bluuebunny/crossref_metadata_2025 huggingface dataset:
Plant‐dependent CH4 emission in a subarctic Canadian fen
Authors: Whiting Gary J. , Chanton Jeffrey P.
Published: February 2008

The importance of vegetation in affecting CH4 emissions was investigated in a Carex‐dominated fen located near Schefferville, Quebec, and in the Experimental Lakes Area, Ontario. Comparison of emission rates with and without the presence of aboveground vegetation indicated that over 90% of the emission was plant‐associated transport. Further evidence of this association was found in a linear correlation of CH4 emission with aboveground plant biomass (R=0.93). To test the importance of aboveground plant photosynthetic production on methane production, aboveground vegetation was clipped from sites continually over the growing season. Both emissions and dissolved pore water CH4 were reduced relative to adjacent vegetated areas. A significant correlation (R=0.93) of CH4 emissions with net CO2 exchange in this fen gives evidence of the close association between new plant production and methanogenesis.
Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands
Authors: Kelly Cheryl A. , Dise Nancy B. , Martens Christopher S.
Published: February 2008

The stable carbon isotopic composition of methane (δ13C) emitted from two peatland sites in the Marcell Experimental Forest in northern Minnesota was investigated during the snow‐free season of 1989–1990. A seasonal range in δ13C values of 13‰ was seen for a forested bog with heavier (13C enriched) methane emitted during the wanner summer months. This shift was correlated with water table level suggesting control by microbial oxidation. Methane from a nearby poor fen transitional to bog dominated byCarex oligospermashowed a similar temporal trend but with a much smaller range of 5‰ during the same time period and with no water table level correlation. The methane emitted from the fen was consistently heavier than that emitted by the bog.
Carbon isotope indicators of catchment vegetation in the Brazilian Amazon
Authors: Bird M. I. , Fyfe W. S. , Pinheiro‐Dick D. , Chivas A. R.
Published: February 2008

Paniculate organic carbon (POC) samples from rivers draining wholly forested catchments in the Amazon Basin exhibit carbon isotope values (δ13C) of generally between −27 and −30‰. These values are distinct from those of POC from rivers draining nonforested (grassland/woodland) catchments, which are generally higher than ‐26‰. The difference is due to the presence in the nonforested regions, of grasses which assimilate carbon via the C4 photosynthetic pathway (average δ13C = ∼−13‰), rather than the C3 pathway (average δ13C = ∼−28‰) utilized by forest vegetation, and to negligible utilization of low‐δ13C respired carbon dioxide in regions of open vegetation. A change in the area of the Basin covered by closed forest will lead to a modification in the carbon isotope composition of exported POC as a result of changing (1) the proportion of C4 carbon in the total biomass (2) the relative proportion of the total POC derived from upstream, high‐altitude (Andean) sources, (3) erosion rates, and (4) productivity. All of these factors will tend to reinforce each other in their effect on the carbon isotope composition of POC exported from the Basin, with a decrease in forest area leading to an increase in the δ13C value of POC (and vice versa). There is a sufficient difference between the δ13C values of POC exported from forested and non‐ forested catchments to elucidate past vegetation and therefore climatic changes in the Amazon Basin (and other tropical river basins), using the δ13C record in POC from either terrestrial or offshore sedimentary sequences. The carbon isotope composition of POC in rivers draining recently deforested regions indicates that the isotopic composition of coarse (>63 μm) riverine POC will respond to vegetation changes in a catchment within years to decades, whereas the δ13C value of fine‐grained (<63 μm) riverine POC may take longer to respond.
Gaseous ammonia fluxes and background concentrations in terrestrial ecosystems of the United States
Authors: Langford A. O. , Fehsenfeld F. C. , Zachariassen J. , Schimel D. S.
Published: February 2008

Ammonia (NH3) is the dominant gaseous base in the atmosphere and the principal neutralizing agent for atmospheric acids, yet remains one of the least well characterized atmospheric trace compounds. In particular, the spatial and temporal distribution of the background concentrations in terrestrial ecosystems and the importance of natural emissions from undisturbed soils and vegetation is poorly understood. This situation persists because of experimental difficulties associated with ammonia measurements, the rapid gas‐to‐particle conversion of ammonia in the atmosphere, and the capacity of native soils and vegetation to act as both source and sink for atmospheric ammonia. In the present paper, we attempt to summarize the current understanding of the natural sources and sinks for gaseous NH3and the importance of natural emissions relative to anthropogenic emissions in the United States. We briefly review the physical and chemical processes that transform NH3in the atmosphere, the major anthropogenic and potential natural sources of atmospheric NH3, and the techniques used to measure low concentrations and fluxes of atmospheric NH3. The available background concentrations and flux measurements of ammonia in natural ecosystems are then described and used to infer upper limits for the emissions of NH3into the atmosphere from these systems. While the magnitude of both anthropogenic and natural emissions of NH3remain uncertain, it appears that unperturbed terrestrial ecosystems are generally more important as sinks rather than sources for atmospheric NH3. However, net emissions are likely from many eastern forests and other ecosystems exposed to large inputs of atmospheric sulfate.
What atmospheric oxygen measurements can tell us about the global carbon cycle
Authors: Keeling Ralph F. , Najjar Raymond P. , Bender Michael L. , Tans Pieter P.
Published: February 2008

