bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of relative humidity on light scattering by mineral dust aerosol as measured in the marine boundary layer over the tropical Atlantic Ocean
Authors: Li‐Jones X. , Maring H. B. , Prospero J. M.
Published: February 2004

Using an integrating nephelometer, we measured aerosol light scattering during a month‐long experiment conducted in April 1994 on Barbados, West Indies (13.17°N, 59.43°W). Concurrent measurements of aerosol composition showed that the trade wind aerosol mass was alternatively dominated by sea salt and by mineral dust transported from North Africa. Dust concentrations ranged from 0.9 to 257 μg m−3, and sea salt ranged from 9.4 to 29 μg m−3. Nonsea‐salt sulfate (nss SO4=) was generally a minor component with concentrations ranging from 0.5 to 4.2 μg m−3. We obtained hygroscopic growth factors (HGF) by calculating the ratio of the aerosol scattering coefficient (ASC) measured at ambient relative humidity (RH), which ranged from 67% to 83%, to the ASC measured with the sample air stream heated so as to reduce the RH to under 40%. Over the course of the experiment, HGF ranged from 1.13 to 1.69, with lower values corresponding to high dust concentration periods. By a variety of procedures, including the chemical apportioning of ASC, we derived the HGF for mineral dust; depending on the technique used, values ranged from 1.0 to 1.1. In contrast, we obtain for sea salt an HGF of 1.8±0.2, a value consistent with previously reported measurements. The low HGF values obtained for dusty periods suggest that only minor amounts of hygroscopic materials are associated with dust. Consequently, the radiative properties of North African dust in the atmosphere will be relatively insensitive to RH changes.
A new instrument for measuring cloud condensation nuclei: Cloud condensation nucleus “remover”
Authors: Ji Qiang , Shaw Glenn E. , Cantrell Will
Published: February 2004

This communiqué describes a new and novel instrument for measuring concentrations of natural cloud condensation nuclei (CCN) from the free atmosphere. It simultaneously provides the size spectrum of nucleated particles, a useful parameter for gaining insight into the composition of the CCN. The supersaturation spectrum N(S), where N is the number concentration of droplets nucleated at supersaturation S, is readily obtained. This parameter is one of the keys to evaluating aerosol influence on cloud microphysics and on the possible modulations of cloud albedo. Laboratory evaluations show that the instrument has good measurement accuracy and precision, even at low CCN concentrations found in the clean troposphere. The instrument is rugged and reliable.
Polar Ozone and Aerosol Measurement (POAM) II stratospheric NO2, 1993–1996
Authors: Randall C. E. , Rusch D. W. , Bevilacqua R. M. , Hoppel K. W. , Lumpe J. D.
Published: February 2004

We describe, for the first time, measurements of stratospheric nitrogen dioxide (NO2) by the Polar Ozone and Aerosol Measurement (POAM II) instrument. Measurements span October 1993 through mid‐November 1996 and cover latitude ranges from 55° to 72°N and from 63° to 88°S. Comparisons with coincident satellite and space shuttle observations show good agreement and confirm the validity of POAMII measurements for scientific investigations. Overall seasonal variations in both hemispheres are qualitatively consistent with standard photochemistry. In the austral late winter/early spring of 1994, however, anomalously high NO2 mixing ratios were observed above 22 km. We conclude that these high NO2 levels resulted from downward transport of NOx‐enhanced air from the mesosphere or thermosphere inside the polar vortex. Enhanced NO2 mixing ratios in 1994 were factors of ∼1.3 and 2.5 larger than at the corresponding times in 1995 and 1996. We conclude that POAM II observations of coincident, localized reductions in ozone of up to 40% were caused by the increased stratospheric NOx via the standard catalytic NOx cycle.
Natural microseepage of methane to the atmosphere from the Denver‐Julesburg basin, Colorado
Authors: Klusman Ronald W. , Jakel M. Emmelyn
Published: February 2004

Methane fluxes were measured at locations across the Denver‐Julesburg (DJ) basin of northeastern Colorado, which is in a semiarid climate. There is a potential for microseepage of thermogenically derived methane and light hydrocarbons to the atmosphere, which during drier or colder seasons may exceed the capacity for methanotrophic oxidation in the shallow soils. Triplicate measurements of methane flux in three seasons across the basin found areas of positive and negative fluxes in a patchy pattern which averaged +0.57 mg CH4 m−2 d−1. The possibility of drier areas of the continents, which are underlain by thick sedimentary units seasonally producing positive methane fluxes to the atmosphere, represents a second‐order effect in the estimation of the terrestrial sources/sinks for atmospheric methane.
Comparing upper tropospheric and lower stratospheric temperatures: Microwave sounding unit, radiosonde, COSPAR International Reference Atmosphere, and National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis monthly mean climatologies
Authors: Shah Kathryn Pierce , Rind David
Published: February 2004

