bluuebunny/crossref_metadata_2025 huggingface dataset:
Bimolecular second‐order reactions in spatially varying flows: Segregation induced scale‐dependent transformation rates
Authors: Kapoor Vivek , Gelhar Lynn W. , Miralles‐Wilhelm Fernando
Published: February 2004

To quantify chemical transformations in natural hydrologic environments, their coupling with spatially varying flows needs to be understood. Consider the elementary bimolecular second‐order irreversible chemical reaction for which the transformation rate at any point is proportional to the product of the concentrations of two distinct species at that point (r(c1, c2) = κc1c2, κ assumed to be constant). A problem arises in incorporating such chemical reactions in hydrologic transport models. On account of a lack of knowledge of all of the details of flows, or computational limitations, these transport models can only provide estimates of spatial averages of the concentration fields
and not the average of the product of the concentration of the different species,
. However, that spatial average needs to be assessed to quantify the chemical transformation rate pertinent to the spatially averaged concentration field
. For an impulse input of one species in a uniform background of the other species, detailed numerical simulations in laminar shear flow and heterogeneous porous media flow show that the spatial variability of flow causes the small‐scale concentration variations
of the different species to be negatively correlated
. This small‐scale segregation of solutes gives rise to a scale‐dependent transformation rate,
, which can be much smaller than what is found by substituting the mean concentration into the transformation rate expression:
. The segregation intensity
is strongly related to both the flow variations and the small‐scale mixing mechanisms (diffusion/local dispersion). After an initial increase in the absolute value of the segregation intensity, it slowly decreases with time due to the cumulative smoothing action of diffusion. As in hydrologic problems, the diffusion timescales characteristic to the flow variability scales can be quite large; substituting mean concentrations into rate expressions determined in well‐mixed batch tests is likely to overestimate the chemical transformation rate.
The partial duration series method in regional index‐flood modeling
Authors: Madsen Henrik , Rosbjerg Dan
Published: February 2004

A regional index‐flood method based on the partial duration series model is introduced. The model comprises the assumptions of a Poisson‐distributed number of threshold exceedances and generalized Pareto (GP) distributed peak magnitudes. The regionalT‐year event estimator is based on a regional estimate of the GP shape parameter. Analytical approximations of bias and variance of the regionalT‐year event estimator are deduced. The performance of the regional method is evaluated in the case of regional heterogeneity and intersite dependence. For small to moderate sample sizes, the regional estimator is superior to the at‐site estimator even in extremely heterogenous regions, the performance of the regional estimator being relatively better in regions with a negative shape parameter. When the record length increases, the relative performance of the regional estimator decreases, but it is still preferable to at‐site estimation in moderately heterogeneous and homogeneous regions for large sample sizes. Modest intersite dependence has only a small effect on the performance of the regional index‐flood estimator.
Comparison of annual maximum series and partial duration series methods for modeling extreme hydrologic events: 1. At‐site modeling
Authors: Madsen Henrik , Rasmussen Peter F. , Rosbjerg Dan
Published: February 2004

Two different models for analyzing extreme hydrologic events, based on, respectively, partial duration series (PDS) and annual maximum series (AMS), are compared. The PDS model assumes a generalized Pareto distribution for modeling threshold exceedances corresponding to a generalized extreme value distribution for annual maxima. The performance of the two models in terms of the uncertainty of theT‐year event estimator is evaluated in the cases of estimation with, respectively, the maximum likelihood (ML) method, the method of moments (MOM), and the method of probability weighted moments (PWM). In the case of ML estimation, the PDS model provides the most efficientT‐year event estimator. In the cases of MOM and PWM estimation, the PDS model is generally preferable for negative shape parameters, whereas the AMS model yields the most efficient estimator for positive shape parameters. A comparison of the considered methods reveals that in general, one should use the PDS model with MOM estimation for negative shape parameters, the PDS model with exponentially distributed exceedances if the shape parameter is close to zero, the AMS model with MOM estimation for moderately positive shape parameters, and the PDS model with ML estimation for large positive shape parameters. Since heavy‐tailed distributions, corresponding to negative shape parameters, are far the most common in hydrology, the PDS model generally is to be preferred for at‐site quantile estimation.
Calibration of time domain reflectometry technique using undisturbed soil samples from humid tropical soils of volcanic origin
Authors: Weitz A. M. , Grauel W. T. , Keller M. , Veldkamp E.
Published: February 2004

Time domain reflectrometry (TDR) is used to measure the apparent dielectric number (Ka) in soils. We studied two soil types (Humitropept and Hapludand) of low bulk density (about 0.7 Mg m−3 at 0.05 m to 0.8 Mg m−3 at 0.3 m depth) and high organic matter content (about 7% at 0.05 m to 4% at 0.3 m depth). Soils are located in a humid tropical environment (average annual soil water content is 0.51 to 0.58 m3 m−3). For calibration, undisturbed soil blocks, with a TDR probe installed in the center, were saturated and then allowed to dry by evaporation. Volumetric water content was calculated from measured Ka values and from gravimetric measurements. Because we used undisturbed soil samples, our calibration accounts for the natural heterogeneity in soils. We tested the suitability of various calibration functions relating Ka to soil water content for our soils. TDR technique underestimated the actual soil water content by 0.05–0.15 m3m−3, when using the widely applied Topp calibration function. A three‐phase mixing model with a geometry parameter, α=0.47, fit our data best. We consider mixing models to be a robust approach for calibration of TDR technique on various soils.
A continental phenology model for monitoring vegetation responses to interannual climatic variability
Authors: White Michael A. , Thornton Peter E. , Running Steven W.
Published: February 2004

