bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characteristics of Energetic Electron Precipitation Estimated from Simulated Bremsstrahlung X‐ray Distributions
Authors: Xu Wei , Marshall Robert A. 
Published: April 2019

AbstractDetermination of the fluxes and spectra of energetic particle precipitation into the Earth's atmosphere is of critical importance for radiation belt dynamics, magnetosphere‐ionosphere coupling, as well as atmospheric chemistry. To improve the assessments of precipitating electrons using X‐ray measurements requires deeper understanding of bremsstrahlung production, transport, and redistribution throughout the atmosphere. Here we use first‐principles Monte Carlo models to explore relativistic electron precipitation events from the perspective of bremsstrahlung X‐rays. The spatial distribution of X‐rays is quantified from the ground level up to satellite altitudes. We then simulate X‐ray images that would be captured using an ideal camera and calculate the energy spectra of X‐rays originating from monoenergetic beams of precipitating electrons. Moreover, we show how these impulse responses to monoenergetic beams can be used to reconstruct the precipitating source using an inversion technique. Modeling results show that space‐borne measurements of backscattered X‐rays provide a promising method to estimate precipitation spatial size, fluxes, and spectra.
Two‐Dimensional Maps of In Situ Ionospheric Plasma Flow Data Near Auroral Arcs Using Auroral Imagery
Authors: Clayton Robert , Lynch Kristina , Zettergren Matt , Burleigh Meghan , Conde Mark , Grubbs Guy , Hampton Don , Hysell David , Lessard Marc , Michell Robert , Reimer Ashton , Roberts T. Maximillian , Samara Marilia , Varney Roger 
Published: April 2019

AbstractSome existing auroral data products are insufficient for ionospheric simulation input on subkilometer spatial scales and high (second) time resolution near the boundaries of arc structures. Ideally, two‐dimensional data maps of the relevant parameters over these small scales would provide models with constraining inputs. Available in situ data have the time and spatial resolution for small‐scale features but only provide a 1‐D cut through the structure. Ground‐based data can provide 2‐D maps but have lower resolution in time and space than is required to accurately interpret the small‐scale structure near an arc. This paper provides a method to construct two‐dimensional maps of auroral parameters from the combination of one‐dimensional in situ data cuts with two‐dimensional ground‐based (and time dependent) camera imagery. Arc boundaries for each image are defined, and the available 1‐D ionospheric flow data are replicated into many 1‐D cuts at different points along the arc, yielding an irregularly sampled 2‐D flow map. These mapped data are fitted to a regular grid via a divergence minimization routine to generate a regularly sampled flow field that is enforced as divergence free. Comparison of the generated 2‐D data maps to available information from camera inversions and other data products is shown, as are assumptions made through the replication process and alternative strategies. Reconstructed flow maps are shown to maintain the small‐scale features near arc boundaries while increasing the dimensionality to 2‐D and to follow the time evolution of the arc structure by comparisons to imagery. The average electric field magnitudes per unit area of the reconstructed and divergence‐minimized flow fields are also calculated and compared between different data sources.
An Alternative Origin of High Vp/Vs Anomalies in Slow Slip Regions: Experimental Constraints From the Elastic Wave Velocity Evolution of Highly Fractured Rock
Authors: Nishimura K. , Uehara S. , Mizoguchi K. 
Published: April 2019

AbstractUnderstanding the mechanism of episodic slow slip events at the deep part of subduction zones is important for seismic hazard mitigation and understanding fault mechanics. It has been thought that highly pressurized fluids play an important role in the occurrence, of which one of the evidences are observations of high Vp/Vs (ratio of P wave to S wave velocity) anomalies in the source regions, combined with laboratory measurements of Vp/Vs for intact rocks. However, it remains unclear whether “high Vp/Vs ratios” straightforwardly mean “high pore pressure conditions” for highly damaged fault rocks constituting subduction plate boundary faults. Here we examined the degree of damage of seafloor metabasalt in the Shimanto accretionary complex at the Nankai trough, a limited proxy of oceanic crust going down to the deep slow slip region while being subjected to successive subduction megathrust faulting. We investigated the pressure dependencies of Vp/Vs for dolerite specimens damaged by thermal stressing, simulating in situ damage conditions. We found that the more intense the rock damage, the higher the Vp/Vs ratio. An extrapolation of our results to the slow slip depth under hydrostatic condition indicates that for rocks with a fracture density >1.5 mm−1 (comparable to the fracture densities of ancient oceanic crust), Vp/Vs > 1.85, as observed at the Nankai trough, can be achieved even when the pore pressure is hydrostatic. We suggest that the contribution of not only fluid pressurization but also progressive fracturing of the plate interface on the emergence of slow slip events could be considered.
Primary and Secondary Red Bed Magnetization Constrained by Fluvial Intraclasts
Authors: Swanson‐Hysell Nicholas L. , Fairchild Luke M. , Slotznick Sarah P. 
Published: April 2019

