bluuebunny/crossref_metadata_2025 huggingface dataset:
Separation of local signals from the regional paleomonsoon record of the Chinese Loess Plateau: A rock‐magnetic approach
Authors: Banerjee Subir K. , Hunt Christopher P. , Liu Xiu‐Ming
Published: February 2008

We propose a method based on thermal unblocking of low‐temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic [Cullity, 1972] fraction (size, d < 30 nm) of magnetite produced by pedogenesis in the Chinese loess plateau [Liu, 1988]. We applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart, but separated by the mountain range Liupan‐shan. Xifeng to the east (35.7°N, 107.6°E) and Baicaoyuan to the west (36.2°N, 105.0°E) currently have humid and arid microclimates, respectively. As expected, the superparamagnetic fraction increases during known warm temperature intervals at each site. Furthermore, the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka. However, during the period 5 to 10 ka ago, the relative humidity at both sites was the same within experimental errors. Bulk grain size evidence confirms the magnetic data, and we suggest that the present easterly summer monsoon in China came from a more southerly direction during this time to flow parallel to Liupan‐shan, resulting in very similar summer humidity at Xifeng and Baicaoyuan.
Analysis of selected nitric oxide observations in the lower thermosphere by HALOE on UARS
Authors: Luo Mingzhao , Russell James M. , Cicerone Ralph J. , Gordley Larry L.
Published: February 2008

Measurements of nitric oxide mixing ratios have been made by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite over an altitude range extending from 15 to 130 km on every spacecraft sunrise and sunset event with the exception of about 8% downtime. Selected NO profiles from the first few months of observations were examined with emphasis on the relation between geomagnetic disturbances and enhanced NO in the lower thermosphere. During a large geomagnetic storm on November 8–9, 1991, HALOE obtained 15 NO sunrise profiles along the ∼50°N latitude circle. A factor of 3 increase in NO mixing ratio above ∼115 km at this latitude and a one to two day decay time were observed. The measured NO longitude/altitude cross section on this day shows a longitudinal asymmetry in the NO distribution with a maximum near the geomagnetic pole. From approximately one month's data, a latitude/altitude cross section of zonal mean NO mixing ratio was calculated. An example of such cross sections demonstrates a strong positive correlation between geomagnetic activity and thermospheric NO globally. This auroral forcing has a larger effect on increasing NO at high latitude than at low latitude.
Ulysses observations of auroral hiss at high Jovian latitudes
Authors: Farrell W. M. , MacDowall R. J. , Desch M. D. , Kaiser M. L. , Stone R. G. , Kellogg P. J. , Lin N. , Cornilleau‐Wehrlin N. , Canu P. , Bame S. J. , Phillips J. L.
Published: February 2008

During the Ulysses flyby of Jupiter, a whistler‐mode emission was periodically detected by the Unified Radio and Plasma wave (URAP) experiment during intervals when the spacecraft extended to high magnetic latitudes. The signal was detected between the local electron plasma frequency and lower hybrid resonance and appears as a funnel‐shaped structure on frequency‐versus‐time spectrograms; these characteristics are very reminiscent of whistler‐mode auroral hiss observed at high latitudes at Earth. Ray tracing of the emission occurrences suggests the emission source is on magnetic field lines extending out to at least 65 RJ. This location associates the emission with the boundary between open and closed field lines — not the Io torus. The emission radiates about 107 W of power. Consequently, the auroral input power derived from the solar wind to drive the emission is believed to be 1010‐12 W (or about 1% of the energy associated with Io torus electrical processes).
The maximum distortional strain energy density criterion for shear fracture propagation With applications to the growth paths of En Échelon faults
Authors: Du Yijun , Aydin Atilla
Published: February 2008

A maximum distortional strain energy density criterion is proposed for shear fracture propagation under mixed mode loading conditions. It is shown that an isolated mode‐II shear fracture would propagate in its own plane. However, the growth paths of two en échelon shear fractures deviate from straight lines due to crack interaction. Using a displacement discontinuity boundary element method, we show that the shear fracture propagation paths of two en échelon mode‐II faults always converge toward each other regardless of the sense of shear and fault step. In contrast, opening (mode‐I) fracture growth paths of two en échelon faults depend on the sense of stepping and shearing. The predicted propagation paths of en échelon shear fractures agree well with those observed in the field.
The cosmic ray produced ³He/21Ne ratio in ultramafic rocks
Authors: Staudacher Thomas , Allègre Claude J.
Published: February 2008

Although production of noble gases in meteorites are well studied since many years, [e.g. Bogard and Cressy, 1973] the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by Kurz and by Craig and Poreda. Later, Marti and Craig (1987) reported cosmic‐ray produced neon in summit lavas from Maui. More data on cosmogenic helium and neon were reported by Staudacher and Allègre (1991) on ultramafic nodules from Asia. The absolute production rate of cosmic‐ray produced helium has been determined by Kurz (1986, 1987) in olivine rich basalts from Hawaii with known K‐Ar or 14C ages. The resulting ³Hec production rate is 3.72×10−18cm³STP/g/a. This allows cosmogenic ³He to be used to determine cosmic ray exposure ages of surface samples. For neon, no such absolute production rate has yet been determined. In order to develop a complementary dating method using cosmogenic 21Ne, Staudacher and Allègre (1991) measured the cosmogenic ³He/21Ne ratio in ultramafic nodules. Their value was 1.4, which is significantly smaller than the value of 3.5, obtained in this work.

