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Showing papers in "Journal of Geophysical Research in 1988"


Journal ArticleDOI
TL;DR: In this paper, a pigment-dependent optical model is developed to predict the propagation of visible radiant energy within the ocean or the backscattered radiation from the upper layer to be predicted as a function of the local phytoplanktonic content.
Abstract: The aim of the present study is to review and tentatively to interpret the optical behavior of oceanic case I waters, those waters for which phytoplankton and their derivative play a predominant role in determining their optical properties. Chlorophyll-like pigment concentration is used as the index to quantify the algal material (living and detrital), and statistical relationships between this index and the depth of the euphotic layer, the spectral values of the attenuation coefficient for downwelling irradiance, or the scattering coefficient are investigated. On the basis of these statistical relationships a pigment-dependent optical model is developed. It allows the propagation of the visible radiant energy within the ocean or the backscattered radiation from the upper layer to be predicted as a function of the local phytoplanktonic content. Other geophysical or geochemical applications are derived which concern the heating rate due to penetrating visible radiations or the rate of energy storage due to photosynthesis. The nonlinear trends observed in the algal biomass-attenuation relationships are analyzed by (1) considering the rather regular change of the living-to-detrital organic carbon ratio which seems to occur in oceanic waters ranging from oligotrophic to eutrophic, and (2) accounting for the respective contributions of absorption (by pigmented cells) and of scattering (by all kind of particulates) in the attenuation process of radiant energy.

1,385 citations


Journal ArticleDOI
TL;DR: In this article, a semianalytical radiance model is developed which predicts the upwelled spectral radiance at the sea surface as a function of the phytoplankton pigment concentration for Morel Case 1 waters.
Abstract: A semianalytical radiance model is developed which predicts the upwelled spectral radiance at the sea surface as a function of the phytoplankton pigment concentration for Morel Case 1 waters. The model is in good agreement with experimental measurements carried out in waters which were not included in the data base used to derive it. It suggests that the observed variability in the radiance is due to variations in the backscattering of plankton and the associated detrital material. The model is extended to include other material in the water, such as dissolved organic material, referred to as yellow substances, and detached coccoliths from coccolithophorids, e.g., Emiliana huxleyi. Potential applications include an improved biooptical algorithm for the retrieval of pigment concentrations from satellite imagery in the presence of interference from detached coccoliths and an improved atmospheric correction for satellite imagery. The model also serves to identify and to interpret deviations from Case 1 waters.

1,268 citations


Journal ArticleDOI
TL;DR: In this paper, topographic data are combined with data on structure magmatism, seismicity, and paleomagnetism to support a simple kinematical model for the late Cenozoic evolution of the central Andes.
Abstract: Topographic data are combined with data on structure magmatism, seismicity, and paleomagnetism to support a simple kinematical model for the late Cenozoic evolution of the central Andes. The model interrelates Andean uplift, a changing geometry of the subducted Nazca plate, and a changing outline of the leading edge of the South American plate. It is argued that the late Cenozoic uplift of the Andes is a result of thermal thinning of the lithosphere and crustal thickening produced by crustal shortening.

1,131 citations


Journal ArticleDOI
TL;DR: In this article, surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.
Abstract: Surface layer coefficients for wind profiles, wind stress, and heat flux in typical open sea conditions are briefly reviewed. Businger-Dyer flux-gradient relationships and a Charnock wind stress formula fit the empirical data and are dimensionally consistent. These have been solved by an iterative method, and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data.

1,101 citations


Journal ArticleDOI
TL;DR: In this article, the global climate effects of time-dependent atmospheric trace gas and aerosol variations are simulated by NASA-Goddard's three-dimensional climate model II, which possesses 8 x 10-deg horizontal resolution, for the cases of a 100-year control run and three different atmospheric composition scenarios in which trace gas growth is respectively a continuation of current exponential trends, a reduced linear growth, and a rapid curtailment of emissions due to which net climate forcing no longer increases after the year 2000.
Abstract: The global climate effects of time-dependent atmospheric trace gas and aerosol variations are simulated by NASA-Goddard's three-dimensional climate model II, which possesses 8 x 10-deg horizontal resolution, for the cases of a 100-year control run and three different atmospheric composition scenarios in which trace gas growth is respectively a continuation of current exponential trends, a reduced linear growth, and a rapid curtailment of emissions due to which net climate forcing no longer increases after the year 2000. The experiments begin in 1958, run to the present, and encompass measured or estimated changes in CO2, CH4, N2O, chlorofluorocarbons, and stratospheric aerosols. It is shown that the greenhouse warming effect may be clearly identifiable in the 1990s.

