bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimates of Europa's ice shell thickness from elastically‐supported topography
Authors: Nimmo F. , Giese B. , Pappalardo R. T.
Published: May 2003

The thickness of Europa's solid ice shell is uncertain, and has important implications for Europa's habitability and thermal history, and the design of future spacecraft missions. Here we obtain an estimate of the ice shell thickness from observations of a plateau SW of Cilix impact crater. Stereo topographic profiles suggest that the plateau is flexurally supported, with an effective elastic thickness te of 6−2+5 km. For a conductive temperature profile this te value implies a solid ice shell thickness of 15−9+20 km; if the shell is convecting, this estimate is a lower bound. Combined with independent estimates, we infer a probable shell thickness of ≈25 km. The shell thickness is likely to be uniform over the entire satellite.
The transient Dst electromagnetic induction signal at satellite altitudes for a realistic 3‐D electrical conductivity in the crust and mantle
Authors: Velímský Jakub , Everett Mark E. , Martinec Zdeněk
Published: May 2003

Magnetometer data from recent low‐orbit satellite missions Ørsted and CHAMP offer an opportunity to improve our knowledge of electrical conductivity of the Earth's mantle. The forward modelling of electromagnetic induction (EM) is conventionally carried out in the frequency domain. We use a time‐domain technique to study the electromagnetic response of a realistic 3‐D electrical conductivity model based on seismic tomography and laboratory measurements. The model is excited by a Dst signal realistic in time evolution but simplified as spatial dipolar shape. The results show measurable response of simulated satellite time series to mid‐mantle conductivity inhomogeneities. The interpretation of satellite magnetic data for the determination of 3‐D mantle conductivity inhomogeneities will, however, require a more precise model of external ionospheric and magnetospheric currents. This can be obtained by simultaneous measurements by several satellites.
Metal size distributions in EH and EL chondrites
Authors: Schneider D. M. , Benoit P. H. , Kracher A. , Sears D. W. G.
Published: May 2003

EL chondrites exhibit larger average metal grain sizes than EH chondrites, a difference attributed by Easton [1983] to metamorphic coarsening, as most EL chondrites are equilibrated, and most EH chondrites are unequilibrated. In this paper, we present metal grain size data for three unequilbrated EL3 chondrites (PCA 91020; ALH 85119; MAC 88180) , and three EH3 chondrites (ALH 84170; PCA 91085; PCA 91238). We find that EL3 chondrites have larger average metal grain sizes than EH3 chondrites, and that grain sizes of the unequilibrated enstatite chondrites are similar to those of equilibrated chondrites of the same class. We thus interpret the metal size distributions of enstatite chondrites as primarily reflecting their pre‐metamorphic distributions. Shock processing appears to have had only minor influence on metal grain size distributions in these meteorites.
Improving El Niño prediction using a space‐time integration of Indo‐Pacific winds and equatorial Pacific upper ocean heat content
Authors: Clarke Allan J. , Van Gorder Stephen
Published: May 2003

Equatorial westerly (easterly) wind anomalies, phase‐locked to the seasonal cycle, typically ‘propagate’ from the eastern equatorial Indian Ocean and into the western Pacific immediately before an El Niño (La Niña). A space‐time integration of this Indo‐Pacific signal yields an index τ that, for 11 out of 12 months of the calendar year, leads the El Niño index NINO3.4 with a correlation of 0.5 or greater for at least some lead in the range 10–15 months. Cross‐validated hindcasts suggest that a linear combination of this atmospheric index and the ocean indices NINO3.4 and (t), the anomalous equatorial Pacific upper ocean heat content, is an excellent predictor of El Niño. It can predict across the nearest spring persistence barrier, but not the one after that. The present El Niño should die over the spring, leaving near neutral conditions for the rest of 2003.
Ocean bottom pressure variations in the southeastern Pacific following the 1997–98 El Niño event
Authors: Fujimoto Hiromi , Mochizuki Masashi , Mitsuzawa Kyohiko , Tamaki Takayuki , Sato Tadahiro
Published: May 2003

We report important new findings from 14 months of observations of ocean bottom pressure variations in the southeastern Pacific. One is a pressure increase starting in December 1997 at almost the same time as the termination of the 1997–98 El Niño. It is also coincident with a remarkable change in the J2 term of the Earth's gravity field. These results suggest that the El Niño might have brought about mass redistribution in the eastern Pacific Ocean. The other feature in the observations is a local pressure variation across the spreading axis of the ultra‐fast spreading southern East Pacific Rise. It is estimated that the seafloor near the spreading axis was depressed at a rate of about 20 mm/month. The result suggests thermal contraction of the crust in the spreading center in the inter‐eruption period.
Analysis of radiosonde‐based lapse rates and the difference between near‐surface and satellite‐based lower‐tropospheric air temperatures over the central United States
Authors: Balling Robert C. , Cerveny Randall S.
Published: May 2003

Global‐scale near‐surface air temperatures have risen in recent decades with respect to lower‐tropospheric temperatures, and as result, the lower atmospheric lapse rate has apparently steepened providing a possible destabilization effect. In this investigation, we examine the relationship between inferred lapse rate variations based on the difference between near‐surface and lower‐tropospheric temperature measurements and actual lapse rates from radiosonde data. We find high correlations between the inferred lapse rate and a range of actual lapse rates (the surface‐to‐70 kPa to the surface‐to‐30 kPa) in the low‐sun season, but insignificant relations during the high‐sun season.
Search for earthquake precursors in well water levels in a localized seismically active area of Reservoir Triggered Earthquakes in India
Authors: Chadha R. K. , Pandey Ajeet P. , Kuempel H. J.
Published: May 2003

