bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of ERS altimetry during the Tandem Mission
Authors: Moore P. , Carnochan S. , Walmsley R. J.
Published: September 2002

Studies of sea‐level change from altimetric missions require accurate calibration if systematic errors are not to be aliased into the solution. For ERS‐2 there is now growing evidence that the altimetric range is biased. Global studies over the period of concurrent operation of ERS‐1, ERS‐2 and TOPEX/Poseidon show that the mean sea‐level rise, including the steric effect, was −4.7 mm/yr and −5.6 mm/yr for ERS‐1 and TOPEX respectively but 9.0 mm/yr for ERS‐2. Further, repeat pass data between ERS‐1 and ERS‐2 yields a systematic difference equivalent to a 11 mm/yr slope. The results show that discontinuities in the repeat pass data correspond to the ERS altimeters being switched off after an instrument anomaly. These discontinuities occur despite utilisation of the correction data available to the user community. Corrections to the long‐term systematic errors in the ERS altimeters have been determined by using dual satellite crossover residuals with TOPEX. These enhancements remove the spurious trend to the extent that the differential slope between ERS‐1 and ERS‐2 is reduced to 0.2 mm/yr. With most of the correction attributable to ERS‐2 it is apparent that this satellite is more problematic than ERS‐1.
Complex Type III‐like radio emissions observed from 1 to 14 MHz
Authors: Reiner M. J. , Kaiser M. L.
Published: September 2002

We present a morphological description of intense, complex type III‐like radio emissions in the frequency range from 1 to 14 MHz. This previously unexplored frequency range, between ground‐based and space‐based observations, corresponds to the transition region (heliocentric distances from ∼2 to 10 R⊙) between the solar corona and interplanetary medium. The complex type III‐like radio emissions, some of which are SA events [Cane et al., 1981], are found to have distinctly different radio signatures above and below ∼7 MHz. Above ∼7 MHz, the radio emissions consists of very rapid frequency‐drifting structures with time scales of <16 seconds. Below ∼7 MHz, the radio emissions generally blend into slower frequency‐drifting type III‐like substructures, with the addition of short‐duration very narrowband features that are either very slowly frequency drifting or nondrifting. In addition, there is generally a clear diminution in the radio intensity near 7 MHz.
A comparison between auroral particle characteristics and atmospheric composition inferred from analyzing optical emission measurements alone and in combination with incoherent scatter radar measurements
Authors: Hecht J. H. , Christensen A. B. , Strickland D. J. , Majeed T. , Gattinger R. L. , Jones A. Vallance
Published: September 2002

On day 59 of 1987 a well calibrated set of photometric observations of auroral emissions (427.8, 630.0, 844.6, and 871.0 nm) was obtained at Sondre Stromfjord, Greenland coincident with incoherent scatter radar measurements of electron density. Ratios of these optical emissions were used to correct for atmospheric extinction and to infer the average energy of the precipitating electrons and the deviations in the atmospheric composition induced by auroral heating. Previously, the radar data taken alone had been used to infer the average energy of the auroral particles. For the first time a comparison is made of the results from this optical ratio technique with those obtained by combining the optical data with the radar data and with results obtained using the radar data alone. Specifically, we compare the average energy inferred by these techniques as well as the changes in the atomic oxygen to molecular nitrogen ratio in the lower thermosphere. This study shows the critical importance of characterizing the atmospheric scattering if reliable results are to be derived. In particular, it is shown that on occasion the optical technique may be even more reliable than using radar and optics together since, because of light scattering, the radar and the optics are sometimes observing different parts of the aurora. Magnetometer data are used to estimate local Joule heating. The combined data are also used to address the question of whether observed composition changes were produced locally.
Three‐dimensional propagation of coronal mass ejections (CMEs) in a structured solar wind flow: 1. CME launched within the streamer belt
Authors: Odstrčil D. , Pizzo V. J.
Published: September 2002

A three‐dimensional (3‐D) numerical hydrodynamic model is used to investigate the temporal and spatial evolution of large‐scale solar wind structures. A tilted‐dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating solar wind (SW) flow is established by dynamic relaxation. Time‐dependent variation of the pressure and velocity at the inner boundary is applied to generate transient structures within the streamer belt. The dynamical interaction of a coronal mass ejection (CME) with the corotating coronal streamer belt flow between 0.14 and 5.04 AU is then investigated. Numerical results show that the motion and appearance of a CME can be strongly affected by its interaction with the velocity and density structure of the background SW. The initial shape and density distribution of the CME is distorted in all dimensions; it is compressed where, the CME is trapped between slow streamer belt and high‐speed coronal hole flows, and it is distended where the CME penetrates into the trailing edge of the preceding high‐speed stream. Thus a given CME can be observed with substantially different properties at different locations; the shock strength as well as the stand‐off distance between the shock front and the CME driver gas can vary considerably across the structure, and the density profile through the disturbance can adopt different forms depending on location. Merging of CME and corotating interaction region shocks and thermodynamic structures as demonstrated in this simulation complicate the interpretation of single‐spacecraft observations.
The 1996 Leonid shower as studied with a potassium lidar: Observations and inferred meteoroid sizes
Authors: Höffner Josef , von Zahn Ulf , McNeil William J. , Murad Edmond
Published: September 2002

