bluuebunny/crossref_metadata_2025 huggingface dataset:
IMF By‐related cusp currents observed from the Ørsted satellite and from ground
Authors: Stauning Peter , Primdahl Fritz , Watermann Jürgen , Rasmussen Ole
Published: February 2004

Ørsted is the first satellite to conduct high‐precision magnetometer observations from low‐altitude noon‐midnight orbits passing through the polar cusp regions. Field‐aligned currents (FAC) derived from Ørsted magnetic field measurements have been combined with ionospheric current patterns inferred from ground‐based magnetic observations to define the structure and location of cusp currents and their dependencies on interplanetary magnetic field (IMF) conditions. Example cases illustrate the close relation between IMF By‐related FAC and horizontal ionospheric currents in the cusp region. Our statistical analysis defines for the noon region the variations in FAC latitude with IMF Bz. Comparisons with the statistical cusp location indicate that the more equatorward region of IMF By‐related FAC is located on field lines closing at the dayside, while the more poleward FAC are on “open” field lines. High‐energy electron measurements from the satellite confirm this result.
Impact of ship emissions on marine boundary layer NOx and SO2 Distributions over the Pacific Basin
Authors: Davis D. D. , Grodzinsky G. , Kasibhatla P. , Crawford J. , Chen G. , Liu S. , Bandy A. , Thornton D. , Guan H. , Sandholm S.
Published: February 2004

The impact of ship emissions on marine boundary layer (MBL) NOx and SO2 levels over the Pacific Ocean has been explored by comparing predictions (with and without ships) from a global chemical transport model (GCTM) against compiled airborne observations of MBL NOx and SO2. For latitudes >15°N, which define that part of the Pacific having the heaviest shipping, this analysis revealed significant model over prediction for NOx and a modest under prediction for SO2 when ship emissions were considered. Possible reasons for the difference in NOx and SO2 were explored using a full‐chemistry box model. These results revealed that for an actual plume setting the NOx lifetime could be greatly shortened by chemical processes promoted by ship plume emissions themselves. Similar chemical behavior was not found for SO2.
Tomographic image of P‐velocity structure beneath Kilauea's East Rift Zone and South Flank: Seismic evidence for a deep magma body
Authors: Haslinger Florian , Thurber Clifford , Mandernach Megan , Okubo Paul
Published: February 2004

We present first results from the analysis of P‐wave arrival time data recorded from November 11 to December 31, 1999, by a temporary 29‐station network installed across Kilauea Volcano's East Rift Zone (ERZ) and South Flank (SF) on Hawaii, augmented by data from the permanent network of the Hawaiian Volcano Observatory. Starting with the inversion result for a minimum 1D velocity model, we use arrival time data from 135 local earthquakes to invert for the 3D P‐velocity structure. The resulting tomographic image shows evidence for a deep magma body beneath the ERZ just east of its southward bend, and a smaller magma body at about 5 km depth beneath the WNW‐ESE trending segment of the ERZ. We also observe a drastic change in velocities south of the Hilina fault system from velocities around 5.5 km/s in the west to 6.5 km/s to the east.
Fast 3D ultra shallow seismic reflection imaging using portable geophone mount
Authors: Bachrach Ran , Mukerji Tapan
Published: February 2004

We present the results of three different seismic experiments designed to develop and evaluate the use of 3D Ultra Shallow Seismic Reflection for high‐resolution near‐surface imaging. A feasibility study and the first implementation of a 2D portable geophone mount for fast and cost‐effective ultra‐shallow 3D seismic data acquisition are discussed. The portable geophone mount is made out of an anelastic base with a frame and an array of 72 geophones spaced with an interval of 0.25m both in the inline and crossline direction. The array enables acquisition of very high‐resolution 3D data cubes with bin size of 12.5 × 12.5cm and wavelets of 350Hz. The time for re‐planting the 72‐channels portable geophone mount is about 5min in the field. Thus, we show that high‐resolution near‐surface 3D images can be obtained fast and in a cost‐effective manner. This has the potential of providing effective solutions for many geotechnical and geoenvironmental applications.
The M6.1 earthquake triggered by volcanic inflation of Iwate Volcano, northern Japan, observed by satellite radar interferometry
Authors: Nishimura Takuya , Fujiwara Satoshi , Murakami Makoto , Tobita Mikio , Nakagawa Hiroyuki , Sagiya Takeshi , Tada Takashi
Published: February 2004

Ground uplift and earthquake swarm activity has continued around Mt. Iwate volcano, northern Japan, since February 1998. On September 3, 1998 a M6.1 earthquake occurred southwest of the volcano. Satellite radar interferometry acquired by JERS‐1 reveals volcanic and coseismic deformation to the west of the summit, which is concordant with displacements observed by GPS. We analyzed geodetic observations including InSAR, GPS, and EDM to model the source of the surface deformation. The inferred model shows a 0.033km³ volumetric increase of an inflation (Mogi) source at a depth of 7.9km from April 1998 to the occurrence of the M6.1 earthquake. We find that the inflation increased Coulomb failure stress by 0.33MPa (7% of the coseismic stress drop) at the hypocenter. This result suggests the inflation promoted the rupture of a known Quaternary fault.
Insolation dependence of the southeastern subtropical Pacific sea surface temperature over the last 400 kyrs
Authors: Calvo Eva , Pelejero Carles , Herguera Juan C. , Palanques Albert , Grimalt Joan O.
Published: February 2004

