bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthetic seismograms through synthetic Franciscan: Insights into factors affecting large‐aperture seismic data
Authors: Lendl Christof , Tréhu Anne M. , Goff John A. , Levander Alan R. , Beaudoin Bruce C.
Published: February 2004

In spite of an order of magnitude increase over the past 15 years in spatial sampling of the wavefield, a major uncertainty in the analysis of active source seismic data remains phase identification. This uncertainty results in part from the wide range of spatial scales of velocity heterogeneity in the crust. Smaller scale variations than those which can be deterministically resolved given the design of a particular seismic experiment can be modeled statistically using geologic constraints. Here we present synthetic seismograms generated from several different realizations of a stochastic model describing the velocity heterogeneity of Franciscan terrane rocks. We compare the results to observed data and to synthetic seismograms generated for a model derived from tomographic inversion of the data in order to obtain qualitative insights into the relative importance of large and small scale velocity heterogeneity. Not surprisingly, the synthetic data for the tomographic model best reproduce observed small‐scale variations in first arrival time, which only occur for particular realizations of the stochastic model. The synthetic seismograms generated for the stochastic models best reproduce the level of signal‐generated noise and suggest that the amplitude of velocity variation locally within the Franciscan is approximately 1 km/s. They also illustrate the effect of a strongly heterogeneous upper and mid‐crust on the amplitude‐versus‐offset pattern of arrivals from the lower crust and upper mantle. These effects may sometimes be interpreted deterministically, leading to biased models or an overly optimistic estimate of lower crustal resolution.
The depth of the earthquake activity in the Central Betics (Southern Spain)
Authors: Morales José , Serrano Inmaculada , Vidal Francisco , Torcal Federico
Published: February 2004

In this study we used relatively velocity independent methods (Wadati diagrams) to estimate and check the focal depth of the seismic activity in a sector of the Betic Cordilleras‐Alboran Sea in Southern Spain. To understand the behaviour of the seismic activity, a total of 335 earthquakes have been analyzed. The most interesting feature found in the analyzed sector is the strong variation of the focal depth. On average, the focal depth distribution in the Granada basin is restricted to h ≤ 20 km. The lower cut‐off in the seismic activity, which is interpreted to mark the brittle‐ductile transition. This cut‐off is coincident with a midcrustal reflector detected in the zone by previous seismic refraction and deep reflection experiments that was interpreted as an extensional detachment. The seismic activity in the Málaga zone reaches depths up to 100 km. In this case the seismic activity affect to the crust and upper mantle. The deepest activity coincides with low values of heat flow, possibly thus indicating a thicking of the lithosphere towards the W. In the centre of the Alborán Sea the activity is shallower and corresponds with a higher heat flow. We think that our results support models that have proposed active delamination towards the West. Such a model may explain the increase in the lithosphere thickness in this direction, which would be congruent with the depth of the earthquakes.
a possible ENSO signal in the Ross Sea
Authors: Ledley Tamara Shapiro , Huang Zhen
Published: February 2004

An examination of the observational record in the Ross Sea over the period 1982–1994 indicates that there is a statistically significant relationship between the Ross Sea sea surface temperature (SST) and the ENSO signal in the tropical Pacific. During this period the Ross Sea SSTs lag the ENSO signal by 2–4 months with the highest correlation coefficient occurring with a lag of 3 months. Observations of the sea ice concentration during 1982–1994 also indicate a relation between the sea ice concentration in the Ross Sea and the ENSO cycle, with reduced sea ice concentrations in El Niño years.
Short term afterslip in the 1994 Sanriku‐Haruka‐Oki Earthquake
Authors: Heki Kosuke , Tamura Yoshiaki
Published: February 2004

The Sanriku‐Haruka‐Oki earthquake, that occurred on December 28, 1994 at the Japan Trench as a typical interplate thrust event, was followed by year‐long afterslip as large as the slip in the high‐speed rupture [Heki et al., 1997]. Here we report on the transition between these, inferred from crustal movements during the five days interval before and after the earthquake. Since this timescale is too long for seismometers but not suitably long for Global Positioning System (GPS), we rely primarily on strainmeter data taken ∼200 km southwest of the epicenter. To confirm that the recorded strain changes are not local disturbances, we compare them with crustal movements derived by high time resolution analyses of GPS data in the same period. The transition to the long term afterslip was gradually achieved by slow fault slip with a time constant of a few hours. The cumulative slow slip in 24 hours amounts to ∼1/3 of the coseismic slip, i.e. we may overestimate the coseismic displacement if we look at GPS data time series composed of daily solutions. The result presented here indicates that a single earthquake could have multiple aspects in its slowness, visible with different seismological and geodetic tools, and we need to use all such data to understand fully such a hybrid earthquake.
A low latitude stratospheric intrusion associated with a cut‐off low
Authors: Kentarchos Anastasios S. , Davies Trevor D. , Zerefos Christos S.
Published: February 2004

