bluuebunny/crossref_metadata_2025 huggingface dataset:
Mid‐tropospheric aerosol backscatter background mode over the Pacific Ocean at 9.1 µm wavelength
Authors: Rothermel Jeffry , Bowdle David A. , Srivastava Vandana
Published: February 2004

The first atmospheric aerosol backscatter measurements at the rare isotope CO2 wavelength of 9.1 µm were obtained in the troposphere over the Pacific Ocean in May–June 1990. A distinct aerosol backscatter background mode was found in both hemispheres with modal value ∼10−10 m²kg−1sr−1 (backscatter normalized by air density), remarkably consistent with ground‐based and airborne backscatter climatologies collected at other wavelengths, locations, and times.
Improvements in the simulation of surface climatology over the European region with a nested modeling system
Authors: Giorgi F. , Marinucci M. R.
Published: February 2004

In this paper we report on the results of a present day climate simulation over the European region using a Regional Climate Model (RegCM) nested within a General Circulation Model (GCM). Precipitation and surface air temperature results over different European subregions are compared with observations and with results from previous simulations by Marinucci and Giorgi [1992] and other modeling groups. Based on a number of measures of simulation skill such as monthly and seasonal biases and spatial correlations, the RegCM performance is substantially improved compared to that of Marinucci and Giorgi [1992], it compares well with that of other recent experiments and it is better than that of the driving GCM. Seasonal precipitation and surface air temperature biases over European subregions of 105–107 km² size are mostly in the range of ±25% of observations and ±2°C, respectively. These biases appear to represent the level of accuracy characteristic of present regional modeling systems over Europe at the seasonal and regional scale.
Detection of and response to a probable volcanogenic T‐wave event swarm on the Western Blanco Transform Fault Zone
Authors: Dziak R. P. , Fox C. G. , Embley R. W. , Lupton J. E. , Johnson G. C. , Chadwick W. W. , Koshi R. A.
Published: February 2004

The East Blanco Depression (EBD), a pull‐apart basin within the western Blanco Transform Fault Zone (BTFZ), was the site of an intense earthquake T‐wave swarm that began at 1317Z on January 9, 1994. Although technically generated earthquakes occur frequently along the BTFZ, this swarm was unusual in that it was preceded and accompanied by periodic, low‐frequency, long‐duration acoustic signals, that originated from near the swarm epicenters. These tremor‐like signals were very similar in character to acoustic energy produced by a shallow‐submarine eruption near Socorro Island, a seamount several hundred km west of Baja, California. The ∼69 earthquakes and ∼400 tremor‐like events at the EBD occurred sporadically, with two periods of peak activity occurring between January 5–16 and 27–31. The swarm‐like character of the earthquakes and the similarity of the tremor activity to the Socorro eruption indicated that the EBD was undergoing an intrusion or eruption episode. On January 27, six CTD/rosette casts were conducted at the site. Water samples from two of the stations yielded anomalous ³He concentrations, with maxima at ∼2800 m depth over the main basin. In June 1994 two camera tows within the basin yielded evidence of pillow‐lava volcanism and hydrothermal deposits, but no conclusive evidence of a recent seafloor eruption. In September 1994, deployments of the U.S. Navy's Advanced Tethered Vehicle resulted in the discovery of an active hydrothermal mound on the flanks of a pillow‐lava volcano. The hydrothermal mound consists of Fe‐rich hydrothermal precipitate and bacterial mats. Temperatures to 60°C were measured 30 cm below the surface. This is the first discovery of active hydrothermal vents along an oceanic fracture zone. Although no conclusive evidence of volcanic activity associated with the T‐wave event swarm was found during these response efforts, the EBD has been the site of recent seafloor eruptions.
Is internal friction friction?
Authors: Savage J. C. , Byerlee J. D. , Lockner D. A.
Published: February 2004

Mogi [1974] proposed a simple model of the incipient rupture surface to explain the Coulomb failure criterion. We show here that this model can plausibly be extended to explain the Mohr failure criterion. In Mogi's model the incipient rupture surface immediately before fracture consists of areas across which material integrity is maintained (intact areas) and areas across which it is not (cracks). The strength of the incipient rupture surface is made up of the inherent strength of the intact areas plus the frictional resistance to sliding offered by the cracked areas. Although the coefficient of internal friction (slope of the strength versus normal stress curve) depends upon both the frictional and inherent strengths, the phenomenon of internal friction can be identified with the frictional part. The curvature of the Mohr failure envelope is interpreted as a consequence of differences in damage (cracking) accumulated in prefailure loading at different confining pressures.
Satellite‐based identification of linked vegetation index and sea surface temperature Anomaly areas from 1982–1990 for Africa, Australia and South America
Authors: Myneni Ranga B. , Los Sietse O. , Tucker Compton J.
Published: February 2004

Meteorological satellite data from 1982 to 1990 were used to identify areas of significant association between tropical Pacific sea surface temperature (SST) and remotely sensed normalized difference vegetation index (NDVI) anomalies, here taken as a surrogate for rainfall anomalies. During this period, large areas of arid and semi‐arid Africa, Australia and South America experienced NDVI anomalies directly correlated to tropical Pacific SST anomalies. The results are limited by the relatively short time period of analysis. However, they confirm the disruptive effects of large‐scale tropical Pacific SST variations on arid and semiarid continental rainfall patterns in Africa, Australia, and South America, as reported previously.
The mid‐latitude total ozone trends in the northern hemisphere
Authors: Chandra S. , Varotsos C. , Flynn L. E.
Published: February 2004

