bluuebunny/crossref_metadata_2025 huggingface dataset:
Apatite thermochronometry of Western Canadian Shield: Implications for origin of the Williston Basin
Authors: Crowley Kevin D. , Kuhlman Steven L.
Published: February 2008

Apatite fission‐track ages of basement from the Superior craton in eastern Manitoba, western Ontario and eastern North Dakota record two periods of Late Proterozoic epeirogeny and erosion. The latter period, which involves basement beneath and adjacent to the eastern flank of the Williston Basin, precedes basin subsidence and roughly coincides in time with breakup of the Proterozoic supercontinent. We speculate that these events are genetically related.
Thermal response of the f‐region ionosphere for conditions of large HF‐induced electron‐temperature enhancements
Authors: Newman A. L. , Carlson H. C. , Mantas G. P. , Djuth F. T.
Published: February 2008

Under current solar minimum conditions at Arecibo, Puerto Rico, large (∼1000–2000 K) enhancements in electron temperature are observed in the winter, nighttime ionosphere when high‐power 3‐MHz radio waves reflect near the F region peak. Computational modeling has been performed to determine the cause of the unusually large temperature enhancements. The results indicate that low collisional cooling rates combined with strong thermal conduction along geomagnetic field lines play a key role in elevating F region temperatures and spreading the electron temperature enhancements far outside of the region where radio wave energy is deposited. Thus, the dramatically large temperature enhancements are attributable primarily to expected low cooling rates of the electron gas, rather than unexpectedly high heating rates.
Iridium abundance maxima in the Upper Cenomanian extinction interval
Authors: Orth C. J. , Attrep M. , Mao X. Y. , Kauffman E. G. , Diner R. , Elder W. P.
Published: February 2008

Two iridium abundance peaks, both 0.11 ppb (whole‐rock basis) over local background of 0.017 ppb, have been found in Middle Cretaceous marine rocks near Pueblo, Colorado. They occur just below the 92‐million‐year‐old Cenomanian‐Turonian (C‐T) stage boundary. No other peaks were found in 45 meters of strata (∼2.5 million years of deposition) above and below the boundary interval. The broad lower peak straddles the first in a series of extinctions of benthic and nektonic macro biota which comprise the C‐T extinction event. The sharp upper peak occurs stratigraphically about 1.2 meters above the lower peak. The excess Ir might be from meteoroid impacts although no microspherules or shocked‐mineral grains have yet been found and several elements not normally associated with meteorites (Sc, Ti, and Mn) are enriched at the Ir peaks. Alternatively, several terrestrial Ir enrichment processes are suggested.
Source of the Australasian Tektite Strewn Field: A possible off‐shore impact site
Authors: Schnetzler C. C. , Walter L. S. , Marsh J. G.
Published: February 2008

Although there is a preponderance of evidence that tektites were formed by asteroid impacts on the Earth, no source crater has been found for the largest and youngest of the strewn fields‐the Australasian strewn field. We have examined a combined Seasat/Geos 3 altimeter data set of sea surface heights in the northern portion of the Australasian strewn field for negative gravity anomalies on the continental shelf and slope which might be related to the source crater for these tektites. A large negative anomaly, which we call the Qui Nhon Slope Anomaly, is a sea surface depression of approximately 1.5 meters over an area of 100 km diameter, centered at 13° 47′N latitude and 110° 37′ E longitude. It corresponds to a gravity anomaly of about −50 mgal. A surface‐ship gravimeter track across most of the structure confirms the satellite measurements. This site is on the continental slope in 1 to 3 km of water. A sea floor topographic depression of more than 200 meters occurs in the southeast quadrant of the anomaly. The center of the structure is 175 km offshore and is less than 300 km from the Muong Nong substrewn field and the prolific Dalat site, perhaps the largest concentration of tektites within the Australasian field. Due to its location, size, and magnitude we propose this anomaly may be due to the impact structure that produced the Australasian strewn field.
The Ree and εNd of 40–70 Ma old fish debris from the west‐African platform
Authors: Grandjean Patricia , Cappetta Henri , Albarède Francis
Published: February 2008

REE concentrations and Nd isotopic compositions have been determined in Late Cretaceous‐Early Cenozoic fish debris from West African and Israeli platform sediments and show a strong regional control independent of time, except for those from the Northern Morocco. It is suggested that, although diagenetic fluids do not contribute REE's directly to the phosphatic debris, they may significantly change the Ce anomaly and the εNd(T) value of the epicontinental seas, which, in turn, control the REE in marine phosphates. It is shown that, over the investigated period, the North Moroccan shelf seawater was progressively flushed by deep North‐Atlantic water.
Trapping of magma at midcrustal density discontinuities
Authors: Glazner Allen F. , Ussler William
Published: February 2008

Seismic anomalies which are interpreted as midcrustal magma bodies are present in several areas in the Basin and Range province of the western United States. In general, the upper boundaries of these inferred magma bodies coincide in depth with prominent midcrustal increases in P‐wave velocity. Crustal density profiles calculated from measured P‐wave velocity profiles indicate that near‐liquidus basalts should be less dense than the crust below the midcrustal velocity increases, and more dense above them. This implies that rising mafic magmas are trapped where they lose buoyancy at midcrustal density discontinuities. Buoyant trapping may generate large silicic magma systems by focussing ascending mafic magmas into a small depth interval in the crust.
First VHF radar observation of midlatitude F‐region field‐aligned irregularities
Authors: Fukao S. , McClure J. P. , Ito A. , Sato T. , Kimura I. , Tsuda T. , Kato S.
Published: February 2008

