bluuebunny/crossref_metadata_2025 huggingface dataset:
Potential impact of iodine on tropospheric levels of ozone and other critical oxidants
Authors: Davis D. , Crawford J. , Liu S. , McKeen S. , Bandy A. , Thornton D. , Rowland F. , Blake D.
Published: February 2004

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low‐productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one‐dimensional model simulations, these three source scenarios resulted in estimated Ix (Ix = I + IO + HI + HOI + 2I2O2 + INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O3 destruction estimated. Total column O3 destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ix source scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging processes.
Regional and seasonal variations in the flux of oceanic carbon monoxide to the atmosphere
Authors: Bates Timothy S. , Kelly Kimberly C. , Johnson James E. , Gammon Richard H.
Published: February 2004

Carbon monoxide (CO) is produced photochemically in the surface ocean and emitted to the atmosphere. To assess the magnitude of this ocean‐atmosphere flux, seawater and atmospheric CO mole fractions were measured on six cruises throughout the Pacific Ocean from 1987 to 1994. The results showed consistent regional and seasonal variations in surface seawater CO concentrations with daily averaged concentrations ranging from 0.1 to 4.7 nM. Based on the concentration fields, the data were divided into four seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean‐Atmosphere Data Set wind and surface seawater temperature data and theWanninkhof[1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from 0.25 to 13 μmol/m2/d. The combined seasonal and zonal fluxes result in a total global flux of 0.46 Tmol CO/y with 2/3 of this flux in the southern hemisphere. The estimated uncertainty in this number is approximately a factor of 2.
Measurements of methane and nitrous oxide distributions by the improved stratospheric and mesospheric sounder: Retrieval and validation
Authors: Remedios J. J. , Ruth S. L. , Rodgers C. D. , Taylor F. W. , Roche A. E. , Gille J. C. , Gunson M. R. , Russell J. M. , Park J. , Zipf E. C. , Erdman P. W.
Published: February 2004

The improved stratospheric and mesospheric sounder (ISAMS) used the pressure modulation technique to observe infrared emissions from the atmosphere at 7.4 μm and 7.8 μm. The target gases, methane and nitrous oxide, both emit at these wavelengths and so their concentrations have been determined by joint retrieval from the available signals. This paper describes the measurement technique, the current retrieval scheme (version 10), and the data set produced. The version 10 data set comprises typically 2600 profiles per day on 180 days between September 26, 1991, and July 29, 1992. Retrieved profiles extend in altitude from 7 mbar to 0.08 mbar for methane and from 7 mbar to 0.8 mbar for nitrous oxide. The precision of the data is better than 20% over much of the range and estimated systematic uncertainties are less than 30%. Comparisons with coincident measurements show that the systematic uncertainties are a reasonable estimate for the methane data but reveal a much larger positive bias for the nitrous oxide data relative to other measurements. Accounting for aerosol contamination effects and a priori biases, ISAMS methane data are recommended for use in scientific studies at altitudes between 7 mbar (5 mbar in the tropics) and 0.1 mbar. ISAMS nitrous oxide data are recommended for use between 7 mbar and 1.0 mbar where relative rather than absolute values are required.
Meteorological conditions associated with vertical distributions of aerosols off the west coast of Africa
Authors: Fuelberg Henry E. , VanAusdall Jonathan D. , Browell Edward V. , Longmore Scott P.
Published: February 2004

Vertical distributions of aerosol backscattering were obtained on Transport and Atmospheric Chemistry Near the Equator‐Atlantic (TRACE A) flights parallel to the west coast of Africa using the airborne differential absorption lidar (DIAL) instrument. Aerosol distributions on the flight of October 15, 1992 (from 22°S to 5.5°S) exhibit strong horizontal and vertical gradients. The top of the aerosol layer ranges from 3.5 to 5.7 km above sea level, while its thickness ranges from 1.4 to 4.5 km. The greatest aerosol loading generally occurs near 4.0‐ to 4.5‐km altitude between 8° and 12°S. Meteorological conditions are found to exert a major influence on the aerosol distributions. Dropsonde data along the flight track indicate numerous temperature inversions and stable layers in each sounding. The top of the aerosol region is associated with strong inversions due to subsidence. Five‐day backward trajectories are calculated along the flight track at vertical intervals of 1 km using global meteorological analyses. Trajectories arriving at most locations of large aerosol loading originate over southern Africa, where biomass burning is occurring and deep surface‐based mixed layers are common. Conversely, the air with less aerosol loading originates over the Atlantic Ocean. The exception is the northernmost segment of the flight above 3.5 km. Although this segment receives flow off Africa at these altitudes, lower level stable layers inhibit transport to higher levels. In addition, trajectories arriving at this part of the flight pass over a portion of Africa with reduced biomass burning and extensive deep convection that penetrates the stable layers.
High‐resolution ammonium ice core record covering a complete glacial‐interglacial cycle
Authors: Fuhrer Katrin , Neftel Albrecht , Anklin Martin , Staffelbach Thomas , Legrand Michel
Published: February 2004