Four cases of coseismic changes are observed in water levels recorded at a close network of wells around seismically active Koyna‐Warna region in western India during 1997–2000. While coseismic increase of 2–7 cm in water levels is observed for earthquakes of M 4.3–4.7 in three cases, a trough like decrease of 0.4–8 cm is observed at 7 wells for an M 5.2 earthquake. All these earthquakes occurred within the network of wells drilled for the study. Our results suggest that decrease in water levels prior to these earthquakes may be premonitory in nature. We show that magnitude of the earthquake and epicentral distance are two vital parameters to record anomalous changes. In our study local earthquakes of M ≥ 4.3 are recorded at a maximum distance of about 24 km. Our studies provide first observations from a unique experiment in India, which will have implications in developing earthquake prediction models.
Seasonal and year‐to‐year variations of rainfall in the Sahara desert region based on TRMM PR data
Authors: Harada Chikako , Sumi Akimasa , Ohmori Hiroo
Published: May 2003

Using TRMM (Tropical Rainfall Measuring Mission) precipitation radar data and NCEP reanalysis data, this paper investigates seasonal and year‐to‐year variations of rainfall over the Sahara desert (20°–30°N, 10°W–30°E) from 1998 to 2001. Most rainfall occurs in the northern (southern) part from October to April (July to September). From May to June, precipitation occurs in both areas. Seasonal migration of the main rainfall area results from changes of rainfall types associated with the change of zonal wind and specific humidity in Northern Africa. While the mid‐latitude westerly at 200 hPa is dominant when rainfall occurs in the northern part, the African Easterly Jet (AEJ) at 600 hPa and the lower layer moisture affect rainfall in the southern part. Year‐to‐year rainfall amount variations are also related to zonal wind changes, such as southward migration of the westerly and northward migration of the AEJ.
Temporal and spatial variation of the surface winds in the Gulf of California
Authors: Parés‐Sierra A. , Mascarenhas A. , Marinone S. G. , Castro R.
Published: May 2003

Satellite‐derived data (NSCAT and Quickscat) are used to study the annual variability of the sea surface winds over the Gulf of California. Our results indicate that the monsoon character of the winds have been exaggerated. Particularly erroneous is the concept that the wind has two symmetrically, gulf‐following, preferred directions. For some years, this has been the main characteristic used to explain the dynamics of the gulf and it has influenced the development of the ocean circulation models of the region. The surface winds have an average direction that follows that of the gulf, but generally track in a south‐eastward direction. The summer reversal (i.e. wind flowing from the south‐east) occasionally occurs only in the southern regions. The magnitude and standard deviation of the winds are largest in winter. There is an important east‐west gradient in their magnitude and the winds on the peninsular side are generally stronger than those of the continental side.
Static stress transfer during the 2002 Nenana Mountain‐Denali Fault, Alaska, earthquake sequence
Authors: Anderson Greg , Ji Chen
Published: May 2003

On 23 October 2002, the Mw 6.7 Nenana Mountain earthquake occurred in central Alaska. It was followed on 3 November 2002 by the Mw 7.9 Denali Fault mainshock, the largest strike‐slip earthquake to occur in North America during the past 150 years. We have modeled static Coulomb stress transfer effects during this sequence. We find that the Nenana Mountain foreshock transferred 30–50 kPa of Coulomb stress to the hypocentral region of the Denali Fault mainshock, encouraging its occurrence. We also find that the two main earthquakes together transferred more than 400 kPa of Coulomb stress to the Cross Creek segment of the Totschunda fault system and to the Denali fault southeast of the mainshock rupture, and up to 80 kPa to the Denali fault west of the Nenana Mountain rupture. Other major faults in the region experienced much smaller static Coulomb stress changes.
The variation of short‐lived NOy species around sunrise at mid‐latitudes as measured by MIPAS‐B and calculated by KASIMA
Authors: Stowasser M. , Oelhaf H. , Ruhnke R. , Kleinert A. , Wetzel G. , Friedl‐Vallon F. , Kouker W. , Lengel A. , Maucher G. , Nordmeyer H. , Reddmann Th. , Fischer H.
Published: May 2003

Vertical profiles of total reactive nitrogen (NOy = NO2 + HNO3 + ClONO2 + 2N2O5 + HO2NO2) along with the source gas N2O up to 38 km were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloonborne version (MIPAS‐B) instrument from Aire sur l'Adour (France, 42°N) on April 30, 1999. Three limb sequences of spectra were recorded 1 hour before, at and 3 hours after sunrise, respectively, allowing to investigate the NOy partitioning under night‐time and ‐ for the first time with the MIPAS‐B instrument ‐ also under day‐time conditions and to study the temporal evolution of the short‐lived species N2O5 and NO2 around sunrise. The MIPAS‐B data are compared to calculations performed with the 3‐D chemical transport model KASIMA (Karlsruhe Simulation model of the Middle Atmosphere). This comparison reveals a high degree of confidence in the model for the HNO3/NOy ratio in the whole altitude region and for the N2O5/NOy and NO2/NOy ratios above about 26 km. Below this altitude the N2O5/NOy ratio is significantly underestimated by the model. Also, the MIPAS‐B measurement suggests a night‐time built up of NO2 which is not reproduced by the model in an altitude region around 22 km.
Clustering of synoptic activity by Indian summer monsoon intraseasonal oscillations
Authors: Goswami B. N. , Ajayamohan R. S. , Xavier Prince K. , Sengupta D.
Published: May 2003

Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active phase of monsoon as compared to the break phase. In addition, the tracks of these synoptic systems are also strongly spatially clustered along the monsoon trough during the active phase of the monsoon. The enhanced (decreased) frequency of occurrence of LPS during active (break) phases is due to modulation of meridional shear of zonal winds and cyclonic vorticity along the monsoon trough by the intraseasonal oscillations (ISO).
Seasonal persistence of midlatitude total ozone anomalies
Authors: Fioletov Vitali E. , Shepherd Theodore G.
Published: May 2003

Temporal autocorrelations of monthly mean total ozone anomalies over the 35–60°S and 35–60°N latitude bands reveal that anomalies established in the wintertime midlatitude ozone buildup persist (with photochemical decay) until the end of the following autumn, and then are rapidly erased once the next winter's buildup begins. The photochemical decay rate is found to be identical between the two hemispheres. High predictability of ozone through late summer exists based on the late‐spring values. In the northern hemisphere, extending the 1979–2001 springtime ozone trend to other months through regression based on the seasonal persistence of anomalies captures the seasonality of the ozone trends remarkably well. In the southern hemisphere, the springtime trend only accounts for part of the summertime trends. There is a strong correlation between the ozone anomalies in northern hemisphere spring and those in the subsequent southern hemisphere spring, but not the converse.
X‐ray tomography characterization of fracture surfaces during dissolution
Authors: Gouze Philippe , Noiriel Catherine , Bruderer Céline , Loggia Didier , Leprovost Richard
Published: May 2003

The changes of fracture surfaces geometry and extend are studied using X‐ray tomography during aperture increase due to CO2‐rich fluid percolation. Dissolution experiments were conducted on two micritic rock samples; one pure calcite end‐member and one with typical composition for marine carbonates (85% calcite). High‐resolution digital images of the fracture geometry allow quantifying the surface properties changes over four spatial scales with a resolution of 4.91 μm. Fracture surfaces are self‐affine with an initial dimension of 2.5. Dissolution of the pure‐calcite sample is clearly a process of homogeneous chemical “erosion” of the surface elevation: fractal dimension and specific surface remains constant (1.5 times the planar surface). Conversely, for the 85% calcite sample, initial topographic surfaces of the fracture walls evolve rapidly toward “non‐topographic” interfaces displaying overhangs due the preferential dissolution of the carbonate grains. In this case, the conventional definition of the effective aperture must be revisited. Such structures can only be assessed from 3D observations. As dissolution progresses, the specific surface increases strongly, more than 5 times the planar surface, and probably faster than the reactive surface.
A role for ocean biota in tropical intraseasonal atmospheric variability
Authors: Gildor Hezi , Sobel Adam H. , Cane Mark A. , Sambrotto Raymond N.
Published: May 2003

We propose that temporal variations within the marine plankton system can induce intraseasonal variations in sea surface temperature (SST) through the effect on solar penetration due to chlorophyll and other optically active organic components. Sensitivity studies with a simple model suggest that these small oscillations in SST may stimulate radiative‐convective oscillations in the atmosphere which amplify them and thus induce or modulate significant variability in the coupled system. Long term bio‐optical measurements in the Western Pacific, where satellite time series are degraded by clouds, would provide a test of our theory and would improve our understanding of the heat balance in this climatically important region.
When faults communicate: Viscoelastic coupling and earthquake clustering in a simple two‐fault system
Authors: Lynch J. C. , Bürgmann R. , Richards M. A. , Ferencz R. M.
Published: May 2003

3‐D finite element models of a simplified northern and southern San Andreas‐type fault system are presented with the goal of better understanding how great earthquakes (M ≥ 7.5) on one major segment of a fault can affect the earthquake cycle on another colinear fault segment separated from the first by an asesimically creeping segment. We find that the earthquake cycles of the two seismogenic fault segments become coupled as the lower crustal viscosity and/or the fault separation distance are decreased. Further, models with a 10%–30% difference in relative fault breaking strengths exhibit a bi‐modal distribution of repeat times for each fault, resulting in earthquakes that appear clustered in time.
Seismicity related to the glacier of Cotopaxi Volcano, Ecuador
Authors: Métaxian Jean‐Philippe , Araujo Sebastián , Mora Mauricio , Lesage Philippe
Published: May 2003

