bluuebunny/crossref_metadata_2025 huggingface dataset:
Radium isotopes in the Orinoco estuary and eastern Caribbean Sea
Authors: Moore Willard S. , Todd James F.
Published: February 2008

Radium isotopes provide a means of identifying the source of freshened waters in the ocean and determining the time elapsed since these waters were in the estuary. We present evidence that during April, waters from the Amazon mixing zone pass within 50 km of the mouth of the Orinoco River. These Amazon waters are characterized by a lower 228Ra/226Ra activity ratio (AR) than are waters from the Orinoco at similar salinities. During autumn, the increased discharge of the Orinoco displaces the freshened Amazon waters seaward, yet the two can be distinguished clearly. Within the Caribbean Sea, waters of Orinoco origin carry a characteristic radium signature including excess activities of 224Ra. This isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments. Current speeds based on 224Ra dating ranged from 15 to 33 cm/s during April. The radium isotopes also provide an assessment of sediment mixing in the estuary. During low discharge (April), considerable mixing of older sediment by physical or biological processes or dredging maintained high activities of 228Ra in the estuary and produced the highest 228Ra/226Ra AR's yet measured in any estuary. During high discharge (September), a large fraction of the 228Ra was derived from desorption from fresh sediment rather than mixing of older sediments. Activities of 224Ra were high in the estuary during both high and low discharge, indicating that considerable mixing of recently introduced sediment must occur during each period. The high 224Ra/228Ra AR's measured in September imply that the same sediments were being resuspended during high discharge, and that there was little exposure of older sediments which had been stored in the estuary for months to years. During April, 224Ra and 228Ra activities in the water were about equal, indicating that most of the sediment being resuspended had been stored in the estuary long enough to reestablish radioactive equilibrium in the 232Th decay series (i.e., 20 years).
Synoptic aspects of Antarctic mesocyclones
Authors: Carleton Andrew M. , Fitch Mark
Published: February 2008

The characteristic regimes (formation and dissipation areas, tracks) and synoptic environments of cold air mesocyclones over Antarctic and Subantarctic latitudes are determined for the contrasting winters (June, July, and August) of 1988 and 1989. Defense Meteorological Satellite Program (DMSP) thermal infrared (IR) imagery is used in conjunction with southern hemisphere pressure/height analyses. Outbreaks of mesocyclones (“active periods”) are frequent in the Ross Sea sector in 1988. They are associated most often with areas of maximum horizontal gradient of the 1000‐ to 500‐mbar thickness. Over higher latitudes of the Southeast Pacific in 1989, mesocyclones develop in association with a “cold pool” that migrates equatorward. The between‐winter differences in mesocyclone frequencies are examined for associations with sea ice conditions and the continental katabatic winds using correlation and “superposed epoch” analysis of temperature data from selected automatic weather stations (AWSs). The results support a katabatic wind‐sea ice extent‐mesocyclone link for key sectors of the Antarctic.
Inferring middle atmospheric ozone height profiles from ground‐based measurements of molecular oxygen emission rates: 2. Comparison with O2(1Δg) (0,1) band measurements at sunset
Authors: Sica R. J. , Lowe R. P.
Published: February 2008

The ability to routinely acquire measurements of the ozone density profile in the mesosphere and lower thermosphere is important for use in chemical‐dynamical models of the middle atmosphere. Zenith measurements of the O2(1Δg) (0,1) band emission rate in the evening twilight were acquired near the spring equinox of 1982 with a Michelson interferometer from London, Ontario, Canada. Knowledge of the change of the O2(1Δg) (0,1) band emission rate at twilight can be related directly to ozone density, since ozone destruction by sunlight is the primary source of O2(1Δg) in the atmosphere. Measurements and calculations have shown that a secondary peak in the ozone density often exists in the middle atmosphere. A model has been developed to infer the ozone profile in the middle atmosphere by simultaneously solving the time‐dependent chemistry of the molecular oxygen atmospheric and atmospheric‐IR bands and O(1D) during twilight. Calculations are presented which show the effect of a secondary peak in the ozone density at various heights on the O2(1Δg) (0,1) band emission rate during twilight. The model is used to demonstrate that the London measurements are consistent with an ozone profile with a secondary peak at 85–90 km.
Diurnal variations of the mobility spectrum of ions and size distribution of fine aerosols in the atmosphere
Authors: Dhanorkar Savita , Kamra A. K.
Published: February 2008

