bluuebunny/crossref_metadata_2025 huggingface dataset:
Polar wind ion dynamics in the magnetotail
Authors: Delcourt D. C. , Sauvaud J. A. , Moore T. E.
Published: February 2008

The circulation of polar wind ions from the high‐latitude ionosphere to the plasma sheet is examined by means of three‐dimensional particle codes. The simulations reveal the possible creation of high‐altitude mirror points for particles traveling in the equatorial magnetotail. It is shown that this feature follows from large centrifugal decelerations in the course of the fast E×B transport, and not from nonadiabatic motion in the local field reversal. Such decelerations yield the confinement (here termed “centrifugal trapping”) of populations with relatively small parallel speed near the tail midplane. It is also demonstrated that the net polar wind plasma supply to the plasma sheet critically depends upon the dynamics of ions which originate from the highest latitudes of the dayside ionosphere. During quiet times, these particles gain access to the distant tail where they experience durable nonadiabatic trapping. Accordingly, it is found that the polar wind forms a prominent source of plasma for the central nightside sector, yielding appreciable density levels (of the order of several tenths of ions cm−3) in the characteristic plasma sheet energy range (from hundreds of electron volts up to a few kiloelectron volts). A lesser contribution is obtained during active times, as the low‐energy ions are confined to low L shells where they essentially travel in an adiabatic manner. Thus, in addition to emphasizing the role of ionospheric ion dynamics inside the neutral sheet, the trajectory simulations suggest that the polar wind efficiency in feeding the plasma sheet depends upon the time evolution of the particles, rather than from an increasing plasma supply from low to high Kp.
Magnetospheric plasma analyzer: Initial three‐spacecraft observations from geosynchronous orbit
Authors: McComas D. J. , Bame S. J. , Barraclough B. L. , Donart J. R. , Elphic R. C. , Gosling J. T. , Moldwin M. B. , Moore K. R. , Thomsen M. F.
Published: February 2008

The first three magnetospheric plasma analyzer (MPA) instruments have been returning data from geosynchronous orbit nearly continuously since late 1989, 1990, and 1991. These identical instruments provide for the first time simultaneous plasma observations from three widely spaced geosynchronous locations. The MPA instruments measure the three‐dimensional velocity space distributions of both electrons and ions with energies between ∼1 eV/q and ∼40 keV/q. MPA capabilities and observations are summarized in this paper. We use the simultaneous observations from three longitudinally separated spacecraft to synthesize a synoptic view of the morphology of the magnetosphere at geosynchronous orbit over a 6‐week interval in early 1992. The MPA observations indicate that the spacecraft encountered seven regions with characteristic plasma populations during this period: (1) the cool, dense plasmasphere (13.1% of the data); (2) a warmer, less dense plasma trough (22.5%); (3) the hot plasma sheet (40.3%); (4) a combination of plasma trough and plasma sheet (18.6%); (5) an empty trough region, devoid of plasma sheet, plasmasphere, or plasma trough populations (4.3%); (6) the magnetosheath and/or low‐latitude boundary layer (0.7%); and (7) the lobe (0.3%). The local time distributions of these regions are examined. For example, as suggested by previous authors, we find that at geomagnetically quiet times (Kp < 2) geosynchronous orbit can lie entirely within the plasmasphere while at more active times only the afternoon to evening portions of the orbit are typically within the plasmasphere. We also find that the plasma convection inside the plasmasphere is generally sunward in the corotating (geosynchronous spacecraft) reference frame, independent of activity level, in contrast to previous studies. In addition to such statistical results, the simultaneous data sets at different local times allow us to at least partially separate spatial from temporal variations. In particular, we use these observations to examine the instantaneous shapes of the plasmapause and magnetopause as they pass over geosynchronous orbit. As expected, the plasmapause is found to have a highly variable shape, at various times showing (1) a stable dusk side bulge, (2) a variable bulge which expands, contracts, and moves, (3) an overall expansion and contraction of the plasmasphere, and (4) even more complicated behavior which is best accounted for by large‐scale structure of the plasmapause and/or disconnected plasma blobs. During the 6 weeks of data examined, the magnetosheath was encountered on several occasions at synchronous orbit, preferentially on the prenoon side of the magnetosphere. For the first time, simultaneous prenoon and postnoon observations confirm this asymmetry and demonstrate that the magnetopause shape can be highly asymmetric about the Earth‐Sun line.
Sunrise effect on dayside Pc pulsations at the dip equator
Authors: Saka O. , Alperovich L.
Published: February 2008

