bluuebunny/crossref_metadata_2025 huggingface dataset:
The Caribbean Anomaly: Short‐wavelength lateral heterogeneity in the lower mantle
Authors: Bokelmann Götz H. R. , Silver Paul G.
Published: February 2008

Data from the Archean‐Proterozoic Transect Experiment 1989 (APT89) are used to constrain smaller‐scale velocity heterogeneity in the lower mantle beneath the Caribbean. We observe large variations in travel time residuals for paths from South America to portable broadband and short‐period stations distributed along a 1500km‐long transect in central North America. After accounting for near‐source and near‐reciever contributions, we find variations as large as 3s for P and 8s for S for rays that bottom around 1200km depth. This rapid spatial variation in travel times suggests that we are seeing the western edge of the Caribbean anomaly, a feature previously observed by several investigators. The gradient appears to be about 300km wide. Assuming a path length of 1500km through the anomaly (suggested from previous studies), we find that δνP/νP=2.1% and δνS/νS=3.6% which for S waves is about three times the estimate from long period body waves. The relative velocity ratio for this anomaly is δlnνS/δlnνP ≈1.7, a value slightly larger than typical laboratory values but lower than previous recent seismological studies of lower mantle heterogeneity. With increasing turning depth the observed anomaly appears to decrease in size, although it continues to be visible for shear waves with bottoming depths as great as 2200km. The presence of multiple direct S phases for several records of one of the events studied suggests multipathing induced by the large observed lateral velocity gradients required by the travel time data.
Haloe Antarctic observations in the spring of 1991
Authors: Russell James M. , Tuck Adrian F. , Gordley Larry L. , Park Jae H. , Drayson S. Roland , Harries John E. , Cicerone Ralph J. , Crutzen Paul J.
Published: February 2008

HALOE observations of O3, CH4, HF, H2O, NO, NO2, and HCℓ collected during the October 1991 Antarctic spring period are reported. The data show a constant CH4 mixing ratio of about 0.25 ppmv for the altitude range from 65 km down to about 25 km at the position of minimum wind speed in the vortex: i.e., the vortex center, and depressions in pressure versus longitude contours of NO, NO2, HF, and HCℓ in this same region. Water vapor, HF, and HCℓ enhancement are also observed in the vortex center region above ∼25 km. Between 10 and 20 km, the expected mixing ratio signatures exist within the vortex, i.e., low ozone and dehydration. The water vapor increased by 50%, and the ozone level doubled inside the vortex between October 11 and 24 in the 15 to 20 km layer. These changes imply a time constant for recovery from ozone hole conditions or 19 and 30 days for O3 and H2O, respectively. The data further show the presence of air inside the vortex between 3 and 30 mb which has mixing ratios characteristic of mid latitudes.
A possible paleoclimatic ENSO indicator in the spatial variation of annual tree‐ring 14C
Authors: Jirikowic John L. , Kalin Robert M.
Published: February 2008

Preliminary investigations show regional differences in atmospheric 14C activity archived in tree‐ring cellulose may reflect oceanic and atmospheric shifts associated with ENSO (El Niño‐Southern Oscillation) events. Much of the shared variation between sites in Washington and Arizona can be attributed to global forcings such as anthropogenic 14C‐depleted CO2 emissions and modulation of 14C production. The contrasted residual variation between the Washington and Arizona data sets correlates with the annual Southern Oscillation Index (SOI) of the previous year. The spatial differences between Δ14C data sets may provide a proxy indicator of ENSO phenomenon.
Domain observations of the magnetic phases in volcanics with self‐reversed magnetization
Authors: Haag M. , Heller F. , Lutz M. , Reusser E.
Published: February 2008

The temperature dependence of magnetic domain pattern in titanomagnetite and titanohematite of self‐reversing andesites from the 1985 eruption of the Nevado del Ruiz volcano (Colombia) has been observed using the Bitter technique. During heating/cooling cycles in the laboratory, the natural remanent magnetization loses the self‐reversal properties between 110°C and 180°C. The titanohematite domains disappear in the same temperature interval. Large grain regions without visible colloid contrast are observed between room temperature and 180°C. Utilizing the high resolution of a magnetic force microscope, it is found that these regions contain either domains with wall spacing of about 3 µm or a buckling pattern. Along the phase boundary between a titanomagnetite and a titanohematite grain but within the titanohematite, a 2 µm wide chemically different domain has been observed. It has a higher iron content than the main titanohematite. The two titanohematite phases represent two phases with higher Curie temperature, Tc, but lower spontaneous magnetization, Ms, and lower Tc but higher Ms, respectively. Negative interaction between these phases is called upon to explain the self‐reversing mechanism.
Regional patterns of hydrothermal alteration of sediments as interpreted from seafloor reflection coefficients, Middle Valley, Juan De Fuca Ridge
Authors: Rohr Kristin M. M. , Schmidt Ulrike , Groschel‐Becker Henrike
Published: February 2008

