bluuebunny/crossref_metadata_2025 huggingface dataset:
Modulation of Jovian and galactic electrons in the heliosphere: 2. Radial transport of a few MeV electrons
Authors: Ferreira S. E. S. , Potgieter M. S. , Burger R. A. , Heber B. , Fichtner H. , Lopate C.
Published: February 2004

Previously, the latitudinal transport of ∼7 MeV electrons was studied by comparing the results of a newly developed three‐dimensional model, based on the numerical solution of Parker's transport equation including the Jovian source, with Ulysses observations. Here, the radial transport of ∼16 MeV electrons is studied by comparing the model results with the −16 MeV electron intensities observed by Pioneer 10 up to ∼70 AU. Electron modulation responds directly to the energy dependence of the diffusion coefficients below ∼500 MeV, in contrast to protons, which experience large adiabatic energy losses below this energy, and in addition, low‐energy electrons do not experience significant drifts. The modulation of low‐energy electrons is therefore a handy tool to find a diffusion tensor suitable to the heliospheric modulation of electrons. It is illustrated that the computed electron intensities are sensitive to the radial dependence of the diffusion coefficients in the inner heliosphere and that compatibility between the model and observations gives an indication as to the radial dependence of the diffusion coefficients. The effects of different perpendicular diffusion coefficients in the radial direction are also shown, and an upper limit for the value of this coefficient is proposed. Finally, the relative contributions of the Jovian and galactic electrons to the total electron intensity are shown along the Pioneer 10 trajectory, illustrating that the Jovian electrons dominate the total electron intensity at these low energies in the inner equatorial regions only up to ∼9 AU. From 15 AU outward, the Jovian contribution becomes less than 20%, decreasing rapidly as a function of increasing distance.
Energy partition between solar wind protons and pickup ions in the distant heliosphere: A three‐fluid approach
Authors: Wang C. , Richardson J. D.
Published: February 2004

The solar wind in the outer heliosphere is fundamentally different from that in the inner heliosphere, with the influence from the local interstellar medium becoming significant. Observational evidence of pickup ions has accumulated as Voyager 2 continues its journey to the interstellar medium. Specifically, the nonadiabatic solar wind proton temperature profile, the slowdown of the solar wind, and an increased pressure associated with pickup ions are the manifestations of pickup ions in the outer heliosphere. In order to understand the solar wind in the outer heliosphere, we extend the three‐fluid model first proposed by Isenberg [1986] to include the energy sharing between the solar wind protons and pickup ions. For simplicity, we introduce a parameter, the energy partition ratio ε, to represent the division of the total energy provided by the pickup process between the solar wind protons and pickup ions. We find that only a small percentage of this total thermal energy is needed to heat the solar wind proton to produce the observed temperature profile. As expected, the higher the interstellar neutral hydrogen density, the smaller the percentage. The energy partition ratio has little effect on the slowdown of the solar wind and the pickup ion density distribution in the outer heliosphere, which is primarily controlled by the interstellar neutral hydrogen density. As far as the slowdown is concerned, a neutral hydrogen density of nH∞ = 0.08 cm−3 at the termination shock gives the best match to the observations [Wang et al., 2000]. In this case, the pickup ion density at 40 AU is also of the order of that estimated by Burlaga et al. [1996], and the energy partition ratio ε = 0.05 gives the best fit to the observed proton temperature profile.
Seasonal variations of the ionospheric total electron content in Asian equatorial anomaly regions
Authors: Tsai Ho‐Fang , Liu Jann‐Yenq , Tsai Wei‐Hsiung , Liu Chao‐Han , Tseng Ching‐Liang , Wu Chin‐Chun
Published: February 2004

The ionospheric total electron contents (TEC) in both northern and southern equatorial anomaly regions are examined by using the Global Positioning System (GPS) in Asian area. The TEC contour charts obtained at YMSM (25.2°N, 121.6°E; 14.0°N geomagnetic) and DGAR (7.3°S, 72.4°E; 16.2°S geomagnetic) stations in 1997, solar minimum, are investigated. It is found that the ionospheric crests manifest remarkable seasonal variations. The TEC values on both northern and southern equatorial anomaly crests yield their maximum values during the vernal and autumnal months, but the winter anomaly does not appear in the southern region. Results show that both crests are fully developed around midday in winter, postnoon in equinoxes and late afternoon in summer, and the two crests move significantly equatorward in winter but slightly poleward in summer and autumn. These phenomena can be fully explained by a combined theory of the transequatorial netural wind, the subsolar point, and the auroral equatorward wind.
Modeling negative ionospheric storm effects caused by thermospheric disturbances observed in satellite UV images
Authors: Daniell Robert E. , Strickland Douglas J.
Published: February 2004

