bluuebunny/crossref_metadata_2025 huggingface dataset:
A role of neutral motions in formation of midlatitude E‐region field‐aligned irregularities
Authors: Kagan Ludmila M. , Ogawa Tadahiko , Fukao Shoichiro , Yamamoto Mamoru
Published: September 2002

To explain observations of the E‐region 3.2‐m and 6.1‐m backscatter measured by the MU and FAR radars over Japan we propose that a source for generation of field‐aligned irregularities (FAI) is neutral gas motions operating over a wide altitude range. Due to an altitude dependence of ionospheric parameters these neutral motions may result in different dominating processes at different altitudes. On the basis of recent theories of neutral wind‐driven gradient drift and thermal instabilities [Kagan and Kelley, 1998; 1999] we show that thermal processes may both support the gradient drift instability and be responsible for sporadic‐E independent backscatter. In this scenario the time of FAI occurrence seems to follow a tidal mode behavior. The nighttime echo occurrence is most probably due to slower recombination.
Statistical aspects of major (Intense) Hurricanes in the Atlantic Basin during the past 49 hurricane seasons (1950–1998): Implications for the current season
Authors: Wilson Robert M.
Published: September 2002

Statistical aspects of major (intense) hurricanes, those of category 3 or higher on the Saffir‐Simpson scale (e.g., having a maximum sustained wind speed of ≥50 m s−1), in the Atlantic basin during the interval of 1950–1998 are investigated in relation to the El Niño‐Southern Oscillation cycle and to the postulated “more” versus “less” activity modes for intense hurricane activity. Based on Poisson statistics, when the hurricane season is simply classified as “non‐El Niño‐related” (NENR), the probability of having three or more intense hurricanes is ≈ 53%, while it is only ≈ 14% when it is classified as “El Niño‐related” (ENR). Including the activity levels (“more” versus “less”), the probability of having three or more intense hurricanes is computed to be ≈ 71% for the “more‐NENR” season, 30% for the “less‐NENR” season, 17% for the “more‐ENR” season, and 12% for the “less‐ENR” season. Because the 1999 hurricane season is believed to be a “more‐NENR” season, the number of intense hurricanes forming in the Atlantic basin should be above average in number, probably about 4±1 or higher.
Low attenuation resonance of a spherical magma chamber: Source mechanism of monotonic volcanic tremor at Asama Volcano, Japan
Authors: Fujita Eisuke , Ida Yoshiaki
Published: September 2002

Some volcanic tremor has the extreme persistence of oscillation that may be attributed to resonance. Our theoretical analysis suggests that a spherical resonator consisting of volcanic fluid has a low attenuation mode (LAM) at some of the lowest frequencies, which can be as low as 0.01 times the attenuation factors of higher modes in some cases. The monotonic waveforms of volcanic tremor (LP‐events) at Asama Volcano, Japan, can be explained by the LAM excitation in the spherical resonator model, which well fits both characteristic frequencies and attenuation factors. The resonator size, velocity, and the density contrasts between the fluid inside and the elastic medium outside are estimated.
Analysis of the far‐field tsunamis generated by the 1998 Papua New Guinea Earthquake
Authors: Tanioka Yuichiro
Published: September 2002

The tsunami generated by the 1998 Papua New Guinea earthquake was observed at several tide gauges and ocean bottom pressure gauges in Japan. The fault model of the 1998 Papua New Guinea earthquake was estimated using those tsunami waveforms observed in Japan and coseismic subsidence observed along the coast of Papua New Guinea near the source region. The numerical simulation of the tsunami using the linear Boussinesq equation was carried out. The tsunami waveforms in Japan are explained by the fault model which is consistent with the Harvard CMT solution including the seismic moment of 3.7 × 1019 Nm. The additional tsunami source, such as a sediment slump, is not needed to explain the observed tsunami in Japan. The mechanism of the earthquake was likely to be a steeply dipping reverse fault. The 1998 Papua New Guinea earthquake was likely to occur within a small accretionary prism near the New Guinea trench.
Detection of a crack‐like conduit beneath the active crater at Aso Volcano Japan
Authors: Yamamoto Mare , Kawakatsu Hitoshi , Kaneshima Satoshi , Mori Takehiko , Tsutsui Tomoki , Sudo Yasuaki , Morita Yuichi
Published: September 2002

To constrain the source of long period tremors (LPTs), we deployed a very dense broadband seismic network consisting of totally twenty‐four stations around the active crater of Aso volcano in Kyushu, Japan. The spatial variation of the observed signal amplitudes reveals that the source of LPTs consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1.8 km. The extension of the crack plane meets the crater chain including the active fumarole at the surface, suggesting that the crack has played an important role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.
First results from the Hawaiian SWELL Pilot Experiment
Authors: Laske G. , Morgan J. Phipps , Orcutt J. A.
Published: September 2002

In a year‐long pilot experiment to the southwest of the Hawaiian Islands, we recorded teleseismic intermediate‐period Rayleigh waves on ocean‐bottom “L‐CHEAPO” instruments using differential pressure gauges as sensors. We analyzed over 70 events and obtained accurate phase velocity estimates at periods between 15 and 70s. The average seismic structure beneath the pilot array does not deviate significantly from a standard seismic model of 100 Myr old oceanic lithosphere. However, we find strong lateral velocity variations across the array with large negative anomalies appearing within roughly 300km of the island chain. We are able to image the edge of the Hawaiian Swell and hence demonstrate the importance of making phase velocity measurements on the ocean floor.
Interplate coupling and deformation in the accretionary prism in the Southwest Japan Subduction Zone
Authors: Furukawa Yoshitsugu
Published: September 2002

