bluuebunny/crossref_metadata_2025 huggingface dataset:
Radiocarbon distributions in Southern Ocean dissolved and particulate organic matter
Authors: Druffel Ellen R. M. , Bauer James E.
Published: September 2002

Dissolved organic carbon (DOC) is the largest actively exchanging pool of organic carbon in the ocean, yet its sources and sinks are not well constrained. The average 14C ages of DOC in the deep N. Atlantic and N. Pacific Oceans are 4,000 [Bauer et al., 1992; Druffel et al., 1992] and 6,000 years [Williams and Druffel, 1987], respectively, and represent the beginning and end of the deep ocean conveyor [Broecker, 1991]. Here we report that the deep Southern Ocean DOC has a 14C age (5,600 y) much closer to that of the deep N. Pacific, but its concentration in seawater (41±2 µM) is nearly equal to that of the deep N. Atlantic. The radiocarbon and concentration data indicate that most, but not all, deep DOC is transported conservatively with the ocean's conveyor. A younger (post‐bomb) source of DOC to the N. Atlantic is the most likely explanation for the large age difference we observe between deep DOC in the Atlantic and Southern Oceans. Other possibilities are a source of older DOC or a smaller microbial sink in the S. Ocean, or perhaps a possible slowdown of S. Ocean deep water formation during the past century [Broecker et al., 1999].
Gradient drift instability in high latitude plasma patches: Ion inertial effects
Authors: Gondarenko N. A. , Guzdar P. N.
Published: September 2002

Nonlinear simulations of the gradient drift instability with inertial effects for the high latitude plasma patches demonstrate that the initial cross‐field elongated structures, driven by the gradient drift instability, are unstable to the Kelvin‐Helmholtz instability and to the generation of sheared flows. The shear flow develops on the scale‐length of the irregularities. Both these processes arise from the nonlinear inertial term. The computed density and velocity fluctuation levels are of the order of the observed values. The combined effect of parallel dynamics as well as secondary instabilities prevent the patch from disintegrating for hours.
The heights of sporadic‐E layer simultaneously observed by the VHF radar and ionosondes in Chung‐Li
Authors: Lee C. C. , Liu J. Y. , Pan C. J. , Igarashi K.
Published: September 2002

Ionosondes and VHF radars have been employed to simultaneously observe the sporadic E (Es) layer. In previous studies, an absence of correlation was found between the height of field‐aligned irregularities (FAI) observed by the radars and the virtual height of the Es layer (h'Es) by their colocated ionosondes. A careful study shows that the low correlation results from the beam broadening effect of the ionosonde. In this paper, the investigation shows a better correlation between the heights of the FAI bottom‐side (hFAIb) observed by the VHF radar and the h'Es by ionosondes in Chung‐Li. It is found that the h'Es is greater than the hFAIb by about 5–10 km. Meanwhile, we demonstrate that h'Es observed by two nearby ionosondes (and/or radars) can be employed to evaluate the velocity of waves in the Es layer.
Comparison of the coseismic rupture with the aftershock distribution in the Hyuga‐nada Earthquakes of 1996
Authors: Yagi Yuji , Kikuchi Masayuki , Yoshida Shingo , Sagiya Takeshi
Published: September 2002

On October 19, 1996, a large underthrusting earthquake (Ms = 6.7), the Hyuga‐nada, Japan, earthquake occurred along the southern end of Nankai trough. About two months later, a second large earthquake (Ms = 6.7) occurred in the adjacent region. We study the source process of the two large earthquakes in the Hyuga‐nada region and compare the coseismic rupture area with aftershock distribution. The main source parameters obtained for the first mainshock are: (strike, dip, rake)= (210°, 12°, 81°); the seismic moment Mo = 2.3×1019 Nm (Mw = 6.8); the rupture area S = 20 × 15 km², and the source duration T = 17 s. For the second main‐shock, (strike, dip, rake) = (210°, 12°, 87°); the seismic moment Mo = 1.5×1019 Nm (Mw = 6.7); the rupture area S = 18 × 18 km², and the source duration T = 15 s. The coseismic rupture areas do not overlap the aftershock area, while the aftershock areas of the two main‐shocks mutually overlap. This implies that the common aftershock region takes a role of barriers to dynamic rupture. It is also seen that the aftershock area expanded during the first one day.
Teleseismic Shear wave splitting and lithospheric anisotropy beneath and across the Altyn Tagh Fault
Authors: Herquel Georges , Tapponnier Paul , Wittlinger Gérard , Mei Jiang , Danian Shi
Published: September 2002

Sixteen 3‐component, 5‐sec stations along a profile crossing the Altyn Tagh fault (ATF) at ∼90°E were used to measure shear wave splitting beneath Tibet's northern edge. Splitting delay times (δt) of 0.3 to 1.3 sec show a marked increase in a narrow zone centered on the fault, implying anisotropy in the lithospheric mantle. Fast Polarization Directions (FPD) (ϕ) swing counterclockwise, from a mean azimuth of ∼110°E in the Qaidam basin south of the fault, to a mean azimuth of ∼60°E in the Altyn mountains. This swing occurs in less than 40 km. Both the δt increase and FPD swing are consistent with ductile sinistral shear along the 40 km‐wide, low P‐wave velocity anomaly (−6–8%) imaged to a depth of ∼140 km beneath the fault by teleseismic tomography. This confirms that the ATF guides the northeastwards extrusion of the Tibet‐Qaidam's lithosphere relative to the Tarim's.
Low‐angle normal fault earthquakes and triggering
Authors: Axen Gary J.
Published: September 2002

