bluuebunny/crossref_metadata_2025 huggingface dataset:
Non‐equilibrium compositions of liquid polar stratospheric clouds in gravity waves
Authors: Voigt Christiane , Tsias Athanasios , Dörnbrack Andreas , Meilinger Stefanie , Luo Beiping , Schreiner Jochen , Larsen Niels , Mauersberger Konrad , Peter Thomas
Published: February 2004

On 25 January 1998 mountain induced gravity waves developed over Scandinavia leading to the formation of mesoscale polar stratospheric clouds (PSCs). Balloon‐borne mass spectrometric measurements of particle composition and optical backscatter measurements were performed above Kiruna/Sweden. PSCs were encountered twice, showing a correlated increase in the condensed phase water, nitric acid and the backscatter ratio. Thermodynamic modeling allows the PSC particles to be unambiguously identified as supercooled ternary solution (STS) droplets, but cannot account for the measured scatter in the particulate HNO3∶H2O mole ratio. Simultaneous temperature measurements show that the particles were subject to rapid atmospheric temperature fluctuations of ±1 K and cooling/heating rates exceeding 1 K/min caused by the gravity waves. Micro‐physical non‐equilibrium modeling of STS droplet distributions reveals that the observed temperature perturbations cause particle compositions in close agreement with the measured HNO3∶H2O variations. Non‐equilibrium compositions of liquid PSC particles are thus a principal stratospheric characteristic related to gravity waves affecting particle evolution.
The 1741 Oshima‐Oshima Eruption: Extent and volume of submarine debris avalanche
Authors: Satake Kenji , Kato Yukihiro
Published: February 2004

Oshima‐Oshima is a small volcanic island southwest off Hokkaido in Japan Sea. The violent eruption on August 29, 1741 caused a sector collapse of the north flank. Very destructive tsunami with 2000 casualties was also documented, but the generation mechanism has been controversial; volume of subaerial sector collapse, about 0.4 km³, was considered too small to generate such a devastating tsunami. We compile and analyze recently‐obtained swath bathymetry data around the island. The volcano is more than 2.3 km high from ocean bottom with its base diameter of 18 km. The total slide volume, both subaerial and submarine, is estimated as 2.4 km³, nearly an order of magnitude larger than the subaerial part. To the north of island, hummocky terrain of debris deposits extends in an area of 69 km² with the total volume of 2.5 km³, a value similar to the total slide volume. It is one of the largest debris avalanches in historic time, and probably responsible to the tsunami generation.
Polarization electric fields sustained by closed‐current dynamo structures in midlatitude sporadic E
Authors: Cosgrove Russell B. , Tsunoda Roland T.
Published: February 2004

The existence of anomalously large midlatitude E region electric fields, occurring at night when sporadic‐E ( Es) is present, has now been verified experimentally. The only known source mechanism requires that the Hall current driven by the polarization electric field be sustained in some way. While current closure in the F layer is possible, we show that closure through nearby dynamo structures that involve Es patches may be more likely. To illustrate the concept, we have considered an analytically tractable problem in which two dynamo regions have formed, one above and one below an ion‐convergent wind‐shear node. We show that the polarization Hall current generated in each region can close through the other region via field‐aligned currents. With this kind of closure, we further show that the polarization electric field can approach the idealized Cowling‐conductivity‐enhanced field strength, which would be large enough to account for the observations that have been made by rocket and radar.
Impact of geothermal heating on the global ocean circulation
Authors: Adcroft Alistair , Scott Jeffery R. , Marotzke Jochem
Published: February 2004

The response of a global circulation model to a uniform geothermal heat flux of 50 mW m−2 through the sea floor is examined. If the geothermal heat input were transported upward purely by diffusion, the deep ocean would warm by 1.2°C. However, geothermal heating induces a substantial change in the deep circulation which is larger than previously assumed and subsequently the warming of the deep ocean is only a quarter of that suggested by the diffusive limit. The numerical ocean model responds most strongly in the Indo‐Pacific with an increase in meridional overturning of 1.8 Sv, enhancing the existing overturning by approximately 25%.
A new look at the Pacific/North American Index
Authors: Rodionov Sergei , Assel Raymond
Published: February 2004

This study examines inconsistencies in the Pacific/North American (PNA) index relative to atmospheric circulation over North America. Two types of atmospheric circulation were found to be associated with high PNA values. The first type is the true PNA pattern characterized by an amplified ridge‐trough system. It appears to be related to the leading mode of sea surface temperature variability in the North Pacific. The second type is observed during El Niño events. It is characterized by a flattening of the polar jet stream and southward shift of the subtropical jet stream. The recognition of these two types of the PNA index improves our understanding of the relative role of to El Niño/Southern Oscillation events and sea surface temperatures in the North Pacific in affecting winter atmospheric circulation over North America.
A possible correlation between satellite‐derived cloud and aerosol microphysical parameters
Authors: Nakajima Teruyuki , Higurashi Akiko , Kawamoto Kazuaki , Penner Joyce E.
Published: February 2004

