bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of sulfur flow emplacement on Io from Galileo data and numerical modeling
Authors: Williams David A. , Greeley Ronald , Lopes Rosaly M. C. , Davies Ashley G.
Published: February 2004

Galileo images of bright lava flows surrounding Emakong Patera have been analyzed and numerical modeling has been performed to assess whether these flows could have resulted from the emplacement of sulfur lavas on Io. Images from the solid‐state imaging (SSI) camera show that these bright, white to yellow Emakong flows are up to 370 km long and contain dark, sinuous features that are interpreted to be lava conduits, ∼300–500 m wide and >100 km long. Near‐Infrared Mapping Spectrometer (NIMS) thermal emission data yield a color temperature estimate of 344 K ± 60 K (≤131°C) within the Emakong caldera. We suggest that these bright flows likely resulted from either sulfur lavas or silicate lavas that have undergone extensive cooling, pyroclastic mantling, and/or alteration with bright sulfurous materials. The Emakong bright flows have estimated volumes of ∼250–350 km3, similar to some of the smaller Columbia River Basalt flows. If the Emakong flows did result from effusive sulfur eruptions, then they are orders of magnitude greater in volume than any terrestrial sulfur flows. Our numerical modeling results show that sulfur lavas on Io could have been emplaced as turbulent flows, which were capable of traveling tens to hundreds of kilometers, consistent with the predictions of Sagan [1979] and Fink et al. [1983]. Our modeled flow distances are also consistent with the measured lengths of the Emakong channels and bright flows. Modeled thermal erosion rates are ∼1–4 m d−1 for flows erupted at ∼140–180°C, which are consistent with the melting rates of Kieffer et al. [2000]. The Emakong channels could be thermal erosional in nature; however, the morphologic signatures of thermal erosion channels cannot be discerned from available images. There are planned Galileo flybys of Io in 2001 which provide excellent opportunities to obtain high‐resolution morphologic and color data of Emakong Patera. Such observations could, along with further modeling, provide additional information to better constrain whether sulfur lavas produced the Emakong flows.
Mountains on Io: High‐resolution Galileo observations, initial interpretations, and formation models
Authors: Turtle Elizabeth P. , Jaeger Windy L. , Keszthelyi Laszlo P. , McEwen Alfred S. , Milazzo Moses , Moore Jeff , Phillips Cynthia B. , Radebaugh Jani , Simonelli Damon , Chuang Frank , Schuster Peter
Published: February 2004

During three close flybys in late 1999 and early 2000 the Galileo spacecraft acquired new observations of the mountains that tower above Io's surface. These images have revealed surprising variety in the mountains' morphologies. They range from jagged peaks several kilometers high to lower, rounded structures. Some are very smooth, others are covered by numerous parallel ridges. Many mountains have margins that are collapsing outward in large landslides or series of slump blocks, but a few have steep, scalloped scarps. From these observations we can gain insight into the structure and material properties of Io's crust as well as into the erosional processes acting on Io. We have also investigated formation mechanisms proposed for these structures using finite‐element analysis. Mountain formation might be initiated by global compression due to the high rate of global subsidence associated with Io's high resurfacing rate; however, our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains. The large fraction (∼40%) of mountains that are associated with paterae suggests that in some cases these features are tectonically related. Therefore we have also simulated the stresses induced in Io's crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses. If this mechanism is responsible for some of Io's mountains, it could also explain the common association of mountains with paterae.
Utopia Basin, Mars: Characterization of topography and morphology and assessment of the origin and evolution of basin internal structure
Authors: Thomson Bradley J. , Head James W.
Published: February 2004

Recently obtained Mars Orbiter Laser Altimeter (MOLA) topography has permitted a new assessment of the morphology, structure, and history of Utopia Planitia. The new topographic data convincingly demonstrate that the Utopia region is an impact basin, as originally proposed byMcGill[1989], whose major topographic expression is a circular, 1–3 km deep depression ∼3200 km in diameter. Utopia Basin is the largest easily recognizable impact structure in the northern hemisphere of Mars and is the only portion of the northern lowlands that exhibits a distinct large‐scale impact signature; its presence there and its ancient age verify that a significant part of the northern lowlands dates back to the Noachian period. Using slope maps derived from gridded topography, we have mapped a series of linear slope anomalies within the basin and have analyzed their origin. These features can be classified into two groups: one oriented dominantly radial to the basin and the other oriented dominantly circumferential. The circumferential features are tens of kilometers wide and hundreds of kilometers long and appear to remain relatively constant with respect to elevation. We explore end‐member hypotheses (1) that the features formed by the direct action of water/ice or (2) that they are tectonic wrinkle ridges. We interpret the majority of these features to be due to tectonic deformation (primarily wrinkle‐ridge formation), modified by later sedimentation associated with the emplacement of the outflow channels and the formation of the Vastitas Borealis Formation. On the basis of the characteristics of these features and their association with polygonal terrain, thumbprint terrain, smoothness of units, and evidence of sedimentary deposits, we find that our results support earlier hypotheses proposing that a significant volume of water occupied Utopia Basin during the late Hesperian period of outflow channel formation. Utopia Basin differs significantly in depth and degradation state from the deeper and fresher‐appearing Hellas Basin, although the two are both Noachian in age. A significant part of this difference seems reasonably interpreted to be the result of the extensive sedimentary and volcanic infilling history of the northern lowlands.
On the possibility of liquid water on present‐day Mars
Authors: Haberle Robert M. , McKay Christopher P. , Schaeffer James , Cabrol Nathalie A. , Grin Edmon A. , Zent Aaron P. , Quinn Richard
Published: February 2004