The total column ozone trends derived from satellite measurements at northern mid‐latitudes during winter and spring seasons appear to be significantly larger than predicted by recent 2‐D photochemical models. Analyses of 13 to 14 years of the Nimbus‐7 TOMS total ozone measurements suggest that satellite‐derived trends during winter and spring months are influenced by interannual variability associated with dynamical perturbations in the atmosphere. Such perturbations cause large longitudinal spread in total ozone trends at mid‐latitudes. For example, the decadal trends in column ozone during February over the 10° latitude band centered at 40° N vary from about −9% to about −4% with error bars of 4 to 6% at the 2σ level. During the same month the zonally averaged trend is about −6±3%. The trends in certain geographical regions or locations, such as the Mediterranean region (35° to 45°N, 0° to 30°E) and Athens, Greece (38°N, 24°E), track the seasonal trends derived from the zonally‐averaged data. Given the large uncertainties of the regional and local trends, such similarities may be fortuitous. By using the lower stratospheric and tropospheric temperatures as indices of dynamical variability in the ozone trend analysis, the total ozone trends are reduced by 1 to 3% per decade. For the winter months (December, January and February), the midlatitude trends (zonally averaged) are respectively −3.7±2.5 and −5.1±2.8 % per decade with and without using the lower stratospheric temperature as an index of dynamical variability.
Substorms with multiple intensifications: Post‐onset plasma sheet thinnings in the morning sector observed by Prognoz‐8
Authors: Popielawska B. , Koperski P. , Lundin R. , Zakharov A. , Grygorczuk J. , Oberc P.
Published: February 2004

We present observations of plasma sheet thinnings in the dawn sector of the near‐Earth tail (8≤R<11 RE) recorded by Prognoz‐8 during 2 substorms with multiple intensifications. In one case (on 23 July 1981), prolonged substorm activity in the compressed magnetosphere (at ∼5nPa solar wind dynamic pressure) was related to persistent strongly southward (Bz=−15 nT) interplanetary magnetic field. Plasma sheet dropouts began ∼one hour after the main onset. Cold plasma populations (the low latitude boundary layer/mantle) were seen in a stretched magnetic configuration deep in the magnetosphere. There were signatures of local acceleration/heating of cold plasma above the retreating/expanding hot plasma sheet. In the case of 27 July 1981, the same phenomena were recorded at a low ∼1 nPa solar wind dynamic pressure. The distribution of the duration of total 13 plasma sheet dropouts was in the range of 1 to 15 minutes, with the majority of events lasting less than 5 minutes. Our data are complementary to the recent reports on GEOS‐2 and CRRES energetic particle dropouts in the morning sector.
Ultraviolet spectrum of CF3OH: Upper limits to the absorption cross sections
Authors: Molina Luisa T. , Molina Mario J.
Published: February 2004

The ultraviolet spectra of gaseous and liquid samples of the species CF3OH have been investigated in the laboratory. Upper limits to the absorption cross sections in the 190–300 nm wavelength range were determined. The species CF3OH is produced in the stratosphere by hydrogen abstraction reactions involving the CF3O radical; the upper limits to the ultraviolet absorption cross sections established in this work imply that photolysis of CF3OH in the stratosphere is too slow to regenerate CF3O radicals at rates significant for ozone depletion.
Modeling of upstream energetic particle events observed by WIND
Authors: Winglee R. M. , Skoug R. M. , McCarthy M. P. , Parks G. K. , Lin R. P. , Anderson K. A. , Carlson C. , Ergun R. , Larson D. , McFadden J. , Lepping R. L. , Szabo A. , Reme H. , Bosqued J. , d'Uston C. , Sanderson T. , Wenzel K.‐P.
Published: February 2004

Energetic particle events observed upstream of the bow shock can have two possible sources, shock acceleration of solar wind particles or leakage from the magnetosphere. Three‐dimensional global fluid simulations in conjunction with particle tracking in global fields are used to investigate the sources of the energetic particles. Acceleration of particles from the bow shock can account for many of the lower intensity events but the magnetosphere appears to be an important source during the more energetic particle events where 2 MeV particles are observed.
The potential role of non‐Newtonian rheology in the resurfacing of Venus
Authors: Weinstein Stuart A.
Published: February 2004

Several analyses of the impact cratering record on Venus have concluded that 80% of Venus was resurfaced sometime in the last 300 m.y.–700 m.y. The cratering record further indicates that the resurfacing took place over a 30 m.y. ‐ 100 m.y. interval. Since this period, there has been little large scale tectonic activity on Venus. This study proposes that such behavior may result from the interaction of phase change induced avalanches with a non‐Newtonian lithosphere. The mechanism proposed here suggests that because of non‐Newtonian rheology, the Venusian lithosphere becomes highly mobile during brief periods when avalanches occur. Afterwards, due to the reduction in stress levels, the lithosphere becomes rigid and large scale tectonics ceases.
The oceans: A source or a sink of methyl bromide?
Authors: Pilinis Christodoulos , King Daniel B. , Saltzman Eric S.
Published: February 2004

