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All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Argon-40/Argon-39 Age Spectra of Apollo 17 Highlands Breccia Samples by Laser Step Heating and the Age of the Serenitatis Basin

We have obtained high-resolution (21-63 steps) Ar-40/Ar-39 age spectra using a continuous laser system on 19 submilligram samples of melt rocks and clasts from Apollo 17 samples collected from the pre-Imbrian highlands in the easternmost part of the Serenitatis basin. The samples include poikilitic melt rocks inferred to have been formed in the Serenitatis basin-forming impact, aphanitic melt rock whose compositions vary and whose provenance is uncertain, and granulite, gabbro, and melt clasts. Three of the poikilitic melts have similar age spectrum plateau ages (72395,96, 3893 +/- 16 Ma (2sigma); 72535,7, 3887 +/- 16 Ma; 76315,150, 3900 +/- 16 Ma) with a weighted mean age of 3893 +/- 9 Ma, which we interpret as the best age for the Serenitatis basin- forming impact. Published Ar-40/Ar-39 age spectrum ages of Apollo 17 poikilitic melts are consistent with our new age but are much less precise. Two poikilitic melts did not give plateaus and the maxima in their age spectra indicate ages of greater than or equal to 3869 Ma (72558,7) and greater than or equal to 3743 Ma (77135,178). Plateau ages of two poikilitic melts and two gabbro clasts from 73155 range from 3854 +/- 16 Ma to 3937 +/- 16 Ma and have probably been affected by the ubiquitous (older?) clasts and by post- formation heating (impact) events. Plateau ages from two of the aphanitic melt 'blobs' and two granulites in sample 72255 fall in the narrow range of 3850 q 16 Ma to 3869 q 16 Ma with a weighted mean of 3862 +/- 8 Ma. Two of the aphanitic melt blobs from 72255 have ages of 3883 +/- 16 Ma and greater than or equal to 3894 Ma, whereas a poikilitic melt clast (of different composition from the 'Serenitatis' melts) has an age of 3835 +/- 16 Ma, which is the upper limit for the accretion of 72255. These data suggest that either the aphanitic melts vary in age, as is also suggested by their varying chemical compositions, or they formed in the 72255 accretionary event about 3.84-3.85 Ga and older relict material is responsible for the dispersion of ages. In any case the aphanitic melts do not appear to be Serenitatis products. Our age for the Serenitatis impact shows, on the basis of the isotopic age evidence alone, that Serenitatis is greater than 20-25 Ma and probably greatr than 55-60 Ma older than Imbrium (less than or equal to 3870 Ma and probably less than or equal to 3836 Ma (Dalrymple and Ryder, 19931). Noritic granulite sample 78527 has a plateau age of 4146 +/- 17 Ma, representing a minimum age for cooling of this sample in the early lunar crust. So far there is no convincing evidence in the lunar melt rock record for basin-forming impacts significantly older than 3.9 Ga.

Dalrymple, G. Brent↗
NASA NTRS2013

Properties of Polycyclic Aromatic Hydrocarbons in the Northwest Photon Dominated Region of NGC 7023. I. PAH Size, Charge, Composition, and Structure Distribution

Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer Infrared Spectrograph spectral map of the northwest photon dominated region (PDR) in NGC 7023 was analyzed exclusively using PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb). The 5-15 micron spectrum at each pixel is fitted using a non-negative-least-squares fitting approach. The fits are of good quality, allowing decomposition of the PAH emission into four subclasses: size, charge, composition, and hydrogen adjacency (structure). Maps tracing PAH subclass distributions across the region paint a coherent astrophysical picture. Once past some 20 seconds of arc from HD 200775, the emission is dominated by the more stable, large, symmetric, compact PAH cations with smaller, neutral PAHs taking over along the lines-of-sight toward the more distant molecular cloud. The boundary between the PDR and the denser cloud material shows up as a distinct discontinuity in the breakdown maps. Noteworthy is the requirement for PANH cations to fit the bulk of the 6.2 and 11.0 micron features and the indication of PAH photo-dehydrogenation and fragmentation close to HD 200775. Decomposition of the spectral maps into "principal" subclass template spectra provides additional insight into the behavior of each subclass. However, the general applicability of this computationally more efficient approach is presently undetermined. This is the first time the spectra of individual PAHs are exclusively used to fit the 5-15 micron region and analyze the spatial behavior of the aromatic infrared bands, providing fundamental, new information about astronomical PAH subpopulations including their dependence on, and response to, changes in local conditions.

