bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of CaCl2‐type and α‐PbO2‐type SiO2 at high pressure and temperature determined by in‐situ X‐ray measurements
Authors: Murakami Motohiko , Hirose Kei , Ono Shigeaki , Ohishi Yasuo
Published: May 2003

Here we report the phase boundary between CaCl2‐type and α‐PbO2‐type silica at high pressure and temperature up to 151 GPa and 2500‐K determined by in‐situ X‐ray measurements in a laser‐heated diamond anvil cell. Amorphous silica was used as starting material, and the sample was heated for more than 1.5 hr to examine the kinetic effects. The results demonstrated that the CaCl2‐type silica is a post‐stishovite phase and that it undergoes further transition to the α‐PbO2‐type structure above 121 GPa at 2400‐K. Present data together with previous first‐principles calculations indicate that the phase boundary between CaCl2‐type and α‐PbO2‐type silica is represented by a linear equation P (GPa) = 98 + (0.0095 ± 0.0016) * T (K). The α‐PbO2‐type silica can be present in the deep portion of the lower mantle in the silica‐saturated bulk compositions. This phase transition might contribute to the seismic wave velocity anomalies observed in the D″ region.
Long‐term decrease of cadmium concentrations in the Canadian Arctic air
Authors: Li Chunsheng , Cornett Jack , Ungar Kurt
Published: May 2003

Average cadmium concentrations in the air (particulate samples) at two Canadian Arctic sites, Resolute (74.7°N, 95.0°W) and Coral Harbor (64.2°N, 83.3°W) declined from about 0.2 ng m−3 to about 0.05 ng m−3 between the early 1970s and 2000. Cadmium concentrations in the air at southern Canada sites were much higher (0.05–1.12 ng m−3) than those measured in the Arctic. The concentrations of cadmium measured at all sites decreased by a factor of three to seven between 1973 and 2000. This decrease correlates with the reduction in anthropogenic emissions from the production of non‐ferrous metals following the introduction of emission control technologies.
Radon short range forecasting through time series preprocessing and neural network modeling
Authors: Pasini Antonello , Ameli Fabrizio
Published: May 2003

In the framework of studies about the relevance of radon progeny measurements for the estimation of the mixing height, here a time series of radon data is analyzed and used for a short range forecasting activity. After a preprocessing of the time series in order to subtract the known periodicities, we perform forecasts of the future values of the residual series by means of neural network modeling. Finally we apply a simple box model to real data and forecast results, and obtain useful predictions of the mixing height during stability conditions.
Isotopic evidence for a marine ammonia source
Authors: Jickells T. D. , Kelly S. D. , Baker A. R. , Biswas K. , Dennis P. F. , Spokes L. J. , Witt M. , Yeatman S. G.
Published: May 2003

Ammonium salts represent a major component of fine mode marine aerosol that play an important role in climate regulation, atmospheric pH control and nutrient supply to the oceans. We report here the results of analyses of aerosol samples collected in remote areas of the Atlantic Ocean. We show the isotopic abundance of aerosol ammonium varies with concentration, with isotopically light ammonium (−5 to −8‰) associated with the lowest ammonium concentrations compared to isotopically heavy ammonium (+10‰) associated with higher ammonium concentrations. We interpret the light ammonium signature as arising from marine emissions of ammonia. These results provide further evidence for the existence of a marine ammonia source and offers a method to define the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and thereby investigate the role of these emissions in climate regulation.
Sound scattering from oceanic turbulence
Authors: Ross Tetjana , Lueck Rolf
Published: May 2003

The first near coincident measurements of acoustic backscatter and temperature/velocity microstructure confirm theoretical predictions that oceanic turbulence scatters sound. Not only are acoustic backscatter at 307 kHz and turbulent microstructure unambiguously correlated on a patch‐by‐patch basis, but measured scattering amplitudes agree with theoretical predictions for scattering from turbulent microstructure. Nearby plankton net‐hauls indicate that there were far too few zooplankton in the turbulent regions to account for the scattering intensity. At an acoustic frequency of 307 kHz, backscatter from salinity microstructure can be as strong as ‐ or stronger than ‐ the signal from a zooplankton scattering layer. There are two important consequences of these strong scattering results. First, they suggest the feasibility of using acoustics to remotely sense oceanic turbulence. Second, they could easily confound acoustic zooplankton biomass estimates in turbulent regions.
X‐ray emission from the terrestrial magnetosheath
Authors: Robertson I. P. , Cravens T. E.
Published: May 2003

X‐rays are generated throughout the terrestrial magnetosheath as a consequence of charge transfer collisions between heavy solar wind ions and geocoronal neutrals. The solar wind ions resulting from these collisions are left in highly excited states and emit extreme ultraviolet or soft X‐ray photons. A model has been created to simulate this X‐ray radiation. Published terrestrial exospheric hydrogen distributions and solar wind speed, density and temperature distributions were used in this model. Simulated images were created as seen from an observation point outside the geocorona. The locations of the bow shock and magnetopause are evident in these images. Perhaps this X‐ray emission can be used to remotely sense the solar wind flow around the magnetosphere. Since similar X‐rays are produced in the heliosphere, the challenge will be, however, to eliminate this background emission.
Impact of high resolution water vapor cross‐sectional data on hurricane forecasting
Authors: Kamineni Rupa , Krishnamurti T. N. , Ferrare Richard A. , Ismail Syed , Browell Edward V.
Published: May 2003

