bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth and high‐resolution paleoenvironmental signals of rhodoliths (coralline red algae): A new biogenic archive
Authors: Halfar Jochen , Zack Thomas , Kronz Andreas , Zachos James C.
Published: February 2004

We investigated rhodoliths (coralline red algae) from a subtropical locality in the Gulf of California (Lithothamnium crassiusculum) and a subarctic locality in Newfoundland (Lithothamnium glaciale) for their potential as paleoenvironmental archives using microanalytical geochemical techniques to measure variations in δ18O, Mg, and Ca. Rhodoliths are potentially well suited as recorders of shallow water paleoenvironmental signals because they (1) have worldwide distribution from the tropics to polar regions, (2) are long lived from decades to centuries, and (3) display well‐developed growth bands. Our results indicate that rhodolith growth bands preserve ultrahigh‐resolution records of paleoceanographic‐paleoclimatic change and likely constitute an important new archive for reconstructing the paleoenvironmental history of littoral‐neritic areas in which these algae are found. The δ18O content of individually sampled rhodolith growth bands ranges from −2.4 to −4.6‰ in L. crassiusculum and from −3.2 to −0.3‰ in L. glaciale. In both cases, the range of δ18O values suggests a slightly lower amplitude of variation in sea surface temperature than that actually measured in the ocean at the two study sites. Both L. crassiusculum and L. glaciale show a negative offset from isotopic equilibrium. Electron microprobe analysis of magnesium and calcium in growth bands reveals cyclic variations with values ranging between 7.7–18.5 mol % MgCO3 in L. glaciale and 13.2–22.5 mol % MgCO3 in L. crassiusculum. In addition, electron microprobe element maps highlight individual growth bands, provide a powerful approach to study rhodolith formation, and indicate that the specimens we analyzed have vertical growth rates of 250–450 μm/yr.
Inherent optical properties imagery of the western North Atlantic Ocean: Horizontal spatial variability of the upper mixed layer
Authors: Hoge Frank E. , Wright C. Wayne , Lyon Paul E. , Swift Robert N. , Yungel James K.
Published: February 2004

Until now, no satellite methods were available for the study of inherent optical properties (IOPs) over wide areas of the world's oceans. Herein the coefficients of phytoplankton absorption, chromophoric dissolved organic matter (CDOM)‐detritus absorption, and total backscattering have been retrieved from Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) 412, 490, and 555 nm reflectances by linear matrix inversion of an oceanic radiance model. The retrieved phytoplankton absorption coefficient from an October 6, 1997, image of the Middle Atlantic Bight (MAB) reveals (1) characteristic patchiness over the shelf, slope, Gulf Stream, and Sargasso Sea, (2) a warm‐core ring being encircled by a spiral streamer of phytoplankton‐containing cooler shelf water, and (3) phytoplankton prominence at South Atlantic Bight capes and remarkable minima between the capes. The retrieved CDOM‐detritus absorption coefficient image readily shows (1) the characteristic offshore 20× decline from ∼0.4 to 0.02−1, (2) a sharp drop at the Gulf Stream NW boundary, (3) striations of CDOM‐detritus within the Gulf Stream, and (4) that during the fall, non‐bloom season, the dominant absorbing constituent over the MAB continental shelf is not phytoplankton but CDOM‐detritus. The retrieved total backscattering coefficient image shows a patchy depression over the middle to outer shelf (compared to the Gulf Stream and inner shelf). A feature common to all three inherent optical property images is the northeasterly extrusion of phytoplankton, CDOM‐detritus, and other constituents from Cape Hatteras coastal/shelf water along a streamer adjacent to the Gulf Stream. No similar evidence of shelf water export was observed over the Diamond Shoals area south of Cape Hatteras where previous researchers have reported episodic offshore advection of shelf water. A second mechanism for carbon export can be seen in the New York Bight where a tongue of outflow from the Hudson River can be seen to cross the shelf and interact with the northern wall of a warm‐core ring. A concurrent 150 m altitude airborne underflight with a calibrated ocean‐viewing 256‐channel spectroradiometer provided supplementary atmospheric correction to the SeaWiFS remote sensing reflectances and validation of the resulting SeaWiFS IOP retrievals. Simultaneous airborne lidar fluorescence was used to confirm further the SeaWiFS CDOM‐detritus and phytoplankton absorption coefficient IOP retrievals. The results strongly suggest that if satisfactory atmospheric correction can eventually be achieved without airborne underflights, then the SeaWiFS data will readily allow global‐scale IOP variability studies.
Validation of two algorithms to retrieve ocean wave spectra from ERS synthetic aperture radar
Authors: Voorrips A. C. , Mastenbroek C. , Hansen B.
Published: February 2004

