bluuebunny/crossref_metadata_2025 huggingface dataset:
Harper‐Dorn Creep: An artifact of low‐amplitude temperature cycling?
Authors: Weertman J. , Blacic J.
Published: February 2008

Harper‐Dorn (HD) creep may be produced by inadvertent low‐amplitude temperature cycling that is inevitable during high‐temperature creep tests. A cyclic temperature change causes a cyclic change in the equilibrium point defect concentration. In turn, a cyclic chemical force on dislocations that produces climb motion is created because the actual point defect concentration can never exactly equal the instantaneous equilibrium value. A cyclic ±1 K can produce a maximum chemical stress of ±5 MPa. When the period of the cycling is neither very large nor very small and the chemical stress is larger than the applied mechanical stress, it, rather than the mechanical stress, determines the dislocation density. In this case, the dislocation density is independent of the applied stress. Therefore, the creep rate is proportional to the applied stress and is independent of grain size, the characteristics of HD creep. If this explanation of HD creep is correct, this type of creep will not occur in the earth's mantle and will not serve as a mechanism for Newtonian mantle viscosity.
Remanent magnetization of a “paramagnetic” composition in the ilmenite‐hematite solid solution series
Authors: Lawson Charles A. , Nord Gordon L.
Published: February 2008

In the ilmenite‐hematite solid solution series, compositions more ilmenite‐rich than Ilm73Hem27 are classically thought to be paramagnetic at room temperature. We have, however, synthesized Ilm80Hem20 samples that acquire hard saturation remanent and thermoremanent moments. From analysis of AC demagnetization data, the source of the measured remanence is believed to be a single‐domain (SD) like material within the Ilm80Hem20 grains themselves. On the basis of transmission electron microscope observations, we suggest that transformation‐ induced domain boundaries, which in part are enriched in hematite component relative to the bulk composition of the grains, could act as the magnetic carrier of the SD‐like remanence.
On viscous remanent magnetization of synthetic multidomain titanomagnetite
Authors: Sporer Hans
Published: February 2008

This paper presents experimental data on VRM of synthetic multidomain titanomagnetites under three aspects: (1) The variability of the acquisition and decay viscosity coefficients, Sa and Sd by the measuring procedure, (2) the dependence of Sd on the titanium content x in the system Fe3−x Tix O4 at 77 K and 300 K, (3) the viscous behavior of magnetization under an uniaxial stress parallel and perpendicular to the direction of the applied field. The experiments indicate a predominant influence of the diffusion after‐effect.
Large scale temporal and radial gradients in the IMF: Helios 1, 2, ISEE‐3, and Pioneer 10, 11
Authors: Slavin J. A. , Smith E. J. , Thomas B. T.
Published: February 2008

Recent investigations using measurements at 1 AU have discovered three types of long term variation in the interplanetary magnetic field: solar minimum decreases, solar maximum enhancements, and small decreases around solar reversal. In this study we have examined the 1972‐1982 Helios 1,2, ISEE‐3, and Pioneer 10,11 observations between 0.3 and 12 AU to further investigate these changes. It was found that all three IMF solar cycle effects are also present in the Helios and Pioneer measurements, confirming that these variations occur throughout the low latitude heliosphere. In addition, the comparison of measurements by identical magnetometers on ISEE‐3, Pioneer 10, and Pioneer 11 has revealed a more rapid decrease in IMF intensity than predicted by classical Parker theory. Causes and ramifications of both the long term variations and steeper than expected radial gradients in the interplanetary magnetic field are discussed.
Acidic sulfate particles in the winter Arctic atmosphere
Authors: Lazrus Allan L. , Ferek Ronald J.
Published: February 2008

Particle samples were collected by aircraft in the vicinity of Barrow, Alaska, the North Pole, and over the North Atlantic north of Bodo, Norway, during March and April, 1983 as part of the Atmospheric Gas and Aerosol Sampling Program. The sulfate concentrations and aerosol acidities from each of the regions were similar. The average sulfate concentration was 3.2 ± 1.3 µg m−3. The aerosol acidity was equivalent to 41 ± 13% of the sulfate.
Trajectories during AGASP
Authors: Harris J. M.
Published: February 2008

Atmospheric trajectories were calculated for various Arctic locations for March and April 1983, during the Arctic Gas and Aerosol Sampling Program (AGASP). Ten‐day back trajectories arriving twice daily were calculated on the 850‐, 700‐, and 500‐mb isobaric surfaces. During AGASP flights 1‐4, trajectories arriving at Barrow show that flow was over the pole from western Asia and Europe, and that the strongest winds and least vertical wind shear were on March 14 at 0000 UT. Surface aerosols and optical depth measured at this time at Barrow reflect a significant haze episode. Trajectories arriving along the flight tracks north of Barrow were frequently very different from those arriving at Barrow, indicating that source areas were in northeast Asia. Trajectories arriving along AGASP flight track 8 near Svalbard showed that the source area changed dramatically in the approximate location of an Arctic aerosol/haze front.
An iridium rich layer at the Cretaceous/Tertiary boundary in the Bidart Section (southern France)
Authors: Bonté Ph. , Delacotte O. , Renard M. , Laj C. , Boclet D. , Jehanno C. , Rocchia R.
Published: February 2008

