bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of ion injections at the dawn and dusk polar edges of the auroral oval
Authors: Stenuit H. , Sauvaud J.‐A. , Delcourt D. C. , Mukai T. , Kokubun S. , Fujimoto M. , Buzulukova N. Y. , Kovrazhkin R. A. , Lin R. P. , Lepping R. P.
Published: February 2004

In the auroral dawn and dusk magnetosphere at altitudes of ∼2–3 RE, three distinct zones of ion and electron precipitation are commonly detected onboard Interball 2, near the polar edge of the auroral oval. From high to low latitudes the satellite encounters (1) magnetosheath/low‐latitude boundary layer (LLBL) like plasma (Zone 1), (2) a mixing region with plasma characteristics between LLBL and plasma sheet (Zone 2), and (3) the auroral plasma sheet precipitation (Zone 3). Further equatorward, the satellite crosses the inner plasma sheet characterized by “ion gaps” in the morning sector. Inside Zones 1 and 2 impulsive ion injections are often detected. They consist of overlapping energy dispersed structures from about 10 keV down to several hundreds of eV with temperature close to that of the magnetosheath. Using trajectory computations backward in time, these dispersions are shown to be cause by time‐of‐flight effect from a distant source located close to the equatorial magnetopause. Whereas Zone 1 is located mainly poleward of region 1 (downward field‐aligned currents at dawn and upward field‐aligned currents at dusk), Zone 2 generally coincides with region 1. It is mainly located on closed field lines, as evidenced from the local detection of bouncing ion clusters. Finally, Zone 3 corresponds with region 2 of upward field‐aligned currents. A statistical study of Zone 1 which is present in about 12% of the satellite passes at dawn and dusk reveals that its probability of occurrence seems to be controlled by the interplanetary magnetic field (IMF). That is, it is formed when the IMF has a northward component and tends to be radially directed. Moreover, it is more frequently encountered during periods of enhanced solar wind pressure. Detailed case studies uncover a remarkable correlation between the onsets of individual injections inside Zone 1 and those of pressure pulses in the magnetosheath. Both have a characteristic period of ∼ 200–250 s, similar to that of Pc5 events associated with these injections and detected on board Interball 2. Altogether, these observations indicate that injections and related Alfvèn waves are driven by magnetosheath pressure pulses associated with a quasi‐parallel bow shock. The plasma penetration mechanism remains to be understood.
Crustal thickness of the Peninsular Ranges and Gulf Extensional Province in the Californias
Authors: Lewis Jennifer L. , Day Steven M. , Magistrale Harold , Castro Raul R. , Astiz Luciana , Rebollar Cecilio , Eakins Jennifer , Vernon Frank L. , Brune James N.
Published: February 2004

We estimate crustal thickness along an east‐west transect of the Baja California peninsula and Gulf of California, México, and investigate its relationship to surface elevation and crustal extension. We derive Moho depth estimates from P‐to‐S converted phases identified on teleseismic recordings at 11 temporary broadband seismic stations deployed at ∼31°N latitude. Depth to the Moho is ∼33 (±3) km near the Pacific coast of Baja California and increases gradually toward the east, reaching a maximum depth of ∼40 (±4) km beneath the western part of the Peninsular Ranges batholith. The crust then thins rapidly under the topographically high eastern Peninsular Ranges and across the Main Gulf Escarpment. Crustal thickness is ∼15–18 (±2) km within and on the margins of the Gulf of California. The Moho shallowing beneath the eastern Peninsular Ranges represents an average apparent westward dip of ∼25°. This range of Moho depths within the Peninsula Ranges, as well as the sharp ∼east‐west gradient in depth in the eastern part of the range, is in agreement with earlier observations from north of the international border. The Moho depth variations do not correlate with topography of the eastern batholith. These findings suggest that a steeply dipping Moho is a regional feature beneath the eastern Peninsular Ranges and that a local Airy crustal root does not support the highest elevations. We suggest that Moho shallowing under the eastern Peninsular Ranges reflects extensional deformation of the lower crust in response to adjacent rifting of the Gulf Extensional Province that commenced in the late Cenozoic. Support of the eastern Peninsular Ranges topography may be achieved through a combination of flexural support and lateral density variations in the crust and/or upper mantle.
Crustal and upper mantle structure beneath Antarctica and surrounding oceans
Authors: Ritzwoller Michael H. , Shapiro Nikolai M. , Levshin Anatoli L. , Leahy Garrett M.
Published: February 2004

