bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modeling Soil Porewater Salinity Response to Drought in Tidal Freshwater Forested Wetlands
Authors: Wang Hongqing , Krauss Ken W. , Noe Gregory B. , Stagg Camille L. , Swarzenski Christopher M. , Duberstein Jamie A. , Conner William H. , DeAngelis Donald L. 
Published: February 2020

AbstractThere is a growing concern about the adverse effects of saltwater intrusion via tidal rivers, streams, and creeks into tidal freshwater forested wetlands (TFFW) due to sea level rise (SLR) and intense and extended drought events. However, the magnitude and duration of porewater salinity in exceedance of plant salinity stress threshold (2 practical salinity units, psu) and the controlling factors remain unclear. In this study, we developed a TFFW soil porewater salinity model, in which the feedback mechanisms between soil salinity and evapotranspiration and hydraulic conductivity were incorporated. We selected sites (upper, middle, lower tidal freshwater forest sites, and oligohaline marsh site) along the coastal floodplains of two rivers, the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA), that represent landscape salinity gradients from tidal influence of the Atlantic Ocean. The model results agreed well with field measurements and revealed that with drought‐induced saltwater intrusion, the mean annual soil porewater salinity and duration of elevated soil porewater salinity (>2 psu) increased significantly compared to the normal (nondrought) condition, posing a threat to the health and ecosystem services of TFFW even in the absence of SLR. Model results also showed more severe salinity stress under drought for the lower forest sites along the two rivers, where soil salinity values have already been at or in exceedance of the 2 psu threshold.
A Global Model for Methylmercury Formation and Uptake at the Base of Marine Food Webs
Authors: Zhang Yanxu , Soerensen Anne L. , Schartup Amina T. , Sunderland Elsie M. 
Published: February 2020

AbstractMonomethylmercury (CH3Hg) is the only form of mercury (Hg) known to biomagnify in food webs. Here we investigate factors driving methylated mercury [MeHg = CH3Hg + (CH3)2Hg)] production and degradation across the global ocean and uptake and trophic transfer at the base of marine food webs. We develop a new global 3‐D simulation of MeHg in seawater and phyto/zooplankton within the Massachusetts Institute of Technology general circulation model. We find that high modeled MeHg concentrations in polar regions are driven by reduced demethylation due to lower solar radiation and colder temperatures. In the eastern tropical subsurface waters of the Atlantic and Pacific Oceans, the model results suggest that high MeHg concentrations are associated with enhanced microbial activity and atmospheric inputs of inorganic Hg. Global budget analysis indicates that upward advection/diffusion from subsurface ocean provides 17% of MeHg in the surface ocean. Modeled open ocean phytoplankton concentrations are relatively uniform because lowest modeled seawater MeHg concentrations occur in oligotrophic regions with the smallest size classes of phytoplankton, with relatively high uptake of MeHg and vice versa. Diatoms and synechococcus are the two most important phytoplankton categories for transferring MeHg from seawater to herbivorous zooplankton, contributing 35% and 25%, respectively. Modeled ratios of MeHg concentrations between herbivorous zooplankton and phytoplankton are 0.74–0.78 for picoplankton (i.e., no biomagnification) and 2.6–4.5 for eukaryotic phytoplankton. The spatial distribution of the trophic magnification factor is largely determined by the zooplankton concentrations. Changing ocean biogeochemistry resulting from climate change is expected to have a significant impact on marine MeHg formation and bioaccumulation.
Foreshock Bubbles at Venus: Hybrid Simulations and VEX Observations
Authors: Omidi N. , Collinson G. , Sibeck D. 
Published: February 2020

