bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling terrestrial13C cycling: Climate, land use and fire
Authors: Scholze M. , Ciais P. , Heimann M.
Published: February 2008

The LPJ terrestrial carbon isotope model, which includes isotopic fractionation of13C during assimilation and a full description of the isotopic terrestrial carbon cycle, has been used to calculate the atmosphere‐biosphere exchange flux of CO2and itsδ13C for the years 1901 to 1998. A transient, spatially explicit data set of C4crops and tropical C4pastures has been compiled. In combination with a land use scheme this allows the analysis of the impact of land use conversion of C3ecosystems to C4cultivation, besides climate, fire disturbances, atmospheric CO2and the isotope ratio of atmospheric CO2, on the terrestrial carbon stable isotope composition. Globally averaged values of modeled leaf discrimination vary between 17.9‰ and 17.0‰ depending on the chosen land use scheme and also the year of the simulation. Results from the simulation experiment prescribing the conversion of C3ecosystems into C4crops and C4pastures show the lowest leaf discrimination. Modeled values of isotopic disequilibrium flux, caused by theδ13C difference between fixed CO2and released CO2, similarly depend on the amount of prescribed C4vegetation and vary between 37.9 Pg C‰ yr−1and 23.9 Pg C‰ yr−1averaged over the years 1985 to 1995. In addition, the effect of fire on the isotopic disequilibrium has been diagnosed; generally wildfires lead to a disequilibrium reduction of ≈10 Pg C‰ yr−1because they shorten the turnover time of terrestrial carbon. If used in a global double deconvolution study, the differences in the results between the standard experiment without any C4cultivation and the experiment including C4crops and pastures could account for a shift of about 1 Pg C yr−1from the inferred terrestrial sources to the ocean fluxes.
Global optical climatology of the free tropospheric aerosol from 1.0‐μm satellite occultation measurements
Authors: Kent G. S. , McCormick M. P. , Schaffner S. K.
Published: February 2008

Measurements of the aerosol/molecular extinction ratio at l‐μm wavelength, obtained from the SAGE I, SAGE II, and SAM II solar occultation satellite experiments between 1978 and 1986, have been used to study the global‐scale behavior of the upper tropospheric aerosol. The distribution of extinction ratio values shows a pronounced mode between about 0.5 and 5 in all data subsets, regardless of latitude and season. Within a given latitude band and season the mode value is nearly constant over the altitude range from about 5 km above Earth's surface to 3 km below the tropopause. The mode shows a distinct seasonal variation, with maxima in local spring and summer, and is significantly enhanced following volcanic injection of material into the stratosphere. South of latitude 20°N, mode values in the absence of volcanic contamination are normally between 0.5 and 1.0. North of 20°N, values up to about 5 are observed, probably associated with aerosol derived from surface dust or anthropogenic sources. A secondary mode, with extinction ratios of 30 or greater and little or no variation of extinction with wavelength, is apparent just below the tropopause. This mode is believed to be associated with thin cloud along the ray path from the Sun to the satellite.
Observations of correlated broadband electrostatic noise and electron cyclotron emissions in the plasma sheet
Authors: Roeder J. L. , Angelopoulos V. , Baumjohann W. , Anderson R. R.
Published: February 2008

Electric field wave observations in the central plasma sheet show the correlated occurrence of broadband electrostatic noise and electrostatic electron cyclotron harmonic emissions. A model is proposed in which the broadband emissions are electron acoustic waves generated by an observed low energy electron beam, and the cyclotron emissions are generated by the hot electron loss cone instability. The high degree of correlation between the two emissions is provided in the model by the presence of the cold electron beam population, which allows both of the plasma instabilities to grow.
Pitch angle scattering of energetic particles by oblique whistler waves
Authors: Inan U. S. , Bell T. F.
Published: February 2008

First order cyclotron or Landau resonant pitch angle scattering of electrons by oblique whistler waves propagating at large angles to the ambient field are found to be at least as large as that due to parallel propagating waves. Commonly observed precipitation of >40 keV electrons in association with ducted whistlers may thus be accompanied by substantial fluxes of lower energy (10 eV–40 keV) electrons precipitated by the nonducted components.
Total ozone measurements and stratospheric cloud detection during the AASE and the TECHNOPS Arctic Balloon Campaign
Authors: Lefèvre F. , Cariolle D.
Published: February 2008

Total ozone fields over Northern Europe and Arctic regions were calculated during the winter 1989 using the TOVS/HIRS2 infra‐red radiances. During that same period ground‐based and airborne ozone and PSCs measurements were performed as part of the AASE and TECHNOPS campaigns. Comparisons have also been made with the TOMS ozone data. They show a generally good agreement, except above northernmost regions where TOMS observations seem affected by the high solar zenith angles encountered near the polar night region. This result is confirmed by the ozone measurements made from Kiruna on the 23 and 30 of January. A major type II PSC event is clearly identified in the TOVS ozone field on January 31. The cloud location is consistent with airborne lidar soundings made on the same day from DC‐8 aircraft, which measured scattering and depolarization ratios characteristic of high concentration of large ice crystals. TOVS observations show that type II PSCs covering large areas may form above the Arctic regions. However, only two major water‐ice PSCs events have been identified from January 3 to February 10. These events may not be frequent enough for the polar air to reach a completely processed state, with high ClO amounts and significant denitrification as it is observed in the Antarctic early spring.
The influlence of polar heterogeneous processes on reactive chlorine at middle latitudes: Three dimensional model implications
Authors: Douglass Anne R. , Rood Richard B. , Kaye Jack A. , Stolarski Richard S. , Allen Dale J. , Larson Edmund M.
Published: February 2008

