bluuebunny/crossref_metadata_2025 huggingface dataset:
Survey of coherent ion reflection at the quasi‐parallel bow shock
Authors: Onsager T. G. , Thomsen M. F. , Gosling J. T. , Bame S. J. , Russell C. T.
Published: February 2008

Ions coherently reflected off the Earth's bow shock have previously been observed both when the upstream geometry is quasi‐perpendicular and when it is quasi‐parallel. In the case of quasi‐perpendicular geometry, the ions are reflected in a nearly specular manner and are quickly carried back into the shock by the convecting magnetic fleld. In the quasi‐parallel geometry, however, near‐specularly reflected ions' guiding center velocities would on the average be directed away from the shock, allowing the ions to escape into the upstream region. The conditions under which coherent reflection occurs and the subsequent coupling of the reflected ions to the incoming solar wind plasma are important factors when assessing the contribution of the reflected ions to the downstream temperature increase and the shock structure. The survey presented in this paper, along with previously reported observations, suggests that near‐specularly reflected ions are indeed an important aspect of energy dissipation at the Earth's quasi‐parallel bow shock. We find that (1) cool, coherent, near‐specularly reflected ion beams are detected over nearly the full range of upstream plasma parameters commonly found at the Earth's bow shock; (2) the beams are typically observed only near the shock ramp or some shock‐like feature; and (3) the observed beam velocities are almost always consistent with what one would expect for near‐specularly reflected ions after only a small fraction of a gyroperiod following reflection. The second and third points indicate that the beams spread very quickly in velocity space. This spread in velocities could be due either to interactions between the beam and incoming solar wind ions or to some initially small velocity spread in the beam. In addition we present an observation of two simultaneous beams, one outgoing from the shock and one incoming towards the shock. They were detected when the local geometry was quasi‐perpendicular, even though the averaged field geometry was quasi‐parallel. This observation indicates that local variations in the magnetic field are in some cases responsible for returning the specularly reflected ions to the downstream region, allowing them to contribute directly to the downstream thermalization.
The configuration of the auroral distribution for interplanetary magnetic field Bz northward: 1. IMF Bx and By dependencies as observed by the Viking satellite
Authors: Elphinstone R. D. , Jankowska K. , Murphree J. S. , Cogger L. L.
Published: February 2008

Viking images obtained throughout 1986 have been utilized in combination with IMP 8 satellite measurements of the interplanetary magnetic fields (IMF) to determine typical northern hemisphere auroral distributions for a variety of IMF Bz positive conditions. Varying By has an effect which is consistent with expected results. That is, By positive implies high‐latitude auroral arcs in the dusk sector while negative By gives dawn sector polar arcs. A new result gives significant importance to the Bx component of the IMF. Bx toward the Sun (By = 0) gives polar arcs on both dawn and dusk with comparatively weak UV emissions. With Bx away from the Sun (By = 0) a single Sun‐aligned morning sector polar arc dominates the auroral distribution. Azimuthal angle changes to the IMF of only 45° seem to affect the global auroral distribution with time scales of less than 2–3 hours. Poleward boundaries of the aurora were found to have a strong dependence on the IMF azimuthal angle which varied according to the magnetic local time investigated.
Satellite and ground‐based observations of a fading transpolar arc
Authors: Pellinen R. J. , Koskinen H. E. J. , Pulkkinen T. I. , Murphree J. S. , Rostoker G. , Opgenoorth H. J.
Published: February 2008

Satellite and ground‐based observations of the end phase of a transpolar arc event on September 25, 1986, are presented. The event was recorded by the UV imager of the Viking spacecraft during the time period from 2000 to 2110 UT. The transpolar arc was formed during a period of more than one hour of northward directed interplanetary magnetic field BZ. The arc started to fade when the IMF turned southward and a localized brightening of the auroral oval west of its nightside foot point was observed. Within a few minutes the activation was strongly enhanced. This enhancement was also recorded by all‐sky cameras in Kevo and Kilpisjärvi. Magnetic disturbances below the activation were relatively weak, but at the EISCAT magnetometer stations a counterclockwise rotating equivalent current system, which can be interpreted as due to an upward directed filamentary field‐aligned current, was observed to last about 10 min. The magnetic disturbance was fairly stable and did not move across the magnetometer chain during its existence. EISCAT observed a clear enhancement of E region electron density after 2100 UT, which was not associated with hard precipitation, as indicated by the absence of riometer absorption over Scandinavia. The EISCAT scan data on ionospheric horizontal ion drift velocity showed a localized region of eastward drift just north of Tromsø, in agreement with the magnetometer recordings. Also the postmidnight auroral oval was brightened in a wide longitudinal range, and a weak westward current was observed flowing in the ionosphere by Soviet magnetometers below the bright region of the oval. Though the magnetic signatures below the activation west of the foot point indicate a cablelike upward field‐aligned current, we do not interpret the phenomenon as a substorm. After the activation discussed here the ionospheric convection increased, leading to the breakup of a strong substorm about one hour later.
Electrical breakdown at low pressure in the presence of a weak magnetic field
Authors: Alport M. J. , Antoniades J. A. , Boyd D. A. , Greaves R. G. , Ellis R. F.
Published: February 2008

