bluuebunny/crossref_metadata_2025 huggingface dataset:
Cyclic jerky opening of magma sheet and caldera formation during the 2000 Miyakejima volcano eruption
Authors: Fujita Eisuke , Ukawa Motoo , Yamamoto Eiji , Okada Yoshimitsu
Published: October 2002

NIED volcanic observation network at Miyakejima volcano, central Japan, repeatedly detected extraordinary step‐like tilt changes during the 2000 Miyakejima eruption. Associated with these tilt steps, broadband seismometers recorded very long pulse waves with a duration of about 50‐s. During this stage, the summit of the volcano collapsed and sank, forming a caldera with a depth of about 400 m and a diameter of about 1.6 km in total. Prior to the tilt steps, long‐period earthquakes were acceleratingly generated in the shallow part of the volcano, and the ground tilt changes suggested inflation of the volcanic body. The analysis of these data from the tiltmeters and broadband seismometers suggests that the tilt steps and the 50‐s pulse packets were caused by the opening of sill(sheet)‐like tensile cracks oriented in a SE‐NW direction beneath the volcano with a depth of about 6–8 km and a volumetric change of about 106–107 m3.
High resolution spectroscopy of the OIO radical: Implications for the ozone‐depleting potential of iodine
Authors: Ashworth Stephen H. , Allan Beverley J. , Plane John M. C.
Published: October 2002

The absorption spectrum of iodine dioxide (OIO), obtained at high resolution between 540 and 605 nm by cavity ring‐down spectroscopy, exhibits no evidence of coarse rotational structure. Furthermore, a laser induced fluorescence spectrum was not observed when pumping the molecule in this region of the visible spectrum. Ab initio quantum calculations were performed on the ground and first excited states of OIO. The rotational envelopes of the observed absorption bands are very satisfactorily simulated if the lifetime of the excited state is 200 ± 50 fs, indicating prompt predissociation. Quantum calculations indicate photolysis to I + O2, rather than O + IO. The estimated photodissociation rate of OIO in daylight ranges from 0.36 to 2.2 s−1, depending on the choice of absolute cross‐section, which explains why OIO has only been observed in the atmosphere after sunset. Photolysis to yield atomic I will enhance the O3‐depleting potential of iodine in the remote marine boundary layer.
The rate of loss of water from mars
Authors: Hodges R. Richard
Published: October 2002

Photolysis of CO2 in the upper atmosphere of Mars results in escaping oxygen and residual CO. Proceeding at a rate of .5–3 × 1010 mole yr−1, this process could reduce all of the atmospheric CO2 to CO in 20–140 Myr. The persistence of CO2 is maintained by re‐oxidation of CO through several processes that duplicate the classical water‐gas reaction, resulting in the destruction of water at a rate of a few mm per Myr. Owing to the continual decay of the solar EUV flux, oxygen escape and water loss have diminished with time roughly as t−2. If, as seems likely, a predominantly CO2 atmosphere appeared within 100 Myr of the formation of the planet, Mars should have lost several hundred meters of water over geologic time. Since very little carbon has escaped, the mass of the atmosphere has not been much greater than at present during the past 3.5 Gyr.
Annual atmospheric torques: Processes and regional contributions
Authors: de Viron Olivier , Dickey Jean O. , Marcus Steven L.
Published: October 2002

All three components of annual atmospheric torque are analyzed with a focus on understanding the contributions from various sources and the physical interactions involved. The annual variations of the equatorial component are dominated by the torque on Earth's ellipticity, with the X component mainly due to an anomaly over the Himalayas, and the Y component associated with pressure anomalies over the North Pacific Ocean. The axial annual component is due to the combined effect of friction and mountain torque, whose amplitudes are at the same order of magnitude with the friction term being larger. Partial cancellation of the mountain torque over Asia and North America is effected by the out‐of phase contribution of the Andes (South America having the opposite seasonal cycle to Asia and North America).
Origin of Earth's ground noise from 2 to 20 mHz
Authors: Nishida Kiwamu , Kobayashi Naoki , Fukao Yoshio
Published: October 2002

Long‐period seismology probes the Earth's interior through the seismic passband in a period range from 50 to 500 s. The limitation of seismic transparency in this band is known but the source of limitation has been a long‐standing puzzle. Here we report evidence that the transparency is limited, over the entire passband, by fundamental Rayleigh waves excited by global and random sources in a thin layer just above the Earth's surface.
Tectonic versus glacial isostatic deformation in Europe
Authors: Marotta Anna Maria , Sabadini Roberto
Published: October 2002

Tectonic deformation induced by the Alpine collision and Glacial Isostatic Adjustment (GIA) in Europe are modelled by means of a finite element scheme and viscoelastic normal modes techniques in order to compare the resulting deformation patterns with the ITRF2000 (International Terrestrial Reference Frame) geodetic dataset. Due to the Alpine collision and to the flow from the equatorial mantle to the deglaciated regions in the northern hemisphere, both tectonics and GIA contribute to the observed widespread shortening, with tectonics playing a major role in the southern part of the modelled domain, from Potsdam to the Alpine front, and GIA affecting the shortening in proximity of the region subject to post glacial rebound. Since the stiff Baltic Shield causes a counterclockwise rotation of the collision velocity away from the shield itself and the GIA induced velocities are directed outward from the deglaciated region, both tectonics and GIA contribute to the observed east‐west baseline extension of 0.8–1.1 mm/yr at the latitude of ONSA.
Multiscale derivative analysis: A new tool to enhance detection of gravity source boundaries at various scales
Authors: Fedi Maurizio
Published: October 2002

