bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo studies of flow and transport in fractal conductivity fields: Comparison with stochastic perturbation theory
Authors: Hassan Ahmed E. , Cushman John H. , Delleur Jacques W.
Published: February 2004

A Monte Carlo simulation of flow and transport is employed to study tracer migration in porous media with evolving scales of heterogeneity (fractal media). Transport is studied with both conservative and reactive chemicals in media that possess physical as well as chemical heterogeneity. Linear kinetic equations are assumed to relate the sorbed phase and the aqueous phase concentrations. The fluctuating log conductivity possesses the power law spectrum of a fractional Brownian motion (fBm). Chemical heterogeneity is represented as spatially varying reaction rates that also are assumed to obey fBm statistics and may be correlated to the conductivity field. The model is based on a finite difference approximation to the flow problem and a random walk particle‐tracking approach for solving the solute transport equation. The model is used to make comparisons with the nonlocal transport equations recently developed by Deng et al. [1993], and Hu et al. [1995, 1997]. The results presented herein support these nonlocal models for a wide range of heterogeneous systems. However, the infinite integral scale associated with the fractal conductivity has a significant effect on the prediction of the nonlocal theories. This suggests that integral scale should play a role in stochastic Eulerian perturbation theories. The importance of the local‐scale dispersion depends to a great extent on the magnitude of the local dispersivities. The effect of neglecting local dispersion decreases with the decrease in local dispersivity.
Determination of effective air‐water interfacial area in partially saturated porous media using surfactant adsorption
Authors: Kim Heonki , Rao P. Suresh C. , Annable Michael D.
Published: February 2004

The effective specific air‐water interfacial area
in a sand‐packed column was measured at several water saturations (Sw) using a surface‐reactive tracer (sodium dodecylbenzene sulfonate (SDBS)) and a nonreactive tracer (bromide). Miscible displacement experiments were conducted under steady water flow conditions to quantify the retardation of SDBS resulting from its adsorption onto the air‐water interface in a sand‐packed column. A consistent trend of increased retardation of SDBS compared with the nonreactive tracer, bromide, was observed with decreasing Sw. The data for air‐water surface tension measured at various SDBS concentrations were interpreted using the Gibbs model to estimate the required adsorption parameters. The retardation factors (Rt) for SDBS breakthrough curves were then used in combination with the estimated SDBS adsorption coefficient to calculate the
values at different Sw. For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from Rt = 1.00 at Sw = 1.00 (Rt < 1 due to SDBS sorption on sand) to Rt = 3.44 at Sw = 0.29 (which corresponds to
). These values are in agreement with theoretical predictions and recently published data. Improvements needed to overcome the experimental limitations of the presented method are also discussed.
A lumped parameter model for nitrogen transformation in the unsaturated zone
Authors: Ling Ge , El‐Kadi Aly I.
Published: February 2004

Prediction of NO3−‐N leaching in the unsaturated zone is difficult owing to the complex nature of processes dominating the fate of N in soils. Although many sophisticated numerical models have been developed for simulating nitrogen behavior in the soil‐plant system, applying them in practice is often difficult, owing to the limited specific‐site information. A lumped parameter model (LPM) for the assessment of nitrate leaching using limited data is presented. An LPM considers nitrogen transformation processes, climate data, and irrigation and fertilizer applications. The model was tested against detailed numerical models and against field data and provided realistic estimates of leachate at the bottom of the root zone. The model can be used for decision making regarding of nitrogen‐N application and can provide an alternative way to estimate leachate loading to the groundwater.
On the sensitivity of drainage density to climate change
Authors: Moglen Glenn E. , Eltahir Elfatih A. B. , Bras Rafael L.
Published: February 2004

Drainage density reflects the signature of climate on the topography and dictates the boundary conditions for surface hydrology. Hence defining the relationship between drainage density and climate is important in assessing the sensitivity of water resources and hydrology to climate change. Here we analyze the equilibrium relationship between drainage density and climate and estimate the relative sensitivity of drainage density to climate change. We conclude that the sign of the resulting change in drainage density depends not only on the direction of the change in climate but also on the prevailing climatic regime.
A nonlinear model for analysis of slug‐test data
Authors: McElwee C. D. , Zenner M. A.
Published: February 2004

While doing slug tests in high‐permeability aquifers, we have consistently seen deviations from the expected response of linear theoretical models. Normalized curves do not coincide for various initial heads, as would be predicted by linear theories, and are shifted to larger times for higher initial heads. We have developed a general nonlinear model based on the Navier‐Stokes equation, nonlinear frictional loss, non‐Darcian flow, acceleration effects, radius changes in the well bore, and a Hvorslev model for the aquifer, which explains these data features. The model produces a very good fit for both oscillatory and nonoscillatory field data, using a single set of physical parameters to predict the field data for various initial displacements at a given well. This is in contrast to linear models which have a systematic lack of fit and indicate that hydraulic conductivity varies with the initial displacement. We recommend multiple slug tests with a considerable variation in initial head displacement to evaluate the possible presence of nonlinear effects. Our conclusion is that the nonlinear model presented here is an excellent tool to analyze slug tests, covering the range from the underdamped region to the overdamped region.
A monthly streamflow model
Authors: Mohseni Omid , Stefan Heinz G.
Published: February 2004

