bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic geometries that carry decoupled transverse or compressional magnetic field oscillations
Authors: Wright Andrew N. , Evans N. W.
Published: February 2008

In general, the eigenmodes of a cold inhomogeneous magnetoplasma have magnetic field perturbations with components both parallel and transverse to the background magnetic field direction. Recently, constraints have been derived which the medium must satisfy for the field perturbation to be either entirely compressional, or entirely transverse. Here these criteria are investigated to yield all planar and axisymmetric magnetic field configurations allowing pure decoupled transverse and compressional oscillations. For example, in an axisymmetric dipole field the azimuthal field perturbations decouple exactly, but solely poloidal transverse ones do not. The magnetic field geometry and background plasma density for the most useful decoupled solutions are listed in a table at the end of the paper.
A comparison of the ion density measured simultaneously by a wave and a particle instrument
Authors: Kessel R. L. , Johnstone A. D. , Brown C. C. , Woolliscroft L. J. C.
Published: February 2008

We have made comparisons between the equivalent electron number density calculated from the ion instrument on the Active Magnetospheric Particle Tracer Explorer United Kingdom Satellite (AMPTE‐UKS) and the electron number densities calculated with the wave instrument on AMPTE‐UKS and the passive sounder on ISEE 1. We make comparisons between the density values determined from the ion and wave instruments according to the location of the spacecraft in the foreshock region. If the spacecraft is on the edge of the electron foreshock where the spectrum is narrow, the emissions are at the plasma frequency so that a direct comparison is possible (region 1). Further into the foreshock region, the spectrum is wider with a low‐frequency cutoff at the plasma frequency. In this region the density from the ion instrument must be compared with the lower edge of the wave band (region 2). Still further downstream of the foreshock boundary, the spectrum intensity is distributed symmetrically above and below the plasma frequency. In this region the density from the ion instrument must be compared with the center of the wave band (region 3). Taking this into account we can make the following conclusions. For low number densities (∼3 cm−3) and medium number densities (∼7 cm−3) the agreement is good, within 5%. For higher number densities (∼20 cm−3) the values from the ion instrument are within 15% of the number density values estimated from the AMPTE‐UKS wave instrument.
Electrostatic noise measurement with a pair of spherical probes near interplanetary shocks
Authors: Solomon Jacques , Touzin Frederic
Published: February 2008

In order to obtain accurate measurements of electrostatic noise spectra on board the ISEE 1 satellite, near interplanetary shock waves, we perform a detailed theoretical and numerical study of an antenna consisting of a pair of spherical probes. We compute the quasi‐thermal electrostatic noise observed theoretically on the antenna by assuming that the solar wind plasma can be properly represented by the sum of two Maxwellian distributions (core and halo). We study the dependence of the electrostatic spectra on the antenna length and on the different plasma parameters, particularly on the density and temperature ratio of the core and of the halo. We show that by also taking into account the instrumental noise and the shot noise on the antenna, a calibration factor can be precisely determined for the antenna that we consider. We display some results obtained from measurements of electrostatic noise spectra behind interplanetary shock waves. Finally, we discuss the real meaning of a specific halo temperature, and we show that, in a first approximation, our theoretical results are only slightly modified when we consider types of distributions other than Maxwellians.
Compressional instability in the solar wind driven by wave dissipation
Authors: Dougherty M. K. , McKenzie J. F.
Published: February 2008

In this paper we examine the stability of a steady solar wind dissipatively heated by Alfvén waves whose relative amplitude is saturated at a given level by nonlinear processes. It is shown that long‐wavelength compressional modes can be driven unstable by dissipative heating arising from short‐wavelength saturated Alfvén waves. Analytic expressions are derived for the marginal stability condition and the growth rates in the unstable region for the case of a moderate to low β plasma. These are supplemented by a numerical solution of the full MHD dispersion equation, including dissipative Alfvénic effects, which confirms the approximate analysis. It is shown that the growth time of the instability can be of the order of 7 times the characteristic period of an Alfvén wave for a wide range of parameters appropriate to the solar wind. The implication is that the compressional instability driven by dissipative Alfvén waves could play a significant role in the large‐scale heating and dynamics of the solar wind, particularly in the supersonic region.
A numerical modeling study of the interaction between the tides and the circulation forced by high‐latitude plasma convection
Authors: Mikkelsen I. S. , Larsen M. F.
Published: February 2008

A spectral, time‐varying thermospheric general circulation model has been used to study the nonlinear interaction at high latitudes between the tides propagating into the thermosphere from below and the circulation induced by magnetospheric forcing and in situ solar heating. The model is discrete in the vertical with 27 layers spaced by half a scale height. In the horizontal, the fields are expanded in a series of spherical harmonics using a triangular truncation at wave number 31, equivalent to a homogeneous global resolution with a minimum wavelength of 1270 km. A hypothetical uniform grid point model would require a horizontal spacing of 417 km to describe the same minimum wavelength. In the high‐latitude F region the tides affect the dusk vortex of the neutral flow very little, but the dawn vortex is either suppressed or amplified dependent upon the universal time and tidal phase. In the E region neutral flow, both the dusk and dawn vortices are shifted in local time by the tides, again as a function of universal time and tidal phase. At dusk a nonlinear amplification of the sunward winds occurs for certain combinations of parameters, and at dawn the winds may be completely suppressed. Below 120 km altitude the magnetospheric forcing creates a single cyclonic vortex which is also sensitive to the high‐latitude tidal structure.
Limits to the lunar atmosphere
Authors: Morgan T. H. , Shemansky D. E.
Published: February 2008