This paper explores the role that measurements of changes in atmospheric oxygen, detected through changes in the O2/N2 ratio of air, can play in improving our understanding of the global carbon cycle. Simple conceptual models are presented in order to clarify the biological and physical controls on the exchanges of O2, CO2, N2, and Ar across the air‐sea interface and in order to clarify the relationships between biologically mediated fluxes of oxygen across the air‐sea interface and the cycles of organic carbon in the ocean. Predictions of large‐scale seasonal variations and gradients in atmospheric oxygen are presented. A two‐dimensional model is used to relate changes in the O2/N2 ratio of air to the sources of oxygen from terrestrial and marine ecosystems, the thermal ingassing and outgassing of seawater, and the burning of fossil fuel. The analysis indicates that measurements of seasonal variations in atmospheric oxygen can place new constraints on the large‐scale marine biological productivity. Measurements of the north‐south gradient and depletion rate of atmospheric oxygen can help determine the rates and geographical distribution of the net storage of carbon in terrestrial ecosystems.
Three‐dimensional numerical modeling of earthquake migration along a northwestern Pacific Subduction Slab
Authors: Loo H. Y. , Gao X. L. , Sun J. X. , Mikumo T. , Hirahara K. , Yoshioka S.
Published: February 2008

Three‐dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern Pacific subduction slab, on the assumption that the lithosphere is a linear visco‐elastic body, but the asthenosphere and the mesosphere respond to the applied stress as non‐linear Newtonian materials obeying the power law of creep. On the basis of a three‐dimensional thermal structure of the northwestern Pacific region, the preliminarily computed stress regimes show that: 1) to induce a stick‐slip type trench event, prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required; 2) the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep‐focus earthquake; and 3) the co‐seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab.
On inertial waves in the Earth's fluid core
Authors: Zhang Keke
Published: February 2008

New classes of inertial oscillation mode in the form of equatorially trapped boundary waves are discovered. The structure and frequency of the waves are hardly affected by the presence of an inner sphere if the azimuthal wavenumber

where η = ri/ro, the radius ratio of a spherical shell. Also discovered is that equatorially symmetric inertial waves belong to particular subclasses of thermal instabilities. The problem of the inertial waves can therefore be understood within the framework of instability theory. The findings provide valuable insights into many geophysical phenomena, not only for the Earth's fluid core but atmospheres and oceans as well.
Galileo and Ulysses dust measurements: Fz Venus to Jupiter
Authors: Grün E. , Baguhl M. , Fechtig H. , Hanner M. S. , Kissel J. , Lindblad B.‐A. , Linkert D. , Linkert G. , Mann I. , McDonnell J. A. M. , Morfill G. E. , Polanskey C. , Riemann R. , Schwehm G. , Siddique N. , Zook H. A.
Published: February 2008

The Galileo and Ulysses spaceprobes carry two similar dust detectors through interplanetary space from Venus to Jupiter. We report here on impacts which correspond to dust particles above a mass threshold of about 10−13g for which we have complete records. Between December 1989 and January 1992 Galileo repeatedly traversed interplanetary space between 0.7 and 2.26 AU and recorded 374 impacts. The observed impact rates ranged from 0.1 to about 3 impacts per day strongly dependent on whether the spacecraft moved towards or away from the Sun. From October 1990 to January 1992 the Ulysses spacecraft had reached a distance of 5.17 AU from the Sun and had recorded 72 impacts at rates between 0.1 and 0.5 per day. Inside about 2 AU the observed fluxes are compatible with a population of interplanetary dust particles moving on low to moderately eccentric (e = 0.1 to 0.5) and low inclination (i = 0 deg. to 30 deg.) orbits. Outside this distance a dust particle population on different orbits is required in order to explain the Ulysses data.
A search for source side mantle anisotropy
Authors: Kaneshima Satoshi , Silver Paul G.
Published: February 2008

Shear wave splitting due to source‐side mantle anisotropy is estimated from broadband teleseismic S waves using a suite of 19 subduction zone events more than 50km depth and recorded at several RSTN stations. We assume that there are at most two anisotropic regions along the ray path, one on the source side and one on the receiver side, and that receiver‐side splitting is known from analysis of SKS. It is found that splitting is not detectable below the slab in most cases. There are, however, two specific regions, one beneath Peru and one beneath Kamchatka that indicate significant source side anisotropy. Based on two nearby intermediate focus events in each region, splitting delay times are found to be 1.0–1.5s with the fast polarization direction perpendicular to the trench. Assuming 4% intrinsic anisotropy, this would correspond to path lengths of 100–170 km below the slab. Both regions are characterized by abrupt changes in along‐strike slab properties, a change in slab dip in the case of Peru, and a cessation of deep focus seismicity in the case of Kamchatka. We speculate that the anisotropy is caused by an associated local disturbance in asthenospheric flow. That the shear‐wave splitting parameters of S and SKS are consistent with each other in most cases, suggests that large‐scale anisotropy in the lower mantle is very small, probably no more than a fraction of a percent.
Numerical simulation of plasma transport driven by the Io torus
Authors: Yang Y. S. , Wolf R. A. , Spiro R. W. , Dessler A. J.
Published: February 2008

