bluuebunny/crossref_metadata_2025 huggingface dataset:
A reconsideration of the effects of terminator ionopause height on the nightward ion transport at Venus
Authors: Kar J. , Mahajan K. K. , Ghosh S. , Brace L. H.
Published: February 2008

The ionopause altitude near the terminator is a crucial parameter for studies dealing with the maintenance of the nightside ionosphere of Venus. It is generally thought that during high solar wind dynamic pressures (Psw) or during solar minimum conditions the ionopause comes down to very low altitudes so that the dayside ionosphere is not able to supply sufficient plasma to maintain the observed nightside densities. However, there are a number of workable definitions of the ionopause. Near the terminator, the altitude of the ionopause differs considerably depending upon the definition. The ionopause deduced from the radio occultation experiment (used to study the nightward transport for solar minimum conditions) as well as the pressure ionopause can be significantly lower than the density ionopause deduced from the Langmuir probe at these locations. The latter refers to the altitude where the electron density falls to 100 cm−3. Using in situ data from the Pioneer Venus Orbiter, it is shown that the density ionopause remains fairly high even for high Psw conditions. Simple quantitative estimates indicate that significant flow of plasma is still possible under these conditions. Thus nightward transport of plasma during high Psw conditions may be more efficient than has been assumed so far. Since such conditions are more prevalent during solar cycle minimum, it is argued that transport may be relevant in the maintenance of nightside ionosphere at that time also.
Physics of pitch angle scattering and velocity diffusion, 1. Theory
Authors: Karimabadi H. , Krauss‐Varban D. , Terasawa T.
Published: February 2008

We present a general theory for the pitch angle scattering and velocity diffusion of particles in the field of a spectrum of waves in a magnetized plasma. We use the test particle theory to analyze the particle motion. We examine the form of diffusion surfaces and give analytical expressions for the resonance width and bounce frequency. The resonance widths are found to vary strongly as a function of harmonic number. The resulting diffusion can be quite asymmetric with respect to pitch angle of 90°. The conditions for the onset of pitch angle scattering and energy diffusion are explained in detail. We also address some of the known shortcomings of the standard quasi‐linear theory and show ways to overcome them. In particular, we find the often stated quasi‐linear gap at 90° to exist only under very special cases. For instance, oblique wave propagation can easily remove the gap. The conditions for the existence of the gap is described in great detail. A new diffusion equation which takes into account the finite resonance widths is also discussed. The differences between this new theory and the standard resonance broadening theory is explained. The emphasis is on the simplicity of the theory and its power in making quantitative predictions.
A time‐dependent model of polar cap arcs
Authors: Zhu L. , Sojka J. J. , Schunk R. W. , Crain D. J.
Published: February 2008

A two‐dimensional time‐dependent model of polar cap arcs has been developed. The electrodynamics of the polar cap arcs are treated self‐consistently in the frame of the coupled magnetosphere‐ionosphere system. The focus of this paper is to introduce the physics and mathematical formulation of the model and describe the features of the spatial structure and temporal evolution of the polar cap arcs. The modeling results indicate that the time constant for the formation of the polar cap arcs is around 10 min. It is found that an initial single‐arc precipitation pattern associated with a polar cap arc tends to split into multiple precipitation regions and leads to a multiple structure of the polar cap arcs. It is also found that strong downward field‐aligned currents can develop near the intensive upward field‐aligned currents and form a pair structure of the field‐aligned current in the polar cap arcs. The model predicts the existence of plasma flow crossing the polar cap arcs, but the amplitude of such a flow is small, and the characteristic time scale of it is much larger than the time constant for the formation of the polar cap arcs. Our results also show that while polar cap arcs are developing, the associated current system tends to become further localized and at steady state the current system in the narrow regions around the arcs is locally self‐closed.
A statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude
Authors: Lopez Ramon E. , von Rosenvinge Tycho
Published: February 2008

In this paper we examine the relationship between geosynchronous magnetic field variations during substorms measured by GOES 5 and the auroral electrojet as measured by AE and Poste de la Baleine. As in previous studies, we find that the more taillike the field prior to the local onset, the greater the dipolarization of the field during the substorm. We also find that the greater the deviation of the field from a dipolar configuration, the larger the change in AE during the event. This implies that stronger cross‐tail currents prior to the substorm are associated with larger substorm‐associated westward electrojets and thus more intense substorms. Previous work has shown that in order to produce the observed taillike fields at geosynchronous altitude, the intense cross‐tail current that builds up during the growth phase must be localized in the near‐Earth (≤10 RE) region. Since the westward electrojet is the ionospheric leg of the substorm current wedge, our result implies that the substorm‐associated westward electrojet is drawn from the near‐Earth region. In fact, we find that most of the current diversion occurs in the near‐Earth magnetotail. Furthermore, we estimate that a diversion about half of the near‐Earth cross‐tail current can account for the current in the northern and southern westward electrojets associated with the substorm current wedge.
EXOS D (Akebono) observations of molecular NO+ and N2+ upflowing ions in the high‐altitude auroral ionosphere
Authors: Yau A. W. , Whalen B. A. , Goodenough C. , Sagawa E. , Mukai T.
Published: February 2008

