bluuebunny/crossref_metadata_2025 huggingface dataset:
The sheath structure around a negatively charged rocket payload
Authors: Neubert T. , Mandell M. J. , Sasaki S. , Gilchrist B. E. , Banks P. M. , Williamson P. R. , Raitt W. J. , Meyers N. B. , Oyama K. I. , Katz I.
Published: February 2008

The sheath structure around a rocket payload charged up to 460 V negative relative to the ambient ionospheric plasma is investigated experimentally and by computer simulations. Our experimental results come from the CHARGE 2 sounding rocket experiment in which the payload was split into two separate sections (mother and daughter) connected with a conducting, insulated tether. In one of the experimental modes, the voltage between the payloads was increased linearly from 0 to 460 V in 2.5 s. In this case the tethered mother/daughter functioned as a double probe, the negative probe (mother) reaching large negative potentials, while the positive probe (daughter) stayed close to the ambient plasma potential. A floating probe array was mounted on the mother with probes located 25, 50, 75, and 100 cm from the rocket surface. The internal impedance of the array was smaller than the probe/plasma impedance, which influenced the potential measurements. However, the measurements contain signatures, which we interpret as resulting from the outward expansion of the ion sheath with increasing negative mother potential. This conclusion is substantiated by NASCAP/LEO computer simulations of space charge limited flow. At high potentials, the observed ion current flowing to the mother increased more strongly with bias potential than found from the simulations. It is suggested that the enhancement of the current is generated by secondary electrons emitted by the ions bombarding the payload skin. The effects of the motion of the mother (540–580 m/s) and of the ambient magnetic field have been assessed by the code. It was estimated that the ion current to the mother was increased by 20% relative to a stationary payload, while the incorporation of a magnetic field had no practical influence on the simulation results.
MHD intermediate shocks in coronal mass ejections
Authors: Steinolfson R. S. , Hundhausen A. J.
Published: February 2008

We consider a simplified model of coronal mass ejections in which at least a portion of the interaction with the background corona involves a shock wave and examine the allowable shock solutions and their compressive signatures. The MHD shock‐jump equations have a maximum of three possible types of solutions with an entropy rise for fixed values of the physical variables (slow, intermediate, and fast shocks). However, one of the three solution classes (the intermediate shock) is widely believed to not occur in nature and is regarded as nonevolutionary or extraneous. Without the intermediate shock, there is no multiplicity of solutions in that only one shock (or none) can occur for given physical values. We consider all three potential shock types and show, solely on the basis of the shock‐jump equations, that intermediate shocks must exist along some segment of the shock front for certain parametric regimes and for conditions that probably occur in some coronal mass ejections. Intermediate shocks arise as a result of cross‐flow interactions in our study; that is, allowable shock solutions at certain locations in the flow field dictate that intermediate shocks occur in adjacent (perpendicular to the flow velocity) regions. Consequently, the flow must be treated as, at least, a two‐dimensional problem. Numerical simulations of the nonlinear, time‐dependent MHD equations in two dimensions verify the formation of intermediate shocks as predicted from arguments based on analytic theory.
Observations and modeling of wave‐induced microburst electron precipitation
Authors: Rosenberg T. J. , Wei R. , Detrick D. L. , Inan U. S.
Published: February 2008

Energy‐time features of X ray microbursts are examined and compared with the predictions of a test particle simulation model of wave‐induced electron precipitation resulting from gyroresonant wave‐particle interactions in the magnetosphere. The observations were obtained on a balloon flight at Siple station, Antarctica (L ≈ 4.2) on December 30, 1980. The energy and time evolution of the microbursts were studied for X rays in the energy range from 25 to 175 keV. An algorithm designed to search the E > 25 keV counting rate data for single isolated microbursts identified 651 events in a 3‐hr interval. The distribution of burst durations ranged from 0.2 to 1.2 s. Approximately two‐thirds of the distribution were narrow bursts (0.2–0.6 s), the rest wide (0.6–1.2 s), with the average burst durations equal to ∼0.4 s and ∼0.7 s, respectively, for the two classes. The precipitation was characterized by exponential electron spectra with e‐folding energies Eo of 25‐50 keV. Individual and superposed microburst profiles show that the X ray energy spectrum is softest (equivalent to a reduction in electron Eo of ≤ 5 keV) near the peak of the energy influx. Computer simulations of the flux‐ and energy‐time profiles of direct and mirrored electron precipitation induced by a whistler‐mode wave pulse of 0.2‐s duration and linear frequency increase from 2 to 4 kHz were performed for plasma, energetic particle and wave parameters appropriate for the location and geophysical conditions of the observations. The wave pulse is representative of the VLF chorus elements that were observed at Siple station and at the conjugate location of Roberval, Quebec during the microburst activity on this day. In both the direct and mirrored cases the combined effect of latitude dependence of the energy of maximum gyroresonant scattering and energy dispersion leads to energy spectrum variations qualitatively consistent with the microburst observations. The average burst durations in the two cases were ∼0.3 s (direct) and ∼0.7 s (mirrored). On this basis we identify the narrow microbursts with electrons which are precipitated directly after undergoing pitch‐angle scattering interactions with the wave pulse; the wide microbursts are identified with electrons which, after interacting with the wave, mirror in the conjugate ionosphere before precipitating into the southern ionosphere. In general, the results provide further support for the gyroresonant test particle simulation model, and for the belief that the observed type of microbursts originates in the vicinity of the magnetic equator in a gyroresonant process involving discrete chorus emissions.
Extreme energetic particle decreases near geostationary orbit: A manifestation of current diversion within the inner plasma sheet
Authors: Baker D. N. , McPherron R. L.
Published: February 2008