Regional phenology is important in ecosystem simulation models and coupled biosphere/atmosphere models. In the continental United States, the timing of the onset of greenness in the spring (leaf expansion, grass green‐up) and offset of greenness in the fall (leaf abscission, cessation of height growth, grass brown‐off) are strongly influenced by meteorological and climatological conditions. We developed predictive phenology models based on traditional phenology research using commonly available meteorological and climatological data. Predictions were compared with satellite phenology observations at numerous 20 km × 20 km contiguous landcover sites. Onset mean absolute error was 7.2 days in the deciduous broadleaf forest (DBF) biome and 6.1 days in the grassland biome. Offset mean absolute error was 5.3 days in the DBF biome and 6.3 days in the grassland biome. Maximum expected errors at a 95% probability level ranged from 10 to 14 days. Onset was strongly associated with temperature summations in both grassland and DBF biomes; DBF offset was best predicted with a photoperiod function, while grassland offset required a combination of precipitation and temperature controls. A long‐term regional test of the DBF onset model captured field‐measured interannual variability trends in lilac phenology. Continental application of the phenology models for 1990–1992 revealed extensive interannual variability in onset and offset. Median continental growing season length ranged from a low of 129 days in 1991 to a high of 146 days in 1992. Potential uses of the models include regulation of the timing and length of the growing season in large‐scale biogeochemical models and monitoring vegetation response to interannual climatic variability.
One‐dimensional modeling of nitrogen and sulfur cycles in the aphotic zones of the Black and Arabian Seas
Authors: Yakushev Evgeniy V. , Neretin Lev N.
Published: February 2004

A coupled one‐dimensional model of the nitrogen and sulfur cycles in suboxic and anoxic conditions is developed and applied to the Black Sea redox zone and the oxygen‐deficient layer of the Arabian Sea. The vertical distributions of nitrogen compounds (total organic nitrogen, ammonium, nitrate, nitrite) and inorganic reduced sulfur compounds (hydrogen sulfide, elemental sulfur, thiosulfate, sulfate) as well as dissolved oxygen agree reasonably well with the observations. The model takes into account the processes of turbulent diffusion, sedimentation, and biogeochemical transformation of compounds. Rates of biochemical processes mediated by bacteria are described by first‐order equations using semiempirical functions of O2 concentration. The free nitrogen production in the model due to denitrification is estimated to be 34 Tg N yr−1 in the Arabian Sea and 1 Tg N yr−1 in the Black Sea; H2S oxidation and sulfate reduction in the Black Sea water column are in a balance of 1.6 mol m−2 yr−1.
Carbon balance of a temperate poor fen
Authors: Carroll Paul , Crill Patrick
Published: February 2004

The dynamic carbon balance of a southeastern New Hampshire wetland was constructed for the 1994 growing season using a light‐dark box sampling method. Net ecosystem exchange (NEE) (n = 414) and ecosystem respiration (n=234) measurements were made at the 1.7 ha Sphagnum spp. dominated poor fen. The NEE rates ranged from −192 to 492 mg C m−2 h−1 and the ecosystem respiration measurements were between −10 and −365 mg C m−2 h−1. The negative values represent a loss of carbon from the wetland system. NEE and respiration data were used to derive photosynthesis rates of the vegetation at the study site. A simple model, using hourly averages of photosynthetically active radiation, and air and soil temperatures to generate hourly rates of photosynthesis and respiration, was constructed to interpolate the carbon cycling rates at this fen through the entire 1994 growing season. Results of the carbon balance model suggest that the wetland lost an estimated 145 g C m−2 for the 9 month modeling period (April through December). The 1994 climate season was warmer (+1.15°C/month)and drier (−12.3 cm) than the 30 year normals for Durham, New Hampshire, the nearest meteorological station. These data suggest that if future climate change brings about warmer temperatures and lower water tables in peatland soils, positive climatic feedback leading to substantial releases of CO2 from boreal and subarctic peatlands is probable.
Long‐term variability of the air‐sea CO2 exchange coefficient: Consequences for the CO2 fluxes in the equatorial Pacific Ocean
Authors: Boutin J. , Etcheto J.
Published: February 2004

Global distributions of the air‐sea CO2 exchange coefficient are deduced from the remotely sensed Geosat and special sensor microwave imager wind speeds from April 1985 to December 1992. The global 8‐year average of the air‐sea CO2 exchange coefficient is 3.15 10−2 mol m−2 yr−1 μatm−1. The peak‐to‐peak interannual variations are as large as the amplitude of the seasonal cycle in the tropical oceans and in the Antarctic Ocean, whereas in the northern high latitudes the seasonal variability dominates. The exchange coefficient time series is combined with air‐sea CO2 partial pressure gradient that we extrapolate between 0°N and 5°S in the equatorial Pacific Ocean, and a zonal distribution of the air‐sea CO2 fluxes is presented from 1985 to 1992. While the maximum of the CO2 partial pressure gradient is located in the eastern Pacific, the maximum of the flux is in the central Pacific because of low exchange coefficients in the eastern Pacific. The largest interannual variation, 3.5 mol m−2 yr−1, occurs in the central Pacific between 1987 and 1989 because the 1986–1987 El Niño event drives ocean partial pressure close to equilibrium with the atmosphere and the 1988–1989 La Niña event brings about both high partial pressure gradient and strong exchange coefficient.
Interannual variation in global‐scale net primary production: Testing model estimates
Authors: Malmström Carolyn M. , Thompson Matthew V. , Juday Glenn P. , Los Sietse O. , Randerson James T. , Field Christopher B.
Published: February 2004

