bluuebunny/crossref_metadata_2025 huggingface dataset:
A preliminary model for Geosat altimeter data errors
Authors: LeSchack A. Richard , Sailor Richard V.
Published: February 2008

Noise power spectral densities (PSDs) are derived from the analysis of GEOSAT altimeter data from segments of nearly collinear repeat tracks. The difference time series, which are analyzed using autoregressive (AR) modeling techniques, contain primarily time‐varying oceanographic signals and altimeter noise, since geoid content is canceled by the differencing process. An average noise PSD, obtained from eight independent repeat‐track PSDs, is fitted closely by a first‐order Markov model with a white‐noise floor. The GEOSAT average noise PSD is compared with analogous spectra derived earlier for GEOS‐3 and SEASAT. In general, the GEOSAT noise PSD is the same as the SEASAT noise spectrum at wavelengths greater than about 100 km, but has a lower noise level at shorter wavelengths.
Mixed state variable friction laws: Some implications for experiments and a stability analysis
Authors: Horowitz Franklin G.
Published: February 2008

Friction laws that depend on both the rate and past history (or state) of a sliding surface have been previously used to describe the sliding friction of rocks and to model some earthquake features. It has been proposed that these laws require an overall velocity weakening for stick‐slip instability to begin. This proposal is based on the assumption that the stress evolution after step changes in slip rate is monotonic. Recent experiments show, however, an irregularly present ‘mixed state variable’ effect in which it appears that a short range state variable is velocity weakening, and a long range state variable velocity strengthening. The stress evolution is clearly not monotonic after imposed step changes in velocity in these cases. It is shown here that instabilities can occur in the presence of overall velocity strengthening. Indeed, mixed state variable laws can explain some paradoxical laboratory observations in a natural way: Slow experiments may be too short to see the effects of the long state variable. Fast experiments may be too fast to resolve the effects of the short state variable. Thus similar experiments can show both velocity strengthening and velocity weakening depending on the total slip length increment measured.
Effects of electron heating on the current driven electrostatic ion cyclotron instability and plasma transport processes along auroral field lines
Authors: Ganguli Supriya B. , Palmadesso Peter J. , Mitchell Horace G.
Published: February 2008

Fluid simulations of the plasma along auroral field lines in the return current region have been performed to show that the onset of electrostatic ion cyclotron (EIC) related anomalous resistivity and the consequent heating of electrons leads to much higher transverse ion temperature than the current driven EIC instability (CDICI) alone would produce. Anomalous resistivity enhances ion heating in two ways: (1) by inhibiting the growth of the critical electron‐ion drift velocity, VcH, which must be exceeded to excite the EIC instability and (2) by increasing the relative drift velocity between the electrons and the ions, VD, through the formation of density cavities due to increased ambipolar electric field. The anomalous resistivity associated with the turbulence is limited by electron heating, so that CDICI eventually saturates, but at a substantially higher transverse temperature than would be the case in the absence of resistivity. This process demonstrates a positive feedback loop in the interaction between CDICI, anomalous resistivity, and parallel large scale dynamics in the topside ionosphere.
Subcritcal dispersive shock waves upstream of planetary bow shocks and at comet Giacobini‐Zinner
Authors: Omidi N. , Winske D.
Published: February 2008

Electromagnetic simulations are used to investigate the nonlinear evolution of ULF (magnetosonic) waves. These waves have been observed upstream of planetary bow shocks and at Comet Giacobini‐Zinner. The simulations show that these waves steepen to form subcritical dispersive shock waves. The steepening process is due to the generation of a broad spectrum of waves on the magnetosonic‐whistler branch. As a result, the waveform spreads in space and the original elliptical polarization of the wave changes to linear. Also, the fastest propagating waves overtake the steepened front and form a circularly polarized precursor. Interpretation of the steepened waves as shocks suggest that Comet Giacobini‐Zinner did not have a single bow shock but instead a transition region consisting of a series of shocks which were convected by the solar wind.
A solar‐wind “trigger” for the outer heliosphere radio emissions and the distance to the terminal shock
Authors: McNutt Ralph L.
Published: February 2008

For roughly five years the Plasma Wave experiments on the two Voyager spacecraft have detected 2 to 3 kHz radio noise in the outer heliosphere. It has been suggested that this noise originates at the terminal shock of the solar wind, and this idea has been used to estimate the distance to the shock. We have examined the solar wind data from the Plasma Science experiment on the Voyager spacecraft to search for correlations with these radio emissions. We find that two anomalous high speed streams passed Voyager 2 before the noise was initially observed. We suggest that the interaction of these streams with the terminal shock may be responsible for the more intense emission. We estimate the distance to the shock using a time‐of‐flight argument as ∼70 A.U. ‐ 140 A.U.; the larger value is consistent with an estimate using Voyager data and a standard pressure balance argument.
A characterization of the spectral density of residual ocean floor topography
Authors: Gilbert Lewis E. , Malinverno Alberto
Published: February 2008

