bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparative study of imaging and broad‐beam riometer measurements: The effect of spatial structure on the frequency dependence of auroral absorption
Authors: Rosenberg T. J. , Detrick D. L. , Venkatesan D. , van Bavel G.
Published: February 2008

Observations of the spatial structure and motion of auroral absorption regions obtained with the 38.2‐MHz imaging riometer for ionospheric studies (IRIS) at South Pole Station, Antarctica, have been used to simulate the absorption that should be measured by the 20.5‐, 30‐, 38.2‐, and 51.4‐MHz broad‐beam riometers also operating at the station. The simulation assumes that the absorption measured by each of the 49 narrow IRIS beams obeys the absorption‐frequency relationship, A(ƒ) ∼ ƒ−2, predicted by generalized magnetoionic theory to apply at altitudes above ∼70 km. Excellent agreement to quite fine detail is obtained between the simulated and measured broad‐beam absorption profiles. The power law exponents derived from absorption ratios, or from least squares fits to the absorption as a function of frequency, range between 1 and 2 and are essentially the same for simulated and measured broad‐beam data, to within the experimental and computational uncertainties. These results indicate that spatial gradients in the absorption regions, corresponding to similar features in the electron precipitation source, are the cause of deviations from the inverse‐square frequency dependence noted previously in broad‐beam riometer measurements and erroneously interpreted as signifying very high energy electron precipitation and enhanced ionization at much lower altitudes.
Solar cycle effects on the structure of the electron density profiles in the dayside ionosphere of Venus
Authors: Kliore Arvydas J. , Luhmann Janet G.
Published: February 2008

Since 1975, United States and Soviet Venus‐orbiting spacecraft have carried out more than 190 radio occultation measurements of the vertical electron density structure of the dayside ionosphere of Venus: 148 of these have been produced by the Pioneer Venus orbiter between 1979 and 1989; Venera 9 and 10 provided 13 measurements in 1975; and Venera 15 and 16 made 31 measurements in 1983. These measurements were made at various times of the solar cycle, including solar maximum and solar minimum. It has therefore been possible to observe the changes in the electron density structure of the dayside ionosphere of Venus that were brought about by changing solar activity. Some of the resulting observations are as follows: (1) The ionopause height is generally low for values of the solar zenith angle below about 50° regardless of the phase in the solar cycle. (2) At solar maximum, and at times of intermediate solar activity, the ionopause height for solar zenith angles greater than about 50° is highly variable, ranging from a minimum of about 200 km to a maximum of more than 1000 km. (3) At times of solar minimum the great majority of all ionopause heights for all solar zenith angles are uniformly low, lying between 200 and 300 km, with the notable exception of three Venera 9 and 10 measurements. The compressed nature of the Venus ionosphere at solar minimum is most likely produced by permeation of the ionosphere by the solar wind magnetic field, which occurs when the solar wind dynamic pressure exceeds the ionospheric plasma pressure. This happens at times of solar maximum at low solar zenith angles and when the solar wind dynamic pressure is exceptionally high. At solar minimum the Venus ionosphere, like the Martian ionosphere at all phases of the solar cycle, does not appear to have sufficient thermal pressure to balance the incident solar wind pressure.
Modeling the ionospheric response to artificially produced density enhancements
Authors: Çakir S. , Haerendel G. , Eccles J. V.
Published: February 2008

The motion of plasma density enhancements (barium clouds) artificially introduced into the postsunset equatorial F region is investigated with a two‐dimensional model incorporating flux tube integrated quantities. The temporal development of the ionosphere, in which the density perturbations are imbedded, is derived from a one‐dimensional set of relations modeling plasma transport and the vertical electric field from initial conditions and a prescribed variation of the horizontal electric field as a function of time. The calculations show that the strong horizontal shear flows existing at the nighttime F region ledge (where the perturbations were placed) reduce the growth of polarization fields associated with the enhancements and adjacent relative depletions of plasma for weak perturbations. The reason is that the perturbations develop a strong tilt with respect to the horizontal. More massive density perturbations lead to stronger drop velocities with respect to the rising ambient plasma. At a later time they develop secondary horizontal density perturbations on the side unstable to E × B drift instability due to the motion of neutral constituents. When rising “bubbles” of low density are produced, they (1) form on the steepened eastward side of the enhancement perturbation, (2) have a width comparable to the scale size of the enhancement perturbation, and (3) are most easily produced when the enhancement perturbation size is comparable to the scale height of the integrated density. These simulations show why experimental efforts of initiating rising bubbles and equatorial spread F have not been successful. The experiments require larger‐scale and stronger density perturbations than what can be achieved with conventional sounding rocket releases of barium vapors.
Prediction of the Ulysses Jovian hectometric observations
Authors: Ladreiter H. P. , Leblanc Y.
Published: February 2008

