bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated auroral precipitation by wave‐particle interactions: Implications for the CRRES satellite mission
Authors: Schriver David , Haerendel Gerhard
Published: February 2008

The interaction of charged particles with plasma waves is one of the mechanisms which can cause the precipitation of energetic particles from the Earth's radiation belts. In the inner plasma sheet of the magnetotail, electromagnetic cyclotron waves can resonate with the bulk of the ambient energetic electron distribution only if an additional cold dense plasma population is present. In an attempt to artificially stimulate auroral electron precipitation, one experiment to be carried out during the Combined Release and Radiation Effects Satellite (CRRES) mission will be to release lithium close to the Earth's equatorial plane near geosynchronous orbit, along field lines that map to the diffuse auroral region. As the lithium is photoionized, a dense cold plasma cloud will be created within the background energetic population causing increased whistler wave growth. To support the CRRES Stimulated Electron Precipitation (STEP) experiment theoretically, a quantitative study is presented examining wave growth, propagation, and trapping of electromagnetic whistler waves within the released cold plasma cloud. Results show that for a background thermal electron temperature anisotropy of T⊥/T∥ > 1.2, whistler wave growth rates are greatly increased by the presence of the cold electrons, and the waves can be trapped in the cloud. Because of a maserlike effect, intense standing waves with amplitudes the order of a few nanotesla will be generated within a frequency range of about 50–500 Hertz. The waves will reach their largest amplitudes about 20 s after release, causing strong electron pitch angle diffusion into the loss cone, which can be detected as enhanced diffuse aurora at Earth.
A new numerical code for simulating current‐driven instabilities on auroral field lines
Authors: Goertz C. K. , Whelan T. , Nishikawa K.‐I.
Published: February 2008

We have studied the microphysical processes that occur on auroral field lines carrying field‐aligned currents by means of a newly developed two‐dimensional electrostatic PIC (particle in cell) numerical code. We are, in particular, interested in the question of whether field‐aligned currents carried by obliquely propagating shear Alfvén waves can lead to the formation of quasi‐stationary field‐aligned electric fields and double layers in order to provide a detailed test of the shear Alfvén wave model for generating small‐scale field‐aligned currents and auroral particle acceleration. Our model includes upward and downward propagating oblique shear Alfvén waves in a phenomenological way through an impedance characterizing the source plasmas outside the simulation box. In this paper we concentrate on the description of the numerical scheme, the testing of it, and a few important results which are obtained from a number of runs. Some of the findings are as follows: the reflection of Alfvén waves at regions containing plasma waves coupling the electron and ion motion causes a reduction of the field‐aligned current and limits it to a critical value which is marginal for the excitation of electrostatic ion cyclotron waves at a height of 1–2 Re. Electron heating due to current‐driven plasma turbulence causes the overall plasma density in the system to decrease, because we do not require the number of particles to remain constant inside the system. This may be related to the formation of the auroral cavity. The plasma density in the region of upward current is enhanced relative to that in the region of downward current, and the current density in upward directed current channels (carried by precipitating electrons) can be larger than in downward directed current channels. We also observe a filamentation of the current in the upward directed current channel. We find upward propagating solitary potential structures in the upward directed current channel and intermittent double layers in both channels.
Self‐consistent gyroviscous fluid model of rotational discontinuities
Authors: Hau L.‐N. , Sonnerup B. U. Ö
Published: February 2008

One‐dimensional steady state equilibrium structures of rotational discontinuities (RDs) have been constructed by use of a nondissipative gyroviscous two‐fluid model in which electron inertia is neglected. The gyroviscous terms in the pressure tensor, due to first‐order finite ion Larmor radius effects, are included, but the plasma pressure is assumed to be isotropic. The gyroviscous two‐fluid RDs presented in this paper satisfy the MHD jump conditions for RDs exactly, i.e., all plasma properties as well as the magnetic field magnitude are identical and uniform on both sides of the layer, and νx, the normal flow speed there, is equal to the Alfvén speed based on the normal magnetic field component Bx. A necessary condition for the existence of RD solutions is derived by performing fixed‐point analysis at the possible upstream and downstream states of RDs. This analysis shows that in the ByBz plane the magnetic structure near the fixed point is either a center or a saddle point. For those upstream conditions that give rise to a center point, no RD structure exists; when the fixed point examined is a saddle point, numerical integration of the one‐dimensional, steady state, nonlinear gyroviscous two‐fluid equations indicates that self‐consistent complete RD structures may sometimes (but not always) exist. These structures involve a rotation of the tangential magnetic field by an angle ϕ that is determined by the angle θ between the upstream (or downstream) magnetic field and the vector normal to the discontinuity, by the plasma beta value, β = 2μop/B², and by the electron to ion temperature ratio Te/Ti. Only electron‐polarized structures have been found to date: within these discontinuities the field magnitude |B| always displays a maximum. If ion‐polarized RDs exist, they would contain a minimum in |B|. However, no such structures have been found. Instead, ion polarization leads to what appears to be chaotic behavior in which a solution “trajectory” emerging from one fixed point (the upstream state, say) is extremely sensitive to initial conditions and integration step length. It does not connect to a second fixed point (the downstream state) but rather winds around indefinitely in solution space. RD structures in the limit of the ordinary two‐fluid model, i.e., including the electron inertia terms as well as the Hall term but excluding gyroviscous stresses, are examined in an appendix. Under certain, rather special conditions, a pulselike two‐fluid RD structure can be found, in which the tangential field rotates in the ion sense by a fixed but now very small angle ϕ. These solutions owe their existence to the electron inertia terms in the generalized Ohm's law; the ion inertia (Hall current) term alone leads to an RD of infinite thickness.
Forbush decreases and interplanetary magnetic field disturbances: Association with magnetic clouds
Authors: Lockwood J. A. , Webber W. R. , Debrunner H.
Published: February 2008

