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At least 1,369 records · Page 76

The Fate of Saharan Dust Across the Atlantic and Implications for a Central American Dust Barrier

Saharan dust was observed over the Caribbean basin during the summer 2007 NASA Tropical Composition, Cloud, and Climate Coupling (TC4) field experiment. Airborne Cloud Physics Lidar (CPL) and satellite observations from MODIS suggest a barrier to dust transport across Central America into the eastern Pacific. We use the NASA GEOS-5 atmospheric transport model with online aerosol tracers to perform simulations of the TC4 time period in order to understand the nature of this barrier. Our simulations are driven by the Modem Era Retrospective-Analysis for Research and Applications (MERRA) meteorological analyses. We evaluate our baseline simulated dust distributions using MODIS and CALIOP satellite and ground-based AERONET sun photometer observations. GEOS-5 reproduces the observed location, magnitude, and timing of major dust events, but our baseline simulation does not develop as strong a barrier to dust transport across Central America as observations suggest. Analysis of the dust transport dynamics and lost processes suggest that while both mechanisms play a role in defining the dust transport barrier, loss processes by wet removal of dust are about twice as important as transport. Sensitivity analyses with our model showed that the dust barrier would not exist without convective scavenging over the Caribbean. The best agreement between our model and the observations was obtained when dust wet removal was parameterized to be more aggressive, treating the dust as we do hydrophilic aerosols.

Nowottnick, E.↗

Nucleotide- and metalloid-driven conformational changes in the arsenite efflux ATPase ArsA

Arsenite (As III ) is toxic to all organisms due to its ability to tightly bind exposed thiols within cells. An important As III resistance mechanism in prokaryotes involves proteins encoded by thearsoperon. A central component of thearsoperon in many bacteria is the cytoplasmic ATPase, ArsA, which orchestrates a series of nucleotide-dependent handoffs, starting with the capture of As III by the ArsD metallochaperone and culminating in its removal from the cell by the ArsB efflux pump. Although the mechanism of ArsA has been widely studied, the molecular details of how nucleotide hydrolysis modulates these events remain unclear. ArsA is an archetypal member of the intradimeric Walker A (IWA) family of ATPases, implicated in a diversity of complex biological functions. Conformational changes typical of IWA ATPases have been postulated to drive these molecular events but have not been demonstrated. We report cryogenic electron microscopy (cryo-EM) structures of ArsA in MgADP-bound and MgATP-boundopenstates, as well as a distinctclosedMgATP-bound state liganded to As III . X-ray absorption spectroscopy (XAS) confirmed three-coordinate binding of As III to the conserved cysteines at the metalloid-binding site of the closed state. Coupled with biochemical characterization, our cryo-EM structures reveal key conformational changes in the ArsA catalytic cycle consistent with other IWA ATPases and provide the structural basis for allosteric activation of nucleotide hydrolysis by As III . This work establishes how the nucleotide state of ArsA transiently creates a high-affinity binding site that can sequester metalloid within the cell, followed by a nucleotide-driven handoff to ArsB for efflux.

Science & Technology - Other Topics↗

Origin of coronal mass ejection and magnetic cloud: Thermal or magnetic driven?

A fundamental problem in Solar-Terrestrial Physics is the origin of the solar transient plasma output, which includes the coronal mass ejection and its interplanetary manifestation, e.g. the magnetic cloud. The traditional blast wave model resulted from solar thermal pressure impulse has faced with challenge during recent years. In the MHD numerical simulation study of CME, the authors find that the basic feature of the asymmetrical event on 18 August 1980 can be reproduced neither by a thermal pressure nor by a speed increment. Also, the thermal pressure model fails in simulating the interplanetary structure with low thermal pressure and strong magnetic field strength, representative of a typical magnetic cloud. Instead, the numerical simulation results are in favor of the magnetic field expansion as the likely mechanism for both the asymmetrical CME event and magnetic cloud.

