Showing papers in "Geophysical Research Letters in 1985"

A theory of magnetic flux transfer at the Earth's magnetopause

Abstract: It is suggested that the flux transfer events (FTE's) observed by ISEE satellites can be the result of multiple X-line reconnection at the dayside magnetopause, which may be caused by the development of a tearing instability. In the presence of the y-component of the magnetic field (By) in the transition region of the magnetopause, the tearing instability leads to the interconnection of the geomagnetic field lines and the interplanetary field lines, and hence to the occurrence of FTE's. Twisted field lines and field-aligned currents are formed as a consequence of the tearing instability. The flow direction of the field-aligned currents depends on By and the results are found to be consistent with satellite observations.

Resonant ULF waves: A new interpretation

Abstract: Many important properties of ultra-low frequency (ULF) magnetic pulsations (∼1 to 10 minute periods) observed in the magnetosphere and by ground-based magnetometers have been interpreted successfully in terms of resonant field line models. These models describe the pulsations as transverse hydromagnetic waves standing on dipole flux tubes with fixed ends in the ionosphere. Signal structure and frequency are consistently explained, but the models fail to provide convincing explanations of the discrete spectra often observed. In this letter we propose that the spectrum of excitations is dominated by frequencies that correspond to eigenfrequencies of the fast (compressional) mode in the outer magnetosphere. In the presence of plasma inhomogeneities, the eigenmodes of the fast mode damp, even in the absence of dissipation, through coupling to transverse waves. We discuss the properties of a damped eigenmode, pointing out the analogy to the properties of damped surface eigenmodes. The coupled waves exhibit field line resonance behavior on the magnetic shell where the transverse mode dispersion relation is satisfied, but the spectrum is dominated by fast mode eigenfrequencies.

Source pulse enhancement by deconvolution of an empirical Green's function

Abstract: Observations of the earthquake source-time function are enhanced if path, recording-site, and instrument complexities can be removed from seismograms. Assuming that a small earthquake has a simple source, its seismogram can be treated as an empirical Green's function and deconvolved from the seismogram of a larger and/or more complex earthquake by spectral division. When the deconvolution is well posed, the quotient spectrum represents the apparent source-time function of the larger event. This study shows that with high-quality locally recorded earthquake data it is feasible to Fourier transform the quotient and obtain a useful result in the time domain. In practice, the deconvolution can be stabilized by one of several simple techniques. In this paper, the method is implemented and tested on high-quality digital recordings of aftershocks of the Jan. 9, 1982 Miramichi (New Brunswick) earthquake. In particular, seismograms from a Jan. 17 aftershock (017 13:33 GMT, local mag.=3.5) exhibit path or site effects which complicate the determination of source parameters. After deconvolution, the apparent far-field source of this event is a simple pulse in displacement with duration ≈ 0.07 second for both P and S.

A diffuse plate boundary model for Indian Ocean tectonics

Abstract: It is suggested that motion along the virtually aseismic Owen fracture zone is negligible, so that Arabia and India are contained within a single Indo-Arabian plate divided from the Australian plate by a diffuse boundary. The boundary is a zone of concentrated seismicity and deformation commonly characterized as 'intraplate'. The rotation vector of Australia relative to Indo-Arabia is consistent with the seismologically observed 2 cm/yr of left-lateral strike-slip along the Ninetyeast Ridge, north-south compression in the Central Indian Ocean, and the north-south extension near Chagos.

Magnetic anisotropy in the Trenton Limestone: Results of a new technique, anisotropy of anhysteretic susceptibility

Abstract: A new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described. The magnetic fabric of a collection of Trenton Limestone specimens has been determined using this method, as well as by conventional anisotropy of magnetic susceptibility. The results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the Trenton in which the bulk magnetic susceptibilities are low. A model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses. The original fabric is inferred to have been isotropic; the anisotropy resides in secondary magnetite of Late Paleozoic age. It is argued that the observed magnetic fabric must therefore be Alleghenian or younger in age. Our method has the potential to determine paleostress directions in carbonates elsewhere, provided our assumptions are correct.

Further observations of X-rays inside thunderstorms

Abstract: A control and an x-ray detector were flown into thunderstorms in 1984. The x-ray detector found statistically significant radiation flux increases in the energy range 5 to > 110 keV for time intervals of several seconds each. Simultaneously the control detector showed no statistically significant changes. The x-ray flux is sometimes seen to increase prior to observed lightning discharges and then return to background levels within 0.1 seconds of a lightning flash initiation. The temporal development of these x-ray bursts is discussed and related to inferring the scale size of the x-ray emitting regions.