northwest photon↗
NASA NTRS2005

Surface characteristics of spacecraft components affect the aggregation of microorganisms and may lead to different survival rates of bacteria on Mars landers

Layers of dormant endospores of Bacillus subtilis HA101 were applied to eight different spacecraft materials and exposed to martian conditions of low pressure (8.5 mbar), low temperature (-10 degrees C), and high CO(2) gas composition and irradiated with a Mars-normal ultraviolet (UV-visible- near-infrared spectrum. Bacterial layers were exposed to either 1 min or 1 h of Mars-normal UV irradiation, which simulated clear-sky conditions on equatorial Mars (0.1 tau). When exposed to 1 min of Mars UV irradiation, the numbers of viable endospores of B. subtilis were reduced three to four orders of magnitude for two brands of aluminum (Al), stainless steel, chemfilm-treated Al, clear-anodized Al, and black-anodized Al coupons. In contrast, bacterial survival was reduced only one to two orders of magnitude for endospores on the non-metal materials astroquartz and graphite composite when bacterial endospores were exposed to 1 min of Mars UV irradiation. When bacterial monolayers were exposed to 1 h of Mars UV irradiation, no viable bacteria were recovered from the six metal coupons listed above. In contrast, bacterial survival was reduced only two to three orders of magnitude for spore layers on astroquartz and graphite composite exposed to 1 h of Mars UV irradiation. Scanning electron microscopy images of the bacterial monolayers on all eight spacecraft materials revealed that endospores of B. subtilis formed large aggregates of multilayered spores on astroquartz and graphite composite, but not on the other six spacecraft materials. It is likely that the formation of multilayered aggregates of endospores on astroquartz and graphite composite is responsible for the enhanced survival of bacterial cells on these materials.

Mars↗
NASA NTRS2003

Foliar Reflectance and Fluorescence Responses for Corn and Soybean Plants Under Nitrogen Stress

We are investigating the use of spectral indices derived from actively induced fluorescence spectra and passive optical spectra. We examined the influence of photosynthetic pigment, carbon (C) and nitrogen (N) content on the spectral fluorescence and passive optical property characteristics of mature, upper leaves from plants provided different N fertilizer application rates: 20%, 50%, 100% and 150% of recommended N levels. A suite of optical, fluorescence, and biophysical measurements were collected on leaves from field grown corn (Zea mays L.) and soybean plants (Glycine max L.) grown in pots (greenhouse + ambient sunlight. Steady state laser-induced fluorescence emission spectra (5 nm resolution) were obtained from adaxial and abaxial surfaces resulting from excitation at single wavelengths (280, 380 or 360, and 532 nm). For emission spectra produced by each of these excitation wavelengths, ratios of emission peaks were calculated, including the red far-red chlorophyll fluorescence (ChlF) ratio (F685/F740) and the far-red/green (F740/F525) ratio. High resolution (< 3 nm) optical spectra (350-2500 nm) of reflectance, transmittance, and absorptance were also acquired for both adaxial and abaxial leaf surfaces. Species differences were demonstrated for several optical parameters. A 'red edge' derivative ratio determined from transmittance spectra [as the maximum first deivative, between 650-750 nm, normalized to the value at 744 nm, or Dmax/D744], was strongly associated with the C/N ratio (r(exp 2) = 0.90, P +/- 0.001). This ratio, calculated from reflectance spectra, was inversely related to chlorophyll b content (r(exp 2) = 0.91, P +/- 0.001) as was the ChlF (F685/F740) ratio obtained with 532 nm excitation (r(exp 2) = 0.76, P +/- 0.01). Discrimination of N treatment groups was possible with specific fluorescence band ratios (e.g., F740/F525 obtained with 380 nm excitation). Higher ChlF and blue-green emissions were measured from the abaxial leaf surfaces. Abaxial surfaces also produced higher reflectances, in general, in the 400-800 nm spectrum.