Geologic evidence for low‐angle normal faulting is overwhelming, and recent seismic, geodetic, and neotectonic studies have identified probable active examples. Thus, apparent low levels of historical seismicity on such faults is enigmatic. However, one large mainshock was a low‐angle normal event, and three others may have been. One earthquake sequence had significant moment release from a triggered low‐angle normal fault, and three earthquakes may have had normal subevents on low‐angle planes. Such (sub)events are hard to recognize, may be underrepresented in catalogs (helping to explain the apparent seismic enigma), and pose increased seismic hazard due to delayed strong motions. Triggered (sub)events may be along strike or down dip from their triggering shock. In cases where the triggered slip is downdip of the steep fault and under its hanging wall, elevated pore fluid pressure or fault weakness may be required.
Mantle transition zone structure beneath Tanzania, east Africa
Authors: Owens Thomas J. , Nyblade Andrew A. , Gurrola Harold , Langston Charles A.
Published: September 2002

We apply a three‐dimensional stacking method to receiver functions from the Tanzania Broadband Seismic Experiment to determine relative variations in the thickness of the mantle transition zone beneath Tanzania. The transition zone under the Eastern rift is 30–40 km thinner than under areas of the Tanzania Craton in the interior of the East African Plateau unaffected by rift faulting. The region of transition zone thinning under the Eastern rift is several hundred kilometers wide and coincides with a 2–3% reduction in S wave velocities. The thinning of the transition zone, as well as the reduction in S wave velocities, can be attributed to a 200–300°K increase in temperature. This thermal anomaly at >400 km depth beneath the Eastern rift cannot be easily explained by passive rifting and but is consistent with a plume origin for the Cenozoic rifting, volcanism and plateau uplift in East Africa.
Temporal changes of the crustal structure associated with the M6.1 earthquake on September 3, 1998, and the volcanic activity of Mount Iwate, Japan
Authors: Nishimura Takeshi , Uchida Naoki , Sato Haruo , Ohtake Masakazu , Tanaka Satoru , Hamaguchi Hiroyuki
Published: September 2002

We detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a M6.1 shallow earthquake. Phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the M6.1 earthquake and Mount Iwate decreased 0.3–1.0% during the three months. Since lager decreases of the seismic velocity were observed at the region close to the focal area and a volcanic pressure source, we conclude that stress change in the upper crust is a most plausible candidate to cause the velocity changes among other candidates. We further apply a coherence analysis to the observed seismic coda to locate regions where short‐wavelength heterogeneity temporally changed. The result indicates a possibility of temporal changes of the heterogeneity beneath the volcano, where a magmatic fluid intrusion is inferred.
Co‐seismic geoelectric potential changes observed in Japan
Authors: Nagao T. , Orihara Y. , Yamaguchi T. , Takahashi I. , Hattori K. , Noda Y. , Sayanagi K. , Uyeda S.
Published: September 2002

Co‐seismic signals of geoelectric potential have been observed for several earthquakes (EQs) in Japan. It has been argued that if there are precursory changes, stronger signals should be at the time of EQ. Our results prove that co‐seismic signals do exist, but they start with the arrival of seismic waves and not at the origin time of EQs. Their amplitude does not scale with the dipole length. Both facts indicate that the changes are local effects of passing seismic waves and not due to electric signals emitted from the seismic source. Observed changes contain two components; oscillatory and offset/decay ones. The former is synchronized with seismic waves, whereas the latter, with a rise time of a few seconds and a decay time of 0.1–1 hour, is suspected to be related to EQ induced subsurface hydrological change. In both cases, the origin of electric potential variations may be electrokinetic.
El Niño during the Last Interglacial Period recorded by a fossil coral from Indonesia
Authors: Hughen Konrad A. , Schrag Daniel P. , Jacobsen Stein B. , Hantoro Wahyoe
Published: September 2002

Measurements of oxygen isotopes and elemental ratios in a fossil coral that grew 124,000 years ago in North Sulawesi, Indonesia, reflect interannual variability in precipitation and sea surface temperature (SST) due to the El Niño/Southern Oscillation (ENSO). This indicates that ENSO was robust during the last interglacial period, a time when global climate was slightly warmer than the present. The pattern of ENSO frequency behavior in the past is similar to variability in modern instrumental records, but distinct from the most recent period since the mid‐1970s, supporting the hypothesis that ENSO behavior in recent decades is anomalous with respect to natural variability.
Hydrological responses to dynamically and statistically downscaled climate model output
Authors: Wilby Robert L. , Hay Lauren E. , Gutowski William J. , Arritt Raymond W. , Takle Eugene S. , Pan Zaitao , Leavesley George H. , Clark Martyn P.
Published: September 2002

Daily rainfall and surface temperature series were simulated for the Animas River basin, Colorado using dynamically and statistically downscaled output from the National Center for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) re‐analysis. A distributed hydrological model was then applied to the downscaled data. Relative to raw NCEP output, downscaled climate variables provided more realistic simulations of basin scale hydrology. However, the results highlight the sensitivity of modeled processes to the choice of downscaling technique, and point to the need for caution when interpreting future hydrological scenarios.
Factors controlling satiated relative permeability in a partially‐saturated horizontal fracture
Authors: Nicholl M. J. , Rajaram H. , Glass R. J.
Published: September 2002

