bluuebunny/crossref_metadata_2025 huggingface dataset:
Neptune's deep atmosphere revealed
Authors: Romani Paul N. , de Pater Imke , Atreya Sushil K.
Published: February 2008

The brightness temperature of Uranus at 20 cm is 260 ± 10K, while for Neptune it is 318 ± 16K. Since NH3 is the dominant absorber at this wavelength we have modeled the microwave spectra of Neptune based upon an assumed deep gaseous mixing ratio of NH3 and subsequent loss into clouds. The difference between the two brightness temperatures implies that the NH3 mixing ratio below the level of cloud formation on Neptune compared to Uranus is lower by nearly 2 orders of magnitude. An alternative explanation is that the 20 cm radiation from Neptune is a combination of thermal plus synchrotron emission as proposed by de Pater and Goertz [1989].
Albedo patterns on Triton
Authors: Stansberry John A.
Published: February 2008

A model for seasonal transport of volatiles based on Trafton [1984] is coupled to considerations of methane photochemistry to infer albedo patterns on Triton's surface at the time of the Voyager encounter. The relatively large southern subsolar latitude at present is predicted to cause the deposition of an extensive fresh frost layer covering approximately the northern half of the planet. The region between 10° and −35° latitude is expected to be devoid of surface frosts and will probably be covered with organic compounds resulting from methane dissociative chemistry. These substances are thought to be rather low in albedo and red in color. Triton's south polar cap is expected to extend from −35° latitude to the southern pole. Sublimation of Triton's south polar ices been occurring since 1930, but has been preferentially occurring near the pole. The sublimation of ices in this region may be concentrating dark organic matter on the surface of the ice or exposing layers of this material which have built up in the ice over many seasonal cycles. The south polar cap may be distinctly darker near the pole than around the equatorward edge.
Altimetry, ship gravimetry, and the general circulation of the North Atlantic
Authors: Zlotnicki Victor , Marsh James G.
Published: February 2008

Gravity accelerations estimated from satellite altimetric mean sea surfaces (Seasat and Geos‐3) are compared to ship gravity measurements. Ship gravity are closer to an estimate based on least squares collocation, orbit perturbations, altimetry and terrestrial gravity than to an estimate based on Fourier transforms, orbit perturbations and altimetry only. Both altimetric estimates yield a smoothed picture of the geostrophic component of sea surface currents in the North Atlantic when gravity acceleration data from only nine cruises are subtracted from the altimetric gravity.
Multi‐trace deconvolution with unknown trace scale factors: Omnilinear inversion of P and S waves for source time functions
Authors: Ruff Larry J.
Published: February 2008

Seismic source and wave propagation theories allow seismologists to compute long period synthetic seismograms that commonly match even subtle details of observed waveshapes, but the overall trace amplitudes must be scaled by arbitrary factors to emphasize the excellent waveshape correspondence. This unexplained amplitude scatter causes considerable difficulties when formally inverting observed waveforms for seismic source parameters. Omnilinear inversion simultaneously determines the linear model parameters and trace scaling factors to minimize the mismatch between observed and synthetic seismograms. Omnilinear inversion is applied to source time function deconvolution from long period P and SH waves. A simulation with synthetic data shows that omnilinear inversion finds the proper scaling factors, and the source time function and focal depth are better determined than for “standard” linear inversion. Omnilinear inversion is then applied to a data set of seven P waves and one SH wave from the underthrusting earthquake of July 5, 1968 (Ms 6.6) in northern Honshu, Japan. Omnilinear inversion produces a best‐fit focal depth of 36 km with a single pulse time function of 6 s duration and seismic moment of 4×1018 N m (Mw 6.4). This focal depth supports the notion that the seismically coupled plate interface extends no deeper than 40 km in northern Honshu.
The interannual variability of polar stratospheric clouds and related parameters in Antarctica during September and October
Authors: Poole Lament R. , Solomon Susan , McCormick M. Patrick , Pitts Michael C.
Published: February 2008

Antarctic polar stratospheric cloud (PSC) sightings by the orbiting SAM II sensor during September and October show a pronounced Quasi‐Biennial Oscillation (QBO) signal, and October sightings have increased markedly over the past 10 years in years of westerly QBO phase. The QBO in PSC frequency is likely to affect the rate of Antarctic heterogeneous chemical processes and, hence, ozone depletion. Studies of the observed long‐term temperature trend suggest that the decadal PSC trend probably results from the ozone decline through its effect on stratospheric heating rates. A more detailed analysis of data from 1986 and 1987 shows that there were more PSCs in 1987 and that they persisted much later into the spring season as compared to 1986. Qualitatively similar behavior was found for the OClO column abundances and 18‐km ozone depletion observed at McMurdo Station during these 2 years. These observations suggest that both the intensity and duration of heterogeneous chemical processes are likely greater during colder, QBO‐westerly phase years.
Seismic detection of a major moho upheaval beneath the Campania volcanic area (Naples, southern Italy)
Authors: Ferrucci F. , Gaudiosi G. , Pino N. A. , Luongo G. , Hirn A. , Mirabile L.
Published: February 2008

