bluuebunny/crossref_metadata_2025_split huggingface dataset:
Variability of the directly observed, middepth subpolar North Atlantic circulation
Authors: Palter Jaime B. , Caron Charles‐André , Law Kara Lavender , Willis Joshua K. , Trossman David S. , Yashayaev Igor M. , Gilbert Denis 
Published: February 2016

AbstractSatellite views of the ocean have suggested a decline of the subpolar North Atlantic surface circulation during the 1990s and 2000s. This was a period of unprecedented observational capacity in the basin, thanks to the presence of many hundreds of profiling floats. We use more than 40,000 subsurface displacements of these floats to characterize the circulation at 1000 m depth, and its evolution from 1997–2013. We show a statistically significant slowdown in the Labrador Sea boundary currents of −0.8 cm s−1 per decade (95% confidence interval of −1.4 to −0.15 cm s—1 per decade, a conservative estimate of the uncertainty). Otherwise, the middepth circulation field was largely stable. Our analysis of the location where the North Atlantic Current crosses the Mid‐Atlantic Ridge shows that profiling floats can reveal steering by bathymetric features, but do not reveal of decadal variability in the position where the current crosses the ridge.
Nitrous oxide in the atmosphere: First measurements of a lower thermospheric source
Authors: Sheese Patrick E. , Walker Kaley A. , Boone Chris D. , Bernath Peter F. , Funke Bernd 
Published: February 2016

AbstractNitrous oxide (N2O) is an important anthropogenic greenhouse gas, as well as one of the most significant anthropogenic ozone‐depleting substances in the stratosphere. The satellite‐based instrument Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer has been observing the Earth's limb since 2004 and derives profiles of N2O volume mixing ratios in the upper troposphere to the lower thermosphere. The resulting climatology shows that N2O is continuously produced in the lower thermosphere via energetic particle precipitation and enhanced N2O is present at all latitudes, during all seasons. The results are consistent with an N2O production source peaking near or above 94 km via low‐energy particles, as well as a polar wintertime source near 70 km via medium energy particles. N2O produced in the polar upper atmosphere descends each winter to as far down as ~40 km.
Simulation of low‐latitude ionospheric response to 2015 St. Patrick's Day super geomagnetic storm using ionosonde‐derived PRE vertical drifts over Indian region
Authors: Joshi L. M. , Sripathi S. , Singh Ram 
Published: February 2016

AbstractIn this paper, we present low‐latitude ionospheric response over Indian longitude to the recent super geomagnetic storm of 17 March 2015, using the Sami2 is Another Model of the Ionosphere (SAMI2) model which incorporates ionosonde‐derived vertical drift impacted by prompt penetration eastward electric field occurring during the evening prereversal enhancement (PRE) in the vertical drift. The importance of this storm is that (1) Dst reaches as low as −228 nT and (2) prompt penetration of eastward electric field coincided with evening hours PRE. The daytime vertical E × B drifts in the SAMI2 model are, however, considered based on Scherliess‐Fejer model. The simulations indicate a significant enhancement in F layer height and equatorial ionization anomaly (EIA) in the post sunset hours on 17 March 2015 vis‐a‐vis quiet day. The model simulations during recovery phase, considering disturbance dynamo vertical E × B drift along with equatorward disturbance wind, indicate suppression of the daytime EIA. SAMI2 simulations considering the disturbance wind during the recovery phase suggest that equatorward wind enhances the ionospheric density in the low latitude; however, its role in the formation of the EIA depends on the polarity of the zonal electric field. Comparison of model derived total electron content (TEC) with the TEC from ground GPS receivers indicates that model does reproduce enhancement of the EIA during the main phase and suppression of the EIA during the recovery phase of the superstorm. However, peculiarities pertaining to the ionospheric response to prompt penetration electric field in the Indian sector vis‐a‐vis earlier reports from American sector have been discussed.
Survey of the frequency dependent latitudinal distribution of the fast magnetosonic wave mode from Van Allen Probes Electric and Magnetic Field Instrument and Integrated Science waveform receiver plasma wave analysis
Authors: Boardsen Scott A. , Hospodarsky George B. , Kletzing Craig A. , Engebretson Mark J. , Pfaff Robert F. , Wygant John R. , Kurth William S. , Averkamp Terrance F. , Bounds Scott R. , Green Jim L. , De Pascuale Sebastian 
Published: February 2016

