bluuebunny/crossref_metadata_2025 huggingface dataset:
Fourier and wavelet analyses of TOPEX/Poseidon‐derived sea level anomaly over the South China Sea: A contribution to the South China Sea Monsoon Experiment
Authors: Hwang Cheinway , Chen Sung‐An
Published: February 2004

We processed 5.6 years of TOPEX/Poseidon altimeter data and obtained time series of sea level anomaly (SLA) over the South China Sea (SCS). Fourier analysis shows that sea level variability of the SCS contains major components with periods larger than 180 days and is dominated by the annual and semiannual components. Tidal aliasing creates 30–180 day components that can be misinterpreted as wind‐induced variabilities. Continuous and multiresolution wavelet analyses show that the SLA of the SCS has monthly to interannual components of time‐varying amplitudes, and the regional slope of SLA is 8.9 mm yr−1, which may be caused by the decadal climate change. Coherences of SLA with wind stress anomalies (WSA) and sea surface temperature anomalies (STA) are significant at the annual and semi‐annual components. At periods of 2–5 years the wavelet coefficients of SLA, WSA, and STA have the same pattern, but WSA leads SLA, and STA follows SLA. The zero crossing of SLA in spring is highly correlated with the onset of the summer monsoon. The interannual variability of SLA is correlated with El Niño‐Southern Oscillation, and most important is that when the El Niño‐like wavelet coefficients of SLA over the warm pool northeast of Australia or the SCS change curvature from negative to positive, an El Nino is likely to develop. This is a contribution to the South China Sea Monsoon Experiment (SCSMEX).
A circumpolar gravest empirical mode for the Southern Ocean hydrography
Authors: Sun Che , Watts D. Randolph
Published: February 2004

Historical hydrographic data from the Southern Ocean are projected into a baroclinic stream function space, generating a three‐dimensional gravest empirical mode (GEM) parameterized by pressure, geopotential height, and longitude. The low‐dimensional GEM projection is intrinsically steady, vertically coherent, and equivalent barotropic. The GEM fields capture more than 97% of the total density and temperature variance in the Antarctic Circumpolar Current region.
Interannual variability of bottom temperatures in Drake Passage
Authors: Rubython Katie E. , Heywood Karen J. , Vassie John M.
Published: February 2004

A continuous 4 year record of in situ bottom temperature measurements at 3690 m within Drake Passage (59°43.7S, 55°29.5W) from November 1992 to November 1996 is presented. The record shows fluctuations of order 0.02°C over monthly periods throughout the 4 years, but the annual mean temperature is the subject under discussion in this work. Between June 1994 and May 1995 the water sampled was of the order of 0.1°C cooler than the water sampled on either side of this time period. This temporal temperature anomaly has only become apparent because of the availability of a continuous data set. Hydrographic data sampled from the same location as part of the World Ocean Circulation Experiment SRl repeat section provide potential temperature and salinity values that enable the bottom temperature record to be linked to variability in Weddell Sea Deep Water (WSDW). Oxygen isotope measurements demonstrate that the year‐long period of cooler WSDW measured in Drake Passage is a signature of Ice Shelf Water in the formation process of deep and bottom waters within the Weddell Sea.
On the zonal and meridional circulation and ocean transports between Tasmania and Antarctica
Authors: Yaremchuk M. , Bindoff N. L. , Schröter J. , Nechaev D. , Rintoul S. R.
Published: February 2004

The large scale circulation of the WOCE hydrographic section SR3 between Tasmania and Antarctica is studied through inversion of hydrographic, wind stress and current meter data acquired between 1993 and 1996. These data are assimilated within the framework of a steady state model that is based on hydrostatics, geostrophy, tracer, and potential vorticity conservation. Posterior variances of the major current systems are estimated via the implicit inversion of the Hessian matrix, associated with the assimilation scheme. Analysis of the residuals shows that averaged water mass properties are conserved and that most of the adjustment by the solution is through the uniform vertical displacement of density surfaces within the water column. The inclusion of the tracer and potential vorticity constraints allows parts of the large‐scale vertical and meridional circulation to be determined. The upwelling over the Antarctic Divergence is driven mainly by the winds, while the downwelling in the Sub‐antarctic Front (SAF) is shown to result from the meridional gradients of the density field caused by the mean Antarctic Circumpolar Current. The shallow meridional and vertical circulation in the SAF is along density surfaces and follows the salinity minimum waters to a depth of 500 dbar. This result suggests a possible mechanism for ventilation of Antarctic Intermediate Water. The total volume transport between Tasmania and Antarctica is estimated as 151±50 Sv, with variations between repeat measurements for this section of about 10 Sv. The cross‐section circulation pattern can be qualitatively subdivided into five persistent features and their transports and errors are given.
Satellite altimetry observations of the Black Sea level
Authors: Korotaev G. K. , Saenko O. A. , Koblinsky C. J.
Published: February 2004

