bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution and persistence of 5‐μm hot spots at the Galileo probe entry latitude
Authors: Ortiz J. L. , Orton G. S. , Friedson A. J. , Stewart S. T. , Fisher B. M. , Spencer J. R.
Published: February 2004

We present a study on the longitudinal locations, morphology, and evolution of the 5‐μm hot spots at 6.5°N latitude (planetocentric) from an extensive Infrared Telescope Facility‐National Science Foundation Camera (IRTF‐NSFCAM) data set spanning more than 3 years, which includes the date of the Galileo probe entry. A probabilistic analysis of the data shows that within periods of several months to even more than a year, there are eight or nine longitudinal areas with high likelihood of containing a 5‐μm hot spot. These areas drift together with respect to System III at a rate which changes only slowly in time, and they are quasi evenly spaced, suggesting a wave feature. A spectral analysis of the radiance data reveals that planetary wavenumbers 8, 9, and 10 are predominant in the data, 10 having more spectral power in several time periods when the speed was 103.5–102.5 m/s, while planetary wavenumber 8 has much more power when the speed is (99.5±0.5) m/s. By using the Galileo probe zonal wind speed [Atkinson et al., 1997] at the level of the main cloud that is opaque to the radiation at 5 μm (∼2 bar), our drift corrections imply a westward phase speed for the proposed wave. The wavenumbers and phase speeds are found to be consistent with an equatorial Rossby wave, and the dispersive properties of this wave can account for the observed simultaneous changes in the dominant wavenumber and drift speed. We take advantage of this interpretation to infer properties of the vertical structure at 6.5°N.
Non‐water‐ice constituents in the surface material of the icy Galilean satellites from the Galileo near‐infrared mapping spectrometer investigation
Authors: McCord T. B. , Hansen G. B. , Clark R. N. , Martin P. D. , Hibbitts C. A. , Fanale F. P. , Granahan J. C. , Segura M. , Matson D. L. , Johnson T. V. , Carlson R. W. , Smythe W. D. , Danielson G. E.
Published: February 2004

We present evidence for several non‐ice constituents in the surface material of the icy Galilean satellites, using the reflectance spectra returned by the Galileo near infrared mapping spectrometer (NIMS) experiment. Five new absorption features are described at 3.4, 3.88, 4.05, 4.25, and 4.57 μm for Callisto and Ganymede, and some seem to exist for Europa as well. The four absorption bands strong enough to be mapped on Callisto and Ganymede are each spatially distributed in different ways, indicating different materials are responsible for each absorption. The spatial distributions are correlated at the local level in complex ways with surface features and in some cases show global patterns. Suggested candidate spectrally active groups, perhaps within larger molecules, producing the five absorptions include C‐H, S‐H, SO2, CO2, and C≡N. Organic material like tholins are candidates for the 4.57‐ and 3.4‐μm features. We suggest, based on spectroscopic evidence, that CO2 is present as a form which does not allow rotational modes and that SO2 is present neither as a frost nor a free gas. The CO2, SO2, and perhaps cyanogen (4.57 μm) may be present as very small collections of molecules within the crystal structure, perhaps following models for radiation damage and/or for comet and interstellar grain formation at low temperatures. Some of the dark material on these surfaces may be created by radiation damage of the CO2 and other carbon‐bearing species and the formation of graphite. These spectra suggest a complex chemistry within the surface materials and an important role for non‐ice materials in the evolution of the satellite surfaces.
Galileo probe measurements of thermal and solar radiation fluxes in the Jovian atmosphere
Authors: Sromovsky L. A. , Collard A. D. , Fry P. M. , Orton G. S. , Lemmon M. T. , Tomasko M. G. , Freedman R. S.
Published: February 2004

The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin‐induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5–0.75 μm in radius) of optical depth 1.5–2 at 0.5 μm. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small‐particle cloud, implying a heterogeneous cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3–4 times ground‐based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5‐bar level. The 5‐μm thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 μm, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 μm, which might be due to unaccounted‐for absorption by NH3 between 0.65 and 1.5 μm.
Classification of surface types using SIR‐C/X‐SAR, Mount Everest Area, Tibet
Authors: Albright Thomas P. , Painter Thomas H. , Roberts Dar A. , Shi Jiancheng , Dozier Jeff , Fielding Eric
Published: February 2004

