bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the precipitation flux in the Jovian auroral zones: 1. The model and its application to the UV auroral emissions
Authors: Prangé Renée , Elkhamsi Mohamed
Published: February 2008

The IUE observatory and the UVS instrument on board the Voyager spacecraft have measured EUV Jovian auroral emission and given evidence for its strong longitudinal dependence. However, limited spatial resolution has not allowed the derivation of any information concerning the latitudinal structure, nor even the exact location of the excitation process. This study is an attempt to relate the atmospheric auroral features to the magnetospheric mechanisms which generate the auroral precipitation of particles and, if possible, to allow the identification of the particle species, energy, L shell of injection, and equatorial anisotropy. Under assumptions consistent with a “diffuse” auroral process, we have estimated the precipitated flux for particles drifting in the distorted Jovian magnetic field, and we have shown that the effect of the magnetic field asymmetry by itself is able to account for the main longitudinal features of the observed emissions through the loss cone variations. We have then analyzed the dependence of the predicted precipitation profiles in both hemispheres on the model parameters, namely the L shell value and the anisotropy of the source. By comparison with the observations, we derive that the bulk of the emissions, almost longitude‐independent, is due to quasi‐isotropic equatorial distributions in the loss cone, depends only slightly on the L shell parameter, and does not depend on the particle species. This implies that the auroral brightness observations cannot by themselves provide enough information on this main component of the magnetospheric losses (beyond its equatorial angular distribution). Conversely, the brightness peaks observed near λIII = 180° in the northern hemisphere and λIII = 0° in the southern one, which are shown to originate from particles with anisotropic angular distributions, very clearly depend on all the model parameters. That is to say that because of the peculiar Jovian magnetic field structure, these rather moderate contributions to the auroral emissions (10 to 25% each) are closely related to the auroral injection mechanisms. Finally, using both brightness profiles from the Voyager observations and recently published EUV color ratios from IUE, we have obtained one possible scenario of the magnetospheric losses into the Jovian atmosphere involving different particle species of different energy injected on various L shells of the middle magnetosphere. In particular the northern peak is interpreted as due to electrons (or perhaps protons depending on the plasma lag to rigid corotation) while the southern peak is attributed to ions. New constraints on the surface magnetic field strength (namely the location of its minima) are also derived which could help to reduce the uncertainties in the higher multipole terms of the magnetic field model.
The altitude of type 3 auroral irregularities: Radar interferometer observations and implications
Authors: Sahr J. D. , Farley D. T. , Swartz W. E. , Providakes J. F.
Published: February 2008

VHF coherent scatter radars at auroral latitudes have observed scatterers with narrow power spectra at sub‐ion acoustic mean Doppler shifts. These spectra have been designated “type 3.” The mean Doppler shift of these waves is often near the atomic (O+) or, less frequently, the molecular (O2+ and NO+) gyro frequencies. These type 3 echoes have been attributed to an electrostatic ion‐cyclotron (EIC) instability in the upper E region (h > 140 km), where the ion collision frequency becomes low enough to permit ion gyromotion. Interferometric analysis of recent coherent radar observations with the CUPRI (Cornell University portable radar interferometer) shows that type 3 events occur at typical electrojet altitudes (100–120 km), however. At these altitudes the ion collision frequency is greater than the ion gyrofrequency, and there can be no cyclotron motion. The cause of the observed type 3 echoes hence remains a mystery.
A kinetic‐magnetohydrodynamic model for low‐frequency phenomena
Authors: Cheng C. Z.
Published: February 2008

A hybrid kinetic‐magnetohydrodynamic (MHD) model is presented for describing low‐frequency phenomena in high‐beta (β ∼ O(1)) anisotropic plasmas that consist of two components: a low‐energy core component and an energetic component with low density. The kinetic‐MHD model treats the low‐energy core component by magnetohydrodynamic description, the energetic component by a kinetic approach such as the gyrokinetic equation, and the coupling between the dynamics of these two components through plasma pressure in the momentum equation. The kinetic‐MHD model optimizes both the physics contents and the theoretical efforts in studying low‐frequency MHD waves and transport phenomena in general magnetic field geometries and can be easily modified to include the core plasma kinetic effects if necessary. It is applicable to any magnetized collisionless plasma system where the parallel electric field effects are negligibly small. In the linearized limit, two coupled eigenmode equations for describing the coupling between the transverse Alfvén type and the compressional Alfvén type waves are derived. The eigenmode equations are identical to those derived from the full gyrokinetic equation in the low‐frequency limit and were previously analyzed both analytically and numerically to obtain the eigenmode structure of the drift mirror instability (Cheng and Lin, 1987) which explains successfully the multisatellite observation of the antisymmetric field‐aligned structure of the compressional magnetic field of Pc 5 waves (Takahashi et al., 1987) in the magnetospheric ring current plasma. Finally, a quadratic form is derived to demonstrate the stability of the low‐frequency transverse and compressional Alfvén type instabilities in terms of the pressure anisotropy parameter τ the magnetic field curvature‐pressure gradient parameter αp as defined in equations (31) and (69), respectively. A procedure for determining the stability of a marginally stable MHD wave due to wave‐particle resonances is also presented.
Charged dust dynamics: Orbital resonance due to planetary shadows
Authors: Horányi M. , Burns J. A.
Published: February 2008

