bluuebunny/crossref_metadata_2025 huggingface dataset:
Transformation superplasticity of water ice and ice containing SiO2 particulates
Authors: Schuh C. A. , Dunand D. C.
Published: December 2002

Transformation superplasticity is a deformation mechanism that occurs during the phase transformation of an externally stressed material. Pressure‐induced transformation superplasticity, which has long been postulated to occur in olivine in the Earth's interior, is shown to take place in water ice. Pure ice specimens were subjected to hydrostatic pressure cycling between 0 and 300 MPa to reversibly induce the I/II transformation (at 220 or 230 K) or the I/III transformation (at 240 K). When a small uniaxial compressive stress was applied during cycling, the specimens exhibited a uniaxial compressive strain (as large as 22% after a single cycle) proportional to the applied stress, in agreement with observations and theory for transformation superplasticity of metals and ceramics, induced by polymorphic thermal cycling. Additionally, specimens of ice containing 10 vol% SiO2 were deformed by this mechanism during the I/II transformation at 230 K. The presence of silica particles was found to enhance the deformation, rather than strengthen the ice. These experimental results are discussed both qualitatively and quantitatively in terms of existing models of transformation superplasticity and implications for the lithosphere rheology of the icy moons of the outer planets.
Barium in hemipelagic sediment of the northwest Pacific: Coupling with biogenic carbonate
Authors: Hyun Sangmin , Han Sang‐Joon , Taira Asahiko
Published: December 2002

Geochemical analyses were performed on three cores of hemipelagic sediment from the northwest Pacific Ocean (eastern edge of Shikoku Basin) dating from the last interglacial and glacial periods. The objective of this work is to delineate the relations between excess sedimentary barium [Ba(ex)] content and biogenic opal, carbonate, and organic carbon contents. Calculated Ba(ex) values show variable degrees of correlation with biogenic sediment fractions. The mass accumulation rate (MAR) of Ba(ex) covaries with the MARs of biogenic fractions, and in particular, with the MAR of biogenic carbonate (r2 = 0.68) and TOC (R2 = 0.50). Variable relations between barium and biogenic fractions in hemipelagic sediment provide detailed insight into the behavior of sedimentary barium. It appears that a significant proportion of the barium is affiliated with the carbonate fractions. Based on this strong correlation, carbonate dissolution rates of the last glacial to interglacial periods are estimated. Assuming that the MARs of carbonate and Ba(ex) covary, variation of the Ba/Ca ratio in sediments reflects the extent of postdepositional carbonate dissolution. The record of sedimentary Ba/Ca ratios exhibits striking difference between sediment of the glacial and interglacial periods, with a higher positive correlation during glacial intervals and a lower correlation during interglacial intervals; this pattern is the result of enhanced carbonate dissolution during interglacial times. Sedimentary Ba/Ca ratios may, therefore, be a useful tool for estimating the relative extent of carbonate dissolution. Ba(ex) curves and patterns in sedimentary Ba/Ca ratios resemble the established Pacific carbonate dissolution record (i.e., enhanced dissolution during interglacial times and reduced dissolution during glacial times) as suggested by previous studies. Variations in sedimentary Ba/Ca ratios strongly support an interpretation of pronounced glacial‐interglacial fluctuation in carbonate dissolution in the northwest Pacific Ocean. An especially pronounced dissolution event is evident at 75 kyr during the transition interval form interglacial to glacial conditions. This suggests that Ba(ex) may be used as a proxy for evaluating carbonate dissolution in hemipelagic settings, but further study of the association between barium content and carbonate dissolution will be required to confirm this.
Three‐dimensional electromagnetic wave polarizations in a plasma
Authors: Dong Bao , Yeh K. C.
Published: December 2002

It is known that electromagnetic waves propagating in a magnetized cold plasma are not necessarily purely transverse waves. The existence of a longitudinal component implies that the normal to the polarization plane is not, in general, parallel to the propagation vector. The offset angle between these two directions is computed for each characteristic wave. Our investigation shows that the offset angle can be large under certain conditions. For a wave field that is a superposition of two characteristic waves, its polarization plane wobbles in space as the field propagates while its normal precesses. Several examples are considered, computed, and given to illustrate the 3D nature of wave polarizations.
Direction finding using a linear three‐element interferometer approach
Authors: Webster A. R. , Jones J. , Ellis K. J.
Published: December 2002

Results are presented from the operation of a system employing a linear three‐element antenna array to determine the angle of arrival of a radio signal in a low‐angle terrestrial situation. Based on a system successfully employed in determining the direction in space of meteor echoes at elevated angles, accurate direction determination was obtained in three widely varied terrestrial environments. While such a system would be susceptible to strong multipath signals, and therefore unsuitable for congested cellular environments, there are applications, for example emergency beacon location, in which this does not present a significant problem.
Autonomous estimation of plasmasphere content using GPS measurements
Authors: Mazzella Andrew J. , Holland Elizabeth A. , Andreasen Angela M. , Andreasen C. Charley , Rao G. Susan , Bishop Gregory J.
Published: December 2002