The Ulysses unified radio and plasma wave (URAP) experiment is well suited to observe the Jovian hectometric radio emission (HOM) and so complement the observations made by the low‐frequency band receiver on Voyager. When the spacecraft encounters Jupiter, in February 1992, observations will be possible from considerably higher magnetic latitudes, in both hemispheres, than those accessible to Voyager. It should also be possible to determine the source location, as URAP can measure the direction of the incoming waves and all four Stokes parameters. Based upon the HOM beaming and source model of Ladreiter and Leblanc (1990a,b), the following predictions can be made: (1) During the inbound trajectory, Ulysses will, at first, be close to the Jovian equatorial plane when equal amounts of right‐handed (RH) and left‐handed (LH) circularly polarized HOM should be observed from the northern hemisphere and the southern hemisphere, respectively. During the three days immediately before closest approach, the spacecraft will move to a higher northerly magnetic latitude where it will be outside the main HOM emission beam so that a relatively weak RH polarized signal should be observed. (2) At closest approach, Ulysses will be very close to the low‐frequency HOM sources and will cross the HOM shadow zone, which extends out to about 20 Jovian radii (RJ) close to the magnetic equatorial plane. (3) During the outbound trajectory, the spacecraft will be at a fairly high southerly magnetic latitude and outside the main HOM emission beam, so that a relatively weak LH polarized signal should be observed.
Interplanetary magnetic flux: Measurement and balance
Authors: McComas D. J. , Gosling J. T. , Phillips J. L.
Published: February 2008

We have developed a new method for determining the approximate magnetic flux content of various solar wind structures in the ecliptic plane, using single‐spacecraft measurements. The two‐dimensional magnetic flux in a region of the solar wind is given by the integral of the radial magnetic field component over an arc perpendicular to the radial. Unfortunately, such measurements cannot be achieved with single (or even several) spacecraft in the solar wind. We will show that the desired two‐dimensional, ecliptic plane magnetic flux integral, at least for regions with simple magnetic topologies, is equivalent to ϕ = ∫ By|v|dt, where By is the ecliptic plane field component perpendicular to the solar wind velocity vector v. Thus ϕ can be determined entirely from measured quantities. In this study we examine variations in the magnetic flux in the ecliptic plane over a 16‐year interval. In addition, we address the question of the opening and closing of interplanetary magnetic flux by comparing the ecliptic plane flux content of both coronal mass ejections (CMEs) and heat flux dropouts (HFDs). If CMEs remain at least partially attached to the Sun, they would serve to open new magnetic flux to the interplanetary medium. In contrast, flux could be closed off by reconnection across helmet streamers in the corona, leading to the release of U‐shaped magnetic structures open to the outer heliosphere at both ends and to the return of closed arches to the Sun. One interpretation of HFDs in the solar wind halo electron population is that they represent the interplanetary signatures of such disconnected U‐shaped structures. The ecliptic plane flux of CMEs is determined to be approximately 4 times greater than that of HFDs over the 18‐month interval studied; complications of making such comparisons are discussed.
The magnetic topology of solar coronal structures following mass ejections
Authors: Kahler S. W. , Hundhausen A. J.
Published: February 2008

The bright radial structures observed in the solar corona for 1–2 days following a coronal mass ejection (CME) have traditionally been interpreted as unidirectional magnetic fields, commonly known as “legs,” at the sides of the ejections. We examine in detail the bright structures following 16 CMEs observed with the coronagraph on the Solar Maximum Mission (SMM) spacecraft and find that these structures can form anywhere within the lateral span of a CME, not only at the sides. We suggest that a more plausible interpretation is that the bright radial structures are the tops of coronal streamers containing magnetic neutral sheets across which the magnetic fields reverse direction. The observational support for this view is that: (1) the bright features left behind at CME sides, when a CME has left the field of view of this instrument, last <10 hours; (2) some bright radial structures form at the sites of preexisting streamers; (3) some such structures have the broad bases characteristic of helmet streamers; and (4) in several cases, narrow radial structures form over H α prominences. A reexamination of the structures observed in the Skylab era, including the well‐studied CME of August 10, 1973, reveals that they are also consistent with the streamer interpretation. This interpretation avoids several difficulties encountered with the “leg” interpretation: (1) the systematic brightening of legs −1 day after the CME; (2) the origin of the energetic electrons characteristic of stationary type IV bursts if they are associated with high‐density, unidirectional‐field structures unfavorable for particle acceleration; and (3) the lack of strong evidence of magnetic reconnection in the coronagraph data following CMEs.
Observations from Millstone Hill during the geomagnetic disturbances of March and April 1990
Authors: Buonsanto M. J. , Foster J. C. , Sipler D. P.
Published: February 2008