Electron trapping in crossed electric and magnetic fields is an important mechanism by which electrical discharges can develop in low pressure gasses. We report observations of discharges produced by this mechanism around a spherical anode in two space simulation chambers, namely the Space Plasma Interaction Experiment (SPIE) at the University of Maryland, and the NASA‐Lewis B‐2 chamber. We have identified two types of discharges in these experiments. In the B‐2 chamber, the breakdown takes the form of a runaway discharge with spherical topology, limited only by the ability of the power supply to provide the current. In the SPIE chamber this type of discharge also occurs, in addition to a low current toroidal discharge which is observed at higher magnetic fields. We present measurements of both types of discharge and show how the trapping effect of the magnetic field together with secondary electron emission by high energy ion bombardment of the chamber walls may initiate and sustains these discharges.
Two‐state ion heating at quasi‐parallel shocks
Authors: Thomsen M. F. , Gosling J. T. , Bame S. J. , Onsager T. G. , Russell C. T.
Published: February 2008

In a previous study of ion heating at quasi‐parallel shocks, we showed a case in which the ion distributions downstream from the shock alternated between a cooler, denser, core/shoulder type and a hotter, less dense, more Maxwellian type. In this paper we further document the alternating occurrence of two different ion states downstream from several quasi‐parallel shocks. Three separate lines of evidence are presented to show that the two states are not related in an evolutionary sense (i.e., the hotter distributions do not evolve from the cooler ones), but rather both are produced alternately at the shock: (1) The asymptotic downstream plasma parameters (density, ion temperature, and flow speed) are intermediate between those characterizing the two different states closer to the shock, suggesting that the asymptotic state is produced by a mixing of the two initial states; (2) examples of apparently interpenetrating (i.e., mixing) distributions can be found during transitions from one state to the other; and (3) examples of both types of distributions can be found at actual crossings of the shock ramp. The alternation between the two different types of ion distribution provides direct observational support for the idea that the dissipative dynamics of at least some quasi‐parallel shocks is non‐stationary and cyclic in nature, as demonstrated by recent numerical simulations. Typical cycle times between intervals of similar ion heating states are ∼2 upstream ion gyroperiods. Both the simulations and the in situ observations indicate that a process of coherent ion reflection is commonly an important part of the dissipation at quasi‐parallel shocks; we suggest that cyclic variations in the fraction of reflected ions could account for the alternating regions of cooler and hotter downstream ion distributions documented in this paper.
Structure and properties of the subsolar magnetopause for northward IMF: ISEE observations
Authors: Song P. , Elphic R. C. , Russell C. T. , Gosling J. T. , Cattell C. A.
Published: February 2008

Detailed magnetopause structure and properties for the magnetic field, electric field and plasma are examined for an ISEE 1 magnetopause crossing which occurred near the subsolar point when the interplanetary magnetic field (IMF) was strongly northward. Because the crossing is slow, the spatial variations in the plasma are clearly resolved. This example illustrates the nature of the steady state interface of two magnetized thermal plasma populations with parallel fields and can serve as a guide to theoretical modeling and simulations. We have found that the magnetopause is composed of three layers, a sheath transition layer, an outer boundary layer and an inner boundary layer. In the sheath transition layer, there is a gradual density decrease without a change in temperature. The transition layer occurs totally within the magnetosheath plasma. The outer boundary layer and the inner boundary layer are dominated by magnetosheath and magnetospheric particles, respectively. In each of the boundary layers, the plasma can be interpreted as simple mixtures of the magnetosheath and magnetospheric populations. No significant heating or cooling is seen across the magnetopause during this crossing. The plasma within each of these layers is quite uniform and their boundaries are sharp, suggesting that there is very little diffusion present. The sharp boundaries between the transition layer, the boundary layers and the magnetosphere are all thinner than an ion gyroradius. Transverse waves with right hand or linear polarization near the ion gyrofrequency are observed in the transition layer. These appear to be generated in the transition layer and to be a common feature of this layer when the IMF is northward.
Beam‐generated upper hybrid noise in Jupiter's outer magnetosphere
Authors: Barbosa D. D. , Kurth W. S.
Published: February 2008

Upper hybrid waves in Jupiter's outer magnetosphere are the subject of this paper which proposes a new theoretical model for their generation. Energetic electrons are assumed to be accelerated in the high‐latitude auroral zone and stream away from the planet to the outer magnetosphere. The electrons travel along magnetic field lines located at the edges of the plasma sheet through a region which has the properties of a plasma sheet boundary layer similar to the terrestrial entity. The kinematic evolution of the electron beam results in a highly collimated field‐aligned distribution which excites quasi‐perpendicular upper hybrid waves through a so‐called beam‐anisotropic heat flux instability. The instability is dominated by anomalous cyclotron harmonic resonances in a parameter regime where the gyroradius of the beam electrons is comparable to the wavelength of the wave. Propagation of the upper hybrid waves across the magnetic field into a plasma density gradient produces a spectrum of Z mode waves which then undergo a linear mode conversion to ordinary mode electromagnetic radiation. The model successfully accounts for the observations of upper hybrid noise, Z mode emissions, and continuum radiation in the Jovian magnetosphere in a self‐consistent coherent manner.
Effects of ionospheric recombination time scale on the auroral signature of substorms
Authors: Zhu L. , Kan J. R.
Published: February 2008