The column aerosol particle number and low cloud microphysical parameters derived from AVHRR remote sensing are compared over ocean for four months in 1990. There is a positive correlation between cloud optical thickness and aerosol number concentration, whereas the effective particle radius has a negative correlation with aerosol number. The cloud liquid water path (LWP), on the other hand, tends to be constant with no large dependence on aerosol number. This result contrasts with results from recent model simulations which imply that there is a strong positive feedback between LWP and aerosol number concentration. Estimates for indirect forcing over oceans derived from the satellite data/model comparison range from −0.7 to −1.7 Wm−2.
Interannual prediction of the Paraná River
Authors: Robertson Andrew W. , Mechoso Carlos R. , Garcia Norberto O.
Published: February 2004

Interannual‐to‐decadal predictability of the Paraná river in South America is investigated by extracting near‐cyclic components in summer‐season streamflows at Corrientes over the period 1904–1997. It is found that oscillatory components with periods of about 2–5, 8 and 17 years are accompanied by statistically significant changes in monthly streamflow. Autoregressive predictive models are constructed for each component. Cross‐validated categorical hindcasts based on the 8‐yr predicted component are found to yield some skill up to four years in advance for below‐average flows. A prediction based upon the 8‐ and 17‐yr components including data up to 1999 suggests increased probability of below‐average flows until 2006.
Biotic feedback extends the life span of the biosphere
Authors: Lenton Timothy M. , von Bloh Werner
Published: February 2004

The Sun is becoming more luminous with time and will eventually overheat the biosphere. However, life cools the Earth by amplifying the rate of silicate rock weathering and maintaining a low level of atmospheric CO2. Recent studies indicate a much stronger biotic weathering effect than in models used to estimate the life span of the biosphere. Here we show that the resulting feedback lengthens the survival of complex life by delaying the loss of CO2 from the atmosphere. The weathering biota can potentially maintain the Earth in a habitable state when otherwise it would be too hot for them. If so, catastrophic warming rather than gradual CO2 starvation will terminate complex life. Despite the possibility of an irreversible collapse, the current biosphere should remain resilient to carbon cycle perturbation or mass extinction events for at least 0.8 Gyr and may survive for up to 1.2 Gyr.
Ice‐core evidence for a small solar‐source of atmospheric nitrate
Authors: Palmer Anne S. , Van Ommen Tas D. , Curran Mark A. J. , Morgan Vin
Published: February 2004

A precision‐dated ice‐core nitrate record from Law Dome, coastal East Antarctica is compared to the timing of known solar proton events and geomagnetic storms over the period 1888 to 1995. We find statistical evidence for a significant (P < 0.03) but small elevation in mean nitrate concentrations following the solar events (∼11% over the 12 months beginning 3 months post‐event). While some solar events are identifiable in the nitrate record, most are not distinguishable from the background noisy signal (which has numerous large peaks), and some solar events show no nitrate elevation above even mean levels.
First observations of flickering auroral roar
Authors: Hughes J. M. , LaBelle J.
Published: February 2004

We present observations of ∼10 Hz pulsations in the amplitude of an auroral roar emission. These pulsations vary in frequency from 7–11 Hz and occur in groups which last for ∼1 sec with adjacent pulsation groups spaced from one to several seconds. We put forth that these pulsations reflect 7–11 Hz modulations in the distribution function of the auroral electrons, similar to those associated with flickering aurora. The radio observations provide an additional means of probing this phenomenon and in particular may put constraints on the form of modulations of the electron distribution function at the auroral roar source height of a few hundred kilometers.
Single and double asperity failures in a large‐scale biaxial experiment
Authors: Yoshida Shingo , Kato Aitaro
Published: February 2004

Stick slip experiments were performed in a direct shear apparatus using large Inada granite blocks. The dimensions of the pre‐existing fault surface were 100 cm (length) × 10cm (width). By applying heterogeneous normal stress using three actuators by which forces could be independently controlled, two asperities with different strengths were formed on a fault plane. Under a certain normal stress distribution, single asperity failure and double asperity failure occurred alternately. The single asperity failure refers to a stick slip event in which a weaker asperity ruptures alone, without triggering the rupture of a stronger asperity. The double asperity failure is an event in which a rupture of a weak asperity triggers the rupture of a strong asperity, resulting in a failure of the whole fault. For triggering of a rupture, it is necessary that stress at the strong asperity is accumulated to a certain level just prior to the failure of the weak asperity.
ozone mini‐hole observed over Europe, influence of low stratospheric temperature on observations
Authors: Allaart Marc , Valks Pieter , van der A Ronald , Piters Ankie , Kelder Hennie , van Velthoven Peter
Published: February 2004

A region of extreme low ozone values passed over North‐western Europe during November 30, and December 1, 1999. The total ozone values were measured from the ground, with a Brewer Spectrophotometer, and from space with the GOME and TOMS satellite instruments. The ozone sonde measurement and the retrieved GOME ozone profile have shown that the main reduction in the ozone column occurred between the ozone maximum (22 km) and the tropopause. The low temperatures found in the stratosphere during this event have significant consequences for the ozone retrieval algorithm, both for satellite retrievals and for the Brewer measurements.
Unusually intense continuing current in lightning produces delayed mesospheric breakdown
Authors: Cummer Steven A. , Füllekrug Martin
Published: February 2004

