bluuebunny/crossref_metadata_2025 huggingface dataset:
Remote sensing of the albedo and detection of the slush line on the Greenland ice sheet
Authors: Greuell Wouter , Knap Wouter H.
Published: September 2002

Advanced very high resolution radiometer images of a section of the Greenland ice sheet (western part between 64.5° and 70.5°N with a size of 667 (N‐S) by 700 (W‐E) km) were processed in order to retrieve the surface albedo. The images were for the summer seasons of 1990–1997 inclusive, and results were compared to simultaneous mass balance measurements carried out along the transect (67°N). The main findings are as follows: First, many images show a distinct transition from low spatial variability of the albedo at higher elevations to higher spatial variability of the albedo at lower elevations. It is argued that these transitions most likely correspond to transitions at the surface (“slush line,” in short) from the area that is uniformly covered by snow to a mosaic of snow patches, slush, and ice. Second, 70% of the interannual variations in the mean specific mass balance can be explained by interannual variations in the maximum elevation of the slush line (i.e., the “slush limit”). However, a higher percentage (82%) can be explained by interannual variations in the sum of positive degree‐days. Third, in 3 years with a relatively large amount of melt the inland migration of the slush line stopped or slowed considerably down at a fixed distance from the ice sheet margin. We believe that this so‐called “maximum slush line” coincides with an abrupt change in the density profile. The maximum slush line forms an important limitation for the use of satellite data to monitor interannual variations in the mass balance. Its position is a climate indicator and was determined for the entire section of the Greenland ice sheet investigated.
A comparison of gravity wave energy observed by VHF radar and GPS/MET over central North America
Authors: Nastrom G. D. , Hansen A. R. , Tsuda T. , Nishida M. , Ware R.
Published: September 2002

Monthly mean values of the kinetic energy at frequencies associated with gravity waves based on wind observations made with the VHF radar at White Sands Missile Range, New Mexico, are compared with potential energies determined using GPS/MET soundings. The monthly mean curves of Ek and Ep are highly correlated, and both show minimum values in the summer season with summer to winter increases of about a factor of 2. The observed ratio Ek/Ep agrees very closely with the prediction of a linear gravity wave model. These observations support previous comparisons of Ek with Ep made using the middle and upper atmosphere radar in Japan.
Near‐infrared absorption cross sections and integrated absorption intensities of molecular oxygen (O2, O2‐O2, and O2‐N2)
Authors: Smith K. M. , Newnham D. A.
Published: September 2002

Near‐infrared absorption cross sections and integrated absorption intensities of gas phase oxygen and nitrogen mixtures have been determined from laboratory measurements using a coolable multipass gas cell and Fourier transform spectroscopy. Spectra were recorded at instrument resolutions of 0.5 and 0.05 cm−1, between 2200–12,500 cm−1 (between 4.5 and 0.8 μm). Monomer and binary cross sections of the a1 Δg ← χ3 Σg− (0,0) and (1,0) bands and underlying continuum absorptions of oxygen centered near 7,850 and 9,400 cm−1 (1.27 and 1.06 μm) were determined for mixtures of 21%, 50%, and 75% oxygen in nitrogen, from measurements at a total pressure of 100 kPa (1000 mbar) and temperatures of 200, 230, and 294 K. Where possible, comparisons are made between the results obtained from this work and those of previous studies.
Effect of water vapor feedback on internal and anthropogenic variations of the global hydrologic cycle
Authors: Hall Alex , Manabe Syukuro
Published: September 2002

Using two versions of the GFDL coupled ocean‐atmosphere model, one where water vapor anomalies are allowed to affect the longwave radiation calculation and one where they are not, we examine the role of water vapor feedback in internal precipitation variability and greenhouse‐gas‐forced intensification of the hydrologic cycle. Without external forcing, the experiment with water vapor feedback produces 44% more annual‐mean, global‐mean precipitation variability than the one without. We diagnose the reason for this difference: In both experiments, global‐mean surface temperature anomalies are associated with water vapor anomalies. However, when water vapor interacts with longwave radiation, the temperature anomalies are associated with larger anomalies in surface downward longwave radiation. This increases the temperature anomaly damping through latent heat flux, creating an evaporation anomaly. The evaporation anomaly, in turn, leads to an anomaly of nearly the same magnitude in precipitation. In the experiment without water vapor feedback, this mechanism is absent. While the interaction between longwave and water vapor has a large impact on the global hydrologic cycle internal variations, its effect decreases as spatial scales decrease, so water vapor feedback has only a very small impact on grid‐scale hydrologic variability. Water vapor feedback also affects the hydrologic cycle intensification when greenhouse gas concentrations increase. By the 5th century of global warming experiments where CO2 is increased and then fixed at its doubled value, the global‐mean precipitation increase is nearly an order of magnitude larger when water vapor feedback is present. The cause of this difference is similar to the cause of the difference in internal precipitation variability: When water vapor feedback is present, the increase in water vapor associated with a warmer climate enhances downward longwave radiation. To maintain surface heat balance, evaporation increases, leading to a similar increase in precipitation. This effect is absent in the experiment without water vapor feedback. The large impact of water vapor feedback on hydrologic cycle intensification does not weaken as spatial scales decrease, unlike the internal variability case. Accurate representations of water vapor feedback are therefore necessary to simulate global‐scale hydrologic variability and intensification of the hydrologic cycle in global warming.
Sampling strategies for the comparison of climate model calculated and satellite observed brightness temperatures
Authors: Engelen Richard J. , Fowler Laura D. , Gleckler Peter J. , Wehner Michael F.
Published: September 2002

