bluuebunny/crossref_metadata_2025 huggingface dataset:
Constraints on magnitudes of extension on Venus from slope measurements
Authors: Connors Chris , Suppe John
Published: February 2004

Slopes related to normal fault scarps on Venus appear in Magellan synthetic aperture radar (SAR) images as tonal bands of increased or decreased radar backscatter relative to adjacent regions. Calculations, through measurements on Magellan stereo SAR, of 170 of these surfaces in plains regions show that their mean topographic slope is 36.4°±1.2°, regardless of height. Because there is little to no erosion on Venus, we infer from this mean slope that most faults have collapsed to talus slopes, approximately at an angle of repose, and thus the effective cohesive strength of the crust near most fault surfaces is low, probably because of secondary faulting and jointing. In areas well imaged by Magellan stereo SAR we show that simple, balanced cross sections through individual grabens on Venus can be made, assuming a Coulomb shear angle for fault orientation, thus constraining slip on causitive faults and extension across the grabens. In rifted areas imaged by one Magellan SAR viewing geometry, constraints on extension across rift zones can be made by measuring the tonal bands representing slopes related to individual normal faults and using the mean topographic slope calculated for similar slopes imaged in stereo SAR. With this technique we show that crustal extension due to faulting and folding for the most recent deformation in the Beta rift zone is <20 km. These modest strains, across one of the best developed rift zones on Venus, put severe limits on tectonic models of Venus that invoke significant lateral displacements of the Venusian crust.
Chryse Planitia, Mars: Topographic configuration, outflow channel continuity and sequence, and tests for hypothesized ancient bodies of water using Mars Orbiter Laser Altimeter (MOLA) data
Authors: Ivanov M. A. , Head J. W.
Published: February 2004

Many of the largest and most prominent outflow channels on Mars debouch into Chryse Planitia. Pre‐Mars Global Surveyor topographic data show Chryse to be a closed depression almost 2000 km in diameter. New Mars Orbiter Laser Altimeter (MOLA) data reveal the following: (1) Chryse is not a locally closed basin but instead opens into the North Polar basin. (2) The highly distinctive morphology of the six largest predominantly Hesperian‐aged channels (Kasei, Maja, Simud, Tiu, Ares, and Mawrth) disappears into the northern lowlands at average elevations that all occur within less than ∼170 m of a mean elevation of −3742 (SD = 153 m), over a lateral distance in excess of 2500 km. (3) The elevations where the distinctive morphology of each channel disappears all fall within ∼190 m of Contact 2, a boundary mapped byParker et al.[1993] and interpreted to represent an ancient shoreline, and the mean elevation values of Contact 2 and circum‐Chryse channel termini fall within 18 m of each other. In contrast, the termini of seven later Amazonian‐aged channels emerging from Elysium into Utopia Planitia are spread over a vertical range of >1500 m. (4) Topographic evidence of the continuation of some of the outflow channels can be observed for distances of 250–450 km into the North Polar basin, but the morphology is subdued and distinctly different. (5) The nature of this less distinctive topography and its crosscutting relationships show that Simud and Tiu are likely to represent the youngest activity (specifically crosscutting Ares Valles). (6) The distinctive change in channel morphology is consistent with rapid loss of energy encountered at base level (subaerial/submarine boundary) and emplacement into a shallow submarine environment. Channel characteristics, lack of distinctive deltas or lobes, and continuation of subdued channel morphology suggest hyperpychnal flow and the possibility of density/turbidity currents. Estimates of the volumes of individual channel events are wide‐ranging. The minimum volume estimates ofCarr[1996] suggest that 46 such events would be required to fill the basin to the level of Contact 2 and thus that the channels may have emptied into an existing standing body of water. Volume estimates ofBaker et al.[1991] assume that single individual events may have filled the basin to the level of Contact 2, thus requiring significant water loss between events and refilling during subsequent events to essentially the same level. In both end‐member cases these observations are consistent with the presence of large standing bodies of water in the northern lowlands in Hesperian‐Early Amazonian times.
A parametric study of SO2 scale height as a probe of the sulfur cycle at the Venusian cloud top
Authors: Yamamoto Masaru
Published: February 2004

At Venus's cloud top (∼70 km) the SO2 scale height HSO2 is one of the important observational quantities to give us scientific information about not only the vertical distribution of SO2 but also the chemical and transport processes of SO2. HSO2 is controlled by the SO2 photodissociation rate and by the concentrations of ClO and SO2 through the SO2 oxidation in the sulfur cycle including the SOx‐ClOx catalysis. The sensitivity of HSO2 in the SOx‐ClOx process is classified into three regimes: inefficient‐oxidation, low‐ClO, and high‐ClO regimes. On the other hand, the sensitivity of HSO2 to the SO2 oxidation and the vertical flow is also classified into three regimes: regime O, where HSO2 is controlled by the SO2 oxidation, regime D, where HSO2 is controlled by the downward flow, and regime U, where HSO2 is insensitive to both the SO2 oxidation and the vertical flow. As a result of exploring the sensitivity of HSO2 in the chemical and transport processes, either strong downward flow or some other SO2 oxidation process in addition to the SOx‐ClOx catalysis is required in order to reproduce the small HSO2 observed at high latitudes.
Predictions of a simple cloud model for water vapor cloud albedo feedback on Venus
Authors: Hashimoto George L. , Abe Yutaka
Published: February 2004

