bluuebunny/crossref_metadata_2025 huggingface dataset:
Field line resonance interference model for multiple auroral arc generation
Authors: Lyatsky W. , Elphinstone R. D. , Pao Q. , Cogger L. L.
Published: September 2002

A simple model for the generation of poleward moving multiple auroral structures in the nightside ionosphere is described. The model is based upon the premise that field line resonance oscillations are responsible for auroral arc generation. The auroral structures are considered to be a result of interference between Alfven field line resonance oscillations on different L shells. Because of the dependence of the Alfven resonance frequency on L shell, any two neighboring L shells which oscillated initially with the same phase, after some time appear to be in opposite phase. That results in the appearance of counter‐directed electric fields at these L shells and a large field‐aligned current between them, which can produce the electron acceleration along the magnetic field and be responsible for auroral arc generation. The out of phasing between the oscillating L shells begins first at low latitudes and then propagates poleward producing poleward moving auroral arcs. The model requires that oscillations be maintained for many cycles of the waves and the source for energy is ultimately derived from magnetospheric convection in the plasma sheet. The field line resonance interference model explains periodic behavior in such auroral activity having frequencies in the Pc5 range. It is significant that the period of oscillation may be greater than that for the Alfven resonant oscillations, which implies that two separate frequencies may appear on the same field line. The poleward propagation of the auroral arcs in this model is a result of the poleward propagation of interference maxima rather than any real plasma or wave propagation.
Low‐frequency surface waves in a structured magnetized dusty plasma
Authors: Ostrikov K. N. , Vladimirov S. V. , Yu M. Y.
Published: September 2002

It is shown that charged dust particles carrying a considerable proportion of the negative charge of a structured magnetized plasma can lead to low‐frequency electromagnetic surface waves which otherwise do not exist. The waves are Alfvén‐like and propagate across the stationary external magnetic field with a frequency below the ion cyclotron but much above the dust cyclotron frequency. The dispersion characteristics of the modes are obtained and applications to space plasmas discussed.
Nonlinear instability of the geomagnetotail current sheet combining the features of tearing and cross‐field current instabilities
Authors: Kropotkin A. P. , Trubachev O. O. , Lui A. T. Y.
Published: September 2002

The substorm onset instability suggested in this paper combines the features of “macroscale” electromagnetic disturbance of the tearing mode type and the “microscale” plasma turbulence, which is generated by the cross‐field current instability and modulated in coherence with the large‐scale disturbance. Both components are involved in a feedback loop. It is assumed that the current sheet (CS) is thin enough for cross‐field current instability (CCI) turbulence to already exist. Current density variations of the large‐scale disturbance result in additional CCI generation; on the other hand, that additional plasma turbulence provides in quasi‐linear approximation a flux of electrons which violates the frozen‐in condition for magnetized electrons and thus destabilizes the tearing mode. The process is nonlinear. However, the threshold amplitude diminishes as the CS thins, so that the instability sets in when the threshold gets smaller than the background fluctuations.
Pitch angle scattering near energy resonances in the geomagnetic tail
Authors: Delcourt D. C. , Martin R. F.
Published: September 2002

In sharp field reversals where the particle Larmor radius is larger than the magnetic field line curvature radius, particles may not conserve their magnetic moment (first adiabatic invariant). We examine the pitch angle scattering which affects the particles under such conditions. We show that this scattering alternatively leads to a clear three‐branch pattern of magnetic moment variations or to unstructured variations. The three‐branch pattern is characterized by systematic enhancements of magnetic moment at relatively small (up to ∼30°) pitch angles, negligible change at large pitch angles and, in between, either enhancement or damping depending on phase. We show that this three‐branch pattern emerges near the energy resonances reported by Burkhart and Chen [1991], where particles escape after transient oscillations inside the reversal. Away from resonance, the three‐branch pattern gradually expands and affects an increasing volume of the velocity space. Such a structuring of pitch angle scattering is similar to that obtained in weak field reversals (i.e., for Larmor radii comparable to the field line curvature radius), even though the particle orbits in sharp and weak reversals are significantly different. This fact allows us to extend the centrifugal impulse model developed for weak reversals to sharp ones. In this model, nonadiabatic behavior is viewed as the result of perturbation of the particle gyromotion by an impulsive centrifugal force. We demonstrate that this model reproduces essential features of the particle dynamics in sharp reversals and, in particular, the energy resonance phenomenon. We show that in a like manner to weak reversals, three‐branch dynamics in sharp reversals has significant implications for particle injection into the loss cone and gyrophase bunching near the magnetotail midplane.
A statistical study of banded magnetospheric emissions
Authors: LaBelle J. , Ruppert D. R. , Treumann R. A.
Published: September 2002