The plasmasphere (also denoted as the protonosphere) is a large toroidal domain of light ionized particles situated above the ionosphere and confined by the Earth's magnetic field. While plasmaspheric charge densities are considerably less than those of the ionosphere, the large extent of the plasmasphere can produce significant charge column densities, or total electron content (TEC), for lines‐of‐sight passing through the plasmasphere. A method for Self‐Calibration of Range Errors (SCORE) has been developed previously both to determine combined bias calibration values for GPS receivers and satellites and to calculate absolute TEC values for the ionosphere. An enhanced SCORE process, described here, retains the “self‐calibration” feature of the original SCORE process, by not requiring any measurements beyond those performed by the GPS receiver system being calibrated. The enhanced SCORE process also determines a characteristic plasmasphere amplitude parameter, thus providing an autonomous determination of both the ionospheric and plasmaspheric TEC. Case studies for a near‐equatorial site are presented, with model parameters derived from 1998 data being applied to determine ionospheric and plasmaspheric TEC for measurements made in 1999.
Conductivity and resistivity tensor rotation for surface impedance modeling of an anisotropic half‐space
Authors: Wilson Glenn A. , Thiel David V.
Published: December 2002

The electromagnetic surface impedance of a half‐space with inclined conductivity anisotropy can be derived from the isotropic half‐space solution provided the conductivity term used in the expressions is the effective horizontal conductivity. For a TM‐mode plane wave incidence, the effective horizontal conductivity must be derived from the tensor rotation of the resistivity tensor and not from the tensor rotation of the conductivity tensor. For a TE‐mode incident with the same geometry as the TM‐mode, the surface impedance is independent upon the inclined anisotropy. This same formulation can then be extended to a multiple layered half‐space where each layer has an inclined anisotropy. For an anisotropic half‐space with coefficient of anisotropy of 4, with a horizontal conductivity of 0.001 S/m inclined at 45° with respect to the horizontal plane, the magnitude of the surface impedance calculated using the resistivity tensor rotation is approximately 53% larger than the magnitude of the surface impedance calculated using the conductivity tensor rotation.
A localized beam representation of high‐frequency wave fields using a Wilson basis
Authors: Arnold J. M.
Published: December 2002

A rigorous orthogonal basis is described for the representation of high‐frequency wave fields using beam superpositions parameterized by both spatial position and spatial direction of each beam. This representation has many desirable features not found in conventional phase‐space‐based expansions such as the discrete or continuous Gabor schemes. The representation is highly efficient in the geometrical optics limit, that is, when it is strongly localized in the phase‐space.
Tidal meanders
Authors: Marani Marco , Lanzoni Stefano , Zandolin Diego , Seminara Giovanni , Rinaldo Andrea
Published: December 2002

Observational evidence is presented on the geometry of meandering tidal channels evolved within coastal wetlands characterized by different tidal, hydrodynamic, topographic, vegetational and ecological features. New insight is provided on the geometrical properties of tidal meanders, with possible dynamic implications on their evolution. In particular, it is shown that large spatial gradients of leading flow rates induce important spatial variabilities of meander wavelengths and widths, while their ratio remains remarkably constant in the range of scales of observation. This holds regardless of changes in width and wavelength up to two orders of magnitude. This suggests a locally adapted evolution, involving the morphological adjustment to the chief landforming events driven by local hydrodynamics. The spectral analysis of local curvatures reveals that Kinoshita's model curve does not fit tidal meanders due to the presence of even harmonics, in particular the second mode. Geometric parameters are constructed that are suitable to detect possible geomorphic signatures of the transitions from ebb‐ to flood‐dominated hydrodynamics, here related to the skewness of the tidal meander. Trends in skewness, however, prove elusive to measure and fail to show detectable patterns. We also study comparatively the spatial patterns of evolution of the ratios of channel width to depth, and the ratio of width to local radius of curvature. Interestingly, the latter ratio exhibits consistency despite sharp differences in channel incision. Since the degree of incision, epitomized by the width‐to‐depth ratio, responds to the relevant erosion and migrations mechanisms and is much sensitive to vegetation and sediment properties, it is noticeable that we observe a great variety of landscape carving modes and yet recurrent planar features like constant width/curvature and wavelength/width ratios.
Long‐term assessment of bias adjustment in radar rainfall estimation
Authors: Borga Marco , Tonelli Fabrizio , Moore Robert J. , Andrieu Hervé
Published: December 2002

A comprehensive analysis of precipitation estimation by weather radar operated over hilly terrain is performed. The study is based on a nearly continuous 3‐year database of weather radar observations and hourly rain accumulations from a dense rain gauge network located in southwest England. Radar rainfall processing scenarios associated with correction for systematic and range dependent errors are investigated. The focus of the study is in demonstrating the impact of radar range effects at short to medium (less than 70 km) distances and determining the significance of coupling mean‐field and range‐related bias adjustment in radar rainfall estimation. The investigation is performed on the basis of climatological radar rainfall statistics for selected radar sectors and radar‐rain gauge comparisons over a densely instrumented medium size (135 km2) basin located at 35–40 km from the radar. The application of the combined adjustment is shown to reduce overall error over the catchment by 24%. The analysis suggests that gauge‐based radar adjustment can be properly applied only when homogeneity in the accuracy of the radar rainfall estimates with respect to range and scanning elevation is ensured.
Extracting low‐resolution river networks from high‐resolution digital elevation models
Authors: Olivera Francisco , Lear Mary S. , Famiglietti James S. , Asante Kwabena
Published: December 2002

