bluuebunny/crossref_metadata_2025 huggingface dataset:
Constraining global air‐sea gas exchange for CO2 with recent bomb 14C measurements
Authors: Sweeney Colm , Gloor Emanuel , Jacobson Andrew R. , Key Robert M. , McKinley Galen , Sarmiento Jorge L. , Wanninkhof Rik
Published: May 2007

The 14CO2 released into the stratosphere during bomb testing in the early 1960s provides a global constraint on air‐sea gas exchange of soluble atmospheric gases like CO2. Using the most complete database of dissolved inorganic radiocarbon, DI14C, available to date and a suite of ocean general circulation models in an inverse mode we recalculate the ocean inventory of bomb‐produced DI14C in the global ocean and confirm that there is a 25% decrease from previous estimates using older DI14C data sets. Additionally, we find a 33% lower globally averaged gas transfer velocity for CO2 compared to previous estimates (Wanninkhof, 1992) using the NCEP/NCAR Reanalysis 1 1954–2000 where the global mean winds are 6.9 m s−1. Unlike some earlier ocean radiocarbon studies, the implied gas transfer velocity finally closes the gap between small‐scale deliberate tracer studies and global‐scale estimates. Additionally, the total inventory of bomb‐produced radiocarbon in the ocean is now in agreement with global budgets based on radiocarbon measurements made in the stratosphere and troposphere. Using the implied relationship between wind speed and gas transfer velocity ks = 0.27〈u102〉(Sc/660)−0.5 and standard partial pressure difference climatology of CO2 we obtain an net air‐sea flux estimate of 1.3 ± 0.5 PgCyr−1 for 1995. After accounting for the carbon transferred from rivers to the deep ocean, our estimate of oceanic uptake (1.8 ± 0.5 PgCyr−1) compares well with estimates based on ocean inventories, ocean transport inversions using ocean concentration data, and model simulations.
Optimizing a process‐based ecosystem model with eddy‐covariance flux measurements: A pine forest in southern France
Authors: Santaren Diego , Peylin Philippe , Viovy Nicolas , Ciais Philippe
Published: May 2007

We design a Bayesian inversion method (gradient‐based) to optimize the key functioning parameters of a process‐driven land surface model (ORganizing Carbon and Hydrology In Dynamic EcosystEms (ORCHIDEE)) against the combination of prior information upon the parameters and eddy covariance fluxes. The model calculates energy, water, and CO2 fluxes and their interactions on a half‐hourly basis, and we carry out the inversion using measurements of CO2, latent heat, and sensible heat fluxes as well as of net radiation over a pine forest in southern France. The inversion method makes it possible to assess the reduction of uncertainties and error correlations of the parameters. We designed an ensemble of inversions with different set ups using flux data over different time periods, in order to (1) identify well‐constrained parameters and loosely constrained ones, (2) highlight some model structural deficiencies, and (3) quantify the overall information gained from assimilating each type of CO2 or energy fluxes. The sensitivity of the optimal parameter values to the initial carbon pool sizes and prior parameter values is discussed and an analysis of the posterior uncertainties is performed. Assimilating 3 weeks of half‐hourly flux data during the summer improves the fit to diurnal variations, but merely improves the fit to seasonal variations. Assimilating a full year of flux data also improves the fit to the diurnal cycle more than to the seasonal cycle. This points out to the key importance of timescales when inverting parameters from high‐frequency eddy‐covariance data. We show that photosynthetic parameters such as carboxylation rates are well‐constrained by the carbon and water fluxes data and get increased from their prior values, a correction that is corroborated by independent measurements at leaf scale. In contrast, the parameters controlling maintenance, microbial and growth respirations, and their temperature dependencies cannot be robustly determined. The CO2 flux data could not discriminate between the different respiration terms. At face value, all the parameters controlling the surface energy budget can be safely determined, leading to a good model‐data fit on different timescales.
ELF waves generated by modulated HF heating of the auroral electrojet and observed at a ground distance of ∼4400 km
Authors: Moore R. C. , Inan U. S. , Bell T. F. , Kennedy E. J.
Published: May 2007

We present calibrated measurements of ELF waves generated by modulated HF heating of the auroral electrojet by the High frequency Active Auroral Research Program (HAARP) HF transmitter in Gakona, Alaska, and detected after propagating more than 4400 km in the Earth‐ionosphere waveguide to Midway Atoll. The magnitude of the 2125 Hz wave received at Midway Atoll is consistent with the radiation from a horizontal dipole located at the altitude of the maximum Hall conductivity variation (created by modulated HF heating) and radiating ∼4–32 W. The HF‐ELF conversion efficiency at HAARP is thus estimated to be ∼0.0004–0.0032% for the 2125 Hz wave generated using sinusoidal amplitude modulation.
Parkfield earthquakes: Characteristic or complementary?
Authors: Custódio Susana , Archuleta Ralph J.
Published: May 2007