To estimate the potential consequences of projected global warming on streamflow, a deterministic model for a monthly timescale has been developed. The model structure is based on the water budget theory and contains deterministic relationships to estimate four components of streamflow: direct runoff, interflow, base flow, and snowmelt. The model inputs are six climate variables for each time step and 10 watershed parameters. The model has four calibration parameters which are related to direct runoff and snowmelt runoff. The model has been applied to the Little Washita River watershed in Oklahoma and the Baptism River watershed in Minnesota. The former is an agricultural watershed with a warm and seasonally dry climate, and the latter is a forested watershed with a cool and humid climate. The model simulates mean monthly streamflow in the Baptism River with a Nash‐Sutcliffe coefficient (NSC) of 0.99 and a correlation coefficient of 0.83. For the Little Washita River the same simulation measures are 0.94 and 0.89, respectively.
Tritium–helium 3 dating under complex conditions in hydraulically stressed areas of a buried‐valley aquifer
Authors: Shapiro Stephanie Dunkle , Rowe Gary , Schlosser Peter , Ludin Andrea , Stute Martin
Published: February 2004

The 3H‐3He dating method is applied in a buried‐valley aquifer near Dayton, Ohio. The study area is large, not all sampling locations lie along well‐defined flow paths, and existing wells with variable screen lengths and diameters are used. Reliable use of the method at this site requires addressing several complications: (1) The flow system is disturbed because of high pumping rates and induced infiltration; (2) tritium contamination is present in several areas of the aquifer; and (3) radiogenic helium concentrations are elevated in a significant number of the wells. The 3H‐3He ages are examined for self‐consistency by comparing the reconstructed tritium evolution to the annual weighted tritium measured in precipitation; deviations result from dispersion, tritium contamination, and mixing. 3H‐3He ages are next examined for consistency with chlorofluorocarbon ages; the agreement is poor because of degradation of CFCs. Finally, the 3H‐3He ages are examined for consistency with the current understanding of local hydrologic processes; the ages are generally supported by hydrogeologic data and the results of groundwater flow modeling coupled with particle‐tracking analyses.
The interstate river compact: Incentives for noncompliance
Authors: Bennett Lynne Lewis , Howe Charles W.
Published: February 2004

Twenty‐one western United States rivers are governed by interstate compacts. This paper examines the issue of compliance with interstate river compacts in the western United States and some of the factors influencing compact compliance. Theoretical arguments and empirical evidence presented in this paper suggest that upper basin states governed by interstate compacts with percentage delivery rules are more likely to comply with compact requirements than states whose rivers are governed by fixed delivery rules. Evidence indicates that both the frequency and level of noncompliance tend to be larger under a fixed allocation rule. Under such a rule the upper basin state bears a greater share of a shortage and experiences greater variability so it would have a greater incentive to cheat. A comparative study of the South Platte and La Plata Rivers is consistent with this hypothesis. Given the large demands imposed on many western United States rivers, our analysis suggests that compliance analysis is likely to be an important component of interstate negotiations and that administration of interstate compacts will become increasingly important.
Parameter space methods in joint parameter estimation for groundwater flow models
Authors: Weiss Richard , Smith Leslie
Published: February 2004

The joint estimation of model parameters using hydraulic head and tracer concentration data is examined using methods based on parameter space analysis. Response surfaces and confidence regions can indicate how multiple data sets interact to reduce parameter uncertainty. Each data set produces a unique response surface and confidence region. If these surfaces are oriented differently for the two data sets, parameter uncertainties are significantly reduced when the second data set is included in the inversion. If these surfaces are similar for the two data sets, the second data set will not reduce the parameter uncertainty significantly. The axes of the linearized confidence ellipsoid are analyzed to determine the difference in orientation of the confidence regions for the two data sets. The use of confidence regions can be extended to predict the value of a second data set in reducing the uncertainty of parameter estimates, before the data are collected. Parameter space approaches are also introduced for selecting the relative weights for the individual data sets in joint parameter estimation. The conventional method based on the analysis of data residuals is compared to three other methods: with the weights selected to maximize parameter stability, to minimize the volume of the confidence region, or to minimize the longest axis of the confidence region. Each criterion can lead to substantially different weights applied to each of the data sets. The application of these methods is demonstrated using hydraulic head measurements and 14C concentrations to calibrate a model of groundwater flow in the San Juan Basin, New Mexico.
Analytical series expressions for Hantush's M and S functions
Authors: Trefry Michael G.
Published: February 2004

M. S. Hantush established relationships between the dynamics of groundwater mounding beneath recharge zones and two integral functions, M and S. Exact algebraic expressions for these functions are developed in terms of a formal power series expansion. This expansion may be reordered to provide two independent analytical partial summations involving elementary functions. The convergence characteristics of these two formulae are discussed and compared with numerical quadratures of M and, hence, S. The algebraic expressions are used to generate identities for related integrals. Compact algebraic approximations to M and S can be deduced from the series expansions with essentially arbitrary accuracy, while retaining valuable functional information. For example, a simple two‐term truncated sum yields a reasonable approximation to M over a useful range of arguments. The results are amenable for use in further theoretical studies of groundwater percolation and mounding where numerical quadratures may be undesirable.
Pressure solution for sequential hydraulic tests in low‐transmissivity fractured and nonfractured media
Authors: Johns R. T.
Published: February 2004