Potential fields generally require to be analyzed by different scales, owning to sources of different depth and extent. Fourier or wavelet filtering not always yield an optimal separation of those different source effects, often introducing some amount of distortion. We propose therefore a new method, the Multiscale Derivative Analysis (MDA), based on the “Enhanced Horizontal Derivative” (EHD) of the field, which does not imply any separation of the effects, but simply enhances those related to a given scale. Such process, based on a weighted sum of increasing order derivatives of the field, converges quickly toward the actual boundaries of the sources. MDA applied to gravity data of the of Thyrrenian Sea (Italia) yields 3 maps of structural lineaments, each one characterized by a different scale. Known, partially known or relatively new structures are described in the maps, allowing a rapid analysis of all the “gravity‐related” structural lineaments of this region.
Fracture properties from seismic data – a numerical investigation
Authors: Toomey Aoife , Bean Christopher J. , Scotti Oona
Published: October 2002

In the upper 5–10 km of the crust fractures may remain partially open, exerting a strong influence over the response of their host material to seismic waves due to their superior deformability. This raises the possibility of using seismic data to determine not only the location but also the properties of fracture networks. We present a particle‐based numerical scheme for modeling rock deformation and wave propagation and show that it accurately captures wave propagation across fractures. We investigate the effect of fracture tensile properties (i.e. fracture cohesion) on the wavefield and demonstrate that seismic data can, in theory, be used to distinguish fracture tensile and compressional properties.
Modelling and analysis of Ørsted total field data over Southern Africa
Authors: Kotzé P. B.
Published: October 2002

Ørsted total intensity data were analysed over the southern African region between 10° South and 45° South in latitude and between 10° East and 45° East in longitude, measured between May 1999 and October 1999. The satellite magnetic field measurements were selected corresponding to magnetic quiet conditions during local day and night times before carrying out an analysis of external fields by subdividing the data into Dst bins 5 nT wide, centred at multiples of 5 nT. The technique of Spherical Cap Harmonic Analysis (SCHA) was then applied to each of these Dst data bins to obtain external field coefficients, after removing the core field by means of the IGRF model. Combining all data sets after removing main ‐ and external fields, enabled the compilation of a total intensity crustal anomaly map over southern Africa at an average satellite altitude of 800 km. The Walvis Ridge and Agulhas anomalies are understood to represent thickened remanently magnetised ocean crust. The Agulhas anomaly however, represents the remnant scar of the process that led to the Gondwana fragmentation.
Influence of source on 2‐D site effects
Authors: Triantafyllidis Petros , Suhadolc Peter , Hatzidimitriou Panagiotis M.
Published: October 2002

The site response has been theoretically estimated along seven 2‐D profiles distributed in the broader urban area of Thessaloniki. The hybrid method employed allows the simulation of various events located at different distances and azimuths around the city. We focus on the results obtained at all points where the profiles intersect. The diversity of the “site response” at each intersection is attributed mainly to the influence of the seismic source, suggesting that the ground response does not depend only on local soil characteristics but also on different earthquake “scenarios” generating the impingent wavefield.
Influence of gas exchange coefficient parameterisation on seasonal and regional variability of CO2 air‐sea fluxes
Authors: Boutin J. , Etcheto J. , Merlivat L. , Rangama Y.
Published: October 2002

We examine the consequences of using four exchange coefficient versus wind speed parameterisations on the air‐sea CO2 flux estimation, using one year of satellite wind speed measurements at global scale. Estimates of the global net CO2 flux to the ocean vary from 1.2 to 2.7 GtC yr−1. Additionnaly, we show 1) that the relative importance of outgassing and absorption fluxes is dependent on the form of the relationships due to a correlation between the direction of the flux and wind speed and 2) that differences on global net air‐sea flux deduced from different K parameterisations are primarily due to parameterisation differences in the wind speed range between 4 and 17 ms−1.
A unified monsoon index
Authors: Li Jianping , Zeng Qingcun
Published: October 2002

There are several monsoon regions in the world. Some monsoon indices have been proposed to describe their variability, but a unified monsoon index suitable for all known monsoon regions has not yet been found. Here we present a unified dynamical index of monsoon, the dynamical normalized seasonality (DNS), and carry out an analysis of observation data over the past 40 years. The analysis shows that the DNS index can characterize the seasonal cycle and interannual variability of monsoons over different areas very well. The South Asia summer monsoon (SASM) sector (5°–22.5°N, 35°–97.5°E) is composed of two independent components, SASM1 (2.5°–20°N, 35°–70°E) and SASM2 (2.5°–20°N, 70°–110°E), with quite different relations with the monsoon rainfall over the South Asia. The African summer monsoon (ASM) is dominated by variability on the decadal time‐scale, and its decadal abrupt decrease in 1967 may be an important cause of the persistent drought over the Sahel region. It is also found that there is a remarkable global correlation pattern between the South China Sea summer monsoon index (SCSSMI) and global precipitation during boreal summer.
Elliptical cross‐section model for the magnetic topology of magnetic clouds
Authors: Hidalgo M. A. , Nieves‐Chinchilla T. , Cid C.
Published: October 2002