Here we discuss the cosmogenic ³He/21Ne ratio in view of new noble gas data on olivine separates of oceanites from Réunion Island and on ultramafic nodules from Mongolia.
VLF imaging of the Venus foreshock
Authors: Crawford G. K. , Strangeway R. J. , Russell C. T.
Published: February 2008

VLF plasma wave measurements obtained from the Pioneer Venus Orbiter Electric Field Detector (OEFD) have been used to construct statistical images of the Venus foreshock. Our data set contains all upstream measurements from an entire Venus year (∼200 orbits). Since the foreshock VLF characteristics vary with IMF orientation we restrict the study to IMF orientations near the nominal Parker spiral angle (25° to 45°). Our results show a strong decrease in 30 kHz wave intensity with both foreshock depth and distance. There is also an asymmetry in the 30 kHz emissions from the upstream and downstream foreshocks. The ion foreshock is characterized by strong emissions in the 5.4 kHz OEFD channel which are positioned much deeper in the fore‐shock than expected from terrestrial observations. No activity is observed in the region where field aligned ion distributions are expected. ULF wave activity, while weaker than at Earth, shows similar behavior and may indicate the presence of similar ion distributions.
Observations of the Kaiser Effect under multiaxial stress states: Implications for its use in determining in situ stress
Authors: Holcomb David J.
Published: February 2008

Experimental tests of the Kaiser effect, the stress‐history dependence of acoustic emission production, show that interactions between principal stresses cannot be ignored as is commonly done when trying to use the Kaiser effect to determine in situ stress. Experimental results obtained under multiaxial stress states are explained in terms of a qualitative model. The results show that the commonly‐used technique of loading uniaxially along various directions to determine stress history must be reevaluated as it cannot be justified in terms of the laboratory experiments. One possible resolution of the conflict between laboratory and field results is that the Kaiser effect phenomenon observed in cores retrieved from the earth is not the same phenomenon as is observed in rock loaded under laboratory conditions.
Rate constants for the reactions of OH with HFC‐134a (CF3CH2F) and HFC‐134 (CHF2CHF2)
Authors: DeMore W. B.
Published: February 2008

Rate constants for the reactions of OH with HFC‐134a (CF3 CH2F) have been measured relative to CH3CCl3, CH4 and HFC‐125 (CF3CHF2). Rate constants for HFC‐134 (CF2HCF2H) were measured relative to CH3CCl3, HFC‐125, and HFC‐134a. The measurements were made in a slow‐flow, temperature controlled photochemical reactor, and were based on relative rates of disappearance of the parent compounds as measured by FTIR spectroscopy. The temperature range was 298–358 K. Hydroxyl radicals were generated by 254 nm photolysis of O3 in the presence of water vapor. Using current NASA/JPL rate constants for the reference compounds, temperature dependent rate constants of both compounds were derived. Rate constants obtained from the different reference compounds are in excellent agreement. The presently recommended rate constant for HFC‐134a [JPL 92–20, 1992; IUPAC, 1992] is too high by about 25%.
Flood basalts and extinction events
Authors: Stothers Richard B.
Published: February 2008

The largest known effusive eruptions during the Cenozoic and Mesozoic Eras, the voluminous flood basalts, have long been suspected as being associated with major extinctions of biotic species. Despite the possible errors attached to the dates in both time series of events, the significance level of the suspected correlation is found here, by an objective, direct method, to be 1% to 4%. Statistically, extinctions lag eruptions by a mean time interval that is indistinguishable from zero, being much less than the average residual derived from the correlation analysis. Oceanic flood basalts, however, must have had a different biological impact, which is still uncertain owing to the small number of known examples and differing physical factors. Although not all continental flood basalts can have produced major extinction events, the non‐correlating eruptions (including most or all of the oceanic flood basalts) may have led to smaller marine extinciton events that terminated at least some of the less catastrophically ending geologic stages. Consequently, the 26 Myr quasiperiodicity seen in major marine extinctions may be only a sampling effect, rather than a manifestation of underlying periodicity.
Hack's relation and optimal channel networks: The elongation of river basins as a consequence of energy minimization
Authors: Ijjasz‐Vasquez Ede J. , Bras Rafael L. , Rodriguez‐Iturbe Ignacio
Published: February 2008

As pointed by Hack (1957), river basins tend to become longer and narrower as their size increases. This work shows that this property may be partially regarded as the consequence of competition and minimization of energy expenditure in river basins.
A field demonstration of variable density diffraction tomography
Authors: Witten Alan
Published: February 2008

Geophysical diffraction tomography (GDT) was first suggested in 1984 as a quantitative method for high‐resolution subsurface imaging. In this early form, the imaged quantity is the so called ”object function” which is linearly related to the square of the complex refractive index. More recently, two algorithms have been proposed for separately imaging the spatial variations of the more fundamental mechanical properties of density and compressibility based on the GDT approach, acoustic measurements, and the exploitation of multiple frequencies. It was concluded in these works that these algorithms would not be sufficiently robust for the inherently noisy field applications. The postulated problems result from the existence of the density gradient appearing in the governing equation.