The paper is devoted to the application of satellite altimetry from the TOPEX/Poseidon and ERS‐1 missions to the Black Sea. We use the NASA Ocean Altimeter Pathfinder Project collinear data set and monthly climatic hydrography to restore the dynamical sea level, that which connected with the Black Sea circulation. Two realizations are created from collinear and grid data sets. The Black Sea hydrographic survey data collected by the Cooperative Marine Science Programme for the Black Sea are used for validation of both products. The estimated rms accuracy of the Black Sea dynamical level is about 3 cm. The grid data product is used for the analysis of seasonal and mesoscale variability of the Black Sea level. The observations show that the Black Sea circulation has strong seasonal variation. It attenuates in summer to autumn and intensifies in winter to spring. This variability is accompanied by a western‐phase propagation of the sea level. A simple two‐layer model of the wind‐induced circulation in the rectangular basin provides an interpretation of these results. It is shown that the seasonal variability of the geostrophic circulation is most likely produced by the annual changes of the wind stress curl. The western phase propagation is treated as the radiation of Rossby waves from the eastern coast of the basin. Typical periods of the mesoscale oscillations of the dynamical sea level are estimated from the TOPEX/Poseidon altimeter time series in different parts of the Black Sea by means of a spectral analysis. The strongest mesoscale oscillations have periods of about 120 days and are located in the southeastern part of the basin, where the Rim Current bifurcates, and off the Crimean peninsula, where the Rim Current meanders as a result of the local orography.
Remotely sensed ocean observations of the Coastal Mixing and Optics site from synthetic aperture radars and advanced very high resolution radiometers
Authors: Porter David L. , Thompson Donald R. , Alpers Werner , Romeiser Roland
Published: February 2004

In this paper, we describe the remote sensing data set collected in support of the Coastal Mixing and Optics experiment (CMO). The data set consists of 33 synthetic aperture radar (SAR) images concentrated during the main CMO experimental period supplemented by nearly concurrent advanced very high resolution radiometer imagery. Discussion of how the two‐dimensional spatial view of the ocean surface provided by the imagery can greatly reduce the difficulty in interpreting in situ data from point sensors or ship transects is presented. After a brief discussion of the imaging physics we examine interesting signatures ubiquitous in SAR imagery of the coastal ocean, such as internal wave packets, flow over variable bottom topography, and boundary layer turbulence. In particular, we demonstrate how the SAR imagery can be used in conjunction with the mooring data to estimate the speed and propagation direction of coastal internal waves within the study area. Furthermore, we use a recently developed technique to show how estimates of the thermocline depth and surface layer density can be obtained using the SAR imagery and a simple two‐layer model of the water column stratification.
Observations of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study
Authors: Colosi John A. , Beardsley Robert C. , Lynch James F. , Gawarkiewicz Glen , Chiu Ching‐Sang , Scotti Alberto
Published: February 2004

Observations are presented of nonlinear internal waves on the outer New England continental shelf during the summer Shelfbreak Primer study conducted between July 26 and August 5, 1996. Current and temperature measurements were made with an upward looking acoustic Doppler current profiler (ADCP) located on the 147 m isobath near the shelfbreak and three vertical thermistor moorings located upshelf. Data from the ADCP and two nearby thermistor chains show energetic internal tides propagating at roughly 0.9 m s−1 to the north‐northwest, nearly perpendicular to the local topography with 10–15 cm s−1 horizontal currents and 15–30 m vertical displacements. These waves evolve rapidly within a 5.8 km range into an undular internal tidal bore. Cross‐isobath barotropic tidal currents, responsible for generating the internal tides are in the 5–12 cm s−1 range. The bore formation is highly variable. There is evidence of a correlation between internal tide steepening and a shelfbreak front jet orientation that is oppositely directed to the internal tide propagation. There is no correlation between steepening and the jet's vertical shear. Statistics of the undular bores show rms travel time fluctuations from 0.8 to 1.7 hours and average tidal bore durations from 12 to 9 hours. The average undular bore speed is 0.9 m s−1, with an rms fluctuation of 0.4 m s−1. The number of high‐frequency waves in the bore varies from 0 to 8 near the shelfbreak and increases to 30 waves 26.7 km upshelf. The observed distribution function of temporal spacing between high‐frequency internal waves is spread between 4 and 20 min.
Turbulent flow over a wavy surface: Neutral case
Authors: Calhoun Ronald J. , Street Robert L.
Published: February 2004