An ozonesonde released from Thessaloniki, Greece (40°N, 23°E) in March 1995 indicated a pronounced stratospheric intrusion into the troposphere. This study represents the first identification and analysis of such an event in southeastern Europe. Total ozone data and potential vorticity values of >6 pvu on the 315 K isentrope over Greece supported a low‐level southwards excursion of stratospheric air. The event was associated with the evolution of a cut‐off low over the Balkans. Confirmation is achieved through potential vorticity and specific humidity fields, as well as METEOSAT‐5 images.
Magnetostratigraphy and paleoclimatic interpretation of a continuous 7.2 Ma Late Cenozoic Eolian sediments from the Chinese Loess Plateau
Authors: Sun Donghuai , Shaw John , An Zhisheng , Cheng Minyang , Yue Leping
Published: February 2004

An almost 300m thick eolian sequence of Late Cenozoic sediments, which includes 162.5m of Quaternary loess‐paleosols and 126m of Late Tertiary eolian Red Clay from the central part of the Chinese Loess Plateau, was investigated to determine the magnetostratigraphy. The results show that eolian dust accumulation, and by inference the related East Asia paleomonsoon, had begun by 7.2Ma. As paleomonsoon are largely controlled by the Tibetan Plateau, this implies that the Plateau had reached some critical elevation by 7.2Ma. The section also documents a rapid increase in eolian dust accumulation in the Late Cenozoic at 3.2Ma that is probably due to the influence of global ice volume on the East Asian monsoon.
Test of tomographic models of D″ using differential travel time data
Authors: Bréger Ludovic , Romanowicz Barbara , Vinnik Lev
Published: February 2004

We compare local measurements of SH‐velocity in D″ under the Pacific Ocean with four recent S‐velocity models derived with different techniques. From the local measurements, we find evidence for both strong fast and slow anomalies with an amplitude sometimes exceeding 10%, as well as strong lateral velocity gradients. The tomographic models underestimate the magnitude of the observed anomalies by roughly a factor of 3. The model that best matches both the sign and the localisation of the anomalous structures is exclusively an SH model. This indicates, in particular, that the presence of anisotropy in D″ may not be ignored and that it is important to separate SH and SV contributions in tomographic studies of the lowermost mantle.
A stratospheric excess of CO2‐due to tropical deep convection?
Authors: Harnisch J. , Bischof W. , Borchers R. , Fabian P. , Maiss M.
Published: February 2004

The ‘age’ of stratospheric air is a measure that characterizes the time scales of stratospheric transport. In this work we present and compare age‐values as determined from measurements of CO2 and SF6 on air‐samples obtained between 1987 and 1995 on six flights of our balloon‐borne cryo‐sampler. In the middle stratosphere for five out of six flights we find age‐values that are up to three years lower for CO2 than for SF6. This CO2‐age anomaly corresponds to an excess of 4–5 ppm CO2. Evidence is presented that this excess‐CO2 originates from rapid convective transport of near surface air. Additionally, we show that globally no compact relationship exists between the age of air and mixing‐ratios of N2O.
Sample bias and the scaling of hydraulic conductivity in fractured rock
Authors: Renshaw Carl E.
Published: February 2004

Compilations of the hydraulic conductivity of fractured rock suggest an increase in conductivity between laboratory and field scales. This scaling is inconsistent with recent suggestions that natural fracture networks are near the percolation threshold as the effective conductivity of networks near the percolation threshold decreases with increasing scale. The predicted decrease in conductivity with increasing scale may not be apparent in laboratory data due to a systematic bias in laboratory scale samples; many laboratory scale samples may not contain fractures that are larger than the size of the sample. When this bias is accounted for, the conductivity of simulated networks near the percolation threshold is consistent with the compiled data and increases with the sample dimension. Therefore, observed scaling in conductivity can be interpreted as consistent with the idea that many fracture networks are near the percolation threshold.
Global hexachlorocyclohexane use trends and their impact on the Arctic atmospheric environment
Authors: Li Y. F. , Bidleman T. F. , Barrie L. A. , McConnell L. L.
Published: February 2004

The relationship between the global technical HCH use trends and their impact on the arctic atmospheric environment has been studied. Two significant drops in global technical HCH usage were identified. In 1983, China banned the use of technical HCH. This represented the largest drop ever in global use rates. In 1990 India stopped technical HCH usage in agriculture and the former Soviet Union banned the use of technical HCH. Since 1990, India has been the biggest user of technical HCH in the world. Significant drops in atmospheric α‐HCH in the arctic were observed between 1982 and 1983, and again between 1990 and 1992. The rapid response in atmospheric concentrations to usage is encouraging; however, since α‐HCH concentrations in the arctic waters have remained relatively unchanged, the decline in atmospheric α‐HCH has reversed the net direction of air‐sea gas flux. The accumulated mass in oceans and large lakes may represent a new source of HCH to the arctic atmosphere.
Evolution of the morphology and microphysics of contrail cirrus from airborne remote sensing
Authors: Spinhirne James D. , Hart William D. , Duda David P.
Published: February 2004

