bluuebunny/crossref_metadata_2025 huggingface dataset:
Rate of helicity production by solar rotation
Authors: Berger Mitchell A. , Ruzmaikin Alexander
Published: September 2002

In recent years, solar observers have discovered a striking pattern in the distribution of coronal magnetic structures: northern hemisphere structures tend to have negative magnetic helicity, while structures in the south tend to have positive magnetic helicity. This hemispheric dependence extends from photospheric observations to in situ measurements of magnetic clouds in the solar wind. Understanding the source of the hemispheric sign dependence, as well as its implications for solar and space physics has become known as the solar chirality problem. Rotation of open fields creates the Parker spiral which carries outward 1047 Mx2 of magnetic helicity (in each hemisphere) during a solar cycle. In addition, rough estimates suggest that each hemisphere sheds on the order 1045 Mx2 in coronal mass ejections each cycle. Both the α effect (arising from helical turbulence) and the Ω effect (arising from differential rotation) should contribute to the hemispheric chirality. We show that the Ω effect contribution can be captured in a surface integral, even though the helicity itself is stored deep in the convection zone. We then evaluate this surface integral using solar magnetogram data and differential rotation curves. Throughout the 22 year cycle studied (1976–1998) the helicity production in the interior by differential rotation had the correct sign compared to observations of coronal structures ‐ negative in the north and positive in the south. The net helicity flow into each hemisphere over this cycle was approximately 4 × 1046 Mx2. For comparison, we estimate the α effect contribution; this may well be as high or higher than the differential rotation contribution. The subsurface helicity can be transported to the corona with buoyant rising flux tubes. Evidently only a small fraction of the subsurface helicity escapes to the surface to supply coronal mass ejections.
Role of collisionless heat flux in magnetospheric convection
Authors: Heinemann M.
Published: September 2002

A fluid model of magnetospheric convection appropriate for the inner magnetosphere, including the effects of heat flux in collisionless plasma, is presented. The plasma is assumed to be isotropic, with the flow speed much less than the thermal speed, and parallel electric fields and loss cone effects are neglected; the effects of slow time variations of the magnetic field are included. The classical transport coefficients are considered and, except for the collisionless heat flux, shown to be negligible in plasma in the inner magnetosphere. Beginning with three‐dimensional two‐fluid equations, we derive two‐dimensional equations for transport of mass and energy mapped to the magnetospheric equator. The equation of mass transport, derived from the mass conservation equations, is equivalent to those obtained in previous studies [e.g., Peymirat and Fontaine, 1994]. The equation of energy transport contains the effects of collisionless heat conduction that represents the transport of energy in the rest frame of the species and has hitherto been neglected in magnetospheric fluid and MHD models. The energy transport equation is shown to be equivalent to that of Peymirat and Fontaine [1994] if the heat flux is neglected. The two equations are coupled first‐order partial differential equations; they can be uncoupled by taking linear combinations. The uncoupled equations show that the effect of the collisionless heat flux is to spread information across the fluid drift paths in a manner quite different from that of fluid flow neglecting heat flux.
A survey of MHD waves in the magnetosheath: International Solar Terrestial Program observations
Authors: Sibeck D. G. , Phan T.‐D. , Lin R. P. , Lepping R. P. , Mukai T. , Kokubun S.
Published: September 2002

We present case and statistical studies of magnetohydrodynamic (MHD) fluctuations observed by the Wind and Geotail spacecraft in the Earth's dawn and dusk magnetosheath. A case study with simultaneous IMP 8 interplanetary magnetic field observations confirms theoretical predictions that most of the fluctuations originate in the solar wind rather than at the magnetopause and that the fluctuations can be almost absent from the outer magnetosheath. A second case study indicates that northward plasma velocity perturbations observed in the prenoon magnetosheath correspond to southward velocity perturbations in the postnoon magnetosheath, implying that the MHD perturbations apply a torque to the magnetosphere. A statistical survey reveals that the fluctuations almost invariably propagate antisunward in the magnetosheath, independent of their propagation direction relative to the interplanetary magnetic field.
Magnetic moment scattering in a field reversal with nonzero BY component
Authors: Delcourt D. C. , Zelenyi L. M. , Sauvaud J.‐A.
Published: September 2002

