bluuebunny/crossref_metadata_2025 huggingface dataset:
Different ERS altimeter measurements between ascending and descending tracks caused by wind induced features over ice sheets
Authors: Legresy Benoît , Remy Frédérique , Schaeffer Philippe
Published: December 2003

The ERS1 satellite provides a high‐resolution radar altimeter measurement up to 81.5° latitude for polar cap studies. Here we examine one particular effect of the interaction between the radar wave and the snowpack surface on height measurements. Measurements from ascending and descending tracks show systematic differences as large as 1 m. Such an artifact to be removed if it can be estimated. The difference appears to be related to the way the altimeter perceives wind‐induced sculpting of the snowpack. Differences in the volume echo weight between ascending and descending passes appear to be the explanation. Seasat scatterometer allowed us to estimate independently the signal that could be inferred by wind features. Our results yield a map of the effect of wind on radar altimetric measurement. Finally, we consider the possibility of long‐term variations in altimeter measurements caused by climatic changes.
Sources of reactive nitrogen in the upper troposphere during SONEX
Authors: Liu S. C. , Yu H. , Ridley B. , Wang Y. , Davis D. D. , Kondo Y. , Koike M. , Anderson B. E. , Vay S. A. , Sachse G. W. , Gregory G. L. , Fuelburg H. , Thompson A. , Singh H.
Published: December 2003

The relationship among NOy, O3, N2O, ultra‐fine condensation nuclei (CN), and other trace gases in the upper troposphere (UT) and lower stratosphere (LS) observed during SONEX are analyzed with the goal to identify and quantify the sources of NOy in the UT. We use N2O to separate upper tropospheric air from stratospheric influenced air and focus our analysis to the former. The distributions of NOy and O3 show remarkable similarity when they are plotted as a function of N2O. The only difference between NOy and O3 is found in upper tropospheric air where a large number of data points have high values of both NOy and the NOy/O3 ratio. Major sources contributing to these high NOy values are found to be emissions from lightning and surface sources transported to the UT by convection.
Can El Nińo amplify the solar forcing of climate?
Authors: Ruzmaikin A.
Published: December 2003

ENSO (El Nińo and the Southern Oscillation) is considered as a stochastic driver that excites the atmospheric anomaly states, such as Pacific North American pattern. This can make 11‐year solar activity forcing of climate feasible through stochastic resonance‐a mechanism that amplifies a weak input to a nonlinear bistable system by the assistance of noise.
Altitude dependence of the ice sheet surface climate
Authors: Krinner G. , Genthon C.
Published: December 2003

The dependence of surface climate on elevation changes is evaluated for Greenland and Antarctica in an atmospheric general circulation model (AGCM) sensitivity study.

The results indicate a rather low temperature sensitivity of ≈ −5 K km−1 near the coasts, which is weaker than the values often used in parameterizations in climate models. In the plateau areas, the sensitivity is stronger (−10 K km−1).

Relative precipitation change is relatively strong (≈ −10%/100m) on the plateaus, while near the coasts, precipitation strengthens when the surface elevation is increased.
The Philippine Sea: New survey results reveal the structure and the history of the marginal basins
Authors: Okino Kyoko , Ohara Yasuhiko , Kasuga Shigeru , Kato Yukihiro
Published: December 2003

The Hydrographic Department of Japan initiated a long‐term geological and geophysical survey of the Philippine Sea in 1983. We here summarize the bathymetry and magnetic anomalies of this area, focusing on the structure and the evolutionary process of the marginal basins. The Shikoku and Parece Vela Basins developed as one backarc system in the later phase of their formation; the significant difference between the two basins, however, is clearest near the extinct spreading centers. The steep Oki‐Daito Escarpment, located in the northern West Philippine Basin, was mapped for the first time during this survey. The N‐S trending spreading fabric north of the escarpment is in striking contrast to the NW‐SE to E‐W lineament in the south. Our data reveal a more complicated history of the western Philippine Sea than has been proposed in previous studies.
GCM simulations of snowball earth conditions during the Late Proterozoic
Authors: Jenkins Gregory S. , Smith Steven R.
Published: December 2003

In order to simulate the Snowball Earth conditions that may have existed during the late Proterozoic we have conducted a series of GCM simulations using a simple 50‐meter slab ocean, a reduced solar constant of 6% and varied CO2 concentrations. In this study, we vary the CO2 concentration from 100 to 3400‐ppmv and use rotation rates corresponding to 18 and 24‐hour day‐lengths. We also examine the effects of increasing the poleward transport of heat by the oceans. Our results show that below a critical value of approximately 1700 ppmv of atmospheric CO2, sea‐ice and sub‐freezing temperatures occur from the poles to the Equator. A global mean annual two meter air temperature of 221°K is found for boundary conditions of 100 ppmv atmospheric CO2, 6% reduction in solar forcing and a rotation rate corresponding to an 18 hour day. These results confirm those of earlier studies suggesting or implying that low‐latitude glaciation occurred during the late Proterozoic. However, since the ocean is the critical factor for low‐latitude glaciation, the results should be view cautiously because of the simple slab‐ocean used in this study.
Mercury in aquatic systems in Antarctica
Authors: Lyons W. Berry , Welch Kathleen A. , Bonzongo Jean‐Claude
Published: December 2003