We report on the observation and analysis of meteor trails that are detected by ground‐based lidar tuned to the D1 fine structure line of K. The lidar is located at Kühlungsborn, Germany. The echo profiles are analyzed with a temporal resolution of about 1 s and altitude resolution of 200 m. Identification of meteor trails in the large archive of raw data is performed with help of an automated computer search code. During the peak of the Leonid meteor shower on the morning of November 17, 1996, we observed seven meteor trails between 0245 and 0445 UT. Their mean altitude was 89.0 km. The duration of observation of individual trails ranges from 3 s to ∼30 min. We model the probability of observing a meteor trail by ground‐based lidar as a function of both altitude distribution and duration of the trails. These distributions depend on the mass distribution, entry velocity, and entry angle of the meteoroids, on the altitude‐dependent chemical and dynamical lifetimes of the released K atom, and on the absolute detection sensitivity of our lidar experiment. From the modeling, we derive the statistical likelihood of detection of trails from meteoroids of a particular size. These bracket quite well the observed trails. The model also gives estimates of the probable size of the meteoroids based on characteristics of individual trails.
Magnetopause crossings observed at 6.6 RE
Authors: Cahill L. J. , Winckler J. R.
Published: September 2002

Thirty‐five boundary crossing events, many with multiple crossings, have been observed in data from the GOES 6 and 7 geostationary satellites in the years 1989, 1990 and 1991. Many of the crossings within these events appear to have been caused primarily by inward boundary motion due to compression of the magnetosphere by the solar wind and some were crossings caused primarily by inward motion due to geomagnetic field erosion. Most crossings appear to be due to some combination of compression and erosion. A rise of the main, northward component of the Earth's magnetic field to approximately twice the dipole level, observed prior to a crossing, is taken as evidence of inward motion due to compression. When a crossing occurs with no prior rise of the northward component, it is taken as evidence of erosion. The magnetosheath field, just outside the boundary, is found to be predominantly southward for almost all of the crossings, including those due to compression as well as those due to erosion. This indicates that, even for the many events noted above for which inward motion by compression appears adequate to account for the boundary crossings, there is almost always some erosion present. In the 3‐year interval, each spacecraft spent 0.3 % of the dayside observation time in the magnetosheath.
Gaussian analysis of two hemisphere observations of galactic cosmic ray sidereal anisotropies
Authors: Hall D. L. , Munakata K. , Yasue S. , Mori S. , Kato C. , Koyama M. , Akahane S. , Fujii Z. , Fujimoto K. , Humble J. E. , Fenton A. G. , Fenton K. B. , Duldig M. L.
Published: September 2002

We have analyzed the yearly averaged sidereal daily variations in the count rates of 46 underground muon telescopes by fitting Gaussian functions to the data. These functions represent the loss cone and tail‐in anisotropies of the sidereal anisotropies model proposed by Nagashima et al. [l995a, b]. The underground muon telescopes cover the median rigidity range 143–1400 GV and the viewing latitude range 73°N–76°S. From the Gaussian amplitudes and positions we have confirmed that the tail‐in anisotropy is more prominent in the southern hemisphere with its reference axis located at declination (δ) ∼14°S and right ascension (α) ∼4.7 sidereal hours. The tail‐in anisotropy is asymmetric about its reference axis, and the observed time of maximum intensity depends on the viewing latitude of the underground muon telescopes. We also find that the declination of the reference axis may be related to the rigidity of the cosmic rays. We show that the loss cone anisotropy is symmetric and has a reference axis located on the celestial equator (δ ∼ 0°) and α ∼ 13 sidereal hours. We have used the parameters of the Gaussian fits to devise an empirical model of the sidereal anisotropies. The model implies that the above characteristics of the anisotropies can explain the observed north‐south asymmetry in the amplitude of the sidereal diurnal variation. Furthermore, we find that the anisotropies should cause the phase of the sidereal semidiurnal variation of cosmic rays to be observed at later times from the northern hemisphere compared to observations from the southern hemisphere. We present these results and discuss them in relation to current models of the heliosphere.
Longitudinal and seasonal variations in nighttime plasma temperatures in the equatorial topside ionosphere during solar maximum
Authors: Venkatraman Sarita , Heelis Rod
Published: September 2002

Latitude profiles of the ion and electron temperatures and total ion concentration across the equatorial region near 800 km altitude are routinely obtained from Defense Meteorological Satellite Program (DMSP) spacecraft. We have examined these profiles at 2100 hours local time to discover the influences of field‐aligned plasma transport induced by F region neutral winds. Such dependencies are readily seen by contrasting observations at different seasons and different longitudes distinguished by different magnetic declinations. These data show strong evidence for adiabatic heating produced by interhemispheric plasma transport. This heating manifests itself as a local temperature maximum that appears in the winter hemisphere during the solstices and is generally absent during equinox. A longitudinal variation in the appearance of this maximum is consistent with the roles of meridional and zonal winds in modulating the field‐aligned plasma velocities. The data also show a local temperature minimum near the dip equator. However, it is not so easy to attribute this minimum to adiabatic cooling since transport of plasma from below and the latitude variation in the flux tube content may also produce such a minimum.
Heating and cooling of protons by turbulence‐driven ion cyclotron waves in the fast solar wind
Authors: Li Xing , Habbal Shadia R. , Hollweg Joseph V. , Esser Ruth
Published: September 2002