Thermoviscous remagnetization in some Appalachian limestones

Abstract: Experimental evidence shows that blocking temperature-relaxation time theory for single-domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in Devonian limestones of New York State. Thermoviscous remagnetization at moderately elevated burial temperatures during the late Paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks. More generally, these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite-bearing rocks. The Prob leto

First composition measurement of the bulk of the storm‐time ring current (1 to 300 keV/e) with AMPTE‐CCE

Abstract: Measurements of the charge state and elemental compositions as well as differential intensities of ring current ions with energies of 1-315 keV/e, made with the Charge-Energy-Mass Spectrometer on the AMPTE/CCE spacecraft, are reported. An analysis of the data suggests that while the storm-time increases in the number densities of ring current H(+) and He(2+) may be roughly accounted for by the decreased volume of the compressed magnetosphere, the large jumps in the number and energy densities of O(+) require injection of energetic ionospheric ions.

Magnetic storm of September 4, 1984 - a synthesis of ring current spectra and energy densities measured with AMPTE/CCE

Abstract: Compositional studies of the equatorial distributions of ring current ions during the September 4, 1984 magnetic storm have been made possible by comprehensive energy, charge state, and mass coverage data from the Charge Composition Explorer satellite. An examination of ion spectra at an L value of about 4 on September 5, in the local evening sector, shows that energy density was dominated by protons, with O ions contributing about 27 percent at the peak of about 150 keV, while He ions contributed less than about 2 percent. September 6 ion spectra, taken during the recovery phase of the storm, indicate that ion densities at more than 20 keV had decreased markedly, and that the ring current energy density was primarily provided by protons.

Geodynamic mixing in the mesosphere boundary layer and the origin of oceanic islands

Abstract: Since the isotopic heterogeneity of ocean island basalts (OIB) is of variable character and requires billions of years to evolve, we propose to explain the observed heterogeneity by mixing between delaminated continental lithosphere, a continental crustal component, depleted mantle and pristine mantle. Such mixing occurs in a boundary layer located at the bottom of the upper mantle from which ascending blobs rise.

Regional stress field of the Indian plate

Abstract: We have calculated the regional stress field in the Indian plate, implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure. The high level of the calculated stress field (order of a few kbar) and the dominance of compression in the plate are consequences of the unique dynamic situation of the present-day Indian plate. The calculated stress field explains the concentration of intraplate deformation in the region of the Ninetyeast Ridge and yields insight into the variations in stress directions in the Australian continent.

In situ sampling of clouds with a droplet to aerosol converter

Abstract: Cloud droplets are separated from atmospheric trace gases, rain drops and submicrometer aerosol particles on the basis of their aerodynamic properties and evaporated by dry air. The evolved trace gases and residual particles can be analyzed by in-situ methods and collected for subsequent chemical analysis.

A mineral physics test of a pyrolite mantle

Abstract: An homogeneous upper mantle chemical composition defined by the pyrolite model is compatible with the observed seismic velocities. Such a test is now possible for depths to 670 km as a result of new mineral elasticity and phase equilibria data. Three specific criteria for comparing the mineralogical velocity model with that of the mantle include i) the velocity jump at the 400 km discontinuity, ii) the velocity increase between 400 and 670 km, and iii) the absolute value of velocity in the 300 to 400 km region (shallower than the discontinuity). The first criterion is sensitive to the olivine content of the mantle and the pressure derivatives of the elastic properties of the high pressure phase. The application of this criterion suggests a mantle with 40 - 65% olivine by volume. The second criterion is sensitive to the details of the pyroxene to garnet transformation and the relative abundances of the pyroxenes and garnet. The third criterion depends on details of this transformation as well as iron content and the temperature of the mantle.

Evidence for the presence of 53Mn in the early solar system

Abstract: Chromium isotopic anomalies appear to be widespread in the Allende inclusions. Clear anomalies are shown in the 53Cr/52Cr isotopic ratio. Deviations from the terrestrial value are correlated with the Mn/Cr ratios indicating that 53Mn (half life T=3.7 × 106y) existed at the time of formation of the meteorite inclusions. The initial 53Cr/52Cr in the inclusions is lower than the terrestrial value. Recent results have shown that chromium isotopic anomalies are frequent on the 54Cr isotope (Birck and Allegre, 1984). At the same time our results suggested the possibility of some anomalies on the 53Cr isotope which can be produced by the decay of 53Mn. The present article presents improved precision measurements on the same inclusions of the Allende meteorite and an extension of the measurements to mineral separates. These results are the first evidence for live 53Mn in the early solar system (noted 53Mn* in the following).