We model the two most recent Mw∼6 Parkfield, California, earthquakes, which occurred in 1966 and 2004, from a nonlinear global inversion of near‐fault strong motion seismograms. Our rupture models are characterized by spatially variable slip amplitude and rake, rupture velocity, and risetime. The rupture models indicate that the two earthquakes generated slip in regions of the fault that are not identical, as earlier suggested. Given the sparse seismic data set available for the 1966 earthquake, we conduct a series of tests to verify our results: (1) we perform synthetic tests in order to study the resolution of the 1966 seismic data set; (2) we perform an inversion of the 2004 earthquake using a data set equivalent to the 1966 earthquake; and (3) we model the 1966 data set under the a priori assumption that it was similar to the 2004 earthquake. All of the tests, as well as independent observations, indicate that slip during the 1966 and 2004 Parkfield earthquakes occurred in different regions of the fault. This result implies that regions of a fault that are frictionally locked may remain locked even during a main shock (moderate‐size earthquake). In this scenario, large earthquakes occur when all the locked regions of a fault are “synchronized” and ready to slip at the same time.
Fine‐resolution satellite‐based daily sea surface temperatures over the global ocean
Authors: Kara A. B. , Barron C. N.
Published: May 2007

The accuracy and relative merits of two sets of daily global sea surface temperature (SST) analyses are examined and compared. The 1/8° Modular Ocean Data Analysis System (MODAS) of the Naval Research Laboratory (NRL) is based only on infrared satellite retrievals. The 1/2° Real‐Time, Global (RTG) SST analysis of the National Centers for Environmental Prediction (NCEP) supplements infrared satellite observations with ship and buoy data. The accuracy of both products is reported, providing potential users of either data set a common basis to assess the strengths and weaknesses of either product. Differences between the two show the impact of horizontal resolution, inclusion of source data streams, and different assumptions regarding error covariances. The global average of the root‐mean‐square (RMS) SST difference between MODAS and RTG is found to be 0.51°C, with almost no mean bias. A global set of yearlong daily SST time series from moored buoys during 2002–2005 provides extensive validation data for this study. Comparisons at the locations of these 420 yearlong time series give a median RMS SST difference of 0.40°C between MODAS and RTG. RMS error relative to the buoy observations is comparable, 0.38°C for MODAS and 0.36°C for RTG. The seasonal cycle of SST is well produced by both products with respect to the buoys with a median correlation coefficient of 0.94 for both products. Overall, higher resolution is an advantage for MODAS in improving pattern of daily SSTs, while including in situ SSTs is an advantage for RTG.
Atmospheric in situ measurement of nitrate radical (NO3) and other photolysis rates using spectroradiometry and filter radiometry
Authors: Stark H. , Lerner B. M. , Schmitt R. , Jakoubek R. , Williams E. J. , Ryerson T. B. , Sueper D. T. , Parrish D. D. , Fehsenfeld F. C.
Published: May 2007

We describe field measurements of nitrate radical photolysis rates, j(NO3), conducted during the International Consortium for Atmospheric Transport and Transformation (ICARTT) study in the summer of 2004 in the northeastern United States on board the NOAA research vessel Ronald H. Brown (RHB) and the NOAA aircraft WP‐3. The photolysis rates of 17 other atmospherically important compounds were also measured. Direct measurements of spectral actinic fluxes using spectroradiometers were conducted on board the WP‐3, which were then converted into photolysis rates. On board RHB, we used filter radiometers that specifically measured j(NO3) and were calibrated before and after the campaign by the spectroradiometers. NO3 photolysis rates ranged from below the detection limit of 10−5 s−1 at twilight to peak values of 0.5 s−1 over clouds at midday. The measurement uncertainties were 9% for the spectroradiometers and 14% for the filter radiometers. A field intercomparison between ship and aircraft instruments showed general agreement, indicating that aircraft data can be used to calculate the ship nadir radiation from the ocean surface. The measurements were used to evaluate the importance of photolysis of nitrate radicals in the troposphere. One result was that because of its spatial correlation with NO, NO3 daytime loss is dominated by reaction with NO in the free troposphere and the marine boundary layer. The tropospheric branching ratio between the two NO3 photolysis channels producing NO and NO2, was found to be (10.8 ± 1.2)% for NO in the lower troposphere.
Aerosol optical properties along the northeast coast of North America during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation 2004 campaign and the influence of aerosol composition
Authors: Wang Wei , Rood Mark J. , Carrico Christian M. , Covert David S. , Quinn Patricia K. , Bates Timothy S.
Published: May 2007

Optical and hygroscopic properties of submicrometer diameter aerosol particles were measured on board the NOAA R/V Ronald H. Brown as part of the NEAQS‐ITCT field campaign. The campaign occurred along the northeast coast of North America during the summer of 2004. A scanning relative humidity (RH) nephelometry system (humidograph) measured total light scattering and backscattering coefficients (σsp and σbsp, respectively) at three wavelengths (λs) and RH = 26% and while RH was scanned between 40% and 85%. These measurements were combined with aerosol light absorption and composition measurements to describe σsp, σbsp, single scattering albedo (ω), Ångström exponent (å), and hemispheric backscatter fraction (b) at a low reference RH of 26 ± 4% and the aerosol's hygroscopic properties based on its optical response up to 85% RH. Humidogram curve structure was dominated by hygroscopic growth without hysteresis (76% frequency). Dependence of the aerosol's σsp values with changes in RH, fσsp(RH, 26), was observed to decrease with increasing mass fraction of particulate organic matter (POM, FO). Statistical analyses indicated that increasing FO resulted in a less hygroscopic aerosol, while increasing molar equivalence ratio (ER) resulted in lower hysteresis factors based on the aerosols' optical responses at a 0.95 confidence level. ω showed little RH dependence while å and b decreased with increasing RH values. Values for å(λ1, λ2), b, and fσsp(82, 26) increased with increasing λ values. Sensitivities of top of the atmosphere aerosol radiative forcing to changes in ω, b, and σsp with RH were also estimated.
Emission, oxidation, and secondary organic aerosol formation of volatile organic compounds as observed at Chebogue Point, Nova Scotia
Authors: Holzinger R. , Millet D. B. , Williams B. , Lee A. , Kreisberg N. , Hering S. V. , Jimenez J. , Allan J. D. , Worsnop D. R. , Goldstein A. H.
Published: May 2007