A major perturbation of the lower crust is revealed by a DSS experiment carried out across the Campania volcanic region (southern Italy). PmP phases, recorded both at constant offsets by two fan‐arrays and along a linear profile intersecting at depth their reflection ranges, provide evidence of a limited structural high in the crust‐mantle boundary, located at the center of the 100 km wide area. Such an abrupt lateral variation in the structure of the crust may relate to the difference in volcanic products and eruptions between the two main volcanic centers of the area, Mt. Vesuvius and Campi Flegrei caldera.
Comparison of stratospheric clouds in the Antarctic and the Arctic
Authors: Hofmann D. J. , Deshler T.
Published: February 2008

Balloonborne measurements of stratospheric clouds using optical particle counters in the Arctic and the Antarctic are compared in terms of particle size distribution. There appear to be two distinct classes of particles, a small mode (r ≥ 0.2 µm) in which at least half of the available condensation nuclei have grown and a large mode (r ≈ 2–3 µm) in which fewer than 1% of the available condensation nuclei have grown. The latter particles appear in thin layers (0.1 to 0.3 km) while the former appear in relatively thicker layers (2 to 5 km). Temperatures dictate a nitric acid trihydrate composition for most of the layers and the inferred HNO3 vapor in the large particles is comparable to that available. While the small particle mode is probably related to fast cooling events such as those associated with mountain lee waves, the origin of the thin layers of large particles is not apparent.
How will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere?
Authors: Roble R. G. , Dickinson R. E.
Published: February 2008

A global average model of the coupled mesosphere, thermosphere and ionosphere is used to examine the effect of trace gas variations on the overall structure of these regions. In particular, the variations caused by CO2 and CH4 doublings and halvings from present day mixing ratios are presented. The results indicate that the mesosphre and thermosphere temperatures will cool by about 10K and 50K respectively as the CO2 and CH4 mixing ratios are doubled. These regions are heated by similar amounts when the trace gas mixing ratios are halved. Compositional redistributions also occur in association with changes in the temperature profile. The results show that global change will occur in the upper atmosphere and ionosphere as well as in the lower atmosphere during the 21st century.
First detection of a noctilucent cloud by lidar
Authors: Hansen G. , Serwazi M. , von Zahn U.
Published: February 2008

During the night of August 5/6, 1989 for the first time a noctilucent cloud (NLC) was detected and measured by a lidar instrument. The observations were made with our ground‐based narrow‐band Na lidar located at Andenes, Norway (69°N, 16°E geographic coordinates). In wavelength the lidar was operated both at the Na D2 resonance line of 589 nm as well as 5 Doppler widths shifted away. The altitude resolution was 200 m. The NLC developed at about 22:20 UT, reached its maximum backscatter cross section at 23:05 UT and became unobservable at around 00:10 UT. During this period the NLC exhibited the following properties: (a) its altitude ranged between 83.4 and 82.2 km; (b) its full width at half maximum ranged between 1.4 and 0.3 km; (c) the ratio of measured backscatter intensity from the NLC to the calculated Rayleigh signal from 82.6 km reached 450; (d) its volume backscatter cross section maximized at 6.5·10−9 m−1 sr−1.
Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties
Authors: Lowrie W.
Published: February 2008

The common ferromagnetic minerals have distinctive, characteristic coercivities and thermomagnetic properties. The analysis of the acquisition curve of isothermal remanent magnetization (IRM) is a useful but often ambiguous diagnostic technique. For a more conclusive interpretation, IRM acquisition must be combined with subsequent thermal demagnetization of the IRM. A modification of this method is proposed as a more powerful analytical technique. Different coercivity fractions of IRM are remagnetized in successively smaller fields along three orthogonal directions. The thermal demagnetization of each orthogonal component of the composite IRM is then plotted separately. This method often gives a clearer interpretation of the ferromagnetic mineral content of a rock. Examples are described for limestone and sandstone samples.
An Automatic Editing Algorithm for GPS data
Authors: Blewitt Geoffrey
Published: February 2008

An algorithm has been developed to edit automatically Global Positioning System data such that outlier deletion, cycle slip identification and correction are independent of clock instability, selective availability, receiver–satellite kinematics, and tropospheric conditions. This algorithm, called TurboEdit, operates on undifferenced, dual frequency carrier phase data, and requires (1) the use of P code pseudorange data and (2) a smoothly varying ionospheric electron content. The latter requirement can be relaxed if the analysis software incorporates ambiguity resolution techniques to estimate unresolved cycle slip parameters. TurboEdit was tested on the large data set from the CASA Uno experiment, which contained over 2500 cycle slips. Analyst intervention was required on 1% of the station–satellite passes, almost all of these problems being due to difficulties in extrapolating variations in the ionospheric delay. The algorithm is presently being adapted for real time data editing in the Rogue receiver for continuous monitoring applications.
Some tests of wet tropospheric calibration for the CASA Uno Global Positioning System Experiment
Authors: Dixon T. H. , Wolf S. Kornreich
Published: February 2008