Reflection coefficients of the seafloor have been calculated from three multi‐channel seismic reflection profiles across Middle Valley of the Juan de Fuca ridge. Seafloor reflection coefficients in this sedimented rift valley are high over an active hydrothermal vent and adjacent to major offset faults. Comparison of our measurements to drilling results from Leg 139 shows that high reflection coefficients over an active vent mound are produced by cemented sediments. Large reflection coefficients adjacent to major faults may have a similar origin and indicate that ongoing faulting creates pathways for hydrothermal fluids which alter the sediments and result in higher densities and velocities. Since 30 Hz seismic energy responds to the top 50 m of sediments, we are looking at the integrated response of hydrothermal alteration over tens of thousands of years. This is the first time seafloor reflection coefficients have been used to identify highly altered sediments in a region of deep‐water hydrothermal activity.
Age of rifting and associated volcanism in Atla Regio, Venus
Authors: Basilevsky A. T.
Published: February 2008

An estimation of the age of rifting and associated volcanism in Atla Regio is carried out based on photogeologic analyses of the stratigraphic relationships between rift‐associated features and impact craters which have associated radar‐dark paraboloids. The paraboloids are believed to be thin mantles of debris derived from the fallout of crater ejecta, and their “parent” craters are considered to be among the youngest 10% of Venus' crater population [Campbell et al., 1992]. In the area under study, there are three impact craters with associated dark paraboloids: 1) Von Schuurman, which is clearly superimposed on the Dali Chasma Rift; 2) Sitwell, which is superimposed on Ganis Chasma and may have been followed by subsequent rift‐associated fracturing; and 3) Luxemburg, which is superimposed on lavas from Ozza Mons and dissected by Dali Chasma fractures which, in turn, are covered by lavas from Maat Mons. This means that at least the last stages of tectonic and volcanic rift‐associated activity in Atla Regio are contemporaneous with the formation of impact craters which are young enough to preserve the associated dark paraboloids. If these craters represent the youngest 10% of the Venus crater population, and the average age of the venusian surface is about 500 m.y. [Schaber et al., 1992], this means that rifting and rift‐associated volcanism in Atla Regio were active as recently as during the last ∼50 m.y. In the current debate over models of Venus resurfacing the results of our study favor the equilibrium resurfacing model [Phillips et al., 1992].
Raman dial measurements of stratospheric ozone in the presence of volcanic aerosols
Authors: McGee Thomas J. , Gross Michael , Ferrare Richard , Heaps William , Singh Upendra
Published: February 2008

Since the eruption of Mt. Pinatubo in June, 1991, measurements of atmospheric species which depend on Rayleigh scattering of radiation, have been severely compromised where the volcanic aerosol cloud exists. For the GSFC stratospheric ozone lidar, this has meant that ozone determination has been impossible below approximately 30 km. The GSFC lidar has been modified to detect Raman scattering from nitrogen molecules from transmitted laser wavelengths. The instrument transmits two laser wavelengths at 308 nm and 351 nm, and detects returns at four wavelengths; 308 nm, 332 nm, 351 nm, and 382 nm. Using this technique in conjunction with the Rayleigh DIAL measurement, ozone profiles have been measured between 15 and 50 km.
Constraints on mantle anisotropy beneath Precambrian North America from a transportable teleseismic experiment
Authors: Silver Paul G. , Kaneshima Satoshi
Published: February 2008

Data from the APT89 transportable teleseismic experiment was examined to characterize the variations in shear‐wave splitting along a 1500 km traverse in North America (see Silver et al, 1993, this issue). Three geologic terranes were sampled: the western Superior Province of the Canadian Shield, the Trans‐Hudson Orogen and the Wyoming Craton. The primary goal was to determine the extent to which the anisotropy was controlled by Archean and Proterozoic tectonic episodes. Of the large suite of teleseismic events recorded, 5 were suitable for determining the fast polarization direction ϕ and delay time δt from the phases SKS and SKKS. The resulting observations reveal two important features. First, the values of ϕ display good geologic coherence. Stations along the northern part of the array in the exposed shield reveal a consistent ENE direction for ϕ that is generally parallel to exposed geologic structures. Within the part of the Superior Province buried beneath Phanerozoic cover, ϕ rotates to a NE‐SW azimuth as do geophysical indicators of geologic fabric. Moving onto the Trans‐Hudson, ϕ changes abruptly to a more EW azimuth. The stations within the Wyoming Craton suggest values with a more northerly direction although the waveforms from these stations are more complex. Second, δt, which ranges between 0.40s and 1.75s, shows systematic variations across the traverse. The largest delay times (approaching 2 s) are found in a 250 km wide band from Red Lake, Ontario to the U.S‐Canada border. δt is reduced to about 1 s or less both north and south of this region. For the stations with detectable splitting, we find an inverse relationship between S isotropic station delays derived from the portable data set and δt, which is most easily explained by the anisotropy being primarily localized in the fast, presumably lithospheric mantle. The splitting observations overall suggest that mantle anisotropy along this traverse is dominated by Precambrian ‘fossil’ anisotropy that is preserved in the subcontinental lithosphere.
Differences of the atmospheric CH4 concentration between the Arctic and Antarctic regions in pre‐industrial/pre‐agricultural era
Authors: Nakazawa T. , Machida T. , Tanaka M. , Fujii Y. , Aoki S. , Watanabe O.
Published: February 2008

