bluuebunny/crossref_metadata_2025 huggingface dataset:
Cloud three‐dimensional effects evidenced in Landsat spatial power spectra and autocorrelation functions
Authors: Oreopoulos Lazaros , Marshak Alexander , Cahalan Robert F. , Wen Guoyong
Published: September 2002

An analysis of nadir reflectivity spatial Fourier power spectra and autocorrelation functions for solar wavelengths and cloudy conditions is presented. The data come from Landsat thematic mapper (TM) observations, while Monte Carlo (MC) simulations are used to aid the interpretation of the observations and to examine sensitivity to various factors. We show that shortwave radiative processes produce consistent signatures in power spectra and autocorrelation functions. Power spectra take a variety of shapes not shown or explained in previous observational studies. We demonstrate that TM spectra can potentially be affected by radiative “roughening” at intermediate scales (∼1–5 km) and radiative “smoothing” at small scales (<1 km). These processes are wavelength‐dependent, with systematic differences between conservative (for cloud droplets) TM band 4 (∼0.8 μm) and absorbing band 7 (∼2.2 μm). Band 7 exhibits more roughening and less smoothing than band 4 and faster decrease in autocorrelation. Roughening is more prevalent at large solar zenith angles due to optical and/or geometrical side illumination and shadowing. MC spectra illustrate that scale invariant optical depth fields can produce complex power spectra that take a variety of shapes under different conditions. Radiative roughening increases with decreasing single scattering albedo and increasing solar zenith angle (as in the observations). For low solar zenith angles, there is a clear shift in the radiative smoothing scale to smaller values as droplet absorption increases. Power spectra also show stronger decorrelations between optical depth and reflectivity when cloud top variations are more pronounced. Finally, it is shown that power spectral analysis is a useful tool for evaluating the skill of novel optical depth retrieval techniques in removing three‐dimensional radiative effects. New techniques using inverse nonlocal independent pixel approximation and normalized difference of nadir reflectivity yield optical depth fields which better match the scale‐by‐scale variability of the true optical depth field.
Airborne observations of dust aerosol over the North Atlantic Ocean during ACE 2: Indications for heterogeneous ozone destruction
Authors: de Reus Marian , Dentener Frank , Thomas Andreas , Borrmann Stephan , Ström Johan , Lelieveld Jos
Published: September 2002

Aerosol size distribution measurements have been performed in the free troposphere during the Second Aerosol Characterization Experiment (ACE 2) near Tenerife, Canary Islands, in July 1997. During one measurement flight, on July 8, a uniform aerosol layer was encountered between 2.5 and 5.5 km altitude, characterized by a relatively low Aitken mode particle number concentration and high concentrations of accumulation and coarse mode particles, resulting in a relatively large aerosol surface area and mass, which is estimated to be about 400 μg m−3. Five‐day backward trajectories indicate that the aerosol in this layer was mineral dust originating from arid regions on the North African continent. The dust layer was associated with reduced ozone mixing ratios. Model simulations have been performed with a photochemical box model including different heterogeneous removal reactions to study the interaction between gas phase chemistry and mineral aerosol. The best agreement between the observed and modeled ozone concentrations was obtained when heterogeneous removal of ozone and precursor gases on dust aerosol were taken into account. Heterogeneous O3 loss is estimated at 4 ppbv O3 per day. Although uncertainties concerning heterogeneous ozone removal remain, in particular related to the reactive uptake coefficient of O3, it is likely that the loss of O3 and precursor gases on mineral dust aerosol significantly reduces the O3 abundance in large‐scale dust plumes.
Stochastic and oscillatory forcing of global atmospheric angular momentum
Authors: Weickmann Klaus M. , Robinson Walter A. , Penland Cécile
Published: September 2002

The temporal variability and forcing of global atmospheric angular momentum (AAM) is studied using a three‐component Markov model derived from observed statistics of global AAM and the global torques. The model consists of stochastic forcing by the mountain (τM) and friction (τ *F) torque plus a pervasive negative feedback on AAM by the friction torque. AAM anomalies are damped at a 30‐day timescale and forced by torques having 1.5‐day ( τM) and 6‐day (τ *F) decorrelation timescales. A large portion of the intraseasonal variance and covariance of AAM, τM, and τF is accounted for by the Markov model. Differences between the modeled and the observed covariances are maximized in the 10‐ to 90‐day band and account for 10–30% of the variance when using data not stratified by season. An especially prominent deviation from the Markov model is the oscillatory forcing of AAM by the frictional torque at 30‐ to 60‐day periods. Additionally, there is greater coherent variance between τF and τM across the entire 10‐ to 90‐day band, with the frictional torque leading the mountain torque. This “feedback” between the global torques results from physical processes not represented in the Markov model. The synoptic characteristics of the stochastic mountain and frictional torques and of the oscillatory Madden‐Julian Oscillation are described.
An analysis of the pilot point methodology for automated calibration of an ensemble of conditionally simulated transmissivity fields
Authors: Cooley Richard L.
Published: September 2002