The Rice Convection Model (RCM) has been modified to a form suitable for Jupiter (RCM‐J) to study plasma interchange motion in and near the Io plasma torus. The net result of the interchange is that flux tubes, heavily loaded with torus plasma, are transported outward, to be replaced by tubes containing little low‐energy (< 1 keV) plasma. The process is numerically simulated in terms of time evolution from an initial torus that is longitudinally asymmetric and with gradually decreasing density outward from Io's orbit. In the simulations, the nonlinear stage of the instability characteristically exhibits outreaching fingers of heavily‐loaded flux tubes that lengthen at an accelerating rate. Our principal finding is that the primary geometrical form of outward transport of torus plasma in Jupiter's magnetosphere is through long, outward‐moving fingers of plasma. In the simulations, the fingers mainly form in the active sector of the Io torus (the heavier side of the asymmetric torus), and they are spaced longitudinally roughly 20° apart.
In ecliptic observations of Jovian radio emissions by Ulysses comparison with Voyager results
Authors: Lecacheux A. , Pedersen B. M. , Zarka Ph. , Aubier M. G. , Desch M. D. , Farrell W. M. , Kaiser M. L. , MacDowall R. J. , Stone R. G.
Published: February 2008

During the Ulysses inbound cruise to Jupiter the Unified Radio and Plasma Wave (URAP) experiment observed a variety of the planet's radio components in the frequency range below 1 MHz. Most of these emissions were already detected by the Voyager Radio Astronomy (PRA) and Plasma Wave (PWS) experiments, however with much less sensitivity and different spectral coverage. We identify these different radio components within the URAP dynamic spectra and compare their appearance with the previous Voyager observations.
The effect of the Pinatubo cloud on hydrogen chloride and hydrogen fluoride
Authors: Wallace L. , Livingston W.
Published: February 2008

Ground‐based spectroscopic observations from Kitt Peak in Arizona showed no enhancement of stratospheric HCl or HF when the plume of the Pinatubo volcano was overhead.
On the potential importance of the gas phase reaction CH3O2 + ClO → ClOO + CH3O and the heterogeneous reaction HOCl + HCl → H2O + Cl2 in “ozone hole” chemistry
Authors: Crutzen P. J. , Müller R. , Brühl Ch. , Peter Th.
Published: February 2008

We call attention to the great importance of the gas phase reaction ClO + CH3O2 → ClOO + CH3O and the heterogeneous reaction HCl + HOCl → Cl2 + H2O on polar stratospheric cloud (PSC) particles. These reactions may accomplish the almost complete conversion of HCl into ClOx radicals, thus leading to rapid destruction of ozone.
Rare Earth element patterns of the Central Indian Basin sediments related to their lithology
Authors: Nath Bejugam Nagender , Roelandts Iwan , Sudhakar Maruthadu , Plüger Walter L.
Published: February 2008

Rare earth element (REE) concentrations have been determined in terrigenous, siliceous (nodule barren and nodule bearing), calcareous, and red clay from the Central Indian Basin. The bulk distribution of REE, and in particular the relative cerium fractionation, in these surface sediments has been studied in relation to bottom water conditions. REE fractionations within a single depositional environment are found to be characteristic for each sediment type, with flat shale‐normalized patterns associated with terrigenous sediments, positive Ce anomalies with siliceous sediments, negative Ce and positive Eu anomalies with calcareous sediments, and LREE‐depleted patterns with pelagic red clays. There is no relation of any REE fractionation to bottom water redox conditions. Our results indicate that the REE signatures in marine sediments are not only related to depositional setting, but also to the lithological variations, and surficial diagenetic processes. Therefore, these further constraints on understanding REE behaviour indicate that REE fractionations for paleotectonic and paleoredox reconstructions should be used with caution.
How much gabbro is in ocean seismic layer 3?
Authors: Swift Stephen A. , Stephen Ralph A.
Published: February 2008

Ocean seismic layer 3 is distinguished from layer 2 by higher velocities, lower variability, and lower gradients in velocity with depth. Based on studies of rocks recovered from ophiolites and walls of fracture zones, most models of lower ocean crust correlate seismic layer 3 with sequences of gabbroic lithologies. The average velocity (6.5 km/s) for a vertical seismic profile in a gabbro sequence at Ocean Drilling Program (ODP) Hole 735B is consistent with refraction values for layer 3. However, the Q (inverse attenuation) obtained from these data, after correction to temperature and pressure conditions in lower crust, are too low to be consistent with amplitudes of signals observed in refraction experiments. Laboratory measurements of Q on gabbros from Hole 735B and ophiolites are also one to two orders of magnitude lower than seismic refraction Q. These results indicate that the gabbro sequence at Hole 735B cannot be typical of seismic layer 3. Based on Q, upper layer 3 may be metadolerite, and lower layer 3 may include interbedded gabbros and ultramafics. Serpentine is highly attenuating and is unlikely to be a major component.
Estimating the Sun's radiative output during the Maunder Minimum
Authors: Lean Judith , Skumanich Andrew , White Oran
Published: February 2008

