bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrothermal convection at mid‐ocean ridges: Influence of permeability anisotropy within a tilted triangular structure
Authors: Aron C. , Pezard P. A. , Angot Ph. , Laverne C.
Published: February 2004

Recent results from seafloor exploration and crustal drilling are used to constrain the structures within which hydrothermal circulation occurs at mid‐ocean ridges (MOR). This hydrothermal activity is modeled numerically using a two‐dimensional triangular domain representing the extrusive layers. The configuration is heated both at an impermeable base and along the intrusion axis. The structure is characterized by dips of a few degrees towards the axis and permeability anisotropy (δ = KZ/KX). The temperature fields are computed for different hydrothermal regimes defined by variable axial heat flow values characterized by variable Rayleigh numbers (Ra*). These computed temperatures are compared to those deduced from basalt alteration mineralogy observed in seafloor samples and drillcores. The mean temperature of discharge zones is considered as limited to values between 60° and 100°C.

In the isotropic case, the chosen geometry and boundary conditions are found to stabilize the convective process, which advocates against a time‐fluctuating behavior as a steady state is reached in an average time of 5000 years up to Ra* values as large as 600. However, Ra* values are unlikely to exceed 400, or to be smaller than 100 due to the mineralogical constraints. With Ra* larger than 100 and δ=1, a mean temperature of 25°C, compatible with celadonite‐nontronite and iron‐hydroxides mineralization, is obtained for the recharge zones. For a given axial heat flow and a similar permeability contrast, larger temperatures are obtained in subhorizontal settings than in subvertical ones. Small changes of permeability anisotropy are confirmed to induce large variations in temperatures and strength of the flow.
High rotational excitation of NO infrared thermospheric airglow: A signature of superthermal nitrogen atoms?
Authors: Hubert Benoit , Gérard Jean‐Claude , Shematovich Valeri I. , Bisikalo Dimitri V.
Published: February 2004

The reaction between superthermal N(4S) atoms produced by exothermic processes and O2 has been proposed to explain observations of highly rotationally excited nitric oxide in the sunlit thermosphere. We examine the importance of this mechanism using a detailed calculation of the fast N(4S) atoms energy distribution. It is shown that the hot thermal N atoms are able to produce rotationally excited NO in the upper thermosphere through the reaction of O2 with N(4S). By contrast, near the NO peak at 110 km, the Maxwellian nitrogen atoms produce substantially less rovibrationally excited NO than the superthermal component. Consequently, the non Maxwellian N(4S) atoms show a clear spectral signature in the (1–0) and (2–1) bandheads at this altitude. The calculated rovibrationally excited NO concentration at 140 km is shown to be consistent with the value derived from the analysis of infrared airglow spectra.
Absorption of solar energy in a cryoconite hole
Authors: Podgorny Igor A. , Grenfell Thomas C.
Published: February 2004

Absorption of solar energy in contaminated sea ice depends not only on contaminant concentrations but also on the way contaminants are distributed in the ice. We used an analytical model of short‐wave radiative transfer in a dirty melt pond to calculate solar energy absorption by contaminants for two cases: (1) contaminants are aggregated sparsely in cryoconite holes within the pond and (2) contaminants are uniformly distributed on the pond bottom. We found that the concentration of contaminants in the holes may result in an enhanced solar energy absorption and the total energy absorbed by the contaminants can exceed the incident irradiance. For particular wavelengths in the visible, the absorbed energy can be as large as 270% of the incident spectral irradiance. The cryoconite hole depths should reach an equilibrium when the hole deepening rate matches the ablation rate of the pond bottom. Based on a pure radiative melting criterion, we estimate equilibrium values between 0.15 and 0.4 m for a typical pond of 0.3 m depth.
Anomalous shear strain at Parkfield during 1993–94
Authors: Gwyther R. L. , Gladwin M. T. , Mee M. , Hart R. H. G.
Published: February 2004

Three borehole tensor strainmeters (BTSM) installed near Parkfield and separated by more than 5km have established a good baseline of data spanning the past eight years. A significant change in the accumulation rate of shear strain (0.5 and 1.0 microstrain per year at two of the sites) commencing approximately in mid‐1993 and continuing to 1995 is evident in these data. Some supporting evidence for a change in the strain rate at that time is seen in dilatometer data, geodetic measurements, fault zone properties and microearthquake locations. There was also an increase in medium level seismic activity (M=4.7 on 10/20/92; M=3.9 on 10/26/92; m=3.5 on 3/13/93; M=4.4 on 4/3/93; M=4.8 on 11/14/93) in the Middle Mountain area. While a rainfall induced strain may have caused the observed strain anomaly, we propose a more likely explanation is that aseismic slip at depth on a section of the fault south of Middle Mountain produced the observed strain anomaly. This anomaly is unique in the current data set.
Slope of the geoid spectrum and constraints on mantle viscosity stratification
Authors: Čížková Hana , Čadek Ondřej , Yuen David A. , Zhou Hua‐wei
Published: February 2004