We report the detection of a class of related oxygenated compounds by proton‐transfer‐reaction mass‐spectrometry (PTR‐MS) that have rarely or never been observed as a group using in situ instrumentation. Measurements were made as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 in Chebogue Point, Nova Scotia. The detected class of compounds discussed here includes acetic acid, formaldehyde, acetaldehyde, tentatively identified formic acid and hydroxyacetone, and unidentified compounds detected at mass to charge ratios 85, 87, 99, 101, 113, 115, and 129. Typical concentrations were 800, 2500, 450, 700, 85, 25, 50, 50, 60, 35, 20, and 25 ppt, respectively. The uniqueness of this class of compounds is illustrated by showing they were poorly related to trace gases found in the US outflow, local pollution, primary biogenic emissions and other oxygenated compounds such as acetone, methanol, and MEK measured by other in situ instrumentation. On the other hand these oxidized volatile organic compounds were related to chemical species in aerosols and their abundance was high during nucleation events. Thus they likely are gas phase species that are formed in parallel to biogenic secondary organic aerosol production. We clearly show these compounds do not originate from local sources. We also show these compounds match the oxidation products of isoprene observed in smog chamber studies, and we therefore suggest they must be mainly produced by oxidation of biogenic precursor compounds.
Sensitivity of mineral dust concentrations to the model size distribution accuracy
Authors: Menut Laurent , Forêt Gilles , Bergametti Gilles
Published: May 2007

Evaluating the impacts of atmospheric dust on marine ecosystems and climate requires the use of three‐dimensional transport models including a size‐resolved bins scheme to describe the evolution of the particle size distribution. Recently, Forêt et al. (2006) proposed an alternative size bins scheme to better account for the physical size‐dependent processes such as dry deposition. This paper evaluates the benefit of using this new bin scheme in three‐dimensional transport models. This is achieved by performing a one‐and‐a‐half month simulation with the CHIMERE‐DUST model forced by the Mesoscale Model, Version 5 (MM5)/National Center for Environmental Prediction (NCEP) analysis fields, for an area covering the northern Atlantic Sea and the western Africa. Compared to a reference run performed using a large number of size bins (40), our results show that the size bins scheme proposed by Forêt et al. (2006) reduces at least by a factor of 2 the numerical errors on the simulated concentrations compared to the classical isolog bins scheme for the same number of size bins. However, with this new bins scheme requiring to define the bins according to a dry deposition velocity corresponding to a given friction velocity, we examine the errors associated to this constraint.
Estimating daytime CO2 fluxes over a mixed forest from tall tower mixing ratio measurements
Authors: Wang Weiguo , Davis Kenneth J. , Cook Bruce D. , Yi Chuixiang , Butler Martha P. , Ricciuto Daniel M. , Bakwin Peter S.
Published: May 2007

Difficulties in estimating terrestrial ecosystem CO2 fluxes on regional scales have significantly limited our understanding of the global carbon cycle. This paper presents an effort to estimate daytime CO2 fluxes over a forested region on the scale of 50 km in northern Wisconsin, USA, using the tall‐tower‐based mixed layer (ML) budget method. Budget calculations were conducted for 2 years under fair‐weather conditions as a case study. With long‐term measurements of CO2 mixing ratio at a 447‐m‐tall tower, daytime regional CO2 fluxes were estimated on the seasonal scale, longer than in earlier studies. The flux derived from the budget method was intermediate among those derived from the eddy‐covariance (EC) method at three towers in the region and overall closest to that derived from EC measurements at 396 m of the tall tower. The dormant season average daytime‐integrated regional CO2 flux was about 0.35 ± 0.18 gC m−2. During the growing season, the monthly averaged daytime‐integrated regional CO2 flux varied from −1.58 ± 0.19 to −4.15 ± 0.32 gC m−2, suggesting that the region was a net sink of CO2 in the daytime. We also discussed the effects on theses estimates of neglecting horizontal advection, selecting for fair‐weather conditions, and using single‐location measurements. Daytime regional CO2 flux estimates from the ML budget method were comparable to those from three aggregation experiments. Differences in results from the different methods, however, suggest that more constraints are needed to estimate regional fluxes with more confidence. Despite uncertainties, our analyses indicate that it is feasible to estimate daytime regional CO2 fluxes on long timescales using tall tower measurements.
A stochastic process for the interannual snow storage and melting dynamics
Authors: Perona P. , Porporato A. , Ridolfi L.
Published: May 2007