Recent work demonstrates that phase displacements within horizontal fractures large with respect to the spatial correlation length of the aperture field lead to a satiated condition that constrains the relative permeability to be less than one. We use effective media theory to develop a conceptual model for satiated relative permeability, then compare predictions to existing experimental measurements, and numerical solutions of the Reynolds equation on the measured aperture field within the flowing phase. The close agreement among all results and data show that for the experiments considered here, in‐plane tortuosity induced by the entrapped phase is the dominant factor controlling satiated relative permeability. We also find that for this data set, each factor in the conceptual model displays an approximate power law dependence on the satiated saturation of the fracture.
measuring stream discharge by non‐contact methods: A Proof‐of‐Concept Experiment
Authors: Costa John E. , Spicer Kurt R. , Cheng Ralph T. , Haeni F. Peter , Melcher Nick B. , Thurman E. Michael , Plant William J. , Keller William C.
Published: September 2002

This report describes an experiment to make a completely non‐contact open‐channel discharge measurement. A van‐mounted, pulsed doppler (10GHz) radar collected surface‐velocity data across the 183‐m wide Skagit River, Washington at a USGS streamgaging station using Bragg scattering from short waves produced by turbulent boils on the surface of the river. Surface velocities were converted to mean velocities for 25 sub‐sections by assuming a normal open‐channel velocity profile (surface velocity times 0.85). Channel cross‐sectional area was measured using a 100 MHz ground‐penetrating radar antenna suspended from a cableway car over the river. Seven acoustic doppler current profiler discharge measurements and a conventional current‐meter discharge measurement were also made. Three non‐contact discharge measurements completed in about a 1‐hour period were within 1% of the gaging station rating curve discharge values. With further refinements, it is thought that open‐channel flow can be measured reliably by non‐contact methods.
On the existence of Lloró (the rainiest locality on Earth): Enhanced ocean‐land‐atmosphere interaction by a low‐level jet
Authors: Poveda Germán , Mesa Oscar J.
Published: September 2002

The department of Chocó, on the Colombian Pacific coast experiences 8,000 to 13,000 mm of average annual precipitation. Lloró (5°30′N, 76°32′W, 120m) has received above 12,700 mm (1952–1960). Using the NCEP/NCAR Reanalysis data, we show that the ocean‐land‐atmosphere interaction over the easternmost fringe of the tropical Pacific, enhanced by the dynamics of a low‐level westerly jet (“CHOCO”), contributes to explain the existence of such record‐breaking hydrological region. Deep convection develops from low‐level moisture convergence by the CHOCO jet, combined with high‐level easterly trade winds, orographic lifting on the western Andes, low surface pressures and warm air. Precipitation is organized in mesoscale convective complexes, in turn dynamically linked to the jet. The strength of the CHOCO jet (centered at 5°N) is associated with the gradient of surface air temperatures between western Colombia and the Niño 1+2 region, thereby exhibiting strong annual and interannual variability, which contributes to explaining Colombia's hydro‐climatology and its anomalies during ENSO.
Enhanced sedimentation along the Equator in the western Pacific
Authors: Higgins Sean M. , Broecker Wallace , Anderson Robert , McCorkle Daniel C. , Timothy David
Published: September 2002

Measurements on cores from the Ontong Java Plateau demonstrate that on‐equator sediment accumulates more 230Th than is produced in the overlying water column. In contrast, cores from 3.8°N accumulate an amount of excess 230Th more nearly equal to that produced in the overlying water column. Taken together with the observation that both the CaCO3 and non‐CaCO3 components of the sediment accumulate at approximately twice the rate in on‐equator than in off‐equator cores [Broecker et al., 1999], this requires either that the rain of the larger amount of organic matter generated above the equatorial upwelling plume entrains excess clay and 230Th during its fall to the sea floor or that physical transport processes, either in the upper ocean or near the sea bed, redistribute sediments in such a way that they preferentially accumulate‐in a narrow (±1° latitude) equatorial‐centered belt.
Photo‐induced fractionation of water isotopomers in the Martian atmosphere
Authors: Cheng Bing‐Ming , Chew Eh Piew , Liu Chin‐Ping , Bahou Mohammed , Lee Yuan‐Pern , Yung Yuk L. , Gerstell M. F.
Published: September 2002

The history and size of the water reservoirs on early Mars can be constrained using isotopic ratios of deuterium to hydrogen. We present new laboratory measurements of the ultraviolet cross‐sections of H2O and its isotopomers, and modeling calculations in support of a photo‐induced fractionation effect (PHIFE), that reconciles a discrepancy between past theoretical modeling and recent observations. This supports the hypothesis that Mars had an early warm atmosphere and has lost at least a 50‐m global layer of water. Likely applications of PHIFE to other planetary atmospheres are sketched.
Water on Mars: Isotopic constraints on exchange between the atmosphere and surface
Authors: Kass David M. , Yung Yuk L.
Published: September 2002