High‐resolution ammonium measurements were performed along the Greenland Ice Core Program (GRIP) deep ice core, covering a complete climatic cycle. No overall anthropogenic increase is observed over the last 300 years; however, springtime concentrations have roughly doubled since 1950. Biomass burning is estimated to be a major source for ammonia emissions for preindustrial times. It contributes between 10% to 40% to the total ammonium deposited on the central Greenland ice sheet during the Holocene. No correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern North America, which is considered to be the main source area for ammonium deposited on the central Greenland ice sheet. This suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core. If unchanged meteorological conditions are assumed for the Holocene, as indicated by the δ18O ice record, a decreasing biomass burning activity toward present time can be derived from the ammonium ice record. Soil and vegetation emissions are responsible for the ammonium background concentrations in the ice. The record therefore may be used to trace back the biomass history of the North American continent. A pronounced decreasing trend in background ammonium is found during the Holocene, reflecting decreasing temperature and therefore lower NH3emissions in the source region. Variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength, which can be related to the extent of the Laurentide ice sheet. The data suggest that the Laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age.
Source‐oriented model for air pollutant effects on visibility
Authors: Eldering A. , Cass G. R.
Published: February 2004

A source‐oriented model for air pollutant effects on visibility has been developed that can compute light scattering, light extinction, and estimated visual range directly from data on gas phase and primary particle phase air pollutant emissions from sources. The importance of such a model is that it can be used to compute the effect of emission control proposals on visibility‐related parameters in advance of the adoption of such control programs. The model has been assembled by embedding several aerosol process modules within the photochemical trajectory model previously developed for aerosol nitrate concentration predictions by Russell et al. [1983] and Russell and Cass [1986]. These modules describe the size distribution and chemical composition of primary particle emissions, the speciation of organic vapor emissions, atmospheric chemical reactions, transport of condensible material between the gas and the particle phases, fog chemistry, dry deposition, and atmospheric light scattering and light absorption. Model predictions have been compared to observed values using 48‐hour trajectories arriving at Claremont, California, at each hour of August 28, 1987, during the Southern California Air Quality Study. The predicted fine particle concentration averages 62 μg m−3 compared to an observed value of 61 μg m−3, while predicted PM10 concentrations average 102 μg m−3 compared to an observed average of 97 μg m−3. The size distribution and chemical composition predictions for elemental carbon, sulfate, and sodium ion agree with observations to within plus or minus a few micrograms per cubic meter, while ammonium and nitrate concentrations are underpredicted by the base case model by 3 to 7 μg m−3 on average. Light‐scattering coefficient values are calculated from the predicted aerosol size distribution and refractive index, and the model predictions agree with measured values on average to within 19%. The advantages and limitations of the modeling procedure are discussed.
Regionalization and spatial properties of Ceará State rainfall in northeast Brazil
Authors: Uvo Cíntia , Berndtsson Ronny
Published: February 2004

The overall daily rainfall pattern of Ceará for 19 years, 1974–1992, was analyzed in terms of physiography and atmospheric circulation. This information was used to suggest a regionalization of the state's rainfall into homogeneous areas. The three large‐scale precipitation mechanisms (Intertropical Convergence Zone, cold fronts, and upper air vortices) interact and create a certain rainfall pattern over the state during different months. An example of this interaction is the opposite behavior that the northern and southern parts of the state tend to have when considering the amount of precipitation during the rainy season; That is, in general, large amounts of rain in the southern part of the state tend to be associated with small amounts of rain in the northern part and vice versa, as influenced by the opposite influence of the cold fronts and the upper air vortices. At the same time, the orographical influence enhances the precipitation in the northern part of the state and decreases precipitation in the central part of the state. The state could be divided into six homogeneous precipitation regions: (1) the northwestern part of the state, locally influenced by the mountain massif of Serra da Ibiapaba; (2)–(3) the northern and northeastern Ceará influenced locally by sea breeze and the slopes of the Serra de Baturité; (4)–(5) the central state, influenced by the Chapada do Apodi and the Serra de Uruburetama; and (6) the southern part of the state, influenced mostly by cold fronts and by the slopes of the Serra do Araripe.
Satellite observations of thermospheric tides: Results from the Wind Imaging Interferometer on UARS
Authors: McLandress Charles , Shepherd Gordon G. , Solheim Brian H.
Published: February 2004