Significant seismic activity is generally recorded on volcanoes covered by an icecap. This work was carried out in order to quantify the role of the glaciers in the generation of seismicity for Cotopaxi volcano. We compared the seismic activity registered on the glacier and on the rock near the snout of the north flank glacier. We focused on the analysis of low frequency events (<5 Hz) similar to volcanic LP events when recorded on rock base. The particle motion analysis helps to estimate source locations, which are distributed in crevasses areas. High incident angles suggest a superficial origin. These events are interpreted as icequakes for which we propose as source mechanism a fluid‐driven crack model triggered by ice cracking or hydraulic transients. The low quality factor values estimated are consistent with the resonance of an ice crack filled with water. This work shows that low frequency icequakes can be confusingly taken as volcanic LP events.
Rock properties of the upper‐crust in Central Apennines (Italy) derived from high‐resolution 3‐D tomography
Authors: Monna Stephen , Filippi Luisa , Beranzoli Laura , Favali Paolo
Published: May 2003

High‐resolution 3‐D P and S‐wave velocity models of a central sector of the Apennines (Central Italy) are computed by inverting first arrival times from an aftershock sequence (September–December, 1997) following the Mw 5.7 and Mw 6.0 Umbria‐Marche earthquakes that occurred on September 26, 1997. The high quality of the data set, especially for the S‐wave, allows us to compute 3‐D variations in Vp, Vp/Vs and Vp · Vs. The anomalies can be interpreted as lateral changes in rock type and fracturing, which control fluid diffusion and variation in pore pressure. This is in agreement with a poro‐elastic view that can be inferred from the spatio‐temporal evolution of the seismic sequence.
Contribution of time tracers (Mg2+, TOC, δ13CTDIC, NO3−) to understand the role of the unsaturated zone: A case study—Karst aquifers in the Doubs valley, eastern France
Authors: Celle‐Jeanton H. , Emblanch C. , Mudry J. , Charmoille A.
Published: May 2003

Time tracers (NO3−, TOC, δ13CTDIC, Mg2+) have been used to define the hydrodynamic behavior of a karst system: high values in NO3− and TOC reflect rapid infiltration and consequently a short residence time within the aquifer, whereas enriched δ13CTDIC and high Mg2+ are expected for “old water”. 9 Springs and 5 boreholes have been sampled during three field campaigns in the Doubs valley karst aquifer: low water, flood and recession periods. A clear differentiation can be highlighted between boreholes, characterized by a long residence time, and springs that show a rapid infiltration. Considering only the springs values, it appears that TOC and δ13CTDIC contents can easily be correlated to the sampling period. We show then the contribution of the unsaturated zone to the discharge during the low‐water period, and the existence of reserves that seem badly connected to the drainage network, and that contribute poorly to the minimal flow.
Gas‐phase chemistry dominates O3 loss to a forest, implying a source of aerosols and hydroxyl radicals to the atmosphere
Authors: Kurpius Meredith R. , Goldstein Allen H.
Published: May 2003

Tropospheric ozone (O3) effectively deposits to forested ecosystems but the fate of O3 within the forest canopy is unresolved. We partitioned total measured ecosystem daytime O3 deposition to a ponderosa pine (Pinus ponderosa) forest into its major loss pathways; stomatal uptake, non‐stomatal surface deposition, and gas‐phase chemistry. Total O3 flux was dominated by gas‐phase chemistry during the summer and by stomatal uptake during winter. O3 loss due to gas‐phase chemistry was exponentially dependent on temperature, with the same functionality as biogenic hydrocarbon emissions, implicating reactions with biogenic hydrocarbons as the likely gas‐phase chemical O3 loss process within the canopy. The reaction of O3 with biogenically‐emitted hydrocarbons leads to both hydroxyl radical formation and secondary aerosol growth with important effects on atmospheric chemistry and climate.
Shock Hugoniot of H2O ice
Authors: Stewart Sarah T. , Ahrens Thomas J.
Published: May 2003

The outcome of impacts onto and between icy planetary bodies is controlled by the material response defined by the shock Hugoniot. New Lagrangian shock wave profile measurements in H2O ice at initial temperatures (T0) of 100 K, together with previous T0 = 263 K data, define five distinct regions on the ice Hugoniot: elastic shocks in ice Ih, ice Ih deformation shocks, and shock transformation to ices VI, VII and liquid water. The critical pressures required to induce incipient melting (0.6, 4.5 GPa) and complete melting (3.7, >5.5 GPa) upon isentropic release from the shock state (for T0 = 263, 100 K) were revised using calculated shock temperatures and entropy. On account of the >40% density increase upon transformation from ice Ih to ices VI and VII, the critical shock pressures required for melting are factors of 2 to 5 lower than earlier predicted. Consequently, hypervelocity impact cratering on planetary surfaces and mutual collisions between porous cometesimals will result in abundant shock‐induced melting throughout the solar system.
Magma degassing as a trigger of bradyseismic events: The case of Phlegrean Fields (Italy)
Authors: Chiodini Giovanni , Todesco Micol , Caliro Stefano , Del Gaudio Carlo , Macedonio Giovanni , Russo Massimo
Published: May 2003

Phlegrean Fields is an active and densely populated caldera near Naples (Italy). Two major unrest episodes characterized its recent history, each leading to remarkable ground uplift and followed by slow subsidence. Fumaroles near the caldera centre underwent important chemical changes during these volcanic crises. Based on published data we show that a correlation exits between ground displacement and gas composition. Numerical modelling of hydrothermal circulation shows that periods of enhanced fluid injection at the base of the hydrothermal system, are consistent with the observed chemical variations. The model predicts an average increase in pore pressure and temperature within the system, suggesting potential effects on ground deformation. Literature data and simulation results show that periods of intense magmatic degassing could explain most of the features characterizing recent bradyseismic crises and should be considered a potential trigger for the unrest at Phlegrean Fields, as well as at other calderas in the world.
The 1994 Java Tsunami earthquake and its “Normal” aftershocks
Authors: Polet Jascha , Thio Hong Kie
Published: May 2003