Wet tropospheric path delay can be a major error source for Global Positioning System (GPS) geodetic experiments. We investigate strategies for minimizing this error using data from CASA Uno, the first major GPS experiment in Central and South America, where wet path delays may be both high and variable. We compared wet path delay calibration using water vapor radiometers (WVRs) and residual delay estimation, with strategies where the entire wet path delay is estimated stochastically without prior calibration, using data from a 270 km test baseline in Costa Rica. Both approaches yield centimeter‐level baseline repeatability and similar tropospheric estimates, suggesting that WVR calibration is not critical for obtaining high precision results with GPS in the CASA region.
The Role of Chlorine Chemistry in Antarctic Ozone Loss: Implications of New Kinetic Data
Authors: Rodriguez Jose M. , Ko Malcolm K. W. , Sze Nien Dak
Published: February 2008

New kinetic data yielding a slower formation rate and larger absorption cross sections of Cl2O2 are incorporated into a photochemical model to reassess the role of chlorine chemistry in accounting for the ozone reductions derived from TOMS observations in different regions of the Antarctic polar vortex during 1987. The model is further constrained by existing measurements from the Airborne Antarctic Ozone Experiment (AAOE) and the National Ozone Expedition (NOZE)‐II. Calculated concentrations of ClO based on the new kinetic data increase by almost a factor of two between the collar and core regions of the vortex during the second half of September. As a result of the higher concentrations of ClO, the calculated ozone reductions in the vortex core appear to be still consistent with the TOMS observations in spite of the slower rate for the self‐reaction of ClO. The agreement in the collar region is poorer. However, uncertainties in mixing, possible spatial inhomogeneities in both ClO and O3 t the lack of more extensive data, and the uncertainties in the rate data and observations preclude determination of whether additional processes are required to account for the observed ozone removal in the collar region.
A comparison of Arctic lower stratospheric winter temperatures for 1988–89 with temperatures since 1964
Authors: Nagatani Ronald M. , Miller Alvin J. , Gelman Melvyn E. , Newman Paul A.
Published: February 2008

Lower stratospheric temperatures during the Airborne Arctic Stratospheric Expedition are compared with temperatures available since January, 1964. January, 1989, was the coldest average January in the last 26 years at high latitude, lower stratospheric levels. There have been other months with temperatures almost as low as the level of January, 1989, and localized temperatures (e.g., minimum polar vortex temperatures) have been lower than that encountered in January 1989. February, 1989, was warmer than average and March, 1989, had some of the highest polar vortex temperatures in the last 26 years. Conditions were therefore not very favorable for Polar Stratospheric Cloud (PSC) formation into early spring.
Observed particle evolution in the polar stratospheric cloud of January 24, 1989
Authors: Dye J. E. , Gandrud B. W. , Baumgardner D. , Chan K. R. , Ferry G. V. , Loewenstein M. , Kelly K. K. , Wilson J. C.
Published: February 2008

Particle size distributions measured from the NASA ER‐2 with the new Forward Scattering Spectrometer Probe (FSSP) 300 in a type I polar stratospheric cloud (PSC) on January 24, 1989 show a volume mode near 0.8 μm diameter. The large increase in particle concentration and volume after cloud entry did not occur until the apparent saturation ratio of nitric acid with respect to nitric acid trihydrate reached 10, but at ratios near 1 subtle changes in the size distribution suggest some type I particles were present. Particle concentrations in cloud of 15 to 17 cm−3 were greater than the CN concentrations of 5 to 7 cm−3 just outside of cloud, suggesting nucleation on more than just sulfate particles. Some particles > 4 μm diameter were observed in a region which was saturated with respect to ice.
Heterogeneous conversion of COF2 to HF in polar stratospheric clouds
Authors: Wofsy Steven C. , Yatteau John H. , Salawitch Ross J. , McElroy Michael B. , Toon Geoffrey C. , Mankin William G. , Coffey Michael T.
Published: February 2008

It is argued that the reaction COF2+H2O → 2HF+CO2 should proceed on surfaces of polar stratospheric clouds (PSCs), based on laboratory evidence for this reaction in condensed phase and on analysis of column observations of HF during the Airborne Arctic Stratospheric Expedition (AASE). If the hypothesis is confirmed, observations of COF2 and HF could provide unambiguous indication of heterogeneous processing of polar air, and could help elucidate the influence of heterogeneous chemistry on concentrations of HCl, ClNO3, and chlorine oxide radicals.
A comparison of ER‐2 measurements of stratospheric water vapor between the 1987 Antarctic and 1989 Arctic airborne missions
Authors: Kelly K. K. , Tuck A. F. , Heidt L. E. , Loewenstein M. , Podolske J. R. , Strahan S. E. , Vedder J. F.
Published: February 2008

Vertical profiles of water vapor inside the Antarctic vortex have been compared with those taken outside it over Punta Arenas (53°S, 71°W). A similar exercise was performed with Arctic vortex profiles and those taken over Stavanger (59°N, 6°E). Residual water, defined as the stratospheric water vapor mixing ratio with the contribution from methane oxidation subtracted, is also shown as profiles inside and outside the vortex for both missions. The Arctic and Antarctic profiles of water vapor and residual water are compared. Locally dehydrated air was evident both inside and outside the Antarctic vortex, but such dehydration was not evident in and around the Arctic vortex. Arctic profiles of residual water are consistent with non‐tropical entry for some air.
Measurements of total reactive nitrogen during the Airborne Arctic Stratospheric Expedition
Authors: Kawa S. R. , Fahey D. W. , Anderson L. C. , Loewenstein M. , Chan K. R.
Published: February 2008