We developed a simple physical model of Venus' cloud to understand the influence of cloud on the evolution of the surface environment and the climate on Venus. For a given atmospheric structure and composition, the cloud structure and its albedo are calculated through our model. We successfully reproduced the present Venus albedo: the calculated value is 0.87 for 550‐nm wavelength, while the observed one is 0.85. The variability of cloud albedo caused by the change of H2O abundance in the Venus atmosphere is examined. Although H2O is merely a minor component of the atmosphere, its influence on the surface environment is found to be quite large. Increase in H2O abundance raises the albedo and cools the surface environment. When H2O abundance is smaller than a critical value of about 0.5 ppmv, complete evaporation of cloud occurs owing to a warming of the atmosphere. When H2O abundance is increased, the H2O abundance in the upper atmosphere is reduced owing to a lower atmospheric temperature produced by high albedo of thick clouds. The cooling of the atmosphere will affect the escape flux of hydrogen from the Venus atmosphere into space.
Mars topography: Lessons learned from spatial and spectral domain comparisons of Mars Orbiter Laser Altimeter and U.S. Geological Survey data
Authors: Bills Bruce G. , Nerem R. Steven
Published: February 2004

The topography of Mars has been mapped several times by a variety of techniques. We compare the recent Mars Orbiter Laser Altimeter version with the earlier U.S. Geological Survey compilation of photogrammetry and Earth‐based radar. We compare them in terms of their respective representations on 1°×1° grids, though both data sets contain substantially higher spatial resolution. We find that the differences between the two Mars topography models, though locally quite large, are generally quite small. The difference variance is 4% of the data variance. The difference between the two models is also quite notably anisotropic, with substantially more variation north‐south than east‐west. The variance in the difference between the models is a very small fraction of the data variance at the longest wavelengths and approaches a constant value of ∼30% at harmonic degrees above 20.
A high‐resolution, three‐dimensional model of Jupiter's Great Red Spot
Authors: Cho James Y‐K. , de la Torre Juárez Manuel , Ingersoll Andrew P. , Dritschel David G.
Published: February 2004

The turbulent flow at the periphery of the Great Red Spot (GRS) contains many fine‐scale filamentary structures, while the more quiescent core, bounded by a narrow high‐velocity ring, exhibits organized, possibly counterrotating, motion. Past studies have neither been able to capture this complexity nor adequately study the effects of vertical stratification LR(z) on the GRS. We present results from a series of high‐resolution, three‐dimensional simulations that advect the dynamical tracer, potential vorticity. The detailed flow is successfully captured with a characteristic value of LR ≈ 2000 km, independent of the precise vertical stratification profile.
Experimental and theoretical studies on the gas/solid/gas transformation cycle in extraterrestrial environments
Authors: Cottin Hervé , Gazeau Marie‐Claire , Chaquin Patrick , Raulin François , Bénilan Yves
Published: February 2004

The ubiquity of molecular material in the universe, from hydrogen to complex organic matter, is the result of intermixed physicochemical processes that have occurred throughout history. In particular, the gas/solid/gas phase transformation cycle plays a key role in chemical evolution of organic matter from the interstellar medium to planetary systems. This paper focuses on two examples that are representative of the diversity of environments where such transformations occur in the Solar System: (1) the photolytic evolution from gaseous to solid material in methane containing planetary atmospheres and (2) the degradation of high molecular weight compounds into gas phase molecules in comets. We are currently developing two programs which couple experimental and theoretical studies. The aim of this research is to provide data necessary to build models in order to better understand (1) the photochemical evolution of Titan's atmosphere, through a laboratory program to determine quantitative spectroscopic data on long carbon chain molecules (polyynes) obtained in the SCOOP program (French acronym for Spectroscopy of Organic Compounds Oriented for Planetology), and (2) the extended sources in comets, through a laboratory program of quantitative studies of photochemical and thermal degradation processes on relevant polymers (e.g., Polyoxymethylene) by the SEMAPhOrE Cometaire program (French acronym for Experimental Simulation and Modeling Applied to Organic Chemistry in Cometary Environment).
Constraints on the depth of magnetized crust on Mars from impact craters
Authors: Nimmo Francis , Gilmore Martha S.
Published: February 2004