The magnetosphere‐ionosphere coupling model of substorms developed by Kan et al. (1988) is improved by including the realistic recombination time scale in the ionosphere and the quiet‐time convection prior to the enhancement of the magnetospheric convection. One of the new features of the improved model is the appearance of highly localized structures of the discrete aurora along the poleward boundary of the diffuse aurora in the evening‐midnight sector. This feature is consistent with the auroral image data of the Viking satellite. Our results show that a discrete auroral arc suddenly brightens near midnight and starts moving westward when the auroral brightness increases to a certain level, which is similar to the substorm onset scenario. The bright discrete aurora has an initial velocity of around 2 km s−1, but gradually slows down and dims. This feature is identified with the westward traveling surge. Following passage of the surge, several localized auroral arcs remain along the auroral oval, and the most active auroral region shifts from premidnight toward the evening sector.
Magnetic field structure of interplanetary magnetic clouds at 1 AU
Authors: Lepping R. P. , Jones J. A. , Burlaga L. F.
Published: February 2008

Interplanetary magnetic clouds, although not dominant, are a relatively common feature of the solar wind at 1 AU. Their diameters at 1 AU fall in the range of 0.2–0.4 AU, and they have enhanced field strength (B ≃ 15–30 nT at 1 AU), and lower plasma temperature and density than the surrounding plasma. The internal field is a magnetic force‐free configuration, and therefore the current density (J) is proportional to B everywhere: J = α B, giving ▽×B = α B. If α is constant throughout the cloud (Burlaga, 1988), then ▽²B = −α²B, which has a cylindrically symmetric field solution that is consistent with observations: the axial field is proportional to the zeroth‐order Bessel function of r, where r is the perpendicular distance from the cloud's axis, the tangential component is proportional to the first‐order Bessel function, and the radial component is zero. We have developed a least squares program that fits magnetic field data within a cloud to these functions and which estimates various properties of the cloud, such as its size, maximum B, and inclination of its axis, as well as closest approach distance of the spacecraft. Results of a study of 18 clouds observed at 1 AU indicate that the most probable direction of the cloud's axis is within 15° of the ecliptic plane and ≃100° from the Sun's direction when it is projected into the ecliptic plane. A broad range of orientations is observed with some extending to 80° from the ecliptic. Other statistical properties are presented, and three cases are discussed in detail.
Numerical study of the current sheet and plasma sheet boundary layer in a magnetotail model
Authors: Doxas I. , Horton W. , Sandusky K. , Tajima T. , Steinolfson R.
Published: February 2008

A test particle code is used to study the response of ensembles of particles to a two‐dimensional, time‐dependent model of the geomagnetic tail and to test the proposition (Coroniti, 1981a, b; Büchner and Zelenyi, 1986; Chen and Palmadesso, 1986; Martin, 1986) that the stochasticity of the particle orbits in these fields is an important part of the physical mechanism for magnetospheric substorms. The realistic results obtained for the fluid moments of the particle distribution with this simple model, and their insensitivity to initial conditions, are consistent with this hypothesis.
The radiative association of N and O atoms
Authors: Du M. L. , Dalgarno A.
Published: February 2008

Calculations are carried out of the rate coefficient for the radiative association of ground state nitrogen oxygen atoms to form nitric oxide as a function of the temperature T. The rate coefficient is given by α(T) = 2.8(300/T)0.5 × 10−17 cm³ s−1. Detailed predictions are presented for the emission spectra accompanying the radiative association process. The widths of the emission profiles vary significantly with temperature.
Perpendicular heating of electrons by upper hybrid waves generated by a ring distribution
Authors: Lin C. S. , Menietti J. D. , Wong H. K.
Published: February 2008

Satellite observations of electron conical distributions with enhanced fluxes just outside the loss cone suggest that electrons have been heated perpendicularly to the magnetic field in the mid‐altitude polar magnetosphere. To understand electron conical distributions, plasma simulations are conducted to examine an upper hybrid wave instability of a ring electron distribution perpendicular to the magnetic field in a cold electron background. The simulations indicate that both the cold and ring distributions are heated perpendicularly during the saturation stage. From the plasma data, a ring distribution can be identified as a trapped distribution function with an enhancement near 90° pitch angle in the phase space density plot. It is suggested that the ring distribution might provide an additional free energy source for generating upper hybrid waves associated with electron conical events.
Quantitative study of the nonlinear formation and acceleration of plasmoids in the Earth's magnetotail
Authors: Otto A. , Schindler K. , Birn J.
Published: February 2008