Including a global river network in the land component of global climate models (GCMs) is necessary in order to provide a more complete representation of the hydrologic cycle. The process of creating these networks is called river network upscaling and consists of lowering the resolution of already available fine networks to make them compatible with GCMs. Fine‐resolution river networks have a level of detail appropriate for analysis at the watershed scale but are too intensive for global hydrologic studies. A river network upscaling algorithm, which processes fine‐resolution digital elevation models to determine the flow directions that best describe the flow patterns in a coarser user‐defined scale, is presented. The objectives of this study were to develop an algorithm that advances the previous work in the field by being applicable at a global scale, allowing for the upscaling to be performed in a projected environment, and generating evenly distributed flow directions.
Burgers' equation: A general nonlinear solution of infiltration and redistribution
Authors: Basha H. A.
Published: December 2002

The general one‐dimensional solution of Burgers equation is developed using a series of transformations and the Green's function method. The solution gives the distribution of the reduced water content in a semi‐infinite and in a finite domain for arbitrary initial moisture distributions. The boundary condition at the soil surface can be either a time‐dependent flux or constant water content while the bottom boundary condition for the finite case is a constant water content value. Explicit results for a uniform, discrete, hydrostatic and steady state initial moisture profiles are derived. An analytical solution that spans the preponding to the postponding period of rainfall is also obtained. Expressions for the time to ponding and infiltration equations are presented for the various cases of initial moisture profiles and a shallow water table. These results allowed the quantification of the error in the various approximations to the time to ponding and the cumulative infiltration in the postponding period.
Contour‐based digital elevation modeling of watershed erosion and sedimentation: Erosion and sedimentation estimation tool (EROSET)
Authors: Sun H. , Cornish P. S. , Daniell T. M.
Published: December 2002

An erosion and sedimentation model, erosion and sedimentation estimation tool (EROSET), was developed and applied to a watershed in Happy Valley, South Australia. The model simulates the dynamics of event runoff, soil detachment, and transport processes. The erosion and sedimentation model is able to predict watershed erosion and deposition for storm events at an element as well as watershed scale. The model was developed and incorporated into an existing rainfall‐runoff model based on a contour‐based digital elevation framework. It combines the use of the USLE data source and extended erosion and transportation modeling into a distributed and intra storm erosion and deposition analysis. This results in storm‐based, time‐variant, distributed erosion and deposition modeling in the watershed for both storm‐based and long‐term sediment estimation. The modeling can better enable land managers to identify the areas in a watershed where erosion and deposition may occur. The modeled processes and results can be related to total storm erosion estimated by MUSLE, although they operate on different temporal and spatial frames. Satisfactory modeling results were obtained with very limited calibration which compares well with other studies.
On the dialog between experimentalist and modeler in catchment hydrology: Use of soft data for multicriteria model calibration
Authors: Seibert Jan , McDonnell Jeffrey J.
Published: December 2002

The dialog between experimentalist and modeler in catchment hydrology has been minimal to date. The experimentalist often has a highly detailed yet highly qualitative understanding of dominant runoff processes; thus there is often much more information content on the catchment than we use for calibration of a model. While modelers often appreciate the need for “hard data” for the model calibration process, there has been little thought given to how modelers might access this “soft” or process knowledge. We present a new method where soft data (i.e., qualitative knowledge from the experimentalist that cannot be used directly as exact numbers) are made useful through fuzzy measures of model simulation and parameter value acceptability. We developed a three‐box lumped conceptual model for the Maimai catchment in New Zealand, a particularly well‐studied process‐hydrological research catchment. The boxes represent the key hydrological reservoirs that are known to have distinct groundwater dynamics, isotopic composition, and solute chemistry. The model was calibrated against hard data (runoff and groundwater levels) as well as a number of criteria derived from the soft data (e.g., percent new water, reservoir volume, etc.). We achieved very good fits for the three‐box model when optimizing the parameter values with only runoff (Reff = 0.93). However, parameter sets obtained in this way showed in general a poor goodness of fit for other criteria such as the simulated new water contributions to peak runoff. Inclusion of soft data criteria in the model calibration process resulted in lower Reff values (around 0.84 when including all criteria) but led to better overall performance, as interpreted by the experimentalist's view of catchment runoff dynamics. The model performance with respect to soft data (like, for instance, the new water ratio) increased significantly, and parameter uncertainty was reduced by 60% on average with the introduction of the soft data multicriteria calibration. We argue that accepting lower model efficiencies for runoff is “worth it” if one can develop a more “real” model of catchment behavior. The use of soft data is an approach to formalize this exchange between experimentalist and modeler and to more fully utilize the information content from experimental catchments.
On the porous‐continuum modeling of gravity‐driven fingers in unsaturated materials: Extension of standard theory with a hold‐back‐pile‐up effect
Authors: Eliassi Mehdi , Glass Robert J.
Published: December 2002

