bluuebunny/crossref_metadata_2025 huggingface dataset:
A dynamic model of Venus's gravity field
Authors: Kiefer Walter S. , Richards Mark A. , Hager Bradford H. , Bills Bruce G.
Published: February 2008

Unlike on Earth, long‐wavelength gravity anomalies and topography correlate well on Venus. Venus's admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either Airy or Pratt isostasy but is consistent with dynamic support from mantle convection. A model using whole mantle flow and a high viscosity near‐surface layer overlying a constant viscosity mantle reproduces this admittance curve. On Earth, the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle. These viscosity estimates may be biased by neglect of lateral variations in mantle viscosity. The different effective viscosity profiles for Earth and Venus may reflect their convective styles, with tectonism and mantle heat transport dominated by hot plumes on Venus and by subducted slabs on Earth.
Characteristics of regional stresses in Alaska and neighboring areas
Authors: Biswas N. N. , Aki K. , Pulpan H. , Tytgat G.
Published: February 2008

Earthquake focal mechanisms for a selected group of crustal earthquakes located in Alaska and neighboring areas have been compiled from various sources and regional characteristics of the horizontal stress have been deduced for the region from the mechanism solutions. The results show that the compressional stress regime of southern coastal areas continues inland through central Alaska and extends in a northeast direction. The western part of Alaska, including Chukotka and the Bering Sea adjoining Alaska and the Aleutian Island chain constitute a tensional stress regime.
Calculated occultation profiles of Io and the hot spots
Authors: McEwen Alfred S. , Soderblom Laurence A. , Matson Dennis L. , Johnson Torrence V. , Lunine Jonathan I.
Published: February 2008

Occultations of Io by other Galilean satellites in 1985 provide a means to locate volcanic hot spots and to model their temperatures. We have computed the expected time variations in the integral reflected and emitted radiation of the occultations as a function of wavelength (visual to 8.7µm). The best current ephemerides were used to calculate the geometry of each event as viewed from Earth. Visual reflectances were modeled from global mosaics of Io. Thermal emission from the hot spots was calculated from Voyager 1 IRIS observations and, for regions unobserved by IRIS, from a model based on the distribution of low‐albedo features. The occultations may help determine (1) the location and temperature distribution of Loki; (2) the source(s) of excess emission in the region from long 50° to 200°; and (3) the distribution of small, high‐temperature sources.
Plasma fluctuations and large‐scale mixing near comet Giacobini‐Zinner
Authors: Baker D. N. , Feldman W. C. , Gary S. P. , McComas D. J. , Middleditch J.
Published: February 2008

Low‐pass filtered data from the ICE electron spectrometer are used to examine relationships between plasma densities, flow speeds, flow directions, and temperatures at distances within ∼105 km of the Comet Giacobini‐Zinner nucleus. The sheath/transition region is characterized by electron temperatures of ∼4 × 105K but with flow speeds and densities that commonly vary by factors of three or more. The largest amplitude density spikes often have significant flow changes associated with them, but a consistent pattern (as might accompany cometary rays, for example) is not found. Power spectral analyses in and near the sheath/transition regions show that density fluctuation levels are enhanced at all detectable frequencies from ∼0 to 21 mHz, consistent with strong density fluctuations on all measured time scales. Marginally significant power peaks are seen at periods of ∼4 min on the inbound pass and at ∼2 min on the outbound pass. Mechanisms such as amplification of convected ion pick‐up waves and cometary rays for producing the large plasma variations are discussed. It is also suggested that a Rayleigh‐Taylor driven mixing mechanism at a mass loading boundary ∼105 km from the nucleus may be operative.
The interaction of heavy ions from comet P/Giacobini‐Zinner with the solar wind
Authors: Sanderson T. R. , Wenzel K‐P. , Daly P. , Cowley S. W. H. , Hynds R. J. , Smith E. J. , Bame S. J. , Zwickl R. D.
Published: February 2008

The encounter between the ICE spacecraft and Comet P/Giacobini‐Zinner was characterised in the solar wind by intense fluxes of heavy ions, measurable over a region 6 × 106 km in extent. The ions are observed with highly anisotropic angular distributions, steep energy spectra, and a change in the energy spectrum at around 80 keV, consistent with a composition predominantly of the water group. Flux versus time profiles follow a general fall off with increasing distance from the comet but with a marked inbound/outbound asymmetry. This asymmetry is due to the higher solar wind velocity on the outbound pass, giving rise to an increased energy gain of the pick‐up ions. The flux versus time profiles are strongly modulated by the rapid changes in the direction of interplanetary magnetic field. We present correlated observations of energetic ions, the interplanetary magnetic field and the solar wind, and compare these observations with theoretical predictions of the ion pick‐up process.
Basalt magma interaction with harzburgite and the formation of high‐magnesium andesites
Authors: Fisk Martin R.
Published: February 2008

The interaction of basalt melt with mantle harzburgite at low pressure might result in silica enrichment of the melt by assimilation of orthopyroxene. Experimental tests of this hypothesis show that silica‐rich liquids (56 wt %) are produced by melt‐orthopyroxene reaction at 1200° to 1250°C. These silica‐rich liquids are enriched in Na2O and K2O over that found in the starting basalt melt which indicates that the alkalis diffused against a concentration gradient into the silica‐rich liquid. The liquids produced in this way are chemically similar to high‐magnesium andesites or boninites.
Moment‐tensor spectra of the 19 Sept 85 and 21 Sept 85 Michoacan, Mexico, earthquakes
Authors: Riedesel Mark A. , Jordan Thomas H. , Sheehan Anne F. , Silver Paul G.
Published: February 2008

