bluuebunny/crossref_metadata_2025 huggingface dataset:
Europa: Stratigraphy and geological history of the anti‐Jovian region from Galileo E14 solid‐state imaging data
Authors: Prockter Louise M. , Antman Amy M. , Pappalardo Robert T. , Head James W. , Collins Geoffrey C.
Published: September 2002

A tidal bulge and associated deformation are predicted to occur in Europa's anti‐Jovian region. Voyager data showed polygonal, low‐albedo bands in this region. It was demonstrated that when the low‐albedo material comprising these bands was removed, the margins could be reconstructed, suggesting that fracturing and lateral separation has occurred. Higher‐resolution Galileo images of a 300,000 km2 portion of the anti‐Jovian region permit recognition and mapping of a variety of features and documentation of the geological history. We identify background ridged plains, several distinctive types of bands and ridges, lenticulae, and regions of chaotic terrain. These images are at relatively low incidence angles; thus albedo variations predominate over topography. This presents a challenge to morphologically based mapping but does not preclude development of a regional stratigraphy. Stratigraphic relationships show that dark bands crosscut the bright ridged plains and are themselves disrupted by lenticulae and chaos, suggesting a geological history beginning with background ridged plains, followed by a period dominated by band formation, with the most recent history predominantly characterized by formation of lenticulae and chaos. The paucity of impact craters in this region indicates that the entire sequence of units is geologically recent.
Comparative terrestrial planet thermospheres: 2. Solar cycle variation of global structure and winds at equinox
Authors: Bougher S. W. , Engel S. , Roble R. G. , Foster B.
Published: September 2002

The present maturity of available planetary databases and modeling capabilities now permits us to extend the comparison of terrestrial planetary thermospheres beyond the limited capability of one‐dimensional models to global multidimensional models [e.g., Bougher and Roble, 1997]. This effort focuses upon the comparison of the solar cycle responses of the thermospheres of Venus, Earth, and Mars using three‐dimensional global models that couple the energetics, dynamics, and neutral‐ion composition above ∼100 km for each planet. Standard solar EUV and UV fluxes are adopted for use in these simulations. The Venus, Earth, and Mars Thermosphere General Circulation Models (TGCMs) each share a common formulation scheme and development heritage making use of the computing facilities of the National Center for Atmospheric Research. The motivation of this research is not only to simulate the observed responses of these individual planets to solar EUV/UV flux variations but also to understand the relative importance of common processes that regulate this unique behavior. The role of O‐CO2 enchanced 15‐μm cooling is investigated in the context of global dynamics and its effect on atomic‐O distributions. It is found that CO2 cooling is an effective thermostat for control of the Venus dayside temperatures, while Mars and Earth are only moderately affected. By contrast, the role of global dynamics in controlling temperature distributions is most pronounced for Mars and the Venus nightside but negligible for Earth. The net effect of these radiative and dynamical processes is to determine that Venus and Mars thermospheres respond rather quickly to solar flux variations (much less than an Earth day), while the Earth thermosphere is more sluggish in its behavior. This work confirms the relative importance of CO2 cooling in the Earth's lower thermosphere. Furthermore, the value of the CO2‐O deactivation rate near 300 K is rather well constrained by these planetary comparisons.
Digital mapping of the Mars Pathfinder landing site: Design, acquisition, and derivation of cartographic products for science applications
Authors: Gaddis L. R. , Kirk R. L. , Johnson J. R. , Soderblom L. A. , Ward A. W. , Barrett J. , Becker K. , Becker T. , Blue J. , Cook D. , Eliason E. , Hare T. , Howington‐Kraus E. , Isbell C. , Lee E. M. , Redding B. , Sucharski R. , Sucharski T. , Smith P. H. , Britt D. T.
Published: September 2002

The Imager for Mars Pathfinder (IMP) acquired more than 16,000 images and provided panoramic views of the surface of Mars at the Mars Pathfinder landing site in Ares Vallis. This paper describes the stereoscopic, multispectral IMP imaging sequences and focuses on their use for digital mapping of the landing site and for deriving cartographic products to support science applications of these data. Two‐dimensional cartographic processing of IMP data, as performed via techniques and specialized software developed for ISIS (the U.S. Geological Survey image processing software package), is emphasized. Cartographic processing of IMP data includes ingestion, radiometric correction, establishment of geometric control, coregistration of multiple bands, reprojection, and mosaicking. Photogrammetric processing, an integral part of this cartographic work which utilizes the three‐dimensional character of the IMP data, supplements standard processing with geometric control and topographic information [Kirk et al., this issue]. Both cartographic and photogrammetric processing are required for producing seamless image mosaics and for coregistering the multispectral IMP data. Final, controlled IMP cartographic products include spectral cubes, panoramic (360° azimuthal coverage) and planimetric (top view) maps, and topographic data, to be archived on four CD‐ROM volumes. Uncontrolled and semicontrolled versions of these products were used to support geologic characterization of the landing site during the nominal and extended missions. Controlled products have allowed determination of the topography of the landing site and environs out to ∼60 m, and these data have been used to unravel the history of large‐ and small‐scale geologic processes which shaped the observed landing site. We conclude by summarizing several lessons learned from cartographic processing of IMP data.
Morphologies of rocks within and near the Rock Garden at the Mars Pathfinder landing site
Authors: Basilevsky A. T. , Markiewicz W. J. , Thomas N. , Keller H. U.
Published: September 2002