Middleton, E. M.↗
NASA NTRS1994

The Titan haze revisted: Magnetospheric energy sorces quantitative tholin yields

We present laboratory measurements of the radiation yields of complex organic solids produced from N2/CH4 gas mixtures containing 10 or 0.1% CH4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton tholin. High-energy magnetospheric electrons responsible for tholin production represents a class distinct from the plasma electrons considered in models of Titan's aiglow. Electrons with E greater than 20 keV provide an energy flux approximately 1 x 10(exp -2) erg/cm/sec, implying from our measured tholin yields a mass flux of 0.5 to 4.0 x 10(exp -14) g/sq cm/sec of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m). This figure is in agreement with required mass fluxes computed from recent radiative transfer and sedimentation models. If, however, theses results, derived from experiments at approximately 2 mb, are applied to lower pressure levels toward peak auroral electron energy deposition and scaled with pressure as the gas-phase organic yields, the derived tholin mass flux is at least an order of magnitude less. We attrribute this difference to the fact that tholin synthesis occurs well below the level of maximum electron energy depositon and to possible contributions to tholis from UV-derived C2-hydrocarbons. We conclude that Tita tholin, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze-a result consistent with the fact that the optical properties of Titan tholin, among all proposed material, are best at reproducing Titan's geometric albedo spectrum from near UV to mid-IR in light-scattering models.

Thompson, W. Reid↗
NASA NTRS1994

Intraseasonal behavior of clouds, temperature, and motion in the tropics

The spectral character of tropical convection is investigated in an 11-yr record of outgoing longwave radiation from the Advanced Very High Resolution Radiometer (AVHRR) to identify interaction with the tropical circulation. Along the equator in the eastern hemisphere, the space-time spectrum of convection possesses a broad peak at wavenumbers 1-3 and eastward periods of 35-95 days. Significantly broader than the dynamical signal of the Madden-Julian oscillation (MJO), this quasi-discrete convective signal is associatd with a large-scale anomaly that propagates across and modulates time mean or 'climatological convection' over the equatorial Indian Ocean and western Pacific. Outside that region the convective signal is small, even though, under amplified conditions, coherence can be found east of the date line and in the subtropics. Having a zonal scale of approximately wavenumber 2, anomalous convection propagates eastward at some 5 m/s and suppresses as well as reinforces climatological in the eastern hemisphere. The convective signal amplifies to a seasonal maximum near vernal equinox and, to a weaker degree, again near autumnal equinox, when climatological convection and warm sea surface temperature (SST) cross the equator. Contemporaneous records of motion from European Center for medium-Range Weather Forecasts (ECMWF) analyses and tropospheric-mean temperature from Microwave Sounding Unit reveal an anomalous component of the tropical circulation that coexists with the convective signal and embodies many of the established properties of the MJO. In the eastern hemisphere, subtropical Rossby gyres and zonal Kelvin structure along the equator flank the convective anomaly as it tracks eastward, giving the anomalous circulation to form of a 'forced response.' In the western hemisphere, the dynamical signal is composed chiefly of wavenumber-1 Kelvin structure, which as the form of a 'propagating response' that is excited in and radiates away from anomalous convection at some 10 m/s. Kelvin structure comprising the propagating response appears in 850-mb and 200-mb zonal winds even when the convective signal is absent, albeit with much smaller amplitude. In contrast, the signal in 1000-mb convergence appears only when accompanied by anomalous convection, which suggests that convergence in the boundary layer is instrumental in achieving strong interaction with the convective pattern.

