bluuebunny/crossref_metadata_2025 huggingface dataset:
On the termination of the closed field line region of the magnetotail
Authors: Birn J. , Yur G. , Rahman H. U. , Minami S.
Published: February 2008

The structure of the distant magnetotail, where the closed field line region terminates, is investigated in the framework of several models and on the basis of laboratory simulations of the interaction of a flowing magnetized plasma with a dipole field. From our simulations we find that this region for northward “interplanetary” field (IMF) resembles that of a quasi‐static tail structure, in which a flaring tail transits into a nonflaring current sheet configuration at a location where the internal pressure is balanced by the external plasma and magnetic pressures only. This structure is associated with a Y‐type magnetic neutral line. The magnetic tail apparently includes an open field line region, despite the fact that this magnetosphere did not have a history of southward IMF, which could have opened the dipole field through frontside reconnection. This is a strong indication of the occurrence of high‐latitude nightside reconnection, suggested by Dungey (1963). For southward model IMF the closed field line region appears to terminate at an X‐type magnetic neutral line, as in the standard open magnetosphere model of Dungey (1961). This configuration is also in qualitative agreement with the simple superposition of a dipole field and a uniform IMF and with the presence of a plasma mantle which expands through drifts from the magnetosphere boundary toward the plasma sheet and becomes partially trapped through magnetic reconnection at the distant X line.
High‐order stimulated ionospheric diffuse plasma resonances: Significance for magnetospheric emissions
Authors: Benson Robert F. , Osherovich Vladimir A.
Published: February 2008

The sequence nature of the diffuse ionospheric resonances Dn stimulated by topside sounders was discovered by Oya (1970) with n ranging from 1 to 4. Osherovich (1987) organized these observations using his earlier theory which predicted a nonequidistant spectrum with frequencies proportional to n1/2 where n = 1, 2, …. This work has since been used to predict the exact spectral position of naturally occurring magnetospheric emissions, and such emissions corresponding to n = 4 and 5 were identified in the IMP 6 data. These results motivated a search for the stimulated ionospheric counterpart at n = 5. We here present Alouette 2 and ISIS 1 topside sounder evidence for the stimulation of Dn resonances with n > 4. The results indicate the close relationship between active ionospheric plasma wave experiments and the interpretation of magnetospheric emissions.
The quiet time polar cap: DE 1 observations and conceptual model
Authors: Burch J. L. , Saflekos N. A. , Gurnett D. A. , Craven J. D. , Frank L. A.
Published: February 2008

Auroral activity increases over the polar caps during quiet times, which are associated with northward interplanetary magnetic field (IMF) components. Polar cap auroras (Sun‐aligned arcs, theta auroras, and horse collar auroras) occur under these conditions. DE 1 data show that the theta and horse collar auroras are generally characterized by sunward convection, closed field lines, auroral hiss, and inverted‐V events. These phenomena are similar to those observed along the auroral oval but have somewhat lower electron energies. Adjacent dark regions of the polar caps appear to contain open field lines and antisunward convection. A conceptual northward IMF merging model containing lobe cells, merging cells, and viscous cells is shown to be consistent with the observations. As the IMF becomes more northward, the polar arc configuration changes from the “horse collar” pattern to the theta aurora pattern in the model, and this is shown to be generally true for the set of published data on these phenomena. The model involves dayside merging both at high latitudes on open field lines and at lower latitudes on closed field lines. The ratio between the merged flux produced by the high‐latitude merging to that produced by the lower‐latitude merging increases as the IMF becomes more northward. Two types of open field lines, equator‐crossing and non‐equator‐crossing, are produced by the higher‐ and lower‐latitude merging, respectively. The equator‐crossing field lines have a strong azimuthal component of convection as they flow around the magnetopause, while the non‐equator‐crossing field lines can convect more or less directly across the polar cap, leading to an antisunward flow channel across the central polar cap. This antisunward flow region grows as the IMF becomes less northward, causing dual polar cap arcs to spread out into the horse collar configuration. The sunward flow segments of the lobe cells are associated with induced electric fields, thus accounting for the particle energization leading to auroral forms.
Plasma and magnetic field parameters at substorm onsets derived from GEOS 2 observations
Authors: Pu Z. Y. , Korth A. , Kremser G.
Published: February 2008

Measurements of energetic particles and the geomagnetic field by the geostationary satellite GEOS 2 were used to investigate the plasma and magnetic field parameters at substorm onset. The distribution of energetic ions and electrons are found to be statistically isotropic at the nightside synchronous orbit. The number density and pressure of electrons Ne and Pe are much lower than Ni and Pi of ions. Electrons and ions increase substantially at substorm onset when compared with magnetically quiet days. Earthward pressure gradients of energetic ions with scale length of ∼10 ri (ri = ion gyroradius) are commonly observed during the substorm growth phase. The magnetic field lines are stretched out tailward with a curvature radius in the equatorial plane being considerably smaller than that in the dipole field. Plasma β values increase to the order of 1. All these parameters are found to be consistent with the development of the ballooning instability in connection with substorm onsets.
Ion temperature profiles in the horns of the plasma sheet
Authors: Suszcynsky D. M. , Gosling J. T. , Thomsen M. F.
Published: February 2008