First, we discuss the dynamics of a weakly charged dust grain orbiting in the equatorial plane of a planet surrounded by a rigidly corotating magnetosphere, where the magnetic field can be represented by a centered aligned dipole field. If the grain has a constant charge, we can introduce an effective one‐dimensional potential, from which we find that the perturbation due to electrostatic forces induces a motion of the pericenter, similar to the effect of the planetary oblateness. However, the electric charge on an orbiting dust grain is generally not constant. Accordingly, we explore the case where the charge varies periodically due to the modulation of the photoelectron current that occurs as the grain enters and leaves the planetary shadow. Thus the electromagnetic perturbation resonates with the orbital period and can modify the orbit's size and eccentricity. We demonstrate this effect both numerically and analytically for small grains comprising the Jovian ring. We show that the resulting changes are periodic, and their amplitude is much bigger than the amplitude of the periodic changes due to light‐pressure perturbation or the secular changes due to resonant charge variations that develop over a comparable time span.
Attractor dimensionality for mesospheric turbulence
Authors: Hall Chris , Blix T. A. , Thrane E. V.
Published: February 2008

We have examined the relative ion density fluctuations measured in situ by a rocket‐based ion probe. These fluctuations are directly related to turbulent intensities, in this case from the region 63 km–72 km altitude. We have identified fractality in these data having dimensions around 5. The result is remarkably similar to that obtained from radar data at 2.75 MHz, and we hypothesize that this might indicate that, for the data sets in question, the radar echoes were due to volume scattering from eddies rather than partial reflections.
Ion distributions and flows in and near the plasma sheet boundary layer
Authors: Nakamura M. , Paschmann G. , Baumjohann W. , Sckopke N.
Published: February 2008

We have studied three‐dimensional ion distribution functions obtained with high time resolution (every 4.5s) in and near the plasma sheet boundary layer with the plasma instrument on AMPTE IRM. This multicase study in the plasma sheet boundary layer reveals that at times, both an earthward and a tailward high‐speed ion component are observed. Comparing these two components, the earthward components have the larger densities, while the tailward components have higher velocities. Typically, the distribution function changes from this two‐component highly anisotropic character to generally isotropic as the spacecraft moves from the lobe, across the plasma sheet boundary layer, and into the central plasma sheet. The high‐speed components often deviate from simple crescent‐shaped distributions and exhibit significant structure. During disturbed times, substantial flows perpendicular to the magnetic field are observed. In several of the reported cases an additional cold ion component of comparable density was observed whose bulk velocity perpendicular to the magnetic field sometimes differed dramatically from that of the high‐speed components. It is speculated that these differences might be a signature of gyrophase bunching.
On the structure of the distant magnetotail: ISEE 3
Authors: Fairfield D. H.
Published: February 2008

ISEE 3 magnetic field and electron plasma data have been used to determine the relative frequency of observation of the magnetosheath and magnetotail in the region where a nominal magnetotail is expected. These observations have been compared with how frequently a tail of a given radius would be expected to be seen, assuming typical variations in the direction of the solar wind flow relative to the radial. Observations match expectations if the average radius of the distant magnetotail is 24 RE. This relatively small radius is consistent with an open magnetotail where field lines are lost both through the magnetopause and also by closing across the equatorial current sheet. Further evidence suggests that the distant tail is seen somewhat less frequently during geomagnetically quiet periods which may mean that the radius is smaller during such conditions. The average solar magnetospheric Bz component of the field in the distant plasma sheet is 0.6 nT during quiet times but zero during disturbed times. This observation suggests that when the polar cap becomes smaller during quiet times, many of the field lines that previously formed the distant tail lobes are converted into closed field lines that cross the equatorial plane earthward of 240 RE. Further indications are that the east‐west dimension may exceed the north‐south dimension. The lobe is observed 4.5 times as often as the plasma sheet, implying an average half thickness of the distant plasma sheet of 3.4 RE.
Current disruptions in the near‐Earth neutral sheet region
Authors: Lui A. T. Y. , Lopez R. E. , Anderson B. J. , Takahashi K. , Zanetti L. J. , McEntire R. W. , Potemra T. A. , Klumpar D. M. , Greene E. M. , Strangeway R.
Published: February 2008

