Showing papers in "Geophysical Research Letters in 1984"

Mineralogy and composition of the upper mantle

Abstract: Seismic velocities are calculated for two petrological models of the upper mantle, an olivine-rich assemblage, pyrolite, and a garnet-clinopyroxene rich, olivine bearing, assemblage, piclogite. These are compared with recent seismic profiles for various tectonic provinces. The shield data is most consistent with a cold olivine and orthopyroxene-rich LID (the seismic lithosphere) extending to 150 km followed by a high temperature gradient and/or a change in mineralogy that serves to decrease the velocity. From 200 to 400 km the velocities follow a 1400°C adiabat. The rise-tectonic mantle is much slower, presumably hotter and is likely to be above the solidus to depths of at least 300 km. The high V_p/V_s ratio of the lower oceanic lithosphere in the western Pacific is most consistent with eclogite.

Structure of the magnetotail at 220 RE and its response to geomagnetic activity

Abstract: Using plasma electron and magnetic field measurements from ISEE 3, 220 RE from earth, we find that the magnetotail at that distance is a coherent structure that evidently waves about through distances comparable to its own lateral scale size. For about one-third of the time it was inside the magnetotail, ISEE 3 was in the plasma sheet. During quiet times the plasma sheet is apparently quite thin, but in response to geomagnetic activity it expands, becoming filled with hot plasma flowing tailward at speeds sometimes exceeding 1000 km/sec, and forces the magnetotail cross-section itself to expand. The plasma sheet’s expansion is delayed typically by about 30 minutes from the onset of the associated geomagnetic activity (often a clearly identified isolated substorm). The magnetic field in the newly-expanded plasma sheet usually exhibits a few-minute steep northward excursion followed by a more prolonged (and often steep) southward excursion. We believe these to be the signatures of arrival of a plasmoid formed and released near the earth at the onset of the corresponding geomagnetic activity. The discreteness of these plasma releases through the magnetotail and their close association with onsets of geomagnetic activity at earth suggest that they are consequences of spontaneous release, probably by magnetic reconnection, of energy and plasma earlier stored in the magnetotail.

Composition of the upper mantle: Geophysical tests of two petrological models

Abstract: The elastic properties of candidate mantle phases are used to test the viability of olivine-rich (pyrolitic) and CaO + Al_2O_3-rich (eclogitic) assemblages for the mantle. High temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties. Both pyrolitic and eclogitic assemblages satisfy the seismic properties between ∼ 200 and 400 km. Between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients. The 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite (“piclogite”) or phase transformations in the olivine + orthopyroxene components of a piclogitic assemblage containing about 16% olivine. High velocity gradients in the transition zone may be explained by the transformation of Ca-rich cpx to majorite garnet. Seismic properties at the top of the lower mantle are consistent with pyrolite, piclogite or perovskite, implying that the 670 km discontinuity may be a chemical boundary.

Flux transfer events: Scale size and interior structure

Abstract: The first direct investigation of the spatial properties of flux transfer events (FTEs) at the earth's dayside magnetopause are reported. Simultaneous magnetometer and plasma data from the ISEE 1 and 2 satellites are combined to show that magnetosheath FTEs can have a scale size of the order of an earth radius in the magnetopause normal direction. It is confirmed that the magnetic field within the events appears to be twisted, this twisting corresponding to a core field-aligned current of a magnitude of a few tens of thousands of A. Also shown is evidence for plasma vorticity in FTEs. The transverse flow and field perturbations accompanying the three events studied obey approximately the Walen relation for a propagating Alfven wave.

Electrostatic hiss and the beam driven electron acoustic instability in the dayside polar cusp

Abstract: In this study it is shown that the upward moving electron beams observed by DE-1 in the dayside polar cusp at 2 to 5 R sub E drive an electrostatic electron acoustic mode, rather than the whistler mode near resonance, as previously concluded. The characteristics of this instability are compared with the properties of hiss in the polar cusp observed by the Dynamics Explorer (DE-1) satellite. The hiss frequencies and funnel shaped frequency-time spectra are consistent with the electron acoustic mode. However, because the hiss often has a fluctuating magnetic field component near the axis of the funnel shape, the hiss is probably composed of both electron acoustic and whistler mode waves.

