bluuebunny/crossref_metadata_2025 huggingface dataset:
Balloon‐borne measurements of total reactive nitrogen, nitric acid, and aerosol in the cold Arctic stratosphere
Authors: Kondo Y. , Aimedieu P. , Matthews W. A. , Fahey D. W. , Murcray D. G. , Hofmann D. J. , Johnston P. V. , Iwasaka Y. , Iwata A. , Sheldon W. R.
Published: February 2008

Total reactive nitrogen (NOy) between 15 and 29 km was measured for the first time on board a balloon within the Arctic cold vortex. Observations of HNO3, aerosol, and ozone were made by instruments on the same balloon gondola which was launched from Esrange, Sweden (68°N, 20°E) on January 23, 1989. The N0y mixing ratio was observed to increase very rapidly from 6 ppbv at 18 km altitude to a maximum of 21 ppbv at 21 km, forming a sharp layer with a thickness of about 2 km. A minimum in the NOy mixing ratio of 5 ppbv was found at 27 km. The measured HNO3 profile shows broad similarities to that of NOy. This observation, together with the observed very low column amount of NO2, shows that NOx had been almost totally converted to HNO3, and that NOy was composed mainly of HNO3. The enhanced aerosol concentration between 19 and 22 km suggests that the maximum abundance of HNO3 trapped in the form of nitric acid trihydrate (NAT) was about 6 ppbv at 21 km. The sampled air parcels were highly supersaturated with respect to NAT. Although extensive denitrification throughout the stratosphere did not prevail, an indication of denitrification was found at altitudes of 27 and 22 km, and between 18 and 15 km.
In situ observations of ClO in the Arctic stratosphere: ER‐2 aircraft results from 59°N TO 80°N latitude
Authors: Brune W. H. , Toohey D. W. , Anderson J. G. , Chan K. R.
Published: February 2008

Large abundances of ClO were observed inside the arctic polar vortex during 14 flights of the NASA ER‐2 aircraft from Stavanger, Norway (59°N, 6°E) to 80°N latitude. Flights were conducted at altitudes between 14 and 20 km when the solar zenith angle was between 79° and 101°. Data are reported for three flights, January 6, January 16, and Febmary 10, that represent the main features observed during the mission. ClO mixing ratios were typically less than 50 parts per trillion by volume (pptv) outside the vortex and exceeded 100 pptv inside the vortex for all flights. ClO mixing ratios were more than 500 pptv for four days in a row in early February, reaching 1130 pptv on February 10, at an altitude of 19 km (potential temperature of 460 K). Peak ClO mixing ratios in early February were ∼ 100 times larger than those observed at mid‐latitudes for all altitudes surveyed between 15 and 19 km. These data, comparable to those obtained in the antarctic ozone hole, indicate that the springtime arctic polar vortex was extensively perturbed by heterogeneous chemistry and contained enough ClO to catalytically destroy ozone rapidly.
Observations of the Mf ocean tide from Geosat altimetry
Authors: Cartwright David E. , Ray Richard D.
Published: February 2008

Zonal averages of the 13.66‐day Mf tide are derived from one year of Geosat altimetry records. The orbit errors are reduced by 1/revolution corrections taken over long (several day) arcs. The short‐period tides are removed using a model previously derived by us from the same data. The Mf zonal averages indicate definite nonequilibrium character at nearly all latitudes. The imaginary admittances indicate a Q of at least 8; such a value is consistent with a simplified theory of coupled gravitational and vorticity modes and suggests a value for Proudman's ‘friction period’ about 123 days.
Molten (Mg0.88Fe0.12)2SiO4 at lower mantle conditions: Melting products and structure of quenched glasses
Authors: Williams Quentin
Published: February 2008

Infrared spectra of quenched magnesium silicate glasses synthesized by fusing olivine at pressures in excess of 50 GPa and temperatures greater than 2500 K demonstrate that silicon is dominantly present in four‐fold coordination with respect to oxygen within these quenched glasses. This low coordination is attributed, by analogy with the structural behavior of glasses compressed at 300 K, to the instability of higher coordinations in glasses of these compositions on decompression. Spectra of glasses formed in a hydrous environment document that water is extensively soluble in melts at these high pressures and temperatures. Also, these results are consistent with the melting of (Mg0.88Fe0.12)2SiO4 compositions to liquids near pyroxene in stoichiometry under these conditions, with iron‐rich magnesiowüstite being the liquidus phase.
Geodetic point positioning with GPS carrier beat phase data from the CASA UNO Experiment
Authors: Malys Stephen , Jensen Peter A.
Published: February 2008

The Global Positioning System (GPS) carrier beat phase data collected by the TI4100 GPS receiver has been successfully utilized by the US Defense Mapping Agency in an algorithm which is designed to estimate individual absolute geodetic point positions from data collected over a few hours. The algorithm uses differenced data from one station and two to four GPS satellites at a series of epochs separated by 30 second intervals. The ‘precise’ GPS ephemerides and satellite clock states, held fixed in the estimation process, are those estimated by the Naval Surface Warfare Center (NSWC). Broadcast ephemerides and clock states are also utilized for comparative purposes.