Salby, Murry L.↗
NASA NTRS1994

The Titan haze revisited: magnetospheric energy sources and quantitative tholin yields

We present laboratory measurements of the radiation yields of complex organic solids produced from N2/CH4 gas mixtures containing 10 or 0.1% CH4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton tholin. High-energy magnetospheric electrons responsible for tholin production represent a class distinct from the plasma electrons considered in models of Titan's airglow. Electrons with E > 20 keV provide an energy flux approximately 1 x 10(-2) erg cm-2 sec-1, implying from our measured tholin yields a mass flux of 0.5 to 4.0 x 10(-14) g cm-2 sec-1 of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m.) This figure is in agreement with required mass fluxes computed from recent radiative transfer and sedimentation models. If, however, these results, derived from experiments at approximately 2 mb, are applied to lower pressure levels toward peak auroral electron energy deposition and scaled with pressure as the gas-phase organic yields, the derived tholin mass flux is at least an order of magnitude less. We attribute this difference to the fact that tholin synthesis occurs well below the level of maximum electron energy deposition and to possible contributions to tholins from UV-derived C2-hydrocarbons. We conclude that Titan tholin, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze--a result consistent with the fact that the optical properties of Titan tholin, among all proposed materials, are best at reproducing Titan's geometric albedo spectrum from near UV to mid-IR in light-scattering models.

Non-NASA Center↗
NASA NTRS2002

Photoionization of the Fe lons: Structure of the K-Edge

X-ray absorption and emission features arising from the inner-shell transitions in iron are of practical importance in astrophysics due to the Fe cosmic abundance and to the absence of traits from other elements in the nearby spectrum. As a result, the strengths and energies of such features can constrain the ionization stage, elemental abundance, and column density of the gas in the vicinity of the exotic cosmic objects, e.g. active galactic nuclei (AGN) and galactic black hole candidates. Although the observational technology in X-ray astronomy is still evolving and currently lacks high spectroscopic resolution, the astrophysical models have been based on atomic calculations that predict a sudden and high step-like increase of the cross section at the K-shell threshold (see for instance. New Breit-Pauli R-matrix calculations of the photoionization cross section of the ground states of Fe XVII in the region near the K threshold are presented. They strongly support the view that the previously assumed sharp edge behaviour is not correct. The latter has been caused by the neglect of spectator Auger channels in the decay of the resonances converging to the K threshold. These decay channels include the dominant KLL channels and give rise to constant widths (independent of n). As a consequence, these series display damped Lorentzian components that rapidly blend to impose continuity at threshold, thus reformatting the previously held picture of the edge. Apparent broadened iron edges detected in the spectra of AGN and galactic black hole candidates seem to indicate that these quantum effects may be at least partially responsible for the observed broadening.

Palmeri, P.↗
NASA NTRS1973

Skylab Student Project: Summary Description

In 1971 the NASA conceived the Skylab student project in an effort to involve the general public in the Skylab program. The primary aim of this project was to stimulate national interest in science and technology. NASA decided to direct the Skylab student project to those young people who have indicated an interest in science and technology and to foster this interest through direct participation in an ongoing program emphasizing as wide a spectrum of science and technology as possible. Skylab, with the opportunity it gives to provide experiments in areas of science and technology provided the ideal opportunity for such participation. In implementing this project, the National Science Teachers Association (NSTA), having an existing, closely associated contact with students, grades 9 through 12, in United States schools, was asked to sponsor, organize, and administer a national competition for high school students. This competition called for individual students (or groups of students) to develop meaningful experiments to be flown on Skylab. To facilitate the organization and administration of this program, the NSTA divided the participating students into their 12 geographical regions with a regional chairmen appointed to receive all proposals for his region. The regional chairmen then appointed a committee of eminent scientists, engineers, and science educators to evaluate each proposal. Some 80,000 applications were requested by teachers and 3409 proposals were submitted. Moreover, because of team proposals, over 4000 students participated and approximately 300 regional winners selected. Each participant received a certification of merit. The 300 winning regional proposals were transmitted to the NSTA headquarters where they were further screened. In March of 1972 twenty-five national winners and 22 special mentions were announced. The 25 winning students were then assigned science advisers at the George C. Marshall Space Flight center (MSFC), the center selected by NASA to be responsible for development of the student project. The Johnson Space Center (JSC) also provided science advisors and valuable support.

