bluuebunny/crossref_metadata_2025 huggingface dataset:
Mapping and energization in the magnetotail: 2. Particle acceleration
Authors: Kaufmann Richard L. , Larson Douglas J. , Lu Chen
Published: February 2008

The selection of magnetic and electric field models specifies the rate at which charged particles must be accelerated throughout the magnetosphere during steady state conditions. This rate is estimated in the mid‐tail region (−60 RE < xsm < −10 RE) using the Tsyganenko (1989) magnetic field model and the assumption that E is approximately uniform in the plasma sheet. The uniform E assumption is found to be consistent with a projection of the Heppner and Maynard (1987) electric field model, and with observations in the magnetotail. The resulting energization rate decreases from 4 and 50 GW/RE at xsm = −10 RE to 0.8 and 5 GW/RE at xsm = −60 RE during quiet and disturbed times, respectively. The total energization rate throughout this entire tail region varies from 100 GW when Kp = 0 to 700 GW when Kp = 5. Ions carry most of the cross‐tail current, and therefore gain most of the kinetic energy within the plasma sheet. Electrons, which are responsible for most auroral phenomena, are primarily accelerated at lower altitudes. For quasi‐static conditions, the electromagnetic energy flow to low altitudes is controlled by the magnitude and location of Birkeland currents, even when the current‐carrying particles have negligible kinetic energy. The typical power deposited in the entire nightside auroral zone during steady conditions is approximately 15% to 20% of the Poynting flux entering the plasma sheet. Induced electric fields are known to produce most of the energetic ring current and radiation belt particles during substorm injection events. The energization rate goes well above 1000 GW during brief intervals. A simple injection model is used to compare the structure of induced and potential fields. This model shows plasma drifting earthward, equatorward, and toward midnight during a localized injection event.
Penetration of polar electric fields to the nightside dip equator at times of geomagnetic sudden commencements
Authors: Sastri J. Hanumath , Rao J. V. S. Visweswara , Ramesh K. B.
Published: February 2008

Measurements of Doppler Velocity, VD of F ‐ region reflections at normal incidence over Kodaikanal (dip 3°N, 77° 28′E) are used to study the nature of perturbations in F ‐ region vertical plasma drift, Vz associated with the geomagnetic sudden commencements (sc) on July 8, 1991, and January 1, 1992. Both the events which occurred in a narrow time window, 1630‐1700 UT (2200‐2230 IST) were of sc* type at middle‐ and high‐ latitude stations in the afternoon sector. At Kodaikanal, which is on the nightside, the sc of January 1, 1992, is characterized by a double‐step main impulse (MI) in H ‐ component (the structure of the sc on July 8 could not be ascertained from normal run magnetograms because of the large amplitude and very small rise time of the sc). It is found that the usual downward motion of F ‐ region plasma during the premidnight hours at Kodaikanal suddenly ceased (and even reversed to upward in one event) for ∼1 min coincident with the preliminary impulse (PI) and was immediately enhanced in association with the MI of the sc*. This pattern which is consistently seen in the two events implies that an eastward electric field prevails near the nightside dip equator at the time of the first impulse of double‐step MI there and the PI of sc* at high latitudes. Our Doppler velocity observations constitute the first and direct experimental evidence of vertical plasma motions due to the sc ‐ associated electric fields in the nighttime dip equatorial ionosphere. They substantiate the view based on theory (Kikuchi and Araki, 1979) and ground‐based magnetic observations (Araki et al, 1985) that the dusk‐to‐dawn electric field imposed on the polar ionosphere with the onset of PI of sc* is instantaneously transmitted to the dip equator on the nightside as on the dayside.
On the altitude of the HF‐enhanced plasma lines
Authors: Kuo S. P. , Huang J. , Lee M. C.
Published: February 2008

The height of the HF‐enhanced plasma lines (HFPLs) has been observed to be greater than that of the photon‐enhanced natural plasma lines by 1 km or so at Arecibo, Puerto Rico. Besides the development of HFPLs affected strongly by that of the short‐scale field‐aligned irregularities (SFAIs) has been indicated in the recent Arecibo heating experiments. These outstanding features of HFPLs can be reasonably understood in terms of the proposed instability process which is elucidated in the present paper. This instability involves a four‐wave interaction process whereby a parallel propagating (i.e.
) Langmuir pump wave excites two obliquely propagating daughter Langmuir waves together with a purely growing field‐aligned mode (density irregularities). The field‐aligned mode can further scatter the propagating Langmuir waves to render even larger propagation angles leading to the detection of HFPLs. The parallel propagating Langmuir waves originate from the excitation of the parametric decay instability by the HF heater wave near its reflection height. Since the HFPLs were detected by the 430‐MHz radar, while the SFAIs by a portable 49.92‐MHz backscatter radar, the thresholds and growth rates of the instability are calculated separately for these two cases. One case is for the excitation of the Langmuir sidebands having a scale length of 42 cm and a propagation angle of 23° (which leads to HFPLs after being scattered off the 1.1‐m field‐aligned density irregularity), and the other case for the generation of the 3‐m SFAIs. The threshold field of the instability leading to the generation of HFPLs is about 0.15 V/m. It is also found that the growth rate of the HFPLs is slightly greater than twice that of the 3‐m SFAIs and that it takes a few milliseconds for the HFPLs to develop. Further, the proposed instability process occurs at an altitude about 1.2 km higher than the matching height determined by the dispersion relation of the HFPLs when the heater wave at 5.1 MHz is employed. Based on these results, the puzzling height difference between the HFPLs and the photon‐ enhanced natural plasma lines as well as the correlation between the HFPLs and SFAIs can be explained consistently.
Acceleration of protons trapped upstream of interplanetary shocks
Authors: Erdős Géza , Balogh André
Published: February 2008