Frictional coupling at the plate interface will control seismicity and tectonics in subduction zones. In this study temperature structure and deformation in the accretionary prism in the Southwest Japan subduction zone are simulated using a two‐dimensional isoviscous corner flow model. Using surface heat flow data the average interplate frictional stress and the effective viscosity of the prism are estimated to be 2∼30MPa and 1020∼21Pa·s, respectively. The low stress value is probably due to high pore pressures caused by dehydration reactions in the subducting crust. Estimated temperature of the landward limit of the seismogenic zone is 350∼500°C; when the frictional stress is considered, the temperature becomes ∼100°C higher than that of the onset of plasticity of granitic rocks (350°C). The brittle‐plastic transition of gabbroic rocks might control the depth extent of the seismogenic zone in lower crustal depths. Flow velocity at the bottom of the prism is estimated to be ∼0.01m/yr, indicating that ∼25% of the conversion has been consumed by the deformation of the accretionary prism in this subduction zone.
Crustal deformation at the Sumatran Subduction Zone revealed by coral rings
Authors: Sieh Kerry , Ward Steven N. , Natawidjaja Danny , Suwargadi Bambang W.
Published: September 2002

Analyses of coral rings grown in the interval 1970–1997 reveal a geographically distinct pattern of interseismic uplift off Sumatra's western coast. At distances less than 110 km from the Sumatran trench, coral reefs are submerging as fast as 5 mm/y. At 130 and 180 km distance from the trench, they are emerging at similar rates. We suggest that a locked, or partially locked patch, located above 30 km depth on the upper surface of the subducting oceanic plate, generates this pattern.
Seismic scaling exponents as a tool in detecting stress corrosion crack growth leading to the September–October 1989 flank eruption at Mt. Etna Volcano
Authors: Vinciguerra Sergio
Published: September 2002

The September–October 1989 flank eruption at Mt. Etna marked a change in the recent eruptive style of the volcano. Seismicity, at mid (order of months) and short (order of days) time, preceding the eruption onset has been analysed by means of D (scaling exponent), inferred from the seismic b value, that represents the scaling exponent of fracture length distribution. Time evolution of D preceding the eruption onset revealed: a general increase of D (9 to 4 months prior the eruption onset), intermittent rates of D decreases and increases (4 to 1 month), steady values of D (1 month) and sharp decreases of D marking the eruption onset. Integration of these results with ground deformation data reveals that at Mt. Etna stress corrosion crack growth, enhanced by fresh magma arrival, is a plausible mechanism for weakening the country rock, by promoting crack growth and the coalescence of major faults.
Monsoon ecosystems control on atmospheric CO2 interannual variability: Inferred from a significant positive correlation between year‐to‐year changes in land precipitation and atmospheric CO2 growth rate
Authors: Yang Xin , Wang Mingxing
Published: September 2002

Previously, large interannual variability were observed of atmospheric CO2 mixing ratio at Mauna Loa Observatory, and were found to be positively correlated with the global near‐surface air temperature variability, notably over El Nino‐Southern Oscillation (ENSO) cycles, and negatively correlated with global precipitation on the decadal time‐scale. By contrast, here we report a significant positive correlation between the interannual variability of CO2 growth rate and the year‐to‐year changes in global land precipitation. The interannual extremes in the rate of CO2 growth coincide with the largest year‐to‐year changes in precipitation over land, particularly Indian summer monsoon. We suggest that this positive correlation results mainly from changes in terrestrial net ecosystem production (NEP) with incoming solar radiation: an increase of clouds concurrent with precipitation reduces seasonal NEP and causes a positive anomaly in CO2 emission from the terrestrial biosphere.
Atmospheric SF6 near a large urban area
Authors: Ho David T. , Schlosser Peter
Published: September 2002

Sulfur hexafluoride (SF6) has the potential to be a valuable transient tracer for dating of young ground water. However, near urban areas, there are typically numerous point sources of SF6 which have a significant impact on the temporal evolution of its atmospheric mixing ratio, thereby complicating its use as an age‐dating tool. Here, we present and discuss a 12‐month record of atmospheric SF6 from a location near New York City. The data were obtained by gas chromatographic analyses performed at intervals of approximately 10 minutes yielding about 40,000 data points for the time series. Nearly all measured SF6 values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. Temporal trends in the baseline data from LDEO indicate that atmospheric SF6 at LDEO decreased at a rate of 0.4 ppt yr−1 over the 12‐month period. The SF6 data are compared to records of CFCs obtained during the same period. Whereas the CFCs show daily, weekly, and seasonal patterns of variability near New York City, the SF6 data exhibit only a daily cycle. The observed SF6 excesses are far greater than those found for CFCs during the same time period. This indicates that in order to use SF6 as an age dating tool of groundwater near source regions, its input function, i.e., its concentration in soil air above the groundwater table, needs to be defined explicitly.
Lunar orbital evolution: A synthesis of recent results
Authors: Bills Bruce G. , Ray Richard D.
Published: September 2002

