bluuebunny/crossref_metadata_2025 huggingface dataset:
Near‐specular reflection of ions at quasi‐parallel shocks
Authors: McKean M. E. , Winske D. , Thomsen M. F. , Onsager T. G.
Published: February 2008

Previous numerical simulations have shown that ions are reflected at quasi‐parallel shocks in a bursty manner. At times, relatively cold and dense beams of ions that appear to be specularly reflected are seen, consistent with observations at the quasi‐parallel portions of the bow shock. The intermittent reflection of ions has been shown to be connected to the periodic disruption and re‐formation of the shock front. Though the connection of the reflected ions to the re‐formation cycle has been studied extensively, the reflection process itself has received little attention. Using one‐dimensional hybrid simulations and a semianalytic model of the shock front, the reflection process is investigated to determine the source regions in incident ion phase space of reflected and transmitted ions, the relative importance of the electric and magnetic forces in the reflection of incident ions, and how these characteristics change in time. We find that the phase space origin of reflected particles and the fraction of incident ions that reflect depend upon the electromagnetic field structure of the shock front at the time the ions encounter it. The reflection fraction is maximized when the electric field along the shock normal (Ex) and the noncoplanar magnetic field (By) are at their maximum values. This happens because the phase space source region of reflected particles lies closer to the center of the incident ion distribution function when those two field components are large than it does when they are small. Also, when Ex and By are large, the reflection process produces a beam that is cooler, more dense, and closer to specular than when they are small. We also find that, in general, the electric field provides most of the force involved in the reflection of incident ions, although the magnetic field is also needed to reflect all but a few ions. When Ex is near zero, almost no ions reflect in the shock ramp.
The solar coronal sheet during the period of sunspot maximum
Authors: Saito T. , Akasofu S. ‐I. , Kozuka Y. , Takahashi T. , Numazawa S.
Published: February 2008

It is demonstrated that the outer corona extends radially in the form of a sheet from the neutral line on the source surface (a spherical surface of 2.5 solar radii) or from an even lower level during the period of sunspot maximum, as well as during the period of sunspot minimum. During the period of sunspot maximum the neutral line lies nearly at right angles to the heliographic equator. Assuming that the long coronal streamers extend from the neutral line (as is known to be the case during the period of sunspot minimum), the expected geometry is constructed and is compared with coronal photographs taken during the eclipse. It resembles the observed coronal photographs seen from the Earth on July 11, 1991. In particular, the expected geometry can explain details of the observed features, including highly inclined (with respect to the heliographic equator) long coronal streamers and polar plumelike features. It is also explained why in the past the corona was believed to be nearly spherical during the period of sunspot maximum.
Low‐frequency mobility response functions for the central plasma sheet with application to tearing modes
Authors: Hernandez J. , Horton W. , Tajima T.
Published: February 2008

The effect of a constant cross‐tail magnetic field By on the collisionless conductivity produced by chaotic scattering and stochastic diffusion of particles in the current sheet for a parabolic geometry is considered. We show that the correlation time scales as (By/Bz)² and from this strong By scaling, we infer a strong tendency toward stabilization of the linear tearing modes with increasing values of By. This effect of increased dawn‐dusk mobility is particularly dramatic when electrons are introduced in the calculation, and is in agreement with the results of kinetic particle simulations. The By‐stabilizing effect complements the earlier Bz‐stabilizing effect reported by Galeev and Zelenyi (1976) and Lembége and Pellat (1982) due to electron compression. The collisionless conductivity is expressed in terms of the ensemble‐averaged power spectrum of the single particle trajectories. This allows us to calculate directly the linear conductivity instead of deriving it from the calculation of the irreversible heating rates as in Horton and Tajima (1991a). The importance of determining the collisionless conductivity (or dawn‐dusk mobility) in terms of the power spectrum cannot be overstated since the power spectrum gives direct evidence of the effect of the chaotic particle dynamics in the generation of a collisionless irreversible resistivity.
The role of ring current nose events in producing stable auroral red arc intensifications during the main phase: Observations during the September 19‐24, 1984, Equinox Transition Study
Authors: Kozyra J. U. , Chandler M. O. , Hamilton D. C. , Peterson W. K. , Klumpar D. M. , Slater D. W. , Buonsanto M. J. , Carlson H. C.
Published: February 2008