In this work we present an extension of a previous model for the topology of magnetic clouds which takes into account their interaction with the solar wind. The main consequence of this interaction is the possible distortion of their cross‐section, that need no longer be circular during the propagation from the Sun to 1AU. The first approach to this deformation is to assume an elliptical geometry for the cross section although we consider a cylindrical shape for the clouds. Once we make the assumptions of an uniform radial component of the plasma current density and a null value for the radial component of the magnetic field vector, the solutions of the Maxwell equations and the continuity equation in stationary conditions, both expressed in elliptical coordinates, incorporate the expansion of the cross‐section of the cloud. To illustrate the model we present the fitting of four selected clouds with different magnetic field profiles. We show in all cases analyzed that the fits are indeed good and have eight free parameters.
Investigation of the shape of noctilucent cloud particles by polarization lidar technique
Authors: Baumgarten Gerd , Fricke K. H. , von Cossart Götz
Published: October 2002

We report on the shape of particles in a noctilucent cloud (NLC) as deduced from measurements by groundbased lidar. The experiment was performed on August 3, 2000 using the ALOMAR Rayleigh/Mie/Raman lidar, located at 69°N, 16°E. Over a period of 74 min, the instrument performed high quality measurements of the polarization state of 532 nm laser light backscattered from NLC particles. From the experiment we derive that the observed depolarization, averaged over the altitude range 84.2 to 85.5 km was δNLC = (1.7 ± 1.0) %. Considering the small ratio of particle size over wavelength this is an unexpectedly large depolarization. The layer of enhanced depolarization was centered 1 km above the maximum of the NLC layer. We compare the observed depolarization with that calculated for cylinder‐shaped NLC particles. The observed depolarization can be explained by the presence of elongated particles with a length‐over‐diameter ratio larger than 2.5.
Current plate motion across the Dead Sea Fault from three years of continuous GPS monitoring
Authors: Pe’eri Shachak , Wdowinski Shimon , Shtibelman Alon , Bechor Noa , Bock Yehuda , Nikolaidis Rosanne , van Domselaar Matthijs
Published: October 2002

The Dead Sea Fault (DSF) is a transform plate boundary between the Arabian plate and the Sinai sub‐plate. The rate of displacement across the fault has been estimated as 1–10 mm/yr. In this study we present a new estimate of the current displacement across the DSF, which is based on three years of continuous Global Positioning System measurements (July 1996 to July 1999). Our analysis of these data shows that relative northward velocity, which is the fault parallel component, on the baseline between Tel Aviv and Katzerin (Golan Heights) and Katzerin and Elat is 1.4 ± 0.3 mm/yr and −1.0 ± 0.5 mm/yr, respectively, assuming a colored noise (white noise plus flicker noise) model for the daily position estimates. By using a simple locked fault model, we estimate that during the three‐year observation period the relative plate motion across the DSF was 2.6 ± 1.1 mm/yr.
Variation of marine aerosol acidity with particle size
Authors: Keene William C. , Pszenny Alexander A. P. , Maben John R. , Sander Rolf
Published: October 2002

pHs were measured in minimally diluted extracts of size‐segregated aerosols sampled under moderately polluted conditions at Bermuda during spring 1997. Extrapolation to aerosol liquid water contents yielded ambient pHs for most super‐μm diameter size fractions in the upper 3s to upper 4s. Aerosols within this size range of each sample approached similar pH. These results are consistent with recent estimates of sea‐salt aerosol pHs based on thermodynamic considerations and model calculations. Most pHs for finer aerosol size fractions were in the 1s and 2s. The H+ + SO42− ↔ HSO4− equilibrium strongly buffered aerosol pH in all size fractions.
Geochemistry of atmospheric aerosols generated from lava‐seawater interactions
Authors: Sansone Francis J. , Benitez‐Nelson Claudia R. , Resing Joseph A. , DeCarlo Eric H. , Vink Sue M. , Heath Jacqueline A. , Huebert Barry J.
Published: October 2002

Trace elements were measured in the aerosol plume produced by lava‐seawater interactions along the shoreline of Kilauea volcano, Hawaii. Plume concentrations were normalized relative to Hawaiian basalt composition and showed a linear log‐log co‐variation with their emanation coefficient (an indicator of element volatility). Normalized aerosol concentrations also consistently covaried with corresponding normalized concentrations in dilute fumarolic gas from Kilauea volcano and fumarolic gas condensates from Kudryavy and Merapi volcanoes, despite different mechanisms of element volatilization. Conservatively estimated regional ocean deposition rates of Cu, Cd, Ni, Pb, Mn, Zn, Fe and P were >50 times background rates. Thus, upper ocean volcanism may be an important source of both toxic and nutrient elements to the surrounding ocean. It appears unlikely, however, that shallow ocean volcanism can exert a significant impact on the global ecosystem, even during massive lava emplacements.
Constraining the crustal thickness on Mercury from viscous topographic relaxation
Authors: Nimmo Francis
Published: October 2002