Observations from the Charge Composition Explorerin 1985 and 1986 revealed fifteen current disruption events in which the magnetic field fluctuations were large and their onsets coincided well with ground onsets of substorm expansion or intensification. These events are of short durations locally (∼1–5 min). They are mostly confined to within ∼0.5 RE of the neutral sheet and 1 hour local time from the magnetic midnight. Over the disruption interval, the local magnetic field can change by as much as a factor of ∼7. In general, the stronger the current buildup and the closer to the neutral sheet, the larger the resultant field change. There is also a tendency for a larger subsequent enhancement in the AE index with a stronger current buildup prior to current disruption. For events with good pitch angle coverage and extended observation in the neutral sheet region we find that the particle pressure increases toward the disruption onset and decreases afterward. Just prior to disruption, either the total particle pressure is isotropic, or the perpendicular component (P⊥) dominates the parallel comment (P∥), the plasma beta is seen to be as high as ∼70, and the observed plasma pressure gradient at the neutral sheet is large along the tail axis. The deduced local current density associated with pressure gradient is ∼27–80 nA/m² and is ∼85–105 mA/m when integrated over the sheet thickness. We infer from these results that just prior to the onset of current disruption, (1) an extremely thin current sheet requiring P∥ > P⊥ for stress balance does not develop at these distances, (2) the thermal ion orbits are in the chaotic or Speiser regime while the thermal electrons are in the adiabatic regime and, in one case, exhibit peaked fluxes perpendicular to the magnetic field, thus implying no electron orbit chaotization to possibly initiate ion tearing instability, and (3) the neutral sheet is in the unstable regime specified by the cross‐field current instability. Subsequent to the disruption onset, enhancement of magnetic noise over a broad frequency range, magnetic field aligned counterstreaming electron beams, ion energization perpendicular to the magnetic field, and current reduction in the amount similar to that of current buildup during the growth phase are observed. These features seem to be compatible with the predicted development of the cross‐field current instability.
Magnetospheric equilibrium with anisotropic pressure
Authors: Cheng C. Z.
Published: February 2008

Self‐consistent magnetospheric equilibria with anisotropic pressure are obtained by employing an iterative metric method for solving the inverse equilibrium equation in an optimal flux coordinate system. A method of determining plasma parallel and perpendicular pressures from either analytic particle distributions or particle distributions measured along a satellite's path is presented. The numerical results of axisymmetric magnetospheric equilibria including the effects of finite beta, pressure anisotropy, and boundary conditions are presented for a bi‐Maxwellian particle distribution. For the isotropic pressure cases the finite beta effect produces an outward expansion of the constant magnetic flux surfaces in relation to the dipole field lines, and along the magnetic field the toroidal ring current is maximum at the magnetic equator. The effect of pressure anisotropy is found to further expand the flux surfaces outward. Along the magnetic field lines the westward ring current can be peak away from the equator owing to an eastward current contribution resulting from pressure anisotropy. As pressure anisotropy increases, the peak westward current can become more singular. The outer boundary flux surface has a significant effect on the magnetospheric equilibrium. For the outer flux boundary resembling the dayside compressed flux surface due to solar wind pressure, the deformation of the magnetic field can be quite different from that for the outer flux boundary resembling the taillike flux surface.
Signatures of transient boundary layer processes observed with Viking
Authors: Woch J. , Lundin R.
Published: February 2008

Transient penetration of plasma with magnetosheath origin is frequently observed with the hot plasma experiment on board the Viking satellite at auroral latitudes in the dayside magnetosphere. The injected magnetosheath ions exhibit a characteristic pitch angle/energy dispersion pattern, earlier reported for solar wind ions accessing the magnetosphere in the cusp regions. In contrast to the continuous plasma entry in the cusp, the events discussed here show temporal features which suggest a connection to transient processes at or in the vicinity of the magnetospheric boundary. A single event study confirms previously published observations that the injected ions flow essentially tailward with a velocity comparable to the magnetosheath flow and that the energy spectra inferred for the source population resemble magnetosheath spectra. Those ion injection structures, which were resolved by the Viking mass spectrometer, consist of protons. Based on a statistical study, it is found that these events are predominantly observed around 0800 and 1600 MLT, in a region populated both by ring current/plasma sheet particles and by particles whose source is the magnetosheath plasma. Magnetic field line tracing based on the Tsyganenko magnetic field model yields a scatter of the source locations around the mid‐latitude region of the magnetospheric boundary. The probability for these events to occur is highest when the interplanetary magnetic field (IMF) is confined to the ecliptic plane. The event occurrence frequency shows a dawn‐dusk asymmetry depending on the azimuthal direction of the IMF. The occurrence frequency is independent of the solar wind velocity but increases with increasing solar wind pressure. For radially directed IMF the probability for observing the events is generally higher than for azimuthally directed IMF. The difference is especially pronounced during times when the solar wind pressure is comparatively low. Some of the most prominent events are obviously associated with significant changes in solar wind plasma density. The connection of the events to transient impulsive solar wind/magnetosphere interaction processes, such as transient reconnection (FTE), impulsive plasma transfer, Kelvin Helmholtz instabilities, and solar wind pressure pulses, is discussed. A relation with transient reconnection can be excluded.
Coordinated measurements made by the Sondrestrom radar and the Polar Bear Ultraviolet Imager
Authors: Robinson R. , Dabbs T. , Vickrey J. , Eastes R. , Del Greco F. , Huffman R. , Meng C. , Daniell R. , Strickland D. , Vondrak R.
Published: February 2008