Substorm associated traveling compression regions in the distant tail: Isee‐3 Geotail observations

Abstract: While in the lobes of the distant magnetotail, ISEE-3 encountered regions of compressed magnetic field at a rate of several per day. The duration of these events was 5 to 20 minutes and they were observed 10 to 30 minutes following the onset of substorm activity near the earth. During each event, the lobe magnetic field tilted first northward and then southward with the inflection point near the time of peak field strength. Following the compression events, the lobe field weakened and retained a southward component for 20 to 40 minutes. It is suggested that these traveling compression regions are the lobe signatures of plasmoids moving rapidly down the tail in the plasma sheet. Comparison of ISEE-3 compression event times with substorm onset times yielded propagation speeds of 350 to 750 km/s.

Mapping convection in the mantle

Abstract: Long-period (100-250 sec.) Love and Rayleigh waves are used to map heterogeneity and azimuthal anisotropy in the upper mantle. Spherical harmonic descriptions of anisotropy up to l = m = 3 and 2θ are derived. Azimuthal anisotropy obtains values as high as 1½%. There is good correlation of fast Rayleigh wave directions with upper mantle return flow directions derived from kinematic considerations. This is consistent with the a-axis of olivine being aligned in the flow direction. The main differences between the flow models and the Rayleigh wave azimuthal variation maps occur in the vicinity of hotspots. Hawaii, for example, appears to perturb the return flow. There is strong correlation of the geoid and surface wave velocity at l = 4 and 5. Slow regions at this scale are associated with geoid highs and high heat flow, consistent with upwelling convective flow or with isostatically compensated regions of low density. The correlation of azimuthal anisotropy with upper mantle return flow directions, rather than with plate directions, suggests that part of the return flow is in the upper mantle and this, in turn, implies a low viscosity channel.

Anisotropy and shear-velocity heterogeneities in the upper mantle

Abstract: Long-period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle. The dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure. The data are corrected for the effect of surface layers and both Love and Rayleigh waves are used. Shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km. The shear wave velocity distribution to 200 km depth correlates with surface tectonics, except in a few anomalous regions. Below that depth the correlation vanishes. Cold subducted material shows up weakly at 350 km as fast S-wave anomalies. In the transition region a large scale pattern appears with fast mantle in the South-Atlantic. S-anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones. The Pacific shows a NW-SE fabric.

A simple model of the 1982-83 Californian "El Nino"

Abstract: Steric height, steric height anomaly, sea level and sea level anomaly show that the 1940-41 and the 1982-83 Californian "El Nino" events were resonant responses of the California Current to the expansion and intensification of the Aleutian low and decrease in strength of the North Pacific high. In both cases, the subsurface anomalies were dynamically produced by onshore transport of Subarctic water and a downward tilt of the inshore thermocline. Sea surface temperature anomalies, however, resulted from thermodynamic as well as dynamic processes. Most previous Californian "El Nino" or "anti-El Nino" events probably were resonant or anti-resonant responses of the seasonal cycle of the California Current to anomalous large-scale atmospheric forcing.

First measurements of supersonic polar wind in the polar magnetosphere

Abstract: Measurements from the Retarding Ion Mass Spectrometer on the Dynamics Explorer have, for the first time, revealed a supersonic polar wind along polar cap field lines. The observations reported were obtained on the nightside (22:30 to 23:30 MLT) from 65 to 81 deg invariant latitude and at altitudes near 2 earth radii. Fitting the data using a thin-sheath model gives a range of temperatures of 0.1 to 0.2 eV with corresponding flow velocities of 25 to 16 km/s over the estimated range of spacecraft potential of +3 to +5 V. For these values the Mach number ranged from 5.1 to 2.6 (with a most likely value of 3). Characteristics of the H(+) flow are in general agreement with those predicted by 'classical' polar wind theory, but high variabilty of the He(+)/H(+) ratio was observed.

