bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionospheric convection in the postnoon auroral oval: Super Dual Auroral Radar Network (SuperDARN) and polar ultraviolet imager (UVI) observations
Authors: Kozlovsky A. , Koustov A. , Lyatsky W. , Kangas J. , Parks G. , Chua D.
Published: December 2002

Super Dual Auroral Radar Network (SuperDARN) observations, ultraviolet imaging from the Polar satellite (ultraviolet imager [UVI]), and particle precipitation data from Defense Meteorological Satellite Program (DMSP) satellites have been used to investigate the electrodynamics of the postnoon auroral oval in the Northern hemisphere. We show that: (1) For negative interplanetary magnetic field (IMF) By, the convection reversal (CR) was colocated with the maximum of auroral luminosity, but during positive IMF By, the convection reversal was poleward of the auroral oval up to several degrees in latitude. (2) Postnoon auroral oval was associated with a large‐scale upward field‐aligned current (FAC) of the order of 6 × 10−7 A m−2 in magnitude (the FAC was inferred from the SuperDARN and UVI data). For negative IMF By, the maximum of the auroral intensity coincides in latitude with the maximum of the upward FAC. However, for positive IMF By, the maximum of the upward FAC was shifted to the poleward edge of the auroral oval. (3) In response to the IMF By turning from positive to negative, the maximum of the auroral luminosity did not change its position noticeably, but the position of the CR changed considerably from 80°–81° to about 76° magnetic latitude (MLAT), and the maximum of FAC moved from 77°–78° to about 76° MLAT. Thus, after the IMF By turns negative, both the FAC maximum and CR tend to coincide with the auroral maximum. (4) The IMF Bz positive deflection was followed by a decrease in both FAC intensity and auroral luminosity. However, the decrease in the auroral luminosity lags behind the FAC decrease by about 12 min. Firstly, these observations allow us to suggest that the IMF By‐related electric field can penetrate into the closed magnetosphere and produce convection and FAC changes in the region of the postnoon auroral oval. Secondly, we suggest that the interchange instability is a promising mechanism for the postnoon auroras.
On the turbulent spectrum of equatorial spread F: A comparison between laboratory and space results
Authors: Kelley M. C. , Franz T. L. , Prasad G.
Published: December 2002

We compare space and laboratory instabilities of the flute mode type. In space the classic gravitationally driven Rayleigh‐Taylor mode is studied. In the laboratory, centrifugal force provides an effective gravity. In both cases we find that classical diffusion is adequate to explain the spectrum but for different reasons. In particular, no drift wave modes need be involved to provide anomalous diffusion in the space case. Owing to the large physical space available in space, the spectrum covers a huge range of k values and drift waves are not generated. In the laboratory case the large fluctuation level is compressed into a small portion of k space. This greatly increases the drift wave growth rate. The space case is typically studied using radar Bragg scales deep in the diffusive subrange, scales which we think are populated by a three‐wave coupling process. In the laboratory, drift waves are generated but are probably part of the overall linkage of large to small scales. Curiously, even though the spectra are quite similar, the evolution of the two systems seems to be quite different.
Possible in situ tests of the evolution of elemental and isotopic abundances in the solar convection zone
Authors: Turcotte S. , Wimmer‐Schweingruber R. F.
Published: December 2002

Helioseismology has shown that the chemical composition of the Sun has changed over its lifetime. The surface abundance of helium and heavy elements is believed to have decreased by up to 10% in relation to their initial values. However, this reduction is too small to be tested by direct observations of the photospheric chemical composition. Here we compare the predicted variations in the solar photospheric composition with precise measurements of abundances in meteorites and the solar wind composition. Although elemental composition ratios can vary by roughly a percent (e.g., for Ca/Mg and Ca/Fe) over the Sun's lifetime, their measurements are rife with uncertainties related to uncertainties in the interpretation of meteoritic measurements, photospheric determinations, and the complex fractionation processes occurring between the upper photosphere and lower chromosphere and the corona. On the other hand, isotopic ratios can be measured much more accurately and are not expected to be affected as much by extrasolar processes, although more work is required to quantify their effect. As the isotopic ratios evolve in the Sun proportionally to the mass ratios of the isotopes, light elements yield the highest variations in isotopic ratios. They are predicted to reach as high as 0.6% for18O/16O and are only slightly lower in the cases of26Mg/24Mg and30Si/28Si. Such a value should be well within the sensitivity of new missions such as Genesis.
Equatorial plasma bubbles observed by DMSP satellites during a full solar cycle: Toward a global climatology
Authors: Huang C. Y. , Burke W. J. , Machuzak J. S. , Gentile L. C. , Sultan P. J.
Published: December 2002