The plasma sheet horns are the low‐altitude extensions of the plasma sheet that lie poleward of the plasmasphere and equatorward of the tail lobes. Within the horns, magnetic field lines of increasing geomagnetic latitudes map to increasing distances into the downtail plasma sheet. Plasma data from the fast plasma experiment on ISEE 2 have been analyzed for 11 outbound crossings of the horns in the premidnight sector of the magnetosphere at typical altitudes of ∼2–4 RE. These crossings typically occurred on time scales of less than 1 hour, providing almost instantaneous snapshots of plasma gradients within the horns. Ion temperatures observed during these crossings generally decreased by a factor of 4–8 as magnetic field lines of increasing geomagnetic latitude were traversed. If we make the reasonable assumption that the ion temperatures are constant along the field lines within the plasma sheet, then this result implies that the ion temperatures in the downtail plasma sheet also commonly decrease by this same factor over the radial range extending from the inner edge of the plasma sheet at the plasmapause boundary to the outer edge at a neutral line in the distant tail.
Initial signatures of magnetic field and energetic particle fluxes at tail Reconfiguration: Explosive growth phase
Authors: Ohtani S. , Takahashi K. , Zanetti L. J. , Potemra T. A. , McEntire R. W. , Iijima T.
Published: February 2008

Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetometer and Medium Energy Particle Analyzer (MEPA) data are used to examine the initial signatures of tail field reconfiguration observed in the near‐Earth magnetotail (< 9 RE). Sixteen events are selected preliminarily from 9 months (January‐September 1985) of magnetometer data according to two criteria, that is, an unambiguous commencement of tail field reconfiguration and a sharp recovery of the north‐south (H) component. The second criterion requires that the satellite was close to the onset region of current disruption. Although these strict criteria result in the small number of events, the magnetic and particle flux signatures of the events are considered to be informative concerning the mechanism of substorm onsets. It is found that these tail reconfiguration events are classified into two types: Type I and Type II. In Type I events a current disruption starts in a flux tube that is inward (earthward/equatorward) of the spacecraft, and consequently, the spacecraft is immersed in a hot plasma region expanding from inward (earthward/equatorward). The other type (Type II) is characterized by a distinctive interval (explosive growth phase) just prior to the local commencement of tail reconfiguration. The duration of this interval is typically 1 min, much shorter than that of the so‐called growth phase. During this interval the north‐south magnetic (H) component is depressed sharply, and the flux of energetic ions increases outward (tailward/poleward) of the spacecraft, suggesting that the cross‐tail current is explosively enhanced. It is also found that the radial magnetic (V) component changes with a distinctive phase relationship relative to the north‐south component, which can also be explained in terms of the explosive enhancement in the cross‐tail current intensity just prior to the current disruption. This enhancement is inferred to be a local process, rather than a result of a current disruption which has occurred somewhere else, although it is possible that the commencement of the H recovery observed is not exactly simultaneous with a substorm onset. The present results contribute significantly to modeling efforts regarding the triggering mechanism of substorms in the magnetotail.
Simulations and qualitative analysis of the AMPTE Experiments
Authors: Kazeminezhad F. , Dawson J. M. , Bingham R.
Published: February 2008

Injection of neutral gases into space plasmas has emerged as a powerful technique in reproducing many of the naturally occurring processes in planetary physics in a controlled way. In this article, we present thorough simulation results aimed at explaining and interpreting the AMPTE experiments. The results show the formation of a diamagnetic cavity and a shocklike structure, intense wave activity upstream of the shock, the sideways deflection of the bulk of the AMPTE cloud, the Rayleigh‐Taylor instability, and the ejection of a tenuous cloud of plasma. All these results are in qualitative agreement with AMPTE observations.
Radial transport of storm time ring current ions
Authors: Lui A. T. Y.
Published: February 2008

Radial transport of energetic ions for the development of the main phase of geomagnetic storms is investigated with data from the medium energy particle analyzer (MEPA) on the Charge Composition Explorer (CCE) spacecraft, which monitored protons (Ep > 56 keV), helium ions (EHe > 72 keV), and the carbon‐nitrogen‐oxygen group, which is mostly dominated by oxygen ions (E0 > 137 keV). From a study of four geomagnetic storms, we show that the flux increase of these ions in the inner ring current region (L ≲ 5) can be accounted for by an inward displacement of the ring current population by ∼0.5 to 3.5 RE. There is a general trend that a larger inward displacement occurs at higher L shells than at lower ones. These results are in agreement with previous findings. The radially injected population consists of the prestorm population modified by substorm injections which occur on a much shorter time scale than that for a storm main phase. It is also found that the inward displacement is relatively independent of ion mass and energy, suggesting that the radial transport of these energetic ions is effected primarily by convective motion from a large electric field or by diffusion resulting from magnetic field fluctuations.
Coupling of magnetopause‐boundary layer to the polar ionosphere
Authors: Wei C. Q. , Lee L. C.
Published: February 2008