Large (diameter greater than ∼500 km) Martian impact basins are associated with observed magnetic fields which are statistically distinct from, and smaller than, fields associated with smaller craters. We suggest that this effect arises because impacts cause shock, heating, and excavation, reducing the magnetization of previously magnetized crust. For a simple, uniformly magnetized model the magnetic field at 100 km altitude is reduced by ∼50% when a crater‐shaped demagnetization zone reaches the base of the magnetized layer. By analogy with terrestrial data, we assume that in Martian craters the zone of demagnetization extends to a depth of 0.04–0.15 crater diameters. On the basis of this assumption, the data suggest that the depth to the base of the magnetized layer on Mars, if uniform, is ∼35 km, with lower and upper bounds of 10 and 100 km, respectively. These bounds imply magnetizations of 5–40 A m−1 and are consistent with likely Mars geotherms at 4 Gyr B.P.
A Serenitatis origin for the Imbrian grooves and South Pole‐Aitken thorium anomaly
Authors: Wieczorek Mark A. , Zuber Maria T.
Published: February 2004

The northwest corner of the Moon's South Pole‐Aitken (SPA) basin contains a high abundance of thorium and a unique Imbrian aged geomorphologic unit that consists of “grooves and mounds” (referred to here as the Imbrian grooves). Because the location of these features are almost antipodal to the Imbrium basin, where high‐thorium ejecta and seismic energy are expected to have converged, an Imbrium origin for these units has long seemed certain. By modeling the deposition of impact ejecta on the Moon, we have investigated whether the convergence of Imbrium's ejecta at its antipode could be the origin of both the Imbrian grooves and SPA thorium anomaly. As a result of the Moon's rotation, our results show that ejecta from this basin should converge more than 12° west of its antipode. Both the Imbrian grooves and thorium anomaly within SPA, however, reside slightly to the east of Imbrium's antipode. In an attempt to reconcile this disparity, the effects of a putative oblique Imbrium impact have been qualitatively investigated. While this model can distribute ejecta in the general vicinity of the Imbrian grooves, the planform of our modeled antipodal ejecta is distinctly different from that which is observed. As an alternative explanation for the origin of these features, we find that the modeled distribution of ejecta from an oblique Serenitatis impact is surprisingly similar to the planform of the Imbrian grooves, with the exception that it is offset directly to the east. This eastward offset is likely to be an artifact of our not being able to properly include the effects of the Moon's rotation in our oblique impact models. We conclude that the Imbrium grooves and SPA thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the Serenitatis basin. If this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined, then this implies that (1) the Serenitatis target contained a high abundance of thorium and (2) the convergence of seismic energy at the antipodes of either the Imbrium or Serenitatis basin was not sufficient to cause substantial surface modification. Extrapolating this result to Mercury suggests that the “hilly and lineated” terrain antipodal to the Caloris basin was formed by the convergence of ejecta, and not seismic waves.
H2O adsorption on smectites: Application to the diurnal variation of H2O in the Martian atmosphere
Authors: Zent Aaron P. , Howard D. Jeanie , Quinn Richard C.
Published: February 2004

Observations of the Martian planetary boundary layer lead to interpretations that are baffling and contradictory. In this paper we specifically address the question of whether or not water vapor finds a substantial diurnal reservoir in the Martian regolith. To address this issue, we have measured H2O adsorption kinetics on SWy‐1, a Na‐rich montmorillonite from Wyoming. The highest‐temperature (273 K) data equilibrate rapidly. Data gathered at realistic H2O partial pressures and temperatures appropriate to early morning show two phenomena that preclude a significant role for smectites in diurnally exchanging a large column abundance. First, the equilibration timescale is longer than a sol. Second, the equilibrium abundances are a small fraction of that predicted by earlier adsorption isotherms. The explanation for this phenomenon is that smectite clay actually increases its surface area as a function of adsorptive coverage. At Mars‐like conditions we show that the interlayer sites of smectites are likely to be unavailable.
On the source region of the lunar mare basalt
Authors: Jafar Arkani‐Hamed , Pentecost Alison
Published: February 2004

The lack of pervasive flooding of the South Pole‐Aitken (SPA) basin raises questions about the mare basalt source region models, whether deep or shallow. This paper investigates the dichotomy between the nearside Imbrium‐sized flooded basins and the unflooded SPA basin by modeling the effects of giant impacts on the dynamics of the lunar mantle. We allow an initially spherically symmetric Moon model to evolve through mantle convection for a few hundred million years and then let an Imbrium‐ or SPA‐sized impact occur. The subsequent thermal convection in the lunar mantle resulting from the thermal energy introduced by an impact is determined for the next 100 m.y. The mantle is assumed to be incompressible, with an infinite Prandtl number and a temperature‐dependent viscosity. It is shown that the mantle circulation induced by an Imbrium‐sized impact can only slightly push aside the radioactive‐rich potassium, rare earth element, and phosphorous (KREEP) layer from beneath the surrounding crust. This concentrates the radioactive elements beneath the low‐thermal‐conductive ejecta blanket and provides a suitable condition for enhancement of radioactive heating and subsequent partial melting and basaltic flow to the basin. On the other hand, a SPA‐sized impact is capable of generating mantle circulation vigorous enough to strip away the KREEP layer from beneath the surrounding crust within a time period of 40 m.y. This removal prevents radioactive heating and partial melting beneath the surrounding crust, thus explaining the lack of flooding of SPA basin. These results are insensitive to the time of impacts but are strongly dependent on their sizes.
Lunar rotational dissipation in solid body and molten core
Authors: Williams James G. , Boggs Dale H. , Yoder Charles F. , Ratcliff J. Todd , Dickey Jean O.
Published: February 2004