In this study, these variable density algorithms were applied to acoustic data acquired at a site known to contain the buried skeletal remains of a dinosaur. For one of the algorithms tested, images of object function and normalized density and compressibility of a buried bone were reconstructed that exhibited image sharpness consistent with the frequencies employed. In addition, the images of this bone, that was subsequently excavated, properly render its size and location and yielded qualitatively correct density and compressibility.
Rotation of stress and blocks in the Lake Mead, Nevada, Fault System
Authors: Ron Hagai , Nur Amos , Aydin Atilla
Published: February 2008

The Coulomb criterion, has been widely used to infer paleostresses from fault slip data, assuming that faults are optimally oriented relative to the tectonic stress direction. Consequently if the stress direction is fixed during deformation so must be the faults. Freund [1974] has shown however that faults, when arranged in sets, must generally rotate as they slip. Nur et al. [1986] showed that large rotations require the development of new faults more favorably oriented to the principal direction of stress. This leads to older faults sets offset by younger ones, both having the same sense of slip. Because stress rotates as well, paleostress analysis must include the possible effect of stress field rotation.

The combined effects of stress field rotation and material rotation were found in the Lake Mead, Nevada Fault System (LMFS). Fault inversion results imply an apparent 60° clockwise (CW) rotation of the stress field since mid‐Miocene time. In contrast, structural data from the Great Basin suggest only a 30° CW stress field rotation. By incorporating paleomagnetic declination anomalies it is inferred that slip on faults caused a local 30° counterclockwise (CCW) rotation of blocks and faults in the Lake Mead area, so that the inferred 60° CW rotation of the stress field in the LMFS is actually only 30°.
On the structure of the magnetotail current sheet
Authors: Ashour‐Abdalla M. , Zelenyi L. M. , Peroomian V. , Richard R. L.
Published: February 2008

Results from modeling ion distribution functions in a two‐dimensional reduction of the Tsyganenko [1989] magnetic field model have enabled us to calculate the full ion pressure tensor inside the model magnetotail. A thin current sheet is formed in the distant tail and the pressure tensor within this sheet has significant off‐diagonal terms. These terms resulting from quasiadiabatic ion trajectories create azimuthally asymmetric distribution functions which are capable of maintaining stress‐balance. Outside the current sheet the off‐diagonal terms disappear and moderate anisotropy builds up with P⟂/P∥ ∼ 0.8. Closer to the Earth rapid isotropization of the distribution occurs.
On generating Kappa‐like distribution functions using velocity space Lévy flights
Authors: Collier Michael R.
Published: February 2008

A simple model using random walk jumps in velocity space whose path lengths are governed by a power law or Lévy flight probability distribution is shown to generate distribution functions that are Maxwellian at low velocities and power law at high velocities.
A general circulation model simulation of El Nino effects in the extratropical northern hemisphere stratosphere
Authors: Hamilton Kevin
Published: February 2008

This paper examines the effects of imposed tropical Pacific sea surface temperature perturbations on the December‐February mean circulation in the Northern Hemisphere stratosphere as simulated by the GFDL “SKYHI” troposphere‐stratosphere‐mesosphere GCM. The results are in impressive agreement with observational studies that have shown that, on average, ENSO events are associated with a mixed zonal wave 1 and 2 perturbation to the stratospheric stationary wave field (phased so that the Aleutian High at midstratospheric levels is intensified). This is a useful test of the sensitivity of the model stratospheric climate to changes in tropospheric forcing. These results for the effects of the Pacific SST anomalies are used in a very rough estimate of the impact of year‐to‐year changes in SST on the interannual variability of the extratropical stratospheric circulation. The results of this calculation suggest that interannual SST variation accounts for only a small fraction of stratospheric variability at high latitudes.
Variations in the thermal output of the subglacial Grímsvötn Caldera, Iceland
Authors: Björnsson Helgi , Guðmundsson Magnús T.
Published: February 2008

