bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport‐theoretic model for the electron‐proton‐hydrogen atom aurora: 1. Theory
Authors: Basu B. , Jasperse J. R. , Strickland D. J. , Daniell R. E.
Published: February 2008

The first self‐consistent transport‐theoretic model for the combined electron‐proton‐hydrogen atom aurora is presented. This is needed for accurate modeling of the diffuse aurora, particularly in the midnight sector, for which a statistical study (Hardy et al., 1989) indicates that the proton contribution to the total auroral energy flux is (on the average) about 20 to 25% of that of the electrons. As a result, the ionization yield as well as the yields of many emission features will be underestimated (on the average) by about the same percentage if the proton‐hydrogen atom contributions are neglected. The model presented here can also be used to study a pure electron aurora or a pure proton‐hydrogen atom aurora by choosing the appropriate boundary conditions, namely, by setting the incident flux of one or the other particle population equal to zero. In the latter case, the new feature of the present model is the rigorous transport‐theoretic treatment of the contributions to ionization rates and to emission rates and yields from the secondary electrons produced by protons and hydrogen atoms. A coupled set of three linear transport equations is presented. Protons and hydrogen atoms are coupled only to each other through charge‐changing (charge exchange and stripping) collisions, while the electrons are coupled to both protons and hydrogen atoms through the secondary electrons that they produce. Source functions for the secondary electrons produced by the three primary particle populations are compared and contrasted, and the numerical methods for solving the coupled transport equations are described. Finally, formulas for calculating pertinent aurora‐related quantities from the particle fluxes are given. In the companion paper (Strickland et al., this issue), the model results are presented.
Simulations of particle acceleration in parallel shocks: Direct comparison between Monte Carlo and one‐dimensional hybrid codes
Authors: Ellison Donald C. , Giacalone J. , Burgess D. , Schwartz S. J.
Published: February 2008

We have made a direct comparison between two different computer simulations of a plane, parallel, collisionless shock including particle acceleration to energies typical of those of diffuse ions observed at the earth bow shock. Despite the fact that the one‐dimensional hybrid and Monte Carlo techniques employ entirely different algorithms, they give surprisingly close agreement in the overall shapes of the complete distribution functions for protons as well as heavier ions. Both methods show that energetic ions emerge smoothly from the background thermal plasma with approximately the same relative injection rate and that the fraction of the incoming plasma's energy flux that is converted into downstream enthalpy flux of the accelerated population (i.e., the acceleration efficiency) is similar in the two cases. The fraction of the downstream proton distribution made up of superthermal particles is quite large, with at least 10% of the energy flux going into protons with energies above 10 keV. In addition, an upstream precursor, produced by backstreaming energetic particles, is present in both shocks, although the Monte Carlo precursor is considerably longer than that produced in the hybrid shock. These results offer convincing evidence that, at least in these ways, the two simulations are consistent in their description of parallel shock structure and particle acceleration, and they lay the groundwork for development of shock models employing a combination of both methods.
Interference patterns in the Spacelab 2 plasma wave data: Lower hybrid waves driven by pickup ions
Authors: Feng Wei , Gurnett Donald A. , Cairns Iver H.
Published: February 2008

During the Spacelab 2 mission the University of Iowa's Plasma Diagnostics Package (PDP) was released from the shuttle to explore the plasma environment around the shuttle. Wideband spectrograms were obtained from the PDP at frequencies from 0 to 30 kHz and distances up to 400 m from the shuttle. The wideband data frequently showed antenna interference patterns when the PDP was on the downstream side of the shuttle. Analysis of these interference patterns allows a determination of the wavelength, the plasma rest frame frequency, the direction of propagation, the power spectrum, and in some cases the location of the source. We concentrate our analysis on interference patterns due to lower hybrid waves: waves which have rest frame frequencies near the lower hybrid frequency and propagate perpendicular to the magnetic field. The waves have an almost flat dispersion relation with frequencies just above the lower hybrid frequency and relatively short wavelengths (1‐4 m). The observed lower hybrid waves depend strongly on the position of the PDP relative to the shuttle and the magnetic field direction. Our results confirm previous suggestions that the lower hybrid waves are generated primarily in the vicinity of the shuttle and that they are driven by a charge exchange interaction between the ambient ionosphere and a H2O cloud around the shuttle.
Equatorial spacecraft‐plasma interaction phenomenon observed with DE 2
Authors: Cragin B. L. , Hanson W. B.
Published: February 2008

Data from the retarding potential analyzer and ion drift meter on Dynamics Explorer 2 indicate that an unusual spacecraft‐plasma interaction phenomenon occurs at times when the spacecraft velocity vector becomes nearly aligned with the local geomagnetic field lines. The primary signature of the interaction is a transient increase in the ion collection currents obtained with these instruments, up to approximately 15% in magnitude and typically a few tens of seconds duration. This signature is indicative of an increase in the net ion and electron currents to the satellite and is accompanied by a small positive increase in the spacecraft potential relative to the plasma. We present here a case study covering six of the strongest such events observed with DE 2 and discuss a possible physical mechanism. We suggest in particular that what might be called a collisional electron “snowplow” effect may be occurring, and we derive a simple numerical model based on this scenario. Least squares fitting is employed to test the model and to derive new estimates of the ambient ion concentration at times when the measurements are being perturbed by the interaction.
Energetic electron power flux deposition at Sanae (L = 4.0) From riometer recordings
Authors: Stoker P. H.
Published: February 2008

