bluuebunny/crossref_metadata_2025 huggingface dataset:
A numerical study of solar wind—magnetosphere interaction for northward interplanetary magnetic field
Authors: Song P. , DeZeeuw D. L. , Gombosi T. I. , Groth C. P. T. , Powell K. G.
Published: September 2002

The solar wind‐magnetosphere interaction for northward interplanetary magnetic field (IMF) is studied using a newly developed three‐dimensional adaptive mesh refinement (AMR) global MHD simulation model. The simulations show that for north‐ward IMF the magnetosphere is essentially closed. Reconnection between the IMF and magnetospheric field is limited to finite regions near the cusps. When the reconnection process forms newly closed magnetic field lines on the day side, the solar wind plasma trapped on these reconnected magnetic field lines becomes part of the low‐latitude boundary layer (LLBL) plasma and it convects to the nightside along the magnetopause. The last closed magnetic field line marks the topological boundary of the magnetospheric domain. When the last closed magnetic field line disconnects at the cusps and reconnects to the IMF, its plasma content becomes part of the solar wind. Plasma convection in the outer magnetosphere does not directly contribute to the reconnection process. On the dayside the topological boundary between the solar wind and the magnetosphere is located at the inner edge of the magnetopause current layer. At the same time, multiple current layers are observed in the high‐altitude cusp region. Our convergence study and diagnostic analysis indicate that the details of the diffusion and the viscous interaction do not play a significant role in controlling the large‐scale configuration of the simulated magnetosphere. It is sufficient that these dissipation mechanisms exist in the simulations. In our series of simulations the length of the magnetotail is primarily determined by the balance between the boundary layer driving forces and the drag forces. With a parametric study, we find that the tail length is proportional to the magnetosheath plasma beta near the magnetopause at local noon. A higher solar wind density, weaker IMF, and larger solar wind Mach number results in a longer tail. On the nightside downstream of the last closed magnetic field line the plasma characteristics are similar to that in the magnetotail, posing an observational challenge for identification of the topological status of the corresponding field lines.
Rocket observations of banded structure in waves near the Langmuir frequency in the auroral ionosphere
Authors: McAdams K. L. , LaBelle J. , Trimpi M. L. , Kintner P. M. , Arnoldy R. A.
Published: September 2002

Using data from the PHAZE II sounding rocket, launched from Poker Flat, Alaska, we present high‐resolution observations of structure in auroral HF waves at and below the local plasma frequency. These observations were made in the altitude range of 390–945 km where the local plasma frequency is below the electron cyclotron frequency. We observe monochromatic, long‐lived, narrowband emissions occuring below the local plasma frequency during times of intense HF wave emission. We have termed these emissions “HF bands” due to their appearance in spectrogram images. These emissions are probably identical to the “spike” emissions identified by previous observers using lower time resolution data from the AUREOL/ARGAD3 satellite which showed a narrow peak spectra below the local plasma frequency. HF bands often occur when the local plasma density is varying and are associated with regions of intense Langmuir wave generation. We investigate the hypothesis that the HF bands are created when a Langmuir wave propagates from a low‐density region into a higher density region. The wave moves onto the whistler mode branch and propagates as an HF band. Theoretical calculations of propagation times of whistler mode waves support this hypothesis.
Two‐point measurements of the magnetopause: Interball observations
Authors: Sibeck D. G. , Prech L. , Safrankova J. , Nemecek Z.
Published: September 2002

The impulsive penetration model predicts that blobs of solar wind plasma penetrate the magnetosphere by becoming detached from the magnetopause. If so, negative radial density gradients should be common near the magnetopause. We present the results of case and statistical studies of simultaneous Interball 1 and Magion 4 omnidirectional plasma sensors (VDP) plasma observations during magnetopause passes. The spacecraft observe very similar features when their interspacing is small. By contrast, the spacecraft located farther radially outward invariably observes densities greater than or equal to those seen at the spacecraft nearer Earth when the interspacing increases. The observations are consistent with the standard interpretation in which a boundary layer of plasma with intermediate densities often lies sandwiched between the magnetosheath and the magnetosphere proper. Blobs of magnetosheath or boundary layer plasma rarely, if ever, become detached from the magnetopause.
Association of electron precipitation with auroral kilometric radiation
Authors: Imhof W. L. , Walt M. , Anderson R. R. , Chenette D. L. , Hawley J. D. , Mobilia J. , Petrinec S. M.
Published: September 2002

