bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy dissipation rates at ridge and transform fault segments—A model of oblique spreading—
Authors: Tanaka Akiko , Fujii Naoyuki
Published: February 2008

We present an energy dissipation model that can easily be applied to oblique transform faults as well as to oblique spreading ridges. By using a simple model of a rising incompressible Newtonian fluid in a vertical conduit of the ridge‐transform fault system, the energy dissipation rates of the ridge and transform fault segments are calculated as a function of the angles between the strikes of segments and the spreading directions, and the spreading rates. The stability is estimated for about 400 present‐day segments from the energy dissipation rates of ridge and transform fault segments. It is concluded that most of the present‐day configurations of the ridge‐transform fault system are nearly stable, even though the strikes of ridge and transform fault segments are oblique to the spreading directions. Several exceptions seem to be associated with extraordinary phenomena, such as the occurrence of an extremely large earthquake and overlapping spreading centers.
Geodetic evidence for aseismic reverse creep across the Teton Fault, Teton Range, Wyoming
Authors: Sylvester Arthur Gibbs , Byrd John Odard Dutton , Smith Robert Baer
Published: February 2008

The valley block (hanging wall) of the central segment of the Teton fault rose 8±0.7mm during 1988 and 1989, relative to the mountain block west of the fault, a displacement opposite to that expected on a normal fault. The height change is based on first‐order leveling data over a 21.2km‐long fault‐crossing line of 42 permanent bench marks established and initially surveyed in 1988 and resurveyed in 1989. The rapid height change took place across a 1200m‐wide zone coincident with the steep escarpment at the base of the range front including the surface trace of the east‐dipping Teton fault, a major, active, range‐front normal fault bounding the east side of the Teton Range at the northeastern edge of the Basin and Range province. The total stratigraphic offset across the fault, as much as 9km, accumulated over the last 7 to 9 million years. Quaternary fault scarps, up to 52m in height, cut Pinedale (about 14,000 yr) glacial and younger fluvial‐alluvial deposits, indicating that the Teton fault has been the locus of several large, scarp‐forming earthquakes in the past 14,000 years, and it exhibits up to 25m of latest Quaternary displacement where crossed by the level line. Although the relative uplift of the hanging wall maybe local and unique to the Teton fault, this unexpected observation of aseismic, reverse creep may have a variety of tectonic and nontectonic causes, including hydrologic effects, aseismic fault creep or tilt, and pre‐seismic dilation.
Internal consistency of the Tsyganenko Magnetic Field Model and the Heppner‐Maynard Empirical Model of the ionospheric electric field distribution
Authors: Donovan E. F. , Rostoker G.
Published: February 2008

We use the Tsyganenko [1987; 1989] magnetic field models to map the electric field distribution of Heppner and Maynard [1987] into the geomagnetic tail and infer the velocity of the CPS convective flow. We obtain qualitatively different convection patterns from the two Tsyganenko models. Results obtained with the 1987 model predict a narrow jet of earthward moving plasma in the center of the CPS, while the 1989 model predicts relatively uniform earthward convection throughout the CPS. Based on in situ observations of CPS convection, we conclude that the 1989 model provides the most realistic mapping. We also suggest that the effects of field‐aligned currents should be incorporated into the magnetic field models to improve the mapping.
Directional site resonances and the influence of near‐surface geology on ground motion
Authors: Bonamassa Ornella , Vidale John E. , Houston Heidi , Schwartz Susan Y.
Published: February 2008

We examine the horizontal motions at close stations from earthquakes in the Loma Prieta and Whittier Narrows sequences to study the shear wave polarizations. We use a dense, six station array recording 10 aftershocks for the former, and use two events and 11 stations across the Los Angeles area for the latter.

We compute the average azimuth of strongest shaking in the shear wave as a function of frequency from 1 to 18 Hz for each record of each earthquake. The direction of shaking at a given frequency often correlates much better with an empirical site resonance direction than with the direction of shaking expected from the focal mechanism of the earthquake. The effect tends to be greatest at the frequencies that are the most amplified. This phenomenon can complicate determination of the earthquake source at frequencies higher than 1 Hz.

Further, since sites only 25 meters apart show different preferred directions, very near‐surface geology is probably responsible. Estimation of directional site resonances may prove useful for seismic design.
Effects of collisions on the nonlinear particle dynamics in the magnetotail
Authors: Holland Daniel L. , Chen James
Published: February 2008

The effects of collisional processes on the nonlinear particle dynamics in the magnetotail are considered. A simple collision operator is developed to model the effects of pitch‐angle and energy scattering. It is found that the phase space partition persists for up to moderate scattering amplitudes in pitch‐angle and energy, and that certain distribution function features are robust even in the presence of large amplitude collisions. It is shown that if the collisions are due to short scale length electrostatic fields, excessively large field amplitudes are required to significantly alter the phase space structures and the resulting distribution function features.
Inductive attenuation of the transpolar voltage
Authors: Sanchez E. R. , Siscoe G. L. , Meng C.‐I.
Published: February 2008