This paper considers the morphology of several rocks and rock fragments within and near the Rock Garden at the Mars Pathfinder landing site. We have analyzed stereo images taken both by the lander IMP camera and by the rover forward cameras. The rocks were found to differ in their roundness/angularity and in their densities of millimeter‐to‐centimter‐sized pits and flute‐like features on the rock facets. No correlation between rock roundness and the degree of rock pitting was found, indicating that rounding and pitting formed by different processes. Pits are either gas bubbles (implying the rocks are volcanic), or resulted from eolian abrasion/deflation (putting no constraints on the nature of the rocks), or both. If pits are of eolian origin, variations in the density of pits on different rocks imply that either the rock surfaces have been exposed for different times or the susceptibility of different rocks to pitting varies significantly from rock to rock, implying differences in composition and/or lithology. The densities of pits and flutes on the rocks show anti‐correlation. This may be connected to their formation but probably is mostly related to issues of observation and definition of what is a pit and what is a flute. Flute‐like features may be a result of eolian abrasion, but in this case, the spatial closeness of fluted rocks and those having delicately pitted textures requires explanation. Other mechanisms producing flute‐like features, such as collisions in the deposit‐forming flows and the breaking of rock in impact cratering, should not be ignored. Our observations agree with the conclusion of Golombek et al. [this issue] that deflational exhumation in this area was minor. The observed, relatively large, rocks typically do not show a concentration of small rock fragments at their feet. This implies diurnal thermal cycles are very ineffective at destructing rocks. The rock, Half Dome, may be a conglomerate (as suggested by Smith et al. [1997a] for this and several other rocks), but this hypothesis remains to be proven. Impact craters of a few meters to a few tens meters in diameter may have played a role in the local geology.
The nature of fossil bacteria: A guide to the search for extraterrestrial life
Authors: Westall Frances
Published: September 2002

In an attempt to establish reliable criteria for the identification of potential fossil life in extraterrestrial materials, the fossilizable characteristics of bacteria, namely, size, shape, cell wall texture, association, and colony formation, are described, and an overview is given of the ways in which fossil bacteria are preserved (as compressions in fine‐grained sediments; preservation in amber; permineralized by silica; replacement by minerals such as silica, pyrite, Fe/Mn oxides, calcite, phosphate, and siderite; or as molds in minerals). The problem of confounding minerally replaced bacteria with non biological structures having a bacterial morphology is addressed. Examples of fossilized bacteria from the Early Archaean through to the Recent are used to illustrate the various modes of preservation and the morphology of fossil bacteria.
The Diversity and Distribution of Modern Planktic Foraminiferal Small Subunit Ribosomal RNA Genotypes and their Potential as Tracers of Present and Past Ocean Circulations
Authors: Darling Kate F. , Wade Christopher M. , Kroon Dick , Brown Andrew J. Leigh , Bijma Jelle
Published: September 2002

Molecular phylogenetic analysis of the small subunit ribosomal RNA gene of planktic spinose foraminifers shows that morphospecies may represent clusters of different and often highly divergent genotypes. In some cases the level of divergence may justify separate taxonomic status as distinct “cryptic” species. Molecular evolution rate estimates, based on fossil record evidence, suggest that the cryptic divergences may have occurred many millions of years ago. An investigation of their distribution in the Caribbean (tropical zone), Coral Sea and Mediterranean Sea (subtropical zone), and Southern California Bight (transitional zone) indicates that genotypes are transported across water mass boundaries, and it is proposed that the direction of gene flow follows the prevailing global ocean surface circulation pattern. At the present time the prevailing currents transport tropical/subtropical genotypes from the Pacific to Atlantic around the South African Cape. Cooler water transitional genotypes may transit from Pacific to Atlantic in gene corridors opened during glacial periods.
U and Th isotope constraints on the duration of Heinrich events H0‐H4 in the southeastern Labrador Sea
Authors: Veiga‐Pires C. C. , Hillaire‐Marcel C.
Published: September 2002