Spectral analysis of a recently obtained high resolution tomographic model, describing the top 1200 km of the mantle, shows a power‐law dependence on the degree, for degrees greater than around 10. The spectrum of recent geoid models is also found to decay in a linear fashion with degree on a log‐log plot. We have employed the logarithmic slope of the geoid between degrees 10 and 25 as a constraint on the viscosity structure of the top 1200 km of the mantle. The constraint of fitting the geoid slope represents a new and independent approach to the determination of the upper‐mantle viscosity structure. From conducting over one million runs in a Monte Carlo inversion, we have found that there are basically three families of viscosity which can fit the geoid slope. They are (1) with a viscosity hill between 660 and 1000 km, (2) with a weak viscosity increase at 660 km, and (3) with a significant viscosity increase at depths between 820 and 1000 km. Below 1000 km the viscosity of the lower‐mantle for all 3 families is larger than that in the upper mantle. These results corroborate the complexity of the mantle viscosity profile between 660 and 1200 km, which would have important ramifications on flows between the upper and lower mantle.
Local Rayleigh and Nusselt Numbers for Cartesian convection with temperature‐dependent viscosity
Authors: Honda S.
Published: February 2004

We present a local treatment of the heat transport efficiency of the base‐heated cartesian convection with a temperature‐dependent viscosity, which allows us to give a clearer interpretation of Nusselt (Nu)‐Rayleigh (Ra) numbers relation. The “local” Nusselt (Nul) and Rayleigh (Ral) numbers are defined by the local values at each boundary layer except the length scale which is the total depth. We find that Nul‐Ral relation obtained for the convection with constant viscosity and free surfaces is nearly coincident with that of the bottom boundary layer. For top boundary layer, we can treat it as a constant viscosity convection with rigid surface for large enough viscosity contrast. This interpretation can be made by choosing an appropriate temperature drop and viscosity for the definitions of Nul and Ral.
Shear wave splitting in the mantle Ps phases
Authors: Vinnik Lev , Montagner Jean‐Paul
Published: February 2004

Distribution with depth of mantle anisotropy is uncertain. To better constrain this distribution, we analyse records of the mantle Ps phases at the GRF array in Germany. Two components of these phases (SV and T) are detected by stacking teleseismic records of events well distributed in azimuth and distance. The detected T component signals are generated not so much by splitting of SV as by conversion from P to SH at the mantle discontinuities. To reconcile these observations with previously known observations of splitting in SKS, at least three anisotropic layers are required. The data favour a model with NNE oriented fast velocity at the top of the mantle. This is consistent with the results of other seismic studies. Beneath this layer the fast direction of anisotropy changes by around 90°. Our analysis suggests as well that a layer at the base of the mantle transition zone (MTZ) is anisotropic. This anisotropy might arise in the convective boundary layer between the upper and the lower mantle.
Rare gas systematics in Red Sea Ridge basalts
Authors: Moreira M. , Valbracht P. J. , Staudacher Th. , Allègre C. J.
Published: February 2004

From south to north, the 4He/³He ratios of submarine basalts from the Red Sea and Gulf of Tadjoura increase from 45,000 (R/Ra=15) in the Gulf of Tadjoura to 84,000 (R/Ra=8.6) in the Mabahiss deep. Our helium isotopic results indicate that the mantle plume proposed in the north of the Red Sea, on the basis of the Sr and Pb isotopic ratios, does not posess a high ³He signature. The neon and Ar isotopic ratios of the Red Sea glasses are explained by mixing between AIR, MORB and a plume with a solar like isotopic Ne signature and low 40Ar/36Ar, similar to that of Loihi.
Mixing zone in the tropical stratosphere above 10 mb
Authors: Dunkerton Timothy J. , O'Sullivan Donal J.
Published: February 2004

Observations of trace constituents obtained from the Upper Atmosphere Research Satellite in late winter 1991/92 reveal a zone of quasi‐horizontal stirring in the tropical stratosphere above 10 mb, equatorward of the subtropical jet. This mixing region was separated from the midlatitude surf zone by a strong gradient of potential vorticity and tracer along the jet axis. The jet and accompanying gradients are climatological features. Mixing equatorward of the jet evidently depends on the phase of the quasi‐biennial oscillation.
TOLEX‐ADCP monitoring
Authors: Hanawa Kimio , Yoshikawa Yasushi , Taneda Takeshi
Published: February 2004

Since February 1991, the Physical Oceanography Group, Tohoku University has conducted a program to monitor the oceanic current field across the Kuroshio over the Izu Ridge, south of Japan, using the Acoustic Doppler Current Profiler (ADCP) system installed on the ferry Ogasawara Maru. This ferry shuttles 58 times in a usual year between Tokyo and Chichijima in the Ogasawara (Bonin) Islands. This ADCP monitoring is regarded as one component of the Tokyo‐Ogasawara Line Experiment (TOLEX). In this paper, we will briefly describe our ADCP system and data processing procedures, and will present several aspects of the current fields using the data obtained through January 1996. During the period, the Kuroshio took the bimodal paths of 34°N and 32°30′N as mean positions on the ship course. In the sea south of the Kuroshio, many cyclonic and anticyclonic eddies with a scale of 200–400 km in diameter were observed with passing time of two–four months.
Estimation of drag coefficient at low wind speeds over the monsoon trough land region during MONTBLEX‐90
Authors: Rao Kusuma G. , Narasimha R. , Prabhu A.
Published: February 2004

