On One of the Possible Formation Mechanisms of Narrow Sporadic Ionosphere Layers
Electron density, and sounding rocket investigation of narrow sporadic ionosphere layers
SOUNDING ROCKET↗
SEARCH · Search NASA
Results for “Formation mechanism”
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.
Accelerator irradiations of minerals - Implications for track formation mechanisms and for studies of lunar and meteoritic materials
Radiation effects of accelerated heavy ions in minerals characteristic of meteorites
Heckman, H.↗
Track formation mechanisms in minerals
Heavy ion track registration in nonconductor minerals, discussing radiation damage and atomic species along trajectory
Maurette, M.↗
Formation mechanism of a silicon carbide coating for a reinforced carbon-carbon composite
Results are presented for a study to determine the mechanisms involved in a high-temperature pack cementation process which provides a silicon carbide coating on a carbon-carbon composite. The process and materials used are physically and chemically analyzed. Possible reactions are evaluated using the results of these analytical data. The coating is believed to develop in two stages. The first is a liquid controlled phase process in which silicon carbide is formed due to reactions between molten silicon metal and the carbon. The second stage is a vapor transport controlled reaction in which silicon vapors react with the carbon. There is very little volume change associated with the coating process. The original thickness changes by less than 0.7%. This indicates that the coating process is one of reactive penetration. The coating thickness can be increased or decreased by varying the furnace cycle process time and/or temperature to provide a wide range of coating thicknesses.
Rogers, D. C.↗
Observational test of a hot flow anomaly formation mechanism
The hot-flow anomalies (HFAs) observed in the vicinity of the earth's bow shock, whose high-temperature plasma is strongly deflected relative to the solar wind and flanked by density and magnetic field enhancements, are presently addressed by a model in which the coupling of ions reflected off the shock with the solar wind would convert the relative streaming energy between the reflected and solar wind ions into thermal energy; the hot plasma would then expand to form the HFA. Attention is given to a simple observational test of this model, which compares the measured temperature and density with the values expected immediately after the assumed coupling and after expansion.
Onsager, T. G.↗
A New Formation Mechanism for the Hottest Horizontal-Branch Stars
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch (EHB) are found in the ultraviolet color-magnitude diagrams (CMDs) of both omega Cen and NGC 2808. In order to investigate the origin of these subluminous stars, we have constructed a detailed set of evolutionary sequences that follow the evolution of low-mass stars continuously from the zero-age main sequence through the helium-core flash to the HB for a wide range in the mass loss along the red-giant branch (RGB). Stars with the largest mass loss evolve off the RGB to high effective temperatures before igniting helium in their cores. Our results indicate that the subluminous EHB stars, as well as the high temperature gap along the EHB of NGC 2808, can be explained if these stars undergo a late helium-core flash while descending the white-dwarf cooling curve. Under these conditions the convection zone produced by the main helium flash will penetrate into the stellar envelope, thereby mixing most, if not all, of the envelope hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the 'born-again' scenario for producing hydrogen-deficient stars following a very late helium-shell flash. This 'flash mixing' of the envelope during a late helium-core flash greatly enhances the envelope helium and carbon abundances and, as a result, leads to a discontinuous increase in the HB effective temperature. We argue that the hot HB gap observed in NGC 2808 is associated with this theoretically predicted dichotomy in the RB properties. Using new helium- and carbon-rich stellar atmospheres, we show that the changes in the envelope abundances due to flash mixing will suppress the ultraviolet flux in the spectra of hot EHB stars. We suggest that such changes in the emergent spectral energy distribution are primarily responsible for explaining the hot subluminous EHB stars in omega Cen and NGC 2808. Moreover, we demonstrate that models without flash mixing lie, at most, only approximately 0.1 mag below the EHB, and hence fail to explain the observations.
Sweigart, Allen V.↗
The Formation Mechanism of Helium-Rich Subdwarf B Stars
This program used the Far Ultraviolet Spectroscopic Explorer (FUSE) to measure the surface abundances of three helium-rich hot subdwarfs in the Galactic field, to test our hypothesis that such stars form via a late helium flash while descending the white dwarf cooling curve. If these stars form via a late He flash, the stellar envelope should be mixed with the interior, enhancing the surface carbon abundance enormously while depleting the surface hydrogen. The first observation of PG1544+488 was incomplete because detector 2 was turned off, but the repeat observations were fully successful. The observations of LB1766 had no problems and were fully successful. The observations of PG1127+019 became infeasible when FUSE lost some of its pointing control, so it was replaced with JL87, which was also observed successfully.