Ocean floor topography can be characterized as a signal related to lithospheric cooling and a residual; the residual can be further modeled by comparing the shape of its power spectrum with that of a fractal process. Power spectra of Seabeam profiles along a flowline in the South Atlantic are calculated; the data are resampled, pre‐whitened by first differencing, tapered with a Hanning window, and passed to an FFT. The effects of the pre‐whitening are removed and the spectra are smoothed. The spectra of ocean floor topographic residuals are curved (power increases less rapidly with increasing wavelength) at wavelengths greater than several 10's of kilometers and straight at shorter wavelengths.
Results of a deep‐tow multichannel survey on the Bermuda Rise
Authors: Gettrust J. F. , Grimm M. , Madosik S. , Rowe M.
Published: February 2008

High‐resolution multichannel seismic data taken with a deep‐towed source and receiving array are used to obtain estimates of thein situcompressional velocity‐depth functions within the upper ∼300 m of deep‐ocean sediments. These data, taken on the Bermuda Rise in an area with a nominal water depth of 5 km, resolve structure consistent with DSDP data from the region. The velocity‐depth functions derived from these data indicate little P‐velocity gradient in the upper 100–170 m of sediments and resolve P‐velocity inversions within the depth range sampled.
Spatial distribution and B value of intermediate‐depth earthquakes beneath central Peru
Authors: Schneider John F. , Sacks I. Selwyn , Huaco Daniel , Ocola Leonidas , Norabuena Edmundo , Flores Antonio
Published: February 2008

We have investigated the spatial distribution of microseismicity at intermediate depth in the Wadati‐Benioff (W‐B) zone beneath central Peru. Within a 100 to 150 km depth range in the subducting Nazca plate, 67 event hypocenters define a horizontally subducting zone (horizontal zone) 450 to 600 km from the trench, and a zone of increasing dip (resubduction zone) 600 to 700 km from the trench. Although the shape of the W‐B zone is generally consistent with previous observations from teleseismic data, the distribution of seismicity is not. In a 550 km wide band extending from 400 to 750 km from the trench, the density of teleseismicity (mb > 4, 1971–1981) increases as a function of distance from the trench, while the density of microseismicity (1.6 < mb < 4.2) decreases. Combining local and teleseismic data, the b value changes from .84 in the horizontal zone to .57 in the resubduction zone. This result suggests that the minimum earthquake size and/or stress drop increases in the resubduction zone. Low b‐values may also be intrinsic to proximity to the seismic cut‐off at 150 km depth.
P to S converted wave observations at Long Valley Caldera
Authors: Steck Lee K. , Prothero William A.
Published: February 2008

We report observations of P to S converted teleseismic body waves recorded with seismic stations located on the resurgent dome of Long Valley caldera. We observe strong conversions occurring at boundaries that could be the tops of small magma bodies at depths of 4–9 kilometers beneath the resurgent dome. This technique could prove to be an extremely powerful tool in the search for magma bodies for geothermal energy development. Our results from a two month deployment of triggered, digital teleseismic recorders are discussed.
Simulation of ion heating in the topside auroral ionosphere
Authors: Chiu Y. T. , Cladis J. B. , Francis W. E.
Published: February 2008

Energization of ions to low energies (≤10 eV) in the topside ionosphere (300 – 2000 km altitude) has been observed for a decade. These ionospheric ions have been invoked as the source population of keV ions energized in the auroral magnetosphere (≥1 RE altitude). If so, the importance of the topside ionosphere as a source of magnetospheric plasma must be assessed quantitatively. To determine the efficiencies of this topside auroral ionospheric source of magnetospheric plasma, Monte Carlo simulations of wave‐particle ion heating under realistic topside conditions have been carried out. It is shown that the source efficiency is greatly affected by (1) copious charge exchange interactions between the ions and thermospheric neutrals at an early stage of heating and (2) the observed existence of downward parallel electric potential drops. The composition of warm ions escaping the topside is predicted to be drastically different from the cold ionospheric composition. This study also predicts the existence of a hot (<10 eV) population of thermospheric neutrals in the high latitude topside ionosphere.
Impulsive electric and magnetic field perturbations observed over South Pole: Flux transfer events?
Authors: Bering Edgar A. , Benbrook James R. , Byrne Gregory J. , Liao Bing , Theall Jeffrey R. , Lanzerotti Louis J. , Maclennan Carol G. , Wolfe Allan , Siscoe George L.
Published: February 2008

Electric and magnetic field data from the cusp latitude station at South Pole were searched for impulsive perturbations. These events may be evidence of the ionospheric signature of flux transfer events (FTE's). In a single X‐line reconnection model, the field aligned current system associated with the convection of the flux tube across the polar cap will produce two convection vortices around the flux tube. The multiple X‐line reconnection model predicts additional co‐axial field‐aligned currents in helical flux tubes. The electric field signatures of these two models are different. The electric field of the towing system attenuates less than the electric field of the twisting system as the field is mapped down to balloon altitude. Combining electric and magnetic field data may determine the relative importance of the two systems. Near local magnetic noon on 7 January 1986, at least three candidate events have been found. One event is shown to approximate the signature expected for reconnected flux tubes passing overhead. The paper brings a new data set to bear on an important phenomenon.
The polar heliospheric magnetic field
Authors: Jokipii J. R. , Kóta J.
Published: February 2008