We have measured Hg concentrations in one of the most remote aquatic systems in the world, Taylor Valley (∼78°S), Antarctica. These measurements, along with the previous work of Vandal et al. [1998], provide baseline concentrations of Hg, as well as an assessment of Hg speciation, in a remote aquatic environment. Dissolved Hg concentrations in the surface waters of these lakes and glacial streams range from 0.10 to 0.44 ngL−1. These values are similar to those reported from other remote locations and suggest these values represent current baseline values globally.
Regularization uncertainty in density models estimated from normal mode data
Authors: Resovsky Joseph S. , Ritzwoller Michael H.
Published: December 2003

Normal mode structure coefficients provide important constraints on the long‐wavelength component of 3‐D mantle density (ρ) structure, but inversions for independent models ofυs,υp,and ρ using normal mode data alone are ill‐posed even at long wavelengths. Ill‐posed inversions typically are regularized by imposinga prioriassumptions on the set of estimated models, but such regularization can introduce important uncertainties in the models. We characterize these uncertainties for ρ models estimated from current normal mode data using a set of 512 different “regularization schemes”. These schemes sample a variety of plausiblea prioriassumptions about the nature and distribution of mantle heterogeneity by specifying allowableυs,υp, ρ, and boundary topography structures. The estimated ρ models are fairly robust with respect to prior constraints onυs,υp, and topography. However, the character and amplitude of the estimated ρ models depend strongly on how ρ is allowed to decorrelate fromυs, and we display several models in which ρ andυs, decorrelate in very different depth intervals. Because these models all result from plausible prior constraints and fit the data equally and acceptably well, inversions of current normal mode data cannot robustly locate the decorrelation of ρ fromυs. It remains possible that reliable ρ models may be obtained in the future as more normal mode measurements are introduced to break the strong tradeoffs between upper and lower mantle ρ structures that characterize current inversions.
Behavior of oceanic crustal magnetization at high temperatures: Viscous magnetization and the marine magnetic anomaly source layer
Authors: Bowles Julie A. , Johnson H. Paul
Published: December 2003

Although the source layer for marine magnetic anomalies has been assumed to be the extrusive basalts of uppermost ocean crust, recent studies indicate that lower crustal rocks may also contribute. Because the temperature at which magnetization of crustal rocks achieves long‐term stability is crucial to any source layer contribution, we undertook high‐temperature VRM (viscous remanent magnetization) experiments on samples of basalt, dike and gabbroic sections. Samples were heated at temperature intervals up to Tc, while a magnetic field was applied for periods between 6 hours and 28 days. Results show that the dike and gabbro samples achieve maximum VRM acquisition near 250°C, well below the Tc of 580°C. The basalt sample shows a peak at 68°C, also well below Tc. Results of this pilot study indicate that the critical isotherm for stable magnetization acquisition is defined by the VRM behavior of the specific crustal section.
Bidirectional reflectance of rough bare soil surfaces
Authors: Despan D. , Bedidi A. , Cervelle B.
Published: December 2003

A BRDF model for rough surfaces using probabilistic expressions for the shadowing factor and the conditional mean slope was previously proposed. We present here its validation by laboratory measurements on bare soil surfaces, using the European Goniometric Facility (EGO). The model could be used to retrieve the statistical characteristics of land profiles in astronomy and terrestial remote sensing.
The influence of clouds and water vapor on atmospheric absorption
Authors: Arking Albert
Published: December 2003

Observations during a field experiment in Oklahoma in the fall of 1995 have been analyzed to show the influence of clouds and water vapor on broadband absorption of solar radiation in the atmosphere. The fraction of solar flux incident at the top of the atmosphere absorbed during 34 days was 0.24±0.04. For a 7‐day subset during which skies were clear, it was 0.22±0.02. Both clouds and water vapor contribute to day‐to‐day variability. The water vapor effect is systematic, increasing absorption by 0.04–0.05 per g cm−2 increase of column water vapor. Clouds, on the other hand, increase or decrease total column absorption, depending on cloud configuration. Most of the average cloud effect is due to water vapor, which was highly correlated with cloud amount during the experiment. Overall, model calculations underestimate atmospheric absorption by 0.06, with 25–55% of the discrepancy associated with water vapor, ∼15% with clouds, and the remaining 30–60% independent of either variable.
3D simultaneous seismic refraction and reflection tomography of wide‐angle data from the Central Chilean Margin
Authors: Zelt Colin A. , Hojka André M. , Flueh Ernst R. , McIntosh Kirk D.
Published: December 2003

We present an application of three‐dimensional (3D) simultaneous seismic refraction and reflection tomography for velocity and interface structure. The inversion technique and method for developing the starting model are specifically designed for relatively sparse wide‐angle data acquired across strongly‐varying structure. The data were recorded in a region of seamount subduction on the Chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90×90 km area providing constraint to 25 km depth. The tomographic method and the final model are assessed through a comparison with the large‐scale geologic features of the margin and a resolution test. The 3D model shows the Valparaiso forearc basin, the accretionary wedge, the subducting plate, and possibly a subducted seamount. Our results show the potential of relatively sparse 3D wide‐angle data.
Spectrometry of pasture condition and biogeochemistry in the central Amazon
Authors: Asner Gregory P. , Townsend Alan R. , Bustamante Mercedes M. C.
Published: December 2003