Brightness temperatures derived from polar‐orbiting satellites are valuable for the evaluation of global climate models. However, the effect of orbital constraints must be taken into account to ensure valid comparisons. As part of the Atmospheric Model Intercomparison Project II climate model comparisons, this study seeks to evaluate the monthly mean simulated brightness temperature differences of possible model output sampling strategies with respect to the exact satellite sampling and whether they can be practically implemented to provide meaningful comparisons with these satellite observations. We compare various sampling strategies with a proxy satellite data set constructed from model output and actual TIROS operational vertical sounder orbital trajectories, rather than with the observations themselves. To a large extent, this enables isolation of the sampling error from errors caused by deficiencies in the modeled climate processes. Our results suggest that the traditional method of calculating brightness temperatures from monthly mean temperature and moisture profiles yields biases from both nonlinear effects and the removal of the diurnal cycle that may be unacceptable in many applications. However, we also find that a brightness temperature calculation every hour of the simulation provides substantially lower sampling biases provided that there are two or more properly aligned satellites. This is encouraging because it means that for many applications modelers need not accurately mimic actual satellite trajectories in the sampling of their simulations. If only one satellite is available for comparison with simulations, more sophisticated sampling seems necessary. For such circumstances, we introduce a simple procedure that serves as a useful approximation to the rather complex procedure required to sample a model exactly as a polar‐orbiting satellite does the Earth. With all sampling methods, removal of biases associated with cloud cover is problematic and deserves further study.
Introduction to special section: Southern Oxidants Study Nashville/Middle Tennessee Ozone Study, Part 2
Authors: Cowling E. B. , Chameides W. L. , Kiang C. S. , Fehsenfeld F. C. , Meagher J. F.
Published: September 2002

The 12 papers in this second Southern Oxidants Study (SOS) special section of the Journal of Geophysical Research‐Atmospheres constitute a significant addition to earlier scientific contributions from the SOS: Fehsenfeld et al. [1993], Chameides and Cowling [1995], first special section “;Southern Oxidants Study” in the Journal of Geophysical Research, 103 (D17), 22, 209‐22,669, 1998, and North Carolina State University [1999].

These 12 papers give special emphasis to advances in scientific understanding of chemical, meteorological, and biological factors that govern accumulation of ozone and other oxidants in the lower troposphere within the 11‐state study area centered over Nashville, Tennessee. The aircraft and ground‐based fieldwork portions of SOS' Nashville/Middle Tennessee Ozone Study were completed during the summers of 1994 and 1995. In‐depth analysis and interpretation of measurements made during those two summers has continued, and will likely continue for some years in the future. Readers of this second SOS special section needing background information on the Nashville/Middle Tennessee Ozone Study should consult the four overview papers published in the first SOS special section [Cowling et al., 1998; Meagher et al., 1998; McNider et al., 1998; Hübler et al., 1998].
Macroscale hydrological modeling using remotely sensed inputs: Application to the Ohio River basin
Authors: O'Donnell Greg M. , Czajkowski Kevin P. , Dubayah Ralph O. , Lettenmaier Dennis P.
Published: September 2002

Predictions of water and energy budgets at the land surface are central to climate simulation and numerical weather prediction, as well as to water resources planning and management. Macroscale hydrological models provide a new tool for simulating surface water and energy balances at the scale of large continental river basins. However, these models are limited by the scarcity of in situ meteorological forcing data. Remote sensing data provide an alternative to in situ data, with observations that are, in some cases, at a higher spatial and temporal resolution than those available from traditional surface sources. Nonetheless, there remain important questions as to whether the accuracy of remotely sensed surface variables is sufficient to serve as forcings for surface hydrological models. This question is addressed through comparison of hydrologic simulations for the Ohio River basin with the variable infiltration capacity (VIC) macroscale hydrology model, using in situ and remotely sensed data. In situ data consist of gridded (at ½ degree latitude‐longitude spatial resolution) precipitation, temperature, and wind, with downward solar and longwave radiation inferred from the diurnal temperature range. Remotely sensed observations include incident solar radiation, air temperature, and vapor pressure deficit inferred from the Geostationary Operational Environmental Satellite (GOES), the advanced very high resolution radiometer (AVHRR) and the TIROS Operational Vertical Sounder (TOVS), respectively. Precipitation, in all cases, is from gridded station data. The modeled streamflows and evapotranspiration rates are quite similar for the two cases. The largest differences in predicted surface hydrology are associated with differences in modeled snow cover accumulation and snowmelt, and result from a warm bias in the remotely sensed temperature data.
Investigating the interactions between an atmosphere and an ejecta curtain: 1. Wind tunnel tests
Authors: Barnouin‐Jha O. S. , Schultz P. H. , Lever J. H.
Published: September 2002