We have examined more than 75,000 latitudinal profiles of plasma densities measured by ion detectors on five Defense Meteorological Satellite Program (DMSP) satellites in the evening local time (LT) sector between 1989 and 2001. This survey established detection frequencies of equatorial bubbles (EPBs) at 840 km over the recent solar cycle. The annual rate of EPB detections decreased by more than an order of magnitude from >1000 during solar maximum to <100 during solar minimum years. EPB data were divided into 24 longitude sectors to determine seasonal and solar cycle variability in rates of encounter by DMSP. During the ascending and descending portions of the solar cycle, each longitude sector showed repeatable seasonal variations. The envelope of seasonally averaged rates of EPB encounters resembles the solar cycle variability for similar averages of the F10.7 index. On both global and longitude sector scale sizes, annual rates of EPB encounters correlate with the yearly averages of F10.7. We also find that throughout the solar cycle the EPB detections were overrepresented during times of high geomagnetic activity signified by Kp ≥ 5. During solar minimum years, about one third of the EPBs occurred when traces of the Dst index had significant negative slopes (dDst/dt ≤ −5 nT/hr). This suggests that electric field penetration of the inner magnetosphere is responsible for driving many EPBs. Comparisons of plasma and neutral density profiles in the evening sector, calculated using the Parameterized Ionospheric Model (PIM) and MSIS‐86 Model, indicate that the height of the bottomside of the F layer is >100 km lower during solar minimum than solar maximum. However, the overall effect is to increase the growth rate of the Rayleigh–Taylor instability at solar maximum in the bottomside F layer only by about a factor of 2. We suggest that the variability of electric fields in the postsunset equatorial ionosphere is the source of the observed discrepancy between EPB detections under solar maximum/minimum conditions.
Global surface wave diffraction tomography
Authors: Ritzwoller Michael H. , Shapiro Nikolai M. , Barmin Mikhail P. , Levshin Anatoli L.
Published: December 2002

We determine the effect of replacing geometrical ray theory in surface wave tomography with scattering theory. We describe a tomographic method based on a simplified version of the scattering sensitivity kernels that emerge from the Born or Rytov approximations in which surface wave travel times are a weighted average of phase or group slowness over the first Fresnel zone of the wave. We apply this “diffraction tomography” to Rayleigh and Love wave group velocity measurements to produce group velocity maps from 20 to 150 s period on a 2° × 2° grid globally. Using identical data and damping parameters, we also produce maps using “Gaussian tomography” which is based on ray theory with intuitive Gaussian smoothing constraints. Significant differences in the amplitude and geometry of the imaged features appear primarily at long periods but exist even in the short‐period maps in regions where average path lengths are large. Diffraction tomography, therefore, is significant in most oceanic regions at all periods, but it is also important on continents at long periods at least. On average, diffraction tomography produces larger velocity anomalies in a period‐dependent band of spherical harmonic degrees, and diffraction and Gaussian tomography maps decorrelate past a critical spherical harmonic degree that also depends on period. The widths of resolving kernels that emerge from diffraction tomography are systematically larger than those from Gaussian tomography. Finally, mantle features inferred from diffraction tomography tend to have larger amplitudes and extend deeper than those from Gaussian tomography.
On the mechanism of dislocation creep of calcite at high temperature: Inferences from experimentally measured pressure sensitivity and strain rate sensitivity of flow stress
Authors: De Bresser J. H. P.
Published: December 2002

Previous laboratory experiments on coarse grained calcite materials at intermediate conditions of stress and temperature have shown low strain rate sensitivity of flow stress, expressed by standard power law stress exponents n ≥ 7. This conflicts with conventional models for creep controlled by dislocation climb (n = 3–4.5), a recovery mechanism widely used to explain steady‐state dislocation creep. This paper addresses the question whether dislocation cross slip rather than climb is the mechanism controlling creep of calcite at intermediate conditions. New uniaxial compression tests were performed on calcite single crystals and Carrara marble at T = 800–1000°C, = 10−4–2 × 10−7 s−1, and P = 100–600 MPa. Emphasis was on the pressure (P) and strain rate () sensitivity of flow stress, because these form potential ways for discriminating between climb and cross slip models. Both single crystals and Carrara marble demonstrated a small increase in flow stress with increasing pressure (∼1.6% per 100 MPa) at constant strain rate and temperature. The low strain rate sensitivity of flow stress was confirmed, though n gradually changes with stress or temperature rather than taking a constant value. Evaluation of the experimental data against microphysical models indicated that cross slip controlled by dissociation of dislocations offers the best explanation for the observed mechanical behavior. If the associated creep equation is used to estimate flow stresses under natural conditions, the resulting values at T < 500°C are substantially less than those following from previous flow laws. The influence of pressure on the behavior of marble, however, can be safely ignored for most practical purposes.
Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW Bohemia
Authors: Hainzl Sebastian , Fischer Tomas
Published: December 2002