The ionospheric behavior over Podkamennaya, Russia, during the geomagnetic storm of October 14, 1981, is modeled subject to the constraints provided by ƒoF2 measurements and by the thermospheric O/N2 ratio deduced from a DE 1 FUV image taken at 0540 UT. The (nonunique) diurnal variation of O/N2 deduced from the model is consistent with the thermospheric composition disturbance zone observed in the DE 1 image having moved over Podkamennaya during the early morning hours, producing a reduction of O/N2 (referred to a column density of N2 of 1017 cm−2) to a value of less than 0.2 before recovering to a value of 0.6 during the afternoon hours. Although the data are insufficient to provide a unique solution, the results demonstrate the utility of satellite‐based FUV images in the study of thermospheric and ionospheric disturbances produced by geomagnetic storms.
Altitude determination of ultraviolet measurements made by the Student Nitric Oxide Explorer
Authors: Merkel A. W. , Barth C. A. , Bailey S. M.
Published: February 2004

The spinning motion of the Student Nitric Oxide Explorer spacecraft allows for the measurement of limb profiles of atmospheric radiation. In order to interpret the observations an accurate altitude reading is necessary. Included in the limb profile is Rayleigh‐scattered solar radiation that can be used to determine altitude. The Rayleigh profile increases exponentially with decreasing altitude until the absorption due to ozone dominates the shape, producing a peak near 53 km. We use the Rayleigh scattering peak to register the measured profile in altitude for each ultraviolet spectrometer observation. A single‐scattering radiance model is developed and used to provide an altitude‐dependent shape of the Rayleigh profile. The altitude registration is done by correlating the data to the model for each measured profile. By systematically shifting the measured profile in small increments and fitting the shifted profile to the model the quality of the fit is determined. The shifted profile that best correlates with the model profile determines the correct shift in altitude to 1.5 km precision.
Six days of thermospheric‐ionospheric weather over the Northern Hemisphere in late September 1981
Authors: Strickland D. J. , Craven J. D. , Daniell R. E.
Published: February 2004

Thermospheric and ionospheric behavior over a 6‐day period beginning on September 24, 1981 (day 267), is discussed using DE 1 far ultraviolet dayglow data and ground‐based ionosonde data. The DE 1 data are processed to produce images of O/N2, the column density of O relative to N2. Ionospheric data are in the form of Nmax, the maximum electron density that is equivalent to NmF2 provided an F2 layer is present. Day‐to‐day variations in these parameters and their relationship to one another are examined in three geographical regions: Siberia/Japan, Europe, and North America. The 6‐day period begins and ends with undisturbed days and includes a 2‐day geomagnetic storm period with onset on day 269. Several disturbances in the form of reduced O/N2 are observed, with the most severe occurring during the storm. For those ionosonde sites located within the disturbed regions, negative ionospheric effects are observed similar to those reported by Strickland et al. [2001] for a different storm period. In Siberia, 4 consecutive days of negative effects are observed that coincide with reductions in O/N2. Middle‐ to low‐latitude enhancements in O/N2 are also observed that are more global and less structured than regions of reduced O/N2. Percentage changes in the enhancement from quiet time values are typically less than ∼20% in contrast to much larger changes in well‐developed regions of reduced O/N2. Several instances of positive ionospheric effects are observed that are in phase with coincident O/N2 enhancements, but the coupling is less convincing than that between negative ionospheric effects and O/N2 reductions. It is, nevertheless, seen often enough in this work to argue that compositional effects in addition to dynamical effects must be considered in addressing the causes of long‐duration positive ionospheric storm behavior.
Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes
Authors: Basu Sunanda , Basu Santimay , Valladares C. E. , Yeh H.‐C. , Su S.‐Y. , MacKenzie E. , Sultan P. J. , Aarons J. , Rich F. J. , Doherty P. , Groves K. M. , Bullett T. W.
Published: February 2004

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)‐based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well‐correlated in the past with rapid changes in the well‐known Dst index (or through its recently available high resolution 1‐min counterpart the SYM‐H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250‐MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground‐based TEC observations with a GPS network and space‐based in situ density and electric field measurements using the Republic of China Satellite‐1 (ROCSAT‐I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.
Generation of atmospheric gravity waves associated with auroral activity in the polar F region
Authors: Oyama S. , Ishii M. , Murayama Y. , Shinagawa H. , Buchert S. C. , Fujii R. , Kofman W.
Published: February 2004