Three dimensional model calculations with the NASA/GSFC chemistry and transport model have been designed to consider the impact of heterogeneous processes occurring on polar stratospheric clouds (PSC's) in the Arctic vortex on the HCl distribution. By examining the HCl concentration for a calculation with PSC's relative to a calculation with gas phase chemistry only, we infer the impact of polar processing on reactive chlorine species at middle latitudes. Results from the chemistry and transport model reproduce basic features of the ClO measurements [Toohey et al., 1991], which were made on the ferry flights of the ER‐2 from Stavanger, Norway to Moffett Field, California via Wallops Island, Virginia on February 20 and 21, 1989. The model indicates that perturbed air which is contained within the polar vortex during winter is not homogeneously mixed, and that the ferry flights were made through air with the largest conversion of HCl to reactive chlorine that is seen at middle latitudes.
Possible ground‐level measurements of solar neutron decay protons during the 19 October 1989 solar cosmic ray event
Authors: Shea M. A. , Smart D. F. , Wilson Margaret D. , Flückiger E. O.
Published: February 2008

The high energy solar proton event of 19 October 1989 had an atypical particle anisotropy during the initial phase with the Deep River, Ottawa and Goose Bay neutron monitors in the Eastern Canadian region recording the intensity increase about 25 minutes before comparable detectors in Europe and Western Canada. Using the asymptotic cones of acceptance for these neutron monitors, we show that this early onset in the particle intensity increase may be the detection of relativistic protons that are the decay product of solar flare generated relativistic neutrons.
Geochemical stratification in the upper mantle beneath NE China
Authors: Nohda Susumu , Chen Hai , Tatsumi Yoshiyuki
Published: February 2008

Compositions of primary magmas were estimated for Cenozoic Mg‐rich basalts from the NE China region based on the olivine maximum fractionation model. These data together with available experimental data gave an estimate for magma segregation depth for each basalt magma, which is inversely correlated with 87Sr/86Sr isotopic ratio and positively correlated with 143Nd/144Nd isotopic composition. This observation may provide convincing evidence that the enriched upper mantle is underlain by more depleted asthenosphere mantle reservoir.
Transition metal partitioning between lower mantle and core materials at 27 GPa
Authors: Ohtani Eiji , Kato Takumi , Ito Eiji
Published: February 2008

Partitioning of transition metals, V, Cr, Mn, Fe, Co, and Ni between Mg‐perovskite, magnesiowustite and metallic iron has been studied at 27 GPa and 1700–1900°C. V, Cr and Mn are strongly enriched in Mg‐perovskite and magnesiowustite, whereas Fe, Co, and Ni are strongly concentrated in metallic iron under the present experimental condition. Using these data on partitioning, we tested models of core‐mantle separation in the primordial earth.

Abundances of V, Cr, and Fe in the upper mantle are consistent with a model of the upper mantle formed by olivine accumulation and/or majorite fractionation in the primordial earth after the core segregation under the core‐mantle equilibrium. However, depletion of Mn, and overabundances of Co and Ni in the mantle remain unexplained by this model. Other processes, such as vaporization of Mn before or during accretion and the equilibrium partitioning of Co and Ni between minerals and iron in the deep lower mantle may account for the observed abundances of these elements in the mantle.
Comparison of “Electrojet” Indices from the northern and southern hemispheres
Authors: Maclennan C. G. , Lanzerotti L. J. , Akasofu S.‐I. , Zaitzev A. N. , Wilkinson P. J. , Wolfe A. , Popov V.
Published: February 2008

A unique data set of digital and digitized analog magnetic recordings from 22 stations in the Antarctic was used to construct the southern hemisphere “equivalent” of the northern hemisphere auroral electrojet index, AE, for two separate intervals of magnetic disturbance (totaling seven days) in June 1982. A second index constructed using only 9 stations between ∼60°–70° geomagnetic south latitude showed only small differences from that using all 22 stations. For the universal time interval (00–11 UT inclusive) when a reasonably good coverage of ground stations exists in the austral auroral zone, it is found that there is a good correlation between the northern and southern hemisphere indices; this is the case even though the southern ionosphere is largely in total darkness during the interval studied. No effect of the north–south direction of the interplanetary magnetic field is found on the correlation.
Coordinated incoherent scatter radar observations of the semidiurnal tide in the lower thermosphere
Authors: Salah J. E. , Johnson R. M. , Tepley C. A.
Published: February 2008