The traditional Richards equation (RE) in combination with standard monotonic properties (constitutive relations and hysteretic equations of state) has been shown to lack critical physics required to model gravity‐driven fingering (GDF). We extend the RE with an experimentally observed hold‐back‐pile‐up (HBPU) effect not captured in the standard porous‐continuum RE formulation. We postulate the HBPU effect is tied to wetting front sharpness and can be mathematically formulated in a variety of ways to include hypodiffusive, hyperbolic, and mixed spatial‐temporal forms involving respectively a Laplacian, a second‐order derivative in time, and a Laplacian of a first‐order derivative in time of the state variables. For each, we can infer an extended flux relation comprised of the Darcy‐Buckingham flux plus an additional component due to the HBPU effect. Extended flux relations that are mathematically similar to each can be found in the single‐phase and multiphase flow literature, however, all with very different underlying conceptualizations of the possible physics.
Input and distribution of photosynthesized carbon in a flooded rice soil
Authors: Lu Yahai , Watanabe Akira , Kimura Makoto
Published: December 2002

Photosynthesis by terrestrial vegetations is the driving force for the C cycling between Earth and the atmosphere. To evaluate the impacts of photosynthates on the dynamics of soil organic matter (SOM) in a rice‐soil system, we conducted an isotopic pulse‐chase labeling experiment in a microcosm system. Pulse labeling of 13C was performed to rice plants at varying ages. Destructive samplings of plants and soils were conducted at the end of the growing season. The 13C distribution in the aboveground mass increased from 45–52% of the net assimilation during the tillering stage of rice to 78–90% of that during the maturing stage of rice, whereas the proportion that was retained in roots decreased from 28 to 2%. About 1–5% of the net assimilation was incorporated into soil and mean 19.5% was lost presumably by respirations of plants and soil microorganisms. The percentage distribution of assimilates to soil was significantly exponentially correlated to the rate of root growth. Over a growing season, the estimated net plant C input into soil was 200 kg C ha−1. Most of the incorporated organic C was present as humin‐like substances at the end of the season. Rice plants also produced 865 kg C ha−1 of root residues which were retained in soil after harvest. It was, therefore, considered that totally about 1065 kg C ha−1 of the atmospheric CO2 was fixed in soil after a growing season of plants. The increase of soil organic C content, however, was less than the net plant C input, suggesting that the mineralization of native SOM occurred concomitantly.
Motion of the Easter hot spot relative to Hawaii and Louisville hot spots
Authors: Steinberger Bernhard
Published: December 2002

Hot spots have been widely used as a fixed reference frame for plate motions; however, it should be expected that the underlying plumes are advected in a dynamic mantle. Here a simple numerical model of hot spot motion due to large‐scale mantle flow is applied to the Easter, Hawaii and Louisville hot spots. Computations are performed for a range of different buoyant plume rising speeds, ages and locations, mantle density and viscosity structures, etc. It is shown that a westward motion of the Easter hot spot at several cm per year relative to Hawaii and Louisville hot spots is a robust result for a large range of model parameters. In order to obtain an appropriate boundary condition for mantle flow near the Easter hot spot and to find out on which plate to expect the Easter hot spot track for a given time, published isochrons were rotated to the past ridge locations in a reference frame that takes the computed motion of Hawaii and Louisville hot spots into account. Calculations that include flow in the mantle and therefore relative movement of hot spots yield a better explanation of the observed age distribution along the Sala y Gomez ridge than assuming hot spot fixity; at the same time they predict a hot spot track on the Nazca plate that is roughly similar to the shape of the Nazca and Sala y Gomez ridges. Furthermore, the Easter hot spot was in an intraplate location on the Nazca plate during the past 43 Ma, somewhat closer to the ridge between ∼43 and 26 Ma than between ∼26 Ma and now. In contrast, if hot spots are assumed fixed, a location close to the ridge and rather on the Pacific plate between ∼43 and 26 Ma is predicted. Based on morphology and gravity signature it is suggested that the Nazca ridge and the eastern Tuamotu Island Plateau were created by plume material that erupted at the East Pacific Rise while the Easter plume was located sufficiently close to enable such an interaction, whereas the Sala y Gomez ridge was formed above the Easter plume in an intraplate setting. This implies that, if hot spots have indeed moved as computed here, during formation of the Nazca ridge (between ∼43 and 26 Ma) plume material erupted at the ridge several 100 km away from the plume, whereas no volcanism occurred directly above the plume. It also implies that any seamounts on the Pacific plate east of the Tuamotu Plateau are unrelated to the Easter plume.
Relationship of the tetra‐unsaturated C37 alkenone to salinity and temperature: Implications for paleoproxy applications
Authors: Sikes Elisabeth L. , Sicre Marie‐Alexandrine
Published: December 2002

