bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of aerosol extinction profiles from lidar and SAGE II data at a tropical station
Authors: Parameswaran K. , Rose K. O. , Murthy B. V. Krishna , Osborn M. T. , McMaster L. R.
Published: February 2008

Aerosol extinction profiles obtained from lidar data at Trivandrum (8.6°N, 77°E) are compared with corresponding Stratospheric Aerosol and Gas Experiment (SAGE) II extinction profiles. The agreement between the two is found to be satisfactory. The extinction profiles obtained by both the experiments showed a prominent peak at 23–24 km altitude in the stratosphere. The study revealed large variability in upper tropospheric extinction with location (latitude).
Surface albedo derived from METEOSAT imagery with an application to Africa
Authors: Nacke Gudrun
Published: February 2008

A method is presented to derive surface albedo from METEOSAT radiances with a special application to Africa for January and June 1983 at 1130 UT. The method is based on radiative transfer simulations followed by a multivariate analysis. A linear relationship was derived between filtered (0.4–1.1 μm) surface albedo, METEOSAT radiance, optical depth at 0.55 μm, water vapor content and anisotropic conversion factor. After consideration of very different reflecting surfaces, anisotropic behavior of surface reflection is found to be dominant compared to atmospheric effects if extreme situations like dust outbreaks can be recognized and excluded. Multivariate analysis also revealed a linear relationship between filtered and unfiltered (0.3–2.6 μm) surface albedo. In this case atmospheric influence and anisotropy can be neglected. Errors of estimate, resulting from the applied regression model, are 0.02 – 0.04 if additional information about atmospheric state and anisotropy are available, for Sun zenith angles up to about 67° and for most of the viewing geometries. Application of the regression model to January and June 1983 at 1130 UT reveals that the surface albedo for Africa can be derived for low Sun zenith angles with reasonable accuracy from satellite radiances alone. The error of estimate is in the range of 0.02–0.03. Results derived with the proposed method are somewhat lower than satellite‐derived surface albedos of the West African Sahel available from the literature. Comparison with in‐situ data measured over six different surfaces in the Sahara shows good agreement in cases of large, homogenous surfaces where point measurements can be assumed to represent larger areas.
Electrical discharges involving the formation of NO, NO2, HNO3, and O3
Authors: Franzblau Edward
Published: February 2008

Laboratory discharge experiments were performed with a Wimshurst electrostatic generator with discharge energies between 0.005 and 0.1 J in air, in an atmosphere of various O2/Ar ratios, and an atmosphere of various O2/N2 ratios in order to determine relative NO, NO2, HNO3, and O3 production from a discharge. The results in air showed that the NO/NOx (NOx = NO + NO2) ratio varied between 0.75 and 0.95, depending on the discharge energies, that little O3 was produced except at very low discharge energies, and that no HNO3 was produced when the air was humidified. The results in an O2/Ar atmosphere showed that O3 production increased with decreasing O2 content of the atmosphere, that its production was kinetically controlled, and that its production probably takes place in the cooler regions of the discharge. In an atmosphere of various O2/N2 ratios the production of NOx was also seen to be kinetically controlled and was observed to reach a maximum when the O2 was 30–40%, although its production, unlike O3, probably takes place in the hot regions of the discharge and results from an ohmic heated channel.
Global average ozone change from November 1978 to May 1990
Authors: Herman J. R. , McPeters R. , Stolarski R. , Larko D. , Hudson R.
Published: February 2008

A recent recalibration and reprocessing of the total ozone mapping spectrometer (TOMS) data have made possible a new determination of the global average (69°S to 69°N) total ozone decrease of 3.5% over the 11‐year period, January 1, 1979, to December 31, 1989, with a 2σ error of 1.4%. The revised TOMS ozone trend data are in agreement, within error limits, with the average of 39 ground‐based Dobson stations and with the world standard Dobson spectrometer 83 at Mauna Loa, Hawaii. Superimposed on the 11‐year ozone trend is a possible solar cycle effect, quasi‐biennial oscillation (QBO), annual, and semiannual cycles. Using solar 10.7‐cm flux data and 30‐mbar Singapore wind data (QBO), a time series has been constructed that reproduces the long‐term behavior of the globally averaged ozone. Removal of the apparent solar cycle effect from the global average reduces the net ozone loss to 2.66±1.4% per decade. The precise value of the global average ozone trend depends on the latitude range selected, with ranges greater than ±69° emphasizing the larger variations at high latitudes.
Measurements of Aitken nuclei and cloud condensation nuclei in the marine atmosphere and their relation to the DMS‐Cloud‐climate hypothesis
Authors: Hegg Dean A. , Radke Lawrence F. , Hobbs Peter V.
Published: February 2008

New airborne measurements provide support for the hypothesis that layers of high concentrations of Aitken nuclei near the tops of marine clouds are due to photochemical nucleation. They also reveal a significant correlation between cloud condensation nucleus (CCN) concentrations in the boundary layer and mean cloud droplet concentration in stratus clouds topping a marine boundary layer. Nonsea salt sulfate mass and the concentration of CCN active at 1% supersaturation are also significantly correlated. These results provide quantitative support for some facets of the DMS‐cloud‐climate hypothesis.
Measurement of mobility spectrum and concentration of all atmospheric ions with a single apparatus
Authors: Dhanorkar Savita , Kamra A. K.
Published: February 2008

