bluuebunny/crossref_metadata_2025 huggingface dataset:
Identification and uncertainty analysis of soil water retention models using lysimeter data
Authors: Chen J. , Wheater H. S.
Published: December 2003

The water retention characteristic is one of the fundamental properties that control water movement in unsaturated soils. Because of the complexity and heterogeneity of soil properties, variability and uncertainty of this property has long been a key issue in understanding the relationship between water content and matric pressure. Although considerable progress has been achieved in understanding spatial and local variability, discussions have primarily been based on deterministic analyses. Misleading conclusions can result since the derived variability cannot often be discriminated from uncertainties in model parameters, structure, and observed data. Using a combined deterministic and stochastic analysis, this paper addresses the variability of the retention characteristic in association with the uncertainty of water retention models. Calibrated against two sets of lysimeter data observed over 2 years, the analysis yields insights into the physical changes in the system, and model performance is significantly improved when these effects are explicitly included.
Determination of recharge in unsaturated soils using temperature monitoring
Authors: Tabbagh Alain , Bendjoudi Hocine , Benderitter Yves
Published: December 2003

Can shallow temperature measurements on vertical profiles be used to determine the recharge? As a first approach to answer this question, we consider here the soil temperature data recorded by meteorological stations. A group of annual cycles allows us to determine the average value of vertical water seepage: in the presence of infiltration the apparent diffusivity deduced from the damping of the amplitude of the temperature between two different depths differs from the apparent diffusivity deduced from the temperature phase shift between the same pair of depths, and the average water flow can be easily deduced from this difference. For the determination of recharge over shorter periods of time it would be better to reconsider the sampling step of the data in both time and space domains. These limitations can be overcome by installing specialized vertical temperature profiles.
Unconventional flow of water from dry to wet caused by hysteresis: A numerical experiment
Authors: Heinen Marius , Raats Peter A. C.
Published: December 2003

We demonstrate with the help of a numerical simulation model for hysteretic water movement that for horizontal redistribution three types of flow can occur. Raats and Van Duijn [1995] previously identified these types as a reaction to a specific analysis by Philip [1991]. Depending on the initial conditions in two parts of an infinitely long soil column, there can be no flow, conventional flow from the wet part to the dry part, or unconventional flow from the dry part to the wet part. When flow occurs, the cumulative wetting and cumulative drying both show the expected linear relationship with the square root of time. Hysteresis is described according to the modified dependent domain model of Mualem [1984]. We also indicate that Kool and Parker [1987], who used this model as well, presented a erroneous expression for the differential moisture capacity of the scanning drying curve.
Predicting saturated hydraulic conductivity from air permeability: Application in stochastic water infiltration modeling
Authors: Loll Per , Moldrup Per , Schjønning Per , Riley Hugh
Published: December 2003

Several relationships exist for predicting unsaturated hydraulic conductivity K(ψ) from saturated hydraulic conductivity Ks and the soil‐water retention curve. These relationships are convenient for modeling of field scale system sensitivity to spatial variability in K(ψ) . It is, however, faster and simpler to measure air permeability ka at ψ = −100 cm H2O, than Ks. This study explores the existence of a general prediction relationship between ka, measured at −100 cm H2O, and Ks. Comparative analyses between ka‐Ks relationships for nine Danish and Norwegian soils, six different soil treatments, and three horizons validated the establishment of a soil type, soil treatment, and depth/horizon independent log‐log linear ka‐Ks relationship. The general ka‐Ks relationship is based on data from a total of 1614 undisturbed, 100‐cm3 core samples and displays general prediction accuracy better than ±0.7 orders of magnitude. The accuracy and usefulness of the general relationship was evaluated through stochastic analyses of field scale infiltration and ponding during a rainstorm event. These analyses showed possible prediction bias associated with the general ka‐Ks relationship, but also revealed that sampling uncertainty associated with estimation of field scale variability in Ks from a limited number of samples could easily be larger than the possible prediction bias.
Detectability of variations in continental water storage from satellite observations of the time dependent gravity field
Authors: Rodell M. , Famiglietti J. S.
Published: December 2003

Continental water storage is a key variable in the Earth system that has never been adequately monitored globally. Since variations in water storage on land affect the time dependent component of Earth's gravity field, the NASA Gravity Recovery and Climate Experiment (GRACE) satellite mission, which will accurately map the gravity field at 2–4 week intervals, may soon provide global data on temporal changes in continental water storage. This study characterizes water storage changes in 20 drainage basins ranging in size from 130,000 to 5,782,000 km2 and uses estimates of uncertainty in the GRACE technique to determine in which basins water storage changes may be detectable by GRACE and how this detectability may vary in space and time. Results indicate that GRACE will likely detect changes in water storage in most of the basins on monthly or longer time steps and that instrument errors, atmospheric modeling errors, and the magnitude of the variations themselves will be the primary controls on the relative accuracy of the GRACE‐derived estimates.
Effect of bias adjustment and rain gauge data quality control on radar rainfall estimation
Authors: Steiner Matthias , Smith James A. , Burges Stephen J. , Alonso Carlos V. , Darden Robert W.
Published: December 2003

