bluuebunny/crossref_metadata_2025 huggingface dataset:
MHD simulations of magnetic reconnection in a skewed three‐dimensional tail configuration
Authors: Birn J. , Hesse M.
Published: February 2008

Using our three‐dimensional MHD code we have studied the dynamic evolution of a non‐symmetric magnetotail configuration, initiated by the sudden occurrence of (anomalous) resistivity. The initial configuration included variations in all three space dimensions, consistent with average tail observations. In addition, it was skewed due to the presence of a net cross‐tail magnetic field component ByN with a magnitude as typically observed, so that it lacked commonly assumed mirror symmetries around the midnight meridian and the equatorial planes. The field evolution was found to be very similar to that of a symmetric configuration studied earlier (e.g., Birn and Hones, 1981), indicating plasmoid formation and ejection. The most noticeable new feature in the evolution of the individual field components is a reduction of By on the reconnected dipole‐like field lines earthward from the reconnection region. The topological structure of the magnetic field, however, defined by the field line connections, shows remarkable differences from the symmetric case, consistent with conclusions by Hughes and Sibeck (1987) and Birn et al. (1989). The plasmoid, which is a magnetically separate entity in the symmetric case, becomes “open”, connected initially with the Earth, but getting gradually connected with the interplanetary field, as reconnection of lobe field lines proceeds from the midnight region to the flanks of the tail. The separation of the plasmoid from the Earth is thus found to take a finite amount of time. When the plasmoid begins to separate from the Earth, a filamentary structure of field connections develops, not present in the spatial variation of the fields; this confirms predictions by Birn et al. (1989). A localization of the electric field parallel to the magnetic field is found consistent with conclusions on general magnetic reconnection (Schindler et al., 1988a,b; Hesse and Schindler, 1988). The effect of E∥, integrated along field lines, is found to be maximal on field lines near the plasma sheet/lobe interface. The “footprints” of these regions at the near‐Earth boundary show a clottiness, reflecting the filamentary structure of the field connections, but not present in the spatial structure of the field itself.
Oblique propagation of nonlinear hydromagnetic waves: One‐ and two‐dimensional behavior
Authors: Malara Francesco , Elaoufir Jawad
Published: February 2008

The one‐ and two‐dimensional behavior of obliquely propagating hydromagnetic waves is analyzed by means of analytical theory and numerical simulations. It is shown that the nonlinear evolution of a one‐dimensional MHD wave leads to the formation of a rotational discontinuity and a compressive steepened quasi‐linearly polarized pulse whose structure is similar to that of a finite amplitude magnetosonic simple wave. For small propagation angles, the pulse mode (fast or slow) depends on the value of β with respect to unity while for large propagation angles the wave mode is fixed by the sign of the initial density‐field correlation. The two‐dimensional evolution shows that an MHD wave is unstable against a small‐amplitude long‐wavelength modulation in the direction transverse to the wave propagation direction. A “two‐dimensional magnetosonic wave” solution is found, in which the density fluctuation is driven by the corresponding total pressure fluctuation, exactly as in the one‐dimensional simple wave. Along with the steepening effect, the wave experiences both wave front deformation and a self‐focusing effect which may eventually lead to the “collapse” of the wave. The results compare well with observations of MHD waves in the Earth's foreshock and at comets.
A method for the retrieval of atomic oxygen density and temperature profiles from ground‐based measurements of the O+(²D ‐ ²P) 7320‐Å twilight airglow
Authors: Fennelly J. A. , Torr D. G. , Richards P. G. , Torr M. R. , Sharp W. E.
Published: February 2008

This paper describes a technique for the retrieval of altitude profiles of the atomic oxygen concentration (n = [O]) and temperature (T) from ground‐based measurements of the O+(²D ‐ ²P) doublet at 7320 and 7330 Å in the twilight airglow. The technique is based on previously demonstrated knowledge that at solar zenith angles (SZA) characteristic of twilight conditions, the upper state of the 7320‐Å doublet transition is produced by photoionization and photoelectron impact ionization of atomic oxygen and lost mainly by radiative decay, thereby providing a sensitive dependence on [O]. We apply inverse problem theory to retrieve the exospheric temperature (T∞), the atomic oxygen concentration at 120 km (n120), the temperature at 120 km (T120) and the temperature profile shape factor (S) using a Bates‐Walker representation of n given approximately by n = (n120T120/T) exp[−z] where T = T∞ ‐ (T∞ ‐ T120)exp[−S(h ‐ h120)], z is the reduced height, and h is the altitude. The algorithm is tested and theoretically verified using synthetic data sets where random errors of measurements are characterized by Poisson noise due primarily to sky background. In the tests that we report here the solar EUV flux is specified. In a separate paper we will report how the solar EUV ionization rate can be independently derived from various twilight emissions. By comparing retrieved with known input values, it is demonstrated that for the altitude range 200 to 500 km the atomic oxygen concentration [O] can be retrieved with relative errors ≃15% and systematic errors of about 25% if the solar EUV is given. Sensitivity of the results to noise, sample size (degrees of freedom), and absolute calibration are quantitatively evaluated. In addition, to demonstrate the validity of the technique experimentally, we utilized the Atmosphere Explorer E (AE‐E) in situ measurements of the solar EUV flux and [O], with the latter taken when perigee was over Arecibo on an occasion when the observatory airglow spectrometer was simultaneously measuring the 7320‐Å emission from the ground during twilight. The results show excellent agreement with the measured [O] values which were ∼ 50% lower than the mass spectrometer incoherent scatter (MSIS‐86) model values at ∼ 300 km on that day, thereby demonstrating the value of the method for monitoring day‐to‐day variations in [O] and the temperature.
Nonthermal atmospheric escape from Mars and Titan
Authors: Lammer H. , Bauer S. J.
Published: February 2008

