bluuebunny/crossref_metadata_2025 huggingface dataset:
Direction of maximum horizontal compression in western Oregon determined by borehole breakouts
Authors: Werner K. S. , Graven E. P. , Berkman T. A. , Parker M. J.
Published: February 2008

Elliptical borehole enlargements or ”breakouts” caused by systematic spalling of a borehole wall were identified in 18 wells drilled in the Coast Range and Willamette Valley of western Oregon. The breakouts generally indicate a NNW to NNE orientation of maximum horizontal compression (σHmax) that agrees with the predominant direction of σHmax determined from earthquake focal mechanisms, from post‐middle Miocene structural features, and from alignments of Holocene volcanic centers in the Pacific Northwest. However, this orientation is inconsistent with the N50°E convergence between the Juan de Fuca and North American plates determined by Riddihough (1984) from Juan de Fuca plate magnetic lineations as young as 730 ka (the Brunhes‐Matuyama boundary). The predominant NNW to NNE orientation of σHmax may be due to the complex interaction of a northwestward moving Pacific plate driving into the Gorda and Juan de Fuca plates and indirectly transmitting N‐S compression across the strongly coupled Cascadia subduction zone into the overriding North American plate (Spence, 1989). Alternatively, the predominant NNW to NNE orientation of σHmax may be due to a landward counterclockwise rotation of the direction of σHmax from N50°E compression offshore to N‐S compression in the Coast Range.
Late Neogene relative motions of the Pacific and North America Plates
Authors: Harbert William
Published: February 2008

In this paper a new set of finite rotations describing the relative motion of the Pacific and North America plates during the last 10 m.y., incorporating recently published studies of the Pacific‐Antarctic, Antarctic‐Africa, and Africa‐North America plate boundaries is presented. These finite rotations show that changes have occurred in Pacific‐western North America motion at 2.48 Ma and between 3.40 and 3.9 Ma, resulting in increased compression along the Pacific‐North America plate boundary. The most significant change in relative motion was of the latter age. During this change the predicted motion of the Pacific plate along the California coast changes from transform to transpressive, due to a clockwise rotation of the relative convergence vector by 12°. This timing of the onset of transpression agrees well with a variety of geologic data along the California plate boundary, including the onset of compressive deformation in onshore and offshore sedimentary basins, formation of reverse faults and anticlines (which are parallel to strike‐slip faults of the previous, more westerly directed azimuth of relative motion), a change in the orientation of the San Andreas fault, and formation of a set of new, more northerly trending strike‐slip faults. In this model this change in relative motion is caused by a change in the absolute motion of the Pacific plate, due to the detachment of a slab beneath the Fiji Basin between 3.4 and 3.9 Ma. The detachment of this slab and the resultant change in overall Pacific plate torque resulted in a noncollisional “orogeny” along the California plate boundary. This study shows that minor adjustments in the motion of large oceanic plates, such as the Pacific plate, can have profound consequences on the preserved geologic record.
Tectonic loading in the Canadian Cordillera as recorded by mass accumulation in the Foreland Basin
Authors: Underschultz J. R. , Erdmer P.
Published: February 2008

The geometry of the Alberta foreland basin is generally the result of lithospheric loading in the adjacent Cordillera. Sedimentation within the Foreland basin was quantified by calculating mass accumulation rates to constrain loading events in time and space. Rates of mass accumulation for Cretaceous units in the basin can be classified into low, moderate, and high categories and may related to deformation events in the Canadian Cordillera. The rates of accumulation show a distinct cyclic pattern when plotted against time. There were two major episodes of rapid sedimentation from early Aptian to late Campanian time, each preceded by a period of relative quiescence. The first lasted from approximately 115 to 108 Ma. It shows a succession of rapid, moderate, and rapid sedimentation events. The second episode of rapid sedimentation lasted from approximately 95 to 90 Ma, and is characterized by three rapid sedimentation events separated by short intervals of quiescence. Order‐of‐magnitude changes in sediment accumulation in the foreland basin can be related to major deformation events in the Cordillera. Furthermore, the deformation can be located in time and space and evaluated in terms of its relative intensity. This delineation of deformation indicates that the docking of foreign terranes does not necessarily coincide with their final accretion and the associated tectonic loading.
Low energy upflowing ion events observed by EXOS‐D: Initial results
Authors: Sagawa E. , Iwamoto I. , Watanabe S. , Whalen B. A. , Yau A. W. , Fukunishi H.
Published: February 2008

Initial observations of low‐energy upflowing ion events from the Suprathermal Ion Mass Spectrometer (SMS) on EXOS‐D are presented. The SMS instrument on EXOS‐D measures the mass composition, energy and angular distributions of thermal and suprathermal ions in the 0.1–4000 eV/q energy‐per‐charge and 0.8–70 amu/q mass‐per‐charge ranges. Two events are examined. The first features predominantly conical ion pitch‐angle distributions and the second has mainly field‐aligned distributions. Both events displayed small scale variations in upward flux, composition, and angular characteristics, and coincided with small‐scale magnetic field fluctuations. In both events, the peak He+ and H+ fluxes were comparable; the O+ flux was a factor of 5–10 larger. The pitch‐angle distribution often switched rapidly between field‐aligned and conical.
What parts of broadband spectra are responsible for ion conic production?
Authors: Ball Lewis , André Mats
Published: February 2008