We describe the first midlatitude F‐region VHF radar observations of small‐scale Field‐Aligned Irregularities (FAI). Looking northward at ∼ 34° elevation the locus of perpendicularity with the geomagnetic field B remains inside the radar beam, such that FAI can be observed, at radar ranges from 350 to over 800 km or F region altitudes from 200 to 500 km, approximately. The preliminary results presented here show the large‐scale (> 20 km) configuration of small‐scale (322.6 cm, half the radar wavelength) F‐region irregularities as well as typical spectral results. Our most important finding is that at least on some occasions there is a quasi‐periodic variation in time and space, on scales of approximately 12 minutes and 50 km, of the observed plasma instabilities.
Elemental and mineralogical analyses using geochemical logs from the Cajon Pass Scientific Drillhole, California, and their preliminary comparison with core analyses
Authors: Anderson Roger N. , Dove Roy E. , Broglia Cristina , Silver Leon T. , James Eric W. , Chappell B. W.
Published: February 2008

Estimates of elemental and mineralogical abundances from geochemical logs are compared to preliminary chemical and modal analyses from cores in the Cajon Pass Scientific Drillhole. Accuracies of log‐computed weight percent oxide and mineralogy determinations range from 10 to 30%.
Paleomagnetic evidence for long‐term (104 year) Memory and periodic behavior in the Earth's core dynamo process
Authors: Lund Steve P. , Liddicoat Joseph C. , Lajoie Kenneth R. , Henyey Thomas L. , Robinson Steve W.
Published: February 2008

A new paleomagnetic secular variation (PSV) record from the late Quaternary Wilson Creek beds of Mono Lake, California contains a distinctive periodic vector waveform that follows and is almost certainly related to the Mono Lake excursion. A comparison of all published PSV results from the Wilson Creek beds suggests that the magnetic field at Mono Lake went through an interval (36,000‐28,000 ybp) of very low amplitude PSV followed by the Mono Lake excursion (28,000‐27,000 ybp) and four subsequent recurrences (27,000‐12,500 ybp) of the excursion waveform with relatively diminished amplitudes (lower than the excursion amplitude but higher than PSV amplitudes at Mono Lake since 12,500 ybp). This pattern clearly suggests that the Mono Lake excursion is related to ‘typical’ PSV; it also suggests that the core dynamo process responsible for PSV is capable of near impulse onset, persistence over a time scale of 15,000 years, and quasi‐periodic behavior that is non‐wave‐length dispersive in time.
Protons and alpha particles in field‐aligned beams upstream of the bow shock
Authors: Ipavich F. M. , Gloeckler G. , Hamilton D. C. , Kistler L. M. , Gosling J. T.
Published: February 2008

We report measurements of H+ and He++ ions in field‐aligned beams (∼ 10 keV/nuc) made with the AMPTECCE spacecraft. The proton beam population has a density ≲ 1% of the solar wind density and a significant thermal anisotropy (ω⟂ > ω∥), in agreement with previous observations. The beam density is found to be anti‐correlated with the angle θBn. The observed beam velocities are in reasonable agreement with the “direct reflection” model, in which a portion of the solar wind is reflected and energized at the earth's bow shock, but are consistently larger than expected from magnetosheath leakage models. The He++ ions in the beams have approximately the same velocity as the H+ ions, but the He++ to H+ density ratio is dramatically smaller than that measured simultaneously in the solar wind. Our beam observations were obtained during several of the time intervals previously analyzed by other workers, using data from the same spacecraft, who attributed the streaming upstream ions to magnetospheric rather than bow shock origin.
Observations of the Earth's bow shock under high Mach number/high plasma beta solar wind conditions
Authors: Winterhalter Daniel , Kivelson Margaret G.
Published: February 2008

Using 221 observations of the earth's bow shock by ISEE‐1 we investigate the magnetic field characteristics of the shock as a function of Mach number and plasma beta. We find that immediately behind the overshoot of the shock large amplitude waves develop preferentially with increasing Alfvén Mach number and/or plasma beta. Their probability of occurrence is very high in the parallel regime (θBN ≤ 10°), and declines to near zero probability when θBN reaches 75°. It is estimated that approximately 10% of the total solar wind energy density is dissipated by these waves.
Aerosol black carbon measurements at the South Pole: Initial results, 1986‐1987
Authors: Hansen A. D. A. , Bodhaine B. A. , Dutton E. G. , Schnell R. C.
Published: February 2008

In December 1986 an aethalometer was installed at the NOAA/GMCC South Pole Observatory to measure concentrations of the combustion effluent tracer species aerosol black carbon (BC) with a time resolution of one hour. We present data covering a 1‐yr period from December 1986 through November 1987. The hourly data show infrequent events in which the concentrations increased greatly for periods of a few hours. We attribute these events to local contamination and identified them as such in the database. The remaining background data then yield daily average BC concentrations generally ranging from 50 pg m−3 to 5 ng m−3, with a minimum in the early austral winter. The results imply long‐range transport of this aerosol species, and suggest a minimum value of the order of 10 pg m−3 for its global background concentration.
Thermal structure of the low velocity zone derived from laboratory and seismic investigations
Authors: Sato Hiroki , Sacks I. Selwyn , Murase Tsutomu , Scarfe Christopher M.
Published: February 2008