Ultra low frequency magnetic field measurements made 500–2000 km from positive lightning discharges show a signature that is consistent with unusually high amplitude cloud‐to‐ground continuing lightning current. The magnitude of this nearly constant current moment is as large as 60 kA km and can last at half this amplitude for longer than 150 ms, thereby moving 640 C or more (assuming a 7 km vertical channel length) to the ground after the return stroke. This total charge transfer is more than an order of magnitude greater than most previously reported continuing currents in positive discharges. Three cases analyzed show this strong continuing current flows before, during, and after sprites that initiate more than 40 ms after the return stroke. Accounting for this continuing current, quantitative analysis shows that the total vertical lightning charge moment changes are large enough to produce mesospheric electrical breakdown and long‐delayed sprites.
Upper mantle transition zone structure beneath the Philippine Sea Region
Authors: Shito Azusa , Shibutani Takuo
Published: February 2004

Seismic structure of the upper mantle transition zone beneath the Philippine Sea region is studied using waveform modeling. Models AZ41 and AZ40 were derived to represent the structure beneath the north central part and northwestern regions, respectively. AZ41 has high‐velocity anomalies from 510 km to 660 km 3% faster than a global model ak135, a depression of the ‘660’ discontinuity to 690 km, and a small (+2%) discontinuity at 510 km. Model AZ40 is the same as AZ41 except there is no depression of the ‘660’. On the other hand, the waveforms which sampled the southern region can be explained with the global model ak135. These results support the interpretation that subducted Pacific lithosphere is stagnant above the ‘660’ in the northern part, whereas in the southern part it penetrates into the lower mantle. In accord with earlier suggestions, the different configuration of the subducted slab suggested above could be due to the migration of the trench.
Parameterization of the radiative properties of midlatitude high and middle level clouds
Authors: Sassen Kenneth , Comstock Jennifer M. , Wang Zhien
Published: February 2004

The clouds of the middle and upper troposphere are difficult to treat in models because of their variable optical properties. Based on extensive cirrus, altostratus, and altocumulus cloud remote sensing measurements and model simulations, we offer parameterizations of visible optical depth and infrared layer emittance as functions of both cloud temperature and vertical thickness to facilitate climate modeling research. We find significant differences in the emittance relations that depend on cloud phase, but quite similar optical depth versus emittance relations. In comparison to previous midlatitude cirrus results, our findings lead to lower atmospheric heating rates.
Propagation analysis of plasmaspheric hiss using Polar PWI measurements
Authors: Santolík O. , Parrot M. , Storey L. R. O. , Pickett J. S. , Gurnett D. A.
Published: February 2004

We have analyzed high‐rate waveform data, taken by the POLAR Plasma Wave Instrument at high altitudes in the equatorial plasmasphere, to study plasmaspheric hiss in the range of frequencies between 100 Hz and several kHz. These emissions are found almost everywhere in the plasmasphere, and their origin is still controversial. Our analysis of several cases shows that most of the waves were propagating more or less parallel to the Earth's magnetic field, but sometimes a few of them were propagating obliquely with their normals near the Gendrin angle. Evidence of amplification was found near the geomagnetic equator. The results suggest that waves with normals both parallel and anti‐parallel to the magnetic field were being amplified by the classical mechanism that involves gyroresonant interaction with energetic electrons.
The ring effect in the cloudy atmosphere
Authors: de Beek R. , Vountas M. , Rozanov V. V. , Richter A. , Burrows J. P.
Published: February 2004

Filling‐in of spectral features observed in scattered sunlight and known as the “Ring Effect” has a significant impact on the retrieval of atmospheric trace constituents from both ground‐based and satellite observations. As clouds also have strong impacts on radiative transfer in the UV/visible spectral range, possible changes of the Ring Effect due to clouds have to be considered. Such investigations require radiative transfer modeling explicitly accounting for both cloud and Ring features. UV/visible ground and satellite based measurements, taken under cloudy conditions, were used to validate Ring and cloud modeling using the radiative transfer model SCIATRAN. As an example results of model applications are presented for both ground‐based and satellite geometry. Two case studies are used to explain the sources of the observed Filling‐in.
Intermediate water mass production controlled by southern hemisphere winds
Authors: Ribbe Joachim
Published: February 2004

It is demonstrated that the production of intermediate water in a coarse resolution ocean general circulation model is controlled by Southern Hemisphere winds. Results from four equilibrium experiments using simplified topography and surface forcing are presented. The first experiment was carried out with no wind forcing, subsequent experiments employed annual mean surface stresses, which were amplified using factors of 0.5, 1, and 2.0 south of 30°S. In all experiments, the salinity minimum characteristic for intermediate water is reproduced. Volume transports are directly proportional to the applied Southern Hemisphere surface stresses. These force an increased export of intermediate water and heat into the South Pacific Ocean northward across 30°S and through Drake Passage into the South Atlantic Ocean. It results in a warming of the South Pacific Ocean, which is at a maximum in the intermediate water density range.
Aliasing of high‐frequency variability by altimetry: Evaluation from bottom pressure recorders
Authors: Gille Sarah T. , Hughes Christopher W.
Published: February 2004

In spectra computed from bottom pressure records, 18% to 63% of non‐tidal energy is associated with frequencies greater than 1 cycle per 20 days, the approximate Nyquist frequency for the TOPEX/Poseidon (T/P) satellite. Since bottom pressure measurements are correlated with coincident sea surface height measurements from the T/P satellite, hourly bottom pressure data are used here to characterize the aliasing expected from T/P. As numerical modeling studies have also demonstrated, spectra computed from data sampled at 10‐day intervals are likely to alias substantial amounts of unresolved high‐frequency variability, particularly at frequencies greater than 0.2 times the Nyquist frequency.
Climatology of intrusions into the tropical upper troposphere
Authors: Waugh Darryn W. , Polvani Lorenzo M.
Published: February 2004

