bluuebunny/crossref_metadata_2025_split huggingface dataset:
Radial Melt Segregation During Extrusion of Partially Molten Rocks
Authors: Quintanilla‐Terminel Alejandra , Dillman Amanda M. , Pec Matej , Diedrich Garrett , Kohlstedt David L. 
Published: May 2019

AbstractWe performed a series of extrusion experiments on partially molten samples of forsterite plus 10 vol% of an anorthite‐rich melt to investigate melt segregation in a pipe‐extrusion geometry and test the predictions of two‐phase flow theory with viscous anisotropy. The employed flow geometry has not been experimentally investigated for partially molten rocks; however, numerical solutions for a similar, pipe‐Poiseuille geometry are available. Samples were extruded from a 6‐mm diameter reservoir into a 2‐mm diameter channel under a fixed normal stress at 1350°C and 0.1 MPa. The melt distribution in the channel was subsequently mapped with optical and backscattered electron microscopy and analyzed via quantitative image analysis. Melt segregated from the center toward the outer radius of the channel. The melt fraction at the wall increased with increasing extrusion duration and with increasing shear stress. The melt fraction profiles are parabolic with the melt fraction at the wall reaching 0.17–0.66, values 2 to 16 times higher than at the channel center. Segregation of melt toward the wall of the channel is consistent with base‐state melt segregation as predicted by two‐phase flow theory with viscous anisotropy. However, melt‐rich sheets inclined at a low angle to the wall, which are anticipated from two‐phase flow theory, were not observed, indicating that the compaction length is larger than the channel diameter. The results of our experiments are a test of two‐phase flow theory that includes viscous anisotropy, an essential theoretical frame work needed for modeling large‐scale melt migration and segregation in the upper mantle.
On the Use of Different Magnetic Field Models for Simulating the Dynamics of the Outer Radiation Belt Electrons During the October 1990 Storm
Authors: Loridan Vivien , Ripoll Jean‐François , Tu Weichao , Scott Cunningham Gregory 
Published: May 2019

AbstractDuring geomagnetic storms, the magnetic field in the outer belt is known to be significantly distorted by the solar wind, such that the outer belt dynamics can be greatly impacted by the magnetic field topology. In this context, we develop a reduced Fokker‐Planck code that includes the effects related to realistic magnetic field models (by using the existing dedicated LANLGeoMag library) through the steps of preprocessing, postprocessing, and, for the first time, during the computation of the reduced Fokker‐Planck diffusion equation itself. We perform the solutions of the reduced Fokker‐Planck equation in the framework of the geomagnetic storm that occurred from 9 October to 15 October 1990. With the use of Combined Release and Radiation Effects Satellite observations, the magnetic field model is shown to strongly affect the way of conciliating theory with observations (processing steps). More specifically, we explain analytically and numerically why the use of a dipole field can lead to misleading interpretations on the local enhancements (attributed to local acceleration) displayed by the electron distribution function, resulting in inaccurate simulations results at large L‐shells. The consideration of a realistic field does not produce any artificial peaks. With such corrected data sets, a great part of the dynamics can be described by radial transport and is thus better reproduced by the simulations. This crucial importance of the field geometry is further emphasized with the calculation of unidirectional, omnidirectional, and integral electron fluxes, and their accuracy is quantified thanks to dedicated metrics.
Evaluating Aerosol Optical Depth From Himawari‐8 With Sun Photometer Network
Authors: Wang Wei , Mao Feiyue , Pan Zengxin , Gong Wei , Yoshida Mayumi , Zou Bin , Ma Huiyun 
Published: May 2019

