bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress drop and frictional heating during the 1994 Deep Bolivia Earthquake
Authors: Bouchon Michel , Ihmlé Pierre
Published: September 2002

We use the slip model of the Bolivian earthquake obtained by Ihmlé [1998] to investigate the spatial distribution of stress drop and calculate the frictional heat generated locally on the fault. We find that the stress drop associated with the earthquake averages between 5 and 10 M Pa over the fault. The total frictional energy is estimated at between 3 × 1017 and 2 × 1018J. If the fault width is as thin as the one produced by transformational faulting in laboratory experiments, where it is about two orders of magnitude smaller than the local shear displacement, melting likely occurred on a significant portion of the fault. In this event, as proposed by Kanamori et al. [1998], once rupture is triggered, shear melting can quickly promote extensive slip.
Models of downdip frictional coupling for the Cascadia Megathrust
Authors: Stanley Dal , Villaseñor Antonio
Published: September 2002

We have developed models of downdip frictional coupling along two transects across the Cascadia subduction zone in northern Washington and central Oregon. The models involve complicated downdip coupling profiles. Although not unique, our models closely predict available GPS shortening rates and vertical uplift data. We are able to explain relatively low horizontal shortening rates along the Washington coast and small vertical uplift rates in central Oregon. Our models depart from previous models by inclusion of a deeply coupled region assumed to be related to mafic rocks in both the upper and lower plates of the thrust.
Relationships between δ18O in precipitation and surface air temperature in the Urumqi River Basin, East Tianshan Mountains, China
Authors: Tandong Yao , Masson Valérie , Jouzel Jean , Stievenard Michel , Weizhen Sun , Keqin Jiao
Published: September 2002

In order to evaluate the dependency of present‐day δ18O in precipitation with temperature, individual precipitation events have been continuously sampled since 1996 at three stations along the Urumqi River Basin, eastern Tianshan mountains, north High China. Both the correlation and the positive slope of δ18O versus temperature increase with elevation for monthly averages, indicating that on long time scales high altitude precipitation δ18O should be a reliable indicator of regional temperature fluctuations, and supporting future ice core drilling.
Simulation of intense monsoons under glacial conditions
Authors: Masson Valérie , Braconnot Pascale , Jouzel Jean , de Noblet Nathalie , Cheddadi Rachid , Marchal O.
Published: September 2002

Paleoenvironmental and paleoclimatological proxy data indicate that strong continental monsoons took place under glacial conditions during marine isotopic stage 6.5 (175 ka BP). So far, no climate model has explored the possible coexistence of glacial conditions at mid and high latitudes and of interglacial monsoons in the tropics. Here we use an atmospheric general circulation model and clearly demonstrate that high insolation can generate increased monsoon activity even with surface glacial conditions. Our experiments show that Indian and African monsoons at 175 ka were stronger than nowadays and induced an increase in outflow of the Nile river to the Mediterranean sea. This freshwater supply combined with the local low glacial evaporation may explain the stagnation of the Eastern Mediterranean sea leading to the deposit of sapropel S6. Our simulations show also increased surface winds in the northern Atlantic and Pacific oceans which may have affected the bioproductivity in these areas.
No glacial‐interglacial cycle in the ice volume simulated under a constant astronomical forcing and a variable CO2
Authors: Loutre M. F. , Berger A.
Published: September 2002

The relative impact of the CO2 concentration on climate over the last 200 kyr is investigated here using the Louvain‐la‐Neuve 2‐dimension Northern Hemisphere (LLN 2‐D NH) climate model forced by different constant insolations and the variable CO2 concentration reconstructed from the Vostok ice‐core record. Under such conditions the 100‐kyr cycle in the Northern Hemisphere ice volume cannot be reproduced. Either a warm orbit prevents the ice sheets from forming or a present‐day and colder orbits do not allow them to melt after being formed. The global hemispheric temperature, however, continues to respond directly to the atmospheric CO2‐concentration changes.
Enhanced circulation during a warm period
Authors: Ravelo Ana Christina , Andreasen Dyke H.
Published: September 2002

The Early Pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today. To characterize thermohaline circulation during this warm period (4.4–3.1 Ma) we combined new benthic foraminiferal isotopic data from the southeast Atlantic with published data, and reconstructed Pacific and Atlantic Ocean nutrient distributions. The data indicate enhanced ventilation of the deep Atlantic and intermediate Pacific during the Early Pliocene. Enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of North Pacific and Atlantic subtropical gyres was probably also enhanced. Future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation.
Global correlation of mesoscale ocean variability with seafloor roughness from satellite altimetry
Authors: Gille Sarah T. , Yale Mara M. , Sandwell David T.
Published: September 2002

Both seafloor bathymetry and eddy kinetic energy at the ocean surface can be estimated by making use of satellite altimeters. Comparing the two quantities shows that in regions of the ocean deeper than about 4800m, surface eddy kinetic energy is greater over smooth abyssal plains than over rough bathymetry, while the opposite is true in shallower waters. Thus in the deep ocean, bottom roughness may dissipate eddy kinetic energy. A simple model indicates that the dissipation rate increases as root‐mean‐squared bottom roughness increases from 0 to 250 m and decreases to negative values (implying eddy generation) for higher roughness.
Electrical structure of the Himalaya of central Nepal: High conductivity around the mid‐crustal ramp along the MHT
Authors: Lemonnier Carole , Marquis Guy , Perrier Frédéric , Avouac Jean‐Philippe , Chitrakar Gyani , Kafle Basantha , Sapkota Som , Gautam Umesh , Tiwari Dilliram , Bano Maksim
Published: September 2002

