bluuebunny/crossref_metadata_2025 huggingface dataset:
The origins of planar magnetic structures in the solar wind
Authors: Neugebauer M. , Clay D. R. , Gosling J. T.
Published: February 2008

The term “planar magnetic structure” (PMS) refers to distinctive features in the solar wind characterized by a series of abrupt changes in the direction of the interplanetary field, with the field remaining nearly parallel to a single plane. Previous suggestions concerning the origin of PMS are (1) closed magnetic field loops arising from newly emerging, nearly parallel magnetic structures in the photosphere, (2) compression and draping of the field around a high‐speed coronal mass ejection (CME), and (3) extended current sheets and/or loops from multiple helmet streamers in the corona. Four years of ISEE 3 data have been examined to establish a larger data base of PMS events and to allow a search for common features that may reveal the causes of these structures. A total of 33 PMS events were found and analyzed. The loop theory of PMS origins was tested by searching for a correlation between the PMSs and the electron or proton counterstreaming characteristic of closed loops or CMEs. It was found that the PMSs tended to precede rather than coincide with the CMEs. Fourteen of the PMS events (42%) were in the sheath between an interplanetary shock and a CME. Other PMS events were found to be related to crossings of the heliospheric current sheet (HCS). Sorting the events according to their association with either HCS or compression/draping ahead of fast streams (of which postshock sheaths are a subset) yields 19 events (58%) associated with compression/draping but not with HCS, 8 events (24%) associated with both compression/draping and HCS, 5 events (15%) associated with HCS but not with compression/draping, and 1 event (3%) associated with neither compression/draping nor the HCS. The conclusion is that planar magnetic structures are not closed loops of plasma from coronal mass ejections or helmet streamers. Instead, a process is postulated in which coronal mass ejections lead to the amplification and alignment of preexisting discontinuities in the ambient wind compressed by the CMEs. A possible cause of the alignment of discontinuities in the vicinity of the heliospheric current sheet is compression by lateral expansion of CMEs in the lower corona.
Plasma energization by the ponderomotive force of magnetospheric standing Alfvén waves
Authors: Allan W.
Published: February 2008

The second‐order force generated by standing Alfvén waves in a dipolar magnetospheric geometry is rederived in a more straightforward and accurate way than previously done. The spatial structure of the resulting ponderomotive acceleration consists of regions in which plasma can be trapped between “diverging” nodes of the acceleration field (where the acceleration vector points away from the node). The “specific parallel energy” (parallel energy per proton mass) of accelerated plasma particles is derived. This quantity maximizes at “converging” nodes of the acceleration field (where the acceleration vector points toward the node). For the fundamental standing mode the converging node is at the equatorial plane, and the maximum specific energy there increases rapidly with L (where L is the McIlwain parameter), becoming significant above L = 5 for typical wave amplitudes. At L ∼ 10, H+ ions should reach energies of ∼ 100 eV and O+ ions energies of ∼ 2keV. Gravity has little effect on the energization, other than to define the magnetic latitude from which acceleration begins. At reasonably high L, almost all the energization occurs at magnetic latitudes above ±45°. For the second harmonic the converging nodes are situated away from the equatorial plane, and accelerated particles starting from rest do not reach that plane. The third harmonic has a converging node at the equatorial plane, and specific energies there are the same as for the fundamental mode. However, the accelerated particles originate from above the diverging nodes at latitudes of about ±18°, passing through a larger acceleration field than for the fundamental mode in that region. Particles having initial energies of a few electron volts near the ionosphere may be able to traverse the diverging nodes of the third harmonic and reach the equatorial plane.
Nonlinear Rayleigh‐Taylor instabilities, atmospheric gravity waves and equatorial spread F
Authors: Huang Chao‐Song , Kelley M. C. , Hysell D. L.
Published: February 2008

Although it is generally accepted that equatorial spread F (ESF) is due to nonlinear evolution of the Rayleigh‐Taylor instability, a number of important properties of the process remain unexplained. In particular, we investigate two as yet unexplained features of ESF: the common dominance of very large scale features (≥20 km) and their large amplitude. Although associated for years with spread F we show here for the first time that atmospheric gravity waves can initiate the Rayleigh‐Taylor instability. In agreement with other analytical theories and computer simulations we find that the initiated instability will be saturated by nonlinear coupling of unstable modes to damped modes if the amplitude of the seed gravity wave is small. However, if the amplitude of the seed gravity wave is large enough, the relative plasma density perturbations can reach 50% or more, implying essentially no saturation. The required initial amplitudes are not unreasonable. In the latter case, significant enhancements and depletions of the plasma density occur within several hundred seconds. The analysis presented here demonstrates the possibility that large‐scale spread F is triggered by gravity waves with the Rayleigh‐Taylor instability as a source of amplification. In a separate calculation we also show that large‐amplitude plasma perturbations can be produced by an explosive instability of the Rayleigh‐Taylor modes. It is found that the condition for explosive growth of the Rayleigh‐Taylor modes can be satisfied in the ionospheric F region. We propose that the explosive mode coupling of the Rayleigh‐Taylor instability is a possible mechanism for production of large‐amplitude bottomside sinusoidal structures.
Electromagnetic ion‐cyclotron instability in space plasmas
Authors: Xue S. , Thorne R. M. , Summers D.
Published: February 2008