The incoherent scatter radars at Arecibo (18.3°N), Millstone Hill (42.6°N), and Sondrestrom (67°N) simultaneously observed the thermal structure and dynamics of the Earth's lower thermosphere during the period September 21–25, 1987, as part of the Lower Thermosphere Coupling Study. Plasma spectral measurements were used to derive ion temperatures and horizontal neutral wind velocities in the altitude region 90–140 km, and semidiurnal harmonics were determined from the data sets. The daily mean temperatures were found to be within 40°K at the three stations, and the largest semidiurnal temperature amplitudes, exceeding 70°K above 115 km, were recorded at Arecibo. Mean winds were about ±30 m/s at most altitudes, shifting from eastward to westward directions at 115 km, and the semidiurnal amplitudes were in the range 20–70 m/s. Phase variations with height reveal reasonable consistency among the three stations, within 3–4 hours, and indicate an average vertical wavelength of 40–70 km. However, a strong variation is seen in the vertical structure in the 110‐ to 130‐km region, suggesting the presence of different tidal modes. Comparison with the most current tidal propagation model by Forbes and Vial (this issue) indicates that the model tends to underestimate the amplitudes for this time period, particularly above 110 km. By virtue of the latitudinal distribution of the three radars their coordinated observations of the lower thermosphere serve to constrain numerical models and improve our understanding of this important yet complex region of the atmosphere.
On the origins of energetic ions in the Earth's dayside magnetosheath
Authors: Fuselier S. A. , Klumpar D. M. , Shelley E. G.
Published: February 2008

Energetic ion events in the Earth's dayside subsolar magnetosheath (0900 ‐ 1300 Local Time) are surveyed using data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) Hot Plasma Composition Experiment. Ion species carrying the signature of their origin (O+ and energetic He2+) are used to distinguish between magnetospheric and solar wind origins for the energetic ion events. The results of this survey indicate that the majority of energetic (10–17 keV/e) H+ and He2+ ions observed in the dayside magnetosheath are accelerated from the solar wind population. The energetic He2+ to H+ density ratio in the magnetosheath is consistent with that predicted from first‐order Fermi acceleration of solar wind ions in the turbulent regions upstream and downstream from the Earth's quasi‐parallel bow shock. Although the majority of the energetic ions appear to be of solar wind origin, magnetospheric O+ is also occasionally present in the magnetosheath. The simultaneous occurrence of both energetic He2+ and magnetospheric O+ indicates that, on occasion, both Fermi acceleration of solar wind ions and leakage of magnetospheric ions occurs in the dayside magnetosheath.
Interaction of a finite‐length ion beam with a background plasma: Reflected ions at the quasi‐parallel bow shock
Authors: Onsager T. G. , Winske D. , Thomsen M. F.
Published: February 2008

The coupling of a finite‐length, field‐aligned, ion beam with a uniform background plasma is investigated using one‐dimensional hybrid computer simulations. The finite‐length beam is used to study the interaction between the incident solar wind and ions reflected from the Earth's quasi‐parallel bow shock, where the reflection process may vary with time. The coupling between the reflected ions and the solar wind is relevant to ion heating at the bow shock and possibly to the formation of hot, flow anomalies and re‐formation of the shock itself. We find that although there are many similarities between the instabilities driven by the finite‐length beam and those predicted by linear theory for an infinite, homogeneous beam, there are also some important differences. Consistent with linear theory, the waves which dominate the interaction are the electromagnetic right‐hand polarized resonant and nonresonant modes. However, in addition to the instability growth rates, the length of time that the waves are in contact with the beam is also an important factor in determining which wave mode will dominate the interaction. Whereas linear theory predicts the nonresonant mode to have the larger growth rate for the parameters we investigate, with a finite‐length beam we find that both the nonresonant and resonant modes contribute to the interaction. We find that the interaction will result in strong coupling, where a significant fraction of the available free energy is converted into thermal energy in a short time, provided the beam is sufficiently dense or sufficiently long.
Chaos in the sunspot cycle: Analysis and prediction
Authors: Mundt Michael D. , Maguire W. Bruce , Chase Robert R. P.
Published: February 2008

The variability of solar activity over long time scales, given semiquantitatively by measurements of sunspot numbers, is examined as a nonlinear dynamical system. First, a discussion of the data set used and the techniques utilized to reduce the noise and capture the long‐term dynamics inherent in the data is presented. Subsequently, an attractor is reconstructed from the data set using the method of time delays. The reconstructed attractor is then used to determine both the dimension of the underlying system and also the largest Lyapunov exponent, which together indicate that the sunspot cycle is indeed chaotic and also low dimensional. In addition, recent techniques of exploiting chaotic dynamics to provide accurate, short‐term predictions are utilized in order to improve upon current forecasting methods and also to place theoretical limits on predictability extent. Finally, we compare the results to chaotic solar‐dynamo models as a possible physically motivated source of this chaotic behavior.
Local characterization of homogeneous turbulence in a space plasma from simultaneous Measurements of field components at several points in space
Authors: Pinçon Jean Louis , Lefeuvre François
Published: February 2008