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp.
Measuring fracture apertures: A comparison of methods
Authors: Renshaw Carl E. , Dadakis Jason S. , Brown Stephen R.
Published: September 2002

Two methods for measuring aperture distributions within rough‐walled fractures are compared; magnetic resonance imaging (MRI) and spectrophotometric analysis (SA) of epoxy replicas. Comparisons with observations of flow through the fracture for Reynolds numbers less than one indicate that the resolution and accuracy of SA is sufficient to predict the effective transmissivity of the fracture using the locally applied cubic law. Observed effective transmissivities for higher Reynolds number flows are lower than predicted using the local cubic law. MRI apertures are generally consistent with SA, except for those less than 100 microns which were not detectable. The lower resolution of the MRI data results in a poor estimate of the effective transmissivity, indicating that despite their low transmissivities, the smallest apertures (<100 microns in this fracture) must still be accurately measured to predict flow through the fracture.
Direct measurements of oceanic crustal density at the Northern Juan de Fuca Ridge
Authors: Gilbert Lisa A. , Johnson H. Paul
Published: September 2002

Density and porosity of oceanic crust are critically important to understanding the circulation of hydrothermal fluid at mid‐ocean ridges. To determine these parameters directly, we completed a fixed, on‐bottom gravity survey on the Endeavour Segment of the Juan de Fuca Ridge, which is a well studied spreading center that is both hydrothermally and tectonically active. A Bell‐Aerospace gravity meter mounted within the submersible ALVIN was used to make gravity measurements from the floor of the axial valley to the summit of the west axial ridge. Analysis of these data gave a density of 2240±150 kg/m³ for the upper 130 m of crust. Comparison of the outcrop density with that of recovered rock samples resulted in a calculated porosity of 38±4%. Forward modeling of the seafloor gravity data indicated that both the valley floor and bounding walls have unusually low density crust that is flanked by higher density crust.
The dependence of permeability on effective stress for an injection test in the Higashi‐Hachimantai Geothermal Field
Authors: Nathenson Manuel
Published: September 2002

A simple inverse‐power relation for the influence of effective stress on permeability is used to explain the flow behavior during an injection test at the Higashi‐Hachimantai geothermal field, Japan. The new analytical expression successfully models data from the experiment involving high‐pressure injection and monitoring at an observation well.
Non‐active dunes in the Acheron Fossae Region of Mars between the Viking and Mars Global Surveyor eras
Authors: Zimbelman James R.
Published: September 2002

Comparison of a high resolution Viking image (422B10; 8 m/pixel) with a Mars Orbiter Camera image (SP2‐502/06; 5.6 m/pixel) of dunes in the Acheron Fossae region of Mars (38°N, 135°W) reveals that the dunes moved <1 pixel during a span of almost 21 Earth years. Very shallow illumination in the MOC image indicates the dunes are <1.5 m high. The images indicate that any movement of these Martian dunes is <0.4 m/yr, a rate that is less than the documented movement of comparable dunes on Earth by a factor of up to 200. The Acheron Fossae dunes occur within a region of low thermal inertia, indicating that the dunes may be stabilized by a pervasive dust cover. Alternatively, the saltation threshold was not exceeded significantly at this location in more than 20 years.
Geodetic constraints on the kinematics of the Alpine Fault in the southern South Island of New Zealand, using results from the Hawea‐Haast GPS Transect
Authors: Pearson Chris , Denys Paul , Hodgkinson Kathleen
Published: September 2002

Repeated GPS measurements at 21 sites along a profile extending 100 km southeast of the Alpine Fault demonstrate that active deformation occurs across much of the South Island of New Zealand and that slip on the Alpine Fault cannot accommodate all of the Pacific‐Australian plate motion. Projecting the velocity field as normal and parallel components to the Alpine Fault, shows that the fault‐parallel component of motion falls on a characteristic dislocation curve that suggests elastic strain accumulation in the upper crust. This type of deformation is associated with deep aseismic slip in the ductile regions below the brittle‐ductile transition. The derived slip rate, of greater than 20 mm/yr, is in reasonable agreement with geological observations. The best fitting locking depth is about 10 km. There is also evidence that approximately 25% of the relative plate motion is accommodated either by distributed deformation or by slip on a second structure located within the Southern Alps, approximately 80 km east of the Alpine Fault.
A new seismic velocity model for the Moon from a Monte Carlo inversion of the Apollo lunar seismic data
Authors: Khan A. , Mosegaard K. , Rasmussen K. L.
Published: September 2002