We investigate ion energization and ion conic production by broadband electric field fluctuations. The roles played by resonant fluctuations (which can satisfy the cyclotron resonance condition), and nonresonant fluctuations well removed from the ion gyrofrequency, are compared. We show that in the resonant case the mean ion energy increases essentially linearly with time, while in the nonresonant case this energy rapidly reaches a maximum. Using observed electric spectra we find that nonresonant energization due to high spectral densities at frequencies well below the ion gyrofrequency may be important for the generation of some heavy ion conics, but confirm that heating by resonant electric fluctuations is sufficient to explain most ion conics.
Application of fluid dynamics principles in tilted permeable media to terrestrial hydrothermal systems
Authors: Criss Robert E. , Hofmeister Anne M.
Published: February 2008

Fluid dynamics principles require that circulation of aqueous fluid will be practically ubiquitous in tectonically active parts of the Earth's crust and upper mantle. Both experiment and theory [e.g. Hart, 1971] demonstrate that flow, generally in the form of unicells (Hadley circulation), always occurs for isothermal tilts above a very small critical angle (∼ 5°), for any non‐zero permeability or Rayleigh number, and even for hot over cold geometries. Interestingly, heat transport rates in the unicellular regime are essentially conductive, so such flow, unlike more vigorous flow at higher Rayleigh number, is not properly termed “convective”. These principles have numerous geological ramifications, including: 1) Many of the hydrothermal systems developed around epizonal intrusions should be dominanty unicellular in nature, which explains their aspect ratios and the smooth and very regular δ18O variations that are produced in the rocks; 2) Large, long‐lived unicells are predicted to occur deep in the Earth's crust wherever Rayleigh numbers are finite and isotherms are substantially inclined, as in zones of batholith intrusion, regional metamorphism, and collision; 3) Unicells with lateral dimensions of several hundred kilometers are predicted to be associated with subduction zones dipping more than 6–12°, with fluid advection into the hot mantle wedge being instrumental in mantle metasomatism and in the generation of andesitic magmas.
Heating and ionization of the lower ionosphere by lightning
Authors: Inan U. S. , Bell T. F. , Rodriguez J. V.
Published: February 2008

Nighttime ionospheric electrons at 90–95 km altitude are found to be heated by a factor of 100–500 during the upward passage of short (< 100 μs) pulses of intense (5–20 V/m at 100 km distance) electromagnetic radiation from lightning. Heated electrons with average energy of 4–20 eV in turn produce secondary ionization, of up to 400 cm−3 at ∼95 km altitude in a single ionization cycle (∼3 μs). With the time constant of heating being 5–10 μs, a number of such ionization cycles can occur during a 50 μs, radiation pulse, leading to even higher density enhancements. This effect can account for previously reported observations of ‘early’ or ‘fast’ subionospheric VLF perturbations.
Geologic implications of the oxygen isotope profile of the Toa Baja Drill Hole, Puerto Rico
Authors: Smith Brian M.
Published: February 2008

The whole‐rock O‐isotopic compositions of volcanic and volcaniclastic samples from the Toa Baja drill hole demonstrate that low‐temperature (<200°C) processes have strongly enriched the island arc materials in 18O. Subsequent to eruption, processes such as subaerial weathering, alteration during transport and deposition in volcaniclastic aprons, submarine weathering, burial diagenesis, and prograde regional metamorphism through the beginning of the prehnite‐pumpellyite facies have raised average whole‐rock δ18O values by ∼4‰ for basalt and andesite lava flows, and by ∼8‰ for volcaniclastic sandstones. These O‐isotopic disturbances were probably caused by oxygen exchange with regionally circulating seawater under rather high water/rock conditions. The processes associated with “ageing” of volcanic and volcaniclastic materials in the oceanic environment are probably more important to the global budgets of the oxygen isotopes than has been assumed in the past. Integration of these results into global models for the oxygen isotopes awaits analysis of more varied oceanic terranes, to determine the generality of the O‐isotopic conclusions preferred here, and to more carefully evaluate the relative sizes of volcanic, volcaniclastic, and oceanic oxygen reservoirs and their variabilities in time.
Diagenesis of Middle Tertiary carbonates in the Toa Baja Well, Puerto Rico
Authors: González Luis A. , Ruiz Héctor M.
Published: February 2008

The Toa Baja Well drilled in northern Puerto Rico to adepth of 2705 m (8872 ft.) penetrated over 550 m (1800 ft.) of Tertiary carbonates. The limestone‐dominated portion of the well consists mostly of shallow‐water backreef carbonates. Metastable carbonates have been either calcitized, dolomitized or dissolved. The pétrographie character of the Tertiary carbonates in the Toa Baja Well, and those reported by Monroe [1980], the cathodoluminescence petrography, and the stable isotopic compositions indicate that these carbonates were rapidly cemented in the marine environment (limiting compaction), that replacement of metastable carbonates by calcite and precipitation of sparry calcite took place mostly in meteoric diagenetic environments and dolomitization probably occurred in a meteoric‐marine mixing zone. Given the history of numerous drainage systems that dissected the Tertiary carbonates throughout their depositional history [Monroe, 1980] and the repeated Cenozoic sea level oscillations [Monroe, 1980; Seiglie and Moussa, 1984], it is likely that alteration of metastable carbonates took place during lowstands. Interstratified fluvial deposits suggest the continued influence of meteoric fluids in local highlands [Monroe, 198O].
Rb‐Sr and Sm‐Nd isotopic characterization of Eocene volcanic and volcaniclastic rocks from Puerto Rico
Authors: Frost Carol D. , Schellekens Johannes H.
Published: February 2008