The present study describes the first sea surface temperature (SST) reconstruction in the southeastern Subtropical Pacific Ocean, offshore the South American coast. The obtained record encompasses the last 400 kyr and follows the characteristic glacial/interglacial pattern defined by global ice volume. However, SST leads the δ18O isotopic record reflecting the role of the low latitudes areas in driving climate change. SST in the Holocene is lower by about 0.5–0.8°C than maximal SST in stages 5e, 7, 9 and 11. SST in stages 2–4 is lower by about 0.6–1.3°C than minimal SST in stages 6, 8 and 10. These features are similar to SST records obtained in the South Atlantic Ocean pointing to a general inter‐basinal behaviour at these low latitudes. For most of the record, the observed long‐term SST evolution is well correlated with the orbital parameter of eccentricity, which modulates the insolation at low latitudes. However, at low eccentricity values (Stages 11–9 and Termination I), SST is driven by obliquity, exhibiting a dependence from high latitude climatic responses.
A systematic test of the hypothesis that the b value varies with depth in California
Authors: Gerstenberger Matt , Wiemer Stefan , Giardini Domenico
Published: February 2004

We spatially map the shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0–5 km depth range to b < 0.8 in the depth range 7–15 km. Our detailed mapping confirms that this pattern can be established at the 99% significance level for about 32% of the entire seismically active crust. About 2% of the crust displays the opposite b‐gradient. One such area is the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations.
Evidence for diurnal period Kelvin waves in the Martian atmosphere from Mars Global Surveyor TES data
Authors: Wilson R. John
Published: February 2004

Midlevel (∼25 km) atmospheric temperatures derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectra indicate the presence of stationary waves and thermal tides. Stationary waves are prominent at middle to high latitudes where westerly zonal flow is indicated by the meridional temperature gradient. Longitudinal variability within 30°S to 30°N is dominated by topographically‐forced nonmigrating thermal tides that have westward and eastward propagating components. The MGS mapping data are available at two fixed local times so that it is not possible to distinguish between these components or isolate the sun‐synchronous tide. A comparison with Mars general circulation model (MGCM) simulations suggests that the observed wave patterns are consistent with the presence of eastward propagating, diurnal period Kelvin waves with zonal wavenumbers one and two. These waves can propagate to great heights and may account for observed zonal variations in thermospheric density.
Isothermal remanence experiments at room and at liquid nitrogen temperature: Application to soil studies
Authors: Bógalo María Felicidad , Heller Friedrich , Osete María Luisa
Published: February 2004

A simple magnetic mineral identification technique is proposed which measures isothermal remanent magnetisation (IRM) acquired in different field strengths at 293 K and 77 K. Magnetic parameters are introduced that use these IRMs in order to distinguish concentration and type of ferromagnetic minerals in different magnetic states. The method determines easily contributions of superparamagnetic ferrimagnetic minerals in environmental magnetism studies. In addition, high coercivity goethite and possibly superparamagnetic hematite are identified. The analysis of the magnetic properties of a soil profile on a Late Pleistocene terrace from the Jarama river in central Spain is given as an example.
Increases in snow season length due to earlier first snow and later last snow dates over north central and northwest Asia during 1937–94
Authors: Ye Hengchun
Published: February 2004

Trends of the first and last snow date and the length of snow season, defined by the time period between first and last snow date, during 1937–94 over northern Asia are examined. The length of snow season over North Central and Northwest Asia has increased by about 4 days per decade during the study period. The increased length of the snow season is due to earlier snowfalls at the beginning of the season and to a lesser extent to later last snowfalls at the end of the season.
Identification of sources of potential fields with the continuous wavelet transform: Application to self‐potential profiles
Authors: Gibert Dominique , Pessel Marc
Published: February 2004

We show how the continuous wavelet transform may be used to quickly localize and characterize the sources of self‐potential anomalies. The method is applied to synthetic examples and to a self‐potential profile crossing a shallow fault zone.
The effect of fault geometry on the 1999 Chi‐Chi (Taiwan) Earthquake
Authors: Oglesby David D. , Day Steven M.
Published: February 2004

The September 20, 1999 M 7.6 Chi‐Chi (Taiwan) earthquake produced enough near‐source seismic data to verify many theoretical predictions of the effects of fault geometry on the physics of the earthquake process. These effects include increased motion on the hanging wall (peaked at the fault trace), a transition from thrust to significant left‐lateral slip as one proceeds northward on the fault, and a mismatch between the near‐field and far‐field estimates of faulting style, energy, and apparent stress. Through rigorous 3‐D dynamic models of this earthquake, all of these features can be seen to be robust consequences of the three‐dimensional, asymmetric fault geometry and its angle with the free surface of the earth. The results of this study imply that for dipping faults that intersect the earth's surface, many important features of earthquakes are controlled by the fault geometry, and in principle might be predicted ahead of time.
Parameterizing tidal dissipation over rough topography
Authors: Jayne Steven R. , St. Laurent Louis C.
Published: February 2004