A reanalysis of the Apollo lunar seismic data and the subsequent application of an inverse Monte Carlo method to P and S‐wave arrival times has resulted in a more detailed lunar velocity structure than previously obtainable. The velocity is seen to increase from the surface down to the base of the crust at 45±5 km depth. The results furthermore indicate a constant velocity upper mantle extending to 560±15 km km depth, separated from a more complex high velocity middle mantle by an increase in velocity of 1.0 km/s. In addition, the moonquake locations have been improved. The shallow moonquakes are found to be located in the depth range 50–220 km. The majority of deep moonquakes are concentrated in the depth range 850–1000 km with an apparently rather sharp lower boundary.
SP coseismic signals observed on an electrodes array in an underground quarry
Authors: Gensane Olivier , Konyukhov Boris , Le Mouël Jean‐Louis , Morat Pierre
Published: September 2002

Electric self‐potentials (SP) and strains were measured on the surface of a pillar of the abandoned underground Meriel limestone quarry (France). The time variations of strains and SP are interpreted in terms of crack instability. The dynamic mapping technique shows that the most intensive SP variations, corresponding to maximum stress variations estimated from strain data, occur in the vicinity of active crack tips or crack intersections. The location of these maxima changes with the stress distribution variations. The synchronous stacking technique allows to detect significant pressure variations through electrodes installed in the fractured zone as well as through outside electrodes close to the medium discontinuities (petrological boundaries and pillar edges).
Acoustic detection of immiscible liquids in sand
Authors: Geller Jil T. , Kowalsky Michael B. , Seifert Patricia K. , Nihei Kurt T.
Published: September 2002

Laboratory cross‐well P‐wave transmission at 90 kHz was measured in a 61 cm diameter by 76 cm tall water‐saturated sand pack, before and after introducing a non‐aqueous phase organic liquid (NAPL) (n‐dodecane). In one experiment, NAPL was introduced to form a lens trapped by a low permeability layer; a second experiment considered NAPL residual trapped behind the front of flowing NAPL. The NAPL caused significant changes in the travel time and amplitude of first arrivals, as well as the generation of diffracted waves arriving after the direct wave. The spatial variations in NAPL saturation obtained from excavation at the end of the experiment correlated well with the observed variations in the P‐wave amplitudes and travel times. NAPL residual saturation changes of 4% were detectable and the 40 to 80% NAPL saturation in the NAPL lens was clearly visible at acoustic frequencies. The results indicate that small NAPL saturations may be more easily detected with amplitude rather than travel time data, but that the relationships between the amplitude changes and NAPL saturation may be more complex than those for velocity.
Fault plane structure of the Antofagasta, Chile Earthquake of 1995
Authors: Sobiesiak Monika M.
Published: September 2002

From a large dataset of aftershocks in the rupture area of the Mw=8.0 1995 Antofagasta earthquake we derived the spatial distribution of hypocenters and a b‐value map along the fault plane. We further examined various focal mechanisms of the aftershock sequence. The b‐value distribution on the fault plane clearly indicates the heterogeneity of the rupture area. We found a positive correlation between the b‐value variations and the main shock source time function determined by Delouis [1996] where zones of high b‐value are related to high slip. With these results we propose to have found evidence for the segmentation of the fault plane and suggest that the mapped inhomogeneities could have played the role of asperities prior to the main shock rupture.
Why are the meanders of the North Atlantic Current stable and stationary?
Authors: Kearns Edward J. , Paldor Nathan
Published: September 2002

A simple, 1.5‐layer, reduced gravity model of an f‐plane jet with a symmetric cross section is employed to explain the stability and stasis of meanders in the North Atlantic Current. The main factors affecting the current's stability are the highly contrasting water masses on both its sides and the current's vertical density structure. The stability and stasis of the NAC results from (1) the near‐surface shoaling of the deep isopycnals inshore of the Subpolar Front and (2) a small internal radius of deformation which is due mainly to the small upper‐to‐lower‐layer density difference. In the absence of unstable modes, the dominant wavelength of the meanders is determined by the bathymetry, so the NAC simply follows those contours.
Effects of intermittency on interplanetary velocity and magnetic field fluctuations anisotropy
Authors: Bruno R. , Bavassano B. , Pietropaolo E. , Carbone V. , Veltri P.
Published: September 2002

Intermittency is a well‐established feature of interplanetary MHD fluctuations and, we show the effect of intermittency on the radial evolution of solar wind velocity and magnetic field fluctuations anisotropy. On one hand we confirm results obtained by previous investigations which showed that magnetic fluctuation anisotropy increases with distance and, on the other hand, we prove that much of this trend is due to intermittency. Once intermittency has been reduced, thanks to a technique based on wavelet transform for the identification of the intermittent events, the radial trend vanishes. Similar analysis performed on velocity fluctuations, showed that intermittency although altering the anisotropy, does not markedly change its radial trend.
The connection between transpolar arcs and magnetotail rotation
Authors: Kullen A.
Published: September 2002