The duration and sequence of events recorded in Heinrich layers at sites near the Hudson Strait source area for ice‐rafted material are still poorly constrained, notably because of the limit and uncertainties of the 14C chronology. Here we use high‐resolution 230Th‐excess measurements, in a 6 m sequence raised from Orphan Knoll (southern Labrador Sea), to constrain the duration of the deposition of the five most recent Heinrich (H) layers. On the basis of maximum/minimum estimates for the mean glacial 230Th‐excess flux at the studied site a minimum/maximum duration of 1.0/0.6, 1.4/0.8, 1.3/0.8, 1.5/0.9, and 2.1/1.3 kyr is obtained for H0 (∼Younger Dryas), Hl, H2, H3, and H4, respectively. Thorium‐230‐excess inventories and other sedimentological features indicate a reduced but still significant lateral sedimentary supply by the Western Boundary Undercurrent during the glacial interval. U and Th series systematics also provide insights into source rocks of H layer sediments (i.e., into distal Irminger Basin/local Labrador Sea supplies).
Latest Miocene‐Pleistocene Productivity and Deep‐Sea Ventilation in the Northwestern Indian Ocean (Deep Sea Drilling Project Site 219)
Authors: Gupta Anil K. , Thomas Ellen
Published: September 2002

Knowledge of the biology of deep‐sea benthic foraminifera was used to interpret the results of multivariate analyses (factor and cluster) on relative abundance data of benthic foraminifera at Deep Sea Drilling Project Site 219 (southeastern Arabian Sea; 1764 m depth) in combination with carbon and oxygen isotope data. Faunal data document major changes in deep‐sea ventilation and productivity over the past 5.5 Ma, including the end of the Miocene‐Pliocene Indo‐Pacific “biogenic bloom” period at ∼4.0 Ma. Interestingly, there is no simple correlation between high productivity and low oxygenation. Productivity fluctuated but became overall more pulsed, whereas overall oxygenation increased. We interpret the records as a combination of local to regional fluctuations in productivity probably caused by changes in monsoonal development, particularly its seasonality, and changes in oxygenation of intermediate depth waters in the Indian Ocean caused by global changes in climate and ocean circulation.
A 1.0 Myr Record of Glacial North Atlantic Intermediate Water Variability from ODP Site 982 in the Northeast Atlantic
Authors: Venz Kathryn A. , Hodell David A. , Stanton Cathy , Warnke Detlef A.
Published: September 2002

Changes in the intermediate water structure of the North Atlantic were reconstructed using benthic foraminiferal δ13C at Ocean Drilling Program (ODP) site 982 for the past 1.0 Myr. During most terminations of the late Pleistocene, melting of icebergs and low‐salinity surface waters caused production of Glacial North Atlantic Intermediate Water to cease, resulting in decreased ventilation of the middepth North Atlantic. Poor ventilation of intermediate water masses lasted well into some interglacial stages until upper North Atlantic Deep Water (NADW) production resumed under full interglacial conditions. The magnitude of benthic δ13C minima and ice‐rafted debris maxima at terminations at site 982 generally match the degree of glacial suppression of NADW inferred from site 607. These processes may be related and controlled by the spatial and seasonal extent of sea ice cover during glaciations in the Nordic Seas.
Organic molecular and carbon isotopic records of the Japan Sea over the past 30 kyr
Authors: Ishiwatari R. , Yamada K. , Matsumoto K. , Houtatsu M. , Naraoka H.
Published: September 2002

The organic and isotopic geochemical study of two sediment cores (KH‐79‐3, L‐3, and KH‐79‐3, C‐3) from the Oki Ridge in the Japan Sea has revealed that total organic carbon (TOC) mass accumulation rates are extremely high in the 12–11 ka (calendar age) interval and TOC in the sections in the 24–17 ka interval is depleted in 13C by 3.5‰ relative to Holocene sediments. Alkenone sea surface temperature (SST) shows a decrease from 18° to 14°C from 17.5 to 11.6 ka and a sharp increase from 14° to 19°C from 11.6 to 11.1 ka. The SST changes are associated with the inflow of cold seawater with the vertical water mixing and the inflowof warm Tsushima Current into the Japan Sea. The δ13C values for both 24‐methylcholesta‐5,22‐dien‐3β‐ol (diatom marker) and dinosterol (dinoflagellate marker), are at their minimum from 24 to 17 ka, while those for long‐chain alkenones are not. The theoretical considerations on δ13C for biomarkers suggest low photosynthetic carbon demand of diatoms and dinoflagellates from 24 to 17 ka.
The isotopic composition of diatom‐bound nitrogen in Southern Ocean sediments
Authors: Sigman Daniel M. , Altabet Mark A. , Francois Roger , McCorkle Daniel C. , Gaillard Jean‐Francois
Published: September 2002