An analysis of the pilot point method for automated calibration of an ensemble of conditionally simulated transmissivity fields was conducted on the basis of the simplifying assumption that the flow model is a linear function of log transmissivity. The analysis shows that the pilot point and conditional simulation method of model calibration and uncertainty analysis can produce accurate uncertainty measures if it can be assumed that errors of unknown origin in the differences between observed and model‐computed water pressures are small. When this assumption is not met, the method could yield significant errors from overparameterization and the neglect of potential sources of model inaccuracy. The conditional simulation part of the method is also shown to be a variant of the percentile bootstrap method, so that when applied to a nonlinear model, the method is subject to bootstrap errors. These sources of error must be considered when using the method.
Hydrologic response of soils to precipitation at Carnation Creek, British Columbia, Canada
Authors: Fannin R. J. , Jaakkola J. , Wilkinson J. M. T. , Hetherington E. D.
Published: September 2002

The extreme hydrologic response of gravelly, sandy soils in the Carnation Creek watershed is examined from observations at 12 standpipe piezometers. The nearly continuous piezometric data are reported as a time series of monthly maximum readings. Ten locations of measurement appear to exhibit an upper limit to the pore water pressure head that is independent of rainfall intensity and duration. Two locations exhibit artesian pressures that appear directly influenced by rainfall characteristics and may last for several hours. We found the impact of individual storms to be highly variable. The spatial variation in hydrologic response is attributed to the influence of preferential flow paths in the soil matrix.
Characterizing multiple timescales of stream and storage zone interaction that affect solute fate and transport in streams
Authors: Choi Jungyill , Harvey Judson W. , Conklin Martha H.
Published: September 2002

The fate of contaminants in streams and rivers is affected by exchange and biogeochemical transformation in slowly moving or stagnant flow zones that interact with rapid flow in the main channel. In a typical stream, there are multiple types of slowly moving flow zones in which exchange and transformation occur, such as stagnant or recirculating surface water as well as subsurface hyporheic zones. However, most investigators use transport models with just a single storage zone in their modeling studies, which assumes that the effects of multiple storage zones can be lumped together. Our study addressed the following question: Can a single‐storage zone model reliably characterize the effects of physical retention and biogeochemical reactions in multiple storage zones? We extended an existing stream transport model with a single storage zone to include a second storage zone. With the extended model we generated 500 data sets representing transport of nonreactive and reactive solutes in stream systems that have two different types of storage zones with variable hydrologic conditions. The one storage zone model was tested by optimizing the lumped storage parameters to achieve a best fit for each of the generated data sets. Multiple storage processes were categorized as possessing I, additive; II, competitive; or III, dominant storage zone characteristics. The classification was based on the goodness of fit of generated data sets, the degree of similarity in mean retention time of the two storage zones, and the relative distributions of exchange flux and storage capacity between the two storage zones. For most cases (>90%) the one storage zone model described either the effect of the sum of multiple storage processes (category I) or the dominant storage process (category III). Failure of the one storage zone model occurred mainly for category II, that is, when one of the storage zones had a much longer mean retention time (ts ratio > 5.0) and when the dominance of storage capacity and exchange flux occurred in different storage zones. We also used the one storage zone model to estimate a “single” lumped rate constant representing the net removal of a solute by biogeochemical reactions in multiple storage zones. For most cases the lumped rate constant that was optimized by one storage zone modeling estimated the flux‐weighted rate constant for multiple storage zones. Our results explain how the relative hydrologic properties of multiple storage zones (retention time, storage capacity, exchange flux, and biogeochemical reaction rate constant) affect the reliability of lumped parameters determined by a one storage zone transport model. We conclude that stream transport models with a single storage compartment will in most cases reliably characterize the dominant physical processes of solute retention and biogeochemical reactions in streams with multiple storage zones.
Fractal identification of supercell storms
Authors: Féral Laurent , Sauvageot Henri
Published: September 2002

The most intense and violent form of convective storm is the supercell storm, usually associated with heavy rain, hail, and destructive gusty winds, downbursts, and tornadoes. Identifying a storm cell as a supercell storm is not easy. What is shown here, from radar data, is that when an ordinary, or multicell storm evolves towards the supercellular organization, its fractal dimension is modified. Whereas the fractal dimension of the ordinary convective storms, including multicell thunderstorms, is observed around 1.35, in agreement with previous results, the fractal dimension of supercell storms is found close to 1.07. This low value is due to the unicellular character of supercells. The present paper suggests that the fractal dimension is a parameter that should be considered to analyse the dynamical organization of a convective field and to detect and identify the supercell storms, either isolated or among a population of convective storms.
Relaxation of the shock wave‐reflected ion beam, and generation of hot electrons
Authors: Strokin N. A. , Indyukov A. E. , Kichigin G. N.
Published: September 2002