An examination of stable auroral red (SAR) arc emissions over the last solar cycle (Slater and Kleckner, 1989) indicates that the strongest emissions, during the lifetime of a particular SAR arc, often occur in association with the main phase of the magnetic storm. Previous observations of thermal and energetic particle populations at high and low altitudes on SAR arc field lines by Kozyra et al. (1987a) indicate that the energy source for these emissions is the O+ in the ring current. The O+ content of the ring current increases with increasing magnetic activity reaching its maximum percentage contribution near minimum Dst for a particular storm. This variation in the O+ content of the ring current is inconsistent with an early main phase enhancement of SAR arc emissions. To investigate the source of main phase enhancements in SAR arc emissions, a study of the September 19‐24, 1984, magnetic storm period during which SAR arc emissions were observed by the ground‐based mobile automatic scanning photometer network in both the main and recovery phases is presented. The emissions associated with the main phase (∼ 400 R) were an order of magnitude greater than those associated with the recovery phase (tens of R). Energetic particle measurements from the DE 1 and AMPTH spacecraft, on field lines that map to the SAR arc position at low altitude, were examined to determine if differences in the energy sources during these time periods were evident. In agreement with previous work, ring current O+ supplied the bulk of the electron heating during storm recovery phase as a result of Coulomb collisions of O+ with the plasmaspheric electrons; contributions by ring current H+ were negligible. A new result of the present work is that an enhancement of the 15‐25 keV H+ component of the ring current during the main phase of the September 19 magnetic storm was responsible for an approximately one order of magnitude increase in the electron heating rate and SAR arc emissions during the main phase compared to the recovery phase. The increase in the H+ flux occurred in association with a ring current “nose event”, a front of ions injected into the inner magnetosphere in response to a discontinuous change in the cross‐tail electric field. The association between nose events and intensifications of SAR arc emissions in the main phase has not previously been explored but is a natural consequence of the injection of significant fluxes of relatively low‐energy ring current ions earthward of the plasmapause during early storm time ring current formation.
Ion cyclotron anisotropy instabilities in the magnetosheath: Theory and simulations
Authors: Gary S. Peter , McKean Michael E. , Winske Dan
Published: February 2008

Recent observations and theoretical studies have established the importance of the tenuous helium ion component in determining instability growth and low‐frequency magnetic fluctuation properties in the terrestrial magnetosheath. Under low‐β, high‐ion‐anisotropy sheath conditions, enhanced fluctuations observed below the proton cyclotron frequency have been attributed to the proton and helium cyclotron anisotropy instabilities. This paper uses second‐order theory and one‐dimensional hybrid computer simulations to examine the nonlinear properties of these two instabilities at relatively weak fluctuation levels and at propagation parallel to the background magnetic field. The simulations confirm the second‐order predictions that both instabilities yield efficient wave‐particle interactions; the rate at which the driving species anisotropy is reduced is much greater than the rate at which that species loses kinetic energy. This result suggests that these instabilities should saturate at relatively low levels; we derive an approximate expression for the fluctuating field energy at saturation of the fastest growing modes and find that it is in fair agreement with three computer simulations. In addition, second‐order theory and simulations concur that the helium component enhances the wave‐particle exchange rate for proton anisotropy reduction by the proton cyclotron instability; theory predicts and simulations confirm that the saturation energy of this mode is substantially lower than it is in the absence of helium.
Optical signatures and sources of prenoon auroral precipitation
Authors: Burke W. J. , Jacobsen B. , Sandholt P. E. , Denig W. F. , Maynard N. C. , Newell P. T.
Published: February 2008

Optical observations from Ny Ålesund, Svalbard, of prenoon auroral structures are compared with simultaneous measurements of energetic particle precipitation and plasma drifts taken during a nearby overpass of the DMSP‐F9 satellite near the 0830 MLT meridian. Close to the time of this pass, a stable band of red‐line emissions was detected from south of Ny Ålesund. Very weak, transient auroral activity was observed from the sky above and north of the station. An automatic classification algorithm applied to the DMSP‐F9 particle‐precipitation data, identified source regions in the central plasma sheet, the boundary plasma sheet, the plasma mantle, and the polar rain. We interpreted the poleward and equatorward parts of the red auroral band as being collocated with regions of mantle and boundary plasma sheet precipitation on tailward and sunward convecting field lines, respectively. However, the mantle part of the red auroral band, as interpreted by the automated algorithm, is difficult to reconcile with simultaneously measured, high‐latitude convection distributions. Another interpretation of the data, which includes magnetic mapping considerations and constraints imposed by the global electric field pattern, suggests that the entire band of red‐line emissions from south of Ny Ålesund is excited by electrons originating in the nightside, boundary plasma sheet. Although neither interpretation is definitive, resolving dilemmas that they pose is critical for understanding this common and simple form of dayside aurora.
Aeronomy of infrared ozone fluorescence measured during an aurora by the SPIRIT 1 rocket‐borne interferometer
Authors: Rawlins W. T. , Woodward A. M. , Smith D. R.
Published: February 2008