Contrail cirrus were observed by airborne lidar and multispectral infrared remote sensing. Results of microphysical analysis were obtained for two cases of persistent contrail cirrus which form in the same area but of differing maturity. The cross section area of the older contrail was 18 times greater than the newer case. In both cases the inferred number of particles per distance along the flight track dimension of contrails was nearly similar, close to 26 ×1011/m. The average liquid water concentration of the two cases was similar, but the amount of water per distance along the flight line dimension was much greater for the older contrail, 34 g/cm and 680 g/cm. In both cases the amount of liquid water in the contrail was much greater than the amount generated by the aircraft which produced the contrail. These results are consistent with a model of contrail evolution where the effect of the jet exhaust is to generate a dense concentration of small ice particles. Mixing then maintains the total number of particles, and particles grow to the extent of the available vapor in the ambient air.
Same night observations of spread‐F by the Jicamarca Radio Observatory in Peru and CUPRI in Alcântara, Brazil
Authors: Swartz Wesley E. , Woodman Ronald F.
Published: February 2004

50‐MHz echoes from equatorial spread‐F were observed on several nights by both the Jicamarca Radio Observatory (JRO) in Peru and the Cornell University Portable Radar Interferometer (CUPRI) in Alcântara, Brazil. Although little detailed correlation is expected between sites separated by such large distances, the night of October 17, 1994 shows some remarkable similarities between Peru and Brazil. On this night spread‐F commenced at both Jicamarca and Alcântara as thin bottomside layers situated near 320 km altitude at nearly the same local times. Later, major plumes erupted that reached to over 1000 km altitude at both sites. Since plumes normally drift west to east, these are obviously not the same structures but the similarities indicate that conditions for spawning them must have been coincidentally very similar on this night. The next two nights which produced plumes over Brazil, but only bottomside layers over Peru, emphasize that local conditions on the same night can be very different at the two locations. The importance of having a sufficiently wide beam for exploring spread‐F over a large altitude range at the Alcântara site is also explored.
Satellite detection of cloud liquid water over land using polarization differences at 85.5 GHz
Authors: Combs Cynthia L. , Greenwald Thomas J. , Jones Andrew S. , Randel David L. , Vonder Haar Thomas H.
Published: February 2004

A physical retrieval method is described that detects cloud liquid water path (LWP) over land using polarization difference measurements from the Special Sensor Microwave Imager (SSM/I). The brightness temperature difference at 85.5 GHz is divided by the surface emissivity polarization difference to define a normalized polarization difference (NPD). The SSM/I NPD observations are compared to coincident observations of liquid water path from upward looking ground‐based microwave radiometers in Oklahoma and Kansas for November and December of 1994. The comparisons reveal a moderate correlation and nonlinear relationship (which is expected from theory) between the 85.5 GHz NPD and liquid water path. The NPD method is anticipated to be valid over a broad region of the U.S., excluding the southern Mississippi Valley where the surface emissivity polarization differences are negligible. The observational results indicate that the NPD method holds promise for future routine satellite detection and retrieval of cloud LWP over land.
On the radiative properties of contrail cirrus
Authors: Liou K. N. , Yang P. , Takano Y. , Sassen K. , Charlock T. , Arnott W.
Published: February 2004

Using the observed ice crystal size distribution in contrail cirrus from SUCCESS, we have carried out the scattering and absorption calculations based on a unified theory for light scattering by ice crystals covering all sizes and shapes. We illustrate the effects of ice crystal size and surface roughness on the scattering phase function features for remote sensing applications. The extinction coefficient and single‐scattering albedo exhibit a minimum feature at 2.85 µm, referred to as the Christiansen effect, which is particularly pronounced for clouds consisting of a significant number of small ice crystals. Based on a line‐by‐line equivalent solar model, we show from spectral curves that cloud reflection increases as ice crystal sizes become smaller, but the cloud absorption increase is only evident for wavelengths longer than about 2.7 µm. The ice crystal shape has a substantial effect on the cloud reflection and absorption for a given size; more complex ice particles reflect more solar radiation. Finally, we propose a contrail cirrus cloud model consisting of a combination of bullet rosettes (50%), hollow columns (30%), and plates (20%), with sizes ranging from 1 to 90 µm in association with radiation perturbation studies.
Transient solar influence on terrestrial temperature fluctuations
Authors: Lawrence J. K. , Ruzmaikin A. A.
Published: February 2004

We argue that the 11‐year periodic solar cycle input assisted by intrinsic climatic noise can produce transient correlations between activity on the Sun and the Earth's temperature. The argument is based on wavelet analysis of simultaneous, 140‐year time series of terrestrial global temperature and solar activity. It is supported by a simple model utilizing the concept of “stochastic resonance,” a unique effect of amplification of a weak, periodic signal by a noisy, nonlinear system.
The role of ammoniated aerosols in cirrus cloud nucleation
Authors: Tabazadeh Azadeh , Toon Owen B.
Published: February 2004