Regional analyses of Amazon cattle pasture biogeochemistry are difficult due to the complexity of human, edaphic, biotic and climatic factors and persistent cloud cover in satellite observations. We developed a method to estimate key biophysical properties of Amazon pastures using hyperspectral reflectance data and photon transport inverse modeling. Remote estimates of live and senescent biomass were strongly correlated with plant‐available forms of soil phosphorus and calcium. These results provide a basis for monitoring pasture condition and biogeochemistry in the Amazon Basin using spaceborne hyperspectral sensors.
Implications of the oceanic thermal skin temperature deviation at high wind speed
Authors: Donlon C. J. , Nightingale T. J. , Sheasby T. , Turner J. , Robinson I. S. , Emergy W. J.
Published: December 2003

Extensive oceanographic and atmospheric observations obtained during three independent experiments in the Atlantic Ocean are used to demonstrate the relationship between wind speed and the temperature deviation ΔT, which is defined as the sea surface skin temperature (SSST) minus the subsurface bulk sea surface temperature (BSST). At wind speeds <6 m s−1, the variability of ΔT increases because thermal stratification complicates the measurement and interpretation of ΔT: extreme ΔT magnitudes of >1.5 K are common during periods of high insolation. The variability of ΔT at night is reduced and extreme cool skin temperatures of < −0.5 K are recorded. In all cases, at wind speeds >6 m s−1, the variability of ΔT is diminished and the mean value of ΔT approximates a cool bias of −0.14 K±0.1 K. We conclude that BSST measurements obtained at wind speeds >6 m s−1, when corrected for a small ( −0.14 K) cool bias, are representative of the SSST and can be used with confidence to validate satellite derived SSST. When the wind speed is <6 m s−1 and the magnitude of ΔT is high, in situ radio metric SSST measurements are mandatory to validate satellite derived SSST.
Enhanced Arctic stratospheric gravity wave activity above a tropospheric jet
Authors: Whiteway James A. , Duck Thomas J.
Published: December 2003

Meteorological balloon measurements were applied to investigate gravity waves in the stratosphere above three weather stations in the Canadian Arctic. It was found that a distinct enhancement in the amount of wave activity occurred at each of the three stations simultaneously while a tropospheric jet stream crossed the Arctic during mid‐November of 1996. An interpretation is proposed in which the enhanced wave activity occurs as the tropospheric jet provides conditions that are favourable for the upward propagation of mountain waves. Further propagation into the stratosphere is then facilitated during winter when the tropospheric and stratospheric jets coincide.
Stochastically induced climate shift of El Niño‐Southern Oscillation
Authors: Flügel Moritz , Chang Ping
Published: December 2003

Observational studies indicate that the characteristics of El Niño‐Southern Oscillation (ENSO) during the 1980s are noticeably different from those during the 1960s and 1970s. Here, we provide modeling evidence that stochastic processes can not only produce a shift in the persistence of sea‐surface temperature (SST) but also causes changes in the predictability of ENSO. Based on this result, we suggest that climate shifts of ENSO, such as that occurred around 1976, could be induced by stochastic processes. Therefore, the stochastic mechanism should be taken as a null hypothesis for climate shifts against which competing mechanisms must be compared.
Simulation of postseismic deformations caused by the 1896 Riku‐u Earthquake, northeast Japan: Re‐evaluation of the viscosity in the upper mantle
Authors: Suito Hisashi , Hirahara Kazuro
Published: December 2003

We examine the postseismic deformations which are induced by an inland earthquake at a subduction zone, and investigate especially the effect of the viscoelastic heterogeneity through Finite Element Method (FEM). We estimate the viscosity of the upper mantle beneath the northeast Japan with realistic structures based on the leveling data which includes the postseismic deformations due to the 1896 Riku‐u earthquake. For this purpose, we construct two 3‐D FEM models with different viscoelastic structures, namely Layered Model and Plate Model. The results show that spatial patterns of the postseismic deformations differ considerably, depending on the two viscoelastic structures. Postseismic deformations are strongly controlled by the effective thickness of the viscoelastic medium above descending plate. At the surface, if the structure is complicated, the relaxation time is outwardly longer and varies significantly with position. Considering the plate structure, the profile of surface deformation and decaying subsidence rate constrain the Maxwell time of the upper mantle to be 5 years, which is shorter than the value of 10 years estimated by Thatcher et al. [1980] assuming the layered structures.
Deep sub‐crustal features in the Bengal Basin: Seismic signatures for plume activity
Authors: Mall D. M. , Rao V. K. , Reddy P. R.
Published: December 2003

The basaltic Rajmahal Traps (117 Ma), India are believed to be due to a mantle plume. The plausible plume path on the continental side has not been traced probably because of the thick sedimentary cover. These Traps are located close to the Bengal basin; therefore, a re‐examination of the deep seismic velocity structure should help in tracing the plume path on the continental side and its possible relationship to plume activity. Data obtained along four seismic profiles in the Bengal basin were re‐examined, which helped in identifying and modeling strong reflections from lower crust and Moho. The results indicate a 7.5 km/s velocity zone as an underplated mantle material injected by the mantle plume or hotspot at the base of the continental crust. The study also indicates the probable trace of plume in the continental region, a NNW‐SSE trending path east of 87°E with an up‐warp in Moho.
Sensitivity of seismic wide‐angle wave‐field and first arrival times to fine scale crustal structure and moho topography
Authors: Hansen Thomas M. , Singh Satish C. , Jacobsen Bo H.
Published: December 2003