Imaging radar is a promising tool for mapping snow and ice cover in alpine regions. It combines a high‐resolution, day or night, all‐weather imaging capability with sensitivity to hydrologic and climatic snow and ice parameters. We use the spaceborne imaging radar‐C/X‐band synthetic aperture radar (SIR‐C/X‐SAR) to map snow and glacial ice on the rugged north slope of Mount Everest. From interferometrically derived digital elevation data, we compute the terrain calibration factor and cosine of the local illumination angle. We then process and terrain‐correct radar data sets acquired on April 16, 1994. In addition to the spectral data, we include surface slope to improve discrimination among several surface types. These data sets are then used in a decision tree to generate an image classification. This method is successful in identifying and mapping scree/talus, dry snow, dry snow‐covered glacier, wet snow‐covered glacier, and rock‐covered glacier, as corroborated by comparison with existing surface cover maps and other ancillary information. Application of the classification scheme to data acquired on October 7 of the same year yields accurate results for most surface types but underreports the extent of dry snow cover.
A multilayer bidirectional reflectance model for the analysis of planetary surface hyperspectral images at visible and near‐infrared wavelengths
Authors: Douté Sylvain , Schmitt Bernard
Published: February 2004

We present a practical, timely, and effective radiative transfer algorithm, suitable for qualitative and quantitative analyses of high‐resolution hyperspectral images of planetary surfaces in the visible and near‐infrared domains. The bidirectional reflectance of a plane parallel, absorbing, scattering, and slightly stratified medium is generated. The local mean properties of scattering and absorption of such media are obtained apart, using semiempirical approaches. The functions which express the diffuse reflection and transmission behaviors of each homogeneous layer are then derived. For the multiple scattering term, we numerically resolve the equations appearing in the H, X and Y function method of radiative transfer, reducing the real phase function to a simplified one which can nevertheless be anisotropic. A better approach to the physical realism is obtained for the single and double scattering contributions, using their real analytical expressions. This contrasts with the Hapke model dedicated to homogeneous and semi‐infinite media, where only an isotropic reduced phase function is adopted and the single scattering correction is applied. The bidirectional reflectance and the derived quantities (albedos) of an optically semi‐infinite homogeneous medium are then easily derived from these quantities. For a stratified medium, a simple adding algorithm based on principles of invariance is presented. Compared to earlier and more complete theoretical developments, this model in most cases reproduces the dependence of the bidirectional reflectance according to the different geometrical and radiative parameters with a maximum of 10% relative error. It leads to important gains of computation time and significantly extends the validity of Hapke's or similar practical approaches.
Spectroscopic measurements of vapor clouds due to oblique impacts
Authors: Sugita Seiji , Schultz Peter H. , Adams Marc A.
Published: February 2004

Vapor clouds generated by oblique impacts were observed spectroscopically. The observations here concentrate on the earliest downrange‐moving component, among multiple components of vapor clouds generated by hypervelocity impacts of quartz projectiles into dolomite targets. The spectrum of impact vapor simultaneously exhibits blackbody radiation, molecular band emission, and atomic line emission, but their relative ratios change with time and space. The spectrum of the earliest component is dominated by the line/band emissions. The strong band/line emissions demonstrate the presence of a gas phase. The ratio of line/band emission to blackbody radiation is higher in near‐vertical impacts than shallower angle impacts. Ratios of normalized intensities of emission lines indicate that a Boltzmann distribution with an equilibrium temperature ranging from 4000 K to 6000 K approximates well the distribution of calcium atoms in energy levels in the impact vapor. Furthermore, the temperature of impact vapor appears to be controlled by the vertical component of the impact velocity. The degree of self‐absorption of a calcium emission line, which comes from vaporized dolomite target, indicates that only a very small mass of impact vapor is involved in the observed atomic radiation process. This observation, as well as the short lifetime and extremely high temperature, suggests that jetting may be the dominant source of the line emissions in the earliest stage vapor component at impact velocities less than 6 km/s. The experimental techniques and analysis methods presented in this study are applicable to other components of impact‐generated vapor and will provide new information on vaporization mechanisms in hypervelocity impacts.
Estimation of Mars radar backscatter from measured surface rock populations
Authors: Baron J. E. , Simpson R. A. , Tyler G. L. , Moore H. J. , Harmon J. K.
Published: February 2004

Reanalysis of rock population data at the Mars Viking Lander sites has yielded updated values of rock fractional surface coverage (about 0.16 at both sites, including outcrops) and new estimates of rock burial depths and axial ratios. These data are combined with a finite difference time domain (FDTD) numerical scattering model to estimate diffuse backscatter due to rocks at both the Lander 1 (VL1) and Lander 2 (VL2) sites. We consider single scattering from both surface and subsurface objects of various shapes, ranging from an ideal sphere to an accurate digitized model of a terrestrial rock. The FDTD cross‐section calculations explicitly account for the size, shape, composition, orientation, and burial state of the scattering object, the incident wave angle and polarization, and the composition of the surface. We calculate depolarized specific cross sections at 12.6 cm wavelength due to lossless rock‐like scatterers of about 0.014 at VL1 and 0.023 at VL2, which are comparable to the measured ranges of 0.019–0.032 and 0.012–0.018, respectively. We also discuss the variation of the diffuse cross section as the local angle of incidence, θi, changes. Numerical calculations for a limited set of rock shapes indicate a marked difference between the angular backscattering behavior of wavelength‐scale surface and subsurface rocks: while subsurface rocks scatter approximately as a cosine power law, surface rocks display a complex variation, often with peak backscattering at high incidence angles (θi = 70°–75°).
Characteristics of the Galileo probe entry site from Earth‐based remote sensing observations
Authors: Orton Glenn S. , Fisher Brendan M. , Baines Kevin H. , Stewart Sarah T. , Friedson A. James , Ortiz Jose Luis , Marinova Milena , Ressler Michael , Dayal Aditya , Hoffmann William , Hora Joseph , Hinkley Sasha , Krishnan Viswanathan , Masanovic Milan , Tesic Jelena , Tziolas Andreas , Parija Kartik C.
Published: February 2004

