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At least 1,315 records · Page 73

Deep Generative Models for Materials Discovery and Machine Learning-Accelerated Innovation

Machine learning and artificial intelligence (AI/ML) methods are beginning to have significant impact in chemistry and condensed matter physics. For example, deep learning methods have demonstrated new capabilities for high-throughput virtual screening, and global optimization approaches for inverse design of materials. Recently, a relatively new branch of AI/ML, deep generative models (GMs), provide additional promise as they encode material structure and/or properties into a latent space, and through exploration and manipulation of the latent space can generate new materials. These approaches learn representations of a material structure and its corresponding chemistry or physics to accelerate materials discovery, which differs from traditional AI/ML methods that use statistical and combinatorial screening of existing materials via distinct structure-property relationships. However, application of GMs to inorganic materials has been notably harder than organic molecules because inorganic structure is often more complex to encode. In this work we review recent innovations that have enabled GMs to accelerate inorganic materials discovery. We focus on different representations of material structure, their impact on inverse design strategies using variational autoencoders or generative adversarial networks, and highlight the potential of these approaches for discovering materials with targeted properties needed for technological innovation.

36 MATERIALS SCIENCE↗

Spectral structure of the field of solar radiation reflected from the ocean/atmosphere system

The statistical characteristics of the spectral structure of the brightness field of the ocean/atmosphere system are determined from the spectra of incident radiation and the radiation reflected from the ocean, obtained from aircraft (Conveyor 990) at heights of 0.3 and 10 km above the Atlantic Ocean. Analysis of the spectral structure reveals a weak correlation between atmospheric brightness variations in the 0.4 to 0.5 micron and 0.55 to 0.70 micron regions of the spectrum. This is attributed to the possible influence of variations of the scattering coefficient or optical thickness on the brightness variations (whose sign depends on the predominance of damping or multiple scattering in a given spectral region).

Malkevich, M. S.↗

Structural Dynamics, Stability, and Control of Helicopters

The dynamic synthesis of gyroscopic structures consisting of point-connected substructures is investigated. The objective is to develop a mathematical model capable of an adequate simulation of the modal characteristics of a helicopter using a minimum number of degrees of freedom. The basic approach is to regard the helicopter structure as an assemblage of flexible substructures. The variational equations for the perturbed motion about certain equilibrium solutions are derived. The discretized variational equations can be conveniently exhibited in matrix form, and a great deal of information about the system modal characteristics can be extracted from the coefficient matrices. The derivation of the variational equations requires a monumental amount of algebraic operations. To automate this task a symbolic manipulation program on a digital computer is developed.

Meirovitch, L.↗

Galactic plane gamma radiation

Analysis of the complete data from SAS-2 accentuates the fact that the distribution of galactic gamma radiation has several similarities to that of other large-scale tracers of galactic structure. The gamma radiation shows no statistically significant variation with direction, and the spectrum seen along the plane is the same as that derived for the galactic component of the gamma radiation at high latitude. This uniformity of the energy spectrum, the smooth decrease in intensity as a function of galactic latitude, and the absence of any galactic gamma ray sources at high latitudes argue in favor of a diffuse origin for most of the galactic gamma radiation, rather than a collection of localized sources. All the localized sources identified in the SAS 2 data are associated with known compact objects on the basis of observed periodicities, except gamma195+5 Excluding those SAS 2 sources observed by COS-B and two other excesses (CG 312-1 and CG333+0) visible in the SAS 2 data associated with tangential directions of spiral arms, thera are eight remaining new sources in the COS-B catalog.

Hartman, R. C.↗

Implications of cavity, topographic and geologic influences on tilt and strain observations

Tilt and strain observations are importantly (pathologically at the 100%, typically at the few 10s% level) affected by cavities, topography, and geological inhomogenities; gravity observation are practically unaffected. The traditional earth tide observatory and abandoned mine or tunnel is a very poor place to measure body tides because of the complicated cavities, topography and geology. Instead, the ideal site for observing the body tide is in flat terrain with horizontally layered, mechanically homogeneous geology. Strain will be measured with long surface- or trench-mounted laser strain meters and tilt with long, surface- or trench mounted liquid levels, or with borehole tiltmeters. Horizontal geological discontinuities can produce large perturbations of the tilt and strain tides, and these perturbations, using the known homogeneous tidal strains and tilts, can be used in exploring local structure in favorable cases and, through possible time variations of tidal admittances, in predicting earthquakes.