When an asteroid impacts a planetary surface, ejecta are excavated along ballistic trajectories whose loci define an inverted cone shape. The outward advancing motion of this curtain displaces atmosphere to generate a ring vortex whose winds can entrain, transport, and deposit ejecta and fine‐grained surface materials. Curtain width and velocity, particle concentration, size distribution and motion parallel to the curtain, and the density, viscosity, and compressibility of the surrounding atmosphere all influence the vortex circulation strength. As analogs to an advancing ejecta curtain, we tested the effect of inclined solid and porous plates on vortex formation in a low‐speed wind tunnel. We found that hydraulic resistance, a measure of energy losses for 1‐D porous flow, governs the position along a porous plate where it becomes effectively permeable and flow separation occurs. The resulting flow field is similar to that over an inclined solid plate of the same effective length. Energy losses through the top, permeable portions of the plate reduce circulation strength by only 7% relative to flow over a solid plate. The two parameters needed to estimate circulation strength, curtain velocity and impermeable height, can thus be determined by coupling an impact model with published hydraulic resistance data. These tests also served to calibrate a numerical model, which we then applied to investigate the influence of atmospheric compressibility and particle motion parallel to the curtain (see Part 2 [Barnouin‐Jha et al., this issue]). These two studies provide a method to predict the curtain‐induced flow and consequent patterns of debris deposition associated with impacts on planets with atmospheres.
Investigating the interactions between an atmosphere and an ejecta curtain: 2. Numerical experiments
Authors: Barnouin‐Jha O. S. , Schultz P. H. , Lever J. H.
Published: September 2002

The locus of ejecta excavated during an impact generates a debris curtain that expands outward. In an atmosphere this advancing curtain acts like a semipermeable barrier that displaces the surrounding gas. The generated flow separates near the top of the curtain to form a vortex ring whose strong winds entrain, transport, and deposit fine‐grained ejecta, affecting the morphology of distal ejecta deposited on planets with atmospheres. We have investigated how the curtain width and velocity, particle concentration, size distribution and velocity parallel to the curtain, and the density, viscosity, and compressibility of the surrounding atmosphere controls the flow strength of these winds. Wind tunnel tests (Part 1 [Barnouin‐Jha et al., this issue]) show that for an ejecta‐like porous plate, the hydraulic resistance, a measure of energy losses for one‐dimensional porous flow, governs the position along the curtain where it becomes effectively impermeable. Combined with suitable cratering models and published hydraulic resistance data, this information allows estimating the flow strength or circulation generated by an advancing curtain. The present study assesses the influence of atmospheric compressibility and particle motion parallel to the curtain surface on the curtain's circulation in order to improve these estimates. Numerical experiments indicate that atmospheric compressibility has little effect on the circulation at Mach number below 0.5, consistent with analytical solutions. Analytical solutions show, however, that this flow circulation should increase significantly at higher Mach numbers. The numerical experiments also show that individual ejecta traveling parallel to the surface of the curtain enhance the induced circulation by 9% to 33%.
Production and damping of turbulence by particles in the solar nebula
Authors: Dobrovolskis Anthony R. , Dacles‐Mariani Jennifer S. , Cuzzi Jeffrey N.
Published: September 2002

We have developed a “one‐equation” model for the turbulent boundary layer surrounding the midplane of the solar nebula and compared it with the “zero‐equation” (Prandtl) model used in our previous work. Unlike the Prandtl model, our current model includes local generation, transport, and dissipation of turbulence. In particular, our derivation of the equation governing the kinetic energy of turbulence explicitly includes the damping of turbulence by particle drag. We have also included the kinetic pressure of the particles in both models, analogous to the turbulent pressure of the gas phase, and refined certain dimensionless constants of the flow. We present numerical results for particles 60 cm in radius. Both the switch to the one‐equation model and the inclusion of turbulence damping by particles tend to increase particle concentrations at the midplane of the nebula, but the particle pressure and the improved flow constants both decrease the particle concentration there. These effects nearly offset one another, supporting our previous conclusion that settling of particles to the midplane of the solar nebula is self‐limiting, so that direct accretion of planetesimals by gravitational instability is inhibited.
The sputtering of an oxygen thermosphere by energetic O+
Authors: Johnson R. E. , Schnellenberger D. , Wong M. C.
Published: September 2002

