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At least 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A line identification study of the IUE SWP high dispersion spectrum of the UV-bright star Barnard 29 of Messier 13

A line identification study was performed of the coaddition of three SWP high dispersion IUE exposures of the UV-bright star Barnard 29 of Messier 13. One of these images took two IUE shifts. The previous study by de Boer and Savage (1983) was extended and stellar lines of C I, C II, C III, C IV, N I, N III, O I, O III, O IV,Al III, Si II, Si III, Si IV, P III, S II, and S III and interstellar lines of C I, C II, C IV, N I, N III, N IV, N V, O I, Al II, Al III, Si II, Si IV, S II, and Fe II were found. These lines are the strongest expected lines in a hot Population II star. Apparent intensity minima corresponding to interstellar features are noted, especially those matching a high velocity cloud found by de Boer and Savage.

Adelman, S. J.↗
NASA NTRS1991

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism.

Brandt, John C.↗
NASA NTRS1991

The last disconnection events in Comet Halley in April 1986

Disconnection event (DE) in cometary plasma physics is the regular loss of the entire plasma tail and the growth of the new one. Analysis of a sequence of DEs that occurred 13-18 April, 1986 shows that they correlate well with a magnetic, sector-boundary crossing and a complex magnetic structure in the solar wind with polarity reversals that occurred about one day later. These events are entirely consistent with sunward, magnetic reconnection as the DE mechanism.

Brandt, John C.↗
NASA NTRS1992

Results of a low power ice protection system test and a new method of imaging data analysis

Tests were conducted on a BF Goodrich De-Icing System's Pneumatic Impulse Ice Protection (PIIP) system in the NASA Lewis Icing Research Tunnel (IRT). Characterization studies were done on shed ice particle size by changing the input pressure and cycling time of the PIIP de-icer. The shed ice particle size was quantified using a newly developed image software package. The tests were conducted on a 1.83 m (6 ft) span, 0.53 m (221 in) chord NACA 0012 airfoil operated at a 4 degree angle of attack. The IRT test conditions were a -6.7 C (20 F) glaze ice, and a -20 C (-4 F) rime ice. The ice shedding events were recorded with a high speed video system. A detailed description of the image processing package and the results generated from this analytical tool are presented.

Shin, Jaiwon↗
NASA NTRS1992

Boundary layer structure over the ocean observed by LEANDRE 1 during a tramontane event

A new airborne backscatter lidar, has been developed by CNRS (Service d'Aeronomie, (SA) Laboratoire de Meteorologie Dynamique (LMD) and the Institut des Sciences de 1'Univers) in the frame of the LEANDRE research program. It has been qualified on board the ARAT in autumn 1989 and spring 1990 and was involved in its first cooperative campaign during PYREX in October and November 1990. During this campaign, lidar observations of the perturbations induced on tropospheric flow and boundary layer structure were performed, and results are presented. A large number of experiments were performed, for synoptic situation description (meteorological radiosoundings, constant level balloons) and local flow analysis (aircrafts, radars, sodars). For the first time in such an experiment, a lidar has been flown on a research aircraft to perform altitude resolved observations of these perturbations, and we will present here results obtained for deflected flow structure. In the presence of a synoptic northerly flow, part of it is deflected to the east by the Pyrenees, and accelerated over the Mediterranean by the mountain surroundings. In this case, a low level wind is generated (the Tramontane) bringing cold and dry air over the Mediterranean Sea. As the sea is still at a warm temperature in November (around 17 degrees), an Internal Marine Boundary Layer rapidly grows over the first tens of kilometers and stabilizes at about 1 km depth, corresponding to an altitude just below the Lifting Condensation Level. The whole Marine Atmospheric Boundary Layer (MABL) is characterized by highly turbulent motions bringing large humid particles from the surface up to its top. The lidar signal due to scattering by these particles is then representative of the turbulent kinetic energy in this layer.

Flamant, C.↗
NASA NTRS1991

On the dynamics and composition of the high-latitude thermosphere

An overview is presented of some of the more recent progress made in the study of the dynamics and composition of the high-latitude thermosphere at the University of Michigan, with emphasis on the interpretation of experimental measurements made from the DE-2 spacecraft and from ground-based observatories in Greenland. Attention is focused on perturbations to the solar-driven upper thermospheric winds, temperatures, and compositional structures due to forcing associated with high-latitude ionosphere-thermosphere coupling processes. The nature of the experimental measurements made on DE-2 is discussed. The vector spherical harmonic model, some sample results, and experiment-model comparisons are described using both satellite and ground-based measurements of high-latitude thermosphere dynamics. Considerations pertaining to compositional perturbations at high latitudes are presented.

