bluuebunny/crossref_metadata_2025 huggingface dataset:
Continuation of the hydrothermal fluid chemistry time series at TAG, and the effects of ODP drilling
Authors: Edmonds H. N. , German C. R. , Green D. R. H. , Huh Y. , Gamo T. , Edmond J. M.
Published: February 2004

New data are presented for major dissolved species in vent fluids from the TAG hydrothermal mound on the Mid‐Atlantic Ridge, sampled in 1993, 1994, and 1995. The latter two cruises bracket the drilling of the mound by the Ocean Drilling Program in September–November 1994. Changes in venting patterns have been observed over this time period, both before and after drilling. However the major ion chemistry of venting fluids appears to be stable, indicating that the drilling did not penetrate the primary plumbing system of the mound.
Genetic algorithm inversion for receiver functions with application to crust and uppermost mantle structure beneath eastern Australia
Authors: Shibutani Takuo , Sambridge Malcolm , Kennett Brian
Published: February 2004

Genetic algorithm (GA) inversion, a nonlinear global optimization technique, has been applied to determine crustal and uppermost mantle velocity structure from teleseismic receiver functions. With a new modelling in which not only the S wave velocity but also the layer thickness and the VP/VS ratio are model parameters, the GA receiver function inversion is able to recover the velocity structure with reasonable accuracy in the whole crust, and particularly well for the surface layer and the crust‐mantle boundary. Structural inversion for receiver functions from a network of broadband seismic stations in eastern Australia allows a comparison of crustal structure from different tectonic environments. The crust‐mantle boundary is shallow and sharp on the craton but deep and transitional along the axis of the fold belt zone in eastern Australia. This suggests crustal thickening in the fold belt by underplating or intrusion of mantle materials into the lower crust.
Atmospheric electric potential gradient measurements of ash clouds generated by pyroclastic flows at Unzen Volcano, Japan
Authors: Miura Toshiro , Koyaguchi Takehiro , Tanaka Yoshikazu
Published: February 2004

Atmospheric electric potential gradient measurements were made at two localities simultaneously during pyroclastic flow events on September 25, 1993 at Unzen volcano. Immediately after generation of the pyroclastic flows, the potential gradients showed a positive anomaly that was followed by a negative anomaly. The magnitude of the positive anomalies were nearly equal at the two measurement localities, whereas the magnitude of the negative anomalies at the measurement locality, close to the axis of the ash cloud, was larger than the magnitude at the off‐axis measurement locality. These differences may be accounted for by the differences in the distance between the ash cloud and each measurement locality. It is suggested that the height to the positive charge was sufficiently larger and the height to the negative charge was sufficiently smaller than the horizontal distance between the two measurement localities. Our results suggest that the upper part of the ash cloud, which contained volcanic gases and/or relatively fine ash, carried a net positive charge and the lower part of ash cloud, which contained relatively coarse ash, carried a net negative charge. This charge distribution in the ash clouds generated by pyroclastic flows is more consistent with the results reported for Vulcanian plumes from Sakurajiama volcano [Lane and Gilbert, 1992; Lane et al., 1995] and Asama volcano [Hatakeyama and Ishikawa, 1946] than any other cases.
The 1994 northern midlatitude budget of stratospheric chlorine derived from ATMOS/ATLAS‐3 observations
Authors: Zander R. , Mahieu E. , Gunson M. R. , Abrams M. C. , Chang A. Y. , Abbas M. , Aellig C. , Engel A. , Goldman A. , Irion F. W. , Kämpfer N. , Michelson H. A. , Newchurch M. J. , Rinsland C. P. , Salawitch R. J. , Stiller G. P. , Toon G. C.
Published: February 2004

Volume mixing ratio (VMR) profiles of the chlorine‐bearing gases HCl, ClONO2, CCl3F, CCl2F2, CHClF2, CCl4, and CH3Cl have been measured between 3 and 49° northern‐ and 65 to 72° southern latitudes with the Atmospheric Trace MOlecule Spectroscopy (ATMOS) instrument during the ATmospheric Laboratory for Applications and Science (ATLAS)‐3 shuttle mission of 3 to 12 November 1994. A subset of these profiles obtained between 20 and 49°N at sunset, combined with ClO profiles measured by the Millimeter‐wave Atmospheric Sounder (MAS) also from aboard ATLAS‐3, measurements by balloon for HOCl, CH3CCl3 and C2Cl3F3, and model calculations for COClF indicates that the mean burden of chlorine, ClTOT, was equal to (3.53±0.10) ppbv (parts per billion by volume), 1‐sigma, throughout the stratosphere at the time of the ATLAS 3 mission. This is some 37% larger than the mean 2.58 ppbv value measured by ATMOS within the same latitude zone during the Spacelab 3 flight of 29 April to 6 May 1985, consitent with an exponential growth rate of the chlorine loading in the stratosphere equal to 3.3%/yr or a linear increase of 0.10 ppbv/yr over the Spring‐1985 to Fall‐1994 time period. These findings are in agreement with both the burden and increase of the main anthropogenic Cl‐bearing source gases at the surface during the 1980s, confirming that the stratospheric chlorine loading is primarily of anthropogenic origin.
Auroral signature of lobe reconnection
Authors: Sandholt P. E. , Farrugia C. J. , Øieroset M. , Stauning P. , Cowley S. W. H.
Published: February 2004