An examination is made of a large body of auroral X ray and auroral kilometric radiation (AKR) data acquired on the Polar satellite in order to investigate the relationship between these two phenomena on a global scale. A detailed study was made of 14 cases selected with strong X ray and AKR emissions and appreciable time variations. In nearly all cases a maximum in the cross‐correlation coefficient of auroral X rays and AKR occurred for X rays emitted slightly before local midnight, indicating that the AKR sources were concentrated in this magnetic local time (MLT) sector. The correlation is quite strong with coefficients between 0.29 and 0.82. The enhancement in the correlation coefficient generally extends over an MLT interval of 6 hours or less, even though electron precipitation above 2 keV extends over a longer MLT interval. This behavior is consistent with the importance of another factor, such as the presence of plasma cavities, being necessary for AKR production. Owing to statistical uncertainties, it was not possible from apogee data to establish whether the X rays or AKR occurred first during the short enhancements. Higher time resolution was achievable when Polar was near perigee, but AKR propagation to Polar was not assured at this location. During a perigee pass on January 13, 1997, Polar observed an auroral X‐ray enhancement in time coincidence with inverted‐V electrons, upward streaming ions, and enhanced AKR. In this event the inverted‐V potential structure extended over at least 2 hours of MLT and lasted for only l min.
Gradient drift instabilities and turbulence in the nighttime equatorial electrojet
Authors: Hu S. , Bhattacharjee A.
Published: September 2002

Linear and nonlinear evolution of gradient drift instabilities, responsible for type II irregularities in the equatorial electrojet, are discussed for the nighttime density profile obtained by rocket flights at Alcantara, Brazil, during the 1994 Guara Campaign. In contrast with the daytime case, the nighttime density profile is quite jagged. The jaggedness implies strong density gradients which excite short‐wavelength instabilities with large growth rates. The most unstable waves have horizontal wavelengths in the range 10 – 20 m. Linear instabilities with much larger horizontal wavelengths, equal to and greater than 1 km, also occur, but their growth rates are smaller. In contrast with short waves that tend to localize themselves in narrow vertical regions where the density gradient is favorable for instability, the long waves have broad vertical extent, spanning regions that have density gradients favorable as well as unfavorable for instability. Despite the larger linear growth rates of the short waves (10 – 20 m), nonlinear numerical simulations exhibit significant kilometer‐scale structures in the turbulently saturated state. The saturated power spectrum is anisotropic in the plane perpendicular to the ambient magnetic field. Kilometer‐scale structures are realized by a balance between the linear instability drive of long waves and the diffusive damping of short waves produced by mode coupling. The ability of the long waves to fill out the vertical layer, spanning regions in which the density gradients are favorable as well as unfavorable for stability, makes them survive the mode competition in the nonlinear regime. Thus the saturated fluctuations in the nighttime show a preponderance 2‐ to 4‐km structures (which are somewhat longer than those seen in the daytime), qualitatively consistent with observations.
The termination shock: 1979–1995
Authors: Whang Y. C. , Lu J. Y. , Burlaga L. F.
Published: September 2002

We study the varying location, jump condition, and shock speed of the termination shock in 1979–1995 taking into account the effects of pickup protons. Pickup protons and neutral hydrogen are calculated using a hot hydrogen model. Plasma and magnetic field data from Voyagers over a 17‐year period from 1978 through 1994 are used as input. The steady state MHD equations are integrated to extrapolate input data from the Voyager trajectory to the upstream of the termination shock. This study continues to show that the location of the termination shock is anticorrelated with the sunspot number, moving inward or outward during the rising or declining phase of the solar cycle. We expect that this anticorrelation would repeat itself during the solar cycle 23 in an extension of this study beyond 1995, and the result can predict the Voyager crossing of the termination shock. If the average location of the termination shock is at 79 AU, the pickup proton effects reduce the average shock strength from 3.87 to 3.15 and reduce the average temperature ratio of the solar wind protons from 378 to 42.1. We use the empirical formula of Whang and Burlaga [1999] to calculate the postshock temperature of pickup protons, the average temperature ratio of pickup protons across the termination shock is ∼4.5.
Observations of traveling Pc5 waves and their relation to the magnetic cloud event of January 1997
Authors: Clemmons J. H. , Pfaff R. F. , Lennartsson O. W. , Mozer F. S. , Singer H. J. , Peterson W. K. , Scudder J. D. , Kletzing C. A. , Chi P. J. , Wallis D. D. , Larson D. E.
Published: September 2002

Measurements of ULF waves in the Pc5 frequency range are presented and discussed. The waves were observed during the magnetic cloud event of January 1997 by instruments on the Polar satellite and ground instrumentation. These large‐amplitude waves are best interpreted as traveling shear Alfvén waves rather than in the usual standing‐wave scenario. Characterization of the associated complex particle environment shows that the waves were largely confined to the plasma trough. Energetic protons modulated by the wave are shown to have caused modulated proton aurora. It is argued that the waves were caused by the interaction of a magnetic hole with the magnetosphere.
Interaction of a strong interplanetary shock with the termination shock
Authors: Lu J. Y. , Whang Y. C. , Burlaga L. F.
Published: September 2002