We examine the nonadiabatic motion of charged particles in a field reversal with nonzero BY. We show that magnetic moment variations are organized into three categories: (1) at small equatorial pitch angles, large magnetic moment enhancements regardless of gyration phase, (2) at intermediate pitch angles, a prominent dependence upon phase with either magnetic moment enhancement or damping, and (3) at large pitch angles, negligible magnetic moment changes. This three‐branch pattern of magnetic moment variations resembles that obtained for zero BY, but it is here more or less developed when particles enter above or below the field reversal. We investigate this effect using the centrifugal impulse model developed for zero BY, whereby nonadiabatic behavior is viewed as the result of perturbation of the gyromotion by an impulsive centrifugal force. We show that nonzero BY leads to a rotation of the centrifugal impulse in the gyration plane and that the nonadiabatic particle behavior is attenuated or enhanced when this rotation opposes or goes together with the gyromotion, respectively. As a consequence of this, particles with opposite charge states or originating from opposite hemispheres behave in quite distinct manners, exhibiting for instance large or negligible magnetic moment changes depending upon their direction of propagation. More generally, we demonstrate that prominent hemispherical differences are obtained as a result of nonzero BY, be it for injection inside the loss cone or gyrophase bunching near the current sheet midplane.
Wind observations of mixed magnetosheath‐plasma sheet ions deep inside the magnetosphere
Authors: Phan Tai D. , Lin Robert P. , Fuselier Stephen A. , Fujimoto Masaki
Published: September 2002

We have analyzed in detail a Wind spacecraft crossing of the high‐latitude magnetosheath, magnetopause, lobe, and the high‐ and low‐latitude plasma sheet on November 27, 1998. The crossing occurred during the first perigee pass of Wind's (new) high‐inclination (45°) petal orbit. Between the lobe and the hot plasma sheet on the duskside, an extended region (xGSM∼ −2.8 to −7.8 RE, yGSM∼ 7.8 RE, zGSM∼ −11.6 to −4.3 RE) of mixed low‐energy magnetosheath and high‐energy plasma sheet ion populations was detected. This region was detected in a region of strong (∼ 40 nT) and steady lobe‐like magnetic field and ceased to exist when the spacecraft approached the neutral sheet. The mixed ion populations are remarkably similar (in their thermal properties) to the distributions detected in the adjacent magnetosheath and plasma sheet proper, except that the mixed ions are nearly stagnant. As the neutral sheet is approached, the low‐energy component of the mixed ions is gradually heated, while the high‐energy component is unmodified. Near the neutral sheet the ion distributions are dominated by a single, high‐energy population. The electron behavior is significantly different from that of the ions. In the mixed ion region the electrons consist of a single population with energy between those of the magnetosheath and the plasma sheet proper. Electron pitch angle information suggests that the entire mixed region is on closed field lines. However, the large reduction of high‐energy plasma sheet electrons in this region may indicate that the field lines threading this regions were once open. The exact path of plasma entry to form the mixed region cannot be discerned with single‐point observations, especially since the mixed ions are stagnant. However, it is possible (for this event) to rule out the mantle as a source for the low‐energy component of the mixed ions. The similarity between the thermal property of the low‐energy component of the mixed ions and the magnetosheath population suggests that the magnetosheath plasma had direct access to this region without significant heating along its path. The presence of nearly unmodified magnetosheath plasma deep inside the magnetosphere raises questions concerning the processes of plasma entry across the magnetopause. Finally, the mixed region in this event was detected during an extended period of persistent northward and duskward interplanetary magnetic field, which makes this event ideal for model comparisons.
High‐ and low‐altitude observations of adiabatic parameters associated with auroral electron acceleration
Authors: Shiokawa K. , Baumjohann W. , Haerendel G. , Fukunishi H.
Published: September 2002

Electron density and temperature, adiabatic thermal current, and field‐aligned conductivity have been estimated on the basis of observations by the Defense Meteorological Satellite Program (DMSP) satellites above the auroral oval and from measurement by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (IRM) satellite in the near‐Earth plasma sheet. We found that the estimated densities are comparable between these two satellites, while the temperatures obtained from an accelerated Maxwellian fitting procedure used on the DMSP spectra are far lower than those measured by IRM in the near‐Earth plasma sheet. From this temperature discrepancy we conclude that the accelerated electrons do not come from the equatorial plane of the magnetosphere but from the region just above the field‐aligned potential difference at an altitude of a few RE. The DMSP data show that a large field‐aligned potential difference, which accelerates auroral electrons downward, is formed in the region with low field‐aligned conductivity. The IRM data show that the field‐aligned conductivity decreases with increasing XGSM distance, increasing AE index, and after earthward high‐speed flow passage. The adiabatic thermal current estimated from the IRM data is found to be not enough to supply typical auroral current. Though the mechanism that produces field‐aligned potential difference has not been identified yet, these results suggest that field‐aligned potential difference is formed to keep the balance between the field‐aligned current generated by magnetospheric processes and the current carried away from the generator region by accelerated electrons.
Energetic magnetosheath ions connected to the Earth's bow shock: Possible source of cusp energetic ions
Authors: Chang S.‐W. , Scudder J. D. , Fennell J. F. , Friedel R. , Lepping R. P. , Russell C. T. , Trattner K. J. , Fuselier S. A. , Peterson W. K. , Spence H. E.
Published: September 2002

Plasma and magnetic field data detected by NASA GGS/Polar and Wind during the May 4, 1998, storm event are analyzed to demonstrate for the first time a causal relation between the magnetosheath energetic ions and bow shock magnetic geometry. Intense magnetosheath energetic ions observed upstream from the cusp are from the quasi‐parallel bow shock and show properties indicating that they are a possible source of cusp energetic ions.
Radio wave scattering in the outer heliosphere
Authors: Armstrong J. W. , Coles W. A. , Rickett B. J.
Published: September 2002