We report specific changes in the dayside auroral morphology in the winter hemisphere which occur in response to sharp transitions between northward and southward‐directed interplanetary magnetic fields (IMF). In two case examples we show how a switch between large negative and large positive IMF BZ component was accompanied by a corresponding switch in the location of the 630.0 nm aurora: the cusp aurora situated at ≈ 74° MLAT disappeared and another form this time situated at ≈ 77–78° MLAT appeared simultaneously (within 1 min.). We suggest that the lower‐ and higher‐latitude auroras correspond to injections of magnetosheath plasma associated with, respectively, magnetic reconnection at low and high magnetopause latitudes. They may be called cusp/LLBL and cusp/mantle auroras, respectively. According to this interpretation the cusp/mantle aurora thus corresponds to reconnection tailward of the cusp, the so‐called lobe reconnection. The auroral signature is observed to last for a few tens of minutes, indicating that lobe reconnection can occur in a quasi‐steady mode. During the 17 December 1992 case event sunward plasma convection in the polar cap was inferred from magnetometer records obtained during the period when the high‐latitude aurora occurred.
UV‐B increases (1979–1992) from decreases in total ozone
Authors: Herman J. R. , Bhartia P. K. , Ziemke J. , Ahmad Z. , Larko D.
Published: February 2004

Increases in ultraviolet fluxes (300 nm to 340 nm) reaching the ground between 1979 and 1992 are estimated using measured stratospheric ozone amounts and reflectivity data from Nimbus‐7/TOMS (Total Ozone Mapping Spectrometer). The UV‐increases are estimated from an ozone data set obtained using a new algorithm incorporating improved in‐flight instrument calibration. The 380 nm radiance data are used to show that there were no changes in ultraviolet atmospheric albedo due to clouds and aerosols from 1979 to 1992 within the 1% uncertainty of the measurements. Linear least squares fits to the monthly and annual increases in UV exposure since 1979 are given for 3 wavelengths (300 nm, 310 nm, and 320 nm) that are strongly, moderately, and weakly absorbed by ozone. The estimated linear changes for the 3 wavelengths become significant (2 standard deviations) poleward of about 40° latitude. In the 45°±5° latitude band, the slope of linear fits to the annual zonally averaged changes for these wavelengths are about 13%, 3%, and 1% per decade in the southern hemisphere, and 10%, 3%, and 1% per decade in the northern hemisphere. Similarly derived values are estimated for DNA, plant, and erythema action spectra. Monthly values of exposure changes are larger towards higher latitudes and during the spring and winter months (e.g., 8.6%, 9.8%, and 5.1% per decade during April at 45°N). Erythemal UV‐increases obtained from TOMS data disagree with previously determined ground based UV‐decreases from an average of 8 U.S. Robertson‐Berger sites.
Simulation of tides with a spectral mesosphere/lower thermosphere model
Authors: Akmaev Rashid A. , Forbes Jeffrey M. , Hagan Maura E.
Published: February 2004

The spectral mesosphere/lower thermosphere model (SMLTM) is an extension into the thermosphere (up to ∼220 km) of the three‐dimensional middle atmospheric (∼15–120 km) model developed by Akmaev et al. [1992]. The model incorporates realistic parameterizations of radiative transfer and dissipation processes including eddy and molecular viscosity and thermal conductivity and ion drag. A discrete multi‐component gravity wave parameterization is optimized with respect to the zonal mean temperature distribution as represented in the CIRA‐1986 empirical model. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The semi‐diurnal tide is primarily generated by ozone absorption of solar UV radiation in the stratosphere. First results of tidal simulations indicate that even without any additional tuning the simulations compare well with recent observations from the Upper Atmosphere Research Satellite (UARS). This means that the background temperature and wind distributions, gravity wave momentum deposition, turbulent diffusion, and tidal forcing all interact self‐consistently and are in agreement with observational data. It is noted that simultaneous UARS wind and temperature measurements, if they become available, will provide key information needed to distinguish the primary mechanism for damping the diurnal tide.
Polarization of crack waves along an artificial subsurface fracture
Authors: Nagano Koji , Sato Kazuhiko , Niitsuma Hiroaki
Published: February 2004

We analyze three‐dimensional (3D) particle motions of crack waves which propagate along a fracture to determine fracture orientation. Crack waves were measured at an artificial subsurface fracture at a depth of approximately 370 m in the Higashihachimantai Hot Dry Rock model field in Japan. Dispersion was observed in crack waves. Velocities of crack waves below 100 Hz are 100 m/s–150 m/s along this artificial fracture. Therefore, crack waves arrived after compressional wave and shear wave. A coherence matrix is used to analyze the 3D particle motions in the frequency domain. Noise, which is outside of the band of the detector, is excluded in the coherence matrix analysis. We examine the polarization of 3D particle motions of crack waves using Principal Component Analysis. The longest axis of an ellipsoid which approximates the 3D particle motions shows a direction consistent with estimates obtained by core‐sample measurements and tectonic stress measurements. The differences in fracture orientation obtained from the crack‐wave analysis and the core sample analysis are less than 16° in azimuth and less than 20° in dip.
On the role of hydrothermal systems in the transfer of volcanic sulfur to the atmosphere
Authors: Oppenheimer Clive
Published: February 2004