It is shown that for a given mobility distribution in an air sample a single universal characteristic curve can be obtained for all Gerdien condensers. An apparatus has been developed which can measure the mobility spectrum and concentration of small, intermediate, and large atmospheric ions simultaneously in much shorter time. Sources of error and some precautions taken to minimize these errors in this apparatus are described. Some representative results showing the validity and usefulness of the apparatus are described.
Orbital signals in the diurnal cycle of radiation
Authors: Cerveny Randall S.
Published: February 2008

Time of day plays a critical role in determining the dominance of the Earth's orbital parameters involving eccentricity, precession, and obliquity. Extension of orbital radiation data to the diurnal cycle permits the recognition of well‐defined, temporally separate signals corresponding to all three major orbital parameters and to combinations of these parameters. Past research involving computed radiation changes at the daily and seasonal scales has not shown significant eccentricity cycles. Results of this study show: (1) at the solstices, a marked change in dominance from precession (for the solar elevation classes associated with early morning/late afternoon) to obliquity (for the elevation class containing solar noon) occurs in solar radiation; (2) at the equinoxes, cycles corresponding to those associated with eccentricity are evident in the radiation data at all individual time intervals of the day. Past studies have shown that the daily radiative total reflects only the influence of precession. A third set of cycles at the equinoxes centered near 11 kyr corresponds to a combination of orbital signals. These results imply that paleoclimatic proxy data displaying strong spectral peaks corresponding to eccentricity (100 kyr) and obliquity (41 kyr), such as records of global ice volume, may be related to radiation changes occurring in the solar elevation class containing solar noon, while paleoclimatic proxy data displaying strong spectral peaks corresponding to precession, for example, sea‐surface temperatures, appear to be showing the dominance of precession on the daily total radiation.
Measurements of pure CO2 absorption at high densities near 2.3 μm
Authors: Brodbeck C. , Van‐Thanh Nguyen , Bouanich J.‐P. , Boulet C. , Jean‐Louis A. , Bezard B. , de Bergh C.
Published: February 2008

Thermal emission from the deep atmosphere of Venus can be detected on the nightside around 2.3 μm. The analysis of this radiation requires a reliable knowledge of the absorption in the far wings of the nearby allowed CO2 bands and of the absorption due to collision‐induced bands. We measured absorption coefficients for pure CO2 at pressures varying from 30 to 60 bars in the frequency range 3910–4570 cm−1 at 297.5 K. Values between 1.0 and 1.6 × 10−7 cm−1 amagat−2 are found in the 4100–4500 cm−1 interval where emission from the Venus nightside occurs. The comparison of experimental results with synthetic spectra calculated from a line by line code demonstrates that the Lorentzian line shape strongly overestimates the observed absorption, whereas the use of a χ factor extrapolated from the 3800–4000 cm−1 region does not provide enough opacity.
Ion irradiation experiments relevant to cometary physics
Authors: Strazzulla Giovanni , Leto Giuseppe , Baratta Giuseppe Antonio , Spinella Francesco
Published: February 2008

During their stay in the Oort cloud (4.6 × 109 years), comets are subject to the flux of galactic cosmic rays. Experimental results on the chemical and physical changes induced by ion and electron irradiation of relevant materials have been recently reviewed with a view to their relevance for cometary astrophysics. In particular, the ability of ion irradiation of simple carbon‐containing ices to produce complex refractory organic materials has been discussed. Here we discuss further experiments carried out in our laboratory whose results are relevant to cometary physics. Experiments on targets much thicker than the penetration range of irradiating ions give support to the hypothesis that the ion‐produced cometary organic crust can “survive” gas ejection from deeper layers and be as old as the comet itself. A study of IR and Raman spectra obtained “in situ” before, during, and after irradiation of frozen benzene has clarified the processes producing, after long enough exposures, a tridimensional network of aromatic molecules linked together by linear chains. We call this material ion‐produced hydrogenated amorphous carbon. Other experimental data demonstrate the transition from crystalline to amorphous water ice induced by ion irradiation at temperatures between 10 and 100 K. Thus the observation of amorphous ice cannot be considered, as it has been up to now, a definitive proof of its formation and permanence at low T.
Water Mass Conversion in the Glacial Subarctic Pacific (54°N, 148°W): Physical Constraints and the Benthic‐Planktonic Stable Isotope Record
Authors: Zahn Rainer , Pedersen Thomas F. , Bornhold Brian D. , Mix Alan C.
Published: February 2008

