bluuebunny/crossref_metadata_2025 huggingface dataset:
Signatures of salinity variability in tropical Pacific Ocean dynamic height anomalies
Authors: Maes Christophe , McPhaden Michael J. , Behringer David
Published: December 2002

The vertical variability of the salinity field, with emphasis on interannual timescales, is examined within the tropical Pacific Ocean (10°N–10°S) using a compilation of the conductivity‐temperature‐depth (CTD) casts for the period 1975–1998. Compared to the vertical dependence of temperature that exhibits a systematic maximum variability at the depth of the main thermocline, the salinity typically shows a maximum variability in the surface layers. One notable region where this rule is violated is the southwestern and central Pacific Ocean. Below the surface, salinity variability is correlated with the strong gradients in mean salinity above and below the subsurface salinity maximum. It is shown that using conventional mean T‐S curves to estimate salinity profiles from temperature observations leads to strong biases because of the large scatter around the T‐S relationships. From the surface to the bottom of the thermocline, the dispersion of the T‐S diagrams is large regardless of whether the conditions are representative of “El Niño” or “La Niña” conditions. Error introduced in computing the dynamic height anomaly (DHA) is larger than 2 dyn. cm (dynamic centimeters) throughout the tropical Pacific if salinity variability is neglected. This error represents 30% of the total variability of the sea surface height in the western Pacific, and more than 50% in the south central Pacific. In order to estimate salinity variability when direct measurements are not available, an empirical orthogonal function analysis of existing temperature and salinity profiles was conducted. It is shown that a relatively low number of dominant modes, typically less than 6, are sufficient to explain 80% or more of the total variance. The separate contributions of the temperature and salinity fields to the DHA are then examined. The salinity contribution is generally smaller than the temperature contribution, though in some instances the two oppose one another, resulting in lowered dynamic height anomalies. These results confirm that salinity variability should not be neglected in ocean analyses that attempt to infer vertical changes in density from sea level fluctuations within the tropical Pacific Ocean.
Cloud susceptibility and the first aerosol indirect forcing: Sensitivity to black carbon and aerosol concentrations
Authors: Chuang Catherine C. , Penner Joyce E. , Prospero Joseph M. , Grant Keith E. , Rau Gregory H. , Kawamoto Kazuaki
Published: December 2002

Present‐day global anthropogenic emissions contribute more than half of the mass in submicron particles primarily due to sulfate and carbonaceous aerosol components derived from fossil fuel combustion and biomass burning. These anthropogenic aerosols increase cloud drop number concentration and cloud albedo. Here, we use an improved version of the fully coupled climate/chemistry models to investigate cloud susceptibility and the first indirect effect of anthropogenic aerosols (the Twomey effect). We examine the correspondence between the model simulation of cloud susceptibility and that inferred from satellite measurements to test whether our simulated aerosol concentrations and aerosol/cloud interactions give a faithful representation of these features. This comparison provides an overall measure of the adequacy of cloud cover and drop concentrations. We also address the impact of black carbon absorption in clouds on the first indirect forcing and examine the sensitivity of the forcing to different representations of natural aerosols. We find that including this absorption does not change the global forcing by more than 0.07 W m−2, but that locally it could decrease the forcing by as much as 0.7 W m−2 in regions where black carbon emissions are pronounced. Because of the nonlinear relationship between cloud drop number and aerosol number concentrations, the total forcing does not equal the sum of the forcing from each individual source. Our estimated total first indirect forcing is −1.85 W m−2, with −0.30 W m−2 associated with anthropogenic sulfate, −1.16 W m−2 associated with carbonaceous aerosols from biomass burning, and −0.52 W m−2 associated with carbonaceous aerosols from fossil fuel combustion. Estimates of forcing by sulfate and total carbonaceous aerosols increase to −0.31 and −1.67 W m−2, respectively, if natural emissions of organic aerosols are only 8.4 Tg yr−1, but decrease to −0.26 and −1.27 W m−2 if they are as large as 42 Tg yr−1. Even larger estimates of forcing are derived if dust and sea‐salt emissions are not included. The effect of aerosol abundance on cloud life cycle may be important but is not treated in this study.
Characterization of the photooxidant formation in the metropolitan area of Milan from aircraft measurements
Authors: Dommen Josef , Prévôt André S.H. , Neininger B. , Bäumle M.
Published: December 2002