The Voyager 1 and 2 plasma wave instruments have observed low‐frequency (1.5–4 kHz) radio waves apparently generated near the heliopause. The waves are found in two distinct bands. Power at the lower frequencies centered on 1.78 kHz shows no modulation when the spacecraft is rolled about the Earth‐spacecraft line, indicating that the radiation is isotropic. Power in the higher band centered on 3.11 kHz shows roll modulation as high as 60%, indicating that the source of the radiation is quite compact, subtending an angle <1 rad at the spacecraft. Simple estimates of the scattering of 3.11 kHz radiation from electron density fluctuations indicate that, if the radiation had originated from the distance of the heliopause, the scattering would be so large that no roll modulation should be observed. Here we show that these earlier scattering estimates were too high because they ignored the latitude variation of scattering and the inner scale of the electron density fluctuation spectrum. With these two effects properly included, the expected scattering is consistent with the observations and the postulate that the radiation originates from near the nose of the heliosphere.
Computer studies of the three‐dimensional magnetic reconnection with the superimposed By component
Authors: Lee Ensang , Min Kyoung‐Wook , Seon Jongho , Lee L. C. , Ryu Dongsu
Published: September 2002

Three‐dimensional magnetic reconnection is studied using magnetohydrodynamic simulations. The initial configuration is based on the two‐dimensional Harris neutral sheet model that lies in the xz plane and is extended in the y direction. Localized anomalous resistivity is applied to the central region, and the subsequent evolution of spontaneous magnetic reconnection is observed. Special attention is given to the results with a finite By superimposed on the Harris model. Significant changes are seen in the reconnection morphology, as the By component causes asymmetries. The reconnected field lines are skewed, and the plasma flows, shock structures, and current flows show the corresponding peculiar asymmetries. The plasma sheet is also seen twisted. A broader region is affected by magnetic reconnection as By increases, and it is seen that energy conversion over the whole simulation domain is more significant when By is larger, unless By is the dominant component of the magnetic field. The field‐aligned component of the current, which initially exists because of the finite By component, is enhanced off the central plane when reconnection develops, while it is reduced on the central plane. The spatial scale of resistivity affects the reconnection rate as in previous studies of By = 0, yielding a small energy conversion for a very localized model resistivity.
Difference in the current‐voltage relationships between dawn and duskside inverted‐V events
Authors: Olsson A. , Janhunen P.
Published: September 2002

We analyze 81 Freja inverted‐V events to study the current‐volt age relationship in different magnetic local time sectors. We use data from the F7 (TESP) electron detector. The events are categorized according to the current‐voltage relationship so that in categories named U, C, and O the current j increases, is constant, or decreases with the voltage V, respectively. We find that the U and C categories are the most common on the eveningside, while the O category dominates the morningside. Globally, category U is the least common. Besides the current density, we also study the behavior of the source plasma density and temperature as a function of the voltage in each category and find that source plasma density is nearly constant in U category, decreases slightly in C, and decreases strongly in O. The source plasma temperature is linearly correlated with the voltage and behaves rather uniformly in all categories. We suggest that there may be an important physical difference between morningside and eveningside inverted‐V events and discuss possible explanations in terms of different auroral potential structure models.
Ringed structural zones with deep roots formed by the Chicxulub impact
Authors: Snyder D. B. , Hobbs R. W.
Published: September 2002

The Chicxulub meteorite/comet impact of 65 Ma occurred on the present northeast coast of the Yucatan peninsula of Mexico and is considered by many to have caused a mass extinction at the end of the Cretaceous period. Impact craters are traditionally described in terms of ring structures. Deeply rooted faults underlying four previously recognized ring zones of the Chicxulub impact crater, as revealed on new deep seismic reflection profiles, demonstrate the catastrophic and diverse deformations that produced these zones. The innermost annular zone at Chicxulub coincides with a central peak ring. The seismic data show that the peak ring is composed of low‐velocity (<5 km s−1), chaotically reflective material, interpreted as impact breccia, lying on top of blocks of pre‐impact strata that were downdropped 4–6 km. Normal faults within the Cretaceous strata and deep, inward dipping reflections characterize two deformation zones at radial distances of 55–65 and 85–98 km. These normal faults accommodated the collapse, during the modification stage of crater development, of large (radial width of 5–25 km) blocks of pre‐impact strata uplifted during the excavation stage. The structures root in the lower crust at depths of 15–30 km. Blind thrusts indicated by monoclines in pre‐impact sedimentary rocks and deep, inward dipping reflections within the crystalline basement mark the outermost ring of deformation. These monoclines correlate with small topographic changes on the Cretaceous‐Tertiary boundary layer at radial distances of 120–135 km. Palinspastic restoration of coherent blocks of downdropped pre‐impact strata enabled improved estimates of the size and shape of the hole formed by the impact in a prominent reflector at 3‐ to 4‐km depth. This hole is slightly elliptical along a SE‐NW major axis (radius of 68 versus 63 km).
Some thoughts on the stability of cratonic lithosphere: Effects of buoyancy and viscosity
Authors: Lenardic A. , Moresi L.‐N.
Published: September 2002