Using observational data acquired during the Monsoon Trough Boundary Layer Experiment (MONTBLEX‐90), the drag coefficient CD is estimated over two land stations, Jodhpur (arid region) and Kharagpur (moist region), by eddy correlation of velocity data from sonic anemometers. At low winds CD varies with wind speed according to a power law, which at Jodhpur takes the form CD=0.04 Ū10−1.06 (Ū10 being the mean wind speed at 10 m). The large values of the drag coefficient at low winds are associated with both near‐neutral and strongly unstable situations. On the other hand, the CD values at Kharagpur are less than at Jodhpur by nearly 50% for wind speeds > 3ms−1, and cannot be explained by the different roughness lengths at the two sites.
Applicability of adaptive grid inversion to imaging thermal anomalies caused by convection
Authors: Honda S.
Published: February 2004

The mantle is most likely to be convecting and the resultant temperature anomaly δT (i.e. the deviation from the horizontally averaged temperature) may be the major cause of seismic velocity anomalies. Convection at high Rayleigh number is characterized by thin thermal boundary layers and plumes. Thus, it is very important to determine their thickness. A simple tomographic inversion of model thermal anomalies caused by mantle convection is presented. It clearly reveals thin plumes and improves the estimate of amplitude anomalies. This could be done by using the adaptive grid method. Our results suggest that our approach has a potential ability of getting a clearer image of mantle convection.
Probing the Earth's bow shock with upstream electrons
Authors: Larson D. E. , Lin R. P. , McFadden J. P. , Ergun R. E. , Carlson C. W. , Anderson K. A. , Phan T. D. , McCarthy M. P. , Parks G. K. , Rème H. , Bosqued J. M. , d'Uston C. , Sanderson T. R. , Wenzel K.‐P. , Lepping R. P.
Published: February 2004

We present detailed measurements from the WIND 3D Plasma and Energetic Particle experiment of electron distributions in the deep foreshock, well away from the foreshock boundaries. Both electrons escaping from the magnetosheath and solar wind electrons reflected by the strong magnetic field of the shock are observed. The loss cone angle that separates the reflected electrons from the escaping magnetosheath electrons can be determined as a function of electron energy. At lower energies, the loss cone angle increases because the cross‐shock potential tends to pull electrons into the magnetosheath. The loss cone angle is typically ≲ 30° at energies ≳ 100 eV, corresponding to peak shock magnetic field to local magnetic field ratio of ≳ 5, while the loss cone variation with energy gives typical shock potentials of ∼85 V. Thus the measurements of back‐streaming foreshock electrons can be used as a tool to remotely probe the characteristics of the bow shock.
Quasi‐Biennial Oscillation effects in the semidiurnal tide of the Antarctic lower thermosphere
Authors: Jarvis M. J.
Published: February 2004

A continuous 37‐year record of the geomagnetic field from Argentine Islands Geomagnetic Observatory at Faraday research station (65°S, 64°W), Antarctica, has been analysed to reveal a 27‐month periodicity in the amplitude of the semidiurnal Sq variation in the H‐component significant at the 98% level. This indicates the presence of a periodicity in the strength of the semidiurnal tide at dynamo region altitudes matching that of the quasi‐biennial oscillation (QBO) classically observed in the zonal stratospheric winds at equatorial latitudes. It adds to similar observations (Olsen, 1994) which demonstrated such an effect in the diurnal Sq range at near‐equatorial latitudes but which showed no evidence for its presence at mid‐latitudes. Faraday research station is in a uniquely advantageous geophysical location for such geographically high‐latitude observations.
Polar vortex conditions during the 1995–96 Arctic Winter: Meteorology and MLS ozone
Authors: Manney G. L. , Santee M. L. , Froidevaux L. , Waters J. W. , Zurek R. W.
Published: February 2004

The 1995–96 northern hemisphere (NH) winter stratosphere was colder than in any of the previous 17 winters, with lower stratospheric temperatures continuously below the type 1 (primarily HNO3) polar stratospheric cloud (PSC) threshold for over 2 1/2 months. Upper tropospheric ridges in late Feb and early Mar 1996 led to the lowest observed NH lower stratospheric temperatures, and the latest observed NH temperatures below the type 2 (water ice) PSC threshold. Consistent with the unusual cold and chemical processing on PSCs, UARS MLS observed a greater decrease in lower stratospheric ozone (O3) in 1995–96 than in any of the previous 4 NH winters. O3 decreased throughout the vortex over an altitude range nearly as large as that typical of the southern hemisphere (SH). The decrease between late Dec 1995 and early Mar 1996 was ∼2/3 of that over the equivalent SH period. As in other NH winters, temperatures in 1996 rose above the PSC threshold before the spring equinox, ending chemical processing in the NH vortex much earlier than is usual in the SH. A downward trend in column O3 above 100 hPa during Jan and Feb 1996 appears to be related to the lower stratospheric O3 depletion.
Comparison of postseismic groundwater temperature changes with earthquake‐induced volumetric strain release: Yudani Hot Spring, Japan
Authors: Kitagawa Yuuichi , Koizumi Naoji , Tsukuda Tameshige
Published: February 2004

We observed temporal variations of the groundwater temperature during two observation periods (May 1983–July 1984, September 1991–August 1995) at Yudani hot spring in Tottori Prefecture, Japan. The groundwater temperature is affected by tides, atmospheric pressure changes and earthquakes. Assuming that volumetric strain change is the cause of these temperature changes, we calculated the respective strain sensitivities of the groundwater temperature. As a result, the sensitivities estimated from theoretical volumetric strain changes by earthquake fault models are more than 1,000 times larger than those from tides and atmospheric pressure. We infer that the actual volumetric strain changes near Yudani hot spring due to the earthquakes are larger than theoretical strain changes calculated by earthquake fault models.
Chemical characteristics of hydrothermal fluids from the TAG Mound of the Mid‐Atlantic Ridge in August 1994: Implications for spatial and temporal variability of hydrothermal activity
Authors: Gamo Toshitaka , Chiba Hitoshi , Masuda Harue , Edmonds Henrietta N. , Fujioka Kantaro , Kodama Yukio , Nanba Hiromi , Sano Yuji
Published: February 2004