The coincidence between the Maunder Minimum of solar magnetic activity from 1645 to 1715 and the coldest temperatures of the Little Ice Age raises the question of possible solar forcing of the Earth's climate. Using a correlation which we find between measured total solar irradiance (corrected for sunspot effects) and a Ca II surrogate for bright magnetic features, we estimate the Sun's radiative output in the absence of such features to be 1365.43 w/m2, or 0.15% below its mean value of 1367.54 W/m2 measured during the period 1980 to 1986 by the ACRIM experiment. Observations of extant solar‐type stars suggest that the Ca II surrogate vas darker during the Maunder Minimum. Allowing for this, we estimate the total solar irradiance to be 1364.28 W/m2 or 0.24% below its mean value for the 1980 to 1986 period. The decrease in the global equilibrium temperature of the Earth due to a decrease of 0.24% in total solar irradiance lies in the range from 0.2° C to 0.6° C, which can be compared with the approximately 1° C cooling experienced during the Little Ice Age, relative to the present.
An implosive component in the seismic moment tensor of a mining‐Induced tremor
Authors: McGarr A.
Published: February 2008

In early 1988, a special study in one of the major South African gold fields yielded seismograms that indicate seismic moment tensors having substantial implosive components. The moment tensor, resulting from the inversion of the ground motion data from the best‐recorded event, was decomposed into isotropic and deviatoric components from which both the coseismic volumetric closure ΔV and the total shear deformation AD, where A is fault area and D is average slip, could be estimated. The finding here that ΔV ∼ AD is consistent with earlier analyses of how the tabular mine stopes interact with the surrounding rock mass to produce seismic deformation.
Deep velocity structure of rifted continental crust, U.S. Mid‐Atlantic Margin, from wide‐angle reflection/refraction data
Authors: Holbrook W. Steven , Purdy G. M. , Collins J. A. , Sheridan R. E. , Musser D. L. , Glover L. , Talwani M. , Ewing J. I. , Hawman R. , Smithson S. B.
Published: February 2008

We present new ocean‐bottom, wide‐angle seismic data, collected during the 1990 EDGE Mid‐Atlantic multichannel seismic experiment, which provide a measurement of the deep velocity structure of rifted Appalachian continental crust beneath the U.S. East Coast continental margin. Reflections from the entire crust and Moho are visible from offsets of zero to 100 km. One‐dimensional inverse traveltime modeling reveals a 34‐km‐thick crust consisting of four layers beneath the post‐rift sediments, with velocities of 5.9, 6.3, 6.4, and 6.85 km/s. This velocity structure is indistinguishable from that found beneath several Appalachian terranes in the northern Appalachians of New England. The average velocity of 6.85 km/s in the lower crust limits the landward extent of the high‐velocity (7.2‐7.4 km/s) lower crust found farther seaward, and supports the interpretation of that layer as the result of rift‐related magmatic underplating, rather than as thinned, pre‐existing lower continental crust.
Upper limit for stratospheric HBR using far‐infrared thermal emission spectroscopy
Authors: Traub W. A. , Johnson D. G. , Jucks K. W. , Chance K. V.
Published: February 2008

We measure an upper limit for stratospheric HBr from three balloon flights. The observations were made with the FIRS‐2 far‐infrared Fourier transform spectrometer. The 1σ upper limits from the 1988, 1989 and 1990 balloon flights are 13 pptv at 35 km, 7 pptv at 32 km, and 3 pptv at 31 km, respectively. Combining all 3 flights, the weighted average 1σ upper limit for HBr is 4 pptv at 32 km. This value is significantly smaller than the only other previously published spectroscopic value of 20 ± 7 pptv (2σ), but is consistent with (albeit well in excess of) a theoretical estimate which predicts roughly 0.4 pptv at this altitude.
Influence of the starting date of model integration on projections of greenhouse‐gas‐induced climatic change
Authors: Fichefet Thierry , Tricot Christian
Published: February 2008

The influence of the starting date of model integration on time‐dependent projections of greenhouse‐gas‐induced climatic change is assessed with a coupled atmosphere‐ocean energy‐balance model. It is shown that the starting date effect depends on the rate of forcing change and on the oceanic thermal inertia. On the other hand, the model results indicate that, for the various cases considered here, reliable projections of green‐house‐gas‐induced climatic change over the next century can be obtained by starting the model integration as late as the year 1960.
Staircase ion signature in the polar cusp: A case study
Authors: Escoubet C. P. , Smith M. F. , Fung S. F. , Anderson P. C. , Hoffman R. A. , Basinska E. M. , Bosqued J. M.
Published: February 2008

On 15 October 1981 Dynamics Explorer 2 crossed the polar cusp at 1015 MLT and observed three distinct ion populations as it was moving poleward. These three populations had peak‐flux energy around 2.7 keV, 850 eV and 360 eV. At the time of observation, the IMF was southward. The first step coincided with a rotation of the flow from westward to poleward and then eastward. The second and third steps showed a flow directed principally poleward. Furthermore, the magnetic and electric perturbations in the first step are well fitted by an elongated flux tube footprint model. These results suggest that three consecutive Flux Transfer Events (FTEs) have injected solar wind plasma into the ionosphere forming the polar cusp. The individual FTE signatures can only be identified by the jumps in the precipitation pattern. The newest reconnected FTE footprint was crossed near the edge of the event while the two oldest ones were crossed around the event center. The small latitudinal size of these FTE footprints (∼ 40 km) and their short recurrence rate (3, 6 min) is consistent with an intermittent reconnection taking place at the subsolar point on a short time scale.
The use of atmospheric cosmogenic 35S and 7Be in determining depositional fluxes of SO2
Authors: Turekian Karl K. , Tanaka Noriyuki
Published: February 2008