A reassessment of ground‐based observations confirms to better than a 98% confidence level that the Galileo probe entered a 5‐μm hot spot, a region of unusual clarity and dryness, some 900±300 km north of its southern boundary. Cloud conditions at that point were similar to those in the center of this region, some 600 km further north. At the time of the probe entry, the region was evolving to a slightly larger size and even thinner cloud conditions, as evidenced by its rapidly brightening appearance at 4.78 μm. The low reflectivity of the region in red light is highly anticorrelated with 4.78‐μm thermal emission, but this correlation breaks down in the blue. In general, the reflectivity of most hot spots is remarkably uniform, although the 4.78‐μm thermal emission is highly variable. A cloud structure most consistent with both the observed reflected sunlight and thermal emission properties consists of two layers: (1) a cloud layer above the 450‐mbar level extending up to the 150‐mbar level that probably consists of submicron sized particles and (2) a tropospheric cloud that is probably below the 1‐bar level, possibly ammonia hydrosulfide, with low optical thickness in the infrared. A population of particles larger than ∼3 μm, clearly present at the NH3 ice cloud level outside hot spots, is absent inside them. The NH3 gas abundance near 300–400 mbar pressure does not appear to be unusually depleted in hot spots. Zonal structures in the tropospheric temperature field near the probe entry site were not correlated with the location of 5‐μm hot spots but moved at speeds closer to the internal rotation rate of the planet. The properties of the tropospheric thermal waves at the probe entry latitude show little correlation to the properties of the 5‐μm hot spot waves. Temperatures at the probe entry site derived from remote sensing are warmer than the Atmospheric Structure Instrument (ASI) experiment results near the tropopause, probably because the low‐temperature ASI features are confined to regions smaller than the ∼6000‐km resolution characteristic of the remote sensing.
Depth and seasonality of alkenone production along the California Margin inferred from a core top transect
Authors: Herbert T. D. , Schuffert J. D. , Thomas D. , Lange C. , Weinheimer A. , Peleo‐Alampay A. , Herguera J.‐C.
Published: February 2004

Alkenone unsaturation indices (Uk'37) of marine sediment could prove particularly useful on organic‐rich continental margins where carbonate dissolution hampers the use of other paleoclimatic proxies [McCaffrey et al., 1990; Kennedy and Brassell, 1992]. Forty core top samples of Recent sediment from a latitudinal transect (23°–40°N) along the California margin yield Uk'37 values that correlate linearly with modern mean annual sea surface temperatures (SSTs) in the range of 12°–23°C. Reproducibility of the unsaturation value in closely spaced cores is near analytical error. Uk'37 data define a relationship to temperature nearly identical to the Prahl et al. [1988] laboratory cultures of Emiliania huxleyi. The close agreement is particularly significant in light of the nannofossil composition of the sediments, where the abundance of the coccolith taxon Gephyrocapsa oceanica (known to synthesize alkenones) equals or exceeds that of E. huxleyi. Comparison with seasonal temperature variations at different depths indicates that little if any alkenone production occurs at depths >30 m along the continental margin (water depths <2 km). Sediments in more pelagic locations exhibit small but consistent biases toward winter and/or subsurface production similar to previously reported sediment trap and core top data from the Oregon margin [ Prahl et al., 1993; Doose et al., 1997].
What level of resolution is attainable in a deep‐sea core? Results of a spectrophotometer study
Authors: Chapman Mark R. , Shackleton Nicholas J.
Published: February 2004

A Minolta CM‐2022 spectrophotometer has been used to characterize downcore fluctuations in sediment lightness and color in core NEAP15K, a 7‐m core collected from the northeast Atlantic Ocean. High‐resolution data series, measured using a 4‐mm‐diameter measurement spot and a 1‐cm sampling interval, were generated along two independent tracks down the core to investigate the statistical significance of fluctuations across the 2–20‐cm‐depth range. This small‐scale variability is characterized by abrupt changes in the lightness and color of the sediment that are several orders of magnitude greater than the instrumental precision. Our results establish that significant information is preserved at the 1‐cm scale in spite of bioturbation effects. These findings demonstrate that high‐resolution studies using conventional paleoclimatic proxies have the potential to recover meaningful century‐scale climate records in regions of the ocean where sedimentation rates exceed 10 cm per thousand years. The coherency of these downcore records also implies that the spectrophotometer is a powerful instrument for establishing precise centimeter‐scale stratigraphic correlations between cores.
Impact of climatic changes on the benthic foraminiferal fauna in the Ionian Sea during the last 330,0000 years
Authors: Schmiedl Gerhard , Hemleben Christoph , Keller Jörg , Segl Monika
Published: February 2004