Benthic (Uvigerina spp., Cibicidoides spp., Gyroidinoides spp.) and planktonic (N. pachyderma sinistral, G. bulloides) stable isotope records from three core sites in the central Gulf of Alaska are used to infer mixed‐layer and deepwater properties of the late glacial Subarctic Pacific. Glacial‐interglacial amplitudes of the planktonic δ18O records are 1.1–1.3‰, less than half the amplitude observed at core sites at similar latitudes in the North Atlantic; these data imply that a strong, negative δw anomaly existed in the glacial Subarctic mixed layer during the summer, which points to a much stronger low‐salinity anomaly than exists today. If true, the upper water column in the North Pacific would have been statically more stable than today, thus suppressing convection even more efficiently. This scenario is further supported by vertical (i.e., planktic versus benthic) δ18O and δ13C gradients of >1‰, which suggest that a thermohaline link between Pacific deep waters and the Subarctic Pacific mixed layer did not exist during the late glacial. Epibenthic δ13C in the Subarctic Pacific is more negative than at tropical‐subtropical Pacific sites but similar to that recorded at Southern Ocean sites, suggesting ventilation of the deep central Pacific from mid‐latitude sources, e.g., from the Sea of Japan and Sea of Okhotsk. Still, convection to intermediate depths could have occurred in the Subarctic during the winter months when heat loss to the atmosphere, sea ice formation, and wind‐driven upwelling of saline deep waters would have been most intense. This would be beyond the grasp of our planktonic records which only document mixed‐layer temperature‐salinity fields extant during the warmer seasons. Also we do not have benthic isotope records from true intermediate water depths of the Subarctic Pacific.
The Lorentz polarization term for magnetospheric whistler propagation
Authors: Strangeways Hal J.
Published: February 2008

If the particles of a dielectric are arranged in a cubic lattice, the effective field acting on a particle is E + lP/εo, where l = 1/3. The second term is the Lorentz polarization term. Appleton believed that this term should be omitted from magneto‐ionic theory, whereas Hartree included it. Various arguments have been presented to prove Appleton's contention that this term is zero for a plasma. Some proofs have considered the perturbation of the individual orbits of the electrons, but careful consideration of these treatments shows that they would predict a nonzero Lorentz term for some velocity distributions. It has also been argued that l ≠ 0 would violate causality but it can be shown that this only applies in the zero frequency limit for positive l. Other proofs that have been presented also possess significant limitations, in particular being limited to rather simpler plasma configurations than exist in the magnetosphere and weak field strengths. The theoretical problem for more general conditions is very difficult and thus an appeal to experiment results is considered relevant. It is shown that a very small nonzero Lorentz term would be significant for whistler‐mode propagation. The observed lower cutoff frequencies of whistlers put a very low limit (l ∼ 10−5) on the maximum value of positive l but put no limit on negative l. It is shown, however, that l < 0 would increase the cutoff frequency for ducted propagation. Thus the upper cutoff frequency is determined for values of l in the range −5×10−6 to −2×l0−3 by ray tracing in a model Gaussian cross‐section duct in a realistic model of the magnetospheric plasma corresponding to winter night conditions. Cutoffs (normalized to the equatorial electron gyrofrequency determined by curve fitting to the ray‐traced whistler spectra) are found to be in the range 0.49 to 0.55 for a 15% enhancement duct. The range of cutoffs for l ∼ −10−4 is in much better agreement with experimentally observed whistler cutoffs than the assumption that l = 0 (no Lorentz term). This provides experimental evidence for a nonzero (negative) Lorentz term for ducted whistler propagation.
Electrostatic image theory for the dielectric sphere
Authors: Lindell Ismo V.
Published: February 2008

The image of a point charge in a conducting sphere, produced by Kelvin's inversion theory, has become well known since its formulation in 1848. The corresponding theory in terms of point charges does not exist for the dielectric sphere. In the present paper, electrostatic image theory for the dielectric sphere, both for the field outside and inside of the sphere, is developed. Simple analytic expressions in terms of a point charge and a line charge are seen to arise in both cases. The theory is extended to give simple images for dipole and line sources. Possible application of the new expressions in simplified quasi‐static analyses of interactions of dielectric spherical particles in artificial dielectric media is pointed out.
Accelerating the iterative inverse scattering algorithms by using the fast recursive aggregate T‐matrix algorithm
Authors: Wang Y. M. , Chew W. C.
Published: February 2008

A review and a convergence analysis of the Born and distorted Born iterative methods are given. The fast recursive aggregate T‐matrix algorithm is reviewed and then applied to the solution of the direct scattering part in the iterative inverse scattering algorithms. Together with the conjugate gradient method in the inversion part of the iterative inverse scattering algorithms, the overall computational complexity for the iterative inverse scattering algorithms is reduced from O(N3) to O(N2), representing a significant reduction of their computational complexity. The resulting distorted Born iterative algorithm is demonstrated to have a high‐resolution ability of reconstructing one‐ or two‐pixel pulse objects.
A numerical assessment of rough surface scattering theories: Vertical polarization
Authors: Kim Yunjin , Rodríguez Ernesto , Durden Stephen L.
Published: February 2008

In a previous paper, we presented a numerical evaluation of the regime of validity for various rough surface scattering theories. The evaluation was done for horizontal polarization by comparing these theories against numerical results obtained by the method of moments. In this paper we address the same issues for vertical polarization. The theories considered here are the small perturbation method, the Kirchhoff approximation, the momentum transfer expansion, the two‐scale expansion, and the unified perturbation method (UPM). All these theories can be derived in a unified manner starting with the extinction theorem. The rough surfaces of interest are oceanlike surfaces, which exhibit height power law spectra. Considering both horizontal and vertical polarizations, the UPM provides best results among all theories considered.
Basement slivers within the New Britain Accretionary Wedge: Implications for the emplacement of some ophiolitic slivers
Authors: Bernstein‐Taylor Barrie L. , Brown Kevin M. , Silver Eli A. , Kirchoff‐Stein Kimberly S.
Published: February 2008

