bluuebunny/crossref_metadata_2025 huggingface dataset:
Retrieving Soil Moisture Over Bare Soil from ERS 1 Synthetic Aperture Radar Data: Sensitivity Analysis Based on a Theoretical Surface Scattering Model and Field Data
Authors: Altese Elio , Bolognani Orsola , Mancini Marco , Troch Peter A.
Published: February 2004

In order to assess the retrieval of soil moisture from ERS 1 (European Remote Sensing Satellite) synthetic aperture radar (SAR) data, an inversion procedure based oft the integral equation model (IEM) [Fung et al., 1992] is developed. First, the IEM is used to analyze the sensitivity of radar echoes (in terms of the backscattering coefficient σ0) to the surface parameters (roughness and dielectric constant) under ERS 1 SAR configuration. Results obtained for random rough bare soil fields show that the effect of surface roughness is very strong, particularly in the case of smooth surfaces, and that the sensitivity of σ0 to dielectric constant is independent of the radar configuration and the roughness conditions. This means that the range of variation of backscattering with respect to the dielectric constant variation of dry to wet soil remains the same (about 5 dB) for any roughness condition and radar configuration. The possibility of applying the inversion procedure to retrieve soil moisture is investigated using a set of data collected in a test site situated near Naples, Italy, during the SeIe Synthetic Aperture Radar experiment (SESAR) campaign (November 1993). Simultaneous with ERS 1 overpasses, dielectric constant and roughness measurements were taken over two flat bare fields. From this analysis it is found that the inversion of backscattering from ERS 1 SAR into soil moisture is not reliable without accurate information on roughness if the surface is smooth. In this case it is observed that the sensitivity to the roughness parameters is much higher than the sensitivity to dielectric constant, so that even a small error in the measurement of this parameter can affect the retrieved value of soil moisture significantly. The inversion procedure provides more reliable soil moisture estimates when surfaces rougher than those analyzed in the field experiment are considered.
On the Relationship Between Catchment Scale and Climatological Variability of Surface‐Runoff Volume
Authors: Seo Dong‐Jun , Smith James A.
Published: February 2004

The relationship between catchment scale and climatological variability of surface‐runoff volume is evaluated through theoretical and empirical analyses. Using a point description of surface‐runoff volume following infiltration excess under the time compression approximation, climatological mean and variance of areal runoff volume over heterogeneous soil are evaluated by integrating second‐order statistics of point runoff volume. Second‐order statistics of rainfall duration and intensity, required for evaluation of those of point runoff volume, are obtained from mean and variance of areal rainfall volume under fractional coverage considerations. Hourly Weather Surveillance Radar‐1988 Doppler rainfall data were used to estimate rainfall parameters following a set of simplifying assumptions on the space‐time correlation structure. Two contrasting climate‐soil systems were considered in specifying soil and soil moisture parameters following a set of assumptions on spatial variability and interdependency of the soil parameters.
Calcium Inputs and Transport in A Base‐Poor Forest Ecosystem as Interpreted by Sr Isotopes
Authors: Bailey Scott W. , Hornbeck James W. , Driscoll Charles T. , Gaudette Henri E.
Published: February 2004

Depletion of Ca in forests and its effects on forest health are poorly quantified. Depletion has been difficult to document due to limitations in determining rates at which Ca becomes available for ecosystem processes through weathering, and difficulty in determining changes in ecosystem storage. We coupled a detailed analysis of Sr isotopic composition with a mass balance at Cone Pond Watershed, New Hampshire, in order to further constrain estimates of these processes. Strontium acted as an analog for Ca in most processes except translocation of nutrients within forest vegetation. Variability in mineralogic and Sr isotopic composition of bedrock and soils complicated assessment of the 87Sr/86Sr ratio released to solution by weathering reactions. By conducting a mass balance on atmospherically derived Ca, it is possible to distinguish Ca weathering losses from Ca leached from ecosystem pools. The calcium weathering rate estimated by this method was less than half of that determined by mass balance assuming steady state conditions.
The Radial Diffusion Method: 1. Using intact cores to determine isotopic composition, chemistry, and effective porosities for groundwater in aquitards
Authors: van der Kamp G. , Van Stempvoort D. R. , Wassenaar L. I.
Published: February 2004

A new method is described for the determination of the isotopic composition and chemistry of groundwater in aquitards. The effective porosities for each component can be determined concurrently. The method is based on diffusive exchange of water and solutes between groundwater in a cylindrical core of aquitard material and water in a reservoir placed along the axis of the cylinder. The method was tested for 2H, Cl, and SO4in an aquitard consisting of clay‐rich glacial till. The δ2H in aquitard core samples as determined by radial diffusion agreed closely with δ2H in groundwater obtained from corresponding piezometers installed in the aquitard. For the concentrations of Cl and SO4 the agreement was less close but still satisfactory. The effective porosities were equal to the total porosities for 2H and were less than the total porosities for Cl and SO4 ions.
Nitrate in Groundwater and Water Sources used by Riparian Trees in an Agricultural Watershed: A Chemical and Isotopic Investigation in Southern Minnesota
Authors: Komor Stephen C. , Magner Joseph A.
Published: February 2004