A continuous time process of alternating stochastic growths and deterministic decays is proposed as a simple model for the interannual dynamics of snow water equivalent (SWE) storage and melting. The related stationary properties are studied, and an integral difference equation for the probability density function of the state variable is derived. The corresponding solution is obtained by both solving the master equation numerically and simulating the process with a Monte Carlo scheme. An analytical solution is also obtained for the inverse problem, i.e., when the pdf of growths that lead to accumulation is reconstructed from that of the process. The model's statistical properties help explain the probabilistic character of the end of summer snowlines and their intrinsic variability with the geographic location and climatic factors. Possible applications to model the interannual SWE cover evolution or to reconstruct the past precipitation characteristics are also discussed.
Seasonal asymmetry in diurnal variation of aerosol optical characteristics over Pune, western India
Authors: Pandithurai G. , Pinker R. T. , Devara P. C. S. , Takamura T. , Dani K. K.
Published: May 2007

There is a large day‐to‐day variability in aerosol optical properties due to varied meteorological conditions that exhibit different diurnal asymmetry in different synoptic seasons. Aerosol optical depth (AOD) and Ångstrom exponent (AE) measured for a period of 3 years at Pune, an urban site in western India, were analyzed to study the diurnal variation and its seasonal dependence. Mean AOD at 500 nm in the premonsoon (March–May) period was about 0.42, while in the winter (December, January, and February) it was ∼0.38. AOD departures from the daily mean showed ±20% variation (which is equivalent to AOD of 0.08) in both seasons. A systematic seasonal asymmetry in diurnal variation was found. AOD is higher in the morning and lower in the afternoon during winter, mainly because of higher relative humidities, calm winds, and associated ground‐based inversions that are conducive for haze, which persists till noon. However, in premonsoon, AODs are lower in the forenoon hours and higher in the afternoon hours because of higher air temperatures, strong convection, and stronger winds raising more dust locally and transporting it from Saudi Arabia and Afghanistan regions. The difference between Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua derived AOD showed similar seasonal asymmetry in forenoon‐afternoon difference in aerosol loading as observed at the experimental site.
Assessment of the local windblown component of dust in the western United States
Authors: Kavouras Ilias G. , Etyemezian Vicken , Xu Jin , DuBois David W. , Green Mark , Pitchford Marc
Published: May 2007

We estimated the contributions of windblown dust from nearby area sources to dust concentrations at Class I areas in the western United States including Alaska and Hawaii. The approach utilized multivariate linear regression of dust concentrations against categorized wind conditions (wind direction and speed) for all 2001–2003 data for 70 Interagency Monitoring of Protected Visual Environments sites. Statistically significant associations between dust concentrations and at least one of the wind variables were found at 41 sites with correlation coefficients as high as 0.97. At some sites, primarily in New Mexico and Texas, windblown dust from nearby sources accounted for up to 3 μg m−3 over the 2001–2003 period. In addition, the impact of local windblown dust sources during the 20% worst visibility days when dust was the major component of visibility reduction (worst dust days) was examined. A total of 608 worst dust days were identified for 2001–2003, mostly at Class I areas in southwestern states during spring and summer with 24‐h average dust concentrations as high as 153 μg m−3. Windblown dust from local sources was present with statistical confidence on many of the worst dust days at sites in New Mexico, Utah, Colorado, southern Texas, and Death Valley in California. A smaller percentage of worst dust days were associated with local windblown dust in Arizona and other sites in southern California, suggesting either nonwindblown or distant sources of dust. The methods discussed can serve as a useful, semiquantitative tool for identifying sites where local wind conditions affect dust concentrations.
A tropospheric assessment of the ERA‐40, NCEP, and JRA‐25 global reanalyses in the polar regions
Authors: Bromwich David H. , Fogt Ryan L. , Hodges Kevin I. , Walsh John E.
Published: May 2007

The reliability of the global reanalyses in the polar regions is investigated. The overview stems from an April 2006 Scientific Committee on Antarctic Research (SCAR) workshop on the performance of global reanalyses in high latitudes held at the British Antarctic Survey. Overall, the skill is much higher in the Arctic than the Antarctic, where the reanalyses are only reliable in the summer months prior to the modern satellite era. In the Antarctic, large circulation differences between the reanalyses are found primarily before 1979, when vast quantities of satellite sounding data started to be assimilated. Specifically for ERA‐40, this data discontinuity creates a marked jump in Antarctic snow accumulation, especially at high elevations. In the Arctic, the largest differences are related to the reanalyses' depiction of clouds and their associated radiation impacts; ERA‐40 captures the cloud variability much better than NCEP1 and JRA‐25, but the ERA‐40 and JRA‐25 clouds are too optically thin for shortwave radiation. To further contrast the reanalyses skill, cyclone tracking results are presented. In the Southern Hemisphere, cyclonic activity is markedly different between the reanalyses, where there are few matched cyclones prior to 1979. In comparison, only some of the weaker cyclones are not matched in the Northern Hemisphere from 1958–2001, again indicating the superior skill in this hemisphere. Although this manuscript focuses on deficiencies in the reanalyses, it is important to note that they are a powerful tool for climate studies in both polar regions when used with a recognition of their limitations.
Fate of inorganic mercury and methyl mercury within the snow cover in the low arctic tundra on the shore of Hudson Bay (Québec, Canada)
Authors: Constant Philippe , Poissant Laurier , Villemur Richard , Yumvihoze Emmanuel , Lean David
Published: May 2007