The results obtained with the collisionless shock wave (CSW) experiments on a laboratory device are presented. Ion beam relaxation regimes both in the region of main jump of the magnetic field (“ramp”) and in the zone occupied by the reflected ion flux are grouped into classes. Ahead of the magnetic front a reflecting hump of the electrostatic potential was recorded, which was formed by the ion beam reflected by potential jump in the magnetic ramp region. An electron acceleration effect in the ion‐ion interaction region ahead of the magnetic front to energies of about 100 initial electron temperatures was detected. Hot electrons propagate anomalously rapidly from this region across the magnetic field in the form of a localized disturbance. In the neighborhood of the ion reflection zone in the ramp region, a nonadiabatic heating of electrons to energies approaching the value of the electrostatic potential within the CSW was detected. Analysis of experimental results suggests that (1) the shock front structure is not in steady state and (2) the transverse magnetosonic shock wave in a collisionless plasma has the following common structural characteristics: hot electron region‐turbulent region, occupied by the reflected ion flux‐diffusion magnetic foot‐jump of plasma parameters and the magnetic field (ramp region)‐relaxation region behind the shock front. Implications of experimental results obtained in this study to the Earth's magnetosphere physics are discussed.
Jovian and terrestrial low‐frequency radio bursts: Possible cause of anomalous continuum
Authors: Kaiser M. L.
Published: September 2002

Observations by the Ulysses Unified Radio and Plasma Wave instrument show that the most intense portion of the Jovian continuum emission appears to emanate from the planet's bow shock or magnetosheath region. This intense component is highly correlated with the Jovian “type III” or quasi‐periodic (QP‐15 and QP‐40) bursts. I suggest that this intense continuum component may be the unresolved merging of the low‐frequency portion of the QP bursts and occasionally the low‐frequency extent of broadband kilometric emissions which have been scattered and dispersed in the magnetosheath. A similar, but much less dramatic, effect happens at Earth with the scattering of LF bursts.
Ion cyclotron waves in the Io torus: Wave dispersion, free energy analysis, and SO2+ source rate estimates
Authors: Huddleston D. E. , Strangeway R. J. , Warnecke J. , Russell C. T. , Kivelson M. G.
Published: September 2002

As the Galileo spacecraft passed through the Io torus, ion cyclotron waves were observed near the sulfur dioxide ion gyrofrequency. The torus plasma is continually replenished by the ionization of neutral particles from Io. It is well known that sulfur dioxide dissociates rapidly, so that the corotating torus plasma consists of predominantly sulfur and oxygen ions. However, for the small fraction of molecules that become ionized before dissociation, the appearance of SO2+ gyroresonant waves near Io indicates that the wave growth timescale (or wave‐particle scattering time) is short compared with the lifetime of these SO2+ ions. Newly created ions initially form “ring”‐type ion distributions which are highly unstable and generate the observed ion cyclotron waves. A warm plasma dispersion analysis finds that growth at the SO2+ gyrofrequency dominates over that at the O+ and S+ gyrofrequencies, partly because the ring energy scales with ion mass, but mainly due to the absence of a thermalized “background” component of SO2+ which would otherwise damp these waves. At the growth rate peak, the wave frequency is just below the SO2+ gyrofrequency (0.4 Hz) and the phase velocity is ∼55 km/s. A free energy analysis for wave‐particle scattering of ions toward a “bispherical” shell‐type distribution suggests that the SO2+ density in the torus falls off steeply with distance from Io wake values. These density estimates obtained from the observed wave power do not rely on assumptions of the exact plasma composition, but require that the SO2+ wave dominates the spectrum and is not strongly damped (as verified by the dispersion analysis). From our density estimates, we infer an ion production rate for SO2+ of ∼8×1026/s, representing the small fraction of sulfur dioxide that survives in molecular form long enough to be ionized and generate waves before dissociation occurs. This is consistent with 5% of the total ion source at the time of the Galileo flyby but is less than 3% of the widely accepted total torus supply rate of ∼3×1028 particles/s from Io.
A global oceanic sediment model for long‐term climate studies
Authors: Heinze C. , Maier‐Reimer E. , Winguth A. M. E. , Archer D.
Published: September 2002

A chemically reactive 10‐layer sediment module was coupled to a geochemical ocean general circulation model (the Hamburg Oceanic Carbon Cycle Model). The sediment model includes four solid sediment components (CaCO3, opal, organic carbon, and clay), and five pore water substances (dissolved inorganic carbon, total alkalinity, PO43−, O2, Si(OH)4) plus corresponding species containing 13C and 14C instead of 12C. The processes, namely, particle deposition, pore water reactions, pore water diffusion and interaction with the open water column, vertical sediment advection, sediment accumulation, and bioturbation, are simulated through basic parametrizations. For the water column part the Si and C cycles are coupled by a formulation of the “rain ratio” Si:C(CaCO3):C(POC), where POC is particulate organic carbon, in biogenic particle export production, with CaCO3 frustrule production growing in parallel to a weakening of opal production during progressing deficiency of dissolved silicate in the surface layer. For two preindustrial velocity fields the model reproduces major features of observed water column and sediment tracer distributions parallel to a correct preindustrial CO2 level close to 280 ppm. The model reacts sensitively to the formulation of the POC flux parametrization, the rain ratio, as well as the solubility of opal but is fairly insensitive to changes in the bioturbation rate as well as the amount of clay deposition. A simulation of the sediment distribution by use of a velocity field, which represents the ocean at conditions during the last glacial maximum, yields realistic glacial‐interglacial changes for the Atlantic Ocean, while discrepancies remain for the Indo‐Pacific region. A significant decrease of the atmospheric pCO2 could be achieved through an additional change of water column inventories by a change in weathering input of Si and alkalinity.
Slab weakening by the exothermic olivine‐spinel phase change
Authors: Brunet David , Machetel Philippe , Yuen David A.
Published: September 2002