Thirty major storms that passed over Goodwin Creek, a small research watershed in northern Mississippi, were analyzed to assess the bias between radar rainfall estimates at rain gauge locations and the gauge amounts. These storms, each contributing at least 10 mm of storm total rainfall, accumulated approximately 785 mm of rain, which corresponds to about half the average annual rainfall amount for the area. The focus of this study was to demonstrate the importance of (1) bias adjustment of the radar rainfall estimates and (2) the quality control of the rain gauge data used for bias adjustment. The analyses are based on Memphis Weather Surveillance Radar—1988 Doppler radar data, tipping‐bucket rain gauge data, and raindrop spectra information collected within the Goodwin Creek catchment. Because of measurement and rainfall estimation uncertainties, radar observations are often combined with rain gauge data to obtain the most accurate rainfall estimates. Rain gauge data, however, are subject to characteristic error sources: for Goodwin Creek, malfunctioning of the tipping‐bucket rain gauges was frequently caused by biological and mechanical fouling, and human interference. Therefore careful quality control of the rain gauge data is crucial, and only good quality rain gauge information should be used for adjusting radar rainfall estimates. By using high‐quality gauge data and storm‐based bias adjustment, we achieved radar rainfall estimates with root‐mean‐square errors (RMSE) of approximately 10% for storm total rainfall accumulations of 30 mm or more. Differences resulting from radar data processing scenarios were found to be small compared to the effect caused by bias adjustment and using high‐quality rain gauge data.
The interpretation of a tracer experiment conducted in a single fracture under conditions of natural groundwater flow
Authors: Lapcevic P. A. , Novakowski K. S. , Sudicky E. A.
Published: December 2003

The development of conceptual models to describe the hydrogeology of sparsely fractured media requires the characterization of the properties of discrete fractures at the field scale. In this study, a tracer experiment conducted under conditions of natural groundwater flow in a discrete fracture in an interbedded shale and limestone sequence is interpreted. This experiment, which was initiated by injecting a small quantity of tracer into the fracture plane, involved monitoring tracer movement using 27 boreholes within a 35×40 m area. Test data revealed a tracer plume which spread both longitudinally and transversely in the direction of mean groundwater flow. The field breakthrough curves were interpreted using a two‐dimensional finite element transport model that incorporated longitudinal and transverse dispersion, diffusion into the rock matrix, and constant fracture aperture. Additional simulations which incorporated the effects of aperture variability were conducted in a Monte Carlo format by varying the spatial correlation and variance of the aperture field. The constant aperture analysis found that the mean aperture determined from the tracer experiment was approximately 20% greater than the mean aperture measured by hydraulic methods. Values of matrix porosity ranging between 1 and 3%, a constant longitudinal dispersivity of 0.1 m, and a large range in transverse dispersivity from 0.01 to 0.22 m were required to simulate the data. Trends of increasing aperture and matrix porosity with distance were observed, suggesting that tracer transport followed increasingly tortuous pathways. Although the variable aperture simulations were not fitted to the field data, the aperture field having an isotropic spatial correlation length of 0.5 m and variance of 10,000 μm2 provided simulated plumes that appeared to be most similar to the shape and concentration of the field plume. Evidence from the hydraulic measurement of the aperture distribution suggests that this is a reasonable estimate of the natural aperture field.
Nonlinear reactions and nonuniform flows
Authors: Chilakapati Ashok , Yabusaki Steve
Published: December 2003

Analytic solutions are developed for the advection‐reaction problem with nonuniform flows and nonlinear reactions. We consider a stratified flow field and a flow field due to the operation of an injection‐extraction pair of wells in a homogeneous medium. In each case we derive the transfer function for transport between a pair of surfaces in the flow field. We consider linear/nonlinear aqueous kinetics and the nonlinear equilibrium adsorption of a Langmuir radionuclide with decay. Using a stream tube concept in conjunction with these transfer functions, we develop analytic solutions to the flux‐averaged concentration response of these reactive solutes. Resident concentrations are obtained by solving a simpler batch reaction system in place of coupled partial differential equations. We use these solutions to evaluate two numerical approaches. One of them is based on an operator‐split approach that employs a conventional finite difference scheme with total variation diminishing fluxes for the advection operator. The other is based on the use of advected particles to compute a numerical approximation of the transfer function. We consider a boundary‐value problem and establish that the particle‐tracking approach yields near‐analytic solutions for both the flux‐averaged and resident concentrations. We consider an initial‐value problem and evaluate the “dilution index” [Kitanidis, 1994] associated with the advective flow field and the numerical scheme. A flow field that highly distorts a plume and increases its surface area exhibits a larger dilution index, and most numerical schemes for transport in such a flow field suffer larger numerical dispersion. For linear reactions the predicted amount of plume degradation is unaffected by plume deformation, while the errors in predictions for nonlinear reactions increase as the plume deformation increases.
Slow two‐phase flow in artificial fractures: Experiments and simulations
Authors: Amundsen Håkon , Wagner Geri , Oxaal Unni , Meakin Paul , Feder Jens , Jøssang Torstein
Published: December 2003