The ultraviolet auroral images obtained with the Viking satellite often show spatially periodic bright spots which resemble “beads” or “pearls” aligned on the postnoon auroral oval, which may be due to the coupling of the plasma vortices formed in the boundary layer to the polar ionosphere. Geomagnetic pulsations in the ULF range observed in the cusp region may also be associated with the boundary layer vortices. To explain the observed auroral bright spots and the cusp geomagnetic pulsations, we have developed a model for the plasma dynamics in the low‐latitude boundary layer and its interaction with the polar ionosphere via the field‐aligned current (FAC). In the presence of a driven plasma flow along the magnetopause, the Kelvin‐Helmholtz instability can develop, leading to the formation and growth of plasma vortices in the boundary layer. On the other hand, the finite ionospheric conductivity leads to the decay of these vortices. The competing effect of the formation and decay of vortices leads to the formation of strong vortices only in a limited region. Several enhanced field‐aligned power density regions associated with the boundary layer vortices and the upward FAC filaments can be found along the postnoon auroral oval. These enhanced field‐aligned power density regions may account for the observed auroral bright spots. In addition, we found that the FACs are stronger in the prenoon sector than in the postnoon sector due to the presence of the field‐aligned potential drop in the postnoon sector. The larger prenoon FACs produce stronger cusp pulsations in the prenoon sector, consistent with observations. The frequency spectrum and polarization of the generated pulsations are also discussed. On the other hand, the vortices in the postnoon boundary layer are found to be stronger than those in the prenoon boundary layer.
Numerical quasi‐linear study of the critical ionization velocity phenomenon
Authors: Moghaddam‐Taaheri E. , Goertz C. K.
Published: February 2008

The critical ionization velocity (CIV) for a neutral barium (Ba) gas cloud moving across the static magnetic field is studied numerically using quasi‐linear equations and a parameter range which is typical for the shaped‐charge Ba gas release experiments in space. For consistency the charge exchange between the background oxygen ions and neutral atoms and its reverse process, as well as the excitation of the neutral Ba atoms, are included. The numerical results indicate that when the ionization rate due to CIV becomes comparable to the charge exchange rate the energy lost to the ionization and excitation collisions by the superthermal electrons exceeds the energy gain from the waves that are excited by the ion beam. This results in a CIV yield less than the yield by the charge exchange process.
Propagation of three‐dimensional Alfvén waves and its nonlinear effects in the solar wind
Authors: Lou Yu‐Qing
Published: February 2008

We consider propagation of three‐dimensional, adiabatic magnetohydrodynamic (MHD) perturbations in a background radial MHD solar wind which is spherically symmetric. In the linear approximation, three‐dimensional propagation of Alfvén waves decouples from three‐dimensional propagation of MHD fast and slow perturbations in the wind. For Alfvén waves, we extend previous analytical WKB, non‐WKB, and asymptotic solutions at large distances as well as numerical results to explicit three‐dimensional versions; as to three‐dimensional MHD fast and slow perturbations, we generalize the prior perturbation scheme for a purely hydrodynamic solar wind by including a magnetic field. In general, Alfvén wave flux in the solar wind is not conserved; we further study the following second‐order nonlinear effects due to a three‐dimensional propagation of Alfvén waves in the wind, namely, (1) the driving of compressible MHD wind flow and thermodynamic variations; (2) the generation of MHD fast and slow waves; (3) the generation of Alfvén waves; and (4) the formation of steady, incompressible vortex and current structures. Therefore in the absence of direct dissipative mechanisms, the energy flux of Alfvén waves in the solar wind is consumed to produce these nonlinear effects. For typical solar wind parameters in the absence of dissipations, the Alfvén wave flux density at the solar coronal base inferred from that observed at 1 AU is thus 1 to 2 orders of magnitude less than what is needed (5 ∼ 8 × 105 ergs cm−2 s−1) if a solar wind stream would be entirely driven and maintained by Alfvén waves. We discuss several relevant heliospheric applications of our results.
CDAW 9 analysis of magnetospheric events on May 3, 1986: Event C
Authors: Baker D. N. , Pulkkinen T. I. , McPherron R. L. , Craven J. D. , Frank L. A. , Elphinstone R. D. , Murphree J. S. , Fennell J. F. , Lopez R. E. , Nagai T.
Published: February 2008

