bluuebunny/crossref_metadata_2025 huggingface dataset:
Kelvin‐Helmholtz instability at the magnetospheric boundary
Authors: Miura Akira
Published: February 2008

A magnetohydrodynamic simulation of Kelvin‐Helmholtz instability at the magnetospheric boundary is performed by including inhomogeneities of plasma and magnetic field at the dayside low latitude magnetospheric boundary. A magnetopause current layer is corrugated highly nonlinearly by the instability and a wide velocity boundary layer is formed within the magnetopause current layer, a result being consistent with observations. Interchange of plasma and flux tubes is accomplished by the instability.
Wind corners in the winter mesosphere
Authors: von Zahn U. , Widdel H.‐U.
Published: February 2008

A series of 18 mesospheric wind profiles with an altitude resolution of about 50 m has been obtained by means of radar tracking of chaff clouds. The experiments were performed during during wintertime at 69°N latitude and they cover mostly the 72 to 86 km altitude range. The observed wind shears frequently concentrate the changes of wind direction α with altitude z into shallow layers, which we designate "wind corners". They are characterized by having large directional gradients dα/dz (⩾ 5°/100 m) and by having a small vertical vertical extent ( ⩽ 500 m). Also wind corners separate altitude layers of a few km thickness in which little change of wind direction occurs. Frequently wind corners coincide with local minima in the horizontal wind speed (≤15 m/s).
MAGSAT scalar anomaly map of India and a part of Indian Ocean‐magnetic crust and tectonic correlation
Authors: Mishra D. C. , Venkatraydu M.
Published: February 2008

Magnetic scalar anomaly maps over India and a part of the Indian ocean are prepared from the MAGSAT data using an average of the observed field in a grid of 1°×1°. The MAGSAT scalar anomaly map thus obtained is characterised by ‘highs’ over the Indian shield and large ‘lows’ over the Himalayas centered over Tibet Plateau. A NE‐SW magnetic trend emanating from NE India and spreading almost over the entire peninsular shield is the single most prominent feature of the map which might be related to the stress system or deepseated structural trends associated with plate motion. The Deccan Traps are reflected by ’nosing’ of the magnetic contours. The Shillong massif is characterised by a separate anomaly which deflects the NE‐SW magnetic trend referred to above. A magnetic crustal map prepared from this MAGSAT scalar anomaly, using the principles of harmonic inversion, yields a thickness of 30 km under the cambay basin and the western part of the Narmada‐Son lineament for a magnetization of 2A/m suggesting a higher subsurface thermal gradient in this area and therefore a tectonically active region.

A maximum thickness of 40‐42 km is found under the regions of Aravallis, Singhbhum and south of the Narmada‐Son Lineament, which are Precambrian stable cratons. Major lineaments and faults are reflected as variations in the thickness of the magnetic crust along these lines.

The Himalayas are chaaracterised by an average thickness of 35‐36 km with a thickening of 2‐4 km under the Tibet plateau and Shillong massif. The MAGSAT anomaly in the northern part of the Indian ocean can be accounted for by a thickening of 8 km in the magnetic crust.

The center of this crustal thickening derived from the MAGSAT anomaly after appropriate correction for the inclination of the earth's magnetic field in this region coincides with the wellknown Indian ocean gravity and geoid lows.
Post‐orogenic evolution of a mountain range: South‐eastern Australian Highlands
Authors: Lambeck Kurt , Stephenson Randell
Published: February 2008

In the post‐orogenic phase of a mountain's life history, erosion is the dominant process shaping the landscape and elevations. As the highland is eroded away, a broad regional isostatic rebound occurs and the base of the mountain is uplifted and rivers cut down in response to new base levels. Absolute elevations of the highland decreases with time. This erosion‐rebound process can be quantified as the unloading of a lithospheric plate overlying a fluid mantle. The adopted rheological model is a viscoelastic plate characterized by an effective flexural rigidity and relaxation time constant. The rate of erosion is assumed to be proportional to the elevation at any time. The erosion‐rebound model has been applied to the southern highlands of Australia. It is consistent with recent geological estimates of Tertiary erosion rates.
Thermoviscous remagnetization in some Appalachian limestones
Authors: Kent Dennis V.
Published: February 2008

Experimental evidence shows that blocking temperature‐relaxation time theory for single‐domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in Devonian limestones of New York State. Thermoviscous remagnetization at moderately elevated burial temperatures during the late Paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks. More generally, these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite‐bearing rocks.
The conversion of N2O5 to HNO3 at high latitudes in winter
Authors: Evans W. F. J. , McElroy C. T. , Galbally I. E.
Published: February 2008

A mechanism for the conversion of stratospheric N2O5 to HNO3 by the reaction of N2O5 with aqueous aerosol in the winter polar stratosphere is proposed to explain the observed deficit amounts of dinitrogen pentoxide and excess amounts of nitric acid compared to the results of calculations based on current gas phase chemistry. The presence of large numbers of aqueous aerosols in the winter polar vortex is associated with the formation of polar stratospheric clouds. An estimate of the rate of the conversion reaction leads to a time constant of about 18 days; model calculations indicate that a conversion process with this time constant is sufficient to convert about 4 ppbv of NO2 into HNO3 in the polar winter vortex.
Plasma waves associated with the AMPTE artificial comet
Authors: Gurnett D. A. , Anderson R. R. , Häusler B. , Haerendel G. , Bauer O. H. , Treumann R. A. , Koons H. C. , Holzworth R. H. , Lühr H.
Published: February 2008

