bluuebunny/crossref_metadata_2025 huggingface dataset:
The thickness and structure of high beta magnetopause current layer
Authors: Le G. , Russell C. T.
Published: February 2004

Previous surveys of the magnetopause current layer indicated that its thickness ranges from 200 to 1800 km and on average is about 900 km or ∼10 ion gyroradii [Elphic and Russell, 1979; Berchem and Russell, 1982]. In the present study, we have used ISEE magnetic field data to examine the magnetopause current layer under high beta magnetosheath conditions. Under high beta conditions, we find that the current layer is thinner than observed under typical conditions. The thickness is typically between 200 to 600 km and the peak of the distribution scales to 2–4 ion gyroradii for the high beta magnetopause. However, the velocity of the magnetopause is found to be highly variable similar to the highly variable distribution observed under more typical conditions. A wave‐like structure is often observed at the foot of the current layer extending from the current layer into the magnetosheath. The presence of this propagating wave structure at the foot of the magnetopause may lead to some confusion in attempts to determine the thickness of the current layer with a single spacecraft.
DE1 observations of polar O+ stream bulk parameters and comparison with a model of the centrifugally‐accelerated polar wind
Authors: Wing Ho C. , Horwitz J. L. , Moore T. E.
Published: February 2004

A survey of bulk parameters of analyzable O+ outward streams in the midaltitude (3–4.7 RE geocentric distance) polar cap magnetosphere is obtained from measurements by the Retarding Ion Mass Spectrometer (RIMS) aboard the Dynamics Explorer 1 (DE1) spacecraft. There is wide scatter in the obtained densities, but they do display discernible trends: the average O+ density in these streams decreases from about 30 ions cm−3 at 3.5 RE to about 1 ion cm−3 at 4.6 RE. The streaming velocities are somewhat more defined, and their average increases from about 8 km s−1 at 3.5 RE to about 12 km s−1 at 4.6 RE. The outward ion flux increases by a factor of 4 with magnetic activity from Kp = 2 to 5. We have further compared these observational trends with model profiles for the centrifugally‐accelerated polar wind as recently described by Horwitz el al. [1994]. The large outflow velocities observed can be understood in part as centrifugally‐driven by convection with ionospheric electric field magnitudes of the order 50–70 mV m−1, perhaps including plasma expansion effects.
Frequency dependence of the visibility and depths of mantle seismic discontinuities
Authors: Helffrich George , Bina Craig R.
Published: February 2004

The visibility and apparent position of a seismic gradient zone depends on the seismic wave frequency at which it is observed, which we illustrate by showing how the visibility and depth of the olivine α→β phase transformation (410 km discontinuity) change as temperature varies. Temperature variations change bottomside reflection coefficients by factors of 2–4 in the 0.5–1.0 Hz frequency band, which can explain spatial variability in short period P′ 410P′ observations and the small apparent variability in observed 410 km discontinuity depths. Discontinuity depths inferred from the time lags of these reflections are also frequency dependent, leading to a frequency dependence of estimates of mantle lateral thermal variability based on discontinuity displacements. The frequency dependent seismic Clapeyron slopes for the olivine α→β phase change we compute are 2.04 MPa K−1 at 0.2 Hz and 2.12 MPa K−1 at 0.05 Hz, and differ significantly from thermodynamic slope for the Mg2SiO4 end‐member.
The response of ClO radical concentrations to variations in NO2 radical concentrations in the lower stratosphere
Authors: Stimpfle R. M. , Koplow J. P. , Cohen R. C. , Kohn D. W. , Wennberg P. O. , Judah D. M. , Toohey D. W. , Avallone L. M. , Anderson J. G. , Salawitch R. J. , Woodbridge E. L. , Webster C. R. , May R. D. , Proffitt M. H. , Aiken K. , Margitan J. , Loewenstein M. , Podolske J. R. , Pfister L. , Chan K. R.
Published: February 2004

The response of ClO concentrations to changes in NO2 concentrations has been inferred from simultaneous observations of [ClO], [NO], [NO2] and [O3] in the mid‐latitude lower stratosphere. This analysis demonstrates that [ClO] is inversely correlated with [NO2], consistent with formation and photolysis of [ClONO2]. A factor of ten range in the concentration of NO2 was sampled (0.1 to 1×109 mol/cm³), with a comparable range in the ratio of [ClO] to total available inorganic chlorine (1% ≤ [ClO]/[Cly] ≤ 5%). This analysis leads to an estimate of [ClONO2]/[Cly] = 0.12 (×/÷2), in the mid‐latitude, lower‐stratospheric air masses sampled.
The comparative behaviors of yttrium and lanthanides in the seawater of the North Pacific
Authors: Zhang J. , Amakawa H. , Nozaki Y.
Published: February 2004