This study assesses the relationship to salinity and temperature of the levels of the tetra‐unsaturated 37‐carbon methyl alkenone (C37:4) in the surface ocean. UK′37, a measure of the relative abundances of the C37:2 and the C37:3 alkenones, has a well constrained linear relationship to temperature in the open ocean [Prahl and Wakeham, 1987] and is a well‐established technique for estimating past sea surface temperatures in the sediments (e.g. [Müller et al., 1998]). Unlike the di‐ and tri‐unsaturated C37 alkenones, the temperature response of the tetra‐unsaturated C37 alkenone is less certain [Sikes et al., 1997], and recent work has suggested a relationship to salinity instead [Rosell‐Melé, 1998; Schulz et al., 2000]. Our study examined 106 surface water and sediment trap samples from the Atlantic, Pacific, and Southern Oceans to assess the relationship of the relative abundance of C37:4 to temperature and salinity. We also examined the relative unsaturation of C37:2 and C37:3 (the parameter UK′37) to the same parameters to place the C37:4 results in context. UK′37 has a strong correlation to salinity in the Atlantic, but the relationship does not hold worldwide, whereas UK′37 shows a strong linear relationship to temperatures in all ocean basins as shown in previous calibrations. The salinity response in the Atlantic does not confirm cause and effect and interpretation of the broader data set suggests any correlation is an artifact of the strong correlation of salinity to temperature in this basin implying salinity has no effect on the unsaturation of the C37:2 or C37:3 alkenones. The C37:4 alkenone shows no discernable relationship to temperature or salinity across the several basins, even when correlations are restricted to cooler temperatures where the tetra‐unsaturated alkenone would be expected to be present. These results indicate that C37:4 alkenone levels in the open ocean do not reflect either salinity levels or temperature but respond most strongly to some other environmental variable, perhaps changes in growth rate, light, or nutrient supply as suggested by culture studies.
The ENSO effect on the tropical Atlantic variability: A regionally coupled model study
Authors: Huang Bohua , Schopf Paul S. , Pan Ziqin
Published: December 2002

A CGCM is used to study the relative roles played by local air‐sea interaction and remote ENSO effects on the tropical Atlantic interannual variability. A regional coupling strategy allows full air‐sea coupling only over the Atlantic Ocean north of 30°S. Two experiments were conducted, respectively with either climatological or real time boundary conditions prescribed over the uncoupled portion of the global domain. The simulations show that major anomalous SST modes in the tropical South Atlantic are mainly caused by coupled ocean‐atmosphere processes within the Atlantic sector. SST anomalies in the northern tropical ocean, however, are substantially affected by the Pacific ENSO, with local coupling accounting for an enhanced effect. The El Niño effect causes warmer SST in the northern tropical Atlantic in the boreal spring season, forced by weakened northeast trade winds over the Atlantic basin starting from boreal winter in the maturing El Niño phase. The warmer SST anomalies then trigger a positive air‐sea feedback within the Atlantic basin, which leads to stronger anomalous cross‐equatorial atmospheric flow in the spring season.
Seasonal predictability of the winter NAO from north Atlantic sea surface temperatures
Authors: Saunders Mark A. , Qian Budong
Published: December 2002

We examine the seasonal predictability of the winter (December–January–February) North Atlantic Oscillation (NAO) from lagged north Atlantic sea surface temperatures (SSTs) for the period 1950/1–2000/1. We identify two lagged modes of SST variability whose principal components (PCs) are correlated significantly to upcoming winter NAO indices. We use linear regression with the PCs as predictors to assess the predictability of the winter NAO from cross‐validation over the full period and from replicated real‐time forecasts over the recent 15 year period 1986/7–2000/1. The model anticipates, in early November, the upcoming winter NAO ‐ for a range of NAO indices ‐ with a correlation between 0.47 and 0.63 for 1950/1–2000/1, and between 0.51 and 0.65 for the replicated real‐time forecast period. The model also anticipates the correct NAO sign in 67% to 75% of the last 51 winters and in 80% to 93% of the last 15 winters.
CO2 hydrate behavior in the deep ocean sediments; phase equilibrium, formation kinetics, and solubility
Authors: Lee Kyung Min , Lee Huen , Lee Jaehyoung , Kang Joo M.
Published: December 2002

Three‐phase equilibria (H‐LW‐V) for CO2 and NaCl solutions containing clay were measured to examine both electrolyte and capillary effect inside interlayer pores. Equilibrium line of 3 wt% NaCl and 10 wt% clay system was a little shifted to left side of 3 wt% NaCl solutions, which indicated that the injection of carbon dioxide into ocean sediments required higher pressure. CO2 hydrate formation kinetics was also attempted at 274.15 K and 30 bar. When clay minerals were added, the final consumption of CO2 decreased, but initial formation rate increased. Two‐phase equilibria of CO2 hydrate and water solution eliminating gas phase particularly measured to simulate more closely actual deep sea condition. The overall results of this study would be used for estimating carbon dioxide injection depth and formation rate in sequestration process and making out solubility tendency in the deep sea.
The glacial thermohaline circulation: Stable or unstable?
Authors: Prange M. , Romanova V. , Lohmann G.
Published: December 2002