Treatment of diatom microfossils from Southern Ocean sediments with hot perchloric acid leaves a “diatom‐bound” N fraction which is 0–4‰ lower in δ15N than the bulk sediment, typically 3‰ lower in recent Antarctic diatom ooze. Results from Southern Ocean surface sediments indicate that early diagenetic changes in bulk sediment N content and δ15N are not reflected in diatom‐bound N, suggesting that diatom‐bound N is physically protected from early diagenesis by the microfossil matrix. A meridional transect of multicores from the Indian sector of the Southern Ocean shows a northward increase in the δ15N of diatom‐bound N, suggesting that diatom‐bound δ15N, like bulk sedimentary δ15N, varies with nitrate utilization in the overlying surface waters. The δ15N of diatom‐bound N is 3–4‰ higher in glacial age Antarctic sediments than in Holocene sediments, supporting the hypothesis, previously based on bulk sediment δ15N, that nitrate utilization in the surface Antarctic was higher during the last ice age. While there are important uncertainties, the inferred range of utilization changes could potentially explain the entire ∼80 ppm amplitude of observed glacial/interglacial variations in atmospheric CO2.
The three‐dimensional solar wind at solar activity minimum
Authors: Neugebauer M.
Published: September 2002

In early 1998 the Ulysses spacecraft made history by completing an orbit around the Sun inclined to the solar equator by 80°. It observed the properties of the high‐latitude heliosphere at a time of low solar activity when the configuration of the solar magnetic field was relatively simple. The solar wind data acquired by Ulysses have placed firm observational constraints on theories of the acceleration of the fast wind from the polar regions. The speed of the polar solar wind was in the range 750–800 km s−1, with a slow increase toward the poles. The polar proton flux of ∼2 × 108 cm−2 s−1 was only two‐thirds the low‐latitude flux but still sufficiently high to require energy sources in addition to the conduction of heat from the hot solar corona. In comparison with the slow, low‐latitude solar wind, the heavy ions in the fast, high‐latitude wind had less elemental fractionation relative to the solar surface and a lower state of ionization. Fine structures in the polar solar wind, named microstreams, exhibited a correlation between speed, proton temperature, and alpha particle abundance. The absence of a latitude dependence of the widths of the microstreams suggests that they are caused by temporal, rather than purely spatial variations at the Sun. Magnetic field observations revealed a latitude‐independent radial field strength of ∼3 nT (normalized to 1 AU) and a large flux of outward propagating Alivén waves. The most probable direction of the interplanetary magnetic field was close to the Archimedes (Parker) spiral predicted on the basis of radial fields close to the Sun combined with the effects of solar rotation. The fluctuations about the average direction were so large, however, that there was little latitudinal variation of the average value of |BR|B, where BR is the radial component of the field with magnitude B. Latitudinal transport of field lines and nonradial field directions near the Sun appear to be other necessary additions to the simple Parker model. Some unexpected differences were found between the low‐ and the high‐latitude properties of the plasma from transient events called coronal mass ejections (CMEs). At high latitudes, the speeds of the CME plasma clouds were close to the speed of the ambient fast wind, shocks were observed both ahead of and behind the CMEs, and enhancements of the helium abundance and of high charge states of heavy ions were not observed, as they often are at low latitudes. The review closes with a summary of arguments in support of the view that the fast solar wind observed by Ulysses at intermediate latitudes has its origin at higher latitudes in the polar coronal holes.
Atmospheric formic and acetic acids: An overview
Authors: Khare Puja , Kumar N. , Kumari K. M. , Srivastava S. S.
Published: September 2002

Interest in the role of organic acids as chemical constituents in troposphere has been growing rapidly over the past couple of decades. In addition to their presence in the atmosphere in a variety of phases, organic acids are important constituents of the global troposphere and contribute a large fraction (∼25%) to the nonmethane hydrocarbon atmospheric mixture. They contribute significantly to the acidity of precipitation and cloud water, especially in remote regions. In this review, we consider the information presently available on concentration distribution of formic and acetic acids in multiple phases and their sources in different geographical locations, i.e., midlatitude continental, tropical continental and marine sites. Photochemical reactions (i.e., ozone‐olefin reaction, isoprene oxidation, gas phase reaction of formaldehyde with HO2, and aqueous phase oxidation of formaldehyde) are important sources of these acids. In midlatitude continental regions, possible sources of formic and acetic acids, in addition to photochemical reactive vehicular emission, are direct emission from vegetation and biomass burning. In tropical continental sites, direct emission from vehicles, ants, soil, vegetation, and biomass burning are the important source of these species. The probable sources at marine locations are photochemical reactions, biogenic emissions, and long‐range transport from continental sites.
Fourier's heat conduction equation: History, influence, and connections
Authors: Narasimhan T. N.
Published: September 2002

The equation describing the conduction of heat in solids has, over the past two centuries, proved to be a powerful tool for analyzing the dynamic motion of heat as well as for solving an enormous array of diffusion‐type problems in physical sciences, biological sciences, earth sciences, and social sciences. This equation was formulated at the beginning of the nineteenth century by one of the most gifted scholars of modern science, Joseph Fourier of France. A study of the historical context in which Fourier made his remarkable contribution and the subsequent impact his work has had on the development of modern science is as fascinating as it is educational. This paper is an attempt to present a picture of how certain ideas initially led to Fourier's development of the heat equation and how, subsequently, Fourier's work directly influenced and inspired others to use the heat diffusion model to describe other dynamic physical systems. Conversely, others concerned with the study of random processes found that the equations governing such random processes reduced, in the limit, to Fourier's equation of heat diffusion. In the process of developing the flow of ideas, the paper also presents, to the extent possible, an account of the history and personalities involved.
Dynamics and anisotropy of traveling ionospheric disturbances as deduced from transionospheric sounding data
Authors: Afraimovich E. L. , Boitman O. N. , Zhovty E. I. , Kalikhman A. D. , Pirog T. G.
Published: September 2002