From minute‐by‐minute and daily‐averaged absorptions of cosmic radio wave noise, deduced from continuous riometer recordings during the last 20 years at Sanae, the annual temporal pattern of energetic (≳40 keV) electron energy flux deposited per second into the atmosphere has been estimated. We conclude that the energy flux deposited per second is closely related to geomagnetic activity, both for auroral precipitation during magnetic active periods and for particles precipitated from the trapping regions. The average energy flux deposited per second is strongly peaked during the equinoxes and has a deep minimum during the summer and winter, related to the equinoctial effect of geomagnetic activity. No difference exists between the energy fluxes deposited per second during the illuminated Antarctic summer and the dark Antarctic winter. It follows then that the daytime deposited flux is larger than the nighttime averaged exposition of energetic electrons. The averaged electron energy flux deposited per second peaked during 1973‐1975 and 1982‐1984, years with persistent solar wind high‐speed streams from low solar latitude coronal holes, and has a lower value in a year of solar polar magnetic reversal, i.e., in 1971 and in 1980, than during solar minimum activity in 1976 and 1986/1987. During the 20 years of investigation the solar flare events of August 1972 and October 1989 are outstanding in flux of electron precipitation, being ∼3 orders of magnitude larger than the annually averaged flux.
Kinetic physics of the mirror instability
Authors: McKean M. E. , Gary S. P. , Winske D.
Published: February 2008

Kinetic mechanisms for the growth and saturation of the mirror instability are described using one‐dimensional hybrid simulations. Two parameter regimes are considered. In the first regime, a relatively small ion anisotropy excites a slowly growing instability that produces small‐amplitude waves; most ions respond to the waves as an adiabatic fluid. In the second regime a large anisotropy excites a rapidly growing instability that generates large‐amplitude waves; the response of many ions in this case is nonadiabatic. The difference in ion response is due to the relative importance of two ion populations, resonant and nonresonant. Resonant ions, those ions with low velocities parallel to the background magnetic field, contribute to the growth of the instability as a result of their gyrointeractions with the noncoplanar component of the wave electric field and respond to the mirror waves nonadiabatically. Nonresonant ions, those with large parallel velocities, respond as an adiabatic fluid. In both regimes, ion anisotropy is reduced by means of the magnetic mirror force; in the second regime, the anisotropy is further reduced as a consequence of the field rotation at magnetic minima. The anisotropy reduction reduces the free energy available for wave growth and leads to the saturation of the mirror instability.
Preliminary results from the Berkeley EUV Airglow Rocket Spectrometer: O I and N2 FUV/EUV Dayglow in the thermosphere and lower exosphere
Authors: Cotton D. M. , Chakrabarti S. , Gladstone G. R.
Published: February 2008

The Berkeley extreme‐ultraviolet airglow rocket spectrometer (BEARS) made spectroscopic measurements of far and extreme UV, atomic oxygen emissions from a Black Brant XII (12.041 WT) sounding rocket launched from Wallops Island, Virginia, on September 30, 1988. BEARS' primary instrument, a near‐normal Rowland mount spectrometer, measured several atomic oxygen and molecular nitrogen dayglow features at high spectral resolution (1.5 Å): O I (989, 1027, 1304, and 1356 Å); and N2 Lyman‐Birge‐Hopfield (4,0) and (3,0) bands at 1325 and 1354 Å. The instrument collected over 800 s of data spanning altitudes of 150‐963 km with look directions between 95° and 125° from zenith. We have analyzed the data using electron and radiative transport models in a forward modeling approach. The model and data are generally in good agreement. However, there are some discrepancies, which are discussed in terms of remote sensing capabilities and improvements to the models. In particular, the data indicate an optically thick cascade contribution of 40% to the O I (1304 Å) emission. There is a significant contribution to the O I (1027 Å) feature due to Lyman β resonant scattering and an inconsistency in the modeled and measured intensities in the lower thermosphere.
Low‐frequency electromagnetic plasma waves at comet P/Grigg‐Skjellerup: Analysis and interpretation
Authors: Neubauer F. M. , Glassmeier K. ‐H. , Coates A. J. , Johnstone A. D.
Published: February 2008