We present and discuss a new model of the crust and upper mantle at high southern latitudes that is produced from a large, new data set of fundamental mode surface wave dispersion measurements. The inversion for a 2°×2° shear velocity model breaks into two principal steps: first, surface wave tomography in which dispersion maps are produced for a discrete set of periods for each wave type (Rayleigh group velocity, 18–175 s; Love group velocity, 20–150 s; Rayleigh and Love phase velocity, 40–150 s) and, second, inversion for a shear velocity model. In the first step, we estimate average resolution at high southern latitudes to be about 600 km for Rayleigh waves and 700 km for Love waves. The second step is a multistage process that culminates in a Monte Carlo inversion yielding an ensemble of acceptable models at each spatial node. The middle of the ensemble (median model) together with the half width of the corridor defined by the ensemble summarize the results of the inversion. The median model fits the dispersion maps at about the measurement error (group velocities, 20–25 m/s; phase velocities, 10–15 m/s) and the dispersion data themselves at about twice the measurement error. We refer to the features that appear in every member of the ensemble as “persistent.” Some of persistent features are the following: (1) Crustal thickness averages ∼27 km in West Antarctica and ∼40 km in East Antarctica, with maximum thicknesses approaching 45 km. (2) Although the East Antarctic craton displays variations in both maximum velocity and thickness, it appears to be a more or less average craton. (3) The upper mantle beneath much of West Antarctica is slow and beneath the West Antarctic Rift is nearly indistinguishable from currently dormant extensional regions such as the western Mediterranean and the Sea of Japan. Our model is therefore consistent with evidence of active volcanism underlying the West Antarctic ice sheet, and we hypothesize that the West Antarctic Rift is the remnant of events of lithospheric rejuvenation in the recent past that are now quiescent. (4) The Australian‐Antarctic Discordance is characterized by a moderately high velocity lid to a depth of 70–80 km with low velocities wrapping around the discordance to the south. There is a weak trend of relatively high velocities dipping to the west at greater depths that requires further concentrated efforts to resolve. (5) The strength of radial anisotropy (vsh − vsv)/vsv in the uppermost mantle across the Southern Hemisphere averages ∼4%, similar to the Preliminary Reference Earth Model. Radial anisotropy appears to be slightly stronger in West Antarctica than in East Antarctica and in the thinner rather than the thicker regions of the East Antarctic craton.
Dynamic modeling of the 1992 Landers earthquake
Authors: Peyrat Sophie , Olsen Kim , Madariaga Raúl
Published: February 2004

We have used observed band‐pass filtered accelerograms and a previously determined slip distribution to invert for the dynamic rupture propagation of the 1992 Landers earthquake. In our simulations, dynamic rupture grows under the simultaneous control of initial stress and rupture resistance by friction, which we modeled using a simple slip‐weakening law. We used a simplified Landers fault model where the fault segments were combined into a single vertical, planar fault. By trial and error we modified an initial stress field, inferred from the kinematic slip distribution proposed byWald and Heaton[1994], until dynamic rupture generated a rupture history and final slip distribution that approximately matched those determined by the kinematic inversion. We found that rupture propagation was extremely sensitive to small changes in the distribution of prestress and that a delicate balance with energy release rate controls the average rupture speed. For the inversion we generated synthetic 0.5 Hz ground displacements using an efficient Green's function propagator method (AXITRA). This method enables us to propagate the radiation generated by the dynamic rupture to distances greater than those feasible using the finite difference method. The dynamic model built by trial‐and‐error inversion provides a very satisfactory fit between synthetics and strong motion data. We validated this model using records from stations used in the slip inversion as well as some which were not included. We also inverted for a complementary model that fits the data just as well but in which the initial stress was perfectly uniform while rupture resistance was heterogeneous. This demonstrates that inversion of ground motion is nonunique.
Multiparameter two‐dimensional inversion of scattered teleseismic body waves 1. Theory for oblique incidence
Authors: Bostock M. G. , Rondenay S. , Shragge J.
Published: February 2004

This is the first paper in a three‐part series that examines formal inversion of the teleseismic P wave coda for discontinuous variations in elastic properties beneath dense, three‐component, seismic arrays. In this paper, we develop the theoretical framework for a migration method that draws upon the tenets of inverse scattering theory and is amenable to practical implementation. The forward problem is formulated for two‐dimensional (2‐D) heterogeneity in observance of formal sampling requirements and currently accessible instrumentation. A ray theoretic Green's function, corresponding to a line source with axial component of forcing, is employed within the 2‐D Born approximation to accommodate planar, incident wave fields at arbitrary back azimuths. Both the forward scattered response generated by the upgoing incident wave field and the backscattered response created by its reflection at the free surface are included within the formulation. In accordance with the high‐frequency and single‐scattering approximations employed in the forward problem the inverse problem is cast as a generalized Radon transform. The resulting back projection operator is well suited to the teleseismic context in several respects. It is tolerant of irregularities in array geometry and source distribution and allows a full complement of global seismicity to be utilized through its accommodation of oblique incidence. By permitting both independent and simultaneous treatment of different scattering modes (reflections, transmissions, conversions) the inversion formula facilitates a direct appraisal of individual mode contributions to the recovery of structure. In particular, it becomes evident that incorporation of backscattered modes leads to (1) a better localization of structure than possible using forward scattered energy and (2) the imposition of complementary constraints on elastic properties.
Kerguelen Plateau crustal structure and basin formation from seismic and gravity data
Authors: Gladczenko Tadeusz P. , Coffin Millard F.
Published: February 2004

