bluuebunny/crossref_metadata_2025 huggingface dataset:
Variations of atmospheric methane supply from the Sea of Okhotsk induced by the seasonal ice cover
Authors: Lammers S. , Suess E. , Mansurov M. N. , Anikiev V. V.
Published: February 2004

Measurements of dissolved methane in the surface waters of the western Sea of Okhotsk are evaluated in terms of methane exchange rates and are used to assess the magnitude of seasonal variations of methane fluxes from the ocean to the atmosphere in this area. Methane concentrations northeast of Sakhalin were observed to range from 385 nmol L−1 under the ice cover in winter to 6 nmol L−1 in the icefree midsummer season. The magnitude of supersaturations indicates that this part of the Okhotsk Sea is a significant source for atmospheric methane. From the seasonal variation of the supersaturations in the surface waters it is evident that the air‐sea exchange is interrupted during the winter and methane from sedimentary sources accumulates under the ice cover. According to our measurements an initial early summer methane pulse into the atmosphere of the order of 560 mol km−2 d−1 can be expected when the supersaturated surface waters are exposed by the retreating ice. The methane flux in July is approximately 150 mol km−2 d−1 which is of the order of the average annual flux in the survey area. The magnitude of the seasonal CH4 flux variation northeast of Sakhalin corresponds to an amount of 7.3 × 105 g km−2 whereby 74% or 5.4 × 105 g km−2 are supplied to the atmosphere between April and July. For the whole Sea of Okhotsk the annual methane flux is roughly 0.13 × 1012 g (terragrams), based on the assumption that 15% of the entire area emit methane. Variations of long‐term data of atmospheric methane which are recorded at the same latitude adjacent to areas with seasonal ice cover show a regional methane pulse between April and July. The large‐scale level of atmospheric methane in the northern hemisphere undergoes an amplitudinal variation of about 25 parts per billion by volume (ppbv) which translates into approximately 36 Tg. Thus the estimated 0.6 Tg of ice‐induced methane dynamics in northern latitudes can hardly explain this seasonal signal. However, the effects of seasonal ice cover on pulsed release of methane appear strong enough to contribute, in concert with other seasonal sources, to characteristic short‐term wobbles in the atmospheric methane budget which are observed between 50°N and 60°N.
Seasonal variability in the phytoplankton community of the North Pacific Subtropical Gyre
Authors: Winn Christopher D. , Campbell Lisa , Christian James R. , Letelier Ricardo M. , Hebel Dale V. , Dore John E. , Fujieki Lance , Karl David M.
Published: February 2004

Time series measurements of in situ fluorescence, extracted particulate chlorophyll a, primary productivity, extracted adenosine 5′‐triphosphate, and fluorescence per cell, as measured by flow cytometry, demonstrate seasonal cycles in fluorescence and chlorophyll concentrations in the North Pacific Subtropical Gyre (22° 45′N, 158° 00′W). Two opposing cycles are evident. In the upper euphotic zone (0–50 m), chlorophyll a concentrations increase in winter, with a maximum in December, and decrease each summer, with a minimum in June or July. In contrast, chlorophyll a concentrations in the lower euphotic zone (100–175 m) increase in spring, with a maximum in May, and decline in fall, with a minimum in October or November. The winter increase in chlorophyll a concentration in the upper 50 m of the water column appears to be a consequence of photoadaptation in response to decreased average mixed‐layer light intensity rather than a change in phytoplankton biomass. In the lower euphotic zone, however, the seasonal cycle in pigment concentration does reflect a change in the rate of primary production and in phytoplankton biomass as a consequence of increased light intensity in summer. These observations have important implications for phytoplankton dynamics in the subtropical oceans and for remote sensing of phytoplankton biomass.
Soil carbon turnover in a recovering temperate forest
Authors: Harrison K. G. , Post W. M. , Richter D. D.
Published: February 2004

Soil radiocarbon measurements show that mineral soil carbon under a recovering temperate forest in South Carolina turns over twice as fast as carbon in undisturbed soil. The observed 12‐year turnover time influences the design and interpretation of CO2 fertilization experiments. Experiments conducted on formerly disturbed sites will show a soil carbon fertilization response considerably faster than experiments conducted on native sites. Calculating the soil carbon CO2 fertilization factor from observed increases in soil carbon requires values for the turnover time and inventory of active soil carbon. We also use the observed turnover time to estimate the rate of atmospheric carbon dioxide sequestration in soil following agricultural abandonment. Although using the observed turnover rate increases estimates of soil carbon uptake on abandoned land, the amount of carbon sequestered globally is minimal because the net area of land being abandoned is small.
The influence of air‐sea exchange on the isotopic composition of oceanic carbon: Observations and modeling
Authors: Lynch‐Stieglitz Jean , Stocker Thomas F. , Broecker Wallace S. , Fairbanks Richard G.
Published: February 2004