Satellite wave instruments commonly detect banded magnetospheric emissions between harmonics of the electron gyrofrequency fce in the outer magnetosphere. To complement an earlier statistical study on the amplitude of these emissions, we investigate the statistical properties of their frequencies by identifying the spectral peaks in 193 sample spectra observed with the Active Magnetospheric Particle Tracer Experiment/Ion Release Module (AMPTE/IRM), normalizing these peaks to fce as determined by the onboard magnetometer, and plotting them against the ratio of the plasma to gyrofrequency fpe/fce where fpe was determined from the onboard particle or wave instrument. A histogram of the spectal peaks normalized by fce reveals well‐defined peaks between harmonics of fce. In this histogram, as well as in individual spectra, the higher‐frequency peaks tend to occur closer to the electron gyroharmonic than do the lower‐frequency peaks. In about 15% of the spectra sampled, more than one peak was observed between gyroharmonics. The statistics also suggest that the banded emissions shift downward in normalized frequency with increasing fpe/fce although this observation is subject to the considerable uncertainty in the measured fpe. These trends may be consistent with several wave generation mechanisms such as excitation of electron Bernstein modes destabilized by non‐Maxwellian magnetospheric electron distributions.
A new look at the >70 Mev cosmic ray radial gradients in the heliosphere measured by spacecraft
Authors: Webber W. R. , Lockwood J. A.
Published: September 2002

In this paper we critically reexamine the interplanetary cosmic ray radial gradients of >70 MeV cosmic rays as observed by the IMP, Voyager, and Pioneer spacecraft between 1978 and 1996. We separate out three distinct effects on the radial gradient which are due to (1) intensity changes, (2) changes in the radial distance, and (3) the influence of the solar magnetic polarity. The new characteristics that we find, which are at odds in several important ways with many earlier studies, can be summarized as follows: First, the value of Gr depends strongly on the cosmic ray intensity in periods of negative solar magnetic polarity (e.g., 1983.0 – 1989.5). This intensity dependence is only weakly evident in periods of positive polarity. During the negative polarity periods this is the dominant effect changing the gradients. It masks the radial effects which show only a weak dependence at this time. Second, the radial dependence of the gradient has two regimes. Beyond 10–20 AU the gradient is only weakly dependent on r and is quite small. Between 1 and 10–20 AU the gradient exhibits a very rapid radial falloff during the minimum modulation periods of 1978, 1987, and 1996. This decrease could be a factor of 10 or more in 1978 and 1996. Third, the values of Gr show a strong solar magnetic field polarity dependence. Inside 10–20 AU this is manifested in a much more rapid radial falloff in the 1978 and 1996 positive polarity periods as compared with the 1987 negative polarity period. Beyond 10–20 AU the average radial gradient in the positive cycle in 1996 is only ∼0.5%/AU or about 1/3 of that (1.5%/AU) in the negative polarity cycle in 1987. In the positive polarity period the gradient actually increases at large r. This increase, which is predicted in some models as one nears the boundary, may provide a new way of estimating the distance from the modulation boundary. Some features of the behavior of the radial gradient that we observe are predicted by current modulation models, but other aspects of this behavior require further modeling for a better understanding.
Resonant amplification of MHD waves in realistic subsolar magnetopause configurations
Authors: De Keyser J. , Roth M. , Reberac F. , Rezeau L. , Belmont G.
Published: September 2002

Broadband ULF fluctuations are routinely observed throughout the magnetosheath; the fluctuation level peaks at the magnetopause and becomes very small in the magnetosphere. The present paper analyzes the propagation of magnetosheath waves and the transport of energy at the subsolar magnetopause by means of a linear perturbation analysis in the limit of the MHD approximation. We examine realistic equilibrium magnetopause configurations with a cold and dense magnetosheath and a hot, tenuous magnetosphere, possibly including a trapped magnetopause population. The effects of magnetic field rotation are examined. Resonant amplification of monochromatic magnetosonic waves at the magnetopause is found to occur under various conditions. For a given frequency, several field lines inside the magnetopause layer can resonate simultaneously.
Quasi‐biennial and higher‐period oscillations in the mean winds in the mesosphere and lower thermosphere over Saskatoon, 52°N, 107°W
Authors: Kane R. P. , Meek C. E. , Manson A. H.
Published: September 2002

The mesospheric zonal and meridional winds over Saskatoon (52°N, 107°W) show significant QBO (quasi‐biennial oscillations), with periods of 2.01, 2.42, 2.30, 2.37, and 2.31 years at 94, 91, 88, 85, and 82 km, respectively, in the meridional (north) component and 2.51, 2.88, 2.52, 2.29, and 2.61 years in the zonal (east) component, mostly not matching with the QBO (∼2.50 years) of equatorial stratospheric winds. Also, significant higher periodicities (∼4 years) are seen in the mesospheric winds, indicating that QBO and higher periodicities in the midlatitude mesosphere could be highly distorted forms of stratospheric wind QBO and/or may have altogether different origins (e.g., El Niño‐Southern Oscillation effects).
The role of nonlinear interaction in the formation of LF whistler turbulence upstream of a quasi‐perpendicular shock
Authors: Balikhin M. A. , Alleyne H. St.‐C. K. , Treumann R. A. , Nozdrachev M. N. , Walker S. N. , Baumjohann W.
Published: September 2002

