bluuebunny/crossref_metadata_2025_split huggingface dataset:
Impact of Background Magnetic Field for EMIC Wave‐Driven Electron Precipitation
Authors: Woodger L. A. , Millan R. M. , Li Z. , Sample J. G. 
Published: October 2018

AbstractWave‐particle interaction between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves is a highly debated loss process contributing to the dynamics of Earth's radiation belts. Theoretical studies show that EMIC waves can result in strong loss of relativistic electrons in the radiation belts (Summers & Thorne, 2003, https://doi.org/10.1029/2002JA009489). However, many of these studies have not been validated by observations. Li et al. (2014, https://doi.org/10.1002/2014GL062273) modeled the relativistic electron precipitation observed by Balloon Array for Radiation belt Relativistic Electron Losses (BARREL) in a single‐event case study based on a quasi‐linear diffusion model and observations by Van Allen Probes and GOES 13. We expand upon that study to investigate the localization of the precipitation region and the effectiveness of EMIC waves as an electron loss mechanism.The model results of BARREL 1I observations on 17 January 2013 show that as the BARREL balloon drifts in local time to regions that map to lower equatorial magnetic field strength, the flux of precipitating electrons increases and peaks at lower energy. The hypothesis that the energy of the precipitating electrons is controlled by background magnetic field strength is further tested by considering observations from balloon campaigns conducted from 2000 to 2014, including BARREL. Consistent with theory for wave‐particle interaction between relativistic electrons and EMIC waves, we find observationally that stronger equatorial magnetic field strength generally correlates with more energetic electron precipitation and further conclude that magnetic field strength can drive the localization and distribution of precipitating electrons.
Small‐Scale Flux Transfer Events Formed in the Reconnection Exhaust Region Between Two X Lines
Authors: Hwang K.‐J. , Sibeck D. G. , Burch J. L. , Choi E. , Fear R. C. , Lavraud B. , Giles B. L. , Gershman D. , Pollock C. J. , Eastwood J. P. , Khotyaintsev Y. , Escoubet Philippe , Fu H. , Toledo‐Redondo S. , Torbert R. B. , Ergun R. E. , Paterson W. R. , Dorelli J. C. , Avanov L. , Russell C. T. , Strangeway R. J. 
Published: October 2018

AbstractWe report MMS observations of the ion‐scale flux transfer events (FTEs) that may involve two main X lines and tearing instability between the two X lines. The four spacecraft detected multiple isolated regions with enhanced magnetic field strength and bipolar Bn signatures normal to the nominal magnetopause, indicating FTEs. The currents within the FTEs flow mostly parallel to B, and the magnetic tension force is balanced by the total pressure gradient force. During these events, the plasma bulk flow velocity was directed southward. Detailed analysis of the magnetic and electric field and plasma moments variations suggests that the FTEs were initially embedded within the exhaust region north of an X line but were later located southward/downstream of a subsequent X line. The cross sections of the individual FTEs are in the range of ~2.5–6.8 ion inertial lengths. The observations suggest the formation of multiple secondary FTEs. The presence of an X line in the exhaust region southward of a second X line results from the southward drift of an old X line and the reformation of a new X line. The current layer between the two X lines is unstable to the tearing instability, generating multiple ion‐scale flux‐rope‐type secondary islands.
Ionospheric and Thermospheric Responses to the Recent Strong Solar Flares on 6 September 2017
Authors: Li Wang , Yue Jianping , Yang Yang , He Changyong , Hu Andong , Zhang Kefei 
Published: October 2018