Laboratory measurements of seismic velocities in upper mantle peridotite can be used to constrain the thermal state in the low velocity zone. The measurements, carried out at high temperature and high confining pressure, can be extrapolated to upper mantle conditions. We compare laboratory velocities with seismic observations, and estimate temperatures and partial melt fractions in the low velocity zone. The comparison gives 1310°C and 7 vol.% partial melt at 45 km depth in 0‐5 my asthenosphere beneath the Iceland Plateau, and 1450°C and 3 vol.% partial melt at 85 km depth in 0‐20 my asthenosphere beneath the Pacific Ocean. The laboratory results show that up to a 6% velocity drop in the low velocity zone (relative to the lithospheric velocity) does not necessarily require the existence of partial melt. This implies that partial melt may not exist in the low velocity zone older than ∼5 my and ∼50 my in the Iceland Plateau and the Pacific Ocean, respectively.
Multiple remagnetizations of lower Paleozoic limestones from the Taconics of Vermont
Authors: Tucker Scott , Kent Dennis V.
Published: February 2008

The natural remanent magnetizations in some upper Cambrian and middle Ordovician limestones from the Taconics of Vermont (∼44°N 287°E) are multicomponent and can be attributed to early Paleozoic, late Paleozoic and Cretaceous‐early Tertiary remagnetizations. A northwesterly and up magnetization found at 2 sampling localities, characterized by maximum unblocking temperatures of about 350°C, has directions prior to tilt correction corresponding to Ordovician reference poles for North America and is likely to represent a Taconic remagnetization. A shallow, southerly magnetization found at a third locality, with unblocking temperatures that range to about 500°C, has directions corresponding to a Permian (Kiaman and Alleghenian) remagnetization. Common to all 3 sampling localities is a low (up to 300°C) unblocking temperature overprint similar in direction to Cretaceous to Paleocene reference poles.
AMPTE/CCE observations of substorm‐associated standing Alfvén waves in the midnight sector
Authors: Takahashi K. , Kokubun S. , Sakurai T. , McEntire R. W. , Potemra T. A. , Lopez R. E.
Published: February 2008

Magnetic field and medium energy particle data acquired with the AMPTE/CCE spacecraft have been used to study substorm‐associated ULF pulsations occurring in the midnight sector at a radial distance of 8 to 9 RE. The particle data are used not only to identify ion injections but also to detect the electric field of ULF waves. A case study of events observed on 23 May 1985 (day 143) shows that the waves have the properties of a fundamental‐mode standing Alfvén wave. The waves started nearly simultaneously with the Pi 2 pulsations observed at the Syowa station (L = 6.1) near midnight local time. However, different frequencies are observed on the ground and in space, which is consistent with the view that the Alfvén waves are excited at the local Alfvén resonance frequency. This CCE observation gives the first unambiguous evidence of substorm‐associated standing Alfvén waves in the nightside magnetosphere.
An artifact in the measurement of N2O from combustion sources
Authors: Muzio L. J. , Kramlich J. C.
Published: February 2008

An artifact in the use of sampling containers for the measurement of nitrous oxide emissions from fossil fuel combustion has been identified. The storage of moist combustion products containing SO2 and NO for periods as short as 2 hours can lead to the formation of several hundreds of parts‐per‐million of N2O in the sample containers where none originally existed. The drying of the gas in a 0°C trap before introduction into the container reduces, but does not eliminate N2O formation. Removing SO2 prior to the sample container appears to eliminate the artifact. This finding may have important consequences with respect to the validity of the existing data base on N2O emissions from combustion sources and the role of combustion generated N2O in the atmospheric N2O balance.
Meridional plasma flow in the outer heliosphere
Authors: Lazarus A. J. , Yedidia B. , Villanueva L. , McNutt R. L. , Belcher J. W. , Villante U. , Burlaga L. F.
Published: February 2008

Voyager 2 observations made in the outer heliosphere near 25 AU and within 2° of the heliographic equatorial plane show periodic variations in the meridional (North/South) flow velocities that are much more prominent than the East/West variations. An autocorrelation analysis shows that the flow variation has an ∼25.5 day period in the latter half of 1986, in approximate agreement with the solar rotation period. The turning of the flow from northward to southward is often accompanied by abrupt increases in solar wind speed and pressure consistent with changes expected at forward shocks, suggesting origins of non‐radial flow in stream dynamics. Approximately 80% of the time, the magnetic field data indicate that the northward flow is seen when Voyager is above the current sheet and southward flow when it is below, but the transitions from northward to southward and vice versa do not occur at the crossings of the current sheet. Thus increased pressure in interaction regions remains the best candidate for the driver of the non‐radial flows.
Temporal features of the outflow of heavy ionospheric ions in response to a high altitude plasma cavity
Authors: Singh Nagendra , Hwang K. S. , Torr D. G. , Richards P.
Published: February 2008

