Showing papers in "Geophysical Research Letters in 1987"

Biharmonic spline interpolation of geos-3 and seasat altimeter data

Abstract: Green functions of the biharmonic operator, in one and two dimensions, are used for minimum curvature interpolation of irregularly spaced data points. The interpolating curve (or surface) is a linear combination of Green functions centered at each data point. The amplitudes of the Green functions are found by solving a linear system of equations. In one (or two) dimensions this technique is equivalent to cubic spline (or bicubic spline) interpolation while in three dimension it corresponds to multiquadric interpolation. Although this new technique is relatively slow, it is more flexible than the spline method since both slopes and values can be used to find a surface. Moreover, noisy data can be fit in a least squares sense by reducing the number of model parameters. These properties are well suited for interpolating irregularly spaced satellite altimeter profiles. The long wavelength radial orbit error is suppressed by differentiating each profile. The shorter wavelength noise is reduced by the least squares fit to nearby profiles. Using this technique with 0.5 million GEOS-3 and SEASAT data points, it was found that the marine geoid of the Caribbean area is highly correlated with the sea floor topography. This suggests that similar applications, in more remote, areas may reveal new features of the sea floor.

Sunspots, the QBO, and the stratospheric temperature in the north polar region

Abstract: There is an association between the polar stratospheric temperature in the northern winter and the solar cycle in the winters when the 50-mb equatorial winds are westerly: The lower the sunspot number in such winters, the lower is the temperature. No major mid-winter warmings occurred in these winters when the sunspot number was below about 100. There is no such relationship in the easterly phase of the QBO. In that phase the temperatures are generally higher than in the westerly phase, and major mid-winter warmings occur regardless of the state of the solar cycle.

Roughness of natural fault surfaces

Abstract: The roughness of fault surfaces is important in the mechanics of fault slip and could play a role in determining whether sliding occurs via earthquakes or fault creep. We have made preliminary measurements of the power spectral density of several fault surfaces over the wavelength range from 10−5 to 1 m, using field and laboratory scale profilimeters. The fault surfaces are strongly anisotropic; profiles parallel to the slip direction have amplitudes about one order of magnitude lower than those perpendicular to the slip direction over most of the wavelength interval measured. Fault roughness perpendicular to the slip direction is similar to the roughness of natural joints; the greater smoothness in the slip direction presumably represents wear resulting from slip. Combining our data with previous measurements of the roughness spectrum of the San Andreas fault shows that fault surfaces are fractal over nearly eleven orders of magnitude in wavelength.

Energetic neutral atom image of a storm-time ring current

Abstract: Energetic neutral atom (ENA) images of a storm-time ring current produced from direct measurements and model simulations are presented and analyzed. An ENA image of the storm-time ring current was obtained with the Medium Energy Particles Instrument (MEPI) mounted on ISEE 1 on September 29, 1978, and a zero-order model of the ring current ion was employed to simulate the ENA image. The simulated instrument response is compared with actual ENA measurements. It is observed that the spatial distribution of singly charged ions detected from the ENA images displays equatorial midnight/noon asymmetry in differential ion intensity with a midnight/noon ratio of greater than or equal to 20:1; peak ion fluxs, H(+) and O(+), in the midnight sector are greater than 10 to the 6th/(sq cm s sr keV); and ENA fluxes from the brighter areas of the image exceed greater than 1000/(sq cm s sr keV). It is noted that the ENA image from MEPI data correlates with the morphology of the simulated response of MEPI calculated from the model.

A note on the description of surface roughness using fractal dimension

Abstract: Self-affine fractals are useful models of the surfaces of rock fractures. The scaling properties of these surfaces are described by two parameters, the fractal dimension and the crossover length. Two methods for estimating the fractal dimension of a profile of a rough surface are compared, the divider method and the spectral method. It is shown that the two methods yield the same results, if the horizontal resolution at which the profile is measured is smaller than the crossover length. However, for resolutions greater than the crossover length, the divider method always gives a fractal dimension close to 1. To guide future work, the crossover length is estimated for typical joint surfaces and for the San Andreas fault. Additionally, a simple method is proposed to obtain the correct fractal dimension without prior knowledge of the crossover length. copyright American Geophysical Union 1987

Hydroxide decomposition of dimethylsulfoniopropionate to form dimethylsulfide

Abstract: The kinetics of DMS production resulting from reaction of OH(-) with DMSP were investigated as a function of hydroxide concentration and temperature. The reaction was first-order with respect to DMSP and OH(-). The second order rate constant at 20+/-1 C is 0.0044/M/sec. The activation energy for this reaction is 14.4 kcal/mode. The investigation indicates that the rate of reaction of DMSP with OH(-) is very slow at the pH of seawater, suggesting that DMSP, which may be a major precursor of DMS in seawater, decomposes in the ocean by other mechanisms. A bacterium which produces DMS from DMSP quantitatively at rates many orders of magnitude higher than indicated by OH(-1) decomposition has been cultured, suggesting that enzymatic processes accelerate the production of DMS from DMSP in seawater.