Regions of upper tropospheric equatorial westerly winds, observed over the Pacific and Atlantic Oceans during northern fall to spring, are important for extratropical‐tropical interactions. This paper focuses on one feature of these “westerly ducts” that has received relatively little attention to date: the occurrence of Rossby wave breaking events that transport tongues of extratropical air deep into the tropics, mix tropical and subtropical air, and can affect deep convection. A climatology of these “intrusion” events formed from 20 years of meteorological analyses shows a strong dependence on the basic‐state flow. Notably, intrusion events are found to occur almost exclusively within westerly ducts, with more events in the presence of stronger equatorial westerlies. It is also found that there is strong interannual variability in the frequency of Pacific events, with fewer events during the warm phases of ENSO (consistent with the changes in the basic flow). Since these intrusion events laterally mix trace constituents and have been linked to tropical convection, their spatial and temporal variability may cause related variability in the distribution of trace constituents and tropical convection.
Nanga Parbat crustal anisotropy: Implications for interpretation of crustal velocity structure and shear‐wave splitting
Authors: Meltzer Anne , Christensen Nikolas
Published: February 2004

The Nanga Parbat‐Haramosh massif represents a unique exposure of mid‐lower continental crust from beneath the Himalayan orogen. Seismic velocity measurements on a suite of quartzofeldspathic gneisses show up to 12.5% velocity anisotropy for compressional waves and up to 21% for shear waves. The degree of anisotropy is a function of mica content and rock fabric strength. Over 30% of the samples have maximum compressional wave velocities of 6.4–6.5 km/s; velocities typically associated with more maficlithologies. These results have implications for the interpretation of crustal velocity structure obtained from wide‐angle seismic surveys where in situ velocity measurements are made from refracted or turning rays that potentially spend a substantial portion of their travel path propagating in the foliation plane. Velocities determined from these surveys may overestimate mean velocities of crustal rocks with well‐developed horizontal fabric. In addition, crustal anisotropy due to the development of pervasive rock fabric has the potential to be a significant contributing factor to shear‐wave splitting observations.
Deformation of a seamount subducting beneath an accretionary prism: Constraints from numerical simulation
Authors: Baba Toshitaka , Hori Takane , Hirano Satoshi , Cummins Phil R. , Park Jin‐Oh , Kameyama Masanori , Kaneda Yoshiyuki
Published: February 2004

We examined the process of seamount subduction via a numerical simulation using the finite element method, applying a frictional force on the plate interface that is proportional to the normal stress. We calculate the incremental stress due to infinitesimal deformation of the seamount associated with subduction, and consider the implications for stress buildup and fracturing of the seamount itself. Our results show that the maximum shear stress concentrates at both flanks of the seamount, which suggests that fracturing will start there. We can surmise that, eventually, the seaward flank may be more apt to break than the landward flank at shallow depth if the confining pressure there is sufficiently low. We consider this to be a possible scenario for the generation of a thrust fault imaged at the seaward flank of the Muroto seamount, which is subducting under the Nankai trough accretionary prism.
Hydroelectric coupling in a clayey material
Authors: Revil A. , Leroy P.
Published: February 2004

The hydroelectric coupling generating self‐potential anomalies recorded on the Earth's surface has received a considerable attention in the last four years. Geophysical applications include the use of this phenomenon for mapping subsurface fluid flow, the study of hydrothermal activity, and the use of self‐potential anomalies to monitor volcanic activity and the possibility, still debated, of seismic activity forecasting through the use of electromagnetic precursors of electrokinetic nature. We model here two key‐parameters relevant of the hydroelectric coupling in clayey porous rocks. These two parameters are the ζ‐potential and the specific surface conductivity, which depend on the electrochemistry of the pore water/mineral interface. They are modeled in the case of a kaolinite saturated by a NaCl brine using a triple layer model (TLM) in which the parameters are optimized using the Simplex algorithm.
Can we resolve 3D density heterogeneity in the lower mantle?
Authors: Romanowicz Barbara
Published: February 2004

We reexamine the possibility of resolving 3D density structure in the mantle, by performing test inversions using a dataset of well constrained degree 2 splitting coefficients. We show that degree 2 in Vs and Vp can be resolved, and confirm that dlnVs/dlnVp > 2.5 at depths >2000km, indicating a chemical component to large scale heterogeneity. Models with ρ positively correlated with Vs in the deepest mantle fit the data at least as well as those in which density and velocities are anti‐correlated. However, some rough bounds on dlnρ/dlnVs can be inferred. Models obtained from geodynamic considerations are acceptable, in contrast to models with large negative (<−0.3) or positive (>0.6) dlnρ/dlnVs throughout the lower mantle. The most robust feature is a marked increase in dlnρ/dlnVs at the top of the lower mantle reaching a maximum around 1500 km.
Considering the third dimension in stress‐triggering of aftershocks: 1993 Klamath Falls, Oregon, Earthquake Sequence
Authors: Crider Juliet G. , Schaff David P. , Pollard David D. , Beroza Gregory C.
Published: February 2004