AbstractIn the absence of a global magnetic field at Venus, its ionosphere is the obstacle to the flow of the solar wind resulting in the formation of a smaller bow shock and foreshock. Spacecraft observations and global hybrid (kinetic ions, fluid electrons) simulations have demonstrated that despite its smaller size, the foreshock at Venus has properties similar to those seen at its much larger terrestrial counterpart. This study employs global hybrid simulations and Venus Express observations to demonstrate the formation of foreshock bubbles (FBs) at Venus and examine their impacts on the Venusian ionosphere. Discovered at Earth, FBs form in response to backstreaming ion beams in the foreshock interacting with solar wind rotational or tangential discontinuities. They exhibit a hot, tenuous, and low magnetic field core bounded by a fast magnetosonic shock wave and the associated plasma sheath. Their sizes scale with the width of the foreshock and they are carried by the solar wind toward the bow shock. The results of the hybrid simulations show that despite the smaller size of the Venusian foreshock, FBs should form at this planet. A preliminary examination of the Venus Express data has shown the presence of a number of FB candidates, one of which is discussed here. Using the results of hybrid simulations we examine the impacts of FBs on the ionosphere and demonstrate their global consequences which exceed those of hot flow anomalies also formed as a result of the interaction between solar wind discontinuities and the Venusian bow shock.
Turbulence Near the Venusian Bow Shock: Venus Express Observations
Authors: Xiao S.D. , Zhang T. L. , Vörös Z. , Wu M. Y. , Wang G. Q. , Chen Y. Q. 
Published: February 2020

AbstractIn terms of composition and size, Venus is an Earth‐like planet without a global intrinsic magnetic field. The Venusian bow shock and magnetosheath, reaching only 10% of the size of the Earth, are formed from the interaction of the solar wind with the Venusian ionosphere. Energy injection from the solar wind to the Venusian space environment can be studied at the bow shock. In this respect, we hypothesize that the spectral scaling features of magnetic fluctuations near the bow shock represent important information. Nine years (2006–2014) of Venus Express magnetic observations are used to study the spectral features of fluctuations near the bow shock over the frequency range from 0.05 to 0.3 Hz. Our results show that in terms of the spectral scaling features of magnetic fluctuations, the dayside‐nightside shock crossings exhibit a clear asymmetry. Magnetic fluctuations at the dayside shock crossings are associated with spectral indices of ~1, resembling the scaling of 1/f noise. At the nightside shock crossings, the spectral indices are closer to 3/2, resembling the magnetohydrodynamic turbulence scaling. We speculate that the noise at the dayside is the consequence of the newborn strong fluctuations near the dayside bow shock, which can mask or destroy the turbulence convected from the solar wind. The spectral scaling features of magnetic fluctuations at bow shock crossings do not exhibit solar cycle dependence. Although the fluctuations differ downstream of quasi‐parallel or quasi‐perpendicular shocks in the magnetosheath, the shock geometry has no influence on the scaling properties of fluctuations at the dayside‐nightside shocks.
Solar Wind and Interplanetary Magnetic Field Influence on Ultralow Frequency Waves and Reflected Ions Near the Moon
Authors: Howard S. K. , Halekas J. S. , Farrell W. M. , McFadden J. P. , Glassmeier K.‐H. 
Published: February 2020

AbstractWith no global magnetic field or atmosphere, the Moon was traditionally seen as a perfect absorber of the incoming solar wind. Recently, it has become apparent that magnetic fields with sources in the lunar crust act to reflect a significant percentage of incoming solar wind particles, which can then interact with the surrounding plasma environment and drive plasma waves. Using data collected by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft, we look for simultaneous observations of reflected ions and 0.01 Hz waves to study the characteristics and conditions under which wave‐particle resonant interactions occur. Analyzing the solar wind and interplanetary magnetic field during these observations reveals particular solar wind and interplanetary magnetic field conditions that favor the generation of these waves. We use an ion tracing program to produce reflected ion distributions for various ambient conditions. These distributions show that the conditions that lead to more ions crossing the equatorial region where ARTEMIS orbits are also those favored for wave observations. Low‐frequency waves, such as those generated by cyclotron resonance with ions, can be heavily Doppler shifted, making it difficult to determine their intrinsic properties. Reflected ion distributions for the same ambient conditions as the observed waves suggest that most of the waves are intrinsically right‐hand polarized.
Modulation of Whistler Mode Waves by Ion‐Scale Waves Observed in the Distant Magnetotail
Authors: Zhao Duo , Fu Suiyan , Parks George K. , Chen Lunjin , Liu Xu , Tong Yuguang , Li Jinxing , Pu Zuyin , Zong Qiugang , Sun Weijie , Li Shouxian , An JianZhu 
Published: February 2020