Snow samples were collected in the seasonal snow cover of the low arctic tundra (Whapmagoostui‐Kuujjuarapik, Québec) during episodic atmospheric mercury depletion events (AMDEs) and in the snowmelt period, long after AMDEs had occurred. Total and methyl mercury analyses were done in order to investigate the critical factors influencing the fate of mercury once deposited in the snowpack. Following AMDEs, snow total mercury (THg) concentrations increased and were inversely proportional to the distance from Hudson Bay. The correlations between MeHg, sulfate (SO42−), and chlorine (Cl−) snow concentrations implicated marine aerosols as a significant source of MeHg, independent of AMDEs. However, the newly deposited MeHg was unstable in the snow cover as 15–56% of the MeHg was demethylated or otherwise “lost” during the nighttime period. In contrast, during the snowmelt period, marine aerosols were not a significant source of MeHg. MeHg snow concentrations higher than 200 pg L−1 were observed when snow's heterotrophic plate counts, total suspended volatile solids, and total suspended solids were higher than 5.0 × 105 CFU L−1, 25 mg L−1, and 90 mg L−1, respectively. During the snowmelt, although the THg snow concentrations remained at 8–9 ng L−1, the proportion of MeHg increased from 2.7 to 7.6%. This is the first report suggestive of the presence of mercury methylation activities within the snow cover of the low arctic tundra.
Sources and sinks of hydroxyl radical in sea‐salt particles
Authors: Anastasio Cort , Newberg John T.
Published: May 2007

We have examined the photochemical formation of hydroxyl radical (•OH) in aqueous extracts of supermicron sea‐salt particulate matter (SS PM) collected from the coast of northern California. All extracts formed •OH during illumination, indicating that this process is widespread in sea‐salt particles. Scaling extract results to SS PM conditions reveals that in situ rates of •OH photoformation are typically 100–1000 μM hr−1 in midlatitude sea‐salt particles (summer, midday, 88% relative humidity). These rates are comparable to calculated rates of partitioning of gas phase •OH to the particles and are 3–4 orders of magnitude greater than •OH photoformation rates in surface seawater. Photolysis of nitrate was a dominant source of •OH in the particle extracts, accounting for an average of 59 ± 25% of its formation. The other sources of •OH have not been identified, but photolysis of organic compounds derived from seawater is likely important. The lifetimes of •OH in the sea‐salt particles are of the order of 10−9–10−8 s and are primarily controlled by reactions with unidentified, but probably organic, compounds. Bromide and chloride are also significant sinks of •OH, typically accounting for approximately 25% of its loss. The rapid formation and destruction of •OH in sea‐salt particles likely significantly affects the chemistry of halides, organic compounds, and other reduced species in SS PM. In turn, these particle reactions probably alter the budgets of gases such as ozone and volatile organic compounds in the marine boundary layer.
Young lava flows on the eastern flank of Ascraeus Mons: Rheological properties derived from High Resolution Stereo Camera (HRSC) images and Mars Orbiter Laser Altimeter (MOLA) data
Authors: Hiesinger H. , Head J. W. , Neukum G.
Published: May 2007

We report on estimates of the rheological properties of late‐stage lava flows on the eastern flank of Ascraeus Mons, Mars. From previous studies it is known that the dimensions of flows reflect rheological properties such as yield strength, effusion rates, and viscosity. Our estimates are based on new high‐resolution images obtained by the High Resolution Stereo Camera (HRSC) on board the European Space Agency's Mars Express spacecraft in combination with Mars Orbiter Laser Altimeter (MOLA) data. Compared to earlier studies, the high spatial resolution of the HRSC and MOLA data allowed us to map 25 late‐stage lava flows and to measure their dimensions, as well as their morphological characteristics, in greater detail. Our estimates of the yield strengths for these flows range from ∼2.0 × 102 Pa to ∼1.3 × 105 Pa, with an average of ∼2.1 × 104 Pa. These values are in good agreement with estimates for terrestrial basaltic lava flows and are comparable to previous estimates derived for a small number of lava flows on Ascraeus Mons. Our investigation indicates that the effusion rates for the studied Ascraeus Mons flows are on average ∼185 m3 s−1, ranging from ∼23 m3 s−1 to ∼404 m3 s−1. These results are higher than earlier findings that indicate effusion rates of 18–60 m3 s−1, with an average of 35 m3 s−1. However, our effusion rates are similar to terrestrial effusion rates of Kilauea and Mauna Loa and other Martian volcanoes. On the basis of our estimates of the effusion rates and the measured dimensions of the flows, we calculated that the time necessary to emplace the flows is on average ∼26 days. Viscosities were estimated on the basis of yield strengths and effusion rates, yielding average values of ∼4.1 × 106 Pa‐s and ranging from ∼1.8 × 104 Pa‐s to ∼4.2 × 107 Pa‐s. On the basis of newly available data sets (e.g., HRSC, MOLA) we are now able not only to identify possible differences in eruptive behavior between Ascraeus Mons and Elysium Mons but also to study such differences over time.
Acheron Fossae, Mars: Tectonic rifting, volcanism, and implications for lithospheric thickness
Authors: Kronberg P. , Hauber E. , Grott M. , Werner S. C. , Schäfer T. , Gwinner K. , Giese B. , Masson P. , Neukum G.
Published: May 2007