936 citations


Journal ArticleDOI
TL;DR: In this paper, the formation and evolution of large Tertiary strike-slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models.
Abstract: The processes which have governed the formation and evolution of large Tertiary strike-slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models. The steady state deformation of plasticine obeys a power creep flow law (e˙=C(T)σn). The stress exponent (n) is between 6 and 9 at 25°C. Uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening, a process observed for strain rates ranging from 3.5×10−5 to 3.6×10−3 s−1. Fault or shear zones form after only 7–10 % bulk strain. Subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine. A series of nine plane strain indentation experiments shows the influence of boundary conditions, as well as that of the internal structure of the plasticine model on the faulting sequence. The ubiquity of strain softening in experimental deformation of a variety of rocks, as well as the widespread occurrence of shear zones in nature suggest that long-term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain. The deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision-induced strikeslip faulting in Asia, particularly to total offsets and rates of movements on the faults. The experiments simulate the evolution of the western ends of the strike-slip faults, which have probably been analogous to trench-fault-fault triple junctions. The experiments also illustrate mechanisms for the formation of extensional basins, such as the South China Sea, North China Basin, and Andaman Sea, near active continental margins. The basins, which appear to absorb terminal offsets along major strike-slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts. The existence of distinct phases of strike-slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous, non-steady state deformation of the continental lithosphere.

781 citations


Journal ArticleDOI
TL;DR: In this paper, a cylindrically symmetric, constant alpha force-free magnetic field model was proposed to explain the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft.
Abstract: Magnetic clouds observed at 1 AU are modeled as cylindrically symmetric, constant alpha force-free magnetic fields. The model satisfactorily explains the types of variations of the magnetic field direction that are observed as a magnetic cloud moves past a spacecraft in terms of the possible orientations of the axis of a magnetic cloud. The model also explains why the magnetic field strength is observed to be higher inside a magnetic cloud than near its boundaries. However, the model predicts that the magnetic field strength profile should be symmetric with respect to the axis of the magnetic cloud, whereas observations show that this is not generally the case.

665 citations


Journal ArticleDOI
TL;DR: In this article, the authors describe a procedure for mapping high-latitude electric fields and currents and their associated magnetic variations, using sets of localized observational data derived from different types of measurements.
Abstract: This paper describes a novel procedure for mapping high-latitude electric fields and currents and their associated magnetic variations, using sets of localized observational data derived from different types of measurements. The technique provides a formalism for incorporating simultaneously such different classes of data as electric fields from radars and satellites, electric currents from radars, and magnetic perturbations at the ground and at satellite heights; the technique also uses available statistical information on the averages and variances of electrodynamic fields. The technique provides a more rigorous way of quantitatively estimating high-latitude electric field and current patterns than other methods and has a capability to quantify the errors in the mapped fields, based on the distribution of available data, their errors, and the statistical variances of the fields. The technique is illustrated by an application to a substorm which was analyzed by Kamide et al. (1982) by an earlier technique.

632 citations


Journal ArticleDOI
TL;DR: In this article, simultaneous ISEE-3 field and plasma data were used to examine interplanetary phenomena associated with 10 major magnetic storms detected from August 16, 1978, to December 28, 1979, in a study of Gonzalez and Tsurutani (1987), and, in particular, to determine the origins of the southward magnetic fields which caused the storms.
Abstract: Simultaneous ISEE-3 field and plasma data were used to examine interplanetary phenomena associated with 10 major magnetic storms detected from August 16, 1978, to December 28, 1979, in a study of Gonzalez and Tsurutani (1987), and, in particular to determine the origins of the southward magnetic fields which caused the storms. In nine of the 10 cases, the responsible interplanetary events were found, as expected, to be associated with the high magnetic fields in the stream-stream interaction regions (sheaths) or driver gases, with the events following the interplanetary shocks. The tenth event was found to be associated not with a high-speed stream, but with a noncompressional density-enhancement event. The results of this study indicate the equal importance of both the sheath fields or draped fields and the driver gas fields for the generation of major geomagnetic storms.