The slow displacement of a wetting fluid by a nonwetting fluid in models of a single fracture was studied experimentally and by computer simulations on identical geometries. The fracture was modeled by the gap between a rough plate and a smooth transparent plate, both oriented horizontally. Two different rough plates were used, a textured glass plate and a polymethyl methacrylate plate with a computer‐generated pattern. A nonwetting fluid (air) was injected slowly through an inlet into the model and displaced a wetting fluid (water) initially filling the model. The aperture fields of the artifical fractures were measured using a light absorption technique. The experiments were simulated using modified invasion percolation models, making use of the measured aperture fields. The simulation models captured invasion bursts and fragmentation and redistribution of the invading air. Experiments and simulations were compared step by step, and good qualitative and quantitative agreement was found.
Empirical comparison of various methods for training feed‐Forward neural networks for salinity forecasting
Authors: Maier Holger R. , Dandy Graeme C.
Published: December 2003

Feed‐forward artificial neural networks (ANNs) are being used increasingly to model water resources variables. In this technical note, six methods for optimizing the connection weights of feedforward ANNs are investigated in terms of generalization ability, parsimony, and training speed. These include the generalized delta (GD) rule, the normalized cumulative delta (NCD) rule, the delta‐bar‐delta (DBD) algorithm, the extended‐delta‐bar‐delta (EDBD) algorithm, the QuickProp (QP) algorithm, and the MaxProp (MP) algorithm. Each of these algorithms is applied to a particular case study, the forecasting of salinity in the River Murray at Murray Bridge, South Australia. Thirty models are developed for each algorithm, starting from different positions in weight space. The results obtained indicate that the generalization ability of the first‐order methods investigated (i.e., GD, NCD, DBD, and EDBD) is better than that of the second‐order algorithms (i.e., QP and MP). When the prediction errors are averaged over the 30 trials carried out, the performance of the first‐order methods in which the size of the steps taken in weight space is automatically adjusted in response to changes in the error surface (i.e., DBD and EDBD) is better than that obtained when predetermined step sizes are used (i.e., GD and NCD). However, the reverse applies when the best forecasts of the 30 trials are considered. The results obtained indicate that the EDBD algorithm is the most parsimonious and the MP algorithm is the least parsimonious. It was found that any impact different learning rules have on training speed is masked by the effect of epoch size and the number of hidden nodes required for optimal model performance.
Episodic lake acidification in the Sierra Nevada, California
Authors: Leydecker Al , Sickman James O. , Melack John M.
Published: December 2003

Seven high‐altitude headwater catchments were studied from 1990 to 1994 to evaluate susceptibility to episodic acid neutralizing capacity (ANC) depression. Dilution (decreasing base cation concentrations) was the primary factor in ANC depression during snowmelt, accounting for 75 to 97% of the ANC reduction. In lakes where acidification (increasing anion concentrations) was noted, nitrate and sulfate were equally important during the first half of snowmelt, while sulfate dominated the latter half. A linear model, based on the relationship between minimum and fall‐overturn ANC for the lakes in our study, estimated that none of the 114 lakes sampled during the 1985 EPA Western Lakes Survey had been episodically acidified (ANC < 0). Modifications of the model were used to predict that approximately 6 and 10% of Sierran lakes will become episodically acidified with increases in nitrate and sulfate deposition of 50 and 150%, respectively. No lakes will be chronically acidified with these depositional increases.
A Probability model of scour and fill depths in gravel‐bed channels
Authors: Haschenburger Judith K.
Published: December 2003

The exponential density function was fitted to frequency distributions of scour and fill depths derived from six single‐threaded gravel‐bed channels. Scour chains and monitors were used to record the depth of bed activity in the 6 to 20 m wide channels. Goodness‐of‐fit analyses indicate that the exponential function is a plausible model. Introduction of dimensionless shear stress (т*) collapses model parameters and the active proportion of the streambed of individual rivers into general trends. An exponential relation between model parameters and т* suggests a means to derive scour and fill depths to a first approximation based only on flow and channel characteristics. Exponentially distributed depths require over 400 observations to estimate the mean to within 10% with 95% confidence.
A reformulation of Horton's Laws for large river networks in terms of statistical self‐similarity
Authors: Peckham Scott D. , Gupta Vijay K.
Published: December 2003

The well‐known Norton's laws are empirical observations on how the means of measurements for river networks and basins vary with Horton‐Strahler order. It is now known that these laws are a consequence of an average‐sense self‐similarity in the bifurcation structure of river networks. In this paper we present a reformulation of Horton's laws which generalizes the familiar scaling of first moments, or means, to scaling of entire distributions. We also present extensive data analysis which supports this reformulation and show that this feature is also exhibited by Shreve's well‐known random topology model.
Best linear unbiased design hyetograph
Authors: Veneziano Daniele , Villani Paolo
Published: December 2003

