bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial budget of organic carbon flux to the seafloor of the northern North Atlantic (60°N–80°N)
Authors: Schlüter Michael , Sauter Eberhard J. , Schäfer Angela , Ritzrau Will
Published: September 2002

The transfer of organic carbon from surface waters to the seafloor was calculated for the northern North Atlantic east of Greenland. This calculation is based on an empirically derived relationship between the rain rate of remineralizable organic carbon, derived by in situ O2 profiles, water depth, and primary production. The reliability of this attempt is supported by the good correspondence of calculated rain rates with an independent data set of particle trap studies and shipboard measurements of O2 profiles. For water depths of > 500 m the total seafloor remineralization rate is 2.7 × 106 tC yr−1 for the northern North Atlantic. Low and nearly similar average rain rates of 0.60 and 0.65 gC m−2 yr−1 have been derived for the deep basins of the Norwegian and Greenland Seas. Therefore, 1.7–1.8% of the primary production is transferred to the seafloor of the basins. A considerably higher average flux of 3.8 gC m−2 yr−1 was calculated for the Iceland Plateau, where ∼3.3% of primary produced organic carbon reaches the seafloor. The sediments of the Iceland Plateau receive 1.0 × 106 tC yr−1 or ∼37% of the organic carbon rain rate to the seafloor derived for the entire northern North Atlantic. The transfer of primary produced organic carbon below water depths of 500 and 1000 m suggests that 10.3 × 106 tC yr−1 and 4.5 × 106 tC yr−1are exported from surface waters. This is 2‐4.4% of the organic carbon produced in the photic zone of the northern North Atlantic east of Greenland.
Methane emissions from beaver ponds: Rates, patterns, and transport mechanisms
Authors: Weyhenmeyer Constanze E.
Published: September 2002

Net methane fluxes as well as methane fluxes via gas bubbles and via molecular diffusion were measured separately at nine different sampling sites throughout a beaver pond located in the low boreal forest region of central Ontario, Canada. In 1990, the average daily methane emission rate was 37.2 ± 30.4 mg CH4 m−2 d−1, yielding a total annual methane efflux of 5.8 g CH4 m−2 of which 1.1 g CH4 m−2 were produced in the wintertime when the pond was ice covered. On average, 65% of the net methane flux was released via gas bubble emission, the remaining 35% via molecular diffusion. Gas bubble flux rates were highly variable in space and time, suggesting that low sampling frequencies, common in many methane emission studies, could introduce large uncertainties to the estimated annual flux rates for different wetland systems. The effects of changes in environmental variables on methane emissions were found to be strongly dependent on which transport mechanism dominated the release of methane from the sediments. While the gas bubble flux volume was mainly affected by changes in atmospheric pressure and sediment temperatures, the diffusive flux component was found to be more sensitive to changes in the pond water level. Overall, magnitudes of methane fluxes from this pond were similar to other beaver ponds and moderate to high when compared to other wetland types in the low boreal forest region.
A model of the iron cycle in the ocean
Authors: Archer D. E. , Johnson K.
Published: September 2002

The IRONEX II in situ fertilization experiment showed decisively that availability of iron limits the growth of phytoplankton in equatorial Pacific surface waters. High‐nutrient low‐chlorophyll (HNLC) surface waters, potentially iron limited, are also found in the North Pacific and the Southern Ocean. A model of seawater iron geochemistry has been incorporated into a global ocean circulation and carbon cycle model and tuned to match the observed ocean Fe distribution. The model reproduces the observed HNLC areas in the equatorial Pacific and Southern Ocean but predicts nutrient depletion in the North Pacific (a region of high dust fluxes from Asia). The model is based on the idea that the concentrations of organic iron‐binding ligands in seawater control the subsurface Fe concentration and predicts that a majority (70–80%) of the global carbon export production can be sustained by upwelling of dissolved iron in seawater rather than by atmospheric deposition. The ocean iron cycle increases the efficiency of iron utilization in the biological pump, buffers new production against interannual fluctuations in iron deposition, and contributes several new intrinsic timescales to the ocean carbon cycle: several hundred years for the ocean iron concentration and perhaps several thousand for the concentration of ligand in the deep sea. Ocean recycling of Fe appears to play a major role in determining the strength of the biological pump in the ocean and the pCO2 of the atmosphere.
Redfield revisited: 2. What regulates the oxygen content of the atmosphere?
Authors: Lenton Timothy M. , Watson Andrew J.
Published: September 2002