Analyses of Lunar Laser ranges show a displacement in direction of the Moon's pole of rotation which indicates that strong dissipation is acting on the rotation. Two possible sources of dissipation are monthly solid‐body tides raised by the Earth (and Sun) and a fluid core with a rotation distinct from the solid body. Both effects have been introduced into a numerical integration of the lunar rotation. Theoretical consequences of tides and core on rotation and orbit are also calculated analytically. These computations indicate that the tide and core dissipation signatures are separable. They also allow unrestricted laws for tidal specific dissipation Q versus frequency to be applied. Fits of Lunar Laser ranges detect three small dissipation terms in addition to the dominant pole‐displacement term. Tidal dissipation alone does not give a good match to all four amplitudes. Dissipation from tides plus fluid core accounts for them. The best match indicates a tidal Q which increases slowly with period plus a small fluid core. The core size depends on imperfectly known properties of the fluid and core‐mantle interface. The radius of a core could be as much as 352 km if iron and 374 km for the Fe‐FeS eutectic composition. If tidal Q versus frequency is assumed to be represented by a power law, then the exponent is −0.19±0.13. The monthly tidal Q is 37 (−4,+6), and the annual Q is 60 (−15,+30). The power presently dissipated by solid body and core is small, but it may have been dramatic for the early Moon. The outwardly evolving Moon passed through a change of spin state which caused a burst of dissipated power in the mantle and at the core‐mantle boundary. The energy deposited at the boundary plausibly drove convection in the core and temporarily powered a dynamo. The remanent magnetism in lunar rocks may result from these events, and the peak field may mark the passage of the Moon through the spin transition.
The pacemaker always rings twice
Authors: Wara M. W. , Ravelo A. C. , Revenaugh J. S.
Published: February 2004

We generated new, long, high‐resolution, climate proxy records from Deep Sea Drilling Project (DSDP) Sites 607 and 609 in the subpolar North Atlantic over the interval 225–970 ka, which have pronounced variability at periods <19 kyr, shorter than any solar forcing due to Earth's changing orbital parameters. This high‐frequency variability results from a transfer of variance from primary orbital frequencies (1/23, 1/40 kyr). Previous work suggesting that high‐frequency climate variability in the region may be paced by variations in tropical Atlantic climate or by insolation forcing at low latitudes is not supported by comparison of our records with either records of dust and CaCO3 accumulation at Ocean Drilling Program (ODP) Site 663 in the subtropical Atlantic or with time series of equatorial insolation. Rather the majority of suborbital scale (1/19 kyr > f > 1/7 kyr) variance in our North Atlantic climate proxy records is shown, using bispectral and cross‐bispectral methods, to be explainable as harmonics and/or combination tones of orbital‐scale climatic variability of the North Atlantic region itself. Thus the timing and amplitude of high‐frequency climate change in the North Atlantic region appears to be a nonlinear function of variations in high‐latitude climate at Milankovitch frequencies.
VHF tropospheric scatterer anisotropy at Resolute Bay and its implications for tropospheric radar‐derived wind accuracies
Authors: Hocking W. K.
Published: February 2004

Knowledge about the anisotropy of VHF radio wave scatterers in the atmosphere is important when VHF wind profiler radars are used to measure atmospheric winds by Doppler beam‐swinging methods, since appropriate correction factors must be employed. In this study, the anisotropy of the scatterers is determined using two different methods, over the course of a full year, for a VHF radar at Resolute Bay in northern Canada (75°N). The first method utilizes direct comparisons of the radar‐derived winds with those of radiosondes launched from close to the site, while the second uses comparisons of powers received on vertical and off‐vertical beams. The study is unique in that a full year of radiosonde data were available from balloons which were launched from a site only 4 km from the radar. The two methods for estimating the anisotropy at first appear to give slightly different conclusions, but by properly considering the errors associated with both radar and radiosonde measurements it is possible not only to reconcile the two sets of measurements but also to give reasonable estimates of the errors associated with both the sonde and radar winds. An important result which arises is that the radar data have very similar accuracies to the radiosonde data. Furthermore, it is demonstrated that in the past, radar wind measurements may have been overcorrected, because the errors in the sonde measurements may not have been properly considered in determining the correct conversions. Seasonal variations in the aspect sensitivity are found but are not large.
Derivation of the ionospheric structure over extended geographical regions from azimuth and elevation scan backscatter ionograms: A modular neural network approach
Authors: Jacquet F. , Caratori J. , Dorizzi B.
Published: February 2004