Killeen, T. L.↗
NASA NTRS1991

Dynamical approach study of spurious steady-state numerical solutions of nonlinear differential equations. I - The dynamics of time discretization and its implications for algorithm development in computational fluid dynamics

Spurious stable as well as unstable steady state numerical solutions, spurious asymptotic numerical solutions of higher period, and even stable chaotic behavior can occur when finite difference methods are used to solve nonlinear differential equations (DE) numerically. The occurrence of spurious asymptotes is independent of whether the DE possesses a unique steady state or has additional periodic solutions and/or exhibits chaotic phenomena. The form of the nonlinear DEs and the type of numerical schemes are the determining factor. In addition, the occurrence of spurious steady states is not restricted to the time steps that are beyond the linearized stability limit of the scheme. In many instances, it can occur below the linearized stability limit. Therefore, it is essential for practitioners in computational sciences to be knowledgeable about the dynamical behavior of finite difference methods for nonlinear scalar DEs before the actual application of these methods to practical computations. It is also important to change the traditional way of thinking and practices when dealing with genuinely nonlinear problems. In the past, spurious asymptotes were observed in numerical computations but tended to be ignored because they all were assumed to lie beyond the linearized stability limits of the time step parameter delta t. As can be seen from the study, bifurcations to and from spurious asymptotic solutions and transitions to computational instability not only are highly scheme dependent and problem dependent, but also initial data and boundary condition dependent, and not limited to time steps that are beyond the linearized stability limit.

Yee, H. C.↗
NASA NTRS1992

The density minimum at the earth's magnetic equator

Observations of the density structure in the plasmapause region reveal the existence of a local minimum in the total electron density at the magnetic equator. Data from the plasma wave instrument and ion mass spectrometer on the DE-1 satellite are used to study this phenomenon. The density depletion typically extends from +/- 5 to +/- 20 deg in latitude and is found at altitudes from 2 to 5 RE. Density depletions of 10-70 percent are found in regions where the off-equator density ranges from 10 to 1000/cu cm. This density structure is associated with equator crossings where the thermal plasma has been heated over normal plasmasphere values. The heated plasma is the equatorially trapped plasma previously reported from DE 1 and the SCATHA satellite. Within the plasmasphere, the drop in total (electron) density corresponds to a decrease in the cold-ion density, in both H(+) and He(+). There is a rough pressure balance provided by the warm tail of the distribution, which is a few percent by density but 1-2 orders of magnitude higher in temperature.

Olsen, R. C.↗
NASA NTRS1992

Dynamics and stability of a tethered centrifuge in low earth orbit

The three-dimensional attitude dynamics of a spaceborne tethered centrifuge for artificial gravity experiments in low earth orbit is analyzed using two different methods. First, the tethered centrifuge is modeled as a dumbbell with a straight viscoelastic tether, point tip-masses, and sophisticated environmental models such as nonspherical gravity, thermal perturbations, and a dynamic atmospheric model. The motion of the centrifuge during spin-up, de-spin, and steady-rotation is then simulated. Second, a continuum model of the tether is developed for analyzing the stability of lateral tether oscillations. Results indicate that the maximum fluctuation about the 1-g radial acceleration level is less than 0.001 g; the time required for spin-up and de-spin is less than one orbit; and lateral oscillations are stable for any practical values of the system parameters.

Quadrelli, B. M.↗
NASA NTRS1992

The polar cap environment of outflowing O(+)

The properties of the core (0-50 eV) and 'energetic' (0-1 keV) ions, plasma waves, and auroral images obtained from Dynamics Explorer 1 (DE-1) and those of electrons, obtained from DE-2, are examined in the context of the polar cap environment. Results indicate the presence of two populations: high-speed (10-30 eV, or higher, streaming energies) polar beams and low-speed (generally less than 10-eV streaming energies) streams. The high-speed polar beams show an auroral connection (i.e., they are observed on or near the field lines threading auroral arcs), while the low-speed streams are on or near the field lines threading the dark polar cap and may be converted from the cleft ion fountain. Compared to the high-speed streams, the low-speed streams are significantly more stable with respect to energy and flux.