AbstractTwo solar flares X2.2 and X9.3 erupted over the active region 2673 on 6 September 2017, and the second flare is the strongest since 2005. In order to investigate the ionospheric and thermospheric responses to the two solar flares, the global total electron content and the critical frequency of F2 layer obtained from GPS stations and ionosondes are used. The results indicate that the ionosphere in the sunlit hemisphere increased significantly with magnitudes of 0.1 and 0.5 total electron content units for the X2.2 and X9.3 solar flares, respectively. The electron density, thermospheric neutral density, and neutral temperature simulated by the Thermosphere‐Ionosphere Electrodynamics Global Circulation Model show that the behavior of ionospheric and thermospheric responses is different. The ionospheric disturbances occurred at the altitude ranges of 150−300 km, and the thermospheric responses occurred at the altitudes of 250−400 km are caused by solar extreme ultraviolet and ultraviolet photons, respectively. Both ionospheric and thermospheric responses are proportional to the height within their corresponding altitude ranges. Observations and simulations reveal that the ionospheric and thermospheric responses are nonlinearly dependent on the solar zenith angle. The disturbances caused by the X2.2 solar flare are symmetric, but the X9.3 solar flare are not. The O/N2 density ratio simulated by Thermosphere‐Ionosphere Electrodynamics Global Circulation Model increases from lev0 to lev5.0 pressure surface with a magnitude of 0.1–1.8, while the ratio decreases in the American sector with a magnitude of −0.6 to −0.3. The longitudinal asymmetry of O/N2 density ratio is a major contributor to the longitudinal asymmetry of ionospheric and thermospheric responses.
Diagnostics of Closed Magnetic Flux Depletion in the Near‐Earth Magnetotail During the Substorm Growth Phase
Authors: Shukhtina M. A. , Gordeev E. I. , Sergeev V. A. , Shinohara I. 
Published: October 2018

AbstractBased on magnetohydrodynamics simulations, it was recently suggested that magnetotail reconfiguration during the substorm growth phase results from combined action of two large‐scale processes: (1) open magnetic flux accumulation (OMFA) in the tail lobes and (2) closed magnetic flux depletion (CMFD) in the near‐Earth tail, caused by flux evacuation to the dayside magnetopause. Simultaneous action of uniform along the tail OMFA and strongly nonuniform CMFD leads to different rates of magnetic flux growth in different tail cross sections. According to Global magnetohydrodynamics simulations CMFD is about 35% of OMFA, the corresponding differences of magnetic flux increase in two‐tail cross sections at −7RE and −20RE being as large as 0.1–0.2 GWb. To study this effect on real data, we applied the recent method of magnetotail flux estimation (Shukhtina et al., ) to simultaneous Cluster and Geotail observations in the tail lobes. By finding such rare observations in the inner and middle magnetotail (at<X > −10 RE and − 21RE) we confirm a larger magnetic flux increase in the midtail, with the ratio between OMFA and CMFD comparable to that obtained in magnetohydrodynamics, although with a large scatter. These results confirm the scenario of two decoupled magnetotail regions with different convection regimes during the growth phase. Convection is depressed in the midtail plasma sheet, showing the pressure crisis in action inherent to tail‐like 2‐D magnetic configuration. At the same time convection is substantially enhanced in the inner magnetosphere, where the pressure crisis is solved due to azimuthal flux transport around the Earth, essentially a 3‐D effect.
Using Fast Marching Eikonal Solver to Compute 3‐D Pds Traveltime for Deep Receiver‐Function Imaging
Authors: Guan Zhe , Niu Fenglin 
Published: October 2018

AbstractIn common‐conversion‐point stacking of receiver functions, most current studies compute 3‐D relative Pds traveltime corrections by integrating traveltime anomalies along 1‐D raypaths. This ray tracing approach is generally time‐consuming and less accurate when prominent velocity anomalies exist and effects of 3‐D raypaths become significant. In this study we introduce a new scheme that utilizes a fast‐marching method eikonal solver to improve both the efficiency and accuracy of 3‐D Pds traveltime computation. We first employ a 1‐D raytracing method and the iasp91 model to calibrate the accuracy of the new scheme and optimize the parameters of the numerical solver. We then apply the new scheme to compute a massive number of Pds traveltimes using two sets of 3‐D synthetic models, one set with a high‐velocity slab and another set with a low‐velocity plume, and compare these 3‐D traveltimes with those computed with the raypath integrating approach. We find 2.7% and 11.8% raypaths in the two slab models and 7.8% and 12.0% raypaths in the two plume models show a 3‐D traveltime difference of ≥0.5 s. We apply the proposed scheme to a subset of transportable array receiver functions that sample the transition zone beneath the Yellowstone hotspot and find that a common‐conversion‐point stack using 3‐D Pds traveltimes computed by the eikonal solver method has the best focused P660s. Finally, we illustrate that computational times can be reduced by 1 to 2 orders of magnitude with the new scheme to compute 3‐D Pds traveltimes of 20,000–200,000 receiver functions.
Last Glacial Maximum and Mid‐Holocene Thermal Growing Season Simulations
Authors: Jiang Dabang , Sui Yue , Lang Xianmei , Tian Zhiping 
Published: October 2018