Recent results obtained during the SUCCESS experiment show that aerosols containing ammonium ion are commonly found in the upper troposphere and therefore it is of great importance to investigate the ice freezing properties of such aerosols. For this purpose we present mathematical expressions, derived from recent model calculations and laboratory data, for calculating the deliquescence relative humidity of (NH4)2SO4, NH4HSO4 and NH4NO3 under upper tropospheric conditions. Using these functions we define the temperature and relative humidity requirements for the deliquescence of dry particles in the upper troposphere. Further using a thermodynamic model of the H2SO4/HNO3/NH3/ H2O system, we calculate the variation in the supercooled aerosol composition and saturation ratios of various ionic salts in solution as a function of temperature and relative humidity. Using these values we show salt crystallization in the upper troposphere may occur in two distinct type of ammoniated aerosol solutions. First (NH4)3H(SO4)2 (letovicite) may crystallize in acidic solutions of (NH4)2SO4. Second double salts of (NH4)2SO4 and NH4NO3 may crystallize in nearly neutralized solutions of (NH4)2SO4 and NH4NO3. Finally we discuss how the presence of crystalline salt surfaces in aqueous solutions may alter the mode of cirrus cloud formation from homogeneous to heterogeneous freezing nucleation.
Sources and chemistry of NOx in the upper troposphere over the United States
Authors: Jaeglé L. , Jacob D. J. , Wang Y. , Weinheimer A. J. , Ridley B. A. , Campos T. L. , Sachse G. W. , Hagen D. E.
Published: February 2004

The origin of NOx in the upper troposphere over the central United States is examined using aircraft observations obtained during the SUCCESS campaign in April–May of 1996. Correlations between NOy (sum of NOx and its oxidation products) and CO at 8–12 km altitude indicate that NOx originates primarily from convective transport of polluted boundary layer air. Lightning and aircraft emissions appear to be only minor sources of NOx. Chemical steady state model calculations constrained by local observations of NO underestimate the measured NOx/NOy concentration ratio at 8–12 km altitude by a factor of two on average. The magnitude of the underestimate is correlated with concentrations of condensation nuclei, which we take as a proxy for the age of air in the upper troposphere. We conclude that the NOx/NOy ratio is maintained above chemical steady state by frequent convective injections of fresh NOx from the polluted boundary layer and by the long lifetime of NOx in the upper troposphere (5–10 days). In contrast to previous studies, we find no evidence for fast heterogeneous recycling from HNO3 to NOx in the upper troposphere.
Airborne scanning lidar observations of aircraft contrails and cirrus clouds during SUCCESS
Authors: Uthe Edward E. , Nielsen Norman B. , Osberg Terje E.
Published: February 2004

An angular scanning backscatter lidar was deployed on the NASA DC‐8 research aircraft as part of SUCCESS. The lidar viewing direction could be continuously scanned from vertically upward to forward to vertically downward. Real‐time pictorial displays generated from lidar signatures were used to locate clouds and contrails above, ahead of, and below the DC‐8; to depict their spatial structure; and to help select DC‐8 altitudes for achieving optimum sampling by onboard in situ sensors. The lidar data are being analyzed to establish their value in the interpretation and extension of the in situ sensor databases. Data examples are presented that illustrate (1) correlation with particulate, gas, and radiometric measurements made by onboard sensors, (2) discrimination and identification between contrails observed by the onboard sensors, (3) a 13.1 km altitude layer that exhibits greatly enhanced vertical backscatter relative to off‐vertical backscatter, and (4) mapping of vertical distributions of individual precipitating ice crystals and their capture by cloud layers.
Modeling errors in diffuse‐sky radiation: Vector vs scalar treatment
Authors: Lacis A. A. , Chowdhary J. , Mishchenko M. I. , Cairns B.
Published: February 2004

Radiative transfer calculations that utilize the scalar approximation of light produce intensity errors as large as 10% in the case of pure Rayleigh scattering. This modeling error, which arises primarily from second order scattering, is greatly reduced for flux and albedo results because of error cancellation brought about by integration over scattering angle. However, polarized light scattered from an underlying ocean surface, or from atmospheric aerosols, interacts with the pattern of Rayleigh scattered polarization to distort the error cancellation and thus incur larger flux and albedo errors. While addition of scattered radiation from clouds, aerosols or ground surface into the Rayleigh atmosphere tends to reduce the magnitude of scalar approximation intensity errors, the scalar errors in fluxes and albedos are not proportionately reduced, but are actually increased.
Symmetry and disorder in garnets on the pyrope (Mg3Al2Si3O12)‐majorite (Mg4Si4O12) join from Cr3+ luminescence spectroscopy
Authors: Quérel G. , Reynard B.
Published: February 2004

Cr3+‐bearing garnets with compositions of majorite, Mj72, Mj23, Mj21 and pyrope were studied by luminescence spectroscopy. The tetragonal distortion of the pure majorite sample is marked by the occurrence of two ²E doublets corresponding to Cr3+ replacing Si and Mg in the Oc1 and Oc2 octahedral sites. The other garnets are cubic and a single ²E doublet is observed. The width of the luminescence lines in the disordered cubic majorites is proportional to the number of Mg, Si, Al random configurations around the Cr3+ sites. In the tetragonal majorite, the linewidth is only slightly higher than that in pyrope, indicating ordering of Mg and Si on the octahedral sites. These results are consistent with previous X‐ray diffraction and NMR spectroscopic studies, and show that Cr3+ luminescence spectroscopy is a potential tool for studying short range order in high‐pressure silicate solid solutions relevant to the Earth's mantle.
Anthropogenic and solar components of hemispheric 14C
Authors: Stuiver Minze , Braziunas Thomas F.
Published: February 2004