An outline of the data corrections applied, the mathematical model and the estimation algorithm are presented. Point positioning results and statistics are presented for a globally‐distributed set of stations which contributed to the CASA UNO experiment. Statistical assessment of 114 GPS point positions at 11 CASA UNO stations indicates that the overall standard deviation of a point position component, estimated from a few hours of data, is 73 centimeters. Solution of the long line geodetic inverse problem using repeated point positions such as these can potentially offer a new tool for those studying geodynamics on a global scale.
Magnetic techniques applied to the study of rock varnish
Authors: Clayton Jeffrey A. , Verosub Kenneth L. , Harrington Charles D.
Published: February 2008

We report the successful application of magnetic techniques to the study of rock varnish, a surficial coating that forms on rocks in arid environments. Using thin wafers of varnished and unvarnished greywacke sandstone, we have detected a stable remanent magnetization that can be unambiguously attributed to the rock varnish. We have also measured several rock magnetic properties of the rock varnish and have shown that the primary magnetic carrier is either magnetite or maghemite. Our results demonstrate the feasibility of applying magnetic techniques to the study of geomorphic surfaces. Subsequent rock magnetic and paleomagnetic studies of rock varnish should provide new information about the geochemical processes involved in the formation of rock varnish and about the validity of a new chronological technique known as rock varnish dating.
Rock magnetism of the Early Pliocene Trubi Formation at Eraclea Minoa (Sicily)
Authors: Van Velzen A. J. , Zijderveld J. D. A.
Published: February 2008

The most important magnetic mineral in the finegrained marls of the Early Pliocene Trubi formation near Eraclea Minoa is magnetite. Some goethite is also present. The magnetite is of high purity and of primary origin.

Hysteresis and low‐temperature measurements show that most magnetite grains are single‐domain. The presence of superpara‐magnetic grains is suspected. Multi‐domain grains are rare. It is stewn that AF demagnetization does not yield correct NRM directions.
Effects of changing water content and atmospheric pressure on radon flux from surfaces of five soil types
Authors: Owczarski P. C. , Holford D. J. , Freeman H. D. , Gee G. W.
Published: February 2008

A computer code, Rn3D, was used to study the effects of varying the water content of five homogeneous soil types (clay, silt, loam, sand, and gravel) and atmospheric pressure on the transport of radon from soil surfaces. Temperature (20°C) and radium content were assumed to be the same for all soils. Surface fluxes and soil pore space concentrations were computed for steady‐state diffusion only, steady‐state diffusion with steady pressure gradients, and sinusoidal (e.g., diurnal) changes in atmospheric pressure. Pressure gradients drive advective radon transport. A steady‐state pressure gradient of −0.5 Pa/m enhanced the total radon surface flux over the diffusive flux from 0.01% for clay to 1000% for gravel at 0% saturation. At 90% saturation the enhancements were one‐tenth as much. The degree of enhancement was approximately proportional to the gradient along the soil column. A net enhancement of surface flux over steady diffusive flux (up to 6%) for sinusoidal surface pressure changes was observed for all five soil types. The study reveals that radon flux is affected as much by varying soil water content as by varying soil type.
Coronal mass ejections and large geomagnetic storms
Authors: Gosling J. T. , Bame S. J. , McComas D. J. , Phillips J. L.
Published: February 2008

Previous work indicates that coronal mass ejection (CME) events in the solar wind at 1 AU can be identified by the presence of a flux of counterstreaming solar wind halo electrons (above about 80 eV). Using this technique to identify CMEs in 1 AU plasma data, we find that most large geomagnetic storms during the interval surrounding the last solar maximum (Aug. 1978 – Oct. 1982) were associated with Earth‐passage of interplanetary disturbances in which the Earth encountered both a shock and the CME driving the shock. However, only about one CME in six encountered by Earth was effective in causing a large geomagnetic storm. Slow CMEs which did not interact strongly with the ambient wind ahead were particularly ineffective in a geomagnetic sense.
Two‐frequency Lidar technique for mesospheric Na temperature measurements
Authors: She C. Y. , Latifi H. , Yu J. R. , Alvarez R. J. , Bills R. E. , Gardner C. S.
Published: February 2008

We describe a new two‐frequency lidar for measuring Na temperature profiles that uses a stabilized cw single‐mode dye laser oscillator (rms frequency jitter < 1 MHz) followed by a pulsed‐dye power amplifier (140 MHz FWHM linewidth) which is pumped by an injection‐locked Nd: YAG laser. The laser oscillator is tuned to the two operating frequencies by observing the Doppler‐free structure of the Na D2 fluorescence spectrum in a vapor cell. The lidar technique and our initial observations of the temperature profile between 82 and 102 km at Ft. Collins, CO (40.6°N,105°W) are described. Absolute temperature accuracies at the Na layer peak of better than ±3 K with a vertical resolution of 1 km and an integration period of approximately 5 min were achieved.
Bremsstrahlung X ray spectra of JUpiter and Saturn: Predictions for future planetary spacecraft
Authors: Barbosa D. D.
Published: February 2008

