bluuebunny/crossref_metadata_2025 huggingface dataset:
The sensitivity of seismic free oscillations to upper mantle anisotropy 1. Zonal symmetry
Authors: Park Jeffrey
Published: February 2008

Rayleigh‐Love coupling is a diagnostic tool for detecting anisotropic lateral structure in the upper mantle. The inference of mantle anisotropy, however, is complicated by the 21 independent parameters to be constrained in the fourth‐order elastic tensor. We simplify the elastic tensor to possess hexagonal symmetry about an arbitrary axis, a reasonable first‐order model for deformed peridotite. The elastic tensor in this case is determined by the orientation of the symmetry axis and five linear coefficients that can be related to the Backus/Crampin equations for the azimuthal dependence of head wave velocity in marine refraction. Three anisotropic parameters B, C, and E govern the P wave cos 2ξ, cos 4ξ, and SV wave cos 2ξ terms, respectively, where ξ is the angle from the symmetry axis. We estimate coupling interaction between pairs of seismic free oscillations using zonally symmetric lateral structure models with angular wavenumber s. Zonally symmetric models afford computational simplicity and emphasize forward scattering interaction. For mode pairs with angular degrees l,l′, coupling interaction tends to peak near s = |l–1′| and s = l+l′. For mode pairs with ƒ≲20 mHz from distinct dispersion branches, particularly fundamental spheroidal and toroidal modes, the sensitivity to smooth isotropic lateral structure is very weak relative to anisotropic lateral structure. Sensitivity to the P wave cos 2ξ coefficient B tends to peak near the crust‐mantle interface, suggesting poor potential for depth resolution. Sensitivity to C and E for fundamental and first overtone modes, however, often peaks at 100‐ to 250‐km depth and is often maximal for symmetry axes oriented between the vertical and horizontal. The parameter C≲0.01 is small and weakly constrained in most estimates from seismic data and mineralogical studies. The parameter E = 0.01 – 0.03 in most estimates, and shear wave splitting studies argue for significant shear anisotropy in the uppermost mantle. Therefore it appears that S wave anisotropy is the likely tool for constraining global‐scale mantle deformation in the deep lithosphere and asthenosphere with coupled‐mode surface wave anomalies.
Introduction to Special Section on Physical Processes in Geodynamics
Authors: Schubert G.
Published: February 2008

On May 16 and 17, 1992, a meeting was held at Cornell University as part of activities to honor Donald L. Turcotte on the occasion of his 60th birthday (which actually occurred on April 22, 1992). The papers in the following special section are a result of that meeting, which included invited presentations and posters representing the diverse areas of geology and geophysics to which Donald Turcotte has contributed.

Three of the papers deal with aspects of mantle convection. A. C. Fowler addresses the problem of initiation of subduction by extending boundary layer theory to a viscoplastic rheology. Turcotte and Oxburgh [1967] pioneered such studies by showing how boundary layer theory could be used to describe the dependence of geophysical observables such as heat flux on distance from a midocean ridge or age of the oceanic crust. D. W. Sparks et al. use a model of three‐dimensional convection beneath a segmented ridge to explore how buoyancy effects and melt production beneath a spreading center influence ridge‐parallel variations in crustal thickness and gravity. Magma migration and the dynamics of midocean ridges have been major focuses of Donald Turcotte's research in papers dating back to 1978 [e.g., Turcotte and Ahern, 1978; Ahern and Turcotte, 1979; Turcotte, 1982a; Kenyon and Turcotte, 1987]. G. A. Glatzmaier and G. Schubert calculate models of three‐dimensional two‐layer mantle convection in a spherical shell and compare with whole layer, spherical shell, three‐dimensional convection models. These models represent the end‐member states of a spectrum of possible modes of mantle convection [Turcotte and Oxburgh, 1972; Oxburgh and Turcotte, 1978; Turcotte, 1979a; Silver et al., 1988].
Deep cross‐equatorial flow in the Atlantic measured with SOFAR floats
Authors: Richardson Philip L. , Schmitz William J.
Published: February 2008

Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6°N and at several sites near 11°N as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7°N to the equator. The average transport per unit depth in the DWBC was estimated to be 13.8 × 103 m2/s. Coupling this value with mean velocities measured in the DWBC by current meters gave a volume transport of 15 × 106 m3/s between depths of 900 m and 2800 m. Approximately 6 × 106 m3/s recirculated northward between the DWBC and the Mid‐Atlantic Ridge, leaving 9 × 106 m3/s as cross‐equatorial transport. No obvious DWBC nor swift equatorial current was observed by the 3300‐m floats; a low mean velocity at this depth lay between F‐11 and higher velocity cores above and below. The 1800‐m trajectories also suggest that at times (February‐March 1989) the North Atlantic Deep Water in the DWBC turned eastward and flowed along the equator and at other times (August‐September 1990) the DWBC crossed the equator and continued southward. The velocity near the equator, calculated by grouping floats in a box along the equator, was eastward at 4.1 cm/s from February 1989 to February 1990 and westward at 4.6 cm/s from March 1990 to November 1990. Thus the amount of cross‐equatorial flow in the DWBC appeared to be linked to low‐frequency variability of the structure of the equatorial current system. Floats in Antarctic Intermediate Water at 800 m revealed a northwestward western boundary current, although flow patterns were complicated. Three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm/s. Six 800‐m floats drifted eastward along the equator between 5°S and 6°N at a mean velocity of 11 cm/s; one reached 5°W in the Gulf of Guinea, suggesting that the equatorial currents at this depth extended at least 35°–40° along the equator. Three of these floats reversed direction near the end of the tracking period.
Importance of convective mixing in seasonal ice margin simulations
Authors: Houssais Marie‐Noëlle , Hibler William D.
Published: February 2008

