2 x 10(exp 17) cm(sup -3). Absorption measurements indicate that the band tailing in the absorption spectra of the alloy was shifted about 0.04 eV towards shorter wavelength as compared to the unalloyed material. Diodes fabricated from the n(+)-GaP/p-ZnSiP2-ZnGeP2-Ge heterostructure at x = 0.05 have a reverse break-down voltage of -10.8 V and a reverse saturation current density of approximately 6 x 10(exp -8) A/sq cm.

Xing, G. C.↗
NASA NTRS1993

Comparative magnetic and thermoanalytical study of two enstatite chondrites: Adhi Kot and Atlanta

With allowance for the discussion of classification of enstatite chondrites and their relation to aubrites, the obtained magnetic and thermoanalytical data is submitted to be considered as additive arguments. Our study covered the Adhi Kot (EH4) and Atlanta (EL6). meteorites belonging to two distinct groups of enstatite chondrites. Applying AF demagnetization the intensity of natural remanent magnetization (NRM) was measured and the mean magnetic susceptibility of the samples was determined. The differential thermal (DTA) and thermogravimetric (TG) curves were obtained for meteorites under study. For measurements of the intensity of NRM, a superconducting cryomagnetometer SQUID (2 G Enterprise, USA), while magnetic susceptibility Kappabridge KLY-2 (Czechoslovakia) were used. The abbreviated magnetic data sheets are given. The values 786 x 10(exp -4)A/mkg and 196.1 x 10(exp -4)A/mkg were obtained as NRM intensities for Atlanta and Adhi Kot respectively, while 17.4 x 10(exp -6) SIu/kg and 43.4 x 10(exp -6) SIu/kg for their susceptibilities. Both meteorites proved to be strongly magnetized. The demagnetization down to 3.2 percent of NMR was received for Atlanta at AF field intensity of 250 Oe. For Adhi Kot at this level rested 13.2 percent of NRM intensity, this sample being demagnetized without change of direction till 750 Oe field. The demagnetization curves are similar to those obtained for Abee (EL4) chondrite by Sugiura and Strangway. Against Abee the Adhi Kot exhibited a little bit steeper downfall, and in both cases dominate one component of magnetization. The DTA and TG curves were obtained with Rigaku-Denki thermoanalytical instrument. The DTA curves exhibit striking similarity in their shape and relatively close temperature values for various features. The same is valid for TG curves. The higher values for TG for Adhi Kot express its higher content of oxydable (Fe, Ni) whose oxidation in air is reached at 1000-1200 C.

Krol, Elizabeth↗
NASA NTRS1993

A Study of Saturn's E-Ring Particles Using the Voyager 1 Plasma Wave Instrument

The flyby of Voyager 1 at Saturn resulted in the detection of a large variety of plasma waves, e.g., chorus, hiss, and electron cyclotron harmonics. Just before the outbound equator crossing, at about 6.1 R(sub s), the Voyager 1 plasma wave instrument detected a strong, well-defined low-frequency enhancement. Initially it was suggested that plasma waves might be responsible for the spectral feature but more recently dust was suggested as at least a partial contributor to the enhancement. In this report we present evidence which supports the conclusion that dust contributes to the low-frequency enhancement. A new method has been used to derive the dust impact rate. The method relies mainly on the 16-channel spectrum analyzer data. The few wide band waveform observations available (which have been used to study dust impacts during the Voyager 2 ring plane crossing) were useful for calibrating the impact rate from the spectrum analyzer data. The mass and, hence, the size of the dust particles were also obtained by analyzing the response of the plasma wave spectrum analyzer. The results show that the region sampled by Voyager 1 is populated by dust particles that have rms masses of up to few times 10(exp -11) g and sizes of up to a few microns. The dust particle number density is on the order of 10(exp -3) m(exp 3). The optical depth of the region sampled by the spacecraft is 1.04 x 10(exp -6). The particle population is centered about 2500 km south of the equatorial plane and has a north-south thickness of about 4000 km. Possible sources of these particles are the moons Enceladus and Tethys whose orbits lie within the E-ring radial extent. These results are in reasonable agreement with photometric studies and numerical simulations.

Tsintikidis, D.↗
NASA NTRS1993

Low-power millimeter-wave radar observations of the atmosphere

Historically, cloud structures, dynamics, and precipitation processes have been observed and measured with sensors from two different spatial resolutions. Laser-based sensors have volume resolutions on the order of 10(exp -4) to 10(exp -2) cubic meters for a 1-s sample. Radar systems operating at wavelengths between 1 and 10 cm have resolutions on the order of 10(exp 4) to 10(exp 7) cubic meters. The resolutions of micro-wave systems depend primarily on the system RF bandwidth and antenna bandwidth. Both resolution regimes were useful in the study of cloud structures and processes - the former for determining resolution on the individual cloud particle scale and the latter for studying the coarse characteristics of cloud dynamics and structure. There are, however, cloud processes and structures that occur on scales that lie between these two regimes: the process of entrainment, where outside air is brought within the cloud boundaries; the mixing of in-cloud particles with different histories; cloud particle coalescence; and ice formation. The use of mm-wave (30- to 300-GHz) radars offers an opportunity to observe cloud processes at these scales and determine their influence on precipitation development, cloud albedos, cloud lifetimes, chemical cycling of tract substances, aircraft icing, and other meteorological phenomena. To determine the usefulness of a 35-GHz radar for observing these precipitation and cloud processes, a research program was initiated. The objectives of this program are to develop a 35-GHz radar, to measure scattering from precipitation and clouds, and to develop a model to compute scattering from clouds using the finite-difference time-domain (FDTD) technique.

Ronnau, James F.↗