Horwitz, J. L.↗
NASA NTRS1992

Precipitation of ions induced by magnetotail collapse

The precipitation of ions triggered by dipolarization of magnetospheric field lines during substorm expansion phase is examined by means of single-particle codes. As large but short-lived electric fields develop and particles experience transient nonadiabatic motions, it is demonstrated that 'de-trapping' of inner plasma sheet populations can be achieved either via damping of magnetic moment or impulsive accelerations in the parallel direction. It is shown that the former precipitation mechanism results from phasing between gyromotion and the surging electric field. It preferentially affects heavy and low-charge state ions which can display large de-energization rates within a cyclotron period. The latter mechanism rather favors the loss of lighter and/or higher-charge state ions which are less sensitive to temporal nonadiabaticity. In this case, injection into the loss cone may result from enhanced curvature related acceleration. As this latter acceleration occurs on the dipolarization time scale, it affects a limited amount of plasma sheet populations, namely those intercepting the region of maximum curvature at the time of peak induced electric field.

Delcourt, D. C.↗
NASA NTRS1992

Thermospheric composition changes seen during a geomagnetic storm

The largest magnitude winds observed using the instruments on board the Dynamics Explorer 2 (DE-2) satellite were measured during the large geomagnetic storm that occurred on November 24, 1982. Neutral temperatures exceeded 2000 K during this storm, and these high temperatures, combined with the very large observed winds and the very full instrumental coverage available in both hemispheres, make it a unique event to study. This paper presents results obtained using these DE-2 data and a time dependent simulation of the event made using the NCAR Thermosphere/Ionosphere General Circulation Model (NCAR-TIGCM). In general, the agreement between model calculations and the data is very good, implying that most of the important physical processes controlling the energetics and dynamics of the thermosphere are reasonably well represented in the model. The modeled summer hemisphere changes in the mass mixing ratio of N2 (Psi(N2)) are in very good agreement with the averaged data in both hemispheres. This paper describes model-experiment comparisons for the November 24, 1982 geomagnetic storm changes.

Burns, A. G.↗
NASA NTRS1992

Formation of quasiparallel Alfven solitons

The formation of quasi-parallel Alfven solitons is investigated through the inverse scattering transformation (IST) for the derivative nonlinear Schroedinger (DNLS) equation. The DNLS has a rich complement of soliton solutions consisting of a two-parameter soliton family and a one-parameter bright/dark soliton family. In this paper, the physical roles and origins of these soliton families are inferred through an analytic study of the scattering data generated by the IST for a set of initial profiles. The DNLS equation has as limiting forms the nonlinear Schroedinger (NLS), Korteweg-de-Vries (KdV) and modified Korteweg-de-Vries (MKdV) equations. Each of these limits is briefly reviewed in the physical context of quasi-parallel Alfven waves. The existence of these limiting forms serves as a natural framework for discussing the formation of Alfven solitons.

Hamilton, R. L.↗
NASA NTRS1992

Initial overview of disconnection events in Halley's Comet 1986

We present an initial overview of the disconnection events (DE's) in Comet Halley in 1986. Although disconnection events are arguably the most spectacular of all dynamic comet phenomena, the mechanisms by which they occur are not fully understood. It is generally believed that the solar wind plays a major role in determining when disconnection events occur, but the details of the solar wind/cometary interactions responsible for initiating the tail disconnection are still under debate. The three most widely accepted models are: (1) high speed streams in the solar wind cause the tail to disconnect due to pressure effects; (2) decreased production of cometary ions in a high speed stream allows magnetic field to slip away from the comet; and (3) the tail disconnects after frontside reconnection of the interplanetary magnetic field (IMF) as the comet crosses a magnetic field sector boundary. We find that the front-side magnetic reconnection model is the best explanation for the DE's we have considered.