Composite distributions of measured total reactive nitrogen (NOy) from the NASA ER‐2 during the Airborne Arctic Stratospheric Expedition (AASE) are presented. The observed features of these distributions are discussed in terms of the controlling dynamical, chemical, and microphysical processes. In the latitudinal profile from 58°N to within about 4° poleward of the polar vortex boundary, NOy conforms closely to predictions of NOy based on N2O measurements. The features of the distribution are apparently dynamically controlled. Poleward of 5° of latitude within the boundary, the average NOy decreases sharply and is significantly lower than that predicted from N2O. This feature is consistent with loss of NOy through sedimentation of particles containing NOy in polar stratospheric clouds. The observed loss is not as systematic as in the Antarctic, consistent with the observed differences in season and meteorological conditions between the two campaigns.
Thermoelasticity and the formation of black smokers
Authors: Lowell Robert P.
Published: February 2008

Darcy's Law flow in a permeable medium, consisting of uniform parallel evenly spaced fractures, is used to elucidate how thermoelastic effects may modify the permeability and flow in fracture‐controlled hydrothermal systems. Some simple permeability models are then used to investigate whether black smoker venting can result from focussing of low velocity porous flow into fractures at shallow depths (≃ 100 m.). The models indicate that: (a) thermoelastic processes may be important in controlling the temporal evolution of hydrothermal upflow zones; (b) permeability structure, not just the bulk value of the permeability, may be critical for the formation of black smokers; (c) a small zone extending to a depth of ≃ 100 m containing a few fractures a factor of 2 or more wider than average may be sufficient to focus upflow into discrete vents provided thermoelastic and chemical effects seal parts of the upper crust.
Comparison of radon in soil over faulted crystalline terranes: Glaciated versus unglaciated
Authors: Gates Alexander E. , Gundersen Linda C. S. , Malizzi Lawrence D.
Published: February 2008

Radon in soil correlates directly with the bedrock geology in unglaciated terranes. In the Hylas shear zone, Virginia, the rock units exhibit a marked contrast in uranium concentration which is reflected in soil radon. The soils are clay‐rich and are derived directly from the underlying bedrock. Even small geologic features such as pegmatite veins and thin shear zones are discernable by differences in soil radon concentrations. This bedrock/soil radon correlation is obscured in areas with thick glacial covers. The Reservoir fault area, New Jersey, is proximal to a terminal moraine. Although the bedrock units exhibit a strong contrast in radioactivity and uranium concentration, soil radon concentrations in the overlying tills reflect the local chemical and physical properties of the till rather than the bedrock.
Regional and temporal relations of radon in soil gas to soil temperature and moisture
Authors: Washington John W. , Rose Arthur W.
Published: February 2008

In deep soil where diffusional loss is negligible, the Rn concentration in soil gas is controlled by Ra, emanation coefficient (E), dry bulk density (ρb), and particle density (ρP), plus the transient properties of fraction water saturation (F) and temperature because the gaseous/aqueous partition coefficient for Rn (KT ) is temperature sensitive. In a typical soil (Ra = 30 Bq/kg, E = 0.2, ρb = 1.5 g/cm3, ρP = 2.65 g/cm3), Rn in soil gas (kBq/m3 ) is calculated to range from 21 (T = 0°C, F = 0.05) to 78 (T = 25°C, F = 0.95).

At sites in Pennsylvania and elsewhere, measured soil gas Rn shows seasonal variations of 2‐ to 10‐fold. Much of this variability may be explained by temperature‐sensitive gaseous/aqueous partitioning. Since temporal soil moisture and temperature patterns vary geographically, temporal patterns in soil gas Rn vary geographically too. Mean soil gas Rn will tend to be higher in regions with warm or moist soils. Soil temperature and moisture are predictable from soil maps.
Lightning over the Gulf Stream
Authors: Biswas Kumud R. , Hobbs Peter V.
Published: February 2008

During the period 15 January–15 March 1986 the average frequency and strength of lightning flashes to the earth's surface over the Gulf Stream off the Carolina Coast was greater than that over the adjacent land.

Near the North Carolina Coast, in prefrontal situations, the mean number of flashes to the earth's surface per lightning day per grid area (1° latitude × 1° longitude) was 20.9 in the vicinity of the Gulf Stream and 4.8 over the adjacent land. In postfrontal situations the corresponding numbers were 18.4 and 8.9. Most (but not all) of the lightning flashes over the Gulf Stream conveyed negative charge to the earth's surface, and the increase in the frequency of lightning over the Gulf Stream was greater for these lightning flashes than for those that carried positive charge to the earth's surface.