The solar wind-magnetosphere-ionosphere current-voltage relationship

Abstract: The global current-voltage relationship for the solar wind-magnetosphere-ionosphere system (SW-M-I) is investigated, restricting the study to strong southward IMF. The dynamo presently identified is on open field lines, and it operates at close to short circuit and at a fraction of the available power output. Control of the dynamo by ionospheric conductivity is discussed. Implications of the simulation results, including the relationship between open and closed field dynamos, the effect of solar wind conditions, the control of reconnection on the bow, the size of the open field line region, and the effects of increased auroral conductivity, are discussed.

The effect of cluster ions on anomalous VHF backscatter from the summer polar mesosphere

Abstract: Radar backscatter at 50 MHz from the summer polar mesosphere is sometimes much stronger than the scatter from other latitudes at any time or from polar latitudes in other seasons. Kelley and Ulwick [1987] suggested that heavy water cluster ions in this uniquely cold region of the atmosphere could be responsible because, in the presence of these ions, the enhanced, turbulence-driven spectrum of the ionization irregularities could extend to considerably shorter wavelengths than that of the neutral turbulence itself. Here we explore this idea further and relate it to past and planned incoherent scatter measurements at EISCAT. These observations should be able to establish whether or not water cluster ions have an anomalously low diffusion coefficient.

Sources of electromagnetic radiation from fracture of rock samples in the laboratory

Abstract: Rock, when fractured in the laboratory, emits electrical signals with a broad maximum in the power density spectrum in the band from 900 Hz to 5 kHz. Quartz-free basalt rocks radiate both light and low-frequency electrical signals as intensely as quartz-bearing rocks, suggesting that the piezoelectric effect of quartz is at most a minor contributor to the total power radiated. Newly created rock surfaces acquire a local net charge distribution when exoelectrons are expelled and removed from the vicinity of the new surfaces by collisional and other processes. The rotational, vibrational, and linear motion of rock fragments with charged surfaces is the major contributor to the observed power density spectrum of the low-frequency electrical signals. Neither optical spectral lines nor high-frequency electromagnetic radiation, characteristic of electric discharges, occur at fracture. Exoelectron bombardment of the ambient fluid surrounding the sample, not electric discharge, is the excitation source for the light emitted at fracture.

Melt migration beneath mid‐ocean spreading centers

Abstract: Mid-ocean ridge volcanism is largely concentrated within a few kilometers of the spreading axis. Geophysical models of upwelling induced melting beneath a ridge axis predict significant melting within a ∼100 kilometer wide region beneath a ridge axis. This provides a strong constraint for testing models of melt migration beneath a spreading center since a successful melt migration mechanism must be able to focus melt from a broad region of melt production to the narrow emplacement zone at the ridge axis. Tensile dike propagation transport of melt within the mantle would move melt away from the ridge axis and thus does not satisfy this constraint. However, porous flow of melt within a viscously deforming asthenosphere is a viable possibility. Models of porous flow are constructed which consider the effects of mantle flow-induced pressure gradients and strain-induced anisotropic permeability on porous melt migration. In order for flow-induced pressure gradients to shape melt migration the sub-ridge asthenospheric viscosity must be 1021 Pa-s, two orders of magnitude larger than currently accepted values. Mantle strain-induced anisotropic permeability variations can explain the focussing of melt towards the ridge axis if the ratio of the directional anisotropic permeabilities is as small as 3. Furthermore, mantle strain-induced anisotropy in rock permeability may be an important mechanism shaping melt migration within the mantle.

Disruption of the magnetotail current sheet observed by AMPTE/CCE

Abstract: An unusual large-amplitude (from less than 10 nT to greater than 40 nT) magnetic oscillation characterized by about-13-sec periodicity and southward turnings of the field was observed by AMPTE/CCE on August 28, 1986. The magnetic field was often stronger southward, with some southward components exceeding 20 nT being noted. The level of the high frequency perturbations was also seen to be enhanced. It is suggested that these observations may be due to the formation of an X-type neutral line and its motion near the spacecraft.