To reconstruct paleoceanographic changes in the eastern Mediterranean during the last 330,000 years, we studied benthic foraminifera in a piston core from the Ionian Sea. The fauna exhibits large fluctuations in foraminiferal number, diversity, and species composition. Interglacials are characterized by low foraminiferal number and diversity indicating oligotrophic conditions. Directly below or above interglacial sapropels, increased numbers of low‐oxygen‐tolerant species indicate a strong reduction of deep water circulation. Glacials are characterized by increased foraminiferal number and diversity and faunas that are dominated by shallow infaunal species indicating mesotrophic conditions. Around glacial sapropel S6 very high foraminiferal numbers and the dominance of shallow and deep infaunal species suggest enhanced organic matter fluxes. These faunal results provide information about changes in the African and North Atlantic climate systems (monsoon and westerlies) controlling the humidity and wind stress in the Mediterranean region.
Effects of zooplankton herbivory on biomarker proxy records
Authors: Grice Kliti , Klein Breteler Wim C. M. , Schouten Stefan , Grossi Vincent , de Leeuw Jan W. , Damsté Jaap S. Sinninghe
Published: February 2004

The stable carbon isotopic compositions of cholesterol, generally the most dominant sterol in the copepod Temora, bears the δ13C “signature” of its dietary precursor sterol when fed on Isochrysis galbana and Rhodomonas sp. The δ13C of cholesterol in the faecal pellets released from Temora longicornis fed on Rhodomonas sp. is identical to the δ13C of the sterols in the diet, indicating that no significant carbon isotopic fractionation effects occur when the copepod modifies eukaryotic precursor sterols to cholesterol. Furthermore, the ratio of long‐chain alkenones and their stable carbon isotopic compositions in I. galbana were identical to those egested in faecal material. Thus Zooplankton herbivory does not invalidate the use of these alkenones as a proxy for sea surface temperature and pCO2.
Use of grain size and magnetic fabric analyses to distinguish among depositional environments
Authors: Joseph Leah H. , Rea David K. , Van der Pluijm Ben A.
Published: February 2004

The combined use of grain size and magnetic fabric analyses provides the ability to discriminate among depositional environments in deep‐sea terrigenous sediments. We analyzed samples from three different depositional settings: turbidites, pelagic or hemipelagic interlayers, and sediment drifts. Results indicate that sediment samples from these different environments can be distinguished from each other on the basis of their median grain size, sorting, as well as the intensity and shape of magnetic fabric as determined from an examination of the anisotropy of magnetic susceptibility. We use these discriminators to interpret downcore samples from the Bermuda Rise sediment drift. We find that the finer grains of the Bermuda Rise (relative to the Blake Outer Ridge) do not result from lower depositional energy (current speed) and so may reflect a difference in the nature of sediment being delivered to the site (i.e., distance from source) between the two locations.
Optimal sites for coral‐based reconstruction of global sea surface temperature
Authors: Evans Michael N. , Kaplan Alexey , Cane Mark A.
Published: February 2004

We determine the structure of a network of sites from which coral‐based, proxy measurements of sea surface temperature (SST) variability minimize the error in a reconstruction of the large‐scale features of the global SST field. For a wide range of coral‐derived SST observational error and several minimization criteria, sites in the equatorial oceans, especially the central and eastern equatorial Pacific, best minimize the error in the reconstruction. If the observational error is sufficiently low, additional optimal sites are in selected subtropical locations. If the observational error is high, the error is minimized by resampling the most important equatorial sites. The marginal return on additional sites diminishes rapidly and is not sensitive to the size of the observational error: two sites reduce the analysis error by half as much as 10 sites, while the first 6–7 sites achieve half the error reduction of all 65 sites in the analysis domain. In the extratropics the reduction of reconstruction error is small and gradual, while error in the tropical Pacific is sharply reduced by 2–3 sites and gradually thereafter. These results suggest that a limited set of redundantly sampled sites with uncorrelated and low observational error (≈ 0.3°–0.6°C) will provide the best approach to reconstruction of large‐scale features of the SST field from coral data for the preinstrumental period.
Non‐double‐couple earthquakes 1. Theory
Authors: Julian Bruce R. , Miller Angus D. , Foulger G. R.
Published: February 2004