The incoherent scatter radars at Millstone Hill operated continuously during the periods March 16–23 and April 6–12, 1990, providing observations of large‐scale ionospheric structure and dynamics over a large portion of eastern North America. Major geomagnetic storms occurred during each of these periods, with deep nighttime ionospheric troughs and large magnetospheric convection electric fields observed equatorward of Millstone. The Millstone observations provide a comprehensive data set detailing storm‐induced ionospheric effects over a 35° span of latitude during both of these intervals. At the latitude of Millstone the ionospheric peak height hmF2 rose above 600 km in the trough on March 22 and 23 and reached ≈500 km at night on April 11 and 12. Increased recombination, apparently due to the strong electric fields, the temperature dependent recombination rate coefficient, and neutral composition changes, greatly depleted the F2 region over a wide latitude range during the day on April 10, 1990. This resulted in an ionosphere dominated by molecular ions, with ionospheric peak heights below 200 km on this day. A number of frictional heating events during the disturbed periods are seen from comparison of ion temperature and velocity measurements. The most intense event took place near 1200 UT (≈0715 LMT) on April 10, 1990, when Kp reached 8. At 0110 UT on March 21, line of sight ion velocities in excess of 500 m s−1 were observed at the extreme southern limit of the Millstone steerable radar's field of view (40° apex magnetic latitude at an altitude of 700 km). These could be due to penetration of magnetospheric electric fields or electric fields associated with ring current shielding in the storm‐time outer plasmasphere. About an hour later, ion outflow was observed just equatorward of Millstone. This is most likely due to heating from a latitudinally confined region of intense westward convection. Neutral meridional winds above Millstone were obtained by three different techniques employing radar and Fabry‐Perot measurements. The latitude variation of the winds was also estimated from radar measurements of hmF2 and electric fields using the servo model method. Strong equatorward nighttime neutral wind surges were found during both the March and April disturbances, which reached the equatorward limit of the observations at F peak heights.
Isolation of major Venus thermospheric cooling mechanism and implications for Earth and Mars
Authors: Keating Gerald M. , Bougher Stephen W.
Published: February 2008

A study of Pioneer Venus orbiter atmospheric drag (OAD) data establishes that there is a weak but clear response of the atmosphere to short‐term variations associated with the 27‐day rotation of the Sun. Thermospheric temperature residuals relative to the mean diurnal variation have been found to be associated with variations in solar activity after correction for the Earth‐Sun‐Venus angle. All of the dayside OAD thermospheric data are found to exhibit this characteristic with peak‐to‐peak temperature variations of approximately 25 K. These results are compared with National Center for Atmospheric Research theoretical models of the Venus thermosphere that include processes not in local thermodynamic equilibrium. It is found that the low amplitude of the 27‐day oscillations, combined with the cooling necessary for observed 300‐K dayside temperatures, may not be explained by eddy conduction cooling and may only be explained by very strong 15‐μm cooling. This presumably results from collisional excitation of the (ν2 = 1) CO2 bending mode by atomic oxygen with subsequent radiation at 15 μm. This mechanism is estimated to be 200–1000 times more effective than self‐collisions of CO2 and is even more effective since increases in EUV lead to higher O/CO2 ratios. Isolation of this cooling mechanism, which cools the atmosphere from approximately 700 K to 300 K, was only possible through a study of 27‐day variations, since the magnitude of 11‐year atmospheric variations is very uncertain because of the absence of in situ data near solar minimum. Implications concerning the decrease in amplitude of the 11‐year Venus thermospheric variability due to the 15‐μm thermostat effect are also examined in this paper. The unexpectedly strong radiative cooling mechanism isolated here should also result in substantial cooling of the Earth's mesosphere and thermosphere as CO2 doubles in the next century with even stronger effects than predicted by Roble and Dickinson (1989). The Mars atmosphere should not be as strongly affected by this thermostatic mechanism as Venus' because of the lower O/CO2 ratios on Mars, and thus larger 11‐year variations are expected.
Amplification of critical velocity ionization by associative ionization
Authors: Lai Shu T. , Murad Edmond , McNeil William J.
Published: February 2008

Space experiments to verify the critical ionization velocity (CIV) theory have for the most part involved the release of barium in the ionosphere. It is suggested in this paper that associative ionization (AI) reactions involving barium (i.e., Ba + O → BaO+ + e) may occur in these experiments, and since the products would be indistinguishable from those generated by electron impact ionization (CIV), it is likely that AI‐generated ions may be mistaken for those generated by CIV. The electrons formed in AI and by electron impact may be energized by the ion beams, generated by both CIV and AI. The energized electrons may ionize, enhancing the CIV process. In this way, the AI‐CIV double process could amplify CIV. The amplification is especially important for sustaining CIV when the CIV energy budget is tight or when the ionization rate due to CIV alone is too low to satisfy Townsend's criterion. The result may have significant implications for the results observed in recent CIV space experiments using barium jets. The AI‐CIV double process may offer a plausible explanation for the observed long distance of Ba+ ionization. Any other ion beam formation mechanism, such as charge exchange, stripping and reflected ambient ions, can also amplify CIV in a manner similar to AI, although comparison of the relative magnitudes of several of these processes indicates that AI may be the most significant. We suggest an AI‐assisted experiment in which a samarium beam could undergo AI, promoting CIV in an accompanying xenon beam.
Time‐dependent localized reconnection of skewed magnetic fields
Authors: Semenov Vladimir S. , Kubyshkin Igor V. , Lebedeva Valentina V. , Sidneva Marina V. , Biernat Helfried K. , Heyn Martin F. , Besser Bruno P. , Rijnbeek Richard P.
Published: February 2008