The global ocean/atmosphere flux of methyl bromide has been estimated from shipboard measurements of the saturation anomaly. When such data are extrapolated globally on the basis of constant saturation anomaly, the ocean is a net sink for methyl bromide [Lobert et al, 1995]. The same data can also be extrapolated on the basis of steady‐state production rate of methyl bromide in the water column, allowing regional and seasonal variations in temperature to affect the saturation anomaly. We have carried out this type of extrapolation, and we found that the oceans are a strong net source of methyl bromide to the atmosphere. The difference arises mainly due to slow degradation rates in water of higher latitudes. A reduction of the applied production rate by more than 35% is needed in order to switch the ocean from a source to a sink of methyl bromide. These results demonstrate the sensitivity of current estimates of oceanic flux to assumptions about methyl bromide production and destruction in the water column.
Kerr microscopy of small synthetic Ti‐rich titanomagnetite grains
Authors: Ambatiello Alex , Soffel Heinrich C.
Published: February 2004

Magnetic domain structure observations have been carried out on small synthetic Ti‐rich titanomagnetite (TM) grains (ulvöspinel content 50%–70%) employing the magneto‐optical Kerr effect (MOKE). On randomly oriented grains simple domain structures with planar domains or complex domain structures with curved domains and reverse spike domains have been observed. In addition to the description of the domain pattern, a qualitative determination of the magnetization direction within the domains is presented. Planes with a magnetization parallel to the surface show broad domains with nearly straight domain walls while planes with a magnetization normal to the surface show narrow domains and more complex patterns. With the help of the observed domain patterns on the randomly oriented grains and the knowledge about the magnetization direction within the domains a three‐dimensional (3‐D) domain structure model for Ti‐rich TM is proposed. These Ti‐rich titanomagnetites have only one easy direction of the magnetization and show a real unaxial character, although the magnetic mineral is cubic.

By analysing the domain structure of more than a hundred TM particles in a range of size between 1.7 and 20 microns the transition from the single‐domain (SD) to the multidomain (MD) state is determined. The fit of the data in this study gives a critical SD size of ≈ 1.1 µm for Ti‐rich TM.
Solar activity forecast for solar cycle 23
Authors: Schatten Kenneth , Myers Daniel J. , Sofia Sabatino
Published: February 2004

In this paper, we predict the next cycle's activity and improve the timing of solar cycle predictions. Dynamobased solar activity prediction techniques rely upon two properties inherent in the solar cycle: that solar magnetism oscillates between poloidal and toroidal components; and that there is a degree of “magnetic persistence” in dynamos, which in the case of the Sun, results in the dependence of many magnetic related quantities (activity related quantities) upon the amount of magnetism embedded below the Sun's surface. Using the SODA (SOlar Dynamo Amplitude) index as a measure of magnetic persistence, we predict that solar cycle #23 will reach a mean smoothed F10.7 peak of 182±30 solar flux units (sfu) and a mean sunspot number Rz of 138±30. This is particularly intriguing because the “folklore” is that odd cycles are larger than the preceding even cycle. Additionally, by tracking the equatorward march of solar activity, the timing of the cycle can be better estimated. From this, we estimate that the next solar maximum will occur near May, 2000 ±9 months.
Control parameters for polar ionospheric convection patterns during northward interplanetary magnetic field
Authors: Taguchi S. , Hoffman R. A.
Published: February 2004

Using the DE 2 electric field data from 191 dawndusk passes, we have sought parameters that distinguish between the various polar ionospheric convection patterns for northward IMF. The passes were divided into the daytime and nighttime sectors, and categorized the convection into three convection patterns for each sector by focusing on flow having a horizontal scale of >150 km. The three daytime sector patterns are convection of reverse polarity, i.e., sunward at the highest latitudes and antisunward on both sides of the sunward flow region, irregular convection having multiple sunward and antisunward flow regions, and convection having a single antisunward flow region at the highest latitude, which is similar to the pattern for southward IMF. The IMF clock (BY/BZ) angle, the Earth's dipole tilt angle, and the IMF BX/BZ angle measured from the dipole axis are the parameters that cause the difference in the three convection patterns. In the nighttime sector three similar convection patterns are identified. The occurrence of these patterns is controlled by the IMF clock angle. Thus, the convection pattern for northward IMF is very sensitive to the direction of the IMF. The magnetospheric sources for the convection patterns are discussed.
SCIFER—Structure of the Cleft Ion Fountain at 1400 km altitude
Authors: Arnoldy R. L. , Lynch K. A. , Kintner P. M. , Bonnell J. , Moore T. E. , Pollock C. J.
Published: February 2004

The SCIFER sounding rocket intersected the cleft ion fountain (CIF) at an altitude of about 1400 km revealing the structure of upward flows and ion acceleration. This altitude range is intermediate, between lower altitude studies using DE‐2 and HILAT data and higher altitude studies using DE‐1 and EXOS‐D data. Within the observed intense ion heating region the ions are consistent with satellite measurements of the cleft ion fountain of DE‐1, or the “ion wall” of EXOS‐D obtained at much higher altitudes. We conclude from the SCIFER data set that the accelerated ion flux levels at 1400 km altitude are similar to the TAI fluxes measured at higher altitudes. Furthermore the transversely accelerated ions are found within separate “events” of 30–40 km latitudinal width and have well‐defined boundaries. The ion distribution function within an event indicates transverse‐heating up to about 40 eV, occurring a few tens of km below the rocket. The SCIFER data set serves to demonstrate that the cleft ion fountain is composed of discrete structures originating poleward of the electron trapping boundary in regions of depleted ionospheric density.
Statistical tests for evaluating earthquake prediction methods
Authors: Riedel Kurt S.
Published: February 2004