Using two Monte Carlo models, we have calculated the ejection of O atoms from an atomic oxygen thermosphere which is bombarded by energetic O+. In these calculations, incident oxygen ions strike atmospheric O, heating the atmosphere and setting O atoms on escape trajectories. For those ion fluxes most relevant to atmospheric sputtering at Mars, the full Monte Carlo model (DSMC model) is found to give the same sputtering yield as the Monte Carlo model in which the atmospheric structure is ignored and only the energetic atoms are tracked. In both models the sputtering yield is reduced by the electronic energy loss by the ions. The yield depends weakly on the details of the interaction potential if the relative amounts of forwardscattering to backscattering are realistic. The ejected atom energy spectra for the mean incident angle 55° are shown to be harder than the standard collision cascade distribution, close to the so‐called “incomplete cascade distribution.” Yields for grazing incidence and for a thin atmosphere such as Europa's are given, and the analytic models used earlier are tested. Because the isotope ratios at Mars are likely to have been determined by atmospheric sputtering, Ar and Ne are included as constituents to verify the analytic models for the sputtering of trace species. The yields calculated here for an atomic O thermosphere can be used to estimate the sputtering of an atmosphere having molecules at the exobase. For Mars, ignoring feedback processes and using the ion fluxes listed byJohnson and Luhmann[1998], the loss is ∼0.45 bar, of which ∼0.15 bar is CO2with the largest uncertainty for atmospheric loss being the incident ion flux.
Mineralogic and compositional properties of Martian soil and dust: Results from Mars Pathfinder
Authors: Bell J. F. , McSween H. Y. , Crisp J. A. , Morris R. V. , Murchie S. L. , Bridges N. T. , Johnson J. R. , Britt D. T. , Golombek M. P. , Moore H. J. , Ghosh A. , Bishop J. L. , Anderson R. C. , Brückner J. , Economou T. , Greenwood J. P. , Gunnlaugsson H. P. , Hargraves R. M. , Hviid S. , Knudsen J. M. , Madsen M. B. , Reid R. , Rieder R. , Soderblom L.
Published: September 2002

Mars Pathfinder obtained multispectral, elemental, magnetic, and physical measurements of soil and dust at the Sagan Memorial Station during the course of its 83 sol mission. We describe initial results from these measurements, concentrating on multispectral and elemental data, and use these data, along with previous Viking, SNC meteorite, and telescopic results, to help constrain the origin and evolution of Martian soil and dust. We find that soils and dust can be divided into at least eight distinct spectral units, based on parameterization of Imager for Mars Pathfinder (IMP) 400 to 1000 nm multispectral images. The most distinctive spectral parameters for soils and dust are the reflectivity in the red, the red/blue reflectivity ratio, the near‐IR spectral slope, and the strength of the 800 to 1000 nm absorption feature. Most of the Pathfinder spectra are consistent with the presence of poorly crystalline or nanophase ferric oxide(s), sometimes mixed with small but varying degrees of well‐crystalline ferric and ferrous phases. Darker soil units appear to be coarser‐grained, compacted, and/or mixed with a larger amount of dark ferrous materials relative to bright soils. Nanophase goethite, akaganeite, schwertmannite, and maghemite are leading candidates for the origin of the absorption centered near 900 nm in IMP spectra. The ferrous component in the soil cannot be well‐constrained based on IMP data. Alpha proton X‐ray spectrometer (APXS) measurements of six soil units show little variability within the landing site and show remarkable overall similarity to the average Viking‐derived soil elemental composition. Differences exist between Viking and Pathfinder soils, however, including significantly higher S and Cl abundances and lower Si abundances in Viking soils and the lack of a correlation between Ti and Fe in Pathfinder soils. No significant linear correlations were observed between IMP spectral properties and APXS elemental chemistry. Attempts at constraining the mineralogy of soils and dust using normative calculations involving mixtures of smectites and silicate and oxide minerals did not yield physically acceptable solutions. We attempted to use the Pathfinder results to constrain a number of putative soil and dust formation scenarios, including palagonitization and acid‐fog weathering. While the Pathfinder soils cannot be chemically linked to the Pathfinder rocks by palagonitization, this study and McSween et al. [1999] suggest that palagonitic alteration of a Martian basaltic rock, plus mixture with a minor component of locally derived andesitic rock fragments, could be consistent with the observed soil APXS and IMP properties.
Lunar polar topography derived from Clementine stereoimages
Authors: Cook A. C. , Watters T. R. , Robinson M. S. , Spudis P. D. , Bussey D. B. J.
Published: September 2002

Clementine stereoimagery has been used to produce digital elevation models of the Moon, at a scale of ∼1 km/pixel. These models cover regions poleward of 60° in both hemispheres and reveal topography beyond that covered by the Clementine laser altimeter or Earth‐based radar. By combining these polar terrain models with the current Clementine laser altimeter data we have produced a global topographic map of the lunar surface. Several topographic features in the new polar topographic data set are described, including three previously unrecognized pre‐Nectarian impact basins. Several known basins have also been mapped, including the southern extent of the South Pole‐Aitken basin, and other previously suspected basins have been verified.
The state, potential distribution, and biological implications of methane in the Martian crust
Authors: Max Michael D. , Clifford Stephen M.
Published: September 2002