We have measured an anomalous iridium concentration in the clays of the Cretaceous/Tertiary boundary in the Bidart section, thus confirming the planetary character of this anomaly. The iridium anomaly is synchronous with a very sharp minimum of the 13C/12C ratio and there is no evidence of any other iridium enrichment in marly concentrations away from the boundary. The integrated iridium content is of the same order of magnitude as that calculated from deep sea cores in the Pacific Ocean. In spite of a careful search, we have found no evidence of any exotic material.
Observations of double layer‐like and soliton‐like structures in the ionosphere
Authors: Boehm M. H. , Carlson C. W. , McFadden J. , Mozer F. S.
Published: February 2008

Two types of large electric field signatures, individual pulses and pulse trains, were observed on a sounding rocket launched into the afternoon auroral zone on January 21, 1982. The typical electric fields in the individual pulses were 50 mV/m or larger, aligned mostly parallel to B, and the corresponding potentials were at least 100 mV (kT ∼0.3 eV). A lower limit of 15 km/sec can be set on the velocity of these structures, indicating that they were not ion acoustic double layers. The pulse trains, each consisting of on the order of 100 pulses, were observed in close association with intense plasma frequency waves. This correlation is consistent with the interpretation of these trains as Langmuir solitons. The pulse trains correlate better with the intensity of the field‐aligned currents than with the energetic electron flux.
First experimental evidence of HF produced electron density irregularities in the polar ionosphere; Diagnosed by UHF radio star scintillations
Authors: Frey A. , Stubbe P. , Kopka H.
Published: February 2008

HF‐produced electron density irregularities with scale sizes of several hundred meters were observed in the polar ionosphere by means of UHF‐scintillations using the new facilities at Tromsø. HF‐power densities as low as 20 µW/m² excited irregularities during over‐dense (HF < penetration frequency) modification.
Poker Flat MST Radar and meteorological rocketsonde wind profile comparisons
Authors: Smith S. A. , Fritts D. C.
Published: February 2008

A critical comparison of two wind measurement techniques (MST radar and rocket‐sondes) is made using data collected at Poker Flat, Alaska. These techniques provide equivalent wind estimates (well within experimental uncertainty) at heights of 60 to 70 km. An isolated difference between the techniques is attributed to a localized gravity wave. Conjunctive use of both systems can provide continuous wind profiles from the ground to 80 km.
A tomographic image of mantle structure beneath Southern California
Authors: Humphreys Eugene , Clayton Robert W. , Hager Bradford H.
Published: February 2008

We determined the variations in seismic structure beneath southern California by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The algorithm employed was a modified form of an Algebraic Reconstruction Technique (ART) used in medical X‐ray imaging. Deconvolution with an empirically estimated point spread function was also used to help in focusing the image.

The inversion reveals two prominent features beneath the region. The first is a thin, vertical wedge directly beneath the Transverse Ranges that is 2‐3% faster than the surrounding region. This feature deepens to the east, attaining a maximum depth of about 250 km beneath the San Bernardino Mountains. The second feature is a major zone of low velocity material that is 2‐4% slow under the Salton Trough rift valley, extending to a depth of about 125 km. Two possible explanations for the spatial association of the Transverse Ranges with the velocity anomaly below are lithospheric subduction or small‐scale sublithospheric convection in the region of the Big Bend of the San Andreas Fault. The low velocity anomaly beneath the Salton Trough is consistent with convective upwelling there.
Field aligned observations of transequatorial bubbles from Rarotonga in 1969‐70
Authors: Malcolm R. , Miles C. , Tinsley B. A.
Published: February 2008

A raster scanning photometer with 0.5° field of view was used in 1969‐70 to make images in 6300 Å and 5577 Å emission from Rarotonga of tropical F region trans‐equatorial ‘bubbles’ or ‘depletions.’ From this site the F region plasma structures in the emission layer can be viewed in a magnetic field aligned configuration. A spatial resolution of approximately 15 km was achieved, and revealed a variety of shapes in the bubble cross sections, including ‘bifurcations’ and ‘pinched off’ bubbles. Eastward drift velocities averaged 125 ms−1. Tilts were usually westward at about 45° in the equatorial plane, but some were near zero and some were eastward. The tops reached apex heights of over 800 km. The structures appeared a little less diffuse in 5577 Å emission than in 6300 Å owing to less diffusion of O¹S with its 0.75 sec. lifetime, than O¹D with its 150 sec. lifetime.
Direct detection of the precipitation of ring current electrons and protons stimulated by artificial VLF emission
Authors: Kovrazhkin R. A. , Mogilevsky M. M. , Moltchanov O. A. , Galperin Yu. I. , Djordjio N. V. , Lissakov Yu. V. , Bosqued J. M. , Rème H.
Published: February 2008