We compare the source spectra of the 1994 Java tsunami earthquake (MW = 7.9) and its two largest aftershocks. The source spectra of the aftershocks, from teleseismic P‐waves, can be fit remarkably well by a standard ω2 model. However, the mainshock shows a lower moment release at periods of 1–20 sec than its aftershocks, although its total moment is 1000 times larger. We analyze this discrepancy by exploring two mechanisms that, in a simple Haskell rupture model, will decrease one of the corner frequencies of the source spectrum: 1. lowering the rupture velocity and 2. lowering the particle (slip) velocity. Both changes maintain an identical spectral drop‐off and static stress drop, and result in the characteristic spectral shape of a “slow” earthquake. These kinematic models can be related to physical rupture parameters by considering the earthquake energy budget. In both cases the radiated seismic energy is lowered, either due to a higher frictional stress during the break‐down phase, increasing the fracture energy (case 1), or by an increase in the sliding stress (case 2). We do not suggest that either of these simplified models is the only possible interpretation, but want to illustrate the danger in directly interpreting “slow” earthquakes as having a low rupture velocity.
Hydrograph for small basins following intense storms
Authors: Criss R. E. , Winston W. E.
Published: May 2003

We show how diffusive fluxes based on Darcy's law define groundwater hydrographs following sharp perturbations on the hydraulic head. The derived normalized hydrograph is based on a single fitting parameter, the time constant “b”, and has great semblance to observed discharge hydrographs for many karst springs and small rivers following rainfall events. Observed time constants increase with basin area but at a given site decrease as peak flows increase. The predicted interval between a pulse in head and the resultant peak flow is equal to 2b/3. Longer delays are observed between actual rainfall events and peak flows because the head perturbation must postdate the rainfall.
Climate change at the 4.2 ka BP termination of the Indus valley civilization and Holocene south Asian monsoon variability
Authors: Staubwasser M. , Sirocko F. , Grootes P. M. , Segl M.
Published: May 2003

Planktonic oxygen isotope ratios off the Indus delta reveal climate changes with a multi‐centennial pacing during the last 6 ka, with the most prominent change recorded at 4.2 ka BP. Opposing isotopic trends across the northern Arabian Sea surface at that time indicate a reduction in Indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south Asia. The 4.2 ka event is coherent with the termination of urban Harappan civilization in the Indus valley. Thus, drought may have initiated southeastward habitat tracking within the Harappan cultural domain. The late Holocene drought cycles following the 4.2 ka BP event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14C production rates. This suggests that solar variability is one fundamental cause behind Holocene rainfall changes over south Asia.
Cyclones, tides, and the origin of a cross‐equatorial dust storm on Mars
Authors: Wang Huiqun , Richardson Mark I. , Wilson R. John , Ingersoll Andrew P. , Toigo Anthony D. , Zurek Richard W.
Published: May 2003

We investigate the triggering mechanism of a cross‐equatorial dust storm observed by Mars Global Surveyor in 1999. This storm, which had a significant impact on global mean temperatures, was seen in visible and infrared data to commence with the transport of linear dust fronts from the northern high latitudes into the southern tropics. However, other similar transport events observed in northern fall and winter did not lead to large dust storms. Based on off‐line Lagrangian particle transport analysis using a high resolution Mars general circulation model, we propose a simple explanation for the diurnal, seasonal and interannual variability of this type of frontal activity, and of the resulting dust storms, that highlights the cooperative interaction between northern hemisphere fronts associated with low pressure cyclones and tidally‐modified return branch of the Hadley circulation.
Summer snow extent heralding of the winter North Atlantic Oscillation
Authors: Saunders Mark A. , Qian Budong , Lloyd‐Hughes Benjamin
Published: May 2003

Winter climate over the North Atlantic and European sector is modulated by the North Atlantic Oscillation (NAO). We find that the summer extent of snow cover over northern North America and northern Eurasia is linked significantly (p < 0.01) to the upcoming winter NAO state. Summers with high/low snow extent precede winters of low/high NAO index phase. We suggest the linkage arises from the summer snow‐associated formation of anomalous longitudinal differences in surface air temperature with the subpolar North Atlantic. Our findings indicate the seasonal predictability of North Atlantic winter climate may be higher and extend to longer leads than thought previously.
Case study of a Chinese dust plume reaching the French Alps
Authors: Grousset Francis E. , Ginoux Paul , Bory Aloys , Biscaye Pierre E.
Published: May 2003