In the last decade, considerable effort has been applied to the characterization of low‐permeability, heterogeneous formations. A primary reason for the increased activity is the importance of estimating formation properties for use in safety assessments at proposed nuclear waste disposal sites, such as those in Switzerland, Germany, Sweden, and the United States. Methods for determining formation properties (e.g., transmissivity, static head, storativity, and flow boundaries) are varied, but hydraulic testing is commonly used. Hydraulic tests in low‐permeability media typically consist of a sequence of multiple test events such as slug, constant pressure, and pulse tests. Each single test event can significantly affect the measured pressure response of subsequent test events. The pressure response can also be affected by borehole mud pressures that occur prior to testing (i.e., pretest pressures) and other factors such as well bore storage, well bore skin, and well bore temperature. We present a new analytical solution that accounts for all of the aforementioned complexities. The solution technique treats a sequence of pretest pressures and multiple test events (slugs, pulses, and shut‐ins) as one test sequence, thereby accounting for the influence of one test event upon another. The solution is derived so that only a kernel function, the constant‐rate pumping test solution, is required for new flow models. Furthermore, the solution is presented for any flow dimension allowing for interpretation in fractured formations where linear, radial, and fractional flow may exist. We demonstrate the use of the solution by inverse modeling to estimate the flow model and parameters for an example hydraulic test conducted in Switzerland.
On water vapor transport in field soils
Authors: Cahill Anthony T. , Parlange Marc B.
Published: February 2004

Measurements of soil volumetric moisture content and temperature were made at 2, 4, 7, 10, and 15 cm below the surface of a bare field soil, over a 1‐week period at 20‐min intervals. The conductive heat and liquid moisture fluxes were calculated for the soil layer 7–10 cm below the surface, and the water vapor flux was then determined from both the energy transfer and mass transfer equations. Water vapor flux in this layer transported a significant amount of the total energy flux ∥up to 50%) and an appreciable amount of the total moisture flux (up to 25%). There was reasonable agreement between the water vapor flux calculated by the mass transfer equation and the vapor flux calculated by the energy equation.
Meridional transport of dissolved inorganic carbon in the South Atlantic Ocean
Authors: Holfort J. , Johnson K. M. , Schneider B. , Siedler G. , Wallace D. W. R.
Published: February 2004

The meridional oceanic transports of dissolved inorganic carbon and oxygen were calculated using six transoceanic sections occupied in the South Atlantic between 11°S and 30°S. The total dissolved inorganic carbon (TCO2) data were interpolated onto conductivity‐temperature‐depth data to obtain a high‐resolution data set, and Ekman, depth‐dependent and depth‐independent components of the transport were estimated. Uncertainties in the depth‐independent velocity distribution were reduced using an inverse model. The inorganic carbon transport between 11°S and 30°S was southward, decreased slightly toward the south, and was −2150 ± 200 kmol s−1 (−0.81 ± 0.08 Gt C yr−1) at 20°S. This estimate includes the contribution of net mass transport required to balance the salt transport through Bering Strait. Anthropogenic CO2 concentrations were estimated for the sections. The meridional transport of anthropogenic CO2 was northward, increased toward the north, and was 430 kmol s−1 (0.16 Gt C yr−1) at 20°S. The calculations imply net southward inorganic carbon transport of 2580 kmol s−1 (1 Gt C yr−1) during preindustrial times. The slight contemporary convergence of inorganic carbon between 10°S and 30°S is balanced by storage of anthropogenic CO2 and a sea‐to‐air flux implying little local divergence of the organic carbon transport. During the preindustrial era, there was significant regional convergence of both inorganic carbon and oxygen, consistent with a sea‐to‐air gas flux driven by warming. The northward transport of anthropogenic CO2 carried by the meridional overturning circulation represents an important source for anthropogenic CO2 currently being stored within the North Atlantic Ocean.
Winter fluxes of CO2 and CH4 from subalpine soils in Rocky Mountain National Park, Colorado
Authors: Mast M. Alisa , Wickland Kimberly P. , Striegl Robert T. , Clow David W.
Published: February 2004

Fluxes of CO2 and CH4 through a seasonal snowpack were measured in and adjacent to a subalpine wetland in Rocky Mountain National Park, Colorado. Gas diffusion through the snow was controlled by gas production or consumption in the soil and by physical snowpack properties. The snowpack insulated soils from cold midwinter air temperatures allowing microbial activity to continue through the winter. All soil types studied were net sources of CO2 to the atmosphere through the winter, whereas saturated soils in the wetland center were net emitters of CH4 and soils adjacent to the wetland were net CH4 consumers. Most sites showed similar temporal patterns in winter gas fluxes; the lowest fluxes occurred in early winter, and maximum fluxes occurred at the onset of snowmelt. Temporal changes in fluxes probably were related to changes in soil‐moisture conditions and hydrology because soil temperatures were relatively constant under the snowpack. Average winter CO2 fluxes were 42.3, 31.2, and 14.6 mmol m−2 d−1 over dry, moist, and saturated soils, respectively, which accounted for 8 to 23% of the gross annual CO2 emissions from these soils. Average winter CH4 fluxes were −0.016, 0.274, and 2.87 mmol m−2 d−1 over dry, moist, and saturated soils, respectively. Microbial activity under snow cover accounted for 12% of the annual CH4 consumption in dry soils and 58 and 12% of the annual CH4 emitted from moist and saturated soils, respectively. The observed ranges in CO2 and CH4 flux through snow indicated that winter fluxes are an important part of the annual carbon budget in seasonally snow‐covered terrains.
Characterizing patterns of global land use: An analysis of global croplands data
Authors: Ramankutty Navin , Foley Jonathan A.
Published: February 2004

