bluuebunny/crossref_metadata_2025 huggingface dataset:
The significance of turbulent flow representation in single‐continuum models
Authors: Reimann Thomas , Rehrl Christoph , Shoemaker W. Barclay , Geyer Tobias , Birk Steffen
Published: August 2011

Karst aquifers exhibit highly conductive features caused from rock dissolution processes. Flow within these structures can become turbulent and therefore can be expressed by nonlinear gradient functions. One way to account for these effects is by coupling a continuum model with a conduit network. Alternatively, turbulent flow can be considered by adapting the hydraulic conductivity within the continuum model. Consequently, the significance of turbulent flow on the dynamic behavior of karst springs is investigated by an enhanced single‐continuum model that results in conduit‐type flow in continuum cells (CTFC). The single‐continuum approach CTFC represents laminar and turbulent flow as well as more complex hybrid models that require additional programming and numerical efforts. A parameter study is conducted to investigate the effects of turbulent flow on the response of karst springs to recharge events using the new CTFC approach, existing hybrid models, and MODFLOW‐2005. Results reflect the importance of representing (1) turbulent flow in karst conduits and (2) the exchange between conduits and continuum cells. More specifically, laminar models overestimate maximum spring discharge and underestimate hydraulic gradients within the conduit. It follows that aquifer properties inferred from spring hydrographs are potentially impaired by ignoring flow effects due to turbulence. The exchange factor used for hybrid models is necessary to account for the scale dependency between hydraulic properties of the matrix continuum and conduits. This functionality, which is not included in CTFC, can be mimicked by appropriate use of the Horizontal Flow Barrier package for MODFLOW.
Infiltration models for semi‐infinite soil profiles
Authors: Basha H. A.
Published: August 2011

Infiltration models for hydrological applications are developed for rational forms of the soil hydraulic conductivity and moisture retention functions. The models are based on the traveling and kinematic wave solutions of the governing equation that proved to be highly accurate in specific flow conditions. The approximate solutions describe the one‐dimensional infiltration into a semi‐infinite medium for uniform initial conditions under either a time‐dependent flux or a constant head boundary condition. They capture the profile development from early to late time accurately due to the proper derivation of the wetting front speed in terms of the time‐varying surface boundary potential and flux rate. Algebraic expressions of theoretical and practical importance are derived in terms of well‐defined soil water parameters and include the effect of the antecedent moisture content in a specific fashion. Practical results include simple expressions for the surface moisture variation for variable rainfall patterns, time‐to‐ponding expressions and infiltration equations that account explicitly for the initial condition, formulas for the depth of the wetting zone, and algebraic equations for parameter estimation of the soil hydraulic parameters by inverse analysis.
Thermal artifacts in measurements of fine‐scale water level variation
Authors: McLaughlin Daniel L. , Cohen Matthew J.
Published: August 2011

The estimation of evapotranspiration (ET) from observed diurnal surface water level fluctuations may be highly sensitive to measurement errors. Available total pressure transducers (TPT) can provide requisite accuracy and precision but require atmospheric pressure compensation with equally accurate and precise barometric pressure transducers (BPTs). The BPT installation location determines the thermal setting, and sensor sensitivity to temperature could affect compensated water levels and any analyses requiring fine‐scale data. We investigated the effects of BPT installation location on compensated surface water levels and estimates of ET and net infiltration using the White method in four isolated forested wetlands in northern Florida. Water levels compensated with two differently positioned BPTs, one in buffered thermal conditions (BPTbuf; dry well at the TPT depth) and one in ambient temperatures (BPTamb; screened wellhead space), resulted in markedly different diurnal signatures. Both displayed the expected diurnal pattern, but compensation with BPTamb amplified diurnal variation by as much as 1.5 cm, greatly overestimated ET, and suggested net exfiltration, while BPTbuf compensation resulted in ET estimates similar to potential ET and suggested net infiltration. BPT temperature sensitivity followed diurnal temperature variation and generated systematic water level bias with the same signature as the expected trend, making these errors difficult to detect and underscoring the importance of proper BPT positioning.
Analysis of time compression approximations
Authors: Hogarth W. L. , Lockington D. A. , Barry D. A. , Parlange M. B. , Sander G. C. , Li L. , Haverkamp R. , Brutsaert W. , Parlange J.‐Y.
Published: August 2011

Time compression approximation (TCA) is a practical and often quite accurate tool to predict postponding infiltration for field applications. A modified approximation (MTCA) can be used just as easily and, in general, will reduce the error by about 50%. This is based on two results: (1) After ponding, TCA and MTCA predict very close infiltration rates; and (2) MTCA, but not TCA, uses the actual cumulative infiltration up to the ponding time. Thus, TCA has an additional error in its prediction of postponding infiltration. Previously, those results, including the 50% reduction in error, were observed numerically for linear and Burger's soils. They are illustrated here numerically with an actual soil (a Grenoble sand). More importantly, we developed a general analytical approximation for this problem and showed that it can provide a very convenient predictive tool which can then be used for arbitrary soil properties.
Stimulation of gross dimethylsulfide (DMS) production by solar radiation
Authors: Galí Martí , Saló Violeta , Almeda Rodrigo , Calbet Albert , Simó Rafel
Published: August 2011