Thermospheric winds measured by the Wind Imaging Interferometer (WINDII) on the upper atmosphere research satellite are analyzed for migrating solar tides. The data cover a 2‐year period commencing February 1992 and are obtained from the atomic oxygen O(1S) 557.7‐nm emission, which provides observations of the 90‐ to 200‐km altitude range during daytime and the 90‐ to 110‐km range at night. The subtropical lower thermosphere is dominated by the diurnal propagating tide which exhibits a vertical wavelength of approximately 22 km, grows in amplitude up to 95 km, and decays rapidly above where molecular diffusion greatly reduces the vertical shears. Although the phase remains fairly uniform throughout the year, a pronounced semiannual oscillation is observed in the diurnal tide amplitude. At both 20°N and 20°S the meridional and zonal wind components attain their maximum values at equinox of approximately 70 and 40 m/s, respectively, while the solstitial minima are nearly a factor of 2 smaller. At 35°N the diurnal tide semiannual amplitude oscillation is still present in the lower thermosphere, but above 100 km it is replaced by an annual cycle with a maximum in July and August. This contrasts with 35°S where the July/August peak is absent and the semiannual oscillation extends to 110 km. At midlatitudes the zonal and meridional winds are of similar magnitude, and no significant hemispheric asymmetries in amplitudes are observed. In the lower thermosphere the semidiurnal tide amplitude exhibits an annual oscillation, with maximum values of 30 to 40 m/s occurring in June/July near 100 km at 35°N, 35°S, and the equator. A bimodal structure in the seasonal variation of the semidiurnal phase is observed. This feature is characterized by rapid equinoctial transitions and is particularly well defined at the equator. Examination of the equatorial middle thermosphere indicates that the semidiurnal tide attains its maximum amplitude at 140 km and exhibits a vertical wavelength of approximately 60 km. These findings indicate the predominance of the antisymmetric (2,3) Hough mode in the tropics.
Measurements of polar vortex air in the midlatitudes
Authors: Newman Paul A. , Lait L. R. , Schoeberl M. R. , Seablom M. , Coy L. , Rood R. , Swinbank R. , Proffitt M. , Loewenstien M. , Podolske J. R. , Elkins J. W. , Webster C. R. , May R. D. , Fahey D. W. , Dutton G. S. , Chan K. R.
Published: February 2004

The Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE) was conducted in the spring of 1993 from Moffett Field, California (NASA Ames Research Center), utilizing the NASA high‐altitude ER‐2 aircraft. These northern midlatitude aircraft flights showed laminae containing high ozone concentrations, traceable to the April 1993 polar vortex breakup and corroborated by laminae of other trace gases such as CFCs, CH4, N2O, and CO2. These laminae are clearly traceable as polar vortex breakup fragments using Rossby‐Ertel's potential vorticity and isentropic trajectory calculations. Laminae in stratospheric ozone profiles are commonly observed in the northern hemisphere from fall to spring, and are hypothesized to originate from very low frequency transverse waves, and/or via Rossby wave breaking. On the basis of these results, the ozone laminae observed during SPADE were a result of Rossby wave breaking during the breakdown of the polar vortex. In addition, it is shown that conventional once‐per‐day meteorological analyses were adequate for representing the transport of this material into the lower stratosphere midlatitudes over the course of the spring vortex breakup.
Meteorological analysis of tropospheric ozone profiles at Bermuda
Authors: Merrill John T. , Moody Jennie L. , Oltmans Samuel J. , Levy H.
Published: February 2004

As part of the North Atlantic Regional Experiment (NARE) a number of ozonesonde profiles were obtained from Bermuda during the spring and summer of 1993. We present meteorological case studies of two instances of elevated O3mixing ratio in the middle and upper troposphere which took place during the summer. The ozonesonde profile of July 9, 1993, indicated ozone mixing ratios exceeding 100 parts per billion by volume (ppbv) from ∼5 km to above 15 km above sea level. A series of profiles for August 2–4 indicated elevated ozone in a layer which initially extended from 8 to 9 km and thickened to cover the 8‐ to 14‐km range. Isentropic trajectories lead back to potential source areas over North America 2 to 5 days prior to the events. Objective analyses based on radiosonde data demonstrate that the air masses, as indicated by the trajectories, pass through areas of elevated isentropic potential vorticity located in troughs in the tropopause‐level geopotential height field. The conformation and temporal development suggest that active planetary wave breaking was responsible for stratosphere‐troposphere exchange in these systems. These results suggest an important role for O3of stratospheric origin in the western North Atlantic Ocean area, transported into the troposphere upstream over or near North America. It is notable that these events took place in the summer months of July and August, when such events had been considered unlikely.
Stratosphere‐troposphere exchange in a midlatitude mesoscale convective complex: 1. Observations
Authors: Poulida Olga , Dickerson Russell R. , Heymsfield Andrew
Published: February 2004