Historically, most quantitative seismological analyses have been based on the assumption that earthquakes are caused by shear faulting, for which the equivalent force system in an isotropic medium is a pair of force couples with no net torque (a “double couple,” or DC). Observations of increasing quality and coverage, however, now resolve departures from the DC model for many earthquakes and find some earthquakes, especially in volcanic and geothermal areas, that have strongly non‐DC mechanisms. Understanding non‐DC earthquakes is important both for studying the process of faulting in detail and for identifying nonshear‐faulting processes that apparently occur in some earthquakes. This paper summarizes the theory of “moment tensor” expansions of equivalent‐force systems and analyzes many possible physical non‐DC earthquake processes. Contrary to long‐standing assumption, sources within the Earth can sometimes have net force and torque components, described by first‐rank and asymmetric second‐rank moment tensors, which must be included in analyses of landslides and some volcanic phenomena. Non‐DC processes that lead to conventional (symmetric second‐rank) moment tensors include geometrically complex shear faulting, tensile faulting, shear faulting in an anisotropic medium, shear faulting in a heterogeneous region (e.g., near an interface), and polymorphic phase transformations. Undoubtedly, many non‐DC earthquake processes remain to be discovered. Progress will be facilitated by experimental studies that use wave amplitudes, amplitude ratios, and complete waveforms in addition to wave polarities and thus avoid arbitrary assumptions such as the absence of volume changes or the temporal similarity of different moment tensor components.
Contribution to the problem of near‐zone inverse Doppler effect
Authors: Ben‐Shimol Yehuda , Censor Dan
Published: February 2004

The existence of an inverse Doppler effect in free space is again scrutinized, following some papers predicting the existence of such phenomena in the near zone of a moving oscillating three‐dimensional dipole. In the present paper the wave scattered from a perfectly conducting thin cylinder moving in the presence of a plane electromagnetic wave is analyzed. The response of such a cylinder may be considered as due to either a two‐dimensional monopole or a two‐dimensional dipole in accordance with the polarization of the incident wave. An intensive numerical spectral estimation based on the fundamental definitions of the terms “frequency,” “spectrum,” and “uncertainty” is performed on the scattered wave at various ranges. An inverse Doppler effect was not found for the two‐dimensional case. The same analysis was applied to the case of a moving three‐dimensional radiating dipole, reconfirming previous results which showed the presence of an inverse Doppler effect in the near zone of the three‐dimensional dipole.
The role of mutual coupling in the performance of synthetic aperture radiometers
Authors: Weissman David E. , Le Vine David M.
Published: February 2004

This paper describes research to evaluate the role of mutual coupling between antennas in the performance of synthetic aperture radiometers. Measurements, taken from the aircraft prototype instrument ESTAR (electronically scanned thinned array radiometer), are presented to demonstrate the importance of mutual coupling. Theory for the ESTAR configuration (an array of dipoles) is presented and checked against the measurements. This theory is then used to assess the effects of small displacements in the elementary antennas in the synthesis array of a hypothetical sensor in space. The measurement errors that result from small displacements (as small as a few hundredths of a wavelength) can be important for applications such as the measurement of soil moisture. Therefore the design approach for future systems should include the control of these and should also include the capability to perform onboard calibration to monitor and remedy detrimental effects. A synthetic aperture radiometer is particularly suited to the measurement of mutual coupling because the basic hardware (a correlation radiometer) measures the product for each antenna pair in the array. Hence, in conclusion, this paper proposes a concept for monitoring changes in mutual coupling to correct for such effects in formation of images.
Prediction of over‐the‐horizon radar clutter using the clutter effects model
Authors: Lauer Carl , Nickisch L. J. , Wortman William
Published: February 2004

We have developed an over‐the‐horizon radar (OTHR ) clutter prediction model to better define equatorial clutter environments. This physics‐based model is referred to as the clutter effects model, or CLEM. It is nominally configured toward simulation of the Navy's Relocatable OTHR (ROTHR). Radar signal propagation geometry is computed by ray tracing through a global ionosphere model (driven by geophysical conditions, time, and location). The equatorial ionosphere has high levels of field‐aligned irregularities which produce strong clutter environments for OTHR. Strengths of modeled ionospheric irregularities are derived from transionospheric scintillation measurement data. Clutter Doppler and delay spreading is analyzed using multiple phase screen forward diffraction techniques. Technical background on an earlier version of CLEM, which performed three‐dimensional (3‐D) ray tracing, is given by Lauer et al. [1995]. The current version, which is under discussion here, has been sped up through the use of 2‐D ray tracing and substitution of a simpler spectral form for the signal delay‐Doppler spectrum; however, the reader may refer to Lauer et al [1995] for background on various aspects of the theory which still pertain, including phase diffraction propagation effects, absorption calculations, and ionospheric structure predictions. This paper describes CLEM models and compares CLEM predictions with clutter measurement data for specific cases.
A stable algorithm for simulation of array induction and measurement‐while‐drilling logging tools
Authors: Tian Xiang , Liu Ce , Shen Liang C.
Published: February 2004