Measurements of mobility spectra of atmospheric ions with mobility between 3.37 × 10−4 and 6.91 × 10−8 m2/(V s), have been made at a height of 1 m above ground at different times of day at Pune (18° 32′ N, 73° 51′ E, 559 m above msl), India. Measurements made for a period of two days only are presented here. Observations demonstrate the presence of all three groups of ions: small, intermediate and large, each having a distinct peak in mobility at all times of day. Diurnal variations of the mobility spectrum show that ion concentrations in all the mobility ranges increase during nighttime and attain their maximum values in the early morning hours. Mobility spectra at these early morning hours suggest the possibility of the presence of ions of mobility even larger than that of 3.37 × 10−4 m2/(V s). Size distribution of atmospheric aerosol particles in the radius range of 0.0023 > r > 0.03 μm, as computed from the mobility spectra, are bimodal in shape throughout the day on either of the two days. One peak that occurs in the nuclei mode, is always observed at 0.003μm, while the position of the other peak which occurs in accumulation mode, changes between 0.01 and 0.03 μm, at different times of the day. The peak in accumulation mode occurs at larger radius when the peak in nuclei mode is higher. Diurnal variation of the concentration of particles ( 0.03 > r > 0.0063 μm) in accumulation mode exhibits single periodicity with a peak in the early morning and that of particles (0.002 < r ≤ 0.0063 μm) in the nuclei mode exhibits double periodicity with the first peak in the afternoon and the second in the early morning. The observations suggest that high concentrations of particles in the nuclei mode at Pune may be due to photochemical activity in the afternoon and to the presence of decay products of radon and the aerosol particles formed by the radiolytic process in the early morning.
Electric field measurements with an airplane: Problems caused by emitted charge
Authors: Jones J. J. , Winn W. P. , Han F.
Published: February 2008

The temporal variation of electric field components deduced from measurements made with airplanes penetrating electrified clouds is often complex, especially when the airplane experiences strong electrical charging. However, unusually simple electric field variations were obtained for penetrations involving severe charging of an airplane on nights over Kennedy Space Center, Florida, on August 19, 1989. Analysis of these results suggests that plumes of ions were emitted from the airplane. The electric field from these plumes was more intense than the ambient field from the cloud at the location of the aft electric field sensor. As a consequence the deduced component of the ambient electric field in the direction of flight was severely distorted. These findings emphasize (1) the importance of careful evaluation of electric field data obtained with airplanes and (2) the need for improved measurements.
The 200‐ to 300‐nm radiation field in the stratosphere: Comparison of models with observation
Authors: Kylling A. , Stamnes K. , Meier R. R. , Anderson D. E.
Published: February 2008

The SABE 1 experiment (Herman and Mentall, 1982) measured the direct and scattered solar radiation at wavelengths between 190 and 320 nm from a balloon floating at 40 km altitude. We use state of the art radiative transfer models together with cross sections and solar flux data commonly used in modeling of the stratosphere to calculate the direct and scattered radiation at 40 km. The comparison between theory and experiment made by Herman and Mentall (1982) is extended. Significant discrepancies in the ratio of the scattered to direct flux not discussed in their work were found in the Schumann‐Runge band and above 280 nm. Possible reasons for the differences are discussed.
In situ evidence of rapid, vertical, irreversible transport of lower tropospheric air into the lower tropical stratosphere by convective cloud turrets and by larger‐scale upwelling in tropical cyclones
Authors: Danielsen Edwin F.
Published: February 2008

The STEP tropical objectives were successfully met during the flight experiments conducted from Darwin, Australia, January 16 to February 16, 1987. Necessary and sufficient measurements were made in, above, and downwind from very cold cirrus clouds, produced by three convective cloud types, to demonstrate irreversible mass transports into and dehydration in the lower tropical stratosphere. The three types are defined and described in terms of the physical processes that produce them and illustrated by examples derived from in situ and remote measurements. Intense solar heating is shown to produce, in addition to the usual vertical, sea breeze circulations normal to the coastline, an unusual pair of continental spanning, horizontal circulations. An upper tropospheric‐lower stratospheric anti cyclonic circulation, inclined upward toward the tropics, contributes to the dehydration of dissipating cirrus anvils and intensifies the upper level, tropical easterlies. The lower tropospheric cyclonic circulation with tropical westerlies and extratropical easterlies is in direct conflict with the normal tropical easterlies and extratropical westerlies. Impulsive switches between these two opposing lower‐level wind systems create conditions favorable for each of these cloud types and explain the summer season's aperiodic variability.
Albedos for a cloud bar array over a reflecting surface
Authors: Smith T. F. , Ehlert J. R.
Published: February 2008

The purpose of this study is to develop a model for the albedo of a cloud bar array over a reflecting surface. The model is based on the discrete‐ordinates method for evaluation of radiant exchange between surfaces separated by a radiatively transparent medium. The model assumes that the clouds are opaque and accounts for the cloud geometry, the separation between the clouds, the height of the clouds above the ground, and the ground reflectance. The cloud bar array is irradiated by direct and diffuse components of solar energy. Cloud albedos are greater than the ground reflectance for ground surfaces with low reflectances and are less than the ground reflectance for highly reflecting ground surfaces but may be greater than the albedo for a plane layer cloud. In view of these findings, care must be exercised to account properly for the effects of cloud geometry and ground reflectance when cloud albedos are determined.
Multiple diffraction of obliquely incident plane waves by a three‐part impedance plane
Authors: Büyükaksoy Alinur , Uzgören Gökhan
Published: February 2008