The formation and dynamical evolution of plasmoids are investigated by two‐dimensional resistive MHD computations. It is shown that a magnetotail equilibrium has to include a distant neutral line for a quantitative evaluation of the relevant dynamical properties of plasmoids. Special attention is given to the acceleration mechanism, which exhibits qualitatively a universal pattern but depends quantitatively on the details of the initial field structure and on the type of the microscopic dissipation. In addition to pressure and magnetic forces, mass and momentum transfer from the surrounding plasma to the plasmoid contributes to the acceleration process. These contributions arise in the presence of nonidealness, i.e., resistivity in the system. In the beginning of the plasmoid formation process, a unique feature of the acceleration is the dominant contribution of momentum transfer and pressure forces. For the cases considered, the pressure forces strongly depend on the chosen initial equilibrium state. Although magnetic forces increase before the plasmoid detaches from the near‐Earth X line, the contribution of mass transfer to the plasmoid becomes dominant, which leads to rapid growth and small deceleration of the plasmoid. When the detachment has occurred, mass and momentum transfer is small, and the relative importance of magnetic forces and pressure forces depends on the chosen initial state and the chosen resistivity. Larger Lundquist numbers and/or a current‐dependent resistivity similar to the one used in the computations seem to enhance magnetic forces. At rather late times an enhanced electric field at the distant neutral line might lead to growing mass transfer by which a plasmoid may be decelerated. The qualitative picture of a slingshot effect, i.e., plasmoid acceleration by the tension of open interplanetary magnetic flux, is shown to be misleading for the cases considered in this paper. For magnetotail configurations that are more realistic than those considered so far, the results confirm that spontaneous magnetic reconnection resulting in plasmoid formation and acceleration represents a sufficiently fast process in order to explain magnetotail dynamics during substorms.
The 9‐ to 12‐µm atmospheric ozone emission observed in the SPIRIT 1 Experiment
Authors: Adler‐Golden S. M. , Matthew M. W. , Smith D. R. , Ratkowski A. J.
Published: February 2008

Spectra of ozone υ3 emission in an auroral nighttime sky were obtained in the Spectral Infrared Interferometric Telescope experiment. High‐quality hot band spectra reveal spectral structure not previously observable and provide a critical test of ozone radiance models. The limb radiance profile is in good agreement with previous data below ∼95 km, but above this it falls off more rapidly.
The longitude dependence of the dayside F region trough: A detailed model‐observation comparison
Authors: Sojka J. J. , Schunk R. W. , Whalen J. A.
Published: February 2008

The nighttime main F region trough extends into the sunlit afternoon sector. This trough feature exhibits both a strong magnetic activity dependence and a longitude (UT) dependence. Whalen (1987), using International Geophysical Year (IGY) ionosonde data, showed that both of these effects are readily extracted from foF2 observations. In this study we show that the longitude effect is the same as that contained in the Utah State University time‐dependent ionospheric model. It arises from the offset of the geomagnetic axis from the geographic axis. The magnetic activity dependency is associated with the westward convection in the afternoon sector. It is also contained in the ionospheric model via the empirical magnetospheric convection model. This one‐to‐one observational‐model agreement is unique; to date no other magnetic activity dependent ionospheric feature has been simulated this systematically.
The average tangential electric field at the noon magnetopause
Authors: Lindqvist P.‐A. , Mozer F. S.
Published: February 2008

The tangential electric field at the magnetopause in the magnetopause frame of reference has been estimated for 205 magnetopause crossings of the ISEE 1 satellite within 2 hours of local noon during 1977–1981. At most of these crossings, time variations of the electric field caused its fluctuations to be large compared with its mean value. This implies both that variations of the field (due, for example, to time varying reconnection) may be of fundamental significance, and that statistical analyses of a large number of crossings are required to determine the average dependence of the tangential electric field at the magnetopause on the magnetosheath magnetic field. A generalized least squares method, which takes into account uncertainties in both the electric and magnetic field measurements, has been developed and applied to the data. The results indicate that, on the average, reconnection occurs at the subsolar magnetopause; that the average reconnection flow speed of magnetosheath plasma toward the magnetopause is 20−6+14 km/s (which is about 15% of the local Alfvén speed); and that the dependence of the average tangential electric field at the magnetopause on the magnetosheath north–south magnetic field is sufficient to account for a major part of the magnetospheric potential and convection.
AMPTE CCE observations of Pc 3–4 pulsations at L = 2–6
Authors: Takahashi Kazue , Anderson Brian J. , Strangeway Robert J.
Published: February 2008

Very few spacecraft observations of magnetic Pc 3–4 pulsations at L < 4 have been reported in the past despite the fact that Pc 3–4 pulsations are a common occurrence on the ground at L < 4. In order to fill this observational gap in the magnetosphere, we have initiated an analysis of magnetic field data acquired by the AMPTE CCE spacecraft. A procedure has been developed to reduce the artificial modulation and noise in magnetic field measurements, with the resultant data useful for studying magnetic pulsations with amplitude less than 1 nT in the 1000 nT (L ∼ 3) background and periods longer than 12 s. From a survey of 41 CCE passes in L = 2–6 and in the postnoon sector, we find that the most commonly observed pulsations in this region are the east‐west oscillations of magnetic field with clearly L‐dependent frequencies. These are identified to be the toroidal mode standing Alfvén waves. From a comparison of CCE and GOES 5 magnetic field measurements there is evidence that the resonances can be excited by the same mechanism as those previously found at geostationary orbit or beyond. Abrupt changes in resonance frequency, as a function of L, are not usually seen, supporting previous reports that the mass or number density profile on the dayside does not have a steep plasmapause structure.
Seeding and layering of equatorial spread F by gravity waves
Authors: Hysell D. L. , Kelley M. C. , Swartz W. E. , Woodman R. F.
Published: February 2008

