bluuebunny/crossref_metadata_2025 huggingface dataset:
New observations of ion beams in the plasma sheet boundary layer
Authors: Parks G. , Chen L. J. , McCarthy M. , Larson D. , Lin R. P. , Phan T. , Reme H. , Sanderson T.
Published: September 2002

The Wind perigee passes covered tail distances from 6–24 RE. By use of bulk quantities and the parent distributions, we have found new features in the PSBL that had been missed previously. The PSBL consists of a unidirectional earthward streaming ion beam at the edge and another unidirectional beam inside this edge streaming in the tailward direction. Bidirectional beams are observed with higher densities, further inside the PSBL. The plasma in the region supporting the tailward streaming beams consists of the beam distribution plus an isotropic component, whereas the earthward streaming beams consists mainly of the beam distribution. These distributions yield fast flows (>400 km/s) in the earthward direction and slower flows (≈ 150 km/s) in the tailward direction. Both regions support counter streaming electron beams superposed on an isotropic component. These new findings are substantially different from previous observations and the interpretation of fast flows and ion beams in terms of a neutral line model needs to be reexamined.
Middle atmosphere cooling trend in historical rocketsonde data
Authors: Dunkerton Timothy J. , Delisi Donald P. , Baldwin Mark P.
Published: September 2002

Data from the historical rocketsonde network demonstrate that significant cooling of the upper stratosphere and lower mesosphere (∼30–60 km) occurred in northern midlatitudes of the western hemisphere and in the tropics during 1962–1991. The downward trend of temperature averaged over this layer was about −1.7 K/decade and temperatures were apparently modulated by the solar cycle with amplitude ∼1.1 K. The trend was a function of height and somewhat larger in the lower mesosphere relative to the middle and upper stratosphere.
A forty year record of Mercury in central Greenland snow
Authors: Boutron Claude F. , Vandal Grace M. , Fitzgerald William F. , Ferrari Christophe P.
Published: September 2002

Hg has been measured using ultraclean procedures in snow deposited in central Greenland from 1949 to 1989. Concentrations range from ≤0.05 to 2.0 pg/g (mean : 0.43 pg/g), i.e. values which are orders of magnitude lower than those obtained in earlier studies. It indicates that earlier data were plagued by major contamination problems. Combined estimated contributions from natural Hg sources cannot explain Hg concentrations observed in the snow. It suggests that Hg deposition to the Greenland ice sheet is now significantly influenced by anthropogenic inputs from North America, Asia and Europe linked especially to coal burning and solid waste incineration. Although our data suggest that Hg concentrations were higher in snow dated from the late 1940s to the mid 1960s than in more recent snow, further studies are needed to clearly assess Hg temporal trends.
Quasi‐biennial modulation of the southern hemisphere stratospheric polar vortex
Authors: Baldwin Mark P. , Dunkerton Timothy J.
Published: September 2002

Observations reveal that the wintertime southern hemisphere stratospheric polar vortex is modulated by the phase of the equatorial quasi‐biennial oscillation (QBO). The high‐latitude southern stratosphere is shown to be slightly colder throughout the winter, and the final warming occurs later, when the QBO is in its west phase. During May–October, the modulation of winds by the QBO is confined to midlatitudes, at the edge of the polar vortex. The difference between west and east phase composites of zonal mean wind during November, at the time of the final warming in the southern hemisphere, exceeded 14 ms−1. This difference is very similar to that in January in the northern hemisphere. While northern hemisphere QBO effects are optimized using equatorial winds near 40 hPa, southern hemisphere effects are best seen using ∼25 hPa winds.
Lunar reflectivity from Extreme Ultraviolet Explorer imaging and spectroscopy of the full moon
Authors: Flynn Brian C. , Vallerga John V. , Gladstone G. Randall , Edelstein Jerry
Published: September 2002

We present new imaging and spectroscopic observations obtained with the Extreme Ultraviolet Explorer (EUVE). Our 200 Å image shows high contrast (33%) and complete albedo reversal, i.e., contrast is reversed relative to the Moon in visible light. The high contrast at 200 Å suggests the potential of the EUV spectral region for surface compositional studies. The 600 Å image is surprisingly bland (<3% contrast). We derive surface reflectivities (geometric albedos) of individual surface regions for the first time with values of 0.11–0.15% at 200 Å, and 2.5% at 600 Å. Reflectivities derived from the medium‐ (MW) and long‐wavelength (LW) spectra (140–380 Å and 280–760 Å) show that lunar reflectivity decreases by a factor of approximately 60 over the 700 to 150 Å range. Our data further suggest that the Moon's brightness vs phase may behave differently than at longer wavelengths, and that surface fluorescence may contribute to the observed EUV brightness.
Hybrid simulation of the curved dayside magnetopause during southward IMF
Authors: Omidi N. , Karimabadi H. , Krauss‐Varban D.
Published: September 2002

