bluuebunny/crossref_metadata_2025 huggingface dataset:
Neutral winds in the thermosphere at mid‐latitudes over a full solar cycle: A tidal decomposition
Authors: Buonsanto M. J.
Published: February 2008

The magnetic meridional component of the thermospheric neutral wind at the F2 peak at Boulder, Colorado, is derived for days with magnetic index Ap ≤ 20 over a full solar cycle (July 1975–June 1986). The servo model technique and ionosonde measurements of hmF2 are used, and the results are corrected for ionization drifts due to electric fields. A Fourier decomposition of the mean daily variation is carried out for each month. This reveals significant components with wave periods of 24, 12, 8, and 6 hours. The importance of ion drag in causing seasonal and solar cycle variations in the winds is inferred. Specifically, the conclusion of previous work that the diurnal amplitude is larger at solar minimum than at solar maximum is confirmed by the present study. The diurnal amplitude is generally in the range 60–80 m s−1 at solar minimum but only 25–45 m s−1 at solar maximum. The diurnal phase maximum of the northward wind lies in the range 1115–1625 LT with latest values in winter and at solar maximum, when the daytime effects of ion drag are greatest. The amplitude of the semidiurnal component shows little solar cycle variation but tends to be smaller in summer compared to other seasons. It has a mean value of 41 m s−1 over the solar cycle. The semidiurnal phase maximum is remarkably close to 0800 LT for all seasons throughout the solar cycle. The amplitudes of the 8 and 6 hour waves are both in the range 4–25 m s−1. The 8 and 6 hour waves both have later phase maxima in summer than in winter. The present results are generally consistent with current models of atmospheric tides, except for the large solar cycle variation in the diurnal component.
Ionosphere‐exosphere coupling through charge exchange and momentum transfer in hydrogen‐proton collisions
Authors: Hodges R. R. , Breig E. L.
Published: February 2008

We examine the implications of a traditional assumption of exospheric physics, that collisions of hydrogen atoms and protons preferentially result in charge exchange with negligible momentum transfer. Initially adopted as a necessary convenience to accommodate limited computer resources in exosphere model calculations, this approximation results in a direct transformation of the proton velocity distribution into a hot component of neutral hydrogen. It leads to significant enhancements in both escape and in the formation of extended planetary coronae. With expanding computational facilities, the need for the approximation has passed. As the first step toward its replacement with a realistic, quantum mechanical model of the H‐H+ collision process, we calculate differential and cumulative cross sections for quantum elastic scattering of indistinguishable nuclei for a fine grid of encounter energies and scattering angles. These data are used to study the nature of ionosphere‐exosphere coupling through H‐H+ collisions, and to demonstrate that the distribution of velocities of scattered H produced in the traditional exospheric charge exchange approximation, as well as that arising from an alternative, fluid dynamics approach, leads to unacceptable abundances of coronal atoms in long‐term, highly elliptic trajectories.
Global upper mantle structure from long‐period differential travel times
Authors: Woodward Robert L. , Masters Guy
Published: February 2008

We have made over ten thousand measurements of PP‐P and SS ‐ S differential travel times from long‐period Global Digital Seismograph Network recordings of all events with mb ≥ 5.5 which occurred during the years 1976 to 1986. We have obtained reasonable global coverage, with very good sampling of several regions of the world. The residuals range from −5 to 5 s for PP‐P and −12 to 12 s for SS‐S and reveal a large‐scale pattern of heterogeneity when plotted at the reflected phase's bounce point. Our experiments indicate that lower‐mantle structure and source‐receiver structure can each contribute approximately ±0.5 s to our measured PP – P residuals so there is considerable signal to be explained. The pattern observed in the PP – P measurements is similar to the pattern observed in the SS‐S measurements, with the SS‐S residuals 2 to 4 times larger in magnitude. Comparisons of our measured residuals to those predicted by the upper‐mantle models of Woodhouse and Dziewonski show that the overall patterns are quite similar but the amplitude of the model residuals is roughly a factor of 2 too small. Comparisons with the predictions of a whole‐mantle model of Tanimoto again shows that the predicted pattern of residuals is reasonably consistent with the observations but now the predicted residuals are too large by about a factor of 2. This variation in predicted amplitudes is probably mainly due to differences in the near‐surface structure of the models. We have also binned our measurements according to the tectonic regionalization GTR1 of Jordan and find a qualitative correlation of average residual with tectonic region. In particular, Precambrian shields show a strong anomaly, and there is a correlation of residual size with the age of oceanic crust at the bounce point. For all tectonic regions the ratio of SS ‐ S to PP ‐ P residuals is approximately 2. This ratio is consistent with a thermal origin for the observed signal. Finally, our measurements show no compelling evidence for azimuthal anisotropy which might be related to fossil spreading direction or the direction of absolute plate motion. Such a signal is probably overwhelmed by the large signal from three‐dimensional structure.
Heat flow in Jane Basin, northwest Weddell Sea
Authors: Lawver Lawrence A. , Della Vedova Bruno , Von Herzen Richard P.
Published: February 2008