The best method was sought for defining locally an electrostatic or an electromagnetic turbulence in a space plasma, based on the simultaneous measurement of several wave field components at several points in space. The turbulent field is supposed to be stationary in time and homogeneous in space. An estimate of a local field energy distribution, denoted F(ω,k), which specifies how the field energy observed by a limited number of satellites is distributed with regard to the angular frequency ω and to the wave number k, is derived from a generalization of the Capon [1969] high‐resolution method. The Quality of this estimate is deemed sufficient, as shown by examples of application to synthetic data.
Propagation of the solar semidiurnal tide in the mesosphere and lower thermosphere at mid latitudes
Authors: Clark R. R. , Salah J. E.
Published: February 2008

The meteor wind radar at Durham and the incoherent scatter radar at Millstone Hill, which are closely located at 43°N, 71°W, were used simultaneously during the first campaign of the Lower Thermosphere Coupling Study in September 1987 to map the structure of the semidiurnal tidal wind from 80 km to 135 km. The results from the two techniques are consistent and indicate overall continuity in the observed propagation of the semidiurnal tide in both amplitude and phase. Northward winds gradually increase from about 30 m/s at 80 km to 60 m/s above 110 km, while the eastward winds are near 20 m/s below 100 km and reach a maximum of 70 m/s at 115 km. Phase variations indicate a vertical wavelength of the propagating tidal mode of about 100 km at all altitudes for the eastward component, but the incoherent scatter results suggest a shorter wavelength for the northward component above 100 km. The results obtained during the campaign are similar to climatological averages obtained from nonsimultaneous data sets from the two radars. Comparison with numerical model results from Forbes and Vial (this issue) indicates that the model underestimates the wind amplitudes and reaches the maximum value about 10 km below the observed peak; the phase variations lag the observations by 2 hours but indicate an overall structure with altitude that is very similar to the data.
Thermospheric tides at equinox: Simulations with coupled composition and auroral forcings: 1. Diurnal component
Authors: Fesen C. G. , Roble R. G. , Ridley E. C.
Published: February 2008

Thermospheric tidal components calculated by the National Center for Atmospheric Research thermospheric general circulation model have been revised by including the effects of self‐consistent composition couplings and auroral processes. Calculations are presented for equinox conditions at solar cycle minimum and maximum. The diurnal neutral winds and perturbation temperatures predicted by the model are compared with radar observations and the MSIS‐86 model at low, mid, and high latitudes. Sensitivity tests indicate that the feedback between composition and dynamics affects the diurnal tidal temperature amplitudes up to 50% and the diurnal winds by about 15%. The calculated phases are within an hour of uncoupled composition calculations. Auroral processes dominate the diurnal waves at high latitudes, where the wind amplitudes may be double or triple those calculated for solar forcing only. The high‐latitude energy and momentum sources must be included in the model formulations in order to reproduce observations of the September 1984 Equinox Transition Study exospheric temperatures.
A morphological study of vertical ionospheric flows in the high‐latitude F region
Authors: Loranc M. , Hanson W. B. , Heelis R. A. , St.‐Maurice J.‐P.
Published: February 2008

We have studied the vertical bulk ion drift data recorded by the DE 2 satellite between 200 and 1000km altitudes. For this data set we have found that field‐aligned ion flows between 100ms−1 and 3km s−1 are a common occurrence in the high‐latitude F region. The flows are predominantly upward near the cusp region and throughout the auroral zone. Strong downward flows of somewhat smaller magnitude are also recorded but mostly over the polar cap. These statements are true for all drift speeds in excess of 50ms−1 and for all altitudes and magnetic activity levels sampled. The morphology of low‐altitude upward flowing ions agrees well with the morphology of outflowing ions, ion beams, and ion conics observed at much higher altitudes, but the low‐altitude fluxes are often considerably greater. This suggests that a large fraction of the upflowing ions actually returns to the ionosphere, to be observed as large downward ion fluxes. We propose that upflowing ion events are generated by sudden large changes in the ion temperature below the neutral exobase, where ion frictional heating dominates the ion energy balance. The sudden changes in temperature occur when the horizontal velocity of a convecting field tube increases rapidly in regions like the cusp.
Observations of plasma wave turbulence generated around large ionospheric spacecraft: Effects of motionally induced EMF and of electron beam emission
Authors: Neubert T. , Sasaki S. , Gilchrist B. E. , Banks P. M. , Williamson P. R. , Fraser‐Smith A. C. , Raitt W. J.
Published: February 2008

We report on observations of plasma wave turbulence generated during electron beam injections, spacecraft potential variations, and neutral gas emissions of the CHARGE 2 sounding rocket experiment. The payload was flown in a mother/daughter configuration, with the two sub‐payloads electrically connected by an insulated, conducting tether. While tethered, the two platforms were separated, drifting apart in a direction perpendicular to both the magnetic field and to the spacecraft velocity, reaching a maximum distance of 426 m at the end of the flight. The mother carried a high‐voltage (HV) system (0–460 V), biasing the mother negative relative to the daughter. The operation of the HV bias system simulated the motional emf induced in larger orbiting space structures like the Tethered Satellite System 1 (TSS 1) space shuttle mission scheduled for the spring of 1992. In addition, the mother carried an electron beam accelerator (1 keV, 0–46 mA). The daughter diagnostics included wave receivers (400 Hz to 10 MHz) connected to a pair of electric dipole antennas. Broadband ELF/VLF turbulence was generated both during electron beam emissions and when the HV bias system was operated, while turbulence in the HF range was observed only during electron beam emissions. As a result of the electrical connection through the tether, the ELF/VLF wave intensities show little variation with separation distance of the two payloads. Two sources of wave turbulence are proposed to account for the observations: (1) wave turbulence generated by the beam/spacecraft system in particular in the HF range, and (2) electron return currents to the daughter stimulating VLF noise during HV operations and electron beam emissions.
Combined incoherent scatter radar and Fabry‐Perot interferometer measurements of frictional heating effects over Millstone Hill during March 7–10, 1989
Authors: Hagan Maura E. , Sipler Dwight P.
Published: February 2008

