bluuebunny/crossref_metadata_2025 huggingface dataset:
VLF emissions in the Venus foreshock: Comparison with terrestrial observations
Authors: Crawford G. K. , Strangeway R. J. , Russell C. T.
Published: February 2008

Enhanced ELF/VLF emissions are observed in the solar wind upstream of the Venus bow shock for periods when the interplanetary magnetic field connects the Pioneer Venus Orbiter spacecraft to the shock surface. Such magnetic connection places the spacecraft in the foreshock region and provides an avenue along which counterstreaming electrons and ions of shock origin escape away from the shock and propagate into the upstream region. Two distinct sets of emissions are observed by the Orbiter electric field detector: Enhanced 30‐kHz emissions are found in the electron foreshock and are identified as parallel polarized electron plasma oscillations while large amplitude 5.4‐kHz and 730‐Hz signatures are measured in the ion foreshock and identified as parallel polarized ion acoustic emissions. The 30‐kHz electron foreshock emissions have peak intensity at the electron foreshock boundary and decrease in intensity with penetration into the foreshock, but the enhanced 5.4‐kHz emissions in the ion foreshock are observed throughout the entire ion foreshock region. Both the electron and ion foreshock emissions show a decrease in intensity with distance away from the shock surface along the field line connecting the spacecraft to the shock surface. The ion foreshock emissions also show a strong asymmetry between the two foreshock regions formed on either side of the foreshock boundary tangent point (the upstream and downstream foreshocks). This asymmetry may be a consequence of differences in the transmission and/or reflection mechanisms which act both to populate and energize ions in the upstream region. Our study shows that the 30‐kHz emissions are tied to the quasi‐perpendicular portion of the shock while the 5.4‐kHz and 730‐Hz emissions are associated predominantly with quasi‐parallel regions on the shock. Comparisons of the Venus observations with equivalent terrestrial measurements show them to be surprisingly similar given the large difference in solar wind interaction for the two planets.
A particle simulation of the westward‐traveling surge
Authors: Yamamoto Takashi , Makita K. , Meng C. ‐I.
Published: February 2008

By numerical simulations, it is shown that the evolution of a westward‐traveling surge can be identified with the aurora deformation due to the appearance of a westward plasma flow at the boundary between the open and closed field lines. It is assumed that the westward flow is generated from nonadiabatic proton acceleration in the region of magnetic field reconnection by enhancement of the westward inductive electric field. The numerical simulation is performed by using the two‐dimensional electrostatic particle code, which is capable of studying plasma dynamics in a plane perpendicular to the geomagnetic field. The present model for the westward‐traveling surge assumes the following background state ((1) and (2)) in the plasma sheet boundary layer magnetically connected to the high‐latitude auroral ionosphere. (1) A well potential (Δϕ < −10 kV) is distributed uniformly in the azimuthal direction, being responsible for a large‐scale inverted‐V structure. (2) A number of azimuthally aligned sheets of dense plasmas (which are called arc sheets) exist in the large‐scale potential well. The electrons in such arc sheets can be accelerated by field‐aligned electric fields distributed at low altitudes, and they can thus produce a number of discrete arcs longitudinally elongating at the ionospheric altitude. Initially, a westward plasma flow appears (near local midnight) at the last closed magnetic shell, inside which a number of the arc sheets are located. The temporal and spatial evolution of the discrete arcs are numerically followed in a plane perpendicular to the geomagnetic field. The simulation results show that some discrete arcs can convolute in a few minutes and form the surge structure. The surge head is found to typically move westward with a speed of ∼ 1 km/s. Convolution of discrete arcs is due to local accumulation of positive space charges carried by the accelerated protons. Such positive charge accumulation in the magnetosphere is not strong so that the auroral electrons can still be accelerated toward the ionosphere by the field‐aligned potential drop resulting from the potential well. Both effects of the diminished field‐aligned potential drop and electron acceleration in the reconnection region may lead to production of the flat and hard differential energy spectrum of precipitating electrons observed inside a surge. A remarkable similarity in dynamics between the numerically simulated surges and the observed ones strongly suggests that the westward‐traveling surge is driven by a local westward plasma flow at the boundary between the open and closed field lines. (Recently, the high‐altitude (∼ 10,000 km) observations from the EXOS‐D satellite (T. Yamamoto et al., manuscript in preparation, 1993) have first revealed the existence of a plasma jet just poleward of a surge.) An expanding auroral bulge can also be reproduced by assuming wider (in latitude) spreading of the particles accelerated in the reconnection process.
Ionization rates for atmospheric and ionospheric breakdown
Authors: Papadopoulos K. , Milikh G. , Gurevich A. , Drobot A. , Shanny R.
Published: February 2008