Testing estimates of year‐to‐year variation in global net primary production (NPP) poses some challenges. Large‐scale, multiyear records of production are not readily available for natural systems but are for agricultural systems. We use records of agricultural yields at selected sites to test NPP estimates produced by CASA, a global‐scale production model driven by both meteorological data and the satellite‐derived normalized difference vegetation index (NDVI). We also test estimates produced by the Miami model, which has underlain several analyses of biosphere response to interannual changes in climate. In addition, we test estimates against tree ring data for one boreal site for which data from both coniferous and deciduous species were available. The agricultural tests demonstrate that CASA can reasonably estimate interannual variation in production. The Miami model estimates variation more poorly. However, differences in NDVI‐processing algorithms substantially affect CASA's estimates of interannual variation. Of the four versions tested, the FASIR NDVI most closely reproduced yield data and showed the least correlation with changes in equatorial crossing time of the National Oceanic and Atmospheric Administration satellites. One issue raised is the source of the positive trends evident in CASA's NDVI‐based estimates of global NPP. The existence of these trends is consistent with potential stimulation of terrestrial production by factors such as CO2 enrichment, N fertilization, or temperature warming, but the magnitude of the global trends seen is significantly greater than suggested by constraints imposed by atmospheric fluxes.
Nutrient fluxes in forests of the eastern Sierra Nevada Mountains, United States of America
Authors: Johnson Dale W. , Susfalk Richard B. , Dahlgren Randy A.
Published: February 2004

Soil nutrient contents and fluxes in semiarid lodgepole (Pinus contorta[Dougl.]) and Jeffrey pine (P. jeffreyi[Grev. and Balf.]) stands of the eastern Sierra Nevada mountains are described and compared to those in the Integrated Forest Study sites [Johnson and Lindberg, 1992]. These Sierran forests, like others in the southwestern United States, have very low N, S, and H+fluxes compared to more humid forests. Base cation fluxes in these Sierran forests are high relative to more humid forests, however, reflecting the high base status of the soils, inputs from nearby desert systems, and high rates of soil weathering. Soil C and N contents in these Sierran forests are low compared to those in more humid forests, probably because of lower primary productivity and more frequent fire. Soil extractable P pools in these Sierran forests vary by 2 orders of magnitude and are strongly influenced by parent material. As in most snow‐dominated systems, pulses of NO3−are released from the melting snowpack each spring in the Sierran forests. Nitrogen released from melting snowpack is retained in the soil in most cases, but there are substantial springtime pulses of NO3−in stream waters during dry years. Budget calculations indicated that N losses during fire (both wildfire and prescribed fire) and N gains associated with postfire N‐fixing vegetation are an order of magnitude greater than N inputs and outputs via solution phase.
Leaching of nitrogen from forested catchments in Finland
Authors: Kortelainen Pirkko , Saukkonen Sari , Mattsson Tuija
Published: February 2004

This study provides an assessment on the spatial variability of the long‐term leaching (8–23 years) of nitrogen and organic carbon from 22 forested catchments (0.69–56 km2). The catchments are located throughout Finland excluding the northernmost regions. The Kruunuoja catchment is located in a national park; the other catchments represent typical Finnish forestry land. The leaching from the 21 forestry land catchments can be considered to represent average leaching from Finnish forestry land since the most important forestry practices (ditching, clear‐cutting, scarification, and fertilization) since the 1960s have affected about 2.4% of the catchment area per year (compare 2.5% in the entire country in 1980 and 2% in 1991). Moreover, the mean annual runoff from the catchments, 230–420 mm yr−1, agree with the mean annual runoff from Finland (301 mm yr−1 from 1931 to 1990). The major part of the nitrogen transported from the catchments consisted of organic nitrogen (on average 79%). The average inorganic nitrogen proportion ((NO3−N + NH4−N)/Ntot) was lowest (7.3%) in the Kruunuoja catchment and was highest (54%) in the southernmost Teeressuonoja catchment located in the highest anthropogenic nitrogen deposition area. The median C/N ratio in the study streams was high, ranging from 34 to 66. Nitrate leaching from the catchments varied between 2.8 (Kruunuoja) and 100 kg km−2 yr−1 (Teeressuonoja) and was negatively related to C/N ratio in stream water and latitude. The stepwise multiple regression model selected C/N ratio and nitrogen deposition which together explained 72% of the variation in NO3−N leaching. Retention of NO3−N deposition (calculated as ((input−output)/input) was high in all catchments, ranging from 0.99 in Kruunuoja to 0.67 in Teeressuonoja.
Missing sinks, feedbacks, and understanding the role of terrestrial ecosystems in the global carbon balance
Authors: Houghton R. A. , Davidson E. A. , Woodwell G. M.
Published: February 2004