Wave spectra that are retrieved from ERS‐1/2 synthetic aperture radar (SAR) wave mode observations with two different algorithms are validated against 6 years of buoy observations. The Max‐Planck Institut für Meteorologie (MPIM) algorithm, which runs operationally at the European Centre for Medium‐Range Weather Forecasts (ECMWF), is found to deteriorate the quality of the WAM spectrum which is used as a first guess. The Semi‐Parametric Retrieval Algorithm (SPRA) does not use a first‐guess spectrum. For wavelengths which are observed by the SAR, it has a skill comparable to WAM. Several causes for the poor performance of the MPIM scheme are suggested. First, despite the fact that the SAR generally does not resolve the wind sea peak, the MPIM scheme allows for independent adjustment of its energy and peak frequency. Second, by using the quasi‐linear approximation in the inversion, the scheme is inclined to interpret the SAR signal at low wave numbers as swell, whereas often it is generated by waves at higher wave numbers via nonlinearities in the SAR mapping. Third, the MPIM scheme is not able to adjust the spectral width of wave systems. The SPRA scheme retrieves swell information only up to a 180° directional ambiguity, and the SPRA retrievals often contain a spectral gap between the shortest waves observed by the SAR and the parameterized wind sea. In conclusion, the retrieval scheme performing best is the SPRA scheme, which has an accuracy comparable to WAM model output for the longer‐swell waves.
Measurements and modeling of swash‐induced pressure gradients in the surface layers of a sand beach
Authors: Baldock Tom E. , Baird Andrew J. , Horn Diane P. , Mason Travis
Published: February 2004

Field measurements of swash‐induced hydraulic (pressure) gradients in the surface layers of a sand beach are presented and compared to a one‐dimensional diffusion model. The model provides a description of flow through a porous, quasi‐saturated sediment and is driven by the time‐varying swash depth on the beach surface. The field measurements broadly show four different types of pressure propagation behavior, which appear dependent on cross‐shore position in the swash zone. The hydraulic gradients within the surface layers of the beach are typically found to be much greater than those likely to be generated by tidally‐induced groundwater flow. In these instances the diffusion model generally provides a good description of the data. However, very large hydraulic gradients are frequently observed just below the beach surface, and these cannot be adequately described by the diffusion model. A mechanism is proposed through which these large hydraulic gradients could be generated.
Internal hydraulic flows on the continental shelf: High drag states over a small bank
Authors: Nash Jonathan D. , Moum James N.
Published: February 2004

Observations of currents, hydrography, and turbulence provide unambiguous evidence for hydraulic control of flow over an isolated three‐dimensional topographic feature on Oregon's continental shelf. The flow becomes critical at the crest of the bank, forming a strong supercritical downslope flow in the lower layer. Farther downstream, internal hydraulic jumps form as the bottom flow becomes subcritical. As a consequence, turbulence is greatly enhanced in the bottom boundary layer, in the sheared interface above the swiftly flowing bottom current, and in the internal hydraulic jump. The dissipation rate of turbulent energy is consistent with the mean energy removal rate for a hydraulic jump in an idealized two‐layer flow. This enhanced turbulence constitutes a “high drag” state of the flow in which the form drag introduced by the topography exerts significant influence on the flow around it and mixing is increased 102–103 times the background values.
Mesoscale variability in biological community structure and biomass in the Antarctic Polar Front region at 170°W during austral spring 1997
Authors: Brown Susan L. , Landry Michael R.
Published: February 2004

The influence of the Antarctic Polar Front (PF) on microbial biomass and community structure was investigated during late spring, October‐November 1997, as part of the U.S. Joint Global Ocean Flux Study Antarctic Environment and Southern Ocean Process Study. In conjunction with SeaSoar sampling, samples for flow cytometry and epifluorescence image analysis were collected across the PF region along a 170°W transect and in two maps involving repeated crossings of the front. Phytoplankton abundance and size estimates clearly showed the influence of the front, with smaller, more numerous cells to the north and larger, less abundant cells to the south. Autotrophic biomass varied substantially across the region, ranging from 8 to 102 μg C L−1. Biomass accumulation, dominated by Phaeocystis spp. and Chaetoceros spp., was particularly apparent in discrete areas downstream of a frontal meander feature. Grazer biomass, ranging from 1 to 31 μg C L−1, was usually much less than 50% of phytoplankton biomass and did not show any spatial trends with regard to the PF. The distribution of heterotrophic bacteria was clearly influenced by the PF, with larger, less abundant cells south of the frontal zone. The developing assemblage of phytoplankton in the frontal meander was biologically distinct and spatially separated from the community sampled at the marginal ice zone. Analysis of phytoplankton biomass increases along PF current streamlines yielded net growth rates of ∼0.08 d−1, pointing to in situ growth, rather than transport, as the primary mechanism for chlorophyll accumulation. The significance of the front on the development of the seasonal phytoplankton increase is evident, yet the spatial heterogeneity of the microbial assemblage indicates a complex physical environment with multiple mesoscale influences.
Inference of tidal elevation in shallow water using a vessel‐towed acoustic Doppler current profiler
Authors: Li Chunyan , Valle‐Levinson Arnoldo , Atkinson Larry P. , Royer Tom C.
Published: February 2004