The TAG hydrothermal mound on the Mid‐Atlantic Ridge (26°08′N, 44°50′W) was revisited in August 1994 with the submersible Shinkai 6500 in order to characterize time‐series fluid chemistry prior to the ODP drilling. Fluid samples were taken from both black smokers and white smokers. Si, pH, alkalinity, H2S, major cations (Na+, K+, Ca2+, Mg2+), major anions (Cl−, SO42−), and minor elements (Li, Sr, B, Fe, Mn, Cu, Zn, Br) as well as Sr isotope ratios were measured. We report the first Br/Cl ratios for the TAG hydrothermal fluids, showing no fractionation between Br and Cl during the fluid‐rock interaction. This study shows small changes in composition of the black smoker fluids from the 1990 data (Edmond et al., 1995). Changes of pH, alkalinity, Fe, K, and 87Sr/86Sr values are suggestive of subsurface FeS precipitation and a decrease of water/rock ratio at a deeper reaction zone. Differences in chemical characteristics between the black and white smoker fluids were similarly observed as in 1990.
Seasonal occurrence of thin metallic ion layers at high latitudes
Authors: Bedey D. F. , Watkins B. J.
Published: February 2004

A year‐long series of incoherent‐scatter radar observations of the high‐latitude ionosphere conducted at Sondrestrom in Greenland has revealed an apparent increase in the rate of occurrence of thin metallic ion layers within the E‐region during the summer months, consistent with the known seasonal effect existing at middle latitudes. Unlike the situation at temperate latitudes, where shears in the neutral wind drive layer formation, layers at high latitudes are often formed by the action of large‐scale electric fields of magnetospheric origin. It is suggested here that a summertime increase in layer occurrence at high latitudes is attributable to a seasonal variation in the structure of the convective electric field, which determines the regions in which the necessary conditions for layer formation are present.
GFZ‐1: A small laser satellite mission for gravity field model improvement
Authors: König Rolf , Schwintzer Peter , Bode Albert , Reigber Christoph
Published: February 2004

The passive laser satellite GFZ‐1 (215 mm diameter, 21 kg mass, 60 retroreflectors) is orbiting the Earth since April 1995. GFZ‐1 was placed in a circular, medium‐inclination orbit with an initial altitude of 390 km, the lowest ever flown for a geodetic satellite. GFZ‐1 is tracked with cm‐accuracy during its expected lifetime of 4 years by some 15 ground stations of the international laser ranging net. The scientific objective of this small satellite mission is the improvement of the current knowledge of the Earth's gravity field by exploiting observed gravitational orbit perturbations, taking advantage of the increased sensitivity due to the very low orbit. A first tentative laser data evaluation resulted in a pronounced contribution of GFZ‐1 data to the spectral terms around orders 31 and 46 of the spherical harmonic expansion of the geopotential when solved in combination with satellite tracking data from 34 other satellites, leading to small but perceptible improvements in the global geoid and gravity field model.
First VHF radar observations of tropical latitude E‐region field aligned irregularities
Authors: Choudhary R. K. , Mahajan K. K. , Singh Sachchidanand , Kumar Keshav , Anandan V. K.
Published: February 2004

Indian MST radar at Gadanki (13.5°N, 79.2°E, 12.5° dip) was operated during July/August 1994, to observe the 3‐m scale size field aligned irregularities associated with the lower E region. Irregularity structure was studied by using height‐time variation of the echo intensity and weighted mean Doppler velocity. In this paper results of three diurnal cycles of observation are presented. Field perpendicular echoes were observed both during daytime and nighttime. A layered irregularity structure extending down to altitude below 86 Km was seen during the nighttime. The daytime structure showed a narrow echoing region with significant downward slope. Doppler velocity was in the range of 20–50 ms−1, both during day and night and was, in general, consistent with the slope of scattering structure observed in the height‐time‐intensity plots.
Photochemical production of carbon monoxide in authentic rainwater
Authors: Zuo Yuegang , Jones Ronald D.
Published: February 2004

Carbon monoxide (CO) was rapidly formed when rainwater samples were exposed to sunlight or light from a solar simulator. The initial photoformation rate of CO for rainwater samples collected in Miami, FL ranged from 22 nM/h to 200 nM/h at summer clear‐day‐noon time. The initial CO formation rate was also strongly correlated with the concentrations of dissolved organic carbon in rainwaters. This correlation indicates that dissolved organic matter is the major substrate for the photochemical production of CO in atmospheric liquids. Carbonyl compounds are suggested to be an important species in the photochemical production of CO in atmospheric waters. When assuming the global average concentration of dissolved organic carbon (DOC) of 2.0 mg liter−1, the global photoproduction rate of CO in atmospheric liquid‐phase is estimated to be of the order of 200–300 Tg per year, which is about 10% of the total global CO emissions from all sources. This newly found source of CO can significantly influence the redox chemistry in the atmosphere, particularly in cloud droplets and in the surrounding air
Measurements of superadiabatic lapse rates in the middle atmosphere
Authors: Sica R. J. , Thorsley M. D.
Published: February 2004