Air samples in ice cores from Site J, Greenland and Mizuho, Antarctica were analyzed to reconstruct the past concentrations of atmospheric CH4 at northern and southern high latitudes. The results showed that the CH4 concentrations were fairly constant in the pre‐industrial/pre‐agricultural era covered by this study, average values being 756±10 and 701±10 ppbv for the Site J and Mizuho cores, respectively, and then increased significantly with time. The pre‐industrial/pre‐agricultural levels of the CH4 concentration obtained from the Site J core were higher by 55±20 ppbv than those from the Mizuho core, suggesting that natural CH4 sources had been stronger in the northern hemisphere than in the southern hemisphere.
Observed thermal structure of a midlatitude mesopause
Authors: She C. Y. , Yu J. R. , Chen H.
Published: February 2008

The Colorado State Na temperature lidar has been in regular nighttime operation since 1991. By the end of January 1993, 94 nights of quality data with more than 4 hrs each night have been collected. Analysis of a total of 22,999 photocount files gives rise to high resolution seasonally and nightly averaged temperature and Na density profiles, from which the thermal structure of a midlatitude mesopause emerges. Contrary to the reference atmosphere of CIRA 1986, two prevailing temperature minima are seen at altitudes 86.3 ± 2.5 km and 99.0 ± 2.9 km with the lower minimum having the dominating effect. The trend of the mean nightly temperature of the lower minimum is observed to vary monotonically between a low of 172 K at summer solstice and a high of 212 K at nearly one month following winter solstice. As in the polar regions, this is a clear signature of a wave‐driven diabatic circulation observed at a midlatitude. The temperature of the upper minimum fluctuates around 189 ± 8 K year‐round. Like the polar mesopause, the mesopause altitude follows a bistable pattern, although the seasonal variation of the mesopause temperature is much more complicated. Based on the seasonal variations of these two temperature minima, the observed patterns of mesopause altitude and temperature can be explained. Although the mechanisms responsible for the formation of double temperature minima are not clear at present, the lower temperature minimum with a robust 40 K annual temperature change plays the role of the mesopause commonly known.
Observations of V = 1–0 emission from thermospheric nitric oxide by ISAMS
Authors: Ballard J. , Kerridge B. J. , Morris P. E. , Taylor F. W.
Published: February 2008

The Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) has made global measurements of emission from the 1→0 band of nitric oxide (NO), using a limb viewing geometry in which the tangent point is scanned from 0 km to >150 km. Vertical profiles of atmospheric radiance often show a peak around 120 km altitude, due to the relatively high temperatures and densities of NO(v=1) found in the lower thermosphere. In this letter we report on some aspects of the radiance from NO(v=1), in particular observations of the lower thermosphere during periods of quiet and intense geomagnetic activity.
Reflected ions near Mars: PHOBOS‐2 observations
Authors: Barabash S. , Lundin R.
Published: February 2008

When the PHOBOS‐2 spacecraft entered the Martian ion foreshock region and was magnetically connected with the bow shock, the mass spectrometer ASPERA recorded enhanced proton fluxes in the perpendicular and antisunward directions. The third elliptical orbit on February 8, 1989 has been analysed in detail in this respect and we have concluded that the observed disturbances might have resulted from gyrating reflected ions and field aligned beams propagating upstream from the bow shock. Comparison with a simple model of specular reflection can partially explain observations. Disturbances recorded inside the foot seem to result from kinetic processes mentioned by Moses et al. [1988]. The density of the backstreaming ions at the Martian bow shock reached 0.4 cm−3 and could account for 60% of the observed solar wind velocity decrease just upstream of the shock. The Martian upstream region is very compact and that results in a shortage of space for different instabilities to develop. So no reflected beam disruptions were observed, only beam‐like distributions were recorded.
Direct detection of solar wind dynamic pressure effect on ground geomagnetic field
Authors: Araki T. , Funato K. , Iguchi T. , Kamei T.
Published: February 2008

The scatter plot of the equatorial Dst index versus the square root of the dynamic pressure of the solar wind (P0.5) showed a clear upper envelope which linearly increases with increasing P0.5. This upper envelope was interpreted as showing the minimum level of the ring current intensity and thus dependence of the Dst index upon P0.5. The similar upper envelope was also found in scatter plots of the nighttime H‐component versus P0.5 at stations in low and middle latitudes. From inclination of these upper envelopes the coefficients for the assumed linear relationship between P0.5 and the ground magnetic field were obtained. The coefficients derived from the envelopes for the Dst index and the nighttime H‐component at different stations agree well each other and the latter showed no latitudinal variation. The scatter plots also showed a clear linear lower envelope with negative inclination. The ground state of the magnetosphere should be specified by an intersection of the two lines showing upper and lower envelopes in the scatter plot of Dst which corresponds to zero IMF‐ Bz and the minimum value of the dynamic pressure of the solar wind.
A comparison of VLBI data with the Ice‐3G Glacial Rebound Model
Authors: James Thomas S. , Lambert Anthony
Published: February 2008

Crustal motion predicted by the ICE‐3G glacial rebound model exhibits a pattern of tangential (horizontal) divergence away from the centres of uplift, which in North America and Europe are located around Hudson Bay and the Gulf of Bothnia. Tangential velocities reach peak magnitudes of 1–2 mm/yr, and must be included when predicting Very‐Long‐Baseline‐Interferometry (VLBI) baseline‐length change rates due to post‐glacial rebound. Out of 18 observed VLBI baselines examined 3 are situated such that their predicted length rates are around their 2σ uncertainties or greater. It is encouraging that 2 of these baselines exhibit predicted length rates within 2σ of the observed rates.
Investigation of nearfield postseismic slip following the Mw 7.3 Landers earthquake sequence of 28 June 1992, California
Authors: Sylvester Arthur Gibbs
Published: February 2008