The SPIRIT 1 rocket‐borne infrared interferometer (4 to 25 µm) was launched from Poker Flat, Alaska, on April 8, 1986‐, and viewed the atmosphere in full‐limb and partial‐limb geometries during nighttime IBC II‐III auroral activity. Analysis of O3(ν3) fluorescence spectra (9 to 12 µm), obtained for tangent altitudes between 70 and 105 km, provides determinations of O3(υ) column density profiles for at least six quanta of O3 stretching vibration. Pure‐rotation transitions from highly rotationally excited OH also contribute to the spectral intensities in this wavelength region. The observed ozone vibrational state populations are generally consistent with predictions of a nonauroral model describing atmospheric O3(υ) excitation in terms of earthshine absorption and three‐body recombination of O and O2. In addition, discrepancies between full‐limb and partial‐limb results indicate an enhancement of O3(υ ≥ 3) in the vicinity of a strong auroral arc, due to either a downward propagation of atomic oxygen near the aurora or a direct auroral excitation process. The derived O3(υ) number density profiles are used to estimate altitude profiles for total O3, O, and H and to examine the response of odd oxygen/odd hydrogen chemistry to auroral bombardment. The results illustrate the potential of spectrally resolved infrared fluorescence for remote sensing of high‐altitude chemical responses on a global basis.
Heat flow effect on the plasma line frequency
Authors: Kofman W. , St‐Maurice J. ‐P. , van Eyken A. P.
Published: February 2008

Recent high‐resolution studies of the electron plasma line show that the difference between the upshifted and the downshifted frequency does not depend only on the electron drift in the plasma and on the electron temperature through the classical dispersion relation using the Maxwellian electron distribution. The difference contains an additional strong nonlinear dependence on the electron temperature. We show here that the origin of this shift in the center frequency between the upshifted and downshifted lines can easily be due to heat flows in the electron gas. We present a new expression for the dispersion relation that includes the heat flow contribution, and we use a recently acquired data set to demonstrate that the heat flow correction produces the right kind of results. For radar frequencies around 1000 MHz and plasma frequencies of the order of 7 MHz, this means that an electron temperature gradient of the order of 1 K/km can produce a difference approaching 500 Hz between the upshifted and downshifted frequencies. This shift in frequency is important, as it is comparable to the shift caused by parallel currents of a few microamperes per square meter.
Deformation of a magnetic dipole field by trapped particles
Authors: Neukrirch T.
Published: February 2008

We investigate the equilibrium and stability of rigidly rotating quasi‐neutral magnetospheres of objects with an aligned magnetic dipole moment, e.g., planetary magnetospheres. We use the Vlasov theory to calculate the equilibria with emphasis on trapped particle populations. Based on a new approach for the description of the trapped particles within the framework of collisionless theory, we calculate self‐consistent axisymmetric magnetospheric models. For the purpose of demonstrating the capability of this approach, we choose a mathematically simple, but nevertheless physically relevant, class of distribution functions. By varying the plasma density, we calculate solution branches and investigate the bifurcation properties for typical cases. Furthermore, the stability of the solutions is tested by applying a sufficient stability criterion to them. We find that in our models there is always a maximum amount of plasma which can be confined in a rigidly rotating magnetosphere within a dipolelike magnetic field. The implications for possible scenarios of magnetospheric activity are discussed on the basis of these results.
Neutral lines and azimuthal “transport” of solar energetic particles
Authors: Kallenrode May‐Britt
Published: February 2008

We discuss properties of solar energetic particle (SEP) events observed by both Helios spacecraft in the time period March 1976 to March 1980, in particular variations of the intensity time profiles with angular distance between the flare and the observer's magnetic footpoint. Special attention is paid to the neutral lines of the large‐scale coronal magnetic field. For individual events we will show that sector boundaries can have a strong influence on intensities and time‐scales of SEP events. This can be seen best in events in which both spacecraft are connected more or less symmetrically around the flare site with a sector boundary between them. In a statistical approach we discuss variations of onset and maximum times as well as maximum intensities with angular distance between the flare and the observer's magnetic footpoint for 39 SEP events. The use of two spaceprobes has the advantage that unknown flare parameters, such as the number of injected particles and the time of acceleration, can be eliminated by studying only relative variations of time and intensity with angular distance. These variations were analyzed under consideration of the sector boundaries of the large‐scale coronal magnetic field. We find (1) particles can be observed on both sides of sector boundaries during both, impulsive and gradual, events, (2) the onset times of ∼0.5‐MeV electrons can be ordered by the occurrence of sector boundaries, inside the flare sector we can define some kind of “azimuthal propagation velocity” Δϕ/Δtons with ≥4°/min, outside the flare sector this velocity is ≤1°/min, (3) the azimuthal propagation of ∼20‐MeV protons seems to be systematically slower, (4) there seems to be no difference in the variations of onset times or maximum intensities with angular distance between impulsive and gradual events, and (5) within the broad scatter the variation of the maximum intensity with angular distance is not systematically influenced by the crossing of sector boundaries, despite the obvious influence of sector boundary crossings in individual events. These observations support the association of the fast propagation region (FPR) with the large‐scale polarity cells of the coronal magnetic field. The “transport” inside the FPR can be understood in terms of an open cone of propagation. The “transport” outside the FPR can be interpreted in terms of a propagation mechanism or acceleration at a shock.
Energy storage and dissipation in the magnetotail during substorms: 2. MHD simulations
Authors: Steinolfson R. S. , Winglee R. M.
Published: February 2008