590 citations


Journal ArticleDOI
TL;DR: In this paper, the variations of the ring current energy density and composition during the great magnetic storm of February 1986 were investigated using particle measurements obtained by the charge-energy-mass instrument on the AMPTE Charge Composition Explorer spacecraft.
Abstract: The variations of the ring current energy density and composition during the great magnetic storm of February 1986 were investigated using particle measurements obtained by the charge-energy-mass instrument on the AMPTE Charge Composition Explorer spacecraft. The ring current composition of this storm, which had a complicated main phase and a minimum Dst of -312 nT on February 9, was followed for five days from the prestorm quiet time to the early recovery phase. Results suggested that the very rapid initial Dst recovery (tau of about 9.3 hrs) in this storm resulted largely from the rapid loss of 75- to 100-keV O(+) via charge exchange in the inner portion of the ring current. It is proposed that a major O(+) + N(+) ring current component generally exists near the maximum phase of great storms.

536 citations


Journal ArticleDOI
TL;DR: The characteristics of haze layers, which were visually observed over the central Amazon Basin during many of the Amazon Boundary Layer Experiment 2A flights in July/August 1985, were investigated by remote and in situ measurements, using a broad range of instrumentation and sampling equipment on board the Electra aircraft as discussed by the authors.
Abstract: The characteristics of haze layers, which were visually observed over the central Amazon Basin during many of the Amazon Boundary Layer Experiment 2A flights in July/August 1985, were investigated by remote and in situ measurements, using the broad range of instrumentation and sampling equipment on board the Electra aircraft. It was found that these layers strongly influenced the chemical and optical characteristics of the atmosphere over the eastern Amazon Basin. Relative to the regional background, the concentrations of CO, CO2, O3, and NO were significantly elevated in the plumes and haze layers, with the NO/CO ratio in fresh plumes much higher than in the aged haze layers. The haze aerosol was composed predominantly of organic material, NH4, K(+), NO3(-), SO4(2-), and organic anions (formate, acetate, and oxalate).

Journal ArticleDOI
TL;DR: In this paper, the magnitude of a hotspot swell is defined as the rate at which new topography is created, expressed as a volume per unit time, and is a measure of the buoyancy and heat fluxes in the underlying plume.
Abstract: Numerical models of plate-scale convection confined to the upper mantle predict large deviations from observed ocean bathymetry, gravity, and geoid, while whole mantle models yield first-order agreement with these observations. The upper mantle models fail because there is insufficient radioactivity in the upper mantle to explain the surface heat flux, the upper mantle must therefore be heated mainly from below, the resulting hot boundary layer generates buoyant material, and when this buoyant material rises to the base of the lithosphere, it generates large positive anomalies in topography (2 km), the geoid (30 m), and gravity (50 mGal). The concept of the magnitude of a hotspot swell is introduced: it is the rate at which new topography is created, expressed as a volume per unit time, and is a measure of the buoyancy and heat fluxes in the underlying plume. By this measure the major Pacific hots pots dominate the Earth's total plume flux, but plumes account for less than 10% of the Earth's total heat flux. This is comparable to the amount of heat likely to be coming from the core and supports the idea that plumes originate at the core mantle boundary and not at the 670-km transition. These results indicate that mantle convection is dominated by a plate-scale flow which penetrates throughout the mantle, with a secondary mode involving plumes rising from a weak thermal boundary layer at the bottom of the mantle.

Journal ArticleDOI
TL;DR: In this paper, the boundary layer chemistry over the Amazon forest during the dry season is simulated with a photochemical model and the 24-hour averaged isoprene emission flux is calculated to be 38 mg/sq m per day.
Abstract: The boundary layer chemistry over the Amazon forest during the dry season is simulated with a photochemical model. Results are in good agreement with measurements of isoprene, NO, ozone, and organic acids. Photochemical reactions of biogenic isoprene and NOx can supply most of the ozone observed in the boundary layer. Production of ozone is very sensitive to the availability of NOx, but is insensitive to the isoprene source strength. High concentrations of total odd nitrogen (NOy) are predicted for the planetary boundary layer, about 1 ppb in the mixed layer and 0.75 ppb in the convective cloud layer. Most of the odd nitrogen is present as PAN-type species, which are removed by dry deposition to the forest. The observed daytime variations of isoprene are explained by a strong dependence of the isoprene emission flux on sun angle. Nighttime losses of isoprene exceed rates of reaction with NO3 and O3 and appear to reflect dry-deposition processes. The 24-hour averaged isoprene emission flux is calculated to be 38 mg/sq m per day. Photooxidation of isoprene could account for a large fraction of the CO enrichment observed in the boundary layer under unpolluted conditions and could constitute an important atmospheric source of formic acid, methacrylic acid, and pyruvic acid.