Relations between auroral activities and the generation of neutral‐wind oscillations in the polar F region (150–300 km) were investigated using data from the European Incoherent Scatter (EISCAT) radar, the all‐sky auroral camera, and the IMAGE (International Monitor for Auroral Geomagnetic Effects) magnetograms. We dealt with two cases: observations on March 1, 1995 (case 1), and on March 29, 1995 (case 2). For both cases the field‐aligned component of the neutral‐wind velocity estimated from EISCAT radar data had dominant oscillation periods of 20–30 min, which are longer than the typical Brunt‐Väisälä period in the polar F region (≃13 min). The observed oscillations showed the downward propagation of the phase with time. These properties on the oscillation period and the phase are general ones of atmospheric gravity waves (AGWs). For case 1 the all‐sky auroral images obtained at Kilpisjärvi showed the auroral arc extending in an almost zonal direction near a distance estimated using wave parameters derived from the equation of the dispersion relation for AGWs applicable to the observed oscillations. This suggested that the auroral arc appeared to be the effective generator of the observed oscillations. The comparison of observed phase lines with predicted ones using models by Francis [1974] and Kato et al. [1977] showed agreements between the two for both cases. The comparison suggests that effective parameters of the wave source in characterizing neutral‐wind oscillations would be the horizontal distribution of the wave source and the distance between the observing point and the source region. It was concluded that geomagnetic activities on March 1 and 29, 1995, in northern Scandinavia significantly related to the generation of the observed oscillations. The conclusion implies that geomagnetic activities at high latitudes are an important source to generate AGWs, as indicated by previous theoretical studies.
Latitudinal velocity structures up to the solar poles estimated from interplanetary scintillation tomography analysis
Authors: Kojima M. , Fujiki K. , Ohmi T. , Tokumaru M. , Yokobe A. , Hakamada K.
Published: February 2004

The Ulysses spacecraft observed high‐speed wind at high latitudes up to 80° and found that the high‐speed solar wind increased in velocity gradually with latitude and that the velocity had asymmetry between Northern and Southern Hemispheres. We have investigated the velocity increase up to the polar regions for the Carrington rotations of 1908–1915 in the year 1996. For this purpose we have made tomographic analyses of the latitudinal structure of the solar wind speed using interplanetary scintillation data obtained at heliocentric distances of 0.1–0.9 AU and latitudes up to 90°. The tomographic analysis method was modified from its previous version [Kojima et al., 1998] so that it could obtain more reliable solutions with better sensitivity in the polar region than the previous method. The results from the observations in 1996 showed that the velocity increased with latitude and had the N–S asymmetry as observed by Ulysses. These features persisted during the period analyzed. Since the asymmetry was found in rather short period observations of several Carrington rotations and at distances within 0.9 AU, it is caused neither by temporal evolution of the solar wind structures nor by interactions in the solar wind in interplanetary space. These global latitudinal velocity structures agree qualitatively with the magnetic flux expansion factor.
Transient development of magnetohydrodynamic wave mode conversion layers
Authors: De Keyser J. , Čadež V.
Published: February 2004

We study the transmission, reflection, and absorption of ultralow frequency waves at density nonuniformities in low beta plasmas, such as plasma structures in the solar corona, inhomogeneities in the solar wind, and, at times, in the magnetopause. We simulate the time‐dependent interaction of a monochromatic magnetohydrodynamic (MHD) wave with a planar plasma transition layer aligned with the magnetic field. When the incident wave front reaches the initially unperturbed transition layer, a resonant sheet starts to develop within a thin layer where the conditions for resonant MHD wave mode conversion are satisfied. In this sheet the wave amplitude is found to grow exponentially until a saturation level is reached due to dissipative effects. Dissipation controls the thickness of the sheet, the saturation level, and the time needed to reach the saturation regime. The resonantly absorbed energy, however, is essentially independent of the dissipation coefficient. The simulations are carried out in the context of linear resistive low beta magnetohydrodynamics. The simulation results are important for the case of the magnetopause as the enhanced wave amplitudes found inside the transition could promote diffusive mass transport across the layer.
Observations of two complete substorm cycles during the Cassini Earth swing‐by: Cassini magnetometer data in a global context
Authors: Khan H. , Cowley S. W. H. , Kolesnikova E. , Lester M. , Brittnacher M. J. , Hughes T. J. , Hughes W. J. , Kurth W. S. , McComas D. J. , Newitt L. , Owen C. J. , Reeves G. D. , Singer H. J. , Smith C. W. , Southwood D. J. , Watermann J. F.
Published: February 2004