Numerous plasma wave effects were detected by the AMPTE/IRM spacecraft during the artificial comet experiment on December 27, 1984. As the barium ion cloud produced by the explosion expanded over the spacecraft, emissions at the electron plasma frequency and ion plasma frequency provided a determination of the local electron density. The electron density in the diamagnetic cavity produced by the ion cloud reached a peak ≳5×105 cm−3, then decayed smoothly as the cloud expanded, varying approximately as t−2. As the cloud began to move due to interactions with the solar wind, a region of compressed plasma was encountered on the upstream side of the diamagnetic cavity. The peak electron density in the compression region was about 1.5×104 cm−3. Later, a very intense (140 mVolt/m) broadband burst of electrostatic noise was encountered on the sunward side of the compression region. This noise has characteristics very similar to noise observed in the earth's bow shock, and is believed to be a shock‐like interaction produced by an ion beam‐plasma instability between the nearly stationary barium ions and the streaming solar wind protons.
Are Uranus' rings black because they are in the solar wind?
Authors: Curtis S. A. , Ness N. F.
Published: February 2008

We show that, for a reasonable range of choices of ionospheric parameters for Uranus and of solar wind parameters at Uranus orbit, the rings of Uranus are for the most part immersed in the solar wind if Uranus lacks an intrinsic magnetic field. If Uranus ionosphere forms the obstacle to the solar wind, then very low upper bounds (1–4 nT) may be obtained for an intrinisic magnetic field at the polar 1 bar level. In addition, from an examination of the geometric albedos of Jupiter's and of Saturn's rings and moons, we show that these bodies have characteristically either high albedos in a low radiation environment or low albedos in a high radiation environment. The only exceptions to this pattern are geologically active moons such as Io. We suggest that the moons of Uranus possess a significantly lower albedo than those in the low radiation environment of Saturn and that the rings of Uranus are black because the rings and moons of Uranus may be relatively depleted in their surface stores of ices. In this scenario, immersion of the rings of Uranus in the solar wind would provide a loss mechanism for ring ices and produce the black rings observed. The black rings of Uranus may thus imply the lack of an intrinsic Uranus magnetic field.
Thermal structure of the Venus ionosphere: A two‐dimensional model study
Authors: Whitten R. C. , Singhal R. P. , Knudsen W. C.
Published: February 2008

A simple, two‐dimensional, spectral model of heat transport in the Venus ionosphere has been constructed. Numerical experiments with the model suggest that a nocturnal heat source about an order of magnitude smaller than the daytime source is required to simulate the observed ion and electron temperatures. The solar zenith angle dependence of the high‐altitude ion temperature appears to be caused by expansion followed by compression of the plasma as it flows from the dayside to the nightside. Thermal‐flux saturation appears to be significant on the nightside, but the formation of a standing shock wave does not seem to be required to explain the broad features of the observed, nocturnal ion temperature.
Long‐term temperature trends in the stratosphere: Possible influence of anthropogenic gases
Authors: Labitzke K. , Brasseur G. , Naujokat B. , De Rudder A.
Published: February 2008

Analysis of northern hemisphere mid‐latitude temperature data at 30 mbar over the last 20 years suggests a long‐term negative trend of about ‐0.24 K/decade when all observations between 10°N and 90°N are considered. This stratospheric cooling which could tentatively be attributed to the increase of the carbon dioxide amount in the stratosphere is in qualitative agreement at 30 mbar with a model calculation of the historical evolution of temperature and chemical composition in the middle atmosphere.
The phase relationship between gyrophase‐bunched ions and MHD‐like waves
Authors: Fuselier S. A. , Thomsen M. F. , Gary S. P. , Bame S. J. , Russell C. T. , Parks G. K.
Published: February 2008

Recent calculations and computer simulations of the right‐hand resonant ion beam instability have indicated that gyrophase‐bunched ion distributions may be generated as a first step in the disruption of field‐aligned beams. Theory predicts a distinct phase angle relationship between the gyrovelocity of the phase‐bunched ions and the wave magnetic field. In this paper, observations of such a relationship between gyrophase‐bunched ions and MHD‐like waves upstream from the Earth's bow shock are presented. The observed phases are in accord with the theoretical predictions of the right‐hand resonant ion beam instability.
Uranian H Ly‐α emission: The interstellar wind source
Authors: Yelle R. V. , Sandel B. R.
Published: February 2008