Continental cratons have not experienced major tectonic disruptions over a timescale of 109 years. The thickness of cratonic lithosphere also appears to have changed little over this timescale. These observations are often attributed to the presence of chemically buoyant and/or highly viscous subcratonic roots. Simple physical scaling relationships are developed to explore the buoyancy and/or viscosity conditions required to stabilize such roots against large‐scale deformation and rapid remixing into the mantle. The scalings are tested using idealized numerical simulations with good general agreement. Applied to Earth, the scalings suggest that (1) buoyancy alone is unlikely to stabilize cratonic roots and (2) if root viscosity is to provide stability into the Archean, then roots must be 103 times as viscous as the mantle. Based on available experimental data, root dehydration cannot account for the required viscosity increase. Temperature‐dependent viscosity can stabilize roots, but it does so at the expense of stagnating the entire mantle lithosphere, i.e., at the expense of sacrificing plate tectonics. This suggests that the plastic yielding properties of rocks at low temperatures will need to be more directly accounted for in future experiments exploring root stability.
Self‐similarity and multifractality of topographic surfaces at basin and subbasin scales
Authors: Veneziano Daniele , Iacobellis Vito
Published: September 2002

Self‐similarity of fluvial erosion topography manifests itself through statistical renormalization properties of drainage networks and river profiles. Results based on these renormalization properties are often inconsistent with those from spectral or variogram analysis of linear transects. We show that the inconsistency is due to two factors: (1) Network and transect analyses are based on different self‐similarity conditions, and (2) elevation along transects depends on the hillslopes, where self‐similarity either does not apply or applies with different parameters. As a result, transect methods produce misleading results when the topography has finite hillslopes. We also consider whether self‐similarity is sufficient to describe the scaling invariance of natural river basin topography or more sophisticated multifractal conditions should be used. The conclusion of multifractality reached in recent studies was based on the use of gradient amplitudes rather than elevation increments. The gradient amplitude method is found to produce spurious multifractality also in cases when the surface is known to be self‐similar. When more appropriate methods are used, topography shows little evidence of multifractality. We support our conclusions through numerical analysis of synthetic and natural river basins.
Influence of borehole overpressurization and plastic yielding on the Stoneley and flexural dispersions
Authors: Sinha Bikash K. , Papanastasiou Panos , Plona Thomas J.
Published: September 2002

During overbalanced drilling, sonic measurements in a borehole are generally made in the presence of overburden, radial, and hoop stresses caused by a borehole fluid overpressure. The associated static deformations of the formation in the borehole vicinity may have both elastic and plastic strains prior to the initiation of tensile fractures. The influence of such static deformations on the borehole Stoneley and flexural dispersions has been calculated from a perturbation model for equations of motion for small dynamic fields superposed on a static bias. The static bias refers to the prestress and associated elastic and plastic strains in the formation prior to any tensile fracture. Computational results for the Castlegate sandstone formation show characteristic differences in the Stoneley and flexural dispersions, notably at higher frequencies. For a borehole of diameter 20 cm (8 inches) with an overpressurization of 10 MPa in a formation with a confining isotropic stress of 5 MPa both the Stoneley and flexural mode velocities increase by ∼5–10% at frequencies larger than 5 kHz compared to model predictions that do not account for the prestress. When the overpressure is increased to 20 MPa, there is a slight increase in the plastic deformation close to the borehole surface. Most of the changes in the Stoneley and flexural dispersions in the intermediate frequency range (3–8 kHz) are caused by an increase in the borehole pressure rather than a small increase in the plastic deformation. These changes in borehole dispersions are measures of formation stress sensitivity and the radial extent of stress‐induced alteration in the borehole vicinity. However, the Stoneley dispersion for frequencies up to 2 kHz and flexural dispersion for frequencies up to 3 kHz are largely insensitive to the near‐field stresses caused by borehole pressures and any associated plastic deformation extending to ∼5 cm into the formation in a 20 cm borehole.
The development of slabs in the upper mantle: Insights from numerical and laboratory experiments
Authors: Becker Thorsten W. , Faccenna Caludio , O'Connell Richard J. , Giardini Domenico
Published: September 2002