The propagation and polarization characteristics of low‐frequency electromagnetic wave fields near comet P/Grigg‐Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted with the following new findings: (1) Broad pickup‐generated wave fields exist in the magnetoplasma around comet P/GS. (2) Outside the bow wave/shock, wave fields can be pictured as plane waves with propagation angles of about 10° with respect to ±B0, where B0 is the background magnetic field. (3) An envelope of very regular wave fields with almost exclusively left‐hand polarization in the spacecraft frame exists around the bow wave/shock. More complex polarization properties are found farther away. (4) Because of the unusual interplanetary magnetoplasma characteristics the ratio MAr of the local ring beam velocity component parallel to the magnetic field in the plasma frame of reference and the Alfvén velocity varies between zero and a little greater than 1. As a consequence, the waves excited in the plasma frame must be predominantly left‐hand Alfvén waves propagating away from the Sun in contrast to the right‐hand waves propagating toward the Sun in the same frame of reference at comets P/Giacobini‐Zinner and P/Halley. (5) it is argued that the regular character of the waves close to but upstream of the bow wave/shock is due to the effects of nongyrotropy and/or nonlinear dispersive effects. (6) Magnetosheath waves are characterized as compressive, nonplanar waves with mixed polarization.
Bernstein waves in the Io plasma torus: A novel kind of electron temperature sensor
Authors: Meyer‐Vernet Nicole , Hoang Sang , Moncuquet Michel
Published: February 2008

During Ulysses passage through the Io plasma torus, along a basically north‐to‐south trajectory crossing the magnetic equator at R ∼ 7.8 RJ from Jupiter, the Unified Radio and Plasma Wave experiment observed weakly banded emissions with well‐defined minima at gyroharmonics. These noise bands are interpreted as stable electrostatic fluctuations in Bernstein modes. The finite size of the antenna is shown to produce an apparent polarization depending on the wavelength, so that measuring the spin modulation as a function of frequency yields the gyroradius and thus the local cold electron temperature. This determination is not affected by a very small concentration of suprathermal electrons, is independent of any gain calibration, and does not require an independent magnetic field measurement. We find that the temperature increases with latitude, from ∼1.3 × 105 K near the magnetic (or centrifugal) equator, to approximately twice this value at ±10° latitude (i.e., a distance of ∼1.3 RJ from the magnetic equatorial plane). As a by‐product, we also deduce the magnetic field strength with a few percent error.
The dusty ballerina skirt of Jupiter
Authors: Horanyi M. , Morfill G. , Grün E.
Published: February 2008

We suggest a model to explain the unexpected recurrent dust events that were observed during the Jupiter encounter by the dust detector on board the Ulysses spacecraft. This model is based on dust‐magnetosphere interactions. Dust particles inside the Jovian magnetosphere collect electrostatic charges and their interaction with the magnetic and electric fields can lead to energization and subsequent ejection. We discuss the dusty regions (ring/halo, “gossamer” ring) and also Io as potential sources for the Ulysses events. This model favors Io as a source. The mass and velocity range of the escaping particles are compatible with the observations, and we also suggest internal periodicities to explain the recurrent nature of the Ulysses dust events.
Crustal structure of the offshore southern Santa Maria Basin and transverse ranges, southern California, from deep seismic reflection data
Authors: Henrys Stuart A. , Levander Alan R. , Meltzer Anne S.
Published: February 2008

Seismic reflection data from the 1986 EDGE/Pacific Gas and Electric experiment together with refraction and gravity data are used to construct an integrated north‐south geophysical cross section along the offshore Santa Maria Basin and across the offshore extension of the Transverse Ranges. Thrust faults offsetting acoustic basement and folding Miocene sediments root in a midcrustal detachment at 12–13 km depth. The detachment coincides with an increase in reflectivity seen on the seismic reflection section and the maximum depth of earthquakes. A high‐velocity oceanic crustal layer at the base of the crust (17–22 km) and Moho (≈ 25 km) is offset by a low‐angle thrust fault with an apparent dip ≈ 10° to the north offshore Point Arguello. Gravity models indicate crustal thickening and a change in the depth to Moho from 25 to 30 km in this region. However, the contribution of elastic flexure from loading of the Transverse Ranges is considered to be minor in this part of the borderland. The cross section constrains Neogene shortening to less than 3% based on faulting and folding of the sedimentary section. Shortening of the upper crust is balanced by a combination of ductile flow and subduction of the oceanic crust beneath the midcrustal detachment.
Predictive modeling of the seismic cycle of the Greater San Francisco Bay Region
Authors: Bufe Charles G. , Varnes David J.
Published: February 2008

The seismic cycle for the San Francisco Bay region is synthesized by a model combining the pre‐and post‐1906 seismic histories. The long‐term acceleration of seismic release (seismic moment, Benioff strain release, or event count) in the seismic cycle and the shorter‐term accelerations preceding the larger earthquakes within that cycle are modeled using an empirical predictive technique, called time‐to‐failure analysis, in which rate of seismic release is proportional to an inverse power of the remaining time to failure. The exponent of time to failure in the accelerating sequences appears to be scale invariant, and the length of the full cycle is estimated at 269 ± 50 years. The 1989 Loma Prieta earthquake, which is the culmination of the first subcycle in the present long‐term seismic cycle, should have been predictable with an uncertainty of 2 years in time and 0.5 in magnitude, although the specific location (at Loma Prieta) was not predictable by this technique. If our model is correct and if the Loma Prieta earthquake is the culmination of a subcycle, the San Francisco Bay region should be entering a relatively long (20–50 years) period of seismic quiescence above magnitude 6. A great earthquake, such as the 1906 San Francisco event, would appear to be more than a century in the future.
Static compression of Mg(OH)2 to 78 GPa at high temperature and constraints on the equation of state of fluid H2O
Authors: Fei Yingwei , Mao Ho‐Kwang
Published: February 2008