On June 28, 1989, a severe thunderstorm over North Dakota developed into a squall line and then into a mesoscale convective complex (MCC) with overshooting tops as high as ∼14 km and a cirrus anvil that covered more than 300,000 km2. In this paper we describe the trace gas concentrations prior to, in, and around the storm; paper 2 presents numerical simulations. Observations of O3 and θeq unaffected by upstream convection for at least 3 days prior to the flights placed the undisturbed tropopause between 10.7 and 11 km. The anvil outflow, sampled at altitudes of 10.8 to 12.2 km, extended well into what used to be the stratosphere. Air inside the anvil was characterized by notably low concentrations of O3 and high CO relative to the out‐of‐cloud environment. Elevated concentrations of NO and NOy, due to lightning and upward transport, were observed in the anvil. A tongue of air with tropospheric characteristics lay above stratospheric air, showing that extensive stratosphere‐troposphere exchange had occurred. The effects of this mechanism on atmospheric budgets of trace species depend on the fate of the air that enters the anvil and on the frequency of MCCs. Assuming that the symmetry was cylindrical and that the material transported during the observations at the east edge of the anvil was representative of the entire cirrus anvil cloud, we estimate a minimum flux of 2 × 1010 g of O3 into the troposphere and a maximum flux of 3–7 × 1013 g of H2O into the stratosphere. This is a greater flux of water than the stratospheric water budget can support, and thus most of this water must return to the troposphere; the ice crystals were of sufficient size to have substantial settling velocity. If, however, even a small fraction of the mass of such anvils remains in the stratosphere, then convective transport of reactive tropospheric trace species such as NOy, CO, and NMHC may dominate the chemistry of the lower stratosphere in this midlatitude region. More detailed estimates of the fluxes, taking into account the rear anvil as well, are presented in the companion paper.
Tropospheric ozone derived from TOMS/SBUV measurements during TRACE A
Authors: Fishman Jack , Brackett Vincent G. , Browell Edward V. , Grant William B.
Published: February 2004

The use of data from Total Ozone Mapping Spectrometer (TOMS) and Solar Backscatter Ultraviolet (SBUV) measurements to develop daily maps of the amount of ozone in the troposphere during Transport and Atmospheric chemistry Near the Equator‐Atlantic (TRACE A) is presented. By comparing these maps with available ozone measurements from sensors aboard the NASA DC‐8, we show that the agreement between the satellite‐derived measurements and the other data is generally very good. The TOMS/SBUV technique successfully captures large‐scale gradients and the derived integrated tropospheric ozone amount is generally within 10–15% of the observed amounts. The largest discrepancy between the two data sets occurs when copious amounts of lower‐tropospheric (1–6 km) aerosols (from intense widespread biomass burning) are present. The amount of ozone found over the TRACE A region for the September–October 1992 period agrees well with the climatological values published previously using TOMS and Stratospheric Aerosol and Gas Experiment (SAGE) measurements and with ozonesonde measurements from several tropical sites established as part of TRACE A. The satellite‐derived ozone maximum observed during TRACE A is ∼1000 km north of the previously published climatological data and it is not obvious whether or not this northward shift is due to normal interannual variability, or if it is a result of systematic data processing differences between the SAGE and the SBUV data sets.
Measurements and steady state calculations of OH concentrations at Mauna Loa Observatory
Authors: Eisele F. L. , Tanner D. J. , Cantrell C. A. , Calvert J. G.
Published: February 2004

In situ OH measurements were made as part of the spring and summer intensives of Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2). These measurements provided an opportunity to compare measured OH concentrations with those predicted by steady state models which use the many other measurements which made up MLOPEX 2 as input parameters. These models thus provide a highly specific real‐time view of the environment in which OH was being measured. Both short‐ and long‐term variations of OH are discussed along with their relation to changes in sources and sinks. In general, agreement between measured and modeled OH concentrations is good during periods when free tropospheric air masses, typically downslope, were present. During periods when certain types of upslope boundary layer air dominated the measurement site, however, measured OH concentration averaged about a factor of 2 lower than the model predictions. Possible reasons for the latter differences are suggested, such as unidentified sinks in the boundary layer air.
Tunable diode laser absorption measurements of H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment
Authors: Mackay G. I. , Karecki D. R. , Schiff H. I.
Published: February 2004