We report observations of molecular upflowing ions in the high‐altitude auroral ionosphere by the EXOS D (Akebono) suprathermal mass spectrometer (SMS). At Akebono altitude, the occurrence of molecular upflowing ions was rare and was often confined in latitudinal extent, in both the dayside and the nightside. In the dayside, molecular upflowing ions were observed up to about 85°Λ, inside and up to ∼10° poleward of the cleft. In the nightside, they were generally located at lower invariant latitudes (∼60° ‐ 75° Λ) and were observed both inside and equatorward of regions of energetic (1‐10 keV) electron precipitation. The observed molecular ions appeared as a minor component of the upflowing ionospheric ion population, with a flux typically less than 5% but at times as high as 15% of the total. They were composed of comparable fluxes of NO+ and N2+ ions; at times, smaller fluxes of O2+ were also observed. The observed NO+/N2+ ratio ranged from 0.3 to 1.3; the observed O2+/N2+ ratio was 0.1 or less. They were invariably accompanied by enhanced N+ ion flux, the N+/O+ ratio being 0.5‐1.0. They were more intense (flux ∼105 cm−2 s−1), had higher density (∼ 0.1‐0.2 cm−3) and lower energy (∼5 to ∼20 eV/q), and were more anisotropic and peaked near the upward field line direction, in the dayside than in the nightside. Data from about 440 Akebono passes in the 7‐month period from November 1989 to May 1990 were surveyed for their occurrence. Molecular ions were found in 14 passes, i.e., 3% of the passes. In most but not all cases, the Kp index was 4 or higher at the time of their observation and in the preceding several hours, suggesting that they typically occur in periods of sustained auroral activity. These observations are compared with previous low‐altitude observations and discussed in terms of the transit and recombination times of the observed molecular ions, the orders‐of‐magnitude enhancement in neutral molecular densities and the corresponding N+, N2+ and NO+ ion production above the F region in periods of prolonged auroral activity, and the moderate energization of the ions immediately after their production.
Plasma response to high potential satellite in Electrodynamic Tether System
Authors: Usui Hideyuki , Matsumoto Hiroshi , Omura Yoshiharu
Published: February 2008

We study plasma responses to a high potential satellite in the electrodynamic tether system with the aid of two‐dimensional electromagnetic particle computer experiments. In the computer experiments, one conducting body representing the satellite is placed in the center of the simulation plane in the vehicle frame. The satellite potential ϕs is fixed to be higher than the space potential of the surrounding plasma. The transient process of the plasma response to the satellite and the spatial profiles of plasma density and of plasma current are examined. Due to the high potential, an electron sheath and an ion cavity are formed around the satellite. The ion cavity is extended downstream by the ambient plasma flow. In the direction perpendicular to B0, the density profile of the plasma flow shows asymmetric distortion caused by the deceleration of the ambient plasma owing to the intense E×B effect. An intense transient current is observed followed by an exponentially decreasing current. The current flows both along and across B0. The cross‐field current is mainly produced by the electron E×B drift motion around the satellite. The current‐voltage characteristics of the satellite in the steady state are examined by changing the satellite potential up to eϕs/kBTe≃10³ and comparing to the classical theories. The average current obtained by the computer experiments is between that predicted by the Langmuir theory and that predicted by the single particle theory. From eϕs/kBTe≃10¹ to 10³, the theory based on the constant density model provides a good estimate of the upper bound of the current flowing from the satellite.
Structure of the dayside reconnection layer in resistive MHD and hybrid models
Authors: Lin Y. , Lee L. C.
Published: February 2008

The structure of reconnection layer at the Earth's dayside magnetopause is studied based on one‐dimensional simulations for the evolution of an initial current sheet. A resistive MHD code and a hybrid code are used in simulations. Initially, the magnetopause current sheet separates two uniform plasma regions: magnetosheath (high plasma density and low magnetic field) and magnetosphere (low plasma density and high magnetic field). In MHD simulations, it is found that steady intermediate shock, time‐dependent intermediate shock (TDIS), Alfvén wave pulse, slow shock, slow expansion wave, or contact discontinuity can be formed as a result of magnetic reconnection. The current density in the magnetopause‐boundary layer region is mainly concentrated at the intermediate shock on the magnetosheath side. High‐speed flow is observed in the boundary layer. The Alfvén wave pulse, which propagates to the magnetosphere, is found to have a large amplitude in plasma flow speed. In hybrid simulations, the reconnection layer has striking differences from that in MHD simulations: (1) Because of the mixing of hot magnetospheric plasma and cold magnetosheath plasma along magnetic field lines, the contact discontinuity does not appear in the reconnection layer. (2) In general cases, the TDIS evolves quickly to a steady rotational discontinuity with a constant width. In the singular case in which the magnetic fields in the magnetosheath and in the magnetosphere are exactly antiparallel, a weak steady intermediate shock is formed. (3) No clear shock front can be identified for the slow shock, and the slow expansion wave is strongly modified as compared with that in the MHD simulations. (4) The boundary layer is a transition region between the magnetosheath and the magnetosphere with a mixing of plasmas from the two sides; the plasma density decreases monotonically from the magnetosheath side to the magnetosphere side, while the total temperature and magnetic field strength increase monotonically toward the magnetosphere side. (5) A decrease in T⊥/T∥ is observed in the boundary layer. Our results indicate that in general cases rotational discontinuities bound the reconnection layer at the dayside magnetopause.
Solar cycle modulation of galactic cosmic rays at high heliographic latitudes: A one‐dimensional simulation
Authors: Perko J. S. , Burlaga L. F.
Published: February 2008