A 69 year record (1922–1991) of heat release has been obtained from the subglacial geothermal system of the Grímsvötn caldera within the Vatnajökull ice cap. The data were derived from in situ measurements of the volume of meltwater accumulated in the caldera lake, subtracting climatically induced melting. The overall fluctuations in the heat flux are closely related to volcanic activity and are dominated by a main pulse of 11,600 MW, caused by a major eruption in 1938, gradually declining to 1600 MW in 1976–1982. Heat extracted from the roof of a magma chamber, with the aid of hydrothermal convection, may have given a basic contribution to the heat flux of 1500 to 2000 MW (an upper bound). The variable part of the heat flux (from 0 to 10,000 MW) was released from magma erupted at the base of the glacier and from shallow intrusions. The total heat released over the period 1922 to 1991 was (8.1±1.6)×1018J, equivalent to the energy released by the solidification and cooling of 2.1±0.4 km³ of basaltic magma. The contribution to the total heat flux was 45% (max.) from a magma chamber, 35% (min.) from shallow intrusions, and 20% from eruptions. This implies that magma at the roof of a chamber solidified and cooled at the rate of 1.2−1.6×107 m³ a−1 or about 1 km³ over the last 69 years. Heat release at Grímsvötn was probably more intense in the 19th century when volcanic eruptions were more frequent.
Mantle convection flow reversals due to continental collisions
Authors: Lowman Julian P. , Jarvis Gary T.
Published: February 2008

Rigidly moving continental and oceanic plates with distinct thermal and mechanical properties, and finite thickness, are included in a two‐dimensional numerical model of mantle convection. Model plates typically span six vertical increments of a finite difference mesh. We model two identical continents being carried towards each other by a pair of underlying, counter‐rotating, mantle convection cells. Upon meeting at the model mid‐plane the model continents form a motionless, rigid, conducting, “supercontinent” along a portion of the upper boundary, while the model oceanic plates continue to move and recycle through the mantle. The resulting changes in the mechanical boundary conditions at the upper surface prove to be important factors in facilitating flow reversal below the supercontinent, leading to a subsequent dispersal of the individual continental blocks. We find the following factors to favour the development of sustained flow reversal below our model supercontinent: wider continents, lower thermal diffusivity of continental plates, thicker plates and lower proportions of internal radiogenic heating within the mantle.
Effect of depth‐dependent viscosity on the pattern of mantle convection
Authors: Cserepes László
Published: February 2008

Three‐dimensional structures of thermal convection are studied by numerical experiments in a rectangular domain of a plane fluid layer. The main question is the joint effect of the heating mode and the depth‐dependent viscosity on the planform of the circulation. Heating mode is defined by the ratio of internal and basal heat inputs. For viscosity‐depth functions, simple models of the mantle viscosity are used. If the Rayleigh number is of the order 105–106 (based on the maximum viscosity), this choice produces cellular flow with a closed network of cold descending sheets around isolated upwellings, independently of the heating mode. The central upwellings are concentrated in narrow cylindrical plumes if there is a significant basal heat input. Convection of the Earth's mantle may occur with descending currents represented by subduction and ascending currents in the form of plumes situated below hotspots.
Dynamical and radiative response to the massive injection of aerosol from Kuwait oil burning fires
Authors: Ferretti Rossella , Visconti Guido
Published: February 2008

The effects of the injection of large amount of soot comparable to that produced in the burning of oil wells in Kuwait were studied using a 2‐D mesoscale model. During the three day numerical simulation the ground‐atmosphere system appears to be strongly perturbed. A surface cooling is produced in the first two days above and downwind of the sources. The cooling, between −10 C over the desert and −0.5 C over the sea is dependent on the surface characteristics. The temperature decrease at the ground results in a stratified troposphere which inhibits convection and perturbs the normal diurnal variability of the boundary layer while the upper levels are driven by the radiative warming of the aerosol layer. In this region after few hours the simulation produces a warming of 0.8 C reaching a maximum of 6 C is after 60 hours. During the last 2 days of simulation the long wave radiation emitted by the low altitude atmospheric layers contribute to mitigate the surface cooling. A detailed discussion of the radiative and the dynamical interactions is given and it is shown that beside the specific interest in the short term effects these results may be useful to parameterize the smoke source for a GCM simulation.
Intercomparison of total ozone measured at low sun angles by the Brewer and Dobson spectrophotometers at Scott Base, Antarctica
Authors: Nichol S. E. , Valenti C.
Published: February 2008

The total ozone measurements made at Scott Base (78°S, 167°E) with Dobson spectrophotometer #17 and Brewer spectrophotometer #50 during the 1991 Antarctic ozone hole season are compared. Due to the high latitude of Scott Base, these measurements have been made using low‐sun measurement techniques, which are considered to be less reliable and less accurate. The differences between the quasi‐simultaneous total ozone measurements, and the mean daily total ozone values, from Dobson instrument 17 and Brewer instrument 50 are generally between ±5%. The Dobson‐Brewer differences can, on occasions, be as great as 10% for measurements made at airmass values between 5 and 6. The Dobson and Brewer ozone data are generally within ±5% of the Total Ozone Mapping Spectrometer (TOMS) value.
Evidence for two different structuring and scattering mechanisms and the associated role of aerosols in the polar summer mesosphere
Authors: Ulwick J. C. , Kelley M. C. , Alcala C. , Blix T. A. , Thrane E. V.
Published: February 2008