Yttrium has long been recognized as an ekalanthanide, because of its chemical contiguity relative to the chemistries of rare earth elements which are, in recent years, intensively utilized for elucidation of metal scavenging processes in the ocean. Here, we present the first detailed depth‐profile of Y(III) in seawater together with the lanthanides in the North Pacific Ocean. The concentrations of Y(III) range 70–290 pmol/kg and show the “nutrient‐like” profile best resembling that of Ho(III) amongst the other rare earth elements. The results agree well with an expectation based on the similarity in their ionic radii and hence stability constants of complexation with carbonate ions. Yet the Ho(III)/Y(III;) ratios in seawater systematically increase with depth, suggesting that Y and Ho are fractionated during scavenging by natural marine particulates. This is likely to result from the different complexation behavior in that Y(III) is more weakly complexed than Ho(III) with soft organic ligands on the surface of particulate matter during scavenging in the surface water but, once released into seawater in the deep sea, Y(III) is complexed with carbonate ions equally or stronger than Ho. The pattern of deep water enrichment in the lanthanide series appears to be consistent with the recent observation of partitioning between suspended particles and seawater. Our precise measurements also indicated that Pr and Tb best resemble Nd and Dy, respectively in their oceanic behavior, whereas Ho and Tm are intermediate between their neighboring rare earth elements.
Analysis of the physical state of one Arctic polar stratospheric cloud based on observations
Authors: Drdla K. , Tabazadeh A. , Turco R. P. , Jacobson M. Z. , Dye J. E. , Twohy C. , Baumgardner D.
Published: February 2004

During the Arctic Airborne Stratospheric Expedition, simultaneous measurements of aerosol size distribution and NOy, (HNO3 + NO + NO2 + 2·N2O5) were made along ER‐2 flight paths. The flow characteristics of the NOy instrument allow us to derive the condensed NOy amount (assumed to be HNO3) present during polar stratospheric cloud (PSC) events. Analysis of the January 24th flight indicates that this condensed HNO3 amount does not agree well with the aerosol volume if the observed PSCs are composed of solid nitric acid trihydrate (NAT), as is generally assumed. However, the composition agrees well with that predicted for liquid H2SO4/HNO3/H2O solution droplets using a new Aerosol Physical Chemistry Model (APCM). The agreement corresponds in detail to variations in temperature and humidity. The weight percentages of H2SO4, HNO3, and H2O derived from the measurements all correspond to those predicted for ternary, liquid solutions.
One‐ to two‐month oscillations in SSMI surface wind speed in western tropical Pacific Ocean
Authors: Collins Michael L. , Stanford John L. , Halpern David
Published: February 2004

The 10‐m wind speed over the ocean can be estimated from microwave brightness temperature measurements recorded by the Special Sensor Microwave Imager (SSMI) instrument mounted on a polar‐orbiting spacecraft. Four‐year (1988–1991) time series of average daily 1° × 1° SSMI wind speeds were analyzed at selected sites in the western tropical Pacific Ocean. One‐ to two‐month period wind speed oscillations with amplitudes statistically significant at the 95% confidence level were observed near Kanton, Eniwetok, Guam, and Truk. This is the first report of such an oscillation in SSMI wind speeds.
Climatic impact of the A.D. 1783 Asama (Japan) Eruption was minimal: Evidence from the GISP2 Ice Core
Authors: Zielinski G. A. , Fiacco R. J. , Mayewski P. A. , Meeker L. D. , Whitlow S. , Twickler M. S. , Germani M. S. , Endo K. , Yasui M.
Published: February 2004

Assessing the climatic impact of the A.D. 1783 eruption of Mt. Asama, Japan, is complicated by the concurrent eruption of Laki, Iceland. Estimates of the stratospheric loading of H2SO4 for the A.D. 1108 eruption of Asama derived from the SO42− time series in the GISP2 Greenland ice core indicate a loading of about 10.4 Tg H2SO4 with a resulting stratospheric optical depth of 0.087. Assuming sulfur emissions from the 1783 eruption were only one‐third of the 1108 event yields a H2SO4 loading value of 3.5 Tg and a stratospheric optical depth of only 0.029. These results suggest minimal climatic effects in the Northern Hemisphere from the 1783 Asama eruption, thus any volcanically‐induced cooling in the mid‐1780s is probably due to the Laki eruption.
New evidence for ozone depletion in the upper stratosphere
Authors: Semenov V. S. , Farrugia C. J. , Biernat H. K. , Lebedeva V. V. , Rijnbeek R. P.
Published: February 2004

We highlight one result of a time‐dependent analysis of reconnection at the terrestrial magnetopause, namely, that the Petschek‐type reconnection process is inevitably accompanied by the generation of surface waves. These waves propagate along the magnetopause at a speed which is generally different from that of the shocks and discontinuities bounding the region of heated and accelerated plasma, the so‐called outflow region of reconnected plasma. The surface waves play a central role in the reconnection process because it is through them that the perturbations of the magnetic field and velocity normal to the magnetopause are transmitted along the current sheet from the source region to the system at large. Since reconnection acts as a source of surface waves, the present general view that transient phenomena at the magnetopause are due either to surface waves (generated by, for example, variations in the solar wind) or to transient reconnection should be revised in the interests of a more unified treatment.
The mechanism of the Deep Bolivia Earthquake of June 9, 1994
Authors: Kikuchi Masayuki , Kanamori Hiroo
Published: February 2004