The continuous charcoal record, interpreted with the aid of the results of combustion experiments, indicates that the mixing ratio of atmospheric oxygen has varied remarkably little over the past 350 Myr. We develop a dynamic feedback model of the coupled P, N, C, and O2 cycles and use perturbation analysis and a case study of the past 40 Myr to test various feedback mechanisms that have been proposed to stabilize atmospheric oxygen. These mechanisms involve alterations in nutrient driven productivity and the subsequent burial flux of organic carbon, which provides the main source of atmospheric oxygen. Suppression of the burial of phosphorus sorbed to iron minerals under anoxic conditions in ocean bottom waters tends to increase the ocean nutrient inventory and provide negative feedback against declining oxygen [Holland, 1994]. However, denitrification is enhanced by anoxia, tending to reduce the nutrient inventory and amplify declining oxygen [Lenton and Watson, this issue]. If organic phosphorus removal from the ocean is also suppressed under anoxic conditions, this improves oxygen regulation [Van Cappellen and Ingall, 1994], as does direct enhancement of organic carbon burial due to reduced oxygen concentration in bottom waters [Betts and Holland, 1991]. However, all of the ocean‐based feedback mechanisms cease to operate under increases in oxygen that remove anoxia from the ocean. Fire frequency is extremely sensitive to increases in oxygen above 21% of the atmosphere, readily suppressing vegetation on the land surface. This should transfer phosphorus from the land to the ocean, causing less carbon to be buried per unit of phosphorus and providing a weak negative feedback on oxygen [Kump, 1988]. However, a new proposal that increases in oxygen suppress the biological amplification of rock weathering and hence the input of phosphorus to the Earth system provides the most effective oxygen regulation of all the mechanisms considered. A range of proxies suggests that the input of available phosphorus to the ocean may have been significantly reduced 40 Myr ago, suppressing new production and organic carbon burial in the model. With only ocean‐based feedback, the atmospheric oxygen reservoir is predicted to have shrunk from ∼26% of the atmosphere 40 Myr ago. However, when land plant mediated negative feedback on phosphorus weathering is added, oxygen is regulated within 19–21% of the atmosphere throughout the past 40 Myr, in a manner more consistent with paleorecords.
Stable carbon and nitrogen isotope systematics in a human‐disturbed watershed (Lanyang‐Hsi) in Taiwan and the estimation of biogenic particulate organic carbon and nitrogen fluxes
Authors: Kao S. J. , Liu K. K.
Published: September 2002

Systematic investigation of carbon and nitrogen isotopic compositions of suspended particulate matters, plants, soils, sediments, and bed rocks was conducted during 1993–1994 in the Lanyang‐Hsi watershed, which is a typical small Oceania river. The dramatic increase in particle concentration and changes in nitrogen isotopic composition from tributaries to main stem indicate a shift in the major sources of particulate matter from soil and plant detritus (biogenic) for tributaries to poorly weathered rock fragments (lithogenic) and soil for the main stem. Using a two end‐member mixing of nitrogen isotopes and the load‐runoff relationship, we estimated the mean yield of biogenic particulate nitrogen (PN) in this watershed to be 0.41 ± 0.19 g N m−2 yr−1, which comprised only 9% of the total PN yield. Assuming the mean atomic C/N ratio (13.6) of soil samples as that for the biogenic end‐member, we estimated the yield of biogenic particulate organic carbon (POC) to be 4.9 ± 2.2 g C m−2 yr−1. If this represents the natural POC yield in Oceania islands, more than half of the POC flux from these islands could have been humanly induced. The flux‐weighted mean δ15N and δ13C values of exported particles were +3.6‰ and −25.3‰. The δ13C value is similar to those observed elsewhere, whereas the δ15 value is considerably heavier, apparently attributable to the high contribution of lithogenic PN induced by human disturbance.
Methane emissions from rice fields: Effect of plant height among several rice cultivars
Authors: Ding A. , Willis C. R. , Sass R. L. , Fisher F. M.
Published: September 2002

The effect of rice cultivar and plant height on methane (CH4) emission from rice fields was studied from data of 8‐year field experiments. Field experiments were conducted from 1989 to 1996 at the Texas A & M University Agricultural Research and Extension Center near Beaumont, Texas. Eleven rice cultivars and three soil types were involved. The results of this study showed the following: 1) Rice cultivar choice slightly influenced the day‐to‐day seasonal pattern of methane emission, but it had a significant effect on the seasonal total methane emission. The average seasonal methane emission is 22.8 g CH4 m−2 for high‐emitting cultivars (Mars and Della), ranging from 8.0 to 41.0 g CH4 m−2 and 17.7 g CH4 m−2 for low‐emitting cultivars (Lemont, Labelle, Lebonnet, Dawn, Katy, IR 36, Cypress, and Jasmine), ranging from 1.7 to 28.4 g CH4 m−2.) Seasonal methane emission has a positive linear correlation with postheading rice plant height. The correlation coefficient r2 is 0.736 (n = 24, and p < 0.05). In this paper we suggest that plant height may be a good criteria in choosing cultivars for mitigation of methane emission. More importantly, it provides a potential method for estimating methane emission from regional or global rice paddies as plant height can be evaluated by remotely sensed imagery.
Hydrogen and carbon kinetic isotope effects during soil uptake of atmospheric methane
Authors: Snover Amy K. , Quay Paul D.
Published: September 2002