Floyd, Henry B.↗
NASA NTRS1995

Anisotropic microturbulence near the Sun

Interplanetary scintillation observations which sample the spatial spectrum of electron density at scales between 10 and 100 km show power-law spectra which are flatter than the Kolmogorov spectra observed at larger scales by spacecraft and also, indirectly, by phase scintillation of coherent radio signals. Furthermore, angular broadening observations of compact radio sources have shown that the microscale density fluctuations are field-aligned and become more anisotropic as R decreases inside 10 solar radius. We present angular broadening observations taken in October of 1992 which were recorded nearly simultaneously on the VLA and VLBA arrays. The VLA samples structure at scales between 3 and 30 km, whereas the VLBA samples scales between 200 and 4000 km. The small scale VLA measurements of the south polar source 1246-075 showed lower turbulence than those of the equatorial source 1256-057, consistent with previous work showing that the density delta N(exp 2)(sub e) is a factor of 10-15 lower in coronal holes. The VLA observations inside of 10 solar radius were anisotropic, as expected. We were not able to measure the equatorial source with the VLBA inside of 10 solar radius because the scattering was too strong, however we did observe the polar source just inside this distance with both the VLA and the VLBA. Significant anisotropy was seen on the smaller scales, but the larger scales were essentially isotropic. This suggests that the process responsible for the anisotropic microturbulence is distinct from the larger scale isotropic turbulence.

Coles, William A.↗
NASA NTRS2016

Thermodynamic Spectrum of Solar Flares Based on SDO/EVE Observations: Techniques and First Results

The Solar Dynamics Observatory (SDO)/EUV Variability Experiment (EVE) provides rich information on the thermodynamic processes of solar activities, particularly on solar flares. Here, we develop a method to construct thermodynamic spectrum (TDS) charts based on the EVE spectral lines. This tool could potentially be useful for extreme ultraviolet (EUV) astronomy to learn about the eruptive activities on distant astronomical objects. Through several cases, we illustrate what we can learn from the TDS charts. Furthermore, we apply the TDS method to 74 flares equal to or greater than the M5.0 class, and reach the following statistical results. First, EUV peaks are always behind the soft X-ray (SXR) peaks and stronger flares tend to have faster cooling rates. There is a power-law correlation between the peak delay times and the cooling rates, suggesting a coherent cooling process of flares from SXR to EUV emissions. Second, there are two distinct temperature drift patterns, called Type I and Type II. For Type I flares, the enhanced emission drifts from high to low temperature like a quadrilateral, whereas for Type II flares the drift pattern looks like a triangle. Statistical analysis suggests that Type II flares are more impulsive than Type I flares. Third, for late-phase flares, the peak intensity ratio of the late phase to the main phase is roughly correlated with the flare class, and the flares with a strong late phase are all confined. We believe that the re-deposition of the energy carried by a flux rope, which unsuccessfully erupts out, into thermal emissions is responsible for the strong late phase found in a confined flare. Furthermore, we show the signatures of the flare thermodynamic process in the chromosphere and transition region in the TDS charts. These results provide new clues to advance our understanding of the thermodynamic processes of solar flares and associated solar eruptions, e.g., coronal mass ejections.