Large‐scale, 2‐D hybrid (fluid electrons, particle ions) simulations have been performed to investigate the macroscopic and the microscopic structure of the dayside (curved) magnetopause during southward IMF. It is found that in addition to the formation of an X‐line at the subsolar point, other X lines are formed at higher latitudes. The locations of these X lines are asymmetric with respect to the magnetic equator, and the resulting plasmoids have different sizes. The thickness of the magnetopause varies considerably as a function of latitude. In addition, the spatial scales associated with the plasma variations across the magnetopause are considerably different than those associated with the field variations. The rotational (current) layer exhibits a linear polarization and does not match the jump conditions of any known discontinuity. Instead, the topology of the magnetic field lines, determines the spatial scale and the polarization of the current layer.
Spatial interpolation of the atmospheric water vapor content between sites in a ground‐based GPS Network
Authors: Emardson T. Ragne , Johansson Jan M.
Published: September 2002

We have studied three methods of spatially interpolating GPS‐estimated wet delay time series. Five months of data from three sites in the Swedish continuously operating GPS network were used for interpolation to a fourth site. Different geometries were used, resulting in interpolation distances of 40–120 km. To assess the quality of the interpolations we compared the obtained time series with estimated wet delay from a GPS station and with radiosonde data. A method based on the theory of atmospheric turbulence showed the lowest errors when compared to both the GPS estimates and the radiosonde data. The rms errors using this method are typically 1 cm.
The global response of relativistic radiation belt electrons to the January 1997 magnetic cloud
Authors: Reeves G. D. , Baker D. N. , Belian R. D. , Blake J. B. , Cayton T. E. , Fennell J. F. , Friedel R. H. W. , Meier M. M. , Selesnick R. S. , Spence H. E.
Published: September 2002

In January 1997 a large fleet of NASA and US military satellites provided the most complete observations to date of the changes in >2 MeV electrons during a geomagnetic storm. Observations at geosynchronous orbit revealed a somewhat unusual two‐peaked enhancement in relativistic electron fluxes [ Reeves et al., 1998]. In the heart of the radiation belts at L ≈ 4, however, there was a single enhancement followed by a gradual decay. Radial profiles from the POLAR and GPS satellites revealed three distinct phases. (1) In the acceleration phase electron fluxes increased simultaneously at L ≈ 4–6. (2) During the passage of the cloud the radiation belts were shifted radially outward and then relaxed earthward. (3) For several days after the passage of the cloud the radial gradient of the fluxes flattened, increasing the fluxes at higher L‐shells. These observations provide evidence that the acceleration of relativistic electrons takes place within the radiation belts and is rapid. Both magnetospheric compression and radial diffusion can cause a redistribution of electron fluxes within the magnetosphere that make the event profiles appear quite different when viewed at different L‐shells.
Diffuse emission of carbon dioxide, methane, and helium‐3 from Teide Volcano, Tenerife, Canary Islands
Authors: Hernández Pedro A. , Pérez Nemesio M. , Salazar José M. , Nakai Shun'ichi , Notsu Kenji , Wakita Hiroshi
Published: September 2002

Diffuse emission of CO2, CH4 and ³He was investigated in the summit crater of Teide volcano, Tenerife, Canary Islands. The results indicate that Teide volcano releases abundant CO2 not only from its active crater, but also from its flanks as diffuse soil emanations. The spatial distribution of these emanations correlates quite closely with that of geothermal anomalies and manifestations. Our flux measurements indicate a total output of 380 ton·day−1 of CO2 and 1.6 on·day−1 of CH4 by diffuse degassing. From the ³He/CO2 ratios and the CO2 flux we estimate a diffuse ³He flux of 0.052 mol·year−1. Isotopic analysis of carbon in soil gas samples from sites of high soil CO2 flux indicates a magmatic origin for the CO2, also reported by high ³He/ 4He ratios of 5.50 and 5.87 Ra. The origin of methane is more uncertain with low δ13C values suggesting a biogenic contribution.
Analysis of BrO measurements from the Global Ozone Monitoring Experiment
Authors: Chance Kelly
Published: September 2002

The Global Ozone Monitoring Experiment (GOME), launched on the ESA ERS‐2 satellite in April 1995, measures solar back scatter spectra from which atmospheric BrO columns may be derived globally with high precision (slant column uncertainties of ±3×1013 cm−2 or less). Obtaining such precision requires additional characterization of GOME spectra, reference spectra that are well calibrated in wavelength, and a fitting method capable of reproducing measurements to several parts in 104. This paper describes these elements of the data processing and gives examples of BrO distributions in the stratosphere and enhanced tropospheric BrO in the polar springtime.
Review of laboratory experiments on Alfvén waves and their relationship to space observations
Authors: Gekelman Walter
Published: September 2002