This paper describes a new real‐time inversion method based upon multifrequency sounding schemes and variable azimuth and elevation angle backscatter ionograms for the derivation of the three‐dimensional structure of the ionosphere. The inversion technique which has been developed for this purpose employs neural network models in a modular structure and is fully explained. Tests on simulated data demonstrate the potential accuracy of the method for reconstructing the electron density profile over remote geographical regions.
Performance of axially magnetized ferrite coupled lines
Authors: Xie Kang , Davis Lionel E.
Published: February 2004

The performance of a pair of axially magnetized ferrite coupled lines (FCLs) is studied numerically, taking into account the reflections at both input and output interfaces for the first time. It is found that the peak power transfer factor (operation frequency) of a microstrip FCL decreases (increases) when the saturation magnetization increases. It is also predicted that broad bandwidth is possible for FCLs built on strip line. The influence of a metal enclosure and other parameters of the FCL such as ferrite thickness, separation of the coupled lines, etc., are also discussed.
Comparison of the properties of long‐lasting and impulsive type IV solar radio bursts with fine structures
Authors: Chernov G. P. , Poquerusse M. , Bougeret J.‐L. , Zlobec P.
Published: February 2004

We consider source positions of fine structures (mainly of zebra pattern) and of continuum emission in some type IV radio bursts with respect to optical and X‐ray features. For completeness, dynamic spectra of fine structures recorded in different observatories were compared with each other and with records of circular left and right polarization at single frequencies. We analyzed three long‐lasting type IV events (events 1989 03 12, 1990 06 05, and 1992 02 17) that were characterized by the absence of microwave bursts and big flares and two impulsive events (events 1990 06 11 and 1994 10 25) formed by type II bursts and by rather short living type IV continuum in close relation to Hα flares and microwave bursts. All those events were connected with active filaments. As an example of more complex events, including both impulsive and long‐lasting type IV bursts, event 1997 11 04 is considered. A general peculiarity of fine structures in the three long‐lasting events is the long‐term evolution of zebra patterns and pulsations (2–3 hours), which testifies to the presence of repeated acceleration of fast particles in vertical current sheets during the restoration of magnetic arches after the escape of coronal mass ejections. The magnetic configurations in the source of the impulsive events must be rather close to oblique flare current sheets. Complex events can have a two‐step evolution: The former regards the reconnection in low flaring loops, while the latter is related to the reconnection in the higher corona.
Type III solar radio bursts in inhomogeneous interplanetary space observed by Geotail
Authors: Kasahara Yoshiya , Matsumoto Hiroshi , Kojima Hirotsugu
Published: February 2004

Characteristics of type III solar radio bursts are studied using high‐frequency resolution of the SFA (sweep‐frequency analyzer) of the Plasma Wave Instrument (PWI) on board the Geotail spacecraft. We often observe abnormal type III bursts, which have separated frequency bands or have prolonged tails at particular frequencies. These observations provide observational clues to detect density inhomogeneities in the upstream interplanetary medium. We propose possible models of interplanetary density structures which can account for some type III spectrum structures observed.
Investigation on horizontal wind estimation around high aspect sensitivity
Authors: Kawano Noriyuki , Fukao Shoichiro
Published: February 2004

In the present paper, simultaneous radar observations in the Doppler beam swinging (DBS), spatial interferometry (SI), and spaced antenna (SA) modes with the middle and upper atmosphere (MU) radar are shown. High aspect sensitivities were found after a passage of a cutoff low and an incidental trough owing to observations in the DBS mode. Because it is known that tilted aspect sensitivities introduce a bias in radial velocity measurements, we corrected the horizontal wind directly measured by oblique beams in the DBS mode by taking account of the arrival angle of the oblique beams estimated by the SI technique. On the other hand, the aspect angle is derived from the echo power ratio of the vertical to the oblique beams with the DBS mode, which allows us to estimate an effective pointing angle, and the Doppler wind is corrected by using this effective angle. In addition, the SA technique is performed simultaneously for estimating the horizontal wind. The above four estimating processes are used to determine the horizontal wind and compared for the aspect sensitivity. Finally, a simple computer simulation is performed for the SA technique for constant and variable wind conditions during incoherent integrations.
Bistatic form of the electric field equations for the scattering of vertically polarized high‐frequency ground wave radiation from slightly rough, good conducting surfaces
Authors: Gill E. W. , Walsh J.
Published: February 2004