We use multichannel seismic data, gravity, and subsidence modeling, in conjunction with plate reconstructions, to evaluate the crustal origin and composition of the Kerguelen Plateau. Predominantly oceanic crust of the southern and parts of the central Kerguelen Plateau appears to include continental fragments related to the breakup of India and Antarctica; these fragments may have been metamorphosed during emplacement of the main plateau. The upper crust is basaltic, the middle crust is intrusive mafic rock and intruded continental crust, and the lower crust is a plagioclase‐rich metamorphic rock. The Labuan Basin crust is predominantly oceanic with stranded Kerguelen Plateau fault blocks. High‐density lower crust in the Labuan Basin is probably composed of serpentinized peridotites formed during slow rifting and spreading. Plate reconstruction models indicate opening between eastern Broken Ridge and southern Kerguelen Plateau at ∼90 Ma, heralding the formation of the Labuan Basin and Diamantina Zone; crustal attenuation and slow accretion of oceanic crust continued until the Australian and Antarctic plates separated at CI8 time (∼40 Ma). Plate reconstructions of the free‐air gravity field indicate that the Naturaliste Plateau fits against Antarctica and that Elan Bank and India were juxtaposed until ∼110 Ma. Both Naturaliste Plateau and Elan Bank are probable microcontinents. A ∼1 km positive residual depth anomaly in the oceanic basins adjacent to the plateau, along with the positive geoid anomaly centered beneath the northern Kerguelen Plateau, imply that the lithosphere is partially dynamically supported by an upwelling hot asthenosphere of the Kerguelen hot spot.
Three‐dimensional shallow crustal emplacement at the 9°03′N overlapping spreading center on the East Pacific Rise: Correlations between magnetization and tomographic images
Authors: Bazin S. , Harding A. J. , Kent G. M. , Orcutt J. A. , Tong C. H. , Pye J. W. , Singh S. C. , Barton P. J. , Sinha M. C. , White R. S. , Hobbs R. W. , Van Avendonk H. J. A.
Published: February 2004

We report a three‐dimensional (3‐D) seismic reflection and tomographic survey conducted at the 9°03′N overlapping spreading center (OSC) on the East Pacific Rise to understand crustal accretion at this feature. Inversions of travel time data from 19 ocean bottom hydrophones provide a 3‐D image of the shallow velocity structure beneath the nontransform offset and associated discordance zone. Seismic analysis indicates that layer 2A thickness varies between 100 and 900 m and averages 430 m throughout the study area. The heterogeneous upper crustal structure at the OSC region contrasts with the simpler symmetric structure flanking the midsegments of the East Pacific Rise. The crust affected by the OSC migration carries evidence for the complex accretion at the axial discontinuity where the overlap basin may act as a lava pond. An area of thick layer 2A covers the southern half of the overlap basin and the propagating ridge tip and shows good correlation with a high magnetization region. Comparison of the magnetic field anomaly derived from the seismic structure model with the observed sea surface magnetic anomaly suggests that a significant portion of the high magnetization can be related to magnetic source thickness variation rather than solely to the geochemistry of the volcanic rocks.
Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation
Authors: Hatzfeld D. , Karagianni E. , Kassaras I. , Kiratzi A. , Louvari E. , Lyon‐Caen H. , Makropoulos K. , Papadimitriou P. , Bock G. , Priestley K.
Published: February 2004

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present‐day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present‐day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.
Effects of normal stress variations on frictional stability of a fluid‐infiltrated fault
Authors: Chambon Guillaume , Rudnicki John W.
Published: February 2004

This paper extends the spring‐slider model of Segall and Rice [1995] for dilating fluid‐infiltrated faults to include geometry‐induced normal stress variations. Because of the coupling between normal and shear stresses on a dipping plane the shear stress drop accompanying slip is associated with a decrease in normal stress for reverse faulting and with an increase for normal faulting. This coupling stabilizes frictional sliding on normal faults and destabilizes it on reverse faults. When a fluid phase is present, these normal stress variations during slip events also trigger, through poroelastic coupling, compensating pore pressure changes: stabilizing pressure decreases for reverse geometry and destabilizing increases for normal geometry. Fault stability is quantitatively studied using two parameters: the critical spring stiffness kcr separating the stable and unstable slip domains and, when numerical simulations exhibit stick‐slip cycles, the stress drop during the slip phases Δτ. As expected, geometric coupling is found to reduce kcr and Δτ for normal faults (in comparison with Segall and Rice's [1995] strike‐slip geometry) and to increase these parameters for reverse faults. Poroelastic coupling shows opposite consequences. The combined effect of shear‐induced dilatancy and poroelastic coupling is stabilizing for reverse geometry but can be either stabilizing or destabilizing for normal geometry, depending on the dilatancy magnitude. Nevertheless, the stability differences between the types of faults are primarily controlled by the geometric coupling between shear and normal stresses: For all the parameters we investigated, normal faults exhibit lower stress drops (and critical stiffnesses) than reverse faults. Finally, extrema appearing in the graphs featuring Δτ or kcr versus the diffusion rate demonstrate that the influence of the fluid effects is not maximum in the undrained case. Surprisingly, their influence is maximum at a point intermediate between the drained and undrained limits, when the diffusion time is roughly equal to the frictional relaxation time associated with stick phases. This phenomenon is presumably caused by subtle second‐order mechanisms involving the instantaneous pore pressure changes induced by the poroelastic coupling in response to instantaneous friction changes and the interseismic pressure recovery process.
Regional seismicity and on‐land deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough
Authors: Fournier Marc , Fabbri Olivier , Angelier Jacques , Cadet Jean‐Paul
Published: February 2004