The effects of parallel propagating nondispersive ion cyclotron waves on the solar wind plasma are investigated in an attempt to reproduce the observed proton temperature anisotropy, namely, Tp⊥ ≫ Tp‖ in the inner corona and Tp⊥ < Tp‖ at 1 AU. Low‐frequency Alfvén waves are assumed to carry most of the energy needed to accelerate and heat the fast solar wind. The model calculations presented here assume that nonlinear cascade processes, at the Kolmogorov and Kraichnan dissipation rates, transport energy from low‐frequency Alfvén waves to the ion cyclotron resonant range. The energy is then picked up by the plasma through the resonant cyclotrou interaction. While the resonant interaction determines how the heat is distributed between the parallel and perpendicular degrees of freedom, the level of turbulence determines the net dissipation. Ion cyclotron waves are found to produce a significant temperature anisotropy starting in the inner corona, and to limit the growth of the temperature anisotropy in interplanetary space. In addition, this mechanism heats or cools protons in the direction parallel to the magnetic field. While cooling in the parallel direction is dominant, heating in the parallel direction occurs when Tp⊥ ≫ Tp‖. The waves provide the mechanism for the extraction of energy from the parallel direction to feed into the perpendicular direction. In our models, both Kolmogorov and Kraichnan dissipation rates yield Tp⊥ ≫ Tp‖ in the corona, in agreement with inferences from recent ultraviolet coronal measurements, and predict temperatures at 1 AU which match in situ observations. The models also reproduce the inferred rapid acceleration of the fast solar wind in the inner corona and flow speeds and particle fluxes measured at 1 AU. Since this mechanism does not provide direct energy to the electrons, and the electron‐proton coupling is not sufficient to heat the electrons to temperatures at or above 106 K, this model yields electron temperatures which are much cooler than those currently inferred from observations.
Numerical simulation of Alfvénic turbulence in the solar wind
Authors: Goldstein M. L. , Roberts D. A. , Deane A. E. , Ghosh S. , Wong H. K.
Published: September 2002

Low‐frequency fluctuations in the solar wind magnetic field and plasma velocity are often highly correlated, so much so that the fluctuations can be thought of as nearly perfect Alfvén waves. Evidence from the Helios and Ulysses spacecraft suggest strongly that these fluctuations emanate from the solar corona with high correlation and flat power spectra (∼f−1). These fluctuations constitute a source of free energy for a turbulent cascade of magnetic and kinetic energy to high wave numbers, a cascade that evolves most rapidly in the vicinity of velocity shears and the heliospheric current sheet. Numerical solutions of both the compressible and incompressible equations of magnetohydrodynamics (MHD) in Cartesian geometry showed that sharp gradients in velocity would decrease substantially the Alfvénicity of initially pure Alfvénic fluctuations; however, the effects of solar wind expansion on this turbulent evolution is, as yet, undetermined. We demonstrate that as was the case in Cartesian geometry, in an expanding volume, velocity shears and pressure‐balanced flux tubes still reduce the Alfvénicity of parallel propagating wave packets. These three‐dimensional spherically expanding simulations include velocity shears separating fast and slow flows, pressure‐balanced flux tubes, and a central current sheet which is the site of magnetic reconnection. Two‐dimensional spectra constructed in the r – θ plane resemble closely those resulting from similar initial conditions in Cartesian geometry.
Nonlinear saturation of Rayleigh‐Taylor instability in the presence of time‐dependent equilibrium
Authors: Basu B.
Published: September 2002

The nonlinear evolution and the saturation of the collisional Rayleigh‐Taylor instability in the presence of a time‐dependent ambient density gradient are studied by using a local, two‐dimensional nonlinear theory. A set of nonlinear equations that describes coupling of the initial unstable mode with its various harmonics, which are self‐consistently generated by the dominant nonlinearity, is derived and then solved to find the saturated amplitudes and the power spectrum of the density fluctuations. In the saturated state the density spectrum is dominated by the fluctuations that are linearly damped but are nonlinearly excited and that have spatial variations only along the direction of the ambient density gradient. It is shown that the time‐dependent (increasing) density gradient enhances the saturated amplitude of the dominant modes by a factor of 2 or more, depending on the altitude of the bottomside F layer at sunset. Analysis also shows that the collisional diffusion damping leads to a two‐component density fluctuation spectrum with a shallow slope at long scalelengths (>1 km) and a steep slope at shorter scalelengths. The calculated values of the spectral index p, for a power law of the form kx−p, are p ≅ 2.0 for the shallow slope and p ≅ 3.0 for the steep slope, and they are close to the values determined from the one‐dimensional measurements.
Properties of He+ beams observed by Geotail in the lobe/mantle regions: Comparison with O+ beams
Authors: Seki K. , Hirahara M. , Terasawa T. , Mukai T. , Kokubun S.
Published: September 2002