A qualitative model of magnetic field reconfiguration as might result from neutral line formation in the central plasma sheet late in a substorm growth phase is considered. It is suggested that magnetic reconnection probably begins before the substorm expansion phase and that cross‐tail current is enhanced across the plasma sheet both earthward and tailward of a limited region near the neutral line. Such an enhanced cross‐tail current earthward of the original X line region may contribute to thinning the plasma sheet substantially, and this would in turn affect the drift currents in that location, thus enhancing the current even closer toward the Earth. In this way a redistribution and progressive diversion of normal cross‐tail current throughout much of the inner portion of the plasma sheet could occur. The resulting intensified current, localized at the inner edge of the plasma sheet, would lead to a very thin plasma confinement region. This would explain the very taillike field and extreme particle dropouts often seen late in substorm growth phases. When the currents cannot be further enhanced within the inner plasma sheet (and when diffuse electron precipitation has sufficiently increased ionospheric conductivities), an intense (≳106 A), very rapid increase of current flow through the auroral ionosphere would be expected. This latter phase, in combination with magnetic reconnection reaching field lines of the plasma sheet boundary layer and the tail lobes, would constitute the substorm expansion phase in this scenario.
Obliquely propagating ion acoustic waves in the auroral E region: Further evidence of irregularity production by field‐aligned electron streaming
Authors: Villain J.‐P. , Hanuise C. , Greenwald R. A. , Baker K. B. , Ruohoniemi J. M.
Published: February 2008

Common volume observations of E region high‐latitude irregularities at decameter wavelengths have been obtained with the JHU/APL HF radar located at Goose Bay, Labrador, and the SHERPA HF radar located at Schefferville, Quebec. In this paper, we analyze an event with characteristics similar to those of a distinctive type of event described by Villain et al. [1987]. The experimental configuration, which combines the azimuthal‐scanning capability of the Goose Bay radar with the frequency‐scanning operation of the Schefferville radar, has provided unambiguous evidence of the existence of two irregularity layers at different altitudes within the E region. The layers, which exhibit different characteristics, can be related to the action of the gradient drift and ion acoustic instability mechanisms. It is shown that the ion acoustic modes have phase velocities in the range of 400 to 550 m/s and are produced in regions of subcritical perpendicular electron Hall drift. We infer that the observed irregularities are produced through a combination of perpendicular and field‐aligned relative electron‐ion drifts. Features previously observed but not satisfactorily explained by perpendicular drift excitation alone can be understood in terms of field‐aligned drift excitation. We conclude that the role of electron‐ion field‐aligned drift may be much more important than previously realized.
On the origin of solar wind MHD turbulence: Helios data revisited
Authors: Grappin Roland , Mangeney Andre , Marsch Eckart
Published: February 2008

We study the fluctuations of density, magnetic and velocity fields in the frequency range from (1 day)−1 ≈ 1.2×10−5 Hz to (2.8 min)−1≈ 6×10−3 Hz, as measured by the primary Helios mission (118 days), at heliocentric distances ranging from 0.3 to 1 AU. We address the question of the existence of nonlinear cascades in the observed turbulence, possibly separate for the two “inward” and “outward” components, corresponding to opposite directions of propagation along the large‐scale magnetic field. We consider energies per unit mass, not per unit volume, in order to work with variables which are not very sensitive to the heliocentric distance variations. We find that while the whole spectrum of total (kinetic plus magnetic) turbulent energy undergoes very large daily variations both in its amplitude and spectral shape the instantaneous spectrum follows a power law in the frequency range 10−4 to 6×10−3 Hz. We show that both the amplitude and the spectral index m depend on the proton temperature, in a monotonic way, so that a large temperature (thermal speed about 60 km/s) leads to a low level of turbulence with a steep, Kolmogorov‐like spectrum (m ≈ −1.8), while a low temperature (thermal speed about 16 km/s) leads to a flatter spectrum (m ≈ −1.2) with a high level of turbulence. This relation is independent from heliocentric distance, at least between 0.3 and 1 AU. Decomposing the turbulent energy into two components, “outward” and “inward,” we find that the spectrum of the outward component also follows very closely the daily proton temperature variations, while the inward component's spectrum is less sensitive to the temperature but also varies with the relative level of rms proton density fluctuations. As a consequence, Alfvénic periods (in which energy is dominated by the outgoing component) occur mainly when density fluctuations are low and temperature is high, which does not contradict the classical view that they are found in the “trailing edges of high‐speed streams” (Belcher and Davis, 1971). The existence of inertial ranges controlled by the level of density fluctuations is not completely new (see the numerical simulations of purely hydrodynamic turbulence by Pouquet and Passot (1987)), but the strong dependence of both turbulent energy level and spectral slope on temperature is a new, unexpected property of solar wind turbulence which remains to be explained.
Broadening of the resonantly scattered H I Lyman alpha line caused by Alfvén waves
Authors: Esser Ruth
Published: February 2008