Static compression data on brucite, Mg(OH)2, to 35 GPa at 300 K and to 80 GPa at 600 K were measured by synchrotron X ray diffraction with a diamond‐anvil cell. A least squares fit yielded bulk modulus KT0 = 54.3(±1.5) GPa and its pressure derivative K′T0 = 4.7(±0.2) at 300 K, and KT = 48.8(±1.0) GPa and KT′ = 4.74(±0.12) at 600 K. The temperature derivative of the bulk modulus, (∂KT/∂KTp, is −0.018(±0.008) GPa/K, assuming that KT linearly depends on temperature. The average thermal expansivity α decreases from 80(±4) × 10−6 K−1 at ambient pressure to 10(±6) × 10−6 K−1 at 80 GPa. The coefficient as a function of pressure P (in gigapascals), can be expressed by α (K−1)= 80 × 10−6(1 + 0.087P)−0.97. The thermal expansion and elastic properties of Mg(OH)2 are highly anisotropic such that the c axis with the linear thermal expansion coefficient αc0 = 59(±4) × 10−6 K−1 and the linear incompressibility Kc0 = 75(±5) GPa, is about 5 times more expansible as well as compressible than the a axis with αa0 = 11(±3) × 10−6 K−1 and Ka0 = 388(±10) GPa. The present equation of state for Mg(OH)2 and previously determined equations of state for MgO and H2O ice VII were used to calculate the breakdown pressures of Mg(OH)2 into MgO plus H2O ice VII in the stability field of H2O ice VII. This equilibrium boundary is used to constrain the dehydration curve of Mg(OH)2 at very high pressure because the intersection point between this boundary and the melting curve of ice VII must be the triple point of Mg(OH)2, MgO plus ice VII, and MgO plus fluid H2O. The calculation shows that the equilibrium boundary between Mg(OH)2 and MgO plus ice VII intersects the melting curve of ice VII at 55(±15) GPa and 850(±20) K. The calculation of the melting curve of ice VII indicates that the existing equation of state of fluid H2O, derived from shock compression and molecular dynamics calculations, may overestimate the volume of fluid H2O at pressures above 50 GPa.
Tensile fracture of rock at high confining pressure: Implications for dike propagation
Authors: Rubin Allan M.
Published: February 2008

Field observations indicate that zones of inelastic deformation produced at the tips of propagating dikes can be much larger than those produced at the tips of tensile cracks in laboratory experiments. This is in direct conflict with the concept that fracture toughness and fracture energy are rock properties, independent of crack size and loading configuration. A Barenblatt model that treats fracture resistance as an internal cohesive stress acting at the crack tip is used to investigate the effect of confining pressure on rock tensile failure. When the confining pressure exceeds the cohesive strength of the rock, as it does at depths greater than several hundred meters, Linear Elastic Fracture Mechanics is inapplicable and the near‐tip stress field of a propagating crack is determined by the crack size and loading configuration as well as by rock properties. As inelastic deformation depends upon the near‐tip stress field, it follows that fracture energy may also depend upon crack size and loading configuration. For a propagating dike, the near‐tip stress field is dominated by the large suction acting within a small (∼several meter) cavity at the tip generated by viscous flow of magma within the dike. Perturbations to the ambient stress are on the order of the cavity suction and act over regions on the order of the cavity length. The tip cavity pressure may be maintained by exsolution of magmatic volatiles or by influx of host rock pore fluids; inelastic deformation is enhanced by the latter. For a tip cavity pressure maintained by influx of pore fluids, the pore pressure exceeds the least compressive stress off the dike plane, even while it equals the least compressive stress at the dike tip. This can lead to tensile failure off the dike plane and the formation of observed dike‐parallel joints. Shear stresses scale with the cavity suction and may produce shear failure off the dike plane; such deformation is generally enhanced if the dike is intruded perpendicular to the least compressive stress. For sills intruded parallel to bedding, shear failure in the form of bedding plane slip can lead to the observed blunting and fingering of the intrusion front. Because the tip cavity grows with dike size, the energy consumed by rock fracture also increases with dike size and is potentially as significant for large dikes as for small dikes, a view not adopted by existing fluid mechanical models of dike propagation.
Earthquake travel time tomography of the southern Santa Cruz Mountains: Control of fault rupture by lithological heterogeneity of the San Andreas Fault Zone
Authors: Foxall W. , Michelini A. , McEvilly T. V.
Published: February 2008