The ninth Coordinated Data Analysis Workshop (CDAW 9) focused upon several intervals within the PROMIS period (March‐June 1986). Event interval C comprised the period 0000‐1200 UT on May 3, 1986, which was a highly disturbed time near the end of a geomagnetic storm interval. A very large substorm early in the period commenced at 0111 UT and had a peak AE index value of ∼1500 nT. Subsequent activity was lower, but at least three other substorms occurred at 2‐3 hour intervals. The substorms on May 3 were well observed by a variety of satellites including ISEE 1, 2, and IMP 8 in the magnetotail plus SCATHA, GOES, GMS, and LANL spacecraft at or near geostationary orbit. A particularly important feature of the 0111 UT substorm was the simultaneous imaging of the southern auroral oval by DE 1 and of the northern auroral oval by Viking. The excellent constellation of spacecraft near local midnight in the radial range 5–9 RE made it possible to study the strong cross‐tail current development during the bstorm growth phase and the current disruption and current wedge development during the expansion phase. We use a time‐evolving magnetic field model to map observed auroral features out into the magnetospheric equatorial plane. There was both a dominant eastward and a weaker westward progression of activity following the expansion phase. A clear latitudinal separation (≳10°) of the initial region of auroral brightening and the region of intense westward electrojet current was identified. The combined ground, near‐tail, and imaging data for this event provided an unprecedented opportunity to investigate tail current development, field line mapping, and substorm onset mechanisms. We find evidence for strong current diversion within the near‐tail plasma sheet during the late growth phase and strong current disruption and field‐aligned current formation from deeper in the tail at substorm onset. We conclude that these results are consistent with a model of magnetic neutral line formation in the late growth phase which causes plasma sheet current diversion before the substorm onset. The expansion phase onset occurs considerably later due to reconnection of lobelike magnetic field lines and roughly concurrent cross‐tail disruption in the inner plasma sheet region.
ExternalSqcurrents in the India‐Siberia region
Authors: Campbell Wallace H. , Arora Baldev R. , Schiffmacher Edward R.
Published: February 2008

TheSqcurrent systems for quiet days of 1976 and 1977 were studied for observatories established near 76° E. longitude in the northern hemisphere. We used a special spherical harmonic analysis technique to separate the internal and external contributions of the quiet field variations and to determine the equivalent external current contours responsible for theSqfield variations on the sixth and twenty‐first of each month. The average ratio of the external to internal spectral power was 6.7 in 1976 and 7.4 in 1977. Focus positions for the India‐Siberia region externalSqcurrent vortex, near 22° to 29°, were found to be at lower geomagnetic latitudes than for Europe and North America. The usual mid‐latitude vortex current, reaching 11.0 to 13.2 × 104A in summertime, disappeared completely during winter months in both the years. TheSqcurrent position was affected clearly by the off‐spin axis position of the north geomagnetic pole. Throughout the 2 years, we also found small current vorticies near 70° to 80° geomagnetic latitude, which we ascribed to auroral region activity on quiet days.
The occurrence rate of magnetospheric‐substorm onsets: Random and periodic substorms
Authors: Borovsky Joseph E. , Nemzek Robert J. , Belian Richard D.
Published: February 2008

Particle‐injection events are monitored on three geosynchronous satellites to determine the occurrences of magnetospheric substorms: for every consecutive pair of substorms found, the time interval Δt between substorm onsets is determined. In this manner, 1001 values of Δt are obtained. A statistical analysis of the Δt values finds that the most‐probable time between substorms onsets is Δt ≈ 2.75 hours: this is interpreted to be the period between substorms when substorms occur cyclically. The statistical analysis of the Δt values also finds a random probability for the occurrence of substorms with a mean time between random substorms of about 5 hours: it is speculated that this random occurrence may be caused by a property of the solar wind that varies randomly with an approximately 5‐hour time scale. About 1500 substorms occur per year: about half are periodic and about half occur randomly.
Inverted vs spanning the cusp boundary layer
Authors: Menietti J. D. , Smith M. F.
Published: February 2008

We present several examples of dayside inverted Vs in which ion convection and energy dispersion appear to span the region of both open and closed field lines. All of these examples occur in the region generally called the low‐latitude boundary layer or the cusp boundary layer and suggest a connection between closed field lines of the dayside auroral region and open field lines of the dayside cusp. One explanation is that plasma in the cusp boundary layer on closed field lines convects across field lines in a smooth fashion into the cusp. Such cross‐field‐line convection may be possible in regions of finite conductivity and strong currents, where the field can diffuse through the plasma. Another explanation for the observations is that they represent the mid‐altitude manifestation of merging at the dayside magnetopause and the opening of previously closed field lines.
Solar cycle variations of geocoronal Balmer α emission
Authors: Nossal S. , Reynolds R. J. , Roesler F. L. , Scherb F. , Harlander J.
Published: February 2008