Climate‐modeling groups are having to assess their models' temperatures at high, thinly observed altitudes in order to investigate near‐tropopause forcings with confidence. To support such analyses, the microwave sounding unit (MSU), GFDL/Oort radiosonde, COSPAR International Reference Atmosphere (CIRA), and new 13‐year National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis Project climatologies are intercompared herein in terms of their monthly mean microwave brightness temperatures Tb. In the lower stratosphere these climatologies agree extremely well. Small differences between NCEP/NCAR and MSU Tb centered at about 80 mbar amount to ≤2 K year round across the tropics and ≤5 K in southern winter polar latitudes. Artificial land‐ocean outlines ≤2 K do appear in maps of the NCEP/NCAR lower stratospheric Tb between 30°S and 30°N. This NCEP/NCAR land‐ocean Tb distinction may be due to sparseness of inputed radiosonde data at lower stratospheric heights not compensated for by satellite retrievals in the NCEP/NCAR assimilation process. In the upper troposphere, good agreement again occurs between the GFDL radiosonde, the CIRA, and the NCEP/NCAR reanalysis Tb. However, the experimental MSU channel 3R climatology has markedly cooler upper tropospheric Tb. Coolness peaks at over 6 K across low latitudes and diminishes to a few kelvins at polar latitudes with little seasonal dependence. The cool bias of the MSU channel 3R is likely due to a combination of channel 3 receiver drift and limb‐darkening problems. Latitudinal correction terms are suggested for this experimental, but valuable, MSU product based on comparisons with calculated NCEP/NCAR reanalysis Tb.
Northern and southern hemisphere ground‐based infrared spectroscopic measurements of tropospheric carbon monoxide and ethane
Authors: Rinsland Curtis P. , Jones Nicholas B. , Connor Brian J. , Logan Jennifer A. , Pougatchev Nikita S. , Goldman Aaron , Murcray Frank J. , Stephen Thomas M. , Pine Alan S. , Zander Rodolphe , Mahieu Emmanuel , Demoulin Philippe
Published: February 2004

Time series of CO and C2H6measurements have been derived from high‐resolution infrared solar spectra recorded in Lauder, New Zealand (45.0°S, 169.7°E, altitude 0.37 km), and at the U.S. National Solar Observatory (31.9°N, 111.6°W, altitude 2.09 km) on Kitt Peak. Lauder observations were obtained between July 1993 and November 1997, while the Kitt Peak measurements were recorded between May 1977 and December 1997. Both databases were analyzed with spectroscopic parameters that included significant improvements for C2H6relative to previous studies. Target CO and C2H6lines were selected to achieve similar vertical samplings based on averaging kernels. These calculations show that partial columns from layers extending from the surface to the mean tropopause and from the mean tropopause to 100 km are nearly independent. Retrievals based on a semiempirical application of the Rodgers optimal estimation technique are reported for the lower layer, which has a broad maximum in sensitivity in the upper troposphere. The Lauder CO and C2H6partial columns exhibit highly asymmetrical seasonal cycles with minima in austral autumn and sharp peaks in austral spring. The spring maxima are the result of tropical biomass burning emissions followed by deep convective vertical transport to the upper troposphere and long‐range horizontal transport. Significant year‐to‐year variations are observed for both CO and C2H6, but the measured trends, (+0.37±0.57)% yr−1and (−0.64±0.79)% yr−1, 1 sigma, respectively, indicate no significant long‐term changes. The Kitt Peak data also exhibit CO and C2H6seasonal variations in the lower layer with trends equal to (− 0.27±0.17)% yr−1and (−1.20±0.35)% yr−1, 1 sigma, respectively. Hence a decrease in the Kitt Peak tropospheric C2H6column has been detected, though the CO trend is not significant. Both measurement sets are compared with previous observations, reported trends, and three‐dimensional model calculations.
Coronagraphic observations of the lunar sodium exosphere near the lunar surface
Authors: Potter A. E. , Morgan T. H.
Published: February 2004

The sodium exosphere of the Moon was observed using a solar coronagraph to occult the illuminated surface of the Moon. Exceptionally dust‐free atmospheric conditions were required to allow the faint emission from sunlight scattered by lunar sodium atoms to be distinguished from moonlight scattered from atmospheric dust. At 0300 UT on April 22, 1994, ideal conditions prevailed for a few hours, and one excellent image of the sodium exosphere was measured, with the Moon at a phase angle of 51°, 81% illuminated. Analysis of the image data showed that the weighted mean temperature of the exosphere was 1280 K and that the sodium column density varied approximately as cosine‐cubed of the latitude. A cosine‐cubed variation is an unexpected result, since the flux per unit area of solar photons and solar particles varies as the cosine of latitude. It is suggested that this can be explained by a temperature dependence for the sputtering of sodium atoms from the surface. This is a characteristic feature of chemical sputtering, which has been previously proposed to explain the sodium exosphere of Mercury. A possible interaction between chemical sputtering and solar photons is suggested.
Desorption of alkali atoms and ions from oxide surfaces: Relevance to origins of Na and K in atmospheres of Mercury and the Moon
Authors: Madey Theodore E. , Yakshinskiy B. V. , Ageev V. N. , Johnson R. E.
Published: February 2004