A sensitivity study of stochastic crustal and Moho models on Moho refracted first arrivals is presented. The crustal model is a self‐affine velocity structure superposed on a linear velocity gradient. The Moho is represented by a first‐order discontinuity with a topography which is also self‐affine. The sensitivity of both travel‐time and wave‐field is considered. We found that the full wave‐field around the Pn arrival is sensitive to crustal structures down to 1 km, and to Moho undulations down to 5 km. However, the first arrivals are only sensitive to crustal structure down to about 3 km, and to Moho structure down to 10 km. Our analysis suggests that the wide‐angle full wave‐field has a sensitivity that is intermediate between the sensitivity of the vertically reflected wave‐field and wide‐angle arrival times. These findings give some justification to the mainstream ray tracing approach to wide‐angle data inversion, where resulting models incorporate only a moderate number of features at relatively large scales.
Remote sensing of cirrus cloud parameters based on a 0.63‐3.7 µm radiance correlation technique applied to AVHRR data
Authors: Ou S. C. , Liou K. N. , King M. D. , Tsay S. C.
Published: December 2003

Using the data gathered from the Advanced Very High Resolution Radiometer (AVHRR) 0.63 and 3.7 µm channels, an algorithm for the inference of cirrus cloud optical depth and mean effective size has been developed for the first time. This scheme is based on the correlation between the 3.7 µm (total) and 0.63 µm radiances that is constructed from radiative transfer calculations involving ice crystal clouds. Application of the algorithm to AVHRR channels has been performed by using data sets that were collected during the First ISCCP Regional Experiment, Phase II, Cirrus Intensive Field Observation (FIRE‐II‐IFO; November–December 1991) at Coffeyville, Kansas. For validation, the in‐situ data collected by the balloon‐borne replicator and airborne 2‐D probes that were collocated with AVHRR pixels were carefully analyzed for five cases involving single and multiple cirrus cloud layers. We demonstrate that the retrieved cirrus cloud optical depths and mean effective sizes compare reasonably well with those determined from the in‐situ analyses.
Sulfate‐induced cooling in the southeastern U.S.: An observational assessment
Authors: Saxena V. K. , Menon Surabi
Published: December 2003

The effect of sulfates on cloud microphysical and optical properties have been studied using data from a rural mountaintop location in the Mt. Mitchell State Park, North Carolina. Although the amended Clean Air Act has limited the sulfur emissions beginning 1990, we found the sulfate concentrations greater during 1993–97 compared to that during 1986–89. Cloud albedo inferred from satellite data and calculated from surface observations does not indicate a monotonic increase with increasing sulfates. The direct and indirect (cloud‐mediated) sulfate forcing are assessed to be 4.8 and −4 W m−2. These values exceed current model predictions as evaluated by other investigators. Surface temperature records of the region during 1949–94 indicate a cooling trend tacitly supporting our assessment.
Hydrous minerals in the mantle wedge and the maximum depth of subduction thrust earthquakes
Authors: Peacock Simon M. , Hyndman Roy D.
Published: December 2003

In many subduction zones the downdip limit of thrust earthquakes approximately coincides with the intersection of the subduction thrust with the forearc mantle. This limit may be explained by aseismic hydrous minerals present in the mantle wedge. During subduction, fluids released from the subducting slab infiltrate the overlying forearc mantle forming serpentine + brucite, especially in cool subduction zones. At the slab interface itself, talc‐rich rocks form in the mantle by the addition of silica transported by rising fluids and by mechanical mixing of mantle and siliceous rocks. In the laboratory, serpentine generally exhibits stable‐sliding aseismic behavior. The behavior of talc, a layered hydrous silicate, and brucite, a layered hydroxide, has not been investigated, but their structures also suggest weak stable‐sliding behavior. We suggest all three layered hydrous minerals promote aseismic behavior and that their presence controls the downdip limit of thrust earthquakes in many subduction zones.
Peculiarities of interannual changes in the mean wind and gravity wave characteristics in the mesosphere over Shigaraki, Japan
Authors: Gavrilov N. M. , Fukao S. , Nakamura T.
Published: December 2003

Results are presented of month by month measurements of the mean wind and intensity and momentum fluxes of internal gravity waves (IGWs) with the Middle and Upper atmosphere radar (MU radar) at the altitudes 65–80 km at Shigaraki, Japan (35°N, 136°E) in 1986–1997. The mean zonal wind is eastward in winter and westward in summer, but the amplitude of these annual variations is different in different years, having a minimum in 1992–1994. The mean meridional wind shows substantial quasi‐bi‐annual variations. IGW intensities and momentum fluxes had a maximum in 1991–1994. Some possible reasons for such interannual changes are discussed.
New observations of heavy‐ion‐rich solar particle events from ACE
Authors: Cohen C. M. S. , Mewaldt R. A. , Leske R. A. , Cummings A. C. , Stone E. C. , Wiedenbeck M. E. , Christian E. R. , von Rosenvinge T. T.
Published: December 2003