By combining reconstruction of airmass back‐trajectories from dust deposition sites in Europe and measurements of the (Nd) isotopic composition of deposited dust particles, potential sources of different Saharan dust events can be identified. The study of “red dust” events collected in France allowed us to identify distinct North African source areas (e.g. Lybia vs. Mauritania). Surprisingly, the airmass trajectory of one dust event (March 6, 1990) was distinct from the others, and revealed a Chinese origin. The Nd isotopic composition of this dust was consistent with the range of isotopic compositions of Chinese loess. Moreover, an atmospheric global model simulation reveals that a dust plume left China before February 25, 1990, flew over North America around the February/March transition and reached the French Alps by March 6, 1990, revealing that intercontinental dust and pollutant transport may occur across the Pacific Ocean and the North Atlantic at the Westerlies latitudes.
Deep Space 1 encounter with Comet 19P/Borrelly: Ion composition measurements by the PEPE mass spectrometer
Authors: Nordholt J. E. , Reisenfeld D. B. , Wiens R. C. , Gary S. P. , Crary F. , Delapp D. M. , Elphic R. C. , Funsten H. O. , Hanley J. J. , Lawrence D. J. , McComas D. J. , Shappirio M. , Steinberg J. T. , Wang J. , Young D. T.
Published: May 2003

We report in situ ion mass spectrometer measurements from the coma of Comet 19P/Borrelly, obtained during the flyby of the Deep Space 1 spacecraft on 22 September 2001. Cometary ions were detected at distances from ∼5.5 × 105 km to 2200 km from the nucleus of the comet. The predominant heavy ions observed during the seven minute interval about closest approach to Borrelly included OH+ (at 57% of the total water‐group ion density), H2O+ (at 29%), O+ (at 13%), CH3+ (at 5%), and CH2+ (at 4%). Of particular note is the small amount of H3O+ (<9%), as this was the most abundant molecular ion observed at closest approach to Comet Halley. This difference is due to the difference in water production rates between the two comets.
Factors controlling large scale variations in methane emissions from wetlands
Authors: Christensen Torben R. , Ekberg Anna , Ström Lena , Mastepanov Mihail , Panikov Nicolai , Öquist Mats , Svensson Bo H. , Nykänen Hannu , Martikainen Pertti J. , Oskarsson Hlynur
Published: May 2003

Global wetlands are, at estimate ranging 115–237 Tg CH4/yr, the largest single atmospheric source of the greenhouse gas methane (CH4). We present a dataset on CH4 flux rates totaling 12 measurement years at sites from Greenland, Iceland, Scandinavia and Siberia. We find that temperature and microbial substrate availability (expressed as the organic acid concentration in peat water) combined explain almost 100% of the variations in mean annual CH4 emissions. The temperature sensitivity of the CH4 emissions shown suggests a feedback mechanism on climate change that could validate incorporation in further developments of global circulation models.
The zonal currents and transports at 35°W in the tropical Atlantic
Authors: Schott Friedrich A. , Dengler Marcus , Brandt Peter , Affler Karina , Fischer Jürgen , Bourlès Bernard , Gouriou Yves , Molinari Robert L. , Rhein Monika
Published: May 2003

The total of 13 existing cross‐equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top‐to‐bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the results is that, on the equator, a mean westward Equatorial Intermediate Current below the Equatorial Undercurrent exists.
Monthly Strontium/Calcium oscillations in symbiotic coral aragonite: Biological effects limiting the precision of the paleotemperature proxy
Authors: Meibom Anders , Stage Morten , Wooden Joseph , Constantz Brent R. , Dunbar Robert B. , Owen Art , Grumet Nancy , Bacon Charles R. , Chamberlain C. Page
Published: May 2003

In thermodynamic equilibrium with sea water the Sr/Ca ratio of aragonite varies predictably with temperature and the Sr/Ca ratio in coral have thus become a frequently used proxy for past Sea Surface Temperature (SST). However, biological effects can offset the Sr/Ca ratio from its equilibrium value. We report high spatial resolution ion microprobe analyses of well defined skeletal elements in the reef‐building coral Porites lutea that reveal distinct monthly oscillations in the Sr/Ca ratio, with an amplitude in excess of ten percent. The extreme Sr/Ca variations, which we propose result from metabolic changes synchronous with the lunar cycle, introduce variability in Sr/Ca measurements based on conventional sampling techniques well beyond the analytical precision. These variations can limit the accuracy of Sr/Ca paleothermometry by conventional sampling techniques to about 2°C. Our results may help explain the notorious difficulties involved in obtaining an accurate and consistent calibration of the Sr/Ca vs. SST relationship.
Thickening the Altiplano crust by gravity‐driven crustal channel flow
Authors: Husson L. , Sempere T.
Published: May 2003

In the Central Andes, crustal thickness is not well correlated to upper crustal shortening. Only little shortening is documented in the upper crust of the 60–65 km‐thick Altiplano plateau, whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent Western and Eastern Cordilleras. Because crustal thickness variations induce horizontal stress gradients and cause crustal flow, a thickness‐dependent channel flow is modeled here and applied to the Central Andes. In situ thickening is assumed for both cordilleras, while the Altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains. A ∼8.1018 Pa s viscosity channel is predicted for crustal thicknesses exceeding 45–50 km to match the estimated topographic evolution of the Central Andes. Thickening in the cordilleras was sufficient to generate a flow of 6.109 m3 per unit length toward the initially ∼30–35 km‐thick Altiplano.
Buoyant disruption of magnetic arcades with self‐induced shearing
Authors: Manchester Ward
Published: May 2003