Using multichannel seismic data from the western Solomon Sea we have imaged several 2.5 to 3‐km‐thick oceanic basement slivers within the New Britain accretionary wedge which may serve as a modern analogue for the detachment of some ophiolitic slivers. The most recently accreted basement sliver is located just over 5 km from the thrust front. An approximate area balance indicates the subjacent section of the downgoing plate from which this sliver originated is between 11 km and 15 km arcward of its present position. Though we have not imaged the basement ramp, sedimentary strata beneath the basement sliver pinch out between 9 and 12 km from its seaward edge, suggesting that detachment occurred in that vicinity. Normal faults reactivated in response to the flexural bending of the downgoing oceanic plate are a prominent feature of this region. Significantly, almost all basement normal faults near the recently accreted basement sliver, including those imaged beneath the toe of the wedge, dip arcward. Based on these data, we suggest that small (1–3 km thick) basement slivers are decoupled along favorably oriented, preexisting zones of weakness formed by normal fault detachments within the oceanic basement.
Natural sulfur emissions to the atmosphere of the continental United States
Authors: Bates Timothy S. , Lamb Brian K.
Published: February 2008

Seasonal emissions of biogenic sulfur gases from marine and terrestrial sources were assessed for 13 regions of the continental United States. The resulting inventory was compared to volcanic and anthropogenic sulfur emissions from the same regions. Terrestrial biogenic sulfur emissions were less than 1% of the total sulfur emissions in each region. However, during the summer, marine biogenic sulfur emissions may be contributing 20–40% of the total sulfur emissions in western coastal regions.
Thermal convection in the Earth's mantle: Mode coupling induced by temperature‐dependent viscosity in a three‐dimensional spherical shell
Authors: Stewart Cheryl A.
Published: February 2008

Temperature‐dependent viscosity places constraints on the spherical harmonic spectrum of thermal convection in the earth's mantle. Viscosity, being a strong function of temperature, gives rise to quantized feed‐back loops among low‐order modes in the governing equations. This nonlinear resonance is proposed as the cause of the dominance of a low‐order convective pattern in the earth's mantle.
Stratospheric ozone profile and total ozone trends derived from the SAGE I and SAGE II data
Authors: McCormick M. Patrick , Veiga Robert E. , Chu William P.
Published: February 2008

SAGE I/II ozone data from the period 1979–1991 have been used to derive global trends in both stratospheric column ozone and as a function of altitude. A statistical model containing quasibiennial, seasonal, and semiannual oscillations, a linear component, and a first‐order autoregressive noise process was fit to the time series of SAGE I/II monthly zonal mean data. The linear trend in column ozone above 17 km altitude, averaged between 65°S and 65°N, is −0.30 ± 0.19%/year or −3.6% over the time period February 1979 through April 1991. The data further show that the column trend above 17 km is nearly zero in the tropics and increases towards the high latitudes with values of −0.6%/year at 60°S and −0.35%/year at 60°N. Both these results are in agreement with the recent TOMS results. Furthermore, the profile trend analyses show the column ozone losses are occurring below 25 km, with most of the loss coming from the region between 17 and 20 km. Negative trend values on the order of −2%/year are found at 17 km in mid‐latitudes.
Strike‐slip earthquakes on quasi‐vertical transcurrent faults: Inferences for general scaling relations
Authors: Romanowicz Barbara
Published: February 2008

The recent occurrence of several large strike‐slip earthquakes provides the opportunity to review and complement available data on the scaling of seismic moment (MO) with length of rupture (L) for large earthquakes, depending on their tectonic setting and mechanism. For strike‐slip earthquakes on quasi‐vertical transcurrent faults, the MO versus L relation has a significant change of slope around MO ∼(0.6–0.8)*1020 N‐m, and for larger earthquakes, MO scales linearly with L. This is compatible with models where slip is controlled by the width of the fault. Also, it appears to be easier to categorize large earthquakes by their mechanism (strike‐slip on vertical transcurrent fault, versus pure thrust/normal) than their tectonic setting (interplate/intraplate).
The spatial distribution of the association between total ozone and the 11‐year solar cycle
Authors: Labitzke K. , van Loon H.
Published: February 2008

The pattern of correlation between the 11‐year solar cycle and heights and temperatures in the lower stratosphere is in all months shaped as a crescent with its axis in the subtropics. The change of total ozone from the solar maximum in 1979–1980 to the minimum in 1985–1986 has the same shape. Although the effect of the solar cycle is said to have been removed from the ozone data, two thirds of the stations which have been used for this purpose lie outside the regions where the stratosphere is significantly correlated with the solar cycle. For this reason it is unlikely that the influence of the cycle has been completely eliminated.
Evidence for substantial seasonal variations in the structure of the mesospheric Fe layer
Authors: Kane Timothy J. , Mui Peter H. , Gardner Chester S.
Published: February 2008