Although the carbon isotopic composition of ocean waters after they leave the surface ocean is determined by biological cycling, air‐sea exchange affects the carbon isotopic composition of surface waters in two ways. The equilibrium fractionation between oceanic and atmospheric carbon increases with decreasing temperature. In Southern Ocean Surface Waters this isotopic equilibration enriches δ13C relative to the δ13C expected from uptake and release of carbon by biological processes alone. Similarly, surface waters in the subtropical gyres are depleted in δ13C due to extensive air‐sea exchange at warm temperatures. Countering the tendency toward isotopic equilibration with the atmosphere (a relatively slow process), are the effects of the equilibration of CO2 itself (a much faster process). In regions where there is a net transfer of isotopically light CO2 from the ocean to the atmosphere (e.g., the equator) surface waters become enriched in 13C, whereas in regions where isotopically light CO2 is entering the ocean (e.g., the North Atlantic) surface waters become depleted in 13C. A compilation of high quality oceanic δ13C measurements along with experiments performed using a zonally averaged three‐basin dynamic ocean model are used to explore these processes.
Predicting the oceanic input of organic carbon by continental erosion
Authors: Ludwig Wolfgang , Probst Jean‐Luc , Kempe Stefan
Published: February 2004

For a large set of major world rivers we established the empirical relations existing between the observed organic carbon fluxes and the climatic, biologic, and geomorphologic patterns characterizing the river basins. These characteristics were extracted from various ecological databases. The corresponding carbon fluxes were taken from the literature. Dissolved organic carbon fluxes are mainly related to drainage intensity, basin slope, and the amount of carbon stored in soils. Particulate organic carbon fluxes are calculated as a function of sediment fluxes, which depend principally upon drainage intensity, rainfall intensity, and basin slope. Although the drainage intensity is mainly related to the amount of precipitation and to mean temperature in the basin, slope is also retained as one of the controlling factors. Our empirical models result in a total organic carbon flux to the oceans of about 0.38 Gt per year globally. About 0.21 Gt carbon (Gt C) enter the oceans in dissolved form and about 0.17 Gt C in particulate form. We further regionalize fluxes with respect to major climates, different continents, and different ocean basins. About 45 % of the organic carbon is discharged from tropical wet regions. The major part of the dissolved organic carbon is discharged into the Atlantic Ocean, while the bulk of the particulate organic carbon is discharged into the Indian and Pacific Oceans.
Simulation of proton radiation belt formation during the March 24, 1991 SSC
Authors: Hudson M. K. , Kotelnikov A. D. , Li X. , Roth I. , Temerin M. , Wygant J. , Blake J. B. , Gussenhoven M. S.
Published: February 2004

The rapid formation of a new proton radiation belt at L ≃ 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole magnetic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20–80 MeV range, primary contribution is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic conservation of the first adiabatic invariant leads to greater energization of protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations.
Geometry of interplanetary magnetic clouds
Authors: Cargill P. J. , Chen J. , Spicer D. S. , Zalesak S. T.
Published: February 2004

Two dimensional magnetohydrodynamic simulations are presented of the distortion of a magnetic flux rope that is being accelerated through ambient solar wind plasma. The flux rope magnetic field has an axial component parallel to the solar wind field and an azimuthal component, which lies in the simulation plane. As the flux rope moves through the solar wind plasma, vortices form on its trailing edge and couple strongly to its interior. If the flux rope azimuthal field is weak, it deforms into an elongated banana‐like shape after a few Alfvén transit times. A strong azimuthal field component tends to inhibit this distortion. If the flux rope is taken to model a magnetic cloud, it is suggested that the shape of the cloud at 1 AU is determined by its distortion in the inner solar wind. Distortion timescales beyond 1 AU are estimated as many days. It is estimated that effective drag coefficients somewhat greater than unity are appropriate for modelling flux rope propagation.
Solar history and paleohydrology during the last two millennia
Authors: Vita‐Finzi Claudio
Published: February 2004

Fluvial paleohydrology provides a link between solar variability and terrestrial climate during the last two millennia by revealing progressive changes in the degree of seasonality which reflect latitudinal shifts in climatic belts. A latitudinally diachronous phase of channel deposition which prevailed in AD 300–1850 in Eurasia between 45° and 25°N indicates more equable stream discharges prompted by a temporary southward displacement of the Atlantic depression zone. A similar effect in central Mexico, between 18° and 28°N, points to changes in the contribution of winter and autumn rains to the annual total which may stem from latitudinal shifts in the high‐pressure cells. The radiocarbon evidence suggests that the displacements coincided with an increase in solar activity that reached a maximum in about AD 600. The proposed association is consistent with evidence from 1948–1966 for increased frontal activity in the North Atlantic during sunspot maxima, although it is not yet clear whether the process was mediated by UV absorption in the lower stratosphere or by the flux of MeV‐GeV particles.
Enhanced cosmic‐ray production of 10Be coincident with the Mono Lake and Laschamp Geomagnetic Excursions
Authors: McHargue Lanny R. , Damon Paul E. , Donahue Douglas J.
Published: February 2004

The cosmogenic isotope 10Be, total Be, and Al were measured in partly varved sediments from the upper 50 m of core 480, leg 64 (DSDP), Gulf of California. The concentration of 10Be from 1 to 50 kyr is in general agreement with estimates of the geomagnetic dipole moment obtained from archaeomagnetic and marine core research. 10Be anomalies were also found at 32 kyr and 43 kyr, contemporaneous with the Mono Lake and Laschamp excursions, respectively. The production of 10Be required to explain these anomalies is too high, particularly for the Mono Lake excursion, to be produced by a combination of decreased geomagnetic field and unprecedented long‐term solar activity. We conclude that the cause is a change in the galactic cosmic‐ray flux consistent with a supernova event. The coincidence with the two excursions remains a paradox.
Thermal conductivity of water‐saturated rocks from the KTB Pilot Hole at temperatures of 25 to 300°C
Authors: Pribnow D. , Williams C. F. , Sass J. H. , Keating R.
Published: February 2004