Bursts of energetic electron and proton precipitation from the magnetosphere and artificially stimulated by a powerful ground‐based Soviet VLF emitter (F = 19.1 kHz, L = 4.0), were detected during the ESSA experiment aboard the AUREOL‐3 satellite (ARCAD‐3 project) at altitudes of 1500 ‐ 2000 km in the MLT morning sector. Stimulated electron and proton precipitations were clearly recorded in 7 overpasses out of 17 during the period 1 ‐ 25 December 1981. The precipitation zone corresponded to the projection on the ionosphere of the ring current region (L = 2.8 to 3.6), and was displaced equatorward with respect to the location of the VLF transmitter. The intensities of the artificial 0.1 ‐ 2 keV electron fluxes ranged from 104 to 105 (cm² sec ster keV)−1 and those of the 60 ‐ 220 keV protons from 100 to 10² (cm² sec ster keV)−1. The 1.8 keV electron bursts were delayed by about 1 sec with respect to the VLF pulses, and the 110 keV proton fluxes were delayed by about 3 ‐ 5 sec. Results are discussed in terms of both Cerenkov and cyclotron resonance effects between VLF nonducted waves and magnetospheric particles.
Rate constants for the gas phase reaction of OH radicals with peroxyacetyl nitrate (PAN) at 273 and 297 K
Authors: Wallington Timothy J. , Atkinson Roger , Winer Arthur M.
Published: February 2008

Recently, peroxyacetyl nitrate (PAN) has been postulated to be a potential tropospheric reservoir of oxides of nitrogen, and to be important in their long‐range transport. To better assess its atmospheric chemistry, absolute rate constants for the reaction of OH radicals with peroxyacetyl nitrate (PAN) have been determined using a flash photolysis resonance fluorescence technique. Rate constants of (1.13 ± 0.06) × 10−13 cm³ molecule−1 s−1 and (1.37 ± 0.05) × 10−13 cm³ molecule−1 s−1, independent of total pressure over the range 25‐100 torr of argon, were determined at 273 ± 2 and 297 ± 2 K, respectively. (The errors limits represent two standard deviations; systematic errors could contribute an additional ∼10% uncertainty.) These rate constants imply that reaction with the OH radical is the most important removal process for PAN in the upper troposphere.
Plasma entry into the distant tail lobes: ISEE‐3
Authors: Gosling J. T. , Baker D. N. , Bame S. J. , Hones E. W. , McComas D. J. , Zwickl R. D. , Slavin J. A. , Smith E. J. , Tsurutani B. T.
Published: February 2008

ISEE‐3 measurements indicate that a broad mantle‐like boundary layer plasma often exists within the distant geomagnetic tail lobes at all latitudes, directly adjacent to the tail magnetopause. The presence of this boundary layer at large tail distances indicates that plasma from the magnetosheath often crosses the magnetopause locally along much of the length of the tail, and is evidence that the tail is "open."
Possible association of stratospheric aerosols and El Nino type events
Authors: Handler Paul
Published: February 2008

To determine whether the 1982/83 El Nino event and the eruption of El Chichon in April 1982 are related, eleven similar eruptions at low latitudes (<20°, tropical) between 1868 and 1980 were analyzed for subsequent warm sea surface temperature anomalies in the eastern tropical Pacific Ocean. The composite result using superposed epoch analysis show a significant warming of the sea surface temperatures in the region 0°‐10°S, 90° ‐180°W. The results are also consistent with the statistical dynamical model calculations of MacCracken and Luther which indicate atmospheric circulation changes similar to those thought to lead to an El Nino type event within a few seasons after the establishment of an El Chichon type aerosol. In addition the composite behavior of twenty eruptions at high latitudes (extratropical, >20°) show just the opposite behavior. They show significant cooling of the sea surface temperatures in this region especially in the fourth and fifth season after the eruption. The association is found to be significant at the 95% level.
Active airborne infrared laser system for identification of surface rock and minerals
Authors: Kahle Anne B. , Shumate Michael S. , Nash David B.
Published: February 2008

Emissivity and reflectivity in the thermal infrared spectral region (8‐13 µm) may be used to discriminate among rocks and minerals. Although considerable success has been achieved in remote sensing classification of rock types based on emissivity measurements made with NASA'S Thermal Infrared Multispectral Scanner (TIMS), classification based on reflectivity offers several advantages: much narrower bandwidths are used, higher signal to noise ratios are possible, and measurements are little affected by surface temperature. As a demonstration, an airborne CO2 laser instrument was flown along the margin of Death Valley, California. Measurements of spectral reflectance collected with this device were compared with emissivity measurements made with the TIMS. Data from either instrument provided the means for recognizing boundaries between geologic units including different rock types and fan surfaces of different ages.
Magma solitons
Authors: Scott David R. , Stevenson David J.
Published: February 2008

Motivated to understand the process of melt migration in the earth’s mantle, we have studied a generalised form of Darcy’s law that describes porous flow in a matrix that can deform by creep. We find a remarkable richness of phenomena, including a new class of solitons. These consist of shape‐preserving waves of high liquid fraction which buoyantly ascend through a stationary matrix. This may have important implications for the morphology and geochemistry of primary igneous processes, and applicability to other porous flow problems.
methane concentrations and source strengths in urban locations
Authors: Blake Donald R. , Woo Van H. , Tyler Stanley C. , Rowland F. Sherwood
Published: February 2008

Higher atmospheric concentrations of methane are found in air samples from urban locations than in contemporary samples at the same latitude in remote locations. Higher concentrations of several trace chlorocarbon gases are also found in the same urban samples than in the corresponding remote samples. The “urban excess”, i.e. urban concentration minus remote concentration, is generally 1000 to 2000 times larger on a molar basis for CH4 than for CCl3F. Because almost all CCl3F is emitted in urban environments, the urban release of CH4 is estimated from the observed molar ratios to be 30 to 60 megatons per year world‐wide.