The role of the nonlinear interaction in developed low‐frequency turbulence has been experimentally studied upstream of the ramp of a quasiperpendicular shock. The study has been carried out by application of the methods of bispectral analysis and wavelet decomposition to the Active Magnetospheric Particle Tracer Explorer Ion Release Module and Interball Tail Probe magnetic field data. It is shown that three‐wave processes play a key role in the formation of the spectrum of the turbulence. Experimental results presented and previous theoretical considerations lead to the conclusion that energy is transfered from a narrow maximum frequency pumped by nonlinear dynamic processes to lower and higher frequencies.
Low‐frequency electromagnetic waves near and below the proton cyclotron frequency at the AMPTE Ba release: Relevance to comets and Mars
Authors: Sauer K. , Dubinin E. , Dunlop M. , Baumgartel K. , Tarasov V.
Published: September 2002

“Nongyrotropic massloading” is a characteristic signature of the AMPTE barium release in the solar wind because of the large pickup radius compared with the size of the interaction region. In the fluid picture this is equivalent to a beam‐plasma configuration where the solar wind protons have a large relative velocity to the heavies, which can be considered as unmagnetized. Dispersion analysis on the basis of bi‐ion HaIl‐MHD equations gives that two types of low‐frequency electromagnetic waves are excited. The “upper” instability occurs in the branch of whistler/magnetoacoustic waves with the remarkable feature that the frequency in the beam (spacecraft) frame of the most unstable wave is close to the proton cyclotron frequency. Its growth rate strongly depends on the electron plasma beta (βe) which preferentially determines the whistler damping at oblique propagation. In consequence, the increase of the growth rate with the propagation angle is limited and an “instability gap” results at intermediate angles (40° ≤ θ ≤ 70°). For β ≈ 1, only small ranges near parallel and transverse propagation remain. For a high beam velocity the “lower” instability is determined by the cutoff frequency of the bi‐ion system. Both lines were clearly seen in the spectra of magnetic field data of the AMPTE Ba release during the transition phase when the Ba ion density decreases below the solar wind density. These results may be relevant for other situations in space: comets with low gas production rates (q ≤ 1028 s−1), nonmagnetized planets (Venus, Mars)and active moons (Phobos, Deimos) including their gas tori.
Evidence for a global disturbance with monochromatic pulsations and energetic electron bunching
Authors: Lessard M. R. , Hudson M. K. , Anderson B. J. , Arnoldy R. L. , Lühr H. , Reeves G. D. , Sato N. , Weatherwax A. T.
Published: September 2002

We present data from a number of ground stations and satellites that reveal an example of a previously unreported type of global event. The event is characterized by the occurrence of monochromatic pulsations of widely varying frequencies in different regions of local times and L shells. The pulsations appear to be modulated with a ∼45 min periodicity. Simultaneously, energetic particle fluxes observed at geosynchronous orbit (with energies resulting in a drift period of ∼45 min) appear to become phase bunched and the pulsations are observed to occur coincident with minima in the particle bunching. Corresponding data from GOES 5 show an increase in the compressional component of the magnetic field with this period. Data from South Pole Station show fluctuations in the east–west component of the magnetic field at half this period, along with a weak auroral signature and riometer absorptions. We conclude that the data show the existence of a global disturbance with a compressional magnetic field signature, and we suggest that this compression induces a radial electric field, based on Faraday's law. Phase bunching of energetic electrons (due to the induction electric field) and monochromatic pulsations are consequences of the global disturbance, although the mechanism responsible for exciting the pulsations is not clear.
Is there evidence for the existence of nonlinear behavior within the interplanetary solar sector structure?
Authors: Brown A. G. , Francis N. M. , Broomhead D. S. , Cannon P. S. , Akram A.
Published: September 2002

Using data from the Sweden and Britain Radar Experiment (SABRE) VHF coherent radar, Yeoman et al. [1990] found evidence for two and four sector structures during the declining phase of solar cycle (SC) 21. No such obvious harmonic features were present during the ascending phase of SC 22. It was suggested that the structure of the heliospheric current sheet might exhibit nonlinear behavior during the latter period. A direct test of this suggestion, using established nonlinear methods, would require the computation of the fractal dimension of the data, for example. However, the quality of the SABRE data is insufficient for this purpose. Therefore we have tried to answer a simpler question: Is there any evidence that the SABRE data was generated by a (low‐dimensional) nonlinear process? If this were the case, it would be a powerful indicator of nonlinear behavior in the solar current sheet. Our approach has been to use a system of orthogonal linear filters to separate the data into linearly uncorrelated time series. We then look for nonlinear dynamical relationships between these time series, using radial basis function models (which can be thought of as a class of neural networks). The presence of such a relationship, indicated by the ability to model one filter output given another, would equate to the presence of nonlinear properties within the data. Using this technique, evidence is found for the presence of low‐level nonlinear behavior during both phases of the solar cycle investigated in this study. The evidence for nonlinear behavior is stronger during the descending phase of SC 21. However, it is not possible to distinguish between nonlinear dynamics and a nonlinearly transformed colored Gaussian noise process in either instance, using the available data. Therefore, in conclusion, we find insufficient evidence within the SABRE data set to support the suggestion of increased nonlinear dynamical behavior during the ascending phase of SC 22. In fact, nonlinear dynamics would seem to exert very little influence within the measurement time series at all, given the observed data. Therefore it is likely that stochastic or unresolved high‐dimensional nonlinear mechanisms are responsible for the observed spectrum complexity during the ascending phase of SC 22.
Microstructure of the auroral acceleration region as observed by FAST
Authors: McFadden J. P. , Carlson C. W. , Ergun R. E.
Published: September 2002