Recent observations show that the evolution of transpolar arcs (TPA's) are often associated with a sign change of the dawn‐dusk component of the interplanetary magnetic field (IMF). It is known that a nonzero IMF By causes a twist of the tail plasma sheet. In this study it is assumed that a sign change of IMF By causes the magnetotail to rotate until the entire tail is twisted in the opposite direction. To examine a possible connection between TPA's and tail rotation the Tsyganenko 1989 model is modified such that the near‐Earth tail and the far tail are twisted in opposite directions. Mapping from the tail current sheet to the ionosphere shows a separated region of closed field lines that extends into the polar cap. This separated part may be interpreted as the mapped region of a TPA.
Identification of widespread turbulence of dispersive Alfvén waves
Authors: Stasiewicz K. , Khotyaintsev Y. , Berthomier M. , Wahlund J ‐E.
Published: September 2002

A revised analysis of Freja observations shows that events interpreted previously as broadband turbulence in the frequency domain, f ∼1–500 Hz represent in fact turbulence of dispersive Alfvén waves (DAW) covering a wide range of wave vectors k⟂. A satellite's motion through broad Δk⟂, low frequency, ω < ωci, Alfvénic structures makes them appear as broadband waves at (Doppler) frequencies Δω = vs · Δk⟂ in the satellite reference frame. The identification is made with the measured ratio of δE/δB fluctuations which is consistent with the theoretical prediction for inertial Alfvén wave spectrum at spatial scales λ⟂ ∼30–7000 m. In addition, we demonstrate that the multipoint measurements with double density probes provide independent confirmation of the spatial nature of the low frequency fluctuations.
Time‐frequency analysis of total solar irradiance variations
Authors: Willson Richard C. , Mordvinov Alexander V.
Published: September 2002

Continuous wavelet analysis of the Total Solar Irradiance (TSI) time series accurately describes its non‐stationary, multi‐scale variations over a wide time‐scale domain. The analysis was also applied to Photometric Sunspot Index (PSI) and full‐disk Calcium II K‐line Photometric Facular Index (PFI) time series to identify the contributions of sunspots and faculae/magnetic‐network to the time‐frequency behavior of TSI. Significant spectral power is found near 27‐days in the time series spectra of TSI, PSI and PFI, confirming sunspot and facular phenomena as the primary causes of TSI variability near the average solar rotation rate for active regions. The spectra of both PFI and TSI exhibit correlative trends to longer periodicities during solar cycles 21 and 22, demonstrating the dominant influence of faculae and magnetic network on TSI during the declining phase of these solar cycles.
Subionospheric VLF signatures of oblique (nonducted) whistler‐induced precipitation
Authors: Johnson M. P. , Inan U. S. , Lauben D. S.
Published: September 2002

Subionospheric very low frequency (VLF) signal perturbations observed at multiple sites in association with individual lightning discharges exhibit onset delays which steadily increase with increasing L‐value. The timing of the onsets and the inferred spatial extent (∼1000 km) of the ionospheric disturbance are consistent with those expected for electron precipitation induced by obliquely propagating (i.e., nonducted) whistlers.
Thermal and driven stochastic growth of Langmuir waves in the solar wind and Earth's foreshock
Authors: Cairns Iver H. , Robinson P. A. , Anderson R. R.
Published: September 2002

Statistical distributions of Langmuir wave fields in the solar wind and the edge of Earth's foreshock are analyzed and compared with predictions for stochastic growth theory (SGT). SGT quantitatively explains the solar wind, edge, and deep foreshock data as pure thermal waves, driven thermal waves subject to net linear growth and stochastic effects, and as waves in a pure SGT state, respectively, plus radiation near the plasma frequency fp. These changes are interpreted in terms of spatial variations in the beam instability's growth rate and evolution toward a pure SGT state. SGT analyses of field distributions are shown to provide a viable alternative to thermal noise spectroscopy for wave instruments with coarse frequency resolution, and to separate fp radiation from Langmuir waves.
First in situ temperature measurements at the Antarctic summer mesopause
Authors: Lübken Franz‐Josef , Jarvis Martin J. , Jones G. Owen L.
Published: September 2002

The Arctic summer mesopause at ∼88 km is the coldest‐known place (∼130 K) in the terrestrial atmosphere and is ∼60 K colder in summer than winter. Indirect evidence has suggested that the summer mesopause temperatures in the Antarctic are a few Kelvin warmer than in the Arctic. However, reliable measurements have not been available at southern high latitudes to verify this. We report the very first in situ temperature observations in the summer mesosphere from Antarctica based on rocket‐borne falling spheres launched from Rothera (68°S, 68°W). The first of 24 successful launches, on 5 January 1998, showed a mesopause temperature of 129 K at 87 km, surprisingly close to northern hemisphere (NH) mean summer values. During January the mesospheric temperatures are similar to the northern summer, but the difference increases to several Kelvin in February.
Multicomponent plasma distributions in the tail current sheet associated with substorms
Authors: Chen LiJen , Larson D. , Lin R. P. , McCarthy M. , Parks G.
Published: September 2002