Following launch of the Advanced Composition Explorer in August 1997, the Solar Isotope Spectrometer measured the composition of nine solar energetic particle events. We have used isotopic measurements of Ne to determine the degree of charge‐to‐mass‐dependent fractionation and infer the charge states of C‐Ni in the four most heavy‐ion‐rich of the nine events. The results indicate a source temperature of ∼4×106 K; this and the measured abundances suggest that these four events are more characteristic of impulsive events than gradual. Although the ³He/4He ratios are not enhanced to the level commonly ascribed to impulsive events, there are sizable enhancements over typical solar wind values measured in three of the events.
Event‐to‐event variations in the isotopic composition of neon in solar energetic particle events
Authors: Leske R. A. , Mewaldt R. A. , Cohen C. M. S. , Cummings A. C. , Stone E. C. , Wiedenbeck M. E. , Christian E. R. , von Rosenvinge T. T.
Published: December 2003

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have measured the isotopic composition of neon at 24–72 MeV/nucleon in nine solar energetic particle events between 4 November 1997 and 16 November 1998. The 22Ne/20Ne ratio varies from ∼0.7 to ∼2 times the solar wind value from event to event, indicating significant mass fractionation in the acceleration and/or transport of these particles. The mass fractionation is strongly correlated with the Fe/O and Na/Mg element abundance ratios, suggesting that elemental and isotopic fractionation are both governed by the ionic charge to mass ratio.
Semidiurnal behavior of quasi‐periodic echoes in the mid‐latitude Es region observed with the Chung‐Li VHF Radar
Authors: Pan C. J. , Tsunoda Roland T.
Published: December 2003

The range rate of the so‐called quasi‐periodic (QP) echoes from nighttime mid‐latitude sporadic‐E regions observed with the Chung‐Li VHF radar during the SEEK (Sporadic‐E Experiment over Kyushu) campaign displayed semidiurnal characteristics. The time periods before ∼2200 LT and after ∼0400 LT were dominated by the negative range rate QP (NQP) echoes. In contrast, a preponderance of positive range rate QP (PQP) echoes occurred between 2200 and 0400 LT. The slopes of NQP echoes found during the evening period steepened with time and became positive around 2200 LT. Similar behavior was also found prior to ∼0400 LT when PQP echoes were replaced by NQP echoes during the SEEK campaign. The sign of the range rate changed around 2200 and 0400 LT on the six consecutive nights. Semidiurnal neutral‐wind variations are proposed to explain the slope sign reversals in the observations. These results, together with previous findings, suggest that semidiurnal tides and long‐period waves (with 30 minute periodicity) may be playing a role, in addition to atmospheric gravity waves, in the formation of QP echoes.
On the nature of radar echoes below 95 km during counter streaming in the equatorial electrojet
Authors: Tsunoda Roland T. , Ecklund Warner L.
Published: December 2003

Using data obtained with a 49.92‐MHz radar on the island of Pohnpei (Federated States of Micronesia), we show that a weak, westward current exists at the base of the normal eastward electrojet. We suggest that this counter stream is driven by a downward‐directed electric field associated with positive‐charge accumulation near the base of the eastward electrojet. To explain the existence of radar echoes from this counter‐streaming region, where conditions are not conducive to irregularity generation by the gradient‐drift instability (GDI), we suggest that “seed” plasma structure may be produced by neutral turbulence. If so, the GDI could act on appropriately directed gradients in plasma density that are associated with the seed structure to generate the secondary type‐2 irregularities responsible for the radar echoes.
Velocity field of around the Sea of Okhotsk and Sea of Japan regions determined from a new continuous GPS network data
Authors: Takahashi H. , Kasahara M. , Kimata F. , Miura S. , Heki K. , Seno T. , Kato T. , Vasilenko N. , Ivashchenko A. , Bahtiarov V. , Levin V. , Gordeev E. , Korchagin F. , Gerasimenko M.
Published: December 2003

To investigate the current crustal movements in and around the Sea of Okhotsk and Sea of Japan regions, we have established a continuous GPS network. By the end of 1997, the network had been expanded to include 12 new stations. Data for the period from July 1995 to November 1997 were analyzed together with data from International GPS Service for Geodynamics (IGS) global stations. To fix the estimated coordinates to the terrestrial reference frame, the Tsukuba IGS station was assumed to be moving westward relative to the stable Eurasian continent at ∼2cm/yr according to Heki's[l996] estimate. We find that: (1) stations in the western margin of the Sea of Japan have eastward velocity vectors, (2) the pole position of the Okhotsk plate is located near Okha, which reconfirms the Okhotsk micro plate, (3) a plate boundary of the Okhotsk and Amurian plates between southen Sakhalin and Hokkaido is suggested.
The locations of magma chambers at Mt. Etna, Italy, mapped by b‐values
Authors: Murru M. , Montuori C. , Wyss M. , Privitera E.
Published: December 2003