Energy flux spectra and particle concentrations of the hot O and N coronae from Mars and Titan, respectively, resulting primarily from dissociative recombination of molecular ions, have been calculated by means of a Monte Carlo method. The calculated energy flux spectra lead to an escape flux ϕesc ≈ 6×106 cm−2 s−1 for Mars and ϕesc 2×106 cm−2 s−1 for Titan, corresponding to a mass loss of about 0.14 kg/s for Mars and about 0.3 kg/s for Titan. (The contribution of electron impact ionization on N2 amounts to only about 25% of Titan's mass loss.) Mass loss via solar and magnetospheric wind is also estimated using newly calculated mass loading limits. The mass loss via ion pickup from the extended hot atom corona for Mars amounts to about 0.25 kg/s (O+) and for Titan to about 50 g/s (N2+ or H2CN+). Thus, the total mass loss rate from Mars and Titan is about the same, i.e., 0.4 kg/s.
Anomalous optical events detected by rocket‐borne sensor in the WIPP Campaign
Authors: Li Ya Qi , Holzworth Robert H. , Hu Hua , McCarthy Michael , Massey R. Dayle , Kintner Paul M. , Rodrigues Juan V. , Inan Umran S. , Armstrong W. C.
Published: February 2008

A photodiode sensor has been designed and flown in an experiment to measure the broadband optical power of lightning. Several such sensors were launched on one rocket‐borne payload (apogee over 400 km) and four balloon‐borne payloads (float altitudes over 30 km) during the Wave Induced Particle Precipitation (WIPP) campaign (Kintner et al., 1987) at Wallops Island during the summer of 1987. Sensors aloft measured the optical power of events occurring below the payloads along with the waveforms of electric field transients. The same transients were recorded simultaneously by a ground‐based VLF receiver with a magnetic (loop) antenna. Rocket and balloon sensors detected a great majority of lightning flashes also recorded by the State University of New York at Albany (SUNYA) lightning locating network. Many other signatures which were similar to those of lightning but were not recorded by SUNYA were also detected. Overall during the 10‐min flight, more than 500 lightning‐related events were detected in the (wide) field of view of the rocket‐borne photodiode sensor. Among these is a class of about 23 events all having an anomalous signature, with obvious clustering of optical impulses or continuous emissions, and resulting durations of several hundred milliseconds. Such durations are much longer than typical for the lightning‐related events recorded at the rocket, which are more frequent overall. Every anomalous optical event (AOE) was accompanied by broadband VLF signals of a distinctive character, signals which were received on the balloon, the rocket, and the ground. No such AOEs were ever detected by the balloon optical photodiode sensor even when the data suggested that a signal from the AOEs also should have been detectable at the balloon. In fact, no such unusually long‐enduring optical events were ever detected by a balloon photodiode sensor on any of six separate flights in 1987 and 1988 under similar experimental conditions. Responses excluded so definitively from detection at the balloons are consistent with events originating primarily at altitudes above the 30 km balloon altitude. In considering possible sources above 30 km we find that the AOEs do not seem to resemble other natural optical phenomena, such as meteors which bum up well above 30 km in the mid‐latitude atmosphere.
Sondrestrom incoherent scatter radar observations during the Lower Thermosphere Coupling Study: September 21–26, 1987
Authors: Johnson R. M.
Published: February 2008

The incoherent scatter radar located at Søndre Strømfjord, Greenland, obtained E and F region measurements during the first Lower Thermosphere Coupling Study (LTCS 1), September 21–26, 1987. Lower thermospheric neutral winds deduced from these measurements show that the neutral dynamics are influenced by both tidal oscillations and magnetospheric forcing. During an interval which was relatively quiet geomagnetically, September 23–24, a semidiurnal oscillation dominated the neutral motion. The model equinox tidal amplitudes and phases of Forbes (1982) for the diurnal tide are roughly in agreement with the observed diurnal oscillation for the first four days of the experiment. Vertical variations in the observed diurnal phases are consistent with the results of Forbes and Hagan (1988) and may provide evidence of dissipation of the propagating (1, 1) tidal mode. The semidiurnal motion observed during this period is not well represented by the recent theoretical results for the amplitude and phase of the semidiurnal tide (Forbes and Vial, this issue). Neutral winds obtained during a geomagnetically active interval, September 25–26, displayed a flow pattern that was significantly distorted from that observed during the preceding, relatively quiet interval. Although the changes in the zonal winds throughout this active interval were consistent with the direction of the ion drag force at 115 km and above, the variations in the meridional winds suggest that other forces, such as pressure gradients driven by Joule heating, need to be considered to explain the observations.
Barium swarm: An ionospheric alternating current generator in CRIT I
Authors: Brenning N. , Kelley M. C. , Providakes J. , Stenbaek‐Nielsen H. C. , Swenson C.
Published: February 2008