In the lobe/mantle regions of Earth's magnetotail (<210 RE), the Geotail spacecraft sometimes observed multicomposition ion flows consisting of both ionospheric (H+/He+/O+) and solar wind (H+/He++) ions. Statistics on the He+ beams show that they were observed under similar conditions to those of the O+ beams reported previously. Namely, the heavy ion beams of ionospheric origin tend to exist during geomagnetically active periods in the mantle‐like regions of high plasma beta, where ions of solar wind origin are the major component. The total duration of identified He+ events amounts to 3% of the total observation time in the lobe/mantle region, while that of O+ events amounts to 13%. A remarkable point is that on a short timescale, the He+ and O+ beams often appear to exist nearly exclusively. That is to say, their densities sometimes vary in an opposite sense, even when He+ and O+ coexist. If the O+ and He+ ions are given the same energy in a source region, the initial distribution function of He+ has twice the peak velocity of that of O+, and their alternating appearance and density anticorrelation in the lobe/mantle are easily explained due to the velocity filter effect. Thus the anticorrelation of their densities may suggest that ionospheric ions have undergone an energization which leads to a different velocity for different ion species. A mechanism leading to the same velocity, on the other hand, would require an alternating enhancement of the He+ and O+ fluxes in a source region to explain the anticorrelation of the densities.
Climatic impact of glacial cycle polar motion: Coupled oscillations of ice sheet mass and rotation pole position
Authors: Bills Bruce G. , James Thomas S. , Mengel John G.
Published: September 2002

Precessional motion of Earth's rotation axis relative to its orbit is a well‐known source of long‐period climatic variation. It is less well appreciated that growth and decay of polar ice sheets perturb the symmetry of the global mass distribution enough that the geographic location of the rotation axis will change by at least 15 km and possibly as much as 100 km during a single glacial cycle. This motion of the pole will change the seasonal and latitudinal pattern of temperatures. We present calculations, based on a diurnal average energy balance, which compare the summer and winter temperature anomalies due to a 1° decrease in obliquity with those due to a 1° motion of the rotation pole toward Hudson Bay. Both effects result in peak temperature perturbations of about 1° Celsius. The obliquity change primarily influences the amplitude of the seasonal cycle, while the polar motion primarily changes the annual mean temperatures. The polar motion induced temperature anomaly is such that it will act as a powerful negative feedback on ice sheet growth. We also explore the evolution of the coupled system composed of ice sheet mass and pole position. Oscillatory solutions result from the conflicting constraints of rotational and thermal stability. A positive mass anomaly on an otherwise featureless Earth is in rotational equilibrium only at the poles or the equator. The two polar equilibria are rotationally unstable, and the equatorial equilibrium, though rotationally stable, is thermally unstable. We find that with a plausible choice for the strength of coupling between the thermal and rotational systems, relatively modest external forcing can produce significant response at periods of 104–106 years, but it strongly attenuates polar motion at longer periods. We suggest that these coupled oscillations may contribute to the observed dominance of 100 kyr glacial cycles since the mid‐Pleistocene and will tend to stabilize geographic patterns that are suitable to glaciations.
Active tectonics along the Wadi Araba‐Jordan Valley transform fault
Authors: Galli Paolo
Published: September 2002

A geological study has been carried out along the 200 km long Wadi Araba following the transform fault that separates the Arabian and Sinai‐African Plates. Recent movements along this structure affect upper Pleistocene‐Holocene deposits and archaeological sites. Kinematic indicators show sinistral strike‐slip and oblique movements, in agreement with the relative motion between the two plates. Other evidence of recent horizontal displacement exists along the Jordan Valley Fault, both on the Dead Sea‐Lake Tiberias segment and on the segment north of Lake Tiberias. A minimum horizontal slip rate of 1 cm yr−1has been estimated for both the southern segment of the Wadi Araba Fault and for the southern Jordan River Fault. The two faults can be roughly subdivided into at least four segments: two in the Wadi Araba (80 km long each), one from the Dead Sea to Lake Tiberias (130 km), and one from Lake Tiberias to the Hula graben (>30 km). Faulting may also occur along shorter subsegments, as shown by bends in the Wadi Araba‐Jordan River Fault and by the growth of local compression and extension features. Instrument‐recorded seismicity appears to be mainly concentrated along some of these subsegments. A comparison between the observed seismic and field‐determined slip rates across the fault indicates possible strain accumulation during the last 2000 years.
A global study ofPnanisotropy beneath continents
Authors: Smith Gideon P. , Ekström Göran
Published: September 2002