We present the results of magnetohydrodynamic (MHD) simulations of the nonlinear development of instabilities of magnetically sheared arcades and show how they relate to plasmoid ejections and coronal mass ejections (CMEs). To model the arcade disruptions, we capitalize on a family of analytical solutions for initial states; these describe magnetic arcades in uniform gravity that are characterized by magnetic shear. Two‐and‐a‐half‐dimensional (2.5‐D) time‐dependent simulations were performed with the ZEUS‐2D code to show the response of the arcades to small velocity perturbations. The model arcades respond by rising and expanding, and most significantly, we find shearing motions naturally arise in conjunction with the instability. This field line shearing is in response to the Lorentz force, which drives large‐amplitude Alfvén waves, which in turn transport magnetic shear from the lower to the upper extremities of the arcades. The self‐induced shear Alfvén waves, coupled with magnetic buoyancy, provide a feedback mechanism that drives the arcades to a loss of equilibrium and disruption following a long period of slow rise and expansion. These simulations of arcade disruptions may be relevant with regard to CME initiation for three major reasons. First, the arcade disruptions are the result of undriven magnetic buoyancy instabilities. Second, magnetic field line shearing is an intrinsic aspect of the instability that occurs spontaneously and is driven only by the magnetic tension force. (No imposed shear motions are specified.) Third, the arcade disruption process has been found to repeat without prompting.
Nonmigrating diurnal tides in the thermosphere
Authors: Forbes Jeffrey M. , Zhang Xiaoli , Talaat Elsayed R. , Ward William
Published: May 2003

Horizontal wind measurements from the HRDI and WINDII instruments on the Upper Atmosphere Research Satellite are analyzed to reveal the most prominent nonmigrating diurnal tidal components at 95 km: the eastward propagating diurnal tide with zonal wave number s = 3 (DE3), the standing (s = 0) diurnal oscillation (D0), and the westward propagating diurnal tide with s = 2 (DW2). The strongest DE3 occurs primarily during Northern Hemisphere summer/fall with maximum eastward winds near the equator of order 15 ms−1 and is a vertical extension of the s = 3 Kelvin wave. The first antisymmetric mode of DE3 dominates during October–April, with maximum meridional winds near the equator of order 8 ms−1. D0 exists during nearly all months and is nonsymmetric about the equator with maximum northward wind amplitudes in the Southern Hemisphere of order 7–10 ms−1. DW2 closely resembles the first symmetric propagating mode from classical tidal theory, with maximum northward wind amplitudes of order 10–12 ms−1 during September through February. The combination of DE3, D0, and DW2 with DW1 gives rise to significant longitude variations in the diurnal tide between ±40° latitude. Forcing in the Global Scale Wave Model (GSWM) is calibrated according to the above observations, thus enabling global estimates of nonmigrating tidal temperatures and other fields. For instance, it is estimated that the temperature and eastward wind perturbations associated with DE3 may be as large as 20K and 35 ms−1 at 145 km and 115 km, respectively, over the equatorial region during July.
The dependence of AKR production on the intensity and energy spectra of auroral bremsstrahlung
Authors: Imhof W. L. , Anderson R. R. , Walt M. , Hawley J. D. , Petrinec S. M. , Mobilia J. , Matsumoto H.
Published: May 2003

Our limited knowledge of the energy dependence of electron precipitation associated with auroral kilometric radiation (AKR) production is improved with the use of bremsstrahlung x‐ray measurements. Simultaneous observations of bremsstrahlung x‐rays above 2 keV from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) on the POLAR satellite are compared with the frequency distributions of AKR waves measured with the Plasma Wave Instrument (PWI) on the GEOTAIL satellite. Various AKR characteristics such as the low‐ and high‐ frequency cutoffs and the frequencies at peak intensity are compared with various x‐ray parameters such as the magnetic local times of emission, the total intensities, and the spectral shapes. In particular, it is found that higher fluxes of x‐rays occur when the various cutoffs are at lower frequency. Also, higher AKR cutoff frequencies are associated with higher e‐folding x‐ray energy spectra.
Implications of the observed anticorrelation between solar wind speed and coronal electron temperature
Authors: Gloeckler George , Zurbuchen Thomas H. , Geiss Johannes
Published: May 2003

Electron temperatures at the source of the solar wind are now obtained routinely and continuously from measurements of charge states of solar wind ions with modern solar wind composition spectrometers over a wide range of solar wind speeds. While the general anticorrelation between solar wind speed and electron temperature was previously noted, the physical processes responsible for this anticorrelation were not well understood, nor were mechanisms proposed that would produce this observed anticorrelation. We present a detailed analysis of solar wind measurements made with the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses over nearly an entire solar cycle and the full latitude range of Ulysses. We show that the electron temperature, T, derived from the O7+/O6+ density ratios, is not only well anticorrelated with measured solar wind speed, Vsw, in general, but also that the dependence of Vsw on 1/T is well represented by the solar wind equation derived by Fisk [2003]. This equation is based on the simple model in which the plasma of both the fast and slow wind is released from magnetic loops, which are opened by reconnection with open field lines. Fitting these SWICS data to the Fisk equation, we infer the dependence on loop height of the ratio of the magnetic field strength to mass density near the base of the loops (at the altitude of reconnection) and infer the solar cycle and latitude dependence of the size of the loops and of the strength of the average open field. We suggest that the same simple mechanism can account for both the fast and slow solar wind and that the final speed of the solar wind is determined primarily by the electron temperature in magnetic loops on the Sun, from which the solar wind originates.
Ion isotropy and ion resonant waves in the solar wind: Corrected Cassini observations
Authors: Kellogg P. J. , Gurnett D. A. , Hospodarsky G. B. , Kurth W. S. , Dougherty M. K. , Forsyth R. J.
Published: May 2003