Observations of the geocoronal Balmer α nightglow have been made from Wisconsin for more than a solar cycle with an internally consistent intensity reference to standard astronomical nebulae. These measurements were made with a double‐etalon, pressure‐scanned, 15‐cm aperture Fabry‐Perot interferometer. The resulting long time line data provides an opportunity to examine solar cycle influence on the mid‐latitude exosphere and to address accompanying questions concerning the degree to which the exosphere is locally static or changing. Our exospheric Balmer α absolute intensity measurements show no statistically significant variations throughout the solar cycle when the variation with viewing geometry is removed by normalizing the data to reference exospheric model predictions by Anderson et al. However, the relative intensity dependence on solar depression angle does show a solar cycle variation. This variation suggests a possible related variation in the exospheric hydrogen density profile, although other interpretations are also possible. The results suggest that additional well‐calibrated data taken over a longer time span could probe low‐amplitude variations over the solar cycle and test predictions of a slow monotonic increase in exospheric hydrogen arising from greenhouse gases.
Long‐term changes in Jovian synchrotron radio emission: Intrinsic variations or effects of viewing geometry?
Authors: Hood Lon L.
Published: February 2008

A long‐term decadal scale variation of Jovian decimetric radio emission has been reported that could indicate intrinsic time variability in the Jovian radiation belts and/or magnetic field. However, the strong beaming of the radiation by trapped relativistic electrons in the complex Jovian magnetic field can also result in apparent decimetric radiation changes due to the evolving viewing geometry of Jupiter during its 11.9‐year orbit. To investigate this problem, radial diffusion models of Jovian trapped electron fluxes at relevant energies are first constructed to simulate possible causes of intrinsic variations. Results indicate several inconsistencies with observations. For example, an increase in diffusion rate with other parameters held constant results in an inward displacement of the peak emission radial distance that is not observed. Second, effects of viewing geometry changes are examined. The possible importance of such effects is suggested by a correlation between the total decimetric radio flux and the Jovigraphic latitude of the Earth, DE, which varies between −3.3° and +3.3° during one Jovian orbital period. Because the Jovian central meridian longitudes where the magnetic latitude passes through zero during a given Jovian rotation change substantially with DE and since significant longitudinal asymmetries exist in both the volume emissivity and the latitudinal profile of the beam, the total intensity should be at least a partial function of DE. Thus variations of viewing geometry associated with Jupiter's orbital motion should be more fully accounted for if long‐term intrinsic variability of Jovian synchrotron radiation is to be unambiguously detected.
Motion of the heliospheric termination shock: A gas dynamic model
Authors: Barnes Aaron
Published: February 2008

The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions; we present a simple quantitative model of this motion. In the model it is assumed that the termination shock is initially a strong gas dynamic shock at rest with respect to the Sun and that upstream of the shock there is a discontinuous increase or decrease in dynamical pressure. This jump is taken to be a contact discontinuity, i.e., an increase (decrease) in density without change in speed at the discontinuity. We then analyze what happens after the discontinuity encounters the shock. The postinteraction configuration is a moving termination shock, a postshock contact discontinuity, and a compression or rarefaction signal propagating into the downstream medium. The analysis is also extended to consider the successive passage of contact discontinuities through the termination shock. On the basis of this model we suggest that the termination shock is constantly in motion and that the following picture emerges: (1) the mean position of the shock is near the mean equilibrium position corresponding to balance between mean solar wind dynamical pressure and mean interstellar pressure; but (2) the shock makes inward and outward excursions over several (or even several tens?) astronomical units and at any given moment its position is determined by the recent (past several month) history of variations of solar wind dynamical pressure. The inward or outward speed of the shock depends on the magnitude of the change in upstream dynamical pressure but is typically of the order of 100 km/s. Therefore the first detection of this shock would be due to the shock moving inward through the spacecraft location rather than the spacecraft reaching a fixed shock location. A kinematic analysis due to Suess (this issue) leads to generally similar conclusions, although his conjecture that the speed of the termination shock may be much larger for outward motion than for inward motion is not supported by our dynamical analysis.
Empirical correlations in thermospheric NO density measured from rockets and satellites
Authors: Smith D. R. , De P. , Adler‐Golden S. , Roth C.
Published: February 2008

The diurnal, latitude, solar flux, and geomagnetic activity variations in thermospheric NO density measured at latitudes as high as 70° have been parameterized with a simple, empirical model. Consistent trends are found in both rocket and satellite measurements, although there are some differences that suggest the presence of systematic errors that add factor of 2 uncertainties to the model predictions. While NO measurements taken with IR emission and γ band fluorescence techniques are in reasonable agreement, evidence suggests that the ion composition and continuum chemiluminescence methods may be less accurate. The current model provides a convenient means for estimating daytime and nighttime NO densities for use in atmospheric chemistry and radiation studies.
High‐pressure and high‐temperature reactions between silicates and liquid iron alloys, in the diamond anvil cell, studied by analytical electron microscopy
Authors: Goarant Francoise , Guyot Francois , Peyronneau Jean , Poirier Jean‐Paul
Published: February 2008