A flux gate magnetometer with an amplitude resolution of 6 pT/LSB was installed at Huancayo, Peru (12°S, 750°W in geographic coordinates, 0.8°N, 335.6° in geomagnetic coordinates) in December 1985 and recorded field variations for 1 year with 3‐s sampling intervals. Horizontal field variations of Pc 3‐4 pulsations (periods from 150 to 30 s) were studied and it was found that the oscillations were observed predominantly in the H component (magnetic north‐south), though oscillations in the D component (magnetic east‐west) are superimposed temporarily at local sunrise. The D component signals, which last for ≈2 hours after local sunrise, bring about a large tilt of the polarization azimuth angles in the H‐D plane without a significant change in the ellipticity. The azimuth at sunrise lies in the NW‐SE quadrant during local summer (November to January), while it lies in the NE‐SW quadrant for local winter (March to September). On the basis of the observed seasonal change of the azimuth angles, we present a model of the ionospheric current system setup by the incident Pc 3‐4 pulsations in the dayside region of the dawn terminator at the dip‐equator.
Ground signatures of convecting reconnected flux tubes
Authors: Chaston C. C. , Hansen H. J. , Menk F. W. , Fraser B. J. , Hu Y. D.
Published: February 2008

The ground magnetic and ionospheric signatures of reconnected flux tubes have been modeled using an expanding and rotating elliptical field‐aligned current system. Assuming a constant total current of 2 × 105 A, initial ground field strengths of ∼100 nT are shown to decay rapidly with distance from the current system and with current system expansion. The associated ionospheric electric field estimates show that the flux tube footprint convects initially eastward or westward with velocity decreasing from 3 to 1 km s−1 and then poleward decreasing from 0.5 to 0.2 km s−1. The signature detected by a ground magnetometer array will be a 0.5 to 1‐cycle oscillation with period of 8 min as the current system convects through the ionosphere above. These signatures are characterized by an initially rapid increase in field strength, with maximum amplitudes being reached within the first 5 min, followed by gradual decay. This modeling emphasizes that the temporal variation of the position of a recording magnetometer with respect to the ground image of the ionospheric footprint determines the form of the magnetic signature. The formalism should be useful in modeling flux tube behavior at high latitudes and may be relevant to dayside auroral observations.
Evidence for non‐local‐thermodynamic‐equilibrium rotation in the OH nightglow
Authors: Pendleton W. R. , Espy P. J. , Hammond M. R.
Published: February 2008

Improved ground‐based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Δν= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the P1(N″) transitions with N″ = 10, 12, and 13, respectively. These measurements indicate that ∼10% of the OH(X, ν′ = 7) column rotational population is typically in rotational levels with N′ ≥7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of ∼1%. The excess high‐N′ population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high‐P1(N″) transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N′ and are typically much more intense than expected on the assumption of complete rotational‐translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R‐T equilibration of OH(X²Π,3≤ν′≤7,J′) on a column basis under typical nightglow conditions.
A radiating one‐dimensional current sheet configuration
Authors: Pritchett P. L. , Coroniti F. V.
Published: February 2008

The structure of the x‐independent (one‐dimensional) forced current sheet including a self‐consistent By component is investigated for the case of small normal field component, Bz/B0 « 1. A hybrid (kinetic ions, massless fluid electrons) simulation model is used to demonstate that such a current sheet has a time‐dependent structure which radiates incompressible Alfvén waves with amplitude of the order of the asymptotic (lobe) field strength B0. The central density enhancement acts as the source of a propagating wavetrain in which Bx rotates into By and back again. One of the characteristic signatures of the radiating current sheet is the presence of a reversal in Bx (or By) without a corresponding increase in density.
Polytropic relationship in interplanetary magnetic clouds
Authors: Osherovich V. A. , Farrugia C. J. , Burlaga L. F. , Lepping R. P. , Fainberg J. , Stone R. G.
Published: February 2008

Interplanetary magnetic clouds are expanding MHD configurations characterized by strong magnetic fields, large rotations of the field vector, and low ion temperatures. In this paper we present high time resolution data from the ISEE 3 and IMP 8 spacecraft on the magnetic field and the proton and electron populations in a number of magnetic clouds. Our objective is to study aspects of the thermodynamics of magnetic clouds and the relation between their thermodynamic and magnetic structures. Our analysis suggests the following features of the thermodynamics of magnetic clouds: (1) The electron and ion populations are not in thermal equilibrium with each other, the electron temperature, Te, being in general up to an order of magnitude higher than the proton temperature, Tp. The temperature ratio Te/Tp in these magnetic clouds is larger than typical values of this quantity in the solar wind at comparable heliocentric distances. (2) For the proton component we find that a polytropic law with γp in the range 1.1 < γp < 1.3 is probably adequate to describe the relation between Tp and density. (3) For the electrons (E < 1.18 keV) the energetics are likewise governed by a polytropic law. Unlike the protons the polytrope that describes the electrons has an index that is less than unity, implying anticorrelation between Te and the number density. For the two clouds analyzed where electron data are available, γe ≈ 0.48 ± 0.2. (4) As a corollary of case 3, the variation of Te with density in magnetic clouds is the reverse of that generally found in the inner heliosphere. (5) Electron temperatures are well correlated with the magnetic field strength, the highest values being reached where the field strength maximizes. We interpret these experimental findings along the following lines. While the magnetic field of the cloud expands, the ions are cooled (though not so effectively as in the adiabatic case (γad = 5/3), indicating some exchange of energy with the ambient solar wind). In contrast, since γe < 1, when the density drops as a result of expansion, Te increases and, consequently, a temperature difference develops between the two species. The hot electrons are trapped by the magnetic field in the core of the magnetic cloud. If magnetic clouds originate in the region near the Sun where Tp < Te, the subsequent expansion accentuates this temperature disparity further. Such conditions are favorable for the generation of ion acoustic waves.
Inputs to the atmosphere from relativistic electrons
Authors: Imhof W. L. , Gaines E. E.
Published: February 2008