Previous studies have established that large merged interaction regions (MIRs) in the outer heliosphere are the main cause of the 11‐year modulation cycle of galactic cosmic rays in the ecliptic. Those simulations were successful despite their use of a constant convection speed. In this study, we show that a large rise in solar wind speed, seen at higher heliographic latitudes during 1985–1987, combined with MIRs, can generate the difference between the Voyager 1 and Voyager 2 count rates in this period. Also, the difference between the starting times of the new cycle at both spacecraft is directly attributable to the difference between the pattern of MIRs measured at the two spacecraft. In addition, the Voyager 2 simulation exhibits the correct behavior of the 11‐year cycle in the cosmic ray radial gradients between points in the outer heliosphere.
Kilometer scale structures in auroral X ray arcs detected by combining satellite X ray images with in situ energetic electron flux profiles
Authors: Datlowe D. W. , Imhof W. L. , Voss H. D.
Published: February 2008

X ray images of the Earth's auroral zone give a large‐scale view of the precipitation patterns of energetic electrons through the bremsstrahlung produced by the electrons stopping in the atmosphere. Here we present X ray maps based on observations by the XRIS imager on the S81‐1 polar orbiting satellite; discrete X ray arcs are the most prominent features in these data. The full width half maximum spatial resolution of XRIS was 40 km, but high time resolution measurements of electrons above 16 keV could detect 2 km spatial variations in one dimension along the track of the satellite. The technique was used to study 26 auroral arcs observed in 4‐40 keV X rays by the XRIS and showed that in 2/3 of the cases the full width of the precipitation region was smaller than 40 km. Furthermore, comparisons with electron data lead to the conclusion that significant spectral variations occur on kilometer size scales.
Absolute spectrum and charge ratio of cosmic ray muons in the energy region from 0.2 GeV to 100 GeV at 600 m above sea level
Authors: De Pascale M. P. , Morselli A. , Picozza P. , Golden R. L. , Grimani C. , Kimbell B. L. , Stephens S. A. , Stochaj S. J. , Webber W. R. , Basini G. , Bongiorno F. , Brancaccio F. M. , Ricci M. , Ormes J. F. , Seo E. S. , Streitmatter R. E. , Papini P. , Spillantini P. , Brunetti M. T. , Codino A. , Menichelli M. , Salvatori I.
Published: February 2008

We have determined the momentum spectrum and charge ratio of muons in the region from 250 MeV/c to 100 GeV/c using a superconducting magnetic spectrometer. The absolute differential spectrum of muons obtained in this experiment at 600 m above sea level is in good agreement with the previous measurements at sea level. The differential spectrum can be represented by a power law with a varying index, which is consistent with zero below 450 MeV/c and steepens to a value of −2.7 ± 0.1 between 20 and 100 GeV/c. The integral flux of muons measured in this experiment span a very large range of momentum and is in excellent agreement with the earlier results. The positive to negative muon ratio appears to be constant in the entire momentum range covered in this experiment within the errors and the mean value is 1.220 ± 0.044. The absolute momentum spectrum and the charge ratio measured in this experiment are also consistent with the theoretical expectations. This is the only experiment which covers a wide range of nearly 3 decades in momentum from a very low momentum.
Association of coronal mass ejections with the heliomagnetic current sheet
Authors: Mendoza Blanca , Pérez‐Enríquez Román
Published: February 2008

The distribution of distances of coronal mass ejections (CMEs) with respect to the heliomagnetic current sheet is studied. The analysis is carried out for different times during the last solar cycle. We studied first the site of occurrence of all the CMEs for 1980 and 1984‐1986. We further considered those CMEs which are associated with interplanetary shocks for the period 1979‐1982. Finally, we considered two‐ribbon long duration soft X ray events not related with interplanetary shocks and presumably associated with CMEs. We found that during solar minimum the CMEs in general seem better associated with the heliomagnetic current sheet than at solar maximum and that the two‐ribbon not shock‐related events are poorly associated with it.
The interplanetary magnetic field By effects on large‐scale field‐aligned currents near local noon: Contributions from cusp part and noncusp part
Authors: Yamauchi M. , Lundin R. , Woch J.
Published: February 2008