A two‐dimensional ice‐ocean model applicable to the Greenland Sea is constructed and used to examine the role of penetrative convection in the seasonal advance and retreat of the sea ice margin. The standard version of the model is based on the full dynamic and thermodynamic equations of sea ice and on the primitive equations discretized on a vertical slab for the ocean. It includes a special treatment of upper ocean vertical mixing based on a parameterization of entrainment at the bottom of the ocean mixed layer. The simulation results with this standard version of the model show that when ice advection is included with ice moving outward over open water, penetrative convection delays and greatly reduces the ice edge advance. The resulting seasonal ice distribution yields better agreement with observation than a simulation that does not allow mixed layer entrainment. When lateral heat transport is neglected in the ocean, the ice margin again moves too a great extent outward. This result demonstrates that in addition to vertical mixing, a heat source must exist in the ocean mixed layer in order to resist and ultimately stop the advection of ice. Finally, in the absence of ice motion, penetrative convection has a less pronounced effect on the ice edge location.
Water vapor correction method for advanced very high resolution radiometer data
Authors: Steyn‐Ross Moira L. , Steyn‐Ross D. A. , Smith P. J. , Shepherd J. D. , Reid J. , Tildesley P.
Published: February 2008

Current sea surface temperature (SST) retrieval algorithms consist of linear equations involving brightness temperatures (of the infrared advanced very high resolution radiometer (AVHRR) channels) multiplied by fixed coefficients ai. These coefficients are derived from atmospheric transmission models and radiosonde profiles or from regression against ocean buoy measurements. Errors in SST retrieval occur when the actual atmospheric conditions differ from those assumed when the ai were originally computed; it is generally agreed that water vapor variations are the primary source of this error. We propose a novel radiance‐based SST algorithm which compensates for temporal fluctuations in the water vapor profile. The method does not use any form of regression; rather, we utilize a single radiosonde seed combined with full radiative transfer theory applied to an atmospheric model. As well as improving the accuracy of the remotely sensed SST, this approach allows us to estimate the total water vapor content for a given AVHRR pixel. We compare the performance of this technique with two standard SST models, using buoy data from the west coast of Tasmania as surface truth. Preliminary results from a sample of 33 NOAA 9 overpasses gathered over a 10‐month period in 1987 are most encouraging: the dynamic water vapor (DWV) algorithm outperforms the standard SST methods in terms of retrieval accuracy, underestimating the buoy temperature by an average of 0.22 K. This result demonstrates that the method has merit and suggests that the total water vapor content returned by the DWV algorithm is probably a reasonable first estimate.
Physical and biological observations of coastal squirts under nonupwelling conditions
Authors: Moore Mike I. , Murdoch Rob C.
Published: February 2008

Offshore‐directed plumes of inshore water, marked by a shallow mixed layer, low salinity, and low temperature during winter, were observed south of the Hokitika Canyon on the South Island west coast, New Zealand. Hydrographic and satellite observations are presented which show that plume time scales were of the order of a few days, and across‐shelf length scales were approximately 50 km. The low‐salinity plumes were found to be associated with the southern edge of an offshore‐directed jet that followed the Hokitika Canyon bathymetry. Decay of the plumes was rapid and appears to be associated with mixing of plume water into offshore surface waters. The occurrence of the plumes is nonseasonal, and they do not appear to be associated with upwelling. Observations suggest that strong southward flow around the head of the Hokitika Canyon, perhaps as a consequence of coastal‐trapped wave activity, is a possible mechanism for plume formation. Biologically, the plumes were important because they stabilized the water column and prevented the mixing of phytoplankton below the photic zone. Although the plumes were found to transport productive nearshore waters offshore, the major impact of the plumes was to lower total water column chlorophyll a in outer shelf regions. The relationship between the Hokitika plumes and the filaments and squirts identified off the Californian coast is discussed, and it is concluded that the plume dynamics resemble those of the squirts found inshore of the seasonal filaments off California. The results of this study indicate that squirts not only influence biological variability in shelf waters but are important mechanisms for the across‐shelf exchange of coastal and oceanic waters.
Mean circulation of the upper layers of the western equatorial Pacific Ocean
Authors: Gouriou Yves , Toole John
Published: February 2008