Garnet-perovskite transformation in CaGeO3: In-situ X-ray measurements using synchrotron radiation

Abstract: The garnet-perovskite phase transformation in CaGeO3 was investigated in the pressure-temperature region to 6.5 GPa and 1200°C using a cubic anvil type of high-pressure apparatus combined with synchrotron radiation. In-situ measurements with an energy dispersive x-ray diffraction system enable us to carry out dynamical observation of the transformation. The equilibrium phase boundary between the garnet and perovskite phases was determined as P(GPa)= 6.9 - 0.0008T(°C). The negative P-T slope definitely established in the present study is in reasonable agreement with the value, −0.0023(8) GPa/°C, that was calculated from the thermochemical data on the enthalpy of transition. The molar volume change accompanied with this transformation was estimated to be about 13% at about 6 GPa and 1000°C.

Magnetospheric coupling of hydromagnetic waves ‐ Initial results

Abstract: Recently, emphasis in modelling ULF pulsations has begun to shift away from steady-state driving mechanisms towards impulsive and non-steady sources. We propose a model allowing numerical solution of the coupled hydromagnetic wave equations with arbitrary azimuthal asymmetry in a cylindrical magnetospheric geometry. General time-dependent stimuli can be applied at the outer (magnetopause) boundary, an arbitrary Alfven speed distribution can be defined within the boundary, and ionospheric Joule dissipation is included. Initial results are presented which show that impulsive stimuli at the magnetopause can set up compressional cavity resonances which drive transverse field-line resonances within the magnetosphere, in general agreement with recent predictions. For a realistic ionospheric dissipation, and azimuthal wavenumber ∼3, it is found that transient transverse mode solutions are of comparable importance to monochromatic solutions near the resonant field line.

First principles equations of state of MgO and CaO

Abstract: The ability of local density functional theory to predict properties of oxide minerals at high pressures is tested by computing some thermodynamic properties of MgO and CaO in the B1 (NaCl) and B2 (CsCl) structures. Both static and dynamic contributions to the equations of state are computed ab initio, thus yielding predictions that are completely independent of data. Except for a possible overestimate of the density of B2 phases, the theoretical room temperature isotherms, shock-wave Hugoniots and zero pressure density and elastic parameters are in excellent agreement with data. The predicted B1-B2 transformation pressure for CaO is 32 GPa, compared to the experimental value of 61 GPa. The corresponding pressure for MgO is 205 GPa and is believed to be a lower bound on the actual transformation pressure.

A time-dependent theoretical model of the polar wind: preliminary results

Abstract: The coupled time dependent continui- which observed supersonic H + (Nagai et al., 1984) ty, momentum and energy equations of a two ion (O + and O + ion flows (Waite et al., 1985). These and H +) quasineutral plasma were solved in order observational results prompted us to develop a new to extend our understanding of polar wind be- hydrodynamical numerical code to study the time havior. This numerical code allows studies of dependent behavior of ionospheric plasma flows the time dependent behavior of polar wind-type along magnetic field lines. In section 2 we flows into and out of the ionosphere. Initial briefly outline the governing equations, the nu- studies indicate that the typical time constants merical method and the parameters selected for our for electron and ion temperature changes are of calculations. Section 3 gives the results of a few the order of minutes and tens of minutes, respec- representative sets of calculations. Finally the tively. The response time of the minor high alti- implications of these preliminary studies are tude ion O + is less than an hour, whereas that of considered in the discussion section. the major ion, H + is many hours The initial test runs also demonstrate the fact that temporary supersonic flows of both O + and H + are possible, especially in the presence of significant ion heating.

Simulation of multiple X‐line reconnection at the dayside magnetopause

Abstract: The multiple X-line reconnection process, which was proposed by Lee and Fu [1985] for the dayside magnetopause, is simulated based on a two-dimensional incompressible MHD model. It is found that when the system length is much larger than the thickness of the current layer, magnetic flux tubes are repeatedly formed and convected out of the system. Impulsive magnetic reconnections associated with the formation of magnetic flux tubes are observed. In the case of the dayside magnetopause, magnetic flux tubes with a cross-section of ∼ 1 RE² are found to be formed repeatedly every 5-10 minutes, consistent with satellite observations.