Measurements of O3, NO2, HCHO, H2O2, and hydrocarbons were performed on MetAir's airborne sampling platform in the area of Milan within the Italian Po Basin. The objective was to characterize the O3 formation process and its dependence on ambient levels of NOx and volatile organic compounds (VOCs). In the metropolitan area and downwind a regional plume developed of the order of 50 km wide and atop of this an urban plume 10–15 km wide. In the Po Basin, ozone mixing ratios were around 80–130 ppb, while in the regional plume, ozone levels were 15–30 ppb higher. In addition, the urban plume adds another 10–30 ppb. Maximum afternoon ozone mixing ratios reached 195 ppb. Downwind of Milan, ozone strongly increased (0.6–1 ppb km−1), whereas peroxide usually decreased. An observation based budget analysis of the photooxidant formation yielded chemical ozone production rates between 5 and 15 ppb h−1, occasionally exceeding 20 ppb h−1. The chemical peroxide production rate was negligible in the urban and regional plume and small in the adjacent areas. Ozone production rates derived from an observation‐driven steady state model were mostly lower than calculated from the budget approach, while peroxide production rates were negligible. Missing VOCs due to incomplete sampling might be responsible for the discrepancies between the model and measurements. The analysis indicates VOC‐sensitive ozone formation within and downwind of the metropolitan area of Milan. This would still be the result of the model calculation in the case of doubled VOC levels.
Aerosol–radiation interaction in the cloudless atmosphere during LACE 98 2. Aerosol‐induced solar irradiance changes determined from airborne pyranometer measurements and calculations
Authors: Wendling P. , Stifter A. , Mayer B. , Fiebig M. , Kiemle C. , Flentje H. , Wendisch M. , Armbruster W. , Leiterer U. , Hoyningen‐Huene W. v. , Petzold A.
Published: December 2002

One of the goals of the aerosol column closure experiment Lindenberg Aerosol Characterization Experiment (LACE 98) that was carried out over the eastern part of Germany in summer 1998 was to reduce uncertainties in the determination of the radiative effects of aerosol particles over land. For this purpose, a variety of closure experiments was carried out during LACE 98 to determine the chemical, physical, optical, and radiative properties of the aerosols during situations of clean and strongly polluted continental air. By coordinated flights of three aircraft, measurements of aerosol microphysical and optical properties, of spectral surface reflectance, and of upwelling and downwelling solar irradiances were performed at altitude levels between 200 m and 11 km. The measurements are used for a comparison with results from a radiative transfer model. The calculations are based on measured aerosol properties such as the total number concentration and particle size distribution. The chemical composition of the aerosol was obtained from an analysis of filter samples in order to obtain the optical properties of the aerosol throughout the relevant spectral range by use of Mie theory. The results for the daily averaged solar radiative forcing due to aerosol particles reveal cooling of the total Earth/atmosphere system ranging from −4 to −13 W m−2 in cloud‐free conditions.
Remotely sensed geology from lander‐based to orbital perspectives: Results of FIDO rover May 2000 field tests
Authors: Jolliff B. , Knoll A. , Morris R. V. , Moersch J. , McSween H. , Gilmore M. , Arvidson R. , Greeley R. , Herkenhoff K. , Squyres S.
Published: December 2002

Blind field tests of the Field Integration Design and Operations (FIDO) prototype Mars rover were carried out 7–16 May 2000. A Core Operations Team (COT), sequestered at the Jet Propulsion Laboratory without knowledge of test site location, prepared command sequences and interpreted data acquired by the rover. Instrument sensors included a stereo panoramic camera, navigational and hazard‐avoidance cameras, a color microscopic imager, an infrared point spectrometer, and a rock coring drill. The COT designed command sequences, which were relayed by satellite uplink to the rover, and evaluated instrument data. Using aerial photos and Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data, and information from the rover sensors, the COT inferred the geology of the landing site during the 18 sol mission, including lithologic diversity, stratigraphic relationships, environments of deposition, and weathering characteristics. Prominent lithologic units were interpreted to be dolomite‐bearing rocks, kaolinite‐bearing altered felsic volcanic materials, and basalt. The color panoramic camera revealed sedimentary layering and rock textures, and geologic relationships seen in rock exposures. The infrared point spectrometer permitted identification of prominent carbonate and kaolinite spectral features and permitted correlations to outcrops that could not be reached by the rover. The color microscopic imager revealed fine‐scale rock textures, soil components, and results of coring experiments. Test results show that close‐up interrogation of rocks is essential to investigations of geologic environments and that observations must include scales ranging from individual boulders and outcrops (microscopic, macroscopic) to orbital remote sensing, with sufficient intermediate steps (descent images) to connect in situ and remote observations.
Sea surface temperature at the Last Glacial Maximum: A reconstruction using the modern analog technique
Authors: Trend‐Staid Melissa , Prell Warren L.
Published: December 2002

We have applied the modern analog technique (MAT) to reconstruct sea surface temperature (SST) at the Last Glacial Maximum (LGM) using a database of 292 planktonic foraminiferal samples from the Atlantic, Indian, and Pacific Oceans. LGM SST maps reveal many important patterns and gradients which reflect the climate and circulation patterns of the LGM. One of the most important results is the 2°–6°C mean annual cooling in the central to eastern tropical Atlantic and up 8°C cooling in the eastern tropical Pacific. These changes resulted in an increased east‐west SST gradient in the equatorial regions of these two basins, compared to modern. SSTs in eastern boundary currents of all three ocean basins were cooler than modern at the LGM. Additionally, results indicate more zonal flow patterns in the Kuroshio/North Pacific Current and Gulf Stream/North Atlantic Drift. These changes indicate more intense midlatitude‐to‐equator SST gradients at the LGM, which likely resulted in more intense circulation and increased zonal winds. SSTs in much of the tropical and northern Indian Ocean were within 2°C of modern temperatures. Consistent with the results of previous studies, results indicate little change in much of the western tropics and the subtropics of all ocean basins.
Combining remote sensing and ground census data to develop new maps of the distribution of rice agriculture in China
Authors: Frolking Steve , Qiu Jianjun , Boles Stephen , Xiao Xiangming , Liu Jiyuan , Zhuang Yahui , Li Changsheng , Qin Xiaoguang
Published: December 2002