Hydrographic (0–1000 dbar) and direct current measurements (0–600 dbar) along 165°E (20°S‐10°N) between January 1984 and July 1991 are used to investigate the mean circulation and its relationships with the distributions of salinity and potential vorticity on isopycnal surfaces. Less well documented mean sections along 142°E and 137°E are used to complement the 165°E analysis. The Equatorial Undercurrent is centered on the equator at 165°E and on 0.5°N at 142°E. No variation in its transport (15.0 × 106 m3 s−1) is found between those longitudes. We find indication of the Equatorial Undercurrent at 137°E‐0.75°N in the geostrophic field. The northern and southern Subsurface Countercurrents are clearly identified by extrema of eastward velocity at 165°E around 3°N and 3°S (250 dbar). No evidence of a southern Subsurface Countercurrent is found at 142°E. At 137°E the northern Subsurface Countercurrent is not characterized by a local extrema of eastward velocity: the North Equatorial Countercurrent seems to extend from the surface to 400 dbar with a southward shift of its core. Analysis of the seasonal variability at 165°E indicates that the Equatorial Undercurrent transport increases by a factor 2 between January (10.7 × 106 m3 s−1) and July (21.5 × 106 m3 s−1) and the Equatorial Intermediate Current transport is halved (6.3 × 106 m3 s−1 in January, 3.5 × 106 m3 s−1 in July). In contrast, the transport of the Subsurface Countercurrents does not vary substantially between those two months. The meridional distributions of salinity and potential vorticity show that the axes of the main eastward currents are associated with strong meridional property gradients, not with property extrema. The eastward currents thus represent a barrier to the northward extension of the high salinity Tropical Water. Relatively weak meridional gradients of salinity and potential vorticity are observed in the westward directed South Equatorial Current and Equatorial Intermediate Current.
Solar ultraviolet irradiance observed from southern Argentina: September 1990 to March 1991
Authors: Frederick J. E. , Soulen P. F. , Diaz S. B. , Smolskaia I. , Booth C. R. , Lucas T. , Neuschuler D.
Published: February 2008

A nearly continuous data set of solar ultraviolet spectral irradiance exists for Ushuaia, Argentina, latitude 54°59′ S, over the period from mid‐September 1990 to mid‐March 1991. This includes a season of prolonged depletion in column ozone over Antarctica, 10° or more in latitude poleward of Ushuaia. Cloudiness provides a major source of variance in the measurements. When this influence is removed, the irradiances at wavelengths between 300 and 310 nm are enhanced relative to clear sky calculations based on a 10‐year ozone climatology. During December the average noontime irradiance at 306.5 nm, which is a good proxy for erythemal irradiance, is 45% larger than the zonal mean climatological prediction. The largest noontime radiation levels observed at Ushuaia are equivalent to moving 20° in latitude closer to the equator at the summer solstice.
Fast low‐level light pulses from the night sky observed with the SKYFLASH program
Authors: Winckler J. R. , Franz R. C. , Nemzek R. J.
Published: February 2008

This paper presents further discussion of and new data on fast subvisual increases in the luminosity of the night sky described in our previous papers. A detailed technical description of the simple telescopic photometers used in the project SKYFLASH and their mode of operation including the detection of polarized Rayleigh‐scattered flashes is provided. Distant lightning storms account for many of the events, and the complex relations between short and long luminous pulses with and without sferics are shown by examples from a new computerized data system, supplemented by two low‐light‐level TV cameras. Of particular interest are the previously observed “long” events having a slow rise and fall, 20‐ms duration, and showing small polarization and no coincident sferic. A group of such events on September 22–23 during the invasion of U.S. coasts by Hurricane Hugo, is discussed in detail. The recently observed “plume” cloud‐top‐to‐stratosphere lightning event is suggested as a possible source type for these flashes. An alternative source may be exploding meteors, recently identified during SKYFLASH observations by low‐light‐level television techniques as the origin of some sky‐wide flash events described herein.
Photoreduction of iron(III) in marine mineral aerosol solutions
Authors: Zhu Xiaorong , Prospero Joseph M. , Savoie Dennis L. , Millero Frank J. , Zika Rod G. , Saltzman Eric S.
Published: February 2008

Although there have been a number of studies of the solubility of Fe in marine mineral aerosols, there have been few studies of the oxidation states of the soluble iron fraction and of the factors that affect the solubility of iron in aerosol solutions. In this paper we present measurements of the concentrations of total Fe (including particulate), total soluble Fe and total soluble Fe(II) in marine aerosol particles. Only 1% of the total Fe and 7.5% of the soluble Fe was in the Fe(II) form. Photolysis experiments were performed with solutions extracted from aerosol filter samples and with solutions of Fe(III) in acidic sodium chloride. In both systems, Fe(II) concentrations increased rapidly when the solutions were exposed to sunlight and they attained steady state within an hour. However, in all cases Fe(II) is only a minor component even when conditions are favorable for photolysis. Fe(II) formation is hindered at the low pH that is believed to be characteristic of marine aerosol solutions. Solutions with added oxalate yielded greatly increased concentrations of Fe(II); this is probably related to the fact that Fe(III) oxalate complexes have strong ligand to metal charge transfer bands in the tropospheric solar UV‐visible region. However, the presence of oxalate also leads to the formation of H2O2; when the radiation level decreased, the Fe(II) was partially or totally oxidized back to Fe(III) due to reactions with H2O2. The photoreduction of Fe(III) to Fe(II) did not appear to significantly increase the dissolution of Fe(III) from the dust mineral matrix.
Operational overview of the NASA GTE/CITE 3 airborne instrument intercomparisons for sulfur dioxide, hydrogen sulfide, carbonyl sulfide, dimethyl sulfide, and carbon disulfide
Authors: Hoell James M. , Davis Douglas D. , Gregory Gerald L. , McNeal Robert J. , Bendura Richard J. , Drewry Joseph W. , Barrick John D. , Kirchhoff Volker W. J. H. , Motta Adauto G. , Navarro Roger L. , Dorko William D. , Owen Dennis W.
Published: February 2008