Magnetic clouds have been proposed as a mechanism to produce Forbush decreases in the cosmic radiation. We have examined the temporal association of magnetic clouds and Forbush decreases and find practically no association of the main phase of the Forbush decrease with the arrival of a magnetic cloud. On the other hand, Forbush decreases generally follow the strong interplanetary shocks which sometimes precede magnetic clouds. The main phase of the cosmic ray decrease occurs 2–5 hours after the shock and during the passage of the region in which the magnetic field is disturbed. It appears that a Forbush decrease is more likely to occur following a shock in which the magnetic field and plasma parameters are strongly enhanced. These results indicate that the decrease of the cosmic ray intensity may be produced by the smaller diffusion coefficient in the region behind the shock. The sweeping effect of the enhanced magnetic field associated with the fast shock also probably contributes to the rapid depression of the cosmic ray intensity seen in some decreases.
Average vertical and zonal F region plasma drifts over Jicamarca
Authors: Fejer B. G. , de Paula E. R. , González S. A. , Woodman R. F.
Published: February 2008

The seasonal averages of the equatorial F region vertical and zonal plasma drifts are determined using extensive incoherent scatter radar observations from Jicamarca during 1968–1988. The late afternoon and nighttime vertical and zonal drifts are strongly dependent on the 10.7‐cm solar flux. We show that the evening prereversal enhancement of vertical drifts increases linearly with solar flux during equinox but tends to saturate for large fluxes during southern hemisphere winter. We examine in detail, for the first time, the seasonal variation of the zonal plasma drifts and their dependence on solar flux and magnetic activity. The seasonal effects on the zonal drifts are most pronounced in the midnight‐morning sector. The nighttime eastward drifts increase with solar flux for all seasons but decrease slightly with magnetic activity. The daytime westward drifts are essentially independent of season, solar cycle, and magnetic activity.
Mid‐latitude E region field‐aligned irregularities observed with the MU radar
Authors: Yamamoto Mamoru , Fukao Shoichiro , Woodman Ronald F. , Ogawa Tadahiko , Tsuda Toshitaka , Kato Susumu
Published: February 2008

Fine structures E region field‐aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E; geomagnetic latitude 25.0°N). The 3.2‐m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field‐perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time‐height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi‐periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi‐periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi‐periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.
Strong plasma turbulence in the Earth's electron foreshock
Authors: Robinson P. A. , Newman D. L.
Published: February 2008

A quantitative model is developed to account for the distribution in magnitude and location of the intense plasma waves observed in the Earth's electron foreshock given the observed rms levels of waves. In this model, nonlinear strong‐turbulence effects cause solitonlike coherent wave packets to form and decouple from incoherent background beam‐excited weak turbulence, after which they convect downstream with the solar wind while collapsing to scales as short as 100 m and fields as high as 2 V m−1. The existence of waves with energy densities above the strong‐turbulence wave‐collapse threshold is inferred from observations from IMP 6 and ISEE 1 and quantitative agreement is found between the predicted distribution of fields in an ensemble of such wave packets and the actual field distribution observed in situ by IMP 6. Predictions for the polarization of plasma waves and the bandwidth of ion‐sound waves are also consistent with the observations. It is shown that strong‐turbulence effects must be incorporated in any comprehensive theory of the propagation and evolution of electron beams in the foreshock. Previous arguments against the existence of strong turbulence in the foreshock are refuted.
Parametric excitation of electrostatic whistler waves by electron plasma waves
Authors: Sharma R. P. , Tripathi Yogesh Kumar , Al Janabi A. Hadi , Boswell R. W.
Published: February 2008