The differences in the ionization rates for RF induced atmospheric breakdown between the analytic formula given by Gurevich et al. [1978] and recent numerical simulations Short et al., 1991; Tsang et al., [1991] were examined and clarified. An updated analytic formula for the ionization rate of air, which includes corrections due to kinetic effects of the electron distribution function, was derived. The formula is valid for arbitrary ratios of RF to electron‐neutral collision frequency.
The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons
Authors: Burgess W. C. , Inan U. S.
Published: February 2008

New experimental evidence suggests that every ducted whistler component may precipitate bursts of radiation belt electrons into geomagnetically conjugate ionospheric regions. Strong spatial and temporal associations are seen between transient ionospheric disturbances observed in conjugate regions and ducted whistlers monitored at Palmer Station, Antarctica. The ionospheric disturbances were detected by their characteristic perturbing effects (“Trimpi events”) on subionospheric VLF, LF, and MF signals recorded at Palmer Station and at northern hemisphere sites. Of 74 such events examined on four different days, all were time‐associated with ducted whistlers. In no case was the arrival azimuth or dispersion of the associated whistlers inconsistent with the locations of the conjugate ionospheric disturbances, which were inferred from the configuration of perturbed signal paths. Other whistlers occurring independently of detected disturbances were found to be either weak or to have arrived from regions where a disturbance would not have been detected for lack of monitored signal paths. Signal perturbation onset behavior was consistent with multiple regions of precipitation induced by components of multipath whistlers and with theoretical predictions for ducted whistler‐induced precipitation. The results not only support the hypothesis that ducted whistlers are responsible for burst precipitation of energetic electrons but imply that such bursts may be induced by every ducted whistler component. Since radio energy from a lightning discharge can excite whistler ducts located 2500 km or more away from the flash, every ducted whistler observed at Palmer Station may indicate the presence of precipitation bursts associated one‐to‐one with excited whistler ducts distributed over a 5000‐km‐wide portion of the Earth's surface, and over its geomagnetic conjugate as well. The estimated effect of this precipitation on 70‐ to 200‐keV radiation belt electron populations for 2 < L < 3 is comparable to that predicted as a result of plasmaspheric hiss, indicating that ducted whistlers may contribute as significantly as hiss to radiation belt equilibrium at those electron energies.
On the origin of auroral fingers
Authors: Liu W. W. , Rostoker G.
Published: February 2008

During the development of large‐scale magnetospheric substorm expansive phase activity, the auroral oval may become very wide. During such episodes, clear discrete arc structures appear from time to time in the equatorward portion of the evening sector oval in a region of space normally thought to feature structureless diffuse auroras. On occasion, these discrete auroral forms acquire a north‐south orientation and remain as identifiable forms for up to several minutes. Most auroral theories attempt to describe the origin of the normally east‐west aligned features. In this paper we propose that the particles responsible for north‐south aligned arc structures originate as plasma blobs injected into the central plasma sheet (CPS) from the adjacent low‐latitude boundary layer, as a result of enhanced wave activity at the interface between the two regions during active times. We show that the characteristic geometric forms of the CPS auroral structures follow from tracking the drift paths of the particles in the injected plasma blob as they convect earthward, heating adiabatically in the process. We also outline outstanding issues pertaining to the proposal as motivation for a continued study.
Spectral structure of stimulated electromagnetic emissions between electron cyclotron harmonics
Authors: Leyser T. B. , Thidé B. , Waldenvik M. , Goodman S. , Frolov V. L. , Grach S. M. , Karashtin A. N. , Komrakov G. P. , Kotik D. S.
Published: February 2008

Experimental results concerning the spectrum of stimulated electromagnetic emissions (SEE) in the sidebands of a powerful high‐frequency electromagnetic ordinary mode pump wave are presented. The pump wave was vertically injected into the ionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, and broad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations of a new wideband emission occurring primarily in the upper sideband. The dependence of the SEE spectral structure on the pump frequency in relation to electron cyclotron harmonics is emphasized. All spectral features in the sidebands of the pump exhibit cyclotron harmonic effects.
A Comparison of in situ measurements of and from Dynamics Explorer 2
Authors: Hanson W. B. , Coley W. R. , Heelis R. A. , Maynard N. C. , Aggson T. L.
Published: February 2008