The 1991 global merged interaction region (GMIR) shock is a strong forward shock observed from Voyager 2 on day 146 of 1991 at 34.6 AU. As the GMIR shock propagated out‐ward in the distant heliosphere, it eventually encounters the termination shock. We study the interaction of the GMIR shock with the termination shock taking into account the influence of interstellar pickup protons on the solar wind and the shocks. Because the location of the termination shock is an unknown parameter, it remains uncertain about when and where the GMIR shock penetrates through the termination shock. This paper carries out calculations for the interaction to take place over a wide range of possible locations between 70 and 110 AU. The interaction can significantly modify the conditions of both shocks. For the GMIR shock, the interaction can cause a decrease of the shock speed by ∼120 km/s, a decrease of the shock Mach number from ∼1.9 to ∼1.4, and a decrease of the shock strength from ∼2.2 to ∼1.6. For the termination shock, the interaction can cause an increase of the shock speed by ∼95 km/s, a decrease of the shock Mach number from ∼3.6 to ∼2.6, and a decrease of the shock strength from ∼3.2 to ∼2.8.
Cusp observations of high‐ and low‐latitude reconnection for northward interplanetary magnetic field
Authors: Fuselier S. A. , Trattner K. J. , Petrinec S. M.
Published: September 2002

Polar/Toroidal Imaging Mass Angle Spectrograph (TIMAS) observations in the cusp reveal evidence of both high‐ and low‐latitude reconnection during high solar wind dynamic pressure, northward interplanetary magnetic field (IMF) intervals. Under these restrictive conditions, the magnetic field observed by the Polar/Magnetic Fields Investigation in the northern cusp often rotates from lobe‐like orientations (BZ<0, Bx>0) to dayside magnetospheric‐like orientations (Bz>0, Bx<0). Proton distributions observed when the magnetic field has a magnetospheric‐like orientation are consistent with either reconnection poleward of the cusp at high‐latitudes or reconnection equatorward of the cusp at lower latitudes. Transitions from one type of proton distribution to another as the field rotates from lobe‐like to dayside magnetospheric‐like orientations indicate sunward (poleward) convection of the reconnected magnetic field for reconnection poleward (equatorward) of the cusp. A survey of 17 intervals of high solar wind dynamic pressure and northward IMF indicates that these two reconnection topologies occur with approximately equal probability and that reconnection equatorward of the cusp favors small angles between the magnetosheath and dayside magnetospheric magnetic fields.
The electron velocity distribution in the high‐speed solar wind: Modeling the effects of protons
Authors: Lie‐Svendsen Øystein , Leer Egil
Published: September 2002

The evolution of the electron velocity distribution function (VDF) in high‐speed solar wind streams is modeled taking the expanding geometry, the polarization electric field, and Coulomb collisions into account. The VDF we find at the orbit of Mercury is composed of an isotropic, collision‐dominated core, a trapped, anisotropic population called “halo” in this study, and a narrow, high‐energy “strahl” that escapes along the magnetic field. The distribution function is very similar to the electron VDF observed in the low‐density, high‐speed solar wind by Pilipp et al. [1987] and Phillips et al. [1989]. The main features of the VDF can be obtained by considering only electron self‐collisions; the effect of proton collisions is to make the distribution function more isotropic. At low energies, collisions with protons dominate the angular scattering, but electron self‐collisions alone are frequent enough to keep the core of the distribution function quite isotropic. The expanding geometry produces an anisotropic halo and a narrow strahl. The angular scattering by protons reduces the anisotropy of the trapped halo particles and broadens the lower‐energy part of the strahl. Along the magnetic field the resulting electron velocity distribution is composed of a relatively cold core and a halo‐strahl spectrum that is “flatter” than the coronal spectrum. The two‐temperature electron distribution function often observed in the solar wind may therefore be produced by Coulomb collisions and should not be taken as a “proof” of a non‐Maxwellian (two‐temperature) distribution function in the corona.
Multisatellite observations of large magnetic depressions in the solar wind
Authors: Chisham G. , Schwartz S. J. , Burgess D. , Bale S. D. , Dunlop M. W. , Russell C. T.
Published: September 2002

Two large depressions in the magnitude of the interplanetary magnetic field, lasting ∼10–20 min, have been observed in the solar wind just upstream of the Earth's bow shock by three spacecraft (Active Magnetospheric Particle Tracer Explorer UK Subsatellite (AMPTE UKS), AMPTE Ion Release Module (IRM), and ISEE 1). The multiple satellite observations show that the depressions are convecting with the ambient solar wind. Analysis of the depression boundaries shows that they are tangential discontinuities with normals aligned approximately parallel to the GSE x direction. The electron distributions measured within the structures are remarkably Isotropic when compared to the more anisotropic distributions found in the ambient solar wind. There is also a reduction in plasma wave activity during the depressions. The depressions exhibit characteristics similar to those of solar wind magnetic holes but are much larger than has been typically observed. The depressions also display similarities with encounters of the heliospheric plasma sheet and heat flux dropouts, both of which are typically observed near sector boundaries, close to the heliospheric current sheet. The nature of these depressions is discussed in the context of magnetic hole and heliospheric plasma sheet observations. A large magnetic hole structure formed from a conglomeration of small holes appears the more likely scenario for the observed depressions.
New method for detecting interplanetary flux ropes
Authors: Shimazu Hironori , Marubashi Katsuhide
Published: September 2002