The behaviour of magmatic volatiles dictates various aspects of a subaerial volcano's eruptive style, morphology, and potential impact on the atmosphere. Typically, when volatiles bleed from nonerupting magma bodies, they encounter superimposed hydrothermal envelopes. If their interaction with hydrothermal fluids is minimal, subaerial degassing may be important, and possibly manifested in high fluxes of gaseous SO2. By contrast, a “wet” volcano, in which there is substantial mixing of magmatic and hydrothermal fluids, scavenges reactive magmatic components: sulfur‐bearing minerals precipitate in crater lakes, in pore spaces created by hydrothermal dissolution of host rock, and in fractures formed by caldera collapse. Measurements of subaerial SO2 fluxes (e.g., by Correlation Spectrometer, COSPEC) therefore say as much about hydrothermal vigour as they do about magma bodies. Major explosive eruptions necessarily entail reaming of conduits through volcanic edifices, and entrainment of hydrothermal sulfur. It is argued here that eruption clouds might thereby derive significant proportions of their SO2 budget from combustion of native sulfur and/or sulfides sequestered by hydrothermal systems during inter‐eruptive periods of >10²–10³ yr.
Stick‐slip statistical mechanics at the bed of a glacier
Authors: Bahr David B. , Rundle John B.
Published: February 2004

The multiple processes which control motion at the bed of glaciers are too heterogeneous to be modeled directly, but a simple stick‐slip slider‐block model of a glacier bed allows a statistical mechanical treatment of the physics beneath thin and shallow sloped glaciers. The technique may reconcile apparently contradictory field observations of the relationship between glacier sliding and basal water pressure. Simulations also predict hysteresis in the relationship between water pressure and basal velocities.
Lithospheric wide‐angle seismic profiles using stacked airgun shots
Authors: Chian Deping , Hall Jeremy , Marillier François
Published: February 2004

The signal‐to‐noise ratio of marine wide‐angle seismic profiles can be significantly enhanced by stacking multiple shots. Signals detected from airgun shots from a seismic ship repeated many times within a small source area (called “stack shots”) can be stacked in a manner somewhat similar to common‐mid‐point processing of reflection seismic data. We collected two such “stack shot” profiles across the eastern margin of Newfoundland. At each shooting site, 36 closely spaced airgun shots were fired consecutively, and recorded along a profile made of about 400 land receivers at offsets of 100 to 455 km. While the data can be stacked in several different ways we show that a two‐step technique or “two‐pass stack” is the most effective. The traces of each receiver gather are first stacked at an aperture of 0.5 km along a slope of 8 km/s (stacking with linear moveout). The stacked traces are then reordered by increasing offset and stacked a second time along the same slope and with an aperture of 0.5 km or less. This technique is superior to a direct stack in which all the traces would be stacked in one pass because it allows improvement in the data quality by the various methods designed for each phase of the two‐pass stack. Our results show that the “stack shot” technique coupled with the “two‐pass stack” is a viable alternative, with less environmental impact, to using large, single explosions at sea.
The H2SO4 component of stratospheric aerosols derived from satellite infrared extinction measurements: Application to stratospheric transport studies
Authors: Lambert A. , Grainger R. G. , Rogers H. L. , Norton W. A. , Rodgers C. D. , Taylor F. W.
Published: February 2004

The ambient water vapour and temperature conditions of stratospheric sulphate aerosol particles govern their composition and thereby influence their infrared extinction properties. This causes problems in the use of the infrared aerosol extinction as a tracer because variations in the aerosol composition modify the changes in extinction that may arise from the transport of aerosols. An improved tracer which can be derived from measurements of the infrared aerosol extinction, temperature and water vapour abundance, is the H2SO4 component of aerosols. The application of this tracer to studies of stratospheric transport is demonstrated using data from instruments on the Upper Atmosphere Research Satellite and the results are compared to a contour advection calculation.
Geoelectrical measurements at the TAG Hydrothermal Mound
Authors: Von Herzen R. P. , Kirklin J. , Becker K.
Published: February 2004

Direct current electrical experiments with electrode arrays deployed by a submersible (ALVIN) were used to determine the resistivity structure of the seafloor below, and vertical potential gradients in sea water above, the hydrothermally active TAG mound. Apparent resistivities of the sulfides at shallow depths (∼10 m) below seafloor over the mound ranged between 0.18–0.21 ohm‐m, compared to nearby basalt pillow apparent resistivities of 2.1–2.4 ohm‐m. The potential differences between heights of about 18 to 27 m above the mound averaged +3.7 mV (voltage increasing with height), with variations between about −2.9 and +12.7 mV.
The geochemistry of Atlantic hydrothermal particles
Authors: Ludford E. M. , Palmer M. R. , German C. R. , Klinkhammer G. P.
Published: February 2004