Using a validated general circulation model, we determine where and for how long the surface pressure and surface temperature on Mars meet the minimum requirements for the existence of liquid water in the present climate system: pressures and temperatures above the triple point of water but below the boiling point. We find that for pure liquid water, there are five “favorable” regions where these requirements are satisfied: between 0° and 30°N in the plains of Amazonis, Arabia, and Elysium; and in the Southern Hemisphere impact basins of Hellas and Argyre. The combined area of these regions represents 29% of the planet's surface area. In the Amazonis region these requirements are satisfied for a total integrated time of 37 sols each Martian year. In the Hellas basin the number of degree days above zero is 70, which is well above those experienced in the dry valley lake region of Antarctica. These regions are remarkably well correlated with the location of Amazonian paleolakes mapped by Cabrol and Grin [2000] but are poorly correlated with the seepage gullies found by Malin and Edgett [2000]. In both instances, obliquity variations may play a role. For brine solutions the favorable regions expand and could potentially include most of the planet for highly concentrated solutions. Whether liquid water ever forms in these regions depends on the availability of ice and heat and on the evaporation rate. The latter is poorly understood for low‐pressure CO2 environments but is likely to be so high that melting occurs rarely, if at all. However, even rare events of liquid water formation would be significant since they would dominate the chemistry of the soil and would have biological implications as well. It is therefore worth reassessing the potential for liquid water formation on present day Mars, particularly in light of recent Mars Global Surveyor observations.
Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor‐fractured craters
Authors: Dombard Andrew J. , Gillis Jeffery J.
Published: February 2004

Historically, two modes of origin for lunar floor‐fractured craters have been proposed: subcrater laccolith emplacement and topographic relaxation. To test the viability of the topographic relaxation model, we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase, an analog to lunar crustal materials. Trials were run under two situations: (1) under extreme conditions (e.g., high heat flow) to determine the maximum degree of relaxation that the model can accommodate, and (2) under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions. We find that in the extreme case, topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor‐fractured craters (>100 km in diameter). Results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor‐fractured crater population. These results indicate that topographic relaxation is not the primary mechanism for the formation of floor‐fractured craters. Because the topographic relaxation model does not apply for the vast majority of floor‐fractured craters (i.e., those with diameters less than ∼60 km), we conclude that the laccolith intrusion model is a more viable alternative.
Morphometric measurements of martian valley networks from Mars Orbiter Laser Altimeter (MOLA) data
Authors: Williams Rebecca M. E. , Phillips Roger J.
Published: February 2004

Morphometric measurements of martian valley networks using Mars Orbiter Laser Altimeter (MOLA) topographic data yield mean valley width values of1548 m and depth values of. Our analysis of changes in valley shape with latitude and wall slope with depth indicate infilling and mass wasting processes have not greatly altered the original valley shape; thus valley network morphometry is dominantly due to channel formation mechanisms. Both U‐ and V‐shaped profiles are observed, commonly within the same valley network system. The average U‐shaped valley is slightly deeper (18 m) and significantly wider (1313 m) than its V‐shaped counterpart. Valley networks have decreasing width‐to‐depth ratios and increasing wall slopes as valley depth increases. Our observations are consistent with a two‐phase valley network formation model. (1) Valleys initially form via surface runoff, a process that creates V‐shaped profiles and quasi‐dendritic form. The observed linear correlation of top width and depth (below 125 m depth) suggests the depth of vertical incisement governs top width. (2) Reactivation of the same valleys by headward extending sapping processes widens the channel to form U‐shaped cross sections in downstream reaches. The availability of liquid water within a few hundred meters of the surface appears to be a necessary requirement for valley network formation. Mean valley depth decreases by ∼50 m from equatorial to higher latitudes (∼50°), contrary to the relationship predicted if the sapping depth was governed by the ice‐water boundary. Deeper equatorial valley networks may result from latitudinal variations in the availability of water or formation efficiency.
Thermal evolution of a thicker KREEP liquid layer
Authors: Hess Paul C. , Parmentier E. M.
Published: February 2004