Alfvén waves propagating outward from the coronal base set the solar wind into oscillatory motion. The effect of this wave motion on the broadening and shape of the spectral lines of the resonantly scattered Ly α radiation emitted by a low‐density coronal hole is investigated at hights above 1.5 Rs from Sun center. Usually it has been assumed that the broadening due to waves can be characterized by a kinetic temperature proportional to the wave velocity amplitude squared, δυ². It is shown that the accuracy of this approximation depends on the wave velocity amplitude and the assumed wave spectrum.
Transport and turbulence modeling of solar wind fluctuations
Authors: Zhou Ye , Matthaeus William H.
Published: February 2008

Magnetohydrodynamic (MHD) activity, including waves and turbulence, has been a focus of attention in solar wind research for several decades, owing to the wealth of available relevant spacecraft observations. Characterizations of the turbulence have generally been based on incompressible homogeneous turbulence theory. However, recent observations show that the fluctuations undergo systematic temporal evolution, suggesting couplings to large‐scale plasma and magnetic field inhomogeneities. Here we present in detail, and further develop, a theory of transport of small‐scale solar wind MHD turbulence, the preliminary results of which have been recently reported (Zhou and Matthaeus,Geophysical Research Letters, 16, 755, 1989). Large‐scale plasma velocity and magnetic fields are specified, and small‐scale incompressible turbulence evolves in response to the associated inhomogeneities, as well as in accordance with modeled local nonlinear couplings. Dynamical equations based on a two length scale expansion are derived, from which the evolution of various wave number spectra may be computed, including magnetic and kinetic energies, cross helicity, induced electric field, and the corresponding helicities. Several simple analytic solutions predict, with increasing heliocentric distance, a decrease of the preponderance of outward traveling type Alfvénic fluctuations and a lowering of the small‐scale kinetic to magnetic energy ratio of small scale. Both of these are consistent with Helios and Voyager observations. The theory provides a basis for further computations that may be compared with statistical features of solar wind observations.
Jovian H2 dayglow emission (1978–1989)
Authors: McGrath M. A. , Ballester G. E. , Moos H. W.
Published: February 2008

Observations of the Jovian equatorial region obtained with the International Ultraviolet Explorer (IUE) satellite over the past 10 years (December 1978 to March 1989) have been analyzed to determine the long‐term temporal variation of the H2 dayglow emission. Lyman band emission in the region of 1500–1700 Å decreased by ≈40–50% between 1981 and 1986 and has increased by more than a factor of 2 between 1986 and March 1989. Observations in late 1988 and early 1989 show the highest levels of Jovian equatorial H2 emission ever recorded by the IUE satellite. Comparison of the observations with simple models indicates a possible relation between the dayglow emission and 11‐year solar cycle activity. The magnitude of the observed variation is closer to that of the solar Ly α flux than that of the 10.7‐cm radio flux. The intensity of the short‐wavelength H2 band emission (1240–1325 Å) is inconsistent with the amount of long‐wavelength emission (1500–1700 Å) but may be reconciled if relatively low‐energy electron excitation or fluorescence of solar radiation is invoked. It is not possible to unambiguously distinguish between these possibilities with the present data. Crude longitudinal binning of the observations reveals no persistent feature analogous to the H I Ly α “bulge,” although there may be enhanced emission at the approximate longitude of the Ly α bulge in the 1988 observations.
A correlative study of Pc 5 magnetic pulsations with substorm onsets
Authors: Pangia M. J. , Lin C. S. , Barfield J. N.
Published: February 2008

This study uses simultaneous observations of storm time Pc 5 events by the GOES 2 and GOES 3 geostationary satellites to study the correlation of Pc 5 waves with substorm onsets. Eighteen Pc 5 events occurring from March to December 1979 are first surveyed. After excluding events with highly varying activity or scarce station coverage, only six events with clear substorm features 2–4 hours before the GOES observation of the Pc 5 events are then analyzed. From the wave propagation speed and the distance from local midnight to the satellite position, the substorm onset time is estimated. For the six events examined, ground magnetograms near local midnight indicate that a substorm onset occurred within 20 min of the estimated substorm onset times. This result suggests that the occurrence of storm time Pc 5 waves is probably correlated with substorm onsets.
A study of the coherence length of ULF waves in the Earth's foreshock
Authors: Le G. , Russell C. T.
Published: February 2008