The presence of sodium and potassium on the Moon implies that other more abundant species should be present. Volatile molecules like H2O are significantly more abundant than sodium in any of the proposed external atmospheric sources. Source mechanisms which derive atoms from the surface should favor abundant elements in the regolith. It is therefore puzzling that the Apollo ultraviolet spectrometer experiment set limits on the density of oxygen of NO < 5×10² cm−3, and that the Apollo Lunar Atmospheric Composition Experiment data imply NO < 50 cm−3 above the subsolar point. These limits are surprisingly small relative to the measured value for sodium. A simple consideration of sources and sinks predicts significantly greater densities of oxygen. It is possible but doubtful that the Apollo measurements occurred during an epoch in which source rates were small. A preferential loss process for oxygen on the darkside of the Moon is considered in which ionization by electron capture in surface collisions leads to escape through acceleration in the local electric field. Cold trapping in permanently shadowed regions as a net sink is considered and discounted, but the episodic nature of cometary insertion may allow formation of ice layers which act as a stabilized source of OH. On the basis of an assumed meteoroid impact source we predict a possible emission brightness of ∼50 R in the OH(A ‐ X)(0,0) band above the lunar bright limb. A very uncertain small comet source of H2O could raise this value by more than two orders of magnitude.
Electrons in the boundary layers near the dayside magnetopause
Authors: Hall D. S. , Chaloner C. P. , Bryant D. A. , Lepine D. R. , Tritakis V. P.
Published: February 2008

Entry of heated solar wind plasma into the magnetosphere is examined using electron distributions measured by AMPTE UKS and HEOS 2. In particular, the angular structure of the electron distributions is studied within the transition region separating the magnetosheath from the inner magnetosphere. The measurements suggest that electrons in the outer part of the transition region originate in the magnetosheath, whilst the population closer to the Earth consists of electrons from the magnetosphere combined with an energized magnetosheath component. This energized component contains “counterstreaming” electrons, which are confined to directions closely parallel and antiparallel to the magnetic field direction. The possibilities, that the energization of the counterstreaming electrons is cumulatively gained from either waves, electric fields perpendicular to the magnetic field, or quasi‐Fermi acceleration, are discussed. It is not possible to identify the topology of the magnetic fields of the outer part of the region, but there is strong evidence that the inner part is on closed magnetic field lines, which map to the day side auroral zone. The outer part of the transition region is a plasma depletion/magnetic field compression layer. The structure of the transition region is similar to that surrounding flux transfer events, which leads to the deduction that the plasma and field signatures of flux transfer events may be the result of displacement of the transition region earthward. Cases where the displacement is such that the field maximum of the depletion/compression region is encountered may well explain “crater” flux transfer event signatures.
Solar tides in the equatorial upper thermosphere: A comparison between AE‐E data and the TIGCM for solstice, solar minimum conditions
Authors: Burrage M. D. , Storz M. F. , Fesen C. G. , Roble R. G. , Abreu V. J.
Published: February 2008

Equatorial thermospheric tidal temperatures and densities inferred from Atmosphere Explorer E (AE‐E) mass spectrometer data are compared with theoretical predictions from the National Center for Atmospheric Research Thermosphere/Ionosphere General Circulation Model (TIGCM) for solar minimum, solstice conditions. The thermospheric diurnal and semidiurnal tides are excited in situ by solar heating and by ion‐neutral momentum coupling. Semidiurnal tides are also generated by upward propagating waves excited by heating in the lower atmosphere. The model calculations include all of these sources. The TIGCM reproduces the gross tidal features observed by the satellite, including the midnight temperature anomaly, and the diurnal phases are in good agreement for the densities of atomic oxygen and molecular nitrogen. However, for the neutral temperature, the predicted phases are 1–2 hours earlier than observed. In addition, the diurnal temperature and density amplitudes predicted by the model are considerably weaker than indicated by the AE‐E measurements. The semidiurnal variations found in the observations agree well with the model for December solstice but not for June. The present results indicate that upward propagating tides from the lower atmosphere are responsible for at least half of the amplitude of the semidiurnal tide in the upper thermosphere.
Generation of energetic proton shells during substorms
Authors: Delcourt D. C. , Sauvaud J. A.
Published: February 2008

