bluuebunny/crossref_metadata_2025 huggingface dataset:
Suction Cup Sampler Bias in Leaching Characterization of an Undisturbed Field Soil
Authors: Brandi‐Dohrn Florian M. , Dick Richard P. , Hess Mario , Selker John S.
Published: February 2004

An accurate assessment of leaching losses in the vadose zone requires measurement of both solute and water flux to compute flux concentrations (CF). Leachate collected at a depth of 1.2 m in 32 passive capillary samplers (PCAPS), which sample soil‐pore water continuously at tensions of 0–50 cm H2O was compared to that collected in 32 suction cup samplers operated under a falling head vacuum of 530‐cm H2O over a 2‐year period. There was evidence that PCAPS collected CF and suction cup samplers collected resident concentrations (CR) as shown by the earlier breakthrough of a bromide tracer in the PCAPS as compared to the suction cup samplers. CR was up to 100% lower than CF during the rising branch of Br tracer breakthrough and up to 78% greater during the declining branch of breakthrough. Br content and water flux into PCAPS were correlated with correlation coefficients changing from positive to negative values with the advancement of the tracer breakthrough peak through the profile indicating the declining importance of preferential flow on Br transport. CR and CF differed significantly (P < 0.05) for 35% of the sampling events for NO3, but seasonal means were mostly insignificantly different for this regularly applied and therefore more uniformly distributed anion. The early breakthrough of Rhodamine WT and Brilliant Blue FCF, which was applied with the Br, was very low with 0.15% and 0.08% of the initial concentration C0, respectively, with CR differing from CF by up to −100%. For all tracers, mass leached using CR is therefore prone to bias for short‐term (<0.6 pore volumes) monitoring.
The Temporally Integrated Monitoring of Ecosystems (TIME) Project Design: 1. Classification of Northeast Lakes Using a Combination of Geographic, Hydrogeochemical, and Multivariate Techniques
Authors: Young Thomas C. , Stoddard John L.
Published: February 2004

This investigation is part of the Temporally Integrated Monitoring of Ecosystems ((TIME) project, an effort to meet the difficult challenge of monitoring surface water quality in the northeastern United States for signs of change in response to the Clean Air Act Amendments of 1990. The overall objective of the study was to develop a unified scheme for classifying lakes in the northeast into relatively homogeneous groups and improve the likelihood of detecting water quality trends in the region. The study approach involved combining the best elements of several procedures recently used for defining regional subpopulations of lakes; these were termed the hydrogeochemical model (HM), geographical model (GM), and multivariate statistical model (MSM). Lake and watershed data from the U.S. Environmental Protection Agency Eastern Lake Survey (ELS) were used to evaluate the classification methods and their modifications. After preliminary comparisons were made of the three classification schemes, it was concluded that the resulting subpopulations indicated that there was meaningful similarity among methods but that the significant dissimilarity reflected distinctive attributes of each classification method. These differences were deemed important; accordingly, integration of the methods entailed efforts to preserve parts of each. This was accomplished by assigning each lake of the ELS data set into a lake cluster that had been defined by jointly applying the HM and GM methods. Subsequently, the jointly classified clusters were aggregated by coupling an application of the MSM (cluster analysis) with subjective judgment regarding termination of the process of cluster formation. This integration of procedures gave rise to nine subpopulations that separated mainly on the basis of hydrogeochemical factors, though geographic influences also were evident in the results. The integrated classification procedure provided an explicit method involving the combination of several kinds of data to yield lake subpopulations. Although the process of integrating this information may stand alone as a useful exercise, the results obtained from the integrated classification model exhibited less dispersion than those formed by the parent procedures. This is an important consideration when subpopulation homogeneity is wanted as a means to improve trend detection. From the view of statistical efficiency, therefore, the integrated procedure may be considered to be more optimal than the parent schemes. When the complexity of the integrated approach is considered, however, we conclude that the next most precise classification scheme, the HM approach, may be preferred over the integrated classification for use in the design of an actual monitoring program.
Field Investigation of Nutrient Pulse Mixing in an in Situ Biostimulation Experiment
Authors: Devlin J. F. , Barker J. F.
Published: February 2004

A permeable wall was installed in a shallow, uncontaminated portion of the Borden aquifer, and periodically flushed with a solution of potassium acetate. The acetate pulses were injected at intervals of 5–7 weeks and were observed to merge within 5–10 m of the injection wall. The chief mechanism for the merging of these pulses was longitudinal dispersion. Geochemical changes in the aquifer, manifested as the development of a sustained reducing environment, indicated that the dispersive mixing occurred at the scale of the microorganisms and was not merely an artifact of the sampling method. This work indicates that large‐period pulsed injections of a substrate solution might be useful in bioremediation programs where the promotion of injected and ambient water mixing is desirable. It is also advantageous, from the standpoint of biofouling prevention, that this mixing occur at some distance from the injection wells.
Predicting the migration rates of subaqueous dunes
Authors: Mohrig David , Smith J. Dungan
Published: February 2004