Large‐scale assessments of the potential for food production and its impact on biogeochemical cycling require the best possible information on the distribution of cropland. This information can come from ground‐based agricultural census data sets and/or spaceborne remote sensing products, both with strengths and weaknesses. Official cropland statistics for China contain much information on the distribution of crop types, but are known to significantly underestimate total cropland areas and are generally at coarse spatial resolution. Remote sensing products can provide moderate to fine spatial resolution estimates of cropland location and extent, but supply little information on crop type or management. We combined county‐scale agricultural census statistics on total cropland area and sown area of 17 major crops in 1990 with a fine‐resolution land‐cover map derived from 1995–1996 optical remote sensing (Landsat) data to generate 0.5° resolution maps of the distribution of rice agriculture in mainland China. Agricultural census data were used to determine the fraction of crop area in each 0.5° grid cell that was in single rice and each of 10 different multicrop paddy rice rotations (e.g., winter wheat/rice), while the remote sensing land‐cover product was used to determine the spatial distribution and extent of total cropland in China. We estimate that there were 0.30 million km2 of paddy rice cropland; 75% of this paddy land was multicropped, and 56% had two rice plantings per year. Total sown area for paddy rice was 0.47 million km2. Paddy rice agriculture occurred on 23% of all cultivated land in China.
Seasonal variations in stable carbon and hydrogen isotope ratios in methane from rice fields
Authors: Marik Thomas , Fischer Horst , Conen Franz , Smith Keith
Published: December 2002

During two successive growing seasons, methane emissions from rice fields in Italy were measured. High‐precision measurements of the methane 13C/12C and D/H ratios were carried out by mass spectrometry and tunable diode laser absorption spectrometry. Significant seasonal variations were found for both δ13C and δD. The results confirm earlier observations by Bergamaschi [1997] in finding a seasonal cycle with isotopically depleted methane in the main growing season and higher values at the beginning and the end of the season during drainage of the field. The measured δ13C diurnal cycles showed a strong correlation with the methane emission rate. The isotopic composition of methane, which depended on the season, can be explained by variations of the different pathways for methane production, oxidation, and release into the atmosphere. A model based on these parameters was able to reproduce the field measurements and indicate the principal causes of observed fluctuations in the isotopic methane composition.
Role of the ocean in the global mercury cycle
Authors: Mason R. P. , Sheu G.‐R.
Published: December 2002

Air‐sea exchange of mercury (Hg) is a critical part of the global Hg cycle as it determines, to a large degree, the response time of the biosphere to changes in mercury inputs. Recent measurements have demonstrated that the cycling of Hg between the ocean and atmosphere is complex, principally because of the enhanced oxidation of elemental Hg (Hgo), and the formation of reactive gaseous Hg (RGHg) in the marine boundary layer. We estimate that the dry deposition of RGHg to the ocean, which has not been previously considered in global budgets, is 35% of the total Hg input to the ocean. A further reevaluation of the global Hg cycle suggests that there is a net transfer of Hg from the terrestrial environment to the ocean and that the deep ocean Hg concentration is increasing by a few percent per year. Similarly, anthropogenic inputs on land have increased Hg on the Earth's surface layer with accumulation in the terrestrial environment accounting for nearly 80% of the net input from man's activities. Dry deposition of RGHg is important for the terrestrial realm but because of its relatively short residence time in the atmosphere, it is the oxidation of Hgo over the ocean, rather than RGHg transport offshore, which is primarily contributing to oceanic RGHg deposition.
A process‐based model of conifer forest structure and function with special emphasis on leaf lifespan
Authors: Osborne Colin P. , Beerling David J.
Published: December 2002

We describe the University of Sheffield Conifer Model (USCM), a process‐based approach for simulating conifer forest carbon, nitrogen, and water fluxes by up‐scaling widely applicable relationships between leaf lifespan and function. The USCM is designed to predict and analyze the biogeochemistry and biophysics of conifer forests that dominated the ice‐free high‐latitude regions under the high pCO2 “greenhouse” world 290–50 Myr ago. It will be of use in future research investigating controls on the contrasting distribution of ancient evergreen and deciduous forests between hemispheres, and their differential feedbacks on polar climate through the exchange of energy and materials with the atmosphere. Emphasis is placed on leaf lifespan because this trait can be determined from the anatomical characteristics of fossil conifer woods and influences a range of ecosystem processes. Extensive testing of simulated net primary production and partitioning, leaf area index, evapotranspiration, nitrogen uptake, and land surface energy partitioning showed close agreement with observations from sites across a wide climatic gradient. This indicates the generic utility of our model, and adequate representation of the key processes involved in forest function using only information on leaf lifespan, climate, and soils.
Factors controlling the flux of organic carbon to the bathypelagic zone of the ocean
Authors: Francois Roger , Honjo Susumu , Krishfield Richard , Manganini Steve
Published: December 2002