Under the conditions of system linearity and stationarity of the precipitation process we derive the hyetograph associated with any given flood discharge Q, using best linear unbiased estimation (BLUE) theory. The BLUE hyetograph depends explicitly on the correlation characteristics of the rainfall process and the instantaneous unit hydrograph (IUH) of the basin. When the correlation time of the rainfall process is short relative to the dispersion of the IUH, the shape of the BLUE hyetograph is the mirror image of the IUH itself. In the opposite case the design hyetograph is the mirror image of the rainfall correlation function. The procedure is extended from lumped to semidistributed and distributed basins. In the latter case we obtain simultaneous design hyetographs for each subbasin or each site in the basin. The analytical findings and the possibility of regionalization are verified using synthetic and recorded rainfall time series, with excellent agreement in both cases. Because of its simplicity, robustness, and theoretical basis the proposed procedure should be appealing for practical use, particularly when not only the peak but the entire flood hydrograph is required.
Evaluation of prediction intervals for expressing uncertainties in groundwater flow model predictions
Authors: Christensen Steen , Cooley Richard L.
Published: December 2003

We tested the accuracy of 95% individual prediction intervals for hydraulic heads, streamflow gains, and effective transmissivities computed by groundwater models of two Danish aquifers. To compute the intervals, we assumed that each predicted value can be written as the sum of a computed dependent variable and a random error. Testing was accomplished by using a cross‐validation method and by using new field measurements of hydraulic heads and transmissivities that were not used to develop or calibrate the models. The tested null hypotheses are that the coverage probability of the prediction intervals is not significantly smaller than the assumed probability (95%) and that each tail probability is not significantly different from the assumed probability (2.5%). In all cases tested, these hypotheses were accepted at the 5% level of significance. We therefore conclude that for the groundwater models of two real aquifers the individual prediction intervals appear to be accurate.
Measurement of fracture aperture fields using transmitted light: An evaluation of measurement errors and their influence on simulations of flow and transport through a single fracture
Authors: Detwiler Russell L. , Pringle Scott E. , Glass Robert J.
Published: December 2003

Understanding of single‐phase and multiphase flow and transport in fractures can be greatly enhanced through experimentation in transparent systems (analogs or replicas) where light transmission techniques yield quantitative measurements of aperture, solute concentration, and phase saturation fields. Here we quantify aperture field measurement error and demonstrate the influence of this error on the results of flow and transport simulations (hypothesized experimental results) through saturated and partially saturated fractures. We find that precision and accuracy can be balanced to greatly improve the technique and present a measurement protocol to obtain a minimum error field. Simulation results show an increased sensitivity to error as we move from flow to transport and from saturated to partially saturated conditions. Significant sensitivity under partially saturated conditions results in differences in channeling and multiple‐peaked breakthrough curves. These results emphasize the critical importance of defining and minimizing error for studies of flow and transport in single fractures.
Modeling uncertainty in reservoir loss functions using fuzzy sets
Authors: Teegavarapu Ramesh S. V. , Simonovic Slobodan P.
Published: December 2003

Imprecision involved in the definition of reservoir loss functions is addressed using fuzzy set theory concepts. A reservoir operation problem is solved using the concepts of fuzzy mathematical programming. Membership functions from fuzzy set theory are used to represent the decision maker's preferences in the definition of shape of loss curves. These functions are assumed to be known and are used to model the uncertainties. Linear and nonlinear optimization models are developed under fuzzy environment. A new approach is presented that involves development of compromise reservoir operating policies based on the rules from the traditional optimization models and their fuzzy equivalents while considering the preferences of the decision maker. The imprecision associated with the definition of penalty and storage zones and uncertainty in the penalty coefficients are the main issues addressed through this study. The models developed are applied to the Green Reservoir, Kentucky. Simulations are performed to evaluate the operating rules generated by the models considering the uncertainties in the loss functions. Results indicate that the reservoir operating policies are sensitive to change in the shapes of loss functions.
Connectivity of joint networks with power law length distributions
Authors: Renshaw Carl E.
Published: December 2003

Over the range of fracture spatial densities commonly observed in the field, existing effective medium models for network connectivity yield inconsistent results because the connectivity depends on both the density and length distribution of the joints. Field observations indicate that joint lengths often follow a power law length distribution. Requiring the frequency of joint lengths to decrease with increasing length and the total area of the joints to be finite suggests approximate upper and lower bounds on the power law exponent which are consistent with observed distributions. The appropriate relationship between connectivity and density is investigated numerically. Results motivate a new measure of fracture intensity, the effective fracture density, and the development of a new approach toward a closed‐form prediction of connectivity using measurable parameters. Although limited, model predictions compare favorably to the conductivities of networks simulated using a geomechanical model and observed in the field.
Estimation in the Pearson type 3 distribution
Authors: Koutrouvelis Ioannis A. , Canavos George C.
Published: December 2003