The circulation of the terrestrial polar wind during dynamical reconfigurations of the geomagnetic field is systematically examined by means of three‐dimensional single‐particle codes. In these calculations the storm time collapse of the geomagnetic tail is simulated by a gradual evolution of the Mead and Fairfield (1975) model from disturbed to ground state geometry. The polar wind is considered to consist primarily of protons, distributed throughout the magnetosphere at substorm onset consistently with the large‐scale plasma convection. The simulations clearly illustrate the “convection surge” accompanying the development of substorms, displaying an overall earthward compression of the H+ populations. Among these, a dense and low‐energy core can be identified at close L shells (within L ∼ 7), which appears as a persistent feature during the dipolarization process. Tailward of these inner regions, the simulations demonstrate a pileup of protons initiated in the mid‐tail (∼10–15 RE), which experience intense (several keV) accelerations and pitch angle diffusion owing to the transient breaking of their adiabatic invariants. Most notably, a detailed analysis of the trajectory results reveals a sharp earthward boundary for these newly created energetic H+, which corresponds to the postdipolarization image of the ion adiabatic “threshold” in the geomagnetic tail. Further nonadiabatic considerations suggest that this mechanism which supports the “substorm injection boundary” concept depends upon particle mass per charge and may consequently organize distinct ion layers at the inner edge of the plasma sheet.
Evolution of magnetic flux ropes associated with flux transfer events and interplanetary magnetic clouds
Authors: Wei C. Q. , Lee L. C. , Wang S. , Akasofu S.‐I.
Published: February 2008

Spacecraft observations suggest that flux transfer events and interplanetary magnetic clouds may be associated with magnetic flux ropes which are magnetic flux tubes containing helical magnetic field lines. In the magnetic flux ropes, the azimuthal magnetic field (Bθ) is superposed on the axial field (Bz). In this paper the time evolution of a localized magnetic flux rope is studied. A two‐dimensional compressible magnetohydrodynamic simulation code with a cylindrical symmetry is developed to study the wave modes associated with the evolution of flux ropes. It is found that in the initial phase both the fast magnetosonic wave and the Alfvén wave are developed in the flux rope. After this initial phase, the Alfvén wave becomes the dominant wave mode for the evolution of the magnetic flux rope and the radial expansion velocity of the flux rope is found to be negligible. Numerical results further show that even for a large initial azimuthal component of the magnetic field (Bθ ≃ 1–4 Bz) the propagation velocity along the axial direction of the flux rope remains to be the Alfvén velocity. Diagnoses show that after the initial phase the transverse kinetic energy equals the transverse magnetic energy, which is characteristic of the Alfvén mode. It is also found that the localized magnetic flux rope tends to evolve into two separate magnetic ropes propagating in opposite directions. The simulation results are used to study the evolution of magnetic flux ropes associated with flux transfer events observed at the Earth's dayside magnetopause and magnetic clouds in the interplanetary space.
Turbulent upwelling of the mid‐latitude ionosphere: 2. Theoretical framework
Authors: Kelley Michael C. , Fukao Shoichiro
Published: February 2008

In a companion paper, data from the MU radar have been presented which show that during sunspot minimum conditions in Japan the mid‐latitude ionosphere is sometimes characterized by regions of rapid and turbulent upwelling. In this paper we explore possible mechanisms for these events. The most likely process seems to be the instability of the equilibrium which occurs when the mid‐latitude plasma is supported against gravity either by an eastward electric field component or by a southward neutral wind, as was proposed by Perkins (1973). We show, for example, that the growth rate determined by Perkins is considerably higher in sunspot minimum conditions than at sunspot maximum for comparable altitudes of the ionospheric F layer. The growth rate is not very large, however, and we argue here that the observed structures must evolve from preexisting undulations of the bottomside of the F region which are generated by gravity waves. That is, the gravity waves create finite amplitude structures which are amplified by the plasma instability. An intriguing feature of the gravity wave role in this process is that the echoing patches detected by the MU radar and the height bands detected by the Arecibo radar, which we believe to be related phenomena, all seem to propagate to the west. This is the same direction reported for the angle‐of‐arrival measurements of classic mid‐latitude spread F by a number of researchers using ionosonde techniques. Since the MU radar detects the upwelling regions from nonthermal 3‐m irregularities there must be mechanism to create such tracers. We propose that secondary structures are created at intermediate scales via the E×B instability operating on the dome of the upwelling structure and by the neutral wind‐driven process on either the west or the east wall of the structure, depending on the direction of the zonal wind. The 3‐m waves are then created in a cascade process which brings energy into a range of k space in which the structures are linearly damped. Finally, we discuss the fine structure of the echoing patches and suggest several plausible mechanisms, two of which involve E region coupling and one which deals with the vertical structure of the gravity wave seeding process.
Lower Thermosphere Coupling Study: Comparison of observations with predictions of the University College London‐Sheffield Thermosphere‐Ionosphere Model
Authors: Fuller‐Rowell T. J. , Rees D. , Parish H. F. , Virdi T. S. , Williams P. J. S. , Johnson R. M.
Published: February 2008

