bluuebunny/crossref_metadata_2025 huggingface dataset:
Energetic ion outflow from the dayside ionosphere: Categorization, classification, and statistical study
Authors: Øieroset Marit , Yamauchi Masatoshi , Liszka Ludwik , Hultqvist Bengt
Published: September 2002

The energetic ion outflow (40 eV – 1.2 keV) from the dayside ionosphere has been investigated using 2 weeks (70 orbits) of Viking ion data when the AE index showed little dependence on the interplanetary magnetic field (IMF) Bz component. The data cover altitudes between 6000 and 13,500 km, 0600–1800 magnetic local time (MLT), and 65°–90° invariant latitude (ILAT). We present an automated algorithm using principal component analysis to categorize and classify the upflowing ions into beams, conies, and hybrids. Conies dominate the ion outflow (number flux) in the cusp region, while beams are the main contributor to the outflow prenoon and postnoon outside the cusp region. Generally, the highest average number flux is found in the cusp region. The outflow intensity and the region of energetic ion outflow are different for positive and negative IMF Bz. However, this difference is not as strong as the difference in outflow intensity and active region observed for AE <50 and AE >200. The results imply that the dayside energetic ion outflow is controlled partly by nightside (driven by the release of stored energy in the magnetotail, as observed in the AE index) and partly by dayside (directly driven, as observed in the polarity in the IMF Bz component) activity. The effect of the directly driven component (IMF Bz) is strongest in the cusp and the postnoon region.
Composition of interstellar gas and dust
Authors: Snow Theodore P.
Published: September 2002

The composition of interstellar dust is normally inferred indirectly, from analyses of gas‐phase abundances and an assumed total interstellar medium composition, providing data on the depletions of elements from the gas. Additional information comes from infrared observations of solid‐state absorption bands (usually applicable only to dense clouds), from X‐ray absorption and scattering measurements, and from the analysis of components of interplanetary dust particles. This review is focused on issues related to interstellar gas‐phase abundances and depletions, and the question of appropriate reference abundance standards. The paper includes an overview of gas‐phase abundance measurements, a summary of depletions as derived from the adoption of solar or stellar reference abundances, comments on the impact of depletion measures on the inferred dust composition, and a discussion of alternatives to the adoption of solar or stellar reference abundances. The paper concludes with a description of new instruments, soon to be in operation, that should help to resolve some of the issues.
Nonlinear interactions between planetary waves in the mesosphere/lower‐thermosphere region
Authors: Pancheva D. , Beard A. G. , Mitchell N. J. , Muller H. G.
Published: September 2002

A meteor radar has been used to measure horizontal winds during 1992 and 1993 in the mesosphere/lower‐thermosphere region over the England at 54.5°N, 3.9°W. Evidence is found of quasi‐periodic modulation of the amplitudes of planetary waves. Bispectral analysis applied to the data reveals a wealth of interactions occurring between different members of the population of planetary waves. The results of the bispectral analysis strongly support the idea that the mechanism of interaction is the quadratic nonlinear process proposed by Teitelbaum and Vial [1991] to account for tidal/planetary‐wave interactions. The interactions are strongest in summer between the 2‐day wave and waves with periods of about 10 and about 16 days. A number of secondary waves are produced in the interactions, and these could explain the apparent splitting of the 2‐day wave into a number of closely spaced frequencies. Considerable interannual variability in the waves and interactions is evident.
On the effects of solar wind dynamic pressure on the anisotropic terrestrial magnetosheath
Authors: Farrugia C. J. , Erkaev N. V. , Biernat H. K.
Published: September 2002

We apply our recent three‐dimensional anisotropic MHD model of magnetosheath flow [Erkaev et al., 1999] to study quantitatively effects of solar wind dynamic pressure (Pd∞ ) and Alfvén Mach number (Ma∞) on the anisotropic magnetosheath and the plasma depletion layer (PDL) in the subsolar region. Given the wide range over which these two parameters vary, their influence on the magnetosheath structure may be significant. Our analysis is applicable to quasi‐steady changes in the interplanetary medium. Following our earlier work, and in general agreement with the data, we define the sunward edge of the PDL by β‖ = 1, where β‖ is the proton beta parallel to the magnetic field. We first discuss changes in Pd∞ occurring under constant Ma∞. In this case, a rescaling of the parameters yields the effects on the magnetosheath. We then study quantitatively a changing dynamic pressure through a varying Alfvén Mach number. We obtain profiles of key magnetosheath parameters and the width of the PDL for Alfvén Mach numbers representative of the solar wind at Earth orbit. Gradients in parameter profiles become steeper and shift toward the magnetopause as Ma∞ increases. We find that PDL width varies as
even in the anisotropic magnetosheath. Using our model to study the magnetosheath location where the electromagnetic ion cyclotron wave (EICW) instability dominates over the mirror instability, we find that this location occurs well inside the PDL. In addition, we estimated the fraction of the PDL width occupied by the EICWs as a function of solar wind Alfvén Mach number. We conclude that the EICW regime is contained in, but is not co‐extensive with, the PDL. Examining critically this issue by changing the PDL identification criterion to others based on a density decrease relative to the bow shock value and a systematic drop toward the magnetopause, we find that this result still holds, but the region where EICWs are destabilized occupies a different fraction of the PDL thus defined. Some model results are compared with documented data from an inbound crossing of the magnetosheath made on December 24, 1994. Good agreement with model predictions are obtained.
Analysis of a deuterium‐rich interplanetary dust particle (IDP) and implications for presolar material in IDPs
Authors: Keller Lindsay P. , Messenger Scott , Bradley John P.
Published: September 2002