A tomographic image of mantle structure beneath Southern California

Abstract: We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X‐ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image. The inversion reveals two prominent features beneath the region. The first is a thin, vertical wedge directly beneath the Transverse Ranges that is 2‐3% faster than the surrounding region. This feature deepens to the east, attaining a maximum depth of about 250 km beneath the San Bernardino Mountains. The second feature is a major zone of low velocity material that is 2‐4% slow under the Salton Trough rift valley, extending to a depth of about 125 km. Two possible explanations for the spatial association of the Transverse Ranges with the velocity anomaly below are lithospheric subduction or small‐scale sublithospheric convection in the region of the Big Bend of the San Andreas Fault. The low velocity anomaly beneath the Salton Trough is consistent with convective upwelling there.

Evidence for slow-mode shocks in the deep geomagnetic tail

Abstract: Plasma and field data from the ISEE-3 space probe provide evidence that the lobe-plasma sheet boundary is the central part of the distant geomagnetic tail is often a slow-mode shock. Such shocks are predicted by Petchek's (1964) model of the reconnection. If this model applies, then the ISEE-3 observations place the general location of the reconnection diffusion region closer to the earth than x of about -100 earth radii.

High temperature stability of maghemite (γ-Fe2O3)

Abstract: The temperature stability of acicular maghemite (γ-Fe2O3) was studied between 20°C and 600°C by x-ray diffraction and magnetic measurements. Synthetic monodomain maghemite was found to be stable with respect to hematite (α-Fe2O3) at temperatures as high as 500°C, inverting to α-Fe2O3 between 510-660°C. However, a complete inversion of the γ to α form for the sample was not achieved even at 660°C, the final product was a two-phase mixture, with 65% of the original γ-Fe2O3 unchanged. Two conclusions are made: (a) the absence of maghemite in a natural sample cannot be proven if heating has been restricted to moderate temperatures (300°-400°C) and (b) the true Curie point of pure maghemite is close to 645°C.

Thermal ion composition measurements of the formation of the new outer plasmasphere and double plasmapause during storm recovery phase

Abstract: Thermal ion composition measurements from several successive dusk sector passes by the DE-1 satellite show the formation of the new outer plasmasphere and double plasmapause following a sharp decrease in geomagnetic activity. In less than one day after the magnetic activity decrease, the outer plasmasphere formed and consisted of cold, essentially Maxwellian plasma with ion composition and thermal energy characteristics generally similar to those of the inner plasmasphere, albeit at significantly lower densities. There is also evidence that at times the thermal O(+) density is comparable to the H(+) density within the plasmasphere.

Limitation of electrostatic charging of dust particles in a plasma

Abstract: It is shown that in a dusty plasma consisting of a plasma (density n and temperature T) and dust grains (density N and radius a) the charge on a grain is not given by its free-space value. Instead, the charge is reduced by a factor 1 + x. Except for the optically thin E and G rings, this factor is large. Usually electromagnetic forces on dust particles in Saturn's ring system are too small to produce observable effects. The current carried by dust particles moving relative to the plasma with a speed w is to a good approximation given by j = NQw. Thus, magnetic perturbations by the F ring should be much smaller than previously estimated.

Henry's law constants for dimethylsulfide in freshwater and seawater

Abstract: Distilled water and several waters of varying salinity were subjected, over a 0-32 C temperature range, to measurements for Henry's law constants for dimethylsulfide. Values for distilled water and seawater of the solubility parameters A and C are obtained which support the concept that the concentration of dimethylsulfide in the atmosphere is far from equilibrium with seawater.

El Nino‐induced onshore transport in the California Current during 1982‐1983

Abstract: Persistent (>9 month) large-scale positive temperature (3-4°C), negative salinity (0.1-0.3‰) and positive dissolved oxygen (0.5-1 ml/l) subsurface anomalies characterize the El Nino-induced onshore transport in the California Current during 1982-83. These anomalies, characteristic diagrams, and sign reversals in the salinity and oxygen anomalies are consistent only with enhanced onshore transport of Subarctic water from the offshore California Current. Onshore transport excludes poleward propagating Kelvin waves as a generation mechanism for the 1982-83 Californian "El Nino". The data, however, support the conclusion (Simpson, 1983) that an anomalous basinwide atmospheric circulation produced the Californian "El Nino".

Arctic haze and the Arctic Gas and Aerosol Sampling Program (AGASP)

Abstract: The Arctic Gas and Aerosol Sampling Program (AGASP) was conducted across the Arctic in the spring of 1983 to study the large scale air pollution phenomenon known as Arctic haze. A component of this program utilized a National Oceanic and Atmospheric Administration WP-3D Orion research aircraft to study the haze in situ. The program and aircraft instrumentation are described.

