bluuebunny/crossref_metadata_2025 huggingface dataset:
Io‐related Jovian decametric arcs
Authors: Wilkinson M. H.
Published: February 2008

In this work an empirical modeling technique is used to analyze certain Io‐caused emission (ICE) structures in the Jovian decametric radio spectrographic data. Taking into account that some of these ICE structures have the appearance of great arcs with internal arclike fine structure, a geometrical model is developed in which the observed emission is seen to result from conical emission beams emanating from source regions carried on corotating field lines which are excited only as they cross an excitation zone centered on the Io flux tube. The model is also applied to the analysis of a limited set of spectrograms of the traditional Io‐related sources Io A, Io B, and Io D. In some of these spectra it was possible to identify multiple‐ICE structures. The model results for these spectra are consistent with a physical model in which emission is occurring from multiple spaced flux tubes positioned ahead of and moving with Io, and lend credence to the multiple Alfvén wave reflection hypothesis.
On Jovian plasma sheet structure
Authors: Khurana Krishan K. , Kivelson Margaret G.
Published: February 2008

We evaluate several models of Jovian plasma sheet structure by determining how well they organize several aspects of the observed Voyager 2 magnetic field characteristics as a function of Jovicentric radial distance. The present study focuses exclusively on the data from the Voyager 2 spacecraft because the magnetosphere was more stable during that flyby than it was during the Pioneer 10 and Voyager 1 flybys. It is shown that in the local time sector of the Voyager 2 outbound pass (near 0300 LT) the published hinged‐magnetodisc models with wave (i.e., models corrected for finite wave velocity effects) are more successful than the published magnetic anomaly model in predicting locations of current sheet crossings. We also consider the boundary between the plasma sheet and the magnetotail lobe which is expected to vary slowly with radial distance. We use this boundary location as a further test of the models of the magnetotail. The plasma‐sheet‐lobe boundary is often identified from the observed field gradients, the gradients in the lobes being much smaller than those in the plasma sheet. We show that the compressional MHD waves have much smaller amplitude in the lobes than in the plasma sheet and use this criterion to refine the identification of the plasma‐sheet‐lobe boundary. When the locations of crossings into and out of the lobes are examined, it becomes evident that the magnetic‐anomaly model yields a flaring plasma sheet with a halfwidth of ∼3 RJ at a radial distance of 20 RJ and ∼12 RJ at a radial distance of 100 RJ. The hinged‐magnetodisc models with wave, on the other hand, predict a halfwidth of ∼3.5 RJ independent of distance beyond 20 RJ. New optimized versions of the two models locate both the current sheet crossings and lobe encounters equally successfully. The optimized hinged‐magnetodisc model suggests that the wave velocity decreases with increasing radial distance. The optimized magnetic anomaly model yields lower velocity contrast than the model of Vasyliunas and Dessler (1981). The hinged‐magnetodisc models are shown to satisfy the constraints on the plasma sheet thickness imposed by MHD theory. The magnetic anomaly model can be made consistent with the expectations from MHD theory only by assuming plasma anisotropy so large that the plasma would be unstable to mirror mode instability. We recognize that the magnetic anomaly model is a comprehensive model designed to account for many features of the Jovian system as observed in multiple flyby missions. The plasma sheet structure for the Voyager 2 epoch is only one aspect of the model's predictions. Nonetheless, we believe that the model's failure to satisfy constraints on plasma sheet structure imposed by MHD equilibrium theory presents a challenge to the magnetic anomaly model as it is currently described.
Dimensionality effects in hybrid simulations of high Mach number collisionless perpendicular shocks
Authors: Thomas V. A.
Published: February 2008

Hybrid simulations of a high Alfvén Mach number (MA≫1) collisionless perpendicular shock are performed in one, two, and three spatial dimensions. The results are compared to highlight the implicit effects of dimensionality in the simulation model. As in previous work, one‐dimensional simulations show that the dissipation mechanism can be identified as reflected ions. Two‐dimensional simulations allow the ion temperature anisotropy associated with the reflected ions to be relaxed via a collisionless electromagnetic instability. In the high MA limit, this instability causes the shock front to have rotating magnetic fields with approximately equal amplitudes in each direction as well as large‐amplitude density fluctuations. The three‐dimensional simulations are similar to the two‐dimensional simulations with the exception that the fluctuations at the shock front are significantly smaller. This difference is due to the implied phase coherence in the two‐dimensional calculations which is absent in three dimensions. The results of the simulations are compared with spacecraft observations of high MA perpendicular shocks.
Origins of enhanced field‐aligned current at the edge of an auroral arc
Authors: Greenspan M. E.
Published: February 2008