This study evaluates processes that affect nitrate concentrations in groundwater beneath riparian zones in an agricultural watershed. Nitrate pathways in the upper 2 m of groundwater were investigated beneath wooded and grass‐shrub riparian zones next to cultivated fields. Because trees can be important components of the overall nitrate pathway in wooded riparian zones, water sources used by riparian trees and possible effects of trees on nitrate concentrations in groundwater were also investigated. Average nitrate concentrations in shallow groundwater beneath the cultivated fields were 5.5 mg/L upgradient of the wooded riparian zone and 3.5 mg/L upgradient of the grass‐shrub zone. Shallow groundwater beneath the fields passed through the riparian zones and discharged into streams that had average nitrate concentrations of 8.5 mg/L (as N). Lateral variations of δD values in groundwater showed that mixing among different water sources occurred beneath the riparian zones. In the wooded riparian zone, nitrate concentrations in shallow groundwater were diluted by upwelling, nitrate‐poor, deep groundwater. Upwelling deep groundwater contained ammonium with a δ15N of 5‰ that upon nitrification and mixing with nitrate in shallow groundwater caused nitrate δ15N values in shallow groundwater to decrease by as much as 19.5‰. Stream water penetrated laterally beneath the wooded riparian zone as far as 19 m from the stream's edge and beneath the grass‐shrub zone as far as 27 m from the stream's edge. Nitrate concentrations in shallow groundwater immediately upgradient of where it mixed with stream water averaged 0.4 mg/L in the wooded riparian zone and 0.8 mg/L near the grass‐shrub riparian zone. Nitrate concentrations increased toward the streams because of mixing with nitrate‐rich stream water. Because nitrate concentrations were larger in stream water than shallow groundwater, concentrated nitrate in the streams cannot have come from shallow groundwater at these sites. Water sources of riparian trees were identified by comparing δD values of sap water, soil water, groundwater, and stream water. Soil water was the main water source for trees in the outer 4 to 6 m of one part of the wooded riparian zone and outer 10 m of another part. Groundwater was a significant water source for trees closer to the streams where the water table was less than about 2.1 to 2.7 m below the surface. No evidence was found in the nitrate concentration profiles that trees close to the streams that took up groundwater through their roots also took up nitrate from groundwater. The lack of such evidence is attributed to the nitrate concentration profiles being insufficiently sensitive indicators of nitrate removal by trees.
A Nordic Seas–Arctic Ocean carbon budget from volume flows and inorganic carbon data
Authors: Lundberg Lena , Haugan Peter M.
Published: February 2004

A carbon budget for the Nordic Seas and Arctic Ocean is compiled by combining volume flows from the literature and carbon chemistry data, mainly from the Transient Tracers in the Ocean/National Academy of Sciences (TTONAS) 1981 expedition. The net advective export of dissolved inorganic carbon from the Nordic Seas and Arctic Ocean is found to be approximately 0.23 *106 moles C s−1 (0.09 Pg C yr−1). Including a tentative estimate of the increase of the carbon inventory in the region, which is due to the supply of anthropogenic CO2, gives an estimate of the net uptake of atmospheric CO2 amounting to 0.11 Pg C yr−1. If the ice‐free area is assumed to be 3.1*1012 m2, this corresponds to a net uptake of 3 moles C m−2 yr−1 over the ice‐free area. Input of carbon to the North Atlantic via the Arctic Ocean and Nordic Seas comprises transport of carbon from the Pacific through the Bering Strait, riverine carbon input, and net uptake of atmospheric CO2. On the basis of data from the literature the input of carbon from the Pacific is found to be approximately 0.63 Pg C yr−1, giving a total flux of carbon to the North Atlantic from the Nordic Seas and Arctic Ocean amounting to approximately 0.8 Pg C yr−1. A prominent feature of the transports is the large gross transport across the Greenland‐Scotland ridge and the concentration difference between the inflowing North Atlantic water and the outflowing deep water, that is, the northern limb of the Atlantic thermohaline circulation. The gross carbon transport associated with deep and intermediate water overflow across the Greenland‐Scotland ridge is estimated to 4.5 Pg C yr−1, which is at least 5% of the global gross exchange between surface and deep ocean. The role of the region in the global carbon cycle may therefore be summarized as a transit area for carbon from the Pacific and from river runoff, uptake of atmospheric CO2, and a substantial transformation of surface water to deep water. A large fraction of the regional storage of anthropogenic CO2 appears to be due to advection rather than enhanced regional air‐sea uptake.
Model estimates of nitrous oxide emissions from agricultural lands in the United States
Authors: Li Changsheng , Narayanan Vijay , Harriss Robert C.
Published: February 2004