We describe and analyze a model for time‐varying, localized reconnection in a current sheet with skewed magnetic field orientations on opposite sides. As in Petschek's description, disruption is initiated in a localized part of the current sheet known as the diffusion region, and the disturbances are subsequently propagated into the system at large through magnetohydrodynamic (MHD) waves. The MHD waves therefore play the dominant role in energy conversion, and collectively they form an outflow for plasma streaming toward the current sheet and a field reversal region joining magnetic field lines from opposite sides. We restrict the analysis to an incompressible plasma, in which case the Alfvén wave and the slow shock merge to form shocks bounding the field reversal or outflow region, and to the case of weak reconnection, which implies that the reconnection electric field is much smaller than the product of the characteristic values of the external field strength and Alfvén speed. It is then possible to perform a perturbation analysis of the MHD equations which govern the plasma and field behavior. The analysis can be formulated as a mixture of three well‐known problems. The problem of determining the appropriate combination of MHD waves corresponds to the Riemann problem, which also specifies the tangential field and flow components in the field reversal region. These results, it is important to note, are not sensitive to variations in the reconnection rate. Reconnection also acts as a source of surface waves, and their analysis determines the behavior of the perpendicular field and flow components and the shape of the shocks. Lastly, the field reversal region can be considered as a thin boundary layer in our treatment, and the external disturbances can therefore be solved in a way similar to the flow around a thin aerofoil. The model presented here can be applied to the Earth's magnetopause, where reconnection is considered to be the dominant process coupling the solar wind and the magnetosphere. In particular, the results can be used to interpret different manifestations of reconnection such as accelerated plasma flows along the magnetopause and flux transfer events.
On type 3 auroral VHF coherent radar backscatter
Authors: Haldoupis C. , Schlegel K. , Nielsen E.
Published: February 2008

Type 3 echoes of radar aurora relate to coherent backscatter signals with narrow power spectra peaking below ion acoustic Doppler speeds in the 100‐ to 300‐m/s range and preferentially close to 200 m/s. The first observations, all made at 50 MHz, suggested that these echoes are due to electrostatic ion cyclotron (EIC) plasma waves generated at altitudes well above the electrojet where ions become magnetized. Most recent evidence, however, based on multifrequency studies and altitude interferometry results in conjunction with theoretical difficulties, placed the EIC interpretation in jeopardy; thus today the type 3 echo question remains unresolved. In the present paper, after a brief review of past studies and an update of the problem, we present new evidence of type 3 auroral scatter obtained at 140 MHz from cross‐beam radar measurements made at optimum magnetic aspect angles. The observations do not favor the EIC mechanism, and they show that type 3 waves resemble in several ways the type 1 waves generated by the two‐stream instability, e.g., they propagate in the same direction inside the same azimuthal sector at small angles to the electrojet E × B flow. This suggests a direct role for the ambient electric field in generating type 3 plasma waves as well. Further, type 3 and type 1 echoes can occur simultaneously over extended ionospheric areas and thus possibly originate at different altitudes. Prompted by the present results, we consider the option that type 3 echoes are simply due to type 1 waves originating in narrow sporadic Es layers located at lower electrojet altitudes, and we investigate the effect of both an enhanced mean ionic mass (because of the predominance of heavy metallic ions) and a large destabilizing electron density gradient component on lowering the phase velocity well below typical ion acoustic speed values. Despite limitations and some controversy, approximate numerical estimates show this option might be feasible and therefore ought to be studied further.
Effects of magnetospheric electrons on polar plasma outflow: A semikinetic model
Authors: Ho C. W. , Horwitz J. L. , Singh N. , Wilson G. R. , Moore T. E.
Published: February 2008