Studies dating back more than 15 years have presented evidence that atmospheric gravity waves play a role in initiating nighttime equatorial F region instabilities. This paper analyzes a spectacular spread F event that for the first time demonstrates a layering which, we argue, is controlled by a gravity wave effect. The 50‐km vertical wavelength of a gravity wave which we have found is related theoretically to a plasma layering irregularity that originated at low altitudes and then was convected, intact, to higher altitudes. Gravity waves also seem to have determined bottomside intermediate scale undulations, although this fact is not as clear in the data. The neutral wind dynamo effect yields wave number conditions on the gravity wave's ability to modulate the Rayleigh‐Taylor instability process. Finally, after evaluating the gravity wave dispersion relation and spatial resonance conditions, we estimate the properties of the seeding wave.
The heliospheric current sheet and modulation of galactic cosmic rays
Authors: Smith Edward J.
Published: February 2008

The relatively abrupt onset of cosmic ray modulation, observed in May 1987 on Earth and in September 1987 by the Pioneer 10 and 11 spacecraft, enables further investigation of the possible effect of the heliospheric current sheet (sector boundary) on modulation. According to the gradient drift theory of cosmic ray transport, modulation is associated with solar cycle changes in the current sheet, whose inclination increases systematically from solar minimum to solar maximum. The measure used in testing this hypothesis is the difference in the maximum latitudinal extent of the current sheet in the northern and southern solar hemispheres. This difference is obtained from contours of the current sheet produced by extrapolating photospheric magnetic field measurements to a solar wind source surface. When the latitude difference, divided by 2 and called the pseudoinclination or pseudotilt angle, is compared with cosmic ray count rates measured on Earth, an excellent correlation is found, as in a previous study of modulation in the years 1976–1986. The abruptness of the onset can be attributed to the greater sensitivity of the cosmic ray intensity to changes in the current sheet when the solar‐heliospheric magnetic field is inward in the northern hemisphere (above the current sheet) as predicted by the model. Thus the observations are found to be consistent with the predictions of the gradient drift model. A preliminary analysis is also carried out to test an alternative, but not necessarily incompatible, hypothesis that large‐scale transient solar wind structures (coronal mass ejections, shocks, etc.) form barriers to the inward transport of cosmic rays. Available spacecraft magnetic field measurements (ICE, Pioneer 11), geomagnetic activity, including storm sudden commencements, and Forbush decreases do not reveal an obvious change in solar wind stream structure within several months of the modulation onset. It appears that in this instance, promoted by the abruptness of the onset, it may be possible to discriminate between the effect on modulation of the heliospheric current sheet and the possible effect of solar wind shocks and/or ejecta.
Microearthquakes located on the Blanco Fracture Zone with sonobuoy arrays
Authors: Johnson S. H. , Jones P. R.
Published: February 2008

Tripartite arrays of sonobuoys were used to investigate microseismicity at two locations on the central portion of the northwest trending Blanco fracture zone 450 km west of Oregon. The two arrays operated for 14 and 12 hours at locations separated by 50 km. Individual sonobuoys were tracked by radar as they drifted across the fracture zone. Epicenters determined for 32 of the 38 events detected show close association with topographic features of the fracture zone, in contrast to teleseismically determined epicenters, which are offset approximately 30 km from the fracture zone. The events at array 3 do not lie in the central trough but along the southern flank of a linear ridge that is parallel to the trend of the fracture zone. The events at array 4 lie within a topographic depression that offsets the center of the fracture zone and are probably related to steep scarps observed on the bathymetry. Magnitudes assigned to 23 events at array 4 range from −0.5 to +0.5.
Mantle circulation with partial shallow return flow: Effects on stresses in oceanic plates and topography of the sea floor
Authors: Schubert Gerald , Yuen David A. , Froidevaux Claude , Fleitout Luce , Souriau Marc
Published: February 2008

Fully two‐dimensional analytic boundary layer solutions are used to model the thermomechanical structure of the oceanic upper mantle when a shallow horizontal return flow helps balance the lithospheric transport of mass from ridge to trench. The following are all incorporated in the solutions: horizontal and vertical advection of heat, vertical heat conduction, viscous dissipation, adiabatic heating and cooling, buoyancy, and the pressure‐ and temperature‐dependent nonlinear rheology of olivine. Depth profiles of horizontal and vertical velocities, temperature, and shear stress are calculated for several ages of the ocean floor. Such solutions are used to construct accurate isotherm and streamline patterns within the rigid lithosphere and high‐shear, return flow asthenosphere of the oceanic upper mantle boundary layer. Ocean floor topography is inferred from the thermal contraction of the cooling lithosphere and asthenosphere and from the adverse horizontal pressure gradient required by the dynamics to drive the shallow return flow. The flattening of the topography of the old ocean floor can be attributed to the retardation of boundary layer cooling by shear heating and/or to the adverse pressure gradient of a shallow return flow. The latter effect would be dominant if a shallow return flow did occur in the earth's upper mantle. Solutions which provide adequate fits to the ocean floor bathymetry data for the Pacific plate can be found if the activation volume for the creep of olivine is small, i.e., about 11 cm3/mol, and if the deep mantle temperature T is high, i.e., if T is about 1500°C at depths of 100–200 km (depending on age) for dry olivine or about 1400°C at similar depths for wet olivine. The upper mantle temperatures required for a shallow return flow to be compatible with bathymetry data imply partial melting in the lower lithosphere and upper asthenosphere from the ridge axis to ages of about 10 m.y. Shear stresses at the base of the rigid lithosphere generally range from about 1 to several tens of bars, and horizontal adverse pressure gradients at great depth vary from about 10 to 100 mbar/km for wide variations in the extent of return flow and deep mantle temperature. For a relatively cold deep mantle, shear stresses in the lower lithosphere and asthenosphere decrease almost linearly with depth, implying horizontal pressure gradients essentially constant with depth. For a hot deep mantle, shear stress versus depth profiles show curvature in the lower lithosphere and upper asthenosphere and a linearly decreasing pattern at greater depths. Under such circumstances, horizontal pressure gradients exhibit considerable depth variation including the possibility of a reversal in sign between the top and the bottom of the asthenosphere.
Precursors to failure in rocks while undergoing anelastic deformations
Authors: Sobolev G. , Spetzler H. , Salov B.
Published: February 2008