Zhang, Gong-Liang↗

Significance of pH and iron-sulfur chemistry for molybdenum sequestration under sulfidic conditions

Molybdenum (Mo), a redox-sensitive trace metal, plays an important role in recording ancient oxygenation and deoxygenation events as a paleoredox proxy. The mobility and reactivity of Mo in aqueous conditions are closely tied to the chemistry of reduced sulfur and iron species. However, our current knowledge on the formation, structure, stability, and condensation pathways of FeMoS clusters in aqueous settings remains limited, which has driven the current study. In this study, we conducted systematic experiments investigating the interactions between dissolved Mo (initially introduced as molybdate, MoO 4 2– , or tetrathiomolybdate, MoS 4 2– ), ferrous iron (Fe 2+ ), and sulfide (ΣH 2 S aq ) in variously defined abiotic sulfidic systems to determine the external conditions (i.e., pH, and reactant concentrations and ratios) necessary for the formation of solid-phase Fe-Mo sulfides. Solution samples of each system were monitored using ultraviolet-visible spectroscopy (UV–vis) to track the degree of thiolation of dissolved Mo species. Precipitates were analyzed using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) to determine their elemental compositions and valences, and structure (i.e., crystalline or amorphous), respectively. All FeMoS precipitates were amorphous and contained 76–90% Mo(IV) and 10–24% Mo(V) with a trend toward lower Mo(IV):Mo(V) ratios with increasing pH. The degree of Mo thiolation, which was strongly dependent on solution pH and Fe 2+ concentrations, greatly affected the amount of Mo sequestered (i.e., an increased degree of Mo thiolation in solution led to an increased amount of Mo in the final FeMoS precipitate). Furthermore, these findings suggest that changes in pH and Fe 2+ concentrations may be responsible for the sulfide-independent variations in Mo behavior observed in euxinic basins.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Characteristics of Bay of Bengal Monsoon Depressions in the 21st Century

The controversy surrounding the observed trends in the frequency of South Asian Monsoon Depressions (MDs)1-4, the association of MDs with the summer season extremes5,6, and a moisture driven future precipitation response in Global Climate Models (GCMs)7,8 have elevated the importance of understanding climate change impacts on the characteristics of MDs. GCMs exhibit a significant relationship between the long-term mean occurrences of MDs and the strength of the background upper (lower) tropospheric meridional (zonal) winds and atmospheric moisture in the core genesis region of MDs. Likewise, the strength of the meridional tropospheric temperature gradient (MTG) in GCMs determines the trajectories of MDs over land. While monsoon dynamics progressively weakens in the future period, atmospheric moisture exhibits a strong increase, limiting the impact of changes in background dynamics on the frequency of MDs. Moreover, the weakening of the future MTG is many times weaker than its inherent underestimation in the GCMs. Hence the 21st century increase in radiative forcing does not significantly reduce the frequency of MDs or impact their trajectories over land in the GCMs. Although more wet extremes are expected to occur during the MD lifecycle, non-depression day extremes dominate the overall increase in extreme precipitation, which may render the monsoon extremes less predictable in the future.

Rastogi, Deeksha↗

Greenland Surface Melt Dominated by Solar and Sensible Heating

The Greenland Ice Sheet is the primary source of global Barystatic sea-level rise, and at least half of its recent mass-loss acceleration is caused by surface meltwater runoff. Previous studies on surface melt have examined various thermodynamic and dynamic drivers, yet their contributions are not compared using unified observations. We use decade-long in-situ measurements from automatic weather stations throughout the ablation zone to assess energy components and identify the leading physical processes in this area. Large melt events exceeding 3σ contribute only ~2% to total surface melt since 2007. The day-to-day variability of all melt is dominated by sensible heat exchange (31 ± 7%) and shortwave radiation (28 ± 5%). Sensible and solar heating correlate with the occurrence of dry and fast gravity-driven winds. These katabatic winds increase sensible heating of the surface mainly by enhancing vertical mixing that reduces the temperature inversion. The concomitant low humidity and clear skies yield increased solar heating.