Although the Pearson type 3 (P3) is one of the basic models in statistical hydrology, its use to model untransformed data has been restrained because of difficulties encountered in fitting this distribution by traditional methods. In this paper an adaptive estimation procedure of mixed moments for the P3 family is introduced which is based on several fractional moments of the exponentially transformed data and the mean of the original data. The procedure is easy to implement in small samples and is valid over the entire parameter space. Explicit formulae for the variances and covariances of parameter estimators and of the variance of the T‐year event are derived. In addition, two variants of the new procedure are compared with two versions of the method of moments and a version of the method of conditional moments via Monte Carlo simulation. With samples generated from P3 populations, it is found that one of the variants of the new procedure is the best overall method in estimating 100‐year flood events, and the other variant is best in estimating the median and 10‐year low‐flow events. The good performance of these two variants is also observed in samples generated from alternatives to P3 distributions. A modification of the procedure is also introduced and investigated when a prior assumption of positive skewness is adopted.
Leveling rod base required for surveying gravel river bed surface elevations
Authors: De Vries Paul , Goold Desmond J.
Published: December 2003

Consistent surveying of bed elevations in natural channels containing gravel and cobble substrates is subject to measurement error if a leveling rod is placed indiscriminately between or on top of individual particles. The influence of surface microtopography decreases with increasing leveling rod base diameter, while the influence of bed form topography increases. Field tests were performed to identify a rod base diameter that minimizes these survey errors. A leveling rod base diameter of 127 mm (5 inches) is suggested to be most appropriate for consistent bed surveys.
Saturated hydraulic conductivity prediction from microscopic pore geometry measurements and neural network analysis
Authors: Lebron I. , Schaap M. G. , Suarez D. L.
Published: December 2003

Traditional models to describe hydraulic properties in soils are constrained by the assumption of cylindrical capillarity to account for the geometry of the pore space. This study was conducted to develop a new methodology to directly measure the porosity and its microscopic characteristics. The methodology is based on the analysis of binary images collected with a backscattered electron detector from thin sections of soils. Pore surface area, perimeter, roughness, circularity, and maximum and average diameter were quantified in 36 thin sections prepared from undisturbed soils. Saturated hydraulic conductivity Ksat, particle size distribution, particle density, bulk density, and chemical properties were determined on the same cores. We used the Kozeny‐Carman equation and neural network and bootstrap analysis to predict a formation factor from microscopic, macroscopic, and chemical data. The predicted Ksat was in excellent agreement with the measured Ksat (R2=0.91) when a hydraulic radius rH defined as pore area divided by pore perimeter and the formation factor were included in the Kozeny‐Carman equation.
Long‐term behavior of the OI 558 nm emission in the night sky and its aeronomical implications
Authors: Deutsch K. A. , Hernandez G.
Published: December 2003

Study of the long‐term behavior of the OI 558 nm emission in the night‐sky, using data encompassing observations at low geomagnetic activity from 1923 to present, gives the result that these long‐term, noisy, geophysical time series need to include nearly 20 cycles of an oscillation before a stable statistically meaningful answer is achieved. Periodogram analysis reveals that the strongest and predominant statistically significant oscillation in the OI 558 nm emission is a yearly variation. When this yearly oscillation is the main oscillation present in the spectrum, it has a fractional amplitude of value near 0.4. A semiannual oscillation is often also found in addition to the annual oscillation and, when significant annual and semiannual components are present, the fractional amplitudes are typically near 0.3 and 0.2 for the annual and semiannual oscillations, respectively. However, the annual fractional amplitude increases with latitude, while the semiannual fractional amplitude decreases with latitude. These results provide an insight on the nature of the seasonal variation of the OI 558 nm emission. Our longest time series are about 12 years long and thus set the lowest frequency to be studied. It is found that the OI 558 nm emission rate increases with solar activity, but in general, the emission is higher during the decreasing phase of solar activity; the apparent relationship changes with different solar cycles. OI 558 nm emission observations have been used to provide observational constraints to global dynamical atmospheric models. However, those constraining observational series have been of limited length and location and are possibly atypical. In the context of a model constrained by such observations, one can infer differences in the dynamical behavior of the atmosphere with latitude in order to account for the observed climatological emission behavior.
Effect of crustal magnetic fields on the near terminator ionosphere at Mars: Comparison of in situ magnetic field measurements with the data of radio science experiments on board Mars Global Surveyor
Authors: Krymskii A. M. , Breus T. K. , Ness N. F. , Hinson D. P. , Bojkov D. I.
Published: December 2003