IDA records of the 19 Sept 85 Michoacan earthquake and its large aftershock of 21 Sept 85 have been used to estimate source‐mechanism spectra
and total‐moment spectra MT(ω) in 1‐mHz bands over the frequency interval 1‐11 mHz. Source mechanisms, obtained by the phase‐equalization method of Riedesel and Jordan, show no significant frequency dependence and no significant non‐double‐couple components. The best average double‐couple solution has a strike, dip and rake of 289° ± 4°, 19° ± 15° and 76° ± 5° for the main event, and 293° ± 3°, 24° ± 12° and 73° ± 4° for the aftershock. Total‐moment spectra, obtained by the power‐spectral method of Silver and Jordan, are parametrized by a total integrated moment MT0 and a characteristic time τc. For the main event, we obtain MT0 = (10.7 ± 2.0) × 1020 N−m and τc = 49 ± 7 s; the corresponding parameters for the aftersnock are MT0 = (2.6 ± 0.6) × 1020 N−m and τc = 30 ± 11 s. Both events are relatively slow; in particular, the aftershock has a larger characteristic time than either the fast‐rupturing 29 Nov 78 Oaxaca or the 14 Mar 79 Petalan earthquakes, although the static moments are about the same. Our results support the model of the main shock as a multiple‐rupture event with moment release occuring as long as 100 s after the initial break.
The convection‐dominated magnetosphere of Uranus
Authors: Vasyliunas Vytenis M.
Published: February 2008

In the magnetosphere of a planet which has its rotation axis aligned with the solar wind flow direction and its magnetic dipole axis significantly tilted to its rotation axis, solar‐wind‐driven magnetospheric convection can transport plasma throughout the magnetosphere; although corotation is present, it does not produce a closed flow region (normally identified with the plasmasphere) and thus does not Impede penetration of convection into the inner magnetosphere. The difference from the conventional theory of plasmasphere formation lies in the fact that, with the assumed geometry, the spatial pattern of magnetospheric convection is steady in the corotating rather than in the fixed frame of reference. Uranus at the present epoch closely approximates to this geometry, and hence magnetospheric convection is expected to be the dominant transport process in the Uranian magnetosphere, leading to transport times comparable to the estimated flow times of 30‐40 hours.
Photodissociation of metastable O2 (a¹Δg): Implications for stratospheric O3
Authors: Dalgarno A. , McElroy M. B.
Published: February 2008

Cross sections for photodissociation of O2 (a¹Δg) are presented as functions of wavelength from 1900 to 3000 Å Contrary to an earlier suggestion, it is shown that the process is unimportant as a source of stratospheric odd oxygen.
Application of 36/40Ar Versus 39Ar/40Ar Correlation diagrams to the 40Ar/39Ar spectra of phlogopites from Southern African kimberlites
Authors: Phillips D. , Onstott T. C.
Published: February 2008

40Ar/39Ar incremental heating analyses were performed on phlogopite mica from the DK‐7 kimberlite pipe in southern Botswana. No age plateau was obtained, as the spectrum yielded a step‐wise increase in age with increasing temperature. The ages appear to be anomalously high, suggesting the presence of excess radiogenic argon components that have been partially released by diffusion during cooling of the kimberlite.

The pattern of data points on an 39Ar/40Ar vs. 39Ar/40Ar correlation plot, however, delineates two types of excess argon, one which is radiogenic‐rich, and one with a 40Ar/36Ar ratio of 366. The latter component may reflect the contamination of the kimberlite xenocrysts by fluid originating from an undegassed mantle source.
A mechanism for tectonic deformation on Venus
Authors: Phillips Roger J.
Published: February 2008

In the absence of identifiable physiographic features directly associated with plate tectonics, alternate mechanisms are sought for the intense tectonic deformation observed in radar images of Venus. One possible mechanism is direct coupling into an elastic lithosphere of the stresses associated with convective flow in the interior. Spectral Green's function solutions have been obtained for stresses in an elastic lithosphere overlying a Newtonian interior with an exponential depth dependence of viscosity, and a specified surface‐density distribution driving the flow. At long wavelengths and for a rigid elastic/fluid boundary condition, horizontal normal stresses in the elastic lid are controlled by the vertical shear stress gradient and are directly proportional to the depth of the density disturbance in the underlying fluid. The depth and strength of density anomalies in the venusian interior inferred by analyses of long wavelength gravity data suggest that stresses in excess of 100 MPa would be generated in a 10 km thick elastic lid unless a low viscosity channel occurring beneath the lid or a positive viscosity gradient uncouples the flow stresses. The great apparent depth of compensation of topographic features argues against this, however, thus supporting the importance of the coupling mechanism. If there is no elastic lid, stresses will also be very high near the surface, providing also that the viscosity gradient is negative.
Interannual variability of total ozone during the breakdown of the Antarctic circumpolar vortex
Authors: Bowman Kenneth P.
Published: February 2008