Wang, Yuming↗
NASA NTRS1996

Spinoff 1996

By their challenging nature, NASA programs are particularly demanding of technological input. Meeting the aeronautical and space goals of the past four decades has necessitated leading edge advancements across a diverse spectrum that embraces virtually every scientific and technological discipline. Technology is simply knowledge and, like other forms of knowledge, it is often broadly applied and transferable. For that reason, the vast storehouse of technology NASA has built is a national resource, a bank of knowledge available for commercial applications and enhancements to the quality of life-"spinoff"-to new products and processes of benefit to the national economy, industrial efficiency and human welfare. Multiple use of technology has never been more important. Budgetary stringency is reducing the amount of government funding available for new research and development, but at the same time intensifying international competition demands increasing technological innovation to strengthen the U.S. posture in the global marketplace. Reuse of technology offers a relatively inexpensive supplementary means of partnering with industry focused on bringing new products and processes to the market. More than a thousand of spinoff products and processes have emerged from reapplication of technology developed for NASA mission programs. Each has Contributed some measure of benefit to the national economy, productivity or lifestyle; some bring only moderate increments of gain, but many generate benefits of significant order with economic values in the millions of dollars. Other technologies with moderate economic return have added measurably to the quality of life of U.S. citizens. Collectively, they represent a substantial dividend on the national investment in aerospace research. By Congressional mandate, it is NASA's responsibility to promote expansion of spinoff in the public interest. Through its Technology Transfer Program, NASA seeks to encourage greater use of its technological resources by providing a link between the technology and those who might be able to put it to advantageous use. The program's aim is to broaden and accelerate the transfer accomplishments and thereby to gain national benefit in terms of new products, services, and new jobs. This publication is an instrument of-and documents the outcome of-that purpose. It is intended to heighten awareness of the technology available for transfer and its potential for public benefit. Spinoff 1996 is organized in three sections: Section 1, summarizes NASA's current mainline programs, whose objectives require development of new technology and therefore replenish and expand the bank of knowledge available for reapplication. Section 2, the focal point of this volume, contains a representative sampling of spinoff products and processes that resulted from secondary application of NASA technology. Section 3, describes the various mechanisms NASA employs to stimulate technology transfer and lists, in an appendix, contact sources for Further information about the Technology Transfer Program.

Haggerty, James J.↗
NASA NTRS1999

An Overview of the NASA Sounding Rockets and Balloon Programs

The U.S. National Aeronautics and Space Administration (NASA) Sounding Rockets and Balloon Programs conduct a combined total of approximately fifty to sixty missions per year in support of the NASA scientific community. These missions are provided in support of investigations sponsored by NASA'S Offices of Space Science, Life and Microgravity Sciences & Applications, and Earth Science. The Goddard Space Flight Center has management and implementation responsibility for these programs. The NASA Sounding Rockets Program has continued to su,pport the science community by integrating their experiments into the sounding rocket payload and providing the rocket vehicle and launch operations necessary to provide the altitude/time required obtain the science objectives. The sounding rockets continue to provide a cost-effective way to make in situ observations from 50 to 1500 km in the near-earth environment and to uniquely cover the altitude regime between 50 km and 130 km above the Earth's surface, which is physically inaccessible to either balloons or satellites. A new architecture for providing this support has been introduced this year with the establishment of the NASA Sounding Rockets Contract. The Program has continued to introduce improvements into their operations and ground and flight systems. An overview of the NASA Sounding Rockets Program with special emphasis on the new support contract will be presented. The NASA Balloon Program continues to make advancements and developments in its capabilities for support of the scientific ballooning community. Long duration balloon (LDB) is a prominent aspect of the program with two campaigns scheduled for this calendar year. Two flights are scheduled in the Northern Hemisphere from Fairbanks, Alaska, in June and two flights are scheduled from McMurdo, Antarctica, in the Southern Hemisphere in December. The comprehensive balloon research and development (R&D) effort has continued with advances being made across the spectrum of balloon related disciplines. As a result of these technology advancements a new ultra long duration balloon project (ULDB) for the development of a 100- day duration balloon capability has been initiated. The ULDB will rely upon new balloon materials and designs to accomplish its goals. The Program has also continued to introduce new technology and improvements into flights systems, ground systems and operational techniques. An overview of the various aspects of the NASA Balloon Program will be presented.

Flowers, Bobby J.↗
DOE OSTI2023

Metastable argon dynamics in a pulsed microplasma at 43 GHz

Short pulses of millimeter wave (MMW) radiation at 43 GHz create microplasma within a photonic crystal for pressures from 40 to 600 Torr (1.3 × 10 3 –8.0 × 10 4 Pa). Gas breakdown occurs within a photonic crystal, which acts as an electromagnetic resonator to create a strong initial electric field. The time response of the argon metastable density is experimentally determined during the pulse and in the afterglow using laser absorption. The metastable density overshoots the steady-state condition at the beginning of the pulse and during the afterglow. Modeling is presented to understand these observations. The overproduction of argon 1s5 at the beginning of each pulse is due to a concurrent overshoot in the MMW electric field within the photonic crystal. This field overheats the plasma electrons and enhances the production of excited states. The burst of argon metastables observed in the afterglow is due to the pooled energy of the plasma stored in electrons, ions, and excited states of argon. As a result, understanding metastable production is an important intermediate step to ionization and is also critical in the study of diode-pumped rare gas lasers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1994