A model is developed for the motion of a low‐β cloud of plasma which starts with uniform velocity Vi across the magnetic field through a larger ambient plasma. The model cloud is a circular cylinder of length L, with the axis aligned with the magnetic field. The density limit above which the cloud is self‐polarized to EP = − Vi × B is determined. At lower cloud densities the internal electric field is tilted at an angle between antiparallel to the injection direction Vi and the self‐polarization direction EP = −Vi × B and becomes close to antiparallel to Vi for thin clouds. The ions in thin clouds perform a collective gyration with decreasing amplitude, which a stationary observer will see as an oscillation involving electric, magnetic, and density perturbations with a frequency close to the ion gyro frequency. The currents perpendicular to B inside the cloud are a combination of electron Hall current, ion current, and plasma displacement current. They combine as magnetic‐field‐aligned currents which are drawn from the ambient ionosphere by a divergence in the perpendicular electric field. These field‐aligned currents give a perturbation of the magnetic field which is related to the electric field perturbation by ΔE/ΔB ≈ VA. The low‐density version of the model is applied to an oscillation observed by a subpayload in the ionospheric rocket experiment CRIT I, which had a frequency close to the barium gyrofrequency, electric field fluctuations of amplitude 10 mV/m, density fluctuations with Δn/n ≈ 0.2, and a time duration of 1 s. All four features can be reproduced by the model, using a density and an injection velocity consistent with other independent observations from CRIT I. In CRIT I the magnetic oscillations were not measured at the subpayload, and the relation ΔE/ΔB ≈ VA could not be tested. However, it has been confirmed for similar oscillations observed in the earlier Porcupine release experiment.
The solar wind density spectrum near the Sun: Results from Voyager radio measurements
Authors: Coles W. A. , Liu W. , Harmon J. K. , Martin C. L.
Published: February 2008

Spacecraft radio propagation data are analyzed to estimate the solar wind density fluctuation power spectrum across five decades of spatial wave number. The data were from radio science observations made during superior conjunctions of the Voyager 1 and Voyager 2 spacecrafts in 1979 and 1980. These data were analyzed to yield the wave structure function, a statistic which is related to, but obtained more readily than, the spectrum itself. Structure functions were obtained from a total of 28 observations spanning the heliocentric distance range 7–22 RS. Each structure function was a composite of two shorter segments estimated, respectively, from phase scintillation and spectral broadening analyses. The composite structure functions tend to be steep (approximately Kolmogorov) at large scales (10³–106 km) and nearly always show some flattening at smaller scales (10‐100 km). The inflection between the steep and flatter regions is abrupt and occurs at scales of 100–300 km. Most of the structure functions also show some turnover at the very smallest scales. These results are in excellent agreement with the spectral shape model proposed by Coles and Harmon [1989]. A few “transients” were seen in the Voyager data which were characterized by an overall increase in power at large scales and a steepening of the structure function at small scales. These transients were similar in appearance to transients reported earlier from Arecibo spectral broadening observations.
Coupled quasi‐linear wave damping and stochastic acceleration of pickup ions in the solar wind
Authors: Bogdan Thomas J. , Lee Martin A. , Schneider Peter
Published: February 2008

Coupled spatially homogeneous quasilinear kinetic equations are derived which describe the evolution of the energetic ion omnidirectional distribution function and the intensities of magnetohydrodynamic waves propagating parallel and antiparallel to the ambient magnetic field. The energetic ions are assumed to be nearly isotropic and possess speeds much greater than the Alfvén speed. For application to pickup ions the equations may also include an energetic ion injection rate and wave excitation or damping caused by isotropization of the newborn ions. The wave kinetic equations may be integrated to yield explicit expressions for the wave intensities, which may be substituted into the ion kinetic equations to yield a single self‐consistent energy diffusion equation for the energetic ions. The theory represents the first treatment of stochastic (second‐order Fermi) acceleration in which the back reaction of the ions on the turbulence is included self‐consistently. Numerical solutions of the kinetic equations are presented for four cases of pickup ions in the solar wind which illustrate the essential features of the evolution: (1) interstellar pickup helium near a heliocentric radial distance of 1 AU; (2) interstellar pickup hydrogen near 10 AU; (3) water group pickup ions downstream of the bow wave of Comet Giacobini‐Zinner for parameters observed during the International Cometary Explorer flyby; (4) water group pickup ions downstream of the bow wave of Comet Halley for parameters observed during the Giotto flyby. The helium calculation reveals some modification of the solar wind wave spectrum and energy diffusion of the ions; although adiabatic deceleration is not included, acceleration rates are qualitatively consistent with the observed spectrum at 1 AU (Möbius et al., 1985). The hydrogen calculation shows extreme damping of the solar wind wave spectrum in the cyclotron‐resonant frequency range and a reduction in the acceleration rate of most of the ions. It is suggested that this behavior is responsible for an underabundance of hydrogen relative to the minor ions in the anomalous cosmic ray component, which is thought to originate from pickup ions accelerated at the solar wind termination shock. Wave damping is small at comet G‐Z, and the calculated energy spectra do not appear to be in quantitative agreement with the observed spectra (Richardson et al., 1987). At Comet Halley, on the other hand, wave damping is substantial and the calculated spectra appear to be in general agreement with the observations (McKenna‐Lawlor et al., 1989).
The ballooning instability in space plasmas
Authors: Hameiri E. , Laurence P. , Mond M.
Published: February 2008