The stress field evolution and the kinematics of opening of the Okinawa Trough are investigated on the basis of earthquake focal mechanisms and structural data in the Ryukyu arc and the Okinawa Trough. Focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions: a regional arc‐perpendicular extension and a local arc‐parallel extension. Both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal σ2 and σ3 axes. Earthquake slip vectors reveal a southward motion of the Ryukyu arc with respect to the south China block. The current pole of opening of the Okinawa Trough is located around 16°N and 50°E. Fracture analysis in Okinawa island allows identification of three episodes of extension: a late Miocene N40°W to N20°E extension (episode I), a late Pliocene to early Pleistocene N20°E extension (episode II), and a latest Pleistocene to present‐day N20°W extension (episode III). Episodes II and III are characterized by permutations between the two horizontal σ2 and σ3 axes. By synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the Okinawa Trough since the late Miocene. The direction of divergence of the Ryukyu arc has rotated clockwise from ∼N150°E in the late Miocene to nearly N‐S today.
Numerical experiments on the movement of a warm‐core ring with the bottom slope of a western boundary
Authors: Itoh S. , Sugimoto T.
Published: February 2004

Numerical experiments are performed to investigate the effects of the bottom slope of a western boundary on realistic warm‐core rings, using a primitive equation sigma coordinate model. Warm‐core rings which initially have baroclinic velocity near a steep bottom slope gradually move northward, but those with barotropic velocity do not. Barotropic velocity on the slope immediately disperses as a topographic Rossby wave, while baroclinic velocity is retained. Since the northward movement is caused by an effect very much like the image effect which is originally the effect of a free‐slip wall, the authors term this effect the equivalent image effect. The equivalent image effect increases as a slope becomes steeper and bottom friction becomes weaker, and vice versa. It seems that a steep slope acts like a wall, and the bottom friction of a steep slope corresponds to the lateral viscous effect of a wall. However, the effect of a slope on a warm‐core ring is different from that of a wall in that ring‐slope interactions take place at the lower part of the ring. Because the velocity of a warm‐core ring is weak in deep layers, ring‐slope interactions are moderate and the leakage of the interior fluid of a ring seen in ring‐wall interactions is not apparent, while the viscous effect is weaker than that of a no‐slip wall as well. These results are applied to the northward movement of Kuroshio warm‐core rings and to warm eddies in the Gulf of Mexico.
Dynamic measurements of the nonlinear elastic parameter α in rock under varying conditions
Authors: Johnson Paul A. , Zinszner Bernard , Rasolofosaon Patrick , Cohen‐Tenoudji Frederic , Van Den Abeele Koen
Published: February 2004

Since the exhaustive work by Adams and Coker at the Carnegie Institute in the early 1900s and the work of F. Birch's group at Harvard University conducted in the 1940s–1950s, it has been well documented that the quasi‐static stress‐strain behavior of rock is nonlinear and hysteretic. Over the past 20 years, there has been an increasing body of evidence suggesting that rocks are highly elastically nonlinear and hysteretic in their dynamic stress‐strain response as well, even at extremely small strain amplitudes that are typical of laboratory measurements. In this work we present a compendium of measurements of the nonlinear elastic parameter α extracted from longitudinal (Young's mode) and flexural‐mode resonance experiments in eight different rock types under a variety of saturation and thermal conditions. The nonlinear modulus α represents a measure of the dynamic hysteresis in the wave pressure‐strain behavior. We believe that hysteresis is the primary cause of nonclassical nonlinear dynamics in rock, just as it is responsible for elastic nonlinear behavior in quasi‐static observations. In dynamics, α is proportional to the wave speed and modulus reduction as a function of wave strain amplitude due to the hysteresis, based on our current model. The rocks tested include pure quartz sandstone (Fontainebleau), two sandstones that contain clay and other secondary mineralization (Berea and Meule), marble (Asian White), chalk, and three limestones (St. Pantaleon, Estaillades, and Lavoux). The values of α range from ∼500 to >100,000, depending on the rock type, damage, and/or water saturation state. Damaged samples exhibit significantly larger α than intact samples (hysteresis increases with damage quantity), and water saturation has an enormous influence on α from 0 to 15–30% water saturation.
Small‐scale variability and model error in tropical Pacific sea level
Authors: Kaplan A. , Cane M. A. , Chen D. , Witter D. L. , Cheney R. E.
Published: February 2004