We have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab. A two‐dimensional finite element code is applied to model Newtonian creep in the numerical experiments. Scaled analog media are used in the laboratory, a sand mixture models the brittle crust, silicone putty simulates creep in the lower crust and mantle lithosphere, and glucose syrup is the asthenosphere analog. Both model approaches show similar results and reproduce first‐order observations of the subduction process in nature based on density and viscosity heterogeneities in a Stokes flow model. Subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold. We find that the subduction velocity and angle are time‐dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670‐km discontinuity. The style of subduction is controlled by the prescribed velocity of convergence, the density contrast between the plates, and the viscosity contrast between the oceanic plate and the mantle. These factors can be combined in the buoyancy number F which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate. Variations in F control the stress in the plates, the speed and the dip of subduction, and the rate of trench retreat, reproducing the contrasting styles of subduction observed in nature. The subduction rate is strongly influenced by the work of bending the lithosphere as it subducts.
Forces driving ridge propagation
Authors: West Brian P. , Lin Jian , Christie David M.
Published: September 2002

Recent multibeam bathymetric and geophysical data from the Southeast Indian Ridge (SEIR) reveal eight propagating ridges (PRs) moving down the regional bathymetry gradient toward the Australian‐Antarctic Discordance (AAD). These PRs are propagating at a nearly uniform rate of 40‐mm yr−1 despite dramatic variations in axial morphology and segment length, and local reversals of the regional bathymetric gradient. Because existing dynamic rift propagation models do not explain the continued, uniform propagation of the diverse SEIR PRs, we have developed a new model that includes variable crustal thickness and thermally‐driven, along‐axis asthenospheric flow, in addition to topographic gradients, as positive contributions to the forces driving rift propagation. Along‐axis asthenospheric flow and variations in crustal thickness along the SEIR appear to provide the first‐order driving force required for ridge propagation, even in the highly segmented, magmatically starved region in and near the AAD where previous models for ridge propagation fail. Further, our model development requires that the rift valley at the propagating tip is a consequence of rifting in the cooler thermal regime of preexisting lithospherc, rather than a consequence of viscous resistance to flow. This explanation of PR tip topography is consistent with gravity data from the SEIR that suggest that the rift tip and associated pseudofaults are at least partially compensated by crustal thinning.
Elastic properties of hydrate‐bearing sediments using effective medium theory
Authors: Jakobsen Morten , Hudson John A. , Minshull Tim A. , Singh Satish C.
Published: September 2002

Accurate and detailed models of the seismic velocity structure of gas hydrate‐bearing sediments may be determined by careful analysis of controlled source seismic data. However, interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulae and/or simple effective medium theory. We develop a rigorous theoretical scheme to relate the seismic properties of a clay‐rich hydrate‐bearing sediment to its porosity, mineralogy, microstructural features and hydrate saturation. We consider separately the two possible end‐members for the distribution of hydrate in the pore space: (1) hydrates are unconnected and located in the pore voids without appreciable grain contact and (2) connected hydrates are forming cement binding around the grains. The scheme is transversely isotropic, to allow for anisotropy due to alignment of clay platelets, and is based on a combination of a self‐consistent approximation, a differential effective medium theory, and a method of smoothing for crystalline aggregates. We have applied the scheme to lithological and seismic velocity data from Ocean Drilling Program Site 995 on the Blake Ridge (southeastern U.S. continental margin) to make estimates of the hydrate saturation. It was found that the hydrates are probably unconnected, and their volume concentration varies between ∼0% at 100 m below the seabed and ∼9% at 400 m depth, just above the “bottom simulating reflector”, if the clay platelet orientation distribution resembles the function we have used.
In situ control of oxygen fugacity at high temperature and high pressure
Authors: Li Heping , Xie Hongsen , Guo Jie , Zhang Yueming , Xu Zuming , Liu Congqiang
Published: September 2002

A new technique to control oxygen fugacity in situ under high‐temperature and high‐pressure conditions has been developed. In this method a special sample assembly constructed of a reference buffer, yttrium‐stabilized zirconia (YSZ) disks, a sample, and an oxygen reservoir was used. Through a driving voltage ε0 between the sample and the oxygen reservoir, oxygen was moved from the reservoir into the sample, or from the sample into the reservoir. The rate of oxygen transport was adjusted by regulating ε0 and the thickness of the YSZ disk d0. The oxygen fugacity in the sample and its variation during the process were monitored in situ by measuring the voltage between the sample and reference buffer. In this way the oxygen fugacity in the sample was controlled independently of temperature, pressure, and composition of the sample. We made experiments at 773–994 K and 1.0–4.0 GPa in the systems Cu‐O, C‐O, and Ni‐O, and the results show that the technique is successful.
Thermal and mechanical aspects of magma emplacement in giant dike swarms
Authors: Fialko Yuri A. , Rubin Allan M.
Published: September 2002

We consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts. For driving pressure gradients inferred for giant dike swarms, thicknesses of <10 m would enable dikes to transport magma laterally over the distances observed in the field (up to thousands of kilometers) without suffering thermal lock‐up. Using time‐dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions, we investigate the possibility that the observed dike thicknesses (of the order of 100 m) result from thermal erosion of the country rocks during dike emplacement. This implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure. It is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts. The presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement. In this case, measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion. Shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width, which implies that thicker dikes may have less tendency to produce consistent fabric alignment. Our results suggest that if the dike was propagating downslope off a plume‐related topographic swell, the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse (implosion) of the shallow magma plumbing system feeding the intrusion. Radial dikes that erupted at the periphery of the topographic uplift might have increased (rather than decreased) extensional stresses in the crust within the topographic uplift upon their solidification.
Coseismic deformation associated with the November 1995, MW = 7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), detected by synthetic aperture radar interferometry
Authors: Baer Gidon , Sandwell David , Williams Simon , Bock Yehuda , Shamir Gadi
Published: September 2002

The November 22, 1995, MW = 7.1 Nuweiba earthquake occurred along one of the left‐stepping segments of the Dead Sea Transform in the Gulf of Elat (Aqaba). Although it was the largest earthquake along this fault in the last few centuries, little is yet known about the geometry of the rupture, the slip distribution along it, and the nature of postseismic deformation following the main shock. In this study we examine the surface deformation pattern during the coseismic phase of the earthquake in an attempt to better elucidate the earthquake rupture process. As the entire rupture zone was beneath the waters of the Gulf, and there is very little Global Positioning System (GPS) data available in the region for the period spanning the earthquake, interferometric synthetic aperture radar (INSAR) provides the only source of information of surface deformation associated with this earthquake. We chose four synthetic aperture radar (SAR) scenes of about 90 × 90 km each spanning the rupture area, imaged by the ERS‐1 and ERS‐2 satellites. The coseismic interferograms show contours of equal satellite‐to‐ground range changes that correspond to surface displacements due to the earthquake rupture. Interferograms that span the earthquake by 1 week show similar fringe patterns as those that span the earthquake by 6 months, suggesting that postseismic deformation is minor or confined to the first week after the earthquake. A high displacement gradient is seen on the western side of the Gulf, 20–40 km south of Elat and Aqaba, where the total satellite‐to‐ground range changes are at least 15 cm. The displacement gradient is relatively uniform on the eastern side of the Gulf, and the range changes are less than 10 cm. To interpret these results, we compare them to synthetic interferograms generated by elastic dislocation models with a variety of fault parameters. Although selecting the best fit fault parameters is nonunique, we are able to generate a group of simplified model interferograms that provide a reasonable fit to the coseismic interferogram and serve to constrain the location of the fault. The present analysis shows that if the rupture reached the Gulf‐bottom surface, the mean sinistral slip along the fault is constrained to about 1.4 m. If surface rupture did not occur, the average sinistral slip is constrained to the range of 1.4–3 m for a fault patch buried 0–4 km below the Gulf‐bottom surface, respectively, with a minor normal component.
Simulation and characterization of fracture patterns in glaciers
Authors: Malthe‐Sørenssen Anders , Walmann Thomas , Jamtveit Bjørn , Feder Jens , Jøssang Torstein
Published: September 2002

During drainage of subglacial lakes the surface of the glacier subsides and fractures, generating a homogeneous, circular fracture pattern on the rim of the resulting depression. We have analyzed several fracture patterns around the Skafta cauldron on Iceland. A regular fracture spacing was observed on the cauldron rim, indicating that the region was subjected to a uniform strain. In this region, standard image analysis techniques were applied to find the lengths L and open areas A of the fractures. We demonstrate that an understanding of sampling biases allows a scaling relation L ∝ Aβ to be established and the exponent β to be determined. This relation provides a quantitative characterization of interactions between fractures in the population. The value of β is higher for that observed for laboratory experiments on clay, but the uncertainties do not rule out a universal behavior, which applies to ice, rock, and all other solids. The size distribution of fractures did not display a similarly simple crossover behavior, and a power law scaling relation could not be established without a better understanding of the crossovers for large fractures. We introduce a simple simulation model that reproduces the most important visual and statistical properties of the glacier fracture pattern. The direct comparison validates the use of the model to simulate geological fracturing processes.
Nature of the Levantine (eastern Mediterranean) crust from multiple‐source Werner deconvolution of Bouguer gravity anomalies
Authors: Khair Kamal , Tsokas Gregory N.
Published: September 2002