54 ENVIRONMENTAL SCIENCES↗

Interfacing electron and neutrino quasielastic scattering cross sections with the spectral function in GENIE

Progress in neutrino-nucleus cross section models is being driven by the need for highly accurate predictions for the neutrino oscillation community. These sophisticated models are being developed within a microscopic description of the nucleus with the goal of encompassing all reaction modes relevant for the accelerator neutrino program. The disconnect between these microscopic models and the event generators that will be used in the next generation of experiments represents a critical obstacle that must be overcome in order to precisely measure the neutrino oscillation parameters. To this end we have developed a hadron tensor interface for lepton-nucleus quasielastic (QE) scattering within the GENIE event generator as a proof of principle, with the broader goal of creating an efficient pipeline for incorporating advanced theoretical models in event generators. As a demonstration of this interface we have implemented the spectral function model into GENIE by connecting theorist provided fortran code through the hadron tensor interface. The spectral function model offers a more complete description of the nuclear ground state, as well as the ability to provide quantifiable theoretical uncertainties. Finally, we validate this implementation and compare its predictions against data and against QE models already available in GENIE.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Climate impact on fluvial-lake system evolution, Eocene Green River Formation, Uinta Basin, Utah, USA

In light of a modern understanding of early Eocene greenhouse climate fluctuations and new highly seasonal fluvial system faces models, the role of climate in the evolution of one classically-cited continental, terminal lake system is re-examined. Detailed stratigraphic description and elemental abundance data from fifteen cores and seven outcrop regions of the Green River Formation were used to construct a ~150 km cross section across the Uinta Basin, Utah, USA. Lake Uinta in the Uinta Basin is divided into five lake phases: (1) post-Paleocene Eocene Thermal Maximum, (2) peak Eocene hyperthermal, (3) waning hyperthermal, Early Eocene Climatic Optimum (EECO), (4) post-hyperthermal, and (5) post-EECO regimes, based primarily on climatically driven changes in fluvial style in combination with sedimentary indicators of lacustrine carbonate deposition, organic matter preservation, salinity, and lake depth. Basinwide siliciclastic dominated intervals were deposited by highly seasonal fluvial systems and record negative organic carbon isotope excursions associated with early Eocene abrupt, transient global warming (hyperthermal) events. Carbonate dominated or organic rich intervals record stable, less seasonal climate periods between hyperthermals, with lower siliciclastic sediment supply allowing the development of carbonate and organic matter preservation. The stratigraphic progression from alternating organic rich and lean zones to the overlying organic rich Mahogany and R8 zones represents the global transition out of the pulsed early Eocene hyperthermal climate regime to a time of sediment starvation and lake stratification, sequestering sedimentary organic carbon. This study provides a novel approach to terrestrial paleoclimate reconstruction that relies largely on unique sedimentary indicators and secondarily on isotopic proxy records within the context of a large basin-wide sedimentologic and stratigraphic data set, thus setting the stage for future detailed geochemical terrestrial paleoclimate proxy development.

Geology↗

Coronal Heating by Magnetic Explosions

We build a case for the persistent strong coronal heating in active regions and the pervasive quasi-steady heating of the corona in quiet regions and coronal holes being driven in basically the same way as the intense transient heating in solar flares: by explosions of sheared magnetic fields in the cores of initially closed bipoles. We begin by summarizing the observational case for exploding sheared core fields being the drivers of a wide variety of flare events, with and without coronal mass ejections. We conclude that the arrangement of an event's flare heating, whether there is a coronal mass ejection, and the time and place of the ejection relative to the flare heating are all largely determined by four elements of the form and action the magnetic field: (1) the arrangement of the impacted, interacting bipoles participating in the event, (2) which of these bipoles are active (have sheared core fields that explode) and which are passive (are heated by injection from impacted active bipoles), (3) which core field explodes first, and (4) which core-field explosions are confined within the closed field of their bipoles and which ejectively open their bipoles.