Rocket and radar observations of auroral arcs indicate that the upward field‐aligned currents associated with such arcs sometimes are larger at their edges than within the arcs. Brüning and Goertz (1985) reported one such set of observations, made by instruments on the Porcupine F4 rocket. Porcupine F4 measured a fourfold increase in the magnitude of the upward field‐aligned current at the edge of an auroral arc over its value in the arc's center, while the energy of the peak in the electron differential flux decreased at the edge of the arc and the electron distribution function remained nearly isotropic in pitch angle throughout the arc. One mechanism which has been proposed to explain the creation of auroral arcs is the adiabatic acceleration of magnetospheric electrons through a field‐aligned electrostatic potential. The upward field‐aligned current which results is a function of the density and parallel and perpendicular temperatures of the magnetospheric electron population as well as the magnitude and altitude of the electrostatic potential drop. If the arc traversed by Porcupine F4 was created in this way, the increase in upward current at the edge of the arc cannot be explained solely by an increase in the altitude of the top of the field‐aligned potential drop. Rather, an increase in the density, a decrease in the perpendicular temperature of the magnetospheric electron population at the edge of the arc, or both are needed to explain the observed increase in upward field‐aligned current.
Combined first‐ and second‐order Fermi acceleration in cometary environments
Authors: Gombosi T. I. , Lorencz K. , Jokipii J. R.
Published: February 2008

Our two‐dimensional (one velocity space and one spatial dimension), time‐dependent model calculations indicate that an interplay between velocity and spatial diffusion may be responsible for the acceleration of implanted heavy ions in the cometary preshock region. Velocity diffusion (second‐order Fermi acceleration) accelerates the pickup ions to moderate energies, thus creating a seed population for the more efficient diffusive‐compressive shock acceleration. Solar wind convection limits the time available for diffusive‐compressive acceleration, therefore the resulting energy spectrum above the pickup energy is a combination of an exponential decrease at lower energies (up to about 100 keV) and a power law spectrum at higher energies. The calculated energy spectra constitute a surprisingly good fit to the published VEGA and Giotto energy spectra above the pickup energy.
Analysis of an extended period of earthward plasma sheet flow at ∼220 RE: CDAW 8
Authors: Schindler K. , Baker D. N. , Birn J. , Hones E. W. , Slavin J. A. , Galvin A. B.
Published: February 2008

Statistical studies of the ISEE 3 deep magnetic tail data (r≳120 RE) show that the bulk electron plasma flow is virtually always tailward during both quiet and geomagnetically active times. This aspect of the magnetotail data has been interpreted as suggesting that ISEE 3 is nearly always tailward of the distant neutral line when the spacecraft is at distances ≳120 RE. During an extended period (∼0900 to ∼1230 UT) on January 29, 1983, however, ISEE 3 observed substantial earthward plasma flows while located in the distant plasma sheet at XGSM = −217 RE. We have analyzed the distant neutral line location within the context of the distant tail, geostationary orbit, auroral zone, and solar wind data for this case based on an extension of the Coroniti and Kennel (1972) flaring tail theory. We conclude from known solar wind conditions that for a typical neutral line location at ∼135 RE, an ∼30% increase of the near‐Earth lobe field strength would be required to cause the distant neutral line to move tailward beyond 220 RE. The available data suggest that this is quite plausible and that, therefore, the earthward flow episode is due to an exceptionally distant neutral line position. The question of why substorms did not terminate the growth phase earlier is also addressed.
Can ion acceleration by double‐cyclotron absorption produce O+ ion conics?
Authors: Ball Lewis
Published: February 2008

The process where an ion in a magnetized plasma simultaneously absorbs two waves whose frequencies sum to the ion gyrofrequency may be referred to as double‐cyclotron absorption. The feasibility of double‐cyclotron absorption as a source mechanism for oxygen (O+) conic distributions in the Earth's magnetosphere is investigated. The appropriate values of the relevant wave and plasma parameters are discussed and the applicability of approximate expressions for the average rate of perpendicular acceleration of O+ ions due to double‐cyclotron absorption, derived by Ball (1989), is reviewed. The acceleration rate which results from the approximations of Ball (1989) is shown to be exactly a factor of 5 larger than the corresponding result of Temerin and Roth (1986). A possible explanation for this discrepancy is presented. Rough numerical estimates of the average rate of perpendicular acceleration of O+ ions along auroral field lines are considered, and it is shown that double‐cyclotron absorption is indeed capable of producing O+ ions at energies comparable to those observed by spacecraft.
Observations of rotational discontinuity—slow expansion fan structure of the magnetotail boundary
Authors: Sánchez E. R. , Siscoe G. L. , Gosling J. T. , Hones E. W. , Lepping R. P.
Published: February 2008

This paper gives the first explicit test of the rotational discontinuity‐slow expansion fan structure predicted by MHD models of the open tail boundary. We analyzed the magnetic field and plasma data collected by IMP 8 during its magnetotail boundary crossings at geocentric distances near 25 Re and high latitudes. Many crossings show the particle and field signatures of the paired MHD tail boundary structure. The openness of the tail is revealed by rotational discontinuities. They often show relatively large normal magnetic field components. Other crossings show properties that suggest a tangential discontinuity and reveal regions where or times when the tail is closed. We give examples of each here.
On the behavior of the Dst geomagnetic index in the vicinity of magnetic cloud passages at Earth
Authors: Wilson Robert M.
Published: February 2008