AbstractWave activities in tailward flows have been explored in the distant magnetotail at ~54 RE, where the coupling between whistler mode waves and ion‐scale waves was observed. The whistler mode waves periodically appeared at each cycle of the ion‐scale waves, and the electron distribution functions associated with the whistler mode waves showed enhancement in the direction parallel to background magnetic field. Wave analyses show that the field‐aligned electron components act as the energy source of the whistler mode waves. The ion‐scale waves are generated by the interaction of the hot ion beam with background ions. A likely candidate of the ion‐scale wave is the kinetic Alfven wave, which can generate the enhanced field‐aligned electron populations by the parallel electric field.
Energized Oxygen in the Magnetotail: Current Sheet Bifurcation From Speiser Motion
Authors: George Don E , Jahn Jörg‐Micha 
Published: February 2020

AbstractOxygen can be a major constituent of magnetospheric plasma, yet the role of oxygen in the magnetosphere is not sufficiently understood. Observations of bifurcated current sheets (BCS) attributed to O
have been made, yet their origin is not well understood. We examine the case of a thinning current sheet prior to the onset of magnetic reconnection using three‐species, 2.5‐D particle‐in‐cell simulations. There currently is no consensus on the formation mechanism of a BCS. Our principal result is to demonstrate that a single population of energized, Speiser orbiting, heavy ions produces a BCS. Previous particle‐in‐cell simulations have studied a three‐species system with a background of thermal O
. Here we energize our O
by applying an initial duskward velocity. While this is a nonphysical initialization, it does place the O
into Speiser orbits streaming duskward, which has been observed in situ. Until now the connection between Speiser orbiting ions and a BCS has only been predicted. While there are two physical peaks in the current, we show that there is no splitting or bifurcation of the current in a BCS. Both current peaks are produced by a single population of ions. This is due to their gyromotion on either side of the current sheet turning them back toward the current sheet null. Additionally, there is no consensus whether magnetic reconnection is required to form a BCS or not. We confirm that magnetic reconnection is not required to produce a BCS.
Dynamical Properties of Peak and Time‐Integrated Geomagnetic Events Inferred From Sample Entropy
Authors: Mourenas D. , Artemyev A. V. , Zhang X.‐J. 
Published: February 2020

AbstractWe provide a comprehensive statistical analysis of the sample entropy of peak and time‐integrated geomagnetic events in 2001–2017, considering different measures of event strength, different geomagnetic indices, and a simplified solar wind‐magnetosphere coupling function. Our investigations reveal the existence of significant correlations between the entropies of,, and, and between such entropies and event strengths, as well as good correlations between peak levels of solar wind‐magnetosphere coupling and ring current () andentropies, suggesting a potential predictability of significantandevents on the basis of appropriate functions of. Sensibly weaker correlations are found withentropy. We further show the presence of several significant entropy correlations between geomagnetic indices, solar wind‐magnetosphere coupling, and trapped or precipitated energetic electron and ion fluxes measured by geostationary and low Earth orbit satellites in the outer radiation belt during the same periods. Entropy correlations betweenand trapped or precipitated 30‐ to 80‐keV ion fluxes at lowand betweenand trapped 40‐keV electron fluxes at geostationary orbit correspond well with ring current properties and substorm‐induced injections, respectively. Entropy correlations betweenand precipitation rates of energetic ion and electron fluxes demonstrate the sensibility ofindex entropy to both low‐energy (5–30 keV) electron injections and ring current. The stronger entropy correlation between solar wind‐magnetosphere coupling andthanlikely stems from the more stochastic behavior of electron injections and fast losses near geostationary orbit.
Characteristics of the Flank Magnetopause: MMS Results
Authors: Haaland S. , Paschmann G. , Øieroset M. , Phan T. , Hasegawa H. , Fuselier S. A. , Constantinescu V. , Eriksson S. , Trattner K. J. , Fadanelli S. , Tenfjord P. , Lavraud B. , Norgren C. , Eastwood J. P. , Hietala H. , Burch J. 
Published: February 2020

AbstractWe have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity.
Latitude Dependence of Interhemispheric Field‐Aligned Currents (IHFACs) as Observed by the Swarm Constellation
Authors: Park Jaeheung , Yamazaki Yosuke , Lühr Hermann 
Published: February 2020