AbstractThe Advanced Himawari Imager (AHI), the primary sensor aboard the Japanese Himawari‐8 geostationary satellite, measures regional aerosol observations with high temporal‐spatial resolution. To improve product quality and scientific applications, we performed a comprehensive evaluation of AHI aerosol products (version 1.0). We compared nearly 2 years (15 July 2015 to 31 June 2017) of AHI aerosol optical depth at 500 nm (AOD500) with AODs from the Aerosol Robotic Network (AERONET) and the Maritime Aerosol Network (MAN). Results showed that, over land, AHI retrievals exhibit a large overall bias of −0.062, with an R of 0.78; over ocean, average bias measured 0.036 (0.051 for MAN), with an R of 0.89 (0.95 for MAN). AHI retrievals collocated with AERONET AODs (τA) showed the following expected AHI AOD errors: (−0.66 × τA + 0.02, −0.34 × τA + 0.16) over land and (−0.24 × τA + 0.03, 0.10 × τA + 0.11) over ocean. AHI retrievals with degraded performance correlated to different regions, angles, aerosol types, and surface types, suggesting that the AHI aerosol algorithm can be improved by changing aerosol optical models, using better cloud filters, and combining multiple methods to estimate ground reflectance. Collocated comparisons of AHI‐MODerate‐resolution Imaging Spectroradiometer‐AERONET demonstrate that, over land, AHI daytime AODs clearly improve when retrievals with a large viewing zenith angle and small scattering angle are excluded.
A Layer‐Averaged Nonhydrostatic Dynamical Framework on an Unstructured Mesh for Global and Regional Atmospheric Modeling: Model Description, Baseline Evaluation, and Sensitivity Exploration
Authors: Zhang Yi , Li Jian , Yu Rucong , Zhang Shixun , Liu Zhuang , Huang Jiahao , Zhou Yihui 
Published: May 2019

AbstractThis paper describes the development and evaluation of a new nonhydrostatic dynamical framework for global and regional atmospheric modeling, with an emphasis on the numerical performance of dry dynamics. The model is formulated in a layer‐averaged manner using a generalized hybrid sigma‐mass vertical coordinate and an unstructured mesh. The mass‐based equations allow a flexible and effective switch between the hydrostatic and nonhydrostatic solvers. The unstructured mesh treats the conventional icosahedral grid and the more general Voronoi polygon in a consistent manner, allowing a flexible switch between quasi‐uniform and variable‐resolution modeling. The horizontal discretization and vertical discretization are formulated in an explicit Eulerian approach, while those terms describing the vertically propagating fast waves are solved implicitly. The model is equipped with physically based Smagorinsky diffusion as a tuning tool. A suite of multiscale test cases from hydrostatic to nonhydrostatic regimes is used to assess the model performance. The general strategies for evaluation focus on two aspects: (i) the nonhydrostatic solver should behave similarly to its hydrostatic counterpart under the hydrostatic regime and (ii) the nonhydrostatic solver should produce unique nonhydrostatic responses under the nonhydrostatic regime. In the context of model evaluation, model sensitivity to numerical configurations is further explored to understand the impact of isolated components, helping to identify appropriate configurations for realistic modeling applications. The present framework is a prototype toward a Global‐Regional Integrated forecast SysTem (GRIST).
Electro‐Thermal Subsurface Gas Generation and Transport: Model Validation and Implications
Authors: Molnar Ian L. , Mumford Kevin G. , Krol Magdalena M. 
Published: May 2019

AbstractGas generation and flow in soil is relevant to applications such as the fate of leaking geologically sequestered carbon dioxide, natural releases of methane from peat and marine sediments, and numerous electro‐thermal remediation technologies for contaminated sites, such as electrical resistance heating. While traditional multiphase flow models generally perform poorly in describing unstable gas flow phenomena in soil, Macroscopic Invasion Percolation (MIP) models can reproduce key features of its behavior. When coupled with continuum heat and mass transport models, MIP has the potential to simulate complex subsurface scenarios. However, coupled MIP‐continuum models have not yet been validated against experimental data and lack key mechanisms required for electro‐thermal scenarios. Therefore, the purpose of this study was to (a) incorporate mechanisms required for steam generation and flow into an existing MIP‐continuum model (ET‐MIP), (b) validate ET‐MIP against an experimental lab‐scale electrical resistance heating study, and (c) investigate the sensitivity of water boiling and gas (steam) transport to key parameters. Water boiling plateaus (i.e., latent heat), heat recirculation within steam clusters, and steam collapse (i.e., condensation) mechanisms were added to ET‐MIP. ET‐MIP closely matched observed transient gas saturation distributions, measurements of electrical current, and temperature distributions. Heat recirculation and cluster collapse were identified as the key mechanisms required to describe gas flow dynamics using a MIP algorithm. Sensitivity analysis revealed that gas generation rates and transport distances, particularly through regions of cold water, are sensitive to the presence of dissolved gases.
Estimation of Surface Turbulent Fluxes From Land Surface Moisture and Temperature Via a Variational Data Assimilation Framework
Authors: Abdolghafoorian Abedeh , Farhadi Leila 
Published: May 2019