Brandt, John C.↗
NASA NTRS1992

The disconnection event of 16.0 March 1986 in comet Halley

From kinematic extrapolation of tail/nucleus distance measurements on photographic images in the International Halley Watch (IHW) archive, we calculated the disconnection time of the 16-19 March 1986 event to be 16.0 (plus or minus 0.1) March. The solar wind conditions around Comet Halley at the time of the DE, inferred by corotation of IMP-8 satellite data to the comet, were such that (1) Comet Halley had just crossed the interplanetary magnetic field (IMF) sector boundary; (2) the solar wind density was approximately 8 cm(exp -3); (3) the solar wind speed was approximately 600 km/sec; (4) the IMF magnitude was approximately 8 nT. Given these conditions, we conclude that the most likely cause of the 16.0 March DE was front-side magnetic reconnection, as described in the model of Niedner and Brandt (1978).

Randall, Cora E.↗
NASA NTRS1993

Shuttle landing facility cloud cover study: Climatological analysis and two tenths cloud cover rule evaluation

The two-tenths cloud cover rule in effect for all End Of Mission (EOM) STS landings at the Kennedy Space Center (KSC) states: 'for scattered cloud layers below 10,000 feet, cloud cover must be observed to be less than or equal to 0.2 at the de-orbit burn go/no-go decision time (approximately 90 minutes before landing time)'. This rule was designed to protect against a ceiling (below 10,000 feet) developing unexpectedly within the next 90 minutes (i.e., after the de-orbit burn decision and before landing). The Applied Meteorological Unit (AMU) developed and analyzed a database of cloud cover amounts and weather conditions at the Shuttle Landing Facility for a five-year (1986-1990) period. The data indicate the best time to land the shuttle at KSC is during the summer while the worst time is during the winter. The analysis also shows the highest frequency of landing opportunities occurs for the 0100-0600 UTC and 1300-1600 UTC time periods. The worst time of the day to land a shuttle is near sunrise and during the afternoon. An evaluation of the two-tenths cloud cover rule for most data categorizations has shown that there is a significant difference in the proportions of weather violations one and two hours subsequent to initial conditions of 0.2 and 0.3 cloud cover. However, for May, Oct., 700 mb northerly wind category, 1500 UTC category, and 1600 UTC category there is some evidence that the 0.2 cloud cover rule may be overly conservative. This possibility requires further investigation. As a result of these analyses, the AMU developed nomograms to help the Spaceflight Meteorological Group (SMG) and the Cape Canaveral Forecast Facility (CCFF) forecast cloud cover for EOM and Return to Launch Site (RTLS) at KSC. Future work will include updating the two tenths database, further analysis of the data for several categorizations, and developing a proof of concept artificial neural network to provide forecast guidance of weather constraint violations for shuttle landings.

Atchison, Michael K.↗
NASA NTRS1992

Upflowing ionospheric ions in the auroral region

Observations of upflowing ionospheric ions are obtained nearly simultaneously by DE 1 and DE 2 over the nightside auroral regions. At low altitudes, the mean value of the net upward ion number flux is of the order of 10 exp 9/sq cm per sec. The ionosphere is predominantly O(+), and the flux of ions with energy greater than 5 eV is a very small fraction (less than 1 percent) of the total ion flux. At high altitudes, the upflowing ions are accelerated by a parallel electric field and heated (with characteristic energies of hundreds of electron volts). Comparing upflowing fluxes at high and low altitudes yields an estimated height of the bottom of the auroral acceleration region of 1400-1700 km for the region of peak potential drop. This low-altitude acceleration could either be from a parallel electric field or from perpendicular acceleration. The fluxes at the edges of the arc are mostly H(+) thus implying a higher-altitude base of the acceleration region at the edges where the potential drop is lower.

Lu, G.↗
NASA NTRS1991

Thermospheric dynamics, energetics, and composition at auroral latitudes

Recent experimental results from the Dynamics Explorer-2 (DE-2) spacecraft and theoretical calculations from the NCAR-TGCM (thermospheric general circulation models) are discussed. DE-2 observations show that the close coupling exists between the magnetosphere and the thermosphere, with magnetospheric energy and momentum sources playing a key role in establishing the thermospheric dynamical and thermal structure via ion-neural collisions. The tight ion-neutral momentum coupling results in the neutral-wind pattern which is strongly dependent on the level of geomagnetic activity and the orientation of the interplanetary magnetic field. Thermospheric temperatures are directly controlled by the Juole and particle heat sources associated with auroral latitudes. It is concluded that the various dynamical, energetic, and compositional variations associated with auroral processes are highly coupled with each other.

Killeen, T. L.↗