bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ measurements of the scattering phase function of stratocumulus, contrails and cirrus
Authors: Gayet Jean‐François , Auriol Frédérique , Oshchepkov Sergey , Schröder Franz , Duroure Christophe , Febvre Guy , Fournol Jean‐François , Crépel Olivier , Personne Pascal , Daugereon Daniel
Published: February 2004

Original measurements were obtained in stratocumulus, contrails and cirrus clouds by using a new optical airborne probe, the ‘Polar Nephelometer’, which is the first airborne instrument to make direct in situ measurements of the scattering phase function of cloud particles over a broad range of sizes (from a few micrometers to about 500 µm diameter). Preliminary measurements show that in stratocumulus water droplet cloud, the measured phase function fits very well with the phase function derived from direct PMS probes measurements. This definitively confirms the reliability of the Polar Nephelometer for airborne measurements. In contrails and natural cirrus, measured scattering phase function indicates major differences with those used in cloud models which assume ice spheres or simple geometric shape of ice particles. These results highlight new potential insights on both modelling of climate processes and methodologies for cloud remote sensing from satellite measurements.
Guided waves in near‐surface seismic surveys
Authors: Roth M. , Holliger K. , Green A. G.
Published: February 2004

Much of the Earth's surface is covered by a thin blanket of unconsolidated sediments or weathered bedrock. Boundaries within or at the base of this surficial layer (e.g. watertable, bedrock surface, transition to unweathered rock) are commonly associated with a strong seismic contrast. Layers contained between the surface and shallowest boundaries may be efficient waveguides and, as such, associated with significant source‐generated seismic noise. Whereas Rayleigh waves (groundroll) can be described in terms of normal modes, we demonstrate that the faster shingled guided waves can be explained in terms of leaking modes. Due to the relatively high Poisson's ratios in the shallow subsurface, guided waves are composed dominantly of multiply reflected P‐waves. We compare our results with those obtained using a conventional acoustic waveguide approximation and discuss methods for extracting information from guided waves for geotechnical and environmental applications.
Effect of wave‐particle interactions on ring current evolution for January 10–11, 1997: Initial results
Authors: Jordanova V. K. , Farrugia C. J. , Quinn J. M. , Thorne R. M. , Ogilvie K. E , Lepping R. P. , Lu G. , Lazarus A. J. , Thomsen M. F. , Belian R. D.
Published: February 2004

We simulate the ring current evolution during the magnetic storm caused by Earth passage of the January 1997 magnetic cloud. Compared to previous studies, we include for the first time energy diffusion caused by wave‐particle interactions. The modeled Dst index agrees reasonably well with the measured one, corrected for magnetopause currents and currents induced in the solid Earth. We compare H+distributions calculated from our model with those measured by the HYDRA instrument on the POLAR spacecraft and find that: a) the agreement between theory and data at large Lshells (L>5.5) is very good; b) although the agreement at low Lshells is improved when scattering by EMIC waves is included, the result is not entirely satisfactory, suggesting that either transport in a more realistic magnetospheric electric field or additional loss processes should be considered.
Energy transfer between the ionosphere and magnetosphere during the January 1997 CME event
Authors: Sánchez E. R. , Thayer J. P. , Kelly J. D. , Doe R. A.
Published: February 2004

This Letter shows that transient changes in convection and precipitation patterns can significantly influence the electromagnetic energy transfer between the magnetosphere and the ionosphere. At the times of the transient events during the 10 January 1997 storm that was produced by a Coronal Mass Ejection (CME)‐related magnetic cloud, the Sondrestrom incoherent scatter radar measured the local electrodynamic response of the high‐latitude ionosphere. Important discrepancies between the net electrodynamic energy flux and the electromagnetic energy flux estimated for a stationary atmosphere suggest that the assumption of the ionosphere as a purely resistive load is often inadequate during these transient events as neutral winds cannot conform to the new orientation of the electric field and thus impact the load/generator characteristics of the ionosphere.
Long‐term‐mean aspect sensitivity of PMSE determined from Poker Flat MST radar data
Authors: Huaman Mercedes M. , Balsley Ben B.
Published: February 2004

Results of a long‐term study of aspect sensitivity of Polar Mesosphere Summer Echoes (PMSE) are presented using archived Poker Flat MST radar (65°08′N; 147°27′W) data. We have used data from the PMSE region (80‐ to 90‐ km) during the months of June and July for the years 1981–84, when three antenna beams were available, one beam pointing vertical and two beams pointing 15° off‐vertical. Comparisons of calibrated vertical and 15°off‐vertical monthly‐average echo power profiles for each of the summer months show that VHF scattering from PMSE is strongly aspect sensitive at the lowest heights (∼80 km), and exhibits a clear decrease of aspect sensitivity with increasing height. Comparison of monthly‐average aspect sensitivity profiles during the four‐year period shows a reasonable consistency throughout the years. The composite four‐year‐average aspect sensitivity profile for the months of June and July data suggests a strong tendency to isotropy at the higher mesospheric heights.
A chronology for the Dome C deep ice‐core site through radio‐echo layer Correlation with the Vostok Ice Core, Antarctica
Authors: Siegert Martin J. , Hodgkins Richard , Dowdeswell Julian A.
Published: February 2004