The characteristics of earthquake swarms can neither be described by simple laws nor are the underlying mechanisms presently understood. Swarm activity is often assumed to be caused by an intrusion of fluids into the seismogenic zone. We have studied the earthquake catalog of the large earthquake swarm that occurred in the year 2000 in Vogtland, SE‐Germany and NW‐Bohemia, an area well known for its episodic swarm generation. We observe a significant decrease of the Gutenberg–Richter b value during the swarm evolution as well as a fractal temporal clustering of the earthquakes. The spatial spreading of the swarm's activity, which is approximately confined to one plane, cannot simply be explained by a process of fluid diffusion. Instead, we observe a simple relationship between the spatial spreading and the seismic moment release, which is in good agreement with empirical relationships derived for tectonically driven earthquakes and theoretical crack growth models. This observation points to a progressively growing main fracture underlying the swarm activity. In addition, we find that the swarm earthquakes themselves trigger aftershocks near the border of their rupture area. The stick‐slip behavior of the rupture propagation can be explained by stress transfers and induced fluid flows due to earthquakes in a fluid‐permeated critically loaded fault zone. However, during the first phase, the temporal behavior is found to be different, pointing to intrusion of fluids initiating the swarm activity.
Tectonic evolution of the Pacific margin of Antarctica 2. Structure of Late Cretaceous–early Tertiary plate boundaries in the Bellingshausen Sea from seismic reflection and gravity data
Authors: Cunningham Alex P. , Larter Robert D. , Barker Peter F. , Gohl Karsten , Nitsche Frank O.
Published: December 2002

Interpretations of multichannel seismic (MCS) reflection and potential field data suggest that some prominent gravity anomalies in the Bellingshausen Sea are associated with plate boundaries that were active during the Late Cretaceous and early Tertiary. Between 83° and 93°W, a belt of negative anomalies extends along the West Antarctic continental slope, which we term the continental slope gravity anomaly (CSGA). MCS profiles show that the CSGA coincides with an acoustically opaque structural high imaged beneath the lower slope. We interpret this structure as the upper part of an accretionary prism which formed during southward subduction of the Phoenix and Charcot plates, before Chatham Rise separated from West Antarctica. MCS profiles crossing the same margin to the northeast show no evidence of an extensive buried accretionary prism, but instead reveal an abrupt northeastward steepening of the continental slope near 78°W. We attribute this change in tectonic style, at least in part, to subduction erosion resulting from subduction of rough oceanic basement which formed at the Antarctic‐Phoenix ridge after an abrupt decrease in spreading rate at chron 23r (52 Ma). Near 95°W, the Bellingshausen gravity anomaly (BGA) consists of a prominent low‐high gravity couple which crosses the West Antarctic continental shelf, slope, and rise. The BGA corresponds to a buried asymmetric basement trough, where Cretaceous oceanic basement dips beneath more elevated basement to the east. The trough probably formed after subduction of Charcot plate ocean floor stalled at the nearby Antarctic Peninsula margin, near the end of the Cretaceous Normal Superchron. Ocean floor to the east of the BGA became attached to the Antarctic Peninsula, and the BGA trough subsequently accommodated a small amount of convergent motion between the Antarctic Peninsula and the ocean floor to the west (initially part of the Marie Byrd Land plate and later part of the Bellingshausen plate). Tectonism probably ceased at the BGA at chron 27 (61 Ma), as a result of a general plate reorganization in the South Pacific.
Intensive atmospheric mercury measurements at Terra Nova Bay in Antarctica during November and December 2000
Authors: Sprovieri F. , Pirrone N. , Hedgecock I. M. , Landis M. S. , Stevens R. K.
Published: December 2002

It is well known that due to its long atmospheric residence time, mercury is distributed on a global scale and aeolian transport is believed to be the major contributor to mercury in polar environments. No measurements of reactive gaseous mercury (RGM) at all have ever been performed in the Antarctic before. Hg0(g) concentrations were in the range 0.29 to 2.3 ng m−3, with an average value of 0.9 ± 0.3 ng m−3. RGM was measured using KCl‐coated annular denuders and a speciation unit coupled to a TGM analyzer; concentrations ranged from 10.5 to 334 pg m−3, with an average of 116.2 ± 77.8 pg m−3. The Hg0(g) measurements are in good agreement with the few data available for such southerly latitudes. The RGM concentrations are as high as those found in some industrial environments; the high concentrations in the absence of local sources (anthropogenic or natural) show that in situ gas phase oxidation of Hg0 is the most important factor influencing RGM production and therefore also Hg deposition. The toxicity of Hg means that the consequences of high concentrations of oxidized and soluble Hg species depositing in the fragile Antarctic environment could be serious indeed.
Evaluation of lightning flash rate parameterizations for use in a global chemical transport model
Authors: Allen Dale J. , Pickering Kenneth E.
Published: December 2002

Model simulations of tropospheric O3 require an accurate specification of the reactive odd nitrogen (NOx) source from lightning that is consistent in time and space with convective transport of O3 precursors. Lightning NOx production in global models is often parameterized in terms of convective cloud top heights (CLDHT). However, a closer relationship may exist between flash rate and other measures of convective intensity. In this study, flash rates are parameterized in terms of CLDHT, convective precipitation (PRECON), and upward convective mass flux (MFLUX) from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). GEOS‐based flash rates are compared to flash rates from the National Lightning Detection Network (NLDN) and Long Range Flash (LRF) network and the Optical Transient Detector (OTD). Overall, MFLUX‐based flash rates are most realistic. PRECON‐ and CLDHT‐based flash rates are too large in the tropics. The MFLUX‐ and PRECON‐based flash rates are a factor of 3 too high (low) over the equatorial western Pacific (central and southern Africa), while CLDHT‐based flash rates are much too low at nearly all marine locations. In many cases, biases in the flash rate distributions can be traced to biases in the GEOS DAS convective fields. Improvements in flash rate parameterizations will be tied closely to improvements in model physics as well as to increases in the amount of tropical data that are available for assimilation. Flash rates calculated from the 6‐hour averaged CLDHTs are much less variable than observed, and O3 production rates calculated using the NOx produced from these flash rates are likely to be larger than observed.
Spectral dependence of radiative horizontal transport in stratocumulus clouds and its effect on near‐IR absorption
Authors: Kassianov E. I. , Kogan Y. L.
Published: December 2002