A semikinetic model for the study of the effect of hot magnetospheric electrons on polar plasma outflow is developed. The model is based on a hybrid particle‐in‐cell approach which treats the ions (H+ and O+) as adiabatic, parallel‐drifting gyro‐centers injected as the upgoing portions of drifting bi‐Maxwellian distributions at 1.6 RE, while the electrons are treated as a massless neutralizing fluid. As a first approach to understanding the effects of hot magnetospheric electrons on the outflow, we consider electron temperature profiles which increase from low temperatures at the ionospheric levels to high temperatures at high altitudes. The electric field is determined by both the electron temperature and its gradient. The electric field produced by the electron temperature alone generally accelerates ions outward while that associated with the electron temperature gradient increases the potential barrier and inhibits the outflow. For typical polar wind stream conditions, electron temperature gradients exceeding 3×104 K/RE cause reflection of much of the ion stream back downward toward the ionosphere. Under these circumstances the H+ outflow forms two counterstreaming beams at altitudes below the reflecting potential barrier and a cooler and faster transmitted beam at high altitudes. Above the potential barrier, the O+ density decreases by 7 orders of magnitude for a very large electron temperature gradient. For the case of an electron temperature profile established by thermal conduction the results show inhibition of polar plasma outflow very near the lower boundary, but continuous acceleration of the escaping ions along most of the flux tube. H+ shows a continuous decrease in net outward flux from 3×107 ions cm−2 s−1 when the electron temperature is isothermal at 4400 K to 1.5×107 ions cm−2 s−1 when the upper boundary temperature is increased to 1×106 K. On the other hand, the flux of O+ exhibits a rise and fall with upper boundary electron temperature with a peak of 1.1×107 ions cm−2 s−1 when the upper boundary electron temperature is approximately equal to 2×105 K.
Deep crustal reflection profiling offshore southern central California
Authors: Meltzer Anne S. , Levander Alan R.
Published: February 2008

Deep crustal reflection profiling offshore southern central California shows that the offshore Santa Maria Basin and adjacent shelf and slope are underlain by oceanic crust which underthrusts the margin. On common‐midpoint (CMP) sections the top of the oceanic crust appears as a strong coherent reflection occurring at 6 s two‐way time (twt). The reflection can be traced continuously more than 50 km from the deep ocean basin eastward beneath the continental shelf. Depth conversion places the top of oceanic crust at 6–16 km depth with a dip of 8° toward the coast. Beneath the Santa Lucia High the crustal reflection breaks up into several east dipping segments. Above the oceanic crust an accretionary wedge increases in thickness from 3 km at the base of the slope to over 12 km beneath the Santa Lucia High. The Neogene sedimentary section above the accretionary prism exhibits relatively little deformation, implying that most of the accretion occurred during pre‐Neogene convergence between the North American plate and oceanic plates located to the west. The offshore Santa Maria Basin exhibits at least two periods of Neogene deformation: an early extensional phase, accompanied by strike‐slip faulting, occurred in the lower to middle Miocene followed by compression and basin inversion in the upper Miocene‐lower Pliocene. The development of structures in the basin began in the northwest and is progressively younger to the southeast. Faults identified in the offshore sedimentary column offset acoustic basement but are not imaged in the Franciscan basement material below 2–3 s twt and do not extend to, or offset, the deep reflections. We suggest that the oceanic crust served as a detachment surface above which the shallow deformation occurred. Neogene sediments in the offshore basins show only 1–3 km of shortening. This implies that shortening, due to Neogene oblique plate motion between the Pacific and North American plates, extends east of the offshore Santa Maria Basin and is distributed across the entire transform margin.
Seismic strain rate and deep slab deformation in Tonga
Authors: Fischer Karen M. , Jordan Thomas H.
Published: February 2008

Seismic strain rates estimated from volume‐averaged moment tensor data are combined with the strike, dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep Tonga slab. Tonga strain rate orientation tensors, constructed from the Harvard Centroid Moment‐tensor catalog, are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650‐km discontinuity. Those which sample lithospheric volumes from 18°S to 25°S at 500–700 km depth indicate that the axis of maximum shortening lies within 15° of the slab's downdip azimuth and plunge. The discrepancy between shortening and slab dip may be larger in northernmost Tonga (17°S–18°S) where slab morphology is less well constrained, but overall the slab is compressing roughly parallel to its trajectory. Tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab. The proportion of along‐arc strain rate decreases from northern to southern Tonga. The scalar rate of seismic strain peaks at certain latitudes, but its values averaged over northern and central Tonga and southern Tonga are roughly equal. Strain rate data may be related (nonuniquely) to constraints on slab structure and finite strain inferred from seismicity and travel time data. Assuming that aseismic and seismic flow have the same geometry, and that the total rate of strain is constant along the arc, the degree of cross‐strike advective thickening increases to the south, while along‐arc strain decreases. If along‐arc strain rate exclusively produces southward shear of the deeper lithosphere, and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the Louisville Ridge, thickening of at least 50% is required in central Tonga. This lower bound improves the resolution of minimum slab width provided by travel time observations. If 50% thickening occurs, an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab. Larger thickening factors or flow in the outer regions of the slab require aseismic deformation. Assuming that the observed travel time residuals are the result of a single, coherent slab, a synthesis of seismicity, strain rate and travel time data indicate a slab structure in northern Tonga which curves sharply to the west at the base of the upper mantle, but which strikes roughly to the north at shallower depths and in the lower mantle. Such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m.y. and the tectonic history of the region.
Late Cretaceous sea level from a paleoshoreline
Authors: McDonough Katie Joe , Cross Timothy A.
Published: February 2008