The 3D plasma instrument on the Wind spacecraft has obtained new features of plasma distributions in the region of the geomagnetic tail current sheet (CS) that have not been previously reported. In addition to the isotropic plasma sheet component, ion beams of energy 1–3 keV are observed. The beams are generally counter streaming but unidirectional beams are also observed streaming parallel and antiparallel to the direction of the magnetic field. In addition, a low energy (a few hundred eV) ion component appears in close association with Bz increases. The electron distributions have bidirectional anisotropy with T∥/T⟂ ≈1.5, and they are often accompanied by counter streaming beams. However, a few keV unidirectional electron beam is also observed with bidirectional anisotropic distributions. These keV electrons are streaming away from the CS, suggesting they have been accelerated in the CS and subsequently ejected. The plasma distribution in the CS is multicomponent and thus requires a kinetic description of the dynamics.
Dispersion imposed limits on atmospheric gravity waves in the mesosphere: Observations from OH airglow
Authors: Swenson Gary R. , Alexander M. Joan , Haque Rezaul
Published: September 2002

Mesospheric airglow perturbations offer a method of observing fast (long vertical wavelengths, λz and short horizontal wavelengths, λh) atmospheric gravity waves (AGWs) propagating through the mesosphere. In a recent study, wave phase speeds (and λh) have been deduced from the airglow observations and Doppler corrected using lidar winds [Swenson et al., 1999]. Combining these measurements with the dispersion relationship, wave momentum and energy fluxes were calculated. The observed waves have intrinsic phase speeds which are not linearly related to λz for short λh as is assumed in the commonly applied approximation to the dispersion relation. This corrected dispersion relationship is graphically presented along with imager measurements of AGWs and an estimate of error in the wave momentum flux related to this term. The observed waves with λz <20 km are found to be near the high‐frequency limit for vertical propagation, suggesting they may be ducted, evanescent, or close to the point of total internal reflection.
Evidence for a solar wind origin of the power law burst lifetime distribution of the AE indices
Authors: Freeman M. P. , Watkins N. W. , Riley D. J.
Published: September 2002

In this paper we examine the claim that the power law distribution of burst lifetimes in the AE index is evidence that the magnetosphere is a Self‐Organized Critical (SOC) system. To do this we compare the burst lifetime distributions of the AU and |AL| indices with those of the υBs and ε solar wind input functions. We show for the first time that both the υBs and ε burst lifetime distributions are of power law form with an exponential cut‐off, consistent with the solar wind being an SOC system. Furthermore, the power law of the ε burst lifetime distribution is not significantly different to that of the AU and |AL| indices, indicating that this scale‐free property of the AE indices could arise from the solar wind input and may not be an intrinsic property of the magnetospheric system. We discuss the implications of this result for SOC theories of the magnetosphere.
The long‐term variation of the Sun's open magnetic flux
Authors: Wang Y. ‐M. , Lean J. , Sheeley N. R.
Published: September 2002

The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φopen can be inferred from current‐free extrapolations of the observed photospheric field. We derive the long‐term variation of Φopen during 1971–1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower‐latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φopen closely follows that of the Sun's total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large‐scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φopen and the IMF strength lasting typically ∼1 yr.
Ionospheric Alfvén resonator as seen by Freja satellite
Authors: Grzesiak Marcin
Published: September 2002

Sharp increase of the Alfvén speed above the ionosphere is expected to create a resonant cavity for the Alfvén waves traveling along the geomagnetic field lines (so‐called Ionospheric Alfvén Resonator). The wavelet transform has been used to analyze magnetic and electric fields recorded on Freja satellite. Cross‐phase spectra between field components in the frequency range 0.5–10 Hz reveal a pattern similar to that predicted theoretically for the resonator. The Fourier cross‐phase spectra fail to show any characteristic features. We conclude that this is due to the spatial structure of the resonator which can be examined only with local analysis techniques.
Elastic thickness of the lithosphere in the Central Canadian Shield
Authors: Wang Yihua , Mareschal Jean‐Claude
Published: September 2002

We have used the coherence between Bouguer gravity and topography to determine the elastic thickness (Te) of the lithosphere in central Canada. In order to improve the resolution, we have estimated the spectra with the maximum entropy method. The variations in elastic thickness delineate three main regions: (1) The Interior Platform where Te varies between 25 and 50 km; (2) The Shield includes several high rigidity cores, in particular in the Trans‐Hudson orogen and in the Hearne Province where Te >80 km; (3) The Hudson Bay basin has an elastic thickness between 50 and 70 km with higher values in the eastern part.

The results suggest that Te is controlled by the thermal regime in the platform. There is no difference in Te between the Proterozoic and Archean Provinces of the Shield. In the platform, the elastic thickness is lowest where sediments are thickest, indicating that low rigidity has amplified the subsidence of the basin.
Gamma‐ray emissions observed in a thunderstorm anvil
Authors: Eack Kenneth B. , Suszcynsky David M. , Beasley William H. , Roussel‐Dupre Robert , Symbalisty Eugene
Published: September 2002

Balloon‐borne gamma‐ray and electric‐field‐change instruments were launched into a daytime summer thunderstorm to evaluate a new experimental design to test hypotheses for the production of transient luminous events (TLE) (eg. sprites, and blue jets) in the mesosphere. While ascending, the instrument triggered many times on the signals from the electric‐field‐change instrument, recording the gamma‐ray background at those times. A greater than three‐fold increase in the gamma‐ray flux was observed as the balloon descended through a thunderstorm anvil where a strong electric field was suspected to be present. These observations suggest that gamma‐ray production in thunderstorms may not be as uncommon as previously believed.
North Atlantic Oscillation modulates total ozone winter trends
Authors: Appenzeller Christof , Weiss Andrea K. , Staehelin Johannes
Published: September 2002