AbstractThe growing season—the number of days per year during which the climate favors plant growth—is an important factor for describing climate. Based on daily data of all available numerical experiments from the Paleoclimate Modeling Intercomparison Project phases two and three, we examine the growing season over global ice‐free land for the last glacial maximum and mid‐Holocene to improve our understanding of glacial and interglacial climates. Compared to the preindustrial period, the last glacial maximum growing season was shortened at midlatitude and high latitude and by 26.1 days globally in terms of nine climate models due to a later start and an earlier end. Growing degree days above 5 °C (GDD5) decreased by 24% due to both cooling and shortening of the growing season, agreeing with pollen‐based reconstructions of globally decreased signal and spatial pattern of GDD5 changes at 88 grid cells. Twenty‐four models indicate that the mid‐Holocene growing season start and end dates and length were similar to preindustrial states globally but featured a latitude‐varying difference. The length increased north of ~50°N and decreased in much of the area from 20–50°N owing to variations in both the start and end dates. In response to orbital forcing, both the growing season temperature and GDD5 increased at northern midlatitude and high latitude but decreased elsewhere. The GDD5 simulations agreed in sign with the reconstructions at 516 grid cells more often than not, though their spatial patterns were different to some extent.
Large Increases in Primary Trimethylaminium and Secondary Dimethylaminium in Atmospheric Particles Associated With Cyclonic Eddies in the Northwest Pacific Ocean
Authors: Hu Qingjing , Qu Keming , Gao Huiwang , Cui Zhengguo , Gao Yang , Yao Xiaohong 
Published: October 2018

AbstractDimethylaminium (DMA+) and trimethylaminium (TMA+) ions in size‐segregated atmospheric particles are measured across the marginal seas of China and the northwest Pacific Ocean (NWPO) in March–May 2014. The concentrations of DMA+ and TMA+ in particles with diameters of 0.056–10 μm (PM0.056–10) collected from the eutrophic seas are 0.22 ± 0.38 nmol/m3 and 0.11 ± 0.23 nmol/m3, respectively. Surprisingly, an average TMA+ concentration that is 1 order of magnitude higher and a slightly higher average DMA+ concentration are observed across the oligotrophic NWPO. However, the concentrations of chlorophyll‐a in the NWPO are approximately 5 times lower than those in the marginal seas. The maximum concentrations of TMA+ (4.39 nmol/m3) and DMA+ (0.92 nmol/m3) in PM0.056–10 are observed close to the largest cyclonic eddy in the NWPO under an average wind speed of 14 m/s. The concentrations of TMA+ increase with decreasing particle size in the sample, whereas those of DMA+ exhibit a condensation mode at 0.2 μm and a droplet mode at 1–2 μm. The bimodal size distribution of DMA+ is conventionally interpreted in terms of secondary reactions in the atmosphere. The unique size distribution of TMA+ suggests that it very likely originates from sea‐spray aerosols. Based on their size distributions in other samples collected over the NWPO, these conclusions may generally apply for TMA+ and DMA+. Moreover, we propose a novel conceptual model to explain how the largely increased primary TMA+ and secondary DMA+ are linked to emissions of sea‐spray aerosols and gaseous precursors from various cyclonic eddies.
Model Intercomparison of Atmospheric 137Cs From the Fukushima Daiichi Nuclear Power Plant Accident: Simulations Based on Identical Input Data
Authors: Sato Yousuke , Takigawa Masayuki , Sekiyama Tsuyoshi Thomas , Kajino Mizuo , Terada Hiroaki , Nagai Haruyasu , Kondo Hiroaki , Uchida Junya , Goto Daisuke , Quélo Denis , Mathieu Anne , Quérel Arnaud , Fang Sheng , Morino Yu , von Schoenberg Pontus , Grahn Håkan , Brännström Niklas , Hirao Shigekazu , Tsuruta Haruo , Yamazawa Hiromi , Nakajima Teruyuki 
Published: October 2018