The conductivitites of selected gneiss (two) and amphibolite (one) core samples have been measured under conditions of elevated temperature and pressure with a needle‐probe. Water‐saturated thermal conductivity measurements spanning temperatures from 25 to 300°C and hydrostatic pressures of 0.1 and 34 MPa confirm the general decrease in conductivity with increasing temperature but deviate significantly from results reported from measurements on dry samples over the same temperature range. The thermal conductivity of water‐saturated amphibolite decreases with temperature at a rate approximately 40% less than the rate for dry amphibolite, and the conductivity of water‐saturated gneiss decreases at a rate approximately 20% less than the rate for dry gneiss. The available evidence points to thermal cracking as the primary cause of the more rapid decrease in dry thermal conductivity with temperature. The effects of thermal cracking were also observed in the water‐saturated samples but resulted in a net decrease in room‐temperature conductivity of less than 3%. These results highlight the importance of duplicating in‐situ conditions when determining thermal conductivity for the deep crust.
Observations of ice‐sheet motion in Greenland using satellite radar interferometry
Authors: Joughin Ian R. , Winebrenner Dale P. , Fahnestock Mark A.
Published: February 2004

We present interferometric observations of ice‐sheet motion in western Greenland based on pairs of ERS‐1 synthetic aperture radar (SAR) images. These observations provide the first detailed regional view of ice motion associated with dynamically supported topography near the margin of an ice sheet. The interferograms of this area are much more complicated than other interferograms of ice sheets presented to date. We devote the largest part of this paper to explaining the source of the complexity in these interferograms. A synthetic interferogram based on a simple model helps to illustrate the effects of different components of the ice velocity field in interferometric data and suggests a method for estimating the large‐scale ice velocity field from such data.
Magma migration and magmatic solitary waves in 3‐D
Authors: Wiggins Chris , Spiegelman Marc
Published: February 2004

Numerical studies of fluid flow in the mantle suggest that magma migration is an inherently time‐dependent process that produces magmatic solitary waves from obstructions in melt flux. Previous work has considered one and two dimensional problems. Here we present the results of three dimensional calculations that utilize a new, efficient multigrid scheme. We demonstrate that one and two dimensional solitary waves are unstable and break up into sets of 3‐D solitary waves which are perfectly spherical when propagating through a uniform porosity medium. While these waves are not solitons, their non‐linear interactions are qualitatively similar. The solitary waves are highly opportunistic and establish efficient pathways for migration by linking up with nearby waves. When the initial condition is a random distribution of porosity, the porosity structure can organize into elongate, time‐dependent channels formed from chains of solitary waves. These results are natural consequences of the assumptions that the matrix is permeable and viscously deformable. We suggest that solitary waves are likely to exist in the mantle and may contribute to the episodicity of mantle magmatism.
Numerical modeling of chemically buoyant mantle plumes at spreading ridges
Authors: Feighner Mark A. , Kellogg Louise H. , Travis Bryan J.
Published: February 2004

The geometry of spreading of plumes beneath midoceanic ridges is investigated by three‐dimensional numerical modeling, with the goal of characterizing the width of the plume along the ridge, or the “waist width”. Chemically buoyant plumes are modeled, in order to compare to previously reported laboratory tank experiments. The plume is generated near the bottom of the box and rises and forms a mushroom‐head with some entrainment of surrounding fluid. The head flattens at a considerable depth beneath the ridge before rising to the surface. Once the plume reaches the surface the head is quickly divided by the diverging plates, and the waist width is found to have reached a steady‐state value, W. The results show that W is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed, which is consistent with previously reported experimental data. Our scaling law gives an independent method for estimating the volumetric flux of mantle plumes. The calculated plume fluxes are two to four times larger than previous estimates. If plume buoyancy is purely of thermal origin, then excess temperatures can be estimated from the fluxes. For Iceland, Azores and the Galápagos, the calculated excess temperatures are 140, 57, and 51°C respectively, in agreement with recent, independent estimates from modeling of gravity and bathymetry.
Results of decametric monitoring of the comet collision with Jupiter
Authors: Carr T. D. , Reyes F. , Phillips J. A. , May J. , Wang L. , Aparici J. , Alvarez H. , Olmos F. , Garcia L. , De Buizer J. M. , Greenman W. B. , Clark T. , Levy J. , Padin S. , Higgins C. A.
Published: February 2004

Decametric observations of Jupiter were made at frequencies from 16.7 to 32 MHz from the Maipu Radio Astronomy Observatory in Chile, the University of Florida Radio Observatory, and the Owens Valley Radio Observatory in California before, during, and after the collision of comet Shoemaker‐Levy 9 with the planet. No significant change in the general level of Jovian decametric activity that might be attributed to the comet was observed. However, single bursts of possibly Jovian origin appeared at two of the 12 fragment impact times during which we were observing. We are attempting to establish more definitely whether these two bursts really were Jovian, and assuming that they were, we are tentatively modeling the circumstances of their emission.
Time dependent response of equatorial ionospheric electric fields to magnetospheric disturbances
Authors: Fejer Bela G. , Scherliess Ludger
Published: February 2004