The circuit that couples the magnetospheric boundary to the ionosphere has a significant self inductance which, in combination with ionospheric resistivity, gives the circuit an appreciable inductive inertia. The inductive inertia attenuates the amplitude of fluctuation of the transpolar ionospheric voltage relative to fluctuations of the transpolar boundary voltage, which drives the ciruit. We adopt a simple electrical circuit analog of the tailward moving part of magnetospheric convection and combine it with statistical representations of the solar wind EMF to obtain a quantitative estimate for the average voltage attenuation at the ionospheric level. We find that in an average three hours interval—the period range in which most magnetospheric phenomena directly related to the transpolar voltage lie—fluctuation amplitudes in the ionosphere reach only about 50% of their boundary values.
Photochemical regulation of CO on Mars
Authors: Chassefière E.
Published: February 2008

The balance between the production and loss rates of CO in the Martian atmosphere is examined in the photochemical frame defined by Parkinson and Hunten (1972). A first important point is that, for any given value of the CO abundance, the balance of CO is roughly reached in a short lapse of time through balances of short lived O, O3 and OH. From this basic fact, we suggest a quite interesting characteristic of the photochemistry of carbon compounds on Mars, implicit in earlier models, but not yet discussed in detail. Because the production of OH is the rate‐limiting step in oxidation of CO, there is in first approximation no feedback process regulating CO at any particular level. A more refined analysis shows that feedbacks exist, although there are weak. Damping processes are identified and their efficiency estimated in a rather general frame. A corollary is the intrinsic instability of the equilibrium abundance of CO, which is very sensitive to fluctuations of its production and loss rates. Nevertheless, because of the long life time of CO, temporal variations (seasonal, solar cycles) are expected to be weak (≈ ±10%). Large latitudinal gradients predicted in static conditions are likely smoothed by dynamical mixing. The contribution of the present ideas to the understanding of the recent Earth‐based and space observations of the Martian CO is finally evaluated. Some new directions of research are proposed.
Aeromagnetic anomalies and discordant lineations beneath the Niger Delta: Implication for new fracture zones and multiple sea‐floor spreading directions in the “meso‐Atlantic” Gulf of Guinea cul‐de‐sac
Authors: Babalola Olufemi Oladapo , Gipson Mack
Published: February 2008

An aeromagnetic contour map compiled over shallow water and onshore portions of the Nigerian continental margin, shows several elongate, long‐wavelength anomaly closures with some alternating polarity, separated by steep gradient, NE lineations. The lineations are interpreted as new fracture zones or extensions of previously mapped ones.

The NE trend in the western delta region is concordant with Ee fracture zone trends of the deeper Gulf of Guinea. Aeromagnetic lineations of the SE Niger Delta Basin however, discordantly trend ENE. Their termination against the former, is interpreted as evidence of early sea‐floor spreading in a ENE‐WSW direction in addition to the well documented NE‐SW spreading of the Gulf of Guinea and the rest of the “meso‐Atlantic” sea‐floor.

The geophysical crustal structure indicate the existence of two Early Cretaceous triple junctions beneath the Niger Delta Basin. The two triple‐junctions further support the hypothesis that the African continent was a multi‐plate system (in the Niger Delta region) during the early opening of the Atlantic.
Effects of depth‐dependent thermal expansivity on mantle circulations and lateral thermal anomalies
Authors: Hansen Ulrich , Yuen David A. , Kroening Sherri E.
Published: February 2008

The influences of depth‐dependent thermal expansivity on the structure of Newtonian thermal convection and lateral thermal anomalies are studied. In an aspect‐ratio ten box the effects of the depth‐dependent expansivity are to produce large aspect‐ratio cells with a few relatively stationary upwellings. The lateral thermal anomalies in the lower mantle are greatly diminished by depth‐dependent expansivity. This together with a decrease in the temperature derivative of seismic velocities in the lower mantle would support the low level of lateral heterogeneities found in recent tomographic studies.
Laboratory simulation of Ionospheric Heating Experiment
Authors: Cros Brigitte , Godiot Jacques , Matthieussent Gilles , Héron Anne
Published: February 2008

Resonant absorption of an electromagnetic wave (f=3.5 GHz) in a multipolar discharge (ne ≃ 1011cm−3) with an adjustable density gradient is studied. Around the resonant location, nonlinear effects appear, associated with the ponderomotive force arising from the plasma wave. These effects are modelled by the Zakharov equations. Experimental and numerical results both exhibit a transition from a nonlinear steady state regime toward chaos when the amplitude of the electromagnetic wave or the gradient length is increased.
Crustal deformation of the Yellowstone Caldera from first GPS measurements: 1987–1989
Authors: Meertens Charles M. , Smith Robert B.
Published: February 2008