In this paper we develop a simple, physically based method for predicting what fraction of sediment moving over the crests of dunes will bypass their lee faces. The bypass fraction is found by calculating a characteristic excursion length for every grain size making up a particular train of dunes. All particles with excursion lengths greater than the downstream span of the lee face of the average dune are assumed not to contribute to dune propagation. Bypass fractions based on distributions of excursion lengths account for the discrepancies between total sediment discharge and dune migration rate measured by Stein [1965] and Guy et al. [1966] in laboratory flumes, as well as by us in the North Loup River of Nebraska. Calculations and these data agree over sediment‐transporting conditions associated with the entire stability field for dunes composed of medium sand. The two laboratory studies show that commonly 30–60% of all sand moving over fully developed dunes is not deposited on slip faces. Measurements from the North Loup River reveal that of all sediment moving over the dune crests there, roughly 45% is not being captured on lee faces even though 99% of sediment is transported within 2 cm of the bed. The method developed herein successfully estimates measured values for the bypass fraction from 0 to 80%. Our analysis indicates that the division between grains that are deposited versus those that are bypassing falls within the range of sand sizes making up the suspended load at dune crests.
A Method to Evaluate the Importance of Interactions Between Land Surface and Atmosphere
Authors: Hu Zhenglin , Islam Shafiqul
Published: February 2004

Using a factorial experiment framework, a systematic relationship is developed between uncoupled and coupled land surface model sensitivities. It is shown that the relative sensitivity of a coupled land‐atmosphere model can be partitioned into the relative sensitivity of the corresponding uncoupled model, a term related to interactive effects, and a product term involving main effects of land and atmospheric processes. A widely used land surface model is used in its uncoupled and coupled modes to illustrate the methodology. The proposed methodology demonstrates the need and utility to combine the traditional one‐factor‐at‐a‐time method (e.g., relative sensitivity analysis) with a factorial experiment framework. The traditional sensitivity analysis cannot evaluate the multifactor effects. The factorial design experiments, on the other hand, can estimate multifactor effects but are not dimensionless and hence depend on the chosen range of parameters. The proposed method offers the best of both approaches by estimating multifactor effects with dimensionless quantities. The proposed framework is used to explain why sensitivities to certain land surface parameters are enhanced or damped when the land surface module is coupled with an atmospheric model. For the tested cases there does not appear to be any systematic trend in sensitivities of the coupled and uncoupled models. It is argued and shown that similarity of relative sensitivities between an uncoupled and coupled model does not necessarily imply that the influence of land‐atmosphere feedback is negligible. By decomposing the relative sensitivity of a coupled model, we have shown that interactions between land and atmosphere cannot be neglected while testing land surface representations.
Adapting geostatistical transmissivity simulations to finite difference flow simulators
Authors: Roth Chris , Chilès Jean‐Paul , de Fouquet Chantal
Published: February 2004

The validity of hydrogeological studies is often dependant on the correct estimation of the flux (or, equivalently, the equivalent transmissivity) of the field. However, depending on the level of fine‐scale heterogeneity of the porous media considered, flow simulators based on the finite differences algorithm can be shown to underestimate the equivalent transmissivity. This result is shown to depend on the value on the intermesh transmissivities and not the mesh transmissivities. When working with geostatistically simulated point support transmissivities, intermesh transmissivities can be directly calculated so that the finite differences result obtained is consistent with the spatial transmissivity distribution.
Reexamining the determination of the fractal dimension of river networks
Authors: Claps Pierluigi , Oliveto Giuseppe
Published: February 2004

In analyzing the literature on the fractal nature of river networks one can recognize several points that require clarification or additional investigations. For instance, many interpretations, with different conclusions, have been given to the important empirical results found in the previous literature on river geomorphology. Similarly, the argument concerning a possible “more likely” value of the fractal dimensions of river networks has not yet found a convincing answer. In an attempt to shed more light on the topic, particularly on the latter point, we have reexamined the basic aspects related to selfsimilarity in river networks using uniquely the fundamental concepts of fractal geometry. This approach leads to relations for the determination of the fractal dimensions which hold exactly for deterministic self‐similar trees and are compatible with related findings reported in literature. Results obtained from application of these relations to several river networks in southern Italy indicate very low variability around the average values of about 1.7, 1.5, and 1.1 for the fractal dimensions of the whole river network, of its topological equivalent, and of its individual streams, respectively. This outcome, in addition to comparable results taken from the literature, provides new arguments to the hypothesis that natural networks tend to have the same fractal dimensions.
Characterization of Water and Solute Transport in the Unsaturated Zone of a Hypersaline Environment
Authors: Ronen Daniel , Yechieli Yoseph , Shatkay Michal
Published: February 2004

We present a methodology for the analysis of chemical profiles from the unsaturated zone where ions from the sediment are extracted by two methods:centrifugation and water addition. The methodology enables one to (1) assess the ion amounts present in two different phases, pore water and minerals; (2) determine the depth in the unsaturated profile where the degree of saturation of each mineral is reached; (3) establish the principal direction of water flow; and (4) differentiate between upward transport of water either as vapor or as a saline solution. The methodology was applied to the unsaturated zone of the Dead Sea coastal area where the original salinity of interstitial water in these sediments was >300 g/L. Our analysis of the field data suggests that reduction of interstitial water salinity is the result of vertical upward transport of fresh water from a confined aquifer at a depth of 7 m. Flushing, up to the potentiometric surface (depth of 3.5 m), is due to the positive pressure head of the aquifer. Above the potentiometric surface, upward water transport is due to capillary forces, and flow is maintained by evaporation at the soil surface. Evaporation leads to an increase in the salinity of the rising interstitial solution and to the sequential deposition of salts such as NaCl and KMgCl3 · 6H2O.
Modeling the dynamics of seal formation and its effect on infiltration as related to soil and rainfall characteristics
Authors: Assouline S. , Mualem Y.
Published: February 2004