Parametric decay instability of the electron plasma wave into another electron plasma wave and an electrostatic whistler wave has been studied taking into account the full matrix elements. The relevance of this decay channel to the generation of whistler mode radiations in the Spacelab 2 experiment has been pointed out.
Auroral resonance line radiative transfer
Authors: Gladstone G. R.
Published: February 2008

A model is developed for simulating the two‐dimensional radiative transfer of resonance line emissions in auroras. The method of solution utilizes Fourier decomposition of the horizontal dependence in the intensity field so that the two‐dimensional problem becomes a set of one‐dimensional problems having different horizontal wavenumbers. The individual one‐dimensional problems are solved for using a Feautrier‐type solution of the differential‐integral form of the radiative transfer equation. In the limit as the horizontal wavenumber becomes much larger than the local line‐center extinction coefficient, the scattering integral becomes considerably simplified, and the final source function is evaluated in closed form. The two‐dimensional aspects of the model axe tested against results for nonresonance radiative transfer studies, and the resonance line part of the model is tested against results of existing plane‐parallel resonance line radiative transfer codes. Finally, the model is used to simulate the intensity field of OI 1304 Å for hard and soft auroras of various Gaussian horizontal widths. The results demonstrate the importance of considering the effects of two‐dimensional radiative transfer when analyzing auroral resonance line data.
Magnetic flipping: Reconnection in three dimensions without null points
Authors: Priest E. R. , Forbes T. G.
Published: February 2008

In three dimensions, magnetic reconnection may take place in a sheared magnetic field at any singular field line, where the nearby field has X‐type topology in planes perpendicular to the field line and where an electric field is present parallel to the field line. In the ideal region around the singular line there will, in general, be singularities in the plasma flow and electric field, both at the singular line and at “magnetic flipping layers,” which are remnants of local magnetic separatrices. In the absence of a three‐dimensional magnetic neutral point or null point, reconnection of field lines can still occur by a process of magnetic flipping, in which the plasma crosses the flipping layers but the field lines rapidly flip along them by magnetic diffusion. Depending on the boundary conditions, there may be two or four flipping layers which converge on the singular line. A boundary layer analysis of a flipping layer is given, in which the magnetic field parallel to the layer decreases as one crosses it while the plasma pressure (or magnetic pressure associated with the field along the singular line) increases. The width of the flipping layer decreases with distance from the singular line.
Observations of short large‐amplitude magnetic structures at a quasi‐parallel shock
Authors: Schwartz Steven J. , Burgess David , Wilkinson William P. , Kessel Ramona L. , Dunlop Malcolm , Lühr Hermann
Published: February 2008

We have conducted a detailed analysis of a set of events termed short large‐amplitude magnetic structures (SLAMS) observed at an encounter of the quasi‐parallel bow shock by the AMPTE UKS and IRM satellites. Both the satellite configuration and the solar wind conditions are favorable for the case study presented here. We have identified isolated SLAMS, surrounded by solar wind conditions, and embedded SLAMS, which lie within or form the boundary with regions of significant heating and deceleration. The duration, polarization, and other characteristics of SLAMS are all consistent with their growth directly out of the ULF wave field, including the common occurrence of an attached whistler as found in ULF shocklets. The plasma rest frame propagation speeds, where they can be determined, and two‐spacecraft time delays for all cases show that the SLAMS attempt to propagate upstream against the oncoming flow, but are convected back downstream. The speeds and delays vary systematically with SLAMS amplitude in the way anticipated from nonlinear wave theory, as do their polarization features. Inter‐SLAMS regions, and boundary regions with the solar wind, contain hot deflected ions of lesser density than within the SLAMS. The amplitude of the SLAMS requires an active growth mechanism. Following earlier inferences about the limited transverse extent of SLAMS, we highlight the importance of determining the thickness of the transition zone over which SLAMS grow and the bulk heating and deceleration is effected. From this case study it appears that, at least under some circumstances, the quasi‐parallel shock cannot be regarded as an undulating, cyclically re‐forming simply connected surface. Instead, the transition zone is better represented as a set of ULF waves, some of which grow to become SLAMS which gradually decelerate and merge to form the downstream state.
A diagnostic model for equatorial spread F 2. The effect of magnetic activity
Authors: Kelley Michael C. , Maruyama Takashi
Published: February 2008