The theory of ballooning modes, which are modes localized to a particular magnetic field line, is extended to configurations relevant to space plasmas. Included are the effects of gravity and rotation and, in particular, boundary effects on magnetic field lines which intersect the plasma boundary. Three types of boundary conditions are considered, corresponding to perfectly conducting, conducting, and insulating boundaries. The interchange instability is also examined and is shown to be a special case of the ballooning instability.
Lower thermospheric neutral densities determined from Søndre Strømfjord Incoherent Scatter Radar during LTCS 1
Authors: Reese K. W. , Johnson R. M. , Killeen T. L.
Published: February 2008

Ion‐neutral collision frequencies determined from measurements obtained by the incoherent scatter radar located at Søndre Strømfjord, Greenland, have been used to derive lower thermospheric neutral densities during the first Lower Thermosphere Coupling Study (LTCS 1), September 21–26, 1987. Periods of Joule and particle heating which might disturb the E region thermal equilibrium were systematically eliminated. The mean profile of neutral density for the period is in good agreement with the mass spectrometer incoherent scatter 1986 (MSIS‐86) model between 92 and 104 km. A tendency to overestimate collision frequencies above 105 km may arise from range‐smearing effects. The results of a tidal analysis performed on the neutral density between 92 and 109 km show that the amplitudes of the diurnal and semidiurnal components of the tides are approximately equivalent. The observations are generally in better agreement with the MSIS‐86 predictions than with the thermosphere‐ionosphere general circulation model (TIGCM) simulation of the LTCS 1 interval. The observed phase of the diurnal component is approximately constant with height above 98 km and is in close agreement with the MSIS‐86 model phases; however, the TIGCM diurnal phases are shifted by 6–8 hours to later local times. The phase of the semidiurnal tide is in good agreement with predictions of the MSIS‐86 model and the TIGCM simulation of this interval, except near 98 km. The observed semidiurnal phase is also consistent with previous high‐latitude results (Kirkwood, 1986). The relative amplitude of the observed semidiurnal oscillation is up to 15% larger than that previously observed at the European Incoherent Scatter facility but is consistent with the amplitudes presented in an earlier study of Millstone Hill measurements (Salah, 1974).
A three‐dimensional model and initial time numerical simulation for an artificial plasma cloud in the ionosphere
Authors: Gatsonis N. A. , Hastings D. E.
Published: February 2008

A fully three‐dimensional model for an artificial plasma cloud in the ionosphere is developed. Such a cloud could be a result of a deliberate release or contamination about a spacecraft. Finite perpendicular and parallel lengths, all elastic collisions between the considered species, finite temperature effects, variable ionospheric densities, variable neutral winds, ambient electric fields and gravity are taken into account. The velocities of the charged species are written with the use of transport coefficient tensors. The current balance equation includes conductive currents, diffusion currents, currents due to the ambient and contaminant neutral winds, and gravitational drift currents. The numerical solution for a plasma sphere immersed in a plasma with an external electric field is obtained and compared with known analytic solutions. The electrodynamical behavior of Gaussian‐like clouds is examined. The neutral cloud simulates an orbital release (U=8 km/s) with various masses. The effects of the central density of the contaminant neutrals and ions are investigated. The perturbation potential increases with increasing ion and neutral density. The perturbation electric field consists of a dipole field due to the action of the neutral wind and a diffusion field due to the density gradients. The dipole field is mapped along the magnetic field lines while the diffusion field is bounded within the density perturbation. With cloud densities of 1015 m−3 the ion cloud drifts almost with the neutral cloud velocity. The effects of the coupling of the lower E and upper F region are also investigated. The ambient plasma is expected to develop image clouds with differential rotation which will stretch the flux tube. The effect of the altitude of the release is examined. It is found that releases at higher altitudes result in faster moving ion clouds. As a result of the nonuniformity of the perturbation electric field, the ion cloud drifts with differential speed which will result in fingerlike shaping.
The physics of the Harang discontinuity
Authors: Erickson G. M. , Spiro R. W. , Wolf R. A.
Published: February 2008