We identified candidate flux ropes by using interplanetary magnetic field data obtained by the Wind spacecraft and selecting the periods when the third derivative of each interplanetary magnetic field component with respect to time was continuously smaller than a certain value. If the data gathered during these periods fit a cylindrical force‐free field model, the candidates were classified as flux ropes. There were too many flux ropes that had a large impact parameter value to be explained by the finite lengths of the flux ropes (or possibly the curvatures of the flux rope axes). We thus excluded flux ropes having a large impact parameter value from our analysis of fitted parameters. We took into account short‐duration flux ropes (2–10 hours), and we found that the number of small flux ropes is much larger than that of large ones. We found that this method works well in identifying flux ropes provided that valid criteria have been established.
Thermospheric disturbance recorded by photometers onboard the ARIA II rocket
Authors: Strickland D. J. , Hecht J. H. , Christensen A. B. , McEwen D. J.
Published: September 2002

An analysis is reported of near zenith viewing brightness observations of O I 844.6 nm, N2+ ING 391.4 nm, N2 2PG 337.1 nm, O I 555.7 nm, and O I 630.0 nm recorded by photometers onboard the ARIA II rocket launched from Poker Flat, Alaska, on February 12, 1994. The rocket passed through diffuse aurora mixed with intermittent soft precipitation on the upleg followed by much more structured precipitation on the downleg. The most meaningful analysis of the data comes from the upleg portion of the flight and is reported here. The primary objective of the investigation was to quantify possible departures of the density profiles of N2 and O from their representations within the MSIS model. To do so requires knowledge of the electron precipitation responsible for the observed emissions. This was obtained by fitting electron transport results to altitude profiles of in situ electron spectrometer data and
391.4 nm photometer data. Good agreement with profile shapes was achieved with precipitation characterized by a Maxwellian distribution having a characteristic energy E0 of 1.75 keV. The strength of the precipitation was determined by the brightness of the
391.4 nm profile which yielded an energy flux Q of 3.08 ergs cm−2 s−1. Simultaneous good fits to the low‐altitude
391.4 nm and O I 844.6 nm data required a scaling of the MSIS‐based O density profile by a factor of 0.45. This factor amounts to a significant reduction in O and is close to that obtained from an analysis of coincident ground‐based optical data [Christensen et al., 1997]. Perhaps more significant is a clear sign of an abrupt composition change as the rocket approached apogee based on the observed brightness ratios of 844.6/391.4 and 557.7/391.4. Both exhibit the same gross structure as well as fine structure arising from intermittent soft precipitation. Their ratios are observed to drop by more than a factor of 2 along a portion of the trajectory as the rocket passed above 218 km. Upon achieving such heights, a close relationship exists between these ratios and the ratio of O to N2 column densities (O/N2). Full consideration of the relationships between the observed brightnesses of O I 844.6 nm, O I 557.7 nm, and
391.4 nm leads to the conclusion that the rocket entered a region of enhanced N2 density with continued depletion of the O density similar to what was observed prior to entering this region. Relative to MSIS, the N2 densities are at least a factor of 2 higher while the O densities are at least a factor of 2 smaller leading to a reduction in MSIS‐based O/N2 of more than a factor of 4.
Dynamical models of mantle volatile evolution and the role of phase transitions and temperature‐dependent rheology
Authors: van Keken P. E. , Ballentine C. J.
Published: September 2002

Helium isotopic variations demand the preservation of a primitive volatile source for ocean island basalts (OIB) in conjunction with a well‐mixed and more radiogenic source for mid‐oceanic ridge basalts (MORB). Dynamical models of the Earth's evolution should be able to predict this basic geochemical observation. We have developed a number of increasingly more realistic models of mantle convection that satisfy present‐day heat loss and plate velocities to study the effects of convective mixing, radiogenic ingrowth, and degassing on the mantle3He/4He evolution. We have included mechanisms that have been proposed to enhance the isolation of individual reservoirs in the mantle such as high lower mantle viscosity, strongly temperature‐ and pressure‐dependent rheology, and an endothermic phase transition at 670 km depth. Although the combination of these mechanisms can produce regions of lower mixing efficiency, our models cannot satisfactorily explain the existence of two distinct OIB and MORB3He/4He sources. If further improvements to the model, such as simulated plates and continents, still fail to explain the geochemical constraints, it may be prudent to consider sources of primitive helium beyond the current paradigm requiring them to be stored within the terrestrial mantle since early in the Earth's history.
Recent volcanic rocks from Jan Mayen: Low‐degree melt fractions of enriched northeast Atlantic mantle
Authors: Trønnes Reidar G. , Planke Sverre , Sundvoll Bjørn , Imsland Pall
Published: September 2002