We suggest that the polar heliospheric magnetic field, at large heliocentric distances, may deviate considerably from the generally accepted Archimedean spiral. Instead, we suggest that the large‐scale field near the poles may be dominated by randomly‐oriented transverse magnetic fields with magnitude much larger than the average spiral. The average vector field is still the spiral, but the average magnitude may be much larger. In addition, the field direction is transverse to the radial direction most of the time instead of being nearly radial. This magnetic‐field structure has important consequences for the transport of cosmic rays. Preliminary model calculations suggest changes in the radial gradient of galactic cosmic rays which may improve agreement with observations.
Interannual temperature changes in the Antarctic lower stratosphere‐A relation to the ozone hole
Authors: Kawahira Kohji , Hirooka Toshihiko
Published: February 2008

Interannual changes in the daily zonal mean temperature in the Antarctic lower stratosphere are analyzed for the past seven years from 1980 to 1986 by using NMC data. It is found that the Antarctic polar region tends to cool not only in spring but also in all seasons, remarkably in winter and spring. Further analysis concerning the meridional gradient of the zonal mean temperature showed that the period per year of the winter pattern (temperature decrease toward the south pole) becomes longer as the ozone hole becomes deeper. From this evidence, the interrelation between the temperature decline and the ozone depletion is discussed.
Numerical simulation of the movement of a Mediterranean water lens
Authors: Beckmann Aike , Käse Rolf H.
Published: February 2008

The evolution of the dynamical signal of an isolated Mediterranean Water lens (‘meddy’) is simulated using a quasigeostrophic model on the β‐plane. The initially circular anticyclonic vortex moves south‐westward at a non‐uniform velocity of about 1 cm/s for over two years. Repeated instability processes (both barotropic and baroclinic) modify the horizontal shape and the radius of the meddy as well as its vertical structure. Particle trajectories inside the lens show strong structural similarities with tracks of ‘SOFAR’ floats deployed in a meddy by Armi et al. (1988). A tracer concentration inside the meddy is transported away from the source region and slowly spreads to the surrounding. ‘SOFAR’ floats injected in nearly zonal tracer tongues produced by the quasi‐barotropic far field behind the meddy moved westward in a narrow band as observed by Price et al. (1986).
Frontal signals east of Iceland from the GEOSAT altimeter
Authors: Robinson A. R. , Walstad L. J. , Calman J. , Dobson E. B. , Denbo D. W. , Glenn S. M. , Porter D. L. , Goldhirsh J.
Published: February 2008

Frontal Sea Surface Height signals were identified in the Iceland‐Faeroe region for height differences from three sets of GEOSAT tracks (July‐May 1987). These signals were verified with in situ measurements and then combined with them for an improved and synoptic estimate of frontal location. The data assimilation context made the weak signals quantitatively useful. Real time analysis of GEOSAT data contributed to a frontal nowcast and forecast scheme.
A magnetohydrodynamic simulation of the formation of magnetic flux tubes at the Earth's dayside magnetopause
Authors: Ogino Tatsuki , Walker Raymond J. , Ashour‐Abdalla Maha
Published: February 2008

We have studied dayside magnetic reconnection by using a three‐dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and the magnetosphere. We found two different mechanisms for the formation of magnetic flux tubes at the dayside magnetopause which depend on the orientation of the interplanetary magnetic field (IMF). The dayside magnetic flux tubes occur only when the IMF has a southward component. A strongly twisted and localized magnetic flux tube similar to magnetic flux ropes appears at the subsolar magnetopause when the IMF has a large By component. When the By component is small, twin flux tubes appear at the dayside magnetopause. Both types of magnetic flux tube are consistent with several observational features of flux transfer events and are generated by antiparallel magnetic reconnection.
New evidence for global tectonic zones on Venus
Authors: Kozak Richard C. , Schaber Gerald G.
Published: February 2008

Venera 15 and 16 spacecraft images show clear evidence of major crustal disruptions on Venus which have been interpreted to indicate crustal divergence. Complementary to the divergent zones are mountain belts that border the continent‐like high terrains. The requisite transcurrent motions appear to be manifested as diffuse shear zones. The rift zones form an interconnected transpolar system which ties in with previously recognized equatorial disruption zones, suggesting a global tectonic network. Several independent lines of evidence suggest that the tectonism may be geologically young.
The effect of Coriolis coupling of free oscillation multiplets on the determination of aspherical Earth structure
Authors: Smith Mark F. , Masters Guy
Published: February 2008

Coriolis coupling between free oscillation multiplets affects the determination of structure coefficients which linearly constrain the earth's aspherical structure. The coupling between fundamental spheroidal and toroidal modes may be modeled quite well by a complex frequency shift to the degenerate multiplet eigenvalues and a perturbation to the degree 2 axisymmetric structure coefficients. Observations of structure coefficients of coupled spheroidal modes clearly show the expected effect of coupling and we develop corrections to compensate for the phenomenon. Our results suggest that the P20 pattern of aspherical structure in the upper mantle may be biased by up to 50% if unconnected structure coefficients are used in an inversion for earth structure.
Determination of the gravitational coefficient of the Earth from near‐Earth satellites
Authors: Ries J. C. , Eanes R. J. , Huang C. , Schutz B. E. , Shum C. K. , Tapley B. D. , Watkins M. M. , Yuan D. N.
Published: February 2008

