Physics of cellular synthesis, growth and division Status report, 1 Jul. - 31 Dec. 1969
Theoretical biology of cellular synthesis, growth, and division
Pollard, E. C.↗
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Results for “Theoretical Physics”
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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Investigations into the Power MOSFET SEGR Phenomenon and its Physical Mechanism
The state of understanding of the destructive SEGR event in power MOSFETs is relatively mature with large published efforts, both experimental and theoretical. However, gasps remain in the uderstanding of the phenomenon, including unexplained anomalies, emperical-only dependencies on some important device and incident ion physical parameters, and limited insight into latent effets.
MOSFET SEGR↗
Viscosity in a magnetized plasma - Physical interpretation
The viscosity of a fully ionized H plasma in a magnetic field is investigated theoretically, revising the derivation proposed by Braginskii (1965) for the viscous-stress-tensor viscosity coefficient eta(0). It is shown that eta(0) terms can be attributed to the tendency of the plasma to produce small thermal anisotropies during its evolution, and hence that they are fully incorporated in the gyrotropic diagonal pressure tensor, neglecting the off-diagonal terms. The role of collisions in preventing anisotropy production (and thus causing irreversible heating) is explored, and the implications of the eta(0) terms for the physics of 1000-10,000-km-scale structures in the solar corona are indicated.
Hollweg, J. V.↗
Remote Sensing of Earth Terrain
Theoretical models that are useful and practical in relating remote sensing data to the important physical parameters characterizing Earth terrain are developed. The development of models that are useful in data analysis and interpretation, scene simulation, and developing new remote sensing approaches and techniques is discussed. Numerous theoretical models that are applicable to the active and passive remote sensing of plowed fields, atmospheric precipitation, vegetation, and snow fields were developed. The radiative transfer theory is used to interpret the active and passive data as a function of rain rate. Both the random medium model and the discrete scatterer model is used to study the remote sensing of vegetation fields. Due to the non-spherical geometry of the scatterers there is strong azimuthal dependence in the observed data. Thus, the anisotropic random medium model and the discrete scatterer model with nonspherical particles was developed. In order to relate the remote sensing data to the actual physical parameters, the scattering of electromagnetic waves from randomly distributed dielectric scatterers was studied. Both the rigorous random discrete scatterer theory and the strong fluctuation theory are used to derive the backscattering cross section in terms of the actual physical parameters and the results agree well with the data obtained from the snow fields.
Kong, J. A.↗
Study of Vacuum Energy Physics for Breakthrough Propulsion
This report summarizes the accomplishments during a three year research project to investigate the use of surfaces, particularly in microelectromechanical systems (MEMS), to exploit quantum vacuum forces. During this project, we developed AFM instrumentation to repeatably measure Casimir forces in the nanoNewton range at 10 6 torr, designed an experiment to measure attractive and repulsive quantum vacuum forces, developed a QED based theory of Casimir forces that includes non-ideal material properties for rectangular cavities and for multilayer slabs, developed theoretical models for a variety of microdevices utilizing vacuum forces, applied vacuum physics to a gedanken spacecraft, and investigated a new material with a negative index of refraction.
Millis, Marc G.↗
Mars - The intermediate between earth and moon
The present work represents a summary of the state of our knowledge concerning the chief physical characteristics of Mars as deduced from existing observational and theoretical evidence. Two main geological provinces are distinguished on Mars: areas of rough terrain and dense cratering, and smooth areas comparable to lunar maria. A thermal model for the Martian poles which takes into account convective transport would indicate that the CO2 frost over the north pole is not permanent, lasting about 420 days with the water frost lasting the remaining 267 days of the Martian year. The meandering appearance of Martian rift valleys are cracks probably produced, as also in the case of lunar rilles, by stresses, perhaps in solidifying lava of impact origin, and not by erosional river beds. Data indicate a strong absorption in the thin Martian atmosphere. The atmospheric blue and violet cap around a Martian pole in winter is interpreted as a 'blue clearing' of the atmosphere increasing transparency so that surface detail of higher albedo shines through in the blue.
Opik, E. J.↗
A standard verification for rainfall estimation from remote platforms
Measurements of reflectivity at horizontal (Zh) and vertical (Zy) polarizations provide adequate information necessary to infer the two parameters of an exponential raindrop size distribution (No, Do) where the distribution is given by N(D) = Noexp(-3.67 D/Do). This distribution enables computation of water content or still air rainfall rates. The physical basis of the radar technique is outlined and illustrated theoretically, and experimental results, comparing radar derived rainfall rates with raingage and disdrometer measurements, are reviewed. The technique is useful for many meteorological and hydrological purposes, including ground truth measurements of rainfall rate over the ocean for comparison with satellite related observations.
