bluuebunny/crossref_metadata_2025 huggingface dataset:
Oscillatory instability of one‐dimensional two‐phase hydrothermal flow in heterogeneous porous media
Authors: Xu Wenyue , Lowell Robert P.
Published: September 2002

We derive a general analytical solution for one‐dimensional steady state two‐phase hydrothermal flow in porous media. The solution indicates that saturation jumps associated with discontinuities in material properties of porous media, such as permeability and thermal conductivity, and phase change boundaries are common in these systems. Using linear stability analysis, we then show that the saturation jump associated with discontinuities in permeability or thermal conductivity of porous media can be unstable under infinitesimal perturbation. We also provide a simple numerical example to show that saturation waves attributed to the instability can propagate away from the discontinuity as relatively undamped oscillations. Because rapid spatial changes in material properties are likely in nature, we suggest that two‐phase hydrothermal systems may often exhibit oscillatory behavior. When observational data on oscillations become available, such data may yield information on permeability and/or thermal conductivity structures of two‐phase hydrothermal systems.
Origin of extensional core complexes: Evidence from the Mid‐Atlantic Ridge at Atlantis Fracture Zone
Authors: Blackman Donna K. , Cann Johnson R. , Janssen Bob , Smith Deborah K.
Published: September 2002

The contrast in geologic structure observed on opposing flanks of the Mid‐Atlantic Ridge, where it is offset by the Atlantis transform fault, illustrates how significant differences in crustal structure can result from tectonic processes that operate near the ends of slow spreading segments. New geophysical and geological data provide information on the nature of large massifs that punctuate the strips of crust formed at the inside corner of ridge‐transform intersections (RTI), as well as of the low‐relief volcanic morphology that typifies the outside corners. The geological relations mapped at the inside corner of the eastern Atlantis RTI are strikingly similar to those seen in the Basin and Range where metamorphic core complexes are unroofed through asymmetric detachment faulting. The core of the eastern RTI massif exposes deep‐seated rocks beneath a shallow‐dipping, corrugated surface which is interpreted as a fault surface. On the median valley side of the massif, this seafloor detachment is overlain by upper crustal blocks bounded by steeper fault scarps. The western side of the 15‐km‐wide massif is characterized by en echelon faults which face away from the ridge axis. Similar features are mapped at two fossil massifs that are interpreted to have formed at the inside comers of each RTI and to have rafted off‐axis as plate spreading proceeded. Analysis of new and preexisting shipboard gravity data indicates that high‐density material is not continuously emplaced at the inside corner. Rather, peaks in the gravity anomaly map are patchily distributed along the transform valley walls. The gravity highs associated with the three massifs (oceanic core complexes) in this area are not centered with respect to their morphology but are located toward their spreading axis and transform sides. Gravity modeling suggests that the western boundary of a high‐density wedge at the eastern RTI massif is steeply dipping, whereas the eastern boundary may dip about 15° toward the median valley. In contrast to the inside corners of the RTIs in our study area, the outside corner seafloor is characterized by volcanic constructions similar to those found on either side of the spreading axis at the center of the segments and inferred to be typical basaltic upper crust. Kinematic analysis at the Mid‐Atlantic Ridge‐Atlantis Transform RTI suggests that the formation of seafloor detachments may occur when the rate of extension not accommodated by magmatic input exceeds about 4 mm/yr. Isolated volcanic ridges that extend into the fracture zone domain, curving as they approach the fault trace, mark times of abundant magma supply at the segment ends. The apparent interplay between magmatic and tectonic strain accommodation at a mid‐ocean ridge, as well as the overall structure of oceanic core complexes, may provide important kinematic constraints on core complex formation and the development of shallow‐dipping detachment faults.
Origin of the Indian Ocean‐type isotopic signature in basalts from Philippine Sea plate spreading centers: An assessment of local versus large‐scale processes
Authors: Hickey‐Vargas Rosemary
Published: September 2002

Basalts erupted from spreading centers on the Philippine Sea plate between 50 Ma and the present have the distinctive isotopic characteristics of Indian Ocean mid‐ocean ridge basalt (MORB), such as high 208Pb/204Pb and low 143Nd/144Nd for a given 206Pb/204Pb compared with Pacific and Atlantic Ocean MORB. This feature may indicate that the upper mantle of the Philippine Sea plate originated as part of the existing Indian Ocean upper mantle domain, or, alternatively, that local processes duplicated these isotopic characteristics within the sub‐Philippine Sea plate upper mantle. Synthesis of new and published isotopic data for Philippine Sea plate basin basalts and island arc volcanic rocks, radiometric ages, and tectonic reconstructions of the plate indicates that local processes, such as contamination of the upper mantle by subducted materials or by western Pacific mantle plumes, did not produce the Indian Ocean‐type signature in Philippine Sea plate MORB. It is more likely that the plate originated over a rapidly growing Indian Ocean upper mantle domain that had spread into the area between Australia/New Guinea and southeast Asia before 50 Ma.
Model of joint spacing distribution based on shadow compliance
Authors: Rabinovitch A. , Bahat D.
Published: September 2002