Terrestrial ecosystems are thought to be a major sink for carbon at the present time. The endeavor to find this terrestrial sink and to determine the mechanisms responsible has dominated terrestrial research on the global carbon cycle for years. Some of the mechanisms advanced to explain the “missing sink” are also negative feedbacks to a global warming. Here we distinguish between mechanisms likely to act as feedbacks to a global warming and other mechanisms consistent with a terrestrial sink that are not feedbacks to a global warming. One of the postulated negative feedback mechanisms that also helps explain the current “missing sink” is based on the theory that carbon should accumulate in vegetation as a result of a warming‐enhanced mineralization of nitrogen in soil organic matter. The theory assumes that mineralized N is neither retained in the soil (through reimmobilization by microbial biomass) nor lost from the ecosystem, but rather becomes available for plant growth. None of these assumptions is supported yet by field data. In contrast, trends across existing climatic gradients suggest that warmer temperatures will lead to a decrease in the C:N ratio of soils (i.e., the mineralized N remains in soil). Data pertaining to temporal variability in the global carbon balance are conflicting with respect to the question of whether increasing temperatures cause a release or storage of terrestrial carbon. The answer seems to depend in part on time scale. Most likely, multiple mechanisms, including some that release carbon and others that accumulate it, account for the present net accumulation of carbon on land. However, a positive feedback between temperature and the release of CO2to the atmosphere by terrestrial respiration seems likely to grow in importance and could change significantly the role that terrestrial ecosystems play in the global carbon balance.
Methane fluxes on boreal peatlands of different fertility and the effect of long‐term experimental lowering of the water table on flux rates
Authors: Nykänen Hannu , Alm Jukka , Silvola Jouko , Tolonen Kimmo , Martikainen Pertti J.
Published: February 2004

Methane (CH4) fluxes were measured at 17 peatland sites with different nutritional and hydrological characteristics in the southern and middle boreal zones in Finland by a static chamber technique. Many of the natural peatlands also had counterparts drained for forestry 30–50 years ago. The mean emissions from May to September were 8.0 g CH4 m−2 for natural ombrogenous bogs and 19.0 g CH4 m−2 for natural minerogenous fens thus being higher than the 2 g CH4 m−2 yr−1 estimated for the Canadian peatlands. Change in the mean water table level had greater effect on CH4 fluxes on natural fens than on natural bogs. The mean CH4 emissions on drained bogs and fens were 3.9 g CH4 m−2 and 0.3 g CH4m−2, respectively. Some drained fens even had CH4 uptake from the atmosphere. The change in the mean water table had the lowest effect on CH4 fluxes on drained peatlands. The CH4 fluxes on peatlands (natural fens and bogs and drained peatlands) are associated with peat aeration, nutrient level, vegetation cover, peat compaction, peat temperature, and finally with microbial processes responsible for the net release of CH4. We could explain 67% of the variation in mean CH4 fluxes on Finnish peatlands by measuring the mean water table, peat bulk density, and peat pH. The present results can be used to predict the possible changes in CH4 emissions on peatlands if the climate is drying in north. For example, lowering of the present water table by 10 cm would induce a 70% reduction in the CH4 emissions from fens and a 45% reduction from bogs.
Controls on annual emissions of nitric oxide from soils of the Colorado shortgrass steppe
Authors: Martin R. E. , Scholes M. C. , Mosier A. R. , Ojima D. S. , Holland E. A. , Parton W. J.
Published: February 2004

Estimates of NOx (NO+NO2) emissions from temperate grasslands range from 0.003 to 101 ng NO‐N m−2 s−1(average 4.17 ng NO‐N m−2 s−1). As a result of this uncertainty, the potential contribution of soil NOx emissions to the global budget and to nitrogen loss for this ecosystem is unclear. The few existing measurements are short‐term observations of growing‐season fluxes. We measured soil NOx emissions from six sites within the shortgrass steppe at the Central Plains Experimental Range in Colorado from June 1994 through October 1995. The soils at these sites provided a range of texture (from a sandy loam to a clay loam) and soil moisture. Mean NOx emissions over the sampling period ranged from 2.6 to 5.7 ng NO‐N m−2 s−1 from the four unfertilized sites. Temperature was the dominant control on seasonal variations in NOx fluxes. Seasonal fluxes were highest in the summers (5.4 to 10.5 ng NO‐N m−2 s−1) and lowest in the winter (0.2 to 1.5 ng NO‐N m−2 s−1). The winter NOx emissions contribute up to 25% to the mean annual flux. Water‐filled pore space (WFPS) alone was a poor predictor of NOx emissions; however, peak NOx emissions were found near the field capacities for these soils (32–35% WFPS for coarse soils and 66% WFPS for fine‐textured soils). Water additions produced large (22–51 ng NO‐N m−2 s−1) but short‐lived (24 hour) pulses Of NOx emissions that were independent of both the amount of water added and the number of antecedent dry days. Short‐term increases in NOx flux stimulated by wetting are significant, and increase the summer estimate of NOx emissions 8 times estimates calculated from periodic sampling. Nitrogen applied in previous studies, 5 to 12 years earlier, increased the average annual NOx emissions approximately 1.5 times. Extrapolating our estimate to similar systems around the world, we estimate that grassland ecosystems, globally, emit 1.0 Tg N yr−1 as NOx. A current estimate of NOx emissions from grassland soils is 0.6 Tg N yr−1 [Davidson, 1991].
The northern edge of the band of solar wind variability: Ulysses at ∼4.5 AU
Authors: Gosling J. T. , Bame S. J. , Feldman W. C. , McComas D. J. , Riley P. , Goldstein B. E. , Neugebauer M.
Published: February 2004