We report new experimental results on mixtures of iron alloys and silicates (Fe + forsterite, Fe + (Mg0.89,Fe0.11)SiO3 enstatite, Fe + (Mg0.89,Fe0.11)2SiO4 olivine, Fe + FeS + (Mg0.83,Fe0.17)2SiO4 olivine, and FeS + (Mg0.83,Fe0.17)2SiO4 olivine) reacted at about 70 GPa and 130 GPa, at high temperature in a laser‐heated diamond anvil cell. The recovered samples were studied by analytical transmission electron microscopy. We found that chemical reactions occurred between molten iron alloys and solid oxides, leading to a dissolution of oxygen in the molten metallic phase, and a drastic depletion of iron in the oxides in contact with the metallic phase. Chromium and manganese partition into the liquid iron. Oxygen is therefore a serious candidate for being a light element in the Earth's core, and oxides which have experienced a high‐pressure contact with molten iron should have a rather low Fe/Fe+Mg ratio.
Micromechanics of rock friction 1. Effects of surface roughness on initial friction and slip hardening in westerly granite
Authors: Biegel R. L. , Wang W. , Scholz C. H. , Boitnott G. N. , Yoshioka N.
Published: February 2008

Frictional behavior of Westerly granite sheared in a rotary apparatus was observed up to 20 MPa normal stress. Before and after each experiment, surface roughness was measured with a profilometer. Macroscopic frictional properties were observed to evolve toward steady state through two distinct stages denoted initial slip and slip hardening. Total slip was less than 200 μm, but some experiments were extended to several centimeters. The instant a shear load was applied, a finite shear stiffness was observed which decreased nonlinearly with displacement. Initial stiffness was roughness dependent; the smoother samples supported equal or higher shear stresses than rough samples at any given displacement. In the shear stress‐displacement data, a distinct yield point was observed which was roughness and normal load dependent. Samples with rougher surfaces and/or higher normal loads required longer displacements to reach a yield point. The yield point marked the start of the slip‐hardening stage during which the shear strength of the surfaces continued to increase at a rate that was roughness and normal load dependent. Rougher surfaces exhibited higher rates of slip hardening than smooth, and over much greater slip distances. During slip hardening, normal closure continued, but at a diminished rate with increasing displacement. The closure rate and slip hardening were correllated: rougher surfaces exhibited higher rates of slip related closure which produced increased asperity interlock and, consequently, increased friction. Beyond 100 to 300 μm displacement, slip hardening and closure nearly ceased, beginning a third stage in which friction is nearly steady state. At steady state, friction of the rough surfaces (0.60 to 0.65) was greater than that of the smooth (0.50 to 0.55).
A comparison of pyroclastic flow and debris avalanche mobility
Authors: Hayashi J. N. , Self S.
Published: February 2008

Many pyroclastic flows have runout distances longer than expected from basic frictional arguments. In these cases the ratio of the height descended H and the runout L are as small as ∼0.2, a feature which has been attributed to fluidization. However, significant fluidization by gases is now considered unlikely during emplacement of debris avalanches, where H/L and volume are inversely related. We determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout. H/L and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits, as well as 15 pyroclastic flow deposits. Systematic differences in the volume versus H/L plots were explored by statistical analysis. Results show an inverse relationship between H/L and volume for both nonvolcanic and volcanic debris avalanche deposits, as expected, and also for pyroclastic flow deposits. Regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non‐volcanic debris avalanche line is parallel to the other two. We argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms. Therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable H/L versus volume, it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism. Based on the data in hand, we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large‐volume debris avalanches.
A seismic study of methane hydrate marine bottom simulating reflectors
Authors: Hyndman R. D. , Spence G. D.
Published: February 2008

Multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island. The reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate. The shallow water depth of 1300 m and the 3600‐m‐long hydrophone array have allowed BSR amplitude‐versus‐offset and high‐resolution velocity analysis, as well as modelling of vertical incidence data. The results of all three types of analysis can be best explained by a 10 to 30‐m‐thick high‐velocity layer located immediately above the BSR about 300 m below the seafloor, having a sharp base and transitional top. In the layer, about one third of the sediment pore spaces must be filled with hydrate “ice”. There is no seismically detectable free gas beneath the BSRs. These results put important constraints on models for the distribution and formation of BSR hydrate.
Global positioning system testing of geoids computed from geopotential models and local gravity data: A case study
Authors: Mainville A. , Forsberg R. , Sideris M. G.
Published: February 2008