Using a new measurement of the D/H fractionation efficiency and new estimates of the water loss, we calculate that Mars has the equivalent of a ∼9 m global water layer in a reservoir that exchanges with the atmosphere. The measured D/H enrichment is about 5 times the terrestrial value, but without exchange, the atmosphere converges on an enrichment of 50 in about 0.5 Ma. Due to the large buffering reservoir and the rapid loss rate (10−3 pr‐µm yr−1), the small atmospheric reservoir, averaging 10 pr‐µm, is unlikely to be in continuous isotopic equilibrium with the full 9 m exchangeable reservoir. Instead, it presumably equilibrates during periods of high obliquity; the atmospheric D/H ratio is expected to be enriched in between such periods. If isotopic exchange with a small (4 mm global layer) reservoir occurs under current conditions, it possible for the atmospheric D/H ratio to be within 10% of its long term equilibrium.
Kunhild and Ereshkigal, an extinct hot‐spot region on Venus
Authors: Herrick Robert R. , McGovern Patrick J.
Published: September 2002

Analysis of imagery, topography and gravity for Kunhild and Ereshkigal, adjacent large volcanoes on Venus, indicates that they are extinct hot spots that were located over a broad mantle upwelling. The volcanoes probably formed contemporaneously, but volcanism persisted at Kunhild after Ereshkigal's extinction. A late event in each structure's formation was sagging of the central region to form a broad depression. Ereshkigal, the shorter of the two structures, has a broader and deeper central depression than Kunhild. We suggest that when a long‐lived plume goes away from beneath a large Venusian shield volcano, removal of dynamic support from the plume stem produces a central sagging analogous to the collapse phase associated with corona formation on Venus.
Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions
Authors: Fiquet G. , Dewaele A. , Andrault D. , Kunz M. , Le Bihan T.
Published: September 2002

A new synchrotron X‐ray diffraction study of MgSiO3 perovskite at high‐pressure and high‐temperature has been carried out in a laser‐heated diamond‐anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P–T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si‐O‐Si angle evolution. In addition, thermoelastic parameters identified from this new pressure‐volume‐temperature data set indicate that a significant amount of magnesiowüstite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.
The influence of vegetation cover on soil organic matter preservation in Antarctica during the Mesozoic
Authors: Beerling D. J.
Published: September 2002

A possible biological component of the explanation for the lack of substantial coal formation on Antarctica between the Late Jurassic and Late Cretaceous is hypothesised. Forcing a process‐based terrestrial biogeochemistry model with climate model simulations for three successive intervals of the Mesozoic [Late Jurassic (150 Ma), mid‐Cretaceous (100 Ma) and Late Cretaceous (65 Ma)], predicts the establishment of productive terrestrial ecosystems with substantial soil carbon pools throughout the land surface of Antarctica between 150 and 65 million years ago. Sensitivity analyses indicate the annual land surface hydrological budget of Antarctica 150–65 Ma was largely balanced due to the effects of canopy transpiration on soil moisture. Post‐depositional erosion and tectonic setting must have played an important role in determining coal preservation on the continent. However, the model results indicate that an active vegetated land surface precluded the preservation of the substantial soil organic carbon inputs by minimising the potential for anaerobic preservation (i.e. water logging).
The role of mobile belts for the longevity of deep cratonic lithosphere: The Crumple Zone Model
Authors: Lenardic A. , Moresi L. , Mühlhaus H.
Published: September 2002

Many Archean cratons are surrounded by Proterozoic mobile belts that have experienced episodes of tectonic re‐activation over their lifetimes. This suggests that mobile belt lithosphere may be associated with long lived, inherited weakness. It is proposed that the proximity of this weakness can increase the longevity of deep Archean lithosphere by buffering Archean cratons from mantle derived stresses. The physical plausibility of this idea is explored through numerical simulations of mantle convection that include continents and allow for material rheologies that model the combined brittle and ductile behavior of the lithosphere. Within the simulations, the longevity of deep cratonic lithosphere does increase if it is buffered by mobile belts that can fail at relatively low stress levels.
Thermal evolution of sedimentary basin formation with variable thermal conductivity
Authors: Starin L. , Yuen D. A. , Bergeron Stephen Y.
Published: September 2002

Understanding the thermal evolution of sedimentary basins is crucial for the geosciences and petroleum genesis. A recent model of thermal conductivity, based on microscopic physics, has shown that the variations of thermal conductivity depend nonlinearly on the temperature, pressure and mineralogy. The time‐dependent temperature equation takes on a nonlinear diffusive character with the temperature‐dependent portion of the thermal conductivity. We have compared the numerical solutions of the temperature fields, following a lithospheric stretching event between constant thermal conductivity and variable thermal conductivity. There is a great sensitivity of the thermal signatures to the parameter governing the influence of mineralogy on the phonon contribution to the thermal conductivity. For an olivine‐like lithosphere, differences on the order of 5% or 60 to 70K in the interior temperature between the two models can be expected. For times younger than 150 Myr, the topography for the variable conductivity model is shallower by around 5 to 10% than for the constant conductivity model, while the surface heat flow remains always lower for variable conductivity and the departure between the two conductivity models can reach 20% in the early stage. Variable thermal conductivity may play a non‐trivial role in thermal‐chemical reactive processes under basins.
chemical composition of martian soil and rocks: Complex mixing and sedimentary transport
Authors: McLennan Scott M.
Published: September 2002