The tunable diode laser absorption spectrometric technique was employed to measure H2O2 and HCHO during the Mauna Loa Observatory Photochemistry Experiment 2 (MLOPEX 2) fall and winter intensives. Comparisons were made between the measurements made with the Tunable Diode Laser Absorption Spectrometer (TDLAS) technique and several other methods. The half‐hourly average mixing ratios varied between 1300 parts per trillion by volume (pptv) and the instrument detection limit of 50 – 100 pptv for H2O2 and between 400 pptv and the instrument detection limit ∼25 pptv for HCHO. Daily average mixing ratios for H2O2 were 550 ± 79 pptv (fall) and 540 ± 80 pptv (winter), and for HCHO were 130 ± 21 pptv (fall) and 200 ± 35 (winter). HCHO showed strong diurnal behaviour with maxima in the 1200 – 1700 time period whereas H2O2 showed essentially no diurnal variation. Clean tropospheric air sampled during downslope conditions between 2000 and 0600 had HCHO mixing ratios averaging 80 pptv in the fall and 160 pptv in the winter. These values are considerably lower than model predictions.
Longitudinal dunes on Mars: Relation to current wind regimes
Authors: Lee Pascal , Thomas Peter C.
Published: February 2004

Longitudinal dunes are extremely rare on Mars, but constitute a substantial fraction of terrestrial desert dunes. We report finding isolated examples of longitudinal dunes on Mars and relate their occurrence to expected sand transport regimes. Terrestrial longitudinal dunes form in bimodal and multimodal transport regimes. General circulation models and streak data indicate that bimodal and multimodal transport of sand should be very rare on Mars. Thus the dearth of longitudinal dunes on Mars is consistent with their apparent formation conditions on Earth.
Interpreting gravity, geoid, and topography for convection with temperature dependent viscosity: Application to surface features on Venus
Authors: Moresi Louis , Parsons Barry
Published: February 2004

While Venus and Earth are similar in size, Venus does not appear to have developed the same system of mobile surface plates which dominates convective heat transfer from the interior of Earth. Numerical models with temperature dependent viscosity do not produce plate boundaries and are more closely reminiscent of Venus than Earth. We demonstrate that many puzzling aspects of surface features on Venus are also found in the analysis of the surface observables in these models. Despite having no strong lithosphere, our calculations exhibit very high geoid/topography ratios and uncompensated short‐wavelength topography. Surface heat flux values are not required to be very different from Earth‐scaled values, and the mantle remains below the likely solidus.
Raman spectroscopy as a method for mineral identification on lunar robotic exploration missions
Authors: Wang Alian , Jolliff Bradley L. , Haskin Larry A.
Published: February 2004

The sharp, nonoverlapping Raman bands for plagioclase, pyroxene, and olivine would be advantageous for on‐surface, active mineralogical analysis of lunar materials. A robust, light‐weight, low‐power, rover‐based Raman spectrometer with a laser exciting source, entirely transmission‐mode holographic optics, and a charge‐coupled device (CCD) detector could fit within a <20 cm cube. A sensor head on the end of an optical fiber bundle that carried the laser beam and returned the scattered radiation could be placed against surfaces at any desired angle by a deployment mechanism; otherwise, the instrument would need no moving parts. A modern micro‐Raman spectrometer with its beam broadened (to expand the spot to 50‐μm diameter) and set for low resolution (7 cm−1 in the 100–1400 cm−1 region relative to 514.5‐nm excitation), was used to simulate the spectra anticipated from a rover instrument. We present spectra for lunar mineral grains, <1 mm soil fines, breccia fragments, and glasses. From frequencies of olivine peaks, we derived sufficiently precise forsterite contents to correlate the analyzed grains to known rock types and we obtained appropriate forsterite contents from weak signals above background in soil fines and breccias. Peak positions of pyroxenes were sufficiently well determined to distinguish among orthorhombic, monoclinic, and triclinic (pyroxenoid) structures; additional information can be obtained from pyroxene spectra, but requires further laboratory calibration. Plagioclase provided sharp peaks in soil fines and most breccias even when the glass content was high.
Ionization cross‐sections for the production of positive ions from H2O by electron impact
Authors: Rao M. V. V. S. , Iga I. , Srivastava S. K.
Published: February 2004

Cross‐section values for the direct and dissociative ionization of H2O by electron impact have been measured and compared with previously published data. The present measurements have been carried out from thresholds to 1 keV by utilizing a crossed electron beam and molecular beam collision geometry and an improved ion extraction technique. To the best of our knowledge, the present results for the double ionization of H2O are the first measurements.
Line profile of H Lyman α from dissociative excitation of H2 with application to Jupiter
Authors: Ajello Joseph M. , Kanik Isik , Ahmed Syed M. , Clarke John T.
Published: February 2004