The contemporary elevation of an ancient strandline provides a measure of eustasy at a single time in the geologic past. If the elevation of a paleoshoreline has not changed since deposition (or if it has changed by a known amount), then the measured (or corrected) elevation equals the sea level at that time. The ideal location for measuring an ancient sea level is a paleoshoreline deposited high on the shoulder of a stable craton where postdepositional elevation changes through tectonic movement, lithospheric compensation to loads, and sediment compaction are minimized. The contemporary elevation of a Late Cenomanian (≈93 Ma) shoreline was determined at five localities along the tectonically stable, eastern margin of the Cretaceous Western Interior Seaway, North America. This shoreline, represented by marine‐to‐nonmarine facies transitions in strata of the Greenhorn sequence (UZA‐2 cycle of Haq et al. (1987)), was identified from outcrop and borehole data. Biostratigraphic zonations constrained the geologic age at each locality. Sequence stratigraphic correlations, based on identifying discrete progradational units and the surfaces that separate them, were used to refine age correlations to better than 100 kyr between localities. A single Cenomanian shoreline was correlated within a single progradational unit, and its elevation was determined at five localities. This paleostrandline occurs 265–286 m above present‐day sea level, at an average elevation of 276 m. Isostatic and flexural corrections were applied to remove the effects of postdepositional vertical movement, including sediment compaction by loading, uplift due to erosion, and glacial loading and rebound. Errors inherent in each measurement and each correction were estimated. Corrections and their cumulative error estimates yield a Late Cenomanian elevation of 269±87 m above present sea level. The corrected elevation approximates sea level at 93 Ma and provides a measure of Late Cenomanian eustasy prior to the Early Turonian highstand. Establishing the absolute value for eustasy at a single point in geologic time provides a frame of reference for calibrating relative sea level curves, as well as constraining the magnitudes of tectonic subsidence, sediment flux, and othervariables that controlled water depth and relative sea level.
Removal of barometric pressure response from water level data
Authors: Quilty Eddie G. , Roeloffs Evelyn A.
Published: February 2008

The removal from water level data fluctuations due to barometric forcing at subtidal frequencies is studied using transfer functions. In a poorly confined well‐aquifer system these transfer functions are, in general, dependent upon air diffusivity through the unsaturated zone and diffusivity from the water table into the unsaturated zone. The stability in time of estimated transfer functions from selected water well sites near Parkfield, California, is examined, and an average transfer function is computed for each site that demonstrates reasonable stability over time. Two of these transfer functions that show marked frequency dependence in the subtidal frequency band are used to filter water level data in this frequency range. A comparison is made with residuals after subtracting the best fitting multiple of band‐passed barometric pressure from the water level data, and transfer function filtering is shown to be superior. Comparing the estimated transfer functions with previously derived theoretical ones shows that in one case the well‐aquifer system can be reasonably well modeled by assuming that the effects of air and water diffusivity are nonnegligible, while the remaining frequency‐dependent transfer function can be better modeled by assuming minimal air diffusivity through the unsaturated zone.
SeaMARC II Survey of a propagating limb of a large nontransform offset near 29°S along the fastest spreading East Pacific Rise segment
Authors: Klaus Adam , Icay Wilfredo , Naar David , Hey R. N.
Published: February 2008

SeaMARC II side scan and bathymetric data, combined with magnetic anomaly and Sea Beam bathymetric data, define a large (∼90–120 km) nontransform offset of the East Pacific Rise (EPR) between the Easter and Juan Fernandez microplates near 29°S, where one or more transforms had been suggested by the seismicity pattern. On the basis of morphology, structural, and tectonic features, three major regions are delineated: an eastern spreading axis, a western spreading axis, and a central transfer zone. These two spreading axes overlap by at least 35 km and possibly as much as 100 km. The eastern axis is characterized by an axial ridge that narrows and deepens toward the tip (28°38′S, 112°08′W). Approaching the tip, the spreading axis azimuth changes from approximately N5°E to N25°W, and axial magnetic anomaly values increase to over 700 nT 37 km south of the rift tip. A pair of deep (>600 m) bathymetric troughs on either side of the ridge axis, trending N11°W and N44°W, converge on the ridge tip and define the outermost boundaries of ridge‐parallel structures. These structures are interpreted as pseudofaults resulting from northward rift propagation. The western spreading axis is approximately 2400 m deep and corresponds to a young volcanic area. The central lithospheric transfer zone, west of the inner pseudofault of the eastern spreading axis, is characterized by oblique abyssal hill and magnetic fabric interpreted to result from shear between overlapping spreading centers and a dispersed pattern of teleseismic earthquake epicenters. The eastern spreading axis is propagating rapidly northward at up to 265 km/m.y, based on a 158 km/m.y spreading rate and the 33° angle between the pseudofaults. However, complex patterns of seafloor fabric, which differ from that predicted by the simple propagating rift model, suggest that multiple propagation events may have occurred. Furthermore, southward propagation of the western limb may be dominant in the long term. The absence of transform faults along this superfast spreading EPR segment documents that transforms do not presently exist at such superfast spreading rates.
Stress propagation and strain rate in subducted lithosphere
Authors: Willemann Raymond J.
Published: February 2008