Key Points

Two clearly different systems of IHFACs exist at low and middle latitudes, respectively, with a boundary near ±35° in magnetic latitudes
The latitudinal and longitudinal structures of IHFACs depend strongly on wave forcing from the lower atmosphere, below 30 km
Midlatitude IHFACs have similar climatology in equinoxes/June solstice, while those of December solstice exhibit different behavior
Role of the Tide on the Structure of the Amazon Plume: A Numerical Modeling Approach
Authors: Ruault Valentin , Jouanno Julien , Durand Fabien , Chanut Jérôme , Benshila Rachid 
Published: February 2020

AbstractThe dynamical balance on the Amazon shelf and its implication on the properties of the Amazon River plume is not fully understood and poorly represented in global‐ and basin‐scale ocean models. In this study, the sensitivity of the Amazon shelf dynamics to tidal forcing is explored with a set of high‐resolution numerical simulations (1/36°) with and without the tide. A comparison of the simulations with sea surface salinity in situ measurements at 5°N (a location where the plume seasonally detaches from the coast and retroflects toward the east) revealed that the explicit resolution of the tide significantly improves the representation of the offshore spread of the river plume. This study further highlights the finding that tidal currents affect the properties of the whole Amazon plume. This sensitivity is explained by a near total collapse of the northwestward alongshore mean flow located near the river mouth, once the tidal forcing is included. This weakening of the ambient flow reduces (i) the dilution ratio between the ambient salty shelf waters and the riverine freshwaters and (ii) the constraint on the cross‐shore extension of the low‐salinity bulge. With tides, the plume is fresher near the river mouth (by up to 5 units), more extended in the cross‐shore direction, and more easily exported offshore by the North Brazil Current at the shelf break.
Variability of the Shallow Overturning Circulation in the Indian Ocean
Authors: Meng Lingsheng , Zhuang Wei , Zhang Weiwei , Yan Chi , Yan Xiao‐Hai 
Published: February 2020

AbstractVariability of the Indian Ocean (IO) shallow overturning circulation during 1958–2017 was investigated, which consisted of a cross‐equator cell (CEC) and a southern subtropical cell (SSTC). Structures of these shallow cells were examined through the velocity sections and the transport steamfunction patterns based on the newly released Ocean Reanalysis System 5 data set, and the results showed that the returning flows, at the subsurface, of the shallow cells mainly presented at the western boundary while absented in the ocean interior. Strengths of the IO shallow cells were then determined according to the annual‐mean meridional streamfunction profiles. Both the CEC and the SSTC exhibited significant variability on interannual to decadal timescales during 1958–2017, and this variability mainly resulted from changes in both meridional Ekman transports and meridional geostrophic flows in the upper layer, and each component could dominate the shallow cells' variations in certain years. Contributions of the geostrophic currents to the shallow cells were likely opposite to that of the Ekman transport in the South Indian Ocean during years when the latter reached the extremes. The CEC and the SSTC were negatively correlated during the research period. The CEC exhibited a decadal weakening during 2000–2015, contributing to the decadal increasing of the IO heat content, while the SSTC only experienced very little strengthening. Here this paper presents a comprehensively study of the interannual to decadal variability of the IO shallow cells and the corresponding reasons and influences and tried to link these variations with other variations in the IO.
Estimates of Regional Source Contributions to the Asian Tropopause Aerosol Layer Using a Chemical Transport Model
Authors: Fairlie T. Duncan , Liu Hongyu , Vernier Jean‐Paul , Campuzano‐Jost Pedro , Jimenez Jose L. , Jo Duseong S. , Zhang Bo , Natarajan Murali , Avery Melody A. , Huey Gregory 
Published: February 2020