Deuterium (D)‐rich interplanetary dust particles (IDPs) are the most primitive extraterrestrial materials available for laboratory studies in terms of their mineralogy, chemistry, and isotopic compositions. Transmission electron microscopy analysis of one such D‐rich IDP shows it to be a highly porous object that consists of crystalline grains (Mg‐rich pyroxene and olivine, and FeNi sulfides) and glass with embedded metal and sulfides (GEMS) enclosed within a matrix of amorphous carbonaceous material. The nonsolar H isotopic anomaly measured in this IDP provides direct evidence for the incorporation of partially preserved cold molecular cloud material that predates the solar system. These results indicate that organic molecules associated with the carbonaceous matrix of the IDP are the most likely D carrier phase in the particle. GEMS are a major constituent of this D‐rich IDP. The physical and chemical characteristics of GEMS show many similarities to the properties of interstellar amorphous silicates inferred from astronomical observations including: size (diameters of 0.1–0.5 um), solar abundances for heavy elements, presence of superparamagnetic FeNi metal, and an amorphous silicate matrix. Infrared transmission spectra from GEMS‐rich IDPs show marked similarities to astronomical data for interstellar silicates. The close association of GEMS and mineral grains to D‐rich matter of probable interstellar origin suggests that these inorganic materials are themselves interstellar grains.
On spatial and temporal structures in the cusp
Authors: Trattner K. J. , Fuselier S. A. , Peterson W. K. , Sauvaud J.‐A. , Stenuit H. , Dubouloz N. , Kovrazhkin R. A.
Published: September 2002

We identify a series of magnetic conjunctions and close encounters of polar‐orbiting satellites to investigate cusp features including flux variations and sudden energy changes of cusp precipitating ions. The variations and changes are generally interpreted as signatures of variations in the reconnection rate and/or changes in the de Hoffman‐Teller velocity at the magnetopause. They are expected to be temporal features which move poleward with convecting field lines. However, our analysis shows that a series of cusp features observed on one satellite are remarkably similar to cusp features observed up to 1.5 hours later by a second satellite. This similarity suggests that these features are spatial structures and not temporal variations in reconnection parameters.
A four‐fluid turbulence‐driven solar wind model for preferential acceleration and heating of heavy ions
Authors: Hu You Qiu , Esser Ruth , Habbal Shadia R.
Published: September 2002

We present for the first time a one‐dimensional, four‐fluid turbulence‐driven solar wind model in order to investigate the preferential acceleration and heating of heavy ions by the resonant cyclotron interaction with parallel‐propagating left‐hand‐polarized ion cyclotron waves. The model contains four species: electrons, protons, alpha particles, and one species of minor ions. A Kolmogorov type of cascade effect is introduced to transfer energy from the low‐frequency Alfvén waves to the high‐frequency ion cyclotron waves, which are assumed to be entirely dissipated by the wave‐particle interaction. The quasi‐linear theory of the wave‐particle interaction is invoked to distribute the dissipated wave energy among the three ion species based on a given power law spectrum of the ion cyclotron waves and the cold plasma dispersion relation. It is found that in terms of the cold plasma dispersion relation, the dispersion generated by all ion species has an appreciable influence on both the behavior of the major species and the preferential acceleration and heating of the minor ions. The larger the number of species included in the dispersion relation is, the stronger preferential acceleration and heating produced by the waves for the heavy ions close to the Sun will be. A detailed comparison is carried out between two cases, one with and the other without the dispersive effect of the minor ions. Although the solutions for the two cases are somewhat different, they predict a more or less similar behavior of the minor ions, which essentially agrees with recent observations from SOHO. This indicates that the resonant cyclotron interaction may be responsible for the preferential acceleration and heating of minor ions in the fast solar wind. Furthermore, the influence of minor ions on the proton‐alpha solar wind is found to be dominated by the dispersive effect of the minor ions. Even though such an influence is exaggerated by the cold plasma dispersion relation, it is still small and remains within the present observational uncertainties. Therefore minor ions may be treated approximately as test particles in the solar wind.
A model for generating relativistic electrons in the Earth's inner magnetosphere based on gyroresonant wave‐particle interactions
Authors: Summers Danny , Ma Chun‐yu
Published: September 2002