Mercury exhibits long‐wavelength topography which has probably survived for ∼4 Ga. Assuming Airy compensation, the survival of the topography indicates that only certain combinations of crustal thickness and thermal structure are allowable. A dry diabase rheology allows a thicker crust than a dry plagioclase rheology. The existence of ancient faults places some constraints on the thermal structure. Unless the crust of Mercury is both as strong as dry diabase and is heated mainly from within, the crustal thickness must be ≤200 km. The faulting evidence implies that the concentration of radiogenic elements in the crust plus mantle of Mercury is at least 80% of the terrestrial value. Combined with previous studies of the long wavelength gravity and topography, the crustal thickness of Mercury is probably 100–200 km. Faults on Mercury are probably several times stronger than terrestrial faults.
Catastrophic erosion of Hellas basin rim on Mars induced by magmatic intrusion into volatile‐rich rocks
Authors: Tanaka Kenneth L. , Kargel Jeffrey S. , MacKinnon David J. , Hare Trent M. , Hoffman Nick
Published: October 2002

Malea and Hesperia Plana form large sectors of the rim of Hellas basin that display partly eroded volcanic shields and plains. These regions have topographic profiles that appear to be several hundred meters lower than those of adjacent rim sectors and lack prominent massifs of remnant basement that would be expected to stand above the lava plains. We interpret that before the volcanic edifices were constructed, these regions were denuded by an early stage of voluminous sill intrusion into friable, volatile‐rich impact breccia. Magma‐volatile interactions may have resulted in catastrophic generation of debris flows deposited into the adjacent basin, particularly if CO2 were involved. Later, lavas covered the eroded terrain; in turn, the lavas were eroded locally by volatile interactions. Across Mars, huge channel systems, erosional features in volcanic terranes, and vast layered deposits may be due to magma‐volatile interactions.
Sanyi‐Puli conductivity anomaly in NW Taiwan and its implication for the tectonics of the 1999 Chi‐Chi earthquake
Authors: Chen Chien‐chih , Chen Chow‐son
Published: October 2002

Based on tensor decomposition technique, we have analyzed the dimensionality of magnetotelluric (MT) data and propose a 2‐D electrical model characterized by a high conductive anomaly beneath Sanyi‐Puli seismic zone, a distinct NW‐SE trending linear seismic zone in the fold‐thrust belt of NW Taiwan. Fluid pressurization indicated by the highly conductive anomaly may result in the active seismicity in Sanyi‐Puli seismic zone and, particularly, may also induce the large slip associated with high static stress drop in the northern part of the fault rupture process for the 1999 Chi‐Chi, Taiwan, earthquake.
Sensitivity of cosmic ray modulation to the correlation length
Authors: Parhi S. , Bieber J. W. , Matthaeus W. H. , Burger R. A.
Published: October 2002

Recent efforts to improve and complete modulation theory are described, emphasizing factors that arise in attempts to understand the diffusion tensor based upon turbulence theory and the theory of charged particle scattering. Direct numerical solutions of the transport equations are used to study sensitivity of cosmic ray modulation to the correlation length. It is shown that modulation is strongly influenced by the radial variation of the correlation length. This variation is still poorly understood, in part owing to the uncertain impact of pickup ion driven turbulence in the outer heliosphere and to practical difficulties in measuring the parallel correlation length in the outer heliosphere.
Scaling law between corner frequency and seismic moment of microearthquakes: Is the breakdown of the cube law a nature of earthquakes?
Authors: Hiramatsu Yoshihiro , Yamanaka Hiroshi , Tadokoro Keiichi , Nishigami Kin′ya , Ohmi Shiro
Published: October 2002

The corner frequency, fC, and the seismic moment, M0, of microearthquakes (local magnitude from −1.3 to 1.3) are estimated from the records of sensors located at an 1800‐m‐deep borehole at the Nojima fault, central Japan. The digital waveforms of each event are recorded with two different sampling rates, 10 kHz and 100 Hz. The source parameters estimated from high‐sampling data satisfy a relationship of M0 ∝ fC−3, corresponding to a constant stress drop law, while those calculated from low‐sampling data follow a relationship of M0 ∝ fC−4. A lack of high frequency components in the 100 Hz sampling data seem to be obvious, indicating that the value of fC and M0 are incorrectly estimated and the relationship of M0 ∝ fC−4 is an artifact caused by the limited frequency band of these data. The scaling relationship of M0 ∝ fC−3 is, thus, also valid for microearthquakes, and the breakdown of the constant stress drop law, reported in the literature, is not well founded.
Lee‐wave clouds and denitrification of the polar stratosphere
Authors: Dhaniyala Suresh , Mckinney Karena A. , Wennberg Paul O.
Published: October 2002

We present a hypothesis that the known formation of nitric acid hydrates in lee‐wave ice clouds produces a “standing crop” of nitric acid trihydrate (NAT) particles, and that growth and sedimentation of these particles efficiently denitrifies the polar stratosphere. Simulations using a simple two‐dimensional model illustrate that the large NAT number concentrations produced in lee waves (>0.1 cm−3) lead to low gas‐phase HNO3 concentration in the cloud layer, limiting subsequent particle growth. Provided the NAT existence temperature is not exceeded, these particles slowly sediment out of the cloud layer, at which point further growth is initiated. As a result of the long residence time and vertical wind shear, NAT particles produced in geographically selective regions over a short duration can cause extensive denitrification throughout the lower stratosphere. Our model illustrates that a single hypothetical lee‐wave nucleation event [10 hours, 1000 km width (along a longitude) × 100 m altitude] is sufficient to produce significant denitrification (∼25%) vortex‐wide.
Global auroral power as an index for geospace disturbances
Authors: Meng C.‐I , Liou K.
Published: October 2002