The Denitrification‐Decomposition (DNDC) model was used to elucidate the role of climate, soil properties, and farming practices in determining spatial and temporal variations in the production and emission of nitrous oxide (N2O) from agriculture in the United States. Sensitivity studies documented possible causes of annual variability in N2O flux for a simulated Iowa corn‐growing soil. The 37 scenarios tested indicated that soil tillage and nitrate pollution in rainfall may be especially significant anthropogenic factors which have increased N2O emissions from soils in the United States. Feedbacks to climate change and biogeochemical manipulation of agricultural soil reflect complex interactions between the nitrogen and carbon cycles. A 20% increase in annual average temperature in °C produced a 33% increase in N2O emissions. Manure applications to Iowa corn crops enhanced carbon storage in soils, but also increased N2O emissions. A DNDC simulation of annual N2O emissions from all crop and pasture lands in the United States indicated that the value lies in the range 0.9–1.2 TgN. Soil tillage and fertilizer use were the most important farming practices contributing to enhanced N2O emissions at the national scale. Soil organic matter and climate variables were the primary determinants of spatial variability in N2O emissions. Our results suggest that the United States Government, and possibly the Intergovernmental Panel on Climatic Change (IPCC), have underestimated the importance of agriculture as a national and global source of atmospheric N2O. The coupled nature of the nitrogen and carbon cycles in soils results in complex feedbacks which complicate the formulation of strategies to reduce the global warming potential of greenhouse gas emissions from agriculture.
In the shadow of 1857‐the effect of the Great Ft. Tejon Earthquake on subsequent earthquakes in southern California
Authors: Harris Ruth A. , Simpson Robert W.
Published: February 2004

The great 1857 Fort Tejon earthquake is the largest earthquake to have hit southern California during the historic period. We investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake. When post‐1857 earthquakes with unknown focal mechanisms were assigned strike‐slip mechanisms with strike and rake determined by the nearest active fault, 13 of the 13 southern California M≥5.5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture. When post‐1857 earthquakes in the Transverse Ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike‐slip, 11 of the 13 earthquakes were encouraged by the 1857 earthquake. These results show significant correlations between static stress changes and seismicity patterns. The correlation disappears around 1907, suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20th century.
Deriving century‐long trends of surface temperature change from borehole temperatures
Authors: Huang Shaopeng , Shen Po Yu , Pollack Henry N.
Published: February 2004

We have employed a few‐parameter estimation (FPE) technique to derive information on past surface temperature variations from present‐day borehole temperatures. Synthetic experiments show that the FPE technique can separate reasonably well information on century‐long trends of surface temperature change from borehole temperatures within a wide range of noise levels. The technique also allows a simple illustration and comparison of the inversion results, both spatially and temporally. We illustrate this technique by analyzing seventeen borehole temperature profiles from eleven sites in northeast USA and southeast Canada, utilizing a ground surface temperature model of five ramps of century‐long duration.
Gravity wave activity associated with tropical convection detected in TOGA COARE Sounding data
Authors: Karoly D. J. , Roff G. L. , Reeder M. J.
Published: February 2004

Upper‐air sounding data collected at Santa Cruz (10°S, 165°E) in the western tropical Pacific Ocean during November 1992–February 1993 have been used to estimate the gravity wave activity in the upper troposphere and lower stratosphere. Large amplitude gravity waves with vertical wavelengths of 2–5 km were found in almost all the soundings. Time series describing the wave activity in two height ranges, 10–17 km (the upper troposphere) and 17–24 km (the lower stratosphere) are presented and compared to simple measures of the convective cloud activity. There is enhanced gravity wave activity associated with increased convection on intraseasonal timescales.
Deduction of temperature profile from MST radar observations of vertical wind
Authors: Revathy K. , Prabhakaran Nair S. R. , Krisha Murthy B. V.
Published: February 2004

From the temporal spectra of vertical wind obtained from MST radar observations, the Brunt‐Vaisala frequency is identified. Altitude profile of temperature in troposphere and lower stratosphere is derived from the altitude profile of the Brunt‐Vaisala frequency thus obtained.
Magnetic fabric development in the Tertiary Accretionary Complex in the Boso and Miura Peninsulas of central Japan
Authors: Kanamatsu T. , Herroro‐Bervera E. , Taira A. , Saito S. , Ashi J. , Furumoto A. S.
Published: February 2004

It has been demonstrated that the anisotropy of magnetic susceptibility (AMS) is a sensitive indicator for distinguishing various processes that had acted during and after the deposition of sediments. Recently, much attention has been focused on proving its usefulness in distinguishing early deformation signatures in sediments which show no visible grain‐scale deformation. Here, we present the AMS results of the Neogene Miura Group of Central Japan, which was formed by arc‐arc collision tectonics with subdivisions of two contrasting tectonic provinces: the southern province, a highly deformed accretionary prism and slope basin, and the northern province, a gently deformed forearc basin. Samples were collected from 21 sites in both tectonic settings. We obtained from axes orientations three typical fabric domains considered to represent sedimentary, tectonically deformed, and their intermediate types. The shape parameters of AMS also revealed distinctions clearly. Sedimentary fabrics are more foliated and anisotropic. Deformed fabrics show a more prolate shape and are closer to being isotropic. The sedimentary fabrics predominate in the northern forearc basin sequence whereas the intermediate and deformed fabrics occur predominantly in the southern province. The general trend of the shape of the magnetic fabric in studied sediments reveals a clear evolution in the pattern of the isotropic tectonic fabric constructed from foliated sedimentary fabric.
Polar cap contraction associated with the leading edge of a magnetic cloud
Authors: Knipp D. J. , Emery B. A.
Published: February 2004