The main physical aspects of soil surface seal formation caused by rainfall are modeled. The proposed model relates the surface sealing to the specific hydraulic and mechanical properties of the initially undisturbed soil as well as the physical characteristics of the rainfall applied, under given initial and boundary conditions defining the flow system. The soil disturbance resulting from the raindrop impacts is expressed in terms of an increase of the initial soil bulk density. The dynamics of seal formation at the soil surface are found to be related to the following variables: the rainfall intensity, the second moment of the drop‐size density distribution, the maximal drop diameter, the compaction limit of the given soil, and its initial shear strength, which depends upon the initial soil bulk density and water content. Following Mualem and Assouline [1989], a nonuniform seal is considered, represented by an exponentially decreasing function of the bulk density with depth. The seal thickness is assumed to be dependent upon the rainfall rate. Its steady state value is assumed to be reached shortly after the beginning of rainfall so that it might be considered as constant during the stage of formation. A calibration procedure is presented on the basis of measured infiltration under sealing conditions. The results show that the proposed model addresses the main factors affecting soil sealing formation and is able to simulate the process of infiltration through sealing soils under saturated as well as unsaturated flow conditions.
Rapid exchange effects on isotope ratios in groundwater systems: 2. Flow investigation using Sr isotope ratios
Authors: Johnson Thomas M. , DePaolo Donald J.
Published: February 2004

Sr isotope ratios were measured in groundwater, whole rock digestions, and cation exchange extracts from a clay‐rich groundwater system at Ernest O. Lawrence Berkeley National Laboratory and were used to constrain flow velocities and search for preferential flow paths. In the Orinda formation siltstone,87Sr/86Sr increases strongly over tens of meters along presumed flow paths, indicating slow groundwater flow. Dissolved Sr is close to isotopic equilibrium with the exchangeable Sr in the clays, and the observed87Sr/86Sr increase is interpreted as a cation exchange front moving slowly through the unit combined with dissolution of minerals with relatively high87Sr/86Sr ratios. The data are inverted using a one‐dimensional transport‐dissolution‐exchange model; the results indicate long‐term average flow velocities of less than 0.2 m/yr which are consistent with14C measurements. The data suggest a lack of strong preferential flow paths through this unit.
Critical flow constrains flow hydraulics in mobile‐bed streams: A new hypothesis
Authors: Grant Gordon E.
Published: February 2004

A new hypothesis predicts that in mobile‐bed river channels, interactions between the channel hydraulics and bed configuration prevent the Froude number (Fr) from exceeding 1 for more than short distances or periods of time. Flow conditions in many steep, competent streams appear to be close to critical. Froude numbers of steep (slope ≈ 0.01) sand‐bed streams with considerable freedom to adjust boundaries oscillate between 0.7 and 1.3 over 20‐ to 30‐s cycles, with an average of 1.0 at the channel thalweg. Critical flow in these streams is maintained by the interaction between the mobile bed and free water surface at high Fr, which results in a cyclical pattern of creation and destruction of bed forms. Field observations support that a similar mechanism of flow–bed form interaction constrains Fr ≤ 1 in active‐bed braided gravel rivers, step–pool streams, laboratory rills, lahar‐runout channels, and even some bedrock channels. Empirical and analytical results show that as slope increases, competent flows tend to asymptotically approach critical flow. An assumption of critical flow would dramatically simplify paleohydraulic flow reconstructions and modeling of flow hydraulics in high gradient streams.
Comparison of nonlocal Eulerian to Lagrangian moments for transport in an anisotropic heterogeneous aquifer with deterministic linear nonequilibrium sorption
Authors: Hu Bill X. , Cushman John H.
Published: February 2004

An Eulerian perturbation scheme is applied to study transport of a reactive chemical experiencing linear nonequilibrium sorption with deterministic rate constants in a heterogeneous porous medium. Exact solutions for the mean concentrations are used to obtain mean values of spatial moments through the third moments. Comparisons are made with results obtained for the identical problem in a Lagrangian frame [Dagan and Cvetkovic, 1993]. If local‐scale dispersion is neglected in the Eulerian analysis and the fully nonlocal flux is retained, then the Eulerian and Lagrangian moments agree. However, if either the Eulerian model is localized or the local‐scale dispersion is retained, then the moments disagree. This disagreement is especially acute in the asymptotic limits. If local‐scale dispersion is included in the first‐order Lagrangian analysis, then one anticipates agreement through the third moments between the two approaches.
Impact of geologic heterogeneity on recharge estimation using environmental tracers: Numerical modeling investigation
Authors: McCord J. T. , Gotway C. A. , Conrad S. H.
Published: February 2004