The spectral dependence of the radiative horizontal transport (the horizontal transport of radiative energy)Eand its effect on the accuracy of spectral and broadband absorption retrieval in the near‐infrared (IR) wavelength range was investigated using a large‐eddy simulation (LES) cloud model with explicit microphysics and a three‐dimensional Monte Carlo radiative transfer model. Two typical types of marine clouds representing inhomogeneous overcast and broken stratocumulus clouds have been simulated. We demonstrate that (1) the basic statistics (e.g., variance and correlation function) of the horizontal transport are wavelength‐dependent and (2) the estimates of spectral and broadband absorption with a given accuracy (e.g.,rmse∼4%) may require the use of different spatial resolutions.
Cloudy and clear stratospheres before A.D. 1000 inferred from written sources
Authors: Stothers Richard B.
Published: December 2002

How can observational information about stratospheric transparency in the preinstrumental era be acquired today? It may be reasonably assumed that a high turbidity in the stratosphere is almost always caused by light‐scattering sulfate aerosols derived from large volcanic eruptions. Historical reports of a dimming of the Sun, red twilight glows, reddish solar haloes, and dark total eclipses of the Moon indicate a high turbidity; contrariwise, a ruddy disk shown by the eclipsed Moon betrays a clear stratosphere. On the basis of an extensive search of primary European and Middle Eastern written sources pertaining to the ancient and early medieval periods, seven stratospheric dry fogs, in addition to the four major ones already known, have been identified by using solar observations, and five smaller ones have been detected from dark total lunar eclipses. Seven of the eight most important dry fogs between 300 B.C. and A.D. 1000 can be either definitely or plausibly correlated with high levels of sulfate acidity observed in Greenland ice cores. An important conclusion is that this sample is probably very nearly complete for major dry fogs during this period. A second conclusion is that the ratio of dark to normal total lunar eclipses during early medieval times (A.D. 400–1000) appears to be equal, approximately, to the ratio that has prevailed for the past 40 years. These conclusions suggest that the frequency of volcanic eruptions, both large and moderate, throughout the world may have remained statistically constant (on a long timescale) since at least 300 B.C.
A simple explanation for the increase in relative humidity between 11 and 14 km in the tropics
Authors: Folkins Ian , Kelly K. K. , Weinstock E. M.
Published: December 2002

We use a very simple model with no adjustable parameters to simulate the increase in relative humidity between 11 and 14 km in the tropics. The rate of increase in relative humidity, in this interval, appears to be largely determined by the shape of the convective detrainment profile, the temperature profile, and the level of zero radiative heating. It appears to be unaffected by evaporative moistening or the complex three‐dimensional structure of convective systems. We also show that the rapid increase in the fraction of supersaturated air parcels above 150 mb (∼14.5 km) is associated with a transition from large scale descent to ascent.
Surface emissivity retrieval from Digital Airborne Imaging Spectrometer data
Authors: Sobrino José A. , Jiménez‐Muñoz Juan C. , Labed‐Nachbrand Jélila , Nerry Françoise
Published: December 2002

A study has been carried out on the most recent algorithms for the estimation of land surface emissivity (ε) using high‐resolution data (Digital Airborne Imaging Spectrometer, DAIS) over the Rhine Valley (France) and Castilla La Mancha (Spain). Three published methods have been applied for extracting absolute spectral emissivity information from images recorded during the DAISEX experiment in 1999. They are NDVI Thresholds Method (NDVITHM), Normalized Emissivity Method (NEM) and Temperature/Emissivity Separation (TES). These lather two methods were originally designed to work over geological surfaces. Five methods have been used for extracting relative spectral emissivity. They are temperature‐independent spectral indices method (TISI), reference channel method (REF), emissivity normalization method (NOR), emissivity re‐normalization method (RE), and alpha emissivity method (ALPHA), respectively. NDVITHM and NEM give the same absolute emissivity values with differences between 1% and 0.2% depending on the thermal channel considered, while NDVITHM and TES give the greatest differences, around 2%. The comparison with in situ values shows that NDVITHM gives the best results for vegetation plots, while NEM gives the best ones for bare soil and water plots. The TISI and NOR relative methods give the same relative emissivity values within less than 0.4%, in accordance with Becker and Li's [1990] conclusion as regards the superiority of these methods compared to the others.
Ice core chemistry of Vestfonna Ice Cap in Svalbard, Norway
Authors: Matoba Sumito , Narita Hideki , Motoyama Hideaki , Kamiyama Kokichi , Watanabe Okitsugu
Published: December 2002