We introduce a methodology to calculate the effects of frictional heating associated with geomagnetic activity using simultaneous incoherent scatter radar and Fabry‐Perot interferometer measurements. Vector measurements of ion drift from radar backscatter and neutral wind from optical shifts in the atomic oxygen red line over Millstone Hill, Massachusetts (43°N) for the nights of March 7–10, 1989 are presented and are characterized by the magnetic storm activity which prevailed. We combine these measurements to calculate differences in the ion and neutral velocity fields which approach 350 m/s during the most geomagnetically active period that we monitored near 01 UT on March 9. This velocity difference results in a 110°K heating of the ion gas at that time.
Plasma convection and ion beam generation in the plasma sheet boundary layer
Authors: Moghaddam‐Taaheri E. , Goertz C. K. , Smith R. A.
Published: February 2008

Because of the dawn‐dusk electric field Edd, plasma in the magnetotail convects from the lobe toward the central plasma sheet (CPS). In the absence of space or velocity diffusion due to plasma turbulence, convection would yield a steady state distribution function ƒ = V−2/3 g(υ²V2/3), where V is the flux tube volume. Starting with such a distribution function and a plasma beta which varies from β > 1 in the CPS to β ≪ 1 in the lobe, we study the evolution of the ion distribution function considering the combined effects of ion diffusion by kinetic Alfvén waves (KAW) in the ULF frequency range (1–10 mHz) and convection due to Edd×B drift in the plasma sheet boundary layer (PSBL) and outer central plasma sheet (OCPS). Our results show that during the early stages after launching the KAWs, a beamlike ion distribution forms in the PSBL and at the same time the plasma density and temperature decrease in the OCPS. Following this stage, ions in the beams convect toward the CPS resulting in an increase of the plasma temperature in the OCPS. We also discuss the effects on the polytropic index γ by simultaneous convection and wave‐induced diffusion, both in the PSBL and CPS. We find that γ is less than the adiabatic value in the OCPS but approaches the adiabatic value in the CPS and in the PSBL.
Electric fields and cold electrons in the vicinity of comet Halley
Authors: Laakso Harri
Published: February 2008

The structure and dynamics of the plasma environment on the sunward side of comet Halley are investigated using measurements during the Vega inbound encounters from a double probe antenna and two Langmuir probes (the APV‐V experiment), within a cometocentric distance of ∼850,000 km. Based upon the behavior of the electric fields and the cold electrons, three regions can be identified in the cometosheath (in particular during the Vega 1 approach): transition layers are passed through at ∼780,000 km (R1) and ∼360,000 km (R2). The outer cometosheath (near and beyond R1) is characterized by large‐scale variations in the cold electron density and the electric field, peaking at ∼1 mHz. The R2 crossing is detected in the plasma wave data as enhanced fluctuations at ∼15 mHz. About 25,000 km downstream of R2, the spacecraft traverses a current layer (thickness ∼10,000 km) indicated by a sharp gradient in the dc electric field and the cold electron density. The continuous, relatively rapid increase in the intensity of plasma waves begins at a cometocentric distance of ∼200,000 km. Simultaneously, the cold electron density starts growing in accordance with the r−2 law. The cometopause crossing at ∼160,000 km produces only moderate effects in the APV‐V measurements. In the cometary plasma region, some interesting features are examined, such as a current layer passed by Vega 1 at 54,000 km and sudden ion density enhancements observed by Vega 2 at ∼35,000 km. On several occasions, the APV‐V measurements are heavily influenced by dust particles striking the spacecraft structure. From these events we can conclude that the dust particles are strongly charged and partially coupled to cometary plasmas (or vice versa).
Marine deep seismic reflection profiles off central California
Authors: Ewing John , Talwani Manik
Published: February 2008