The nature of the Levantine (eastern Mediterranean) crust has been the subject of controversy for many years, revolving around two hypotheses: the continental crust hypothesis and the oceanic crust hypothesis. The proponents of the first hypothesis suggest that the Levantine (eastern Mediterranean) basin is characterized by a thick sedimentary succession overlying thinned crust of continental origin, through which a number of aborted Mesozoic rifts were etched. However, multiple‐source Werner deconvolution (MSWD) estimates and other geophysical data, integrated with earlier geological and geophysical results, provide further support to the second hypothesis (oceanic crust) and lead to the following conclusions: (1) The depth to Moho ranges from about 20 km to about 28 km below sea level, with an average crustal thickness of about 22 km. (2) The large thickness (about 10 km) of Phanerozoic section leaves only about 12 km of thickness for the igneous/metamorphic (basement) complex. (3) The northern boundary of the Levantine (easternmost Mediterranean) lithosphere is delineated by an arcuate belt of seismic activities along the southern margin of Cyprus. (4) The formation of the Phoenician and Latakia basins and the Iskenderun Bay is probably controlled by the counterclockwise rotation of the lithospheric slices southeast of Cyprus. (5) The apparent absence of magnetic anomaly lineations (reversals) is due probably to the thick Phanerozoic cover, and/or the formation of the oceanic crust during a long magnetic chron.
Record of seamount production and off‐axis evolution in the western North Atlantic Ocean, 25°25′–27°10′N
Authors: Jaroslow Gary E. , Smith Deborah K. , Tucholke Brian E.
Published: September 2002

Using multibeam bathymetry, we identified 86 axial and 1290 off‐axis seamounts (near‐circular volcanoes with heights ≥70 m) in an area of 75,000 km2 on the western flank of the Mid‐Atlantic Ridge (MAR), 25°25′N to 27°10′N, extending ∼400 km from the inner rift valley floor to ∼29 Ma crust. Our study shows that seamounts are a common morphological feature of the North Atlantic seafloor. Seamount‐producing volcanism occurs primarily on the inner rift valley floor, and few, if any, seamounts are formed on the rift valley walls or the ridge flank. The high abundance of off‐axis seamounts is consistent with 1–3 km wide sections of oceanic crust being transferred intact from the axial valley to the ridge flank on crust >4 Ma. Significant changes in seamount abundances, sizes, and shapes are attributed to the effects of faulting between ∼0.6 and 2 m.y. off axis in the lower rift valley walls. Few seamounts are completely destroyed by (inward facing) faults, and population abundances are similar to those on axis. However, faulting reduces the characteristic height of the seamount population significantly. In the upper portions of the rift valley, on 2–4 Ma crust, crustal aging processes (sedimentation and mass wasting), together with additional outward facing faults, destroy and degrade a significant number of seamounts. Beyond the crest of the rift mountains (>4 Ma crust) faulting is no longer active, and changes in the off‐axis seamount population reflect crustal aging processes as well as temporal changes in seamount production that occurred at the ridge axis. Estimates of population density for off‐axis seamounts show a positive correlation to crustal thickness inferred from analysis of gravity data, suggesting that increased seamount production accompanies increased magma input at the ridge axis. We find no systematic variations in seamount population density along isochron within individual ridge segments. Possible explanations are that along‐axis production of seamounts is uniform or that seamount production is enhanced in some regions (e.g., segment centers), but many seamounts do not meet our counting criteria because they are masked by younger volcanic eruptions and low‐relief flows.
Thermodetrital and crystallodetrital magnetization in an Icelandic hyaloclastite
Authors: Goguitchaichvili Avto , Prévot Michel , Dautria Jean‐Marie , Bacia Maria
Published: September 2002

An Icelandic hyaloclastite, mostly composed of millimetric fragments of basaltic glass, that is fresh at the bottom of the unit but largely palagonitized in the upper part, has been studied by petrologic, mineralogical, and magnetic means, with the aim of determining the nature and characteristics of the natural remanent magnetization (NRM). The NRM was generally found to consist of two components: a thermodetrital remanent magnetization (thermo‐DRM) and a crystallodetrital remanent magnetization (crystallo‐DRM). Thermo DRM and crystallo‐DRM are defined here as the remanences acquired as a result of the deposition of magnetic particles of detrital origin individually carrying either a TRM (thermoremanent magnetization) or a CRM (crystallization remanent magnetization), respectively. Regardless of the chemistry and size and of these particles, Thellier experiments carried out on samples carrying a thermo‐DRM provide apparent paleointensities close to the expected geomagnetic paleointensity, which suggests that in the present case the fractional alignment of individual magnetic moments is similar for DRM and TRM. In the upper part of this outcrop, grain growth CRM was acquired by individual grains of magnetite which crystallized as a result of palagonitization of basaltic glass at low temperature (<100°C). No systematic difference could be observed between the directions of characteristic remanence in the layers richest in magmatic magnetic grains and the layer where secondary magnetite is the main remanence carrier. In both cases, the overall remanence exhibits a large inclination error (∼20°), and the samples have a marked anisotropy of magnetic susceptibility which is typical of sedimentary fabrics. Thus, secondary magnetite probably formed prior to the deposition of particles, and the bulk remanence in the palagonitized layers is a crystallo‐DRM rather than a CRM. Throughout the entire stratigraphic thickness, Thellier paleointensity data are of good or excellent quality regardless of the nature of the primary remanence. In agreement with previous theoretical inferences and experimental results the layers carrying a crystallo‐DRM provide a much lower (by a factor of 2) apparent paleointensity than the layers where the remanence is a thermo‐DRM. This suggests that palagonitized basaltic glasses should not be used for paleointensity determinations. Our study shows that application of the Thellier thermal paleointensity method to sedimentary rocks can be a useful tool for distinguishing crystalline versus thermal blocking of the magnetic moments of the individual particles which, after deposition, carry a DRM. More generally, discrepancies between relative paleointensities obtained from sediments can be expected if rocks with different proportions of thermo‐DRM, crystallo‐DRM, or CRM are compared.
Global postseismic deformation: Deep earthquakes
Authors: Boschi Lapo , Piersanti Antonio , Spada Giorgio
Published: September 2002