During the recovery phase of a magnetic storm, fluxes of relativistic (>1 MeV) electrons in the inner magnetosphere (3 ≤ L ≤ 6) increase to beyond prestorm levels, reaching a peak ∼4 days after the initiation of the storm. In order to account for the generation of these “killer electrons” a model is presented primarily on the basis of the stochastic acceleration of electrons by enhanced whistler mode chorus. In terms of a quasi‐linear formulation a kinetic (Fokker‐Planck) equation for the electron energy distribution is derived comprising an energy diffusion coefficient based on gyroresonant electron‐whistler mode wave interaction and parallel wave propagation, a source term representing substorm‐produced (lower‐energy) seed electrons, and a loss term representing electron precipitation due to pitch angle scattering by whistler mode waves and electromagnetic ion cyclotron (EMIC) waves. Steady state solutions for the electron energy distribution are constructed and fitted to an empirically derived relativistic Maxwellian distribution for the high‐energy “hard” electron population at geosynchronous orbit. If the average whistler amplitude is sufficiently large, for instance, 75–400 pT, dependent on the values of the other model parameters, and assuming a background plasma density of N0 = 10 cm−3 outside the plasmasphere, then a good fit to the empirical distribution is obtained and corresponds to a timescale for the formation of the high‐energy steady state distribution of 3–5 days. For a lower representative value of the background plasma density, N0 = 1 cm−3, smaller whistler amplitudes, in the range 13–72 pT, can produce the high‐energy distribution in the required time frame of several days. It is concluded from the model calculations that the process of stochastic acceleration by gyroresonant electron‐whistler mode wave interaction in conjunction with pitch angle scattering by EMIC waves constitutes a viable mechanism for generating killer electrons during geomagnetic storms. The mechanism is expected to be particularly effective for the class of small and moderate storms possessing a long‐lasting recovery phase during which many substorms occur.
Spectral properties of low‐frequency electrostatic waves in the ionospheric E region
Authors: Krane B. , Pécseli H. L. , Trulsen J. , Primdahl F.
Published: September 2002

The spectral properties of low frequency electrostatic waves in the polar cap E region over northern Scandinavia were studied experimentally by instruments on the Rocket and Scatter Experiment (ROSE) rockets. Fluctuations in plasma density were detected as well as potential differences between boom‐mounted probes. By comparison of the spectral index for fluctuations in the potential signal and plasma density, evidence is obtained for deviations from Boltzmann distributions in the electron dynamics, which would predict fluctuations in density and potential to be proportional, with the same constant of proportionality at all frequencies. Investigations of the cross correlation between density and potential signals demonstrate that the phase between the two increases approximately linearly with frequency. Empirical relations are obtained for the frequency dependence of the amplitude and phase relations between fluctuations in density and potential.
Porosity‐formation factor and porosity‐velocity relationships in Barbados prism
Authors: Erickson Stephanie N. , Jarrard Richard D.
Published: September 2002

Ocean Drilling Program Leg 171A collected logging while drilling (LWD) data to investigate the porosity and velocity structure of Barbados accretionary complex. Porosity and velocity measurements were needed for interpretation of prism dynamics. However, the LWD tool string did not measure velocity, and previous velocity data from Barbados prism are scarce and generally of low quality. Furthermore, most density‐based porosities are subject to errors associated with variations in smectite content. To determine porosities more accurately, we use smectite‐corrected, density‐based porosities to calibrate a porosity‐formation factor (FF) transform, then calculate porosities from resistivity logs. We used cation exchange capacity (CEC) measurements and LWD logs to evaluate the possibility that resistivity is affected by clay conduction. The CEC data warn of potential clay conduction, especially in smectite‐rich zones, but LWD data indicate that clay conduction is minor in these high‐porosity sediments. Instead, tortuosity is the secondary control of FF, after porosity. By comparing resistivity‐based porosities to velocities from vertical seismic profiles, we determine a porosity‐velocity transform applicable to Barbados. We then use this transform to calculate velocity logs from resistivity‐based porosity logs.
Application of wave‐theoretical seismoacoustic models to the interpretation of explosion and eruption tremor signals radiated by Pavlof volcano, Alaska
Authors: Garces Milton A. , McNutt Stephen R. , Hansen Roger A. , Eichelberger John C.
Published: September 2002

Tremor and explosion signals recorded on September 29 during the Fall 1996 Pavlof eruption are interpreted using video images, field observations, and seismic data. Waveform analysis of tremor and explosions provided estimates of the melt's volcano‐acoustic parameters and the magma conduit dimensions. Initial mass fractions of 0.25% water and 0.025% carbon dioxide in the melt can explain the resonance characteristics of the tremor and explosion pulses inferred from seismic data. The magma conduit is modeled as a three‐section rectangular crack. We infer that the tremor‐radiating region consists of the lowermost two sections, both with cross‐sectional areas of ∼10 m2. The deeper section is 43 m long, with magma sound speed of 230 m/s, density of 2600 kg/m3, and viscosity of 1.0×106 Pa s. The section above it, defined by the water nucleation depth, is 64 m long, with magma sound speed of 91 m/s, density of 2000 kg/m3, and viscosity of 1.4×l06 Pa s. An average magma flow velocity of 1.2 m/s, with superposed random oscillations, acts as the tremor source. Explosions are postulated to occur in the uppermost part of the magma conduit after water comes out of solution. The explosion source region consists of a 15 m long section, with cross‐sectional area of 20 m2, sound speed of 51 m/s, density of 1000 kg/m3, and viscosity of 1.5×103 Pa s. A burst pressure of 220 MPa at 14 m depth would generate an acoustic pulse whose amplitude and character match the observed signal. Waveform analysis of the explosion pulses shows that the explosive event may be preceded by a long‐period fluid transient which may trigger the metastable magma‐gas mixture. The modeling procedure illustrates the synergy of fluid dynamic, seismic, and acoustic models and data with geological and visual observations.
Spherical versus flat models of coseismic and postseismic deformations
Authors: Nostro Concetta , Piersanti Antonio , Antonioli Andrea , Spada Giorgio
Published: September 2002