A strong reflection horizon at two‐way travel time of approximately 6 s is observed in a deep seismic profile across the outer continental shelf of central California. It is interpreted as the seismic image of subducted oceanic crust emplaced prior to the change from principally convergent to principally transcurrent motion between the Pacific and North American plates during the late Paleogene. The reflector dips landward at a very shallow angle and is at a depth of 14–15 km under Santa Lucia Bank. The reflection is not observed, or at best is very discontinuous, under the inner shelf (Santa Maria Basin). This suggests that tectonic or other processes have produced significantly different structural styles or compositions on the two sides of the Santa Lucia Bank fault. Under the outer shelf a prominent, apparently deeper (later arrival time) horizon dips more steeply and diverges from the 6‐s reflector. The deep horizon is at least partially composed of diffracted energy but is nearly linear after migration. Possible interpretations are that the horizon indicates crustal imbrication or out‐of‐plane diffractions. Alternatively, it is a relict feature imparted to the crust at the now inactive Pacific‐Farallon spreading ridge. Reflective zones at intermediate depths are observed in apparently accreted sediments in parts of the Santa Lucia and Santa Maria basins. These features could represent technically induced fabrics within the accretionary complex, or they could be coherent depositional sequences.
The high‐pressure phase diagram of Fe0.94O: A possible constituent of the Earth's core
Authors: Knittle Elise , Jeanloz Raymond
Published: February 2008

Electrical resistivity measurements to pressures of 83 GPa and temperatures ranging from 300 K to 4300 K confirm the presence of both crystalline and liquid metallic phases of FeO at pressures above 60–70 GPa and temperatures above 1000 K. By experimentally determining the melting temperature of FeO to 100 GPa and of a model core composition at 83 GPa, we find that the solid‐melt equilibria can be described by complete solid solution across the Fe‐FeO system at pressures above 70 GPa. Our results indicate that oxygen is a viable and likely candidate for the major light alloying element of the Earth's liquid outer core. The data suggest that the temperature at the core‐mantle boundary is close to 4800 (±500) K and that heat lost out of the core accounts for more than 20% of the heat flux observed at the surface.
Inversion of full seismogram envelopes based on the parabolic approximation: Estimation of randomness and attenuation in southeast Honshu, Japan
Authors: Scherbaum Frank , Sato Haruo
Published: February 2008

The envelope shape around the arrival of the direct waves of seismic signals traveling in a randomly inhomogeneous medium has been predicted, based on the parabolic approximation, to be a measure of the long‐wavelength components of the randomness as well as the attenuation properties of the medium. We use a nonlinear Marquardt‐Levenberg inversion technique in order to model the SH wave envelopes of 119 earthquakes in the frequency band 2–6 Hz for lapse times less than 1.5 times the shear wave travel time. We attempted to obtain the ratio of the mean square fractional velocity fluctuation to the correlation length (ε2V/a) estimates as well as estimates of attenuation Qs−1. For the majority of the events we found a good correlation between the envelope shape and the hypocentral distances. The resultant ε2V/a of 10−3.27±0.32 km−1 is independent of frequency. It agrees well with the choice of the Gaussian autocorrelation function for the long‐wavelength components of the random velocity fluctuations. The resultant attenuation Qs−1 is roughly proportional to the reciprocal of frequency. We may interpret it as either the scattering loss due to short‐wavelength components of randomness or intrinsic loss. We have performed numerical simulations of the inversion process to quantify the model parameter uncertainties and to obtain a better understanding of the model parameter resolution. By modeling the wave envelopes as a superposition of noise‐free wave envelopes and band‐pass‐filtered Gaussian noise we were able to reproduce the visual appearance of the observed envelopes as well as the observed features in the model parameter dependency. We find that for long hypocentral distances the envelope shape is controlled by the attenuation coefficient, while for short hypocentral distances the velocity fluctuations contribute dominantly.
Faulting associated with the olivine to spinel transformation in Mg2GeO4 and its implications for deep‐focus earthquakes
Authors: Burnley Pamela C. , Green Harry W. , Prior David J.
Published: February 2008

Samples of metastable Mg2GeO4 olivine fail by faulting when loaded at temperatures where the incoherent nucleation and growth of spinel (the stable phase) just becomes possible. At lower temperatures, where the transformation is inhibited, metastable olivine is ductile and strong, while at higher temperatures, where the transformation happens very quickly, samples are ductile and weak. Microfaults that have just been created and have slipped very little contain only very fine‐grained spinel. Those that have slipped more contain a mixture of fine‐grained spinel and larger, broken, olivine fragments. A new type of transformation microstructure that develops only under nonhydrostatic stress occurs in all specimens that failed; the spinel nucleates on grain boundaries and on subgrain boundaries within olivine and grows as fine‐grained, lens‐shaped bodies with a strong preferred orientation normal to maximum compression. The correlation between the onset of transformation, the new microstructure, and the mechanical instability suggests a cause and effect relationship. The grain size of the spinel within the microfaults suggests that these zones have superplastic properties; in this case the instability should not be suppressed by increasing pressure as normal frictional processes are suppressed. Analogous conditions exist in down going slabs of lithosphere in the mantle, and we postulate that the same mechanism that causes faulting in our samples operates in the mantle causing deep‐focus earthquakes.
A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana
Authors: Gunderson Jay A. , Sheriff Steven D.
Published: February 2008