Observations of the H Lyman α (Ly α) emission from Jupiter have shown pronounced emissions, exceeding solar fluorescence, in the polar aurora and equatorial “bulge” regions. The H Ly α line profiles from these regions are broader than expected, indicating high‐energy processes producing fast atoms as determined from the observed Doppler broadening. Toward understanding that process a high‐resolution ultraviolet (UV) spectrometer was employed for the first measurement of the H Ly α emission Doppler profile from dissociative excitation of H2 by electron impact. Analysis of the deconvolved line profile reveals the existence of a narrow central peak of 40±4 mÅ full width at half maximum and a broad pedestal base about 240 mÅ wide. Two distinct dissociation mechanisms account for this Doppler structure. Slow H(2p) atoms characterized by a distribution function with peak energy near 80 meV produce the peak profile, which is nearly independent of the electron impact energy. Slow H(2p) atoms arise from direct dissociation and predissociation of singly excited states which have a dissociation limit of 14.68 eV. The wings of H Ly α arise from dissociative excitation of a series of doubly excited states which cross the Franck‐Condon region between 23 and 40 eV. The profile of the wings is dependent on the electron impact energy, and the distribution function of fast H(2p) atoms is therefore dependent on the electron impact energy. The fast atom kinetic energy distribution at 100 eV electron impact energy spans the energy range from 1 to 10 eV with a peak near 4 eV. For impact energies above 23 eV the fast atoms contribute to a slightly asymmetric structure of the line profile. The absolute cross sections of the H Ly α line peak and wings were measured over the range from 0 to 200 eV. Analytic model coefficients are given for the measured cross sections which can be applied to planetary atmosphere auroral and dayglow calculations. The dissociative excitation process, while one contributing process, appears insufficient by itself to explain the line broadening observed at Jupiter.
Monte Carlo interior models for Uranus and Neptune
Authors: Marley Mark S. , Gómez Percy , Podolak Morris
Published: February 2004

To explore a greater realm of model phase space than has previously been mapped, we generate randomly chosen interior models for Uranus and Neptune. The models are constrained by the observed mass, rotation rate, radius, and gravitational moments of the two planets. New models with a composition gradient in the outer third of the planetary radius are found for both planets. Such composition gradients may play a significant role in controlling the internal heat flow. In agreement with previous models, we find that the pressure‐density relation for the central third of both planets is consistent with either ice or a mixture of materials with a similar equation of state. While the range of allowed models is now somewhat greater, the principal conclusions drawn from traditional interior modeling techniques are robust.
Gravity wave drag in three‐dimensional atmospheric models of Mars
Authors: Joshi M. M. , Lawrence B. N. , Lewis S. R.
Published: February 2004

Gravity waves are disturbances whose restoring force is buoyancy. In the atmosphere they are made possible by the variation of potential temperature with height. They can be excited by various mechanisms including flow over orography. The momentum carried by these disturbances can exert significant drag forces on the mean flow in the terrestrial mesosphere. One parameterization of this effect, the Lindzen gravity wave drag scheme, is inserted into two simplified general circulation models (GCMs) of the Martian atmosphere, and its effects are described. The influence of large‐amplitude, planetary‐scale topography on wave breaking altitudes is discussed. We show that above 60 km in altitude, gravity wave drag dominates the momentum balance of the Martian atmosphere on the largest scales. This indirectly alters the thermal state of the atmosphere through the thermal wind relation. Model winds at this level are consistent with other, simpler studies, as well as with Earth‐based spectroscopic observations. Below 60 km, the effects of planetary wave‐mean flow interaction, as well as heating associated with Hadley circulation descent, are significant in determining the wind strength. Eddy diffusion coefficients derived from the Lindzen parameterization above 50 km are consistent with values implied from chemical modeling of the Martian middle atmosphere, as well as with simpler models of gravity wave drag.
Gravitational and topographic isotropy of the Earth, Moon, Mars, and Venus
Authors: Bills Bruce G. , Lemoine Frank G.
Published: February 2004

The internal structures and surface shapes of the terrestrial planets are expected to be dominantIy isotropic. We attempt to test this hypothesis by developing a criterion for recognizing statistical isotropy of spherical scalar fields and applying that test to the nonhydrostatic components of the gravitational and topographic fields of the Earth, Moon, Mars, and Venus. The criterion for topographic or geoidal surfaces is that the directional variance should be independent of azimuth. That is, the mean square east‐west slopes should be equal to the mean square north‐south slopes. All four planets generally pass the criteria, with a few minor exceptions. Most of the exceptions are likely to be artifacts of the current generation of planetary gravity and topography models rather than real features of the planets. Of the several broad‐band departures from isotropy which we found, the only one likely to be real is the tendency for the Earth's gravity and topography to have only ∼80% as much variance in a north‐south direction as in an east‐west direction.
Composition and exposure age of the Apollo 16 Cayley and Descartes regions from Clementine data: Normalizing the optical effects of space weathering
Authors: Fischer Erich M. , Pieters Carlé M.
Published: February 2004