AbstractA model intercomparison of the atmospheric dispersion of cesium‐137 (137Cs) emitted after the Fukushima Daiichi Nuclear Power Plant accident in Japan was conducted to understand the behavior of atmospheric 137Cs in greater detail. The same meteorological data with a fine spatiotemporal resolution and an emission inventory were applied to all models to exclude the differences among the models originating from differences in meteorological and emission data. The meteorological data were used for initial, boundary, and nudging data or offline meteorological field. Furthermore, a horizontal grid with the same resolution as that of the meteorological data was adopted for all models. This setup enabled us to focus on model variability originating from the processes included in each model, for example, physical processes. The multimodel ensemble captured 40% of the atmospheric 137Cs events observed by measurements, and the figure of merit in space for the total deposition of 137Cs exceeded 80. The lower score of the atmospheric 137Cs than that of the deposition originated from the difference in timing between observed and simulated atmospheric 137Cs. Our analyses indicated that meteorological data were most critical for reproducing the atmospheric 137Cs events. The results further revealed that differences in 137Cs concentrations among the models originated from deposition and diffusion processes when the meteorological field was simulated reasonably well. The models with small deposition fluxes produced higher scores for atmospheric 137Cs, and those with strong diffusion succeeded in capturing the high 137Cs concentrations observed; however, they also tended to overestimate the concentrations.
A Systematic Method for Classifying and Grouping Late Noachian and Early Hesperian Rock Targets Analyzed by the Mars Exploration Rover Opportunity at Endeavour Crater, Mars
Authors: Bouchard Michael C. , Jolliff Bradley L. 
Published: October 2018

AbstractThe Mars rover Opportunity has collected in situ compositional data with the Alpha Particle X‐ray Spectrometer at almost 500 sites. To analyze these data, hierarchical clustering analysis and an error‐weighted similarity index are applied to a subset of 57 APXS target compositions and selected Martian meteorites. Hierarchical clustering provides a rapid first approximation of compositional relationships, whereas the error‐weighted similarity index provides an in‐depth and quantifiable comparison of individual composition pairs. These analyses are combined into a statistical grouping model that provides insight into lithologic relationships and is critically informed by examination of Panoramic Camera and Microscopic Imager images. Major lithologies are (1) the Burns formation sulfate sandstones; (2) Shoemaker impact breccias (Endeavour crater ejecta/rim deposits); (3) the morphologically distinct Grasberg formation, associated with Endeavour crater rim deposits; (4) the Matijevic formation, an exposure interpreted to be Endeavour crater target rocks; and (5) erratics or other rocks that do not cluster with groups 1–4. The Grasberg formation is more similar to the Shoemaker formation than any other formation, and thus likely incorporated eroded Shoemaker material. The lowest Shoemaker member (Copper Cliff breccia) may contain material from the preimpact Matijevic formation. The Matijevic formation is the most chemically distinct formation and is most similar to the volcanic erratic rock “Marquette Island.” Clustering and similarity index values also show that regolith breccia Martian meteorites (represented by the NWA 7475/7034 paired meteorites) are similar in bulk composition to Mars surface materials at Meridiani, especially the Matijevic formation.
Survivability of 1‐Chloronapthalene During Simulated Early Diagenesis: Implications for Chlorinated Hydrocarbon Detection on Mars
Authors: Royle Samuel H. , Tan Jonathan , Kounaves Samuel P. , Sephton Mark A. 
Published: October 2018

AbstractAll missions to Mars which have attempted to detect organic molecules have detected simple chlorohydrocarbons, the source of which has yet to be firmly established. This study assessed the likelihood of these chlorinated molecules being indigenous to the sedimentary units in which they were detected or if they were chlorinated during analysis. The survivability of 1‐chloronapthalene was examined via hydrous pyrolysis experiments and its dechlorination kinetics were determined. The results of these experiments were used to model the survivability of this simple chlorohydrocarbon under Mars‐relevant diagenetic conditions using the Sheepbed mudstone unit as a case study. It was found that 1‐chloronapthalene was rapidly dechlorinated under Noachian conditions, and thus, the detected Martian chlorohydrocarbons are unlikely to be ancient and probably formed within the rover's sample handling chain during analysis.
Using Persistent Scatterer Interferometry to Map and Quantify Permafrost Thaw Subsidence: A Case Study of Eboling Mountain on the Qinghai‐Tibet Plateau
Authors: Chen Jie , Liu Lin , Zhang Tingjun , Cao Bin , Lin Hui 
Published: October 2018