We consider the thermal evolution of the thickened potassium, rare earth element, and phosphorous (KREEP) liquid layer that extends over a significant fraction of the lunar nearside with a prescribed thickness and heat production and beneath an insulating anorthosite crust. This layer represents a subcrustal heat source beneath the Procellarum‐Imbrium terrane and may be responsible for the localized and anomalous enrichments of highly incompatible elements in this terrane. In some models this thickened liquid layer is the thermal source for the remelting of mare basalts and also produces the parent magmas of the magnesian suite. Indeed, a KREEP liquid layer thickened from 5 to 10 km with more than 200 times chondritic heat‐producing elements does not continue to cool but undergoes a reheating and grows in thickness by dissolving several times its mass of anorthosite and ultramafic cumulates. However, such a liquid layer cannot give rise to the parent magmas to the magnesian suite, because it cannot account for the extraordinarily high incompatible element content, the primitive major element content, or the positive εnd of the magnesian suite. This growing layer will also form an impenetrable barrier to the eruption of mare basalts and is generally inconsistent with a number of geophysical constraints of the Moon, specifically, the existence of mascons subsequent to the basin filling by mare basalts. A thickened KREEP liquid layer can, however, explain the asymmetry in the distribution of KREEP‐rich rock on the lunar crust. Such a KREEP layer must be efficiently cooled and be quickly solidified to avoid some of the difficulties described above. A thinned anorthosite crust would allow efficient heat loss and induce crystallization of such a layer early in lunar history.
Integration of lunar polar remote‐sensing data sets: Evidence for ice at the lunar south pole
Authors: Nozette Stewart , Spudis Paul D. , Robinson Mark S. , Bussey D. B. J. , Lichtenberg Chris , Bonner Robert
Published: February 2004

In order to investigate the feasibility of ice deposits at the lunar south pole, we have integrated all relevant lunar polar data sets. These include illumination data, Arecibo ground‐based monostatic radar data, newly processed Clementine bistatic radar data, and Lunar Prospector neutron spectrometer measurements. The possibility that the lunar poles harbor ice deposits has important implications not only as a natural resource for future human lunar activity but also as a record of inner solar system volatiles (e.g., comets and asteroids) over the past billion years or more. We find that the epithermal neutron flux anomalies, measured by Lunar Prospector, are coincident with permanently shadowed regions at the lunar south pole, particularly those associated with Shackleton crater. Furthermore, these areas also correlate with the β = 0 circular polarization ratio (CPR) enhancements revealed by new processing of Clementine bistatic radar echoes, which in turn are colocated with areas of anomalous high CPR observed by Arecibo Observatory on the lower, Sun‐shadowed wall of Shackleton crater. Estimates of the extent of high CPR from Arecibo Observatory and Clementine bistatic radar data independently suggest that ∼10 km2 of ice may be present on the inner Earth‐facing wall of Shackleton crater. None of the experiments that obtained the data presented here were ideally suited for definitively identifying ice in lunar polar regions. By assessing the relative merits of all available data, we find that it is plausible that ice does occur in cold traps at the lunar south pole and that future missions with instruments specifically designed to investigate these anomalies are worthy.
The roughness of natural terrain: A planetary and remote sensing perspective
Authors: Shepard Michael K. , Campbell Bruce A. , Bulmer Mark H. , Farr Tom G. , Gaddis Lisa R. , Plaut Jeffrey J.
Published: February 2004

We examine the various methods and parameters in common use for quantifying and reporting surface topographic “roughness.” It is shown that scale‐dependent roughness parameters are almost always required, though not widely used. We suggest a method of standardizing the parameters that are computed and reported so that topographic data gathered by different workers using different field techniques can be directly and easily intercompared. We illustrate the proposed method by analyzing topographic data from 60 different surfaces gathered by five different groups and examine the information for common features. We briefly discuss the implications of our analysis for studies of planetary surface roughness, lander safety, and radar remote sensing modeling and analysis.
UV shielding of NH3and O2by organic hazes in the Archean atmosphere
Authors: Pavlov Alexander A. , Brown Lisa L. , Kasting James F.
Published: February 2004

The late Archean atmosphere was probably rich in biologically generated CH4and may well have contained a hydrocarbon haze layer similar to that observed today on Saturn's moon, Titan. Here we present a detailed model of the photochemistry of haze formation in the early atmosphere, and we examine the effects of such a haze layer on climate and ultraviolet radiation. We show that the thickness of the haze layer was limited by a negative feedback loop: A haze optical depth of more than ∼0.5 in the visible would have produced a strong “antigreenhouse effect,” thereby cooling the surface and slowing the rate at which CH4was produced. Given this climatic constraint on its visible optical depth, the amount of UV shielding provided by the haze can be estimated from knowledge of the optical properties and size distribution of the haze particles. Contrary to previous studies [Sagan and Chyba, 1997], we find that when the finite size of the particles is taken into account, the amount of UV shielding provided by the haze is small. Thus NH3should have been rapidly photolyzed and should not have been sufficiently abundant to augment the atmospheric greenhouse effect. We also examine the question of whether photosynthetically generated O2could have accumulated beneath the haze layer. For the model parameters considered here, the answer is “no”: The upper limit on ground level O2concentrations is ∼10−6atm, and a more realistic estimate for pO2during the late Archean is 10−8atm. The stability of both O2and NH3is sensitive to the size distribution and optical properties of the haze particles, neither of which is well known. Further theoretical and laboratory work is needed to address these uncertainties.
Tracking nutrient and productivity variations over the Last Deglaciation in the Arctic Ocean
Authors: Schubert Carsten J. , Stein Ruediger , Calvert Stephen E.
Published: February 2004