Sm‐Nd and Rb‐Sr data are presented for six volcanic and volcaniclastic samples from the Eocene belt and the Toa Baja drill hole of Puerto Rico in order to provide a preliminary isotopic description of Greater Antilles arc rocks. The lava samples have trace and rare earth element compositions typical of island arc rocks. The isotopic compositions of the volcaniclastic samples are displaced from those of the lavas, and suggest that they include an authigenic or detrital crustal component. The similarity of Sr and Nd isotopic ratios of Puerto Rican lavas and northern Lesser Antilles volcanic rocks suggests that they may have been produced from similar magma sources.
The influence of mantle water and heterogeneity on the viscous relaxation of Venus impact craters
Authors: Rovetta Mark R.
Published: February 2008

Empirical flow laws for peridotite and diabase were used to model the viscous relaxation of large impact craters on Venus, The models were then compared to the depths of the three largest impact craters (≥60 km diameter) measured by Venera 15/16 radar altimetry. The maximum crater retention times predicted by models that assumed ≥ 0.03% wt. water in the upper mantle were inconsistent with the observed depths of these craters and a mean minimum age of 0.5 Ga. Models that assumed an upper mantle water content of ≤ 0.003% wt. were consistent with the observations and a maximum crater retention time of 1.0 Ga. This result was insensitive to up to 60% by volume diabase mixed into the upper mantle as inclusions at the several kilometer scale and to the presence of a basalt‐eclogite or melting transition at a depth of 100 km.
The simplest, unsteady surface flow of a frozen‐flux core that exactly fits a geomagnetic field model
Authors: Benton Edward R. , Celaya Michael A.
Published: February 2008

This paper reconsiders a fundamental ambiguity encountered during inversion of the radial component of the MHD induction equation evaluated at the top of a spherical, inviscid, frozen‐flux core. Specifically, we demonstrate that when the input model of the radial main geomagnetic field at any fixed point on the coremantle boundary evolves as an even degree polynomial in time, then there exists a unique unsteady fluid motion, that simultaneously evolves as a lowest degree polynomial in time and exactly generates the radial secular variation at that point. This result is illustrated by calculating the flow which evolves as a quartic in time and exactly fits the recent 1900–1980 field model of Bloxham and Jackson (1989), whose time dependence includes terms up through degree ten.
Inversion of teleseismic travel time residuals for velocity structure in the Larderello Geothermal Field, Italy
Authors: Foley John E. , Toksoz M. Nafi , Batini Fausto
Published: February 2008

The analysis of teleseismic travel time residuals recorded on the Larderello Seismic Network of central Italy has revealed a sharp low velocity zone (LVZ) in the center of the geothermal production area. Residuals from 101 teleseismic events are inverted to produce an image of a region of anomalously low velocities which we have interpreted to be a series of intrusive magmatic bodies. The top of this anomalous region is constrained to be below 6 km in depth from local earthquake seismicity patterns and extends to a depth of greater than 20 km. The LVZ broadens with increasing depth and may extend into the upper mantle. The observed travel time residual anomaly correlates spatially with observed gravity lows, regions of high heat flow, and the shallowing of a dominant upper crustal reflection horizon. This body has elevated the temperatures in the region to over 350° C at 2 km and appears to be the origin of the geothermal energy.
Scattering of regional Pn by Moho Topography
Authors: Kværna T. , Doornbos D. J.
Published: February 2008

The often observed relatively large amplitudes in the later part of the Pn signal cannot be explained with the traditional interpretation of Pn in 1‐D crust‐mantle models. To determine the cause of these characteristics we have analyzed in some detail the NORESS array records of Pn from a suite of quarry blasts in S.W. Norway. Application of wide‐band frequency‐wavenumber analysis to these records confirms that there is a slowness and azimuth anomaly associated with the dominant part of the wavetrain and that it is confined to a particular frequency range. Moreover, the scattering source of the anomaly is determined to be at Moho depth, which is consistent with the concept of scattering by topographic relief. We demonstrate the viability of this concept by means of numerical experiments, showing that for realistic models of topography, the energy flux of scattered P can dominate the specular flux (i.e., the flux in the direction defined by the ray crossing a plane interface) for incidence angles approaching the critical angle of Pn. Since the effects appear to be systematic, we have the possibility to calibrate the Pn parameters for event location and velocity determination purposes.
Coincident imaging and spectrometric observations of zenith OH nightglow structure
Authors: Taylor M. J. , Turnbull D. N. , Lowe R. P.
Published: February 2008