AbstractThe Asian Tropopause Aerosol Layer (ATAL) represents an accumulation of aerosol in the upper troposphere and lower stratosphere associated with the Asian Summer Monsoon. Here we simulate the ATAL for summer 2013 with the GEOS‐Chem chemical transport model and explore the likely composition of ATAL aerosols and the relative contributions of regional anthropogenic sources versus those from farther afield. The model indicates significant contributions from organic aerosol, nitrate, sulfate, and ammonium aerosol, with regional anthropogenic precursor sources dominant. The model underestimates aerosol backscatter in the ATAL during summer 2013, compared with Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite instrument. Tests of a more physically based treatment of wet scavenging of SO2 in convective updrafts raise sulfate, eliminating the low bias with respect to CALIPSO backscatter in the ATAL, but lead to an unacceptable high bias of sulfate compared with in situ observations from aircraft over the United States. Source apportionment of the model results indicate the dominance of regional anthropogenic emissions from China and the Indian subcontinent to aerosol concentrations in the ATAL; ~60% of sulfate in the ATAL region in August 2013 is attributable to anthropogenic sources of SO2 from China (~30%) and from the Indian subcontinent (~30%), twice as much as in previously published estimates. Nitrate aerosol is found to be a dominant component of aerosol composition on the southern flank of the Asian Summer Monsoon anticyclone. Lightning sources of NOx are found to make a significant (10–15%) contribution to nitrate in the ATAL for the case studied.
Distribution Characteristics of Dimethylated Sulfur Compounds and Turnover of Dimethylsulfide in the Northern South China Sea During Summer
Authors: Zhai Xing , Song Yu‐Chen , Li Jian‐Long , Yang Jian , Zhang Hong‐Hai , Yang Gui‐Peng 
Published: February 2020

AbstractSpatial distributions of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP), and dimethyl sulfoxide (DMSO) in the northern South China Sea were determined in the summer of 2015. The mean concentrations of DMS, particulate DMSP (DMSPp), dissolved DMSP (DMSPd), dissolved DMSO (DMSOd), and particulate DMSO (DMSOp) in the surface seawater were 2.8 ± 1.0, 6.8 ± 1.5, 14.0 ± 3.1, 12.2 ± 3.2, and 9.2 ± 2.6 nmol L−1, respectively. The contributions of three principal removal pathways of DMS were investigated. The average biological production and consumption rates of DMS in the surface seawater were 6.15 ± 1.12 and 4.63 ± 1.32 nmol L−1 day−1. Deck incubation experiments showed that the photooxidation rates of DMS for different radiation bands followed the order: KUVB > KUVA > Kvisible light. The DMS photooxidation rates were higher in the coastal waters than in the open ocean, which might be associated with the higher concentrations of dissolved organic matter of terrigenous input. The sea‐to‐air fluxes of DMS ranged from 1.6 to 50.3 μmol m−2 day−1 with an average value of 15.6 ± 11.6 μmol m−2 day−1. Based on the respective turnover times of DMS, the contributions of biological consumption, photooxidation, and sea‐to‐air exchange to the sink of DMS were 32.6 ± 11.0%, 23.2 ± 8.9%, and 44.1 ± 19.2%, respectively, suggesting that the three removal pathways were all critical to the migration and transformation of DMS in the surface seawater during summer.
WISHE‐Moisture Mode in a Vertically Resolved Model
Authors: Fuchs‐Stone Željka 
Published: February 2020

AbstractThe Madden‐Julian oscillation is a global phenomenon, a wave with the wavelength of the circumference of the Earth. A theoretical model presented by Fuchs and Raymond (2007, https://doi.org/10.1111/j.1600-0870.2007.00230.x) obtained a mode called the eastward propagating wind‐induced surface heat exchange (WISHE)‐moisture mode that had its largest instability for the longest wavelengths. That model imposed the first baroclinic mode vertical structure. This paper expands the Fuchs and Raymond model by developing a vertically resolved model, a model in which all the variables except the heating vertical profile are calculated. The heating profile is assumed to have a vertical structure of the first baroclinic mode. The model uses the upper boundary radiation condition and thus includes the effects of the stratosphere. The convective parametrization includes wind‐induced surface heat exchange, cloud‐radiation interactions, and moisture closure. The vertically resolved model produces one unstable mode whose dispersion relation characteristics closely resemble the eastward propagating WISHE‐moisture mode. For the longest wavelengths it is more unstable and its phase speed is slightly larger than in the Fuchs and Raymond model. The calculated vertical structure reveals a similar vertical structure to the first baroclinic mode with small differences that to some extent agree with observations.