We use extensive radar measurements of F region vertical plasma drifts and auroral electrojet indices to determine the storm time dependence of equatorial zonal electric fields. These disturbance drifts result from the prompt penetration of high latitude electric fields and from the dynamo action of storm time winds which produce largest perturbations a few hours after the onset of magnetic activity. The signatures of the equatorial disturbance electric fields change significantly depending on the relative contributions of these two components. The prompt electric field responses, with lifetimes of about one hour, are in excellent agreement with results from global convection models. The electric fields generated by storm time winds have longer lifetimes, amplitudes proportional to the energy input into the high latitude ionosphere, and a daily variation which follows closely the disturbance dynamo pattern of Blanc and Richmond [1980]. The storm wind driven electric fields are responsible for the larger amplitudes and longer lifetimes of the drift perturbations following sudden decreases in convection compared to those associated with sudden convection enhancements.
Oxidation of volcanic SO2: A sink for stratospheric OH and H2O
Authors: Bekki S.
Published: February 2004

The oxidation of volcanic SO2 to sulphate in the stratosphere is investigated for an anomalously large volcanic sulphur loading (∼200 Mt) similar to that generated by the Tambora eruption in 1815. Model calculations suggest that stratospheric OH levels can be severely reduced by the gas‐phase oxidation of SO2. One implication of this is that dense volcanic SO2 clouds may last substantially longer than previously thought. SO2 oxidation is also found to lead to significant stratospheric dehydration because approximatively three molecules of H2O are consumed for each molecule of SO2 converted to sulphate. Therefore it is possible to view major volcanic eruptions as a potential sink for stratospheric H2O.
Microwave dielectric properties of snow: Modeling and measurements
Authors: Surdyk S. , Fujita S.
Published: February 2004

We investigated the dielectric loss tangent of dry snow in the 30–40 GHz range as a function of crystal size and frequency. Measurements were carried out with an open resonator method of semiconfocal type. To validate a strong fluctuation model [Stogryn, 1986], measurements and simulated results are compared. As predicted by the model, the loss tangent increases with the frequency and the crystal size. The comparison is satisfactory for snow crystal sizes below 1 mm. Above 1 mm, more measurements are needed.
Preliminary results from the Sprites94 Aircraft Campaign: 2. Blue jets
Authors: Wescott E. M. , Sentman D. , Osborne D. , Hampton D. , Heavner M.
Published: February 2004

Initial observations of a newly documented type of optical emission above thunderstorms are reported. “Blue jets,” or narrowly collimated beams of blue light that appear to propagate upwards from the tops of thunderstorms, were recorded on B/W and color video cameras for the first time during the Sprites94 aircraft campaign, June‐July, 1994. The jets appear to propagate upward at speeds of about 100 km/s and reach terminal altitudes of 40–50 km. Fifty six examples were recorded during a 22 minute interval during a storm over Arkansas. We examine some possible mechanisms, but have no satisfactory theory of this phenomenon.
Decadal‐to‐centennial‐scale climate variability: Insights into the rise and fall of the Great Salt Lake
Authors: Mann Michael E. , Lall Upmanu , Saltzman Barry
Published: February 2004

We demonstrate connections between decadal and secular global climatic variations, and historical variations in the volume of the Great Salt Lake. The decadal variations correspond to a low‐frequency shifting of storm tracks which influence winter precipitation and explain nearly 18% of the interannual and longer‐term variance in the record of monthly volume change. The secular trend accounts for a more modest ∼1.5% of the variance.
On modeling migrating solar tides
Authors: Hagan M. E. , Forbes J. M. , Vial F.
Published: February 2004

Recent updates and extensions to a steady‐state two‐dimensional linearized model of global‐scale atmospheric waves have facilitated improved calculations of the subset of those waves which are subharmonics of a solar day and propagate with the apparent motion of the sun. The model improvements are briefly described and some updated predictions of the migrating solar diurnal component are highlighted. The latter represent the first numerical modeling effort to examine the seasonal variability of the migrating diurnal harmonic as it propagates into the mesosphere and lower thermosphere.
Flow fields within the Tonga Slab determined from the moment tensors of deep earthquakes
Authors: Holt William E.
Published: February 2004