We have reconstructed the surface water environment of the Arctic Ocean over the last ∼ 50,000 years using measurements of the organic nitrogen and carbon isotope ratios, carbonate and total organic carbon concentrations (TOC), and terrestrial biomarkers (lignin and long‐chain n‐alkanes) in four multicores. Variations in nitrogen isotope ratios that are concordant with TOC and carbonate concentrations (representing foraminifera and excluding ice‐rafted‐debris) reflect differences in relative nutrient utilization of phytoplankton in the surface waters. However, δ15N variations also appear to be dependent on the stratification of the water column and therefore potentially track the exchange of nutrients between deep and surface waters. Low Last Glacial Maximum (LGM) δ15N values and higher Holocene values are opposite to those recorded in the Southern Ocean. The Arctic Ocean with higher nutrient utilization today compared to the LGM therefore acts as a counterpart to the Southern Ocean, although the global impact on carbon dioxide variations compared to the Southern Ocean is probably low.
Stochastic resonance in the North Atlantic
Authors: Alley R. B. , Anandakrishnan S. , Jung P.
Published: February 2004

Large, abrupt changes between warm and cold modes of North Atlantic climate exhibit spectral power at ∼1500 years, yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic. We hypothesize that a weak periodic forcing has combined with “noise” from ice sheet‐related events to cause the observed mode switches. This stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models. The ice isotopic data from central Greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested. We thus support arguments for the existence of a periodicity of ∼1500 years in the North Atlantic climate system and for the importance of ice sheet events in forcing North Atlantic changes.
Sedimentation rates in the Makarov Basin, central Arctic Ocean: A paleomagnetic and rock magnetic approach
Authors: Nowaczyk Norbert R. , Frederichs Thomas W. , Kassens Heidi , Nørgaard‐Pedersen Nils , Spielhagen Robert F. , Stein Rüdiger , Weiel Dominik
Published: February 2004

Three long sediment cores from the Makarov Basin have been subjected to detailed paleomagnetic and rock magnetic analyses. Investigated sediments are dominated by normal polarity including short reversal excursions, indicating that most of the sediments are of Brunhes age. In general, the recovered sediments show only low to moderate variability in concentration and grain size of the remanence‐carrying minerals. Estimations of relative paleointensity variations yielded a well‐documented succession of pronounced lows and highs that could be correlated to published reference curves. However, together with five accelerator mass spectrometry 14C ages and an incomplete 10Be record, still two different interpretations of the paleomagnetic data are possible, with long‐term sedimentation rates of either 1.3 or 4 cm kyr−1. However, both models implicate highly variable sedimentation rates of up to 10 cm kyr−1, and abrupt changes in rock magnetic parameters might even indicate several hiatuses.
Monitoring Termination II by environmental magnetic records from subtropical South Atlantic sediments
Authors: Schmidt Andrea M.
Published: February 2004

Termination II, the transition from oxygen isotope substages 6.2 to 5.5 at around 130 ka, is the penultimate drastic change from extreme glacial to interglacial climatic conditions, invoking shifts in terrigenous and biogenous influx to the deep ocean. Environmental magnetic methods are applied to reveal these shifts in an ocean‐spanning transect of the subtropical South Atlantic. Magnetic mineralogy, grain size, and coercivity reflect intensified eolian inflow of volcanogenic material from the Andes due to enhanced continental aridity and wind intensity during substage 6.2. An African dust supply is only observable in continental vicinity. On the Santos Plateau, magnetic attributes indicate fluvial influx of eroded flood basalts. Layers of reductive magnetic mineral diagenesis were identified only at the westernmost sites and appear to reflect paleoredox boundaries related to enhanced Corg deposition at the termination and during stage 5.5. In all unaltered sections, stage 5.5 is magnetically similar to the Holocene.
Sediment focusing in the central equatorial Pacific Ocean
Authors: Marcantonio Franco , Anderson Robert F. , Higgins Sean , Stute Martin , Schlosser Peter , Kubik Peter
Published: February 2004