The Jane Basin is a marginal basin situated immediately to the east of the South Orkney microcontinent in the Northwest section of the Weddell Sea. Thirty‐five heat flow measurements made in the Jane Basin ranged in value from 67.5±4.3 to 92.1±3.0 mW/m2. Excluding values that were corrected for tilting or were on the very edge of the basin, the remaining 28 values range between 75.0±8.0 and 84.6±9.1 mW/m2. Magnetostratigraphy on the recovered core from Ocean Drilling Program hole 697, which was drilled in Jane Basin to a depth of 322 m, allowed sedimentation rates to be calculated back to 4.5 Ma. Single‐channel seismic reflection data from RSS Shackleton allowed estimations of total sediment thickness for the Jane Basin to be made. We calculate that the measured heat flow is only 86–89% of the actual heat flow as a result of sedimentation. Heat generation in the sediments contributes 1.5–1.9 mW/m2to the total heat flow. The corrected heat flow gives an age for the Jane Basin of between 25 and 32 Ma from age‐versus‐heat flow comparisons, similar to the age determined from basement depth. The Scotia Sea, located to the north of Jane Basin and the South Orkney microcontinent, has been dated as anomaly 10 (30 Ma) and younger. Our calculated age for the Jane Basin would indicate that it may have been created prior to the initiation of seafloor spreading in the Scotia Sea. Evidence from major plate motions indicate that Antarctica began to rotate clockwise away from South America at about 65 Ma. Such motion may have triggered subduction along the southeast side of Jane Bank and the opening of Jane Basin as a back arc basin. Subduction at Jane Bank ended at anomaly 6A time (22 Ma) as evidenced by the age of the identified magnetic anomalies on the Antarctic plate found immediately to the east of Jane Bank. We conclude that Jane Basin opened prior to the opening of the Scotia Sea and that the spreading center that opened Jane Basin may have jumped to the Scotia Sea and produced the seafloor spreading there.
Paleomagnetism of Late Mesozoic granitic intrusions in Hong Kong: Implications for Upper Cretaceous reference pole of south China
Authors: Chan Lung S.
Published: February 2008

Two Late Jurassic‐early Cretaceous granite plutons in Hong Kong were sampled at 16 sites for paleomagnetic determination. The two granites show differences in magnetic properties and mineralogy but similar natural remanent magnetization (NRM) directions. Multiple magnetizations and dual polarity have been detected in the samples. Isothermal remanent magnetism, magnetic susceptibility, and coercivity measurements reveal multidomain titanomagnetite as the principal magnetic carrier. A rating of paleomagnetic results from individual sites was based on the number of specimens, efficacy of the demagnetization procedures, and within‐site variance of NRM directions. The pole position from 12 high quality sample sites is 171.9°E, 78.2°N, A95=10.6°. The dual polarity and cooling ages of the granites suggest a Late Cretaceous acquisition of the remanent magnetism. Based on four different studies, the Upper Cretaceous reference pole of South China is located at 194.3° longitude, 73.5°N latitude, A95=10.4°. Paleomagnetic results from North China, South China, and Eurasia show no large‐scale Cenozoic relative movements among the three blocks. But the small separation between the paleomagnetic poles of South China and Eurasia probably implies a minor relative movement since the Late Cretaceous.
On the along‐track acceleration of the LAGEOS satellite
Authors: Scharroo R. , Wakker K. F. , Ambrosius B. A. C. , Noomen R.
Published: February 2008

The Earth‐induced Yarkovsky thermal drag is known to be the principal drag mechanism for LAGEOS. In combination with charged and neutral particle drag this thermal drag can account for all of the observed average along‐track acceleration as well as its long periodic variation, if we allow for an alignment of the satellite's spin axis with the Earth's polar axis, decreasing the colatitude of the spin axis by 50% every 6 years. This motion is probably due to the interaction of eddy currents in the satellite and the Earth's magnetic field. The spin axis appears not to precess in longitudinal direction, indicating that gravitational torques hardly affect the satellite. Calculations confirm that the related principal moments of inertia of the spacecraft differ much less than generally assumed. A combination of two other force models, operating only during the partly eclipsed orbital periods, is likely to explain most of the anomalous residual fluctuations in the along‐track acceleration. The first model is the Yarkovsky‐Schach effect, a variant of the aforementioned thermal drag, involving heating by the Sun. The second embodies anisotropic reflection of sunlight by LAGEOS' surface; one hemisphere of the satellite probably has a specular albedo, which is about 5% larger than the other. By applying these two models, in addition to particle and Yarkovsky thermal drag, the standard deviation of the residual acceleration could be reduced by 61%.
Deterministic frequency‐wavenumber methods and direct measurements of rupture propagation during earthquakes using a dense array: Theory and methods
Authors: Goldstein Peter , Archuleta Ralph J.
Published: February 2008