The seismic strain rates and instantaneous velocity fields within the plane of the Tonga slab are investigated using 450 Harvard centroid moment tensor (CMT) solutions from the period 1977–1992. Down‐dip seismic strain rates at the bottom of the slab are around 1 × 10−15 s−1. Spline functions are fitted to the distribution of seismic strain rates to recover the velocity gradient tensor within the plane of the slab. Solutions show a northward motion of the 200 km depth contour in the Tonga slab at rates of 8±2 mm/yr relative to the 670 km depth contour of the slab. Relative to the deep, southern portion of the Tonga slab, down‐dip velocities increase from near zero in southern Tonga to 10±2.5 mm/yr in northern Tonga, consistent with the increased subduction rate from south to north described by the Pacific‐Australia pole of rotation. Moreover, patterns of strain and velocity obtained from all events show a remarkable similarity with the patterns of strain and velocity obtained using only events with scalar moment values less than 1 × 1025 dyn cm. Along‐strike extensional strain rates in the deep zone of northern Tonga are approximately 0.5 × 10−15 s−1. This is of the same order of magnitude as the extensional strain rates associated with changes in curvature produced by the subduction of a spherical plate, suggesting that the seismic strain rates within Tonga represent a large fraction of the total strain rate budget within the seismogenic zone. An estimated maximum of 60% of the relative motion between the surface and 670 km is absorbed as strain within the slab, indicating considerable slab thickening and penetration of the slab into the lower mantle.
Further evidence of fracture‐zone induced tectonic segmentation of the Antarctic Peninsula from detailed aeromagnetic anomalies
Authors: Johnson A. C. , Swain C. J.
Published: February 2004

Aeromagnetic anomaly data collected between 67°S and 70°S crossing the Antarctic Peninsula and adjacent offshore areas show a prominent NW‐SE trend in the magnetic fabric. Apparent lateral offsets, previously recognized in the Pacific Margin Anomaly, have been mapped in detail and are shown to be much smaller than previously suggested. A 35 km wide zone of subdued magnetic anomalies at the Western edge of the Pacific Margin Anomaly, bounded by these apparent offsets, is interpreted as a downfaulted block of the mafic‐intermediate batholith thought to be responsible for the Pacific Margin Anomaly. The trends of both fracture zones and magnetic lineaments strongly support the link between faulting in the Antarctic Peninsula magmatic arc and offshore tectonics.
Toroidal/poloidal partitioning of global post‐seismic deformation
Authors: Sabadini Roberto , Piersanti Antonio , Spada Giorgio
Published: February 2004

An Earth model subject to dislocations is studied in order to investigate the toroidal/poloidal content of global post‐seismic deformation. Differently from previous analyses, our approach allows us to deal with some of the main complexities of the real Earth, such as sphericity, self‐gravitation, and rheological stratification. The time‐dependent ratio between toroidal and poloidal displacements is evaluated in the near and far‐field of both strike and dip‐slip lithospheric sources characterized by a finite length. Our findings show that post‐seismic deformation changes its nature from poloidal to toroidal (and viceversa) on time‐scales of several decades and over distances comparable with the dimensions of the lithospheric plates. The effects on horizontal velocities in North America of both poloidal and toroidal components induced by the Alaska 1964 earthquake is comparable with the signatures due to postglacial rebound; these two geodynamical phenomena are currently detectable by variations along baselines connecting VLBI stations.
Production of dicarboxylic acids in the Arctic atmosphere at polar sunrise
Authors: Kawamura Kimitaka , Kasukabe Hideki , Yasui Osamu , Barrie Leonard A.
Published: February 2004

An homologous series of low molecular weight dicarboxylic acids (C2–C11) have been measured in the arctic aerosol samples collected from July 1987 to June 1988 in Alert. Oxalic acid (C2) was found as the dominant species followed by malonic (C3) or succinic (C4 acid. At the time of polar sunrise, certain dicarboxylic acids in arctic haze aerosols became 5 to 20 times more abundant than in the preceding dark winter months. Furthermore, they are correlated with tropospheric ozone depletion events marked by particulate bromine. Here, we report, for the first time, observations of dicarboxylic acids in the polar atmosphere and discuss the formation of these organic acids in terms of photochemical oxidation of pollutant precursors that reach the Arctic by long range atmospheric transport from mid‐latitudes.
The transition from weakening to strengthening in dehydrating gypsum: Evolution of excess pore pressures
Authors: Ko Suz‐chung , Olgaard David L. , Briegel Ueli
Published: February 2004

Dehydration reactions can induce excess pore‐fluid pressures that are high enough to affect the mechanical strength and ductility of rocks. Laboratory experiments have been conducted to investigate this phenomenon in dehydrating rocks with porosities that are initially negligible but increase as the reaction progresses. Polycrystalline gypsum samples were first heated under pressure to induce dehydration, then axially loaded after different amounts of reaction. Within the first 1% of reaction, the ultimate strength of the sample decreased and embrittlement was observed. The ultimate strength recovered within 3% of reaction and eventually exceeded that of pure gypsum. Our results indicate that the initial weakening and embrittlement correspond to a pulse of excess pore pressure in the sample interior. Subsequent strengthening is caused by a decrease in pore pressure and the higher strength of the product phase.
The collision of comet Shoemaker‐Levy 9 with Jupiter: Detection and evolution of HCN in the stratosphere of the planet
Authors: Marten A. , Gautier D. , Griffin M. J. , Matthews H. E. , Naylor D. A. , Davis G. R. , Owen T. , Orton G. , Bockelée‐Morvan D. , Colom P. , Crovisier J. , Lellouch E. , de Pater I. , Atreya S. , Strobel D. , Han B. , Sanders D. B.
Published: February 2004