Chemical compositions of Martian soil and rocks indicate complex mixing relationships. Mixing of rock and soil clearly takes place and explains some of the chemical variation because sulfur, chlorine, magnesium, and perhaps iron are positively correlated due to their control from a secondary “sedimentary” mineralogy (e.g., Mg‐ and possibly Fesulfate; Fe‐oxides) that is present within the soils. Certain deviations from simple soil‐rock mixing are consistent with mineralogical fractionation of detrital iron and titanium oxides during sedimentary transport.
Crossover plots: A useful method for plotting SIRM data in paleomagnetism
Authors: Symons D. T. A. , Cioppa M. T.
Published: September 2002

The effective grain‐size of magnetic minerals, and therefore domain type, controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create “crossover” plots for paleomagnetic specimens, where %SIRM is plotted as a function of the field applied, using a logarithmic scale. This paper presents templates of such plots for magnetite, pyrrhotite, hematite and goethite, onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain‐size.
A new fiber optic gradiometer for 4‐D absolute differential gravity
Authors: Brown J. M. , Niebauer T. M. , Klopping F. J. , Herring A. T.
Published: September 2002

We introduce the concept of absolute differential gravimetry using fiber optic‐based laser interferometry. Absolute differential gravimetry provides an instantaneous determination of the difference in gravity between two or more points on or beneath the surface of the earth. Applications include enhancing the resolution of the gravity field, as well as subsurface fluid and structure monitoring. With a network of sensors, observation of time dependent variations of the gravity field is also possible. Results from a proof‐of‐concept prototype instrument using fiber‐optic coupler technology represent the first time absolute laser interferometric differential gravity data have been collected. Implications of the new measurement method in terms of geophysical studies and future research plans are discussed.
Unusual ocean‐atmosphere conditions in the tropical Indian Ocean during 1994
Authors: Behera S. K. , Krishnan R. , Yamagata T.
Published: September 2002

The southeastern tropical Indian Ocean (SETIO) was characterized by unusually cold sea surface temperature (SST) and strong northwestward alongshore surface winds during 1994. Using multi‐source data sets including ocean model simulation, two key processes are identified for the SETIO cooling. Entrainment cooling produced most of the negative SST anomaly near the coast whereas evaporative cooling dominated the process away from the coast. Convection was anomalously suppressed over SETIO and the divergence of moist air from the region helped the local evaporative process. This also led to anomalous moisture transports that explain the enhanced convection over the central equatorial Indian Ocean, India and East Asia. The positive feedback between the enhanced and suppressed convection regions in turn helped maintain the surface wind anomalies. These evidences clearly indicate the existence of an ocean‐atmosphere coupled phenomenon in the Indian Ocean during 1994.
Slip‐parallel seismic lineations on the Northern Hayward Fault, California
Authors: Waldhauser Felix , Ellsworth William L. , Cole Alex
Published: September 2002

A high‐resolution relative earthquake location procedure is used to image the fine‐scale seismicity structure of the northern Hayward fault, California. The seismicity defines a narrow, near‐vertical fault zone containing horizontal alignments of hypocenters extending along the fault zone. The lineations persist over the 15‐year observation interval, implying the localization of conditions on the fault where brittle failure conditions are met. The horizontal orientation of the lineations parallels the slip direction of the fault, suggesting that they are the result of the smearing of frictionally weak material along the fault plane over thousands of years.
Eddies and the annular modes of climate variability
Authors: Limpasuvan Varavut , Hartmann Dennis L.
Published: September 2002

The dominant modes of month‐to‐month variation in the troposphere are realistically simulated in a general circulation model whose lower boundary is prescribed with realistic topography and seasonally varying, climatological sea surface temperatures. In both hemispheres, these internal modes describe a nearly zonally symmetric, North‐South shifting of the zonal jets as anomalous westerlies vacillate between high (50°–60°) and low (30°–40°) latitudes. The eddy structures evolve with the jets, and the corresponding eddy momentum forcings support the shifts in jet position. Stationary (synoptic) wave variations mainly account for the total eddy forcings in the Northern (Southern) Hemisphere. Pronounced eddy variations occur over the North Atlantic sector.
Atmospheric gravity wave signatures in the infrared hydroxyl OH airglow
Authors: Frey H. U. , Mende S. B. , Arens J. F. , McCullough P. R. , Swenson G. R.
Published: September 2002

Atmospheric gravity wave induced airglow fluctuations of the hydroxyl OH Meinel and other bands are routinely observed using CCD imagers operating in the near infrared wavelength region. Farther into the infrared, the intensity of the OH emissions is much greater. Recently, a new IR imaging camera was modified for infrared viewing of the night sky with approximately 1 steradian field of view. The filter had a center wavelength of 1467 nm and a bandwidth of 177 nm to include components of the OH (Δv = 2) band. The images show good signal‐to‐noise ratios of 180 with 10 second exposures. Cross‐spectral and wavelet analysis were used to obtain spatial and temporal information about observed airglow fluctuations, and horizontal wavelengths and wave periods as short as 3.7 km and 3 min, respectively, were determined.
Parallel electric fields in dispersive shear Alfvén waves in the dipolar magnetosphere
Authors: Rankin R. , Samson J. C. , Tikhonchuk V. T.
Published: September 2002