The scattering of high‐frequency (HF) electromagnetic radiation from slightly rough, good conducting surfaces for the case of bistatic reception is examined. In this work, the scattering surface is considered to be time invariant and to be representable by a two‐dimensional Fourier series. The source is taken to be a continuously excited elementary vertical dipole whose current distribution is arbitrary. Thus the basis is provided for the introduction of any desired source waveform. The convolution integrals resulting from earlier analyses are treated asymtotically, primarily via stationary phase techniques. The physical relevance of the stationary points derived from the first 2 orders of scatter is discussed. The various field components presented for the time‐invariant surface may be easily extended to time‐varying surfaces and subsequently to deducing the high‐frequency cross sections of highly conducting surfaces such as that of the ocean.
An analysis of the scattering of high‐frequency electromagnetic radiation from rough surfaces with application to pulse radar operating in backscatter mode
Authors: Walsh J. , Gill E. W.
Published: February 2004

The scattering of high‐frequency (HF) electromagnetic radiation from slightly rough, good conducting surfaces is presented. The analysis is based on a decomposition of the relevant space using generalized functions. The fundamental analysis incorporates a general source and involves all scattering orders for the normal component of the field. Subsequently, derivation of the scattered electric field (to third order in scatter) using a pulsed dipole source is effected. The first 2 orders are used to deduce an estimate of radar cross sections of bounded regions or targets when operation is carried out in the backscatter mode. Conditions of small height and small slope are imposed. Application is made to the determination of the first‐order cross section of a perfectly conducting sphere (within the limits of the imposed contraints) and of an exponential boss. The results are shown to be consistent with Rayleigh scattering theory.
Inverse scattering of dielectric cylinders by using radial basis function neural networks
Authors: Rekanos Ioannis T.
Published: February 2004

In this paper, a new on‐line inverse scattering methodology, which is based on radial basis function neural networks, is presented. The construction of these networks is implemented by means of the orthogonal least squares algorithm. By applying this training algorithm we can calculate the values of the free parameters of the network and also define its structure. Thus a trial‐and‐error strategy concerning the definition of the network size is avoided. In particular, the network is constructed to perform the mapping from scattered‐field measurements to electromagnetic and geometric properties of the scatterer. Although this approach can be applied to various inverse scattering applications, we focus on the reconstruction of cylindrical dielectric scatterers from simulated measurements of the scattered electric field, while transverse magnetic illuminations are used. The objective is to estimate the relative dielectric constant, the size, and the position of the scatterer. In numerical results an investigation of the performance of the network is carried out. After the completion of the training procedure the network can rapidly estimate the scatterer properties, without extreme storage demands. Finally, the robustness of the proposed methodology in inverting measurements that are corrupted by additive white Gaussian noise is examined.
A neural network technique for inversion of atmospheric observations from microwave limb sounders
Authors: Jiménez Carlos , Eriksson Patrick
Published: February 2004

The limb sounder radiometer on board the satellite Odin will be the first instrument measuring emission from space in the sub‐millimeter region to map atmospheric species. Nonlinear inversions of Odin spectra by traditional iterative approaches will be computationally very intensive, so this paper proposes a faster inversion technique based on training neural networks. The technique is described first in general terms and then applied to invert simulated observations in two of the Odin bands, around 501.4 GHz and 544.6 GHz. To deal with the large dimension of the measured spectra, a data reduction based on the eigenvectors of the measured space is first applied. The reduced spectra are then input to a set of multilayer perceptrons that, after training with a set of simulated spectra, do the inversions. The same spectra are also inverted by optimal estimation, and the performance from both techniques is compared. The neural network technique retrieves species profiles with errors and vertical resolutions comparable to optimal estimation, it can be made very robust against the uncertainties of the a priori information by including different learning terms during the training, and it is faster than optimal estimation if nonlinear inversions are required. Although final conclusions on processing time have to wait until Odin is operational, the simulations show that the neural network technique has the potential to make the Odin nonlinear inversions at least 1 order of magnitude faster than they are made by optimal estimation.
GPS‐determination of along‐strike variation in Cascadia margin kinematics: Implications for relative plate motion, subduction zone coupling, and permanent deformation
Authors: Miller M. Meghan , Johnson Daniel J. , Rubin Charles M. , Dragert Herb , Wang Kelin , Qamar Anthony , Goldfinger Chris
Published: February 2004

High‐precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along‐strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades‐long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3–7 mm/yr) to northern and central Oregon (∼9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin‐parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north‐westward velocities reflect the influence of Pacific‐North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.
One‐, two‐, and three‐dimensional root water uptake functions for transient modeling
Authors: Vrugt J. A. , van Wijk M. T. , Hopmans J. W. , Šimunek J.
Published: February 2004