Vessel‐towed acoustic Doppler current profilers (ADCPs) have been widely used to measure velocity profiles. Since the instrument is usually mounted on a catamaran floating on the surface, previous studies have used the water surface as the reference level from which the vertical coordinate for the velocity profile is defined. However, because of the tidal oscillation, the vertical coordinate thus defined is time‐dependent in an Earth‐coordinate system, which introduces an error to the estimated harmonic constants for the velocity. As a result, the total transport will also be in error. This is particularly a problem in shallow waters where the tidal elevation is relatively large. Therefore tidal elevation needs to be resolved to make a correct harmonic analysis for the velocity. The present study is aimed at resolving the tidal elevation change in shallow water using a vessel‐towed ADCP. Semidiurnal and diurnal tidal elevations across the lower Chesapeake Bay have been determined using a vessel‐towed ADCP. Data from four cruises ranging from 25 to 92 hours in 1996 and 1997 are used. Water depth averaged every 30 s by the ADCP is studied by harmonic and statistical analysis. By selecting only the data within a narrow band (∼320 m) over the planned transect, we are able to improve the reliability of the data. We then grid the depth data along the 16 km transect into 200 equal segments and use harmonic analysis to resolve the semidiurnal and diurnal tidal variations within each segment. We find that (1) the depth data from the ADCP contain both semidiurnal and diurnal signals that can be resolved, from which the surface elevation can be inferred, (2) the major error appears to come from spatial variation of the depth, (3) the semidiurnal and diurnal tidal variations of elevation inferred over flat bottom topography account for almost 100% of the total variability, while those measurements over large bottom slopes account for a much lower percentage of the total variability, (4) at least 70% of the variability of depth can be explained by semidiurnal and diurnal tides if the bottom slope is smaller than 0.006, and (5) the spatial variation of both amplitude and phase of the elevation along the transect appears to be small with a slightly lower tidal amplitude at the south of the Chesapeake Bay entrance, consistent with the Coriolis effect. The inferred elevations from the ADCP readings are consistent with sea level measurements at a tide station 10 km inside the estuary.
Photomineralization of fluorescent dissolved organic matter in the Orinoco River plume: Estimation of ammonium release
Authors: Morell Julio M. , Corredor Jorge E.
Published: February 2004

We present field data documenting near‐surface chlorophyll a enhancement associated with the Orinoco River outflow and experimental evidence identifying photomineralization of riverine fluorescent dissolved organic matter (DOM) as a potential fertilization mechanism capable of sustaining phytoplankton biomass up to several hundred kilometers from the river delta. Low‐salinity waters from the Gulf of Paria in the Orinoco River estuary with high content of fluorescent DOM were observed to decrease their fluorescence yield while releasing ammonium upon experimental exposure to natural sunlight. Our field data collected during two cruises across the eastern Caribbean Sea evidence the disappearance of DOM fluorescence beyond that expected from dilution, thus indicating that during the seasonal extension of the Orinoco River plume throughout the eastern Caribbean Sea, ammonium release from DOM constitutes a major source of inorganic nitrogen to phytoplankton communities in the river plume. We propose that this mechanism, along with nitrogen retention through recycling, explains enhanced surface phytoplankton biomass of up to 1000 km from the river delta.
Bio‐optical properties and remote sensing ocean color algorithms for Antarctic Peninsula waters
Authors: Dierssen H. M. , Smith R. C.
Published: February 2004

Increasing evidence suggests that bio‐optical properties of Antarctic waters are significantly different than those at temperate latitudes. Consequently, retrieval of chlorophyll concentrations from remotely sensed reflectance measurements using standard ocean color algorithms are likely to be inaccurate when applied to the Southern Ocean. Here we utilize a large bio‐optical data set (>1000 stations) collected in waters west of the Antarctic Peninsula in conjunction with the Palmer Long Term Ecological Research program to assess ocean optical properties and associated ocean color algorithms. We find that the remote sensing reflectance spectrum as a function of chlorophyll concentrations appears significantly different from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) Bio‐optical Algorithm Mini‐workshop data set collected from other regions of the world's oceans. For Antarctic waters, remote sensing reflectance is significantly higher in the blue region and lower in the green region of the spectrum for high chlorophyll concentrations (> 1 mg Chl m−3). Therefore applying general processing algorithms for both Coastal Zone Color Scanner and SeaWiFS in these Antarctic waters results in an underestimate of chlorophyll by roughly a factor of 2. From modeled estimates of absorption and backscattering we hypothesize that both low chlorophyll‐specific absorption and low backscattering contribute to the high reflectance ratios.
Phytoplankton pigment distribution in relation to upper thermocline circulation in the eastern Mediterranean Sea during winter
Authors: Vidussi Francesca , Claustre Hervé , Manca Beniamino B. , Luchetta Anna , Marty Jean‐Claude
Published: February 2004