During the first Lower Thermosphere Coupling Study (LTCS), September 21–25 1987, data were recorded from the incoherent scatter radar sites at EISCAT, Millstone Hill, Sondrestrom, and Arecibo. These experimental facilities measured ionospheric parameters (Ne, Te, Ti, and plasma velocity) in the E and the F regions which have been used to determine the E region neutral wind and infer the neutral temperature in the height range 100–150 km. Propagating tides are clearly visible in some of the parameters, and the latitude structure and phase variations with height indicate the presence of at least the (2,2) and (2,4) global tidal Hough modes. The influence of geomagnetic forcing is also clearly present at high latitudes. The University College London‐Sheffield University three‐dimensional coupled thermosphere‐ionosphere model has been used to simulate this period of observation, by imposing tidal forcing at the lower boundary and magnetospheric forcing at high latitudes, in an attempt to interpret and understand the experimental data. Model simulations are able to predict where the signature of a particular tidal mode is likely to be observed in the respective responses of the temperature and wind structure. The numerical simulations predict the range of observed tidal amplitudes at mid and high latitudes, provided the tidal forcing functions imposed near the lower boundary of the model are larger (400 m geopotential height variation) than those inferred from linear tidal models. At high latitudes, the semi‐diurnal ion temperature response is driven by geomagnetic heating, rather than lower atmospheric tides and, as low as 110 km altitude, there is a large difference between ion and neutral temperatures during active conditions. Temperature and zonal wind tidal amplitudes observed at the low‐latitude site of Arecibo are consistently larger than those produced by any of the simulations.
Relationship between solar and geomagnetic activity
Authors: Venkatesan D. , Ananth A. G. , Graumann H. , Pillai Suresh
Published: February 2008

The relationship between the solar and geomagnetic activity has been investigated. The results show clearly that the geomagnetic activity represented by Kp index reveals two maxima for each single maximum of solar activity as represented by sunspot numbers and indicate that the geomagnetic activity has two discrete components attributed to solar flare effects and corotating streams.
Plasma expansion characteristics of ionized clouds in the ionosphere: Macroscopic formulation
Authors: Schunk R. W. , Szuszczewicz E. P.
Published: February 2008

The field‐aligned expansion of Ba+, Li+ and Ba+‐Li+ plasma clouds in the upper F region was modeled with a macroscopic hydrodynamic formulation in order to study the early‐time (t ≲ 20 s) plasma expansion characteristics. Simulations were conducted for a range of Ba+‐Li+ mixtures, cloud sizes, electron/ion temperatures, and ratios of cloud/background densities and flow velocities. The expansion scenarios were chosen to be similar to previous small‐scale (Debye length), short‐duration (plasma period) numerical simulations of collisionless plasma clouds. Since both the spatial and temporal scales differ by four orders of magnitude in the macroscopic and small‐scale simulations, the comparison of results not only elucidates a broader domain of early‐time plasma expansion characteristics and their extreme sensitivity to initial conditions but sheds light on the applicability of small‐scale simulations to expanding plasma clouds in the ionosphere. The macroscopic simulations led to the following results: (1) An expanding Ba+ cloud acts as an electrostatic snowplow, creating a hole in the ionosphere (factor of 10) as it pushes O+ density bumps (factor of 1.8) ahead of it along the geomagnetic field. (2) For the same cloud half width a decrease in the cloud density leads to a weaker snowplow. (3) An initially weaker, longer lasting snowplow ultimately produces a larger hole in the ionosphere than a short‐lived strong snowplow. (4) Elevated electron temperatures act both to speed the plasma cloud expansion and to strengthen the electrostatic snowplow. (5) A bulk velocity component along the magnetic field resulting from a rocket‐ or satellite‐borne chemical release has several important effects on the plasma cloud expansion and the ionospheric response. For a Ba+ cloud with a 2 km/s release velocity, a deep asymmetric hole is created in the ambient O+ and electron densities, and a large O+ bump is pushed ahead of the moving Ba+ cloud while a small O+ bump propagates away from the back of the cloud. For a 6 km/s release velocity the Ba+ ions create an initial ionospheric perturbation but then move rapidly through the O+ plasma, leaving behind a fossil O+ hole with little‐to‐no Ba+ ions. (6) A Li+ cloud expansion is qualitatively similar to a Ba+ expansion but it is faster. (7) Without Li+‐O+ collisions the Li+ snowplow is weaker than the comparable Ba+ snowplow, but with collisions the reverse is true at early times. (8) The presence of a minor cloud ion does not affect the cloud‐ionosphere interaction. (9) Light minor Li+ ions are significantly affected by the electric fields associated with the major ion density profiles (Ba+ and O+). These and other macroscopic expansion features are in general agreement with those obtained from the small‐scale numerical simulations.
Seasonal and diurnal variations of the latitude of the westward auroral electrojet in the nightside polar cap
Authors: Wu Q. , Rosenberg T. J. , Lanzerotti L. J. , Maclennan C. G. , Wolfe A.
Published: February 2008