Moore, Ronald L.↗

Gusev Crater Paleolake: Two-Billion Years of Martian Geologic, (and Biologic?) History

Ancient Martian lakes are sites where the climatological, chemical, and possibly biological history of the planet has been recorded. Their potential to keep this global information in their sedimentary deposits, potential only shared with the polar layered-deposits, designates them as the most promising targets for the ongoing exploration of Mars in terms of science return and global knowledge about Mars evolution. Many of the science priority objectives of the Surveyor Program can be met by exploring ancient Martian lake beds. Among martian paleolakes, lakes in impact craters represent probably the most favorable sites to explore. Though highly destructive events when they occur, impacts may have provided in time a significant energy source for life, by generating heat, and at the contact of water and/or ice, deep hydrothermal systems, which are considered as favorable environments for life. In addition, impact crater lakes are changing environments, from thermally driven systems at the very first stage of their formation, to cold ice-protected potential oases in the more recent Martian geological times. Thus, they are plausible sites to study the progression of diverse microbiologic communities.

Cabrol, N. A.↗

Fine-Tuning the Accretion Disk Clock in Hercules X-1

RXTE ASM count rates from the X-ray pulsar Her X-1 began falling consistently during the late months of 2003. The source is undergoing another state transition similar to the anomalous low state of 1999. This new event has triggered observations from both space and ground-based observatories. In order to aid data interpretation and telescope scheduling, and to facilitate the phase-connection of cycles before and after the state transition, we have re-calculated the precession ephemeris using cycles over the last 3.5 years. We report that the source has displayed a different precession period since the last anomalous event. Additional archival data from CGRO suggests that each low state is accompanied by a change in precession period and that the subsequent period is correlated with accretion flux. Consequently our analysis reveals long-term accretion disk behaviour which is predicted by theoretical models of radiation-driven warping.

Still, M.↗

Asian Outflow and Trans-Pacific Transport of Carbon Monoxide and Ozone Pollution: An Integrated Satellite, Aircraft, and Model Perspective

Satellite observations of carbon monoxide (CO) from the Measurements of Pollution in the Troposphere (MOPITT) instrument are combined with measurements from the Transport and Chemical Evolution Over the Pacific (TRACE-P) aircraft mission over the northwest Pacific and with a global three-dimensional chemical transport model (GEOS-CHEM) to quantify Asian pollution outflow and its trans-Pacific transport during spring 2001. Global CO column distributions in MOPITT and GEOS-CHEM are highly correlated (R(exp 2) = 0.87), with no significant model bias. The largest regional bias is over Southeast Asia, where the model is 18% too high. A 60% decrease of regional biomass burning emissions in the model (to 39 Tg/yr) would correct the discrepancy; this result is consistent with TRACE-P observations. MOPITT and TRACE-P also give consistent constraints on the Chinese source of CO from fuel combustion (181 Tg CO/yr). Four major events of trans-Pacific transport of Asian pollution in spring 2001 were seen by MOPITT, in situ platforms, and GEOS-CHEM. One of them was sampled by TRACE-P (26-27 February) as a succession of pollution layers over the northeast Pacific. These layers all originated from one single event of Asian outflow that split into northern and southern plumes over the central Pacific. The northern plume (sampled at 6-8 km off California) had no ozone enhancement. The southern subsiding plume (sampled at 2-4 km west of Hawaii) contained a 8 - 17 ppbv ozone enhancement, driven by decomposition of peroxyacetylnitrate (PAN) to nitrogen oxides (NOx). This result suggests that PAN decomposition in trans-Pacific pollution plumes subsiding over the United States could lead to significant enhancements of surface ozone.

Heald, Colette L.↗

fmdtools Tutorial: Intro to Resilience Modelling, Simulation, and Visualization in Python With fmdtools

This workshop will cover the basics of using the fmdtools package for the simulation of hazardous scenarios for resilience simulation. The fmdtools simulation package is an open-source python toolkit for simulating the dynamic response of a system to internal and externally-driven hazardous scenarios, including faults and environmental conditions, that can be used to analyze the risks related to these hazards. Prior to the development of fmdtools, researchers had to either adapt an (often limited) propriety toolkit or develop their own design/simulation/analysis codes to develop their models of hazardous events, a significant technical burden to both (1) leveraging resilience modeling methodologies and (2) extending these methodologies with their own contributions. The fmdtools package provides a number of model constructs and simulation and analysis methods to enable the designer to focus to solely on their modeling case-study while still enabling a significant degree of model expressiveness and adaptability via Python-based model definition. This tutorial will present the setup of fmdtools and a high-level overview of its use, as well as some simple examples for understanding how to leverage its modelling, simulation, and analysis capabilities. Familiarity with jupyter notebook and basic python will be assumed.