By using a numerical model we have studied electric field effects on the generation of equatorial spread F (ESF) in the postmidnight hours. Equatorial electric fields are zonally westward during late evening and postmidnight hours on normal days, while they sometimes reverse to eastward in magnetically disturbed days. The quiet time westward configuration and typical low altitude of the layer results in stable bottomside gradient or at most one with a very low generalized Rayleigh‐Taylor growth rate. The model calculation shows that the growth rate becomes significant when the electric field reverses to eastward and may remain so even when the electric field disturbance weakens due to the upward altitude excursion of the layer. These results account for the correlation of ESF occurrence and high Kp values in the postmidnight hours when taken in conjunction with the characteristics of the penetration of magnetospheric electric fields during such magnetic activity. The results also show the crucial role of layer height in the physics. We have calculated the growth rates for specific nights in which perturbation of electric fields and plumes were observed at Jicamarca. These case studies suggest that a quantitative requirement for the generation of plumes is that about 10 e‐folding times of the linear instability growth occur. We also suggest that a statistical study using ionosonde data could be very valuable in a testing whether a local version of this quantitative measure of event intensity could be useful in predicting plume activity.
Coupling of Fast and Alfvén modes in realistic magnetospheric geometries
Authors: Wright Andrew N.
Published: February 2008

The resonant coupling of linear fast and Alfvén modes is considered in a cold plasma permeated by a curl‐free background magnetic field. The medium is assumed to possess an invariant coordinate (e.g., slab or axisymmetric geometry). We present the problem in terms of a set of orthogonal wave functions which describe the wave fields. Perturbations are Fourier analyzed along the invariant direction with wave number kβ, which is subsequently employed as a second expansion parameter with which to expand the governing equations. These equations are then solved using time‐dependent perturbation theory, familiar in quantum mechanics. Our calculations provide generalizations to the results of previous authors, such as the excitation of resonant Alfvén waves and the damping of the fast mode. To test our novel formulation we compare our results with numerical solutions of the ‘box’ magnetosphere. For small azimuthal wave numbers (<3–4) our lowest‐order estimates of the cavity mode damping rates are in excellent agreement with previous calculations.
Bursty bulk flows in the inner central plasma sheet
Authors: Angelopoulos V. , Baumjohann W. , Kennel C. F. , Coroniti F. V. , Kivelson M. G. , Pellat R. , Walker R. J. , Lühr H. , Paschmann G.
Published: February 2008

High‐speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990)) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from ≈ 9 to 19 RE in the Earth's magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on 2 years of data. The high‐speed flows organize themselves in 10‐min time scale flow enhancements which we call bursty bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y‐z GSM direction of the order of 5×1019 ergs/RE².
A statistical study of Pc 1–2 magnetic pulsations in the equatorial magnetosphere: 1. Equatorial occurrence distributions
Authors: Anderson B. J. , Erlandson R. E. , Zanetti L. J.
Published: February 2008

A study of AMPTE CCE magnetic field data covering the frequency range 0.1–4.0 Hz using all data obtained during the first complete local time precession of the satellite orbit major axis (7500 hours of observations) has been made to evaluate the occurrence of transverse, narrowband Pc 1–2 emissions, identified as electromagnetic ion cyclotron (EMIC) waves, in the equatorial magnetosphere from L = 3.5 to L = 9 at all local times. We report this work in a pair of papers of which this is the first. A set of example events illustrates the pattern of Pc 1–2 occurrence: events occur primarily for L > 7, and a radial separation of several RE between low (<6) and high L (>6) pulsations is observed. Statistically, the highest concentration of events occurred near apogee in the afternoon. The L > 6 and L < 6 event populations appear to be radially separated in the morning but merge together in the afternoon. We construct a normalized occurrence distribution throughout the equatorial plane from L = 3.5 to L = 9 of Pc 1–2 with peak to peak amplitudes greater than 0.8 nT. The occurrence distribution exhibits a number of properties: for L > 7, Pc 1–2 occur at any given place in the early afternoon (1200–1500 MLT) with 10–20% probability and in the morning (0300–0900 MLT) with ≈3% probability; the L = 6–7 local time distribution reproduces results obtained previously from data at geostationary orbit; L < 5 events occur with a probability of ≤1% and a relatively uniform local time distribution. The predominance of L > 7 events implies that plasma sheet ions develop sufficient temperature anisotropy to generate EMIC waves on a routine basis in their drift from the nightside to the dayside and that plasma sheet ions on open drift paths rather than ring current ions on closed drift paths present the greatest source of equatorially generated EMIC waves. Consideration of the maximum convective growth rate of EMIC waves in a pure proton plasma versus cold plasma density and magnetic field strength illustrates how the predominance of L > 7 events and radial gap structure can be understood in terms of radial profiles of linear convective growth rates.
Direct determination of the local ionospheric hall conductance distribution from two‐dimensional electric and magnetic field data
Authors: Inhester B. , Untiedt J. , Segatz M. , Kürschner M.
Published: February 2008