This paper begins with a brief survey of the literature dealing with the adsorption and desorption of alkalis on oxide surfaces. Emphasis is on desorption phenomena: thermal desorption, electron‐ and photon‐stimulated desorption, and ion‐induced desorption (sputtering). Then the relevance of these data to the desorption of alkalis from mineral surfaces and to the origins of alkali vapors in tenuous planetary atmospheres is discussed. The data presented for Na and K indicate that desorption processes initiated by thermal or electronic excitations do not depend strongly on whether the Na returns to the surface or diffuses up through the regolith, and that neutral yields dominate ion yields in all cases. Although the desorbed neutral energy distributions are not well approximated by Maxwell‐Boltzmann distributions, the mean energies of the desorbed neutral Na and K are seen to be consistent with the temperatures extracted for the “hot” component of the lunar atmosphere. This suggests that the “hot” component may be produced by electronically stimulated desorption (e.g., electron‐stimulated desorption and/or photon‐stimulated desorption). If this is the case, a possible “size effect” may be operative, in which desorbed neutral K atoms are somewhat more energetic than desorbed Na. In such desorption processes a low‐energy component may be generated by scattering of desorbing atoms in the porous regolith; thermal desorption can also generate low‐energy atoms. The data further indicate that thermal desorption should be rapid in the equatorial regions of Mercury, possibly depleting this region of alkalis, whereas thermal desorption should be less efficient on the Moon. Surface charging may be important at the surface of the Moon, by accelerating the solar electrons to energies above the threshold for initiating alkali desorption. Suggestions are made for future laboratory work.
Characterization of Etnean soils and application to Mars
Authors: Buemi A. , Cimino G. , Strazzulla G.
Published: February 2004

We present the results of a study of some petromineralogical, chemical, and spectral properties of Etnean palagonitic soils. These soils exhibit highly variable mineralogy although they are considered among the best candidates as Martian soil analogs. Five specimens of Etnean volcanic soil were collected at different altitudes. Four of these are weathered and reworked pyroclasts from the historic eruptive activity; the fifth represents an ancient paleosoil (5420–2370 years B.P.). The chemical composition is hawaiitic. Petrography and mineralogy of grain mounts show they mainly consist of glass particles (black and red‐orange) containing microphenocrystals of plagioclase, clinopyroxene, olivine, and Ti‐magnetite, in order of abundance. One sample also contains rare amphiboles. IR (2.3–25 μm) transmittance and UV‐visible‐near IR (0.4–2.6 μm) diffuse reflectance spectra were obtained and compared with those of Martian dust.
Cloud structure and atmospheric composition of Jupiter retrieved from Galileo near‐infrared mapping spectrometer real‐time spectra
Authors: Irwin P. G. J. , Weir A. L. , Smith S. E. , Taylor F. W. , Lambert A. L. , Calcutt S. B. , Cameron‐Smith P. J. , Carlson R. W. , Baines K. , Orton G. S. , Drossart P. , Encrenaz T. , Roos‐Serote M.
Published: February 2004

The first four complete spectra recorded by the near infrared mapping spectrometer (NIMS) instrument on the Galileo spacecraft in 1996 have been analyzed. These spectra remain the only ones which have been obtained at maximum resolution over the entire NIMS wavelength range of 0.7–5.2 μm. The spectra cover the edge of a “warm” spot at location 5°N, 85°W. We have analyzed the spectra first with reflecting layer models and then with full multiple scattering models using the method of correlated‐k. We find that there is strong evidence for three different cloud layers composed of a haze consistent with 0.5‐μm radius tholins at 0.2 bar, a cloud of 0.75‐μm NH3 particles at about 0.7 bar, and a two‐component NH4SH cloud at about 1.4 bars with both 50.0‐ and 0.45‐μm particles, the former being responsible for the main 5‐μm cloud opacity. The NH3 relative humidity above the cloud tops is found to decrease slightly as the 5‐μm brightness increases, with a mean value of approximately 14%. We also find that the mean volume mixing ratio of ammonia above the middle (NH4SH) cloud deck is (1.7±0.1) × 10−4 and shows a similar, though less discernible decrease with increasing 5‐μm brightness. The deep volume mixing ratios of deuterated methane and phosphine are found to be constant and we estimate their mean values to be (4.9±0.2) × 10−7 and (7.7±0.2) × 10−7, respectively. The fractional scale height of phosphine above the 1 bar level is found to be 27.1±1.4% and shows a slight decrease with increasing 5‐μm brightness. The relative humidity of water vapor is found to be approximately 7%, but while this and all the previous observations are consistent with the assumption that “hot spots” are regions of downwelling, desiccated air, we find that the water vapor relative humidity increases as the 5‐μm brightness increases.
Analysis of Jupiter north equatorial belt hot spots in the 4–5 μm range from Galileo/near‐infrared mapping spectrometer observations: Measurements of cloud opacity, water, and ammonia
Authors: Roos‐Serote M. , Drossart P. , Encrenaz T. , Lellouch E. , Carlson R. W. , Baines K. H. , Kamp L. , Mehlman R. , Orton G. S. , Calcutt S. , Irwin P. , Taylor F. , Weir A.
Published: February 2004