Fracture spacing is analyzed with a special reference to the functional shape of a shadow, which allows some presence of joints in the proximity of existing ones. A new term, the “shadow compliance” α, is mathematically defined. A good agreement is obtained between this model and several fracture populations. We obtained α ∼ 1 (α = 0.884 ± 0.070) for a spacing distribution that occurred in layers that were connected to their neighbors with major differences in mechanical properties across the lithologic boundaries. In this situation, different strains along the boundaries influenced joint spacing according to the Cox‐Hobbs model. Their theory asserts that stress release behaves as a first power of distance, implying, in our theory, α = 1. On the other hand, we obtained α ≈ 3 (α = 2.904 ± 0.156) for a spacing distribution where fracture occurred in layers of uniform elastic properties, which were disconnected from their neighboring ones by fractures, so that the stress reduction was not dependent on material properties along the boundaries. In this case the Pollard and Segall model was the appropriate one for analyzing joint spacing. Their theory asserts that stress release behaves as a third power of distance, which indeed implies, for our theory, α = 3. Our model shows that in a given joint set, α is not sensitive to variations in strains that were caused by a “normal mechanism.” However, a mechanism of very intense jointing can drastically change α either to an unrealistic range or to a state of being indefinable.
Perturbation to the lithosphere along the hotspot track of La Réunion from an offshore‐onshore seismic transect
Authors: Gallart Josep , Driad Lynda , Charvis Phillippe , Sapin Martine , Hirn Alfred , Diaz Jordi , de Voogd Béatrice , Sachpazi Maria
Published: September 2002

A 250 km long NE‐SW lithospheric transect spanning the 40 km wide island of La Réunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high‐velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island‐crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material.
Investigation of multifrequency/multipolarization radar signatures of rain cells over the ocean using SIR‐C/X‐SAR data
Authors: Melsheimer Christian , Alpers Werner , Gade Martin
Published: September 2002

Radar signatures of rain cells are investigated using multifrequency/multipolarization synthetic aperture radar (SAR) images acquired from the space shuttleEndeavourduring the spaceborne imaging radar‐C/X‐band SAR (SIR‐C/X‐SAR) missions in April and October 1994. In SIR‐C/X‐SAR images, radar signatures of rain cells over the ocean usually consist of irregularly shaped bright and dark patches that strongly depend on radar frequency and polarization. The radar signatures of rain cells observed in SIR‐C/X‐SAR imagery of the ocean originate from (1) the scattering and attenuation of the microwaves by raindrops and ice particles in the atmosphere and (2) the modification of the sea surface roughness induced by the impact of raindrops and by wind gusts associated with rain cells. Raindrops impinging on the sea surface generate ring waves, which enhance the sea surface roughness, but they also generate turbulence in the upper water layer, which reduces the sea surface roughness. Depending on the radar wavelength, ocean areas struck by rain can have higher or lower normalized radar cross section (NRCS) than the surrounding rain‐free area; in ocean areas where heavy rain is impinging on the sea surface, the X‐ and C‐band NRCS is usually enhanced, and the L‐band NRCS is reduced. From the phase difference between the horizontally and vertically copolarized signals, estimates of the rain rate are obtained. The present analysis shows further that the presently used wind speed retrieval algorithms for the scatterometers aboard the ERS and ADEOS satellites may yield biased wind fields if several rain cells lie within a scatterometer resolution cell.
Sea surface height variations in the Yellow and East China Seas: 1. Linear response to local wind stress
Authors: Jacobs G. A.
Published: September 2002

The sea surface height (SSH) variations observed by the TOPEX/POSEIDON satellite in the Yellow and East China Seas are examined over the 3 years from 1993 through 1995 assuming a linear response to wind stress across the region. The transfer function relating wind stress to SSH is modeled by a series of increasingly complex functions that gradually allow wind stress over successively broader regions and time periods to influence the SSH solution at a particular point. The SSH response to instantaneous wind stress at the same point in space implies an Ekman transport, but the response explains only a small fraction of the SSH variability. Relating SSH to wind stress averaged over two subregions and including both instantaneous and 12‐hour lagged wind stress indicates that a significant portion of SSH variability responds to winds that are remote in both space and time. An optimal estimation of the transfer function by minimizing the error variance leads to an extended empirical orthogonal function (EEOF) analysis of the wind stress field. The SSH response to each wind mode is determined. The analysis shows the principle variations in the wind stress to consist of northerly wind bursts during winter, which produce large SSH drops in the Bohai Bay and northern Yellow Sea region. The principal wind mode also is significantly related to variations along the shelf break near the Kuroshio Current. The third and fourth wind stress modes indicate the passage of typhoons.
A third‐generation wave model for coastal regions: 1. Model description and validation
Authors: Booij N. , Ris R. C. , Holthuijsen L. H.
Published: September 2002