The precipitation rates into the atmosphere of electrons >1 MeV during a weak relativistic enhancement in the spring of 1979 have been derived from electron fluxes measured on a low‐altitude satellite, P78‐1. The inputs to the atmosphere were obtained from drift loss cone fluxes of electrons >1 MeV by using measurements with fine angular resolution at lower energies of the ratios of the direct precipitation rates to the fluxes of locally trapped electrons. The analyses indicated that often the inputs to the atmosphere may be primarily from precipitating electrons at pitch angles near the edge of the trapped fluxes, illustrating the inadequacy of broad angular resolution measurements for precipitation studies. The data showed a strong L dependence for the precipitation of relativistic electrons extending from L ≈ 2.5 to L ≈ 8.5, indicating the limitations of obtaining inputs to the atmosphere from high‐altitude measurements alone over a limited L shell range, such as at geosynchronous altitude (L ≈ 6.6). When summed over all longitudes and latitudes in the southern hemisphere, the total input energy rate of electrons >1 MeV precipitating into the atmosphere near the peak of the event on May 25‐30, 1979, was ∼5 × 1019 ergs d−1.
Plasma waves as indicators of the termination shock
Authors: Kurth W. S. , Gurnett D. A.
Published: February 2008

The plasma wave receivers on the Voyager spacecraft will likely provide indicators of both the actual crossing of the termination shock as well as precursors of the shock crossing. Langmuir waves are commonly observed in the electron foreshock regions associated with planetary bow shocks; hence we expect to observe similar emissions ahead of the termination shock. Since the electron foreshock can extend considerable distances upstream of the termination shock, the detection of these waves can provide as many as several weeks warning that a crossing of the termination shock is imminent. Electrostatic turbulence associated with planetary bow shocks themselves is also an expected feature of the solar wind termination shock and will provide an important signature with which to identify the shock and to provide information on its thickness and fundamental processes. Since this turbulence is collocated with the shock, it cannot provide any advanced warning of the shock. Both upstream Langmuir waves and electrostatic wave turbulence can often be found in conjunction with interplanetary shocks, although the generally weaker nature of these shocks often leads to weaker plasma wave signatures than observed at planetary bow shocks. We demonstrate with Voyager observations that the amplitudes expected for each of these phenomena are well within the range of detectability by the Voyager plasma wave receiver even for termination shock distances exceeding 100 AU.
Monte Carlo Study of the transition region in the polar wind: An improved collision model
Authors: Barghouthi I. A. , Barakat A. R. , Schunk R. W.
Published: February 2008

A Monte Carlo simulation was used to study the steady state flow of the polar wind protons through a background of O+ ions. The simulation region included a collision‐dominated region (barosphere), a collisionless region (exosphere), and the transition layer embedded between these two regions. Special attention was given to using an accurate collision model, i.e., the Fokker‐Planck expression was used to represent H+ ‐ O+ collisions. The model also included the effects of gravity, the polarization electric field, and the divergence of the geomagnetic field. For each simulation, 105 particles were monitored, and the collected data were used to calculate the H+ velocity distribution function fnof;H+, density, drift velocity, parallel and perpendicular temperatures, and heat fluxes for parallel and perpendicular energies at different altitudes. The transition region plays a pivotal role in the behavior of the H+ flow. First, the shape of the distribution function is very close to a slowly drifting Maxwellian in the barosphere, while a “kidney bean” shape prevails in the exosphere. In the transition region, the shape of fnof;H+ changes in a complicated and rapid manner from Maxwellian to kidney bean. Second, the flow changes from subsonic (in the barosphere) to supersonic (in the exosphere) within the transition region. Third, the H+ parallel and perpendicular temperatures increase with altitude in the barosphere due to frictional heating, while they decrease with altitude in the exosphere due to adiabatic cooling. Both temperatures reach their maximum values in the transition region. Fourth, the heat fluxes of the parallel and perpendicular energies are positive and increase with altitude in the barosphere, and they change rapidly from their maximum (positive) values to their minimum (negative) values within the transition region. The results of this simulation were compared with those found in previous work in which a simple (Maxwell‐molecule) collision model was adopted. It was found that the choice of the collision model can alter the results significantly. The effect of the body forces was also investigated. It was found that they can also alter the results significantly. Both the body forces and collision model have a large effect on the heat flux, while they have only a small quantitative effect on the lower‐order moments (density, drift velocity, and temperature).
Heating and radiation production by neutralized ring current particles
Authors: Noël S. , Prölss G. W.
Published: February 2008