During the ALOHA‐90 campaign a novel comparative study was made between near infrared wave structure imaged in the zenith using a CCD camera and that detected at infrared wavelengths by a Fourier Transform Spectrometer. Coincident measurements were made briefly on several occasions and for an extended period on 31 March. The temporal variations imaged in the near infrared structure during this night almost completely matched those detected in the OH (3,1) band spectrometer data when similar viewing fields were compared. However, the image data also displayed small scale wave forms that were not resolved by the larger field instrument. These structures exhibited significant changes in brightness and position on a time scale much shorter than the local Brunt‐Väisälä period indicating that very high resolution measurements are necessary to investigate short period (<20 min) upper atmospheric wave motions.
Characteristics of AKR sources; A statistical description
Authors: Hilgers A. , Roux A. , Lundin R.
Published: February 2008

A description of plasma properties within the sources of the Auroral Kilometric Radiation (AKR) is given. It is based on data collected during ∼50 AKR source crossings in the altitude range between 4000 and 9000 km by the Swedish spacecraft Viking. The following results are obtained. (i) The frequency of the lowest frequency peak of the AKR fpeak is found to be very close to fce, the electron gyrofrequency ((fpeak–fce)/fce ≤ 0.08), on the average, (ii) The lower cutoff frequency fLC is on the average at fce ((fLC–fce)/fce ≃ 0), (iii) in the sources the density is typically less than 1.5 cm−3, which is of the order of the density of hot electrons and (iv) the source is located within an acceleration region, as evidenced by electrons accelerated above and ions accelerated below.
Inferences of mantle viscosity from tectonic plate velocities
Authors: Forte A. M. , Peltier W. R. , Dziewonski A. M.
Published: February 2008

The present‐day velocities of the Earth's tectonic plates provide direct constraints on the absolute value of the mantle viscosity. We present the results of a formal inversion of plate motion data and demonstrate its utility in constraining the depth variation of absolute viscosity.
Three‐dimensional simulation of hydrothermal circulation at mid‐ocean ridges
Authors: Travis Bryan J. , Janecky David R. , Rosenberg Nina D.
Published: February 2008

Results of numerical simulations of three dimensional hydrothermal convection in a layered medium representative of mid‐ocean ridge environments are presented. Regional‐scale three dimensional flow patterns naturally arise from reasonable assumptions about permeability distributions and magnitude of base heating. These flow patterns are characterized by broad downwelling but restricted regions of upwelling. Magmatic intrusions along the ridge axis induce vigorous convection restricted to the vicinity of the ridge‐axis, whose character changes during the intrusion cooling phase.
Excitation of artificial airglow by high power radio waves from the “SURA” Ionospheric Heating Facility
Authors: Bernhardt P. A. , Scales W. A. , Grach S. M. , Keroshtin A. N. , Kotik D. S. , Polyakov S. V.
Published: February 2008

The SURA facility for generation of high power radio waves, located near the village of Vasil'sursk USSR, operates between 4.5 and 9.0 MHz and has a maximum effective radiated power (ERP) of 300 MW. Nonlinear interactions between the HF radio waves and F‐layer plasma occur near the electromagnetic wave reflection point. Energetic electrons are accelerated out of the interaction regions by the electrostatic waves. Ambient oxygen atoms collisionally excited by these suprathermal electrons yield enhanced airglow. Low‐light‐level, optical measurements were made at SURA during September 1990. Images of enhanced red‐line (630 nm) emissions were recorded during radio wave transmissions at 4.786, 5.455, and 5.828 MHz. The antenna radiation pattern, ionospheric irregularities, and the magnetic field orientation affected the shape of the observed airglow structures. The airglow clouds drifted across the night sky, disappeared, and reformed at the zenith of the antenna array. This has been interpreted in terms of radio beam refraction in drifting plasma irregularities and bifurcation when the beam is split between two density cavities. Subject to clear skies, our experience indicates that the low‐light‐level‐imaging technique is a reliable method to study large scale irregularities and electron acceleration with high‐power HF transmitting facilities.
Chaotic jumps in the generalized first adiabatic invariant in current sheets
Authors: Brittnacher M. J. , Whipple E. C.
Published: February 2008

In attempting to develop a fluidlike model of plasma dynamics in a current sheet, kinetic effects due to chaotic non‐adiabatic particle motion must be included in any realistic description. Using drift variables, derived by the Kruskal averaging procedure, to construct distribution functions may provide an approach in which to develop the fluid description. However, the drift motion is influenced by abrupt changes in the value of the generalized first adiabatic invariant J. In this letter, we indicate how the changes in J derived from separatrix crossing theory can be incorporated into the drift variable approach to generating distribution functions. In particular, we propose a method to determine distribution functions for an ensemble of particles following interaction with the tail current sheet by treating the interaction as a scattering problem characterized by changes in the invariant.
How large is the retrograde annual wobble?
Authors: King N. E. , Agnew Duncan Carr
Published: February 2008