Particles were collected from the dilute portion of neutrally buoyant hydrothermal plumes from four Mid‐Atlantic Ridge sites (MARK, 23°N; TAG, 26°N; Broken Spur, 29°N; Lucky Strike, 37°N). Comparison of data from proximal portions of the TAG (Atlantic) [German et al., 1991; this study] and North Cleft (Pacific) [Feely et al., 1994] plumes show that oxyanion (e.g., V) scavenging is more efficient at TAG, possibly due to a higher proportion of Fe removed as sulfides at North Cleft and/or the more vigorous mixing in the high energy TAG buoyant plume. Chalcophile elements (e.g., Cu) show two stage removal. They are precipitated as sulfides during initial mixing of vent fluids with seawater and are sedimented from the buoyant plume. In the dilute plume they are scavenged from seawater by Fe oxyhydroxides. The REE show continued scavenging in the neutrally buoyant plume and lower levels in 1993 samples, compared to 1988 samples [German et al., 1990] suggesting that the amount of scavenging is related to particle recycling.
Reduced crustal magnetization beneath Relict Hydrothermal Mounds: TAG Hydrothermal Field, Mid‐Atlantic Ridge, 26°N
Authors: Tivey Maurice A. , Rona Peter A. , Kleinrock Martin C.
Published: February 2004

Submersible magnetic profiles over the MIR and ALVIN relict hydrothermal zones of the TAG hydrothermal field on the Mid‐Atlantic Ridge at 26°N show magnetic anomalies that are consistent with the presence of reduced crustal magnetization. These magnetic anomalies are similar to the anomaly found associated with the TAG active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones. Magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems.
The role of microphysical and chemical processes in prolonging the climate forcing of the Toba Eruption
Authors: Bekki S. , Pyle J. A. , Zhong W. , Toumi R. , Haigh J. D. , Pyle D. M.
Published: February 2004

The mega‐eruption of Toba, Sumatra, occurred around 73 Ka ago, during the onset of a glaciation of the Late Quaternary. This coincidence combined with the unprecedented amount of sulphur released by this volcano has led to the hypothesis that Toba sulphate aerosols caused a transient surface cooling which may have contributed to a shift of the climate system. Because of the self limiting effect of gravitational sedimentation, the climatic impact of extremely large sulphur injections into the stratosphere are thought to be rather limited. Here we present model calculations combining microphysical and chemical feedbacks which show that the eruption could instead have led to the formation of a long‐lasting volcanic aerosol layer. Although the concentrations of radiatively active species such as O3 or SO2 could also have been considerably perturbed, the resulting forcings should have only slightly moderated the aerosol cooling effect during the first few years following the eruption. According to our results, extremely high stratospheric sulphur loading could lead to a more prolonged effect on the climate than previously assumed.
The importance of heterogeneous bromine chemistry in the lower stratosphere
Authors: Tie XueXi , Brasseur Guy
Published: February 2004

Heterogeneous conversion of bromine reservoirs (BrONO2, HOBr) on the surface of aerosol particles in the lower stratosphere has limited effect on the concentrations of reactive bromine radicals (Br, BrO). However, the heterogeneous hydrolysis of BrONO2 on sulfate aerosol particles, which produces HOBr, leads to the enhancement of the abundance of hydroxyl radicals, and consequently modifies the partitioning between inorganic chlorine compounds. Further, because of the enhanced abundance of HOBr, the heterogeneous reaction between HOBr and HCl(s) converts significant amounts of inactive chlorine (HCl) into reactive chlorine (ClO), especially during post‐volcanic periods. These processes could partially explain the unexpected high ClO/HCl and ClONO2/HCl concentration ratios at 20 km reported after the eruption of Mt. Pinatubo (1991), and could have contributed to the substantial ozone depletion observed after the large volcanic eruptions of the 1980's and 1990's. Our model simulations suggest that, owing to the heterogeneous hydrolysis of BrONO2 and the reaction HOBr+HCl(s) on sulfate aerosols, the increase in total bromine loading from 1960 (12 pptv) to 1990 (21 pptv) has led to enhanced concentrations of ClO, and consequently to an ozone depletion of 2–3% in the lower stratosphere at mid and high latitudes under post‐volcanic conditions.
Prediction of bathymetry in French Polynesia constrained by shipboard data
Authors: Sichoix L. , Bonneville A.
Published: February 2004

The south central Pacific Ocean is well covered with satellite‐derived gravity data (ERS‐1, GEOSAT, SEASAT and Topex/Poseidon) but sparsely covered with shipboard depth soundings and gravity measurements. We present an improved method of bathymetric prediction from gravity using both shipboard and satellite data. We employ a condensation model with two layers for the crust to attempt bathymetric prediction using a 2‐D spectra approach. Parameters such as flexural wavelength and oustal density are the major constraints for the model They are determined on a limited area by minimizing the difference between observed bathymetry ship profiles and the model predictions. Then, in regions where there are no ship data, these parameters are used to predict the seafloor topography from ship gravity data combined with satellite grid points. The prediction has an horizontal resolution limit of 8 km and the depth error is usually less than 300 m. This method is better constrained than the ones using satellite altimetry data only. Results on the Western Tuamotu archipelago and on Marquesas Islands, French Polynesia, are presented and discussed.
Gases and helium isotopes in high temperature solutions sampled before and after ODP Leg 158 drilling at TAG Hydrothermal Field (26°N, MAR)
Authors: Charlou Jean L. , Donval Jean P. , Jean‐Baptiste Philippe , Dapoigny Arnaud , Rona Peter A.
Published: February 2004