Particle fluxes measured with time series sediment traps deployed below 2000 m at 68 sites in the world ocean are combined with satellite‐derived estimates of export production from the overlying water to assess the factors affecting the transfer of particulate organic matter from surface to deep water. Multiple linear regression is used to derive an algorithm suggesting that the transfer efficiency of organic carbon, defined as the settling flux of organic carbon normalized to export production, increases with the flux of carbonate and decreases with water depth and seasonality. The algorithm predicts >80% of the organic carbon transfer efficiency variability in diverse oceanic regions. The influence of the carbonate flux suggests that the ballasting effect of this biogenic mineral may be an important factor promoting export of organic carbon to the deep sea by increasing the density of settling particles. However, the lack of a similar effect for biogenic opal suggests that factors other than particle density also play a role. The adverse effect of increasing seasonality on the transfer efficiency of carbon to the deep sea is tentatively attributed to greater biodegradability of organic matter exported during bloom events. In high latitude opal‐dominated regions with high f‐ratios and seasonality, while a higher fraction of net production is exported, a higher fraction of the exported organic matter is remineralized before reaching bathypelagic depths. On the other hand, in warm, low latitude, carbonate‐dominated regions with low f‐ratios and seasonality, a higher fraction of the exported organic matter sinks to the deep sea.
Global response of the terrestrial biosphere to CO2and climate change using a coupled climate‐carbon cycle model
Authors: Berthelot M. , Friedlingstein P. , Ciais P. , Monfray P. , Dufresne J. L. , Le Treut H. , Fairhead L.
Published: December 2002

We study the response of the land biosphere to climate change by coupling a climate general circulation model to a global carbon cycle model. This coupled model was forced by observed CO2emissions for the 1860–1990 period and by the IPCC SRES‐A2 emission scenario for the 1991–2100 period. During the historical period, our simulated Net Primary Production (NPP) and net land uptake (NEP) are comparable to the observations in term of trend and variability. By the end of the 21st century, we show that the global NEP is reduced by 56% due to the climate change. In the tropics, increasing temperature, through an increase of evapotranspiration, acts to reduce the soil water content, which leads to a 80% reduction of net land CO2uptake. As a consequence, tropical carbon storage saturates by the end of the simulation, some regions becoming sources of CO2. On the contrary, in northern high latitudes, increasing temperature stimulates the land biosphere by lengthening the growing season by about 18 days by 2100 which in turn leads to a NEP increase of 11%. Overall, the negative climate impact in the tropics is much larger than the positive impact simulated in the extratropics, therefore, climate change reduce the global land carbon uptake. This constitutes a positive feedback in the climate‐carbon cycle system.
A circumpolar perspective on fluvial sediment flux to the Arctic ocean
Authors: Holmes Robert M , McClelland James W. , Peterson Bruce J. , Shiklomanov Igor A. , Shiklomanov Alexander I. , Zhulidov Alexander V. , Gordeev Viatcheslav V. , Bobrovitskaya Nelly N.
Published: December 2002

Quantification of sediment fluxes from rivers is fundamental to understanding land‐ocean linkages in the Arctic. Numerous publications have focused on this subject over the past century, yet assessments of temporal trends are scarce and consensus on contemporary fluxes is lacking. Published estimates vary widely, but often provide little accessory information needed to interpret the differences. We present a pan‐arctic synthesis of sediment flux from 19 arctic rivers, primarily focusing on contributions from the eight largest ones. For this synthesis, historical records and recent unpublished data were compiled from Russian, Canadian, and United States sources. Evaluation of these data revealed no long‐term trends in sediment flux, but did show stepwise changes in the historical records of two of the rivers. In some cases, old values that do not reflect contemporary fluxes are still being reported, while in other cases, typographical errors have been propagated into the recent literature. Most of the discrepancy among published estimates, however, can be explained by differences in years of records examined and gauging stations used. Variations in sediment flux from year to year in arctic rivers are large, so estimates based on relatively few years can differ substantially. To determine best contemporary estimates of sediment flux for the eight largest arctic rivers, we used a combination of newly available data, historical records, and literature values. These estimates contribute to our understanding of carbon, nutrient, and contaminant transport to the Arctic Ocean and provide a baseline for detecting future anthropogenic or natural change in the Arctic.
Petrology and geochemistry of the lower ocean crust formed at the East Pacific Rise and exposed at Hess Deep: A synthesis and new results
Authors: Coogan L. A. , Gillis K. M. , MacLeod C. J. , Thompson G. M. , Hékinian R.
Published: December 2002

The geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle, how these melts mix and crystallize, and the role of hydrothermal circulation in this portion of the crust. Unfortunately, lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes. At Hess Deep, crust formed at the East Pacific Rise (EPR) is exposed due to the propagation of the Cocos‐Nazca spreading center westward. Here we review our state of knowledge of the petrology of lower crustal material from Hess Deep and document new mineral major and trace element compositions, amphibole‐plagioclase thermometry, and plagioclase crystal size distributions. Samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive (moderate Cr, high Mg#) orthopyroxene and very calcic plagioclase. Because primitive mid‐ocean ridge basalts (MORBs) are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle. These apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction. The occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal MORB in the axial magma chamber (AMC), but rather that some melts partially crystallize in isolation within the lower crust. However, evidence that primitive melts fed the AMC, along with steep fabrics in shallow gabbros (from near the base of the dyke complex), provides support for models in which crystallization within the AMC followed by crystal subsidence is also an important process in lower crustal accretion. More evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust. Amphibole‐plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high‐temperatures in the shallow gabbros (Tave 713°C) and show that the temperature of amphibole formation was similar in deeper gabbros (Tave 722°C). This thermometry also suggests that fracture and grain boundary permeability for seawater‐derived fluids was open over the same temperature interval.
Determining the inputs to the Mariana Subduction Factory: Using core‐log integration to reconstruct basement lithology at ODP Hole 801C
Authors: Barr S. R. , Révillon S. , Brewer T. S. , Harvey P. K. , Tarney J.
Published: December 2002

Calculating elemental mass balance across subduction zones enhances our understanding of global geochemical budgets and large‐scale Earth processes. However, to accurately constrain the input flux, it is critical to know the lithological diversity and chemical characteristics of the downgoing oceanic plate. The west Pacific altered ocean crust that was drilled during Ocean Drilling Program (ODP) Leg 185 represents a significant component of the input to the Mariana Subduction Factory. The lithological sequence in Hole 801C consists of aphyric basalt, occurring as thick massive units, pillow units, and breccia units. The shallowest basalts are intercalated with sediments and two hydrothermal deposits. Core recovery was good for a basement hole (average 47%); however, over half the lithological section was unaccounted for. Downhole logging data provide a continuous record of physical, chemical, and structural properties of the rocks at the borehole wall, thus, when calibrated using available cored material, they can be used to reconstruct lithology in unrecovered intervals. Core‐log integration results reveal a significant bias in core recovery, with infrequent retrieval of delicate breccia units and preferential recovery of more massive, competent, and less altered flow units. This is important because the breccia units are host to many of the key tracer elements used in mass balance calculations. The massive basalts exhibit high density, resistivity and velocity values and low porosity and gamma ray values. Formation MicroScanner (FMS) images of massive basalts are bright (reflecting their resistive nature) with a homogenous texture and regular fracture pattern. Breccia or pillow basalts are characterised by low resistivity, density and velocity, and high porosity and gamma ray values and unrecovered intervals displayed these same characteristics. The reconstructed log‐based lithological sequence consists of thick massive flow units (27.4%), pillow units (33%), breccia units (31%), sediments (1.4%), and hydrothermal deposits (1.3%), with 5.9% unclassified due to unreliable tool response in intervals where hole conditions were poor. These findings have a significant bearing on the Subduction Factory recycling equation. The proportion of pillow basalts doubled and the amount of breccia increased six‐fold from that reported using core description alone, demonstrating convincingly that core‐log integration is essential to provide an accurate representation of the input flux. The log‐based stratigraphy reconstructed for Hole 801C represents the first example of Jurassic fast‐spread (160 km/m.y.) ocean crust and provides constraints on the relationships between crustal structure, age, alteration, and spreading rate.
“Dawn‐dusk asymmetry” in solar coronal arcade formations
Authors: Isobe Hiroaki , Shibata Kazunari , Machida Shinobu
Published: December 2002

Solar flares/arcades and magnetospheric substorms are believed to have a common physical process: energy release via magnetic reconnection. In substorms, it is known that the reconnection point tends to occur more in the dusk side of the magnetotail than in the dawn side. This asymmetry is called the dawn‐dusk asymmetry. However, no one has examined so far the existence of such asymmetry in the solar corona. We investigated the directions of arcade formation and −vin × B electric field for 32 events, and found that more arcades successively formed in the direction of the electric field than in the opposite direction. This may be a solar analogue of dawn‐dusk asymmetry. We also found that velocity of successive formation of arcades increases with the aspect ratio of the arcade.
Statistical Characteristics of Sprite Halo Events Using Coincident Photometric and Imaging Data
Authors: Miyasato R. , Taylor M. J. , Fukunishi H. , Stenbaek‐Nielsen H. C.
Published: December 2002