Repeated precise surveys of six nearfield trilateration arrays reveal that minor horizontal displacements occurred across the several fault ruptures in the five months following the Landers earthquake sequence. The displacements range from 0±2 mm on the Camp Rock and Emerson faults, to 3±2 mm on the Landers fault, to 9plusmn;2 mm on the Johnson Valley fault, to 40±5 mm on the Eureka Peak fault, consistent in magnitude and duration with GPS and creepmeter measurements by other investigators. The relative lack of afterslip at all sites except across the Eureka Peak fault is probably due to the lack of a thick alluvial cover. Concomitant precise resurveys of level lines across the San Andreas fault in Coachella Valley and of a trilateration array at Durmid Hill, south easternmost San Andreas fault, failed to reveal vertical or horizontal displacements greater than the allowable uncertainty (<1 mm for leveling; 2 mm±2 ppm for trilateration). Therefore the Landers earthquake sequence did not perturb the San Andreas in any way that was evident from our measurements of near‐surface strain.
Remote detection of HCl and SO2 in volcanic gas from Unzen Volcano, Japan
Authors: Mori Toshiya , Notsu Kenji , Tohjima Yasunori , Wakita Hiroshi
Published: February 2008

A telescope‐attached FT‐IR spectral radiometer was applied for remote measurement of chemical composition of the volcanic gas of Unzen volcano, Japan, which started erupting in November 1990. The observations were carried out at the site about 1.3 km away from the gas‐emitting region of the volcano in March and July 1992. Absorption features of SO2 (Vl) and HCl were identified and column amounts of SO2 and HCl of the volcanic gas plume were estimated using a nonlinear least‐squares method. The SO2/HCl ratio was in the range between 0.7 and 1.8 for both March and July observations. This is the first remote ground‐based observation of HCl in volcanic gas.
Longitudinal structure in atomic oxygen concentrations observed with WINDII on UARS
Authors: Shepherd G. G. , Thuillier G. , Solheim B. H. , Chandra S. , Cogger L. L. , Duboin M. L. , Evans W. F. J. , Gattinger R. L. , Gault W. A. , Hersé M. , Hauchecorne A. , Lathuilliere C. , Llewellyn E. J. , Lowe R. P. , Teitelbaum H. , Vial F.
Published: February 2008

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, began atmospheric observations on September 28, 1991 and since then has been collecting data on winds, temperatures and emissions rates from atomic, molecular and ionized oxygen species, as well as hydroxyl. The validation of winds and temperatures is not yet complete, and scientific interpretation has barely begun, but the dominant characteristic of these data so far is the remarkable structure in the emission rate from the excited species produced by the recombination of atomic oxygen. The latitudinal and temporal variability has been noted before by many others. In this preliminary report on WINDII results we draw attention to the dramatic longitudinal variations of planetary wave character in atomic oxygen concentration, as reflected in the OI 557.7 nm emission, and to similar variations seen in the Meinel hydroxyl band emission.
Diurnal and semi‐diurnal temperature variability of the middle atmosphere, as observed by ISAMS
Authors: Dudhia A. , Smith S. E. , Wood A. R. , Taylor F. W.
Published: February 2008

Some preliminary results are presented showing that the ISAMS instrument on board the UARS satellite has detected sun‐synchronous variations in atmospheric temperature between 15–80 km altitude. These are interpreted as the westward‐migrating diurnal and semi‐diurnal tides. Zonal mean maps of the amplitudes of these components are shown, determined from 40 days' observations during the northern hemisphere winter of 1991/92. The results show maximum amplitudes ∼5 K, with the diurnal tide strongest over the equator while the semi‐diurnal tide is strongest at mid‐latitudes. Comparisons are made with previous observations from the LIMS instrument and the lidar at Biscarrosse, France. While the ISAMS and LIMS results are consistent, several significant differences from the lidar results are noted. The conclusion is that information on the global diurnal and semi‐diurnal temperature tides in the middle atmosphere may be obtained from a relatively simple treatment of ISAMS data.
On the escape of oxygen and hydrogen from Mars
Authors: Fox J. L.
Published: February 2008

Escape rates of oxygen atoms from dissociative recombination of O2+ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by Viking. If it is assumed that the ions are not lost from the gravitational field of the planet, as required by an analysis of nitrogen escape, the computed average O escape rate is 3 × 106 cm−2 s−1, much less than half the H escape rates inferred from measurements of the Lyman alpha dayglow, which are in the range (1 – 2) × 108 cm−2 s−1. Suggestions for restoring the relative escape rates of H and O to the stoichiometric ratio of water are explored. The final resolution of this problem may have to await a future aeronomy mission to Mars.
Visco‐elastic response of joints to transmission waves
Authors: Yoshioka Naoto , Kikuchi Masayuki
Published: February 2008