In the early hours of 14 January 1988 a large magnetic cloud passed the earth. In this brief article we show evidence of a sudden impulse associated with the leading edge of the main body of the magnetic cloud and we map the evolution of the convection patterns which are indicative of polar cap contraction associated with the subsequent, large, northward‐directed interplanetary magnetic field. We provide a unique, step‐by‐step view of the high latitude events, simultaneously tracing both day side and nightside responses to the solar wind variations. Our maps provide clear global‐scale support for polar cap expansion/contraction theory. The final state of convection in the northern hemisphere was one of extreme quiescence with the suspension of both dayside and nightside reconnection and a near total loss of motional electric fields mapping into the ionosphere. We discuss how this state developed.
Determination of the location of faulting beneath Kobe During the 1995 Hyogo‐Ken Nanbu, Japan, Earthquake from near‐source particle motion
Authors: Sekiguchi Haruko , Irikura Kojiro , Iwata Tomotaka , Kakehi Yasumaro , Hoshiba Mitsuyuki
Published: February 2004

We constrained the location of faulting beneath Kobe during the 1995 Hyogo‐ken Nanbu (Kobe), Japan, earthquake by investigating the particle motion of strong motion records from sites close to the causative fault. The intersection of the earth's surface and the extension of buried fault plane is consistent with known active fault lines but not with the severely damaged zone. The intersection line also agrees with geodetic data.
Large amplitude solar modulation cycles of 10Be in Antarctica: Implications for atmospheric mixing processes and interpretation of the ice core record
Authors: Steig E. J. , Polissar P. J. , Stuiver M. , Grootes P. M. , Finkel R. C.
Published: February 2004

10Be concentrations in an ice core at Taylor Dome, Antarctica show greater variation over the last 75 years than similar 10Be time‐series from Greenland. Like the Greenland records, the new Antarctic data exhibit a strong periodicity which follows expected changes in the production rate of 10Be over the 11‐year solar cycle. Noting that the amplitude of production‐rate variation is both latitude and altitude dependent, we estimate the relative contribution of 10Be from different atmospheric reservoirs. The calculations yield a relatively small (<35%) contribution from the low‐to‐mid latitude stratosphere, suggesting a weak coupling between Antarctic 10Be and geomagnetic field intensity.
On the formation of electron depletions at the summer polar mesopause
Authors: Klostermeyer J.
Published: February 2004

In the summer polar mesopause region, rocket and radar observations have revealed layers of deep electron depletions for which electron scavenging by ice particles with sizes less than 10 nm appears to be the most plausible explanation. The lack of significant ion depletions and the absence of any relationship between the occurrence of polar mesospheric clouds and auroral activity suggest that the ice particles are generated by heterogeneous nucleation on meteoric dust. Then the Kelvin effect, predicting an increase of the equilibrium water vapor pressure with decreasing particle radius, strongly affects the formation of electron depletions. In particular, weak perturbations of the temperature and water vapor concentration associated with small‐scale internal gravity waves can produce time‐dependent multiple layer structures with steep gradients characterized by length scales of a few hundred meters, in good agreement with observational results.
Geomagnetic secular variation through the last 3500 years as recorded by Lake Aslikul sediments from eastern Europe (Russia)
Authors: Nurgaliev Danis K. , Borisov Anatoly S. , Heller Friedrich , Burov Boris V. , Jasonov Pavel G. , Khasanov Damir I. , Ibragimov Shamil Z.
Published: February 2004

A paleomagnetic record of geomagnetic secular variation during the last 3500 yr has been obtained from three lake sediment cores from lake Aslikul in the western Pre‐Ural region (Russia). Pollen data provide a time‐scale and relative paleotemperatures which can be compared with radiocarbon dated paleotemperature data for this region. Magnetic susceptibility measurements have been used for correlation between cores and reconstruction of composite core data. The paleosecular variations which have been recorded in the lake Aslikul sediments are consistent with results from West‐European lakes. Westward shifting inclination features are observed between the more western European and the Pre‐Ural regions which have apparent age differences between 200 and 600 years resulting in drift rates which are of the order of 0.1 degrees/year.
Impact of the heterogeneous hydrolysis of BrONO2 on calculated ozone changes due to HSCT aircraft and increased sulphate aerosol levels
Authors: Randeniya L. K. , Vohralik P. F. , Plumb I. C. , Ryan K. R. , Baughcum S. L.
Published: February 2004

The heterogeneous hydrolysis of BrONO2 via sulphate aerosols has been included in 2‐D model calculations of the impact of projected fleets of supersonic aircraft on atmospheric ozone. Calculations have been performed for aerosol levels ranging from zero to 16× the lower limit in WMO [1992]. The results show that the addition of this reaction has a major effect when the heterogeneous hydrolysis of N2O5 has reached saturation in regions where the night length is short. At 4× the lower limit of aerosols, the additional calculated change in aircraft impact due to the inclusion of BrONO2 hydrolysis is of the same order as the impact calculated when this reaction is not included. Calculations for background atmospheres at high aerosol levels show that the inclusion of this reaction significantly increases the predicted ozone depletion resulting from volcanically‐enhanced aerosol levels.
Trajectories of crustal deformation of western Greece from GPS observations 1989–1994
Authors: Kahle Hans‐Gert , Müller Max V. , Veis George
Published: February 2004