Ulysses observations reveal that the northern edge of the low‐latitude band of solar wind variability at ∼4.5 AU was located at N30° in the latter part of 1996 when solar activity was at a minimum. This edge latitude is intermediate between edge latitudes found during previous encounters with the band edge along different portions of Ulysses' polar orbit about the Sun. Corotating interaction regions, CIRs, near the northern edge of the band were tilted in such a manner that the forward and reverse shocks bounding the CIRs were propagating equatorward and poleward, respectively, providing definite confirmation that CIRs have opposed tilts in the opposite solar hemispheres. No shocks or coronal mass ejections, CMEs, were detected during the ∼1.5 y traverse of the northern, high‐latitude northern hemisphere; however, at the northern edge of the band of variability an expanding CME was observed that was driving a shock into the high‐speed wind.
Reactive uptake and hydration experiments on amorphous carbon treated with NO2, SO2, O3, HNO3, and H2SO4
Authors: Rogaski C. A. , Golden D. M. , Williams L. R.
Published: February 2004

The reactivity and hydration properties of amorphous carbon were studied in a low‐pressure Knudsen cell reactor at room temperature (298 K). Reactions of NO2 (γ=0.11±0.04) and HNO3 (γ=0.038±0.008) were observed and may be important for nitrogen partitioning in the atmosphere. Water uptake was measured before and after exposure to various gases. Treating the amorphous carbon with NO2 and O3 does not alter the H2O uptake, while treatment with SO2, HNO3, and H2SO4 significantly increases the H2O uptake. The experimental results support current assumptions in jet aircraft plume models that sulfuric acid condensation is involved in the activation of soot particles as condensation nuclei.
Multiple timescales for neutralization of fossil fuel CO2
Authors: Archer David , Kheshgi Haroon , Maier‐Reimer Ernst
Published: February 2004

The long term abiological sinks for anthropogenic CO2 will be dissolution in the oceans and chemical neutralization by reaction with carbonates and basic igneous rocks. We use a detailed ocean/sediment carbon cycle model to simulate the response of the carbonate cycle in the ocean to a range of anthropogenic CO2 release scenarios. CaCO3 will play only a secondary role in buffering the CO2 concentration of the atmosphere because CaCO3 reaction uptake capacity and kinetics are limited by the dynamics of the ocean carbon cycle. Dissolution into ocean water sequesters 70–80% of the CO2 release on a time scale of several hundred years. Chemical neutralization of CO2 by reaction with CaCO3 on the sea floor accounts for another 9–15% decrease in the atmospheric concentration on a time scale of 5.5–6.8 kyr. Reaction with CaCO3 on land accounts for another 3–8%, with a time scale of 8.2 kyr. The final equilibrium with CaCO3 leaves 7.5–8% of the CO2 release remaining in the atmosphere. The carbonate chemistry of the oceans in contact with CaCO3 will act to buffer atmospheric CO2 at this higher concentration until the entire fossil fuel CO2 release is consumed by weathering of basic igneous rocks on a time scale of 200 kyr.
New constraints on source processes of volcanic tremor at Arenal Volcano, Costa Rica, using broadband seismic data
Authors: Benoit John P. , McNutt Stephen R.
Published: February 2004

Broadband seismic data recorded 2.3 km from the active vent of Arenal Volcano, Costa Rica, provide new constraints on tremor source processes. Arenal's tremor contains as many as seven harmonics, whose frequencies vary temporally by up to 75 percent, from initial values of 1.9 Hz for the first peak immediately following explosive eruptions to 3.2–3.5 Hz several minutes later. We infer that gas bubble concentration is variable within the conduit and also changes as a function of time, thereby changing the acoustic velocity. We infer that the source is a shallow, 200–660 m‐long, vertically oriented 1‐D resonator with matched boundary conditions, radiating seismic energy from a displacement antinode. Polarization analyses show that particle motion azimuths abruptly rotate, which may be explained by a decrease of the incidence angle. We suggest that energy is radiated predominantly from a displacement antinode that is changing position with time. Tremor consists mainly of transverse waves that travel at speeds of about 800 m/s. P waves in the magma conduit will couple very efficiently into S waves in the surrounding medium when there is virtually no impedance contrast between the two media for these two types of waves. The tremor at Arenal is similar to tremor at nine other volcanoes.
Breakdown in power‐law scaling in an analogue model of earthquake rupture and stick‐slip
Authors: Hamilton T. , McCloskey J.
Published: February 2004