The North Atlantic Oscillation (NAO) is modulating the Earth's ozone shield such that the calculated anthropogenic total ozone decrease is enhanced over Europe whereas over the North Atlantic region it is reduced (for the last 30 years). Including the NAO in a statistical model suggests a more uniform chemical winter trend compared to the strong longitudinal variation reported earlier. At Arosa (Switzerland) the trend is reduced to −2.4% per decade compared to −3.2% and at Reykjavik (Iceland) it is enhanced to −3.8% compared to 0%. The revised trend is slightly below the predictions by 2D chemical models. Decadal ozone variability is linked to variations in the dynamical structure of the atmosphere, as reflected in the tropopause pressure. The latter varies in concert with the NAO index with a distinct geographical pattern.
Constraining uncertainties in climate models using climate change detection techniques
Authors: Forest Chris E. , Allen Myles R. , Stone Peter H. , Sokolov Andrei P.
Published: September 2002

Predictions of 21st century climate by different atmosphere‐ocean general circulation models depend on the sensitivities of the models to external radiative forcing and on their rates of heat uptake by the deep ocean. This study constrains these properties by comparing radiosonde‐based observations of temperature trends in the free troposphere and lower stratosphere with corresponding simulations of a fast, flexible climate model, using objective techniques based on optimal fingerprinting. Parameter choices corresponding either to low sensitivity, or to high sensitivity combined with slow oceanic heat uptake are rejected provided the variability estimates used from the HadCM2 control run are correct. Nevertheless, the range of acceptable values is significantly wider than that usually quoted. The IPCC's range of possible sensitivities, 1.5 to 4.5 K, corresponds at best to only an 80% confidence interval. Therefore, climate change projections based on current general circulation models do not span the range of possibilities consistent with the recent climate record.
Halogen atom concentrations in the Arctic Troposphere derived from hydrocarbon measurements: Impact on the budget of formaldehyde
Authors: Rudolph Jochen , Ru Fu Ban , Thompson Alex , Anlauf Kurt , Bottenheim Jan
Published: September 2002

As part of the Polar Sunrise Experiment in 1998 measurements of hydrocarbons were made at the Canadian Arctic station Alert. Halogen atom concentrations play a key role in determining formaldehyde mixing ratios. Formaldehyde mixing ratios observed during ozone depletion episodes agree with those calculated from time integrated halogen atom concentrations. Formaldehyde is the most important loss mechanism for active bromine and at the same time an important source for HOx radicals. Via these reactions formaldehyde will indirectly influence chlorine chemistry and thus feedback mechanisms involving halogen atom concentrations and formaldehyde are likely to play a major role in the development of tropospheric ozone depletion episodes during polar sunrise.
Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Last Glacial and the transition to the Holocene
Authors: Dällenbach A. , Blunier T. , Flückiger J. , Stauffer B. , Chappellaz J. , Raynaud D.
Published: September 2002

Significant variations in the interpolar difference of atmospheric CH4 concentration over the Holocene period were observed by Chappellaz et al., [1997]. Here we extend this study to the Last Glacial and the transition to the Holocene. We observe a gradient of −3±4 parts per billion by volume (ppbv) during the Last Glacial Maximum. It increases to 26±10 ppbv during the Bølling/Allerød and remains at 26±9 ppbv during the Younger Dryas cold period. On average, we find an interpolar difference of 14±4 ppbv during the cold phases and of 37±10 ppbv during the warm periods of the Last Glacial. With a three‐box model we derive from the measured gradients the contributions of methane from the Tropics and the mid‐to‐high latitudes of the northern hemisphere. The Tropics have been the largest source in all glacial epochs. The contribution by the northern latitudes have been very small during the last glacial maximum but surprisingly large during the earlier part of the glacial epoch. The model result suggests completely unexpected, that the higher atmospheric CH4 concentration during the warm Dansgaard/Oeschger events are caused by a higher source strength of the northern latitudes and not of the Tropics.
Nitrogen deposition, terrestrial carbon uptake and changes in the seasonal cycle of atmospheric CO2
Authors: Erickson David J.
Published: September 2002

Observational evidence indicates an increasing trend in the amplitude of the seasonal cycle of atmospheric CO2 over the last several decades. Here, the influence of nitrogen deposition on the seasonal cycle of atmospheric CO2 is investigated using a global carbon cycle model embedded in a 3‐D general circulation model. We employ a recently published estimate of the enhancement of carbon dioxide uptake induced by the atmospheric deposition of NOy and NHx. We partition the carbon sink related to nitrogen deposition over the seasonal cycle of CO2 uptake. The modeled increase in the amplitude of the seasonal cycle of CO2 in the Northern Hemisphere related to the simulated nitrogen deposition alone is 25%–50% of observed. At Mauna Loa the increased amplitude in the CO2 seasonal cycle due to nitrogen deposition is 50– of that observed. The subtle interaction between 3‐D atmospheric transport, atmospheric nitrogen deposition, and seasonal CO2 uptake results in significant changes in the amplitude of the seasonal cycle of atmospheric CO2. The magnitude of these nitrogen deposition‐induced changes in the atmospheric behavior of CO2 is comparable to other processes that are thought to influence global carbon cycle dynamics.
Winter clouds over the North Martian Polar Cap
Authors: Pettengill Gordon H. , Ford Peter G.
Published: September 2002

