bluuebunny/crossref_metadata_2025 huggingface dataset:
Spatial variability in methane emissions from a Texas rice field with some general implications
Authors: Sass Ronald L. , Fisher Frank M. , Andrews Jeffrey A.
Published: October 2002

This study addresses the issue of the effect of spatial variability within a single rice field on methane emissions observed using static flux boxes. The experimental data were collected from a commercially farmed field located near Richmond, Texas. The field was managed according to the normal practice of the area, including continuous flooding and the use of only inorganic fertilizer. Twenty‐four experimental plots were employed to measure daily and seasonal methane emission, seasonal aboveground biomass, and soil texture. A statistical analysis of the data provided predictive information on the precision with which static flux box measurements can determine the actual methane emission value in a particular field. The results of this study are related to the general global question of spatial variability in rice fields by comparisons with experimental and calculated methane emissions from several comparably managed rice fields around the world. Results from the present study indicate that methane emission values obtained from static flux box measurements are within ±20% of the actual field values within a 95% confidence interval. A comparison of this study with other reported results and related model calculations indicate that static flux box measurements are within ±24% of the actual field values.
Study of the 1997 Borneo fires: Quantitative analysis using global area coverage (GAC) satellite data
Authors: Wooster M. J. , Strub N.
Published: October 2002

In 1997 a drought commenced in Southeast Asia, this being directly related to the then ongoing El Niño‐Southern Oscillation (ENSO) event. Interaction between land clearance activities and this drought led to massive, uncontrolled vegetation fires that burned large areas of forest and agricultural land, most severely on the Indonesian island of Borneo. A similar situation in 1982–1983 led to the largest uncontrolled forest fire ever documented, damaging around 50,000 km2of Borneo's forest. This paper investigates the extent to which nighttime advanced very high resolution radiometer (AVHRR) global area coverage (GAC) data can be used to detail the spatial and temporal evolution of these large‐scale Indonesian fire events, and particularly the 1997 activity. GAC data are a spatial subsample of the full resolution local area coverage (LAC) AVHRR data, but unlike LAC, they have been archived globally on a daily basis for almost 20 years. Despite the extreme subsampling involved in GAC data production, simulation modeling and analysis of real data indicates that surprisingly reliable fire statistics can be obtained for the 1997 Borneo fire event from GAC imagery. For three dates in October 1997 where coincident LAC and GAC data are available, once numerical adjustment for the GAC subsampling procedure is included, fire counts extracted from LAC and GAC data consistently agree to within 0.15–13%. Time series analysis of the 1997 GAC data indicates that fires began in July in south Kalimantan, principally at the interface between cleared, cultivated land and lowland rain forest. Fire activity moved generally southward and peaked in September 1997. GAC data show the equivalent of 9733 LAC pixels as containing one or more active fires on 2 September. Fire activity declined significantly in November owing to the onset of the monsoon rains, with all fires ceasing by December 1997. Analysis of the cumulative fire map distinguishes peat swamp forest as the most severely affected ecosystem on Borneo, with >20% of this land cover category being identified as directly impacted by active fires. If the performance of GAC data can be shown to extend to other El Niño years and also to less intense periods of burning, then the GAC archive offers a tool for analyzing long‐term changes in the spatiotemporal pattern of fire activity in the region. Any trend can then be investigated for its relation to variations in agriculture, El Niño‐related climate, and other phenomena relevant to regional change in Southeast Asia.
Landscapes as patches of plant functional types: An integrating concept for climate and ecosystem models
Authors: Bonan Gordon B. , Levis Samuel , Kergoat Laurent , Oleson Keith W.
Published: October 2002

While most land models developed for use with climate models represent vegetation as discrete biomes, this is, at least for mixed life‐form biomes, inconsistent with the leaf‐level and whole‐plant physiological parameterizations needed to couple these biogeophysical models with biogeochemical and ecosystem dynamics models. In this paper, we present simulations with the National Center for Atmospheric Research land surface model (NCAR LSM) that examined the effect of representing vegetation as patches of plant functional types (PFTs) that coexist within a model grid cell. This approach is consistent with ecological theory and models and allows for unified treatment of vegetation in climate and ecosystem models. In the standard NCAR LSM the PFT composition and leaf area for each grid cell are obtained by classifying grid cells as 1 of 28 possible biomes. Here, we develop a data set from 1‐km satellite data that provides each model grid cell a unique PFT composition and leaf area for each PFT. Global simulations at 3° × 3° spatial resolution showed that ground temperature, ground evaporation, and northern high‐latitude winter albedo exhibited direct responses to these landscape changes, which led to indirect effects such as in soil moisture and sensible and latent heat fluxes. Additional simulations at 2° × 2° and 1° × 1° spatial resolution showed that low‐resolution simulations masked landscape heterogeneity in both approaches but the satellite‐based, continuous representation of vegetation reduced model sensitivity to resolution. It is argued that the use of spatially continuous distributions of coexisting PFTs is a necessary step to link climate and ecosystem models.
Dissolved organic carbon export with North Pacific Intermediate Water formation
Authors: Hansell Dennis A. , Carlson Craig A. , Suzuki Yoshimi
Published: October 2002