This paper presents a numerical modeling approach for assessing the impacts of geologic heterogeneity on groundwater recharge estimates derived from environmental tracers. Common to many of the environmental tracer methods used to infer recharge in arid environments is an assumption of one‐dimensional, vertical downward flow of water and solutes. However, in recent years there has been a growing recognition that fluid flux rates through geologic materials can be spatially variable owing to heterogeneities in porous media properties. Previous studies have suggested that local flow directions are dependent upon the stratigraphic and sedimentologic characteristics of the medium and may not be vertical even when application of water at the surface is spatially uniform and hydraulic gradients are vertical. Consequently, environmental tracer movement will also be spatially variable, and the one‐dimensional assumption invoked to interpret unsaturated environmental tracer concentration profiles may be unrealistic. Two different numerical simulation experiments were performed to investigate the problems with using environmental tracers to infer recharge. The first involves simulation of flow and tracer transport through heterogeneous flat‐lying media, and the second considers dipping layered anisotropic media. The results of these simulations show that recharge inferred using environmental tracer methods is also highly spatially variable and that recharge estimates obtained by tracer profiles tend to overestimate recharge and should be considered accurate only to within an order of magnitude, particularly in situations with significant media heterogeneity. Consequently, recharge estimates obtained from tracer profiles should be critically evaluated with regard to impacts of spatially variable flow.
High‐speed measurements of three‐phase flow using synchrotron X rays
Authors: DiCarlo D. A. , Bauters T. W. J. , Steenhuis T. S. , Parlange J.‐Y. , Bierck B. R.
Published: February 2004

Accurate monitoring of flow instabilities, which can occur when nonaqueous phase liquids (NAPLs) flow through porous media, is an important component of predicting the transport and fate of these compounds in the subsurface. In particular, flow situations in which three mobile phases (such as water, NAPL, and air) exist in the porous media are inherently complex. Unfortunately, the relatively low source intensities and the nontunable source energies make traditional dual gamma techniques unsuitable to study flow instabilities which can change within seconds. We present an alternate technique, which uses synchrotron X rays from the Cornell High Energy Synchrotron Source (CHESS) to measure three‐phase fluid saturations on the time scale of seconds. Using the harmonic content resulting from X ray diffraction, we obtained a high‐intensity X ray beam consisting of distinct tunable energies. Three‐phase saturations were measured on 5‐s timescales during fingering of light NAPL into regions of dry and water wet sandy soil. In the water wet soil the oil finger was less saturated, slower, and wider than the same finger in the dry soil. The results yield insights into the nature of three‐phase preferential flow.
Stochastic modeling of transect‐to‐transect properties of Great Basin rangeland streams
Authors: Myers Thomas J. , Swanson Sherman
Published: February 2004

We developed a transect‐to‐transect stochastic model of stream width and pools. The model included Markov chains for transect‐to‐transect transitions among pools, nonpools, and part pools, a two‐parameter beta distribution for pool fraction when the transect is a partial pool, and a two‐parameter gamma distribution to model standardized widths. The model accurately reproduced the mean and standard deviation of both width and transect‐based measures of pool area. However, the model smoothed the distributions thereby reducing the skewness of the observed data. The physical tendency for streams to have very few transects that are almost, but not totally, all pool or nonpool implies that stochastic models based on continuous distributions will have more density in the tails than observed on the stream. The model represented odd‐shaped empiric distributions of pool area which indicates it is very flexible and can accurately model a variety of streams.
Nitrogen cycling in forested catchments: A Chapman Conference
Authors: Church M. Robbins , Driscoll Charles T.
Published: February 2004

The American Geophysical Union sponsored a Chapman Conference on “Nitrogen Cycling in Forested Catchments” on September 16–20, 1996. This conference brought together scientists from many disciplines to share recent observations and to discuss advances in the study of nitrogen (N) cycling in forests. Conference presentations and discussions focused on mechanisms controlling the retention and losses of N in forests and on the effects of atmospheric deposition, land use, and climate on watershed N loss. The importance of dissolved organic nitrogen in the biogeochemistry of N was a subject of considerable discussion. Several critical issues were identified for future research on N cycling in forested catchments.
Effects of pasture management on N2O and NO emissions from soils in the humid tropics of Costa Rica
Authors: Veldkamp Edzo , Keller Michael , Nuñez Marvin
Published: February 2004