Journal ArticleDOI
TL;DR: In this article, the seafloor is modeled as a stationary, zero-mean, Gaussian random field completely specified by its two-point covariance function, and the second moments are used as data functionals.
Abstract: At scale lengths less than 100 km or so, statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes, off-ridge tectonics and vulcanism, sedimentation, and postdepositional transport. We seek to develop methods for the estimation of seafloor statistics that take into account the finite precision, resolution, and sampling obtained by actual echo sounding systems. In this initial paper we restrict our attention to the problem of recovering second-order statistics from data sets collected by multibeam devices such as Sea Beam. The seafloor is modeled as a stationary, zero-mean, Gaussian random field completely specified by its two-point covariance function. We introduce an anisotropic two-point covariance function that has five free parameters describing the amplitude, orientation, characteristic wave numbers, and Hausdorff (fractal) dimension of seafloor topography. We formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder, in particular the along-track autocovariance functions of individual beams and the cross-covariance functions between beams of arbitrary separation. Using these second moments as data functionals, we then pose the inverse problem of estimating the seafloor parameters from realistic, noisy data sets with finite sampling and beamwidth, and we solve this inverse problem by an iterative, linearized, least squares method. The inversion method is applied to Sea Beam transit data from both the Pacific and Atlantic oceans. Sea Beam system noise stands out as a sharp spike on the along-track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle. The five parameters in our second-order model can be estimated from the inversion of data sets comprising ∼100–200 km of track length. In general, the cross-track wave number is the most poorly determined, although uncertainties in the assumed Sea Beam response may bias the values of the fractal dimension. Using the assumed beamwidth, the measured noise values, and the seafloor parameters recovered from the inversion, we generate Sea Beam “synthetics” whose statistical character can be directly compared with raw Sea Beam data. For most of the track segments we have processed thus far the synthetics are similar to the data. In the case of one Atlantic profile, however, the comparison clearly indicates the necessity of incorporating higher-order statistics. The space domain procedures described in this paper can be extended for this purpose.

Journal ArticleDOI
TL;DR: In this article, the relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NOx or nonmethane hydrocarbons (NMHCs), was studied by using a box model with particular emphasis on the nonlinearity problem of the relationship with respect to the concentration of the precursor.
Abstract: The relationship of photochemical ozone production versus photochemical loss of an ozone precursor, that is, either NOx or nonmethane hydrocarbons (NMHCs), is studied by using a box model with particular emphasis on the nonlinearity problem of the relationship with respect to the concentration of the precursor. Model calculations indicate that the composition of NMHCs, the ratio of NMHCs to NOx, and the background concentrations of natural hydrocarbons, CO, and CH4 all play important roles in determining the nonlinearity of O3 production with respect to the loss of NOx. In addition, influences on the nonlinearity due to radical loss via reactions of HO2 with RO2, exchanges between PAN and NO2, and inclusion of nighttime NOx loss processes are also investigated. Mechanisms that contribute to the nonlinearity are discussed. The nonlinear property of O3 production versus loss of hydrocarbons and CO is different from that of NOx. When the sum of CO and all hydrocarbons, including CH4, natural NMHCs, and anthropogenic NMHCs, is used as the reference O3 precursor, the nonlinearity is much less pronounced for ambient conditions usually found in rural air.

Journal ArticleDOI
TL;DR: In this article, the authors considered the concept of magnetic reconnection from a general point of view, with special consideration given to magnetic reconnections in nonvanishing magnetic fields, and showed that the electric field component parallel to the magnetic field plays a crucial physical role in finite-B reconnection.
Abstract: This paper considers the concept of magnetic reconnection from a general point of view, with special consideration given to magnetic reconnection in nonvanishing magnetic fields, i.e., finite-B reconnection as distinct from zero-B reconnection. It is shown that the electric field component parallel to the magnetic field (E-parallel) plays a crucial physical role in finite-B reconnection. Two theorems involving E-parallel are proposed. The first defines a necessary and sufficient condition of E-parallel for global reconnection to occur. The second theorem is concerned with the change of magnetic helicity to E-parallel for cases where the electric field vanishes at large distances. The general magnetic reconnection concept is tested by applying it to the process of plasmoid formation process.