A Super Arcas rocket, MISTI B, was launched as part of the PMSE salvo to measure electron density irregularities using rf and dc probes. Large and small scale structures in the electron density were measured on rocket ascent and descent at the altitudes of 86.5 and 88.5 ±.5 km. Since the rocket apogee was 89 km, the rocket was in the height range 88.5 ±.5 km for 30 seconds giving us an unusual measurement of horizontal structure over a distance of 5.5 km. A power spectrum of the fluctuations for the upper layer gives further evidence that turbulent mixing is an important process in PMSE. The power spectrum of the lower layer, however, gives evidence that this layer is characterized by a form of partial or Fresnel scattering. Both spectra are in excellent agreement with similar analysis of electron fluctuation spectra measured in the same layers on the TURBO‐B rocket flown 12 minutes later and the analysis of CUPRI data by Cho et al., 1993A. Thus two different structuring and scattering mechanisms exist at altitudes only 1 km apart. Using the simultaneous dc and rf probe measurements of electron depletions and sharp gradients in the lower layer, we speculate on the role of aerosols in creating these depletions and gradients.
Solubility of HCL in sulfuric acid at stratospheric temperatures
Authors: Williams Leah R. , Golden David M.
Published: February 2008

The solubility of HCl in sulfuric acid was measured using a Knudsen cell technique. Effective Henry's law constants are reported for sulfuric acid concentrations between 50 and 60 weight percent and for temperatures between 220 and 230 K. The measured values indicate that very little HCl will be dissolved in the stratospheric sulfate aerosol particles.
Ulysses observations of a recurrent high speed solar wind stream and the heliomagnetic streamer belt
Authors: Bame S. J. , Goldstein B. E. , Gosling J. T. , Harvey J. W. , McComas D. J. , Neugebauer M. , Phillips J. L.
Published: February 2008

Near‐ecliptic solar wind observations by Ulysses on its way to the polar regions of the Sun, compared with those from IMP 8 at 1 AU, showed that high‐speed streams decay and broaden with heliocentric distance from IMP 8 to Ulysses, as expected. In July 1992 while travelling south at ∼13°S and 5.3 AU, Ulysses encountered a recurrent high‐speed stream, that may also have been observed at IMP 8. The stream has been observed a total of 14 times, once in each solar rotation through June 1993 at ∼34°S. The source of the high‐speed stream is an equatorward extension of the south polar coronal hole. From July 1992 through June 1993, averages of solar wind peak speed increased while density decreased with heliographic latitude. Both the stream and a low‐speed, high‐density flow, presumably associated with the heliomagnetic (coronal) streamer belt encircling the heliomagnetic equator, crossed Ulysses with the solar rotation period until April 1993 when the spacecraft was at ∼29°S heliographic latitude. After this time, as the spacecraft climbed to higher latitudes, the central portion of the streamer belt with lowest speed and highest density disappeared. Therefore, at its maximum inclination, the belt was tilted at ∼29° to the heliographic equator at this point in the solar cycle.
Effects of Pinatubo aerosol on stratospheric ozone at mid‐latitudes
Authors: Weaver A. , Loewenstein M. , Podolske J. R. , Strahan S. E. , Proffitt M. H. , Aikin K. , Margitan J. J. , Jonsson H. H. , Brock C. A. , Wilson J. C. , Toon O. B.
Published: February 2008

Mid‐latitude ozone data from ER‐2 aircraft measurements in 1989, 1991 and 1992 were examined to determine how sulfate aerosols from the eruption of Mt. Pinatubo had affected ozone at about 18 km. N2O was used as a tracer to help distinguish between chemical and dynamical aerosol effects. At 20–45°N in February 1992, ozone was about 10–20% lower than February 1989 and 1991, with respect to N2O. Data from Aug. 1991‐Mar. 1992 showed changes in ozone with respect to N2O, but the magnitude of those changes was not correlated with the magnitude of the changes in aerosol surface area density.
Properties and decay of stratospheric aerosols in the Arctic following the 1991 eruptions of Mount Pinatubo
Authors: Stone Robert S. , Key Jeffrey R. , Dutton Ellsworth G.
Published: February 2008

Sunphotometer observations made from an aircraft several months after the June 1991 eruptions of Mount Pinatubo are used to quantify the spectral opacity of the Arctic stratosphere. Ancillary surface‐based measurements are presented in support of the aircraft data that show large increases in stratospheric optical depth attributed to the presence of volcanic aerosols. Visible optical depths greater than 0.2 were observed during flight segments flown above the tropopause. An inversion algorithm and the optical depth data are used to infer effective aerosol size distributions. The distributions tend to be bimodal, having a large‐particle mode radius of about 0.50 µm and a small‐particle mode of higher concentration with radii less than 0.18 µm. Surface measurements made during spring 1992 and 1993 are also used to estimate a time constant (e‐folding time) of about 13.5 months assuming that the Arctic stratosphere's opacity decays exponentially; this estimate is larger than decay times observed following other major volcanic eruptions. Our results suggest that any climate perturbations in the Arctic caused by the eruptions of Pinatubo may be significant and will very likely persist longer than any volcanically‐induced changes observed there during the past century.
On the penetration of the 660 km phase change by mantle downflows
Authors: Bercovici David , Schubert Gerald , Tackley Paul J.
Published: February 2008