Lidar measurements of mesospheric Fe were conducted at Urbana, IL (40°N, 88°W) during the fall of 1989 and the spring through fall of 1991. The Fe layer abundance, centroid height and rms width varied substantially during the observation period with minimum values for all three parameters in summer. The abundance varied from 3.3×109 ‐ 25×109 cm−2, the centroid height from 86.7 – 90.4 km and the rms width from 2.1 – 3.9 km. Sporadic Fe (Fes) layers were also frequently observed. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana. The mean Fe column abundance is more than twice the average Na column abundance. The Fe layer is also on average approximately 3.6 km lower and 32 % narrower than the Na layer. The seasonal behavior of the Fe layer is discussed in light of our preliminary temperature measurements of the mesopause region as well as a simple chemical model.
The spectrum of the tropical oxygen nightglow observed at 3 Å resolution with the Hopkins Ultraviolet Telescope
Authors: Feldman P. D. , Davidsen A. F. , Blair W. P. , Bowers C. W. , Durrance S. T. , Kriss G. A. , Ferguson H. C. , Kimble R. A. , Long K. S.
Published: February 2008

Ultraviolet spectra of the tropical oxygen night‐glow in the range of 830 to 1850 Å (in first order) at 3 Å resolution were obtained with the Hopkins Ultraviolet Telescope during the Astro‐1 space shuttle mission in December 1990. We present here the data obtained on a setting celestial target as the zenith angle of the line‐of‐sight varied from 77 to 95°. The dominant features in the spectrum (other than geocoronal hydrogen) are OIλλ1304 and 1356 and the radiative recombination continuum near 911 Å. The continuum is resolved and found to be consistent with an electron temperature in the range 1000 – 1250 K. The observed ratio of the brightness of OIλ1356 to the continuum suggests that O+ ‐ O− mutual neutralization contributes about 40% to the 1356 Å emission. The dependence of the optically thin emissions on zenith angle is consistent with a simple ionospheric model. Weak OIλ989 emission is also detected, but there is no evidence for any similarly produced atomic nitrogen emissions.
Whistler‐mode chorus and morningside aurorae
Authors: Inan U. S. , Chiu Y. T. , Davidson G. T.
Published: February 2008

Quasi‐electrostatic ELF/VLF chorus emissions immediately above the equatiorial half‐gyrofrequency, observed to propagate with wave normal angle (Ψ) within 0.4–1.2° of the resonance cone (Ψr) [Muto et al., 1987], are shown to efficiently resonate with 0.1–10 keV electrons. These waves may thus be important in driving the relatively low energy (<10 keV) component of pulsating aurorae and the morning‐side diffuse aurorae (∼1 keV).
Tracer exchange between tropics and middle latitudes
Authors: Rood Richard , Douglass Anne , Weaver Clark
Published: February 2008

The interaction between the tropics and middle latitudes is studied using a tracer emitted at 50 hPa along a great circle route between Los Angeles, USA and Sydney, Australia. Though designed to examine the impact of stratospheric aircraft, the study more generally addresses the transport between tropics and middle latitudes for a three month period from January through March 1989. The results show that air is transported from the tropics to middle latitudes by planetary scale and tropospheric cyclonic scale waves. Except for intrusions by these wave events, the tropics are substantially isolated throughout the lower stratosphere. These waves draw material out of the tropics which ends up in the middle latitude westerly jets, with little material entering the winter polar latitudes prior to the springtime transition. The summer southern hemisphere is characterized by tracer being drawn out in streamers that extend from north and west to south and east. The material in the tropics is zonally asymmetric. The material that reaches the troposphere comes down in the synoptic scale eddies and is concentrated in the middle latitude jet stream. These characteristics are similar to those observed during the dispersion of volcanic clouds.
Observation of a strong surface radar duct using helicopter acquired fine‐scale radio refractivity measurements
Authors: Babin Steven M. , Rowland John R.
Published: February 2008

A very strong surface duct was observed near San Nicolas Island, California, using a unique helicopter‐based data acquisition system developed for making fine‐scale measurements of radio refractivity in the lower marine troposphere. Such fine‐scale measurements are used in a computer model (TEMPER) for predicting microwave propagation in the first 400 m above the ocean surface. The helicopter system makes measurements of atmospheric temperature, pressure, relative humidity, and radar altitude. These measurements are acquired by an on‐board computer, which derives and displays real‐time plots of modified refractivity versus altitude. Data is stored on floppy disks for later use in the TEMPER (Tropospheric Electromagnetic Parabolic Equation Routine) computer model for microwave propagation prediction. The helicopter measurements are presented and compared with TEMPER computer model microwave propagation predictions using this helicopter data and a standard atmosphere profile.
Predictions of the solar wind interaction with Comet Grigg‐Skjellerup
Authors: Huddleston D. E. , Coates A. J. , Johnstone A. D.
Published: February 2008

The planned encounter of the Giotto spacecraft with comet Grigg‐Skjellerup on 10th July 1992 promises to extend our knowledge of the solar wind interaction with comets substantially. While there have been spacecraft missions to comets before now, this mission is exploratory in the sense that the target comet is much older and therefore it has a much lower gas production rate than comets Halley (by a factor ∼200) or Giacobini‐Zinner (factor ∼10). Here we present theoretical predictions for the location of the bow shock and contact surface features, and compare similar predictions with the observed features at the previous encounters. We discuss the applicability of fluid‐type theory which these models employ, in the case of strong and weak comets in the solar wind.
Physical properties of 110 MA oceanic crust at Site OSN‐1: Implications for emplacement of a borehole seismometer
Authors: Goldberg David , Moos Daniel
Published: February 2008