This paper presents the analysis of hot spot observations in the Jovian North Equatorial Belt obtained with the near‐infrared mapping spectrometer (NIMS) instrument on the Galileo spacecraft. The data were acquired during the closest approach sequences between June 1996 and April 1997. We focus on the spectral window between 4.5 and 5.2 μm determining the cloud opacity above 2 bar, the water vapor relative humidity, and the ammonia abundance between 4 and 8 bar. We find a linear relationship between the cloud opacity and the continuum level of the spectrum. For a given radiance level of an individual spectrum, significant variations in the water vapor relative humidity are seen. However, no clear evidence for a relationship between the cloud opacity and the water relative humidity is seen. A cloud structure similar to that measured by the Galileo entry probe, with no significant cloud opacity below 2 bar, is adequate. The air in the hot spots is found to be overall dry, consistent with the probe measurements. None of the considered spectra show water vapor relative humidities exceeding 10%. Significant spatial variations of the water vapor relative humidity are found, and the distribution over the observed hot spot regions is complex. Because of a low sensitivity of the NIMS spectra to ammonia, uncertainties in the derived ammonia abundance are much higher than for water. There is, however, a possible trend in all the observed hot spots toward more ammonia inside than outside the hot spots at the sounded pressure levels.
The long lava flows of Elysium Planita, Mars
Authors: Mouginis‐Mark Peter , Yoshioka Michelle Tatsumura
Published: February 2004

Viking orbiter images are used to study the distribution and morphology of 59 lava flows in the Elysium Planitia region of Mars. Average widths for these flows range from 3 to 16 km, and many of the flows exceed 100 km in length. The aspect ratio (flow length to average width) is highly variable (>40:1 to <5:1), all flows that erupted <200 km from the summit are short (<70 km long), and the 11 longest flows all have vents >294 km from the summit of Elysium Mons. An unusual attribute of five flows is their segmented nature, with up to 13 individual segments on a single flow, which each have a surface area from 25 to 250 km2. Photoclinometry is used to derive an average thickness of 40–60 m for these flows, indicating that individual flow volumes range from 17.7 to 68.1 km3. Plausible values for the effusion rate (101–104 m3 s−1) suggest that individual eruptions could have lasted for a few months to several decades and could have injected between 1.36 and 2.04 × 1012 kg of water vapor into the atmosphere, assuming a 1 wt % water content for the parental magma. A total volume for all 59 flows is estimated to be 817–1226 km3, which would have released ∼2.04–3.01 × 1013 kg of water vapor into the atmosphere or ∼0.2% of the amount previously calculated [Plescia, 1993] as the amount released from the Cerberus flows in SE Elysium Planitia.
Landed XRD/XRF analysis of prime targets in the search for past or present Martian life
Authors: Vaniman D. , Bish D. , Blake D. , Elliott S. T. , Sarrazin P. , Collins S. A. , Chipera S.
Published: February 2004

Mars landers seeking evidence for past or present life will be guided by information from orbital mapping and from previous surface exploration. Several target options have been proposed, including sites that may harbor extant life and sites most likely to preserve evidence of past life. These sites have specific mineralogic characteristics. Extant life might be gathered around the sinters and associated mineral deposits of rare active fumaroles, or held within brine pockets and inclusions in a few evaporite‐mineral deposits. Possibilities for fossilization include deltaic and lake‐bottom sediments of once‐flooded craters, sinters formed by ancient hot‐spring deposits, and the carbonate deposits associated with some evaporite systems. However, the highly varied mineralogy of fossil occurrences on Earth leads to the inference that Mars, an equally complex planet, could host a broad variety of potential fossilizing deposits. The abundance of volcanic systems on Mars and evidence for close associations between volcanism and water release suggest possibilities of organism entrapment and mineralization in volcaniclastic deposits, as found in some instances on Earth. Thus the targets being considered for exploration include a wide variety of unique deposits that would be characterized by silica or various nonsilicate minerals. Beyond these “special” deposits and in the most general case, an ability to distinguish mineralized from uncemented volcanic detritus may be the key to success in finding possible fossil‐bearing authigenic mineralogies. A prototype miniaturized X ray diffraction/X ray fluorescence (XRD/XRF) instrument has been evaluated with silica, carbonate, and sulfate minerals and with a basalt, to examine the capabilities of this tool in mineralogic and petrologic exploration for exobiological goals. This instrument, CHEMIN (chemical and mineralogical analyzer), is based on an innovative low‐power X ray tube, transmission geometry, and CCD collection and discrimination of diffracted and fluoresced X rays. The ability to accumulate and integrate the entire circumference of each complete Debye diffraction ring compensates for poor powder preparations, as might be produced by robotic sampling systems. With CHEMIN, a wide range of minerals can be uniquely identified. Using Rietveld analysis of the XRD results, mineral quantification is also possible. Expanded capabilities in phase analysis and constrained data solutions using quantitative XRD and XRF are within reach.
Thermophiles, early biosphere evolution, and the origin of life on Earth: Implications for the exobiological exploration of Mars
Authors: Farmer Jack
Published: February 2004