Meitin, J. G., Jr.↗
Solar variability
The variability in the total radiant energy flux, or solar irradiance, is discussed. Direct techniques of measuring irradiance, including ground-based, balloon and rocket-borne, and spacecraft-based measurements are compared; the latter type has led to dramatic advances in accuracy since it eliminates the need for corrections for atmospheric absorption. Correlations of the measured irradiance with solar activity are described. Indirect techniques that monitor other solar parameters such as photospheric conditions or the solar diameter are reviewed, and theoretical studies which attempt to interpret the measurements are discussed. Physical mechanisms which can lead to changes in the solar luminosity are addressed, as is global time-dependent modeling of the response of the sun to structural perturbations. Finally, findings from all techniques are summarized, their weak and strong points are assessed, and suggestions for future research are made.
Sofia, S.↗
The dynamics of planetary rings
The physical processes that occur in planetary rings are discussed. The theoretical arguments leading to the conclusion that Saturn's rings are solid particles in nonuniform rotation are summarized, and the optical depth, thickness, and particle size of the rings are discussed. The influence of nearby satellites on the rings is analyzed, and asymmetries in the rings are briefly discussed. What is known of the rings of Uranus and of Jupiter's ring is summarized. Some of the dynamical processes and influences that are expected to be incorporated in more advanced theories of planetary rings are reviewed in detail, including radiation drag, plasma drag, interparticle collisions and viscosity, resonances with external satellites, shepherd satellites and moonlets. Finally, the orbital evolution of the shepherd satellites caused by the rings is estimated.
Goldreich, P.↗
Observations and theory of the solar semidiurnal tide in the mesosphere of Venus
The 1978 Pioneer Venus mission, in addition to producing strong IR radiometer experiment evidence for the existence of a semidiurnal solar tide in the Venus mesosphere, has yielded radiative flux and cloud particle distribution data about the structure of solar heating and IR cooling which facilitate calculations of tidal forcing functions and estimates of radiative dissipation rates. It is presently shown that a physically reasonable mean flow can be found which leads to theoretically predicted tides that are in excellent agreement with those observed. The zonal mean flow obtained exhibits a maximum velocity of about 130 m/sec at 70 km altitude, decreasing to about zero at 88 km, together with a jet-like meridional structure that strongly deviates from solid body rotation.
Fels, S. B.↗
Numerical simulation of boundary layers. Part 2: Ribbon-induced transition in Blasius flow
The early three-dimensional stages of transition in Blasius boundary layers are studied by numerical solution of the Navier-Stokes equations. A finite-amplitude two-dimensional wave and random low-amplitude three-dimensional disturbances are introduced. Rapid amplification of the three-dimensional components is observed and leads to transition. For intermediate amplitudes of the two-dimensional wave the breakdown is of subharmonic type, and the dominant spanwise wave number increases with the amplitude. For high amplitudes the energy of the fundamental mode is comparable to the energy of the subharmonic mode, but never dominates it; the breakdown is of mixed type. Visualizations, energy histories, and spectra are presented. The sensitivity of the results to various physical and numerical parameters is studied. Agreement with experimental and theoretical results is discussed.
Spalart, P.↗
Computer models of the spacecraft wake
Until recently, computations of space plasma flow over a spacecraft have been unstable for ratios of spacecraft dimension to Debye length typical of the low Earth orbit environment. Calculations are presented of the spacecraft/environment interaction based on two computer codes, MACH and POLAR. MACH, an inside-out particle tracking code, was developed for the purpose of validating the physics of POLAR in regimes where these are no comprehensive theoretical or experimental results. While the spacecraft which can be treated by MACH are restricted to simple geometries, the methodology is more fundamental than POLAR. MACH generates self-consistent solutions within the context of quasisteady Vlasov plasma flow and achieves Debye ratios previously unobtainable. POLAR uses a three-dimensional finite-element representation of the vehicle in a staggered mesh. The plasma sheath is modeled by outside-in particle tracking. Solutions for the plasma flow, wake and vehicle charging are obtained by Vlasov-Poisson iteration; charge stabilization techniques make the results virtually insensitive to the Debye ratio. POLAR reproduces the Laframboise static plasma solutions for sperical probes and fits the Makita-Kuriki probe data for spheres in a flowing plasma in regions where comparisons are valid. POLAR and MACH solutions for the particle and electrostatic potential structure of the wake of a charged disk in a low-altitude flow are shown for Mach numbers 4, 5, and 8. New features of the solutions include ion focussing in the wake and a definitive determination of the sheath edge in the wake which shows that the sheath is not an equipotential.