This paper addresses the impact of moisture profile data sets derived from the Lidar Atmospheric Sensing Experiment (LASE) for the assimilation and forecasts of hurricanes. LASE is a vertical profiler for moisture measurements that was developed at NASA Langley Research Center. LASE was deployed on board the NASA DC‐8 research aircraft and provided vertical profiles of moisture in a hurricane environment during the Convection and Moisture Experiment‐4, which occurred in August and September 2001. In this preliminary study we show results from two types of experiments (with and without LASE data) that show improvements in track and intensity forecasts of hurricanes, Erin, Humberto and Gabrielle. When the LASE moisture profiles are assimilated, track errors are reduced by 100 km and intensity errors are reduced by 20–25% for 3 day forecasts.
Assessment of global climate model land surface albedo using MODIS data
Authors: Oleson Keith W. , Bonan Gordon B. , Schaaf Crystal , Gao Feng , Jin Yufang , Strahler Alan
Published: May 2003

Land surface albedo from the Community Land Model is compared to white‐sky (diffuse) and black‐sky albedo (direct at local solar noon) from MODIS. Generally, comparisons are more favorable in summer than winter, for visible waveband than near‐infrared in regions without snow cover, and for black‐ than white‐sky. In regions with extensive snow cover, the model overestimates white‐ and black‐sky albedo by up to 20% absolute. The snow‐free visible and near‐infrared black‐sky albedo is simulated quite well with biases within ±5% over most of the land surface. However, a large negative model bias was found for the Sahara Desert and Arabian Peninsula, particularly in the near‐infrared. The poorer simulation of white‐ compared to black‐sky albedo in vegetated areas implies that the model may be overestimating the increase of albedo with solar zenith angle. These results identify several areas that should have priority in further evaluating and improving albedo in the model.
Further evidence of hemispheric differences in the MLT mean wind climatology: Simultaneous MF radar observations at Poker Flat (65°N, 147°W) and Davis (69°S, 78°E)
Authors: Kishore P. , Namboothiri S. P. , Igarashi K. , Murayama Y. , Vincent R. A. , Dowdy A. , Murphy D. J. , Watkins B. J.
Published: May 2003

Results of a comparative study of the wind fields in the mesosphere and lower thermosphere over Poker Flat (65°N, 147°W) and Davis (69°S, 78°E) are presented. The study utilized the MF radar wind data collected simultaneously during the period 1999–2000. Mean winds during this period are examined to study the hemispheric features. Emphasis has been given to the climatological variations over these two high‐latitude stations. The results suggest large hemispheric differences in the mean circulation. Further, the observed mean wind characteristics are compared with results obtained from other high‐latitude stations.
Inertial instability flow in the troposphere over Suriname during the South American Monsoon
Authors: Fortuin J. Paul F. , Kelder Hennie M. , Sigmond Michael , Oemraw Radchis , Becker Cor R.
Published: May 2003

Weekly sonde observations in Suriname, supported by ECMWF analyses and a linear stability analysis, are used to analyze the recurrence of inertial instability as the ITCZ migrates over land during the South American Monsoon. A layer of cool moist air from the Atlantic Ocean is then advected southward over Suriname in the shape of a cold front, displacing the warmer air over the continent. The return flow northward, by the upper branch of the Hadley cell, is a region where inertial instability pervades due to cross‐equatorial advection of anticyclonic vorticity and the proximity of the subtropical jet as it migrates closer to the Equator. This unstable region evidently leads to the episodical formation of a meridional sub‐cell below the tropopause, where the damping is calculated to be strong enough to stabilize flow at smaller vertical scales, and yet weak enough to allow the observed cell recurrence ‐ at approximately the inertial frequency of the underlying latitudes. This instability should also contribute to the Hadley cell formation through northward acceleration in the upper branch. The moist saturated conditions in the lower troposphere do allow inertial instability here, but the high damping values within the boundary layer suggests that the observed southward acceleration in the lower branch of the Hadley cell has a cause other than inertial instability.
Bow shock motions observed with CLUSTER
Authors: Maksimovic M. , Bale S. D. , Horbury T. S. , André M.
Published: May 2003

The Cluster mission allows the determination not only of the bow shock crossing position but also, with a simple timing method and a reasonable confidence, the shock normal and the velocity along this normal. We apply this technique to a series of eleven consecutive bow shock crossings which occurred during a time interval of approximatively two and a half hours on 31 March 2001. We fit, on a distance versus time frame, the position of the bow shock subsolar point by imposing that the time derivatives at the crossings be equal to the shock speeds we determine. The curve we obtain this way represents global oscillations of the bow shock with a typical amplitude that compares quite well to the prediction of standard gasdynamic models which take into account the upstream solar wind plasma conditions.
Mineral aerosol and cloud interactions
Authors: Mahowald Natalie M. , Kiehl Lisa M.
Published: May 2003