Absent a source of energetic ions at the flanks of the tail, the westward gradient/curvature drift of E×B‐convecting plasma results in the depletion of energetic ions from the dawnside of the plasma sheet. This dawnside depletion effect means that, on average, the duskside of the plasma sheet will have higher ion temperatures, pressures, and flux tube contents, and hence, stronger westward cross‐tail drift current than the dawnside. The resulting cross‐tail divergence of drift current must find closure by means of Birkeland currents connecting to the ionosphere. Tailward and poleward of the inner edge region, the divergence of cross‐tail current requires upward current from the ionosphere. In the ionosphere, current closure requires electric fields that are directed toward the center of the upward current, i.e., directed equatorward on the poleward side, poleward on the equatorward side. This is precisely the nature of the Harang discontinuity. The region poleward of the Harang discontinuity maps well out into the plasma sheet and provides an eastward component of E×B drift to oppose the westward gradient/curvature drift of the ions. This helps keep the flow of plasma sheet ions directed toward the inner plasma sheet, rather than toward the dusk flank of the tail, a point originally made by Atkinson. The region equatorward of the Harang discontinuity maps close to the inner edge of the plasma sheet and results in westward E×B drift, increasing the westward flow of plasma azimuthally around the duskside of the inner magnetosphere and toward the dayside magnetopause. Although this scenario can be understood qualitatively, runs were carried out using the Rice convection model (RCM) to examine the ionospheric‐magnetospheric coupling implications of this dawnside depletion effect. These runs confirm the above scenario, generally. They show that dawnside ion depletion results in a band of upward Birkeland current in the central auroral zone on the nightside, similar to what has been consistently observed by Iijima and Potemra and others. These currents modify the nightside, auroral electric field distribution to produce a strong reversal in the meridional electric field, similar to the classically observed Harang discontinuity. Inclusion of dawnside ion depletion also results in a major improvement in the agreement between the RCM and observations with regard to the latitudinal distribution of Birkeland currents. Finally, the dawnside depletion effect results in a reduction of plasma sheet pressures in the near‐Earth, midnight sector of the plasma sheet. However, this reduction is significantly less than that suggested by Kivelson and Spence.
On the response of exospheric temperature on Venus to solar wind conditions
Authors: Kar J. , Paul R. , Kohli R. , Mahajan K. K. , Kasprzak W. T. , Niemann H. B.
Published: February 2008

Exospheric temperatures (Tex) inferred from in‐situ measurements of neutral densities by the Pioneer Venus Orbiter have been analyzed to examine any possible dependence on solar wind dynamic pressure (Psw). We do not find any linear relationship between Psw and Tex. However, on one occasion when Psw was unusually high for a prolonged period, an unmistakable temperature enhancement was observed. Minor enhancements of Tex were also observed in a few other cases, with sustained but moderately high Psw.
Inhomogeneity and nonlinearity effects on stop bands of Alfvénic ion cyclotron waves in multicomponent plasma
Authors: Nekrasov A. K. , Mursula K. , Kangas J. , Pikkarainen T. , Maltseva N. F.
Published: February 2008

We study the existence of stop bands for left‐handed Alfvénic ion cyclotron waves propagating in the direction of magnetic field in a multicomponent plasma. Three effects are discussed: finite ion temperature, inhomogeneous magnetic field, and nonlinear wave amplitude. All of them affect the existence of stop bands and set critical bounds on the relevant physical parameters, particularly on the density of heavy ions. Using the model of a linearly varying, longitudinally inhomogeneous magnetic field, we calculate the critical lower bound on heavy ion density as a function of longitudinal inhomogeneity, and we discuss the difference between the truly inhomogeneous and essentially homogeneous situations. Typical values for the wave vector in the resonance region are derived. We also find the upper bound on the wave amplitude for the linear theory to be applicable, and we derive the condition for stop band formation in the nonlinear case. The obtained results are applied to the magnetospheric environment and are shown to lead to relevant modifications. For example, the formation of stop bands by thermal heavy ions is strongly dependent on the local inhomogeneity of the magnetic field.
Foreword and Dedication [to Special Section on Lower Thermosphere Coupling Study (LTCS)]
Authors: Forbes Jeffrey M.
Published: February 2008

The Lower Thermosphere Coupling Study (LTCS) is a coordinated investigation of the lower thermosphere (90–150 km) combining observational (optical and radar) and numerical modeling efforts with the ultimate goal of better understanding the dynamic and electrodynamic processes coupling the mesosphere, lower thermosphere, and upper thermosphere regions. The study, which is being conducted under the auspices of the National Science Foundation CEDAR (Coupling, Energetics, and Dynamics of Atmospheric Regions) Program, is intended to address such questions as, What is the relative importance of upward propagating tides, planetary waves, and gravity waves, related accelerations of the mean flow, and heating and momentum sources of magnetospheric origin, in determining neutral dynamical fields and ionospheric structures as a function of height between 90 and 150 km? How do these different spatial and temporal scales of motion interact with each other, and how do they influence the vertical, seasonal‐latitudinal, and diurnal variations in turbulent diffusivity and transport of minor chemical constituents? What are the relative roles of magnetospheric currents and the ionospheric wind dynamo in determining ionospheric electric fields?
On the auroral current‐voltage relationship
Authors: Lu G. , Reiff P. H. , Burch J. L. , Winningham J. D.
Published: February 2008