A variable solar (helio‐magnetic) modulation of the cosmic ray flux causes atmospheric 14C to change on a decadal to century timescale. Ocean‐atmosphere exchange rates and atmospheric circulation patterns play a role in Northern‐Southern Hemispheric 14C offsets and regional 14C differences that are significant for radiocarbon dating. Time dependent radiocarbon age differences, relative to Washington, were determined for Alaska (A.D. 1884–1932), South Chile (A.D. 1850–1952 and A.D. 1670–1722), Tasmania (A.D. 1895–1950) and Siberia (A.D. 1545–1715). Twentieth century fossil fuel CO2, lacking 14C and mostly released in the Northern Hemisphere (N), has entered the Southern Hemisphere (S) by atmospheric N‐S exchange. Regional 14C time series show that initial (19th century) positive N‐S 14C offsets switch to negative values by AD 1940. First order carbon reservoir modeling predicts such crossovers.
Three tails of comet Hale‐Bopp
Authors: Wilson J. K. , Baumgardner J. , Mendillo M.
Published: February 2004

We report on the imaging detection of the sodium tail of comet Hale‐Bopp using large field‐of‐view, multi‐wavelength observations from the McDonald Observatory in March 1997. We subtract off‐band images from sodium‐filter images to obtain the atomic sodium tail, and we find it to be more extended in width than either the dust or ion tails. The cross‐tail integrated brightness of the sodium tail increases with distance from the nucleus for at least 8 million km. The direction of the sodium tail in our data is different from the ion tail, the dust tail, and the sodium tails imaged at different times by other observers. To account for these characteristics, there must be an extended source of sodium atoms tailward of the comet nucleus. We propose that the sodium tail in March 1997 was produced by the release of sodium atoms from grains in the dust tail.
The correlation of AKR waves with precipitating electrons as determined by plasma wave and X‐ray image data from the POLAR spacecraft
Authors: Imhof W. L. , Chenette D. L. , Datlowe D. W. , Mobilia J. , Walt M. , Anderson R. R.
Published: February 2004

Simultaneous measurements are presented of the intensities of AKR waves in the 60 kHz to 800 kHz range and the local time distributions of the fluxes of electrons precipitating into the atmosphere using wave and bremsstrahlung x‐ray data acquired on the POLAR spacecraft. The images of 2 to 12 keV x‐ray emission measured with the PIXIE spectrometer were used to obtain the positions and intensities of the electron precipitation regions. Electric field wave measurements of the AKR were obtained with the Plasma Wave Instrument (PWI). Data are presented from a satellite pass on 19 April 1996 in which there were several short term enhancements in the intensities of both waves and x‐rays. The AKR electric field strengths were correlated with x‐rays emitted over a six hour local time range in the pre‐midnight sector with a correlation coefficient of 0.51. At the times of some of the rapid increases in the x‐ray fluxes the AKR intensities exhibited similar sharp onsets.
Variations of total electron content during geomagnetic disturbances: A model/observation comparison
Authors: Lu G. , Pi X. , Richmond A. D. , Roble R. G.
Published: February 2004

This paper studies the ionospheric response to the major geomagnetic storm of October 18–19, 1995, using the NCAR TIE‐GCM simulations and the global ionospheric maps (GIM) of total electron content (TEC) observations from the worldwide network of Global Positioning System (GPS) receivers. The TIE‐GCM results show a good agreement with the GPS‐GIM in terms of simulating storm‐time TEC disturbances over the polar regions. The model indicates that the increase of electron density in the high‐latitude E and lower F regions below 200 km is directly related to the magnetospheric energy input through auroral precipitation to the ionosphere, while the decrease of TEC is mainly due to the increase in O2 and N2 densities in the upper F region above 200 km. During the recovery phase, both the TIE‐GCM and GPS‐GIM reveal a distinct hemispheric asymmetry in the TEC integrated above |50°| magnetic latitude, with a 20% decrease in the southern (summer‐like) hemisphere and a 30% increase in the northern (winter‐like) hemisphere.
Characteristics of the Indo‐Pacific Throughflow in the eastern Indian Ocean
Authors: Bray N. A. , Wijffels S. E. , Chong J. C. , Fieux M. , Hautala S. , Meyers G. , Morawitz W. M. L.
Published: February 2004

Five hydrographic sections across the Indonesian throughflow from the World Ocean Circulation Experiment (WOCE) and the Java Australia Dynamic Experiment (JADE) are used to examine seasonal water mass and density distributions, including semiannual variation of the South Java Current. Altimetric sea surface height variations match dynamic height along each WOCE cruise track remarkably well. They also reveal energetic eddies and meanders of the current system south of Java with intraseasonal (60‐day) changes in surface dynamic height comparable in magnitude to seasonal and interannual variations. Through the formation of discrete eddies and meanders of the large‐scale zonal flows, intraseasonal fluctuations account for much of the variation in water mass structure between cruises.
Observations of tidally coherent diurnal and semidiurnal variations in the geocenter
Authors: Watkins Michael M. , Eanes Richard J.
Published: February 2004