Experiments of wave transmission across a joint were carried out to investigate the visco‐elastic response of the joint. Two models for wave transmission were examined with the experimental data. The results show that the velocity discontinuity model which includes the effect of viscosity is in better agreement with the experimental observations for both P‐ and S‐waves under dry conditions than the displacement discontinuity model. This suggests that at the real asperity contacts, the solid material behaves visco‐elastically to transmission waves in the high frequency range. A possible mechanism for the visco‐elastic behavior is plastic flow at the asperities caused by high local pressure.
Discrimination of earthquakes and explosions in the eastern United States using regional high‐frequency data
Authors: Kim Won‐Young , Simpson D. W. , Richards P. G.
Published: February 2008

High‐frequency regional records from small earthquakes (1.3 < magnitude < 4), and comparable magnitude explosions, are analyzed to find a reliable seismic discriminant in the eastern U. S. Over 500 digital, vertical‐component seismograms recorded by the New York State Seismic Network in the distance ranges 10 to 610 km are used. Mean P/Lg spectral ratios in the band 1–25 Hz are about 0.5 and 1.25 for earthquakes and explosions, respectively, in the eastern United States. We find that the high‐frequency P/Lg spectral amplitude ratio in the frequency band 5–25 Hz is a reliable and robust discriminant for classifying these events. A linear discriminant function analysis indicates that the P/Lg spectral amplitude ratio method provides discrimination power with a total misclassification probability of about 1%. Single‐hole instantaneous explosions and ripple‐fired quarry blasts have somewhat different P/Lg spectral ratios, but as a group are distinctly different from earthquakes.
Cloud‐to‐ground lightning in the blizzard of ′93
Authors: Orville Richard E.
Published: February 2008

In the 48‐hour period of 06 UT March 12 to 06 UT March 14, 1993, the eastern United States experienced one of the most severe storms of the century, This winter storm produced over 59,000 cloud‐to‐ground flashes with a peak lightning rate exceeding 5,100 flashes per hour. Flash densities exceeded 0.16 flashes per km2 in an area just south of Tampa, Florida. The overall percentage of positive flashes averaged 13.1% and positive flash percentages exceeded 60% in the northern portion of the storm as it moved over northern Florida and southern Alabama and Georgia. Median peak currents for negative flashes were 30 kA and for positive flashes 52 kA. Flash density contours followed the Gulf Stream off the coast of the Carolinas. Few lightning flashes to ground were recorded north of the Carolinas.
Earthquake location applied to a mini‐array: k‐spectrum versus correlation method
Authors: Cansi Yves , Plantet Jean‐Louis , Massinon Bernard
Published: February 2008

Two different methods devoted to automatic location of both regional and teleseismic events recorded on a mini‐array are compared in terms of location accuracy. The first method belongs to the frequency‐wavenumber methods family; the second one uses signal processing to compute accurate time‐delays and then derive the event parameters. In the latter method a careful study of each time‐delay set is performed in order to remove ambiguity errors using Chasles relationship. It leads to an accuracy of about 10 degrees on the azimuth determination for distances smaller than 80 degrees, and 3 degrees for regional distances. Finally, a criterion derived from these relations is proposed for phase detection and identification.
Cupri observations of PMSE during Salvo C of NLC‐91: Evidence of a depressed mesopause temperature
Authors: Miller Clark A. , Swartz Wesley E. , Cho John Y. N.
Published: February 2008

The Cornell University Portable Radar Interferometer (CUPRI) observed two extremely strong layers of Polar Mesosphere Summer Mesosphere (PMSE) thirty minutes prior to the launch of Salvo C of the NLC‐91 campaign. The lower layer exhibited a S/N ratio of 42 dB (the second strongest event of NLC‐91), vertical velocities of a few m/s, and a narrow spectral width, suggesting that it was the result of partial reflections. The upper layer, in contrast, exhibited sinusoidal structures in vertical velocity with peak amplitudes greater than ± 10 m/s and wide spectral widths. These structures were observed to grow and steepen with altitude until they broke and produced turbulent radar scattering. We conclude that the rapid rate of growth of the wave with altitude was the result of a depressed mesopause temperature and a nearly adiabatic temperature gradient at PMSE heights and that the simultaneous measurement of both a low mesopause temperature and strong PMSE supports recent theories that find the presence of charged aerosols to be the key to the unique radar cross sections associated with PMSE.
Low‐level jet streams associated with atmospheric cold fronts: Seven case studies from the Fronts 87 Experiment
Authors: Kotroni Vassiliki , Lagouvardos Konstantinos
Published: February 2008

Sounding data collected during the European experiment FRONTS87 are used to investigate the dynamic structure of mid‐latitude low‐level jets associated with atmospheric cold fronts. These jets are identified as strong currents of relative warm and moist air, confined in the lower layers of the atmosphere (within the first two kilometers) ahead of cold fronts. The importance of the low‐level jets lies on the fact that they can be responsible for the occurrence of strong winds near the surface which sometimes can be disastrous, especially near coasts where frictional deceleration due to the land is minimum. Furthermore they are often associated with a narrow region of ascending motions just ahead of the frontal surface which can account for strong precipitation at ground. The data to be discussed in this paper are provided from the special upper air sounding stations deployed at Brittany, France, during FRONTS87. A comparison of seven case studies permitted to give a detailed picture of the jets characteristics and to better understand their dynamic structure.
Global surface temperatures and the atmospheric electrical circuit
Authors: Price Colin
Published: February 2008