The hydrogen and carbon kinetic isotope effects (KJEs) occurring during uptake of atmospheric methane (CH4) by soils were measured using in situ static flux chambers in a native grassland and a temperate forest in Washington State. The hydrogen KIE was αDsoil =k(CH4)/k(CH3D) = 1.099 ± 0.030 and 1.066 ± 0.007 for the grassland and forest, respectively. The carbon KIE of αCsoil =k(12CH4)/k(13CH4) = 1.0173 ± 0.0010 and 1.0181 ± 0.0004 for the grassland and forest, respectively, compares well to previous determinations in other ecosystems. Local spatial variability in αsoil was as large as the between‐ecosystem variability. The dependence of αsoil on αox and the KIE during diffusion is described. The apparent KIE associated with microbial oxidation, αox, was determined from αsoil and the relative rates of CH4 oxidation and diffusion in the soil column, derived from observed steady state profiles of soil air CH4 concentration. The apparent αox ranged from 1.094 to 1.209 for αDox and from 1.0121 to 1.0183 for αCox. These are the first determinations of the hydrogen KIEs during soil uptake of atmospheric CH4 and during aerobic microbial oxidation of CH4 at or below atmospheric concentrations. The KIE during uptake of atmospheric CH4 by soils is significantly different than the KIEs associated with the other sinks of atmospheric CH4. The interhemispheric asymmetry in the strength of the soil sink of atmospheric CH4 suggests a difference of ∼6‰ between the overall hydrogen KIEs in the two hemispheres. Modeling studies of the global atmospheric CH4 budget using deuterium as a tracer must therefore include αDsoil.
The recommended dissociation constants for carbonic acid in seawater
Authors: Lee Kitack , Millero Frank J. , Byrne Robert H. , Feely Richard A. , Wanninkhof Rik
Published: September 2002

A coherent representation of carbonate dissociation constants and measured inorganic carbon species is essential for a wide range of environmentally important issues such as oceanic uptake of anthropogenic CO2 and carbon cycle depictions in ocean circulation models. Previous studies have shown varying degrees of discordance between calculated and measured CO2‐system parameters. It is unclear if this is due to errors in thermodynamic models or in measurements. In this work, we address this issue using a large field dataset (15,300 water samples) covering all ocean basins. Our field data, obtained using laboratory‐calibrated measurement protocols, are most consistent with calculated parameters using the dissociation constants of Mehrbach et al. [1973] as refit by Dickson and Millero [1987]. Thus, these constants are recommended for use in the synthesis of the inorganic carbon data collected during the global CO2 survey during the 1990s and for characterization of the carbonate system in seawater.
Strong inertial currents and marginal internal wave stability in the central North Sea
Authors: van Haren Hans , Maas Leo , Zimmerman J. T. F. , Ridderinkhof Herman , Malschaert Hans
Published: September 2002

Solar insolation stabilizes the water column and suppresses vertical exchange. Observations from the central North Sea clearly show that increased heating in summer is accompanied by enhanced de‐stabilizing vertical current differences (shear), surprisingly to such extent that the equilibrium state is marginally stable. Under calm weather conditions, the shear is predominantly generated by near‐inertial motions while the internal wave spectrum primarily results from nonlinear interaction between the dominating tidal and near‐inertial motions. In terms of the associated enhanced vertical mixing across the largest vertical temperature gradients, shelf seas are not different from the abyssal ocean, despite the proximity to energy sources near boundaries in the former. By the lack of sufficiently strong wind‐ and tidal‐mixing this internal mixing is considered to be responsible for the diapycnal transport of nutrients leading to the observed increase in near‐surface values and triggering the late‐summer phytoplankton bloom.
The intermediate depth circulation of the western South Atlantic
Authors: Boebel O. , Davis R. E. , Ollitrault M. , Peterson R. G. , Richardson P. L. , Schmid C. , Zenk W.
Published: September 2002

The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally‐buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 106m³s−1), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.
Validation of AVHRR Pathfinder SST's over the Mediterranean Sea
Authors: D'Ortenzio F. , Marullo S. , Santoleri R.
Published: September 2002

Pathfinder Sea Surface Temperatures (SST) have been validated for the Mediterranean Sea using independent data obtained from MEDATLAS CTD and XBT casts. The Mean Bias is, in most of the case less than 0.5° C, with a mean value of 0.2 °C. The bias was uncorrelated to the number of Mediterranean matchups points used to evaluate the coefficients of the SST Pathfinder algorithm. This implies that Pathfinder data over the Mediterranean Sea represents a coherent time series of temperature that can be used for climatological applications. As an example of such an application the time series of the mean Mediterranean SST is discussed for the entire period of available data (1985–1996). The analysis shows evidence of interannual variability but does not suggest an evident increasing trend for SST as suggested by previous works. Comparison with corresponding meteorological data (COADS) confirms the close correlation between meteorological forcings and SST field.
Subinertial response of the Gulf Stream System to Hurricane Fran of 1996
Authors: Xie Lian , Pietrafesa Leonard J. , Zhang Chen
Published: September 2002

The evidence of subinertial‐frequency (with periods from 2 days to 2 weeks) oceanic response to Hurricane Fran of 1996 is documented. Hurricane Fran traveled northward across the Gulf Stream and then over a cool‐core trough, known as the Charleston Trough, due east of Charleston, SC and in the lee of the Charleston Bump during the period 4–5 September, 1996. During the passage of the storm, the trough closed into a gyre to form an intense cool‐core cyclonic eddy. This cool‐core eddy had an initial size of approximately 130 km by 170 km and drifted northeastward along the Gulf Stream front at a speed of 13 to 15 km/day as a subinertial baroclinic wave. Superimposed on this subinertial‐frequency wave were near‐inertial frequency, internal inertia‐gravity waves formed in the stratified mixed‐layer base after the passage of the storm. The results from a three‐dimensional numerical ocean model confirm the existence of both near‐inertial and subinertial‐frequency waves in the Gulf Stream system during and after the passage of Hurricane Fran. Model results also showed that hurricane‐forced oceanic response can modify Gulf Stream variability at both near‐inertial and subinertial frequencies.
A Comparison Between the World Ocean Atlas and Hydrobase Climatology
Authors: Chang C. ‐W. June , Chao Yi
Published: September 2002