We explore the use of two cosmogenically produced nuclides in the atmosphere, 35S (half‐life = 87 days) and 7Be (half‐life = 53 days), as tracers of gaseous SO2 and aerosol SO4 and Be removal processes to the Earth's surface. Based on 35S and 7Be wet precipitation fluxes and 35SO2, 35SO4 and 7Be concentrations in air samples in the planetary boundary layer at New Haven, Connecticut (USA) we determine coefficients for incloud scavenging of 35SO2, oxidation of 35SO2 to 35SO4 in both the planetary boundary layer and the free troposphere, and dry deposition. In addition, the distributions of 35SO2, 35SO4 and 7Be between the planetary boundary layer and the free troposphere are determined as well as the air exchange coefficient between the two reservoirs. Application to stable S yields a dry deposition flux to total flux ratio of about 0.20 for August, 1990.
Broadband seismic observation at the Sakurajima Volcano, Japan
Authors: Kawakatsu Hitoshi , Ohminato Takao , Ito Hisao , Kuwahara Yasuto , Kato Takashi , Tsuruga Kayoko , Honda Satoru , Yomogida Kiyoshi
Published: February 2008

We installed a portable broadband seismometer (Streckeisen STS‐2) at the Sakurajima volcano, which has been very active in the recent years. The recorded seismograms show a wide variety (both in temporal and spectral contents) of seismic events, from explosions to tremors, and exhibit the importance of such broadband seismometry at volcanos. We present examples of seismograms to show the potential of broadband seismic observation in monitoring volcanic activities.
Ultraviolet imaging of the Jovian aurora with the Hubble Space Telescope
Authors: Dols V. , Gérard J. C. , Paresce F. , Prangé R. , Vidal‐Madjar A.
Published: February 2008

We present here for the first time a Lyman‐α image of the north polar region of Jupiter obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope a few hours after the encounter of the ULYSSES spacecraft with Jupiter. The presence of high latitude regions of enhanced emission is clearly observed. A comparison with the location of the “UVS oval”, the Io (L = 6) and high‐latitude field‐line footprints shows that the best agreement is obtained with the L ≥ 15 footprint and the UVS oval which are close to each other for the particular longitudinal sector (30° < λIII < 210°) observed. These two L‐shells correspond to two possible sources of precipitation: particles originating respectively from the region of the plasma torus of Io in a distorted magnetic field or particles from the distant magnetosphere by analogy with the terrestrial aurora. The first direct determination of the latitudinal extent of the oval and of its intensity is made and compared with previous estimates.
Broadband waveform modelling of anomalous strong ground motion in the 1989 Loma Prieta Earthquake using three‐dimensional geologic structures
Authors: Lomax Anthony J. , Bolt Bruce A.
Published: February 2008

Strong motion recordings of the 1989 Loma Prieta earthquake show a large, coherent SH wave displacement pulse across much of the San Francisco Bay area. This distinctive ground motion has been modelled using approximate, broadband synthetics for a double‐couple centroid source in a 3‐D crustal model for the region. The synthetics indicate that (1) horizontal bending and focusing of SH waves by the lateral crustal velocity contrast across the San Andreas fault and (2) the source radiation pattern, contribute significantly to the observed azimuthal variation of the SH pulse and hence to the regional variations of strong ground shaking in the period range of 1 to 8 sec. In addition, the scattering of surface‐reflected shear waves by deep basins under the Santa Clara Valley may contribute to increased coda amplitude and duration observed for sites around Oakland.

The wave modelling also supports the earlier conclusion that an increase in intensity in portions of San Francisco and Oakland is in part the result of energy reflected from the base of the crust [Somerville and Yoshimura, 1990].
Contribution to the cloud droplet effective radius parameterization
Authors: Pontikis Constantin , Hicks Elizabeth
Published: February 2008

An analytic cloud droplet effective radius expression is derived and validated by using field experiment microphysical data. This expression shows that the effective radius depends simultaneously upon the cloud liquid water content, droplet concentration and droplet spectral dispersion. It further suggests that the variability in these parameters present at all scales, due to turbulent mixing and secondary droplet activation, could limit the accuracy of the effective radius parameterizations used in climate models.
Drainage, topographic, and gravity anomalies in the Lake Superior Region: Evidence for a 1100 Ma mantle plume
Authors: Allen David J. , Hinze William J. , Cannon William F.
Published: February 2008

A topographic dome, radial drainage pattern, and regional negative gravity anomaly, all centered on Lake Superior, are vestiges of a 1100 Ma mantle plume that formed the Midcontinent Rift System of central North America. The topographic and gravimetric relations suggest that the dome is maintained isostatically by fundamental changes imprinted on the lithosphere by the plume, including magmatic underplating of the crust and depletion of the upper mantle. These changes occurred at distances of up to 500 km or more from the center of the plume, well beyond the margins of the rift.
Limits on plasma anisotropy in a tail‐like magnetic field
Authors: Hill T. W. , Voigt G.‐H.
Published: February 2008