A large power‐aperture product Rayleigh‐scatter lidar system has been successfully built and over 175 nights of middle atmosphere temperature measurements have been obtained. The high signal‐to‐noise ratio of these measurements allows the stability of the air in the upper stratosphere and mesosphere to be determined. A detailed methodology has been developed to attempt to differentiate between lapse rate variations due to photon counting errors and actual geophysical variations. On nights when the geophysical variations are large compared to the photon counting errors, regions of convective stability and instability can be determined at a reasonably high confidence level. Both statistics of the layers and an “image” of the layers is presented for the night of May 31, 1996. The measured percentage of unstable layers is in agreement with the predictions of Hines (1991), as is the apparently sporadic formation and distribution of the unstable regions.
Geographic analysis of differences in trends between nearsurface and satellite‐based temperature measurements
Authors: Balling Robert C.
Published: February 2004

The purpose of this article is to assess geographical differences that exist in the differential trends between the satellite‐based lower‐tropospheric temperature measurements and the near‐surface temperatures measured by the land and sea thermometer network. Over the period 1979–1994, a highly significant difference of 0.28°C exists between the two global data sets. Analyses presented in this paper show that most of this difference comes from tropical ocean areas, with a much smaller contribution from the mid‐latitude land areas of the Northern Hemisphere. The implications of these results to the many explanations for the differential trend between the data sets are discussed.
The relationship between methyl bromide and chlorophyll α in high latitude ocean waters
Authors: Moore R. M. , Webb M.
Published: February 2004

We present a set of measurements of methyl bromide concentrations along with chlorophyll a data made in the high latitude, biologically productive, waters of the Labrador Sea in July 1995. Methyl bromide concentrations are found not to show a positive linear correlation with chlorophyll a above a chlorophyll concentration of ca. 0.7 mg L−1. Production rates of methyl bromide, calculated from a steady‐state balance with ocean‐atmosphere exchange, chemical degradation and downward mixing, are also found to have no positive linear correlation with chlorophyll a. If chlorophyll levels higher than ca. 0.7 mg L−1 are selected, a negative linear correlation is found between methyl bromide production rates (calculated using climatological wind speeds) and chlorophyll a. Labrador Sea waters were found to be undersaturated with methyl bromide, an observation which, when taken with evidence for a negative correlation between calculated methyl bromide production rate and the higher chlorophyll values, points to the existence of a biological consumption process. We conclude that models depending on an assumed positive linear correlation between methyl bromide and chlorophyll cannot be used to infer the source strength of methyl bromide in high latitude, productive waters.
Correlations between ozone loss and volcanic aerosol at altitudes below 14 km over McMurdo Station, Antarctica
Authors: Deshler Terry , Johnson Bryan J. , Hofmann David J. , Nardi Bruno
Published: February 2004

Ozone and aerosol profiles over McMurdo Station, Antarctica (78°S), have been measured August–October for the years 1986–1995. This spans the development and decay of the recent perturbation to stratospheric aerosol caused by Pinatubo. Volcanic aerosol surface areas, in the 11–14 km region, peaked near 100 µm² cm−3 in 1991, decaying to 20–30 µm² cm−3 in 1992, 15–25 µm² cm−3 in 1993, and to background levels of 4–8 µm² cm−3 in 1994. Based on these measurements the volcanic aerosol signal persisted over Antarctica for three austral springs, implying an exponential decay rate of about 14 months. The aerosol below 14 km was correlated with previously unobserved ozone loss at these altitudes. Ozone loss rates of 5–15 ppb dy−1 (0.3–0.5 DU dy−1) were observed in the 10–12 and 12–14 km layers. Beginning in 1994, when the aerosol approached its pre‐Pinatubo level, ozone loss diminished in the 12–14 km layer, and was not observed in the 10–12 km layer.
How rigid is the stable interior of the North American Plate?
Authors: Dixon Timothy H. , Mao Ailin , Stein Seth
Published: February 2004

We analyze data from eight permanent GPS stations broadly distributed through the interior of the North American plate, and use the resulting velocities to estimate an Euler vector describing motion of “stable” North America as a single rigid plate. The site velocities fit the single plate model with a mean residual of 1.3 mm/yr. The residuals do not appear to reflect post‐glacial rebound, and tests for differential motion between eastern and western North America at the New Madrid seismic zone show no resolvable motion within uncertainties. The residuals likely reflect observational error, and thus our estimate of the stability of the plate interior is likely an upper bound.
Observations of ENA in and near a proton arc
Authors: Søraas Finn , Aarsnes Kjell
Published: February 2004

A rocket equipped with solid state detectors sensitive to ions/protons and ENA with energies above 20 keV was launched into the poleward leap phase of the auroral substorm. The rocket reached an altitude of 454 km, that is into the region of intense charge exchange for precipitating ions. At low altitudes the pitch angle distribution of the precipitating particles was observed symmetric with respect to the magnetic field line. At higher altitudes this changed, the particle flux did not exhibit symmetry around the magnetic field as would be expected for a charged particle beam. This asymmetry and the fact that it only occurs at high altitudes is interpreted to indicate the detection of ENA, most likely hydrogen in our case. The ENA arrive from a specific azimuthal direction. The observations are consistent with calculations of the atmospheric spreading of the precipitating protons due to charge exchange. The importance of charge transfer collisions in the ion‐outflow problem is considered.
Recent progress in rock magnetism
Authors: Courtillot Vincent
Published: February 2004