Numerical modelling of mantle convection, with lateral variations of viscosity, and both the 670 km depth endothermic and the 400 km exothermic phase changes, reveals that downwellings are weakened upon crossing the exothermic phase change. A high temperature/low viscosity section is developed within the downgoing current, owing to both the olivine to spinel phase transition and the slowing down of the flow due to the 670 km endothermic transition. This mechanism may cause the creation of a weak zone at 400 km that results in a preferential location for horizontal or vertical slab tearing as revealed by seismic gaps in many slabs.
Optical characteristics of blue jets produced by runaway air breakdown, simulation results
Authors: Yukhimuk V. , Roussel‐Dupré R. A. , Symbalisty E. M. D. , Taranenko Y.
Published: September 2002

The results of numerical calculations of the intensity and spectra of optical emissions from blue jets produced by runaway air breakdown in the atmosphere are presented. It is found that a positive runaway streamer develops in the altitude range 20–34 km following an intracloud discharge that possesses a continuing current of ∼1.7 kA. The ionization front of the runaway streamer propagates upward with a velocity ∼90 km/s and produces optical emissions with a maximum intensity ∼400 kR and a duration ∼53 ms. The comparison between theory and observation yields good agreement for such important blue jet characteristics as maximum intensity of optical emissions, color, front velocity, duration, maximum radius and vertical dimensions and supports the viability of runaway air breakdown as a driving mechanism for this particular type of high altitude discharge.
Constraining the heterogeneous loss of O3 on soot particles with observations in jet engine exhaust plumes
Authors: Gao R. S. , Kärcher B. , Keim E. R. , Fahey D. W.
Published: September 2002

In situ measurements in the engine exhaust of a Concorde supersonic aircraft in the lower stratosphere are used to constrain heterogeneous reaction rates on soot particles in a plume model. Upper limit values are obtained for the product of the reactive uptake coefficients of O3 and NO2 and the mean surface area of individual soot particles using the model and the well‐established O3‐N2O correlation in the lower stratosphere. The low reactivity value obtained for O3 implies that soot reactions cannot account for ozone trends in the lower stratosphere.
Investigations of the possible relationship between PMSE and tides using a VHF MST radar
Authors: Barabash V. , Chilson P. , Kirkwood S. , Réchou A. , Stebel K.
Published: September 2002

The possible relationship between Polar Mesospheric Summer Echoes (PMSE), tidal winds and vertical wind shear associated with the tidal winds is examined. PMSE were observed by the Esrange VHF MST Radar (ESRAD ) 52 MHz radar at 67°56′N, 21°04′E during May–August 1997. Three data series in the beginning, middle and the end of the PMSE season are considered. The seasonal variation of PMSE shows a steep increase in occurrence at the end of May and fade‐out in mid‐August. Daily variation of echo power in the middle of the season has a minimum around 20–22 UT. Wind measurements reveal terdiurnal, semidiurnal and diurnal tides. Between 82–87 km we found a strong wind shear which tends to increase through the season, but does not show significant correlation with SNR (signal‐to‐noise ratio). It seems likely that PMSE appearance is conditioned by small scale dynamics and tidal effects on aerosols, and not by large scale wind shear.
Low energy anomalous ions at northern heliolatitudes
Authors: Maclennan C. G. , Lanzerotti L. J.
Published: September 2002

Measurements of the anomalous component of nitrogen, oxygen, and neon ions (∼0.5–8 MeV/nucl.) at northern heliolatitudes by the HI‐SCALE instrument on the Ulysses spacecraft show that the energy spectra of these ions are essentially flat from the highest heliolatitudes (80° N) to near the ecliptic plane. For energies from 1–8 MeV/nucl., the northern hemisphere O fluxes are about a factor of two larger than those measured earlier at high southern latitudes. During ∼150 days (∼5 solar rotations) in late 1996, solar activity added low energy (∼0.5–1 MeV/nucl.) N, O, Ne, and Fe ions to the high latitude interplanetary medium. The northern hemisphere measurements are consistent with essentially no latitude gradient for particles in this energy range at this time. These results call into some question the importance of acceleration of anomalous cosmic rays of this energy by corotating interaction regions in the inner heliosphere.
A magnetic cloud with unusual structure and corresponding bow shock movement observed on May 13, 1995
Authors: Takeuchi T. , Araki T. , Lepping R. P. , Yamamoto T. , Kokubun S. , Nagai T. , Iyemori T.
Published: September 2002