Scatter‐free acceleration of energetic protons by quasi‐perpendicular interplanetary shocks is investigated. Pitch angle distributions of 35‐384 keV protons have been determined upstream of four interplanetary shocks that passed through the ISEE‐3 spacecraft during 1978‐1979. Bidirectional, loss‐cone‐type angular distribution of particles observed at events with the closest to perpendicular field line to shock normal angle suggests that the magnetic field line may have crossed the shock front several times, forming traps on the upstream side. The model was tested by numerical trajectory calculations, resulting in a qualitatively similar angular distribution to those observed. The loss cones, appearing in the observed as well as in the simulated pitch angle distributions imply that the first adiabatic invariant (magnetic moment) is at least approximately conserved during the shock interaction process. The violation of magnetic moment conservation was investigated in more detail by simulating the trajectories of particles in various trap geometries. It was found that the invariance is fairly well satisfied when the magnetic field line grazes the shock surface. The conservation of the second (longitudinal) adiabatic invariant was also studied numerically. A possible role which the longitudinal adiabatic invariant may play in the control of the particle release from the trap is pointed out. It is suggested that the fluctuations in the energetic particle flux, observed frequently on the downstream side, could be explained by the nonuniform speed of trap collapse.
Hale cycle effects in cosmic ray east‐west anisotropy and interplanetary magnetic field
Authors: Ahluwalia H. S.
Published: February 2008

We have reanalyzed diurnal anisotropy data obtained with the shielded ion chamber (IC) at Cheltenham/Fredericksburg and the neutron monitor (NM) at Swarthmore/Newark. IC data are for the 1936‐1977 period and NM data are for the 1965‐1988 period. These data have been studied previously by Bieber and Chen [1991a]. However, we have corrected IC data for the diurnal temperature effect. Application of this correction results in a better agreement between IC and other data sets, thereby making it possible to study the long‐term changes in the diurnal anisotropy using IC data. The behavior of the annual mean east‐west anisotropy is studied for 53 years of observations. The period encompasses more than two solar magnetic (Hale) cycles. Its amplitude undergoes the expected 11 and 22 year variations [Ahluwalia et al., 1965], with the largest changes occurring near solar activity minima. Moreover, the data indicate the presence of the subsidiary maxima [Ahluwalia, 1992b] for the entire 53‐year period, following the solar polar field reversals, during the declining phases of activity cycles when high‐speed solar wind streams are present in the heliosphere [Ahluwalia et al., 1990]. The data suggest that the amplitude of the subsidiary maximum is large when the solar polar magnetic field points toward the Sun in the northern hemisphere, and radial anisotropy is absent. These could also be the epochs when the annual mean magnitude of interplanetary magnetic field (IMF) is the largest in the heliosphere, indicating a possible Hale cycle variation of IMF flux. We have calculated the values of the important modulation parameter λ∥Gr for the 1968‐1970 period, over a wide range of primary rigidities. The time period covers a part of the negative solar magnetic polarity epoch. The computed mean value is (1.085 ± 0.015)%.
A kinetic model for dust acoustic waves applied to planetary rings
Authors: Melandsø Frank , Aslaksen Torsten K. , Havnes Ove
Published: February 2008

We have derived a kinetic model for the propagation of low‐frequency waves in a dusty plasma containing dust particles and drifting plasma particles. The model includes Landau damping or growth and damping from charge variation on the dust particles, and is applied to dust‐acoustic waves in planetary rings. Analytic expressions for the dispersion function are used to examine the stability of this wave mode. The dispersion properties are also found numerically for dense dust clouds or large drift velocities, where the analytical expressions are not applicable. We show how the stability condition depends on the density of dust particles and the wavelength, for plasma and dust parameters which may apply to Saturn's F ring, G ring, and E ring, and to Jupiter's ring.
Fractal behavior of cosmic ray time series: Chaos or stochasticity?
Authors: Aglietta M. , Alessandro B. , Arneodo F. , Bergamasco L. , Fauth A. Campos , Castagnoli C. , Castellina A. , Cattadori C. , Chiavassa A. , Cini G. , Piazzoli B. D'Ettorre , Fulgione W. , Galeotti P. , Ghia P. L. , Mannocchi G. , Morello C. , Navarra G. , Osborne A. R. , Riccati L. , Saavedra O. , Serio M. , Trinchero G. C. , Vallania P. , Vernetto S.
Published: February 2008