Laser range, doppler and altimeter data collected from 14 near‐Earth satellites have been combined to determine the value of the geocentric gravitational coefficient (GM) of the Earth. A long‐arc solution using 3 years of laser range data to Lageos was used in a separate determination in which the effects of general relativity were investigated. The value of GM (including the mass of the atmosphere) was determined to be 398600.4405 km³/sec² when all corrections for general relativity are taken into account. The uncertainty (1‐σ) in the value of GM is estimated to be 0.001 km³/sec².
Structures of Alfvén shocks: S‐shaped magnetic hodogram observed at the magnetopause
Authors: Lyu L. H. , Kan J. R.
Published: February 2008

The Alfvén shock structure is obtained by a two‐fluid simulation code including dissipations due to resistivity, viscosity and heat conductivity. The hodogram of the tangential magnetic field component of the simulated Alfvén shock is S‐shaped with a 180° angular rotation. This is the first simulation result to show the S‐shaped hodogram associated with Alfvén shocks. The S‐shaped hodograms have been observed at the magnetopause [Berchem and Russell, 1982]. On the basis of this result, we propose that the magnetopause is an Alfvén shock where and when the hodogram is S‐shaped and rotates 180°. The upstream portion of the hodogram is always right‐hand polarized; the downstream portion is always left‐handed. The thickness of the Alfvén shock transition layer is about a few ion inertial lengths, which tends to increase with decreasing shock normal angle θbn. For θbn = 80°, the thickness is estimated to be about 5 c / ωpi.
a forced baroclinic vortex around a hydrothermal plume
Authors: Speer Kevin G.
Published: February 2008

A steady, frictional flow around a hydrothermal plume is described which is forced by the entrainment of water into the buoyant part of the plume as it rises above its source. The circulation is anticyclonic (clockwise in the northern hemisphere) near the equilibrium density level or spreading level of the plume, and cyclonic below this level. The strength of this flow is adequate to trap anomalous properties from the hydrothermal fluid near the source in the presence of weak background flow. Some observational support for such a flow is presented.
Sea ice thickness data: The many vs. the few
Authors: Eicken Hajo , Lange Manfred A.
Published: February 2008

During May 1988 a 1230‐m ice thickness profile was drilled across a floe in Fram Strait. The probability density function of this data set corresponds closely to extensive ice draft measurements from submarines and the thickness distribution of floes sampled randomly in the area. The observed similarity is attributed to the topography of the pack ice in Fram Strait where each floe is composed of a spectrum of smaller components. This may imply that mutual independence and representativeness of thickness measurements can be satisfied to some degree even for short profiles.
Coda Q in the southern Appalachians
Authors: Chapman M. C. , Rogers Melissa J. B.
Published: February 2008

The attenuation of seismic coda waves in the Southern Appalachian region was studied. Signal sources were five local and regional earthquakes and one explosion located in Ohio, Virginia and South Carolina. The data were recorded digitally by a network of seven stations in western Virginia. Joint least squares estimates of the quality factor Q, relative recording site responses and coda source amplitudes were obtained in five frequency bands using the single scattering model, and assuming body wave geometrical spreading. The results indicate a frequency dependent coda Q, represented by Q = 811 f0.42, 1.5 ≤ f ≤ 12 Hz, for lapse times in the range 15 to 250 seconds. Site responses determined relative to the average network response differ by less than a factor of 2 in the range 3 to 12 Hz at five of the seven stations. However, the differences in the response of the other two sites increase with frequency, amounting to a factor of 5 at 12 Hz. Estimated relative coda source amplitude spectra exhibit similar shapes, consistent with the assumption that the scattering process is independent of path in the study region.
Effect of a low viscosity layer on long‐wavelength topography, upper mantle case
Authors: Davies Geoffrey F.
Published: February 2008

Numerical models of upper mantle convection incorporating both plates and a substantial low viscosity layer show that long‐wavelength topography is only moderately reduced compared with a model without the low viscosity layer. In particular, topography due to hot upwelling from a putative hot boundary layer at 670 km depth is still large. In the case of a migrating spreading center, the symmetry and square‐root‐of‐age dependence of seafloor topography would be destroyed. These results strengthen the case against mantle models in which heat transport across the 670 km seismic discontinuity is predominantly by conduction. A substantial mass flux through 670 km is implied.
Lake level observations to detect crustal tilt: San Andreas Lake, California, 1979–1989
Authors: Mueller R. J. , Johnston M. J. S. , Myren G. D. , Murray T.
Published: February 2008