A magnetic cloud embedded in a corotating interaction region was observed by Wind 244 Re upstream of Earth on May 13, 1995. This cloud was unusual as it had at least two internal structures and the profile of the magnetic field strength was far from that predicted by a simple force‐free flux rope model, although the field direction fit well. About 90 min later, this cloud was detected by the IMP 8 and Geotail satellites in front of Earth's bow shock. Geotail observed a sunward movement of the bow shock 8 min after the passage of the front boundary. An earthward movement of the bow shock was observed about 3 hours later. From the normal vector of the bow shock front, we deduce that the shape of the expanding bow shock was distorted from an axially symmetric geometry. The shape was consistent with distortion due to the observed inclination of the front boundary of the magnetic cloud.
Vertical structure in the topside sodium layer
Authors: Clemesha B. R. , Batista P. P. , Simonich D. M.
Published: September 2002

It is shown that the assumption that sodium can be used as a passive tracer for studying atmospheric waves in the region above 90 km is not always valid. An analysis of wave structures observed in the sodium layer by lidar shows that the associated oscillations in the mixing ratio of sodium are frequently too large to be produced by vertical displacements in the presence of mixing ratio gradients. On some occasions these large amplitude oscillations appear to be associated with the formation of sporadic sodium layers, and it is suggested that both phenomena are most probably related to the windshear concentration of sodium ions.
The hydrolysis of ClONO2 on sub‐micron liquid sulfuric acid aerosol
Authors: Ball Stephen M. , Fried Alan , Henry Bruce E. , Mozurkewich Michael
Published: September 2002

The reactive uptake of ClONO2 on liquid sulfuric acid has been measured for size‐selected, sub‐micron aerosol in the composition range 36–54% H2SO4 by weight at ≈ 250 K. A comparison with results from studies employing bulk surfaces at similar temperatures and compositions shows no evidence for a dependence of the reaction probability, γ, on size. We use this observation to estimate an upper limit to the diffuso‐reactive length of 26 nm for the ClONO2 + H2O reaction on 43 weight % aerosol at 248 K. But, the present values of γ are consistently lower than bulk values observed at lower temperatures and similar composition, suggesting that γ is temperature dependent.
Ultra‐slow tails of sprite‐associated lightning flashes
Authors: Füllekrug Martin , Fraser‐Smith Antony C. , Reising Steven C.
Published: September 2002

We describe the terrestrial excitation of horizontal magnetic field variations in the Pc1 frequency range (0.2–5.0 Hz) by tropospheric, sprite‐associated lightning flashes, measured ∼1900 km west from the source. These variations, which we call ultra‐slow tails, exhibit amplitudes on the order of tens of pT, they have a duration of ∼3 seconds, and they occur immediately following the initial pulse of the sprite‐associated lightning flash. The ultra‐slow tails exhibit two peaks in the frequency domain at 0.67 Hz and 1.67 Hz. The mean polarization ellipses at these two frequencies are oriented ∼45° clockwise from geographic north and exhibit right‐hand and left‐hand polarization respectively with a weak ellipticity of ∼0.1. The horizontal magnetic intensity of the initial pulse is related to the horizontal magnetic intensity of the ultra‐slow tail, in agreement with the interpretation of ultra‐slow tails as ionospheric Alfven resonances.
Differences in the O7+/O6+ ratio of magnetic cloud and non‐cloud coronal mass ejections
Authors: Henke T. , Woch J. , Mall U. , Livi S. , Wilken B. , Schwenn R. , Gloeckler G. , von Steiger R. , Forsyth R. J. , Balogh A.
Published: September 2002

On its trajectory to Jupiter and over the poles of the Sun the Ulysses spacecraft has observed a considerable number of Coronal Mass Ejection (CME) transients in slow and in fast solar wind streams. The analysis of the magnetic field topology and the O7+/O6+ charge state ratio of 56 of these events has yielded strong evidence for a systematic connection between the two features. Coronal mass ejections with magnetic cloud structure have an increased O7+/O6+ ratio with respect to the ambient solar wind whereas non‐cloud CMEs do not show enhanced O7+/O6+ ratios. We discuss possible mechanisms based on the freezing‐in concept that can account for the observation.
Implications of upper stratospheric trace constituent changes observed by HALOE for O3 and ClO from 1992 to 1995
Authors: Siskind David E. , Froidevaux L. , Russell J. M. , Lean J.
Published: September 2002