Natural space plasmas generally exhibit a pronounced high‐energy tail distribution that can best be modeled by a generalized Lorentzian (kappa) distribution. We employ the recently introduced modified plasma dispersion function [Summers and Thorne, 1991] to obtain the dispersion relation for field‐aligned electromagnetic waves in such a plasma, and use this to study the instability properties of R mode and L mode waves in the solar wind and in planetary magnetospheres. We demonstrate for a wide range of plasma parameters that the growth of R mode waves in the solar wind can be significantly enhanced by the presence of a pronounced high‐energy tail; previous studies based on a Maxwellian distribution could therefore be seriously in error. The corresponding enhancement in the growth rate of L mode waves in planetary magnetospheres is less dramatic, but the kappa distribution tends to produce significant wave amplification over a broader range of frequency than a Maxwellian distribution with comparable bulk properties. At frequencies comparable to the ion gyrofrequency wave growth is primarily caused by cyclotron resonance with ions. Hot anisotropic electrons can nevertheless influence the instability as a result of changes in the wave phase velocity. This modulating effect is most important for a Maxwellian plasma and becomes less significant as the spectral index of the kappa distribution is reduced.
Electron generation of electrostatic waves in the plasma sheet boundary layer
Authors: Onsager T. G. , Thomsen M. F. , Elphic R. C. , Gosling J. T. , Anderson R. R. , Kettmann G.
Published: February 2008

Broadband electrostatic noise (BEN) is a common feature of the plasma sheet boundary layer in the Earth's magnetotail. Observationally, BEN has been shown to occur in conjunction with ion beams. Consequently, extensive research has been done investigating the possible ion beam‐related instabilities that could generate the observed wave spectra. It has also been demonstrated that unstable electron distribution functions are sometimes measured in the plasma sheet boundary layer. We present simultaneous observations of ion and electron distribution functions and electric field wave spectra measured by ISEE 1 and ISEE 2 in the Earth's magnetotail. For this study we have used particularly slow traversals of the boundary layer where the distinct electron and ion boundaries can be clearly resolved. As the spacecraft moved from the tail lobe toward the plasma sheet, the first indication of boundary layer plasma was seen in the electron distributions, followed some minutes later by the detection of boundary layer ions. The onset of large‐amplitude electrostatic waves at frequencies up to the electron plasma frequency was coincident with the onset of the boundary layer electrons. This evidence suggests that broadband electrostatic waves may often be generated by unstable electron distributions in the plasma sheet boundary layer, particularly the high‐frequency portion of the wave spectrum. The lower‐frequency waves are found to be polarized perpendicular to the background magnetic field, consistent with previous observations. The higher‐frequency waves (near the electron plasma frequency) had perpendicular polarization when the amplitudes were low and parallel polarization when the amplitudes were high. The observed changes in the electron distribution functions indicate that the plasma was not heated locally by the waves.
Field‐aligned flow in a centrifugally confined magnetodisc
Authors: Liu W. William
Published: February 2008

Significant magnetic field‐aligned flows were detected by Voyager 1 during its encounter with Jupiter. We try to understand the origin and dynamics of the field‐aligned flow in the presence of a slow corotating convection. We find that the field‐aligned flow is a necessary part in the preservation of the magnetodisc geometry and mass conservation in the disc. We further investigate the dynamical origin of the flow on the basis of a phenomenological equation, which indicates that the field‐aligned dynamics is likely to be chaotic.
A multisatellite study of a pseudo‐substorm onset in the near‐Earth magnetotail
Authors: Ohtani S. , Anderson B. J. , Sibeck D. G. , Newell P. T. , Zanetti L. J. , Potemra T. A. , Takahashi K. , Lopez R. E. , Angelopoulos V. , Nakamura R. , Klumpar D. M. , Russell C. T.
Published: February 2008