At four sites in the central equatorial Pacific Ocean the flux of extraterrestrial ³He, determined using the excess 230Th profiling method, is 8 × 10−13 cm³ STP cm−2 ka−1. This supply rate is constant to within 30%. At these same sites, however, the burial rate of ³He, determined using chronostratigraphic accumulation rates, varies by more than a factor of 3. The lowest burial rates, which occur north of the equator at 1°N, 139°W are lower than the global average rate of supply of extraterrestrial ³He by 20% and indicate that sediment winnowing may have occurred. The highest burial rates, which are recorded at the equator and at 2°S, are higher than the rate of supply of extraterrestrial ³He by 100%, and these provide evidence for sediment focusing. By analyzing several proxies measured in core PC72 sediments spanning the past 450 kyr we demonstrate that periods of maximum burial rates of 230Th, ³He, 10Be, Ti, and barite, with a maximum peak‐to‐trough amplitude of a factor of 6, take place systematically during glacial time. However, the ratio of any one proxy to another is constant to within 30% over the entire length of the records. Given that each proxy represents a different source (234U decay in seawater, interplanetary dust, upper atmosphere, continental dust, or upper ocean), our preferred interpretation for the covariation is that the climate‐related changes in burial rates are driven by changes in sediment focusing.
Empirical relationship between pH and the boron isotopic composition of Globigerinoides sacculifer: Implications for the boron isotope paleo‐pH proxy
Authors: Sanyal Abhijit , Bijma Jelle , Spero Howie , Lea David W.
Published: February 2004

Culture experiments were carried out with the planktonic foraminifera Globigerinoides sacculifer at three different pH values (7.6±0.05, 8.20±0.05, and 8.60±0.05) in order to establish the pH dependence of the boron isotopic fractionation between the calcitic tests of G. sacculifer and seawater. Additional objectives were to compare the pH dependences of boron isotopic composition of G. sacculifer with that of Orbulina universa and inorganic calcite in order to evaluate the robustness of the boron isotope paleo‐pH proxy. The relationship between the boron isotopic composition of G. sacculifer and the culture water pH shows heavier boron isotopic values at higher pH. This empirical relationship is similar but offset (by ∼ 3‰) from that of O. universa, indicating the presence of biogenic influence (“vital effect”) on the boron isotopic composition of foraminifera. However, comparison of the boron isotopic composition of O. universa and G. sacculifer with that of inorganic calcite grown over a common pH range shows that inorganic processes are the dominant control on the boron isotopic composition of foraminifera and the “vital effects” play a small but not insignificant role. The results imply that in spite of a small biogenic influence the variation in boron isotopic composition of foraminiferal shells with pH (at least for O. universa and G. sacculifer) is consistent with that for inorganic calcite, demonstrating the potential of this isotopic signature in foraminifera as a reliable paleo pH proxy.
The mathematics of early diagenesis: From worms to waves
Authors: Boudreau Bernard P.
Published: February 2004

The changes that sediments undergo after deposition are collectively known as diagenesis. Diagenesis is not widely recognized as a source for mathematical ideas; however, the myriad processes responsible for these changes lead to a wide variety of mathematical models. In fact, most of the classical models and methods of applied mathematics emerge naturally from quantification of diagenesis. For example, small‐scale sediment mixing by bottom‐dwelling animals can be described by the diffusion equation; the dissolution of biogenic opal in sediments leads to sets of coupled, nonlinear, ordinary differential equations; and modeling organisms that eat at depth in the sediment and defecate at the surface suggests the one‐dimensional wave equation, while the effect of waves on pore waters is governed by the two‐ or three‐dimensional wave equation. Diagenetic modeling, however, is not restricted to classical methods. Diagenetic problems of concern to modern mathematics exist in abundance; these include free‐boundary problems that predict the depth of biological mixing or the penetration of O2 into sediments, algebraic‐differential equations that result from the fast‐reversible reactions that regulate pH in pore waters, inverse calculations of input functions (histories), and the determination of the optimum choice in a hierarchy of possible diagenetic models. This review highlights and explores these topics with the hope of encouraging further modeling and analysis of diagenetic phenomena.
Oxygen‐isotopic evolution of the solar nebula
Authors: Wasson John T.
Published: February 2004

Studies of the three oxygen isotopes in chondrite meteorites demonstrate that diverse O reservoirs (characterized by their Δ17O values) were present in the solar nebula. The discovery that some chondritic materials have O‐isotopic compositions that cannot be explained by mass‐dependent fractionation of an initially well mixed reservoir has important implications for the history of the solar nebula. On a plot of δ17O versus δ18O (or 17O/16O versus 18O/16O), terrestrial samples (with the main exception of stratospheric ozone) fall along a single line (the terrestrial fractionation, or TF, line) having a slope ∼0.52, an indication that the parental reservoir was homogenized. A convenient measure of O‐isotopic heterogeneity is the deviation from the TF line, Δ17O = δ17O − 0.52 × δ18O. A popular model to explain the evolution of chondritic oxygen compositions during nebular and asteroidal aqueous alteration processes calls for the nebula to have formed from solids having δ18O = −40‰ and δ17O = −41‰ (Δ17O = −20‰) and a gas having a composition roughly estimated to be δ18O = 30‰ and δ17O = 24‰ (Δ17O = 9‰). This model encounters serious difficulties when examined in detail; in particular, it cannot readily account for the O‐isotopic compositions of both chondrules and refractory inclusions from individual chondrite groups, or for the differences between groups, particularly in the compositions of chondrules. Magnetite (Fe3O4) is a key phase for O‐isotope studies because during its formation by the oxidation of ferrous metal or FeS, all O comes from the oxidant, probably H2O. It appears that most (and, possibly, nearly all) magnetite formed during aqueous alteration processes that occurred in asteroids. In all chondrite groups studied to date, Δ17O of the magnetite is greater than or equal to that of the chondrule silicates. If the H2O originated in the ambient (local) nebula, then at the time of accretion the Δ17O of the nebular gas was generally higher than that of the solids. An alternative view is that the heterogeneity in the O‐isotopic composition of chondrites indicates that the nebula formed from diverse batches of presolar materials, the precise O‐isotopic composition of the mix varying during the accretion history of the nebula. The large compositional gaps between groups suggest that agglomeration of nebular dust to form chondrites did not proceed at a constant rate but that periods with turbulence levels low enough to allow agglomeration were punctuated by periods of high turbulence during which the composition of the inner nebula sampled by chondrites changed appreciably.
Current controversies in magnetospheric physics
Authors: Lui A. T. Y.
Published: February 2004