Uniform asymptotic high‐frequency solution is developed for the diffraction of obliquely incident plane waves by a three‐part impedance plane. The multiple interaction up to third order between the edges of the three‐part impedance plane is obtained to yield a uniform scattered field. The employed formulations are based on a spectral iteration techique consisting of taking the Fourier integral representation of the n.tuply diffracted field by one junction as the incident field for the two‐part plane where the (n+1)th diffraction will occur. Upon using the field components normal to the three‐part surface, the (n+1)th diffracted field is obtained through the solution of a pair of uncoupled Wiener‐Hopf equations. The contribution of the surface wave fields to the secondary as well as triply diffraction is included in the analysis.
Radar reflectivity of the clear air at wavelengths of 5.5 and 33 cm
Authors: Rogers R. R. , Knight C. A. , Tuttle J. D. , Ecklund W. L. , Carter D. A. , Ethier S. A.
Published: February 2008

Collocation of a 5.5‐cm weather radar and a 33‐cm wind profiler during the Hawaiian Rainband Project of 1990 presented several opportunities to compare vertical profiles of reflectivity in the atmosphere at the two wavelengths. On three occasions the beams of both systems were pointed vertically as light rain drifted overhead. The atmospheric reflectivity structure was then dominated by rain at low altitudes but included an elevated layer at the height of the trade wind inversion. Allowing for minor uncertainties in radar sensitivity and range, we found close agreement in the patterns of reflectivity factor Z in the rain as measured by the two systems. The reflectivity factor of the inversion echo at 33 cm then exceeded that at 5.5 cm by approximately 29 dB on average, an amount predicted by the theory of scattering by spatial fluctuations in the clear‐air refractive index. These observations are consistent with the assumptions that (1) the inversion echo is a clear‐air echo, uncontaminated by raindrops or other Rayleigh scatterers and (2) the reflectivity of the inversion at both wavelengths is explained by Bragg scattering from refractive index fluctuations induced by mixing in the inertial subrange of homogeneous turbulence.
RING: An integral equation algorithm for HF‐VHF radio wave propagation over irregular, inhomogeneous terrain
Authors: Ott R. H.
Published: February 2008

A modification in the mathematical derivation of an integral equation for propagation over irregular, inhomogeneous terrain and a numerically stable algorithm, RING, is developed to extend the frequency range of program WAGNER to frequencies in the VHF region. The solution to the integral equation is compared to residue theory and Fock theory and excellent agreement is obtained.
A note on oblique spaced antenna method
Authors: Kudeki Erhan , Rastogi Prabhat K. , Sürücü Fahri
Published: February 2008

Biases in oblique spaced antenna (OSA) horizontal wind estimates due to flow field inhomogeneities are shown to preclude reliable estimation of the vertical component of wind vorticity with the OSA method.
Fate of methane in the Hudson River and Estuary
Authors: de Angelis Marie A. , Scranton Mary I.
Published: February 2008

Methane (CH4) concentrations and oxidation rates were measured throughout the Hudson River Estuary in March and August of 1991. Methane concentrations ranged from 50 to 940 nM and were supersaturated with respect to the atmosphere along the entire length of the river, with generally higher CH4 values in the lower, saline portion of the estuary. A seasonally averaged diffusive flux to the atmosphere from the Hudson River was estimated to be 5.6 mg CH4 m−2 d−1, corresponding to an annual flux of 0.76 × 109 g CH4. The Hudson River Estuary also releases approximately 0.2 × 109 g CH4 annually to nearshore marine waters. Diffusive flux across the air/river interface was the dominant removal mechanism for Hudson River CH4 in March. In August, CH4 oxidation was the dominant CH4 sink in freshwater and brackish (<6‰) sections of the river, removing up to 70% of ambient CH4 per day compared to maximum daily removal rates of 13% in March. Thus methane oxidation can play a major role in limiting releases of CH4 to the atmosphere from rivers and other freshwater environments. Methane oxidation activity decreased rapidly as salinity increased, with less than 2% of ambient CH4 being oxidized per day at salinities greater than 25‰. Addition of NaCl or seawater to freshwater samples resulted in comparable inhibition of methanotrophic activity. Budget calculations showed that flux to the atmosphere and CH4 oxidation in March removed less water column CH4 than was supplied to the Hudson River over its entire length. In August, however, CH4 removal approximately equaled CH4 supply with the result that there was no net accumulation of CH4 over the length of the river.
Evidence for proton cyclotron waves near comet Giacobini‐Zinner
Authors: Tan L. C. , Mason G. M. , Tsurutani B. T.
Published: February 2008

We have computed frequency spectra of power density and polarization parameters of magnetohydrodynamic waves from observations on board the ICE spacecraft as it flew past comet Giacobini‐Zinner on September 11, 1985. Since the spectral parameters are frequency dependent, we find that the analysis is best carried out in a ‘wave’ reference frame where one of the major axes is along the wave normal direction for each frequency component. The power density along the wave normal direction shows a systematic peak structure which we identify as belonging to cyclotron wave harmonics of pick‐up ions (H+, H2+, and O+) near the comet. The fundamental harmonics of the cyclotron waves are also consistent with the gyrofrequencies calculated from the magnetic field data.
Diffuse upstream ions: A comparison between one‐ and two‐dimensional hybrid simulations
Authors: Kucharek H. , Fujimoto M. , Scholar M.
Published: February 2008