Neutralized ring current particles are thought to be responsible for a number of different disturbance effects in the upper atmosphere. Here the energy deposition and radiation production by these particles are investigated. It is found that the particle heating is not sufficient to explain the geomagnetic activity effect at low latitudes. In contrast, the Hα and Hβ emission rates predicted by our model calculations are above the general background level and compare well with the observations.
Effects of a parallel electric field and the geomagnetic field in the topside ionosphere on auroral and photoelectron energy distributions
Authors: Min Q. ‐L. , Lummerzheim D. , Rees M. H. , Stamnes K.
Published: February 2008

The consequences of electric field acceleration and an inhomogeneous magnetic field on auroral electron energy distributions in the topside ionosphere are investigated. The one‐dimensional, steady state electron transport equation includes elastic and inelastic collisions, an inhomogeneous magnetic field, and a field‐aligned electric field. The case of a self‐consistent polarization electric field is considered first. The self‐consistent field is derived by solving the continuity equation for all ions of importance, including diffusion of O+ and H+, and the electron and ion energy equations to derive the electron and ion temperatures. The system of coupled electron transport, continuity, and energy equations is solved numerically. Recognizing observations of parallel electric fields of larger magnitude than the baseline case of the polarization field, the effect of two model fields on the electron distribution function is investigated. In one case the field is increased from the polarization field magnitude at 300 km to a maximum at the upper boundary of 800 km, and in another case a uniform field is added to the polarization field. Substantial perturbations of the low energy portion of the electron flux are produced: an upward directed electric field accelerates the downward directed flux of low‐energy secondary electrons and decelerates the upward directed component. Above about 400 km the inhomogeneous magnetic field produces anisotropies in the angular distribution of the electron flux. The effects of the perturbed energy distributions on auroral spectral emission features are noted.
Determining exospheric hydrogen density by reconciliation of Hα measurements with radiative transfer theory
Authors: He Xiangqun , Kerr Robert B. , Bishop James , Tepley Craig A.
Published: February 2008

The singly scattered resonant fluorescent component of the geocoronal Balmer‐alpha (Hα) emission is obtained by subtracting the multiply scattered component of Hα, determined by a radiative‐transfer (RT) model, from Hα measurements. Exospheric column abundance then follows a straightforward single‐scattering calculation. The reality of that abundance depends upon the validity of the model exosphere used by the RT model, upon the validity of the estimated solar Ly‐β flux, and upon the statistical measurement errors. Iterative adjustments of the hydrogen density ([H]) profile input to the RT model, converging to a best fit to Hα observations, generate a unique atmosphere best matching model intensity to measured Hα, brightnesses. Applying this method to ground‐based measurements of the geocoronal Hα emission made at Arecibo, Puerto Rico, in 1988, we find that the best estimated multiply and singly scattered components of Hα are about 85 and 60%, respectively, in excess of the initial model values, while the derived [H] is generally in agreement with the initial model [H] at lower altitudes and about 35% higher than the initial model [H] at higher altitudes. Examination of the iteration technique sensitivity indicates that the neutral temperature prescribed by the MSIS‐83 model and the solar Ly‐β flux estimated by a correlation relation are the parameters with the principal impact on the determined [H]. In addition, the examination shows that these parameters impact the RT‐calculated intensity components. Because convergence of the iteration technique is insensitive to the initial model guess for most cases, the technique is capable of accurately determining [H] from Hα measurements, particularly if the temperature is simultaneously measured and the line center solar Ly‐β flux is known.
Generation mechanism of plasmaspheric ELF/VLF hiss: A statistical study from GEOS 1 data
Authors: Cornilleau‐Wehrlin Nicole , Solomon Jacques , Korth Axel , Kremser Gerhard
Published: February 2008

ELF/VLF hiss is whistler mode waves that interact with energetic electrons in the magnetosphere. One of the consequences of these interactions is the precipitation of energetic electrons into the atmosphere due to pitch angle diffusion. One related question that is still controversial is how these waves can reach the intensities that are measured. A study of ELF/VLF hiss in the plasmasphere (Solomon et al., 1988) has shown that the observed plasmaspheric hiss intensities can be explained by the presence of a relatively high level of anisotropic energetic electron fluxes, with the wave growth rate γ being directly proportional to the flux level. Sonwalkar and Inan (1989) have argued, however, that this result was based on a limited number of cases only and that lightning‐generated whistlers may be a more important source for plasmaspheric hiss. In order to clear up this problem we have performed a statistical study using 75 orbits of GEOS 1, providing 48 events. All of them occurred on the day side at 4 < L < 5, where L is the McIlwain parameter, in the vicinity of the geomagnetic equator during relatively quiet geomagnetic conditions. These events have the usual characteristics of plasmaspheric hiss (frequency range and intensity). Simultaneous measurements of hiss and electron fluxes show on a statistical basis that (1) there is some trend for the strongest wave events to correspond to the largest electron fluxes, (2) the observed fluxes for the 48 events are generally much above the “typical” values used in previous theoretical studies, and (3) in agreement with the suggestion of Solomon et al. (1988) the statistical results are consistent with wave generation in the equatorial region, the plasmaspheric hiss being amplified during a single crossing of the waves through this region.
Jovian electron propagation in three dimensions of the heliosphere: The Ulysses Investigations
Authors: Simpson J. A. , Smith D. A. , Zhang M. , Balogh A.
Published: February 2008