Simultaneous measurements of magnetic field variations at the nominally conjugate sites of South Pole station, Antarctica, and Iqaluit, Canada, have been used to study latitude differences in the location of corresponding westward electrojets in the nightside polar cap. Substorm‐related auroral events were selected from 1986 data with the criterion that the magnitude of the negative bay exceed 100 nT in at least one hemisphere and that the electrojet, oriented mainly in the magnetic east–west direction, pass overhead of this station. The resulting data base comprised 60 events distributed seasonally as follows: austral summer (17); fall (6); winter (18); spring (19). The interhemispheric latitude difference in the locations of the electrojets was obtained by examining the ratio of the vertical and horizontal component magnetic variations. Seasonal and diurnal variations of the latitude of westward electrojets were evident during solstice periods. In particular, it is noted that the substorm westward electrojet flows at higher latitudes in the winter hemisphere than in the summer hemisphere, after 2000 MLT. The latitude difference between the two hemispheres maximizes at ∼4° near local midnight. The latitude difference becomes smaller away from midnight and reverses for local times prior to ∼2000 MLT. The observed seasonal and diurnal variations are generally consistent with predictions of the effect of the magnetic dipole tilt on the location of last closed field lines. However, because on the nightside the field lines originating from South Pole station and Iqaluit reach deep into the magnetotail, they are likely to be influenced by the plasma sheet. Thus the implications of an asymmetric distribution of the plasma sheet with respect to the neutral sheet have also been considered. Such a postulated distribution can account for the seasonal variations while conserving magnetic flux and maintaining pressure balance across the tail.
Modeling the large‐scale structure of the solar corona
Authors: Bagenal F. , Gibson S.
Published: February 2008

The aim of this study is to find a quantitative description of both the magnetic field and the distribution of plasma in the lower corona that matches the white light images of the K‐corona. We use the magnetostatic model of Bogdan and Low (1986) and data obtained by the High Altitude Observatory Mark III K‐Coronameter stationed at Mauna Loa, Hawaii. To start with, we take the simplest, solar minimum case when the corona is approximately longitudinally symmetric. By varying parameters in the Bogdan and Low model we are able to quantitatively match the general characteristics of the lower corona at solar minimum: power law radial profiles of coronal brightness; enhanced brightness at the equator; uniform density depletion at the pole. Further, we find the set of parameters that best fit the data and investigate how well the model parameters are resolved by the data.
Spatial and spectral characterization of laboratory shuttle glow simulations
Authors: Swenson G. R. , Leone A. , Holtzclaw K. W. , Caledonia G. E.
Published: February 2008

Laboratory experiments designed to uncover mechanistic information about the spectral and spatial characteristics of shuttle glow were conducted. Pulsed oxygen atoms traveling at orbital velocities were directed toward a substrate which was previously dosed with NO molecules. Heterogeneous recombination of the O and NO species resulted in NO2* exiting the surface of the sample, and an associated emission was found to extend from the sample plane. In the experiments the materials investigated were Z306 Chemglaze® (a common baffle black paint), aluminum, and nickel. The sample temperatures were varied from 300 to 77 K, and the oxygen atom velocity was varied from 5 to 10 km s−1. The experimental results include the measure of (1) an effective NO2* lifetime of 185 μs, (2) complete surface thermal accommodation of the formed NO2*, (3) a large NO2* emission brightness which was inversely related to surface temperature, and (4) a spectral shape which indicates a red shifting to distance from the sample as well as (5) a slight spectral shift which appears to be material related. The preliminary experimental data from this experiment were presented by Caledonia et al. (1990).
Comparison of observed and calculated implanted ion distributions outside comet Halley's bow shock
Authors: Gombosi T. I. , Neugebauer M. , Johnstone A. D. , Coates A. J. , Huddleston D. E.
Published: February 2008

This paper compares calculated and measured energy spectra of implanted H+ and O+ ions on the assumption that the pickup geometry is quasi‐parallel and about 1% of the waves generated by the cometary pickup process propagate backward (toward the comet). The model provides a good description of the implanted O+ and H+ energy distribution near the pickup energies. The thickness of the implanted ion velocity distribution shells was nearly constant between 2.50×106 km and 1.20×106 km (just outside the shock) along the inbound Giotto trajectory. The explanation is that the velocity diffusion coefficient and characteristic diffusion time vary approximately as 1/r and r, respectively, and therefore their product (which determines the velocity shell thickness) remains nearly constant.
Source mechanism of the 1987 Vatnafjöll Earthquake in south Iceland
Authors: Bjarnason Ingi Th. , Einarsson Páll
Published: February 2008