During the Earth swing‐by of the Cassini spacecraft, a worldwide program of data‐gathering was undertaken to define the prevailing interplanetary and geophysical conditions. This included observations of the interplanetary medium, outer magnetosphere, geostationary orbit, UV aurora, geomagnetic disturbance, and ionospheric flow. These data show that during the Cassini outbound passage through the geomagnetic tail the magnetosphere underwent two complete “classic” substorm cycles. The global data are used to set the Cassini data into a context which allows a much fuller interpretation. The pass took place through the dawn tail, where previous Geotail observations indicate that the plasma sheet usually remains ‘stationary’ at expansion phase onset. Reconnection and plasmoid formation are typically dusk and midnight phenomena. The Cassini observations nevertheless show a marked response of the plasma sheet both to the growth phase (thinning), and expansion phase onset (expansion without heating). Subsequent impulsive expansion of the substorm into the Cassini sector, however, resulted in the prompt appearance of reconnection‐related phenomena (earthward flowing plasma and strongly disturbed fields), though the spacecraft footprint remained poleward of the intense UV auroras. Due to continuing strong southward‐directed IMF during the expansion phases, a quasi‐equilibrium appears to have formed between dayside and near‐Earth nightside reconnection for ∼30 min after onset. Auroral zone recovery began about ∼10 min after a northward turn of the interplanetary magnetic field (IMF) to nearer‐zero values in each case. A net closure of open flux then ensued, leading to deflation of the tail lobe field, ‘dipolarization’ of the near‐Earth tail plasma sheet field, simultaneous reduction in the earthward plasma sheet flow and the flow in the nightside ionosphere, and displacement of the ground‐based disturbance to high latitudes.
Magnetic bubbles and kinetic Alfvén waves in the high‐latitude magnetopause boundary
Authors: Stasiewicz K. , Seyler C. E. , Mozer F. S. , Gustafsson G. , Pickett J. , Popielawska B.
Published: February 2004

We present a detailed analysis of magnetic bubbles observed by the Polar satellite in the high‐latitude magnetopause boundary. The bubbles which represent holes or strong depressions (up to 98%) of the ambient magnetic field are filled with heated solar wind plasma elements and observed in the vicinity of strong magnetopause currents and possibly near the reconnection site. We analyze the wave modes in the frequency range 0‐30 Hz at the magnetopause (bubble) layer and conclude that the broadband waves in this frequency range represent most likely spatial turbulence of kinetic Alfvén waves (KAW), Doppler‐shifted to higher frequencies (in the satellite frame) by convective plasma flows. We present also results of a numerical simulation which indicate that the bubbles are produced by a tearing mode reconnection process and the KAW fluctuations are related to the Hall instability created by macroscopic pressure and magnetic field gradients. The observed spatial spectrum of KAW extends from several ion gyroradii (∼ 500 km) down to the electron inertial length (∼ 5 km).
Significance of short‐scale irregularities for radar diagnostics of HF‐driven Langmuir turbulence in the ionosphere
Authors: Mjølhus E. , Helmersen E. , DuBois D. F.
Published: February 2004

In experiments where ground‐based VHF/UHF radars are used to diagnose Langmuir turbulence driven by powerful HF waves transmitted from the ground into the ionosphere, there has been a severe observability aspect: The excited Langmuir wave vector spectrum must contain a component satisfying the actual radar Bragg condition. In order to explain earlier observations, in particular those made by the Arecibo facility, it was proposed long ago that the actual wave number spectra were widened by refraction in small‐scale irregularities, or ducts. This mechanism of improved observability is discussed in a broad approach in this paper. The main contribution is a mathematical derivation of the theory of parametric decay instability in the presence of magnetic‐field‐aligned ducts. This implies that the decay is into discrete duct modes. The threshold is slightly higher and the growth rate is slightly lower than for a corresponding homogeneous medium. Moreover, this tendency is more pronounced for the higher duct modes, which are those that may satisfy conditions for observability. The mathematical derivation is based on the driven and damped Zakharov model with ducts included. The model is run numerically, confirming the approximate decay instability theory and demonstrating improved observability. The numerics also demonstrate higher Langmuir energy density inside the ducts than outside.
Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound
Authors: Brocher Thomas M. , Parsons Tom , Blakely Richard J. , Christensen Nikolas I. , Fisher Michael A. , Wells Ray E.
Published: February 2004

A new three‐dimensional (3‐D) model shows seismic velocities beneath the Puget Lowland to a depth of 11 km. The model is based on a tomographic inversion of nearly one million first‐arrival travel times recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher‐resolution mapping of subsurface structures than previously possible. The model allows us to refine the subsurface geometry of previously proposed faults (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures (Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of the Tacoma basin; we informally refer to this structure here as the Tacoma fault zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low‐velocity basins (Tacoma, Seattle, Everett, and Port Townsend) separated by regions of higher velocity ridges that are coincident with fault‐bounded uplifts of Eocene Crescent Formation basalt and pre‐Tertiary basement. The shapes of the basins and uplifts are similar to those observed in gravity data; gravity anomalies calculated from the 3‐D tomography model are in close agreement with the observed anomalies. In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high‐velocity uplift, locally with a velocity “overhang” that suggests a basin vergent thrust fault boundary. Crustal fault zones grow from minor folds into much larger structures along strike. Inferred structural relief across the Tacoma fault zone increases by several kilometers westward along the fault zone to Lynch Cove, where we interpret it as a zone of south vergent faulting overthrusting Tacoma basin. In contrast, structural relief along the Seattle fault zone decreases west of Seattle, which we interpret as evidence that the N‐S directed compression is being accommodated by slip transfer between the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east‐west trending, pop‐up structures underlying Puget Lowland from the Black Hills to the San Juan Islands.
Geodetic measurements in Greenland and their implications
Authors: Wahr John , van Dam Tonie , Larson Kristine , Francis Olivier
Published: February 2004