In 1986 and 1987 the Sondrestrom incoherent scatter radar in Greenland was operated routinely in coordination with selected overpasses of the Polar Bear satellite. For these experiments the auroral ionospheric remote sensor on Polar Bear obtained images of auroral emissions in two far ultraviolet wavelength bands centered at approximately 136 and 160 nm and one visible band centered at 391.4 nm. Measurements at these three wavelengths were extracted from the images for comparison with the coincident radar measurements. Model calculations have shown that for Maxwellian incident electron distributions the ratio between the 136‐nm luminosity and 391.4‐nm luminosity can be used to estimate the mean energy of precipitating electrons. Once the mean energy is known, then either of the two emissions can be used to determine the total energy flux. This procedure is used to determine the properties of the incident electron distribution during three midnight sector auroral events over Sondre Stromfjord. The incident electron flux is then used to calculate the expected height profile of electron density which is compared with the simultaneous and coincident radar measurements. The results show that the derived profiles agree well with the measured profiles both in the peak electron density and the altitude of the peak. Differences between the measured and derived profiles below the peak are attributed to deviations between the actual spectral shape and the Maxwellian shape assumed in the modeling. Above the peak the observed differences may be due to the longer times required for the electron density to reach its steady state value. In spite of these differences the accuracy with which the peak of the profile is predicted by this technique is such that many important ionospheric parameters can be reliably inferred from remote measurements, including, for example, the height‐integrated electrical conductivities.
The distribution of atomic hydrogen in the magnetosphere of Saturn
Authors: Shemansky D. E. , Hall D. T.
Published: February 2008

Three sets of previously unpublished Voyager ultraviolet spectrometer (UVS) observations in H Ly α emission reveal a complex three‐dimensional distribution of atomic hydrogen in the Saturn system. Voyager 1 observations during the postencounter period have provided a map of the distribution looking down on the equatorial plane. The reduced data show a nonuniform distribution in local time with a preponderance of emission on the duskside. The emission extends radially inward to the top of the Saturn atmosphere with stronger signals appearing close to the planet strongly suggesting that the principal source is the sunlit Saturn atmosphere. In the subsolar sector of the magnetosphere no excess emission in the vicinity of Titan's orbit is detectable. But a peak in the emissions near 20 Saturn radii (RS) appears in the antisolar sector suggesting a significant contribution from Titan. Voyager 1 and 2 preencounter observations also show H Ly α emissions increasing monotonically toward the planet but with a distinctive dawnside excess. The preencounter and postencounter results may be reconciled if there exists a complex H gas distribution with significant radial and azimuthal variations. We conclude that the preliminary results reporting a toroidal hydrogen distribution with a cavity inside 8 RS (Broadfoot et al., 1981) are invalid because of limited spatial resolution and poor statistics. The fact that the hydrogen distribution joins continuously into the atmosphere of Saturn and shows significant azimuthal variations indicates that the dominant source inside 8 RS is in the Saturn exosphere. The local time character and magnitude of the distribution could be produced by ballistic and escaping atoms originating in the sunlit exosphere. The energies required for the source atoms can be obtained by electron excitation of H2 within a scale height of and above the exobase. The total rate of production of atomic hydrogen may be as much as 3 times larger than that inferred earlier from the observed brightness of H2 EUV radiation from the equatorial atmosphere. The deposition of heat at the top of the thermosphere by the atomic hydrogen source can account for the measured temperature. We have examined the impact of the conditions imposed by the measured content of neutrals and ions in the inner magnetosphere through model calculations. We predict in a preliminary analysis that the neutral gas in the low latitude region of the inner magnetosphere is dominated by water products. A typical calculation shows [O] ∼ 400 cm−3, [OH] ∼ 40 cm−3, [H] ∼ 130 cm−3, [O+] ∼ 30 cm−3, [H+] ∼3 cm−3, in the plasma sheet near 4.5 RS; a mix of neutrals and ions of this order is required to balance the energy budget.
Ion velocity distribution function in the upper auroral F region: 1. Phenomenological approach
Authors: Kinzelin E. , Hubert D.
Published: February 2008