Calculations of X ray spectra due to bremsstrahlung from precipitating auroral electrons at Jupiter and Saturn are presented. The model assumes that a field‐aligned potential drop accelerates a primary beam of electrons into the atmosphere where a population of secondary electrons having a power law energy dependence is generated. The spectrum at Jupiter is normalized to the soft X ray observations of Metzger et al. (1983) at the low‐energy end and constrained at the high‐energy end by UV auroral energy requirements. The spectrum at Saturn is constructed by analogy to the Jovian case allowing for variation of the beam energy, energy flux, and scale size of the Saturnian aurora. The results indicate that a significant flux of X rays is emanating from both planets which may serve as a basis for conducting planetary X ray astronomy as a part of future spacecraft missions to the planets.
Hybrid simulations of tangential discontinuities
Authors: Cargill P. J.
Published: February 2008

One dimensional hybrid simulations of tangential discontinuities (TDs) are presented. Ion kinetic effects are shown to play a crucial role in determining the TD structure. When the density ratio across a TD is large, magnetic profiles with either overshoots or monotonic gradients are obtained. When there is a bulk plasma flow perpendicular to the magaetic field, so that the motional electric field is directed into the moving plasma, the field profiles exhibit similar behavior when the density ratio differs from unity. In each of these cases, the final width is between 2 and 5 ion Larmor radii, roughly what is seen at the earth's magnetopause.
Ion leakage, ion reflection, ion heating and shock‐front reformation in a simulated supercritical quasi‐parallel collisionless shock
Authors: Lyu L. H. , Kan J. R.
Published: February 2008

Ion reflection, ion leakage, ion heating and shock‐front reformation are studied based on the simulation results of a supercritical quasi‐parallel shock. The backstreaming ions upstream of the simulated shock are predominantly leakage ions. The leakage of downstream ions is regulated by the large‐amplitude waves in the shock transition region. These waves not only can act as a filter to reduce the leakage ion number density, but also can energize the backstreaming ions to result in the suprathermal upstream ions. The average temperature of the leakage ions is about 100 times that of the upstream incoming ions. The average leakage ion density is about 2% ∼ 3% of the upstream ion density. Ion reflections occur intermittently due to coherent ion scattering by the large‐amplitude whistler waves near the shock front. Most of the coherently reflected ions are subsequently scattered back toward the downstream to contribute to the ion heating. Only a small fraction of the reflected ions can escape upstream with an average number density about 0.1% ∼ 0.2% of the upstream ion density. The shock‐front appears to reform as characterized by the decay and reappearance of the large‐amplitude whistler waves near the shock ramp. We wish to propose a possible explanation for the shock‐front reformation process. Insufficient dissipation in the supercritical shock can lead to a highly‐steepened shock ramp, which in turn can emit large‐amplitude whistler waves to result in the ion reflection event. Since ion reflections can provide additional dissipation, to counter balance the nonlinear steepening and thereby reduce the wave amplitude and turn off the ion reflection event. The cycle of under‐dissipation followed by over‐dissipation is proposed to be a possible cause for the shock‐front reformation process.
Variation in global tropospheric temperature after adjustment for the El Nino influence, 1958–89
Authors: Angell J. K.
Published: February 2008

Data from a 63‐station radiosonde network are used to estimate the variation in global tropospheric (850–300 mb) temperature between 1958 and 1989. The annual temperature was a maximum in 1988, 0.42°C above the 1958–88 mean. However, the 1989 value is indicated to be only 0.12°C above this mean. During 1958–88, there has been a correlation of 0.76 (significant at the 0.1% level) between annual values of global tropospheric‐temperature deviation and annual values of sea‐surface temperature (SST) deviation in eastern equatorial Pacific two seasons earlier. The associated linear‐regression line indicates that an annual SST deviation of 1°C in the region 12°S‐2°N, 180°‐90°W has been related on average to an annual global tropospheric‐temperature deviation of 0.36°C. The annual values of global tropospheric temperature have been adjusted based on this regression. With this adjustment, the year‐to‐year variability in global tropospheric temperature is halved, the increase in decadal mean temperature between the 1960's and 1980's is reduced from 0.33°C to 0.24°C, the annual temperature is a maximum in 1989 (0.39°C above the 31‐year mean) rather than in 1988, and there is much more convincing evidence that the eruptions of Agung in 1963 and El Chichon in 1982 decreased global tropospheric temperatures by 0.2–0.3°C for about 3 years.
Transient magmatic convection prolonged by solidification
Authors: Brandeis Geneviève , Marsh Bruce D.
Published: February 2008

Magma chambers cool from all sides, with expected vigorous convection. Strong changes in viscosity with crystallization, however, lock up within an inwardly propagating crust a great deal of buoyancy otherwise available to drive convection. With sufficiently small degrees of superheat, the Rayleigh number (Ra) is small and convection is transient and never becomes fully developed. In order to understand this transient stage of convection, we have performed some fluid dynamic experiments on the solidification of a paraffin layer. Except in the growing crust and its thin forward boundary layer, the interior remains isothermal and the temperature decreases uniformly until it is locked at the convective liquidus. The crystals are fine and hair‐like dendrites and no major differentiation of the fluid composition is observed. The time‐scale of the convective stage is predictable and is very short relative to the time of complete solidification. We also report measurements over time of crust thickness, convective velocity, and heat transfer. When the interior temperature reaches the convective liquidus, convection wanes, although the calculated Ra, based on an arbitrary small temperature contrast of 0.2°C, is still large (∼107). A small (±0.2°C) uncertainty in ΔT (i.e., 0≤ΔT≤0.2°C) produces a large uncertainty in Ra (i.e., 0≤Ra≤107). This uncertainty in Nature may preclude an accurate determination of Ra in a magma chamber residing at a temperature very close to the convective liquidus.
Information on stress conditions in the oceanic crust from oval fractures in a deep borehole
Authors: Morin Roger H.
Published: February 2008