Human activities have shaped significantly the state of terrestrial ecosystems throughout the world. One of the most direct manifestations of human activity within the biosphere has been the conversion of natural ecosystems to croplands. In this study, we present an analysis of the geographic distribution and spatial extent of permanent croplands. This analysis represents the area in permanent croplands during the early 1990s for each grid cell on a global 5 min (∼10 km) resolution latitude‐longitude grid. To create this data set, we have combined a satellite‐derived land cover data set with a variety of national and subnational agricultural inventory data. A simple calibration algorithm was used so that the spatial land cover data were generally consistent with nonspatial agricultural inventory data. The spatial distribution of croplands represented in this analysis presents a quantitative depiction of global agricultural geography. The regions of the world known to have intense cultivation (e.g., the North American corn belt, the European wheat‐corn belt, the Ganges floodplain, and eastern China) are clearly portrayed in this analysis. It also captures the less intensely cultivated regions of the world, usually surrounding the regions mentioned above, and regions characterized by subsistence agriculture (e.g., Sahelian Africa). Data generated from this kind of analysis can be used within global climate models and global ecosystem models to assess the importance of permanent croplands on environmental processes. In particular, these data, combined with models, could help evaluate the role of changing land cover on regional climate and carbon cycling. Future efforts will need to concentrate on other land use systems, including pastures and regions of shifting cultivation. Furthermore, land use and land cover data must be extended to include an historical dimension so as to evaluate the changing state of the biosphere over time. This article contains supplementary material.
A stochastic appraisal of the annual carbon budget of a large circumboreal peatland, Rapid River Watershed, northern Minnesota
Authors: Rivers J. S. , Siegel D. I. , Chasar L. S. , Chanton J. P. , Glaser P. H. , Roulet N. T. , McKenzie J. M.
Published: February 2004

The probable limits of the carbon budget of the Rapid River Watershed, within the greater Glacial Lake Agassiz Peatland in northern Minnesota, were evaluated using a Monte Carlo simulation approach. Carbon enters the peatlands in groundwater, precipitation, and primary productivity. Carbon leaves the peatlands by groundwater and surface water outflow and by the outgassing of methane. Results of the simulations of the carbon budget show that the peatland is now probably a sink for carbon, supported by field data showing peat is, in fact, accumulating at the rate of about 1 mm yr−1[Glaser et al., 1997]. Excluding extreme values, Monte Carlo simulation results indicate that the Rapid River Peatland stores between −28.98 g C m−2yr−1(release) and 50.38 g C m−2yr−1(storage) with a mean accumulation of 12.74 g C m−2yr−1over the 1,506,200 m2watershed. The peatland appears to be delicately poised with respect to net gain or loss of carbon.
Quantifying the emission of light‐absorbing particles: Measurements tailored to climate studies
Authors: Bond Tami C. , Charlson Robert J. , Heintzenberg Jost
Published: February 2004

The emission rate of light‐absorbing aerosols, which contribute to climate forcing, has previously been calculated using mass emission factors combined with fuel‐use inventories. Several assumptions made in this calculation lead to overestimates that are significant, but as yet unevaluated. We propose a new measurement approach that augments, and may be preferable to, the mass‐based method for modeling radiative forcing by aerosols: direct measurement of the source strength of absorption. This quantity, in units of absorption cross‐section per unit time, may be used directly in models of atmospheric dispersion and transport to predict the three‐dimensional, time‐dependent distribution of the absorption of visible light by aerosols. In demonstration measurements made at a coal‐burning plant, the emitted absorption is an order of magnitude lower than would be inferred from the previous method of calculation.
Sources of HOx and production of ozone in the upper troposphere over the United States
Authors: Jaeglé L. , Jacob D. J. , Brune W. H. , Tan D. , Faloona I. C. , Weinheimer A. J. , Ridley B. A. , Campos T. L. , Sachse G. W.
Published: February 2004

The sources of HOx (OH+peroxy radicals) and the associated production of ozone at 8–12 km over the United States are examined by modeling observations of OH, HO2, NO, and other species during the SUCCESS aircraft campaign in April–May 1996. The HOx concentrations measured in SUCCESS are up to a factor of 3 higher than can be calculated from oxidation of water vapor and photolysis of acetone. The highest discrepancy was seen in the outflow of a convective storm. We show that convective injection of peroxides (CH3OOH and H2O2) and formaldehyde (CH2O) from the boundary layer to the upper troposphere could resolve this discrepancy. More generally, the data collected over the central United States during SUCCESS suggest that local convection was a major source of HOx and NOx to the upper troposphere. The OH and HO2 observations together with the observations of NO allow us to directly calculate the ozone production in the upper troposphere and its dependence on NOx. We find an average net ozone production of 2 ppbv day−1 between 8 and 12 km over the continental United States in the spring. Ozone production was NOx‐limited under essentially all the conditions encountered in SUCCESS. The high levels of HOx present in the upper troposphere stimulate ozone production and increase the sensitivity of ozone to NOx emissions from aircraft and other sources.
Retrieval of effective microphysical properties of clouds: A wave cloud case study
Authors: Ackerman S. A. , Moeller C. C. , Strabala K. I. , Gerber H. E. , Gumley L. E. , Menzel W. P. , Tsay S. ‐C.
Published: February 2004