We propose an expression for the O+ atomic oxygen ion velocity distribution in the upper part of the auroral F layer when the ions are subjected to an intense convection electric field. We assume that this layer is made up of neutral O atomic oxygen and atomic O+ ions. In order to obtain the O+ ion velocity distribution function, we use the Boltzmann equation with a modified relaxation collision term. The evolution of this function is governed by the convection electric ionospheric fields and by the ion‐neutral and ion‐ion collisions. To investigate these two collision effects, we use a new phenomenological and self‐consistent model. We obtain the non‐Maxwellian ion distribution function as a linear combination of a toroidal anisotropic contribution and of a Maxwellian isotropic contribution. The coefficients of this linear combination depend on the ion‐neutral and ion‐ion collision frequencies νii and νin. We self‐consistently calculate the expressions and values of these two frequencies. We note that the νii/νin ratio increases when the electric field increases and that this ratio is greater in our model than for the Maxwellian steady state. At 500 km the O+ ion velocity distribution function is almost isotropic, but at 300 km the distribution is anisotropic when the magnitude of the ionospheric electric field is very large.
Effects of plasma turbulence on electron collection by a high‐voltage spherical probe in a magnetized plasma
Authors: Palmadesso P. J. , Ganguli G.
Published: February 2008

We study the effects of electron trapping and plasma turbulence on the current‐voltage relation for a positively charged high‐voltage spherical probe in a magnetized plasma. In a force field consisting of an attractive radial electric field and a uniform magnetic field, the electric field provides a potential well in which electrons can be bound if they have insufficient energy to escape and cannot be captured by the sphere (by virtue of angular momentum conservation and/or the effects of the magnetic field). Electrons entering such a well from infinity may become trapped if they lose energy as a result of some scattering process acting within the well, such as inelastic collisions with neutral atoms or wave‐particle interactions due to plasma collective effects. When the neutral density is low enough so that ionization can be ignored, our results indicate that the sheath structure and the current‐voltage relation of a charged sphere configured as in the originally planned SPEAR I experiment (with an operating plasma contactor) would be substantially influenced by the combined action of trapped electrons and plasma instabilities. Our method is a modification of Linson's (1969) method in which potential contours are taken to be elliptical rather than cylindrical, space charge is not assumed to be constant within the sheath, and the controlling parameter is a scattering rate. We find reduced levels of current collection relative to Linson's model. Further, variation of the scattering parameter over the whole range of physically realizable values results in a range of predicted current collection values which is a small fraction of the interval between the Parker‐Murphy and Langmuir‐Blodgett limits.
Results From improved Monte Carlo calculations of auroral ion velocity distributions
Authors: Winkler E. , St‐Maurice J. ‐P. , Barakat A. R.
Published: February 2008

We have computed ion velocity distributions for the auroral ionosphere with the use of a Monte Carlo computation scheme that includes a detailed description of the interaction between ions and neutrals. This means that both the speed and angular dependence of the interaction are accounted for in a self‐consistent manner. We have used the model to deal with several types of interactions all at once and to describe quantitatively how the velocity distribution evolves in the presence of a mixture of neutral constituents. The model also contains an ad hoc procedure to deal with the effect of chemistry, which is important for periods during which N2+ ions are abundant. We have found that the shape of NO+ ion velocity distributions reaches an asymptotic limit for electric fields reaching about 125 mV/m, that the shape of O+ and N2+ ion velocity distributions may be very difficult to model analytically for very large electric field strengths, but that conversely, analytical theories should do an adequate job for fields up to 100 or 150 mV/m for these two ion species. We have, among other things, presented our results in terms of an ion temperature anisotropy, showing how this anisotropy, or the partition parameters that are sometimes used to describe it, change with electric field strength. We have found in the process that the anisotropy of O+ ions is noticeably greater than what has been assumed to be the case in the past. Finally, we have found that the shape of the distribution is sensitive to the presence of a secondary neutral constituent. For example, the presence of just 25% atomic oxygen is sufficient to give the O+ distribution a detectably greater toroidal character.
Wide‐angle seismic reflections from two‐dimensional random target zones
Authors: Levander Alan R. , Gibson Bruce S.
Published: February 2008