The fraction of world‐wide methane release occurring in the urban environment can be estimated from the concentration ratios, urban to remote, for CH4 vs. CCl3F. About 8% to 15% of the atmospheric methane release is observed to occur in urban locations.
Eruption of basalt and andesite lava degasses 222Rn and 210Po
Authors: Gill J. , Williams R. , Bruland K.
Published: February 2008

Activities of 222Rn and 210Po were measured in a September, 1983, basic andesite lava from Arenal and a November, 1983, basalt from Kilauea, starting 3 and 1 days after eruption, respectively. In both cases, in‐growth patterns show that all Rn volatalized during eruption. Po degassing also was complete at Kilauea but only 84±10% at Arenal.
Paleomagnetism of the Dunn Point Formation (Nova Scotia): High paleolatitudes for the Avalon Terrane in the Late Ordovician
Authors: Van der Voo Rob , Johnson Rex J. E.
Published: February 2008

Volcanic flows of the Late Ordovician Dunn Point Formation contain a generally univectorial magnetization which passes the fold test. Its steep direction (D = 335°, I = −61°, k = 79, α95 = 4.2°, paleopole at 2°S, 136°E) is unlike any known for North America or for the Avalon terrane for post‐Ordovician time, so that a primary age of the magnetization appears to be very likely. The paleolatitude (42°S) for the Avalon terrane derived from this result is much higher than that predicted for the area on the basis of the cratonic North American apparent polar wander path, and a substantial post‐Ordovician displacement (>3500 km) of Avalon with respect to the craton can be deduced. In all likelihood the Avalon terrane did not collide with North America until Middle Devonian time. This collision produced the Acadian orogeny. High Ordovician paleolatitudes have also been obtained for northern Africa and for several localities in Hercynian Europe (the Armorica plate) and it is inferred that these areas may have drifted together until Late Ordovician time.
Particles upstream of the pre‐dawn bow shock: ISEE‐3 observations
Authors: Terasawa T. , Scholer M. , Ipavich F. M. , Hovestadt D. , Klecker B. , Gloeckler G. , Sanderson T. R. , Wenzel K.‐P. , Smith E. J.
Published: February 2008

The first observations of energetic ions (≳ 30 keV) in the region upstream of the pre‐dawn bow shock (0 ≳ X ≳ ‐ 60 Re) are presented. The intensity in this region is controlled by the direction of the interplanetary magnetic field (IMF) and is maximized when the IMF is around the spiral direction. The particle distributions are highly anisotropic with the anisotropy directed perpendicular to the magnetic field. In the E × B frame this perpendicular anisotropy is conserved and it is argued that the distribution is pancake‐like. This indicates that the energetic particles in the pre‐dawn upstream region have their origin in the near‐earth upstream region, from where they are convected by the solar wind perpendicular to the magnetic field. It is therefore concluded that acceleration occurs mainly near the nose of the bow shock and particle acceleration at the distant bow shock is weak.
Detection of the response of ozone in the middle atmosphere to short‐term solar ultraviolet variations
Authors: Keating G. M. , Brasseur G. P. , Nicholson J. Y. , De Rudder A.
Published: February 2008

The response of ozone to solar UV variation is determined in the middle atmosphere between the heights of 10 and 0.2 mb. The definitive isolation of the smaller variations associated with short‐term solar variability is accomplished only after removal of the larger changes of ozone related to temperature variations. Using this approach the correlation coefficients between detrended ozone (Nimbus 7 LIMS) and short‐term 205 nm solar variation (Nimbus 7 SBUV) are found to be much higher (0.9) than achieved in previous studies. The theoretical response time and amplitude of response of ozone in the middle atmosphere to observed short‐term solar UV variations is found to be in good agreement with observations, except near 0.2 mb. The corresponding long‐term response over the solar cycle is also estimated.
Earthquake prediction by means of change of chemical composition in mineral spring water
Authors: Koizumi Naoji , Yoshioka Ryuma , Kishimoto Yoshimichi
Published: February 2008

Since March 1977 we have measured Cl− concentration in the groundwater almost every day at Shiota mineral spring near the Yamasaki fault, which is situated in the northern Kinki and Chugoku Districts, Southwest Japan, to see if the Cl− concentration shows any variations which may be related to seismic activity near Shiota mineral spring. From March 1977 through December 1984 there occurred 6 earthquakes of M3.5 or greater near Shiota mineral spring and two of M6.0 or greater in the northern Kinki and Chugoku Districts. The Cl− concentration anomalously changed four times before these earthquakes though the pattern of preseismic changes of the Cl− concentration was not uniform. We conclude that measurement of Cl− concentration at a carefully chosen mineral spring near an active fault should give information useful for earthquake prediction.
On the excitation of the Earth' polar motion
Authors: Chao B. Fong
Published: February 2008

One of the conclusions reached by recent studies of Barnes et al. and later Hide was “that atmospheric excitation alone was sufficient to account for the observed polar motion over (the studied) period, that there is apparently no need to invoke substantial excitation either by the fluid core or …earthquakes.” The purpose of the present paper is to point out that their argument that led to the above conclusion is unjustifiable (hence whether the conclusion is in reality true or not is still an open question). I demonstrate this through a physical “thought” experiment and a numerical simulation. In essence, they show that if we want to compare a geophysically observed excitation function ψa(t) with the excitation function deduced (via deconvolution) from the polar motion observation m(t), we should do so directly (the “direct approach”). To compare m(t) with the polar motion computed (via convolution) from ψa( t) (the “integration approach”), as Barnes et al. and Hide did, is misleading.
High‐resolution auroral observations of the OI(7774) and OI(8446) multiplets
Authors: Hecht J. H. , Christensen A. B. , Pranke J. B.
Published: February 2008