To monitor future global temperature trends, it would be extremely useful if parameters nonlinearly related to surface temperature could be found, thereby amplifying any warming signal that may exist. Evidence that global thunderstorm activity is nonlinearly related to diurnal, seasonal and interannual temperature variations is presented. Since global thunderstorm activity is also well correlated with the earth's ionospheric potential, it appears that variations of ionospheric potential, that can be measured at a single location, may be able to supply valuable information regarding global surface temperature fluctuations. The observations presented enable a prediction that a 1% (≈3K) increase in global surface temperatures may result in a 20% (≈50KV) increase in ionospheric potential.
Satellite detection of volcanic sulphuric acid aerosol
Authors: Baran A. J. , Foot J. S. , Dibben P. C.
Published: February 2008

The major eruption of Mt Pinatubo (15.1°N, 120.4°E) on June 15th 1991 ejected 20 megatons of sulphur dioxide into the stratosphere at altitudes of 20–30 km. The sulphur dioxide converts to sulphuric acid vapour in 30–40 days, the vapour thus formed nucleates, condensing with water to form sulphuric acid aerosol, the resident time of this aerosol is several years and significantly increases the planetary albedo. A new method is presented for the detection of sulphuric acid aerosol utilising observations made by polar orbiting satellites of brightness temperature differences at two thermal wavelengths. Time series of this difference shows an increase of 2.4 K after the eruption, a value in agreement with radiative transfer calculations based on in‐situ observations of the sulphuric acid aerosol. This new method can be used to globally map the distribution of the sulphuric acid aerosol and obtain an estimate for the mass loading, necessary information for climate models attempting to study the impact of the Pinatubo eruption.
Rupture process of the June 28, 1992 Big Bear Earthquake
Authors: Jones L. E. , Hough S. E. , Helmberger D. V.
Published: February 2008

The June 28, 1992 Big Bear earthquake in southern California was assumed to have ruptured along a northeast‐trending plane, as suggested by long‐term aftershock distribution. No surface rupture was found, however, and mainshock locations determined from both strong motion and TERRAscope data are mutually consistent and do not lie on the assumed fault plane. An integrated study involving waveform modeling, directivity and seismicity analyses suggests a complex rupture pattern, with significant short‐ and long‐period energy propagating northwest along the presumed conjugate fault‐plane.
Noctilucent clouds and regions with polar mesospheric summer echoes studied by means of rocket‐borne electron and ion DC‐probes
Authors: Blix T. A. , Thrane E. V.
Published: February 2008

During the Noctilucent Cloud Campaign from Esrange, Kiruna, Sweden in July‐August 1991, detailed in situ mesurements were performed by means of plasma probes in the TURBO rocket payload. When launched through NLC and PMSE layers, these probes indicate that the NLC layer was associated with a one kilometer thick region containing heavy positive ions. Embedded in this layer were structures only a few tens of metres in vertical extent. These structures exhibited strongly enhanced ion current and corresponding sharp decreases of electron current. The observations indicate that the structures contain enhanced densities of light positive ions as well as heavy, negative charge carriers in the form of ions or particles. VHF radar measurements from the same site showed Polar Summer Mesospheric Echoes from a layer that at first was located just above the NLC layer, but the rocket results indicate that it later merged with the NLC‐layer.
How strong is the invisible component of the magnetic field in the Earth's core
Authors: Zhang Keke , Fearn D. R.
Published: February 2008

The maximum strength of toroidal magnetic field in the Earth's core is estimated on the basis of the stability of the magnetic field. In our model we take our basic magnetic field BO to be composed of both toroidal and poloidal axisymmetric decay modes of lowest order. While the strength of the poloidal component, BP is taken consistent with observation, the maximum strength of the toroidal field, |BT|max, is regarded as a parameter of the model. By demonstrating that viscous dissipation is of secondary importance and therefore that the results are independent of the parameter associated with viscosity, our model is eventually dependent on only one parameter: the ratio A of the maximum strength of the invisible toroidal field to the strength of the poloidal field at the pole of the core‐mantle boundary. It is shown that |BT|max < 10|BP(θ = 0, r = ro)| in order that the basic magnetic field BO = BP + BT is stable, giving an estimated upper bound on strength of the invisible toroidal field of order 50 gauss.
Improving flow laws and piezometers for quartz and feldspar aggregates
Authors: Gleason Gayle C. , Tullis Jan
Published: February 2008

We have modified the molten salt cell (MSC) in order to measure the flow stress of quartz and feldspar aggregates deforming by dislocation creep. The flow stress of Cararra marble measured in the MSC is within 5 MPa of that measured in a gas apparatus. Quartz and feldspar aggregates deformed at 900°–1100°C and confining pressures of 1000–1500 MPa in copper and calcite solid confining pressure media are about twice as strong as those deformed in the MSC, whereas quartz aggregates deformed in talc and solid NaCl have about the same strength as that measured in the MSC. These results suggest that the stress exponent in the flow law is larger and the dynamically recrystalized grain size is more sensitive to stress than previously reported.
Shallow fluid pressure transients caused by seismogenic normal faults
Authors: Fleischmann Karl Henry
Published: February 2008