The Fast Auroral Snapshot (FAST) explorer satellite was designed to investigate the microscale structure of the auroral acceleration region that was unresolved by previous satellites. This paper will present highlights from the first 2 years of the FAST mission and compare them with previous observations and auroral models. In particular, we find good agreement with the overall field‐aligned current systems previously discovered; however, we present evidence that the downward currents are often carried by energetic upgoing electrons. These upgoing electrons are correlated with diverging electrostatic shocks, indicating that quasi‐static parallel electric fields are responsible for their energization. Some of these field‐aligned fluxes contain large‐amplitude fast solitary waves which produce strong modulations of the electrons. Observations in inverted‐V arcs show that the parallel acceleration region contains narrow (∼10 km) fingers of potential that extend along the magnetic field. On these narrow kilometer scales, ion beam energies are found to agree with the inferred potential determined from the perpendicular electric field or from the widening of the electron loss cone, implying acceleration is typically quasi‐static on ion transit times. We also find evidence that both the ion and electron upgoing beams produced by the parallel electric fields have plateaued parallel distribution functions. Fast solitary waves are a prime candidate to stabilize the electron beams and may provide the resistance that allows the downward directed parallel electric fields to form in the highly conducting ionosphere. Intense ion cyclotron waves and ion solitary waves are often observed during ion beams, but the stabilizing waves have not been identified. In addition, intense electromagnetic ion cyclotron waves are also observed in inverted‐V arcs, along with strongly modulated electron fluxes, indicating that turbulent acceleration is occurring in addition to simple acceleration by a static potential drop.
Laboratory simulation of broadband ELF waves in the auroral ionosphere
Authors: Koepke M. E. , Carroll J. J. , Zintl M. W.
Published: September 2002

Space‐relevant observational signatures, such as frequency spectra, phase velocities, excitation thresholds, and ion heating, associated with electrostatic ion‐cyclotron waves excited by the inhomogeneous energy‐density driven (IEDD) instability in a laboratory experiment are presented. A comparison is made between these waves and the broadband ELF waves recently observed in the auroral ionosphere with sounding rockets and satellites. The measurements of broadband spectra, electron‐Landau‐resonant phase velocities, low excitation threshold value of parallel electron drift speed, and significant perpendicular ion heating suggest that attributing the broadband ELF waves to the IEDD instability mechanism is justified.
Effects of magnetospheric precipitation and ionospheric conductivity on the ground magnetic signatures of traveling convection vortices
Authors: Zhu L. , Schunk R. W. , Sojka J. J.
Published: September 2002

By using an improved TCV model (Zhu et al., 1997), a quantitative study of the effects of magnetospheric precipitation and ionospheric background conductivity on the ground magnetic signatures of traveling convection vortices (TCVs) has been conducted. In this study the localized conductivity enhancement associated with the TCVs is present and the ratio of the Hall and Pedersen conductances vary both spatially and temporally according to the hardness of the TCV precipitation. It is found that a strong conductivity enhancement associated with hard TCV precipitation can significantly distort the TCV current closure in the ionosphere and lead to ground magnetic disturbance patterns with strong asymmetry in E–W direction. The asymmetry of the ground magnetic patterns is characterized by a stronger magnetic disturbance on the side of the upward field‐aligned currents (clockwise convection cell) and a possible rotation of the whole magnetic patterns. Specifically, the modeling results predict that when the characteristic energy of the TCV precipitation is below 500 eV, the asymmetry of the ground magnetic patterns is minimal (less than 1%) and may not be detectable. When the characteristic energy of the precipitation is about 7 keV, the asymmetry of the magnetic patterns can be well above 30%. It is also found that a low ionospheric background conductivity favors the appearance of strong asymmetry in the ground magnetic patterns of the TCVs, while a high ionospheric background conductivity favors the appearance of strong ground magnetic disturbances but with less asymmetry. We concluded that the most favorable condition for the appearance of strong asymmetry in the TCV ground magnetic signatures is the condition of winter, solar minimum, and hard precipitation.
Large‐amplitude transient growth in the linear evolution of equatorial spread F with a sheared zonal flow
Authors: Flaherty J. P. , Seyler C. E. , Trefethen L. N.
Published: September 2002