Journal ArticleDOI
TL;DR: In this article, it is shown that the intensity profiles of solar protons depend on the heliolongitude, and it is suggested that the major controlling agent is the existence of an interplanetary (IP) shock.
Abstract: Data from the Goddard particle experiments on IMPs 4, 5, 7, and 8, and ISEE 3 are used to model particle intensity profiles including prompt solar particles and the effects of shocks. It is shown that the intensity profiles of solar protons depend on the heliolongitude, and it is suggested that the major controlling agent is the existence of an interplanetary (IP) shock. Shocks are strongest when observed along the radial from the source region, with the highest shock-associated intensities generally observed from central meridian flares. Using a recently derived model for the large-scale structure of IP shocks, the change in shape as a function of heliolongitude is explained.

Journal ArticleDOI
TL;DR: In this paper, the ion and electron differential energy spectra with the predictions of theoretical models were analyzed in detail, comparing the ion-electron differential energy spectrum with the predicted theoretical models.
Abstract: ISEE-1 charged-particle measurements obtained during eight plasma temperature transitions (PTTs) in 1978-1979 are compiled in tables and graphs and analyzed in detail, comparing the ion and electron differential energy spectra with the predictions of theoretical models. PTTs are defined as approximately 1-h periods of low bulk plasma velocity and steadily increasing or decreasing thermal energy. A Maxwellian distribution is found to be inadequate in describing the PTT energy spectra, but velocity-exponential and kappa distributions are both successful, the latter especially at higher energies. The power-law index kappa varies from PTT to PTT, but the high-energy spectral index and overall shape of the distribution remain constant during a PTT; both spatial and temporal effects are observed.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the distinction between the low-altitude cusp and the cleft (with the latter identified as the ionospheric signature of low-latitude boundary layer (LLBL)) on both a statistical and a case study basis.
Abstract: Particles of roughly magnetosheath energies precipitate at low altitudes throughout the dayside, in a band referred to as the cusp or cleft. Recently it has been suggested that the cusp proper is a more limited region of the cleft localized near noon, although the criteria for distinguishing between the two regions have been unclear. An investigation into the distinction between the low-altitude cusp and the cleft (with the latter herein identified as the ionospheric signature of the low-latitude boundary layer (LLBL)) was performed on both a statistical and a case study basis. One year of DMSP F7 electron and ion data, comprising in all 5609 individual dayside passes, was employed. It was found that the average energy of precipitating particles allows for a clear morphological distinction between the cusp proper and the cleft/LLBL. Often both regions are observed on a given pass at the same MLT, each with its own characteristic properties. The probability of observing the cusp was found to be sharply peaked at 1200 MLT, while the probability of observing the cleft/LLBL was near unity away from noon and had a minimum at noon. The cusp was found to be 0.8°–1.1° magnetic latitude (MLAT) thick essentially independent of MLT, whereas the cleft was thinnest at noon and widened rapidly at local times away from noon. The ion number flux in the cusp was statistically 3.6 times higher than in the cleft. The peak flux within the cusp was located on average closer to the equatorward than to the poleward boundary. Yearly average composite spectrograms of precipitation in the two regions as a function of local time show that the properties of the cusp change comparatively little with local time, but that the peak ion energy flux in the cleft increases smoothly from roughly magnetosheath values close to noon to about plasma sheet boundary layer values near 0600 MLT.

Journal ArticleDOI
TL;DR: A model for the estimation of total dust production for the United States is discussed in this paper, where the model is based on a dust emission function derived theoretically and verified by experiment and is used in the inventory of alkaline elements for use in acid/base balance studies of atmospheric precipitation by the National Acid Precipitation Assessment Program (NAPAPAP).
Abstract: A model for the estimation of total dust production for the United States is discussed. Its primary use will be in the inventory of alkaline elements for use in acid/base balance studies of atmospheric precipitation by the National Acid Precipitation Assessment Program (NAPAP). The model is a summation of the expected dust production caused by wind erosion for individual sampling units of the detailed soil and land use inventory of the National Resources Inventory, compiled by the U.S. Department of Agriculture. The model is based on a dust emission function derived theoretically and verified by experiment. An extremely important parameter is the threshold velocity for dust production; this parameter is dependent on effects of vegetative residue, roughness of the soil, live standing plants, soil texture and the effect of atmospheric precipitation. Experimentation has supplied values of this parameter for the calculation. Wind data used in the model were obtained from the Wind Energy Resource Information System (WERIS). The model was calibrated with dust emission data for the area, including the panhandles of Texas and Oklahoma.