One of the important objectives of the SUbsonic aircraft: Contrail and Cloud Effecs Special Study (SUCCESS) field campaign data analysis is to investigate retrieving cloud microphysical properties using remote sensing observations. This paper presents the results of an infrared based retrieval of effective particle radius Re using brightness temperature observations near 8.5, 11 and 12µm. The retrieval method relies on comparing 8.5–11 and 11–12µm observed brightness temperature differences to theoretical simulations. During SUCCESS, conducted in April–May 1996 out of Salina KS, the MODIS Airborne Simulator (MAS) on the NASA ER‐2 made observations of contrails, cirrus and mountain lee wave clouds. Observations indicate that contrail and cirrus clouds are distinguishable by their radiative properties. Retrieval of Re for the lee wave cloud case on 2 May agrees with in situ observations from probes mounted on a NASA DC‐8 aircraft, validating the infrared retrieval. The addition of the 8.5µm information to the Re retrieval greatly enhances the sensitivity of the retrieval at small particle sizes (< 10µm)
Effects on stratospheric ozone and temperature during the Maunder Minimum
Authors: Wuebbles Donald J. , Wei Chu‐Feng , Patten Kenneth O.
Published: February 2004

The period of low solar activity between 1645 and 1715, known as the Maunder Minimum, corresponds to a period in which the ultraviolet spectral irradiance was significantly smaller than found in the current atmosphere [Lean et al., 1995a]. In this study, we use our two‐dimensional chemical‐radiative‐transport model of the global atmosphere to examine the effect of this reduced solar flux on the distribution of stratospheric ozone and temperature relative to that normally expected for the pre‐industrial atmosphere. A decrease in total global ozone of about 3% was determined for the Maunder Minimum period. Lean et al. [1995b] have examined the implications of the direct solar irradiance effects on climate during that period. The changes in stratospheric ozone may further have influenced the radiative forcing on climate during that period. In addition, such events are lilely to happen again with resulting decreases in ozone.
in situ measurements of HOx in aircraft exhaust plumes and contrails during SUCCESS
Authors: Tan D. , Faloona I. , Brune W. H. , Weinheimer A. , Campos T. , Ridley B. , Vay S. , Collins J. , Sachse G.
Published: February 2004

In situ flight measurements of the hydroxyl (OH) and hydroperoxyl (HO2) radicals were performed in the exhaust plume of a Boeing 757 and in the contrail of a Douglas DC‐8 flying under typical cruise conditions in the upper troposphere and lower stratosphere during the SUCCESS mission of April and May 1996. Measurements were made in a range of exhaust plume ages from roughly 10 seconds to 20 minutes. OH mixing ratios were found to be greatly enhanced in the plumes, to 2–5 pptv from a background of 0.3–0.5 pptv in young plumes, and to 0.6 pptv from a background of ≤0.2pptv in older contrails. HO2 mixing ratios were heavily suppressed in the plumes, to 2–4 pptv from a background of 10–20 pptv, for both young plumes and older contrails.
Mantle discontinuities under southern Africa from precursors to P′ P′df
Authors: Xu Fei , Vidale John E. , Earle Paul S. , Benz Harley M.
Published: February 2004

We investigate the reflection properties of upper‐mantle discontinuities beneath southern Africa using precursors to the df branch of PKPPKP (P′ P′). The P′ P′df branch is weaker than the ab and bc branches, but it does not have the complication of a caustic and appears across a wider distance range. Stacks from hundreds of short‐period seismograms recorded in California from the March 9, 1994 Tonga earthquake (Mw = 7.6) show an ∼5% reflection (at 3.5 s dominate period) from 660‐km depth indicating a sharp “660” under southern Africa. A 3.5 s period reflection from 410‐km depth is also visible in these stacks, but only ∼2% the strength of P′ P′df. This result contrasts with the observation of the “410” and the “660” reflecting comparable amounts of high‐frequency energy under the Indian Ocean [Benz and Vidale, 1993a], indicating either a diffuse “410” boundary under southern Africa or global variations in the impedance change across the “410”. A 1.5 s period reflection may indicate the existence of fine‐scale heterogeneity near 320‐km depth. Reflectivity synthetic seismograms also show that a previously claimed reflection from 785‐km depth has the more likely explanation as PcPPKP.
Ionospheric effects of the solar eclipse of March 9, 1997, as deduced from GPS data
Authors: Afraimovich E. L. , Palamartchouk K. S. , Perevalova N. P. , Chernukhov V. V. , Lukhnev A. V. , Zalutsky V. T.
Published: February 2004