This paper reports the multisatellite and ground observations of two pseudo‐substorm onset events that occurred successively at 0747 UT and 0811 UT, May 30, 1985, with more attention to the 0747 UT onset. The distinguishing features of the 0747 UT event are as follows. (1) The substorm‐associated tail reconfiguration started in a very localized region in the near‐Earth magnetotail. (2) The magnitude of the current disruption decreased markedly as the disruption region expanded tailward. (3) On the ground the onset of a very small negative bay (∼ 40 nT) was observed simultaneously with the onset of the current disruption, but over a much wider local time sector than the near‐Earth tail reconfiguration. Positive bay onsets at mid‐latitudes also had a longitudinally wide distribution. From these features we infer that in the present event the current disruption took place filamentarily near AMPTE/CCE at ∼8.8 RE. It is also inferred that pseudo‐substorm onsets are distinguished from standard substorm onsets by the absence of a global expansion of the current disruption, and that the spatial scale of the onset region in the magnetosphere is not a major difference between the two. The present study suggests that the spatial distribution of the magnetic distortion before onsets is an important factor to determine the expansion scale of the current disruption. It is also suggested that the current disruption is basically an internal process of the magnetosphere.
Computations of local ion energy balance in the auroral zone
Authors: Sica R. J. , St.‐Maurice J. ‐P. , Hernandez G. , Romick G. J. , Tsunoda R.
Published: February 2008

An investigation of the coupling between the ionosphere and the thermosphere is made using coordinated measurements obtained simultaneously with the Chatanika incoherent‐scatter radar and two Fabry‐Perot spectrometers at College, Alaska, during a 2 hr period in February 1982. One of the Fabry‐Perot spectrometers measured the line shape of the oxygen 630.0‐nm emission, and the other measured the line shape of the oxygen 557.7 nm emission. The measurements were used to obtain the neutral and ion velocities, the neutral, ion, and electron temperatures; and the electron densities in the E and F regions. Interpreting these measurements using the ion energy equation for the F region yields the height(s) at which heat exchange between the ions and neutrals balances the sum of frictional heating due to differences in the ion and neutral velocities and heat exchange between the electrons and ions. This height can be interpreted as the location of the oxygen 630.0‐nm emission profile centroid, thus providing an estimate of the height of the neutral temperature measurement. Furthermore, the centroid location and extent in altitude depend on the processes producing O(1D) in aurorae. A unique balance height was not typically found, but balance often occurred over a wide range of altitudes, depending on the assumption of a pure O+ or a pure NO+ ionosphere, which is consistent with model calculations of the 630.0‐nm emission rate profile. Though the coordinated optical‐radar measurements available are limited, the initial results demonstrate that important questions relating to ion and neutral composition, chemistry, and dynamics can be examined by the methodology employed in this study.
Observations of an intermediate shock in interplanetary space
Authors: Chao J. K. , Lyu L. H. , Wu B. H. , Lazarus A. J. , Chang T. S. , Lepping R. P.
Published: February 2008

An interplanetary intermediate shock is identified from the bulk velocity, number density, and temperature of the solar wind protons and the three components of the interplanetary magnetic field observed by Voyager 1 on May 1 (day 122), 1980, when the spacecraft was at a distance of about 9 AU from the Sun. It is shown by a best fit procedure that the measured plasma and magnetic field on both sides of the discontinuity satisfy the Rankine‐Hugoniot relations for a magnetohydrodynamic (MHD) intermediate shock. This shock satisfies the following conditions. (1) The normal Alfvén‐Mach number (MA = Vn*/VA) is greater than unity in the preshock state and less than unity in the postshock state. (2) Both the fast‐mode Mach number (Mf = Vn*/Vf) in the preshock state and the slow‐mode Mach number (Msl = Vn*/Vsl) in the postshock state are less man unity, but the slow‐mode Mach number is greater than unity in the preshock state. (3) The projected components of the magnetic fields in the shock front for the pre‐ and postshock states have opposite signs. (4) The magnitudes of the magnetic fields decrease from the preshock to the postshock states. In the above expression, VA is the Alfvén speed based on the magnetic field component normal to the shock front, Vn* is the component of the bulk velocity normal to the shock front and measured in the shock frame of reference, and Vf and Vsl are the speeds of the fast‐ and slow‐mode magnetosonic waves in the direction of the shock normal, respectively. The discontinuity event in our discussion cannot be a rotational discontinuity because the Walen's relation is not satisfied. The identified intermediate, shock has MA = 1.04, θBn = 37°, and β = 0.56, where θBn is the angle between the preshock magnetic field and the shock normal direction and β is the ratio of thermal to magnetic energy densities. Using these parameters, a numerical solution of the MHD equations for the shock is obtained. The simulated profiles of the bulk velocity, number density, temperature, and magnetic fields of the pre‐and postshock states agree with those of the observed values. The same parameters are used to simulate an intermediate shock using a hybrid numerical code in which full ion dynamics is retained while electron inertial force is neglected. The results of this simulation are compared with high‐resolution magnetic field data with a time resolution of 1.92‐s averages. The shock thickness of about 70 c/ωpi predicted from the hybrid code agrees with the observations. The general behavior of the magnetic field in the shock transition region is also very similar for the simulated and observed results. The macro‐ and microstructures of the intermediate shock obtained from the MHD and hybrid models resemble the observed structures.
Two‐dimensional simulations of supercritical quasi‐parallel shocks: Upstream waves, downstream waves, and shock re‐formation
Authors: Scholer M. , Fujimoto M. , Kucharek H.
Published: February 2008