From two‐dimensional ground magnetic and ionospheric electric field observations it should in principle be possible to estimate the two‐dimensional distribution of the ionospheric conductance. In the past this was achieved using auroral zone data by modeling the ionospheric conductances on a trial‐and‐error basis until the observed magnetic variations were reproduced. No information about the existence or the uniqueness of the solution was available this way. We have developed a method to directly deduce the Hall conductance distribution ΣH from ground magnetic and ionospheric electric field observations based upon some assumption for the ratio of the Hall to the Pedersen conductivity. In general, the solutions are shown not to be unique because for a specific solution the value of ΣH on certain parts of the boundary of the two‐dimensional domain has to be specified. However, in many situations, especially in the presence of strong and isolated field‐aligned currents, these boundary values become less influential, and the solution can be shown to be practically unique over a large area of the domain. In some cases, a rather restrictive relation between the electric field and the equivalent height‐integrated current density is shown to hold that could be used to cross‐check the quality of the observations. As a specific example, we apply our formalism to the observation of a Harang discontinuity obtained in northern Scandinavia simultaneously by the STARE coherent radar system and the IMS Scandinavian Magnetometer Array.
Electron acceleration by Alfvén waves in the magnetosphere
Authors: Hui C. ‐H. , Seyler C. E.
Published: February 2008

The self‐consistent electron kinetics of Alfvén waves on the electron inertial scale is studied using a two‐dimensional hybrid‐kinetic description. The ions follow a fluid description for Alfvén waves at frequencies below the ion cyclotron frequency. The parallel electron dynamics are treated kinetically using particle‐in‐cell techniques. In this model the electron plasma mode is eliminated and only the physics of the Alfvén waves is retained. At sufficiently large amplitudes, it is found that oblique Alfvén waves break due to finite electron inertia in a cold plasma. The consequence of wave breaking is the formation of an electron beam which can be unstable to the beam‐plasma instability. The electrons supporting the parallel current thermalize into a non‐Maxwellian distribution with an energetic tail up to several keV, assuming a reasonable magnetospheric Alfvén speed. In hot plasma simulations, electron trapping is the principal mechanism of electron acceleration. It is proposed that wave breaking or electron trapping of oblique Alfvén waves at 1 RE can result in electron acceleration and may explain some observed auroral phenomena.
The decompensative gravity anomaly and deep structure of the region of the Rio Grande Rift
Authors: Cordell Lindrith , Zorin Yuliy A. , Keller G. Randy
Published: February 2008

An isostatic correction is commonly made to Bouguer anomaly gravity data to remove the gravity effect of isostatic compensation of topographic loads. In the USSR a “decompensative” correction has then been made to the isostatic gravity anomaly to remove the gravity effect of isostatic compensation of geologic loads as well. Under an hypothesis of local (as opposed to regional) compensation the gravity effect of a shallow geological body can be separated from that of its inferred deep compensating root by deconvolution. By contrast with the procedure originally defined by Zorin, we employ here calculations in the wave number domain, leading to an efficient and exact solution. The decompensative transfer function acts to increase amplitude of the isostatic anomaly at all wavelengths, but especially at the low end of the isostatic anomaly spectrum in the 200–600 km wavelength region, where anomaly amplitudes may increase by as much as a factor of 2. In a 1200×1200 km region centered on the Rio Grande rift the decompensative correction ranges from about −35 to +25 mGal. The rift is not particularly apparent in the decompensative correction nor is the rift any more apparent in the decompensative anomaly than in the isostatic anomaly. The decompensative anomaly, however, highlights an arcuate gravity low and a system of gravity highs inferred to reflect prerift welts of mass concentration which have indirectly influenced the position of the rift and its segmentation and zones of accommodation. Under the assumptions made, if the decompensative anomaly is subtracted from the Bouguer anomaly, then the residual is the gravity anomaly field of deep structure, without gravity effects of shallow sources in the upper crust. We suppose this gravity field to be dominated by the gravity effects of the density contrast across the crust‐mantle boundary (Moho) and the density contrast at the lithosphere‐asthenosphere boundary. Using available seismic data to (weakly) constrain the Moho surface, we invert the residual gravity field for topography of the base of the lithosphere. Lithosphere is found to be 200 km thick in the High Plains; 40–50 km in the eastern Great Basin; 75–100 km in the Colorado Plateau, and as thin as 40 km in the southern Rio Grande rift. In the area studied, the thickness of the lithosphere is everywhere greater than that of the crust. An asthenosphere bulge closely tracks the axis of the rift and is symmetrical to it, by contrast with asymmetry that might be inferred from broad features of the Bouguer anomaly. The separation of gravity effects made possible by the decompensative correction shows how the rift is fundamentally controlled by thinning of the lithosphere, yet in detail is deflected by long‐lived tectonic welts in the shallow, brittle crust.
The velocity field along the San Andreas Fault in central and southern California
Authors: Lisowski M. , Savage J. C. , Prescott W. H.
Published: February 2008