The gridded 1/4° World Ocean Atlas (WOA, Boyer and Levitus, 1998) is compared with the 1° WOA (Levitus et al., 1994) over the North Atlantic Ocean. The temperature and salinity fields from both the 1/4° and 1° WOA are projected on two isopycnal surfaces and compared with the Hydrobase climatology (Lozier et al., 1995). Geostrophic current maps diagnosed from hydrographic temperature and salinity are also presented. Results show that the 1/4° WOA, with a smaller smoothing radius, greatly improves the 1° WOA in several important aspects. In particular, sharp frontal features associated with the western boundary current which are absent in the 1° WOA are apparent in the 1/4° WOA. The 1/4° WOA describes similar small‐scale mid‐depth recirculation gyres reported by Lozier (1997) based on Hydrobase. However, “artifacts” in the property fields created by the isobaric averaging are still evident in the 1/4° WOA.
Sea surface temperature signals from satellites—An update
Authors: Strong A. E. , Kearns E. J. , Gjovig K. K.
Published: September 2002

Polar satellite‐derived observations of sea surface temperatures (SSTs) have been used routinely since 1982 to provide a complete monitoring of our planet, covering all comers of the oceans (unless covered by clouds) twice each day. In 1992, an initial glimpse was published (Strong, 1992) of some tendencies that had been observed during the 1980s. Now that seven additional years of NOAA satellite SST data have become available, the earlier time‐series (Strong, 1992) has been up‐dated, hi this analysis of the global nighttime SSTs, care was taken to avoid the anomalous conditions found during the 1982–83 El Chichón aerosols, 1991–92 Mt. Pinatubo aerosols, and the strong El Niño of 1997‐98. Evidence of warming is found to be present throughout much of the Tropics and in the mid‐latitude Northern Hemisphere. Estimates from the Southern Hemisphere, while strongly indicative of compensatory cooling in the region, are found to be not as reliable.
The equatorial west Pacific fresh pool at the end of the La Niña event in October/November 1996
Authors: Ioualalen M. , Holfort J. , Siedler G. , Zenk W. , Hénin C. , Picaut J.
Published: September 2002

The salinity, temperature and current distributions have been measured during the TROPAC cruise (Oct./Nov. 1996) at two sections, i.e. 143°E and 150°E, during the final phase of the 1995/1996 La Niña. The results present evidence that the fresh pool and the salinity front at its eastern boundary had moved far to the west, and that a barrier layer existed in that phase. The observed currents support the idea that advective processes play an essential role in creating the thermohaline structure during this ENSO phase. In relation with this process, it is found that the westward subduction mechanism of relatively dense eastern equatorial waters may apply during that phase.
Temperatures Using radar‐meteor decay times
Authors: Hocking W. K.
Published: September 2002

Experimental studies of the temperature and pressure dependence of the ambipolar diffusion coefficient in the mesopause region have been undertaken by studying meteor trail decay times with radars at a variety of sites in North America, with latitudes between 75N and 35N. The site at Resolute Bay, Canada, has proven especially useful, due to the wide range of mesopause temperatures experienced at that site between summer and winter. Theoretical predictions have been confirmed, and an algorithm is outlined which permits meteor decay times to be used to determine absolute measurements of mesospheric temperatures.
Excitation and decay of magnetospheric lobe cell convection and its associated aurora
Authors: Sandholt P. E. , Farrugia C. J. , Cowley S. W. H. , Lester M. , Moen J. , Lybekk B. , Trondsen E.
Published: September 2002

We discuss multi‐instrument observations of the excitation and decay within a ∼10 min interval of a convection pattern typical of lobe cells, centered at ∼1230 MLT in the winter hemisphere. The plasma convection and its associated aurora were triggered by a rapid northward turning of the interplanetary magnetic field (IMF) monitored by near‐Earth spacecraft. The IMF stayed northward oriented ( Bx = −4 nT; By = 2–3 nT; Bz = 2–4 nT; clock angle = 30°–70°) for 8 min, before rotating back south. The optical instruments recorded the activation of an east‐west aligned auroral form near the latitude of the pre‐existing cusp aurora at ∼ 73° MLAT, which was followed by a smooth, 10‐min long poleward advance at an average speed of 0.4 km s−1, reaching 75° MLAT at its most expanded phase. Simultaneous CUTLASS radar and local magnetic data confirm the occurrence of a large reconfiguration of plasma convection involving a change from antisunward to sunward flow components in the vicinity of the cusp, consistent with the presence of a lobe cell.
On the diurnal variation of polar mesosphere summer echoes
Authors: Klostermeyer J.
Published: September 2002

During quiet solar and geomagnetic conditions, the observed mean diurnal variation of the reflectivity of polar mesosphere summer echoes is characterized by a distinct maximum near noon and a deep minimum near 20 LT, so that it appears to contain a relatively strong semidiurnal component. Theoretical considerations predict a nonlinear relationship between the reflectivity, the tidal temperature variation, and the electron production by solar radiation and energetic‐particle precipitation. Least squares fits based on model predictions of atmospheric tides, on a Chapman function, and on cosmic noise absorption data indicate that the observed diurnal variation of the reflectivity is predominantly determined by the electron production. Its semidiurnal component is essentially caused by the fact that the ionization rates due to the solar radiation and the energetic‐particle precipitation have maxima at noon and near midnight, respectively.
Localized reconnection and substorm onset on Dec. 22, 1996
Authors: Ashour‐Abdalla Maha , El‐Alaoui Mostafa , Peroomian Vahé , Walker Raymond J. , Zelenyi Lev M. , Frank Louis A. , Paterson William R.
Published: September 2002