A stress diffusion model is adapted for application to subducted lithosphere by including a gravitational body force parallel to the direction of diffusion. In the model a sequence of step function boundary displacements represents a history of interplate thrust earthquakes. The response of subducted lithosphere to these displacements is approximated by considering the transient response of the slab to be purely elastic and the response of the surrounding mantle to be purely viscous. The model indicates that following even the largest interplate thrusts, the tension due to the weight of the slab is only partially relaxed and the preseismic stress orientation is maintained outside of the immediate vicinity of the interplate thrust. In contrast, others have previously found that the focal mechanisms of intermediate depth earthquakes sometimes change from T downdip to P downdip orientation following great interplate thrusts. Thus some focal mechanisms in subducted lithosphere may have an orientation in conflict with the mean stress in the slab. This apparent conflict could result from seismic failure in zones of heterogeneous stress or strength. The stress diffusion model shows that the orientations of mean stress and mean strain rate in subducted lithosphere can differ, at least transiently. If strength is heterogeneous, then focal mechanisms will be consistent with mean strain rate rather than mean stress.
On the period and damping of polar motion
Authors: Groten E. , Lenhardt H. , Molodensky S. M.
Published: February 2008

The theory of the Chandler wobble is an old and rather complicated branch of mathematical geophysics which includes both the methods of the elastic and hydrodynamic equations solutions, describing the reaction of mantle, liquid core and ocean to the variable centripetal force. This theory has recently attracted increased attention in view of (1) the substantial improvement of geodetic measurements (such as very long base line interferometry, satellite measurements, etc.) requires a corresponding refinement of the theory and (2) the last achievement in modeling the Earth's inner structure gave the possibility to calculate the period and damping of Chandler wobble with much higher accuracy. The comparison of these theoretical results with the results of measurements can isolate mantle anelasticity effects, thus permitting the determination of the Earth's anelastic properties at extremely low frequencies. Obviously, information of such kind is very important for modeling of many geodynamical processes. In this paper the influence of the mantle's anelasticity, the Earth's liquid core and oceanic pole tides on the Chandler wobble is considered. In section 2 different approaches to determine the effects of mantle anelasticity are compared with each other. It is shown that the results obtained by use of complex Love Numbers and by consideration of energy dissipation are in a good agreement; numerical results are presented. The consideration of the liquid core effects (in section 3) is based on the work by Molodensky and Zharkov (1982) an approach which takes into account the torque acting between liquid core and deformable mantle instead of the consideration of momentum equations for the system of mantle plus core. This approach gives the possibility to use hydrostatical approximation and to avoid the essential difficulties which are connected with the dynamical equations in the liquid core. The errors of hydrostatical approximation are estimated and are found to be small. The influence of the oceanic pole tides, both in statical and dynamical approximations, are considered in section 4. The attention is drawn to the fact that the solutions of Laplace's tidal equations at low frequencies significantly depend on terms of the order of the ratio of the frequency σ to the angular velocity of the Earth's rotation ω. For the case σ=0 there is an infinite set of solutions, satisfying the fixed boundary conditions; the solutions become unstable in the limit σ/ω→0 hence even very small errors of numerical integration significantly affect the final results. From our point of view, this circumstance can explain some disagreements, arising in the dynamical theory of pole tides; it is necessary to take it into account in the numerical modeling.
Application of Cantor's Method for fractal analysis of fractures in the Toyoha Mine, Hokkaido, Japan
Authors: Merceron Thierry , Velde Bruce
Published: February 2008

Cantor's method has been used for fractal analysis of fractures in the Toyoha mine, Hokkaido, Japan. The analyses were performed on a two‐dimensional area ranging from 30 cm2 to 220 km2. Cantor's analysis predicts the probability of fracture occurrence in rocks in a given distance. The probability of finding a fractured zone of length x is given by P = x−D where D is the fractal dimension. In the Toyoha mine the samples collected in Sugo and Izumo veins are characterized by D values of 0.25 and 0.10–0.24, respectively. The distribution of fractures in galleries (200 m long) is very similar (D = 0.17). At a kilometer scale the disposition of veins varies according to the directions of observation: D = 0.20 (E‐W) and 0.25–0.28 (NW‐SE). At a larger scale (Landsat map) the fault distribution appears very similar in all directions (0.22 < D < 0.25). In the vicinity of the Toyoha mine the fracture disposition is fractal at all scales of observation. Despite a scale factor that can vary by 106 the distribution of fractures remains fractal with similar fractal numbers (average D = 0.25). Moreover, D values gradually change according to the distance from the Toyoha mine, suggesting that the type of rupture and consequently the nature of stress are different far from the Toyoha mine.
On the measurement of body wave dispersion
Authors: Correig Antoni M.
Published: February 2008