We report submillimeter heterodyne observations of Jupiter taken with the JCMT during and after the infall of Comet Shoemaker‐Levy 9 into the planet. We detected the J = 4 ‐ 3 and J = 3 ‐ 2 rotational transitions of HCN in emission at many of the impact sites. Measurements suggest for fragment G a mixing ratio of ∼ 5 × 10−8 above the 0.5‐mbar pressure level and a total HCN mass of 6 × 1011g. Subsequent observations, made in September and November 1994, reveal that HCN is still present but that the lines now appear in absorption. This results from a cooling of the stratospheric thermal profile between July and September. Chemical implications of the observed persistence of HCN in the Jovian stratosphere for over 6 months are discussed.
Severe convective weather in the context of a nighttime global warming
Authors: Dessens Jean
Published: February 2004

The recent finding that most of the global warming observed these past decades is due to an increase of the nighttime temperature may have important implications on severe storms occurrence. Indeed, the daily minimum temperature which is generally recorded in the early morning is an approximation of the wet bulb potential temperature observed during the following afternoon, which is a storm predictor. A mean minimum temperature increase in a region will then probably be accompanied by an increase in the storm frequency. This hypothesis is tested in France for the years 1946 to 1992 with the annual mean minimum temperature in summer being compared to a yearly hail severity index deduced from hail insurance data. The two elements are found to be year‐to‐year correlated, and the correlation gives a 40% increase in the hail damage for a 1°C increase in the mean minimum temperature.
Spatial symmetry breaking in rapidly rotating convective spherical shells
Authors: Zhang Keke , Schubert Gerald
Published: February 2004

Many problems in geophysical and astrophysical convection systems are characterized by fast rotation and spherical shell geometry. The combined effects of Coriolis forces and spherical shell geometry produce a unique spatial symmetry for the convection pattern in a rapidly rotating spherical shell. In this paper, we first discuss the general spatial symmetries for rotating spherical shell convection. A special model, a spherical shell heated from below, is then used to illustrate how and when the spatial symmetries are broken. Symmetry breaking occurs via a sequence of spatial transitions from the primary conducting state to the complex multiple‐layered columnar structure. It is argued that, because of the dominant effects of rotation, the sequence of spatial transitions identified from this particular model is likely to be generally valid. Applications of the spatial symmetry breaking to planetary convection problems are also discussed.
An extended plasma model for Saturn
Authors: Richardson John D.
Published: February 2004

The Saturn magnetosphere model of Richardson and Sittler [1990] is extended to include the outer magnetosphere. The inner magnetospheric portion of this model is updated based on a recent reanalysis of the plasma data near the Voyager 2 ring plane crossing. The result is an axially symmetric model of the plasma parameters which is designed to provide accurate input for models needing either in situ or line‐of‐sight data and to be a useful tool for Cassini planning.
Time‐domain approach for the transient responses in stratified viscoelastic Earth models
Authors: Hanyk L. , Moser J. , Yuen D. A. , Matyska C.
Published: February 2004

We have developed the numerical algorithm for the computation of transient viscoelastic responses in the time domain for a radially stratified Earth model. Stratifications in both the elastic parameters and the viscosity profile have been considered. The particular viscosity profile employed has a viscosity maximum with a contrast of O(10²) in the mid lower mantle. The distribution of relaxation times reveals the presence of a continuous spectrum situated between O(10²) and O(104) years. The principal mode is embedded within this continuous spectrum. From this initial‐value approach we have found that for the low degree harmonics the non‐modal contributions are comparable to the modal contributions. For this viscosity model the differences between the time‐domain and normal‐mode results are found to decrease strongly with increasing angular order. These calculations also show that a time‐dependent effective relaxation time can be defined, which can be bounded by the relaxation times of the principal modes.
Porosity of natural fracture networks
Authors: Montemagno Carlo D. , Pyrak‐Nolte Laura J.
Published: February 2004

Volumetric imaging is used to determine the three‐dimensional topology of natural fracture networks in coal subjected to reservoir conditions. Computerized X‐ray tomographic data and bulk porosity measurements are combined with image processing techniques to quantify the porosity distribution of three‐dimensional fracture networks in two coal cores. Under in‐situ conditions, the porosity of the fracture networks in the cores is less than one‐tenth of a percent. In addition, the porosity varies by ±10% over depth intervals of approximately 16 mm.
Dynamical consequences on fast subducting slabs from a self‐regulating mechanism due to viscous heating in variable viscosity convection
Authors: Larsen Tine B. , Yuen David A. , Malevsky Andrei V.
Published: February 2004

We have studied 2‐D time‐dependent convection for a rheology which is both non‐Newtonian and temperature‐dependent. Strong effects associated with viscous heating are found in the downwelling sheets, which are heated on both sides with an intensity around O(10²) times the chondritic value. The magnitude of viscous heating increases strongly with the subduction speed. The slab interior is weakened by viscous heating and slab breakoff then takes place. This process provides a self‐regulating mechanism for governing the speed of intact slabs able to reach the deep mantle. Timescales associated with viscous heating are quite short, a few million years. Internal heating by radioactivity decreases the amount of shear heating.
A note on the recent increase of Solar UV‐B radiation over northern middle latitudes
Authors: Zerefos C. S. , Bais A. F. , Meleti C. , Ziomas I. C.
Published: February 2004