Although solutions of multidimensional transient water flow can be obtained by numerical modeling, their application may be limited as root water uptake is generally considered to be one‐ or two‐dimensional only. This is especially the case for trees. The first objective of this paper is to test the suitability of a three‐dimensional root water uptake model for the simultaneous simulation of transient soil water flow around an almond tree. The soil hydraulic and root water uptake parameters were optimized by minimizing the residuals between measured and simulated water content data. Water content was measured in a three‐dimensional grid around a sprinkler‐irrigated almond tree for a 16 day period, following irrigation. A second objective was to compare the performance and results of the three‐dimensional flow model with one‐ and two‐ dimensional root water uptake models. For this purpose, measured water contents were aggregated in the x and y direction in the one‐dimensional case and in the radial direction for the two‐dimensional uptake model. For the estimation of root water uptake model parameters a genetic algorithm was used to estimate the approximate global minimum of the parameter space, whereas final parameters were determined using the Simplex optimization algorithm. With the optimized root water uptake parameters, simulated and measured water contents during the 16‐day period were in excellent agreement for all root water uptake models. Most significantly, the spatial variation in flux density below the rooting zone decreased when reducing multidimensional root water uptake to fewer dimensions, thereby justifying the proposed multidimensional approach.
Analytical modeling of the influence of denitrifying sediments on nitrate transport in aquifers with sloping beds
Authors: Hantush Mohamed M. , Mariño Miguel A.
Published: February 2004

Denitrification is a significant process for the removal of nitrate transported in groundwater drainage from agricultural croplands to streams and has important ecological consequences. In this paper, analytical solutions are developed for advective‐reactive and nonpoint sources contaminant transport in a two‐layer unconfined aquifer with a sloping bed. In particular, we investigate the impact of a sloping bed of denitrifying sediments on nitrate in groundwater discharge to streams. Nitrate discharge to streams is considered to be the result of mixing of waters from two geochemically distinct zones in the aquifer. Groundwater flow is assumed to be steady, and the effect of dispersion on solute transport is neglected. First, the problem of steady state groundwater flow in an unconfined aquifer with a sloping bed subject to uniform recharge is solved exactly and approximately using a regular perturbation technique. The transport equations, which describe advective‐reactive transport in a two‐layer aquifer, are then developed based on mass balance and solved in the Lagrangian viewpoint for the case of complete denitrification in the lower layer (or redox zone), using the perturbation technique. Application of the methodology to paired agricultural watersheds in the mid‐Atlantic coastal plain predicts that the sloping bed of the surficial aquifer affects both groundwater discharge and nitrate loading to two creeks in the watersheds. Because of the sloping bed of the surficial aquifer, groundwater discharged to Morgan Creek is estimated to be dominated by discharge from the lower layer and therefore shows greater potential for losses by denitrification at the base of the aquifer than those predicted at the Chesterville Branch. Simulation results show that a relatively low nitrate discharge to Morgan Creek may be also the result of historically low input of nitrogen at the source. Further, a numerical experiment based on the solutions shows that dilution with nitrate‐free groundwaters in a riparian buffer strip could be misinterpreted as nitrate losses. Lumped parameter indices are derived, which estimate the capacity of watersheds and riparian zones for nitrate removal by denitrification and/or other loss pathways, and their variation with aquifer bed slope, stream‐aquifer hydraulic and geometric properties, and groundwater recharge is illustrated.
Toward improved streamflow forecasts: value of semidistributed modeling
Authors: Boyle Douglas P. , Gupta Hoshin V. , Sorooshian Soroosh , Koren Victor , Zhang Ziya , Smith Michael
Published: February 2004

The focus of this study is to assess the performance improvements of semidistributed applications of the U.S. National Weather Service Sacramento Soil Moisture Accounting model on a watershed using radar‐based remotely sensed precipitation data. Specifically, performance comparisons are made within an automated multicriteria calibration framework to evaluate the benefit of “spatial distribution” of the model input (precipitation), structural components (soil moisture and streamflow routing computations), and surface characteristics (parameters). A comparison of these results is made with those obtained through manual calibration. Results indicate that for the study watershed, there are performance improvements associated with semidistributed model applications when the watershed is partitioned into three subwatersheds; however, no additional benefit is gained from increasing the number of subwatersheds from three to eight. Improvements in model performance are demonstrably related to the spatial distribution of the model input and streamflow routing. Surprisingly, there is no improvement associated with the distribution of the surface characteristics (model parameters).
A chaotic approach to rainfall disaggregation
Authors: Sivakumar Bellie , Sorooshian Soroosh , Gupta Hoshin Vijai , Gao Xiaogang
Published: February 2004