Mars holds great interest for exobiology and presently stands center stage in our plans to explore the inner solar system for signs of past or present life. With the spectacular success of the Pathfinder mission [Golombek, 1997] and upcoming mapping efforts by the Mars Global Surveyor orbiter [Albee et al, 1998], it is an especially appropriate time to revisit some important issues in Mars exobiology. The papers making up this Journal of Geophysical Research‐Planets special issue on Mars were the outgrowth of an American Geophysical Union special session entitled “Exploring for a Martian Biosphere,” which was presented at the fall meeting in San Francisco in 1995 [Farmer, 1995a]. Together these papers provide a context for evaluating some fundamental issues in the exobiological exploration of Mars. Topics covered fall into the following broad categories: (1) environments and conditions required for the origin of life (hydrothermal environments as sites for prebiotic chemical synthesis [Shock and Schulte, this issue]), (2) early Martian environments as a context for the evolution and diversification of life on Mars (early impact hazards [Sleep and Zahnle, this issue], early volatile and climate history of Mars [Haberle, this issue], and ice‐covered lakes as habitats for early life [Doran et al., this issue]), (3) exploring for extant Martian life (results of the Viking missions [Klein, this issue], environmental extremes for Martian life [Clark, this issue], and planetary protection and Mars sample return [DeVincenzi et al., this issue]), and (4) exploration strategies (phosphate minerals as targets for chemofossils [Mojzsis and Arrhenius, this issue] and the role of robotic surface missions [Stoker, this issue]).
Lunar regional dark mantle deposits: Geologic, multispectral, and modeling studies
Authors: Weitz Catherine M. , Head James W. , Pieters Carle M.
Published: February 2004

Clementine five‐channel UV‐visible (UVVIS) data have been used to study seven regional dark mantle deposits (DMDs) on the Moon. The DMDs were mapped in distribution to determine their extent and stratigraphic relationship to other geologic units. Based upon the spectral properties for each DMD, the crystallization of the beads in each deposit was inferred and used to estimate cooling rates in the volcanic plumes that emplaced the deposits. Deposits with a high concentration of glasses reflect volcanic plumes that had low optical densities and high cooling rates, whereas deposits dominated by crystallized beads indicate plumes with slower cooling rates due to higher optical densities. Spectral data from each of the regional DMDs show that their glass:crystallized bead ratio can be estimated based upon their 415/750 and 750/950 nm values and comparison to laboratory spectra for the beads. Patches of young dark mantle in the Sinus Aestuum DMD represent one extreme with the bluest color (highest 415/750) and weakest glass band absorption (lowest 750/950) of all the DMDs. At the other extreme is the Aristarchus Plateau DMD with the reddest color and strongest glass band absorption. The other nearside DMDs, including Taurus‐Littrow, Sulpicius Gallus, Rima Bode, and Mare Vaporum lie between these two extremes due to intermediate mixtures of the crystallized beads and glasses with other local soils. The Orientale Ring DMD on the western limb is dominated by volcanic glasses and is spectrally similar to the localized DMDs found in Alphonsus crater. We have identified a central vent for the Orientale Ring DMD and model the eruption as degassing of a near‐surface dike to produce a 20‐km‐high umbrella‐shaped plume with ejection velocities of 360 m/s and deposition of the glasses at an average radius of 80 km from the vent. Although we cannot identify the exact sources for the other regional DMDs (probably because they are buried beneath younger mare), the eruptions most likely resulted from dikes breaching the surface and producing a volcanic plume dominated by larger (greater than submillimeter) hot clasts that formed mare and sinuous rules. The small percent of clasts that form the volcanic beads are carried by the expanding gas cloud to large distances, in some cases >100 km, to produce the observed continuous regional DMDs. The lack of basalt samples that can be petrologically related to the volcanic glasses may be a result of their spatial separation, with the basalts flowing into the basins while the beads are deposited both into the basins and on the adjacent highlands.
Global deep‐sea burial rate of calcium carbonate during the Last Glacial Maximum
Authors: Catubig Nina R. , Archer D. E. , Francois Roger , deMenocal Peter , Howard Will , Yu Ein‐Fen
Published: February 2004

Global databases of calcium carbonate concentrations and mass accumulation rates in Holocene and last glacial maximum sediments were used to estimate the deep‐sea sedimentary calcium carbonate burial rate during these two time intervals. Sparse calcite mass accumulation rate data were extrapolated across regions of varying calcium carbonate concentration using a gridded map of calcium carbonate concentrations and the assumption that accumulation of noncarbonate material is uncorrelated with calcite concentration within some geographical region. Mean noncarbonate accumulation rates were estimated within each of nine regions, determined by the distribution and nature of the accumulation rate data. For core‐top sediments the regions of reasonable data coverage encompass 67% of the high‐calcite (>75%) sediments globally, and within these regions we estimate an accumulation rate of 55.9±3.6×1011 mol yr−1. The same regions cover 48% of glacial high‐CaCO3 sediments (the smaller fraction is due to a shift of calcite deposition to the poorly sampled South Pacific) and total 44.1±6.0×1011 mol yr−1. Projecting both estimates to 100% coverage yields accumulation estimates of 8.3 × 1012 mol yr−1 today and 9.2 × 1012 mol yr−1 during glacial time. This is little better than a guess given the incomplete data coverage, but it suggests that glacial deep sea calcite burial rate was probably not considerably faster than today in spite of a presumed decrease in shallow water burial during glacial time.
Dust response to seasonal atmospheric forcing: Proxy evaluation and calibration
Authors: Clemens Steven C.
Published: February 2004