In this work we present the results from the analysis of regular solar UV‐B spectral measurements obtained at Thessaloniki (40°N) during a period of three years, from November 1990 through November 1993. The results presented here refer to both clear‐skies (cloudiness ≤3/8) as well as to all‐skies conditions, and to solar zenith angles of 63°±1°. These results support earlier findings of increased solar UV‐B irradiances, under all‐skies conditions, associated with observed ozone decline at the same period. The observed acceleration of the ozone decline‐of about 5% per year‐during the three‐year period resulted to increased solar UV irradiance levels at Thessaloniki by 12.3% per year at 305 nm and 2.7% per year at 325 nm. These figures are slightly modified under all‐skies conditions, and they are in agreement with findings reported independently by other scientists.
Galileo infrared observations of the Shoemaker‐Levy 9 G Impact Fireball: A Preliminary report
Authors: Carlson R. W. , Weissman P. R. , Segura M. , Hui J. , Smythe W. D. , Johnson T. V. , Baines K. H. , Drossart P. , Encrenaz Th. , Leader F. E.
Published: February 2004

The Galileo spacecraft was fortuitously situated for a direct view of the impacts of the fragments of comet Shoemaker‐Levy 9 in Jupiter's atmosphere. The Galileo Near Infrared Mapping Spectrometer (NIMS) instrument observed several of the impact events in several discrete bands and with a temporal resolution of roughly five seconds. Data have been received for the G impact showing two phases of strong infrared emission. The first phase is approximately one minute in duration and corresponds to the initial fireball and early plume development. This is followed six minutes later by the onset of heating by plume ejecta falling back on the upper atmosphere. This report provides a preliminary description of the fireball phase. The first detection of the G fireball occurred at 07:33:37 UT on July 18, 1994, approximately five seconds after the initial signal recorded by the Galileo Photopolarimeter‐Radiometer (PPR) instrument. The preceding NIMS measurement, occurring approximately one second before the initial PPR signal, showed no evidence of fireball emission. The detected duration of the fireball at 4.38 µm was 70 seconds. Spectra in the first half of this period show blackbody‐like emission, with absorption features from overlying methane and molecular hydrogen. The strengths of these features place the fireball in the upper troposphere and lower stratosphere, above the ammonia cloud layer. The emitting surface rises and accelerates, achieving a velocity of 2–3 km/sec after 25 seconds, in qualitative agreement with that expected for an explosion in an inhomogeneous atmosphere.

The Galileo spacecraft, en route to Jupiter, was in a position to obtain a direct view of the impacts of Comet Shoemaker‐Levy 9 fragments on the nightside of Jupiter, providing an opportunity to investigate the early temporal evolution of the impact events. It was predicted that the comet fragments would produce high temperature bolides as they entered the atmosphere and then explode, producing hot fireballs which would rise, expand, and cool (Sekanina, 1993; Zahnle and Mac Low, 1994; Chevalier and Sarazin, 1994; Ahrens et al., 1994; Boslough et al., 1994). Much of the predicted radiation occurs in the infrared region, and time‐resolved infrared spectral observations, obtained over a broad wavelength range, are ideal for studying these phenomena.
Convectively induced transition in mantle rheological behavior
Authors: van den Berg Arie P. , Yuen David A.
Published: February 2004

Mantle flow occurs through both linear and nonlinear creep mechanisms and the style of convection is strongly influenced by the dominant creep mechanism in a composite rheology. We show here by numerical modelling of convection with a composite flow law that the nonlinear, strain‐rate dependence of mantle rheology would dominate over the linear (Newtonian) portion for sufficiently vigorous convection such as might have prevailed in the early Earth. This rheological transition occurs at a Rayleigh number value which increases dramatically with higher values of the activation energy. For activation energy values for dislocation and diffusion creep of about 100 and 70 kJ/mole respectively we find a transition between Rayleigh numbers 106 and 107. Corresponding values of the transition‐stress agree with estimates for the lower mantle from empirical data for perovskite analogue. These results suggest that early in the Earth's thermal history a transition would have taken place from a primarily non‐Newtonian flow to a Newtonian, temperature‐dependent style of mantle convection. With a high activation energy, greater than 125 kJ/mole, the mantle would have been in a Newtonian regime throughout most of its thermal history.
Ponderomotive saturation of magnetospheric field line resonances
Authors: Rankin R. , Frycz P. , Tikhonchuk V. T. , Samson J. C.
Published: February 2004

Compressional Alfvén waves in the terrestrial magnetospheric cavity constitute a discrete spectrum of global modes which can resonate with specific components of the continuum spectrum of shear Alfvén wave (SAW) field line resonant frequencies. We investigate the effect of the ponderomotive force (PF) of excited standing SAWs on the nonlinear saturation of field line resonances (FLRs). In low β plasmas FLR saturation occurs due to a nonlinear phase slip between the fast mode driver and SAW field. Ponderomotive forces also lead to density depletions at the ionospheres, nonlinear narrowing of the FLR and meridional scales comparable to those embedded within temporally modulated discrete auroral arcs. Observational features relating to these effects are discussed.
Cenozoic plate driving forces
Authors: Lithgow‐Bertelloni C. , Richards Mark A.
Published: February 2004