We present an approach for measuring the spatial extent, duration, directions and speeds of rupture propagation during an earthquake using array signal processing techniques. Unlike most previous approaches, no assumptions are made regarding the directions and speeds of propagation. The solution we obtain is unique, and its accuracy, precision and resolution can be estimated. We use preprocessing and coprocessing techniques that improve the accuracy/precision of existing frequency‐wavenumber methods. Subarray spatial averaging modifies frequency‐wavenumber techniques so that they are sensitive to multiple deterministic (transient) as well as stochastic (stationary) signals. This technique is also advantageous because it ensemble averages the cross spectral matrix and reduces the variance of the frequency‐wavenumber spectrum. Seismogram alignment is a preprocessing procedure that accounts for nonplanar wavefronts due to lateral velocity variations beneath an array. This procedure also allows us to study signals of interest using much shorter time windows than has previously been possible. The use of short time windows is important for reducing uncertainty in estimates of arrival times of seismic phases. We tested subarray spatial averaging and seismogram alignment with a variety of frequency‐wavenumber techniques and found that the multiple signal classification (MUSIC) method gave the best resolution of multiple signals. We also note that standard theoretical estimates of uncertainties in peak locations of frequency‐wavenumber spectra are much smaller than those typically observed using seismic arrays and we present a different formula that more accurately describes observed uncertainties. Using synthetic P and S body‐wave seismograms from extended earthquake sources we show that the above array signal processing techniques can be combined with ray theory to obtain accurate estimates of the locations and rupture times of an earthquake's high‐frequency seismic sources. We show how to estimate uncertainties in source locations and rupture times due to limitations of the data, uncertainties in source parameters, and uncertainties in velocity structure. We find that the size of the uncertainties can be very sensitive to a fault's geometry relative to an array and we suggest criteria for optimizing an array's location. We show that high‐frequency source locations and their associated rupture times can be used to estimate an earthquake's spatial extent, duration, directions and speeds of rupture propagation.
Recent ground movement and seismic activity in Campi Flegrei, southern Italy: Episodic growth of a resurgent dome
Authors: Dvorak John J. , Berrino Giovanna
Published: February 2008

Leveling surveys in Campi Flegrei, an active volcanic caldera in southern Italy, show that this caldera has undergone both subsidence and uplift of a few meters during the past 85 years. A single source located at a depth of about 3 km beneath the center of the caldera can account for almost all elevation changes recorded in Campi Flegrei since the first leveling survey conducted in 1905/1907. This includes leveling surveys that span periods of subsidence (such as between 1905/1907 and 1919 and between January 1985 and October 1986) and periods of rapid uplift (between March 1970 and October 1971 and between January 1981 and December 1984). A possible geometry for the source of uplift and subsidence beneath Campi Flegrei is a rectangular horizontal sheet located at a depth of about 3 km and centered a few hundred meters east of the city of Pozzuoli. The horizontal dimensions of this sheet are about 1.5 km by 3.0 km; the long axis is oriented N70°W. This sheet lies adjacent to the most intense area of seismicity that occurred from mid‐1983 to December 1984. During an uplift episode this sheet may represent the top of a magma body undergoing internal pressure changes, possibly by intrusion of additional magma. Subsidence is caused by groundwater removal from the water‐saturated deposits that fill this caldera to a depth of a few kilometers. A temporary increase in subsidence rate after an uplift episode is caused by increased rate of groundwater removal from newly fractured regions that lie immediately above the magma body. The close proximity of this water‐saturated region and the magma body makes it impossible to separate the depth to these two sources from the results of leveling surveys. Two observations indicate that uplift has occurred in the center of Campi Flegrei for at least the past several thousand years. First, marine deposits in the center of Campi Flegrei were uplifted at least 40 m between 4000 and 8000 years ago. Second, continuous seismic profiles of the undersea portion of Campi Flegrei have identified only one region where rocks are fractured and faulted: adjacent to the uplifted marine deposits that surround the point of maximum uplift indicated by recent leveling surveys. On the basis of these observations and the historical record of activity we suggest that Campi Flegrei caldera is in a stage of growing a resurgent dome. The present diameter of the resurgent dome is about 4 km, and the maximum amount of vertical uplift during the past 10,000 years has probably not exceeded 100 m. The episodes of rapid uplift and shallow earthquake swarms during the past 20 years are probably related to the growth of this resurgent dome.
Constraints on the strength of the upper crust from stress inversion of fault slip data
Authors: Reches Ze'ev , Baer Gidon , Hatzor Yossef
Published: February 2008

The coefficient of friction of small faults in the field are estimated here by stress inversion of fault slip data. The small faults that were measured in Israel and the Grand Canyon, Arizona, are considered as representing natural friction experiments. The stresses associated with the faulting are determined by a stress inversion method which incorporates the Coulomb failure criterion [Reches, 1987]. The coefficients of friction determined for 27 fault clusters in limestone, sandstone, and basalt range from 0.0 to 1.3 with mean value of 0.58 ± 0.37. These values are in general agreement with the friction of 0.6–0.85 determined from laboratory experiments. The magnitudes of the calculated principal stresses are compared with in situ stress measurements in similar tectonic environments.
Absolute gravity measurements in California, 1984–1989
Authors: Sasagawa Glenn , Zumberge Mark A.
Published: February 2008

