bluuebunny/crossref_metadata_2025_split huggingface dataset:
Particle flocculation: One possible control on the magnetization of deep‐sea sediments
Authors: Ellwood Brooks B. 
Published: February 2008

Anisotropy of magnetic susceptibility measurements have been used to describe the orientations of magnetic grains in sediments from the South Atlantic Ocean. It has been observed that magnetic grain long axis alignment is normal to the expected bottom current direction in the Vema Channel. Traction transport is the inferred mechanism for this alignment. It is proposed here that one possible explanation for this process is sediment aggregation into large floccules (>16 µm), which are of low density due to entrapped water and can therefore become elogated and aligned due to shear produced by bottom currents. The resulting poorly compacted bottom sediment compresses after accumulation of small amounts of overburden. Below ∼ 2 meters the compressed magnetic fabric, residing in the larger magnetic grains, is consistent with expected current deposited primary sedimentary fabric.The process of particle aggregation, current controlled deposition, compaction, and particle dewatering and breakup can explain the remanent magnetic characteristics observed in some deep‐sea sediments. A DRM inclination error of ∼11°, found at the top of core CH115‐61, disappears with depth in the core; RM inclinations steepen to −50° consistent with an axial geocentric magnetic field inclination for the site. It is suggested that orientations of very small magnetic grains are initially constrained within large floccules and that a DRM inclination error is produced during deposition. Later, floccule disaggregation and dewatering allows realignment of these very small grains along the geomagnetic field direction.
Large scale three dimensional P velocity structure beneath the western U.S. and the lost Farallon Plate
Authors: Romanowicz Barbara A. 
Published: February 2008

The results of a recent large scale three‐dimensional study of P velocity beneath North America are analyzed from the point of view of the search for the fossil Farallon plate in the mantle beneath the western edge of the North American continent.The large scale velocity structure obtained in the western U.S. is compatible with independent finer scale seismic studies and suggests that the Farallon plate can be traced to depths of 300 to 450 km along the entire North‐South extent of the area. At greater depth, no clear evidence is found for its presence: it has either not reached it or is no longer distinguishable from the surrounding mantle.On the basis of the 3‐D velocity study we also venture to predict that, whereas in the central and southern part of the western U.S. the top of the plate seems to have sunk to depths in excess of 200 km, it should still be present at shallower depths beneath the Northwestern region. This is in agreement with the progressive migration to the North of the Mendocino transform‐transform‐trench triple junction and its present day position.
Lidar measurements of atmospheric lithium
Authors: Jegou Jean‐Pierre , Chanin Marie‐Lise , Mégie Gérard , Blamont Jacques E. 
Published: February 2008

The Haute‐Provence Observatory (44° N, 6°E)lidar facility has been extended to study the atmospheric lithium and its isotopic ratio Li7/Li6 in the upper atmosphere. The experimental features have been improved in order to lower the detection limit to 3.105 cm−2 for the total abundance with a resolution of 1.2 km in the altitude profile. The night‐time measurements have been carried out from November 1977 to December 1979 and show some characteristic trends.The seasonal variation of the total abundance is similar to the one observed for sodium. The altitude profile is variable during the night. Total abundance fluctuations and peaks with small scale heights (1.5 km) are observed on numerous occasions. The layer is sensitive to high‐altitude artificial releases, like CAMEO in November 1978, and to impinging meteor showers. It is concluded that study of the behavior of atmospheric lithium and sodium should further our understanding of the alkali‐metals chemistry.
Meteorological effects on the seasonal variations of the rotation of Mars
Authors: Cazenave Anny , Balmino Georges 
Published: February 2008

We have investigated the meteorologically induced seasonal variations in the rotation rate of Mars. As on Earth, meteorological effects are expected to produce fluctuations in the rotation of Mars but unlike on Earth, where zonal winds are the major contributors, on Mars seasonal variations are mainly the result of exchange of CO2 between atmosphere and polar caps through a variation of the polar moment of inertia.From Viking pressure data and zonal winds models by Haberle et al. [1979] we predict an annual and a semi‐annual cycles in the relative rotation rate of Mars of amplitude ∼ 0.2 × 10−8 and 0.1 × 10−8 respectively. On the length of the martian day these seasonal changes are 0.17 ms and 0.09 ms respectively. Between the fall equinox and winter solstice (northern hemisphere), the maximum change in the martian l.o.d. amounts to 0.44 ms.
Radium‐226 and radon‐222 in southern California groundwaters: Spatial variations and correlations
Authors: Chung Yu‐Chia 
Published: February 2008