The Mars Orbiter Laser Altimeter (MOLA) experiment, carried on the Mars Global Surveyor spacecraft, has observed echoes from cloud tops on roughly 2.5% of the total data taken above 70°N over the northern winter Martian polar hood from March to June 1998. Sloping wavefronts are commonly seen at latitudes above 70°N, implying the presence of propagating buoyancy waves. Since these wavefronts frequently extend from the surface up to 10 km at a time when CO2 is known to be condensing on the polar surface, it seems likely that the laser‐scattering particles consist primarily of CO2 ice, and that the near‐surface temperature lapse rate is 0.85 K km−1, set by the vapor pressure of dry ice. From the observed wavelengths, where available, we have calculated the corresponding phase velocities; some of these waves appear to be correlated with surface discontinuities, and may represent lee waves.
Precipitable water column retrieval from GOME data
Authors: Maurellis Ahilleas N. , Lang Rüdiger , van der Zande Wim J. , Aben Ilse , Ubachs Wim
Published: September 2002

We investigate the retrieval of terrestrial precipitable water columns using a new spectral fitting method applied to Global Ozone Monitoring Experiment (GOME) data. The method is an optical absorption spectroscopy technique and employs a new approach to the opacity sampling of absorption line spectra which we apply to a little‐studied visible band between 585 and 600 nm. The GOME‐retrieved columns are compared with data from the European Center for Medium‐Range Weather Forecasts for different orbits and show good agreement. The new retrieval algorithm is sensitive to the temperature and pressure dependence of absorption lines in general and may be easily applied to spectra of trace gases other than water vapor.
P‐wave deep velocity structure of the Southern Tyrrhenian Subduction Zone from nonlinear teleseismic traveltime tomography
Authors: Cimini Giovanni B.
Published: September 2002

A new 3‐D model for the P‐wave velocity structure of the Southern Tyrrhenian Subduction Zone (STSZ) is determined from nonlinear inversion of relative arrival times of teleseismic events. The data used in the imaging are the travel time residuals of both direct, P and PKPdf and secondary pP, sP, PcP, PKPbc phases, computed with respect to the global 1‐D velocity model ak135. 2308 teleseismic waveforms were collected for this study from 109 events recorded by the Italian National Seismic Network (RSNC) during 1988–1998. The velocity perturbation field is reconstructed gradually by means an iterative sequence of linearized inversions, incorporating a 3‐D minimum travel time ray tracing. The tomographic images reveal a broad high‐velocity zone dominating the pattern of lateral variations beneath the Southern Tyrrhenian Sea and Calabria. This fast structure extends laterally for a maximum of ∼350 km, from northern Sicily to southern Campania, and vertically for at least 400 km, from the uppermost mantle down to 500 km depth. Below 350 km the geometry of the depicted slab is characterized by horizontal deflection of the subducting lithosphere towards the central Tyrrhenian basin.
An interpretation of the results from atmospheric general circulation models forced by the time history of the observed sea surface temperature distribution
Authors: Bretherton C. S. , Battisti D. S.
Published: September 2002

Recent studies using atmospheric general circulation models forced by the observed time history of global sea surface temperature anomalies have been used to hind‐cast the temporal history of the North Atlantic Oscillation. They find that the mean of a large ensemble of integrations using slightly different initial atmospheric conditions reproduces the observed variability surprisingly well, especially on time scales longer than a few years. However, they also find that amplitude of the atmospheric variability is considerably reduced and the air‐sea heat fluxes are of the reverse sign to those observed. Here, a linear model of midlatitude atmosphere/ocean interaction forced only by high‐frequency atmospheric stochastic variability is shown to reproduce all of these findings. This model suggests that despite the hind‐cast skill, the useful predictability associated with midlatitude SST anomalies may be limited to one or two seasons.
Wide‐band magnetotelluric measurements across the Taupo Volcanic Zone, New Zealand‐Preliminary results
Authors: Ogawa Yasuo , Bibby Hugh M. , Caldwell T. Grant , Takakura Shinichi , Uchida Toshihiro , Matsushima Nobuo , Bennie Stewart L. , Tosha Toshiyuki , Nishi Yuji
Published: September 2002

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by intensive geothermal activity and frequent rhyolitic volcanism. Sixteen wide‐band (0.01–1,800 s) magnetotelluric soundings were measured along a 110 km‐long profile approximately perpendicular to the strike of the TVZ. A model obtained from 2D inversion of the soundings shows two near‐surface regions of high conductance which correspond to low density volcaniclastic sediments, up to 3 km thick, which infill a sequence of collapse calderas. At deeper levels (approximately 5–10 km) a resistive layer underlies the entire TVZ. Modelling shows other conductive zones occur beneath the TVZ, with the shallowest lying below the central part at a depth of 10–15 km. Given the high heat flux and volcanic history of the TVZ, the high conductivity at depth may indicate the presence of connected melt. At greater depth (20–30 km) the upper mantle beneath the TVZ appears to be anomalously conductive, consistent with observed high seismic attenuation.
The impact of the parameterization of heterogeneous vegetation on the modeled large‐scale circulation in CCM3‐BATS
Authors: Burke Eleanor J. , Shuttleworth W. James , Yang Z. ‐Liang , Mullen Steven L. , Arain M. Altaf
Published: September 2002

