bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of decay phases of solar energetic particle events at 1 and 5 AU from the Sun
Authors: Dalla S. , Balogh A. , Heber B. , Lopate C. , McKibben R. B.
Published: December 2002

The slow decay phase of gradual solar energetic particle (SEP) events has been interpreted as an indication of particle reservoirs being established in the inner heliosphere. The same phenomenon is sometimes termed spectral invariance and explained in terms of a magnetic bottle effect, whereby a barrier impedes particle escape. In alternative to the above picture, decay‐phase SEPs have been ascribed to (1) continuous acceleration at an interplanetary shock front, (2) interplanetary scattering, or (3) leakage from the solar atmosphere over several days. In this paper we investigate two large gradual SEP events characterized by comparable signatures at 1 AU from the Sun. We use measurements at 1 AU made by the ACE and IMP8 spacecraft, and at 5.2 AU by the Ulysses spacecraft. At 5.2 AU, the ∼MeV proton intensities during the decay phase of the two events are found to have strikingly different profiles, showing in one case a long‐duration smooth decay and in the other a depletion in particle intensity. We discuss how the four above mentioned models on the origin of decay‐phase SEPs would interpret the observations.
Multistage substorm expansion: Auroral dynamics in relation to plasma sheet particle injection, precipitation, and plasma convection
Authors: Sandholt Per Even , Farrugia Charles J. , Lester Mark , Cowley Stan , Milan Steve , Denig William F. , Lybekk Bjørn , Trondsen Espen , Vorobjev Vjacheslav
Published: December 2002

We present observations of the auroral expansions during two substorms, focusing on multistage intensifications and the morphology of the poleward boundary, and relate these auroral observations to the local plasma convection and plasma sheet dynamics. The observations are made by meridian scanning photometers and an all‐sky camera (ASC) at Ny Ålesund, Svalbard (76° magnetic latitude (MLAT)), an ASC in Lovozero, Russia (64° MLAT), the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer chain in Svalbard and Scandinavia, the HYDRA instrument on Polar located at the inner edge of the plasma sheet, particle detectors on DMSP F13 and DMSP F14 traversing the ionospheric projection of the plasma sheet, and the CUTLASS Finland HF radar. In each substorm the aurora between 70° and 80° MLAT consisted of two branches separated by ∼5° in MLAT. The higher‐latitude branch (at ∼75°–78°MLAT) was subject to a sequence of short‐lived (∼1–2 min) intensifications, so‐called “poleward boundary intensifications” (PBIs), recurring at ∼3‐min intervals. Subsequent to each brightening, auroral forms traveled equatorward at a speed of ∼1.0–1.5 km s−1. On Polar the PBIs are related on a one‐to‐one basis with injections of electrons in the 5‐ to 20‐keV energy range at the inner edge of the equatorial plasma sheet with predominantly a trapped distribution, delayed by ∼5 min. Electron precipitation within 60°–77° MLAT, corresponding to a large radial extent of the plasma sheet, is documented by DMSP flights in the 1800–2000 magnetic local time (MLT) sector. In discussing the branches of the high‐latitude aurora within the context of current understanding of the relation of bursty bulk flows to substorm expansion phase dynamics, we note the following: (1) the initial auroral breakup located at 63°–64° MLAT near the equatorward edge of plasma sheet precipitation, which was followed by (2) two successive brightenings/auroral expansions appearing within 72°–74° MLAT/∼2100 MLT, separated by 14 min, (3) a 20‐min‐long brightening sequence in the poleward auroral branch (75°–78° MLAT), consisting of six discrete events (PBIs) within the boundary plasma sheet precipitation, and (4) the presence of auroral vortex motion/strong field‐aligned current sheets in some of these PBIs, which were accompanied by (5) electron injections at the inner edge of the plasma sheet, (6) brightenings of the lower‐latitude auroral branch when equatorward moving auroral forms (EMAFs/streamers) arrive there, and (7) localized bursts of equatorward ionospheric convection at speeds of 0.5–1 km s−1 in the latitude range of the EMAFs/streamers. The documented associations between PBIs/EMAFs, plasma sheet injections, and the local convection events are explained in terms of a substorm scenario involving bursty bulk flows in the late expansion phase.
Electromagnetic ion cyclotron waves at proton cyclotron harmonics
Authors: Chaston C. C. , Bonnell J. W. , McFadden J. P. , Ergun R. E. , Carlson C. W.
Published: December 2002

Waves with frequencies in the vicinity of the proton cyclotron frequency and its harmonics are commonly observed from the Fast Auroral Snapshot spacecraft when traversing regions of auroral particle acceleration. In areas of upward current, large‐amplitude electromagnetic waves with frequencies within 5% of the local proton gyrofrequency Ωp and its harmonics are often observed where upstreaming ion beams exist. These waves have electric field (E1) and magnetic field (B1) amplitudes of up to 1 V m−1 and 2 nT with the ratio E1/B1 as small as c. The waves occur in the low‐altitude portion of the primary auroral acceleration potential, where plasma densities are ≤1 cm−3. It is shown how these waves grow through inverse Landau resonance with a cold field‐aligned electron beam superimposed on an accelerated and magnetically mirrored plasma sheet electron component in the absence of any significant plasma densities at energies below ∼100 eV. Significantly, the drift velocity of the cold beam (voeb) is several times larger than its thermal velocity veb, and it is this feature that allows the wave to become electromagnetic at cyclotron harmonics while simultaneously giving rise to broadband electrostatic emissions spanning the first few cyclotron harmonics as is observed.
Mechanical compaction of sand/clay mixtures
Authors: Revil A. , Grauls D. , Brévart O.
Published: December 2002