Using a hydrodynamic model for the plasma, it is demonstrated that the auroral plasma cavity is capable of drawing an appreciably large flux of oxygen ions, which are normally gravitationally bound. This escape mechanism of O+ does not involve any additional heating or acceleration of the plasma in the ionosphere. The temporal evolution of the outflow shows that it starts near the cavity and penetrates into the ionosphere with a flux front moving down with the ion‐acoustic speed. A steady outflow is reached in a few hours for a cavity at a height of one Re if the cavity is maintained. This time reduces to < 1 hour for cavity heights < 3000 km. During the transient state (< 1 hour) the outflux can appreciably exceed that in the steady state. In view of the horizontal convection, the transient outflow is of special significance. As the cavity descends, the ionospheric flux of O+ (Γo+) increases; for the lower edge of the cavity in the altitude range 3 ‐ 10 × 10³ km, the steady flux is found to be in the range 109 > Γo+ (cm−2s−1) > 107. An analysis for the steady state shows that outflow is controlled by the density scale height in the cavity.
The horse‐collar aurora: A frequent pattern of the aurora in quiet times
Authors: Hones E. W. , Craven J. D. , Frank L. A. , Evans D. S. , Newell P. T.
Published: February 2008

Reported here are DE 1 auroral imager observations of an auroral configuration which is given the name “horse‐collar aurora.” The horse‐collar pattern comprises the total area of auroral emissions from a single hemisphere and derives its name from the shape of the emitting area. The pattern is found in images recorded during quiet geomagnetic conditions and is possibly related to the theta aurora, another quiet time configuration of the auroras. This initial report of the DE 1 observations illustrates the horse‐collar aurora with a 2‐hour image sequence that displays its basic features and shows an example of its evolution into a theta‐like auroral pattern. The interplanetary magnetic field was northward during this image sequence and there is some evidence for IMF By influence on the temporal development of the horse‐collar pattern. A preliminary statistical analysis found the horse‐collar pattern appearing in one‐third or more of image sequences recorded during quiet conditions; it did not appear during disturbed conditions. Further study is required to establish more fully the characteristics of the horse‐collar aurora and to determine its implications concerning solar wind‐magnetosphere coupling when the IMF Bz is northward.
Structure and dating errors in the geologic time scale and periodicity in mass extinctions
Authors: Stothers Richard B.
Published: February 2008

Structure in the geologic time scale reflects a partly paleontological origin. As a result, ages of Cenozoic and Mesozoic stage boundaries exhibit a weak 28‐Myr periodicity that is similar to the strong 26‐Myr periodicity detected in mass extinctions of marine life by Raup and Sepkoski. Radiometric dating errors in the geologic time scale, to which the mass extinctions are stratigraphically tied, do not necessarily lessen the likelihood of a significant periodicity in mass extinctions, but do spread the acceptable values of the period over the range 25–27 Myr for the Harland et al. time scale or 25–30 Myr for the DNAG time scale. If the Odin time scale is adopted, acceptable periods fall between 24 and 33 Myr, but are not robust against dating errors. Some indirect evidence from independently‐dated flood‐basalt volcanic horizons tends to favor the Odin time scale.
Radioactivity of the ocean floor and marine phosphorite deposits: Observations with a new deep‐towed scintillometer
Authors: Jones E. J. W.
Published: February 2008

A new bottom‐towed scintillometer has been deployed in the tropical Atlantic to investigate whether lithological changes on the deep‐sea bed can be determined from variations in its natural radioactivity. Although count rates over the blanket of recent pelagic carbonates fall within the seawater background, high rates of gamma ray emission were detected on the crest of a seamount near 9° N, 21° W. Seismic profiles, bottom sampling and photography demonstrate that the radiometric anomalies coincide with areas in which the veneer of pelagic ooze is locally absent and phosphatized limestones of Cenozoic age (P2O5 > 14%) are exposed. The contrast in the U contents of the phosphorites (∼30 ppm) and pelagic carbonates (<3 ppm) appears to be responsible for producing the prominent radiometric signatures. Since phosphorites on continental margins and oceanic plateaus are generally more enriched in U than the seamount deposits (often >100 ppm), profiling the levels of seabed radioactivity would be a relatively rapid means of defining the distribution of phosphorite in many parts of the oceans.
Cross‐fault triggering in the November 1987 Superstition Hills Earthquake Sequence, southern California
Authors: Hudnut K. W. , Seeber L. , Pacheco J.
Published: February 2008

Two large strike‐slip ruptures 11.4 hours apart occurred on intersecting, nearly orthogonal, vertical faults during the November 1987 Superstition Hills earthquake sequence in southern California. This sequence is the latest in a northwestward progression of earthquakes (1979, 1981, and 1987) rupturing a set of parallel left‐lateral cross‐faults that trend northeast between the Brawley seismic zone and Superstition Hills fault, a northwest trending main strand of the San Jacinto fault zone. The first large event (MS=6.2) in the 1987 sequence ruptured the Elmore Ranch fault, a cross‐fault that strikes northeasterly between the Brawley seismic zone and the Superstition Hills main fault. The second event (MS=6.6) initiated its rupture at the intersection of the cross‐fault and main fault and propagated towards the southeast along the main fault. The following hypotheses are advanced; (1) slip on the cross‐fault locally decreased normal stress on the main fault, and triggered the main fault rupture after a delay; and (2) the delay was caused by fluid diffusion. It is inferred that the observed northwestward progression of ruptures on cross‐faults may continue. The next cross‐fault expected to rupture intersects both the San Andreas fault and the San Jacinto fault zone. We hypothesize that rupture of this cross‐fault may trigger rupture on either of these main faults by a mechanism similar to that which occurred in the Superstition Hills earthquake sequence.
The effects of Spall on regional seismograms
Authors: Taylor Steven R. , Randall George E.
Published: February 2008