In an earlier paper [Kellogg et al., 2001], observations of electric field fluctuations in the range 0.5 to 25 Hz were reported. We have recently found that the data presented in that paper are seriously contaminated by broadband interference which appears to be generated in the spacecraft wake. Wake instabilities have been found by others, but the plasma conditions are quite different from those of any other observations of wake instabilities of which we are aware. In this paper, some characteristics of the wake interference are investigated, and an attempt is made to find conditions when it is not present. We believe that we have done this successfully and present a new spectrum and characteristics for electric field fluctuations in the solar wind. However, essentially no uncontaminated data were obtained nearer the Sun than 4.5 AU, and so the new results pertain only to the outer region. In the outer heliosphere the conditions are expected to be sufficiently different (large plasma beta) that the results may not pertain to the inner heliosphere.
Energy distribution and lifetime of magnetospherically reflecting whistlers in the plasmasphere
Authors: Bortnik J. , Inan U. S. , Bell T. F.
Published: May 2003

Using ray tracing and Landau damping calculations based on recent data on suprathermal particle fluxes from the HYDRA instrument aboard the POLAR satellite, we estimate the energy distribution and the lifetimes of 0.2–10 kHz whistler mode waves in the plasmasphere. The rays are injected at 1000 km altitude and latitudes of 25°, 35°, 45°, and 55° to represent whistler wave energy originating in lightning discharges occurring in middle to low‐latitude thunderstorms. The lifetime is defined as the time at which the wave power is reduced by 10 dB. Results indicate that the lifetime of whistler waves at lower frequencies is dramatically larger than those at higher frequencies and that rays injected at lower latitudes generally persist longer than those injected at higher latitudes in agreement with previous studies. An important characteristic of magnetospherically reflected (MR) whistlers is the strong tendency for whistler wave components at each frequency to eventually migrate to and settle into a multiply reflecting pattern at a specific (determined only by the wave frequency) L‐shell, at which the wave energy would persist indefinitely in the absence of Landau damping and other losses. Consideration of this behavior together with the typical power spectrum of a single, vertical, cloud‐to‐ground lightning stroke, allows the estimation of the relative MR whistler wave energy in the inner magnetosphere as a function of L‐shell. Results indicate that MR whistler energy deposition is maximized at the location of the slot region, suggesting that such MR whistlers launched by lightning discharges may be responsible for the enhanced diffusion rates and may play a more significant role than previously assumed in the formation and maintenance of the slot region between the inner and outer radiation belts.
Magnetic reconnection between a magnetic cloud and the solar wind magnetic field
Authors: Schmidt J. M. , Cargill P. J.
Published: May 2003

This paper presents a numerical study of magnetic reconnection processes between magnetically organized structures in the solar wind (e.g., magnetic clouds or plasma bubbles) and the solar wind magnetic field. Magnetic reconnection occurs at points along the surface bounding the two flux systems where antiparallel field lines are forced together. For a magnetic cloud moving outward in an unidirectional radial magnetic field, the reconnection occurs asymmetrically, the location depending on the sense of rotation of the cloud's magnetic field. When a cloud moves in a current sheet, reconnection occurs either symmetrically along both sides of the interface or not at all, depending on the relative orientation of the cloud and solar wind fields. The effectiveness of the reconnection process increases with increasing relative speed between the cloud and the solar wind. When the cloud moves faster (slower) than the solar wind, reconnection takes places at the leading (trailing) edge of the cloud. While important, reconnection does not appear to be able to destroy the overall magnetic cloud structure over distances of 1 AU.
Properties of localized, high latitude, dayside aurora
Authors: Frey H. U. , Immel T. J. , Lu G. , Bonnell J. , Fuselier S. A. , Mende S. B. , Hubert B. , Østgaard N. , Le G.
Published: May 2003

The FUV instrument on the IMAGE spacecraft frequently observes intense ultraviolet (UV) emission from a localized dayside region poleward of the general auroral oval location. One type of these emissions has been described as the signature of direct proton precipitation into the cusp after lobe reconnection during northward interplanetary magnetic field (IMF) and high solar wind dynamic pressure periods [Frey et al., 2002]. Here we describe a completely different type of high latitude aurora, which does not show any signature of precipitating protons. It also occurs during northward IMF conditions however, only during periods of very low solar wind dynamic pressure. It occurs at a much higher geomagnetic latitude than the normal cusp location and only during periods of positive IMF By. The intensity of the UV emission is somewhat anti‐correlated with the solar wind dynamic pressure, much in contrast to the cusp emission. The brightness of the localized emission changes rapidly on time scales between 30 and 70 minutes without corresponding changes in solar wind properties. Coincident measurements by the FAST spacecraft verify that this is not the cusp, that ion precipitation is absent in these regions, and that strong precipitation of field‐aligned accelerated electrons causes the aurora. We interpret this aurora as the optical signature of electron precipitation in the upward leg of a current system which closes the downward leg of the current system into the cusp in the ionosphere.