A simple model of collisionless plasma kinetic theory [Knight, 1973] has been used to predict the relationship between the upward parallel current and the parallel potential drop. The DE 1/DE 2 pair offers a unique opportunity to test this relationship because the DE 1 spacecraft can measure the high altitude plasma parameters without contamination from auroral heating. The field‐aligned current density from Knight's formula is almost linear in the ratio of j∥/eΦ∥ for values of eΦ∥/kT between 1 and 10. We find, using measured values of J∥ (mapped to the surface) and Φ∥ (inferred from measurements either at high altitudes or low altitudes), the ratio of J∥/eΦ∥ varies considerably but with a mean value about 0.5∼2.2×10−9 mho/m². Using either the full Knight's formula or the linear simplification, we can predict the parallel potential drop (given the high‐altitude density, temperature, current, and magnetic mirror factor) to test against the measured potential drop, or we can predict the current given the potential drop and the high‐altitude plasma parameters. The agreement is generally quite good and the linear simplification is reasonable; however, the Knight current, since it does not include upward ionospheric electrons, is not valid in the downward return current region. We also show that suprathermal electron bursts are observed in the diffuse aurora at the same invariant latitudes, both at high and at low altitudes. Thus we suggest that these “bursts” are more properly described as a spatial rather than temporal phenomenon.
Formation of intermediate shocks in both two‐fluid and hybrid models
Authors: Wu C. C. , Hada T.
Published: February 2008

Intermediate shocks are studied in a two‐fluid model that includes finite ion inertia dispersion and in a hybrid model in which the full ion dynamics is retained while the electrons are treated as a massless fluid. We show that in both models intermediate shocks can be formed through wave steepening, meaning that they are stable and possess shock structures.
A comprehensive magnetohydrodynamic model of the Venus ionosphere
Authors: Shinagawa H. , Kim J. , Nagy A. F. , Cravens T. E.
Published: February 2008

The MHD Venus ionospheric model developed by Shinagawa and Cravens (1988) has been improved by including the energy equations for ions and electrons in a self‐consistent manner. This new model reproduces observed electron density and magnetic field profiles very well, and the basic MHD processes of the Venus ionosphere, as described by Shinagawa and Cravens (1988), remain virtually unchanged. The results indicate that including energetics does not significantly alter the density and magnetic field profiles. Under unmagnetized conditions, it is necessary to impose heat fluxes for both ions and electrons in order to reproduce the observed plasma temperature profiles, which are consistent with the studies by Cravens et al. (1979, 1980) and Kim et al. (1990). In the magnetized ionosphere, it is likely that a heat source for the ions is present at higher altitudes. On the other hand, the observed very high electron temperatures can be reproduced with a reduced conductivity or with a heat source at high altitudes. It is also found that heating processes do not play a significant role in the dynamics at low altitudes. Thus a nearly supersonic downward velocity layer in the lower ionosphere of Venus, proposed by Cloutier et al. (1987), is unlikely, suggesting that their flow/field model is not applicable to the solar wind‐Venus interaction and other unmagnetized bodies in magnetized plasma flows.
Tracing the subducted oceanic crust beneath the Central California Continental Margin: Results from ocean bottom seismometers deployed during the 1986 Pacific Gas and Electric EDGE Experiment
Authors: Tréhu Anne
Published: February 2008

Large aperture seismic data were collected on several ocean bottom seismometers (OBS) deployed along a deep crustal seismic profile that was shot across the central California continental margin. The line of shots extends from the oceanic crust seaward of the Santa Lucia Escarpment to the California coast near Morro Bay and crosses the Santa Lucia Basin, Santa Lucia Bank, Santa Maria Basin, and Hosgri fault zone. The OBS data permit us to trace the subducted oceanic crust from seaward of the Santa Lucia Escarpment to beneath the central part of the Santa Maria Basin. A complementary study using onshore recordings of the same shots extends the crustal model onshore. Just seaward of the Santa Lucia Escarpment, the oceanic crust is subhorizontal and covered by a thin layer of low‐velocity sediment. The velocity (4.5 km/s) and gradient (1.20–1.25 km/s/km) of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern Pacific. Beneath the Santa Lucia Escarpment and Santa Lucia Basin, the oceanic crust dips approximately 16° to the east. It is overlain by material with a velocity that increases from 4.8 to 6.4 km/s at a depth of 1.7–5.5 km below the seafloor beneath the Santa Lucia Basin. A low‐velocity zone may be sandwiched between the subducted crust and this shallow high‐velocity material, which we interpret to represent obducted oceanic crustal material. Beneath the eastern edge of Santa Lucia Basin, the dip of the subducted oceanic crust decreases to less than 2°. The configuration of the subducted crust in this region is consistent with imbrication of the subducted crust, as suggested by others based on common midpoint reflection data. Beneath the central Santa Maria Basin, the top of the subducted oceanic crust is at a depth of about 14–16 km and the Moho is at 19–21 km.
A review of the regional geophysics of the Arizona Transition Zone
Authors: Hendricks J. D. , Plescia J. B.
Published: February 2008