AbstractThe magnetization of hematite‐bearing sedimentary rocks provides critical records of geomagnetic reversals and paleogeography. However, the timing of hematite remanent magnetization acquisition is typically difficult to constrain. While detrital hematite in sediment can lead to a primary depositional remanent magnetization, alteration of minerals through interaction with oxygen can lead to the postdepositional formation of hematite. In this study, we use exceptionally preserved fluvial sediments within the 1.1‐billion‐year‐old Freda Formation to gain insight into the timing of hematite remanence acquisition and its magnetic properties. This deposit contains siltstone intraclasts that were eroded from a coexisting lithofacies and redeposited within channel sandstone. Thermal demagnetization, petrography, and rock magnetic experiments on these clasts reveal two generations of hematite. One population of hematite demagnetized at the highest unblocking temperatures and records directions that rotated along with the clasts. This component is a primary detrital remanent magnetization. The other component is removed at lower unblocking temperatures and has a consistent direction throughout the intraclasts. This component is held by finer‐grained hematite that grew and acquired a chemical remanent magnetization following deposition resulting in a population that includes superparamagnetic nanoparticles in addition to remanence‐carrying grains. The data support the interpretation that magnetizations of hematite‐bearing sedimentary rocks held by >400‐nm grains that unblock close to the Néel temperature are more likely to record magnetization from the time of deposition. This primary magnetization can be successfully isolated from cooccurring authigenic hematite through high‐resolution thermal demagnetization.
Deep Seismic Structure Across the Southernmost Mariana Trench: Implications for Arc Rifting and Plate Hydration
Authors: Wan Kuiyuan , Lin Jian , Xia Shaohong , Sun Jinlong , Xu Min , Yang Hongfeng , Zhou Zhiyuan , Zeng Xin , Cao Jinghe , Xu Huilong 
Published: April 2019

AbstractThe southernmost Mariana margin lacks a mature island arc and thus differs significantly from the central‐north Mariana and Izu‐Bonin margins. This paper presents a new P wave velocity model of the crust and uppermost mantle structure based on a 349‐km‐long profile of wide‐angle reflection/refraction data. The active source seismic experiment was conducted from the subducting Pacific plate to the overriding Philippine plate, passing through the Challenger Deep. The subducting plate has an average crustal thickness of ~6.0 km with Vp of 7.0 ± 0.2 km/s at the base of the crust and low values of only 5.5–6.9 km/s near the trench axis. The uppermost mantle of the subducting plate is characterized by low velocities of 7.0–7.3 km/s. The overriding plate has a maximum crustal thickness of ~18 km beneath the forearc with Vp of ~7.4 km/s at the crustal bottom and 7.5–7.8 km/s in the uppermost mantle. A zone of slight velocity reduction is imaged beneath the Southwest Mariana Rift that is undergoing active rifting. The observed significant velocity reduction in a near‐trench crustal zone of ~20–30 km in the subducting plate is interpreted as a result of faulting‐induced porosity changes and fracture‐filling fluids. The velocity reduction in the uppermost mantle of both subducting and overriding plates is interpreted as mantle serpentinization with fluid sources from dehydration of the subducting plate and/or fluid penetration along faults.
Hydraulic and Poroelastic Rock Properties From Oscillating Pore Pressure Experiments
Authors: Hasanov Azar K. , Dugan Brandon , Batzle Michael L. , Prasad Manika 
Published: April 2019