Two‐dimensional hybrid (particle ions, massless fluid electrons) simulations of quasi‐parallel collisionless shocks are carried out in order to investigate the upstream wave properties, the shock re‐formation process, and the downstream turbulence. The two‐dimensional simulations confirm the results of earlier one‐dimensional simulations. When backstreaming diffuse ions are retained re‐formation of a shock with an upstream magnetic field ‐ shock normal angle of ΘBno = 30° occurs as a result of upstream low‐frequency waves which steepen, become pulsationlike structures and take over as the re‐formed shock. The upstream waves are initially aligned with the shock normal; later in the run the waves become more and more aligned with the upstream magnetic field. However, when approaching the shock, the wave vectors are refracted in the region of increasing diffuse ion density into the shock normal direction so that shock re‐formation is again coherent along the shock surface. In addition, re‐formation on a smaller scale and out of phase along the shock front is due to more or less specularly reflected ions. Re‐formation of a ΘBno = 10° shock is due to locally at the shock ramp emerging waves. These are attributed to the so‐called interface instability in the region of partial overlap between the incident cold solar wind and part of the hot downstream distribution. These waves emerge in phase along the shock surface and thus re‐formation is in this more parallel case also coherent along the shock. At medium Alfvén Mach number (MA ∼ 5) shocks, upstream waves which are aligned with the upstream magnetic field are convected into the shock and produce ripples on the shock surface. At higher Mach number (MA ∼ 9) the shock surface becomes less coherent and the local value of the shock normal ‐ magnetic field angle varies greatly. The re‐formation length scale is larger than in the lower Mach number case. The turbulence downstream reflects the two mechanisms of shock reformation: in the ΘBno = 30° case the upstream pulsations are mode converted when convected through the shock layer. In the ΘBno = 10° case the downstream turbulence results from the local instability at the shock front.
Comprehensive simulation study on local and global development of auroral arcs and field‐aligned potentials
Authors: Watanabe Tomohiko , Oya Hiroshi , Watanabe Kunihiko , Sato Tetsuya
Published: February 2008

Extensive three‐dimensional computer simulations of the magnetosphere‐ionosphere (M‐I) coupling are performed to study self‐excitation of an auroral arclike structure with special emphasis on (1) nonlinear evolution of the feedback instability in the M‐I coupling system, (2) controlling mechanisms of the arc structure, (3) formation of a field‐aligned electric potential structure in association with the development of the feedback instability, and (4) effects of the parallel potential generation on the development of the arclike structure. The present study takes the first step toward the theoretical understanding of the M‐I coupling system with parallel potentials. As was already shown by Sato [1978] and Watanabe and Sato [1988], it is reconfirmed that the feedback instability produces a longitudinally elongated, latitudinally striated structure where the upward field‐aligned current and the ionospheric density are locally enhanced. On top of this the present extended study reveals the following important new features: The global distribution of the striation structure is primarily governed by the magnetospheric convection pattern and the ionospheric density distribution. There appears a significant dawn‐dusk asymmetry in the arc formation, even though the apparent geometrical relationship is symmetric. This dawn‐dusk asymmetry reflects the geometrical fact that the ionospheric Pedersen current closing the magnetospheric current is antisymmetric with respect to the noon‐midnight plane, while the self‐closed Hall current is symmetric. The recombination effect plays a significant role in the global, as well as local, development of the arc structure. The nonlinearity of recombination, in conjunction with the closure of an arc‐associated local field‐aligned current system, acts to destroy an old arc and creates a new arc in a different but adjacent position. This results in a peculiar dynamic evolution of the arclike structure. A V‐shaped field‐aligned potential structure is created in association with an arc structure, when we introduce the parallel anomalous resistivity. The nonlinear phase mismatching due to the parallel resistivity reduces the growth rate of the feedback instability, and suppresses the growth of the arc structure. When the effect of precipitating hot electrons is taken into account, the ionospheric density is considerably enhanced locally at the foot of the field lines where the field‐aligned potential is generated. An oscillatory behavior is observed in the development of the ionospheric density, field‐aligned current and potential. The period seems to be governed by the Alfvén bounce time and the ionospheric density.
Decay of equatorial ring current ions and associated aeronomical consequences
Authors: Fok M. ‐C. , Kozyra J. U. , Nagy A. F. , Rasmussen C. E. , Khazanov G. V.
Published: February 2008