Measurements from the Halogen Occultation Experiment (HALOE) of the upper stratosphere show increases in HCl and H2O and decreases in CH4 and O3 during the period 1992–1995. These changes all coincide with the decline of solar cycle 22. Using a simple photochemical model, we find that 4 major components contribute to the O3 decrease: 1) an increase in total chlorine as indicated by increasing HCl 2) an additional increase in reactive chlorine due to repartitioning of Cly by the decreasing CH4 3) a decrease in odd oxygen production due to decreased solar flux and 4) an increase in odd hydrogen loss due to increasing H2O. At 2 mbar, the Cly repartitioning is the largest cause of O3 changes. Because the Cly repartitioning coincides with decreasing solar flux, some recent observational estimates of the long‐term response of upper stratospheric O3 to solar UV irradiance variability may be too large. Compared with the HALOE O3 data, the model O3 exhibits a larger negative trend. This appears to be because the model O3 is more sensitive to the increased H2O than is the observed O3. The 20–30% decline in CH4 also implies a large increase in ClO which exceeds that expected from CFCs. Observations of ClO from the Microwave Limb Sounder (MLS) support this inference.
Ground based millimeter‐wave observations of Arctic Ozone depletion during winter and spring of 1996/97
Authors: Sinnhuber Björn‐Martin , Langer Jens , Klein Ulf , Raffalski Uwe , Künzi Klaus , Schrems Otto
Published: September 2002

Ground based millimeter‐wave measurements of Arctic stratospheric ozone in the winter 1996/97 are presented. The measurements have been performed at one of the primary Arctic stations of the Network for the Detection of Stratospheric Change (NDSC) in Ny‐Ålesund, Spitsbergen (78.9°N, 11.9°E). Over the period 11 February to 26 April the measurements show an ozone mixing ratio decrease of 1.3 ppm in the lower stratosphere. Correspondingly, stratospheric ozone column densities decreased by more than 50 DU. Taking into account the transport of ozone due to diabatic decent, we estimated chemical ozone loss rates of 22 ppb/day in February decreasing to 15 ppb/day in late April 1997.
The ITRF96 realization and its associated velocity field
Authors: Sillard Patrick , Altamimi Zuheir , Boucher Claude
Published: September 2002

This text intends to present and to analyse the quality of the new 1996 edition of the International Terrestrial Reference System, called the ITRF96. The ITRF96 represents a key step forward in the activity of the IERS terrestrial system determinations: a new physical combination model has been developed as well as a new stochastic model. For the first time, the level of accuracy has reached 1 cm (or better) in position for about 50% of ITRF96 stations and 30% of the stations have their velocities determined, free of any aditional tectonic plate model assumption, with an uncertainty better than 3 mm/yr. The ITRF96 also provides a more complete solution: a new software has been developed which makes available the ITRF96 full variance‐covariance matrix whereas this information was restricted to station positions in the previous ITRF94 edition. Moreover, the network coverage is clearly the largest set of globally distributed terrestrial Space Geodesy stations that have ever been computed: 521 stations located among 290 sites. In addition, station velocities are also determined at a very accurate level allowing possible determination of kinematic plate motion models free of any geological assumption.
Refined coseismic displacement modeling for the 1994 Shikotan and Sanriku‐oki earthquakes
Authors: Cummins Phil R. , Hirano Satoshi , Kaneda Yoshiyuki
Published: September 2002

We consider refined modeling of GPS measurements of coseismic displacement for two large earthquakes occurring in the Japan and Kurile Trenches. Calculations are performed for a layered sphere and the homogeneous half‐space more commonly used to model such data, using fault geometry and slip distributions from previously published studies. We show that the difference between the layered sphere and homogeneous half‐space calculations can be significant, as expected from previous theoretical studies. However, it does not appear large enough to seriously affect results from previously published studies of these earthquakes. Nevertheless, the fact that the effects of sphericity and especially layering are measurable in coseismic observations of great earthquakes at moderate distances indicates that these effects should be taken into account in order to fully exploit the precision of modern GPS measurements.
Cooler sea surface west of the Sahara Desert correlated to dust events
Authors: Schollaert Stephanie E. , Merrill John T.
Published: September 2002

Atmospheric particles scatter incoming solar radiation and cause regional cooling for areas in which they increase the net backscattering. Stratospheric aerosols have been widely observed to have this cooling effect on the Earth's surface following major volcanic eruptions. A smaller effect is expected to result from the regular entrainment of aerosols in the troposphere from natural and anthropogenic surface sources. Over long time scales, even a small cooling by tropospheric aerosols could be significant in offsetting warming caused by greenhouse gases. We looked for indications of this effect at the ocean's surface, when the albedo is raised in the presence of tropospheric aerosols. To test whether a correlation exists between tropospheric aerosol outbreaks and a cooler ocean surface, we compared satellite‐derived aerosol optical depth (AOD) measurements to in situ sea surface temperatures (SSTs) over a four year period. We selected a subtropical North Atlantic region where the upwind mobilization of desert dust results in an aerosol signal large enough to outweigh other factors which cause cooling of the sea surface. In this location, the differing seasonal variations of aerosol amount, mixed layer depth, and cloudiness permit a glimpse of the causal relationship between aerosols and temperature changes at the sea surface. Although we were limited by in situ SST coverage, we found temperature perturbations correlated to large aerosol outbreaks from the Sahara Desert.
Lens‐effect in Santa Monica?
Authors: Alex Carmen M. , Olsen Kim B.
Published: September 2002