A Global Positioning System (GPS) network was established in 1987 in order to measure crustal deformation across the Yellowstone Plateau (centered on the Yellowstone caldera) and adjacent Teton, Wyoming, and Hebgen Lake, Montana, fault zones. Forty‐four GPS stations in the Yellow‐stone network were re‐surveyed in 1989 and station coordinates using broadcast orbits were obtained. These results were used to assess the precision and accuracy of the GPS surveys and to identify areas of rapid crustal deformation on the Yellowstone Plateau. The GPS measured displacements between 1987 and 1989 were calculated relative to a station at the eastern edge of the 100 km‐wide network. The relative horizontal displacements have an average magnitude of 15±10 mm. The relative vertical displacements are generally less than 25 mm about an average near zero and with an uncertainty of ±20 mm. Comparisons with leveling and trilateration measurements, taken nearly simultaneously, suggest that the accuracy of these GPS measurements is at the level of the above uncertainties. GPS‐measured vertical displacements across the northern part of the Yellowstone caldera and 1st‐order leveling results show up to 20 mm of subsidence between 1987 and 1989. This subsidence is part of a six year period of caldera deflation that began in 1985. In addition, up to 66 mm of uplift is observed with GPS at the northwest edge of the Yellowstone caldera and may be associated with processes following an intense earthquake swarm in 1985–86 and with caldera subsidence.
Long wavelength topography, seafloor subsidence and flattening
Authors: Cazenave Anny , Lago Bernard
Published: February 2008

Seafloor subsidence effects due to cooling of the oceanic lithosphere have been removed from bathymetry data. The corrected ocean floor topography presents long wavelength highs, in particular over western Pacific. We show in this study that this long wavelength residual topography can be interpreted as either a term of seafloor flattening at old ages or a dynamic response to large‐scale convection. Whatever its origin, this long wavelength residual topography is dominated by a degree 2 pattern highly correlated with geoid, lower mantle heterogeneities and plate age.
On the source time function of the October 13, 1963 Kurile Islands Earthquake
Authors: Mocquet Antoine , Fukao Yoshio
Published: February 2008

The temporal evolution of the source mechanism is retrieved for the October 13, 1963 Kurile islands earthquake by applying centroid moment tensor inversions to successively different narrow frequency windows between 2 and 12.5mHz. The lateral heterogeneity effect is corrected using model M84A of Woodhouse and Dziewonski [1984]. To the first approximation the frequency‐dependent scalar moment follows an ω−2 decay law and the origin time shift is frequency independent, which can be simply explained by an isoscele source time function 140s long and a total moment release of 62 × 1027 dyne.cm. The expansion of the moment rate functions in terms of spline functions enables to retrieve a smooth temporal change of the moment tensor. This second inversion yields a total moment of 71 × 1027 dyne.cm with a peak activity at about 70s after the onset in a total source time of 250s. The mechanism has remained unchanged during the main stage. The occurence of back‐slip to the amount of 6.5% of the total moment is implied for the last 80s.
Chaos driven by kinetic Alfven waves
Authors: Liu W. W.
Published: February 2008

We present a simple model describing the effect of a kinetic Alfvén wave on the motion of a test electron along a closed magnetic field line. We show that the electron motion can be strongly chaotic for a wide range of conditions. The chaos exhibits a broad‐band power spectrum in the frequency domain, although only three principal frequencies are involved in the model. Phase space density of electrons exhibits complex but physically interesting evolution in response to the KAW. Typical of a chaotic situation, the scale of the electron distribution in phase space descends rapidly over the scale hierarchy to give rise to a growing set of irregularities, which may in turn generate strong electrostatic turbulences. This chain of events suggests a fundamental mechanism that couples physical processes of different scales.
Velocities and compressibilities of komatiitic melts
Authors: Secco Rick A. , Manghnani Murli H. , Liu Teh‐ching
Published: February 2008

Ultrasonic measurements of compressional wave velocities for six komatiitic melts containing 13–30 wt% MgO are reported in the frequency range 3–20 MHz and in the temperature range 1280–1661°C. Melts with less than about 20 wt% MgO have velocities that are 6% lower and relaxed compressibilities, βr, that are 15% higher than melts with more than about 20 wt% MgO. The general trend of increasing velocity with increasing MgO content correlates well with increasing depolymerization (decreasing molar volume) of the melts as reflected in the NBO/T, (CaO + MgO)/Al2O3, and Al/(Al+Si) ratios. The value of dβr/dw (w=wt% MgO) for the high MgO melts is more than four times greater than dβr/dw for the low MgO melts.
Non thermal nitrogen atoms in the Earth's thermosphere 2. A source of nitric oxide
Authors: Gérard J.‐C. , Shematovich V. I. , Bisikalo D. V.
Published: February 2008

The role of the non maxwellian nitrogen atoms in the odd nitrogen thermospheric chemistry is investigated with a numerical model. This one‐dimensional model solves the continuity equation, including molecular and turbulent transport for the coupled NO‐N(4S) system. It is shown that the very fast reaction between hot N(4S) atoms and O2 provides an additional source of nitric oxide. At equatorial latitudes, this contribution amounts to 6–30% of the other classical production sources of NO for solar minimum activity conditions. The cross section for elastic collisions with ambient gas introduces the largest uncertainty on this estimate. It is concluded that, although it does not drastically alter the conclusions of previous models of NO, the effect of hot N atoms must be considered in future models of the NO‐N system chemistry.
Picked‐up protons near Mars: Phobos observations
Authors: Barabash S. , Dubinin E. , Pissarenko N. , Lundin R. , Russell C. T.
Published: February 2008

The measurements carried out by the plasma spectrometer, ASPERA, onboard the PHOBOS‐2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes ≤7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the MAGMA magnetometer. Analysis of the plasma data shows that weak pitch‐angle diffusion of a ring‐distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived.
Laboratory measurements of direct ozone loss on ice and doped‐ice surfaces
Authors: Dlugokencky Edward J. , Ravishankara A. R.
Published: February 2008