The scientific ascent of humankind comes in two forms: incremental progress with traditional thinking, and paradigm transition. The latter typically engenders intense debate, often long lasting, before the transition of the paradigm is deemed acceptable. The magnetospheric discipline is no exception, especially in view of the sparse observations available to reach conclusive theory‐data closure. Scientific controversies serve an important role in identifying significant unresolved issues for progress to be made. This review highlights key issues on four controversies present in magnetospheric physics: (1) the on‐and‐off debate on whether the magnetic field combined with the plasma bulk flow or the electric field combined with the current density is the primary quantity in treating magnetospheric problems; (2) the proper interpretation of transient dayside magnetospheric phenomena, i.e., whether they are related to flux transfer events, plasma transfer events, or solar wind pressure pulses; (3) the physical processes responsible for substorm onset; and (4) justifications for substorms or enhanced magnetospheric convection as the cause of magnetic storms. In issue 1 the merits and limitations of the two approaches are expounded. In issue 2 the predicted similarities and differences between the three interpretations are summarized. In issue 3 the strengths and weaknesses of prominent existing substorm models are elaborated on. In issue 4 the two contributors to storms are recognized and combined. Each controversy has an element of a paradigm transition. The resolutions of these controversies appear to also have one common element in that the presumption of only one theory to be correct may not be valid; a synthesis of existing theories may provide a better understanding of all features associated with the phenomenon.
Middle atmosphere dynamical studies at Resolute Bay over a full representative year: Mean winds, tides, and special oscillations
Authors: Hocking W. K.
Published: February 2004

Wind motions in the 80–98‐km altitude region above Resolute Bay, Nunavut, Canada (75°N, 95°W), are presented for a representative year. The measurements were made using an interferometric meteor radar during 1997, 1998, 1999, and 2000. Results presented include a comprehensive study of monthly mean winds, using all of the available data from these 4 years, as well as running tidal studies and power spectra over the time frame 1998–1999 and studies of special persistent oscillations which appear from time to time. All studies are accompanied with a detailed consideration of the effects of the spectral windows used. A superposed epoch analysis is used to investigate a dominant 15‐hour oscillation which appeared in November 1999, and evidence of period changes, associated with changes in resonance conditions, is demonstrated. Other periodic motions are also seen, especially at periods in the range 9–12 hours.
Thermal model calculations of enclosures for millimeter wavelength radio telescopes
Authors: Greve A. , MacLeod G.
Published: February 2004

Millimeter wavelength radio telescopes require a high surface and pointing precision, which in steerable open‐air telescopes is obtained by a stiff or homologous construction and elaborate thermal control. The perturbing thermal loads and wind loads can also be reduced by protecting the telescope by a radome or astrodome, as applied on several operating telescopes. However, for the new generation telescopes of 30‐m to 50‐m diameter the radome or astrodome is very large and costly, and it is a priori not certain to which extent the internal thermal climate must be controlled, for instance, by forced air‐conditioned ventilation, to obtain a telescope of good radio performance. We present data from the Onsala and Haystack radomes to illustrate agreement between measurements and thermal model calculations, and we use similar calculations to investigate the thermal behavior of a very large astrodome and radome. The accuracy of the calculations is sufficient for design and operational purposes.
Fast inhomogeneous plane wave algorithm for the analysis of electromagnetic scattering
Authors: Hu Bin , Chew Weng Cho , Velamparambil Sanjay
Published: February 2004