Journal ArticleDOI
TL;DR: In this article, an analytic expression for the deformation field resulting from the inflation of a finite prolate spheroidal cavity in an infinite elastic medium is given, which is equivalent to that generated by a parabolic distribution of double forces and centers of dilatation along the sphroid generator.
Abstract: Exact analytic expressions are given for the deformation field resulting from inflation of a finite prolate spheroidal cavity in an infinite elastic medium. The field is equivalent to that generated by a parabolic distribution of double forces and centers of dilatation along the spheroid generator. Approximate, but quite accurate, solutions for a dipping spheroid in an elastic half-space are found using the half-space double force and center of dilatation solutions. We compare results of the surface deformation field with those generated by the point source ellipsoidal model of Davis (1986). In the far field both models give identical results. In the near field the finite model must be used to calculate displacements and stresses within the medium. We also test the limits of applicability of the finite model as it approaches the surface by comparing the surface displacement field from a vertical spheroid with that calculated from the finite element method. We find the model gives a satisfactory approximation to the finite element results when the minimum radius of curvature of the upper surface is less than or equal to its depth beneath the free surface. Comparison of surface displacements generated by the point and finite element models gives good agreement, provided this criterion is satisfied. We have used the finite model to invert deformation data from Kilauea volcano, Hawaii. The results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.

Journal ArticleDOI
TL;DR: In this paper, a 4 ½-day time series on the equator was used to obtain profiles of currents, density, and microstructure in the Pacific Ocean on and near the Equator at 140°W in late 1984.
Abstract: Profiles of currents, density, and microstructure were obtained in the Pacific Ocean on and near the equator at 140°W in late 1984 as part of the Tropic Heat program. During a 4½-day time series on the equator, the shear zone above the core of the undercurrent had very low mean Richardson numbers, Ri, between ¼ and ½. Average turbulent dissipation rates, e, were high in this zone, ≈ 10−7 W kg−1, and varied by a factor of 100 between minima in the early afternoon and maxima at night. The signal reached to 90 m, well into the stratified zone. Eddy coefficients, K, were low at high Ri, and gradually increased as Ri decreased, until they rose dramatically near Ri = ½. The observed K(Ri) relations are specific to time, location, and temporal and spatial resolution of the data.

Journal ArticleDOI
TL;DR: Using Bruggeman's effective medium theory, the hydraulic permeability and dc electrical conductivity of sandstone are both expressed in terms of the ratio of two microscopic lengths, which are the dimensions of a characteristic pore throat and pore chamber, respectively as discussed by the authors.
Abstract: Using Bruggeman's effective medium theory, the hydraulic permeability and dc electrical conductivity of sandstone are both expressed in terms of the ratio of two microscopic lengths, which are the dimensions of a characteristic pore throat and pore chamber, respectively. The theory is consistent with Kozeny-Carman formulas in the limit of a perfectly microscopically homogeneous pore structure. Based on the effective medium approximation, the transport properties of Fontainebleau sandstone are predicted from a quantitative study of the pore space morphology. A series of epoxy-impregnated thin sections of Fontainebleau sandstone was prepared from cores with porosity ranging between 5 and 22%. Using an image analyzer, throat and pore size distributions were constructed from the digitized and segmented microsections. For each sample the transport coefficients are calculated from the characteristic lengths, which are estimated directly from the experimental size histograms. The changes of both permeability and conductivity are predicted within a factor of 3 over the whole range of porosity. The variations of transport coefficients with porosity are interpreted from the contrasting evolution of pores and throats during diagenesis. Large pore chambers alternate with narrow passages in Fontainebleau sandstone. With decreasing porosity, some of the large pores remain stable, while the throats gradually shrink and are finally eliminated. The microscopic inhomogeneity of the pore geometry of Fontainebleau sandstone implies that its flow properties deviate from Kozeny-Carman predictions.