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two locations along the edge of the ice sheet: Kellyville, located about one third of the way up the western ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crustal uplift rates of −5.8±1.0 mm/yr at Kellyville and −2.1±1.5 mm/yr at Kulusuk. There have not yet been enough absolute gravity occupations to permit useful secular gravity solutions at either location. The negative uplift rate at Kellyville is consistent with independent archeological and historical evidence that the southwestern edge of the continent has been subsiding over the last 3000 years, but it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice during the early Holocene, which predict that Kellyville is likely to be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy between the observed and predicted uplift rates is too large to be caused by loading from present‐day changes in nearby ice. However, it is consistent with independent suggestions that the western ice sheet margin in this region may have advanced by ≈50 km during the past 3000–4000 years. If this advance did occur and if the crustal subsidence it induces is not removed from altimeter measurements of Greenland ice sheet elevations, then the altimeter solutions could underestimate the true snow/ice thickness change by 5–10 mm/yr along portions of the western margin of the ice sheet.
Thermal erosion by laminar lava flows
Authors: Kerr Ross C.
Published: February 2004

Thermal erosion by hot lavas during their laminar flow over cold ground is investigated both theoretically and experimentally. An analysis of the steady forced convective heat transfer by a laminar channel flow at large Péclet numbers is presented, which is used to determine the final steady state erosion velocity as well as the thicknesses and timescales of the associated thermal boundary layers in both the lava and ground. The initial transient period involving the growth and remelting of a basal chill layer is also quantified. Laboratory experiments are described in which hot, molten wax flowed over and thermally eroded an underlying bed of solidified wax. The experimental observations of chill formation and thermal erosion are found to be in excellent agreement with the theoretical predictions. The theory is applied to predict rates of thermal erosion consistent with those observed in recent flows of basaltic lava in Hawaii and of carbonatite lava in Tanzania.
Can surface pressure be used to remove atmospheric contributions from GRACE data with sufficient accuracy to recover hydrological signals?
Authors: Velicogna Isabella , Wahr John , Van den Dool Huug
Published: February 2004

The Gravity Recovery and Climate Experiment (GRACE) satellite mission will resolve temporal variations in gravity orders of magnitude more accurately and with considerably higher resolution than any existing satellite. Effects of atmospheric mass over land will be removed prior to estimating the gravitational field, using surface pressure fields generated by global weather forecast centers. To recover the continental hydrological signal with an accuracy of 1 cm of equivalent water thickness down to scales of a few hundred kilometers, atmospheric pressure must be known to an accuracy of 1 mbar or better. We estimate errors in analyzed pressure fields and the impact of those errors on GRACE surface mass estimates by comparing analyzed fields with barometric surface pressure measurements in the United States and North Africa/Arabian peninsula. We consider (1) the error in 30‐day averages of the pressure field, significant because the final GRACE product will average measurements collected over 30‐day intervals, and (2) the short‐period error in the pressure fields which would be aliased by GRACE orbital passes. Because the GRACE results will average surface mass over scales of several hundred kilometers, we assess the pressure field accuracy averaged over those same spatial scales. The atmospheric error over the 30‐day averaging period, which will map directly into GRACE data, is generally <0.5 mbar. Consequently, analyzed pressure fields will be adequate to remove the atmospheric contribution from GRACE hydrological estimates to subcentimeter levels. However, the short‐period error in the pressure field, which would alias into GRACE data, could potentially contribute errors equivalent to 1 cm of water thickness. We also show that given sufficiently dense barometric coverage, an adequate surface pressure field can be constructed from surface pressure measurements alone.
Three‐dimensional attenuation variations in southern California
Authors: Schlotterbeck Beth A. , Abers Geoffrey A.
Published: February 2004