This study uses observations from the Wind, Geotail and Interball spacecraft together with global MHD simulation to investigate the onset of a substorm on Dec. 22, 1996. At 1240 UT, during the growth phase, a small localized neutral line formed in the dawn sector at x ∼ −10 RE and initially extended less than 3 RE in azimuth. The formation of this neutral line was associated with the total Poynting flux being focused in the region close to the neutral line. During the growth phase the neutral line expanded in azimuth and moved tailward. At the onset of the expansion phase lobe field lines began to reconnect, and a second small, localized neutral line formed in the dusk magnetotail at ∼1256 UT. Lobe reconnection at this neutral line corresponded to a second intensification of the substorm at ∼1316 UT.
Energy dispersion features in the vicinity of the cusp
Authors: Karra Maria , Fritz Theodore A.
Published: September 2002

Highly energetic particles have been observed by the Polar spacecraft in the vicinity of the cusp. The energy of these particles, which often exceeds 1 MeV, is frequently characterized by dispersion. Emphasis is given to the data obtained by Polar in the time interval March 1996–May 1997. Time‐Energy Dispersive (TED) features are observed almost every day in the vicinity of the dayside cusp (and usually multiple times in one day), from 66° to 78° invariant latitude, while no TEDs are detected at latitudes lower than 60° or higher than 84°. In addition, TEDs are observed mostly around the time period between 12:00 and 15:00 MLT, and they can last from a few minutes up to three hours.
On the SuperDARN autocorrelation function observed in the ionospheric cusp
Authors: André R. , Pinnock M. , Rodger A. S.
Published: September 2002

The SuperDARN radars are proving to be a very powerful experimental tool for exploring solar wind‐magnetosphere‐ionosphere interactions. They measure the autocorrelation function (ACF) of the transmitted signal backscattered from the ionospheric irregularities, and derive parameters such as the Doppler velocity and the spectral width. The associated spectra have a specific behaviour inside the cusp, a strong temporal and spatial evolution of the velocity and spectral width, and a high value of the spectral width. Although no studies have explained these characteristics, they are routinely used to detect the cusp in the radar data and estimate the open/closed field line boundary. Both satellite and magnetometer data show in the same region broadband wave activity in the Pc1, Pc2 frequency band. In this study, we evaluate how such wave activity modifies the radar's ACF, and conclude that it explains the spectra seen in the cusp.
A simple model of diurnal variations in the mesospheric oxygen nightglow
Authors: Ward W. E.
Published: September 2002

Local time variations in the OH, and O2(b¹Σ g+) and O(¹S) nightglow have recently been reported in observations by the Wind Imaging Interferometer (WINDII) and the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) and attributed to dynamical effects associated with the migrating diurnal tide. A simple model involving vertical advection, a background atmosphere from MSIS, and a latitudinal variation in amplitude given by the (1,1) Hough mode is developed and found to duplicate many of the features of the satellite observations. The model demonstrates that vertical advection is the prime process causing these airglow variations. The observed variability in the relative phases and intensities between these nightglow signatures are most likely due to variations in the phase, amplitude and/or vertical wavelength of the tide, and/or the background oxygen mixing ratio. Given the ubiquitous nature of the tidal influence in the mesopause region any attempt to determine background atmospheric parameters from nightglow signatures must take the dynamical effects of the diurnal tide into account.
The spatial size of ion events measured far upstream of the Earth's bow shock by ACE/ULEIS and WIND/STEP
Authors: Dwyer J. R. , Mason G. M. , Desai M. I. , Mazur J. E. , von Rosenvinge T. T.
Published: September 2002

Using ACE/ULEIS and WIND/STEP >40 keV ion measurements, we have calculated the probability of simultaneously observing ion events upstream of the earth's bow shock as a function of the lateral separation (in YGSE and ZGSE) of the two spacecraft, thus providing a quantitative measurement of the spatial size of upstream ion events. Our survey consisted of 202 days in 1997 and 1998 when the ACE and WIND spacecraft were near the first Lagrangian point, approximately 200 RE upstream of the earth. We find that for lateral separations of up to 70 RE, the coincidence rate of events at the two spacecraft remains high and conclude that the source region of the upstream ions is large when compared to the size of the bow shock.
Multi‐color photometric measurements of ionization and energies in sprites
Authors: Armstrong R. A. , Suszcynsky D. M. , Lyons W. A. , Nelson T. E.
Published: September 2002

Recent time‐resolved multi‐color photometric data obtained on one class of lightning‐related transient upper‐atmospheric electromagnetic events called sprites have confirmed an impulsive ionization emission during the sprite initiation. Data have also been obtained on some sprites which do not exhibit observable tendrils and which exhibit ionization emission that, if present, is below our detection limit. This suggests that some sprite events exhibit strong ionization while others do not. These results indicate that conditions causing sprite optical emissions are highly variable.
How many waves are in the gravity wave spectrum?
Authors: Sica R. J. , Russell A. T.
Published: September 2002