Compaction of siliciclastic sediments is of interest for the study of numerous transport processes occurring in sedimentary basins. Mechanical compaction of sand/clay mixtures depends on the clay content, the effective stress history, and both the mechanical compaction coefficients and the depositional porosities of the two end‐members (clean sand and pure shale). The porosity/depth profiles of siliciclastic sediments result from the superposition of two kinds of spatial variations. The first component corresponds to compaction of the mixture with depth of burial. The second component corresponds to small‐scale variations of the clay content during deposition and clay infiltration processes. The porosity/depth data are bounded by a minimum porosity/depth envelope corresponding to clay content at the limit between the shaly sand domain and the sandy shale domain. There are two possible upper bounds corresponding to small or large clay contents. The porosity of the clean sand end‐member decreases with the depth of burial until a critical porosity of 0.25–0.40 is reached. This critical porosity corresponds to the porosity of random assemblages of more or less spherical grains and depends on the grain‐size distribution of the sand grain assemblage. The critical porosity of the shale end‐member is much smaller because of the high aspect ratio of the clay particles. The model developed here is applied to downhole measurements made in a borehole that penetrates 3 km of Mio‐Pleistocene shaly sand series. The compaction model agrees well with the observed compaction profile.
On the causes of biased estimates of seismic moment for earthquakes in central Asia
Authors: Patton Howard J. , Randall George E.
Published: December 2002

Comparisons of seismic moment M0 in Harvard's centroid moment tensor (CMT) catalog with M0 estimated from regional surface waves reveal a systematic discrepancy of about a factor of 2 for earthquakes located in western China. Causes of the discrepancy are investigated with analyses of observations and with calculations of Green's function responses for relevant Earth models. The PREM model used in CMT inversions is characterized by a thin, high‐velocity crust, unlike tectonic structures in western China, while ASIA, used for modeling surface waves, was developed for this region. Analyses of teleseismic body wave synthetics for models similar to PREM and ASIA show that medium properties, source radiation patterns, and the presence of interfering phases introduce systematic amplitude variations. Inversions of synthetic data were carried out to simulate features of observed M0 residuals, log[M0(reg)/M0(CMT)], which are (1) a bias of −0.24 ± 0.04 log units for dip‐slip events with mb 6 to 6.8, (2) a tendency for bias to increase with decreasing mb, and (3) a mechanism dependence, where residuals are large for high‐angle, dip‐slip events and small for vertical strike‐slip events. Residuals for small events scatter more, probably due to larger errors in source depth and mechanism affecting both M0 estimates. Results of inverting synthetics mimic magnitude and mechanism dependences but underestimate the bias in feature 1. Rather than impugn the integrity of CMT catalogs, this study sheds light on uncertainties introduced by limitations of the methodology.
Effects of thermal history on crystal nucleation in silicate melt: Numerical simulations
Authors: Davis Mark J. , Ihinger Phillip D.
Published: December 2002

We have used a numerical treatment to investigate time‐dependent phenomena associated with transient crystal nucleation in a model silicate melt. Our approach is the first to explicitly account for both thermodynamic (surface and bulk free energies) and kinetic (equilibrium and nonequilibrium transport) driving forces in molten silicates. The degree of undercooling, transient nucleation behavior, and structural relaxation effects are included in our model. Our model can simulate a variety of natural processes, ranging from the rapid cooling of fragmenting melt during a volcanic eruption to the slow devitrification of a chilled lava flow. To illustrate the utility and generality of our approach, we present numerical solutions for T–t paths that mimic the conditions of experimental studies on the one‐component lithium disilicate system [Davis et al., 1997]. Our numerical simulations suggest that the formation of a finite number of crystalline clusters occurs on quench of starting materials. Nucleation is enhanced initially by preexisting nuclei when quenched glasses are subjected to a nucleation treatment. The simulations further suggest that the effect of preexisting nuclei, cooling rate, and nonequilibrium viscosity becomes negligible for longer nucleation treatments (>1000 min). We also provide simulation results for the geologically relevant physical model of the devitrification of a lava flow following burial by a new flow, including the calculation of the crystal size distribution (CSD). This example exhibits several interesting features, indicating multiple nucleation events, negative nucleation rates, and widely disparate final CSDs depending on the position within the flow.
Interaction between faulting and sedimentation in the Sea of Marmara, western Turkey
Authors: Parke J. R. , White R. S. , McKenzie D. , Minshull T. A. , Bull J. M. , Kuşçu I. , Görür N. , Şengör C.
Published: December 2002