We present a simple analytic model of the interaction of cold convective downwelling currents with an endothermic phase change. The model describes the ponding and lateral spreading of downflows along the phase transition interface. A simple comparison of the vertical forces on the ponding material provides a necessary condition for a downflow to penetrate the phase boundary. This condition is fundamentally dependent on the geometry of the downflow. For planar downwellings, the model predicts a minimum ponding time before the structure can penetrate the phase boundary. For columnar (axisymmetric) downflows, there is no minimum time of spreading required before penetration can proceed. The model thus provides an explanation for the observation that in numerical models of three‐dimensional convection with an endothermic phase change, cylindrical downflows penetrate the phase interface while planar ones do not. Since descending slabs in the Earth‧s mantle display a wide spectrum of geometries between planar and cylindrical (given various trench curvatures, as well as intersections of two or more subduction zones), this phenomenon may explain, in part, why some slabs appear to extend into the lower mantle while others are deflected at the 660 km discontinuity.
Experiments on convection in a rotating hemispherical shell: Transition to chaos
Authors: Cordero Sergio
Published: February 2008

Thermal convection in a self‐gravitating rotating fluid shell is modeled using a hemispherical fluid shell that can be rotated about its axis of symmetry. In this apparatus, a tertiary convective state begins to exist at a Rayleigh number approximately equal to 2.1 times the critical value for the onset of convection (RC1). This state is characterized by the coexistence of three waves. In this tertiary state noise is always present. At a slightly higher Rayleigh number, a strong interaction was observed to develop. Frequency locking takes place at 2.4 RC1. Later, the flow exhibits chaotic behavior as shown by the broad band Fourier spectra of the temperature records. Planetary implications of these findings are discussed.
An empirical model of the size and shape of the near‐Earth magnetotail
Authors: Petrinec S. M. , Russell C. T.
Published: February 2008

A model of the size and shape of the near‐earth magnetotail which depends upon distance downtail, ρv²sw, and IMF Bz has been developed. This empirical model was developed by first using observations of the magnetic field in the lobe by ISEE‐1 and simultaneous observations of the magnetic field and plasma properties of the solar wind by IMP‐8 to determine the flaring angle of the near‐earth magnetotail in the region −22.5 Re ≤ x ≤ −10 Re. These studies show that the flaring angle depends on x, ρv²sw and IMF Bz. In particular, the median flare angle increases linearly with decreasing Bz when the IMF is southward, while the flare angle is constant when the IMF is northward. The empirical function derived for the flaring angle of the magnetotail is used to determine a relation for the radius of the tail.
Microseismicity derived fault‐Planes and their relationship to focal mechanism, stress inversion, and geologic data
Authors: Urbancic Theodore I. , Trifu Cezar‐Ioan , Young R. Paul
Published: February 2008

General stress and faulting trends have been retrieved through the analysis of mining induced microseismic events (M < 0) at two sites, related to an mN 2.6 rockburst, and an excavation at depth. A comparison of results obtained through principal component analysis (PCA) of seismicity, focal mechanism, and stress inversion, with in‐situ measurements of stress and structural mapping data, show that: under stable stress conditions, the P, B, T, and stress inversion axes are consistent with in‐situ measurements of stresses; stress inversion and PCA fault‐planes lie within 10 to 20° of the most significant mapped features at the sites; and the PCA technique provides a robust approach for the determination of fault‐planes.
The theoretical response of sedimentary layers to ambient seismic noise
Authors: Field Edward , Jacob Klaus
Published: February 2008

For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earth's free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half‐space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal‐component noise spectrum by that of the vertical component. Nakamura's estimate applied to both observed and predicted noise‐spectra was also successful in identifying the fundamental resonance, with a slight (<10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.
The sidereal rotation period of Neptune
Authors: Lecacheux A. , Zarka Ph. , Desch M. D. , Evans D. R.
Published: February 2008

The two main, low frequency radio components discovered at Neptune by the Planetary Radio Astronomy experiment carried aboard Voyager 2 have a well defined periodicity at about 16.1 hour. By analyzing all the available data, i.e. about 60 days around the closest approach for the “Burst” component and 15 days for the “Smooth” component, we determine, for each component, the best estimate of the radio period. We conclude that the two estimates are not statistically different. While the two kinds of radio emissions have very different characteristics (in particular their frequency ranges and beaming properties), and probably correspond to different emission processes, their modulation is very likely due to the rotation of the planetary magnetic field tied to the core of the planet, as it has already been assumed for the other giant planets. The deduced estimate of the sidereal rotation period of Neptune is 16.108 ± 0.006 (or 16h06.5m ± 00.4m).
Influence of microstructure on rock elastic properties
Authors: Berge Patricia A. , Berryman James G. , Bonner Brian P.
Published: February 2008