The traditional model of tidal dissipation is based on a frictional bottom boundary layer, in which the work done by bottom drag is proportional to a drag coefficient and the velocity cubed. However, away from shallow, coastal regions the tidal velocities are small, and the work done by the bottom boundary layer can account for only weak levels of dissipation. In the deep ocean, the energy flux carried by internal waves generated over rough topography dominates the energy transfer away from barotropic flow. A parameterization for the internal wave drag over rough topography is included as a dissipative mechanism in a model for the barotropic tides. Model results suggest that the inclusion of this dissipation mechanism improves hydro‐dynamical models of the ocean tide. It also substantially increases the amount of modeled tidal dissipation in the deep ocean, bringing dissipation levels there into agreement with recent estimates from TOPEX/Poseidon altimetry data.
Analysis of direct comparison of cloud top temperature and infrared split window signature Against independent retrievals of cloud thermodynamic phase
Authors: Giraud V. , Thouron O. , Riedi J. , Goloub P.
Published: February 2004

An accurate determination of cloud particle phase is required for retrieval of other cloud properties. The main purpose of this letter is to demonstrate that the cloud phase assumption made in previous cirrus clouds retrievals using infrared split window signatures is not relevant. Coincident observations of the Earth surface from ERS‐2 and ADEOS‐1 satellites give us the opportunity to compare clouds infrared signatures, as observed by ATSR‐2 onboard ERS‐2, with cloud thermodynamic phase derived from POLDER onboard ADEOS‐1. We find out, firstly, that large Brightness Temperature Differences (BTD), estimated between 11 and 12 µm, may occured for cold liquid water clouds consisting of small supercooled droplets (i.e. altostratus). These BTDs have no longer been attributed to cirrus clouds as previous studies have been. Secondly, the probability to observe ice or water clouds with respect to their cloud top temperature is quantified for our data set. A sharp transition between the ice and water phase is shown for cloud top temperatures ranging between 240 and 260 K. The probability for a cloud to be composed of super‐cooled liquid droplets is shown to be higher over land than over ocean. This points out that ice activation susceptibility is more efficient in maritime air mass than in continental one.
Physical‐biological coupling in the California Current during the 1997–99 El Niño‐La Niña Cycle
Authors: Bograd Steven J. , Lynn Ronald J.
Published: February 2004

The rapid transition from strong El Niño to strong La Niña conditions in the equatorial Pacific in 1998 was accompanied by considerable environmental variability in the southern California Current System (CCS). The evolution of this transition in the CCS is investigated based on hydrographic and biological data collected on 25 cruises over a 45‐month period (February 1996 to October 1999). The El Niño period was characterized by high steric heights and a deep nutricline in the coastal regions, which effectively limited biological production. This was followed by a period of increased cross‐shore dynamic height gradients, a significant shoaling of the nutricline, and a dramatic rebound in primary and secondary production. The observed physical and biological response in the CCS is remarkably similar to that observed in the tropical Pacific, but with a lag of several months.
Signatures of mode conversion and kinetic Alfvén waves at the magnetopause
Authors: Johnson Jay R. , Cheng C. Z. , Song P.
Published: February 2004

It has been suggested that resonant mode conversion of compressional MHD waves into kinetic Alfvén waves at the magnetopause can explain the abrupt transition in wave polarization from compressional to transverse commonly observed during magnetopause crossings [Johnson and Cheng, 1997b]. We analyze magnetic field data for magnetopause crossings as a function of magnetic shear angle (defined as the angle between the magnetic fields in the magnetosheath and magnetosphere) and compare with the theory of resonant mode conversion. The data suggest that amplification in the transverse magnetic field component at the magnetopause is not significant up to a threshold magnetic shear angle. Above the threshold angle significant amplification results but with weak dependence on magnetic shear angle. Waves with higher frequency are less amplified and have a higher threshold angle. These observations are qualitatively consistent with theoretical results obtained from the kinetic‐fluid wave equations.
A composite rate‐ and state‐dependent law for rock friction
Authors: Kato Naoyuki , Tullis Terry E.
Published: February 2004

Existing constitutive laws of rock friction cannot describe both observed strengthening in the absence of slip and frictional behavior around steady‐state sliding. Combining existing rate‐ and state‐dependent friction laws, we propose a new friction law, which expresses logarithmically time‐dependent healing at sliding velocities smaller than a cut‐off velocity Vc and approaches the so‐called slip law at velocities larger than Vc. Numerical simulations show that the new friction law adequately explains experimental results of velocity stepping tests and slide‐hold‐slide tests, which have widely been used to examine velocity‐, time‐, and displacement‐dependent property of rock friction. Because the new friction law has a simple form and appears appropriate for a wide range of conditions, it is useful for application to numerical modeling of seismic cycles.
The North Atlantic Oscillation and its relationship with near surface temperature
Authors: Slonosky Victoria C. , Yiou Pascal
Published: February 2004