The Aegean region is one of the most rapidly deforming continental areas in the world. In the Sea of Marmara, interaction between strike‐slip and extensional faulting has led to a complicated style of local deformation. To the east, the continental plate containing eastern and central Turkey is moving westward with respect to Eurasia, with little internal deformation. Tectonic deformation is restricted to a narrow zone along the dominantly strike‐slip North Anatolian fault. However, in the vicinity of the Sea of Marmara, both surface mapping of faults and earthquake fault plane solutions indicate that extensional motions, presumably related to back arc extension behind the Hellenic subduction zone, create a much wider zone of deformation which reaches hundreds of kilometers in width. We use a regional grid of high‐resolution seismic reflection profiles to map the fault systems as they cross the Sea of Marmara and show how they divide the Sea of Marmara into separate deep basins with distinctive sediment sources and depositional styles. In the southern Sea of Marmara these basins are half graben, formed on north dipping fault planes, which have trapped sediment coming into the Sea of Marmara from the south. No evidence is found in the data set for the existence of a single strike‐slip fault through the Sea of Marmara or for the existence of a northern boundary fault along the Tekirdağ and Central Marmara basins. We conclude that a single strike‐slip fault would be incapable of accommodating the relative motion and extension observed between western Turkey and Eurasia.
Toward global inverse solutions for current and past ice mass variations: Contribution of secular satellite gravity and topography change measurements
Authors: Wu Xiaoping , Watkins Michael M. , Ivins Erik R. , Kwok Ronald , Wang Ping , Wahr John M.
Published: December 2002

We investigate the accuracy and resolution of estimating global ice mass variations using secular gravity and ice elevation change observations by the planned Gravity Recovery and Climate Experiment (GRACE) and ICESat missions. The gravity and altimetry data combination can determine the total bedrock uplift rate with root‐mean‐square (RMS) errors of 0.2–1.3 mm/yr when Gaussian averaged over a half scale of 250 km. Moreover, covariance analyses indicate that the mean snow compaction error can be constrained to the level of 3 mm/yr in equivalent Antarctic ice thickness change. The mean Antarctic present‐day ice mass change could then be determined to about 4 mm/yr. For a known viscosity profile, such data sets provide a spatial estimate of a linear combination of the late Pleistocene deglaciation and a more recent past trend (last few thousand years) in the Greenland and Antarctica mass balance. For example, when the linear rate of the recent past trend is estimated solely for ice history, the RMS errors of the 450‐km Gaussian averages range between 1 and 3 cm/yr. Although contaminated by the signature of present‐day hydrological variations over the deglaciated areas, the 450‐km Gaussian‐averaged Last Glacial Maximum height of the ancient ice sheets can be determined to a RMS level of 300 m. The mean recovery errors are less than 100 m. However, the gravity/altimeter data cannot resolve the load history and lower mantle viscosity simultaneously. Additional data such as relative sea level records must be invoked to overcome this difficulty.
Intensity of the geomagnetic field in western Europe over the past 2000 years: New data from ancient French pottery
Authors: Genevey Agnès , Gallet Yves
Published: December 2002

We studied 14 groups of French pottery fragments dated between the 4th and 16th centuries. The potsherds were analyzed using the Thellier and Thellier [1959] method, revised by Coe [1967]. Intensity values were corrected for thermoremanent magnetization (TRM) anisotropy and cooling rate dependence of TRM acquisition. We first analyzed modern ceramics produced following ancient techniques and fired in a wood‐burning kiln inside of which field intensity was measured. The recovered mean intensity is within ∼3% of the expected value, which proves the reliability of our experimental procedure. Thermal experiments carried out at rapid and slow cooling rates clearly indicate that the cooling rate correction is critical in archeointensity studies. Our data indicate that large variations in intensity occurred in France over the last 2000 years. Two relative maxima in intensity are observed, one between the 8th and 10th centuries and the second between the 14th and 15th centuries. Similarities are observed between the archeointensity data from France and Ukraine, yielding some evidence for eastward drift of geomagnetic sources between western and eastern Europe from A.D. 800 to A.D. 1700. We also show that the dipole moment evolution proposed by McElhinny and Senanayake [1982] and Yang et al. [2000] for the last two millennia is likely biased toward higher values, mainly because of the absence of correction for the cooling rate dependence of TRM acquisition in most published archeointensity studies. We finally underline a possible relationship, valid at least in western Europe, between changes in direction and intensity of the geomagnetic field.
Active source tomography at Mt. Vesuvius: Constraints for the magmatic system
Authors: Di Stefano Raffaele , Chiarabba Claudio
Published: December 2002

Recently, Mt. Vesuvius has been the subject of renewed seismological studies designed to allow the definition of the subsurface structure, focusing on the location and geometry of a presumed shallow magma chamber. The high volcanic risk associated with the activity of Mt. Vesuvius motivates detailed studies to trace a possible magmatic supply at shallow depth that could feed future eruptions. In 1994 and 1996 two active seismic surveys were carried out by an international team of scientists. Body waves from 17 shots executed around the volcano were recorded by dense arrays of seismic stations located across the volcano and extending to the surrounding Apenninic belt. These experiments encountered logistic difficulties due to the highly populated area; nevertheless, a good‐quality data set of waveforms was collected. In this work we present the three‐dimensional P wave velocity structure of the upper crust beneath Mt. Vesuvius, obtained through tomographic inversion of data collected during the TomoVes active seismic surveys. Due to the geometry of the problem, we used a tomographic technique that allows ray tracing in a complex model with both sharp velocity discontinuities and velocity gradients. The shot and station distribution and the presence of a very shallow velocity discontinuity, producing head wave first arrivals, limited the analysis to the upper 3 km of the crust. The images we obtained show a well‐resolved, main axial high‐Vp body interpreted as a magmatic intrusion remnant of recent eruptions from the central crater. Lateral branches depart from the central anomaly, possibly indicating the path followed by the magma during lateral eruptions that occurred in the past.
The Mw 8.2, 17 February 1996 Biak, Indonesia, earthquake: Rupture history, aftershocks, and fault plane properties
Authors: Henry C. , Das S.
Published: December 2002