Hannes Alfvén predicted the existence of a hydrodynamic wave in a perfectly conducting fluid in 1942. It took 6 years before this discovery was accepted and 10 years before Alfvén waves were first observed in the laboratory. Now it is widely recognized that these waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Alfvén waves have been observed in the solar wind, are thought to be prevalent in the solar corona, may be responsible for parallel electric fields in the aurora, and can cause particle acceleration over large distances in interstellar space. They have also been considered as a candidate for heating thermonuclear plasmas and are potentially dangerous to confinement. Alfvén waves have been difficult to observe in basic laboratory experiments because of their low frequencies and long wavelengths. In this paper we present a review of plasma Alfvén wave experiments performed in recent years. The quality of the laboratory data have paralleled advances in plasma sources and diagnostics. In the past few years the quantum jump in data collection on the Freja and FAST missions have lead to the reevaluation of the importance of these waves in the highly structured plasma that was probed. Recent laboratory experiments have examined, in great detail, shear waves generated by filamentary currents in both spatially uniform and striated plasmas. Tone bursts, short pulses, and interference effects have been studied with emphasis on structures of the order of the skin depth, c/ωpe. These are features of significant interest to the space community. In fact, it appears that the phenomena observed in laboratory experiments show striking similarities to what has been observed in space. A comparison of these results will be given.
Global observation of the ionospheric electronic response to solar events using ground and LEO GPS data
Authors: Hernández‐Pajares M. , Juan J. M. , Sanz J. , Solé J. G.
Published: September 2002

We present in this work the temporal evolution of the three‐dimensional electron density at global scale during two ionospheric storms (October 18–19, 1995, and January 10, 1997) computed using only actual Global Positioning System data. The tomographic model is solved by means of a Kaiman filtering with a filter updating time of 1 hour in a Sun‐fixed reference frame, and with a resolution of 10 × 10 deg in latitude/local time and 100 km in height including also a protonospheric component (eight layers). The data set contains the data from the International GPS Service IGS (with more than 100 ground GPS stations worldwide distributed) and the GPS/MET low orbiting GPS receiver (both positive and negative elevation observations are used). This means for each storm 1,000,000 of delays, 400 occultations and 3000 unknowns per batch. The International Reference Ionosphere and data coming from the ionosonde of the Ebre observatory are used to show the reliability of the results.
Ion and electron distribution functions in the distant magnetotail: modeling of Geotail observations
Authors: Smets R. , Delcourt D. , Fontaine D.
Published: September 2002

Ion and electron distribution functions measured by the Geotail spacecraft in the distant magnetotail are investigated considering single‐particle dynamics in sharp field reversals. The observed H+ distribution functions exhibit two components, namely, a low‐energy core and a high‐energy population propagating tailward. In agreement with previous studies, we show that ion dynamics in a two‐dimensional (2‐D) reconnection field geometry naturally leads to such distributions and that the neutral line location can be analytically inferred from the velocity space structure. As for electron distribution functions, Geotail observations reveal characteristic “flattop” profiles with enhanced flux in the parallel direction at low energies and in the perpendicular direction at high energies. We interpret these distributions as the result of the electron chaotic behavior in the reconnection region. This chaotic regime, which occurs when the particle Larmor radius is comparable to the magnetic field line curvature radius, can lead to prominent pitch angle scattering. Near the neutral line, we show that at a given energy, this regime yields flux depletion in the parallel direction and enhancement in the perpendicular one. The flattop profile arises from this behavior affecting only the (relatively) high‐energy part of the electron distribution. Conversely, the threshold energy above which electrons exhibit such a behavior contains information on the field reversal characteristics. The electron distribution functions obtained numerically are in agreement with the observations, which support this interpretation framework and allow us to estimate the thickness of the neutral sheet.
Propagation analysis of electromagnetic waves between the helium and proton gyrofrequencies in the low‐altitude auroral zone
Authors: Santolík Ondřej , Parrot Michel
Published: September 2002

We present a statistical study of electromagnetic emissions with a lower frequency limitation observed in a broadband below the local proton gyrofrequency. An extensive data set recorded by the low‐orbiting Freja satellite is examined. The emissions occur outside the plasmasphere, mainly at auroral latitudes. The maximum wave power is observed before the local noon, and no events have been detected during the local night. In the majority of cases we can assume the presence of a single plane wave. Further analysis shows a nearly circular and right‐handed polarization. The statistics of wave vector directions reveal a downward and nearly field aligned propagation. An experimental estimation of the phase velocity modulus is in rough agreement with the cold plasma theory. The origin of the emissions could be possibly explained by the tunnelling of the downgoing ELF hiss below the two‐ion cross‐over frequency.
Dayside auroral configurations: Responses to southward and northward rotations of the interplanetary magnetic field
Authors: Sandholt Per Even , Farrugia Charles J. , Moen Jøran , Cowley Stanley W. H.
Published: September 2002