Oval images etched into the vail of a deep borehole were detected in DSDP (Deep Sea Drilling Project) Hole 504B, eastern equatorial Pacific Ocean, from analysis of an acoustic televiever log. Initial speculation as to the origin of these remarkable features considered the intersection of pillow basalts with the wellbore. However, a systematic inspection of these ovals has identified intriguing consistencies in appearance that cannot be explained satisfactorily by a random, coincidental distribution of pillow lavas. As an alternative hypothesis, Mohr‐Coulomb failure criterion is used to account for the generation and orientation of similarly curved, stress‐induced fractures. Consequently, these oval features can be interpreted as fractures and related directly to stress conditions in the oceanic crust at this site. The azimuth of the oval center corresponds to the orientation of maximum horizontal principal stress (SH), and the oval width, which spans approximately 180° of the borehole, is aligned vith the azimuth of minimum horizontal principal stress (Sh). The oval height is controlled by the fracture angle and thus is a function of the coefficient of internal friction of the rock.
Coseismic stress changes induced by the 1989 Loma Prieta, California Earthquake
Authors: Michael Andrew J. , Ellsworth William L. , Oppenheimer David H.
Published: February 2008

Earthquake focal mechanisms from before and after the 1989 Loma Prieta, California earthquake are used to infer the coseismic stress change. Before the main shock, most earthquakes correspond to right lateral slip on planes sub‐parallel to the San Andreas fault, and imply a generally N‐S most compressional stress axis and a vertical intermediate stress axis. Aftershocks within the main shock rupture zone, however, display almost every style and orientation of faulting, implying an extremely heterogeneous stress field. This suggests that the main shock relieved most, if not all, of the shear stress acting on its fault plane. Aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states, but only when considered in three groups: north, south, and above the main shock rupture area. In each of these areas the stress state may reflect stress transfer by the main shock.
Recent secular variations in the extent of Northern Hemisphere snow cover
Authors: Robinson David A. , Dewey Kenneth F.
Published: February 2008

Northern hemisphere snow cover during 1988 and 1989 was at its lowest extent since the advent of reliable satellite snow‐cover monitoring in 1972; running some 8–10% below the eighteen‐year annual mean of 25.7 million km2. Monthly minima for the period of record occurred six times during these two years. In general, the last nine years of the satellite record had less extensive cover than the 1972–80 interval. Negative anomalies during the 1980s were largest over Eurasia in all seasons, and in the Spring over North America. Hemispheric seasonal means for the most recent nine years were 3.7% to 8.4% lower than those between 1972 and 1980. Results are based on analyses of National Oceanic and Atmospheric Administration weekly snow charts, which are produced from visible satellite imagery.
A model of dust production in the Neptune ring system
Authors: Colwell Joshua E. , Esposito Larry W.
Published: February 2008

Voyager 2 images of the Neptunian ring system show it to be relatively dusty with the fraction of micron‐sized dust particles in the rings between about 0.2 and 0.7 (Smith et al 1989). We apply a numerical model developed for the dust bands (Colwell and Esposito, Icarus (in press)) to the Neptune system. Our results show that 1) the suggested distribution of dust fractions in the rings is not related to ring optical depth and may be due to different ring particle size distributions. 2) Collision velocities between the macroscopic ring particles must be ∼ 1 m/sec if they are to produce im fractions above 1/2. 3) The small moons of Neptune have regoliths for meteoroid excavation to produce the dust population at Voyager ring plane crossing. 4) Dust optical depths interior to 1989N3R are negligible unless there are macroscopic source bodies in that region.
The coupled orbital and thermal evolution of Triton
Authors: Ross Martin N. , Schubert Gerald
Published: February 2008

The coupled orbital and thermal evolution of Triton is simulated with a model of the satellite having temperature dependent k/Q (k is the second degree Love number and Q is the tidal dissipation factor). Large changes in orbital and interior properties occur in a short time span of less than 108 years. The peak tidal heating rate exceeds the radioactive heating rate by a factor of at least 103. The ability of tidal heating to overwhelm solid state convection and cause global melting is unequivocally demonstrated. Triton's initial thermal state and composition are shown to control the length of time that passes between capture from heliocentric orbit and the occurrence of swift and dramatic changes in orbital and internal properties. This time interval could have been several billion years.
The marine geochemistry of actinium‐227: Evidence for its migration through sediment pore water
Authors: Nozaki Yoshiyuki , Yamada Masatoshi , Nikaido Hirofumi
Published: February 2008