Interactions between aerosols and clouds are the subject of considerable scientific research, due to the importance of clouds in controlling climate. Here we consider the relationship between mineral aerosols and cloud properties over North Africa and the North Atlantic using monthly mean observations of mineral aerosols and clouds over 16 years. The results of this study are consistent with mineral aerosols suppressing precipitation in thin low altitude clouds and changing cloud amounts in ice phase clouds. Because we cannot eliminate either spurious correlations, or that the cloud and dust changes are both driven by the same meteorological conditions, we cannot provide definitive conclusions. However, these results suggest complicated and tantalizing feedbacks between mineral aerosols and climate. This question becomes crucial as we note that mineral aerosols from North Africa have increased substantially since the 1960s for reasons which are poorly understood but that may be linked to human activity.
The nature of the crust in southern India: Implications for Precambrian crustal evolution
Authors: Gupta Sandeep , Rai S. S. , Prakasam K. S. , Srinagesh D. , Bansal B. K. , Chadha R. K. , Priestley Keith , Gaur V. K.
Published: May 2003

We present crustal thickness and Poisson's ratio determinations from receiver function analyzes at 32 sites on the Archaean and Proterozoic terrains of South India. The crustal thickness in the late Archaean (2.5 Ga) Eastern Dharwar Craton varies from 34–39 km. Similar crustal thickness is observed beneath the Deccan Volcanic Province and the Cuddapah basin. The most unexpected result is the anomalous present‐day crustal thickness of 42–51 km beneath the mid‐Archaean (3.4–3.0 Ga) segment of the Western Dharwar Craton. Since the amphibolite‐grade metamorphic mineral assemblages (5–7 Kbar paleopressures) in this part of Western Dharwar Craton equilibrated at the depths of 15–20 km, our observations suggest the existence of an exceptionally thick (57–70 km) crust 3.0 Ga ago. Beneath the exhumed granulite terrain in southernmost India, the crustal thickness varies between 42–60 km. The Poisson's ratio ranges between 0.24–0.28 beneath the Precambrian terrains, indicating the presence of intermediate rock type in the lower crust. These observations of thickened crust suggest significant crustal shortening in South India during the Archaean.
Vegetation index trends for the African Sahel 1982–1999
Authors: Eklundh Lars , Olsson Lennart
Published: May 2003

Pathfinder AVHRR NDVI data have been analyzed for the African Sahel to study recent trends in vegetation greenness. A strong increase in seasonal NDVI was observed over large areas in the Sahel during the period 1982–1999. The increase is interpreted as a vegetation recovery from the drought periods of the 1980's. Although strong shifts in satellite overpass times have led to shifting solar zenith angles (SZA) over the time period, only minimal influence of SZA's on the Pathfinder NDVI was found in our data. A preliminary analysis of rainfall data indicates increasing rainfall during the period. The observed trends may have important implications to the Sahel including changes to the water cycle, energy exchange and carbon storage.
Source model for the 2001 flank eruption of Mt. Etna volcano
Authors: Lundgren Paul , Rosen Paul A.
Published: May 2003

Using interferometric synthetic aperture radar (InSAR) we constrain the deformation sources for the July–August 2001 flank eruption of Mt. Etna volcano, Italy. InSAR data from ascending and descending passes of the ERS2 satellite reveal a pattern of deformation that cannot be explained by a dike intrusion alone. In addition to a vertical dike beneath the south rift zone, the spatially large (10 km scale, 15–20 cm in range) negative range displacement lobes across the western (descending data) and eastern (ascending data) flanks require a nearly symmetric set of shallowly dipping normal faults to each side of the central dike. Complexity in the observed InSAR surface displacements constrains an additional dike intrusion beneath its NE flank. Long‐term deformation of Etna's eastern and southern flanks is well established through field and InSAR observations. Therefore, the relative symmetry of motion beneath both the western and eastern flanks during the 2001 eruption is surprising. Our model of symmetric flank motion suggests that on the short time scales of a large dike intrusion volcanoes can deform differently from their long‐term deformation.
A monitoring design for the Atlantic meridional overturning circulation
Authors: Hirschi J. , Baehr J. , Marotzke J. , Stark J. , Cunningham S. , Beismann J.‐O.
Published: May 2003

Current hydrographic data can provide snapshots but no continuous timeseries of the meridional overturning circulation (MOC). Using output from two eddy‐permitting numerical ocean models we test the feasibility of a monitoring system for the MOC in the North Atlantic. The results suggest that a relatively simple arrangement, using moorings placed across a longitude‐depth section and the zonal wind stress, is able to capture most of the MOC strength and vertical structure as a function of time. Being closely related to the transport of energy to the North Atlantic, measuring the MOC would open the prospect of having continuous information about a key element of northern hemisphere climate.
Sea ice formation on a very cold surface
Authors: Smedsrud L. H. , Saloranta T. M. , Haugan P. M. , Kangas T.
Published: May 2003

In this study laboratory experiments of sea ice formed on a vertical surface with initial temperature of −30 to −50°C are presented. The ice formation is rapid, and in 300 s >5 mm of sea ice is formed. Ice formation cooled and salinified the water, and induced a vertical down wards flow of ∼5 mm/s with a boundary layer about 5 mm thick. This ice has a structure with columnar crystals that have small circular cross sections (0.2–1.0 mm) and sea ice salinities are between 24 and 32. A simple model approach indicate that the thermal conductivity of such ice is lower than for other types of sea ice.
On the seasonal circulation within the Panama Bight derived from satellite observations of wind, altimetry and sea surface temperature
Authors: Rodríguez‐Rubio Efraín , Schneider Wolfgang , Abarca del Río Rodrigo
Published: May 2003