We mapped b‐values in three dimensions in a cube of the crust with approximately 20 km dimensions under Mt. Etna. The earthquake catalog used contains 450 events with M≥2.5 for 1990‐1997.9. Comparison of b‐values derived from samples of N=50 events, extracted from volumes with radii ranging from 2.5 to 6 km, showed statistically highly significant differences between b=1.5±0.3 in most of the crust and b=3.0±0.5 in two volumes with r≤5 km, one located 2 km E of the summit of Etna and at 10±3 km depth and the other one WSW of Etna at a depth of about 3±2 km below sea level. We interpret these high b‐value anomalies as an expression of two active magma chambers at those locations.
A new model for volcanic earthquake at Unzen Volcano: Melt Rupture Model
Authors: Goto Akio
Published: December 2003

Small earthquakes occurred inside the flowing magma during the 1991–1995 activities of Unzen Volcano. This paradoxical phenomenon can be explained when we consider that even a flowing magma fails brittly under the condition that the product of viscosity and strain rate of magma exceeds a certain threshold value. The estimated strain rate of magma in the conduit leads us to the conclusion the brittle failure is expected when the viscosity is over 1011.5 Pa s. Laboratory measurements of samples show that this viscosity range can be attained when the temperature is under 850°C in dry condition, and 800°C with 0.6 wt.% water. Considering the temperature from Fe‐Ti oxide geothermometer (780–880°C) and the gas temperature (800°C), the brittle failure condition could be satisfied inside the conduit. The melt failure by shear flow could be the source of the earthquakes occurred at the summit area.
The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM Climate Change Experiment
Authors: Dixon Keith W. , Delworth Thomas L. , Spelman Michael J. , Stouffer Ronald J.
Published: December 2003

The mechanism by which the model‐simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi‐century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.
Diagnosing the excitation mechanisms of Pc5 magnetospheric flank waveguide modes and FLRs
Authors: Mann Ian R. , Wright Andrew N.
Published: December 2003

Using the theoretical results of Mann et al. [1999], we show how the characteristics of multiple field line resonances (FLRs), driven by waveguide mode harmonics, can be strongly local time dependent and how these characteristics can be used to differentiate between waves driven by solar wind stimuli or by the development of magnetopause shear flow instabilities. Using observations previously reported in the literature, we show examples of this diagnostic analysis and conclude that the dawn/dusk asymmetry in Pc5 FLR characteristics may be linked to different dominant excitation mechanisms operating either side of noon. In particular, we suggest there is a preference for dawn‐side FLRs to be excited by waveguide modes which are amplified through the action of over‐reflection at the magnetopause.
Reconstruction of monthly NAO and EU indices back to AD 1675
Authors: Luterbacher Jürg , Schmutz Christoph , Gyalistras Dimitrios , Xoplaki Eleni , Wanner Heinz
Published: December 2003

Instrumental station pressure, temperature and precipitation measurements and proxy data were used to statistically reconstruct monthly time series of the North Atlantic Oscillation (NAO) and the Eurasian (EU) circulation indices back to 1675. Systematic testing of the reconstruction procedure indicated generally reliable reconstructions throughout the entire period, except for summertime before about 1750. Predictive skill varied for different sub‐periods depending on data availability. It was highest for autumn and winter and was generally better for the EU than for the NAO index. Wavelet analysis suggested significant low‐frequency variability, especially for the spring, summer and annual averaged indices. The co‐variability between the NAO and EU indices was found to exhibit large decadal to century timescale variations, indicating that climate variability over the continent is temporarily decoupled from the NAO.
Solar cycle length and 20th Century northern hemisphere warming: Revisited
Authors: Damon Paul E. , Peristykh Alexei N.
Published: December 2003

It has been suggested that the length of the solar cycle (SCL) is related to solar forcing of global climate change [Friis‐Christensen and Lassen, 1991]. Although no physical mechanism had been proposed, the relation seemed to be supported by interesting correlations with several paleoclimate records and, separately, with the 20th century Northern Hemisphere instrumental record. Actually, what has been correlated is the quasi‐sinusoidal Gleissberg cycle which is slightly greater in the 18th century than in the 20th century. Using the pre‐industrial record as a boundary condition, the SCL‐temperature correlation corresponds to an estimated 25% of global warming to 1980 and 15% to 1997.
Fast and accurate depth and source mechanism estimation using P‐waveform modeling: A tool for special event analysis, event screening, and regional calibration
Authors: Goldstein Peter , Dodge Doug
Published: December 2003

We have developed, validated, and applied a fast and accurate, interactive, easy to use tool for estimating seismic event depth and mechanism by modeling the initial P‐wave bundle. Significant features of this tool include its applicability over a broad band of frequencies (f≤2 Hz) and magnitudes (mb≥4.5), its ability to provide accurate high‐resolution depth estimates, and its ability to provide constraints on event mechanism including isotropic contributions. It also provides estimates of random and model errors and can be used to test hypotheses and investigate sensitivity to and trade‐offs between model parameters.