An evaluation of DOC export with the formation of North Pacific Intermediate Water is presented. Data from sites representing the relevant North Pacific water masses (subtropical, subpolar, and subtropical transitional) are used to demonstrate the process. We suggest that dissolved organic carbon (DOC)‐replete Oyashio (subpolar) water mixes with DOC‐depleted Kuroshio (subtropical) water in the mixed water region east of Japan. The new intermediate water, formed at a rate of 2–5 Sv, exports DOC at 13 ± 6 Tg DOC yr−1. On the basis of these findings we present an alternative model to explain the 14C‐DOC age gradient recently reported for intermediate depths of the North Pacific.
Hydrological alterations with river damming in northern Sweden: Implications for weathering and river biogeochemistry
Authors: Humborg Christoph , Blomqvist Sven , Avsan Ellinor , Bergensund Yvonne , Smedberg Erik , Brink Jenni , Mörth Carl‐Magnus
Published: October 2002

This case study tests the hypothesis that damming leads to a depletion of major elements in river systems. It determines the effect of river dams on the weathering regime, and thus on dissolved silicate (DSi) fluxes from land to the Sea by comparing two headwater areas in northern Sweden. In the pristine River Kalixälven, major dissolved elements are enriched within a few kilometers downstream from a high mountainous provenance, coming from an area low in vegetation and a thin active soil layer to a forested landscape. Also, alkalinity increased from 30 μeq L−1 to 110 μeq L−1, compared to 240 μeq L−1 measured at the river mouth. In the headwater of the River Luleälven, regulations led to inundation of the river valley and associated loss in vegetated soils. In reaches between the reservoirs, underground channeling of water and a reduction of water level fluctuations result in further decrease in soil‐water contact, and consequently diminishing weathering rates. The ratio of forest area to lentic area in the headwater was reduced dramatically with damming, from 2.65 to 0.84. As a consequence, geochemical variables in the river water show uniformity in space and in time. Alkalinity values at the River Luleälven mouth (155 μeq L−1) remained unchanged from the two main mountainous storage reservoirs (161 μeq L−1 and 166 μeq L−1). These results indicate that loss of vegetated soils through damming in river headwater critically reduces weathering fluxes and also suggest that changes in vegetation coverage in the Quaternary have altered DSi inputs significantly to the global Ocean.
Controls on suppression of methane flux from a peat bog subjected to simulated acid rain sulfate deposition
Authors: Gauci Vincent , Dise Nancy , Fowler David
Published: October 2002

The effect of acid rain SO42− deposition on peatland CH4 emissions was examined by manipulating SO42− inputs to a pristine raised peat bog in northern Scotland. Weekly pulses of dissolved Na2SO4 were applied to the bog over two years in doses of 25, 50, and 100 kg S ha−1 yr−1, reflecting the range of pollutant S deposition loads experienced in acid rain‐impacted regions of the world. CH4 fluxes were measured at regular intervals using a static chamber/gas chromatographic flame ionization detector method. Total emissions of CH4 were reduced by between 21 and 42% relative to controls, although no significant differences were observed between treatments. Estimated total annual fluxes during the second year of the experiment were 16.6 g m−2 from the controls and (in order of increasing SO42− dose size) 10.7, 13.2, and 9.8 g m−2 from the three SO42− treatments, respectively. The relative extent of CH4 flux suppression varied with changes in both peat temperature and peat water table with the largest suppression during cool periods and episodes of falling water table. Our findings suggest that low doses of SO42− at deposition rates commonly experienced in areas impacted by acid rain, may significantly affect CH4 emissions from wetlands in affected areas. We propose that SO42− from acid rain can stimulate sulfate‐reducing bacteria into a population capable of outcompeting methanogens for substrates. We further propose that this microbially mediated interaction may have a significant current and future effect on the contribution of northern peatlands to the global methane budget.
Upscaling methane emissions from rice paddies: Problems and possibilities
Authors: van Bodegom P. M. , Verburg Peter H. , Denier van der Gon Hugo A. C.
Published: October 2002