This paper presents results on the fractal and statistical behavior of cosmic ray time series detected in an air shower experiment located at 2000‐m altitude above the underground Gran Sasso Laboratory, Italy. We consider single particles (muons), corresponding to primary energies of ≥10 GeV, and air showers, corresponding to primary energies of ≥80 TeV. For all time series the analysis indicates a clear stochastic monofractal, non‐Gaussian character; comparing these results with those obtained for underground muons and for neutron monitors, we conclude that these properties likely belong in general to cosmic ray time series, irrespective of the nature of the particles and the energies of their progenitors. In particular, the air shower time series from high‐energy primaries have a fractal dimension larger than the single‐muon time series originating from low‐energy primaries.
Competing Processes of Electrostatic Plasma Waves Excited by Auroral Electron Beams: Comparison of EXOS D Observation Results With Computer Simulations
Authors: Watanabe Tomohiko , Oya Hiroshi
Published: February 2008

In the regions of auroral particle precipitation the plasma wave phenomena detected by the EXOS D (Akebono) satellite show remarkable differences corresponding to the plasma condition and precipitating particle energy condition. In the region of the high plasma density where fnof;p > fnof;c, for the plasma frequency fnof;p and the electron cyclotron frequency fnof;c, the upper hybrid waves are dominantly generated, while the quasi‐electrostatic whistler mode waves are generated under the condition fnof;p < fnof;c. By computer simulation, based on the electrostatic particle code, the competing processes between the quasi‐electrostatic whistler mode wave and the upper hybrid waves are investigated for their growth; the results show that the generation of the electrostatic plasma waves are mainly controlled by the plasma parameter fnof;p/fnof;c and that the upper hybrid waves become dominant only at less oblique wave normal angles (≤30°) when fnof;p > fnof;c. The simulation results then give interpretation for the evidences observed by the EXOS D satellite.
Excitation of two O(1;D) atoms in the collision of ground state oxygen atoms
Authors: Sun Y. , Dalgarno A.
Published: February 2008

The collisional process O(3P) + O(3P) → O(1D) + O(1D) is investigated. It is found that the excitation cross section is comparable to that of the single‐excitation process O(3P) + O(3;P) → O(3P) + O(1D). The process enhances the intensity of the 6300‐Å red line emission arising from the deposition into the atmosphere of energetic O+ ions which undergo charge transfer to produce fast oxygen atoms.
EXOS D observations of enhanced electric fields during the Giant Magnetic Storm in March 1989
Authors: Okada T. , Hayakawa H. , Tsuruda K. , Nishida A. , Matsuoka A.
Published: February 2008

We study spatial and temporal variations of the intense electric field that is observed in middle latitudes in the evening sector. The observations were made by the EXOS D (AKEBONO) satellite at altitudes of 2000‐6000 km during and after the great storm event of March 13‐14, 1989. During the main phase of the storm a sharp peak was observed in the poleward electric field. The strength of this field was ∼100 mV/m when projected to the ionosphere, and this corresponds to the plasma drift speed of 1.8 km/s. The position of the peak moved toward higher latitudes during the recovery phase. Comparison with the electron density profile deduced from the wave measurements (Oya et al., 1990) shows that the peak was located in the low‐density region outside the plasmapause. An intense electric field of the same kind was observed also on March 15‐17 when the geomagnetic field was only moderately disturbed.
Electron ‐ whistler interaction at the Earth's bow shock: 2. Electron pitch angle diffusion
Authors: Veltri P. , Zimbardo G.
Published: February 2008

The effect of whistler waves on the electron distribution function is considered for the November 7, 1977, bow shock crossing. Order of magnitude estimates of the diffusion times due to whistler waves and to electrostatic noise shows that whistler waves are also effective in shaping the electron distribution function fnof;e, causing pitch angle diffusion in the limit of low frequencies. A Monte Carlo simulation of the electron dynamics, which includes electrostatic as well as whistler random terms, is then set up. A study of the diffusion coefficients, together with the use of the experimental data on electromagnetic noise, allows us to assess spatial and velocity profiles for the random terms of the simulation. The moments of fnof;e, including the perpendicular temperature and the heat flux densities, are reproduced satisfactorily by the numerical results. The resulting scenario for electron heating in quasi‐perpendicular shocks can be described as follows: (1) the shock steady electric field in the deHoffman‐Teller frame energizes the electrons, but the parallel temperature is too high and the perpendicular temperature too low, and a hole in fnof;e is formed; (2) parallel diffusion due to the electrostatic noise fills the hole in fnof;e and creates a flat‐topped distribution, thus cooling the electrons, but the perpendicular temperature remains too low; (3) pitch angle diffusion due to whistler waves transfers energy from parallel to perpendicular, increasing the perpendicular temperature up to the observed values.
Hybrid simulations of the effects of interstellar pickup hydrogen on the solar wind termination shock
Authors: Liewer P. C. , Goldstein B. E. , Omidi N.
Published: February 2008