This paper presents the diurnal and seasonal statistics for the dynamics and anisotropy of medium‐scale traveling ionospheric disturbances (MS TIDs) in comparison with the dynamics of small‐scale irregularities (SSIs), using data from an annual run of ETS 2 radio signal polarization, angle‐of‐arrival, and scintillation measurements at 136 MHz in Irkutsk, Russia (52°N, 104°E). In daytime, phase interference patterns have the form of traveling waves with the mean propagation direction ψ equal to 160° and rms of 68°, and phase velocity of 133 ± 62 m/s. The mean value of ψ increases in a clockwise direction from southeastward to southward and opposite to the calculated direction of the neutral wind, with the spread not exceeding 10°–30°. This result is in agreement with the hypothesis of neutral wind‐induced filtering of atmospheric gravity waves (AGW) and MS TIDs caused by them. It was found that at night, MS TIDs are characterized by a strong anisotropy; the direction of elongation (modulo 180°) is, on average, 25° less than for SSIs. This difference can be explained by assuming that medium‐scale phase disturbances detected at night are caused by magnetic‐field‐aligned irregularities located in the plasmasphere.
Analysis of the interference due to differential rain attenuation induced by an adjacent path on a double‐site diversity system using frequency reuse
Authors: Kanellopoulos J. D. , Livieratos S. N. , Vazouras C. N.
Published: September 2002

One of the main propagation effects on interference between adjacent Earth‐space paths is considered to be the differential rain attenuation. In the present paper a method to predict differential rain attenuation statistics, valid for single‐site communication systems, is extended to include double‐site interfered systems operating under the dual‐polarization mode. The extended procedure is again based on a model of convective rain cells as well on the lognormal model for point rainfall rate statistics. Numerical results are presented referring to realistic diversity systems operating under the hypothesis of using both single and dual polarization and suffering from differential rain attenuation. Some very useful conclusions are deduced on the reliable and economic design of future communication systems where orbital and frequency congestion are quite expectable.
Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz
Authors: Ishimaru Akira , Kuga Yasuo , Liu Jun , Kim Yunjin , Freeman Tony
Published: September 2002

Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass. However, it is noted that conventional SAR using C‐band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P‐band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ−4. In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant. This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere. First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D/2 where D is the antenna aperture size. Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity. Third, the dispersion also creates broadening of the pulse. In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening. Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P‐band. Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values.
Profiles of radio refractive index and humidity derived from radar wind profilers and the Global Positioning System
Authors: Gossard Earl E. , Gutman Seth , Stankov B. Boba , Wolfe Daniel E.
Published: September 2002

It has often been pointed out that the Bragg backscatter of radar waves from elevated turbulent layers is very highly correlated with the height gradient of radio refractive index (RI) through these layers. However, many users need the profiles of RI, or the associated humidity, rather than profiles of their gradients. Simple integration of the gradients is usually not feasible because of ground or sea clutter and because biological scatterers such as insects and birds often severely contaminate the lower range gates. We show that if the total height‐integrated RI is independently available (say, from the Global Positioning System (GPS)), and if the surface value of RI is known, the profiles of RI are retrievable with good accuracy. For those profiler systems equipped with a radio acoustic sounding system to measure temperature, the humidity is also retrievable. The method is demonstrated with data collected in southern California, where 7 hours of profiler data were recorded at 449 MHZ along with GPS data. Three radiosonde balloons were launched during the period, and the profiles of RI from the balloon and the profiler are compared. The advantages of the system are its invulnerability to nonprecipitating clouds (at frequencies of 449 MHZ or lower) and that it uses only facilities that will soon be deployed globally. Simulations are used to assess errors from various factors such as loss of sign of the gradient of the potential RI (important especially during some frontal events) and the presence of biological contaminants in some geographical areas (such as coastal zones and some agricultural areas at night).
Radar backscattering from artificial field‐aligned irregularities
Authors: Franz T. L. , Kelley M. C. , Gurevich A. V.
Published: September 2002