Dynamics Explorer‐2 provided the first opportunity to make a direct comparison of in situ measurements of the high‐latitude convection electric field by two distinctly different techniques. The vector electric field instrument (VEFI) used antennae to measure the intrinsic electric fields and the ion drift meter (IDM) and retarding potential analyzer (RPA) measured the ion drift velocity vector, from which the convection electric field can be deduced. The data from three orbits having large electric fields at high latitude are presented, one at high, one at medium, and one at low altitudes. The general agreement between the two measurements of electric field is very good, with typical differences at high latitudes of the order of a few millivolts per meter, but there are some regions where the particle fluxes are extremely large (e.g., the cusp) and the disagreement is worse, probably because of IDM difficulties. The auroral zone potential patterns derived from the two devices are in excellent agreement for two of the cases, but not in the third, where bad attitude data may be the problem. At low latitudes there are persistent differences in the measurements of a few millivolts per meter, though these differences are quite constant from orbit to orbit. This problem seems to arise from some shortcoming in the VEFI measurements. Overall, however, these measurements confirm the concept of “frozen‐in” plasma that drifts with velocity
within the measurement errors of the two techniques.
A wave‐wave interaction in whistler frequency range in space plasma
Authors: Trakhtengerts V. Y. , Hayakawa M.
Published: February 2008

The quantitative theory is developed for the frequency spectrum broadening and sideband structure of VLF signals in the upper ionosphere. The theory is based on a common theory of wave‐wave interaction in plasma and includes the transformation effects of a quasi‐monochromatic whistler wave into quasi‐electrostatic whistler mode waves (l waves) near the resonance cone with their interaction with field‐aligned plasma density (electric field) inhomogeneities. Such inhomogeneities can be ion‐acoustic or ion cyclotron electrostatic waves. We consider both cases of regular (deterministic plases) and irregular (random phases) inhomogeneities. The spectral characteristics and interaction length of l waves and some nonstationary effects are considered, which permit us to obtain the quantitative interpretation of main experimental data. The conditions of high coherence of interacting waves are investigated, which are compared with the results of bicoherence analysis of sideband structure of VLF signals. The estimations of bicoherence value are given for such type of wave‐wave interaction.
Transport‐theoretic model for the electron‐proton‐hydrogen atom aurora: 2. Model results
Authors: Strickland D. J. , Daniell R. E. , Jasperse J. R. , Basu B.
Published: February 2008

In the companion paper (Basu et al., this issue), a self‐consistent transport‐theoretic model for the combined electron‐proton‐hydrogen atom aurora was described. In this paper, numerical results based on the model are presented. This is done for the pure electron aurora, the pure proton‐hydrogen atom aurora, and finally for the combined aurora. Adopting commonly used types of energy distributions for the incident particle (electron and proton) fluxes, we give numerical solutions for the precipitating electron, proton, and hydrogen atom differential number fluxes. Results are also given for ionization yields and emission yields of the following features: N2+ first negative group (3914 Å), N2 second positive group (3371 Å), selected N2 Lyman‐Birge‐Hopfield bands (1325 Å, 1354 Å, 1383 Å, 1493 Å, and all bands between 1700 and 1800 Å), O I (1356 Å), Lα (1216 Å), Hβ (4861 Å), and Hα (6563 Å). The yield at 1493 Å also contains a contribution from N I (1493 Å), which in fact dominates LBH emission. A major new result of this study is that the secondary electron flux produced by the proton‐hydrogen atom aurora is much softer than that produced by the electron aurora. This increased softness is due to the fact that (for energies of auroral interest) cross sections for secondary electron production by proton and hydrogen atom impact decrease exponentially with increasing secondary electron energy, whereas the cross sections for electron impact decrease as an inverse power law with increasing secondary energy. In our study of the pure electron aurora (no primary protons or hydrogen atoms present) and the pure proton‐hydrogen atom aurora (no primary electrons present), two important results obtain. First, certain emission features (for example, 3371 Å) are excited in completely different ways for the two kinds of aurora. Second, the "eV per electron‐ion pair" as a function of the characteristic energy E0 is nearly constant for the pure electron aurora, with a value of about 34, but varies by about 20% for the pure proton‐hydrogen atom aurora. In our study of the combined electron‐proton‐hydrogen atom aurora, two additional results obtain. Since the proton‐hydrogen atom contribution to the total incident energy flux in the midnight sector is, on the average, about 20 to 25% of that of the electrons, we find that when it is neglected, the ionization yield as well as the yields of many emission features will be underestimated, on the average, by about the same percentage. We also find that in the morning sector of the combined aurora, a double bump in the altitude profile of the E region electron density is possible for certain auroral conditions.
Solar cycle variation of interplanetary disturbances observed as Doppler scintillation transients
Authors: Woo Richard
Published: February 2008