Dip slip on a fault of finite dimension produces a static stress change that varies considerably in 3‐D. The 1993 Klamath Falls (Oregon, USA) earthquake sequence shows a shallow, off‐fault cluster of aftershocks with mixed dip‐slip and strike‐slip focal mechanisms. Modeled static stress change and location of aftershocks show a misleading correspondence when viewed in the customary 2‐D presentation. Full 3‐D analysis shows that shallow right‐lateral slip is consistent with the local stress perturbation following the dip‐slip main shocks.
The Tropospheric Biennial Oscillation and Indian Monsoon rainfall
Authors: Meehl Gerald A. , Arblaster Julie M.
Published: February 2004

An analysis of observed data shows that the tropospheric biennial oscillation (TBO, with roughly a 2–3 year period) encompasses most ENSO years (with their well‐known biennial tendency) as well as additional years that contribute to biennial transitions in the Indian monsoon. Three conditions (related to anomalous surface temperatures) postulated to contribute to TBO transitions are examined for the northern spring season before the Indian monsoon: 1;) anomalous tropical Pacific SSTs, 2) atmospheric circulation‐related anomalous south Asian land temperatures and resulting meridional temperature gradients, and 3) anomalous SSTs in the Indian Ocean. We quantify these three interannual transition conditions on a year by year basis, thus taking into account intermittent influences and secular variations in the strength of any particular linkage in any given year. The more transition conditions that are included in the analysis, the greater the TBO amplitude in terms of spectral power in the TBO periods of 2–3 years.
Detailed coseismic slip distribution of the 1944 Tonankai Earthquake estimated from tsunami waveforms
Authors: Tanioka Yuichiro , Satake Kenji
Published: February 2004

Coseismic slip distribution on the fault plane of the 1944 Tonankai earthquake is estimated from inversion of tsunami waveforms. Three improvements from a previous study [Satake, 1993] are made. These are: (1) smaller subfaults are used to resolve detailed slip distribution; (2) the sub faults fit better to the plate interface geometry; and (3) finer and more accurate bathymetry data is used. The inversion result shows that a maximum slip of about 3 m occurred on the plate interface off Shima peninsula. The total seismic moment is estimated to be 2.0 × 1021 Nm (Mw 8.2). The result confirms that the 1944 Tonankai earthquake did not rupture the plate interface beneath the Tokai region and supports the existence of the seismic gap in the Tokai region suggested by Ishibashi [1981]. The slip of about 1.5 m on the plate interface beneath Atsumi peninsula, northeast of the large slip area, is necessary to explain the observed tsunami waveforms, although no seismic moment release was estimated from strong motion data by Kikuchi et al. [1999]. This may suggest that the rupture beneath Atsumi peninsula was slow.
Variational inversion of the ocean acoustic tomography data using quadratic approximation to travel times
Authors: Yaremchuk Maxim I. , Yaremchuk Alexey I.
Published: February 2004

A non‐linear method for inverting oceanic acoustic tomography data is presented. The algorithm takes into account the quadratic term of the travel time expansion in the powers of the reciprocal sound speed perturbations. The method is tested within a 3D variational inversion scheme by means of twin data experiments and against the real data acquired in the Kuroshio Extension region.
Melting relations of hydrous pyrolite in CaO‐MgO‐Al2O3‐SiO2‐H2O System at the transition zone pressures
Authors: Litasov Konstantin , Ohtani Eiji , Taniguchi Hiromitsu
Published: February 2004

Phase relations and melt compositions in CaO‐MgO‐Al2O3‐SiO2‐pyrolite under hydrous (+2% of H2O) and anhydrous conditions have been determined at 13–20 GPa and 1600–2220°C. Liquidus and solidus temperatures for the hydrous system are about 50–100°C and 180–240°C lower than those for the dry system, respectively. Majorite is a liquidus phase of the hydrous pyrolite from 13 to 20 GPa. Olivine is a liquidus phase at 13 GPa and both periclase and majorite are the liquidus phases at 20 GPa in the dry pyrolite. We observed expansion of the stability field of anhydrous phase B in hydrous experiments. Compositions of partial melts at 13–20 GPa are generally similar in dry and hydrous systems, but hydrous melts contain more SiO2 at 13–17 GPa. The melts formed by low degree of melting have Al2O3‐depleted and CaO‐rich compositions. Trends of hydrous melt compositions are generally consistent with those of aluminum‐depleted komatiite magmas.
Diapycnal flow through the Georges Bank Tidal Front: A Dye Tracer Study
Authors: Houghton Robert W. , Ho Cheng
Published: February 2004

A fluorescent dye tracer, Fluorescein, injected into the bottom mixed layer at the seaward edge of the tidal front on Georges Bank has provided the first quantitative measurement of an on‐bank diapycnal Lagrangian flow through the front. From the warming of the dye patch, 2.5 × 10−6 °C/s and 7.8 × 10−6 °C/s on the south flank and northeast peak respectively, as it passed through the frontal temperature gradient we infer an on‐bank flow of 1.9 cm/s on the south flank and 3.2 cm/s on the northeast peak. The heat flux required for this warming is predominantly due to vertical mixing within the tidal front. These observations further demonstrate the utility of direct Lagrangian measurements and provide quantitative estimates of the cross‐frontal exchange on Georges Bank, the focus of the U.S GLOBEC Northwest Atlantic/Georges Bank Phase III program.
Upgrading a synthetic seismicity model for more realistic fault ruptures
Authors: Robinson Russell , Benites Rafael
Published: February 2004