Ice at the new Dome C drill site is correlated directly with the Vostok ice‐core record using internal radio‐echo layering in the Antarctic Ice Sheet. Layering observed on 60 MHz radar records at ice depths greater than 1000 m is a result of electro‐magnetic wave reflections from acidic ice layers formed, it is assumed, by volcanic eruption‐derived aerosols. These acidic layers represent isochronous surfaces within the ice sheet. We are able to trace five prominent layers for over 500 km across the Antarctic Ice Sheet, between Vostok and the new ice‐core site at Dome C. This correlates the stratigraphy and depth‐age relationship between the two sites. The thickness of ice deposited over the last glacial cycle (120,000 years) at Dome C is 300 m greater than at Vostok and, at comparable depths, the ice at Dome C is between about 10,000 and 25,000 years younger than at Vostok.
Evidence for nonlinear interaction between internal waves of inertial and semidiurnal frequency
Authors: Mihaly Steven F. , Thomson Richard E. , Rabinovich Alexander B.
Published: February 2004

Observational evidence is presented for pervasive nonlinear interaction between inertial and M2 frequency oscillations in the northeast Pacific. Enhanced currents at the sum frequency (termed the “fM2 frequency”) have the rotary spectral properties of freely propagating waves suggesting permanent energy exchange from the inertial and semidiurnal bands to the fM2 band. The interaction may be responsible for the greater than predicted decay of inertial energy observed in the ocean and may contribute significantly to oceanic mixing.
An interannual Antarctic sea‐ice ‐ocean mode
Authors: Stössel Achim , Kim Seong‐Joong
Published: February 2004

By means of a global coupled sea‐ice‐ ocean model we propose an oscillating mechanism in the Antarctic region with the same frequency as that associated with the Antarctic Circumpolar Wave (ACW) [White and Peterson, 1996]. Since our model is forced from the atmosphere with climatological monthly data with an annual repeat cycle, our oscillation suggests an internal mode of the sea‐ice‐ ocean system rather than one which is dictated by global links through the atmosphere. This mode is robust for a wide range of parameterisation and forcing of Southern Ocean sea‐ice, except for the case where higher rates of new‐ice production and thus Antarctic Bottom‐Water (AABW) formation are enforced. The present results imply that the sea‐ice ‐ocean system itself may produce regional oscillations at a 4‐year period in climatic conditions of relatively weak rates of AABW formation, which may contribute to regionally sustaining the ACW pattern.
Effect of quartz inclusions on plastic flow in marble
Authors: Dresen Georg , Evans Brian , Olgaard David L.
Published: February 2004

To investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks, we produced synthetic marbles with a grainsize of 5–25 µm, containing quartz of similar size, with volume fractions of 0, 5.1, and 20.4 vol.%, and between 3 to 9% porosity. These rocks were deformed in triaxial compression at a strain‐rate of 3 × 10−5 s−1 confining pressure of 200 MPa, and 600°C. Marbles containing 20.4 vol.% quartz were almost 5 times stronger than those containing no quartz. The work‐hardening rate at given strain increased with quartz content. Marbles containing 20.4 vol.% quartz work‐hardened up to 10% strain whereas in single‐phase samples work hardening was negligible beyond 3% strain. The dislocation density in the calcite increased with increasing quartz content. The data are consistent with models for the strength of ceramic matrix composites, or with the hypothesis that strength is increased by decreasing porosity.
The association of electrostatic ion cyclotron waves, ion and electron beams and field‐aligned currents: FAST observations of an auroral zone crossing near midnight
Authors: Cattell C. , Bergmann R. , Sigsbee K. , Carlson C. , Chaston C. , Ergun R. , McFadden J. , Mozer F. S. , Temerin M. , Strangeway R. , Elphic R. , Kistler L. , Moebius E. , Tang L. , Klumpar D. , Pfaff R.
Published: February 2004

FAST particle and wave data for a single nightside auroral zone crossing are utilized to examine the free energy source for electrostatic ion cyclotron (EIC) waves. Comparisons of the unstable wave modes, obtained by an electrostatic linear dispersion relation solver, to the observed waves for two intervals with upflowing ion beams and two with upflowing electron beams are consistent with the conclusion that the observed waves near the cyclotron frequencies are EIC which are driven by the electron drift both in the upgoing ion beam regions and in the upgoing electron regions. A limitation is that the drifting bi‐Maxwellian model used in the dispersion relation is not a good match to the observed upflowing electron distributions. The observed ion beams do not drive EIC waves; however, the relative drift of the various ion species comprising the ion beam can drive low frequency (<∼50 Hz) waves unstable. The electron drift, during some intervals, also destabilizes electron acoustic waves.
Interpretation of high mixing ratios of O3 observed in the upper troposphere over Syowa Station, Antarctica using a trajectory analysis
Authors: Murayama Shohei , Yamazaki Koji , Nakazawa Takakiyo , Aoki Shuji
Published: February 2004