AbstractPermafrost thaw subsidence, a key indicator of permafrost degradation, remains poorly quantified or understood. It is particularly challenging to detect and measure surface subsidence due to the loss of subsurface ice over a large area because it usually develops gradually, over several years or decades. Here we utilize the persistent scatterer interferometric synthetic aperture radar (PSI) approach to remotely measure gradual surface subsidence on Eboling Mountain in the northeastern region of the Qinghai‐Tibet Plateau, where thermal erosion gullies are well developed. Most of the previous multitemporal interferometric synthetic aperture radar studies on permafrost used the small baseline subset method. By contrast, the PSI approach benefits from the full spatial resolution and is less affected by temporal or geometric decorrelation. In the PSI analysis, we incorporate a piecewise elevation change model that includes periodic subsidence/uplift because of its seasonally varying components as well as its linear subsidence trends. Applying this permafrost‐designated PSI algorithm to 17‐L band ALOS‐1 PALSAR images taken between 2006 and 2011, we find that both the thermal erosion gullies and the surrounding regions (within about 300 m) subside gradually. The subsidence trends range from 0.3 to 3 cm/yr. This suggests that permafrost areas near the gullies are more vulnerable to gradual thawing and degradation. This study demonstrates the potential of using PSI to study permafrost thaw processes and of assessing its impacts over vast areas on the Qinghai‐Tibet Plateau and in the Arctic.
Weathering of Reactive Mineral Phases in Landslides Acts as a Source of Carbon Dioxide in Mountain Belts
Authors: Emberson R. , Galy A. , Hovius N. 
Published: October 2018

AbstractBedrock landsliding in mountain belts can elevate overall chemical weathering rates through rapid dissolution of exhumed reactive mineral phases in transiently stored deposits. This link between a key process of erosion and the resultant weathering affects the sequestering of carbon dioxide through weathering of silicate minerals and broader links between erosion in active orogens and climate change. Here we address the effect on the carbon cycle of weathering induced by bedrock landsliding in Taiwan and the Western Southern Alps of New Zealand. Using solute chemistry data from samples of seepage from landslide deposits and river discharge from catchments with variable proportions of landsliding, we model the proportion of silicate and carbonate weathering and the balance of sulfuric and carbonic acids that act as weathering agents. We correct for secondary precipitation, geothermal, and cyclic input, to find a closer approximation of the weathering explicitly occurring within landslide deposits. We find highly variable proportions of sulfuric and carbonic acids driving weathering in landslides and stable hillslopes. Despite this variability, the predominance of rapid carbonate weathering within landslides and catchments where mass wasting is prevalent results at best in limited sequestration of carbon dioxide by this process of rapid erosion. In many cases where sulfuric acid is a key weathering agent, a net release of CO2 to the atmosphere occurs. This suggests that a causal link between erosion in mountain belts and climate change through the sequestration of CO2, if it exists, must operate through a process other than chemical weathering driven by landsliding.
Ventilation of Northern and Southern Sources of Aged Carbon in the Eastern Equatorial Pacific During the Younger Dryas Rise in Atmospheric CO2
Authors: Bova Samantha C. , Herbert Timothy D. , Altabet Mark A. 
Published: October 2018

AbstractAtmospheric carbon dioxide (CO2) levels rose by ~90 ppmv during the last deglaciation, but the source of this carbon remains unknown. One popular hypothesis suggests carbon accumulated in the deep Southern Ocean, becoming increasingly radiocarbon (14C) depleted during the last glacial period, and was released into Antarctic Intermediate Water (AAIW) and, subsequently, the atmosphere during deglaciation. Detection of extremely 14C depleted carbon in the intermediate‐depth tropical oceans during periods of atmospheric CO2 rise was initially considered the smoking gun for the hypothesis, but attempts to reproduce the anomalies closer to the Southern Ocean source have largely failed. Here we present new 14C records from four cores recovered at intermediate depths in the eastern equatorial Pacific (EEP). In the context of additional geochemical records, including benthic foraminiferal stable isotopes and sedimentary nitrogen isotopes, we demonstrate that the extreme 14C anomalies observed during the last deglaciation do not reflect the radiocarbon content of AAIW. We show that although AAIW likely transported modestly 14C depleted carbon to the EEP subsurface, the extreme 14C signatures might reflect a distinct source of aged carbon arriving from the north, suggesting the North Pacific helped transport deep ocean carbon to the atmosphere during the last deglaciation. In the EEP, additional local hydrothermal inputs of 14C‐free carbon near the Galapagos Islands magnified the already low‐14C signatures. Finally, regardless of arrival route (North/South Pacific or hydrothermal), 14C‐depleted carbon was released from the EEP subsurface during a late deglacial pulse of renewed upwelling, likely contributing to the Younger Dryas rise in atmospheric CO2.