The present rate of tidal dissipation in the Earth‐Moon system is known to be anomalously high, in the sense that the implied age of the lunar orbit is only 1.5×109 years, though other evidence suggests an age closer to 4×109 years. To assess how long the anomalous dissipation has persisted, we use published estimates of lunar orbital configurations derived from (a) fine grained sediments containing tidal laminations and (b) numerical ocean models averaged over varying ocean geometries. The implied histories of the lunar semimajor axis are surprisingly consistent over the past 109 years. The ocean models imply, on average, reduced dissipation in the past because of a spatial mismatch between tidal forcing and oceanic normal modes of higher frequencies. Webb's ocean model suggests that the “anomalous” oceanic dissipation began about 109 years ago and has been increasing since then.
Stable gas plume composition measured by OP‐FTIR spectroscopy at Masaya Volcano, Nicaragua, 1998–1999
Authors: Horrocks Lisa , Burton Mike , Francis Peter , Oppenheimer Clive
Published: September 2002

We report non‐eruptive plume gas composition data collected by open‐path Fourier transform infrared (OP‐FTIR) spectroscopy at the summit of Masaya Volcano, Nicaragua, in 1998 and 1999. Masaya's plume is characterised by SO2/HCl and HCl/HF mass ratios of 2.8–3.0 and 8.1–8.2 respectively. When combined with COSPEC SO2 emission rate data, these ratios indicate average emission rates of HCl and HF of 7.3 kgs−1 and 0.9 kgs−1. The gas composition is remarkably consistent, showing little temporal variation on the scale of days, weeks or even between the two observation periods, one year apart. Such striking uniformity is best explained by steady‐state open system degassing.
Tomographic low‐velocity anomalies in the uppermost mantle around the northeastern edge of Okinawa Trough, the backarc of Kyushu
Authors: Sadeghi Hossein , Suzuki Sadaomi , Takenaka Hiroshi
Published: September 2002

A simultaneous inversion for three‐dimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of Kyushu, southwestern Japan. The data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide region from western Japan to eastern China. Additional data are provided by including arrival times of 357 teleseismic earthquakes. We got the high resolution of the uppermost mantle images for P waves at least in the depth range to 150 km, the latitude range of 30°–34°N and the longitude range of 128°–132°E. Results of inversion reveal a major zone of strong low velocity under west off Kyushu Island in the depth from 40 km to 100 km. This feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending Okinawa trough.
Acid gas and metal emission rates during long‐lived basalt degassing at Stromboli Volcano
Authors: Allard Patrick , Aiuppa Alessandro , Loyer Henri , Carrot Francine , Gaudry André , Pinte Guy , Michel Agnès , Dongarrà Gaetano
Published: September 2002

The discharge of acid gases and metals from Stromboli is determined from airborne and ground‐based filter sampling of particulate matter in the volcanic plume, combined with COSPEC measurements of SO2 fluxes. Smaller particle sizes and high enrichment factors distinguish the most volatile elements (by order: S, Se, Br, Cl, Cd, Bi, In, As, Sb, Sn, F, Au, Pb, Cr, Cu) from those strictly (Fe, Mn, REE, Sc, Sr, Th, Ti, V) or mainly (Al, Ba, Ca, Co, K, Na, U) derived from volcanic ash. Time‐averaged volatile fluxes show that Stromboli is a representative arc emittor, producing 1–2% of the global volcanic budget of sulfur, halogens and several trace metals, while 15–25% of volcanic emissions of Bi, Cd, Cs, Pb and Sn in southern Italy. Subaerial degassing of its S‐Cl‐rich shoshonitic magma over the last 2 ky of similar activity may have released as much copper and gold as is encountered in magma‐derived high‐sulfidation ore deposits.
High ³He/4He ratio in xenoliths from Takashima: Evidence for plume type volcanism in southwestern Japan
Authors: Sumino Hirochika , Nakai Shun'ichi , Nagao Keisuke , Notsu Kenji
Published: September 2002

Cenozoic basalts distributed along the southwestern Japan arc system have geochemical features different from typical island‐arc basalts, suggesting that the volcanism might have been triggered by a mantle plume. To identify the source materials in these volcanics, noble gas isotopic compositions of olivines separated from dunite xenoliths in alkaline basalts from Takashima, northwestern Kyushu, were measured. Most samples contain MORB‐type helium. However, some samples showed ³He/4He ratios significantly higher than the MORB value, from 9.4 up to 16.6 times the atmospheric ratio of 1.4×10−6. This finding strongly supports the contribution of a mantle plume to the volcanism of this region.
Observations of annual variations of the Earth's gravitational field using satellite laser ranging and geophysical models
Authors: Nerem R. S. , Eanes R. J. , Thompson P. F. , Chen J. L.
Published: September 2002