Chemical analyses were done on samples from a 2.2‐m deep snow pit and a 211‐m deep ice core that came from the top of the Vestfonna Ice Cap in Nordaustlandet, Svalbard, Norway. The snow pit samples showed that the temperature increased during the winter owing to advection of warm air from the south, and δ18O did not show a seasonal cycle. Chronology of the ice core was determined by tritium analysis and comparison to the signal from the volcanic eruption of Laki; the resulting accumulation rate was 0.34–0.35 m water eq. · yr−1 for the last 400 years. The δ18O profile in the ice core revealed a warm period from 1920 up to the present in Svalbard. According to nonseasalt (nss) SO42− and NO3− concentrations, anthropogenic substances had increased from 1940s but declined from 1970s to the present. The variation of nss SO42− and the increase of NO3− can be explained by an increase of SO2 and NOx emissions from Eurasia. The decline of NO3− is assumed to be caused either by a change in the transport process or by a change of NOx emissions from Europe.
Airborne measurements of atmospheric carbonaceous aerosols during ACE‐Asia
Authors: Mader Brian T. , Flagan Richard C. , Seinfeld John H.
Published: December 2002

Airborne levels of carbonaceous aerosols were measured using the Twin Otter aircraft during the Aerosol Characterization Experiment (ACE)‐Asia. Particles were collected using a newly developed honeycomb denuder sampler and organic carbon (OC), elemental carbon (EC), and carbonate (CC) carbon levels determined using a thermal–optical carbon analyzer. During some flights, atmospheric layers could be identified as marine boundary, pollution dominated, or mineral dust dominated. Ångstrom exponent (å) values, calculated based on data from an onboard three‐wavelength nephelometer, were used to discern the nature of some individual layers. Values of å for individual layers ranged from 0.2 to 2, corresponding to dust‐ and pollution‐dominated layers, respectively. OC and EC concentrations below 3 km ranged from 0.58 to 29 μg C m−3 and from 0.20 to 1.8 μg C m−3, respectively. In general, for a given type of atmospheric layer, higher levels of total carbon (TC) were observed during ACE‐Asia than those observed during ACE‐2, Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) and Indian Ocean Experiment (INDOEX). Mixed layers of dust and pollution were found on some occasions. CC was detected in samples taken from layers in which å = 1.6, indicating that significant amounts of dust can be present even though å > 0.2. A linear regression of light absorption coefficient σap (Mm−1) versus EC concentration had an r2 of only 0.50, indicating that parameters other than the mass of EC significantly affected the value of σap. The mass absorption coefficient Eabs (m2 g−1) varied by as much as a factor of 8 between sampling events, and the average value of 11 m2 g−1 (±5.0) agrees well with previous published values.
Statistical derivation of the evolution equation of liquid water path fluctuations in clouds
Authors: Ivanova K. , Ausloos M.
Published: December 2002

How to distinguish and quantify deterministic and random influences on the statistics of turbulence data in meteorology cases is discussed from first principles. Liquid water path (LWP) changes in clouds, as retrieved from radio signals, upon different delay times, can be regarded as a stochastic Markov process. A detrended fluctuation analysis method indicates the existence of long range time correlations. The Fokker‐Planck equation which models very precisely the LWP fluctuation empirical probability distributions, in particular, their non‐Gaussian heavy tails is explicitly derived and written in terms of a drift and a diffusion coefficient. Furthermore, Kramers‐Moyal coefficients, as estimated from the empirical data, are found to be in good agreement with their first principle derivation. Finally, the equivalent Langevin equation is written for the LWP increments themselves. Thus rather than the existence of hierarchical structures, like an energy cascade process, strong correlations on different timescales, from small to large ones, are considered to be proven as intrinsic ingredients of such cloud evolutions.
Potential for photochemical ozone formation in the troposphere over the North Atlantic as derived from aircraft observations during ACSOE
Authors: Reeves Claire E. , Penkett Stuart A. , Bauguitte Stephane , Law Kathy S. , Evans Mathew J. , Bandy Brian J. , Monks Paul S. , Edwards Gavin D. , Phillips Gavin , Barjat Hannah , Kent Joss , Dewey Ken , Schmitgen Sandra , Kley Dieter
Published: December 2002

In this paper, ozone (O3), water vapor (H2O), carbon monoxide (CO), and peroxide concentrations and photolysis rates measured in the troposphere over the North Atlantic during two Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) aircraft field campaigns are used to calculate the concentration of nitric oxide (NO) required for net photochemical O3 production (nPO3) to be positive (NOcomp). NOcomp tended to show a decrease with altitude, although it was sometimes found to be low in the marine boundary layer (MBL) where H2O concentrations were high and O3 concentrations were low. nPO3 was calculated for the spring when NO data were available and was found to be mostly negative and generally increased from about −0.5 to −0.2 ppbv hr−1 in the MBL to +0.04 ppbv hr−1 at about 7–8 km altitude. The results suggest that much of the lower and middle troposphere over the eastern North Atlantic during spring is in a state of slow net photochemical O3 destruction. However, in the upper troposphere, the system changes to one of net photochemical production, which results from the drier environment and higher NO concentrations. Furthermore, examples of net O3 production were also observed in the lower and middle troposphere associated with either in situ sources of NO or long‐range transport of pollution. The paper also illustrates the sensitivity of this O3 production/loss state to H2O and NO concentrations, photolysis rates, and temperatures.
Water adsorption and dynamics on kerosene soot under atmospheric conditions
Authors: Ferry D. , Suzanne J. , Nitsche S. , Popovitcheva O. B. , Shonija N. K.
Published: December 2002