Isotopic and trace element analyses of recent alkaline volcanic rocks from Jan Mayen were used to characterize their mantle source. The samples, ranging in composition from ankaramites to trachytes, are isotopically homogeneous with 11 of 12 samples within the following ranges: 87Sr/86Sr 0.7035–0.7036, 143Nd/144Nd: 0.51285–0.51290, 206Pb/204Pb: 18.54–18.76, 207Pb/204Pb 15.45–15.49, and 208Pb/204Pb: 38.19–38.51. Geophysical data indicate that the Jan Mayen Ridge represents a microcontinent rifted off the east Greenland continental margin at 36 Ma. The isotopic homogeneity and poor correlation between the degree of magmatic differentiation and Sr‐Nd isotopic composition show that the recent Jan Mayen magmas escaped significant contamination by Precambrian or Lower Paleozoic continental crust. A pervasive Tertiary to recent magmatic infrastructure may explain the lack of contamination with old crustal material. The Jan Mayen alkali basalts have similar trace element and isotopic composition to other oceanic plume basalts and are dominated by low‐degree melts from an enriched mantle component prevalent in the NE Atlantic. The Icelandic alkali basalts are derived from similar sources but are more diluted with picritic or tholeiitic melt fractions formed by progressive melting at shallower levels. Minimum dilution of the incipient low‐degree melts occurs when the melting column is truncated by a thick lithosphere. The mantle source sampled by the Jan Mayen and other alkaline volcanic systems in the NE Atlantic is strongly enriched in high field strength elements like Nb, has a relatively high μ value, and has 207Pb/206Pb ratios below and 208Pb/206Pb ratios above the Northern Hemisphere Reference Line. Such a Pb isotopic composition fits well with an old mantle domain with a time‐integrated low μ value, enriched by a Paleozoic high μ (HIMU) component. The young HIMU‐component in the NE Atlantic upper mantle could represent recycled oceanic crust entrained in the ancestral Iceland plume and distributed laterally by the ancestral plume head.
Two‐dimensional velocity models of the Nazca Plate subduction zone between 19.5°S and 25°S from wide‐angle seismic measurements during the CINCA95 project
Authors: Patzwahl Regina , Mechie James , Schulze Albrecht , Giese Peter
Published: September 2002

The interpretation of seismic refraction/wide‐angle reflection data from the 1995 Crustal Investigations off‐ and on‐shore Nazca/Central Andes (CINCA95) project has resulted in the derivation of nine E‐W two‐dimensional (2‐D) velocity cross sections for the region between the Peru‐Chile trench and the coast between 19.5°S and 25°S, with three of the cross sections extending a farther 100–200 km inland. These sections define the major lithospheric structures of the upper South American plate and the lower Nazca plate down to uppermost mantle depths of 30–60 km beneath this part of the present‐day forearc region. In addition to showing the Nazca plate subducting at an increasing angle of 9–25° down to 30–50 km depth near the coast, these cross sections show a portion of the Moho dipping eastward from 43–50 km near the coast to 55–64 km up to about 240 km inland. Owing to a gap of almost 50 km in the data coverage of the Moho near the coast, it is uncertain from this data set alone whether the Moho defined east of the coast should be correlated with the lower oceanic Nazca plate or the upper continental South American plate. However, a comparison with other seismological data suggests that the Moho identified here east of the coast defines the base of the continental crust of the upper South American plate immediately behind the downgoing Nazca plate. Thus the hypothesis of Delouis et al. [1996] that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported. The cross sections also show between 20°S and 22.5°S, a boundary extending from upper crustal levels down to 25–30 km depth subparallel to and 5–10 km above the top surface of the subducting Nazca plate. The discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate.
A steady state conductive geotherm for the north central Slave, Canada: Inversion of petrological data from the Jericho Kimberlite pipe
Authors: Russell James K. , Kopylova Maya G.
Published: September 2002

Mantle xenoliths carried by kimberlite provide direct evidence for the rock types that constitute the roots to ancient cratons and provide critical constraints on the thermal state of the lithosphere. We derive a steady state conductive geotherm for the north central Slave craton based on published data for xenoliths from the Middle Jurassic Jericho kimberlite. The preferred model geotherm assumes a two‐layer lithosphere. The upper layer is D km thick, has constant thermal conductivity, and an exponential decrease in heat production with depth (Ao e−z/D); Ao is a fixed value based on the bedrock geology. Below D, the lithosphere has a higher, constant thermal conductivity and a constant value of radiogenic heat production (Am). Inverting the thermobarometric data produces model estimates of surface heat flow (qo) and the depth‐scale parameter (D) of 54.1 mWm−2, and 25.8 km, respectively. The value of D corresponds to the thickness of upper to middle crust as defined by seismology for the southern Slave craton. On the basis of our analysis the Slave craton is characterized by high surface heat flow, relative to other Archean cratons, which is consistent with a single field measurement from the southern Slave craton. The high surface heat flow results from the high concentrations of radiogenic heat producing elements found in Slave crustal rocks. Despite the relatively high surface flow, the underlying lithosphere is cool relative to mantle beneath the rest of North America and other Archean cratons. The model geotherm predicts a thermal gradient in the mantle (100–200 km) of between 5.0 and 3.7 (°C/km) and a mantle heat flow of 15.9–11.9 mWm−2 that is consistent with other estimates of mantle heat flow beneath Precambrian continental crust.
Geochemical and Sr‐Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin
Authors: Shinjo Ryuichi , Chung Sun‐Lin , Kato Yuzo , Kimura Masaaki
Published: September 2002