As part of the radon and helium monitoring program for earthquake prediction studies in Southern California, concentrations of radium‐226 in groundwaters have been measured along the major fault zones including the Palmdale area. Radium was measured in order to obtain information on its spatial variation as well as baseline levels. Several measurements were made at selected sites to examine their temporal variations. Radium concentrations are used to define the portion of radium supported radon in the observed groundwater radon. Radium levels vary spatially by 4 orders of magnitude (0.015 to 146 dpm/kg), but temporally by a factor of 2 or less. Similar spatial and temporal variations are observed in groundwater radon (60 to 46,700 dpm/kg). The radon/radium activity ratio ranges from 10 to 105. These values indicate that radon is in large excess over radium in groundwaters because of radon diffusion from the ambient rocks. The spatial variations of radon, radium and the activity ratio are probably due to different chemical and physical properties of the rocks through which groundwaters flow.The dependence of groundwater radium concentrations on the water temperature and conductivity is explored. It appears that radium increases in general with conductivity but not necessarily with temperature. Radium is probably leached more readily into the groundwater when the water is more conductive with more dissolved chemical constituents.
Polyhedral modeling of the elastic properties of corundum (α‐Al2O3) and chrysoberyl (Al2BeO4)
Authors: Au Andrew Y. , Hazen Robert M. 
Published: February 2008

Coordination polyhedra in a crystal structure are considered to be the structural units in a modified rigid‐ion model. In the polyhedral model the elastic constants of coordination polyhedra are used as input parameters to calculate the single‐crystal elastic constants of corundum and chrysoberyl. The average deviation of the calculated elastic constants from the measured elastic constants is within 6% for corundum and within 1% for chrysoberyl.The corundum crystal structure is composed entirely of AlO6 octahedra, which are also major polyhedral constituents of the chrysoberyl crystal structure. The nonelectrostatic contribution of the bulk compressibility of an ideal (regular) AlO6 octahedron is found to be 10.5 ± 0.2 Mbar, an invariant in these two crystal structures. The polyhedral elastic constants and the associated perturbational parameters are not universal, however, as demonstrated in the two crystal structures studied.
Source characteristics of the 1985 Michoacan, Mexico Earthquake at periods of 1 to 30 seconds
Authors: Houston Heidi , Kanamori Hiroo 
Published: February 2008

Source characteristics of the Sept. 19, 1985 Michoacan, Mexico earthquake and its aftershock on Sept. 21 were inferred from broadband and short‐period teleseismic GDSN records. We Fourier‐transformed the P waves, corrected for instrument response, attenuation, geometrical spreading, and radiation pattern (including the depth phases), and then averaged to obtain the teleseismic source spectrum from 1 to 30 s. The Michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events. Source spectra for the Sept. 21 aftershock, 1981 Playa Azul, 1979 Petatlan, and 1978 Oaxaca events follow a trend similar to that of the 1985 Michoacan event. This spectral trend may characterize the Mexican subduction zone.A station‐by‐station least‐squares inversion of the Michoacan earthquake records for the source time function yielded three source pulses, which we interpreted as events on the fault plane. The first two are similar in moment, and the third contains only 20% of the moment of the first. Directivity is evident in the timing. At each station, we measured the time differences between the pulses, and performed a least‐squares nonlinear estimation of the strike, distance, and time separation between the events to locate them relative to one another. The second event occurred 26 s after the first, and 82 km southeast of it, indicating southeastward rupture along the trench. The third event occurred 21 s after the second, and about 40 km seaward of it. The two large events are also seen in the near‐field strong motions.The mainshock records, spectrum, and time functions contain less high frequency radiation than those of the 1985 Valparaiso, Chile earthquake. Apparently, the Michoacan earthquake ruptured two relatively smooth, strong patches which generated large 30 s waves, but small 1 to 10 s waves. Such behavior contrasts with the Valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy. This difference is consistent with the higher near‐field accelerations recorded for the Valparaiso event.
Viking in the plasma universe
Authors: Alfvén H. 
Published: February 2008

Realizing that the magnetosphere cannot be understood until the electric field is mapped, Viking has placed emphasis on electric field measurements. Because energy is transferred from the solar wind mainly through electric currents in the active plasma regions, Viking's orbit is selected so as to include as much of the active regions as possible. The real‐time presentation of the results makes it possible to modify the observations as soon as interesting events are seen. As several of the project leaders have experience in plasma laboratory experiments, Viking can be characterized as a new type of space plasma experiment.Some of the results of the Viking mission are important as a basis for drawing the circuit of energy transfer from the solar wind to the magnetosphere and ionosphere (Figure 1) and for understanding the general properties of collisionless plasmas in space (Table 1). They will constitute a platform for a systematic exploration of the plasma universe (Figure 2), which will have an impact on our views on cosmogony and cosmology.
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.
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.
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.
Computer studies of the dynamic evolution of the geomagnetic tail
Authors: Birn J. 
Published: February 2008