Interplanetary disturbances characterized by plasma that is more turbulent and/or moves faster than the background solar wind are readily detected as transients in Doppler scintillation measurements of the near‐Sun solar wind. Systematic analysis of over 23,000 hours of Pioneer Venus Orbiter Doppler measurements obtained inside 0.5 AU during 1979‐1987 have made it possible for the first time to investigate the frequency of occurrence of Doppler scintillation transients under solar minimum conditions and to determine its dependence on solar cycle. On the basis of a total of 142 transients, Doppler scintillation transient rates vary from a high of 0.22 in 1979 (one every 4.6 days) to a low of 0.077 transients/d in 1986 (one every 13 days), a decrease by almost a factor of 3 from solar maximum to solar minimum. This solar cycle variation, the strongest yet of any solar wind Doppler scintillation property, is highly correlated with both solar activity characterized by sunspot number and the coronal mass ejection rates deduced from Solwind and Solar Maximum Mission (SMM) coronagraph observations. These results indicate that coronal mass ejections and Doppler scintillation transients are closely related not just during solar maximum, as occasional individual comparisons have shown in the past, but throughout the entire solar cycle, and strengthen the notion that the Doppler scintillation and optical transients are different manifestations of the same physical phenomenon. The magnitudes of the transients, as described by the ratio of peak to pretransient scintillation levels (EF for enhancement factor), and their distribution with heliocentric distance also vary with solar cycle. While EF tends to diminish with increasing heliocentric distance during high solar activity, it is more evenly distributed during low solar activity. EF is also lower during solar minimum, as 13% of the transients during solar maximum have values exceeding 23, the highest EF observed during solar minimum. These results are consistent with the fact that occasional major fast‐moving interplanetary shocks that are observed during solar maximum are very rare during solar minimum.
Sounding rocket observation of a hot atomic oxygen geocorona
Authors: Cotton Daniel M. , Gladstone G. Randall , Chakrabarti Supriya
Published: February 2008

A sounding rocket measurement of the ultraviolet, atomic oxygen dayglow reveals an excess of emission compared to standard thermospheric model calculations at exospheric altitudes. We explore two explanations for this discrepancy: a breakdown of the radiative transfer model due to nonlocal thermal equilibrium (non‐LTE) conditions above the exobase and a hot atomic oxygen geocorona. In particular, the effects of non‐LTE on the ³P2,1,0 sublevel populations are modeled, and a hot O component in the upper thermosphere and lower exosphere is added to investigate the effects on the modeled emissions. For both cases, the data are reanalyzed and compared with the results using a standard LTE model. A hot O geocorona having a peak density of 106 cm−3 at 550 km and a temperature of 4000 K is consistent with the data and appears to be the most reasonable explanation of the high‐altitude enhanced emissions observed in the data.
Modeling the Europa plasma torus
Authors: Schreier Ron , Eviatar Aharon , Vasyliūnas Vytenis M. , Richardson John D.
Published: February 2008

The existence of a torus of plasma generated by sputtering from Jupiter's satellite Europa has long been suspected but never yet convincingly demonstrated. Temperature profiles from Voyager plasma observations indicate the presence of hot, possibly freshly picked‐up ions in the general vicinity of the orbit of Europa, which may be interpreted as evidence for a local plasma torus. Studies of ion partitioning in the outer regions of the Io torus reveal that the oxygen to sulfur mixing ratio varies with radial distance; this may indicate that oxygen‐rich matter is injected from a non‐Io source, most probably Europa. We have constructed a quantitative model of a plasma torus near the orbit of Europa which takes into account plasma input from the Io torus, sputtering from the surface of Europa, a great number of ionization and charge exchange processes, and plasma loss by diffusive transport. When the transport time is chosen so that the model's total number density in consistent with the observed total plasma density, the contribution from Europa is found to be significant although not dominant. The model predicts in detail the ion composition, charge states, and the relative fractions of hot Europa‐generated and (presumed) cold Io‐generated ions. The results are generally consistent with observations from Voyager and can in principle (subject to limitations of data coverage) be confirmed in more detail by Ulysses.
Ulysses plasma observations in the Jovian magnetosheath
Authors: Phillips J. L. , Bame S. J. , Thomsen M. F. , Goldstein B. E. , Smith E. J.
Published: February 2008

The solar wind plasma experiment aboard the Ulysses spacecraft, including separate ion and electron instruments, measured the plasma properties of the Jovian magnetosheath during the February 1992 encounter with Jupiter. Seven separate magnetosheath intervals were observed, as well as four bow shock crossings and numerous encounters with the magnetopause and its boundary layer. We present an overview of ion and electron bulk parameters and a sampling of distribution shapes for the magnetosheath and adjacent plasma regions. Plasma flows are generally appropriate for slowing and deflection of the solar wind flow about a relatively stationary obstacle, with the notable exception of the first inbound sheath transit, when an expanding magnetosphere resulted in sunward flow just above the magnetopause. The existence of a planetary depletion layer is suggested by trends in plasma density for some magnetopause encounters. The magnetopause boundary layer is characterized by a combination of sheathlike and magnetospheric distributions of both ions and electrons. The ion population in the sheath is observed to include a significant population of suprathermal protons. Electron distributions have a distinctive shape previously observed in the terrestrial magnetosheath, with fluxes parallel to the magnetic field dominating at thermal energies and perpendicular fluxes dominating at higher energies. Trends in electron temperature near the bow shock indicate that shock motion plays an important role in heating the electrons. In general, the plasma characteristics of the Jovian magnetosheath are quite similar to those in its terrestrial counterpart, but the compressible nature of the Jovian magnetosphere accentuates the importance of boundary motions.
Quaternary fold deformation associated with blind thrust faulting, Los Angeles Basin, California
Authors: Bullard Thomas F. , Lettis William R.
Published: February 2008