The effects of spall on regional signals are investigated using synthetic seismograms to frequencies of 20 Hz. Depending on the scaling relationships used, it is found that spall may have an important effect on regional body waves and especially surface waves. The frequency range in which the spall effect dominates is dependent upon the characteristic times of the spallation process, but generally occurs in the frequency range of about 0.2 to 2 Hz. Because of the peaked nature of the radiated spall spectra, effects at frequencies outside the 0.2 to 2 Hz frequency range will be dominated by the explosion source. The frequency range and strength of the spall effect depend in a complicated manner upon yield, depth of burial, near‐source structure, and material strength. Uncertainties in the spall scaling relationships need to be better understood before the effects of spall on regional discrimination and yield estimation can be properly quantified.
A diurnal modulation of the lunar tide in the upper atmosphere
Authors: Stening R. J.
Published: February 2008

Analysis of geomagnetic data from several stations in the equatorial region shows that the amplitude of the lunar semimonthly tide frequently does not have the expected symmetry about noon in its diurnal variation. There is a distinct tendency for larger amplitudes in the morning. At many stations this effect is largest in equinoctial months, though there are regional differences. Analysis of data from different years confirms its presence. The interaction between solar and lunar tides in the middle atmosphere may be responsible for the observed modulation.
Solar cycle variation of the upper atmosphere temperature of Mars
Authors: Bauer S. J. , Hantsch M. H.
Published: February 2008

Neutral gas temperatures inferred from topside plasma scale heights of electron density distributions obtained from observations by US and USSR Mars missions imply a much higher dependence on solar activity (expressed by the 10.7 cm radio flux F10.7) than that found for Venus. This dependence, however, does not appear to be consistent with the observed solar cycle dependence of ionospheric peak plasma densities. The reason for this discrepancy seems to lie in the fact, that photochemical equilibrium applies only to altitudes below 170 km, whereas topside scale heights are usually derived for a much greater altitude range and thus may be modified by transport and other processes. If scale heights are obtained by matching a Chapman‐layer to plasma density profiles near the ionosphere peak, the derived neutral temperatures show a much weaker dependence on F10.7, in fact one essentially the same as for Venus. Thus, the response of the upper atmosphere of Mars, at least near the ionospheric peak, appears to be virtually identical to that of Venus.
Core modes of the Earth's free oscillations and structure of the inner core
Authors: Fukao Y. , Suda N.
Published: February 2008

Core modes are spheroidal modes of the Earth's free oscillations whose energy is dominantly partitioned into the inner core as shear energy. We analyzed a total of 87 IDA records of the Earth's free oscillations to detect three core modes 2S2, 6S2 and 7S3 at periods of about 1050, 410 and 320s, respectively. We used a new autoregressive technique of spectral analysis that determines the periods and Q simultaneously. The Q of these core modes are consistently high, about 2400, 1500 and 1900, respectively. These high values directly demonstrate that the inner core is least dissipative within the Earth for long‐period shear waves. Our eigen‐frequency data are remarkably consistent with velocity model 1066A [Gilbert and Dziewonski, 1975]. Our attenuation data can be fully explained if the shear wave Q of the inner core is 1500 in the upper 200km and 3800 in the rest of about 1000km.
The adjustment of mantle plumes to changes in plate motion
Authors: Griffiths Ross W. , Richards Mark A.
Published: February 2008

The relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle, causing horizontal advection of mantle plumes as they rise toward the lithosphere. Consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered. Thus the shape of bends in the surface tracks of hotspots, resulting from changes in plate motion, will reflect the plume adjustment. Laboratory experiments, as well as computations using a simple theory developed in Richards & Griffiths [1988] for the dynamics of continuous plume conduits, demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit. Thus the sharpness of the bend at an age of 43Ma in the Hawaiian‐Emperor volcanic chain implies that the deflection of the underlying plume in that case was small (<200 km). This small deflection is expected for plumes carrying large buoyancy fluxes, and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability.
Diffusion and location of hydrochloric acid in ice: Implications for polar stratospheric clouds and ozone depletion
Authors: Wolff E. W. , Mulvaney R. , Oates K.
Published: February 2008

We have carried out experiments to study the incorporation and movement of HCl within the structure of ice. These involved freezing HCl solutions, and observing them in a scanning electron microscope fitted with an X‐ray microanalysis system. We are able to show that HCl is not easily incorporated into ice crystals, but is strongly partitioned towards the grain boundaries. Furthermore, the diffusion of HCl through ice crystals is slow. These results contradict the interpretation of earlier experiments. They mean that if HCl is to be available for reaction on polar stratospheric cloud particles, as required by current theories of Antarctic ozone depletion, then it must be present in some form other than a solid solution.
Systematic lidar measurements of the stratospheric ozone vertical distribution
Authors: Godin S. , Mégie G. , Pelon J.
Published: February 2008