High resolution observations of the OI(7774) and OI(8446) multiplet auroral emissions were made using a Fabry‐Perot spectrometer at Churchill, Canada during March, 1984. The intensity of each of these lines, relative to N2+ (4278) may depend on atmospheric composition. The OI(8446) emission linewidths are under 40 mÅ while the OI(7774) emission linewidths have two components; one narrow (∼ 40 mÅ) and one broad (∼ 275 mÅ). The broad to narrow intensity ratio increases with decreasing red to blue ratio. These results are consistent with electron impact excitation of O being the dominant source of both emissions at high altitudes. However, at lower thermospheric altitudes, dissociative excitation of O2 molecules may be an important source of OI(7774) emission.
Garnet‐perovskite transformation in CaGeO3: In‐situ X‐ray measurements using synchrotron radiation
Authors: Susaki J. , Akaogi M. , Akimoto S. , Shimomura O.
Published: February 2008

The garnet‐perovskite phase transformation in CaGeO3 was investigated in the pressure‐temperature region to 6.5 GPa and 1200°C using a cubic anvil type of high‐pressure apparatus combined with synchrotron radiation. In‐situ measurements with an energy dispersive x‐ray diffraction system enable us to carry out dynamical observation of the transformation. The equilibrium phase boundary between the garnet and perovskite phases was determined as P(GPa)= 6.9 ‐ 0.0008T(°C). The negative P‐T slope definitely established in the present study is in reasonable agreement with the value, −0.0023(8) GPa/°C, that was calculated from the thermochemical data on the enthalpy of transition. The molar volume change accompanied with this transformation was estimated to be about 13% at about 6 GPa and 1000°C.
Cometary rays: Magnetically channeled outflow
Authors: Wolff R. S. , Siscoe G. L. , Sibeck D. G. , Neugebauer M. M.
Published: February 2008

A model of the structure and evolution of cometary rays is described which considers the rays to be the channeled outflow of cometary ions from the ionosphere. Ions in the central tail ray flow down the channel formed by the tailward closing of the draped interplanetary field. Ions in the side rays flow through magnetic flux ropes which become embedded in the cometary ionosphere through Kelvin‐Helmholtz instability of the cometary ionopause. The flux ropes tunnel through the ionosphere, penetrate the ionopause at two points, and extend out into the solar wind on both sides of the comet. As the flux ropes are dragged through the ionosphere by the solar wind, they are continually resupplied with ionospheric plasma through Bohm diffusion driven by ion‐neutral collisions. Differences in observed cometary ray structure are attributed to differences in the radial distance and ecliptic latitude of different comets at the time of observation, as well as to variations in the interplanetary magnetic field direction and the gas production rate of the comet.
Paleomagnetic evidence for crustal and thin‐skinned rotations in the European Hercynides
Authors: Bachtadse Valerian , Van der Voo Rob
Published: February 2008

Devonian and Carboniferous paleomagnetic data from the European Hercynides show a coherent pattern in inclination but significant dispersion in declination. The declination anomalies with respect to expected declinations for stable Europe indicate a strong correlation with the regional changes in the structural trend of the mountain belt. Detailed analysis of the paleomagnetic and structural data revealed the highest degree of correlation between changes in strike and declination anomalies for external parts of the orogen, where thin‐skinned nappe emplacement has been most prevalent. The correlation between changes in strike and declination anomalies from interior parts of the Hercynides is less well defined and displays a greater degree of scatter. Nevertheless the partly secondary character of the arcuate shape can be demonstrated. Thus the internal Hercynides are at least in part a true orocline with secondary bending in the sense of S.W. Carey. We infer that the bending of the European Hercynides resulted from the combination of multiple deep‐reaching deformations of the European lithosphere as well as the effects of thin‐skinned rotations. The decreasing degree of rotations toward the crystalline interior of the belt appears to favor a geodynamic model involving indentations and/or buttressing effects. The indentation of Hercynian Europe by a microplate or an African promontory during the Hercynian orogeny might be one of the principal causes for the secondary bending and the associated nappe rotations.
Comet‐solar wind interaction: Dynamical length scales and models
Authors: Mendis D. A. , Smith E. J. , Tsurutani B. T. , Slavin J. A. , Jones D. E. , Siscoe G. L.
Published: February 2008

While the ICE magnetometer measurements at Comet Giacobini‐Zinner led to a number of unexpected discoveries such as the intense hydromagnetic turbulence, the large‐scale structure of the induced cometary magnetosphere that it delineated was close to theoretical predictions. While the strong draping of the magnetic field lines to form a magnetotail was anticipated, the structure of this magnetotail also indicated the existence of other predicted features. These included a weak shock or compression wave, an ionopause which at least partially impeded the penetration of the solar wind into the cometary ionosphere, and a magnetic barrier region where the magnetic field was piled up ahead of the ionopause. The inferred positions of these features enabled the estimation of the production rate of the neutrals from the nucleus (≈ 4×1028 mols/sec), as well as the strength of the coupling between the inflowing solar wind ions and the outflowing cometary neutrals.
Strong hydromagnetic turbulence associated with comet Giacobini‐Zinner
Authors: Tsurutani Bruce T. , Smith Edward J.
Published: February 2008

The turbulence surrounding comet Giacobini‐Zinner reached intensities three orders of magnitudes above that of “intermediate” solar wind conditions, was characterized by Δ B/Bo almost equal to one, and extended to cometocentric distances beyond 106 km. The strong spectral peak at f ∼ 10−2 Hz and the general intensity profile of the turbulence (a lack of emissions close to the nucleus [< 2.5 × 104 km], peak intensities near the bow wave, and a spatial extend > 106 km) are in excellent agreement with generation by plasma instabilities associated with the pickup of H2O+ group ions.