Oceanic gross DMS production (GP) exerts a fundamental control on the concentration and the sea‐air flux of this climatically‐active trace gas. However, it is a poorly constrained process, owing to the complexity of the microbial food web processes involved and their interplay with physical forcing, particularly with solar radiation. The “inhibitor method”, using dimethyldisulfide (DMDS) or other compounds to inhibit bacterial DMS consumption, has been frequently used to determine GP in dark incubations. In the work presented here, DMDS addition was optimized for its use in light incubations. By comparing simultaneous dark and light measurements of GP in meso‐ to ultraoligotrophic waters, we found a significant enhancement of GP in natural sunlight in 7 out of 10 experiments. Such stimulation, which was generally between 30 and 80% on a daily basis, occurred throughout contrasting microbial communities and oceanographic settings.
Nighttime nitric oxide densities in the Southern Hemisphere mesosphere–lower thermosphere
Authors: Sheese P. E. , Gattinger R. L. , Llewellyn E. J. , Boone C. D. , Strong K.
Published: August 2011

Observations of NO2 continuum nightglow emissions from OSIRIS, on board the Odin satellite, have been used to derive an eight‐year time series of Southern Hemispheric nitric oxide densities, [NO]. OSIRIS is one of the very few current satellite instruments deriving ground state [NO] during the polar winter. The production of NO in this region is strongly dependent on energetic particle precipitation (EPP), and densities are therefore observed to vary with solar activity. Between 2003 and 2009, mean Antarctic winter [NO] in the mesosphere–lower thermosphere decreased by a factor of 3.7, and now, after the prolonged solar minimum that spanned 2008–2009, [NO] in this region is on the rise. As solar activity increases in the next few years, the production of NO is expected to increase. As downward advection readily transports NOx into the lower mesosphere and stratosphere, such an increase will lead to a greater potential for stratospheric ozone loss.
Two successive slow slip events evidenced in 2009–2010 by a dense GPS network in Guerrero, Mexico
Authors: Walpersdorf Andrea , Cotte Nathalie , Kostoglodov Vladimir , Vergnolle Mathilde , Radiguet Mathilde , Santiago José Antonio , Campillo Michel
Published: August 2011

A large slow slip event (SSE) had been expected for the Guerrero gap for 2010. It was actually observed with an onset in July 2009. Comparison with the preceding large SSEs, which occurred in 2002 and 2006, highlights both persistent characteristics of the Guerrero SSEs (e.g. the localization of slip in the seismogenic part of the subduction interface), and also particularities of the 2009/2010 event (namely two distinct slip patches on the fault interface moving consecutively). The long GPS time series and the density of the GPS network provide evidence that the Guerrero SSEs, like classical earthquakes, have complex features. Despite having very short and relatively regular repeat times (∼4 yr), Guerrero SSEs appear aperiodic. A shorter loading time before the 2009/2010 event than before the 2006 SSE seems to produce consistently reduced surface displacements for a group of stations in a core zone.
Forecasting volcanic eruptions and other material failure phenomena: An evaluation of the failure forecast method
Authors: Bell Andrew F. , Naylor Mark , Heap Michael J. , Main Ian G.
Published: August 2011

Power‐law accelerations in the mean rate of strain, earthquakes and other precursors have been widely reported prior to material failure phenomena, including volcanic eruptions, landslides and laboratory deformation experiments, as predicted by several theoretical models. The Failure Forecast Method (FFM), which linearizes the power‐law trend, has been routinely used to forecast the failure time in retrospective analyses; however, its performance has never been formally evaluated. Here we use synthetic and real data, recorded in laboratory brittle creep experiments and at volcanoes, to show that the assumptions of the FFM are inconsistent with the error structure of the data, leading to biased and imprecise forecasts. We show that a Generalized Linear Model method provides higher‐quality forecasts that converge more accurately to the eventual failure time, accounting for the appropriate error distributions. This approach should be employed in place of the FFM to provide reliable quantitative forecasts and estimate their associated uncertainties.
Phytoplankton behavior affects ocean mixed layer dynamics through biological‐physical feedback mechanisms
Authors: Sonntag S. , Hense I.
Published: August 2011