The stability of the glacial thermohaline circulation (THC) with respect to North Atlantic freshwater input is examined using a global ocean general circulation model. It is found that the quasi‐equilibrium hysteresis behaviour is much less pronounced under glacial conditions than under present‐day conditions, and the existence of multiple equilibria requires an anomalous freshwater inflow. The results may help to assess the effect of iceberg invasions and meltwater events, suggesting that the THC is prone to instability during a deglaciation phase when the Atlantic meridional overturning is weakened. Under full glacial conditions, however, the THC is mono‐stable and even extreme freshwater pulses are unable to exert a persistent effect on the conveyor.
Gulf of Aden eddies and their impact on Red Sea Water
Authors: Bower Amy S. , Fratantoni David M. , Johns William E. , Peters Hartmut
Published: December 2002

New oceanographic observations in the Gulf of Aden in the northwestern Indian Ocean have revealed large, energetic, deep‐reaching mesoscale eddies that fundamentally influence the spreading rates and pathways of intermediate‐depth Red Sea Water (RSW). Three eddies were sampled in February 2001, two cyclonic and one anticyclonic, with diameters 150–250 km. Both cyclones had surface‐intensified velocity structure with maxima ∼0.5 m s−1, while the equally‐energetic anticyclone appeared to be decoupled from the surface circulation. All three eddies reached nearly to the 1000–2000 m deep sea floor, with speeds as high as 0.2–0.3 m s−1 extending through the depth range of RSW. Comparison of salinity and direct velocity measurements indicates that the eddies advect and stir RSW through the Gulf of Aden. Anomalous water properties in the center of the anticyclonic eddy point to a possible formation site in the Somali Current System.
Thermal models of the Middle America Trench at the Nicoya Peninsula, Costa Rica
Authors: Harris Robert N. , Wang Kelin
Published: December 2002

We present thermal models of the 24 Ma Cocos plate subducting under the Caribbean plate at the Nicoya Peninsula, Costa Rica. Our modeling incorporates the effect of hydrothermal cooling of the seafloor and the absence of an accretionary prism. Initial geotherms of the incoming plate, representing different effective cooling depths of hydrothermal circulation, affect the thermal structure and temperatures along the main subduction thrust. Excluding the deforming prism results in a higher heat flow immediately landward of the trench than previous models and therefore significant frictional heating is not required. Conductive plate cooling over‐predicts heat flow across the frontal forearc. With no or very low frictional heating, models with hydrothermal cooling to depths of 2 km under‐predicts the heat flow. Comparisons with recently determined hypocenter locations indicate that the updip limit of seismicity is consistent with temperatures between 100°–150°C, but there are large along‐strike variations in both the seismicity and thermal regime.
Positive feedbacks of fire, climate, and vegetation and the conversion of tropical savanna
Authors: Hoffmann William A. , Schroeder Wilfrid , Jackson Robert B.
Published: December 2002

We combine general circulation modeling (GCM), remote sensing, and field results to identify a positive feedback loop in which clearing of tropical savannas results in warmer and drier climate, accelerated fire frequencies, and further tree cover loss. The GCM simulations indicate that ongoing clearing of tropical savannas increases temperatures and wind speeds and decreases precipitation and relative humidity, substantially increasing fire frequency. Using NOAA‐12 satellite images and meteorological data, we estimate that complete savanna clearing will increase fire frequency by 42%. Combining these data with long‐term fire studies, we demonstrate that this fire‐mediated feedback may already be contributing to declining tree densities in the world's savannas and will become increasingly important as vegetation change continues in the coming century.
An explanation for the shape of Earth's gravity spectrum based on viscous mantle flow models
Authors: Steinberger Bernhard , Holme Richard
Published: December 2002

The Earth's gravity spectrum can be used as an observational constraint on geophysical modelling. Here we show how the spectrum up to degree l = 31 can be explained to a large part by viscous mantle flow in combination with a very simple model of random mantle density anomalies. Efforts to make the calculation more “realistic” by considering effects of thermal boundary layers, or using density anomalies based on tomography, or geodynamic modelling, tend to worsen the fit. Results are rather sensitive to assumptions on density anomalies in the upper thermal boundary layer. We suggest that, in combination with other observations, the shape of the Earth's gravity spectrum can serve to better constrain radial viscosity structure, density anomalies and flow in the Earth's mantle. Appropriate treatment of the lithosphere and of lateral viscosity variations will be the main challenges in modelling this spectrum.
Small‐scale experimental testing of fire radiative energy for quantifying mass combusted in natural vegetation fires
Authors: Wooster Martin J.
Published: December 2002

Combustion of forest and grassland vegetation contributes to atmospheric pollution and rising greenhouse gases concentrations. Remotely measuring the energy radiated during natural fires has been suggested as a method for enhancing current emissions estimates. When made from satellites, such measures can potentially provide important new information on large‐scale biomass combustion rates, which relate directly to the production of emissions. EOS‐MODIS now makes such observations globally, multiple times per day. Using small experimental fires observed with a field spectro‐radiometer we present the first evaluation of the relationship between time‐integrated fire radiative energy and total mass of vegetation combusted. Results indicate a linear relationship (r2 = 0.78) for fire sizes varying over almost two orders of magnitude. Further information on the rate and intensity of burning is contained within the emission spectra. The results support the continued investigation of fire radiative energy as a new tool to enhance biomass burning emissions inventories.
High concentration of water in stishovite in the MORB system
Authors: Chung Jung In , Kagi Hiroyuki
Published: December 2002