High O3 mixing ratios were observed above 6 km over Syowa Station (SYO), Antarctica in mid‐September 1989 when the tropopause was found to be higher than 10 km. From the results of a 3‐dimensional trajectory analysis, it is suggested that the observed high O3 was caused by vertical transport of O3 from the lower stratosphere to the upper troposphere. To examine general features of the transport process associated with high O3 mixing ratios observed in the upper troposphere over SYO, the same trajectory analysis was further performed for high O3 observations at 350 hPa over SYO in 1987–1994. The results imply that the transport of stratospheric air descending in the polar region to SYO plays a very important role in the increase of upper tropospheric O3 over SYO, especially in spring and autumn, while the downward transport of stratospheric O3 in the circumpolar region is important in summer.
Subsonic aircraft: Contrail and cloud effects special study (SUCCESS)
Authors: Toon Owen B. , Miake‐Lye Richard C.
Published: February 2004

The Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) was a multi‐aircraft field campaign which took place in April and May of 1996, over the Central and Western United States. The goal of SUCCESS was to learn more about Earth's radiative heat balance, cirrus clouds, aircraft exhaust, heterogeneous atmospheric chemistry and their interactions with each other. The SUCCESS results will be addressed in a series of Special Sections of Geophysical Research Letters over the next few months. Here we provide an overview of these Special Sections.
FAST observations in the downward auroral current region: Energetic upgoing electron beams, parallel potential drops, and ion heating
Authors: Carlson C. W. , McFadden J. P. , Ergun R. E. , Temerin M. , Peria W. , Mozer F. S. , Klumpar D. M. , Shelley E. G. , Peterson W. K. , Moebius E. , Elphic R. , Strangeway R. , Cattell C. , Pfaff R.
Published: February 2004

Observations of plasma particles and fields by the FAST satellite find evidence of acceleration of intense upgoing electron beams by quasi‐static parallel electric fields. The beam characteristics include a broad energy spectrum with peak energies between 100 eV and 5 keV, perpendicular temperatures less than 1 eV, and fluxes greater than 109/cm²sec. Diverging electrostatic shocks associated with the beams have integrated potentials that match the beam energy. These beams are found in regions of downward Birkeland current and account for the total field‐aligned current when they are present. The most energetic ion conics in the auroral zone are found coincident with these beams, in agreement with the model for “trapped” conics. The measured particle densities of the electron beams and associated ion conics are approximately equal and typically range from 1 to 10 cm−3, with no evidence for additional cold density. The beams are seen frequently at altitudes between 2000 and 4000 km in the winter auroral zone. Their probability of occurrence has a strong dependence on season and altitude and is similar to that for upgoing ion beams in the adjacent upward current regions. This similarity suggests that the density and scale height of ionospheric ions play an important role in the formation of both types of beams.
Electron modulation and ion cyclotron waves observed by FAST
Authors: McFadden J. P. , Carlson C. W. , Ergun R. E. , Chaston C. C. , Mozer F. S. , Temerin M. , Klumpar D. M. , Shelley E. G. , Peterson W. K. , Moebius E. , Kistler L. , Elphic R. , Strangeway R. , Cattell C. , Pfaff R.
Published: February 2004

New observations from the FAST satellite demonstrate strong wave‐particle interactions between energetic electrons and H+ EMIC waves in inverted‐V arcs. The intense waves are shown to occur in strong upward current regions which contain intense downgoing field‐aligned electron fluxes. Electrons near the inverted‐V spectral peak have large, factor of 2 to 10, coherent flux modulations at or near the wave frequency. The electron modulations are typically centered at about fCH+/2 where fCH+ is the local H+ cyclotron frequency. The EMIC waves are broadbanded, extending from about 0.3fCH+ to 0.7fCH+. These waves also accelerate cold secondary electrons, forming counterstreaming field‐aligned electrons at energies up to about 300 eV. In addition, electron modulations at fCH+ are observed in the density cavities associated with upgoing ion beams. Intense waves at fCH+ are simultaneously detected and shown to have a magnetic component similar to the EMIC waves.
On the rapid postseismic uplift along Turnagain Arm, Alaska following the 1964 Prince William Sound Earthquake
Authors: Cohen Steven C.
Published: February 2004

An analysis of leveling data obtained along Turnagain Arm, Alaska in the aftermath of the 1964 Prince William Sound earthquake shows that the postseismic uplift rate was constant during the first year following the earthquake. The maximum 1‐yr peak‐to‐trough uplift was about 15 cm, while the maximum uplift relative to a tide gauge site in Anchorage was about 9 cm. There is some evidence for a modest decrease in the uplift rate between May and October of 1965, although the evidence is weakened by the lack of an appropriate reference point. The longer term temporal evolution of uplift rates, as compiled from leveling, GPS, tide gauge, and VLBI observations by Cohen [1996], can be modeled by an Omori‐like or exponential transient decay curve with a time constant of several years. The geodetic data, taken in conjunction with seismological constraints, can be explained by aseismic‐slip‐at‐depth along the tectonic plate interface with an initial slip rate of 35 cm/yr and a slip depth of 25–37 km.
Sounding rocket observation of sporadic‐E layer electron‐density irregularities
Authors: Mori Hirotaka , Oyama Koh‐ichiro
Published: February 2004