The impact of including postcursors in the null hypothesis test is discussed. Unequal prediction probabilities can be included in the null hypothesis test using a generalization of the central limit theorem. A test for determining the enhancement factor over random chance is given. The seismic earthquake signal may preferentially precede earthquakes even if the VAN methodology fails to forecast the earthquakes. We formulate a statistical test for this possibility.
Corotating shock accelerated particles guided by wavy spiral magnetic fields in the solar wind at high heliographic latitudes
Authors: Lou Yu‐Qing
Published: February 2004

We propose the persistent presence of randomly fluctuating, large‐amplitude, long‐timescale wavy spiral magnetic fields in the solar wind at all heliographic latitudes. These Alfvénic fluctuations can be generated by gradual evolution of solar coronal holes or by interplanetary Kelvin‐Helmholtz instabilities occurring around the two conical interfaces at ∼±30° where strong wind shears appear quasi‐periodically during the solar minimum. Using this scenario, we offer interpretations for several recent high‐latitude observations of quasi‐periodic (i.e., ∼26.6 days) energetic particle and cosmic ray modulations from Energetic Particle Composition instrument and Cosmic Ray and Solar Particle Investigation sensor systems aboard Ulysses spacecraft.
The relationship between ELF‐VHF waves and magnetic shear at the dayside magnetopause
Authors: Zhu Z. , Song P. , Drake J. F. , Russell C. T. , Anderson R. R. , Gurnett D. A. , Ogilvie K. W. , Fitzenreiter R. J.
Published: February 2004

ELF‐VLF waves within the current layer of the dayside magnetopause are studied using ISEE‐1 data. The database consists of 272 current layer crossings at the dayside magnetopause from 1977 to 1979. For each crossing, the average intensity of ELF‐VLF waves inside the current layer is obtained and the magnetic shear angle across the current layer is calculated from the magnetometer data. It is found that the wave amplitudes (both electric and magnetic fields), after normalization by the average magnetic field strength in the current layer, are proportional to the local magnetic shear angle, i.e. large magnetic shear corresponds to strong wave emission and vice versa. From the dispersion relation of the waves for different shear angles, the phase velocity of the waves increases with the magnetic shear and peaks around 700 Hz to 1 kHz. The dispersion curve of the waves is consistent with that of whistler modes.
Stabilization of the Kelvin‐Helmholtz instability by the transverse magnetic field in the Magnetosphere‐Ionosphere Coupling System
Authors: Miura Akira
Published: February 2004

A sheared flow equilibrium in the magnetosphere‐ionosphere (M‐I) coupling system and its stability against the Kelvin‐Helmholtz (K‐H) instability are investigated within the ideal MHD by using a box‐shaped magnetospheric model. Without forcing, the unperturbed transverse (convection) electric field responsible for the E×B drift declines exponentially with time due to the ionospheric Joule dissipation and the decay (e‐folding) time is larger than one‐half of the Alfvén bounce period. The restoring force due to the line bending associated with the transverse magnetic field is responsible for the existence of a critical height‐integrated Pedersen conductivity ΣPc∼(µoVA)−1, where VA is the average Alfvén velocity along the field line, above which the K‐H instability in the magnetosphere is suppressed completely.
ATSR sea surface temperature data in a global analysis with TOPEX/POSEIDON altimetry
Authors: Knudsen P. , Andersen O. B. , Knudsen T.
Published: February 2004

Along Track Scanning Radiometer (ATSR) data from the ERS 1 satellite mission are used in a global analysis of the surface temperature of the oceans. The data are the low resolution 0.5° by 0.5° average temperatures and cover about 24 months. At global scales a significant seasonal variability is found. On each of the hemispheres the surface temperatures reach their maximum after summer heating. The seasonal sea level variability, as observed from TOPEX/POSEIDON, reaches its maximum 1.1–1.4 months later.
Atmospheric Propagation heterogeneities revealed by ERS‐1 interferometry
Authors: Tarayre H. , Massonnet D.
Published: February 2004

Spaceborne Synthetic Aperture Radar (SAR) repeat‐pass interferometry is a technique based on the exploitation of the interference pattern of two SAR images acquired in similar geometric conditions, at two different dates. This measure contains geometric, atmospheric, topographic and land‐deformation information. In this paper we focus on the atmospheric information. First, we show theoretically that an adequate processing is sufficient to remove homogeneous atmospheric effects and that heterogeneities, as tropospheric turbulences, rain falls and ionospheric phenomena can be detected with SAR interferometry. Then, we interpret several observations and we present interferometric images of cumulus and, turbulences due to relief. SAR interferometry could be a new remote sensing tool for the study of tropospheric turbulences at a high horizontal resolution (40 m)
Differential flow between solar wind protons and alpha particles: First WIND observations
Authors: Steinberg J. T. , Lazarus A. J. , Ogilvie K. W. , Lepping R. , Byrnes J.
Published: February 2004