Luers, Jeannine N.↗
Performance of Heterogeneous Sulfate Aerosol Formation Mechanisms Against Aircraft and Ground-Based Observations in Seoul, South Korea
Haze events are a common occurrence in East Asia, where local meteorology combines with anthropogenic emissions under certain conditions to create rapid increases in secondary aerosol. Often these events are driven by rapid production of secondary sulfate-nitrate-ammonium (SIA) aerosol. Models have typically included simple parameterizations of the production of sulfate during these events which vary widely in their magnitude. There are a range of different mechanistic descriptions of the chemical processes occurring during these events in large part due to poor constraints on aerosol pH and transition metal content, both of which are important to determining the key pathway(s) driving aerosol production. Here, we use the GEOS-Chem chemical transport model to interpret detailed ground and aircraft observations from May and June 2016 during the Korea-United States Air Quality (KORUS-AQ) field study to evaluate the mechanistic pathways governing sulfate production during a haze event. We apply our model of sulfate production mechanisms to ground-based speciated aerosol measurements in winter, summer, and fall in Seoul.
Katherine R Travis↗
Morphometrics of polygonal ground on Earth and Mars: Assessing formation mechanisms from orbit
Explore the source record for details and available documents.
Derek A. Berman↗
The formation mechanisms for mid-latitude ice scarps on Mars
Explore the source record for details and available documents.
K.E. Williams↗
Mechanism of Pinhole Formation in Membrane Electrode Assemblies for PEM Fuel Cells
The pinhole formation mechanism was studied with a variety of MEAs using ex-situ and in-situ methods. The ex-situ tests included the MEA aging in oxygen and MEA heat of ignition. In-situ durability tests were performed in fuel cells at different operating conditions with hydrogen and oxygen. After the in-situ failure, MEAs were analyzed with an Olympus BX 60 optical microscope and Cambridge 120 scanning electron microscope. MEA chemical analysis was performed with an IXRF EDS microanalysis system. The MEA failure analyses showed that pinholes and tears were the MEA failure modes. The pinholes appeared in MEA areas where the membrane thickness was drastically reduced. Their location coincided with the stress concentration points, indicating that membrane creep was responsible for their formation. Some of the pinholes detected had contaminant particles precipitated within the membrane. This mechanism of pinhole formation was correlated to the polymer blistering.
Stanic, Vesna↗
A study of Type I polar stratospheric cloud formation
Mechanisms for the formation of Type I (nitric acid-based) polar stratospheric clouds (PSCs) are discussed. If the pre-existing sulfate aerosols are liquid prior to PSC formation, then nitric acid particles (Type Ib) form by HNO3 dissolution in aqueous H2SO4 solution droplets. This process does not require a nucleation step for the formation of HNO3 aerosols, so most pre-existing aerosols grow to become relatively small HNO3-containing particles. At significantly lower temperatures, the resulting supercooled solutions (Type Ib) may freeze to form HNO3 ice particles (Type Ia). If the pre-existing sulfate aerosols are initially solid before PSC formation, then HNO3 vapor can be deposited directly on the frozen sulfate particles. However, because an energy barrier to the condensation exists a nucleation mechanism is involved. Here, we suggest a unique nucleation mechanism that involves formation of HNO3/H20 solutions on the sulfate ice particles. These nucleation processes may be highly selective, resulting in the formation of relatively small number of large particles.