Plasma‐density structures related to a night‐time sporadic‐E layer were measured by a fixed‐bias Langmuir probe during the flight of the S310‐25 sounding rocket, launched at 2300:00 JST on August 26, 1996 as part of the SEEK campaign. Large plasma‐density fluctuations were observed during the upleg of the rocket trajectory at altitudes of 90 to 130 km. Intense electron‐density irregularities with kilometer scales were found to be well correlated with strong polarization dc electric fields measured simultaneously by double probes. Meter‐scale irregularities measured with the Langmuir probe were found to exist in a similar altitude range as the quasi‐periodic (QP) echoes detected with a frequency agile radar operated at 24.5 MHz. However, the detailed altitude structures differed from one another. The preliminary results suggest that the regions probed by rocket and radar contained different spatial structures.
Connectivity of aqueous fluid in the Earth's upper mantle
Authors: Mibe Kenji , Fujii Toshitsugu , Yasuda Atsushi
Published: February 2004

The geometrical distribution of the aqueous fluid in textural equilibrium with forsterite has been investigated by measurements of the dihedral angle, θ, at pressures of 3 and 5 GPa and at 1000°C. The measured θ at 3 and 5 GPa are 48° and 40° to 42°, respectively. Since a value of θ smaller than 60° indicates that the aqueous fluid can form an interconnected network along the three‐grain and through the four‐grain junctions, the existing aqueous fluid in the mantle can migrate by percolation even at the small volume fractions in the pressure range. Combining these results with previously published results at lower pressure, it is suggested that the interconnected network of aqueous fluid is stable only at higher pressure than 2 GPa for the commonly accepted water content of the upper mantle. The present results show that the large scale transport of aqueous fluid is possible above 2 GPa, and that the physical properties of the upper mantle may change drastically at that pressure.
Seismic anisotropy in the lowermost mantle beneath the Pacific
Authors: Ritsema Jeroen , Lay Thorne , Garnero Edward J. , Benz Harley
Published: February 2004

Onset time differences of up to 3 s are observed between transverse (SSH) and longitudinal (SSV) components of broadband S waves at distances of 85° to 120° for paths traversing the lowermost mantle (D”) beneath the Pacific. After correction for upper mantle anisotropy, SSH usually arrives earlier than SSV with the splitting increasing with distance from 100° to 120°. The data yield two possible models of anisotropy: (1) anisotropy may vary laterally, with transverse isotropy existing in higher‐than‐average shear velocity regions beneath the northeastern Pacific, or (2) anisotropy may vary with depth, with transverse isotropy concentrated in a thin (100 km) thermal boundary layer at the base of D”. A few recordings at distances less than 105° show that SSV arrives earlier than SSH, indicating that general anisotropy likely exists in shallower regions of D”.
The SEEK (Sporadic‐E Experiment over Kyushu) Campaign
Authors: Fukao Shoichiro , Yamamoto Mamoru , Tsunoda Roland T. , Hayakawa Hajime , Mukai Toshifumi
Published: February 2004

The SEEK (Sporadic‐E Experiment over Kyushu) campaign was conducted in late August 1996 from the southern region of Kyushu, Japan, to investigate the mechanism for the generation of quasi‐periodic (QP) radar backscatter from field‐aligned irregularities imbedded in nighttime sporadic‐E (Es) layers. SEEK was designed to determine in‐situ small‐scale electrodynamical properties using two sounding rockets and large‐scale dynamics and electrodynamics using ground‐based sensors, which included a transportable radar and other radio and optical instruments deployed in the vicinity of the rocket range. It was observed by this campaign that Es layers existed in a convergent wind shear region, where large electric fields were induced and when active atmospheric gravity waves existed in the mesosphere. However, there was little evidence which positively supported the hypothesis that Es layers were deeply modulated in altitude.
Confrontation of mantle seismic anisotropy with two extreme models of strain, in central Asia
Authors: Griot Daphné‐Anne , Montagner Jean‐Paul , Tapponnier Paul
Published: February 2004

Although most authors agree that convergence is accommodated by a large shortening in Asia, two radically different modes of shortening have been put forward. In one approach, the mechanism of shortening is considered to be homogeneous, producing diffuse deformation in the crust and the mantle. In the second one, motions are localized along faults which are supposed to extend into deep lithospheric shear zones, inducing a heterogeneous deformation pattern. Seismic anisotropy is one observable manifestation of large scale deformation. Large scale seismic anisotropy field are inferred from the two extreme competing models. 3D‐mapping of anisotropy, inferred from surface waves, reveals a better coherence on average with filtered heterogeneous strain models between 100 and 200 km depths. It suggests a strong coupling between surface and deep motions down to at least 200 km. At larger depths, the coherence with homogeneous strain models suggests that mantle flow becomes more homogeneous.
Dynamics of Rocky Mountain lee waves observed during SUCCESS
Authors: Dean‐Day J. , Chan K. R. , Bowen S. W. , Bui T. P. , Gary B. L. , Mahoney M. J.
Published: February 2004

On two days during SUCCESS, the NASA DC‐8 flew carefully designed flight tracks to sample wave clouds downstream of the Rocky mountains. Wave morphology was deduced by applying linear perturbation theory to in‐situ measurements of the Meteorological Measurement System (MMS) and remotely sensed isentrope profiles of the Micro‐wave Temperature Profiler (MTP). Vertical winds from the MMS were consistent with updraft velocities derived from air parcel displacements estimated from the MTP. Derived wave characteristics and cooling rates are useful inputs to cloud microphysics models.
Are polar cap gravity waves a heat source for the high‐latitude thermosphere?
Authors: Innis J. L. , Greet P. A. , Dyson P. L.
Published: February 2004