Hybrid (kinetic ions/fluid electrons) plasma simulations are used to study the effects of a population of energetic interstellar pickup hydrogen ions on the solar wind termination shock. The pickup hydrogen is treated as a second ion species in the simulations, and thus the effects of the pick‐ups on the shock, as well as the effects of the shock on the pickups, are treated in a fully self‐consistent manner. For quasi‐perpendicular shocks with 10‐20% pickup hydrogen the pickup ions manifest themselves in a small foot ahead of the shock ramp caused by pickup ion reflection. For oblique shocks with smaller angles between the field and the shock normal (θBn ≈ 40°‐60°) a large fraction of the pickup ions are reflected and move back upstream where they excite large amplitude magnetosonic waves which steepen into shocklets. These backstreaming pickup ions may provide advance warning of a spacecraft encounter with the termination shock.
Voyager and Pioneer missions to the boundaries of the heliosphere
Authors: Pesses M. E. , Jones W. V. , Forman M.
Published: February 2008

The interaction of the solar wind with the interstellar medium is expected to result in a complex, probably dynamic outer heliospheric boundary region. In increasing distance from the Sun the boundary region includes the solar wind termination shock, the heliopause, and perhaps a heliobow‐shock. The continuing missions of Voyager 1 and 2 and Pioneer 10 and 11 provide a unique opportunity to make in situ particle and field observations of the boundaries of the heliosphere and associated phenomena. Observations already made by these spacecraft suggest that the termination shock may be located 60‐100 AU from the Sun. Voyager 1 will reach 60 AU in 1995 and 100 AU in 2006. NASA's Space Physics Division is planning with the Voyager and Pioneer Projects to ensure timely and maximum scientific return from spacecraft encounters with the solar wind termination region. Initial emphasis is on being able to reliably determine when an encounter is imminent, so that spacecraft tracking and spacecraft and instrument configurations can be changed to encounter modes. It is anticipated that multiple encounters with heliospheric boundaries will occur due to motion of the boundaries in response to large variations in the solar wind pressure.
Analytic model of aurorally coupled magnetospheric and ionospheric electrostatic potentials
Authors: Cornwall John M.
Published: February 2008

This paper describes modest but significant improvements on earlier studies of electrostatic potential structure in the auroral region, using the adiabatic auroral arc model. This model has crucial nonlinearities (connected, for example, with aurorally produced ionization) which have hampered analysis; earlier work has either been linear, which I will show is a poor approximation or, if nonlinear, either numerical or too specialized to study parametric dependencies. With certain simplifying assumptions I find new analytic nonlinear solutions fully exhibiting the parametric dependence of potentials on magnetospheric (e.g., cross‐tail potential) and ionospheric (e.g., recombination rate) parameters. No purely phenomenological parameters are introduced. The results are in reasonable agreement with observed average auroral potential drops, inverted‐V scale sizes, and dissipation rates. The dissipation rate is quite comparable to tail energization and transport rates and should have a major effect on tail and magnetospheric dynamics. This paper gives various relations between the cross‐tail potential and auroral parameters (e.g., total parallel currents and potential drops) which can be studied with existing data sets.
The scattering of VLF waves by localized ionospheric disturbances produced by lightning‐induced electron precipitation
Authors: Poulsen William L. , Bell Timothy F. , Inan Umran S.
Published: February 2008

A three‐dimensional model of the scattering of VLF waves in the Earth‐ionosphere waveguide by localized disturbances in the lower ionosphere is examined for typical disturbances expected to be produced by lightning‐induced electron precipitation events. Results indicate that the scattering is generally independent of the conductivity and permittivity of the Earth's surface immediately beneath the disturbed region except for extremely low conductivities such as that found over deep ice caps. Thus the scattered signal is principally a function of the ionospheric perturbation. For typical disturbances characterized by altitude profiles of enhanced ionization expected for 1.4 ≤ L ≤ 3, most of the measurable wave energy scatters within a fairly narrow angular region (15‐dB beam width of ±7° for a disturbance radius of 100 km) centered on the forward scatter direction. Thus moderate‐ to large‐scale disturbances (radius 50‐200 km) must be located within <250 km of a moderate‐length path (3000‐16,000 km) in order to scatter a measurable signal to the receiver. These two findings suggest that the scattered signals can be used with confidence as a diagnostic tool to determine the characteristics of the energetic electron precipitation.
On the electric field model for an open magnetosphere
Authors: Wang Zhi , Ashour‐Abdalla Maha , Walker Raymond J.
Published: February 2008