The geomagnetic response to magnetic clouds is investigated for the 34 magnetic clouds observed between 1973 and 1982, in particular, according to cloud type: southward turning clouds as compared to northward turning clouds (a southward turning cloud is one whose Bz near cloud onset at Earth is directed northward, while a northward turning cloud is one whose initial Bz is directed southward). While, on average, the most negative Dst value associated with interplanetary magnetic clouds is the one for northward turning clouds, the difference in the depths of decrease (onset Dst value minus the most negative Dst value during cloud passage) for the two cloud types does not appear to be statistically important; hence northward turning and southward turning clouds have depths of decrease (negative excursions in the Dst index) of comparable size, differing primarily in the time of most negative Dst occurrence (relative to cloud onset at Earth). For northward turning clouds, the most negative Dst value usually occurs within 12 hours of cloud onset at Earth, while for southward turning clouds it is delayed until after 12 hours from cloud onset. Such behavior is consequential in that it is directly related to the occurrence of southward Bz.
Large‐scale high‐latitude dayside auroral emissions
Authors: Murphree J. S. , Elphinstone R. D. , Hearn D. , Cogger L. L.
Published: February 2008

Observations made with the Viking UV imager reveal that on the dayside of the auroral distribution there often occur emission features poleward of the normal continuous auroral distribution when the interplanetary magnetic field (IMF) is northward and in an away garden hose configuration. The position of these features, while consistent with the expected location of cusp aurora, does not exhibit the strictly diffuse character of cusp emissions which is generally the signature of direct magnetosheath particle access. Further the strong IMF dependence tends to suggest a separate auroral process from that which results in the discrete features commonly found in the cleft region. This coupled with the observation that large‐scale polar arcs are seen to frequently connect to these dayside features argues that in fact these auroral emission features are the result of processes occurring on the front surface of the magnetotail (i.e., on the magnetopause poleward of the cusp). This is consistent with the discovery that these features seem to exist predominantly when the IMF By component is positive, Bx negative, and Bz is northward, which implies that merging on the front surface of the magnetotail is involved in producing these emissions. Evidence is presented which distinctly links the location of merging lines due to dipole tilt and magnetic activity with the location of these high‐latitude dayside features.
Wave intensifications near the electron cyclotron frequency within the polar cusp
Authors: Farrell W. M. , Gurnett D. A. , Menietti J. D. , Wong H. K. , Lin C. S. , Burch J. L.
Published: February 2008

As DE 1 flew through the polar cusp, enhanced narrowband electrostatic waves were sometimes observed just above the electron cyclotron frequency, fce. In this report, we present wave and particle measurements from three representative cusp transits in order to characterize these signals and understand the conditions that favor their generation. In these representative cases, narrowband emission intensifications occurred at frequencies between 1.1 to 1.3 fce. The emission intensities ranged between 5×10−14 to 10−12 V²/(m² Hz), such waves being 50 to 1000 times greater than the narrowbanded cyclotron‐related signal levels detected in adjacent regions. Simultaneously occurring with the wave enhancements were energetic cusp electrons with energies extending up to about 500 eV. It was found that the form of the local cusp electron velocity distribution had a direct influence on the wave spectral character. A preliminary study indicates that electron beams in the cusp can generate the enhanced signals, although generation by an anisotropic warm component cannot be ruled out. Based on an examination of many cusp transits, the occurrence of these enhanced signals appeared to have some dependency on Kp index, indicating that increased particle flows (associated with increased geomagnetic activity) seem to affect their generation. Although the exact wave/particle coupling mechanism responsible for these enhancements is difficult to identify, it is evident that the generation is directly related to the energetic cusp electrons.
Non‐solar UV produced ions observed optically from the “CRIT I” Critical Velocity Ionization Experiment
Authors: Stenbaek‐Nielsen H. C. , Wescott E. M. , Rees D. , Valenzuela A. , Brenning N.
Published: February 2008

A critical velocity ionization experiment was carried out with a heavily instrumented rocket launched from Wallops Island at dawn on May 13, 1986. Two neutral barium beams were created by explosive shaped charges released from the rocket and detonated at 48° to B at altitudes near 400 km and below the solar UV cutoff. Critical velocity ionization was expected to form a detectable ion jet along the release field line, but instead an ion cloud of fairly uniform intensity was observed stretching from the release field line across to where the neutral barium jet reached sunlight. The process creating these ions must have been present from the time of the release and the efficiency is estimated to be equivalent to an ionization time constant of 1800 s. This ionization is most likely from collisions between the neutral barium jet and the ambient atmospheric oxygen, and if so, the cross section for collisional ionization is 9×10−18 cm². A critical velocity ionization process may have been present during the first few tenths of a second after release, but its efficiency cannot have exceeded an equivalent ionization time constant of about 1800 s.
Mars ionopause during solar minimum: A lesson from Venus
Authors: Mahajan K. K. , Mayr H. G.
Published: February 2008

The ion densities measured by the Viking landers (Hanson et al., 1977) do not show an abrupt falloff with height, giving the false impression that Mars has no ionopause. On the basis of knowledge gained from the solar wind interaction at Venus during solar minimum, we demonstrate that the observed O2+ profile above about 160 km on Mars is a distributed photodynamical ionosphere and can produce an ionopause at around 325 km, similar to that observed on Venus during solar minimum. We conclude that the solar wind interacts directly with the Mars ionosphere, suggesting that the planet does not have an intrinsic magnetic field of any consequence.
Analysis of substorm expansion and surge development
Authors: Lyons L. R. , De la Beaujardiere O. , Rostoker G. , Murphree J. S. , Friis‐Christensen E.
Published: February 2008