Ground based measurements of thermospheric temperatures from Mawson, Antarctica, often show a marked spatial gradient, with data taken poleward of the discrete auroral oval up to 200 K or more warmer than measurements taken in or equatorward of the oval. This region of increased temperature is identified as the polar cap. We suggest a possible contribution to polar cap heating may be from thermospheric gravity waves.
Widespread tropical atmospheric drying from 1979 to 1995
Authors: Schroeder Steven R. , McGuirk James P.
Published: February 2004

A daily gridded data base of tropical precipitable water (PW) from 1979–95 shows 3% average drying from 1979–87 to 1989–95. Spatial patterns of the change are consistent with strengthening of the thermally direct overturning circulation. Moistening in most of the Inter‐Tropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and the Americas is more than offset by drying in oceanic subtropical highs and desert land areas.
Tidal signatures in mesospheric kinetic energy dissipation rates determined by EISCAT
Authors: Hall C. M.
Published: February 2004

By examining profiles of vertical velocity in the arctic mesosphere determined by the European Incoherent Scatter (EISCAT) VHF radar, it is possible to estimate rates of dissipation of kinetic energy. These rates are intended as first approximations to the turbulent energy dissipation rate. The values are averaged in such a way as to obtain diurnal variations for each of four seasons and periodicity emerges which we associate with tides.
Self‐absorption by vibrationally excited H2 in the Astro‐2 Hopkins Ultraviolet Telescope spectrum of the Jovian aurora
Authors: Wolven Brian C. , Feldman Paul D.
Published: February 2004

Observed H2 band intensities in the spectrum of the northern Jovian aurora, obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro‐2 mission in March 1995, are shown to be consistent with attenuation due to absorption by vibrationally excited H2 (ν≤2) in the overlying atmosphere. Strong Werner band transitions near 1050 and 1100 Å, in a region of the spectrum not accessible to IUE or HST, have intensities significantly weaker than those predicted by the theoretical branching ratios for optically thin emission. Absorbed photons escape primarily by fluorescent scattering in transitions which connect to unpopulated vibrational levels at longer wavelengths. A spectrum of the Jovian day glow (produced by a combination of solar and electron‐impact fluorescence) obtained a few days earlier displays considerably less attenuation in the 1050–1100 Å region. We present synthetic H2 fluorescence spectra which include the effects of self‐absorption and demonstrate the ability to reproduce the features observed in the HUT spectra.
Global variation of column droplet concentration in low‐level clouds
Authors: Han Qingyuan , Rossow William B. , Chou Joyce , Welch Ronald M.
Published: February 2004

Cloud droplet concentration is a very important parameter in model studies. However, no global observation is available because it is hard to retrieve by current satellite remote sensing techniques. This study introduces another parameter, column droplet concentration, which can be retrieved by satellite data and used in models. The column droplet concentration (Nc) is the product of cloud geometrical thickness and droplet volume number concentration. This paper presents a method and the results of retrieving column droplet concentration for low‐level clouds. The first near‐global survey (50°S to 50°N) of Nc reveals more clearly the effect of aerosol concentration variations on clouds. The survey shows the expected increase of column droplet concentrations between ocean and continental clouds and in tropical areas during dry seasons where biomass burning is prevalent. Therefore, column droplet concentration is demonstrated as a good indication of available CCN populations in certain areas.
The Longitudinal Variation of the Color Ratio of the Jovian Ultraviolet Aurora: A Geometric Effect?
Authors: Gérard Jean‐Claude , Grodent Denis , Dols Vincent , Waite J. Hunter
Published: February 2004

A three‐dimensional model is used to assess the role of the viewing geometry on the auroral color ratio. The simulations show that both an auroral arc with a geometry deduced from images obtained with the Hubble Space Telescope (HST) and a uniform polar cap emission produce no modulation or a minimum absorption when the longitude of the Jovian central meridian (CML) is close to 200°. This result is in contrast with the statistical measurements made with the International Ultraviolet Explorer (IUE) spectrograph that the hydrocarbon optical depth above the auroral emission maximizes for CMLs about 180°. In the frame of this simplified model, we examine a possible way to reconcile the model with the IUE data. An intrinsic longitudinal dependence of the column of methane above the level of the auroral emission is introduced in the simulation. It may result from a combination of a vigorous upwelling in sectors of strong and stable precipitation and/or a longitudinal dependence of the characteristic energy of the auroral particles.
A strong CME‐related magnetic cloud interaction with the Earth's Magnetosphere: ISTP observations of rapid relativistic electron acceleration on May 15, 1997
Authors: Baker D. N. , Pulkkinen T. I. , Li X. , Kanekal S. G. , Ogilvie K. W. , Lepping R. P. , Blake J. B. , Callis L. B. , Rostoker G. , Singer H. J. , Reeves G. D.
Published: February 2004

A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT). SAMPEX data showed that relativistic electrons (E ≳ 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly “injected” from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for ∼2 hrs) by remarkably strong low‐frequency wave activity as seen by CANOPUS ground stations and by the GOES‐8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high‐energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi‐point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi‐MeV energies deep in the radiation belts on timescales of tens of minutes.
The effects of topography on magma chamber deformation models: Application to Mt. Etna and radar interferometry
Authors: Williams Charles A. , Wadge Geoff
Published: February 2004