We analyze the rupture history of this earthquake, the largest thrust earthquake since 1977, primarily using broadband SH wave seismograms. We show that the earthquake occurred on a shallow dipping thrust fault (strike 109°, dip 9°, rake 72°) with a seismic moment of 2.7 × 1021 N m. The rupture propagated bilaterally at an average speed of ∼90% of the shear wave speed, on a fault extending 180 km west and 50 km east of the hypocenter, with very variable width ranging from 30 to 100 km at different locations along strike. The mean slip over a 230 km × 100 km fault area was 4 m and the mean stress drop 1.9 MPa. The slip distribution is very nonuniform over the fault, with the largest slip of ∼12 m being near the hypocentral depth (∼10 km). The rupture is very complex, propagating first to the west and then, after a ∼15‐s delay, to the east. We interpret this delay as being due to the existence of a inhomogeneous barrier just east of the hypocenter, which initially acted as a barrier to rupture propagation toward the east, but subsequently failed due to stress increase on it generated by the rupture to its west and with the rupture then continuing to propagate eastward. This barrier acted as a stress concentrator before the earthquake, giving rise to foreshocks and was also the initiator of rupture for this earthquake. The aftershock zone closely corresponds to the region in which rupture occurred, the area with greatest aftershock density lying entirely within the area of highest moment release.
Physical properties of liquid Fe alloys at high pressure and their bearings on the nature of metallic planetary cores
Authors: Sanloup C. , Guyot F. , Gillet P. , Fei Y.
Published: December 2002

Sulfur and silicon are among the expected alloying light elements in planetary liquid iron cores. Structural properties of Fe‐27 wt % S and Fe‐17 wt % Si liquid alloys at high pressure and high temperature (0–5 GPa/1400–2300 K) are measured by synchrotron X‐ray diffraction. Sulfur strongly modifies the local structure of liquid iron whereas silicon has only small structural effects. Fe‐27 wt % S melts are indeed poorly ordered which explains a higher compressibility compared to pure liquid Fe. These results point out the necessity to consider the strong effect of S on liquid Fe properties while modeling planetary interiors. They imply a low S content in the Earth's outer core, leaving Si as a strong candidate, and argue for a present‐day Martian solid core when combined with previous global chemical models.
Deformation of polycrystalline MgO at pressures of the lower mantle
Authors: Merkel Sébastien , Wenk Hans Rudolf , Shu Jinfu , Shen Guoyin , Gillet Philippe , Mao Ho‐kwang , Hemley Russell J.
Published: December 2002

Room temperature investigations on the shear strength, elastic moduli, elastic anisotropy, and deformation mechanisms of MgO (periclase) are performed in situ up to pressures of 47 GPa using radial X‐ray diffraction and the diamond anvil cell. The calculated elastic moduli are in agreement with previous Brillouin spectroscopy studies. The uniaxial stress component in the polycrystalline MgO sample is found to increase rapidly to 8.5(±1) GPa at a pressure of 10(±1) GPa in all experiments. Under axial compression, a strong cube texture develops which was recorded in situ. It is probable that the preferred orientation of MgO is due to deformation by slip. A comparison between the experimental textures and results from polycrystal plasticity suggest that the {110} is the only significantly active slip system under very high confining pressure at room temperature. These data demonstrate the feasibility of analyzing elastic moduli, shear strength, and deformation mechanisms under pressures relevant for the Earth's lower mantle. Implications for the anisotropy and rheology of the lower mantle are discussed.
Morphostructural development of the Eritrean rift flank (southern Red Sea) inferred from apatite fission track analysis
Authors: Abbate Ernesto , Balestrieri Maria Laura , Bigazzi Giulio
Published: December 2002

The Red Sea is one of the best exposed young rift basins in the world. Its flanks on both the African and Arabian sides are characterized by basement uplifts parallel to the margins and by active erosion. Through the integration of 37 new apatite fission track (FT) analyses and regional geology, we elucidate the uplift and denudational history of the Eritrean continental margin along the southern Red Sea and, in particular, the development, timing, and past and present morphostructural features of its onshore portion. FT data indicate that at around 20 Ma, the Eritrean margin was affected by a crustal cooling event due to a postrifting accelerated phase of denudation. This cooling has the same age as those already detected on the conjugate Arabian margin (Yemen and Saudi Arabia). FT ages increase from 10–20 to 300–400 Ma with increasing distance and elevation from the coastal areas toward the interior. This trend indicates a diminishing amount of eroded section in the same direction. We use FT and structural data to define the position of the main border fault along the margin where the original scarp was located. We estimate rates of vertical denudation of 190–200 m/Myr at the border fault and of ≤60–70 m/Myr on the plateau. On the basis of these new data, we argue against an Oligocene tectonic unroofing of the margin through a low‐angle detachment assumed by previous authors, but we maintain the essential role of the erosional denudation in the development of present margin morphology.
Changes in west Antarctic ice stream velocities: Observation and analysis
Authors: Joughin Ian , Tulaczyk Slawek , Bindschadler Robert , Price Stephen F.
Published: December 2002