We report specific types of daytime auroral activities occurring in response to certain rotations of the interplanetary magnetic field (IMF). The IMF rotations we focus on consist of a sharp southward turning from an initial northward orientation, followed by slower rotations back to an intermediate state, which is dominated by the By component (small Bz). When the IMF turned from north to south, the day side auroral configuration changed from one typical of northward conditions to one typical of southward conditions. The type 2 cusp aurora in the north (∼78°–79°magnetic latitude) was replaced by activation of a sequence of moving auroral forms (type 1 cusp aurora) at lower latitudes, each form appearing at the equatorward boundary of the previous one, resulting in ∼100‐ to 200‐km total displacements of the auroral equatorward boundary in the cusp region during intervals of ∼5‐ to 10‐min duration. This auroral activity is accompanied by enhancement of ionospheric convection and onset of magnetic bays on the ground. Since southward rotations of the IMF are expected to give rise to enhancements of the magnetopause reconnection rate, we conclude that these auroral/convection events represent distinct ionospheric signatures of such reconnection rate enhancements. When the IMF turned partially northward again, into an intermediate state, the aurora responded as expected, i.e., the equatorward auroral displacement was halted or reversed, and the type 2 aurora reappeared. The coexistence of the types 1 and 2 cusp auroras in the intermediate state is interpreted in terms of the simultaneous occurrence of reconnections at low and high magnetopause latitudes. The dynamical configurations of particle precipitation and ionospheric convection in the cusp region in such cases are discussed.
On the combined effects of vertical shear and zonal electric field patterns on nonlinear equatorial spread F evolution
Authors: Sekar R. , Kelley M. C.
Published: September 2002

In order to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large‐scale plume events, an investigation is carried out by means of a nonlinear numerical simulation model using different background ionospheric conditions. The investigation revealed that the development of long wavelength (200 km) perturbations can only be confined to the bottomside of the ionosphere in a localized region with the combined effects of shear in the zonal plasma drift and a nighttime westward electric field. The variabilities in the occurrence characteristic are shown to depend on the variabilities in the prereversal enhancement in the zonal electric field. Further, the resurgence of a plume event at a later time, as seen by the Jicamarca radar, is explained on the basis of confined (preseeded) structures subject to a fresh instability source. First evidence is presented that a source of fresh instability may actually be a wind and/or a temperature disturbance launched hours earlier by a small substorm.
The motion of ions in the auroral ionosphere
Authors: Fujii Ryoichi , Nozawa Satonori , Buchert Stephan C. , Matuura Nobuo , Brekke Asgeir
Published: September 2002

The ion motion in the E and F regions relative to the local magnetic field line and the horizontal plane has been determined from the European incoherent scatter (EISCAT) common program 1 version I (CP‐1‐I) data. The ion motion highly depends on altitude and magnetospheric electric field strength. Under the condition of a strong electric field (|E| > 25 mV/m), ions move perpendicularly to the local magnetic field line both in the F region and at the upper part of the E region (≥117 km). This suggests the ions moving by the E × B drift, which is expected when the ion‐neutral collision frequency is much smaller than the ion gyrofrequency. At lower altitudes, however, the ions no longer move perpendicularly to the magnetic field line but move horizontally, as expected from a stronger interaction between the ions and neutrals. Under the condition of a weak electric field (5 mV/m < |E| < 10 mV/m) the neutral winds tend to move the ions horizontally everywhere in the E region and even in the F region. The main cause of these ion motions and their dependence on altitude and electric field strength comes from the relative importance of the magnetospheric electric field and the large‐scale neutral wind drag to the ionospheric ions.
Properties of interplanetary magnetic sector boundaries based on electron heat‐flux flow directions
Authors: Kahler S. , Crooker N. U. , Gosling J. T.
Published: September 2002

We have used the solar wind electron heat flux flow directions to determine the interplanetary magnetic field polarities for the ISEE 3 period from 1978 to 1982. This technique assumes that the heat flux electrons flow away from the Sun along magnetic field lines. It provides the field polarities independently of the field directions. The resulting distribution of sector durations and the changes in that distribution with solar activity cycle are presented for four 1‐year periods. The large‐scale sectors expected from extrapolation of the Stanford source surface maps are present along with a population of small‐scale sectors with a peak in the time range of 9 hour to 1 day. About half the small‐scale sectors contain bidirectional electron (BDE) flows, suggesting their origins in coronal mass ejections. We also examine cases of false polarities, in which the directions of the fields imply polarities opposite to those determined from the heat‐flux directions. These constitute only 6 to 8% of all the hourly averages of the data. The majority (78%) of these false polarity regions were not associated with BDEs, and 75% were of only 1 or 2 hour durations. False polarity regions tended to lie nearly orthogonal to the spiral field angles and at relatively high latitudinal angles. While multiple (≥ 3 in 24 hours) current sheet crossings are common, we find no cases consistent with a wavy current sheet.
Coronal mass ejections, flares, and geomagnetic storms
Authors: Landi R. , Moreno G. , Storini M. , Antalová A.
Published: September 2002