Using a sampling grid of 67 stations, the influence of basin‐wide and subbasin‐scale circulation features on phytoplankton community composition and primary and new productions was investigated in the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment‐based proxy of the ƒ ratio (new production/total production). For the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m−2, and estimated primary and new production were 0.27 and 0.04 g C m−2 d−1, respectively, when integrated between the surface and the depth of the productive zone (1.5 times the euphotic layer). Nanophytoplankton and picophytoplankton (determined from the pigment‐derived criteria) were the dominant size classes and contributed to 60 and 27%, respectively, of Tchl a, while microphytoplankton contributed only to 13%. Subbasin and, to a certain extent, mesoscale structures (cyclonic and anticyclonic gyres) were exceptions to this general trend. Anticyclonic gyres were characterized by low Tchl a concentrations (18.8±4.2 mg m−2, with the lowest value being 12.4 mg m−2) and the highest picophytoplankton contribution (40% of Tchl a). In contrast, cyclonic gyres contained the highest Tchl a concentration (40.3±15.3 mg m−2) with the highest microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyre (cyclonic) region show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplankton. We estimate that the Rhodes gyre is a zone of enhanced new production, which is 9 times higher than that in adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the eastern Mediterranean and emphasize the role of subbasin and mesoscale dynamics in driving phytoplankton biomass and composition and, finally, biogeochemical cycling in this area.
Low‐salinity pools at Barbados, West Indies: Their origin, frequency, and variability
Authors: Kelly Peter S. , Lwiza Kamazima M. M. , Cowen Robert K. , Goni Gustavo J.
Published: February 2004

A vertical array of conductivity‐temperature recorders moored off the west coast of Barbados, West Indies, from May 1996 to November 1997 revealed a heterogeneous and variable salinity pattern punctuated by five pools of low‐salinity water (<34.5 practical salinity units (psu)) entering the region. A typical pool extended to 30‐m depth and lasted‐25 days, although one pool extended to 47 m and lasted 94 days. Water samples taken from a pool in May 1997 have radium 228/226 activity ratios of ∼1, consistent with previous measurements in Barbados of water that originated in the Amazon River mixing zone. The Amazon water likely was translated to Barbados in rings spawned from the North Brazil Current. Analysis of sea height anomaly and residual derived from the TOPEX/Poseidon satellite supports this conclusion and reveals that, contrary to previous studies, rings are shed throughout the year, mostly during spring. The pools of low‐salinity water and their associated velocities dramatically changed the already variable flow in our study area. We believe the complex salinity and flow we observed represented the disorganized remnants of North Brazil Current rings that were at or near the ends of their lives. The changes we observed in the velocity and water structure are interesting in their own right as evidence of the Barbados region as a mixing zone and for their influence on recruitment of larval fishes to the reef along the island's west coast.
Antarctic Circumpolar Current response to zonally averaged winds
Authors: Gille Sarah T. , Stevens David P. , Tokmakian Robin T. , Heywood Karen J.
Published: February 2004

Coherence analysis techniques are used to compare Southern Ocean wind forcing with Antarctic Circumpolar Current transport. Winds are derived from five different products: ERS winds that have been bin‐averaged, weekly gridded ERS winds produced by the Centre ERS d'Archivage et de Traitement, 5 day winds from the Special Sensor Microwave Imager, analysis winds from the European Centre for Medium‐Range Weather Forecasts, and reanalysis winds from the National Centers for Environmental Prediction. Barotropic transport is estimated from the pressure difference between bottom pressure gauges deployed on either side of Drake Passage by Proudman Oceanographic Laboratory as part of the World Ocean Circulation Experiment. Surface transport is estimated from TOPEX altimetry. Results indicate that transport and wind forcing are coherent over a broad range of frequencies, corresponding to time periods of roughly 10–256 days. Highest coherences occur for winds at latitudes on the south side of Drake Passage. Barotropic ocean transport lags wind forcing not by a constant time interval but by a constant phase lag of about one eighteenth of a cycle at a broad range of frequencies, suggesting that the oceanic response to wind is controlled by both the tendency term and a frequency‐dependent viscous process. Surface transport lags by a longer phase interval. Wind stress curl north of Drake Passage is more coherent with transport than is wind stress curl in the latitudes of Drake Passage. Ocean transport lags wind stress curl, suggesting that transport fluctuations are not governed by a simple Sverdrupian vorticity balance. Like the observations, general circulation model transports from the Parallel Ocean Program and from the Parallel Ocean Climate Model are coherent with wind stress from the south side of Drake Passage and with wind stress curl from latitudes north of Drake Passage. Unlike the observations, model transport and bottom pressure vary almost simultaneously with the wind and do not replicate the observed phase lags, implying that the effective model viscosity may be too large.
A study of the forenoon ionospheric F2 layer behavior over the middle and upper atmospheric radar
Authors: Zhang Shun‐Rong , Oliver W. L. , Fukao Shoichiro , Otsuka Yuichi
Published: February 2004