Journal ArticleDOI
TL;DR: In this article, a general method for constructing simple dynamical models to approximate complex dynamical systems with many degrees of freedom is described, which can be applied to interpret sets of observed time series or numerical simulations with high-resolution models, or to relate observation and simulations.
Abstract: A general method is described for constructing simple dynamical models to approximate complex dynamical systems with many degrees of freedom. The technique can be applied to interpret sets of observed time series or numerical simulations with high-resolution models, or to relate observation and simulations. The method is based on a projection of the complete system on to a smaller number of “principal interaction patterns” (PIPs). The coefficients of the PIP expansion are assumed to be governed by a dynamic model containing a small number of adjustable parameters. The optimization of the dynamical model, which in the general case can be both nonlinear and time-dependent, is carried out simultaneously with the construction of the optimal set of interaction patterns. In the linear case the PIPs reduce to the eigenoscilations of a first-order linear vector process with stochastic forcing (principal oscillation patterns, or POPs). POPs are linearly related to the “principal prediction patterns” used in linear forecasting applications. The POP analysis can also be applied as a diagnostic tool to compress the extensive information contained in the high-dimensional cross-spectral covariance matrix representing the complete second-moment structure of the system.

Journal ArticleDOI
Abstract: A dynamic source model is presented, in which a three-dimensional crack containing a viscous compressible fluid is excited into resonance by an impulsive pressure transient applied over a small area ΔS of the crack surface. The crack excitation depends critically on two dimensionless parameters called the crack stiffness, C = (b/μ)(L/d), and viscous damping loss, F = (12ηL)/(ρƒd2α), where b is the bulk modulus, η is the viscosity, ρƒ is the density of the fluid, μ is the rigidity, α is the compressional velocity of the solid, L is the crack length, and d is the crack thickness. The first parameter characterizes the ability of the crack to vibrate and shapes the spectral signature of the source, and the second quantifies the effect of fluid viscosity on the duration of resonance. Resonance is sustained by a very slow wave trapped in the fluid-filled crack. This guided wave, called the crack wave, is similar to the tube wave propagating in a fluid-filled borehole; it is inversely dispersive, showing a phase velocity that decreases with increasing wavelength, and its wave speed is always lower than the acoustic velocity of the fluid, decreasing rapidly as the crack stiffness increases. The source spectrum shows many sharp peaks characterizing the individual modes of vibration of the crack; the variation of spectral shape, both in the number and width of peaks, is surprisingly complex, reflecting the interference between the lateral and longitudinal modes of resonance, as well as nodes for these modes. The far-field spectrum is marked by narrow-band dominant and subdominant peaks that reflect the interaction of the various source modes. The frequency of the dominant spectral peak radiated by the source is independent of the radiation direction. The frequency, bandwidth, and spacing of the resonant peaks are strongly dependent on the crack stiffness, larger values of the stiffness factor shifting these peaks to lower frequencies and decreasing their bandwidth. The excitation of a particular mode depends on the position of the trigger and on the extent of the crack surface affected by the pressure transient. Fluid viscosity decreases the amplitudes of the main spectral peaks, smears out the finer structure of the spectrum, and greatly reduces the duration of the radiated signal. The energy loss by radiation is stronger for high frequencies, producing a seismic signature that is marked by a high-frequency content near the onset of the signal and dominated by a longer-period component of much longer duration in the signal coda. Such signature is in harmony with those displayed by long-period events observed on active volcanoes and in hydrofracture experiments. The very low velocity which is possible in a crack with high stiffness (C ≥ 100) also provides an attractive explanation for very long period tremor, such as type 2 tremor at Aso volcano, Japan, without the requirement of an unrealistically large magma container. The standing wave pattern set up on the crack surface by the sustained resonance in the fluid is observable in the near field of the crack, suggesting that the location and extent of the source may be estimated from the mapping of the pattern of nodes and antinodes seen in its vicinity. According to the model, the long-period event and harmonic tremor share the same source but differ in the boundary conditions for fluid flow and in the triggering mechanism setting up the resonance of the source, the former being viewed as the impulse response of the tremor generating system and the latter representing the excitation due to more complex forcing functions.

Journal ArticleDOI
TL;DR: Tropospheric concentrations of formic and acetic acids in the gas, the aerosol, and the rainwater phases were determined in samples collected 1-2 m above ground level at an open field site in eastern Virginia.
Abstract: Tropospheric concentrations of formic and acetic acids in the gas, the aerosol, and the rainwater phases were determined in samples collected 1-2 m above ground level at an open field site in eastern Virginia. These acids were found to occur principally (98 percent or above) in the gas phase, with a marked annual seasonality, averaging 1890 ppt for formate and 1310 ppt for acetate during the growing season, as compared to 695 ppt and 700 ppt, respectively, over the nongrowing season. The data support the hypothesis that biogenic emissions from vegatation are important sources of atmospheric formic and acetic acid during the local growing season. The same time trends were observed for precipitation, although with less defined seasonality. The relative increase of the acetic acid/formic acid ratio during the nongrowing season points to the dominance of anthropogenic inputs of acetic acid from motor vehicles and biomass combustion in the wintertime.