Hydrothermal fluids were sampled for dissolved gases at TAG (26°N‐MAR) during two Alvin dive series, in April–May 1993 and March 1995, respectively 17 months before and 4 months after Ocean Drilling Program Leg 158. Total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude, even if some increase in H2S, CO2 and CH4 was noticeable in the 1995 samples. No significant difference was observed in helium concentration and helium isotopic ratio (³He/4He = 8.2 ±0.1 Ra). The CH4/³He ratio found in black smokers sampled in 1993 is around 9 × 106, close to ratio found in MAR basalts. In 1995, this ratio is around 4 times higher, due to the CH4 increase. The δ13C in CO2, measured on two black smokers sampled in 1993, is uniform at −8.4 to −8.8‰ (versus PDB), while a δ13C value of −13‰ is measured at a new site sampled in 1995, located 40 meters east of the main Black Smokers Complex (BSC). CH4/³He and 13C values both point to a mainly magmatic (abiogenic) origin of the carbon species in the system. Overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium, CO2 and CH4 of magmatic origin, even if some thermogenic CH4 contribution seems likely in the 1995 samples collected in the new active area. We can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the TAG mound and temporarily enhanced the hydrothermal circulation, by opening new pathways favouring transient input of CH4 rich fluids, before coming back to a new steady state.
Shear wave splitting across the Rocky Mountain Front
Authors: Savage Martha Kane , Sheehan Anne F. , Lerner‐Lam Arthur
Published: February 2004

We search for evidence of seismic anisotropy by determining shear‐wave splitting parameters using teleseismic phases from stations placed across the Rocky Mountain Front. Three features are striking: 1) fast polarization orientations are consistent within small geographic regions, but vary rapidly across the network, 2) a large number of high quality “null” measurements indicate that little transverse anisotropy with horizontal symmetry axis is present, and 3) stations with well‐constrained but inconsistent parameters for rays from different sources imply that a single layer of anisotropic material with a horizontal symmetry axis is an inadequate parameterization. The general pattern suggests fast axes pointing toward (or away from) a central region of little anisotropy. A model of asthenospheric flow converging on or diverging from the central uplifted region is postulated. Within the southern Rocky Mountains, parameters are similar to those in the nearby northern Rio Grande Rift, suggesting that a similar mechanism causes the anisotropy in both regions.
Light at deep‐sea hydrothermal vents
Authors: Van Dover Cindy Lee , Reynolds George T. , Chave Alan D. , Tyson J. Anthony
Published: February 2004

Ambient light spectral data were acquired at two deep‐sea hydrothermal vents with a temperature of ∼350°C: the Hole‐to‐Hell site on the East Pacific Rise at 9°N and the Snake‐Pit site on the Mid‐Atlantic Ridge. Measurements were made with a simple, multi‐channel photometer which simultaneously detected light in four 100 nm‐wide bands over the wavelength range of 650–1050 nm. Most of the light detected is near‐infrared (750–1050 nm), but there is a 19x greater photon flux than expected from thermal radiation alone at shorter wavelengths (650–750 nm) at the Hole‐to‐Hell vent. At Snake Pit, more light in the 750–850 nm band was observed 10 cm above the orifice where the temperature was 50–100°C than at the 351°C vent opening. These data suggest the presence of non‐thermal light sources in the vent environment. Some possible non‐thermal mechanisms are identified, but further data will be required to resolve them.
A numerical model of the Hellenic Subduction Zone: Active stress field and sea‐level changes
Authors: Giunchi C. , Kiratzi A. , Sabadini R. , Louvari E.
Published: February 2004

A 2‐D viscoelastic finite element model for subduction in the southern Aegean Sea allows us to study the distribution of stress and surface velocities in a vertical cross section through the island of Crete. Recent tomographic results constrain the geometry of the slab and the structure of the crust, lithosphere and mantle. Two models are considered, the first for passive subduction driven solely by slab pull, and the second one where active convergence between the African and Aegean plates is also included. Both models predict subsidence of the Hellenic trench and of Cretan trough; uplift of Crete and of the Aegean volcanic arc. The island of Crete is subject to differential uplift, highest in the southern part, with maximum rates of 1.5 and of 3.2 mm/yr for the two models, respectively. These results agree with the tectonic uplift inferred from Holocene sea‐level changes in the island. The style of stress within the modeled African and Aegean plates is in general agreement with the focal mechanisms of the largest earthquakes along the subduction zone.
A flexible, IMF dependent model of high‐latitude electric potentials having “Space Weather” applications
Authors: Weimer D. R.
Published: February 2004

A method has been developed to derive the electric potentials in the high‐latitude ionosphere resulting from any arbitrary combination of the interplanetary magnetic field (IMF) magnitude and orientation, solar wind velocity, and dipole tilt angle. This model is based on spherical harmonic coefficients that were derived by a least error fit of measurements from multiple satellite passes. These harmonic coefficients have been found to have systematic variations that can be reproduced by a combination of a Fourier series and a multiple linear regression formula. Examples of the model output are shown. In principal, this technique could be used as a fundamental building block to forecast geomagnetic disturbances, or “space weather”, from satellite measurements in the upstream solar wind.
The 1995 Grevena (northern Greece) Earthquake: Fault model constrained with tectonic observations and SAR interferometry
Authors: Meyer B. , Armijo R. , Massonnet D. , de Chabalier J. B. , Delacourt C. , Ruegg J. C. , Achache J. , Briole P. , Papanastassiou D.
Published: February 2004