AbstractWe present a statistical survey of the latitudinal structure of the fast magnetosonic wave mode detected by the Van Allen Probes spanning the time interval of 21 September 2012 to 1 August 2014. We show that statistically, the latitudinal occurrence of the wave frequency (f) normalized by the local proton cyclotron frequency (fcP) has a distinct funnel‐shaped appearance in latitude about the magnetic equator similar to that found in case studies. By comparing the observed E/B ratios with the model E/B ratio, using the observed plasma density and background magnetic field magnitude as input to the model E/B ratio, we show that this mode is consistent with the extra‐ordinary (whistler) mode at wave normal angles (θk) near 90°. Performing polarization analysis on synthetic waveforms composed from a superposition of extra‐ordinary mode plane waves with θk randomly chosen between 87 and 90°, we show that the uncertainty in the derived wave normal is substantially broadened, with a tail extending down to θk of 60°, suggesting that another approach is necessary to estimate the true distribution of θk. We find that the histograms of the synthetically derived ellipticities and θk are consistent with the observations of ellipticities and θk derived using polarization analysis. We make estimates of the median equatorial θk by comparing observed and model ray tracing frequency‐dependent probability occurrence with latitude and give preliminary frequency dependent estimates of the equatorial θk distribution around noon and 4 RE, with the median of ~4 to 7° from 90° at f/fcP = 2 and dropping to ~0.5° from 90° at f/fcP = 30. The occurrence of waves in this mode peaks around noon near the equator at all radial distances, and we find that the overall intensity of these waves increases with AE*, similar to findings of other studies.
Evidence of m = 1 density mode (plasma cam) in Saturn's rotating magnetosphere
Authors: Goldstein J. , Waite J. H. , Burch J. L. , Livi R. 
Published: February 2016

AbstractCassini field and plasma data measured in the rotating Saturn Longitude System 3 (SLS3) coordinate system show positive evidence of structure whose dominant azimuthal wave number is m = 1: a long‐lived, nonaxisymmetric, cam‐shaped, global plasma distribution in Saturn's magnetosphere. Previous studies have identified evidence of this plasma cam in wave‐derived electron density data and in Cassini Plasma Spectrometer (CAPS) W+ ion counts data. In this paper we report the first comprehensive analysis of CAPS ion moments data to identify the m = 1 density cam. We employ a multiyear, multispecies database of 685,678 CAPS density values, binned into a 1 RS by 4.8° discretized grid, spanning 4–19 RS. Fourier (harmonic) analysis shows that at most radial distances the dominant azimuthal mode is m = 1, for both W+ and H+ ion distributions. The majority (63%) of m = 1 ion peaks are clustered in an SLS3 quadrant centered at 330°. The plasma cam's existence has important implications for the global interchange‐driven convection cycle and is a clue to solving the mystery of the rotational periodicities in Saturn's magnetosphere.
The impact of sunlight on high‐latitude equivalent currents
Authors: Laundal K. M. , Gjerloev J. W. , Østgaard N. , Reistad J. P. , Haaland S. , Snekvik K. , Tenfjord P. , Ohtani S. , Milan S. E. 
Published: February 2016

AbstractGround magnetic field measurements can be mathematically related to an overhead ionospheric equivalent current. In this study we look in detail at how the global equivalent current, calculated using more than 30 years of SuperMAG magnetometer data, changes with sunlight conditions. The calculations are done using spherical harmonic analysis in quasi‐dipole coordinates, a technique which leads to improved accuracy compared to previous studies. Sorting the data according to the location of the sunlight terminator and orientation of the interplanetary magnetic field (IMF), we find that the equivalent current resembles ionospheric convection patterns on the sunlit side of the terminator but not on the dark side. On the dark side, with southward IMF, the current is strongly dominated by a dawn cell and the current across the polar cap has a strong dawnward component. The contrast between the sunlit and dark side increases with increasing values of the F10.7 index, showing that increasing solar EUV flux changes not only the magnitude but also the morphology of the equivalent current system. The results are consistent with a recent study showing that Birkeland currents indirectly determine the equivalent current in darkness and that Hall currents dominate in sunlight. This has implication for the interpretation of ground magnetic field measurements and suggests that the magnetic disturbances at conjugate points will be asymmetrical when the solar illumination is different.
Reconstructing the magnetosphere from data using radial basis functions
Authors: Andreeva Varvara A. , Tsyganenko Nikolai A. 
Published: February 2016