227Ac with a half life of 21.8 years has a potential utility as a tracer of deep water circulation and mixing studies on time scales less than 100 years. Here we present the first measurement of 227Ac profile in the pore water of Northwest Pacific deep‐sea sediment and in the ∼ 10,000 m long water column of Izu‐Ogasawara Trench. The results clearly show that 227Ac is supplied from the sediment to the overlying water through migration in the pore water. The model calculation indicates that the molecular diffusion alone through sediment porewater can support only a half of the standing crop of excess 227Ac in the water column and the enhanced supply of 227Ac by particle mixing is necessary to account for the remainder. Thus, bioturbation in the deep sea plays an important role in controlling the flux of some short‐lived radionuclides such as 227Ac and 228Ra across the sediment‐water interface.
The carbonate profile of two recent Ionian Sea cores: Evidence that the sedimentation rate is constant over the last millennia
Authors: Castagnoli G. Cini , Bonino G. , Caprioglio F. , Provenzale A. , Serio M. , Guang‐Mei Zhu
Published: February 2008

We confirm and extend the results previously reported on the carbonate profile of the GT14 Ionian Sea core [Cini Castagnoli et al., 1990]. A second, much longer core (2.81 meters), named GT89/3, has been taken about 1 km apart from the previous one. The carbonate profiles of the two cores are impressively similar; the details of the CaCO3 variations in the two sediments match on the scale of the sampling interval Δd = 2.5 mm used for both cores. We show that Δd corresponds to the mud deposited in a time interval Δt = 3.87±0.04 years, a value which is constant throughout the entire length of the cores. This precision is achieved by the tephroanalysis of the two cores. In this approach the markers of well‐known historical eruptions in the Vesuvius area are recognized (Pompei, AD 79, Pollena, AD 472, Ischia, AD 1301), providing a precise dating which accurately tunes that obtained by the radiometric method. The correlation between the carbonate profile of the GT14 core and the tree‐ring radiocarbon record has been discussed in [Cini Castagnoli et al., 1990]; here we extend these results and show that the same correlation holds at least up to 1690 BC. Due to the longer length of the GT89/3 time series, we also show that three periodic components at about 206 yr, 228 yr and 179 yr may now be resolved in the carbonate series, in close agreement with the results already found for the radiocarbon record.
Low frequency electrical response of polycrystalline olivine compacts: Grain boundary transport
Authors: Tyburczy James A. , Roberts Jeffery J.
Published: February 2008

The complex electrical impedance of sintered, polycrystalline compacts of San Carlos olivine (grain size 75–180 μm) has been determined over the frequency range 104 to 10−4 Hz, at temperatures between 800 and 1400°C, at the fO2 of the wustite‐magnetite buffer. Complex plane plots (Im Z vs. Re Z) show separate effects of grain interior and grain boundary conductivity. Grain interior transport controls the response above ∼102 Hz, whereas grain boundary transport dominates between ∼102 and 10−2 Hz. Overall response of the compacts is modeled with an equivalent circuit in which parallel RC circuit elements representing grain interior and grain responses act in series. Bulk DC conductivity is lower than either grain interior or grain boundary conductivity alone. Inversion of magnetotelluric profiles that employ lower bulk conductivities of the grain interior‐grain boundary system yield upper mantle temperature estimates 30 to 50°C higher than those based on single crystal values.
The transition from the magnetosheath to the magnetosphere
Authors: Hall D. S. , Hapgood M. A. , Bryant D. A.
Published: February 2008

Electron measurements are studied in the transition region where the magnetosheath merges with the magnetosphere. The outer part of the transition is found to be a depletion region, in which a reduction in the electron thermal pressure coincides with compression of the magnetic field. It is suggested that the compression is associated with the draping of the interplanetary field around the inner part of the region, which is a region of closed magnetic field lines. Implications for the topology of boundary layers throughout the magnetosphere are discussed.
Crystal structure and thermal expansion of (Mg,Fe)SiO3 perovskite
Authors: Parise J. B. , Wang Y. , Yeganeh‐Haeri A. , Cox D. E. , Fei Y.
Published: February 2008

High‐resolution x‐ray diffraction data were collected from 10 to 433K on a homogeneous polycrystalline specimen of Mg0.9Fe0.1SiO3‐perovskite. Rietveld structural refinement and x‐ray absorption fluorescence measurements demonstrate that Fe substitutes for Mg and not Si. The thermal expansion behavior is anisotropic, with the orthorhombic distortion becoming less with increasing temperature. The volumetric thermal expansion, which is dominated by the decrease in octahedral tilts, is 1.9×l0−5 K−1 between 150 and 373K. No evidence of conversion to enstatite could be found up to 873 K.
Melt migration in a silicate liquid‐olivine system: An experimental test of compaction theory
Authors: Riley G. N. , Kohlstedt D. L. , Richter P. M.
Published: February 2008

To investigate the kinetics of porous flow in partially molten peridotite, a melt migration couple – formed from a disc of fine‐grained olivine plus ∼15% of a synthetic K‐Al silicate glass and a disc of poly crystalline olivine – was heated at 1255°C under a confining pressure of 300 MPa for 2 hr. Driven by capillary forces, silicate liquid in the source disc infiltrates along three‐grain junctions into the sink disc. To analyze the resulting melt migration profile in terms of compaction theory, the equations developed by McKenzie [1984] to describe porous flow of a liquid in a deformable matrix were modified by replacing the buoyancy force term with a capillary force term. The governing equations were then solved numerically with the initial and boundary conditions specified by the experimental design. Comparison of the melt migration profile obtained from the experiment with those generated by numerical simulation demonstrates that compaction theory provides a good description of the experimental results provided that the permeability of these solid‐liquid materials increases linearly with increasing liquid fraction.
Microwave radar measurements of ocean wave propagation —Initial results
Authors: Poulter E. M. , Smith M. J. , McGregor J. A.
Published: February 2008