After the 1995 Grevena Ms=6.6 event in northern Greece, we mapped the earthquake fault break in detail. The surface break is small (8–12 km long, 4 cm slip) compared to the moment release of the event. However, the morphologic and tectonic study of the active faults, in the field and using the SPOT satellite imagery, suggests that the earthquake ruptured part of a much larger fault system including interconnecting segments. We used SAR interferometry of the satellite ERS‐1 imagery to characterize the coseismic displacement field. This shows a kidney‐shaped zone of subsidence reaching 30 cm flanked by an uplift zone reaching 5 cm. We reproduce this field using dislocations in an elastic half‐space and our observations of the fault system. This requires 1 m slip from 4 to 15 km depth on a main normal fault segment dipping NNW. Our preliminary model includes significant NE‐dipping scissors faulting at the eastern end of the rupture, clearly seen in the interferograms.
Nocturnal variations of geomagnetic horizontal field at equatorial stations
Authors: Rastogi R. G. , Chandra H. , James M. E.
Published: February 2004

The nocturnal variations of the horizontal component (H) of the geomagnetic field at equatorial stations Koror, Trivandrum, Addis‐Ababa and Ibadan during any of the seasons of the period 1958–59 showed a steady decrease after sunset throughout the nighttime hours. However, at Huancayo a steady decrease after sunset to sunrise is seen only during the D‐months. During the J and E months decrease is seen after sunset till about 22–23 hr and a slow and steady increase from midnight hours to sunrise. The nocturnal variations of H field at Huancayo show a remarkable similarity with the corresponding variations of the horizontal electric field in the ionosphere as determined by the Doppler shifts of backscatter echoes from the E and F layers over Jicamarca. Thus, genuine changes in the equatorial electric field and current in the ionosphere over the American sector during the nighttime hours are clearly indicated.
Coronal hole‐active region‐Current sheet (CHARCS) Association with intense interplanetary and geomagnetic activity
Authors: Gonzalez W. D. , Tsurutani B. T. , McIntosh P. S. , Clúa de Gonzalez A. L.
Published: February 2004

Intense geomagnetic storms (Dst ≤ −100 nT) have been associated with interplanetary structures involving large‐intensity (Bs ≥ 10 nT) and long‐duration (T ≥ 3 hours) values of the southward component of the IMF. We show that near solar maximum, the solar origin of such structures seems to be associated with active regions* (involving flares and/ or filament eruptions) occurring close to the streamer belt and to growing low‐latitude coronal holes. It is also shown that low‐latitude coronal holes had a dual‐peak solar cycle distribution during solar cycle 21, similar to that previously reported for the above mentioned interplanetary and geomagnetic phenomena.
Ice core record of fatty acids over the past 450 years in Greenland
Authors: Kawamura Kimitaka , Suzuki Ikuko , Fujii Yoshiyuki , Watanabe Okitsugu
Published: February 2004

Fatty acids have been studied in the ice core taken from Site‐J, Greenland using a capillary gas chromatography and mass spectrometry. Their molecular distributions showed a strong even/odd carbon number predominance with two maxima at C16 and C24 or C22 acids. Unsaturated fatty acids such as oleic acid (C18∶1) were also detected in the ice samples as major species, being in contrast to the remote marine aerosols in which unsaturated fatty acids are depleted by photochemical degradation. This suggests that organic aerosols derived from marine and terrestrial biological sources are transported long distances in the atmosphere over the south Greenland ice sheet without serious photochemical transformation. Total concentrations of fatty acids were relatively constant (ca. 10 µg/kg‐ice) in the 16th to 19th centuries, however, they significantly increased in the 20th century with maxima in the 1930s–1950s and 1980s (up to ca. 100 µg/kg‐ice), suggesting an enhanced sea‐to‐air emission of organic matter and subsequent transport over the Greenland. The increased concentrations of fatty acids in this century were found to be consistent with an increased arctic temperature. During warmer periods, atmospheric circulation is enhanced and more lipids which are enriched in the sea surface microlayers are emitted to the atmosphere by bubble bursting mechanisms and imprinted in the ice core of south Greenland.
Limitations on suprathermal tails of electrons in the lower solar corona
Authors: Ko Y.‐K. , Fisk L. A. , Gloeckler G. , Geiss J.
Published: February 2004

Measurements of the charge states of solar wind ions by the SWICS instrument on Ulysses are used to place limits on the extent to which the velocity distribution of electrons in the lower solar corona deviates from a Maxwellian distribution by having a suprathermal tail. It is found that the data are consistent with a suprathermal tail which is not large, and which has a minor influence on the location and magnitude of the maximum of the electron temperature. This result has implications for theories which propose that suprathermal tails of electrons in the solar corona can have a strong influence on the formation of the solar corona and the dynamics of the solar wind.
Modeling 3D surface topography by finite‐difference method: Kobe‐JMA Station Site, Japan, Case Study
Authors: Pitarka Arben , Irikura Kojiro
Published: February 2004

We propose a simple approach for treating non‐planar free‐surface boundary conditions that can be easily implemented into an existing finite‐difference code. The approach is based on the three‐dimensional staggered‐grid finite‐difference method (FDM, hereafter). The proposed scheme gives very good results when compared with other numerical methods. Its accuracy depends on how well the topography is matched by the 3D‐FD grid. Based on the proposed technique, we made a qualitative evaluation of the local topography effect on ground motion at KOB‐JMA site, Kobe, Japan. The local topography at JMA station amplifies the horizontal ground motion in the frequency range 1–3 Hz by a factor of less than two, and deamplifies it at frequencies around 5 Hz by a factor of five.
Seismic anisotropy beneath the Northern Apennines (Italy) and its tectonic implications
Authors: Margheriti L. , Nostro C. , Cocco M. , Amato A.
Published: February 2004