Lidar measurements of the ozone vertical distribution have been performed on a routine basis at the Observatoire de Haute‐Provence, starting in October 1986. They provide a unique data base which is used to derive the observed seasonal behaviour of ozone in various altitude ranges from 25 to 45 km with a height resolution much higher than previous operational systems. Comparison with Umkehr measurements performed at the same location show a rather good agreement in layers 5 and 6 with unexpected differences appearing in the uppermost layers.
Waveform complexity in teleseismic broadband SH displacements: Slab diffractions or deep mantle reflections?
Authors: Lay Thorne , Young Christopher J.
Published: February 2008

Transverse component bodywave ground displacements from deep focus earthquakes exhibit complexities which are not accounted for by standard radially symmetric Earth models. This anomalous behavior is examined using 123 broadband teleseismic SH displacement recordings, reconstituted from short‐ and long‐period or broadband seismograms for 11 events in the Kurile subduction zone. The greatest complexity, involving an extra arrival between the S and ScS phases at epicentral distances greater than 72°, has a strong distance dependence and little azimuthal variation with respect to the source region, favoring an interpretation as a deep mantle triplication rather than a near‐source slab diffraction.
Simulating the core‐mantle boundary: An experimental study of high‐pressure reactions between silicates and liquid iron
Authors: Knittle Elise , Jeanloz Raymond
Published: February 2008

Experiments with the laser‐heated diamond cell show that (Mg,Fe)SiO3 perovskite reacts chemically with liquid iron at pressures greater than 70 GPa and temperatures above 3700 K. X‐ray diffraction analyses of quenched samples demonstrate that the reaction products include SiO2 stishovite and iron alloys (Fe,Mg)xO and FexSiy, at the interface between the Fe and silicate. Our results suggest that similar chemical reactions occur at the Earth's core‐mantle boundary, forming chemical heterogeneities composed of silicate‐rich and iron alloy‐rich regions in the D′ layer. This scenario is consistent with seismological observations.
The 11/25/88, M=6 Saguenay Earthquake near Chicoutimi, Quebec: Evidence for anisotropic wave propagation in northeastern North America
Authors: Hough S. E. , Jacob K. H. , Friberg P. A.
Published: February 2008

On November 25, 1988, a magnitude 6 earthquake occurred in the province of Quebec, Canada. This earthquake triggered nine digital strong motion instruments in New York and Maine at epicentral distances of 200 to 820 km which were installed as part of an effort by the National Center for Earthquake Engineering Research (NCEER) to study ground motions and wave propagation in eastern North America. We calculate Q(f) at discrete frequencies from 0.6 to 26 Hz, assuming that geometrical spreading causes a l/r0.5 decay in spectral amplitudes.

Of the nine stations, four are in the Adirondack Mountains in New York and three are in eastern Maine. If we calculate Q(f) for these two clusters of stations separately, we obtain higher values for the Adirondack stations. The Quebec‐Adirondack path is along the strike of the predominant structural trends in northeastern North America, in the Grenville Province crust, while the Quebec‐Maine path is at high angle to the structural grain and crosses the boundary between the Grenville and the Appalachian provinces. We thus have instrumental data in support of earlier observations based on contours of intensity from historic earthquakes: Seismic wave propagation in northeastern North America is more efficient along the predominantly NE‐SW striking geological trends. We address possible biases due to site effects.
Seismic detection of a low‐velocity layer beneath the southeast flank of Mauna Loa, Hawaii
Authors: Thurber Clifford H. , Li Yingping , Johnson Carl
Published: February 2008

We have identified seismic phases reflected off the top and bottom of a low velocity layer (LVL) by analysis of seismograms from six small earthquakes in Hawaii. These events occurred almost directly beneath station AIN of the Hawaiian Volcano Observatory (HVO) seismic network, located within the Kaoiki seismic zone on the southeast flank of Mauna Loa. The polarity reversals of the first reflected phases provide clear evidence for the existence of a LVL beneath this station. The LVL is estimated to be at a depth of about 11.5 km, with a thickness of about 800 m. The estimated depth to the top of the LVL is consistent with the depth distribution of low‐angle thrust faulting events from the aftershock sequence of the 1983 Kaoiki earthquake. Presumably, this zone is the marine sediment layer buried beneath the volcanic pile, along which the volcanic edifice may slip easily to cause large earthquakes. Mapping the interface between the volcanic pile and oceanic crust and thus determining the depth and thickness of the buried sediment layer is essential for understanding the tectonics of large earthquakes in Hawaii.
140 MHz auroral backscatter: Evidence for characteristic phase velocities below and above the ion acoustic speed
Authors: Haldoupis C. , Nielsen E.
Published: February 2008

The capability of Stare (Scandinavian twin auroral radar experiment) to perform cross beam measurements over a large area in the E region, is used to study prominent Doppler spectrum features of 140 MHz auroral backscatter. By analysing an extended spectral data base, two less common but distinct spectrum types were identified, in addition to the well known and commonly observed type 1 and type 2 spectra. The evidence shows that, at times, meter scale irregularities may propagate with preferential phase velocities not only near but also below and above the ion acoustic speed always inside well defined ranges from 150 to 250 m/s and from 900 to 1200 m/s, respectively. The new 140 MHz phase velocity values compare well with similar values reported previously with 50 MHz radars; this supports the notion of characteristic plasma wave phase velocities and not that of characteristic angular frequencies as it is for electrostatic ion cyclotron waves. These results are indicative of new plasma precesses in the auroral E region that need to be analytically developed and understood.
Paleomagnetic evidence for tertiary anticlockwise rotation in southwest Puerto Rico
Authors: Van Fossen Mickey C. , Channell James E. T. , Schellekens Johannes H.
Published: February 2008