The total energy of the magnetic turbulence near f ∼ 10−2 Hz is estimated to be ∼ 5 × 1015 joules and that of ion energization due to solar wind pickup ∼6 × 1013 watts. It thus takes a minimum of ∼ 100 s to build up the magnetic turbulence. This scale is approximately the linear growth rate of the ion pickup instability, giving a consistent picture of the energy source of the turbulence.
Statics and dynamics of Giacobini‐Zinner magnetic tail
Authors: Siscoe G. L. , Slavin J. A. , Smith E. J. , Tsurutani B. T. , Jones D. E. , Mendis D. A.
Published: February 2008

We subject the data ICE collected in its traverse of the tail of Giacobini‐Zinner to the requirements of stress balance. The result provides estimates for unmeasured quantities. We note that unlike planetary magnetotails, the comet's tail is embedded in an ionosheath that has a static pressure nearly equal to the solar wind stagnation pressure. This leads to a large lobe field strength. Transverse pressure balance reveals a systematic variation in the ion temperature across the tail which in turn tells how the pick‐up velocity of new ions varies across the pick‐up region. We find the pick‐up velocity increases from 14 km s−1 in the plasma sheet to 46 km s−1 in the ionosheath. This is the same velocity shear that stretches the magnetic field to form the tail. Imposing axial stress balance leads to an expression for the strength of the lobe field which reveals that it varies weakly with axial distance. The expression also gives an estimate of 24 km s−1 for the plasma sheet velocity at ICE's location. We find that the plasma takes between 7 and 13 minutes to flow through the coma to the position of ICE in the plasma sheet.
Groundbased infrared imaging of Comet Giacobini‐Zinner: The distribution of dust during the ICE flyby
Authors: Campins H. , Telesco C. M. , Decher R. , Mozurkewich D. , Thronson H. A.
Published: February 2008

A thermal‐infrared image of comet Giacobini‐Zinner has been used to determine the spatial distribution of dust grains in the Comet's coma. In the present paper we emphasize the results of this imaging which bear on the expected impact rate of dust grains on the International Cometary Explorer (ICE). the expected impact rate is calculated for a range of grain radii. For particle sizes comparable to those inferred for most comets, the impact rate is on the order of several hundred per second. This rate is in agreement with the predictions by Divine (1985), but it is more than two orders of magnitude larger than the rate observed by ICE if the observed impacts were on the spacecraft body. However, the observed impact rate is roughly comparable to that inferred from our infrared imaging if, as suggested by Gurnett et al. (this issue), the impacts were on the ICE antennae, which had a much smaller cross‐sectional area than the spacecraft body.
Arctic Ocean chronology confirmed by accelerator 14C dating
Authors: Clark David L. , Andree Michael , Broecker Wallace S. , Mix Alan C. , Bonani Georges , Hofmann H. J. , Morenzoni Elvezio , Nessi Marzio , Suter Martin , Woelfli Willy
Published: February 2008

The role of the Arctic Ocean in Earth's climatic and oceanographic development is significant but has become controversial because of disagreements concerning a reliable Arctic chronology. The first Arctic Ocean chronology based on U‐Th and 14C data was in agreement with magnetostratigraphy developed later but these data have been challenged by recent amino acid diagenesis dates. New accelerator 14C data support the earlier U‐Th‐magnetostratigraphic dates and confirm the reliability of the established Arctic Ocean chronology.
A remanent and induced magnetization model of Magsat vector anomalies over the west African craton
Authors: Toft P. B. , Haggerty S. E.
Published: February 2008

Scalar and vector Magsat anomalies over the west African craton are analysed by forward and inverse models. A forward model of the Man shield is based on Liberia. Induced magnetization contrasts due to sporadic iron‐formations and to regional metamorphic rocks, and a contrast in remanent magnetization within the lower crust are included. This combination reproduces the location, magnitude and adopted local zero level of anomalies in the initial Magsat maps. An inverse model of the Reguibat shield estimates the magnetization contrast of its, lithosphere, and when magnetism is restricted to shallower than 75 km both shields can be represented by a susceptibility contrast of +0.02. A residual anomaly between the shields involves a relative deficiency of induced magnetization along with other causes.
Bimodality of the solar cycle
Authors: Rabin Douglas , Wilson Robert W. , Moore Ronald L.
Published: February 2008