IUE observation of Uranian emissions in hydrogen Lyman alpha (H Ly‐α over, the past four years have recently been summarized by Clarke et al. [1985]. Over this time period they find an average H Ly‐α brightness of 1260 R which they estimate is composed of 200 R of solar scattered radiation and 1060 R from a collisional source. A third component, not considered by previous authors, is the reflection of H Ly‐α emissions from the interstellar wind. Hydrogen in the interstellar wind forms an extended source of H Ly‐α whose importance relative to the solar flux increases with distance from the sun. We demonstrate that scattering of interstellar H Ly‐α is more important than scattering of solar H Ly‐α for reasonable values of H column abundance and, in fact, may make up 10 ‐ 40% of the observed signal. Large H column abundances are still required to explain the H Ly‐α brightness solely on the basis of resonant scattering; therefore it is likely that the emissions are due in part to collisional sources and in part to the scattering of interstellar H Ly‐α with solar scattering playing a minor role.
Paleomagnetism of Silurian‐Ordovician sediments from the Valley and Ridge Province, northwest Georgia
Authors: Morrison Jean , Ellwood Brooks B.
Published: February 2008

The remanent magnetism (RM) for 93 specimens has been determined in 34 independently oriented samples from 20 sites at Ringgold Gap, Georgia, in the Valley and Ridge Province of the Southern Appalachians. Lithologies sampled include Middle Ordovician through Silurian fossiliferous limestones and hematitic sandstones and siltstones. The section is confined within a single thrust slice. Thermal demagnetization results indicate that hematite is the dominant RM carrier although detrital magnetite has also been identified optically in some specimens.

Both normal and reversed polarities are present and the resulting paleopole is 28°N, 142°E (α95 = 7.3°). This pole falls within the cluster of published Lower Paleozoic Valley and Ridge poles but rather than clarifying the distribution of poles it augments the scatter. Three factors are thought to contribute to this scatter: (1) unresolved components of rotation, possibly introduced during folding and thrust faulting, which result in declination anomalies, (2) true vertical axis tectonic rotations during deformation, and (3) insufficient knowledge of the timing of magnetization. In light of these uncertainties, caution should be exercised in constructing a Lower Paleozoic apparent polar wander path with paleopoles from the Valley and Ridge Province.
Plasma flow and critical velocity ionization in cometary comae
Authors: Haerendel Gerhard
Published: February 2008

The plasma flow in the cometary coma is described by a simple relation which results from the balance of magnetic stresses and drag forces, the latter being mainly due to ion mass loading. Inertial and pressure effects are not explicitly considered, but lumped into a correcting factor of order unity. The relation is used to infer the ionization rate due to the critical velocity effect. The limits, rcrit and rlim, between which the effect dominates over photoionization are evaluated. Critical velocity ionization stabilizes the plasma flow speed at η−1/2 vcrit, where η is the overall efficiency of energy transfer to the ionizing electrons. This speed is estimated to be close to 20 km/s. At the outer limit, rlim, there is a strong rise of plasma density as seen from outside. Inside rcrit, recombination and ion‐neutral friction come into play. The contact surface is found to be largely determined by ion‐neutral collisions. A second neutral component of the order of 10² cm−3 is predicted to originate from recombination inside rlim. It is characterized by an antisunward flow with ∼ 20 km/s. Numerical values for the radial profile of ni and of the critical boundaries (except the bow shock) are given. The high plasma density found by ICE at the closest approach to comet Giacobini‐Zinner is interpreted as a result of the critical velocity effect.
AKR signal increases caused by triggering
Authors: Farrell W. M. , Calvert W. , Gurnett D. A.
Published: February 2008

This paper presents a study of the amplitude increases which accompany the triggering of auroral kilometric radiation (AKR) by Type‐III solar radio bursts. IMP‐8 data were used to determine the signal increases observed during one‐hour periods before and after Type‐III bursts at 100 kHz, 178 kHz and 500 kHz and these were compared with similar observations when the Type‐III bursts were absent. The results indicate that between 8 to 16% of the Type‐III bursts caused statistically significant intensity increases and that infrequent large signal increases of sometimes 20 dB or more tended to characterize the triggered AKR, rather than a large proportion of small increases.
Three component plasma electron distribution in the intermediate ionized coma of comet Giacobini‐Zinner
Authors: Zwickl R. D. , Baker D. N. , Bame S. J. , Feldman W. C. , Fuselier S. A. , Huebner W. F. , McComas D. J. , Young D. T.
Published: February 2008

On 11 September, 1985, in the interval from 1056 to 1110 UT surrounding closest approach (i.e. within ∼1.2 × 104 km), ICE encountered a region characterized by an unusually low electron temperature, a high density, a bulk flow speed V < 30 km/s, and a relatively low level of plasma turbulence. We call this region the intermediate ionized coma, IIC. Electron velocity distribution functions in the IIC are composed of three distinct components: cold, mid, and hot. We believe that the cold population represents electrons produced close to the comet nucleus by ionization of cometary matter and subsequent cooling by Coulomb collisions. The mid distribution also appears to be composed of electrons produced by photoionization of cometary neutrals, but sufficiently far from the comet nucleus that the distributions are largely unaffected by Coulomb interactions. Thus, the mid population is not in thermal equilibrium with the cold distribution. The temperature of the mid distribution compares well with the average energy of electrons resulting from the ionization of H2O, which leads us to conclude that the composition of the ion population at this distance is consistent with the interpretation that H2O is the dominant species. The hot component is most likely a population of electrons of solar wind origin. Throughout the IIC the electrostatic potential of the spacecraft was very low (< 0.8 eV), implying ICE generated very little impact‐produced plasma during its passage.
Influence of a quasi‐geostrophic flow on the deepening of the wind‐mixed layer
Authors: Klein Patrice , Le Saos Jean‐Pierre
Published: February 2008