A new NAO index is presented here using homogenized surface pressure data from Reykjavik and Gibraltar (for November to March) and Reykjavik and Ponta Delgada (for April to October). This index suggests that the positive trend in recent years is not unprecedented, as the early 20th century was also a period of persistently positive NAO values. The relationship between the secular warming trend and the influence of the NAO on near‐surface temperatures over the North Atlantic region and surrounding land masses is examined on a seasonal basis using standardized temperature anomalies since 1900. The near‐surface temperature field separates into two independent modes, which we designate a “warming” mode and dynamic (“NAO”) mode, with distinct seasonal cycles.
Winter and spring observations of stratospheric chlorine monoxide from Ny‐Ålesund, Spitsbergen, in 1997/98 and 1998/99
Authors: Klein Ulf , Lindner Kai , Wohltmann Ingo , Künzi Klaus F.
Published: February 2004

Observations of stratospheric chlorine monoxide (ClO) were performed at the Arctic station of the ‘Network for the detection of Stratospheric Change” (NDSC). A ground based millimeter wave radiometer was routinely operated in Ny‐Ålesund, Spitsbergen (78.9°N, 11.9°E). In 1998 a period with enhanced ClO in a layer around 22 km altitude was detected from February 16 to March 9. Calculations using the three dimensional chemical transport model SLIMCAT are consistent with the observations. In 1999 no significant chlorine activation was observed while SLIMCAT calculations predicted a rather strong chlorine activation in February 1999, when the polar vortex was situated above Ny‐Ålesund. Deviations of SLIMCAT calculations for 1999 from the observations could be attributed to differences in temperatures obtained from the United Kingdom Meteorological office (UKMO) used in the model, and temperatures obtained from other sources. UKMO temperatures were compared to temperatures obtained from the National Center for Environmental Predictions (NCEP), the European Centre for Medium Range Weather Forecast (ECMWF) and radio soundings in Ny‐Ålesund. A low bias of 1 to 2 K in UKMO temperatures was detected during the preconditioning phase of the vortex air at the altitude were the ClO activation was modeled. No systematic deviation from the radio sonde temperatures were found in the other data sets. In UKMO data temperature differences reach up to 8 K during the investigated time period.
Enhanced new production observed from the diurnal cycle of nitrate in an oligotrophic anticyclonic eddy
Authors: Zhang Jia‐Zhong , Wanninkhof Rik , Lee Kitack
Published: February 2004

A diurnal study in an anticyclonic eddy provides the first evidence of nutrient dynamics consistent with the observed trends in solar radiation, oxygen concentration changes, and estimates of the eddy diffusive flux of nitrate from nitracline. A new production rate of 24 mmol C m−2 d−1 was determined from nitrate inventory changes at nM levels in the mixed layer using a liquid waveguide technique combined with eddy diffusion estimates across the base of the mixed layer from temporal changes in the vertical penetration of SF6. The new production supported by nitrate from deepening of the mixed layer after storm events is two times larger than that from the daily diffusive flux. Our results demonstrate that new production in the oligotrophic ocean can be enhanced by a supply of nitrate through the eddy turbulence‐induced diffusive flux and entrainment during storms.
Cloud optical depths and TOA fluxes: Comparison between satellite and surface retrievals from multiple platforms
Authors: Trishchenko Alexander P. , Li Zhanqing , Chang Fu‐Lung , Barker Howard
Published: February 2004

Performances of two cloud property retrieval schemes are assessed by comparison with each other. The study is limited to liquid phase clouds. Two parameters are assessed: cloud optical depth in the visible band and broadband shortwave (SW) flux at the top‐of‐atmosphere (TOA). Retrievals are based on look‐up tables for a variety of conditions using an adding‐doubling code coupled with LOWTRAN‐7 transmittance models. Comparisons of cloud optical depths retrieved from ground measurements with those from ISCCP DX data agree better than previously reported comparisons with the original ISCCP CX data. Likewise, good agreement is obtained between retrieved and inferred TOA SW fluxes. Differences fall within uncertainties of input parameters, as well as shortcomings in the use of a plane‐parallel radiative transfer model and in the inversion schemes themselves.
Divine‐Garrett Model and Jovian synchrotron emission
Authors: Bolton Scott J. , Levin Steven M. , Gulkis Samuel L. , Klein Michael J. , Sault Robert J. , Bhattacharya Bidushi , Thorne Richard M. , Dulk George A. , Leblanc Yolande
Published: February 2004

Simulations of synchrotron emission from relativistic electrons trapped in Jupiter's magnetic field are used to evaluate the energetic electron distribution of the Divine‐Garrett Jupiter radiation belt model at radial distances less than 4 Jovian radii. The fundamental characteristic of synchrotron emission, narrow beaming from gyrating electrons, provides the basis for constraints on both the magnetic field and the distribution of particles in the inner magnetosphere. A comparison between model results and observations is presented. The results suggest the Divine Garrett model significantly underestimates the number of relativistic electrons (>1 MeV) present in Jupiter's inner magnetosphere. The results also indicate that the pitch angle distribution of relativistic electrons in the Jovian radiation belts is different than assumed in the Divine‐Garrett model. These results have important implications for the development of self‐consistent models of Jupiter's magnetosphere and the planning of future missions requiring close flybys of Jupiter.
Observations of mesospheric summer echoes at VHF in the polar cap region
Authors: Rüster R. , Röttger J. , Schmidt G. , Czechowsky P. , Klostermeyer J.
Published: February 2004