A quasi‐static synthetic seismicity model for multiple interacting faults of any orientation is upgraded to include a more realistic rupture model. In the original model entire ruptures, covering many fault patches, occur instantaneously and rupture propagation is ignored. Now time is introduced into the rupture process via a patch interaction delay in proportion to distance/shear wave velocity. Also, to try to mimic observed rise times for real earthquakes, a simple time dependent friction law is used. Comparison of results for the old and new models for a simple case of two strike‐slip faults show that clustering of large events is somewhat reduced in the new model. It remains to be seen if this conclusion will be verified in more complex fault networks. The new model takes about twice as much computation time, but the results may now be used to calculate synthetic seismograms.
Electron densities determined by the HIRAAS Experiment and comparisons with ionosonde measurements
Authors: Dymond K. F. , Budzien S. A. , Thonnard S. E. , McCoy R. P. , Thomas R. J.
Published: February 2004

We present electron density profiles derived by inversion of ultraviolet limb scans made by the High Resolution Airglow and Aurora Spectroscopy (HIRAAS) experiment on the Advanced Research and Global Observing Satellite (ARGOS). The ultraviolet limb scans were inverted using an iterative algorithm based on discrete inverse theory. We present two comparisons with nearly coincident ionosonde measurements of the F‐region peak density and peak height. Our observations took place on 24 November 1999 when the 10.7 cm radio flux was 181 × 10−22 Watt m−2 Hz−1 and the daily ap was 21, indicating moderate geomagnetic activity. The retrieved peak electron density and peak height were in good agreeent with the ionosonde measurements, demonstrating the accuracy of the ultraviolet technique for sensing the ionospheric state.
On the contribution of lightning to ultrafine aerosol formation
Authors: Yu Fangqun , Turco Richard P.
Published: February 2004

We propose that locally high concentrations of new ultrafine aerosols may be generated within lightning channels, in which the air is initially highly ionized by discharge currents. In the channel, temperatures are sufficiently elevated to ensure that preexisting condensed materials are vaporized, generating aerosol precursors at concentrations much higher than would be found in ambient air. As the channel expands and cools, ions that survive neutralization attract condensable vapors and grow rapidly. Detailed microphysical simulations indicate that ultrafine particles (>3 nm in diameter) may achieve concentrations of 105–106/cm³ in the channel within 10 seconds after a flash, leading to an effective “production index” of 1013–1014 particles per meter of channel. While lightning may generate very high local concentrations of ultrafine particles, and occur worldwide, the overall contribution to global aerosol production is estimated to be small.
Thin‐shell modeling of neotectonics in the Azores‐Gibraltar Region
Authors: Jiménez‐Munt Ivone , Bird Peter , Fernàndez Manel
Published: February 2004

We applied the thin‐shell neotectonic modeling method to study the neotectonics of the Africa/Eurasia plate boundary in the Azores‐Gibraltar region. The plate boundary consists of a simple fault system running from Azores to the Gorringe Bank where it branches along the Betics and Rift‐Tell thrust fronts. Major faults in west Iberia and NW Africa have also been incorporated. Results are compared with seismic strain rates, fault slip rates and stress orientations. The best estimate for the fault friction coefficient is 0.1–0.15 meaning that the plate‐boundary is only about 1/4 as strong as the adjacent lithosphere. The largest fault slip rates (>1.5 mm/yr) are obtained along the Gloria fault (strike‐slip), and the Betic (transpressive) and Rif‐Tell (compressive) thrust systems. Whereas tectonic activity in the Atlas region is comparable to that obtained along the plate boundary, the fault slip rates in the west Iberia fault systems are one order of magnitude less.
Electron bombardment of Europa
Authors: Paranicas C. , Carlson R. W. , Johnson R. E.
Published: February 2004

Energetic electrons trapped in Jupiter's magnetic field are a principal source of energy for driving chemistry on the surface of Europa. Here we show that energetic (10 keV–10 MeV) electrons precipitate primarily on the trailing hemisphere, and we give the spatial distribution of the dose‐rate deposited in Europa's surface. Based on this we propose that energetic electrons are a primary agent in determining the hemispherical differences in Europa's albedo, which have been attributed to differences in the incident ion flux. We compare the spatial distribution in the dose‐rate deposited by the energetic electrons to the spatial distribution in the hydrate (likely frozen, hydrated sulfuric acid) determined by Galileo's near‐infrared mapping spectrometer. This comparison supports the idea that radiolysis by the energetic electrons contributes significantly to producing the hydrate.
NAO‐induced long‐term changes in nutrient supply to the surface waters of the North Atlantic
Authors: Oschlies Andreas
Published: February 2004