Global methane emission estimates depend highly on the models, techniques, and databases used. Since emissions cannot be measured directly at large scales, it is impossible to judge which estimate is more realistic. In this paper, different aspects of uncertainty in upscaling methane emissions from rice paddies are discussed. These aspects are visualized by a case study on the spatial upscaling of methane emissions from the island of Java, Indonesia. The first aspect concerns process information. An approach to incorporate this information in a simplified but process‐based way in predictive models is discussed. Sources of uncertainty include the methane emissions measurements, processes quantification, process simplification, and the use of data transfer functions. Data availability of input parameters, the second aspect, is uncertain because of differences between different data sources, the use of data sources for purposes not originally planned for, and the scale at which data are available. Data interpolation in combination with nonlinear model responses introduces scaling errors, the third aspect. Data accuracy introduced the highest uncertainties in emission estimates but is rarely accounted for in the estimation of global emissions.
Regional variability in the vertical flux of particulate organic carbon in the ocean interior
Authors: Lutz Michael , Dunbar Robert , Caldeira Ken
Published: October 2002

Carbon transport within sinking biogenic matter in the ocean contributes to the uptake of CO2 from the atmosphere. Here we assess the extent to which particulate organic carbon (POC) transport to the ocean’s interior can be predicted from primary production or export flux. Relationships between POC flux and depth are generally described by a uniform power law or rational decrease with depth, scaled to new or total primary production of POC. While these parameterizations of flux are used in most quantitative biogeochemical models, they are based on data sets from a limited geographic and depth range. We examine these relationships through a review of parameters derived from 14C uptake experiments, regional remote sensing, 234Th studies, nitrogen balances, and sediment trap records. Ocean regions considered include sites studied by the Joint Global Ocean Flux Study, Hawaii Ocean Time‐series, and Bermuda Atlantic Time‐series Study programs and involve observed and radiochemically corrected flux to depth. We demonstrate regional variability in the efficiency of the biological pump to transport organic carbon from surface waters to the ocean’s interior. Commonly applied flux relationships, while representative of some areas of the ocean, generally overestimate flux to depth. We estimate that the fraction of carbon transported as POC to depths greater than 1.5 km ranges between 0.10 and 8.8% (1.1% average) of primary production and between 0.28 and 30% (5.7% average) of export from the base of the euphotic zone. We develop empirical parameterizations of flux to depth using region‐specific constants. Using a one‐dimensional ocean model, we predict that the residence time of biogenic carbon may vary by up to 2 orders of magnitude depending on the regional efficiency of export and vertical transport.
Distribution of surface carbon dioxide and air‐sea exchange in the upwelling system off the Galician coast
Authors: Borges Alberto V. , Frankignoulle Michel
Published: October 2002

Data on the distribution of the partial pressure of CO2 (pCO2) were obtained during six cruises off the Galician coast, a region characterized by a seasonal upwelling. The values of pCO2 over the continental shelf are highly variable and range between 265 and 415 μatm during the upwelling season and between 315 and 345 μatm during the downwelling season. Both the continental shelf and off‐shelf waters behave as significant net sinks of atmospheric CO2. The computation of the air‐sea fluxes of CO2 over the continental shelf yields a net influx in the range of −2.3 (±0.6) to −4.7 (±1.0) mmol C m−2 d−1 during the upwelling season and −3.5 (±0.8) to −7.0 (±1.5) mmol C m−2 d−1 on an annual basis. During the upwelling season and on an annual basis, although the observed air‐sea gradients of CO2 over the continental shelf are significantly stronger than those in off‐shelf waters, the computed air‐sea CO2 fluxes are not significantly different because of the important incertitude introduced in the calculations by the estimated error on wind speed measurements. The presence of upwelling filaments increases the influx of atmospheric CO2 in the off‐shelf waters. During summer, important short‐term variations of pCO2 are observed that are related to both upwelling and temperature variations. During winter the cooling of water causes important undersaturation of CO2 related to the effect of temperature on the dissolved inorganic carbon equilibrium constants.
Characterizing patterns of agricultural land use in Amazonia by merging satellite classifications and census data
Authors: Cardille Jeffrey A. , Foley Jonathan A. , Costa Marcos Heil
Published: October 2002