A third‐generation numerical wave model to compute random, short‐crested waves in coastal regions with shallow water and ambient currents (Simulating Waves Nearshore (SWAN)) has been developed, implemented, and validated. The model is based on a Eulerian formulation of the discrete spectral balance of action density that accounts for refractive propagation over arbitrary bathymetry and current fields. It is driven by boundary conditions and local winds. As in other third‐generation wave models, the processes of wind generation, whitecapping, quadruplet wave‐wave interactions, and bottom dissipation are represented explicitly. In SWAN, triad wave‐wave interactions and depth‐induced wave breaking are added. In contrast to other third‐generation wave models, the numerical propagation scheme is implicit, which implies that the computations are more economic in shallow water. The model results agree well with analytical solutions, laboratory observations, and (generalized) field observations.
The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño
Authors: Stoens Anne , Menkès Christophe , Radenac Marie‐Hélène , Dandonneau Yves , Grima Nicolas , Eldin Gérard , Mémery Laurent , Navarette Claudie , André Jean‐Michel , Moutin Thierry , Raimbault Patrick
Published: September 2002

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high‐resolution Ocean General Circulation Model forced with satellite‐derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September‐October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate‐depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.
Photochemical ozone production in the rural southeastern United States during the 1990 Rural Oxidants in the Southern Environment (ROSE) program
Authors: Frost G. J. , Trainer M. , Allwine G. , Buhr M. P. , Calvert J. G. , Cantrell C. A. , Fehsenfeld F. C. , Goldan P. D. , Herwehe J. , Hübler G. , Kuster W. C. , Martin R. , McMillen R. T. , Montzka S. A. , Norton R. B. , Parrish D. D. , Ridley B. A. , Shetter R. E. , Walega J. G. , Watkins B. A. , Westberg H. H. , Williams E. J.
Published: September 2002

Extensive measurements of ozone and its photochemical precursors and coproducts were made in the 1990 Rural Oxidants in the Southern Environment (ROSE) program. Peroxy radical production, loss, and partitioning are described at a rural site in Alabama, showing the important role that biogenic organic compounds play in ozone production. Estimates of the peroxy radical concentration obtained by four methods along with the measured nitric oxide level are used to predict the instantaneous rate of photochemical ozone production at the site. The four methods agree on the diurnal behavior of peroxy radicals and ozone production rates, while consistent discrepancies between the methods generally are within their combined uncertainties. Selected aircraft measurements are used to derive ozone production rates above the ground site, with the highest rates occurring in the boundary layer and in industrial plumes. The dependences of peroxy radical concentration and ozone production rate on the level of nitrogen oxides exhibit good agreement between the various methods and are consistent throughout the lower troposphere. Surface deposition and entrainment are shown to be as important as photochemical production in determining the diurnal evolution of ozone at this site.
Distribution of hydrogen peroxide and methylhydroperoxide over the Pacific and South Atlantic Oceans
Authors: O'Sullivan D. W. , Heikes B. G. , Lee M. , Chang W. , Gregory G. L. , Blake D. R. , Sachse G. W.
Published: September 2002

The gas phase hydrogen peroxide and methylhydroperoxide concentrations were measured in the troposphere over the tropical Pacific Ocean as a component of NASA's Global Tropospheric Experiment/Pacific Exploratory Mission‐Tropics A field campaign. Flights on two aircraft covered the Pacific from 70°S to 60°N and 110°E to 80°W and South Atlantic from 40°S to 15°N and 45°W to 70°E, and extending from 76 to 13,000 m altitude. H2O2 and CH3OOH have the highest concentrations at a given altitude at the equator and decrease with increasing latitude in both the northern and southern hemispheres. Above 4 km the gradient is substantially reduced for both H2O2 and CH3OOH with latitude, and at altitudes in excess of 8 km there is no latitudinal dependence. H2O2 and CH3OOH exhibit maximum mixing ratios between 1 and 2 km at all latitudes. The mean mixing ratio of H2O2 at the equator was 1600±600 parts per trillion by volume (pptv) decreasing to 500±250 pptv at latitudes greater than 55° north and south between 1 and 2 km altitude. CH3OOH at the equator was 1400±250 pptv, decreasing to 330±200 pptv at high latitudes at altitudes between 1 and 2 km. The concentration of peroxides at high latitudes in the northern hemisphere was generally a factor of 2 higher than at corresponding latitudes in the southern hemisphere. The ratio of H2O2 to CH3OOH was between 1 and 2 from 45°S to 35°N at altitudes below 4 km. Between 5° to 15°N the ratio is less than 1, due to preferential removal of H2O2 in the Intertropical Convergence Zone.
Two new methods for deriving tropospheric column ozone from TOMS measurements: Assimilated UARS MLS/HALOE and convective‐cloud differential techniques
Authors: Ziemke J. R. , Chandra S. , Bhartia P. K.
Published: September 2002