Late Cretaceous and Eocene paleomagnetic results from calcareous mudstone, limestone, and volcaniclastic rocks in southwest Puerto Rico yield a significant anticlockwise discordance in magnetization direction relative to North American reference data. Progressive thermal demagnetization has revealed stable, characteristic high blocking temperature magnetization components that display an intrasite bipolarity. The direction of this stable magnetization in tilt‐corrected coordinates is consistently either northwest and down, or southeast and up. The pole position (39.4° N.Lat.; 209.6° E.Long.; dp = 9.3°; dm = 16.7°; N = 10) suggests post Eocene anticlockwise rotation of the sampling region relative to North America by 45° about a vertical axis. A rotating block domain model in which north‐south compression is superimposed on WNW‐ESE strike‐slip faults in SW Puerto Rico can explain the inferred rotation.
A simulation study of electron hole distributions
Authors: Ashour‐Abdalla Maha , Richard Robert L. , Coroniti Ferdinand V.
Published: February 2008

Narrowband electrostatic noise (NEN) is frequently observed in the Earth‧s distant magnetotail. NEN exhibits a spectral peak between 100 and 316 Hz and is very impulsive, having a peak amplitude in the range 10−4−10−3V/m√Hz. It has been suggested that NEN represents a plasma wave mode which results from a hole in the low energy electron velocity distribution. In this paper, it is shown by means of numerical simulation that a wave mode is excited in the presence of a modest sized electron hole. In the simulations the mode saturates at low amplitudes, in agreement with observations, without destroying the hole. It is suggested that the burst‐like behavior of NEN is due to the persistence of these nonstandard modes and their ability to be excited by a variety of weak free energy sources.
Venus ionopause during solar miminum
Authors: Mahajan K. K. , Mayr H. G.
Published: February 2008

During solar minimum, the Venus ionosphere is weak and the solar wind depresses the ionopause to a limiting altitude [Knudsen et al., 1987]. With the knowledge gained from the ion composition measurements on Pioneer Venus during solar maximum and during conditions of high solar wind pressure, we argue that the typical topside electron density profile at Venus during solar minimum has two distinct regimes; one from about 140 km (the altitude of peak electron density) to 180 km and the other above 180 km. While the former is dominated by O2+ ions which are in photochemical equilibrium, the latter is dominated by O+; ions which are strongly disturbed by the solar wind induced plasma transport. The disturbed ionosphere is formed in the photo‐dynamical regime and has a scale height which is several times smaller than that expected under undisturbed conditions when the ionosphere is in diffusive equilibrium. The small scale height of the disturbed ionosphere is nearly equal to that of the ionizable constituent, atomic oxygen, and is only slightly larger than the chemical equilibrium scale height of the underlying chemical equilibrium region. While the photodynamical ionopause occurs rarely during solar maximum and only when the solar wind pressure is large, we believe that this kind of ionopause is observed much more frequently during solar minimum. We find evidence for this in the radio occultation data from Pioneer Venus, Mariner 10 and Venera 9 and 10.
Neptune's visual albedo variations over a solar cycle: A pre‐Voyager look at ion‐induced nucleatlon and cloud formation in Neptune's troposphere
Authors: Moses Julianne I. , Allen Mark , Yung Yuk L.
Published: February 2008

High‐energy galactic cosmic rays can penetrate to deep levels within Neptune's atmosphere to form a substantial ionospheric layer in the lower stratosphere and upper troposphere of the planet. Because cosmic‐ray modulation in the interplanetary medium creates an inverse relationship between cosmic‐ray intensity and solar activity, the ionization rate in the lower atmosphere will vary with the 11‐year solar cycle in such a way that maximum ionization will occur at sunspot minimum and minimum ionization at sunspot maximum. This variable ionization may, by the process of ion‐induced nucleation, regulate the formation and optical properties of an upper tropospheric haze in the atmosphere of Neptune and could thus provide a mechanism for modulating the planet's visual brightness over a solar cycle. We estimate the rates of cosmic‐ray ionization at solar maximum and minimum for a range of proposed Neptune dipole magnetic field strengths; discuss the physics of formation of ion clusters and eventual aerosol particles; and, using a simple radiative transfer model, discuss the ways in which this variable aerosol formation can modulate the geometric albedo of Neptune over a solar cycle. If Voyager spacecraft observations reveal that Neptune's dipole magnetic field is about 1 Gauss or less, then our modeling indicates that variations in an aerosol layer due to differences in cosmic ray ionization may help explain some of the observed brightness variation of Neptune over a solar cycle.
Observations of teleseismic shear‐wave splitting in the basin and range from portable and permanent stations
Authors: Savage M. K. , Silver P. G. , Meyer R. P.
Published: February 2008