Existing theories do not explain large parallel electric fields that are associated with keV electron precipitation in auroral arcs. The MHD electron response results in an electrical conductivity in the low altitude magnetosphere that is two orders of magnitude greater than is required. We suggest a new mechanism for forming parallel electric fields based on a nonlocal electron response to standing shear Alfvén waves on dipole magnetic fields. Electron trapping is the primary cause of a significant reduction in the collisionless electron conductivity and consequent enhancement of parallel electric fields in the 1–4 mHz frequency range.
A neutral sheet surface observed on ISEE‐2, IMP‐8, AMPTE/IRM, and INTERBALL Satellites
Authors: Li L. , Xu R. L.
Published: September 2002

A neutral sheet model has been developed, with parameters optimized based upon a large number of magnetometer data from the ISEE‐2, IMP‐8, AMPTE/IRM, and INTERBALL satellites. In this model, the equatorial‐neutral sheet is best fitted by a smooth curved surface in the region up to 40Re, with no slope discontinuities between the neutral sheet and the equatorial plane, or on the flanks of the neutral sheet. The cross‐section of the magnetotail is divided into approximately equal areas above and below the equatorial‐neutral sheet inside the magnetopause of the Sibeck model. An exponential dependence on Xgsmis adopted to express the influence of the tilted equatorial plane on the position of the neutral sheet, since it is found this influence weakens with increasing |Xgsm|.
Homologous sudden disappearances of transequatorial interconnecting loops in the solar corona
Authors: Khan Josef I. , Hudson Hugh S.
Published: September 2002

We have found a remarkable sequence of homologous disappearances of transequatorial X‐ray loops linking active regions. Each disappearance was closely associated with a major flare and coronal mass ejection (CME). In each case the flarings precede the disappearances and the CMEs. Mass estimates for the X‐ray loops are similar to CME masses. This, the timing of the disappearances, their morphology, and the homology of the events in the sequence, provide direct evidence for a new class of CME origins in the low corona. We also briefly report observations of features which we infer to be the soft X‐ray counterparts of shock waves emanating from the flare region. The inferred shocks appeared to play a vital role in the disappearances. Our results suggest that flare‐generated shock waves may destabilize large transequatorial loops, causing them to erupt.
The Faint Young Sun Paradox: An observational test of an alternative solar model
Authors: Gaidos Eric J. , Güdel Manuel , Blake Geoffrey A.
Published: September 2002

We report the results of deep observations at radio (3.6 cm) wavelengths of the nearby solar‐type star π01 Ursa Majoris with the Very Large Array (VLA) intended to test an alternative theory of solar luminosity evolution. The standard model predicts a solar luminosity only 75% of the present value and surface temperatures below freezing on Earth and Mars at 4 Ga, seemingly in conflict with geologic evidence for liquid water on these planets. An alternative model invokes a compensatory mass loss through a declining solar wind that results in a more consistent early luminosity. The free‐free emission from an enhanced wind around nearby young Sun‐like stars should be detectable at microwave frequencies. Our observations of π01 UMa, a 300 million year‐old solar‐mass star, place an upper limit on the mass loss rate of 4–5 × 10−11 M⊙yr−1. Total mass loss from such a star over 4 Gyr would be less than 6%. If this star is indeed an analog of the early Sun, it casts doubt on the alternative model as a solution to the faint young Sun paradox, particularly for Mars.
Is the dynamic magnetosphere an avalanching system?
Authors: Lui A. T. Y. , Chapman S. C. , Liou K. , Newell P. T. , Meng C. I. , Brittnacher M. , Parks G. K.
Published: September 2002

The analogy between the dynamic magnetosphere and an avalanche system is examined by using the global auroral energy deposition obtained by the Polar UVI as a measure of the energy output of the magnetosphere. The analysis reveals two types of energy dissipation: those internal to the magnetosphere occurring at all activity levels with no intrinsic scale, and those associated with active times corresponding to global energy dissipation with a characteristic scale. The internal events exhibit the same power law index in both active and quiet times. These features are consistent with a simple avalanche (‘sandpile’) numerical model.
Sferic clusters associated with early/Fast VLF events
Authors: Johnson M. P. , Inan U. S.
Published: September 2002

Characteristic subionospheric signal perturbations referred to as ‘early/fast’ VLF events are found to consistently be associated with the simultaneous occurrence of unusual clusters of radio atmospherics as observed in wideband VLF data. A burst of sferic activity occurs at or before the onset of the early/fast events and lasts from several hundred milliseconds to several seconds. The cluster energy attenuates with distance more rapidly than cloud‐to‐ground (CG) lightning, suggesting that these clusters may be produced by intracloud (IC) lightning, and that the charge movement within the cloud may thus play a role in the causative mechanism of early/fast VLF events.
Importance of oceanic decadal trends and westerly wind bursts for forecasting El Niño
Authors: Perigaud Claire M. , Cassou Christophe
Published: September 2002