Data from the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus 7 satellite are compared with Dobson spectrophotometer measurements from the Antarctic, where a large decrease in October total ozone has been observed over the last decade. It has been hypothesized that the decrease is caused by local photochemical effects of man‐made chlorofluorocarbons. The TOMS data, which start in 1978, agree well with the Dobson data, confirming the low values of recent years. However, during the southern hemisphere spring total ozone in the Antarctic is strongly affected by transport associated with the breakdown of the stratospheric vortex. The strength and timing of the vortex breakdown are highly variable and can have a large effect on monthly‐mean total ozone values. The TOMS data suggest that variations in the stratospheric circulation have contributed to the long‐term decrease. Large variations in total ozone are not confined to the Antarctic. Zonal‐mean ozone values have also decreased in the northern hemisphere spring during the TOMS observing period.
total ozone variations at Syowa, Antactica
Authors: Chubachi Shigeru , Kajiwara Ryoichi
Published: February 2008

Monthly mean total ozone data from Syowa (69°S, 40°E), Antarctica, for 1961‐1985 show ozone to have been decreasing in October months, particularly after about 1980. Data For 1974‐1985 exhibit total ozone decrease during September throughout March months, with particularly large decrease occurring October and November months. (Observation were not made during April through August months.)
Global ozone minima in the historical record
Authors: Rood Richard B.
Published: February 2008

The magnitude and structure of the global total ozone minimum between 1958 and 1962 is similar to that observed between 1979 and 1983. Analysis of the single station data that exhibit the most pronounced minima suggest that the spatial structure of the global minimum is different from the currently observed reduction. Very low north polar values were observed, but there is no indication of anomalously low ozone in Antarctica. The temporal relationship to the sun spot cycle is similar in both time periods. Rather than solar terrestrial interaction, however, a more likely explanation of the early 1960's reduction is normal climatology caused by a persistent period of planetary wave activity. Such a natural explanation may also be appropriate for the current depletion.
On the relationship between the thermal structure of the stratosphere and the seasonal distribution of ozone
Authors: Tung Ka Kit
Published: February 2008

An attempt is made to put the phenomenon of Antarctic ozone hole in a global perspective by pointing out that there is a much larger and more persistent "hole" over the tropics. The thermal structure of the Antarctic lower stratosphere in winter is more similar to the tropical lower stratosphere than the Arctic lower stratosphere in winter. The dynamical transport that is responsible for creating the two column ozone minima has similar relationship to the thermal structure in the lower stratosphere in the two regions.
Antarctic ozone decreases: A dynamical cause?
Authors: Mahlman J. D. , Fels S. B.
Published: February 2008

A hypothesis is advanced that natural dynamical processes might explain much of the observed late winter ozone decreases over Antarctica. For this to be the case, sometime after 1979 there must have been a substantial reduction of the wintertime planetary‐scale disturbance activity in the Southern Hemisphere troposphere. The expected stratospheric response to such a natural process is to reduce wintertime polar ozone, prolong the life of the polar vortex, reduce the transport of ozone out of the middle stratosphere, and to increase the possibility of polar rising motion shortly after the return of the sun to high latitudes. All of these effects are in qualitative agreement with the observed ozone changes.
Scales of heterogeneity near the core‐mantle‐boundary
Authors: Lavely Eugene M. , Forsyth Donald W. , Friedemann Phyllis
Published: February 2008

We have mapped shear velocity variations near the base of the mantle with ScS‐S differential travel times. Beneath the North Atlantic, there is a systematic variation in velocity over a distance of 1500 km or more, with a pronounced low velocity anomaly centered roughly beneath Madeira island. Beneath the northern Indian Ocean, there is no similar long‐wavelength anomaly. The average ScS‐S residual beneath the Indian Ocean is 3.6 ± 0 .7 s more negative than the Atlantic residual, indicating faster average velocities near the base of the mantle.
Roughness of natural fault surfaces
Authors: Power W. L. , Tullis T. E. , Brown S. R. , Boitnott G. N. , Scholz C. H.
Published: February 2008

The roughness of fault surfaces is important in the mechanics of fault slip and could play a role in determining whether sliding occurs via earthquakes or fault creep. We have made preliminary measurements of the power spectral density of several fault surfaces over the wavelength range from 10−5 to 1 m, using field and laboratory scale profilimeters. The fault surfaces are strongly anisotropic; profiles parallel to the slip direction have amplitudes about one order of magnitude lower than those perpendicular to the slip direction over most of the wavelength interval measured. Fault roughness perpendicular to the slip direction is similar to the roughness of natural joints; the greater smoothness in the slip direction presumably represents wear resulting from slip. Combining our data with previous measurements of the roughness spectrum of the San Andreas fault shows that fault surfaces are fractal over nearly eleven orders of magnitude in wavelength.
Calculation of elasticity and high pressure instabilities in corundum and stishovite with the Potential Induced Breathing Model
Authors: Cohen Ronald E.
Published: February 2008