We examined shear wave splitting in SKS and S phases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split‐shear wave (ϕ) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ϕ in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW‐SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ϕ oriented about E‐W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E‐W post‐orogenic extension observed at the surface.
Autocatalytic release of bromine from Arctic snow pack during polar sunrise
Authors: Tang T. , McConnell J. C.
Published: February 2004

Measurements and modeling studies strongly suggest that spring time depletion of ozone in the Arctic planetary boundary layer (PBL) is due to catalytic destruction by bromine atoms. However, the source of the bromine is uncertain. In this note, we propose that the source of the bromine at polar sunrise is the snow pack on the ice covering Arctic ocean and that it is released auto‐catalytically, stimulated by a bromine seed from one of the brominated organic compounds, such as CHBr3, by photolysis. In this manner ∼100 pptv of bromine can be transferred to the atmosphere where it can reside in the gas phase or, by scavenging, be partitioned in the aerosol or ice crystal phase. Moreover, it appears that heterogeneous recycling of bromine may be a process that self‐terminates as ozone depletes to low levels. We also have included chlorine chemistry in the model in order to simulate inferred levels of chlorine atoms. This is important as it results in the production of HCHO which acts to convert post ozone depletion active bromine into HBr which is then returned to the snow pack or scavenged by aerosols or ice crystals.
Upper atmosphere wind systems during reverse equatorial electrojet events
Authors: Stening R. J. , Meek C. E. , Manson A. H.
Published: February 2004

Days of reversed equatorial electrojet were identified at Trivandrum, India, and, in order to seek a global wind change in the dynamo region responsible for these events, upper atmosphere winds at a height of 99 km at Saskatoon, Canada, were examined. During northern winter, and also in April and September, the reversed electrojets were accompanied by reversals of the eastward wind at Saskatoon. The winter events generally occurred during a stratwarm. No changes in the winds were detected during summer reversed electrojets.
Quasiclassical trajectory study of the N(4S)+NO(X2Π)→N2(X1Σ+g)+O(3P) reaction rate coefficient
Authors: Duff J. W. , Sharma R. D.
Published: February 2004

Rate coefficients for the N+NO→N2+O reaction are calculated over the temperature range 100–1000 K by a quasiclassical trajectory calculation on the ³A″ potential energy surface (PES) based on the semiempirical London‐Eyring‐Polanyi‐Sato (LEPS) formalism, neglecting spin‐orbit coupling. The calculated results are only slightly temperature dependent and are in excellent agreement with available experimental data and the JPL recommended rate coefficient [DeMore et al., 1994] at room temperature. The present results do not support either the rate coefficient arrived at by Siskind and Rusch [1992] from modeling of the terrestrial thermosphere or the one arrived at by Fox [1994] from modeling of the Martian thermosphere. Possible causes of the discrepancies between the calculated rate coefficients and those arrived at from modeling studies may be additional NO production (loss or loss and production) mechanisms in the Martian (terrestrial) thermosphere.
Long‐term trends and interannual variability in tropical cyclone activity over the western North Pacific
Authors: Chan Johnny C. L. , Shi Jiu‐en
Published: February 2004

While Landsea et al. (1996) show that the number of intense Atlantic hurricanes is on the decrease, the trend of tropical cyclone activity over the western North Pacific is found to be just the opposite. For the period 1959 to 1994, the number of tropical storms and typhoons went through a period of decrease and then an increase, showing an obvious second‐order variation such that the activity has been on a generally upward trend since the late 1980s. A similar trend is found for the number of typhoons. All these time tendencies are statistically significant. In addition to a trend, shorter‐term oscillations of two, five and seven years can also be identified after the trend is removed.
Single crystal 40Ar‐39Ar dating of a Late Quaternary paroxysm on Kos, Greece: Concordance of terrestrial and marine ages
Authors: Smith P. E. , York D. , Chen Y. , Evensen N. M.
Published: February 2004

The late Quaternary plinian eruption forming the Kos Plateau Tuff on the island of Kos, Greece, was one of the largest in the Mediterranean. A precise date for this eruption would significantly improve the chronology of the deep‐sea tephra layer W‐3, because reliable correlation of W‐3 with the KOS Plateau Tuff has been previously established. We have dated sanidine from the eruption at 161.3±1.1 ka using the laser‐fusion single crystal 40Ar‐39Ar technique. This age represents a significant refinement over both conventional and highly precise (Cassignol method) K‐Ar ages, and the 40Ar‐39Ar age is shown to be in remarkably good agreement with the age of W‐3 inferred from recent astronomically calibrated oxygen isotope chronologies.
The Kelvin‐Helmholtz instability: Finite Larmor radius magnetohydrodynamics
Authors: Huba J. D.
Published: February 2004