The presence of a mean flow vorticity field is found to affect significantly mixed‐layer deepening.
Excitation of the Earth's rotational axis by recent glacial discharges
Authors: Gasperini P. , Sabadini R. , Yuen D. A.
Published: February 2008

We study the effects of present‐day glacial discharges and the growth of the Antarctic ice sheet on exciting the Earth' rotational axis. Glacial forcing could cause a maximum change in J2 of about one‐third of the observed amount, for the Maxwell rheology and for Burgers’ body models with a long‐term, lower‐mantle viscosity greater than about 1023P. For transient rheologies the amount of excitation due to glacial melting decreases. Polar wander is not much excited by recent glacial melting for the various types of rheologies examined.
The effect of depth‐dependent viscosity on convective mixing in the mantle and the possible survival of primitive mantle
Authors: Gurnis Michael , Davies Geoffrey F.
Published: February 2008

The effect depth‐dependent viscosity has on convective mixing and sampling (or degassing) of primitive mantle beneath ridges is explored in two‐dimensional models. Higher relative viscosities in the deep mantle decrease convection velocities and strain rates and prolong the residence time of material in the deep mantle. If the average viscosity of the lower mantle is at least 100 times the viscosity of the upper mantle, then some mantle material could have survived from very early in the earth's history. If, in addition, the depth of degassing under ridges has been less than 75 km, on average over earth history, then helium isotopic systematics are qualitatively consistent with whole mantle convection.
The Korteweg‐deVries equation from laboratory conduit and magma migration equations
Authors: Whitehead John A. , Helfrich Karl R.
Published: February 2008

Wave equations that govern porous flow in a matrix and analogous equations for fluid conduits can be reduced to the Korteweg‐deVries equation in the limit of small perturbations.
First two‐station results for long‐period surface waves velocity from the Geoscope Stations in Africa
Authors: Hadiouche O. , Jobert N. , Romanowicz B.
Published: February 2008

Rayleigh and Love phase and group velocity dispersion measurements are performed along a purely african path between the Geoscope stations TAM and AGR, for the Leewards Isl. earthquake of March 16, 1985. This regional study is important as it brings deep‐seated structure information in a seismologically blank region of Africa. It enables to improve the spatial resolution of larger scale models. For periods longer than 160 s. and up to 250 s., the fundamental Rayleigh wave dispersion is in agreement with prediction by global models. For shorter periods, the results fall below this prediction, which is an indication of lithospheric S‐wave velocities lower than those of the models. The results are also compared with previous regionalizations. The Rayleigh second higher mode is extracted at AGR, and the phase velocity dispersion obtained in the period range 50‐140 s is in agreement with the theoretical dispersion for PREM.
Plasmasphere formation in arbitrarily oriented magnetospheres
Authors: Selesnick R. S. , Richardson J. D.
Published: February 2008

We investigate the formation of plasmaspheres in planetary magnetospheres with arbitrary orientations of the rotation and magnetic dipole axes. A traditional plasmasphere with closed orbits inside the plasmapause and open trajectories outside it only occurs for the limiting case of aligned rotation and dipole axes. A time variable plasmapause exists if the rotation axis is perpendicular to the solar wind flow direction. In any other case no definite plasmapause exists. Solar wind driven convection transports plasma throughout the magnetosphere with an effectiveness which increases as the orientation becomes further from one of the two limiting cases of strict plasmapause formation. Our analysis is applied to Earth and Uranus using the actual orientations of the rotation and dipole axes. Particle trajectories at Earth deviate only slightly from those obtained with traditional models. Uranus has no plasmasphere, and plasma convects sunwards throughout the inner magnetosphere.
Observation of several chlorine nitrate (ClONO2) bands in stratospheric infrared spectra
Authors: Zander R. , Rinsland C. P. , Farmer C. B. , Brown L. R. , Norton R. H.
Published: February 2008

Four of the most prominent and sharpest infrared absorption features of chlorine nitrate at 780.2, 807.7, 809.4 and 1292.6 cm−1 have been observed in a series of infrared solar spectra obtained at an unapodized spectral resolution of 0.01 cm−1, using the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument from on‐board Spacelab 3.

A quantitative analysis of the ν4 Q branch at 780.2 cm−1 has provided insight into the concentration of ClONO2 between 19 and 40 km altitude. While the mean profile deduced from 3 sunset occultations near 30° N latitude exhibits a shape close to that predicted by model calculations, its conconcentrations in the 20 to 32 km altitude range are, however, about 30% larger, reaching a peak concentration of 9 × 108 molecules/cm³ at 25 km. The concentrations above 32 km, deduced from one sunrise occultation at 47° S, are even larger than the corresponding sunset values at 30° N latitude. Some of these discrepancies may be caused by the rather large uncertainty in the assumed Q branch strength.