VHF radar measurements have been carried out at high polar latitudes using the SOUSY‐Svalbard‐Radar in the summer of 1999 and spring of 2000 at Longyearbyen/Norway (78°N, 16°E). The vertical and temporal variabilities of polar mesospheric summer echoes and mean winds in the mesopause region have been studied for the first time at such high latitudes in the polar cap region. The typical height variation of the received signal is characterized by a pronounced double layer structure during June and July, which is much less pronounced at latitudes 10° further south. Within late summer to equinox conditions, however, the two peaks merge and one layer is dominating. It is suggested that these features together with observed changes in the mesospheric wind system are related to the temperature variation in the mesopause region.
Dynamical and cloud‐radiation feedbacks in El Niño and greenhouse warming
Authors: Jin F.‐F. , Hu Z.‐Z. , Latif M. , Bengtsson L. , Roeckner E.
Published: February 2004

An El Niño‐like steady response is found in a greenhouse warming simulation resulting from coupled ocean‐atmosphere dynamical feedbacks similar to those producing the present‐day El Niños. There is a strong negative cloud‐radiation feedback on the sea surface temperature (SST) anomaly associated with this enhanced eastern equatorial Pacific warm pattern. However, this negative feedback is overwhelmed by the positive dynamical feedbacks and cannot diminish the sensitivity of the tropical SST to enhanced greenhouse gas concentrations. The enhanced eastern‐Pacific warming in the coupled ocean‐atmosphere system suggests that coupled dynamics can strengthen this sensitivity.
Had the Eastern Mediterranean Transient an impact on the new production in the southern Adriatic?
Authors: Civitarese Giuseppe , Gacic Miroslav
Published: February 2004

The Southern Adriatic represents a key area for the entire Eastern Mediterranean, being a site where the thermohaline cell originates through open‐ocean deep water formation processes. The nutrient pool of the basin is mainly controlled by the inflow of nutrient‐rich intermediate water from south and by the autotrophic consumption during the spring bloom. In this work, two new production estimates are given: the first from the post‐convection decrease of the nutrient pool, and the second from surface nitrate consumption. The new production does not seem to be affected by the increase of the nutrient amount imported from south during the last decade as a result of the new deep circulation pattern in the Eastern Mediterranean. On the other hand, the winter climatic conditions act as year‐to‐year controlling mechanisms in determining the magnitude of the convective mixing, and, ultimately of the amount of new nutrients available to the autotrophs.
Modeling Jupiter's synchrotron radiation
Authors: Levin Steven M. , Bolton Scott J. , Gulkis Samuel L. , Klein Michael J. , Bhattacharya Bidushi , Thorne Richard M.
Published: February 2004

We have constructed a computer model to simulate synchrotron emission from relativistic electrons trapped in Jupiter's magnetic field. The computer program generates the four Stokes parameters of the synchrotron emission for assumed electron distributions and magnetic field models. The resulting two dimensional Stokes parameter maps can be compared directly with ground based observations. We use magnetic field models derived from spacecraft measurements, and tailor the electron distributions to fit synchrotron observations. The gross features of data from both VLA and single‐dish observations are fit by a longitudinally symmetric particle distribution. We suggest that higher order terms in the magnetic field, coupled with relativistic beaming effects of synchrotron radiation, are primarily responsible for the observed rotational asymmetry.
A numerical simulation of Martian atmospheric convection with a two‐dimensional anelastic model: A case of dust‐free Mars
Authors: Odaka Masatsugu
Published: February 2004

A numerical simulation of convection in the Martian atmosphere driven by surface heating is performed with a two‐dimensional anelastic model employing radiative process appropriate for clear sky conditions. The model diurnal surface temperature variation is specified, based on typical observed diurnal variations. The simulated convection extends up to about 10 km and the maximum values of vertical and horizontal wind speeds are about 30 m/sec and 20 m/sec, respectively. The instantaneous values of calculated surface stress frequently exceed the minimum threshold value for raising dust from the surface. These results suggest that kilometer‐scale surface stresses associated with convection forced by diurnally‐varying surface heating may strongly contribute to dust injection in the Martian atmosphere.
Stochastic resonance in the thermohaline circulation
Authors: Vé1ez‐Belchí P. , Alvarez A. , Colet P. , Tintoré J. , Haney R. L.
Published: February 2004

A wide variety of climate records have revealed the existence of sudden a recurrent climatic changes. An important part of this variability might be related to transitions between stable equilibrium states of the thermohaline circulation. Here, we employ a box model of the ocean thermohaline circulation to show that in the presence of environmental fluctuations, a subthreshold periodic perturbation in the fresh water fluxes can induce quasiperiodic transitions between the stable states of the thermohaline circulation. This enhanced response occurs for a wide range of frequencies, including the Milankovic orbital forcing, and amplitudes. The mechanism that allows such response of the system under small perturbations arise from a nonlinear cooperation between the periodic perturbations and the fluctuations. Through this nonlinear mechanism, called stochastic resonance, significant climatic variability may be originated due to small perturbations enhanced by environmental noise and dynamics.
Small‐scale Martian polygonal terrain: Implications for liquid surface water
Authors: Seibert Nicole M. , Kargel Jeffrey S.
Published: February 2004