Biologically induced changes in physical oceanic properties through phytoplankton provide potential positive and negative feedback loops. In particular, surface floating cyanobacteria, which are expected to be favored from future environmental conditions and can form large surface mats, can increase light absorption and the surface albedo and decrease momentum input from the atmosphere by wind. In this work we study the effect of a changing phytoplankton community composition to one dominated by buoyant cyanobacteria on the physical oceanic properties. We use the water column model General Ocean Turbulence Model and set up an idealized biological model taking into account the phytoplankton species' characteristics as well as the effects of biology on physics. The model results show that an increase of buoyant cyanobacteria leads to substantial changes in the seasonal cycle of the mixed layer. The results furthermore indicate that the effects due to altered absorption and biologically induced reduction of the wind drag are larger than contrary effects due to changes in the surface albedo. Overall, our model results suggest that the development of cyanobacterial surface blooms and their feedbacks on light absorption and wind drag need to be taken into account in ocean models used for climate scenarios in order to capture changes in the dynamics of the upper ocean.
Capillary heterogeneity trapping of CO2 in a sandstone rock at reservoir conditions
Authors: Krevor Samuel C. M. , Pini Ronny , Li Boxiao , Benson Sally M.
Published: August 2011

The storage of CO2 in deep subsurface porous rocks is being developed worldwide for the mitigation of emission from large industrial sources such as power plants and steel manufacturing. A main concern of this technology is in ensuring that the upwardly buoyant CO2 does not migrate to the surface. Simulation studies suggest that substantial amounts of CO2 can be trapped within permeable sections of a reservoir by capillary forces and intra‐reservoir heterogenities, but there is little experimental observation of these phenomena. We report the results of CO2 core flooding experiments at high pressure and temperature performed to investigate the impact of natural capillary heterogeneity in a sandstone rock on CO2 saturation buildup and trapping. CO2 and water were injected through a Mt. Simon sandstone core at 9 MPa pore pressure and 50°C. The core had two regions of distinct capillarity: An upstream 10 cm long region of the core consisted of a relatively high permeability and homogenous sand. A downstream 3 cm long region of the core consisted of a low permeability region characterized by significant cross‐bedding and a high capillary entry pressure for CO2. During a drainage process of CO2 displacing water, CO2 builds up upstream of the capillary barrier. Once in place, CO2 on the upstream side of the barrier cannot be displaced during 100% water flooding leading to trapped saturations that are a factor 2–5 times higher than what would be expected from residual trapping alone.
El Niño and its relationship to changing background conditions in the tropical Pacific Ocean
Authors: McPhaden M. J. , Lee T. , McClurg D.
Published: August 2011

This paper addresses the question of whether the increased occurrence of central Pacific (CP) versus Eastern Pacific (EP) El Niños is consistent with greenhouse gas forced changes in the background state of the tropical Pacific as inferred from global climate change models. Our analysis uses high‐quality satellite and in situ ocean data combined with wind data from atmospheric reanalyses for the past 31 years (1980–2010). We find changes in background conditions that are opposite to those expected from greenhouse gas forcing in climate models and opposite to what is expected if changes in the background state are mediating more frequent occurrences of CP El Niños. A plausible interpretation of these results is that the character of El Niño over the past 31 years has varied naturally and that these variations projected onto changes in the background state because of the asymmetric spatial structures of CP and EP El Niños.
Energetics of a global ocean circulation model compared to observations
Authors: Thoppil Prasad G. , Richman James G. , Hogan Patrick J.
Published: August 2011

The majority of the eddy kinetic energy (EKE) in the ocean is found on scales of 50 km to 500 km, encompassed by mesoscale eddies and the meanders and rings of the boundary currents. Mesoscale eddies play a critical role in the dynamics of the ocean circulation with instabilities of the strong mean currents generating eddies in the upper ocean. Interactions between eddies transfer energy from the upper ocean to the deep ocean where eddies interact with bottom topography to generate abyssal mean flows and eddies transfer momentum back to the mean currents. The kinetic energy in a global Hybrid Coordinate Ocean Circulation Model (HYCOM) is compared with long‐term observations from surface drifters, geostrophic currents from satellite altimetry, subsurface floats and deep current meter moorings. HYCOM, configured at 1/12.5° (∼9 km, typical of the present generation of high resolution models), is deficient in EKE in both the upper and abyssal ocean (depths greater than 3000 m) by ∼21% and ∼24% respectively compared to surface drifting buoys and deep current meters. Increasing the model resolution to 1/25° (∼4.4 km) or injecting mesoscale eddies through the assimilation of surface observations in a 1/12.5° model increases the surface and the abyssal EKE to levels consistent with the observations. In these models, the surface (abyssal) EKE is increased by 23% (51%) and 15% (46%) for the higher resolution or data‐assimilative models, respectively, compared to the 1/12.5° non‐assimilative model. While data assimilation increases the EKE in both the upper and abyssal ocean, the kinetic energy of the mean flow in the upper ocean is decreased in the data‐assimilative hindcast.
Simultaneous Cassini VIMS and UVIS observations of Saturn's southern aurora: Comparing emissions from H, H2 and H3+ at a high spatial resolution
Authors: Melin H. , Stallard T. , Miller S. , Gustin J. , Galand M. , Badman S. V. , Pryor W. R. , O'Donoghue J. , Brown R. H. , Baines K. H.
Published: August 2011