Alpha particle and proton measurements in the solar wind made using the SWE Faraday cup detectors on the WIND spacecraft are reported. Some overall trends observed confirm past observations: the ratios of alpha particle to proton density Nα/NP, thermal speed Wα/WP, as well as the differential velocity Vα‐VP (hereafter VαP) are generally correlated with bulk solar wind flow speed. The detailed WIND measurements enable us to investigate instances when the alpha‐proton differences deviate from these overall general trends. Occasionally, difference velocities as large as 80 km/s were seen, with the ratio of |VαP| to the Alfvén speed VA near unity, characteristics more typical of observations at solar distances less than 1 AU. An example is presented where |Vα|‐|VP| reverses sign while |VαP| stays nearly constant. Comparison of the vector velocities and the magnetic field suggests that the speed reversal is associated with a localized kink in the magnetic field. Finally we show an instance where |VαP| exceeds the observed wave speed for Alfvénic fluctuations (Vwave=B0ΔV/ΔB) resulting in alpha particle velocity fluctuations that anti‐correlate with the wave. Though this phenomenon has been previously reported in high‐latitude measurements beyond 1 AU, it is shown here to also occur at 1 AU in the ecliptic.
Source region of 0.2 to 1.0 Hz geomagnetic pulsation bursts
Authors: Anderson B. J. , Erlandson R. E. , Engebretson M. J. , Alford J. , Arnoldy R. L.
Published: February 2004

Geomagnetic pulsations from 0.2 to 5.0 Hz (Pc 1) observed on the ground could be used to monitor the magnetospheric processes generating them if their source regions were known. We report simultaneous observations of Pc 1 bursts at South Pole Station and in space at 9 Re geocentric distance near the magnetic equator showing that the subsolar outer magnetosphere, earthward of the low latitude boundary layer, is the source region of Pc 1 bursts. The source extends 1 to 2 hours azimuthally and 1 to 2 Re radially. Correlation of magnetopause displacements and bursts at South Pole indicates that the bursts are stimulated by compressions of the dayside magnetosphere. In space the bursts occur above the He+ gyrofrequency implying that the H+‐He+ bi‐ion resonance does not prevent the signals from reaching the ionosphere.
On the growth of ternary system HNO3/H2SO4/H2O aerosol particles in the stratosphere
Authors: Hamill Patrick , Tabazadeh A. , Kinne S. , Toon O. B. , Turco R. P.
Published: February 2004

We present a study of the growth of ternary solution (nitric acid, sulfuric acid and water) droplets in the stratosphere. The growth mechanism is hetero‐molecular condensation in which the particle is assumed to be in equilibrium with environmental water vapor. Model results are in reasonable agreement with the averaged extinction ratio obtained by the SAM II satellite system.
Surficial evidence of fluid expulsion from the Costa Rica Accretionary Prism
Authors: Kahn L. M. , Silver E. A. , Orange D. , Kochevar R. , McAdoo B.
Published: February 2004

The nature and distribution of authigenic carbonates, chemosynthetic bacterial mats, and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes. This paper describes the authigenic carbonates, chemosynthetic fauna, and fluid venting observed at the four tectonic regions of the Costa Rica accretionary wedge in February 1994 during an ALVIN diving program of 20 submersible dives. We found no surficial evidence of highly focused fluid venting at the toe of the prism (outermost 3 km), as implied by the absence of authigenic carbonates and chemosynthetic fauna. The absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent, incoming Cocos plate (Langseth and Silver, this issue), suggests diffuse, rather than focused flow through the toe of the prism. Twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out‐of‐sequence‐thrusts, implicating fracture permeability as the fluid conduit in the lower slope region (but upslope from the toe). Vast authigenic carbonates and seven active and relict vent sites marked by large, dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid‐slope region. Fluid expulsion appears to be more restricted on the upper slope, with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon.
Is the 670 km phase transition able to layer the Earth's convection in a mantle with depth‐dependent viscosity?
Authors: Monnereau Marc , Rabinowicz Michel
Published: February 2004

The effect of a viscosity stratification on phase change dynamics have been investigated with axi‐spherical convection models. As in previous studies with a constant viscosity mantle an intermittent layering appears for a Clapeyron slope from −2 MPa/K to −3 MPa/K. A viscosity increase in lower mantle requires a more negative Clapeyron slope to produce the layering. This shift is sensitive to the mechanical boundary condition. With a viscosity contrast of 30, a no‐slip top condition does not lead to layering in the range of the possible values for the Clapeyron slope. With a free‐slip condition, the threshold is at −4 MPa/K. Just below this threshold, a whole mantle circulation driven by a cylindrical hot plume coexists with layered mantle domains over several billion years.
Introduction to the Special Section on Fluid Flow in the Costa Rica Accretionary Prism
Authors: Silver Eli A.
Published: February 2004