We have analyzed 6 years of satellite laser ranging (SLR) data to the Lageos 1 & 2 satellites to determine the annual variation of a set of spherical harmonic coefficients of the Earth's gravity field complete to degree and order 4 (half‐wavelength resolution of ∼5000 km). We have compared these results to a suite of geophysical models describing annual variations of the gravity field due to changes in the distribution of mass in the atmosphere, in the ocean, and continental hydrology (soil moisture and snow). We find that spherical harmonic coefficients derived from the satellite‐observations and the aggregate of these geophysical models agree to about 1 mm RMS in geoid height, and have degree correlations that generally exceed the 90% confidence limit. We found that the SLR data could distinguish between two different hydrologic models, but were unable to distinguish between competing models of atmosphere and ocean mass variation, probably due to the small magnitude of the differences in these models (and the small total magnitude of the ocean signal) at the long wavelengths that can be observed by the satellite data. The satellite results should improve considerably in 2001 with the launch of the Gravity Recovery and Climate Experiment (GRACE), which will allow the determination of the time variations of the Earth's gravity field with a spatial resolution of about 300 km.
Origin of air‐like noble gases in oceanic basalts
Authors: Mohapatra Ratan K. , Murty S. V. S.
Published: September 2002

Noble gas studies of oceanic basalts show the ubiquitous presence of an air‐like component, which is usually attributed to a simple atmospheric contamination. However, simultaneous nitrogen and argon isotopic study shows the presence of heavy nitrogen with an isotopic composition (δ15N) ∼ +18‰, accompanied by 40Ar/36Ar values much lower than the mantle value. Such a component can be attributed to recycled materials from subduction zones, which are incorporated into these basaltic magmas during their ascent through a shallow enriched region in the mantle. This suggests that a part of the air‐like noble gases in oceanic basalts is due to recycled atmospheric gases. Thus the air‐like noble gases in the oceanic basalts, hitherto considered a hindrance, provide useful clues for understanding the geochemical evolution of these samples.
Interannual sea level changes in the tropical Pacific associated with ENSO
Authors: Merrifield Mark , Kilonsky Bernard , Nakahara Shikiko
Published: September 2002

A regression analysis involving tide gauge and altimeter sea level data is used to construct an estimate for interannual changes in sea level associated with the El Niño Southern Oscillation in the tropical Pacific Ocean. The method provides an assessment of the island regions most at risk of anomalously high sea levels during peak El Niño and La Niña events.
Short‐period oceanic circulation: Implications for satellite altimetry
Authors: Tierney Craig , Wahr John , Bryan Frank , Zlotnicki Victor
Published: September 2002

Atmospherically forced, high‐frequency oceanic variability is investigated using different configurations of an ocean general circulation model. At periods less than 20 days, the dynamic response of the sea surface to pressure loading exceeds that due to wind stress, and is mostly barotropic. Energy at these periods aliases into satellite altimeter measurements of sea surface height (SSHT). The global variance of collinear (≈10 day) differences of this modelled aliased SSHT is between (2 cm)² and (3.5cm)², depending on the model configuration used. The local variance can reach (14cm)² at some high latitude locations. We use the ocean model predictions to remove the high‐frequency signals from TOPEX/Poseidon (T/P) observations. We obtain a global variance reduction in collinear differences of up to (2cm)², about 7% of the T/P signal. Our model has difficulty in predicting the variability at periods less than 5 days.
Small‐amplitude bipolar flows in the near‐Earth tail
Authors: Petrukovich A. A. , Wanliss J. , Mukai T. , Kokubun S. , Yamamoto T.
Published: September 2002

Geotail observations in the near‐Earth magnetotail at XGSM=−11RE revealed the small‐amplitude bipolar (first tailward then earthward) ion flow signature. Tailward‐flow part contained spikes of negative GSM Bz magnetic field, while earthward‐flow part included transient Bz increase and subsequent dipolarization. This and similar structures had maximal velocities less than 200 km/s, duration of about one minute and were associated with a stretched magnetotail configuration and substorm activity. We interpret them as tailward‐propagating reconnection events involving only magnetic field lines adjacent to the neutral sheet and therefore having low merging rates. Such structures might also be important because they increase the normal magnetic field component and stability in the neutral sheet.
Coqui 2: Mesospheric and lower thermospheric wind observations over Puerto Rico
Authors: Larsen M. F.
Published: September 2002

During the 1998 Coqui 2 sounding rocket campaign, three chemical release rockets were launched from Puerto Rico to measure the wind profiles in the mesosphere and lower thermosphere region. The first release took place on February 19, and the next two releases were five days later and four hours apart. All three measurements show large winds and wind shears in the altitude range between 95 and 110 km. The largest wind speeds occurred near 100 km with horizontal flow velocities close to 150 m s−1. Taken in isolation, such features may appear to be unusual, but in fact, a large number of earlier rocket measurements show wind features similar to those observed in the Coqui 2 campaign. Such large winds are therefore not an isolated but rather a common feature of the wind profiles at midlatitudes in an altitude range that is critical for the electrodynamics of the ionosphere. The wind profile characteristics described here are not generally well known since the earlier more extensive chemical release data sets were only published in reports with a rather limited distribution.
Small‐scale magnetic flux ropes in the solar wind
Authors: Moldwin M. B. , Ford S. , Lepping R. , Slavin J. , Szabo A.
Published: September 2002