We have developed a new canonical separator line type magnetospheric magnetic field and electric field model for use in magnetospheric calculations, we determine the magnetic and electric field by controlling the reconnection rate at the subsolar magnetopause. The model is applicable only for purely southward interplanetary magnetic field (IMF). We have obtained a more realistic magnetotail configuration by applying a stretch transformation to an axially symmetric field solution. We also discuss the Stern singularity in which there is an electric field singularity in the canonical separator line models for By ≠ 0 by using a new technique that solves for the electric field along a field line directly instead of determining it by a potential mapping. The singularity not only causes an infinite electric field on the polar cap, but also causes the boundary conditions at plus infinity and minus infinity in the solar wind to contradict each other. This means that the canonical separator line models do not represent the open magnetosphere well, except for the case of purely southward IMF.
The shapes of galactic cosmic ray intensity maxima and the evolution of the heliospheric current sheet
Authors: Cliver E. W.
Published: February 2008

The observation of a triangular‐shaped galactic cosmic ray (GCR) intensity maximum during the mid‐1980s solar minimum was successfully predicted by drift models of cosmic ray modulation. The assumption underlying this prediction was that the evolution of solar activity, represented in drift models by the time development of the ”tilt” angle of the heliospheric current sheet (HCS), does not vary greatly from cycle to cycle. Tilt angles derived from coronal brightness distributions in the 1970s show that this assumption, seemingly supported by the successful prediction, may not be valid. The evolution of the HCS during the 1970s, when the 11‐year GCR maximum exhibited a broad peak, may have been significantly different (with a broader and less regular period of low tilt angles) from that inferred for the preceding mid‐1960s solar minimum and from that determined by the Wilcox Solar Observatory during the following mid‐1980s minimum. Had the 1970s corresponded to an A < 0 epoch (in the drift formulation), it appears that the resultant GCR intensity maximum would have been double peaked, with a deep rift corresponding to the 1974 minicycle. The different sensitivities of GCR intensity to tilt angle changes in A positive and A negative solar cycles, as demonstrated by other authors, provide support for drift models of modulation. The point we make here is that nonsystematic evolution of the HCS from one cycle to the next can be an additional source of variation in the shapes of consecutive GCR intensity maxima.
Intermittent turbulence in large‐scale velocity fluctuations at 1 AU near solar maximum
Authors: Burlaga L. F.
Published: February 2008

The large‐scale fluctuations of the bulk speed at 1 AU near solar maximum, from day 1 through day 365, 1979 exhibit multifractal scaling over the range from 8 hours to 2.7 days. The observations are consistent with the binomial cascade model of intermittent turbulence proposed by Meneveau and Sreenivasan (1987a). The intermittency exponent computed from these observations is µ, = 0.19 ± 0.02, which is consistent with the best estimate µ = 0.025 ± 0.05 obtained from earth‐based measurements of intermittent turbulence (Sreenivasan and Kailasnath, 1993). Thus the complicated speed fluctuations observed over this range of periods at 1 AU near solar maximum are described by a simple relation for the inertial range of intermittent turbulence. The multifractal structure of these fluctuations differs from that associated with the large‐scale fluctuations observed during 1974 when corotating streams were dominant. In both cases the distribution of speed differences was exponential. There appears to be a solar cycle variation in the spectral exponent describing the large‐scale speed fluctuations.
Magnetic field‐aligned electrodynamics of Alfvén/ion cyclotron waves
Authors: Mauk B. H. , McIlwain C. E.
Published: February 2008

We report on the first experimental determination of the magnetic field‐aligned electric field (E∥) of an Alfvén/ion cyclotron (A/I) wave. The wave was observed propagating in the geostationary regions of the Earth's magnetosphere with a frequency of 1/3 the proton cyclotron frequency. Electron oscillations parallel to (B) are used in the determination of E∥. E∥ is found to be a factor of ∼18 greater than anticipated from cold plasma theory (i.e., electron inertial effects). The inclusion of electron pressure and pressure anisotropy brings reasonable agreement between the observations and linear wave theory.
Light scattering by spherical particles with hematite and goethitelike optical properties: Effect of water impregnation
Authors: Bedidi Ali , Cervelle Bernard
Published: February 2008

The interpretation of reflectance spectra (through remote sensing or laboratory research) requires spectral reflectance models that can be inversed to determine the more fundamental physical properties of these surfaces. One of the main hypotheses in reflectance models is that a soil or a powder are generally considered as a collection of particles or aggregates with averaged optical properties. Verification of this hypothesis is of great interest. This paper deals with the study of the scattering and absorption of light by a single mineral particle where diameter ranges from 0.01 to 50 μm. The particles are assumed to be spherical, and their complex refractive index is assumed to be equal to that of hematite or goethite (ordinary indices). These two minerals are responsible for the nonlinear and wavelength‐dependent behavior of lateritic soils reflectance spectra upon moistening. The absorption and scattering efficiencies, single scattering albedo, scattering diagram, and opening angle were computed for a single sphere. Different scenarios were considered: first, a sphere in the air, second, a sphere in water, and third, a sphere surrounded by a water layer of different thickness. All the computed spectra (400–700 nm) are classified into three diameter groups (0.01–0.1, 0.1–2, and 2–50 μm). The behavior of the calculated spectra (second and third scenarios) depends both on the particle diameter and the wavelength. A simplified Hapke's model was then used to simulate the reflectance spectra of powders of hematite and goethite. The main result is the existence of an optical elementary arrangement cell which is for the soil what a unit cell is for a crystal.
Determination of tectonic stress field from fault slip data: Toward a probabilistic model
Authors: Yin Z.‐M. , Ranalli G.
Published: February 2008