Seismic attenuation (1/Q) in southern California varies substantially over distances of tens of kilometers. These variations are apparent in high‐frequency spectral decay of body waves. We measure the spectral decay from earthquake seismograms at regional distances by estimating a (frequency‐independent) decay parameter t* for both P and S waves, for frequencies of 0.5–25 Hz, and invert the resulting 1138 t* values to show spatial 1/Q variations in the crust and upper mantle. The P and S results agree closely with each other, and both correlate with regional tectonics. The upper crust (0.5–14 km depth) exhibits high 1000/Q (≥2.0; Q<500) in the Los Angeles Basin and Transverse Ranges, compared to low 1000/Q (≈1.0; Q∼1000) in the Mojave Desert. High 1/Q of the Los Angeles Basin probably reflects high porosity in basin sediments. The lower crust exhibits areas of high 1/Q beneath the Salton Trough (1000/QP≈1.5 and 1000/Qs≈2.3, or QP∼700 and Qs∼400) and beneath the San Gabriel Mountains (1000/QP≈2.2 and 1000/Qs≈1.7), values 2–4 times larger than other areas. High 1/Q beneath the Salton Trough may reflect elevated temperature or partial melt resulting from active rifting. The high 1/Q beneath the San Gabriel Mountains may represent elevated temperature associated with active mountain building, as seen in similar tectonic settings elsewhere. Attenuation varies less in the mantle, perhaps because resolution is poorer. Relatively high QP/Qs (∼1.5) beneath the Salton Trough may similarly indicate high temperatures. These measurements show large variation in 1/Q across the region, relevant to any complete model for ground shaking in southern California.
Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models
Authors: Barckhausen Udo , Ranero Cesar R. , von Huene R. , Cande Steven C. , Roeser Hans A.
Published: February 2004

The oceanic Cocos Plate subducting beneath Costa Rica has a complex plate tectonic history resulting in segmentation. New lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the East Pacific Rise from lithosphere formed at the Cocos‐Nazca spreading center. They also define two early phase Cocos‐Nazca spreading regimes and a major propagator. In addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of Cocos Plate over the Galapagos hot spot. The subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab. Newly identified seafloor‐spreading anomalies show oceanic lithosphere formed during initial breakup of the Farallon Plate at 22.7 Ma and opening of the Cocos‐Nazca spreading center. A revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the Cocos‐Nazca spreading center. At 19.5 Ma a major ridge jump reshaped its geometry, and after ∼14.5 Ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere. A suspected cause of ridge jumps is an interaction of the Cocos‐Nazca spreading center with the Galapagos hot spot.
Laboratory determination of the full permeability tensor
Authors: Renard Philippe , Genty Alain , Stauffer Fritz
Published: February 2004

Hydraulic conductivity (or intrinsic permeability) determined in a standard permeameter is biased if the anisotropy is not taken into account. This bias can be a significant source of error in the characterization of any type of aquifer or reservoir. Theoretical arguments show that it is possible to determine the complete permeability tensor of a sample by measuring the average filtration velocity and the average gradient vectors during steady state flow experiments. The full permeability tensor is calculated with a linear least squares algorithm. To date, a prototype has been built that shows promising results, but the level of accuracy of the measurements is not yet sufficient to fully demonstrate its applicability. The primary advantages of this new technique are that no preliminary assumptions with respect to the principal directions of anisotropy are required prior to testing and that it does not require sophisticated test equipment.
Mechanics of extension and inversion in the hanging walls of listric normal faults
Authors: Erickson S. Gregg , Strayer Luther M. , Suppe John
Published: February 2004

Fault shape, material properties and bedding anisotropy determine the style of deformation in the hanging walls of listric normal faults. We use numerical models to study this deformation in both extension and inversion during displacement on a variety of master fault shapes. Elastic‐plastic material properties in the models allow the development of shear bands, which simulate secondary faults within the hanging wall. If the master fault is composed of a planar ramp and planar flat separated by a sharp fault bend, a series of antithetic normal shear bands develops in the hanging wall, propagating up from the fault bend. Each shear band is progressively abandoned as displacement on the master fault moves it away from the fault bend. Displacements on the antithetic faults produce the limb of a hanging wall monocline and bound one side of a graben, the other side of which is bounded by the master fault. The antithetic shear bands are not rotated, and layering within the graben remains subhorizontal. On the other hand, if the fault bend is curved rather than sharp, symmetrical nested graben develop in the upper part of the hanging wall above the base of the ramp. Displacement on the master fault moves these shear bands away from the fault bend, after which they are abandoned in favor of new shear bands. Early formed synthetic shear bands become shallower and concave upward because of the folding and rotation of the hanging wall. With increasing radius of curvature of the master fault, localization into shear bands decreases and, with a large radius of curvature, shear bands do not develop. If weak bedding layers are included within the hanging wall, they become sites of bedding‐parallel shear bands that accommodate flexural slip folding and replace the synthetic shear bands that develop in homogeneous models. If extension is followed by shortening and inversion, normal shear bands that developed during extension are reactivated as reverse shear bands but are also crosscut by new reverse shear bands. The models produce results that are similar to both natural structures and analogue models and provide explanations for many observations of deformation in seismic profiles through extensional terranes.
Stability field and thermal equation of state of ε‐iron determined by synchrotron X‐ray diffraction in a multianvil apparatus
Authors: Uchida Takeyuki , Wang Yanbin , Rivers Mark L. , Sutton Stephen R.
Published: February 2004