We apply a two‐station method of velocity determination toPnarrival times reported in the International Seismological Centre Bulletin during the period 1964–1993. The azimuthal variation of these velocities provides evidence ofPnanisotropy as large as 9.6%. Anisotropic regions are identified by examination of the azimuthal variation of thePnvelocities within circular caps of radius 1.5° and 3°, spaced at 1° intervals across the Earth. This procedure leads to 255 geographically distributed estimates of anisotropy. The length scale over which the anisotropy varies indicates that uppermost mantle anisotropy beneath continents is controlled by regional orogenic strains and is inconsistent with a general interpretation in terms of plate‐scale mantle flow. In addition, the correlation betweenPnanisotropy and surficial tectonic features indicates that the anisotropy is primarily the result of the most recent deformation event. We compare ourPnanisotropy results (fast directions and magnitudes) with those ofSKSsplitting studies to attempt to constrain the depth extent of the anisotropy. This comparison suggests that in some areas, where the fast directions agree,SKSsplitting observations may arise from a layer beginning just below the Moho, whereas in others it must have a deeper origin. In these areas, the relative magnitudes of the measured anisotropy are consistent with a thin (100–150 km) lithospheric layer of anisotropy.
Noise in GPS coordinate time series
Authors: Mao Ailin , Harrison Christopher G. A. , Dixon Timothy H.
Published: September 2002

We assess the noise characteristics in time series of daily position estimates for 23 globally distributed Global Positioning System (GPS) stations with 3 years of data, using spectral analysis and Maximum Likelihood Estimation. A combination of white noise and flicker noise appears to be the best model for the noise characteristics of all three position components. Both white and flicker noise amplitudes are smallest in the north component and largest in the vertical component. The white noise part of the vertical component is higher for tropical stations (±23° latitude) compared to midlatitude stations. Velocity error in a GPS coordinate time series may be underestimated by factors of 5–11 if a pure white noise model is assumed.
Seasonal variations in low degree zonal harmonics of the Earth's gravity field from satellite laser ranging observations
Authors: Cheng Minkang , Tapley Byron D.
Published: September 2002

Seasonal variations in the Earth's gravity field were determined using satellite laser ranging (SLR) observations from multisatellite. The time series for the variations of the even zonal harmonics, Jl(l = 2, 4, 6, and 8), were determined using the SLR data from the geodetic satellites, including Starlette, Ajisai, Stella, LAGEOS I and LAGEOS II, during the period from October 1993 to December 1996. Owing to uncertainties in the eccentricity excitation for LAGEOS I and II, the variations of J3 and J5 were determined using only the SLR data from Starlette, Ajisai, and Stella. The seasonal variations of J2, J4, J6 and J8 become separable using the existing multisatellite SLR data sets collected in 15‐day time intervals. The amplitude (normalized and in units of 10−10) and phase (in a cosine conversion and in units of degrees) for the annual variation in Jl (l= 2, 3, 4, 5, 6, and 8) are estimated to be (1.25 ± 0.1, 140 ± 10), (2.16 ± 0.21, 341 ± 19), (1.07 ± 0.1, 338 ± 15), (1.12 ± 0.16, 152 ± 16), (0.26 ± 0.17, 337 ± 9), and (1.03 ± .016, 209 ± 10), respectively. The observed annual variations of J3 and J5 are essentially opposite in phase. This phenomenon results in a different lumped sum effect for various satellites. For example, the lumped sum of J3 and J5 annual variation from LAGEOS I is twice as large as that from Starlette. The excitation due to the mass redistribution in the atmosphere and ocean and the changes in continental water storage were considered in this study using the available global geophysical data, which included the European Centre for Medium‐Rang Weather Forecasts atmospheric surface pressure, the TOPEX/Poseidon altimetry derived sea surface anomalies, and the World Monthly Surface Station Climatic Data. Overall, the variation in the even zonal coefficients due to the atmospheric mass redistribution is responsible for 30% to 60% of the observed annual variations in the node residual for Starlette and LAGEOS I. Comparison indicates that the oceanic mass movement, in particular, the continental water change produce comparable contributions to the seasonal zonal variations. The amplitude of the observed annual variation of J2 is found to fall between the values predicted from the models of the surface water, ocean, and atmosphere with and without the inverted barometer (IB) oceanic response, but the phase is in good agreement with the IB models. The nontidal mass redistributions in atmosphere, ocean, and continental water change can only account for ∼13% of the semiannual variation in J2 but are the primary excitation sources for semiannual variations in the higher‐degree zonal terms.
Single‐domain critical sizes for coercivity and remanence
Authors: Newell Andrew J. , Merrill Ronald T.
Published: September 2002

It is usually assumed that magnetic parameters such as coercivity and saturation remanence are single‐domain (SD) over the same size range. In reality, there is a different SD size range for each parameter. We define critical sizes LSDcoerc for coercivity and LSDrem for remanence. In general, LSDcoerc ≤ LSDrem. Up to L = LSDrem, the saturation remanent state is single‐domain. If a sufficiently large reverse field is applied, a conventional SD state would reverse by uniform rotation. However, the mode of reversal is nonuniform if the grain size is between LSDcoerc and LSDrem, so in this size range the SD state is less stable. To calculate the critical sizes, we use rigorous nucleation theory and obtain analytical expressions. The analytical form allows us to explore the effect of grain shape, stress, crystallographic orientation and titanium content in titanomagnetites. We adapt the theory to cubic anisotropy with K1 < 0, which allows us to apply the expressions to titanomagnetites. We find that the size range for SD coercivity is always small. The size range for SD remanence can vary enormously depending on the anisotropy. If the easy axes are oriented favorably, the SD state can occur in large x = 0.6 titanomagnetite grains. Ensembles of magnetite grains with aspect ratios greater than 5 have SD‐like remanence but low coercivity. However, most synthetic magnetite grains are nearly equant, and the predicted size range for SD remanence is small to nonexistent. This, rather than grain interactions, may be the reason they have properties such as saturation remanence that do not agree well with standard SD theory.
Melting depths and mantle heterogeneity beneath Hawaii and the East Pacific Rise: Constraints from Na/Ti and rare earth element ratios
Authors: Putirka Keith
Published: September 2002