The occurrence of equatorial spread F on the bottomside of the Flayer is likely the result of a process often referred to as the collisional interchange instability or generalized Rayleigh‐Taylor instability. The traditional approach to the analysis of this instability with sheared zonal flow has been to calculate the eigenvalues of the linearized system. It is well documented that the introduction of shear has a stabilizing effect on the eigenvalues and significantly increases the wavelength corresponding to the fastest growing eigenmode. In this paper it is argued that the well‐accepted conclusions drawn from eigenvalue analyses are not correct for cases of geophysical interest. A calculation of the ε pseudospectra demonstrates that the system is highly nonnormal and that large‐amplitude transients may exist even when all of the eigenmodes are decaying with time. Transient effects are shown to be of fundamental importance to the evolution of the system over a wide range of horizontal wavelengths. This viewpoint is consistent with some aspects of VHF radar observations of F region gravitational interchange dynamics and may explain the presence of irregularities which appear to be confined to the bottomside of the F layer. The techniques discussed are of general interest to the analysis of nonnormal linear systems.
Experimental and theoretical fracture mechanics applied to Antarctic ice fracture and surface crevassing
Authors: Rist M. A. , Sammonds P. R. , Murrell S. A. F. , Meredith P. G. , Doake C. S. M. , Oerter H. , Matsuki K.
Published: September 2002

Recent disintegration of ice shelves on the Antarctic Peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally. In this paper we present a fracture criterion, incorporating new experimental fracture data, coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the Filchner‐Ronne Ice Shelf. We have developed experiments that have enabled us to quantify, for the first time, quasi‐stable crack growth in Antarctic ice core specimens using a fracture initiation toughness, Kinit, for which crack growth commences. The tests cover a full range of near‐surface densities, ρ = 560–871 kg m−3 (10.9–75.7 m depth). Results indicate an apparently linear dependence of fracture toughness on porosity such that Kinit = 0.257 ρ‐80.7, predicting a zero‐porosity toughness of Ko = 155 kPa m1/2. We have used this data to test the applicability to crevassing of a two‐dimensional fracture mechanics criterion for the propagation of a small sharp crack in a biaxial stress field. The growth of an initial flaw into a larger crevasse, which involves a purely tensile crack opening, depends on the size of the flaw, the magnitude of Kinit and the nature of the applied stress field. By incorporating the criterion into a stress map of the Filchner‐Ronne Ice Shelf derived from a depth‐integrated finite element model of the strain‐rate field, we have been able to predict regions of potential crevassing. These agree well with satellite imagery provided an initial flaw size is assumed in the range 5–50 cm.
Carbon‐enhanced electrical conductivity during fracture of rocks
Authors: Roberts J. J. , Duba A. G. , Mathez E. A. , Shankland T. J. , Kinzler R.
Published: September 2002

Changes in electrical resistance during rock fracture in the presence of a carbonaceous atmosphere have been investigated using Nugget sandstone and Westerly granite. The experiments were performed in an internally heated, gas‐pressure vessel with a load train that produced strain rates between 10−6 and 10−5 s−1. Samples were deformed at temperatures of 354° to 502°C and pressures of 100 to 170 MPa in atmospheres of Ar or mixtures of 95% CO2 with 5% CO or 5% CH4, compositions that are well within the field of graphite stability at the run conditions. In experiments using Nugget sandstone, resistance reached a minimum value when the maximum temperature was achieved and good electrode contact was made. The resistance then increased as the experiment continued, probably due to dry out of the sample, a change in the oxidation state of the Fe‐oxide associated with the cement, or destruction of current‐bearing pathways. At approximately 200‐MPa end load, the rock sample failed. Plots of load and resistance versus time show several interesting features. In one experiment, for example, as the end load reached about 175 MPa, resistance stopped increasing and remained fairly constant for a period of approximately 0.5 hour. During loading, the end load displayed small decreases that were simultaneous with small decreases in resistance; when the end load (and the displacement) indicated rock failure, resistance decreased dramatically, from ∼150 MΩ to 100 MΩ. In a single experiment, the Westerly granite also showed a decrease in resistance during dilatancy. The nature and distribution of carbon in the run products were studied by electron microprobe and time‐of‐flight secondary‐ion mass spectroscopy (TOP‐SIMS). Carbon observed by mapping with the former is clearly observed on micro‐cracks that, based on the microtexture, are interpreted to have formed during the deformation. The TOF‐SIMS data confirm the electron‐probe observations that carbon is present on fracture surfaces. These observations and experimental results lead to the hypothesis that as microfractures open in the time leading up to failure along a fracture, carbon is deposited as a continuous film on the new, reactive mineral surfaces, and this produces a decrease in resistance. Subsequent changes in resistance occur as connectivity of the initial fracture network is altered by continued deformation. Such a process may explain some electromagnetic effects associated with earthquakes.
The oldest magnetic anomalies in the Australian‐Antarctic Basin: Are they isochrons?
Authors: Tikku Anahita A. , Cande Steven C.
Published: September 2002