The 1989 Loma Prieta earthquake occurred along the stretch of the San Andreas fault zone within the southern Santa Cruz Mountains that last failed as a major earthquake in 1906. The southeastern end of the 1989 rupture marks the transition from stable, aseismic slip on the central creeping section of the San Andreas fault to unstable failure on the locked 1906 segment. We investigate this transition and the rupture characteristics of the 1989 earthquake using a three‐dimensional P wave velocity model of the southern Santa Cruz Mountains section of the fault zone. The velocity model was determined by joint progressive inversion of 5422 P wave arrival times from 173 earthquakes, which included aftershocks of the 1989 earthquake and premainshock “background” events, recorded by the U.S. Geological Survey central California network. This velocity model was used to relocate 424 background earthquakes, the Loma Prieta mainshock, and 292 aftershocks. The velocity model is calibrated in terms of lithology using laboratory data for California Coast Ranges rock types and available seismic refraction profiles and is shown to be in good agreement with the surface geology. The model images a large anomalous high‐velocity body at midcrustal depths within the rupture zone of the 1989 earthquake that the available evidence suggests might have gabbroic or other mafic composition. On the basis of the relationship of the lithological features interpreted from the velocity model to the seismicity and surface creep we propose a model in which the high‐velocity body is primarily responsible for the transition from stable to unstable fault slip at Pajaro Gap. The active plane of the San Andreas fault cuts throughout the body. The fault system attempts to circumvent this barrier by transferring slip to secondary faults, including splay faults that have propagated along the frictionally favorable contact between the high‐velocity rock mass and Franciscan country rocks. However, the near arrest of stable sliding causes stress to concentrate within the body, and the high‐strength, unstable contact within it evolves from a barrier to the asperity that failed in the 1989 earthquake. The general features of the 1989 rupture predicted by this asperity model agree with several rupture histories computed for the earthquake. The model implies that as proposed by other workers, the Loma Prieta earthquake did not involve a repeat of the 1906 slip, which has an important bearing on earthquake recurrence estimates for the Santa Cruz Mountains segment of the fault.
Paleomagnetism of the Mississippi Valley‐type Newfoundland zinc deposit: Evidence for Devonian mineralization and host rock remagnetization in the northern Appalachians
Authors: Pan H. , Symons D. T. A.
Published: February 2008

Newfoundland Zinc Mines (NZM) are exploiting sphalerite‐rich Mississippi Valley‐type (MVT) zones in platformal carbonate rocks of the Humber Zone in western Newfoundland. The zones are stratabound within the upper part of the Lower Ordovician St George group. Geologic studies in the mine area simply require ore genesis to postdate the Early to Middle Ordovician Taconic orogeny. Thermal and alternating field (AF) demagnetization reveal that both the ore and adjacent host rocks at the NZM have similar characteristic remanence magnetizations (ChRM) with normal polarity. The ChRM direction from both ore and host rocks is at 9°/30° (declination/inclination) (N = 16 sites, α95 = 6°, k = 46) which gives a pole position of 22°N, 113°E (dp = 3°, dm = 6°). This pole falls on the boundary between the Middle and Late Devonian on the North American apparent polar wander path and corresponds in time to the Acadian orogeny. Paleomagnetic breccia tests show that the host rocks were remagnetized after the Middle Ordovician and that this remagnetization coincides with or postdates MVT mineralization. The ChRMs paleoinclination indicates that both remagnetization and mineralization are not younger than Late Devonian. The similar magnetization characteristics of the adjacent and remote host rocks in the Humber Zone and the authigenic nature of the magnetic carriers revealed by scanning electron microscopy with energy dispersive analysis suggest that the NZM host rock has a secondary magnetization. From the pervasive nature of both the secondary remagnetization and the trace MVT mineralization in the northern Humber Zone, we argue that both events are coeval. We suggest that Acadian orogenic uplift forced hot basinal brines into the porous and fractured Ordovician shelf carbonates of the northern Humber Zone.
Paleomagnetism and tectonic interpretations of the Taos Plateau Volcanic Field, Rio Grande Rift, New Mexico
Authors: Brown Laurie L. , Caffall Nancy M. , Golombek Matthew P.
Published: February 2008

The tectonic response of the Taos Plateau volcanic field in the southern San Luis basin to the late stage extensional environment of the Rio Grande rift was investigated using paleomagnetic techniques. Sixty‐two sites (533 samples) of Pliocene volcanic units were collected covering four major rock types with ages of 4.7 to 1.8 Ma. Twenty‐two of these sites were from stratigraphic sections of the lower, middle and upper Servilleta Basalt collected in the Rio Grande gorge at two locations 19 km apart. Flows from the lower and middle members in the southern gorge record reversed polarities, while those in Garapata Canyon are normal with an excursion event in the middle of the sequence. The uppermost flows of the upper member at both sites display normal directions. Although these sections correlate chemically, they seem to represent different magnetic time periods during the Gilbert Reversed‐Polarity Chron. Alternating field demagnetization, aided by principal component analysis, yields 55 sites with stable directions representing both normal and reversed polarities, and five sites indicating transitional fields. Mean direction of the normal and inverted reversed sites is I = 49.3° and D = 356.7° (a95 = 3.6°). Angular dispersion of the virtual geomagnetic poles is 16.3°, which is consistent with paleosecular variation model G, fit to data from the past 5 m.y. Comparison with the expected direction indicates no azimuthal rotation of the Taos Plateau volcanic field; inclination flattening for the southern part of the plateau is 8.3° ± 5.3°. Previous paleomagnetic data indicate 10°–15° counterclockwise rotation of the Espanola block to the south over the past 5 m.y. The data suggest the Taos Plateau volcanic field, showing no rotation and some flattening in the south and east, has acted as a stable buttress and has been downwarped by overriding of the southeastern end of the plateau by the Picuris Mountains, which make up the northern corner of the counterclockwise rotating Espanola block.
Measuring rapid ocean tidal earth orientation variations with very long baseline interferometry
Authors: Sovers O. J. , Jacobs C. S. , Gross R. S.
Published: February 2008