The velocity field within a 100‐km‐broad zone centered on the San Andreas fault between the Mexican border and San Francisco Bay has been inferred from repeated surveys of trilateration networks in the 1973–1989 interval. The velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the San Andreas fault in the Transverse Ranges of southern California. Across‐strike profiles of the fault‐parallel component of velocity exhibit the expected sigmoidal shape, whereas across‐strike profiles of the fault‐normal component of velocity are flat and featureless. No significant convergence upon the fault is observed even along the big bend sector of the fault. Simple dislocation models can explain most of the features of the observed velocity field, but those explanations are not unique. About 35 mm/yr of relative plate motion is accounted for within the span of the trilateration networks. Geologic studies indicate that the secular slip rate on the San Andreas fault is about 35 mm/yr. The agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes. The relative motion observed across the San Andreas fault (35 mm/yr) plus that observed across the Eastern California shear zone (8 mm/yr) accounts for most (43 mm/yr) of the observed North America‐Pacific relative plate motion (47 mm/yr).
Eruption dynamics and magma withdrawal during the Plinian Phase of the Bishop Tuff Eruption, Long Valley Caldera
Authors: Gardner James E. , Sigurdsson Haraldur , Carey Steven N.
Published: February 2008

Variations in eruption column height, magma discharge (intensity), and geochemistry have been documented for the plinian phase of the Bishop Tuff eruption from the Long Valley caldera, California. Although exposures of the complete Bishop fall deposit are sparse, their widespread relationships, knowledge of the plinian vent location, and a reasonable estimate of the main dispersal axis allow for the construction of minimum clast isopleths. Column height and intensity are estimated to have increased from 23 km and 7×107 kg s−1 to a peak of 45 km and 7.5×108 kg s−1 at the end of the plinian phase. Variations in geochemistry of the plinian deposit reveal complex magma withdrawal processes during the Bishop plinian phase. Concentrations of many incompatible trace elements increase from the base to midlevel and then decrease to the top of the plinian deposit. This trend is mirrored by the variation in pumice density, which in turn reflects vesicularity. Based on the physical volcanology and compositional variations in the plinian deposit, a model is proposed for the temporal progression of magma withdrawal during the plinian phase. Relatively less‐evolved magma was first tapped by the plinian phase from the side of the magma chamber, with the conduit location being controlled by the Hilton Creek fault system. As eruption intensity increased, more‐evolved magma from the chamber roof was drawn down and tapped. Less‐evolved magma was again tapped for the duration of the plinian phase, possibly as withdrawal from the roof zone became less efficient. Renewed eruption of slightly more evolved magma by the first post‐plinian ignimbrites suggests that the vent migrated along the ring fault system to an area that had not experienced previous magma withdrawal.
On the origin of deviatoric stresses in the lithosphere
Authors: Bai Wuming , Vigny Cristophe , Ricard Yanick , Froidevaux Claude
Published: February 2008

Mantle circulation models capable of predicting satisfactory plate velocities and geoid are tested for their ability to generate adequate lithospheric stresses. The model Earth mantle is driven by mass heterogeneities defined by seismic tomography and slab distribution. Its surface is divided into 11 freely moving plates. The mantle circulation produces shear stresses at the base of each plate. These are used to compute a dynamical component of the lithospheric stresses which is added to another component induced by variations of the Moho depth inside the lithosphère itself. The extensional state of the continents caused by their thick crust is more than compensated by a compressional tendency related to their high seismic velocity roots, interpreted as dense roots. Strong upwellings beneath equatorial regions with geoid highs could also generate anomalous topography and extensional stresses. The long‐wavelength stress pattern proposed by our models cannot easily be compared with available data: the internal loads are still poorly defined, the tested Earth model may be too simple, and the observations are inadequately distributed.
Tertiary minette and melanephelinite dikes, Wasatch Plateau, Utah: Records of mantle heterogeneities and changing tectonics
Authors: Tingey David G. , Christiansen Eric H. , Best Myron G. , Ruiz Joaquin , Lux Daniel R.
Published: February 2008