On the 3-dimensional structure of plasmoids

Abstract: The hypothesis that the IMF penetrates plasmoids causing them to be three- rather than two-dimensional is tested by comparing observations of By within plasmoids and related tail structures to upstream IMF By data. The magnetic topologies that result from the mergings of closed plasma sheet flux tubes and open tail lobe flux tubes at a near-earth neutral line, and merging near the tail flanks are described and studied. The particle signals and isotropic electron distributions are examined. It is observed that the IMF By penetrates plasmoids and that their structure is three-dimensional. In the three-dimensional model of plasmoids the reconnected plasma sheet field lines form a magnetic flux-ropelike structure. The three-dimensional model is utilized to analyze stagnant, slowly moving and earthward moving structures.

Mesospheric temperature inversion and gravity wave breaking

Abstract: More than 500 nightly mean temperature profiles have been obtained since 1981 by two Rayleigh lidars located in south of France 550 km apart. In the mesosphere a temperature inversion, a feature persisting for several days, is frequently observed simultaneously at both sites. From a statistical study of the inversion characteristics, it is shown that its altitude ranges from 55-72 km in winter to 70-83 km in summer, while its probability of occurrence presents a semi-annual variation with a maximum higher in winter than in summer and a minimum lower in May than in September. The seasonal variation of the temperature inversion is shown to be very similar to that of the MST radar echoes in the mesosphere associated to turbulent layers generated by the breaking of gravity waves. A crude estimate of the amplitude growth with height of a gravity wave in the presence of an inversion layer indicates that the wave will break preferably inside and above this layer. The persistence of a temperature inversion during several days, with amplitude as large as 40 K, is tentatively explained by the heating of the turbulent layers generated by the continuous breaking of gravity waves above the secondary minimum of temperature.

Ozone isotope measurements in the stratosphere

Abstract: Mass spectrometer measurements of ozone made during two balloon flights included its heavy isotopes at mass 49 and 50 Both flights were flown during the day and during summer from Palestine, TX At float altitudes above 42 km the enrichments in heavy ozone were 41 percent and 23 percent, respectively The enrichment appears to be mass independent since, at high altitudes, both 49 and 50 show the same enhancement During the descent the enrichment in heavy ozone decreased, faster during the first flight than during the second, reaching values between 15 and 20 percent above 30 km Near and below this altitude another increase is observed During a night flight, previously reported, an enhancement in heavy ozone of over 40 percent at 32 km was found, decreasing both toward higher and lower altitudes

Field line draping about fast coronal mass ejecta: A source of strong out‐of‐the‐ecliptic interplanetary magnetic fields

Abstract: Fast coronal mass ejecta interact strongly with the ambient interplanetary plasma and magnetic field into which they propagate. A shock forms in front of an ejection, and the slower moving ambient plasma ahead is accelerated and deflected from its path. It is argued that such flow accelerations and deflections of the ambient plasma must produce a draping of the ambient interplanetary magnetic field about the ejected material similar to that which occurs in the magnetosheath surrounding the earth's magnetosphere. The draping pattern should depend upon the overall size and shape of the ejection, its speed relative to the ambient plasma ahead, the orientation of the ambient magnetic field, and the position where the shocked plasma is sampled. At some locations upstream from an ejection draping leads to an enhancement of the out-of-the-ecliptic field component B(Z) at the expense of the ecliptic components. It is suggested that draping plays an important role in producing intervals of strong and prolonged negative B(Z) in the ecliptic plane at 1 AU, and thus may be an important factor in stimulating geomagnetic activity.

The derivation of 40Ar/39Ar age spectra of single grains of hornblende and biotite by laser step-heating

Abstract: We present the first examples of 40Ar/39Ar age spectra for hornblende and biotite derived by the laser step-heating of individual grains. It is straightforward to use a continuous laser to produce detailed (>20 steps), reproducible and precise age spectra in this way. An infra-red microscope is used to monitor the sample temperature changes so that a judicious heating schedule can be chosen. Such a system can be readily automated. The biotite spectra shown exhibit large age differences (∼60 m.y.) between co-existing flakes differing in mass by a factor of ∼70.