Small‐scale magnetic flux ropes have been discovered in the solar wind at 1 AU in observations from the IMP 8 and WIND spacecraft. These small magnetic structures (diameter of 270 RE, on average) have some similar properties to magnetic clouds (diameters of 0.2–0.3 AU or about 6000–8000 RE), which are well known large‐scale magnetic flux ropes, but have durations of 10s of minutes as opposed to many hours or days for most magnetic clouds. The presence of these small helical field structures suggests that solar wind flux ropes may have a wide‐range of scale sizes, or possibly have a bimodal size distribution, and are perhaps more common than previously estimated. Similarities and differences with magnetic clouds will be discussed. We suggest that these small scale magnetic flux ropes are signatures of magnetic reconnection in the solar wind as opposed to in the solar corona.
Multiple‐spacecraft observation of a narrow transient plasma jet in the Earth's plasma sheet
Authors: Sergeev V. A. , Sauvaud J. ‐A. , Popescu D. , Kovrazhkin R. A. , Liou K. , Newell P. T. , Brittnacher M. , Parks G. , Nakamura R. , Mukai T. , Reeves G. D.
Published: September 2002

We use observations from five magnetospheric spacecraft in a fortuitous constellation to show that narrow transient plasma flow jets of considerable length formed in the tail can intrude into the inner magnetosphere and provide considerable contribution to the total plasma transport. A specific auroral structure, the auroral streamer, accompanied the development of this narrow plasma jet. These observations support the ‘boiling’ plasma sheet model consisting of localized underpopulated plasma tubes (bubbles) moving Earthward at high speeds as a realistic way to resolve the ‘convection crisis’ and to close the global magnetospheric circulation pattern.
Multiple discrete‐energy ion features in the inner magnetosphere: Observations and simulations
Authors: Li Xinlin , Baker D. N. , Temerin M. , Peterson W. K. , Fennell J. F.
Published: September 2002

Recent measurements from the ion composition sensors (CAMMICE/MICS and TIMAS) on the Polar satellite often show multiple discrete‐energy peaks in ion‐energy spectra, which are seen as multiple bands in energy‐time plots. The most striking feature is that these multiple bands occur over a large range of L (L=3‐8) and energy (a few keV to hundreds of keV) independent of the mass of the ions. These events are more likely to be observed during quiet times following substorm activity. Using an event observed on Feb. 9, 1998 we interpret the observed ion bands as the result of a time‐of‐flight effect of the particle's drift around the Earth. We demonstrate by test‐particle simulations that these multiple bands across a large range of L and energy are ion drift echoes that can be injected into the inner magnetosphere from the plasmasheet by a single earthward propagating time‐varying field associated with substorm dipolarization. The existence of ion drift echoes even after only moderate substorm activity shows that localized time‐varying electric and magnetic fields such as modeled here can and do penetrate deep into the inner magnetosphere.
Evidence provided by Galileo of ultra low frequency waves within Jupiter's middle magnetosphere
Authors: Wilson R. J. , Dougherty M. K.
Published: September 2002

The Voyager and Ulysses spacecraft measurements from the dayside of Jupiter's middle magnetosphere provided evidence for ultra low frequency waves in the magnetometer data, in particular waves of periods 10–20 minutes. Galileo is the first orbiter mission around Jupiter and the first to extensively study the nightside magnetosphere. Similar waves are discernible when Galileo passed through the equatorial current sheet with very few features being noticed outside of the current sheet regions. An overview of the data shows that only 12% of crossings do not have any wave signatures in any component of magnetic field in the 10–20 minute period regime. The majority of these waves are found in the perpendicular field component. These waves are thus prevalent over the nightside magnetosphere and when combined along with the earlier work on the dayside seems to suggest that these ultra low frequency waves are a global phenomenon.
Determination of gas diffusivity in polar firn: Comparison between experimental measurements and inverse modeling
Authors: Fabre Adeline , Barnola Jean‐Marc , Arnaud Laurent , Chappellaz Jerome
Published: September 2002

To study air diffusion in polar firns, and thus evaluate the mean age of air constituents as a function of depth, it is necessary to know the effective diffusivity profile for the studied gas and firn. In this work we compare effective diffusivity profiles obtained by two methods: first, by experimental measurements on firn samples, second, using an inverse gas diffusion model. Results are obtained for three gases, CO2, CH4 and SF6, in firns exposed to different climatic conditions. While measurements give similar results for the different sites, the inverse model shows variations depending on the climatic conditions. We therefore conclude that diffusivities measured on small firn samples cannot be directly used to determine the tortuosity profile of real firn and that diffusion models have to be used.
On the origin of SO2 above northern Greece
Authors: Zerefos C. , Ganev K. , Kourtidis K. , Tzortziou M. , Vasaras A. , Syrakov E.
Published: September 2002

This paper describes the sources contributing to two seasonal peaks in columnar SO2 amounts measured with a Brewer spectrophotometer at Thessaloniki, Northern Greece since 1982. The SO2 Brewer measurements combined with those at ground level, meteorological analysis and numerical simulations provide estimates on the contribution of local and remote sources to the SO2 column. It is shown that more than 50% of the observed SO2 column can be attributed to lignite‐burning sources in Bulgaria, Romania and former Yugoslavia, this percentage rising to 70% at periods with NE flow at 850 hPa. Winds from the NW‐N‐NE contribute around 60% to the observed mean SO2 column during winter and 75% during the summer. When including all wind directions at 850 hPa, the Greek sources, including the lignite‐burning power plant complexes to the WSW of the city, contribute around 40% to the SO2 column. These results are in qualitative agreement with independent observations from inversion of GOME measurements.
Power spectrum of a gravity wave propagating in a shearing background wind
Authors: Pulido Manuel , Caranti Giorgio
Published: September 2002