Sprite halos are brief, diffuse flashes, which occur at the top of a sprite and precede the development of streamer structures at lower altitudes. We have investigated the characteristics of sprite halos in detail using coincident photometric and imaging data obtained during the Sprites'96 and '99 campaign in Colorado and Wyoming, USA. It is found that the average altitude of the centroid of the halo emission and the mean horizontal diameter of the halo events are ∼80 and ∼86 km, respectively, while the average speed of the descending motion of the sprite halos was ∼4.3 × 107 m/s. It was also found that the peak current intensity of the causative CG decreases with time delay from the onset of the sferics.
The influence of vortex ozone depletion on Arctic ozone trends
Authors: Andersen S. B. , Knudsen B. M.
Published: December 2002

Satellite measurements over the Arctic region (63°–90° N) have shown that the amount of ozone in the atmospheric column decreased and exhibited increased variability during the last decade. We here show that for the period 1992–2000 the major part (75%) of the observed ozone decrease in March in the Arctic and most of the variability may be explained by winter/spring ozone depletion in the polar vortex. A good correlation (0.96) between the Arctic ozone decreases and the polar vortex winter/spring ozone depletion is found, which is partly a result of coupling between dynamical and chemical effects.
Global IMAGE/HENA observations of the ring current: Examples of rapid response to IMF and ring current‐plasmasphere interaction
Authors: Brandt P. C:son , Mitchell D. G. , Ebihara Y. , Sandel B. R. , Roelof E. C. , Burch J. L. , Demajistre R.
Published: December 2002

Energetic neutral atom (ENA) images in the energy range 16–50 keV obtained by the High‐Energy Neutral Atom (HENA) imager on board the IMAGE satellite are presented from the period 23–26 May 2000. Two events during this period are investigated: (1) A rapid decrease on 23 May of the entire dayside ring current fluxes and (2) an unexpected local time distribution of the ring current during the storm commencing on 24 May. In the first event, ENA fluxes from the dayside ring current decrease rapidly for 10 min. The start of the decrease is associated with a sharp, southward turning of the interplanetary magnetic field (IMF) and a steep increase of the cross polar cap potential. Simulated equatorial ion fluxes are able to reproduce the event using a kinetic ion drift model. The observations are explained in terms of rapidly changing drift paths during a reconfiguration of the convectional electric field in the magnetosphere. The reconfiguration timescale (∼10 min) is too short to modify the pitch angle distribution (PAD). The results show that the solar wind electric field over the dayside magnetosphere penetrates to the inner magnetosphere within a few minutes. In the second event an enhanced ring current develops during the storm on 24 May. Below 27 keV at L < 4 the ring current extends from midnight clockwise through noon toward the dawn sector. At energies above 27 keV the images show an enhancement in the ENA emissions coming from the noon region and a lack of ENA emissions from the duskside. Apart from the main phase of the storm the convection through the magnetosphere was steady throughout the 3‐day recovery phase. Images in extreme ultraviolet (EUV) of the plasmasphere reveal a spatial relation between the ENA emissions and a plasmaspheric tail. Data from the Polar satellite together with preliminary ENA simulations suggest that the lack of ENA emissions is an effect of ion PADs peaked around 90°. We discuss possible explanations for the modification of the PADs, such as adiabatic drifts, charge exchange, and wave‐particle interaction.
Radiation belt electron flux dropouts: Local time, radial, and particle‐energy dependence
Authors: Onsager T. G. , Rostoker G. , Kim H.‐J. , Reeves G. D. , Obara T. , Singer H. J. , Smithtro C.
Published: December 2002

The radiation belt electrons in Earth's magnetosphere exhibit substantial variability driven by changing solar wind conditions. The electron dynamics are due to a number of different adiabatic and nonadiabatic processes that can result in rapid increases and decreases in the particle flux levels. In this paper we present observations of abrupt flux decreases driven by a moderate geomagnetic storm. The particle dynamics are found to have significant local time and energy dependence that developed over roughly a 10‐hour period beginning with the onset of the storm. The electrons with energies greater than 2 MeV dropped fairly abruptly at various local times, but not simultaneously at different local times. It is shown that the initial flux dropout was due to the development of local taillike magnetic field stretching, rather than due to more global processes such as ring current buildup or large‐scale radial diffusion. It is also found that while the lower energy electrons (E < 300 keV) fully recovered by the end of the storm, the >2 MeV electrons were lost from the magnetosphere and did not recover. These results indicate that the initial dropout of the radiation belt electrons at geostationary orbit was controlled by the adiabatic response to localized changes in the geomagnetic field that develop over many hours, but that eventually nonadiabatic processes acted to cause the loss of electrons from the magnetosphere. It is also shown that during geomagnetically quiet conditions, the energetic electron flux can remain at nearly constant levels for as long as 1 week, suggesting that in the absence of geomagnetic activity either the outer radiation belt electron loss rate becomes quite small or the loss and growth rates are balanced.
An investigation of ionospheric responses, and disturbance thermospheric winds, during magnetic storms over South American sector
Authors: Pincheira X. T. , Abdu M. A. , Batista I. S. , Richards P. G.
Published: December 2002