AbstractA new method is proposed to derive from data magnetospheric magnetic field configurations without any a priori assumptions on the geometry of electric currents. The approach utilizes large sets of archived satellite data and uses an advanced technique to represent the field as a sum of toroidal and poloidal parts, whose generating potentials Ψ1 and Ψ2 are expanded into series of radial basis functions (RBFs) with their nodes regularly distributed over the 3‐D modeling domain. The method was tested by reconstructing the inner and high‐latitude field within geocentric distances up to 12RE on the basis of magnetometer data of Geotail, Polar, Cluster, Time History of Events and Macroscale Interactions during Substorms, and Van Allen space probes, taken during 1995–2015. Four characteristic states of the magnetosphere before and during a disturbance have been modeled: a quiet prestorm period, storm deepening phase with progressively decreasing SYM‐H index, the storm maximum around the negative peak of SYM‐H, and the recovery phase. Fitting the RBF model to data faithfully resolved contributions to the total magnetic field from all principal sources, including the westward and eastward ring current, the tail current, diamagnetic currents associated with the polar cusps, and the large‐scale effect of the field‐aligned currents. For two main phase conditions, the model field exhibited a strong dawn‐dusk asymmetry of the low‐latitude magnetic depression, extending to low altitudes and partly spreading sunward from the terminator plane in the dusk sector. The RBF model was found to resolve even finer details, such as the bifurcation of the innermost tail current. The method can be further developed into a powerful tool for data‐based studies of the magnetospheric currents.
Two hundred thirty years of relative sea level changes due to climate and megathrust tectonics recorded in coral microatolls of Martinique (French West Indies)
Authors: Weil‐Accardo Jennifer , Feuillet Nathalie , Jacques Eric , Deschamps Pierre , Beauducel François , Cabioch Guy , Tapponnier Paul , Saurel Jean‐Marie , Galetzka John 
Published: February 2016

AbstractWe sampled six coral microatolls that recorded the relative sea level changes over the last 230 years east of Martinique, on fringing reefs in protected bays. The microatolls are cup‐shaped, which is characteristic of corals that have been experiencing submergence. X‐ray analysis of coral slices and reconstructions of the highest level of survival (HLS) curves show that they have submerged at rates of a few millimeters per year. Their morphology reveals changes in submergence rate around 1829 ± 11, 1895, and 1950. Tide gauges available in the region indicate a regional sea level rise at a constant mean rate of 1.1 ± 0.8 mm/yr, which contrasts with our coral record, implying additional tectonic subsidence. Comparing our coral morphology with that of synthetic corals generated with Matlab by using the Key West tide gauge record (Florida), we show that their growth was controlled by tectonics and that a sudden relative sea level increase drowned them around 1950. Simple elastic models show that this sudden submergence probably occurred during the 21 May 1946 earthquake, which ruptured the plate interface in front of Martinique, in the mantle wedge, in an area of sustained seismic activity. The 1839M8+ earthquake probably occurred in the same area. Long‐term subsidence of microatolls indicates that this deep portion of the megathrust is probably locked down to 60 km depth during the interseismic period. Our oldest coral recorded a long‐lasting period (50 years) of stable relative sea level after the 1839 earthquake, indicating that transient interseismic strain rate variations may occur in the Lesser Antilles.
CMIP5 permafrost degradation projection:A comparison among different regions
Authors: Guo Donglin , Wang Huijun 
Published: February 2016