Since the late 1980s extensive observational campaigns like the Joint Global Ocean Flux Study (JGOFS) and the World Ocean Circulation Experiment (WOCE) have helped to considerably improve our understanding of marine biogeochemistry. By chance, this period corresponded to a phase of a positive swing of the North Atlantic Oscillation (NAO), whereas earlier studies in the North Atlantic generally took place during more negative phases of the NAO (Figure 1). This study demonstrates by means of a coupled ecosystem‐circulation model that the long‐term change in the NAO between the 1960s and 1990s may have induced significant regional changes in the upper ocean's nutrient supply. These include a decrease of nitrate supply by about 30% near Bermuda and in mid‐latitudes, and a simultaneous 60% increase in the upwelling region off West Africa. The results suggest that a synthesis of biogeochemical observations taken during the past decades must take into account NAO‐related climate variability.
Comparing P and S wave heterogeneity in the mantle
Authors: Saltzer R. L. , van der Hilst R. D. , Kárason H.
Published: February 2004

From the reprocessed data set of Engdahl and co‐workers we have carefully selected matching P and S data for tomographic imaging. We assess data and model error and conclude that our S model uncertainty is twice that of the P model. We account for this in our comparison of the perturbations in P and S‐wavespeed. In accord with previous studies we find that P and S perturbations are positively correlated at all depths. However, in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not. In particular, below 1500 km depth ∂ln Vs/∂ln Vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins. This inference is substantiated by wavespeed analyses with random realizations of the slab/non‐slab distribution. Through much of the mantle there is no significant correlation between bulk sound and S‐wave perturbations, but they appear to be negatively correlated between 1700 and 2100 km depth, which is also where the largest differences in ∂ln Vs/∂ln Vp occur. This finding supports convection models with compositional heterogeneity in the lowermost mantle.
Rapid aseismic moment release following the 5 December, 1997 Kronotsky, Kamchatka, Earthquake
Authors: Bürgmann R. , Kogan M. G. , Levin V. E. , Scholz C. H. , King R. W. , Steblov G. M.
Published: February 2004

Model inversions of displacements of continuously operating GPS stations in Kamchatka show that aseismic afterslip during 2 months following the Mw=7.8, 5 December, 1997 Kronotsky subduction earthquake released as much moment as the earthquake itself. The rapidly decaying transient slip on the subduction underthrust occurred near the downdip edge of the coseismic rupture and extended laterally away from it, including a region of vigorous foreshock activity. A logarithmic decay function fit to the cumulative afterslip curve has a relaxation time of about 3 days. Rapid afterslip can contribute significantly to the moment release of partially coupled subduction zones.
The electron and proton aurora as seen by IMAGE‐FUV and FAST
Authors: Frey H. U. , Mende S. B. , Carlson C. W. , Gérard J.‐C. , Hubert B. , Spann J. , Gladstone R. , Immel T. J.
Published: February 2004

The Far Ultraviolet Instrument (FUV) on the IMAGE spacecraft observes the aurora in three different channels. One of them (SI12) is sensitive to the signal from precipitating protons, while the other two (WIC and SI13) observe auroral emissions which are not only excited by precipitating electrons, but also by protons. We examine a period when in‐situ particle measurements by the FAST spacecraft were available simultaneously with global imaging with FUV. The measured electron and proton energy spectra are used to calculate the auroral brightness along the FAST orbit. The comparison with the FUV/IMAGE observations shows good quantitative agreement and demonstrates that under certain circumstances high proton fluxes may produce significant amounts of auroral FUV emission.
Seismic evidence for partial lithospheric delamination model of Colorado Plateau Uplifts
Authors: Lastowka Lynda A. , Sheehan Anne F. , Schneider Jeffrey M.
Published: February 2004

The lithospheric structure beneath the Colorado Plateau and Great Basin of the western United States is investigated using Pn and surface wave data from the Colorado Plateau ‐ Great Basin broadband seismic experiment. Average Pn velocities of 8.0 km/s in the Colorado Plateau and 7.8 km/s in the eastern Great Basin are found. Teleseismic Rayleigh waves are also measured, and eastern Great Basin phase velocities are found to be 3–4% higher than Colorado Plateau phase velocities in the 20–35 s range, but indistinguishable at periods greater than 40 s. The apparent Pn/surface wave discrepancy is resolved with models that include a thicker crust and faster mantle lid for the Colorado Plateau than the eastern Great Basin. Models for both regions require a mantle low velocity zone. The slow mantle deep beneath the Colorado Plateau may suggest a thermal origin of Plateau uplift, which has not yet penetrated the mantle lid.
CRISTA‐2 observations of the South Polar Vortex in winter 1997: A new dataset for polar process studies
Authors: Spang Reinhold , Riese Martin , Offermann Dirk
Published: February 2004

During the second mission of the Cryogenic Spectrometers and Telescopes for the Atmosphere (CRISTA) in August 1997 considerable and highly variable cloud cover in the stratosphere was seen over Antarctica. Two large areas of polar stratospheric clouds (PSC) could be observed. One region of PSCs near the vortex edge was caused by a meteorological situation as it is typical for an ozone minihole event and dissolved within two days. The temperature dependence of the HNO3 measured by CRISTA suggests that outside of their center the clouds consists of type I particles. However, a further discrimination between nitric acid trihydrate (NAT) or supercooled ternary solutions (STS) is not possible, due to the uncertainties in the meteorological temperatures and the amount of water vapor.
High‐latitude ground‐based observations of the thermospheric ion‐drag time constant
Authors: Kosch M. J. , Cierpka K. , Rietveld M. T. , Hagfors T. , Schlegel K.
Published: February 2004