We have produced a map of velocity covering much of the Siple Coast ice streams. The map confirms earlier estimates of deceleration on Whillans Ice Stream. Comparison with bed elevation data indicates that subglacial topography and the location of consolidated sediment play a strong role in determining the location of the tributaries feeding the ice streams. Force balance estimates based on these data indicate that the tributaries have beds nearly an order of magnitude stronger than those beneath many of the ice streams. We have used a theoretical analysis to examine the controls on fast flow. This analysis suggests that ice plains (very wide ice streams) are inherently unstable. This instability may be responsible for the current deceleration on the Ice Plain of Whillans Ice Stream and the shutdown of Ice Stream C 150 years ago. Thinning‐induced reductions in driving stress may also explain some of the observed deceleration, particularly in upstream areas. The active portions of Ice Stream C coincide well with the areas where we estimate that melt should be taking place. Current topography and inferences of large thickening following a shutdown suggest the upstream migration of a stagnation front that initiated at the ice plain. Uncertainty remains about the basal conditions on Ice Stream D, while the basal resistance on Ice Stream E is large enough to ensure basal melting.
The40Ar‐39Ar laser analysis of K‐feldspar: Constraints on the uplift history of the Grenville Province in Ontario and New York
Authors: Streepey Margaret M. , Hall Chris M. , van der Pluijm Ben A.
Published: December 2002

A comprehensive geochronologic database describes a period of late extension across the Metasedimentary Belt (MB) of the Grenville Province in northeastern North America. Because extension continued through much of the young unroofing history of the region, these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes. Analysis of K‐feldspar in the MB by the40Ar‐39Ar method can give information on the very latest perturbations in the area since K‐feldspars have multiple, low closure temperatures, ranging from about 150° to 300°C. These multiple diffusion domains require temperature‐time modeling to fully interpret argon release spectra. This modeling requires precise knowledge of the temperature of degassing of each step, which usually requires the use of a resistance furnace for sample analysis. We establish a first‐order relationship between laser power and temperature, which can be used for multidiffusion domain modeling of K‐feldspars. Comparison of samples analyzed by both methods reveals virtually no difference between the systems, supporting the validity of K‐feldspar analyses by laser step heating. Our data suggest that using the laser for K‐feldspars can give results that are geologically reasonable, precise, and easier to collect. In this case, these data are then applied to the cooling history of the North American Grenville Province. Our K‐feldspar analyses show that the latest extensional motion along shear zones in the MB is after 900 Ma, and the region is uplifting as a uniform block by 780 Ma.
A multicomponent coupled model of glacier hydrology 2. Application to Trapridge Glacier, Yukon, Canada
Authors: Flowers Gwenn E. , Clarke Garry K. C.
Published: December 2002

A new glaciohydraulic model is tailored to Trapridge Glacier, Yukon, Canada, where long‐term measurements of subglacial water pressure provide parameter constraints and a platform for model evaluation. Using digital elevation models and meteorological data as input, we investigate hydrology on diurnal and seasonal timescales. Quantitative comparisons of simulated and observed pressure records allow us to select a reference model objectively, from which we gain first insights into the gross spatial characteristics of the drainage system. Equilibrium simulations highlight target areas for water storage and potential sticky spots, both of which have established relevance to glacier dynamics. Seasonal simulations capture the key signatures of spring and autumn transitions and corroborate our understanding of the processes involved. Successfully modeled features of the spring transition include increased hydraulic coupling between the surface and bed with time, temporary augmentation of subglacial storage, and rapid subglacial transitions from a hydraulically unconnected to connected state. Autumn transitions are characterized by deterioration of the basal drainage network, drawdown of subglacial storage reservoirs, and heightened sensitivity to precipitation and surface refreezing. Finally, we draw attention to the important role of groundwater transport in cold glacial environments where permafrost can impede drainage from the ice margin.
Episodic thermal perturbations associated with groundwater flow: An example from Kilauea Volcano, Hawaii
Authors: Hurwitz Shaul , Ingebritsen Steven E. , Sorey Michael L.
Published: December 2002