The importance of high‐resolution rainfall data to understanding the intricacies of the dynamics of hydrological processes and describing them in a sophisticated and accurate way has been increasingly realized. The last decade has witnessed a number of studies and numerous approaches to the possibility of transformation of rainfall data from one scale to another, nearly unanimously pointing to such a possibility. However, an important limitation of such approaches is that they treat the rainfall process as a realization of a stochastic process, and therefore there seems to be a lack of connection between the structure of the models and the underlying physics of the rainfall process. The present study introduces a new framework based on the notion of deterministic chaos to investigate the behavior of the dynamics of rainfall transformation between different temporal scales aimed toward establishing this connection. Rainfall data of successively doubled resolutions (i.e., 6, 12, 24, 48, 96, and 192 hours) observed at Leaf River basin, in the state of Mississippi, United States of America, are studied. The correlation dimension method is employed to investigate the presence of chaos in the rainfall transformation. The finite and low correlation dimensions obtained for the distributions of weights between rainfall data of different scales indicate the existence of chaos in the rainfall transformation, suggesting the applicability of a chaotic model. The formulation of a simple chaotic disaggregation model and its application to the Leaf River rainfall data provides encouraging results with practical potential. The disaggregation model results themselves indicate the presence of chaos in the dynamics of rainfall transformation, providing support for the results obtained using the correlation dimension method.
Estimation of surface evaporation map over Southern Great Plains using remote sensing data
Authors: Jiang Le , Islam Shafiqul
Published: February 2004

We propose a simple scheme to estimate surface evaporation over large heterogeneous areas using remote sensing data. Our approach is based on an extension of the Priestley‐Taylor equation and a relationship between remotely sensed surface temperature and vegetation index. The required parameters for this approach are derived from advanced very high resolution radiometer NOAA 14 data over the southern Great Plains (SGP) in 1997. Compared to ground flux observations by the Atmospheric Radiation Measurement program, our six case studies varying from early spring to late summer over the SGP domain show that the proposed method provides better estimation accuracy for surface evaporation than the original Priestley‐Taylor method. It appears that the proposed method can estimate evaporation over large areas with significantly less complexity than that of the traditionally used residual method based on aerodynamic resistance. The uncertainty in the estimation of surface evaporation for the proposed approach is closely related to the inaccuracy in deriving net radiation and soil heat flux from remote sensing data. Results suggest that the proposed approach is more reliable and easily applicable for operational estimation of evaporation over large areas where ground‐based data are not readily available.
Overpressures in the Uinta Basin, Utah: Analysis using a three‐dimensional basin evolution model
Authors: McPherson Brian J. O. L. , Bredehoeft John D.
Published: February 2004

High pore fluid pressures, approaching lithostatic, are observed in the deepest sections of the Uinta basin, Utah. Geologic observations and previous modeling studies suggest that the most likely cause of observed overpressures is hydrocarbon generation. We studied Uinta overpressures by developing and applying a three‐dimensional, numerical model of the evolution of the basin. The model was developed from a public domain computer code, with addition of a new mesh generator that builds the basin through time, coupling the structural, thermal, and hydrodynamic evolution. Also included in the model are in situ hydrocarbon generation and multiphase migration. The modeling study affirmed oil generation as an overpressure mechanism, but also elucidated the relative roles of multiphase fluid interaction, oil density and viscosity, and sedimentary compaction. An important result is that overpressures by oil generation create conditions for rock fracturing, and associated fracture permeability may regulate or control the propensity to maintain overpressures.
A model of water streaking down a wall
Authors: Benson David A.
Published: February 2004

Rainwater streaking down a vertical wall exhibited the characteristics of “fingered flow.” The wall serves as an analogy to film flow in a relatively smooth and wide aperture fracture. A digital photograph of the wall suggests that the vertical distribution of water is heavy‐tailed. A fractional‐order advection‐dispersion equation is used as a model.
Use of NMR relaxation times to differentiate mobile and immobile pore fractions in a wetland soil
Authors: Culligan Patricia J. , Sinfield Joseph V. , Maas Werner E. , Cory David G.
Published: February 2004

A method is presented that makes use of nuclear magnetic resonance (NMR) measurements to differentiate between mobile and immobile pore fractions in an organic peat specimen. The method is based on the T1 relaxation mechanism and uses a paramagnetic reagent to mark the effective (mobile) pore space in the specimen under examination. A setup has been developed that allows controlled fluid flow through an NMR specimen while it is positioned in a magnet. Results from tests using fluid‐filled glass tubes illustrate the application of the method and verify its accuracy. Results from a test using a peat moss specimen demonstrate the use of the method during a flow experiment in a wetland soil.
Climate elasticity of streamflow in the United States
Authors: Sankarasubramanian A. , Vogel Richard M. , Limbrunner James F.
Published: February 2004

Precipitation elasticity of streamflow, ϵP, provides a measure of the sensitivity of streamflow to changes in rainfall. Watershed model‐based estimates of ϵP are shown to be highly sensitive to model structure and calibration error. A Monte Carlo experiment compares a nonparametric estimator of ϵP with various watershed model‐based approaches. The nonparametric estimator is found to have low bias and is as robust as or more robust than alternate model‐based approaches. The nonparametric estimator is used to construct a map of ϵP for the United States. Comparisons with 10 detailed climate change studies reveal that the contour map of ϵP introduced here provides a validation metric for past and future climate change investigations in the United States. Further investigations reveal that ϵP tends to be low for basins with significant snow accumulation and for basins whose moisture and energy inputs are seasonally in phase with one another. The Budyko hypothesis can only explain variations in ϵP for very humid basins.