Monthly interannual anomalies of tropical Pacific sea level height from TOPEX/Poseidon altimetry are compared with simulation and assimilation products from a variety of models, ranging from a simple linear long wave approximation to ocean general circulation models. Major spatial similarities in the error patterns are identified. These include zonally elongated maxima in the northwest and southwest tropical Pacific Ocean, a band of high values near 10°N, slightly inclined toward the equator from the Central American coast, and low values on the equator and in the southeastern tropical Pacific. These features are also present in the pattern of small‐scale variability (SSV) of sea level height. Spatial and temporal components of this SSV are analyzed for predominant variability types. Monte Carlo experiments identify the areas where high SSV is wind‐driven, caused by a similar pattern of variability in the wind stress. Model products systematically underestimate signal variance in such areas. Variability in other areas is due to the instability of ocean currents. The major component of uncertainty in the gridded satellite altimeter analyses is due to sampling error, for which estimates are developed and verified.
Lidar observations of polar mesospheric clouds at Rothera, Antarctica (67.5°S, 68.0°W)
Authors: Chu Xinzhao , Nott Graeme J. , Espy Patrick J. , Gardner Chester S. , Diettrich Jan C. , Clilverd Mark A. , Jarvis Martin J.
Published: January 2004

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at Rothera (67.5°S, 68.0°W), Antarctica in the austral summer of 2002–2003. The Rothera PMC are much weaker, less frequent, and not as high as the PMC observed at the South Pole. The mean PMC altitude is 83.74 ± 0.25 km, which is approximately 1.3 km lower than the South Pole clouds. A comparison of numerous cloud observations indicates that southern hemisphere PMC are about 1 km higher than northern clouds at similar latitudes. Lidar measurements also show that the mesopause region temperatures at Rothera in late January are warmer than at the South Pole, while the Fe layer at Rothera has higher density and a lower peak altitude compared to the summertime Fe layer at the South Pole. These Fe density and temperature observations are qualitatively consistent with the PMC observations.
Back front of seismicity induced after termination of borehole fluid injection
Authors: Parotidis M. , Shapiro S. A. , Rothert E.
Published: January 2004

We study a phenomenon occurring by hydraulic fracturing stimulations, where events are triggered during injection and also after the end of injection, sometimes over hours or even days. We assume that pore‐pressure diffusion is the main triggering mechanism. Based on the theory of linear poroelasticity, an equation is derived, that describes the distance from injection point at which seismicity is terminated at a given time after the end of fluid injection. We call this distance the back front of induced seismicity. Using numerical modeling and data from three case studies we show that the back front is observed in reality. The existence of the back front is an important phenomenon supporting the idea of the pore‐pressure diffusion nature of fluid injection triggered seismicity. Moreover, it provides an additional means of rock characterization allowing the estimation of the scalar hydraulic diffusivity of the seismically active area.
Iron charge state distributions as an indicator of hot ICMEs: Possible sources and temporal and spatial variations during solar maximum
Authors: Lepri S. T. , Zurbuchen T. H.
Published: January 2004

It has been shown that Fe charge states greater than and equal to 16 are often observed in the presence of interplanetary coronal mass ejections (ICMEs) at 1 AU. These elevated charge states, representative of hot source regions on the Sun, provide an identifier of ICMEs independent of expansion processes and heliocentric distance. Using this new identifier, we present a comparative study of Fe charge state distributions as a function of latitude and time. We compare Fe charge state data from the ACE spacecraft in the ecliptic plane with that from the Ulysses spacecraft along its orbit from low to high latitudes. We discuss the frequencies of high Fe charge state distributions through a range of latitudes on the approach toward solar maximum. Close to solar maximum, we find almost an order of magnitude fewer hot ICMEs at higher latitudes than in the ecliptic. We offer an explanation for the existence of high Fe charge states in hot ICMEs as a result of magnetic connectivity to flaring regions.
Greenline dayglow emission under equinox conditions
Authors: Singh Vir , Upadhayaya Arun Kumar
Published: January 2004

The present paper deals with the morphological (volume emission rate as a function of altitude and latitude) study of greenline dayglow emission under equinox conditions. The morphology is obtained from the emission profiles of greenline dayglow emission using the/glow model. The/glow model is updated by incorporating newly discovered temperature dependent rate coefficient of the reaction N2 (A3Σu+) + O. The recently proposed correction for the atomic oxygen density in MSIS 90 neutral atmosphere model is also included at mesospheric altitudes to achieve the consistency with the WINDII measurements in this altitude region. The modeled morphology of greenline dayglow emission is presented between 50°S and 50°N latitudes for the months of March and April. It has been found that this emission shows asymmetry between the northern and the southern hemispheres. The uneven transport of atomic oxygen due to dynamical processes in both the hemispheres plays an important role in producing the asymmetry in the mesospheric region. The asymmetry in the thermospheric region is likely due to the changing contributions with altitude of the various sources which are responsible for the production of O(1S) during the daytime.
Further evidence of wave refraction associated with extended rarefaction events in the solar wind
Authors: Smith Charles W. , Mullan Dermott J. , Ness Norman F.
Published: January 2004