The seasonal evolution of geostrophic currents in the Panama Bight is derived from satellite‐borne altimeter data. Current literature describes the circulation in the Bight as being cyclonic only. Our study, instead, reveals two distinct seasonal patterns. In summer the circulation in the Panama Bight is anticyclonic, with a coastal current to the south, whereas in winter, the circulation reverses and is cyclonic with a coastal current to the north, and an oceanic upwelling center in the middle of the Bight. A seasonally changing wind field caused by the meridional migration of the Intertropical Convergence Zone, from 8–10°N in summer, to about 2°N in winter, controls the circulation of the basin. In summer southeasterly trade winds dominate the region, but in winter northeasterly trade winds of the North Atlantic, by means of the Panama Jet, enter the region via the Isthmus of Panama.
Nucleation of gas hydrate in marine environments
Authors: Zatsepina O. Y. , Buffett B. A.
Published: May 2003

An extrapolation of parameters from an experimental study of CO2‐hydrate nucleation in porous media is used to describe the nucleation of CH4 hydrate under conditions found below the seafloor. The requirements for nucleation can significantly delay the onset of hydrate formation when CH4‐bearing liquids rise into the hydrate stability zone. Hydrate may not form at all if the vertical extent of the stability zone is too narrow to achieve sufficient cooling below equilibrium temperature. This is particular relevent in shallow water or at times in the Earth's past where the ocean floor was warmer than today.
Evidence for recent changes in a surface‐air warming singularity in late winter over central North America
Authors: Blasing T. J. , Scott Daria , Kaiser Dale
Published: May 2003

Evidence for an earlier onset of spring in recent years is supported by daily temperature data from central North America. From 1976–1999, mean temperatures during February 15th through 24th were appreciably higher (5°C or more over large areas) than during 1952–1975. This is associated with a strengthened Aleutian Low for those calendar days during 1976–1999, with negative pressure anomalies extending well into North America. This apparently reflects a decrease in strong, cold, high‐pressure systems moving from western Canada into the north central United States during late February. These changes appear to be related to conditions in and over the North Pacific Ocean, possibly in conjunction with global warming, although the latter signal is not clearly obvious. Comparison of time series of daily minimum temperatures for the earlier period (1952–1975) and the recent period (1976–1999) indicates other times of the year when an appreciable warming has occurred; such warming is largely confined to the first half of the calendar year.
The aqueous phase yield of alkyl nitrates from ROO + NO: Implications for photochemical production in seawater
Authors: Dahl Elizabeth E. , Saltzman Eric S. , de Bruyn Warren J.
Published: May 2003

Alkyl nitrates have been observed in remote oceanic regions of the troposphere and in the surface ocean. The mechanism for their production in the oceans is not known. A likely source is the reaction of ROO + NO (where R is an alkyl group). Steady‐state laboratory experiments show that alkyl nitrates are produced in the aqueous phase via this reaction, with branching ratios of 0.23 ± 0.04, 0.67 ± 0.03, and 0.71 ± 0.04 for methyl, ethyl, and propyl nitrate respectively. The branching ratios in aqueous solution are significantly higher than in the gas phase. Irradiation of surface seawaters yield rates of alkyl nitrate production on the order of 10−18 mol cm−3 s−1, suggesting that the reaction of ROO and NO is an important source of alkyl nitrates in seawater.
The Sea Level at Port Arthur, Tasmania, from 1841 to the Present
Authors: Hunter J. , Coleman R. , Pugh D.
Published: May 2003

Observations of sea level at Port Arthur, Tasmania, southeastern Australia, based on a two‐year record made in 1841–1842, a three‐year record made in 1999–2002, and intermediate observations made in 1875–1905, 1888 and 1972, indicate an average rate of sea level rise, relative to the land, of 0.8 ± 0.2 mm/year over the period 1841 to 2002. When combined with estimates of land uplift, this yields an estimate of average sea level rise due to an increase in the volume of the oceans of 1.0 ± 0.3 mm/year, over the same period. These results are at the lower end of the recent estimate by the Intergovernmental Panel on Climate Change of global average rise for the 20th century. They provide an important contribution to our knowledge of past sea level rise in a region (the Southern Hemisphere) where there is a dearth of other such data.
Decadal‐scale variability of upper ocean heat content in the tropical Pacific
Authors: Hasegawa Takuya , Hanawa Kimio
Published: May 2003

Decadal‐scale variability of upper ocean heat content (OHC) in the tropical Pacific is investigated and is compared with that of ENSO scale. The decadal‐scale OHC anomaly with a period of about 13 years shows anticlockwise propagations in the tropical North Pacific like as that of ENSO scale. It is also shown that the entire equatorial OHC anomaly leads Niño‐3 index anomaly by about a quarter (about 3 years) of the period of the decadal variability in the tropical Pacific. This time lag of a quarter of the period is consistent with the idea of the “recharge oscillator” model for ENSO dynamics. Further it is shown that the magnitude of leading OHC anomalies in the entire equatorial Pacific is linearly related to the subsequent magnitudes of Niño‐3 and Niño‐3.4 indices. This relationship is also similar with that of ENSO scale, but there are several different points between them. Particularly, in contrast with the ENSO scale, the amplitude is larger in central equatorial Pacific (Niño‐3.4 region) than in the eastern region (Niño‐3 region).
Structure‐based forest biomass from fusion of radar and hyperspectral observations
Authors: Treuhaft Robert N. , Asner Gregory P. , Law Beverly E.
Published: May 2003