The Okinawa Trough is an incipient intracontinental back arc basin that has developed behind the Ryukyu arc‐trench system. To explore its magmatic evolution and the nature of the mantle source, we present major and trace element and Sr‐Nd isotopic data for mafic volcanic rocks from the Middle and Southern Okinawa Trough and the Central Ryukyu Arc. Two episodes of activity formed the latter: older (∼21–13 Ma) and younger (∼6–4 Ma). Although Quaternary basalts from the Middle Okinawa Trough (MOT) have major element compositions comparable to basalts from intra‐oceanic back arc basins, they are characterized by relative enrichments of large ion lithophile elements and Pb and by depletions of high field strength elements analogous to those observed in Ryukyu Arc volcanics. Two components have been involved in the generation of MOT basalt, which we identify as an E‐MORB type (or oceanic island basalt type) upwelling asthenospheric mantle and a “subduction component.” Quaternary basalts from the Southern Okinawa Trough (SOT) have uniform Nd but heterogeneous Sr isotopic ratios and incompatible trace element compositions. This may be ascribed to more complicated tectonic and magmatic processes in the SOT compared with the MOT, such as oblique subduction of the Philippine Sea Plate and interaction with postcollisional extension in the northern Taiwan orogenic belt. Integrating geological information available from nearby regions, we emphasize that the SOT is an “atypical” back arc basin because its development essentially occurred synchronous with or even prior to development of the arc‐trench system.
Models of ground deformation from vertical volcanic conduits with application to eruptions of Mount St. Helens and Mount Etna
Authors: Bonaccorso Alessandro , Davis Paul M.
Published: September 2002

We present simple analytical models of ground deformation from inflation of vertical volcanic conduits or pipes. We compare three cases corresponding to (1) a pressurized pipe with closed end at the top that resists the internal pressure, (2) a pressurized pipe with top open for which the internal deformation is mainly dislocation of the cylindrical walls, and (3) a pipe‐shaped region that dilates. For the closed pipe we use Eshelby's inclusion theory to model deformation from a thin, pressurized, elongated, prolate ellipsoid in a full‐space, which we express in terms of double forces. To satisfy surface boundary conditions, we superimpose image solutions by using double forces derived from Mindlin's half‐space solution for the point force. For the open pipe we apply the Volterra integral to dislocation across a cylindrical surface and generalize it to the half‐space using Mindlin's point force solution. These two solutions show marked differences from the line of dilatation solution. Ratios of maximum horizontal deformation to maximum vertical deformation for the pipe models are significantly greater than values for the center or line of dilatation models. This, and the more gradual fall off of the deformation with distance may be used as diagnostics for discriminating between pipe‐like sources and dilatational sources on volcanoes, where both components of the deformation field are available. As examples, we compare the closed pipe model with deformation associated with dome building on Mount St. Helens volcano, and tilt predicted by the open pipe model with tilt measured during a period of explosive eruptive activity on Mount Etna, Sicily, in 1995. Comparison between model values and the measurements suggests that the effective elastic moduli of the volcanic cones are very low.
Abiogenic methane in deep‐seated mid‐ocean ridge environments: Insights from stable isotope analyses
Authors: Kelley Deborah S. , Früh‐Green Gretchen L.
Published: September 2002

In this paper we examine geochemical processes that control volatile chemistry at depth in mid‐ocean ridge environments by focusing on CO2‐CH4‐H2O‐H2 fluids entrapped in plutonic rocks from the Southwest Indian Ridge (SWIR), Ocean Drilling Program Hole 735B. Compositional and isotopic analyses of CO2‐CH4‐H2O and CH4‐H2O‐H2 fluids show that methane production involved two phases of magma‐hydrothermal activity, which spanned supersolidus to greenschist facies metamorphic conditions. The first phase of methane generation is characterized by fluid inclusions that contain up to 30–50 mol % CO2 and 43 mol % CH4. Isotopic analyses of CO2, CH4, and H2O released at >900°C yields δ13C(CO2) values of −24‰ to −2‰, δ13C(CH4) values of −30‰ to −19‰, δD(CH4) values of −244‰ to −128‰, and average δD(H2O) values of −43±6‰. Phase equilibria and isotopic data strongly indicate that the CO2‐CH4‐H2O fluids reflect Rayleigh distillation of evolved magmatic CO2, subsequent closed‐system respeciation, and attendant graphite precipitation at temperatures of ∼500–800°C, and at fO2 from −3 log units below, to close to quartz‐fayalite‐magnetite oxygen fugacity (QFM) conditions. The second phase of CH4 production involves CH4‐H2O±H2±C‐fluids that contain >40 mol % CH4. Phase equilibria indicate that the CH4‐H2O fluids were trapped under equilibrium conditions at 400°C, very near to QFM conditions. Our study suggests that in the absence of CO2 as a stable fluid component, extensive distillation fractionation or alteration processes are required to form this later generation of methane. The mean δ13C values of methane extracted at 500°C from the gabbros (−25±4.4‰) are remarkably similar to the range of light carbon observed in studies of mantle rocks. We conclude that the presence of reduced carbon species in oceanic gabbros and mantle peridotites is a potential source of carbon in hydrothermal fluids and that serpentinization processes play a key role in the production of methane at greenschist facies conditions. Although total methane concentrations are low (0.3–0.6 mmol/kg) in the SWIR samples, on a global scale, plutonic layer 3 comprises ∼60% of the oceanic crust and thus represents a potentially immense reservoir (∼1019 gCH4) for abiogenic methane in mid‐ocean ridge hydrothermal systems. Production of methane and hydrocarbon species should be a common process in mid‐ocean ridge systems where high‐temperature fluids interact with mafic mineral phases. This is particularly significant because carbon‐bearing fluids may provide sustenance to subsurface‐and vent‐associated microbial communities and therefore represent an important link between deep‐seated hydrothermal systems and more shallow crustal environments.
Seismic velocities of major silicate and oxide phases of the lower mantle
Authors: Karki Bijaya B. , Stixrude Lars
Published: September 2002