A pair of precision lake level gauging stations, installed in 1978, have been monitoring differential crustal uplift (crustal tilt) at San Andreas lake, California, near the suspected epicenter on the San Andreas fault of the M=8.3, 1906 San Francisco earthquake. The stations are installed in the lake with a 4.2 km station separation parallel to the San Andreas fault. The gauging stations use quartz pressure transducers that are capable of detecting intermediate to long‐term vertical displacements greater than 0.4 mm relative to a fluid surface. Differencing data from the two sites reduces the noise contributed by atmospheric pressure, temperature, and density changes, and isolates the relative elevation changes between the ends of the lake. At periods less than 20 minutes, the differenced data are dominated by lake seiches which have a fundamental mode at a period of 13±0.3 minutes. These seiche harmonics can be filtered or predicted and removed from the data. Wind shear, typically lasting several days, can generate apparent short term tilt of the lake and large seiche amplitudes. The tilt noise power spectrum obtained from these data decreases by about 15 dB/decade of frequency. Monthly averages of the data between 1979–1989 indicate a tilt rate of 0.02±0.08 microradians/yr (down S34°E). No measurable horizontal tilt has apparently occurred in this region of the San Andreas fault during the last decade, however, measurements of trilateration networks show this region to be undergoing a horizontal strain of 0.6±0.2 µstrain/yr.
Myriametric radiation and the efficiency of linear mode conversion
Authors: Rönnmark Kjell
Published: February 2008

Linear mode coupling is often invoked to explain the conversion of electrostatic upper hybrid waves into terrestrial myriametric radiation. The efficiency of this conversion process can be studied by recently developed methods, which are based on the continuity equation for the wave distribution function. These estimates show that the efficiency of linear mode conversion is at least three orders of magnitude too low to account for the myriametric radiation observed in the magnetosphere.
Intermediate‐term variations of chromospheric and coronal solar flux during high solar cycle 21 activity
Authors: Tobiska W. Kent , Bouwer S. David
Published: February 2008

The solar Lyman‐α emission, the MgII core‐to‐wing ratio, R(MgIIc/w), the 10.7‐cm radio flux, F10.7, and the 1–8 Å X rays are compared during high solar cycle 21 activity from 1981–83. Daily variations of the Mg II and Lyman‐α ultraviolet (UV) lines are highly correlated. There is moderate linear correlation between these lines and F10.7 and poor linear correlation with 1–8 Å X rays. Power spectral analysis indicates that all four fluxes have 27‐day periodicities due to solar rotation while the R(MgIIc/w) and Lyman‐α have noticeable 13‐day periods in the datasets. F10.7 moderately represents the 27‐day solar UV variations and represents to a lesser degree variations shorter than or longer than rotation variations. X rays are not represented by F10.7 on intermediate‐term or shorter timescales.
Measurements of enhanced springtime ultraviolet radiation at Palmer Station, Antarctica
Authors: Lubin Dan , Frederick John E. , Booth C. Rocky , Lucas Timothy , Neuschuler David
Published: February 2008

Measurements of ultraviolet solar spectra from Palmer Station, Antarctica have defined the surface radiation environment of the region during the Austral spring of 1988. At wavelengths where absorption by ozone is negligible, 335–345 nm, the noontime irradiances show the expected gradual increase from the first day of measurements, 19 September, through 21 December. Large variations related to cloudiness are imposed on this background. At wavelengths less than 310 nm the influence of the 1988 ozone “hole” is apparent. The noontime irradiance observed in the wavelength band 295–305 nm on 19 October, two months prior to summer solstice, exceeded any value measured through 21 December.
Testing the Global Warming Hypothesis
Authors: Tsonis A. A. , Elsner J. B.
Published: February 2008

The temperature record for the global surface air temperature indicates that six of the warmest years occurred in the period 1980–1988. Here we address the question on the likelihood that such an arrangement is simply a manifestation of the natural variability of the system. Our results indicate that the probability that such an arrangement will arise naturally is between 0.010 and 0.032.
A solar‐ocean relation: Fact or fiction?
Authors: Barnett Tim P.
Published: February 2008

In the course of studying Quasi‐biennial (QB) variations in the global sea surface temperature (SST) field, an apparent solar signal was detected. Specifically, the amplitude of variations in the QB frequency band appeared to be modulated at the 11 year solar cycle at least over the last four cycles. The sense of the relation is that high solar activity goes with low amplitude variations in the QB frequency band. In a series of recent papers, van Loon and Labitzke (1988a,b) and Libitzke and van Loon (1988) have found in the same frequency band a similar amplitude modulation in atmospheric fields of both Hemispheres also over the last four solar cycles. In an effort to study the apparent relation over more solar cycles, the analysis of the time series of SST was extended back to 1884. The solar‐SST relation found above was found to hold back to about 1925. Before that time, remnants of the SST signal can still be observed but the relation to the solar cycle becomes more confused. This lack of consistency through the 10 solar cycles examined here may be due to the fact that the apparent relationships are fictitious. They may also be partially due to poor SST data before 1920; the major failure in the solar‐SST relation occurring in the data sparse 1915–1920 period. It may also be that the weak solar variability at the turn of the century could not force an observable response in the SST field.
Geoid anomalies at fracture zones and thermal models for the oceanic lithosphere
Authors: Wessel Pål , Haxby William F.
Published: February 2008

Geoid anomalies derived from the Geosat altimeter data set have been used to model the change in the geoid height across four Pacific fracture zones. After accounting for geoid signals from the flexural response to differential subsidence and thermal bending stresses, we find that the estimates of geoid steps across the fracture zones are compatible with existing conductive models for the oceanic lithosphere. If flexure is not accounted for, the geoid steps will be underestimated by 30–40%. Departures from the predicted conductive trends may be explained by small‐scale convection and uncertainties in the crustal ages.
Dynamical consequences of compositional and thermal density stratification beneath spreading centers
Authors: Sotin C. , Parmentier E. M.
Published: February 2008

Extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers. This compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow. The relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity. For a low enough mantle viscosity, buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone. For a mantle viscosity of 1022 Pa s, upwelling due to plate spreading is dominant, and crustal thickness is predicted to be a strong function of spreading rate. However, for a mantle viscosity of 1019 Pa s, buoyancy dominates mantle upwelling, and the calculated crustal thickness, like the oceanic crust, is nearly independent of spreading rate. Thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling. For a mantle viscosity of 1020 Pa s and a low 1 cm/yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single, well defined period. For even slightly lower mantle viscosity, the frequency increases, and crustal thickness oscillations become more complex with two or more frequencies present. While numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers, we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure.
Stability fields of Mg‐pumpellyite composition at high pressures and temperatures
Authors: Liu Lin‐gun
Published: February 2008

Mg‐pumpellyite was observed to break down to the assemblage pyrope + kyanite + coesite + (H2O) in the pressure range 60–80 kbar, to the assemblage pyrope + kyanite + stishovite + (H2O) in the range 100–140 kbar, and to the asemblage pyrope + corundum + stishovite + (H2O) in the range 160–220 kbar at 800–1000°C in a diamond‐anvil cell coupled with laser heating. Mg‐pumpellyite may exist in the coldest part of a subducting lithosphere down to a depth of about 300 km.
Daytime observation of the sodium layer with a Fabry‐Perot spectrometer at Mawson, Antarctica
Authors: Greet P. , Conde M. , Jacka F.
Published: February 2008

The first ground based measurements of temperature in the sodium layer at about 90 km altitude in the Antarctic have been made at Mawson; a mean value of 153 ± 7 K for January 1989 was obtained. A dual, scanning Fabry‐Perot spectrometer was used to record the D2 spectral line resonantly scattered from sunlight; the Doppler width and shift of the line measures the kinetic temperature and wind velocity in the emitting region. The technique is shown to be promising for study of the dynamics of the 90 km region which is otherwise difficult to observe.
Slow shock characteristics as a function of distance from the X‐line in the magnetotail
Authors: Lee L. C. , Lin Y. , Shi Y. , Tsurutani B. T.
Published: February 2008

Both particle and MHD simulations are performed to study the characteristics of slow shocks in the magnetotail. The particle simulations indicate that switch‐off shocks exhibit large amplitude rotational wave trains, while magnetotail slow shocks with an intermediate Mach number MAn < Mc ≃ 0.98 do not display such rotational wave trains. The MHD simulations show that the spontaneous reconnection process in the near‐earth plasma sheet leads to the formation of a pair of slow shocks tailward of the reconnection line (X‐line). The properties of slow shocks are found to vary as a function of the distance from X‐line due to the formation of plasmoid. Slow shocks in most regions of the magnetotail are found to be non‐switch‐off shocks with MAn < 0.98. The present results are used to discuss the lack of large amplitude rotational wave trains at slow shocks in the deep magnetotail.
The ionosphere of Neptune
Authors: Shinagawa H. , Waite J. H.
Published: February 2008

Limited knowledge of ionospheric processes of the outer planets and practically no information on the upper atmosphere of Neptune result in a range of possible ionospheres for Neptune. Various cases are investigated in order to establish a theoretical framework of the ionospheric structure against which the radio occultation measurements can be evaluated. The peak electron densities could range from 10³ cm−3 to 106 cm−3 depending on ion loss processes and particle ionization processes. The scale height could also range from 300 km to 2000 km depending on the exospheric temperature. Although various assumptions must be made, we predict that an auroral ionosphere with large electron densities (∼ 105 cm−3) and a large scale height (∼2000 km) will be observed by the ingress radio occultation measurement, and that a compressed ionosphere with small electron densities (order of 10³ cm−3) will be seen during the egress measurement.
Neptune may have polar rings
Authors: Dobrovolskis Anthony R. , Steiman‐Cameron Thomas Y. , Borderies Nicole J.
Published: February 2008

Perturbations from Neptune's highly inclined satellite Triton can maintain rings passing nearly over Neptune's poles. These hypothetical polar rings are nearly perpendicular to Triton's orbit as well, and lie within several degrees of the plane of Voyager II's trajectory through the Neptunian system. Polar rings can coexist with equatorial rings at different radii. A randomly oriented torus of debris around Neptune has a probability of several percent to settle into a polar ring. Voyager II stands a significant chance of encountering a polar ring.
Monte‐Carlo Modeling of polar wind photoelectron distributions with anomalous heat flux
Authors: Yasseen F. , Retterer J. M. , Chang Tom , Winningham J. D.
Published: February 2008

In situ measurements above the polar cap by the DE satellites show asymmetric, field‐aligned electron velocity distributions in the photoelectron energy range (5–60 eV). The formation of these anisotropic distributions is described by a collisional kinetic equation that includes the effects of Coulomb collisions (via a Fokker‐Planck operator), those of a parallel electric field, and the magnetic mirror effects of the geomagnetic field. We solve this equation using a Monte Carlo simulation method. The simulation illustrates the formation of the portion of the observed electron distributions originating in the ionosphere, and permits us to evaluate the significance of the suprathermal electrons at higher altitude. Both observations and calculations indicate that these electrons possess an energy flux which, by its effect on the polar wind electric field, may influence the dynamics of the polar wind outflow.
3D particle motion trajectories: Direct observation of Love‐Rayleigh coupling
Authors: Tajima Fumiko , Kawasaki Ichiro
Published: February 2008