This paper presents the results of a study of the ionospheric and thermospheric response to magnetic storm disturbances for four stations covering a wide range of latitude in the South American region. The study is based on the F layer critical frequency (foF2) and the peak height (hmF2) from Fortaleza (FZ), which is equatorial, Cachoeira Paulista (CP), which is low latitude, Concepción (CON), a midlatitude site, and King George Island (KGI), a middle to high latitude site. Equivalent neutral winds are extracted from the measured hmF2 by employing the Field Line Interhemispheric Plasma (FLIP) model. The Equatorial Ionization Anomaly (EIA) can undergo enhancement due to magnetospheric disturbance electric field that penetrates to low latitudes during the growth phase of a storm/substorm, whereas EIA inhibition occurs more often under disturbance dynamo (DD) electric field. Season‐dependent meridional/transequatorial winds can significantly alter the EIA response to disturbance electric fields. Large amplitudes of DD electric field is observed during evening and morning hours in equinoctial, but not in winter, months. Over the Brazilian low latitude of large westward magnetic declination angle, large equatorward meridional wind surges seem to be helped by the presence of disturbance zonal (westward) winds. The low latitude disturbance can, in some cases, be traced to specific disturbance fronts originating promptly from auroral activity enhancements. This study confirms earlier studies that show that the quiet time meridional wind increases with increase of latitude and that over midlatitude it shows decreases with increase of solar flux due to the effect of the ion drag force. This study further shows that the disturbance meridional wind and the intensity of DD electric field increase with increasing intensity of high latitude energy input. The negative ionospheric storm phase over middle latitude, observed in equinox and not in winter, is also intensified with the increasing intensity of high latitude energy input.
Large‐amplitude Alfvén waves in open and closed coronal structures: A numerical study
Authors: Grappin R. , Léorat J. , Habbal S. Rifai
Published: December 2002

We present the first simulations of the coronal response to Alfvén wave injection using transparent boundaries in a classical one‐fluid, isothermal axisymmetric model including both closed and open magnetic field structures. The aim of the work is first to study how Alfvén waves change the contrast between the equatorial and high‐latitude wind, and second, how they modify the geometry of the wind and its global stability. We integrate the full time‐dependent MHD equations, and inject large‐amplitude (150 km/s), low‐frequency (20 min period) waves at 1.8 Rs, both in open and in closed field line regions, except within narrow regions around the poles and the equator. The domain considered extends up to 16 Rs. Our principal results are the following: (1) The assumption of a latitude‐independent Alfvén wave amplitude compatible with observations leads to a large acceleration both of the high‐latitude and equatorial wind; as a consequence, the contrast between slow and fast wind speeds at 16 Rs is not as large as the observed values if extrapolated to 1 AU, a result which could potentially change with the use of better resolved, less dissipative simulations; (2) an initial delay in the Alfvén flux onset in one hemisphere generates a stable global circulation in the closed loops region, which after a long enough time produces a global north‐south asymmetry and changes the structure of the corona as a whole.
Four‐point Cluster application of magnetic field analysis tools: The discontinuity analyzer
Authors: Dunlop M. W. , Balogh A. , Glassmeier K.‐H.
Published: December 2002

The Cluster spacecraft have collected 3‐D information on magnetic field structures at small to medium scales (100–1000 km) in the Earth's dayside magnetosphere, magnetosheath, and near solar wind. We focus here on the first application of the discontinuity analyzer analysis technique (determination of the geometry and motion of magnetic discontinuities, using interspacecraft timing and variance information) for the case of boundaries that are planar over the four‐spacecraft array. This multipoint technique uses minimum variance of the magnetic field to determine the boundary normals at each spacecraft. Identification of the four spacecraft crossings requires accurate timing of encounters at each spacecraft, which depends on key features in each time series being stationary in the frame of the boundary. The technique is tested for selected boundary crossing events (magnetopause and bow shock, and one TD in the magnetosheath), for which the independently determined normals are closely (∼1–2°) colinear. Closely aligned normals, on spatial scales of ∼600–1000 km, were found for most crossings studied. For such a planar geometry, the motion can be determined unambiguously and shows that significant acceleration (up to ∼10 km s−2) of the boundary over the spacecraft is nearly always present. This has the consequence that previous estimates of boundary scales, based on constant motion between two spacecraft, may be in error by factors of two or three and that implied variations in boundary thickness could therefore be predominantly due to variations in speed. We comment on the error introduced through assumptions of constant motion.
Dynamical coupling of neutrals and ions in the high‐latitude F region: Simultaneous FPI and HF radar observations at Syowa Station, Antarctica
Authors: Sakanoi Takeshi , Fukunishi Hiroshi , Okano Shoichi , Sato Natsuo , Yamagishi Hisao , Yukimatu Akira S.
Published: December 2002