Availability of affordable high‐performance computers has spurred research into the mathematical modelling of magnetic domain structures, stability of magnetic remanences and their experimental verification. Further, a recently substantially increased amount of observations of magnetic minerals other than magnetite in natural rocks has intitiated studies of their fundamental magnetic properties. To provide a forum for discussion of the latest developments covering these important subjects, two symposia were organized at the XXI General Assembly of the International Union of Geodesy and Geophysics (Boulder, Colorado, USA, July 2–14, 1995): New Approaches in Rock Magnetism (convened by S.L. Halgedahl and F. Heider) and Properties of minor magnetic minerals (convened by MJ. Dekkers and E. McClelland). In total 62 contributions were presented. This special section of Geophysical Research Letters comprises 19 papers, meeting, hopefully some of the most significant. The four convenors assisted me as associate‐editors in preparing this special issue, and I would like to thank them. The time taken by many reviewers is also appreciated. I hope the reader will get a feeling of the excitement that was evident during the Boulder meeting and will find this a useful collection of articles for later use.
Effect of Hall conductivity on time‐dependent convection
Authors: Liu W. William
Published: February 2004

Magnetospheric convection can be seen as a form of interchange motions, but unlike in an ideal MHD system, is subject to the ionospheric boundary condition. For general time‐dependent interchanges, the Hall current in the ionosphere generates a field‐aligned current independent of any background gradients. The effect of the Hall conductivity on time‐dependent convection is isolated through a simple box model where pressure gradient and field line curvature are suppressed. Mathematical analysis suggests that the Hall conductivity can destabilize interchange modes with long longitudinal wavelength. The finding comes at the expense of a high degree of geometrical idealization. More realistic computations are needed to test the theory.
An overview of POAM II aerosol measurments at 1.06 µm
Authors: Randall C. E. , Rusch D. W. , Olivero J. J. , Bevilacqua R. M. , Poole L. R. , Lumpe J. D. , Fromm M. D. , Hoppel K. W. , Hornstein J. S. , Shettle E. P.
Published: February 2004

We present aerosol measurements by the Polar Ozone and Aerosol Measurement (POAM) II instrument, from November 1993 through February 1996. Aerosol extinction profiles are in good agreement with coincident SAGE II measurements from 15 to 24 km. Extinction is continuing to decline since the eruption of Mt. Pinatubo. Seasonal variation in extinction in both hemispheres is examined on constant ground‐referenced and tropopause‐referenced altitude surfaces. Aerosol extinction peaks in the winter in the north outside the vortex. This also appears to occur in the south, but cannot be followed throughout the winter since POAM II measurements occur entirely inside the vortex at this time. A summer maximum at 19 km and above in the south is observed when the extinction ratio outside the vortex is plotted at a constant tropopause‐referenced altitude. This is not true for the north. We conclude that in late spring in the south POAM II is sampling low extinction, vortex‐processed air, or that an unidentified mechanism operates in the south, but not in the north. Signatures of polar stratospheric clouds (PSCs) are prevalent in the southern hemisphere data. PSCs are also observed in the northern hemisphere, but much less frequently. Evidence of enhanced diabatic descent in the polar vortex in both hemispheres is observed at 500 and 600 K. At 400 K the distinction between extinction ratios inside and outside the vortex in early winter is not as clear as at higher altitudes.
Simultaneous measurements of O+ and H+temperatures in the topside ionosphere over Arecibo
Authors: Sulzer Michael P. , González Sixto A.
Published: February 2004

We have measured both proton (TH+) and oxygen ion (TO+) temperatures simultaneously from spectra obtained in the lower topside ionosphere using the Arecibo incoherent scatter radar (ISR). This is the first time that two ion temperatures and three ion compositions have been measured from the same IS spectrum, and, as far as we know, one of the first measurements of both ion temperatures in the ionosphere.

First we describe our spectral‐domain analysis technique, emphasizing the ability to determine goodness of fit, which allows of the need for and effect of including a parameter to be numerically evaluated. We then show that common TH+ and TO+ allow insufficient freedom, and that separate temperatures are sufficient.

A small negative bias in [He+] parameter is removed when the second temperature is allowed to be free while the H+ fraction is not significantly affected. We also show that the measurement cannot be made at altitudes where little O+ is present. The difference between TH+ and TO+ is zero (or at least very small) during the night, and rises to several hundred degrees at about 500 km shortly after sunrise. During the middle of the day the temperature difference declines (presumably due to higher densities) and then rises again before sunset.
Modeling the Gulf Stream System: How far from reality?
Authors: Chao Yi , Gangopadhyay Avijit , Bryan Frank O. , Holland William R.
Published: February 2004

Analyses of a primitive equation ocean model simulation of the Atlantic Ocean circulation at 1/6° horizontal resolution are presented with a focus on the Gulf Stream region. Among many successful features of this simulation, this letter describes the Gulf Stream separation from the coast of North America near Cape Hatteras, meandering of the Gulf Stream between Cape Hatteras and the Grand Banks, and the vertical structure of temperature and velocity associated with the Gulf Stream. These results demonstrate significant improvement in modeling the Gulf Stream system using basin‐ to global‐scale ocean general circulation models. Possible reasons responsible for the realistic Gulf Stream simulation are discussed, contrasting the major differences between the present model configuration and those of previous eddy‐resolving studies.
The detection of enhanced carbon monoxide abundances in remotely sensed infrared spectra of a forest fire smoke plume
Authors: McMillan W. W. , Strow L. L. , Smith W. L. , Revercomb H. E. , Huang H. L.
Published: February 2004