The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E×B and gradient drifts. They also experience losses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time‐dependent convection electric field model. The drift‐loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low‐energy (< 500 eV) ion population as a result of energy degradation caused by Coulomb collisions of the ring current ions with the plasmaspheric electrons; this population may be one source of the low‐energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 Å (stable auroral red [SAR] arc) emission in the subauroral region. The calculated energy‐deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.
Midlatitude observations of the night airglow: Implications to quenching near the mesopause
Authors: Johnston J. E. , Broadfoot A. L.
Published: February 2008

Midlatitude observations of the night airglow from 3000 to 9200 Å were made with a ground‐based panchromatic intensified charge coupled device (ICCD) spectrograph over a period of 4 months. Intensities of the Herzberg I, Chamberlain, Herzberg II, and atmospheric systems of molecular oxygen, the Meinel bands of hydroxyl, and the atomic oxygen 5577‐Å emission were measured. Synthetic spectra and integration were used to determine the emission intensities of these features. The results were used to constrain a middle atmospheric model. Output from this model showed that quenching processes severely depopulate the states to the extent that electronic, vibrational, and rotational populations do not retain the signatures of the excitation reactions or the radiative depopulation processes. The observed coupling between the levels of the O2(A³Σu+,A′3Δu,c1Σu−) states and the observed vibrational distribution of the OH(X2II) levels also supported this heavy quenching hypothesis. The average intensities for a 6.5‐hour integration period on March 16, 1991, of the O2Herzberg I, Chamberlain, Herzberg II, and atmospheric (0‐1) emissions were 350±30 R, 120±15 R, <120 R, 350±20 R, respectively; the OH Meinel (9‐4), (8‐3), (7‐2), (7‐3), (6‐2), (5‐1), and (4‐0) intensities were 450±50 R 450±20 R, 90±20 R, 1620±200 R, 970±50 R, 680±150 R, 190±20 R; the O I(5577 Å) intensity was 320±10 R; the average OH Meinel (6‐2) temperature was 200° K.
Stress transfer during three sequential moderate earthquakes along the central Calaveras Fault, California
Authors: Du Yijun , Aydin Atilla
Published: February 2008

Seismic and geodetic information on three sequential moderate earthquakes, the 1979 M5.9 Coyote Lake, the 1984 M6.2 Morgan Hill, and the 1988 M5.1 Alum Rock, along the central Calaveras fault, is used to examine how tractions on fault segments are transferred from one fault segment to the other. Geodetic data obtained before and after each earthquake are inverted for slip at depth. The perturbed stress field induced by each earthquake is calculated using 3‐D elastic dislocation theory. Although only long‐wavelength features of the slip distributions are well resolved, the model suggests that the distribution of slip is nonuniform. The largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter. According to the model, the Coyote Lake earthquake induced 0.1–1 bar (0.01–0.1 MPa) shear stress across the northern part of the Morgan Hill segment and only about 0.1 bar at the hypocenter of the Morgan Hill mainshock. A shear stress of 0.5 bar was induced by the Morgan Hill earthquake at the hypocenter of the Alum Rock mainshock. The average stress drops for the Coyote Lake, Morgan Hill, and Alum Rock earthquakes are estimated to be 30, 25, and 15 bars, respectively. Thus the earthquake‐induced shear stress of 0.1–0.5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes. However, the induced shear stress can trigger the adjacent earthquakes, in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer. It is inferred that fault geometry may influence the local stress field, thereby initiation and termination of the rupture, but not the regional stress accumulation.
Dynamic feedback between a continentlike raft and thermal convection
Authors: Zhong Shijie , Gurnis Michael
Published: February 2008