This paper presents data from first measurements of total electron content (TEC) and its gradient during the solar eclipse of March 9, 1997, obtained with the GPS‐radio interferometer at Irkutsk. The interferometer consists of three receivers (one Turbo‐Rogue SNR‐8000, and two Ashtech Z‐12) located at the vertices of a triangle and spaced by about 3–5 km. The measured TEC variations are indicative of profound changes in the ion production process in the ionosphere attendant on the solar eclipse, simultaneously in a large volume of space with a radius of at height 300 km at 300 km altitude. The delay of a minimum value of TEC with respect to the maximum phase of eclipse at 300 km altitude was about 10 min, and the depression depth of TEC growth varies from l÷3×1016m−2. By analyzing the data on TEC gradient variations, one is led to conclude that the depression of TEC growth during the eclipse is essentially independent of the longitude and of the latitude (within the observation ranges 52±6° N and 104±11° E).
On the inversion of Sd particle motion for seismic anisotropy in D'
Authors: Vinnik Lev , Bréger Ludovic , Romanowicz Barbara
Published: February 2004

In order to make accurate estimates of anisotropy in D', as measured from observations of Sdi f f (Sd) it is necessary to correct the observed waveforms for anisotropy outside of the deepest mantle as well as side refraction along the path, and to pick arrivals of SVd and SHd as accurately as possible. We show that corrections based on estimates of upper mantle anisotropy inferred from SKS/SKKS splitting measurements are sometimes insufficient and propose a master event method to directly obtain more accurate corrections on specific source‐station paths. Our study illustrates the complexity of structure that needs to be taken into account when addressing issues of D' properties using seismic body wave methods. We also describe a method to accurately measure the arrival time of the corrected SV d wave.
An in situ X ray diffraction study of the α‐β transformation kinetics of Mg2SiO4
Authors: Kubo Tomoaki , Ohtani Eiji , Kato Takumi , Morishima Hideaki , Yamazaki Daisuke , Suzuki Akio , Mibe Kenji , Kikegawa Takumi , Shimomura Osamu
Published: February 2004

This is the first report of the direct measurement of α‐β transformation kinetics in Mg2SiO4, a major constituent in the Earth's mantle, through in situ X ray diffraction experiments at high pressure. The experiments were conducted using the sintered diamond cubic‐anvil apparatus with synchrotron radiation (MAX 80). Although the present work is still preliminary, the kinetic data were analyzed using a nucleation and growth model. The growth rate of β‐phase is determined to be 3.3 (1.6∼4.9) × 10−10 m/s at 13.0 GPa and 900°C, 7.1 (3.4∼10) × 10−10 m/s at 14.1 GPa and 885°C, and 1.5 (0.7∼2.3) × 10−9 m/s at 14.9 GPa and 875°C.
Transient current collection and closure for a laboratory tether
Authors: Stenzel R. L. , Urrutia J. M.
Published: February 2004

The motion of tethered electrodes creates time‐varying plasma currents. A similar current system has been studied in a large laboratory plasma with a stationary, pulsed tether. It is shown that in the appropriate electron MHD regime, the transient current is transported in the whistler mode. The current exhibits helicity. The insulated tether induces electron Hall currents which provide the cross‐field current closure. The transient current increases with voltage and exhibits no saturation at the Langmuir limit due to electron heating. Superposition of delayed pulses emitted from displaced tether positions predicts that tethered satellite currents close along a short whistler wing rather than a long phantom loop.
A new sulfate‐mediated reaction: Conversion of acetone to trimethylbenzene in the presence of liquid sulfuric acid
Authors: Duncan Joanna L. , Schindler Liesl R. , Roberts Jeffrey T.
Published: February 2004

A new heterogeneous reaction, namely the sulfate‐mediated conversion of acetone to 4‐methyl‐3‐penten‐2‐one and trimethylbenzene, is reported. The reaction products were observed in their absorbed states by Fourier transform infrared spectroscopy and in the gas phase by mass spectrometry. In addition, protonated, absorbed acetone was observed under certain conditions. The reaction exhibits a strong concentration and / or temperature dependence, with non‐reactive uptake as protonated acetone dominating at low temperatures and sulfate concentrations and reactive uptake occurring above 200 K and 75 wt. % H2SO4. Under conditions where reactive uptake occurs, the steady‐state acetone loss probability is roughly 40%. It was not possible in this study to assess the relative effects of temperature and concentration on the reactivity of sulfate toward acetone.
Ion temperature anisotropies in the Earth's high‐latitude magnetosheath: Hawkeye observations
Authors: Tan L. C. , Fung S. F. , Kessel R. L. , Chen S.‐H. , Green J. L. , Eastman T. E.
Published: February 2004

We present here for the first time observations of the inverse correlation between the ion temperature anisotropy and plasma beta in the Earth's high‐latitude magnetosheath. Hot proton data with energies of 0.3–8 keV were obtained from magnetosheath passages by the Hawkeye spacecraft which had a polar orbit with an apogee of 20–21 RE. A newly developed technique has been used to calculate the distribution functions of protons in their non‐streaming frame in which their first‐order anisotropy is absent. The ion‐energy dependence of distribution functions indicates the existence of two hot ion components. Thus the correlation has been examined for each hot ion component separately. We have analyzed three Hawkeye magnetosheath passes during which the magnetosheath's magnetic field was close to the spacecraft spin plane, so that the two‐dimensional Hawkeye sensor can adequately sample temperature anisotropies. Results of our analyses are consistent with the theoretical prediction given by Gary et al. [1994; 1995] that a universal inverse‐correlation relationship exists between the temperature anisotropy and plasma beta of hot ions.
Ionospheric plasma bubble generated by Arecibo heater
Authors: Lee M. C. , Riddolls R. J. , Burke W. J. , Sulzer M. P. , Klien E. M. C. , Rowlands M. J. , Kuo S. P.
Published: February 2004