We propose to use global auroral power (in gigawatt) as a new geospace activity index, GAP. The auroral power is inferred from auroral luminosity in the N2 Lyman‐Birge‐Hopfield bands (λ ∼1700 ± 100 Å) observed by the ultraviolet imager on the Polar satellite. We demonstrate, on the basis of one‐day's worth of data, that GAP correlates well with the 1‐min quick‐look auroral electrojet AEQL index (r = 0.85). We also propose a local‐time auroral keogram (LAK) to illustrate dynamic evolution of global auroral energy deposition. The combination of GAP and LAK provides a powerful tool for morphological study of aurora on a global scale. The importance of these new indices is demonstrated by an analysis of substorm onset timing in comparison with the AE index. It is found that the auroral onset often precedes the magnetic onset, indicated by the AEQL index.
The western undersea section of the Ischia volcanic complex (Italy, Tyrrhenian sea) inferred by marine geophysical data
Authors: Bruno Pier Paolo G. , de Alteriis Giovanni , Florio Giovanni
Published: October 2002

Ischia Island represents only the remnant of a once larger volcanic complex. To prove this hypothesis about 600 km of new marine geophysical data were analysed. Results reveal that the western offshore of the island consists of previously unreported E‐W and NE‐SW volcanic ridges that follow the regional structural patterns. The match between sea‐borne magnetic anomalies and bathymetric/seismic data indicates the presence of a large, cooled magmatic system with deeper intrusions along the NE‐SW direction. Some of the newly discovered submerged volcanoes are dissected by fault scarps that offset, with a strong vertical component, recentmost (possibly Holocene) marine and volcanoclastic deposits. Our data suggest the importance of regional tectonics (in particular along the NE‐SW trend) on the patterns of magma uprising and on the recent local deformation in the Neapolitan volcanic district.
Interannual variability in the NCEP Reanalysis 1948–1999
Authors: Ribera Pedro , Mann Michael E.
Published: October 2002

We examine horizontally and vertically distributed climate fields in Northern Hemisphere NCEP reanalysis data (1948–1999) for joint modes of spatiotemporal variation. Our analysis is based on the “MTM‐SVD” methodology to isolate significant spatially‐coherent variability with narrowband, oscillatory character. Interannual tropospheric climate variability associated with the ENSO signal is identified in distinct quasibiennial (2–3 yr) and quasiquadrennial (4–6 yr) frequency bands, with evidence of evolution in amplitude over time. From this dataset, there is no clear evidence to support the existence of any other significant oscillatory signal in the troposphere.
Impact of a coccolithophorid bloom on the CO2 system in surface waters of the eastern Bering Sea shelf
Authors: Murata Akihiko , Takizawa Takatoshi
Published: October 2002

A bloom of the coccolithophorid, E. huxleyi, occurred on the eastern Bering Sea shelf during September–October 2000. We examined the impact of this bloom on the CO2 system in the surface water. Drawdowns of total alkalinity (TAlk) from the values predicted by the TAlk‐salinity conservative mixing relationship reached a maximum of 82.0 μmol kg−1, but was confined to latitudes 57.0°N–61.0°N. Surface water partial pressures of CO2 (pCO2) in excess of 400 μatm, depletion of nitrate + nitrite and low concentrations of silicate were also found, together with the TAlk drawdowns. The relationship between salinity‐normalized TAlk and total CO2 suggests that the ratio of calcification to photosynthesis during the bloom was approximately 1.0, implying that any CO2 produced from calcification was balanced by photosynthesis. We discuss the possible cause of the observed high surface water pCO2 in the TAlk‐drawdown (bloom) area.
Relationship of D″ structure with the velocity variations near the inner‐core boundary
Authors: Luo Sheng‐Nian , Ni Sidao , Helmberger Don
Published: October 2002

Variations in regional differential times between PKiKP (i) and PKIKP (I) have been attributed to hemispheric P‐velocity variations of about 1% in the upper 100 km of the inner core (referred to as HIC). The top of the inner core appears relatively fast beneath Asia where D″ is also fast. An alternative interpretation could be the lateral variation in P velocity at the lowermost outer core (HOC) producing the same differential times. To resolve this issue, we introduce the diffracted PKP phase near the B caustic (Bdiff) in the range of 139–145° epicenter distances, and the corresponding differential times between Bdiff and PKiKP and PKIKP as observed on broadband arrays. Due to the long‐wavelength nature of Bdiff, we scaled the S‐wave tomography model with k values (k ≡ dlnVs/dlnVp) to obtain large‐scale P‐wave velocity structure in the lower mantle as proposed by earlier studies. Waveform synthetics of Bdiff constructed with small k's predict complex waveforms not commonly observed, confirming the validity of large scaling factor k. With P‐velocity in lower mantle constrained at large scale, the extra travel‐time constraint imposed by Bdiff helps to resolve the HOC‐HIC issue. Our preliminary results suggest k > 2 for the lowermost mantle and support HIC hypothesis. An important implication is that there appears to be a relationship of D″ velocity structures with the structures near the inner core boundary via core dynamics.
Assessing the climate impact of trends in stratospheric water vapor
Authors: Forster Piers M. de F. , Shine K. P.
Published: October 2002