We have examined high time resolution magnetic field data for different separations of ISEE 1 and 2 in the Earth's ion foreshock region to study the coherence length of upstream ULF waves. By examining the correlation coefficients of the low frequency waves as a function of separation distance, we find that the correlation coefficient depends mainly on the separation distance of ISEE 1 and 2 transverse to the solar wind flow. It drops to about 0.5 when the transverse separation is about 1 RE, a distance much larger than the proton thermal gyroradius in the solar wind. Thus the coherence length of the low‐frequency waves is about one Earth radius, which is of the order of the wavelength, and is consistent with that estimated from the bandwidth of the waves.
Alpha particle heating in hot diamagnetic cavities
Authors: Galvez Miguel , Fuselier Stephen A. , Gary S. Peter , Thomsen Michelle F. , Winske Dan
Published: February 2008

The heating of solar wind alpha particles in hot diamagnetic cavities (HDCs) is examined both through data analysis and computer simulation. The Lockheed Plasma Composition Experiment on ISEE 1 shows more than an order of magnitude increase in the alpha temperature between the solar wind and the HDCs, analogous to the strong solar wind proton heating evidenced in the same transition. A one‐dimensional electromagnetic hybrid computer simulation is used to study alpha heating by ion/ion instabilities, where the initial condition is a cool field‐aligned proton beam streaming relative to the solar wind protons and alpha particles. Low beam densities excite the proton/proton right‐hand resonant instability which pitch angle scatters the beam without significantly heating the alphas. At larger beam densities, the proton/proton nonresonant instability saturates by strong trapping of all three ion components; after saturation the large amplitude magnetic fluctuations lead to stochastic scattering and heating of the ions such that the final temperatures of the alphas are typically greater than the final proton temperatures. These results provide further support to the hypothesis of Thomsen et al. (1988) that the nonresonant instability is the primary source of ion heating in hot diamagnetic cavities.
Substorm electrodynamics
Authors: Stern David P.
Published: February 2008

The electrodynamics of a substorm are analyzed using a crude one‐dimensional model. The discussion starts from the “standard interpretation“ scenario in which the plasma sheet collapses into a neutral sheet and magnetic merging occurs between the two tail lobes. Plasma then flows into the neutral sheet from both the lobes and the sides and is accelerated in the dawn‐dusk direction according to a “type 2 merging” scenario. The lobes supply the process with magnetic energy, but most particles come from the original plasma sheet population, as evidenced by the relatively low acceleration; Alfvén's theory for this is modified to accommodate both particle sources. The process is further modified by the tendency of accelerated plasma to unbalance charge neutrality. To maintain such neutrality, it is assumed that the ionosphere absorbs any excess of electrons and provides electrons to neutralize any excess of ions. It is this exchange of electrons with the ionosphere, rather than some unspecified “current disruption,” that accounts for the “current wedge” observed to accompany substorms. The cross‐tail current is weakened by the diversion, and this reduces the adjacent lobe field intensity, but there are no drastic consequences, either in the wedge circuit or in the somewhat similar diversion of region 1 Birkeland currents; at most the tail boundary may expand very slightly. The accelerating electric field of the merging region is inductive and could in principle be completely confined to the tail. The ionosphere then comes into play only because the wedge currents flow through it and produce a resistive voltage drop, a true electric potential which unlike the inductive field propagates along field lines and ultimately affects the acceleration region. The system is self‐stabilizing to some extent, because if the ionospheric potential blocks off plasma sheet inflow into the merging region, it also cuts off the wedge currents which produce this potential. However, Alfvénic delays modify this feedback, and a blocking potential might well explain bursts of high‐energy particles that are occasionally observed.
Heating of the polar wind due to ion beam instabilities
Authors: Chen Margaret W. , Ashour‐Abdalla Maha
Published: February 2008