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TL;DR: In this paper, the theoretical background of magnetic reconnection including systems where the magnetic field does not vanish (general magnetic reconnections) is discussed and a critical review of the concept of magnetic merging is given, and it is shown that in certain situations there may be ideal plasma flow across a separatrix which does not involve nonideal effects.
Abstract: This paper deals with the theoretical background of magnetic reconnection including systems where the magnetic field does not vanish (general magnetic reconnection). The magnetic field is represented by Euler potentials. The time evolution of the Euler potentials as seen by the moving plasma provides an appropriate formulation of the properties of general magnetic reconnection. A critical review of the concept of magnetic merging is given, and it is shown that in certain situations there may be ideal plasma flow across a separatrix which does not involve nonideal effects. It is shown that such problems do not arise within the concept of general magnetic reconnection. It is demonstrated that general magnetic reconnection with nonvanishing magnetic field crucially depends on properties of the parallel electric field.

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TL;DR: In this article, a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle was proposed.
Abstract: Stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure/temperature environment where partial melting occurs. The amount of melt liberated is, however, insufficient to account for the thickness of igneous crust emplaced near the continent-ocean boundary at some passive margins. These igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide. Seismic studies also suggest that the transition from continental to oceanic crust on these “volcanic margins” is relatively abrupt. Their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading. To account for these observations, we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle. Deep, hot mantle material is transported upward by the convection, and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling. Numerical calculations show that “convective partial melting” can provide substantial additions to melting due to passive upwelling alone. Using this model, we propose an evolutionary scheme for “volcanic” passive margins, emphasize the contrast with “nonvolcanic” margins, and describe conditions which might lead to the development of both types.

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TL;DR: In this paper, the authors show that the flexural rigidity of the crust (or the apparent elastic thickness) provides the main control of the width of a structure and that real structures are associated with very low flexural rigidities.
Abstract: In many places, earthquakes with similar characteristics have been shown to recur. If this is common, then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures. Following this paradigm, we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault. In these models an elastic layer containing the fault overlies a viscous half-space with a different density. Fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored. Deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion. In this paper, the parameters that control the growth of dip-slip structures are identified. We find that the flexural rigidity of the crust (or the apparent elastic thickness) provides the main control of the width of a structure. The loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault. The flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character. We find that, in general, real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities.

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TL;DR: Paleomagnetic data from the African plate from the Early Jurassic onward are critically re-evaluated and updated, resulting in a set of 30 paleopoles, from which an apparent polar wander path is derived using 20 Ma intervals as mentioned in this paper.
Abstract: Paleomagnetic data from the African plate from the Early Jurassic onward are critically re-evaluated and updated, resulting in a set of 30 paleopoles, from which an apparent polar wander path is derived using 20 Ma intervals. This path is then used, in conjunction with kinematic models of the Indian and Atlantic oceans, to place other continents bordering the Indian Ocean (Eurasia, India), islands and plateaus (Madagascar, Seychelles-Mascarene), and blocks involved in the collision of India with Eurasia (south Tibet, Indochina) in an “absolute” paleogeographic frame fixed with respect to Africa. Paleogeographic maps are presented for the Early Jurassic (pre-breakup), the Jurassic-Cretaceous boundary (anomaly Ml5), the Early Cretaceous (M2), the Late Cretaceous (A 34), the Cretaceous-Tertiary boundary (A 29), the Early Eocene (A 24), the middle Eocene (A 21) and the Oligocene-Miocene boundary (A 6). Observed and predicted paleomagnetic directions for landmasses other than Africa (i.e., for western Europe, China, Korea, India, and Australia, which are not used in deriving the maps) are found to be in very satisfactory agreement. The quality of reconstructions, as measured by rms discrepancies between observed and predicted paleolatitudes, is of the order of 5°. The main features of the paleogeographic maps are discussed, with the aim of relating oceanic and on-land geodynamic events. The constraints provided by Tibetan data on the India-Asia collision, and the 20 Ma reorganization of geology and plate boundaries in a band extending from the south China Sea, through Tibet, to the Red Sea system are discussed in more detail. We emphasize the episodic nature of the breakup of Gondwanaland and formation of the Indian Ocean, and subsequent interaction with Eurasia, in which six major discrete events are identified, separated by ∼30 Ma phases of rather uniform plate motion.