Eolian grain size and flux were measured on samples from 11 Arabian Sea sediment traps deployed 200‐1250 km offshore. The timing of increased grain size is coincident with the onset of strong summer monsoon winds and dust storm activity over the Arabian Peninsula and Middle East. Data spanning a full annual cycle show that eolian grain size is highly correlated with barometric pressure (r=−0.91) and wind speed (r=0.84), enabling calibration of the downcore record in terms of these primary meteorological variables. Eolian flux is highly correlated with organic carbon flux (r=0.80); both increase 6–8 weeks after the grain size increase and summer monsoon onset. This lag, and the low correlation between eolian grain size and eolian flux (r=0.36), likely result from the differential sinking rates of large and small dust particles in the surface waters as well as biological scavenging associated with monsoon‐induced productivity.
Superswells
Authors: McNutt Marcia K.
Published: February 2004

Geophysicists have long sought direct, incontrovertible evidence from surface observables for convection in Earth's mantle. One of the best candidates for a phenomenon of convective origin is the South Pacific Superswell, a broad area of uplifted seafloor containing numerous volcanoes in French Polynesia. Other proposed examples of the superswell phenomenon include seafloor now located in the North Pacific that was anomalously shallow back in Cretaceous time and the present‐day high topography of eastern and southern Africa. Researchers suggest that superswells form over mantle that is hotter than the global average on account of long‐term absence of cooling by subducting lithosphere. Superswell mantle is melt rich, as evidenced by a fourfold increase in the rate of volcanism as compared with that of normal lithosphere. Seismic tomography suggests that the source for this diffuse, radiogenically enriched volcanism without long age progressions is a hot layer above the transition zone, rather than numerous deep‐mantle plumes. Dynamic upwelling of this buoyant material in a low‐viscosity zone immediately beneath the plate is responsible for both the shallow seafloor and the dip in the elevation of the Earth's sea level equipotential surface over French Polynesia. Thermal models consistent with the seismic tomography, depth, and geoid data predict extremely minor perturbations to the temperature structure in the upper 50 km of the lithosphere and thus unresolvable anomalies in both heat flow and the stiffness of the elastic plate supporting the volcanoes. Superswell volcanism is distinct from other types of volcanism by not being immediately attributable to plate separation, plate convergence, or deep‐mantle plumes.
Non‐double‐couple earthquakes 2. Observations
Authors: Miller Angus D. , Foulger G. R. , Julian Bruce R.
Published: February 2004

Most studies assume that earthquakes have double‐couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many well‐recorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single‐force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high‐pressure, high‐temperature fluids. In mines, tunnels are cavities that may close. Deep‐focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick‐slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non‐DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non‐DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results.
Toward an improved understanding of multiphase flow in porous media
Authors: Muccino Julia C. , Gray William G. , Ferrand Lin A.
Published: February 2004

Physical description of multiphase flow in porous media ideally should be based on conservation principles. In practice, however, Darcy's law is employed as the foundation of multiphase flow studies. Darcy's law is an empirical surrogate for momentum conservation based on data obtained from experimental study of one‐dimensional single‐phase flow. In its original form [Darcy, 1856], Darcy's law contained a single, constant coefficient that depended on the properties of the medium. Since 1856, Darcy's relation has been heuristically and progressively altered by allowing this coefficient to be a spatially dependent, nonlinear function of fluid and solid phase properties, particularly of the quantities of these phases within the flow system. The shortcoming of this approach is that the governing flow equation is obtained by enhancing a simple empirical coefficient with complex functional dependencies rather than by simplifying general conservation principles. As a result, some of the important physical phenomena are not properly accounted for. Also, some assumptions intrinsic to the equations are overlooked, making accurate simulation more of an art than an entirely scientific exercise. A more general and more theoretically appealing approach to the derivation of conservation principles for multiphase flow has been evolving over the last 30 years. This approach employs a mathematical procedure for deriving conservation principles at the length scale of interest, followed by imposition of thermodynamic constraints to restrict the generality of these expressions. The product of this approach is a set of balance equations that provides a framework in which the assumptions inherent in a hypothesized model of multiphase flow are clearly stated. Requirements for more comprehensive and physically complete models can then be specified.
Thermal plasma and neutral gas in Saturn's magnetosphere
Authors: Richardson John D.
Published: February 2004