Ocean tidal effects on universal time and polar motion (UTPM) are investigated at four nearly diurnal (K1, P1, O1, and Q1) and four nearly semidiurnal (K2, S2, M2, and N2) frequencies by analyzing very long baseline interferometry (VLBI) data extending from 1978 to 1992. We discuss limitations of comparisons between experiment and theory for the retrograde nearly diurnal polar motion components due to their degeneracy with prograde components of the nutation model. Estimating amplitudes of contributions to the modeled VLBI observables at these eight frequencies produces a statistically highly significant improvement of 7 mm to the residuals of a fit to the observed delays. Use of such an improved UTPM model also reduces the 14–30 mm scatter of baseline lengths about a time‐linear model of tectonic motion by 3–14 mm, also with high significance levels. A total of 28 UTPM ocean tidal amplitudes can be unambiguously estimated from the data, with resulting UT1 and PM magnitudes as large as 21 μs and 270 microarc seconds (μas) and formal uncertainties of the order of 0.3 μs and 5 μas for UT1 and PM, respectively. Empirically determined UTPM amplitudes and phases are compared to values calculated theoretically by Gross from Seller's global ocean tide model. The discrepancy between theory and experiment is larger by a factor of 3 for UT1 amplitudes (9 μs) than for prograde PM amplitudes (42 μas). The 14‐year VLBI data span strongly attenuates the influence of mismodeled effects on estimated UTPM amplitudes and phases that are not coherent with the eight frequencies of interest. Magnitudes of coherent and quasi‐coherent systematic errors are quantified by means of internal consistency tests. We conclude that coherent systematic effects are many times larger than the formal uncertainties and can be as large as 4 μs for UT1 and 60 μas for polar motion. On the basis of such realistic error estimates, 22 of the 31 fitted UTPM ocean tidal amplitudes differ from zero by more than 2σ.
Through thick and thin: A new model for the Colorado Plateau from seismic refraction data from Pacific to Arizona Crustal Experiment
Authors: Wolf Lorraine W. , Cipar John J.
Published: February 2008

We have constructed a model for the crust and uppermost mantle beneath the Colorado Plateau by combining newly acquired seismic refraction/wide‐angle reflection data from the 1989 Pacific to Arizona Crustal Experiment (PACE) cross profile with data collected by Roller (1965). A combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases. The salient features of the model for the Colorado Plateau (hereafter referred to as the plateau) are (1) an upper crust having an average velocity of 6.1 km/s with a low vertical velocity gradient, (2) midcrustal discontinuities at depths of 30 to 38 km beneath the plateau, (3) a lower crust that varies in thickness (5 to 16 km) and has an average velocity of 6.8 km/s, (4) a transitional lower crust/upper mantle boundary, and (5) a crustal thickness of 45 ± 3 km. A comparison of our new model for the Colorado Plateau with recent models for the Basin and Range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker, particularly in the plateau's central region. These observations are consistent with the idea that the Basin and Range could have formed from crust like that of the Colorado Plateau.
Geologic implications of great interplate earthquakes along the Aleutian Arc
Authors: Ryan H. F. , Scholl D. W.
Published: February 2008

The Aleutian arc is one of the most seismically active regions of the world. It consists of a number of segments or blocks that are rotating clockwise and translating westward along the North American‐Pacific plate boundary. Two of the largest recorded earthquakes have occurred along the interplate boundary south of the Andreanof Islands in 1957 and the Rat Islands in 1965. The large magnitudes of these earthquakes implied that the probability of the recurrence of a large‐magnitude event at these same locations was low. However, the occurrence of a great earthquake south of the Andreanof Islands in 1986 less than 30 years after the 1957 event has prompted a reevaluation of the seismic potential of the arc. We present new marine geophysical observations and synthesize previous geologic interpretations of the Aleutian arc to show that the epicenters of these great thrust‐type earthquakes coincide with upper plate segments of the arc characterized by a coherent forearc structural fabric. These segments, which are associated with large plutonic masses, are mechanically stronger and are related to the location of asperities positioned at depth along the main thrust zone; they appear to be more strongly attached to the North American plate than adjacent blocks. The tectonically controlled locations of these asperities thus appear to influence the seismicity along the arc. The Andreanof and Rat segments of the arc are separated by the Delarof segment, which is considered to have a high probability of a large‐magnitude earthquake in the near future based on probabilistic estimates of seismic potential (Nishenko, 1991). The Delarof segment of the arc, however, is highly disrupted by arc‐transverse faults. This segment is structurally weak and does not appear to be capable of accumulating the interplate strain necessary to generate a great, subduction zone earthquake. We propose that variations in upper plate structural strength and mobility affect the mechanical properties of the interplate thrust zone and need to be considered in localizing interplate asperities. Forearc tectonic segmentation associated with the partitioning of strike‐slip and thrust motions may exert long‐term controls on the rates of seismic moment release.
Lateral variations and azimuthal isotropy of Pn velocities beneath Basin and Range Province
Authors: Zhao Lian‐She
Published: February 2008