We used finite‐difference simulations of 10‐Hz P‐SV and SH waves to estimate the contribution from the deep basin structure to the large ground motion amplification in Santa Monica, California, observed for seismic waves incident from the north. Our simulations of a 17‐km deep Northridge aftershock with epicenter 30 km north of Santa Monica show that focusing at the lens‐shaped boundary of the sediments can only account for less than half the amplification observed in the area that was heavily damaged during the 1994 M 6.7 Northridge earthquake. The focusing in the simulations caused amplification of up to 1.6 times in a zone 0.65–2.4 km south of the Santa Monica fault and de‐amplification at sites just south of the fault where some of the largest amplification is observed in the data.
One‐sided velocity distributions at comet P/Grigg‐Skjellerup
Authors: Krauklis I. , Coates A. J.
Published: September 2002

During the hour following the bow shock of the outbound leg of Giotto's encounter with comet P/Grigg‐Skjellerup, asymmetric cometary ion distributions were observed in velocity space. Various explanations for the observed asymmetry include sudden changes in the magnetic field or magnetic mirroring. Several workers have noted that the parallel Alfvénic Mach number was close to unity during this interval, a result of the near orthogonal orientation of the interplanetary magnetic field with respect to the solar wind. Newly formed cometary ions would be implanted into the upstream diffusion gap and would therefore be more likely to diffuse through resonant interactions with downstream propagating waves. Starting with this observation, a simple finite difference representation of Boltzmann's equation has been utilized to model the velocity space distributions. The results of the simulation show that the asymmetry is a consequence of the geometry of the implanted ion distribution and the lifetime of the cometary ions in the solar wind and that no other mechanisms are needed to explain the observations.
Magnetic noise bursts near the interplanetary shock associated with the coronal mass ejection event on February 21, 1994: The Geotail observations
Authors: Zhang Y. , Matsumoto H.
Published: September 2002

A large coronal mass ejection (CME) event was detected by both the Geotail and IMP 8 satellites on February 21, 1994. Magnetic noise bursts (MNB) associated with the CME event are studied with the Geotail observations. An interplanetary shock associated with the CME reached the Geotail satellite, which was in the solar wind, around 0903 UT with a speed of about 1000 km/s. Before the arrival of the shock, the MNB with frequencies above 10 Hz are found to be produced by the whistler mode waves. These whistler mode waves propagate in a direction quasi‐parallel to the ambient magnetic field (B0). After the arrival of the shock, the MNB are produced by two different kinds of waves. The first kind are whistler mode waves with frequencies from 10 Hz to about 200 Hz. The whistler mode waves‐propagate in a single direction nearly parallel to the ambient magnetic field. A stable ambient magnetic field is a necessary condition but not a sufficient condition for the detection of the whistler mode waves. These facts suggest that the sources of the whistler mode waves are in the nonlocal plasmas (such as the shock). The second are lower hybrid waves with frequencies around or less than the lower hybrid frequency. The lower hybrid waves mainly propagate in a direction nearly perpendicular to the ambient magnetic field. They are associated with a rapid change of B0. The large gradients in both the ambient magnetic field and the plasma density may trigger the lower hybrid drifting instability thus exciting the lower hybrid waves.
Bursts of whistler mode waves in the upstream of the bow shock: Geotail observations
Authors: Zhang Y. , Matsumoto H. , Kojima H.
Published: September 2002

Bursts of narrowband and short‐lived electromagnetic waves are frequently observed in the upstream (the electron foreshock and the pure solar wind) by the waveform capture instrument on board Geotail satellite. The electromagnetic waves with frequencies above 10 Hz are all right‐hand circularly polarized and identified as whistler mode waves. We call the waves narrowband and short‐lived whistlers (NSW). Nearly all the NSW in the electron foreshock propagate in a downstream direction parallel to the ambient magnetic field (Bo) with an average θkB of 16°, where θkB is an angle between NSW wave vector and the Bo. Their frequencies cover a range from 0.05 to 1.0Ωe with an average of 0.35Ωe, where Ωe is a local electron cyclotron frequency. Their amplitudes range from a few picoteslas to 100 pT with an average of 23 pT. Because of their parallel propagation, the NSW must be excited by electron cyclotron resonance. These features of the NSW suggest existence of electron beams which travel in an upstream direction parallel to the Bo and which have a temperature anisotropy. Kinetic energies of the beams range from a few eV to about 200 eV (28 eV on average). All these characteristics of the electron beams revealed from the NSW are consistent with ISEE and WIND particle observations. The competition between electrostatic and whistler instabilities and the finite size of the beams are very likely the reasons why the NSW are short‐lived. These NSW can be well explained by a modeled electron beam with a losscone distribution in the electron foreshock. The NSW in solar wind are very similar to those in the electron foreshock. However, they have larger amplitudes (34 pT on average), lower frequencies (0.24Ωe on average), and higher cyclotron resonant energy (100 eV on average). They are very likely excited by halo electrons or nonlocal sources in the solar wind.
Variations of solar UV irradiance related to short‐term and medium‐term changes of solar activity
Authors: Troshichev O. A. , Gabis I. R
Published: September 2002