We have used simultaneous magnetogram, radar, and Viking imager data from evening local time to study several substorm expansion phase onsets and an expansion phase auroral surge that formed near the radar field of view. At the onsets, auroral brightenings developed within the region of the electric field gradient of the Harang discontinuity, and if not already present, a westward electrojet began poleward of the eastward electrojet. Immediately following onsets, the poleward boundary of the westward electrojet moved poleward along with the poleward boundary of the aurora; however, no consistent motion of the Harang discontinuity was found in the ionosphere. The auroral surge formed along the poleward boundary of the westward electrojet (and of the aurora). We argue that the distortion of the poleward boundary of the aurora associated with the surge was a manifestation of a distortion of the separatrix between open and closed magnetic field lines, the distortion being caused by the field‐aligned currents within the head of the surge. We suggest that rapid surge development may result from a tapping of the westward electrojet current at the surge head. We also suggest that the separatrix distortion could lead to the escape of trapped plasma sheet particles along open field lines in the tail.
Solar minimum Lyman α sky background observations from Pioneer Venus Orbiter Ultraviolet Spectrometer: Solar wind latitude variation
Authors: Ajello J. M.
Published: February 2008

Measurements of interplanetary H I Lyman α over a large portion of the celestial sphere were made at the recent solar minimum by the Pioneer Venus orbiter ultraviolet spectrometer. These measurements were performed during a series of spacecraft maneuvers conducted to observe Halley's comet in early 1986. Analysis of these data using a model of the passage of interstellar wind hydrogen through the solar system shows that the rate of charge exchange with solar wind protons is 30% less over the solar poles than in the ecliptic. This result is in agreement with a similar experiment performed with Mariner 10 at the previous solar minimum.
Bremsstrahlung X rays from Jovian auroral electrons
Authors: Barbosa D. D.
Published: February 2008

The spectrum of X rays from the planet Jupiter is calculated according to an auroral electron beam model. The electrons are assumed to be accelerated by a field‐aligned potential drop and penetrate into the atmosphere as a Maxwellian beam of primaries which are scattered, degraded in energy, and merged with a population of ionization secondaries having a power law energy distribution. The soft X rays observed by the Einstein Observatory satellite are due to bremsstrahlung from the secondary electrons in the H2 atmosphere. A good match to the X ray data is obtained if the power law spectral index of the secondary electrons, γe, is ≃2, yielding a power law slope for the photon flux γX = γe + 1 ≃ 3. The X ray intensity is best reconciled with a beam of primaries having a characteristic energy 30–100 keV and penetrating the homopause with an auroral energy flux typically of 10–20 ergs cm−2 s−1 but no greater than 50 ergs cm−2 s−1.
Ground observations of magnetospheric boundary layer phenomena
Authors: McHenry Mark A. , Clauer C. Robert , Friis‐Christensen Eigil , Newell Patrick T. , Kelly J. D.
Published: February 2008

Several classes of traveling vortices in the dayside ionospheric convection have been detected and tracked using the Greenland magnetometer chain (Friis‐Christensen et al., 1988, McHenry et al., 1989). One class observed during quiet times consists of a continuous series of vortices moving generally anti‐sunward for several hours at a time. The vortices' strength is seen to be approximately steady and neighboring vortices rotate in opposite directions. Sondrestrom radar observations show that the vortices are located at the ionospheric convection reversal boundary. Low altitude DMSP observations indicate the vortices are on field lines which map to the inner edge of the low latitude boundary layer. Because the vortices are conjugate to the boundary layer, repeat in a regular fashion and travel antisunward, we argue that this class of vortices is caused by the Kelvin‐Helmholtz instability of the inner edge of the magnetospheric boundary layer.
Space‐time evolution of whistler mode wave growth in the magnetosphere
Authors: Carlson C. R. , Helliwell R. A. , Inan U. S.
Published: February 2008

A new model is developed to simulate the space‐time evolution of a propagating coherent whistler mode wave pulse in the magnetosphere. The model is applied to the case of single frequency (2–6 kHz) wave pulses injected into the magnetosphere near L ≃ 4, using the VLF transmitting facility at Siple Station, Antarctica. The mechanism for growth is cyclotron resonance between the circularly polarized waves and the gyrating energetic electrons of the radiation belts. Application of this model reproduces observed exponential wave growth up to a saturated level. Additionally, the model predicts the observed initial linear increase in the output frequency versus time. This is the first time these features have been reproduced using applied wave intensities small enough to be consistent with satellite measurements. The center velocities of the electrons entering the wave pulse are selected in a way which maximizes the growth rate. The results show the importance of the transient aspects in the wave growth process. The growth established as the wave propagates toward the geomagnetic equator results in a spatially advancing wave phase structure due mainly to the geomagnetic inhomogeneity. Through the feedback of this radiation upon other electrons, conditions are established which result in a linearly increasing output frequency with time.
The ionospheric signatures of flux transfer events and solar wind dynamic pressure changes
Authors: Lockwood M. , Cowley S. W. H. , Sandholt P. E. , Lepping R. P.
Published: February 2008