Laboratory measurements were carried out to test for the possible direct loss of ozone on ice surfaces and solid solutions of nitric acid, sulfuric acid, and sodium sulfite over the temperature range 195 to 262 K. Experiments were performed in a vertical flow tube where the reactor wall was coated with the desired surface. Results indicate that loss of ozone on an ice surface is insignificant. A discussion of the possible importance of these results in the stratosphere is given.
Weak, quasiparallel profiles of Earth's bow shock: A comparison between numerical simulations and ISEE 3 observations on the far flank
Authors: Greenstadt E. W. , Coroniti F. V. , Moses S. L. , Tsurutani B. T. , Omidi N. , Quest K. B. , Krauss‐Varban D.
Published: February 2008

Over 200 crossings of the distant downwind flanks of Earth's magnetosonic bow shock by ISEE 3 included many cases of weak, or low Mach number, quasiparallel shocks. A consistent feature of the magnetic field profiles was the presence of large amplitude, near periodic to irregular transverse oscillations downstream from even the weakest Q∥ shocks. Large downstream perturbations with whistler‐like features similar to those of the observations appear in one‐dimensional simulations when MA > 2.5 but not when MA = 2.1. The observed cases with downstream waves also occurred when MA > ≈ 2.5, suggesting the importance of the Alfvén as opposed to magnetosonic Mach number in determining the signature of weak, Q∥ shocks.
Source parameters of the Sierra Madre Earthquake from regional and local body waves
Authors: Dreger Douglas , Helmberger Donald
Published: February 2008

We inverted the three‐component, long‐period data recorded by the TERRAscope array for the June 28, 1991 Sierra Madre, California event to determine the seismic moment and source orientation of the mainshock (ML=5.8). Remarkably four of the six stations were located 159.2±0.7 km from the epicenter. Variations in absolute traveltime were smaller than 2% and 3% for Pn and Sn, whereas the surface waves displayed greater variation. Similarities in the waveforms and the small variance in traveltime suggest that a common Green's function can be used to invert the data. We used the whole waveforms excluding the fundamental surface waves at the more distant stations to invert for source parameters. The results of the inversion indicated that this event was predominately a thrust type earthquake, where the strike, rake and dip were determined to be 235°, 74° and 50°, respectively. The seismic moment was determined to be 2.5 × 1024 dyne‐cm. The source duration was found to be about 1.0 seconds by direct measurement of the S‐wave recorded at the Pasadena station (located 21 km from the epicenter). Considering circular rupture we obtain a stress drop of 460 bars.
A scale‐invariant cellular‐automata model for distribited seismicity
Authors: Barriere Benoit , Turcotte Donald L.
Published: February 2008

In the standard cellular‐automata model for a fault an element of stress is randomly added to a grid of boxes until a box has four elements, these are then redistributed to the adjacent boxes on the grid. This redistribution may result in one or more of these boxes having four or more elements in which case further redistributions are required. On the average added elements are lost from the edges of the grid. We have modified this model so that the boxes have a scale‐invariant distribution of sizes. The objective is to model a scale‐invariant distribution of fault sizes. When a redistribution from a box occurs it is equivalent to a characteristic earthquake on the fault. A redistribution from a small box (a foreshock) may trigger an instability in a large box (the main shock). A redistribution from a large box always triggers many instabilities in the smaller boxes (aftershocks). The frequency‐size statistics for both main shocks and aftershocks satisfy the Gutenberg‐Richter relation with b = 0.835 for main shocks and b = 0.635 for aftershocks. Model foreshocks occur 28% of the time.
Alphonsus Crater: Floor fracture and dark‐mantle deposit distribution from new 3.0‐cm radar images
Authors: Zisk Stanley H. , Campbell Bruce C. , Pettengill Gordon H. , Brockelman Richard
Published: February 2008

The lunar crater Alphonsus is characterized by numerous fractures or graben, and by endogenic dark‐halo craters. Existing maps of fractures from analysis of lunar photography may be biased by the east‐west solar illumination. This paper presents new high‐resolution, dual‐polarization 3.0‐cm wavelength radar images of Alphonsus with radar illumination from northerly directions, and uses these data to better map the locations of both the graben and a variety of dark‐mantle deposits. The distribution of fractures, and several graben which cut the crater floor and central ridge, are cited as possible evidence for simultaneous, post‐Imbrium uplift of both structures. Some of the endogenic dark halo deposits are more extensive in depolarized radar images than in photographs; these extensions are attributed in some cases to more distant emplacement of pyroclastic material, and in others to fortuitous connections with smoother, less cratered portions of the Alphonsus floor.
Observations of the lunar sodium exosphere
Authors: Potter A. E. , Morgan T. H.
Published: February 2008