We re‐analyze the effect on the spectral tail of a gravity wave propagating in a shearing background. The contribution to the spectrum of horizontal wind perturbations for low vertical wavenumbers comes from the Doppler shifting with a −1 slope and for high wavenumbers from the leakage effect with slopes ranging between −4 to −2. If leakage is not present, it becomes necessary to consider the termination of the wave in order to account for the spectral tail. We show that the decreasing of the wave amplitude when the overturning starts, leads to a spectral slope of −3, and an amplitude proportional to N² (the square of the Brünt Väisälä frequency).
Mechanism of slab thickening near 670 km under Indonesia
Authors: Das Shamita , Schöffel Hans‐Joachim , Gilbert Freeman
Published: September 2002

We examine a new data set of reliably relocated earthquakes deeper than 400 km under Indonesia, developed by [Schöffel and Das, 1999]. The accuracy of this data allows us to demonstrate that the slab thickens, shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone. We also show that the earthquakes in the lower portion of the Wadati‐Benioff zone width are generally fewer and smaller than those in the central portions, with only 8% of the total moment released in the period 1976–1996 being due to earthquakes on the lower side.
On the mix‐down times of dynamically active potential vorticity filaments
Authors: Esler J. Gavin , Polvani Lorenzo M. , Plumb R. Alan
Published: September 2002

A simple model is used to study the evolution of potential vorticity filaments, viewed in cross‐section, subject to steady shear and deformation flows representative of the large‐scale atmospheric circulation. It is found that the balanced, ageostrophic circulation induced by the anomalous potential vorticity can cause the evolution of a dynamically active filament to differ substantially from that of a dynamically passive filament in a similar background flow. It is suggested that estimates of the mix‐down time of material contained in atmospheric filaments need to be corrected to allow for this effect.
Antarctic polar descent and planetary wave activity observed in ISAMS CO from April to July 1992
Authors: Allen D. R. , Stanford J. L. , Nakamura N. , López‐Valverde M. A. , López‐Puertas M. , Taylor F. W. , Remedios J. J.
Published: September 2002

Antarctic polar descent and planetary wave activity in the upper stratosphere and lower mesosphere are observed in ISAMS CO data from April to July 1992. CO‐derived mean April‐to‐May upper stratosphere descent rates of 15 K/day (0.25 km/day) at 60°S and 20 K/day (0.33 km/day) at 80°S are compared with descent rates from diabatic trajectory analyses. At 60°S there is excellent agreement, while at 80°S the trajectory‐derived descent is significantly larger in early April. Zonal wavenumber 1 enhancement of CO is observed on 9 and 28 May, coincident with enhanced wave 1 in UKMO geopotential height. The 9 May event extends from 40 to 70 km and shows westward phase tilt with height, while the 28 May event extends from 40 to 50 km and shows virtually no phase tilt with height.
Trends and interannual oscillations in the main sea‐level surface pressure patterns over the Mediterranean 1955–1990
Authors: Ribera P. , Garcia R. , Diaz H. F. , Gimeno L. , Hernandez E.
Published: September 2002

The evolution of the main sea level pressure patterns built from Rotated Principal Component Analysis (RPCA) over a domain centered on the Mediterranean for the period 1955–1990 is examined. The annual cycle and the interannual variations are analyzed using Singular Spectrum Analysis (SSA) and three methods of spectral analysis, to detect trends and oscillations in the main patterns. The dominant patterns show oscillations characteristic of NAO and SO. This paper shows that NAO and SO seem to be correlated in a band centered in 6–8 years and that SLP behaviour over the region is dominated by QBO (which controls the oscillation frequency) while NAO modulates the amplitudes.
Global distribution of cloud top phase from POLDER/ADEOS I
Authors: Riedi J. , Doutriaux‐Boucher M. , Goloub P. , Couvert P.
Published: September 2002

The eight months of data acquired by the POLDER instrument have now been processed. This dataset provides daily information on the global distribution of cloud top phase. We present here the results of a statistical analysis of ice and liquid phase occurrence frequencies at the global scale. Temporal variation of these frequencies above land and ocean are analyzed. These results are compared with ISCCP data and the consistency of the POLDER phase product is demonstrated.
Temporal and spatial distribution of the stratospheric aerosol optical thickness at 1013 nm derived from the ORA Experiment after the Pinatubo eruption
Authors: Vanhellemont Filip , Fussen Didier , Bingen Christine
Published: September 2002

In this article, we present aerosol optical thickness data, computed from the aerosol extinctions measured by the ORA experiment at 1013 nm. Temporal and latitudinal dependences are discussed for the period Aug. 1992 ­ May 1993. A comparison with SAGE II optical thickness data is presented, and shows good agreement.
Evidence for a recent change in the link between the North Atlantic Oscillation and Arctic Sea ice export
Authors: Hilmer Michael , Jung Thomas
Published: September 2002

Evidence for a recent change in the link between the North Atlantic Oscillation (NAO) and Arctic sea ice export through Fram Strait during wintertime (DJFM) is presented from the analysis of simulated Arctic sea ice and observations for the period 1958–1997. Whereas no correlation between the two time series is found from 1958 to 1977 (r=0.1), the correlation increased significantly thereafter (r=0.7, 1978–1997). The increased coherency between the NAO and Arctic sea ice export through Fram Strait during the last two decades resulted from anomalous meridional wind components near Fram Strait which were associated with the more easterly position of the NAO's centers of interannual variability compared to 1958–1977.
Tidal Evolution by elongated primaries: Implications for the Ida/Dactyl System
Authors: Hurford Terry A. , Greenberg Richard
Published: September 2002