An extensive suite of downhole geophysical logs was acquired through a 70 m thick section of basalt drilled by the Ocean Drilling Program (ODP) into 110 Ma oceanic crust 225 km SW of Hawaii. These data include standard sonic, electrical, and nuclear logs, as well as electrical and acoustic images of the wellbore, and enable a vertically continuous characterization of the physical properties of shallow, old oceanic crust. Fracture porosity and alteration increase with depth in this hole, and considered together with results obtained elsewhere in both young and old (5 to 110 Ma) crust, suggest that physical properties gradients in the uppermost few hundred meters of oceanic basement are controlled by crustal morphology and alteration and are ubiquitously variable. As this hole was drilled primarily for pilot experiments at the first site of the Ocean Seismic Network, evaluation of the log data demonstrates the importance of careful in‐situ data collection in order to emplace a broadband monitoring device below the seafloor for long‐term seismic measurements.
Impingement of mantle plumes on the lithosphere: Contrast between Earth and Venus
Authors: Erickson S. Gregg , Arkani‐Hamed Jafar
Published: February 2008

The penetration, ascent, melt production, cooling and solidification of a mantle plume as it impinges on the base of the lithosphere are modelled as functions of the lithospheric temperature and thickness. The models indicate that, if mantle and plume temperatures and plume dimensions of Venus are similar to those of Earth, the higher lithospheric temperature of Venus causes higher ascent of plumes into the lithosphere and a greater amount of lithospheric melting. However, the rates of intraplate volcanism on Venus and Earth appear to be similar, indicating that mantle or plume temperatures may be less or plumes may be smaller in Venus than in Earth.
Magnetic blowout during collisionless reconnection
Authors: Drake J. F. , Burkhart G. R.
Published: February 2008

Simulations and analytic calculations of the steady‐state structure of the dissipation region during collisionless reconnection are presented. Ions are treated as particles and electrons as a dissipative fluid. The mechanism for electron/ion momentum coupling during collisionless reconnection is elucidated. A limiting reconnection rate results as the ions “blowout” the magnetic field in the dissipation region. The implications for understanding substorm dynamics are discussed.
The relationship between the height of a volcano and the depth to its magma source zone: A critical reexamination
Authors: Wilson Lionel , Head James W. , Parfitt Elisabeth A.
Published: February 2008

It is commonly assumed that hydrostatic pressure balance arguments can be used to establish a relationship between the maximum height to which a volcanic edifice is able to grow and the depth at which the partial melts providing its magma supply are formed. Such a relationship has been used to infer various aspects of the thermal and stress state of the lithosphere beneath volcanic constructs on Earth, Mars, Io and Venus. We examine the assumptions behind this relationship (which are that: (1) a continuous pressure connection exists between source and summit, (2) the pressure around the magma source is the local hydrostatic pressure dictated by the depth below the geoid, and (3) the melt erupting at the summit has a net positive buoyancy), and show that many of them require geologically unreasonable conditions. We then critically assess the evidence cited in the literature for the relationship and find that there are other factors that may explain the observations. We conclude that volcano heights on the terrestrial planets cannot be related in any simple way to lithospheric thickness or depth to the magma source zone and we review the range of other factors controlling volcano height.
Nitrosyl sulfuric acid and stratospheric aerosols
Authors: Burley Joel D. , Johnston H. S.
Published: February 2008

From information found in the atmospheric and chemical literature, we propose that nitrosyl sulfuric acid (NSA), NOHSO4, may play an important role in stratospheric chemistry. In one study, NSA was observed as a slurry of crystals in about a third of the aerosol particles collected between 15 and 20 km. From the chemical literature, we find that NSA is formed from several reactions between NOy species (averaging +3 oxidation state) and sulfuric acid, if the acid is above 70 wt % at room temperature and above 60 % at stratospheric temperatures. NSA is a non‐volatile ionic solid (NO+HSO4−), melting at 346 K and boiling with decomposition at 633 K. Over time, it could accumulate as a solution in the liquid aerosol, and upon reaching solubility saturation (about 3 % under lower stratospheric conditions) precipitate out as solid crystals, which permits further accumulation. At room temperature, HCl gas reacts with NSA, either the solid or in sulfuric acid solution, to form gaseous nitrosyl chloride, ClNO, which is rapidly photolyzed in the atmosphere to form nitric oxide and atomic chlorine. The overall catalytic cycle for this process can be written in a number of ways, depending upon the NOy species responsible for NSA formation; with NSA production from HONO, the cycle is

The net reaction is thermodynamically favorable for at least three modes of NSA formation. If the reaction probability of forming NSA per collision of NO with a sulfuric acid aerosol is 10−3, 1% of the sulfuric acid would be converted to NSA within 1 day at 20 km altitude. If NSA in the sulfate aerosols is 1% of the sulfuric acid and if the second order rate constant for (HCl + NOHSO4 = H2SO4 + ClNO) in sulfuric acid solution is greater than 10−18 cm3 molec.−1 s−1 under conditions at 20 km altitude, the rate of HCl + NSA is faster than the rate of HO + HCl. In this case, this heterogeneous catalysis is expected to affect the balance of NOy, Cly, and ozone in the lower stratosphere.
Geoid effects in a convecting system with lateral viscosity variations
Authors: Ritzert M. , Jacoby W. R.
Published: February 2008