The enhancement in lightning activity over the Gulf Stream is attributed to the recurrent convective rainbands that form in this region.
Magnetic anisotropy of the Trenton limestone revisited
Authors: Jackson Mike
Published: February 2008

Previous studies have shown clear differences between the anisotropies of low‐field magnetic susceptibility (LFS) and of anhysteretic remanent magnetization (ARM) in samples of the Ordovician Trenton limestone from Trenton Falls, NY. ARM was found to be significantly more anisotropic than LFS, and to exhibit a well‐defined east‐northeasterly lineation which was not apparent in anisotropy of magnetic susceptibility (AMS). Because of significant local variation in mean LFS (by approximately a factor of two) at this locality, it has been possible to reanalyze the AMS of these samples by the method of Henry [1983], which in some cases allows mathematical differentiation of the ferromagnetic and paramagnetic susceptibility tensors. The ferromagnetic susceptibility tensor obtained in this way shows the same tectonic shortening apparent in the ARM anisotropy, but surprisingly, it shows no evidence of horizontal foliation. The paramagnetic tensor is horizontally foliated, with no lineation. The difference between the ferromagnetic susceptibility and ARM tensors suggests different ferromagnetic sources of ARM and LFS.
Mantle flow tectonics and the origin of Ishtar Terra, Venus
Authors: Bindschadler D. L. , Schubert G. , Kaula W. M.
Published: February 2008

It is proposed that western Ishtar Terra formed due to compression and crustal thickening above a cylindrical mantle downwelling. A model for crustal deformation due to downwelling successfully reproduces many observed characteristics of western Ishtar. Although axisymmetric downwellings occur in numerical models of constant‐viscosity mantle convection, there is no evidence for their existence in Earth's mantle, where downwellings are sheet‐like. Either modes of downflow in Venus and Earth are fundamentally different, or differences in near‐surface conditions and material behavior selectively emphasize surface expressions of the different downwelling modes.
Western Eisila Regio, Venus: Radar properties of volcanic deposits
Authors: Campbell Bruce A. , Campbell Donald B.
Published: February 2008

1988 Arecibo Observatory dual‐polarization radar images are presented for Western Eisila Regio, Venus. The polarization information and Pioneer‐Venus Orbiter reflectivity and altimetry data are analyzed for volcanic deposits on two 400–500 km radius constructs, Sif and Gula Montes. Many of the large effusive deposits studied appear to require superposed flows or multiple vents to explain the observed progression of roughness along their length. High Fresnel reflectivity material may be present along the summit region of Gula Mons and in an embayed tessera‐like region to the N. Radar‐dark units on the flanks of Sif Mons are inferred to be pyroclastic deposits, but radar‐dark features near the summits of both edifices are more consistent with very smooth lava flows. Higher spatial resolution Magellan data will be useful in testing these predictions.
Consequences of a hydrous mantle for Venus tectonics: Predictions for Magellan
Authors: Williams David R. , Pan Vivian
Published: February 2008

We will consider the consequences of a Venus mantle enriched in water relative to the Earth. If the Moon‐forming impact hypothesis is correct, Earth may have lost substantial volatiles. Venus, however, experienced no such impact and is therefore likely to be relatively more water‐rich. We demonstrate that a higher abundance of water in the Venus mantle has the effect of inhibiting the rise of crustforming melts and volatiles to the surface, which results in a thin stable crust, a dry, stiff upper mantle, and restricted magmatism. Conduction through a shallow thermal lithosphere dominates the removal of heat from the interior, and flow in the mantle is strongly coupled to the surface deformation. Current observations of Venus are consistent with this model. A review of the consequences of this model allows us to make predictions for the Magellan radar images. These include: 1) No evidence for plate tectonic processes; 2) younger apparent surface ages and enhanced crater relaxation for highland areas; 3) limited quantity of hot‐spot volcanism, and 4) occasional evidence of localized magmatism (diatremes) associated with a volatile‐rich mande source. The model has implications to the Earth as well: in the absence of the Moon‐forming impact, crustal formation in the water‐rich Earth mantle may have been arrested, and plate recycling would have ceased early in the planet's history.
Geodetic estimate of coseismic slip during the 1989 Loma Prieta, California, Earthquake
Authors: Lisowski M. , Prescott W. H. , Savage J. C. , Johnston M. J.
Published: February 2008

Offsets in the relative positions of geodetic stations resulting from the Loma Prieta earthquake can be explained with a dislocation model that includes buried oblique slip on a rupture surface extending 37 km along the strike of the San Andreas fault, dipping 70° to the SW, and extending from a depth of about 5 to 17.5 km. Assuming uniform slip on a rectangular surface, the mean values for a range of reasonable fault geometries are 1.6 ± 0.3 m right‐lateral strike slip and 1.2 ±0.3 m reverse slip. Slip on an adjacent extension of the rupture to the southeast, recorded in the aftershock sequence, is not well constrained by the geodetic data. The geodetic data clearly preclude rupture extending near the surface.
Geodynamic aspects of the Loma Prieta Earthquake
Authors: Furlong Kevin P. , Langston Charles A.
Published: February 2008

The Loma Prieta earthquake differs from most of the observed seismicity along the San Andreas in northern California in its depth and style of rupture. The combination of crustal and thermal structure in the Loma Prieta region allowed strain accumulation to 18–20 km in the crust, leading to rupture at that depth during the event. A discrepancy in displacement direction of crust and mantle components of the Pacific plate near Loma Prieta led to a decoupling of the crust from the mantle within the ductile lower crust. This played a key part in the rupture initiation at the base of the seismogenic layer.
A laboratory study of the dependence of elastic wave velocities on pore scale fluid distribution
Authors: Knight Rosemary , Nolen‐Hoeksema Richard
Published: February 2008

Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone. Compressional wave velocity (Vp) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with the level of water saturation (Sw) during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically. When Sw was decreased, pronounced saturation‐induced hysteresis was observed in the region 03 < Sw < 0.9, with Vp measured during drainage greater than Vp measured during imbibition. Similar results were obtained for Vs versus Sw, with Vs during drainage greater that Vs during imbibition in the saturation range Sw > 0.4.