Clastic dikes, induced by paleo‐seismic slip along the Jonesboro Fault, can be used to estimate the magnitude of shallow fluid pressure transients. Fractures show evidence of two phases of seismically induced dilation by escaping fluids. Initial dilation and propagation through brittle rocks was caused by expulsion of trapped reducing fluids from beneath a clay cap. Second phase fluids were thixotropic clays which flowed vertically from clay beds upwards into the main fracture. Using the differential dilation and fracture trace lengths, the fluid pressure pulse is estimated to have ranged from 0.312 – 0.49 MPa, which is approximately equal to the vertical load during deformation. Field observations in adjacent rocks record evidence of large‐magnitude seismic events, which are consistent with the large nature of the fluid pressure fluctuation.
Venus resurfacing rates: Constraints provided by 3‐D Monte Carlo simulations
Authors: Bullock Mark A. , Grinspoon David H. , Head James W.
Published: February 2008

The range of surface ages on Venus and the rate of recent volcanism are two major questions that can be addressed through an analysis of Magellan images. We report on a 3‐D Monte Carlo resurfacing model that seeks to explain the resurfacing history of Venus by simulating the evolving surface under the competing processes of impact cratering and volcanism. By incorporating the observed size distribution of volcanic forms on Venus, and the assumptions of constant volcanic and impactor fluxes, we seek a solution that is consistent with the observed impact crater population. In the Monte Carlo simulations, a volcanic flux of 0.37 km³/yr, operating for 550 m.y. on initially craterless surfaces, yields surfaces that are statistically similar to Venus'. It is shown that the statistics of the impact cratering record are inconsistent with an equilibrium resurfacing model.
Hydrocarbon ratios and photochemical history of air masses
Authors: McKeen S. A. , Liu S. C.
Published: February 2008

An effective and commonly used technique for studying the sources, photochemistry, and even the “photochemical age” of trace species is to examine ratios of hydrocarbons by assuming the ratio is independent of transport processes. We present results from mesoscale model calculations that suggest a significant effect by atmospheric mixing on the ratio. We also show that the photochemical age of an air mass derived from the ratio of hydrocarbons is a function of both photochemistry and atmospheric transport. Without additional information, it is not possible to derive a unique value for the age of an air mass from hydrocarbon ratios alone.
Seismic discontinuity at the top of D″: A world‐wide feature?
Authors: Nataf Henri‐Claude , Houard Sylvain
Published: February 2008

Considering the growing number of observations of a seismic discontinuity at the top of D″, some 300 km above the core‐mantle boundary, we investigate the scenario of whether this feature could be global. We show that the mean first arrivals of P‐waves, as used to build global models, can be fit by the two branches of a discontinuous model. We argue that the intermittence of the detection of the reflector is due to lateral variations of a global discontinuity. These observations put strong constraints on the type of chemical layering that could explain the discontinuity. We speculate that the properties of the D″ discontinuity are better explained in terms of a new phase transition.
The effects of phase transition kinetics on subducting slabs
Authors: Daessler Rolf , Yuen David A.
Published: February 2008

The effects of kinetics on the non‐equilibrium aspects of the olivine to spinel transition in a descending slab have been studied numerically. A one‐dimensional model consisting of the kinetic equations and the heat‐diffusion equation with latent‐heat release has been constructed. Numerical results show that the position and sharpness of the kinetic phase boundary is determined by the surface tension and the activation volume. For slow slab velocities, less than 6 cm/yr, near equilibrium conditions are found. Finger‐like structures emanating from the phase boundaries are obtained in this regime. These phase boundary prolusions may cause earthquakes. For slab velocities of around 10 cm/yr the metastable olivine region may be pushed down to a depth of about 600 km, with a sharp phase boundary. A direct transition from olivine to perovskite may be feasible.
Small‐scale structure of O2+ and proton hydrates in a noctilucent cloud and polar mesospheric summer echo of August 9/10 1991 above Kiruna
Authors: Balsiger F. , Kopp E. , Friedrich M. , Torkar K. M. , Wälchli U.
Published: February 2008

A novel mass spectrometer designed to measure simultaneously positive ion composition in the mesosphere, was successfully launched during the NLC‐91 project. Instruments supporting the mass spectrometer were a probe to measure both electrons and positive ions as well as a wave propagation experiment. The location of the Noctilucent Clouds (NLC) was determined by a particle impact sensor to detect secondary electrons and ions from the impact of NLC particle. The density of proton hydrates and of the related total ions is depleted in the NLC region at 83 km. An improved detection limit of 5 × 104 m−3 for positive ions and improved height resolution revealed for the first time large gradients in the O2+, H+(H2O)2 and H+(H2O)6 densities within a small height range of the order of 50 m. Such gradients at the altitude of NLC and Polar Mesospheric Summer Echoes (PMSE) are associated with strong variability of mesospheric water vapor, temperature and neutral air density.
Zonal mean winds in the equatorial mesosphere and lower thermosphere observed by the High Resolution Doppler Imager
Authors: Lieberman R. S. , Burrage M. D. , Gell D. A. , Hays P. B. , Marshall A. R. , Ortland D. A. , Skinner W. R. , Wu D. L. , Vincent R. A. , Franke S. J.
Published: February 2008