This paper reports the overall experimental design and gives a brief overview of results from the third airborne Chemical Instrumentation Test and Evaluation (CITE 3) mission conducted as part of the National Aeronautics and Space Administration's Global Tropospheric Experiment. The primary objective of CITE 3 was to evaluate the capability of instrumentation for airborne measurements of ambient concentrations of SO2, H2S, CS2, dimethyl sulfide, and carbonyl sulfide. Ancillary measurements augmented the intercomparison data in order to address the secondary objective of CITE 3 which was to address specific issues related to the budget and photochemistry of tropospheric sulfur species. The CITE 3 mission was conducted on NASA's Wallops Flight Center Electra aircraft and included a ground‐based intercomparison of sulfur standards and intercomparison/sulfur science flights conducted from the NASA Wallops Flight Facility, Wallops Island, Virginia, followed by flights from Natal, Brazil. Including the transit flights, CITE 3 included 16 flights encompassing approximately 96 flight hours.
Ozone depletion at northern and southern latitudes derived from January 1979 to December 1991 Total Ozone Mapping Spectrometer data
Authors: Herman J. R. , McPeters R. , Larko D.
Published: February 2008

Long‐term ozone depletion rates (percentage change) have been computed from 13 years of Nimbus 7/Total Ozone Mapping Spectrometer (TOMS) data as a function of latitude, longitude, and month for the period January 1, 1979, to December 31, 1991. In both hemispheres the amount of ozone has decreased at latitudes above 30° by amounts that are larger than predicted by homogeneous chemistry models for the 13‐year time period. The largest rates of ozone decrease occur in the southern hemisphere during winter and spring, with partial recovery during the summer and autumn. Outside of the Antarctic ozone hole region, the 12‐year ozone depletion rates reach 8–10% per decade during the winter and spring at 55°S. Ozone depletion rates in excess of 7% per decade occur over populated regions in the southern hemisphere poleward of 45°S (southern Argentina and the tip of New Zealand) for 7 months of the year. Similar rates of decrease occur during northern winter and spring (6–8% at 55°N) over large populated regions. The enhanced zonal average ozone depletion rates at northern mid‐latitudes (40–50°N) during January, February, and March, that correspond to five geographically localized regions of high ozone depletion rates, are probably associated with long‐term dynamical or temperature changes. Only the equatorial band between ±20° shows little or no long‐term ozone change since January 1979. Ozone time series data have been examined for effect of volcanic eruptions on stratospheric ozone observed by TOMS, with only the Mount Pinatubo stratospheric aerosol injection affecting ozone amounts for a few months after the eruption in June 1991. The TOMS data show no ozone perturbation after the El Chichon eruption, or after any of the other smaller equatorial eruptions, that cannot be explained by interference effects between the annual, El Nino/Southern Oscillation, and Quasi‐Biennial Oscillation cycles. The effect of the stratospheric aerosol scattering phase function is clearly seen in the high spatial resolution TOMS ozone data after El Chichon and Mount Pinatubo. Errors caused by the short‐term presence of stratospheric aerosols in the TOMS zonally averaged ozone data are less than 1% before correction, and have no significant effect on ozone trend determination.
An intercomparison of aircraft instrumentation for tropospheric measurements of carbonyl sulfide, hydrogen sulfide, and carbon disulfide
Authors: Gregory Gerald L. , Davis Douglas D. , Thornton Donald C. , Johnson James E. , Bandy Alan R. , Saltzman Eric S. , Andreae Meinrat O. , Barrick John D.
Published: February 2008

This paper reports results of NASA's Chemical Instrumentation and Test Evaluation (CITE 3) during which airborne measurements for carbonyl sulfide (COS), hydrogen sulfide (H2S), and carbon disulfide (CS2) were intercompared. Instrumentation included a gas chromatograph using flame photometric detection (COS, H2S, and CS2), a gas chromatograph using mass spectrometric detection (COS and CS2), a gas chromatograph using fluorination and subsequent SF6 detection via electron capture (COS and CS2), and the Natusch technique (H2S). The measurements were made over the Atlantic Ocean east of North and South America during flights from NASA's Wallops Flight Center, Virginia, and Natal, Brazil, in August/September 1989. Most of the intercomparisons for H2S and CS2 were at mixing ratios <25 pptv and <10 pptv, respectively, with a maximum mixing ratio of about 100 pptv and 50 pptv, respectively. Carbonyl sulfide intercomparisons were at mixing ratios between 400 and 600 pptv. Measurements were intercompared from data bases constructed from time periods of simultaneous or overlapping measurements. Agreement among the COS techniques averaged about 5%, and individual measurements were generally within 10%. For H2S and at mixing ratio >25 pptv, the instruments agreed on average to about 15%. At mixing ratios <25 pptv the agreement was about 5 pptv. For CS2 (mixing ratios <50 pptv), two techniques agreed on average to about 4 pptv, and the third exhibited a bias (relative to the other two) that varied in the range of 3–7 pptv. CS2 mixing ratios over the ocean east of Natal as measured by the gas chromatograph‐mass spectrometer technique were only a few pptv and were below the detection limits of the other two techniques. The CITE 3 data are used to estimate the current uncertainty associated with aircraft measurements of COS, H2S, and CS2 in the remote troposphere.
Aerosol selenium at Bermuda and Barbados
Authors: Ellis W. G. , Arimoto R. , Savoie D. L. , Merrill J. T. , Duce R. A. , Prospero J. M.
Published: February 2008