During recent experiments ionospheric plasma bubbles were excited by the upgraded HF heater at Arecibo. These plasma bubbles were observed by radar in the midnight sector with the entire flux tube in darkness. A simple model is outlined to explain the dynamics of density depletions generated during O‐mode wave heating of the F layer. We suggest that thermal expansion of plasma away from the heated volume leads to enhanced recombination along the flux tube. In the absence of photoionization sources, density depletions develop along the excited flux tube. The discontinuity of gravity‐driven currents at the walls of the depleted region requires development of polarization electric fields. Eastward polarization electric fields of ∼2.5 mV/m within the flux tube caused an observed plasma bubble to drift vertically at a speed of 70 m/s.
The standard deviation of column ozone from the zonal mean
Authors: Jiang Yibo , Yung Yuk L. , Douglass Anne R. , Tung K. K.
Published: February 2004

The standard deviation of column ozone from the zonal mean (COSDZ) provides a measure of the longitudinal inhomogeneity in ozone and dynamical wave activities in the atmosphere. Using the Total Ozone Mapping Spectrometer (TOMS) data, we obtain latitude‐season maps of COSDZ that are representative of a dynamically quiet year (1987) and a dynamically active year (1992). The spatial and temporal patterns of COSDZ show considerable similarity to the standard deviation of the 50 mb geopotential heights from the zonal mean. We point out that the simulation of this quantity by three‐dimensional (3‐D) models could provide a sensitive check of wave activities in the stratosphere that are responsible for ozone transport. Comparison between the observed COSDZ and the simulations of the GSFC 3‐D model of ozone reveals major discrepancies between data and model in the tropical stratosphere.
Geomagnetic field variations at low and high latitude during the January 10–11, 1997 magnetic cloud
Authors: Villante U. , Francia P. , Lepidi S. , De Lauretis M. , Pietropaolo E. , Cafarella L. , Meloni A. , Lazarus A. J. , Lepping R. P. , Mariani F.
Published: February 2004

On Jan. 10–11, 1997 a wide magnetic cloud reached the Earth triggering intense geomagnetic activity. Observations performed at low and very high latitude show that the same features appear simultaneously in correspondence to different changes in the solar wind conditions. In particular, highly polarized modes are simultaneously observed at the same discrete frequencies after the passage of the high density solar wind region following the cloud. SI's and ULF waves polarization are also examined in a wide latitudinal and longitudinal extent.
Sensitivity of simulated CFC‐11 distributions in a global ocean model to the treatment of salt rejected during sea‐ice formation
Authors: Caldeira Ken , Duffy Philip B.
Published: February 2004

We show that simulated oceanic absorption of an atmospheric gas is very sensitive to the representation of a process that occurs beneath sea ice. As sea ice forms, salt is rejected, locally increasing surface sea‐water density. This dense water can sink to the pycnocline at the base of the mixed‐layer. Previous studies have not considered the impact of this subgrid‐scale process on transient tracers in the ocean. To assess the potential importance of this process to the oceanic absorption of atmospheric gases, we performed two idealized simulations: a Control simulation in which salt rejected during sea‐ice formation is placed in the model's 25 m thick surface layer; and a Test simulation in which salt rejected during sea‐ice formation is distributed uniformly through the upper 160 m beneath the forming sea ice. Our treatment of rejected salt is highly idealized, and is intended to demonstrate the need for a physically‐based parameterization of subgrid‐scale convection for use in ocean general circulation models that takes into account the subgrid‐scale heterogeneity of surface buoyancy forcing. Distributing rejected salt more deeply during periods of ice formation helps to maintain vertical density gradients, inhibiting grid‐scale convection, especially in the Southern Ocean. This greatly diminishes simulated ocean uptake of CFC‐11, and generally improves simulated CFC‐11 and salinity fields. The modeled global ocean inventory of CFC‐11 for year 1990 is about 30% lower, and modeled column inventories in the Southern Ocean are up to 90% lower, in our Test simulation relative to our Control simulation. We infer that a more detailed treatment of subgrid‐scale processes occurring beneath sea ice may diminish simulated oceanic absorption of anthropogenic CO2, especially in the Southern Ocean.
In situ control of oxygen fugacity at high temperature and high pressure: A Ni‐O system
Authors: Li Heping , Xie Hongsen , Guo Jie , Zhang Yueming , Xu Zuming , Xu Ji‐an
Published: February 2004

To control in situ oxygen fugacity in a sample at high temperature and high pressure, the oxygen pump that is usually used at ambient pressure was introduced into a high‐pressure system in the present work. Inside the sample assembly, an oxygen buffer was used as an oxygen reservoir. By means of our high‐pressure oxygen pump, oxygen could be pumped from the sample into the oxygen reservoir or from the reservoir into the sample at high temperature and high pressure. Meanwhile, the oxygen fugacity in the sample could be monitored in situ by inserting an oxygen sensor inside the sample assembly. Our experiments using Ni‐O system show that the oxygen fugacity in the sample at high temperature and high pressure can be conveniently controlled and monitored in situ independently of temperature and pressure.
An empirical evolution law of fractal size frequency of fault population and its similarity law
Authors: Otsuki Kenshiro
Published: February 2004