The generation of flow and current vortices in the dayside auroral ionosphere has been predicted for two processes ocurring at the dayside magnetopause. The first of these mechanisms is time‐dependent magnetic reconnection, in “flux transfer events” (FTEs); the second is the action of solar wind dynamic pressure changes. The ionospheric flow signature of an FTE should be a twin vortex, with the mean flow velocity in the central region of the pattern equal to the velocity of the pattern as a whole. On the other hand, a pulse of enhanced or reduced dynamic pressure is also expected to produce a twin vortex, but with the central plasma flow being generally different in speed from, and almost orthogonal to, the motion of the whole pattern. In this paper, we make use of this distinction to discuss recent observations of vortical flow patterns in the dayside auroral ionosphere in terms of one or other of the proposed mechanisms. We conclude that some of the observations reported are consistent only with the predicted signature of FTEs. We then evaluate the dimensions of the open flux tubes required to explain some recent simultaneous radar and auroral observations and infer that they are typically 300 km in north–south extent but up to 2000 km in longitudinal extent (i.e., roughly 5 hours of MLT). Hence these observations suggest that recent theories of FTEs which invoke time‐varying reconnection at an elongated neutral line may be correct. We also present some simultaneous observations of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (observed using the IMP8 satellite) and the ionospheric flow (observed using the EISCAT radar) which are also only consistent with the FTE model. We estimate that for continuously southward IMF ( ≈ 5 nT) these FTEs contribute about 30 kV to the mean total transpolar voltage (∼30%).
Vortex‐induced tearing mode instability as a source of flux transfer events
Authors: Pu Z. Y. , Hou P. T. , Liu Z. X.
Published: February 2008

A two‐dimensional MHD simulation is performed to investigate the evolution and the asymptotic quasi‐static state (AQSS) of the vortex‐ induced tearing mode instability (VITM) at the magnetospheric boundary. The original VITM model is extended by including By, a magnetic field component along the invariant direction, and varying N, the number density, which makes the model applicable to a wide region on the day side magnetopause. The AQSS is found to be characterized by a large‐scale magnetic island together with a colocated vortex tube. The structure of the magnetic island is studied and compared with observations. The possible role of the VITM in the formation of flux transfer events is discussed.
Saturation of the auroral electrojet current and the polar cap potential
Authors: Weimer D. R. , Reinleitner L. A. , Kan J. R. , Zhu L. , Akasofu S.‐I.
Published: February 2008

It is shown that the auroral electrojet index AE tends to saturate, or approach a limiting value, for large southward interplanetary magnetic field, namely, BZ < −15 nT. The saturation also occurs during individual major geomagnetic storms. The saturation of AE is the result of a saturation in the polar cap potential drop. The polar cap potential has a limiting value due to the nonlinear nature of the magnetosphere‐ionosphere (M‐I) coupling. Results of a M‐I coupling model show that the saturation level of the polar cap potential is lowered as the diffuse auroral conductance increases. The data presented also show an increase in AE for large BZ values, indicating increased auroral activity for BZ > 20 nT.
Direct comparison of 1‐m irregularity phase velocities and ion acoustic speeds in the auroral E region ionosphere
Authors: Haldoupis C. , Schlegel K.
Published: February 2008

Simultaneous coherent and incoherent scatter measurements from a common ionospheric volume are used to investigate the relationship between the phase velocity Vph of meter wavelength irregularities generated by the two‐stream instability and the ambient ion acoustic speed Cs in the auroral electrojet plasma, with the purpose of testing the widely accepted conviction that Vph is always limited at Cs. This study is representative of typical VHF electrojet backscatter at magnetic aspect angles nearing perpendicularity and electric fields varying in the 20‐ to 50‐mV/m range. Since it is not clear if the charged particles behave isothermally or adiabatically, we used several options for Cs suggested in the literature, including (1) isothermal behavior for both ions and electrons, (2) isothermal ions and adiabatic electrons with 3 degrees of freedom, and (3) adiabatic electrons with 3 degrees of freedom and adiabatic ions with 1 degree of freedom. The comparison shows that option 3, in which γe = 5/3 and γi = 3, is rather unlikely. On the other hand, we find that option 2 (γe = 5/3, γi = 1) is more appropriate for use in the ion acoustic speed for meter wavelength waves. The analysis shows that on the average, phase velocities are limited to values which are ∼15% higher than Cs in line with the nonlinear kinetic model of the modified two‐stream instability developed recently by Robinson and Honary (1990). Finally, in spite of the great deal of uncertainty involved in the Vph estimates, there is convincing evidence in the data showing that at times another mechanism, other than anomalous electron gas heating, is involved which allows for Vph to receive values well above Cs.
Permeability changes during the flow of water through westerly granite at temperatures of 100°–400°C
Authors: Summers R. , Winkler K. , Byerlee J.
Published: February 2008