We compare recent measurements of polar motion (wobble) made by space‐geodetic methods with older optical measurements. Multitaper spectra of these data show that the retrograde (clockwise) annual wobble is much larger in the older data than the newer data, implying systematic errors in the older techniques. Two additional analyses show evidence for a small retrograde motion in the newer data, which appears to be consistent between data types. This implies that the excitation of the retrograde wobble must be about half of the prograde excitation. None of the available estimates of the excitation shows full agreement with these observations.
The Apollo 17 Ultraviolet Spectrometer: Lunar atmosphere measurements revisited
Authors: Feldman Paul D. , Morrison Daniel
Published: February 2008

Ultraviolet resonance fluorescence of solar radiation provides the most sensitive means of detecting the expected major constituents of the tenuous daytime lunar atmosphere. Such an experiment was carried out with the Apollo 17 Ultraviolet Spectrometer in December 1972 [Fastie et al, 1973b] and produced only upper limits to the number density of H, H2, O, C, N, CO and two noble gases near the surface of the moon. The complete data set of 47 terminator crossing observations, which were not utilized in the earlier analysis, has been re‐examined and more stringent upper limits to the column emission rates for several species have been derived. These results, together with most recent values for the atomic and molecular fluorescence efficiencies, lead to more definitive limits on the content of the lunar atmosphere. The revised upper limit on atomic oxygen density allows for the possibility of comparable O/Na ratios in the atmosphere of both the Moon and Mercury.
Geomagnetic field control of 14C production over the last 80 Ky: Implications for the radiocarbon time‐scale
Authors: Mazaud A. , Laj C. , Bard E. , Arnold M. , Tric E.
Published: February 2008

We have used a recent estimate of the geomagnetic dipole field strength over the last 80 ky to examine the extent to which changes in geomagnetic field intensity have affected the radiocarbon time scale. The magnetic record was converted into a 14C production signal which was then used to produce a synthetic atmospheric Δ14C record using a two box model for the carbon cycle. This calibration suggests that the geomagnetic field has been the main factor governing the production of cosmogenic 14C atoms over the period of time considered, and that Δ14C changes due to other causes, such as climatic changes, have been of lesser importance. The results indicate that the radiocarbon ages are 2 to 3 ky younger than true age in the interval 18–40 ky b.p., in good agreement with recent U‐Th and 14C age comparisons. Moreover the results predict that a negligible difference between true and 14C ages should be observed around 45–50 ky.
Sub‐milliarcsecond determination of pole position using Global Positioning System data
Authors: Herring Thomas A. , Dong Danan , King Robert W.
Published: February 2008

We discuss the determination of pole‐position variations from the analysis of 23 days of Global Positioning System (GPS) data continuously collected with a globally distributed, 21‐station network. The comparison of the estimated pole positions from these data with results from 11 very long baseline interferometry (VLBI) experiments conducted during this same interval yields weighted root‐mean square differences about the mean differences of 0.5 mas for both X‐ and Y‐pole position estimates from an analysis using single‐day arcs, and 1.0 and 0.8 mas from an analysis using three weekly multiday arcs. These results are of sufficient quality that GPS could provide an alternative technique to VLBI and satellite laser ranging (SLR) for regular monitoring of high‐time‐resolution variations in the position of the Earth's rotation axis.
Attractor dimensions in non‐linear fluctuations of length of the day (LOD) variations
Authors: Tiwari R. K. , Negi J. G. , Rao K. N. N.
Published: February 2008

Recent developments in the theory of non‐linear dynamical systems have been used to find dimensionality of the chaotic behaviour of the short term fluctuations in the Length of Day (LOD). Here we report, for the first time, the measure of attractor dimensions in the latest LOD data spanning over a period of approximately 15 years (1976–1990). Our analysis of daily variations of LOD indicate an evidence of low dimensional attractor of the system in the range 5 to 7. It can be inferred that the underlying dynamics has limited degrees of freedom and influenced by non‐random atmospheric and oceanic processes. We believe that the results may help to constrain models of linkages between earth's rotation and air‐sea interactions.
Source mechanism of the Luzon, Philippines Earthquake of July 16, 1990
Authors: Yoshida Yasuhiro , Abe Katsuyuki
Published: February 2008

The Luzon, Philippines earthquake of July 16, 1990, with Ms=7.8 was generated by left‐lateral faulting in the central Luzon Island. Ground breakage was observed along a linear trace extending 110 km. The maximum horizontal offset as measured on the fault was as much as 6 m. A fault area of 120 × 20 km2 was inferred from a field study and aftershock data. This event is one of the largest strike‐slip earthquakes ever recorded in the world. For long‐period surface waveform analysis, we used very broadband records from 14 stations of the GDSN, GEOSCOPE, IRIS and ERIOS networks. The Centroid Moment Tensor (CMT) solution from surface waves represents almost pure left‐lateral strike‐slip faulting on a steep fault with strike = 155°, dip = 88° and rake = −7°. The seismic moment is 3.9 × 1020 Nm (× 1027 dyn.cm), which gives Mw=7.7. The average dislocation on the fault is 5.4 m and the stress drop is 5.2 MPa (52 bars). Teleseismic P and SH waves from 14 stations are used to model the spatiotemporal distribution of subevents. Five subevents are detected in an interval of 44 sec. Their sequence indicates a bilateral rupture. The mean velocity of rupture toward the north is 2.7 km/sec. The total seismic moment of the subevents is 3.6 × 1020 Nm. The largest aftershock (Ms=6.6) occurred on July 17 near the recognized northern end of the main fault. The CMT solution for the aftershock represents a reverse faulting with moment of 6.4 × 1019 Nm (Mw=6.5). This aftershock was probably caused by compression produced by block motion of the eastern block of the main fault.
The distribution of olivine in the Crater Copernicus
Authors: Lucey Paul G. , Hawke B. Ray , Horton Keith
Published: February 2008