Rubin, A. G.↗
Search techniques for near-earth asteroids
Knowledge of the near-earth asteroids (Apollo, Amor, and Aten groups) has increased enormously over the last 10 to 15 years. This has been due in large part to the success of programs that have systematically searched for these objects. These programs have been motivated by the apparent relationships of the near-earth asteroids to terrestrial impact cratering, meteorites, and comets, and their relative accessibility for asteroid missions. Discovery of new near-earth asteroids is fundamental to all other studies, from theoretical modeling of their populations to the determination of their physical characteristics by various remote-sensing techniques. The methods that have been used to find these objects are reviewed, and ways in which the search for near-earth asteroids can be expanded are discussed.
Helin, E. F.↗
Synergistic use of optical and microwave data in agrometeorological applications
Remotely sensed optical and microwave data can be synergistically used to infer land surface properties. Optical data can be used to estimate surface albedo, radiation absorption by vegetation canopies and their photosynthetic efficiencies. Vegetation canopy reflectance at red and near-infrared wavelengths can be used to correct for vegetation effect on microwave emissivities at low frequencies for estimating soil moisture. Optical data can also provide information about surface and air temperatures, precipitable water vapor, cloud top temperature and its water content. This information can be utilized to correct microwave data for atmospheric effects. These points are illustrated with theoretical analyses and by application to satellite data. The basic physical mechanisms operative at the various wavelengths are also discussed.
Myneni, R. B.↗
Scientific study in solar and plasma physics relative to rocket and balloon projects
The goals of this research are to provide scientific and technical capabilities in the areas of solar and plasma physics contained in research programs and instrumentation development relative to current rocket and balloon projects; to develop flight instrumentation design, flight hardware, and flight program objectives and participate in peer reviews as appropriate; and to participate in solar-terrestrial physics modeling studies and analysis of flight data and provide theoretical investigations as required by these studies.
Wu, S. T.↗
The TSS-1R Electrodynamic Tether Experiment: Scientific and Technological Results
The Tethered Satellite System (TSS) program was designed to provide the opportunity to explore certain space plasma-electrodynamic processes (associated with high-voltage bodies and electrical currents in space) and the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether. A unique data set was obtained during the TSS-1 R mission in which the tether emf and current reached values in excess of 3500 volts and 1 amp, respectively. The insight this has allowed into the current collection process and the physics of high-voltage plasma sheaths is significant. Previous theoretical models of current collection were electro-static assuming that the orbital motion of the system, which is highly sub-sonic with respect to electron thermal motion, was unimportant. This may still be acceptable for the case of relatively slow-moving sounding rockets. However, the TSS-1 R results show that motion relative to the plasma does affect current collection and must be accounted for in orbiting systems.
Stone, Nobie H.↗
Interface Pattern Selection in Directional Solidification
The central focus of this research is to establish key scientific concepts that govern the selection of cellular and dendritic patterns during the directional solidification of alloys. Ground-based studies have established that the conditions under which cellular and dendritic microstructures form are precisely where convection effects are dominant in bulk samples. Thus, experimental data can not be obtained terrestrially under pure diffusive regime. Furthermore, reliable theoretical models are not yet possible which can quantitatively incorporate fluid flow in the pattern selection criterion. Consequently, microgravity experiments on cellular and dendritic growth are designed to obtain benchmark data under diffusive growth conditions that can be quantitatively analyzed and compared with the rigorous theoretical model to establish the fundamental principles that govern the selection of specific microstructure and its length scales. In the cellular structure, different cells in an array are strongly coupled so that the cellular pattern evolution is controlled by complex interactions between thermal diffusion, solute diffusion and interface effects. These interactions give infinity of solutions, and the system selects only a narrow band of solutions. The aim of this investigation is to obtain benchmark data and develop a rigorous theoretical model that will allow us to quantitatively establish the physics of this selection process.
Trivedi, Rohit↗
Accurate universal parameterization of absorption cross sections
We present a simple universal parameterization of total reaction cross sections for any system of colliding nuclei valid for the entire energy range from a few A MeV to a few A GeV. The universal picture presented here treats the proton-nucleus collision as a special case of the nucleus-nucleus collision, where the projectile has charge and mass number one. The parameters are associated with the physics of the collision system. In general terms Coulomb interaction modifies cross sections at lower energies and the effects of Pauli blocking are important at higher energies. The agreement between the calculated and experimental data is better than all earlier published results.
NASA Discipline Radiation Health↗