A model based on probability theory is proposed to determine the orientation of the tectonic stress field from fault slip and earthquake focal mechanism data. On the assumptions that (1) slip and shear stress direction on the fault plane coincide, and (2) measured slip direction and fault plane orientation are random variables following the von Mises and the Fisher distribution, respectively, the effects of measurement errors in both variables on stress field inversion are considered. The distribution of shear stress direction on the fault plane can also be approximately modeled by the von Mises distribution, with the concentration parameter varying from infinity to zero according to the orientation of the fault plane, the direction of the principal stress axes, and the stress shape parameter. Measurement errors in both slip direction and fault plane orientation give rise to misfit angles between the predicted shear stress and the measured slip direction even if the tectonic stress field is uniform. A new inversion method is proposed that takes both measurement errors simultaneously into account. This method makes it possible to evaluate statistically the extent and significance of the deviations of the stress field from uniformity, i.e., the likelihood that a given fault population originates from a constant tectonic stress state. For sufficiently large sample size, misfit angles should belong to a von Mises family if the stress field is uniform. A significant deviation from a von Mises distribution implies that the tectonic stress field is not uniform. A few examples of application of the new inversion method are presented, together with a comparison with the results obtained from different methods and an assessment of the uncertainties in the inverted stress field.
Modeling of volcanic tremor as explosive point sources in a single‐layered, elastic half‐space
Authors: Gordeev Evgenii
Published: February 2008

I investigate path effects on the waveform of a volcanic tremor using a model that consist of an impulsive dilatational point source embedded in a single‐layered, elastic half‐space. Such a source may arise during a variety of volcanic conditions, such as a sequence of rapid explosions at a volcanic vent as well as gas‐piston activity observed in basaltic lava lakes. Using acoustic signals recorded during the 1975 Tolbachik eruption, the 1983 Klyuchevskoy eruption, and the 1989 Tokachi‐dake eruption, I propose a general impulsive time function for the tremor source, consisting either of a single impulse or a sequence of identical impulses. Computed seismograms illustrate the influence of a low velocity, surface layer on the shape and spectra of the waveforms. For the same layered model, a single impulse produces a dispersive wave, while a sequence of identical impulses produces a wave of multiple harmonics. For a single impulse, the peak in the seismograms shifts to higher frequencies when the surface layer is very thin. For multiple impulses, spectral peaks shift to higher frequencies as the time between successive impulses increases, indicating an increasing importance of higher‐order harmonics.
Eruption of rhyolite at the Honeycomb Hills, Utah: Cyclical tapping of a zoned silicic magma reservoir
Authors: Byrd Bradford J. , Nash William P.
Published: February 2008

The Honeycomb Hills volcanic complex, erupted at 4.7 Ma in west central Utah, consists of a 12.5‐m‐thick tephra deposit that is predominantly fallout tuff, overlain by a 0.15 km3 dome of rhyolitic lava. The Honeycomb Hills rhyolite contains unusually high fluorine (up to 3.4 wt %) and lithophile trace elements (Rb to 1900 ppm). The dominantly fallout tephra deposit has been examined in detail, quantifying textural and compositional parameters. Three distinct eruption phases produced the tephra deposit. Phases I and II display similar changes in clast size, clast speciation, crystal content, pumice vesicularity, bedding style, and glass and mineral composition. The final phase of eruption, phase III, exhibits similar compositional changes while texturally recording the transition from explosive to effusive eruption style. Two‐feldspar temperatures yield an average temperature of 570°C with a weakly developed gradient from 550 to 600°C upward through the tephra. ƒH2O/ƒHF increases upsection from 560 near the base of the tuff to a maximum of 2400 in the vitrophyre at the top of the pyroclastic section. A corresponding increase exists in the ƒHCl/ƒHF ratio from 0.17 to 5.85. The mineral assemblage topaz‐anorthite‐annite‐fluorite‐quartz yields fugacities of ƒH2O from 250 to 1200 bars and ƒHF from 0.2 to 0.8 bars. Fugacity ratios, as well as major and minor elements in glass and biotite, exhibit cyclic variation within each eruptive phase whose boundaries are defined by physical parameters. The cyclicity is attributed to dynamic relaxation of the magma reservoir during brief pauses between eruptive phases. The patterns of varying eruption energy are most strongly correlated with vent widening, crystal content, and gas fugacity ratios. Initially, increasing vent radius and tapping of deeper, higher ƒH2O regions strongly increased eruption vigor, evident in phases I and II. During phase III, continuing crystallization, loss of volatiles, increasing viscosity, and growing conduit size from vent erosion led to the transition from explosive to effusive eruption, culminating in the extrusion of the lava dome.
Plate Motions in the north Andean region
Authors: Freymueller Jeffrey T. , Kellogg James N. , Vega Victor
Published: February 2008