The effects of temporal and spatial variations in the plasma resistivity on the evolution of the magnetosphere during substorms are examined with numerical solutions of the two‐dimensional magnetohydrodynamic (MHD) equations. The global MHD simulations self‐consistently consider the interaction of the solar wind with the dayside magnetosphere as well as the evolution of the tail region. These solutions are used to study how various solar wind states generate conditions in the tail, such as pressure gradients and cross‐tail currents, that have the potential of leading to a substorm. Although the MHD formalism does provide information on the large‐scale evolution, the essential mechanism for substorm development may involve microscopic or particle processes not present in an MHD approach. As a result, this MHD study is carried out in association with particle simulations (Winglee and Steinolfson, this issue). Since one connection between the MHD and particle approaches is through the resistivity, the effects of various resistivity distributions on the global MHD configuration are examined. The resistivity distributions considered here are (1) a temporally constant and spatially uniform resistivity, (2) a resistivity proportional to the square of the local current density, and (3) a resistivity proportional to the square of the local magnetic field strength. The latter distribution is suggested by the above particle simulations and represents effects produced by the increased magnetization of particles and the differential motion between electrons and ions. For all three cases a plasmoid is formed and ejected tailward. However, when the resistivity depends on the field strength, considerably more energy is stored in the tail prior to plasmoid formation, and plasmoid formation is delayed relative to the results for the other two resistivity distributions. Furthermore, when the plasmoid is eventually ejected, it moves down the tail with a higher speed. The MHD results are consistent with those from the particle simulations.
Numerical simulation of plasma processes driven by transverse ion heating
Authors: Singh Nagendra , Chan C. B.
Published: February 2008

Numerical simulation is performed to study the plasma processes driven by transverse ion heating in a diverging flux tube. It is found that the heating drives a host of plasma processes, in addition to the well‐known phenomenon of ion conics. The additional processes include formation of a density cavity topped by a density enhancement, formation of a reverse and forward shock pair with a “double‐sawtooth” structure in the flow velocity. The downward electric field near the reverse shock generates a doublestreaming situation consisting of two upflowing ion populations with different average flow velocities. A double streaming also occurs above the forward shock, where the ions energized by the heating are overtaking the relatively slow ions in the ambient polar wind. The energized ions appear as “elevated” ion conics with a low‐energy cutoff depending on the distance from the heating region. The parallel electric fields generated by the transverse ion heating have the following noteworthy features; the electric field near the forward shock is essentially unipolar, and it points upward, and for the heating localized in both space and time, the field has the features of a weak double layer. The electric field in the reverse shock region is modulated by the ion‐ion instability driven by the multistreaming ions. The oscillating fields in this region have the possibility of heating electrons. The results from the simulations are compared with results from a previous study based on a hydrodynamic model. Effects of spatial resolutions afforded by simulations on the evolution of the plasma are discussed, demonstrating how a crude resolution can miss out plasma instabilities, affecting the plasma flow.
Scaling of density fluctuations with Mach number and density‐temperature anticorrelations in the inner heliosphere
Authors: Klein L. , Bruno R. , Bavassano B. , Rosenbauer H.
Published: February 2008

We present observations of field and plasma fluctuations of duration of a few hours or less of spacecraft time frame as observed by Helios 2 in the inner heliosphere. Two distinct turbulent plasma regimes are found. In the regions between high‐speed streams, small‐amplitude density fluctuations show an excellent correlation with plasma frame Mach number (M), and they are often anticorrelated with small‐scale proton temperature fluctuations. In high‐speed flows, however, density fluctuations have several scales, both M and M2, and density and temperature fluctuations are more positively correlated. Our observations cast in a new light the distinctly different nature of high‐speed flows from coronal holes and flows from other equatorial regions. They provide a characterization of small‐scale fluctuations in the inner heliosphere, and we interpret the results in terms of the nearly incompressible fluid description of the solar wind (Zank and Matthaeus, 1990). Additionally, we compare our results with Voyager observations in the outer heliosphere (Matthaeus et al., 1991).
Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus
Authors: Kelley Deborah S. , Robinson Paul T. , Malpas John G.
Published: February 2008