A comparison between one‐ and two‐dimensional hybrid simulations of a collisionless quasi‐parallel shock has been done in order to investigate the influence of the spatial dimensionality on the upstream and downstream spectra of the diffuse ions. The simulations are done for a shock Mach number of MA = 4.6 and a magnetic field ‐ shock normal angle of ΘBn = 10°. Within the same time period (t = 70Ωci−1) the diffuse ions reach in the one‐dimensional simulation, on average, higher energies than in the two‐dimensional simulation. This manifests itself in a considerably flatter upstream spectrum at the high energy end. At lower energies the intensity in the 2‐D case is an order of magnitude higher than in the 1‐D case. The spectral shape exhibits in the 2‐D case a more gradual transition from the solar wind thermal distribution to the diffuse distribution. In both simulations the particles drift about the same distance along the shock. The difference in spectra is due to a smaller motional electric field at the shock (in the shock frame) in the two‐dimensional case as compared to the one‐dimensional case.
External and internal influences on the size of the dayside terrestrial magnetosphere
Authors: Petrinec S. M. , Russell C. T.
Published: February 2008

ISEE‐1 and ‐2 observations of the position of the magnetopause during solar cycle 21 are used to determine quantitatively the role of both internal and external influences. We find that the magnetopause is a function of the direction of the IMF and the magnitude of the solar wind momentum flux but depends only weakly on the ring current intensity. The magnetopause stand‐off position is found to lie at a distance similar to that expected from theory for northward IMF, but is found to lie 0.5 Re closer to the Earth for southward IMF. When the position of the magnetopause is normalized for variations in the solar wind dynamic pressure, the magnetopause is found to move earthward 1 Earth radius for every 7.4 nT southward IMF, but remains at nearly a constant distance for northward IMF.
A conceptual model for the aperiodicity of interannual variability in the tropics
Authors: Krishnamurthy V. , Goswami B. N. , Legnani Roberto
Published: February 2008

We propose that the broad spectrum of interannual variability in the tropics with a peak around four years results from an interaction between the linear low frequency oscillatory mode of the coupled system and the nonlinear higher frequency modes of the system. We illustrate this using a conceptual model where the higher frequency modes, represented by a nonlinear low‐order model, are coupled to a low frequency linear oscillator.
Estimation of the size distribution of Pinatubo volcanic dust from Bishop's ring simulations
Authors: Asano Shoji
Published: February 2008

Since the middle of July 1991, approximately one month after the explosive eruptions of Mt. Pinatubo in the Philippines, Bishop's ring phenomena were occasionally observed over Japan under favorable conditions of clear skies. On the basis of the single‐scattering approximation, the chromaticity of Bishop's rings was simulated. It was found that Bishop's rings are observable only when a sufficient amount of aerosols exist that have an unusually narrow mono‐modal size distribution and a mode radius between 0.57μm and 0.63μm, with an effective variance of less than 0.05.
On the relation between ozone and potential vorticity
Authors: Allaart M. A. F. , Kelder H. , Heijboer L. C.
Published: February 2008

As ozone mixing ratio correlates well with potential vorticity, column integrated ozone (“total ozone”) should correlate with column integrated potential vorticity. Daily maps of these fields are analyzed, and it is shown that this correlation holds well, notably during the winter and spring. During a few short periods however, the correlation breaks down.
Metastable He 1083 nm intensities in the twilight: A reconsideration
Authors: Bishop James , Link Richard
Published: February 2008

The 1083 nm airglow emission arising from the resonant scattering of solar photons by metastable He(2³S) atoms is potentially valuable for remote sensing of total photoelectron fluxes and neutral helium densities at altitudes above the F2 peak. In this study, we adopt the diffusion picture used in all previous studies and point out that even within this crude approach, intensities consistent with the winter observations (≳1000 R) collected in Tinsley and Christensen [J. Geophys. Res., 81, 1253, 1976] are obtained when the correct parameter values for He(2³S) production, loss, and transport are applied and line‐of‐sight geometries are properly taken into account.
Hybrid simulations of collisionless ion tearing
Authors: Hesse Michael , Winske Dan
Published: February 2008

Ion tearing in a thin collisionless neutral sheet is investigated using hybrid (particle ion, massless fluid electron) simulations in which dissipation effects are included through a model of the full electron pressure tensor in which self‐consistently generated anisotropies are reduced via an isotropization time factor. The linear growth rates and ion dynamics are in qualitative agreement with linear theory and one‐component simulations. The reconnection rate increases with the isotropization time.
Triassic paleomagnetic results from the Yangtze Block, S.E. China
Authors: Dobson Jon Paul , Heller Friedrich
Published: February 2008

In an effort to contribute to the refinement of the APWP for the Yangtze Block, samples for paleomagnetic analyses were recovered from early Triassic sediments in Hunan Province in southeast China. The samples were taken from opposite limbs of a large syncline in order to apply the fold test and rock magnetic experiments were performed to determine the carriers of remanence. However, instead of refining the APWP, the results present some problems.