For sunspot cycles 1‐20 (1755‐1976), all cycles occurred in strings (two to six cycles in length) during which the period remained longer or shorter than the sample mean period. These strings have coincided with long‐term trends of growth or decay in the amplitude of the cycle. In six out of six cases, the period of the cycle has switched from long to short (or the reverse) in coincidence with turning points in the long‐term trend. This suggests that the solar dynamo has two modes with different mean periods. In the short‐period mode, the amplitude of the cycle grows; in the long‐period mode, the amplitude decays. The transition between modes has occurred at irregular intervals. A persistence of the long‐period mode would eventually produce a grand minimum such as the Maunder minimum; a persistence of the short‐period mode would produce a grand maximum. Unless the present interval between transitions turns out to be shorter than any previously observed interval, the present cycle (cycle 21) is part of a long‐period, decaying trend and will be of longer‐than‐average duration (>133 months).
Fine Structure of a Postfailure Wadati‐Benioff Zone
Authors: Stolte Christian , McNally Karen C. , González‐Ruiz Jaime , Simila Gerry W. , Reyes Alfonso , Rebollar Cecilio , Munguía Luis , Mendoza Luis
Published: February 2008

The hypocentral distribution of locally recorded aftershocks of the great (Ms=8.1) Michoacan, Mexico, earthquake of September 19, 1985, defines a narrow Wadati‐Benioff zone structure, roughly 10 km thick, dipping 14° at N23°E. This is in good agreement with the source geometry obtained by waveform modeling of the 1985 Michoacan mainshock and the large 1979 Petatlán earthquake in the adjoining region. We inverted for the crustal velocity structure in the epicentral region by applying the Levenberg‐Marquardt non‐linear least squares algorithm to our local aftershock data. The velocity model consists of a layer with linearly increasing velocity in depth overlying a dipping, constant velocity halfspace. Our hypocentral location program uses a velocity model of the same form together with ray tracing. The earthquake hypocentral resolution obtained with this program is significantly better than that from conventional approaches (HYPO) and looks very promising for use in velocity structures with an important dipping interface like subduction zones.
Detection of cometary pickup ions up to 107 km from comet Halley : Suisei observation
Authors: Terasawa T. , Mukai T. , Miyake W. , Kitayama M. , Hirao K.
Published: February 2008

Cometary pickup protons were observed as far as 9.7 × 106 km away from the cometary nucleus, while the pickup cometary ions of water‐group (expected to be mainly O+) were observed as far as 4.0 × 106 km. Observation of these cometary ions is based on the identification of a specific feature in the phase space distribution of ions (ion‐pickup shell). We found that the pickup shell is usually filled only partially (∼50%). This observation suggests that the efficiency of the energy diffusion is comparable to that of the pitch angle diffusion. From the spatial asymmetry in the cometary ion density observed between inbound and outbound parts of the spacecraft orbit, it is suggested that there was a difference of a factor of 2‐3 between the ion production rates around the inbound and outbound orbit.
Numerical BGK‐solutions of large scale electrostatic potential in auroral plasmas
Authors: Washimi Haruichi , Katanuma Isao
Published: February 2008

Formation of large scale and large amplitude electrostatic potential along magnetic field lines due to a high‐speed plasma flow in auroral plasma is discussed. It is shown that the potential profile which is consistent with observations, i.e., which increases very slowly near the plasmasheet and abruptly near the topside ionosphere, can be obtained by introducing a population of electrons which are trapped between the magnetic mirror and the electrostatic potentials.
Direct measurement of stick‐slip and shear fracture energy of granite at elevated pressures
Authors: Shi X. J. , Guo Z. Q. , Wang C. Y. , Hasegawa T.
Published: February 2008

The dynamics of frictional sliding in stick‐slip for a precut Westerly granite is studied at elevated pressures. Piston displacement measured outside the pressure vessel shows a small but significant delay (∼ 2 ms) during a stick‐slip. As a result, most part of the stress‐drop occurs with little piston displacement while most piston displacement occurs while the stress has already dropped to a residual level, giving a peculiar "L"‐shape in the stress‐displacement curve. On the other hand, displacement measured using an internal transducer mounted directly across the rock fracture shows that most slip on the rock fracture occurs concurrently with stress‐drop. Based on the directly measured stress‐displacement histories, we estimate a critical slip‐weakening displacement of 200 µm and a shear‐fracture energy of 300 J/m² at a normal stress of 85 MPa. This result shows that the discrepancy between the extrapolated values for the shear fracture energy [Okubo and Dieterich, 1984] and the estimated values given here and by Rice [1980] cannot be accounted for by different modes of measuring displacement.
Satellite interferometric measurements of auroral kilometric radiation
Authors: Baumback M. M. , Gurnett D. A. , Calvert W. , Shawhan S. D.
Published: February 2008

The first satellite interferometric measurements of auroral kilometric radiation (AKR) were performed by cross‐correlating the waveforms detected by the ISEE 1 and ISEE 2 spacecraft. Such correlations were measured at 125 and 250 kHz for projected baselines perpendicular to the source direction ranging from 20 to 3868 km. High correlations were found for all projected baselines, with little or no tendency to decrease even for the longest baselines. These results must be interpreted differently for incoherent and coherent radiation. For incoherent radiation, the correlation as a function of the baseline is the Fourier transform of the source brightness distribution, and this implies an average source region diameter for all of the bursts analyzed of less than about 10 km. For such small source diameters, the required growth rates are too large to be explained by existing incoherent theories, strongly indicating that the radiation must be coherent. For coherent radiation, an upper limit to the source region diameter can be inferred instead from the angular width of the radiation pattern. The close similarity of the spectra at the longest baselines indicates that the angular width of the radiation pattern must be at least 2.5°, implying that the diameter of the source must be less than about 20 km. At present, the proposed closed‐loop radio lasing model is the only known mechanism for providing sources this small.
Kinetics of the reaction ClO+ClO→products and its potential relevance to Antarctic ozone
Authors: Hayman G. D. , Davies J. M. , Cox R. A.
Published: February 2008