Previous 2D and 3D seismic reflection studies off the Nicoya Peninsula, on the Pacific margin of Costa Rica indicated a reversed polarity décollement beneath the toe of the prism (Shipley et al., 1990), consistent with elevated fluid pressures. In addition, Shipley and Moore (1986) reported a decrease in thickness of underthrust sediment beneath the toe of the wedge of up to 50% over the outer 3–5 km, implying massive dewatering through the toe. Because such dewatering must pass through the toe of the wedge, we suspected the presence of significant fluid venting. During February 1994, we carried out 20 ALVIN dives in search of indicators of fluid venting on the Costa Rica prism. As the trench off Costa Rica reaches depths of 4500m, the ALVIN went through a process of refit and Navy clearance to dive to depths of up to 4500m, and our deepest dive reached that depth. In addition to the dives, we mapped a broad area of the slope with SeaBeam, obtained a series of “pogo” heat flow transects, and recovered a series of piston cores and giant gravity cores for the study of pore water geochemistry. Here we report on the structures observed in the dive program (McAdoo et al.), the nature and locations of fluid vents (Kahn et al.), the heat flow results (Langseth and Silver), the pore water studies (Zuleger et al.), and a reinterpretation of surface amplitude data from 3D seismic records, finding a remarkable association between amplitude anomalies and regions of fluid vents (Mclntosh and Silver).
The isostatic state of Martian topography revisited
Authors: Frey H. V. , Bills B. G. , Nerem R. S. , Roark J. H.
Published: February 2004

We present a spatial analysis of free air gravity and topography for Mars using new, higher resolution gravity data and the latest digital elevation data, both represented by spherical harmonics up to degree and order 50 and, separately, for harmonic degrees 4–19. Interpreting the gravity vs gravity‐from‐topography scatter diagram in terms of isostatic compensation for small regions, we find that most of Mars older than Amazonian is fully compensated at shallow depths (< 50 km). Exceptions to this include the ancient Tempe Terra region and the central portion of the Isidis impact basin (a known mascon). Most Hesperian‐age and Amazonian‐age volcanic plains not located near Tharsis and Elysium volcanoes are also compensated isostatically at shallow depths, but Amazonian‐age plains near the young volcanic constructs, like Olympus Mons and the Tharsis and Elysium volcanoes themselves, are not. Both Olympus Mons and Alba have isostatic compensation depths >300 km.
Coexistence of Earth‐origin O+ and solar wind‐origin H+/He++ in the distant magnetotail
Authors: Seki K. , Hirahara M. , Terasawa T. , Shinohara I. , Mukai T. , Saito Y. , Machida S. , Yamamoto T. , Kokubun S.
Published: February 2004

In the lobe/mantle region at ∼159 RE away from the Earth during a geomagnetically disturbed period, we have found the coexistence of three ion species, H+, He++, and O+, streaming tailward with nearly the same flow velocity ∼200–500 km/s. Both H+ and O+ ions are detected almost continuously from near plasma sheet to near magnetopause region. From a positive correlation between the proton density and their velocity component parallel to the magnetic field VH+║, we conclude that most of protons have come from the solar wind. The existence of He++ further supports this conclusion, which implies the importance of solar wind contribution to the magnetotail. The existence of O+, on the other hand, suggests that the ions of ionospheric origin have mixed with those of solar wind origin. The lack of positive correlation between O+ density and VO+║ is consistent with the idea that O+; ions have some source mechanism different from that of protons. Simultaneously, curious velocity differences are also observed: VO+¶ appears to be often faster than VH+║; by ΔV¶ = 20–30 km/s. This observation may provide a key for further discussion.
Chemical remanent magnetization in oceanic sheeted dikes
Authors: Worm Horst‐Ulrich , Bach Wolfgang
Published: February 2004

Marine lineated magnetic anomalies often exhibit anomalous skewness that indicates non‐vertical boundaries between normally and reversely magnetized crust. The acquisition of secondary magnetization components well after formation of the crust has been suspected as one possible cause for anomalous skewness. A petrographic and rock magnetic study on sheeted dikes recovered from the lower part of the deepest drill hole in the oceanic crust, ODP (Ocean Drilling Program) Hole 504B, demonstrates that these rocks carry a secondary chemical remanent magnetization (CRM) and not a thermoremanent magnetization (TRM). Primary titanomagnetites have mostly been altered to non‐magnetic phases while the remanent magnetization resides in secondary magnetite that formed below its Curie temperature (∼600°C at ambient pressure) mainly by the alteration of olivine. The natural remanent magnetization (NRM) has been compared with laboratory anhysteretic remanent magnetization (ARM) and TRM. The intensities relate NRM < ARM < TRM, each by a factor of ≈ two. Because previous studies have shown that CRMs are much lower in intensity than TRMs and because of the common presence of secondary magnetite we conclude that the NRMs of the lower Hole 504B dikes are CRMs. The dikes exhibit the same reversed magnetization as the overlying extrusive basalts, and, thus, the timing of CRM acquisition is most likely confined to the time span between the crustal age of Hole 504B and the next younger field reversal, i.e. 0.25 m.y. after emplacement.
A statistical study of intense low‐altitude electric fields observed by Freja
Authors: Karlsson T. , Marklund G. T.
Published: February 2004

Extremely intense (up to 2 V/m) and fine‐structured (1–5 km) electric fields have been observed by the Freja satellite at altitudes of 1400–1770 km. To study the occurrence and characteristics of these intense electric field events, a database was set up by searching 7 months' worth of Freja data for events with peak values greater than 200 mV/m. The intense electric field events are distributed over all local times, but they are mostly concentrated to the midnight and early morning sector of the auroral oval. The events are seen to be associated with low ambient ionospheric conductivities, but are not activity dependent. The most intense events (located in the midnight sector) are associated with the smallest scale sizes, whereas the less intense events on the day side have larger scale sizes.