It is now apparent that observed increases in stratospheric water vapor may have contributed significantly to both stratospheric cooling and tropospheric warming over the last few decades. However, a recent study has suggested that our initial estimate of the climate impact may have overestimated both the radiative forcing and stratospheric cooling from these changes. We show that differences between the various estimates are not due to inherent problems with broadband and narrow‐band radiation schemes but rather due to the different experimental setups, particularly the altitude of the water vapor change relative to the tropopause used in the radiative calculations. Furthermore, we show that if recent estimates for the observed water vapor trends are valid globally they could have contributed a radiative forcing of up to 0.29 Wm−2 and a lower‐stratospheric cooling of more than 0.8 K over the past 20 years, with these values more than doubling if, as has been suggested, the trend has persisted for the last 40 years. This 40 year radiative forcing is roughly 75% of that due to carbon dioxide alone but, despite its high value, we find that the addition of this forcing into a simple model of climate change still gives global mean surface temperature trends which are consistent with observations.
Hydroacoustic monitoring of seismicity at the slow‐spreading Mid‐Atlantic Ridge
Authors: Smith Deborah K. , Tolstoy Maya , Fox Christopher G. , Bohnenstiehl DelWayne R. , Matsumoto Haru , J. Fowler Matthew
Published: October 2002

In February 1999, long‐term hydroacoustic monitoring of the northern Mid‐Atlantic Ridge (MAR) was initiated. Six autonomous hydrophones were moored between ∼15°N and ∼35°N on the flanks of the MAR. Results from the first year of data reveal that there is significant variability in along‐axis event rate. Groups of neighboring segments behave similarly, producing an along‐axis pattern with high and low levels of seismic activity at a wavelength of ∼500 km. This broad scale pattern is likely influenced by the axial thermal regime. Several earthquake sequences with variable temporal characteristics were detected, suggesting fundamental differences in the cause of their seismicity. Off‐axis, most seismic faulting occurs within a zone < 15 km from the axis center.
How far ahead could we predict El Niño?
Authors: Collins Matthew , Frame David , Sinha Bablu , Wilson Chris
Published: October 2002

The upper limit of the predictability of the El Niño Southern Oscillation (ENSO) is examined using ensembles of simulations of a coupled ocean‐atmosphere global circulation model which has a relatively realistic ENSO cycle. By making small perturbations to the initial conditions and measuring the rate of divergence of nearby trajectories, it is found that given a perfect model and near perfect initial conditions, ENSO could be usefully predicted, on average, up to 8 months in advance (where “useful” is defined for a forecast with an anomaly correlation coefficient of greater than 0.6). This is at the low end of potential predictability estimates obtained using intermediate models. Some ensemble experiments do show potential predictability beyond 12 months, but in others small “errors” in the initial conditions can saturate in less than 6 months. The physical mechanisms which underlie the high and low predictable states are briefly examined with a view to predicting the reliability of ENSO forecasts.
Demagnetization signatures of lunar impact craters
Authors: Halekas J. S. , Mitchell D. L. , Lin R. P. , Hood L. L. , Acuña M. H. , Binder A. B.
Published: October 2002

We investigate the crustal magnetic signatures of lunar craters using Lunar Prospector (LP) electron reflectometer data. Craters of all ages often have associated magnetic lows, showing that crustal fields were present even in pre‐Nectarian times (≳3.9 Ga). Magnetic lows extend to ∼2–4 crater radii, suggesting shock rather than thermal demagnetization. Younger craters are more likely to have clear and complete demagnetization signatures, suggesting that many older magnetic lows have been subsequently obscured. No size dependence is found for craters larger than 50 km in diameter, suggesting that demagnetization effects for all craters in this size range completely penetrate the magnetized layer. If shock demagnetization is responsible, this suggests an upper limit of ∼50 km for the depth of magnetization. Evidence of edge effects due to magnetization contrasts may show that strong far‐side crustal fields are coherent at scales of ∼25 km.
Indirect relationship between surface water budget and wetland extent
Authors: de Noblet‐Ducoudré Nathalie , Poutou Estelle , Chappellaz Jérôme , Coe Michael , Krinner Gerhard
Published: October 2002

We used a suite of two models: a global climate model, and a hydrological routing scheme, to estimate the changes in the surface water budget and extent of natural wetlands, at the last interglacial (126000 years ago) and at the last glacial maximum (21000 years ago). At both time periods, in northern tropical Africa as well as in northern South America, our simulations exhibit, in many places, an indirect relationship between the surface water budget and the extent of natural wetlands. In relatively moist regions, decreasing (increasing) rainfall and runoff at the last glacial maximum (last interglacial) result in increased (decreased) wetland area due to the reduction (increase) in lake depth. This counter‐intuitive result has never been hypothesized before and may shed a new light on the interpretation of past changes in atmospheric methane, as derived from ice core analyses. It also points to the importance of using a bottom‐up modelling approach in this field of study.
Relationship between the Polar‐Night Jet Oscillation and the Annular Mode
Authors: Kuroda Yuhji
Published: October 2002