Daily aerosol samples collected at Bermuda and Barbados as part of the Atmosphere/Ocean Chemistry Experiment (AEROCE) were analyzed to evaluate the sources for selenium (Se). Isentropic trajectories were used to further evaluate the sources suggested by the chemical information. At Bermuda, the major source for Se was anthropogenic emissions from North America. Daily Se concentrations were highly correlated with antimony (Sb) which originated from a pollution source. However, approximately 6% of the Sb was associated with mineral dust from northern Africa that reached Bermuda during the summer months. The Bermuda aerosol data showed a weak correlation between Se and methane sulfonate (MSA), but the intercept of the Se versus Sb regression indicated that a marine biogenic source of Se accounted for ∼45% of the mean concentration at Bermuda. At Barbados the variability of Se concentrations could not be explained solely by pollution sources. Correlations between Se and Sb, nitrate (NO3−), non‐sea‐salt sulfate (nssSO4=), and MSA were significant. The correlations of Se with Sb, NO3−, and nssSO4= suggested continental sources. The correlation with MSA indicated there was a significant marine biogenic source. Our results indicated that a marine biogenic source of Se accounted for 60–80% of the Se at Barbados. Analyses of seasonal relationships indicated that the marine biogenic source was most important during the summer.
Distribution of ice particles in wintertime thunderclouds detected by a C band dual polarization radar: A case study
Authors: Maekawa Yasuyuki , Fukao Shoichiro , Sonoi Yasuo , Yoshino Fumio
Published: February 2008

Distributions of radar echo intensities (ZH) and differential reflectivity factors (ZDR) for ice particles in Japan's wintertime thunderclouds are investigated using C band dual polarization radar observations conducted on January 12, 1990, as a case study. At the moment of lightning strokes, the percentage of low‐ZDR data points is increased around the cloud center, while that of high‐ZDR data points is increased near the cloud edge. These radar echoes indicate the backscattering properties from graupel particles and ice crystals, both of which play a critical role in thunderstorm electrification. The ice crystal echoes are primarily enhanced 5–15 min before the lightning stroke. At the moment of lightning stroke; however, they are slightly decreased, and the graupel echoes gradually increase the intensity. These characteristics are well described by the convective motion of the dominant ice particles in this observational case. In addition, graupel echoes of more than 40 dBZ are found for these thunderclouds on January 12, whereas the clouds which produced no lightning, but indicated the evidence of comparable electrification, did not show more than 40 dBZ on January 18, 1990. The lightning locations are seen along the edge of the cloud center with ZH ∼ 30 dBZ, where graupel and ice crystal echoes distribute at short range. Their distributions are then estimated by counting both low‐ and high‐ZDR data points on the constant altitude plan position indicator display, combined with their large and small echo intensities, respectively. Using polarimetric information (ZDR, ZH), the product numbers of graupel and ice crystal echoes in each 5 × 5 km area are found to be fairly well correlated with the lightning location. These results are thus obtained for the one case of the present thunderstorm event, but they may give some insights on evolution of thunderclouds and lightning discharges.
A three‐dimensional numerical model of cloud dynamics, microphysics, and chemistry: 3. Redistribution of pollutants
Authors: Wang Chien , Chang Julius S.
Published: February 2008

The evolution of chemical species during the life cycle of a severe local storm has been simulated using our three‐dimensional cloud chemistry model (Wang and Chang, 1993a, b). The importance of dynamical transport was found to be different for different gases, depending on their solubility. For insoluble gases with a comparatively long chemical lifetime, dynamical transport was a major factor in the evolution of in‐cloud gas phase concentration. The three‐dimensional divergence (convergence) and eddy mixing accounted for nearly 100% of the total variation of the in‐cloud O3 amount. For the highly soluble gases such as HNO3 and H2O2 the dominant sources and sinks were dissolution and evaporation. The dissolution of HNO3 and H2O2 accounted for more than 44% of the total change. The extensive condensation occurring in selected parts of the storm caused sharp decreases in the HNO3 and H2O2 mixing ratios. The dissolution of SO2 can exceed the influence of dynamical processes for a short time period, corresponding to a newly born convective cell. For other times, dynamics was the major factor influencing the variation of the SO2 in‐cloud amount. The in‐cloud amounts of the gas phase pollutants were reduced significantly during the development of the storm. The net decrease of SO2 is approximately equal to 4 ppt on the average throughout the entire cloud volume during the simulation, or approximately −3 ppt/h in rate. For O3 the total change is approximately equal to −1.1 ppb/h on the average. The net decrease of HNO3 is equal to 20 ppt on the average, or −15 ppt/h in rate. The H2O2 decreasing rate is approximately equal to −5.9 ppb/h on the average, while the average net change is about 7.8 ppb in the cloud volume and averaged through the duration of the simulation.
Lidar observations of atmospheric dynamics in the troposphere and lower stratosphere over Arecibo
Authors: Meriwether John W.
Published: February 2008