Emissions of nitrous oxide (N2O) and nitric oxide (NO) from agricultural soils in the tropics are important in the global budgets of these trace gases. We made monthly measurements of N2O and NO emissions from pastures with three different management systems on volcanic soils in northwestern Costa Rica: traditional (no N input from fertilizer or legumes), pastures with a grass‐legume combination, and pastures fertilized with 300 kg N ha−11 yr−1. Average annual N2O emissions were 2.7 ng N cm−2h−1from the traditional pastures, 4.9 ng N cm−2h−1from the grass‐legume pastures, and 25.8 ng N cm−2h−1from the fertilized pastures. Average annual NO emissions were 0.9, 1.3, and 5.3 ng N cm−2 h−1 from traditional, grass‐legume and fertilized pastures, respectively. In a separate experiment the effects of ammonium, nitrate, and urea‐based fertilizer mixtures on nitrogen oxide fluxes were compared. We measured nitrogen oxide fluxes following four different fertilization events. Nitrogen oxide fluxes were among the highest ever measured. The difference in soil water content between the fertilization events had a far greater effect on N2O and NO emissions than the effect of fertilizer composition. We conclude that the concept of “emission factors” for calculating N2O and NO emissions from different types of N fertilizer is flawed because environmental factors are more important than the type of N fertilizer. To estimate fertilizer‐induced N2O emission in tropical agriculture, stratification according to soil moisture regime is more useful than stratification according to fertilizer composition.
Ulysses observations of short‐period (≤30 Days) modulation of the galactic cosmic rays
Authors: Blake J. B. , Looper M. D. , Keppler E. , Heber B. , Kunow H. , Quenby J. J.
Published: February 2004

A frequency analysis of the short‐period modulation of the GCR as seen by instruments aboard Ulysses shows interesting temporal variations. In particular, during an approximate five‐month period from December 1993 to May 1994, the modulation was not the usual 26 days but more than 29 days. The CIR‐associated MeV ions did not exhibit this period shift.
Chlorophyll natural fluorescence response to upwelling events in the Southern Ocean
Authors: Letelier Ricardo M. , Abbott Mark R. , Karl David M.
Published: February 2004

Variability of solar‐induced (natural) fluorescence and chlorophyll were measured on scales of hours to weeks in the upper layer of a cyclonic eddy located south of the Antarctic Polar Front using a free‐floating drifter. The fluorescence signal was analyzed both in terms of chlorophyll concentration and as an indicator of energy distribution in the photosynthetic apparatus. Long‐term trends in fluorescence parallel changes in chlorophyll concentration. Considering a significant positive correlation between fluorescence and the relative depth of the eddy upper layer we hypothesize that the observed short‐term variations in natural fluorescence are a physiological response of phytoplankton to changes in the supply of limiting nutrients. This interpretation is consistent with the Southern Ocean iron limitation hypothesis.
Low‐frequency oscillations in temperature‐Proxy records and implications for recent climate change
Authors: Mahasenan Natesan , Watts Robert G. , Dowlatabadi Hadi
Published: February 2004

The global‐mean surface temperature has inherent variability on all time scales. Distinguishing natural climatic variability from the observed ∼0.5°C warming since the mid nineteenth century is crucial to understanding the response of the climate system to the enhanced greenhouse effect. We perform singular spectrum analysis (SSA' of several long‐term (173–1481 years' proxy records of temperature. We find significant variability on the century time‐scale in the records, with the most predominant modes being oscillations of time period ∼160 and ∼80 years. The findings remain essentially unchanged when the portion of the records after 1900 are excluded to remove any notable anthropogenic influence. These oscillations are of sufficient magnitude to account for a significant portion of the warming trend observed in instrumental records of temperatures from the 1850s until the 1970s. Our findings suggest that attribution of anthropogenically induced climate change may be more difficult than currently believed.
Lidar observations of a breaking mountain wave associated with extreme turbulence
Authors: Ralph F. Martin , Neiman Paul J. , Levinson David
Published: February 2004

Observations from a Doppler lidar, which are enhanced by stratospheric aerosols from Mount Pinatubo's eruption, provide unique measurements of mountain waves over the Rocky Mountains, including nearly instantaneous observations of wave breaking in the lower stratosphere. The wave breaking is revealed by a flow‐reversal aloft that marks a localized critical layer created when the wave amplitude was great enough to cause overturning. The altitude, spatial scale, and duration of this wave‐induced critical layer are documented. The mountain waves led to turbulence affecting commercial aircraft, including one accident in which a cargo jet lost an engine and part of a wing in extreme turbulence at the same time the lidar was making observations nearby. Although earlier observations of mountain‐wave drag assumed wave stationarity over 3–5 h, significant changes over less than 2 h are documented here. Such nonstationarity could contribute to discrepancies remaining between observations and simulations of mountain‐wave drag.
WIND, GEOTAIL, and GOES 9 observations of magnetic field dipolarization and bursty bulk flows in the near‐tail
Authors: Slavin J. A. , Fairfield D. H. , Lepping R. P. , Szabo A. , Reiner M. J. , Kaiser M. , Owen C. J. , Phan T. , Lin R. , Kokubun S. , Mukai T. , Yamamoto T. , Singer H. J. , Romanov S. A. , Buechner J. , Iyemori T. , Rostoker G.
Published: February 2004

The WIND, GEOTAIL, and GOES 9 spacecraft were all in the nightside magnetosphere and Interball was in the solar wind when a substorm began at ∼07:25 UT on April 18, 1996. WIND and GEOTAIL were located at downstream distances of X=−12 to −14 Re. The separation between these spacecraft was ∼10 Re in the Y direction and less than ∼1 Re in Z. While the northward field component at GOES 9 began to increase immediately, little or no dipolarization was observed at WIND and GEOTAIL until a series of very rapid Bz increases were observed ∼25–30 min later. About ∼1–2 min prior to each dipolarization event at WIND and GEOTAIL an earthward flow burst with peak speeds of 100–500 km/s was initiated. The duration of these bursty bulk flows were 1–7 min. These unique observations are interpreted as strong evidence for the existence of spatially localized, but sometimes temporally overlapping flow bursts in the near‐tail during substorm expansion phase which result, cumulatively, in sufficient earthward magnetic flux transport to produce the well known dipolarization at geosynchronous orbit.
Kinematic parameters of the tremor wave field at Mt. Etna (Sicily)
Authors: Wegler Ulrich , Seidl Dieter
Published: February 2004