Changes in permeability have been studied during the flow of water through granite for periods of time up to 17 days at temperatures of 100°, 200°, 300°, and 400°C with a constant confining pressure of 500 bars, differential stresses of 0–3500 bars, inlet pore pressure of 275 bars, and outlet pressure of 1 bar. In all cases the initial permeability at elevated temperatures was found to be higher by 1–2 orders of magnitude than the permeability at room temperature, perhaps because of thermal stress cracking. The high initial permeability did not persist with time and in nearly all cases decreased significantly during the first ½ day of water flow. Dissolution of plagioclase and quartz was concentrated near the inlet, where the pore pressure was highest. Precipitation occurred throughout the samples because of oversaturation as the pore pressure dropped, causing significant reduction in permeability. The final permeability after 10 days was less at 300°C than at lower temperatures, and measurable flow stopped in most of the 400°C samples.
A porous flow model for magma migration in the asthenosphere
Authors: Turcotte D. L. , Ahern J. L.
Published: February 2008

It is generally accepted that a large fraction of the earth's volcanism is due to pressure release melting of mantle rock in the ascending limbs of mantle convection cells. A problem that remains largely unsolved is the migration of the melt from the asthenosphere to the earth's surface. We have modeled the early stages of this process by treating the asthenosphere as a porous medium. We assume that mantle rock is moving upward at a constant velocity. The onset of partial melting provides liquid; the liquid creates a liquid‐filled porosity, and the interconnected porosity results in permeability. The lighter liquid rises through the solid matrix according to Darcy's law. Some liquid, however, is necessary to provide the necessary permeability. The volume fraction of liquid present is by definition the porosity and is primarily a function of the magma viscosity. Once a critical value is reached, the volume fraction of liquid present does not increase with further partial melting; the liquid produced by melting migrates upward as rapidly as it is produced. At low mantle ascent velocities, less than about 1 mm/yr, the melting takes place on an interface, and upstream conduction of heat is important. At higher velocities, melting takes place over a depth range that lies on the Clapeyron curve. The melt fraction, the magma velocity, and the degree of partial melting within the melting zone are given.
On trapped noble gases in ureilites
Authors: Göbel R. , Ott U. , Begemann F.
Published: February 2008

Bulk samples of Goalpara, Haverö, Novo Urei, and carbon‐rich veins from all seven ureilites have been analyzed for He, Ne, Ar, Kr, and Xe by mass spectrometry by employing the stepwise heating technique. In all cases, trapped rare gases of the fractionated ‘planetary’ type are found to be strongly enriched in the vein material, up to at least 600‐fold over the silicates. Gas contents of the vein material from different ureilites vary considerably; the range in concentrations increases from a factor of 7 for He and Ne to a factor of 70 for Xe. Consequently, the elemental abundance ratios vary as well: 23 ≤ 4He/132Xe ≤ 500; 0.35 ≤ 20Ne/132Xe ≤ 10; 119 ≤ 36Ar/132Xe ≤ 1090, and 0.91 ≤ 84Kr/132Xe ≤ 3.05. The isotopic compositions, on the other hand, are indistinguishable from one another in all cases except for 40Ar/36Ar. Some characteristic ratios are 20Ne/22Ne = 10.70 ± 0.25 (for 0.03 ≤ 21Ne/22Ne ≤ 0.05), 36Ar/38Ar = 5.26 ± 0.06, and 129Xe/132Xe / 1.035 ± 0.005. The lowest measured 40Ar/36Ar ratio is (1.17 ± 0.20) × 10−3, which, after correction for the extraction blank, yields a new upper limit for the primordial 40Ar/36Ar ratio of (2.9 ± 1.7) × 10−4. Of the main constituents of carbon‐rich veins graphite, diamonds, and kamacite— graphite is shown to be void of trapped gases, while samples which show only the X ray diffraction lines of diamond have very high gas contents. In addition, there must be present at least one more carrier containing noble gases with a nuclide abundance pattern distinctly different from that observed for the diamonds and with gas contents even higher than in the diamonds. Its identity is not known at present. Physical adsorption and ambipolar diffusion are discussed as potential mechanisms responsible for the fractionation pattern observed. For adsorption to make the observed abundances compatible with the predicted ones, heavy neon losses (≈99%) must have occurred. If they were accompanied by an isotopic fractionation, it is possible that the neon initially was of solar composition. Ambipolar diffusion predicts the abundance ratios observed for (T ≈ 8500 K) provided that a means can be found to incorporate the ions into the carrier without any significant additional fractionation. In this case the isotope ratios observed would correspond to the ones in the partially ionized medium where the incorporation occurred.
The Molucca Sea Collision Zone, Indonesia
Authors: Silver Eli A. , Moore J. Casey
Published: February 2008