Here, for the first time, temporally coincident and spatially overlapping Cassini VIMS and UVIS observations of Saturn's southern aurora are presented. Ultraviolet auroral H and H2 emissions from UVIS are compared to infrared H3+ emission from VIMS. The auroral emission is structured into three arcs – H, H2 and H3+ are morphologically identical in the bright main auroral oval (∼73°S), but there is an equatorward arc that is seen predominantly in H (∼70°S), and a poleward arc (∼74°S) that is seen mainly in H2 and H3+. These observations indicate that, for the main auroral oval, UV emission is a good proxy for the infrared H3+ morphology (and vice versa), but for emission either poleward or equatorward this is no longer true. Hence, simultaneous UV/IR observations are crucial for completing the picture of how the atmosphere interacts with the magnetosphere.
Performance of convection‐permitting hurricane initialization and prediction during 2008–2010 with ensemble data assimilation of inner‐core airborne Doppler radar observations
Authors: Zhang Fuqing , Weng Yonghui , Gamache John F. , Marks Frank D.
Published: August 2011

This study examines a hurricane prediction system that uses an ensemble Kalman filter (EnKF) to assimilate high‐resolution airborne radar observations for convection‐permitting hurricane initialization and forecasting. This system demonstrated very promising performance, especially on hurricane intensity forecasts, through experiments over all 61 applicable NOAA P‐3 airborne Doppler missions during the 2008–2010 Atlantic hurricane seasons. The mean absolute intensity forecast errors initialized with the EnKF‐analysis of the airborne Doppler observations at the 24‐ to 120‐h lead forecast times were 20–40% lower than the National Hurricane Center's official forecasts issued at similar times. This prototype system was first implemented in real‐time for Hurricane Ike (2008). It represents the first time that airborne Doppler radar observations were successfully assimilated in real‐time into a hurricane prediction model. It also represents the first time that the convection‐permitting ensemble analyses and forecasts for hurricanes were performed in real‐time. Also unprecedented was the on‐demand usage of more than 23,000 computer cluster processors simultaneously in real‐time.
Initial effect of the Fukushima accident on atmospheric electricity
Authors: Takeda M. , Yamauchi M. , Makino M. , Owada T.
Published: August 2011

Vertical atmospheric DC electric field at ground level, or potential gradient (PG), suddenly dropped by one order of magnitude at Kakioka, 150 km southwest from the Fukushima Dai‐ichi nuclear power plant (FNPP) right after the plant released a massive amount of radioactive material southward on 14 March, 2011. The PG stayed at this level for days with very small daily variations. Such a long‐lasting near‐steady low PG has never been observed at Kakioka. The sudden drop of PG with one‐hour time scale is similar to those associated with rain‐induced radioactive fallout after nuclear tests and the Chernobyl disaster. A comparison with the PG data with the radiation dose rate data at different places revealed that arrival of the radioactive dust by low‐altitude wind caused the PG drop without rain. Furthermore, the PG might have reflected a minor release several hours before this release at the distance of 150 km. It is recommended that all nuclear power plant to have a network of PG observation surrounding the plant.
The unique 2009–2010 El Niño event: A fast phase transition of warm pool El Niño to La Niña
Authors: Kim WonMoo , Yeh Sang‐Wook , Kim Joo‐Hong , Kug Jong‐Seong , Kwon MinHo
Published: August 2011

The latest El Niño event in 2009–2010, which is classified as warm pool El Niño, holds a unique ground in that it marks the strongest warming signal in the central Pacific but rapidly decays to strong La Niña. The strong eastward‐propagating cold anomaly at the subsurface level is found to be a key factor, and two possible mechanisms are suggested that resulted in the fast phase transition of the 2009–2010 event: 1) The anomalously warm Indian Ocean induces a surface easterly over the western edge of the Pacific, which generates the forced Kelvin waves. 2) The record‐breaking high sea surface temperature in the central Pacific excites a strong Rossby response that is to be reflected as an upwelling Kelvin wave at the western boundary. The strong subsurface anomaly then propagates eastward and results in an unusually fast phase transition of the 2009–2010 warm pool El Niño event.
Quantitative estimation of the underwater radiance distribution
Authors: Lewis Marlon R. , Wei Jianwei , Van Dommelen Ronnie , Voss Kenneth J.
Published: August 2011

Within the Radiance in a Dynamic Ocean (RaDyO) program, we have created and deployed a high dynamic range camera that can resolve the full spherical radiance distribution at the ocean surface and at depth. We present here the first results from deployments of the camera in near‐surface water in eutrophic, mesotropic, and oligotropic environments. The instrument resolves the dynamics and fine structure of both the downwelling and upwelling radiance distribution and its variation with depth in these optically diverse water types. The various irradiances (Ed, Eu, Eo, Eou, and Eod) are computed by integration. The distribution functions (e.g., the average cosines) are computed directly, as are the various diffuse attenuation coefficients. The fully specified radiance field therefore provides all the pertinent information to derive not only all of the apparent optical properties but, in principle, the inherent optical properties such as the absorption coefficient and the phase function as well. Comparison of the measured radiance field to independent measurements has shown very good agreement.
Polarized light field under dynamic ocean surfaces: Numerical modeling compared with measurements
Authors: You Yu , Kattawar George W. , Voss Kenneth J. , Bhandari Purushottam , Wei Jianwei , Lewis Marlon , Zappa Christopher J. , Schultz Howard
Published: August 2011