A stable numerical method to simulate array induction and measurement‐while‐drilling (MWD) logs is presented. The algorithm is based on a horizontal eigenmode expansion method. Mode propagation in the vertical direction is described by three‐layer modules. Within each module, general transmission and reflection coefficients are obtained by tracing the propagation of the modes. The multilayer cases are treated as a cascade of such modules. Owing to the mode‐tracing algorithm, numerical stability and efficiency are superior to the other methods. This numerical algorithm is applied to the simulation of array induction and MWD logs in formations with both vertical and horizontal layers.
Inferring rough surface parameters from average scattering data using approximate scattering models: 1. Gaussian spectrum
Authors: Marchand Roger T. , Brown Gary S.
Published: February 2004

The quasi‐specular and two‐scale models for the average radar cross section of randomly rough surfaces have seen extensive use in inversion algorithms to estimate surface roughness parameters. This paper and a companion paper report the results of a numerical study on the accuracy of these models which was initiated with the goal of understanding what scattering mechanisms limit their applicability. In this paper, surfaces which are Gaussian distributed in height and have Gaussian spectra are examined. It is shown that the quasi‐specular model is limited by multiple scattering and diffraction due to surface curvature. The effects of polarization and the role of the surface dielectric constant are examined at length. Approximate conditions for which the quasi‐specular model is valid are provided.
FMCW waveform generator requirements for ionospheric over‐the‐horizon radar
Authors: Earl G. F.
Published: February 2004

The purpose of this paper is to discuss the significance of FMCW waveform generator spectral purity with regard to its propensity to limit over‐the‐horizon (OTH) radar performance. The fundamental principle of operation of FMCW OTH radar is first described and then further developed in order to yield results in terms of several parameters of a synoptic database of OTH radar propagation data acquired with the Australian project Jindalee OTH radar frequency management system. The method of data analysis is addressed in detail and followed by the presentation of the results of the analysis of 5 years of data. Finally, the high demands of waveform generator spectral purity are emphasized in the sense of avoiding radar performance degradation due to this potential form of internal noise limitation.
Analysis of data‐driven parametric models of the vertical ionospheric profile for use in oblique propagation studies
Authors: Barnes Rod I. , Braendler S. A. , Coleman C. J. , Gardiner‐Garden R. S. , Hoang T. V.
Published: February 2004

This paper compares parameterization techniques used for modeling the vertical profile of the ionosphere. In particular, comparisons of three‐layer parameterizations driven by ionosonde data are performed. Quasi‐parabolic, Chapman, and polynomial vertical profile models of the ionosphere are investigated. Optimization techniques applied at varying stages of the sensor output, ranging from direct inversion from the digital ionogram image to fitting to true height output for the estimation of parameters, are described. A Kalman‐based filter is optionally used to track and filter the parameters. The performance of the various methods in producing the parameterized profile is analyzed through the comparison of predicted versus measured values of the F2 O‐mode maximum observable frequency (MOF) on a 1250‐km midlatitude oblique path. As a further benchmark, the monthly median model, FIRIC, is also used to generate oblique ionograms. It is found that three‐layer parameterized models perform well and can increase prediction accuracy over the median model, with the ability to reduce the rms error in MOF estimation from 2.38 MHz to 0.83 MHz.
Total electron content measurements in the southern hemisphere using GPS satellites, 1991 to 1995
Authors: Breed A. M. , Goodwin G. L. , Silby J. H.
Published: February 2004

The seasonal, diurnal, and latitudinal variations of total electron content (TEC) were determined using Global Positioning System (GPS) satellite signals over approximately 5 days per month during almost half a sunspot cycle (July 1991 to June 1995) at Salisbury (latitude 34.77°S, longitude 138.63°E), South Australia. These are the only such extensive southern hemisphere data till 1995 that have been recorded and analyzed. A selection of the data is presented, discussed, and compared with other workers' observations. Examples of Australian Surveying and Land Information Group (AUSLlG) data in the Australian region are presented. Some model predictions for northern hemisphere ionospheric TEC are compared with the GPS southern hemisphere observations of the combined TEC of the ionosphere and protonosphere. (The two models employed are the international reference ionosphere (IRI‐90) and the paramaterized ionospheric model (PIM)(version 1.4, February 1996)). They are considered to be global models, even though the IRJ model is based primarily, but not exclusively, on northern hemisphere TEC data, and PIM is based on a theoretical model and is thus not directly based on TEC data.
Simultaneous meteor echo observations by large‐aperture VHF and UHF radars
Authors: Zhou Q. H. , Perillat P. , Cho J. Y. N. , Mathews J. D.
Published: February 2004