Temperature measurements in deep drill holes on volcano summits or upper flanks allow a quantitative analysis of groundwater induced heat transport within the edifice. We present a new temperature‐depth profile from a deep well on the summit of Kilauea Volcano, Hawaii, and analyze it in conjunction with a temperature profile measured 26 years earlier. We propose two groundwater flow models to interpret the complex temperature profiles. The first is a modified confined lateral flow model (CLFM) with a continuous flux of hydrothermal fluid. In the second, transient flow model (TFM), slow conductive cooling follows a brief, advective heating event. We carry out numerical simulations to examine the timescales associated with each of the models. Results for both models are sensitive to the initial conditions, and with realistic initial conditions it takes between 750 and 1000 simulation years for either model to match the measured temperature profiles. With somewhat hotter initial conditions, results are consistent with onset of a hydrothermal plume ∼550 years ago, coincident with initiation of caldera subsidence. We show that the TFM is consistent with other data from hydrothermal systems and laboratory experiments and perhaps is more appropriate for this highly dynamic environment. The TFM implies that volcano‐hydrothermal systems may be dominated by episodic events and that thermal perturbations may persist for several thousand years after hydrothermal flow has ceased.
Seasonal and ENSO variations of sea surface salinity and temperature in the South Pacific Convergence Zone during 1976–2000
Authors: Gouriou Yves , Delcroix Thierry
Published: December 2002

Sea surface salinity (SSS) and temperature (SST) data collected from voluntary observing ships over 25 years (1976–2000) are analyzed in the Southwestern Tropical Pacific (10°S–24°S/160°E–140°W). This region lies under the South Pacific Convergence Zone (SPCZ), at the southern edge of the western Pacific warm pool between Tahiti and Darwin, the two places whose atmospheric sea level pressure difference is used to define the Southern Oscillation Index (SOI). Complementary data such as precipitation are used to assist in the analysis. The mean and seasonal variations of these parameters are described. An empirical orthogonal function (EOF) analysis of low‐pass filtered time series is then performed to extract the interannual variability. All parameters show an interannual signal which correlates well with the SOI. The Southwestern Tropical Pacific Ocean is saltier and colder during El Niño than during La Niña events. In the southwestern part, there is a shortage (excess) in precipitation during El Niño (La Niña) events. The greatest anomalies appeared during the last La Niña, in 1996 and 1999 as regards SST and in 1999 and 2000 as regards SSS. SST and precipitation El Niño–Southern Oscillation (ENSO)‐related anomalies are an order of magnitude smaller than seasonal anomalies, while the SSS ENSO‐related signal is twice as strong as the seasonal signal. These facts reflect the northeastward (southwestward) shift of the SPCZ during El Niño (La Niña) events. While consistent with precipitation changes, the ENSO‐related variability in SSS can also be partly explained by the displacement of the salinity front that separates fresh, warm pool waters from salty subtropical waters. Computation of surface geostrophic current anomalies from Geosat (1987–1988) and TOPEX/Poseidon (1993–2000) indicates that westward current anomalies developed during the 1987/88 and 1997/98 El Niño and are linked to the displacement of the salinity front. The Southwestern Tropical Pacific salinity front moves westward (eastward) in contrast to the equatorial salinity front which moves eastward (westward) during an El Niño (La Niña) event.
N uptake conditions during summer in the Subantarctic and Polar Frontal Zones of the Australian sector of the Southern Ocean
Authors: Elskens Marc , Baeyens Willy , Cattaldo Thierry , Dehairs Frank , Griffiths Brian
Published: December 2002

N uptake data obtained from 15N tracer techniques were surveyed in surface waters of the Subantarctic and Polar Frontal Zones southwest of Tasmania during summer 1998. The N flux rates and f ratio were computed using inverse modeling. The approach provides a general framework for the handling of random and systematic variations. Furthermore, factorial experiments based on controlled ammonium additions were used to study the sensitivity of the f ratio relative to perturbations of the regenerated nutrient supply. This was achieved in distinct regions: the Subtropical Convergence Zone, the Subantarctic Zone, the Subantarctic Front, and the Polar Frontal Zone. Overall, the stability of the f ratio appeared closely related to the availability of both dissolved iron and ammonium. The effect of one factor depended on the level of the other one, and the interaction influenced the f ratio patterns. The relationship between f ratio and ammonium supplementation ranged from linear to convex curves, with the steepest slopes in water masses rich in dissolved iron and with low ammonium content. The magnitude of this variation may have important ecological and geochemical consequences. Our results suggest that only versatile systems are able to exhibit events of high new production.
A case of an intense anticyclonic eddy in the Balearic Sea (western Mediterranean)
Authors: Pascual Ananda , Buongiorno Nardelli Bruno , Larnicol Gilles , Emelianov Mikhail , Gomis Damià
Published: December 2002

Sea surface topography from TOPEX/Poseidon and ERS satellites, SST images derived from advanced very high resolution radiometer (AVHRR) sensors, and conductivity‐temperature‐depth (CTD) data collected during an oceanographic survey are examined to investigate an intense mesoscale anticyclonic eddy observed at the northern boundary of the Balearic Sea (western Mediterranean). This eddy, which constitutes the strongest signal detected in the Balearic basin since altimetric data are available, appeared in September 1998 to the northwest of Menorca Island and remained almost in the same location until its disappearance in March 1999. The paper focuses mainly on identifying the conditions that led to the eddy formation and on the description of its evolution and sudden disappearance. Derived variables such as geostrophic velocity or vorticity are computed to complement the hydrographic and dynamical information provided by observed variables. Also, meteorological data are examined related to the main hypothesis made for the origin of the eddy.
Near‐surface circulation and tidal currents of the Okhotsk Sea observed with satellite‐tracked drifters
Authors: Ohshima Kay I. , Wakatsuchi Masaaki , Fukamachi Yasushi , Mizuta Genta
Published: December 2002