The findings are consistent with the interpretation that a majority of the intense electric field events observed by Freja are associated with the small‐scale, east‐west‐aligned, low‐conductivity bands devoid of auroral emissions, known as black auroral bands, or with the curls that may develop when these bands go unstable. Several observations of diverging electric fields of around 1 V/m at 800 km altitude, within the southern auroral oval, may serve as support for the prediction that intense electric fields should exist also at ionospheric altitudes.
The subsolar magnetosheath and magnetopause for high solar wind ram pressure: WIND observations
Authors: Phan T. D. , Larson D. E. , Lin R. P. , McFadden J. P. , Anderson K. A. , Carlson C. W. , Ergun R. E. , Ashford S. M. , McCarthy M. P. , Parks G. K. , Rème H. , Bosqued J. M. , D'Uston C. , Wenzel K.‐P. , Sanderson T. R. , Szabo A.
Published: February 2004

On a rapid inward pass through the subsolar magnetosheath (MSH) and magnetopause (MP), the WIND spacecraft initially encountered a moderately‐compressed low‐magnetic shear MP (at a radial distance of 8.6 RE), followed by multiple crossings of a high‐shear MP (at 8.2 RE). The large shear resulted from a southward turning of the external MSH field. Strong magnetic field pile‐up, a plasma depletion layer (PDL), and plasma flow acceleration and rotation to become more perpendicular to the local magnetic field were observed in the MSH on approach to the low‐shear MP. At the high‐shear MP, magnetic reconnection flows were detected, and there are some indications that plasma depletion effects were weak or absent in the adjacent MSH. We attribute the changes in the MP and MSH properties to the sudden rotation of the MSH field direction. In essence, the structure of the MP regions under the unusually high solar wind ram pressure condition in this case does not seem to be qualitatively different from that observed under more typical (less compressed) conditions. Also similar to previous observations, the mirror mode is marginally unstable in the MSH proper, but is stable in the PDL. In this region, the proton temperature anisotropy is inversely correlated with βp∥. Finally, the electron distributions are observed to be anisotropic (Te⟂/Te∥ ∼1.3) throughout the entire MSH.
MHD theory of Earth's magnetosheath for an axisymmetric model
Authors: Grabbe Crockett L.
Published: February 2004

An analysis is made of the Earth's magnetosheath along the Sun‐Earth line under conditions that the IMF (Interplanetary Magnetic Field) is nearly parallel or anti‐parallel to the solar wind flow. MHD conservation equations in temporally‐averaged steady‐state form for the mass, momentum and energy density are combined with the magnetic divergence and induction equations, a hard conducting‐sphere model for the magnetopause, and an adiabatic equation of state in the magnetosheath. The equations are integrated from the nose of the bow shock to the magnetopause and reduced to a set of nonlinear‐coupled equations for the magnetosheath thickness and average magnetosheath parameters, which are then used to obtain a new equation for the thickness of the magnetosheath. This is the first analytical equation for the magnetosheath thickness that has been derived, and it exhibits an interesting functional dependence on Alfven and sonic Mach number MA and Ms,the angle θ0 between the bow shock normal and the IMF, the Chapman‐Ferraro constant k0 at the magnetopause, the polytropic index γ, and the thermal conductivity Qr at the bow shock. The thickness is found to decrease for decreasing Ms, but increase with decreasing MA It exhibits the qualitative feature found in both gasdynamic and θ0 ≥ 45° MHD simulations of an approximate linear variation of the magnetosheath thickness with the density jump ratio X across the bow shock, but it also exhibits a unique negative slope and an offset that is a function both of MA and of K0.
Reply to “Statistical evaluation of the VAN Method using the historic earthquake catalog in Greece,” by Richard L. Aceves, Stephen K. Park and David J. Strauss
Authors: Varotsos P. , Lazaridou M.
Published: February 2004

The pioneering calculation by Aceves et al. [1996] shed light on the main question of this debate, i.e., on whether “VAN predictions can be ascribed to chance.” Aceves et al. [1996] conclude that “the VAN method has resulted in a significantly higher prediction rate than randomly sampling a PDF (probability density function) map generated from a 25 year history of earthquakes.” After investigating the totality of VAN predictions issued during the period 1987–1989, Aceves et al. [1996] found: “The prediction rate for the VAN method clearly exceeds that from the random model at all time lags between 5–22 days. At a 5 day time lag, the VAN prediction rate of 35.7% has a P‐value of less than 0.06%. This means that a random model does as well as does the VAN method less than 0.06% of the time. At 22 days, the prediction rate of 67.9% has a P‐value of less than 0.07%.” These conclusions basically coincide with those of Hamada [1993] although Aceves et al. [1996] followed different procedures. They are also in fundamental agreement with the results of Honkura and Tanaka [1996]. Another important conclusion of Aceves et al. [1996] is that, after declustering the earthquake catalog and prediction list from aftershocks, “;VAN method is still formally significant.”
Reply to “Statistical tests of VAN earthquake predictions: Comments and reflections,” by Y. Kagan and D.D. Jackson
Authors: Varotsos P. , Lazaridou M.
Published: February 2004