Seismic observations of the mantle, which include long‐wavelength structure, a k−1 dependence of heterogeneity on harmonic k, and a heterogeneous upper boundary layer, and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection. We have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry. The azimuthal interconnectivity of this geometry is vital to resolve the two‐way dynamics between rafts and convection. Computations show that (1) raft motion is periodic, (2) long‐wavelength thermal structure is significant within both thermal boundary layers and the fluid interior, and (3) the large‐scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion. These three results, which are observed for a range of Rayleigh numbers, internal heating rates, and raft sizes, are a direct consequence of the dynamic interaction between the raft and convection. The physical processes for a model with a Rayleigh number of 105 are representative: when the raft is stationary, due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer, heat accumulates beneath the raft and results in long‐wavelength thermal anomalies. The long‐wavelength thermal anomalies enhance raft motion. Accompanying the enhanced raft movement, the long‐wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest. Since convection models without rafts generate less long‐wavelength heterogeneity compared to the models with rafts, or continental plates, we suspect that continental plates may play a crucial role in mantle dynamics. Interestingly, raft motion with a period of about 10 transit times is usually significant; 10 transit times is about 600 m.y. if scaled to the Earth. This is close to the observed 300–500 m.y. period of supercontinent aggregation and dispersal.
Early dolomitization of platform carbonates and the preservation of magnetic polarity
Authors: McNeill Donald F. , Kirschvink Joseph L.
Published: February 2008

Results from a combination of techniques are presented to evaluate the nature of magnetization in shallow‐water platform carbonates which have undergone recrystallization during early calcification and dolomitization. Magnetic grain separates, coercivity spectra, modified Lowrie‐Fuller tests, magnetization efficiency, and magnetostratigraphic constraints indicate that the ultrafine‐grained magnetite is preserved during early burial geochemical regimes, inversion from aragonite/high‐magnesium calcite to low‐magnesium calcite, and even pervasive dolomitization. These single‐domain crystals are thought to occur as interacting multigrain clusters, some of which may exceed 1 μm in diameter. These large clusters may help prohibit magnetic reorientation during diagenesis. Furthermore, during both fabric preserving and fabric destructive dolomitization, the ultrafine‐scale replacement process restricts reorientation of the clusters, thus preserving depositional or early postdeposition magnetic orientation. This early dolomitization (matrix stabilization) may even help protect and extend the subsurface lifespan of the original polarity.
Estimation of depth‐dependent fault slip from measured surface deformation with application to the 1906 San Francisco Earthquake
Authors: Matthews Mark V. , Segall Paul
Published: February 2008

We estimate the depth distribution of slip in the 1906 San Francisco earthquake by applying new inversion methods to triangulation data collected near Point Arena, California. Modeling crustal deformation from antiplane slip in a half‐space, we define three regularizing functionals to produce minimum‐norm inverse problems. These functionals measure components of the coseismic change in elastic strain energy, stress magnitude, and stress variability and are all quadratic in slip. Orthogonality conditions define finite‐dimensional representations of slip estimates. Coefficients of basis functions in these representations are estimated by damped least squares with damping parameter and faulting depth chosen by cross validation. The best resulting estimates exhibit right lateral slip to depths of 15 to 20 km with about 6 m of surface slip. This is consistent with directly observed offsets of cultural landmarks at the surface and indicates deeper faulting than has been previously inferred for the 1906 earthquake.
Crustal thickening versus lateral expulsion in the Indian‐Asian continental collision
Authors: Houseman Gregory , England Philip
Published: February 2008

Since the beginning of the continental collision between India and Asia there has been about 2500 km of convergence, and the northward movement of India has been accommodated by major internal deformation of the Asian lithosphere. The crustal thickening in and around the Tibetan Plateau is clearly a direct consequence of this collision, but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern Asia and southern China. Numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere. We employ a thin viscous sheet model of the lithosphere with a depth‐averaged nonlinear viscous rheology described by a stress‐strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless Argand number Ar. The eastern boundary for the collision region is described as a lithostatic boundary; the precollision normal stress is determined by static balance, and that constant stress is applied throughout the collision. The experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1/4 of the indentation rate, with only minor variation due to geometry or rheology. Crustal thickening rates in the region of the plateau are reduced by between 10% and 25% of the corresponding rates determined for similar experiments with a rigid eastern boundary. To generalize from the results of these experiments, the total north‐south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3:1 and more probably 4:1.
Relict hydrothermal zones in the TAG Hydrothermal Field, Mid‐Atlantic Ridge 26°N, 45°W
Authors: Rona Peter A. , Bogdanov Yury A. , Gurvich Evengy G. , Rimski‐Korsakov Nick A. , Sagalevitch Anatoly M. , Hannington Mark D. , Thompson Geoffrey
Published: February 2008