We validated this tool and its algorithms by showing that our estimated depths and mechanisms are in excellent agreement with ground‐truth depths and mechanisms based on local and regional data. We find that depth can usually be estimated to an accuracy of a few km with a small number of stations. Although complete mechanisms are sometimes poorly resolved, we find that two out of three of the mechanism parameters (strike, dip, rake) are usually well constrained. We illustrate the potential of this technique for regional calibration, special event analysis, and event screening by comparing estimated depths and mechanisms of events in the Middle East and North Africa with those of the National Earthquake Information Center (NEIC) and by comparing our depth and mechanism estimates for the May 11, 1998 India nuclear explosion and a November 8, 1991 earthquake that occurred in the same region.
Wave generation by ion horseshoe distributions on auroral field lines
Authors: Bingham R. , Kellett B. J. , Cairns R. A. , Dendy R. O. , Shukla P. K.
Published: December 2003

Recent observation by Freja satellite of ions in the auroral zone at the boundary of the plasma sheet and tail lobe reveal the presence of a ring or horseshoe type distribution function. The horseshoe type distribution function measured by the satellites has a field aligned component which is moving in the case of the Freja data in the down‐going direction with a peak velocity of about 400km/sec. We show that the horseshoe type distribution is unstable to predominantly perpendicular ion waves with frequencies in the lower‐hybrid range.
3D multi‐fluid MHD studies of the solar wind interaction with Mars
Authors: Liu Yifan , Nagy Andrew F. , Groth Clinton P. T. , DeZeeuw Darren L. , Gombosi Tamas I. , Powell Kenneth G.
Published: December 2003

The interaction of the solar wind with planets with no or only very weak intrinsic magnetic fields, such as Mars and Venus, involves their ionospheres. Single fluid MHD models, which incorporate some of the important mass‐loading processes, have been useful in reproducing numerous observed features at these planets, such as the location of the bow shock, but they do have obvious limitations. Our present 3D MHD model is a two‐fluid one, which considers protons and the dominant heavy ions in the ionosphere, separately. We have used this model to study the interaction processes at Mars. The model results are in general agreement with the average observed bow shock shape and position and predict reasonable locations for the ionopause. The calculated oxygen escape flux down the tail was estimated to be 2.7×1025s−1, which is consistent with Phobos‐2 estimates.
Sensing atmospheric structure using small‐scale space geodetic networks
Authors: Elósegui P. , Davis J. L. , Gradinarsky L. P. , Elgered G. , Johansson J. M. , Tahmoush D. A. , Rius A.
Published: December 2003

We describe two ways in which horizontal atmospheric structure affects GPS observations. For a single site, such structure results in azimuthal variations in the atmospheric propagation delay. For a network of sites, the structure will induce an intersite variability in the atmospheric propagation delays. The former effect is more sensitive to high‐ than low‐altitude atmospheric gradients above the site, whereas the latter is insensitive to the altitude of the gradient above the network. This difference in sensitivity can be utilized to probe the local vertical structure of the atmosphere. We demonstrate this technique using GPS data from a small network on the Swedish west coast. We infer for one observing session the presence of a strong horizontal gradient which varies with height.
Robustness of collective behaviour in strongly driven avalanche models: Magnetospheric implications
Authors: Watkins N. W. , Chapman S. C. , Dendy R. O. , Rowlands G.
Published: December 2003

The hypothesis of self‐organised criticality (SOC) predicts that certain open dissipative systems evolve to a critical state where all energy release statistics display power law distributions for event occurrence, size and duration. This has motivated “sandpile” simulations of magnetospheric energy confinement and release events (“avalanches”), previous examples of which have taken the limit where energy inflow (“fuelling”) is slow relative to dissipation, and either uniform or random. However the magnetospheric system has both slow and fast periods mixed together in observations, and naturally modulated fuelling. We have developed an avalanche model with variable, modulated fuelling rate. The power law form for the distribution of energy release events is the least ambiguous current indicator of SOC; we show that this is preserved for the large avalanches in such a system under both constant and varying loading and so such systems are remarkably efficient at eliminating small scale information about their fuelling.
Wetlands: A potentially significant source of atmospheric methyl bromide and methyl chloride
Authors: Varner Ruth K. , Crill Patrick M. , Talbot Robert W.
Published: December 2003

Tropospheric methyl bromide (CH3Br) and methyl chloride (CH3Cl) are significant sources of ozone (O3) destroying halogens to the stratosphere. Their O3 depletion potential (ODP) can be determined from atmospheric lifetimes and therefore their atmospheric budgets, both of which are out of balance with known sink terms larger than identified sources. We have discovered a new source of CH3Br and CH3Cl emissions to the atmosphere at two wetland sites in the Northeastern United States. We have reason to believe that these compounds are biologically produced in situ. Our measurements indicate that the global annual flux of CH3Br and CH3Cl from wetlands could be as high as 4.6 Gg yr−1 Of CH3Br and 48 Gg yr−1 of CH3Cl. These are preliminary estimates based on measurements made during the end of the 1998 growing season, a time period of decreased emissions of other trace gases such as methane (CH4).
Effect of plasma compression on plasma sheet stability
Authors: Lee D.‐Y.
Published: December 2003