In order to quantitatively assess the composition of the lunar surface from reflectance spectroscopy, it is necessary to be able to differentiate between the optical effects due to composition and those due to exposure to space weathering processes. Laboratory analyses of returned lunar soils are used to show that limited quantitative compositional information and the exposure age of the lunar surface can be determined using straightforward calculations from remotely acquired reflectance data sampled with a small number of spectral channels. Exposure age, approximated by Is/FeO (where Is is the characteristic ferromagnetic signal from single‐domain iron metal), is estimated using a ratio of the reflectance at a wavelength outside of the 1‐μm ferrous iron absorption band to the reflectance at a wavelength within the band. The iron plus titanium content of a soil can then be calculated from its Is/FeO and reflectance. These combined techniques are applied to Clementine UVVIS multispectral data of the lunar highlands in the vicinity of the Apollo 16 landing site. The Is/FeO and FeO + TiO2 contents are calculated for this region. Materials associated with recently formed impact craters and recent and/or ongoing downslope movement are easily detected as nonmature. The FeO + TiO2 content of the Descartes Mountains is found to be 1–2 wt% less than the Cayley plains west of the Apollo 16 landing site. Localized areas of compositional heterogeneity associated with some small impact events are also revealed. A model for normalizing the optical contribution due to exposure to space weathering processes is applied to the Clementine UVVIS data in order to bring the data to an equivalent mature exposure state. This approach allows residual spectral differences between the observed materials to be confidently interpreted as being due to compositional differences alone rather than to differences both of composition and of exposure. The exposure‐normalized data reveal that this region of the highlands is characterized by limited large‐scale compositional heterogeneity that is detectable by a small number of spectral bandpasses. The Descartes Mountains are identified as being more anorthositic than the Cayley plains units. Most small impacts in the region excavated material compositionally similar to material surrounding them, with the notable exceptions of the impacts that created South Ray Crater and an unnamed crater on the floor of Abulfeda.
Electron density fluctuations in the nightside Venus ionosphere: Role of gravity waves
Authors: Singhal R. P.
Published: February 2004

A one‐dimensional, two‐ion (O+, O2+) model is described for studying fluctuations in electron density, electron temperature, and magnetic field produced by atmospheric gravity waves in the low‐altitude (<200 km) ionosphere of Venus just past the terminator (solar zenith angle 100°). Plasma velocity and self‐consistent horizontal magnetic field are calculated utilizing the observed data on plasma densities and temperatures. Effect of the Hall and Pedersen conductivity on the gravity wave dispersion relation is investigated. A gravity wave of time period 33 min and horizontal wavelength 700 km is considered for the case study. Amplitude and phase of electron density, electron temperature, and magnetic field fluctuations relative to the fluctuations in neutral atomic oxygen density are calculated. Results are discussed in the light of observed wave structure and correlations in these parameters. It is found that the fluctuations observed in the ionospheric and magnetic parameters in the nightside ionosphere of Venus can be produced by internal gravity waves.
Statistical characteristics of extrasolar planetary transits
Authors: Heacox William D.
Published: February 2004

Extrasolar planets can be detected photometrically by their transits across the parent star, in a small percentage of systems fortuitously oriented with respect to the line of sight. This paper develops statistical models to connect observed transit statistics to the underlying planetary system characteristics. Monte Carlo simulations of the solar system provide a guide for transit surveys of solar‐type stars: about 1.2% of all systems will show an individually detectable transit, usually within 1 year; 0.07–0.14% will show transits of two or more planets; 7–10% of all systems showing a large terrestrial planet transit will subsequently show a gas giant transit. Transit statistics for large numbers of planetary systems can reveal characteristics important to formation: statistical distributions of orbital semimajor axes and planetary sizes, mean relative orbital inclination, and mean number of planets per system. Orbital eccentricities of the order of those characterizing the solar system will have little discernible effect on transit statistics.
A comparative study of electron cooling rates in the Venus ionosphere during magnetized and unmagnetized conditions
Authors: Mahajan K. K. , Upadhyay H. O. , Ghosh Suchita
Published: February 2004