Electron density profiles of the ionosphere of Mars derived from radio occultation data obtained by the radio science Mars Global Surveyor (MGS) experiment have been compared with the crustal magnetic fields measured by the MGS Magnetometer/Electron Reflectometer (MAG/ER) experiment. In the northern hemisphere the crustal fields are rather weak and usually do not prevent direct interaction between the solar wind (SW) and the Martian ionosphere/atmosphere. Exceptions do occur in the isolated mini‐magnetospheres formed by the crustal anomalies. Much stronger crustal fields are located in limited regions in the southern hemisphere and lead to the formation of large‐scale mini‐magnetospheres. A study of the electron density profiles obtained when the mini‐magnetosphere regions are near the terminator has been conducted using the magnetic field measurements at altitudes 170–180 km and 400 km. The magnitude of the electron density peak and the effective scale‐height of the electron density for two altitude ranges, 145–165 km and 165–185 km, have been derived for each of the 326 selected profiles studied. In each hemisphere, the longitudinal variations of these derived parameters have been studied. A significant difference between the large‐scale mini‐magnetospheres and regions outside of them has been found. The neutral atmosphere is cooler inside the large‐scale mini‐magnetospheres. The variations of the magnitude of the electron density peak indicate that the electrons are usually “hotter” inside a large‐scale mini‐magnetosphere than outside. It appears that the mini‐magnetospheres formed by strong crustal magnetic fields prevent additional heating of the neutral atmosphere due to direct interaction of the SW and also confine the hotter electrons created by photo ionization.
Magnetospheric response to sudden changes in solar wind dynamic pressure inferred from polar cap index
Authors: Lukianova R.
Published: December 2003

We have used the polar cap (PC) index, which is a measure of magnetic disturbance caused by the transpolar portion of ionospheric currents associated with global convection, to investigate the effect of sudden changes in the solar wind dynamic pressure (PSW). We find that during a period of steady interplanetary magnetic field (IMF), the PC index shows an enhancement that is a direct response to an enhancement in PSW. Examination of the response associated with the passage of interplanetary clouds accompanied by magnetic storms shows that a PSW pulse can cause changes in the PC index that are as large as the effects due to the changes in the interplanetary electric field. Responses of the PC index, AE index, and low‐latitude horizontal component of magnetic field are observed to be similar, though during a magnetic storm, the influence of a significant ring current can distort the dayside low‐latitude response. Sudden increases in PSW lead to an initial negative spike in the PC index that is followed by the main increase. Sudden decreases in PSW are found to have the opposite effect: an initial positive spike that is followed by a decrease in the PC index. The spike of ∼3 min duration is found to initiate simultaneously with the sudden impulse or commencement and an AE spike on the ground and the same time that geosynchronous particle fluxes start to increase sharply in response to magnetic field compression. It likely reflects a temporal response of convection to an abrupt change in PSW that is initially of the opposite sign to the more prolonged response, and it may be associated with short time scale current vortices that have been observed in the ionosphere near noon. The above results show that both PSW and IMF play a significant role in controlling the strength of magnetospheric convection.
The Solar System in a dense interstellar cloud: Implications for cosmic‐ray fluxes at Earth and 10Be records
Authors: Florinski V. , Zank G. P. , Axford W. I.
Published: December 2003

The structure of the heliosphere is investigated for a period when the Sun enters a cold dense unmagnetized interstellar cloud. Under these conditions the heliospheric interface resembles the two‐shock standard model with the termination shock located close to 23 AU in the upwind direction. Anomalous and galactic cosmic ray fluxes are calculated and compared with their present day intensities in the inner heliosphere. A tenfold increase in anomalous cosmic ray and a twofold increase in galactic cosmic ray intensities are possible if heliosheath modulation is reduced, in this high‐density case. On the basis of our heliospheric modeling, we show that a possible explanation of the enhancement of cosmogenic elements in ice and sediment samples 35,000 and 60,000 years BP might be an encounter of the Sun with a dense interstellar cloud.
Correlated solar wind speed, density, and magnetic field changes at Voyager 2
Authors: Richardson J. D. , Wang C. , Burlaga L. F.
Published: December 2003

The character of the solar wind plasma data observed by Voyager 2 recently changed to a regime in which the speed, density and magnetic field magnitude are positively correlated. In the inner heliosphere, the density and speed are generally anti‐correlated. As streams interact while propagating outward, this correlation weakens. Outside 65 AU, Voyager observed large, in‐phase, fluctuations of speed, density, and magnetic field magnitude with time scales of 6–12 months. The dynamic pressure varies by a factor of ten in these fluctuations, which should produce motions in the termination shock. We use ACE data from 1 AU as input to a 1‐D MHD model which includes pickup ions to model the radial evolution of the solar wind. The model reproduces the basic character (but not the details) of the observations, predicting correlated variations in speed, density, and magnetic field with time scales similar to those observed.
The return of the heliospheric 2–3 kHz radio emission during solar cycle 23
Authors: Gurnett D. A. , Kurth W. S. , Stone E. C.
Published: December 2003