The tremor wave‐field at Mt.Etna was recorded with an array of three 3‐component broadband seismometers. The analysis of its polarisation and dispersion properties leads to the conclusions that (1) the wave‐field consists mainly of surface waves and that (2) two of the observed spectral peaks are caused by a wave guide effect along the propagation path.
Correlation of changes in the outer‐zone relativistic‐electron population with upstream solar wind and magnetic field measurements
Authors: Blake J. B. , Baker D. N. , Turner N. , Ogilvie K. W. , Lepping R. P.
Published: February 2004

A study has been made of the correlation of the population of relativistic electrons in the outer‐zone magnetosphere with the properties of the solar wind (speed, density, magnetic field) during a solar minimum period. The study is based upon observations made in the Spring of 1995 with sensors aboard 1994‐026 and WIND. It is found that a large relativistic electron enhancement depends upon a substantial solar‐wind speed increase associated with precursor solar‐wind density enhancement, and, in particular, upon a southward turning of the interplanetary magnetic field.
IUE's view of Callisto: Detection of an SO2 absorption correlated to possible torus neutral wind alterations
Authors: Lane Arthur L. , Domingue Deborah L.
Published: February 2004

Observations taken with the International Ultraviolet Explorer (IUE) detected a 0.28 µm absorption feature on Callisto's leading and Jupiter‐facing hemispheres. This feature is similar to Europa's 0.28 µm feature, however it shows no correlation with magnetospheric ion bombardment. The strongest 0.28 µm signature is seen in the region containing the Valhalla impact. This absorption feature also shows some spatial correlation to possible neutral wind interactions, suggestive of S implantation (rather than Sx) into Callisto's water ice surface. Indications of possible temporal variations (on the 10% level) are seen at other wavelengths between the '84–'86 and the '96 observations.
Magnetism of the Keweenawan Age Chengwatana Lava Flows, northwest Wisconsin
Authors: Kean W. F. , Williams I. , Chan L. , Feeney J.
Published: February 2004

The Chengwatana Volcanics are subaerial plateau basalts which appear to be stratigraphically above the North Shore Volcanic Group of the Lake Superior Region. Sixty five oriented cores were collected at six sites near St. Croix Falls, Wisconsin. Magnetic cleaning of the samples was primarily step‐wise alternating field (A.F.) demagnetization to 100 mT. Thermal demagnetization and hysteresis property measurements indicate the dominant magnetic mineralogy is multidomain magnetite. However, a stable remanence is present which we attribute to grains with single domain behavior. The mean remanent direction is D=287.5°, I=45.8°, α‐95=8.0°, with a mean pole at 186.1°E., 30.9°N. This result is consistent with the 1.1 Ga. apparent pole for the North American craton. Two sites mapped as stratigraphically equivalent show distinct reversed directions. Both A.F. and thermal demagnetization isolate the same reversed direction which is interpreted as being primary. These particular flows record a short reversal in an otherwise normal sequence. This strengthens the evidence for more than one reversal in the Middle Keweenawan, and may be the same short reversal noted at Mamainse Point.
A new global gridded radiosonde temperature data base and recent temperature trends
Authors: Parker D. E. , Gordon M. , Cullum D. P. N. , Sexton D. M. H. , Folland C. K. , Rayner N.
Published: February 2004

We present a new analysis of global radiosonde temperature data. From 1979 onwards, the data from the Australasian region have been corrected for instrument‐related discontinuities with the help of comparisons with collocated retrievals from satellite‐based Microwave Sounding Units (MSU) and metadata: in future work, adjustments will be applied worldwide and extended to earlier years. The data are stored as monthly anomalies from a 1971–1990 reference period on a 5° latitude × 10° longitude grid at 8 levels from 50 hPa to 850 hPa. Seasonal and annual temperature anomalies have also been created on a 10° × 20° grid using an eigenvector reconstruction method to filter noise. Latitude‐height profiles of zonal‐mean temperature changes since the 1960s show significant cooling in the lower stratosphere, especially in middle and high latitudes of the Southern Hemisphere, but the cooling over Australasia is less than shown by unadjusted data. Warming dominates the troposphere but is not a maximum in the tropical upper troposphere. In the annual mean, tropospheric warming is greatest around 45°N and possibly in the data‐sparse high latitudes of the Southern Hemisphere.
Seasonal circulation in the south Indian Ocean
Authors: Ffield Amy , Toole John , Wilson Doug
Published: February 2004