Mapping alluvial fans in Death Valley, California, using multichannel thermal infrared images

Abstract: Alluvial fans have been mapped in Death Valley, California using NASA's 8-12 micron six-channel airborne Thermal Infrared Multispectral Scanner (TIMS). Both composition and relative age differences were recognized. Age unit boundries are generally consistent with those obtained by conventional mapping. Composition was verified by field investigation and comparison with existing geologic maps. Bedrock and its young derived fan gravels have similar emissivities. The original composition of the fans is modified by differential erosion and weathering, permitting relative age mapping with TIMS.

Gravity anomalies and the structure of western Tibet and the southern Tarim Basin

Abstract: Gravity anomalies across the western part of the Tarim Basin and the Kunlun mountain belt show that this area is not in local isostatic equilibrium. These data can be explained if a strong plate underlying the Tarim Basin extends southwestward beneath the belt at least 80 km and supports part of the topography of northwest Tibet. This corroborates Norin's inference that late Tertiary crustal shortening has occurred in this area by southward underthrusting of the Tarim Basin beneath the Kunlun. This study places a lower bound on the amount of underthrusting.

A fundamental magnetosphere‐Ionosphere coupling mode involving field‐aligned currents as deduced from DE‐2 observations

Abstract: From the magnetic and electric field observations on the DE-2 satellite it is deduced that the perturbation magnetic field and the electric field in the field-aligned current regions are, as a rule, orthogonal to each other and are highly correlated. This deduction is equivalent to the statement that the Poynting vector calculated from these magnetic and electric field components is equal to the ionospheric energy dissipation and that the height-integrated Hall current is divergence free. The state described is interpreted as a frequently prevailing mode of the magnetosphere-ionosphere coupling involving field-aligned currents.

The resolving power of cross‐borehole tomography

Abstract: The data collected in a cross-borehole tomography problem is inherently incomplete, since the boreholes do not completely surround the media under study. The ability of the experiment to resolve structure is therefore limited by poor experimental geometry. We use the method of Backus and Gilbert to investigate the ability of this technique to resolve variations in acoustic velocity and attenuation between the two boreholes. Both integrals that appear in the Backus-Gilbert formulation can be performed analytically, thus expediting the calculations. The resolving kernels have significant horizontal width, demonstrating that the horizontal position of structures can be only poorly resolved.

Spectral analysis of 10m resolution scalar velocity profiles in the stratosphere

Abstract: Vertical profiles of scalar horizontal winds have been measured at high resolution (10m) in the 13 to 37 km region of the stratosphere. This resolution (at that range of altitude) represents the state-of-the-art, and is unique. Our goal was to ascertain whether or not the internal waves of the stratosphere behave consistently with the Garrett-Munk model which was originally created for oceanic internal waves. The power spectral densities (PSD's) of five profiles are presented and it is found that (a) they closely fit a straight line on a log-log graph even to wavelengths as small as 40m, and (b) the average slope is −2.7 ± .2 (standard error = 0.1). We conclude that (a) stratospheric internal waves obey the Garrett-Munk model for vertical wave numbers, and (b) there is no statistically significant evidence for a break in the curve at high wave numbers when due allowance is made for aliasing effects.

Electric field evidence on the viscous interaction at the magnetopause

Abstract: Dusk-to-dawn electric fields just inside the magnetospause have been suggested to be the products of either a viscous interaction or other processes occurring in a closed magnetosphere. Attention is accordingly given to electric field measurements conducted during 28 crossings of the magnetopause by the ISEE 1 satellite, within 2 hours of local dusk, in 1980. Although dusk-to-dawn fields containing potentials of a few kV were often observed, it is concluded that less than 10 percent of the typical magnetospheric potential resulted from viscous interaction or similar processes.