We report simultaneous meteor echo observations using the Arecibo 430‐MHz and 46.8‐MHz radars. Using identical data‐taking and meteor selection criteria, 1868 and 367 meteors were found in the 430‐MHz and 46.8‐MHz beams, respectively, while 145 were found in both beams during the 7 hours of observation. Of the 367 VHF echoes, there were only 10 trail echoes, while the rest were head echoes, which was quite contrary to expectation. The smaller number of meteors detected by the VHF system and its wider beam width show that UHF meteors are far smaller than the VHF meteors. We estimate that VHF head echoes have a typical effective scattering cross section of the order of 10−3 m2, while the accompanying UHF echoes have an effective scattering cross section of the order of 10−6 m2. The paucity of VHF trail echoes observed leads us to suggest that the ratio of head echo power to the trail echo power increases with decreasing meteor size. When a meteor is too small, a radar can observe the head echo but not the trail echo. Of the 145 meteors observed by both radars, the powers received by the two systems were not correlated. Although antenna beam pattern contributes to the lack of correlation, it is also possible that UHF and VHF echoes may be enhanced by different scattering mechanisms.
Electromagnetic energy dissipation and propagation of an ultrawideband plane wave pulse in a causally dispersive dielectric
Authors: Smith Paul D. , Oughstun Kurt E.
Published: February 2004

The dissipation of electromagnetic energy from an ultrawideband plane wave pulse that is propagating through a causally dispersive, attenuative dielectric is described. Because the precursor fields, which are a characteristic of the dispersive medium, propagate with the least attenuation in the medium, the peak in the Brillouin precursor decaying only as z−1/2, they carry a significant fraction of the input electromagnetic energy much deeper into the lossy material. These precursor fields then generate heat at propagation depths where virtually none is generated by the main signal at the input carrier frequency. The implications of this ultrawideband phenomenon to bioelectromagnetics and radiation dosimetry may be significant.
An improved probability function to predict the F1 layer occurrence and L condition
Authors: Scotto C. , Radicella S. M. , Zolesi B.
Published: February 2004

A new probability function to predict the occurrence of the F1 layer and L condition is here proposed and compared with the previous one. A test of the two expressions is performed.
Quantifying uncertainty in tracer‐based hydrograph separations
Authors: Genereux David
Published: February 2004

A method is presented for quantifying the uncertainty in two‐ and three‐component tracer‐based hydrograph separations. The method relates the uncertainty in computed mixing fractions to both the tracer concentrations used to perform the hydrograph separation and the uncertainties in those concentrations. A two‐component example and a three‐component example illustrate the application of the method. The three‐component example yields uncertainty results very similar to those from a previously published Monte Carlo analysis and requires less computation.
Extraction and representation of nested catchment areas from digital elevation models in lake‐dominated topography
Authors: Mackay D. Scott , Band Lawrence E.
Published: February 2004

This paper presents a new method for extracting flow directions, contributing (upslope) areas, and nested catchments from digital elevation models in lake‐dominated areas. Existing tools for acquiring descriptive variables of the topography, such as surface flow directions and contributing areas, were developed for moderate to steep topography. These tools are typically difficult to apply in gentle topography owing to limitations in explicitly handling lakes and other flat areas. This paper addresses the problem of accurately representing general topographic features by first identifying distinguishing features, such as lakes, in gentle topography areas and then using these features to guide the search for topographic flow directions and catchment marking. Lakes are explicitly represented in the topology of a watershed for use in water routing. Nonlake flat features help guide the search for topographic flow directions in areas of low signal to noise. This combined feature‐based and grid‐based search for topographic features yields improved contributing areas and watershed boundaries where there are lakes and other flat areas. Lakes are easily classified from remotely sensed imagery, which makes automated representation of lakes as subsystems within a watershed system tractable with widely available data sets.
Flow modeling and permeability estimation using borehole flow logs in heterogeneous fractured formations
Authors: Paillet F. L.
Published: February 2004

A numerical model of flow in the vicinity of a borehole is used to analyze flowmeter data obtained with high‐resolution flowmeters. The model is designed to (1) precisely compute flow in a borehole, (2) approximate the effects of flow in surrounding aquifers on the measured borehole flow, (3) allow for an arbitrary number (N) of entry/exit points connected to M < N far‐field aquifers, and (4) be consistent with the practical limitations of flowmeter measurements such as limits of resolution, typical measurement error, and finite measurement periods. The model is used in three modes: (1) a quasi‐steady pumping mode where there is no ambient flow, (2) a steady flow mode where ambient differences in far‐field water levels drive flow between fracture zones in the borehole, and (3) a cross‐borehole test mode where pumping in an adjacent borehole drives flow in the observation borehole. The model gives estimates of transmissivity for any number of fractures in steady or quasi‐steady flow experiments that agree with straddle‐packer test data. Field examples show how these cross‐borehole‐type curves can be used to estimate the storage coefficient of fractures and bedding planes and to determine whether fractures intersecting a borehole at different locations are hydraulically connected in the surrounding rock mass.
On second‐order correction to the effective conductivity of two‐dimensional anisotropic media
Authors: Spector M. , Indelman P.
Published: February 2004