Oithometric height differences obtained from a combination of global positioning system (GPS) ellipsoidal height differences and gravimetric geoid predictions have the potential of replacing costly and time‐consuming spirit leveling, especially in remote unsurveyed areas like the Canadian north. In this paper 88 GPS stations along a 900‐km‐long first‐order leveling line around the Great Slave Lake area, Northwest Territories, Canada, are used as control for intercomparison of geoid predictions which used various geopotential models and gravimetric techniques. The global spherical harmonic models tested include satellite‐only solutions, such as GEM‐9, GEM‐L2, GEM‐Tl and GEM‐T2, as well as combination solutions, such as GEMlOB, RAPP78, RAPP81, GPM2, OSU86F, OSU89A, OSU89B and OSU91 A. These models fit the geoid with standard errors of 30 to 50 cm. Local gravimetric geoid predictions based on available gravity data were carried out using various forms of Stokes integration, fast Fourier transform methods and least squares collocation. For the first two methods, gravity data were gridded on a 5 km by 5 km Cartesian grid and on a 5′ by 10′ geographical grid. No terrain reductions were applied due to the flatness of the topography. Results show that the differences between control and predicted geoid heights have standard deviations at the 20‐to 25‐cm level, with differences between methods at 2 to 8 cm (1σ). By fitting a four‐parameter model to the geoid predictions (corresponding to a GPS, leveling or geoid datum shift and scale change), long‐wavelength errors were significantly reduced, with all methods yielding geoid fits of about 7 cm (1σ). The relative accuracies achieved were of the order of 2 ppm for baselines shorter than 200 km, and of the order of 1 ppm for baselines with lengths between 200 km and 700 km. A discussion of possible error sources concludes the paper. Some errors in the original GPS data were detected in the initial phase of the project, and a GPS resurvey verifyed these errors.
Variation of the Gutenberg‐Richter b values and nontrivial temporal correlations in a Spring‐Block Model for earthquakes
Authors: Christensen Kim , Olami Zeev
Published: February 2008

We show that a two‐dimensional spring‐block model for earthquakes is equivalent to a continuous, nonconservative cellular automaton model. The level of conservation is a function of the relevant elastic parameters describing the model. The model exhibits power law distributions for the energy released during an earthquake. The corresponding exponent is not universal. It is a function of the level of conservation. Thus the observed variation in the b value in the Gutenberg‐Richter law could be explained by a variation in the elastic parameters. We address the problem of the boundary conditions and display results for two extreme possibilities. Furthermore, we discuss the correlation in the interoccurrence time of earthquakes. The model exhibits the features of real earthquakes: the occurrence of small earthquakes is random, while the larger earthquakes seem to be bunched. Primarily, the results of our work indicate that the dynamic of earthquakes is intimately related to the nonconservative nature of the model, which gives birth to both the change in the exponent and the correlations in interoccurrence time.
Pore pressure, stress increase, and fault weakening in low‐angle normal faulting
Authors: Axen Gary J.
Published: February 2008

Low‐angle (dip < 30°) normal faults accommodate much extension of the continental crust. They apparently move under low resolved shear stress and are anomalously weak, characteristics that they share with the San Andreas fault. Structural, textural, and geochemical arguments suggest that low‐angle normal faults are weak in both the ductile and brittle regimes, partly or totally due to elevated pore fluid pressure. High pore pressure in detachment zones may be contained by upper‐plate strata, mineral precipitation in their hanging walls, formation of low‐permeability microbreccia layers, threshold pressure gradients, and low‐permeability mylonites below chlorite breccia. Mechanical analysis shows that fault weakening may preclude equality of the regional and fault‐zone stress tensors, and predicts reorientation and increase of principal stresses in weak fault zones. These changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks. Fault weakening focuses extension in the upper crust onto low‐angle normal ductile‐brittle shear zones in the midcrust, promoting propagation of low‐angle brittle normal faults into the upper crust.
Theory of dike intrusion in a saturated porous solid
Authors: Elsworth Derek , Voight Barry
Published: February 2008

A study of the pore pressure fields that develop around intruding dikes is described under the motivation that intruded geometry may be determined from pore pressures recorded in relatively remote monitoring wells. The pore pressure fields induced around cylindrical and planar intrusions are described as analogues to moving point or line dislocations within an infinite saturated porous elastic medium. The resulting transient pressure fields reduce to an equivalent steady state when viewed from the advancing front. Solutions for the moving point and line dislocations yield a dependence on common dimensionless groupings. Thus, dimensionless pressure rise accompanying intrusion, and recorded at a static location, may be uniquely referenced to the dimensionless parameters representing emplacement velocity and time. The resulting transient pressure response may be divided into two groups, representing fast and slow emplacement. Where hydraulic parameters representing the host porous medium are known a priori, both emplacement location and the cross‐sectional area may be determined uniquely. For slow emplacement, the intrusion rate may also be determined; only a lower limiting intrusion rate may be discerned for fast emplacement. Field data are rare, but two intrusive events at Krafla, Iceland are examined using the proposed moving dislocation models. Predicted location and lower limiting intrusion rate of the Krafla dikes compare favorably with field observation despite more than a 9 km separation to the monitoring well.
Tomographic imaging of P and S wave velocity structure beneath northeastern Japan
Authors: Zhao Dapeng , Hasegawa Akira , Horiuchi Shigeki
Published: February 2008