As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i.e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air‐filled or water‐filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, “crack‐like” pores, these variations in fluid distribution cause related variations in velocities.
Plasma convection in Neptune's magnetosphere
Authors: Selesnick R. S.
Published: February 2008

The magnetosphere of Neptune changes its magnetic configuration continuously as the planet rotates, leading to a strong modulation of the convection electric field. Even though the corotation speed is considerably larger, the modulation causes the small convection speed to have a cumulative effect, much like the acceleration of particles in a cyclotron. A model calculation shows that plasma on one side of the planet convects out of the magnetosphere in a few planetary rotations, while on the other side it convects slowly planetward. The observation of nitrogen ions from a Triton plasma torus may provide a test of the model.
Aurora and airglow on the night side of Neptune
Authors: Sandel B. R. , Herbert F. , Dessler A. J. , Hill T. W.
Published: February 2008

We have examined the latitude‐longitude distribution of emissions detected by the Voyager ultraviolet spectrometer on the dark hemisphere of Neptune, The emissions have two significant geographic features: (1) a broad peak near longitude 60° W that extends rather uniformly over the entire range of observed latitudes (55° S to 50° N); and (2) a brighter, narrower peak near the south and 240° W. We interpret the first peak as due to excitation of the night side atmosphere by photoelectrons from the magnetically conjugate, sunlit atmosphere. The second peak can plausibly be attributed to a southern aurora; the field geometry would then seem to require a conjugate (and probably brighter) northern aurora that escaped detection poleward of the latitude range sampled by the UVS data. The power for such an aurora could be extracted from Neptune's rotation by the injection of plasma at Triton's orbit at a rate dm/dt ∼ 1 kg/s.
The atmosphere of Neptune: Results of radio occultation measurements with the Voyager 2 spacecraft
Authors: Lindal G. F. , Lyons J. R. , Sweetnam D. N. , Eshleman V. R. , Hinson D. P. , Tyler G. L.
Published: February 2008

Radio tracking data acquired during Voyager 2's occultation by Neptune have been used to study the thermal structure and composition of Neptune's troposphere and stratosphere. The occultation began near 62° North planetographic latitude and aided near 45°South latitude. The data cover an altitude interval of about 250 km. A comparison with infrared observations indicates that the gas at the tropopause, which was observed near the 100 mbar level, consists of 77 to 85 % hydrogen by number density with the remainder being mostly helium. A layer with a small refractivity scale height that was detected approximately 50 km below the tropopause, may be a region where the CH4 mixing ratio is decreasing with increasing altitude due to condensation effects. This interpretation leads to a CH4 mixing ratio of 1 to 2 % by number density at the base of the layer where the pressure is approximately 1.7 bars. The depth of the occultation measurements was limited by microwave absorption. By assuming that the absorption was caused by saturated NH3 vapor, one obtains a pressure, temperature, and NH3 mixing ratio of 6 bars, 130 K, and 600 parts per billion, respectively, at the lowest level where the radio link was detected. For internal consistency, this solution requires an atmospheric rotation period of 14.2 ± 0.5 hours near 60° North latitude which corresponds to a prograde zonal wind velocity of 170 ± 50 m/sec relative to the magnetic field. The 1 bar isobaric surface has an equatorial radius of 24,764 ± 15 km and a polar radius of 24,341 ± 30 km. The corresponding oblateness, (Req‐Rp)/Req, is 0.0171 ± 0.0014.
SPECTRAL GEOMETRIC ALBEDO AND BOLOMETRIC BOND ALBEDO OF NEPTUNE'S SATELLITE TRITON FROM VOYAGER OBSERVATIONS
Authors: Nelson R. M. , Burattl B. J. , Wallis B. D. , Smythe W. D. , Horn L. J. , Lane A. L. , Mayo M. J. , Simmons K. E.
Published: February 2008

We have calculated the spectral geometric albedo and bolometric Bond albedo of Triton by combining the published data from the Voyager spacecraft Photopolarimeter (PPS) experiment and the Voyager imaging science (ISS) experiment. We have used the PPS ultraviolet (0.25 μm) and infrared (0.75 μm) filter data and the ISS violet (0.41 μm) and green (0.56 μm) filter data to produce geometric albedos and phase integrals at each of the four wavelengths. The resulting spectral geometric albedo is not inconsistent with the presence of weak absorption feature in Triton's spectrum near 0.75 μm. The phase integrals were determined by Russell's approximation and by 2‐point Gaussian quadrature. The geometric albedo at the uv, v, g and ir filters is 0.59, 0.68, 0.81, and 0.75 respectively. The resulting bolometric Bond albedo (∼0.65) is· consistent with the 38 K, unit emissivity, daytime surface temperature for Triton reported as a best fit to the data of the Voyager Infrared Specrotrometer and Radiometer (IRIS). Our results are also in agreement with the 37.5 K temperature of nitrogen at an inferred basal pressure of 14 μbar as reported by the Voyager Ultraviolet Spectrometer (UVS) investigation.