This study introduces two new approaches for determining tropospheric column ozone from satellite data. In the first method, stratospheric column ozone is derived by combining Upper Atmosphere Research Satellite (UARS) halogen occultation experiment (HALOE) and microwave limb sounder (MLS) ozone measurements. Tropospheric column ozone is then obtained by subtracting these stratospheric amounts from the total column. Total column ozone in this study include retrievals from Nimbus 7 (November 1978 to May 1993) and Earth probe (July 1996 to present) total ozone mapping spectrometer (TOMS). Data from HALOE are used in this first method to extend the vertical span of MLS (highest pressure level 46 hPa) using simple regression. This assimilation enables high‐resolution daily maps of tropospheric and stratospheric ozone which is not possible from solar occultation measurements alone, such as from HALOE or Stratospheric Aerosols and Gas Experiment (SAGE). We also examine another new and promising technique that yields tropospheric column ozone directly from TOMS high‐density footprint measurements in regions of high convective clouds. We define this method as the convective cloud differential (CCD) technique. The CCD method is shown to provide long time series (essentially late 1978 to the present) of tropospheric ozone in regions dominated by persistent high tropopause‐level clouds, such as the maritime tropical Pacific and within or near midlatitude continental landmasses. In this our first study of the CCD and MLS/HALOE methods we limit analyses to tropical latitudes. Separation of stratospheric from tropospheric column ozone in the eastern Pacific tropics for January 1979 to December 1997 shows that the dominant interannual variability of stratospheric ozone is the quasi‐biennial oscillation (QBO), whereas for tropospheric ozone it is driven by El Niño events. For validation purposes, both the CCD and assimilated UARS MLS/HALOE results are compared with ozonesonde data from several southern tropical stations. Despite all three measurements being distinctly different in sampling and technique, all three show good qualitative agreement.
A new family Jacobian solver for global three‐dimensional modeling of atmospheric chemistry
Authors: Zhao Xuepeng , Turco Richard P. , Shen Mei
Published: September 2002

We present a new technique to solve complex sets of photochemical rate equations that is applicable to global modeling of the troposphere and stratosphere. The approach is based on the concept of “families” of species, whose chemical rate equations are tightly coupled. Variations of species concentrations within a family can be determined by inverting a linearized Jacobian matrix representing the family group. Since this group consists of a relatively small number of species the corresponding Jacobian has a low order (a minimatrix) compared to the Jacobian of the entire system. However, we go further and define a super‐family that is the set of all families. The super‐family is also solved by linearization and matrix inversion. The resulting Super‐Family Matrix Inversion (SFMI) scheme is more stable and accurate than common family approaches. We discuss the numerical structure of the SFMI scheme and apply our algorithms to a comprehensive set of photochemical reactions. To evaluate performance, the SFMI scheme is compared with an optimized Gear solver. We find that the SFMI technique can be at least an order of magnitude more efficient than existing chemical solvers while maintaining relative errors in the calculations of 15% or less over a diurnal cycle. The largest SFMI errors arise at sunrise and sunset and during the evening when species concentrations may be very low. We show that sunrise/sunset errors can be minimized through a careful treatment of photodissociation during these periods; the nighttime deviations are negligible from the point of view of acceptable computational accuracy. The stability and flexibility of the SFMI algorithm should be sufficient for most modeling applications until major improvements in other modeling factors are achieved. In addition, because of its balanced computational design, SFMI can easily be adapted to parallel computing architectures. SFMI thus should allow practical long‐term integrations of global chemistry coupled to general circulation and climate models, studies of interannual and interdecadal variability in atmospheric composition, simulations of past multidecadal trends owing to anthropogenic emissions, long‐term forecasting associated with projected emissions, and sensitivity analyses for a wide range of physical and chemical parameters.
Ozone formation and transport in southeastern United States: Overview of the SOS Nashville/Middle Tennessee Ozone Study
Authors: Meagher James F. , Cowling Ellis B. , Fehsenfeld Fred C. , Parkhurst William J.
Published: September 2002