A swarm of minette and melanephelinite dikes is exposed over 2500 km2 in and near the Wasatch Plateau, central Utah, along the western margin of the Colorado Plateaus in the transition zone with the Basin and Range province. To date, 110 vertical dikes in 25 dike sets have been recognized. Strikes shift from about N80°W for 24 Ma dikes, to about N60°W for 18 Ma, to due north for 8–7 m.y. These orientations are consistent with a shift from east‐west Oligocene compression associated with subduction to east‐west late Miocene crustal extension. Minettes are the most common rock type; mica‐rich minette and mica‐bearing melanephelinite occurs in 24 Ma dikes, whereas more ordinary minette is found in 8–7 Ma dikes. One melanephelinite dike is 18 Ma. These mafic alkaline rocks are transitional to one another in modal and major element composition but have distinctive trace element patterns and isotopic compositions; they appear to have crystallized from primitive magmas. Major, trace element, and Nd‐Sr isotopic data indicate that melanephelinite, which has similarities to ocean island basalt, was derived from small degree melts of mantle with a chondritic Sm/Nd ratio probably located in the asthenosphere, but it is difficult to rule out a lithospheric source. In contrast, mica‐bearing rocks (mica melanephelinite and both types of minette) are more potassic and have trace element patterns with strong Nb‐Ta depletions and Sr‐Nd isotopic compositions caused by involvement with a component from heterogeneously enriched lithospheric mantle with long‐term enrichment of Rb or light rare earth elements (REE) (epsilon Nd as low as −15 in minette). Light REE enrichment must have occurred anciently in the mid‐Proterozoic when the lithosphere was formed and is not a result of Cenozoic subduction processes. After about 25 Ma, foundering of the subducting Farallon plate may have triggered upwelling of warm asthenospheric mantle to the base of the lithosphere. Melanephelinite magma may have separated from the asthenosphere and, while rising through the lithosphere, provided heat for lithospheric magma generation. Varying degrees of interaction between melanephelinite and small potassic melt fractions derived from the lithospheric mantle can explain the gradational character of the melanephelinite to minette suite.
Lateral extrusion of lower crust from under high topography in the isostatic limit
Authors: Bird Peter
Published: February 2008

Where there is isostasy, the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load, whether compensation is in crustal roots or in the mantle. The result is a Poiseuille flow (planar channel flow) in the weak lower crust, which removes crust from under mountains and smooths and levels the topography. Assuming cubic power law creep, the flux of crust is proportional to the third power of the topographic gradient, to the usual Arrhenius term, to the tenth power of the absolute temperature of the lower crust, and to the negative fifth power of the geothermal gradient. The result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped, with a flat central plateau and steeper flanks spreading outward. When adjacent “pancakes” merge, most of the relief is eliminated. Flow may either roughen or smooth the Moho, depending on whether there are lateral density contrasts in the mantle lithosphere or not. Although it is difficult to find analytic solutions for this evolution, it is easily simulated numerically. Using various published flow laws for plausible lower crustal rocks, lateral extrusion is shown to be insignificant under the oceans, marginally significant under shields and platforms, important under elevated plains, and dominant beneath high plateaus and/or hot, delaminated regions. In particular, the Basin and Range province of the western United States cannot maintain short‐wavelength (100 km) Moho relief of more than 1 km for times greater than 10–20 m.y. at most. If the Tibetan Plateau of China has been delaminated, as recently suggested, then it may flatten even faster, reducing 100‐km‐wavelength topographic features to 200 m relief in no more than 0.03–0.13 m.y., and leaving only features with wavelengths over 400 km at the present. Therefore the lower crust of Tibet may resemble a hydraulic reservoir, as previously suggested. Because the flatness of the Moho is self‐maintained in these regions, information on past tectonics is continually lost, and the present Moho shape cannot be used to balance cross sections for times in the past. The flatness of the Moho today is not a constraint on the mechanisms of extension or shortening in these regions, nor is it evidence for underplating by intrusions. It also follows that very intense and inhomogeneous dilational strains may have occurred locally, as in the metamorphic core complexes of the Basin and Range province.
Dynamo models with permanent dipole fields and secular variation
Authors: Lee Hagee V. , Olson Peter
Published: February 2008

The long‐term behavior of azimuthally averaged core magnetic fields is examined using numerical calculations of nonlinear, axisymmetric, kinematic α2and α2ω dynamos. The important elements of the model are convective turbulence, parameterized by an α effect, and a toroidal shear flow, providing the ω effect. Magnetic fields produced by this model exhibit several styles of behavior. ForRαRω< 0, whereRαandRω, are the amplitudes of the α effect and ω effect in the northern hemisphere, the dipolar component of the field is found to be oscillatory (ac). ForRαRω> 0, we identify three regimes of field behavior corresponding to the magnitude ofRωSmallRωvalues produce nonoscillatory (dc) dipolar fields with zero quadrupolar components, while large values ofRω, produce ac dipole and ac quadrupole fields. Intermediate values ofRωproduce fields with dc dipole components coexisting with poleward propagating αω dynamo waves in the quadrupole family. This regime of field behavior is most consistent with the observed behavior of the Earth's paleomagnetic field on the 103–104year time scale of paleomagnetic secular variation.
Ridge forces, absolute plate motions, and the intraplate stress field
Authors: Richardson Randall M.
Published: February 2008