We examine the turbulence and low‐frequency wave properties of three extended rarefaction intervals observed by the Advanced Composition Explorer (ACE) spacecraft in 1999 and 2002 while in orbit at L1. The proton densities drop to values lower than 1 cm−3 and in one case reach 0.1 cm−3. During each interval, the amplitudes of magnetic fluctuations fall to unusually low levels. Moreover, the fluctuations that are present are more Alfvénic in nature than in the typical low‐latitude wind. The constant and near average interplanetary magnetic field during these times results in elevated Alfvén speeds. We suggest that the relative isolation of the rarefaction intervals from sources of turbulence‐generating transients allows wave refraction to become a significant dynamic controlling the evolution of the magnetized plasma. We present analyses of the intervals that support the hypothesis that wave refraction explains the reduced fluctuation levels and enhanced Alfvénicity during these times.
Aerosol optical properties over east Asia determined from ground‐based sky radiation measurements
Authors: Kim Do‐Hyeong , Sohn Byung‐Ju , Nakajima Teruyuki , Takamura Tamio , Takemura Toshihiko , Choi Byoung‐Cheol , Yoon Soon‐Chang
Published: January 2004

Aerosol optical properties (aerosol optical thickness, Ångström exponent, size distribution, and single scattering albedo) over east Asia were examined using long‐term measurements of sky radiation at Mandalgovi, Dunhuang, Yinchuan, and Sri‐Samrong sites of the Skyradiometer Network (SKYNET). Also included were sky radiation measurements at Anmyon, Gosan in Korea, and Amami‐Oshima in Japan during April for examining optical properties of Asian dust. Results show that the seasonal average of aerosol optical thickness (AOT) generally exhibits a maximum in spring and a minimum in autumn over east Asia. At Sri‐Samrong and Yinchuan, relatively distinct seasonal cycles are noted, in comparison to the arid desert regions of Dunhuang and Mandalgovi. In general, aerosol size distributions are characterized by a bimodal pattern, with a fine mode around 0.2 μm and a coarse mode around 2−5 μm. Similar to AOT and α, volume spectra are also much dependent on geographical location and season. Dunhuang mostly shows coarse mode particles in all seasons, while Mandalgovi and Sri‐Samrong show large seasonal variations in the total volume of fine mode particles. The single scattering albedos of dust particles over east Asia are around 0.9 at 0.5 μm, which are larger than the previously known values of 0.63–0.89 but similar to those found in the Aerosol Robotic Network (AERONET) analysis. It is noted that the optical properties of Asian dust around Korea and Japan are quite similar to those found in dust source regions such as Dunhuang and Mandalgovi. However, the single scattering albedo appears to be smaller than those observed in Dunhuang and Mandalgovi. Furthermore, single scattering albedo tends to become smaller during the dust outbreak period. Considering that aerosols in Korean and Japanese areas are much influenced by anthropogenic aerosols emitted in China particularly under the westerly conditions, the mixing processes between different aerosol species may be the cause of the different optical properties of Asian dust.
Export of NOy from the North American boundary layer: Reconciling aircraft observations and global model budgets
Authors: Li Qinbin , Jacob Daniel J. , Munger J. William , Yantosca Robert M. , Parrish David D.
Published: January 2004

Fossil fuel combustion accounts for >50% of the global atmospheric emission of NOx, but this source is concentrated in the polluted continental boundary layer (CBL) and only a small fraction is exported as NOy (NOx and its oxidation products) to the global troposphere. Better quantification of this export efficiency is needed because of its implications for global tropospheric ozone. A recent Lagrangian analysis of the NOy‐CO correlations observed from the North Atlantic Regional Experiment in September 1997 (NARE'97) aircraft campaign downwind of eastern North America (September 1997) indicated a NOy export efficiency of <10%, with <10% of the exported NOy present as NOx. In contrast, previous three‐dimensional (3‐D) model Eulerian budget analyses for the North American boundary layer indicated NOy export efficiencies of 25–30%, with 30–35% of the exported NOy present as NOx. We investigated this apparent discrepancy by simulating the NARE'97 aircraft observations with a global 3‐D model of tropospheric chemistry (GEOS‐CHEM) and using the model to calculate the NOy export efficiency both through a Lagrangian analysis of the NOy‐CO correlations along the aircraft flight tracks and through an Eulerian budget analysis for the North American boundary layer. The model reproduces the variability and NOy‐CO correlations observed in the aircraft data and also at the Harvard Forest surface site in the northeastern United States. We show that the previous Lagrangian analyses of the NOy export efficiency during NARE'97 were probably biased low because of underestimation of the CO background. Correcting for this bias, we find a NOy export efficiency of 17 ± 7% in the model and 15 ± 11% in the observations. A similar NOy export efficiency (20%) in the model is obtained from the Eulerian budget analysis, demonstrating that the Lagrangian and Eulerian approaches are in fact consistent. Export efficiencies of NOy in previous 3‐D model Eulerian budget analyses were probably too high because of insufficient scavenging out of the CBL. Model results indicate that only 6% of the exported NOy is present as NOx along the aircraft flight tracks, in agreement with the observations, but that 40% of the NOy export flux is present as NOx, in agreement with the previous 3‐D model analyses. This result reflects the fast oxidation of NOx between the point of exit from the CBL and the point of sampling by the aircraft. The eventual ozone production in the global troposphere due to exported NOx and peroxyacetylnitrate (PAN), with equal contributions from each, is comparable in magnitude to the direct export of ozone pollution from the North American boundary layer.
Reconstructing the phytoplankton community of the Cariaco Basin during the Younger Dryas cold event using chlorin steryl esters
Authors: Dahl Kristina A. , Repeta Daniel J. , Goericke R.
Published: January 2004