Two World Ocean Circulation Experiment hydrographic cruises in March and June 1995, along with Topex‐Poseidon altimeter data and National Meteorological Center wind data are used to estimate seasonal changes in the South Indian Ocean subtropical gyre. Mean annual curves derived from the altimeter and wind data reveal strengthening of the anticyclonic gyre in March and September, and weakening in June and December. The seasonal changes correspond to variations in the wind field south of 30°S at the equinoxes and solstices. In addition, the wind‐driven gyre is further north in July, and further south in March. These variations in strength and location of the South Indian Ocean gyre may influence inter‐ocean transports south of Africa. Despite the inferred mean annual seasonal variations in the South Indian Ocean gyre, volume transports estimated in 1995 from the hydrographic data are close to mean values. Apparently, a mesoscale eddy in March disrupts the stronger fall gyre, whereas in June the weaker winter gyre is delayed by 1 month.
Wavevector observations of the two‐stream instability in the daytime equatorial electrojet
Authors: Pfaff Robert F. , Marionni Paul A. , Swartz Wesley E.
Published: February 2004

Detailed measurements of electric field wavevectors of the Farley‐Buneman two‐stream instability in the daytime equatorial electrojet were gathered on a sounding rocket flown from Alcântara, Brazil. Primary two‐stream waves were detected in the altitude region of 104–108 km, where the current density and polarization electric field maximized and where intense 3 m vertical echoes were observed by a 50 MHz backscatter radar. The in‐situ measured wave components are predominantly oriented in the horizontal, zonal direction. Spaced receivers reveal wavelengths of 11–14 m with phase velocities of about 370 m/s at the peak wave power near 106.5 km. These velocities are constant from 1–15 m, indicative of dispersionless flow. Although the neutral wind was not measured, the in‐situ phase velocity appears to be below that predicted by linear theory.
The effect of collapse structures on ground deformations in calderas
Authors: De Natale Giuseppe , Petrazzuoli Stefano M. , Pingue Folco
Published: February 2004

Ground deformations related to unrest episodes in calderas are generally interpreted, like in other volcanic environments, in terms of increased pressure within a magma chamber embedded in a continuous, elastic medium, In this framework, the depth of the pressure source is inferred from the size of the deformed area. This scheme works quite well for individual volcanoes (for instance Hawaiian shields) where a good correspondence between inflation events and eruptive episodes has been observed. Ground deformation in calderas, however, show unusual features that are difficult to interpret within such a scheme, because considerable positive deformation (in the order of several meters) and microfracturing (i.e. intense seismicity) may occur without eruptions, altough the estimated depth for the magma source is sometime very shallow (less than 1–2 km). It has been recently suggested that the contact zones at the border of collapsed calderas, could act as stress‐strain discontinuity zones and thus bias modeling results obtained for continuous elastic media. We use both observations and theoretical modeling based on 3‐D finite element techniques to show that ground deformations in collapsed calderas are strongly influenced by the caldera structure, giving a new perspective in related geological and geophysical observations in such areas.
Heliospheric plasma sheet and coronal streamers
Authors: Bavassano Bruno , Woo Richard , Bruno Roberto
Published: February 2004

Helios 2 measurements of solar wind plasma and magnetic field are used to investigate the structure of the heliospheric plasma sheet between 0.3 and 1 AU. In agreement with previous observations at 1 AU, the plasma sheet thickness is much larger than that of the embedded current sheet. The plasma sheet appears surrounded by a density halo, a region of slightly raised density. High‐time resolution data show that decreases in relative helium abundance coincide with the plasma sheet boundaries, reinforcing the notion that the solar wind within the plasma sheet is of a different nature (with different solar origins) than that outside it. Although radio occultation measurements of the corona were not available at the time of the Helios data, a synthesis of recent results on coronal streamers shows that there is a remarkable similarity between their major features and those of plasma sheets, demonstrating that the coronal counterpart of the plasma sheet is the stalk of the coronal streamer. These measurements also suggest that the density halo seen in the Helios data is associated with the radial extension of the boundaries of the streamer observed in the extended corona before the streamer narrows to a stalk.
Length‐scale dependence of relief along the south eastern border of Massif Central (France)
Authors: Lucazeau Francis , Hurtrez Jean Emmanuel
Published: February 2004

Relief has been calculated over four orders of length‐scales (75 m to 800 km) for several drainage basins along the south‐eastern border of Massif Central, an asymmetric plateau edge feature. Relief is not uniform over the length‐scale domain, but rather appears as smooth at small and large scales, and rough at meso‐scale. This heterogeneous behaviour is interpreted as a consequence of the different processes that shape topography at such ranges of scales. The threshold between small and meso scales is rather well constrained for all basins (400–500 m), and may represent the limit where channel networks appear, while the threshold between meso‐scale and large scale varies with geological context: where relief is fault controlled (Mediterranean drainage basins), the threshold value is of the order of 15–20 km. Where topography is flexurally controlled (Atlantic drainage basins), the threshold value is different (40–80 km).
Three regimes of mantle convection with non‐Newtonian viscosity and stagnant lid convection on the terrestrial planets
Authors: Solomatov V. S. , Moresi L.‐N.
Published: February 2004