In this paper we report the detection of a new heliospheric 2–3 kHz radio emission event by the Voyager 1 spacecraft, the first to be observed during solar cycle 23. The new event started on Nov. 1, 2002, and is believed to be associated with a strong interplanetary shock that originated from a period of intense solar activity in early April 2001. Following previous interpretations of events of this type, we assume that the radio emission is produced when the interplanetary shock interacted with the heliopause, which is the boundary between the solar wind and the interplanetary medium. From the onset time of the radio emission and a simple model for the propagation speed of the interplanetary shock, the heliocentric radial distance to the nose of the heliopause can be calculated, and is about 153 to 158 AU, depending on the parameters used. From computer simulations that give the ratio of the radial distance to the termination shock to the radial distance to the heliopause, the distance to the termination shock can also be calculated and is estimated to be about 101 to 108 AU.
ζ potential measurement of volcanic rocks from Aso caldera
Authors: Hase H. , Ishido T. , Takakura S. , Hashimoto T. , Sato K. , Tanaka Y.
Published: December 2003

We deduced the ζ potential of various rocks in Aso caldera from streaming potential measurements in laboratory. In contrast to the conventional observation that the ζ potential of crustal rocks is mostly negative under typical geologic conditions, almost half of the samples equilibrated with dilute KCl solution show positive values of ζ potential at pH 4.5–5.5. The samples showing positive ζ are characterized by relatively low SiO2 content and abundance of elements having high isoelectric points, and found to be localized around the summit of Mt. Takadake where the positive correlation between self‐potential (SP) and altitude is observed. We emphasize that measurements of ζ potential of volcanic rocks are very important to give more appropriate interpretations of SP data from volcanic fields.
Simultaneous ionospheric variability on Earth and Mars
Authors: Mendillo Michael , Smith Steven , Wroten Joei , Rishbeth Henry , Hinson David
Published: December 2003

Ionospheric physics deals with the basic structure and variability of plasma within the upper atmospheres of the Earth and planets. Comparative studies foster both exploration and synthesis within diverse settings. In this paper we examine observations of the ionospheric profiles on Mars obtained on 9–27 March 1999 by the radio science experiment onboard the Mars Global Surveyor satellite, and compare their day‐to‐day variability with same‐day observations on Earth. Using photochemical‐equilibrium arguments applicable to the peak electron density layer on Mars and the E‐layer on Earth, we find basic agreement in scaling laws between the planets, and in the details of correlations with simultaneous solar flux variations during a period of pronounced solar activity. We ascribe the residual variabilities (larger on Mars than Earth) to both observational uncertainties and to nonsolar mechanisms in need of further study on both planets.
Merged interaction regions at 1 AU
Authors: Burlaga L. , Berdichevsky D. , Gopalswamy N. , Lepping R. , Zurbuchen T.
Published: December 2003

We discuss the existence of large, complex merged interaction regions (MIRs) in the solar wind near Earth. MIRs can have configurations that cause more prolonged geomagnetic effects than a single flow structure. A MIR or successive MIRs can produce relatively long lasting Forbush decreases at 1 AU. We illustrate MIRs at 1 AU with two examples (MIR‐1 and MIR‐2) seen by WIND and ACE in the interval from 18 March through 29 March 2002. We determined the probable structure and origin of each in terms of interacting flows and shocks using in situ and solar observations, but we emphasize that there are uncertainties that cannot be resolved with these data alone. The MIRs were relatively large structures with radial extent ≈2/3 and 3/4 AU, respectively. MIR‐1 was formed by interactions related to at least two complex ejecta, a magnetic cloud, and two shocks. MIR‐2 was related to a corotating stream, the heliospheric plasma sheet (HPS), two complex ejecta, a magnetic cloud and at least two shocks. A MIR can evolve significantly while it moves to 1 AU, and memory of the conditions near the Sun is lost in the process. Thus one cannot unambiguously determine the structure of a MIR and the manner in which it formed using observations from a single spacecraft at 1 AU. The magnetic field strength profiles in MIRs are not correlated with the speed and density profiles so that one cannot infer the magnetic field strength in MIRs from remote sensing observation that give density and speed information. It will be possible to better understand the dynamical processes leading to the formation of MIRs with remote sensing observations, but they cannot measure the magnetic fields in MIRs.
A statistical study of compressional waves in the tail current sheet
Authors: Volwerk M. , Nakamura R. , Baumjohann W. , Treumann R. A. , Runov A. , Vörös Z. , Zhang T. L. , Asano Y. , Klecker B. , Richter I. , Balogh A. , Rème H.
Published: December 2003

A statistical study of the Cluster FGM data is performed on compressional waves for 68 selected 12 min intervals when the spacecraft are in or near the neutral sheet. The data are well distributed in local time between 2100 and 0300 LT. The compressional wave power is strongly dependent on the maximum plasma flow velocity (based on the CIS‐CODIF data) during the 12‐min interval. Up to a perpendicular flow velocity of ∼400 km/s the spectral power increases with velocity; at higher velocities the spectral power saturates. The turbulence measured in the frequency range between 0.08 and 1 Hz shows that on that scale the magnetotail has fully developed quasi two‐dimensional turbulence. We relate the quasi two‐dimensional (2‐D) turbulence to the finite extension of the flow channel, with a streaming instability as its source. There is an asymmetry in wave power between the premidnight and midnight/postmidnight sectors. The plasma flow direction splits the data into two distinct categories, the Earth/duskward and the tail/dawnward flows.
Substorm inner plasma sheet particle reduction
Authors: Lyons L. R. , Wang C.‐P. , Nagai T. , Mukai T. , Saito Y. , Samson J. C.
Published: December 2003