In the western Pacific, Westerly Wind Bursts (WWB) usually show up in the boreal fall‐winter season, depending on the activity of the Madden and Jullian oscillation. It is demonstrated with a coupled ocean‐atmosphere model that WWB have an impact on the coupled system in El Niño forecasting that depends on the ocean preconditioning. Sea level data reveal decadal changes of the Ocean Heat Content of the entire tropical Pacific (the OHC), with a minimum in the mid‐1980s relative to highs before 1982 and after 1996. Adding the WWB in the forecast system does not degrade the successful predictions when the OHC is low whereas WWB are necessary in winter 1981–82 and 1996–97 to successfully predict the forthcoming warm events. Thus, when the OHC is high in addition to the other traditional conditions being favorable for a warm growth, WWB contribute to having a strong El Niño event the following year.
Factors, especially sunspot number, causing variations in surface air concentrations and depositions of 7Be in Osaka, Japan
Authors: Megumi Kazuko , Matsunami Tadao , Ito Norio , Kiyoda Shunji , Mizohata Akira , Asano Takeyoshi
Published: September 2002

We found that increasing sunspot number caused a significant negative effect on monthly and yearly average air concentration and yearly deposition of 7Be. For more than 15 years monthly average surface air concentration and monthly deposition of 7Be was measured in Osaka, Japan. The maximum monthly average surface air concentration of 7Be was 1.6 times higher than the minimum concentration through the fifteen year measurement span. Increase of several percent in the monthly averaged surface air concentration and monthly deposition of 7Be were observed in the monthly mean for February‐March‐April 1987 period following the 23 February supernova explosion in the Large Magellanic Cloud. The seasonal variation pattern which includes the lowest surface air concentration of 7Be in summer is due to the transport of air masses from low latitudes carrying low 7Be concentrations into the middle latitude region.
Spatio‐temporal analysis of stress diffusion in a mining‐induced seismicity system
Authors: Marsan David , Bean Christopher J. , Steacy Sandy , McCloskey John
Published: September 2002

The spatio‐temporal correlation of micro‐earthquakes occuring in a mining‐induced seismic system (Creighton mine, Ontario, Canada) is investigated. It is shown that, when considering only the after‐events correlated to a main event, i.e., not accounting for the uncorrelated regime of ‘background’ activity, the spatial distribution of these after‐events occuring at t after the main event change with t. This change takes the form of an expanding pattern, characterized by a typical scale Lc( t) varying as Lc( t) ∼ tH, H being estimated to 0.18. This diffusion exponent is found to increase when considering only a subset of the most energetic events as mainshocks. We interpret this result as the indication of a stress (sub‐)diffusion mechanism, involving propagation on the heterogeneous fractal fault network.
Results of airborne vector (3‐d) gravimetry
Authors: Jekeli Christopher , Kwon Jay H.
Published: September 2002

Gravity field modeling using airborne vertical component gravimetry has made significant strides over the last decade. We demonstrate the feasibility of extending this to three‐dimensions using data from inertial navigation systems (INS) and the Global Positioning System (GPS). A significant advantage of measuring the horizontal gravity components is that the geoid can be determined in profiles by direct along‐track integration, thus not only adding strength to conventional methods, but reducing the required area of survey support, especially along model boundaries. As such, the ultimate limitation of the method is in the quality of the INS and GPS sensors. In our test case, all three components of the gravity vector were determined over a profile in the Canadian Rocky Mountains. Differences between available truth data and the computed gravity components have standard deviations of 7–8 mGal (horizontal) and 3 mGal (vertical). These standard deviations include uncertainties in the truth data (<5 mGal, for horizontal; 1.3 mGal, for vertical). The resolution in the computed values is about 10 km. These analyses have demonstrated for the first time that the total gravity vector can be determined from airborne INS and GPS to reasonable accuracy and resolution, without any external orientation information, nor prior statistical hypothesis on the gravity signature, using medium‐accuracy INS and geodetic quality GPS receivers.
Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in‐situ and ADEOS‐POLDER measurements
Authors: C. ‐Labonnote Laurent , Brogniez Gérard , Doutriaux‐Boucher Marie , Buriez Jean‐Claude , Gayet Jean‐François , Chepfer Hélène
Published: September 2002

An Inhhomogeneous Hexagonal Monocrystal (IHM) model is used to simulate light scattering by randomly oriented hexagonal ice crystals containing air bubbles. This model based on a combination of ray‐tracing, Mie theory and Monte‐Carlo techniques, allows to retrieve the scattering phase function. In‐situ measurements of the light scattering diagram in natural cirrus clouds with an airborne nephelometer have been performed. The results given by the IHM model have been favorably adjusted with these measurements. This agreement provides an opportunity to use this model in order to analyze ADEOS‐POLDER reflectance measurements over cirrus clouds. POLDER uses an original concept to measure, for a given scene, total and polarized reflectances under several viewing directions. A first analysis of cirrus cloud spherical albedoes for the 10th November 1996 shows a rather good agreement between measurements and calculations.
Shock wave and fracture propagation in water ice by high velocity impact
Authors: Arakawa Masahiko , Shirai Kei , Kato Manabu
Published: September 2002

In order to clarify the elementary processes of impact disruption, we conducted impact experiments with water ice at an impact velocity of 3.6 km/s and observed shock wave and fracture propagation in it by means of ultra‐high speed photography. We observed that a region in which HEL (Hugoniot elastic limit) followed the elastic precursor wave, expanded with a velocity of 3–2.5 km/s until the pressure fell below 240 MPa. Below that pressure, a damage region appeared 0.8–3 µs after the passage of precursor wave. In this region, dynamic shear strength of water ice was estimated to be 21 MPa. Below 80 MPa, the several radial cracks proceeded toward the rear surface and broke the sample before the tensile fracture caused by reflection waves from an antipodal point became visible. Therefore, the main mechanism to cause the largest fragment is the radial crack growth rather than a spallation at the rear.
Comparison of ground‐based 3‐dimensional lightning mapping observations with satellite‐based LIS observations in Oklahoma
Authors: Thomas Ronald J. , Krehbiel Paul R. , Rison William , Hamlin Timothy , Boccippio Dennis J. , Goodman Steven J. , Christian Hugh J.
Published: September 2002