From previous studies, it has been conclusively demonstrated that F‐region thermospheric winds follow, but generally lag behind, the ion drift pattern of magnetospheric convection. Analysis of the ion‐neutral momentum exchange equation shows that ion‐drag and thermal pressure are the major contributors to neutral momentum forcing at F‐region heights with relatively minor effects from coriolis, advection and viscous forces. An ion‐neutral coupling time constant (e‐folding time) has been defined which describes the time taken for the neutral gas velocity to approach the ion velocity after a step change in convection. In this study, F‐region ion drift and neutral winds have been observed by the EISCAT tristatic incoherent scatter facility and a ground‐based Fabry‐Perot interferometer, respectively, from northern Scandinavia. The e‐folding time varies in the range 0.5–6.5 hours, with an average of 1.8 and 3.3 hours for a geomagnetically active and quiet period, respectively, which compares well with previous satellite measurements of 0.5–3.5 hours.
X‐ray imaging of the solar wind—Mars interaction
Authors: Holmström Mats , Barabash Stas , Kallio Esa
Published: February 2004

Wherever the solar wind meets a neutral atmosphere, X‐rays are emitted by charge‐exchange processes between heavy solar wind ions and the neutrals. Using an empirical model of the proton flow near Mars we present computer simulations of X‐ray emissions from this charge‐exchange process in Mars' exosphere. We also discuss implications for remote sensing. Here we show that the total X‐ray luminosity near Mars from charge‐exchange is greater than 1025 eV/s for typical solar wind conditions. This flux is large enough to be detected by an X‐ray satellite in Earth orbit. Thus, Mars belongs to a new class of X‐ray objects in the sky, together with other non‐magnetized planets, such as Venus.
Evolution of electron phase‐space holes in 3D
Authors: Oppenheim M. M. , Vetoulis G. , Newman D. L. , Goldman M. V.
Published: February 2004

Electron phase‐space holes are regions of depleted electron density commonly generated during the nonlinear stage of the two‐stream instability. Recently, bipolar electric field structures—a signature of electron holes —have been identified in the acceleration region of the auroral ionosphere. This paper compares the evolution of electron holes in 2‐D and 3‐D using massively‐parallel PIC simulations. In 2‐D, the holes decay after hundreds of plasma periods while emitting electrostatic whistler waves. In the 3‐D simulations, electron holes also go unstable and generate whistlers but, due to physical processes not present in 2‐D, energy flows out of the whistlers and into highly perpendicular lower hybrid modes. As a result of this difference, 3‐D holes do not decay as far as 2‐D holes. The differences between 2‐D and 3‐D evolution may have important implications for hole longevity and wave generation in the auroral ionosphere.
Effect of depth‐dependent shear modulus on tsunami generation along subduction zones
Authors: Geist Eric L. , Bilek Susan L.
Published: February 2004

Estimates of the initial size of tsunamis generated by subduction zone earthquakes are significantly affected by the choice of shear modulus at shallow depths. Analysis of over 360 circum‐Pacific subduction zone earthquakes indicates that for a given seismic moment, source duration increases significantly with decreasing depth (Bilek and Lay, 1998; 1999). Under the assumption that stress drop is constant, the increase of source duration is explained by a 5‐fold reduction of shear modulus from depths of 20 km to 5 km. This much lower value of shear modulus at shallow depths in comparison to standard earth models has the effect of increasing the amount of slip estimated from seismic moment determinations, thereby increasing tsunami amplitude. The effect of using depth dependent shear modulus values is tested by modeling the tsunami from the 1992 Nicaraguan tsunami earthquake using a previously determined moment distribution (Ihmlé, 1996a). We find that the tide gauge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution. Because excitation of seismic waves also depends on elastic heterogeneity, it is important, particularly for the inversion of short period waves, that a consistent seismic/tsunami shear modulus model be used for calculating slip distributions.
Comparison of cloud liquid water paths derived from in situ and microwave radiometer data taken during the SHEBA/FIREACE
Authors: Lin Bing , Minnis Patrick , Fan Alice , Curry Judith A. , Gerber H.
Published: February 2004

Mean cloud liquid water path LWP derived from microwave measurements using the standard ARM retrieval technique is nearly twice as large as coincident in situ aircraft data taken over the SHEBA ice camp in the Arctic during FIRE ACE. Using an algorithm adopted from satellite remote sensing that more completely accounts for the temperature dependence of water absorption and atmospheric gas absorption results in a 25 to 45 % reduction in LWP values relative to the standard ARM estimates. If possible precipitation cases are excluded, the mean results from the new technique differ by only 3% from the in situ data. Greater differences for heavier clouds may result from in situ probe uncertainties. This algorithm should provide accurate LWP retrievals for a variety of cloud conditions from the tropics to the highly supercooled Arctic clouds.
Variation of sea salt aerosol pH with relative humidity
Authors: von Glasow Roland , Sander Rolf
Published: February 2004

Calculations of the pH of sea salt aerosol particles show that the pH decreases with increasing relative humidity and therefore increasing liquid water content of the particles. We show that this puzzling effect is a result of the high concentration of chloride in the particles and the phase partitioning of HCl between the gas and aqueous phase, that makes the gas phase act as a reservoir for acidity for the sea salt particles. Implications for the marine boundary layer are briefly discussed.