Modeling of geological processes in the laboratory is often quite difficult. Stress accumulation prior to an earthquake occurs over a long time and is associated with large anelastic deformation. We have chosen pyrophyllite as a model material because of its large anelastic deformation (several percent) before brittle failure. Samples of pyrophyllite were stressed to failure in a biaxial press. Photography through transparent pistons, optical holography, microscopy, and sound velocity measurements were used as tools to study the processes leading to brittle failure. At about 50% of ultimate failure strength a zone of preferential deformation forms around the incipient fault, and the seismic velocities decrease perpendicular to the axis of maximum principal stress. At higher stress levels the intense deformation zone becomes very narrow and the velocities increase again before failure. En echelon cracks form in the most narrow part of the intense deformation zone and precede the formation of a macrocrack and a stress drop. The intense deformation zone remains very narrow in front of the macrocrack. Further en echelon crack formation within the intense deformation zone appears before a further advance of the macrocrack. The above observations, especially the velocity reversal before failure, are rate and moisture dependent. They were clearly detectable at a strain rate of 3 × 10−8/s but barely noticeable at 5 × 10−7/s.
Statistical model for gravity, topography, and density contrasts in the Earth
Authors: Jordan Stanley K.
Published: February 2008

A statistical model of topographic heights and density contrasts within the earth is inferred from the empirical worldwide, free air gravity anomaly covariance function and the Bullen and Haddon earth model HB1. Models for relief and loading at the crust‐mantle and mantle‐core boundaries are also inferred. A worldwide Bouguer anomaly model is derived by suppressing terrain‐related portions of the free air anomaly model. Self‐consistent models are given for the cross‐covariance functions between topography, free air gravity anomalies, Bouguer gravity anomalies, and density contrasts within the earth.
A seismic refraction experiment in the central Banda Sea
Authors: Purdy G. M. , Detrick Robert S.
Published: February 2008

A seismic refraction experiment in the central Banda Sea is interpreted by using both slope intercept and delay time function methods. The crustal structure is shown to be oceanic, with velocities (4.97, 6.47, 7.18, and 7.97 km/s) typical of oceanic layers 2, 3A, and 3B and the mantle. Individual layer thicknesses vary systematically along the line, though the range of thicknesses observed for layers 2 (1.5–2.0 km) and 3A (2.0–3.5 km) falls well within the range observed for normal oceanic crust. Layer 3B is unusually thick (2.5–4.6 km), the result being slightly greater than normal depths to Moho of 9–10 km below the sea floor. Shear head waves from layers 3A and 3B are identified on two receivers. In both cases, shear wave conversion occurred at the sediment/layer 2 interface. The observed shear wave velocities and intercepts indicate a Poisson's ratio of 0.25–0.28 in layer 3 and ∼0.33 in layer 2. These and earlier results from the southern Banda basin indicate that the entire Banda Sea is underlain by oceanic type crust.
Lunar tidal acceleration obtained from satellite‐derived ocean tide parameters
Authors: Goad Clyde C. , Douglas Bruce C.
Published: February 2008