The results reported here constitute a significant input towards understanding the chemistry prevailing in the stratosphere as well as for model calculations predicting the secular evolution of our atmosphere.
Energetic ion properties observed near the periphery of the mass‐loaded flow region surrounding comet P/Giacobini‐Zinner
Authors: Tranquille C. , Richardson I. G. , Cowley S. W. H. , Sanderson T. R. , Wenzel K.‐P. , Hynds R. J.
Published: February 2008

Measurements made by the Energetic Particle Anisotropy Spectrometer (EPAS) on the International Cometary Explorer (ICE) spacecraft show that relatively abrupt changes occur in energetic heavy ion properties near the periphery of the mass‐loaded flow region surrounding comet P/Giacobini‐Zinner. Corresponding features are observed both inbound and outbound from the comet. As the distance to the comet decreases, a change in the ion distribution takes place over approximately 5 minutes, indicative of flow deflection away from the comet‐sun axis. Shortly afterwards, the ion angular distribution broadens over a further 6 minutes, indicative of a slowing of the ion bulk flow by about 100 km s−1. Taken together with magnetic field and plasma data, these observations are suggestive of a shock wave standing in the flow.
IPS observations of the solar wind in late March 1979 (CDAW‐6)
Authors: Watanabe Takashi , Kakinuma Takakiyo , Kojima Masayoshi
Published: February 2008

Solar wind data from three‐station IPS (interplanetary scintillation) observations are analyzed in order to study solar wind features during the CDAW‐6 intervals (March 22 and March 31 ‐ April 1, 1979). The empirical model of the interplanetary shock wave disturbance relevant to the ssc of 0826 UT, March 22, 1979 is determined. The layer of enhanced density fluctuations is identified to have been the shocked plasma with highly fluctuating interplanetary magnetic field.
Spherical Earth modelling of the scalar magnetic anomaly over the Indian Region
Authors: Rajaram Mita , Singh B. P.
Published: February 2008

MAGSAT anomaly maps over the Indian region are prepared after removal of core and external current contributions. To examine the physical reality of the map, we calculate the geomagnetic scalar field at MAGSAT height in spherical co‐ordinates, using the equivalent point source distribution of magnetic dipoles. The calculation assumes: (i) the source field of the anomaly to lie within the crust; (ii) only induced magnetisation exists; (iii) the susceptibility is constant over the whole region. Crustal thickness is deduced from Bouguer gravity anomaly maps. Evaluation of the anomaly is done numerically using the Gauss‐Legendre quadrature integration method. The computed values agree reasonably well with the observed anomalies. This suggests that the crustal component is quite accurately discernible in the MAGSAT data. Further, anomalies over India seem to be accountable with induced magnetisation alone and the Curie isotherm appears to coincide with the crustal depth.
Continental alkali basalts as mixtures of kimberlite and depleted mantle: Evidence from Kilbourne Hole Maar, New Mexico
Authors: Feigenson Mark D.
Published: February 2008

A depleted‐mantle nodule from Kilbourne Hole, New Mexico, has been analyzed in detail to test a recent model for the formation of alkali basalts by two‐component mixing of depleted mantle and enriched (kimberlite) mantle. The bulkrock chemistry of the nodule can be reconstructed from its constituent phases, except for calcium, sodium and several incompatible elements. It is found that acid‐leachates derived from the surfaces of the minerals contain the "missing" complement of these elements. Furthermore, major element compositions indicate that each leachate may be a mixture of nodule mineral plus kimberlite. It is suggested that the host alkali basalt lava could have formed by the same mixing process, as the lava and enclosed nodule are in approximate Nd isotopic equilibrium, and calculated mineral/melt partition coefficients for the rare earth elements (REE) between nodule minerals and host lava match those determined experimentally.
Steady state plasma transport in a corotation‐dominated magnetosphere
Authors: Pontius D. H. , Hill T. W. , Rassbach M. E.
Published: February 2008

We present a model for plasma transport in a rotation‐dominated magnetosphere (e.g., Jupiter) containing an internal plasma source (e.g., Io). We make a distinction between magnetic flux tubes filled with outward moving plasma and less dense flux tubes that move in to compensate. In the outer region where the heavier tubes are effectively isolated, their motion is described by a pair of coupled differential equations which have analytic solutions in the small velocity approximation. In a steady state, conservation of flux‐tube plasma content requires that the fraction of mass‐loaded flux tubes be a function of the radial outflow speed. Similar equations govern the motion in the inner region where heavy flux tubes predominate and lighter tubes are isolated. This raises the possibility of net outward radial transport even when the radial gradient of average flux‐tube content is positive. In the outer region where heavy flux tubes are isolated, the average steady‐state flux shell content decreases outward as the inverse square of radial distance.
Predictability of slip events along a laboratory fault
Authors: King Chi‐Yu
Published: February 2008

A "long" sequence of stick‐slip events generated along a laboratory fault, which consists of eight spring‐connected masses that are elastically driven to slide on a frictional surface, has been examined to check whether the "large" events are predictable. The large events are found to recur at intervals of very different durations, although the elastic and frictional properties along the fault are quite uniform. The recurrence intervals are, however, approximately proportional to the displacements of the preceding events, but not of the following events. Thus the occurrence time of a large event is approximately predictable, whereas its displacement is not. The size (logarithmic energy release) distribution of all the events of different sizes can be fitted by a maximum‐entropy model, which has an upper bound in size corresponding to the complete release of the maximum strain energy stored along the fault.
Paleomagnetism of the Upper Devonian Catskill Formation from the southern limb of the Pennsylvania Salient: Possible evidence of oroclinal rotation
Authors: Miller John D. , Kent Dennis V.
Published: February 2008