AbstractAccurate estimation of surface turbulent heat fluxes is important in numerous hydrological, meteorological, and agricultural applications. Recently, several studies have focused on estimating these fluxes via assimilation of land surface temperature (LST) observations into a surface energy balance model following the variational data assimilation (VDA) scheme. However, current VDAs suffer from the following issues: (1) they do not consider the inherent coupling between water and energy in the soil‐plant‐atmosphere continuum, (2) they tend to be ill‐posed, and (3) they do not explicitly compute the uncertainty of estimates. The goal of this study is to enhance the current VDAs in two major ways: (i) coupling water and energy balance equations, assimilating soil moisture (SM) data in addition to LST, and constraining the VDA estimates by the moisture diffusion equation in addition to the heat diffusion equation; and (ii) analyzing the second‐order information that guides toward a well‐posed estimation problem and provides uncertainty of parameters. The performance of the proposed VDA is examined through a set of experiments based on a synthetic data set. The results show that simultaneous assimilation of SM and LST improves the estimation of heat fluxes and reduces the sensitivity of VDA to the initial guess of parameters. Furthermore, by adding moisture diffusion equation as an additional constraint, the correlation between the estimated parameters is reduced and the VDA scheme is oriented toward well posedness. The feasibility of extending the proposed VDA in estimating large‐scale turbulent fluxes using spaceborne SM and LST data is examined, and promising results are obtained.
Degradation of Internal Organic Matter is the Main Control on Pteropod Shell Dissolution After Death
Authors: Oakes R. L. , Peck V. L. , Manno C. , Bralower T. J. 
Published: May 2019

AbstractThe potential for preservation of thecosome pteropods is thought to be largely governed by the chemical stability of their delicate aragonitic shells in seawater. However, sediment trap studies have found that significant carbonate dissolution can occur above the carbonate saturation horizon. Here we present the results from experiments conducted on two cruises to the Scotia Sea to directly test whether the breakdown of the organic pteropod body influences shell dissolution. We find that on the timescales of 3 to 13 days, the oxidation of organic matter within the shells of dead pteropods is a stronger driver of shell dissolution than the aragonite saturation state of seawater. Three to four days after death, shells became milky white and nano scanning electron microscope images reveal smoothing of internal surface features and increased shell porosity, both indicative of aragonite dissolution. These findings have implications for the interpretation of the condition of pteropod shells from sediment traps and the fossil record, as well as for understanding the processes controlling particulate carbonate export from the surface ocean.
Xenon Hydrate as an Analog of Methane Hydrate in Geologic Systems Out of Thermodynamic Equilibrium
Authors: Fu Xiaojing , Waite William F. , Cueto‐Felgueroso Luis , Juanes Ruben 
Published: May 2019