The Acheron Fossae region is an ancient crustal block exposed in the northwestern Tharsis province on Mars. It is named after the Acheron Fossae, a particularly well‐developed system of extensional tectonic grabens. This study is the first to describe the tectonic inventory in detail, using topographic and image data from the Global Surveyor and Mars Express missions. We see the extensional tectonics along the east–west trending topographic high of Acheron Fossae as a surface expression of upwelling asthenospheric material that initiated regional uplift, crustal extension and breakup, and associated volcanic activity in Noachian time. The tectonic architecture and the dimensions of the extensional structures are comparable to those of terrestrial continental rifts. An area of elevated topography and strong erosion, which constitutes a regional landmark in the eastern Acheron Fossae region, is interpreted as a rift‐related center of volcanism, caused by local magmatic uprise and involvement of the lithosphere. The extension across the Acheron Fossae reaches values between 1.2 km and 8.7 km, comparable to young continental rifts on Earth. Crater statistics indicate an absolute cratering model age between ∼3.9 Ga and 3.7 Ga for the rifting. The uplift observed on the flanks of the Acheron Fossae indicates a fairly thin and thus hot lithosphere. Using flexural analysis, we have constrained the elastic lithosphere thickness at the time of rifting to 8.9–11.3 km, corresponding to thermal gradients between 28 and 41 K km−1. These heat flows are substantially larger than Noachian heat flow values previously reported, but are consistent with the presence of rift‐related volcanism and a magmatically very active environment.
Century‐scale discharge stagnation and reactivation of the Ross ice streams, West Antarctica
Authors: Hulbe C. , Fahnestock M.
Published: May 2007

Flow features on the surface of the Ross Ice Shelf, West Antarctica, record two episodes of ice stream stagnation and reactivation within the last 1000 years. We document these events using maps of streaklines emerging from individual ice streams made using visible band imagery, together with numerical models of ice shelf flow. Forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration. According to our analysis, Whillans Ice Stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and MacAyeal Ice Stream either stopped or slowed significantly between 800 and 700 years ago, restarting about 150 years later. Until now, ice‐stream scenarios emphasized runaway retreat or stagnation on millennial timescales. Here we identify a new scenario: century‐scale stagnation and reactivation cycles, as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains. This introduces uncertainty into predictions for future sea‐level withdrawals by the West Antarctic Ice Sheet, which are based in part on recent slowing of Whillans Ice Stream and the stagnant condition of Kamb Ice Stream.
On the spectrum of soil moisture from hourly to interannual scales
Authors: Katul Gabriel G. , Porporato Amilcare , Daly Edoardo , Oishi A. Christopher , Kim Hyun‐Seok , Stoy Paul C. , Juang Jehn‐Yih , Siqueira Mario B.
Published: May 2007

The spectrum of soil moisture content at scales ranging from 1 hour to 8 years is analyzed for a site whose hydrologic balance is primarily governed by precipitation (p), and evapotranspiration (ET). The site is a uniformly planted loblolly pine stand situated in the southeastern United States and is characterized by a shallow rooting depth (RL) and a near‐impervious clay pan just below RL. In this setup, when ET linearly increases with increasing root zone soil moisture content (θ), an analytical model can be derived for the soil moisture content energy spectrum (Es(f), where f is frequency) that predicts the soil moisture “memory” (taken as the integral timescale) as β1−1 ≈ ηRL/ETmax, where ETmax is the maximum measured hourly ET and η is the soil porosity. The spectral model suggests that Es(f) decays at f−2−α at high f but almost white (i.e., f0) at low f, where α is the power law exponent of the rainfall spectrum at high f (α ≈ 0.75 for this site). The rapid Es(f) decay at high f makes the soil moisture variance highly imbalanced in the Fourier domain, thereby permitting much of the soil moisture variability to be described by a limited number of Fourier modes. For the 8‐year data collected here, 99.6% of the soil moisture variance could be described by less than 0.4% of its Fourier modes. A practical outcome of this energy imbalance in the frequency domain is that the diurnal cycle in ET can be ignored if β1−1 (estimated at 7.6 days from the model) is much larger than 12 hours. The model, however, underestimates the measured Es(f) at very low frequencies (f ≪ β1) and its memory, estimated from the data at 42 days. This underestimation is due to seasonality in ETmax and to a partial decoupling between ET and soil moisture at low frequencies.
Simple predictions of maximum transport rate in unsaturated soil and rock
Authors: Nimmo John R.
Published: May 2007