It has been shown recently that the effective conductivity Keff of anisotropic media depends on the shape of correlations [Indelman and Abramovich, 1994]. This property results from the analysis of the second‐order expression of Keff in the log conductivity variance σ2, which is derived in terms of quadratures and calculated by numerical integration for two‐ and three‐dimensional flows and for exponential and Gaussian covariances. The present paper is a continuation of this study and aims at deriving an analytical closed‐form expression of Keff at order σ4 for the case of two‐dimensional flow in lognormal anisotropic media of Gaussian log conductivity correlation.
Preferential transport of nitrate to a tile drain in an intermittent‐flood‐irrigated field: Model development and experimental evaluation
Authors: Mohanty B. P. , Bowman R. S. , Hendrickx J. M. H. , Simunek J. , van Genuchten M. T.
Published: February 2004

A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field‐scale flow and transport in the vadose zone. The field data were analyzed in terms of a two‐dimensional numerical model based on the Richards equation for variably saturated water flow, convection‐dispersion equations with first‐order chemical decay chains for solute transport, and bimodal piecewise‐continuous unsaturated hydraulic functions to account for preferential flow of water and nitrate‐nitrogen (NO3‐N; loosely used as NO3‐) following flood irrigation events at the experimental site. The model was tested against measured NO3‐ flux concentrations in a subsurface tile drain, several monitoring wells and nested piezometers, and against resident NO3‐ concentrations in the soil profile (obtained at 52 spatial locations and four depths along a transect). NO3‐ transport at the field site could be described better with the bimodal hydraulic functions than using the conventional approach with unimodal van Genuchten‐Mualem type hydraulic functions. Average resident nitrate concentrations measured across the soil profile were predicted reasonably well. However, NO3‐ flux concentrations in the subsurface tile drain and piezometers at the field site were occasionally underestimated or overestimated depending upon the irrigation sequence in three field benches, probably reflecting unrepresented three‐dimensional regional flow/transport processes. Limiting the capture zone to a region closer to the tile drain did lead to a better match with observed sharp increases and decreases in predicted NO3‐ flux concentrations during the irrigation events. On the basis of this result we inferred that the preferential flow intercepted by the tile drain was generated in close proximity of the drain and essentially oriented vertically. In summary, our study suggests that irrigation scheduling in adjacent field plots, drainage design (e.g., spacing between tiles, drain depth, drain diameter) and effectiveness (e.g., drain blockage), preferential flow in (horizontal) surface‐opened shallow cracks and (vertical) macropores, and transient regional groundwater flow can add significant uncertainty to the predictions of (local‐scale) flow and transport to a tile drain.
Hydrogeologic Constraint on the origin of deep subsurface microorganisms within a Triassic Basin
Authors: Tseng Hsin‐Yi , Person Mark , Onstott T. C.
Published: February 2004

The thermal history of the Taylorsville basin indicates that the thermophilic anaerobic bacteria extracted from Triassic strata at 2800 m below the land surface within the basin probably migrated to their current location. In order to ascertain the most probable scenario for microbial transport, a two‐dimensional transient fluid flow and heat transport model of the Taylorsville basin was developed using the finite element method. The numerical model was constrained by tectonic and thermal histories of the basin, coupled with the permeability and porosity data extracted from core samples. The model demonstrates that a topographically driven groundwater flow system, caused by the tectonic uplift and erosion during the Jurassic, can explain the observed differential cooling of the basin. The computed groundwater flow rates during this period were on the order of 1 to 100 mm/yr. Combined with the cooling history of the microbially sampled strata, such rates provided a possible mechanism for the introduction of ancient surface or subsurface microorganisms to the deep subsurface. This tectogenetic mechanism may explain the origins of deep subsurface microorganisms for other regions of the Earth.
Scaling of near‐saturated hydraulic conductivity measured using disc infiltrometers
Authors: Shouse P. J. , Mohanty B. P.
Published: February 2004

A function relating unsaturated soil hydraulic conductivity K and soil water pressure head h is most important for understanding water flow and chemical transport in the vadose zone. Furthermore, the K(h) function near saturation is critical for describing flow in macropores and other structural voids. The usefulness of similar media scaling and functional normalization to describe the near‐saturated hydraulic conductivity function K(h) measured in situ at 296 spatial locations across a heterogeneous agricultural field was tested. Disc (ponded and tension) infiltrometers were used to measure K(h) at different field positions (corn row, no traffic interrow, and traffic interrow) cutting across different soil types (Nicollet and Clarion loam derived from glacial till material). The K(h) data ranged several orders of magnitude for different field positions and soil types and were found to be statistically different between different field positions. Using a Gardner type K(h) function, relative hydraulic conductivity values, and a hybrid of similar media scaling and functional normalization concepts, all disc infiltrometer data sets were coalesced to a single reference curve. Poor to moderately correlated K and h scale factors did not show any significant spatial structure across the field. A novel finding is that saturated hydraulic conductivities (Ksat) could be successfully used as the scale factor for the near‐saturated K(h) functions (e.g., 0–15 cm soil water tension) under all field positions and soil types at the experimental field.