Mantle melting calculations are presented that place constraints on the mineralogy of the basalt source region and partial melting depths for oceanic basalts. Melting depths are obtained from pressure‐sensitive mineral‐melt partition coefficients for Na, Ti, Hf, and the rare earth elements (REE). Melting depths are estimated by comparing model aggregate melt compositions to natural basalts from Hawaii and the East Pacific Rise (EPR). Variations in melting depths in a peridotite mantle are sufficient to yield observed differences in Na/Ti, Lu/Hf, and Sm/Yb between Hawaii and the EPR. Initial melting depths of 95–120 km are calculated for EPR basalts, while melting depths of 200–400 km are calculated for Hawaii, indicating a mantle that is 300°C hotter at Hawaii. Some isotope ratios at Hawaii are correlated with Na/Ti, indicating vertical stratification to isotopic heterogeneity in the mantle; similar comparisons involving EPR lavas support a layered mantle model. Abundances of Na, Ti, and REE indicate that garnet pyroxenite and eclogite are unlikely source components at Hawaii and may be unnecessary at the EPR. The result that some geochemical features of oceanic lavas appear to require only minor variations in mantle mineral proportions (2% or less) may have important implications regarding the efficiency of mantle mixing. Heterogeneity required by isotopic studies might be accompanied by only subtle differences in bulk composition, and material that is recycled at subduction zones might not persist as mineralogically distinct mantle components.
Magnitude and timing of New Hebrides Arc rotation: Paleomagnetic evidence from Nendo, Solomon Islands
Authors: Musgrave Robert J. , Firth John V.
Published: September 2002

Paleomagnetic results from Nendo record a total rotation of the northwestern corner of the New Hebrides Arc of 52° ± 12° clockwise. Reversed magnetic polarity and a nannofossil assemblage constrain the age of the Nendo pole to 4.29 ‐ 3.58 Ma. Paleomagnetic data from Malekula, farther south in the same arc, imply a rotation of only about 28° since the late Miocene, probably because these poles have been affected by rotation of Malekula in response to collision of the arc with the D'Entrecasteaux Zone aseismic ridge. A new late Miocene reconstruction of the New Hebrides and Vitiaz arcs based on the paleomagnetic evidence requires a rotation of 39° around the point of intersection of the two arcs. The same rotation restores both the northwestern and southeastern segments of the New Hebrides Arc, although rotation of the southeastern segment commenced earlier. A series of sequential movements of segments of the arc, occurring at different times but acting to keep the arc roughly collinear, may be driven by a mechanism which minimizes deformation of the subducted plate. Propagation of spreading into the northwestern corner of the North Fiji Basin after 4.3 Ma is implied by the paleomagnetic results, and is consistent with spreading anomaly patterns in the basin. When the New Hebrides and Vitiaz Arcs are restored to their position relative to the Lau Ridge at 10 Ma, the Fiji Platform can be accommodated after clockwise rotation of 140°, to a position in line with the New Hebrides Arc. The large rotation for Fiji is consistent with paleomagnetic evidence indicating at least 115° of rotation.
Shallow subduction tectonics in the Raukumara Peninsula, New Zealand, as illuminated by earthquake focal mechanisms
Authors: Reyners Martin , McGinty Peter
Published: September 2002

The Raukumara Peninsula affords an excellent opportunity to study the subduction process, as subduction of the buoyant Hikurangi Plateau on the Pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust. Here we report on the focal mechanisms of 117 earthquakes of ML 2.4–4.9 and shallower than 80 km, recorded during a 5‐month deployment of 36 portable seismographs on the peninsula. Mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes. Downdip tensional strain predominates in the subducted plate, with T axes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone. Trenchward extensional strain is seen in the uppermost part of the overlying Australian plate, in line with geodetic and geological results. This can be related to extension and gravity sliding of surficial rocks due to uplift of the Raukumara range resulting from underplating of subducted sediment. There is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface. A concentration of low‐angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest. This along‐strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically.
Influence of the shape of the friction law and fault finiteness on the duration of initiation
Authors: Ionescu Ioan R. , Campillo Michel
Published: September 2002