Geotail measurements from within the equatorial inner plasma sheet at r ∼ 10–13 RE are used to show that a reduction in equatorial plasma pressure within the current wedge is a general feature of substorms and that this decrease is simultaneous with the well‐known expansion phase increase in energetic particle fluxes. The decrease in pressure is found to be due to a decrease in particle fluxes at lower energies that gives a significant decrease in plasma density. We also find that when viewed as a function of the adiabatic energy invariant for the plasma sheet λ, there is a reduction in the number of particles for all λ despite the enhancements seen when energetic particle fluxes are viewed at fixed particle energy. These observations demonstrate that the expansion phase is associated with a significant reduction in flux tube content of particles within the current wedge and that this reduction leads to the expansion‐phase reduction in equatorial plasma pressure and cross‐tail current.
Inverse modeling of seasonal drought effects on canopy CO2/H2O exchange in three Mediterranean ecosystems
Authors: Reichstein Markus , Tenhunen John , Roupsard Olivier , Ourcival Jean‐Marc , Rambal Serge , Miglietta Franco , Peressotti Alessandro , Pecchiari Marco , Tirone Giampiero , Valentini Riccardo
Published: December 2003

We present a two‐criteria inverse modeling approach to analyze the effects of seasonal drought on ecosystem gas exchange at three Mediterranean sites. The three sites include two nearly monospecific Quercus ilex L. forests, one on karstic limestone (Puéchabon), the other on fluvial sand with access to groundwater (Castelporziano), and a typical multispecies shrubland on limestone (Arca di Noè). A canopy gas exchange model Process Pixel Net Ecosystem Exchange (PROXELNEE), which contains the Farquhar photosynthesis model coupled to stomatal conductance via the Ball‐Berry model, was inverted in order to estimate the seasonal time course of canopy parameters from hourly values of ecosystem gross carbon uptake and transpiration. It was shown that an inverse estimation of leaf‐level parameters was impossible when optimizing against ecosystem H2O or CO2 fluxes alone (unidentifiable parameters). In contrast, a criterion that constrained the optimization against both H2O and CO2 fluxes yielded stable estimates of leaf‐level parameters. Two separate model inversions were implemented to test two alternative hypotheses about the response to drought: a reduction in active leaf area as a result of patchy stomatal closure or a change in photosynthetic capacities. In contrast to a previously tested hypothesis of classical (uniform) stomatal control, both hypotheses were equally able to describe the seasonality of carbon uptake and transpiration on all three sites, with a decline during the drought and recovery after autumn rainfall. Large reductions of up to 80%, in either active leaf area or photosynthetic capacities, were necessary to describe the observed carbon and water fluxes at the end of the drought period. With a threshold‐type relationship, soil water content was an excellent predictor of these changes. With the drought‐dependent parameter changes included, the canopy model explains 80–90% of the variance of hourly gross CO2 uptake (root mean squared error (RMSE): 1.1–2.6 μmol m−2 s−1) and 70–80% of the variance of hourly transpiration (RMSE: 0.02–0.03 mm h−1) at all sites. In addition to drought effects, changes in leaf photosynthetic activity not related to water availability, i.e., high spring activity, were detected through the inverse modeling approach. Moreover, our study exemplifies a kind of multiconstraint inverse modeling that can be profitably used for calibrating ecosystem models that are meant for global applications with ecosystem flux data.
Modelers and Users Meet at CCMC Workshop
Authors: Lanzerotti Louis J.
Published: December 2003

Descriptive and predictive models of the Earth's space environment, from the Sun to the Earth's surface, are critical for operational users. The large solar events during the last week of October 2003 coincided with the many stimulating presentations and vigorous discussions during the second biennial workshop for the Community Coordinated Modeling Center (CCMC) that was held at the Maui (Hawaii) High Performance Computing Center.
Space shuttle based global CO measurements during April and October 1994, MAPS instrument, data reduction, and data validation
Authors: Reichle Henry G. , Anderson Bruce E. , Connors Vickie S. , Denkins Todd C. , Forbes David A. , Gormsen Barbara B. , Langenfelds Ray L. , Neil Doreen O. , Nolf Scott R. , Novelli Paul C. , Pougatchev Nikita S. , Roell Marilee M. , Steele L. Paul
Published: December 2003

The Measurement of Air Pollution From Satellites (MAPS) experiment flew as a payload aboard the space shuttle during April and October 1994. The instrument and the data reduction procedure were modified from earlier flights in 1981 and 1984. The modifications to both are described, and selected portions of the data are compared to concurrent aircraft borne direct measurements that had been carefully intercalibrated. In addition, the data acquired in 1984 were reprocessed using the new data reduction procedure, and the reprocessed data were compared to aircraft data acquired in 1984. The results of these comparisons indicate that the large bias error in the 1984 MAPS data has been reduced to about 10%.