The search for life on Mars has recently focused on its potential survival in deep (>2 km) subpermafrost aquifers where anaerobic bacteria, similar to those found in deep subsurface ecosystems on Earth, may have survived in an environment that has remained stable for billions of years. An anticipated by‐product of this biological activity is methane. The detection of large deposits of methane gas and hydrate in the Martian cryosphere, or as emissions from deep fracture zones, would provide persuasive evidence of indigenous life and confirm the presence of a valuable in situ resource for use by future human explorers.
Irradiated interplanetary dust particles as a possible solution for the deuterium/hydrogen paradox of Earth's oceans
Authors: Pavlov Alexander A. , Pavlov Anatoli K. , Kasting James F.
Published: September 2002

Determining the source of Earth's oceans is a longstanding problem in planetary science. Possible sources of water include water ice or water of hydration of silicate minerals in the original material from which the bulk Earth accreted and water brought in by late‐arriving planetesimals during the heavy bombardment period (4.5–3.8 Gyr ago) [Chyba, 1989, 1991]. Comets are an attractive source of water because their origin in the outer solar system is consistent with the long timescale for heavy bombardment. However, the high deuterium/hydrogen (D/H) ratio of the three comets that have been studied, Halley, Hyakutake, and Hale‐Bopp, indicates that Earth must have had a source with a low‐D/H ratio as well. Here we suggest that solar wind‐implanted hydrogen on interplanetary dust particles (IDPs) provided the necessary low‐D/H component of Earth's water inventory.
Geologic evolution of southern Rusalka Planitia, Venus
Authors: DeShon Heather R. , Young Duncan A. , Hansen Vicki L.
Published: September 2002

Geologic mapping of southern Rusalka Planitia, Venus, reveals interactions of volcanism, tectonism, and topography. We recognize three regional plains units (prR1, prR2, and prR3) based on crosscutting structural relations, embayment patterns, radar brightness, and surface roughness data. Delineation of secondary (tectonic) structures allows us to constrain the relative temporal relations between the three material units. Unit prRl, a radar dark smooth unit exposed in local topographic highs, hosts NE trending extension fractures. Low‐viscosity lava flows of prR2, the most areally extensive unit, fill local topographic lows and the NE trending fractures. A shield‐sourced lava unit, prR3, overlies prR2 on the basis of embayment relations and radar brightness. NW trending wrinkle ridges deform all three plains units and record regional contraction. Locally, flood lava flows that fill NE trending fractures are structurally inverted to form short, stepped NE trending wrinkle ridges. Map patterns indicate that prR2 comprises a thin layer (<50 m thick), much thinner than previous estimates of 1–3 km. Therefore previously proposed estimates of plains flood lava flow volumes and effusion rates are much too high. The local geologic history of southern Rusalka Planitia is inconsistent with global stratigraphy models. Our study supports the view of plains evolution occurring through discrete volcanic processes working at local and regional (but not global) scales.
Determination of habitable zones in extrasolar planetary systems: Where are Gaia's sisters?
Authors: Franck Siegfried , von Bloh Werner , Bounama Christine , Steffen Matthias , Schönberner Detlef , Schellnhuber Hans‐Joachim
Published: September 2002

A general modeling scheme for assessing the suitability for life of extrasolar planets is presented. The scheme focuses on the identification of the “habitable zone” in main sequence star planetary systems accommodating Earth‐like components. Our definition of habitability is based on the long‐term possibility of photosynthetic biomass production under geodynamic conditions. Therefore all the pertinent astrophysical, climatological, biogeochemical, and geodynamic processes involved in the generation of photosynthesis‐driven life conditions are taken into account. Implicitly, a cogenetic origin of the central star and the orbiting planet is assumed. A geostatic model version is developed and investigated in parallel for demonstration purposes. The numerical solution of the advanced geodynamic model yields realistic lookup diagrams for convenient habitability determination. As an illustration, the MACHO‐98‐BLG‐35 event is scrutinized. It is shown that this event is definitely not tantamount to the discovery of one of Gaia's sisters.
Radiative transfer models for light scattering from planetary surfaces
Authors: Cheng Andrew F. , Domingue Deborah L.
Published: September 2002

New, accurate numerical solutions of the radiative transfer equation are compared with the Hapke [1981, 1984, 1986] analytic approximation, which is widely used in planetary data analyses. The numerical solutions use the Ambartsumian invariance principle as do the well‐known Chandrasekhar [1960] H function solutions. The invariance principle has been reexpressed in a form which allows high order‐accurate numerical integrations without any required interpolations. The new numerical solutions reproduce the Chandrasekhar H function solutions for Legendre phase functions but also allow single‐scattering phase functions of arbitrary form. Accurate numerical solutions of the radiative transfer equation for Henyey‐Greenstein phase functions reveal that the errors in the Hapke model for estimating bidirectional reflectance values range from <2% rms for dark surfaces like the Moon to <7% rms for bright surfaces such as Europa. Further comparisons demonstrate that Hapke model fitting procedures estimate single particle scattering albedo values to within <3% for both dark and bright surfaces.
Oxygen cycle of the Martian atmosphere‐regolith system: Δ17O of secondary phases in Nakhla and Lafayette
Authors: Farquhar James , Thiemens Mark H.
Published: September 2002