The geodynamic processes in the Alpine‐Mediterranean area are reflected in the recent crustal movements determined by satellite geodesy. There is counterclockwise rotation of Africa relative to Eurasia, with complex deformation in between. We report repeated GPS measurements carried out between SE Italy and the island of Crete in the period 1989 to 1994. The data reveal distinct crustal motion of the central Ionian Islands and SW Greece: Over the time span of five years Western Greece has moved 200mm±10mm to the SW relative to SE Italy and Epiros. The trajectories constructed from the repeated observations show that the boundary zone against the sites of no significant motion relative to South Italy coincides with the Kephalonia fault zone (KFZ). This intense deformation accounts in part for the earthquakes that have frequently devastated the central Ionian Islands.
Elastic properties of silicate melts up to 2350 K from Brillouin scattering
Authors: Vo‐Thanh Dung , Polian Alain , Richet Pascal
Published: February 2004

Compressional wave velocities (Vp) have been determined up to 2350 K on CaSiO3, MgSiO3, CaMgSi2O6, CaAl2Si2O8 and Ca3Al2Si3O12 glasses and melts from Brillouin‐scattering experiments made with a 180° backscattering geometry. At the glass transition, the decrease of Vp with increasing temperatures becomes much stronger and the width of the Brillouin lines begins to increase markedly. At the highest temperatures investigated, Vp is similar to the relaxed values determined in acoustic measurements for several of these melts. This indicates that the configurational degrees of freedom of these liquids have become accessible within the very short timescale of Brillouin scattering experiments: equilibrium compressibilities of silicate melts can thus be determined with this technique. Our measurements also suggest that the shear modulus at infinite frequency of silicate melts could vary with either temperature or composition more strongly than assumed currently.
Radial modes from the great 1994 Bolivian earthquake: No evidence for an isotropic component to the source
Authors: Okal Emile A.
Published: February 2004

We investigate a possible isotropic component in the source of the Bolivian earthquake of 09 June 1994 through a study of the radial modes pS0 (p = 0, 1) excited by the event. Our study departs from previous similar attempts by making use of a time series of sufficient duration (over 116 days) to sample the shape of the spectral line of the fundamental radial mode 0S0, and by combining measurements of two radial modes, thereby lifting the trade‐off between deviatoric and isotropic sources of excitation. Our result (an isotropic component of 0.5±1.3% of the main double‐couple) fails to identify a significant change in volume in the source of this, the largest deep earthquake ever recorded.
Observations of plasma waves during a traversal of the Moon's wake
Authors: Kellogg Paul J. , Goetz Keith , Monson Steven J. , Bougeret J.‐L. , Manning Robert , Kaiser M. L.
Published: February 2004

The Wind satellite passed through the solar wind wake of the moon at a distance of about 8 lunar radii on 27 Dec. 1994. Several different wave modes were observed, which we describe. Some of these modes are not certainly identifiable with modes of a uniform plasma, and are probably associated with the density discontinuity, and possibly with a velocity shear, at the boundary of the wake. Modes which are identifiable, at least tentatively, include electromagnetic waves at both fp and 2 fp from outside the wake, and a mode at the local plasma frequency of the wake.
Sharp 400‐km discontinuity from short‐period P reflections
Authors: Neele Filip
Published: February 2004

P‐wave data from the short‐period (around 1 Hz) Southern California Seismic Network reveal pre‐critical reflections from the 410‐km discontinuity below the Gulf of California, appearing as a clear and continuous feature in the data to epicentral distances as small as 10°. Deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving P waves. The reflections are used to estimate the width of the gradient zone near 410 km depth, constraining the velocity jump across the discontinuity using critically refracted waves. The velocity jump across the 410‐km discontinuity is 6±2.5 %, which is in agreement with existing estimates of the velocity jump for this region. The clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km. Synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km, with most of the velocity increase occurring over about 4 km or less, for the reflections to be visible in short‐period data. Applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least 410 km depth.
Partial melting beneath a Mid‐Atlantic Ridge Segment detected by teleseismic PKP delays
Authors: Forsyth Donald W.
Published: February 2004

An array of six ocean‐bottom seismometers deployed across a segment of the Mid‐Atlantic Ridge recorded PKP phases from four teleseismic earthquakes. After correcting for normal moveout and elevation differences, there is a systematic variation of anomalous travel time with distance from the ridge. Stations at the ridge‐axis are slowest. The average anomaly at a station 16 km from the axis is −0.36 s relative to the axial stations. Most of this anomaly is probably caused by the presence of partial melt beneath the ridge axis, perhaps concentrated primarily in the upper 30 km of the mantle. The rapid variation across‐axis suggests that the melt is either associated with a narrow zone of buoyant upwelling or a region of veins and dikes forming a narrow transport zone connecting a deep melt‐production region to the crust.
Effects of near‐surface waveguides on shallow high‐resolution seismic refraction and reflection data
Authors: Robertsson J. O. A. , Holliger K. , Green A. G. , Pugin A. , De Iaco R.
Published: February 2004