In situ synchrotron X‐ray diffraction measurements have been carried out on Fe using a “T cup” multianvil high‐pressure apparatus up to 20 GPa and 1500 K. The stability field of the hexagonal phase (ε‐Fe) is characterized by the triple point of the body‐centered cubic (bcc) (α), ε, and face‐centered cubic (fcc) (γ) phases, located at 8.0(±0.3) GPa and 680(±50) K with the slope of the phase boundary between the ε and γ phases being 36±3 K GPa−1. Pressure‐volume‐temperature (P‐V‐T) data for the ε‐Fe enable us to extract thermal equation of state (EOS) parameters accurately. Least squares fit of a combination of our room temperature data with previous results using the diamond anvil cell (DAC) to the third‐order Birch‐Murnaghan EOS yieldsKT,0= 135±19 GPa,K′T,0= 6.0±0.4, andV0= 22.7±0.3 Å3, whereKT,0,K′T,0andV0are zero‐pressure isothermal bulk modulus, its pressure derivative, and zero‐pressure volume, respectively. Volume data at high temperatures are fit with various high‐temperature EOSs. A fit using the high‐temperature Birch‐Murnaghan EOS yields the temperature derivative of the bulk modulus (∂KT,0/∂T)P= −4.48 ±0.56 × 10−2GPa K−1, with the zero‐pressure thermal expansivity in the form αT,0=a+bT−cT−2, where α = 3.98 ± 0.24 × 10−5K−1,b= 5.07 ± 0.88 × 10−8K−2, andcis nonresolvable from 0. The thermal pressure approach based on the Mie‐Grüneisen‐Debye theory gives (αT,0KT,0) and (∂2P/∂T2)vto be 6.88 ± 0.30 × 10−3GPa K−1and 4.63 ± 0.53 × 10−6GPa K−2, respectively. The thermoelastic parameters obtained from various EOSs are mutually consistent. The edge lengths (aandc) for the ε‐Fe are also fit with the Mie‐Grüneisen‐Debye EOS based on fictitious volumes (a3andc3, respectively) to obtain pressure and temperature dependence ofc/a. Linear thermal expansivity for thecaxis is slightly larger than that of theaaxis while incompressibilities are similar. Thus pressure dependence ofc/aat each temperature is quite similar, although absolute values ofc/abecome higher with increasing temperature. Below 20 GPa, no new phase between the ε‐ and γ‐Fe stability fields was observed, and no anomaly in thec/aratio was detected. Under the assumption that ε‐Fe is stable at the correspondingPandTconditions of the Earth's inner core, the density of ε‐Fe is significantly higher than that of the Preliminary Reference Earth Model, indicating light element(s) must be present not only in the outer core but also in the inner core.
Phanerozoic history of the Mahanadi region, India
Authors: Lisker Frank , Fachmann Stefan
Published: February 2004

The Mahanadi Basin is a NW‐SE stretching Gondwana basin perpendicular to the east coast of India. The metamorphic basement to the basin mainly comprises rocks of the Eastern Ghats Mobile Belt (EGMB) which experienced a complex thermotectonic evolution involving several episodes of deformation and magmatism. It can be inferred from40Ar/39Ar ages on hornblende (845 Ma) and biotite (622 and 593 Ma) from basement rocks that the last high‐grade metamorphism in the western part of the EGMB (i.e., the Western Khondalite Zone) occurred during the late Grenvillian, with any Pan‐African overprint only reaching anchimetamorphic conditions. Only pseudotachylite intrusions and their related thermal imprint with ages around 515 Ma display a direct Pan‐African signature. Fission track analyses on nine zircon (419–313 Ma) and 49 apatite (443–119 Ma) samples indicate a rift‐related formation of the Mahanadi Basin as an asymmetrical half graben in two stages during the Permo‐Carboniferous and the Middle Triassic. Moreover, the Gondwana breakup in eastern India is signified by dyke intrusions and the thermal overprint of thrusts and faults in the Mahanadi region and the onset of ocean floor production in the Mahanadi offshore basin at around 120 Ma.
Spatial extent of a hydrothermal system at Kilauea Volcano, Hawaii, determined from array analyses of shallow long‐period seismicity: 2. Results
Authors: Almendros Javier , Chouet Bernard , Dawson Phillip
Published: February 2004