The middle and upper atmosphere (MU) radar incoherent scatter observations of electron density Ne profiles for 1986–1995 at Shigaraki (34.85°N, 136.10°E) show a forenoon density depression (bite‐out) in summer both on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F2 peak for low solar activity and around the F2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon Ne depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature Te, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce Ne and result in a negative ∂Ne/∂t in the morning hours in summer. Among those processes the morning enhancement of Te is found to be most important for the presence of the forenoon bite‐out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite‐out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O]/[N2] in summer leads to a smaller daytime background Ne and thus to an enhanced Te, and the after‐sunrise photochemical contributions (mainly the O+ production) are so small that ∂Ne/∂t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite‐out can be explained basically in terms of the low‐electron‐density background. (4) The height of formation of the forenoon bite‐out is also related to the sunrise effect on photoionization.
Mapping prenoon auroral structures to the magnetosphere
Authors: Ober Daniel M. , Maynard N. C. , Burke W. J. , Moen J. , Egeland A. , Sandholt P. E. , Farrugia C. J. , Weber E. J. , Scudder J. D.
Published: February 2004

All‐sky auroral images acquired at Ny‐Ålesund were used in conjunction with observations of a Polar overflight on November 30, 1997, to determine where prenoon, 0900–1000 magnetic local time (MLT) auroral structures map to the outer magnetosphere. Polar observations at midaltitudes are used to constrain the mapping between the aurora and the magnetosphere. The Tsyganenko 96 magnetic field model (T96), driven by interplanetary conditions and DST, is used for that mapping. When the T96 model is driven by conditions on this day, the open/closed field line boundary maps 2–3 degrees lower in latitude than observed for this day. By making an ad hoc adjustment to match the location of the model open/closed field line boundary to observations, we find that the agreement with other aspects of the observations is also improved. These include (1) Polar observations of the dayside extension of the boundary plasma sheet (BPS) (structured low‐energy electrons, detected in a region of sunward convection and region 1 field‐aligned currents) mapped to the ionosphere in a region of discrete aurora, (2) Polar observations of the central plasma sheet (high‐energy electron precipitation) maps to diffuse green auroral emissions equatorward of the discrete aurora, and (3) Polar electric field observations are consistent with convective motions of the auroral forms and changing interplanetary magnetic field (IMF) conditions. When the all‐sky images are mapped into the magnetosphere, we find that the discrete aurora identified with the BPS mapped to the outer dawnside edge of the magnetosphere. This mapping indicates that earlier Geotail reports of a sunward flowing mixing region in the equatorial magnetosphere [Fujimoto et al., 1998] is really within the dayside extension of the BPS, compatible with both the low‐altitude BPS observations of Newell and Meng [1992] and type 4 dayside aurora of Sandholt et al. [1998]. Since the Polar observations place the sunward flowing mixing region as a source of region 1 currents for this IMF BY positive case [Farrugia et al., this issue], these results extend the source of the region 1 currents from the antisunward flowing low‐latitude boundary layer into the sunward flowing BPS, commensurate with Yamauchi et al. [1998] and Sonnerup [1980].
Kinetic aspects of the Jovian current sheet
Authors: Yin L. , Coroniti F. V. , Pritchett P. L. , Frank L. A. , Paterson W. R.
Published: February 2004

In this work, self‐consistent two‐dimensional particle simulations are used to examine the structure and dynamics of the current sheet in the middle and outer regions of the Jovian magnetotail. A plasma source, representing plasma transported outward from Io, is injected into the simulation system of a corotating magnetodisc configuration that is initially at hydromagnetic equilibrium. Simulations show that the corotating magnetodisc equilibrium evolves into a new configuration in which plasma outflow is present and the ion and electron outflows are different. The initial hydromagnetic equilibrium also develops a cross‐sheet charge separation, an electrostatic potential, and a polarization electric field normal to the sheet. We observe a relatively stable current sheet and a steady, moderate outflow inside the Alfvén radius. Outside the Alfvén radius, a corotation lag spiral magnetic field, radial outward currents carried by the ions, and electron diamagnetic drifts in the polarization field are found. In this region the centrifugally slung plasma may break the magnetic field topology through driven reconnection. During strong reconnection the magnetic field configuration in the plasmoid region is observed to change from corotation lag to corotation lead; the magnetodisc plasma escapes into a super‐Alfvénic outflow, and fast‐shock‐like layers form. Ion heating and non‐Maxwellian velocity distributions are found in these regions.
An optical search for small comets
Authors: Mutel R. L. , Fix J. D.
Published: February 2004