Plant Responses to Increased UV-B Radiation: A Research Project

Ozone decrease implies more ultraviolet-B (UV-B) radiation reaching the surface of the Earth. Increased UV-B radiation triggers responses by living organisms. Despite the large potential impacts on vegetation, little is known about UV-B effects on terrestrial ecosystems. Long-term ecological studies are needed to quantify the effects of increased UV radiation on terrestrial ecosystems, asses the risks, and produce reliable data for prediction. Screening pigments are part of one of the protective mechanism in plants. Higher concentrations of screening pigments in leaves may be interpreted as a response to increased UV radiation. If the screening effect is not sufficient, important molecules will be disturbed by incoming radiation. Thus, genetics, photosynthesis, growth, plant and leaf shape and size, and pollen grains may be affected. This will have an impact on ecosystem dynamics, structure and productivity. It is necessary to monitor selected terrestrial ecosystems to permit detection and interpretation of changes attributable to global climate change and depleted ozone shield. The objectives of this project are: (1) To identify and measure indicators of the effects of increased solar UV-B radiation on terrestrial plants; (2) to select indicators with the greatest responses to UV-B exposure; (3) to test, adapt or create ecosystem models that use the information gathered by this project for prediction and to enhance our understanding of the effects of increased UV-B radiation on terrestrial ecosystems. As a first step to achieve these objectives we propose a three-year study of forest and steppe vegetation on the North slope of the Brooks Range (within the Arctic circle, in Alaska), in the Saguaro National Monument (near Tucson, Arizona) and in the forests and steppes of Patagonia (Argentina). We selected (1) vegetation north of the Polar Circle because at 70N there is 8% risk of plant damage due to increased UV-B radiation; (2) the foothills of Catalina Mountains because there is anecdotal evidence of plant damage on the saguaros that has been linked to increased UV radiation, and (3) the forests of Nothofagus spp. and the steppe of Patagonia where the risk of plant damage at 35S is 5% and increases to as much as 15% at 55S due to increased UV-B radiation. Measurements of UV-B radiation impinging on the surface at 55S largely exceed the predicted UV-B radiation values at 50 latitude and 0% ozone depletion. Preliminary HPLC analyses of UV-B absorbing compounds in Nothofagus antartica, N. pumilio, N. betuloides and Rumex sp. in natural conditions show species-specific patterns. The spectrum of N. antartica grown at 38S differs significantly from that of N. antartica in natural conditions in Ushuaia (55S). These results suggest that the selected main area (Patagonia) is appropriate for assessing the problem and its magnitude and that Nothofagus is appropriate for our study.

DAntoni, H. L.↗
NASA NTRS2013

Suomi NPP VIIRS Prelaunch and On-orbit Geometric Calibration and Characterization

The Visible Infrared Imager Radiometer Suite (VIIRS) sensor was launched 28 October 2011 on the Suomi National Polarorbiting Partnership (SNPP) satellite. VIIRS has 22 spectral bands covering the spectrum between 0.412 m and 12.01 m, including 16 moderate resolution bands (M-bands) with a spatial resolution of 750 m at nadir, 5 imaging resolution bands (I-bands) with a spatial resolution of 375 m at nadir, and 1 day-night band (DNB) with a near-constant 750 m spatial resolution throughout the scan. These bands are located in a visible and near infrared (VisNIR) focal plane assembly (FPA), a short- and mid-wave infrared (SWMWIR) FPA and a long-wave infrared (LWIR) FPA. All bands, except the DNB, are co-registered for proper environmental data records (EDRs) retrievals. Observations from VIIRS instrument provide long-term measurements of biogeophysical variables for climate research and polar satellite data stream for the operational communitys use in weather forecasting and disaster relief and other applications. Well Earth-located (geolocated) instrument data is important to retrieving accurate biogeophysical variables. This paper describes prelaunch pointing and alignment measurements, and the two sets of on-orbit correction of geolocation errors, the first of which corrected error from 1,300 m to within 75 m (20 I-band pixel size), and the second of which fine tuned scan angle dependent errors, bringing VIIRS geolocation products to high maturity in one and a half years of the SNPP VIIRS on-orbit operations. Prelaunch calibration and the on-orbit characterization of sensor spatial impulse responses and band-to-band co-registration (BBR) are also described.