A periodic wave structure with a vertical wave length typically 2 km between 14 and 23 km altitudes has been identified in lidar measurements of relative density profiles at 589 nm that were made by the University of Illinois group during the March/April AIDA 1989 campaign effort. The equivalent temperature fluctuations are several degrees in magnitude. Also seen are stratified aerosol layers at altitudes near 25 km with a vertical extent of ∼1 km. Both of these phenomena show a downward phase speed between 1 and 3 cm s−1. The lower periodic structure is believed to be the same phenomenon seen previously in 430‐MHz measurements of winds obtained at Arecibo Observatory. The source of this structure remains uncertain. Cornish and Larsen (1989) and Sato and Woodman (1982a) considered this structure to be a quasi‐inertial long period wave. Hines (1989) proposed instead the identification of this structure with a stationary wave system generated by orographic waves created through the interaction of the subtropical trade winds with the mountainous terrain of Puerto Rico. The higher altitude feature seen in the AIDA lidar observations is believed to be an aerosol layer with a scattering ratio of about 1.15. An alternative explanation is clear air turbulence, but the signature of the 10 K fluctuation deduced from the lidar observations was not found in the San Juan balloon measurement of the temperature profile for the lower stratosphere.
Constraints on the thermal evolution of Venus inferred from Magellan data
Authors: Arkani‐Hamed J. , Schaber G. G. , Strom R. G.
Published: February 2008

One interpretation of the Magellan data suggests that the cratering record on Venus was erased by a global resurfacing event, or events, the latest ending about 500 m.y. ago. In this global‐resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts. A thermal evolution model of Venus that can explain these observations is one in which a deformable lithosphere, capable of being incorporated in mantle circulations, provides an almost stress‐free condition at the surface. Mantle convection with an almost stress‐free boundary at the surface cools the interior more efficiently. Rapid cooling decreases the Rayleigh number of mantle convection below a transition value required for oscillatory convection, and the vigor of convection diminishes as the mantle changes to a quasi‐steady circulation after about 500 m.y. ago.
Corvid meteoroids are ejecta from the Giordano Bruno impact
Authors: Hartung Jack B.
Published: February 2008

A Corvid meteor shower was observed once, in late June of 1937. On June 18, 1178, the upper horn of a crescent Moon was reported to have split, with fire and sparks emanating from the division point. This report was interpreted to be a description of events related to the formation of the lunar crater, Giordano Bruno. Corvid meteoroids are judged to be Giordano Bruno ejecta fragments based on similarities in their heliocentric positions, directions of motion, and speeds. Commensurability relationships between meteoroid/ejecta and Earth‐Moon system orbital periods enable definitive predictions of Corvid showers in the years 2003 and/or 2006.
The ancient oxygen exosphere of Mars: Implications for atmosphere evolution
Authors: Zhang M. H. G. , Luhmann J. G. , Bougher S. W. , Nagy A. F.
Published: February 2008

The evolution of the Martian atmosphere, particularly with regard to water, is influenced by (1) “nonthermal” escape of oxygen atoms created by dissociative recombination and (2) by oxygen ion pickup by the solar wind. Both processes depend on the intensity of solar EUV radiation, which affects atmosphere temperatures (scale heights) and photoionization rates, and thereby the exosphere and the fluxes of escaping atoms and ions. This study involves the calculation, by the two‐stream model method of Nagy and Cravens (1988), of the exospheric hot oxygen densities for “ancient” atmospheres and ionospheres (e.g., for different EUV fluxes), and the associated neutral escape fluxes. The ion production rates above nominal ionopause altitudes are also computed and are considered to be the upper limits to losses by direct solar wind pickup. Since we do not consider the pickup ion precipitation and additional neutral escape due to the sputtering process described by Luhmann and Kozyra (1991), the results presented here represent conservative estimates of the neutral escape fluxes, but somewhat generous estimates of ion loss rates. We find that when the inferred increased solar EUV fluxes of the past are taken into account, oxygen equivalent to that in several tens of meters of water, planet‐wide, should have escaped to space over the last 3 Gyr.
Martian surface simulations
Authors: Gaskell Robert W.
Published: February 2008

Computer generated surfaces have been created to aid in imaging, landing and rover studies for Mars and the moon. They are also being applied to the study of cratering histories. The surfaces are generated in steps which attempt to mimic geologic episodes. Surface roughness is realized fractally, while craters and other specific features have shapes and distributions dictated by observation. Surface materials are assigned appropriate albedos, making the images more realistic. With the inclusion of correlations between crater and rock distributions, the simulations are beginning to acquire a predictive capability.
An H chondrite stream: Identification and confirmation
Authors: Dodd Robert T. , Wolf Stephen F. , Lipschutz Michael E.
Published: February 2008