Detailed mapping of Quaternary deposits and geomorphic surfaces and quantitative morphometric analyses of landscape elements within the Montebello Hills and Monterey Park Hills are used to characterize the style and patterns of Quaternary deformation associated with the blind thrust fault considered responsible for the 1987 Whittier Narrows earthquake (ML 5.9) in the northeastern Los Angeles Basin, California. Results of these studies show that the eastern end of the Elysian Park anticlinorium above the thrust fault consists of two right en echelon asymmetric Quaternary anticlines and that there is a progressive north‐to‐south migration of the fold front; incipient Holocene folds are developing along the southern margin of the Monterey Park and Montebello Hills. Southward migration of the fold front reflects probable southward vergence and propagation of the underlying thrust fault. Pleistocene and Holocene fluvial terraces and geomorphic surfaces are folded across the crest of both anticlines indicating late Quaternary uplift of at least 150 m and tilting of 10°–30°. Based on available age data for the terraces and geomorphic surfaces, these amounts of deformation provide estimated rates of Quaternary uplift of 0.1–0.2 mm/yr and tilting of 0.2–0.5 μrad/yr, roughly an order of magnitude greater rate of deformation than that reported by earlier workers in the region. Based on differences in rate of fold deformation, changes in fold axis orientation, and clear spatial separation of the folds, we infer that the underlying thrust fault consists of two distinct 6‐ to 8‐km‐long segments with independent deformation histories. In addition to Quaternary folding, we identify two surface faults, one transverse to and one parallel to the fold axes. Smaller, discontinuous transverse faults also occur in the en echelon stepover region between the two folds and may reflect tear faulting between segments of the underlying thrust fault. Deformation in the Whittier Narrows area occurs in a transitional zone between right‐lateral strike‐slip deformation on the Whittier‐Elsinore fault system and left‐oblique contractional deformation on the Santa Monica‐Raymond fault system. Crustal shortening in the Whittier Narrows area represents either (1) an area of local compression within a series of progressively smaller left en echelon restraining stepovers in a regional right‐lateral strike‐slip system or (2) progressive onset of convergence as lateral slip along the Whittier‐Elsinore fault system dies out northward and is accommodated by more westerly trending reverse and left‐oblique slip faults along the southern margin of the Transverse Ranges.
The interaction of plume heads with compositional discontinuities in the Earth's mantle
Authors: Manga Michael , Stone Howard A. , O'Connell Richard J.
Published: February 2008

The effects of compositional discontinuities of density and viscosity in the Earth's mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique. Three specific problems are considered: (1) a plume head rising away from a deformable interface, (2) a plume head passing through an interface, and (3) a plume head approaching the surface of the Earth. For the case of a plume attached to a free‐surface, the calculated time‐dependent plume shapes are compared with experimental results. Two principal modes of plume head deformation are observed: plume head elongation or the formation of a cavity inside the plume head. The inferred structure of mantle plumes, namely, a large plume head with a long tail, is characteristic of plumes attached to their source region, and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high‐ to low‐viscosity region. As a rising plume head approaches the upper mantle, most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle. If the plume head moves from a high‐ to low‐viscosity region, the plume head becomes significantly elongated and, for the viscosity contrasts thought to exist in the Earth, could extend from the 670 km discontinuity to the surface. Plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry. The dynamic surface topography induced by plume heads is bell‐shaped when the top of the plume head is at depths greater than about 0.1 plume head radii. As the plume head approaches the surface and spreads, the dynamic topography becomes plateau‐shaped. The largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical, and the surface expression of these stresses is likely to be dominated by radial extension. As the plume spreads, compressional stresses on the surface are produced beyond the edges of the plume; consequently, extensional features will be produced above the plume head and may be surrounded by a ring of compressional features.
Dislocation recovery in olivine under deep upper mantle conditions: Implications for creep and diffusion
Authors: Karato Shun‐Ichiro , Rubie David C. , Yan Hong
Published: February 2008