We employed multispectral imaging in the visible and near‐IR at four wavelengths (0.73, 0.96, 1.45, and 1.99µm) of Copernicus crater in order to map the distribution of olivine‐rich, pyroxene‐poor material known previously to occur in the central peak complex. We identified three additional portions of the crater to exhibit spectral characteristics similar to those of the central peaks strongly suggesting the presence of material similar to that exposed in the central peaks. These areas are a scarp forming a portion of the northern rim of Copernicus, and two slump blocks in the north wall which may have been derived from the same portion of the ejecta now exposed in the rim scarp. These occurrences decrease the minimum allowable depth for this unusual material in the Copernicus target site though still represent some of the deepest material exposed by Copernicus.
Strato‐meso‐thermospheric coupling at mid‐latitudes in the course of mid‐winter strat‐warmings during Dyana
Authors: Cevolani Giordano
Published: February 2008

Wind observations in the meteor region carried out at the CNR radar station near Bologna (45°N; 12°E) in the winter of 1989–90 during the DYANA (DYnamics Adapted Network for the Atmosphere) project, give evidence of significant strato‐meso‐thermospheric coupling in the course of two strat‐warmings recorded at the end of January and in mid‐February 1990. Amplification of 10–12 day long‐period waves just a few days before the temperature peak at the 10 hPa stratospheric level, and the strong phase variations of the semidiurnal tide (ST) in the 80–110 km region during the two separate events could be of interest for our knowledge of the evolution and influence of a strat‐warming in the overall middle atmosphere. Possible interference processes and nonlinear mixing between planetary waves with different periodicities could account not only for aspects of wave amplification but also for the presence of subsidiary peaks of long period waves in the amplitude spectra of the observed zonal winds.
Evaluation of the sensitivity of reflectance ratios to mafic minerals in the lunar regolith
Authors: Larson Stephen M. , Johnson Jeffrey R. , Singer Robert B.
Published: February 2008

The 950/560 nm spectral ratio has been traditionally used as a qualitative indicator or lunar soil maturity because the 950 nm band falls within the major Fe2+ mineral absorption bands near 1 µm characteristic of relatively immature, crystalline surfaces. Since there are several other processes unrelated to maturation that may affect this ratio value, we have evaluated the 950/730 nm ratio as a potentially better measure of the relative 1‐µm band strength. We find that the 950/730 nm ratio appears to be much more sensitive to the 1 µm band depth, but feel that other unknown variables preclude using these single ratios to provide meaningful quantitative results at this time. Because of problems with near‐IR scattering in vidicon detectors used in previous studies, we have measured the scattering properties of our two CCDs and conclude that any scattering is negligible.
Wind interaction with falling ejecta: Origin of the parabolic features on Venus
Authors: Vervack Ronald J. , Melosh H. Jay
Published: February 2008

Unusual parabolic features associated with impact craters have been observed by Magellan on Venus. A strong correlation exists between the orientation of the features and the zonal winds on Venus. We propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater. As the ejecta particles fall through the atmosphere, the winds transport them downwind from their entry point, smaller particles being transported a greater distance. Since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater, the winds blow the particles on the upwind side back upon one another, leading to a pile‐up of material. On the downwind side, the winds disperse the ejecta particles and no pile‐up occurs. The resulting thickness distribution on the Venusian surface matches the observed parabolic features closely. The dual parabolic feature associated with the crater Carson is also explained by this model.
TH isotope and U‐series disequilibria in some alkali basalts
Authors: Williams R. W. , Gill J. B.
Published: February 2008

The activities of 238U‐series nuclides and 232Th have been measured by alpha spectrometry for nine alkali basalts from Heard island, Mt. Cameroon, Nyamuragira, and Nyiragongo. Large 226Ra enrichments are inferred to be absent from all, suggesting the absence of subsolidus fluids before magma genesis. Th isotope ratios are consistent with deep metasomatism forming carbonates beneath Nyiragongo, versus shallower metasomatism forming amphibole beneath Nyamuragira. In contrast, the Th/U ratio in the Nyamuragira source has increased with time, which is used to confirm that the Th/U ratio of the mantle is about 4.2.
Airborne lidar observations of the Pinatubo volcanic plume
Authors: Winker D. M. , Osborn M. T.
Published: February 2008

A dual‐polarization lidar aboard the NASA Electra aircraft was used in July 1991 to survey the stratospheric plume from the recent Mt. Pinatubo eruption. Many distinct layers were observed, ranging from 17 to 26 km in altitude. Peak scattering ratios of as high as 80 at 532 nm were recorded. Total particle mass of the plume 27 days after the eruption is estimated from these measurements to be on the order of 8 megatonnes, with perhaps half the original SO2 converted to aerosol at that point.
On the contribution of the thermospheric neutral wind to high‐latitude energetics
Authors: Thayer J. P. , Vickrey J. F.
Published: February 2008