The geoid signal and the flow patterns of two‐dimensional steady state convection models with exponential temperature‐ and depth dependent viscosity are compared with results for an equivalent stratified viscosity structure. In analogy to Richards and Hager [1989], the latter are computed by a “dynamic response” approach. The flow fields obtained with this approach are quite different from the full solution; the geoid signals are similar but the amplitudes differ significantly. The differences are analysed in the horizontal wavenumber domain and in the spatial domain. They may lead to an overestimation of the viscosity contrast of the earth's mantle derived by modeling the earth's geoid with dynamic response functions. There are implications on the interpretation of observed geoid anomalies in terms of up‐ or downwelling flow structures.
Comparison of observed diurnal and semidiurnal tropospheric winds at Christmas Island with tidal theory
Authors: Williams Christopher R. , Avery Susan K. , McAfee John R. , Gage Kenneth S.
Published: February 2008

Four years of hourly averaged atmospheric wind measurements from the Christmas Island VHF wind profiler were processed into typical days to show the vertical phase structure of the diurnal and semidiurnal tropospheric zonal winds. The winds from the lowest resolvable height were compared with both the surface winds calculated from a three year record of surface pressure using the equations of motion on a rotating, frictionless sphere, and the winds from a tidal model. It is concluded that at the lowest height, the observed diurnal and semidiurnal zonal winds have tidal (1,1) and (2,2) mode structures, respectively. Further analysis is necessary to evaluate the tidal mode(s) of the zonal wind throughout the troposphere.
40Ar/39Ar laser dating of biotite inclusions in a single crystal of magnetite
Authors: Özdemir Özden , York Derek
Published: February 2008

The iron‐oxide mineral magnetite is widely used to study the past geomagnetic field because of its ability to record the ancient field at the time of its crystallization (or cooling), and because of its common occurrence in continental and oceanic rocks. We present here the 40Ar/39Ar laser dating of one half of a 2 mm octahedral crystal of magnetite from an ultramafic rock near Bush's Corner, Harford County, Maryland, in the Appalachian Piedmont. The other half was examined by scanning electron microscopy (SEM) to characterize the inclusions in the crystal. Laser step‐heating separates the argon isotopes released into three distinct phases. A three‐dimensional isotope correlation plot illustrates the existence of a pure radiogenic component corresponding to an age of 323±5 Ma. This is the age of argon closure of minute biotite inclusions identified in the magnetite by SEM, and therefore is a precise estimate of the minimum age of the magnetite.
Compressional to shear wave velocity ratio of granular rocks: Role of rough grain contacts
Authors: Palciauskas Victor V.
Published: February 2008

Experimental data indicate that although the compressional vp and shear vs wave velocities of unsaturated, monomineralic granular sedimentary rocks vary with grain mineralogy, degree of compaction (porosity), and confining pressure, their ratio vp/vs is to various degrees insensitive to these factors. A rough contact model for the elastic properties of a granular aggregate is presented which is consistent with these observations. The relevant assumptions of the model are: (1) the medium is a random aggregate of monomineralic, elastically isotropic, equal size grains, and (2) the intergrain contacts are rough and their elastic properties are described by the results of Yoshioka and Scholz [1989].

The model predicts that vp/vs, the bulk to shear frame modulus ratio
, and the Poisson's ratio,
, of the aggregate are independent of porosity and confining pressure, and only weakly dependent on mineral Poisson's ratio. As the mineral Poisson's ratio varies between 0.08 and 0.40, vp/vs and
are essentially constant at 1.55 and 1.07 respectively, and the Poisson's ratio,
, varies between 0.141 and 0.144. These values are in good agreement with experimental data on dry, monomineralic granular rocks.
Resolving volcanic activity of 20MA ago with relative accuracy of 1 yr from tree rings of petrified woods
Authors: Kumagai Hiroyuki , Fukao Yoshio
Published: February 2008

Tree rings of petrified woods were analyzed to resolve a volcanic activity of 20 Ma ago with a relative accuracy of 1 yr. Samples are those in the volcanic breccia of the Early Miocene formation of volcaniclastic rocks and tuffaceous lake sediments in central Japan. Field observations indicate that these petrified woods are the fossils of the trees that were carried away from their growth sites with volcanic breccia ejected upon explosive eruption and included in it. Using a dendrochronological procedure we measured the relative years of the last rings of the petrified woods. The relative years of the last rings are concentrated in 17 yrs, where the ring‐width patterns show an anomalous behavior similar to one observed in firs growing in the area devastated by Mt. St. Helens. We suggest that the last‐formed tree ring of a petrified wood is a record of volcanic eruption and that the activity of volcano continued over a period of 17 yrs, in which eruption occurred at least five times. High resolution regarding the activity period demonstrates uniqueness of tree‐ring study to investigate volcanism in the geologic past.
ISEE‐1 and ‐2 observations of an isolated diamagnetic event: An earthward‐moving plasma bulge or a tail‐aligned flux rope?
Authors: Ohtani S. , Elphic R. C. , Russell C. T. , Kokubun S.
Published: February 2008