Waveguides formed between the earth's free‐surface and sharp subsurface velocity discontinuities, such as the watertable, may have a significant impact on high‐resolution seismic refraction and reflection data. Energy trapped in such waveguides may propagate over long distances with little loss due to geometrical spreading. The dispersive nature of the guided waves results in pronounced shingling of the first arrivals. This narrows the “optimum reflection window” between the first arrivals and the ground roll, and may lead to mispicking of first breaks and difficulties in identifying and processing shallow reflections.
Hourly variances of fluctuations in the heliospheric magnetic field out of the ecliptic plane
Authors: Forsyth R. J. , Horbury T. S. , Balogh A. , Smith E. J.
Published: February 2004

Magnetic field fluctuations at high heliographic latitudes are discussed in terms of the latitude and radial dependence of their variances, as observed by the Ulysses spacecraft. The increased level of transverse fluctuations in fast solar wind flows from the polar coronal holes is similar to that found in high speed streams near the ecliptic. However, continuous observations in the fast solar wind both southward and northward of the solar equatorial plane has allowed the determination of the radial dependence of fluctuations on hourly and shorter timescales, free from additional energy input from stream‐stream interactions or from coronal mass ejections and in the presence of a slowly evolving, low frequency Alfvénic population. The conclusion is that fluctuations in the inertial range in the homogeneous polar coronal flows decay faster than predicted by the WKB approximation. However, even minor compression effects can increase the energy in these fluctuations.
Dynamic inversion of ground deformation data for modelling volcanic sources (Etna 1991–93)
Authors: Bonaccorso Alessandro
Published: February 2004

The 1991–93 Etna eruption, that represented the most important recent eruptive event both in terms of duration (472 days) and total volume of erupted lava (ca. 250 · 106 m³), caused marked ground deformation measured by using different geodetic techniques such as EDM, GPS, levelling, and tiltmetry. An inversion, termed “dynamic” because it took into account the sequence of events considering both the separate and cumulative effects of two sources, (a shallow tensile crack and a deeper ellipsoidal deflating source), was performed. It was based on 163 data collected by the different techniques plus 2 geometrical conditions for constraining the geometry of the crack on the surface. The final solution furnished a good fit for all measurements, defining a shallow tensile crack located inside the volcanic edifice, which represents the effect of the intrusion, and a depressuring body with centre at ca. 3000 m b.s.l., which could represent the centre of a generalized and more complex deflation of the volcanic edifice during the course of the eruption.
Ocean bottom microseisms from a distant supertyphoon
Authors: Sutton George H. , Barstow Noel
Published: February 2004

In this paper we describe microseisms and swell produced by a supertyphoon in the western Pacific, near the Philippine Islands, recorded from an ocean‐bottom seismic ‘station’ (OBSS) located about 200 km west of San Francisco, California at a depth of about 4 km and from a pressure sensor installed in shallow water near Pt. Arena, California. The microseisms recorded on OBSS are generated by the swell in the vicinity of the station and narrow‐band spectral peaks observed with frequency increasing slowly with time from the ocean‐wave dispersion correlate well, at twice the frequency, with swell near Pt. Arena. Similar, previously reported, observations at OBSS from storms in the northeastern Pacific are consistent with fundamental Rayleigh wave propagation, toward the coast in that case and away in the present, possibly related to a large difference in the estimated width of the zone of interaction between incident and reflected swell along the California coast.
P‐SV wave propagation in the Earth's mantle using finite differences: Application to heterogeneous lowermost mantle structure
Authors: Igel Heiner , Weber Michael
Published: February 2004

We solve the elastic wave equation in spherical coordinates {γ,φ,θ} by a high‐order finite‐difference (FD) scheme. The Earth model and the required fields are defined on a staggered grid, independent in φ, and thus rotationally symmetric with respect to the axis θ = 0,π. This scheme allows us to model P‐SV wave propagation in a heterogeneous two‐dimensional Earth model. Since a uniform grid spacing in γ and θ is used, the maximum depth and epicentral distance that can be modeled is limited. Comparison with seismograms obtained by the Reflectivity Method (RM), and the Direct Solution Method (DSM), demonstrates the accuracy of the FD scheme. We use this algorithm to study the effects of heterogeneities in the core‐mantle transition zone, the D″ layer, on long‐period P‐waves. Long‐wavelength topography of a reflector in D″ produces significant focusing and defocusing. Random fluctuations (maximum perturbation ±10%) in a D″ layer of 300 km thickness produce a wave‐field similar to that of a sharp discontinuity only 200 km above the coremantle boundary (CMB) at the dominant period considered (15 seconds). Maps of global variations of D″ thickness determined with long‐period data may therefore be severely biased.
Sprites as luminous columns of ionization produced by quasi‐electrostatic thundercloud fields
Authors: Pasko V. P. , Inan U. S. , Bell T. F.
Published: February 2004