In the “classical” theory of the polar wind, there is a significant outflow of cold supersonic H+ and limited O+ from the polar ionosphere. Recent DE 1 observations in the polar cap region report upflowing field‐aligned H+ and O+ beams (∼10 eV) which may interact with the polar wind. The source of these ion beams may be energetic ions in the dayside polar cusp that E×B convect into the polar cap region. These ion beams provide free energy to the polar wind which will alter its “classical” description. In the study described here, the linear as well as the nonlinear effects of O+ and H+ ion beams on an O+ and H+ polar wind are investigated. The O+ and H+ polar wind ions are modeled by isotropic Maxwellian distributions while the electrons, O+ beam ions, and H+ beam ions are modeled by drifting Maxwellian distributions. The electrostatic dispersion relation is solved for a range of O+/H+ beam densities, Te/Ti, and ion beam speeds. There is a parameter regime consistent with observations in which O+ beam modes such as the slow O+ acoustic and slow O+ cyclotron modes are excited with growth rates γ ∼ ΩO while H+ beam modes are damped. Simulations show that these O+ beam modes heat both the O+ and H+ polar wind ions in the perpendicular direction. In fact, the O+ ions are preferentially heated over the H+ ions. This preferential heating of O+ ions has important implications because it enhances the outflow of O+ ions along polar cap field lines by effectively increasing the O+ scale height.
Shear flow beneath oceanic plates: Local nonsimilarity boundary layers for olivine rheology
Authors: Yuen David A. , Tovish Aaron , Schubert Gerald
Published: February 2008

The principle of local similarity, which has been used to model the two‐dimensional boundary layers in the oceanic upper mantle, permits calculation of the temperature, velocity, and stress fields with essentially analytic techniques. Finite difference numerical methods are hard pressed to resolve the detail required by the large variation of viscosity between the lithosphere and the asthenosphere. In this paper the local similarity approximation has been justified by quantitatively evaluating the effect of nonsimilarity due to viscous heating, nonlinear temperature‐ and pressure‐dependent rheology, buoyancy, adiabatic cooling, etc. Nonsimilar effects produce only small modifications of the locally similar boundary layers; important geophysical observables such as surface heat flux and ocean floor topography are given to better than 10% by the locally similar solution. A posteriori evaluations of the terms neglected in the boundary layer simplification of the complete equations have been conducted on the locally similar temperature and velocity profiles close to the spreading ridge. The boundary layer models are valid to depths of 100 km at 3 m.y. and 10 km at 0.3 m.y.
Structure of the East Pacific Rise crest from multichannel seismic reflection data
Authors: Herron T. J. , Ludwig W. J. , Stoffa P. L. , Kan T. K. , Buhl P.
Published: February 2008

Computer‐processed multichannel seismic reflection profiles from the crestal zone of the East Pacific Rise near the Siqueiros Fracture Zone have revealed distinct layering in the structure not previously observed in single‐channel reflection records. After processing, which included common depth point gathering, normal move‐out and stack, time‐varying predictive deconvolution, and wave equation migration, three distinct crustal layers emerge as coherent events across the rise crest. The base of the third layer appears to be at a depth of about 2 km below the sea floor and may represent the top of a low‐velocity magma chamber, as postulated by Orcutt et al. (1975, 1976) from the results of ocean bottom seismometer refraction data. This layer appears to thicken away from the rise crest toward the edge of the raised axial block that characterizes the East Pacific Rise (Anderson and Noltimier, 1973). Illustrated in this article is the ability of modern processing techniques as applied to multichannel seismic data to enhance weak arrivals, to minimize sound source bubble‐pulse reverberations, and to remove the diffraction patterns caused by rough topography.
Characteristics of back arc spreading in the Mariana Trough
Authors: Karig D. E. , Anderson Roger N. , Bibee L. D.
Published: February 2008

Although sea floor spreading of some sort has created marginal basins, it is not known whether this spreading is similar to that at midocean ridges. Back arc spreading in the Mariana Trough occurs along an axial topographic high, but there is rapid variation in style along strike and less well defined symmetry than there is at midocean ridges. The topography is very rough, despite an apparently fast spreading rate. This lack of correlation between spreading rate and topographic relief also occurs in other marginal basins of the western Pacific. In addition, the apparently different curvatures between the frontal and remnant arcs of some marginal basins, such as the Mariana Trough, might result from nonrigid deformation of the frontal arc. There is no single finite pole of rotation about which the Mariana trough can be closed without seriously violating observed fracture zone directions and the geology at both ends of the arc. South of 14°N the frontal arc is truncated by the east‐west fractures so that marginal basin crust is juxtaposed against the Pacific plate near the possible triple junction between the Philippine and the Pacific plates and the Mariana platelet. The present data suggest that the Mariana Trough has opened as if the frontal arc were a horizontal beam pinned at its northern end and deforming laterally so that fracture zones parallel to the spreading direction have themselves undergone minor extension. That is, the Mariana Trough has pulled loose from the Philippine plate at its southern end and has, been bent and fractured so that extension has occurred not only parallel but also perpendicular to the direction of principal rotation.
The upper mantle P velocity structure of the western United States
Authors: Burdick L. J. , Helmberger Donald V.
Published: February 2008