We have used a three‐dimensional elastic finite element model to examine the effects of topography on the surface deformation predicted by models of magma chamber deflation. We used the topography of Mt. Etna to control the geometry of our model, and compared the finite element results to those predicted by an analytical solution for a pressurized sphere in an elastic half‐space. Topography has a significant effect on the predicted surface deformation for both displacement profiles and synthetic interferograms. Not only are the predicted displacement magnitudes significantly different, but also the map‐view patterns of displacement. It is possible to match the predicted displacement magnitudes fairly well by adjusting the elevation of a reference surface; however, the horizontal pattern of deformation is still significantly different. Thus, inversions based on constant‐elevation reference surfaces may not properly estimate the horizontal position of a magma chamber. We have investigated an approach where the elevation of the reference surface varies for each computation point, corresponding to topography. For vertical displacements and tilts this method provides a good fit to the finite element results, and thus may form the basis for an inversion scheme. For radial displacements, a constant reference elevation provides a better fit to the numerical results.
Electric potential variations associated with yearly lake level variations
Authors: Perrier Frédéric , Trique Michaël , Lorne Benoit , Avouac Jean‐Philippe , Hautot Sophie , Tarits Pascal
Published: February 2004

Electric potential variations have been recorded from November 1995 to February 1996 and continuously since October 1996 at 14 measurement points on a one km wide ridge separating two lakes in the French Alps. The levels of the lakes vary by several tens of meters on a yearly cycle, inducing stress variations and fluid percolation. At one point, unambiguous variations as large as 120 mV are observed over a year, linearly correlated with the levels of the lakes with a magnitude of 2 mV per meter of water level change. This particular measurement point lies at the edge of a SP anomaly, which supports the presence of a localized zone of ground water flow forced by the lake level, suggesting an electrokinetic mechanism. The observed correlation implies a ζ‐potential of the order of ‐8 mV for a 60 Ωm electrolyte, in agreement with laboratory measurements.
Ozone depletion at mid‐latitudes: Coupling of volcanic aerosols and temperature variability to anthropogenic chlorine
Authors: Solomon S. , Portmann R. W. , Garcia R. R. , Randel W. , Wu F. , Nagatani R. , Gleason J. , Thomason L. , Poole L. R. , McCormick M. P.
Published: February 2004

Satellite observations of total ozone at 40–60°N are presented from a variety of instruments over the time period 1979–1997. These reveal record low values in 1992–3 (after Pinatubo) followed by partial but incomplete recovery. The largest post‐Pinatubo reductions and longer‐term trends occur in spring, providing a critical test for chemical theories of ozone depletion. The observations are shown to be consistent with current understanding of the chemistry of ozone depletion when changes in reactive chlorine and stratospheric aerosol abundances are considered along with estimates of wave‐driven fluctuations in stratospheric temperatures derived from global temperature analyses. Temperature fluctuations are shown to make significant contributions to model calculated northern mid‐latitude ozone depletion due to heterogeneous chlorine activation on liquid sulfate aerosols at temperatures near 200–210K (depending upon water vapor pressure), particularly after major volcanic eruptions. Future mid‐latitude ozone recovery will hence depend not only on chlorine recovery but also on temperature trends and/or variability, volcanic activity, and any trends in stratospheric sulfate aerosol.
Structural and seismological features of the 1989 Syn‐Eruptive NNW‐SSE fracture system at Mt. Etna
Authors: Bianco Francesca , Castellano Mario , Ventura Guido
Published: February 2004

During the September–October 1989 eruption at Mt. Etna, structural (faults, folds, fractures) and seismological data (locations, focal mechanisms, shear wave splitting) on the opening of a 6.5km long NNW‐SSE fracture system were collected. The fracture system reflected the surface deformation associated to the emplacement of a blade‐like dyke at shallow depth. Seismic activity occurred only at the southern tip of the fracture system and at about 1–1.5km depth, i.e. at the interface between the less stiff clay basement and the stiffer volcanic pile. Structural data indicate that fractures formed in response to an ENE‐WSW striking σ3=σhmin. In contrast, the NNW‐SSE compressive structures, which developed only at the southern tip of the main fracture system, were related to an ENE‐WSW σHmax. The inversion of focal mechanisms shows a thrust‐type deformation related to an ENE‐WSW striking σ1=σHmax. The association of compressive structures is consistent with experimental models of dykes stopping at a less stiff‐stiffer interface. S‐wave splitting parameters have been measured during the fracture system propagation. No change was observed in the qS1 polarization eigendirections (NE‐SW), while the TD values seemed to vary with time. The results of seismological and structural analyses suggest that the dyke stopping mechanism produces: i) a seismic swarm associated to a local compressive stress field; and (ii) dyke‐induced stress generating shallow aligned secondary fractures (EDA cracks).
Warming of the Arctic Ocean by a strengthened Atlantic Inflow: Model results
Authors: Zhang Jinlun , Rothrock D. Andrew , Steele Michael
Published: February 2004