The center of mass of the Earth, about which a satellite orbits, is determined by the mass distribution of the solid Earth (including the Earth's interior), the oceans, and the atmosphere. The tracking sites, however, are all located on the lithosphere only, and so the network origin will not generally coincide with the center of mass. Changes in the vector offset between these two points, referred to as apparent geocenter motion, are due to global scale movement of mass. In this study we have extended the approach used by space geodetic techniques for determination of tidally coherent nearly diurnal and semidiurnal variations in the Earth's orientation to include geocenter variations due to ocean tides. We demonstrate the weaker observability of nearly diurnal retrograde motions in x and y, and hence the utility of their separation from the other equatorial terms. We find statistically significant terms at the 99.5% level due to O1, P1, K1, and M2. The solutions, which indicate geocenter motions at the few millimeter level per tide, compare well with predicted values from theoretical ocean tide models and from TOPEX/Poseidon radar altimetry.
Improving the vertical resolution of ionospheric tomography with GPS Occultations
Authors: Rius A. , Ruffini G. , Cucurull L.
Published: February 2004

We combine GPS/MET data from 29 occultations and IGS ground data collected from 160 stations around the world to perform stochastic tomography of the ionosphere with a 4×20×20 global grid of voxels extending from 200 to 650 km above the mean surface of the Earth. A correlation functional approach that limits the spatial high frequency content of the images is used, and a Kalman filter is applied in the time direction. The combination of ground and occultation data and the use of smoothing techniques is robust enough for vertical resolution in this four‐layer model analysis. We discuss the role of noise on the choice of the correct range of eigenvalues in the inversion problem, and the impact of occultation data, showing that ground data alone is insufficient for vertical resolution even in a three‐layer, noise‐less simulation.
X‐ray spectral observations of a solar active region corona
Authors: Sterling Alphonse C.
Published: February 2004

We study the X‐ray flux and electron temperature variation of the corona above a solar active region. Electron temperatures are determined using data from the S xv channel of the Bragg crystal spectrometer (BCS) instrument on board the Yohkoh satellite. The active region, designated NOAA AR 7999, rotated from just inside the west solar limb to beyond the limb over the period December 2–4, 1996, allowing us to map the electron temperature as a function of height in the active region's corona. Consistent with previous findings, we find two components to the active region coronal temperature, with a hotter (Te ≳5.0 MK) component due to transient flares and microflares, and a cooler component present in between the times of the microflares. There is a steady decrease in the frequency of occurrence of flares and microflares as the occultation progresses, implying that the flaring structures are low lying. For the cool component, the average electron temperature is about 4.5 MK when the region is just inside the limb, and gradually decreases to under 3.0 MK as the region rotates around the limb, indicating that temperature decreases with height.
Solar‐wind acceleration by energetic particles
Authors: Florinski V. , Jokipii J. R.
Published: February 2004

We present results from a two‐fluid model of the solar wind which includes the thermal gas and a non‐thermal particle population. Low‐energy solar cosmic rays (10 to 100 keV) can help accelerate the wind by transferring some of their momentum to comoving magnetic irregularities. We find that increases in the wind velocity of the order of 100 km/s are not uncommon when a small amount (approximately 0.01 of the total pressure at the Sun) of cosmic rays is added in the solar corona. We discuss possible cosmic ray spectra and interplanetary diffusion ranges necessary to produce this effect. Cosmic ray spectra obtained from our model are consistent with experimental data at the 1 AU distance.
Seismic velocity structure of the Puget Sound Region from 3‐D Non‐linear tomography
Authors: Symons Neill P. , Crosson Robert S.
Published: February 2004

Seismic P‐wave velocity structure is estimated for the Puget Sound basin region using iterative, non‐linear tomographic inversion based on finite‐difference travel‐time calculations. Local earthquakes and explosion arrival times are used to image the three‐dimensional velocity structure to a depth of approximately 60 km. Our earthquake data set comprises approximately 3000 well‐located digitally recorded earthquakes collected by the Pacific Northwest Seismograph Network (PNSN) over the 17 year period from 1980 to 1996. Additional constraint on velocity is provided by including travel‐times, with known locations and origin times, from controlled‐source seismic profiles and also by including model interpretations along one profile directly in the inversion. We find good correspondence between the prominent features imaged by this study and previous geological and geophysical interpretations. The method is effective in resolving high resolution structure by combining earthquake, explosion, and active source interpretations into a single velocity structure model.
Anomalously low ozone over the Arctic
Authors: Newman Paul A. , Gleason James F. , McPeters Richard D. , Stolarski Richard S.
Published: February 2004

Total ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instruments during March 1997 reveal an extensive region of low column densities in the Arctic region centered near the north pole. Values were below 250 Dobson units for nearly a two week period during this period, and were correlated with the position of the northern lower stratospheric polar vortex. The March 1997 average total ozone column densities were more than 30% lower than the average of column densities observed during the 1979–1982 March period.
Predicting decadal variations in Atlantic tropical cyclone numbers and Australian rainfall
Authors: Walsh Kevin J. E. , Kleeman Richard
Published: February 2004