Although the neutral wind's contribution to ionospheric electrodynamics is well‐established at low latitudes, this electrical energy source has been largely ignored at high latitudes, owing to the assumed dominance of the magnetospheric dynamo contribution. Yet, the potential for exchange of electrical energy between the neutral wind dynamo and the magnetospheric dynamo is a direct consequence of the coupling between the two regions by highly conducting magnetic field lines. The integral nature of this coupling precludes the direct separation of the neutral wind and solar wind contributions to the observed electrodynamics. Therefore, to gain some insight into their relative importance, we have performed a simple numerical experiment in which the two dynamos are individually connected to a fixed load and their energetics evaluated separately. To determine the electrical energy flux supplied by the magnetosphere, we treat it as a voltage generator and the ionosphere as a resistive load. The available electrical energy flux generated by the neutral wind dynamo is determined from the mechanical energy stored within an established neutral wind field. This exercise has led to a number of conclusions, including: i) The neutral wind dynamo contributes significantly to high‐latitude energetics, particularly in the central polar cap; and ii) In the region near the plasma convection reversal boundary, the amount of energy flux available from the neutral wind dynamo can exceed that provided by the magnetospheric dynamo.
Acoustic imaging of hydrothermal plumes, East Pacific Rise, 21°N, 109°W
Authors: Rona P. A. , Palmer D. R. , Jones C. , Chayes D. A. , Czarnecki M. , Carey E. W. , Guerrero J. C.
Published: February 2008

We report the first observations based on acoustic imaging of large‐scale structure and time variability of buoyant plumes emanating from black smoker‐type seafloor hot springs. Three‐dimensional plume reconstructions were made from a digital data set of acoustic backscattering information recorded on a prototype submersible‐mounted sonar system. The acoustic images of two adjacent black smokers depict volume and show zones of flow organization (meters to tens of meters) in the lower 40 m of the buoyant plumes. The two plumes coalesce, bend in the prevailing current, exhibit short‐term (minutes) variation in cross section, and rapid (seconds) turbulent eddy variations at small scales (< 1 m). The plume imaging system is being developed for determination of plume dynamics, flux determinations when combined with chemical and thermal measurements, and long‐term monitoring of the activity of seafloor hydrothermal fields.
The auroral radiating plasma cavities
Authors: Hilgers A.
Published: February 2008

The electron density profile of the nightside high latitude region has been determined from a geocentric distance 1.5 RE to 3 RE by the use of the Viking Langmuir probe. Inside this region, density depletions are observed. Most of them coincide with acceleration structure crossings. Generation of Auroral Kilometric Radiation (AKR) is observed in the strongest depletions between 1.5 and 2.5 RE. A threshold on the ratio plasma to electron gyrofrequency for AKR generation to occur is estimated at 0.14. This is in good agreement with the cyclotron maser instability theory for AKR generation.
Implications of recent total atmospheric ozone measurements for biologically active ultraviolet radiation reaching the Earth's surface
Authors: Madronich Sasha
Published: February 2008

Recent satellite measurements of total atmospheric ozone were analyzed to deduce the changes in biologically active ultraviolet (UV) radiation reaching the Earth's surface from 1979 to 1989. The calculated increases are on average substantially larger than earlier estimates, particularly at mid and high latitudes of both hemispheres.
Neutral wind and temperature in the upper mesosphere above South Pole, Antarctica
Authors: Hernandez G. , Smith R. W. , Conner J. F.
Published: February 2008

The neutral wind and temperature of the upper mesosphere above Amundsen‐Scott Station (South Pole) have been optically determined from the emission of naturally‐occurring OH tracer molecular species. The results show that quasi‐periodic oscillations occur on these fields at near semidiurnal frequencies. In addition, the steady component of the winds indicates that the center of the polar vortex, at mesospheric heights, is displaced from the geographic pole. Examination of the data suggests that the observed oscillations are caused by the transport of lower‐latitude tidal oscillations by ducting processes.
Large perturbations of ammonium and organic acids content in the summit‐Greenland Ice Core. Fingerprint from forest fires?
Authors: Legrand M. , De Angelis M. , Staffelbach T. , Neftel A. , Stauffer B.
Published: February 2008

Biomass burning is influencing the atmospheric chemistry by emitting large amounts of reactive species such as hydrocarbons, organic acids and nitrogen compounds [Andreae et al., 1988]. Polar ice cores provide a unique record of precipitation whose chemistry reflects the atmospheric composition at the time of deposition. The analysis of such ice samples therefore allows an estimate to be made of the concentration of atmospheric impurities in the past. During the first season of the deep drill operation (GRIP) at Summit, Central Greenland (72° 34' N, 37° 38'W) continuous ammonium (NH4+) measurements were performed between 100 and 600 m depth covering the time period from 330 to 2500 years B.P. The NH4+ concentrations show seasonal variations between 1–20 ng.g−1 with some sporadic high values up to 600 ng.g−1 in narrow layers. The chemical fingerprint points to biomass burning causing the high ammonium peaks.
Transport of gyration‐dominated space plasmas of thermal origin: 1. Generalized transport equations
Authors: Gombosi Tamas I. , Rasmussen Craig E.
Published: February 2008