The Southern Oxidants Study (SOS) is a public‐private partnership collectively engaged, since 1989, in a coordinated program of policy‐relevant research to improve scientific and public understanding of tropospheric ozone pollution. In the summers of 1994 and 1995, SOS implemented the Nashville/Middle Tennessee Ozone Study. This effort, the second SOS urban intensive study, conducted a series of integrated, process‐oriented airborne and surface measurement experiments to better understand the chemistry and meteorology associated with the production, transport, and impact of tropospheric ozone. Specific technical objectives addressed (1) the role of biogenic VOC and NOxemissions on local and regional ozone production, (2) the effect of urban‐rural exchange/interchange on local and regional ozone production, (3) sub‐grid‐scale photochemical and meteorological processes, and (4) the provision of a high‐quality chemical and meteorological data set to test and improve observation‐ and emission‐based air quality forecast models. Some of the more significant findings of the 1994–1995 studies include the following: (1) Ozone production in Nashville was found to be close to the transition between NOx‐sensitive and VOC‐sensitive chemistry. (2) Ozone production efficiency (OPE) in power plant plumes, molecules of ozone formed per molecule of NOxemitted, was found to be inversely proportion to NOxemission rate, with the plants having the greatest NOxemissions exhibiting the lowest OPE. (3) During stagnant conditions, nighttime winds dominated pollutant transport and represent the major mechanism for transporting urban pollutants to rural areas. Ultimately, results provided by this research will allow improved assessment of existing ozone management strategies and provide better scientific tools for the development of future management strategies.
Measurements of OH during PEM‐Tropics A
Authors: Mauldin R. L. , Tanner D. J. , Eisele F. L.
Published: September 2002

Results are presented for measurements of the concentration of OH performed aboard the NASA P3‐B aircraft using the Selected Ion Chemical Ionization Mass Spectrometry technique during the Pacific Exploratory Mission‐Tropics (PEM) study. Typical midday boundary layer OH concentrations of 6–8 × 106 molecules cm−3 were observed. Diurnally averaged boundary layer OH values obtained near Christmas Island, Kiribati, combined with SO2 and filter measurements of non‐sea‐salt sulfate reveal that only 12% of the sulfate found in the particulate form can be attributed to gas phase H2SO4 production/condensation. Measurements performed over clouds yielded [OH] 2–3 times larger than those typically observed under similar cloudless conditions.
A high‐spectral resolution study of the near‐infrared solar flux disposition in clear and overcast atmospheres
Authors: Ramaswamy V. , Freidenreich S. M.
Published: September 2002

The sensitivity of the near‐infrared spectral atmospheric and surface fluxes to the vertical location of clouds is investigated, including a study of factors (drop‐size distribution, drop optical depth, solar zenith angle, cloud geometrical thickness, atmospheric profiles) which govern this dependence. Because of the effects of the above cloud, in‐cloud and below‐cloud water vapor the atmospheric absorbed flux in each spectral band depends critically on the cloud location, with a high cloud resulting in lesser absorption and greater reflection than a low one having the same drop optical depth. The difference between a high and a low cloud forcing of atmospheric absorption increases with drop optical depth. For any optical depth, clouds with larger drops cause a greater forcing of the spectral atmospheric absorption than those with smaller ones, so high clouds can even cause an increase rather than a decrease of the atmospheric absorption relative to clear skies. In contrast, the spectral and total surface fluxes are relatively insensitive to cloud vertical location. Instead, they are determined by the drop characteristics, notably drop optical depth. This near‐invariance characteristic is attributable to the fact that most of the insolation reaching the surface is in the weak water vapor spectral absorption regions; here drops dominate the radiative interactions and thus there is little dependence on cloud height. In addition, the overlap of the drop spectral features with the moderate‐to‐strong vapor absorption bands ensures that insolation in these regimes fails to reach the surface no matter where the cloud is located; instead, these bands contribute the most to atmospheric absorption. The near‐invariant behavior of the spectral and total surface flux holds separately for a wide variety of conditions studied. As a consequence, the difference in reflection, between two columns containing clouds with the same optical depth but located at different altitudes, is approximately balanced in magnitude by the difference in the atmospheric absorption; this holds for every spectral interval whether it be a weak, moderate, or strong vapor/drop absorption band. It also follows that the net fluxes at the top and surface of overcast atmospheres do not have a general, unambiguous relationship; this is in sharp contrast to a linear relation between them in clear skies. However, under certain overcast conditions (e.g., specific vertical location of clouds and solar zenith angle), a simple linear relationship is plausible.
Tropospheric ozone above the southwestern Indian Ocean is strongly linked to dynamical conditions prevailing in the tropics
Authors: Taupin F. G. , Bessafi M. , Baldy S. , Bremaud P. J.
Published: September 2002