We have analyzed the shot gather seismic response of a buried zone of isotropic random velocity fluctuations. Scattering analysis predicts that events at offsets greater than the depth of the target zone will appear more continuous and coherent than events at zero and small offsets due to an effective dip filter inherent to the source‐target‐receiver geometry. The scattering theory is confirmed with finite difference synthetic seismograms calculated for an idealized crustal model. The closely spaced synthetic seismograms show bright events having the appearance of refraction arrivals which suggest a horizontally layered structure in the target zone. These effects appear to be independent of the magnitude of the velocity fluctuations, provided that the magnitude of the fluctuations is small (σ< 3.0% of the mean velocity). We have made a second finite difference calculation of a crustal scale model to reinterpret previously published wide‐angle data from the Black Forest, Germany [Sandmeier and Wenzel, 1986]. The response of the lower crust seen in the conventionally recorded field data can be explained by a lower crustal model of isotropic, small‐amplitude, random velocity fluctuations. We present this in contrast to a laterally homogeneous model of a finely layered lower crust of alternating high and low velocities proposed by Sandmeier and Wenzel [1986]. The synthetic studies and the scattering analysis demonstrate that the geometry of the wide‐angle experiment produces signals which appear to be from layered target zones, even where no layering exists. They also underscore the importance of conducting seismic surveys which make use of closely spaced receivers and which integrate vertical incidence and wide‐angle recording.
Basic magmatism associated with Late Cenozoic extension in the western United States: Compositional variations in space and time
Authors: Fitton J. Godfrey , James Dodie , Leeman William P.
Published: February 2008

Widespread basic magmatism across much of the western United States in the late Cenozoic followed the cessation of subduction along the Pacific coast. This volcanism accompanied lithospheric extension, block faulting and regional uplift In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. The data were divided into seven sets representing the main tectonomagmatic provinces: Basin and Range (BR), Western Great Basin, Transition Zone (TZ), Colorado Plateau, Snake River Plain, Southern Rocky Mountains, and Great Plains. It was further divided into relatively recent (<5 Ma) and older (>5 Ma) subsets on the basis of field relations and K‐Ar data. The <5 Ma subset shows striking chemical differences between the provinces. AU the BR lavas and some of the TZ lavas are indistinguishable from ocean island basalt (OIB) and therefore appear to have a source within the asthenosphere. In contrast, lavas from the other provinces generally show enrichments in Ba and depletions in Nb and Ti compared to OIB. This is accompanied by higher 87Sr/86Sr and lower 143Nd/144Nd than in the BR lavas. The older (>5 Ma) subset shows no great differences between the BR and the other tectonomagmatic provinces; all have high La/Nb and Ba/Nb. Crustal contamination alone cannot be responsible for these variations. We conclude that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. Pelagic sediment, returned to the mantle by subduction, is a possible agent for fluids rich in Ba, radiogenic Sr and unradiogenic Nd, but very poor in Nb. At least some of this enrichment must have accompanied the formation of the Proterozoic crust. It appears that subduction‐enriched lithospheric mantle was involved in the generation of all extension‐related basic magmas across the western United States until relatively recently. Only in the younger BR and parts of the TZ have asthenosphere‐derived magmas, uncontaminated by lithosphere, reached the surface. These observations conflict with models in which uplift and extension are caused by the replacement of mantle lithosphere by asthenosphere. Triey are best explained by the progressive erosion of the lithospheric manue over a plume currently located beneath the Southern Rocky Mountains.

Supplemental data are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B91‐001; $2.50. Payment must accompany order.
The Gravina Sequence: Remnants of a Mid‐Mesozoic oceanic arc in southern southeast Alaska
Authors: Rubin Charles M. , Saleeby Jason B.
Published: February 2008

Fragments of Upper Jurassic to Lower Cretaceous volcanic and basinal strata constitute the Gravina belt in southeast Alaska. In the Ketchikan area the Gravina belt is made up of two lithotectonic units. The lower unit consists of coarse marine pyroclastic and volcaniclastic strata, mafic flows, breccia, and fine‐grained tuff which are locally intruded by hypabyssal bodies of diorite and quartz diorite. The volcanic rocks are characterized by tholeiitic arc basalts, lack felsic volcanic strata, and overlie Upper Triassic and older strata of the Alexander terrane. Augite and/or hornblende‐bearing porphyritic rocks are common and locally intrude the Alexander terrane basement, where they are thought to represent the intrusive equivalents of lavas within the section. Age constraints for the volcanic unit, based on structural and stratigraphic relations with adjacent units, are late Middle to Late Jurassic. The Gravina belt upper unit consists of fine‐ to coarse‐grained turbidites and related conglomeratic channel‐fill deposits. The basinal rocks unconformably overlie Permian and Triassic rocks of the Taku terrane and remnants of the lower volcanic part of the Gravina sequence which overlie the Alexander terrane. The conglomerate units contain mostly volcanic and plutonic lithic clasts, some of which yield Pb‐U zircon ages of 154–158 Ma. The predominance of pyroclastic deposits interbedded with massive flows, tuff, breccia, and argillaceous turbidites, and the lithologic and chemical composition of the volcanic rocks indicate a submarine volcanic arc setting for the Gravina sequence. The basinal pyroclastic rocks are inferred to have been shed from submarine stratovolcanos during the Late Jurassic. Epiclastic rocks were deposited as submarine fans, derived in part from erosion of a magmatic arc. The presence of fine‐grained tuffaceous turbidites implies ongoing, but distant, volcanism. The pyroclastic and volcaniclastic rocks represent remnants of a Late Jurassic oceanic arc constructed on a composite basement consisting of the Alexander and Taku terranes. The strata accumulated in an intra‐arc basin on the eastern edge of the Alexander terrane. The volcanic and basinal rocks were deformed during a major mid‐Cretaceous intra‐arc contractional event, in conjunction with the emplacement of a distinctly younger, arc‐related plutonic suite.
Paleomagnetism of the Moenkopi and Chinle Formations in central New Mexico: Implications for the North American Apparent Polar Wander Path and Triassic magnetostratigraphy
Authors: Molina‐Garza Roberto S. , Geissman John W. , Van der Voo Rob , Lucas Spencer G. , Hayden Steve N.
Published: February 2008