Depending on details of the composite microstructure, different theories may be needed to obtain good agreement with measured elastic properties. This observation is especially pertinent whenever the composite is porous, as is normally true for rocks. Predictions of three theories are compared to data for porous glass samples. The differential effective medium (DEM) theory and Hashin's composite spheres assemblage (H) do a good job of predicting elastic behavior of a porous foam composed of glass. The self‐consistent (SC) effective medium theory does equally well at predicting behavior of a sintered glass‐bead sample. The realizable microstructure of each theoretical model is a good analog of the microstructure for one or the other of these two very different porous glasses. Velocities of granular rocks such as sandstones may be estimated accurately using the SC theory, whereas velocities of rocks such as basalts having isolated cracks and pores may be better estimated using either the DEM theory or Hashin's model.
Electrodynamic aspects of the first tethered satellite mission
Authors: Dobrowolny M. , Melchioni E.
Published: February 2008

The tethered satellite system (TSS) project was developed, starting from 1984, as a unique system to make active experiments in space plasmas. This paper aims at providing a thorough description of the basic electrodynamic aspects of a TSS orbiting in the ionosphere. As will be seen, a wide variety of plasma physics phenomena are associated with the interaction of a conducting tether with the ionospheric plasma and, as a consequence, numerous basic science objectives can be addressed with a TSS system. Besides reviewing the various electrodynamic phenomena and our present knowledge on them, we provide also a description of the scientific payload and the nominal science objectives of the first tethered satellite project (TSS‐1). A brief account, as well as a first evaluation, are also given of the mission itself, which was flown with the shuttle Atlantis launched on July 31, 1992.
Resonant Alfvén wave excitation in two‐dimensional systems: Singularities in partial differential equations
Authors: Thompson Michael J. , Wright Andrew N.
Published: February 2008

The resonant excitation of Alfvén waves is considered in a cold plasma embedded in a uniform magnetic field . All wave fields are assumed to vary as exp[i(λy ‐ ωt)], and the background medium is invariant in y. The background density distribution ρ0(x, z) is otherwise completely arbitrary. Regular and singular solutions for the waves are derived systematically in the vicinity of a resonance by considering a generalized Frobenius series, and we are able to recover many results found in earlier studies. Some new features of our work include a generalization of the overlap integral determining the efficiency with which any resonance may be excited, and the possibility that ξy ∝ 1/xN (N = 1,2,3 …) at the resonance. Hitherto only the solution with N = 1 has been considered.
Particle simulations of ionospheric injection experiments: Comparison with CRIT II
Authors: Bolin O. , Brenning N.
Published: February 2008

A one‐dimensional particle‐in‐cell code has been used to study the barium release experiment CRIT II. The electrodynamic response of the ionosphere to these injections has been modeled by magnetic‐field‐aligned currents, using the concept of Alfvén conductivity. After the active phase, when the fast part of the neutral barium cloud passed the payload, the particle detector 5AB in the CRIT II experiment, aimed almost perpendicular to the ambient magnetic field B in the direction toward the release, showed two clearly defined types of peaks having different durations, frequencies, and energy distributions. This particle energy spectrum has been reproduced in the numerical simulations. It is shown that geometrical effects and phase shifts of the ions are of secondary importance for the creation of the double‐peaked structure. A qualitative model, based on the selective acceleration of barium ions due to electric fields set up by the injection process, is developed to explain the observations.
A global magnetosphere‐ionosphere coupling model of substorms
Authors: Kan J. R.
Published: February 2008

A global model of substorms is proposed on the basis of observational synthesis and theoretical modeling. Since the theoretical basis of the present model is the magnetosphere‐ionosphere coupling (MIC) process, it will be called the MIC model of substorms. Substorms can occur in the MIC model without a new X line formed in the near‐Earth plasma sheet, in contrast to the highly popular near‐Earth neutral line (NENL) model of substorms. Following enhanced dayside reconnection, the ionosphere overloads both the solar wind on open field lines and the plasma sheet on closed field lines. The solar wind responds to the overload by providing more driven energy from the dynamo action on open field lines. The plasma sheet responds to the overload by collapsing itself, i.e., dipolarizing its field configuration to form the substorm current wedge. The explosive intensification during the expansion phase is powered by releasing the magnetic energy stored on closed field lines in the plasma sheet. The stored energy is released by the unloading instability driven by a positive feedback in the substorm current wedge.
Signatures of shock drivers in the solar wind and their dependence on the solar source location
Authors: Richardson I. G. , Cane H. V.
Published: February 2008