The condition of magnetohydrostatic equilibrium implies tight constraints on the degree of anisotropy that is supportable in a magnetotail field geometry. If the plasma pressure tensor is assumed to be gyrotropic at the tail midplane (z=0), then equilibrium requires that it also be nearly isotropic there, with P⟂o/P//o in the range 1 ± δ2 where δ∼0.1 is the ratio of the normal field component at the symmetry plane to the field strength in the tail lobe. The upper and lower limits are essentially equivalent, respectively, to the marginal mirror and firehose stability conditions evaluated at z=0, which have been invoked previously to limit the degree of anisotropy in the plasma sheet.
Distribution function of precipitating ion beams with velocity dispersion observed near the poleward edge of the nightside auroral oval
Authors: Saito Y. , Mukai T. , Hirahara M. , Machida S. , Kaya N.
Published: February 2008

Using low energy ion data obtained by Akebono satellite, we have calculated distribution functions of velocity‐dispersed ion beams observed at the poleward edge of the auroral electron precipitation region. The calculated distribution functions can well be fitted by one‐dimensional shifted‐Maxwellians, whose bulk energy and temperature are several keV and several hundreds of eV, respectively. The bulk energy and temperature show a positive correlation, which may indicate that when the ions are accelerated to higher energy, they are heated to higher temperature simultaneously. We have also found a relation between the invariant latitude width of the observed ion beams divided by the square root of the temperature and their bulk velocity, which indicates that the source region of the ion beam is compact. These ion beams are obtained with high occurrence probability, suggesting that they are supplied from a steady X‐type neutral line in the earth's magnetotail.
A rupture model for the 28 June 1992 Landers, California, Earthquake
Authors: Campillo Michel , Archuleta Ralph J.
Published: February 2008

We have modeled displacement time histories to infer the large‐scale rupture process of the 28 June 1992 M 7.4 Landers, California, earthquake. We have used integrated accelerograms from four TERRAscope stations at distances between 70 and 150 km. The earthquake process consists of unilateral rupture propagation, 3 km/s, on two distinct segments with different strikes: N10°W and N40°W. The two segments are 20 and 30 km long with constant slip of 3.5 m and 5.2 m, respectively. The excitation of surface waves, resulting from a low‐velocity surficial layer, plays a critical role in matching the synthetic waveforms to the observed displacements. The displacements, due to both body waves and surface waves, are strongly affected by directivity. Matching the synthetics to the data requires a one second delay between the end of rupture on the first segment and the initiation of rupture on the second segment. The seismic moment of the subevents are 2.8 × 1019 N‐m and 8.4 × 1019 N‐m, leading to a total moment of 1.12 × 1020 N‐m (M=7.4).
Seismic anisotropy in the Guerrero Gap, Mexico: Preliminary results
Authors: Vázquez‐Contreras A. , Booth D. C. , Zúñiga F. R. , Chesnokov E. , Crampin S. , Meissner R.
Published: February 2008

Three component seismograms of low magnitude earthquakes (Md < 3) recorded at the central station Tetitlan of the seismic network located in the Guerrero gap, Mexico show evidence of shear wave splitting. The data set contains several months of activity from the installation of the network in 1987 until late 1989. Even though the sampling rate is only 75 samples/sec and the signal to noise ratio is not very large, it was possible to estimate in most cases the polarization direction of the first shear wave as well as shear‐wave splitting time delays. Most polarizations show a direction which correlates with the main compressive stress in the area due to subduction of the Cocos Plate beneath the North American Plate. Two groups of hypocenters can be distinguished: a shallow group up to 25 Km in depth and a deeper group from 32 to 45 Km. As there is no difference in the polarization direction pattern of both groups and the time delays do not increase with depth, the shear wave splitting is interpreted in terms of an anisotropic layer above 25 km in depth. Filtering the seismograms to enhance the peak amplitude frequency band does not change the observed polarization directions but helps identify shear wave splitting.
High pressure and high temperature in situ X‐ray observation of MgSiO3 Perovskite under lower mantle conditions
Authors: Funamori Nobumasa , Yagi Takehiko
Published: February 2008

High pressure and high temperature in situ X‐ray observations of MgSiO3 perovskite were carried out using a newly developed Drickamer‐type apparatus combined with synchrotron radiation. Variations of the unit cell parameters of the perovskite were observed as a function of temperature up to about 1900 K at 36 GPa. The orthorhombic distortion changed very little and no indication of transformations to higher symmetry phases was observed up to 1900 K. This result indicates that the orthorhombic perovskite is the stable phase of MgSiO3 in the most part of the lower mantle. The thermal expansivity of the perovskite is determined to be larger than 1.7(2) × 10−5 /K at 36 GPa and room temperature. This value will give an important constraint to the chemical composition of the lower mantle.
Solar EUV index for aeronomical studies at Earth from Langmuir probe photoelectron measurements on the Pioneer Venus Orbiter
Authors: Hoegy W. R. , Mahajan K. K.
Published: February 2008