The relationship between the Polar‐night Jet Oscillation (PJO) and the Annular Mode (AM) is examined based on a correlation analysis of winter in both the Northern Hemisphere (NH) and Southern Hemisphere (SH). The PJO becomes a nearly zonally symmetric structure from the troposphere to the upper stratosphere when a pattern similar to the AM appears at the troposphere. If the signal of the PJO is removed, the signal of the AM exists only in the troposphere, and the persistency of the signal is largely reduced in the stratosphere. This suggests that the AM is essentially a tropospheric process that tends to strongly interact with the PJO in the active season.
Kinetic model of electron heating by turbulent electric field in the E region
Authors: Milikh G. M. , Dimant Y. S.
Published: October 2002

Numerous radar observations demonstrate anomalously strong electron heating in polar electrojets during magnetic storms. The effect correlates with strong convection electric field. Anomalous heating of electrons is caused by turbulent electric fields developing in the E region due to the Farley‐Buneman instability. A quantitative model of the effect is proposed. Numerical computations are based on a kinetic code. A good agreement of the theoretical results with the existing observations supports the physical ideas underlying the model.
Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone
Authors: Sinogeikin Stanislav V. , Bass Jay D.
Published: October 2002

High seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals, such as majorite‐garnet. Here we present Brillouin scattering measurements of the elasticity of polycrystalline Mg‐majorite (Mj100, Mg4Si4O12) and a Mj50Py50 majorite ‐ pyrope solid solution to pressures similar to those at the top of the transition zone (15.1(1) GPa and 12.8(1) GPa, respectively). The pressure derivatives of the adiabatic bulk (KS) and shear (μ) moduli of both Mj50Py50 and Mj100 are 4.2(3) and 1.4(2), respectively, and are equal to those of pyrope within the experimental uncertainties. We conclude that neither the adiabatic compression of majorite‐garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically. High velocity gradients are consistent with gradual transformations between minerals with very different elastic properties. The transformation of ∼25% or more of clinopyroxene into majorite above 520 km, and the formation of Ca silicate perovskite below 520 km could produce the observed seismic gradients.
Tropical cooling at the last glacial maximum and extratropical ocean ventilation1
Authors: Liu Zhengyu , Shin Sang‐Ik , Otto‐Bliesner Bette , Kutzbach John E. , Brady Esther C. , Lee DongEun
Published: October 2002

The NCAR CSM is used to simulate the climate at Last Glacial Maximum. The CSM simulates a tropical SST cooling of 2.1°C. It is shown that about half of this tropical cooling is associated with the upper ocean circulation, especially the ventilation of thermocline and intermediate waters from the mid/high latitude South Pacific.
Nonlinear interaction of outer zone electrons with VLF waves
Authors: Albert J. M.
Published: October 2002

Analytical expressions have been obtained for three different types of cyclotron‐resonant interaction of a test particle with a monochromatic electromagnetic wave: diffusion, phase bunching, and phase trapping. All three lead to changes in particle energy and pitch angle. They are evaluated for VLF waves and electrons at L = 5.5, to try to account for observed electron acceleration following magnetic storms. The results are that diffusion may be too slow, while phase bunching leads to deceleration. A maximal estimate of phase trapping, considered in isolation, leads to the acceleration of at least some 0.1 MeV electrons to ∼1 MeV in about 1 minute.
Mapping low‐mode internal tides near Hawaii using TOPEX/POSEIDON altimeter data
Authors: Dushaw Brian D.
Published: October 2002

Objective maps of M2 internal‐tide variability are derived from TOPEX/POSEIDON altimeter data obtained near the Hawaiian Ridge. An estimate of the wavenumber spectrum shows that the radiation is dominated by a few spectral peaks at wavenumbers consistent with the theoretically expected value for mode‐1 internal tides. The data are not always consistent with the traveling wave model. Because the spacing of the altimeter tracks is larger than the internal‐tide wavelength, the objective map underestimates the energy between the altimeter tracks. With this caveat, the maps are used to determine that 2.6 GW of power is radiated from a 1700 km section of the Hawaiian Ridge. Time series of hydrographic data obtained at the HOT site north of Oahu, Hawaii show that changes in mode‐1 phase speed, caused by variations in stratification, have only a small effect on the propagation of the mode‐1 internal tides.
The solar wind composition throughout the solar cycle: A continuum of dynamic states
Authors: Zurbuchen T. H. , Fisk L. A. , Gloeckler G. , von Steiger R.
Published: October 2002

Variations in the speed and elemental and ionic charge composition of the solar wind are reported throughout the solar cycle, as observed by the SWICS instrument on Ulysses. The apparent bimodal nature of the solar wind during the solar minimum does not persist throughout the solar cycle. Rather, with increasing solar activity, a continuum of solar wind speeds and charge states is observed. The exception is the elemental composition which is noticeably less enhanced in elements with low first ionization potential (FIP) in material from coronal holes throughout the solar cycle. These observations are consistent with theories in which the solar wind originates from coronal loops that reconnect with open magnetic field lines.
Mantle flow modeling of the anomalous subsidence of the Silurian Baltic Basin
Authors: Daradich A. , Pysklywec R. N. , Mitrovica J. X.
Published: October 2002