Size (displacement) frequencies of 10 fault populations follow a power‐law associated with cut‐off in the range smaller than a characteristic size. They are depicted by five geometrical parameters of fault population; cut‐off size, sharpness of cut‐off, fault number density, fault displacement density and the power‐law exponent. The former three are rock properties related to dissipative energy density and constitute the similarity law of the size frequency. The fault displacement density is a measure of total input energy density. The power‐law exponent decreases as the − 1/4 power of total input energy density normalized by dissipative energy density.
A global modeling study of solid rocket aluminum oxide emission effects on stratospheric ozone
Authors: Jackman Charles H. , Considine David B. , Fleming Eric L.
Published: February 2004

Recent laboratory measurements [Molina et al. 1997] have indicated that the heterogeneous chlorine activation reaction ClONO2+HCl ⟶ HNO3+Cl2 has a reaction probability of about 0.02 on aluminum oxide particles (alumina). Since alumina is among those substances emitted by solid rocket motors (SRMs), we have assessed the heterogeneous chemical impact of SRM‐emitted alumina on stratospheric ozone using the Goddard Space Flight Center two‐dimensional photochemistry and transport model. Historical launch rates of the Space Shuttle, Titan III, and Titan IV rockets were used in time‐dependent and steady‐state model calculations. Variations in the temporal ozone decreases reflected the fluctuation in launch rate frequency. The annually averaged global total ozone (AAGTO) is computed to decrease by 0.025% by the year 1997. About one‐third of this AAGTO change results from the SRM‐emitted alumina while about two‐thirds is due to SRM‐emitted hydrogen chloride.
Evidence for increasing ultraviolet irradiance at Point Barrow, Alaska
Authors: Gurney Kevin Robert
Published: February 2004

This paper presents analysis of noontime ultraviolet irradiance measurements made at Point Barrow, Alaska (71.2° N, 159° W) between the years 1991 and 1996. Eighteen wavelengths of the solar irradiance spectrum between 290 nm and 340 nm were examined for multiyear trends. This spectral region is particularly critical as these wavelengths are known to cause biological damage. After reducing the effects of clouds, 305 nanometer wavelength increases ranging between 3 and 10% per year were found in all daylit months except June which exhibited an annual average decline of approximately −3% per year at this wavelength. The correlation between the absorption cross‐section of ozone and the spectral character of the UV‐B changes suggest that declines in column ozone are responsible for the dramatic 305 nm increases. The unexpected decline in June 305 nm irradiance may be due to an increase in column ozone over this location in June.
Crustal thickness variations across the northern Tien Shan from teleseismic receiver functions
Authors: Bump Hedwig A. , Sheehan Anne F.
Published: February 2004

Variations in crustal thickness from the Kazakh Shield to the north‐central Tien Shan are determined by receiver function analysis of broadband teleseismic P waveforms recorded on the Kyrgyzstan Seismic Telemetry Network (KNET). The receiver functions are calculated using a time‐domain deconvolution approach, and are interpreted in terms of a single crustal layer, with thickness determined by a grid‐search comparison of observed receiver functions with synthetics. Crustal thicknesses average 42 km in the southern Kazakh Shield and increase to approximately 60 km in the Tien Shan. Together with mass balance calculations, these results are consistent with the hypothesis that the compensation of the central Tien Shan relative to the Kazakh Shield are accommodated by an Airy‐type crustal root, with a modest (300 kg/m³) density contrast across the Moho.
High‐pressure transformation of pyrope (Mg3Al2Si3O12) in a sintered diamond cubic anvil assembly
Authors: Ito E. , Kubo A. , Katsura T. , Akaogi M. , Fujita T.
Published: February 2004

Transformation of pyrope garnet was investigated at 1600°C and at pressures higher than 35 GPa using a sintered diamond cubic anvil assembly in a double‐staged split‐sphere apparatus. Pyrope transformed into orthorhombic perovskite at ca. 37 GPa, whereas a small amount of corundum coexisted with perovskite at lower pressures. The unit cell dimensions of perovskite were determined to be a = 4.771(2) Å, b = 4.947(2) Å, c = 6.950(3) Å, and V = 164.0(2) Å³. A trace of stishovite observed in the run products may be evidence of nonstoichiometry in the perovskite. These results support the view that perovskite is the major host phase of Al2O3 in the lower mantle.
Azimuthal pressure gradient as driving force of substorm currents
Authors: Shiokawa K. , Haerendel G. , Baumjohann W.
Published: February 2004

We have studied the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9–15 RE. The pressure gradient is statistically estimated for the interval when the magnetic field shows a dipolar configuration (elevation angle >45°). It is found that by this criterion, most data are obtained during and after the passage of high‐speed ion flow in the vicinity of the neutral sheet during magnetically active times. We show that there is an azimuthal gradient of plasma pressure in the dipolar field region. The pressure gradient can drive a substantial amount of field‐aligned current (4.1×105 A per 2‐hour local time sector). We suggest that this current is a source of the substorm current system after the high‐speed ion flow stops.