AbstractKnowledge of hydraulic and poroelastic properties is essential for simulating fluid flow in porous media. Accurate constraints on these properties have impacts on production forecasts and economics. In this paper we document an improvement of the oscillating pore pressure experiment by simultaneously measuring hydraulic and poroelastic properties of reservoir rocks. Measurements were carried out for four conventional reservoir rock quality samples at oscillation frequencies of 0.025–1 Hz and effective pressures of 3.5–62 MPa. Estimated permeability values decreased with increasing effective pressure and increased sharply above a frequency range of 0.3–0.4 Hz. We established that hydraulically measured storage capacities are overestimated by almost an order of magnitude when compared to elastically derived ones. Biot coefficient was estimated both from hydraulic and strain measurements, and comparison of two data sets reveals high uncertainty of the hydraulic specific storage measurements. We documented grain crushing and pore collapse event in a dolostone sample, observed as a permanent and drastic decrease of permeability and bulk modulus. We validated our method by detecting irreversible microstructural changes independently by hydraulic, elastic, X‐ray microtomography (μCT) and nuclear magnetic resonance (NMR) measurements. Our approach can be used to constrain and to improve the estimation of specific storage and thus leads to better model inputs and forecasts.
Spatial Scales of Turbulent Flow Structures Associated With Interacting Barchan Dunes
Authors: Bristow N. R. , Blois G. , Best J. L. , Christensen K. T. 
Published: April 2019

The spatial coherence of turbulent flow structures throughout the flow field associated with a collision between a smaller upstream barchan laterally offset from a larger downstream barchan is investigated using inhomogeneous, two‐point correlation coefficients of fluctuating streamwise velocity, from which the distribution, size, and orientation of the large‐scale motions in the flow are analyzed. Measurements were made with fixed‐bed models in a refractive‐index‐matched flume environment allowing uninhibited optical access where the flow field is captured using particle‐image velocimetry in both streamwise‐wall‐normal and streamwise‐spanwise planes. The shear layer of a barchan produces flow structures of smaller length scale, yet still on the order of the barchan height, and stronger positive streamwise fluctuations near the bed as compared to the incoming boundary layer, and these effects prevail far downstream. Analysis of the orientations of the flow structures suggests secondary flow motions induced by the barchan horns, and interdune flow modification through collision stages indicates changing flow interaction regimes with decreasing interdune separation. The combination of enhanced positive streamwise fluctuations near the bed and significant spatial coherence indicates that for a field‐scale barchan of sufficient size, the coherence of flow structures produced in its wake is of comparable scale to the characteristic drag length associated with aeolian transport. As length scales nominally scale with barchan height, the morphodynamics of collisions between barchans of disparate size can be partially explained through this paradigm of flow structure coherence.
Carbon Uptake in Surface Water Food Webs Fed by Palaeogroundwater
Authors: Mazumder Debashish , Saintilan Neil , Hollins Suzanne , Meredith Karina , Jacobsen Geraldine , Kobayashi Tsuyoshi , Wen Li 
Published: April 2019

AbstractThe use of 14C to elucidate sources of carbon within freshwater aquatic ecosystems is challenging the assumption that modern autochthonous carbon dominates energy flows. We measured the uptake of old carbon through several trophic levels of a wetland fed by groundwater of the Great Artesian Basin, Australia, the largest artesian basin in the world. Stable isotopes (δ13C and δ15N) and radiocarbon (14C) were used to quantify food chain links and connection between groundwater and surface water food webs. Our results suggest that old groundwater was the dominant carbon source even at the highest trophic levels, with predatory fish returning apparent carbon ages of up to 11 ka. Stable isotope analysis (δ13C and δ15N) identified trophic links between fish, aquatic insects, and algae with smaller contributions from particulate organic matter to the food webs. As natural mound springs and associated wetlands are the only source of reliable water during dry periods over vast areas of the western Great Artesian Basin, the result has potential implications for the interpretation of archaeological artifacts associated with indigenous passage within the interior.
A Ca. 2.8‐Ga Plume‐Induced Intraoceanic Arc System in the Eastern North China Craton
Authors: Gao Lei , Liu Shuwen , Zhang Bo , Sun Guozheng , Hu Yalu , Guo Rongrong 
Published: April 2019