Amazonia has been under considerable development pressure as croplands and pasture are established in areas formerly occupied by tropical forest and cerrado. Although this region is an important part of several important planetary biogeochemical cycles, the location and impact of human land use are not well understood. In particular, there is no existing satellite‐based map of agriculture across the Amazon or Tocantins river drainage basins. Recent efforts have classified land cover across this vast region, although they disagree on the location and amount of cropland and do not directly address pasture, a land use that has grown in importance in the last 2 decades. Here we present an analysis of land cover and land use practices over the Amazon and Tocantins basins of South America. In this study, we demonstrate how satellite imagery and agricultural censuses can be merged in order to provide a geographically explicit, fine‐scale description of land cover and land use practices. The result depicts the fraction of each 5‐min (9 × 9 km) grid cell that was devoted to agricultural activity during the mid‐1990s. The resultant map retains many of the characteristics of the agricultural census data, but with a much finer spatial resolution. During the mid‐1990s, cultivated area is estimated to have been 1.7 × 107 ha (2.5% of the basin), natural pasture is estimated at 3.3 × 107 ha (4.9% of the basin), and planted pasture is estimated to cover 3.3 × 107 ha (4.9% of the basin). Perhaps more important than the quantities, however, is that these data sets provide a new blend of ground‐based and satellite‐based spatially explicit data. This snapshot can be used as a basis to project either forward or backward in time, as a new check of finer scale land use classifications or as a driver of ecosystem models.
Estimation of global NH3volatilization loss from synthetic fertilizers and animal manure applied to arable lands and grasslands
Authors: Bouwman A. F. , Boumans L. J. M. , Batjes N. H.
Published: October 2002

One of the main causes of the low efficiency in nitrogen (N) use by crops is the volatilization of ammonia (NH3) from fertilizers. Information taken from 1667 NH3volatilization measurements documented in 148 research papers was summarized to assess the influence on NH3volatilization of crop type, fertilizer type, and rate and mode of application and temperature, as well as soil organic carbon, texture, pH, CEC, measurement technique, and measurement location. The data set was summarized in three ways: (1) by calculating means for each of the factors mentioned, in which findings from each research paper were weighted equally; (2) by calculating weighted median values corrected for unbalanced features of the collected data; and (3) by developing a summary model using linear regression based on weighted median values for NH3volatilization and by calculating global NH3volatilization losses from fertilizer application using 0.5° resolution data on land use and soils. The calculated median NH3loss from global application of synthetic N fertilizers (78 million tons N per year) and animal manure (33 million tons N per year) amount to 14% (10–19%) and 23% (19–29%), respectively. In developing countries, because of high temperatures and the widespread use of urea, ammonium sulfate, and ammonium bicarbonate, estimated NH3volatilization loss from synthetic fertilizers amounts to 18%, and in industrialized countries it amounts to 7%. The estimated NH3loss from animal manure is 21% in industrialized and 26% in developing countries.
Carbon isotopic discrimination and control of nighttime canopy δ18O‐CO2 in a pine forest in the southeastern United States
Authors: Mortazavi Behzad , Chanton Jeffrey P.
Published: October 2002

Canopy and soil CO2 concentration and isotopic measurements were conducted in a slash pine forest during a progressive drought in the southeastern United States (1) to determine and compare variations in δ13C of foliage (δCf), soil (δCs), and ecosystem‐respired CO2 (δCr) and (2) to evaluate the usefulness of a two end‐member oxygen isotope ratio (δ18O of CO2) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. The δCf was enriched by 2.2 ± 0.3 (standard error) ‰ during the extreme drought in May relative to September when precipitation was above normal. The enrichment in δCf exceeded changes in δCr and δCs for the same time period (1.6 ± 0.5 and 1.0 ± 0.4‰, respectively). Lower variations in the 13C of soil‐respired CO2 relative to the variations of the autotrophic component can buffer changes in δCr, which integrates the 13C signature of canopy and soil respired CO2. Application of a two end‐member model to canopy δ18O‐CO2 indicated that soil CO2 contribution to nighttime CO2 buildup within the canopy decreased from 100% at the soil surface to 0% within the canopy in September. In May, during the extreme drought period, soil respiration rate was 2.7 times lower than the rate in September despite similar soil temperatures (23° in May versus 19°C in September). Soil and aboveground respiration equally contributed to the nocturnal CO2 buildup within the canopy in May. Our model was limited in that it produced an upper limit value for soil respiration and neglected respiration by woody tissue.
Heterogeneous reactions in aircraft gas turbine engines
Authors: Brown R. C. , Miake‐Lye R. C. , Lukachko S. P. , Waitz I. A.
Published: October 2002