Images from the Mars Orbiter Camera (MOC) through August 1999 were analyzed for the global distribution of small‐scale polygonal terrain not clearly resolved in Viking Orbiter imagery. With very few exceptions, small‐scale polygonal terrain occurs at middle to high latitudes of the northern and southern hemispheres in Hesperian‐age geologic units. The largest concentration of this terrain occurs in the Utopia basin in close association with scalloped depressions (interpreted as thermokarst) and appears to represent an Amazonian event. The morphology and occurrence of small polygonal terrain suggest they are either mud desiccation cracks or ice‐wedge polygons. Because the small‐scale polygons in Utopia and Argyre Planitiae are associated with other cold‐climate permafrost or glacial features, an ice‐wedge model is preferred for these areas. Both cracking mechanisms work most effectively in water‐ or ice‐rich fine‐grained material and may imply the seasonal or episodic existence of liquid water at the surface.
Monsoonal moisture sources revealed using temperature, precipitation, and precipitation stable isotope timeseries
Authors: Wright William E. , Long A. , Comrie A. C. , Leavitt S. W. , Cavazos T. , Eastoe C.
Published: February 2004

Results of analyses using timeseries of mean temperature, precipitation amount, and stable isotopes from precipitation from July–August in Tucson, Arizona, have revealed atmospheric circulation patterns related to the North American Monsoon in the U.S. Southwest. The isotope timeseries and Tucson air temperatures and precipitation amount are significantly correlated. The temperature and isotope timeseries also correlate significantly with regional and extra‐regional specific humidity, and with Eastern Pacific SSTs near the Mexican coast, evidence for a dominantly Pacific/Gulf of California summer moisture source for the period 1983–1999. Separation of extra‐regional wind vector datasets into groups of years matching relative isotopic depletion or enrichment of the Tucson July–August precipitation seasonal means for the stable isotope timeseries (usually the extreme years in the Tucson seasonal temperature means) suggest circulation patterns entraining more tropical moisture in cooler/isotopically depleted years, and entraining less tropical moisture in hotter/isotopically enriched years.
A magneto‐variation array study in the Central Andes of N Chile and SW Bolivia
Authors: Soyer Wolfgang , Brasse Heinrich
Published: February 2004

During several campaigns, long‐period electromagnetic variation measurements were carried out in the central Andes between 19.5°S and 22°S in the convergence zone of the oceanic Nazca Plate and the continental South American plate. A robust multiple‐station analysis has been used to investigate local differences of geomagnetic variation as an indication of subsurface conductivity distribution. For this purpose, spatially interconnected arrays of simultaneously running field stations have been further combined to form a large synthetic array. The analysis reveals a pronounced NS‐striking anomaly with a maximum at a period of about hundred seconds in the Chilean Precordillera, which is likely due to the presence of the rather complex Precordillera fault system. While this anomaly vanishes towards longer periods, strong anomalous currents in the Bolivian Altiplano can be observed within the whole period range, indicating a great body of enhanced conductivity underneath.
The magnetopause at high time resolution: Structure and lower‐hybrid waves
Authors: Lucek E. A. , Cargill P. , Dunlop M. W. , Kistler L. M. , Balogh A. , Baumjohann W. , Fornacon K.‐H. , Georgescu E. , Haerendel G.
Published: February 2004

Equator‐S made many magnetopause crossings between 06 00 and 10 40 LT, taking magnetic field data at 64 or 128 vectors/s. This allows structure within the shortest crossings to be examined, and examples having an exceptionally smooth ramp in |B| are presented. These crossings show no significant structure at sub‐kilometre scales. Strong electromagnetic wave activity did not occur within the ramp, although some crossings have extremely low amplitude compressional waves in part of the field ramp close to the magnetosphere, with frequencies close to the lower hybrid frequency. These initial results suggest that physical processes on sub‐km scales play no role in determining the structure of the current layer.
Forcing of the Mediterranean Sea by atmospheric oscillations over the North Atlantic
Authors: Tsimplis Michael N. , Josey Simon A.
Published: February 2004

Measurements in the Mediterranean Sea made over the last century reveal a link between sea level variability and the North Atlantic Oscillation (NAO). The link arises from the combined effects of atmospheric pressure anomalies and changes in evaporation and precipitation. The strengthening of the NAO from the 1960s to the 1990s explains a significant proportion of the reduction in Mediterranean Sea level over this period. This finding highlights the need to take atmospheric variability into account when looking for the signature of anthropogenic climate change in the ocean. The change in the freshwater flux in the basin, caused by the consistently higher NAO during the 1990s is linked to the appearance of the Eastern Mediterranean Transient.
Secular variations of OI 5577 Å Airglow in the mesopause region induced by transient gravity wave packets
Authors: Hickey Michael P. , Walterscheid Richard L.
Published: February 2004