While the sites pass the fold test, the declinations deviate clockwise with respect to the reference APWP for the block but agree well with other reported results. Due to the paucity of reliable Triassic results from the Yangtze Block and to the small number of sites from this study, it is difficult to determine whether these sites have been rotated or the APWP is incorrect. Both possibilities are explored.
Atmospheric energy input and ionization by energetic electrons during the geomagnetic storm of 8–9 November 1991
Authors: Chenette D. L. , Datlowe D. W. , Robinson R. M. , Schumaker T. L. , Vondrak R. R. , Winningham J. D.
Published: February 2008

The Atmospheric X‐ray Imaging Spectrometer (AXIS) of the Particle Environment Monitor investigation aboard the Upper Atmosphere Research Satellite monitors energy input to the upper atmosphere due to energetic electrons. Analysis of the AXIS data from the major geomagnetic storm of 8–9 November 1991 is presented. During the November storm, electrons above a few keV flowing into a substantially expanded auroral zone provided the bulk of the ionizing power to the upper atmosphere. At the peak of the disturbance the total AXIS‐observed power reached 40 GW. On 9 November the whole day average atmospheric ionization rate in the auroral zone at 80 km altitude exceeded the rate due to solar UV and solar x‐rays by a factor of over 10 to 100.
Measurements of strain at plate boundaries using space based geodetic techniques
Authors: Robaudo Stefano , Harrison Christopher G. A.
Published: February 2008

We have used the space based geodetic techniques of Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) to study strain along subduction and transform plate boundaries and have interpreted the results using a simple elastic dislocation model. Six stations located behind island arcs were analyzed as representative of subduction zones while 13 sites located on either side of the San Andreas fault were used for the transcurrent zones. The length deformation scale was then calculated for both tectonic margins by fitting the relative strain to an exponentially decreasing function of distance from the plate boundary. Results show that space‐based data for the transcurrent boundary along the San Andreas fault help to define better the deformation length scale in the area while fitting nicely the elastic half‐space earth model. For subduction type boundaries the analysis indicates that there is no single scale length which uniquely describes the deformation. This is mainly due to the difference in subduction characteristics for the different areas, and the use of an elastic stress model then helps us understand some of these differences.
Wide‐angle imaging of reflectors in Mesozoic oceanic crust
Authors: Minshull Timothy A.
Published: February 2008

Two‐ship synthetic aperture data from Mesozoic oceanic crust in the western North Atlantic contain wide‐angle reflections and refractions from oceanic layer 2 at the longer offsets (6–9 km). With appropriate velocity filtering to remove cross‐cutting reflections from the sediment column, the wide‐angle data may be stacked to produce images of the upper oceanic crust. Some dipping events are imaged both near normal incidence and at wide‐angle, and are most likely due to zones of anomalously high impedance. Such zones may be generated by hydrothermal alteration of material in fault zones generated near the spreading center.
An evaluation of the rate of absorption of solar radiation in the O2(X3Σg→b1Σg) transition
Authors: Mlynczak Martin G.
Published: February 2008

The rate at which molecular oxygen absorbs radiation in the O2(X3Σg→b1Σg) transition is calculated using a line‐by‐line radiative transfer model. This rate is critical to the determination of the population of the O2(b1Σg) state required for studies of the O2(b1Σg→X3Σg) dayglow, the O2(a1Δg→X3Σg) dayglow, and possibly the rates of oxidation of H2 and N2O. Previous evaluations of this rate (which is sometimes called the g‐factor) have significantly overestimated its value. The rate is tabulated as a function of altitude, pressure, and solar zenith angle.
Ozone absorption cross section measurements in the Wulf bands
Authors: Anderson Stuart M. , Hupalo Peter , Mauersberger Konrad
Published: February 2008

A tandem dual‐beam spectrometer has been developed to determine ozone absorption cross sections for 13 selected wavelengths between 750 and 975 nm at room temperature. The increasingly pronounced structure in this region may interfere with atmospheric trace gas transitions that are useful for remote sensing and complicate the measurement of aerosols. Ozone concentrations were determined by absorption at the common HeNe laser transition near 632.8 nm using the absolute cross section reported previously. The overall accuracy of these room temperature measurements is generally better than 2%. A synoptic near‐IR spectrum scaled to these measurements is employed for comparison with results of previous studies.
Electrical conductivity of polycrystalline olivine containing a silicate glass
Authors: Wanamaker B. J. , Duba A. G.
Published: February 2008

The complex impedance of a polycrystalline sample of San Carlos olivine including 5 vol% of a synthetic silicate glass containing sodium, magnesium, and iron has been measured as a function of temperature under controlled oxygen fugacity at a.c. frequencies of 10², 10³, and 104 Hz. Below ∼800°C, the conductivity of the sample is at least 5 to 10 times higher than that of single crystal and polycrystalline San Carlos olivine, respectively, and has an activation energy half that of the single crystal. The sample conductivity increases dramatically as a function of the measurement frequency. Hysteresis and time‐dependent changes in sample conductivity are observed above the liquidus of the glass (∼900°C). The data suggest that the electrical conductivity of partially molten samples is characterized by parallel conduction of the solid and melt phases and is a sensitive indicator of melt distribution as well as the kinetics of melt re‐equilibration.
Sensitivity of supersonic aircraft modelling studies to HNO3 photolysis rate
Authors: Jones A. E. , Bekki S. , Pyle J. A.
Published: February 2008