A new compressional velocity profile for the western or technically active portion of the United States has been constructed by matching long‐period body wave forms with synthetic seismograms. The wave form data come from shallow California earthquakes with very well constrained source mechanisms. The known source pulses from the events were convolved with delta function responses for velocity models computed by using a Cagniard‐deHoop algorithm to produce the synthetic seismograms. By using this technique it was possible to identify and model strong arrivals from both upper mantle velocity discontinuities in the wave form data. The new model fits the travel time, dT/dΔ, and short‐period wave form data as well as or better than the velocity profile previously proposed for the region. Nonetheless, it is basically simpler in form than this earlier model. Below the low‐velocity zone the velocity gradients are very nearly linear with depth, and the two discontinuities are simple first‐order jumps. The first discontinuity occurs at 400‐km depth and entails a 5% velocity jump, and the second occurs at 670 km, entailing a 4% jump. The velocity inflection near 550 km has been eliminated.
Elastic moduli of two‐component systems
Authors: Chatterjee A. K. , Mal A. K. , Knopoff L.
Published: February 2008

Self‐consistent results on the properties of inhomogeneous materials are compared with results based on multiple scattering calculations for a medium containing either spherical inclusions or cylindrical inclusions. Inconsistencies are found in terms of order c2, where c is the concentration of inclusions. Thus the claims of O'Connell and Budiansky and others regarding the validity of self‐consistent calculations for properties of inhomogeneous materials at nondilute concentrations are not supported.
Ray method for elastodynamic radiation from a slip zone of arbitrary shape
Authors: Achenbach J. D. , Harris J. G.
Published: February 2008

A ray method is presented to analyze elastodynamic radiation generated by slip over a fault plane. With this method, either the discontinuities at the wave fronts or, equivalently, the high‐frequency components of the fields can be calculated. The method is applied to the computation of ground motions generated by crack propagation models for earthquake faulting. It is very suitable for computing strong motions produced by abrupt changes of the speed of the rupture front. Basic to the method are two canonical solutions for elastodynamic radiation which correspond to sudden changes of the rupture speeds of semiinfinite cracks propagating in the mode 2 and mode 3 fracture modes, respectively. When the rupture front of a slip zone suddenly changes speed, each point on the rupture front produces fans of rays of longitudinal and transverse motion, respectively. The method appropriately combines the mode 2 and mode 3 canonical solutions on the diffracted rays, and it provides corrections for curvature of the rupture front. Particular attention is devoted to the focusing of motion due to curvature of the rupture front. The results relate the intensity of the radiated fields to the nature of the slip just behind the rupture front.
Universal time: Results from lunar laser ranging
Authors: King R. W. , Counselman C. C. , Shapiro I. I.
Published: February 2008

Lunar laser ranging observations from the McDonald Observatory have been analyzed by least squares to estimate universal time (UT) simultaneously with parameters representing the locations of McDonald and the lunar retroreflectors, the orbits of the earth and the moon, and the moon's physical libration. The root‐mean‐square (rms) of the postfit range residuals for the 5‐year period from October 1970 to November 1975 is 28 cm. The rms of the differences between our determinations of UT and those of the Bureau International de l'Heure is 2.1 ms, after removal of the mean difference. The differences between our determinations and those of Stolz et al. (1976), who analyzed most of the same ranging data with a different approach, have an rms of 0.8 ms and, in addition to high‐frequency ‘noise,’ exhibit low‐frequency variations of ≲ 1 ms in magnitude which result at least in part from differences in models used for the lunar orbit. Thus problems in modeling the moon's motions, especially those of long period, make difficult the determination of UT with the accuracy inherent in the ranging observations.
Crustal structure in central Israel from the inversion of magnetotelluric data
Authors: Shoham Y. , Ginzburg A. , Abramovici F.
Published: February 2008

An iterative inversion procedure for magnetotelluric data based on the generalized inverse of an arbitrary matrix is applied to information measured along a traverse of Israel. The resolution and information matrices, a double eigensystem, and the effective number of degrees of freedom are used for the assessment of the set of data, the set of model parameters obtained, and the global relationship between the two sets. The method is used for the interpretation of several magnetotelluric sites, and bounds for the variance of each of the model parameters are computed. The results are in agreement with borehole data and other independent geophysical and geological information. A crustal cross section of Israel is presented. Some new deep structural information is revealed, such as the existence of a conductor at a roughly uniform depth of 23 km along the section.
Paleomagnetism of the Devonian Catskill red beds: Evidence for motion of the coastal New England‐Canadian maritime region relative to cratonic North America
Authors: Kent Dennis V. , Opdyke Neil D.
Published: February 2008