Torque poles are calculated for a variety of possible forces acting on the plates, including ridge push, slab pull, and collisional resistance. These torque poles are then compared to the directions of absolute plate motions. There is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the North American, South American, Pacific, Cocos, and Eurasian plates. Simple slab pull torques correlate well with absolute motion azimuths for the Pacific, Nazca, and Cocos plates and moderately well with the absolute motion azimuth of the IndoAustralian plate. Collisional resistance torque poles correlate with the absolute motion azimuth of the Eurasian plate only. The correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves. Torque poles for various forces are also compared with several long‐wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions. In general, ridge torque directions agree well with the orientations of maximum horizontal stresses for stable North America, western Europe, and South America and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag. Collisional resistance forces can also explain the alignment of stresses in western Europe. For the IndoAustralian plate, the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate. Other processes, in addition to collisional resistance, must be important for the Indo‐Australian plate. Ridge push forces may account for a significant portion of the long‐wavelength features of the intraplate stress field, especially away from continental collisions. Such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism. As previously suggested, slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates.
Miocene synextension sedimentation patterns, Whipple Mountains, southeastern California: Implications for the geometry of the Whipple Detachment System
Authors: Beratan Kathi K.
Published: February 2008

A sedimentary basin formed in the southern and eastern Whipple Mountains during early to middle Miocene detachment faulting. Stratigraphic relations and facies patterns indicate that the shape and size of the sedimentary basin was controlled both by fault activity and by sedimentary processes. High‐angle normal faults formed the southwestern, and perhaps also the northeastern boundary of the half graben, whereas a transfer fault probably formed the southeastern boundary. The transfer fault influenced the earliest synextension sedimentation in the basin. The location of this fault and similar faults in the Mohave Mountains and Aubrey Hills of Arizona is inferred to have been controlled by the position of primary irregularities or corrugations of the detachment fault. Shifts in facies patterns, particularly a shift in the position of the basin depocenter, reflect changes in basin geometry. Comparison with adjacent areas is necessary to distinguish between tectonic and nontectonic causes. A shift in depocentral position parallel to regional transport direction in the Whipple Mountains basin is interpreted as resulting from a change in the location of the high‐angle normal fault forming the southwestern margin of the basin, whereas a shift perpendicular to transport direction probably resulted from overtopping of topographic barriers within a fault block. The most active phase of faulting occurred prior to deposition of stratigraphic units younger than the 18.5 Ma Peach Springs Tuff. Relative uplift of the core of the Whipple Mountains, probably as a result of isostatic rebound following tectonic denudation, influenced sedimentation patterns after deposition of the Peach Springs Tuff.
The Flat Tops Volcanic Field: 1. Lower Miocene open‐system, multisource magmatism at Flander, Trappers Lake
Authors: Gibson S. A. , Thompson R. N. , Leat P. T. , Morrison M. A. , Hendry G. L. , Dickin A. P.
Published: February 2008

The lower Miocene lavas (23–20 Ma) of the Flat Tops volcanic field, NW Colorado, are one of the earliest and most voluminous phases of magmatic activity associated with the development of the Rio Grande rift. Flow‐by‐flow collections of the lavas at Flander, Trappers Lake, provide evidence for complex open‐system magma chamber processes. Geochemically, the lavas range from basalts to shoshonites and have concentrations of major and compatible trace elements that are buffered at higher levels than expected for simple fractional crystallisation, cyclic repetitions (at least five) in the geochemical stratigraphy of the Flander lavas suggest that they are the result of replenishment fractional crystallization (RFC). Variations in some incompatible trace element ratios, such as Ta/Yb and LaN /YbN, and radiogenic isotopes (87Sr/86Sr= 0.70458–0.70607 143Nd/ 144Nd= 0.51226–0.51241) suggest that the lavas have also been contaminated by approximately 10% of Proterozoic 87Sr‐rich upper‐crust. In addition, anomalously K‐rich lava flows with low 87Sr/86Sr occur close to the base and at the top of the succession. It is argued that these are the products of mixing between magmas within the Flat Tops reservoir system and an influx of strongly potassic melt from lithospheric mantle; without subsequent geochemical overprint by the effects of crustal contamination and RFC. All of the lavas have incompatible trace element ratios that mostly resemble those of calc‐alkaline basalts; e.g., their chondrite‐normalized patterns have troughs at Nb and Ta. These features could result from several alternative processes, such as melting within convecting mantle above a low‐angle subducted slab; reaction between various asthenospheric melts and overlying lithospheric mantle; and fusion of metasomatized lithospheric mantle as a result of decompression during regional extension.