The age of asteroid 243 Ida's satellite Dactyl is < 100 myr according to the conventional formula for the rate of tidal evolution outward from Ida, contrary to estimates based on the likely age of the asteroid itself (∼ 1 byr). We investigate whether this discrepancy may be due to the conventional formula for tidal evolution being based on a spherical primary, whereas Ida is actually highly elongated. A model for Ida consisting of three spheres connected by damped springs is used to estimate what effects the elongation may have on the tidal dissipation. In fact, our model gives torques and energy dissipation similar to that in an equivalent sphere, indicating that the spherical model gives reasonable results even when applied to an elongated or irregular body.
A physically‐based treatment of elemental carbon optics: Implications for global direct forcing of aerosols
Authors: Jacobson Mark Z.
Published: September 2002

To date, global models of direct radiative forcing have treated elemental carbon (EC) as completely externally mixed or well‐mixed internally. No global study has treated EC as a core in an internal mixture. It is hypothesized that the well‐mixed treatment is unphysical and reality lies between the externally‐mixed and core treatments. It is also suggested, but not proven, that most EC particles are coated to some degree; hence, the core treatment may be more representative than the external‐mixture treatment. Global simulations with the core treatment resulted in EC forcing 50% higher and 40% lower than forcings obtained with the externally‐mixed and well‐internally‐mixed treatments, respectively. In the core case, EC's positive forcing more than offset negative forcing due to all other anthropogenic aerosol components combined. Further studies are needed to understand the mixing state of EC and determine the accuracy of the core treatment.
Disagreement between predictions of the future behavior of the Arctic oscillation as simulated in Two different climate models: Implications for global warming
Authors: Zorita Eduardo , González‐Rouco Fidel
Published: September 2002

Two global climate models (HadCM2 and ECHAM) forced with the same greenhouse‐gas scenario (IS92a) are found to disagree in their simulated long‐term trends of the intensity of the Arctic Oscillation (AO), an atmospheric circulation pattern of the Northern Hemisphere. The simulated AO trends are strongly dependent on the model and on the initial conditions of the simulations. The simulated winter temperature increase averaged over the Northern hemisphere is very similar in both models. However, the effect of the different AO trends on temperature causes clear differences in the predicted regional warming, which are reduced if the effects of the AO is linearly discounted. The uncertainty in the predictions of circulation changes has impacts on the estimation of regional temperature changes.
Implications of changes in the northern hemisphere circulation for the detection of anthropogenic climate change
Authors: Gillett Nathan P. , Hegerl Gabriele C. , Allen Myles R. , Stott Peter A.
Published: September 2002

The first principal component of Northern Hemisphere sea level pressure, known as the Arctic Oscillation (AO) index, has increased significantly in recent winters, and this change is associated with ∼30% of Northern Hemisphere January–March warming. We examine the AO in a model used to detect anthropogenic influence on climate, and find that it exhibits no systematic trend in response to greenhouse gas, sulphate aerosol, or ozone forcing. To test the significance of this discrepancy for anthropogenic climate change detection, we include the spatio‐temporal pattern of temperature change associated with the observed AO in the set of forcing‐response “fingerprints” used to account for observed changes, thus separating temperature change associated with the AO from a residual. We find that the detection of a global response to both anthropogenic greenhouse gases and sulphate aerosols is robust to this exclusion of AO‐related warming.
Constraint on the S‐wave velocity at the base of the mantle
Authors: Stutzmann Eléonore , Vinnik Lev , Ferreira Ana , Singh Satish
Published: September 2002

Some recent seismic studies have suggested the presence of a thin ultra‐low P‐wave velocity layer (ULVZ) at the base of the mantle, which is interpreted to be due to presence of partial melting. Partial melting would lead to a strong decrease of the S‐wave velocity for which there is no seismic evidence. Such a decrease in the S‐wave velocity would produce a strong precursor to SKS phase from the conversion of S to P at the upper boundary of the layer. We analyze records of events from the subduction zones in the south‐west Pacific region obtained at stations in North America. At the source side, the converted phases propagate in the region, where the ultra‐low P‐wave velocity has been found earlier. Our analysis demonstrates that either the S‐wave velocity drop in this layer is much smaller than predicted by the hypothesis of melting, or the layer is so thin (less than about 10–15 km) that it can not be detected with our technique.
Nucleation rates of nitric acid dihydrate in 1∶2 HNO3/H2O solutions at stratospheric temperatures
Authors: Salcedo Dara , Molina Luisa T. , Molina Mario J.
Published: September 2002

Nucleation rates of nitric acid dihydrate (NAD) in 1∶2 HNO3/H2O solutions between 194 and 183 K were measured by observing the freezing behavior of liquid drops with 7 – 33 µm diameter and with concentration of 64 weight percent HNO3 using an optical microscopy technique. The nucleation rate increases from 3.1×105 cm−3s−1 at 194 K to 1.3x109 cm−3s−1 at 183 K. These results suggest that NAD might not nucleate homogeneously in binary or quasi‐binary nitric acid stratospheric aerosols in relevant time scales.
Interannual variability of equatorial Pacific CO2fluxes estimated from temperature and salinity data
Authors: Loukos H. , Vivier F. , Murphy P. P. , Harrison D. E. , Le Quéré C.
Published: September 2002