Nadir looking infrared spectra of a forest fire smoke plume off the south shore of Long Island, New York, were obtained from a NASA ER‐2 aircraft during two spatially coincident over‐flights on the morning of August 25, 1995. These spectra exhibit enhanced CO column densities at the same geographic locations over the smoke plume on both over‐flights with a peak CO column density ∼2.6 × 1018cm−2, ∼6σ above the clear air background. Meteorological conditions suggest the smoke plume was confined to the planetary boundary layer (PBL), pressures ≥ 850 mb, and perhaps to a thin region near the top of the PBL. Constraining the excess CO to the PBL yields a CO mixing ratio ∼1,400 ppbv. Further constraining the CO to the model layer nearest the top of the PBL, 852–878 mb, yields‐4,300 ppbv. From the spatial overlap of the spectra, the estimated width of the CO rich portion of the plume is ≤ 2.8 km vs. a plume width of ∼5 km in GOES‐8 satellite visible images.
Monitoring potential photochemical interference in laser‐induced fluorescence Measurements of atmospheric OH
Authors: Dubey Manvendra K. , Hanisco Thomas F. , Wennberg Paul O. , Anderson James G.
Published: February 2004

In situ laser‐induced fluorescence measurements of atmospheric OH are susceptible to interference from laser generated OH, particularly in the troposphere. To quantify this interference we implement the addition of perfluoropropene, C3F6, for the chemical removal of OH from the ambient air. The removal rate of OH by C3F6 is determined in the laboratory using the discharge flow technique. Over the temperature range 249 to 296 K the rate constant is (6.0±0.8) × 10−13 exp[(370±40)/T] cm³ molecule−1 s−1, independent of pressure. In situ measurements using C3F6 addition are performed in both aircraft‐borne and ground‐based experiments. These studies show that laser excitation of the ²Σ+(v=1)←²Π(v=0) transition (282 nm) at high pulse repetition rates and low peak power can provide reliable and sensitive measurements of tropospheric OH.
Latitudinal variation of the heliospheric magnetic field during solar minimum
Authors: Bravo S. , Stewart G. A.
Published: February 2004

Ulysses' observations of the heliospheric magnetic field have shown that, above a narrow low‐latitude region, the intensity of the radial component of the field is independent of latitude. Here we present a theoretical explanation of this behaviour based on an MHD model of the solar wind applicable at solar minimum, which corresponds to the Ulysses' pole to pole pass. A discussion of other models' predictions is also presented.
Implications of a metal‐bearing chemical boundary layer in D″ for mantle dynamics
Authors: Manga Michael , Jeanloz Raymond
Published: February 2004

At lower mantle conditions, the thermal conductivity of non‐metallic crystals is an order of magnitude lower than that of dense metallic minerals such as FeO, FeSi and Fe, which may be present in a chemical boundary layer at the base of the mantle. Because the core‐mantle boundary is nearly isothermal, variations in the thickness of a metal‐bearing layer induce lateral temperature variations of several hundred kelvin, which in turn affect the pattern of mantle convection. Upwellings should occur where the layer is thickest; the resulting stability of the metal + silicate layer with respect to the flow may also stabilize the pattern of mantle convection, and reduce its time‐dependence.
Two examples of the use of SAR interferometry on displacement fields of small spatial extent
Authors: Carnec Claudie , Massonnet Didier , King Christine
Published: February 2004

Interferometric combination of pairs of SAR images acquired by the European ERS‐1 satellite maps deformation fields associated with two phenomena, both of small spatial extension and located in SE France: the one is rapid terrain deformation caused by a landslide near the city of Saint Etienne de Tinée, and the other is slower subsidence caused by underground coal mining near Gardanne. Unlike interferometric measurement of wide‐field deformation, atmospheric propagation heterogeneity is not an accuracy‐limiting factor. Although the radar data confirm prior knowledge concerning the landslide, such an application of SAR interferometry appears difficult under normal conditions of observation using current spaceborne radar systems. The study of soil subsidence, however, can be generalized and improves prior knowledge of the displacement field, which has here been modeled assuming elastic deformation in a half‐space from several sources. The two examples help to understand the limits of the interferometric technique.
Fabry Perot observations of helium 10830 Å emission at Millstone Hill
Authors: Kerr R. B. , Noto J. , Lancaster R. S. , Franco M. , Rudy R. J. , Williams R. , Hecht J. H.
Published: February 2004

The He(23S) triplet at 10830 Å is observed using a Fabry‐Perot interferometer optimized for the near infrared. These observations, made at the zenith at the Millstone Hill Optical Facility during evening twilight, are the first to resolve the individual members of the helium triplet from the surrounding OH(5‐2) lines. The introduction of an aeronomically unique germanium detector Noto et al., [1993] was crucial to this achievement. The instrument is configured with a spectral resolution of 1/2 the helium line width at 1000°K, assuring that the line widths accurately reflect the velocity distribution of metastable helium. Although the intensities observed are within a factor of two of helium 10830 Å intensities observed by [Tinsley, 1968, Tinsley & Christensen et. al, 1971], the observed temperatures are different from the expected behavior of a neutral species in the upper‐thermosphere during twilight.
Compression directions north of the San Fernando Valley determined from borehole breakouts
Authors: Kerkela Stacy , Stock Joann M.
Published: February 2004