Oxygen isotope fractionations among silicates, carbonates, and sulfate from Nakhla and Lafayette can be used to resolve how multiple oxygen isotope reservoirs formed and evolved on Mars and to gain insight into the environment and processes that led to the formation of the SNC carbonates and other secondary minerals. Carbonates and sulfate from Nakhla and Lafayette carry an imprint of atmospheric chemistry analogous to that documented on Earth but reflect an inhospitable environment where ozone, hydrogen peroxide, and odd oxygen compounds transferred their signature of atmospheric oxygen isotope fractionations to water, ice, and minerals that formed in the regolith at low temperature. Such an environment would not be conducive to the preservation of easily oxidized organic compounds. Its existence provides an abiotic, isotopic benchmark. Biotic processes should produce isotopic fractionations that lie on a coherent mass fractionation line and not the mass‐independent fractionation relationships observed in the present study.
Rock coatings and aeolian abrasion on Mars: Application to the Pathfinder landing site
Authors: Kraft Michael D. , Greeley Ronald
Published: September 2002

Rock coatings can be used to constrain the rate of abrasion by wind on Mars. The susceptibility to abrasion for potential rock coatings on Mars (salt/salt‐cemented coatings, rock varnish, and amorphous silica) were determined experimentally. Rock coatings generally abrade more easily than the host rock, although amorphous silica is an exception. If coatings exist on rocks at the Mars Pathfinder landing site, then the rate of abrasion is extremely low, consistent with previous studies of erosion rates. Alternatively, rock varnish or silica coatings could protect the rocks from sandblasting, although this may require coating formation under current to recent conditions. The Pathfinder area may lack efficient abrasive particles, and the observed duneforms may be composed of sand‐size dust aggregates rather than crystalline particles.
An exospheric perspective of isotopic fractionation of hydrogen on Venus
Authors: Hodges R. Richard
Published: September 2002

Data from Pioneer Venus provide a basis for estimating current escape rates of H and D from Venus, but there is no consensus in the existing estimates. In an attempt to clarify the effects of exospheric dynamics on escape, models of the distributions of H and D in the upper thermosphere and exosphere of Venus, and mutually consistent distributions of H+ and D+, have been calculated for several scenarios at solar maximum. The model results indicate that the combined (neutral and ion) planet average escape fluxes for H and D are 4.7×107 cm−2 s−1 and 1.4×105 cm−2 s−1, respectively. Allowing for uncertainties in the modeling procedure, the exospheric escape flux of H is inexplicably greater than current estimates of its thermospheric counterpart derived from Hunten's limiting flux theorem.
Martian fluvial‐thermal erosion: Laboratory simulation
Authors: Costard F. , Aguirre‐Puente J. , Greeley R. , Makhloufi N.
Published: September 2002

Wide outflow channels occur both in Siberia and on the planet Mars. In Siberia, thermal erosion results from ground thawing produced by the heat transfer from the flow of water to the frozen ground. We suggest that relatively warm floods on Mars could enlarge outflow channels by a combination of thermal and mechanical erosion along frozen river banks. A one‐dimensional model is proposed to estimate the thermal erosion efficiency. A first test of this model is a comparison of results with experiments carried out in a cold chamber. A hydraulic channel allows measurements of the thawing line propagation, as well as the thermal erosion rate, in simulated ground ice that is subjected to warm water flow. Various laboratory simulations demonstrate the validity of the mathematical model for the range of laboratory conditions. This model is then applied to a range of possible current and ancient Martian conditions.
Deep circulation changes in the Labrador Sea since the Last Glacial Maximum: New constraints from Sm‐Nd data on sediments
Authors: Fagel Nathalie , Innocent Christophe , Stevenson Ross K. , Hillaire‐Marcel Claude
Published: September 2002

Sm‐Nd concentrations and Nd isotopes were investigated in the fine fraction of two Labrador Sea cores to reconstruct the deep circulation patterns through changes in sedimentary supply since the last glacial stage. Three sources are involved: the North American Shield, Palaeozoic rocks from northeastern Greenland, and mid‐Atlantic volcanism. The variable input of these sources provides constraints on the relative sedimentary supply, in conjunction with inception of deep currents. During the last glacial stage a persistent but sluggish current occurred inside the Labrador Basin. An increasing discharge of volcanic material driven by the North East Atlantic Deep Water is documented since 14.3 kyr, signaling the setup of a modern‐like deep circulation pattern throughout the Labrador, Irminger, and Iceland basins. During the last deglacial stage the isotopic record was punctually influenced by erosion processes related mainly to ice‐sheet instabilities, especially 11.4, 10.2, and 9.2 kyr ago.
Analysis of optical waveguide devices using the FD‐TD method based on the principles of multidimensional wave digital filters
Authors: Ikuno Hiroyoshi , Naka Yoshihiro , Yata Akira
Published: September 2002