We have conducted an extensive optical search for small comets with the characteristics proposed by Frank et al. [1986] and Frank and Sigwarth [1993, 1997]. The observations were made using the 0.5‐m reflector of the Iowa Robotic Observatory between September 1998 and June 1999. The search technique consisted of tracking a fixed point in the ecliptic plane at ±9° geocentric solar phase angle. The telescope scan rate was chosen to track objects moving prograde at 10 km s−1 relative to the Earth at a distance of 55,000 km. The camera was multiply shuttered to discriminate against trails caused by cosmic rays and sensor imperfections. Of 6143 total images, we selected 2713 which were suitable for detection of objects with a magnitude 16.5 or brighter with 120 pixel trails. The sensitivity and reliability of the visual detection scheme were determined by extensive double‐blind tests using synthetic trails added to over 500 search images. After careful visual inspection of all images, we found no trails consistent with small comets. This result strongly disagrees with previous optical searches of Yeates [1989] and Frank et al. [1990], whose detection rates and magnitudes, when converted to the present search, predict 65 ± 22 detections. We conclude that at 99% confidence, the number density of any prograde objects in the ecliptic plane brighter than magnitude 16.5 with speeds near 10 km s−1 have a number density less than 5% of the small‐comet density derived by Frank et al. [1990]. Any object fainter than this magnitude limit with a mass corresponding to the small‐comet hypothesis (M > 20,000 kg) must have either an implausibly low geometric albedo (p < 0.01) or a density larger than that of water.
Auroral kilometric radiation at substorm onset
Authors: Liou K. , Meng C.‐I. , Lui A. T. Y. , Newell P. T. , Anderson R. R.
Published: February 2004

The temporal relationship of substorm signatures that occur in different regions of space is an important part of substorm research. In the present study we investigate the relative timing between sharp enhancement of auroral kilometric radiation (AKR) and auroral breakups. It is found, on the basis of 136 isolated substorm events identified with global auroral images from the Polar ultraviolet imager (UVI), that (1) 70% (83%) of the time AKR enhancements were detected within ±1 (±2) min of the auroral breakups; (2) AKR onset tends to occur, on an average, slightly later (0.36 min) than the corresponding auroral breakup; and (3) similar to previous study results, substorm‐associated AKR has a forbidden area at Polar altitude in the noon sector. These results suggest that when the satellite is suitably located, the enhancement of AKR is a good substorm onset indicator and it can be used to time substorm onsets adequately within the ∼1 min of uncertainties from the UVI observations, especially if confirmed by other substorm onset identifiers. The average frequency for the enhancement of AKR at substorm onset is found to be ∼300 kHz, corresponding to an upper limit of the AKR source altitude of ∼4700 km. The entire AKR frequency band expands to ∼64–650 kHz within a few minutes after onset, corresponding to a wider source altitude of ∼2100–12,000 km, a typical zone for auroral electron acceleration. We suggest that the fast expansion of the AKR source region along the local magnetic field lines, equivalent to the expansion of the auroral acceleration region, should be considered as one of the fundamental signatures in the substorm expansion phase.
Modulation effects of anisotropic perpendicular diffusion on cosmic ray electron intensities in the heliosphere
Authors: Ferreira S. E. S. , Potgieter M. S. , Burger R. A. , Heber B.
Published: February 2004

The modulation of cosmic ray electrons provides a useful tool to study the diffusion tensor applicable to heliospheric modulation. Electron modulation responds directly to the assumed energy dependence of the diffusion coefficients below ∼500 MeV in contrast to protons which experience large adiabatic energy losses below this energy. As a result of this and because drifts become unimportant for electrons at these low energies, conclusions can be made about the appropriate diffusion coefficients. Using a modulation model, we illustrate the role of anisotropic perpendicular diffusion on electron modulation. In general, we find that perpendicular diffusion dominates electron modulation below ∼100 MeV. Enhancing it in the polar direction typically produced an increase in modulation for both the A > O (e.g., ∼1990 to ∼2000) and A < 0 (e.g., ∼1980 to ∼1990) solar magnetic polarity cycles. It also causes the radial dependence of the intensity to become more uniform throughout the heliosphere, and causes a significant reduction in the latitude dependence of the intensities at all radial distances, with the largest effects in the inner heliosphere and at low energies. This agrees with studies of cosmic ray protons, which suggest that perpendicular diffusion enhanced in the polar direction of the heliosphere is required in conventional drift models to explain the small latitudinal gradients observed for protons on board the Ulysses spacecraft. The role of enhanced perpendicular diffusion was further investigated by examining electron modulation as a function of the “tilt angle” α of the wavy current sheet. In general, a reduction occurred between the modulation differences caused by drifts as a function of α for both polarity cycles. This work illustrates that anisotropic perpendicular diffusion has profound effects on the modulation of galactic cosmic ray electrons during both polarity cycles.
The May 1997 SOHO‐Ulysses quadrature
Authors: Suess S. T. , Poletto G. , Romoli M. , Neugebauer M. , Goldstein B. E. , Simnett G.
Published: February 2004