Based on atmospheric data and models, the tropical CO2source anomaly reaches up to 2 GtC yr−1, but the respective contributions of the terrestrial biosphere and the oceans to this flux are difficult to quantify. Here we present a new method for estimating CO2fluxes from oceanic observations based on the surprisingly good predictive skill of temperature and salinity for surface dissolved inorganic carbon. Using historical temperature and salinity data, we reconstruct the basin scale CO2flux to the atmosphere in the equatorial Pacific from 1982 to 1993. We find that interannual anomalies do not exceed 0.4±0.2 GtC yr−1which suggests that up to 80% of the tropical CO2source anomaly is due to the terrestrial biosphere.
Enhanced Atlantic freshwater export during El Niño
Authors: Schmittner Andreas , Appenzeller Christof , Stocker Thomas F.
Published: September 2002

Using two reanalysis data sets, the influence of El Niño/Southern Oscillation (ENSO) on the surface freshwater balance of the Atlantic ocean is examined. We present evidence that the transport of water vapour out of the tropical Atlantic is enhanced during warm ENSO phases, while it is reduced during cold phases. These tropical changes alter the freshwater budget of the entire Atlantic drainage basin. The difference in freshwater export between La Niña and El Niño years is on the order of 0.1 Sv (1Sv=106 m³s−1) for the entire Atlantic. Results from an idealized coupled ocean‐atmosphere model suggest that this amplitude is of significance for the strength of the North Atlantic thermohaline ocean circulation, if the persistence of the anomaly is on the order of decades.
On the use of neural networks for dendroclimatic reconstructions
Authors: D'Odorico Paolo , Revelli Roberto , Ridolfi Luca
Published: September 2002

This paper investigates if the use of neural networks can improve the accuracy of tree‐ring based paleoclimatic reconstructions with respect to some of the commonly used methods. A three layers feedforward model of neural network is shown to be very efficient in explaining a high percentage of the variance of the instrumental climatic record both for calibration and validation. Some traditional statistics have been estimated to evaluate the accuracy of the reconstruction. These results have been finally compared with those of a regression‐based model, showing the higher accuracy of the neural network reconstruction.
Paleoclimatic data from 74KL and Guliya Cores: New insights
Authors: Rangarajan Govindan , Sant Dhananjay A.
Published: September 2002

We analyze multi‐proxy climatic records from the 74KL marine core and compare it with the δ18O record from the Guliya ice core. These records have important implications for the evolution of the Southwest Monsoon System. We observe three distinct climatic events (viz. 19 ka to 13.5 ka, 13.5 ka to 10 ka and 3.4 ka to present) from the last glacial phase to the present. Even though the first two events are well documented in the literature, our combined analysis of data from both the cores suggests alternative mechanisms which could have complemented and strengthened these events. The third, most recent event (occurring between 3.4 ka to present) does not seem to have been well documented earlier. We propose a possible mechanism to explain this event. We identify representative elements which better capture the above events as compared to more commonly used elements.
Lyman‐α imaging of the SO2 distribution on Io
Authors: Feldman Paul D. , Strobel Darrell F. , Moos H. Warren , Retherford Kurt D. , Wolven Brian C. , McGrath Melissa A. , Roesler Fred L. , Woodward R. Carey , Oliversen Ronald J. , Ballester Gilda E.
Published: September 2002

Imaging spectroscopy of Io in the ultraviolet (1160–1720 Å) was carried out with the Space Telescope Imaging Spectrograph on HST on three dates in October 1997 and August 1998. Among the initial results was the observation of concentrated regions of Hi Lyman‐α flux near the poles of Io that exhibited a morphology and temporal variability different from those of the atomic oxygen and sulfur emission regions seen near the equatorial limbs. We examine the suggestion that the primary source of Lyman‐α emission is surface reflected solar radiation that penetrates the thin polar atmosphere, but is strongly absorbed by the thicker SO2 atmosphere near Io's equator. Spectral and spatial analyses lead to derived SO2 column densities that are in good agreement with those derived from earlier HST observations of Io's albedo in the 2000–2300 Å wavelength range. The Lyman‐α images clearly illustrate features of Io's atmosphere that have been deduced from previous observations and theoretical modeling: a non‐uniformity with respect to the sub‐solar point dominated by a freezing out of the SO2 near the poles and variation with both longitude and time due to the variability of the sources of the atmospheric gas. Lyman‐α imaging is demonstrated to be an extremely powerful and direct way to globally map the dynamic atmosphere of Io.
The 1976 Friuli (NE Italy) thrust faulting earthquake: A reappraisal 23 years later
Authors: Aoudia A. , Saraó A. , Bukchin B. , Suhadolc P.
Published: September 2002

We revisit the 1976 Friuli earthquake sequence by combining hypocenters relocation, long period surface wave inversion, field geology and strong motion modelling. We show that fault‐related folding is the main active deformation by which the seismic energy was released during the main shock (Ms=6.5) and that some of the surface effects reported in 1976 correspond to widespread bedding planes displacements induced by flexural‐slip folding. The fault evolved from blind to semi‐blind along strike showing the control of the inherited structural geology on the fault surface break and rupture arrest. Our fault model produces waveforms that fit the accelerograms recorded in the area.