Direct calculation of elasticity in the Potential Induced Breathing (PIB) model is developed. This allows calculation of elastic constants and acoustic velocities of oxides and silicates without any experimental data other than the values of fundamental constants, such as Planck's constant. The PIB model allows for spherical charge relaxation in response to the long‐range electrostatic potential. This feature is not present in any other lattice dynamical model, and leads to many‐body forces that break the Cauchy conditions for centrosymmetric crystals (i.e. C44=C12) without the introduction of explicit angle bending forces. The elastic properties are calculated here for corundum and stishovite. High pressure elastic instabilities are found for both corundum (α‐Al2O3) and stishovite (SiO2). Softening of C44 for corundum is calculated to begin at approximately 120 GPa. This may require reconsideration of the ruby fluorescence pressure scale and recent ultra‐high pressure diamond cell experiments. The high pressure instability in stishovite leads to a low pressure dependence for the aggregate shear wave velocity.
Comment on “Heavy ozone in the stratosphere”
Authors: Anderson Stuart M. , Kaye Jack A.
Published: February 2008

It has recently been suggested (Bates [1986]) that the rate constant for the O + O2 recombination reaction to form ozone should be surprisingly dependent on the isotopic composition of the reactants and products through their symmetry numbers. The argument neglected the fact that isotope exchange and recombination reactions share a common energized collision complex, which can decay by dissociation to products which are isotopically distinct from the original reactants. Proper accounting for this additional exit channel in isotopically labeled O3* eliminates this apparent "symmetry number" effect.
Comment on Letter “On the excitation of the Earth's polar motion”
Authors: Hide Raymond
Published: February 2008

The use of modern global meteorological data in the investigation of the extent to which atmospheric processes contribute to the observed short‐term and other changes in polar motion (and length of the day) and the determination, by subtraction, of changes in the Earth's rotation of non‐meteorological origin was initiated several years ago. It is unfortunate that Chao (1985), in a confused article reviewing some of that work, has succeeded in misrepresenting not only its scope and objectives but also the methods used, principal findings and other aspects, thereby confusing the recent literature on fluctuations of the Earth's rotation.
Heat flow and basement radioactivity in Maine: First‐order results and preliminary interpretations
Authors: Decker Edward R.
Published: February 2008

New heat flow values (Q) for 70 boreholes in Maine are in the range 33‐83 mWm−2. New radioactive heat productions (A) for 45 sites range from 0.08 to 8.74 µWm−3. The combined data for plutonic rocks determine least‐squares lines with slopes and intercepts in the ranges 5.0‐6.5 km and 35‐39 mWm−2, respectively. These lines provide continued support for linear relations between Q and A in continental terranes, and confirm that normal reduced flux is the characteristic geothermal parameter of all of the New York‐New England region. Gravity, seismic and thermal modeling suggest that complex distributions of pluton thicknesses and near‐surface radioactivity occur in Maine. Thus, a single Q‐A line for this region is difficult to explain with simple distributions of vertically uniform or exponentially decreasing crustal radiogenic heat in some areas. Normal reduced flux in Maine implies that the lithosphere has equilibrated since thermotectonic events in Paleozoic and Jurassic times. Moho temperature changes associated with 8‐10 km crustal thickness changes in Maine and central New Hampshire are not likely to exceed 150°C, and effects on mantle properties would be small.
Mafic xenoliths from I‐ and S‐type granitoids: Evidence for variations in deep crustal radioactive heat production
Authors: Sawka Wayne N. , Chappell Bruce W.
Published: February 2008

Mafic xenoliths from S‐type granitoids in the Lachlan Fold Belt (LFB), Australia, have radioactive heat productions which may exceed those of the host pluton. Mafic xenoliths from the LFB I‐type granitoids contain low to moderate radioactive heat productions compared to the host plutons. These LFB granitoid mafic xenoliths indicate that large variations in radioactive heat production could occur beneath the plutons in this area. In the Sierra Nevada batholith (SNB), California, two I‐type granitoid groups occur. These are, granitoids having inherently low radioactive heat productions in the western SNB and granitoids having inherently high radioactive heat productions in the eastern SNB. Mafic xenoliths from the eastern SNB granitoids contain radioactive heat productions that in some specimens exceed that found in the western SNB granitoids. Therefore, the low radioactive heat productions of the western SNB granitoids can not be used to constrain heat production deep beneath the eastern SNB.
Non‐double‐couple earthquake mechanisms in the Nazca Plate Subduction Zone
Authors: Kubas Agnieszka , Sipkin Stuart A.
Published: February 2008

Earthquakes that occur within the Nazca plate subduction zone tend to fall into two populations: the generally smaller, nearly pure double‐couple events, and the usually larger, non‐double‐couple earthquakes. In fact, there appears to exist a direct relationship between the proportion of the non‐double‐couple component in the earthquake solution and the scalar moment. A similar trend is not observed on the global scale. Furthermore, most of the earthquakes with large non‐double‐couple components occur in the locations where the greatest amount of structural complexity is expected. We therefore hypothesize that the non‐double‐couple mechanisms in this region are, in general, related to the break‐down of the assumption of slip on a planar fault‐surface, one of the fundamental assumptions in obtaining moment tensor source mechanisms in accord with the double‐couple model. This failure is, in turn, related to the extreme structural complexity in this region.
A model for the harmonic of compressional Pc 5 waves
Authors: Takahashi K. , Zanetti L. J. , Potemra T. A. , Acuña M. H.
Published: February 2008