We present a revised tectonic interpretation of Australia and Antarctica incorporating new magnetic data off of Wilkes Land, Antarctica, for the earliest period of seafloor spreading on the Southeast Indian Ridge, from the Late Cretaceous to Early Tertiary. Reconstructions based on our revised anomaly identifications are characterized by a surprisingly large amount of continental overlap, involving the South Tasman Rise, Tasmania, and the Victoria Land region of Antarctica. The overlap is due to continental extension and/or deformation of oceanic crust in the Australian‐Antarctic Basin. The second hypothesis would imply that the magnetic anomalies older than chron 31 in the Australian‐Antarctic Basin are not isochrons. We also find evidence for a previously unrecognized period of extremely slow spreading on the Southeast Indian Ridge from chron 31 (68.7 Ma) to chron 24 (53.3 Ma) in the Australian‐Antarctic Basin. In the western end of the basin we have tentatively identified a hiatus in the generation of crustal accretion associated with a minimum sustainable threshold half spreading rate of 1.5 mm/yr. In addition, we recognize a new fracture zone on the Antarctic plate, the Vincennes fracture zone, as the conjugate to the Perth fracture zone on the Australian plate, in the western Australian‐Antarctic Basin. These fracture zones record the initial NW‐SE opening direction between Australia and Antarctica.
Rupture processes of large deep‐focus earthquakes from inversion of moment rate functions
Authors: Antolik Michael , Dreger Douglas , Romanowicz Barbara
Published: September 2002

We determine the source time histories of five recent (1994–1996) large, deep‐focus earthquakes using a method that inverts for fault slip from far‐field moment rate functions. The moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from Global Seismic Network (GSN) and GEOSCOPE stations. Tests of this method on synthetic data indicate that it is successful in determining the low‐frequency rupture characteristics of deep earthquakes under a variety of complicating conditions. We find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the June 9, 1994, great Bolivian earthquake are also found for other events. Comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep‐focus earthquakes is probably occurring within the active slab interior. This provides further evidence that temperature‐controlled mechanisms (such as transformational faulting) play a large role in deep earthquake faulting. Most of the events studied also show a tendency for horizontal rupture propagation, suggesting that isobaric processes may be an important factor in controlling progression of the rupture. Large gaps in the slip distributions point to possible occurrence on multiple fault planes.
Regional phase excitation and propagation in the Lop Nor region of central Asia and implications for P/Lg discriminants
Authors: Xie Jiakang , Patton Howard J.
Published: September 2002

Pn and Lg spectra for Lop Nor explosions and nearby earthquakes were analyzed for source and propagation effects using a modified inversion method of Xie [1993]. Spectral analyses were performed on waveform data from Incorporated Research Institution for Seismology, Chinese Digital Seismic Network, Kazakhstan, and Krgyzstan networks. Pn amplitude variations of a factor of 20 are observed across the small‐aperture (< 200 km) Kyrgyzstan network, and are probably related to three‐dimensional Moho structure close to the network. Comparisons of Pn amplitudes and spectral ratios provide evidence for nonisotropic radiation pattern effects in the case of earthquakes and overshoot in explosion time histories. Complex source effects are born out in our inversion results: (1) logarithm of seismic moment (M0) from Pn and Lg spectra scale linearly with body wave magnitude (mb) for explosions (5.0 ≤ mb ≤ 6.0) and for earthquakes (4.0 ≤ mb ≤ 5.5); (2) Pn M0 estimates are similar to Lg M0 for earthquakes but are higher than Lg M0 for explosions; (3) corner frequency fc scale as quarter root of M0 for explosions and cube root for earthquakes; (4) at the same M0 level, Pn fc are 4–5 times higher than Lg fc for earthquakes and explosions; and (5) Pn source spectra are characterized by significant overshoot for explosions. Our results suggest that the cause of maximum separation between earthquake and explosion populations on plots of P/Lg spectral ratios, which typically occurs for frequencies above explosion Pn fc (between 2 and 4 Hz for explosions in this study), may be related to overshoot in the reduced velocity potential.
Numerical simulations of granular shear zones using the distinct element method: 2. Effects of particle size distribution and interparticle friction on mechanical behavior
Authors: Morgan Julia K.
Published: September 2002