As part of the Radiance in a Dynamic Ocean (RaDyO) program, we have developed a numerical model for efficiently simulating the polarized light field under highly dynamic ocean surfaces. Combining the advantages of the three‐dimensional Monte Carlo and matrix operator methods, this hybrid model has proven to be computationally effective for simulations involving a dynamic air‐sea interface. Given water optical properties and ocean surface wave slopes obtained from RaDyO field measurements, model‐simulated radiance and polarization fields under a dynamic surface are found to be qualitatively comparable to their counterparts from field measurements and should be quantitatively comparable if the light field measurement and the wave slope/water optical property measurements are appropriately collocated and synchronized. This model serves as a bridge to connect field measurements of water optical properties, wave slopes and polarized light fields. It can also be used as a powerful yet convenient tool to predict the temporal underwater polarized radiance in a real‐world situation. When appropriate surface measurements are available, model simulation is shown to reveal more dynamic features in the underwater light field than direct measurements.
Dielectric signatures of adsorbed and salty liquid water at the Phoenix landing site, Mars
Authors: Stillman David E. , Grimm Robert E.
Published: August 2011

The real part of the dielectric permittivity of the Martian regolith was measured by the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix lander. We interpret these data using laboratory measurements of permittivity as a function of H2O and salt content, soil type, and temperature. Due to variability in sensor coupling, we focus on data taken at one locality (“Vestri”) three separate times, spanning multiple sols. A daytime increase in permittivity suggests progressive melting of a heterogeneous, disconnected, salty ice with a eutectic temperature of ∼239 K, which is close to the eutectic temperatures of NaClO4 or MgCl2. We found no evidence for Mg(ClO4)2. NaClO4 and MgCl2 are consistent with precipitation by freezing following a prior epoch of high obliquity. The evaporation of diurnal briny meltwater is inhibited by surface tension in small pores. An increase in permittivity occurred on the night of sol 70 that coincided with surface frost and measurement of a decrease in atmospheric water vapor. The permittivity jump can be matched by an increase in adsorbed H2O from ∼1 monolayer to 3 monolayers in an analog soil with a Viking‐like specific surface area (17 m2/g). However, the amount of adsorbed H2O is an order of magnitude larger than that inferred to have precipitated during the night. We suggest that the electrical signature of adsorbed water on Mars is stronger than we measured in the laboratory, possibly due to radiation damage of the regolith.
Sea surface temperatures did not control the first occurrence of Hudson Strait Heinrich Events during MIS 16
Authors: Naafs B. David A. , Hefter Jens , Ferretti Patrizia , Stein Ruediger , Haug Gerald H.
Published: August 2011

Hudson Strait (HS) Heinrich Events, ice‐rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial‐scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(‐like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice‐rafting debris (IRD), we detect the first HS Heinrich(‐like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high‐resolution alkenone‐based SST record demonstrates that the first HS Heinrich(‐like) Event did not coincide with low SSTs. Thus, the HS Heinrich(‐like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid‐Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.
Ionospheric structures correlated with Anatolian surface features
Authors: Garner T. W. , Slack C. M. , Mehta K. , Scholze A. , Mahrous A. M.
Published: August 2011

A UHF/VHF beacon receiver located in Helwan, Egypt, frequently observes structures in ΔTEC/Δt measurements (where TEC is total electron count), where the F region (300 km) intercept of the radio rays crosses the steep topographic gradients associated with the Anatolian Plateau. There are three classes of structures: bumps, ripples and waves. A bump is defines as a single spatial ΔTEC/Δt peak with a peak‐to‐trough amplitude of at least 0.01 TECU/s (1 TEC unit (TECU) = 1016 electrons/m2) that is at least 1° wide in F region latitude. A ripple is a bump with smaller structures on either side of the central bump. Finally, waves have amplitudes ≥0.01 TECU/s with several roughly equal peaks. These features were observed repeatedly in a number passes from 31 August to 30 November 2008. Over half of passes had either a bump (34.6%), a ripple (18.2%) or a wave (6.3%). Most of these structures occur near areas with large orographic gradients. The prevailing surface wind blows across the mountains when bumps and ripples are observed. These correlations suggest that the local ionosphere is affected by the ground topography, most likely through the orographic lifting and the associated gravity waves.
Estimating ionospheric delay using kriging: 1. Methodology
Authors: Sparks Lawrence , Blanch Juan , Pandya Nitin
Published: August 2011