Travel time tomography was used to obtain the lateral variations in Pn velocities and Pn velocity azimuthal anisotropy beneath the Basin and Range Province. We used 1226 Pn arrivals from 184 earthquakes recorded by 113 stations. The tomographic image shows the lateral variation in Pn velocity from 7.9 km s−1 in the northern Province to 7.6 km s−1 in the southern Province. Azimuthal variations in Pn velocities are less than one percent as determined from the travel time residuals after taking out the contribution of the lateral velocity variations of the Pn tomography. Although the standard errors of the azimuthal variations are ∼ 2%, there is no reason to believe the existence of the Pn azimuthal anisotropy in the uppermost mantle beneath the Basin and Range Province. Beghoul and Barazangi's (1990) suggestion of more than 3% azimuthal anisotropy is the result of the lateral heterogeneity and mantle velocity gradient. Before tomographic inversion, we refined the Pn travel times by considering the lateral variations in the crustal thickness and velocities in the region based on the numerical experiment and theoretical development given in this paper. These refinements are crucial in obtaining an accurate velocity image. The average velocity is 7.72 ± 0.16 km s−1, and an average vertical mantle velocity gradient is 8.0 ′ 10−4 s−1. The resolution for Pn velocity image is generally within 2° to 5°.
Electron microscope studies of methane sulfonic acid in individual aerosol particles
Authors: Qian Gong‐Wang , Ishizaka Yutaka
Published: February 2008

The electron microscope has been used in studies of the nature of methane sulfonic acid (MSA) particles collected from the air of Sakushima Island in Japan, near the Pacific Ocean. Particles were impacted on electron microscope screens for morphological analysis and thin‐film chemical tests. MSA particles could be identified on the basis of their distinctive morphology on the collection surface. In contrast to MSA, sulfuric acid has a similar morphology on a carbon film, but a different morphology on a nitron‐barium chloride film. It was found that MSA‐containing particles were dominant in the coarse‐particle fraction under very humid conditions. The examination also suggested that the MSA particles were present as mixed particles. The number concentration of MSA particles varied from 0 cm−3 to 10−1 cm−3. Possible mechanisms for the formation of MSA‐containing particles are discussed in light of the field measurements.
The Mediterranean water tongue and its chlorofluoromethane signal in the Iberian Basin in early summer 1989
Authors: Hinrichsen H.‐H. , Rhein M. , Käse R. H. , Zenk W.
Published: February 2008

Hydrographic observations from the Iberian Basin demonstrate the variability of water masses in upper and intermediate layers. The surveyed area embraces the internal front between water masses from higher latitudes and the Mediterranean outflow, exhibits several isolated Mediterranean eddy (meddy) structures at middepth, and displays the virtual source region for the Mediterranean Water (MW) tongue off the Portuguese continental slope. The description is enhanced by additional chlorofluoromethane measurements, which show anomalously high concentrations at middepth, due to mixing of MW with the overlying Atlantic waters in the Gulf of Cadiz. The geostrophic stream function shows several meddylike features that not only are remarkably extended in the depth range of the MW, but are also correlated with surface height anomalies.
Determination of ocean surface heat fluxes by a variational method
Authors: Roquet H. , Planton S. , Gaspar P.
Published: February 2008

A new technique of determination of the “nonsolar” heat flux (sum of the latent, sensible, and net infrared fluxes) at the ocean surface is proposed. It applies when oceanic advection remains weak and thus relies on a one‐dimensional modeling approach. It is based on a variational data assimilation scheme using the adjoint equation formalism. This allows to take advantage of all observed data with their error estimates. Results from experiments performed with station Papa (Gulf of Alaska) and Long‐Term Upper Ocean Study (LOTUS, Sargasso Sea) data sets are discussed. The temperature profiles assimilation allows the one‐dimensional model to reproduce correctly the temperature evolution at the surface and under the oceanic mixed layer at the two sites. The retrieved fluxes are compared to the fluxes calculated through classical empirical formulae. The diurnal dependence of the fluxes at the LOTUS site is particularly investigated. The results are also compared with those obtained using a simpler technique based on an iterative shooting method and allowing the assimilation of the only sea surface temperature. This second comparison reveals that the variability of the retrieved fluxes is damped when temperature in the inner ocean are assimilated. This is the case for the diurnal cycle at the LOTUS mooring. When the available current data at this site are assimilated, the diurnal variability of the retrieved fluxes is further decreased. This points out a model discrepancy in the representation of mixing processes associated to internal wave activity. The remaining part of the diurnal cycle is significant and could be due to a direct effect of air‐sea temperature difference.
Possible forcing effects of net evaporation, atmospheric pressure, and transients on water transports in the Mediterranean Sea
Authors: Harzallah Ali , Cadet Daniel L. , Crepon Michel
Published: February 2008