Repeated absolute gravity measurements have been made at 12 sites in California between 1984.3 and 1989.7. As determined in laboratory tests, the instrument used has an estimated accuracy of 10 μGal (approximately 10−8 g). The repeatability of the measurements is consistent with this accuracy assessment. No gravity changes above the limits set by instrumental uncertainty and environmental noise are observed in California during this period; the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1–2 cm/yr.
Arc deformation and marginal basin opening: Japan Sea as a case study
Authors: Jolivet Laurent , Huchon Philippe , Brun Jean Pierre , Le Pichon Xavier , Chamot‐Rooke Nicolas , Thomas Jean Charles
Published: February 2008

We discuss the opening mechanism of the Japan Sea in Miocene time using (1) tectonic and published paleomagnetic data along the eastern margin from the north of Hokkaido Island to Sado Island, (2) a mechanical model which is tested by small‐scale physical modeling, and (3) crustal structure and bathymetric features in the Japan Sea which constrain our kinematic model and preopening reconstructions. Our main conclusions are the following. The eastern margin of the Japan Sea was, as a whole, a dextral shear zone about 100 km wide. This conclusion is supported by the existence of a ductile dextral shear zone in Central Hokkaido (Hidaka Mountains) and associated brittle deformation in western Hokkaido and northeastern Honshu. The stress field during the opening (which ended about 12 Ma ago at the end of the middle Miocene) changes from right‐lateral transpression in the north to right‐lateral transtension in the south. The western margin, along the Korean peninsula, during the same period, also was an active dextral shear zone. Paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north. We propose a model of right‐lateral pull‐apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one. Small‐scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary (Pacific side) is free of stress. The opening stopped and compression subsequently began about 12 Ma ago. Finally, we show that the dextral shear, which is distributed over the whole Japan Sea area, is accommodated by N‐S trending right‐lateral faults and rotation of blocks located between these right‐lateral faults.
Deterministic frequency‐wavenumber methods and direct measurements of rupture propagation during earthquakes using a dense array: Data analysis
Authors: Goldstein Peter , Archuleta Ralph J.
Published: February 2008

Data recorded by the SMART 1 array in Taiwan are used to make the first direct measurements of two‐dimensional earthquake rupture propagation. Using frequency‐wavenumber techniques and ray theory, we obtain estimates of the spatial extent, duration, and directions of rupture propagation during two earthquakes located offshore Taiwan. Our estimates include uncertainties due to errors in fault plane location and orientation, velocity structure, and slowness (ray parameter) measured at the array. We find that the January 29, 1981, ML = 6.3 Taiwan earthquake ruptured unilaterally updip and toward the west on a 60° dipping, 109° striking reverse fault. Using P waves, we find its fault length and duration to be 25±18 km and 7.4±3.4 s, respectively. S waves indicate a fault length and duration of 27±15 km and 9.4±3.6 s, respectively. We suggest that the November 14, 1986, ML = 7.0 Hualien, Taiwan earthquake was triggered by a shallow (h ≈ 14 km) foreshock ML = 5.4 which occurred on a subparallel or splay fault approximately 5 s before the mainshock. Based on P waves, rupture propagation during the foreshock is unilateral towards the northeast and slightly downdip on a 58° dipping, 38° striking reverse fault. Its spatial extent and duration are approximately 23 km and 5.4 s. The mainshock initiated at greater depth (h ≈ 34 km) than the foreshock with its primary direction of rupture unilateral toward the northeast and slightly updip on a 57° dipping, 43° striking reverse fault. Using P waves, we found a spatial extent and duration of 68±15 km and 12.4±2.2 s. S waves indicate a similar extent, duration, and direction of propagation for the mainshock. However, we found it difficult to distinguish the foreshock and mainshock 5 waves. We show, by example, that these results represent an improvement over what can be obtained using long‐period (T > 5 s) teleseismic methods. Comparison of our results with those obtained using teleseismic data suggests either a short duration slip‐time function or that most slip occurs near the rupture front. Our results indicate that much of the complexity in the seismograms recorded during these events, by the SMART 1 array, is due to spatial and temporal variations of the source. This suggests that, at least in some cases, predictions of both the phase and amplitude of strong ground motion, as a function of time, would require a priori knowledge of source properties. Finally, we recommend siting small (≈ 10 station) dense arrays on land above offshore subduction zones, which are considered seismic gaps, because they can provide more detailed measurements of rupture propagation than is presently possible.
Tide gage measurements of uplift along the south coast of Alaska
Authors: Savage James C. , Plafker George
Published: February 2008