Stishovite synthesized from natural basalt showed the highest concentration of hydrogen ever reported. The water concentration obtained from the polished single crystal of stishovite in this study was up to a maximum of 5628 ± 291 atoms of hydrogen per 106 silicon atoms (844 ± 44 ppm water). The concentration of hydrogen increased with increasing substitution of trivalent cations, which is almost aluminum, and temperature has a significant effect on the incorporation of hydrogen into stishovite. The result of this study suggests that stishovite in subducted basaltic crust could be an important water‐containing phase and contribute to the transport of hydrogen to the lower mantle.
Direct comparison of pulsating aurora observed simultaneously by the FAST satellite and from the ground at Syowa
Authors: Sato N. , Wright D. M. , Ebihara Y. , Sato M. , Murata Y. , Doi H. , Saemundsson T. , Milan S. E. , Lester M. , Carlson C. W.
Published: December 2002

We have made a direct comparison of a pulsating aurora observed simultaneously from the ground at Syowa in Antarctica and onboard the FAST satellite (∼3100 km altitude). The auroral form appeared as east‐west‐aligned bands consisting of two different types: a poleward moving pulsation and a standing mode pulsation, each with a period of ∼5 sec. The aurora occurs within the region of an inverted‐V structure of lower energy (0.1–1 keV) electron precipitation. The two different types of pulsating aurora are separated in space by a narrow gap in the inverted‐V potential structure. Spatial and temporal variations of the down‐going high‐energy (>5 keV) electron flux show a one‐to‐one correspondence with the optical pulsating aurora. The down‐going high‐energy (1–10 keV) ion flux modulation is out of phase (anti‐correlated) with the high‐energy electron flux modulation. These features suggest that the precipitating high‐energy electrons, which produce the pulsating aurora, are modulated by the oscillation of the field‐aligned electric field located above FAST.
A model study of total ozone evolution 1979–2000 — the role of individual natural and anthropogenic effects
Authors: Geller Marvin A. , Smyshlyaev Sergei P.
Published: December 2002

The SUNY‐SPB 2‐D model is used to model ozone behavior during the period 1970–2000. The roles of different natural and human‐induced impacts (solar variability, volcanoes, changes in dynamics, increasing chlorine and bromine concentrations) on the column ozone are studied separately as well as in combination. We found that variations in solar UV radiation produce global total ozone fluctuations that are comparable to those due to volcanic eruptions and increasing concentrations of chlorine and bromine during this period. We also suggest that the physical reason for the different observed time behavior of total ozone in the Northern and Southern Hemisphere middle latitudes during the 1990s may be found in the greater importance of mixing of polar processed air into middle latitudes in the Southern hemisphere.
Climate forcing of subsonic aviation: Indirect role of sulfate particles via heterogeneous chemistry
Authors: Pitari G. , Mancini E. , Bregman A.
Published: December 2002

The aerosol budget in the upper troposphere and lower stratosphere may be perturbed by subsonic aviation through direct particle emission or, indirectly, following oxidation of emitted SO2 into sulfuric acid. These additional (or larger) particles produce a radiative forcing (RF) of climate by interacting with solar and planetary radiation, and may also favour formation of high‐altitude clouds [Fahey et al., 1999]. The present modeling study focuses on an additional indirect effect of aircraft generated sulfate, namely heterogeneous chemistry on liquid sulfuric acid particles. For the 1992 subsonic aviation case we show that the NOx‐driven RF decreases globally from 0.018 Wm−2 to about zero when emission of SO2 and sulfuric acid particles is also included. This decrease is mostly produced by: (1) a smaller ozone increase when the enhancement of sulfuric acid particle surface area density is taken into account (70%), and (2) a direct negative solar RF of aircraft generated sulfate mass (30%). No indirect effects on high‐altitude clouds and contrails are considered here.
Persistence of thin ice regions in Europa's ice crust
Authors: Buck Leon , Chyba Christopher F. , Goulet Marc , Smith Alex , Thomas Paul
Published: December 2002

Extensive data from planetary spacecraft as well as celestial mechanics models support the existence of a subsurface ocean on Europa ∼100 km thick, maintained by a tidal heat flux. Models in which the overlying ice crust is less than 20 km thick permit breaches in the ice due to impact events or thermal plumes from the tidally heated core. We apply a two‐dimensional thermal model to the analysis of the refreezing of a hole in the ice crust following a breach event. Our model incorporates heat produced by tidal heating of Europa in two ways: a basal heat flux from Europa's silicate and iron core, together with volumetric heating of the ice shell. We compare our refreezing timescales to those obtained from a model where viscous flow in the base of the ice crust fills the hole. We find that catastrophic breaches in Europa's ice crust may produce regions of relatively thin ice persisting up to ∼1 My. These breaches are closed by viscous flow when radii are small (<10–50 km) and by conductive refreezing for larger radii, especially if Europa's crust has a high basal heat flow due to a hot core. Detection of the ice/ocean interface by orbital detection of the temperature anomalies or radar sounding would be most probable in the vicinity of these events.
Solar wind dependence of the occurrence and motion of polar auroral arcs: A statistical study
Authors: Kullen A. , Brittnacher M. , Cumnock J. A. , Blomberg L. G.
Published: December 2002