The May 25, 1987, Vatnafjöll earthquake occurred on the Mid‐Atlantic plate boundary, at a junction between a transform (the South Iceland Seismic Zone) and a spreading segment (the Eastern Volcanic Zone). The South Iceland Seismic Zone has had the most destructive earthquakes in the history of Iceland with average intervals of 80–100 years and estimated maximum magnitudes around 7. The 1987 Vatnafjöll earthquake was the largest earthquake (mb = 5.8; MS = 5.8; Mw = 5.9) in south Iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region. Foreshocks and aftershocks of the Vatnafjöll earthquake were located with relatively good accuracy. They define an elongated N‐S epicentral area, 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km. We determined centroid source parameters of the Vatnafjöll earthquake by inverting long‐period teleseismic P and SH waves. The centroid source mechanism and foreshock and aftershock distribution indicate N‐S right‐lateral strike‐slip faulting on a near‐vertical plane. The rupture initiated near the base of the crust and propagated upward along an approximately 10‐km‐long fault but did not reach the surface. The centroid depth is well constrained and is between 5 and 8 km depth at the 95% confidence level. The seismic moment is 9.1×1024dyn cm. The source has relatively short duration of 3 s, which may indicate higher stress drop than global average of earthquakes of this size. The South Iceland Seismic Zone is a 70‐km‐long E‐W striking transform zone between two overlapping rift zones, the Eastern and Western Volcanic Zones. It is characterized by “bookshelf” type of tectonics; that is, there is no surface faulting in the direction of the transform. Instead, N‐S right‐lateral strike‐slip faults are distributed along and perpendicular to the transform zone. The Vatnafjöll earthquake has characteristics of earthquakes in the South Iceland Seismic Zone and is probably not related directly with magmatic activity in the Eastern Volcanic Zone. The transform zone thus extends farther into the rift zone than was previously recognized.
Nonlinear elastic waveform inversion of land seismic reflection data
Authors: Crase Edward , Wideman Charles , Noble Mark , Tarantola Albert
Published: February 2008

Data from 53 shot records collected in a conventional, land, seismic reflection survey are inverted to obtain subsurface maps of short‐wavelength variations in elastic parameters (impedance). The inversion method employs a robust, nonlinear, iterative, gradient algorithm which inverts waveform data; thus there is no picking of travel times or amplitudes. The algorithm is also computationnally expensive as it relies heavily upon elastic finite difference modeling. In addition, because a gradient search is employed to minimize the misfit between observed and modeled data, we attempt to solve only the weakly nonlinear problem associated with the short wavelenghts of imedance (information on the short‐wavelengths of impedance is mainly derived from amplitude variations of the data). Resolving the long wavelengths of velocity (background) is thought to be a highly nonlinear problem, at least for waveform inversion, and is not attempted herein. For the inversion of this data set, long wavelenghts have been derived from well logs located along the seismic reflection profile. Although the data set is dominated by unconverted P wave arrivals, we obtain maps of the short wavelenghts of both P impedance and S impedance. Three well logs in the survey area confirm the validity of the P impedance map; however, no S velocity well log are available from this area. There are channel sands of economic interest which have a thickness of approximately 10 m in one well and are very thin or absent in the other two wells. The 10 m occurrence of channel sands is quite visible in both impedance maps. We note that because inversion in three dimensions of a realistically sized data set is simply not possible on present‐day computers, all modeling was performed in two dimensions, and a rather crude conversion from spherical to cylindrical energy spreading was applied to the original field data before inversion. Other seismic reflection data in the survey area suggest that there is little lateral variation in the subsurface except for the channel sands; thus we feel that a two‐dimensional inversion with energy compensation is justified.
Disentangling Middle Paleozoic sea level and tectonic events in cratonic margins and cratonic basins of North America
Authors: Bond Gerard C. , Kominz Michelle A.
Published: February 2008

The cratonic margins and basins of North America contain evidence of distinct changes in relative sea level, one of the most intriguing of which occurred in middle Paleozoic time. The change in relative sea level began in Frasnian time (Late Devonian) and continued through Visean time (Middle Mississippian) in the Cordilleran miogeocline, in the Southern Oklahoma Aulacogen, in the Appalachian miogeocline and in the Michigan, Illinois, and Williston basins. The synchroneity and wide geographic distribution of this event are striking and would seem to argue for an eustatic mechanism. An estimate of the middle Paleozoic sea level rise relative to the stable craton in Iowa suggests that while a large sea level rise occurred, it is smaller than the magnitude of subsidence in the cratonic basins and margins. Flexural foreland basin models do not appear to account for the all of the events in the cratonic margins, and thermal subsidence mechanisms do not seem appropriate for the subsidence in the cratonic basins. The middle Paleozoic stratigraphic record from the North American craton and its margins, therefore, poses a basic problem of identifying a mechanism for producing a large‐amplitude rise in sea level relative to the stable craton at the same time as a synchronous onset of tectonic subsidence in widespread basinal and marginal settings of diverse tectonic origin. One plausible mechanism for the tectonic subsidence in the basins and margins is a pulse of intraplate compressive stress. The origin of the large sea level rise relative to the stable craton could reflect an unusually large eustatic sea level change, but we cannot eliminate the possibility of a small component of subsidence or change in dynamic topography of the North American craton. The synchroneity of the sea level rise relative to the craton with the subsidence of basins and margins may be fortuitous, but it is also predicted by recent mantle convection models for the early stages of accretion of supercontinents.
Fluid volcanism on Miranda and Ariel: Flow morphology and composition
Authors: Schenk Paul M.
Published: February 2008