A microwave radar has been developed to measure the surface orbital velocities of ocean waves, and hence estimate the wave height spectra. The Doppler radar exploits the frequency modulated continuous wave (FMCW) technique to provide good spatial resolution (much less than the ocean wavelengths of interest) making it suitable for ocean wave propagation studies. We present initial data demonstrating the radar's capabilities to deduce near‐shore ocean wave properties. The spatial and temporal variations of the orbital velocities reveal the wave propagation, and the deduced wave height spectra show wave growth as they approach the shore.
A 2‐d dynamical model of mesospheric temperature inversions in winter
Authors: Hauchecorne Alain , Maillard Alexandre
Published: February 2008

A 2‐D stratospheric and mesospheric dynamical model including drag and diffusion due to gravity wave breaking is used to simulate winter mesospheric temperature inversions similar to those observed by Rayleigh lidar. It is shown that adiabatic heating associated to descending velocities in the mesosphere is the main mechanism involved in the formation of such inversions. Sensitivity tests are performed with the model and confirm this assumption. It is also explained why other previous similar studies with 2‐D models did not show mesospheric inversion layers.
A mechanism for bursty radio emission in planetary magnetospheres
Authors: Wong H. K. , Goldstein M. L.
Published: February 2008

Bursty radio emissions are often observed from the polar magnetospheres of the Earth, Jupiter, Saturn, and Uranus in addition to the smooth radio emissions commonly detected. We show that in plasma regimes in which the electron plasma frequency is less than the electron cyclotron frequency, anisotropic electron beams or gyrating electron beams can excite directly broadband electromagnetic radiation. The largest growth is for right‐hand X‐mode radiation with frequencies above the electron cyclotron frequency. This instability can produce bursty, broadband emission, consistent with some of the properties of the radiation observed from the magnetized planets.
Equilibrium constant for the reaction ClO + O2 <−> ClO · O2
Authors: DeMore W. B.
Published: February 2008

Photolysis of Cl2/Cl2O/O2 mixtures at 197 K has been used to place an upper limit of 4×l0−18 cm3 on the equilibrium constant for the ClO + O2 <––> ClO·O2 reaction at that temperature. This value is three orders of magnitude below the current NASA recommendation [DeMore et al., 1990]. Estimating S°(ClO·O2 ) as 73 cal/mol‐K, the bond energy for the complex is less than 7.7 kcal/mole.
Summertime formation of Depth Hoar in central Greenland
Authors: Alley R. B. , Saltzman E. S. , Cuffey K. M. , Fitzpatrick J. J.
Published: February 2008

Summertime solar heating of near‐surface snow in central Greenland causes mass loss and grain growth. These depth hoar layers become seasonal markers which are observed in ice cores and snow pits. Mass redistribution associated with depth‐hoar formation can change concentrations of immobile chemicals by as much as a factor of two in the depth hoar, altering atmospheric signals prior to archival in ice. For methanesulfonic acid (MSA) this effect is not significant because the summer maximum does not coincide with the density minimum, and the amplitude of the annual (MSA) signal is more than a factor of ten.
On the choice of boundary conditions in continuum models of continental deformation
Authors: Wdowinski Shimon , O'Connell Richard J.
Published: February 2008

Recent studies of continental deformation have treated the lithosphere as a viscous media and investigated the time evolution of the deformation caused by tectonic and buoyancy forces. We examine the differences between continuum models that keep velocity boundary conditions (b.c.) constant with time and models that keep stress b.c, constant with time and demonstrate these differences by using a simple example of a continental lithosphere that is subjected to horizontal compression. Our results show that in the case of constant stress b.c., the indentation velocity decreases with time, while in the case of constant velocity b.c., the indentation velocity remains constant with time. Observations from the Tibetan plateau and from the Andes indicate that the rate of indentation decreases with time. This suggests that when buoyancy forces are comparable in magnitude to tectonic forces, constant stress b.c. are more appropriate for time evolutionary models of continental lithosphere. We propose a simple procedure that translates velocity b.c., which are easy to estimate, into stress b.c. at the initial stage, and keep the stress b.c. constant along the time progression of the calculations.
Topographically driven crustal flow and its implication to the development of pinned oroclines
Authors: Hsui Albert T. , Wilkerson M. Scott , Marshak Stephen
Published: February 2008

Pinned oroclines, a type of curved orogen which results from lateral pinning of a growing fold‐thrust belt, tend to resemble parabolic Newtonian curvature modified by different degrees of flattening at the flow front. We propose that such curves can be generated by Newtonian crustal flow driven by topographic variations. In our model, regional topographic differences create a regional flow which produces a parabolic flow front upon interaction with lateral bounding obstacles. Local topographic variations modify the parabolic curves and yield more flat‐crested, non‐Newtonian‐like curvatures. The degree of modification depends on the relative strength of the local driving potential, which in turn is dependent on rock type and fluid pressure. On the basis of a finite difference, thin‐skin, tectonic simulation, we demonstrate that both Newtonian‐like and non‐Newtonian‐like curved orogens can be produced within a Newtonian crust. In effect, the shape of curved orogens in plan may provide insight into the rheology of the Earth's upper crust over geological time scales.
Evaluation of GPS estimates of relative positions from central California, 1986–1988
Authors: Larson Kristine M.
Published: February 2008