We present a finite difference time domain (FD‐TD) method with the perfectly matched layers (PMLs) absorbing boundary condition based on the multidimensional wave digital filters (MD‐WDFs) for discrete‐time modeling of Maxwell's equations and show its effectiveness. First we propose modified forms of Maxwell's equations in the PMLs and their MD‐WDFs representation by using the current‐controlled voltage sources. Second we evaluate the numerical errors about phase velocity in the plane wave propagation problem by examining the numerical dispersion relation and show an advantage of the FD‐TD method based on MD‐WDFs over the Yee algorithm. As a numerical example, we analyze two typical optical waveguide devices and design a low‐insertion‐loss polarizer composed of a multiple‐quantum‐well waveguide and a right‐angle bend waveguide using a photonic crystal whose size is on the order of the wavelength of light.
A methodology for modeling and simulating target echoes with a moving polarimetric bistatic radar
Authors: Airiau Olivier , Khenchaf Ali
Published: September 2002

We model the signal received by a FM pulse radar, considering the general case where the transmitter and the receiver are separated and are moving. The electromagnetic wave polarization state change generated by the observed body is also taken into account in the model proposed in this paper. The expression of the received signal is written as the voltage detected on the receiving antenna and is given as a function of time by taking into account the antenna radiation (transmitting antenna and receiving antenna), the polarimetric behavior of the target, and the mobiles velocity effect. Numerical results are presented considering a canonical target or a slightly rough surface using a monostatic or a bistatic configuration to show the accuracy of the theoretical results and to study the velocity effect of the mobiles.
Laboratory measurement of material electrical properties: Extending the application of lumped‐circuit equivalent models to 1 GHz
Authors: Levitskaya Tsylya M. , Sternberg Ben K.
Published: September 2002

For measurements of material electrical properties in a frequency range from 1 kHz to 1 GHz, we used a laboratory method based on the concept of lumped R, L, and C circuit elements. While this method has typically been used at frequencies of less than 100 MHz, we have extended its application up to 1 GHz. The complex electrical parameters of a material, such as resistivity, conductivity, and dielectric permittivity were obtained by measuring magnitude Z and phase ϕ of the sample impedance Z. We relate the material electrical parameters to either series or parallel lumped‐circuit equivalent models. Depending on the frequency range, two different designs of the sample holder can be used: (1) a parallel‐plate capacitor with disk electrodes, for low frequencies (from 1 kHz to 100 MHz), and (2) a coaxial capacitor, for a broad band up to higher frequencies (from 1 kHz to 1 GHz). Measured values of the sample impedance usually include errors due to effects from the sample holder and its connections to the instrument. These effects, caused by the inductance, resistance, and stray capacitance of the measuring system, are taken into account. Our measurements of several standard materials, including air, Teflon, octanol, butanol, and methanol, showed that the relative standard deviation from the mean for the dielectric permittivity (in the range where it is frequency independent) is typically less than 1%. The difference between our mean values and previously published values for these standard materials is also less than 1%.
Fast inhomogeneous plane wave algorithm for electromagnetic solutions in layered medium structures: Two‐dimensional case
Authors: Hu Bin , Chew Weng Cho
Published: September 2002

An accurate and efficient algorithm is proposed for solving two‐dimensional electromagnetic scattering problems in a layered medium. As a natural extension of the previously developed fast inhomogeneous plane wave algorithm (FIPWA), this approach has several inherent merits, such as being simple, versatile, and error controllable. The basic idea is first to express the spatial Green's function, encountered in layered medium studies, as Sommerfeld‐type integrals. By observing the similarity of these integrals with the integral representation of the free space Green's function, FIPWA can be applied with ease to accelerate the matrix‐vector multiplication. The multilevel scheme has been implemented, and the computational complexity ofO(NlogN) is achieved. It is noted that the proposed approach is more accurate and more efficient than the existing fast multipole method coupled with the discrete complex image method.
On the ionosphere calibration in GPS radio occultation measurements
Authors: Syndergaard Stig
Published: September 2002

Elimination of the ionospheric contribution to atmospheric delays in GPS radio occultation measurements is a key issue in the retrieval of accurate pressure and temperature profiles above the tropopause. The traditional so‐called “ionosphere‐free” combination of the L1 and L2 phase paths, eliminating the first‐order ionospheric effects, is not sufficient because of the nonnegligible bending of the two ray paths. Because of the dispersive nature of the ionosphere the L1 and L2 signals will follow slightly different paths, giving rise to an ionospheric residual, in this paper referred to as the “dispersion” residual. Other higher‐order ionospheric effects contribute to the total residual, but the dispersion residual is the most dominant. A linear combination of the L1 and L2 bending angles gives better results in most cases and has in recent years become the method of choice. In this paper a new phase path correction method is presented, dealing with the problem of L1 and L2 ray path separation. Formulas are derived showing how the dispersion residual can be evaluated using measurements of the satellite‐to‐satellite total electron content. The residuals for various conditions in the ionosphere are estimated numerically and compared to analytic estimates. A mathematical formulation of the difference between this improved phase path correction method and the bending angle correction method is obtained, showing that the two methods are nearly equivalent.
Influence of instrumental effects upon the full correlation analysis
Authors: Holdsworth David A.
Published: September 2002