AbstractThe considerable impact of permafrost degradation on hydrology and water resources, ecosystems, human engineering facilities, and climate change requires us to carry out more in‐depth studies, at finer spatial scales, to investigate the issue. In this study, regional differences of the future permafrost changes are explored with respect to the regions (high altitude and high latitude, and in four countries) based on the surface frost index (SFI) model and multimodel and multiscenario data from the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Results show the following: (1) Compared with seven other sets of driving data, Climatic Research Unit air temperature combined with Climate Forecast System Reanalysis snow data (CRU_CFSR) yield a permafrost extent with the least absolute area bias and was thus used in the simulation. The SFI model, driven by CRU_CFSR data climatology plus multimodel mean anomalies, produces a present‐day (1986–2005) permafrost area of 15.45 × 106 km2 decade−1, which compares reasonably with observations of 15.24 × 106 km2 decade−1. (2) The high‐altitude (Tibetan Plateau) permafrost area shows a larger decreasing percentage trend than the high‐latitude permafrost area. This indicates that, in terms of speed, high‐altitude permafrost thaw is faster than high‐latitude permafrost, mainly due to the larger percentage sensitivity to rising air temperature of the high‐altitude permafrost compared to the high‐latitude permafrost, which is likely related to their thermal conditions. (3) Permafrost in China shows the fastest thaw, which is reflected by the percentage trend in permafrost area, followed by the United States, Russia, and Canada. These discrepancies are mainly linked to different percentage sensitivities of permafrost areas in these four countries to air temperature change. (4) In terms of the ensemble mean, permafrost areas in all regions are projected to decrease by the period 2080–2099. Under representative concentration pathway (RCP)4.5, permafrost retreats toward the Arctic, and the thaw in every region mainly occurs at the southern edge of the permafrost area. Under RCP8.5, almost no permafrost is expected to remain in China, the United States, and the Tibetan Plateau. Permafrost in Russia will remain mainly in the western part of the east Siberian Mountains, and permafrost in Canada will retreat to the north of 65°N. Possible uncertainties in this study are primarily attributed to the climate model's coarse horizontal resolution. The results of the present study will be useful for understanding future permafrost degradation from the regional perspective.
Transport of nitrogen oxides through the winter mesopause in HAMMONIA
Authors: Meraner Katharina , Schmidt Hauke 
Published: February 2016

AbstractWe analyze the importance of individual transport processes for the winter polar downward transport of nitrogen oxides (NOx) from the thermosphere to the mesosphere. The downward transport of NOx produced by energetic particle precipitation induces chemical alterations in the middle atmosphere and influences ozone chemistry. However, it remains unclear how much each transport process contributes to the downward transport. We use simulations of the atmospheric general circulation and chemistry model HAMMONIA (Hamburg Model of Neutral and Ionized Atmosphere) for the extended winter 2008/2009 with a passive tracer. The model enables us to separate the contributions of advection, eddy and molecular diffusion on the total transport by switching off processes. The results show that molecular diffusion and resolved vertical mixing due to advection effectively transport NOx to the mesosphere. While the impact of molecular diffusion on the transport rapidly decreases below 0.001 hPa, the impact of advection increases. Around the central date of the sudden stratospheric warming in January 2009, advection is strongly enhanced in the thermosphere and mesosphere and the downward transport through the mesopause region is almost entirely driven by advection. Eddy diffusion has limited impact on the transport in the upper mesosphere and negligible impact on the transport in the thermosphere. If eddy diffusion is enhanced as suggested by observations, it can potentially have a larger impact on transport through the mesopause than was previously assumed.
Overlap determination for temperature measurements from a pure rotational Raman lidar
Authors: Chen Hao , Chen Siying , Zhang Yinchao , Guo Pan , Chen He , Chen Binglong 
Published: February 2016

AbstractWe propose a new method to calibrate the effect of overlap for temperature measurements made with a pure rotational Raman lidar. This method is based on the construction of a factor in the signal intensity ratio, which has an approximately linear relationship with altitude within the troposphere and can be obtained from radiosonde temperature measurements. Using this relationship, the effect of overlap on the signal intensity ratio can be calibrated. The method has been verified by simulations and an experiment. Comparisons with results obtained from using the existing calibration method show that the overlap determined using the new method is more accurate.
Relativistic electrons from sparks in the laboratory
Authors: Østgaard N. , Carlson B. E. , Nisi R. S. , Gjesteland T. , Grøndahl Ø. , Skeltved A. , Lehtinen N. G. , Mezentsev A. , Marisaldi M. , Kochkin P. 
Published: February 2016