The kinetics of dislocation recovery in olivine have been studied up to 10 GPa and 1500°C using a multianvil apparatus. Dislocations in the (010)[100] slip system were generated by high‐temperature deformation (at room pressure), and then the specimens were statically annealed at high pressures and temperatures. The rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions. Microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion. A small activation volume (V* = 6 ± 1 cm3/mol) was obtained, which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport‐limiting species (oxygen or silicon). Combined with the previous observation of a small activation volume for Mg‐Fe diffusion, we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle. We have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters (such as stress exponents, grain‐size exponents and activation volume for dislocation creep). The results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle.
Seismicity and tectonics of the southeastern Caribbean
Authors: Russo R. M. , Speed R. C. , Okal E. A. , Shepherd J. B. , Rowley K. C.
Published: February 2008

We present 33 new focal mechanisms for SE Caribbean earthquakes (1963–1988). We use these mechanisms, in conjunction with 28 previously available mechanisms, to distinguish between two models of plate boundary zone interaction in the SE Caribbean: the trench‐trench transform and hinge faulting model, and the right oblique collision model. Shallow (0–70 km) and intermediate (70–200 km) depth earthquakes occur in the study region; we focus on the tectonic causes of these events and the motions they delineate. The shallow earthquakes are in a broad linear zone which trends NE from the Paria Peninsula of Venezuela towards Barbados. Intermediate depth earthquakes cluster beneath and NW of the Peninsula, and deepen to the NW, perpendicular to the NE‐trending shallow events. The vertical distribution of the earthquakes suggests a slab with steep NW dip. Shallow, dextral strike slip on E‐striking faults is restricted to a 60‐km‐wide linear zone between the Gulf of Cariaco and the western margin of the Gulf of Paria. Dextral strike slip is active only as far east as the Gulf of Paria, and not within or east of Trinidad. Shallow thrust events with ENE‐striking planes, distributed between the Araya Peninsula and the Gulf of Paria, indicate collision at crustal levels between South America and Caribbean, and that folding and thrusting are still active over a 60‐km interval south of the Araya‐Paria isthmus. Active thrusting in Venezuela corroborates predictions of transpression between Caribbean and South America and discounts transtensional motions between the two plates in the SE Caribbean. The conjunction of shallow thrust, strike slip, and normal earthquakes in the Gulf of Paria at around 62.3° may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone. Intermediate (165 km > h > 70 km) depth thrust and dip slip events within the NW‐dipping slab indicate that oceanic lithosphere, probably originally attached to South America, subducts to the NW beneath the Caribbean plate. Shallow normal faulting events E and NE of Trinidad are expressions of plate bending about near‐horizontal axes parallel to the Lesser Antilles subduction zone. We conclude that the earthquake mechanisms provide strong support for the right oblique collision model of Caribbean‐South American plate interaction.
Dielectric properties of ice containing acid and salt impurity at microwave and low frequencies
Authors: Moore John C. , Fujita Shuji
Published: February 2008

We compare the microwave and low‐frequency (LF) dielectric conductivity of natural and artificial ice as a function of impurity concentration and temperature. We find a linear dependence of conductivity on acid concentration that is independent of the type of acid. There appears to be no evidence of significant dielectric dispersions between LF and 10 GHz in acid‐doped ice. The results are well fitted by a model in which concentrated liquid acid at three‐grain boundaries forms a network, earlier proposed as an explanation for the DC conductivity of polar ice. In contrast, evidence from ice with sea salt impurity shows large discrepancies between the microwave response of low‐salinity sea ice, and both the LF and microwave responses of ice with salinities typical of meteoric ice. These discrepancies may be attributed to sea salt chloride being largely incorporated within the ice lattice in meteoric‐type ice, while in higher‐salinity ice most of the sea salt is contained in platelike inclusions of liquid brine.
Plate dynamics near divergent boundaries: Geophysical implications of postrifting crustal deformation in NE Iceland
Authors: Heki Kosuke , Foulger G. R. , Julian B. R. , Jahn C.‐H.
Published: February 2008

The bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average. On the other hand, movements are highly episodic at plate boundaries. We study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in Northeast Iceland 1987–1990. These observations were made about 10 years after an episodic divergent movement between the North American and Eurasian plates 1975–1981. The horizontal displacement field fits well a two‐dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of Newtonian viscosity. This analysis indicates values of about 10 m2/s for the stress diffusivity and 0.3–2 × 1018 Pa s for the Newtonian viscosity of the lower layer. However, no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations. Assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically, we simulate the spatial transition from episodic to continuous plate movements. The plate “boundary zone” where movements are episodic or quasi‐episodic is of the order of 100 km wide, depending on the stress diffusivity (which may be fairly uniform throughout the world) and the frequency of episodic movements.
New paleomagnetic data from the Antarctic Peninsula and their tectonic implications
Authors: Grunow A. M.
Published: February 2008