Quasi‐electrostatic (QE) fields which exist above thunderclouds after lightning discharges can lead to the formation of columnar channels of breakdown ionization and carrot‐like vertical luminous structures with typical transverse dimension ∼5–10 km spaning an altitude range from ∼80 km to well below ∼50 km. The carrot‐like forms closely resemble those observed in sprites. Results indicate that the appearance of optical emissions can be significantly delayed in time (∼1–20 ms) with respect to the causative lightning discharge.
Lateral variations of Pn propagation in Italy: Evidence for a high‐attenuation zone beneath the Apennines
Authors: Mele Giuliana , Rovelli Antonio , Seber Dogan , Barazangi Muawia
Published: February 2004

Pn phases recorded by 40 stations of the Italian seismic network are analyzed using the spectral ratio technique to estimate the Q structure of the uppermost mantle beneath the Italian peninsula and nearby Adriatic Sea. A total of 344 digital waveforms are analyzed from 22 earthquakes that occurred within distances of 300 to 1600 km. The calculated apparent Q values are grouped into two categories: Q > 800 characterizes the Adriatic side of the Italian peninsula, indicating that the Adriatic lithosphere is very efficient in propagating Pn phases through the uppermost mantle; Q < 600 characterizes the uppermost mantle beneath the Apennines and western Italy, indicating less efficient wave propagation. The presence of asthenospheric mantle material at shallow depths beneath the Apennines can explain the observed Q.
P‐velocity structure of the core‐mantle boundary region inferred from PKP(AB)‐PKP(BC) differential travel times
Authors: Sylvander Matthieu , Souriau Annie
Published: February 2004

PKP(AB)‐PKP(BC) differential travel time residuals from the bulletins of the International Seismological Centre are inverted to retrieve the P‐velocity anomalies in the lowermost 300 km of the mantle. We show that a realistic core‐mantle boundary (CMB) topography may have only a marginal contribution to the residuals. The block parametrization used allows us to discard poorly sampled areas prior to a least‐squares inversion. The main features are negative anomalies beneath Eurasia and North America and a positive anomaly beneath the Southwest Pacific. The inferred map is not consistent with other maps resulting from previous P‐ and S‐velocity studies, which are correlated at long wavelengths with each other. The preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the CMB.
SCIFER‐Dayside auroral signatures of magnetospheric energetic electrons
Authors: Lorentzen D. A. , Deehr C. S. , Minow J. I. , Smith R. W. , Stenbaek‐Neielsen H. C. , Sigernes F. , Arnoldy R. L. , Lynch K.
Published: February 2004

The SCIFER sounding rocket was launched over the dayside aurora, at 10 hr Magnetic Local Time (MLT) on January 25, 1995. Meridian‐scanning photometers (MSP) and all‐sky television (ASTV) systems were operated at Longyearbyen (LYR) and Ny‐Ålesund (NYA) on Svalbard under the flight apogee to facilitate the launch decision and identify the ionospheric signatures of the various energetic particle populations observed at the rocket. The characteristics of the 0.3 eV–15.5 KeV electron populations producing the observed aurora were identified from the time, energy and pitch angle electron spectrometer data throughout the SCIFER flight. The optical data clearly showed the location of the trapping boundary seen by the SCIFER energetic electron spectra. Equatorward of the boundary, pulsating patches of auroral luminosity corresponded in pulsation period and location to the >4 keV energy‐dispersed electrons observed at SCIFER. The 10 eV electron flux rather uniformly distributed equatorward from the trapping boundary is accounted for by photoelectrons from the conjugate region, producing most of the 6300 Å [OI] emission observed south of the zenith in the MSP data. Poleward of the trapping boundary, relatively bright, discrete arcs and bands typical of the dayside auroral oval were observed. These corresponded to the inverted V and field‐aligned electron populations which were observed at SCIFER to be < 1 keV characteristic energy and to range from 2 to 5 ergs cm−2 sec−1. The electron populations and the resulting arcs were remarkably similar to those observed on the high latitude nightside (poleward of the trapping boundary) but with obviously different source region. There was no persistent 6300 Å [OI] auroral emission observed poleward of the trapping boundary consistent with the particle observations. Hydrogen emissions were associated with the discrete aurora, indicating that the main proton energy flux was poleward of the trapping boundary.
Viking observations of a reverse convection cell developing in response to a northward turning of the interplanetary magnetic field
Authors: Henderson M. G. , Murphree J. S.
Published: February 2004

Observations of a developing reverse convection cell on September 23, 1986 are examined using auroral images acquired with the UV imager on board the Viking Spacecraft. The cell possesses a counter‐clockwise rotational sense and evolves on the duskside of the northern auroral distribution between the duskside oval and a complex duskside transpolar arc system. As the structure evolves, it grows and appears to displace the transpolar arc toward the dawn. We interpret these observations in terms of a model proposed by Burch et al., [1992] in which open field lines produced by merging on the high latitude magnetopause produce reverse convection cells between the oval and the transpolar arcs associated with horse‐collar or teardrop auroral patterns.
Structuring of the transition region (plasma mantle) of the Martian magnetosphere
Authors: Dubinin E. M. , Sauer K. , Lundin R. , Baumgärtel K. , Bogdanov A.
Published: February 2004