Near‐surface circulation of the Okhotsk Sea was observed with ARGOS drifters from August 1999 to February 2000. The drifters clearly revealed the existence of the southward boundary current (the East Sakhalin Current) off the east Sakhalin. This current is strongly controlled by bottom topography and confined to the region shallower than 1000 m. The volume transport of this current is estimated to be in the range of 4–9 Sv (1 Sv = 106m3 s−1), with the increase from summer to winter. The current appears to consist of two cores: one exists near the coast (50–150 m depths) with typical speeds of 0.3–0.4 m s−1 and the other exists over the shelf slope (300–900 m depths) with typical speeds of 0.2–0.3 m s−1. A part of this southward current continues as far as the southern tip of Sakhalin Island, while another part turns to the east around Terpeniya Bay (∼48°N) flowing eastward as far as Bussol' Strait. Most of the drifters that survived in the Okhotsk Sea went out to the Pacific through Bussol' Strait by less than a half year, suggesting that Bussol' Strait is the main pathway for the surface water to flow out from the Okhotsk Sea. Mesoscale anticyclonic eddies with diameters of 100–200 km are dominant in the Kuril Basin. Diurnal shelf waves were identified over the shelf in the northern part of the east Sakhalin shelf. Large amplification of diurnal tidal currents (order of 1.0 m s−1) was observed over Kashevarov Bank along with the residual anticyclonic circulation. Both diurnal and semidiurnal tidal currents were found to be enhanced in and around Bussol' Strait.
Recent changes in the surface salinity of the North Atlantic subpolar gyre
Authors: Reverdin Gilles , Durand Fabien , Mortensen John , Schott F. , Valdimarsson H. , Zenk W.
Published: December 2002

Sea surface salinity (SSS) was measured since 1896 along 60°N between Greenland and the North Sea and since 1993 between Iceland and Newfoundland. Along 60°N away from the shelves, and north of 53°N, the amplitude of the seasonal cycle is comparable to or less than interannual variability. In these parts of the North Atlantic subpolar gyre, large‐scale deviations from the seasonal cycle correlate from one season to the next. This suggests that in these regions, summer and autumn surface data are useful for monitoring changes in upper ocean salinity best diagnosed from less common winter surface data. Further south near the subarctic front, the Labrador Current or near shelves where seasonal variability is strong, this is not the case. Along 60°N, the multiannual low‐frequency variability is well correlated across the basin and exhibits fresher surface water since the mid 1970s than in the late 1920s to 1960s. SSS in the Irminger Sea along 60°N lags by 1‐year SSS farther east in the Iceland Basin. Variability between Iceland and Newfoundland within the Irminger Sea north of 54°N presents similar characteristics to what is observed along 60°N. Variability near the northwest corner of the North Atlantic Current (52°N/45°W) is larger and is not correlated to what is found further north. Maps of SSS were constructed for a few recent seasons between July 1996 and June 2000, which illustrate the fresh conditions found usually during that period across the whole North Atlantic subpolar gyre, although this includes an episode of higher salinity. The SSS anomaly maps have large uncertainties but suggest that the highest SSS occurred before the spring of 1998 in the Iceland Basin, and after that, in the Irminger Sea. This is followed by fresher conditions, first in the Labrador and Iceland Basin, reaching recently the Irminger Sea.
Dynamically constrained projection for subsurface current velocity
Authors: Shen Colin Y. , Evans Thomas E.
Published: December 2002

An approach is described that projects surface current observations downward to obtain subsurface current structure consistent with the interior current dynamics. The projection approach is unrestricted by water depth. However, this study emphasizes well‐mixed constant density flow in water depths appropriate for mid and outer shelf, where viscous and inertial processes are comparably important. By means of twin experiments, in which the projected current profiles are compared to the known simulated current profiles, it is shown that both the projection time step and the time domain affect the accuracy of the projection. At minimum, the time domain needs to span the dominant period of current oscillation while the time step resolves this oscillation. When both are achieved, the projected current profiles converge to the simulated profiles, and this convergence can be faster than that by assimilating surface observations to correct model spin‐up from an inaccurately known initial condition. Furthermore, the projection is shown to be robust in the presence of data noise, and with appropriate weighting of the data constraints, the effect of noise on the projection accuracy can be minimized. In the present projection problem, the sea surface slope is assumed unknown and obtained together with the current profile. The use of variable eddy viscosity in velocity projection is illustrated as well with an iterative procedure.
A numerical study of the seasonal variability of Arabian Sea high‐salinity water
Authors: Prasad T. G. , Ikeda M.
Published: December 2002