Annual mean sea levels along the south coast of Alaska are used to measure uplift along the Alaska‐Aleutian subduction zone. Oceanographic effects are removed from the observed annual mean sea levels by subtracting a correction that is proportional to the sea level fluctuations observed in southeast Alaska. That correction is effective in reducing fluctuations in the observed, annual mean sea level as far west as the tip of Alaska peninsula. Additional corrections to remove the eustatic rise in sea level and the apparent fall in sea level due to postglacial isostatic rebound of the land are introduced. This corrected sea level record should provide a measure of tectonic subsidence. In the area affected by the 1964 Alaska earthquake, postseismic uplift occurs where coseismic subsidence was observed, and postseismic subsidence occurs where coseismic uplift was observed. The immediate postseismic response is damped out within the first decade, and the subsequent uplift rates appear to be steady over the 1974–1989 interval. However, some of those rates seem to be too high to be sustained over the ∼1000 year earthquake recurrence interval appropriate to this area if the interseismic deformation is only to recover the coseismic displacement. Thus a long‐term ( ∼100 years) relaxation in uplift rates is postulated. The immediate (time constant ∼5 years) postseismic relaxation is attributed to postseismic slip on the plate interface directly downdip from the coseismic rupture. The long‐term (time constant ∼100 years) relaxation is attributed to flow in the asthenosphere.
Oceanic crust of the Grenada Basin in the Southern Lesser Antilles Arc Platform
Authors: Speed R. C. , Walker J. A.
Published: February 2008

Seismic refraction data permit the southern Lesser Antilles arc and surrounding regions to be divided by the velocity of their basement. We propose that high‐velocity basement of the arc platform beneath the Grenadine islands and below a part of the Tobago Trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the Grenada Basin. Low‐velocity basements of the Tobago terrane and the arc platform from St. Vincent north lie south and north, respectively, of the high‐velocity basement of the arc platform. An oceanic origin of this high‐velocity crust in the Grenadines is argued to be more plausible than an origin as unroofed lower arc crust. The segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc, mainly in late Neogene time, relative to the Grenada Basin and Tobago Trough. Accepting the proposition of shallow oceanic crust in the Grenadines, early middle Eocene and possibly older pillow basalts of Mayreau, the oldest rock unit of the southern Lesser Antilles arc platform, may be an exposure of such basement. Major and minor element compositions of Mayreau Basalt are indicative of a spreading rather than arc origin. The stratigraphy of the pillow basalts indicates extrusion in an open marine environment, distant or shielded from sources of arc or continental sediment, followed by a period of pelagic sedimentation above the carbonate compensation depth. The Eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc‐derived turbidites and pelagic sediments accumulated over the succeeding 25–30 ma. Such basalts thus indicate a probable spreading origin of the Grenada Basin and an age of cessation of spreading in the region of Mayreau in Eocene time. The configuration of the Eocene basin and the direction of spreading, however, are unknowns. Regional structural relationships imply the spreading was probably backarc, an origin also hinted at by Ce/Zr ratios of Mayreau Basalt. Although the Aves Ridge probably was the remnant arc, the existence and position of an early middle Eocene frontal arc are unresolved. The early Paleogene tectonic architecture of the southeastern Caribbean differed substantially from that of the Neogene.
Stress field determinations in France by hydraulic tests in boreholes
Authors: Cornet F. H. , Burlet D.
Published: February 2008

The stress field has been determined at eight different sites in France (four in crystalline or metamorphic rocks, four in sedimentary formations) by hydraulic tests in boreholes. For seven of these sites the complete stress field has been determined using the hydraulic tests on pre‐existing fractures (HTPF) inversion method. Validity of the results is demonstrated by the good fit between a priori and a posteriori values for the data and by the low values of the a posteriori standard deviation on the unknowns. For four sites, results have been compared with those derived according to the hydraulic fracturing theory. The two methods yield comparable results for the orientation of the maximum horizontal principal stress σH, a generally satisfactory fit for the magnitude of the minimum horizontal principal stress σh, but a very poor agreement for the magnitude of σH. This latter misfit has been attributed to the effect of fluid percolation prior to the actual opening of the fractures. Below a depth of 500 m, a homogeneous σH direction (N150°E) has been determined at the crystalline sites (Auriat, Echassières, Le Mayet de Montagne, Chassoles), all located in the northern Massif Central. For three of these sites the vertical stress is significantly lower than the weight of overburden as computed from the rock density and the depth of the corresponding measurement. At Auriat and Echassières the stress field is consistent with a mostly strike‐slip faulting regime. At Le Mayet de Montagne and Chassoles the maximum stress is vertical but nearly equal to σH. The stress field has been found to be much more heterogeneous at two of the four sites in sedimentary rocks because of the large variability in mechanical rock properties. In such heterogeneous formations, inversion with the HTPF method must be limited to those data which pertain to the same rock horizon. However, because of its structure this heterogeneity does not restrict the possibility of defining large‐scale uniform principal stress directions. All the results are consistent with the local seismotectonics.
Paleomagnetic data from the Philippine Sea Plate and their tectonic significance
Authors: Haston Roger B. , Fuller M.
Published: February 2008