Solar wind and energetic ion observations following 40 interplanetary shocks with well‐established solar source locations have been examined in order to determine whether signatures characteristic of the coronal material forming the shock driver are present. The signatures considered include magnetic‐field‐aligned bidirectional ion flows observed by the ISEE 3 and IMP 8 spacecraft; bidirectional solar wind electron heat fluxes; solar wind plasma proton and electron temperature depressions; low‐β plasma; enhanced, low‐variance magnetic fields; and energetic ion depressions. Several shock driver signatures are commonly observed following shocks originating from within ∼50° of central meridian and are generally absent for other events. We conclude that shock drivers generally extend up to ∼100° in longitude, centered on the solar source longitude. A particular shock driver signature is typically found following ∼80% of shocks from near‐central meridian events. However, only ∼20% of shocks from near‐central meridian events are associated with the bidirectional 35‐1000 keV ion events discussed by Marsden et al. (1987). Since shocks from central meridian events are not usually associated with all the shock driver signatures examined, the absence of a driver cannot be confirmed from consideration of one of these signatures alone. We also find evidence that a few bidirectional energetic ion and solar wind electron heat flux events following shocks (in particular from far eastern sources) may occur on open field lines outside of shock drivers.
Intercalibration of solar wind instruments during the international magnetospheric study
Authors: Petrinec S. M. , Russell C. T.
Published: February 2008

Systematic discrepancies have been observed between ion density measurements taken by the electrostatic analyzers on ISEE 3 and the Faraday cup and electrostatic analyzer on board the IMP 8 spacecraft during the International Magnetospheric Study. We have quantified these discrepancies as a function of solar wind bulk velocity and ion temperature, and have developed correction algorithms that should bring all density measurements to a common base. The electron densities from ISEE 3 are used as the intercalibrating base, since these measurements of the solar wind density should not depend on the ion velocity or temperature.
Evidence for a spheroidal structure of magnetic clouds
Authors: Vandas M. , Fischer S. , Pelant P. , Geranios A.
Published: February 2008

We have analysed 14 cases of magnetic clouds identified by R. M. Lepping et al. [1990], who have fitted them with the cylindrical model. We treated cloud magnetic field profiles and compared them with the spheroidal models. We argue that all cases exhibit features of spheroidal clouds, namely, the complete sinusoidal profile of magnetic field components inside the cloud, double‐peak and/or plateau‐type magnetic field magnitude profiles.
Heliographic latitude dependence of the Sun's irradiance
Authors: Schatten Kenneth H.
Published: February 2008

It has recently been reported that the total radiative emission variations from solar type stars exceeds the currently observed solar constant variations (from spacecraft over the last decade) by a factor near 4. Aside from other remote alternatives, this suggests three clear possibilities: (1) the Sun may undergo irradiance variations several times larger than any we have seen; (2) our Sun is highly unusual with regard to its radiative output; or (3) our terrestrial position in the heliosphere provides a special vantage point which reduces the observed solar irradiance variations. We investigate the last possibility by considering the influence of observer latitude upon calculated irradiance variations using a simple model for emission from solar contrast features. We consider modeled sunspots, faculae, and network structures. As the latitude angle of the observer rises relative to the heliographic equator, sunspot deficit contributions diminish and facular plus network contributions escalate. We find that the observing latitude can influence the irradiance variations by a factor near 6. When we integrate the irradiance variations, over the celestial sphere, they average to 3 times the terrestrial effect, suggesting that the solar cycle luminosity variations are proportionally, 3 times larger than the solar constant variations. Thus we suggest the Sun's luminosity output varies even more strongly with the solar cycle than is apparent in the solar constant variations. The influence of the observer viewing angle relative to stellar spin axis, studied here, may be possible to investigate with a thorough statistical examination of other solar type stars. Additionally, the rotational modulation due to active regions (as a function of observer viewing angle) may also be a valuable area for future investigation.
The flow of the contaminated solar wind at comet P/Grigg‐Skjellerup
Authors: Flammer K. R. , Mendis D. A.
Published: February 2008

On July 10, 1992, the Giotto spacecraft encountered a second comet, namely, P/Grigg‐Skjellerup. Despite the small production rate of this comet (approximately 2 orders of magnitude smaller than that of P/Halley and 1 order of magnitude smaller than that of P/Giacobini‐Zinner) the plasma flow showed several distinct flow transitions. We have applied theoretical models, previously developed to explain the existence and location of the flow boundaries at comets P/Giacobini‐Zinner and P/Halley to this new data set for comet P/Grigg‐Skjellerup. The observed bow shock crossings are used to evaluate the production rate of the cometary neutrals. Using this production rate, the position of the collisionopause and the ionopause are estimated. The observed location of the magnetic pile‐up boundary corresponds well to the theoretical calculations. We also find the reason that a magnetic field‐free region was not encountered was because the ionopause, while being sharp, was inside the spacecraft trajectory which had an estimated closest approach distance greater than 120 km. Finally we have used the observed spatial profile of the solar wind magnetic field together with a simple one‐dimensional, multispecies chemical model to calculate the radial density profiles of the H2O group ions outside the bow shock. Comparison of the calculated total H2O group ion density profile with observations indicates a good fit in the inner region where the H2O+ ion dominates. While the corresponding fit in the outer region, where the O+ ion dominates, becomes progressively less satisfactory with increasing cometocentric distance, it is noted that the observations themselves are believed to be less reliable at these large distances.