We analyzed the IRIS broad‐band seismograms for the June 9, 1994, Bolivia earthquake, the largest deep‐focus earthquake ever recorded. We inverted the record sections with a duration of 330 sec which include P, pP and sP phases, and obtained a dip‐slip mechanism: (dip(δ), rake(λ), strike(ф)) = (13°, −4°, 1°) or (89°, −103°, 95°), with a scalar moment of Mo=2.9×1021 Nm (Mw=8.2). This mechanism is similar to that obtained from long‐period (175 to 250 sec) Rayleigh and Love waves: (δ, λ, ф) =(83°, −100°, 90°); Mo=3.0×1021 Nm ( Mw=8.3). The main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1.2×1020 Nm (Mw=7.3). The main rupture starts at about 20 km east or northeast of the initial break, and propagates horizontally in the east to northeast direction. The source duration is 40 sec, and the rupture dimension is about 40km. If a fault area of 40×40km² is assumed, the stress drop is 110 MPa. An inversion of body waves (P, pP, and sP) with 6 basis moment tensors yielded a best‐fit solution with a 10% implosive component. This result is of marginal significance and its confirmation would require more detailed information on the structures near the source region, pP bounce points and stations. The observed amplitude of the radial mode, 0S0, (1226 sec) is consistent with a double couple model with less than 2 % isotropic component.
High‐resolution δ13C measurements of oak show a previously unobserved spring depletion
Authors: Ogle Neil , McCormac F. G.
Published: February 2004

We have subdivided single tree rings into between 24 and 35 contiguous samples and measured the δ13C value for each. These high‐resolution measurements show a previously unobserved spring depletion as part of a pattern of enrichment‐depletion‐enrichment‐depletion through a single growing season as opposed to the depletion‐enrichment‐depletion cycle previously thought to exist. The earlywood/latewood transition does not coincide with identifiable isotopic boundaries and therefore is not a suitable demarcation point for sampling as part of palaeoclimatic study. Latewood and post spring depletion values of δ13C were separately correlated with geophysical parameters. Post spring depletion wood yielded better correlations and is therefore more suitable for palaeoclimatic study.
The spectrum of LF/MF/HF radio noise at ground level during substorms
Authors: LaBelle J. , Weatherwax A. T. , Trimpi M. L. , Brittain R. , Hunsucker R. D. , Olson J. V.
Published: February 2004

During the 1993–1994 Winter, simultaneous measurements were conducted in Alaska of the radio spectrum from 0.1 to 4.8 MHz and geomagnetic activity. Night‐time geomagnetic disturbances, consisting of bays in the DC magnetic field and rapid variations recorded with the induction coil, were found to be correlated with prompt absorption features in the LF/MF/HF radio spectrum and with three types of natural emissions at LF/MF/HF frequencies. During February–March, 1994, 66 out of 73 magnetic signatures meeting certain criteria were associated with LF/MF/HF absorption events and 30 of the 73 were associated with natural emissions. The radio spectrum was sometimes observed to be sensitive to relatively weak magnetic activity.
Towards an MHD theory for the standoff distance of Earth's bow shock
Authors: Cairns Iver H. , Grabbe Crockett L.
Published: February 2004

An MHD theory is developed for the standoff distance as of the bow shock and the thickness Δms of the magnetosheath, using the empirical Spreiter et al. relation Δms = kX and the MHD density ratio X across the shock. The theory includes as special cases the well‐known gasdynamic theory and associated phenomenological MHD‐like models for Δms and as. In general, however, MHD effects produce major differences from previous models, especially at low Alfven (MA) and sonic (MS) Mach numbers. The magnetic field orientation, MA, MS, and the ratio of specific heats γ are all important variables of the theory. In contrast, the fast mode Mach number need play no direct role. Three principal conclusions are reached. First, the gas‐dynamic and phenomenological models miss important dependances on field orientation and MS and generally provide poor approximations to the MHD results. Second, changes in field orientation and MS are predicted to cause factor of ∼ 4 changes in Δms at low MA. These effects should be important when predicting the shock's location or calculating γ from observations. Third, using Spreiter et al.'s value for k in the MHD theory leads to maximum as values at low MA and nominal MS that are much smaller than observations and MHD simulations require. Resolving this problem requires either the modified Spreiter‐like relation and larger k found in recent MHD simulations and/or a breakdown in the Spreiter‐like relation at very low MA.
Climatic implications of the seasonal variation of upper troposphere water vapor
Authors: Del Genio Anthony D. , Kovari William , Yao Mao‐Sung
Published: February 2004

Satellite observations indicate that the humidity of the upper troposphere is higher in summer than in winter. We use general circulation model (GCM) simulations to explore the processes that maintain upper troposphere water vapor and determine its seasonal cycle. In the subtropics, drying by Hadley cell subsidence and stratiform condensation is offset primarily by moistening by eddies, with moist convection playing a minor role. Elsewhere, both mean meridional circulation and eddies moisten the upper troposphere and are balanced primarily by stratiform condensation drying. The effect of the seasonal shift of the Hadley cell is limited to latitudes equatorward of 30°. At higher latitudes where the largest observed summer moistening occurs, eddy moisture fluxes are primarily responsible despite the eddies being weaker in summer than winter. The same mechanism causes upper level humidity to increase in GCM climate warming simulations. The observed seasonal variation may thus be a good proxy for decadal climate change. This suggests that upper troposphere water vapor feedback is positive at all latitudes, consistent with GCM predictions.
Thermoviscous remanent magnetism of Columbia River basalt blocks in the cascade landslide
Authors: Smith Rebecca Tyson , Verosub Kenneth L.
Published: February 2004