In the last few years the possibility of a second generation of supersonic aircraft flying mainly in the stratosphere has been discussed. This, and the increasing number of longhaul subsonic aircraft flying in the lower stratosphere, has caused the issue of possible ozone depletion due to nitrogen oxides emitted in the aircraft exhaust gases to be re‐opened. Model calculations have indicated that significant ozone loss could occur if a large, economically viable fleet of supersonic aircraft were to be built. However, the results are sensitive to a number of assumptions and also to uncertainties in photochemical data. We consider the sensitivity with respect to HNO3 photolysis rates, which are dependent upon the assumed photochemical data. There is also considerable variability between models in the calculated photolysis rates. If temperature dependent absorption cross sections for HNO3 are used in model simulations of supersonic aircraft exhaust impact, the calculated ozone loss at high latitudes is significantly reduced, and an ozone increase rather than a decrease is calculated for low to mid‐latitudes. The result emphasizes our current uncertainty about the impact of future supersonic aircraft on ozone chemistry and, more generally, about processes operating in the lower stratosphere.
Counterstreaming suprathermal electron events upstream of corotating shocks in the solar wind beyond ∼2 Au: Ulysses
Authors: Gosling J. T. , Bame S. J. , Feldman W. C. , McComas D. J. , Phillips J. L. , Goldstein B. E.
Published: February 2008

Enhanced fluxes of suprathermal electrons are commonly observed upstream of corotating forward and reverse shocks in the solar wind at heliocentric distances beyond ∼2 AU by the Los Alamos plasma experiment on Ulysses. The average duration of these events, which are most intense immediately upstream from the shocks and which fade with increasing distance from them, is ∼2.4 days near 5 AU. These events are caused by the leakage of shock‐heated electrons into the upstream region. The upstream regions of these shocks face back toward the Sun along the interplanetary magnetic field, so these leaked electrons commonly counterstream relative to the normal solar wind electron heat flux. The observations suggest that conservation of magnetic moment and scattering typically limit the sunward propagation of these electrons as beams to field‐aligned distances of ∼15 AU. Although it seems unlikely that these shock‐associated events are an important source of counterstreaming events near 1 AU, remnants of the backstreaming beams may contribute importantly to the diffuse solar wind halo electron population there.
On the interaction of isotropic pickup ions with oblique Alfvén waves
Authors: Isenberg Philip A.
Published: February 2008

We consider the oblique Alfvén wave instability for isotropic shells of pickup ions which was introduced by Wu and Yoon. We provide some further physical insight into this instability but show that the results published to date have not been correct. Our analysis indicates that the instability disappears when the thickness of the shell in velocity space becomes comparable to the average radius of the shell. Thus the generation of these waves will not lead to a filling‐in of the shell.
Lifetime measurements of auroral scatterers
Authors: Hall G. E. , Andre D. , Danskin D. W. , Sofko G. J. , Koehler J. A.
Published: February 2008

A method for directly measuring the lifetimes of auroral radar scatterers from continuous wave time series is introduced. This method depends on the sharp phase and/or amplitude changes which occur between individual scattering bursts. Six different periods, with examples of type I, type III and type IV radar auroral echoes, were examined to find scatterer lifetimes. Two main observations were noted. Frequently, there were scatter signal bursts with no phase or amplitude discontinuities, indicating the presence of only a single coherent scattering region or a small number of such regions. Second, the lifetimes of the scatterers were strongly anticorrelated with their Doppler speeds.
Laboratory experiments on ion waves associated with parallel electric fields
Authors: Jin Z. , Hamila J. , Allen R. C. , Meassick S. , Chan C.
Published: February 2008

We have performed laboratory experiments to study the property of waves along a quasi‐static parallel electric field in a plasma column. The parallel electric field is setup by allowing two different sources of plasmas to interpenetrate each other. Since all ions accelerated by the electric field will gain the same energy across the potential drop, the lighter ion species will acquire higher velocities. The wave modes associated with a single ion species (Ar+) and two ion species (He+ and Ar+) are quite different. In the plasma with only one species, the fast mode of the ion‐electron acoustic waves is observed in our experiment. This mode is always damped, and the results are analyzed by the theory. When the plasma consists of two ion species, the linear theory of ion acoustic waves indicates that the velocity difference between two ion species of different masses can excite ion‐ion acoustic waves via the two‐stream instability. Experimentally, unstable waves generated by the costreaming He+ and Ar+ ions are found to exist. By actively launching waves near the unstable frequencies at the plasmas, we observe that the waves grow at a certain range of distance. The results are quantitatively explained with the kinetic dispersion relations for the ion acoustic waves.
A kinetic model of the plasma sheet: Isotropic nonuniform plasma temperature
Authors: Voronina V. A. , Kan J. R.
Published: February 2008