We investigate the role of the coronal mass ejections (CMEs) (by using soft X ray solar emission as proxy data) in producing the nonrecurrent geomagnetic storms in the period 1969–1974. The linkage between these phenomena is confirmed: however, it turns out that CMEs associated with chromospheric flares, accompanied by type 4 radio emission, are the most effective in perturbing the geomagnetic field.
Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Images and spectral analysis
Authors: Brandt Pontus C:son , Barabash Stas , Norberg Olle , Lundin Rickard , Roelof Edmond C. , Chase Christopher J.
Published: September 2002

Observations of energetic neutral atoms (ENA) in the energy range 26–52 keV are reported from four occasions during geomagnetically disturbed periods. The data were acquired by the ENA imager flown on the Swedish microsatellite Astrid in a 1000 km circular orbit with 83° inclination. The ENA imager separates charged particles from neutrals through an electrostatic deflection system in the energy range between 0.1 and 114 keV. ENA images obtained from vantage points in the polar cap and in the afternoon magnetic local time (MLT) hours looking into the antisunward hemisphere show intense ENA fluxes (∼ 104 (cm2 sr s)−1 over 26–37 keV) coming from the dusk region and low altitudes (∼300 km). The morphology shows no relation to local magnetic field excluding the possibility of charged particle detection. It is concluded that the source of these ENAs are precipitating/mirroring ions from the ring current/trapped radiation interacting with the exobase on auroral L‐shells and in the dusk region. The observed ENA fluxes show a relation with Kp and Dst geomagnetic indices. The observed ENA spectrum from a geomagnetic storm on February 8, 1995, is investigated in more detail and compared to the parent ion spectrum obtained by the Defense Meteorological Satellite Project (DMSP) satellite, F12, during the same period on L=6±2 around dusk. The observed ENA spectral slope is used to derive the parent ion spectral temperature. The derived ion temperatures range is 3.0–6.0 keV for H and 4.5–8.5 keV for O. The higher of these ion temperatures comes closest in agreement to the extrapolated DMSP spectrum leading us to favor O over H as the species of the detected ENAs. It is shown that the detected ENAs must have been produced at L ≥6 to reach the detector without atmospheric attenuation and that the main energy dependence of the ENA spectrum, apart from the parent ion spectrum, is governed by the energy dependence of the charge exchange cross section between ions and exospheric oxygen.
Characteristics of broadband ULF magnetic pulsations at conjugate cusp latitude stations
Authors: Posch J. L. , Engebretson M. J. , Weatherwax A. T. , Detrick D. L. , Hughes W. J. , Maclennan C. G.
Published: September 2002

Although cusp latitude pulsation studies have for the most part focused on narrowband waves, analysis of magnetometer data from the Arctic has shown that the most common type of dayside long‐period ULF wave activity at very high latitudes is broadband noise (Pi1‐2), and that its occurrence and intensity is largely controlled by solar wind velocity [Engebretson et al., 1995]. However, the origin of temporal variations in the intensity of these waves is not understood. In order to further investigate these broadband waves and their origins, we present a similar data set from another season, data from a roughly conjugate site, and multi‐instrument data. Comparison of conjugate station data revealed that there was a substantial fraction of days during which there was significant temporal disagreement between hemispheres, but the solar wind velocity still appears to control overall daily intensity in broadband power. The coincidence of increased riometer absorption from conjugate locations with strong broadband ULF wave power suggests that precipitating energetic particles are responsible for much of the broadband ULF noise, and further suggests that high solar wind velocity plays a role in precipitation of significant fluxes of energetic particles. Quantitative estimates based on riometer and photometer observations also indicate that modulated electron precipitation is sufficient to drive the broadband pulsations. We review possible source mechanisms for these broadband waves and the precipitating electrons associated with them. Finally, the clear temporal association between these waves and Pc5 waves on closed field lines may suggest a causal connection via modulations of a three‐dimensional current system.
A generation mechanism for the polar cap boundary layer broadband plasma waves
Authors: Lakhina G. S. , Tsurutani B. T.
Published: September 2002

Recent Polar observations indicate the presence of broadband plasma waves in the frequency range of ∼ 101 Hz to about 104 Hz on magnetic field lines mapping into the polar cap boundary layer, at altitudes of about 6 to 8 RE (where RE is the Earth radius). These waves are quite similar to the broadband plasma waves observed in the low latitude boundary layer, and they appear to be a mixture of electromagnetic and electrostatic modes. A linear theory for the generation of these waves is developed. The theory is fully electromagnetic and takes into account the free energy available due to the presence of field‐aligned currents, and gradients in the currents, plasma densities, and magnetic fields. A generalized dispersion relation for the coupled lower hybrid, whistler, and current convective modes is obtained. It is found that the presence of density gradients, the current convective modes develop a finite real frequency, but at the same time their growth rates are reduced. On the other hand, sharp density gradients can lead to the excitation of a lower hybrid drift instability when the hot ions are present in the boundary layer. In general, the current convective and lower hybrid drift modes are coupled, and the dispersion relation has to be solved numerically. The inclusion of electromagnetic effects leads to the reduction of the growth rates.
Potential wells, the cyclotron resonance, and ion heating in coronal holes
Authors: Hollweg Joseph V.
Published: September 2002