We perform an exhaustive study of coseismic and postseismic surface deformations induced by shear dislocations using flat and spherical Earth models. Our aim is to examine the effects of the spherical geometry, the vertical layering, and the self‐gravitation on surface displacement field. For a 100 km long fault, spherical and flat models produce comparable coseismic deformations up to a distance of ∼300 km from the epicenter. This distance is sensibly reduced in the postseismic regime and when infinitely long strike‐slip faults are considered. The differences between predictions based on flat and spherical models are due both to their global geometry and the effect of the gravity forces. Self‐gravitation has a minor role with respect to that of sphericity for surface coseismic deformations, while in the postseismic regime its effects increase considerably. As a case study, we consider the coseismic and postseismic deformations due to the great 1960 Chilean earthquake. The results of the spherical stratified model differ sensibly from those of a flat uniform model. Moreover, within the framework of spherical Earth models, the rheological stratification plays a major role in determining the pattern of the displacement field. We show that the present‐day rates of vertical and horizontal deformations are considerably large (∼10−2 m yr−1) for an asthenospheric viscosity ranging from 1019 to 1020 Pa s. These rates, which could possibly be detected by geodetic investigations, are found to be also sensitive to the rheological properties of the mantle beneath the asthenosphere.
Regional velocity structure in northern California from inversion of scattered seismic surface waves
Authors: Pollitz Fred F.
Published: September 2002

Seismic surface waves recorded by the Berkeley Digital Seismic Network have been analyzed in order to constrain three‐dimensional lateral heterogeneity of the upper mantle under northern California. A total of 2164 seismograms from 173 teleseismic events were windowed for the fundamental mode Rayleigh wave, followed by estimation of complex amplitude spectra over the period range 16 to 100 s using a multiple‐taper method. Since Rayleigh waves at shorter periods, particularly below 35 s, suffer from serious multipathing or “non‐plane” wave arrivals, these amplitude spectra have been interpreted as the product of wavefront distortion along the teleseismic propagation path and seismic structure beneath the network. The amplitude spectra are first modeled in terms of non‐plane incoming wavefields and structural phase velocity perturbations period by period. After corrections for Moho and surface topography, the phase velocity maps are inverted for three‐dimensional shear velocity perturbations δνs down to a depth of 200 km. The δνs maps are in good agreement with the results of body studies over a broad spatial scale. The dominant signals are associated with the thermal effects of the active Gorda and fossil Farallon subducted slab stretching from Mount Shasta through the western Sierran foothills to the southern Great Valley and asthenospheric upwelling beneath the northern Coast Ranges. The southern Sierra Nevada Range is characterized by fast δνs down to ∼50 km and slow velocities between ∼60 and 120 km depth, in agreement with independent inferences of a cold crust and warm upper mantle, which may provide the buoyancy forces necessary to support the elevation of the range.
Small scatterers in the lower mantle observed at German broadband arrays
Authors: Thomas C. , Weber M. , Wicks C. W. , Scherbaum F.
Published: September 2002

Seismograms of earthquakes from the South Pacific recorded at a German broadband array and network show precursors to PKPdf. These precursors mainly originate from off‐path scattering of PKPab or a nearby PKPbc to P (for receiver‐side scattering) or from scattering of P to PKPab or PKPbc on the PKPdf path (for source‐side scattering). Standard array processing techniques based on plane wave approximations (such as vespagram or frequency‐wavenumber analysis) are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 × 300 km for short‐period waves. We therefore develop a migration method to estimate the location of scatterers in the mantle, at the core‐mantle boundary and at the top of the outer core. With our method we are able to find isolated scatterers at the source side and the receiver side, although the depth of the scatterer is not well constrained. However, from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place (scattering volume), we believe that the location of the scatterers is in the lowermost mantle. Since we have detected scatterers in regions where ultralow‐velocity zones have been discovered recently, we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle. Comparing results from broadband and band‐pass‐filtered data the detection of small‐scale structure of the ultralow‐velocity zones becomes possible.
Tomographic images of the upper crust from the Los Angeles basin to the Mojave Desert, California: Results from the Los Angeles Region Seismic Experiment
Authors: Lutter William J. , Fuis Gary S. , Thurber Clifford H. , Murphy Janice
Published: September 2002