Plasma sheet stability to the ballooning mode is analyzed using several physical formulations: ideal MHD, stochastic theory, fast‐MHD, and the Kruskal‐Oberman formulation. It is shown that the major difference among them lies in the plasma compression expression. Explicit computations using the corresponding ballooning equations were performed for two different types of model field lines. For very high β field lines that are excessively stretched where the stochastic description is most appropriate, the ballooning mode is found to be stable or at best weakly unstable. For field lines that are not too much stretched but even rather round, the ballooning instability can be triggered, within both ideal MHD and the stochastic theory, when βe > βec: Here the threshold value of the equatorial beta, βec, is roughly less than unity and practically set by the ideal MHD limit. Also in contrast to a recent suggestion, the fast‐MHD description where the time scale of interest is too short to allow plasma parallel motion is shown to be more stable than ideal MHD. It indicates no instability in all the equilibria that were tested. The Kruskal‐Oberman description is even more stable than fast‐MHD.
Collisionless slow shocks in magnetotail reconnection
Authors: Cremer Michael , Scholer Manfred
Published: December 2003

The kinetic structure of collisionless slow shocks in the magnetotail is studied by solving the Riemann problem of the collapse of a current sheet with a normal magnetic field component using 2‐D hybrid simulations. The collapse results in a current layer with a hot isotropic distribution and backstreaming ions in a boundary layer. The lobe plasma outside and within the boundary layer exhibits a large perpendicular to parallel temperature anisotropy. Waves in both regions propagate parallel to the magnetic field. In a second experiment a spatially limited high density beam is injected into a low beta background plasma and the subsequent wave excitation is studied. A model for slow shocks bounding the reconnection layer in the magnetotail is proposed where backstreaming ions first excite obliquely propagating waves by the electromagnetic ion/ion cyclotron instability, which lead to perpendicular heating. The T⟂/T∥ temperature anisotropy subsequently excites parallel propagating Alfvén ion cyclotron waves, which are convected into the slow shock and are refracted in the downstream region.
Comment on “A reexamination of the ‘Stratospheric Fountain’ Hypothesis” by A. E. Dessler
Authors: Vömel H. , Oltmans S. J.
Published: December 2003

Dessler (1998) analyzed 60,000 radiosonde profiles to re‐examine an analysis done nearly 20 years earlier by Newell and Gould‐Stewart (1981) (hereafter: NGS). Contrary to NGS, Dessler finds that the mean tropical tropopause saturation mixing ratio (SMR) is sufficient to explain stratospheric dryness and that there is no need to assume a seasonal or regional preference for water vapor entering the stratosphere. However, in using the SMR to compute the water vapor amount, he implicitly assumes that the air is saturated, since only under this condition are temperature and water vapor physically connected. If the air is not saturated, the average computed by Dessler will overestimate the amount of water crossing the tropical tropopause ([H2O]e), while supersaturation and the presence of ice particles will underestimate [H2O]e. These processes, which are likely to have different regional and temporal distributions, may have fortuitously canceled each other in Dessler's analysis. NGS studied the distribution of tropopause temperatures and focused on the regions and seasons, in which dehydration is more likely to take place. Recent studies indicate that the tropical tropopause has been cooling over the last 25 years and show that the years used by Dessler have the coldest tropical tropopause temperatures. Thus his conclusion may have been different if other years had been studied. These differing viewpoints emphasize the need for a detailed understanding of the stratospheric dehydration mechanism.
Climatic trends and interdecadal variability from south‐central Pacific coral records
Authors: Boiseau Muriel , Ghil Michael , Juillet‐Leclerc Anne
Published: December 2003

We study climate variability over interdecadal time scales from δ13C and δ18O records, measured in a Porites lutea coral core drilled in the Moorea lagoon. Previously, we demonstrated that the annual δ18O variations reflect those in sea‐surface temperature (SST), while the annual δ13C variations record primarily cloud‐cover changes. We apply two distinct spectral analysis methods to the δ13C and δ18O time series from 1853 to 1989. Our analysis of the δ18O record shows a secular warming trend and a 35‐year oscillation in temperatures over the last 137 years. A trend and a 34‐year cycle are also found in the δ13C record. The latter trend may be caused by either anthropogenic effects or a cloud‐cover increase in this region of the Pacific Ocean, while the interdecadal oscillation appears to be related to changes in cloud cover, as well as in precipitation.
Acetone in the free troposphere and lower stratosphere: Aircraft‐based CIMS and GC measurements over the North Atlantic and a first comparison
Authors: Wohlfrom K.‐H. , Hauler T. , Arnold F. , Singh H.
Published: December 2003

Acetone was measured in the free troposphere and lower stratosphere over the North Atlantic by aircraft‐based CIMS (Chemical lonization Mass Spectrometry) and GC (Gas Chromatography) instruments in fall 1997 within the framework of the POLINAT‐2/SONEX campaigns. The measurements included 28 flights covering altitudes from 2.4 to 12.5 km, and geographic latitudes from 20 to 69°N. The data are the most extensive measurements of acetone in the free troposphere and lower stratosphere published to date. A first comparison of the CIMS and the GC method is presented. The acetone data obtained by both instruments in the same air masses agree within 50% which is consistent with the uncertainty limits quoted. An inspection of all flights reveals upper tropospheric acetone to vary very substantially between 200 and 2200 pptv with a mean value of about 600 pptv. By contrast, in the lower stratosphere the acetone abundance is much less variable and is mostly around 100 to 200 pptv. Acetone rich air masses with volume mixing ratios exceeding 1000 pptv and reaching up to 2200 pptv were surprisingly frequently encountered in the upper troposphere. Probably these air masses had recent contact with the continental planetary boundary layer where they took up acetone and/or acetone precursor gases from ground‐level sources.