Two dimensional (2‐D) numerical simulations were conducted using the distinct element method (DEM) to explore the influences of particle size distribution (PSD), defined by 2‐D power law exponent D, and of interparticle friction μp on mechanical behavior and strength of granular shear zones. The value of D ranged from 0.81 (a coarse breccia) to 2.60 (fine‐grained gouge); μp was assigned to 0.10, 0.50, or 0.75; normal stresses on the shear zone walls, σn, varied from 40 to 140 MPa; assemblages were sheared to 200% strain. Fault friction, defined as the ratio of shear to normal stress, μf = τ/σn, was quite low for all experiments. Low μp suites yielded μf ≈ 0.20–0.25, while higher μp values resulted in only slightly higher values for μf ≈ 0.25–32. The stress‐strain response of the latter experiments was similar to that of overconsolidated granular assemblages: a peak strength was reached by about 10% strain, followed by a period of strain weakening to 30–50% strain, and finally stabilizing at a residual strength for the rest of the experiment. The transitional phases were accompanied by increasing shear zone dilation of up to about 1.5%. The low μp suites behaved more as normally consolidated assemblages; they showed no noticeable strain weakening and relatively minor dilation of about 0.2%. The anomalously low strengths of the simulated assemblages can be explained largely by high degrees of particle rolling. Periodic drops in shear strength during residual deformation phase of the experiments correlated directly with reduced rates of dilation and the localization of strain. Fault strength also showed second‐order variations with D: the low μp suites showed a steady decline in maximum residual strength μfmax with increasing D due to the importance of interparticle sliding in all configurations; in the higher μp suites, μfmax decreased for D values less than a characteristic value of 1.60, then leveled out for increasing D. This may be explained by the increasing importance of particle rolling as small particles became more abundant with increasing D; the particles began to self‐organize and strain became more localized. Although the simulations lack particle fracture, they offer insight into how micromechanics control the mechanical evolution of granular shear zones.
Interpretation of new aeromagnetic anomaly data from the central Antarctic Peninsula
Authors: Johnson Ashley C.
Published: September 2002

New high‐resolution aeromagnetic anomaly data from the central Antarctic Peninsula reveal a magnetic signature typical of a magmatic arc system. Analysis of the new aeromagnetic map, together with susceptibility data and 2.5‐dimensional modelling support the interpretation of the Pacific margin anomaly as reflecting a mid crustal batholithic province consisting of magnetite‐rich plutons. Examination and analysis of the magnetic anomaly map shows variations in magnetic character along the Antarctic Peninsula. These variations, in conjunction with geological and other geophysical data sets, are interpreted as indicating distinct segments of continental crust. Of particular importance is the identification of three distinct areas of basement on the eastern side of the Antarctic Peninsula, of which the central area (66°S to ∼68°) appears to be gneissic basement at a relatively high crustal level. To the north of 66°S, the basement is formed of late Palaeozoic‐?Triassic accretionary material, and to the south of ∼68°S the basement is unknown. Segmentation is a common feature of arcs worldwide, and previous studies of the Antarctic Peninsula have linked the continental segmentation to the arrival of a series of ridge crests at the subducting Pacific margin. This study indicates that the major segmentation of the continental crust is linked to the pre‐Cretaceous crustal structure of the peninsula, which was subsequently modified by extension during the Cenozoic. It is suggested that this Cenozoic extension was caused by a reduction in plate convergence rates at two distinct times, rather than the arrival of ridge segments at the trench, as previously proposed.
Small‐scale convection and divergent plate boundaries
Authors: Boutilier R. R. , Keen C. E.
Published: September 2002

This paper explores the suggestion that observations of large igneous crustal thickness at rifted volcanic margins may in part be explained by small‐scale convection in the upper mantle. This may increase the delivery of magma to the overlying lithosphere without the need for anomalously high mantle temperatures. This concept is quantitatively assessed by numerical modeling of the flow, caused by divergent plate motions, in a viscous, temperature‐ and pressure‐dependent, nonlinear fluid. Significant time‐dependent small‐scale convection is generated at the lower model viscosities (and/or higher temperatures, and/or sharper rift geometries). These models can provide the required thick igneous crust observed at volcanic margins. However, they also give excessive variations in the thickness of igneous crust within the ocean basin and are therefore unacceptable. An additional factor we have explored is the recent suggestion that the oceanic mantle may become dehydrated when melt is generated and removed. This increases its viscosity by 2 to 3 orders of magnitude in the melting zone, above ˜80 km. The high‐viscosity lid stabilizes the flow and provides a more uniform oceanic crustal thickness, while at the same time allowing for vigorous small‐scale convection during the early history of the rift and the delivery of thick igneous crust against the margin. This factor, coupled to the models for flow described above, allow the main features of volcanic margins to be simulated. Besides the thick igneous crust predicted at the margins, these models suggest that time‐dependent small‐scale convection below the margin may persist for tens of million years following the onset of seafloor spreading and that there may be coupling between this flow at the margin and that at the ridge axis. The periodicity of this time‐dependent convection is primarily dictated by the model viscosities; the models suggest that episodicities of tens of million years are reasonable. These episodicities may be reflected in the record of vertical motions at rifted margins.
Investigation of the relationships between basin morphology, tectonic uplift, and denudation from the study of an active fold belt in the Siwalik Hills, central Nepal
Authors: Hurtrez J.‐E. , Lucazeau F. , Lavé J. , Avouac J.‐P.
Published: September 2002