The Wide Area Augmentation System (WAAS) is an augmentation of the Global Positioning System (GPS) that provides safe and reliable use of GPS signals for airline navigation over much of North America. Ever since WAAS was first commissioned in July of 2003, the vertical delay estimate at each node in the WAAS ionospheric grid has been determined from a planar fit of slant delay measurements, projected to vertical using an obliquity factor specified by the standard thin shell model of the ionosphere. In a future WAAS upgrade (WAAS Follow‐On Release 3), however, the vertical delay will be estimated by an established, geo‐statistical technique known as kriging. When compared to the planar fit model, the kriging model is generally found to match better the observed random structure of the vertical delay. This paper presents the kriging methodology to be used to estimate the vertical delay and its uncertainty at each ionospheric grid point. In addition, it provides examples of the improvement in delay accuracy achieved. Under disturbed conditions, the implementation of kriging reduces the magnitude of the root mean square fit residual by up to 15%.
Time domain near‐field to near‐field transformation using a spherical‐multipole approach
Authors: Klinkenbusch L.
Published: August 2011

The time domain electromagnetic field of an arbitrary localized radiating structure can be efficiently obtained by means of a time domain spherical‐multipole expansion valid outside a minimum sphere enclosing all radiating elements. The method is based on the Fourier transform of the frequency‐domain spherical‐multipole expansion and on a finite expansion of the spherical Hankel function of the 2nd kind leading to a triple sum of multipoles instead of the well‐known double sum in case of the frequency‐domain multipole expansion. It is shown that those time domain multipole amplitudes which are relevant only in the near‐field can be recursively deduced from the time domain amplitudes dominant in the far field. The latter can be obtained by a recently proposed spherical‐multipole based time domain near‐field to far‐field algorithm which has been shown to be particularly suited for the Finite‐Difference Time Domain (FDTD) method.
A projection penalization approach for the high‐order DG‐FEM in the time domain
Authors: Gjonaj E. , Weiland T.
Published: August 2011

We investigate the Discontinuous Galerkin‐Finite Element Method (DG‐FEM) for the solution of Maxwell's equations in the time domain. The charge conservation laws are known to be violated by this method which consequently gives rise to spurious numerical solutions. It is, however, possible to introduce additional constrains in the formulation which impose charge conservation either exactly or approximately. In this work, a new approach based on a topological orthogonal projector into a high‐order H(curl)‐conforming approximation space is proposed. Using this approach we derive a constrained DG‐FEM formulation which is strictly free of spurious modes. Furthermore, we construct a time domain penalization scheme which allows to separate the spurious modes related to unphysical charges from the physical solutions while maintaining accuracy and energy conservation.
A new approach for identifying ionospheric backscatter in midlatitude SuperDARN HF radar observations
Authors: Ribeiro A. J. , Ruohoniemi J. M. , Baker J. B. H. , Clausen L. B. N. , de Larquier S. , Greenwald R. A.
Published: August 2011

The Super Dual Auroral Radar Network (SuperDARN) is a network of HF radars that are traditionally used for monitoring phenomena in the Earth's ionosphere at high latitudes. The radar backscatter is due primarily to reflections from plasma irregularities in the ionosphere, known as ionospheric scatter, and to signal reflected from the ground, known as ground scatter. In recent years, SuperDARN has expanded to midlatitudes to provide improved coverage of the auroral region during times of enhanced geomagnetic activity. In addition to high‐speed auroral flows, the radars commonly see a variety of low‐velocity plasma drift associated with the quiet time midlatitude ionosphere. The traditional method of distinguishing between scatter types in SuperDARN data was developed for high latitudes and depends solely on the Doppler velocity and Doppler spectral width of each data point. This method has proven inadequate for identifying quiet time midlatitude ionospheric scatter. In this paper, we present a new technique for the classification of SuperDARN data, which operates on a distributed range time basis and involves procedures similar to “depth first search.” Using the new method for classification of ground and ionospheric scatter, we show a dramatic improvement in the determination of ionospheric scatter within extended (>1 h) events. Compared to the traditional method, the number of ionospheric measurements resolved increases by more than 50%. The new classification algorithm identifies discrete events of ionospheric scatter and can be applied to statistical analysis of event occurrence and characteristics.
Extremely anisotropic boundary conditions and their optical applications
Authors: Alù Andrea , Engheta Nader
Published: August 2011

Following the idea of realizing an extremely anisotropic scatterer by combining positive and negative permittivity materials in a composite sphere, we discuss here some of the features and potential applications of extreme boundary conditions in optics. We analyze the scattering features and surface boundaries of an isolated sphere composed of two hemispheres with oppositely signed real parts of permittivity, whose local surface impedance can vary homogeneously from zero to infinity upon a simple 90 degree rotation of the impinging electric polarization, and we theoretically explore how combinations of these particles and other related geometries may tailor and boost the optical radiation of localized sources, e.g., quantum dots and molecular emitters.
A newly developed UHF radiotelescope for interplanetary scintillation observations: Solar Wind Imaging Facility
Authors: Tokumaru M. , Kojima M. , Fujiki K. , Maruyama K. , Maruyama Y. , Ito H. , Iju T.
Published: August 2011