One‐dimensional flow models and unity probability heterogeneous rate parameters are used to estimate the maximum effect of heterogeneous reactions on trace species evolution in aircraft gas turbines. The analysis includes reactions on soot particulates and turbine/nozzle material surfaces. Results for a representative advanced subsonic engine indicate the net change in reactant mixing ratios due to heterogeneous reactions is <10−6 for O2, CO2, and H2O, and <10−10 for minor combustion products such as SO2 and NO2. The change in the mixing ratios relative to the initial values is <0.01%. Since these estimates are based on heterogeneous reaction probabilities of unity, the actual changes will be even lower. Thus, heterogeneous chemistry within the engine cannot explain the high conversion of SO2 to SO3 which some wake models require to explain the observed levels of volatile aerosols. Furthermore, turbine heterogeneous processes will not effect exhaust NOx or NOy levels.
Large sensitivity to initial conditions in seasonal predictions with a coupled ocean‐atmosphere general circulation model
Authors: Ploshay J. J. , Anderson J. L.
Published: October 2002

An ensemble of one‐year forecasts differing only in details of the atmospheric initial conditions was produced with a coupled ocean‐atmosphere general circulation model (GCM) in order to investigate the predictability of the coupled system. For some ocean initial conditions, the evolution of the tropical Pacific ocean thermal structure seems to be relatively deterministic for lead times out to one year. However, there are other ocean initial conditions, mostly in the mid 1990's for which coupled model forecasts of the tropical Pacific are much more sensitive to details of the atmosphere initial conditions. In some cases, the ensemble forecasts appear to split, with some ensemble members predicting El Niño‐like conditions, and others predicting La Niña. Very large ensembles were run for several of these cases. Very slight perturbations added to the atmospheric initial conditions led to large spread in predicted SST anomalies in some years. These are model results, however, they do suggest the possibility that seasonal predictions of the coupled tropical system may be highly non‐deterministic in some years.
Continuum emission and broadband electrostatic noise at the low latitude boundary layer: A diagnostic of boundary layer dynamics
Authors: Farrell W. M. , Fitzenreiter R. J. , Kaiser M. L. , Goetz K. , Maksimovic M. , Reiner M. J.
Published: October 2002

We examine the plasma and radio waves near the inner edge of the low latitude boundary layer (LLBL) during a period of time when the layer is in a “pulsed” oscillating configuration. Previous work suggests there are numerous possible explanations for such oscillations including a Kelvin‐Helmholtz instability, flux transfer events, and a boundary response to solar wind pressure oscillations. We demonstrate that the inner edge of the LLBL is indeed in motion, based upon the influence this motion has on the incident freely‐propagating continuum emission. We also demonstrate that the inner boundary contains impulsive broadband events that appear, in high resolution, as a series of bipolar solitary pulses. These are the result of a kinetic electron beam instability occurring in association with the boundary fluid motion. We suggest that the large‐scale fluid motion drives the kinetic instability via particle evacuation near the oscillating boundary.
On supercharging: Electrostatic aspects
Authors: Lai Shu T.
Published: October 2002

Charged‐beam emissions control the spacecraft potential if the beam current is higher than the ambient plasma currents. There are important questions as to the upper limit of the beam current that can be emitted into the ambient plasma and the upper limit of the spacecraft potential that can be induced. The usually accepted view is that the beam current emitted reaches its upper limit when the spacecraft potential energy reaches the beam energy. In this view any attempt to emit a current beyond the critical value would result in partial beam return, while both the effective beam current emitted and the spacecraft potential would remain unaffected. Surprisingly, results from three recent beam experiments in space show that the spacecraft potential energy can exceed the beam energy when very high beam currents are emitted. This phenomenon is called supercharging. The experimental results show that the expected limits on the beam current emitted and the spacecraft potential achieved appear to be incorrect. We offer two physical explanations. In the first one the beam energy is higher than expected. For example, the extra energy can be due to the beam space charge or the nonmonoenergetic beam energy distribution. For an unneutralized beam the beam space charge near the beam exit point can be significant. The potential energy of the beam space charge is converted into kinetic energy as the beam diverges. The total beam particle kinetic energy becomes more than its initial value. Scattering of beam particles with each other can spread the beam energy to lower and higher values. The beam particles with higher energies than the spacecraft potential escape. The second explanation concerns a common measurement technique using long booms for measuring spacecraft potential. The measurement represents the potential difference between the tip of a boom and that of a spacecraft. The tip of a long boom is commonly assumed to be at the ambient plasma potential. We stress that the boom can be charged, however, especially during high beam current emissions. As a result of boom charging, the measurement exceeds the true potential difference between the spacecraft and the ambient plasma. Both explanations concern possible misinterpretations of the measurements.
Diagrammatic method to describe the self‐inductive response of the magnetosphere‐ionosphere‐atmosphere‐Earth electromagnetically coupled system as a quasi‐particle excitation
Authors: Yoshikawa Akimsa , Yumoto Kiyohumi , Shinohara Manabu , Itonaga Masahiro
Published: October 2002