Saturn's magnetosphere contains plasma and neutral particles from Saturn's atmosphere, the rings, the inner icy satellites, and Titan. This paper reviews the observations of plasma and neutrals near Saturn. Plasma conditions were observed during the Pioneer 11 and Voyager 1 and 2 flybys of Saturn. Neutral H was observed by the Voyagers; neutral OH has been observed by the Hubble Space Telescope. The attempts which have been made to understand the physical processes behind the data are also discussed. Saturn's magnetosphere is dominated by neutrals, with a neutral to plasma density ratio of about 10. The neutrals are probably produced by bombardment of the moons by micrometeorites and heavy ions, although there is a discrepancy between predicted sputtering rates and the amount of sputtering necessary to produce the observed neutral densities. Transport of plasma out of the magnetosphere must be very fast, of the order of a few days. Models of the processes occurring in Saturn's magnetosphere are used to determine the densities of neutral and plasma species which cannot be directly observed. These results will be tested when Cassini arrives at Saturn in 2004.
Observations of the Florida Current with two over‐the‐horizon radars
Authors: Georges T. M. , Harlan J. A. , Lee T. N. , Leben R. R.
Published: February 2004

We show the space and time variability of the Florida Current in the Southern Straits of Florida using imagery derived from measurements made with two high‐frequency over‐the‐horizon (ionospheric) radars. Between March and August 1997, ten surface current maps show how the Florida Current meanders in response to mesoscale eddies propagating eastward through the Straits. Satellite altimetry aids in the interpretation of the radar imagery by showing upstream eddy development in the Gulf of Mexico.
MST radar measurement of ionospheric F region winds: The “layer wind” technique
Authors: Oliver W. L. , Otsuka Y. , Fukao S.
Published: February 2004

We develop an improved application of the mesosphere‐stratosphere‐troposphere (MST) radar technique for the measurement of winds at ionospheric F region altitudes. While the experiment is basically one of incoherent scatter (IS), which yields very weak signals at the usual operating frequencies of MST radars, we develop an analysis technique that allows the signals to be integrated over the entire F region of the ionosphere to gain sufficient statistical accuracy to compute the “layer wind” accurately. This method is contrasted with the normal IS method of determining winds at discrete heights. The method is applied to data collected with the middle and upper atmosphere radar in Japan, though it is applicable to any MST radar having a comparable or even somewhat smaller power‐aperture product and may also be adapted for improved results from mainline IS radars. Using this method, we are able to discuss confidently the geophysical behavior of the wind on individual days, whereas previously we were reluctant to discuss anything but climatological features from data averages.
Weighted mean tropospheric temperature and transmittance determination at millimeter‐wave frequencies for ground‐based applications
Authors: Ingold T. , Peter R. , Kämpfer N.
Published: February 2004

A simple model for the radiometric determination of tropospheric transmittance is based on an isothermal troposphere. In this model the key parameter is the weighted mean tropospheric temperature Tm, which characterizes the radiation and temperature properties of the troposphere. Statistical approaches in modeling this parameter are presented here by using ground temperature, ground relative humidity, and radiometer data. In order to determine the statistical coefficients for Tm modeling and the parameters used in the transmittance retrieval algorithm, radiosonde data were used in a millimeter‐wave propagation model for a site in the Swiss central plane and an Alpine site. Various observing geometries at different millimeter‐wave frequencies were considered. A determination of Tm from ground temperature was achieved with a rms error between 4–5 K for the low‐altitude site and 3–4 K for the high‐altitude site. By incorporating relative humidity or radiometer data, an improvement of up to 25% relative to these values results, depending on frequency and site. The zenith transmittance estimations for the low‐altitude site with our best model have a rms error of 0.5% at 38 GHz, 1% at 94, 110, and 142 GHz, 1.5% at 115 GHz, 2% at 204 GHz, and 3.5% at 279 GHz, whereas for the high‐altitude site all rms errors are below 1%. The inclusion of radiometric information at 20 and 31 GHz did not provide any additional improvement, which was confirmed by actual measurements at 142 GHz.
Applications of high‐frequency radar
Authors: Headrick J. M. , Thomason J. F.
Published: February 2004

Efforts to extend radar range by an order of magnitude with use of the ionosphere as a virtual mirror started after the end of World War II. A number of HF radar programs were pursued, with long‐range nuclear burst and missile launch detection demonstrated by 1956. Successful east coast radar aircraft detect and track tests extending across the Atlantic were conducted by 1961. The major obstacles to success, the large target‐to‐clutter ratio and low signal‐to‐noise ratio, were overcome with matched filter Doppler processing. To search the areas that a 2000 nautical mile (3700 km) radar can reach, very complex and high dynamic range processing is required. The spectacular advances in digital processing technology have made truly wide‐area surveillance possible. Use of the surface attached wave over the oceans can enable HF radar to obtain modest extension of range beyond the horizon. The decameter wavelengths used by both skywave and surface wave radars require large physical antenna apertures, but they have unique capabilities for air and surface targets, many of which are of resonant scattering dimensions. Resonant scattering from the ocean permits sea state and direction estimation. Military and commercial applications of HF radar are in their infancy.
HF‐over‐the‐horizon radar ship detection with short dwells using clutter cancelation
Authors: Root Benjamin T.
Published: February 2004