Daniel Hulse↗

Density Effects on Lock Exchange Flow During a DCC

The High Temperature Test Facility (HTTF) is a scaled, electric model at Oregon State University (OSU) of the Modular High Temperature Gas Reactor (MHTGR) by General Atomics. There are a few major differences between the design and the OSU facility. In the HTTF the reactor pressure vessel and the reactor cavity are in two separate tanks connected by a crossover duct similar to the crossover duct that would be seen between the reactor core and the steam generator in the MHTGR. This feature allows for the facility to open two main break valves simulating a Double-Ended Guillotine Break (DEGB) of the cross over duct, synonymous to a crossover duct break in the MHTGR. The density gradient driven flow phenomena in the HTTF duct can then be studied further. This break would cause a depressurization of the reactor and a loss of flow. It is called the Depressurized Loss of Forced Convection Accident (D-LOFC), also known as the Depressurized Conduction Cool-Down Event for High Temperature Gas Reactors (HTGRs) because heat removal is by conduction through the core and radiation out of the vessel. The HTTF was built and scaled to maintain the highest accuracy for the DCC. Prototypical temperatures with an inlet temperature of 259 deg. C and an outlet temperature of 687 deg. C are maintained in the HTTF. The temperature profile is also maintained in the smaller volume by using a ceramic core. The HTTF uses graphite heaters because the resistive heat produced does not change significantly with temperature . In order to produce a facility of similar temperatures within the allotted budget, sacrifices had to be made in other areas. The HTTF is scaled one-fourth in diameter, one-fourth in height and one-eighth in pressure of the MHTGR. There are also a few limitations for the HTTF. While in the MHTGR, the cavity is filled with air, the HTTF uses highly pure Nitrogen and does not have the ability to simulate a full depressurization. The DCC event is traditionally comprised of multiple stages: depressurization, molecular diffusion and natural circulation. However, this traditional interpretation ignores lock exchange flow which will occur immediately following depressurization and could have significant consequences in the reactor. It was found that the traditional model underestimates the amount of air ingress by at least 660 times according to Oh and Kim in their theoretical research of air ingress. Molecular diffusion also decreases with stratification. Any model that specifies the gases are stratified has further underestimated the air ingress of the DCC event. In addition, molecular diffusion is a much slower process, allowing the reactor to shut down and cool off considerably from operating temperatures. Thus, understanding lock exchange flow, the air ingress concentrations and the time scale is critical to MHTGR research and licensing. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

Electric actuator modification on a turbine driven feed pump at Calvert Cliffs nuclear power plant - 296

Feedwater systems are a leading contributor to unit SCRAMs as indicated by the 2015 INPO data, where 9 of the 51 SCRAMs recorded were a result of feedwater issues. Digital feedwater control systems and enhanced feed pump control modifications can greatly improve performance and control of the plant feedwater systems and help drive down the potential for reactivity management events and unit SCRAMs. Westinghouse has a strong background in the implementation of digital feedwater control systems, and in planning for a recently completed feedwater control system upgrade at Calvert Cliffs, the Westinghouse team worked with Calvert Cliffs to design a solution to a number of common operational issues. Primarily, improving feedwater control over the entire operating range, eliminating single points of vulnerability (SPV) in the feedwater/feed pump systems and eliminating operational issues with the feed pump steam admission valve hydraulic control oil system. A robust first-of-a-kind design and implementation program was undertaken to split the existing hydraulic valve gear actuator with separate electric actuators. Westinghouse worked with Calvert Cliffs and Emerson Process Management (EPM) to install and commission General Electric (GE) feed pump turbines with separate electric actuators for the high-pressure (HP) and low-pressure (LP) steam admission valves. This successful mechanical and control modification was a first for both the nuclear and non-nuclear power generation industries. Westinghouse is working with a number of other utilities to implement similar modifications for PWR and BWR feed pump applications. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