We present results from the May 1997 SOHO‐Ulysses quadrature (SOHO‐Sun‐Ulysses angle=90°), near sunspot minimum. Ulysses was at 5.1 AU, 10° north of the solar equator, and off the east limb. It was also at the very northern edge of the streamer belt. Nevertheless, the Solar Wind Observations Over the Poles of the Sun instrument (SWOOPS) detected only slow, unusually smooth wind and there was no direct evidence of fast wind from the northern polar coronal hole or of mixing with fast wind. The Large‐Angle and Spectrometric Coronagraph (LASCO) images show that the streamer belt at 10°N was narrow and sharp at the beginning and end of the 2 week observation interval, but broadened in the middle. A related change in density, but not flow speed, occurred at Ulysses. Under these conditions it was possible to show that densities derived from the Ultraviolet Coronagraph Spectrometer (UVCS) in the lower corona are closely related to those in the solar wind, both over quiet intervals and in transient events on the limb. Density and velocity in one small transient observed by both LASCO and UVCS are analyzed in detail.
Cosmic radiation in the heliosphere at successive solar minima: 5. Modulation of anomalous cosmic ray helium during the three consecutive solar minimum periods of 1977/1978, 1987, and 1997
Authors: Reinecke J. P. L. , McDonald F. B. , Moraal H.
Published: February 2004

A renewed attempt is made to model anomalous cosmic ray He+ spectra observed during the solar minimum periods of 1977/1978, 1987, and 1997, from 1 to 66 AU in the heliosphere. This model improves on the one in our paper 3 in this series [Reinecke et al., 1996], because it now contains the new 1997 observations, and because a physically justified input spectrum for anomalous species on the solar wind termination shock is used. This input spectrum is based on the work of Steenberg [1998] and Steenberg and Moraal [1999]. For simplicity, only anomalous He+ is considered. This is partially justified because the properties of other anomalous species, such as O+, are quite similar. This paper extends the work of paper 4 in the series [Reinecke et al., 2000], in which similar fits to galactic He++ were done. It is shown that it remains impossible to find a single set of modulation parameters that fit the 1977, 1987, and 1997 anomalous and galactic spectra simultaneously. In addition, the new 1997 observations introduce further constraints that make it impossible to even find a single set of parameters for 1977 and 1997 with the current model, although these two periods are in the same so‐called drift cycle.
Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects
Authors: Peymirat C. , Richmond A. D. , Kobea A. T.
Published: February 2004

The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere‐Thermosphere‐lonosphere‐Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well‐known overshielding phenomenon corresponding to a sudden reversal of the low‐latitude electric field. For quasi‐periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential‐drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region‐2 field‐aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.
Electrodynamic coupling of high and low latitudes: Observations on May 27, 1993
Authors: Kobea A. T. , Richmond A. D. , Emery B. A. , Peymirat C. , Lühr H. , Moretto T. , Hairston M. , Amory‐Mazaudier C.
Published: February 2004

The penetration of disturbance electric fields from the polar region to the magnetic equator on the dayside of the Earth is examined with geomagnetic data on May 27, 1993. First, we examine a dayside equatorial disturbance that followed the rapid recovery of magnetic activity from a storm and that has the characteristics of overshielding caused by persistent region‐2 field‐aligned currents. It lasted ∼ 3 hours. Second, we analyze a series of fluctuations with periods of 25–75 min, to determine the variations of amplitude and phase with magnetic latitude and magnetic local time. The fluctuations were highly coherent at all latitudes between the magnetic equator and the auroral zone, but the coherency decreased in the polar cap. A northward fluctuation at the equator during midday hours accompanied auroral zone fluctuations that were southward before noon, eastward around noon, and northward after noon. The amplitudes decreased away from the auroral zone toward midlatitudes but were amplified under the equatorial electrojet. No detectable phase differences are found, indicating that any temporal lags which might be induced by persistence in the region‐2 field‐aligned currents are less than 1 min for fluctuations having periods like those examined here. A synoptic inversion analysis of the high‐latitude magnetic data to estimate the time‐varying high‐latitude electric potential patterns shows that fluctuations of the high‐latitude east–west potential gradient tended to be concentrated around midday, where they were in phase with fluctuations in the midday east–west potential gradient at the magnetic equator.
A three‐dimensional experimental study of lower hybrid wave interactions with field‐aligned density depletions
Authors: Rosenberg Steve , Gekelman Walter
Published: February 2004