New paleomagnetic data presented here further constrain the relative motion of the Antarctic Peninsula relative to East Antarctica during the Mesozoic development of the southern ocean basins. The Antarctic Peninsula (AP) is one of four crustal blocks that define West Antarctica, the others being the Ellsworth‐Whitmore Mountains (EWM), Thurston Island‐Eights Coast (TI), and Marie Byrd Land (MBL). A Jurassic pole (∼155Ma) (124°E, 64°S, A95=7.1°, N=10 VGPs) was obtained from the AP block which suggests that the AP block rotated clockwise between ∼175 and ∼155 Ma due to significant early opening in the Weddell Sea basin. A new Early Cretaceous paleomagnetic pole (182°E, 74S, A95=5.9°, N=6 site mean VGPs) indicates that the AP block was in or near to its present‐day position with respect to East Antarctica by ∼130 Ma. Between ∼155 and 130 Ma, counterclockwise rotation of the AP‐TI blocks, together with the southward motion of East Gondwanaland, probably resulted in subduction of Weddell Sea ocean floor beneath the southern AP block and initiated the Palmer Land deformational event. A ∼130 Ma pole from the TI block requires clockwise rotation of the TI and possibly the EWM blocks between 130 and ∼110 Ma producing sinsistral strike‐slip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI‐EWM blocks and the AP block. New AP block ∼110 Ma and ∼85 Ma poles from this study (199°E, 74°S, A95=6.9°, N=13 VGPs; 152°E, 86°S, A95 = 7.5°, N=6 VGPs, respectively) are similar to equivalent age poles from East Antarctica and suggest little or no relative motion between the Antarctic Peninsula and East Antarctica. Northern and southern Antarctic Peninsula mid‐Cretaceous poles are very much alike suggesting that the “S” shape of the Antarctic Peninsula is not due to oroclinal bending since ∼110 Ma.
Precise U‐Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: Geochronological insights to physical, petrogenetic, paleomagnetic, and tectonomagmatic processes associated with the 1.1 Ga Midcontinent Rift System
Authors: Paces James B. , Miller James D.
Published: February 2008

Precise resolution of the timing of igneous activity is crucial to understanding the dynamic processes associated with continental rifting. Mafic intrusive rocks constitute a major portion of the exposed 1100 Ma (Keweenawan) Midcontinent Rift system in northeastern Minnesota; however, prior to this study, geochronological data were insufficient to allow rigorous interpretation of intrusive histories and their relationships to extrusive suites. Eight anorthositic and gabbroic intrusives were chosen to represent both the temporal and spatial ranges of plutonic activity that formed the Duluth Complex and related mafic intrusions. U‐Pb isotopic analyses from zircons and baddeleyites result in U‐Pb concordant ages with little or no ambiguity introduced by inherited components, Pb loss or common Pb. The earliest Keweenawan plutonism exposed in Minnesota occurs along the northeastern flank of the Duluth Complex as a series of layered gabbros (Nathan's layered series) emplaced at 1106.9 ± 0.6 Ma. This sequence of gabbro sheets shares temporal, spatial, and compositional similarities with the nearby Logan sills in Ontario. Four Duluth Complex anorthositic and troctolitic series samples from widely separated areas have unresolvable ages between 1099.3 ± 0.3 and 1098.6 ± 0.5 Ma, indicating a very short duration for peak intrusive activity (0.5–1 m.y.). The unresolvable ages between anorthositic and troctolitic plutons suggest that these two magma series are more closely related than previously modeled and argue strongly for the need to reexamine their fundamental petrogenetic relationships. These dates also imply that the major reverse‐to‐normal magnetic polarity switch, used throughout the rift system as an important correlation tool, occurred prior to 1099 Ma. This date is several million years earlier than previously suspected and emphasizes the need for further paleomagnetic and geochronological data from the overlying volcanics. Much of the hypabyssal intrusive suite within the volcanic pile overlying Duluth Complex plutons may be significantly younger than the main pulse of plutonic activity. Two hypabyssal bodies, the Sonju Lake intrusion and gabbro at Silver Bay, were emplaced at 1096.1 ± 0.8 Ma and 1095.8 ± 1.2 Ma, respectively. Dates reported here and in previous studies support the concept of episodic tectonomagmatic rift development where magmatism was apparently concentrated in episodes of short duration (<1–3 m.y.) interspersed with longer hiatuses (2–8 m.y.).
Regional permeability estimates from investigations of coupled heat and groundwater flow, north slope of alaska
Authors: Deming David
Published: February 2008