Repeated geodetic measurements with the Global Positioning System (GPS) provide direct measurements of displacements due to plate motions and active crustal deformation in Central America and northern South America, an area of complex interaction of the Nazca, Cocos, Caribbean and South American plates. The displacement rates for the period 1988–1991, obtained from the results of the first three Central And South America (CASA) GPS campaigns, are in general agreement with the predictions of the NUVEL‐1 plate motion model, but there are differences in detail between the observations and the model. The Nazca‐North Andes convergence rate vector measured by GPS is different from the NUVEL‐1 vector at 95% confidence. The difference implies that the North Andes are moving northward relative to South America. The measured convergence between the Caribbean plate and the North Andes suggests that the southern margin of the Caribbean plate is located in the South Caribbean deformed belt. The April 1991 Costa Rica earthquake and the Cocos‐Caribbean convergence rate determined by GPS suggest the possibility of significant ongoing deformation between Central America and the stable interior of the Caribbean plate. Our GPS results are consistent with deformation of the overriding plates at the convergent margins of Central and South America and confirm that active convergence is occurring around much of the southern margin of the Caribbean plate, from Colombia west to Costa Rica. Costa Rica and Panama are not part of the stable Caribbean plate. Instead, the South Caribbean deformed belt and the North Panama fold belt probably represent the southern margin of the Caribbean plate.
Nonlinearity of the magnitude‐frequency relation in the Hellenic Arc‐Trench System and the characteristic earthquake model
Authors: Papadopoulos Gerassimos A. , Skafida Helen G. , Vassiliou Ioanna T.
Published: February 2008

Magnitude‐frequency (G‐R) relations of shallow mainshocks have been examined in five separate segments of the Hellenic Arc‐Trench (HA‐T) system, in its western and eastern megasegments as well as in the HA‐T system considered as an entity. Data sets of Ms ≥ 4.5 and Ms ≥ 6.0 covering the 22‐year and 85‐year intervals of 1964–1985 and 1901–1985, respectively, have been used. Frequencies of the 22‐year interval have been normalized to the 85‐year interval so that to obtain frequencies N compatible with the real frequencies, Nr, of the 85‐year interval for magnitudes of Ms ≥ 6.0. Straight G‐R lines determined for N of small and moderate shocks as a rule do not fit, when extrapolated, the larger magnitude frequencies N and Nr. Magnitude gaps, bulge effects, and trends toward the vertical are the geometrical expressions of the nonlinear structure of G‐R in the large magnitude range. Magnitude gaps ranging from 0.5 to 0.9 in the Nr plots and in a wider magnitude spectrum in the N plots, of four out of five separate segments, indicate that the stress fails to release with ruptures of moderate size, thus implying that the characteristic earthquake model may provide an adequate explanation for the G‐R nonlinearity. The Ionian Islands segment constitutes an obvious exception. No magnitude gap in Nr appears and the G‐R linearity is well shaped. Characteristic events may not occur in that segment. When two or more segments are examined together as megasegments, the magnitude gap disappears because of overlapping of separate gaps. The nonlinear structure of G‐R shows the general need for properly designed models of seismic hazard determination.
Ductile deformation and the origin of layered gabbro in ophiolites
Authors: Quick James E. , Denlinger Roger P.
Published: February 2008

Layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3–6 km deep. Although large, steady state magma chambers provide a possible explanation for the limited diversity of mid‐ocean ridge basalts, their existence is not supported by geophysical observations. Sinton and Detrick (1992) recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small (1–4 km wide) magma chambers perched on large (∼8 km wide) zones of low‐velocity material, which are interpreted to consist of partially molten crystal cumulates. In this paper, we compare the geology of the gabbroic rocks of the Samail ophiolite, which is thought to have formed by fast spreading, with results of a finite element analysis of the Sinton‐Detrick model for fast spreading centers. Our results demonstrate that flow, similar to that postulated by Sleep [1975], within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the Samail gabbros. As a consequence of this deformation, (1) layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center, (2) the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime, let alone the magma chamber; (3) feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering, but those that invade farther from the ridge will be less deformed and retain crosscutting relationships; and (4) total deviatoric strain should increase down section toward the Moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros. Upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state.
Azimuthal shear wave velocity anisotropy in the Basin and Range Province using moho Ps converted phases
Authors: McNamara Daniel E. , Owens Thomas J.
Published: February 2008