A preliminary theoretical and computational study of the Kelvin‐Helmholtz instability in an inhomogeneous plasma is presented using finite Larmor radius magnetohydrodynamic (FLR MHD) theory. We show that FLR effects (1) can increase or decrease the linear growth rate, (2) cause the nonlinear evolution to be asymmetric, and (3) allow plasma ‘blobs’ to detach from the boundary layer. The asymmetric growth and nonlinear evolution depend on the sign of B · Ω where B is the magnetic field and Ω = ▽ × V is the vorticity. The simulation results are qualitatively consistent with the hybrid simulations of Thomas and Winske (1991, 1993) and Thomas (1995). These results suggest that FLR MHD can capture important physical processes on length scales approaching the ion Larmor radius.
North Pole ice thickness and association with ice motion history 1977–1992
Authors: Shy Timothy L. , Walsh John E.
Published: February 2004

Ice drafts measured by U.S. Navy submarine sonar near the North Pole from 1977 to 1992 were used together with Arctic Ocean drifting buoy data from 1979 to 1992 to investigate North Pole ice thickness changes during this 15‐year period. A primary objective was to determine dynamical forcing mechanisms and associated time scales relevant to ice thickness variation. North Pole ice thickness showed much interannual variability, and no systematic decrease of ice thickness was observed. Changes in ice thickness were closely associated with long‐term directional deviation of ice motion from geostrophic wind‐forcing, and with ice convergence and divergence at the North Pole during the 1–2 weeks prior to submarine measurement. The geographic origin of ice within the Arctic Ocean was not associated with its eventual thickness at the North Pole.
Environmental magnetic implications of Greigite (Fe3S4) Formation in a 3 m.y. lake sediment record from Butte Valley, northern California
Authors: Roberts Andrew P. , Reynolds Richard L. , Verosub Kenneth L. , Adam David P.
Published: February 2004

Authigenic greigite (Fe3S4) has been identified in several horizons of lake beds in a 102‐m core from Butte Valley, northern California, using mineral magnetic methods and x‐ray diffraction analysis. The presence of greigite has several implications for the paleoenvironmental record from Butte Valley. First, its occurrence in 2.5–3.0 Ma strata confirms that greigite can persist in the geological record for long periods of time. Second, the detrital mineral magnetic record may be partially obscured by the presence of authigenic greigite and care must be taken in interpreting magnetic variations in the greigite‐bearing zones as paleoclimate proxies. Third, differences in the timing of remanence acquisition for authigenic and detrital phases may compromise studies of high‐frequency geomagnetic field variations. Fourth, greigite may also be significant as a paleoenvironmental indicator of lake and sediment chemistry. The magnetic detection of greigite may therefore provide important information about paleolimnological conditions.
Time evolution of broadband seismic noise during the French Pilot Experiment OFM/SISMOBS
Authors: Beauduin R. , Montagner J. P.
Published: February 2004

During the French pilot experiment OFM/SISMOBS (April 22–May 20, 1992) in the Atlantic Ocean, two sets of Guralp CMG‐3 broadband seismometers were successfully installed and recovered. The first set, OFP (Observatoire Fond de Puits: Borehole Observatory), was installed inside the DSDP borehole 396B at 296m below the seafloor and the second one, OFM (Observatoire Fond de Mer: Ocean Bottom Observatory), on the sea bottom close to the hole at a depth of 4450 m. A first analysis showed that the broadband seismic noise (5 sps) on the seafloor had the same magnitude as compared with the long period seismic noise (1 sps) recorded at SSB, a continental station of the GEOSCOPE network. The noise recorded in the borehole was disappointedly high. The seismic data obtained during the experiment have been reanalyzed and it is shown that the seismic noise in the borehole is decreasing significantly with time whereas the instrument on the seafloor displays normal variations. A long term experiment (at least one month) is necessary to assess the final noise in the borehole.
Simulation of the strong lightning pulse penetration into the lower ionosphere
Authors: Sukhorukov A. I. , Rudenchik E. A. , Stubbe P.
Published: February 2004

Lightning electromagnetic pulse penetration into the night‐time lower ionosphere and plasma modification are simulated using the large particle method. The collisions of the electrons with the neutral species, both elastic and related to the excitation, ionization of and attachment to the neutrals are modeled using a Monte‐Carlo technique. The dynamics of the impulse electron heating and breakdown in the lower ionosphere above the strong lightning discharge are analysed. The level of the induced ionization strongly depends on the amplitude, form and duration of the incident pulses. The selfconsistent fields on the time scale of 10² µs are subjected to strongly nonlinear damping during the passage through the D‐region of the ionosphere.
The Rainbow Hydrothermal Plume, 36°15′N, MAR
Authors: German C. R. , Klinkhammer G. P. , Rudnicki M. D.
Published: February 2004

The Rainbow hydrothermal plume was discovered during a recent geophysical survey along 200 km of the Mid‐Atlantic Ridge (MAR), SW of the Azores Triple Junction, in which at least seven new sites of hydrothermal activity were identified. Here, we present the first hydrographic study of the Rainbow plume, 36°15′N, the strongest of the features located during that survey. The plume is detectable from real‐time in situ nephelometer anomalies and extends 10–15km, W‐E, in a non‐transform discontinuity (NTD) between two adjacent ridge‐segments. Maximum anomalies in the Rainbow plume indicate particle enrichments at least as large as those seen directly above the Trans Atlantic Geotraverse (TAG) vent‐field (26°N, MAR). Analysis of hydrographic data indicates a vent source at Rainbow with a thermal output of up to 98 MW, representing 140% of the thermal output previously attributed to TAG. Both lines of evidence indicate the Rainbow plume to be the strongest such feature yet found on the MAR.