In June of 1992 a NASA sounding rocket was fired into the Arecibo heater beam to provide in situ observations of artificially induced ionospheric irregularities. In this paper we provide a radar scattering calculation based on in situ data and compare the same with previous remote sensing experiments and with theory. The calculated backscatter cross section is in good agreement with prior observations over the Arecibo heater at 50 MHz. More important, when we scale the observed in situ power spectrum appropriately and compare it with multiradar cross‐sectional results from the Platteville, Colorado experiments, we find a remarkably similar radar frequency dependence, albeit one shifted to smaller scales over the higher‐latitude site. Even though the rms fluctuation level is almost the same over the Arecibo and Platteville heaters, the shift in scales toward smaller structures over Platteville explains the much larger VHF radar cross section measured there. Comparison of our waveform and its power spectrum with similar predictions from a recent theory shows excellent agreement for k values up to about 5 times the breakpoint in the spectrum of the theoretical prediction. Taken together, these results give very strong evidence for the production of needle‐like solitary structures as the dominant final state when high‐power radio waves reflect from a magnetized plasma. The organization of these structures by as yet unexplained processes may explain the scales between 10 m and 10 km which occur in the heated volume. Finally, the dominant needle‐like field‐aligned density depletions seem to support a second source of smaller‐scale irregularities. This creates a second break in the power law slope from its one‐dimensional value of k−4.3 predicted by theory to one more nearly characterized by k−3. The multiradar results from Platteville show a similar break, and we speculate that a density and/or temperature‐gradient‐driven instability such as the drift wave is operating.
Radiation Q of dipole‐generated fields
Authors: Grimes Dale M. , Grimes Craig A.
Published: September 2002

The radiation Q of several dipole fields in free space are determined using the time‐dependent Poynting theorem. Earlier works on this subject, recently summarized by McLean [1996], are based upon the complex Poynting theorem. It was previously shown [Grimes and Grimes, 1997] that the full complex Poynting theorem is correct only for single‐mode radiation fields. The time‐dependent theorem shows that three numbers are necessary to completely specify time‐varying power, and complex numbers supply but two; the third piece of information, a phase, is discarded when complex notation is formed. Omissions inherent in the complex Poynting theorem affect the calculated value of standing energy about an antenna and hence the calculated value of Q. To avoid such omissions, we develop a method of determining Q based upon the time‐dependent Poynting theorem that builds upon and extends our earlier work [Grimes and Grimes, 1997]. The purposes of this paper are to (1) provide a time domain basis for calculating Q in mixed modal radiation fields, (2) determine the Q of electric and magnetic dipoles, alone and in combination, and (3) demonstrate how source structure and relative phasing affect the physics of several combinations of electric and magnetic dipole radiation fields. The primary conclusion of this work is that the minimum possible Q of a radiation source established by Chu [1948] does not extend to properly mixed and phased multimodal radiation fields. A radiation source is presented for which, by our analysis, the radiation Q is zero.
A flow‐through Hankel transform technique for rapid, accurate Green's function computation
Authors: Raiche Art
Published: September 2002

Layered Earth Green's functions play a central role in modeling the electromagnetic (EM) response of mineral exploration targets, both as the implicit basis of distributed source field computations and for integral equation and hybrid three‐dimensional (3‐D) formulations. They are computed as the Hankel transforms of analytic kernels. Computational precision can depend upon the choice of algebraically equivalent forms by which these kernels are expressed. Since 3‐D modeling can require up to millions of Green's function evaluations, computational time can be significantly reduced by replacing direct point by point evaluation with 1‐D cubic spline representations. Linear digital filters have proven to be a fast and accurate method of computing these Hankel transforms at the spline knots. For a given knot density, accuracy can be maximized by choosing a filter whose evaluation points correspond with the spline knots, which also allows the reuse of kernel evaluations. A new flow‐through Hankel transform technique can further reduce computation times through the optimum use of previously evaluated kernels.
FORTE observations of lightning radio‐frequency signatures: Capabilities and basic results
Authors: Jacobson Abram R. , Knox Stephen O. , Franz Robert , Enemark Donald C.
Published: September 2002

The FORTE satellite, launched on August 29, 1997, carries both radio‐frequency‐receiver and optical (imaging and photometric) payloads for the study of lightning. The radio‐frequency (RF) data for the first 7 months of operation are described, both to illustrate the satellite's capabilities and to explain the basic statistical findings so far. FORTE's multichannel RF trigger system represents a significant advance in space‐based monitoring of lightning emissions. We are able to observe even rather weak and diffuse RF emissions from lightning and are no longer limited to the brightest known events, “transionospheric pulse pairs,” or TIPPs. We do see TIPPs, and we show that the FORTE observations of TIPPs are consistent only with the second pulse's being due to a ground reflection. We find that TIPPs are basically bimodal in character, one type having a steep roll‐off of power from 38 to 130 MHz and the other being essentially flat‐spectrum in that range. The steep‐spectrum TIPPs cluster together in the manner of most RF emissions from lightning, while the flat‐spectrum events tend to maintain a wider spacing (>0.1 s) between recurrent emissions.
Underplating process from melange formation to duplexing: Example from the Cretaceous Shimanto Belt, Kii Peninsula, southwest Japan
Authors: Hashimoto Y. , Kimura G.
Published: September 2002