Using a level 2 three‐dimensional turbulent closure model, the physical processes affecting the upper‐ocean salinity budget in the Arabian Sea have been studied with particular focus on the seasonal variability of the Arabian Sea High‐Salinity Water (ASHSW). The volume integrated salt budget terms for four subdomains in the Arabian Sea (south of 10.5°N, north of 10.5°N, east of 62.5°E, and west of 62.5°E) shows a clear balance between the horizontal advection and evaporative freshwater loss in the annual mean. The horizontal advection dominates the seasonal cycle in salinity. The seasonal balance is clear, when the terms of the salt budget are integrated between 10.5°N and 2.5°N and east of 62.5°E. In these subdomains the two major episodes of horizontal advection during winter and summer are both associated with the seasonal reversal of the Arabian Sea circulation. Model estimation of the upper‐ocean salinity budget terms integrated between the surface and 100 m for the winter and summer monsoons agree with a qualitative description of the ASHSW variability. After formation in the northern Arabian Sea during the winter monsoon, the ASHSW spreads predominantly southward along the eastern boundary during the summer monsoon. Advection causes salinity to increase by 1.0 kg m−2 day−1 along these regions. At this time, the poleward advection of low‐salinity water from the south (and upwelling regions) by the Somali current causes the salinity along the western Arabian Sea to decrease by 0.6 kg m−2 day−1. During the winter monsoon, westward advection of the fresher Bay of Bengal water by the North Equatorial current (NEC) causes a salinity decrease by ∼1.0 kg m−2 day−1 in the region south of 10.5°N. The combined effect of precipitation and evaporation tends to increase the salinity in the Arabian Sea, and horizontal advection is found to be important for maintaining the observed seasonal cycle.
An Oceanic General Circulation Model (OGCM) investigation of the Red Sea circulation, 1. Exchange between the Red Sea and the Indian Ocean
Authors: Sofianos Sarantis S. , Johns William E.
Published: December 2002

The mechanisms involved in the seasonal exchange between the Red Sea and the Indian Ocean are studied using an Oceanic General Circulation Model (OGCM), namely the Miami Isopycnic Coordinate Ocean Model (MICOM). The model reproduces the basic characteristics of the seasonal circulation observed in the area of the strait of Bab el Mandeb. There is good agreement between model results and available observations on the strength of the exchange and the characteristics of the water masses involved, as well as the seasonal flow pattern. During winter, this flow consists of a typical inverse estuarine circulation, while during summer, the surface flow reverses, there is an intermediate inflow of relatively cold and fresh water, and the hypersaline outflow at the bottom of the strait is significantly reduced. Additional experiments with different atmospheric forcing (seasonal winds, seasonal thermohaline air–sea fluxes, or combinations) were performed in order to assess the role of the atmospheric forcing fields in the exchange flow at Bab el Mandeb. The results of both the wind‐ and thermohaline‐driven experiments exhibit a strong seasonality at the area of the strait, which is in phase with the observations. However, it is the combination of both the seasonal pattern of the wind stress and the seasonal thermohaline forcing that can reproduce the observed seasonal variability at the strait. The importance of the seasonal cycle of the thermohaline forcing on the exchange flow pattern is also emphasized by these results. In the experiment where the thermohaline forcing is represented by its annual mean, the strength of the exchange is reduced almost by half.
Observations and predictions of fluid velocities in the surf and swash zones
Authors: Raubenheimer B.
Published: December 2002

Surfzone and swashzone water level fluctuations and cross‐shore fluid velocities observed at seven cross‐shore locations for 139 51‐min periods on a low‐sloped, fine‐grained sandy beach are shown to be modeled well by the depth‐averaged, nonlinear shallow water equations. The model, initialized with observations about 150 m from the shoreline in 300 cm water depth, accurately predicts the observed decrease in sea swell wave heights and orbital velocities with decreasing water depth across the surf zone and the observed maxima in infragravity orbital velocities and offshore‐directed mean flows near the outer edge of the swash zone. In the surf zone both the observed and predicted velocity fluctuations are dominated by sawtoothed‐shaped, pitched forward (asymmetrical) sea swell waves. In the swash zone, infragravity waves contribute substantially to the total velocity variance and asymmetry. Shoreward of the outer swash zone, observed and predicted total and infragravity velocity asymmetry decrease onshore, changing sign from positive to negative in the upper swash.
Online analysis of atmospheric particles with a transportable laser mass spectrometer during LACE 98
Authors: Trimborn A. , Hinz K.‐P. , Spengler B.
Published: December 2002

A novel mobile mass spectrometric acquisition and evaluation system for on‐line analysis of single airborne particles and characterization of particle populations (aerosols), laser mass analyzer for particles in the airborne state (LAMPAS 2), was employed during the Lindenberg Aerosol Characterization Experiment 1998 in July and August 1998 (LACE 98). The major aim of this experiment was to perform a size‐resolved determination of the chemical composition of atmospheric particles within populations in order to evaluate the climate forcing created by anthropogenic aerosol particles. During the field campaign, the LAMPAS 2 instrument continuously analyzed size and composition of individual particles from five size ranges with a time course resolution of 1 hour, recording several ten thousand single‐particle spectra. By using a semiautomated data processing unit, the amount of data could be significantly reduced from several hundreds of particle mass spectra to only a few particle classes in every size range. Statistical evaluation of the mass spectral data measured during LACE 98 resulted in the determination of 10 basic classes of particles, yielding detailed information on the aerosol composition. Chemical interpretation of these particle classes and correlation to meteorological data and physicochemical particle properties (optical parameters) were performed.