Parametric modelling of density perturbation measurements obtained with the University of Western Ontario's Purple Crow Lidar on 5 nights are used to infer that the typical vertical wavenumber spectrum in the upper stratosphere is dominated by a few quasi‐monochromatic waves. In general only 2 of these waves, with growth or decay rates on the order of 1/(14 km) or less, carry most of the spectral energy. These waves are present about half the time on the nights studied. When analyzed using classical spectral techniques these waves appear to form a continuous spectrum. These results may help explain why general circulation models obtain reasonable climatologies when gravity wave parameterization schemes based on a small number of propagating gravity waves are employed.
Field‐aligned currents generated in magnetotail reconnection: 3D Hall‐MHD simulations
Authors: Yamade Y. , Fujimoto M. , Yokokawa N. , Nakamura M. S.
Published: September 2002

Understanding the field aligned current (FAC) system in substorms is one of the central themes of magnetospheric physics. Three‐dimensional (3D) magnetohydrodynamic (MHD) simulations of magnetotail reconnection has shown that FAC structures similar to the wedge‐current system are indeed reproduced either with or without a dipolar region situated at the earthward end of a current sheet. In this study, however, by noting that the ion inertia length is not negligibly small compared to the current sheet width in the magnetotail, we investigate the FAC generation mechanism in the Hall‐MHD system. For an idealized current sheet geometry without the dipolar region, we find that the results obtained to be totally different from the MHD results.
Testing models for traveling convection vortices: Two case studies
Authors: Sibeck D. G. , Korotova G. I.
Published: September 2002

We use geosynchronous magnetic field observations to discriminate between two models for traveling convection vortices (TCVs) generated by pressure variations applied to the magnetosphere. According to the model of Glassmeier [1992], an azimuthal gradient in the pressure generates a single field‐aligned current into the high‐latitude ionosphere and a single TCV. According to the model of Southwood and Kivelson [1990], an azimuthal gradient in the pressure generates a pair of oppositely directed currents and TCVs. We reexamine two well‐documented examples of TCVs and show that their signatures correspond better to the predictions of the Southwood/Kivelson model than to the predictions of the Glassmeier model.
The pitch angle distribution transition of energetic particles at substorm onset observed by GEOS‐2
Authors: Chen Z. X. , Korth A. , Pu Z. Y. , Mouikis C.
Published: September 2002

We discuss here pitch angle distributions of energetic ions and electrons at substorm onset. The transition signatures of ion pitch angle distributions imply that energetic ions experience field‐aligned acceleration after substorm onset which is directed either toward or away from the equator. Our studies indicate that the two often mentioned acceleration mechanisms—parallel electric field and “convection surge” acceleration—cannot explain the observed field‐aligned acceleration signatures. An analysis of the parallel Lorentz equation of ions suggests that under the premise that there exist large temporal variations of the magnetic field, two magnetic field topologies are responsible for the observed field‐aligned acceleration.
The thermal evolution of an Earth with strong subduction zones
Authors: Conrad Clinton P. , Hager Bradford H.
Published: September 2002

It is commonly supposed that plate tectonic rates are controlled by the temperature‐dependent viscosity of Earth's deep interior. If this were so, a small decrease in mantle temperature would lead to a large decrease in global heat transport. This negative feedback mechanism would prevent mantle temperatures from changing rapidly with time. We propose alternatively that convection is primarily resisted by the bending of oceanic lithosphere at subduction zones. Because lithospheric strength should not depend strongly on interior mantle temperature, this relationship decreases the sensitivity of heat flow to changes in interior mantle viscosity, and thus permits more rapid temperature changes there. The bending resistance is large enough to limit heat flow rates for effective viscosities of the lithosphere greater than about 1023 Pa s, and increases with the cube of plate thickness. As a result, processes that affect plate thickness, such as small‐scale convection or subduction initiation, could profoundly influence Earth's thermal history.
Three‐dimensional imaging from 2½‐dimensional reflection measurements
Authors: Witten Alan
Published: September 2002

The characterization of three‐dimensional inhomogeneities from reflection measurements is typically done in a three‐dimensional measurement geometry where sources and receivers are independently deployed over some measurement surface. While such a geometry is theoretically sound for characterizing three‐dimensional inhomogeneities, it is resource‐intensive since it either requires a two‐dimensional array of receivers and associated data acquisition and storage instrumentation or, when limited data acquisition resources are available, it is extremely time comsuming. An alternative to full three‐dimensional data acquistion is to employ the so called 2½‐dimensional reflection geometry where sources and receivers are deployed over a line (a two‐dimensional measurement geometry) and this line is sequentially move in a direction perpendicular to the measurement line. Here it is shown that the 2½‐dimensional reflection measurement geometry is, with appropriate processing, rigorously valid for characterizing three‐dimensional weak inhomogeneities.
An empirical relationship between seismic attenuation and velocity anomalies in the upper mantle
Authors: Roth Erich G. , Wiens Douglas A. , Zhao Dapeng
Published: September 2002