The fast inhomogeneous plane wave algorithm has been developed to accelerate the solution of three‐dimensional electromagnetic scattering problems in free space. By expanding the kernel of the Green's function using the Weyl identity and choosing a proper steepest descent path, the diagonalization of the translation matrix is achieved after the interpolation and extrapolation techniques are applied. The proposed algorithm is implemented on top of the scalable multipole engine, a portable implementation of the dynamic multilevel fast multipole algorithm for distributed‐memory computers. The computational time per matrix vector multiplication is reduced to O(NlogN) and the memory requirement is reduced to O(N), where N is the number of unknowns in the discretized integral equation. The algorithm is validated by applying it to the solution of the electromagnetic scattering from the perfect electric conducting scatterers. This approach can be easily extended to more general problems with complicated Green's function expressed in terms of the plane wave spectral integrals, such as the ones encountered in the multilayered medium studies.
Feed array synthesis for reflector antennas in contoured beam applications via an efficient and novel Gaussian beam technique
Authors: Chou Hsi‐Tseng , Pathak Prabhakar H. , Burkholder Robert J.
Published: February 2004

A relatively rapid procedure is developed, which combines a novel Gaussian beam approach for the fast analysis of reflector antenna radiation patterns with an iterative optimization method, to synthesize feed array distributions for parabolic reflectors in contoured beam applications. Here, relatively few Gaussian beams are employed as electromagnetic basis functions to represent the field radiated by the feed array in a highly efficient manner. Closed form expressions are used to find the reflection and diffraction of each of these Gaussian beams when they are incident on the parabolic reflector after they are launched from appropriate locations on the feed array plane. The coefficients (or initial launching amplitudes) of the Gaussian beams used in this expansion for the radiation from the feed array are found via an efficient iterative optimization procedure based on the method of successive projections. These coefficients are then directly related to the strengths of the array element source distributions (or array weights). Numerical examples of practical interest are presented to illustrate the utility and the speed of the present approach, which is found to be substantially faster than conventional approaches for these cases.
Characteristic‐based time domain method for antenna analysis
Authors: Jiao Dan , Jin Jian‐Ming , Shang J. S.
Published: February 2004

The characteristic‐based time domain method, developed in the computational fluid dynamics community for solving the Euler equations, is applied to the antenna radiation problem. Based on the principle of the characteristic‐based algorithm, a governing equation in the cylindrical coordinate system is formulated directly to facilitate the analysis of body‐of‐revolution antennas and also to achieve the exact Riemann problem. A finite difference scheme with second‐order accuracy in both time and space is constructed from the eigenvalue and eigenvector analysis of the derived governing equation. Rigorous boundary conditions for all the field components are formulated to improve the accuracy of the characteristic‐based finite difference scheme. Numerical results demonstrate the validity and accuracy of the proposed technique.
Characteristics of radio transmitter fingerprints
Authors: Ellis K. J. , Serinken N.
Published: February 2004

In this paper, the characteristics of radio transmitter fingerprints will be examined by analyzing both the amplitude and phase information obtained from complex envelope recordings of transmitter turn‐on transients. The interest in the analysis of such transients is related to the identification of malfunctioning or illegally operated radio transmitters in support of radio spectrum management practices. Of the 28 VHF radios considered in this study, many were found to produce fingerprints having uniquely distinctive features which could be used for identification purposes. Unfortunately, some of these radios were found to have fingerprints that were virtually indistinguishable from each other, making the identification process more difficult, if not impossible. Details of the equipment, analyses, and data collection procedures will be presented along with a discussion of the experimental results. The merits of this technique over others currently in use will also be presented.
A comparison of tropospheric VHF Doppler beam steering and full correlation analysis measurements of aspect sensitivity
Authors: Hobbs Bridget G. , Reid Iain M. , Holdsworth David A.
Published: February 2004

Tropospheric VHF radar data collected using both spaced antenna (SA) and Doppler beam steering (DBS) methods are presented. The SA data are analyzed using full correlation analysis (FCA) on vertical beam data, while DBS analysis is applied to vertical beam data and off‐zenith beam data at 3.6°, 7.2°, and 10.8°. A comparison of meridional velocities using SA FCA and DBS analysis is presented. These results indicate good agreement between the DBS and FCA velocities on all off‐zenith beams, particularly at 10.8°. The main aim of the study is to compare the aspect sensitivity parameter θs,using the spatial correlation method for the FCA data [e.g., Lesicar and Hocking, 1992] and the power method of Hocking et al. [1986] for the DBS data. This comparison reveals good general agreement between isotropic values of θs with both methods, and aspect sensitive values of θs values with both methods, although there are times when one method produces isotropic values and the other produces aspect sensitive values and visa versa. During aspect sensitive periods the FCA values agree best with the DBS method using the 0.0°–3.6° beam combination.
Antenna compression using binary phase coding
Authors: Woodman Ronald F. , Chau Jorge L.
Published: February 2004