The natural remanent magnetizations of reddish clay stones, siltstones, and sandstones from the nearly flat lying Middle to Upper Devonian Catskill sequence of southeastern New York were analyzed with thermal, alternating field, and chemical demagnetization techniques. After removal of a low blocking temperature component along the present geomagnetic field direction a characteristic direction of magnetization was isolated: D = 172.3°, I = 1.0°, k = 116, and α95 = 4.7° for N = 9 sites (43 samples), giving a paleomagnetic north pole at 46.8°N, 116.7°E, dp = 2.4°, and dm = 4.7°. The combined demagnetization analyses show this to be the only stable component of magnetization present in these rocks. The derived pole position agrees well with the poles reported for some Devonian limestones in Ohio, all falling near the Permian poles for North America, but disagrees with Devonian results from eastern Maine‐New Brunswick and eastern Massachusetts which give poles at lower latitudes. A similar geographical grouping with similar directions is also apparent for Lower Carboniferous (Mississippian) paleomagnetic poles for North America. We interpret these and other late Paleozoic paleomagnetic data to show that the coastal Canadian Maritime‐New England region was not an integral part of cratonic North America until about the Late Carboniferous, Geological considerations suggest that the Carboniferous relative motion was along transcurrent shear zones.
Mantle convection and the thermal structure of the plates
Authors: Parsons Barry , McKenzie Dan
Published: February 2008

Though a simple thermal model of plate creation matches the observed bathymetry and heat flow with considerable accuracy, it only does so because a constant temperature is imposed as a lower boundary condition. A more realistic model is developed here, based on the idea that the thermal structure of the plate becomes unstable and leads to the development of small‐scale convection. Convective heat transport then supplies the heat flux needed to match the observations rather than an artificial constant temperature boundary condition. The temperature dependence of the rheology is represented in a simple manner. Below a given temperature the material is assumed to move rigidly, defining an upper mechanical boundary layer. Beneath this rigid layer, where the temperatures are greater, the material is assumed to have a constant Newtonian viscosity. The part of the viscous region where there are significant vertical temperature gradients, immediately below the mechanical boundary layer, forms a thermal boundary layer. As the plate cools, both the mechanical and thermal boundary layers increase in thickness. A local critical Rayleigh number criterion is used to test the stability of the thermal boundary layer. On this basis a convective instability is predicted, its occurrence coinciding with the breakdown of the linear dependence of the depth of the ocean floor on the square root of age. Though the small‐scale convection which develops from this instability modifies the thermal structure, the basic observational constraints are shown to be satisfied. The stability criterion is further tested in two different laboratory experiments. These experiments also illustrate a possible form for the instability, with cold dense material breaking away from the base of the plate and being replaced by hotter material from below.
Present‐day plate motions
Authors: Minster J. Bernard , Jordan Thomas H.
Published: February 2008

A data set comprising 110 spreading rates, 78 transform fault azimuths, and 142 earthquake slip vectors has been inverted to yield a new instantaneous plate motion model, designated Relative Motion 2 (RM2). The model represents a considerable improvement over our previous estimate, RM1 [Minster et al., 1974]. The mean averaging interval for the spreading rate data has been reduced to less than 3 m.y. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. The model systematically misfits data along the India‐Antarctica and Pacific‐India plate boundaries. We hypothesize that these discrepancies are manifestations of internal deformation within the Indian plate; the data are compatible with northwest‐southeast compression across the Ninetyeast Ridge at a rate of about 1 cm/yr. RM2 also fails to satisfy the east‐west trending transform fault azimuths observed in the French‐American Mid‐Ocean Undersea Study area, which is shown to be a consequence of closure constraints about the Azores triple junction. Slow movement between North and South America is required by the data set, although the angular velocity vector describing this motion remains poorly constrained. The existence of a Bering plate, postulated in our previous study, is not necessary if we accept the proposal of Engdahl and others that the Aleutian slip vector data are biased by slab effects. Absolute motion models are derived from several kinematical hypotheses and compared with the data from hot spot traces younger than 10 m.y. Although some of the models are inconsistent with the Wilson‐Morgan hypothesis, the overall resolving power of the hot spot data is poor, and the directions of absolute motion for the several slower‐moving plates are not usefully constrained.
A dynamic, scaled model of accretion at trenches and its implications for the tectonic evolution of subduction complexes
Authors: Cowan Darrel S. , Silling Rose Mary
Published: February 2008

Popular models of accretion at trenches, based on seismic reflection profiles and geological data from active convergent margins, are that accretionary prisms consist of thrust‐bound wedges of material successively scraped off the descending plate at the base of the inner wall. Geological studies of ancient subduction complexes indicate that deeply buried, metamorphosed rocks, including blueschists, are tectonically elevated at landward margins of accretionary prisms, but such uplift is not easily reconciled with models in which underplated wedges merely rotate arcward as new wedges are added beneath them. We constructed a dynamic, scaled model of a subduction zone to study the large‐scale rheological behavior of accreted materials. Model ratios of λ = 10−5, τ = 2.3 × 10−11, and μ = 1 were applied to a prototype subducting oceanic lithosphere covered with 1 km of sediments at a convergence rate of 1 cm yr−1 for 10 m.y. Water‐based clay of density 2.7 g cm−3 was appropriately scaled to represent materials in the entire prototypal accretionary prism. Successive thrust‐bound lobes formed at the ‘trench,’ but appreciable amounts of ‘sediment’ were carried beneath the lobes to the interior of the developing prism where they rose impressively upward along the landward margin of the entire accretionary prism. This pattern, involving the upward emplacement of deeply buried materials, can be theoretically modeled for the two‐dimensional flow of a viscous fluid through a right‐angle or acute corner in response to sliding a rigid plate beneath another fixed plate. Our kinematic model predicts that subduction‐driven macroscopic flow is a reasonable mechanism for the upward tectonic transport of metamorphic rocks in accretionary complexes.
Keweenawan turbidites in Michigan (deep borehole red beds): A foundered basin sequence developed during evolution of a Protoceanic Rift System
Authors: Fowler John H. , Kuenzi W. David
Published: February 2008