New paleomagnetic data from Guam and Saipan have been obtained and are used, in conjunction with previously published results from around the Philippine Sea plate, to interpret the origin and history of the Philippine Sea plate. A lower Miocene direction (I=3.5°, D=54.5°, a95=17.4°, k=20.3) and a middle Oligocene direction (I=5.3°, D=68.4°, a95=12.8°, k=52.4) have been obtained from the Palau Islands. Only one reliable result, a middle Oligocene direction (I=15.1°, D=66.1°, a95=11.1°, k=20.1), has been obtained from Guam. Many characteristic remanent magnetic directions have been obtained from the lower Eocene Facpi Formation; however, the results are too scattered (k=4.5) to be considered reliable. Two reliable directions have been obtained from Saipan: a middle Miocene result (I=30.7°, D=28.1°, a95=8.4°, k=64.6), and a lower Eocene result (I=−12.0°, D=43.0°, a95=12.5°, k=16.0). Two lower Eocene results from Chichijima (I=9.5°, D=105.3°, a95=12.6°, k=6.1) and Anijima (I=−0.8°, D=91.7°, a95=16.9°, k=4.9) are the reliable results from the Bonin islands. There are two end‐member interpretations for these data: (1) small scale local rotation of blocks along the plate margin, or (2) rotation of the Philippine Sea plate as a whole. To distinguish between these interpretations, we reconstructed the East Philippine Sea province back to its prerifting configuration. The rotation caused by deformation can be estimated by comparing the paleomagnetism of Guam, Saipan, and the Bonin islands to Palau. Prior to reconstruction, the older declination values are scattered. Closing the marginal basins reduces the declination scatter, and all of the islands show increasing clockwise deflection with time. This suggests that the Philippine Sea plate has rotated up to 80° clockwise and moved northward around 20° since the Eocene, and deformational rotation is a second‐order feature. The finite pole of rotation of the Philippine Sea plate relative to Eurasia must be somewhere near the southeast comer of the plate, because this is the only location that is compatible with both the change in orientation and paleolatitude for the plate. These data cannot distinguish between a backarc origin or trapped crust origin for the West Philippine Sea province. The existing models require major modification to be consistent with the paleomagnetic data.
A theory for plastic deformation and textural evolution of olivine polycrystals
Authors: Ribe Neil M. , Yu Yang
Published: February 2008

Seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation. In order to understand better the origin of seismic anisotropy, we present a simple theory for plastic deformation and textural evolution of olivine polycrystals. Each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems, whose hardnesses and stress exponents are known from experiments. The evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local (crystal) deformation and the global (aggregate) deformation subject to the constraint of global strain compatibility. The axial compression texture predicted by our model agrees well to first order with that determined experimentally by Nicolas et al. (1973), although there are significant discrepancies in the details. Our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain, such that the crystallographic axes [100], [010], and [001] are concentrated around the finite strain axes a, c, and b, respectively (a > b > c). This result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy.
Heat flux at the ice‐ocean interface
Authors: Wettlaufer J. S.
Published: February 2008

The role of the oceanic heat flux Fw in the sea ice energy and mass balance is discussed, and a method is described for determining Fw from measurements of the temperature and thickness of sea ice. Results obtained using this method and data collected in the fall of 1988 during a drift in the eastern Arctic northeast of Fram Strait are presented. Estimates of Fw ranged from 0 to 37 W m−2 and varied in both time and space. The gradual variation over the course of the drift reflects transformation of the West Spitsbergen Current into Arctic water masses. A significant intersite variability on horizontal scales of 10 to 100 m is ascribed to the interaction between under‐ice topographic features and turbulent energy and mass transfer in the boundary layer. In addition, there is a variability in the ice growth rate on horizontal scales of 0.1 to 1 m which is ascribed to the instability of the ice‐ocean phase boundary in the presence of turbulent flow. These observations have implications for modeling and future experiments.
Air‐sea fluxes and surface layer turbulence around a sea surface temperature front
Authors: Friehe C. A. , Shaw W. J. , Rogers D. P. , Davidson K. L. , Large W. G. , Stage S. A. , Crescenti G. H. , Khalsa S. J. S. , Greenhut G. K. , Li F.
Published: February 2008

The response of the lower marine atmospheric boundary layer to sharp changes in sea surface temperature was studied in the Frontal Air‐Sea Interaction Experiment (FASINEX) with aircraft and ships measuring mean and turbulence quantities, sea surface temperature, and wave state. Changing synoptic weather on 3 successive days provided cases of wind direction both approximately parallel and perpendicular to a surface temperature front. For the wind perpendicular to the front, both wind over cold‐to‐warm and warm‐to‐cold surface temperatures occurred. For the cold‐to‐warm case, the unstable boundary layer was observed to thicken, with increased convective activity on the warm side. For the warm‐to‐cold case, the surface layer buoyant stability changed from unstable to neutral or slightly stable, and the sea state and turbulence structure in the lower 100 m were immediately altered, with a large decrease in stress and slowing of the wind. Measurements for this case with two aircraft in formation at 30 and 100 m show a slightly increased stress divergence on the cold side. The turbulent velocity variances changed anisotropically across the front: the streamwise variance was practically unchanged, whereas the vertical and cross‐stream variances decreased. Model results, consistent with the observations, suggest that an internal boundary layer forms at the sea surface temperature front. The ocean wave, swell, and microwave radar backscatter fields were measured from several aircraft which flew simultaneously with the low‐level turbulence aircraft. Significant reductions in backscatter and wave height were observed on the cold side of the front.
Basin‐scale tomography: A new tool for studying weather and climate
Authors: Spiesberger John L. , Metzger Kurt
Published: February 2008