We consider the two‐dimensional (2‐D) elastic problem of slip instability under slip dependent friction. This paper concentrates on the parameters that determine the duration of the initiation phase, that is, the delay between an initial small perturbation of the system at the metastable equilibrium and the onset of dynamic rupture propagation. We first consider the case of a homogeneous fault (i.e., with infinite length) with a slip dependent friction with varying weakening rate. We show that different laws associated with the same values of stress drop and critical slip lead to a broad range of initiation duration. The duration is mainly governed by the slope of the friction law at the origin. We interpret these results using the analytical solution proposed by Campillo and Ionescu [1997] for the case of a constant weakening rate. These late results suggest a definition of a characteristic length associated with the rate of weakening at the origin. When the region of slip is limited to a finite weak patch, we found that the duration of initiation varies rapidly when the fault length is of the order of the characteristic length. Under these conditions the initiation duration increases extremely rapidly with decreasing fault length up to about 100 s in the numerical experiments we carried out. These results suggest that very simple elastic models with slip dependent friction and realistic values of the parameters could explain a broad range of delay of the onset of rupture propagation.
Control of crater morphology on flow path direction of Soufrière‐type pyroclastic flows
Authors: Lagmay Mahar A. , Pyle David M. , Dade Brian , Oppenheimer Clive
Published: September 2002

We present a model of fountain collapse for small‐scale (Soufrière‐type) explosive eruptions that relates the asymmetry of a volcanic crater (e.g., the presence of crater notches) with the emplacement direction of pyroclastic flows. Analysis of two‐dimensional simulations of compressible fluids emanating from asymmetric nozzles shows that under sonic to supersonic conditions, the streamline of a jet can become tilted. The inclination of the streamline increases with greater slant angle of the nozzle and with increasing exit pressure. Using the two‐dimensional simulations as analogues to volcanic eruptions, we propose that pyroclastic ejecta within the inner core of an erupting jet column can become asymmetrically focused before collapsing at fountain heights of a few hundred meters above the crater exit. The ensuing pyroclastic flows associated with fountain collapse thus become directional in character with flow orientation controlled by crater geometry and eruption exit pressure. The model applies to volcanoes with vertical conduits and crater to vent geometries that act as effective sonic to supersonic jet nozzles. We propose that the 1984 eruption of Mayon volcano fits this model. In the second phase of this eruption a prominent crater notch imparted a southeastward tilt to the basal gas thrust region of the eruption column. In turn, this led to the discharge of pyroclastic flows onto the southeast flank of the volcano.
Age constraints on crustal recycling to the mantle beneath the southern Chile Ridge: He‐Pb‐Sr‐Nd isotope systematics
Authors: Sturm Marnie E. , Klein Emily M. , Graham David W. , Karsten Jill
Published: September 2002

Basalts from the four southernmost segments of the subducting Chile Ridge (numbered 1–4 stepping away from the trench) display large variations in Sr, Nd, Pb, and He isotope and trace element compositions. Klein and Karsten [1995] showed that segments 1 and 3 display clear trace element evidence for recycled material in their source (e.g., low Ce/Pb). The uniformly mid‐ocean ridge basalt (MORB)‐like 3He/4He and modest variations in Pb, Sr, and Nd isotopes of segment 1 (nearest the trench) suggest recent (<20 Ma) introduction of a contaminant into its source, consistent with recycling of material from the adjacent subduction zone. In contrast, segment 3 lavas display a dramatic southward increase in enrichment, extending to highly radiogenic Pb and Sr isotopic compositions (e.g., 206Pb/204Pb = 19.5) and the lowest 3He/4He yet measured in MORB (3.5RA). The segment 3 variations are most readily explained by ancient (∼2 Ga) recycling of terrigenous sediment and altered crust, but we cannot rule out more recent recycling of material derived from a distant continental source. The similarity in isotopic signatures of segment 4 lavas to Indian Ocean MORB extends the Dupal anomaly to the Chile Ridge. Like Indian Ocean MORB, the segment 4 isotopic variations are consistent with contamination by anciently recycled pelagic sediment and altered crust and require a complex history involving at least three stages of evolution and possibly a more recent enrichment event. Southern Chile Ridge MORB reflect the extensive degree of heterogeneity that is introduced into the depleted upper mantle by diverse processes associated with recycling. These heterogeneities occur on a scale of ∼50–100 km, corresponding to transform‐ and propagating‐rift‐bounded segmentation, and attest to the presence of distinct chemical domains in the mantle often bounded by surficial tectonic features that maintain their integrity on the scale sampled by melting.
Spatial and temporal strain rate variations at the northern Hikurangi margin, New Zealand
Authors: Árnadóttir Thora , Thornley Steve , Pollitz Fred F. , Darby Desmond J.
Published: September 2002