A method is devised to measure body wave dispersion in terms of the arrival time of a narrow band‐pass filtered signal. The arrival time is defined as the expectation of the arrival time of the wave front minus half the duration of the first pulse. Body wave dispersion is related to an absorption band, and its transfer function is found. By interpreting the absorption band as a filter and identifying the dispersion as the (frequency dependent) group delay of the filter the measured dispersion is interpreted as due to a cascade of filters composed of the following elements: intrinsic attenuation, source time function, and finiteness of the fault.
Review of the Earth tidal models and contribution of Earth tides in geodynamics
Authors: Dehant V.
Published: February 2008

The attraction of the Moon and the Sun on the Earth induces deformations of the Earth body. At the surface of the Earth, these effects are described using tidal parameters which are observed by tidal instruments and estimated using an Earth model. These tidal parameters are indeed computed from a linear combination of the displacement field, the stress field, and the potential due to the mass readjustment associated with the tides. The theoretical tidal parameter values depend crucially on the model used for the interior of the Earth. The system of equations used in the computation contains a stress‐strain relationship. This equation is the only one which changes when considering mantle inelasticity instead of elasticity. For an inelastic mantle, the stress‐strain relationship is a Hooke law with complex and frequency dependent rheological parameters. The numerical values of these rheological parameters depend on the hypotheses formulated on the Earth body. Even after the introduction of mantle inelasticity in the equations, there are still some differences between the theory and the observations. By increasing the complexity of the model, scientists want to decrease the difference between the theory and the observations. This comparison brings answers to very important points in geophysics. In particular, the resonances at the free core nutation frequency and at the free inner core nutation frequency in the tidal response give the values of these Earth normal mode periods and the comparison between the observed and the computed period constrains the core flattening and the jump in the density at the inner core‐outer core boundary, respectively.
Diffusional isotopic exchange across an interlayered marble‐schist sequence with an application to Tinos, Cyclades, Greece
Authors: Ganor Jiwchar , Matthews Alan , Paldor Nathan
Published: February 2008

Marble‐schist contacts are frequently characterized by fine‐scale interlayering which prevents the application of semi‐infinite layer width assumptions to advection/diffusion models of isotope profiles. Using a finite layer width boundary condition, we show that the minimum width (w) of a layer required in order that diffusional exchange at both boundaries will not mutually interfere is given by the relation w>k (Dt)1/2, where D is the effective diffusion coefficient, t is the duration of the diffusion and k is a constant determined by the initial compositional step and the noise (i.e., natural homogeneity and/or analytical accuracy). When this condition is not satisfied, a solution of the diffusion equation for a multilayer transition zone containing several schist and marble layers of different, finite, widths is applicable. Application of this refinement of the diffusion model to an oxygen isotope profile from Tinos (Cyclades) Greece gives 10−4≤Dt≤10−2 m2. This Dt range, and a Dt value of 10−1 m2 previously determined for Sifnos, indicates that rocks undergoing the Oligocene‐Miocene greenschist facies overprint of Eocene high‐pressure metamorphic rocks in the Cyclades were characterized by extremely low porosities. Permeability perpendicular to marble‐schist contacts was sufficiently low to prevent advection into the marbles; correspondingly, it is suggested that the layer‐parallel schistosity permitted infiltration of the fluids required for the hydration reactions of the metamorphic overprint.
The statistical geometry of amorphous silica at lower mantle pressures: Implications for melting slopes of silicates and anharmonicity
Authors: Rustad James R. , Yuen David A. , Spera Frank J.
Published: February 2008

Coordination changes occurring in amorphous silica have been studied by analysis of Voronoi and Delaunay tessellations of structures generated by molecular dynamics simulations up to pressures characteristic of the core‐mantle boundary. These calculations predict the coexistence of a remarkable variety of silicon coordination environments at pressures above the mantle transition zone. The postoctahedral compression paths identified through the Voronoi and Delaunay constructions and the increase in geometrical entropy that would be expected to be associated with the loss of short‐range order may explain the persistence of the low melting slopes determined for many silicates at pressures from 30 to 60 GPa. The simulated glass transforms from fourfold to sixfold coordination asymmetrically, with a significant number passing through a five‐coordinated state. Paths of structural change producing sevenfold‐ and eightfold‐coordinated silicon atoms do not necessarily involve coordination changes in unit increments; for example, sixfold coordination states tend to change to eightfold coordination states without passing through the sevenfold state. The large assortment of coordination types precludes a simple, comprehensive model for compression in amorphous silica at high pressures. Anharmonic properties, such as thermal expansivity, do not appear to be affected by changes in geometrical (Voronoi) coordination in the same way that they are dependent on physical coordination defined in terms of average bond lengths.