Tabazadeh, A.↗
Lunar Floor-Fractured Craters: Classification, Distribution and Implications for Magmatism and Shallow Crustal Structure
Floor-fractured craters (FFCs) are a class of lunar craters defined by their distinctly shallow, often plate-like floors, and combinations of radial, con-centric, and polygonal floor-fractures; a variety of other interior features are often observed, such as moats, ridges, small dark-haloed pits, and patches of mare material. They were first classified by Schultz [1] , who recognized eight overall types of floor-fractured crater. These eight subtypes have widely differing appearances, a factor that could provide insight into formation mechanisms (different manifestations of the same mechanism, or indicators of varying formation mechanisms). Two formation mechanisms for FFCs were initially proposed: 1) magmatic intrusion [1], in which magma rising toward the surface in dikes encountered low-density breccia lenses beneath crater floors and spread laterally to form sills, raising and fracturing the crater floor. 2) viscous relaxation [2], in which the properties of the crust permitted viscous flow in the vicinity of the crater, causing long-wavelength relaxation of the topography and uplift and fracturing of the crater floor. Critical to distinguishing between these two end-member hypotheses and identifying others is a quantitative assessment of the topography of FFCs and knowledge of their regional and local settings. The purpose of this study is to use newly available Lunar Reconnaissance Orbiter (LRO) Lunar Orbiter Laser Altimeter (LOLA) altimeter and Lunar Reconnaissance Orbiter Camera (LROC) image data to provide an updated global catalog of the locations, classes, morphometric and morphologic characteristics of all lunar floor-fractured craters. We use the excellent 8-class system initially described in Schultz [1] as a starting point for classification and the enhanced LOLA/LROC data sets to examine and categorize all FFCs; we found evidence for a new FFC class, discernably different from the previously existing types. Our approach, and the global categorization of all FFCs, permits the spatial distribution of each FFC-subtype to be plotted and assessed allowing for further investigation into FFC formation mechanisms. Upon completion, the data set will be made available on our web site at http://www.planetary.brown.edu/html_pages/data.htm.
Jozwiak, L. M.↗
Shock tube study of the fuel structure effects on the chemical kinetic mechanisms responsible for soot formation
Soot formation in toluene-, benzene-, and acetylene-oxygen-argon mixtures was investigated to study soot formation in a combustion environment. High concentrations of oxygen completely suppress soot formation. The addition of oxygen at relatively low concentrations uniformly suppresses soot formation at high pressures, while at relatively lower pressures it suppresses soot formation at higher temperatures while promoting soot production at lower temperatures. The observed behavior indicates that oxidation reactions compete with ring fragmentation. The main conclusion to be drawn from the results is that the soot formation mechanism is probably the same for the pyrolysis and oxidation of hydrocarbons. That is, the addition of oxygen does not alter the soot route but rather promotes or inhibits this route by means of competitive reactions. An approach to empirical modeling of soot formation during pyrolysis of aromatic hydrocarbons is also presented.
Frenklach, M.↗
On the mechanism of formation of loop prominences
A mechanism is suggested for the formation of loop-type prominences in solar-active regions following flare events. The mechanism is based on the already existing idea of compression of a coronal plasma element resulting in enhanced radiation and consequent cooling of the element. A model is suggested for such a compression based on the concept of a contracting, force-free filamentary structure. If the current in a filament increases with time, then there is a radial contraction of the filament. Since the coronal plasma is frozen into the magnetic field lines of the filament, a contraction of the filament causes a compression of the filamentary plasma. This model of compression is shown to be in approximate qualitative and quantitative agreement with observations.
De, B. R.↗
Investigation on the Surface Vortex Formation During Mechanical Stirring with an Axial-Flow Impeller Used in an Aluminum Process
The present study investigated the mechanism of surface vortex formation in an aluminum melt vessel stirred by an axial-flow impeller mechanically. The oxide film is formed at the aluminum melt/air interface, and the movement of the interface entrains the oxide film and inclusions. Hence, the transient movement of melt–air interface is significant. The present study conducted a water model experiment and numerical simulation focusing on the movement of gas–liquid interface. The present study found that the oxide film can be entrained by two phenomena: (1) local surface vortex and (2) sloshing near the vessel wall. The local surface vortex is formed due to the pressure distribution around the impeller, and the sloshing is caused by macroinstabilities, which is generated by the discharged flow of axial-flow impeller. Besides, the shape of gas–liquid interface is dependent on the impeller shape. The axial-flow impeller gives rise to steeply curved shape of gas–liquid interface near the impeller shaft.
36 MATERIALS SCIENCE↗