Compressional Pc 5 magnetic waves in the magnetosphere are a unique phenomenon showing a nonsinusoidal waveform in spite of a well‐defined period. Although the waveform can be Fourier‐decomposed into the fundamental and the second harmonics, the phase between the two is kept constant from event to event, implying that we cannot regard the waveform as the result of a chance superposition of two magnetospheric eigenmodes. We offer a phenomenological explanation to this waveform using a field‐line configuration model that is a modified version of a previously proposed antisymmetric standing wave. In this model, the location of the equatorial node of field‐line displacement is assumed to oscillate with the wave, with a peak‐to‐peak amplitude greater than 10% of the wavelength of the standing wave. The predicted waveform at various magnetic latitudes is found to be in excellent agreement with an observation taken near the magnetic equator by the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer spacecraft.
A turbulent mechanism for substorm onset in the Earth's magnetotail
Authors: Esarey Eric , Molvig Kim
Published: February 2008

Turbulence within the plasma sheet of the magnetotail is shown to destabilize the tearing mode even for cases where the equilibrium magnetic field has a normal component within the sheet. In the absence of turbulence, a finite normal magnetic field suppresses the electron wave‐particle resonance and stabilizes the tearing mode. Turbulence leads to spatial diffusion of the electron orbits, thus allowing for resonance between the electrons and the tearing perturbation. This provides the dissipation necessary for instability and hence, magnetic field line reconnection. Furthermore, by accounting for the finite extent of the tearing perturbation in the cross‐tail direction, the existence of a threshold for instability is derived. This threshold predicts instability for sufficiently small values of the normal magnetic field. Hence, substorm onset proceeds when the turbulent plasma sheet becomes sufficiently elongated.
Rocket observation of the NII 2143 A emission in an aurora
Authors: Siskind David E. , Barth Charles A.
Published: February 2008

The NII 2143 A doublet has been observed in the aurora with a rocket spectrometer. An altitude profile of the NII doublet has been obtained and compared with the profiles of the N2 Lyman‐Birge‐Hopfield (LBH) bands and N2+ 3914 A. At high altitudes the ratio of the NII 2143 A emission to LBH = 0.022 ± 0.004, and the ratio of NII 2143 A to N2+ 3914 A = 0.095 ± 0.04. From these ratios we find that the peak excitation cross section of N+(5S) equals 1 ‐ 2 × 10−18 cm². At low altitudes the NII doublet is quenched by molecular nitrogen. The deduced value of the quenching coefficient equals 5.5 × 10−10 cm³ s−1.
The radiation impedance of an electrodynamic tether with end connectors
Authors: Hastings Daniel E. , Wang J.
Published: February 2008

Electrodynamic tethers are wires deployed across the earth's geomagnetic field through which a current is flowing. The radiation impedance of a tether with end connectors carrying an AC current is computed from classical antenna theory. This simulates the use of a tether on a space structure. It is shown that the current flow pattern at the tether connector is critical to determining the overall radiation impedance. If the tether makes direct electrical contact with the ionosphere then radiation impedances of the order of several thousand Ohms can be expected. If the only electrical contact is through the end connectors then the impedance is only a few Ohms for a DC current rising to several tens of Ohms for an AC current with frequencies in the whistler range.
Freezing rate determination by the isotopic composition of the ice
Authors: Souchez Roland , Tison Jean‐Louis , Jouzel Jean
Published: February 2008

The distribution of an isotopic species, HDO (deuterium) or H218O (oxygen), in ice formed by the migration of a well‐defined freezing front in water is dependent on the freezing rate. Development of a box diffusion model combined with the boundary layer concept leads to a possibility of prediction of the freezing rate in nature by the determination of the isotopic composition of the ice in δD or δ18O. The isotopic distribution in ice formed during experiments conducted with variable freezing rates gives results in accordance with the model. Investigation on lake ice formed under well‐known climatic conditions provides a further test of the model.
An estimate of equatorial gravity from terrestrial and satellite data
Authors: Rapp Richard H.
Published: February 2008

Equatorial gravity has been estimated from terrestrial gravity data and from satellite implied data. The terrestrial estimate was based on the analysis of 43271 1°×1° mean free air gravity anomalies. The value estimated was 978032.35±0.5 mgals. This value may be slightly (0 to 1.5 mgals) too low because of the neglect of the downward continuation of surface free air anomalies to the geoid. An alternate estimate based on the comparison of terrestrial anomalies and altimeter derived anomalies yields 978032.6 mgals. Equatorial gravity from space data depends on GM from Lageos and lunar laser data and on a new estimate (6378136.2±0.5 m) of the equatorial radius based on Doppler tracking station positions and a new spherical harmonic expansion of the earth's gravity field to degree 360. The resultant equatorial gravity is 978032.78±0.2 mgals. The difference between the two terrestrial estimates is 0.4 and 0.2 mgals (or 0.4 ppm or 0.2 ppm). The agreement is within the accuracy of the two separate estimates. These comparisons can put constraints on parameters of non‐Newtonian force models.
Volcanism and massive sulfide formation at a sedimented spreading center, Escanaba Trough, Gorda Ridge, northeast Pacific Ocean
Authors: Morton Janet L. , Holmes Mark L. , Koski Randolph A.
Published: February 2008