The present paper examines the influence of evaporation fluxes over the Mediterranean Sea on water flows through the straits of Gibraltar and Sicily. Evaporation fluxes are estimated from European Centre for Medium‐Range Weather Forecasts atmospheric analyzed fields for the period 1981–1985. An aerological approach is followed to evaluate the net water exchanged between the sea and the atmosphere. The resulting Mediterranean Sea circulation is then deduced from water budget in the sea. Net evaporation is found to be the major forcing for the circulation through the straits. Mean flows through the Strait of Gibraltar are found to be slightly weaker than commonly accepted values but appear to be close to recent estimations. Temporal variations of net evaporation show a relatively important annual cycle which may induce significant annual cycle of water flows through the straits. For each month of the studied period, fluxes are separated into two parts: a steady state flux and transients. Transient atmospheric events generate a net water loss in the Mediterranean and may induce a net water inflow through Gibraltar. The latter flow is suggested to be related to Atlantic/Mediterranean water interface fluctuations. Water sinking is also investigated. It is mainly related to the mean component of atmospheric events in the eastern Mediterranean and to transient events in the western Mediterranean. Atmospheric pressure effect shows a significant variability, especially during winter, and may induce a slight water flow through the straits.
Static and dynamic response of natural multicomponent oceanic surface films to compression and dilation: Laboratory and field observations
Authors: Bock Erik John , Frew Nelson M.
Published: February 2008

The quasi‐static elasticity (Gibbs' elasticity) of sea surface films collected in North American coastal waters has been examined using a large number of surface pressure‐area isotherms. The films examined show considerable variability in elastic properties. Differences among films appear to be the result of chemical variability due to differences in source; they are also affected by dynamic physical processes (e.g., film compression). Elasticities of adsorbed films from subsurface and microlayer water are observed to exhibit a bimodal distribution at 0.5 mN m−1 reference spreading pressure; films from organized banded slicks are shown to have generally higher elastic moduli. Mechanisms of surface selectivity and competitive adsorption are suggested to explain this effect; results of film aging and work hardening studies are presented in support. Significant hysteresis effects are observed for cyclical compression and dilation of the films suggesting the prevalence of relaxation processes that lead to compositional changes and higher elastic moduli. Field measurements of capillary and ripple spectra are presented to infer information on wave damping and dynamic viscoelasticity. Results obtained in regions where surface convergence is absent imply damping enhancement over a broad frequency range consistent with theory for films of low elastic moduli. Films found in surface convergence zones, the result of surface upwelling, exhibit damping enhancements consistent with higher elastic moduli. Damping maxima are observed in multiple frequency bands; this banding is not readily explained with current theories. Results similar to these have been observed in laboratory studies with other chemical systems. In both the laboratory and field experiments, the tendency of the films to group into a few subsets with similar surface characteristics implies that natural films might be modeled by a small number of representative end members. This would facilitate remote sensing applications by providing a basis for determining the effects of films on surface waves under various conditions (e.g., diurnal variation, history of variability of wind stress).
Hydrographic variability off the Rías Baixas (NW Spain) during the upwelling season
Authors: Alvarez‐Salgado X. A. , Rosón G. , Pérez F. F. , Pazos Y.
Published: February 2008

During the Galicia X cruise, from May to October 1989 an intensive collection of hydrographic data was carried out at a single station on the shelf off the western coast of Galicia. It allows us to follow the response of the water column to the intermittent equatorward wind stress during the upwelling season. Upwelling events occur with biweekly periodicity, bringing Eastern North Atlantic Water (ENAW) to the subsurface layer at our station. A trend of the thermohaline properties of the upwelled water to increase in time was observed. This seems to be mostly due to the southwestward displacement of the origin of this water mass during the year. Although the saltier and warmer ENAW is less nutrient‐rich, nutrient levels increase because of the rapid remineralization of organic matter from the Rías, which takes place in the bottom water on the shelf.
Formation and stability of shelf break fronts
Authors: Condie Scott A.
Published: February 2008

An analytical, inviscid two dimensional model is used to examine the formation of surface‐to‐bottom density fronts over shelf slope topography. Previous studies have examined the geostrophic adjustment of a vertical front near the shelf break. However, our approach makes no such assumption about the initial density field, requiring only that all upper‐layer fluid originate over the shelf. The main limitation of the study is that the lower‐layer velocities are assumed to be negligible. The simple model demonstrates that geostrophy and potential vorticity conservation are sufficient conditions to ensure that a front will intersect the bottom close to the shelf break. Aspects of the theory have been compared with results from simple laboratory experiments. The experiments also reveal the influence of frontal processes such as instability and wave formation that are not included in the theoretical model. In particular, they provide clear confirmation of the stabilizing influence of steep topography below fronts.