We employ a 2‐dimensional, time‐dependent, fully nonlinear model of minor species in the mesopause region and our Spectral Full‐Wave Model to simulate the response of minor species and the OI 5577 Å airglow to a gravity wave packet in the mesopause region. Gravity waves affect the time‐averaged distribution of minor species in the mesosphere and lower thermosphere (MLT) region through constituent fluxes induced by violation of the non‐acceleration conditions due to wave transience and dissipation. In addition, wave perturbed chemistry can induce a flux of chemically active species. Simulations are performed with nominal values of eddy diffusion coefficients in the MLT region, and also using very small values in order to assess the comparative effects of diffusion and wave fluxes. The wave‐driven secular variation leads to a strong increase in airglow brightness over a time period of ∼2 hrs. In contrast to this, diffusion of minor species leads to a decrease in airglow brightness. The combined effects of the two can lead to secular variations of airglow brightness that resemble those for a long period (>6 hr) wave. These results have important implications for the interpretation of airglow variations observed in the MLT region.
Ion isotropy and ion resonant waves in the solar wind: Cassini observations
Authors: Kellogg Paul J. , Gurnett Donald A. , Hospodarsky George B. , Kurth William S.
Published: February 2004

Electric fields in the solar wind, in the range of one Hertz, are reported for the first time from a 3‐axis stabilized spacecraft. The measurements are made with the Radio and Plasma Wave System (RPWS) experiment on the Cassini spacecraft [Gurnett et al., 2000]. Kellogg [2000] suggested that such waves could be important in maintaining the near‐isotropy of solar wind ions and the validity of MHD for the description of the solar wind. The amplitudes found are larger than those estimated by Kellogg from other measurements, and are due to quasi‐electrostatic waves. These amplitudes are quite sufficient to maintain isotropy of the solar wind ions.
First confirmation that water ice is the primary component of polar mesospheric clouds
Authors: Hervig Mark , Thompson Robert E. , McHugh Martin , Gordley Larry L. , Russell James M. , Summers Michael E.
Published: February 2004

Polar mesospheric clouds (PMCs) have been measured in the infrared for the first time by the Halogen Occultation Experiment (HALOE). PMC extinctions retrieved from measurements at eight wavelengths show remarkable agreement with model spectra based on ice particle extinction. The infrared spectrum of ice has a unique signature, and the HALOE‐model agreement thus provides the first physical confirmation that water ice is the primary component of PMCs. PMC particle effective radii were estimated from the HALOE extinctions based on a first order fit of model extinctions.
Upper Mantle Shear and Compressional Velocity Structure of the Central US Craton: Shear Wave Low‐Velocity Zone and Anisotropy
Authors: Rodgers Arthur , Bhattacharyya Joydeep
Published: February 2004

One‐dimensional upper mantle velocity structure is estimated by modeling P, SV and SH body‐waveforms sampling the central United States. The resulting model features a thick mantle lid with a positive velocity gradient. A significant shear wave low‐velocity zone (6% velocity reduction) is required below the lid however the compressional velocities are not reduced. The paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion. We observe splitting of upper mantle refracted shear‐waves, with the transverse component slow relative to the radial component. In order to fit the shear wave arrivals and satisfy teleseismic shear‐wave splitting observations we infer azimuthal anisotropy (maximum 1.6%) distributed throughout the mantle lid and low‐velocity zone, with increasing anisotropy with depth.
Element partitioning constraints on the light element composition of the Earth's core
Authors: Li J. , Agee C. B.
Published: February 2004

Partitioning of sulfur (S), oxygen (O) and silicon (Si) between liquid Fe‐Ni‐alloy (LA) and liquid silicate (LS) was studied with a multi‐anvil apparatus at pressures between 2 and 25 GPa and temperatures between 2073 and 2623 K. We found that S has a strong affinity for liquid Fe‐Ni‐alloy at all the experimental conditions investigated. Partition coefficient of S between liquid Fe‐Ni‐alloy and liquid silicate (DSLA/LS) increases with pressure and decreases with temperature. At the proposed pressure and temperature of core‐mantle equilibrium during early Earth differentiation [Li and Agee, 1996; Righter et al., 1997], S is highly siderophile (DSLA/LS ∼ 1000), while Si and O are highly lithophile (DSiLA/LS, DOLA/LS < 0.03). Our study suggests that if the core composition was largely established by equilibrium partitioning in a deep magma ocean, then S is the most plausible principle light element in the core.
Evidence against an independent solar wind density driver of the terrestrial ring current
Authors: O'Brien T. P. , McPherron R. L.
Published: February 2004

The interplanetary electromagnetic fields are generally considered the drivers of the storm‐time terrestrial ring current. Recently, the solar wind density has been advocated as an additional driver. Solar wind density partially determines the density in the plasma sheet, which, in turn, is the likely source for the ring current. Therefore, the solar wind density may drive the ring current by enhancing the ring current source plasma. Some studies, using a few years of data, have found a strong statistical signal for a solar wind density driver with a few hours lag prior to the maximum ring current intensity, as measured by a minimum in Dst. However, we show, using a much larger database of storms, that no consistent role for density alone is evident on time scales of a few hours. The previously reported statistical signal seems to be isolated to a particular interval from November 1994 through September 1995.
Mechanisms limiting the southward extent of the South American Summer Monsoon
Authors: Chou Chia , Neelin J. David
Published: February 2004