Past studies of plate driving forces have concluded that the forces due to subducted slabs in the upper mantle and those due to the thickening of the oceanic lithosphere are the principal driving forces. We reexamine the balance of driving forces for the present‐day and extend our analysis through the Cenozoic, using an analytical torque balance method which accounts for interactions between plates via viscous coupling to the induced mantle flow. We use an evolving mantle density heterogeneity field based on the last 200 Myr. of subduction to drive plate motions, an approach which has proven successful in predicting the present‐day mantle heterogeneity field. We find that for plausible upper mantle viscosities the forces due to subducted slabs in the Cenozoic and Mesozoic account for in excess of 90% of plate driving forces and those due to lithospheric thickening for less than 10%.
Evidence of active extension in Quaternary volcanoes of central Italy from breakout analysis and seismicity
Authors: Montone P. , Amato A. , Chiarabba C. , Buonasorte G. , Fiordelisi A.
Published: February 2004

We present active stress directions obtained from borehole breakout analysis performed on 15 geothermal wells located in the western coastal regions of Central Italy. The study area (a 200 km by 50 km NW‐elongated area bordering the Apennines) includes several Quaternary high‐K alkaline volcanoes active mainly after 0.6 Ma. We analyzed both paper logs and digital data to detect breakout directions, the two techniques yielding similar results. The breakout results show a predominant ENE direction of SHmin, with local deviations in one region (Sabatini volcano) where no seismicity is observed. The comparison of breakout data with stress directions inferred from inversion of microearthquake (M<4) focal mechanisms computed in three of the four volcanoes suggests that the whole area is presently undergoing NE to ENE extension.
Direct observations of the comet Shoemaker‐Levy 9 fragment G impact by Galileo UVS
Authors: Hord C. W. , Pryor W. R. , Stewart A. I. F. , Simmons K. E. , Gebben J. J. , Barth C. A. , McClintock W. E. , Esposito L. W. , Tobiska W. K. , West R. A. , Edberg S. J. , Ajello J. M. , Naviaux K. L.
Published: February 2004

The Galileo Ultraviolet Spectrometer (UVS) team has detected the Shoemaker‐Levy 9 fragment G impact on Jupiter in data recently played back from the spacecraft tape recorder. A 20% brightening of the disc‐integrated signal of Jupiter was detected at 292 nm during a swath across Jupiter that lasted 1.6 sec and was centered at 1994‐July 18 (day 199)/07:33:31 UT (all times in this paper are corrected to be the time of the event as seen from Earth). The emission brightness, when combined with simultaneous Photopolarimeter Radiometer (PPR) measurements at 945 nm, is consistent with thermal radiation at a temperature of 7800 (+500, −600) K emitted over an area of 40 (+60, −25) km². No excess signal was seen during swaths 5 1/3 sec before and after the detection swath.
Estimates of comet fragment masses from impact crater chains on Callisto and Ganymede
Authors: McKinnon William B. , Schenk Paul M.
Published: February 2004

Chains of impact craters, or catenae, have been identified in Voyager images of Callisto and Ganymede. Although these resemble in some respects secondary crater chains, the source craters and basins for the catenae cannot be identified. The best explanation, proposed by Melosh and Schenk, is a phenomenon similar to that displayed by former comet P/Shoemaker‐Levy 9: tidal (or other) breakup close to Jupiter followed by gradual orbital separation of the fragments and collision with a Galilean satellite on the outbound leg of the trajectory. Because the trajectories must pass close to Jupiter, this constrains the impact geometry (velocity and impact angle) of the individual fragments. For the dominant classes of impactors, short‐period Jupiter‐family comets and asteroids, velocities at Callisto and Ganymede are dominated by jovian gravity and a satellite's orbital motion, and are insensitive to the pre‐fragmentation heliocentric velocity; velocities are insensitive to satellite gravity for all impactor classes. Complex crater shapes on Callisto and Ganymede are determined from Voyager images and Schmidt‐Holsapple scaling is used to back out individual fragment masses. We find that comet fragment radii are generally less than ∼500 m (for ice densities), but can be larger. These estimates can be compared with those for the Shoemaker‐Levy 9 impactors.
Global equatorial variability of 850 and 200 hPa zonal winds from rawinsondes between 1963 and 1989
Authors: Moron Vincent , Fontaine Bernard , Roucou Pascal
Published: February 2004

The longitude‐height‐time variability of 3‐month averaged zonal wind anomalies at 850 and 200 hPa over the equatorial area (5°N‐5°S) is analyzed using a three‐dimensionnal dataset constructed from rawinsonde data (1963–1989). The first mode, closely related to the Southern Oscillation Index, suggests a strong vertical coupling associated with a horizontal out‐of‐phase pattern between the central/western Pacific and the remainder of the equatorial belt. The vertical coupling appears to be phase‐locked to the annual cycle with strongest intensities found over South America and near the maritime continent early in the calendar year and over the Pacific basin and Africa during the second half of the year. This mode of variability can be viewed as a standing pattern superimposed with an eastward‐migrating component, coherent with the annual cycle. This westerly moving mode originates near the maritime continent during the northern autumn, and tends to precede El Nino/Southern Oscillation events in the central and eastern Pacific Ocean. Variance is mainly concentrated in the 3–8 year low‐frequency time scale throughout the tropics, and in the 2–3 year quasi‐biennal band from 110°–120°E to 180°.