In this Reply we restrict ourselves to those points that are again raised by Kagan and Jackson [1996], although we discussed them in our separate Replies to the papers by Kagan [1996] and Jackson [1996]. For example, we show, with precise quotations, that their claim: “…most of the VAN parameters have been adjusted after the 1987–1989 test period, i.e., retroactively” does not hold. On the other hand, we do agree with their main conclusion that “the diversity of views on these subjects suggests that the seismological community needs a general strategy for evaluating earthquake prediction methods.” Such a strategy, can now be followed for the evaluation of the VAN predictions published to date (which cover the continuous 9 year period 1987–1995), instead of restricting ourselves to the period 1987–1989 discussed in this debate. Kagan and Jackson [1996] also proceeded to a comparison of test results by Aceves et al. [1996] and by Kagan [1996], and concluded that both tests agree that VAN outperforms a Poisson null hypothesis, if aftershocks are left in the catalog or only partially removed. On the other hand, Kagan and Jackson [1996] claim that the VAN method does not outperform the Poisson null hypothesis “if aftershocks are more fully removed.” However, we clarify that the following happens in the “declustering” procedure of the latter case: successful predictions of main shocks are also eliminated, and furthermore, we have conversion of successful predictions to failures; our remarks are in basic agreement with those made by Aceves et al. [1996].
Site and propagation effects on the spectra of an S‐to‐S reflected phase recorded from a set of microearthquakes in the Northern Apennines (Italy)
Authors: Zollo Aldo , Iannaccone Giovanni
Published: February 2004

We use recordings at a single station of a shallow crustal S‐to‐S reflected phase generated by microearthquakes (ML, 2.5–3.5) in the Northern Apennines (Italy) to analyze source, path attenuation and site effects. A site resonance phenomenon in a narrow frequency band (6–8 Hz) has been clearly detected in the shear wave acceleration spectra, consistent with similar observations from records of microseismic noise. The non‐linear inversion of acceleration spectra (which have been corrected for an average site response) yields estimates of apparent corner frequencies that are approximately constant with moment, with a mean value of 2 Hz. A significant increase with seismic moment of the spectral decay parameter at high frequencies is also observed. The anomalous moment dependence of these spectral parameters is found to be related to the existence of a cut‐off high frequency limit, close to the smallest event corner frequency, which could be originated by combined site and travel path effects rather than source characteristics in the frequency range of observations (0.5–15 Hz). Independent geological, well soundings and seismic data for this site in the Apennines, suggest that the spectral shape on short‐period records is mainly controlled by the damping and resonance effects of a surficial layer of alluvium Quaternary sediments (100–200 m thick) characterized by low values of shear velocity and attenuation parameter.
Dispersion of regional body waves at 100–150 km depth beneath Alaska: In situ constraints on metamorphism of subducted crust
Authors: Abers Geoffrey A. , Sarker Golam
Published: February 2004

Phase delays at high frequencies are observed in body waves that travel in the Alaska slab, along its strike at 100–150 km depth. The delays, between 2–6 Hz energy and the direct 0.5–1 Hz arrival, are 0.5–1.5 s for P waves and 1.5–4 s for S waves. Such dispersion suggests a waveguide structure that parallels the slab, perhaps near its top. A channel that is 2–6 km thick and 2.5–5% slower than surrounding mantle can explain the observations. The thickness of the layer is comparable to that of subducted oceanic crust or somewhat thinner. The layer may be crust that is slow at these depths. The required velocity anomaly is too small to be due to a continuous layer of metastable gabbro yet too large to represent an eclogite layer. It may indicate a mixture of the two, or persistence of hydrated mineral assemblages to depth.
Evidence for rapid and widespread emplacement of volcanic plains on Venus: Stratigraphic studies in the Baltis Vallis Region
Authors: Basilevsky Alexander T. , Head James W.
Published: February 2004

Geological mapping of the area around Baltis Vallis, the longest (6,800 km) canali‐type channel on Venus, has led to the determination of stratigraphic relations which appear to be consistent over this entire region comprising ∼20×106 km² or ∼5% of the total surface of Venus. The most abundant unit in the study area, plains with wrinkle ridges (Pwr), consists of two subunits separated by the formation of Baltis Vallis, a feature which is interpreted by most workers to have formed essentially geologically instantaneously. The wrinkle ridges deform both subunits of Pwr, while ridge belts are embayed by the Pwr plains. Consistent age relationships between the Pwr subunits, wrinkle ridges, Baltis Vallis and ridge belts along the entire 6,800 km channel length is evidence that unit Pwr is a real stratigraphic marker unit and not a mosaic of plains emplaced at significantly different times, at least in the ∼5% of the surface of Venus comprising the study area. The Baltis stratigraphic sequence was compared with stratigraphic relations found in earlier studies of thirty‐six 1,000×1,000 km sites randomly distributed over the surface of Venus [Basilevsky and Head, 1995] and was found to be comparable. This supports the interpretation that these stratigraphic units represent generally globally quasi‐synchronous geologic events rather than the products of geologic activity in different areas of the planet at different times.