We present the first experimental results demonstrating the use of acoustic tomography to observe the large‐scale (>500 km) temperature fluctuations in ocean basins. Use of the tomographic technique, as presented here, will eventually lead to a better understanding of weather, climate, ocean circulation, and the distributions of marine organisms. From June to September 1987, tomographic signals were transmitted between four sources and nine receivers over a 3000 by 4000 km region (the basin‐scale) in the northeast Pacific. This paper discusses five issues related to transmissions across one 3000 km section. First, basin‐scale tomography is feasible because multipaths are stable in the presence of ocean fluctuations and the multipaths can be understood with ray theory (and its extensions). Second, the tomography problem can be initiated from climatological estimates. It is unnecessary to initiate with conventional measurements during the experiment (a prohibitively expensive proposition). Third, a description of the tomographic reconstruction procedure (Kalman filtering) is presented. The filter imposes temporal and spatial constraints on the solution for the thermal field. Fourth, the sensitivity of the estimated thermal field is examined with respect to variations of the constraints. Thermal maps are not very sensitive to variations of the constraints. Fifth, tomography is used to observe significant changes from month‐to‐month in the large‐scale thermal field in the ocean, including the seasonal thermocline where week‐to‐week resolution can be obtained. In contrast, available point measurements appear to be insufficient for observing monthly changes in the large‐scale thermal field in the ocean. These acoustic measures of weather and climate in the ocean cannot be verified with other data (e.g., satellites, point measurements) and thus provide an orthogonal perspective by which weather and climate may be observed and understood.
Forced ocean response during the Frontal Air‐Sea Interaction Experiment
Authors: Weller R. A. , Rudnick D. L. , Eriksen C. C. , Polzin K. L. , Oakey N. S. , Toole J. W. , Schmitt R. W. , Pollard R. T.
Published: February 2008

A central hypothesis of the Frontal Air‐Sea Interaction Experiment (FASINEX) was that the strong gradients in the upper ocean associated with ocean fronts would spatially modulate the response of the ocean to local atmospheric forcing. To test this hypothesis, measurements of near‐surface meteorology and upper ocean variability were made for 5 months from an array of moorings and, for one of those 5 months, from two ships. A number of fronts passed through the moored array, and the moored meteorological and current meter data were used to examine the response of the upper ocean to atmospheric forcing at subinertial and near‐inertial frequencies. At low frequencies an Ekmanlike, frictionally driven component in the mixed layer and a more deeply penetrating component thought to be forced by Ekman pumping were found. Strong near‐inertial response was found in the mixed layer. The near‐inertial motion showed considerable spatial variability in the seasonal thermocline; here it was evident that the structure of the front influenced the downward propagating, near‐inertial internal waves. Data were also obtained by profiling instruments deployed along cross‐frontal sections. These data were used to examine the spatial and temporal variability of the internal waves, microstructure, and dissipation, and the relation of that variability to oceanic fronts and atmospheric events. The spectral densities of the components of the internal waves that rotated clockwise with increasing depth were higher in regions of negative relative vorticity than those predicted by Garrett and Munk (Munk, 1981), and estimates of clockwise and total shear were highest in regions of negative relative vorticity. In the strong shear of the ocean front, significant anisotropy of the internal wave field was observed and kinetic energy dissipation levels relative to the rate of work by the wind were higher than previously reported.
The modeling of CO2lidar backscatter from stratospheric aerosols
Authors: Kent G. S. , Yue G. K.
Published: February 2008

Stratospheric aerosol properties, measurement techniques, and long‐term data sets are reviewed in the context of the modeling of aerosol characteristics at CO2laser wavelengths. The techniques for modeling infrared scattering characteristics, both from in situ particle size distribution measurements and from remote optical measurements at other wavelengths, are summarized, and the problems associated with this modeling are discussed. Calculations based on measured size distributions show the backscatter coefficient at wavelength 10.6 μm, β10.6, to vary by a factor of the order of 100 between background and immediate postvolcanic conditions. The calculated value may also be quite dependent upon the scheme used to interpolate the number concentration between the measured particle radii. The ratio of backscattering at 10.6 μm to that at 0.694 μm, β10.6/β0.69was found to vary by a factor of 5 for the same range of particle size distributions. A description is given of the rather limited amount of modeling at CO2laser wavelengths and of data intercomparisons that have been published. A comparison of measured backscattering coefficients at 10.6 μm with those modeled from shorter wavelengths shows agreement within a factor of 2 or better. Areas of disagreement and uncertainty are identified together with the requirements for improving our knowledge of CO2laser backscattering from stratospheric aerosols.
Ozone climatology at high elevations in the southern Appalachians
Authors: Aneja Viney P. , Businger Steven , Li Zheng , Claiborn Candis S. , Murthy Anuradha
Published: February 2008