Harrison, J. C.↗

The extension of a uniform canopy reflectance model to include row effects

The effect of row structure is assumed to be caused by the variation in density of vegetation across rows rather than to a profile in canopy height. The calculation of crop reflectance using vegetation density modulation across rows follows a parallel procedure to that for a uniform canopy. Predictions using the row model for wheat show that the effect of changes in sun to row azimuth are greatest in Landsat Band 5 (red band) and can result in underestimation of crop vigor.

Suits, G. H.↗

Extension of a uniform canopy reflectance model to include row effects

The reflectance model for a uniform crop canopy is extended to include the effects of crops planted in rows. The effect of row structure is assumed to be caused by the variation in density of vegetation across rows rather than to a profile in canopy height. The calculation of crop reflectance using vegetation density modulation across rows follows a parallel procedure to that for a uniform canopy. Predictions using the row model for wheat show that the effect of changes in sun to row azimuth are greatest in Landsat Band 5 (red band) and can result in underestimation of crop biomass.

Suits, G. H.↗

Remote sensing and geologic studies of the planetary crusts

Dark haloed craters and regions of the Moon which were sites of ancient volcanism were remotely sensed as well as KREEP deposits in the Inbrium region. The relationship between geology and geochemistry in the Undarum/Spumans region was also examined. Results are summarized for observations of the Reiner Gamma formation, studies of impact cratering mechanics and processes, spectral variations of asteroidal surfaces, albedo and color variations on Ganymede, and studies of lunar impact structures.

Hawke, B. R.↗

Interim Reference Ozone Models for the Middle Atmosphere

A set of models was generated based on six satellite experiments of the monthly latitudinal variations in total column ozone and the vertical structure of ozone from 20 mb to 0.003 mb. Generally, interannual variability in monthly zonal means is only a few percent. Comparisons of measurements using various techniques to measure global ozone reveal very good agreement between the techniques. Agreement between individual satellite experiments and the reference model of monthly zonal means is generally within 10% below altitudes of 0.4 mb. This has allowed the first global model of ozone measurements to be constructed from multiple sets of satellite measurements. The ozone measurements based on the satellite data are in excellent agreement with previous midlatitude mean annual model based on rocket and balloon data. Also, models are provided of the relation between total ozone and vertical structure.

Keating, G. M.↗

An infrared study of the NGC 1977 H II region/molecular cloud interface

The results of an infrared study of the H II region NGC 1977 and the adjacent dense molecular cloud are reported. Extensive far-infrared maps with 45 arcsec resolution allow the spatial structure of the dust temperature and optical depth variations across the ionization front to be delineated. Analysis of the dust energetics indicates that the only significant energy source is the B1 V star HD 37018 which ionizes the H II region. This result, together with a favorable geometry, provides a good opportunity to determine the ratio of ultraviolet absorption efficiency to far-infrared emission efficiency, 790 + 460 or - 180. Analysis of the gas energetics indicates that collisions with warm dust grains can explain the observed gas temperatures.

Makinen, P.↗

Very-large-array observations of a complex gradual solar burst at 6 cm wavelength

A 6-cm VLA observation of a burst from a solar active region is reported. The burst shows a normal impulsive rise and fall superimposed on a gradual component. The peak brightness temperature of the burst remains relatively constant, and the flux variations are due to changes in the source structure. It appears as though an arcade of loops is gradually heated or filled with hot plasma. The burst is weakly polarized but unipolar; the possible reasons for this structure are considered. Depolarization of optically thin emission due to twisted magnetic fields within the source can produce significant depolarization, but is probably not active in this event. Instead, it is suggested that the flare emission is optically thick, and that hot overlying material preferentially absorbs one polarization.