We use recent P wave attenuation (Qα−1) and velocity (Vp) tomographic models of the Tonga/Fiji region to obtain an empirical relation between attenuation and velocity for the upper mantle. The attenuation and velocity anomalies show a strong inverse correlation, allowing the determination of an empirical relationship. The relationship is similar to a predicted relation derived from laboratory experiments on peridotite and dunite, assuming the observed anomalies result from temperature variations. The results here suggest that the observed anomalies are generally consistent with the effect of temperature, and demonstrate that the combination of velocity and attenuation data provides powerful constraints on the physical state of upper mantle materials. We find that the chosen geotherm and activation energy has a strong effect on the position of the solidus within the seismic data.
Implicit noise reduction and trace interpolation in wavefield depth extrapolation
Authors: Chen How‐Wei , Chang Che‐Wei
Published: September 2002

Various types of noise can obscure reflections and refractions on deep crustal seismic data. This study presents a simple but elegant approach to true wavefield processing where noise reduction and trace interpolations are applied implicitly during depth extrapolation. No velocity and source/receiver information is required and the proposed approach can be combined with conventional or available processing algorithms. Prestack processing is successfully applied to a complicated OBS data set, demonstrating an improvement in noise reduction and effective trace interpolation.
New constraints on current deformation in Asia from continuous GPS measurements at Ulan Baatar, Mongolia
Authors: Calais Eric , Amarjargal Sharavyn
Published: September 2002

On the basis of 3 years of continuous GPS measurements (1995–1998) show a velocity of 6.4±1.6 mm/yr in a N125±30° azimuth with respect to Eurasia at a permanent site in Ulan Baatar, Mongolia. Together with recent GPS results in Asia, this result indicates motion of the Amurian/North China block at 6–10 mm/yr to the east to southeast relative to Eurasia, 0 to 5mm/yr slower than south China (10–11 mm/yr eastward). The small differential motion between these two blocks is in agreement with deformation models of Asia where crustal thickening dominates lateral extrusion. The fact that the velocity of the Amurian/North China block is significantly faster than the predictions of models based on the hypothesis that deformation in Asia is enterely driven by the India/Eurasia collision suggests that other mechanisms, such as far‐field effects of subduction processes and gravitational forces could significantly contribute to intracontinental deformation.
An analytic study on the evolution of the vertical profile of rain water concentration
Authors: Wacker Ulrike
Published: September 2002

This contribution hints at a serious problem in modelling the vertical profile of rain water concentration, which evolves under the effect of sedimentation, when cloud microphysical processes are represented in terms of a familiar one‐variable parameterization. If the parameterized precipitation flux increases faster than linearly with rain water concentration, as e.g. in the original Kessler‐concept, then the prognostic equation for this concentration takes the form of a quasi‐linear advection equation. The analytic solution is a transient ‘signal’ that ultimately breaks at some time and location and the solution becomes multivalued, that is physically meaningless. If, however, the problem is treated according to shock waves in nonlinear wave dynamics, a weak solution can be constructed. In that case, the multivalued solution is replaced by a discontinuous one.
A thick pipe‐like heat‐transfer mechanism in the mantle: Nonlinear coupling between 3‐D convection and variable thermal conductivity
Authors: Dubuffet Fabien , Yuen David A.
Published: September 2002

We have investigated the dynamics and heat‐transfer of 3‐D mantle convection with a realistic temperature‐and pressure‐dependent conductivity in a constant viscosity medium. Variable thermal conductivity is found to stabilize greatly the interior flow and the boundary‐layer dynamics, as compared to constant conductivity solutions for Rayleigh number up to 5 × 105. Due to the nonlinear interaction between variable conductivity and convective flow, a great deal of heat can be transported coherently through the mantle within thick pipe‐like structures with a radius of around 500km. These pipe‐like solutions can be understood from the intermediate asymptotic nature of the nonlinear heat diffusion equation. We suggest that these thick plume‐like structures in the lower mantle imaged recently by tomography may be a manifestation of this nonlinearity in the conductivity of the temperature equation.
Electrical conductivity of orthopyroxene and its high pressure phases
Authors: Xu Yousheng , Shankland Thomas J.
Published: September 2002

Electrical conductivity of orthopyroxene, clinopyroxene, and ilmenite + garnet with the same starting material San Carlos orthopyroxene (Mg0.92Fe0.08)SiO3 containing 2.89% Al2O3 by weight were measured within their respective stability fields in a multi‐anvil press. In the formula for conductivity σ=σ0 exp(−ΔH/kT), pre‐exponential factors σ0 of orthopyroxene, clinopyroxene, and ilmenite + garnet are 5248, 1778, 2239 S/m, respectively, and the activation enthalpies are 1.80, 1.87 and 1.66 eV, respectively. These results are required for defining a laboratory‐based conductivity‐depth profile of the upper mantle.
Airborne detection of diffuse carbon dioxide emissions at Mammoth Mountain, California
Authors: Gerlach T. M. , Doukas M. P. , McGee K. A. , Kessler R.
Published: September 2002

We report the first airborne detection of CO2degassing from diffuse volcanic sources. Airborne measurement of diffuse CO2degassing offers a rapid alternative for monitoring CO2emission rates at Mammoth Mountain. CO2concentrations, temperatures, and barometric pressures were measured at ∼2,500 GPS‐referenced locations during a one‐hour, eleven‐orbit survey of air around Mammoth Mountain at ∼3 km from the summit and altitudes of 2,895–3,657 m. A volcanic CO2anomaly 4–5 km across with CO2levels ∼1 ppm above background was revealed downwind of tree‐kill areas. It contained a 1‐km core with concentrations exceeding background by >3 ppm. Emission rates of ∼250 t d−1are indicated. Orographic winds may play a key role in transporting the diffusely degassed CO2upslope to elevations where it is lofted into the regional wind system.
Spontaneous melt localization in a deforming solid with viscosity variations due to water weakening
Authors: Hall Chad E. , Parmentier E. M.
Published: September 2002