I report estimates of long‐term precision and accuracy of interstation vectors using the Global Positioning System (GPS). Precision is estimated from three experiments conducted in central California over a period of 1.2 years. Horizontal precision is 3 to 8 mm for baseline vectors between 100 and 450 km in length, and the vertical precision ranges from 11 to 33 mm. Accuracy is assessed by comparing VLBI estimates with results from 6 GPS experiments conducted at Mojave and Palos Verdes over a period of 2.2 years. Rates for all vector components agree with VLBI within one standard deviation.
Magnetostratigraphy of the Toarcian Ammonitico Rosso limestone at Iznalloz, Spain
Authors: Galbrun Bruno , Baudin Francois , Fourcade Eric , Rivas Pascual
Published: February 2008

The Iznalloz section (Betic cordillera, Spain) provides a continuous outcrop (20 m thick) of Toarcian ammonitico rosso limestones with all the ammonite zones defined in the Betic realm. The characteristic magnetization direction for each sample was obtained by detailed thermal demagnetization and was stable from about 450 to 560°C. The magnetic polarity sequence extends from the Polymorphum to the Aalensis ammonite zone, and covers the whole Toarcian stage. Thus this section presents a good pattern of the reversal succession during Toarcian times. Considering hiatuses of sampling and excluding single‐sample magnetozones, there does not seem to be any important change in the reversal rate during this period.
Magnetic susceptibility of Chinese loess and its bearing on paleoclimate
Authors: Wang Y. , Evans M. E. , Rutter N. , Ding Z.
Published: February 2008

Magnetic susceptibility measurements are reported from 468 samples spanning a 160 m‐thick section of loess near the city of Baoji, Shaanxi Province. An independent absolute chronology is provided by geomagnetic polarity reversals, which indicate that the sediments represent the geological interval 0 to 2.5 million years ago. Fourier analysis of the susceptibility time series reveals strong spectral peaks at quasi‐periods of 100,000 and 40,000 years in agreement with the astronomical theory of climatic forcing; the longer period is due to the eccentricity of the Earth's orbit, the shorter to the obliquity of the ecliptic. The expected peak near 20,000 years (due to precession) appears only weakly. However, we do find strong evidence for a 400,000 year quasi‐periodicity, particularly prior to 1.32 million years ago; this is firmly predicted by astronomical calculations, but has rarely been recognized in geological data.
Bidirectional anisotropies in solar cosmic ray events: Evidence for magnetic bottles
Authors: Palmer I. D. , Allum F. R. , Singer S.
Published: February 2008

The occurrence of bidirectional anisotropies in low energy solar proton and electron events has been investigated in the period 1967–1973 by using data obtained from five satellites, viz., Explorer 34 and 41 and Vela 5B, 6A, and 6B. Some 16 observations of bidirectional distribution were detected in this time period, including the observation of bidirectionality in three solar electron events. The characteristics of two of these electron events are discussed in detail. The average duration over all events is 9 hours, implying a spatial scale of ≈0.13 AU, and the events show a strong correlation with the minimum of a Forbush decrease. The two maxima in the particle distributions are aligned with the interplanetary magnetic field. The origin of the bidirectional distributions is shown to be an interplanetary phenomenon rather than an effect of the bow shock. A magnetic configuration consisting of large‐scale loops in the interplanetary field (Gold bottle model) is suggested to account for the observations. The bidirectional flow can be produced by particle mirroring and by adiabatic expansion. The rarity of two‐way flow in solar electrons implies that special conditions must exist for its development. Our observations suggest that 1 out of 10 loop‐shaped coronal expulsions is detectable via two‐way cosmic ray particle flow near 1 AU. Bidirectional anisotropy observed in the event of August 4–5, 1972, in exact coincidence with the minimum of a striking Forbush decrease, suggests a shell1 of looped and well‐ordered field lines at the leading edge of the driver gas. The identification of this cavity, in which the galactic intensity is depressed, contradicts some aspects of published models of solar and galactic cosmic ray behavior.
Linear and nonlinear amplification in the magnetosphere during a 6.6‐kHz transmission
Authors: Dowden R. L. , McKay A. D. , Amon L. E. S. , Koons H. C. , Dazey M. H.
Published: February 2008