We studied sixteen basalt samples from a landslide in the Columbia River Gorge to determine if they had acquired a thermoviscous remanent magnetism (TVRM) since the slide was emplaced about 800 years ago. All samples were thermally demagnetized at 20 heating steps until 360°C, where a large change in susceptibility was noted. Analysis of the directional changes during demagnetization indicates that the samples contain up to four TVRM components, in addition to an NRM component. The TVRM components with the lowest blocking temperatures are tightly clustered around the present field direction while the NRM directions are consistent with a random distribution, as expected for a landslide deposit. Measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single‐domain magnetite. The temperature at which the first TVRM component was removed ranges from 70°C to 100°C. This result can be compared to nomographs that relate the time and temperature used to demagnetize a TVRM with the time and temperature at which the TVRM was acquired. Our results are more consistent with the nomograph of Pullaiah et al. [1975] than with the nomograph of Middleton & Schmidt [1982].
Anomalous mantle structure at the Proterozoic‐Paleozoic boundary in northeastern US
Authors: Levin Vadim , Lerner‐Lam Arthur , Menke William
Published: February 2004

Using teleseismic P wave tomography we detect very large (∼5%) variation in the compressional velocity of the upper mantle beneath the Proterozoic‐Paleozoic boundary in the Northeastern US. In this region, the upper mantle is heterogeneous on the 100 km scale to a depth of 300 km. Below this depth, the mantle appears to be laterally homogenous, at least on the scale of 200–250 km. Given the tectonic history of the region, the large amplitude, short wavelength heterogeneities can not be caused by temperature variations, but may be due to chemical heterogeneity or variations in the amount and orientation of flow‐induced seismic anisotropy.
Far‐infrared spectroscopy of CO2 clathrate hydrate with Martian applications
Authors: Landry Joseph C. , England Anthony W.
Published: February 2004

It has been suggested that CO2 clathrate hydrate forms in the Martian polar regions in winter. A large deposit of hydrate would affect the polar energy budget because of its low thermal conductivity and its sizeable latent heat of dissociation, and it could also be important to the total CO2 budget. In this paper we present the far‐infrared spectrum of CO2 hydrate at 150 K. The spectrum exhibits a broad absorption centered near 17 cm−1 (0.59 mm) that is probably due to a rattling transition. Our radiative transfer model predicts that the absorption gives rise to a distinctive emission spectrum for densely packed particles having average diameters greater than 100 µm.
Precursor to equatorial spread‐F in OI 630.0 nm dayglow
Authors: Sridharan R. , Raju D. Pallam , Raghavarao R. , Ramarao P. V. S.
Published: February 2004

A unique OI 630.0 nm dayglow photometer operated from Waltair (10.0° N dip lat.), a‐low‐latitude station in India, in a bidirectional mode, i.e., over zenith and at 20° elevation pointing north, has revealed features associated with the evolution of the Equatorial Ionization Anomaly (EIA) The estimated strength of the EIA on a particular day based on these features reveal that significant differences exist in the EIA contribution to OI 630.0 nm on equatorial spread‐F (ESF) and non‐ESF days. There exists a precursor in the OI 630.0 nm dayglow which enables the prediction of ESF at least 3 hours prior to its actual occurrence and hence points to the significant control of daytime EIA‐related processes in the triggering of the post‐sunset ESF.
Moment‐tensor analysis using regional data: Application to the 25 March, 1993, Scotts Mills, Oregon, Earthquake
Authors: Nábělek John , Xia Ganyuan
Published: February 2004

In this paper we outline a procedure we use for routine moment‐tensor analysis of regional data from broadband seismic stations in northwestern North America and apply it to the moment magnitude 5.5, March, 1993, Scotts Mills, Oregon, earthquake. The results compare favorably with those obtained from teleseismic data. We found that the earthquake occurred at a depth of 13–15 km and had a mechanism with approximately equal amounts of reverse and right‐lateral strike‐slip components. The estimated stress drop of 40 bar is average on a world‐wide basis, supporting the view that the rather large damage was caused primarily by poor construction and not by exceptional properties of the source. The Scotts Mills earthquake is most likely related to the Mt. Angel Fault. This fault is a part of the Gales Creek‐Mt. Angel structural lineament (GCMAL) extending about 150 km across the Willamette Valley. At present data are not sufficient to estimate the likelihood of an earthquake involving the entire GCMAL, but given its length an earthquake of magnitude 7 is conceivable. The results of this study, together with investigations of other earthquakes, suggest that sparse broadband networks can be used efficiently for determining source parameters of earthquakes of magnitude greater than 4.0 in regions with infrequent seismicity.
Relative motions of the Australian, Pacific and Antarctic Plates estimated by the Global Positioning System
Authors: Larson Kristine M. , Freymueller Jeff
Published: February 2004

GPS measurements spanning ∼3 years have been used to determine velocities for 7 sites on the Australian, Pacific and Antarctic plates. The site velocities agree with both plate model predictions and other space geodetic techniques. We find no evidence for internal deformation of the interior of the Australian plate. Wellington, New Zealand, located in the Australian‐Pacific plate boundary zone, moves 20±5 mm/yr west‐southwest relative to the Australian plate. Its velocity lies midway between the predicted velocities of the two plates. Relative Euler vectors for the Australia‐Antarctica and Pacific‐Antarctica plates agree within one standard deviation with the NUVEL‐1A predictions.
Variations in water‐column ³He/heat ratios associated with the 1993 CoAxial event, Juan de Fuca Ridge
Authors: Lupton J. E. , Baker E. T. , Massoth G. J. , Thomson R. E. , Burd B. J. , Butterfield D. A. , Embley R. W. , Cannon G. A.
Published: February 2004