The reaction ClO + ClO → products has been studied by a molecular modulation technique as a function of temperature from 268 to 338 K using pressures from 2 to 700 Torr. The reaction is found to have a strong pressure dependence and to be second order under the conditions used. The data have been interpreted in terms of three pressure‐independent channels and a fourth, pressure‐dependent channel in which the ClO dimer is formed. The termolecular rate constant for the formation of the dimer is The title reaction is then discussed in the light of the large ozone depletions seen in the Antarctic spring. A catalytic cycle involving the ClO dimer is presented which may account for these observations.
Evidence for inner core anisotropy from free oscillations
Authors: Woodhouse John H. , Giardini Domenico , Li Xiang‐Dong
Published: February 2008

Previous hypotheses concerning the cause of anomalous splitting in free oscillation spectra have led to models which are difficult to accept from the physical point of view ‐ involving either substantial heterogeneity in the fluid outer core or large topographic variations in the core‐mantle boundary and the inner core boundary and heterogeneity of several percent in inner core properties. Furthermore, other seismological evidence, some of it pre‐existing and some of it very recently discovered, militates against these models. Within the framework of isotropic earth models there appears to be no acceptable explanation of the modal observations. Here we show that anisotropy in the inner core can produce an effect which is of the correct magnitude and which varies from mode to mode in approximately the observed manner. The data currently available are insufficient to objectively map the anisotropy, but the simple assumption of a constant elastic tensor which is invariant under rotations about the Earth's rotation axis (i.e. is transversely isotropic in the plane of the equator) matches well the gross features of the modal observations. This model does not entirely reconcile the modal data with traveltime observations; it is argued, however, that there exists an anisotropic model which will do so.
A search for lateral heterogeneity in the inner core from differential travel times near PKP‐D and PKP‐C
Authors: Cormier Vernon F. , Choy George L.
Published: February 2008

A search is made for lateral heterogeneity in the earth's inner core by analyzing the differential travel times between PKP‐DF, PKP‐CD and PKP‐Cdiff. These are P waves that are respectively transmitted through, reflected by, and diffracted around the inner core. From this data, heterogeneity in and near the inner core is found to be small. Residuals from PREM in the 134° to 157° range are bounded by ±0.8 sec., consistent with a ±0.9% variation in P velocity in the upper 200 km. of the inner core. The regional pattern of velocity variation inferred from the differential times is consistent with those obtained from global inversions of absolute PKP‐DF times.
Modeling seismic P‐Waves with cellular automata
Authors: Rothman Daniel H.
Published: February 2008

Cellular automata are arrays of discrete variables that follow local interaction rules and are capable of modeling many physical systems. A successful recent application has been in fluid dynamics, in which the Navier‐Stokes equations are solved by creating a model in which space, time, and the velocity of particles are all discrete. Acoustic waves can be obtained from these fluid models when perturbations of the idealized fluid are small. Because seismic P‐waves can be approximated by the acoustic wave equation, cellular automata can be adapted for seismic wave computations. This study shows how to model P‐waves in two dimensions by using a modified form of the cellular‐automaton rules for fluids. Propagation, reflection, and the computation of synthetic seismograms are demonstrated. Because no arithmetic calculations are needed and each lattice site can be updated simultaneously, this method is well‐suited for implementation on massively parallel computers. Among the many potential advantages are unconditional stability, no round‐off errors, and the possibility for devising novel approaches for modeling waves in inhomogeneous media.
On the current rate of formation of impact craters of varying sizes on the Earth and Moon
Authors: Baldwin Ralph B.
Published: February 2008

In Baldwin [1985] absolute ages were derived for 359 post‐Imbrium lunar craters. There is a gradual increase in frequency of crater formation in the last 3.2×109y. From updated analyses of these ages and trends, the frequency of formation of craters of different sizes on the moon at the present rate was determined.
The far ultraviolet vehicle glow of the S3‐4 satellite
Authors: Conway R. R. , Meier R. R. , Strobel D. F. , Huffman R. E.
Published: February 2008

Nadir‐viewing far ultraviolet observations obtained on the S3‐4 satellite show that during the daytime the N2 Lyman‐Birge ‐Hopfield (LBH) emission exhibits a change in vibrational distribution and becomes substantially brighter than predicted from photoelectron theory when the satellite is near perigee. At night, the intensity depends on altitude and no emission is detected when the vehicle is above 230 km. The altitude variation of the signal is proportional to the cube of the N2 density or the product of the square of the N2 density and the O density. We conclude that there is an LBH emission which is excited by the interaction of the vehicle with the ambient atmosphere, and that it is this emission which was previously interpreted as nightglow. The vibrational distribution is sharply peaked at v′ = 0. The altitude variation of the emission suggests a three stage excitation process in which N2 is adsorbed, collisionally excited, and finally electronically excited and desorbed by ambient N2 collisions, but physically reasonable reaction rates fail to account for the observed intensities.