Morphology, microstructure and surface chemistry of laboratory made kerosene soot used as an aircraft soot surrogate have been studied to establish the correlation between the porosity and the mechanism of water adsorption on the soot surface. The quasielastic neutron scattering (QENS) technique has been used to characterize the dynamics of water confined in the soot pores network. Spectra above and below the water triple point Tm describe the translational and rotational diffusion of water molecules adsorbed in 0.5 nm micropores, 2 nm supermicropores and ≥2 nm mesopores. Below Tm an appreciable amount of liquid water exists in the soot micropores down to the lowest tropospheric temperatures. The depression in freezing temperature is related to the pore dimension. Water confined in the micropores appears to freeze completely only at T below 200 K showing that the nucleation process depends on the specific microporosity. At the saturation plume conditions ≅30% of adsorbed water has been transformed into ice. These results show that, in the upper troposphere, soot particles presenting the above‐mentioned properties will contain stable water/ice components inside the pores with 25% of unfrozen water.
Chemical, microphysical, and radiative effects of Indian Ocean aerosols
Authors: Satheesh S. K. , Ramanathan V. , Holben B. N. , Moorthy K. Krishna , Loeb N. G. , Maring H. , Prospero J. M. , Savoie D.
Published: December 2002

Extensive and long‐term multistation measurements of aerosol properties and radiative fluxes were carried out in the haze plume off the South Asian continent. These experiments are carried out at Kaashidhoo Climate Observatory (KCO) (4.95°N, 73.5°E), Minicoy (8.5°N, 73.0°E), and Trivandrum (8.5°N, 77.0°E). In addition, the top of the atmosphere fluxes were measured simultaneously by the CERES radiation budget instrument. Long‐term observations (more than 15 years) over Trivandrum show that there is a gradual increase in aerosol visible optical depth from ∼0.2 in 1986 to ∼0.4 in 1999. Pre‐ and post‐monsoon aerosol characteristics are examined to study the seasonal variations. The impact of aerosols on short‐wave radiation budget is estimated using direct observations of solar radiation using several independent ground‐based radiometers and satellite data as well as from modeled aerosol properties. It was observed that “excess absorption” is not needed to model diffuse fluxes. The lower atmospheric heating due to absorbing aerosols was as high as ∼20 W m−2 which translates to a heating rate perturbation of ∼0.5°K/day. The effect of aerosol mixing state (internally and externally) on aerosol forcing appears to be negligible. A sensitivity study of the effect of aerosols over land in contrast to that over the ocean shows an enhancement in lower atmosphere heating by about 40% simultaneous with a reduction of ∼33% in surface cooling. Increasing sea‐surface winds increase aerosol cooling due to increased sea salt aerosol concentrations, which can partly offset the heating effect due to absorbing aerosols.
An analysis of multimodel ensemble predictions for seasonal climate anomalies
Authors: Peng Peitao , Kumar Arun , van den Dool Huug , Barnston Anthony G.
Published: December 2002

In this paper the potential advantages and relative performances of different techniques for constructing multimodel ensemble seasonal predictions are examined. Two commonly used methods of constructing multimodel ensemble predictions are analyzed. Particular emphasis is placed on the analysis of the schemes themselves. In the first technique—simple multimodel ensemble (SME) predictions—equal weights are assigned to the ensemble mean predictions of each of the atmospheric general circulation models (AGCM). In the second approach—optimal multimodel ensemble (OME) predictions—the weights are obtained using a multiple linear regression. A theoretical analysis of these techniques is complemented by the analyses based on seasonal climate simulations for 45 January–February–March seasons over the 1950–1994 period. A comparison of seasonal simulation skill scores between SME and OME indicates that for the bias corrected data, i.e., when the seasonal anomalies of each of the AGCMs are computed with respect to its own mean state, the performance of seasonal predictions based on the simpler SME approach is comparable to that of the more complex OME approach. A major reason for this result is that the data record of historical predictions may not be long enough for a stable estimate of weights at individual geographical locations to be obtained. This problem can be reduced by extending the multiple linear regression approach to include the spatial domain. However, even with this algorithm change, the performance of OME in seasonal predictions does not improve over that using the SME approach. Results, therefore, indicate that the use of more sophisticated techniques for constructing multimodel ensembles may not be any more advantageous than the use of simpler approaches. Results also show that on average the skill scores for the predictions based on multimodel ensemble prediction techniques are only marginally better than those of the best AGCM. However, an advantage of multimodel ensemble prediction techniques may be that they retain the best performance of each AGCM on a regional basis in the merged forecasts.
Diffusion in sparsely connected pore spaces: Temporal and spatial scaling
Authors: Ewing Robert P. , Horton Robert
Published: December 2002