The Gutenberg‐Richter relationship for earthquakes in both global and fault‐specific seismicity has been interpreted as an indication of self‐similarity for events of all magnitudes. It is observed, however, that the linear extrapolation of this relationship to high‐magnitude often fails to predict the frequency of occurrence of large events; earthquake hazard cannot, generally, be predicted solely by examination of microseismicity. It has been postulated that this break in power‐law scaling is due to a change in the rupture mechanism as the size of the rupture becomes larger than the thickness of the seismogenic layer. Here we examine the scaling of earthquake‐like failures in an analogue model consisting of an elastic solid confined and driven by a steel stressing apparatus. Sensors embedded in the solid close to the interface with a rough substrate measure local strain, and allow the examination of rupture scaling and moment release. The frequency‐magnitude distribution for events is a power‐law at low magnitude but exhibits enhancement for high‐energy events which is consistent with a characteristic earthquake model. We show, unambiguously, that this break in scaling corresponds to ruptures whose diameter is equal to the smaller dimension of the model fault.
Response of the near‐Earth magnetotail to a northward turning of the IMF
Authors: Nishida A. , Mukai T. , Yamamoto T. , Saito Y. , Kokubun S. , Lepping R. P.
Published: February 2004

The response of the near‐earth magnetotail to a sharp and isolated northward turning of the IMF observed at the WIND spacecraft is studied in the near‐Earth plasma sheet at x=−19 Re. Because of the possible time coincidence this northward turning could be causally related to an onset of the substorm expansion phase where Bz increased and the earthward convection intensified in the plasma sheet. The changes in the flow velocity and Bz to lower values occurred about 30 min later but they were not associated with an expansion phase onset. Thus the observations do not support the suggestion that the northward IMF turning produces a tailward sweep of the reduced dawn‐to‐dusk electric field and thereby triggers the substorm onset.
New evidence of size and composition of polar stratospheric cloud particles
Authors: Goodman J. , Verma S. , Pueschel R. F. , Hamill P. , Ferry G. V. , Webster D.
Published: February 2004

A NASA Ames ER‐2 aircraft encountered polar stratospheric cloud particles on July 28 and July 30, 1994 during the ASHOE/MAESA deployment. Stratospheric particles were collected by impaction techniques on specially treated substrates. For the first time, Nitron‐ nitrate reaction spots were detected on the ice crystal replicas, indicating the presence of NO3− ions on/in ice. Because the reaction spots were detected only on very small crystals (r < 1 µm' and never on larger crystals, suggest that the amount of NO3− coating sufficient to initiate reactions will accumulate during ice crystal evaporation in ice subsaturated air. This may slow down the evaporation rate and enable ice crystals to survive longer in subsaturated environment. That provides an explanation of why on both days ice crystals were replicated at temperatures above the frost point, when their appearance and size suggest substantial evaporation.
Temperature Dependence of the Collisional Removal of O2(A³Σu+, υ=9) with O2 and N2
Authors: Hwang Eunsook S. , Copeland Richard A.
Published: February 2004

The temperature dependence of the collisional removal of O2 molecules in the υ=9 level of the A³Σu+ electronic state has been studied for the colliders O2 and N2, over the temperature range 150 to 300 K. In a cooled flow cell, the output of a pulsed dye laser excites the O2 to the υ=9 level of the A³Σu+ state, and the output of a time‐delayed second laser monitors the temporal evolution of this level via a resonance‐enhanced ionization. We find the thermally averaged removal cross section for O2 collisions is constant (∼10 Å²) between room temperature and 200 K, then increases rapidly with decreasing temperature, doubling by 150 K. In contrast, the N2 cross section at 225 K is ∼8% smaller and gradually increases to a value at 150 K that is ∼60% larger than me room temperature value. The difference between the temperature dependence of the O2 and N2 collision cross section implies that the removal by oxygen becomes more important at the lower temperatures found in the mesosphere, but removal by N2 still dominates.
Velocity models for the highly extended crust of Death Valley, California
Authors: Louie John N. , Pullammanappallil Sathish K. , Honjas William
Published: February 2004

We employ nonlinear optimization to obtain shallow velocity structure from COCORP seismic reflection profiles in the highly extended Death Valley region, California. We also use group‐velocity analysis of regional earthquake surface waves that have traversed in and near the extended domain. When compared with regional models for surrounding provinces, our results show no evidence for significant uplift of the middle crust or removal of the upper crust. We do not find fundamental upper‐crustal variations between regions proposed to have widely varying extensional histories. These observations constrain previously proposed extensional mechanisms toward less than extreme amounts of extension.
Effect of Venus surface illumination on photographic image texture
Authors: Campbell Bruce A. , Shepard Michael K.
Published: February 2004

Radar images from Magellan have provided tremendous opportunities to study the geology and bulk dielectric properties of Venus, but many questions can only be addressed through direct photography or imaging spectroscopy of the surface. The thick atmosphere creates a situation of nearly isotropic illumination, so true shadows cannot exist for even very rough terrain. We carried out ray‐tracing simulations for synthetic surfaces under both isotropic and directional illumination conditions. Relative to similar surfaces viewed under directional illumination, venusian terrain will exhibit an equal or greater range of mean brightness with changing roughness, but a concurrent reduction in the maximum possible standard deviation of brightness across any region of given roughness. The local variability of an image is often referred to as texture, and these results indicate that Venus geologic units will appear more homogeneous in brightness than the same surfaces viewed under favorable directional illumination. In addition, the lack of a preferred lighting direction makes identification of surface structural patterns difficult. A similar situation may occur on Titan.
Cautionary note on magnetic grain‐size estimation using the ratio of ARM to magnetic susceptibility
Authors: Yamazaki Toshitsugu , Ioka Noboru
Published: February 2004