A review of existing geophysical information and new data presented in this special section indicate that major changes in crustal properties between the Basin and Range and Colorado Plateau occur in, or directly adjacent to, the region defined as the Arizona Transition Zone. Although this region was designated on a physiographic basis, studies indicate that it is also the geophysical transition between adjoining provinces. The Transition Zone displays anomalous crustal and upper mantle seismic properties, shallow Curie isotherms, high heat flow, and steep down‐to‐the‐plateau Bouguer gravity gradients. Seismic and gravity studies suggest that the change in crustal thickness, from thin crust in the Basin and Range to thick crust in the Colorado Plateau, may occur as a series of steps rather than a planar surface. Anomalous P wave velocities, high heat flow, shallow Curie isotherms, and results of gravity modeling suggest that the upper mantle is heterogeneous in this region. A relatively shallow asthenosphere beneath the Basin and Range and Transition Zone contrasted with a thick lithosphere beneath the Colorado Plateau would be one explanation that would satisfy these geophysical observations.
Compensation of Cretaceous Seamounts of the Darwin Rise, northwest Pacific Ocean
Authors: Wolfe Cecily J. , McNutt Marcia K.
Published: February 2008

We estimate the elastic thickness Te for the Pacific plate at the time of volcanism for approximately 60 guyots of Cretaceous age in the Japanese, Wake, and Mid‐Pacific Mountain seamount groups of the northwest Pacific. The values of Te are constrained by comparing synthetic gravity and deflection of the vertical computed from digital bathymetry assuming regional compensation models to potential field data derived from Seasat radar altimetry. The amplitude of the gridded Seasat gravity field over the seamounts is consistent with Te values between 5 and 15 km, but these values represent only lower bounds since the 100‐km spacing of satellite tracks may undersample the gravity for seamounts lying between adjacent passes. Direct modeling of the deflection of the vertical along individual satellite tracks avoids this distribution bias but can lead to errors if the bathymetric feature is mislocated with respect to the satellite coordinate system. Nevertheless, for no seamount does the root‐mean‐square (RMS) difference between observed and predicted deflection of the vertical show a distinct minimum for elastic plate thickness greater than 15 km. However, for some features the RMS minimum is unconstrained, allowing Te greater than 15 km. Given the possible bias in the modeling of satellite data, the low values for elastic plate thickness were confirmed for the Japanese and Wake group using shipborne gravity data and multibeam bathymetry collected during the Roundabout leg 10 expedition, supplemented with published Navy sonar array sounding system (SASS) bathymetry. The analysis of shipborne data constrains most elastic plate thickness values to between 10 and 15 km for crustal densities between 2600 and 2800 kg/m3. The low values for elastic plate thickness for these Cretaceous guyots that formed in the area of the “Darwin Rise” suggest either that they formed on lithosphere less than 40 m.y. old or that older lithosphere was reheated near the time of volcanism. While the first possibility is permitted by age data from the Mid‐Pacific Mountains, radiometric dates and magnetic lineations from the Japanese and Wakes are more consistent with formation of these seamount groups on older, thermally anomalous lithosphere, such as that which is thought to exist presently beneath the volcanoes of French Polynesia.
The high‐pressure melting curve of iron: A technical discussion
Authors: Williams Quentin , Knittle Elise , Jeanloz Raymond
Published: February 2008

The melting curve of iron is reliably determined to 105 GPa using the laser‐heated diamond cell. In particular, these and other melting data obtained with the same technique are in close agreement with independent measurements using piston‐cylinder and large‐volume presses (5–20 GPa range), as well as shock wave experiments (50–250 GPa range). In order to obtain reliable melting data from the internally heated diamond cell, whether by laser heating or by Joule (resistance) heating, temperature gradients across the sample must be quantitatively measured. An analysis of results derived from Joule heating inside the diamond cell, and consideration of the resulting heat flow and temperature distribution, documents that such “wire heating” experiments can lead to significant underestimates of the melting temperature and its pressure dependence. The best estimate of the high‐pressure melting curve of iron, as derived from the laser‐heated diamond cell and Hugoniot temperature measurements, yields melting temperatures of 4800 (±200) K and 6700 (±400) K at 133 GPa and 243 GPa, respectively. These values are compatible with the Lindemann relation. We conclude that the triple point between the γ, ε, and liquid phases of iron lies in the vicinity of 300 GPa and 7500 K; no other as yet unobserved high‐temperature, high‐pressure phases need be invoked based on the presently available data.
Applications of perturbation theory to acoustic logging
Authors: Ellefsen K. J. , Cheng C. H. , Toksöz M. N.
Published: February 2008