The bolometric Bond albedo that is determined from tile phase integral and spherical albedo reported originally by the ISS investigation yields a surface temperature which is consistent with the PPS, IRIS and UVS temperatures only for a surface with a substantially lower emissivity. There are differences between the PPS and the ÎSS phase curves which cause the differences in bolometric bond albedo reported in the two studies.
Electron plasma oscillations in the Venus foreshock
Authors: Crawford G. K. , Strangeway R. J. , Russell C. T.
Published: February 2008

Plasma waves are observed in the solar wind upstream of the Venus bow shock by the Pioneer Venus Orbiter. These wave signatures occur during periods when the interplanetary magnetic field through the spacecraft position intersects the bow shock, thereby placing the spacecraft in the foreshock region. The electron foreshock boundary is clearly evident in the data as a sharp onset in wave activity and a peak in intensity. Wave intensity is seen to drop rapidly with incresing penetration into the foreshock. The peak wave electric field strength at the electron foreshock boundary is found to be similar to terrestrial observations. A normalized wave spectrum was constructed using measurements of the electron plasma frequency and the spectrum was found to be centered about this value. These results, along with polarization studies showing the wave electric field to be field aligned, are consistent with our interpretation of the waves as electron plasma oscillations.
Hypervelocity dust particle impacts observed by the Giotto Magnetometer and Plasma Experiments
Authors: Neubauer F. M. , Glassmeier K.‐H. , Coates A. J. , Goldstein R. , Acuña M. H. , Musmann G.
Published: February 2008

We report thirteen very short events in the magnetic field of the inner magnetic pile‐up region of comet Halley observed by the Giotto magnetometer experiment together with simultaneous plasma data obtained by the Johnstone plasma analyzer and the ion mass spectrometer experiments. The events are due to dust impacts in the milligram range on the spacecraft at the relative velocity between the cemetery dust and the spacecraft of 68 km/sec. They are generally consistent with dust impact events derived from spacecraft attitude perturbations by the Giotto camera [Curdt and Keller, private communication]. Their characteristic shape generally involves a sudden decrease in magnetic field magnitude, a subsequent overshoot beyond initial field values and an asymptotic approach to the initial field somewhat reminiscent of the magnetic field signature after the AMPTE releases in the solar wind. These observations give a new way of analyzing ultra‐fast dust particles incident on a spacecraft.
The electron edge of low latitude boundary layer during accelerated flow events
Authors: Gosling J. T. , Thomsen M. F. , Bame S. J. , Onsager T. G. , Russell C. T.
Published: February 2008

Magnetosheath plasma entering the Earth's magnetosphere to populate the low latitude boundary layer, LLBL, is often accelerated to speeds considerably greater than are observed in the adjacent magnetosheath. Measurements made during such accelerated flow events separate electron and ion edges to the LLBL, with the electron edge being found earthward of the ion edge, electron velocity distributions observed at the edge of the LLBL are often highly structured, exhibiting large asymmetries parallel and antiparallel, as perpendicular, to the local magnetic field. These can consistently be interpreted as time‐of‐flight effects on recently reconnected field lines, and thus are evidence in support of the reconnection interpretation of accelerated plasma flow events.
Electron distributions in the plasma sheet boundary layer: Time‐of‐flight effects
Authors: Onsager T. G. , Thomsen M. F. , Gosling J. T. , Bame S. J.
Published: February 2008

The electron edge of the plasma sheet boundary layer lies lobeward of the ion edge. Measurements obtained near the electron edge of the boundary layer reveal low‐speed cutoffs for earthward and tailward‐flowing electrons. These cutoffs progress to lower speeds with deeper penetration into the boundary layer, and are consistently for the earthward‐directed electrons than for the tailward‐directed electrons. The cutoffs and their variation with distance from the edge of the boundary layer can be consistently interpreted in terms of a time‐of‐flight effects on recently reconnected magnetic field lines. The observed cutoff speeds are used to estimate the downtail location of the reconnection site.
Pc1 pulsations observed by AMPTE/CCE in the Earth's outer magnetosphere
Authors: Anderson B. J. , Takahashi K. , Erlandson R. E. , Zanetti L. J.
Published: February 2008

Electromagnetic ion cyclotron waves in the Pc1–2 frequency range (0.1–5 Hz) have been observed in Earth's equatorial magnetosphere over the range L=3 to 9 by the AMPTE/CCE magnetic fields experiment. We report detailed analysis of data from two orbits together with preliminary occurrence statistics from 31 days. One example occurred during disturbed geomagnetic conditions (maximum AE>1000 nT) and exhibited intense Pc1 from L=5 to the magnetopause (L≈7.5). The second example occurred during moderate to quiet conditions (average AE≈300 nT) and displayed Pc1 in two distinct L ranges: low (L≈4) and high (L>7,5). Statistical results show that Pc1 are ≈4 times more common for L>7 than for L<6 and occur primarily in the afternoon. The discovery of a statistically dominant outer magnetospheric source region implies that ring current interaction with the plasmasphere is not the primary source of Pcl excitation but suggests that plasma sheet ions may be a significant energy source for Pc1. This result is consistent with the strong dependence of the linear convective growth rate on magnetic field strength.
Time series and correlation of pulsations observed simultaneously by two aircraft
Authors: Ochadlick Andrew R.
Published: February 2008