This letter reports evidence of an unexpected large change in the planetary‐scale circulation as a consequence of modifying the land cover within the Community Climate Model (CCM3). Three 10‐year simulations were analyzed using (i) default land cover, i.e., a single dominant vegetation in each land grid cell; (ii) aggregate land cover that includes a representation of subpixel heterogeneity; and (iii) aggregate land cover for aerodynamic properties, but default land cover otherwise. Simulations (ii) and (iii) were similar, indicating that aerodynamic properties are influential at the planetary scale. Comparing these two runs with (i), there are significant differences in the boreal summer. These include a northward shift of the Northern Hemisphere jet that relates to a decrease in the zonally averaged aerodynamic roughness around 60°N, and a perturbation of the Southern Hemisphere jet. Field significance tests suggest these changes are likely a remote influence of the Northern Hemisphere perturbation, not a sampling fluctuation.
In situ observations of a reduction in effective crystal diameter in cirrus clouds near flight corridors
Authors: Kristensson A. , Gayet J. ‐F. , Ström J. , Auriol F.
Published: September 2002

Measurements of the crystal size distribution performed over Southern Germany show that the effective diameter decrease 10–30% in cirrus clouds perturbed by aircraft. The reduction is more pronounced at higher cloud water content and is not observed at temperatures warmer than −30°C.
Atmospheric hydrogen cyanide (HCN): Biomass burning source, ocean sink?
Authors: Li Qinbin , Jacob Daniel J. , Bey Isabelle , Yantosca Robert M. , Zhao Yongjing , Kondo Yutaka , Notholt Justus
Published: September 2002

The observed seasonal amplitude of atmospheric HCN concentrations implies an atmospheric lifetime of only a few months for HCN, much shorter than is commonly assumed from oxidation by OH (a few years). We propose that ocean uptake provides the missing sink, and show with a global 3‐D model simulation that the observations of atmospheric HCN can be roughly reproduced in a scenario where biomass burning provides the main source (1.4–2.9 Tg N yr−1) and ocean uptake provides the main sink (HCN atmospheric lifetime of 2–4 months). Such a budget implies that HCN is a sensitive tracer of biomass burning on large scales, of particular value because it is readily observed from space. The ocean sink hypothesis can be tested with measurements of HCN concentrations in marine air and seawater.
An interpretation of Martian thermospheric waves based on analysis of a general circulation model
Authors: Joshi Manoj M. , Hollingsworth Jeffery L. , Haberle Robert M. , Bridger Alison F. C.
Published: September 2002

Planetary‐scale longitudinal variations in density observed by the Mars Global Surveyor accelerometer in the 125 km region can be qualitatively reproduced by the NASA Ames Mars general circulation model in the 80 km altitude region, but only when locations having specific local times are used in the analysis. If the model results are averaged over all local times, the high‐altitude longitudinal variations nearly disappear, leaving only a small stationary wave 1 pattern, consistent with theory and previous modeling studies. This analysis suggests that the observed wavelike structures are a result of sampling tidal modes at a limited range of local times, rather than by topographically forced Rossby waves as previously suggested.
Atmospheric CO2 concentration from 60 to 20 kyr BP from the Taylor Dome Ice Core, Antarctica
Authors: Indermühle Andreas , Monnin Eric , Stauffer Bernhard , Stocker Thomas F. , Wahlen Martin
Published: September 2002

A high‐resolution record of the atmospheric CO2 concentration from 60 to 20 thousand years before present (kyr BP) based on measurements on the ice core of Taylor Dome, Antarctica is presented. This record shows four distinct peaks of 20 parts per million by volume (ppmv) on a millennial time scale. Good correlation of the CO2 record with temperature reconstructions based on stable isotope measurements on the Vostok ice core (Antarctica) is found.
Aerosol optical depth over Europe in August 1997 derived from ATSR‐2 data
Authors: Gonzalez C. Robles , Veefkind J. P. , de Leeuw G.
Published: September 2002

Data from the Along Track Scanning Radiometer 2 (ATSR‐2) on board the European ERS‐2 satellite have been used to derive the spatial distribution of the aerosol optical depth (AOD) over Europe for August 1997. The AOD was retrieved in cloud‐free areas using the dual view algorithm. The results agree with co‐located ground based sun‐photometer data within 0.1. The AOD from ATSR‐2 with a resolution of 1×1 km² were averaged on a grid of 0.1° × 0.1°, to produce daily maps of the spatial aerosol distribution over Europe. A composite map of AOD over Europe was constructed by averaging all daily maps for August 1997. This composite map shows large spatial AOD gradients with variations of a factor of 3 over a few hundreds of kilometers. The AOD at 0.555 µm for relatively clean areas is around 0.1 while in strong industrialised areas the AOD can be 0.5 or higher.