Recent reconstructions of the Silurian Baltic Basin indicate a long‐wavelength component of sediment deposition. This deposition extends over 400 km beyond basins predicted on the basis of supracrustal loading in the region; it has been suggested that the discrepancy may be due to dynamic topography associated with mantle flow linked to Silurian plate subduction under Baltica. We test this suggestion using mantle convection simulations, and find that dynamic topography is capable of reconciling the anomalous far‐field deposition. These simulations constrain the paleo‐dip of the subduction to 40°–60°, with the range reflecting an uncertainty in the location of the paleo‐trench. We furthermore demonstrate that this mantle loading component provides 40–85% of the reconstructed near‐field deposition. This result indicates that the slab‐induced mantle flow mechanism is a potentially important contributor to the development of the foreland basin, and that previous estimates of the `orogenic' (i.e., supracrustal) load associated with plate convergence may have been overestimated.
Symmetric suprathermal electron depletions on closed field lines in the solar wind
Authors: Gosling J. T. , Skoug R. M. , Feldman W. C. , McComas D. J.
Published: October 2002

Strong, order of magnitude, depletions in suprathermal electron distribution functions, centered on and roughly symmetric about 90° pitch angle, are relatively common on closed field lines threading coronal mass ejection, CME, material in the solar wind. Such depletions are observed within at least portions of most CME events identified in data obtained from the ACE spacecraft. The symmetry of the depletions about 90° pitch angle is independent of the width or depth of the depletions or the strengths and angular widths of the field‐aligned, counterstreaming beams that characterize closed field lines in the solar wind. We suggest that these symmetric depletions may be largely a consequence of double magnetic connection to the Sun and a relative lack of electron scattering from the counterstreaming beams to large pitch angles.
2D plasma sheet ion density and temperature profiles for northward and southward IMF
Authors: Wing Simon , Newell Patrick T.
Published: October 2002

We previously developed a method for inferring plasma sheet ion temperature and density from ionospheric observations. Using this method and DMSP observations for 1992, we present 2D profiles of the equatorial plasma sheet ion density and temperature for southward and northward IMF for b2i > 64° or Kp < ∼3. During periods of northward IMF, cold dense ions can be found plentifully along the plasma sheet flanks. However, during periods of southward IMF, the presence of these cold dense ions is noticeably diminished, especially at the dusk flank where the density peak is less discernable. These cold dense ions have been previously interpreted in terms of magnetosheath ion entry into the plasma sheet. Our result suggests that any mechanism proposed to transport magnetosheath ions from dusk LLBL to the plasma sheet along the dusk flank during periods of northward IMF would have to be able to do so efficiently over a large spatial scale.
A weekly cycle in atmospheric carbon dioxide
Authors: Cerveny Randall S. , Coakley Kevin J.
Published: October 2002

We present a new statistic called the “Mean Symmetrized Residual” (MSR) for detection and quantification of a weekly cycle in measured daily atmospheric carbon dioxide (CO2). At the Mauna Loa Observatory in Hawaii, we conclude that CO2 concentrations, on average, are significantly lower (0.022 parts per million by volume, ppmv) on weekends (Saturday–Sunday) than during the rest of the week. Over the past twenty‐five years, the variation of the mean values of MSR (as a function of day of the week) has been relatively stable. We speculate that the observed weekday/weekend variation in CO2 at Mauna Loa is the result of anthropogenic emissions on Hawaii and nearby sources. We do not detect a weekly cycle in daily CO2 concentration measured at South Pole, Antarctica. This methodology has applicability to a variety of datasets.
On the importance of offshore data for magnetotelluric studies of ocean‐continent subduction systems
Authors: Evans Rob L. , Chave Alan D. , Booker John R.
Published: October 2002

The presence of water in the mantle wedge overlying a subduction zone is expected to significantly enhance electrical conductivity, raising it above that of the cold subducting slab, or surrounding regions of dehydrated mantle. This suggests that magnetotelluric (MT) transects across subduction systems, measuring regional electrical conductivity structure, might be able to indirectly trace the pathways of water migration into the mantle. For ocean‐continent subduction, it is logistically simpler to collect MT transects on the continental side of the system. However, we show that such land data are relatively insensitive to details of the electrical connections between the ocean and mantle. In contrast, seafloor measurements on the landward and seaward side of the trench are very sensitive to these electrical connections, and are essential to understanding the electrical structure of the entire subduction system. In particular, the conductivity structure of the hydrated mantle wedge overlying the slab can only be studied using offshore MT data. We demonstrate this result using a model of an ocean‐continent system, although our results can be generalised to other subduction geometries.
Enhanced lifetime of methane bubble streams within the deep ocean
Authors: Rehder Gregor , Brewer Peter W. , Peltzer Edward T. , Friederich Gernot
Published: October 2002

We have made direct comparisons of the dissolution and rise rates of methane and argon bubbles experimentally released in the ocean at depths from 440 to 830 m. The bubbles were injected from the ROV Ventana into a box open at the top and the bottom, and imaged by HDTV while in free motion. The vehicle was piloted upwards at the rise rate of the bubbles. Methane and argon show closely similar behavior at depths above the methane hydrate stability field. Below that boundary (∼520 m) markedly enhanced methane bubble lifetimes are observed, and are attributed to the formation of a hydrate skin. This effect greatly increases the ease with which methane gas released at depth, either by natural or industrial events, can penetrate the shallow ocean layers.