Mirror instability in the magnetosphere of comet Halley

Abstract: High resolution Vega 1 and 2 magnetic measurements in the cometary magnetosphere and magnetosheath of Halley reveal the presence of fluctuations with the signature expected for the mirror instability. This instability is a mechanism by which homogeneously produced cometary ions with a large perpendicular temperature anisotropy can be concentrated into discrete linear features. Thus, the mirror instability may provide a new mechanism for the generation of cometary rays. To our knowledge the presence of this instability in the cometary plasma was not predicted.

On the nature of pressure‐induced coordination changes in silicate melts and glasses

Abstract: Progressive decreases in the Si‐O‐Si angles between corner‐shared silicate tetrahedra in glasses and melts with increasing pressure can lead to arrangements of oxygen atoms that can be described in terms of edge‐ or face‐shared octahedra. This mechanism of compression can account for the gradual, continuous increases in melt and glass densities from values at low pressure that indicate dominantly tetrahedral coordination of Si to values at several tens of GPa that suggest higher coordination. It also can explain the unquenchable nature of octahedrally coordinated Si in glasses, the absence of spectroscopically detectable octahedrally coordinated Si in glasses until they are highly compressed, the gradual and reversible transformation from tetrahedral to octahedral coordination in glasses once the transformation is detectable spectroscopically, and the fact that this transformation takes place in glass at room temperature. It may also have relevance to pressure‐induced transformations from crystalline to glassy phases, the difficulty in retrieving some metastable high pressure crystalline phases at low pressure, and the observed differences between the pressures required for phase transformations in shock wave experiments on glasses and crystals.

Experimental evidence bearing on the stability of monazite during crustal anaatexis

Abstract: A major host for LREE and the heat producing elements Th and U in peraluminous granites is monazite, and the stability of monazite will affect the redistribution of these elements within the crust during anatexis. The compositional stability of monazite in "granitic" melts ranging from peraluminous to metaluminous compostions has been investigated under water-saturated conditions between 850-1000°C, at 8 Kb. Monazite solubility (concentration of LREE's in saturated melts) increases with the values of M > 1.3 [=(2Ca+Na+K)/(AlXSi) in the melt], and is constant for M < 1.3. The temperature dependence of monazite saturation has been determined by Rapp and Watson [1986] for a melt of M = 1.3 and model should be applicable to all peraluminous magmas. Application of the monazite solubility model to crustal anatexis indicates that for fusion temperatures at or below 800°C, anatexis producing peraluminous magmas may lead to an enrichment of monazite in the restitic lower crust depending upon the initial LREE concentration in the source. At temperatures approaching 850°C, enrichment of monazite in the lower crust by anatexis is unlikely due to the high monazite-saturation concentrations.

A revised estimate of Pacific‐North America motion and implications for Western North America Plate boundary zone tectonics

Abstract: Marine magnetic profiles from the Gulf of Californa are studied in order to revise the estimate of Pacific-North America motion. It is found that since 3 Ma spreading has averaged 48 mm/yr, consistent with a new global plate motion model derived without any data. The present data suggest that strike-slip motion on faults west of the San Andreas is less than previously thought, reducing the San Andreas discrepancy with geodetic, seismological, and other geologic observations.

Plasma energization on auroral field lines as observed by the Viking spacecraft

Abstract: High-resolution particle measurements obtained from the VIKING spacecraft have provided interesting new observations of accelerated and heated plasma on auroral field lines. We present observations of upward ion and electron beams and conies which indicate that the acceleration/heating process is so variable that many different physical processes have to be taken into account. As a consequence of this variability a new classification scheme will be adopted.

Locating microearthquakes induced by hydraulic fracturing in crystalline rock

Abstract: Microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3.5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced. The induced microseismicity was monitored by a network of five vorehole seismic stations; a total of about 800 induced events were reliably located from arrival times. Event locations show a tabular distribution that strikes 350/sup 0/ and dips 65/sup 0/ east, subparallel to the injection well. The injection was intended to produce a fracture system that would hydraulically connect two subparallel wells. A lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore. The 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties. The injected fluid appears to have stimulated a zone of rock, rather than simply a single fracture. The distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment.

A possible relationship between interannual variability in Antarctic ozone and the quasi-biennial oscillation

Abstract: Extra-tropical temperatures, geopotential heights, and total ozone abundances are known to vary with the tropical Quasi-Biennial Oscillation (QBO), although the details of the causal mechanisms linking the two regions are still a subject of debate. It is shown that total ozone amounts observed in Antarctica during 1979-1986 exhibit year-to-year fluctuations that may be due to the influence of the QBO, with relatively low (high) values occurring during the westerly (easterly) phase. This suggests that the QBO should be taken into account when evaluating the amplitude and continuation of the long-term trend in Antarctic total ozone. In particular, if the tropical QBO is in its westerly phase during October, 1987, an increase in total ozone at that time may be inconsistent with the continuation of the secular decrease observed since the 1970’s.