Accretionary complexes are considered to form through off scraping and underplating, which results in the lateral and vertical growth of an accretionary prism. Accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process, although the origin of melange is considered controversial. In order to better understand the origin of melange and its role in accretion, in particular during underplating, we have studied the relationship between the formation of melange and duplexing in the Miyama Assemblage. Here the melange is composed dominantly of shale, basalt, chert, and sandstone. This study stresses the following features: (1) the structure of the Miyama Assemblage appears to be a composite suite of landward and seaward dipping duplexes; (2) the melange fabric displays a systematic asymmetry, which may have resulted through shearing along the decollement; and (3) facing of horses within duplexes changes from north to south, coinciding with a change in the sense of shear inferred from the melange fabric, which suggests that it formed prior to duplexing.
Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian Shield
Authors: Cook Frederick A. , van der Velden Arie J. , Hall Kevin W. , Roberts Brian J.
Published: September 2002

Lithoprobe deep seismic reflection data from the northwestern Canadian Shield provide images of structures to the base of the lithosphere between the Archean Slave Province on the east and the Cordillera on the west. Mantle reflections dip eastward from the lower crust to about 100 km depth beneath the ∼1.88–1.84 Ga Great Bear magmatic arc and almost certainly represent a subduction surface associated with arc development. Where the mantle reflections flatten into the lower crust, they merge with prominent west dipping crustal reflections thus delineating a lithospheric‐scale wedge that formed as a result of Proterozoic plate convergence between the Slave craton on the east and the ∼1.84 Ga Fort Simpson terrane on the west. The crust throughout the 725 km survey is highly reflective, and the Moho remains at a constant level between about 11.0 and 12.0 s beneath both Archean craton and Proterozoic accreted rocks; the most significant Moho deflection occurs beneath the ∼1.8–0.7 Ga Fort Simpson basin where it appears to rise to about 9.0 s (about 27 km). Within the crust of the Slave Province, east dipping, low‐angle reflections near the surface beneath the Yellowknife basin may be shallow detachments, and lower crustal geometry is consistent with low‐angle imbrication during Late Archean (∼2.65 Ga) tectonism associated with development of the Yellowknife basin volcanic rocks. West of the Slave craton, accreted Proterozoic crust is characterized by gently folded upper crustal layers overlying apparent thrust duplexes above detachment surfaces that flatten near the Moho. These lower crustal rocks were likely inserted as a tectonic wedge above the Moho and beneath the Slave Province during the contractional phase (∼1.90–1.88 Ga Calderian orogeny) of the Wopmay orogen. On the western end of the profile, the Fort Simpson extensional basin has up to 20–25 km of Proterozoic (meta) sedimentary rocks and sills(?) that are younger than 1.84 Ga.
Chronology of deformation within the turbidite‐dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana
Authors: Foster David A. , Gray David R. , Bucher Martin
Published: September 2002

Ar‐Ar data from fabric‐forming white mica in slates, syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism, deformation, and exhumation in the Lachlan orogen, Australia. These data also help define the tectonic evolution of the Tasmanides during Paleozoic time. The Lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to Gondwana during the closing of a small marginal ocean or back arc basin. This tectonic setting was similar to the present western and southwestern Pacific region. Accretion of the Lachlan orogen to Gondwana occurred by closing of the basin system by subduction‐accretion processes and some translation. The process is typified in the western Lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement/melange zone and is recorded by Ar‐Ar mica ages ranging from ∼455 Ma in the western part to ∼390 Ma in the eastern part. In the central Lachlan orogen, deformation migrated southwestward from ∼440–430 to 405 Ma away from the high‐temperature Wagga‐Omeo metamorphic complex, where deformation/metamorphism occurred between >440 and 400 Ma. In the north, ∼400 Ma mica ages record deformation and inversion of structures in the Cobar basin. In the eastern Lachlan orogen, Ar‐Ar mica dates range from 450 to 340 Ma. Ages of 455–445 Ma are yielded by the Narooma accretionary complex, 405–390 Ma ages are found along the major thrust faults bounding high‐grade metamorphic complexes, and 360–340 Ma cooling ages are found in the inverted extensional basins (e.g., Hill End) and related structural zones. The Ar‐Ar results also document periods of reactivation on early‐formed structures during later deformation elsewhere in the orogen.
Lateral ramps as an aid to the unroofing of deep‐crustal rocks: Seismic evidence from the Grenville province
Authors: Hynes Andrew , Eaton David
Published: September 2002

We suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust. Our suggestion is based on a study of the distribution of high‐pressure metamorphosed continental rocks in the Mesoproterozoic Grenvillian orogen in eastern Quebec. These rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed. They are currently exposed in a 100 km × 60 km lobate embayment north of the Manicouagan Reservoir. Using a combination of surface structural data and seismic reflection data from Lithoprobe Line 55, we reconstruct the three‐dimensional form of the high‐pressure rocks. Our reconstruct‐ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile. They make extensive use of the fact that much of the seismic reflectivity on Line 55 comes from outside the vertical plane of the seismic line, because of the sinuous nature of the line and its obliquity to tectonic strike. The validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram. Our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high‐pressure rocks at the surface. The lateral ramp probably provided a favorable pathway for the extrusion of deep‐seated rocks from the deep crust, and their transport onto the foreland.
Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: Experimental evidence and hypothesis development
Authors: Wraith Jon M. , Or Dani
Published: September 2002