The convex north trending Sangihe and Halmahera volcanic arcs face each other and are underlain by oppositely dipping Benioff zones whose surface intersection in the Molucca Sea forms a zone of crustal collision. Seismic reflection profiles across the Talaud‐Mayu ridge between the arcs demonstrate that this topographic high consists exclusively of deformed rocks. Reflection profiles further indicate that this deformed mass, or collision complex, is being obducted onto the flanking arc aprons on both east and west sides along gently dipping thrust faults which crop out in troughs adjacent to the arcs. The near‐surface layers of the arc apron are being incorporated into the collision complex by either plastic deformation or thrusting of coherent sediment wedges. Preliminary gravity and seismic refraction interpretation indicate that at least 8–10 km of low‐density material underlies the Talaud‐Mayu ridge. The total length of lithosphere subducted by the colliding arcs is at least 1000 km, the length of the Benioff zones. The geometry of the collision zone suggests an obductive mechanism for emplacement of mélange and ophiolite belts against island arcs or continental margins. The central part of the mostly submarine Talaud‐Mayu ridge stands 1–3 km higher than the flanking troughs, wherein the thrusts crop out. Gravitational flow of material outward and downslope away from the high translates at the edges to thrusting up over the adjacent arcs. The reconstructed history of the collision predicts at least two opposing vergence directions developed in the deformed rocks of the collision complex. The first, developed during subduction, verges away from the arcs, whereas the second, developed during the present phase of collision, verges towards the arcs. Thus vergence indicators alone are probably equivocal determiners of subduction polarity in ancient rocks.
Seismic properties of a shallow magma reservoir in Kilauea Iki by active and passive experiments
Authors: Aki Keiiti , Chouet Bernard , Fehler Michael , Zandt George , Koyanagi Robert , Colp John , Hay Richard G.
Published: February 2008

The use of multiple methods is essential for determination of seismic properties of a complex structure like a partially frozen lava pond. In our experiment with Kilauea Iki during March 1976 (1) the spatial distribution of seismic events originating from the crust best defined the lateral location of the magma lens, (2) the S waves transmitted through the magma lens and dispersion of Love waves generated by explosive sources in the Iki floor constrained the S wave velocity structure, and (3) P waves from explosions revealed an extremely low P velocity zone below the crust. From Love and S wave data we infer a rather thin (less than 10 m) magma lens, which, in response to our weak seismic signal, behaves like a viscous liquid with apparent viscosity of about 107 P and apparent shear velocity of about 0.2 km/s. Apparent high viscosity at low stress level was reported by Shaw et al. (1968), who made an in situ measurement of viscosity at Makaopuhi, Hawaii, and attributed its possible source to the presence of vesicles. Liquid containing vesicles apparently behaves like a Bingham body: a solid below and liquid above a threshold stress. The presence of vesicles of a few volume percent in the melt can also reduce the apparent bulk modulus to as low as the apparent rigidity inferred from Love and S wave data. The P velocity of about 0.3 km/s is possible in the melt with 5% vesicles. The observed refraction data require the P velocity in the lower crust to be as low as 0.9 km/s. This low velocity may be attributed to dry cracks unfilled with liquid magma.
Elastic behavior near phase transitions with negative dP/dT
Authors: Shaw George H.
Published: February 2008

Sound velocities have been measured in polycrystalline samples of silver iodide at 122°C to pressures of 6 × 108 N/m2. The data cover three different polymorphs within their respective stability fields. The low‐pressure phase (B4 structure) has pronounced negative pressure derivatives of both velocities, probably due to defects. The intermediate pressure phase (B23 structure) stable from ∼2 to ∼3.5 × 108 N/m2, has anomalously low velocities and elastic moduli for its density. The high‐pressure phase (B1 structure) has anomalously high dVp/dP and dK/dP, also possibly due to defects. The velocity and bulk modulus shifts at these transitions, especially the B4‐B23 transition, are in accordance with a previous prediction of anomalous velocity shifts to be expected at a transition with a negative dP/dT.
Preseismic fault slip and earthquake prediction
Authors: Dieterich J. H.
Published: February 2008

It is proposed that preseismic fault creep may be the underlying process that is responsible for observations of earthquake precursors. The assertion that fault creep precedes earthquakes is supported by evidence from at least some earthquakes and by analogy with detailed laboratory observations. Laboratory observations of stick slip reveal that at least two stages of preseismic slip are an intrinsic part of the process leading to seismic slip on preexisting faults with inhomogeneous stress or strength. During the slowly propagating first stage of creep it is assumed that the length of the creeping fault segment is proportional to the source length of the subsequent earthquake. The data giving the well‐known relationship between precursor time and earthquake magnitude are closely satisfied if the rate of propagation of the first stage of creep is independent of fault length. Long‐term precursors may arise because of stress‐strain variations during the first stage of fault creep. Observations of short‐term precursors immediately prior to earthquakes may be related to the second short‐lived state of preseismic fault slip seen in stick slip experiments.
Buoyant ocean floor and the evolution of the Caribbean
Authors: Burke Kevin , Fox P. J. , Şengör A. M. C.
Published: February 2008

Ocean floor in the Caribbean was formed mainly during the Jurassic and Cretaceous. Subduction of the young Caribbean ocean floor started by Late Jurassic times and persisted until the Eocene on its northern and southern borders. By the late Eocene the Caribbean area was occupied by a buoyant mass of abnormally thick and shallow oceanic crust modified from more normal crust by the intrusion of great thicknesses of Late Cretaceous sills. At the beginning of the Oligocene a cordon of extinct arcs surrounded the buoyant oceanic area, and transform motion replaced subduction along the northern and southern edges. Subduction was restricted to boundaries with the American and Cocos plates that were being consumed on the east and the west. Convergence at about 1 cm/yr between the North and the South American plate since 38 m.y. ago appears to have been accommodated largely by internal deformation of the Caribbean plate producing a pattern of faults with an intriguing, though unexplained, resemblance to the slip lines in a modified Prandtl cell.
Skagi and Langjökull Volcanic Zones in Iceland: 1. Petrology and structure
Authors: Sigurdsson H. , Schilling J.‐G. , Meyer P. S.
Published: February 2008