We report investigations of Jovian relativistic electrons in the interplanetary medium that provide new insights into both the physical processes by which the Jovian magnetosphere releases its trapped, relativistic electrons into the interplanetary medium, and the modes of their interplanetary propagation. These studies were dependent on the unique postencounter trajectory for Ulysses. The spacecraft remained close to the radial distance of Jupiter (∼5.2 AU) and moved southward on the duskside by only ∼12° in heliographic latitude and <8° in heliographic azimuth relative to Jupiter for the period of ∼100° days of this study. During this period the nominal Parker spiral interplanetary magnetic field with its alternating polarities sector structure established direct magnetic field line connections frequently between Jupiter and the spacecraft. These unique conditions made it possible to investigate in detail, for approximately four solar rotations, both the Jovian electron burst phenomenon and the continuous, diffusive interplanetary propagation of Jovian electrons. The Jovian electron burst phenomenon in the energy range 1‐20 MeV, with flux increasing up to 50 times the diffusive Jovian electron flux are now discovered to occur in both multiples of the planetary rotation period (∼10 hours) and occasionally in half rotation periods, namely, in sequences of 5‐, 10‐, 20‐, or 40‐hour intervals. Electrons in the burst are shown to stream from the magnetosphere and not the magnetotail to the spacecraft during times when the interplanetary magnetic field vector direction includes both Jupiter and the spacecraft. Over 90% of all electron bursts lasting ≥6 hours occurred during the positive magnetic field sector. The few bursts occurring in a negative sector (<10%) were associated with rapid changes in magnetic field orientation near a sector boundary. We found that the time for these burst sequences corresponded to either the dawnside or the duskside tilt of the magnetic dipole. This dawn‐dusk dipole tilt is in a plane approximately orthogonal to the Sun‐Jupiter line and aligned with the average interplanetary magnetic field direction at ∼5 AU. To interpret these measurements, we assumed that the release mechanism is intermittent reconnection of the interplanetary magnetic field with the opposite polarity of a component of the polar magnetospheric field. These experimental results appeared to exclude some models for electron release such as the magnetic anomaly model where the electrons are released into the magnetotail. The diffusive Jovian electron flux continuously present in the interplanetary medium confirmed the “10‐hour clock” variations of the differential energy spectral index found by Pioneer 10 in 1973. Present evidence indicates that this continuum of Jovian electrons with its 10‐hour rocking spectrum may be associated with the same release mechanism as the electron bursts.
Transfer faults and pull‐apart model in the rhinegraben from analysis of multisource data
Authors: Chorowicz J. , Deffontaines B.
Published: February 2008

From analysis of gravimetric data, comparisons of levelings, seismic profiling, paleostress analysis, and seismicity together with remotely sensed and morphological data, the Rhinegraben may be divided into several rhomb‐shaped basins, separated by ENE trending basement highs. These data express neotectonic movements and slight spatial differences in the subsidence rate. These basins are asymmetrical: on one side they are bounded by a large (several kilometers of vertical throw) normal fault zone; on the opposite side there is a megaflexure, cut by smaller normal faults shaping tilted blocks. Since the middle Miocene, extensional directions are subparallel to the ENE trending basement highs that consequently may be interpreted as strike‐slip fault zones of transfer type. Across the Erstein basement high, there is a reversal in the rift asymmetry (major bordering fault on the western side to the south, on the eastern side to the north). When considered in the context of the West European Rift, the Rhinegraben is interpreted in terms of a pull‐apart depression formed during the horizontal movement of blocks guided by large ENE trending transcurrent faults. The normal faults that are curved shaped in map view may mark the boundaries between areas where the maximum component (σ1) of the paleostress field developed in pull‐apart patterns was changing from horizontal to vertical.
Mapping of the high‐frequency source radiation for the Loma Prieta Earthquake, California
Authors: Zeng Yuehua , Aki Keiiti , Teng Ta‐Liang
Published: February 2008

Waveform inversion has been successfully applied to map the spatial distribution of earthquake fault slip from strong motion records. But its application has been restricted to a relatively lower frequency range of the data (ƒ< 2.0–3.0 Hz) essentially due to the difficulties in calculating the high‐frequency Green's function; these difficulties arise specifically from the small‐scale complexity of source, path and station site, and insufficient numbers of observations. However, we can avoid these difficulties by simply looking at the seismic wave energy envelopes instead of the complicated high‐frequency wave phases. By assuming that the slip responsible for high‐frequency wave (e.g.,ƒ> 5 Hz) generation is incoherent between neighboring patches on the fault, we formulated an analytical model for calculating the envelope time history of the squared high‐frequency displacement seismogram. This envelope time history is found through the line integration of the squared energy radiation intensity over the isochron. Using a recursive stochastic inversion scheme, we determined both the lower‐frequency slip distributions from the low‐pass‐filtered strong motion displacement data and the higher‐frequency energy radiation intensity from the envelope of the squared high‐frequency displacement data on an extended fault for the 1989 Loma Prieta earthquake, respectively. Our resulting lower‐frequency slip model shows that there are two main slip patches and little slip directly updip of the hypocenter. One major slip zone located 10 km northwest of the hypocenter with predominant dip‐slip and the other located 12 km southeast of the hypocenter with predominant oblique slip. Our preliminary results of the high‐frequency energy radiation intensity inversion indicate three large high‐frequency sources and few smaller ones located on the outer periphery of the large slip zones. This suggests that the high‐frequency energy sources are located along or near the boundaries of large slip zones, which is consistent with the theoretical consideration that high frequencies are primarily generated from the rupture stopping areas or places with large slip variation.
Evolution of the Indian Ocean Triple Junction between 65 and 49 Ma (anomalies 28 to 21)
Authors: Dyment Jérôme
Published: February 2008