The effect of the interplanetary magnetic field (IMF) By component on the distribution of dayside large‐scale field‐aligned currents (FACs) is modeled. The study is done by dividing these FACs into a “cusp part” and a “noncusp part” according to the coexisting plasma. We propose that the location of the cusp part FACs shifts in the longitudinal direction whereas the location of the noncusp part FACs shifts in both longitudinal and latitudinal directions in response to the IMF By. If combined, the noncusp part region 1 FAC can be found poleward of the cusp part FAC systems when the IMF By is strong. Since these two FACs (the cusp region FAC and the poleward‐leaped region 1 FAC) flow in the same direction poleward of the cusp, they reinforce each other in building a strong FAC which is called DPY‐FAC. The strong IMF By effect on the dayside FACs near local noon is thus explained by this model. Another important result is that the polewardmost part of the DPY‐FAC (or NBZ current under strong IMF By) flows on closed field lines even though it is located where the polar cap conventionally exists. Relative distortions of noncusp part FAC compared to the polar cap size are expected to be less significant when the IMF is southward as compared to IMF northward, yet the distortion itself is expected even for southward IMF as long as |By| > |Bz|. The present model is supported by Viking particle and magnetic field data.
Single‐crystal elastic properties of omphacite‐C2/c by Brillouin spectroscopy
Authors: Bhagat Snehal S. , Bass Jay D. , Smyth Joseph R.
Published: February 2008

Single‐crystal Brillouin scattering measurements of elastic moduli were performed on an eclogitic omphacite (space group C2/c) with chemical composition close to the diopside (CaMgSiO2O6, 29.9 mol %)‐jadeite (NaAlSi2O6, 57.8 mol %) join. Prom the 13 elastic moduli measured under ambient conditions we calculate the adiabatic bulk modulus, KS = 130.8 ±0.5 GPa, and the shear modulus, μ = 79.2 ± 0.2 GPa. The magnitudes of all longitudinal elastic moduli (C11, C22, and C33) are inferred to be influenced primarily by the MgO6 and CaO8, or M polyhedra. Although the silica tetrahedra are extremely stiff, the great rotational freedom of these polyhedra yields a mechanical unit (tetrahedral chains) which is relatively compliant. Virtually all of the omphacite moduli are intermediate in value to those for diopside and Jadeite. Most of the moduli for omphacitic pyroxene can be estimated to within several percent by interpolation between the end member elastic moduli weighted according to the molar percentages of diopside and Jadeite.
Limitations of quantitative analysis of deep crustal seismic reflection data: Examples from GLIMPCE
Authors: Lee M. W. , Hutchinson D. R.
Published: February 2008

Amplitude preservation in seismic reflection data can be obtained by a relative true amplitude (RTA) processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally, after compensating for amplitude distortion by near‐surface effects and propagation effects. Quantitative analysis of relative true amplitudes of the Great Lakes International Multidisciplinary Program on Crustal Evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression. Processing techniques such as deconvolution, F‐K filtering, and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance. Thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless. The relative strength of individual events, however, is preserved on curves generated at different stages in the processing. We suggest that qualitative comparisons of relative strength, if used carefully, provide a meaningful measure of variations in reflectivity. Simple theoretical models indicate that peg‐leg multiples rather than water bottom multiples are the most severe source of noise contamination. These multiples are extremely difficult to remove when the water bottom reflection coefficient is large (>0.6), a condition that exists beneath parts of Lake Superior and most of Lake Huron.
Shear source multiazimuth near‐ and far‐offset vertical seismic profiles, Asal Rift, Republic of Djibouti
Authors: Hirn Alfred , Magnier Sophie‐Adélaïde
Published: February 2008

A three‐component borehole seismic data set obtained in the Asal rift zone shows that azimuthal anisotropy may be present with the fast propagation direction aligned with the present‐day rift axis. This is seen on the near‐ and far‐offset vertical seismic profile (VSP) data from the two source points lying parallel to the rift axis. The third orthogonal source point gives propagation times characteristic of near‐horizontal paths along the detected slow direction. The reflected VSP wave field contains both P and S energy. The main P and S reflections correlate well and correspond to thin clay and siltstone layers interspersed within the lava flow series. Some evidence exists for a change with depth of the amount or direction of the anisotropy found in the uppermost 400 m.
Paleomagnetism of the Resurrection Peninsula, Alaska: Implications for the tectonics of southern Alaska and the Kula‐Farallon Ridge
Authors: Bol Alan J. , Coe Robert S. , Grommé C. Sherman , Hillhouse John W.
Published: February 2008