On MHD intermediate shocks

Abstract: Contrary to the usual belief that MHD intermediate shocks are extraneous, we show in this paper by numerical solutions of resistive MHD equations that at least some of the intermediate shocks are admissible and can be formed through nonlinear steepening from a continuous wave. The result is of importance not only to the MHD shock theory but also to studies such as magnetic field reconnection models.

An ultraviolet auroral imager for the Viking spacecraft

Abstract: The V5 ultraviolet auroral imager is one of a complement of experiments launched into polar orbit aboard the Swedish spacecraft Viking in February of 1986. The imager consists of two intensified wide-angle f/1 cameras, identical except in passband, with electronic despinning providing exposure times of one second. Critical to electronic despinning is the use of tapered fiber optic bundles to remove distortions generated when the spherical focal surface of the Burch configuration camera is projected onto the plane of the charge coupled detector. Data acquired by V5 are telemetered to a ground station in Kiruna, Sweden for archiving and real time display using a VAX11/750 system.

Highly relativistic magnetospheric electrons: A role in coupling to the middle atmosphere?

Abstract: Twenty-seven-day periodic enhancements of the relativistic electrons are observed in association with concurrently measured solar wind streams, and a numerical transport code is used to investigate the coupling of these high-energy electrons to earth's upper and middle atmosphere. When precipitated, these electrons are found to show a large energy deposition at 40-60 km altitude which is 3-4 orders of magnitude greater than the galactic cosmic ray or solar EUV energy deposition at these altitudes. It is suggested that this electron population could play a role in coupling solar wind and magnetospheric variability to the middle atmosphere through a modulating effect on lower D-region ionization and possibly on upper level ozone chemistry.

Waveform inversion using secondary observables

Abstract: A new method is proposed for inverting single-station surface-wave seismograms. Instead of inverting directly the time-signal, use is made of secondary observables built up from a set of time-frequency images. This allows us to enhance the signal-to-noise ratio and to make more linear the relationship between the model parameters and the inverted observables. The feasibility of the method is checked on multimode synthetics prior to present the inversion of a long-period multimode seismogram related to the Pacific Ocean.

Unusually deep earthquakes in East Africa: Constraints on the thermo-mechanical structure of a continental rift system

Abstract: Shudofsky [1985] has established that earthquakes associated with the East African rift system have well-constrained focal depths as great as 25–30 km. Using published heat flow measurements as a guide to the local geotherm, we find through simple stress envelope calculations that the deepest earthquakes probably occur in the lower crust in a region where the lithosphere is strong. These results are at odds with the commonly held idea that seismicity in zones of continental extension is limited to the shallow upper crust because of elevated temperatures accompanying lithospheric thinning. Any model of the rifting process in East Africa must account for the fact that these regions exhibit considerable strength down to lower crustal levels.

Forced release of sodium from upper atmospheric dust particles

Abstract: The almost explosive growth of narrow sodium layers at altitudes near 95 km was observed by ground-based LIDAR located at 69°N, 16°E. Starting from typical conditions at these altitudes (sodium density approximately 1 to 2× 10³ cm−3) the density is observed to increase by large factors within a few minutes in a narrow layer, the half height of which is typically 1 km only. One of the more spectacular cases involved an increase by a factor 25 within 5 min and a layer width of 0.8 km (full width at half maximum). We propose that the sodium forming these sudden layers is released from upper atmospheric dust by energetic auroral particles. Prior to the auroral bombardment the dust particles were concentrated into a narrow layer by appropriate meteorological processes. Our observations thus imply that considerable amounts of sodium are stored on the surface of upper atmospheric dust.

Frequency domain interferometry: A high resolution radar technique for studies of atmospheric turbulence

Abstract: A new radar technique suitable for high resolution studies of narrow scattering layers in the atmosphere is introduced. The positions of the layers within the radar scattering volume are deduced from the phase differences between radar returns at two distinct transmitter frequencies. Initial mesospheric observations demonstrate the applicability of the technique, and show that the scattering layers are often a few hundred meters thick and widely separated in the mesosphere. Their vertical motions are generally consistent with vertical Doppler velocity estimates.