The ratio of anhysteretic remanent magnetization (ARM) to magnetic susceptibility (χ) is widely used for rapid estimation of magnetic grain‐size variations. On interpreting ARM/χ variations, no significant influence of interactions among magnetic grains on ARM acquisition is usually assumed implicitly, although some previous experiments using artificial sediments and biogenic magnetites opposed this assumption. Our rock‐magnetic study of deep‐sea surface sediments from the Pacific ocean showed that ARM/χ depends strongly on χ, the concentration of magnetic minerals, and on the R value of Cisowski [1981], an index of the strength of magnetic interactions. This result demonstrates that ARM/χ can be influenced significantly by magnetic interactions in natural sediments. We corrected for the influence of the magnetic interactions using the observed concentration dependence of ARM/χ. Regional variations of ARM/χ values before the correction were geologically unnatural in terms of magnetic grain‐size variations. Previous interpretations of ARM/χ variations on magnetic grain‐size without consideration of magnetic interactions may not be valid.
Experimental determination of the dispersion of waves observed upstream of a quasi‐perpendicular shock
Authors: Balikhin M. A. , de Wit T. Dudok , Alleyne H. St. C. K. , Woolliscroft L. J. C. , Walker S. N. , Krasnosel'skikh V. , Mier‐Jedrzejeowicz W. A. C. , Baumjohann W.
Published: February 2004

Highly‐coherent waves in the frequency range 1–15 Hz are usually observed upstream of the ramp of supercritical quasi‐perpendicular shocks. A few models have been proposed to explain their origin. In the present paper the wave vectors of these waves are determined using AMPTE UKS and AMPTE IRM data in order to differentiate between theoretical models.
A Western Pacific Oscillator Paradigm for the El Niño‐Southern Oscillation
Authors: Weisberg Robert H. , Wang Chunzai
Published: February 2004

A data‐based hypothesis is presented on the mechanism of the El Niño‐Southern Oscillation (ENSO), a major determinant of interannual global climate variability. The hypothesis emphasizes the importance of off‐equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western Pacific. These winds compete with westerly winds over the equatorial central Pacific enabling the coupled ocean‐atmosphere system to oscillate. Consistent with this hypothesis, an analogical oscillator model is constructed that produces ENSO‐like oscillations. The proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean‐atmosphere system to oscillate.
Satellite‐derived maps of Arctic and Antarctic sea ice motion: 1988 to 1994
Authors: Emery W. J. , Fowler C. W. , Maslanik J. A.
Published: February 2004

Standard image processing methods applied to 85.5 GHz Special Sensor Microwave/Imager (SSM/I) data provide previously unseen details about sea ice motion in the Arctic and Southern oceans. Means calculated from daily ice motions for 1988–1994 confirm the basic circulation patterns established from drifting stations and buoys, but extend these observations to illustrate less well‐known transport patterns in the Arctic coastal zones and sub‐Arctic seas. In the Antarctic, the gridded motion fields show a nearly continuous westward transport along the Antarctic coast, with well‐defined regions of exchange between this East Wind Drift and the Antarctic Circumpolar Current. This detailed view of mean ice motion in the Southern Ocean is unique and presents a comprehensive overview not previously available for Antarctic ice motions.
Model of red sprite optical spectra
Authors: Milikh G. M. , Valdivia J. A. , Papadopoulos K.
Published: February 2004

A synthetic spectrum of red sprites due to electron energization by the electric field from lightning is computed by using the electron energy spectrum obtained from a Fokker‐Planck code, which includes various inelastic losses. The results are compared with observed sprite spectra. Implications to models of red sprites are presented.
Radiocarbon content of pre‐bomb marine mollusks and variations in the 14C Reservoir age for coastal areas of the Barents and Kara Seas, Russia
Authors: Forman Steven L. , Polyak Leonid
Published: February 2004

Fourteen mollusks, collected alive between 1900 and 1945 from the Russian Barents and Kara seas, were analyzed by AMS 14C dating to evaluate variations in the 14C marine reservoir for arctic coastal sites, which is important for correcting ages in paleoenvironmental time‐series and advancing understanding of the exchange of carbon. The 14C ages on the mollusks reveal a range of marine reservoir values (R(t)) from 159 14C yr to 764 14C yr. The oldest R(t) values of 764 to 620 14C yr are for the bivalve Portlandia arctica, which often inhabit cold and low salinity waters and muddy substrates. The depleted 14C content for this bivalve reflects possibly the incorporation of old carbon from freshwater inputs and/or the consumption of old organic matter from the underlying sediments and pore waters. Other mollusks with sessile habitats and pelagic food sources gave significantly lower R(t) values between 159 and 344 14C yr. The youngest R(t) values indicate enrichment in 14C and may partially reflect enhanced transfer of 14C‐enriched CO2 from the atmosphere to the ocean surface with wind‐generated wave agitation. This study underscores that a variety of processes can lead to variable 14C depletion and enrichment of surface waters yielding a ca. 600 year age span for contemporaneous arctic mollusks. There may be added uncertainty in the 14C reservoir correction for deposit‐feeder species such as Portlandia sp. and perhaps for certain benthic foraminifera (e.g. Nonion labradoricum) because these taxa often incorporate old organic matter from the substrate. A reservoir correction of ≥700 years may be more appropriate for infaunal, deposit‐eater species, particularly in glacier‐dominated environments. Mollusks and foraminifera with sessile habits and pelagic food sources should be selected preferentially for 14C dating, because their shells may more closely reflect the 14C content of the global‐ocean mixed layer.
Excitation of the ionospheric Alfven resonator by strong lightning discharges
Authors: Sukhorukov A. I. , Stubbe P.
Published: February 2004