Array data from a seismic experiment carried out at Kilauea Volcano, Hawaii, in February 1997, are analyzed by the frequency‐slowness method. The slowness vectors are determined at each of three small‐aperture seismic antennas for the first arrivals of 1129 long‐period (LP) events and 147 samples of volcanic tremor. The source locations are determined by using a probabilistic method which compares the event azimuths and slownesses with a slowness vector model. The results show that all the LP seismicity, including both discrete LP events and tremor, was generated in the same source region along the east flank of the Halemaumau pit crater, demonstrating the strong relation that exists between the two types of activities. The dimensions of the source region are approximately 0.6×1.0×0.5 km. For LP events we are able to resolve at least three different clusters of events. The most active cluster is centered ∼200 m northeast of Halemaumau at depths shallower than 200 m beneath the caldera floor. A second cluster is located beneath the northeast quadrant of Halemaumau at a depth of ∼400 m. The third cluster is <200 m deep and extends southeastward from the northeast quadrant of Halemaumau. Only one source zone is resolved for tremor. This zone is coincident with the most active source zone of LP events, northeast of Halemaumau. The location, depth, and size of the source region suggest a hydrothermal origin for all the analyzed LP seismicity.
Spatial extent of a hydrothermal system at Kilauea Volcano, Hawaii, determined from array analyses of shallow long‐period seismicity: 1. Method
Authors: Almendros Javier , Chouet Bernard , Dawson Phillip
Published: February 2004

We present a probabilistic method to locate the source of seismic events using seismic antennas. The method is based on a comparison of the event azimuths and slownesses derived from frequency‐slowness analyses of array data, with a slowness vector model. Several slowness vector models are considered including both homogeneous and horizontally layered half‐spaces and also a more complex medium representing the actual topography and three‐dimensional velocity structure of the region under study. In this latter model the slowness vector is obtained from frequency‐slowness analyses of synthetic signals. These signals are generated using the finite difference method and include the effects of topography and velocity structure to reproduce as closely as possible the behavior of the observed wave fields. A comparison of these results with those obtained with a homogeneous half‐space demonstrates the importance of structural and topographic effects, which, if ignored, lead to a bias in the source location. We use synthetic seismograms to test the accuracy and stability of the method and to investigate the effect of our choice of probability distributions. We conclude that this location method can provide the source position of shallow events within a complex volcanic structure such as Kilauea Volcano with an error of ±200 m.
Evaluation of instability in fractured rock masses using numerical analysis methods: Effects of fracture geometry and loading direction
Authors: Zhang Xing , Sanderson David J.
Published: February 2004

Numerical modeling, using two‐dimensional distinct element methods, is used to examine the effect of stress on the stability of a fractured rock mass. The critical stress state depends on the differential stress, mean stress, and fluid pressure and is represented by a surface which bounds all stable stress states. By examining the critical stress states under different loading conditions it is possible to define the stability/instability in terms of the far‐field differential stress and effective mean stress. Thus the strength of fractured rock can be represented by a macroscopic frictional component (μz) and a cohesion (Cz), which differ from the corresponding parameters for individual fractures. A series of simulations are used to examine the effects of fracture network geometry, such as fracture density, fracture length, and fracture network anisotropy, on the instability strength. A steady decrease in equivalent frictional strength (μe) with increasing fracture density was found. For the same fracture density, rock masses with fewer, but larger, fractures had lower instability strength. As networks became more anisotropic, the orientation of the fractures in relation to the loading direction had a considerable impact on the instability strength and deformation pattern. The effects of loading direction in relation to fracture set orientation have been examined for two fracture networks with different anisotropy coefficients. Where the directions of the principal stresses were parallel to the fracture sets, extensional deformation was observed. Otherwise, dilational shear deformation modes develop, within which sliding, opening, and block rotation occur.
Mechanical analyses of the emplacement of laccoliths and lopoliths
Authors: Zenzri Hatem , Keer Leon M.
Published: February 2004

Elastic deformations of host rocks during the emplacement within the Earth's crust of magmatic intrusions, such as laccoliths and lopoliths, are analyzed. The present analysis is built upon semianalytic elastic solutions for a pressurized horizontal crack buried between an overburden and a semi‐infinite base. The current work improves upon recent analyses by including several important features inherent to the development of laccoliths and lopoliths. First, both elongated intrusions (plane strain case) and circular intrusions (axisymmetric case) are described by the current models. Second, the effect of the difference in elastic moduli between the overburden and the substrate is considered. This difference in elastic moduli is characterized by one of the Dundurs parameters. Third, the stress intensity factor at the tip of the crack is assumed to be zero. Thus the stresses are finite, and the obtained laccolithic shapes are doubly hinged with horizontal slopes at the peripheries, as observed in the field. Fourth, unlike plate bending models, which are commonly used for major laccoliths, the current models describe a full range of ratios of intrusion width to overburden thickness. Numerical results are obtained for different combinations of the geometrical parameters, the material contrast and the driving pressure distributions. Graphs of these results confirm the plate bending model for large laccoliths, support Gilbert's concept pertaining to small laccoliths, permit the prediction of the sill‐laccolith transition, allow the analysis of the asymmetry in the vertical displacements above and below the intrusion, and lead to a possible mechanism of deep lopoliths emplacements. Some of the obtained results are compared to the available field data from the Henry Mountains.