The 1993 volcanic/hydrothermal event on the CoAxial segment, northern Juan de Fuca Ridge, generated a complex set of event plumes and steady‐state type plumes with a pattern of ³He/heat ratios very similar to that observed during the 1986 megaplume event on the southern JdFR. The event plumes residing ∼600 m above the seafloor had “normal” ³He/heat ratios averaging 0.34 × 10−12 cm³STP cal−1, while the deeper, steady‐state plumes had elevated ³He/heat values ranging from 0.6 to 4.5 × 10−12 cm³STP cal−1. These results generally confirm that elevated ³He/heat ratios in some of the water‐column plumes are a reliable indicator of a recent magmatic/tectonic event. However, in contrast to the 1986 JdFR event, there was no evidence for hydrothermal activity on the CoAxial segment prior to 1993, thereby requiring revision of our models for event plume generation.
Hydrochloric acid and the chlorine budget of the lower stratosphere
Authors: Webster C. R. , May R. D. , Jaeglé L. , Hu H. , Sander S. P. , Gunson M. R. , Toon G. C. , Russell J. M. , Stimpfle R. M. , Koplow J. P. , Salawitch R. J. , Michelsen H. A.
Published: February 2004

Concentrations of HCl measured in the lower stratosphere in 1993 by the ALIAS instrument on the ER‐2 aircraft reveal that only 40% of inorganic chlorine (Cly, inferred from in situ measurements of organic chlorinated source gases) is present as HCl, significantly lower than model predictions. Although the sum of measured HCl, ClO, and ClONO2, the latter inferred from measurements of ClO and NO2, equals Cly to within the uncertainty of measurement, it is systematically less than Cly by 30–50%. This discrepancy suggests that concentrations of ClONO2 may exceed those of HCl near 20 km altitude, consistent with a slower photolysis rate for ClONO2 than calculated using recommended cross sections. Comparison of profiles of HCl measured during 1992 and 1993 at mid‐latitudes by balloon (BLISS and MARKIV), space shuttle (ATMOS), and satellite (HALOE) instruments with the aircraft data reveal an apparent pressure dependence to the HCl to Cly ratio, consistent with a factor of 3–10 reduction in the photolysis rate for ClONO2 at ER‐2 altitudes. However, the diurnal variation of ClO is well‐simulated by models using the recommended photolysis rate, and simultaneous measurements of ClONO2 and HCl at mid‐latitudes by ATMOS and MARKIV report HCl/(HCl+ClONO2) ratios ≥ 50%. Preliminary measurements by ALIAS in the southern hemisphere report HCl/Cly values of about 75%. It is unclear to what extent elevated aerosol surface area, uncertainties in the estimation of Cly, or in the photolysis rates, or missing reactive pathways contribute to our inability to balance the budget of inorganic chlorine using the aircraft measurements.
Spectral analysis of stratospheric field variables
Authors: Rögnvaldsson Ö. E. , Bjarnason G. G.
Published: February 2004

The application of an advanced spectral estimation procedure, the multitaper method, to time series of stratospheric field variables is explored. New information on inherent cyclic variations is extracted from the time series of total ozone at Reykjavík, local 30 hPa temperatures and the equatorial zonal winds. Apart from the annual component, strong signatures of 8.6 months periodicity appear in the Dobson data for the period 1978 to 1990. The same harmonic component is successfully detected in the data from the total ozone mapping spectrometer (TOMS) and the longer term equatorial zonal winds, implying a low latitude dynamically induced cyclic variation of global significance. A spurious semi‐annual signal detected in the Dobson data is due to the non‐homogeneous sampling rate of the instrument throughout the year. The amplitude and phase of both the annual and 8.6 months line components of the Dobson and TOMS time series show excellent agreement.
Geoid lineations of 1000 km wavelength over the central Pacific
Authors: Cazenave A. , Parsons B. , Calcagno P.
Published: February 2004

Altimeter profiles of the ERS‐1 and Topex‐Poseidon satellites have been used to compute a geoid surface from which we have extracted medium‐wavelength geoid anomalies over the central Pacific. In this region, the geoid shows prominent elongated anomalies of 20–40 cm in amplitude, with a spacing of ∼1000 km and oriented N60°W. In the south central Pacific, the Polynesian hotspot swells seem to be located on the linear geoid highs. However, the latter extend much farther eastward, preceeding the active hotspots. To the north, other geoid lines are visible but they do not coincide with known tectonic features. The wavelet transform applied to raw geoid data clearly detects a strong signal at the 1000–1200 km wavelength. The amplitude of the lineations increases with age, by a factor of 3 between 10 and 50 my. Analysis of seafloor topography corrected for age and sediments reveals topography anomalies correlated with the geoid lineations. Admittance and coherence calculations give high coherence (0.9) in the 1000–1200 km waveband and admittance values of 1.5 m/km at 10 my and 3 m/km at 60 my. The mechanism producing the lineations is unclear. Their characteristics however are not incompatible with a convective origin.
A quasi‐permeable barrier in the equatorial lower stratosphere
Authors: Chen Ping , Holton James R. , Swinbank Richard
Published: February 2004