We present observations of severe particle motion distortion due to Love‐Rayleigh wave coupling (e.g., between 0L and 1R) using a display of 3D particle motion trajectory for records obtained at GDSN station HON. Though effects of lateral heterogeneity are yet to be examined carefully, phase velocity dispersion curves, calculated for a multilayered oceanic model with azimuthal anisotropy, predict this type of coupling in the period range between 20 and 30 s depending on the propagation direction of surface waves.
Importance of the reference model in linearized tomography and images of subduction below the Caribbean Plate
Authors: van der Hilst Rob D. , Spakman Wim
Published: February 2008

In linearized delay time tomography the choice of a one‐dimensional reference model is very important for correct mapping of seismic anomalies. We demonstrate that the use of an inadequate background model results in artifacts in tomographic images which may give rise to erroneous interpretations. Particularly the absence or presence of upper mantle discontinuities in the reference model, influences the mapping of structures (e.g. subducting slabs) at the transition between upper and lower mantle. New tomographic images of the mantle below the Caribbean Region are presented. These show the extent of the Lesser Antilles subduction zone down to 600 km which is well below the seismic zone. A high velocity anomaly is imaged from the Mid American trench to lower mantle depths (1400 km).
Global Three‐Dimensional Model Calculations of the Budgets and Present‐Day Atmospheric Lifetimes of CF2ClCFCl2 (CFC‐113) and CHClF2 (CFC‐22)
Authors: Golombek Amram , Prinn Ronald G.
Published: February 2008

The annual percentage increases in concentrations of the chlorofluorocarbons CF2ClCFCl2 (CFC‐113, an industrial solvent) and CHClF2 (CFC‐22, a refrigerant) are the highest among major chlorofluorocarbons in the atmosphere today. We have computed the present‐day atmospheric lifetimes for these species using a global three‐dimensional dynamical‐chemical model. The present‐day lifetimes of both are long (15.5 years for CHClF2, and 136 or 195 years for CF2ClCFCl2 depending on assumed O2 absorption cross‐sections) underscoring the need to decrease their emissions in order to minimize their future role in ozone destruction and greenhouse warming.
Simultaneous measurements of wind shear and temperature gradient spectra in the stratosphere
Authors: Barat Jean , Cot Charles
Published: February 2008

We present in this paper the first high resolution analysis of wind shears and temperature gradient measured over 25 m in the low stratosphere. Their power spectral densities deduced by two different methods show that for vertical wavelengths greater than 500 m the behaviors of the temperature and vertical velocity fluctuating field are significantly different from the saturated wave model predictions.
Paleointensity of the Earth's magnetic field and K‐Ar dating of the Louchadiere volcanic flow (central France): New evidence for the Laschamp excursion
Authors: Chauvin Annick , Duncan Robert A. , Bonhommet Norbert , Levi Saul
Published: February 2008

We report paleointensity results of the Earth's magnetic field from a Late Pleistocene lava flow (Louchadière, Central France), which recorded an intermediate geomagnetic field direction (5 sites, mean declination=114.1°, inclination=58.2°, k=130, α95=6.7°). New K‐Ar age determinations confirm that this flow is contemporaneous with the Laschamp and Olby flows, and that this excursion occurred around 45ka ago. Using the Thellier double heating method, reliable paleointensities have been obtained for ten samples from three different sites, providing an average field strength of 12.9 µT (±3.3). This low value and previous results of the Laschamp excursion from France and Iceland confirm that the Earth's magnetic field was in an intermediate state during the Laschamp excursion.
Pluto‐Charon mutual events
Authors: Binzel Richard P.
Published: February 2008

Since 1985, planetary astronomers have been working to take advantage of a once‐per‐century apparent alignment between Pluto and its satellite, Charon, which has allowed mutual occultation and transit events to be observed. These events, which will cease in 1990, have permitted the first precise determinations of their individual radii, densities, and surface compositions. In addition, information on their surface albedo distributions can be obtained.
ULF waves in the Mercury magnetosphere
Authors: Russell C. T.
Published: February 2008

ULF waves are present in the magnetic records returned by the Mariner 10 spacecraft. An analysis of samples of these data show that near the magnetopause the waves are right‐hand circularly polarized and about 3 nT in amplitude. These waves weaken and become linearly polarized as the spacecraft moves inward. Deep in the magnetosphere narrowband waves with 2‐second periods are found. These waves may be standing waves on field lines anchored in the core.
Stratospheric H218O distribution from far infrared observations
Authors: Guo J. , Abbas M. M. , Nolt I. G.
Published: February 2008