We present a solar EUV index for aeronomical studies at Earth, obtained from the Langmuir probe measurement of photoelectron current on the Pioneer Venus orbiter (PVO). When the probe is in the solar wind and is negatively biased, the current, Ipe, is mainly due to the impact of solar EUV photons with energies above the work function of the probe. Ipe thus measures the total EUV flux over the wavelength interval from about 30 nm to Lyman alpha; on Venus this solar flux, VEUV, was defined by Brace et al. (1988). When translated to the solar longitude of Earth, for Earth use, it is called EEUV. To examine the potential of EEUV as a solar EUV flux index, we study the behavior of ionospheric parameters ƒ0E, ƒ0F1, and ƒ0F2 at mid‐latitude stations and compare their relationship with EEUV and with the 10.7‐cm solar radio flux. We find ƒ0F1 and ƒ0F2 to be better correlated with EEUV than with the 10.7‐cm flux. However, ƒ0E is better correlated with the 10.7‐cm flux, because the 10.7‐cm flux is also a proxy for soft X rays, which are an important ionizing source in the E region. We also provide a table of the EUV index, EEUV, for the period February 12, 1979, through most of 1991 which extends in time the table (of Venus values) published earlier by Brace et al. (1988). The EUV index will continue to grow until the end of the PVO mission in late 1992.
Equatorial bubbles updrafting at supersonic speeds
Authors: Aggson Thomas L. , Burke William J. , Maynard Nelson C. , Hanson William B. , Anderson Philip C. , Slavin James A. , Hoegy Walter R. , Saba Jack L.
Published: February 2008

We present plasma and electric field observations from two satellite encounters with equatorial plasma bubbles updrafting at velocities of ∼2 km/s. These large, upward velocities are consistent with an adaptation of Chandrasekhar's model for the motion of plasma blobs supported against gravity by a magnetic field; that is, Vz ≈ −g. Vector magnetic field measurements, available during one of the bubble encounters show a perturbation of ∼150 nT, directed radially outward from the Earth, near the western wall of deepest plasma depletion. This magnetic variation is too large to be caused by simple shunting of the g × B current along the bubble's edge. Rather, it is Alfvénic in nature, radiating from a generator located near the magnetic equator, in the plasma outside the bubble's leading edge. A heuristic model of a depleted flux tube with constant circular cross section moving upward through a background plasma predicts most of the measurements' qualitative features.
Onset conditions for equatorial spread F
Authors: Mendillo Michael , Baumgardner Jeffrey , Pi Xiaoqing , Sultan Peter J. , Tsunoda Roland
Published: February 2008

The problem of day‐to‐day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two‐night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all‐sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300‐Å airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh‐Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low‐latitude thermospheric winds.
Semiannual fluctuation and efficiency factors in Sun‐weather relations
Authors: Baranyi T. , Ludmány A.
Published: February 2008

The changing nature of the Sun‐weather relations over a 119‐year period has been studied by using the geomagnetic aa index. The analysis yields consistent results on several time scales. The most important fluctuation found is a semiannual variation which can be related to the well‐known variation of the geomagnetic activity. A threshold value of about aa≅19–20 nT is necessary to detect any solar influence on the weather. The role of the aa index (i.e., corpuscular influences) seems to be determinant in certain cases.
Velocity shear generation of solar wind turbulence
Authors: Roberts D. Aaron , Goldstein Melvyn L. , Matthaeus William H. , Ghosh Sanjoy
Published: February 2008

We use a two‐dimensional, incompressible MHD spectral code to establish that shear‐driven turbulence is a possible means for producing many observed properties of the evolution of the magnetic and velocity fluctuations in the solar wind and, in particular, the evolution of the cross helicity (“Alfvénicity”) at small scales. We find that large‐scale shear can nonlinearly produce a cascade to smaller scale fluctuations even when the linear Kelvin‐Helmholtz mode is stable and that a roughly power law inertial range is established by this process. While the fluctuations thus produced are not Alfvénic, they are nearly equipartitioned between magnetic and kinetic energy. We report simulations with Alfvénic fluctuations at high wave numbers, both with and without shear layers and find that it is the low cross helicity at low wave numbers that is critical to the cross helicity evolution, rather than the geometry of the flow or the dominance of kinetic energy at large scales. The fluctuations produced by shear effects are shown to evolve similarly but more slowly in the presence of a larger mean field and to be anisotropic with a preferred direction of spectral transfer perpendicular to the mean field. The evolution found is similar to that seen in some other simulations of MHD turbulence, and thus seems in many respects to be a instance of a more generic turbulent evolution rather than due to specific conditions in the solar wind.
Loss of ring current O+ ions due to interaction with Pc 5 waves
Authors: Li Xinlin , Hudson Mary , Chan Anthony , Roth Ilan
Published: February 2008