Rocket observations of spatially localized bursts of intense lower hybrid wave radiation have been made for decades. As the ability of rocket observations to measure small‐scale structure increases, scientists are better able to understand the physics responsible for the observations. However, rocket measurements are limited in their ability to fully diagnose the phenomenon they seek to measure. Therefore a laboratory experiment was undertaken at the Large Plasma Device (LAPD) at the University of California at Los Angeles to study the interaction of lower hybrid waves with a field‐aligned density depletion. The laboratory parameters were chosen to mimic the physics in the ionosphere. Laboratory experiments allow one not only to study the effect of different interaction parameters, like wave frequency or striation size, but also to fully diagnose the wave fields in three spatial dimensions and time. These experiments have shown that large‐amplitude wave fields are localized to the areas of largest density gradient. The field pattern is independent of striation size and shape. The experimental results relate strongly to space‐based observations. The laboratory results, therefore, can act as a guidepost to assist the understanding of the more complex and difficult‐to‐understand plasmas encountered in the ionosphere.
Global role of E‖ in magnetopause reconnection: An explicit demonstration
Authors: Siscoe G. L. , Erickson G. M. , Sonnerup B. U. Ö. , Maynard N. C. , Siebert K. D. , Weimer D. R. , White W. W.
Published: February 2004

We use a global MHD simulation to compute the distribution of E‖ on the face of the magnetopause as represented by the last closed field line surface. In MHD codes, E‖ is a proxy for magnetic reconnection. Integrating E‖ along the topological separator line between open and closed magnetic field lines gives the global reconnection rate at the magnetopause. In the case studied here, where the interplanetary magnetic field (IMF) is precisely duskward, we find the global reconnection rate to be ∼49 kV, comparable to potentials inferred from measurements made in the polar cap. The exercise demonstrates an application of a general reconnection theorem that, in effect, equates reconnection with E‖. It prepares the way for MHD imaging of reconnection in terms of contours of E‖ on the magnetopause. The result also illustrates a property of parallel potentials in the global context that is not generally recognized. Nearly the full magnetopause reconnection voltage exists on some closed field lines between the northern and southern polar caps, so that they leave the dawn, southern hemisphere with a sizable positive polarity and enter the dusk, northern hemisphere with a sizable negative polarity. An unexpected finding is a substantial parallel potential (between 10 and 15 kV) between the magnetopause and the ionosphere in northern dawn and southern dusk sectors. (Interchange “dawn” and “dusk” for dawnward IMF.) This potential has the polarity that accelerates electrons into the ionosphere in the dusk sector and, so, might be the origin of the “hot spot” seen there in precipitating electrons.
Acceleration of electrons at type II shock fronts and production of shock‐accelerated type III bursts
Authors: Dulk George A. , Leblanc Yolande , Bastian T. S. , Bougeret Jean‐Louis
Published: February 2004

We present evidence of electron acceleration by type II‐burst‐emitting shocks in the corona. Some of the electrons travel outward along open magnetic field lines and produce “shock‐accelerated type III bursts” (or SA type III bursts) along their paths. The SA type III bursts are evident in dynamic spectra that cover part or all of the range from metric to kilometric wavelengths. The unique feature of our observations is the complete or near‐complete frequency coverage from about 2 GHz to < 0.1 MHz, that is, ≲ 1.01 Ro to 1 AU. A sample of eight events is presented. All would be classified as “shock accelerated events” at hectometric wavelengths, as first defined by Cane et al. [1981]. Our complete spectra frequently show several to many type III‐like bursts emanating from near the type II burst toward low frequencies, with no trace of emission at frequencies higher than that of the type II burst. The drift rates of these SA type III bursts are similar to those of normal type III bursts, and the exciting electrons have speeds of order 0.1c to 0.2c, or energies of 3–10 keV and higher. Their intensity at hectometer wavelengths is similar to that of normal type III bursts. They often persist to the lowest frequencies observable, near the local plasma frequency at 1 AU. In most of the events examined, there were no microwave bursts from the low corona whose intensity profiles were similar to the hectometric profiles. We therefore conclude that these SA type III bursts originate in type II shocks and are caused by energetic electrons accelerated at the shocks. Of the eight events analyzed, three contain only SA type III bursts. For the remainder, normal type III bursts predominate initially, followed by SA type III bursts later in the event. We emphasize the need for spectra with near‐continuous coverage, especially from decametric to kilometric wavelengths, to identify SA type III bursts unambiguously and to distinguish between the contributions of normal and SA type III bursts.