A large‐aperture radiotelescope called the Solar Wind Imaging Facility (SWIFT) has been developed at the Toyokawa Observatory of the Solar‐Terrestrial Environment Laboratory (STEL), Nagoya University. The SWIFT is dedicated to interplanetary scintillation (IPS) observations of the solar wind at 327 MHz, the same frequency as that of the existing STEL IPS system. The aim of this instrument is to improve the spatial and temporal resolutions of tomographic reconstructions from STEL IPS observations by increasing the number of usable lines of sight within a given time period. The SWIFT consists of a pair of asymmetric cylindrical parabolic reflector antennas with an aperture size of 108 m (N‐S) by 19 m (E‐W), and a 192‐element phased array receiver system which forms a single beam steerable between 60°S and 30°N with respect to the zenith. Since the antenna beam is fixed in the local meridian, IPS observations are taken around the time of meridian transit for each source. The performance of the SWIFT has been tested using preliminary observations for strong discrete sources and diffuse galactic background.
Beam broadening effect on Doppler spectral width of wind profiler
Authors: Chen Meng‐Yuan , Chu Yen‐Hsyang
Published: August 2011

In this study, the beam broadening effect on Doppler spectrum was investigated with theoretical derivation and numerical simulation. It is found that vertical wind may cause a beam broadening spectrum not in exact Gaussian shape. Moreover, the azimuth angle variation of the beam broadening spectral width in the presence of vertical wind is more significant than that in the absence of the vertical wind. The widths of the beam broadening spectra for oblique beams pointing in the opposite directions are different. Further, wind shear effect on the azimuth anisotropy of the beam broadening spectral width is also studied in this article. In the context of three‐dimensional wind field with vertical shear of horizontal wind, an analytic expression of the beam broadening spectral width is proposed. A comparison between the analytic expression and simulation result suggests that the expression is applicable in general.
Transient analysis of microwave Gunn oscillator using extended spectral element time domain method
Authors: Xu Kan , Chen Rushan , Sheng Yijun , Fu Ping , Chen Chuan , Yan Qingshang , Yu YanYan
Published: August 2011

In this study, the microwave Gunn oscillator is analyzed by a hybrid electromagnetic circuit simulator, which is based on the spectral element time domain (SETD) method. The Gunn diode within the oscillator is treated as a lumped element, while the passive distributed part of the oscillator is modeled using the SETD method. In order to incorporate the contribution of the Gunn diode into the SETD context, the SETD method is extended by introducing a lumped current term into the second‐order vector wave equation. When Galerkin's method is used for the space discretization and the central difference scheme is used for time stepping, a global SETD system involving the Gunn diode is assembled. Furthermore, the global system matrix is block‐diagonal and the inversion of this matrix can be easily implemented, and thus the extended SETD method is a fully explicit solver and CPU time can be significantly reduced. By virtue of this method, the strong nonlinear feature of the Gunn oscillator is well characterized in the time domain, such as the phenomenon of injection locking. Numerical results demonstrate the ability and effectiveness of the extended SETD method for the fast analysis of microwave Gunn oscillator.
Design and analysis of low frequency strut‐straddling feed arrays for EVLA reflector antennas
Authors: Harun M. , Ellingson S. W.
Published: August 2011

A new feed system is designed for operation below 100 MHz. The only existing system on the EVLA operating below 100 MHz is the “4 m” (74 MHz) system which uses crossed half‐wave dipoles located in front of the Cassegrain subreflector as the feed. However, the dipole feeds of this system introduce blockage, and a reduction in system sensitivity (estimated to be ∼6% at 1.4 GHz) is observed at higher frequency bands; hence the dipoles are removed most of the time. An alternative feed concept is therefore proposed in this paper. The proposed system appears to reduce sensitivity degradation at 1.4 GHz by 3% and thus might be permanently mounted. Moreover, the new system has sensitivity comparable to the existing system at frequencies below 100 MHz. The feed for this system consists of dipoles mounted between the adjacent struts of the reflector and is thus referred to as a strut‐straddling feed array. This design and the analysis methodology used in this paper should be applicable in meeting the contiguous frequency coverage requirement (50–470 MHz) of the new low frequency system proposed for the EVLA. Also, it may be applied in the modification of other existing large reflector antennas for low frequency operation.
Characterization of system calibration parameters for high gain dual polarization satellite beacon diagnostics of ionospheric variations
Authors: Anderson James P. , Phelps Eric B. , Erickson Philip J. , Lind Frank D. , Coster Anthea J. , Langston Glen I.
Published: August 2011