We have recently investigated the elastic properties of five compounds generally considered as potential consituents of the Earth's lower mantle. Full elastic constant tensors of MgSiO3and CaSiO3perovskites, MgO, CaO, and SiO2have been determined as a function of pressure up to 140 GPa and at zero temperature using the first principles plane wave pseudopotential method within the local density approximation. We report comprehensive comparisons of the calculated high‐pressure elastic wave velocities of these phases with seismically derived lower mantle properties. The effects of iron and temperature are estimated in the case of Mg‐silicate perovskite based on available experimental data. Comparison of the P and S wave velocity profiles of perovskite along a series of high‐temperature isotherms to the lower mantle supports the view that this region is primarily composed of Mg‐rich silicate perovskite. Our study also suggests that among the candidate phases usually considered, none are invisible. Minor phases which may be present, including magnesiowüstite, CaSiO3perovskite, and the high‐pressure polymorphs of SiO2and CaO, have elastic wave velocities that differ significantly from those of Mg‐rich silicate perovskite and may be detectable seismically.
On equivalence of dual‐porosity poroelastic parameters
Authors: Bai M.
Published: September 2002

This paper elicits an in‐depth discussion on equivalence of current dual‐porosity poroelastic (DPP) models in terms of their parametric compatibilities, which are shown to break down when physical implications of each model are further probed. The majority of existing DPP models envision the elastic parameters either as those characterizing the matrix blocks only or as the lumped ones delineating for both the matrix blocks and fractures. The flaw of the former characterization is that it neglects the important impact of fracture deformation which should be substantially larger than the matrix deformation because of its greater compressibility; the drawback of the latter delineation is that it omits the significant difference between the matrix and fracture deformations which cannot offer a clear vision for the relevant experimental studies. As a remedy to these approaches, modifications of the parametric relationships in the existing DPP models are provided to accommodate the compatibility requirements. In addition, as an effort to minimize conceptual conflicts in the associated DPP models, sensible formats of the effective stress laws for the dual‐porosity media are presented. The revised effective stress law facilitates the extension of the DPP model to multiporosity poroelastic formulations.
Earthquake‐related water‐level changes at 16 closely clustered wells in Tono, central Japan
Authors: King C.‐Y. , Azuma S. , Igarashi G. , Ohno M. , Saito H. , Wakita H.
Published: September 2002

Water‐level/pressure data recorded at a dense network of 16 wells of depths ranging from 23 to 201 m within 400 m of the Tono Mine in Gifu prefecture, central Japan, have been studied in search of possible earthquake‐related changes. The study area is traversed by the east‐west trending Tsukiyoshi fault, which is ordinarily impermeable, sustaining a significantly higher groundwater pressure on the north side of the fault than the south side. On March 16, 1997, a local earthquake of magnitude 5.8 occurred about 50 km south of Tono. The related water‐level/pressure changes recorded at different sites showed different features. At three deeper wells on the north side of the fault the water level and pressure showed large coseismic drops (up to 29 cm) followed by larger rises (of as much as 1.8 m). Two days before the earthquake the water level at another deeper well began to show a total drop of about 3 m during the next 2 months. Most of the shallow wells on the north side of the fault showed water‐level drops after the earthquake. On the south side, however, only a small delayed surge and a subsequent gradual increase were recorded at a deeper well. Long‐term data since 1989 at one of the deeper wells on the north side showed coseismic drops and recoveries for 25 moderate local and large distant earthquakes up to about 1000 km away. The different observed earthquake‐related water‐level/pressure changes cannot be explained by the static strain field calculated on the basis of poroelastic dislocation models; the changes require earthquake‐related permeability changes at the monitoring sites. The high sensitivity of some of the water wells to seismic shaking may be due to the near‐critical condition of the tapped aquifer/barrier system beginning several years prior to the March 16 earthquake, such that the local seismic shaking caused by each of the above mentioned earthquakes was sufficient to produce some quickly recoverable fissures in the system, resulting in the observed water‐level changes. The sensitivity appears to be variable, showing an apparent decrease during a 1‐year period after the earthquake.
Small‐scale lateral shear velocity and anisotropy heterogeneity near the core‐mantle boundary beneath the central Pacific imaged using broadband ScS waves
Authors: Russell Sara A. , Lay Thorne , Garnero Edward J.
Published: September 2002