A steady state kinetic model of the plasma sheet is formulated without the electric field. The electron and ion distribution functions in the model are two‐component Maxwellians. The bi‐Maxwellian distribution function is needed to allow the temperature to be nonuniform. The present model of the plasma sheet is constructed based on the exact constants of the particle motion. Therefore the ion and electron dynamics, including the nonadiabatic particle behavior in the plasma sheet, are fully self‐consistent with the magnetic field of the model. The magnetic field configuration and the ion temperature predicted by the model are in good agreement with observations in the plasma sheet under quiet conditions. The number density and the current density in the model are monotonically increasing toward Earth, which makes the present model inapplicable to the inner edge of the plasma sheet.
Are existing magnetospheric models excessively stretched?
Authors: Peredo Mauricio , Stern David P. , Tsyganenko Nikolai A.
Published: February 2008

The distribution of the average north‐south magnetic field component 〈Bz〉 in the vicinity of the neutral sheet has been investigated. This component is crucial for mappings between the nightside polar ionosphere and the equatorial magnetosphere. Data sets consisting of ∼0.5 RE averages of magnetic field observations by the IMP/HEOS and ISEE spacecraft have been compared to the field predicted by the Tsyganenko models T87 and T89. In agreement with recent studies by C. Huang (data with 4‐s resolution), it was found that both T87 and T89 underestimate 〈Bz〉 in the near tail region (xGSM = −10 to −22 RE) by as much as a factor of 2. Modified versions of the T87 model, incorporating plasma sheet warping, were obtained by fitting the model parameters via nonlinear least squares to the ISEE data set and yielded 〈Bz〉 values in agreement with the ISEE data. The study reveals an enormous scatter among the observed baseline values of 〈Bz〉 (on a time scale of 10‐20 min), as well as intrinsic biases imposed by the mathematical structure of tail models, and these two factors (especially the first one) greatly limit the accuracy of model predictions of tail 〈Bz〉. In order to reduce the second factor, a localized tail model has been developed and was used to explore the spatial variations of 〈Bz〉. Comparisons between observations and models, both global and local, are presented. Finally, suggested guidelines for use of existing models and limitations of their use are discussed.
Temporal variations in the termination shock distance
Authors: Suess S. T.
Published: February 2008

A kinematic analysis is made of solar wind driven temporal variations in the heliospheric termination shock distance. This has become possible because the large‐scale dynamics of, and temporal variations in the distant solar wind are now well enough known from a combination of in situ and remote measurements. Conversely, nothing is known of the corresponding properties in the local interstellar medium, and hence these will be ignored. Given specific assumptions for how the termination shock responds to solar wind fluctuations, it is shown that the termination shock is very agile, moving in and out by up to a few AU in a month. This conclusion holds for a broad range of shock response assumptions. Because a spacecraft moves slowly in comparison to the termination shock, the shock will sweep back and forth over the spacecraft once the shock is first encountered.
Heliospheric hydrogen beyond 15 AU: Evidence for a termination shock
Authors: Hall D. T. , Shemansky D. E. , Judge D. L. , Gangopadhyay P. , Gruntman M. A.
Published: February 2008

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman α radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman α intensities in the upstream direction fall as r−0.75±0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r−1.07±0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. Radiative transfer calculations using the hot H model predict that upstream intensities should fall more rapidly as a function of heliocentric distance than downstream intensities, precisely opposite to the observed trends. The Voyager H Lyman α intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r−1, whereas between 30 and 35 AU they fall as r−0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond ≈25 AU. A simple analysis suggests that the density distribution changes from nearly uniform between 15 and 20 AU, to r0.65 dependence between 30 and 35 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman α observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.
Detection of the local interstellar cloud from high‐resolution spectroscopy of nearby stars: Inferences on the heliospheric interface
Authors: Bertin P. , Lallement R. , Ferlet R. , Vidal‐Madjar A.
Published: February 2008

High‐resolution spectroscopic observations of the local interstellar medium between the Sun and nearby stars have been recorded with the Aurelie spectrometer of the Observatoire de Haute Provence (France). They have been used to identify and characterize the local interstellar cloud (LIC) in which the Sun is embedded. The “Doppler triangulation” shows that the local cloud flows toward λ = 74.9°; β = −7.8° at a velocity of 25.7 ± 1.0 kms−1 with respect to the Sun. This is strongly confirmed by Hubble space telescope (HST) UV measurements toward the star Capella. Furthermore, the recent detection of interstellar neutral helium inside the heliosphere, found to flow at 26.0 ± 1.0 kms−1 with the Ulysses Neutral Gas Experiment, brings a definitive proof of this identification. The common interval for the local cloud temperature from these three different types of measurements is 7000 ± 200 K. As a consequence of the LIC velocity determination, the interstellar neutral hydrogen found to flow at 20 kms−1 through the solar system is shown to be decelerated by 6 kms−1 and slightly heated up to 8000 K due to coupling with the plasma at the heliospheric interface. Further high‐resolution spectroscopic observations with the HST‐Goddard high‐resolution spectrometer (provided the short wavelength part can be repaired) should supply the local interstellar electronic density. These informations are of crucial importance with respect to the understanding of the heliospheric cavity.
Lyman α observations as a possible means for the detection of the heliospheric interface
Authors: Quémerais Eric , Lallement Rosine , Bertaux Jean‐Loup
Published: February 2008