North America's apparent polar wander path has been poorly defined between the mid‐Cretaceous and Paleocene reference pole positions. Existing data allowed 13° of apparent polar motion over about 22 m.y. (87–65 Ma) roughly coinciding with the beginning of Laramide deformation (∼80 Ma). We report on a paleomagnetic study of the Adel Mountain Volcanic rocks to refine the North American apparent polar wander path for this interval. The shonkinite rocks of the Adel Mountain Volcanic field are on the eastern edge of the Cretaceous‐Paleocene fold and thrust belt; some of these structures disturb the western edge of the volcanic pile. We obtained two new K‐Ar dates from the Adel rocks. One date, on biotite ( from a shonkinite dike that crosscuts most of the volcanic rocks, is 71.2±2.7 Ma. The other, a whole rock date from a flow deep in the volcanic pile, is 81.1±3.5 Ma. We collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows, dikes, and laccoliths. Positive fold and conglomerate tests, along with alternating field and thermal demagnetization, indicate that our characteristic remanent directions are primary magnetizations acquired before Late Cretaceous to Paleocene thrust belt deformation. Averaging the virtual geomagnetic poles from 26 reliable sites, all of normal polarity, yields a paleopole at 82.2°N, 209.9°E (α95 = 6.80°, k = 18.38). This pole is concordant with the Paleocene reference pole (82.0°N, 170.2°E, α95 = 3.5°, k = 18.6 (Diehl et al., 1983)) and is 11.6° from the Globerman and Irving (1988) mid‐Cretaceous pole at 71°N, 196°E. The youngest information in the Cretaceous stillstand pole is from the Niobrara Formation (Shive and Frerichs, 1974) at about 85–89 Ma. If we take the average age of the Adel Mountain Volcanics to be 76 Ma, then ∼12° of apparent polar motion occurred between 87 Ma and 76 Ma. Thus, rapid apparent polar motion correlates well with the onset of Laramide deformation.
Paleomagnetic constraints on the geometry and timing of deformation at Yucca Mountain, Nevada
Authors: Rosenbaum J. G. , Hudson M. R. , Scott R. B.
Published: February 2008

Paleomagnetic data from Miocene ash flow sheets indicate that southern Yucca Mountain has undergone about 30° of clockwise vertical‐axis rotation since emplacement of the Tiva Canyon Member of the Paintbrush Tuff at about 13 Ma. Declinations of remanent magnetization from 32 sites in the reversely magnetized Tiva Canyon Member display a systematic southward increase over the 25 km north–south extent of Yucca Mountain. Probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites. Analysis of tilt‐corrected directions of remanent magnetization from the Tiva Canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts. Attitudes determined from map patterns apparently minimize effects of (1) initial dips, (2) poorly defined compaction foliation, and (3) tilting that occurred after compaction but before blocking of remanence; however, such attitudes cannot accurately represent tilting related to unmapped local structures. Rotations implied by data from three older ash flow sheets, the Topopah Spring Member of the Paintbrush Tuff (seven sites), and the Prow Pass (four sites) and Bullfrog (three sites) Members of the Crater Flat Tuff are generally consistent with the amount and sense of rotation documented by data from the Tiva Canyon Member. Uncertainties in the declinations of tilt‐corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the Tiva Canyon Member.
Revisions to the stratigraphy and volcanology of the post‐0.5 Ma units and the volcanic section of VC‐1 core hole, Valles Caldera, New Mexico
Authors: Self S. , Wolff J. A. , Spell T. L. , Skuba C. E. , Morrissey M. M.
Published: February 2008

Reexamination of the Continental Scientific Drilling Project VC‐1 core, new field data, and new isotopic ages necessitate revision of the previously described geology of the youngest eruptive units from the Valles caldera. The Banco Bonito rhyolite lava flow and minor attendant pyroclastic, autoclastic, and reworked deposits about 0.9 km3 dense rock equivalent volume (DRE), are recognized as the product of the youngest eruption from the caldera. The best estimate of the age of this event is 170–240 ka (1σ). A slightly earlier eruption produced the El Cajete pumice fall deposit, followed by the Battleship Rock ignimbrite and the VC‐1 Rhyolite, totalling about 1.8 km3 DRE. We suggest that the VC‐1 Tuffs, ignimbrites recognized only in the VC‐1 core hole, are not a separate unit as previously proposed but are, in fact, the Battleship Rock ignimbrite. A complex volcaniclastic breccia dating from about the time of the South Mountain Rhyolite lava flow (529 ka) is the lowermost volcanic unit in the VC‐1 core. The young, post‐500 ka eruption products from the SW part of the Valles caldera are all composed of very similar low‐silica rhyolite and are thus distinct from earlier, postcaldera high‐silica rhyolites included in the Valles Rhyolite Formation. This distinction marks a major change in the magmatic evolution of the Valles caldera. We propose that recognition of this change necessitates revisions to the stratigraphy of the upper part of the Tewa Group.
Experiments on the interaction of thermal convection and compositional layering at the base of the mantle
Authors: Olson Peter , Kincaid Christopher
Published: February 2008