A large ozone profile data set obtained at Réunion Island (21°S, 55°E) since 1992 is presented and linked to observed dynamical conditions in the region. The average seasonal variation of column integrated tropospheric ozone reaches 25 Dobson Units and peaks at 55 Dobson Units during the month of October. Two thermal inversions and high values of ozone mixing ratio during biomass burning activity in southeastern Africa are observed. The marine boundary layer (MBL) is characterized by high relative humidity values and low ozone mixing ratios. The decrease in the water vapor content above 2 km is strongly linked to seasonally evolving dynamical conditions. The first inversion largely hinders vertical exchanges, and low water vapor values in the free troposphere favor the photochemical production of ozone. The second thermal inversion is linked to the zonal wind inversion. Above this second inversion a large ozone enhancement is observed that occurs in parallel the biomass‐burning season in southeastern Africa and Madagascar. The season extents from August to December and peaks in October. The ozone maximum concomitantly results from the transport of ozone and its precursors westward from the biomass‐burning zones. High ozone values recorded in the upper troposphere during austral winter are likely to originate from additional stratosphere‐troposphere exchanges. Winter anticyclonic conditions further favor the photochemical production of ozone. The low tropospheric ozone mixing ratios observed across the entire troposphere during austral summer could result from the heterogeneous destruction of ozone and from less favorable conditions for its photochemical production, because of the development of deep convection.
Ganymede's magnetosphere: Magnetometer overview
Authors: Kivelson M. G. , Warnecke J. , Bennett L. , Joy S. , Khurana K. K. , Linker J. A. , Russell C. T. , Walker R. J. , Polanskey C.
Published: September 2002

Ganymede presents a unique example of an internally magnetized moon whose intrinsic magnetic field excludes the plasma present at its orbit, thereby forming a magnetospheric cavity. We describe some of the properties of this mini‐magnetosphere, embedded in a sub‐Alfvénic flow and formed within a planetary magnetosphere. A vacuum superposition model (obtained by adding the internal field of Ganymede to the field imposed by Jupiter) organizes the data acquired by the Galileo magnetometer on four close passes in a useful, intuitive fashion. The last field line that links to Ganymede at both ends extends to ∼2 Ganymede radii, and the transverse scale of the magnetosphere is ∼5.5 Ganymede radii. Departures from this simple model arise from currents flowing in the Alfvén wings and elsewhere on the magnetopause. The four passes give different cuts through the magnetosphere from which we develop a geometric model for the magnetopause surface as a function of the System III location of Ganymede. On one of the passes, Ganymede was located near the center of Jupiter's plasma disk. For this pass we identify probable Kelvin‐Helmholtz surface waves on the magnetopause. After entering the relatively low‐latitude upstream magnetosphere, Galileo apparently penetrated the region of closed field lines (ones that link to Ganymede at both ends), where we identify predominantly transverse fluctuations at frequencies reasonable for field line resonances. We argue that magnetic field measurements, when combined with flow measurements, show that reconnection is extremely efficient. Downstream reconnection, consequently, may account for heated plasma observed in a distant crossing of Ganymede's wake. We note some of the ways in which Ganymede's unusual magnetosphere corresponds to familiar planetary magnetospheres (viz., the magnetospheric topology and an electron ring current). We also comment on some of the ways in which it differs from familiar planetary magnetospheres (viz., relative stability and predictability of upstream plasma and field conditions, absence of a magnetotail plasma sheet and of a plasmasphere, and probable instability of the ring current).
Disentangling electron temperature and density in the Io plasma torus
Authors: Herbert Floyd , Hall Doyle T.
Published: September 2002

One of the torus characteristics of most interest for understanding torus energization is its electron temperature Te. Yet deriving Te has always been difficult because the measured quantity (emission brightness) is controlled jointly by Te and other unknowns, such as ion and electron density. In order to solve this problem, we have used a new technique to estimate Te from spectral images of the Io plasma torus in the 350 to 700 Å region obtained by the Extreme Ultraviolet Explorer (EUVE). Because of the lack of information available on the collision strengths of important lines between 350 and 600 Å, we have simultaneously attempted to constrain the unknown collision strengths and also to deduce the time‐varying torus characteristics by fitting analytic models which exploit the both the commonalities and the variations among the observations. However, because of present limitations of the data set, we can only deduce relative variations in torus Te, total electron number Ne (a proxy for total torus mass), and ionic composition. In the 1993–1995 data set, Te and Ne were anticorrelated, while total torus luminosity remained steadier than either Te or Ne. One interpretation of the anticorrelation of Ne and Te is that torus luminosity may be primarily determined by a relatively constant power‐limited energy supply, so that as Ne increases (decreases), Te sags (surges) in response. This anticorrelation is a constraint on theories of torus energization and transport. There also seems to have been an abrupt 20% decrease in Ne at about the time of the comet Shoemaker‐Levy 9 impacts on Jupiter, as though a magnetospheric disturbance had increased the convective loss rate of the torus, but this may well be a coincidence.
Plasma injection near Io
Authors: Hill T. W. , Pontius D. H.
Published: September 2002