AbstractDischarge experiments were carried out at the Eindhoven University of Technology in 2013. The experimental setup was designed to search for electrons produced in meter‐scale sparks using a 1 MV Marx generator. Negative voltage was applied to the high voltage (HV) electrode. Five thin (1 mm) plastic detectors (5 cm2 each) were distributed in various configurations close to the spark gap. Earlier studies have shown (for HV negative) that X‐rays are produced when a cloud of streamers is developed 30–60 cm from the negative electrode. This indicates that the electrons producing the X‐rays are also accelerated at this location, that could be in the strong electric field from counterstreamers of opposite polarity. Comparing our measurements with modeling results, we find that ∼300 keV electrons produced about 30–60 cm from the negative electrode are the most likely source of our measurements. A statistical analysis of expected detection of photon bursts by these fiber detectors indicates that only 20%–45% of the detected bursts could be from soft (∼10 keV) photons, which further supports that the majority of detected bursts are produced by relativistic electrons.
Constraints on the formation and properties of a Martian lobate debris apron: Insights from high‐resolution topography, SHARAD radar data, and a numerical ice flow model
Authors: Parsons Reid , Holt John 
Published: February 2016

AbstractLobate debris aprons (LDAs) are midlatitude deposits of debris‐covered ice formed during one or more periods of glaciation during the Amazonian period. However, little is known about the climate conditions that led to LDA formation. We explore a hypothesis in which a single, extended period of precipitation of ice on the steep slopes of Euripus Mons (45°S, 105°E—east of the Hellas Basin) produced a flowing ice deposit which was protected from subsequent ablation to produce the LDA found at this location. We test this hypothesis with a numerical ice flow model using an ice rheology based on low‐temperature ice deformation experiments. The model simulates ice accumulation and flow for the northern and southern lobes of the Euripus Mons LDA using basal topography constrained by data from the Shallow Radar (SHARAD) and a range of ice viscosities (determined by ice temperature and ice grain size). Simulations for the northern lobe of the Euripus LDA produce good fits to the surface topography. Assuming an LDA age of ∼60 Myr and an expected temperature range of 200 to 204 K (for various obliquities) gives an ice grain size of ≈2 mm. Simulations of the southern section produce poor fits to surface topography and result in much faster flow timescales unless multiple ice deposition events or higher ice viscosities are considered.
In situ characterization of forest litter using ground‐penetrating radar
Authors: André Frédéric , Jonard François , Jonard Mathieu , Lambot Sébastien 
Published: February 2016

AbstractDecomposing litter accumulated on the soil surface in forests plays a major role in several ecosystem processes; its detailed characterization is therefore essential for thorough understanding of ecosystem functioning. In addition, litter is known to affect remote sensing radar data over forested areas and their proper processing requires accurate quantification of litter scattering properties. In the present study, ultrawideband (0.8–2.2 GHz) ground‐penetrating radar (GPR) data were collected in situ for a wide range of litter types to investigate the potential of the technique to reconstruct litter horizons in undisturbed natural conditions. Radar data were processed resorting to full‐wave inversion. Good agreement was generally found between estimated and measured litter layer thicknesses, with root‐mean‐square error values around 1 cm for recently fallen litter (OL layer) and around 2 cm for fragmented litter in partial decomposition (OF layer) and total litter (OL + OF). Nevertheless, significant correlations between estimated and measured thicknesses were found for total litter only. Inaccuracies in the reconstruction of the individual litter horizons were mainly attributed to weak dielectric contrasts amongst litter layers, with absolute differences in relative dielectric permittivity values often lower than 2 between humus horizons, and to uncertainties in the ground truth values. Radar signal inversions also provided reliable estimates of litter electromagnetic properties, with average relative dielectric permittivity values around 2.9 and 6.3 for OL and OF litters, respectively. These results are encouraging for the use of GPR for noninvasive characterization and mapping of forest litter. Perspectives for the application of the technique in biogeosciences are discussed.