Earlier studies of terrestrial heat flow in the North Slope Basin, Alaska, found that heat flow varies systematically in a trend perpendicular to the strike of basin strata and the neighboring Brooks Range. Heat flow (∼±20%) increases from a low of 27 mW/m2 in the foothills of the Brooks Range in the south to a high of 90 mW/m2 on the coastal plain to the north. The thermal pattern can be explained by a regional‐scale (∼330 km) groundwater flow system which transports heat by advection from regions of high elevation in the Brooks Range and its foothills to lower elevations on the Arctic coastal plain. Permeability data from 2031 core measurements made parallel to bedding and 15 well tests were compiled for 10 geologic units. Arithmetic‐mean permeabilities derived from measurements on core samples range from 2.2 × 10−13 m2 for sandstones of the Endicott Group to 1.1 × 10−16 m2 for limestones of the Lisburne Group. The arithmetic‐mean permeability derived from all 2031 core measurements made parallel to bedding is 6.1 × 10−14 m2. A numerical model of coupled heat and fluid flow in the North Slope Basin was constructed and a series of model simulations were conducted. A model simulation incorporating permeability data obtained from core measurements resulted in a good match to observed heat flow data, apparently suggesting that permeability in the North Slope Basin does not increase significantly from the core scale (∼10−2‐ 10−1 m) to the basin scale (∼105 –106 m). This inference, however, is complicated by the possible effects of factors such as sample bias in measurements and choice of an appropriate averaging algorithm. A further series of model simulations were done in which the specified model permeability was homogeneous and anisotropic. Comparisons of heat flow predicted by these simulations with heat flow determined in field studies suggested that the effective basin‐scale permeability parallel to bedding (kx) is in the range of 2.5 × 10−14 ≤ Kx ≤ 2.5 × 10−13 m2 and permeability perpendicular to bedding (kz) is in the range of 1.0 ×10−16 ≤ kz ≤ 5.0 × 10−16 m2. These constraints depend upon the explicit assumption that groundwater flow is the only mechanism responsible for heat flow variations across the North Slope Basin.
Three‐dimensional elastic wave velocity structure of the western and central Tien Shan
Authors: Roecker S. W. , Sabitova T. M. , Vinnik L. P. , Burmakov Y. A. , Golvanov M. I. , Mamatkanova R. , Munirova L.
Published: February 2008

Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central Tien Shan are used to determine one‐ and three‐dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one‐dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three‐dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km‐thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk‐Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk‐Kul is overthrusting the surrounding ranges. The low‐velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso‐Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low‐velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere.
Quaternary uplift of southern Italy
Authors: Westaway Rob
Published: February 2008

Dramatic coastline changes demonstrate rapid Quaternary uplift of Calabria in southern Italy. Because most of the west (Tyrrhenian Sea) coast is normal fault bounded, previous work has asserted that its uplift is local footwall uplift related to extension. However, the east (Ionian Sea) coast is also uplifting but is not normal fault bounded. This reanalysis, based on original fieldwork and reinterpretation of the literature, reaches the following conclusions. First, although radiometric dating of only one marine terrace sequence in Calabria is available, on the east coast at Crotone, terraces occur at similar elevations, and can thus be correlated, throughout this coast and the west coast of northern and central Calabria. Uplift rate at both coasts and across the region in between is the same, 1.0 ± 0.1 mm yr−1, provided my correlations are correct. Second, the oldest raised shorelines date from the 0.9 Ma marine highstand and have uplifted ∼700 m since ∼0.7 Ma. Regional uplift rate earlier was minimal, possibly zero. Third, localities on this west coast show marginally (up to ∼5%) higher uplift rates, indicating some local footwall uplift (at ∼0.05 mm yr−) associated with slow extension (at ∼0.1 mm yr−1). Fourth, the 12‐m Holocene marine terrace formed around 7 ka. Allowing for a 3‐m range of wave and tidal action, it is interpreted as the result of 7 m of tectonic uplift and 2 m of eustatic sea level fall during ∼6–4 ka. Finally, the west coast of southern Calabria shows significant elevation changes caused by active normal faulting, as well as regional uplift. Footwall localities have uplifted up to 1300 m since 0.9 Ma. The estimated maximum present‐day uplift rate of 1.67 mm yr−1 is interpreted as 1 mm yr of regional uplift plus 0.67 mm yr of local footwall uplift, indicating much faster extension than farther north. It is suggested that the Tyrrhenian Benioff zone detached from beneath Calabria shortly before 0.7 Ma. The regional uplift of the overlying landmass is thus explained as its transient isostatic response to removal of this load. Order‐of‐magnitude calculations suggest that 1 mm yr−1 uplift rate is reasonable, with ∼2 km total uplift expected, indicating that uplift will continue for > ∼1 Myr into the future. Extension of Calabria appears to have begun at ∼11 Ma, at the same time as formation of the Tyrrhenian Sea and the related subduction at its Benioff zone. Extension rate in southern Calabria abruptly increased from ∼0.1 to ∼1 mm yr−1 around 0.9–0.7 Ma., when the slab detached and the regional uplift began; extension in northern and central Calabria continues at the same ∼0.1 mm yr−1 rate as before.