Coherent sea‐clutter cancelation is shown to be an effective method for the exposure of ships with HF OTH (high‐frequency over‐the‐horizon) sky wave radar. A sinusoidal model has been successfully used to approximate and subtract the first‐order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s). (Short dwells are required for aircraft surveillance over the vast HF radar coverage area.) The new clutter canceling algorithm has been successfully applied to both synthetic and real OTH data from the U.S. Navy's Relocatable Over‐the‐Horizon radar (ROTHR) (AN/TPS‐71), and results are presented.
Incoherent scatter radar observations of horizontal F region plasma structure over Sondrestrom, Greenland, during polar cap patch events
Authors: Pedersen T. R. , Fejer B. G. , Doe R. A. , Weber E. J.
Published: February 2004

Horizontal profiles of F region plasma density from the Sondrestrom incoherent scatter radar facility (66.99°N latitude, 50.95°W longitude) are used to determine the large‐scale structure and statistical distribution of ionospheric plasma during polar cap patch events. Largescale latitudinal gradients in average electron density were observed coincident with primarily meridional flow, suggesting the plasma decayed with time constants of the order of 1 hour. Plasma density distributions for each of the five nights in the data set were characterized by peaks at low plasma densities representing the background plasma, with the presence of patches typically resulting in a “tail” in the high‐density regime of the distribution. Observed patch sizes averaged ∼400 km, with a most probable value in the 200‐ to 300‐km range. Patch edges were found to be characterized by gradient scale lengths of ∼100 km, significantly smaller than the typical 400‐km scale length of background gradients. Gradients interior to patches were intermediate in magnitude, with average scale lengths of ∼275 km. The general characteristics of the patches in these data, including their occurrence in latitude and local time, are consistent with an origin in a tongue of ionization. The observed distributions of patch scale sizes and density enhancement ratios verify the general features of earlier results from satellite measurements. The relatively short decay time constants inferred from this data set suggest that patches do not survive long enough to make multiple passes through the convection pattern and that their impact as a source of auroral zone blobs may be mostly limited to the nightside ionosphere.
Phenomenology of transionospheric pulse pairs: Further observations
Authors: Massey Robert S. , Holden Daniel N. , Shao Xuan‐Min
Published: February 2004

We report on further observations of transionospheric pulse pairs (TIPPs), which are the most powerful transient radio signals observed by the Blackbeard broadband digital radio receiver on the ALEXIS satellite. The source of these signals is unknown but appears to be associated with thunderstorm activity. The signals do not resemble those reported for known lightning processes. We have previously reported observations of these events in the frequency band 28‐95 MHz. In this paper we report observations of TIPPs in the 117‐ to 166‐MHz band, with the subsatellite point situated over the contiguous United States. The main results are that the measured pulse parameter statistics are nearly the same as reported for the low‐frequency events, with the exception that the pulse separation distribution is biased toward smaller values in the high‐frequency observations. The radiated power does not drop off appreciably even at 166 MHz, which further constrains the possible size and timescale of the source(s). We also report results of experiments designed to measure the apparent reflectivity of dry, flat ground at frequencies around 100 MHz. We find that the apparent reflectivity can exceed 90%. This result helps to explain how the second pulse in a TIPP can have so much energy relative to the first.
Crustal‐scale structure and evolution of an arc‐continent collision zone in the southern Urals, Russia
Authors: Brown D. , Juhlin C. , Alvarez‐Marron J. , Pérez‐Estaún A. , Oslianski A.
Published: February 2004

The outcropping geology of the southern Urals contains a well‐preserved accretionary complex related to the Paleozoic collision that took place between the Magnitogorsk arc and the former East European Craton. The crustal‐scale structure of the accretionary complex has been determined from outcropping field geology that is integrated with three reflection seismic profiles. The reflection profiles show the accretionary complex to be highly reflective, allowing direct comparison of many reflections with surface geological features. We interpret the accretionary complex to be a thrust stack that is composed of shallowly subducted continental shelf and rise material, syncollisional sediments derived from the arc, deeply subducted high‐pressure gneisses that are intercalated with eclogites and blueschist, and, at the highest structural level, ophiolite complexes. It is bound at the base by a thrust and at the rear by a highly deformed zone (the Main Uralian fault) adjacent to the backstop (the Magnitogorsk arc). Deposition of the Late Devonian volcaniclastic sediments of the Zilair Formation appears to be related to collision, uplift, and erosion of the arc, possibly following the arrival of the full thickness of the East European Craton continental crust at the subduction zone. With the arrival of the continental crust at the subduction zone, offscraping and underplating of Paleozoic slope and platform material took place at the base of the accretionary complex. Uplift of the arc was followed by its collapse and the unconformable deposition of Lower Carboniferous shallow water carbonates on top of it. A time lag of 10 – 15 Myr occurred between the high‐pressure metamorphism and the subsequent arrival of the East European Craton at the subduction zone.