Extraction of melt is influenced by the rheology of the two phase medium in which it is generated. The viscosity of mantle containing a small melt fraction is significantly increased as the concentration of water dissolved in olivine decreases. Upon melting, water fractionates into melt, where it is transported by both advection and diffusion. Advection transports water into melt rich regions, resulting in higher olivine water contents, lower solid viscosity, and lower pressure locally. Melt flows into these low pressure regions, resulting in self‐induced localization of melt. Diffusive transport of water opposes advection, creating a preferred wavelength of melt localization. An estimated wavelength of melt localization in the range of 100's of meters to a few kilometers may lead to the formation of high melt fraction channels which preserve chemical disequilibrium between melt and residual mantle.
Detection of groundwater conduits in limestones with gravity surveys: Data from the area of the Chicxulub Impact Crater, Yucatan Peninsula, Mexico
Authors: Kinsland Gary L. , Hurtado Manuel , Pope Kevin O.
Published: September 2002

Small negative gravity anomalies are found in gravity data from along the northwestern shoreline of the Yucatan Peninsula. These anomalies are shown to be due to elongate, shallow anomalous porosity zones in the Tertiary carbonates. These zones are caused primarily by groundwater solution and are presently active conduits for groundwater flow. The association of these small gravity anomalies with known topographic and structural features of the area, which partially overlies the Chicxulub Impact crater, indicates their development was influenced by structures, faults and/or fractures, within the Tertiary and pre‐Tertiary carbonates.
In situ determination of the phase boundary between Wadsleyite and Ringwoodite in Mg2SiO4
Authors: Suzuki Akio , Ohtani Eiji , Morishima Hideaki , Kubo Tomoaki , Kanbe Yuichi , Kondo Tadashi , Okada Taku , Terasaki Hidenori , Kato Takumi , Kikegawa Takumi
Published: September 2002

The phase boundary between wadsleyite and ringwoodite in Mg2SiO4 composition was determined by in situ observation using synchrotron X‐ray and multi anvil apparatus in KEK, Tsukuba, Japan. An energy dispersive method was employed using the Ge solid state detector and the white X‐ray beam from the synchrotron radiation source. The pressure was determined by the equation of state of NaCl. The stability field was identified by the change in intensities of diffraction lines of each phases. As a result, the phase boundary is expressed as a linear equation P=10.32(28)+0.00691(9)×T, where P is pressure in gigapascals and T is temperature in degrees Celsius.
Substantial thinning of a major east Greenland outlet glacier
Authors: Thomas Robert H. , Abdalati Waleed , Akins Torry L. , Csatho Beata M. , Frederick Earl B. , Gogineni Siva P. , Krabill William B. , Manizade Serdar S. , Rignot Eric J.
Published: September 2002

Aircraft laser‐altimeter surveys in 1993 and 1998 over Kangerdlugssuaq Glacier in east Greenland reveal thinning, over the 5‐year interim, of several meters for all surveyed areas within 70 km of the seaward ice front, rising to 50 meters in the final 5 km. Such rapid thinning is best explained by increased discharge velocities and associated creep thinning, most probably caused by enhanced lubrication of the glacier bed. The calving ice front over the past decade has occupied approximately the same location as in 1966. Velocity estimates for 1995/96 are about the same as those for 1966 and 1988, but significantly less than for 1999, suggesting that major thinning began after 1995.
The tropical intraseasonal oscillation in a coupled ocean‐atmosphere general circulation model
Authors: Gualdi Silvio , Navarra Antonio , Fischer Martin
Published: September 2002

In this study we report the simulation of the intraseasonal oscillation of the tropical atmosphere (TIO) in the coupled model GIOTTO. The model produces a zonal wave number one eastward propagating disturbance with a mean period of 42 days, which is in good agreement with the observed TIO periodicity. The simulated TIO, however, seems to be influenced by errors in the model mean state. In particular, the errors in the sea surface temperature of the tropical Pacific appear to have a significant impact on the oscillation, affecting its spatial distribution and producing an excess of TIO activity over the eastern Pacific. Despite of these shortcomings, the model seems to be able to reproduce some of the air‐sea interaction mechanisms which characterize the observed TIO.
Fresnel zone: A pitfall in seismic imaging of mid‐ocean ridge magma lenses
Authors: Ponce‐Corrêa Gustavo J. , Mutter John C. , Carbotte Suzanne
Published: September 2002

We present estimates of the Fresnel zone diameter for the axial magma chamber lens along the southern and northern East Pacific Rise and compare them to the magma lens widths derived from migrated seismic sections. The mean values of that diameter are 996m and 1052m and the mean values of the magma lens width are 680m and 1056m respectively. Most individual estimates of the magma lens width fall below the Fresnel zone diameter. Similar results are presented for three other mid‐ocean ridges. The registered signal is therefore a mix of responses from the magma sill and the encasing rock and lateral resolution is not achieved. We conclude that data analysis procedures that assume a laterally homogeneous Earth, cannot provide reliable estimates of the magma lens physical properties, including whether it is liquid or not.