AbstractThe conversion of Archean tectonic regimes from mantle plume to plate tectonic and the mechanism of subduction initiation are two crucial issues in the current Earth sciences. A typical komatiite‐tholeiite sequence consisting of ~2,850–2,800‐Ma komatiites and ~2,780–2,710‐Ma basaltic and andesitic to dacitic volcanic rocks is well preserved in the Archean Western Shandong terrane of the eastern North China Craton. These volcanic rocks were derived from the partial melting of a depleted mantle source with subsequent fractional crystallization, a metasomatized subarc mantle wedge, and a descending oceanic slab. Structural and kinematic observations suggest that the Qixingtai‐Yanlinguan‐Panchegou granite‐greenstone belt experienced ~2.85–2.80‐Ga extensional deformation (D1) by upwelling of a mantle plume and NE‐SW‐oriented shortening (D2) during ~2.78–2.71 Ga. The petrogenesis of the Mesoarchean to early Neoarchean metamorphosed volcanic rock series and regional deformation indicate that a plume‐induced self‐sustaining intraoceanic arc system with SW‐dipping subduction polarity operated ~2.8 Ga, followed by the evolution from initial to mature subduction. An increasing pull caused by the descending slab generated gravitational instability in the surrounding oceanic lithosphere and formation of a new initial subduction zone adjacent to the preexisting arc system. Similar to the process in the Western Shandong terrane, plume activity generally culminated in subduction events in many other Archean greenstone belts, revealing that plume‐arc interaction may have been a crucial mechanism for global Archean tectonic regime conversion from mantle plume to initial plate subduction in the crust of the early Earth.
Improved Aerosol Retrievals Over Complex Regions Using NPP Visible Infrared Imaging Radiometer Suite Observations
Authors: Yang Yikun , Zhao Chuanfeng , Sun Lin , Wei Jing 
Published: April 2019

AbstractAs a new generation of polar‐orbiting satellites, NPP VIIRS (National Polar‐orbiting Partnership Visible Infrared Imaging Radiometer Suite) provides important supports for the studies of local and global scale climate change, environmental monitoring, and radiant energy balance. This paper proposed an improved deep blue aerosol retrieval algorithm over complex regions based on NPP VIIRS observations. Two improvements were proposed and implemented to the traditional aerosol optical depth (AOD) retrieval algorithm in order to improve the retrieval accuracy, including the construction of the surface reflectance conversion model and aerosol optical parameter acquisition. The retrieval results were extensively evaluated. First, the accuracy of the algorithm was evaluated using AErosol RObotic NETwork ground‐based observations, including conditions over different underlying surfaces, different seasons, and different AOD values. Then, the cross validation was carried out between our AOD retrievals and NPP VIIRS aerosol products. The verification results show that our AOD retrievals agree very well with the AErosol RObotic NETwork AOD with coefficient of determination (R2) ~ 0.85. The improved deep blue algorithm performs well overall under different surface conditions and seasons, while the quality of performance gradually decreases with the increase of AOD. Moreover, the improved algorithm is robust with less bias, root‐mean‐square error, and relative mean bias in the AOD retrievals and can provide more detailed aerosol information compared with the NPP VIIRS aerosol products. These results suggest that the improved deep blue algorithm can obtain high‐precision AOD information, providing better data basis for related researches on air pollution and climate change.
Mono‐Model Parameter Joint Inversion by Gramian Constraints: EM Methods Examples
Authors: Ogunbo J. N. 
Published: April 2019

AbstractJoint inversions of coincident geophysical data are usually constrained to produce more reliable subsurface models. Structural, petrophysical, model parameter correlation, empirical, and transforms are some of the published constraints. The Gramian constraint provides a broad mathematical framework for implementing the aforementioned constraints. The Gramian constraint is formed from the determinant of the inner products of the model parameters involved. Previous works have used the Gramian constraint to invert multimodal parameters of different geophysical methods. But there has not been any extension of Gramian‐constrained joint inversion to mono‐model parameter from similar geophysical methods, for example, a similar conductivity or resistivity model from time‐ and frequency‐domain airborne electromagnetic methods. I implement the Gramian‐constrained joint inversion of time‐ and frequency‐domain airborne EM (AEM) data. This implementation allows the Gramian constraint to enhance the linear correlation of the model parameter between the two methods as the number of iterations increases. Improvement of the final joint inversion results over the standalone models is noticeable for both 3% noise‐contaminated synthetic and field data experiments. The field data jointly inverted are the high moment time‐domain SkyTEM data and frequency‐domain RESOLVE helicopter EM data acquired over the salinized Bookpurnong Irrigation District in South Australia in 2006 and 2008, respectively.
Effects of Increasing Aridity on Ambient Dust and Public Health in the U.S. Southwest Under Climate Change
Authors: Achakulwisut Pattanun , Anenberg Susan C. , Neumann James E. , Penn Stefani L. , Weiss Natalie , Crimmins Allison , Fann Neal , Martinich Jeremy , Roman Henry , Mickley Loretta J. 
Published: April 2019