Kundu, M. R.↗

Collisional interchange instability. I - Numerical simulations of intermediate-scale irregularities

Numerical simulations of the generalized Rayleigh-Taylor instability are presented. The model and simulations are applicable to bottomside and topside spread F, unstable barium cloud dynamics, and collisional interchange instability in general. The principal result is that the evolution of the instability tends to an anisotropic state consisting of nearly sinusoidal (quasi-periodic) variation along the effective electric field, and shocklike structures propagating perpendicular to E(eff) along the extrema of the quasi-periodic structures. The spectral properties of the nonlinear state are analyzed using one-dimensional power spectra calculated along spatial trajectories for selected angles to E(eff). In this way a direct comparison to in situ probe data can be made. The inherent anisotropy of the nonlinear state is reflected in major qualitative differences between the spectra taken parallel to and perpendicular to E(eff). The fundamental finding of the present work is that anisotropy in interchange dynamics is much greater than had been previously reported. This strong anisotropy can explain much of the spectral and spatial structural characteristics of both bottomside and topside spread F. In a companion paper a comparison of the simulation results to various in situ data sets is given.

Zargham, S.↗

The relationship of satellite-inferred stratospheric aerosol extinction to the position of the 50-mb north polar jet stream

The relationship between stratospheric aerosols and the location of the north polar night stratospheric jet stream was investigated for selected periods of four successive winters (1979-1982), using measurements from SAM II (Stratospheric Aerosol Measurement II) and SAGE I (Stratospheric Aerosol and Gas Experiment I) satellite-borne sun photometers and corresponding meteorological observations. Each period investigated included a polar stratospheric warming during which major dynamic meteorological changes are known to have perturbed the structure of the polar vortex. The analysis of variations in aerosol extinction mixing ratio patterns among winters and during major stratospheric warming events within separate winters showed a well-defined positive gradient in extinction mixing ratio and temperature across the jet stream from the cyclonic side to the anticyclonic side at altitudes between 20 and 30 km during each winter period. Estimates of extinction mixing ratio profiles measured near the center of the polar vortex suggest that a gradual subsidence took place within the polar vortex during at least three of the four winter periods.

Livingston, John M.↗

Transverse ion energization and low-frequency plasma waves in the mid-altitude auroral zone - A case study

Evidence of transverse ion energization at altitudes of several earth radii in the auroral zone was reexamined using several hundred hours of high-sensitivity and high-resolution plasma data obtained by the Dynamics Explorer 1 satellite. The data on particle environment encountered at midaltitudes in the auroral zone disclosed rapid variations in the values of total density, thermal structure, and composition of the plasma in the interval measured; the modes of low-frequency plasma waves also varied rapidly. It was not possible to unambiguously identify in these data particle and wave signature of local transverse ion energization; however, many intervals were found where local transverse ion heating was consistent with the observations.

Peterson, W. K.↗

False-color display of special sensor microwave/imager (SSM/I) data

Displays of multifrequency passive microwave data from the Special Sensor Microwave/Imager (SSM/I) flying on the Defense Meteorological Satellite Program (DMSP) spacecraft are presented. Observed brightness temperatures at 85.5 GHz (vertical and horizontal polarizations) and 37 GHz (vertical polarization) are respectively used to 'drive' the red, green, and blue 'guns' of a color monitor. The resultant false-color images can be used to distinguish land from water, highlight precipitation processes and structure over both land and water, and detail variations in other surfaces such as deserts, snow cover, and sea ice. The observations at 85.5 GHz also add a previously unavailable frequency to the problem of rainfall estimation from space. Examples of mesoscale squall lines, tropical and extra-tropical storms, and larger-scale land and atmospheric features as 'viewed' by the SSM/I are shown.

Negri, Andrew J.↗

Use of satellite data and modeling to asses the influence of stratospheric processes on the troposphere

The research is comprised of the following tasks: use of simple analytical and numerical models of a coupled troposphere-stratosphere system to examine the effects of radiation and ozone on planetary wave dynamics and the tropospheric circulation; use of satellite data obtained from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) instrument and Solar Backscattered Ultraviolet (SBUV) experiment, in conjunction with National Meteorological Center (NMC) data, to determine the planetary wave vertical structures, dominant wave spectra, ozone spectra, and time variations in diabatic heating rate; and synthesis of the modeling and observational results to provide a better understanding of the effects that stratospheric processes have on tropospheric dynamics.

Nathan, Terrence R.↗