Borehole breakouts in 4 nearly vertical oil wells, and several other deviated holes, in the San Fernando Valley and Santa Susana Mountains suggest a maximum horizontal compressive stress direction (SH) of N49°W. These wells provide information about the stress field from 1974 to 1983 in the uppermost crust (<3 km depth) near the aftershock zones of the January 1994 Northridge earthquake and the February 1971 Sylmar earthquake. This direction of SH is anomalous with respect to the N to NNE directions of SH seen in other regional data, but is consistent with the structural complexity of this zone, including local changes in strike of major thrust fault zones and the presence of lateral ramps in both the Santa Susana and San Fernando faults.
Structural asymmetry of the TAG Rift Valley: Evidence from a near‐bottom survey for episodic spreading
Authors: Kleinrock Martin C. , Humphris Susan E.
Published: February 2004

Comprehensive high‐resolution sidescan sonar imagery, swath bathymetry, and photographic data from the TAG rift valley (Mid‐Atlantic Ridge, 26°N) suggest significant asymmetry in crustal accretion. Discontinuities and asymmetry in structural and volcanic morphology and age distribution about the spreading axis suggest a recent episode of preferential accretion of crust to the east throughout the central 10 km of this spreading segment. Such episodes may contribute to the long‐term spreading asymmetry in the area.
Predicted age‐depth scales for Siple Dome and Inland WAIS Ice Cores in west Antarctica
Authors: Nereson N. A. , Waddington E. D. , Raymond C. F. , Jacobson H. P.
Published: February 2004

Geophysical data are used with ice flow models and generalized accumulation histories to estimate age and annual layer thickness versus depth for two anticipated ice core sites in West Antarctica: Siple Dome (81.65°S, 148.81°W) and an inland site on the West Antarctic Ice Sheet (WAIS). This modeling experiment predicts that 104 year‐old ice is at ∼50% depth and 105 year‐old ice is at ∼90% depth at both sites. Both of these cores could contain climate information through the last glacial cycle with annual resolution through the Holocene. The predicted similarity in resolution and record length between the two cores suggests that they could be compared in detail to obtain both spatial and temporal information about the paleoclimate and history of the West Antarctic ice sheet.
Measurements of atmospheric electric field and conductivity in the locality of a gas well flame
Authors: Gopalakrishnan V. , Deshpande C. G. , Kamra A. K.
Published: February 2004

Measurements of the atmospheric electric field and conductivity of both polarities are made at the Earth's surface 1.5 kms from a very large flame produced due to a gas well blowout. The results show a single periodic diurnal variation of electric field with a maximum in the afternoons and very low values of electric field during nights. The observation is explained with the help of Vonnegut et al.'s [1995] results that a flame burning under the influence of the fair weather electric field releases negative charge into the atmosphere. Possible implication of the results in explaining the development of inverted dipole in forest fire clouds is discussed.
Relationship between the present‐day stress field and plate boundary forces in the Pacific Northwest
Authors: Geist Eric L.
Published: February 2004

The relationship between plate boundary forces and the observed stress field in the Pacific Northwest is established using numerical models of continental deformation. Because the orientation of the greatest horizontal principal stress throughout the Pacific Northwest differs considerably from the direction of convergence between the Juan de Fuca and North American plates, the relationship between the stress field and forces acting along the subduction zone has been unclear. To address this relationship, a two‐dimensional finite element model developed by Bird [1989] is used that incorporates critical aspects of continental deformation such as a stratified rheology and interaction between thermal and mechanical components of deformation. Boundary conditions are specified in terms of either velocity or shear traction, depending on whether the computed shear stress at the plate boundary is less than or exceeds, respectively, a prescribed limit. Shear‐stress limits on the subduction and transform plate boundaries are independently varied to determine the relative effect of forces along these boundaries on intraplate deformation. Results from this study indicate that the shear stress limit of both subduction and transform boundaries is low, and that the intraplate stress field is attributed, in part, to the normal component of relative plate motion along the transform boundaries. However, the models also indicate that although the subduction zone fault is weak, a minimum shear strength (≥ 10 MPa) for the fault is necessary to explain the observed stress field. The balance among forces along the tectonic boundaries of North America results in a surprising degree of variation in the present‐day stress field.
Global warming in a coupled climate model including oceanic eddy‐induced advection
Authors: Hirst Anthony C. , Gordon Hal B. , O'Farrell Siobhan P.
Published: February 2004

The Gent and McWilliams (GM) parameterization for large‐scale water transport caused by mesoscale oceanic eddies is introduced into the oceanic component of a global coupled ocean‐atmosphere model. Parallel simulations with and without the GM scheme are performed to examine the effect of this parameterization on model behavior under constant atmospheric CO2 and on the model response to increasing CO2. The control (constant CO2) runs show substantial differences in the oceanic stratification and extent of convection, similar to differences found previously using uncoupled ocean models. The transient (increasing CO2) runs show moderate differences in the rate of oceanic heat sequestration (less in the GM case), as expected based on passive tracer uptake studies. However, the surface warming is weaker in the GM case, especially over the Southern Ocean, which is contrary to some recent supposition. Reasons for the reduced warming in the GM case are discussed.