Numerical simulations of convection with strongly temperature‐dependent viscosity suggest that non‐Newtonian viscosity convection (dislocation creep) passes through three convective regimes similar to those observed for Newtonian viscosity convection (diffusion creep): the small viscosity contrast regime, the transitional regime and the stagnant lid regime. For realistic viscosity contrasts, mantle convection is in the stagnant lid regime characterized by formation of a very viscous, slowly creeping lid on top of an actively convecting mantle. This explains the tectonic style observed on the terrestrial planets and the Moon. On the other hand, this eliminates the possibility that the plates on Earth could be mobile due to non‐Newtonian viscosity. The nature of the mobility of lithospheric plates on Earth has yet to be explained.
Theory of super‐diffusion for the magnetopause
Authors: Treumann Rudolf A.
Published: February 2004

The diffusivity in a plasma is enhanced over classical diffusion when the particles undergo Lévy flight interactions caused by scale‐invariant turbulence. In this case classical parallel diffusion (heat flux) is modified. Perpendicular magnetized diffusion coefficients obtained assume a scaling D ∝ kBT/B1+δ where δ < 1. This scaling is closer to Bohm‐scaling than conventional theories predict. The non‐classical diffusion coefficients depend on time. As a consequence, the expected density profiles will not exhibit the usually assumed diffusive shapes. It is suggested that in regions of strong plasma turbulence enhanced super‐diffusivity may contribute to the population of the inner LLBL.
Inter‐monsoonal changes in the T‐S properties of the near‐surface waters of the Northern Arabian Sea
Authors: Morrison John M.
Published: February 2004

The Arabian Sea experiences extremes in atmospheric forcing that lead to the greatest seasonal variability in any ocean. During 1995, 6 cruises were carried out within the northern Arabian Sea. The data collected represent the first consistent dataset covering an entire monsoonal cycle. Ocean soundings of temperature and salinity from these cruises are used to characterize variability of water masses in the surface layers associated with monsoonal forcing. This paper documents the role of advection versus local water mass modification on seasonal extremes in T‐S in near‐surface waters of the Arabian Sea. Seasonal changes are associated with seasonal heating, advection of waters from the Oman upwelling zone, varying of mixed‐layer depth due to Ekman pumping in the central basin and mixing of high‐salinity waters from Persian Gulf.
A seismic refraction study of the north basin of the Dead Sea, Israel
Authors: Ginzburg Avihu , Ben‐Avraham Zvi
Published: February 2004

A seismic refraction experiment involving the use of 9 OBS and 11 portable seismic land stations was conducted along a profile in the north basin of the Dead Sea. The seismometers were deployed along a north‐south profile in the lake and the adjacent land area. The interpretation of the recorded seismograms indicates the presence of two large Pliocene salt diapirs in the young basin fill. The basement lies at a relatively shallow depth (6–8 km) under the north basin. Further south along the profile a major fault affecting the basement was detected. The apparent sense of the faulting is down to the southeast, representing the boundary of the south basin. This major faulting with a downthrow of some 4–5 km has depressed the crystalline basement and the overlying Cretaceous and pre‐Cretaceous sediments. The faulting was followed by the deposition of over 8 km of Recent and Tertiary sediments resulting in a 14 km thick sedimentary sequence in the south basin of the Dead Sea.
Deep sections through the South Australian Basin and across the Australian‐Antarctic Discordance
Authors: Schodlok Michael P. , Tomczak Matthias , White Neil
Published: February 2004

Fronts and water masses in the Australian Basin are described, based on three sections made by R/V Franklin during 1994 as part of the WOCE One‐time Hydrographic Survey. The Subtropical Front is found closer to the Australian continent than previously thought. Convection of Subantarctic Mode Water is seen to reach 700 m depth. A locally formed variety of Antarctic Intermediate Water with a temperature near 6°C is found near 120°E. Antarctic Bottom Water enters the Indian Ocean through the Discordance at 125°37′E and 127°E but not at 120°E, where the deep passage seen in the topography may be closed for Bottom Water inflow.
Aftershock sequences of moderate‐sized intermediate and deep earthquakes in the Tonga Subduction Zone
Authors: Wiens Douglas A. , Gilbert H. J. , Hicks B. , Wysession M. E. , Shore P. J.
Published: February 2004

Intermediate and deep focus earthquakes are generally thought to produce few aftershocks. The two year operation of a network of broadband seismographs in the Tonga‐Fiji region provides an opportunity to monitor the aftershock activity of moderate‐sized earthquakes. In addition to the March 9, 1994 (Mw 7.6) event that shows 144 aftershocks, we found that all nine intermediate and deep earthquakes with Mw > 6.0 showed some aftershock activity, suggesting that aftershock sequences are ubiquitous. On average, deep Tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow California earthquakes of similar moment. The largest aftershocks of each sequence have an mb that is 2.0 units smaller on average than the Mw of the mainshock, whereas the California earthquakes show a mean difference of 1.1. Since b‐values of the deeper sequences appear to be similar to shallow sequences, the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks. The most prolific sequences show an activity level not far below shallow earthquakes of the same moment. This is in contrast with observations of little aftershock activity for deep earthquakes in South America, Japan, and Kuriles, and suggests that the Tonga subduction zone is unusually prolific in aftershock production, perhaps due to cold slab temperatures.