Analyses of global isentropic maps of potential vorticity from the United Kingdom Meteorological Office data assimilation system reveal that there is a region of strong meridional potential vorticity gradients in the equatorial lower stratosphere. A semi‐Lagrangian tracer transport model is used to show that this region of strong PV gradients acts as a quasi‐permeable barrier to cross‐equator mass exchange on isentropic surfaces.
Decline of hexachlorocyclohexane in the Arctic atmosphere and reversal of air‐sea gas exchange
Authors: Bidleman T. F. , Jantunen L. M. , Falconer R. L. , Barrie L. A. , Fellin P.
Published: February 2004

Hexachlorocyclohexanes (HCHs) are the most abundant organochlorine pesticides in the arctic atmosphere and ocean surface water. A compilation of measurements made between 1979–93 from stations in the Canadian and Norwegian Arctic and from cruises in the Bering and Chukchi seas indicates that atmospheric concentrations of α‐HCH have declined significantly (p < 0.01), with a time for 50% decrease of about 4 y in summer‐fall and 6 y in winter‐spring. The 1992–93 levels of about 100 pg m−3 are 2–4 fold lower than values in the mid‐1980s. The trend in γ‐HCH is less pronounced, but a decrease is also suggested from measurements in the Canadian Arctic and the Bering‐Chukchi seas. HCHs in ocean surface water have remained relatively constant since the early 1980s. The decline in atmospheric α‐HCH has reversed the net direction of air‐sea gas exchange to the point where some northern waters are now sources of the pesticide to the atmosphere instead of sinks.
Rare earth element deposition in pelagic sediment at the Cenomanian‐Turonian Boundary, Exmouth Plateau
Authors: Dickens Gerald R. , Owen Robert M.
Published: February 2004

ODP Site 762 (eastern Indian Ocean) includes a section of sediment that spans the Cenomanian‐Turonian Boundary (CTB) and was deposited along a continental margin during a period of widespread oceanic O2 deficiency. The rare earth element (REE) content of pre‐ and post‐boundary sediment is similar to that of present‐day continental slope material deposited in well‐oxygenated seawater, whereas the CTB section is characterized by significantly depleted REE abundances, a bulk Ce anomaly that increases to maximum of 1.0, and a REE pattern that resembles that of present‐day fluvial material. We suggest the change in REE patterns reflects release of scavenged REEs upon reductive dissolution of authigenic Fe‐Mn oxyhydroxides, such that sediment deposited during the CTB is dominated by the lithogenous REE fraction. These results are consistent with recent models concerning pervasive fractionation of Mn and Fe and redirection of Mn in pelagic CTB waters.
Regional estimation of electric fields and currents in the polar ionosphere
Authors: Sato Mariko , Kamide Y. , Richmond A. D. , Brekke A. , Nozawa S.
Published: February 2004

A new technique is presented to estimate electric fields and currents in a localized region of the high‐latitude ionosphere by combining two magnetogram‐inversion algorithms. This paper describes the concept and practical procedures of the method as well as the first results of our efforts in which this new scheme is applied to northern Scandinavia, computing the ionospheric parameters on a small scale. Examining latitudinal profiles of these parameters and precipitating particles, it is found that the region of the most intense precipitation in the morning sector is located equatorward of the region of the strongest electric field. To evaluate the relative importance of ionospheric and magnetospheric effects, the field‐aligned current is divided into two components: (∇Σ) · E and Σ∇ · E. These two components give often the opposite directions in the resultant field‐aligned currents. The relative strength of the two components appears to vary considerably with latitude.
A large self‐potential anomaly on Unzen Volcano, Shimabara Peninsula, Kyushu Island, Japan
Authors: Hashimoto T. , Tanaka Y.
Published: February 2004

Self‐potential (SP) observation was carried out in the summit area of Unzen, one of the active volcanoes in Kyushu island, Japan. We found a positive SP anomaly in the vicinity of the newly extruded lava dome. The potential difference across the anomaly exceeds 1000mV per 500m. Streaming potentials associated with subsurface hydrothermal convection seem to be the most reasonable mechanism for the positive anomaly. In association with the first emergence of the lava dome a sharp increase of SP was detected, which is considered to be a result of the growth of the hydro‐thermal system.
Ion gyroradius‐sized structures and artificial ion conics generated by the Arecibo ionospheric heater
Authors: Keskinen M. J. , Rowland H. L. , Bernhardt P.
Published: February 2004

A model for ionospheric density structures at the O+ ion gyroradius induced by the Arecibo heater during the recent El Coqui campaign has been developed. We find that (1) these density structures can be accounted for by the parametric decay of the high frequency high power Arecibo heater wave into a low frequency wave with frequency at or near the ion cyclotron frequency and high frequency sideband modes (2) the growth rate of this process peaks at scale sizes corresponding to the O+ ion gyroradius (3) computed density fluctuations, using a strong turbulence model, associated with the ion cyclotron modes, in the nonlinear regime, are in the range of 2–12% and (4) anisotropic ion‐conic‐like heating will result from this process. The results of this model are in good agreement with recent El Coqui observations.
Radial evolution of the solar wind from IMP 8 to Voyager 2
Authors: Richardson John D. , Paularena Karolen I. , Lazarus Alan J. , Belcher John W.
Published: February 2004