This paper presents analyses of mesospheric and lower thermospheric zonal mean winds observed by the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). Monthly averages of the equatorial zonal mean zonal winds are presented for January 1992 through June 1993. Equatorial zonal winds in the 70–90 km region are dominated by a semiannual oscillation (SAO), ranging from 30 m s−1 (westerly) to −100 m s−1 (easterly). At high latitudes the zonal wind variations are predominantly annual. Above 90 km, the low‐latitude flow is easterly at all times, punctuated by a small semiannual variation. This behavior may be related to the deposition of momentum by the diurnal tides.
Laboratory evidence for a possible non‐LTE mechanism of stratospheric ozone formation
Authors: Rogaski C. A. , Price J. M. , Mack J. A. , Wodtke A. M.
Published: February 2008

Experimental evidence is given that supports the possibility of a previously unknown non‐LTE mechanism for stratospheric ozone formation,

which could have a significant impact on the stratospheric ozone budget even if the quantum yield for production of highly vibrationally excited O2 in reaction (1), (averaged over all wavelengths shorter than 243nm) were as low as 0.2%. Stimulated emission pumping enabled preparation of individual vibrational states of O2(X³Σg−,19≤v≤27) and laser induced fluorescence was used to follow the time evolution of the prepared states and thereby determine the vibrational‐state‐specific total‐removal rate‐constants for relaxation by O2 and N2 at 295K. Self‐relaxation shows a sharp threshold for enhanced relaxation near the energy of O2(X³Σg−, v=26) which is coincident with the energetic threshold for reaction (2). The magnitudes of the self‐relaxation rate constants for O2(X³Σg−, v=26 and 27) are quantitatively consistent with the kinetic parameters of reaction (2). Relaxation by N2, while important for lower O2 vibrational states, is shown to be about 10 and 200 times slower than self relaxation for v=26 and v=27, respectively. These are the first two vibrational states of O2 that could form O3 via reaction (2).
Theoretical structure of a magnetospheric plasma boundary: Application to the formation of discrete auroral arcs
Authors: Roth M. , Evans D. S. , Lemaire J.
Published: February 2008

In the framework of a kinetic theory for tangential discontinuities we modeled the electrical structure of the sheath that separates magnetospheric particle populations of different densities and temperatures. The model can equally be applied to the plasma sheet boundary layer in the tail or to the boundary of some plasma sheet cloud immersed in the central plasma sheet. With plasma parameters typical of the Earth's outer magnetosphere and plasma sheet, we obtain results bearing many features pertinent to magnetospheric processes, specifically the origin of discrete auroral arcs. Creation of a space‐charge separation electrostatic potential in a direction normal to the magnetic field results from the contact of the two plasma populations. When the large‐scale solar wind potential difference is further imposed across the transition layer, the potential gradients are locally much enhanced, to give rise to large electric fields (several hundreds millivolts per meter) appearing over small distances perpendicular to the magnetic field—just the situation needed for the creation of an auroral arc. The transition itself is characterized by two scale lengths of the plasma and fields variables: the average electron Larmor radius ϱe (or some multiple of ϱe) for thin embedded electron‐dominated layers which generate the sharpest potential gradients, and the ion gyroradius ρp (or some multiple of ρp) for the broader ion‐dominated layers located at the outer edges of the transition. The larger‐scale sizes are appropriate to auroral arcs dimensions. The generated electric potential differences, consistent with the energy acquired by the precipitated electrons associated with discrete aurora, are identified with the source of the electromotive force (EMF) required for the auroral current circuit. Wave particle interactions are likely to scatter the electrons into the atmospheric loss cone, establishing the current system threading both the EMF and the ionosphere by means of field‐aligned currents. The half lifetime of the transition is at least of the order of 1000 s. This is also the time interval during which dissipative processes will not alter significantly the available potential gradients of the initially unloaded EMF.
Ion cyclotron harmonic wave generation by ring protons in space plasmas
Authors: McClements K. G. , Dendy R. O.
Published: February 2008

Spacecraft such as the Active Magnetospheric Particle Tracer Explorers, Giotto and the Combined Release and Radiation Effects Spacecraft have shown that suprathermal protons with ringlike or shell‐like distributions in velocity space exist in many space plasmas. Examples include the radiation belts, the auroral zones, the bow shock, and the solar wind. Ring proton distributions may excite obliquely propagating fast Alfvén waves at harmonics of the ion cyclotron frequency. In this paper we carry out an analytical study of this instability, restricting our attention to strictly perpendicular propagation. In the case of a monoenergetic proton ring in a cold plasma, we show that cyclotron harmonics can have a higher growth rate than parallel‐propagating ion cyclotron waves, depending on the ratio of the proton ring speed to the Alfvén speed. Analytical stability boundaries in parameter space are determined, indicating that the threshold for the growth of cyclotron harmonics depends critically on the ion plasma beta, βi. If βi ≪1, for example in the radiation belts or the auroral zones, the concentration of ring protons required for instability is very low. If, on the other hand, βi ∼ 1, for example in the bow shock or the solar wind, instability will only occur if the ring protons constitute a large fraction of the total ion density. In the bow shock case, the required concentrations may indeed occur. Growth rates are also calculated for the case of a ring proton distribution with a Gaussian spread of velocities, and it is shown that the instability persists even if the velocity dispersion is comparable to the ring speed itself. Our analysis is consistent with the observed excitation of perpendicular‐propagating ion cyclotron harmonics in the vicinity of the Earth's plasmapause.