Index of variability of the solar ultraviolet (UV) radiation, the composite Mg II core‐to‐wing ratio, has been used to study relationship between the short‐term (τ < 27 days) changes of solar activity and solar UV irradiance. Such manifestations of the solar activity have been examined, as the solar central meridian passage of active regions, the solar proton events, and the central meridian passage of hypothetical regions responsible for the Forbush decrease in the galactic cosmic rays. Our results show that all these short‐term changes of the solar activity are accompanied by an increase of the solar UV irradiance. The interplanetary magnetic field sector structure is also related to changes in the UV irradiance. After a proper adjustment of the dates of the sector boundary occurrence for the solar disk, the irradiance was found to be maximal on the toward/away boundary and minimal on the away/toward boundary. It has been found that the UV irradiance undergoes quasi‐biennial periodicity (QBP), reaching maximum in years of the east QBP phase and decreasing in years of the west QBP phase. Superposition of the quasi‐biennial periodicity and effects connected with short‐term variations in the solar activity account for the change of the Mg II index up to 2% of the mean level. Thus a new very important agent was found to be responsible for a short‐term and medium‐term influence of the solar activity upon atmospheric processes and hence on the weather and climate.
Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar
Authors: Otsuka Y. , Kawamura S. , Balan N. , Fukao S. , Bailey G. J.
Published: September 2002

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85°N, 136.10°E, magnetic latitude 25°N) in Japan are studied using the electron and ion temperature (Te and Ti, respectively) data measured by the radar during nearly a full solar cycle (1986–1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of Te and Ti is presented for the altitude range 200–550 km. The temperatures Te and Ti are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season, while daytime Te is highest in summer and lowest in equinox, daytime Ti is highest in equinox and lowest in summer. With solar activity, while daytime Te decreases, the corresponding Ti increases. The diurnal variation of Te is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in Te are illustrated through model calculations using the Sheffield University plasmasphere‐ionosphere model.
First determination of the silicon isotopic composition of the solar wind: WIND/MASS results
Authors: Wimmer‐Schweingruber Robert F. , Bochsler Peter , Kern Olivier , Gloeckler George , Hamilton Douglas C.
Published: September 2002

Silicon is a common material in the solar system. For instance, Si accounts for about 10% of the material in primitive meteorites (CI chondrites). Since silicon is a refractory element, we expect the meteoritic isotopic composition to be very similar to that of the Sun. The isotopic composition of Si in meteorites is well known and varies little. Thus the three stable isotopes of Si may serve as powerful indicators to test fractionation of isotopes in the transition from the solar atmosphere into the solar wind. We present, for the first time, measurements of the isotopic composition of Si in the solar wind. The data were obtained with the MASS instrument aboard the WIND spacecraft and accumulated in exceedingly cold and slow wind. Such wind is often associated with large superradial expansion factors and with current sheet crossings which in turn are associated with the most efficient isotopic fractionation processes in the solar wind acceleration region. We detect little or no isotopic fractionation between the solar surface assumed to be of meteoritic composition and the solar wind. This constrains solar wind acceleration models and puts stringent limits on possible secular changes in the isotopic composition of the outer solar convective zone, the solar atmosphere, and the solar wind.
Fast auroral snapshot observations of bouncing ion distributions: Fieldline length measurements
Authors: Boehm M. H. , Klumpar D. M. , Möbius E. , Kistler L. M. , McFadden J. P. , Carlson C. W. , Ergun R. E.
Published: September 2002

Observations of 0.01–10 keV ions at a discrete set of velocities, as expected for ions bouncing repeatedly between hemispheres, have been previously reported both at low altitudes and at geosynchronous orbit. The following two possible models of the ion source have been suggested: for the geosynchronous observations, an equatorial‐acceleration model involving temporally confined ion acceleration “events” and for the low‐altitude observations, a model involving a “heating wall” in the auroral acceleration region from which ions subsequently drift to other latitudes. In the latter case, the spatially bounded source appears temporally bounded from the point of view of one field line drifting over it. We report here Fast Auroral Snapshot (FAST) observations of multienergy events near local noon at several thousand kilometer altitude and show that these events orginated in a temporally localized, spatially extended, equatorial source. The dispersion pattern for individual ion bands over latitude is consistent with such a source, and not with the heating wall hypothesis, the observed energy being proportional to the square of the modeled field line length. The latter condition on the dispersion is much more precise than any imposed by a latitudinal drift model. A fit of this model to Akebono and DMSP F8 data previously published by Hirahara et al. (1997) is also shown. The fit to two Akebono events is accurate at lower latitudes, where the magnetic field model is expected to be reliable. The fit to DMSP data corresponding to one of the Akebono events involves additional field‐aligned potentials of roughly 100 eV in the auroral zone. Observations of dispersion events of this type are expected to be useful more generally. The “nonlocal” determination of the magnetic field line length can provide a check on magnetic field models. A match of the observed field line length to the modeled one provides for greatly increased confidence in magnetic mapping.