Forest biomass was estimated from the remotely sensed profiles of leaf area density. Biomass estimated from forest structure profiles may be more accurate than that determined from microwave power or optical radiance measurements. Multialtitude, airborne, C‐band, radar interferometry produced relative density profiles, which were normalized by leaf area indices from airborne hyperspectral optical imagery, yielding the forest canopy leaf area density for 11 structurally diverse stands in Central Oregon. Fits of field biomass measurements to model functions of remotely sensed leaf area density produced agreement between the field and remotely sensed biomasses at the level of 25 tons/ha, or 16% of the average stand biomass. The errors in the field and remote sensing observations indicated that this level of agreement was significant with greater than 99.5% confidence. These results suggest that further demonstrations may lead to a set of model functions that enable global, structure‐based biomass remote sensing.
Improved mesospheric temperature, water vapor and polar mesospheric cloud extinctions from HALOE
Authors: McHugh Martin , Hervig Mark , Magill Brian , Thompson R. Earl , Remsberg Ellis , Wrotny Jonathan , Russell James
Published: May 2003

We present a new retrieval technique for mesospheric measurements from the Halogen Occultation Experiment (HALOE). Previous HALOE retrievals did not account for polar mesospheric clouds (PMCs) and were biased whenever PMCs were in the sample volume. In the new algorithm we eliminate this bias by retrieving and correcting for PMC extinction, and we optimize the algorithm for this region of the atmosphere. We have reprocessed over ten years (1991–2002) of HALOE data with the improved algorithm, yielding nearly 75,000 vertical profiles of temperature, H2O, O3, NO and extinction at five infrared wavelengths. In this paper we describe the new algorithm in detail, present several examples of the results and discuss some initial validation and quality assessments.
The origin of chondrites: Metal‐silicate separation experiments under microgravity conditions ‐ II
Authors: Moore S. R. , Franzen M. , Benoit P. H. , Sears D. W. G. , Holley A. , Meyer M. , Godsey R. , Czlapinski J.
Published: May 2003

In order to understand mechanical processes likely to be occurring on the surfaces of asteroids, we have performed experiments with mineral mixtures on NASA's KC‐135 microgravity facility. The behavior of sand and iron filing mixtures, the sand and metal being in the proportions and with grain sizes of ordinary chondrite meteorites, was observed with digital cameras as gas was flowed upward through the mixtures and the plane went through cycles of positive, zero, negative and normal gravity. Partial separation of gas and sand occurred readily under gas flow and microgravity conditions and sometimes survived subsequent turbulence. This behavior is consistent with predictions of the Ergun equation derived from terrestrial experiments. The data have implications for understanding chondrite genesis, for understanding the spectra of asteroid surfaces, and for the design of equipment to function on asteroid surfaces.
Detailed analysis of substorm observations using SuperDARN, UVI, ground‐based magnetometers, and all‐sky imagers
Authors: Bristow W. A. , Sofko G. J. , Stenbaek‐Nielsen H. C. , Wei S. , Lummerzheim D. , Otto A.
Published: May 2003

A case study of a small‐magnitude isolated substorm is presented. The substorm was observed by a variety of instruments including the Super Dual Auroral Radar Network (SuperDARN), the Polar Ultra Violet Imager (UVI), CANOPUS magnetometers, the Alaska chain magnetometers, the Poker Flat meridian‐scanning photometer, and the Poker Flat all‐sky imager. It was determined that the initial brightening was centered over the combined fields of view of the various instruments. Temporal and spatial relationships among plasma flows, auroral luminosity, and magnetometer perturbations are investigated. It is found that the initial substorm signature was observed in the plasma flows, followed by the auroral brightening, and finally followed by the magnetometer perturbation. Enhanced plasma flows were observed in a spatially confined region near the auroral oval for a period of ∼5 minutes prior to the brightening. After the brightness peaked, the plasma flow velocity decreased back to a preenhancement level.
Io‐related Jovian auroral arcs: Modeling parallel electric fields
Authors: Su Yi‐Jiun , Ergun Robert E. , Bagenal Fran , Delamere Peter A.
Published: May 2003