Measurements of sodium resonance emission showed that the sodium exosphere of the Moon extends up to 1500 km. altitude above the equatorial limb. The line‐of‐sight sodium atom densities at various altitudes were derived from the measured sodium emission intensities, using a calibration based on brightness of the lunar limb. The data were fit to a simple two‐parameter model of the exosphere. Above the equatorial limb, this yielded apparent temperatures of 950–1150°K., and surface densities of sodium atoms of 29–46 atoms/cm³. Above the north pole, adjacent to the polar terminator, the temperature was much less, 450°K., and the surface density rose to 65 atoms/cm³. Sodium emission could not be detected above the equatorial terminator, indicating a decrease to levels below about 200 Rayleighs, difficult to distinguish from the sodium nightglow. Solar wind impact on the surface does not seem to be an important factor, because the sodium density changed very little when the Moon was shielded from the direct solar wind by the Earth's magnetotail. This suggests that solar photon radiation plays a major role in maintenance of high kinetic temperatures in the sodium exosphere.
Thermodynamic models of the chemistry of lunar volcanic gases
Authors: Fegley Bruce
Published: February 2008

Thermodynamic models and mass balance arguments are used to constrain the chemistry of lunar volcanic gases. The results predict that lunar gases were dominated by reduced C and S gases such as CO, COS, CS2, S2. The more oxidized gases CO2 and SO2 were also important, but only in limited temperature ranges. Gases such as Cl2, CCl4, and CF4 were more abundant than HF and HCl, which were the two major H compounds in the lunar gases. Chlorides and fluorides were important species for transporting many volatile and ore‐forming metals, and the implications for fractionating and concentrating metals into lunar ‘ore‐deposits’ merit further study.
Convection planforms in a fluid with a temperature‐dependent viscosity beneath a stress‐free upper boundary
Authors: Weinstein Stuart A. , Christensen Ulrich
Published: February 2008

This study presents the results of a joint numerical and laboratory study of 3‐D planforms of time‐dependent convection in a Newtonian fluid layer with a temperature‐dependent viscosity. Planforms are found for the case in which the fluid is bounded from above by a stress‐free boundary and below by a rigid boundary and compared to the planforms found when the fluid layer is bounded by rigid boundaries. All of the laboratory and numerical experiments were performed with nearly the same values of the Rayleigh number Ra1/2 (1 × 105) and viscosity ratio µr (50), and viscosity law to allow direct comparisons. When the fluid layer is bounded by rigid boundaries the observed planforms are the spoke patterns, which are an interconnected network of descending and ascending sheets. However, when the upper boundary is stress‐free, the morphology of the downwellings change from spokes to a dendritic network of descending sheets and the wavelength of the planform is more than 3 times as large as the spoke pattern planform. In both cases the numerical and laboratory experiments are in good agreement.
Surface scattering properties estimated from modeling ASAS multiple emission angle reflectance data over the Lunar Crater Volcanic Field, Nevada
Authors: Guinness Edward A. , Arvidson Raymond E. , Irons James R. , Harding David J.
Published: February 2008

ASAS multispectral data were acquired over Lunar Lake playa at 12 different lighting and viewing geometries. Atmospheric models and field‐based data demonstrate that atmospheric contributions to ASAS data were relatively small (<15%) and were thus ignored in analyses of surface photometric properties. ASAS data of an artificially roughened playa and cobble‐strewn surfaces exhibit trends of increasing reflectance as phase angle decreases. The reflectance of a natural playa surface exhibits Lambertian scattering properties that cannot be replicated with the Hapke [1986] photometric function. The Hapke model does fit the rough and cobble‐strewn playa data with single scattering albedo, asymmetry factor, and roughness terms consistent with known properties of these surfaces. No published photometric model explains the reflectance variations of all three surfaces.
Coseismic changes of subsurface gas compositions disclosed by an improved seismo‐geochemical system
Authors: Nagamine Koichiro , Sugisaki Ryuichi
Published: February 2008

An automatic system for seismo‐geochemical observation has been improved to monitor small changes in subsurface gas composition. This system is computer‐based and consists of an on‐site monitoring unit for gas analyses and a telemetry unit for transmitting the observation data to our laboratory. The analytical data with this system showed three coseismic anomalies in spring gas compositions at a monitoring station for these 11 months. These anomalies were associated with a far (and large) earthquake or two small (and near) ones, respectively. The gas anomaly as a spike‐like signal appears only for a short period and can be detected on the basis of temporally dense data provided by the new system. The result shows that the behavior of subsurface gases is sensitive to crustal movements.
The bliss measurements of NO2: Some new insights
Authors: Lary D. J. , Pyle J. A. , Webster C. R. , May R. D.
Published: February 2008

The diurnal variation of NO2 measured near 30 km by the BLISS in‐situ laser spectrometer is compared with calculations from a photochemical model that includes a detailed description of multiple scattering.
The tectonic evolution of Western Ishtar Terra, Venus
Authors: Lenardic A. , Kaula W. M. , Bindschadler D. L.
Published: February 2008

The origin and evolution of Ishtar Terra is modeled using numerical simulations of crust/mantle interaction on Venus. Based on our modeling, we favor a two phase evolutionary sequence for Ishtar. The first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling. The second phase, a result of the thermal perturbation exerted on the mantle by the region of thickened crust, is driven by the sinking of a lithospheric slab. This slab‐driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block. Throughout its evolution Ishtar remains coupled to flow within the deep mantle, due to the lack of an asthenosphere, and ultimately bulk mantle flow comes to dominate as the effects of the slab‐driven pulse wane.
Rapid Post‐Miocene tectonic rotation associated with the San Gregorio Fault Zone in central California
Authors: Holm Eric J. , Horns Daniel M. , Verosub Kenneth L.
Published: February 2008