The behavior of ring current ions in low‐frequency geomagnetic pulsations is investigated analytically and numerically. We focus primarily on ring current O+ ions, whose flux increases dramatically during geomagnetic storms and decays at a rate which is not fully explained by collisional processes. This paper presents a new loss mechanism for the O+ ions due to the combined effects of convection and corotation electric fields and interaction with Pc 5 waves (wave period: 150–600 s) via a magnetic drift‐bounce resonance. A test particle code has been developed to calculate the motion of the ring current O+ ions in a time‐independent dipole magnetic field, and convection and corotation electric fields, plus Pc 5 wave fields, for which a simple analytical model has been formulated based on spacecraft observations. For given fields, whether a particle gains or loses energy depends on its initial kinetic energy, pitch angle at the equatorial plane, and the position of its guiding center with respect to the azimuthal phase of the wave. The ring current O+ ions show a dispersion in energies and L values with decreasing local time across the day side, and a bulk shift to lower energies and higher L values. The former is due to the wave‐particle interaction causing the ion to gain or lose energy, while the latter is due to the convection electric field. Our simulations show that, due to interaction with the Pc 5 waves, the particle's kinetic energy can drop below that required to overcome the convection potential and the particle will be lost to the dayside magnetopause by a sunward E × B drift. This may contribute to the loss of O+ ions at intermediate energies (tens of keV) observed during the recovery phase of geomagnetic storms.
Observations of plasma waves in the solar wind interaction region of comet Giacobini‐Zinner at high time resolution
Authors: Moses S. L. , Coroniti F. V. , Greenstadt E. W. , Tsurutani B. T.
Published: February 2008

High‐time‐resolution spectra of plasma wave emissions detected in the interaction region of comet Giacobini‐Zinner with the solar wind reveal a wave phenomenology much more complicated than first reported. Spectra often exhibit three or more independent peaks, which become more prominent the deeper into the interaction region the spacecraft traversed. The main peaks correspond to whistler emissions below the electron cyclotron frequency, a midfrequency peak near the maximum Doppler shift frequency for waves with kλD= 1, a high‐frequency peak above the Doppler shift maximum frequency, and electron plasma oscillations at the plasma frequency. Similar multipeaked spectra are also observed downstream from weak shocks at Earth, which suggests that the plasma wave generation mechanisms responsible need not require particle populations created by photoionization.
O mode decay and upshifted electromagnetic emissions near cyclotron harmonics in the ionosphere
Authors: Tripathi V. K. , Liu C. S.
Published: February 2008

Near upper hybrid resonance an O mode launched from a ground‐based transmitter undergoes two possible parametric decay instabilities: (1) a resonance instability producing field‐aligned dc density striations and an upper hybrid wave and (2) a parametric decay instability producing lower hybrid and electron Bernstein modes. A nonlinear coupling between the lower and upper hybrid waves produces O mode sideband radiation. The lower sideband is severely cyclotron damped, while the upper sideband is weakly damped and hence observed on the ground. This explains some of the experimental results reported from the Tromso facility.
A simple model of Birkeland currents
Authors: Stern David P.
Published: February 2008

A simple representation of the global circuit of Birkeland currents is developed, based on a representation of the current density j in terms of Euler potentials (αχ). The underlying magnetic field, which shares with j the potential α, is assumed to be dipolar, making the model applicable mainly to region 2 Birkeland currents, although a similar approach could also be used for region 1. A form of j is chosen that gives a current sheet with peak outflow at dawn and peak inflow at dusk (or vice versa), connected across a flat polar cap sheet. The current is further assumed to flow in a thin layer enclosing the dipole fieldline surface L=10, and to close symmetrically across noon and midnight. To produce the asymmetry expected of the partial ring current, a (full) ring current jRC is added to the configuration, and a suitable function G(γ) defining the currents was selected. The field of the current sheet is then numerically derived by Biot‐Savart integration and the magnetic scalar potential Ψ, which consists of a cosϕ component and an axisymmetrical one, is approximated in three regions of space. It is found that the superposition of harmonics of the same type, centered at different “foci,” provided a flexible and powerful representation of harmonic functions, accurate here within less than 1%. Appendix A discusses the Euler potentials of the magnetic field B and Appendix B develops an interpolation formula by which current sheets of finite width could be consistently represented. Future plans are described for deforming configurations of this type to represent region 1 currents and for bypassing the tedious Biot‐Savart integration.
Diurnal variation in Jovian subsolar magnetopause position
Authors: Engle Irene M.
Published: February 2008

The calculation of the idealized Jovian magnetosphere surface and magnetic fields arising from surface currents has been published (Engle, 1991). That calculation assumed the axis of all the interior sources of magnetic field to be perpendicular to the planet's orbital plane and hence to the incident solar wind. The rotation axis of Jupiter makes a small angle (3.13°) with its orbital plane; the magnetic axis is aligned approximately 10° with respect to the planet's rotation axis. The consequences are that the magnetic axis will precess about the planetary rotation axis during each diurnal rotation of period 9 hours and 55 min. The goal of the present work was to calculate the primary effects of this diurnal motion. In this paper are presented the results of calculations which have yielded global magnetopause models with the interior sources' axis tilted ±10° (in the noon‐midnight plane) with respect to the normal to the “plane” of the incident solar wind (which is here approximated to be coincident with the planet's equatorial plane). The consequential diurnally cyclical range in predicted dynamically stable magnetopause positions in the neighborhood of the subsolar point is reported. The magnetic fields arising from the magnetohydrodynamically stable currents on the magnetopause have been calculated. Convenient expressions for those fields' total contributions for points in the inner magnetosphere are included.