We consider the motions of protons and O5+ ions in coronal holes. We first consider the effects of a potential well, which arises from the combination of gravity, the electrostatic electric field, and the mirror force. We show that if the potential well is time dependent, then ions which are initially trapped will undergo a time‐averaged energy gain. They can eventually gain enough energy to escape out of the potential well and be ejected out of the corona. The process is analogous to Fermi acceleration of cosmic rays by reflections off of moving magnetic clouds, except here the trapped ions can be regarded as reflecting off of moving walls. There is evidence that the trajectories of the particles are chaotic. However, the timescales are long, the potential wells are not very deep, and the process is probably not important for coronal heating. We also point out that the potential wells can provide a population of particles which are moving inward relative to waves which are propagating outward from the Sun. These particles are the ones which can interact most strongly with ion cyclotron waves, since they resonate with the lowest frequency waves which have the highest phase speeds and presumably the most power. We present some simple arguments, invoking energy‐conserving pitch angle scattering in the wave frame, which show how O5+ ions can in principle acquire perpendicular temperatures which are more than mass‐proportionally hotter than the protons. The basic principles are demonstrated by calculating trajectories for average particles interacting with dispersive ion cyclotron waves. We also present a strongly driven case which gives perpendicular energies and parallel flow speeds qualitatively resembling those believed to exist in coronal holes, but there are significant differences between the model results and the SOHO/UVCS data. In this case the particles are not trapped in a potential well.
Springtime transition in lower thermospheric atomic oxygen
Authors: Shepherd G. G. , Stegman J. , Espy P. , McLandress C. , Thuillier G. , Wiens R. H.
Published: September 2002

Observations from three optical ground stations and the wind imaging interferometer on the upper atmosphere research satellite have been combined to describe a “springtime transition” in atomic oxygen. At each station the transition is characterized by a rapid 2‐day rise in the night‐time oxygen airglow emission rate by a factor of between 2 and 3, with a subsequent decrease by a factor of about 10 in the same period of time. This latter state of extremely weak oxygen airglow indicates a depletion of atomic oxygen that persists for many days. The characteristic signature is similar at both mid‐latitude and high‐latitude stations and is also observed in the hydroxyl airglow, except that immediately following the enhancement, the hydroxyl emission rate does not fall below the value it had before the event. Airglow rotational temperatures behave coherently with the emission rate. WINDII data show that the airglow emission rate perturbation is a planetary scale feature associated with strong vertical motions and that the event may be associated with the winter‐to‐summer zonal wind reversal. Data from the northern springtimes of 1992 and 1993 are reported upon in detail, with additional data from 1995 to confirm the persistence of the phenomenon.
Lithospheric‐scale structure across the Bolivian Andes from tomographic images of velocity and attenuation for P and S waves
Authors: Myers Stephen C. , Beck Susan , Zandt George , Wallace Terry
Published: September 2002

We have developed a three‐dimensional, lithospheric‐scale model across the Bolivian Andes at ∼20°S, based on tomographic images of velocity and attenuation for both P and S waves. Observations of travel time and attenuation for this study are from regional, mantle earthquakes in the subducted Nazca plate recorded on a portable, broadband seismic array (Broadband Andean Joint Experiment and Seismic Exploration of the Deep Andes) in Bolivia and Chile. The shallow mantle under the Altiplano from ∼18°S to ∼21°S is high‐velocity and moderately high Q (Vp ≈ 8.3,Vs ≈ 4.7, Qp ≈ 500, and Qs ≈ 200), suggesting lithospheric mantle. High‐velocity material in the Altiplano extends to a depth of ∼125–150 km. The shallow mantle of the Western Cordillera is characterized by high Vp/Vs (∼1.83), suggesting a correlation between Vp/Vs and arc volcanism. Seismic velocity in the Western Cordillera mantle is, on average, only slightly reduced from global averages; however, velocity and attenuation anomalies are locally strong ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 200, and Qs ≈ 100), consistent with partial melt conditions. Under the Los Frailes volcanic field, in the Eastern Cordillera, shallow mantle velocity and Q decrease drastically from the neighboring Altiplano ( Vp ≈ 7.8, Vs ≈ 4.3, Qp ≈ 300, Qs ≈ 100); however, high Vp/Vs is not as pervasive as it is in the Western Cordillera. We believe that slab‐derived water, and perhaps other volatiles, strongly influence the Western Cordillera, while the Eastern Cordillera low‐velocity region is more affected by partial melt and/or compositional changes. Average velocity and Q in the shallow mantle across the Bolivian Andes, where the tomographic images are best resolved, are significantly higher than in most mantle wedge environments where corresponding images are available. This is likely the result of a compressional “back arc” setting in the Andes. This implies that lithospheric shortening and thickening associated with the formation of the Andes has profoundly influenced the shallow mantle structure across the range. Shallow mantle structure is locally influenced by the subduction processes, particularly under the Western Cordillera; however, the differing volcanism and seismic character under the two Cordilleras suggest that the volcanic process in the Eastern Cordillera may be distinct from arc volcanism. Tertiary volcanism in the Eastern Cordillera is located in the region where mantle shortening is suspected to be greatest. Both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere.
Mantle stratigraphy and evolution of the Slave province
Authors: Bostock M. G.
Published: September 2002