The rocks of the Resurrection Peninsula compose an ophiolite (57±1 Ma) within the vast accretionary prism of the Chugach‐Prince William terrane. Paleomagnetic data from pillow basalt and sheeted dikes of the ophiolite yield a mean paleolatitude of 54°±7°, which implies 13±9° of poleward displacement. The characteristic component of the remanence resides in a Ti‐poor titanomagnetite with a blocking temperature close td 580°C. A positive fold test and the petrology of the magnetic minerals lead us to conclude that the characteristic remanent magnetization (ChRM) in the sheeted dikes originated as a primary thermoremanent magnetization (TRM). We hypothesize that the ChRM of the pillow basalt originated as a chemical remanent magnetization (CRM) acquired soon after formation of the rocks during hydrothermal metamorphism. Structural data indicate polyphase deformation with folding about vertical and horizontal axes. The paleomagnetic data pass a fold test only when we incorporate this interpretation in our structural correction. Geologic relationships lead us to interpret the Resurrection Peninsula ophiolite as a fragment of the extinct Kula‐Farallon ridge that accreted to the Chugach‐Prince William terrane soon after formation. The paleomagnetic paleolatitude indicates that the ridge intersected the North American continental margin in the vicinity of northern Washington in late Paleocene time. The paleostrike of the sheeted dikes (N26°E±12°) shows that the ridge trended NNE. According to current plate tectonic models, if the Chugach‐Prince William terrane moved north with the full coastwise displacement of the Kula plate, the Resurrection Peninsula ophiolite would have arrived at its present position at 45 Ma. This age is perhaps slightly later than that of the terranes that lay immediately inboard (Peninsular and Wrangellia terranes), indicating some relative displacement between those terranes and the Chugach‐Prince William terrane. Such displacement, in turn, agrees with studies which imply a Coast plutonicmetamorphic complex provenance for the Chugach‐Prince William terrane.
Oblique convergence and deformation along the Kuril and Japan Trenches
Authors: DeMets Charles
Published: February 2008

The hypothesis that present‐day deformation within the southern Kuril forearc is driven by oblique subduction of the Pacific plate is tested using 397 horizontal slip directions derived from shallow‐thrust earthquakes from the Kuril and Japan trenches for the period 1963–1991. A simple two‐plate model fits the 397 slip vectors significantly worse than a model that permits strike‐slip motion of the southern Kuril forearc relative to the overlying plate. Weighted, mean slip directions along the southern Kuril trench are systematically rotated toward the direction orthogonal to the trench, which implies that the net convergence is partitioned into less oblique subduction and trench‐parallel displacement of the southern Kuril forearc. The angular discrepancy between the observed slip direction and the direction predicted by the NUVEL‐1 Pacific‐North America Euler vector implies that the southern Kuril forearc translates 6–11 mm yr−1 to the southwest relative to the overlying North American plate. These results are consistent with geologically, geodetically, and seismologically observed convergence at the leading edge of the forearc sliver in southern Hokkaido and with previously inferred extension at the trailing edge of the sliver, which is located at the Bussol Strait at 46°N. The percentage of the oblique component of the rigid plate convergence vector that is transferred into forearc strike‐slip motion is only 30%, which may be evidence that the Hokkaido/Honshu bend in the trench hinders southwest forearc motion or that the greater strength of oceanic forearcs impedes motion along forearc strike‐slip faults.
Forced nutations of the Earth: Contributions prom the effects of ellipticity and rotation on the elastic deformations
Authors: Buffett B. A. , Mathews P. M. , Herring T. A. , Shapiro I. I.
Published: February 2008

We determine the deformation produced by the lunisolar tidal potential in a rotating, spheroidal model Earth. We proceed by decomposing the equations of motion into separate, though coupled, equations for the nutational and deformational parts of the Earth's response. Using this scheme, we derive a simpler set of equations for the deformational displacements, where the driving forces include not only the tidal terms but also inertial forces and gravitational perturbations associated with the nutational motions. We show that the deformations are affected only to a very small extent by the Earth's asphericity and rotation. This fact is exploited to set up a perturbative procedure, whereby the equation governing the deformation is separated into equations of zeroth and first orders in the perturbation. In the initial calculation (the zeroth order), the influences of the Earth's asphericity and the inertial forces associated with the deformation are neglected, while the forces arising from the nutational motions are taken into account. The resulting calculation for the quasi‐static deformation is equivalent to the so‐called spherical approximation used by Sasao et al. (1980), although the solutions obtained here are physically more insightful. This zeroth‐order calculation is used to determine the compliances defined in the work of Mathews et al. (1991a), which characterize the deformability of the Earth. In the second step of the calculation, the solutions obtained under the spherical approximation are used to determine corrections to the deformation for the omitted effects of ellipticity and inertia (including the Coriolis force). Corresponding corrections to the zeroth‐order compliances used by Mathews et al. (1991b) are found to be nominally O(ϵ) smaller than the zeroth‐order compliances, where ϵ is the geometric ellipticity (surface flattening) of the Earth. As a consequence of these corrections to the compliance parameters, changes in the nutation amplitudes as computed by Mathews et al. (1991b) are produced, which amount to −0.18, 0.46, and 0.26 milliarcseconds, in the prograde semiannual, and the retrograde annual and 18.6‐year terms, respectively. Additional corrections are introduced if we require the theoretical value of the retrograde annual nutation to match the determination made using very long baseline interferometry. The procedure presented here to account for the effects of ellipticity and rotation could also be used to determine corrections to nutations for the effects of anelasticity in the mantle and inner core or for the effect; of lateral heterogeneity in the Earth's density and elastic properties.
Paleomagnetism and geochronology of 23 Ma gabbroic intrusions in the Keku Strait, Alaska, and implications for the Alexander Terrane
Authors: Haeussler Peter J. , Coe Robert S. , Renne Paul
Published: February 2008