In contrast with the extreme variability expected for water and contaminant fluxes in the unsaturated zone, evidence from 64 field tests of preferential flow indicates that the maximum transport speed Vmax, adjusted for episodicity of infiltration, deviates little from a geometric mean of 13 m/d. A model based on constant‐speed travel during infiltration pulses of actual or estimated duration can predict Vmax with approximate order‐of‐magnitude accuracy, irrespective of medium or travel distance, thereby facilitating such problems as the prediction of worst‐case contaminant traveltimes. The lesser variability suggests that preferential flow is subject to rate‐limiting mechanisms analogous to those that impose a terminal velocity on objects in free fall and to rate‐compensating mechanisms analogous to Le Chatlier's principle. A critical feature allowing such mechanisms to dominate may be the presence of interfacial boundaries confined by neither solid material nor capillary forces.
Statistical analysis of hydroclimatic time series: Uncertainty and insights
Authors: Koutsoyiannis Demetris , Montanari Alberto
Published: May 2007

Today, hydrologic research and modeling depends largely on climatological inputs, whose physical and statistical behavior are the subject of many debates in the scientific community. A relevant ongoing discussion is focused on long‐term persistence (LTP), a natural behavior identified in several studies of instrumental and proxy hydroclimatic time series, which, nevertheless, is neglected in some climatological studies. LTP may reflect a long‐term variability of several factors and thus can support a more complete physical understanding and uncertainty characterization of climate. The implications of LTP in hydroclimatic research, especially in statistical questions and problems, may be substantial but appear to be not fully understood or recognized. To offer insights on these implications, we demonstrate by using analytical methods that the characteristics of temperature series, which appear to be compatible with the LTP hypothesis, imply a dramatic increase of uncertainty in statistical estimation and reduction of significance in statistical testing, in comparison with classical statistics. Therefore we maintain that statistical analysis in hydroclimatic research should be revisited in order not to derive misleading results and simultaneously that merely statistical arguments do not suffice to verify or falsify the LTP (or another) climatic hypothesis.
Test of the innermost inner core models using broadband PKIKP travel time residuals
Authors: Cao A. , Romanowicz B.
Published: May 2007

Two different innermost inner core (IMIC) models have recently been proposed, each with different radii and different orientation and strength of anisotropy. In order to test these IMIC models, we have systematically assembled a high quality dataset of broadband PKIKP data in the epicentral distance range of 150° to 180°. Our new dataset consists of ∼1,100 individually measured absolute PKIKP arrival times. Also, PKIKP bottoming points and ray paths in the inner core are well distributed in the east‐west and north‐south hemispheres. We invert this dataset for two layer models of anisotropy in the inner core and compare the fits obtained with those predicted from the existing IMIC models. Our results show that, if there is an IMIC, its radius is most likely around ∼500 km rather than the ∼300 km originally proposed, and is in better agreement with the layering in the inner core based on PKIKP waveform modeling.
On the motion of small heavy particles in an unsteady flow
Authors: Dodin Z. , Elperin T.
Published: May 2007

Motion of a small spherical heavy particle subjected to the Stokes drag force and the force of gravity in a nonstationary fluid flow is studied. An analytical solution of the equation of the particle motion at a short-time interval and conditions for validity of this solution are obtained. For the case of a long-time motion the velocity and the acceleration of the particle are found. The velocity is obtained up to the second-order terms with respect to the Stokes response time and the acceleration is obtained up to the first-order terms.
Mutual interference on transition of wake of circular cylinder
Authors: Ko N. W. M. , Law C. W. , Lo K. W.
Published: May 2007

An experimental investigation was carried out in the very near wake of a circular cylinder over the Reynolds number range of 260<Re<1.8×103. The aspect ratio of the cylinder was 4. The present study concerns the regular shedding mode of the wake, in which the large-scale primary vortex sheet and the small-scale individual and pairing secondary vortices are present. Based on a new conditionally sampling technique, there is mutual interference between the high-energy small-scale secondary vortices and the development of large-scale primary vortex sheet and of the strain field of primary vortex sheet on individual secondary vortices. The pairing vortices, however, do not significantly affect the large-scale vortices. In the transition regimes within this low Reynolds number range, in which transition of two-dimensional large-scale primary vortices to three dimensionality occurs, the disturbances of the individual and pairing secondary vortices excite the separated shear layer at the most amplified mode. Coupled with mutual interference, they play an important role in the transition. Correlation with the available and additional results in the upper transition regime and in the supercritical and transcritical regimes suggests the same mechanism of transition. The mutual interference of different types of vortices in the very near wake is the key factor for transition.
Lindstedt series solutions of the Fermi-Pasta-Ulam lattice
Authors: Dooling David C. , Hammerberg James E.
Published: May 2007

We apply the Lindstedt method to the one dimensional Fermi-Pasta-Ulam β lattice to find fully general solutions to the complete set of equations of motion. The perturbative scheme employed uses ϵ as the expansion parameter, where ϵ is the coefficient of the quartic coupling between nearest neighbors. We compare our nonsecular perturbative solutions to numerical solutions and find striking agreement.
Magnetic properties of Th2Fe17Cx compounds (x=,0.6,0.9,1.1)
Authors: Isnard O. , Pop V. , Buschow K. H. J.
Published: May 2007