The present study investigates correlations between an extensive range of geomorphic properties that can be estimated from a digital elevation model and the uplift rate on geological timescales. The analysis focuses on an area in the Siwalik Hills (central Nepal), where lithology and climate can be considered as uniform. This area undergoes rapid tectonic uplift at rates of up to 15 mm yr−1, which are derived from the geometric pattern of a fault‐bend model of fold growth. The selected geomorphic properties can be divided in two categories, depending on whether or not the vertical dimension is taken into account. None of the planar properties are significantly correlated to uplift rate, unlike those that include the vertical dimension, such as the mean elevation of basins, hypsometric curve, and hypsometric integral, and relief defined by the amplitude factor of length scaling analysis. Correlation between relief and uplift rate is observed for all length scales of topography shorter than 600 m, which suggests that all orders of the streams are able to adjust to the tectonic signal. Simple mass balance considerations imply that the average elevation is only 10% of surface uplift, suggesting that a dynamic equilibrium has been reached quite rapidly. Using a simple two‐process model for erosion, we find that fairly high diffusion coefficients (order of 10 m2 yr−1) and efficient transport of the material by rivers are required. This unusually high value for mass diffusivity at small length scales may be obtained by either a very efficient linear diffusion or by landsliding. Actually, both processes may be active, which appears likely given the nature of the unconsolidated substratum and the favorable climatic conditions. Local relief in the study area may therefore be used to predict either uplift or denudation, but the prediction is calibrated only for that specific climatic and lithologic conditions and cannot be systematically applied to other contexts.
Kinematics of the Pacific‐North America Plate Boundary Zone, northern California
Authors: Freymueller Jeffrey T. , Murray Mark H. , Segall Paul , Castillo David
Published: September 2002

We measured motions of 54 sites in an east‐west transect across northern California at 38°–40° north by Global Positioning System (GPS) observations over a 4 year. We estimate the total slip rate on the San Andreas fault system to be 39.6−0.6+1.5mm/yr (68.6% upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts). Slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters. Our best fitting model fits the fault‐parallel velocities with a mean square error of 1.04 and the following estimated fault slip rates (all in mm/yr): San Andreas 17.4−3.1+2.5, Ma'acama 13.9−2.8+4.1, and Bartlett Springs 8.2−1.9+2.1. The data are fit best by models in which the San Andreas fault is locked to 14.9−7.1+12.5km, the Ma'acama fault locked to 13.4−4.8+7.4km except for shallow creep in the upper 5 km, and the Bartlett Springs fault creeping at all depths. The Ma'acama fault most likely poses a significant seismic hazard, as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake. We find little evidence for contraction across Coast Ranges, except at western edge of Great Valley where 1–3 mm/yr of shortening is permitted by the data. No strain is observed within the Great Valley or Sierra Nevada except that associated with right‐lateral strike slip on the San Andreas fault system. This is consistent with models of the Pacific‐North America plate boundary zone in which the relative plate motion is partitioned into two domains, one strike‐slip and one dominantly extensional, separated by the elastically deforming Sierra Nevada‐Great Valley block.
Similar power laws for foreshock and aftershock sequences in a spring‐block model for earthquakes
Authors: Hainzl Sebastian , Zöller Gert , Kurths Jürgen
Published: September 2002

We introduce a crust relaxation process in a continuous cellular automaton version of the Burridge and Knopoff [1967] model. The most important model parameters are the level of conservation and the ratio of the crust relaxation time to the tectonic reloading time. In correspondence with the original spring‐block model, the modified model displays a robust power law distribution of event sizes. The principal new result obtained with our model is the spatiotemporal clustering of events exhibiting several characteristics of earthquakes in nature. Large events are followed by aftershock sequences obeying the Omori [1894] law and preceded by localized foreshocks, which are initiated after a time period of seismic quiescence. While we observe a considerable variability of precursory seismicity, we find that the rate of foreshocks increases on average, according to a power law with an exponent q, which is in good agreement with the exponent p of the Omori law. In contrast to other events, the distribution of foreshock sizes is characterized by a significantly smaller Richter B value. Our model reproduces simultaneously the empirically observed values of the power law exponents, the Richter B, p and q, and their variability.
Formation and spreading of Arabian Sea high‐salinity water mass
Authors: Kumar S. Prasanna , Prasad T. G.
Published: September 2002

The formation and seasonal spreading of the Arabian Sea High‐Salinity Water (ASHSW) mass were studied based on the monthly mean climatology of temperature and salinity in the Arabian Sea, north of the equator and west of 80°E, on a 2° × 2° grid. The ASHSW forms in the northern Arabian Sea during winter and spreads southward along a 24 sigma‐t surface against the prevailing weak zonal currents. The eastern extent of the core is limited by the strong northward coastal current flowing along the west coast of India. During the southwest monsoon the northern part of the core shoals under the influence of the Findlater Jet, while the southern part deepens. Throughout the year the southward extent of the ASHSW is inhibited by the equatorial currents. The atmospheric forcing that leads to the formation of ASHSW was delineated using the monthly mean climatology of heat and freshwater fluxes. Monsoon winds dominate all the flux fields during summer (June‐September), while latent heat release during the relative calm of the winter (November‐February) monsoon, driven by cool, dry continental air from the north, results in an increased density of the surface layer. Thus excess evaporation over precipitation and turbulent heat loss exceeding the radiative heat gain cool the surface waters of the northern Arabian Sea during winter and drive convective formation of ASHSW.