The “full correlation analysis” (FCA) of spaced antenna (SA) data is sometimes found to be susceptible to the “triangle size effect” (TSE), whereby the magnitude of the FCA “true” velocity increasingly underestimates the actual wind velocity with decreasing antenna spacing. The TSE has been attributed to a number of instrumental effects, including coarsely digitized data, receiver characteristic differences, noise, and antenna coupling. In this paper the sources of the TSE are discussed, and the influence of various previously undocumented instrumental effects upon the FCA are investigated using simulated data. These additional effects are also shown to produce the TSE. The possibility that these instrumental effects may contribute to the lack of agreement exhibited between winds estimated using medium frequency SA radars employing the FCA and alternative in situ, satellite, and other radar techniques is discussed.
The rigorous analysis of a monopole antenna fed by a circumferential slot in a coaxial line
Authors: Yung Edward K. N. , Xie Z. M. , Chen R. S. , Han Y. F.
Published: September 2002

A thick monopole antenna fed by a circumferential slot cut on the outer conductor of a coaxial cable is studied with rigorous field theory in this paper. In our analysis we consider not only the higher‐order modes inside the coaxial cable but also the nonuniform distribution of the electric field in the slot. The coupled integrodifferential equations are formulated for the evaluation of the current distribution on the antenna surface as well as the electric field distribution in the slot. The restriction of the slot width and the radii of the antenna are removed from the analysis, The effect of the slot width on the electric field distribution in the slot, the current distribution on the outer conductive surface of the antenna, and the input admittance viewed in the coaxial cable are presented and discussed. The calculated input admittance is in good agreement with the experimental result.
Diurnal and annual variations in 10‐kHz radio noise
Authors: Clilverd Mark A. , Watkins Nicholas W. , Smith Andy J. , Yearby Keith H.
Published: September 2002

It is shown that the investigation of lightning activity levels, using very low frequency radio emissions near 10 kHz, needs to take into account the significant influences of subionospheric propagation. Ten‐kilohertz spectral power measurements made at Halley, Antarctica (76°S, 26°W), show at least two significant periodicities. The largest variation (10 dB) is due to the diurnal cycle and is strongly dominated by South American thunderstorm activity levels. The effect of African thunderstorm activity is observable in the diurnal variation plots but is strongly reduced by disadvantageous propagation conditions. The analysis suggests that given two identical tropical source regions, one situated east and one situated west, of a polar receiver site, the region to the west will tend to dominate the diurnal variation. The smaller annual variation at Halley (3 dB) can be explained by the improved subionospheric propagation conditions during the Southern Hemisphere winter counterbalancing the seasonal effects of decreased source activity in South America.
VLF scattering from red sprites: Application of numerical modeling
Authors: Rodger Craig J. , Nunn David
Published: September 2002

Red sprites were discovered by chance in 1989 when a low‐light TV system was pointed above an active thunderstorm. Red sprites are associated with columns of ionization in the Earth‐ionosphere waveguide, from above the thunderstorm into the D region of the ionosphere. The ionized columns have been detected through “VLF sprites,” perturbations of the phase and/or amplitude of subionospheric VLF transmissions, which can be used to study the electrical properties of red sprites. There is extensive experimental evidence that VLF sprites may involve wide scattering angles and can produce back scattered radiation. Here we present a numerical and theoretical study of the scattering of subionospheric VLF transmissions caused by the plasma columns associated with red sprites. Comparison of the VLF scattering from sprites is made between a non‐Born rigorous model which assumes the sprites are infinite columns of constant conductivity, and a three dimensional Born scattering code. Both formulations show excellent agreement with one another. The formulations predict VLF sprites similar to those experimentally observed for all scattering angles. This shows that the conclusions of previous studies into VLF sprites making use of the non‐Born formulations of Rodger et al [1997a, b] are valid. The modeling provides strong evidence that red sprite plasma is highly ionized in comparison with the ambient nighttime ionosphere, being nearly 5 orders of magnitude greater than the ambient at some heights.
Determination of the minimum convex source region from the electromagnetic far‐field pattern
Authors: Yaghjian A. D. , Hansen T. B. , Devaney A. J.
Published: September 2002

In a previous paper, a simple method was derived for determining the minimum convex closed surface in which the sources of a given scalar acoustic far‐field pattern can be located. In the present paper, we derive the corresponding method for determining the minimum convex surface for a vector electromagnetic far‐field pattern. We apply the method to find the minimum source region for the far‐field pattern of a circular‐disk antenna.