A mechanism for the nonlinear excitation of the ionospheric Alfven resonator by elves‐ or/and sprites‐produced lightning discharge is proposed. The source of the time‐varying nonlinear current, located at altitudes below 95 km, is due to the large impulse electron heating and breakdown of the atmosphere by a strong tropospheric discharge. The discussed mechanism may be responsible for anomalously large ULF events observed onboard the satellites above atmospheric weather systems.
On interseismic elevation changes and strain accumulation for great thrust earthquakes in the Nepal Himalaya
Authors: Gahalaut V. K. , Chander R.
Published: February 2004

Estimates of interseismic ground elevation change rates from Nepal Himalaya based on repeat levelling observations between 1977 and 1990 in the meizoseismal zone of the great thrust earthquake of 1934 have been reported byJackson and Bilham[1994]. We explain them using the model ofSavage[1983] for seismic strain accumulation and release along plate boundary faults (PBF) at convergent plate margins. Thus the section of the PBF under the Nepal Himalaya that ruptured during the 1934 earthquake may be currently locked and the hanging wall rocks may be experiencing downdip drag at the local plate convergence rate. Three independent estimates of 150, 100 and 50 km for downdip rupture width during the 1934 earthquake are available. If we use them as estimates of the current width of the locked zone in Nepal, then values of 18, 14 and 12 mm/year respectively for the plate convergence rate are obtained. Although all these values compare favourably with the estimated long term average rate of 18±7 mm/year for plate convergence accommodated at the Himalaya, the rms error is the least for the model with 150 km width of the locked zone.
Land subsidence caused by the East Mesa Geothermal Field, California, observed using SAR interferometry
Authors: Massonnet Didier , Holzer Thomas , Vadon Hélène
Published: February 2004

Interferometric combination of pairs of synthetic aperture radar (SAR) images acquired by the ERS‐1 satellite maps the deformation field associated with the activity of the East Mesa geothermal plant, located in southern California. SAR interferometry is applied to this flat area without the need of a digital terrain model. Several combinations are used to ascertain the nature of the phenomenon. Short term interferograms reveal surface phase changes on agricultural fields similar to what had been observed previously with SEASAT radar data. Long term (2 years) interferograms allow the study of land subsidence and improve prior knowledge of the displacement field, and agree with existing, sparse levelling data. This example illustrates the power of the interferometric technique for deriving accurate industrial intelligence as well as its potential for legal action, in cases involving environmental damages.
The 1991–1992 atmospheric methane anomaly: Southern hemisphere 13C decrease and growth rate fluctuations
Authors: Lowe David C. , Manning Martin R. , Brailsford G. W. , Bromley A. M.
Published: February 2004

Measurements of atmospheric methane from 1989–1996 at Baring Head, New Zealand, and at Scott Base, Antarctica show a seasonal cycle in the mixing ratio with a peak to peak amplitude of 28 ppb. This is superposed on a trend varying between 16 ppb yr−1 and near zero. δ13C values also show a seasonal cycle, with an amplitude of 0.1–0.3‰, approximately 6 months out of phase with the mixing ratio cycle. A pronounced negative anomaly in δ13C occurred in 1992 with annual average values dropping from −47.08‰ to −47.28‰. From 1992 to 1996, average δ13C values recovered slowly at an average rate of about 0.04‰ yr−1. The simultaneous changes in the mixing ratio growth rate and δ13C together with the rapid drop and slow recovery in the latter provide a stringent test of possible causes. Although a combination of causes cannot be ruled out, decreased emissions from an isotopically heavy source such as biomass burning best meet the constraints of the data.
Early multiplatform results from the International Solar Terrestrial Physics/Global Geospace Science (ISTP/GGS) Program
Authors: Desch M. , Ogilvie K. , Acuna M. , Fairfield D. , Hoffman R.
Published: February 2004

This issue of the Geophysical Research Letters is the third† to contain a special section devoted to science results from the Global Geospace Science (GGS) Program [Acuna et al, 1995], a key component of the International Solar Terrestrial Physics Program. Designed to provide coverage of key regions of geospace, the GGS suite of spacecraft (WIND, POLAR, and GEOTAIL), along with other currently operational spacecraft, and related groundbased and theoretical support assets are intended to make a significant improvement in our understanding of the flow of energy, mass, and momentum in the solar‐terrestrial environment.

With the WIND spacecraft monitoring the upstream interplanetary medium and providing the solar wind input to the magnetosphere, with POLAR measuring plasma input into the magnetosphere and monitoring the Earth's auroral signature, and GEOTAIL observing the geomagnetic tail response, the principal regions of geospace where energy is transported and stored are sampled by an advanced array of instruments. Simultaneous groundbased investigations and theoretical and global modelling round out the synergistic approach to answering key questions concerning how individual parts of this closely‐coupled system work together. The papers in this special section of GRL emphasize this multi‐spacecraft, multi‐discipline approach to solar‐terrestrial observations. The ISTP/GGS program works best when all the elements are working together, and in this issue the value of coordinated, simultaneous observations with multiple platforms is amply demonstrated.