Using two aircraft as mobile geomagnetic observatories to study the similarity of pulsations (2 – 25 s) as a function of aircraft separation (1.6 – 4600 km), an analysis appears to show that pulsation correlation decreased dramatically at a separation corresponding roughly to the heigh of the ionosphere.
Chemical remanent magnetization of oceanic crust
Authors: Verhoef J. , Arkani‐Hamed J.
Published: February 2008

The effects of chemical remanent magnetization (CRM) of oceanic crust on the anomalous skewness of sea‐floor spreading magnetic anomalies are investigated. Considering a realistic constraint that the actual magnetization at anomaly M0 is reversed, the CRM of layer 2A basalts fails to explain the anomalous skewness of the magnetic anomalies. The CRM of the deeper layers does contribute to the anomalous skewness of anomalies 33/34, but the major contribution comes from thermal remanent magnetization.
A latitudinal pattern in the structure of the outermost liquid core, revealed by the travel times of SKKS‐SKS seismic phases
Authors: Souriau Annie , Poupinet Georges
Published: February 2008

Differential travel times of SKS and SKKS seismic phases are analysed in order to specify the properties of the outermost liquid core. The data have been selected in order to obtain a good sampling of the Earth, they are mainly intermediate period GDSN and broadband GEOSCOPE and NARS records of events at epicentral distances 83–130°. The SKKS‐SKS travel times are referred to the mean Earth model PREM (Dziewonski and Anderson, 1981) and are corrected for ellipticity. A general baseline shift and slope with respect to PREM is also removed from the data. The SKKS‐SKS residuals indicate a departure from spherically symmetric structure in the liquid core. In particular, they exhibit a clear latitude dependence with lower values at the poles and higher values at the equator, indicating that heterogeneities with cylindrical symmetry are present in the most external liquid core. This latitudinal pattern exhibits some similarities with that of the zonal flow in the outermost liquid core deduced from the secular variations of the magnetic field. This suggests that the distribution of the seismic velocities in the outermost liquid core may be influenced by the circulation of the fluid beneath the core‐mantle boundary, or that both are similarly influenced by the Earth rotation.
Observations of the density profile in the magnetosheath near the stagnation streamline
Authors: Song P. , Russell C. T. , Gosling J. T. , Thomsen M. , Elphic R. C.
Published: February 2008

An enhancement in the plasma density occurs in more than half of the ISEE‐1 and 2 magnetosheath passes near the stagnation streamline just in front of the magnetopause. On average, this structure is about 0.4 Re thick and clearly separated from the magnetopause. The average density enhancement is about 44% above the ambient magnetosheath density. The anticorrelation of the magnetic field and density is that expected for the MHD slow mode. Strong fluctuations in the density within the structure are usually accompanied by fluctuations in velocity.
On the consequences of strong stable stratification at the top of Earth's outer core
Authors: Bloxham Jeremy
Published: February 2008

We consider the consequences of strong stable stratification at the top of the Earth's fluid outer core, concentrating on the generation of the geomagnetic secular variation. We assume that the core near the coremantle boundary is both strongly stably stratified and free of Lorentz forces: we find that this set of assumptions severely limits the class of possible motions, none of which is compatible with the geomagnetic secular variation. Relaxing either assumption is adequate: tangentially geostrophic flows are consistent with the secular variation if we relax the assumption that the core is strongly stably stratified (but retain the assumption that Lorentz forces are negligible); purely toroidal flows (consisting in this case of zonal flows together with horizontally polarised MAC‐waves) may explain the secular variation if we include Lorentz forces. We are unable to discriminate between these two cases.
Changes in the intensity of the geomagnetic field
Authors: Walton D.
Published: February 2008

A new, more reliable technique has been used to determine the ancient intensity of the earth's magnetic field using samples of baked clay from eight sites in the Eastern Mediterranean. The resultant power spectrum reveals a single strong component with a period of 1430 years. The reproducibility of the results obtained with the new method also makes limited archaeomagnetic dating with a precision of between about one and two hundred years between 900 BC and AD 400 possible.
Friction, overpressure and fault normal compression
Authors: Byerlee J.
Published: February 2008

More than twenty‐five years ago Miller and Low reported the existence of a threshold pore pressure gradient below which water would not flow through clay. Recent experimental observations of the shear strength of structured water on biotite surfaces have provided a physical basis for understanding this threshold gradient. The existence of this phenomenon has profound implications for the rheological properties of mature fault zones, such as the San Andreas, that contain large thicknesses of fault gouge. For example, a clay–filled fault zone about 1 km wide at the base of the seismogenic zone decreasing to zero width at the surface could support core fluid pressure equal to the maximum principal stress over the entire seismogenic zone. As a result, the fault would have near‐zero strength and the maximum principal stress measured on the flanks of the fault, would be oriented normal to the fault surface. Another consequence of the threshold gradient is that normal hydrostatic fluid pressures outside the fault zone could coexist with near‐lithostatic fluid pressures in the interior of the fault zone without the need for continual replenishment of the overpressured fluid. In addition, the pore pressure at any point should never exceed the local minimum principal stress so that hydrofracture will not occur.