Samples of Tertiary gabbro from 24 sites in the Keku Strait, Alaska, help constrain the displacement history of the Alexander terrane. Step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40Ar/39Ar age of 23.1 ± 1.7 Ma. The characteristic magnetization resides in magnetite, is easily isolated by thermal and alternating field demagnetization, and has both normal and reversed polarities. The mean paleomagnetic pole, with no structural correction, is latitude 87.1°N, longitude 141.6°E, A95 = 10.1°, with N = 20 sites. This pole indicates insignificant tectonic displacement (0.5° ± 8.2° southward) and rotation (0.6° ± 15.2° counterclockwise). We therefore conclude that any northward displacement or vertical axis rotation of the Alexander terrane, and/or tilting in the vicinity of the Keku Strait must have occurred before 23 Ma.
Pressure dependence of the electrical conductivity of (Mg0.9Fe0.1)SiO3 perovskite
Authors: Li Xiaoyuan , Ming Li‐Chung , Manghnani Murli H. , Wang Yanbin , Jeanloz Raymond
Published: February 2008

The electrical conductivity of (Mg0.9Fe0.1)SiO3 perovskite has been measured at pressures of 0–8 GPa and ambient temperature using a DIA‐6 cubic‐anvil apparatus. At ambient conditions, the electrical conductivity of the (Mg0.9Fe0.1)SiO3 perovskite is found to be 7.6 (±2.0) ×10−9 S/m. The conductivity of the perovskite increases with increasing pressure, and the activation volume for electrical conduction is determined to be −0.12 (±0.04) cm3/mol. As a cross‐check with previous electrical conductivity measurements, an assemblage of the perovskite and magnesiowüstite synthesized from the (Mg0.89Fe0.11)2SiO4 olivine has been examined in the present study using a laser‐heated diamond anvil cell at ∼ 48 GPa and in the temperature range of 295–3500 K. The results for this assemblage are consistent with those for the pure (Mg0.9Fe0.1)SiO3 perovskite. The present experimental data show that the electrical conductivity of the crystalline (Mg0.9Fe0.1)SiO3 perovskite remains below 10−5 S/m at 300 K and lower mantle pressures.
Magnetic pore fabric analysis: Verification through image autocorrelation
Authors: Pfleiderer S. , Halls H. C.
Published: February 2008

The pore fabric of sandstones is studied by impregnating porous samples with a magnetic suspension (ferrofluid) and subsequently measuring their anisotropy of magnetic susceptibility (AMS). Similar to the use of natural AMS for estimation of grain fabric, we determine average orientation and shape anisotropy of pore bodies in three dimensions. This new approach overcomes restrictions of previous methods of pore structure characterization. The interpretation of AMS in terms of pore fabric anisotropy is tested by comparison with pore shape observed in thin section. An image analytical technique is used to derive statistically the elongation direction of digitized pore cross sections. By computing the two‐dimensional autocorrelation function we obtain section ellipses reflecting average pore shape and orientation. Combination of ellipses of three mutually perpendicular images leads to fabric ellipsoids comparable to AMS. Comparison of the two anisotropies shows a close correspondence between the principal axis directions of AMS and fabric ellipsoids. Owing to the bedded nature of the samples, the ellipsoids are of oblate shape with minimum axes perpendicular to bedding. However, even small anisotropies within the bedding plane can be detected successfully. Axial ratios of AMS and fabric ellipsoids correlate closely if rock sample AMS is corrected for pore directional scattering. The method represents a powerful tool to analyze pore fabric of sedimentary rocks and to study its effects on petrophysical properties.
How similar were the 1934 and 1966 Parkfield Earthquakes?
Authors: Segall Paul , Du Yijun
Published: February 2008

Bakun and McEvilly (1984) showed that surface waves from the 1934 and 1966 Parkfield earthquakes are remarkably similar. This observation, together with other seismic and geologic data, led to the forecast of a repeat of a “characteristic” Parkfield earthquake by 1993. In this paper we model near‐field geodetic measurements collected before and after the 1934 and 1966 Parkfield earthquakes. The slip in 1934 on the northwestern half of the rupture is poorly constrained due to the limited station coverage. In this part of the rupture, near the hypocenter, the geodetic measurements permit the two earthquakes to have the same slip distribution. On the other hand, the data clearly show that the 1966 earthquake ruptured through the right step in the surface trace of the fault, while the 1934 earthquake stopped northwest of the step. If we assume only that the slip distribution is to some degree smooth, we find that the centroid of the 1934 slip distribution could be as much as 10 km northwest of the centroid of the 1966 slip. The data are consistent with, but do not demand, the largest slip in 1934 occurring in a region of low slip in 1966. The geodetically determined moments of the two earthquakes from smooth inversions are indistinguishable (M0 = 4.4 ± 0.4 × 1018 N m). We also examine inversions in which we assume that the two earthquakes start identically and estimate how far down the fault the two earthquakes could have remained the same. We find that the 1934 event could have had the same slip as in 1966 up to the fault step. The 1934 earthquake must have stopped at the step, whereas the 1966 event slipped southeast of the step. In this “quasi‐characteristic” model, M0 of the 1934 quake is only 60% of that in 1966, the difference presumably occurring as afterslip in 1966. If the 1934 geodetic moment was indeed less than in 1966 and the loading rate was constant, this observation could explain the “early” occurrence of the 1934 earthquake.
Ill‐posed and well‐posed formulations of the reflection travel time tomography problem
Authors: Delprat‐Jannaud Florence , Lailly Patrick
Published: February 2008