We apply inversion methods to first arriving P waves from explosive source seismic data collected along line 1 of the Los Angeles Region Seismic Experiment (LARSE), extending northeastward from Seal Beach, California, to the Mojave Desert, in order to determine a seismic model of the upper crust along the profile. We use resolution information to quantify the extent of blurring in the LARSE images and to smooth a damped least squares (DLS) image by postinversion filtering (PIF). Most of the original data fit is preserved while minimizing model artifacts. We compare DLS, PIF, and smoothing constraint inversion images using both real and synthetic data. A preferred PIF image includes larger‐scale features in the smoothing constraint inversion image and finer‐scale features in the DLS inversion image that are consistent with geologic information. We interpret principal model features in terms of geology, including faulting. The maximum depth of low‐velocity sedimentary and volcanic rocks in the Los Angeles basin is 8–9 km and in the San Gabriel Valley is 4.5–5 km. A horst‐like uplift of basement rocks occurs between these basins. The northeastern boundary of the San Gabriel Valley is imaged as a tabular, moderately north dipping low‐velocity zone that projects to the surface at the southernmost trace of the Sierra Madre fault system. In the central and southern San Gabriel Mountains, velocity‐depth profiles are consistent with intermediate‐velocity mylonites overlying lower‐velocity Pelona Schist along a shallowly southwest dipping Vincent thrust fault. Tomography does not provide a definitive dip for the San Andreas fault but, combined with other LARSE results, is consistent with a vertical to steep northeast dip.
The electrical conductivity of the lower mantle phase magnesiowüstite at high temperatures and pressures
Authors: Dobson David P. , Brodholt John P.
Published: September 2002

Experimental measurements of magnesiowüstite electrical conductivity at Fe/Fe + Mg 0.05 to 0.2 and Fe3+/Fe3+ + Fe2+ 0.01 to 0.7 at high pressure and high‐temperature are presented. Below 1000 K, conduction occurs by a small‐polaron process of electron hopping between ferric and ferrous sites, but above 1000 K there is a change in mechanism. This high‐temperature mechanism is postulated to be a large‐polaron process in which holes are promoted in the oxygen valence band via the reactions: 1/2 O2 = OOx + VMg″ + 2h• and FeFe• = FeFex + h•. The hole and its associated polarization field are free to move in the valence band until trapped by a ferrous ion. Activation energies for the low‐temperature, small‐polaron regime are ∼0.3 eV across the range of Fe/Fe + Mg and Fe3+/Fe3+ + Fe2+ studied, in agreement with previous studies. The high‐temperature, large‐polaron activation energy decreases with increasing Fe/Fe+Mg and decreasing Fe3+/Fe3+ + Fe2+, ranging, from 0.4 to 1.1 eV. Both regions show a small, negative activation volume (ΔVlt = −0.33 (19) cm3 mol−1; ΔVht = −0.26(69) cm3 mol−1), consistent with previous high pressure studies of electronic conduction mechanisms. A compilation of the available data shows a discrepancy between measurements at low and high‐temperatures, consistent with the new results presented here. At the temperature of the lower mantle, the dominant conduction mechanism in magnesiowüstite will be the more mobile large‐polaron process. This is less sensitive to iron content than small‐polaron conduction at Fe/Fe+Mg < 0.17 (the likely compositional range of lower mantle magnesiowüstite) and has a different temperature dependence from the low‐temperature process.
Insolation changes caused by combination of amplitude and frequency modulation of the obliquity
Authors: Liu Han‐Shou
Published: September 2002

A forcing function is derived from the insolation signal with a bipolar pulse modulation train that is a function of Earth's obliquity. From this obliquity‐forced insolation, there is a calculated variation of about ±9% in the energy flux at the top of the atmosphere. Pulsations in the incoming solar radiation, induced by the amplitude‐frequency coupling effect of the obliquity, correlate well with continental biogenic silica data and the marine oxygen isotope record. The theory lends a new perspective to the physical mechanism involving climate change studies. Particularly, it provides a physical explanation of the 100‐kyr cycle in glaciation, which is distinct from the commonly considered variations in orbital eccentricity.
Permeability of upper oceanic basement on the eastern flank of the Juan de Fuca Ridge determined with drill‐string packer experiments
Authors: Becker Keir , Fisher Andrew T.
Published: September 2002

We report drill‐string packer measurements of permeabilities of uppermost basement at three sites drilled during Leg 168 of the Ocean Drilling Program in 0.9‐ to 3.6‐m.y.‐old crust on the thickly sedimented eastern flank of the Juan de Fuca Ridge. Each of the holes was cased completely through the sediment section and penetrated a few meters to tens of meters into basaltic oceanic basement. The transmissivities and average permeabilities of the open‐hole basement sections were assessed using the packer to seal the hole and controlled pumping from the drillship to apply pressure pulse and constant rate injection tests. The results, namely average permeabilities of the order of 10−14 to 10−11 m2, are generally consistent with prior packer measurements in other young ocean crustal settings, but two new findings are important. First, our new results include very high values in the youngest crust yet tested, suggesting a noteworthy age dependence of the bulk permeability of uppermost basement (at the scale measured with the short‐term packer tests), ranging from about 3×1011 m2 at 0.9 m.y. to about 2×10−14 m2 at 3.6 m.y. Second, significant scale dependence of permeability is suggested by comparison to higher values inferred from (1) thermal indications of uphole flow of formation fluids at one of our sites and (2) in situ pressures and tidal loading effects recorded in long‐term formation pressures measured after the three sites were sealed with hydrologic observatories immediately following the packer measurements.
An efficient, probabilistic neural network approach to solving inverse problems: Inverting surface wave velocities for Eurasian crustal thickness
Authors: Devilee R. J. R. , Curtis A. , Roy‐Chowdhury K.
Published: September 2002