Several types of volcanic units have been recognized on the icy Uranian satellites Miranda and Ariel. On Miranda, ridges characterized by crest grooves are up to 10 km wide and 500 m high. A continuous flat‐topped flow band also 10 km wide and 500 m high forms the outer southern margin of Elsinore Corona. Elsinore Coronae at least appears to comprise coalesced flow bands and ridges. On Ariel, in addition to at least one ridge unit similar to these on Miranda, flood plains material has covered the floors of deep chasmata (grabens) and an irregular depression. Flows on both satellites are characterized by linear vent geometries and distinct topographic margins, which indicate extrusion of a relatively viscous material. The topography of the flows can be used to estimate flow viscosity or yield strength using a Bingham plastic model. Extrusion viscosity estimates, incorporating plausible volcanologically based emplacement time scales and a rigid crust correction, range from 107 to 109 P (1013 P in the unlikely absence of a chilled crust). Viscosity estimates are dependent on the assumed emplacement time scale, however, and could be as high as 1016 P, if a solid‐stale‐based time scale is assumed. Bingham models indicate that it is yield strength that controls the final flow width and thickness that we see. Yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale. Yield strength estimates range from 10−1 to 10−2 bars, consistent with yield strengths of partially crystallized silicate melts on other planets. Nevertheless, both the viscosities and yield strengths are well within the range of (and in some cases lower than) silicate flows. Silicate compositions are not possible for these flows because of the ice‐rich composition of these satellites. Measured properties of ammonia‐water liquid‐crystal mixtures are consistent with the observed and estimated flow rheologic properties, however, and extrusion of such material(s) is a viable alternative to solid‐state extrusion for the origin of these flow features.
Improving GLORIA images using Sea Beam data
Authors: Mitchell N. C.
Published: February 2008

GLORIA and Sea Beam data are processed together with a number of benefits. The Sea Beam data allow an improved slant‐ to ground‐range projection of the imagery that results in less of the distortions, such as layover and foreshortening, that are produced when the seafloor is assumed to be flat. A calculation originally intended to quantify side scan sonar data has also been found to improve the images in an image enhancement sense. This calculation involves estimating the seafloor's backscatter strength from GLORIA side scan sonar data using a simple physical model and the characteristics of the GLORIA instrument. The backscatter strength is the ratio of backscattered to incident sonar intensity on the sea bottom, normalized to the surface area covered by the acoustic pulse. It is computed by correcting the GLORIA data for propagation losses, beam directivity pattern, time‐varied amplifier gain and other factors. The calculation is improved by accounting for refraction in the water column and for some topographic effects using Sea Beam data. A GLORIA image from a mid‐ocean ridge crest is shown to have a large 60‐dB range that is due to the effect of the system response, such as the beam pattern, on the images. The corresponding image of the seafloor backscatter strength has an almost Gaussian distribution of picture element digital numbers with a narrow range, 20 dB, showing that most of the system effects have been removed. Cross sections through the backscatter strength image show flat trends superimposed with finer variations due to the seafloor geology. The conversion of GLORIA data into seafloor backscatter strength serves two purposes; it provides quantitative information that can be used to determine the likely causes of particular features in the images and it performs what is effectively a deterministic “shading correction”.
Late Cenozoic tectonic evolution of southwestern California
Authors: Sedlock Richard L. , Hamilton Douglas H.
Published: February 2008

We synthesize available geologic and geophysical data from southern California and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern Coast Ranges, western Transverse Ranges, and borderland regions of California since 30 Ma. Premises include specified relative plate motions for times prior to 10.5 Ma and after 3 Ma, a mid‐Tertiary bight in the continental margin of southern California, midcrustal detachment faults under much of California and adjoining regions, rotation of the western Transverse Ranges about an eastern pivot, and specified fault displacement histories. Prior to 18 Ma, about 90% of the tangential component of Pacific‐North America relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope. From 18 Ma to 5.5 Ma, dextral slip was accommodated predominantly on the offshore system but also on a second, inboard system that included the San Andreas fault. Fault‐bounded crustal blocks between the offshore and inboard fault systems underwent translation, rotation, and extension from 18 Ma to 11.5 Ma but not from 11.5 Ma to 5.5 Ma. Since 5.5 Ma, dextral slip has been accommodated predominantly on the inboard fault system; dextral slip ceased on the offshore fault system north of the Transverse Ranges but continues on faults west of or within Baja California. The San Gregorio‐San Simeon‐Hosgri fault system has accommodated less than 5 km of the late Cenozoic dextral slip. The distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction, and, to a lesser degree, changes in magnitude of Pacific‐North America relative motion. Based on our reconstuction, the tangential component of Pacific‐North America relative motion was ∼ 90mm/yr for the period 10.5 Ma to 5.5 Ma and ∼ 60mm/yr for the period 5.5 Ma to 3 Ma.
Introduction to the Special Section on Measurement, Causes, and Consequences of Long‐Term Sea Level Changes
Authors: Sahagian D. L. , Watts A. B.
Published: February 2008

A large portion of the sea level research community in academia, government, and industry met in April 1989 at the AGU Chapman Conference on Causes and Consequences of Long Term Sea Level Changes at Snowbird, Utah in an attempt to gain a new perspective on the measurement, causes, and consequences of long‐term sea level change. This section is a compilation of several of the papers presented at the conference. The manuscripts were solicited after the conference in the hopes that interdisciplinary insights gained at the conference would be incorporated into this special section. It became clear at Snowbird that a great deal of research is being conducted in each of the various independent methods of measurement of past sea levels, mechanisms of sea level change, and its geologic, oceanographic, biologic, and atmospheric consequences.
Thermal history of Michigan Basin and Southern Canadian Shield from apatite fission track analysis
Authors: Crowley Kevin D.
Published: February 2008