We introduce the first antenna “compression” scheme for coherent radars. The idea is to transmit with a large array of phase‐coded antennas at full power and later synthesize by linear superposition and proper phasing, the equivalent of a small transmitting antenna, similar to a single antenna module of the array. The full “decoding” is done by software by adding the power and cross‐power estimates of the signals of each code, so no extra burden is added other than the summations. This approach allows the use of all the available power in phased array systems where either high‐power transmitters cannot be used with small antennas or the transmitted power is distributed among the antenna elements. The wider beams are particularly important in meteor systems and, recently, in some radar studies of the neutral atmosphere, especially when multiple receiving antennas are used. Our scheme is based on complementary binary phase coding of antenna elements and works in a similar fashion to complementary phase coding used in pulse compression. We have implemented this idea at the Jicamarca Radio Observatory using a two‐dimensional approach; thus four two‐dimensional complementary codes were derived. The performance of this implementation has been tested using imaging techniques on an interferometer of four receiving antenna modules to study the angular characteristics of the well‐known equatorial electrojet irregularities. In addition, in this mode we have made successful observations of the upper troposphere and lower stratosphere.
Reciprocity theorems for electromagnetic or acoustic one‐way wave fields in dissipative inhomogeneous media
Authors: Wapenaar C. P. A. , Dillen M. W. P. , Fokkema J. T.
Published: February 2004

Reciprocity theorems have proven their usefulness in the study of forward and inverse scattering problems. Most reciprocity theorems in the literature apply to the total wave field and are thus not compatible with one‐way wave theory, which is often applied in situations in which there is a clear preferred direction of propagation, like in electromagnetic or acoustic wave guides and in seismic exploration. In this paper we review the theory for one‐way wave fields (or bidirectional beams), and we extensively discuss the symmetry properties of the square root operator appearing in the one‐way wave equation. Using these symmetry properties, it appears to be possible to derive reciprocity theorems of the convolution type and of the correlation type for electromagnetic or acoustic one‐way wave fields in dissipative inhomogeneous media along the same lines as the usual derivation of the reciprocity theorems for the total wave field. The one‐way reciprocity theorem of the convolution type provides a basis for representations of scattered one‐way wave fields in terms of generalized Bremmer series expansions or generalized primaries. The one‐way reciprocity theorem of the correlation type finds its application in reflection imaging based on inverse one‐way wave field propagators.
High‐order, leapfrog methodology for the temporally dependent Maxwell's equations
Authors: Young J. L.
Published: February 2004

A high‐order, leapfrog integrator is presented and analyzed for Maxwell's time domain equations. The integrator is shown to be quite robust in terms of its efficiency and memory demands. Moreover, the integrator retains the original simplicity of the traditional second‐order, leapfrog integrator. Rigorous Fourier analyses are presented to quantify the dispersion, dissipation, and stability properties of the integrator. To limit the scope of the presentation, the 2×2, 2×4, 4×2, and 4×4 schemes are examined and compared. Here the first digit denotes temporal accuracy; the second digit denotes spatial accuracy. Numerical demonstrations, using the three‐dimensional rectangular waveguide as the object under test, are provided to further substantiate the theoretical analysis.
Performance of HF modems on high‐latitude paths using multiple frequencies
Authors: Jodalen Vivianne , Bergsvik Torgeir , Cannon Paul S. , Arthur Paul C.
Published: February 2004

The high‐latitude HF channel has been measured and characterized in terms of Doppler spread, delay spread, and signal‐to‐noise ratio. The performance of data modems has been determined over a comprehensive range of simulated channel conditions. A comparison is made between the channel measurements and the modem characterizations. Thereby the percentage of time that the modem works satisfactorily (availability), during the measured channel conditions, is determined. Two 75 bits/s modems (compliant with STANAG 4415 and STANAG 4285), a 2400 bits/s modem (compliant with STANAG 4285), Morse code, and voice have been selected for analysis. The two 75 bits/s modems show 60–75% higher availability than the 2400 bits/s modem on the measured paths. The 2400 bits/s modem is degraded in performance during a geomagnetic disturbance whereas the 75 bits/s modems maintain approximately the same performance level. The paper also addresses the necessary frequency pool (at a specific time) required for an HF circuit to achieve the best possible communications availability. For the 75 bits/s modems, independent of path length, a one‐frequency pool is all that is required during summer and winter when the ionospheric conditions are quiet. A four frequency pool is necessary during disturbed ionospheric conditions. For the 2400 bits/s waveform, three to four frequencies are needed on the 200 km west‐east path, and four to six frequencies are required on the north‐south 2000 km path. This applies to all seasons and geomagnetic conditions analyzed.
A high‐resolution study of quasiperiodic radio emissions observed by the Galileo plasma wave instrument
Authors: Menietti J. D. , Christopher I. , Granroth L. J.
Published: February 2004

We have conducted a study of quasiperiodic emission observed by the plasma wave instrument on board the Galileo spacecraft. These emissions appear as broadband bursts with dominant periods ranging from 10 min to over 40 min. For these emissions we have explicitly analyzed the high‐resolution (waveform) data to determine the presence of impulsive, solitary signatures. Our investigations have indicated that the broadband bursts, as well as the background more narrowband continuum emission, are composed of a highly turbulent spectrum. Within the broadband burst, however, there are higher‐frequency components present, but no impulsive electrostatic signatures. Also significantly, the broadband bursts show no low‐frequency dispersion. We conclude that the bursts are consistent with a distant, electromagnetic source, probably in the near‐Jupiter vicinity.