Significant relationships on time scales longer than 5 years are demonstrated between northern Pacific sea surface temperatures, North Atlantic tropical cyclone numbers, and northeastern Australian rainfall. In agreement with previous work, the global nature of these correlations suggests that they are indications of an intrinsic interdecadal mode of oscillation in the global ocean‐atmosphere system. The persistence in time of the sea surface temperature variations may enable forecasts to be made of North Atlantic tropical cyclone numbers several years in advance. A similar capability for the prediction of north‐eastern Australian rainfall is also shown. Thus these relationships may be useful as an aid in long‐term planning decisions.
An estimate of the effect of unresolved structure on modeled ozone loss from aircraft observations of ClO
Authors: Sparling L. C. , Douglass A. R. , Schoeberl M. R.
Published: February 2004

High resolution aircraft observations of ClO are used to estimate how much the local ozone loss rate due to the ClO dimer reaction is underestimated when small scale structure in ClO is not fully resolved. A local resolution error in the ozone loss rate, for a model with grid scale L, is defined in terms of the variance of the observations within data segments of length L. Errors in the northern and southern winter polar regions are similiar in magnitude, and are largest for gridboxes near the vortex edge. At the edge of the Arctic vortex, the resolution error increases linearly from 5–35% as the grid scale increases from 100–500 km. Errors are much smaller inside the vortex where [ClO]≥0.7 ppbv, increasing from 2–10% over the same range of grid scales.
Ionospheric effects of magnetopause reconnection observed using ionospheric tomography
Authors: Walker I. K. , Moen J. , Mitchell C. N. , Kersley L. , Sandholt P. E.
Published: February 2004

This paper presents key signatures in the spatial distribution of ionospheric electron density attributed to solar wind particles injected following magnetopause reconnection. Using established criteria, optical auroral emissions have been used to identify the reconnection process. A simultaneous image of electron density shows an enhanced region with a northward gradient in F‐layer peak height that may be linked to dispersion of precipitating soft ions. In addition, an E‐region enhancement at lower latitude can be associated with precipitation of pitch‐angle scattered ring current electrons on closed field lines. A narrow density enhancement extending into the high topside ionosphere may indicate the signature of a field‐aligned current. Taken together, the features enable identification in the ionosphere of the open/closed field‐line boundary.
The symmetry of the heliospheric current sheet as observed by Ulysses during the Fast Latitude Scan
Authors: Erdős Géza , Balogh André
Published: February 2004

The position of the heliospheric current sheet is investigated, based on Ulysses observations during the fast latitude scan from the South pole (in September, 1994) to the North pole (in July, 1995). The location of the current sheet on the source surface was calculated by mapping back the magnetic field vectors measured by Ulysses along the solar wind flowline. The areas on the source surface occupied by the positive and negative sectors have been determined from the observations and no significant difference in the areas has been found. Thus, during the Ulysses fast latitude scan, the heliospheric current sheet had no significant latitudinal offset; the assumption of such an offset had been used on the interpretation of galactic cosmic ray fluxes measured aboard Ulysses. We use the previously published findings concerning the heliolatitude‐independence of the radial component of the heliospheric magnetic field and its approximately constant magnitude in the two solar hemispheres to argue that any north‐south asymmetry in the sector structure is unlikely, as there must be a flux balance between the different polarity sectors. The results are compared to the predictions of the Standford University models, based on photospheric observations, concluding that coronal models should be revised to account for Ulysses observations.
Variational assimilation of Geosat altimeter data into a two‐layer quasi‐geostrophic model over the Newfoundland ridge and basin
Authors: Cong L. Z. , Ikeda M. , Hendry R. M.
Published: February 2004

A variational data assimilation model is examined as a method for reconstructing ocean current fields from sea surface dynamic heights observed with the Geosat altimeter. A two‐layer, eddy‐resolving, quasi‐geostrophic model with realistic bottom topography is used. The space of control variables is composed of initial as well as boundary conditions in both layers. A cost function is defined as a misfit between the altimeter data and the assimilation solution. Regularization terms are also included in the cost function in a form of second‐order spatial and temporal derivatives of the stream function. A preparation step is taken to construct a first guess for the variational method; prior to the full assimilation, the lower‐order assimilation is carried out with fewer control variables included. Focus is put on three potential problems: (1) the horizontal resolution of the data, (2) the vertical projection of sea surface information into the lower ocean, and (3) nonlinear behavior of the mesoscale variability. This method is applied to the energetic mesoscale features in the Gulf Stream extension over the Newfoundland ridge and basin. The ocean current field constructed in the model through the data assimilation is significantly correlated with the velocity field directly observed with current meters, both near the surface and at subsurface (near the bottom). The near‐surface velocity is highly correlated between the data and the model, while the correlation coefficients of the subsurface velocity components are less than 0.5, with nearly unity regression slopes. Thus it is clear that the data contain part unexplained by the quasi‐geostrophic model. The horizontal data resolution is adequate for describing the mesoscale variability near the surface, and it is feasible to extract the quasi‐geostrophic component in the lower ocean velocity field. Once nonlinearity becomes very intense, local minima in the cost function prevent convergence to a unique solution. We show that remotely collected satellite data can be utilized for monitoring the global ocean at all depths.
Paleoclimatic variability inferred from the spectral analysis of Greenland and Antarctic ice‐core data
Authors: Yiou R. , Fuhrer K. , Meeker L. D. , Jouzel J. , Johnsen S. , Mayewski P. A.
Published: February 2004