Recent observations of auroral arcs on Jupiter suggest that electrons are being accelerated downstream from Io's magnetic footprint, creating detectable emissions. The downstream electron acceleration is investigated using one‐dimensional spatial, two‐dimensional velocity static Vlasov solutions under the constraint of quasi‐neutrality and an applied potential drop. The code determines self‐consistent charged particle distributions and potential structure along a magnetic field flux tube in the upward (with respect to Jupiter) current region of Io's wake. The boundaries of the flux tube are the Io torus on one end and Jupiter's ionosphere on the other. The results indicate that localized electric potential drops tend to form at 1.5–2.5 RJ Jovicentric distance. A sufficiently high secondary electron density causes an auroral cavity to be produced similar to that on Earth. Interestingly, the model results suggest that the proton and the hot electron population in the Io torus control the electron current densities between the Io torus and Jupiter and thus may control the energy flux and the brightness of the aurora downstream from Io's magnetic footprint. The parallel electric fields also are expected to create an unstable horseshoe electron distribution inside the auroral cavity, which may lead to the shell electron cyclotron maser instability. Results from our model suggest that in spite of the differing boundary conditions and the large centrifugal potentials at Jupiter, the auroral cavity formation may be similar to that of the Earth and that parallel electric fields may be the source mechanism of Io‐controlled decametric radio emissions.
A statistical survey of low‐frequency electric field fluctuations around the dayside cusp/cleft region
Authors: Miyake Wataru , Matsuoka Ayako , Hirano Yumi
Published: May 2003

Intense low‐frequency (0.2 Hz–4.0 Hz) electric field fluctuations are frequently observed over the dayside auroral region. We studied the statistical properties of the spatial occurrence of the intense fluctuations and their dependence on interplanetary conditions based on electric field observations from the polar‐orbiting Akebono (Exos‐D) satellite. We found that the intense fluctuations are most frequently observed around the prenoon cusp/cleft region at low altitude (5000 km–8000 km) and that the intensity of the fluctuations tends to decrease with increasing altitude, which is less evident on the afternoon oval. The occurrence of the intense fluctuations depends on the spiral angle of the interplanetary magnetic field and on the solar wind velocity. The fluctuations are interpreted in terms of Alfvén waves centered on the cusp/cleft region penetrating down to lower altitude without any significant dissipation and quasi‐static structures associated with parallel potential drops decoupling the intense high‐altitude electric fields from the ionosphere in the afternoon region of the auroral oval.
A pseudo‐breakup observation: Localized current wedge across the postmidnight auroral oval
Authors: Partamies N. , Amm O. , Kauristie K. , Pulkkinen T. I. , Tanskanen E.
Published: May 2003

This study presents a detailed description and analysis of a pseudo‐breakup, which took place in the field of view (FOV) of the Magnetometers–Ionospheric Radars–All‐sky Cameras Large Experiment (MIRACLE) network (IMAGE magnetometers, Scandinavian Twin Auroral Radar Experiment [STARE] radars, and all‐sky cameras) on 3 November 1997 at 2212 UT. The activation lasted ∼10 min, occurred during geomagnetically quiet conditions (Dst ≥ −10 nT), and was followed by a global‐scale substorm about half an hour later (at 2246 UT). Both the pseudo‐breakup and the substorm had onsets at times corresponding to the Wind satellite observations of interplanetary magnetic field (IMF) BZ northward turnings as delayed to the subsolar magnetopause. The auroral display of the pseudo‐breakup was a mesoscale spiral with a counterclockwise winding direction. Simultaneously, STARE recorded clockwise plasma flow in the spiral surroundings. Applying the method of characteristics to the MIRACLE observations revealed that the spiral was associated with a localized current wedge (less than a few hours in MLT and about 4° in latitude), tilted in the northeast‐southwest direction. The maximum upward current density at the western edge of the wedge was about 6 A/km2, which clearly exceeds the threshold of spiral buildup (2.5 A/km2) suggested by earlier theoretical studies. About an hour before the pseudo‐breakup, the energy input from the solar wind to the magnetosphere (quantified by the ϵ parameter) exceeded the substorm threshold and the ionospheric conductivity was high enough to support substorm activity. Two likely reasons for the activation to remain localized were its unfavorable location in the morning side of the Harang discontinuity region and the abrupt decrease of the solar wind energy input at its onset time.
Plasma depletion layer: Event studies with a global model
Authors: Wang Y. L. , Raeder J. , Russell C. T. , Phan T. D. , Manapat M.
Published: May 2003

The plasma depletion layer (PDL) is a layer on the sunward side of the magnetopause with lower plasma density and higher magnetic field compared to the corresponding upstream magnetosheath values. In this study we use global simulations of the solar wind‐magnetosphere‐ionosphere system to compare with Wind PDL observations on the flanks of the magnetopause on 12 January 1996 and 1 January 1999. The consistency between model results and observations shows that the magnetohydrodynamic (MHD) description of the plasma is sufficient to describe the PDL process observed in this region. The simulation also shows that the PDL is stable as long as the interplanetary magnetic field (IMF) has no major variations. Temporal‐spatial ambiguities are analyzed for the 1996 event, and a significant difference between the time series and the instantaneous PDL spatial structure is found. A much smoother instantaneous PDL spatial structure is found than observed in the simulated spacecraft time series, which is highly modulated by solar wind and IMF variations. The local time and latitude dependence of the PDL for the 1996 event are also obtained from the model that predicts that a thinner PDL occurs near the subsolar point and a thicker PDL exists farther away from that point.
Acceleration of the solar wind as a result of the reconnection of open magnetic flux with coronal loops
Authors: Fisk L. A.
Published: May 2003