A data set of 1033 three‐component, P wave seismograms from five broadband stations at the Yellowknife Array is assembled to investigate mantle structure below the southern Slave province in Canada's Northwest Territories. Following wave field decomposition, seismograms are source‐normalized through simultaneous deconvolution to estimate the near‐receiver impulse response as a function of epicentral distance and back azimuth. Images of impulse response reveal a well‐developed mantle stratigraphy, anisotropic in part, extending from the Mohorovicic discontinuity to the transition zone, A layer of depth‐localized anisotropy (±5%), termed H, is situated between ∼70 and 80 km depth with an average shear velocity comparable to that of the ambient mantle and a sharp upper boundary less than 100 m in transition width. The absence of free surface crustal reverberations on the transverse component affords a window into the upper mantle between 100 and 200 km depth. A sequence of at least two layers between 120–150 km depth, collectively termed X, is most clearly evident to the north and is underlain by a second structure L which dips from 170 km in the west to 230 km into the center of the Slave province. The deepest interface above the transition zone W marks a shear velocity inversion near 350 km depth whose signature is restricted to the SV component signalling a dominantly isotropic response. Consideration of these observations in light of data acquired in a recent LITHOPROBE seismic reflection traverse and in petrological studies of kimberlite xenoliths prompts speculation into the role of subduction in craton stabilization. It is suggested that the proto‐Slave craton was assembled through processes of shallow subduction resulting in a near‐horizontal mantle stratigraphy (i.e., H, X) both compositional and rheological in nature. Interpretation of L as the continuation of dipping reflectors on the seismic reflection profile argues for a final phase of craton assembly involving oblique underplating of subducted lithosphere in the Proterozoic. Subsequent modification of the lithosphere, as manifest by Phanerozoic kimberlite volcanism, may be related to W if an interpretation as the top of a layer containing a dense silicate melt fraction is invoked.
High‐pressure transformations in MgAl2O4
Authors: Funamori Nobumasa , Jeanloz Raymond , Nguyen Jeffrey H. , Kavner Abby , Caldwell Wendel A. , Fujino Kiyoshi , Miyajima Nobuyoshi , Shinmei Toru , Tomioka Naotaka
Published: September 2002

X ray diffraction and transmission electron microscopy on laser‐heated diamond cell samples show that with increasing pressure MgAl2O4 spinel transforms first to Al2O3 corundum + MgO periclase, then to the CaFe2O4‐structured phase, and finally to a new phase having the CaTi2O4 structure above ∼40 GPa. The CaFe2O4 and the CaTi2O4 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe2O4 to the CaTi2O4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle.
A numerical model for temporal variations during explosive central vent eruptions
Authors: Folch A. , Martí J. , Codina R. , Vázquez M.
Published: September 2002

An axisymmetrical numerical model has been developed in order to find the temporal evolution of pressure, the position of the exsolution level, the velocity field, the eruption rate, and the amount of erupted material of a shallow, volatile‐rich, felsic magma chamber during a Plinian central vent eruption. The overpressure necessary to trigger the eruption is assumed to result from crystallization‐driven volatile oversaturation. We solve the resulting set of equations using a finite element method. The results obtained show that the pressure at the conduit entrance decreases exponentially as the eruption proceeds. This produces a shifting of the exsolution level, so that deeper parts of the chamber become progressively volatile oversaturated during the eruption. We assess the influence of chamber geometry and the physical properties of the magma on the computed parameters using several numerical examples. The results are also compared with those predicted by previous models from the literature and are found to be in good agreement with documented eruptions. The model constitutes a first attempt to numerically model the dynamics and the temporal evolution of the most relevant physical parameters during withdrawal from a closed magma chamber.
Seismotectonics of the Loma Prieta, California, region determined from three‐dimensional Vp, Vp/ Vs, and seismicity
Authors: Eberhart‐Phillips Donna , Michael Andrew J.
Published: September 2002

Three‐dimensional Vp and Vp/Vs velocity models for the Loma Prieta region were developed from the inversion of local travel time data (21,925 P arrivals and 1,116 S arrivals) from earthquakes, refraction shots, and blasts recorded on 1700 stations from the Northern California Seismic Network and numerous portable seismograph deployments. The velocity and density models and microearthquake hypocenters reveal a complex structure that includes a San Andreas fault extending to the base of the seismogenic layer. A body with high Vp extends the length of the rupture and fills the 5 km wide volume between the Loma Prieta mainshock rupture and the San Andreas and Sargent faults. We suggest that this body controls both the pattern of background seismicity on the San Andreas and Sargent faults and the extent of rupture during the mainshock, thus explaining how the background seismicity outlined the along‐strike and depth extent of the mainshock rupture on a different fault plane 5 km away. New aftershock focal mechanisms, based on three‐dimensional ray tracing through the velocity model, support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression. The subvertical (or steeply dipping) San Andreas fault and the fault surfaces that ruptured in the 1989 Loma Prieta earthquake are both parts of the San Andreas fault zone and this section of the fault zone does not have a single type of characteristic event.
Subglacial electrical phenomena
Authors: Blake Erik W. , Clarke Garry K. C.
Published: September 2002