Two relict hydrothermal zones were delineated between water depths of 3400 and 3500 m at the lower part of the east wall of the rift valley of the Mid‐Atlantic Ridge in the TAG hydrothermal field using a deep‐towed side scan sonar tow and a camera‐temperature tow along the northern 3 km of the wall, and a submersible transect. Named the North and MIR relict zones, they are located about 4 km and 2 km northeast, respectively, of the known active high‐temperature sulfide mound between water depths of 3625 and 3670 m on the rift valley floor near the base of the east wall. The North zone extends about 2 km along the northern end of the lower east wall. The zone includes two moundlike features up to 30 m high by 200 m in diameter imaged by side scan sonar within a 2‐km‐long line of discontinuous hydrothermal deposits comprising inactive toppled and standing chimneys, layered material, and patchy dark stains on sediment photographed by the camera‐temperature tow. Several other moundlike features were imaged with the side scan sonar outside of the photographic coverage. The MIR relict hydrothermal zone 2 km south of the North zone, named after the MIR submersible used to investigate it, consists of three subzones: (1) a 200‐m‐wide area of diverse types of hydrothermal materials exposed by normal faulting at its western margin; (2) a 400‐m‐wide by 700‐m‐long central area of discrete groups of toppled and standing inactive sulfide chimneys up to 25 m high on a substrate of red metalliferous sediment and carbonate lutite; spires sampled on the highest chimneys are composed of coarse‐grained, recrystallized sulfides dominated by pyrite and chalcopyrite which contain the first primary, free gold grains (2–3 μm diameter) found at a hydrothermal site on a mid‐ocean ridge; and (3) a 150‐m‐wide hummocky area of layered hydrothermal material with the appearance of low temperature precipitates and carbonate lutite with patchy dark stains at its eastern margin. The active sulfide mound, the North zone, and the MIR zone are each located on the fractured western margins of gentle, dome‐shaped areas of pillow flows typically 500 m in diameter interpreted as summits of volcanic centers that may have supplied heat to drive adjacent hydrothermal activity. The distribution and size of the active and inactive hydrothermal zones of the TAG field, the chronology, and the characteristics of relict samples recovered indicate a history of at least 100 ×103 years of high‐temperature hydrothermal episodes with multiple overprinting stages of mineralization accompanied by alteration.
A model for the motion of the Philippine Sea Plate consistent with NUVEL‐1 and geological data
Authors: Seno Tetsuzo , Stein Seth , Gripp Alice E.
Published: February 2008

We investigate angular velocity vectors of the Philippine Sea (PH) plate relative to the adjacent major plates, Eurasia (EU) and Pacific (PA), and the smaller Caroline (CR) plate. Earthquake slip vector data along the Philippine Sea plate boundary are inverted, subject to the constraint that EU‐PA motion equals that predicted by the global relative plate model NUVEL‐1. The resulting solution fails to satisfy geological constraints along the Caroline‐Pacific boundary: convergence along the Mussau Trench and divergence along the Sorol Trough. We then seek solutions satisfying both the CR‐PA boundary conditions and the Philippine Sea slip vector data, by adjusting the PA‐PH and EU‐PH best fitting poles within their error ellipses. We also consider northern Honshu to be part of the North American plate and impose the constraint that the Philippine Sea plate subducts beneath northern Honshu along the Sagami Trough in a NNW‐NW direction. Of the solutions satisfying these conditions, we select the best EU‐PH as 48.2°N, 157.0°E, 1.09°/m.y., corresponding to a pole far from Japan and south of Kamchatka, and PA‐PH, 1.2°N, 134.2°E, 1.00°/m.y. Predicted NA‐PH and EU‐PH convergence rates in central Honshu are consistent with estimated seismic slip rates. Previous estimates of the EU‐PH pole close to central Honshu are inconsistent with extension within the Bonin backarc implied by earthquake slip vectors and NNW‐NW convergence of the Bonin forearc at the Sagami Trough.
A multichannel seismic investigation of upper crustal structure at 9°N on the East Pacific Rise: Implications for crustal accretion
Authors: Harding Alistair J. , Kent Graham M. , Orcutt John. A.
Published: February 2008

Reprocessed multichannel seismic profiles from the 9°N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide‐angle reflections from the base of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3–0.45 s within 1–2 km of the axis, corresponding to an increase in layer thickness of 200–600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along‐axis profile, apparent along‐axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is constant.
Mechanics of oblique convergence
Authors: Platt J. P.
Published: February 2008