Reinterpretation of newly published geophysical data (Kamesh‐Raju and Ramprasad, 1989) and older profiles of the Central Indian Basin, associated with similar studies of the Madagascar and Crozet basins, shows that the Indian Ocean Triple Junction trace on the Indian plate corresponds, at anomalies 23 and 22, to a N38°E offset of the magnetic lineations, oblique to both the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) spreading directions. The conjugate Triple Junction trace on the African plate identified in the Madagascar Basin is associated with a roughly north‐south offset, parallel to the Southwest Indian Ridge (SWIR) fracture zones. In order to account for these observations and the velocity triangle of the Indian, African, and Antarctic plates close to the Triple Junction, a ridge‐fault‐fault mode is proposed, with a propagatorlike SEIR‐CIR offset. The Triple Junction jumped between anomalies 24 and 23 and between anomalies 22 and 21, restoring a ridge‐ridge‐ridge configuration which immediately turned to a pseudo‐ridge‐ridge‐fault and later to a true ridge‐fault‐fault configuration. After the Triple Junction jump at anomaly 21, the former SEIR‐CIR offset was accommodated by a new CIR fracture zone. The lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo‐ridge‐ridge‐fault or an unstable ridge‐ridge‐ridge configuration prevailed before anomaly 24, in agreement with the velocity triangles which predict more unstable Triple Junction modes. Both modes support the creation of numerous SWIR fracture zones, presently observed between 52°30′E and 59°30′E, as a consequence of the Triple Junction evolution between anomalies 29 and 24. This result suggests that the physiography of the SWIR records the history of the Triple Junction.
Fast and slow spreading ridges: Structure and hydrothermal activity, ultramafic topographic highs, and CH4 output
Authors: Bougault Henri , Charlou Jean‐Luc , Fouquet Yves , Needham Hubert D. , Vaslet Nathalie , Appriou Pierre , Baptiste Philippe Jean , Rona Peter A. , Dmitriev Leonid , Silantiev Sergei
Published: February 2008

Different parts of the world ridge system have quite different morphologies, which reflect different constructional processes. It appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust. This hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes. The TAG (26°N) and Snake Pit (23°N) hydrothermal sites on the Mid‐Atlantic Ridge (MAR) display Mn/CH4 ratios in overlying seawater similar to those of nonsedimented East Pacific Rise sites, i.e., about 0.2 mol/L. In contrast, large methane anomalies with very low Mn/CH4 ratios of 0.005 mol/L are associated with ultramafic topographic highs near 15°N, close to the axis and on ultramafic walls of the rift valley. The association of ultramafic bodies and CH4 anomalies in seawater indicates active serpentinization processes. CH4 is produced during serpentinization according to the Fisher Tropsch reaction (Charlou et al., 1991). Many ultramafic rock assemblages have been sampled on the MAR away from fracture zones, in particular by the Deep Sea Drilling Project. Changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges.
Seismic structure of oceanic crust in the western North Atlantic
Authors: Morris Ellen , Detrick Robert S. , Minshull Timothy A. , Mutter John C. , White Robert S. , Su Wusi , Buhl Peter
Published: February 2008