Paleomagnetic measurements of samples from the Mio‐Pliocene Purisima Formation demonstrate that the Pomponio tectonic block of central coastal California has rotated clockwise by approximately 35° to 55° within the last 2.5 million years. The most likely interpretation of this data is that the Pomponio block is broken into several small blocks which have rotated by various amounts. The data suggest that rotations contribute to vertical deformation and secondary faulting within the central San Andreas Fault System, and that they play an important role in the accommodation of shear along the fault system.
Borehole fluid chemistry of DSDP Holes 395A and 534A results from Operation Dianaut
Authors: Gieskes Joris M. , Magenheim Andrew J.
Published: February 2008

Fluid samples were obtained in two Deep Sea Drilling Project boreholes, during wireline re‐entry logging of Operation DIANAUT in the Atlantic Ocean. Chemical compositions of borehole fluids in Hole 395A suggest the presence of bottom water in the entire borehole, with no evidence of interaction with the borehole wall or of exchange with formation waters. Fluids from Hole 534A show compositional changes relative to seawater, suggesting an input of fluid from the formation into the borehole. The borehole fluid data, in combination with data on the chemistry of the pore fluids of the sediments at this site, suggest the presence of an aquifer at a depth below 700 mbsf, the depth of a blockage of the hole.
Measurements and modelling of intermediate, descending, and sporadic layers in the lower ionosphere: Results and implications for global‐scale ionospheric‐thermospheric studies
Authors: Wilkinson P. J. , Szuszczewicz E. P. , Roble R. G.
Published: February 2008

We demonstrate that conventional ionosondes can provide long‐term observations of intermediate, descending, and transitional layers in the 100–200 km altitude region of the ionosphere. Using 15 consecutive days of observations at Townsville, Australia, during the SUNDIAL campaign of September 1989, we tracked the “birth” of the layers at altitudes above 150 km and their systematic downward motion to the 110 km region at rates between 4 and 5 km/hr. The observations are compared with NCAR TIGCM simulations, and the results show: (i) that the layering process is identifiable with meridional wind‐shear‐node convergence of ions; (ii) that zonal wind controls of the layers are insignificant under the prevailing conditions; and (iii) that electric fields play an important role in the effectiveness of the ion‐convergence and downward transport processes at altitudes above 125 km. The measurement and modelling comparisons are the first of a kind, providing insight into the relative roles of winds and electric fields, and opening possibilities for determining the global characteristics of the layers and their cause‐effect roles in the dynamics of the lower ionospheric‐thermospheric domain.
The relationship of periodic structures in auroral luminosity in the afternoon sector of ULF pulsations
Authors: Rostoker Gordon , Jackel Brian , Arnoldy Roger L.
Published: February 2008

Preliminary studies of Viking imager data by Lui et al, [1987] have revealed the presence, on occasion, of bright auroral structures periodically arrayed along the auroral oval in the post noon quadrant. It was suggested that these auroral forms reflect the action of a Kelvin‐Helmholtz instability. In this paper we report the results of such a study of periodically arrayed auroral structures with concurrent ULF pulsations. Assuming a common origin of the two phenomena to be the Kelvin‐Helmholtz instability, we make quantitative estimates of some key physical properties of the source region for the ULF and auroral disturbances. We further report the observation of a westward traveling surge in the vicinity of 18 MLT, and argue on the basis of the mapping factor from the ionosphere to the magnetotail why surges closer to midnight do not appear to exhibit a significant westward propagation velocity.
Potential climate impact of Mount Pinatubo eruption
Authors: Hansen James , Lacis Andrew , Ruedy Reto , Sato Makiko
Published: February 2008

We use the GISS global climate model to make a preliminary estimate of Mount Pinatubo's climate impact. Assuming the aerosol optical depth is nearly twice as great as for the 1982 El Chichon eruption, the model forecasts a dramatic but temporary break in recent global warming trends. The simulations indicate that Pinatubo occurred too late in the year to prevent 1991 from becoming one of the warmest years in instrumental records, but intense aerosol cooling is predicted to begin late in 1991 and to maximize late in 1992. The predicted cooling is sufficiently large that by mid 1992 it should even overwhelm global warming associated with an El Nino that appears to be developing, but the El Nino could shift the time of minimum global temperature into 1993. The model predicts a return to record warm levels in the later 1990s. We estimate the effect of the predicted global cooling on such practical matters as the severity of the coming Soviet winter and the dates of cherry blossoming next spring, and discuss caveats which must accompany these preliminary simulations.
Global tracking of the SO2 clouds from the June, 1991 Mount Pinatubo eruptions
Authors: Bluth Gregg J. S. , Doiron Scott D. , Schnetzler Charles C. , Krueger Arlin J. , Walter Louis S.
Published: February 2008