Sawtooth segmentation and deformation processes on the southern San Andreas Fault, California

Abstract: Five contiguous 12-13 km fault segments form a sawtooth geometry on the southernmost San Andreas fault. The kinematic and morphologic properties of each segment depend on fault strike, despite differences of strike between segments of as little as 3 degrees. Oblique slip (transpression) of fault segments within the Indio Hills, Mecca Hills and Durmid Hill results from an inferred 8:1 ratio of dextral slip to convergence across the fault zone. Triggered slip and creep are confined almost entirely to transpressive segments of the fault. Durmid Hill has been formed in the last 28 + or - 6 ka by uplift at an average rate of 3 + or - 1 mm/a.

The neutral wind “flywheel” as a source of quiet‐time, polar‐cap currents

Abstract: The neutral wind pattern over the summer polar cap can be driven by plasma convection to resemble the convection pattern. For a north-south component of the interplanetary magnetic field Bz directed southward, the wind speeds in the conducting E-region can become approximately 25 percent of the electric field drift speeds. If convection ceases, this neutral wind distribution can drive a significant polar cap current system for approximately 6 hours. The currents are reversed from those driven by the electric fields for southward Bz, and the Hall and field-aligned components of the current system resemble those observed during periods of northward Bz. The current magnitudes are similar to those observed during periods of small, northward Bz; however, observations indicate that electric fields often contribute to the currents as much as, or more than, the neutral winds.

Ram vehicle glow spectrum; Implication of NO2 recombination continuum

Abstract: An experiment was operated on several Space Shuttle missions to provide spatial and spectral distributions of a ram glow associated with the Orbiter. The most recent data featured resolved spectrum and imagery of the glow with spectroscopic resolution of 34 A FWHM between 4000 and 8000 A. The spectrum of the glow on the Shuttle tail pod could be clearly separated from spectrum of the reflected light from the Orbiter. Analysis and comparison have been performed which strongly suggest the emission originates from recombination continuum of NO2. Both fast recombination (high temperature) and the spectral dependence in lifetime can describe the spectral difference. If the recombined NO2 retains 25 percent of the kinetic energy of the ram OI, the thickness of the glow layer can be explained by the lifetime of NO2 (2B1) recombination emission.

Lattice dynamics and structural distortions of CaSiO3 and MgSiO3 perovskites

Abstract: The phonon spectra, distortion mechanisms, and thermoelastic properties of CaSiO3and MgSiO3 in the cubic perovskite structure are investigated as a function of pressure using a lattice dynamic approach. The bonding forces are derived from a parameter-free rigid-ion electron-gas formulation. At low pressures, CaSiO3 is found to be dynamically stable in the cubic perovskite structure; however, the phonon spectrum exhibits soft modes at the Brillouin zone boundary which ultimately result in a dynamic instability of the lattice near 80 GPa. The computed phonon spectrum of cubic MgSiO3 perovskite exhibits complex frequencies along parts of the zone boundary at all densities investigated. These vibrational instabilities include coupled octahedral rotations which produce the observed distorted structure of MgSiO3 perovskite. The measured bulk modulus of MgSiO3 perovskite compares well with our calculated value for the cubic structure.

The effects of transient rheology on the interpretation of lower mantle viscosity

Abstract: The role played by transient rheology in the interpretation of mantle viscosity is reexamined. The investigation has been carried out by comparing the amplitude responses with the data of secular variation of J(2), the relative sea-level histories at sites well within the ice margins and at the ice margin like the city of Boston. A linear Burgers body rheology has been assumed in ther lower mantle. The data near the edge of the ice load proves most sensitive to the transient viscosity structure. The non-monotonic behavior of sea-level data near Boston can be explained both by a steady-state lower mantle viscosity of 10 to the 22nd P with a thick lithosphere and by a transient lower mantle rheology but with a thin lithosphere. The long-term viscosity of the lower mantle in this second model has a steady-state value of around 5 x 10 to the 23rd P.