There are compelling observations of a clear anticorrelation between solar wind flow speed and coronal electron temperature, as determined from solar wind ionic charge states. A simple theory is presented which can account for these observations, including the functional form of the correlation: Solar wind flow speed squared varies essentially linearly as the inverse of the coronal electron temperature. In this theory, magnetic field lines in the corona that open into the heliosphere reconnect with coronal loops near their base. This process displaces the open field line and disturbs and imparts energy into the overlying corona, thereby determining the Poynting vector into the corona. This process releases mass from the loop into the corona and determines the mass flux of the solar wind. The Poynting vector and mass flux into the corona determine the final speed of the solar wind and yield a relationship that provides an excellent fit to observations. The reconnection of open field lines with coronal loops, and their subsequent displacements, also results in a diffusive transport of open field lines, which influences the configuration of the heliospheric magnetic field.
Evidence for electrostatic decay in the solar wind at 5.2 AU
Authors: Thejappa G. , MacDowall R. J. , Scime E. E. , Littleton J. E.
Published: May 2003

The Unified Radio and Plasma wave Experiment (URAP) on the Ulysses spacecraft provides in situ observations of Langmuir waves and ion‐acoustic waves in the solar wind. The observations presented in this paper were obtained at 5.2 AU from the Sun. Low‐frequency (20–200 Hz in the spacecraft frame) electric field signals are observed coincident in time with the most intense Langmuir waves. The low‐frequency wave signals are identified as long‐wavelength ion‐acoustic waves. These observations provide evidence for the decay of Langmuir waves into daughter Langmuir and ion‐acoustic waves (the electrostatic decay process) in the solar wind.
Kinetic and space charge control of current flow and voltage drops along magnetic flux tubes: Kinetic effects
Authors: Boström Rolf
Published: May 2003

The current–voltage characteristic of magnetospheric magnetic flux tubes is studied under rather general conditions, assuming the current to be carried by collision‐free particles experiencing inertia, electric forces, and magnetic mirror forces. The approach is that of the powerful kinetic “orbital motion” theory of current collectors in plasma as developed by H. M. Mott‐Smith and I. Langmuir in 1926. When C. Davisson's 1925 condition on particle access is fulfilled, the total voltage drop is uniquely determined from the current by kinetic/geometric considerations only, without considerations of the actual space charge and electrostatic potential distribution. For any velocity distribution that is isotropic within the source cone, the current–voltage characteristic is linear for sufficiently small currents. The nature of the corresponding global flux tube electric resistance is discussed. The current–voltage characteristic is given for a variety of distribution functions (where the Maxwellian case corresponds to that studied by S. Knight in 1973) including distributions with a reduced (nonisotropic) flux in the source cone. The current–voltage characteristic is also discussed for cases where the Davisson condition is not fulfilled, when the solution method provides a lower bound on the voltage for a given current. Making assumptions on the altitude extent of the voltage drop, we can also define an upper bound on the voltage for a particular current. The magnetospheric source of particles should be taken to be located at the top of the acceleration region in order to fulfill the Davisson condition. This may limit the effective magnetic mirror ratio, such that the applicability of the linear approximation to the current–voltage characteristic is more restricted, and the saturation current is moderately small, which may lead to overvoltages, a redistribution of the current, or an increased extent of the acceleration region. Joining solutions that piecewise fulfill the Davisson condition, we can derive the current flow for a rather wide range of prescribed potential distributions. Earlier works on the Knight mechanism and related theories for determining the voltage drop along a flux tube are discussed. Determination of the electrostatic potential distribution, and verification of the Davisson condition, requires considerations of the space charge (Poisson equation), to be discussed in a second paper.
Equatorial electrojet from Ørsted scalar magnetic field observations
Authors: Ivers David , Stening Robert , Turner Jon , Winch Denis
Published: May 2003

Previous studies of the longitudinal variation of the local noon electrojet have yielded doubtful results either because of the poor data quality or because the local times of equatorial crossings occurred in the early morning or late afternoon. The recent launch of the Ørsted satellite in a near‐circular orbit with slow drift in local time of equatorial crossing has provided the opportunity for researchers to study the electrojet more accurately. Most studies remove the main field using a spherical harmonic model and then search the daytime equatorial passes for the distinctive electrojet trough in total intensity. The present study examines the electrojet for two consecutive 6‐month periods and consequently two local time ranges. Pure signal processing is used to remove the main field directly. The residuals are binned separately for night and day passes on a 1° by 1° grid to enhance the signal to noise ratio and are bin centered by a least squares fitted linear model to compensate for the variations in satellite altitude. Thereafter, for each period the compensated night and day binned values are subtracted from each other to produce a difference set. Global plots of the subsequently spatially filtered difference sets reveal an almost constant electrojet 1/e half width of 3°, as seen at satellite altitude apart from a region in the western Pacific. There are four maxima in the electrojet amplitude at 0°–30°E, 90°–120°E, 180°–220°E, and 260°–290°E in each local time range.
Evidence for a discrete spectrum of persistent magnetospheric fluctuations below 1 mHz
Authors: Lessard M. R. , Hanna J. , Donovan E. F. , Reeves G. D.
Published: May 2003