Simultaneous measurements of neutral winds and plasma drifts have been carried out to clarify the local neutral‐ion coupling processes in the auroral F region. High‐resolution maps of thermospheric winds and plasma drifts were made using data obtained by the Fabry‐Perot Doppler Imaging System (FPDIS) and by the Syowa‐South HF radar, respectively, both installed at Syowa Station (66.4° in magnetic latitude), Antarctica during the solar minimum period in 1996. We made case studies for two nights: one on 13 May 1996 during a rather quiet period including the growth phase of a substorm, and the other on 24–25 April 1996 during a moderately active period. From 2000 to 2200 UT on 13 May, the velocities of neutral winds and plasma drifts showed similar time variations: a negative excursion followed by a positive excursion. However, the variations of plasma drifts preceded those of neutral winds with a time lag of ∼30 min, and plasma drift velocities were about 7–10 times greater than neutral wind velocities. Therefore, it is suggested that the variations of neutral winds were driven by the ion drag force. To interpret results of the 13 May case, we simulated the relationship between neutral winds and ion drifts by including the effect of auroral heating, such as Joule and particle heating. The results suggest that strong neutral winds were driven by the ion drag and the pressure gradient force induced by auroral heating. In contrast, in the case of 24–25 April, no similarity was found between neutral winds and plasma drifts. Although it is difficult to specify sources for the discrepancy, some possible origins are proposed. For example, if the pressure gradient force is much greater than the ion drag force as in the case of disturbed period, neutral wind variations become independent of plasma drift variations. The discrepancy is also caused by the existence of vertical winds associated with active aurora.
Mutual nonlinear interaction between two kinetic Alfven waves and its effect on filamentation: Applications to solar wind and coronal heating
Authors: Shukla Anita , Sharma R. P.
Published: December 2002

This paper presents the model equations governing the mutual nonlinear interaction between two small but finite amplitude kinetic Alfven waves using two fluid models when the β of the plasma is less than unity (for high‐β cases one has to use the Boltzmann equation). The source of nonlinearity is the nonlinear electron heating and ponderomotive force‐driven nonlinear modification in the background density. The solutions of these model equations have been obtained by using analytical and numerical methods to study the effect of their mutual interaction on filament (hot spot) formation. It is concluded that the threshold field required for filament formation and their critical size is significantly affected by the mutual nonlinear interaction. Its effect on solar wind turbulence and coronal heating is also pointed out.
Magnetospheric electron density model inferred from Polar plasma wave data
Authors: Denton R. E. , Goldstein J. , Menietti J. D. , Young S. L.
Published: December 2002

Observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March 1996 to September 1997, during which time the Polar orbit sampled all magnetic local time (MLT) values three times. Using the entire data set, we bin the ne values with respect to maximum field line radius LRE and radius R to generate an average density model for 3.5 ≤ L ≤ 7.8 and 3RE ≤ R ≤ LRE. While our method does not assume any particular functional form for the density dependence along field lines, we find that the dependence can be roughly described by the power law form ne = ne0 (LRE/R)α with α having on average a value 0.2–0.9 in the plasmasphere (inner L region, 3.5 ≤ L ≤ 5.5), and a value 1.6–2.1 in the plasmatrough (outer L region, L ≥ 7). For instance, α = 0.8 ± 1.2 at L = 4.4 and α = 2.1 ± 1.4 at L = 7 (where the errors noted are overestimates). This result is similar to a previous result based on the Polar PWI data but using a different method. Alternately, the scale length for field line variation of density Lα ≡ (LRE)/ = 5.5 ± 4.1RE across the entire region 4.4 ≤ L ≤ 7.8 (where the error noted is an overestimate). The data is also separated according to MLT, and it is shown that in the dawn sector the density is depleted and the radial dependence is steeper (larger α) than for other MLT sectors. A similar result holds when the averaged Kp value is large (>2).
Theory of azimuthally small‐scale Alfvén waves in an axisymmetric magnetosphere with small but finite plasma pressure
Authors: Mager Pavel N. , Klimushkin Dmitri Yu.
Published: December 2002

An analytical and numerical study is made of the influence of plasma finite pressure on the structure of Alfvén waves with large numbers of the azimuthal wave number, m ≫ 1, as well as of the conditions under which the waves can be poloidally polarized. The study is based on using the axisymmetric model of the magnetosphere assuming the background plasma inhomogeneity along field lines and across magnetic shells, the curvature of field lines, and an equilibrium electric field. The poloidality condition of the mode can imply that in the region where many azimuthal wavelengths fit in the region where the propagation of the wave is possible (the transparent region). The broader is the transparent region, the less stringent conditions are imposed on azimuthal wave numbers of the poloidally polarized Alfvén wave. In this paper it is shown that with nonzero plasma pressure, the transparent region is much broader when compared with cold plasma case. For instance, if finite pressure for the second standing harmonic along a field line of the wave (N = 2) is accounted for, the width of the transparent regions several thousand kilometers, and poloidal Alfvén waves can exist when m ≫ 10. This is consistent with experimental data on radially polarized Pc4 pulsations in the magnetosphere which have m from 50 to 150, whereas in the case of zero pressure the transparent region does not exceed a few tens of kilometers; therefore, in β = 0 case, poloidally polarized waves must have too large values of m in excess of 1000, which is not observed. A further important result of this study implies that near the ring current maximum there must exist a cavity, and high‐m oscillations enclosed within this cavity must be standing oscillations across magnetic shells.