Paleoclimate variations occur at various time scales, between a few centuries for the Heinrich events and several hundreds of millenia for the glacial to interglacial variations. The recent ice cores from Greenland (Greenland Ice Core Project and Greenland Ice Sheet Project 2) and Antarctica (Vostok) span at least one glacial oscillation and provide many opportunities to investigate climate variations with a very fine resolution. The joint study of cores from both hemispheres allows us to distinguish between the sources of variability and helps to propose mechanisms of variations for the different time scales involved. The climate proxies we analyze are inferred from δ18O and δD for temperature and chemical species (such as calcium) for the joint behavior of the major ions in the atmosphere, which yield an estimate of the polar circulation index. Those data provide time series of climatic variables from which we extract the information on the dynamics of the underlying system. We used several independent spectral analysis techniques, to reduce the possibility of spurious results. Those methods encompass the multitaper spectral analysis, singular‐spectrum analysis, maximum entropy method, principal component analysis, minimum bias spectral estimates, and digital filter reconstructions. Our results show some differences between the two hemispheres in the slow variability associated with the astronomical forcing. Common features found in the three ice‐core records occur on shorter periods, between 1 and 7 kyr. The Holocene also shows recurrent common patterns between Greenland and Antarctica. We propose and discuss mechanisms to explain such behavior.
New approaches to CO2 analysis in polar ice cores
Authors: Wilson A. T. , Long Austin
Published: February 2004

Gases trapped in polar ice sheets provide a means of obtaining samples of Earth's atmosphere from times in the past. We describe geochemical processes affecting gases trapped in ice and discuss how these mechanisms affect the composition of gas extracted from polar ice cores. We review the methods for recovering samples of paleo‐atmospheres from ice cores and discuss the suitability of each method in different applications. We conclude that extracting gas by mechanical disintegration (e.g., crushing, grinding, shaving, or milling) is most easily performed on relatively young ice in which the content of the bubbles is still representative of the original atmosphere. For ice recovered from greater depth, where some or all of the gases have been driven into the ice matrix itself, more care must be taken in interpreting the analysis of the extracted gases.
Chlorine 36 fallout in the Summit Greenland Ice Core Project ice core
Authors: Baumgartner S. , Beer J. , Suter M. , Dittrich‐Hannen B. , Synal H.‐A. , Kubik P. W. , Hammer C. , Johnsen S.
Published: February 2004

The natural cosmogenic radioisotope 36Cl has been measured in the Greenland Ice Core Project ice core from Summit, Greenland. Generally, a good correlation between 36Cl and δ18O was found, even in glacial periods with strong δ18O oscillations. The 36Cl flux is calculated assuming an exponential relationship between accumulation rate and δ18O. A prominent excursion is detected between 35 and 40 kyr B.P., which coincides with a similar 10Be peak measured in the same samples. This sheds light on possible causes of this excursion. In the lowest part of the ice core the flux decreases smoothly. This can be interpreted as radioactive decay of 36Cl (T1/2=300 kyr).
Seasonal cycle of the East Australian Current
Authors: Ridgway K. R. , Godfrey J. S.
Published: February 2004

The seasonal circulation associated with the East Australian Current is examined using a set of steric heights derived from the historical hydrology and expendable bathythermograph data collected in the region. The data are separated into a network of regional bins allowing for known oceanographic and topographic features and a two‐harmonic best fit to the seasonal cycle is obtained in each bin. Maps of the annual‐frequency component of the surface and depth‐integrated steric heights (h and P) show the development and progression of the EAC flow regime through a complete seasonal cycle. The EAC has a strong seasonal cycle from 25°S to 45°S, with strongest southward flow in austral summer. The seasonal cycle in surface flow over the continental shelf is documented by two independent methods, geostrophically, using cross shelf sea level gradients derived from coastal tide gauge data and steric heights at the continental shelf edge, and directly from merchant ship observations. The two estimates are in good agreement. The seasonal cycle in the EAC is more pronounced than in other midlatitude western boundary currents for which data are available. At 28°S, the strength of the total Tasman Sea transport (southward flow) varies between a minimum transport of 7 Sv in winter (July) to a maximum of 16 Sv in summer. The semiannual frequency components of h and P is important near 30°S near the EAC outflow, but not elsewhere. The seasonal cycle of the EAC is not due to strong seasonal variations in Tasman Sea wind stress curl east of the region of interest. Seasonally reversing zonal flows occur offshore north of 25°S, which are apparently locally forced by reversing wind stress curls; but if these flows were fed from the south by the EAC current system, the EAC would have to be weaker in summer, not stronger. The Leeuwin Current Extension along Australia's west and south coasts may pass up the east coast of Australia, providing an important contribution to the enhanced southward flow of the EAC in summer. The vigorous anticyclonic eddies of the EAC also show a marked seasonal cycle, and this is probably an important part of the mechanism for the strong seasonal cycle of the EAC south of 25°S. The location of the strongest anticyclonic eddy in the EAC moves steadily southward throughout the summer season, and the phase of the coastal EAC appears also to move southward, contrary to the expectations of linear theory and to the hypothesis that the Leeuwin Current Extension is the major cause of the seasonal cycle.