AbstractMethane hydrate occurs naturally under pressure and temperature conditions that are not straightforward to replicate experimentally. Xenon has emerged as an attractive laboratory alternative to methane for studying hydrate formation and dissociation in multiphase systems, given that it forms hydrates under milder conditions. However, building reliable analogies between the two hydrates requires systematic comparisons, which are currently lacking. We address this gap by developing a theoretical and computational model of gas hydrates under equilibrium and nonequilibrium conditions. We first compare equilibrium phase behaviors of the Xe·H2O and CH4·H2O systems by calculating their isobaric phase diagram, and then study the nonequilibrium kinetics of interfacial hydrate growth using a phase field model. Our results show that Xe·H2O is a good experimental analog to CH4·H2O, but there are key differences to consider. In particular, the aqueous solubility of xenon is altered by the presence of hydrate, similar to what is observed for methane; but xenon is consistently less soluble than methane. Xenon hydrate has a wider nonstoichiometry region, which could lead to a thicker hydrate layer at the gas‐liquid interface when grown under similar kinetic forcing conditions. For both systems, our numerical calculations reveal that hydrate nonstoichiometry coupled with hydrate formation dynamics leads to a compositional gradient across the hydrate layer, where the stoichiometric ratio increases from the gas‐facing side to the liquid‐facing side. Our analysis suggests that accurate composition measurements could be used to infer the kinetic history of hydrate formation in natural settings where gas is abundant.
Daytime Evolution of Equatorial Plasma Bubbles Observed by the First Republic of China Satellite
Authors: Kil Hyosub , Paxton Larry J. , Lee Woo Kyoung , Jee Geonhwa 
Published: May 2019

AbstractPlasma bubbles in the equatorial F region are thought of as nighttime phenomena because they develop at night and are assumed to vanish after sunrise. However, bubbles occasionally persist throughout the night and into the day. This study investigates the origin of daytime irregularities and their evolution using data from the first Republic of China satellite. Our results show that daytime irregularities occur in the longitudes where bubbles have developed on previous nights. A newly reported feature is the observation of the temporal variation of the locations of daytime irregularities; daytime irregularities are concentrated near the magnetic equator early in the morning, but the location gradually shifts to higher latitudes with time. This phenomenon is explained in terms of the latitudinal redistribution of fossil bubbles by the ionospheric fountain effect.
Evidence of Electron Acceleration at a Reconnecting Magnetopause
Authors: Fu H. S. , Peng F. Z. , Liu C. M. , Burch J. L. , Gershman D. G. , Le Contel O. 
Published: May 2019

AbstractIt is still unknown nowadays whether magnetic reconnection—a process occurring both in the magnetotail and at the magnetopause—can intrinsically accelerate energetic electrons. Observations in the Earth's magnetotail usually indicate strong electron acceleration during magnetic reconnection, while observations at the Earth's magnetopause rarely show such features. With the recently launched Magnetospheric Multiscale (MMS) mission, here we report the first evidence of energetic‐electron acceleration at a reconnecting magnetopause. We find that the acceleration of electrons, with energy up to 70 times their thermal energy, occurs in the magnetosheath side of the ion diffusion region and is associated with strong whistler waves. Such acceleration—not contaminated by the magnetospheric population—is attributed to nonadiabatic wave‐particle interactions, as supported by analyses of the resonance condition. It manifests that energetic‐electron acceleration can happen at the reconnecting magnetopause, like that in the tail.
Characteristics of the Flank Magnetopause: THEMIS Observations
Authors: Haaland S. , Runov A. , Artemyev A. , Angelopoulos V. 
Published: May 2019

AbstractThe terrestrial magnetopause is the boundary that shields the Earth's magnetosphere on one side from the shocked solar wind and its embedded interplanetary magnetic field on the other side. In this paper, we show observations from two of the Time History of Events and Macroscales Interactions during Substorms (THEMIS) satellites, comparing dayside magnetopause crossings with flank crossings near the terminator. Macroscopic properties such as current sheet thickness, motion, and current density are examined for a large number of magnetopause crossings. The results show that the flank magnetopause is typically thicker than the dayside magnetopause and has a lower current density. Consistent with earlier results from Cluster observations, we also find a persistent dawn‐dusk asymmetry with a thicker and more dynamic magnetopause at dawn than at dusk.
High‐Speed Drag Measurements of Aluminum Particles in Free Molecular Flow
Authors: DeLuca Michael , Sternovsky Zoltan 
Published: May 2019