Measurements made by the ASPERA plasma instrument onboard the Phobos‐2 spacecraft have shown that the transition from the Martian magnetosheath occupied mainly by the shocked solar wind to the boundary layer dominated by planetary plasma occurs by a sequence of step‐like drops and jumps of the proton bulk velocity and the magnetic field value. The plasma is stratified on alternating layers occupied either by protons or by heavy ions. The resemblance with shock‐like structures is emphasized by the fact of proton temperature enhancements which replicate drops of the bulk velocity. The observed striations are considered as peculiar discontinuities typical for multi‐ion plasma, or flute‐like modes which may be excited at the boundary of the Martian magnetosphere.
Refractive indices of aerosols in the upper troposphere and lower stratosphere
Authors: Baumgardner Darrel , Dye James E. , Gandrud Bruce , Barr Keith , Kelly Ken , Chan K. Roland
Published: February 2004

A new instrument for simultaneously measuring aerosol diameter from 0.4–10 µm and the refractive index between 1.30–1.60 has recently been flown on the NASA ER‐2 aircraft during a stratospheric measurement campaign. Average stratospheric refractive indices varied from 1.40 to 1.42 over a latitude range from 70°S to 50°N and from 1.34 to 1.46 over a vertical range from 4–20 km. The measured stratospheric refractive indices do not agree well with theoretical predictions and vertical profiles suggest the presence of non‐spherical or absorbing particles in the altitude range of 7–9 km.
Dehydration of the upper troposphere and lower stratosphere by subvisible cirrus clouds near the tropical tropopause
Authors: Jensen Eric J. , Toon Owen B. , Pfister Leonard , Selkirk Henry B.
Published: February 2004

The extreme dryness of the lower stratosphere is believed to be caused by freeze‐drying of air as it enters the stratosphere through the cold tropical tropopause. Previous investigations have been focused on dehydration occurring at the tops of deep convective cloud systems. However, recent observations of a ubiquitous stratiform cirrus cloud layer near the tropical tropopause suggest the possibility of dehydration as air is slowly lifted by large‐scale motions. In this study, we have evaluated this possibility using a detailed ice cloud model. Simulations of ice cloud formation in the temperature minima of gravity waves (wave periods of 1–2 hours) indicate that large numbers of ice crystals will likely form due to the low temperatures and rapid cooling. As a result, the crystals do not grow larger than about 10 µm, fallspeeds are no greater than a few cm‐s−1, and little or no precipitation or dehydration occurs. However, ice clouds formed by large‐scale vertical motions (with lifetimes of a day or more) should have fewer crystals and more time for crystal sedimentation to occur, resulting in water vapor depletions as large as 1 ppmv near the tropopause. We suggest that gradual lifting near the tropical tropopause, accompanied by formation of thin cirrus, may account for the dehydration.
Antarctic Ice Sheet melting in the southeast Pacific
Authors: Jacobs Stanley S. , Hellmer Hartmut H. , Jenkins Adrian
Published: February 2004

The first oceanographic measurements across a deep channel beneath the calving front of Pine Island Glacier reveal a sub‐ice circulation driven by basal melting of 10–12 m yr−1. A salt box model described here gives a melt rate similar to that of ice balance and numerical models, 5–50 times higher than averages for the George VI and Ross Ice Shelves. Melting is fueled by relatively warm Circumpolar Deep Water that floods the deep floor of the Amundsen and Bellingshausen Sea continental shelves, reaching the deep draft of this floating glacier. A revised melt rate for ice shelves in the Southeast Pacific sector raises circumpolar ice shelf melting to 756 Gt yr−1. Given prior estimates of surface accumulation and iceberg calving, this suggests that the Antarctic Ice Sheet is currently losing mass to the ocean.
Can hotter mantle have a larger viscosity?
Authors: Solomatov V. S.
Published: February 2004

In the diffusion creep regime which is believed to be appropriate for the lower mantle of the Earth, the viscosity of rocks depends on the grain size. The grain size is normally assumed to be constant in mantle convection models. However, it is controlled by the kinetics of polymorphic phase transformations and post‐transformation grain growth and can vary substantially. A simple scaling analysis demonstrates one particular feature of the interaction of rheology, convection and kinetics: the mantle resembles a temperature‐dependent viscosity fluid with an extremely unusual apparent activation enthalpy for the viscosity. It can take almost any value including negative so that hotter regions can have a larger viscosity or the averaged viscosity of the mantle can increase during Earth's secular cooling. This affects mantle dynamics and evolution and should also be taken into account in interpretating the viscosity variations in the mantle.
Effects of shear stress applied to surfaces in stationary contact on rock friction
Authors: Nakatani Masao , Mochizuki Hiromine
Published: February 2004

In this paper we show how the applied shear stress affects the frictional strength of granite sliding surfaces initially held in stationary contact. We develop an experimental scheme to perform a slide and hold friction test whereby the shear stress is servocontrolled during the hold period of the test. A series of experiments conducted at a single normal stress of 5 MPa, show the following effects. First, unloading the shear stress during the hold period increases frictional resistance by up to 10–20%; a decrease in shear stress linearly increases the frictional resistance of the interface. Secondly, the static friction coefficient increases with the logarithm of contact time. The friction coefficient increases by 0.01 per decade of contact time at near zero shear stress levels, and by more than 0.02 at shear stress levels that are greater than half the sliding friction.