We study the global viscoelastic deformations associated with a shear dislocation on a fault embedded in a viscoelastic mantle. To address this problem, we extend the theory of the quasi‐static deformations of a Maxwell, spherical, N‐layer incompressible Earth, previously limited to the modeling of the effects of earthquakes occurring only within the elastic lithosphere. Permanent postseismic deformations, which can be generated by lithospheric sources, cannot be sustained if the source region is viscoelastic; however, the transient response of the mantle strongly depends on the viscosity of the source region. We use the technique developed here to investigate thoroughly the quasi‐static surface deformations induced by seismic events of variable depth. We show that owing to the combined effect of sphericity and viscoelastic mantle relaxation, the surface displacements do not systematically decrease as the depth of the source increases. Instead, with increasing source depth we predict an increasing efficiency of mantle relaxation in triggering postseismic deformations of large size. Most of our results are based on a simple four‐layer model, with a 100‐km‐thick lithosphère, upper and lower mantle separated by the 670‐km discontinuity, and a fluid core; the last part of this work is devoted to a study of the effects of a low‐viscosity asthenosphere on the rates of deformation detected at the Earth's surface. The findings reported here may be useful for the interpretation of the transient motions of the Earth's surface in response to deep‐focus earthquakes.
Lateral variations at the base of the mantle from profiles of digital Sdiff data
Authors: Valenzuela Raul W. , Wysession Michael E. , Neustadt Martha O. , Butler Jessica L.
Published: September 2002

Average long‐wavelength shear velocity structure in several regions at the base of the mantle is determined using the profiles of core‐diffracted SH waves (SHdiff). There is significant lateral variation in the Sdiff apparent ray parameters, suggesting considerable lateral shear velocity anomalies in D″, the region just above the core‐mantle boundary (CMB). Apparent ray parameters are determined by least squares fits through the pulse maxima of the instrument‐deconvolved ground displacements, and heterogeneities are quantified through comparisons with reflectivity synthetic ray parameters. Corrections are applied to the data to account for the effects of the Earth's ellipticity and of mantle heterogeneities along the SHdiff upswing paths. A total of 161 SHdiff profiles were obtained, greatly expanding the number of ray parameter measurements reported in previous studies. Most of the observed slownesses fall in the range between 8.2 and 8.8 s/deg. These correspond to slowness anomalies of ±3.5%. The most robust and extensive feature resolved is under the northern and northeastern Pacific Ocean. The slowest values (Δp = +4%) occur toward the southeast, and there is a trend toward fast velocities moving to the northern and eastern rims of the Pacific. A model of D″ shear velocities is obtained by converting the slownesses to velocity anomalies, superposing the D″ path profiles onto the CMB, and applying a weighted moving cap spatial average. This model agrees well with many current tomographic models, both at large (∼5000 km) and intermediate (∼1000 km) scales. The fact that this occurs with a different type of data and technique of analysis suggests that we are now able to recognize some of the smaller‐scale lateral variations at the base of the mantle. Locations of fast and slow velocity anomalies at the CMB are consistent with the model of cold paleoslabs ponding at the CMB and forcing D″ rock laterally to form hot aggregates that give rise to plumes in the mantle and hot spots at the surface.
Micromagnetic models of pseudo‐single domain grains of magnetite near the Verwey transition
Authors: Muxworthy A. R. , Williams W.
Published: September 2002

Domain structures of small pseudo‐single domain (PSD) magnetite near the Verwey transition (Tυ) at ≈120 K were modeled using an unconstrained three‐dimensional micromagnetic algorithm. The single‐domain (SD) threshold (d0) for the monoclinic phase below Tυ was calculated to be ≈0.14 μm at 110 K. However, it is postulated that as a result of the very high energy barriers in the monoclinic phase, grains near d0 in size and in vortex states are unlikely to denucleate domain walls to become SD. Low‐temperature cycling of saturation isothermal remanence (SIRM), thermoremanence (TRM), and partial TRM (pTRM) through Tυ was simulated. Domain structures were found to align along the monoclinic “easy” magnetocrystalline anisotropy axis, i.e., the c axis, on simulated cooling through Tυ. This process was found to “destroy” SIRM structures giving rise to demagnetization; however, for TRM and pTRM structures only “closure” domains were removed increasing magnetostatic leakage giving rise to a reversible anomaly in rough agreement with experimental studies. SIRM displayed a smaller anomaly at Tυ, in agreement with experimental studies.