AbstractThe U.S. Southwest is projected to experience increasing aridity due to climate change. We quantify the resulting impacts on ambient dust levels and public health using methods consistent with the Environmental Protection Agency's Climate Change Impacts and Risk Analysis framework. We first demonstrate that U.S. Southwest fine (PM2.5) and coarse (PM2.5‐10) dust levels are strongly sensitive to variability in the 2‐month Standardized Precipitation‐Evapotranspiration Index across southwestern North America. We then estimate potential changes in dust levels through 2099 by applying the observed sensitivities to downscaled meteorological output projected by six climate models following an intermediate (Representative Concentration Pathway 4.5, RCP4.5) and a high (RCP8.5) greenhouse gas concentration scenario. By 2080–2099 under RCP8.5 relative to 1986–2005 in the U.S. Southwest: (1) Fine dust levels could increase by 57%, and fine dust‐attributable all‐cause mortality and hospitalizations could increase by 230% and 360%, respectively; (2) coarse dust levels could increase by 38%, and coarse dust‐attributable cardiovascular mortality and asthma emergency department visits could increase by 210% and 88%, respectively; (3) climate‐driven changes in dust concentrations can account for 34–47% of these health impacts, with the rest due to increases in population and baseline incidence rates; and (4) economic damages of the health impacts could total $47 billion per year additional to the 1986–2005 value of $13 billion per year. Compared to national‐scale climate impacts projected for other U.S. sectors using the Climate Change Impacts and Risk Analysis framework, dust‐related mortality ranks fourth behind extreme temperature‐related mortality, labor productivity decline, and coastal property loss.
Induced Seismicity Reduces Seismic Hazard?
Authors: Barbour Andrew J. , Pollitz Fred 
Published: April 2019

AbstractEarthquakes caused by human activities have been observed for decades. Often these are related to industrial activities pumping fluids into deep geologic formations, like with wastewater disposal. The simplest theory connecting these processes to earthquakes is straightforward: injection leads to fluid pressure changes that either reduce the strength of preexisting faults or generate new faults. In practice, the conditions that lead to induced earthquakes are not always clear in ways that can be generalized. Kao et al. (2018, https://doi.org/10.1029/2018GL079288) show how the distribution of induced earthquakes in Western Canada relate to natural rates of deformation in the crust. Using these new results, they discuss an intriguing paradox: induced seismicity can cause short‐term increases in the seismic hazard that are followed by a period of reduced seismic hazard. Such hazard‐reducing scenarios are plausible but hinge upon simplifying assumptions about how the crust stores and releases strain energy in the form of earthquakes.
Satellite‐Based Estimation of Cloud Top Radiative Cooling Rate for Marine Stratocumulus
Authors: Zheng Youtong , Rosenfeld Daniel , Zhu Yannian , Li Zhanqing 
Published: April 2019

AbstractCloud top radiative cooling rate (CTRC) is the leading term in the energy budget of a marine boundary layer capped by stratocumulus. It plays a significant role in the formation, evolution, and maintenance of the stratocumulus cloud system. This study demonstrates the feasibility of estimating the CTRC, with high accuracy, from passive satellite data only. The estimation relies on a radiative transfer model with inputs from satellite‐retrieved cloud parameters in combination with reanalysis sounding that is revised, in a physically coherent way, by satellite data. The satellite‐based estimates CTRC agree with ground‐based ones to within ~10%. The high accuracy largely benefits from the good capability of satellite data in constraining parameters of most influence to the CTRC such as free‐tropospheric sounding, cloud top temperature, and cloud optical depth. Applying this technique, we generate a climatology of CTRC during summer over the Southern Hemisphere tropical and subtropical oceans.