Many studies have shown the strong dependence of the solution of reflection tomography (or travel time inversion) in the model discretization interval: such solutions are purely numerical artifacts. The cause lies in the formulation of the reflection tomography which turns out to be an ill‐posed problem unless properly regularized. Adequate regularizations are those which make use of derivatives of the functions describing the model (velocity distributions and reflector geometries). The order of the derivatives to use depends on the space dimension (two or three dimension). Of practical interest is curvature regularization: it works both in two and three dimensions with minimum a priori knowledge on the substratum under survey. We give theoretical arguments to prove that, with such a regularization, the computed solution is unique, stable, and almost independent of model discretization, provided that it is fine enough. These theoretical results are confirmed by numerical tests on a two‐dimensional model with a velocity field varying both vertically and horizontally: they show the practical interest of the proposed regularization. However, mathematical stability is not sufficient, and a priori information has to be used in order to decrease the size of the set of possible solutions. As a consequence of our theoretical study of regularization techniques, only a priori information involving derivatives of the functions describing the model yields stability. In other words, except for some pathological examples, stability is obtained only if we try to determine a smooth model of the subsurface. These conclusions remain valid for three‐dimensional models and for transmission tomography. As a by‐product of our study, we suggest a preconditioning which makes much easier the solution of the linear systems involved in the Gauss‐Newton optimization method.
Bumps on the core‐mantle boundary: Are they facts or artifacts?
Authors: Pulliam R. Jay , Stark Philip B.
Published: February 2008

Large structure in tomographic images of the core‐mantle boundary (CMB) found by fitting low‐degree spherical harmonic models to International Seismological Centre (ISC) travel time data by linear least squares may be artifacts of the fitting procedure, rather than necessary features of Earth. Two numerical experiments show that gaps in the data coverage and the use of low‐degree models tend to yield spurious topography. We fit a large set of summary PKP and PcP rays for 23 years of ISC arrival times (for events relocated with the iasp91 model) to obtain least squares CMB “reference models” of maximum degree 4, 8, and 12. Least squares CMB models derived from subsets of the data with new artificial gaps comparable in size to the gaps in the original data set can have significantly different structure than the reference models, both globally and within the artificial gaps. In spite of these differences, the correlation of models with different sized gaps is high: correlation is not a good measure of model similarity nor of reliability of the fitted models in this problem. We generated synthetic data from the degree 12 reference model rotated relative to the source‐receiver frame. Least squares fits of low‐degree models to the synthetic data show significant features not present in the reference model, in approximately the same place relative to the gaps in the original observations. The spurious features of the models with new gaps and with rotated synthetic data are qualitatively indistinguishable from large features in the reference models.
S wave velocity structure of the Northern Cascadia Subduction Zone
Authors: Cassidy John F. , Ellis Robert M.
Published: February 2008

Teleseismic receiver functions from an array of portable broadband seismograph stations located in southwestern British Columbia are interpreted to estimate the S wave velocity structure to upper mantle depths across the northern Cascadia subduction zone. At our westernmost station on central Vancouver Island, a prominent low‐velocity zone (ΔVs = −1 km s−1) with a high Poisson's ratio is estimated at 36–41 km depth. This feature correlates with the reflective “E” zone observed in LITHOPROBE reflection data, a region that also exhibits high electrical conductivity and has recently been interpreted as a fluid‐saturated shear zone above the subducting Juan de Fuca (JdF) plate. Analysis of data at our stations to the NE permits this zone to be mapped into the upper mantle, 54 km beneath the British Columbia mainland, and approximately 250 km from the locus of subduction. The subducting oceanic crust is imaged at 47–53 km depth dipping 15°± 5° in the direction N30°E ± 20° beneath central Vancouver Island. The dip angle increases to 22° ± 5° at a depth of 60–65 km beneath the Strait of Georgia. The results of this analysis provide the first definitive evidence for the location of the subducting plate in this region and indicate that the seismicity at depth occurs within the oceanic crust. The dip direction of N30°E at each station provides new evidence that the JdF plate is arched upward as it subducts in this region. A low‐velocity zone that coincides with the “C” reflectors beneath Vancouver Island is interpreted to extend east to the British Columbia mainland near the base of the North American crust. The top of this zone, near 20–26 km depth, lies near the lower limit of shallow seismicity. The continental Moho is estimated to be 36 km below the Strait of Georgia.
Anticyclonic eddy X91 in the southern Bay of Biscay, May 1991 to February 1992
Authors: Pingree R. D. , Le Cann B.
Published: February 2008