Ozone measurements are reported for two high‐elevation sites located in the Mt. Mitchell State Park in North Carolina (site 1 on Mt. Gibbs, ∼2006 m, and site 2 on Commissary Ridge, ∼1760 m). These measurements are also compared to those from a nearby, low‐elevation site (Fairview, ∼850 m). The measurements were made from May through September during the years 1986 and 1988 and from May through October during 1987, at sites 1 and 2. Measurements were also made from May through September 1989 at site 1 only. During the monitoring season at site 1 the mean ozone concentrations were 50, 51, 66, and 52 ppbv for 1986, 1987, 1988, and 1989 field seasons, respectively, while at site 2 the mean ozone concentrations were 49, 49 and 52 ppbv for 1986, 1987, and 1988. (It has been shown that exposure to ozone concentrations of ≥50 ppbv is sufficient to cause damage to certain species of vegetation.) The daily maximum, 1‐hour average, and 24‐hour average concentrations were found to be greatest during summer months (late May through early July), with lower concentrations during fall (August and September), suggesting a correlation with the seasonal photochemical cycle. It is suggested that excess hydrocarbons released during budbreak may contribute to the seasonal signal in the ozone data. During the 1988 monitoring season there were three periods of very high ozone levels (>80 ppbv) which lasted over 100 hours. During these long episodes there were 48 hours during which ozone concentrations exceeded the current National Ambient Air Quality Standard (NAAQS) of 0.12 ppmv. High hourly averaged SO2 (∼25 ppbv) and NOx (∼11 ppbv) levels were also found during these episodes. Meteorological analyses show an association between periods of high ozone concentrations and synoptic‐scale patterns. Such high gaseous pollutant concentrations were not observed during the previous two field seasons. Also, no exceedances of the NAAQS were observed during the field season of 1989. No discernible diurnal cycle in the ozone concentrations was observed at site 2; however, a reversed diurnal cycle (nighttime maximum) was evident at site 1. Also, average ozone concentrations increased from lower elevations to higher elevations. Evidence suggests that the relationship between the mountaintop and the height of the mixing layer, coupled with horizontal transport of ozone in the lower troposphere, may be important in explaining the nocturnal maximum at site 1 and the observed altitudinal gradient in ozone.
Cloud area determination from AVIRIS data using water vapor channels near 1 μm
Authors: Gao Bo‐Cai , Goetz Alexander F. H.
Published: February 2008

Fractional cloud area is derived from spectral images collected by the airborne visible‐infrared imaging spectrometer (AVIRIS). AVIRIS covers the spectral region from 0.4 to 2.5 μm with 224 channels and has a ground instantaneous field of view of 20×20 m from an altitude of 20 km. The derivation is made by ratioing radiances near the 0.94‐ and the 1.14‐μm water vapor band centers against those in the intermediate atmospheric window regions. The derivation makes use of the facts that (1) the reflectances of most ground targets vary approximately linearly with wavelength in the 0.94‐ and the 1.14‐μm water vapor band absorption regions, and (2) the peak absorptions of the water vapor band over cloudy areas are smaller than those over nearby clear surface areas because of the rapidly decreasing atmospheric water vapor concentration with height. The band ratioing technique effectively discriminates among clouds and surface areas having similar reflectance values. Such discrimination is not possible using standard radiance thresholding techniques. It is expected that the use of water vapor channels in the near‐infrared region in future satellites will improve the ability to determine cloud cover over land.
Stratospheric cloud observations during formation of the Antarctic ozone hole in 1989
Authors: Hofmann D. J. , Deshler T.
Published: February 2008

The results of six balloon flights at McMurdo Station, Antarctica, under varying temperature conditions, are used in a study of polar stratospheric clouds during September 1989. A new particle counter, with size resolution in the 0.5 μm radius region, indicated that size distributions observed in the clouds were bimodal. Mode radii ranging from 0.05 to 0.10 μm were observed for the small particle mode, representing the sulfate layer or condensational growth enhancements of it. Mode radii generally ranged from 1.5 to 3.5 μm for the large particle mode at concentrations 3 to 4 orders of magnitude lower than the small particle mode. The large particle mode, when observed at temperatures above the water ice point, is believed to be the result of nitric acid trihydrate (NAT) condensation on larger particles of the sulfate layer. In this case the HNO3 condensed mass mixing ratios were 1 to 5 ppbv for most of the cloud layers. Generally, the large particle NAT concentrations were higher in the lower stratosphere, indicating the redistribution of HNO3 through particle sedimentation. On several occasions, distributions were observed with mode radii as high as 7 μm, and correspondingly large inferred mass, indicating water ice clouds in the 12 to 15 km region. On other occasions, absence of such clouds at very low temperatures inferred water vapor mixing ratios of less than 3 ppmv.