Evaluation of effective eddy diffusive coefficients using radar observations of turbulence in the stratosphere

Abstract: Radar observations show that thin, persistent layers of turbulence occur sporadically in the troposphere and stratosphere Two probabilistic approaches are used to show that the vertical eddy diffusivity due to such layers is of the order of 02-03 sq m/sec in the lower stratosphere An actual realization of turbulent layers, derived from the radar observations at Arecibo, is used in a numerical approach to obtain a profile of eddy diffusivity It is suggested that turbulence plays a significant role in the vertical transport of trace constituents in the stratosphere

Magnetotail flux ropes

Abstract: The structures of flux ropes in the distant magnetotail are inferred from correlated ISEE-3 wave, high energy particle and electron plasma data from December 1982 through March 1983. The spacecraft was then travelling in a figure-8 pattern which took it into the region between the moon and 230 earth radii. The ropes were identified by a bipolar variation along one magnetic component while the spacecraft was in the north lobe. The double peaked magnetic signature included a strong axial field, a weaker circumferential field, and uniformity across the axis. The rope was imbedded in the lobe fields and contained hot, tailward streaming plasma. Finally, a correlation was derived between the presence of the ropes and a high Kp value.

Detection of bump-on-tail reduced electron velocity distributions at the electron foreshock boundary

Abstract: Reduced velocity distributions are derived from three-dimensional measurements of the velocity distribution of electrons in the 7-500 eV range in the electron foreshock. Bump-on-tail reduced distributions are presented for the first time at the foreshock boundary consistent with Filbert and Kellogg's proposed time-of-flight mechanism for generating the electron beams. In a significant number of boundary crossings, bump-on-tail reduced distributions were found in consecutive 3 sec measurements made 9 sec apart. We conclude that, although the beams are linearly unstable to plasma waves according to the Penrose criterion, they persist on a time scale of 3-15 sec.

Experiments on TRM intensity dependence on cooling rate

Abstract: Palaeointensity studies have an important role to play in understanding the origin and time variation of the geomagnetic field. However, theoretical considerations indicate that the intensity of magnetization recorded by a rock or archaeological sample will be dependent on the rate at which it cooled as well as field magnitude. Experiments on synthetic magnetite and titanomagnetites of varying grain sizes indicate that for acicular magnetite at low concentration, slower cooled TRM is stronger than faster cooled TRM, but for all other samples slower cooled TRM is weaker. This is the predicted behaviour. However the magnitude of the effect is much greater than the predicted kinetic effects and indicates that the magnetization of sub-microscopic magnetite-rich inclusions in the titanomagnetites strongly controls the TRM intensity of the matrix.

Brominated organic species in the Arctic atmosphere

Abstract: Measurements are reported of four gas-phase, brominated organic species found in the Arctic atmosphere during March and April 1983. Volume mixing ratios for CH3Br, CH2BrCH2Br, CHBr3, and CH2Br2 were determined by gas chromatography/mass spectrometry analysis from samples taken Arctic wide, including at the geographic North Pole and during a tropopause folding event over Baffin Bay near Thule, Greenland. Methyl bromide mixing ratios were reasonably constant at 11 plus or minus 4 pptv, while the other three brominated organics showed a high degree of variability. Bromoform (2 to 46 pptv) was found to be the dominant contributor to gaseous organic bromine to the Arctic troposphere at 38 plus or minus 10 percent followed by CH2Br2 (3 to 60 pptv) at 29 plus or minus 6 percent. Both CH3Br and CH2BrCH2Br (1 to 37 pptv) reservoirs contained less than 20 percent of the organically bound bromine. Stratospheric samples, taken during a tropopause folding event, showed mixing ratios for all four species at levels high enough to support a stratospheric total volume mixing ratio of 249 pptv Br (888 ngBr/SCM).

Frame modulus reduction in sedimentary rocks: The effect of adsorption on grain contacts

Abstract: The adsorption of vapor on the internal surface of a sandstone reduces its frame moduli. A theoretical model has been developed which predicts the weakening as a function of fluid chemistry and effective pressure. The model is based on well established contact theory and surface chemistry. Frame moduli are directly proportional to the stiffness of the grain contacts. The contact stiffness is dependent on surface energy. We show how the adsorption of various chemicals onto quartz surfaces lowers the surface energy, thereby weakening the contact stiffness. The predicted variation of frame moduli with the partial pressure and chemical species is qualitatively consistent with recent experimental results. The calculations suggest that the chemical effect is only significant at low effective pressures. At high pressures, the elastic loading dominates the contribution due to surface energy.