We present complementary methods for calibrating the dual‐polarization feed of a ground‐based tracking antenna used for ionospheric measurements. Several calibration parameters are defined which describe the state of the measurement equipment. These key parameters then form a transformational Mueller matrix which may be used to remove system bias in data received from known beacons. Several strategies are developed to quantify these time‐dependent distortions for the Green Bank 140′ diameter antenna and receiver, although these methods could be applied to other similar systems. One approach quantifies the receiver biases in terms of amplitude and gain differentials. This is accomplished by measuring polarimetric differences between channels using a test signal with a known amplitude and phase. Due to variations over time periods on the scale of a beacon track, this procedure is most effective when the calibration signal is injected concurrently with the beacon measurements. To determine antenna feed distortion, including cross‐polarization and ellipticity, data is recorded from an external celestial source. An example shows the effectiveness of this calibration technique by comparing calibrated data to data without correction. Error estimates of the calibration parameters in the example establish an upper bound of 0.22 TEC units for measurements at 150 MHz along track.
Validation of emission measurement site above 1 GHz
Authors: Tosaka Toshihide , Yamanaka Yukio
Published: August 2011

The International Special Committee on Radio Interference (CISPR) (CISPR 16‐1‐4) measured the site voltage standing wave ratio (SVSWR) to determine the site imperfections of a radiated emission measurement site above 1 GHz. However, the technical background of the SVSWR measurement parameters is not included in this standard. In this paper, we first propose a simple propagation model to evaluate the relation between the SVSWR parameters and the measurement values. The calculated SVSWR values of the total displacement defined in CISPR 16‐1‐4 (0.4 m) were almost the same as those obtained when the transmitting antenna was moved up to 2.0 m from the reference point. In addition, the SVSWR values obtained with a 50 MHz frequency resolution, as defined in CISPR 16‐1‐4 were almost the same as those values obtained with a 1 MHz resolution. From these results, we confirmed that the CISPR measurement parameters are reasonable and effective.
A locally one‐dimensional finite difference time domain method for the analysis of a periodic structure at oblique incidence
Authors: Wakabayashi Y. , Shibayama J. , Yamauchi J. , Nakano H.
Published: August 2011

An implicit finite difference time domain (FDTD) method is developed to analyze periodic structures at oblique incidence. The split‐field technique is applied to the locally one‐dimensional (LOD) FDTD method with the periodic boundary condition. In addition, the dispersion control parameters are introduced to reduce the numerical dispersion error. The effectiveness of the present method is investigated through the analysis of a photonic band gap structure. It is shown that the computational time is reduced to ≅16% of that of the explicit FDTD method with acceptable results being maintained. As an application, a broadband mirror consisting of a subwavelength grating is analyzed and discussed.
Point‐to‐point ionospheric ray tracing by a direct variational method
Authors: Coleman Christopher J.
Published: August 2011

While the Haselgrove ray tracing equations are well suited to situations where the ray launch direction is known, they are less effective for situations where only the end points of the ray are known. In such cases, many rays must be traced from the launch point in order to home in on the landing point. An alternative approach is to directly solve the variational principle from which the Haselgrove equations are derived. Such an approach is well suited to the point‐to‐point ray tracing, but poses several technical difficulties. In this paper we overcome these difficulties and show that a direct approach can indeed provide an effective means of point‐to‐point ray tracing.
Detrital zircon geochronology of pre‐Tertiary strata in the Tibetan‐Himalayan orogen
Authors: Gehrels G. , Kapp P. , DeCelles P. , Pullen A. , Blakey R. , Weislogel A. , Ding L. , Guynn J. , Martin A. , McQuarrie N. , Yin A.
Published: August 2011

Detrital zircon data have recently become available from many different portions of the Tibetan–Himalayan orogen. This study uses 13,441 new or existing U‐Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan–Qilian Shan–Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet–Himalaya region during Neoproterozoic to Mesozoic time. First‐order conclusions are as follows: (1) Most ages from these crustal fragments are <1.4 Ga, which suggests formation in accretionary orogens involving little pre‐mid‐Proterozoic cratonal material; (2) all fragments south of the Jinsa suture evolved along the northern margin of India as part of a circum‐Gondwana convergent margin system; (3) these Gondwana‐margin assemblages were blanketed by glaciogenic sediment during Carboniferous–Permian time; (4) terranes north of the Jinsa suture formed along the southern margin of the Tarim–North China craton; (5) the northern (Tarim–North China) terranes and Gondwana‐margin assemblages may have been juxtaposed during mid‐Paleozoic time, followed by rifting that formed the Paleo‐Tethys and Meso‐Tethys ocean basins; (6) the abundance of Permian–Triassic arc‐derived detritus in the Lhasa and Qiangtang terranes is interpreted to record their northward migration across the Paleo‐ and Meso‐Tethys ocean basins; and (7) the arrival of India juxtaposed the Tethyan assemblage on its northern margin against the Lhasa terrane, and is the latest in a long history of collisional tectonism.