Anticyclonic eddy X91 was observed (by remote sensing) from May 1991 to February 1992 in the southern Bay of Biscay. Two drogued Argos buoys deployed in the center in September showed that particle separation rates in the core of the eddy at a depth of 100 m were of the order of 100 m/day. Similar values were found for the mean radial velocity component of the buoys with respect to the eddy center. The center of the eddy core (∼10‐km radius) was in near‐solid‐body rotation with a period approaching 3.5 days and relative vorticity of −0.4ƒ. Deep expendable bathythermograph profiles (to 2000 m) in September gave an eddy diameter of about 100 km with geostrophic velocities falling to half values at a depth of 800 m. The geostrophic azimuthal transport was ∼10 Sv; the kinetic energy was ∼1.4 × 1014 J. X91 was resurveyed in December, and the azimuthal currents extended to greater depths (∼1 km). The transport was still ∼10 Sv but the kinetic energy was reduced to ∼75% of the September value. Drogued buoys showed a slower (∼10%) rotation period in December. X91 is a good example of a coherent vortex structure, and the structural similarity between X91 and F90a (the slope water oceanic eddy observed in 1990) suggests that X91 is also a swoddy.
Detailed structure of currents and hydrography on the northern side of Georges Bank
Authors: Loder John W. , Brickman David , Horne Edward P. W.
Published: February 2008

A suite of observations from July 2–3, 1988, is used to describe the spatial structure and temporal evolution over the tidal period of currents and hydrography across the northern side of Georges Bank in summer under light winds. The data set includes moored current and hydrographic observations at four cross‐bank positions, fast response thermistor chain observations at two of the sites, a conductivity‐temperature‐depth section, 10 repeated sections over the tidal period using a towed Batfish and ship‐mounted acoustic Doppler current profiler, and two surface drifter trajectories. The observations provide a detailed description of previously identified features such as the strong semidiurnal tidal currents, an internal tide, a tidal front, and a residual current jet and also reveal a hierarchy of energetic smaller‐scale structures. These include an internal hydraulic jump during off‐bank tidal flow and subsequent internal waves propagating onto the bank and also a surface convergence in the frontal zone. The physical oceanographic regime on the northern side of Georges Bank during spring‐fall can be conceptualized as a hybrid of a stratified shelf break with strong tidal advection and a tidal(mixing) front. Key factors to the regime are the strong tidal currents and abrupt topographic variation over the bank's side. The result is a nonlinear and baroclinic tide‐topography interaction at the bank edge and a frontal zone with strong variability over the tidal period associated with tidal advection and large‐amplitude internal waves. The along‐bank transport in the residual jet is estimated to be 0.91 Sv, indicating that it may be the largest summertime transport feature on the northwestern Atlantic shelf between Cape Hatteras and the Grand Bank.
Subtidal sea level fluctuations in a large fjord system
Authors: Mofjeld Harold O.
Published: February 2008

A cross‐spectral analysis is made of subtidal (0.01–0.5 cpd) sea level (SSL), atmospheric pressure and wind fluctuations observed during 1966–1988 in the southern reaches of the fjord system formed by the Straits of Juan de Fuca/Georgia and Puget Sound. It shows that observed SSL can be largely explained by quasi‐steady inverse barometer compensation to atmospheric pressure and the propagation of adjusted sea level fluctuations from the nearby continental shelf, with some local weather forcing in summer. Variations in coherence, amplitudes and relative phase between seasons, locations and frequencies support previous work that both barotropic and baroclinic processes have important influences on SSL within the fjord system.
Effects of wind, density, and bathymetry on a one‐layer southern ocean model
Authors: Klinck John M.
Published: February 2008

Steady solutions from a one‐layer, wind‐driven, primitive equation model are analyzed to determine the importance of wind forcing, pressure gradient force due to the climatological density distribution and bottom form drag on circulation in the Southern Ocean. Five simulations are discussed: three wind‐forced simulations with differing bathymetry (flat bottom, 15% bathymetry, and full bathymetry), one case with full bathymetry forced with the density‐induced pressure force, and one case with full bathymetry forced by both wind and density‐induced pressure gradients. The simulations presented here confirm the previous speculation (Munk and Palmén, 1951) that form drag is effective in balancing the driving force due to the surface wind stress. In fact, it has such a strong effect that bathymetry with only 15% of the true amplitude reduces the transport from over 480 × 106 m3 s−1 to about 190 × 106 m3 s−1. If the true bathymetry is used, the total transport is reduced to a value around 20 × 106 m3 s−1. Analysis of the zonally integrated momentum in the unblocked latitudes of the Southern Ocean shows that the bottom form drag balances the surface forcing, even for simulations that have viscosities that are in the upper range of acceptable values. The vertically integrated pressure gradient due to the climatological density distribution produces a body force that accelerates the Antarctic Circumpolar Current, producing a transport of about 250 × 106 m3 s−1. Therefore the pressure gradient produced by the density structure of the Southern Ocean is an integral part of the dynamics of the Antarctic Circumpolar Current. It forces the flow across bathymetry that would, in the absence of the spatially varying density field, block the circulation. This result is in contrast to mid‐latitude gyres in which the steady, wind‐driven circulation is insulated from the influence of bathymetry by stratification (Anderson and Killworth, 1977).