Multiple components of magnetization were isolated in the natural remanent magnetization of samples of the Upper Devonian Catskill Formation red beds taken from the southern limb of the Pennsylvania Salient. The dominant, thermally distributed component (SF), previously thought to predate folding, is demonstrably synfolding in origin. The mean direction for SF based on data from the current study and a previous study is Declination/Inclination = 161.6°/7.9°, a95 = 3.9° (pole position 127.3°E, 43.1°N, A95 = 3.1°, N = 14 sites). Although the remagnetization is clearly synfolding in most areas, the relative ages of folding and remagnetization vary locally. A subordinate high unblocking temperature component (HT) has a mean tilt corrected direction of 160°/36°, a95 = 16° (pole position 123.5°E, 26.1°N, A95 = 15.4°, n = 7 samples). Comparison of HT with the prefolding magnetization isolated in the northern limb of the salient suggests that the paleolatitude of this part of North America was about 16°S and that part of the curvature of the salient was acquired during orogenesis.
Ozone and aerosol‐measurements in the springtime Antarctic stratosphere in 1985
Authors: Hofmann D. J. , Rosen J. M. , Harder J. A. , Rolf S. R.
Published: February 2008

Balloonborne measurements of ozone and aerosol concentrations at McMurdo Station in Antarctica in November 1985, indicated large variations, apparently associated with movement of the polar vortex. Extremely low ozone concentrations were observed, on one occasion to altitudes in excess of 30 km. Low ozone was accompanied by the absence of optically active aerosol and an enhancement of condensation nuclei while the opposite was the case with normal ozone. The ozone measurements are consistent with a springtime Antarctic vortex ozone minimum (popularly called the “ozone hole”) and variations in the aerosol size distribution appear to be associated with the thermal history of the relevant air parcel.
Stratospheric NO2 and O3 in Antarctica: Dynamic and chemically controlled variations
Authors: Keys J. G. , Johnston P. V.
Published: February 2008

Ground based measurements of stratospheric NO2 made from Arrival Heights, Antarctica, are compared with TOMS satellite total ozone measurements and upper air meteorological data. The observations show the seasonal effects of partitioning on NO2, and the influence of wave driven transport on both the NO2 and O3 column in spring. We conclude that stratospheric air appearing over our site in spring during low pressure conditions is representative of the cold core of the polar vortex, which has effectively been isolated from lower latitude air. Within this core, photochemical processes influencing the ozone column amount and NO2 levels can proceed without marked influence from transport effects.
Chlorine chemistry in the Antarctic stratosphere: Impact of OClO and Cl2 O2 and implications for observations
Authors: Rodriguez José M. , Ko Malcolm K. W. , Dak Sze Nien
Published: February 2008

Theories have been proposed to relate the reduction of O3 during antarctic spring to catalytic cycles involving chlorine and bromine species. A necessary condition for any chlorine‐catalyzed scheme is that a large fraction of the chlorine must be in the form of ClO in the lower stratosphere. It has been suggested that these high levels of ClO could be maintained by fast heterogeneous reactions, whose rates are not known at present. Model calculations based on the above mechanisms predict considerable amounts of OClO and Cl2O2, particularly during the night. We present results of calculations of the diurnal variations of ClO, OClO, and Cl2O2 during antarctic spring, for different cases. Results from our calculations suggest that coincident measurements of the total column abundance and diurnal variation of ClO and OClO may help constrain key aspects of the proposed chemical mechanisms. Removal of O3 by the catalytic cycle involving Cl2O2 could be as important as that involving BrO for present levels of chlorine, provided that Cl2O2 photolyzes rapidly to yield Cl and ClO2. We show that there is no synergy between these two cycles, since they both compete for the available ClO.
The simulation of Antarctic ozone with chemical and dynamical effects
Authors: Evans W. F. J. , Boville B. W. , McConnell J. C. , Henderson G. S.
Published: February 2008

We have conducted several numerical 1‐D model experiments in an attempt to simulate the behaviour of Antarctic O3 throughout the year and in particular to account for the observed downward trend of O3 in October. The vertical and horizontal transports were parameterized by a vertical velocity formulation. We find that the observed low October O3 can be simulated by an increase in the intensity of the vertical transport in the polar night and early spring or by a delay in the transition from the winter to summer thermal regimes. In addition, the effect of atmospheric transports on the total O3 is shown to be important throughout the year. We speculate that dynamical changes in the winter vortex, possibly caused by stratospheric cooling due to the greenhouse effect, may be even more important than photochemical changes in the striking enhancement of the Antarctic O3 hole.
Antarctic ozone depletion: 2‐D model studies
Authors: Isaksen Ivar S. A. , Stordal Frode
Published: February 2008