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high‐temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures >450–600°C resulted in entrapment of brine‐rich aqueous fluids with salinities of 36–61 wt % NaCl equivalent. Homogenization of the brine inclusions by haute dissolution, the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz‐hosted, high‐salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep‐seated, high‐temperature portion of the hydrothermal system. In localized areas, the high‐temperature brines (NaCl±KCl±CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low‐temperature, low‐salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high‐temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures >200–400°C. Hydration reactions under greenschist facies conditions, or limited mixing with brine‐rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low‐salinity inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid‐ocean ridge spreading centers. The low‐temperature fracture networks may represent an extensive deep‐seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.
Petrology of basaltic sills from Ocean Drilling Program sites 794 and 797 in the Yamato Basin of the Japan Sea
Authors: Thy P.
Published: February 2008

The basement rocks drilled at Ocean Drilling Program sites 794 and 797 in the Yamato Basin of the Japan Sea are composed dominantly of aphyric, plagioclase phyric, and olivine and plagioclase phyric, tholeiitic sills intruded into wet sediments. The rocks can be divided into an incompatible element depleted type, and an enriched type, based on their mineral and bulk rock compositions. Site 794 drilled an upper suite of dominating aphyric, enriched, basaltic sills; the upper part of which contains relatively calcic plagioclase phenocrysts (An85–90), and the lower part of which contains occasionally olivine phenocrysts. A lower suite at site 794 is composed of dominating aphyric, depleted sills, with plagioclase and olivine microphenocrysts occasionally in chilled margins. Site 797 recovered an upper suite of sparsely plagioclase and olivine phyric to aphyric, depleted sills, and a lower suite of sparsely plagioclase phyric and aphyric, enriched sills. The groundmass phases of all suites suggest a systematic enrichment in FeO, TiO2, Na2O, and K2O during solidification. In addition, there is evidence that olivine ceases to crystallize approximately when augite appears during groundmass crystallization. Irrespective of the enriched versus the depleted magma type, the rock compositions reveal two contrasting evolution trends, which can be related to fractionation processes in crustal magma chambers. A tholeiitic evolution trend, with increasing FeO and TiO2 and decreasing Al2O3, can be modelled by fractional crystallization of 40% to 50% plagioclase, olivine, and augite. A mildly calc‐alkalic evolution trend, with decreasing FeO, increasing Al2O3, and nearly constant TiO2, can be modelled by 8% to 12% olivine fractionation. One‐ atmosphere, anhydrous melting experiments on a basalt from hole 797C produced liquid compositions very similar to the basalts that exhibit the FeO enrichment trend. There is mineralogical evidence that the contrasting trends may be related to the effect of small amounts of water during crystallization. Relatively high water activity will suppress plagioclase crystallization and thus produce the iron depletion trend, while nearly anhydrous crystallization involves the early appearance of plagioclase, which is responsible for the iron enrichment trend. The tholeiitic suites occur in lower parts of the drill cores, while the calcalkalic suites occur in the upper parts. This suggests a magmatic and tectonic evolution of the Japan Sea, perhaps reflecting a transition between calc‐alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back arc spreading. Furthermore, any model must be able to account for the range in parental magmas from depleted to enriched types throughout the magmatic history of the Yamato Basin.
Effects of periodic atmospheric pressure variation on radon entry into buildings
Authors: Tsang Y. W. , Narasimhan T. N.
Published: February 2008

Using a mathematical model, we have investigated the temporal variations of radon entry into a house basement in the presence of time‐dependent periodic variations of barometric pressure as well as a persistent small steady depressurization within the basement. The tool for our investigation is an integral finite difference numerical code which can solve for both diffusive and advective flux of radon in the soil gas which is treated as a slightly compressible fluid. Two different boundary conditions at the house basement are considered: (1) a dirt floor basement so that diffusion is equally or more important than advective transport, and (2) an “impermeable” cement basement except for a 1‐cm‐wide crack near the perimeter of the basement floor; in which case, advective transport of radon flux dominates. Two frequencies of barometric pressure fluctuation with representative values of amplitudes, based on a Fourier decomposition of barometric pressure data, were chosen in this study: one with a short period of 0.5 hour with pressure amplitude of 50 Pa, the other a diurnal variation with a period of 24 hours with the typical pressure amplitude of 250 Pa. For a homogeneous soil medium with soil permeability to air between 10−13 and 10−10 m2, we predict that the barometric fluctuations increase the radon entry into the basement by up to 120% of the steady radon inflow into the basement owing to a steady depressurization of 5 Pa. If soil permeability heterogeneity is present, such as the presence of a thin layer of higher permeability aggregate immediately below the basement floor, radon flux due to atmospheric pumping is further increased. Effects of pressure pumping on radon entry are also compared to diffusion‐only transport when the steady depressurization is absent. It is found that contribution to radon entry is significant for the basement crack configuration. In particular, for pressure pumping at 0.5‐hour period and for a homogeneous medium of permeability of 10−10 m2, the radon entry is a factor of 10 larger than that predicted by the diffusion‐only transport. This may help to explain indoor radon concentrations during times of low steady state driving force. Extending beyond radon transport, the results of this case study establish the importance of transient advective transport resulting from atmospheric pressure variation. These results may have relevance in the estimations of the transfer of trace gases such as methane and nitrous oxide across the soil‐atmosphere interface and their impact on global climate changes.
Isostatic deformations and polar wander induced by redistribution of mass within the Earth
Authors: Ricard Yanick , Sabadini Roberto , Spada Giorgio
Published: February 2008