AbstractHigh‐speed drag in a free molecular flow is still poorly understood despite playing an important role in a variety of physical situations, including meteor ablation in the upper atmosphere, the orbits of satellites, and the dynamics of cosmic dust grains. To measure drag at high speeds, small aluminum spheres 0.1–2.1μm in radius were shot at 1–10 km/s into air, N2, Ar, and CO2using an electrostatic dust accelerator. The measured drag coefficient in air isΓ = 1.29 ± 0.13, with similar values for the other gases. The measurement constrains the heating coefficient toΛ = 0.58 ± 0.37in air assuming the gas molecules reflect diffusely from the particles' surfaces. The drag appears to be independent of the molecular structures of the four gases tested but has a slight dependence on molecular mass. The drag is also higher than frequently assumed, which has implications for the modeling of high‐speed objects in free molecular flows.
Kinetic Properties of Solar Wind Discontinuities at 1 AU Observed by ARTEMIS
Authors: Artemyev A. V. , Angelopoulos V. , Vasko I. Y. 
Published: May 2019

AbstractBecause solar wind plasma flow transports the entire spectrum of magnetic field fluctuations (from low‐frequency inertial range to electron kinetic range), it is a natural laboratory for plasma turbulence investigation. Among the various wave modes and coherent plasma structures that contribute to this spectrum, one of the most important solar wind elements is ion‐scale solar wind discontinuities. These structures, which carry very intense current, have been considered as a free energy source for plasma instabilities that contribute to solar wind heating. Investigations of such discontinuities have been mostly focused on their magnetic field signatures; much less is known about plasma kinetics around them. Using statistics of such discontinuities observed around the Earth (∼1 AU) by two spacecraft from the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) mission, we demonstrate that they are accompanied by density and temperature variations with a spatial scale of tens of ion inertial lengths. Inversely correlated density and temperature variations suggest the discontinuity configuration is nearly force free. The discontinuities are typified by two spatial scales, an intense current layer (>1 nA/m2), and a much broader current layer structure enveloping them. The magnetic field rotation occurs at the large scale, whereas at the small (inner) scale the plasma pressure gradients provide the pressure balance. We find that the electron kinetic behavior around the discontinuities strongly depends on electron energy: Whereas the pitch‐angle distribution of near‐thermal electrons (10–30 eV) changes significantly across discontinuities, hot (100–1,000 eV) electrons retain their properties on the two sides, suggesting they are able to cross the discontinuities freely. Exploring these and other kinetic characteristics of discontinuities, we discuss possible mechanisms of formation/evolution of these structures.
Empirical Orthogonal Function Analysis and Modeling of the Topside Ionospheric and Plasmaspheric TECs
Authors: Zhong Jiahao , Lei Jiuhou , Yue Xinan , Wang Wenbin , Burns Alan G. , Luan Xiaoli , Dou Xiankang 
Published: May 2019

AbstractThe topside total electron contents (TECs) observed by the MetOp‐A (above 832 km) and TerraSAR‐X (above 520 km) satellites for multiple local times during 2008–2018 have been used to develop global topside ionospheric and plasmaspheric TEC models, based on empirical orthogonal function (EOF) analysis. Gridded TEC maps are first obtained from the original quiet time TECs, and then, they are decomposed into EOF basis modes and associated amplitude coefficients. The EOF analysis well captures the latitudinal, longitudinal, and seasonal variations of the topside ionosphere and plasmasphere, and the associated dependence on solar EUV forcing. The first five EOFs, which can account for more than 98.8% of the total variance, are applied in model construction. The comparison with two empirical models indicates that the EOF TEC models can reproduce the observations well, including TEC magnitudes and longitudinal variations. Moreover, the annual anomaly is obviously seen in the topside TECs. The global averaged TECs are greatest in December and smallest in June. With increasing solar EUV forcing, the global averaged TECs always increase, but the winter hemisphere TECs do not change much and even decrease, especially at night and in the Southern Hemisphere. The overall increasing rate with solar EUV forcing is larger under higher solar activity conditions. The direct plasma diffusion through ionosphere‐plasmasphere coupling may not be able to explain some of the features seen in the topside TECs.