We examine the seismic structure of Cretaceous‐aged ocean crust north of the Blake Spur fracture zone in the western North Atlantic by using a combination of multichannel seismic and wide‐angle reflection/refraction data. Although the oceanic crust in this area is characterized by a relatively uniform thickness and seismic velocity structure, it displays large variations in crustal reflectivity on both ridge‐normal and ridge‐parallel profiles. The upper crust is highly reflective and contains both subhorizontal (or shallow dipping) events and more steeply dipping reflectors. Subhorizontal reflectors are typically present between 1.5 and 2 km depth and are, in some cases, laterally continuous for distances of 15 to 20 km. Generally, these events do not correlate with the depth of the seismic layer 2/layer 3 boundary determined from refraction data, and their origin is still poorly understood. The steeply dipping events are generally confined to the upper 2.5 km of the crust. We interpret these reflectors as the subsurface expression of ridge‐parallel extensional faults commonly mapped at mid‐ocean ridges. The lower crust in this area exhibits alternating regions of high and low reflectivity. The highly reflective zones are made up of packages of linear or concave‐upward dipping reflectors that flatten out in a diffuse zone of high reflectivity near the base of the crust. Some of these dipping reflectors appear to cut through the whole crustal section, although most fade out in the acoustically transparent midcrust. On ridge‐normal profiles the majority of these events dip to the east, toward the paleo‐spreading center, whereas on ridge‐parallel profiles the events typically dip south, toward the Blake Spur fracture zone. The base of the crust is generally not associated with a strong, discrete Moho reflection but is a comparatively indistinct boundary usually associated with a 1‐ to 2‐km‐thick band of diffuse high reflectivity. We interpret the lower crustal and whole crustal dipping events as the subsurface expression of major fault systems that have ruptured the entire crustal section down to depths of 8–10 km. With the available data we cannot unambiguously determine the geometrical relationship between the dipping lower crustal reflectors on the ridge‐normal and ridge‐parallel profiles. We may be imaging a single major detachment surface that dips both toward the ridge axis and away from accommodation zones linking major boundary faults. Alternatively, these events may represent two different classes of fault systems that form during different stages of the emplacement and aging of oceanic lithosphere.
Morphology of the Valu Fa Spreading Ridge in the southern Lau Basin
Authors: Wiedicke Michael , Collier Jenny
Published: February 2008

Multibeam echosounder surveys and seismic reflection profiling were conducted in the southern Lau back arc basin (SW Pacific). The bathymetric survey covered a 130‐km‐long section of the active spreading ridge (3 cm/yr half spreading rate), the N‐S trending Valu Fa Ridge (VFR), while seismic profiles concentrated on a 35‐km‐long ridge section. Analysis of these data shows a morphologically segmented ridge which is underlain by a continuous magma chamber. The ridge shows similarities, but also differences, to the segmentation model developed for mid‐ocean ridges. Analogous to this model the section of the VFR studied displays third‐ and fourth‐order segmentation. The amplitudes of the along‐axis undulations, however, are a factor of 5 to 10 greater than at mid‐ocean ridges. Additionally, the VFR has numerous small volcanic cones along the axis, which have previously only been observed at much slower spreading ridges. In cross section the VFR has a very steep, narrow triangular shape which is thought to result from the high viscosity of its andesitic lavas. The axial magma chamber appears to vary in width (1–4 km) and location relative to the axis beneath different morphological segments. Perhaps the most significant deduction from our combined data is that the magma chamber is widest beneath the most prominent axial discontinuity, a small overlapping spreading center (OSC). This is counter to observations from mid‐ocean ridges. Off‐axis traces of ridge segments allow us to reconstruct the evolution of this OSC for the past 220 kyr. We conclude that this back arc spreading axis shows a segmentation pattern which is similar to mid‐ocean ridges, but it is influenced and modified by the geochemistry of its lavas and its proximity to the island arc.
The mission link between the Hayward and Calaveras Faults
Authors: Andrews D. J. , Oppenheimer David H. , Lienkaemper James J.
Published: February 2008

The left stepover between the Calaveras and Hayward faults gives rise to compressive strain that is accommodated by vertical deformation, producing the highest hills bordering the east side of San Francisco Bay (Monument Peak to Mission Peak). The projection of the active trace of the Hayward fault is separated 3.5 km from the junction of the northern and southern segments of the Calaveras fault at Calaveras reservoir. For pure strike slip on the Hayward and Calaveras faults, the stepover region is compressed in area at a rate given by the separation times the long‐term slip rate of 9 mm/yr on the Hayward fault. Assuming a seismogenic depth of 10 km, the horizontal compression accumulated over 150 years is equivalent to a dip‐slip seismic moment of 2.8 × 1018 N m. This moment, if released in a single event, would generate an earthquake of magnitude 6.3 due to dip slip alone. Dip slip may occur on buried faults throughout the stepover region. The base of the southwest face of Monument Peak suggests a buried thrust or reverse fault dipping northeast. The prominent scarp on Mission Peak has the same strike and sense of slip. Current microearthquakes between the Calaveras and Hayward faults align parallel to the Mission fault, but show only horizontal slip on a vertical fault plane. A dipping fault is needed to satisfy the geometric constraint of the convergence. Therefore, the Mission fault zone must consist of two or more fault surfaces inclined at different dip angles, as is observed in the San Andreas fault zone in the Loma Prieta area. Horizontal slip is building up stress on the inclined fault, which is currently locked. Growing compressive stress will increasingly impede horizontal slip until a thrust or reverse‐slip event occurs, allowing then more horizontal slip to be transferred from the Calaveras to the Hayward fault. The dip‐slip fault might be the unstable element governing slip in the larger system.
Magmatic and tectonic development of the western Aleutians: An oceanic arc in a strike‐slip setting
Authors: Yogodzinski G. M. , Rubenstone J. L. , Kay S. M. , Kay R. W.
Published: February 2008