The explosive June 1991 eruptions of Mount Pinatubo produced the largest sulfur dioxide cloud detected by the Total Ozone Mapping Spectrometer (TOMS) during its 13 years of operation: approximately 20 million tons of SO2, predominantly from the cataclysmic June 15th eruption. The SO2 cloud observed by the TOMS encircled the Earth in about 22 days (∼21 m/s); however, during the first three days the leading edge of the SO2 cloud moved with a speed that averaged ∼35 m/s. Compared to the 1982 El Chichón eruptions, Pinatubo outgassed nearly three times the amount of SO2 during its explosive phases. The main cloud straddled the equator within the first two weeks of eruption, whereas the El Chichón cloud remained primarily in the northern hemisphere. Our measurements indicate that Mount Pinatubo has produced a much larger and perhaps longer‐lasting SO2 cloud; thus, climatic responses to the Pinatubo eruption may exceed those of El Chichón.
Calculations of the radial seismic velocity/density ratio for MgO and MgSiO3‐perovskite at high pressure
Authors: Parker S. C. , Wall A.
Published: February 2008

The experimental values of the ratio of the radial seismic velocity perturbation with respect to the radial density perturbation, (R)T, for MgO at pressures of up to about 20 GPa show a conflicting pressure dependence. However, (R)T, calculated using free energy minimisation at 1000 K and pressures of up to 160 GPa for the lower mantle minerals MgO and MgSiO3‐perovskite, is predicted to decrease with increasing pressure. Previous calculations of the elastic constants of MgO using the Potential Induced Breathing model also predict that (R)T decreases as the pressure increases.

The ratio (dlnνs/dlnνp)T for both MgO and MgSiO3−perovskite is also predicted to decrease with increasing pressure.
The time‐domain behavior of power‐law noises
Authors: Agnew Duncan Carr
Published: February 2008

The power spectra of many geophysical phenomena are well approximated by a power‐law dependence on frequency or wavenumber. I derive a simple expression for the root‐mean square variability of a process with such a spectrum over an interval of time or space. The resulting expression yields the power‐law time dependence characteristic of fractal processes, but can be generalized to give the temporal variability for more general spectral behaviors. The method is applied to spectra of crustal strain (to show what size of strain events can be detected over periods of months to seconds) and of sea level (to show the difficulty of extracting long‐term rates from short records).
Influence of spatially variable instrument networks on climatic averages
Authors: Willmott C. J. , Robeson S. M. , Feddema J. J.
Published: February 2008

Instrument networks for measuring surface air temperature (T) and precipitation (P) have varied considerably over the last century. Inadequate observing‐station locations have produced incomplete, uneven, and biased samples of the spatial variability in climate and, in turn, terrestrial and global scale averages of T and P have been biased. New high‐resolution climatologies [Legates and Willmott, 1990a; 1990b] are intensively sampled and integrated to illustrate the effects of these nontrivial sampling biases. Since station networks may not represent spatial climatic variability adequately, their ability to represent climate through time is suspect.
Stratospheric temperature increases due to Pinatubo aerosols
Authors: Labitzke K. , McCormick M. P.
Published: February 2008

Northern hemisphere stratospheric temperatures at 30 and 50 mb beginning in June 1991 are compared with 20‐year (1965–1984) and 26‐year (1964–1989) monthly means. Significant temperature increases are shown in July, August, September, and October for latitudes from approximately 30°N to the equator. In September and October, +2σ to +3σ deviations are observed for large areas between the equator and 30°N, with temperature increases as high as +3.5°C occurring at some locations. The monthly averaged zonal mean 30‐mb temperatures at 20°N in September and October were approximately 2.5°C higher than the 26‐year mean, with some daily zonal mean increases of almost 3°C. Higher values occurred equatorward of 20°N. We believe these warmings are due to absorption of radiation by the new aerosols produced from the June eruptions of the volcano Pinatubo (15.1°N, 120.4°E) in the Philippines. We also expect stratospheric warmings to be occurring simultaneously at southern latitudes, especially from the equator to about 20°S, based on satellite and lidar measurements of the locations of the new aerosol layers. These localized temperature increases should decrease in magnitude and become more global as the cloud disperses globally and spreads in altitude.
A comparison of GCM simulations of Arctic climate
Authors: Walsh John E. , Crane Robert G.
Published: February 2008

As the atmosphere, ocean, and sea ice components of global climate models are made increasingly interactive, systematic errors or biases in one component can adversely affect the other model components. The fidelity of the component interactions is especially important in the polar regions, where many atmospheric General Circulation Models (GCMs) project an amplified climatic response to increasing concentrations of greenhouse gases. In comparing the Arctic performance of five atmospheric GCMs (GFDL, GISS, NCAR, OSU, and UKMO), we illustrate key differences in the fields most relevant to sea ice/ocean forcing: surface air temperature and sea level pressure (surface wind stress). While the amplitude of the seasonal cycle of simulated air temperature is generally realistic, biases of up to 5–10°C relative to observations are apparent over much of the Arctic. The simulated sea‐level pressure pattern varies widely from model to model, and in some cases is incompatible with the observed wind‐forcing of sea ice from the Arctic Basin to the North Atlantic via Fram Strait. The implications that these differences have for transports of salinity are significant.
Amplitude anomalies of surface waves from the July 16, 1990, Philippine Islands Earthquake
Authors: Tsuboi S.
Published: February 2008