Neutral motions in the polar thermosphere for northward interplanetary magnetic field

Abstract: During the rare occurrence of November 24, 1982, when the north-south B(z) component of the IMF became positive for a period of about 11 hours and reached a steady value of about 25 nT, the DE-2 spacecraft simultaneously measured the ionic and neutral species of the high latitude F-region along the track of the polar-orbiting satellite. Data from two northern winter polar passes of DE-2 illustrate the response of the neutral F-region to ion drag forcing that arises from a configuration of ion convection characteristics of strongly northward IMF. The multicellular ion drift pattern associated with positive B(z) is observed to drive a similar, but less structured and weaker, neutral wind configuration in the winter polar cap. While major features of the ion drift pattern are mimicked by the neutral circulation, smaller scale ion flow structures are not. These results demonstrate that the sunward flow of neutral gas can be established and maintained by ion drag in the central polar cap, for positive B(z).

A new mechanism for excitation of electrostatic ion cyclotron waves and associated perpendicular ion heating

Abstract: A new mechanism for exciting the kinetic ion cyclotron waves in the presence of a nonuniform electric field perpendicular to the external magnetic field is given. Application of this instability to various space plasmas is discussed. The new instability mechanism may provide a more efficient agent for perpendicular ion heating than other EIC generation processes, since the linear growth rate is insensitive to the temperature ratio.

Ab initio structural and thermoelastic properties of orthorhombic MgSiO3 perovskite

Abstract: We present results of an ab initio study of the structural and thermoelastic properties of orthorhombic (Pbnm) MgSiO3 perovskite as a function of temperature and pressure. Self-consistent free energies are computed using quasiharmonic lattice dynamics with interatomic potentials derived from an electron-gas formulation. At high temperatures the orthorhombic phase undergoes successive second-order transitions to tetragonal and cubic phases. The transition temperatures increase with pressure such that the orthorhombic phase is stable throughout most of the lower mantle although the higher symmetry phases could occur near the top of the lower mantle. At zero pressure the calculated bulk modulus and thermal expansion are in excellent agreement with the available data. We find that for high-temperature isotherms, a finite-strain decompression of the high-pressure equation of state of perovskite substantially overestimates its zero-pressure density and bulk modulus; hence, constraints on the chemical composition of the lower mantle, based on a comparison of the decompressed seismological properties with zero-pressure laboratory data may significantly overestimate the proportion of perovskite in the lower mantle.

Origin of mantle peridotite and komatiite by partial melting

Abstract: Theoretical calculations and some preliminary experimental observations show that the liquidus and solidus temperatures for each of a wide range of upper mantle peridotite and komatiite compositions become coherent at high pressures. This suggests that these materials may have been liquids in eutectic-like equilibrium (rather than peritectic-like) with mantle assemblages. It is shown that the major element geochemistry of 83 mantle peridotites and 61 komatiites define a trend which is not primarily due to an olivine control process; rather it is interpreted to represent the pressure-induced compositional trace of eutectic liquids in equilibrium with mantle assemblages from 4 to 15 GPa. If the bulk Earth is chondritic in composition these phase equilibrium constraints would imply that the upper mantle had formed from the whole mantle as an ultrabasic partial melt, the transition zone and lower mantle being the complementary eclogite and pyroxenite residua. This is consistent with mineralogical interpretations of seismic data for the present-day Earth which call for a peridotite upper mantle and pyroxene-like transition zone and lower mantle.

The structure and evolution of slow mode shocks

Abstract: The development and long time behavior of slow mode shocks is studied by means of computer simulation. It is shown that a well defined shock can be generated and maintained for hundreds of ion gyroperiods. The damping length of the trailing magnetic wavetrain increases with time toward the fluid limit. A population of ions with density a significant fraction of the upstream density is observed to flow back upstream from the shock along the magnetic field. The speed of these backstreaming ions relative to the upstream flow is about twice the Alfven speed, which may be sufficient to excite the right hand resonant electromagnetic ion beam instability.

Time‐space variation of seismic S‐wave coda attenuation (Qc−1) and magnitude distribution (b‐values) for the Petatlan Earthquake

Abstract: Average S-wave coda-Q (Qc) at 6 Hz and to about 10 sec after the S-wave arrival shows: (1) temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 Petatlan, Mexico earthquake; (2) spatial variation through the aftershock area, positively correlated with b, high Qc -b values at the two activity centers, intermediate values in between and landward, and low values toward the Middle America Trench, which remain low for two weeks after the main shock; and (3) five-foreshock average Qc = 135±1.6% and eleven-aftershock average Qc= 175±1.3% for the eastern asperity. The b values—and possibly Qc values—seem to correlate with the level of aftershock activity.