Polar UV images from a 3‐month period in winter 1998–1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral oval, and which are detectable by the UV imager. The arcs are examined with respect to their spatial and temporal behavior as well as to a possible connection to solar wind parameters using ACE satellite data. It is found that the majority of polar arcs appear during northward IMF, strong IMF magnitude, and high solar wind speed. A modified Akasofu‐Perreault epsilon parameter with a cosine function instead of a sine function (vB2 cos4(θ/2)(l02/μ0)) combines these results. It correlates well with the occurrence frequency of polar arcs for long timescales. The location of polar arcs is strongly dependent on the sign of the IMF By component. Static polar arcs occur in the Northern Hemisphere on the dawn (dusk) side of the oval for negative (positive) IMF By, whereas poleward‐moving arcs separate from the opposite side of the oval, and then move in the direction of IMF By. All polar arcs are sorted into five different categories according to their spatial structure and evolution: oval‐aligned, bending, moving, midnight, and multiple arcs. Each polar arc type occurs for a characteristic combination of solar wind parameters. IMF clock angle changes seem to have a strong influence on what type of arc occurs. Oval‐aligned arcs appear mainly during steady IMF, bending arcs after an IMF Bz sign change, and moving arcs after an IMF By sign change. For the rare midnight and multiple arc events, no characteristic IMF clock angle dependence has been found. The different types of clear polar arcs are discussed in the context of existing observational studies and transpolar arc models.
Longitudinal dependence of the daily geomagnetic variation during quiet time
Authors: Le Sager P. , Huang T. S.
Published: December 2002

The daily variation of the geomagnetic field during quiet time at equinox features a longitudinal dependence, which is observed in the dip‐latitude/longitude frame. Two explanations are considered: (1) the longitudinal changes of the geomagnetic latitude at constant dip‐latitude and (2) the longitudinal variations of the main field, which modulates the ionospheric dynamo e.m.f. and determines and constrains the field‐aligned currents (FAC) that flow between ionospheric hemispheres to maintain the current continuity. The Prairie View ionospheric dynamo model simulates this inter‐hemispheric coupling within the International Geomagnetic Reference Field (IGRF) by using magnetic coordinates, and is used for this investigation. Two distinct types of magnetic variation observed at midlatitudes in African and American sectors are explained by the differences in geomagnetic latitude. IGRF longitudinal effects are also in evidence. They account for one singular feature (a large positive excursion of the declination around 4 LT at Plateau). At African midlatitudes, they improve the horizontal component amplitude and the morning decrease of the declination, which are better reproduced with IGRF than with a dipolar main field. FAC, which generate a variation of the geomagnetic field of up to 40% of the horizontal current contribution around midday, are essential to simulate these three cases.
Generation of near‐Earth reconnection by divergent flows in the plasma sheet
Authors: Lin Y. , Swift D. W.
Published: December 2002

A global two‐dimensional hybrid simulation is carried out to study the evolution of the near‐Earth plasma sheet. It is shown that reconnection X lines and plasmoids can be generated by diverging flows in the plasma sheet. These diverging flows can be caused by variations in the Maxwell stress due to variations of Bx and Bz components of the magnetic field outside the current sheet. In addition, the flow acceleration/deceleration in the plasma sheet can also be caused by variations in the local plasma mass density. Comparison runs have been made with a Harris model current sheet with zero Bz, and no evidence of reconnection is found. The results indicate that divergent plasma flow is a sufficient condition for reconnection even in a dissipation‐less plasma.
Comparing a spherical harmonic model of the global electric field distribution with Astrid‐2 observations
Authors: Eriksson S. , Blomberg L. G. , Weimer D. R.
Published: December 2002

Electric field measurements provided by the double probe instrument on the Astrid‐2 satellite are compared with the empirical Weimer electric field model for all magnetic local times, except between 11 and 13 MLT, and poleward of 55° corrected geomagnetic latitude (CGLat). We focus the model evaluation on its ability to predict the latitudinal locations of the convection reversal boundaries for two‐cell convection patterns and to estimate the magnitude of the electric field above 55° CGLat. A total number of 780 polar cap passes are employed from the Northern Hemisphere between January and July 1999. The measured average electric field magnitude in the dawn‐dusk meridian plane above 55° CGLat is generally 25% larger than the predicted field independent of the interplanetary magnetic field (IMF) direction. The model shows a better correspondence with the observed electric field for southward IMF than for northward IMF, with most cases centered around Bz = −1.5 nT and r = 0.88. However, the agreement for northward IMF is promising, and a few examples are shown to corroborate this fact. The observed and predicted convection reversal boundary locations along the satellite track for southward IMF are on the average found 2–3° CGLat apart in the dawn‐dusk meridian plane but may be as far apart as 9° CGLat. An initial investigation of the relative timing of a 20‐min averaging window for the IMF along the 20–25 min polar cap crossing suggests that a time‐dependent transfer function may be found that applies a higher weight to the input solar wind data early in the pass and a lower weight later in the pass for an IMF window that corresponds to the first half of the crossing and the opposite weight versus time dependence for an IMF window corresponding to the last half of the crossing.