The effects at the surface of the Earth of time‐dependent mantle mass anomalies are analyzed within the framework of a viscoelastic mantle with Maxwell rheology. The implications for the Earth's rotation are developed using the linearized Liouville equations valid for small polar displacements. Our approach is appropriate for a simplified modeling of subduction. The displacement of the Earth's axis of rotation, called true polar wander, is very sensitive to the viscosity profile of the mantle and to the nature of the 670‐km seismic discontinuity. Phase change models generally yield a huge amount of polar wander, except for large viscosity increases. For chemically stratified models, true polar wander is drastically reduced as a consequence of dynamic compensation of the mass anomalies at the upper‐lower mantle interface. When the source is embedded in the upper mantle in the proximity of the chemical density jump, the polar wander stops after a finite time controlled by the slowest isostatic mode.
Magnetic domain wall visibility in bitter pattern imaging
Authors: Williams Wyn , Enkin Randolph J. , Milne Glenn
Published: February 2008

A theoretical analysis of the imaging properties of the Bitter pattern method is made for both Bloch and Neel domain walls in ferromagnetic grains. We have formulated an analytic expression for the magnetic fields due to Néel and Bloch walls and determined the colloid density (and hence the optical contrast) that collects on the grain surface due to these fields. The results show that 180° Néel and Bloch walls are equally well imaged for walls of equal width, but since Néel walls are generally wider than Bloch walls, they should be significantly more visible. Furthermore, the optical contrast produced by accumulation of colloid decreases as the angle through which the magnetization rotates becomes smaller (an effect which is more pronounced for Néel walls than Bloch walls). We find also that both Néel and Bloch walls become less visible as depth of domain wall decreases, so that very shallow domain walls may be difficult to see compared with domain walls representative of the grain body domain structure.
Short‐period constraints on the proposed transition zone discontinuity
Authors: Jones Laura E. , Mori Jim , Helmberger Donald V.
Published: February 2008

Short‐period P wave data from several events which occurred off the coast of northern Mexico and were recorded on the Southern California Seismic Network are used to investigate the existence and nature of the proposed transition zone discontinuity at 520 km. Synthetically, the 520 discontinuity produces a characteristic signature at 19° to 22° epicentral distance, seen as a small triplication which adds constructively to the first arriving phase to produce a large‐amplitude first arrival. Convincing evidence for such an amplitude pattern is not present in record sections. Thus, the possibility of a diffuse discontinuity at 520 km is explored: several models with 1%, 2%, and 2.5% velocity contrasts distributed over zones from 10 to 30 km are tested. Again, evidence for this feature in our data set is negligible and does not justify its inclusion in our model. However, a model with a simple, sharp density contrast at 520 km produces subcritical angle reflections similar to those observed by other authors, and does not violate the short‐period triplication data.
Constraints on magnetic energy and mantle conductivity from the forced nutations of the Earth
Authors: Buffett Bruce A.
Published: February 2008

Observations of the Earth's nutations are used to explore the possible presence of a conducting layer at the base of the mantle. The existence of such a layer is suggested by recent experiments on iron and silicate mixtures at high pressures and temperatures (Knittle and Jeanloz, 1986, 1989), which indicate that iron may react with silicate minerals to produce metallic compounds. Nutations provide an ideal tool for investigating the existence of such a layer because they are sensitive to the electrical properties within a thin magnetic boundary layer along the core‐mantle boundary (CMB). Evidence in favor of a conducting layer is found in the effects of ohmic dissipation which cause the amplitude of the Earth's nutation to be out‐of‐phase with tidal forcings. Although out‐of‐phase components in the Earth's nutations are conventionally attributed to the dissipative influences of ocean tides and mantle anelasticity, these effects appear incapable of explaining nutation amplitudes determined from very long baseline interferometry (VLBI) observations. The additional effects of ohmic dissipation, enhanced by the presence of a thin conducting layer at the CMB, are sufficient to reconcile theory and observation. The largest mismatch between observations and theory, taking account of ocean tides and mantle anelasticity, is a discrepancy of 0.39 ± 0.04 milliarcseconds in the out‐of‐phase component of the retrograde annual nutation. This discrepancy can be eliminated if the lowermost 200 m of the mantle has a conductivity of 5 × 105S m−1, and the magnetic energy in the spherical harmonic components of the field at the CMB in degrees l > 12 is approximately 4 times greater than the energy inferred in degrees l ≤ 12 from surface observations. Unfortunately, the VLBI observations do not uniquely resolve the mantle conductivity and magnetic energy, so that trade‐offs exist between these two parameters. Nevertheless, a high mantle conductivity is favored on the grounds that the alternative, involving high levels of magnetic energy, yields an excessive amount of ohmic heating. Other interpretations of the VLBI discrepancy are also discussed. One such alternative involves viscous dissipation at the CMB, requiring a kinematic viscosity of ν = 0.1m2s−1.
A dual origin for the hydrothermal component in a metalliferous sediment core from the Mid‐Atlantic Ridge
Authors: Mills Rachel , Elderfield Henry , Thomson John
Published: February 2008