Accumulation and survival of compositional heterogeneity at the base of the convecting mantle is investigated using laboratory experiments with temperature dependent viscous fluids. Two models of heterogeneity in the D” layer are considered: (1) a reservoir of dense material above the core‐mantle boundary and (2) continuous recharge of the D” layer by subduction of cold, negatively buoyant high viscosity slabs of oceanic lithosphere. Survival of a dense reservoir depends on Rp, the ratio of stabilizing compositional buoyancy to destabilizing thermal buoyancy. Rapid mixing occurs for Rp ≤ 1; for Rp ≥ 1, mixing occurs by slow entrainment only. The reservoir model predicts D” layer heterogeneity is concentrated beneath lower mantle upwellings. Differentiation of composite slabs, consisting of dense crustal and light depleted components, occurs in a hot thermal boundary layer. Composite slabs pile up in the basal thermal boundary layer, and upon heating the light component is ejected by Rayleigh‐Taylor instabilities. The dense component accumulates in the boundary layer, arrayed in a pattern of interconnected spokes, and is slowly entrained into thermal plumes. According to these experiments the time required for conversion of slabs to plumes in D” is 1–2 Gyr. The correlation between lateral variations in D” layer thickness and lateral variations in lower mantle seismic velocity suggests that D″ may consist of dense material, perhaps former oceanic lithosphere, which has sunk through the lower mantle.
Effect of iron content on the electrical conductivity of Perovskite and Magnesiowüstite assemblages at lower mantle conditions
Authors: Li Xiaoyuan , Jeanloz Raymond
Published: February 2008

We have measured the electrical conductivity of (Mg0.76Fe0.24)SiO3 perovskite and of an assemblage of (Mg0.89Fe0.11)SiO3 perovskite + (Mg0.70Fe0.30)O magnesiowüstite at pressures of 45–80 GPa and temperatures from 295 to 3600 K. The apparent activation energy for electrical conduction is 0.24 (± 0.10) eV for the perovskite and 0.20 (± 0.08) eV for the perovskite + magnesiowüstite assemblage. Comparing our present results with those derived previously for Fe‐poor samples, we find that the electrical conductivities of both the silicate perovskite and the perovskite + magnesiowüstite assemblage depend strongly on iron content. Thus the electrical conductivity distribution inside the Earth could provide an important constraint in modeling the composition of the lower mantle.
Paleomagnetic study of Mesozoic continental sediments along the northern Tien Shan (China) and heterogeneous strain in central Asia
Authors: Chen Yan , Cogne Jean‐Pascal , Courtillot Vincent , Avouac Jean‐Philippe , Tapponnier Paul , Wang Gongque , Bai Meixiang , You Hongzi , Li Ming , Wei Chunsheng , Buffetaut Eric
Published: February 2008

A paleomagnetic study of rocks from the northern foot of the Tien Shan and the southern border of the Dzungar Basin, east of Urumqi (44.2°N, 86.0°E), spanning ages from middle Jurassic to early Tertiary was carried out to constrain the tectonic evolution in central Asia since Mesozoic time. Five middle Jurassic sites reveal a remagnetized direction close to the present Earth field in geographic coordinates: D = 6.6°, I = 72.6° (α95 = 7.4°). Thirteen out of 17 upper Jurassic and lower Cretaceous sites yield a characteristic direction (stratigraphic coordinates) of D = 12.7°, I = 48.6° (α95 = 5.5°). Nine of 16 upper Cretaceous and lower Tertiary sites provide a characteristic direction of D = 12.5°, I = 51.3° (α95 = 6.9°). The latter two directions pass fold and reversal tests. The pole positions are close to each other and to the Besse and Courtillot [1989, 1990] Eurasian apparent polar wander path, for ages ranging from 130 to 70 Ma. However, the difference in paleolatitudes amounts to about 5.9°±3.7°, which could indicate significant continental shortening in the Altai Mountains and perhaps further north, subsequent to India‐Asia collision. The pole positions from the Dzungar Basin are close to those found for the Tarim [Li et al., 1988a], leading to an insignificant paleolatitude difference (3.0°±6.9°), but showing a larger difference in declination (8.6°±8.7°). These paleomagnetic results are compatible with a model of heterogeneous deformation in the western part of the collision zone between India and Siberia. A significant shortening in the Altai, a slight counterclockwise rotation of the Dzungar block, the westward‐increasing shortening in the Tien Shan with attendant clockwise rotation of the Tarim block are all consistent with this model, in which Tibet, the Tien Shan and the Altai undergo differential strain along strike in a relay fashion, with the total India‐Siberia convergence remaining approximately constant.
A comparison of Ku‐band Doppler measurements at 20° incidence with predictions from a time‐dependent scattering model
Authors: Thompson D. R. , Gotwols B. L. , Keller W. C.
Published: February 2008

The analysis of coherent Ku‐band radar data measured from the Chesapeake Bay Light Tower as part of the SAXON Experiment in September‐October 1988 is discussed. We concentrate on a 20‐min data segment collected at an incidence angle of 20° for both V‐V and H‐H polarization. Average Doppler spectra are estimated for each second of the data. These Doppler spectra show a large variation in center frequency and relative power due to the influence of the longwave surface on the backscattered field. The basic assumptions and approximations used in the formulation of a time‐dependent scattering model are briefly presented, and concurrent measurements of the longwave surface spectrum at the tower are input into this model in order to calculate the expected mean Doppler spectra. These calculated spectra show very good agreement with the measurements. Implications of the measurements and calculations concerning future experiments and refinements to the model are also discussed.