Antarctic ozone depletion is estimated in a 2‐D global model as a result of increase in chlorine levels from 1 ppb to 2.7 ppb and an increase in bromine levels from 15 ppt to 30 ppt. The adopted increase is assumed to be the anthropogenic influence on the stratospheric concentrations of chlorine and bromine species in 1985. Heterogeneous formation of ClO from the reaction between ClONO2 and HCl is included in the calculations, hypothesizing this is the dominant reaction path for chlorinated species on particle surfaces. A total ozone depletion as high as 14‐20% may have occurred during the month of October at 80°S when maximum depletion is obtained. Both the magnitude and the time period when the maximum depletion occurs, agree with observed depletion over the last 8 years.
A kinetics study of the homogeneous and heterogeneous components of the HCl + ClONO2 reaction
Authors: Friedl Randall R. , Goble James H. , Sander Stanley P.
Published: February 2008

The kinetics of the reaction HCl + ClONO2 → Cl2 + HNO3 were investigated at 298 K using a flow reactor with FTIR analysis to assess the importance of this reaction for stratospheric chemistry. The observed reaction was characteristic of a heterogeneous process; an upper limit of 5 × 10−18 cm³ molecule−1 s−1 was obtained for the homogeneous gas phase rate constant. From calculations of the first order wall rate constant, estimates were made of the reaction rate on stratospheric aerosols. Because both HCl and ClONO2 need to be adsorbed on the particle surface, the reaction will be of negligible importance under most stratospheric conditions.
An assessment of the long‐term drift in SBUV total ozone data, based on comparison with the Dobson network
Authors: Fleig Albert J. , Bhartia Pawan K. , Silberstein David S.
Published: February 2008

Six years of total ozone data derived from the SBUV instrument on Nimbus 7 are compared with results from 41 Dobson stations. Results indicate that the SBUV measurements have drifted against the Dobson stations with a roughly linear trend of .38% ± .13% per year. Several possible reasons for this relative drift are discussed.
In situ observations of midlatitude stratospheric ClO and BrO
Authors: Brune William H. , Anderson James G.
Published: February 2008

A balloon‐borne experiment to measure midlatitude stratospheric [BrO] and [ClO] by NO chemical conversion/atomic resonance fluorescence was flown from Palestine, Texas, on 20 May 1986. In this first study of BrO, no signal attributable to BrO was detected, and upper limits (2σ uncertainty) between 35 and 24 km altitude give BrO mixing ratios less than 15 pptv. Current models predict mixing ratios that are 1.7 times larger. Measurements of [ClO] were obtained at less than .2 km altitude resolution from 41 to 22 km. The smoothly varying altitude profile lies within the range of 2‐D model calculations.
Thermally activated viscous remanence in some magnetite‐ and hematite‐bearing dolomites
Authors: Jackson Mike , Van der Voo Rob
Published: February 2008

A Brunhes‐age viscous remanent magnetization (VRM) carried by magnetite and hematite in the El Paso and Montoya dolomite formations has unblocking temperatures significantly higher than those predicted by single‐domain theory. The samples appear to contain a substantial fraction of pseudo‐single domain magnetite, suggesting transdomain processes may be responsible for the stable VRM. A strong inverse correlation between the occurrence of high unblocking temperatures and high isothermal remanence fractions acquired above 0.3 Tesla suggests that the hematite grain size may also be important in determining the range of observed temperatures. Larger grains are inferred to have lower coercivities but higher unblocking temperatures. An older, reverse‐polarity component is also observed, and appears to be a record of late Cretaceous‐Tertiary tectonic activity.
Eocambrian paleomagnetism of the Boston Basin: Evidence for displaced terrane
Authors: Wu Fang , Van der Voo Rob , Johnson Rex J. E.
Published: February 2008

A paleomagnetic study of the Late Precambrian Roxbury formation, consisting of extrusives and clastic sediments, has yielded a steeply downward directed and pre‐folding characteristic magnetization (D/I = 219°/71°, α95 = 5°, k = 311, paleopole at 13°N, 267°E, N = 4 sites), which indicates a significantly higher paleolatitude (55°) than would be expected if the Boston basin were part of the equatorial North American craton in the latest Precambrian and earliest Paleozoic. This characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test. A ubiquitous post‐folding late Paleozoic overprint is present in nine sites (D/I = 183°/14°, α95 = 8°, k = 42), with a paleopole at 41°S, 285°E. The pre‐folding magnetization resides in hematite, which is inferred to have formed during early oxidation of the rocks; the high stability of this hematite may have prevented its magnetization from being reset during the late Paleozoic chemical event responsible for the magnetic overprint. The Boston basin has a marked geological similarity to the Avalon basement terranes in Nova Scotia and Newfoundland, as well as the Armorican Massif in France, and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity; a likely paleolocation is near the northwestern margin of Gondwana which was located at the southpole in the latest Precambrian and Early Cambrian.
Coupling of the core dynamo and mantle: Thermal or topographic?
Authors: Gubbins David , Richards Mark
Published: February 2008

Static features in the earth's magnetic field and fixed sources of secular variation suggest the dynamo is anchored to the lowermost mantle via either thermal or topographic coupling. Here we show a correspondence with dynamic core topography, which is related to mantle temperature, and suggest further effects due to topography associated with subducting slabs. The two coupling mechanisms may occur on different time scales, providing a potential means for discriminating between them.
Mantle plumes and the periodicity of magnetic field reversals
Authors: Loper D. E. , McCartney K.
Published: February 2008