Grad's 20‐moment set of transport equations has been examined in the limit of strong external magnetic fields. In this approximation, transport perpendicular to field lines is assumed to follow E×B convection paths, and the original 20‐moment set of equations reduces to a set of six partial differential equations. This simplified set of equations describes the transport of mass and parallel momentum as well as the transport of parallel and perpendicular energy and heat flow in the magnetic field direction. However, since convection perpendicular to field lines is consistently carried throughout the derivation of the reduced 20‐moment set, the transport of mass, momentum, energy, and heat flow perpendicular to the magnetic field is also explicitly maintained in this formulation. The effect of collisions was calculated assuming a modified relaxation model. Wave speeds and normal modes of the simplified set of equations were examined for an ion and electron gas. It was found that four of the 10 normal modes are electron thermal‐heat waves which approximately decouple from the six ion waves in the system. When low‐frequency waves are considered (slow‐wave approximation), this allows the electron energy and heat flow equations to be solved separately from the ion equations and in a time‐independent fashion. When this was done, it was found that under certain conditions these equations predict an infinite electron perpendicular temperature, Te⊥, in the collisionless regime. This occurs whenever Te⊥ is greater than the parallel temperature, Te∥ at any point along collisionless and diverging magnetic field lines. This nonphysical result calls into question the validity of generalized transport equations in the collisionless regime whenever Te⊥ is greater than Te∥. However, when applied appropriately, the significantly simplified set of equations derived here are well suited for application to a large variety of problems in planetary ionospheres and magnetospheres. We have also demonstrated that the heat flow must be limited to values smaller than the pressure times the thermal speed or otherwise the fundamental assumptions of the 20‐moment truncation are violated. The double adiabatic approximation for hypersonic situations in the presence of heat flow was also examined.
The Venus ultraviolet aurora: A soft electron source
Authors: Fox J. L. , Stewart A. I. F.
Published: February 2008

The Pioneer Venus Orbiter Ultraviolet Spectrometer has recorded continuous but variable emissions of atomic oxygen at 1304 and 1356 Å in images of the nightside of Venus. We show that the observed intensities are consistent with the presence of precipitation of soft electrons into the nightside thermosphere. Model calculations are presented in which upper and lower limits to the magnitude of the electron flux necessary to produce the observed intensities are derived. Constraints are imposed on the energy spectrum of the electrons by the measured ion densities and by the predicted intensities of other emissions that have not been detected.
Ionospheric frequency spread and its relationship with range spread in mid‐latitude regions
Authors: Bowman G. G.
Published: February 2008

The distinction between range spread and frequency spread as seen on mid‐latitude ionograms is discussed. A classification of these two types of spread F is proposed in terms of different arrangements of the duplicate traces which provide the basic trace structures of mid‐latitude spread F ionograms. Experimental results are presented to support the idea that frequency spread results from multiple ray paths (associated with a shallow ripple structure in the isoionic contours) close to the direction of the zenith position, so that each ray path has a range approximately equal to that of its neighbor. Furthermore, a horizontal gradient of maximum electron density is an additional requirement to create frequency spread. Atmospheric conditions (involving ionospheric F2 region heights and upper atmosphere neutral particle densities) which seem to favor the generation of frequency spread are discussed.
Probing the magnetic topologies of magnetic clouds by means of solar energetic particles
Authors: Kahler S. W. , Reames D. V.
Published: February 2008

Magnetic clouds are large (<0.25 AU) interplanetary regions with topologies consistent with those of magnetic loops. They are of interest because they may be an interplanetary signature of coronal mass ejections. Clouds have been identified in solar wind data by their magnetic properties and by the presence of bidirectional particle fluxes. Two possible closed magnetic topologies have been considered for clouds: (1) an elongated bottle with field lines rooted at both ends in the Sun and (2) a detached magnetic bubble or plasmoid consisting of closed field lines. The inferred topologies are also consistent with open field lines that converge beyond 1 AU. We have used solar energetic particles (SEPs) as probes of the cloud topologies. The rapid access of SEPs to the interiors of many clouds indicates that the cloud field lines extend back to the Sun and hence are not plasmoids. The small modulation of galactic cosmic rays associated with clouds also suggests that the magnetic fields of clouds are not closed.
Role of Hall conductivity in the ionospheric dynamo
Authors: Takeda Masahiko
Published: February 2008

Effects of the Hall conductivity (σ2) on the ionospheric dynamo were examined. Simulations for the ionospheric dynamo by the (1,−2) mode tidal winds were made for the cases with and without σ2. It was found that existence of σ2 does not modify global morphology of the height‐integrated ionospheric currents much, although it enhances the total currents measured by the intensity of the equivalent current vortex by 2.1 times and diminishes the total potential difference by 1.4 times. This can be understood by the extended Cowling mechanism in which σ2 makes secondary polarization field and the field drives the Hall current which enhances the original Pedersen currents. It was also shown that the equatorial electrojet and an undulated latitudinal profile of the geomagnetic H field variation near the equatorial electrojet can be understood by the latitudinal variation of the extended Cowling effect.