Voyager 2 and IMP 8 data from 1977 through 1994 are presented and compared. Radial velocity and temperature structures remain intact over the distance from 1 to 43 AU, but density structures do not. Temperature and velocity changes are correlated and nearly in phase at 1 AU, but in the outer heliosphere temperature changes lead velocity changes by tens of days. Solar cycle variations are detected by both spacecraft, with minima in flux density and dynamic pressure near solar maxima. Differences between Voyager 2 and IMP 8 observations near the solar minimum in 1986–1987 are attributed to latitudinal gradients in solar wind properties. Solar rotation variations are often present even at 40 AU. The Voyager 2 temperature profile is best fit with a R−0.49±0.01 decrease, much less steep than an adiabatic profile.
Regional viscosity variations, small‐scale convection and slope of the depth‐age1/2 curve
Authors: Eberle Michael A. , Forsyth Donald W.
Published: February 2004

Cooling plate and cooling half‐space models are commonly employed to describe the depth‐age1/2 curve of seafloor. These conductive models are not able to reproduce the large range of subsidence rates that are observed with reasonable changes in the physical parameters. We examine the influence of small‐scale convection in the mantle on subsidence rates with a one‐dimensional, parameterized‐convection model. The parameterization is based on a recent experimental study of thermal convection in a fluid with temperature‐dependent viscosity. In our model, convection in the asthenosphere begins at the ridge axis, enhancing heat transfer out of the asthenosphere and increasing the subsidence rate above that for simple conductive cooling. Variations in deformation mechanism, temperature and water content of the mantle cause changes in viscosity that affect the vigor of convection and may be responsible for large variations in subsidence rates. Average subsidence rates can range from ∼250–500 m/Ma1/2 when small‐scale convection is considered.
The anomalous component of cosmic rays: Oxygen latitudinal gradient
Authors: Trattner K. J. , Marsden R. G. , Sanderson T. R. , Wenzel K.‐P. , Klecker B. , Hovestadt D.
Published: February 2004

We have studied the latitudinal evolution of the anomalous cosmic ray oxygen component at ULYSSES during its journey to the south pole of the sun. Observations from the COSPIN Low Energy Telescope at ULYSSES are compared with observations in the vicinity of the Earth, provided by the HILT sensor on the SAMPEX spacecraft. We present results in two oxygen energy ranges from 8 to 12 MeV/n and from 12 to 16 MeV/n over a period from August 1992 until end of May 1994. Only quiet time periods which contain no solar energetic particles are taken into account. The data set covers a latitudinal range of about 50° and show for both energy channels a decrease of the SAMPEX to ULYSSES flux ratio equivalent to a positive latitudinal gradient of ∼ 2%/degree.
Earthquake source parameter determination using genetic algorithms
Authors: Zhou Ran , Tajima Fumiko , Stoffa Paul L.
Published: February 2004

Determination of earthquake source parameters is a non‐linear and multi‐dimensional problem in which trade‐off problems between the parameters can be significant and complicated. Accordingly global optimization is desirable to find the best fit parameters in a large model parameter space. We apply Genetic Algorithms (GA) to search for optimal source parameters in a multiparameter space. Results show that the GA method can rapidly explore a large multiparameter model space and find optimal solutions for the source parameters with a relatively small number of function evaluations. Results with the data of the 1988 Nepal‐India border earthquake indicate that GA determined source parameters better explain the observed waveforms.
Response time of the polar ionospheric convection pattern to changes in the north‐south direction of the IMF
Authors: Hairston Marc R. , Heelis Roderick A.
Published: February 2004

A three‐day period from January 27 through January 29,1992 is analyzed using one minute resolution solar wind data from the IMP‐8 satellite and the ionospheric convection pattern data derived from the four operational DMSP satellites. During this period there were several clear reversals of the sign of the Z component of the interplanetary magnetic field (IMF) which is known to have a direct effect on the convection patterns observed in the polar ionosphere. Polar convection patterns observed by the frequent passes of four DMSP satellites are examined following each sign change to determine the time lag between the change in the IMF at the magnetopause and the establishment of a new global convection signature in the ionosphere. After removing the transit time for the IMF to travel from the position of the IMP‐8 satellite to the magnetopause, a further time lag of about 17 to 25 minutes is observed for the five cases where the IMF turned from northward to southward. A longer lag of between 28 and 44 minutes is observed for the two cases where the IMF turned from southward to northward. These lags are interpreted as the inertial response time of the ionosphere in reacting to the change in the IMF.
Three‐dimensional υp and υp/υs structure of the Hengill Triple Junction and Geothermal Area, Iceland, and the repeatability of tomographic inversion
Authors: Foulger G. R. , Miller A. D. , Julian B. R. , Evans J. R.
Published: February 2004

We investigate the crustal structure of the Hengill triple junction in southwestern Iceland, applying tomographic methods to local earthquake data recorded in two field experiments with different network geometries and instrumentation. Data from the two experiments enable us to derive three‐dimensional models of the compressional‐wave speed υp and the wave‐speed ratio υp/υs. Well resolved high‐υp bodies correlate with sites of gabbroic intrusions. A small reduction in υp/υs associated with the high‐temperature part of the geothermal area is probably due to mineral alteration or supercritical fluids. The RMS difference between the two υp models, about 0.26 km s−1, indicates the approximate repeatability that may be expected of good tomographic inversions.
Climatic, eustatic, and tectonic controls on Quaternary deposits and landforms, Red Sea Coast, Egypt
Authors: Arvidson Raymond , Becker Richard , Shanabrook Amy , Luo Wei , Sturchio Neil , Sultan Mohamed , Lotfy Zakaria , Mahmood Abdel Moneim , El Alfy Zeinhom
Published: February 2004