Apatite fission track ages and confined‐length distributions were collected from 38 basement outcrop and 5 basement drillcore samples in order to reconstruct the Phanerozoic thermal history of the Michigan Basin and southern Canadian Shield. The apatite data indicate two periods of thermal activity in the region: Triassic heating/cooling that affected the basin and adjacent shield and Cretaceous or post‐Cretaceous heating/cooling that primarily affected the basin. The magnitude, timing, and cause of Cretaceous thermal activity cannot be identified with the present data. Model calculations suggest that some of the shield samples and probably all of the basin samples were heated to temperatures of at least 90°C just prior to relatively rapid cooling in the Triassic. Available stratigraphic and geochemical constraints suggest that these elevated temperatures were the result of burial by an additional 2–5 km of late Paleozoic (probably Pennsylvanian and Permian) sediments. It is likely that the basin was buried during the Alleghenian Orogeny as observed for the adjacent Appalachian Basin.
Magnetic fabric, flow directions, and source area of the Lower Miocene Peach Springs Tuff in Arizona, California, and Nevada
Authors: Hillhouse John W. , Wells Ray E.
Published: February 2008

We have used anisotropy of magnetic susceptibility (AMS) to define the flow fabric and possible source area of the Peach Springs Tuff, a widespread rhyolitic ash flow tuff in the Mojave Desert and Great Basin of California, Arizona, and Nevada. The tuff is an important stratigraphic marker from the Colorado Plateau to Barstow, California, a distance of 350 km; however, the location of its source caldera is unknown. Dated at 18.5 Ma by 40Ar/39 Ar, the tuff erupted during the early stages of Miocene extension along the lower Colorado River. The thicker accumulations (>100 m) occur at Kingman, Arizona, and in the Piute Mountains, California, on opposite sides of the Colorado River extensional corridor. Our AMS studies produced well‐defined magnetic lineations in 30 of 42 sites distributed throughout the tuff. Typical ratios of the principal AMS axes are 1.01 for the magnetic lineation (kmax/kint) and 1.02 for the foliation (kint/kmin); the bulk magnetic susceptibility of the Peach Springs Tuff averages 2.0×10−3 in the SI unit system. The subhorizontal lineations, which presumably parallel the flow directions, form a pattern radiating outward from the approximate center of the outcrop area. Magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff. The lineation and imbrication indicate a source region near the southern tip of Nevada. Defining the best intersection of the AMS lineations required restoration of major extension, strike‐slip faulting, and associated tectonic rotation in the disrupted tuff. The optimum intersection of magnetic lineations lies in the southern Black Mountains of Arizona on the eastern side of the Colorado River extensional corridor. No caldera structures are known from that area, but the area contains thick sections of the Peach Springs Tuff above a silicic volcanic center. The caldera may be buried under younger deposits in the Mohave Valley of Arizona. Tertiary granite in the Newberry Mountains may represent a deeper level of the Peach Springs Tuff vent that has been exhumed by detachment faulting.
Anistropy of X ray absorption in a slate: Effects on the fabric and march strain determination
Authors: Chen Richard T.
Published: February 2008

The preferred orientation of phyllosilicates in a sample of slate from North Wales causes anisotropy of X ray absorption. The linear absorption coefficient μ should be replaced by what is probably a second‐rank tensor quantity, μij, with principal values of [1.31±0.08, 1.34±0.06, 1.57±0.10]×10−2. The maximum absorption is normal to the slaty cleavage, the intermediate absorption axis lies in the cleavage and is normal to the lineation, and the least absorption is parallel to the lineation. Based on March's theory, transmission pole figure X ray data, corrected for anisotropy of X ray absorption and for the irradiated volume, variable with sample orientation, give principal strains of [0.79±0.03, 0.30±0.02, −0.57±0.00]. This differs from empirically corrected results of [0.81, 0.38, −0.60] but little from the results, [0.80, 0.31, −0.57], of measurements on ellipsoidal strain indicators.
Comparative study of spinel compounds: A pseudopotential periodic Hartree‐Fock calculation of Mg2SiO4, Mg2GeO4, Al2MgO4, and Ga2MgO4
Authors: D'Arco Philippe , Silvi Bernard , Roetti Carla , Orlando Roberto
Published: February 2008

The geometries of Mg2SiO4, Mg2GeO4, Al2MgO4, and Ga2MgO4 spinels have been correctly calculated using the periodic Hartree‐Fock method at core pseudopotential level. The bonding properties are studied on the basis of electron population analysis and electron density. The band structure and density of states have been computed and are discussed. The bulk modulus and its pressure derivative of each compound have been derived. The structures yielded by our calculations are a little bit too stiff. Different compression mechanisms are proposed for, on the one hand, Mg2SiO4 and Mg2GeO4 and, on the other hand, Al2MgO4 and Ga2MgO4.