The dynamic evolution of a stretched magnetotail configuration is investigated numerically by means of a two‐dimensional time‐dependant nonlinear resistive MHD code. The initial configuration is shown to be stable in ideal MHD. The unstable evolution is initiated by imposing a finite resistivity.Rather independently of the boundary conditions, the following phenomena develop spontaneously: plasma sheet thinning, topological changes of the magnetic field configuration, the formation of neutral lines with an almost stationary X line, large induced cross‐tail electric fields, conversion of magnetic to kinetic energy resulting in strong earthward and tailward flow close to the neutral sheet, and outside the neutral sheet a significant part of the flow parallel to the magnetic field. This flow reverses sign across the plasma sheet boundary tailward of the X line. The results support the tearing theory of magnetospheric substorms. At the later stages the configuration around the X line is similar to that obtained from steady state reconnection theory; in particular we found that two current layers form downstream of the magnetic field separatrix, which may represent structures developing into slow shock waves.
Topography and tectonics of Mid‐Oceanic Ridge axes
Authors: Sleep Norman H. , Rosendahl Bruce R. 
Published: February 2008

Numerical fluid dynamic models of mid‐oceanic ridge axes were constructed using distributions of material properties constrained by seismic studies and thermal calculations. The calculations indicate that spreading is passive except for forces caused by density differences due to thermal expansion and partial melt. Except for geometric differences due to temperature distribution, one set of mechanical properties can adequately explain central rifts of slow ridges and central peaks of fast ridges. Viscous head loss in the upwelling material dominates at low spreading rates where material ascends through a narrow conduit. Thermal expansion and partial melting dominate at high spreading rates where a wide low viscosity crustal magma chamber is present. The preferred rheology is 1020 poise for the upwelling lithosphere; less than 5×1017 for the crustal magma chamber and axial intrusion zone at fast ridges, and a yield stress of 200 bars for the lithosphere. The calculation correctly predicts the existence of central peaks at ‘hot‐spot’ ridges, where seismic evidence indicates a large magma chamber.The appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Flordia Avenue, N.W., Washington, D.C. 20009. Document J79‐003; $1.00. Payment must accompany order.
Introduction to Special Section on Open Magmatic Systems
Authors: Hildreth Wes , Grove Timothy L. , Dungan Michael A. 
Published: February 2008

The idea that magmatic systems are open to intermittent gains and losses of mass and energy is hardly new. Eruptive, conductive, and hydrothermally convective losses are obvious, and both wall rock assimilation and mixing of discrete magma batches are petrological ideas with long and distinguished conceptual lineages. New ideas of the last decade are largely outgrowths of the mounting evidence that mixing and assimilation are ubiquitous, that heterogeneity and mixing in mantle source regions are common, and that few igneous rock suites are likely to reflect strictly closed‐system fractionation of single magma batches.The evidence has accumulated quickly on many fronts, owing in part to the wider availability of high‐precision analytical tools and the development of sophisticated methods for modeling the wealth of chemical and isotopic data. Detailed studies of zoned ash flow sheets, zoned and layered plutons, and macroscopically mixed igneous rocks have made petrologists aware that dynamic and nonequilibrium aspects of magma transport play important roles in the evolution of many magmatic systems. Technological advances have permitted routine experimentation over a wide range of pressures, providing kinetic data and phase equilibrum constraints essential to understanding both mantle source processes and magmatic evolution in crustal reservoirs. Finally, the plate tectonic synthesis and seafloor sampling programs have respectively provided the impetus for a detailed assessment of geochemical heterogeneity in a dynamic mantle and an important avenue for accomplishing that assessment.
Positive cloud‐to‐ground lightning in summer thunderstorms
Authors: Fuquay Donald M. 
Published: February 2008

This paper describes cloud‐to‐ground lightning that lowers positive charge to earth over mountainous terrain in the northern Rocky Mountain region. A sample of 14 visually confirmed ground flashes provided criteria for identifying positive ground flashes by using only measured electric fields. According to these criteria, 75 positive ground flashes occurred within the 2830 km2 sampling area during three consecutive summer seasons. About 3% of all ground flashes lowered positive charge. The area density of positive ground flashes is about 0.01/km2 per summer season compared to an area density of negative ground flashes of 0.3/km2 per summer season.Positive ground flashes occurred within the sampling area on 16 of 48 thunderstorm days. Daily occurrences ranged from 1 to 11. When lightning could be associated with individual thunderstorms, the positive flashes usually occurred in sequence during the final 30 min of the storm. The 75 positive flashes are very similar. Each has a single R stroke followed by a continuing current interval. The continuing current duration distribution is approximately log normal, with range 5 to 160 ms and mean of about 60 ms. The prestroke intervals, the total period of field change preceding the first R stroke, are similarly distributed over the range 65 to 210 ms with mean of about 130 ms. Leader field change durations, the latter portion of the prestroke interval, range from 40 to 70 ms with mean of about 50 ms.The positive ground flashes occurred over mountainous and forested terrain free of man‐made structures. Available data indicate that the positive ground flashes for this region may, in most instances, be initiated by positive downward leaders following a horizontally elongated out‐of‐cloud path to ground.