An eight‐station large aperture seismometer array was deployed in the Basin and Range province of west‐central Nevada for the 1988–1989 Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Passive‐source seismic experiment. During the 10 months of data collection a total of 100 teleseismic events were recorded (Δ > 30°). Source‐equalized P wave receiver functions show waveform variations in timing, amplitude, and polarity in the Moho Ps phase that is diagnostic of shear wave velocity anisotropy. Information regarding the crustal anisotropic component can be recovered through the Ps phase since it is generated at the Moho and is sensitive to shear wave splitting in the crust only. We utilize single‐event, stacked, and array‐beam‐formed receiver functions to minimize the influence of structural variations and scattering on the observed converted phases. Analysis of shear wave splitting of the Ps conversion from the crust‐mantle boundary indicates a fast azimuth of anisotropy oriented approximately NW‐SE with observed time delays of 0.20 s. This strongly suggests coherent crustal anisotropy with a fast direction perpendicular to the strike of fault block mountain ranges. This fabric is most likely due to the preferred crystallographic orientation of seismically anisotropic minerals in the middle to lower crust rather than the distribution of Basin and Range upper crustal fracture systems.
Imaging crustal structure in southwestern Washington with small magnetometer arrays
Authors: Egbert Gary D. , Booker John R.
Published: February 2008

We use data from a series of small (three to five stations) overlapping magnetovariational (MV) arrays to image variations of vertically integrated electrical conductivity in the crust of southwestern Washington. Two principal structures are revealed: a large north‐south trending anomaly (the southern Washington Cascades Conductor (SWCC), which has been detected by several previous induction experiments), and a smaller anomaly which branches off of the SWCC just north of Mount St. Helens and trends westward beneath the Chehallis Basin. A weaker east‐west trending anomaly is evident farther to the north beneath southern Puget Sound. The MV results concerning the SWCC are reasonably consistent with the model of Stanley et al. (1987), who interpret the anomaly as a suture zone of mid‐late Eocene age, but the array data allows us to map the horizontal extent and complex three‐dimensional character of the SWCC in greater detail. We suggest that the SWCC represents a section of the early Cenozoic subduction zone which is analogous to the present‐day Olympic Peninsula. In the region west of the Cascades, the array data show that crustal conductivity is distinctly three dimensional, consisting of highly resistive blocks (crystalline rocks) separated (in the upper 5–10 km at least) by interconnected narrow regions of higher conductivity (sedimentary units). This pattern of conductivity variations is consistent with the inferred origin of the region as a seamount complex, which was subsequently broken into discrete blocks which have been thrust together during and after accretion to the North American continent. The distribution of anomalous electric currents and our model for crustal conductance are in striking agreement with a variety of other geophysical constraints, including gravity, magnetics, present crustal seismicity, and the pattern of recent volcanic vents. The St. Helens seismic zone (SHZ), which coincides with the western edge of the broad southern portion of the SWCC, is abruptly terminated in the north by the smaller east‐west trending conductive zone. North of the SHZ near Mount Rainier, seismicity is concentrated in a narrow band coincident with the very narrow northern portion of the SWCC. In addition, volcanic vents are concentrated around the edges of the SWCC but are rare in the interior of the zone of high conductivity. The magnetometer array data thus suggest that present patterns of crustal deformation and volcanism are in part controlled by the complex tectonic history (and resulting crustal structure) of the region.
Do lower crustal rocks record reversals of the Earth's magnetic field? Magnetic petrology of oceanic gabbros from Ocean Drilling Program hole 735B
Authors: Pariso Janet E. , Johnson H. Paul
Published: February 2008

Ocean Drilling Program (ODP) Leg 118 at the Southwest Indian Ridge recovered the first vertically oriented, continuously cored gabbros from in situ ocean crust. Magnetic properties and oxide petrography of drillcore samples were analyzed in order to determine the processes which control the remanent magnetization of oceanic layer 3 and the ability of gabbros to record reversals of the Earth's magnetic field. Primary, igneous magnetite is common only in evolved, Fe‐Ti oxide‐rich gabbros and averages, by volume, 2% of the rock, although substantially higher contents are common. Secondary magnetite, produced by high‐temperature exsolution and hydrous alteration of olivine and pyroxene, is observed in all gabbros studied and is the most important magnetic phase in the majority of recovered gabbros (cumulate olivine gabbros). Because of the high temperatures associated with this alteration, it is likely that the primary magnetization of this crustal section was a thermal remanent magnetization, or a very high temperature chemical remanent magnetization acquired very near the ridge crest. Hysteresis loop parameters indicate that all of the studied samples are capable of carrying significant, stable remanent magnetization. Finally, previously published drilling results and field studies from ODP Site 735B indicate that although this crustal section has been tectonically uplifted, it formed at a mid‐ocean ridge spreading center, similar to much of the oceanic crust. Therefore the results of this magnetic study can be extrapolated to include the oceanic gabbro section, in general, and argue strongly that layer 3 crustal rocks are capable of recording reversals of the Earth's magnetic field.