Many geological phenomena show long term cyclical behavior. Of particular interest are mass extinctions and geomagnetic‐field reversals, which may have correlated periodicities of approximately 30 myr. We propose that these cycles are governed by periodic instability of the thermal boundary layer (D″) at the base of the mantle. As this mobile layer thickens by thermal diffusion, it becomes dynamically unstable and hot material erupts from it. The layer, now thinner, extracts heat from the core at a greater rate, increasing the energy supply and hence the reversal frequency of the dynamo. The hot material from the D″ layer rises via mantle plumes to the surface where it causes increased volcanic activity which affects the climate and biota.
Anomalous difference traveltimes and amplitude ratios of SKS and SKKS from Tonga‐Fiji Events
Authors: Schweitzer J. , Müller G.
Published: February 2008

Difference traveltimes SKKS‐SKS for 12 Tonga‐Fiji events observed from 96‐117° at North and South American WWSSN and CSN stations exceed the difference times of standard earth model PREM by 2‐3 s. Also the amplitude ratios SKS/SKKS at American stations from 96‐107° are lower than the PREM ratios. No such anomalies occur in directions from Tonga ‐ Fiji towards Eurasia and Africa and for a Sea of Japan event and paths to North America and Africa. The regional variations in the data point to pronounced lateral variations in earth structure. The anomalous difference times imply either an advance of SKS or a delay of SKKS relative to PREM. The interpretation of the anomalies is not unique. Our preferred explanation involves S‐velocity anomalies in the lower mantle on the source side, east of Tonga‐Fiji.
Three dimensional reconstruction of Pangea
Authors: Tanimoto Toshiro
Published: February 2008

The three dimensional seismic velocity structure from the surface of the Earth to a depth of about 900 km was determined from fundamental and higher mode surface waves. Velocity anomalies near the surface correlate well with the age of the surface rocks. Reconstruction of the ancient continent Pangea connects all fast velocity anomalies near the center of Pangea. Fast velocity anomalies extend to 200‐400 km. Considering the resolving power of long period surface waves, this indicates that Pangea was a continent with a lithospheric thickness of about 200 (±100) km or larger, which broke up right in the middle (Central Atlantic). Examinations of seismic velocity sections with respect to depth show different depth penetration of fast velocity anomalies under various continents such as North America, South America, Eurasia and Africa. We speculate that continents possessed different thicknesses before Pangea was formed and had moved over the surface of the Earth independently.
Redistribution of U and TH in shallow plutonic environments
Authors: Gosnold William D.
Published: February 2008

Radioactive heat production in the Idaho Batholith and in the Alamosa River Stock in the San Juan Mountains correlates with the degree of hydrothermal alteration of plutons by meteoric groundwater. Fission‐track mapping shows that as much as half of the U in whole‐rock samples of the most‐intensively‐altered zone of the Alamosa River Stock is associated with secondary minerals. The hydrothermally‐altered, shallow plutons in the Idaho Batholith contain significant amounts of U in secondary minerals whereas unaltered, deeply‐emplaced plutons contain virtually all of their U in primary accessory minerals. The relative distributions of U and Th measured by gamma‐ray spectrometry, the associations of U observed by fission‐track mapping and a depth‐dependence in thermally‐induced hydraulic fracturing suggest a depth‐dependent geochemical process that could produce an upward increase in the abundance of U and Th in batholiths. The process adds U and Th to chemically‐receptive secondary minerals during alteration of plutons by convecting groundwater.
The modelled occurrence of non‐thermal plasma in the ionospheric F‐region and the possible consequences for ion outflows into the magnetosphere
Authors: Lockwood M. , Fuller‐Rowell T. J.
Published: February 2008

A global, time‐dependent, three‐dimensional, coupled ionosphere‐thermosphere model is used to predict the spatial distribution of non‐thermal plasma in the F‐layer. It is shown that, even for steady‐state conditions with Kp as low as 3, the difference between the ion and neutral velocities often exceeds the neutral thermal speed by a factor, D', which can be as large as 4. Theoretically, highly non‐Maxwellian, and probably toroidal, ion velocity distributions are expected when D' exceeds about 1.5. The lack of response of the neutral winds to sunward ion drifts in the dawn sector of the auroral oval cause this to be the region most likely to contain toroidal distributions. The maximum in D' is found in the throat region of the convection pattern, where the strong neutral winds of the afternoon sector meet the eastward ion flows of the morning sector. These predictions are of interest, not only to radar scientists searching for non‐thermal ionospheric plasma, but also as one possible explanation of the initial heating and upward flows of ions in the cleft ion fountain and nightside auroral oval, both of which are a major source of plasma for the magnetosphere.
Oval intensifications associated with polar arcs
Authors: Murphree J. S. , Cogger L. L. , Anger C. D. , Wallis D. D. , Shepherd G. G.
Published: February 2008

The existence of large scale auroral forms in the high latitude region poleward of the auroral oval has generated much interest in recent years because of the implications these forms have for magnetospheric topology. Often these so called polar cap arcs are observed to be connected to auroral regions which are identified with plasma sheet particle populations. These connection points are sometimes associated with intensifications in the auroral distribution. Observations with the Viking UV Imager show that these intensifications are highly dynamic on time scales of minutes, being sometimes suggestive of substorms. The polar arcs may move in response to the dynamics of the intensification or else remain spatially fixed.