This paper reports a transient strong diamagnetic event observed with the ISEE‐1 and ‐2 spacecraft in the near‐Earth magnetotail. Magnetic signatures associated with the event resemble those of flux rope events found in the distant magnetotail. The present event should be distinguished from those in several previous reports based on the following points: (1) The event is observed in the near‐Earth tail (X ∼−17 RE), not in the distant tail. (2) The tail field changes from a stressed configuration to a more dipolar one in the course of the event, and the plasma data suggest that the heating and the resultant expansion of the plasma take place simultaneously. (3) The minimum of the magnetic field strength BT is ∼1 nT; such a strong depression is rarely observed in the near‐Earth tail. (4) An earthward plasma jet (300 to 500 km/s) continues for about 3 min, including a period of very weak magnetic field; the duration of the jet is exceptionally long compared to previously reported high‐speed flows in the plasma sheet. These magnetic field and plasma signatures are discussed in terms of two models: an earthward‐moving plasma bulge and a tail‐aligned flux rope.
On the existence of a Lorenz Strange Attractor in magnetospheric convection dynamics
Authors: Summers Danny , Mu Jian‐Lin
Published: February 2008

We modify the corotating convection model usually applied to the Jovian magnetosphere by including a term that simulates the effects of microdiffusion. By applying a technique of Fourier analysis followed by three‐mode truncation, we recover precisely the well known Lorenz equations of meteorology. The implications are that under certain restrictions plasma motion in the Jovian magnetosphere, which is modulated by ionospheric ohmic drag through the Pedersen ionospheric conductivity Σ, can be expected to be chaotic if Σ is less than a critical value Σc; if Σ is greater than Σc the plasma motion is steady‐state convection. The value of Σc depends on a parameter q which characterizes an assumed stationary radial profile (∝ L−q) in plasma mass per unit magnetic flux.
The formation of the wall region: Consequences in the near Earth magnetotail
Authors: Ashour‐Abdalla Maha , Zelenyi Lev M. , Bosqued Jean‐Michel , Peroomian Vahé , Wang Zhi , Schriver David , Richard Robert L.
Published: February 2008

This paper discusses important new findings obtained from global kinetic simulations of magnetotail plasma. A region of strongly non‐adiabatic ion acceleration (known as the “wall” region) exists in the near Earth tail and demarcates two very different regimes of ion motion: adiabatic and quasiadiabatic. A strong enhancement of the cross‐tail current occurs on the tailward side of the wall. Our comparison of numerical and adiabatic pressure profiles indicates that non‐adiabatic processes operating in this region may contribute significantly to a pressure balance relief in the course of quasisteady magnetospheric convection.
Empirical low‐order ENSO dynamics
Authors: Bauer Sonia T. , Brown Michael G.
Published: February 2008

Time series of sea surface temperature (SST) in the eastern equatorial Pacific Ocean are analyzed using singular spectrum analysis as a means of testing the hypothesis that the El Niño/Southern Oscillation (ENSO) system can be modelled as a deterministic low‐order nonlinear dynamical system. The results suggest, with some ambiguity owing to the relatively short and noisy SST record, that the main features of the ENSO system can be captured in a model of this type.
Plasmoid observations in the distant plasma sheet boundary layer
Authors: Moldwin Mark B. , Hughes W. Jeffrey
Published: February 2008

Substorm associated large amplitude bipolar magnetic events occurred when ISEE 3 was in the distant geotail's plasma sheet boundary layer (PSBL). The characteristics of these events, their substorm association and their possible source mechanisms are examined. We propose that these PSBL events are signatures of a passing plasmoid in the plasma sheet, analogous to the traveling compression region model in the geomagnetic lobes.
Diurnal variation in vertical motion over the central equatorial Pacific from VHF wind‐profiling Doppler radar observations at Christmas Island (2° N, 157° W)
Authors: Gage Kenneth S. , McAfee John R. , Reid George C.
Published: February 2008

Wind‐profiling Doppler radars measure vertical motions along a vertically‐directed beam. By averaging over long periods it is possible to measure very small mean vertical motions. Long‐term measurements at Christmas Island have been analyzed to determine the diurnal variation in the vertical motion field. The outstanding feature that emerges from the analysis is a pronounced diurnal cycle in the subsiding motion of the main troposphere (4–10 km) and an equally pronounced diurnal cycle in upward motion centered around the altitude of the tropopause. The possibility of radiative forcing of the observed diurnal cycle is considered in the context of the diurnal variation of atmospheric diabatic heating and cooling rates.
Solar EUV irradiance from the San Marco Assi: A reference spectrum
Authors: Schmidtke Gerhard , Woods Thomas N. , Worden John , Rottman Gary J. , Doll Harry , Wita Claus , Solomon Stanley C.
Published: February 2008

The only satellite measurement of the solar EUV irradiance during solar cycle 22 has been obtained with the Airglow Solar Spectrometer Instrument (ASSI) aboard the San Marco 5 satellite flown in 1988. The ASSI in‐flight calibration parameters are established by using the internal capabilities of ASSI and by comparing ASSI results to the results from other space‐based experiments on the ASSI calibration rocket and the Solar Mesospheric Explorer (SME). A solar EUV irradiance spectrum derived from ASSI observations on November 10, 1988 is presented as a reference spectrum for moderate solar activity (F10.7≈ 150) for the aeronomy community. This ASSI spectrum should be considered as a refinement and extension of the solar EUV spectrum published for the same day by Woods and Rottman [1990]. This new reference solar spectrum covers the wavelength range from 20 to 200 nm and is presented in a format similar to the 37 wavelength intervals defined by Torr and Torr [1985].