An ice‐ocean model is used to examine the behavior of the Arctic Ocean in response to recent changes in Arctic climate. The model shows that, starting about 1989, there has been a significant warming and salinification in the Arctic Ocean, in agreement with recent observations. The warming and salinification occur mainly in the upper ocean owing to a sustained increase of Atlantic inflow both at Fram Strait and, most significantly, via the Barents Sea. The increased incoming warm and salty Atlantic Water “flushes” out cold and fresh Arctic Water, thus increasing the temperature and salinity of the upper ocean and resulting in more oceanic heat flux to the mixed layer and ice cover. Concomitantly, the model shows a continuing decrease in ice volume beginning in 1987.
High altitude trapping of beam ions by self‐generated plasma waves in interhemispheric plasma flows
Authors: Singh Nagendra
Published: February 2004

Counterstreaming ion beams originating from the ionosphere are a natural product of interhemispheric plasma flow in empty plasmaspheric flux tubes after geomagnetic storms. Owing to the prevailing collisionless conditions during the early stage of the flow, trapping of the plasma in the flow at relatively high altitudes has remained a mystery. Using 2.5‐D particle‐in‐cell simulations, a study of the ion‐beam driven plasma instabilities indicates that the fastest growing wave mode is an effective source for the pitch angle scattering of the ions transferring part of the parallel energy in the magnetic field‐aligned flows into the ions' perpendicular energy. The pitch angle scattering is facilitated by the anomalous cyclotron resonance of the beam ions with the fastest growing waves driven by the interaction between the Doppler‐shifted cyclotron modes of the two counterstreaming ion beams; the interaction generates purely growing waves at nearly zero frequency. The Landau resonance of the Doppler‐shifted cyclotron mode of one of the beams with the other beam, and vice versa, also generates waves which are relatively slow growing at frequencies ω ≅nΩi /2, where n is an integer and Ωi is the ion cyclotron frequency. Since these waves appear at ω ≅Ωi in the rest frame of the beams, they transversely accelerate the ions. Both the pitch angle scattering and the transverse acceleration contribute to the trapping of the flow in the flux tubes, but the former being a much faster and more effective process than the latter. This collisionless trapping at high altitudes, facilitated by the self‐generated waves, can be a major process in plasmaspheric refilling.
MHD simulations of oppositely propagating Alfvén waves in the magnetosheath and solar wind
Authors: Cable S. , Lin Y.
Published: February 2004

Recent WIND and GEOTAIL observations indicate that Alfvén waves can reverse their direction of propagation relative to the ambient magnetic field when they cross the bow shock [Sibeck et al., 1997]. To investigate this effect, we use three‐dimensional magnetohydrodynamic (MHD) simulations to study an Alfvén wave (AW) propagating into the magnetosheath. Our results are consistent with the observations. The AW is set to propagate with a wave vector k pointing anti‐sunward and largely opposite to the magnetic field direction, i.e. B · k < 0. The interplanetary magnetic field (IMF) is given a typical Parker spiral angle. The primary Alfvén wave that is transmitted into the magnetosheath (AW′) continues to propagate anti‐sunward. In the dusk magnetosheath, AW′ largely maintains this propagation direction through its entire passage through the magnetosheath. On the dawn side, however, AW′ changes propagation direction relative to the magnetic field so that it propagates along the magnetic field instead of against it, i.e. B · k becomes positive. This effect is strongest in the region of the magnetosheath where field line draping forces the x component of the ambient IMF to change sign. The reversal of B · k is the end result of a phase shift between the velocity and magnetic field perturbations that occurs smoothly as the wave propagates toward the magnetopause.
The sensitivity of the radical amplifier to ambient water vapour
Authors: Mihele C. M. , Hastie D. R.
Published: February 2004

The radical amplifier is an instrument used to measure radical concentrations in the troposphere. The critical parameter in determining the sensitivity is the chain length, which is shown to decrease with increasing water vapour in the reactor. When compared to measurements in dry air, this decrease is a factor 2 at a relative humidity of 40%. This suggests that field measurements using the radical amplifier may be underestimating the ambient radical concentration by a similar factor. One source of this deterioration in performance is an increase in the loss of radicals to the walls of the reactor, although there also appears to be a contribution from a water dependence on the gas phase chemistry.
Melting relations of hydrous and dry mantle compositions and the genesis of komatiites
Authors: Asahara Y. , Ohtani E. , Suzuki A.
Published: February 2004

The hydrous phase relations of primitive upper mantle model compositions in the CaO‐MgO‐Al2O3‐SiO2‐H2O system with 1, 2 and 5 wt.% H2O have been investigated with a multianvil high pressure apparatus at 6.5 GPa and temperatures from 1550°C to 2050°C. Liquidus temperatures decrease by about 100°C and the temperature interval to produce about 60% partial melting reduces to less than 50°C in the 1–2 wt.% H2O‐bearing systems. At an H2O content of about 5 wt.%, garnet is the first dissolved phase and the stability field of orthopyroxene expands. Aluminum undepleted komatiite magmas can be formed by melting at 200 km depth in a hydrous mantle at significantly lower temperatures than under dry conditions. Thus, komatiite magmas cannot constrain temperatures in the Archean mantle. In addition, cratonic peridotites may have formed as residues of partial melting under various H2O contents.
The accuracy of satellite radar altimeter data over the Greenland Ice Sheet determined from airborne laser data
Authors: Bamber J. L. , Ekholm S. , Krabill W.
Published: February 2004