The stratospheric distribution of H218O in the 22 to 37 km altitude range has been obtained from balloon‐based measurement of far infrared thermal emission spectra with an unapodized spectral resolution of 0.0033 cm−. The analysis is based on the observed spectra of 9 isolated spectral lines in the 46 to 72 cm−1 region obtained from four complete limb sequences. Comparison of the results with the statistically expected values indicates enhancement in the H218O concentrations increasing from 27 ± 14 % at 29 km to 40 ± 23 % at 37 km. No significant enhancement at lower altitudes was observed.
Periodic behaviors in the observed vertical column abundances of atmospheric hydroxyl
Authors: Burnett Elizabeth Beaver , Burnett Clyde R. , Minschwaner Kenneth R.
Published: February 2008

OH vertical column abundances measured at Fritz Peak Observatory, Colorado, from 1977 through 1988 reveal semi‐annual and annual cycles which are amplitude modulated over the 11‐year data base. The modulation of the OH seasonal behavior is in phase with solar activity as described by the sunspot number index. No mechanism for the observed covariance of this OH behavior with solar activity has yet been identified.
Roughness constraints in surface wave tomography
Authors: Park Jeffrey
Published: February 2008

A set of inversion experiments is described that tests the ability of global surface wave tomography to retrieve long‐wavelength upper mantle velocity structure in the presence of a modest level of rough structure. We use upper‐mantle model M84A, augmented with randomly‐generated lateral structure to smax = 20, to calculate coupled‐mode synthetic seismograms by means of the subspace projection method. The data kernels for path‐integral data observables do not belong to the model space of allowable phase velocity perturbations, suggesting that constraints on model roughness in the inversion should lead to a better agreement with the long‐wavelength part of the ‘true‧ model than do the commonly‐used constraints on model size. Coherence C between the inverted and the input model is enhanced by using roughness constraints. Estimation of perturbation size is more stable using roughness constraints, but is subject to a positive bias, especially for periods T > 200 s. We can achieve, with a data set of 672 R1 and R2 phase delays, C ≥ 0.9 only for s ≤ 6, suggesting ‘leakage‧ bias caused by the breakdown of the pure‐path assumption.
A ten‐year decrease in the atmospheric helium isotope ratio possibly caused by human activity
Authors: Sano Yuji , Wakita Hiroshi , Makide Yoshihiro , Tominaga Takeshi
Published: February 2008

The isotope ratio of terrestrial air helium is believed to be constant on a global scale since the mixing time for the atmosphere is significantly shorter than the residence time for helium. Recently, the ³He/4He ratio at Ueno Park, central Tokyo, Japan, was found to be significantly lower than previously measured values. We report here a change in the atmospheric helium isotope ratio from 1.362 × 10−6 in December 1977 to 1.339 × 10−6 in September 1988 or a decrease of ∼1 × 10−9/year. Although this change could be due to local/nonglobal effects, minor changes in the atmospheric flux balance for helium or experimental artifacts, the observations are also consistent with a significant influx of a low ³He/4He ratio source to the atmosphere. The magnitude of the flux, 0.48 ‐ 2.9 × 1016 cm³STP He/year, is compatible with estimates of the anthropogenic release of crustal helium from gas and oil production from the solid Earth. Because of the inert chemistry of helium, a quantification of this change may provide a marker against which to calibrate the absolute flux and retention of anthropogenic CO2 in the atmosphere.
Detection of HOCL in the Antarctic stratosphere
Authors: Toon G. C. , Farmer C. B.
Published: February 2008

An integrated vertical column abundance of 1.5±0.4×10+14 molec.cm−2 of HOCl has been inferred from high resolution infrared solar spectra measured by the JPL MkIV interferometer from the NASA DC‐8 aircraft during flights over Antarctica in September 1987. This result was obtained by averaging spectra recorded at different times, dates and locations, but may be considered a mid‐morning measurement at a solar zenith angle of 88.2 degrees from 79°S, 83°E on September 20. This result poses an important constraint on the amount of HOx inside the Antarctic winter vortex and on the contribution of the HOCl catalytic cycle to the observed springtime ozone depletion.
Magma zonation: Effects of chemical buoyancy and diffusion
Authors: Spera Frank J. , Oldenburg Curtis M. , Yuen David A.
Published: February 2008

Numerical simulations and scale analysis are used to assess the viability of the marginal box‐filling mechanism for producing compositional zonation in magma bodies. Scale analysis and two‐dimensional numerical experiments both show that box‐filling occurs provided a critical ratio of compositional to thermal buoyancy (Rρcrit) is exceeded. This critical ratio depends on the ratio of thermal to chemical diffusivity (Le). For box‐filling to occur in the simplified dike‐like model system, Rρ > 2/9 Le2/3. Application of this result to magma bodies suggests that box‐filling may occur for components with relatively high chemical diffusivities such as water. However, box‐filling will not produce significant zonation for components with small chemical diffusivities such as silica, unless diffusive coupling increases silica diffusivity.
On the anomalous splitting of Earth's free oscillations
Authors: Mochizuki Eiji , Ohminato Takao
Published: February 2008

The spectrum of Earth's free oscillations is split not only by the rotation and ellipticity but by unknown structure within the Earth. The anomalous splitting is a phenomenon that the splitting is larger than predicted by the rotation and ellipticity. Although the cause of the anomalous splitting has been uncertain, Woodhouse et al. [1986] have proposed an interpretation in terms of the anisotropy in the inner core. In this letter, we examine an alternative interpretation in terms of the rigidity in the outer core.