Fall data indicate that a significant, elongate cluster of co‐orbital H chondrite falls in May between 1855 and 1895 (H Cluster 1) records encounters with two or three closely spaced and probably related meteoroid stream components, each of which was met near its perihelion. Although meteorites included in the Cluster vary widely in petrographic type (3–6), shock facies (a‐d), and 21Ne exposure age (<5 to 50 Ma), they have a distinct labile trace element signature that confirms a common thermal history and, thus, a common source region within an H chondrite parent body. Hence, meteorites selected by one criterion (fall parameters) as distinguishable from all other H chondrites, are distinguished from them by another completely different criterion (contents of labile trace elements).
Noble gases in ancient asteroidal atmospheres
Authors: Swindle Timothy D.
Published: February 2008

The atmospheres of rocky asteroids are unlikely to have ever been anything more than tenuous exospheres. However, it is possible that the densities of radiogenic heavy noble gases might have once been high enough to have implanted observable quantities in the regoliths that became meteorites. For this to have happened, a significant fraction of these species must have been photoionized and accelerated by the electromagnetic fields associated with the solar wind, rather than escaping thermally. Then, some fraction of the photoions would be accelerated into the asteroid's surface. Analytical and numerical results presented here suggest that acceleration of photoions by the solar wind motional field is a significant loss process for Xe on asteroids about 200 km in radius or larger, if the Xe is thermalized by its interactions with the surface. For Ar, photoion acceleration can only become important for asteroids nearly 500 km in radius. Thus photoion acceleration, previously invoked for lunar samples, could be responsible for excess fission‐produced Xe found associated with solar wind Xe in howardite meteorites. The lack of such Xe in other types of meteorites may reflect either smaller parent bodies or later times of regolith exposure. Similarly, the failure to observe solar‐wind‐associated radiogenic 40Ar in meteorites is consistent with the much smaller likelihood that Ar will be photoionized.
Introduction to the Galileo special section
Authors: Head James W.
Published: February 2008

In December 1990 the Galileo spacecraft encountered the Earth‐Moon system in the first of two flybys that are part of a sequence of planetary gravity assists that will deliver the spacecraft to Jupiter in 1995. The geometry of the first lunar flyby was particularly fortuitous in that the western near side and portions of the far side of the Moon were illuminated, a condition not available during the Apollo missions because of the constraints on illumination during landing and surface operations. The Solid State Imaging System on board the Galileo spacecraft was thus able to obtain the first spacecraft multispectral images of the Moon since Mariner 10 snapped a few vidicon images on its way by the Moon to Venus and Mercury in 1973. These images provided very important new data on the characteristics of the Moon on the western near‐side limb, in the area of the Orientale basin, and on the far side, particularly in the region of the gigantic South Pole‐Aitken basin [Belton et al., 1992].
High‐Resolution Stable Isotope Stratigraphy from Bulk Sediment
Authors: Shackleton N. J. , Hall M. A. , Pate D. , Meynadier L. , Valet P.
Published: February 2008

A high‐quality δ18O record has been obtained from a deep sea core in the Somali Basin in a water depth of 4020 m, well below the foraminiferal lysocline, by analyzing bulk sediment instead of picked foraminifera. In some areas this may be a valuable tool for extending the applicability of the δ18O stratigraphy, for obtaining data by a less labor‐intensive procedure, and for conserving valuable core material. Additionally, we draw attention to the need for more data on δ13C in bulk carbonate if we are to fully utilize the ocean δ13C record either in relation to long‐term global carbon budget, or in relation to changes in carbon cycling in the ocean and its effect on atmospheric pCO2.
Narrow‐band plasmapause hiss observed by ISIS satellites
Authors: Ondoh T.
Published: February 2008

Typical frequency‐time spectra of narrow‐band hiss obtained near plasmapause latitude from ISIS VLF electric field data (50 Hz to 30 kHz) telemetered at Syowa Station, Antarctica, under quiet conditions, are similar to those of narrow‐band 5‐kHz hiss observed at mid‐latitudes and low‐latitudes on the ground. This hiss in the topside ionosphere first appeared as a thin, bar‐shaped emission around 5 kHz at geomagnetic invariant latitude of about 56° and grew into a wider band between 3 and 6 kHz at invariant latitudes around 60° which is the average plasmapause latitude. Then, at higher latitudes, the band narrowed somewhat and disappeared around 65°. The hiss had no lower‐frequency cutoff, and its center frequency remained approximately constant at 5 kHz, between about 56° and 65°. This narrow‐band hiss is completely different from the electrostatic LHR hiss which is also observed above the ionosphere with an electric antenna, but which has latitude dependent lower‐frequency cutoff. Since the narrow‐band 5‐kHz hiss occurs often around plasmapause latitude (60°), hereafter we will call this hiss narrow‐band plasmapause hiss. Latitude and local time distributions of the occurrence rate for narrow‐band plasmapause hiss were obtained by analyzing six‐frequency narrow‐band data processed from the wideband VLF signals received at Syowa Station from ISIS 1 and ISIS 2 during 507 passes between December 1976 and January 1983. Although when taken over the whole latitude range, 55° to 64°, the occurrence rate of narrow‐band plasmapause hiss under various geomagnetic conditions (Kp = 0–6) is 0.83, the rate at any given latitude is below 0.35. Thus it is rather a rare phenomenon as compared with LHR hiss. The peak occurrence rate lies at 58° to 59°, under geomagnetically quiet and moderate conditions (Kp = 0–3) and extends over the somewhat broader range of 56° to 60° under disturbed conditions (Kp = 4–6). The occurrence rate under disturbed conditions is about one third that under quiet and moderate conditions. The magnetic local time (MLT) distribution of the occurrence rate has a significant maximum at 1800–2100 hours MLT under quiet and moderate conditions, and a maximum at 0000–0300 hours MLT under disturbed conditions. The similarity between frequency‐time spectra of narrow‐band hiss observed by the ISIS satellites and on the ground suggests that the narrow‐band hiss generated in the magnetosphere propagates toward the Earth in the whistler mode. The peak occurrence rates of the hiss at invariant latitude 60° (average plasmapause latitude) in the late evening sector under quiet and moderate conditions, and in the midnight sector under disturbed conditions, suggest that narrow‐band plasmapause hiss is generated by cyclotron resonant instability of energetic electrons which are injected from the magnetotail and drift eastward around the equatorial plasmapause.
Applications of the nonlinear finite difference time domain (NL‐FDTD) method to pulse propagation in nonlinear media: Self‐focusing and linear‐nonlinear interfaces
Authors: Ziolkowski Richard W. , Judkins Justin B.
Published: February 2008