Observations of shear‐wave splitting were obtained from temporary and permanent stations in western Nevada and southern California. Measurements of fast polarization azimuth ϕ and delay time δt were made on both SKS and direct S phases. For stations in the northern Basin and Range the results were consistent, yielding an average of (ϕ, δt) = (+75 ± 8°, 0.9 ± 0.3 s). One station in the southern Basin and Range gave (+40 ± 11°, 0.4 ± 0.1 s) and one in the Mojave Desert gave (−54 ± 3°, 1.2 ± 0.1 s). δt = 0.9s is consistent with a 100km‐thick mantle layer characterized by 4% anisotropy. For the northern stations, the anisotropy appears unrelated to the present‐day extension or absolute plate motion. Rather, we suggest that it is ‘fossil’ anisotropy associated with pre‐Miocene extension, whose direction is about +68°. For the California station ϕ is nearly parallel to the strike of the San Andreas Fault system, and is attributed to shear strain associated with relative plate motion.
Ray tracing of broadband bursty radio emissions from Uranus
Authors: Curran D. B. , Menietti J. D. , Wong H. K.
Published: February 2008

To determine the source position of the broad‐band bursty emission, we traced rays of X‐mode emission from source positions along magnetic field lines with footprints that form a large grid centered approximately on the south magnetic pole of Uranus. We assumed the emission is just above the RX cutoff frequency, and we included sources only if the ratio of plasma to gyrofrequency was less than about 0.25. Contrary to the results of others who have assumed straight‐line propagation, we found, for large wave normal angles, source regions different from those producing b‐smooth emission, and along field lines with footprints in a relatively large area associated with the south magnetic pole. The emission which was observed prior to closest approach has a source along field lines that are distinct from those which generate emissions observed after closest approach. In order to understand these findings and the bursty nature of the emission we speculate the plasma distribution in the source region and/or the source mechanism is distinct from that which generates the b‐smooth emission.
Computer modeling of current collection by the Charge‐2 mother payload
Authors: Mandell M. J. , Lilley J. R. , Katz I. , Neubert T. , Myers Neil B.
Published: February 2008

The three‐dimensional computer codes NASCAP/LEO and POLAR have been used to calculate current collection by the mother payload of the CHARGE‐2 rocket under conditions of positive and negative potential up to several hundred volts. For negative bias (ion collection) the calculations lie about 25 percent above the data, indicating that the ions were less dense, colder, or heavier than the input parameters. For positive bias (electron collection) NASCAP/LEO and POLAR calculations show similar agreement with the measurements at the highest altitudes. This agreement indicates that the current is classically magnetically limited, even during electron beam emission. However, the calculated values fall well below the data at lower altitudes. We suggest that beam‐plasma‐neutral interactions are responsible for the high values of collected current at altitudes below 240 km.
Distribution of Tessera Terrain on Venus: Prediction for Magellan
Authors: Bindschadler D. L. , Kreslavsky M. A. , Ivanov M. A. , Head J. W. , Basilevsky A. T. , Shkuratov Yu. G.
Published: February 2008

Tessera terrain is the dominant tectonic unit in the northern hemisphere of Venus and is characterized by complex sets of intersecting structural trends and distinctive radar properties due to a high degree of meter and sub‐meter scale (5 cm to 10 m) roughness. Based on these distinctive radar properties, a prediction of the global distribution of tessera can be made using Pioneer Venus (PV) reflectivity and roughness data. Where available, Venera 15/16 and Arecibo images and PV diffuse scattering data were used to evaluate the prediction. From this assessment, we conclude that most of the regions with prediction values greater than 0.6 (out of 1) are likely to be tessera, and are almost certain to be tectonically deformed. Lada Terra and Phoebe Regio are very likely to contain tessera terrain, while much of Aphrodite Terra is most likely to be either tessera or a landform which has not yet been recognized on Venus. This prediction map will assist in targeting Magellan investigations of Venus tectonics.
Measurements of condensation nuclei in the Airborne Arctic Stratospheric Expedition: Observations of particle production in the polar vortex
Authors: Wilson J. C. , Stolzenburg M. R. , Clark W. E. , Loewenstein M. , Ferry G. V. , Chan K. R.
Published: February 2008

The ER‐2 Condensation Nucleus Counter (ER‐2 CNC) was operated in the Airborne Arctic Stratospheric Expedition (AASE) in January and February 1989. The ER‐2 CNC measures the mixing ratio of particles, CN, with diameters from approximately 0.02 μm to approximately 1 μm. The spatial distribution of CN in the Arctic polar vortex was found to resemble that measured in the Antarctic in the Spring of 1987. The vertical profile of CN in the vortex was lowered by subsidence. At altitudes above the minimum in the CN mixing ratio profile, CN mixing ratios correlated negatively with that of N2O, demonstrating new particle production. CN serve as nuclei in the formation of Polar Stratospheric Clouds (PSC's) and the concentration of CN can affect PSC properties.
Observations of polar stratospheric clouds in the Arctic winter 1989 at 79 N.
Authors: Krüger Bernd C.
Published: February 2008

Throughout January 1989 polar stratospheric clouds were frequently observed over Ny‐Ålesund/Svalbard by means of a lidar instrument. The aerosol layer was located between 17 and 24 km altitude and showed signs of a downward movement. The presence of PSC's suggests that the same chemical processes, that lead to the springtime ozone depletion in the southern hemisphere may go on during the Arctic polar night.