Ultralow‐frequency waves in the magnetosphere have been observed and widely reported in the literature. One important class of such waves includes field‐line resonances, having fundamental frequencies as low as ∼1.3 mHz. Fluctuations below this frequency have been reported infrequently, although a few studies note oscillations with periods of approximately 30 min. The nature of these waves is especially interesting because the expected wavelength that would be associated with them should be larger than the scale size of the magnetosphere. In fact, the majority of these observations have been acquired using satellites located in the fairly distant magnetotail. In one of these studies, the only one which finds a discrete spectrum, data was obtained between XGSM = −8 and −18RE. In this paper, we show evidence for the existence of a discrete spectrum of oscillations within the magnetosphere that can persist for up to 2 days and perhaps longer. These observations were acquired at geosynchronous orbit and were observed at all local times simultaneously with a signature that is clearest in energetic electron data but can also be seen in the GOES magnetometer data. These facts suggest that a global‐scale magnetic fluctuation, perhaps one that originates in the magnetotail, resonates with energetic electron drift orbits via an E × B force.
Observations of the phases of the substorm
Authors: Voronkov I. O. , Donovan E. F. , Samson J. C.
Published: May 2003

Following the database of large‐scale vortices during pseudo‐breakup and breakup registered by the Gillam All‐Sky Imager, we selected one event (19 February 1996) for a detailed consideration. This event is a sequence of pseudo‐breakup and local substorm, and breakup followed by the large substorm, which is isolated from the previous pseudo‐breakup by the second growth phase. Commencement of these elements of auroral activity was clearly seen above the Churchill line of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS; pseudo‐breakup was completely covered by the field of view of the Gillam All‐Sky Imager). Geotail was located at ∼19 RE in the equatorial plane of midnight sector, which, along with supporting observations from two geostationary satellites (GOES 8 and 9), allowed for a comparison of ground‐based, geostationary orbit and midtail signatures. The pseudo‐breakup consisted of two distinct stages: a near‐exponential arc intensity growth and a poleward vortex expansion that started simultaneously with dipolarization in the inner magnetosphere. The latter corresponded to explosive onset of short‐period (tens of millihertz) pulsations observed at geostationary orbit and on the ground in the vicinity of the arc. No significant disturbances poleward of the vortex were observed. Pseudo‐breakup was followed by the second growth phase, which involved a significant thinning of the plasma sheet. Breakup was of a similar two‐stage character as the pseudo‐breakup. Full onset of the expansive phase that followed breakup was seen simultaneously by all instruments including Geotail, which detected strong perturbations in the midtail. The expansive phase onset launched the second postbreakup package of Pi2 pulsations that were of larger amplitude. Finally, during the substorm recovery phase, the poleward boundary intensifications (PBIs) were observed as long‐period, on the order of 10 min, pulses of electron precipitation. PBI commencement coincided with bursty flows and pulses of plasma energization in the midtail. Observed features support recent ideas claiming that we are dealing with processes (breakup, full onset of the expansive phase, and PBIs) of a distinct physical nature that require different commencement thresholds, namely, the inner plasma sheet instability (pseudo‐breakup and breakup), midtail reconnection (expansive phase onset), and further magnetotail dynamics during the recovery phase (PBIs).
Determination of the solar wind slowdown near solar maximum
Authors: Wang C. , Richardson J. D.
Published: May 2003

The magnitude of the solar wind speed decrease from the inner to the outer heliosphere provides important information about the interaction of the heliosphere with the local interstellar medium (LISM), leading to better estimates of the size and shape of the heliosphere and of the LISM density. In this study we combine observations from multiple spacecraft (ACE, Ulysses, and Voyager 2) near solar maximum with a multifluid one‐dimensional numerical model to examine the solar wind slowdown. We find a decrease in speed of 53–62 km s−1 at Voyager 2 (∼60 AU) with respect to ACE and Ulysses measurements or a 12–14% slowdown, the largest yet reported. The model results agree well with observations if we assume an interstellar neutral hydrogen density of 0.09 cm−3 at the termination shock.
Reconstruction of two‐dimensional structures in the magnetopause: Method improvements
Authors: Hu Qiang , Sonnerup Bengt U. Ö.
Published: May 2003

A recently developed method for recovering two‐dimensional (∂/∂z ≈ 0), time‐stationary magnetic field structures in the magnetopause is improved and further applied to two complete magnetopause crossings by the spacecraft AMPTE/IRM. The technique consists of solving the Grad‐Shafranov (GS) equation for magnetohydrostatic structures as a Cauchy problem, using measured data along a spacecraft trajectory through the structure as spatial initial values. We have implemented the following improvements: (1) allowing for different functional behavior of the transverse pressure, Pt = p + Bz2/2μ0, versus A, the calculated magnetic potential, on the magnetosphere and the magnetosheath side of the magnetopause; (2) allowing for change in the velocity of the magnetopause structures as they move past the spacecraft by using a time‐varying deHoffmann‐Teller (HT) frame velocity; (3) suppressing numerical instabilities and thus extending the integration domain by constraining the behavior of the total pressure. Results from two AMPTE/IRM events show layered and asymmetric structures on the two sides of the magnetopause owing to double‐branched behavior in p(A) as well as Bz(A). A time‐varying HT frame results in a curved spacecraft trajectory through the magnetopause. The reconstructed field structures show important changes in size but only slight changes in configuration. The use of a time‐varying HT frame for determination of time‐varying magnetopause motion is discussed.