A highly metalliferous sediment core collected from TAG, 26°N Mid‐Atlantic Ridge has been analyzed for uranium series isotopes, rare earth elements (REE), and radiogenic isotopes. Two types of deposition of hydrothermal material have been identified and characterized. The first is sulfide material which has been rapidly deposited and is sometimes clearly turbiditic. This material exhibits high U/Fe and Eu/Sm ratios and low 230Th/Fe and REE/Fe ratios. It is proposed that U is incorporated into the sulfides by diffusion through the oxidized coating with reductive fixation near the sulfide/oxide boundary. The second hydrothermal component is particulate fallout from the neutrally buoyant plume which is diluted by the background pelagic sedimentation. This component exhibits U/Fe ratios around 2 × 10−5, an order of magnitude greater than reported plume particulate levels, but the 230Th/Fe, REE/Fe and Eu/Sm ratios are similar to plume values. Nd and Pb isotopes have been used to differentiate the hydrothermal and pelagic end‐members within this sediment core.
Velocity‐porosity relationships in the upper oceanic crust: Theoretical considerations
Authors: Berge Patricia A. , Fryer Gerard J. , Wilkens Roy H.
Published: February 2008

We consider here the application of rock physics theories to investigate relationships between seismic velocities and porosities in the shallow oceanic crust. Classical Hashin‐Shtrikman limits ignore void shapes and are too broad to provide useful constraints on velocities and porosities. Making some assumptions about the distribution of void shapes improves the constraints. Theories which ignore crack‐crack interactions underestimate the effects of porosities on velocities, thus providing upper bounds on velocities and porosities. “Self‐consistent” theories overestimate crackcrack interactions and so provide lower bounds. At the high porosities required to reduce basalt from a P velocity of 7km/s in massive form to the 2.2km/s observed in zero‐age oceanic crust, however, the bounds are too far apart to be useful. The theories are strictly valid only for very small porosities. Using an algorithm proposed by Cheng for iteratively building up porosity to create a highly porous medium, analogous to differential computation methods traditionally used to improve upon the self‐consistent approach, we have devised two hybrid theories, which we term extended Walsh and extended Kuster‐Toksöz. These two theories remain approximately valid at the high porosities of oceanic crustal layer 2A to provide useful upper and lower bounds on velocity for a given porosity and pore aspect ratio distribution. We attempt the inverse problem, determining porosity from a given velocity, using on‐bottom refraction data collected on the flank of the East Pacific Rise. For 120ka material with a P velocity of 2.5km/s, if our assumptions regarding the aspect ratio distribution are correct, porosity lies somewhere between 24 and 34%. Resolution on slower, zero‐age crust (2.2km/s) is poorer: there we predict a porosity between 26 and 43%. Use of shear‐wave information would tighten these bounds.
Depth distribution of coseismic slip along the Nankai Trough, Japan, from joint inversion of geodetic and tsunami data
Authors: Satake Kenji
Published: February 2008

Coseismic slip distribution on the fault plane, particularly in the downdip direction, associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data. Two large earthquakes, the 1944 Tonankai earthquake (Mw=8.1) and the 1946 Nankaido earthquake (Mw=8.3), occurred on the Nankai trough, southwestern Japan, where the Philippine Sea plate is subducting beneath the Eurasian plate. The source areas of these events extended over both land and ocean. Coseismic crustal movements on land were measured by leveling, while those in ocean were recorded as tsunami waveforms on tide gauges. The coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts. This indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low. The result has implications for other subduction zones having a similar tectonic environment such as for the Pacific Northwest region of the United States. Although the possibility of a large earthquake there is still debated, should a large subduction earthquake occur in this region, coseismic slip on the shallow part could be large, and the potential for large tsunamis is high.
An evaluation of temporal geochemical evolution of Loihi Summit Lavas: Results from Alvin submersible dives
Authors: Garcia Michael O. , Jorgenson Beth A. , Mahoney John J. , Ito Emi , Irving Anthony J.
Published: February 2008