An intermediate atmospheric model coupled with a simple land‐surface model and a mixed‐layer ocean model is used to examine effects that determine the southward extension of summer precipitation over South America. The extent of the continental convergence zone is mainly determined by two mechanisms, which we term ventilation and the “interactive Rodwell‐Hoskins mechanism”. Ventilation refers to the import into South America of low moist static energy air from the cooler ocean, primarily the Pacific. In the interactive Rodwell‐Hoskins mechanism, Rossby‐wave‐induced subsidence to the west of the diabatic heating interacts with the convection zone. Because of the shape of South America, the interactive Rodwell‐Hoskins mechanism is of comparable importance to ventilation. Soil moisture feedback also helps limit poleward movement of the continental convergence zone, but its effect is relatively weak compared to the above two effects. The characteristic northwest‐southeast tilt of the continental convergence zone appears to be due to a combination of ventilation and the interactive Rodwell‐Hoskins mechanism.
The Adriatic lithosphere is a promontory of the African Plate: Evidence of a continuous mantle lid in the Ionian Sea from efficient Sn propagation
Authors: Mele Giuliana
Published: February 2004

Evidence of a continuous lithosphere between the northern margin of Africa and Italy is given from the analysis of regional waveforms recorded by the Italian Seismic Network. Frequency and amplitude of shear waves Sn propagating over the Ionian Sea and the western part of the Levantine Sea are examined in this study. An important observation is that Sn waves are efficiently transmitted without exception at epicentral distances ranging from 3° to 22.5°. The very efficient propagation of uppermost mantle shear waves implies that mantle lithosphere is continuous in this part of the Mediterranean region. This in turn implies that the lithosphere underlying the Adriatic Sea is attached to the African lithosphere and can be considered a promontory of the major African plate. The regional shear phase Lg, that typically propagates within the continental crust, is not transmitted in this area. This observation is consistent with the presence of oceanic crust in the eastern Mediterranean, as hypothesized by previous studies.
Existence of gasdynamic subshocks in Hall magnetohydrodynamics
Authors: Lee L. C. , Wu B. H.
Published: February 2004

We report the existence of oblique gasdynamic subshocks in Hall‐MHD. These gasdynamic subshocks propagate at a finite angle θ with respect to the local magnetic field and are embedded in a rotating magnetic field. Across these subshocks, the plasma density, velocity and pressure satisfy the Rankine‐Hugoniet (R‐H) jump conditions in ordinary gasdynamics. In addition, there is no jump in magnetic field across the subshocks. The existence of these gasdynamic subshocks require a finite viscosity, the rotation of magnetic field, and the presence of Hall term in the generalized Ohm's law. These gasdynamic subshocks can be formed in the transition regions of rotational discontinuities (RDs) and intermediate shocks.
Mid‐latitude reflection of ion upflows during substorm dipolarization
Authors: Delcourt D. C. , Dubouloz N. , Sauvaud J.‐A. , Popescu D. , Bouhram M.
Published: February 2004

Reconfiguration of the magnetospheric current system during the expansion phase of substorms leads to dipolarization of the magnetic field lines in the inner plasma sheet. This process is characterized by inward field line motion in the equatorial region and outward motion at mid‐latitudes. We investigate the parallel motion of ions during such dynamical reconfigurations. We demonstrate that, in contrast to equatorial latitudes where ions are centrifugally jetted toward the midplane and experience prominent energization, ions at mid‐latitudes are subjected to a downward (earthward) oriented acceleration. We accordingly show that ions with small parallel speeds such as those originating from the topside ionosphere may be decelerated upon reaching mid‐latitudes and return to low altitudes. Field line dipolarization during substorms may thus go together with substantial downward flow of low‐energy ions, as evidenced from measurements of the low‐altitude orbiting Interball‐Auroral spacecraft.
Atmospheric COCl2 measured by solar occultation spectrometry
Authors: Toon G. C. , Blavier J.‐F. , Sen B. , Drouin B. J.
Published: February 2004

Vertical profiles of atmospheric COCl2 (phosgene, carbonyl dichloride) have been measured remotely by the JPL MkIV interferometer during recent balloon flights. The v5 band of COCl2 in the 830 to 860 cm−1 spectral region was used for the analyses, despite its overlap with the stronger v4 band of CCl3F. Upper troposphere COCl2 volume mixing ratios of around 20 pptv were retrieved, increasing to 25 pptv by 15 km altitude, in good agreement with previous aircraft measurements made by in situ techniques. At mid‐latitudes the COCl2 peaks at around 30 pptv at 22 km altitude, above which it rapidly drops, reaching zero by 28 km altitude.
ENSO, volcanoes and record‐breaking temperatures
Authors: Wigley T. M. L.
Published: February 2004

Monthly‐mean, near‐surface temperatures are examined for records and trends after accounting for the influences of ENSO and the volcanic eruptions of Agung, El Chichón and Pinatubo. Of the 16 consecutive record‐breaking months in 1997–98, at least six can be attributed to ENSO‐associated warmth. The period is still unusual, but no more unusual than 1990–91 when an equal number of records occurred in the ENSO‐adjusted data. The warming trend over the past two decades is also shown to be unusual, but is found (in the raw data) to be not significantly different from the warming trend of the 1910s–1930s. When ENSO and volcanic effects are removed, however, the recent warming trend increases (to 0.25°C/decade) and becomes highly significant compared to the earlier period.