To study the normal modes generated during acoustic logging, we have developed a method to calculate (1) phase velocities when the formation has slight, general anisotropy, (2) partial derivatives of either the wave number or frequency with respect to either an elastic modulus or density, (3) group velocities, and (4) phase velocities when the cross section of the borehole is slightly irregular. The method, which is based upon perturbation theory, relates first‐order perturbations in frequency, wave number, elastic moduli, densities, and locations of interfaces for a general model with many fluid and solid layers which have any cross‐sectional shape and any type of anisotropy. To demonstrate the use of this method, formulas for the four applications are developed for a relatively simple model consisting of a fluid‐filled borehole through a transversely isotropic solid with its symmetry axis parallel to the borehole, and some sample calculations are performed. Using these examples, formulas for more complicated borehole models that may be appropriate for some field situations could readily be developed. The significance of this work is that three applications can be used to study the normal modes when the borehole environment is complicated and the other can be used in an inversion for formation properties.
The effects of porosity on the brittle‐ductile transition in sandstones
Authors: Scott Thurman E. , Nielsen K. C.
Published: February 2008

Sandstones with porosities of 6.0%, 13.9%, 19.9%, 20.9%, 22%, and 27.6% were deformed under triaxial conditions either as whole core samples or 35° sawcut samples at confining pressure conditions of 0.1, 5, 10, 30, 50, 70, 90, 110, and 130 MPa. Whole cores exhibited three types of deformational behavior: (1) thoroughgoing shear fractures, (2) brittle‐ductile transitional behavior characterized by cataclastic shear zones, and (3) pervasive ductile cataclastic flow. Sawcut cores exhibited either (1) brittle behavior accomplished by reactivation of the sawcut or by the development of a crosscutting shear fracture, or (2) penetrative ductile cataclastic flow which obliterated the sawcut. The 35° sawcut cores lack the development of multiple shear zones associated with transitional brittle‐ductile behavior in whole cores. The brittle‐ductile boundary in both sawcut and whole cores occurs at similar confining pressures and is dependent upon initial porosity of the sandstone. Increasing the confining pressure has the effect of shifting the brittle‐ductile boundary to lower porosities. A comparison of frictional strengths and fracture strengths confirms Byerlee's (1968) observation that the change from brittle to ductile behavior is related to frictional properties of the material.
Magnetotelluric evidence for crustal suture zones bounding the Southern Great Valley, California
Authors: Park Stephen K. , Biasi Glenn P. , Mackie Randall L. , Madden T. R.
Published: February 2008

A geoelectric section inferred from a regional magnetotelluric study across the Coast Ranges, the Great Valley, and the Sierra Nevada reveals significant variations in electrical resistivity. Zones of lower resistivity interpreted at depths from 10 km to at least 30 km lie near mapped geologic boundaries between the Coast Ranges and the Great Valley and beneath the eastern side of the Great Valley. The former boundary is inferred by others to separate the subduction complex of the Coast Ranges from the mafic basement of the Great Valley. The lower resistivities are most likely associated with metasediments trapped between the Coast Ranges ophiolite and the former oceanic crust beneath the Great Valley. The latter boundary is problematic, but may be evidence for a deep metasedimentary section trapped between the ophiolites beneath the Great Valley and the granitic rocks of the Sierra Nevada. The lack of change in the magnetotelluric phase across the Great Valley indicates that a suture zone marked by lower resistivities is unlikely to be present beneath the valley. However, this does not preclude the existence of a resistive suture zone.
Rupture process of the February 4, 1965, Rat Islands Earthquake
Authors: Beck Susan L. , Christensen Douglas H.
Published: February 2008

The great Rat Islands underthrusting earthquake (Mw = 8.7), of February 4, 1965, represents subduction of the Pacific plate beneath the North American plate along a 600‐km segment of the western end of the Aleutian Islands. Body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the Rat Islands earthquake. We have deconvolved World‐Wide Standard Seismograph Network long‐period teleseismic P wave seismograms to obtain source time functions. Directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s, unilateral rupture in the direction 300°, fault length of 420 km, and average rupture velocity of 2.5 km/s. The three pulses of moment release are located along the fault, and these regions of high moment release are interpreted as asperities. The first asperity extends from the epicenter to 100 km to the WNW. This is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s. The second pulse of moment release is very jagged, is less coherent between stations, and is centered ∼200 km WNW of the epicenter. The third pulse of moment release extends from 360 to 420 km WNW of the epicenter. Although the aftershock area is ∼600 km in length, we can not resolve any moment release from the P waves beyond ∼420 km WNW of the epicenter. The Rat Islands event was closely followed by a large tensional outer‐rise event on March 30, 1965, (Ms = 7.5), which is located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. Detailed analysis of the P waves for this large outer‐rise event confine the depth extent to the upper 30–5 km of the crust. The spatial and temporal association between the February 4 mainshock and the March 30 tensional outer‐rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins (Geist et al., 1988). We suggest that the central undeformed parts of the blocks have the strongest coupling with the down‐going plate and hence are the sites of the largest moment release during an underthrusting earthquake. The three asperities determined from the P waves correspond to the Rat, Buldir, and Near tectonic blocks respectively. Hence the P wave seismic moment release of the Rat Islands earthquake is controlled by the lateral segmentation of the overriding plate.