Magnetic measurements are carried out on polycrystalline Th2Fe17Cx (x=0,0.6,0.9,1.1) samples between 4 and 850 K and magnetic field up to 7 T. This wide temperature range covers both the ferromagnetically ordered region and the paramagnetic region. The ordering temperature, but also the magnetization is found to be very sensitive to the carbon concentration. The evolution of the magnetic properties versus the carbon content is discussed. The Curie temperature and the Fe-Fe exchange interactions increase when the carbon concentration increases. The mean ordered iron magnetic moment deduced from saturation magnetization is also found to depend upon the carbon concentration. Mean magnetic moment values of 1.79 and 1.98 μB per iron atom are observed for Th2Fe17 and Th2Fe17C1.1, respectively. The effective magnetic moment is found to decrease from 4.40 to 4.09 μB per iron atom for Th2Fe17 and Th2Fe17C1.1, respectively. The increase of the carbon concentration induces a progressive evolution toward a more localized Fe magnetic moment. The evolution of the effective magnetic moment is discussed on the basis of the spin fluctuations theory. The iron magnetic state in the Th2Fe17Cx compounds present similarities with that reported for the Fe3C compound.
Interaction potential for silicon carbide: A molecular dynamics study of elastic constants and vibrational density of states for crystalline and amorphous silicon carbide
Authors: Vashishta Priya , Kalia Rajiv K. , Nakano Aiichiro , Rino José Pedro
Published: May 2007

An effective interatomic interaction potential for SiC is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole-dipole (van der Waals) interactions. The covalent characters of the Si–C–Si and C–Si–C bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger-Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline (3C), amorphous, and liquid states of SiC for several densities and temperatures. The structural energy for cubic (3C) structure has the lowest energy, followed by the wurtzite (2H) and rock-salt (RS) structures. The pressure for the structural transformation from 3C-to-RS from the common tangent is found to be 90 GPa. For 3C-SiC, our computed elastic constants (C11, C12, and C44), melting temperature, vibrational density-of-states, and specific heat agree well with the experiments. Predictions are made for the elastic constant as a function of density for the crystalline and amorphous phase. Structural correlations, such as pair distribution function and neutron and x-ray static structure factors are calculated for the amorphous and liquid state.
Robust anti-synchronization of a class of delayed chaotic neural networks
Authors: Meng Juan , Wang Xing-yuan
Published: May 2007

This paper deals with the anti-synchronization problem of a class of delayed neural networks. Based on the Lyapunov stability theory and the Halanay inequality lemma, a kind of controller is designed. It is proved that this kind of controller can achieve anti-synchronization of neural networks with delays. Numerical simulations demonstrate the effectiveness and robustness of the proposed anti-synchronization scheme.
Electrical and deep-level characterization of GaP1−xNx grown by gas-source molecular beam epitaxy
Authors: Kaneko M. , Hashizume T. , Odnoblyudov V. A. , Tu C. W.
Published: May 2007

We have investigated electrical properties and deep levels of n-GaP1−xNx (x=0%−0.62%) grown on (100) n-GaP substrates by gas source molecular beam epitaxy. The x-ray photoelectron spectroscopy results showed no significant effects on the chemical bonding status of the host Ga-P matrix by the incorporation of small amounts of N atoms. In the Raman spectra, the zone-edge GaP-like vibration was observed at 387 cm−1, originating from alloy disorder or local distortion of the GaP1−xNx lattice. The electrical properties of the GaP1−xNx surfaces were characterized using a Schottky contact structure. An ideality factor of 1.10–1.15 and a Schottky barrier height of 1.1 eV were obtained from the current-voltage characteristics of Ni∕GaP1−xNx diodes, indicating good interface quality. The thermal admittance spectroscopy clearly detected the Si donor level with an activation energy of 84±4 meV in GaP and GaP1−xNx. For the GaP1−xNx samples, we observed deep levels probably associated with N-induced defects such as N-N pairs, N clusters, and an N-containing complexes.
Mechanism of solid phase crystallization of prepatterned nanoscale α-Si pillars
Authors: Cho Hyun-Jin , Greene Brian J. , Hoyt Judy L. , Plummer James D.
Published: May 2007

We have investigated the mechanism of crystallite nucleation and growth in prepatterned nanoscale α-Si pillars using transmission electron microscopy. The number of α-Si pillars that crystallize during annealing depends upon the pillar diameter and the density of nucleation sites at the α-Si/oxide interface, in the as-deposited film. These nucleation sites are presumed to be clusters of atoms exhibiting short-range order that are formed during the initial deposition of α-Si. Their density depends upon the specific deposition conditions. The density of nucleation sites is extracted from the measured pillar crystallization statistics using a Poisson distribution model. It is also observed that the orientation dependence of the crystal growth rate enhances the formation of a single grain inside each pillar. Significant reduction of defect density is achieved with high temperature annealing of sub-100-nm pillars where the surface to bulk ratio is high.
Three-dimensional theory of the Cerenkov free-electron laser
Authors: Andrews H. L. , Brau C. A.
Published: May 2007

We describe an analytical theory for the gain of a Cerenkov free-electron laser including diffraction of the optical beam in the direction transverse to the electron beam, parallel to the surface of the dielectric. Since the width of the optical beam depends on the gain, the usual cubic dispersion relation of two-dimensional slow-wave structures is replaced by a 5∕2-power dispersion relation, but three of the five roots are extraneous. The results show that for a narrow electron beam, the optical beam is much wider than the electron beam. This reduces the gain by an order of magnitude. Moreover, in the three-dimensional theory the allowed roots of the dispersion relation have positive real parts, so they correspond to slow waves; when transverse diffraction is included, fast waves are forbidden.