A 1538‐m pre‐Mt. Simon (Upper Cambrian) red bed sequence, encountered in the Gratiot County deep borehole, consists of undeformed recurring beds of submature and immature arkosic sandstone, 5–85 cm thick, that grade upward into mudstone units, 3 cm to 3.8 m thick. Similar facies are recorded by a red bed sequence penetrated in a well on Beaver Island, 260 km to the north, and both sequences occur within the mid‐Michigan gravity high. We suggest penecontemporaneous deposition by sediment gravity flows, low‐velocity clear water currents, and pelagic fallout on submarine fans forming in an elongate, broadly subsiding protoceanic basin which foundered isostatically following arrestment of Keweenawan rifting and production of oceanic lithosphere. We further suggest that protoceanic rifting in the Lake Superior region and that along the mid‐Michigan gravity anomaly were approximately contemporaneous and that both areas underwent a similar sequence of thermotectonic volcanism, deformation, and sedimentation, followed by passive foundering and shallow marine and basinal turbidite sedimentation.
An analysis of isostasy in the world's oceans 1. Hawaiian‐Emperor Seamount Chain
Authors: Watts A. B.
Published: February 2008

Cross‐spectral techniques have been used to analyze the relationship between gravity and bathymetry on 14 profiles of the Hawaiian‐Emperor seamount chain. The resulting filter or transfer function has been used to evaluate the state of isostasy along the chain. The transfer function can be best explained by a simple model in which the oceanic lithosphere is treated as a thin elastic plate overlying a weak fluid. The best‐fitting estimate of the elastic thickness of the plate is in the range 20–30 km. Analysis of individual profiles shows significant differences in the elastic thickness along the seamount chain. Relatively low estimates of the elastic thickness were obtained for the Emperor Seamounts north of 40°N, and relatively high estimates for the Emperor Seamounts south of 40°N and the Hawaiian Ridge. These differences cannot be explained by a simple model in which there is a viscous reaction to the seamount loads through time. The best explanation is a simple model in which the elastic thickness depends on age and hence temperature gradient of the lithosphere. The low values can be explained if the Emperor Seamounts north of 40°N loaded a relatively young hot plate, and the high values can be explained if the Emperor Seamounts south of 40°N and the Hawaiian Ridge loaded a relatively old cold plate. These estimates of the elastic thickness along with determinations from other loads on the Pacific lithosphere suggest that the elastic thickness corresponds closely to the 450 ± 150°C isotherm, based on simple cooling models. Thus the large deformations and associated flexural stresses (>1 kbar) at seamount loads do not appear to change appreciably through time. This conclusion is in agreement with subsidence data along the seamount chain and with some gravity observations in the continents.
Regional free air gravity anomalies and tectonic observations in the United States
Authors: McGinnis L. D. , Wolf M. G. , Kohsmann J. J. , Ervin C. Patrick
Published: February 2008

Four free air gravity maps of the United States have been constructed from mean free air values in areas of 6×102 km2 (one 15′ quadrangle), 7.5×103 km2 (12 quadrangles), 6×104 km2 (99 quadrangles), and 6×105 km2 (945 quadrangles). The technique of averaging free air values in successively larger areas is a simple filter which eliminates short wavelengths. The maps derived from land data can be used for comparison with the long‐wavelength free air field derived from satellite data. An excellent correlation is observed between the 6×105‐km2 map and recent satellite maps. The dominant regional anomalies in the United States are the following: (1) the arcuate positive extending from the Appalachians, through the Gulf Coast inner continental shelf and northward along the Rio Grande Rift and Front Range, (2) the sinuous, midcontinent low extending from Wisconsin through Texas, (3) the Columbia Plateau minimum, (4) the Basin and Range and Salton Sea minimum, and (5) the positive over the Colorado Plateau. We suggest that the local gravity anomalies displayed by Bouguer gravity maps and the free air maps of 1‐ and 12‐quadrangle averages are caused by mass distribution in the lithosphere of some antiquity, whereas the regional anomalies are caused by deformation of the lithosphere which is in turn related to contemporary plate dynamics.