We first summarize our current knowledge of the flow of interstellar H and He gas through the solar system. Both come from the same direction, but the H velocity is 20 km/s, whereas the He velocity is 26 km/s as recently determined by Ulysses, identical to the velocity found for the local interstellar cloud (LIC) as recently determined from absorption lines detected in the spectrum of nearby stars. This velocity difference may be assigned to the deceleration of H atoms at the heliospheric interface through coupling with the partially ionized interstellar plasma. Seen from the inner solar system, the Lyman α emission pattern of H atoms (resonance scattering of solar photons) is quite compatible with a standard model including no interaction for H at the heliopause and therefore cannot be used to characterize such an interaction. We investigate three other types of Lyman α observations which could bear the signature of the heliopause. First, it is shown from Monte Carlo modeling of the interface perturbation that the Lyman α line profile (accessible through high resolution spectroscopy from Earth orbit) varies in a different way from upwind to downwind direction, whether there is a perturbation or not at the heliopause. Indeed, some Prognoz data show such a behaviour, and the potential of planned future observations is discussed (HST and SOHO). Second, since some interaction models predict a decrease of H density at heliopause crossing along the wind (or H increase when cruising upwind), the slope of the radial antisolar Lyman α intensity between 15 and 50 AU, characterized by γ, such as I ≈ k rγ, is computed for various models of H density. It is shown that (1) the value of γ recorded along downwind trajectories depends too much on solar parameters (radiation pressure and ionization) to be very useful to detect a departure from a standard model; (2) the value of γ along upwind trajectories is much less sensitive to solar parameters but radiative transfer of Lyman α in the interplanetary medium must absolutely be taken into account for a correct interpretation; (3) if taken at face value, the value γ = − 0.78 reported for Voyager data between 15 and 50 astronomical units cannot be explained with any standard model, and calls for an increase of density somewhere further upwind. A good fit is obtained with a doubling of the density at 54 astronomical units (hydrogen wall), but this is not a unique solution. Finally, the use of Lyman α intensity maps recorded at large distances (30 ‐ 50 AU) is considered. The shape of the map is different when a hydrogen wall is present, and this method is less prone to instrumental drift and solar Lyman α changes than the preceding one. The first method works whatever is the distance of the heliopause, whereas the two others require observations from upwind region, not too far from the heliopause (a few tens of astronomical units at most).
Upper bound estimates of anomalous ion production in space‐based critical ionization velocity experiments
Authors: Biasca Rodger , Hastings Daniel , Cooke David
Published: February 2008

The critical ionization velocity (CIV) is an anomalous ionization mechanism first proposed by Alfvén. Experiments have confirmed the existence of a critical velocity in laboratory experiments, but sounding rocket experiments have been ambiguous as to the existence of the critical velocity in the ionosphere. The purpose of this paper is to produce upper bound estimates of anomalous ion production in space‐based experiments of the critical ionization velocity. The analysis relies on the results of implicit particle‐in‐cell simulations and a simple rate model to predict the number of ions produced as a neutral cloud traverses a point in space. The model assumes a point release of neutral gas in the ionosphere which is meant to represent a typical sounding rocket experiment. The results of the model suggest why strong evidence of CIV is not observed in space‐based experiments. Space‐based experiments require the anomalous ionization process to be initiated through seed ionization provided by charge exchange or other mechanisms. This seed ionization process is too slow in space experiments to ignite CIV. The results also indicate that some of the space based results can be accounted for by assuming a large barium‐oxygen charge exchange cross section instead of invoking anomalous ionization mechanisms.
Effect of solar EUV variation on the nightside ionosphere of Venus observed on Langmuir Probe at solar maximum
Authors: Ghosh S. , Mahajan K. K. , Brace L. H.
Published: February 2008

The antisunward ion flow across the terminator is believed to be the principal source of nightside ionosphere at solar cycle maximum. The controlling factor for the nightward flow is the altitude of the terminator ionopause which is determined by the combined effect of solar wind pressure (Psw) and solar EUV flux. The long‐term effects of EUV flux are now known since the plasma density in the nightside ionosphere changes by an order of magnitude between solar maximum (SCmax) and solar minimum (SCmin). However, Brace et al. (1990) from Langmuir probe measurements on Pioneer Venus have observed a slight negative response of electron density (Ne) to solar EUV flux on the main nightside ionosphere at SCmax. By using the same measurements, we find that nightside ionosphere does exhibit the signature of day‐to‐day EUV variations, provided we can remove the effects of influencing parameters other than EUV flux. In our analysis we delete orbits which contain holes or have data gaps or correspond to high Psw conditions. We observe that main nightside ionosphere above 200 km shows positive response to variations in the solar EUV flux. This result supports the transterminator flow mechanism for the maintenance of the main nightside ionosphere. However, we also find that electron density below 200 km does not respond to solar rotation changes in EUV flux, a result consistent with Brace et al. (1990), who obtained a slight negative correlation. This indicates that the transterminator flow is not the sole source for the maintenance of the nightside ionosphere below 200 km.