The distribution and magnitude of the strike‐parallel component of velocity in an obliquely converging thrust wedge or accretionary prism are determined by the geometry and mechanical properties of the wedge. A mechanical analysis based on the assumption of a critical or stable geometry of the wedge, for which the rate of cross‐strike deformation is zero, leads to the following conclusions for different bulk rheologies. (1) In a linear viscous wedge, the strike‐parallel motion relative to the underthrust slab decreases exponentially away from the rear and is effectively concentrated in a shear zone with a width comparable to the thickness of the wedge at the rear. The wedge also deforms by corner flow, producing a circulation in the cross‐strike plane. The strike‐parallel and corner flow velocities depend on the thickness and viscosity of the wedge and on the shear stresses applied to its lower and rear boundaries. Convergence at the wedge front is normal to strike. (2) A critically tapered perfect plastic wedge moves coherently without internal deformation. For low and moderate obliquities of the convergence vector, the wedge moves at the same velocity as the backstop (upper plate). For high angles of obliquity, the wedge moves laterally relative to the underthrust slab at a maximum velocity dependent on its dimensions and the stress conditions on its boundaries, so that it is separated from the upper plate by a strike‐slip fault, defining a forearc sliver. No geometrical configuration exists that allows the strike‐parallel motion to be distributed through the wedge. (3) A noncohesive Coulomb wedge behaves in much the same way as a plastic wedge, but the geometry and velocity depend only on its mechanical properties and the shear stresses on its boundaries, and they are independent of scale.
Defining the role of magmatism in extensional tectonics: Helium 3 fluxes in extensional basins
Authors: Torgersen Thomas
Published: February 2008

If (as is suggested by both observation and laboratory studies) mantle degassing of 3He occurs primarily with melt formation, then the 3He/ 4He ratios and 3He fluxes observed in continental extensional basins (e.g., Basin and Range, Pannonian Basin, Rhine Graben) require the input of magma from the mantle. The processes governing the relations between heat and helium fluxes associated with magmatic processes are reviewed and evaluated with respect to simplified mechanisms of enhanced heat flow in extensional basins. The observed heat flux enhancements from underplating (large‐scale accretion of a melt to the base of the crust) that might accompany extension would require large fluxes of 3He and very high 3He/4He ratios to occur in characteristic crustal fluids with a time delay of hundreds of millions of years. The mapping to date of continental 3He/4He in characteristic crustal fluids suggests that this is not the case; high 3He/3He ratios and high 3He fluxes are not dominant in older inactive regions. Furthermore, since high 3He/4He ratios are observed in Cenozoic extensional basins, it is suggested that (1) minor mantle magma intrusion to shallow levels in the crust is responsible for the (small) 3He fluxes observed in extensional terranes on a geologically short time scale and (2) stretching, lateral flow of lower crustal material, erosion, and/or other mechanisms are responsible for the bulk of the higher heat flow (and isostatic compensation) in extensional basins. Combined geophysical and geochemical studies of the 3He flux in extensional basins of all geologic ages and measurements of the effective transport of helium in the crust will better define the role for, e.g., stretching, magmatism, underplating, and lateral inflow during continental extensional tectonics.
Fault growth and fault scaling laws: Preliminary results
Authors: Scholz C. H. , Dawers N. H. , Yu J.‐Z. , Anders M. H. , Cowie P. A.
Published: February 2008

We report progress made in the last few years on the general problem of the mechanism of fault growth and the scaling laws that result. Results are now conclusive that fault growth is a self‐similar process in which fault displacement d scales linearly with fault length L. Both this result and the overall nature of along‐strike fault displacement profiles are consistent with the Dugdale‐Barenblatt elastic‐plastic fracture mechanics model. In this model there is a region of inelastic deformation near the crack tip in which there is a breakdown from the yield strength of the unfractured rock to the residual frictional strength of the fault over a breakdown length S and displacement d0. Limited data also indicate that S and d0 also scale linearly with L, which implies that fracture energy G increases linearly with L. The scaling parameters in these relationships depend on rock properties and are therefore not universal. In our prime field locality, the Volcanic Tableland of eastern California, we have collected data over 2 orders of magnitude in scale range that show that faults obey a power law size distribution in which the exponent C in the cumulative distribution is ∼1.3. If the fault is growing within the brittle field, the zone of inelastic deformation consists of a brittle process zone which leaves a wake of fractured rock adjacent to the fault. Preliminary results of modeling the process zone are consistent with observations now in hand both in predicting the preferred orientation of cracks in the process zone wake and the rate of falloff of crack density as a function of distance from the fault. The preferred orientation of these cracks may be used to infer the mode and direction of propagation of the fault tip past the point in question. According to the model, the width of the process zone wake may be used to infer the length of the fault at the time its tip passed the measurement point, but data have not yet been collected to verify this prediction. If the fault displacement has been accumulated by repeated seismic slips, each of these will sweep the fault with a crack tip stress field of a smaller spatial extent than that of the fault tip stress field, producing an inner, more intensely fractured, process zone wake. This may be the mechanism that creates the cataclasite zone, rather than simple frictional wear, as has been previously supposed.