The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discontinuities (SVDs) and with three‐dimensional variations in the velocities in the modeling space. An efficient three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate‐depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. High‐resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6% for P wave and 10% for S wave are revealed in the crust and upper mantle. In the crust low‐velocity (low‐V) zones exist beneath active volcanoes. In the upper mantle the low‐V zones dip toward the west from the volcanic front. A high‐velocity (high‐ V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double‐planed deep seismic zone are found to occur in relatively high‐V areas. The obtained tomographic images are also found to explain other seismological observations well.
Numerical study of quarry generated Rg as a discriminant for earthquakes and explosions: Modeling of Rg in southwestern New England
Authors: Saikia Chandan K.
Published: February 2008

In this study, we modeled 0.5–2.0 s period Rayleigh waves (Rg) generated by quarry and construction blasting for the shallow crustal velocity and intrinsic Q structure in southwestern Connecticut and southeastern New York. In spite of the Rg waves traversing through different geological formations, the Rg dispersion observed along individual paths, namely, the Clinton Point‐BCT, the Clinton Point‐BPT, the North Brandford‐BCT, the New Britain Plainville‐BCT and the Reeds Gap‐BCT paths, are remarkably similar. The dispersion is similar to the Rg dispersion observed in other parts of southern New England. For a representative shallow crustal velocity structure of the region, we therefore inverted the average dispersion curve observed along the Clinton Point‐BPT path. The inverted crustal model, with shear velocity varying between 2.85 and 3.65 km/s down to a depth of 3 km, is similar to the crustal models published earlier for the other parts of southern New England. High‐frequency Rg waves are found to be highly sensitive to anelasticity within the near‐surface crustal layers. The observed Rg waves require Qβ (shear wave quality factor) as low as 15 to produce the best agreement between data and synthetic. A thin till layer affects the Rg waves significantly provided the layer thickness is of the order of Rg wavelength. Theoretical study shows that the delay times between charges caused by the temporal finiteness of arrays of explosions during a quarry blast are found to have produced a significant effect on the high‐frequency Rg waves. We also investigated theoretical seismograms to discriminate between earthquake and quarry explosions. We found that Rg waves are a successful discriminant only for earthquakes at depths greater than 3 km. The shallow earthquakes show strong Rg waves for dip‐slip faults but are preceded by high‐frequency shear arrival, especially in the range of 50–125 km. Beyond 126 km, pSmS and its multibounces become critical with distance and contribute strongly. These waves appear on the explosion seismograms as shear waves. So, this shear wave may provide a diagnostic in the range between 50 and 125 km for discriminating earthquake seismograms from the small quarry explosions.
Detection of precursory relative quiescence before great earthquakes through a statistical model
Authors: Ogata Yosihiko
Published: February 2008

Relative quiescence is defined as a significant decrease of earthquakes compared with the occurrence rate expected from a point‐process model for ordinary seismic activity. This definition is linked with the idea that aftershocks, as a useful signal for the investigation of seismicity, should not be removed from original occurrence data. The Epidemic Type Aftershock‐Sequences (ETAS) model is proposed to identify patterns in an earthquake sequence in an area, using occurrence time and magnitude data. Even in an active stage of seismicity, a decrease from the expected level can take place, and the significance and size of such a quiescence can be graphically shown using time‐changed occurrence data which are transformed based on the estimated ETAS model. This procedure permits detection of a clear, relatively quiet stage before great earthquakes in Japan and elsewhere in the world. Such relative quiescence lasts a number of years prior to the occurrence of great earthquakes, which occasionally take place in the recovering stage. No significant relative quiescences were seen in the current seismicity of a few wide regions, in and around Japan, including several source regions of expected great earthquakes.
Wave propagation in random anisotropic media
Authors: Jannaud L. R. , Adler P. M. , Jacquin C. G.
Published: February 2008

A method to determine the characteristic lengths of anisotropic two‐dimensional random media is developed which is based on the linear theory of Aki and Chouet (1975) and Sato (1982). The correlation lengths of an equivalent anisotropic Gaussian medium are extracted from the spectrum of the codas. It is proved that for Gaussian correlation functions and weak velocity fluctuations the geometric anisotropy of the velocity distribution does not induce any propagation anisotropy. The spectral power of codas for Boolean media is also analyzed.