Seismic‐reflection profiles over the sediment‐filled Escanaba Trough at the southern Gorda Ridge reveal a series of volcanic centers that pierce the sediment. The volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough. The volcanic intrusions have uplifted the sediment as much as 100 m, and in places fresh basaltic flows are exposed at the sea floor. Unoxidized pyrrhotite‐rich massive‐sulfide samples were obtained from the flanks of one of these volcanic centers. The composition and form of the sulfide samples suggest significant interaction between hydrothermal fluids and sediment at depth, and deposition of sulfide within the sediment pile.
A possible relationship between interannual variability in Antarctic ozone and the quasi‐biennial oscillation
Authors: Garcia Rolando R. , Solomon Susan
Published: February 2008

Extra‐tropical temperatures, geopotential heights, and total ozone abundances are known to vary with the tropical Quasi‐Biennial Oscillation (QBO), although the details of the causal mechanisms linking the two regions are still a subject of debate. It is shown that total ozone amounts observed in Antarctica during 1979‐1986 exhibit year‐to‐year fluctuations that may be due to the influence of the QBO, with relatively low (high) values occurring during the westerly (easterly) phase. This suggests that the QBO should be taken into account when evaluating the amplitude and continuation of the long‐term trend in Antarctic total ozone. In particular, if the tropical QBO is in its westerly phase during October, 1987, an increase in total ozone at that time may be inconsistent with the continuation of the secular decrease observed since the 1970’s.
Isotopic fractionation in ozone decomposition
Authors: Bhattacharya S. K. , Thiemens Mark H.
Published: February 2008

The oxygen isotopic fractionation, which occurs during ozone decomposition, has been determined, complementing earlier studies on the fractionation during ozone formation. Oxygen produced in the decomposition is depleted in heavy isotopes in a mass‐independent fashion. The mechanism for the mass‐independent effect is unknown, though the fractionation magnitude information is required for modelling the steady‐state isotopic composition of stratospheric ozone.
Electrostatic forces in planetary rings
Authors: Goertz C. K. , Linhua‐Shan  , Havnes O.
Published: February 2008

We calculate the average charge on a particle in a particle‐plasma cloud, the plasma potential inside the cloud and the Coulomb force acting on the particle. The net repulsive electrostatic force on a particle depends on the plasma density, temperature, density of particles, particle size and the gradient of the particle density. In a uniformly dense ring the electrostatic repulsion is zero. We also show that the electrostatic force acts like a pressure force and that even a collision‐less ring can be stable against gravitational collapse and a finite ring thickness does not necessarily imply a finite velocity dispersion. In this paper we derive a simple criterion for the importance of electrostatic forces in planetary rings which involves the calculation of the vertical ring thicknes which would result if only electrostatic repulsion were responsible for the finite ring thickness. Electrostatic forces are entirely negligible in the main rings of Saturn and the E and G rings. They may also be negligible in the F ring. However, the Uranian rings and especially Jupiter's ring seem to be very much influenced by electrostatic repulsion. In fact electrostatic forces could support a Jovian ring which is an order of magnitude more dense than observed.
A model for solar constant secular changes
Authors: Schatten Kenneth H.
Published: February 2008

Although the direct influences of sunspots upon the solar constant leads to short‐term decreases, an opposite nearly in phase 11 year variation in the solar constant can be modelled. This opposite behavior results primarily from global faculae (polar, network, and active region). The model attributes the observed variations in the solar constant entirely to magnetic features in the solar atmosphere. Hence, the model does not require any indirect effect of the magnetic cycle on the brightness of non‐magnetic areas of the Sun. The model uses the well studied, active region influences upon the solar constant, together with the influences of global faculae, associated with polar and general solar magnetism, studied by Sheeley. Using the present model, together with the sunspot prediction of Schatten and Sofia (1987), we predict the average solar constant to the year 1997, normalized to the mean Active Cavity Radiometer (ACRIM) and Earth Radiation Budget (ERB) observations. The model also provides a relation ΔSO=0.007 ΔRm max, for the very long term solar constant variations due to the secular changes in solar activity. We obtain a change of ∼1 W/m² for the difference between the late twentieth century solar constant and the 17th century solar constant. This supports Eddy's view that this difference could give rise to the glacial increase during the little ice age of the 17th century.
Seismic rupture and arc segmentation within the Shumagin Islands Seismic Gap, Alaska
Authors: Boyd Thomas M. , Taber J. John , Lerner‐Lam Arthur L. , Beavan John
Published: February 2008

The locations of teleseismically well‐recorded earthquakes suggest that up to 80% of the Shumagin Gap has ruptured seismically during the last 90 years. An earthquake in 1917 (Ms=7.9) nucleated near the gap's eastern edge and appears to have ruptured as much as a 200 km long section of the gap, and an event in 1948 (Ms=7.5) appears to have ruptured the deepest portion of the main thrust‐zone (> 30 km) in the central section of the gap. The rupture extents of these earthquakes suggest that the gap can be divided into two or three smaller arc segments. These observations do not preclude the possibility of failure of the entire region in a single great earthquake (Mw=8.5) in the near future. The two historically documented recurrence intervals for the eastern portion of the gap are 59 and 70 years. If the average recurrence interval for large earthquakes near the Shumagin Islands is 65 years, the conditional probability of a large earthquake rupturing this portion of the arc in the next 20 years is between 62 and 96%.
Dome craters on Ganymede
Authors: Moore Jeffrey M. , Malin Michael C.
Published: February 2008