When the solar wind dynamic pressure (Psw) is high, the “top” of the Venus ionosphere (defined by the altitude where the steep density gradient begins) moves down to altitudes between 200 and 300 km. Thick ionopauses are formed in the region between 200 and 400 km and here the electron density decreases with a scale height of about 20 km. Further, these regions get permeated by strong horizontal magnetic fields. However, during conditions of low Psw, the “top” of the ionosphere moves to higher altitudes and the region between 200 and 400 km is nearly free of magnetic fields. This region then forms a part of the “main” ionosphere, where electron density decreases with a scale height of about 200 km. In this paper, we study the electron cooling processes and their rates in the region between 200 and 400 km for these two conditions, namely, (1) when this region is fully magnetized and (2) when this region is unmagnetized. We use Langmuir probe measurements of electron density and electron temperature from the Pioneer Venus Orbiter for these studies. We find that the dominant electron cooling process in the ionopause region is due to electronic excitation of the ground state of O to the O(1D) level. In the main ionosphere, the major processes are cooling due to the fine structure of O, and electron ion Coulomb collisions. Further, the cooling rates are lower in the ionopause region than in the ionospheric region. Only if thermal conduction is assumed to be inhibited in the presence of a horizontal magnetic field, the magnetized orbits can then be used to estimate heating rates by equating these to cooling rates (i.e., local equilibrium). Under this assumption, we find that heating rates so estimated are far smaller than those used by several workers in the heat balance models. However, these rates are closer to model values which include inhibition of photoelectron transport in the presence of a horizontal magnetic field.
Paleo sea surface salinities in the low‐latitude Atlantic: The δ18O record of Globigerinoides ruber (white)
Authors: Wang L. , Sarnthein M. , Duplessy J.‐C. , Erlenkeuser H. , Jung S. , Pflaumann U.
Published: February 2004

On the basis of the comparison of δ18O values of Globigerinoides ruber (white) (δ18OG. ruber) from modern sediments and measured sea surface temperatures (SST) and salinity (SSS), δ18OG. ruber values most clearly record summer SST and SSS of the uppermost 50 m of water in the low‐latitude Atlantic. A new transfer equation is presented for estimating local summer paleo SSS at 0–50 m depth using δ18OG. ruber, paleo SST, and the global δ18O ice effect of sea water, with the standard error reaching ±0.72 ‐ ±0.77‰ SSS. The equation was applied to a 35 kyr long δ18OG. ruber record and paleo SST estimates based on planktonic foraminiferal assemblages to reconstruct the paleo SSS changes along the east Atlantic margin off the northwest Sahara. The resulting local SSS show a general increase by 0.3–1.4‰ during the last glacial maximum (LGM) and large parts of glacial Termination I, suggesting a general increase in the upwelling of the highly saline North Atlantic Central Water and/or a reduced lateral advection of less saline Canary Current water. Two short salinity lows (up to 0.35‰ less than today) during and after the LGM are coeval with the Heinrich meltwater events 1 and 2 in the North Atlantic, indicating the breakdown of coastal upwelling and meltwater advection from the north. Both events are followed by an extreme SSS maximum (38.15‰) along with the restoration of the salinity conveyor belt. At 9.0–5.5 ka the salinity minimum (up to 0.65‰ less than today) reflects the continental runoff linked to the phase of strong North Saharan humidity during the early Holocene climatic optimum.
Has climate changed the Earth's tilt?
Authors: Rubincam David Parry
Published: February 2004

Have the ice ages secularly altered the obliquity (axial tilt) of the Earth over geologic time? The waxing and waning of ice caps in response to obliquity oscillations plus mantle adjustment to the weight of the caps alter the Earth's dynamical flattening. This affects the lunar and solar torques on the Earth so as to secularly change the Earth's axial tilt. This mechanism is dubbed “climate friction,” since analogous to tidal friction, it arises from lags in the Earth's response to cyclic external forcing. But the existence of two processes, ice cap waxing and waning and mantle viscous flow, can lead to either an increase or decrease in the obliquity. Evidence indicates that the growth and decay of the ice caps greatly lag the orbital forcing; this causes the axial tilt to increase with time. But the ice cap effects are partly canceled by viscous compensation in the mantle. Low mantle viscosities (about 1021 Pa·s) lead to rapid compensation and have only a slight effect on obliquity. High viscosities (about 1022 Pa·s) slow the compensation enough so that there could be significant secular change in the obliquity over geologic time, perhaps explaining all of the present tilt of 23.5°. However, in addition to the current uncertainty as to the effective viscosity of the mantle, knowledge of past ice ages is incomplete, so that the amount of obliquity increase presently remains unknown.
Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ13C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau
Authors: McCorkle Daniel C. , Martin Pamela A. , Lea David W. , Klinkhammer Gary P.
Published: February 2004

Core‐top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong‐Java Plateau (1.6–4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ13C and δ18O). We also measured bottom water δ13C, Cd, and Ba at these sites. Foraminiferal δ13C values remain roughly constant over the entire depth range while bottom water δ13C values increase slightly, such that the δ13C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about −0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ13C values is comparable to the uncertainty in published δ13C calibration studies. We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core‐top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution‐driven decrease in benthic foraminiferal δ13C values.