The subglacial electrical environment is surprisingly complex owing to the variety of processes that promote spatial and temporal variability. Possible sources of variability are the chemical evolution of subglacial water and the structural and morphological evolution of the drainage configuration. Such changes contribute to changes in the apparent resistivity of the glacier bed. Less obvious, but possibly more interesting, are those electrical phenomena associated with the natural transport of water and charge in the subglacial drainage system. Streaming potentials are generated by water flow through subglacial sediment, and these potentials are closely associated with the subglacial water pressure field. To explore these effects, we have monitored induced and natural electrical responses in the deforming material beneath Trapridge Glacier, Yukon, Canada. Several arrays of electrodes were placed at the ice‐bed interface, and data were recorded over a period of several years. Data from these experiments indicate that natural potentials can be used to monitor water pressure gradients and that the bulk resistivity of the glacier bed can be altered by changing the hydraulic regime. We present evidence that temporal variations in streaming potentials can indicate fluctuations in subglacial water flow rate and that spatial variations can be related to variations in bed elevation, water pressure, and hydraulic connectivity.
Coseismic slip resolution along a plate boundary megathrust: The Nankai Trough, southwest Japan
Authors: Sagiya Takeshi , Thatcher Wayne
Published: September 2002

Geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 Tonankai (Mw=8.1) and 1946 Nankaido (Mw=8.3) earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust. Data used include 798 angle changes from triangulation surveys, 328 leveling section differences, and 5 coseismic tidal gage offsets. Many of the nominally coseismic triangulation data span ∼50 years, nearly half the earthquake cycle, and correction for interseismic deformation using post‐1950 observations is applied. Microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous, multisegment fault model. Because the onshore geodetic data have very limited resolving power for offshore fault segments, offshore coseismic slip was constrained by Satake's [1993] estimation based on tsunami data. The majority of the coseismic slip occurs between 15 and 25 km depth. Although resolution declines toward the trench axis, it is sufficiently good to define two distinct high‐slip regions, one off southeastern Shikoku Island (11 m maximum) and the other offshore of Kii Peninsula (3 m maximum). The slip magnitude off southeastern Shikoku, coupled with the plate convergence rate, would imply an recurrence interval of about 270 years, much longer than the average repeat time of ∼120 years for historical great earthquakes on the Nankai Trough. However, the maximum coseismic slip is sensitive to the assumed fault geometry, and slippage on trough‐parallel splay faults could significantly decrease the maximum slip to about 6 m.
New Early Devonian paleomagnetic data from NW France: Paleogeography and implications for the Armorican microplate hypothesis
Authors: Tait J.
Published: September 2002

To constrain the Early Devonian paleogeography of the Armorican Massif, and determine the tectonic relationships between the Armorican and Bohemian Massifs (the Armorican microplate hypothesis), a paleomagnetic study of Lochkovian/Pragian sediments from the Pointe de l'Armorique, Brittany, NW France has been carried out. After removal of a present‐day overprint, a higher unblocking temperature component of magnetization is identified (the B direction) which yields an overall mean direction of 222°/+35°, k = 121.6, α95=4.2° after bedding correction, 11 sites. No fold test is possible, but the presence of a thin sill allowed sampling for a contact test, the results of which are positive. Thus the B direction of magnetization is considered to be primary in origin and closely bracketed with the rock age. The resulting paleopole (315°E, 13°S) corresponds to a paleolatitude of 19°S for the Armorican Massif, placing it adjacent to the southern margin of Baltica and Avalonia. This clearly demonstrates for the first time that this sector of the Rheic Ocean had closed by Early Devonian times, and that the Armorican Massif was in its present‐day orientation with respect to Baltica. These results are significantly different to existing paleomagnetic data from Bohemia which indicate large scale (˜140°) rotation between the earliest and latest Devonian, and thus provide important information for the pre‐ and syncollisional tectonic relationships of these Massifs. As it is unlikely that such large scale differential crustal rotations could have occurred in an intraplate setting, it is proposed that the Armorican and Bohemian Massifs did not form a single coherent microplate, but belonged to a fairly loose mosaic of terranes prior to collision with Avalonia. Thus, the concept of an Armorican microplate is no longer tenable, and the term Armorican terrane assemblage (ATA) is proposed in reference to the Gondwana derived Paleozoic terranes now situated within the European Variscan fold belt south of the Rhenohercynian Zone (Avalonia).