In central New Mexico, red sedimentary rocks unconformably overlying Permian carbonates of the San Andres Formation have been correlated with the Early‐Middle Triassic Moenkopi and Late Triassic Chinle Formations of the Colorado Plateau. Paleomagnetic samples from Triassic sections exposed on basement cored uplifts along both the east and west side of the Rio Grande rift near Albuquerque yield, upon thermal and chemical demagnetization, well‐defined, high unblocking temperature, dual‐polarity magnetizations carried by hematite. The characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding‐parallel magnetization polarity zonation. The Moenkopi and lowermost Chinle formations produced paleomagnetic poles respectively at 57.6°N‐100.3 °E (N=36 sites, K=74.1, A95=2.8°) and 60.8°N‐88.9°E (N=17 sites, K=130.3, A95= 3.1°). These data plus previously published and additional results from the underlying Permian strata suggest that portions of central New Mexico have experienced a small clockwise rotation (i.e., less than 10°) similar to that of the Colorado Plateau with respect to the North American craton. The paleomagnetic directions of the Chinle Formation and related strata in eastern New Mexico document about 12° (great circle distance) of rapid apparent polar wander during mid‐Carnian to late Norian times along a track which contains other cratonic poles of similar age. We present a preliminary magnetic polarity time scale for the Triassic that incorporates the present New Mexico data and previously published data, mostly form continental red bed sequences. This magnetic polarity scale provides a basic framework which can be tested with future data from Triassic sections where additional biostratigraphic control exists.
Estimating aquifer parameters from analysis of forced fluctuations in well level: An example from the Nubian Formation near Aswan, Egypt: 1. Hydrogeological background and large‐scale permeability estimates
Authors: Evans Keith , Beavan John , Simpson David
Published: February 2008

This is the first of three papers concerning the extraction of aquifer parameters through the application of time series analysis to water level data from boreholes. The analysis is performed on data from six wells located in the Nubian desert near the shores of Lake Nasser, Egypt. These wells are ideally suited to this study since they are remote from most sources of hydrological noise and hence respond primarily to forcing functions that can be measured or modeled. For four of the wells, which were part of a monitoring system installed prior to impoundment of Lake Nasser, monthly (at least) water level measurements began in 1964. In 1985, two additional wells were drilled, and high precision piezometers that sampled data every 0.2 hour were installed in five of the wells. Thus data are available which span the period band 23 years to 0.4 hour. This paper provides background information and stratigraphy relevant to assessing the significance of the aquifer parameters derived using spectral methods. A regional‐scale three‐dimensional hydrogeological model for the study area is developed, based on the 23‐year record of water level changes that occurred at the various well sites in response to the changing level of Lake Nasser. The model that best fits the data, and is consistent with recent stratigraphic studies of the Nubian formation in the area, consists of three horizontal units of total thickness 400 m. The lowermost unit, overlying the granitic basement, is an aquifer of fluvial sandstones and has a large‐scale (several kilometer) horizontal permeability of 0.32–0.43 μm2 The middle unit is an aquaclude that extends unbroken under the lake and “leaks” at periods longer than several years. Thus the lower aquifer is confined up to periods of several years. The uppermost unit is the water table aquifer, which is composed of 25–30% porosity sandstones with interspersed claystone lenses and has a large‐scale permeability of 1.0–1.5 μm2 Of the wells studied, two are screened in the basal sandstone and four are screened in the water table aquifer. Analysis of tidally and atmospherically forced fluctuations in five of these wells is presented in the companion papers.
Magsat magnetic anomaly contrast across Labrador Sea passive margins
Authors: Bradley Lauren M. , Frey Herbert
Published: February 2008