A record of the downcore distribution of chlorin steryl esters (CSEs) through the Younger Dryas was produced from Cariaco Basin sediments in order to assess the potential use of CSEs as recorders of the structure of phytoplankton communities through time. Using an improved high‐performance liquid chromatography method for the separation of CSEs, we find significant changes in the distribution of CSEs during the Younger Dryas in the Cariaco Basin. During the Younger Dryas, enhanced upwelling in the Cariaco Basin caused an increase in the diatom population and therefore an increase in the relative abundance of CSEs derived from diatoms. In contrast, the dinoflagellate population, and therefore CSEs derived from dinoflagellates, decreased in response to the climate change during the Younger Dryas. These community shifts agree well with the shifts observed in the present day on a seasonal basis that result from the north‐south migration of the Intertropical Convergence Zone over the Cariaco Basin. We also identify changes in the abundance of several CSEs that seem to reflect rapid warming and cooling events. This study suggests that CSEs are useful proxies for reconstructing phytoplankton communities and paleoenvironments.
Optimal design of groundwater remediation systems using fuzzy set theory
Authors: Guan Jiabao , Aral Mustafa M.
Published: January 2004

In this study we use fuzzy sets to describe uncertainty in aquifer parameters and demonstrate the use of this information in optimal design of pump‐and‐treat systems. On the basis of uncertainty in aquifer parameters, two optimization models are formulated, where pumping rates are represented as either deterministic or fuzzy decision variables. Both models with fuzzy constants and fuzzy variables are then transformed into computationally efficient algorithms through the application of fuzzy set theory. The resulting models are solved by optimization algorithms such as golden section search, genetic algorithm (GA), and direct comparison methods. Several examples are given to demonstrate the effectiveness of the models and algorithms presented in this study. The numerical results, which are also compared with the results obtained from probabilistic analysis, show that the optimal solutions obtained from both models provide flexible and reliable pump‐and‐treat strategies and may increase the effectiveness of the remediation system under uncertainty.
Plasma sheet ion energization during dipolarization events
Authors: Delcourt D. C. , Sauvaud J. A.
Published: February 2004

The energization of plasma sheet particles during storm‐time dipolarization of the magnetospheric field lines is examined by means of three‐dimensional particle codes. In the direction perpendicular to the magnetic field, owing to appreciable temporal variations of the field within a cyclotron turn, particles can experience large magnetic moment changes. A detailed analysis of this effect reveals an “effective pick‐up velocity” at substorm onset, which depends upon the magnitude of the magnetic transition and delineates particle motions at nearly constant or highly variable magnetic moment. In the parallel direction, the large but short‐lived electric field yields an intense acceleration of the particles traveling in the equatorial region. It is demonstrated that significant decelerations can also be achieved off‐equator if the particles drift against the surging electric field. Such decelerations occur in regions where the curvature of the E × B paths substantially exceeds that of the magnetic field lines (namely, at relatively large L‐ shells). Systematic calculations of model plasma sheet distribution functions clearly exemplify the earthward injection of energetic particles during dipolarization events. Owing to phasing between particle motion and the transient electric field, the computed spectrograms display the formation of characteristic bouncing ion patterns with fine structures on fast time scales. To some extent, the simulations support the convection surge mechanism of particle acceleration put forward by Mauk [1986]. However, distinct collapse characteristics as well as the inclusion of nonadiabatic effects result in significantly higher energization rates, particularly at high mass‐to‐charge ratios.
The O+ 834‐Å dayglow: Revised cross sections
Authors: Link R. , Evans J. S. , Gladstone G. R.
Published: February 2004

This study assesses the impact of new O(3P) photoionization and N2 photoabsorption cross sections, and O+ oscillator strengths and transition probabilities, on O II 834‐Å airglow calculations. The 834‐Å intensities computed using the new emission parameters are in good agreement with rocket measurements obtained in 1978 and 1980. The present study does not support a suggested reduction in the N2 photoabsorption cross section based on an earlier analysis of the rocket data. This study also explores the problem of determining F region electron densities from satellite limb scans of the O+ 834‐Å emission. Our results indicate that electron density profiles inferred from limb scans are not necessarily unique; estimates of NmF2 and hmF2 can vary by at least a factor of 2 and 50 km, respectively.
Measurement of auroral rays from the space shuttle
Authors: Jack Todd M. , Hallinan Thomas J.
Published: February 2004