3‐dimensional lightning mapping observations obtained in central Oklahoma by the New Mexico Tech Lightning Mapping Array (LMA) have been compared with optical observations of the discharges from space obtained by NASA's Lightning Imaging Sensor (LIS). Excellent spatial and temporal correlations were obtained between the two sets of observations. All lightning discharges seen by LIS were mapped by the LMA. Most of the detected optical events were associated with lightning channels that extended into the upper part of the storm. Cloud‐to‐ground discharges that were confined to mid‐ and lower‐altitudes were less well detected than intracloud discharges, and tended to be detected late in the discharge. Intracloud discharges were readily detected by LIS as soon as they extended into the upper part of the storm, and often extensively illuminated the cloud at the end or part way through the discharge. The extensive illumination was impulsive in nature and was also seen at the end of some cloud‐to‐ground discharges.
A case study on the biomass burning in southeast Asia and enhancement of tropospheric ozone over Hong Kong
Authors: Chan L. Y. , Chan C. Y. , Liu H. Y. , Christopher S. , Oltmans S. J. , Harris J. M.
Published: September 2002

Substantial enhancement of ozone was frequently observed in the lower troposphere (2.5–6.0 km) over Hong Kong (22.2°N, 114.3°E). Most of these enhanced ozone layers (with peak concentration up to 138 ppbv) feature moderate and high relative humidity (50–100%) and a temperature inversion at the bottom of the layer. These phenomena predominantly occur in spring (late February to mid April) coinciding with the dry season in Southeast Asia. Back air trajectories showed that these ozone‐rich air masses had passed through the Indo‐Burma region of Southeast Asia, where large‐scale fires were recorded at the time. A case study is presented and evidence from the fire count derived from AVHRR satellite images and carbon monoxide concentration derived from space shuttle during the MAPS experiment was used. We have illustrated with this case study that biomass‐burning emissions are the source of the elevated ozone observed.
Thinning of the Arctic sea‐ice cover
Authors: Rothrock D. A. , Yu Y. , Maykut G. A.
Published: September 2002

Comparison of sea‐ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water portion of the Arctic Ocean, from 3.1 m in 1958–1976 to 1.8 m in the 1990s. The decrease is greater in the central and eastern Arctic than in the Beaufort and Chukchi seas. Preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s.
Shear‐wave velocities under the Transantarctic Mountains and terror rift from surface wave inversion
Authors: Bannister S. , Snieder R. K. , Passier M. L.
Published: September 2002

Rayleigh wave data between 20 and 120s period recorded in the western Ross Sea region of Antarctica are inverted using the waveform inversion technique described in Passier and Snieder [1995]. From the data we extract estimates of local S velocity structure beneath the Transantarctic Mountains and the Terror Rift. The shear wave velocities found are up to 6% slower than the PREM model at between 40 and 160 km depth, depending on the propagation path, providing evidence of an anomalously warm upper mantle beneath the Terror Rift and, to a lesser extent, beneath the front of the Transantarctic Mountains.
Speciation and rate of photochemical NO and NO2 production in Antarctic snow
Authors: Jones A. E. , Weller R. , Wolff E. W. , Jacobi H. ‐W.
Published: September 2002

Measurements were made of NO and NO2, in controlled experiments to investigate their production from snow. Throughout a diurnal cycle, measurements were made of ambient air and air from inside a snowblock. Enhanced concentrations of NO and NO2 (up to 15 pptv and 32 pptv respectively) were measured inside the snowblock. The production rate inside the block varied with intensity of incident radiation, and reached a maximum of 1.1×106 molecs/cm³/s for NO and 2.1 × 106 molecs/cm³/s for NO2. A second experiment, in which the snowblock was alternately exposed to sunlight and then shaded, confirmed that the diurnal production was driven by photochemistry rather than some other diurnally varying factor. Concentrations of nitrate in the snowblock did not change as a result of 50 hours of experiments, confirming that if nitrate is the source reservoir, it can not be rapidly depleted. Snowpack production may contribute significantly to NOx concentrations in the Antarctic lower troposphere.
Global tropical cyclone activity: A link to the North Atlantic Oscillation
Authors: Elsner James B. , Kocher Bethany
Published: September 2002

An index of global tropical cyclone activity (GTCA) is extracted from an interbasin dispersion matrix composed of correlations in the frequency of tropical cyclones among the five principal basins of activity over the period 1966–1998. The extraction is performed using a single‐factor common factor analysis model. Factor scores provide the index of GTCA. The time series of factor scores is matched with annual series' of the El Niño‐southern oscillation (ENSO) and the North Atlantic oscillation (NAO) revealing a statistical link to the NAO, but not to the ENSO. Results suggest a greater role of high latitude atmospheric and oceanic processes in modulating tropical cyclone climate than previously considered.