Electron scattering and neutrino physics

Abstract A thorough understanding of neutrino–nucleus scattering physics is crucial for the successful execution of the entire US neutrino physics program. Neutrino–nucleus interaction constitutes one of the biggest systematic uncertainties in neutrino experiments—both at intermediate energies affecting long-baseline deep underground neutrino experiment, as well as at low energies affecting coherent scattering neutrino program—and could well be the difference between achieving or missing discovery level precision. To this end, electron–nucleus scattering experiments provide vital information to test, assess and validate different nuclear models and event generators intended to test, assess and validate different nuclear models and event generators intended to be used in neutrino experiments. Similarly, for the low-energy neutrino program revolving around the coherent elastic neutrino–nucleus scattering (CEvNS) physics at stopped pion sources, such as at ORNL, the main source of uncertainty in the evaluation of the CEvNS cross section is driven by the underlying nuclear structure, embedded in the weak form factor, of the target nucleus. To this end, parity-violating electron scattering (PVES) experiments, utilizing polarized electron beams, provide vital model-independent information in determining weak form factors. This information is vital in achieving a percent level precision needed to disentangle new physics signals from the standard model expected CEvNS rate. In this white paper, we highlight connections between electron- and neutrino–nucleus scattering physics at energies ranging from 10 s of MeV to a few GeV, review the status of ongoing and planned electron scattering experiments, identify gaps, and lay out a path forward that benefits the neutrino community. We also highlight the systemic challenges with respect to the divide between the nuclear and high-energy physics communities and funding that presents additional hurdles in mobilizing these connections to the benefit of neutrino programs.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Forward-backward multiplicity fluctuation and longitudinal harmonics in high-energy nuclear collisions

In this paper, an analysis method is proposed to study the forward-backward (FB) multiplicity fluctuation in high-energy nuclear collisions, built on the earlier work of Bzdak and Teaney [Phys. Rev. C 87, 024906 (2013)]. The method allows the decomposition of the centrality dependence of average multiplicity from the dynamical event-by-event (EbyE) fluctuation of multiplicity in pseudorapidity. Application of the method to AMPT (A Multi-Phase Transport model) and HIJING (Heavy Ion Jet INteraction Generator) models shows that the long-range component of the FB correlation is captured by a few longitudinal harmonics, with the first component driven by the asymmetry in the number of participating nucleons in the two colliding nuclei. The higher-order longitudinal harmonics are found to be strongly damped in AMPT compared to HIJING, due to weaker short-range correlations as well as the final-state effects present in the AMPT model. Two-particle pseudorapidity correlation reveals interesting charge-dependent short-range structures that are absent in HIJING model. Lastly, the proposed method opens an avenue to elucidate the particle production mechanism and early time dynamics in heavy-ion collisions. Future analysis directions and prospects of using the pseudorapidity correlation function to understand the centrality bias in p + p, p + A, and A + A collisions are discussed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Modelling of Ablatant Deposition from Electromagnetically Driven Radiative Pellets for Disruption Mitigation Studies

The Electromagnetic Particle Injector (EPI) concept is advanced through the simulation of ablatant deposition into ITER H-mode discharges with calculations showing penetration past the H-mode pedestal for a range of injection velocities and granule sizes concurrent with the requirements of disruption mitigation. As discharge stored energy increases in future fusion devices such as ITER, control and handling of disruption events becomes a critical issue. An unmitigated disruption could lead to failure of the plasma facing components resulting in financially and politically costly repairs. Methods to facilitate the quench of an unstable high current discharge are required. With the onset warning time for some ITER disruption events estimated to be less than 10 ms, a disruption mitigation system needs to be considered which operates at injection speeds greater than gaseous sound speeds. Such an actuator could then serve as a means to augment presently planned pneumatic injection systems. The EPI uses a rail gun concept whereby a radiative payload is delivered into the discharge by means of the JxB forces generated by an external current pulse, allowing for injection velocities in excess of 1 km/s. The present status of the EPI project is outlined, including the addition of boost magnetic coils. These coils augment the self-generated rail gun magnetic field and thus provide a more efficient acceleration of the payload. The coils and the holder designed to constrain them have been modelled with the ANSYS code to ensure structural integrity through the range of operational coil cu

disruption↗