Many passive margins not complicated by nearby anomalous crustal structure have satellite elevation crustal magnetic anomaly contrasts across them that are recognizable in reduced‐to‐pole versions of the Magsat and POGO data. In the Labrador Sea region this contrast is particularly well developed with strong positive anomalies overlying the continental crust of Greenland and eastern Canada and prominent negative anomalies situated over the Labrador Sea and Baffin Bay. We use forward modeling of the large‐scale crustal bodies in this region (continental, oceanic, passive margin, several anomalous structures) to show that the Magsat anomaly contrast is due simply to the change in crustal susceptibility and thickness at the continental/oceanic crustal transition. Because the thickness varies more than the average susceptibility from continental to oceanic crust, the strong anomaly contrast is essentially an edge effect due mostly to the change in crustal structure.
Seismicity and shear strain in the Southern Great Basin of Nevada and California
Authors: Gomberg Joan
Published: February 2008

This study examines the relationship between the distribution of small earthquakes (ML≤4.3) and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events, the Southern Great Basin Seismic Network (SGBSN) region. The Great Basin is a unique continental extensional province characterized by normal and strike‐slip faulting, high heat flow, crust of thin to normal thickness, and high elevations. The SGBSN is operated to provide data to address suitability issues pertaining to Yucca Mountain, Nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository. Suitability issues include estimation of the probability of occurrence of future damaging earthquakes, the characterization of the mechanisms that drive hydrologic flow, and the identification of fractures (faults) that might act as flow conduits or barriers. This study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field; where strain concentrates there should be a greater number of small earthquakes. Strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field. These strain field models are then interpreted with regard to the regional tectonics and site suitability issues. Modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults; clusters in a wide band along and extending northeast of the northern half of the Furnace Creek fault may correspond to elevated shear strains along the fault and a broad cluster in the Pahranagat Shear Zone may be associated with shear strain arising from a distribution of smaller localized faults. The relatively large number of small earthquakes in the southern and eastern portions of the Nevada Test Site is consistent with the strain field models. A minimum in shear strain at Yucca Mountain is predicted by all models consistent with an almost total lack of earthquakes observed there. The region to the west of the Death Valley/Furnace Creek fault system, the portion of the study area with the most active deformation but few small earthquakes, is an area of low shear strain. A possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep. While there is also a relative lack of earthquakes at Yucca Mountain, this may be indicative of a lack of accumulating strain energy and thus, a lower potential for a large earthquake.
Dynamics of a geyser eruption
Authors: Dowden John , Kapadia Phiroze , Brown Geoff , Rymer Hazel
Published: February 2008

Geyser action is characterised by the eruption of a mixture of steam, water and, in some cases, mud to substantial heights above surface level. The role of vaporisation in the ejection process is investigated with the aid of a mathematical model which treats geyser eruptions as isolated, steady state, one‐dimensional phenomena; they are treated from an Eulerian point of view. It is found that large scale boiling of water occurs close to the surface, even though the instabilities that initiate the cycle may occur much lower down. Somewhat different models characterise superficially similar geyser eruptions from narrow confined tubes and from pools such as crater lakes. In the latter case the model predicts a root zone for steam flash at a depth of a few metres below the water surface, compared to a depth of tens of metres below the exit plane when eruption is from a confining tube, for normal eruptions ten to one hundred metres high. Relationships are obtained between geyser height, water content, depth of vaporisation, and such parameters as temperature and pressure at the onset of vaporisation. These relationships are used to provide a simple model of an eruption of Old Faithful, which reflects its main characteristics. They are also used to discuss geysering in the crater lake of Poás volcano in Costa Rica. A triggering mechanism which depends on progressive compression of a volume of steam is considered briefly. The system ultimately becomes unstable and results in the ejection of a volume of fluid.
A multiple‐crack model of brittle fracture: 2. Time‐dependent simulations
Authors: Lockner David A. , Madden Theodore R.
Published: February 2008

A numerical multiple‐crack interaction model is developed to simulate the failure process in brittle solids containing significant populations of flaws. The model, which is two dimensional, allows for the growth of microcracks on a regular array of potential crack sites. Individual cracks may be oriented vertically, horizontally, or at 45° to the sample axes. Quasi‐static equilibrium equations are expressed in terms of finite difference approximations and are solved by applying a renormalization group theory approach. In this paper the results presented in the companion paper are extended to include time‐dependent processes. In particular, both subcritical mode I crack growth and stress‐sensitive stable slip on existing crack surfaces are included. Crack growth rate and shear slip rate are defined by the local stress conditions. By including these two simple rules in the crack model, constant stress creep simulations are performed which reproduce the classic creep response, including all three modes of creep. In addition, the model predicts the proper stress sensitivity of creep rate. This result supports the interpretation that room temperature creep in crystalline rock in compression is dominated by the same stress corrosion process that occurs during subcritical crack growth observed in tension experiments. The process of crack localization, characteristic of the tertiary creep phase and culminating in failure, is discussed. The relation between inelastic strain and acoustic emission is also examined.