Reception at Dunedin of the magnetospheric signal at 6.6 kHz transmitted from Anchorage, Alaska, showed both linear and nonlinear amplification during an event lasting some 20 min near local midnight. Linear amplification of the transmitter signal was ∼20 dB. Natural whistlers were also amplified but often at frequencies sharply limited to those from the transmitter frequency upward. Nonlinear amplification (NLA) produced a signal positively offset from the transmitter frequency by 20–150 Hz at amplitudes over 40 dB above the unamplified transmitter signal. This signal appeared as a largely self‐sustaining embryo emission (EE) under the control of the transmitter signal. The phase of this EE signal in each NLA event was tracked with respect to a recorded phase reference. These phase studies showed that the accumulating phase of the offset EE signal is frequently interrupted by negative phase steps (‘N events’) which tend to reduce the offset frequency. Five of the NLA events during key‐down transmission were quenched by whistlers which themselves triggered free emissions at ½ƒBO. The theory of nonlinear wave‐wave interaction between the transmitter or input wave (IW) and the embryo emission is developed to explain these features. It is shown that coupling depends on the offset frequency δƒ and the ‘control frequency’ Fc: for δƒ > Fc the emission is effectively free; for δƒ < Fc, EE is controlled by IW. Curiously, Fc is determined by the EE amplitude (Bw) as Fc ∞ BW1/2, and is almost independent of IW amplitude. This control applies whether the emission was originally generated by IW or captured by it. Fc is determined from Bw measurement to be 60–120 Hz, which fits the observed behavior quite well. For δƒ < Fc a fraction of the phase‐bunched electrons are trapped by IW as they are detrapped by EE in the growth region. This superimposes a strong component oscillating in phase which can produce N events, effectively phase locking the low‐amplitude end of EE to IW. Amplitude fluctuations, δƒ, and Fc are interrelated in a complicated way which gives rise to short‐term instabilities and somewhat longer‐term stabilizing influences.
Transmission of Alfvén waves through the Earth's bow shock: Theory and observation
Authors: Hassam Adil B.
Published: February 2008

Alfvén waves, propagating along the magnetic field, are considered to be interacting with strong fast MHD shocks. The wave‐associated magnetic field perturbation is taken to lie in the magnetic field‐shock normal plane. The interaction perturbs the shock surface and gives rise to transmitted waves consisting of a fast magnetoacoustic wave, forward and backward slow magnetoacoustic waves, and an entropy wave. To zeroth order in inverse upstream Mach numbers the wave vectors of the latter three waves are collinear and bend toward the normal, while the fast wave bends away from the normal, propagating parallel to the shock surface for incident angles of 61°. To the same order, for incident angles of less than 61° and for γ = 5/3, the magnetic field amplitude associated with one of the slow waves is roughly the same size as the incident amplitude, while that associated with the other slow wave is amplified 3 times.

Magnetic field and plasma data from Explorer 35, taken in the vicinity of 14 bow shock crossings, are analyzed. On spacecraft time scales of 12–20 min the data suggest the presence of hydromagnetic waves on both sides of all the shocks; the presence of predominantly Alfvénic fluctuations is demonstrated in the upstream regions of at least six of the crossings. Magnetic field fluctuations with frequencies of > 1/100 Hz are found to be amplified across the shock by factors ranging from 0.9 through 2.5. It is suggested that the discrepancy in the observed and theoretical field amplifications is due to strong damping of any transmitted magnetoacoustic modes with relatively little damping of any transmitted Alfvén waves (all of which may be excited for a more general incidence).
Plasmapause signatures in the ionosphere and magnetosphere
Authors: Foster J. C. , Park C. G. , Brace L. H. , Burrows J. R. , Hoffman J. H. , Maier E. J. , Whitteker J. H.
Published: February 2008

Isis 2 observations of a variety of topside ionospheric ‘signatures’ of the plasmapause are compared with simultaneously acquired equatorial electron density profiles obtained by the whistler technique. The satellite data were acquired at ∼1400‐km altitude at dusk and dawn in the sunlit northern hemisphere summer ionosphere within ∼15° longitude of the VLF receiver. Results suggest that the dynamics of plasma coupling between the ionosphere and plasmasphere dominate the topside data and obscure the location of the equatorial plasmapause field line. The total density and light ion troughs begin 2°–10° equatorward of the field line through the equatorial plasmapause and are not clear plasmapause signatures. The invariant latitude of the region of steep spatial gradient in thermal plasma density, i.e., the plasmapause, appears to increase with altitude. Thus measurements of its position at different altitudes may give different results. Plasma sheet electrons, however, are observed on field lines just outside the equatorial plasmapause at both dawn and dusk. Their low‐latitude extent at 1400‐km altitude can be used as a signature of the equatorial plasmapause position.
O I (7774 Ă) and O I (8446 Ă) emissions in aurora
Authors: Christensen A. B. , Rees M. H. , Romick G. J. , Sivjee G. G.
Published: February 2008

Simultaneous ground‐based photometric measurements of auroral O I (6300 Ă), O I (7774 Ă), O I (8446 Ă), and N2+ (4278 Ă) emissions and model calculations of their intensities are presented. Detailed examination of the variations in the observed brightness of these emissions indicates a decrease in the I(8446)/I(7774) ratio as the red (I(6300)) to blue (I(4278)) ratio becomes smaller. This is contrary to the constant I(8446)/I(7774) ratio expected from electron impact excitation of the O I (7774 Ă) and O I (8446 Ă) multiplets, which are not quenched at auroral altitudes. The observed relation between the intensities of the O I (6300 Ă) and O I (8446 Ă) emissions suggests that the dominant mechanism for the production of O I (8446 Ă) is electron impact excitation of atomic oxygen. An additional excitation mechanism for O I (7774 Ă) operating at relatively low altitudes in aurora is required to explain both the measured changes in the I(8446)/I(7774) ratio and the O I (7774 Ă) intensity.