The 336 days of the geodetic phase of ERS‐1 provides dense coverage, by satellite radar altimetry, of the whole of the Greenland ice sheet. These data have been used to produce a digital elevation model of the ice sheet. The errors present in the altimeter data were investigated via a comparison with airborne laser altimeter data an absolute accuracy typically in the range 2–10 cm ± 10 cm. Comparison of differences between the radar and laser derived elevations, showed a correlation with surface slope. The difference between the two data sets ranged from 84 cm ± 79 cm for slopes below 0.1 degrees, to 10.3 m ± 8.4 m for a slope of 0.7 degrees (the half power beam‐width of the ERS‐1 radar altimeter). An explanation for the behaviour of the difference as a function of surface slope is given in terms of the pattern of surface roughness on the ice sheet.
An estimate of the global emissions of methyl bromide from automobile exhausts
Authors: Baker J. M. , Reeves C. E. , Penkett S. A. , Cardenas L. M. , Nightingale P. D.
Published: February 2004

An estimate of the annual global methyl bromide (CH3Br) emissions from automobile exhausts has been determined by extrapolating the results of a field study conducted in the United Kingdom (UK). A strong linear correlation was observed between the CH3Br and carbon monoxide (CO) concentrations of roadside air in three cities. This correlation and knowledge of the UK CO emissions was used to estimate the source strength of CH3Br from automobile exhausts in the UK (0.04 ktonnes yr−1). Further extrapolations lead to a value of 1.5 ktonnes yr−1 (with an upper limit of 3.0 ktonnes yr−1) of CH3Br released globally to the atmosphere from automobile exhausts.
Coordinated heat removal from the equatorial Pacific during the 1986–87 El Niño
Authors: Sun De‐Zheng , Trenberth Kevin E.
Published: February 2004

Utilizing radiation data from Earth Radiation Budget Experiment (ERBE), circulation statistics from NCEP reanalysis, and assimilated ocean data for the tropical Pacific basin, we show that the surface ocean warming during the 1986–87 El Niño is not only accompanied by significant increases in the cloud reflection of the solar radiation, but also by marked increases in the poleward energy transport in both the atmosphere and ocean. Measured over the equatorial region, the feedback from the ocean dynamics is twice as large as from the atmospheric dynamics which in turn is twice as large as the feedback from the cloud albedo. The three feedbacks constitute a strong regulatory effect upon the equatorial SST. The results reveal a prominent role of El Niño in the heat removal from the equatorial Pacific.
Observed changes in the diurnal dewpoint cycles across North America
Authors: Schwartzman Peter D. , Michaels Patrick J. , Knappenberger Paul C.
Published: February 2004

We analyzed the record of hourly dewpoint data from 92 climatological stations in North America. The data were centered with respect to sunrise and sunset and divided into four geographic regions of approximately equal area in an attempt to uncover regional trends. Few statistically significant regional trends in mean dewpoint temperatures were found. However, the within‐day variations were very well behaved and consistent across regions. They showed a relative decline in dewpoints in the late afternoon at most stations, and small declines just after sunrise. The results are consistent with urbanization signals described by Oke [1987]. An observed rise in midday dewpoint, followed by a fall in late afternoon, indicates some regional enhancement of convection, which may in part be from the urban effect, but other causes are possible. Our dewpoint changes are somewhat consistent with precipitation changes that have described by other researchers.
Tentative identification of S2 in the IUE spectra of comet Hyakutake (C/1996 B2)
Authors: Laffont Céline , Boice D. C. , Moreels G. , Clairemidi J. , Rousselot P. , Andernach H.
Published: February 2004

Spectra of Comet Hyakutake were obtained with the International Ultraviolet Explorer (IUE) satellite on February 19, 1996, (20 days after its discovery on January 31), and later during its closest approach to the Earth at the end of March, with a geocentric distance of 0.1 AU. The cometary spectra are compared with a synthetic fluorescence spectrum of S2 On March 27, we find that both synthetic and cometary spectra are reasonably correlated. This suggests that S2 was probably present in the cometary spectrum with a production rate between 6.5×1024 and 1.0×1025 molec/s and an abundance of 0.005 to 0.009% relative to water.
Depletion rates of sulfur dioxide in tropospheric volcanic plumes
Authors: Oppenheimer Clive , Francis Peter , Stix John
Published: February 2004

The tropospheric sulfur cycle has been closely studied from the standpoint of anthropogenic emissions but less so with respect to volcanic sources. We document here evidence for wide variation in lifetimes of volcanic SO2 in the troposphere. At one extreme, our observations of the plume associated with the lavadome eruption of Soufriere Hills volcano (Montserrat) suggest loss of gaseous SO2 at rates exceeding 10−3 s−1. While this efficent SO2 depletion reflects unusual environmental factors, published measurements for Mount Etna's plume approach these rapid rates. In such cases, source strengths of SO2 must significantly exceed apparent fluxes measured several kilometres downwind. This implies that meteorological and geographic factors, as well as volcanic degassing rates, should be considered in the interpretation of fluxes of SO2 measured beyond source vents, with ramifications for volcano monitoring and eruption prediction. Furthermore, since most calculations of global volcanic fluxes of sulfur (and other species) to the troposphere are based on extrapolation of SO2 flux data they may underestimate true source strengths. Finally, we consider that fast sulfur chemistry may also prevail in convecting eruption columns, resulting in partial tropospheric scrubbing of sulfur from stratosphere‐bound plumes.