We have calculated shear strain rates in the northeastern part of the North Island of New Zealand, using geodetic data collected in the mid‐1920s, and 1995. In addition we use observations from a part of the network across Raukumara Peninsula measured in 1976, to look for temporal variation in shear strain rates in that area. The strain rates were estimated using a simultaneous reduction method, assuming uniform strain in space and time within each subnetwork. We find the largest spatial variation in shear strain rates from the mid‐1920s to 1995 occurs across Raukumara Peninsula, increasing from 0.06 ± 0.04 ppm/yr in the west to 0.18 ± 0.03 ppm/yr in the east (1σ confidence). The orientation of the azimuth of relative extension shows significant temporal variation across Raukumara Peninsula. From the mid‐1920s to 1976 the maximum shear strain rate was 0.22 ± 0.05 ppm/yr with the azimuth of relative extension oriented at 129° ± 7°. From 1976 to 1995 the maximum shear strain rate was 0.14 ± 0.05 ppm/yr with the azimuth of relative extension oriented at 13° ± 12°. We show that a model where the subduction interface is locked down to 15 or 30 km depth, and slipping at NUVEL‐IA plate rates below that depth, can explain the strain rates and orientation observed during 1976–1995. To fit the observations from the mid‐1920s to 1976, we suggest that additional dip slip occurred on the shallow part of the subduction zone, above 30 km. From this we conclude that the plate boundary below Raukumara Peninsula is locked down to 15 or 30 km depth and the aseismic shallow slip episode that occurred during the mid‐1920s–1976 was an isolated event, possibly induced byM≥ 7 historical earthquakes in the area.
Self‐organization and scaling relationships of evolving river networks
Authors: Pelletier Jon D.
Published: September 2002

The power spectra S of linear transects of Earth's topography is often observed to be a power law function of wave number k with exponent close to −2: S(k) ∝ k−2. In addition, river networks are fractal trees that satisfy several power law relationships between their morphologic components. A model equation for the evolution of Earth's topography by transport‐limited erosional processes which produces fractal topography and fractal river networks is presented, and its solutions are compared in detail to real topography. The model is the diffusion equation for sediment transport on hillslopes and channels with the diffusivity constant on hillslopes and proportional to the three‐halves power of discharge in channels. The dependence of diffusivity on discharge is consistent with sediment rating curves. We study the model in two ways. In the first analysis the diffusivity is parameterized as a function of elevation, and a Taylor expansion procedure is carried out to obtain a differential equation for the landform elevation which includes the spatially variable diffusivity to first order in the elevation. The solution to this equation is a self‐affine or fractal surface with linear transects that have power spectra S(k) ∝ k−1.8, independent of the age of the topography, consistent with observations of real topography. The hypsometry produced by the model equation is skewed such that lowlands make up a larger fraction of the total area than highlands as observed in real topography. In the second analysis we include river networks explicitly in a numerical simulation by calculating the discharge at every point. We characterize the morphology of real river basins with five independent scaling relations between six morphometric variables. Scaling exponents are calculated for seven river networks from a variety of tectonic environments using high‐quality digital elevation models. River networks formed in the model match the observed scaling laws and satisfy Tokunaga side‐branching statistics.
Picophytoplankton dynamics in the equatorial Pacific: Growth and grazing rates from cytometric counts
Authors: André J.‐M. , Navarette C. , Blanchot J. , Radenac M.‐H.
Published: September 2002

During a 7 day time series in the central equatorial Pacific (0°, 150°W, October 1994) flow cytometry measurements were performed four times per day throughout the surface layer. Cell abundance of the major algal groups, Prochlorococcus, picoeukaryotes, and Synechococcus, exhibited a well‐marked diel rhythm within the mixed layer (50–60 m deep) whereas the signal became unclear below. Cell numbers were minimum at the midday or dusk stations and maximum in the middle of the night. The amplitude of the diel variations in the mixed layer, as observed, was of the order of 40% of the daily minimum and varied significantly during the time series. For each cell group the abundance variations implied that each day, cell division was rather tightly synchronized and that grazing was efficiently competing growth. Assuming that abundance variations were only due to these two processes, a simple model was designed to estimate their rates. The division rates averaged 0.53 (±0.18) d−1 for Prochlorococcus, 0.42 (±0.13) d−1 for the picoeukaryotes, and 0.56 (±0.21) d−1 for Synechococcus in the mixed layer and decreased rapidly below. The rates varied significantly along the time series for all groups whereas growth and grazing closely balanced at the day scale. The estimates compared well with those obtained using various methods during the time series and previously in the equatorial Pacific. Primary production was tentatively predicted from the growth rates. Prochlorococcus, the picoeukaryotes, and Synechococcus contributed 57%, 33%, and 10% of the picoplankton total, and the predictions were consistent with the 14C measurements during the time series.
The time‐mean circulation in the Agulhas region determined with the ensemble smoother
Authors: Van Leeuwen Peter Jan
Published: September 2002

The time‐mean circulation in the Agulhas Retroflection area is determined by combining TOPEX/POSEIDON data and a two‐layer quasi‐geostrophic model using the ensemble smoother. By taking the time‐mean circulation as the unknown in the data assimilation procedure, the time‐varying altimeter signal is used to constrain the time‐mean field. The quasi‐geostrophic model is applied as a strong constraint, with only the time‐mean circulation containing errors. Inspection of the posterior penalty function showed that the inversion was successful. The errors in the time‐mean sea surface topography reduced from about 10 to about 3 cm. A cyclonic recirculation cell over the Agulhas Plateau was found, related to the northward meander of the Agulhas Return Current. Another cyclonic recirculation cell was found west of Africa, probably related to the passage of anticyclonic Agulhas Rings south of it. The new field is compared with advanced very high resolution radiometer infrared satellite data, confirming the northward meander of the Agulhas Return Current.