Stratigraphically controlled sequences of in situ lavas were collected from Loihi Seamount using the Alvin submersible to evaluate the volcano's temporal geochemical evolution. Three sections with up to 370 m of relief were sampled from the two pit craters at the summit of Loihi. All of the analyses were done on glass separates. Our results indicate that tholeiitic and alkalic volcanism at the summit of Loihi has been coeval. The tholeiitic and alkalic lavas have similar incompatible element patterns and O, Pb, Sr, and Nd isotope ratios but are distinct in some incompatible element ratios. These results are consistent with the different Loihi rock types being derived by variable degrees of melting from a common source. The crossing and light‐rare‐earth‐enriched rare earth element patterns and variable Sc/Yb ratios of the tholeiites indicate that their source was a garnet lherzolite. The relatively low δ18O values (∼4.9 ‰) for Loihi lavas are interpreted to be characteristic of the Hawaiian plume.
Heat flow and thermal models of the Barbados Ridge Accretionary Complex
Authors: Ferguson I. J. , Westbrook G. K. , Langseth M. G. , Thomas G. P.
Published: February 2008

The pattern of heat flow from the Barbados Ridge accretionary complex has been derived from marine surveys with heat flow probes, from measurements in drill holes, and from the depths of bottom‐simulating seismic reflectors caused by gas hydrate. The heat flow from the accretionary complex has been simulated using a finite‐difference model to investigate how heat flow responds to changes in the cross‐sectional shape of the complex and the rate of convergence, and to variations in pore‐fluid pressure within the complex and along the decollement at its base. In the south of the complex, heat flow decreases westward from the toe of the wedge, towards the island arc, because the downward movement of subducting lithosphere beneath the wedge and the active thickening of the accreted sedimentary sequence reduce the geothermal gradient more rapidly than thermal diffusion can maintain it. Localised high anomalies, attributed to the flow of warm fluids along fault zones, are superimposed on the conductive heat flow pattern. Farther west, heat flow increases arcward, produced partly by thermal diffusion re‐establishing a steeper thermal gradient where the rate of thickening is decreased, but mainly by the arcward increase of frictional heating along the base of the wedge caused by the increase of slip rate and increased normal stress arising from thickening of the wedge. In the north of the complex, around the ODP Leg 110 drill sites, the conductive model cannot reproduce the anomalously high heat flow measurements, which can only be explained by the advection of warm pore fluids.
A study of the Earth's free core nutation using international deployment of accelerometers gravity data
Authors: Cummins Phil R. , Wahr John M.
Published: February 2008

In this study we consider the influence of the Earth's free core nutation (FCN) on diurnal tidal admittance estimates for 11 stations of the globally distributed International Deployment of Accelerometers network. The FCN causes a resonant enhancement of the diurnal admittances which can be used to estimate some properties of the FCN. Estimations of the parameters describing the FCN (period, Q, and resonance strength) are made using data from individual stations and many stations simultaneously. These yield a result for the period of 423–452 sidereal days, which is shorter than theory predicts but is in agreement with many previous studies and suggests that the dynamical ellipticity of the core may be greater than its hydrostatic value. The estimate of 3035–49,689 for the Q falls in between the incompatible results of previous estimates based on tidal gravity and very long baseline interferometry observations, but the formal uncertainties are large enough for this result to be compatible with all of the previous studies. More careful consideration of the four‐dimensional confidence interval, however, indicates that one reason for our relatively large error bars is a strong trade‐off between the real and imaginary parts of the eigenfrequency and the real and imaginary parts of the resonance strength, respectively. This trade‐off should probably be considered in future work in order to facilitate comparisons between different studies. Finally, we point out that observation of the freely excited wobble in the diurnal surface gravity spectrum does not seem likely based on the most recent estimates of its nutation amplitude.
Energy‐density effects on seismic decoupling
Authors: Glenn L. A.
Published: February 2008

The teleseismic amplitude resulting from an underground explosion is proportional to the asymptotic value of the reduced displacement potential (ϕ∞) or, in physical terms, to the permanent change in volume measured anywhere beyond the range at which the outgoing wave has become elastic. It is known that ϕ∞ decreases with increasing initial cavity size (r0) until the cavity is large enough to preclude inelastic behavior in the surrounding rock, at which point no further decrease occurs. Earlier numerical calculations suggested that ϕ∞ was not a monotonic function of the initial energy density and that the seismic amplitude might actually be decreased, in certain cases, by decreasing the initial cavity size. We have examined this question from an analytical point of view and derived the seismic response for a simple linear‐elastic, perfectly plastic medium as r0 → 0. In this limit, an exact, power law relationship is found between ϕ∞/W and r0W−⅓, where W is the yield, a result which implies that ϕ∞ vanishes altogether for an explosion in which the initial cavity radius is vanishingly small. The physical explanation for this curious behavior is shown to derive from the unique inability of a Hooke's law medium to generate thermal pressure. A similar, but less dramatic, effect is demonstrated with more realistic material models. The significance of these results is that the estimation of yield from measurement of seismic amplitude may be a less accurate process than previously suspected.