Love waves from the 1990 Philippine Islands earthquake show large amplitude anomalies at broadband seismograph stations in Japan, while Rayleigh waves do not. We calculate synthetic seismograms using the fully coupled eigensolutions obtained by a variational method to see if this anomalous behavior of surface waves is explained by the laterally heterogeneous earth model M84A of Woodhouse and Dziewonski. We show that the anomalous long‐period surface wave amplitude behavior can be reproduced relatively well by M84A.
The magnetic barrier at Venus
Authors: Zhang T. L. , Luhmann J. G. , Russell C. T.
Published: February 2008

The magnetic barrier at Venus is a region within which the magnetic pressure dominates all other pressure contributions. The barrier is formed in the inner region of the dayside magnetosheath to transfer solar wind momentum flux to the ionosphere. Passes through the dayside magnetosheath and ionopause with Pioneer Venus have allowed us to probe the magnetic barrier directly. These passes have been used to construct altitude profiles of the barrier. Here we define the ionopause as the lower boundary of the barrier. The upper boundary is defined as the altitude where the magnetosheath magnetic pressure is equal to half of the upstream solar wind dynamic pressure corrected by the boundary normal angle. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north‐south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator, which is comparable with the so‐called “mantle.” We find that the magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model is found to predict the correct bow shock location if the magnetic barrier is treated as the obstacle.
Near‐equatorial pitch angle diffusion of energetic electrons by oblique whistler waves
Authors: Villalón Elena , Burke William J.
Published: February 2008

The pitch angle scattering of trapped, energetic electrons by obliquely propagating whistler waves in the equatorial regions of the plasmasphere is investigated. Storm‐injected electrons moving along field lines near the equator interact with electromagnetic waves whose frequencies are Doppler‐shifted to some harmonic of the cyclotron frequency. The wave normals are distributed almost parallel to the geomagnetic field. Waves grow from the combined contributions of a large reservoir of energetic electrons that are driven into the loss cone by the highest‐harmonic interactions permitted to them. Relativistic, quasi‐linear theory is applied to obtain self‐consistent equations describing the temporal evolution of waves and particles over time scales which are longer than the particle bounce time and group time delay of the waves. The equilibrium solutions and their stability are studied, considering the reflection of the waves by the ionosphere and the coupling of multiple harmonic resonances. The contributions of nonlocal wave sources are also included in the theory. Numerical computations based on our theoretical analysis for regions inside the plasmasphere (L ≤ 2) and near the plasmapause (L ∼ 4.5) and for the first three harmonic resonances are presented.
Magnetic properties of the high‐latitude tail boundary: Draping of magnetosheath field lines and tail‐aligned current
Authors: Ohtani S. , Kokubun S.
Published: February 2008

Magnetic properties of the high‐latitude tail boundary are examined with IMP 8 magnetometer data. The high‐latitude tail boundary separates the tail lobe from the magnetosheath. Magnetic fields are stable in the tail lobe, but very irregular in the magnetosheath. Boundary crossings are marked by the rotation of magnetic components parallel to the boundary plane. The magnetic component normal to the boundary, if any, is very small in comparison to this rotational change. Despite large magnetic fluctuations in the magnetosheath, the magnetosheath‐side field orientation is consistent with the draping of the interplanetary magnetic field (IMF) against the magnetotail. The boundary current has a component parallel to the lobe field (tail‐aligned current), as well as a circumferential component. The IMF orientation controls the sheath‐side BY, while the lobe‐side field has a more rigid configuration flaring antisunward. Accordingly, the polarity of the tail‐aligned current is primarily determined by the sign of IMF BY; in other words, the tail‐aligned current shields the magnetotail against the IMF Y component. The current polarity is antisunward for positive IMF BY and sunward for negative IMF BY in the northern hemisphere. The tail‐aligned current is also discussed in terms of the cusp (mantle) current system.
The diurnal variation of NO, N(²D), and ions in the thermosphere: A comparison of satellite measurements to a model
Authors: Rusch D. W. , Gérard J.‐C. , Fesen C. G.
Published: February 2008

A data base derived from Atmosphere Explorer C measurements of parameters important to the understanding of the diurnal variation of odd‐nitrogen and ions in the thermosphere near equinox, at low latitudes, and for quiet geomagnetic conditions is presented. The data were taken during the period from August to September 1974, when perigee was near the equator and reached an altitude of about 135 km. The data base contains profiles of the odd‐nitrogen species NO, and N(²D); the ions NO+, O2+, O+, N2+, and N+; the total ion density; O an N2; and the neutral, ion, and electron temperatures. The measured time‐dependent variations of NO, N(²D), O2+, and NO+ compare favorably to the results of a time dependent, photochemical, diffusion model for odd‐nitrogen and ion chemistry. In particular, the model reproduces the rapid increase in NO density in the morning hours, the mid‐afternoon maximum, and the late afternoon decrease. The model also predicts the measured absolute densities for NO to within 20% over most of the diurnal cycle but falls below the measured density by as much as a factor of 2 in the late afternoon. The agreement between the data and the model indicates that the most important chemical processes for the production and loss of odd‐nitrogen in the Earth's thermosphere are included in the model.