Plasma and field observations obtained by the Galileo spacecraft during its first close encounter with Io indicate a much stronger electrodynamic interaction than anticipated. The strength of this interaction can be explained if one assumes that a significant fraction of the Io torus plasma is ionized in the immediate vicinity of the satellite. We adopt a simple model to calculate the strength and pattern of the flow perturbation resulting from a localized equatorial ionization source with dimensions much less than the radial thickness of the torus itself. Comparison with Galileo observations suggests that, when due allowance is made for the oversimplifications in the model, the observations are consistent with the hypothesis that the torus mass loading rate is highly localized near Io, and that Galileo encountered this region of concentrated mass loading at its closest approach altitude of about 0.5 Io radius. This hypothesis may also help explain two otherwise puzzling properties of the torus that were known before the Galileo encounter, the low ion temperature (relative to corotation energy) and the existence of a bright, radially confined “ribbon” feature.
Photogrammetric analysis of horizon panoramas: The Pathfinder landing site in Viking orbiter images
Authors: Oberst J. , Jaumann R. , Zeitler W. , Hauber E. , Kuschel M. , Parker T. , Golombek M. , Malin M. , Soderblom L.
Published: September 2002

Tiepoint measurements, block adjustment techniques, and sunrise/sunset pictures were used to obtain precise pointing data with respect to north for a set of 33 IMP horizon images. Azimuth angles for five prominent topographic features seen at the horizon were measured and correlated with locations of these features in Viking orbiter images. Based on this analysis, the Pathfinder line/sample coordinates in two raw Viking images were determined with approximate errors of 1 pixel, or 40 m. Identification of the Pathfinder location in orbit imagery yields geological context for surface studies of the landing site. Furthermore, the precise determination of coordinates in images together with the known planet‐fixed coordinates of the lander make the Pathfinder landing site the most important anchor point in current control point networks of Mars.
The color of Mars: Spectrophotometric measurements at the Pathfinder landing site
Authors: Maki J. N. , Lorre J. J. , Smith P. H. , Brandt R. D. , Steinwand D. J.
Published: September 2002

We calculate the color of the Martian sky and surface directly using the absolute calibration of the Mars Pathfinder (MPF) lander camera, which was observed to be stable during the mission. The measured colors of the Martian sky and surface at the Pathfinder site are identical to the Viking sites, i.e., a predominantly yellowish brown color with only subtle variations. These colors are distributed continuously and fall into five overlapping groups with distinct average colors and unique spatial characteristics: shadowed soil, soil, soil/rock mixtures, rock, and sky. We report that the primary difference between the sky color and the color of the rocks is due to a difference in brightness. Measurements of the sky color show that the sky reddens away from the Sun and toward the horizon and that the sky color varies with time of day and is reddest at local noon. We present a true color picture of the Martian surface and color enhancement techniques that increase image saturation, maximize color discriminability while preserving hue, and eliminate brightness variations while preserving the chromaticity of the scene. Although Mars has long been called the “red” planet, quantitative measurements of the surface color from telescopic and surface observations indicate a light to moderate yellowish brown color. The Pathfinder camera measurements presented here support the claim that the red planet is not red but indeed yellowish brown.
Mars Pathfinder spectral measurements of Phobos and Deimos: Comparison with previous data
Authors: Murchie Scott , Thomas Nick , Britt Daniel , Herkenhoff Ken , Bell James F.
Published: September 2002

The Imager for Mars Pathfinder (IMP) acquired four spectra of parts of the sub‐Mars hemispheres of Phobos and Deimos. The measured region of Phobos is expected to be a mixture of the two spectral units identified on that satellite from Phobos 2 data, and the IMP spectra of Phobos are intermediate to the two units as expected. The derived geometric albedo is consistent with the value for that part of Phobos determined from Viking imagery. The IMP spectrum of Deimos is generally consistent with previous measurements acquired from the ground and from the Hubble Space Telescope (HST), but the signal‐to‐noise ratio is lower than that of the Phobos data. The spectral contrast between the two moons is similar to that seen in HST and Phobos 2 data. Mars Pathfinder measurements therefore substantiate recent results which indicate that Phobos and Deimos are not, as previously believed, analogous to C‐type asteroids. They also provide some indications for an absorption near 700 nm, perhaps like that seen in other low‐albedo asteroids. Both Martian moons are redder than most asteroids, and most closely resemble two analog materials believed to have undergone very dissimilar histories: primitive D‐like asteroids, and highly space‐weathered, mafic‐rich assemblages, such as are present in lunar mare soils.
Surface modification of olivine by H+ and He+ bombardment
Authors: Dukes C. A. , Baragiola R. A. , McFadden L. A.
Published: September 2002