Diffusion both disperses and retards mass movement of contaminants through porous media. The retardation generally involves mass transfer of contaminants from faster to slower flow paths, making prediction more difficult and remediation slower. In ordinary (Fickian) diffusion, root‐mean‐square displacement of a population of diffusing molecules increases with the square root of the elapsed time, and diffusivity (D, L2T−1) of the porous medium is constant in time. The diffusivity can be measured using time‐dependent methods such as pulsed field gradient (PFG) magnetic resonance (MR), or steady state diffusion experiments. However, it is known that diffusion behaves anomalously in sparsely connected pore spaces at the percolation threshold. We conducted a series of Monte Carlo simulations using random walks, designed to mimic measurement of D using both time‐dependent and steady state methods. In the time‐dependent simulations, diffusivity scaled as D ∼ T−0.48 at the percolation threshold. Above the percolation threshold, D decreased at early times, then became constant, with the time required for constant D being greater when the medium was less interconnected. In the steady state simulations we examined the effect of sample size on apparent diffusivity. Above the percolation threshold, diffusivity decreased, then became constant as sample size increased, but at the percolation threshold, diffusivity scaled as D ∼ L−1.83. The drop in diffusivity with sample size was caused by both a decrease in effective porosity and an increase in tortuosity, each of which also followed scaling laws. A single measurement will not suffice to determine whether a sample's pore space is at the percolation threshold, necessitating measurement of diffusivity at more than one scale in time or space. Using a scale‐dependent measurement to make predictions at a different scale can result in serious overestimation or underestimation of diffusive mass transfer and consequent retardation.
The order of discourse in surveys of public understanding of science
Authors: Kallerud Egil , Ramberg Inge
Published: December 2002

This paper discusses how insights from constructivist studies on the place of context, reflexivity, and framing in studies of public understanding of science may be applied within a survey framework. By emphasizing the role that images or ideologies of science play in science policy discourse, surveys are conceptualized as a means to map the social pervasiveness of such ideologies. It is argued that survey results may contribute to a broadening of the framework of science policy, if questions that explicitly reflect civic perspectives on science are included in survey questionnaires. An experiment with three non-standard questions in a Norwegian survey in 1999 on public understanding of science is seen to provide some empirical support for the viability of this approach.
Accomplishments and challenges in literature data mining for biology
Authors: Hirschman Lynette , Park Jong C. , Tsujii Junichi , Wong Limsoon , Wu Cathy H.
Published: December 2002

We review recent results in literature data mining for biology and discuss the need and the steps for a challenge evaluation for this field. Literature data mining has progressed from simple recognition of terms to extraction of interaction relationships from complex sentences, and has broadened from recognition of protein interactions to a range of problems such as improving homology search, identifying cellular location, and so on. To encourage participation and accelerate progress in this expanding field, we propose creating challenge evaluations, and we describe two specific applications in this context.

Contact: wuc@georgetown.edutsujii@is.s.u-tokyo.ac.jppark@nlp.kaist.ac.krlynette@mitre.orglimsoon@lit.org.sg

* To whom correspondence should be addressed.
Computational antisense oligo prediction with a neural network model
Authors: Chalk Alistair M. , Sonnhammer Erik L. L.
Published: December 2002

Motivation: The expression of a gene can be selectively inhibited by antisense oligonucleotides (AOs) targeting the mRNA. However, if the target site in the mRNA is picked randomly, typically 20% or less of the AOs are effective inhibitors in vivo. The sequence properties that make an AO effective are not well understood, thus many AOs need to be tested to find good inhibitors, which is time consuming and costly. So far computational models have been based exclusively on RNA structure prediction or motif searches while ignoring information from other aspects of AO design into the model.

Results: We present a computational model for AO prediction based on a neural network approach using a broad range of input parameters. Collecting sequence and efficacy data from AO scanning experiments in the literature generated a database of 490 AO molecules. Using a set of derived parameters based on AO sequence properties we trained a neural network model. The best model, an ensemble of 10 networks, gave an overall correlation coefficient of 0.30 (p=10−8). This model can predict effective AOs (>50% inhibition of gene expression) with a success rate of 92%. Using these thresholds the model predicts on average 12 effective AOs per 1000 base pairs, making it a stringent yet practical method for AO prediction.

Availability: A prediction server is available at http://www.cgb.ki.se/AOpredict

Contact: alistair.chalk@cgb.ki.se

* To whom correspondence should be addressed.
Comparing expression profiles of genes with similar promoter regions
Authors: Park Peter J. , Butte Atul J. , Kohane Isaac S.
Published: December 2002

Motivation: Gene regulatory elements are often predicted by seeking common sequences in the promoter regions of genes that are clustered together based on their expression profiles. We consider the problem in the opposite direction: we seek to find the genes that have similar promoter regions and determine the extent to which these genes have similar expression profiles.