The degree to which local climatic variations, eustatic sea level fluctuations, and tectonic uplift have influenced the development of Quaternary marine and fluvial landforms and deposits along the Red Sea coast, Eastern Desert, Egypt was investigated using a combination of remote sensing and field data, age determinations of corals, and numerical simulations. False color composites generated from Landsat Thematic Mapper and SPOT image data, digital elevation models derived from steieophotogrammetric analysis of SPOT data, and field observations document that a ∼10‐km‐wide swath inland from the coast is covered in many places with coalescing alluvial fans of Quaternary age. Wadis cutting through the fans exhibit several pairs of fluvial terraces, and wadi walls expose alluvium interbedded with coralline limestone deposits. Further, three distinct coral terraces are evident along the coastline. Climatic, eustatic, and tectonic uplift controls on the overall system were simulated using a cellular automata algorithm with the following characteristics: (1) uplift as a function of position and time, as defined by the elevations and ages of corals; (2) climatic variations driven by insolation changes associated with Milankovitch cycles; (3) sea level fluctuations based on U/Tb ages of coral terraces and eustatic data; and (4) parameterized fluvial erosion and deposition. Results imply that the fans and coralline limestones were generated in a setting in which the tectonic uplift rate decreased over the Quaternary to negligible values at present. Coralline limestones formed during eustatic highstands when alluvium was trapped upstream and wadis filled with debris. During lowstands, wadis cut into sedimentary deposits; coupled with continuing uplift, fans were dissected, leaving remnant surfaces, and wadirelated terraces were generated by down cutting. Only landforms from the past three to four eustatic sea level cycles (i.e., ∼300 to 400 kyr) are likely to have survived erosion and deposition associated with fluvial processes.
Simultaneous solution for core magnetic field and fluid flow beneath an electrically conducting mantle
Authors: Voorhies Coerte V. , Nishihama Masahiro
Published: February 2004

The effects of laterally homogeneous mantle electrical conductivity have been included in steady, frozen‐flux core surface flow estimation along with refinements in method and weighting. The refined method allows simultaneous solution for both the initial radial geomagnetic field component at the core‐mantle boundary and the subadjacent fluid motion; it also features Gauss' method for solving the nonlinear inverse problem associated with steady motional induction. The trade‐off between spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models is studied for various mantle conductivity profiles. For simple flow and a fixed initial geomagnetic condition a fairly high deep‐mantle conductivity performs better than either insulating or weakly conducting profiles; however, a thin, very high conductivity layer at the base of the mantle performs almost as well. Simultaneous solution for both initial geomagnetic field and fluid flow reduces the misfit per degree of freedom even more than does changing the mantle conductivity profile. Moreover, when both core field and flow are estimated, the performance of the solutions and the derived flows become insensitive to the conductivity profile.
Mapping the megathrust beneath the northern Gulf of Alaska using wide‐angle seismic data
Authors: Brocher Thomas M. , Fuis Gary S. , Fisher Michael A. , Plafker George , Moses Michael J. , Taber J. John , Christensen Nikolas I.
Published: February 2004

In the northern Gulf of Alaska and Prince William Sound, we have used wide‐angle seismic reflection/refraction profiles, earthquake studies, and laboratory measurements of physical properties to determine the geometry of the Prince William and Yakutat terranes, the Aleutian megathrust, and the subducting Pacific plate. In this complex region, the Yakutat terrane is underthrust beneath the Prince William terrane, and both terranes are interpreted to be underlain by the Pacific plate. Wide‐angle seismic reflection/refraction profiles recorded along five seismic lines are used to unravel this terrane geometry. Modeled velocities in the upper crust of the Prince William terrane (to 18 km depth) agree closely with laboratory velocity measurements of Orca Group phyllites and quartzofeldspathic graywackes (the chief components of the Prince William terrane) to hydrostatic pressures as high as 600 MPa (6 kbar). A landward dipping reflector at depths of 16–24 km is interpreted as the base of the Prince William terrane. This reflector corresponds to the top of the Wadati‐Benioff zone seismicity and is interpreted as the megathrust. Immediately beneath the megathrust is a 4‐km‐thick 6.9‐km/s refractor, which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in Eocene age oceanic crust of the underthrust Yakutat terrane. Wide‐angle seismic data, magnetic anomaly data, and tectonic reconstructions indicate that the Yakutat terrane has been underthrust beneath the Prince William terrane for at least a few hundred kilometers. Wide‐angle seismic data are consistent with a 9° to 10° landward dip of the subducting Pacific plate beneath the outer shelf and slope, distinctly different from the inferred average 3° to 4° dip of the overlying 6.9‐km/s refractor and the Wadati‐Benioff seismic zone beneath the inner shelf. Our preferred interpretation of the geophysical data is that one composite plate, composed of the Pacific plate of a fairly uniform thickness and the Yakutat plate of varying thickness, is subducting beneath southern Alaska.
Characterization of the North Sea chalk by multifractal analysis
Authors: Muller Jiri
Published: February 2004

Scanning electron microscope pictures of thin sections of North Sea chalk samples have been digitized and analyzed by multifractal statistics. It is demonstrated that such analysis can be useful for characterizing different geological chalk formations. In particular, it is observed that the shape of the multifractal spectrum of pore space could be related to permeability values measured from the corresponding core plugs.