In an effort to meet an ever increasing demand for more accurate and realistic integrated photonics simulations, we have developed a multidimensional, nonlinear finite difference time domain (NL‐FDTD) Maxwell's equations solver. The NL‐FDTD approach and its application to the modeling of the interaction of an ultrashort, optical pulsed Gaussian beam with a Kerr nonlinear material will be described. Typical examples from our studies of pulsed‐beam self‐focusing, the scattering of a pulsed‐beam from a linear‐nonlinear interface, and pulsed‐beam propagation in nonlinear waveguides will be discussed.
Excitations of ion‐Bernstein waves in ionospheric modification experiment
Authors: Sharma R. P. , Kumar Atul , Kumar Raj
Published: February 2008

In the present paper the parametric decay instability of an extraordinary electromagnetic wave into an upper hybrid wave and a low‐frequency electrostatic ion‐Bernstein wave at different harmonics is considered. Expressions have been derived for homogeneous growth rates, thresholds, and convective thresholds at different harmonics of ion‐Bernstein wave for this instability. Applications of this investigation to ionospheric modification experiment and polar cusp region of Earth's magnetosphere have also been pointed out. It has been found that the present decay instability is more important than the stimulated Brillouin scattering in homogeneous plasmas.
Seasonal variations of energetic electron precipitation by cyclotron resonance with VLF waves from a ground‐based transmitter
Authors: Datlowe Dayton W. , Imhof William L.
Published: February 2008

We report here for the first time the detection of a seasonal variation in the occurrence of peaks in the spectra of energetic ∼ 100‐keV electrons precipitating from the inner radiation belt as the result of cyclotron resonance between the waves and the electrons. The spectra reported here were observed from the low‐altitude polar orbiting satellite P78‐1 at midnight local time. The data, acquired over the 5‐year period from March 1979 to July 1984, compose the first multiyear data set used to study cyclotron resonance peaks. The geographical distribution of the events is strongly clustered around the 22.3‐kHz VLF transmitter NWC in Australia. Binning the data into seasons, we find that the events occur 2.5 times more often during the 6 months from day 80 to 263 than during the remainder of the year and that the seasonal difference is statistically significant at a high level. Since the observed change is in the frequency of occurrence of the peaks and not in the mean energy of the peaks, we attribute the effect to a seasonal change in ionospheric transmission of VLF waves rather than a seasonal change in the equatorial cold plasma density.
Uniform formulas for infinite antenna current
Authors: Chen Kenneth C. , Warne Larry K.
Published: February 2008

A uniform asymptotic expansion of the time‐harmonic antenna current with large parameter In [2kz/(ka)2] is given. As usual, the expansion is based on Hallén's filament integral and is restricted by the condition z ≫ a but is otherwise uniform in kz. The use of averaging leads to the vanishing of the second term of the asymptotic series. Two different averaging procedures are used to derive two accurate formulas: The first one, which is very simple, is useful for practical applications. The second formula is of theoretical interest. An improved asymptotic series for large kz is derived to relate the averaging results to existing asymptotic results. Comparison of these two formulas and the exact antenna integral demonstrates that these formulas are more accurate than any other existing formulas for infinite cylindrical antenna current. The Hallén filament integral is found to approximate closely the exact integral even when the condition z ≫ a is violated. Discrepancies in the literature are discussed.
Kelvin transformation for a dielectric‐coated conducting sphere
Authors: Pyati Vittal P.
Published: February 2008

The problem of a conducting sphere with a dielectric coating under the influence of an electrostatic field (primary) is solved by employing Kelvin transformation or the method of inversion. The primary field is assumed to be regular inside the body but otherwise arbitrary. The solution consists of linear combinations of the primary field, its inverted version, and weighted integrals thereof. Physical interpretation of the solution in terms of Neumann images reveals that there are point charges and line charges. The strengths of these charges depend on several parameters, including the orientation angle of the observer, a feature attributable to the layered nature of the object. This aspect of the solution is investigated in some detail, and numerical results are presented. The Kelvin solution presented here should converge faster than the conventional one consisting of an infinite sum of spherical harmonics. It is further believed that extension to other geometries such as a two‐layered dielectric sphere should pose no new difficulties.