Nonlinear inverse problems usually have no analytical solution and may be solved by Monte Carlo methods that create a set of samples, representative of the a posteriori distribution. We show how neural networks can be trained on these samples to give a continuous approximation to the inverse relation in a compact and computationally efficient form. We examine the strengths and weaknesses of this approach and use it to determine the full a posteriori distribution of crustal thickness from surface wave velocities. The solution to this inverse problem shows significant asymmetry and large uncertainties due to trade‐off with shear velocity structure around the Moho. We produce maps of maximum likelihood crustal thickness across Eurasia which are in agreement with current knowledge about the crust; thus we provide an independent confirmation of these models. In this application, characterized by repeated inversion of similar data, the neural network algorithm proves to be very efficient.
Low‐temperature magnetic behavior of ferrihydrite
Authors: Zergenyi R. S. , Hirt A. M. , Zimmermann S. , Dobson J. P. , Lowrie W.
Published: September 2002

Two samples of six‐line ferrihydrite (5Fe2O3·9H2O) were precipitated from an Fe(III) solution. The purity of the samples was verified by X‐ray diffraction. Scanning electron microscope pictures indicate that the grains have good crystallinity and variable grain size (<0.5 to 100 μm). Mössbauer spectra show a broad asymmetric doublet without split components between 150 K and 250 K. Hyperfme magnetic splitting can be observed at 130 K, and a sextet spectrum is apparent below 100 K. The mass susceptibility of the ferrihydrite was measured between 10 K and 300 K. The in‐phase mass susceptibility χ′ has a value of 1.0×10−6 m3 kg−1 at room temperature. At low temperatures, χ′ depends on the frequency of the measurement field, increasing with increasing temperature from 10 K to a maximum at ∼90 K and then decreasing smoothly at higher temperatures. Over the same temperature range the quadrature mass susceptibility rises to a peak at 65 K then decreases to a low background value above 120 K. These characteristics suggest the presence of superparamagnetic grain sizes below 120 K. Cubic spline interpolations of the χ′ versus T curves show that the temperature of the in‐phase susceptibility peak increases with increasing frequency of the field from 86 K at 40 Hz to 94 K at 4000 Hz. The value of χ′ was independent of the field amplitude in the measurement range below 2.5 mT. Hysteresis loops, coercivity, and isothermal remanence at low temperature agree with the susceptibility measurements and suggest that ferrihydrite is antiferromagnetic with a parasitic ferromagnetic moment and a Néel temperature around 120 K.
On scaling relations in time‐dependent mantle convection and the heat transfer constraint on layering
Authors: Butler S. L. , Peltier W. R.
Published: September 2002

We present a series of simulations of the mantle convection process based upon an axisymmetric numerical model and highlight a wide range of results in which scaling emerges. For the more challenging simulations it was found necessary to employ a finite difference mesh with uneven grid spacing in the radial coordinate, and we present the appropriate transformed field equations that are required to implement a model of this kind. The statistics of mass flux events transiting the 660‐km phase transition are calculated for a large number of high‐resolution calculations, and some of these are shown to display scale invariance properties in the high Rayleigh number regime. We also present a new parameterized model of convection and demonstrate its success in predicting the manner in which many of the bulk properties of the convection process scale with convection control parameters. Results are also presented which demonstrate that quantities such as heat flow, characteristic velocity, and thermal boundary layer thickness scale with the mean viscosity even in time‐dependent simulations in which the effects of phase transitions, depth‐varying viscosity, and internal heating are active. The heat flow scaling exponent is seen to decrease in magnitude with increasing internal heating rate and Clapeyron slope of the 660‐km phase transition, but it is shown to be insensitive to depth variation of viscosity. Heat flow is seen to be reduced only modestly as the degree of layering increases unless layering is extreme. These calculations clearly demonstrate that in order for the surface heat flow predicted by the model to equal that characteristic of Earth, the mean viscosity of the mantle that controls the convection process must be considerably higher than the viscosity inferred on the basis of postglacial rebound and/or the flow must be significantly layered by the endothermic phase transition at 660 km depth. If mantle viscosity may be assumed to be Newtonian, in which case the creep resistance that controls rebound and convection must be the same, this constitutes a strong argument for the importance of layering. The force of this argument depends upon the existence of an accurate estimate of the temperature at the core‐mantle boundary which has only recently become available.
Crustal structure of NW British Columbia and SE Alaska from seismic wide‐angle studies: Coast Plutonic Complex to Stikinia
Authors: Hammer Philip T. C. , Clowes Ron M. , Ellis Robert M.
Published: September 2002