Analysis of 100 sets of mean elements of Geos 3 computed at 2‐day intervals has yielded observation equations for the M2 ocean tide from the long periodic variations of the inclination and node of the orbit. If the second‐degree Love number is given the value k2 = 0.30 and the solid tide phase angle is taken to be 0°, the values are (3.99″ ± 0.4) × 10−2sin [σ(τ) + 327° ± 4°] = 1.26″/cm × 10−2C22+ sin [σ(τ) + ε22+] ‐ 0.32″/cm × 10−2C42+ sin [σ(τ) + ε42+] + … for the inclination and (2.73″ ± 0.7) × 10−2 cos [σ(τ) + 291° ± 13°] = −0.24″/cm × 10−2C22 + cos [σ(τ) + ε22+] − 3.38″/cm × 10−2C42+ cos [σ(τ) + ε42+] + … for the node, where σ(τ) = 2Ω − 2M* − 2ω* − 2Ω*; M*, ω*, and Ω* are the lunar mean anomaly, argument of perigee, and right ascension of the ascending node, respectively; and Ω is the Geos 3 right ascension of the ascending node. Combining these equations with the result obtained by Goad and Douglas (1977) for the satellite 1967‐92A gives the M2 ocean tide parameter values C22+ = 3.23 ± 0.25 cm, ε22+ = 331° ± 6°, C42+ = 0.87 ± 0.19 cm, and ε42+ = 113° ± 6°. Under the assumption of zero solid tide phase lag the lunar tidal acceleration is mostly (85%) due to the C22+ term in the expansion of the M2 tide with additional small contributions from the O1 and N2 tides. Using Lambeck's (1975) estimates for the latter, we obtain for the tidal acceleration in lunar longitude the value
= −27.4 ± 3 arc sec/(100 yr)2, in excellent agreement with the most recent determinations from ancient and modern astronomical data. The mean elements of Geos 3 are also presented in tabular form.
Manicouagan Impact Melt, Quebec, 1, Stratigraphy, petrology, and chemistry
Authors: Floran R. J. , Grieve R. A. F. , Phinney W. C. , Warner J. L. , Simonds C. H. , Blanchard D. P. , Dence M. R.
Published: February 2008

Within the moderately eroded Manicouagan structure a sheet of clast‐laden impact melt 230 m thick and 55 km in diameter forms an annular plateau surrounding an uplift of shocked anorthosite. The melt sheet is divided into three vertically gradational units based on decreasing clast abundance and coarsening of the melt above the base. A very fine‐grained lower unit, rich in millimeter‐ and centimeter‐sized inclusions, thickens radially outward but is overlapped and replaced toward the center by a coarser middle unit containing fewer, larger inclusions. The upper unit is medium grained, virtually clast free, and texturally the most homogeneous of the three melt units. Within the lower and middle units a variety of textures are present. Textural development is a function of the cooling rate determined by stratigraphic position and the degree of supercooling determined by initial clast content. The mineralogy of the melt rocks is similar in all units and consists of zoned plagioclase, sanidine, Ca‐poor pyroxene, augite, quartz (rare in the lower unit), magnetite‐ilmenite intergrowths, smectite, and apatite, Pseudomorphs after olivine, and pigeonite in various stages of inversion to hypersthene, are widespread only in the upper unit, while minor biotite and hornblende are confined to the lower and middle units. Replacement of olivine and much of the Ca‐poor pyroxene by smectite, and alteration of iron oxides occurred during late stage crystallization and subsolidus cooling. The melt rocks as a group are chemically homogeneous, with a bulk composition similar to that of latite. No statistically significant regional chemical variations were found as a function of vertical, lateral, or radial position in the melt sheet. A local mafic variant represented by two samples with poikilitic texture indicates that the melt is not completely chemically homogeneous. The poikilitic rocks texturally resemble some Apollo 17 impact melt rocks and are inferred to have had a similar origin and thermal history.
The influence of pore pressure on the mechanical properties of Kayenta sandstone
Authors: Dropek R. K. , Johnson J. N. , Walsh J. B.
Published: February 2008

New experimental techniques are presented for the measurement of pore pressure as an additional dependent variable under undrained triaxial test conditions. Experimental data are presented for the variation of pore pressure in Kayenta sandstone at various degrees of water saturation. We find that the following are in good agreement with theoretical predictions: (1) the rate of increase of pore pressure pw with externally applied pressure p, (2) the compressibility of fully saturated samples, and (3) the volume strains at which pore pressure begins to build up. The strength of samples taken to failure is found to be a unique function of p ‐ pw; i.e., the effective stress law is found to hold for this material.
Mars topography harmonics and geophysical implications
Authors: Bills Bruce G. , Ferrari Alfred J.
Published: February 2008

An improved model of Martian global topography has been obtained by fitting a sixteenth‐degree harmonic series to occultation, radar, spectral, and photogrammetric measurements. The existing observations have been supplemented in areas without data by empirical elevation estimates based on photographic data. The mean radius is 3389.92 ± 0.04 km. The corresponding mean density is 3.9331 ± 0.0018 g cm−3. The center of figure is displaced from the center of mass by 2.50 ± 0.07 km toward 62° ± 3°S, 272° ± 3°W. The geometric flattening (fg = (6.12 ± 0.04) × 10−3) is too great and the dynamic flattening (fd = (5.22 ± 0.03) × 10−3) is too small for Mars to be homogeneous and hydrostatic. It is confirmed that the low‐degree gravity harmonics are produced primarily by surface height variations and only secondarily by lateral density variations. Maps of the data distribution, global topography, and Bouguer gravity anomaly are presented. These are interpreted in terms of a crustal thickness map which is consistent with gravity, topography, and recent preliminary Viking seismic results. From plausible density contrasts and an assumed zero crustal thickness at Hellas, the inferred minimum mean crustal thickness is 28 ± 4 km.
Behavior of Grüneisen's parameter of some metals at high pressures
Authors: Ramakrishnan J. , Boehler Reinhard , Higgins Gary H. , Kennedy George C.
Published: February 2008