Geolocation↗
DOE OSTI2024

The Use of High-Density UN Fuel in Heat-Pipe Microreactors

Heat-pipe microreactors (HPMRs) are very small-scale nuclear reactors that employ heat pipes (HPs) for heat removal. HPMRs can be easily integrated with other forms of renewable energies, can be used for emergency responses to disaster relief zones, can be deployed in remote locations not connected to the grid, and can be removed from sites and replaced by new ones. HPMRs can also be used for space missions as HPs do not rely on gravity for heat transfer. Conventional fuel materials, such as uranium oxide (UO 2 ) and uranium oxycarbide (UCO), are currently considered in most existing HPMR designs, but ceramic uranium nitride (UN) fuel that has high uranium density, high thermal conductivity, and high melting point may become a better fuel candidate. Through neutronics calculations, this paper assesses the impact of using UN fuel in HPMRs with two different neutron spectra (fast and thermal) and two different fuel forms [traditional solid fuel pellets and TRi-structural-ISOtropic (TRISO) fuel compacts]. It was concluded that retrofitting HPMRs with UN fuel has the potential to reduce the initial 235 U enrichment requirement by ~3 wt% (to keep the same cycle length) or increase the cycle length (by keeping the same initial 235 U enrichment), which enables more compact and transportable HPMR core designs. However, using UN fuel decreases the control element worth [by up to 20% for the Special Purpose Reactor (SPR) and 5% for HP-MR] and is up to 80% more costly. Increasing 15 N enrichment can further decrease the initial 235 U enrichment requirement and increase the control element worth but is more costly. In conclusion, compared to fast-spectrum HPMRs fueled with solid pellet fuels, retrofitting UN fuel is more suitable for thermal-spectrum HPMRs fueled with TRISO fuel compacts, where the neutron spectrum hardening caused by using UN is less significant.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Extreme sparsification of physics-augmented neural networks for interpretable model discovery in mechanics

Data-driven constitutive modeling with neural networks has received increased interest in recent years due to its ability to easily incorporate physical and mechanistic constraints and to overcome the challenging and time-consuming task of formulating phenomenological constitutive laws that can accurately capture the observed material response. However, even though neural network-based constitutive laws have been shown to generalize proficiently, the generated representations are not easily interpretable due to their high number of trainable parameters. Sparse regression approaches exist that allow for obtaining interpretable expressions, but the user is tasked with creating a library of model forms which by construction limits their expressiveness to the functional forms provided in the libraries. Here, in this work, we propose to train regularized physics-augmented neural network-based constitutive models utilizing a smoothed version of $L^0$-regularization. This aims to maintain the trustworthiness inherited by the physical constraints, but also enables interpretability which has not been possible thus far on any type of machine learning-based constitutive model where model forms were not assumed a priori but were actually discovered. During the training process, the network simultaneously fits the training data and penalizes the number of active parameters, while also ensuring constitutive constraints such as thermodynamic consistency. We show that the method can reliably obtain interpretable and trustworthy constitutive models for compressible and incompressible hyperelasticity, yield functions, and hardening models for elastoplasticity, using synthetic and experimental data. This work aims to set a new paradigm for interpretable machine learning models in the broad area of solid mechanics where low and limited data is available along with prior knowledge of physical constraints that the learned maps need to obey. This paradigm can potentially be extended to a broader spectrum of scientific exploration.

Data-driven constitutive models↗