The Paleogene Basement Series on Attu Island in the Western Aleutians is dominated by tholeiitic basalts chemically similar to mid‐ocean ridge basalts (MORB; La/Yb ∼ 2, ϵNd > +10.0, 206Pb/204Pb < 18.4). These basalts evolved in “open” magma systems. Contemporaneous rhyolites and albite granites are chemically analogous to silicic volcanic rocks of modern ocean ridges (e.g., Iceland, Galapagos Spreading Center). The occurrence of chemically arc‐type tholeiitic basalts, with high Th/La and La/Ta relative to MORB, suggests that a depleted MORB‐like mantle source was variably modified by a subduction component. The Attu Basement Series is not allochthonous. Geological and tectonic constraints imply formation in arc‐adjacent, transtensional rifts that developed in a strike‐slip regime that was established ∼43 m.y. ago. Rifting of this kind may have produced much of the Western Aleutian crust between ∼43 and 15 Ma. Subsequently, small volumes of crystal‐rich andesite and dacite were erupted on Attu Island as well as throughout the Western Aleutians. These strongly calcalkaline rocks (FeO*/MgO∼1.1, CaO/Al2O3 ∼0.35 at 65% SiO2) are chemically akin to magnesian andesites of Piip Volcano, a hydrothermally active seamount that overlies small dilational structures within the broadly transpressional regime of the modern western arc. This transpressional setting is inferred to have been established throughout the Western Aleutians approximately 15 m.y. ago. The switch from tholeiitic magmatism in a transtensional regime to strongly calcalkaline magmatism in a transpressional regime may have resulted from clogging of the Aleutian‐Kamchatka junction with buoyant, subduction‐related terranes. These terranes probably originated to the east and were transported by strike‐slip motion along the western arc.
Patterns of hot spot volcanism
Authors: Marzocchi Warner , Mulargia Francesco
Published: February 2008

The aim of this paper is to identify the characteristic features, in terms of geophysically measured parameters, of the hot spot sites over the Earth's surface. We use a pattern recognition approach which identifies associations as well as single parameters, and our statistical algorithm allows us to work at a given significance level, avoiding any overfit. We use the following parameters: absolute plate velocity of the site, minimum distance between neighboring hot spots, minimum distance from a ridge, minimum distance from a trench, geoid anomaly of harmonic degrees 2–10 and 11–36, area of the host plate, lithospheric thickness and stress state. The hot spots, generally defined as volcanoes not obviously related to plate boundaries, are found to occur in sites characterized by either one of two main significant patterns: (1) positive values of geoid anomalies with harmonic degree between 2 and 10, which is characteristic of most of the hot spots with a “track” of extinct volcanoes; (2) clustered volcanoes (distance between volcanoes less than ∼900 km) in slow moving plates (velocity <2.5 cm/yr). A third pattern, representative of isolated volcanoes (distance between volcanoes greater than ∼900 km) anomalously close to mid‐ocean ridges (distance from the ridge less than ∼600 km) can also be tentatively identified. This result implies the existence of two main types of hot spot volcanism, the first one of sublithospheric origin with dynamics which is not influenced by surface tectonics, and the second one mainly due to a favorably “soft” lithosphere. A third type would probably consist of “anomalous” and magmatically very productive parts of mid‐ocean ridges.
A model of the geomagnetic 60‐year variation
Authors: Yokoyama Yukiko , Yukutake Takesi
Published: February 2008

A model of the geomagnetic 60‐year variation is proposed. The 60‐year variation is regarded as a magnetohydrodynamic oscillation originating in a thin layer at the top of the core. Two types of magnetic field and two types of fluid motion are mainly considered in the layer, which are poloidal magnetic mode (1, 0), toroidal magnetic mode (2, 0), toroidal velocity mode (1, 0), and poloidal velocity mode (2, 0). When the magnetic force and the Coriolis force are dynamically dominant, these four fields make a closed system and cause magnetohydrodynamic oscillations in the layer. Steady magnetic fields of higher modes generated in the deeper part of the core induce fluctuating fields when they pass through the layer. The induced magnetic fields have a common period and two different types of phase. Observed amplitudes of these fields are mostly explained when the maximum velocity in the layer is 10−4 m/s.
Joint pattern development: Effects of subcritical crack growth and mechanical crack interaction
Authors: Olson Jon E.
Published: February 2008

Fracture networks are examined in the light of subcritical crack growth theory. Examples of equilibrium crack geometries are generated using a fracture mechanics model that explicitly tracks the propagation of multiple fractures. It is determined that propagation velocity as modeled using a subcritical fracture growth law exerts a controlling influence on fracture length distributions and spacing. Velocity is modeled as proportional to the n‐th power of the mode I stress intensity. Numerous, closely spaced, similar length fractures result for n=1, with many en echelon arrays forming due to fracture interaction. Increasing the value of n results in the growth of fewer fractures that are more widely spaced. Fractures tend to cluster in narrow zones, with limited fracture growth in the intervening areas. The spacing between zones is controlled by the stress shielding effects of longer fractures on shorter ones. The amount of time required for fracture pattern development is also influenced by the subcritical velocity exponent, n. At low n, patterns take seconds to minutes to develop, while patterns generated at higher n can require hundreds of years or more.