Videotapes of auroras from two space shuttle missions were analyzed to determine the locations of auroral rays. One hundred and forty auroral ray locations were determined by triangulating from pairs of images taken several seconds (20‐100 km) apart. The rays, observed over a range of 18 hours in local magnetic time between 62° and 85° magnetic latitude, were mostly confined to the Feldstein oval in the evening and midnight sectors but were uniformly scattered well into the polar cap in the morning sector. Plots of ray border altitude versus magnetic time show a steady increase in lower border altitude, suggesting a decrease in energy from near 10 keV in the early evening to near 2 keV by midmorning. This variation for the energy in discrete rayed arcs is opposite to previous reports for the average energy of auroral electrons and suggests that the acceleration mechanism within discrete arcs differs from that pertaining to the auroral oval in general. Comparison of upper and lower border altitudes suggests a nearly monoenergetic electron flux shortly before midnight and a more extended distribution at both earlier and later times. The apparently independent variation in maximum energy and energy spread is difficult to reconcile with models involving acceleration of auroral electrons by parallel electric fields. There was only a slight dependence of border altitudes on magnetic latitude within the auroral oval, while rays in the polar cap (northward Bz) were significantly higher.
One‐dimensional multispecies hydrodynamic models of the wakeside ionosphere of Titan
Authors: Keller C. N. , Cravens T. E.
Published: February 2004

The passage of Voyager 1 through the wakeside region of Titan's ionosphere provided an interesting probe of this plasma environment. A one‐dimensional multispecies hydrodynamic model was developed to study the wakeside ionosphere for radial distances ranging from 1.3 to 4.5 RT and included the ion species H2CN+, CnHm, C2H5+, CH5+, H+, H2+, and H3+. Plasma transport only along magnetic field lines was included. The only source of ion production was from electron impact ionization of Titan's atmosphere by radially streaming Satumian magnetospheric electrons (i.e., the nightside wakeside ionosphere). We found that ion production rates due to hot (200 eV) magnetospheric electrons impacting Titan's neutral atmosphere along radial field lines were comparable to the dayside ion production rates due to solar EUV flux. The magnetic flux tube area was varied along with electron temperature, and the results were compared to the ion densities and velocities measured during the Voyager 1 encounter. We found outward ion flows of about (2‐7) × 106 cm−2 s−1, resulting in a total ion loss from Titan into Saturn's magnetosphere of about (6 ‐ 20) × 1024 s−1.
Plasma sheet expansion at r = 15 ‐ 22 RE: A recovery phase or expansion phase phenomenon?
Authors: Lyons L. R. , Huang C. Y.
Published: February 2004

It has been widely accepted that at radial distances r ∼ 15‐20 RE the plasma sheet abruptly thins at the onset of the substorm expansion phase and later expands during the substorm recovery phase. This dynamical behavior is dramatically different from that observed at r ≲ 15 RE and is a fundamental component of substorm models. Recent results, however, have shown an expansion, rather than a thinning, of the plasma sheet at onset within the range r = 15‐22 RE. Here we reanalyze previously published data that led to the earlier conclusion. We find that it too shows an expansion of the plasma sheet at onset, and we find no evidence for a change in plasma sheet behavior at r ∼ 15 RE. We suggest that satellite location relative to the plasma sheet boundary, effects of multiple onsets, and previous selection of events that preceded extended quiet periods all contributed to the difference between our results and the earlier results. Our interpretation of these findings is that the substorm‐associated dynamics of the plasma sheet is very similar from near synchronous orbit to at least r = 22 RE. We suggest that the dynamics beyond 15 RE can be explained in terms of an extension into this region of a portion of the substorm current wedge, the response to the current wedge being very similar to that which has been well documented at lower r.
Ion transport and loss in the Earth's quiet ring current: 2. Diffusion and magnetosphere‐ionosphere coupling
Authors: Sheldon R. B.
Published: February 2004

We have studied the transport and loss of H+, He+, and He++ ions in the Earth's quiet time ring current (1‐300 keV/e, 3‐7RE, Kp<2+, |Dst|<11, 70°‐110° pitchangles, all LT) comparing the standard radial diffusion model developed for the higher‐energy radiation belt particles with measurements of the lower energy ring current ions in a previous paper (Sheldon and Hamilton, 1993). Large deviations of that model, which fit only 50% of the data to within a factor of 10, suggested that another transport mechanism is operating in the ring current. Here we derive a modified diffusion coefficient corrected for electric field effects on ring current energy ions that fit nearly 80% of the data to within a factor of 2. Thus we infer that electric field fluctuations from the low‐latitude to midlatitude ionosphere (ionospheric dynamo) dominate the ring current transport, rather than high‐latitude or solar wind fluctuations. Much of the remaining deviation may arise from convective electric field transport of the E<30 keV particles. Since convection effects cannot be correctly treated with this azimuthally symmetric model, we defer treatment of the lowest‐energy ions to a another paper. We give χ2 contours for the best fit, showing the dependence of the fit upon the internal/external spectral power of the predicted electric and magnetic field fluctuations.