A diagrammatic method to intuitively describe the inductive response of wave fields in the magnetosphere‐ionosphere‐atmosphere‐Earth (MIAE) electromagnetically coupled system is developed. The coupling process of magnetohydrodynamic (MHD) waves and atmospheric electromagnetic waves, which interact with an anisotropically conducting thin sheet ionosphere, can be understood in terms of a process of redistribution of the induced current of wave modes. On the basis of the current conservation law and Faraday's law of electromagnetic induction, a redistribution process of the source‐induced current of wave modes to the secondary wave is simply illustrated. The purpose of this paper is to propose a methodology to describe the inductive response of a complex electromagnetically coupled system that is divided by a conducting boundary layer. As an example, we derive the reflection coefficient and the mode conversion ratio of MHD waves at an anisotropic, inductive ionosphere by using the concept of induced current redistribution. We also demonstrate that the redistribution process of wave modes under the law of conservation of induced current also satisfies an energy conservation law in the MIAE system. However, the true merit of this method is its applicability to the highly inhomogeneous region, in which many types of waves exist and interact through the arbitrary boundary layers.
Observations of low‐frequency electromagnetic plasma waves upstream from the Martian shock
Authors: Brain D. A. , Bagenal F. , Acuña M. H. , Connerney J. E. P. , Crider D. H. , Mazelle C. , Mitchell D. L. , Ness N. F.
Published: October 2002

We have analyzed magnetic field data returned from Mars Global Surveyor (MGS) for signatures of electromagnetic plasma waves upstream from the Martian bow shock. We discuss two recurring wave features in the data. Left‐hand polarized waves (0.04–0.10 Hz) observed near the local proton gyrofrequency (PCWs) propagate at small to moderate angles to the magnetic field and have amplitudes that decrease with distance from Mars. They are concentrated in two locations upstream of the Martian shock. PCWs were reported from Phobos 2 observations and can be attributed to solar wind pickup of Mars' hydrogen exosphere. Higher‐frequency waves (0.4–2.3 Hz) are observed when MGS is magnetically connected to the Martian shock. These waves have not been reported at Mars before, but have been reported at many solar system bodies, and are attributed to whistler waves generated at the shock and propagating upstream. The sense of polarization (left‐handed or right‐handed) of the whistler waves observed in the spacecraft frame depends upon the angle between the magnetic field and the solar wind flow direction. The whistler waves at Mars follow the trends in frequency, amplitude, propagation angle, and eccentricity observed at other solar system bodies.
Ring current decay rates of magnetic storms: A statistical study from 1957 to 1998
Authors: Dasso Sergio , Gómez Daniel , Mandrini Cristina H.
Published: October 2002

We perform a statistical study of the decay times for the recovery phase of the 300 most intense magnetic storms that occurred from 1 January 1957 to 31 December 1998. The Dst index in the decaying stage has been fitted by an exponential function, and a very good correlation has been obtained for most of the storms. Statistically representative values for the decay time (τ) are obtained by averaging the most reliable τ values, which resulted from applying a least squares method to the Dst index time series during every recovery phase. The mean value of τ turned out to be ∼14 ± 4 hours. We have also found that for very intense storms (Dstmin < −250 nT) the values of τ tend to decrease as the intensity of the storm increases.
Comparing ground magnetic field perturbations from global MHD simulations with magnetometer data for the 10 January 1997 magnetic storm event
Authors: Shao X. , Guzdar P. N. , Milikh G. M. , Papadopoulos K. , Goodrich C. C. , Sharma A. , Wiltberger M. J. , Lyon J. G.
Published: October 2002

A model has been developed to calculate the perturbed magnetic field at various ground‐based magnetometer sites using the output of the ionospheric currents from the Lyon‐Fedder‐Mobarry Global MHD code. The model uses the computed ionospheric currents and the height dependence of the electrojet current above the ground, by evaluating the penetration depth of the precipitating energetic electrons, to calculate the perturbed magnetic field on the ground using Biot‐Savart's law. By applying the model to the 10 January 1997 magnetic storm event we have calculated the perturbed magnetic field for four magnetometers and compared with observations. The comparison shows reasonable agreement between observations and simulations. The model including the dependence of the current sheet height on the precipitating electron energy reduces 10% error compared to the model with fixed current sheet height. The limitations of the global MHD model in calculating perturbed ground magnetic field are also discussed.
Simultaneous observations of earthward flow bursts and plasmoid ejection during magnetospheric substorms
Authors: Slavin J. A. , Fairfield D. H. , Lepping R. P. , Hesse M. , Ieda A. , Tanskanen E. , Østgaard N. , Mukai T. , Nagai T. , Singer H. J. , Sutcliffe P. R.
Published: October 2002

Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A one‐dimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst‐plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X ∼ −15 to −18 RE, respectively, about 2–5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X ∼ −9 RE. The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high‐latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high‐latitude inner magnetosphere, ∼103 km s−1. In summary, the results of our investigation strongly support the modern near‐Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed ∼2–5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within ∼1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, ∼10–20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.
A comparison study of the auroral lower thermospheric neutral winds derived by the EISCAT UHF radar and the Tromsø medium frequency radar
Authors: Nozawa S. , Brekke A. , Manson A. , Hall C. M. , Meek C. , Morise K. , Oyama S. , Dobashi K. , Fujii R.
Published: October 2002

A comparative study of the neutral wind in the polar upper mesosphere/lower thermosphere is conducted using two radars, the European Incoherent Scatter (EISCAT) UHF radar (ν = 931 MHz) and the Tromsø MF radar (ν = 2.8 MHz) colocated in northern Scandinavia. Comparisons of winds are made in case studies as well as on a statistical basis. On the basis of simultaneous observations obtained by the two radars, comparisons over a height range from ∼90 to ∼100 km are made for about 20 days between February and October 1999. In these comparisons we directly compare temporal wind variations. On the other hand, mean winds as well as diurnal and semidiurnal components are derived using 1‐month‐averaged wind data obtained by the medium frequency (MF) radar for 1999. They are compared with those derived from 56 days of EISCAT data studied by Nozawa and Brekke [1999a, 1999b]. In the case studies, we have also utilized altitude profiles of electron density obtained by the EISCAT radar at and above 62 km height to determine the total reflection height as well as to estimate the effect of group retardation for 2.8 MHz radio wave. It can be seen that the effect of particle precipitation sometimes penetrates into the D region (down to ∼84 km). Generally, wind velocities derived by EISCAT with the monostatic method exhibit significant scatter with time below 100 km, while the number of the MF radar wind data sharply decreases above 90 km except for summer. These facts make the comparison difficult, but no significant departures between different radar winds are identified on an instantaneous basis. In the statistical studies, generally both altitude profiles are well connected, or continuous, but significant disagreements are observed for the mean wind and the tidal components for summer. On the basis of these comparisons, we raise strong concerns regarding the use of summer wind data above 91 km obtained by MF radar located at high latitude under high solar activity conditions. The wind values during the winter and equinox observed by the two radars are complementary. Thus here is a good opportunity to utilize wind data from the two different radars through a year, which then enables us to cover a wide height range (approximately 70–120 km). This wide range gives us good opportunities to examine the mesosphere/lower thermosphere coupling processes based on observational data.
Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones
Authors: Lawrence Róisín M. , Gee Jeffrey S. , Karson Jeffrey A.
Published: October 2002

Mantle‐derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid‐Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep‐seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1° +0.8°/−1.4°) that is shallower than either the expected geocentric axial dipole inclination (40.7°) or present‐day inclination (41.9°) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (>350°C) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5°) more closely approximates the time‐averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350°C. Accounting for the anisotropy‐related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional‐scale fault zones on the western median valley wall.
Investigations of ambient light emission at deep‐sea hydrothermal vents
Authors: White Sheri N. , Chave Alan D. , Reynolds George T.
Published: October 2002

The Ambient Light Imaging and Spectral System (ALISS) was used to image ambient light from black smokers, flange pools, and a beehive on the East Pacific Rise and the Juan de Fuca Ridge. ALISS is a low‐light digital camera with custom‐designed optics. A set of nine lenses, each covered by an individual band‐pass filter, allows an identical scene to be imaged simultaneously in nine wavelength bands spanning the range of 400–1000 nm. Thus information about both the location and the spectral character of emitting regions is obtained. The primary source of light at deep‐sea vents is thermal radiation due to the high temperature of the hydrothermal fluid. This thermal light peaks in the infrared with a tail that extends into the visible. Data suggest that flange pools have an emissivity of ∼0.9 and black smoker fluids have an emissivity of ∼0.3. Thermal radiation dominates at wavelengths >700 nm (with a photon flux that increases from ∼106photon cm−2 s−1 sr−1at 700 nm to ∼1010 photon cm−2s−1 sr−1 at 1000 nm). Temporally variable light is observed in the 400–600 nm region of the spectrum that is orders of magnitude greater than predicted for a thermal source (i.e., on the order of 104 rather than 10−2–103photons cm−2 s−1 sr−1). This light is probably caused by mechanisms associated with turbulence, mixing and precipitation, such as vapor bubble luminescence, chemiluminescence, crystalloluminescence and triboluminescence. While biological responses to vent light are not yet known, observed light levels are too low to support obligate photosynthesis.
Thermal evolution of Earth: Models with time‐dependent layering of mantle convection which satisfy the Urey ratio constraint
Authors: Butler S. L. , Peltier W. R.
Published: October 2002