Reports on temperature (T) effects on time domain reflectometry (TDR) measurements of soil water content (θ) are contradictory and often exhibit conflicting trends. We imposed step T changes on sealed columns of four soils having variable θ, while monitoring bulk apparent dielectric constant (or permittivity ϵb) and bulk electrical conductivity (σb) using TDR. Measured ϵb increased substantially with increasing temperature for one silt loam soil, for all θ. For another silt loam soil and for an Oxisol, measured ϵb increased with increasing T at relatively low θ but decreased with increasingT at higher θ. For a sandy loam soil, measured ϵb decreased with increasing T for all θ. The experimental results led to the hypothesis that TDR‐measured ϵb is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric constant of bulk water with increased T; and (2) the increase in TDR‐measured ϵb with increased T due to release of bound water. TDRmeasured ϵb is thus dependent on solid surface area and wetness. Our results have implications for routine use of TDR in fine‐textured and organic soils and potentially for microwave remote sensing of soil water status.
Disturbances of temperature‐depth profiles due to surface climate change and subsurface water flow: 2. An effect of step increase in surface temperature caused by forest clearing in southwest western Australia
Authors: Taniguchi Makoto , Williamson David R. , Peck Adrian J.
Published: September 2002

A series of type curves is presented for estimating vertical groundwater fluxes under the condition of a step increase in surface temperature. The depths of minimum groundwater temperature in temperature‐depth profiles are used as an indicator of the magnitude of the downward groundwater fluxes. The type curve method has been applied to the subsurface thermal regime observed in experimental catchments in the Collie River Basin in southwest Western Australia to estimate the vertical groundwater fluxes under the condition of surface warming caused by land cover change from forest to agriculture. The groundwater fluxes obtained from the type curves and the depths of minimum groundwater temperature in temperature‐depth profiles agree well with the values obtained from the other studies.
Water flow and solute spreading in heterogeneous soils with spatially variable water content
Authors: Harter Thomas , Zhang Dongxiao
Published: September 2002

Although the spatial variability of water content is expected to be significant in heterogeneous, unsaturated media, its effect on solute transport has been neglected in most stochastic analyses. In this work we develop, evaluate, and numerically verify an analytical model describing the statistical moments of water content, soil water flux, soil water velocity, and solute spreading in the unsaturated zone under conditions of variable soil water content. Results are presented for a wide range of soil conditions. It is shown that soil water content variability increases with soil water tension and decreases with soil textural tortuosity parameter m. As expected, the water content variability also increases with those of soil texture and saturated hydraulic conductivity. However, at or above field capacity (near saturation) the water content variability is generally negligible. The water content variability is very large under dry conditions, even if the soil is relatively homogeneous. In the same soil, longitudinal macrodispersivity is generally smaller when including than when neglecting the variability in water content. Variable water content enhances longitudinal macrodispersivity only in soils with large horizontal anisotropy or soils with m < 2. In this case the enhancement is more significant in drier soils and at the early stages of plume displacement (less than 10 λƒ). With travel distance the longitudinal macrodispersivity in the presence of variable water content converges to that in the absence of it.
Evaluation of the atmosphere as a source of volatile organic compounds in shallow groundwater
Authors: Baehr Arthur L. , Stackelberg Paul E. , Baker Ronald J.
Published: September 2002

The atmosphere as a source of volatile organic compounds (VOCs) in shallow groundwater was evaluated over an area in southern New Jersey. Chloroform, methyl tertbutyl ether (MTBE), 1,1,1‐trichloroethane, tetrachloroethylene (PCE), and carbon disulfide (not a VOC) were detected frequently at low‐level concentrations in a network of 78 shallow wells in the surficial Kirkwood‐Cohansey aquifer system. The atmosphere was sampled for these compounds and only MTBE concentrations were high enough to potentially explain frequent detection in shallow groundwater. A mathematical model of reactive transport through the unsaturated zone is presented to explain how variations in unsaturated properties across the study area could explain differences in MTBE concentrations in shallow groundwater given the atmosphere as the source. Even when concentrations of VOCs in groundwater are low compared to regulatory concentration limits, it is critical to know the source. If the VOCs originate from a point source((), concentrations in groundwater could potentially increase over time to levels of concern as groundwater plumes evolve, whereas if the atmosphere is the source, then groundwater concentrations would be expected to remain at low‐level concentrations not exceeding those in equilibrium with atmospheric concentrations. This is the first analysis of VOC occurrence in shallow groundwater involving colocated atmosphere data.