Results: We use the data sets from experiments on Saccharomyces cerevisiae. Our similarity measure for the promoter regions is based on the set of common mapped or putative transcription factor binding sites and other regulatory elements in the upstream region of the genes, as contained in the Saccharomyces cerevisiae Promoter Database. We pair up the genes with high similarity scores and compare their expression levels in time-course experiment data. We find that genes with similar promoter regions on the average have significantly higher correlation, but it can vary widely depending on the genes. This confirms that the presence of similar regulatory elements often does not correspond to similarity in expression profiles and indicates that finding transcription factor binding sites or other regulatory elements starting with the expression patterns may be limited in many cases. Regardless of the correlation, the degree to which the profiles agree under different experimental conditions can be examined to derive hypotheses concerning the role of common regulatory elements. Overall, we find that considering the relationship between the promoter regions and the expression profiles starting with the regulatory elements is a difficult but useful process that can provide valuable insights.

Contact: peter_park@harvard.edu

* To whom correspondence should be addressed.
Robust estimators for expression analysis
Authors: Hubbell Earl , Liu Wei-Min , Mei Rui
Published: December 2002

Motivation: We consider the problem of estimating values associated with gene expression from oligonucleotide arrays. Such estimates should linearly track concentration, yield non-negative results, have statistical guarantees of robustness against outliers, and allow estimates of significance and variance.

Results: A hierarchy of simple models is used to design robust estimators meeting these goals for both stand alone and comparative experiments. This algorithm has been validated against an extensive panel of known spike experiments, and shows comparable performance to existing standards.

Availability: Algorithms available commercially as part of the MAS 5.0 software package. Data sets available from the Affymetrix website.

Contact: earl_hubbell@affymetrix.com

Supplemental Information: http://www.affymetrix.com/community/publications/affymetrix/index.affx

* To whom correspondence should be addressed.
Analysis of high density expression microarrays
with signed-rank call algorithms
Authors: Liu W.-m , Mei R. , Di X. , Ryder T. B. , Hubbell E. , Dee S. , Webster T. A. , Harrington C. A. , Ho M.-h. , Baid J. , Smeekens S. P.
Published: December 2002

Motivation: We consider the detection of expressed genes and the comparison of them in different experiments with the high-density oligonucleotide microarrays. The results are summarized as the detection calls and comparison calls, and they should be robust against data outliers over a wide target concentration range. It is also helpful to provide parameters that can be adjusted by the user to balance specificity and sensitivity under various experimental conditions.

Results: We present rank-based algorithms for making detection and comparison calls on expression microarrays. The detection call algorithm utilizes the discrimination scores. The comparison call algorithm utilizes intensity differences. Both algorithms are based on Wilcoxon's signed-rank test. Several parameters in the algorithms can be adjusted by the user to alter levels of specificity and sensitivity. The algorithms were developed and analyzed using spiked-in genes arrayed in a Latin square format. In the call process, p-values are calculated to give a confidence level for the pertinent hypotheses. For comparison calls made between two arrays, two primary normalization factors are defined. To overcome the difficulty that constant normalization factors do not fit all probe sets, we perturb these primary normalization factors and make increasing or decreasing calls only if all resulting p-values fall within a defined critical region. Our algorithms also automatically handle scanner saturation.

Availability: These algorithms are available commercially as part of the MAS 5.0 software package.

Contact: wei-min_liu@affymetrix.com

* To whom correspondence should be addressed.

† Current address: Mail Code CR145, Oregon Health Sciences University, Portland, OR 79201
Between-group analysis of microarray data
Authors: Culhane Aedín C. , Perrière Guy , Considine Elizabeth C. , Cotter Thomas G. , Higgins Desmond G.
Published: December 2002

Motivation: Most supervised classification methods are limited by the requirement for more cases than variables. In microarray data the number of variables (genes) far exceeds the number of cases (arrays), and thus filtering and pre-selection of genes is required. We describe the application of Between Group Analysis (BGA) to the analysis of microarray data. A feature of BGA is that it can be used when the number of variables (genes) exceeds the number of cases (arrays). BGA is based on carrying out an ordination of groups of samples, using a standard method such as Correspondence Analysis (COA), rather than an ordination of the individual microarray samples. As such, it can be viewed as a method of carrying out COA with grouped data.

Results: We illustrate the power of the method using two cancer data sets. In both cases, we can quickly and accurately classify test samples from any number of specified a priori groups and identify the genes which characterize these groups. We obtained very high rates of correct classification, as determined by jack-knife or validation experiments with training and test sets. The results are comparable to those from other methods in terms of accuracy but the power and flexibility of BGA make it an especially attractive method for the analysis of microarray cancer data.

Availability: The methods described are implemented in ADE-4 which runs under MacOS and Windows, and is freely available at http://pbil.univ-lyon1.fr/ADE-4/. All scripts are available on request.

Contact: A.Culhane@ucc.ie

Supplementary information: Supplementary figures and tables are available at http://bioinfo.ucc.ie/BGA/.

* To whom correspondence should be addressed.