We present a new set of Earth thermal history calculations in which the effect of increasing mantle layering with convection Rayleigh number is included in a parameterized mantle convection model. We demonstrate that the inclusion of this effect results in strong buffering of the upper mantle temperature and surface heat flow. Models of this type deliver the observed surface heat flow when geochemically constrained internal heating rates (Urey ratios) are assumed with reasonable initial core temperatures. The surface heat flow is also relatively unchanged for the last 3 Gyr of Earth history in models of this kind, in accord with geological inferences concerning ancient geotherms derived from the study of Archean continental materials. In contrast, models with constant degrees of layering spanning the range from whole mantle to fully layered convection are shown to require unreasonably high initial core temperatures in order to meet the surface heat flow constraint. All successful models require that the coupling of heat flow between reservoirs be smaller than would be expected if mantle viscosities are those inferred on the basis of postglacial rebound (PGR) observations. This may indicate that viscosity for convection is significantly greater than that for rebound and hence that mantle rheology is non‐Newtonian and that the PGR process is governed by transient rather than steady state creep.
Estimated effects of the vertical structure of atmospheric mass on the time‐variable geoid
Authors: Swenson Sean , Wahr John
Published: October 2002

The GRACE satellite mission is designed to map the Earth's gravity field at a resolution of a few hundred kilometers every 30 days beginning in 2002. At these timescales, much of the change in the gravity field may be attributed to processes involving the redistribution of water on the surface of the Earth. Contributions from continental water storage, the oceans, and the atmosphere will all be present in the time‐varying gravity solutions. Isolating the hydrological and oceanographic signals will first require the removal of the atmospheric component of the gravity field estimates provided by GRACE. The vertical distribution of mass in the atmosphere is typically neglected when calculating the atmospheric gravity signal. We examine the accuracy of this approximation, as well as the accuracies of models which determine idealized atmospheric vertical structure from surface values of temperature and pressure. Using isobaric geopotential height data from a global forecast center to characterize the true atmospheric density distribution, we compute an exact atmospheric gravity signal with which to compare the gravity signal of each of these models. In addition, we examine the effects of including the aspherical component of the Earth's shape when calculating the atmospheric component of the gravity field. Because gravity estimates from GRACE will have limited spatial resolution, we average our results over regions of 200 to 500 km. At these length scales, our results show that using models based solely on surface data can introduce errors in the time variable surface mass signal inferred from GRACE as large as a few millimeters equivalent water thickness, with a global RMS of about 1 mm.
Properties of large ruptures and the dynamical influence of fluids on earthquakes and faulting
Authors: Miller S. A.
Published: October 2002

A model that couples large‐scale plate motion loading with the dominant mechanical effects of fluids within faults shows that the system always evolves to a high degree of disorder in the fault zone stress state. This disorder comes about because of the additional degree of freedom in stress space allowed by including coseismic changes in the fault zone pore pressure. For a given set of initial conditions the long‐term fault‐averaged stress state equilibrates, resulting in effective properties of the system while maintaining continuous complexity. Stress state disorder organizes in physical space into spatially correlated regions of incipient failure. If the spatial extent of incipient failure is limited, small events occur and the system continues forward. Aperiodically, incipient failure covers a large portion of the fault, and failure of a single cell cascades into very large model earthquakes. A switch from static to dynamic friction at the onset of slip results in a slip‐weakening model from stress redistribution during a (quasi‐static) propagating elastic dislocation. The complexity inherent in the model is quantified with simple relationships for the average stress drop, average slip, and seismic moment of large events. The characteristic length relating average slip to stress drop is shown to be asymptotic and a function of the aspect ratio of the rupture and the degree of fault overpressure. The model shows that average slip increases with rupture length but at a reduced rate with ongoing slip, showing a smooth transition from L model to W model mechanics for very large events. Model catalogs are compared with the historical catalog of strike‐slip events [Wells and Coppersmith, 1994], and the good agreement between the model and observations of seismic moment and fault area shows that the model successfully predicts the average slip over rupture areas up to about 2500 km2.