Voyager observations of Ganymede show broad, high‐albedo, topographic domes situated within the central pits of some impact craters, referred to in this study as “dome craters”. Of 56 dome craters identified on Ganymede, all but two can be placed into one of two classes, based on the ratio of dome diameter to crater rim diameter. Two new hypotheses for the origin of the domes involving diapirism as an agent of dome formation are offered. Implicit in both hypotheses are possible regional heat flux variations. Under these scenarios, “relaxation” of crater relief may not be homogeneous in space and/or time, and crater morphology may not be a consistent indicator of crater age. Plutonic intrusions within the upper lithospheres of Ganymede (and Callisto) may have played a far more important role in heat transport on these satellites than previously noted.
Magnetic flux transfer at the magnetopause based on single X line bursty reconnection
Authors: Scholer Manfred
Published: February 2008

A model of flux transfer events (FTEs) at the dayside magnetopause is proposed, which is based on non‐stationary reconnection along a single X line over a large longitudinal segment. An individual field line coming from the magnetospheric side constitutes after reconnection a loop‐like structure in the magnetopause current layer and leaves into the magnetosheath. In the presence of a By component of the magnetic field in the transition region of the magnetopause each loop is twisted in the y direction. Whereas traditionally FTEs are considered to be localized flux tubes with a helical field inside, in the present model field lines come from the magnetospheric side over a large longitudinal segment, have a loop‐like structure in the magnetopause resulting in the FTE signatures, and leave over the same segment in longitude into interplanetary space.
A study of multiple X line reconnection at the dayside magnetopause
Authors: Shi Y. , Wu C. C. , Lee L. C.
Published: February 2008

Interaction between the interplanetary magnetic field and the earth's geomagnetic field at the dayside magnetopause is studied by a global two‐dimensional incompressible MHD simulation. It is found that for a large magnetic Reynolds number (Rm ≥400), the multiple X line reconnection (MXR) becomes the major dayside reconnection process, while for a small magnetic Reynolds number (Rm ≤100), the classical single X line reconnection takes place. Since the magnetic Reynolds number at the dayside magnetopause is probably very high (Rm >10³), it may be suggested that MXR will dominate the dayside reconnection pattern. As a result of the MXR process, magnetic islands are formed on the dayside magnetopause surface. The dependence of the dayside reconnection geometry on the solar wind Alfvén Mach number is also examined.
Global surface air temperatures: Update through 1987
Authors: Hansen James , Lebedeff Sergej
Published: February 2008

Data from meteorological stations show that surface air temperatures in the 1980s are the warmest in the history of instrumental records. The four warmest years on record are all in the 1980s, with the warmest years in our analysis being 1981 and 1987. The rate of warming between the mid 1960s and the present is higher than that which occurred in the previous period of rapid warming between the 1880s and 1940.
The physical isolation and identification of carriers of geologically stable remanent magnetization: paleomagnetic and rock magnetic microanalysis and electron microscopy
Authors: Geissman John Wm. , Harlan Steve S. , Brearley Adrian J.
Published: February 2008

Paleomagnetic and rock magnetic microanalysis of fully oriented microspecimens provides a more direct understanding of the source and origin of geologically stable remanent magnetizations. Scanning transmission electron microscopy allows direct observation and characterization of magnetic phases. We provide a brief history of micromagnetic research, describe a functional microspecimen preparation method, evaluate instrumentation, and present results of three combined micromagnetic/STEM studies.
Temperature dependence of magnetic domains in magnetite crystals
Authors: Heider Franz , Halgedahl Susan L. , Dunlop David J.
Published: February 2008

Synthetically grown and natural multidomain magnetite crystals (d ≥ 30 µm) have strongly temperature‐dependent magnetic domain structures. The unblocking of domain walls at moderate temperatures (T < 120°C) makes these magnetite grains unsuitable as paleomagnetic recorders. Approximately 15% of the smaller hydrothermally grown magnetite crystals (d ≤ 10 µm) show stable domain patterns when heated to 350°C. These magnetites with temperature‐independent domain structures can possibly account for paleo‐magnetic remanences with high blocking temperatures.
Displacements and tilts from dip‐slip faults and magma chambers beneath irregular surface topography
Authors: McTigue D. F. , Segall P.
Published: February 2008

Surface displacements and tilts due to buried deformation sources are influenced by topography. The leading‐order corrections due to topography of arbitrary profile, but small slope, are determined for dip‐slip faults (edge dislocations) and magma bodies (lines of inflation) in a two‐dimensional elastic medium. The vertical displacement correction is simply the product of the topography and the horizontal normal strain due to the source in a flat half‐space. The correction for a normal fault beneath an idealized basin‐and‐range topography is a slight increase in the uplift on the range side. The effect of topography for a line of inflation beneath a symmetric volcano is to reduce the central uplift. Failure to account for topographic influences can bias estimates of source depth and geometry.
The depth of seismic faulting and the upper transition from stable to unstable slip regimes
Authors: Marone Chris , Scholz C. H.
Published: February 2008