Core‐reflected ScS waves from 49 large (M≥5.1) deep Tonga‐Fiji events, recorded in western North America are used to study a localized region of the core‐mantle boundary (CMB) under the central Pacific Ocean. A total of 248 observations from the Berkeley Digital Seismic Network (BDSN), Caltech/United States Geological Survey (USGS) TERRAscope and the Incorporated Research Institutions for Seismology (IRIS) broadband arrays span epicentral distances of 73°–85°. ScS reflection points sample a CMB patch southeast of the Hawaiian islands at latitude 4° to 16° N and longitude −156° to −144° W, near the proposed source location of the Hawaiian plume. Highly variable ScS travel times, amplitudes, waveforms, and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially. ScSH‐SH differential travel times are on average 4 s larger than predicted by the Preliminary Reference Earth Model (PREM). This is due to delayed ScS arrivals and is largely accounted for by a model with a strong velocity decrease in D″. The ScSH‐SH residuals also show a spatial trend not accounted for by a radial model, indicating a lateral decrease in lower mantle shear velocity to the northeast. This lateral velocity gradient appears to cause focusing of ScS energy. ScSH/SH amplitude ratios are larger than predicted by PREM or by a model with a strong negative gradient with depth in D″, which enhances ScS. ScS splitting, corrected for lithospheric anisotropy beneath the receivers, indicates spatial variations in D″ anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path (fast ScSH) to parallel to the ray path (fast ScSV) along a northeast traverse. These spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers, which may be related to dynamical flow near the near the root of the large Hawaiian plume.
Across‐arc geochemical trends in the Izu‐Bonin arc: Constraints on source composition and mantle melting
Authors: Hochstaedter Alfred G. , Gill James B. , Taylor Brian , Ishizuka Osamu , Yuasa Makato , Monta Sumito
Published: September 2002

Across‐arc geochemical trends can constrain subduction zone dynamics by providing clues to the source composition and melting systematics of subduction‐related magmas over a wide portion of the mantle wedge. The Izu‐Bonin arc contains an active volcanic front, an active extensional zone, and a series of 3–9 Ma southwest trending acrossarc seamount chains. The volcanic front (VF) contains one of the most depleted suites of any volcanic arc, with basalt containing 0.2–0.7 ppm Nb, 25–50 ppm Zr, Nb/Zr < 0.015, and Zr/Y < 2.5. Ratios and concentrations of mantle‐derived elements change significantly across the Izu‐Bonin arc. The westernmost portions of the across‐arc seamount chains (WS) contain much higher incompatible element concentrations and associated ratios: 1–8 ppm Nb, 50–130 ppm Zr, Zr/Y = 2–7, and Nb/Zr = 0.02–0.1. The extensional zone contains intermediate concentrations and ratios of these elements. Trace element modeling shows that VF and WS compositions cannot be produced by different degrees of melting of a homogeneous source. Instead, heterogeneous sources are required, implying that enriched source material must exist in the back arc regions of the Izu‐Bonin arc. Melt extraction of fractional melts from the WS source may produce a residual, depleted source capable of generating VF magma. Age dating studies show that the VF and WS suites have retained similar compositions over the last 15 million years, implying that steady state processes have continuously produced these diverse suites of magmas.
Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)
Authors: Nicolas Adolphe , Boudier Françoise , Meshi Avni
Published: September 2002

The Mirdita ophiolite in Albania occupies a N‐S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be excluded. Classically, the western massifs have been considered “lherzolitic” and the eastern massifs “harzburgitic”. Detailed structural mapping reveals that the deep mantle section was harzburgitic and that the major differences between the two are restricted to the uppermost mantle and lower crustal section. These are typically “ophiolitic” in the eastern massifs, being composed of a thick dunitic transition zone rich in basaltic impregnations and chromite deposits overlain by a lower crust of layered gabbros. In contrast, in the western massifs the uppermost mantle is composed of highly strained to mylonitic lherzolites which originate from more depleted harzburgites by impregnation and tectonic dispersion of melt during deformation occurring at 1000°–800°C. Layered gabbros are locally absent, and the crust can be reduced to diabase dikes or sills and extrusives. The diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites. The contrast between the eastern and western massifs is ascribed to successive episodes of magmatic and amagmatic spreading in a slow spreading environment. The low‐T, high strain deformation of the western massifs is localized in the dome‐shaped envelopes of these massifs. This structure, and even the present‐day topography of the western massifs, evoke the “turtleback” domes described along the Mid Atlantic Ridge (MAR) and explained by mantle denudation [Tucholke et al., 1998].