Crustal structure beneath the transition from the Coast belt to the Intermontane superterrane of the northern Canadian Cordillera is interpreted from the inversion of refraction and wide‐angle reflection seismic data. The profile traverses an accretionary suture zone (Coast Plutonic Complex) to continental crust deformed by the transpressive collision (Stikine terrane). Using data acquired by the Accrete onshore/offshore experiment and by a partially overlapping Lithoprobe onshore experiment, P wave travel time inversion and forward amplitude modeling are employed to determine crustal velocity structure. The model exhibits a well‐defined transition throughout the crust that distinguishes the Coast Plutonic Complex (CPC) from Stikinia. Average crustal velocities beneath the CPC (6.45 km/s) are considerably faster than those beneath Stikinia (6.25 km/s). Crustal thickness also changes across the transition; thin crust beneath the Coast belt (30–32 km) thickens beneath Stikinia (35–37 km). The observations within the Coast belt are consistent with a tectonic history most recently dominated by extensional deformation. Primary structural control could be associated with either Neogene extension and/or the processes that are responsible for exhuming the Coast belt during the early Paleogene and have been inferred from geological studies. Slow mantle velocities (7.8–7.9 km/s) beneath the entire profile are indicative of high upper mantle temperatures. Comparison with the southern Cordilleran Coast belt reveals similar velocity structure within the massive plutonic complexes. However, substantial differences between the northern and southern Coast belts emphasize along‐strike variations in terranes, orogen geometry and postorogenic tectonics.
Joint inversion for inner core rotation, inner core anisotropy, and mantle heterogeneity
Authors: Song Xiaodong
Published: September 2002

Observational evidence for differential rotation of the inner core has recently been reported from time‐dependent travel times of the seismic inner core phase, PKP(DF). These analyses are hindered by potential biases from the mantle heterogeneity and systematic earthquake location errors, which have raised doubts about the reality of the inferred differential rotation. Here we report time‐dependent PKP(DF)‐PKP(BC) differential travel times of a total of 611 measurements obtained from 92 earthquakes in the South Sandwich Islands (SSI) over the past nearly half a century recorded at the College, Alaska station (COL) and Alaska Seismic Network (ASN). The events are relocated using the joint hypercenter determination (JHD) technique [Dewey, 1971]. Travel time residuals at COL from SSI earthquakes over a period of 45 years and at three additional stations from SSI earthquakes over periods of about 22 years show steady increases with time; the temporal trends are demonstrated to be statistically very significant. With dense samples of the pathway at different time periods, we use a joint inversion technique that allows us to separate the time‐dependent inner core structure from the time‐independent mantle biases. Surprisingly, the inner core rotation seems resolvable even from digital data observed at ASN in the 1990s (spanning over 8 years) alone in such a joint inversion. The results provide strong support for a differential inner core rotation. The inferred rotation rate ranges from 0.3° to 1.1° per year faster than the mantle when we take into account the uncertainty in the tilt of the anisotropy axis of the patch of the inner core that was sampled. A westward inner core rotation can be ruled out.
Monte Carlo simulation of seismogram envelopes in scattering media
Authors: Yoshimoto Kazuo
Published: September 2002

The analysis of the seismogram coda envelopes of local and regional earthquakes is one of the most effective strategies for investigating the heterogeneous lithospheric structure characterized by the seismic scattering and attenuation. In order to synthesize the coda envelope we introduce a numerical scheme called the direct simulation Monte Carlo method, which has been used in the field of the kinetic theory of gases. Because of the simplicity of the algorithm the method has several advantages over previous methods in terms of the flexibility of the numerical calculation to incorporate various factors required to construct realistic seismogram envelopes. On the basis of coda envelope simulations, including multiple scattering, we show that an increase of seismic velocity with depth severely affects the shape of the coda envelope. The effects of ray bending due to the velocity increase at the Moho and the reflection at the free surface are clearly found in the synthesized envelope for a shallow earthquake. Our simulation demonstrates that the amplitude of the envelope is magnified by stagnation of seismic energy at shallow depths due to the positive velocity gradient with depth. Because of this effect, for an a priori assumption of a homogeneous velocity model the measurement of the scattering coefficient by conventional methods may be overestimated.
The role of winter intermediate waters in the spring‐summer circulation of the Balearic Sea: 1. Hydrography and inverse box modeling
Authors: Pinot J.‐M. , Ganachaud A.
Published: September 2002

Hydrographic transects performed across the Balearic Sea (western Mediterranean) in June 1993 are used to estimate the absolute geostrophic flow by means of a linear inverse model, ensuring conservation of water properties (mass, salt, and heat) in closed boxes. The computed fluxes are supporting the concept that most of the northern waters that flow into the basin over the Spanish peninsula slope (≈1.1 Sv) recirculate back to the north in a cyclonic pattern while southern waters that enter through the Ibiza Channel, mainly modified Atlantic waters (≈0.5 Sv), transit to the north over the Balearic slope. Results point out that winter intermediate waters (WIW), distributed in several coherent mesoscale structures, play an active role in these dynamics. A major anticyclonic WIW eddy (“weddy”) lying in the southern basin obstructs the water exchange through the Ibiza Channel and deflects the Continental Current back to the north. A similar though smaller weddy, strained by the Balearic Current into an elongated along‐slope WIW filament, controls the circulation through the Mallorca Channel. All these results are discussed in the light of data collected during past years.
Spatial and temporal characteristics of the mesoscale circulation of the California Current from eddy‐resolving moored and shipboard measurements
Authors: Chereskin T. K. , Morris M. Y. , Niiler P. P. , Kosro P. M. , Smith R. L. , Ramp S. R. , Collins C. A. , Musgrave D. L.
Published: September 2002