Laboratory simulations of solar wind irradiation of olivine, a constituent of ordinary chondritic meteorites and S‐type asteroids, show a dramatic chemical reduction of surface iron and, to a lesser extent, silicon. Earth's atmosphere oxidizes any reduced iron instantaneously, whereas irradiated material on the surface of an asteroid would remain reduced. Changes in optical reflectance, due to metallization of the surface, provide a possible explanation for differences in the near‐infrared spectra of ordinary chondrite meteorites and their likely parent bodies, the S(IV) asteroids.
Electromagnetic scattering and penetration into a filled cavity through an aperture in a conducting plane
Authors: El‐Hajj Ali , Kabalan Karim Y. , Rayes Asaad
Published: September 2002

Electromagnetic scattering from apertures in an infinite conducting plane backed by a filled cavity is investigated. The boundary conditions are used to obtain a set of equations to solve for the magnetic current. The boundary equations are solved using the characteristic modes theory. Numerical computations are performed to illustrate the scattering behavior in terms of the aperture and cavity sizes.
Improved 20‐ to 32‐GHz atmospheric absorption model
Authors: Pol Sandra L. Cruz , Ruf Christopher S. , Keihm Stephen J.
Published: September 2002

An improved model for the absorption of the atmosphere near the 22‐GHz water vapor line is presented. The Van Vleck‐Weisskopf line shape is used with a simple parameterized version of the model fromLiebe et al. [1993] for the water vapor absorption spectra and a scaling of the model fromRosenkranz[1993] for the 20 to 32‐GHz oxygen absorption. Radiometrie brightness temperature measurements from two sites of contrasting climatological properties, San Diego, California, and West Palm Beach, Florida, were used as ground truth for comparison with in situ radiosonde‐derived brightness temperatures under clear‐sky conditions. Estimation of the new model's four parameters, related to water vapor line strength, line width and continuum absorption, and far‐wing oxygen absorption, was performed using the Newton‐Raphson inversion method. Improvements to the water vapor line strength and line width parameters are found to be statistically significant. The accuracy of the new absorption model is estimated to be 3% between 20 and 24 GHz, degrading to 8% near 32 GHz. In addition, the Hill line shape asymmetry ratio was evaluated on several currently used models to show the agreement of the data with Van Vleck‐Weisskopf based models and to rule out water vapor absorption models near 22 GHz given byWaters[1976] andUlaby et al. [1981], which are based on the Gross line shape.
Electron density and height at the F1 region minimum gradient at high solar activity for an equatorial station
Authors: Adeniyi J. O. , Radicella Sandro M.
Published: September 2002

The occurrence of ƒoF1 at high solar activity is very scanty. The occurrence of the L condition is what predominates during this solar epoch. Electron density N F1 and the height hF1 at the point of minimum gradient in ionospheric profiles at high solar activity were studied for an equatorial station. N F1 is dependent on solar zenith angle (x). The cos(x) indexes for winter, spring, summer, and autumn were found to be 0.78, 0.59, 0.76, and 0.65, respectively. Height h F1 showed no diurnal or seasonal effects and has an average value of 167 km. The results obtained were compared with those of low solar activity from a previous study.
Topside sounder proton‐cyclotron echo generation from a plasma memory process and electron Bernstein‐wave propagation
Authors: Muldrew D. B.
Published: September 2002

Evidence is presented for the radiation of an electron Bernstein wave from a radio frequency pulse simulated by proton‐cyclotron motion. The protons are energized by an ionospheric topside sounder transmitter pulse and take part in a plasma memory process that simulates the original transmitter pulse. These echoes appear on topside ionograms as thin traces. At frequencies below the electron cyclotron frequency fH, the proton echo delay is a multiple of the proton‐cyclotron period tp. At frequencies above fH, there can be an additional delay to tp depending on two factors: 1) the antenna orientation and 2) the closeness of the proton echo frequency f to fH. A model is presented here in which protons passing within a few tens of centimeters of the antenna are energized and modulated by the RF pulse by an amount depending on the RF phase. At multiples of tp later, these protons return to almost the same field line they were on at the time of the RF pulse, although they may have moved some tens of meters parallel or antiparallel to the field line. At multiples of tp, ntp, the protons are thus located in a plane defined by the linear dipole antenna (73 m long tip to tip) at the time of the pulse and the direction of the Earth's magnetic field B. At ntp the protons produce a field which simulates the original field of the RF pulse, and, if wave propagation in the plasma is possible, a pulse is produced which propagates to the new position of the antenna which has moved in the plasma at the satellite velocity Vs. For f > fH, the perpendicularly propagating electron Bernstein waves are responsible for the propagation. No such Bernstein waves exist for f < fH, and proton echoes can be observed at these frequencies if Vs is approximately in the plane of the energized protons at ntp so that wave propagation is not necessary to produce a signal that appears as an echo. For the second factor above, as the wave frequency approaches the electron cyclotron frequency, the group velocity of the Bernstein waves approaches zero, and consequently the propagation time to the antenna increases. This change in group velocity explains the observed increase in tp, which can be as much as several percent.