Changes of sample temperature associated with small adiabatic pressure changes were measured for the metals aluminum, copper, indium, iron, and lead, and the thermodynamic Grüneisen parameter γ is calculated from the relation γ = (KS/T) (∂T/∂P)S where KS is the adiabatic bulk modulus. Measurements were carried out to 33 kbar under hydrostatic conditions in a fluid cell with Bridgman's unsupported seals using an end‐loaded piston cylinder apparatus.
A detailed study of two earthquakes seaward of the Tonga Trench: Implications for mechanical behavior of the oceanic lithosphere
Authors: Chen Thomas , Forsyth Donald W.
Published: February 2008

Two nearby earthquakes seaward of the Tonga trench, one normal faulting and one thrust faulting, were studied in detail by using comparative event, surface wave techniques. The focal depth for the thrust‐faulting event was found to be 49 km below sea level, consistent with the focal depth estimated from the pP‐P delay time. The normal‐faulting event had a depth of 14 km. These are the first precise depth determinations for earthquakes which are associated with the bending of the oceanic lithosphere before subduction. The pattern of horizontal deviatoric tension near the surface and compression within the interior of the lithosphere is consistent with the stresses predicted by an elastic plate model, although the depth of the thrust event suggests that either the elastic layer begins near the base of the crust or the rheological model should be modified to include a plastic layer of finite strength.
The Caracas, Venezuela, earthquake of July 1967: A multiple‐source event
Authors: Rial J. A.
Published: February 2008

A general study of Caribbean plate tectonics is first focused on the determination of fault parameters and source processes of the Caracas (Venezuela) earthquake of July 29, 1967 (mb = 6.5, Ms = 6.7). Synthetic seismograms which closely reproduce the observed P, SH, and Love wave seismograms were generated using generalized ray and mode theories. The results indicate a complicated faulting process, consisting of at least three separated sources aligned along a Nl0°W trending ‘en echelon’ vertical left lateral strike slip system of three faults that ruptured from north to south, at three discrete places with an extreme separation of 90 km. The process of rupture progressed southward with a mean velocity of 3 km/s. The focal depths of the individual sources varied between 8 and 27.5 km. The total dislocation was calculated as 120 cm along the direction N 10°W, and the total average moment as 4 × 1026 dyn cm. The multiple character of the event severely constrains the number of suitable source models that can be inferred, thus facilitating the process of inversion. Tectonic implications are briefly discussed, and local geology is successfully invoked to support the source model.
A deep borehole in the Michigan Basin
Authors: Sleep Norman H. , Sloss L. L.
Published: February 2008

Heat flow, in situ stress, in situ S wave and P wave velocities, and in situ density from borehole gravity were successfully measured in a 5.3‐km‐deep borehole in the Michigan basin. Precambrian (?) red beds and two altered basic igneous bodies were encountered beneath the known Cambrian succession of the basin. Petrological and paleomagnetic studies were undertaken on oriented samples obtained from the igneous body at the base of the hole and on other samples. The igneous body appears to have undergone greenschist metamorphism, probably about 600–800 m.y. ago. The red beds are correlated with the Keweenawan ‘series’ of northern Michigan on the basis of paleomagnetism and lithology. No evidence was found of a post‐Cambrian heating event which might be expected if thermal contraction had caused basin subsidence in a manner analogous to the subsidence of the midoceanic ridges.
Microcracks in crystalline rock from 5.3‐km depth in the Michigan Basin
Authors: Wang H. F. , Simmons G.
Published: February 2008

Laboratory velocity and differential strain analysis measurements indicate that the gabbroic core from 5310‐ and 5315‐m depth beneath the center of the Michigan Basin contains few open microcracks. Approximate agreement between the in situ velocities and the intrinsic laboratory velocities suggests as one hypothesis that these rocks may be crack free in situ. Stress relief, which occurred when the core was brought from depth to the surface, may have created the open microcracks. The majority of the cracks have a closure pressure of 1450 bars, approximately equal to the in situ vertical stress. Open cleavage cracks are observed in chlorite by scanning electron microscopy. The apparent tensile nature of these cracks supports the stress relief hypothesis. Optical microscopy shows that numerous formerly open microcracks are now completely healed or sealed.
Crustal stress in the Michigan Basin
Authors: Haimson Bezalel C.
Published: February 2008

We conducted four successful hydrofracturing tests in a 5325‐m deep well in Gratiot County, near the center of the Michigan Basin. The top three tests were run through perforations and were limited by the availability of thoroughly cemented cased zones. The controlled manner of hydrofracturing and the repeatability of results during repressurizations suggest that the perforation tests were probably as reliable as the open hole test, at least with respect to the least compressive stress. Data analysis shows a steady increase with depth of both horizontal principal stresses between the depths of 1230 and 5105 m. However, the least horizontal compressive stress at 1230 m (295 bars) is considerably higher than was expected from extending the linear curve fitting the stress‐depth relationship for the three deeper values (−265 bars + 0.24 bars/m). This is in agreement with a regional trend for high horizontal stresses in the top kilometer of the crust. The vertical stress variation with depth, based on the gravity log, was estimated to be 0.26 bars/m, quite similar to the least horizontal stress in the 2800 to 5100‐m range. No fracture orientations were obtained, but on the basis of other measurements throughout the upper Midwest the direction of the largest horizontal compressive stress is expected to lie within the N45°E–N90°E range.