Moored observations of currents and temperatures made in the upper 600 m on eddy‐resolving scales over a 2‐year period are used to examine the spatial and temporal characteristics of the California Current mesoscale circulation. The observations were made at three principal longitudes: 124°W, 126°W, and 128°W in the vicinity of Point Arena. They bracket the 600‐km‐wide band of high mesoscale variability found along the eastern boundary of the North Pacific. At all locations, the mesoscale variability was larger than the mean flow, and the spatial modes of variability as determined from empirical orthogonal function analysis consisted of an alongshore mode, a cross‐shore mode, and a rotational mode. Observations made near the continental slope (124°W) were dominated by the poleward flowing California Undercurrent, with mesoscale eddies and meanders superposed. The nearshore eddy kinetic energy peaked in a band centered around 60 days. Observations made at 128°W, near the offshore boundary between the energetic mesoscale band and the “eddy desert” of the northeast Pacific, were characterized by small means, fewer eddy events, and a peak in eddy kinetic energy at 120–180 days. The good horizontal resolution of the current meter arrays allowed us to estimate the relative vorticity, horizontal divergence, and Rossby number and therefore to evaluate the relative strength and occurrence of anticyclones and cyclones. We found the mesoscale eddy field to be strongly nonlinear, with Rossby numbers ranging from 0.1 to 0.5. All of the eddies observed at the offshore site were nonlinear, deep, warm anticyclones. Shipboard hydrography revealed the origin of one of these anticyclones to be the California Undercurrent, and this eddy retained its strong anomalies after several months and several hundred kilometers of propagation. Despite the lower incidence of eddies as one moves west from the coast, the eddies that we observed offshore provide evidence for propagation and transport of properties from the coast to the central North Pacific across the California Current System.
Surface turbulence in natural waters: A comparison of large eddy simulations with microstructure observations
Authors: Sander Johannes , Simon André , Jonas Tobias , Wüest Alfred
Published: September 2002

Turbulent properties of the surface layer of stratified lakes have been quantified from temperature microstructure measurements made during convective conditions. Large eddy simulations have been performed using corresponding surface boundary conditions. We compare rates of dissipation of turbulent kinetic energy and dissipation of temperature variance obtained from observations with those obtained from large eddy simulations. While profiles of the rate of dissipation of turbulent kinetic energy can be reproduced very well by the model, the rate of dissipation of temperature variance exhibits smaller values in the calculations. Other statistical characteristics of turbulence agree for both properties. We show that part of the discrepancy derives from large‐scale lateral advection, which is not represented by the model, while turbulence on the microscale is reproduced well.
Tracing the flow of North Atlantic Deep Water using chlorofluorocarbons
Authors: Smethie William M. , Fine Rana A. , Putzka Alfred , Jones E. Peter
Published: September 2002

Chlorofluorocarbon (CFC) and hydrographic data collected in the North Atlantic in the late 1980s and early 1990s are used to confirm and add to earlier work on the large‐scale circulation pathways and timescales for the spreading of North Atlantic Deep Water (NADW) components and how these components relate to the hydrographic structure. Throughout the western North Atlantic, high CFC concentrations are coincident with newly formed NADW components of Upper Labrador Sea Water (ULSW), Classical Labrador Sea Water (CLSW), and Overflow Waters (OW). ULSW is marked by a CFC maximum throughout the western subtropical and tropical Atlantic, and CLSW is marked by a CFC maximum north of 38°N in data collected in 1990–1992. Iceland‐Scotland Overflow Water (ISOW) splits into two branches in the eastern basin, with one branch entering the western basin where it mixes with Denmark Strait Overflow Water (DSOW) and the densest branch flows southward along the bottom in the eastern basin. DSOW contributes the largest portion of the CFC signal in OW. It is estimated that these NADW components are at 60–75% equilibrium with the CFC concentration in the atmosphere at the time of formation. The large‐scale data set confirms that NADW spreads southward by complex pathways involving advection in the Deep Western Boundary Current (DWBC), recirculation in deep gyres, and mixing. Maps of the CFC distribution show that properties within the gyres are relatively homogeneous, particularly for OW, and there is a profound change at the gyre boundaries. The density of the core of ULSW increases in the equatorward direction because of entrainment by overlying northward flowing Upper Circumpolar Water and at the equator, ULSW has the same density as CLSW in the subtropics but is warmer and saltier. The density of OW decreases between the subpolar region and the subtropics. This is caused by the least dense part of OW exiting the subpolar region in the DWBC, while the densest component recirculates in the subpolar basins. Some variability is observed in OW density in the subtropics and tropics because of variability in mixing with Antarctic Bottom Water and changes in the subtropics that are probably related to the transport of different vintages of DSOW. Ages derived from CFC ratios show that the NADW components of northern origin spread throughout the western North Atlantic within 25–30 years. This corresponds to a spreading rate of 1–2 cm s−1 and is comparable to the time a climate anomaly introduced into the subpolar North Atlantic will take to penetrate the entire western North Atlantic Ocean.