A number of observations indicate that an upper stability transition occurs along well‐developed faults, such as the San Andreas, as a result of unconsolidated gouge within shallow regions of these faults. These observations include the depth distribution of seismicity along faults with and without well‐developed gouge zones, correlations between seismicity and shallow crustal structure, and modeling of coseismic and post‐seismic slip. In addition, recent experimental friction studies indicate that thick layers of simulated gouge exhibit a positive slip‐rate dependence of frictional resistance (velocity strengthening) and thus inherently stable slip, whereas bare rock surfaces and thin gouge layers exhibit potentially unstable velocity weakening behavior. Subduction zones with large accretionary wedges also exhibit an upper stability transition in that slip is aseismic within the accretionary wedge. A stability transition due to the presence of unconsolidated material can also be invoked in this case.
Remote westerly wind forcing of the eastern equatorial Pacific; Some model results
Authors: Harrison D. E. , Giese Benjamin S.
Published: February 2008

An ocean circulation model is used to examine modes of eastern Pacific ocean response to an episode of surface westerly winds in the western Pacific. This episode resembles the May 1986 tropical cyclone pair event. The ocean changes resulting from this episode are isolated by subtracting the circulation that would otherwise have been present. First and second mode baroclinic Kelvin pulses have comparable surface eastward flow, and cause local warming when the surface temperature gradient is negative. The Kelvin pulses also introduce modulations of the eastern Pacific instability waves, which initially increase the mid‐ocean warming and then damp it out. South American coastal warming is substantial, but occurs many months after the wind event because it results primarily from the second baroclinic mode response. Recent analyses of the ocean response to the cyclone event have concentrated on the first mode Kelvin pulse response; this work indicates that only a very incomplete view of the response is possible from this perspective.
Laboratory studies of the nitric acid trihydrate: Implications for the south polar stratosphere
Authors: Hanson David , Mauersberger Konrad
Published: February 2008

Vapor pressures of HNO3 and H2O have been measured over the trihydrate crystal, formed by vapor deposit on a glass surface. In the temperature range 190 K to 205 K the two phase‐equilibrium trihydrate/vapor was studied by adding and removing H2O. Coexistence equilibria vapor pressures of trihydrate/solid solutions of HNO3 in ice (‘ice’) and of mono‐/trihydrate were also measured. Results show that for typical mixing ratios of H2O and HNO3 found in the lower stratosphere (3 ppm H2O, 5 ppb HNO3) the trihydrate would start to form at temperatures about 7 K higher than the ice point. The pressure of atmospheric HNO3 would rapidly decrease as the atmosphere cools without large changes in partial pressures of H2O. These laboratory results provide information on the formation of polar stratospheric clouds containing H2O and HNO3.
Comparative morphology of the vertical ozone profile in the Antarctic spring
Authors: Gardiner Brian G.
Published: February 2008

The form and development of the vertical distribution of ozone are examined during the Antarctic winter and spring by comparing profiles selected from a series of balloon‐borne ozonesonde ascents which were launched from Halley Bay in 1987. The progress of the ozone depletion is followed from its onset in late August until the deep minimum in mid‐October, when nearly all the ozone has been destroyed between 90 and 50 mbar. The presence of structure in the unperturbed profile is discussed, and it is shown that individual profiles are susceptible to the effects of vertical motion and wind shear. After the spring equinox, the vertical distribution of ozone consistently displays a minimum in mixing ratio, in contradiction to theories which purport to explain the spring development by transport alone. The depletion process is restricted to a finite vertical range of some 12 km, with a sharp upper boundary at about 20 mbar.
Lithostratigraphy determined from discriminant analysis of geochemical well logs from the Cajon Pass Scientific Drillhole, California
Authors: Anderson Roger N. , Broglia Cristina , Pezard Philippe A. , Williams Colin F.
Published: February 2008

Geochemical well logs recorded in the Cajon Pass Scientific drillhole provide an estimate of the elemental concentrations of Si, Al, Fe, Ca, K, Th, Ti, S, and the HREE Gd. The continuous geochemical profiles were used to define a lithostratigraphy in the cystalline basement from 501 to 1,829 m. The interval between the faulted top of basement and a thrust fault at 689 m consists of six alternating slivers of granite and granodiorite. Interlayered granitic and granodioritic units were then encountered to another fault at 1290 m, whereas gneisses of granitic to tonalitic composition were logged in the basal portion of the hole. Linear discriminant analysis of major element chemistry, as well as variations in Gd, Th, Ti, and S, were used to demonstrate that each of the major lithostratigraphic units is chemically distinct. Different trace element signatures and variable degrees of metamorphism indicate a complex thermal and hydrological history for the basement rocks encountered in the Cajon Pass well.
Stress orientation inferred from shear wave splitting in basement rock at Cajon Pass
Authors: Li Y. ‐G. , Leary P. C. , Henyey T. L.
Published: February 2008

S‐waves generated by mode conversion of P‐waves from surface compressional sources and recorded by oriented geophones at depths between 1300 and 1820 meters in the Cajon Pass scientific drillhole arrive at higher apparent velocities when polarized along the direction N70°E than when polarized in the orthogonal direction. The observed polarization dependent velocities are consistent with seismic anisotropy due to in situ fractures or cracks that are preferen tially aligned N70°E (subnormal to the San Andreas fault). This result is consistent with the maximum principal horizontal stress direction at Cajon Pass also being subnormal to the San Andreas fault.