The compositional variation of basalts in a 220‐km‐long profile through the length of the northward extension of the western volcanic zone of Iceland shows an abrupt discontinuity at 65° 10′N, separating high iron‐titanium type light‐rare‐earth‐element (REE)‐enriched tholeiitic basalts in the Skagi zone (2.5–0.5 m.y.) to the north from Mg‐Al‐rich olivine tholeiites with REE patterns unfractionated in relation to chondrites in the Langjökull zone (0.7 m.y. to Recent) to the south. Geophysical evidence indicates that the chemical discontinuity may coincide with steepening of presently existing geothermal gradients and thickening of the Iceland crust from south to north. The two basalt groups exhibit striking petrographic contrasts. Skagi basalts contain phenocrysts of augite, plagioclase, olivine, and minor Fe‐Ti oxides and have liquidus temperatures ranging from 1150° to 1170°C. Langjökull basalts are predominantly olivine and plagioclase‐phyric, with minor chromian spinels and liquidus temperatures near 1230°C. Petrologic mixing calculations indicate that major element dispersion within Skagi can be accounted for by 15–17% extraction of plagioclase, augite, and olivine in proportions of 4:3:1. The observed major element range in the Langjökull group is consistent with 33–55% extraction of these same phases in the proportions 2.5:1.5:1. On the basis of major element chemistry alone the Skagi magma can be derived by 41–43% fractional crystallization of Langjökull magma, ascribed to filtering through a thicker crust. However, variable degrees of melting related to the thermal gradients are also required for genesis of these magmas to account for the REE variations observed (Schilling et al., 1978); combination of both processes is therefore invoked.
Active tectonics of Tibet
Authors: Molnar Peter , Tapponnier Paul
Published: February 2008

From an interpretation of Landsat imagery of Tibet the most recent structures appear to be normal faults that trend approximately north‐south. Fault plane solutions of 14 earthquakes in the central part of the Tibetan plateau indicate large components of normal faulting. The solutions are not well constrained, but for the most reliable ones the T axes are oriented approximately east‐west. Only on the margins of the high plateau, where elevations are lower, do fault plane solutions show active thrust faulting. These observations imply an east‐west extension of most of Tibet at the present time. We relate this pattern to the collision of India and Eurasia and to deformation of an especially weak Tibetan crust and upper mantle. India applies a pressure to Eurasia that maintains Tibet at a high uniform altitude, and the hydrostatic head caused by this elevation transmits the pressure northward. Accordingly, Tibet is the pressure gauge of Asia. At the same time a small east‐west strain (or flow) of the lower crust and upper mantle of Tibet would stretch the surface layer. Tibet's position relative to India at the present time is analogous to that of a ‘dead’ triangle in a plastic material indented by a rigid indenter (India).
Paleomagnetic results from igneous and sedimentary rocks from the Michigan Basin borehole
Authors: Van Der Voo Rob , Watts Doyle R.
Published: February 2008

The results of thermal and alternating field demagnetization of 13 specimens from a fully oriented mafic igneous core taken from a depth of 5.32 km near the center of the Michigan basin give a pole position at 9°S, 159°E (k = 69, α95 = 5°). This pole is close to late Hadrynian and Cambrian pole positions from the North American craton. Furthermore, one specimen was cut from each of three vertical red bed core segments, unoriented in declination, from depths of 4.59, 4.09, and 4.08 km. Thermal demagnetization revealed a large secondary magnetization in these samples, which was fully eliminated at 480°C. Further thermal demagnetization revealed closely grouped characteristic inclinations that averaged 8°. By assuming the secondary magnetizations to be parallel to the present‐day geomagnetic field direction it was possible to obtain characteristic declinations as well. The pole from the red beds is located at 7°N, 174°E and agrees well with the pole obtained from the late Keweenawan sediments (Freda sandstone and Nonesuch shale) of the Upper Peninsula in Michigan. These results suggest that the red beds acquired their characteristic magnetization before the igneous basement rock. If the igneous rock is an extrusive body of Keweenawan age, its characteristic magnetization must be entirely secondary.
Gravimeter survey in the Michigan Basin deep borehole
Authors: Hinze William J. , Bradley James W. , Brown Alan R.
Published: February 2008

Borehole gravimetric measurements were made at major Phanerozoic formation contacts and within the Precambrian sedimentary rocks to a depth of 4 km in the McClure‐Sparks No. 1–8 drill hole. The inplace densities of these units were calculated from the corrected observations. The densities of the Phanerozoic rocks vary considerably owing to the presence of evaporites contrasting with the clastic and carbonate rocks, and the mean density of the Phanerozoic rock column is 2.61 g/cm3. The borehole gravimeter density of the Precambrian clastic rocks is 2.77 g/cm3, 0.05 g/cm3 greater than the density obtained from sample cuttings. The source of this discrepancy is unknown, but one possible explanation is the gravimetric effect of mafic igneous rocks, which may be more common within the Precambrian sedimentary rocks than is indicated by intersection of these rocks in the drill hole.