In many geophysical flows the passage of a turbulent fluid over a wavy surface significantly affects quantities of interest, such as momentum flux, mixing, and transport of scalars. Large‐eddy simulation is used here to investigate neutrally stratified flow over a wavy bed. A wavy surface tends to organize the turbulent flow by imposing its wavelength and phase on the near‐wall field. Both instantaneous turbulent structures and turbulence statistics provide evidence of this organization. For example, a vortex identification method applied to the instantaneous flow field reveals streamwise vortices that are locked into a phase relationship with the wavy boundary. These vortices may be linked to a Görtier instability mechanism caused by the wavy wall. Statistical analyses are also presented to gain insights into the effect of the wavy surface. In particular, we compare the effects of three different hill heights. Finally, a brief analysis of the components of drag is given.
Lateral dispersion over the continental shelf: Analysis of dye release experiments
Authors: Sundermeyer Miles A. , Ledwell James R.
Published: February 2004

Lateral dispersion over the continental shelf was examined using dye studies performed as a part of the Coastal Mixing and Optics experiment. Four experiments performed at intermediate depths, each lasting 2.5–5 days, were examined. In some cases the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment. In other cases, significant patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations showed that the dye distributions were significantly influenced by shearing and straining on scales of 5–10 m in the vertical and 1–10 km in the horizontal. Superimposed on these larger‐scale distortions were simultaneous increases in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3–4.9 m2 s−1. Analysis of the dye data in concert with shear estimates from shipboard acoustic Doppler current profiler observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water masses cannot account for the observed diffusive spreading of the dye patches. This result suggests that some other mechanisms provided an additional diffusivity of order 0.2–4.6 m2 s−1. An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, may explain the observed dispersion in some cases.
Vertical structure of currents in western Lake Constance
Authors: Boehrer Bertram , Ilmberger Johann , Münnich Karl Otto
Published: February 2004

Simultaneous measurements of current velocity and density profiles at the Sill of Mainau (western Lake Constance) enabled the observation of the internal response of the lake and the calculation of gradient Richardson number over the entire water column at the sampling site. Periodic changes of current direction corresponded with predicted periods of the two‐layer oscillation and could be confirmed with thermocline oscillation data. In general, the current profiles confirmed a two‐layer structure. However, an activating wind of speeds higher than 4 m/s produced three‐layer profiles for a period of several hours at the sampling site, when the velocities due to the two‐layer oscillation traversed zero. At times the implied current shear produced supercritical values of gradient Richardson number on a vertical scale of a few meters. This occurred during winds in the epilimnion and after strong (> 4 m/s) winds in the lower thermocline and the hypolimnion. In the 20 m above the lake bed, topographical conditions helped the gradient Richardson number to go supercritical sporadically. Across the thermocline, no supercritical gradient Richardson numbers could be confirmed at the measuring site. A lower boundary for vertical mixing in the measuring site due to current shear could be evaluated.
Morphologic properties derived from a simple cross‐shore sediment transport model
Authors: Plant N. G. , Ruessink B. G. , Wijnberg K. M.
Published: February 2004

This paper builds on the now classical discussions by Bowen [1980] and Bailard [1981] on the applicability and implications of Bagnold's [1963] sediment transport model to nearshore profile modeling. We focus on the morphologic implications of both the strengths and weaknesses of Bagnold's model, isolating the transport terms that are well predicted (i.e., mean flow terms) from those that are not well predicted (i.e., transport due to correlations between flow and sediment load). We factor Bagnold's model into a dimensional transport magnitude and a nondimensional term. The nondimensional term describes the relative importance of transport due to undertow, gravity, and correlations between flow and sediment load. The transport magnitude largely determines the response time of nearshore profiles. For typical nearshore environments this response time was estimated to vary as a function of incident rms wave height (Hrms) from ∼500 years (Hrms ∼ 0.5 m) to 2 years (Hrms ∼ 3 m). The relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height (defined as the ratio of the rms wave height to the local depth). Simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles. Several differences from previously computed equilibrium profiles were noted. First, because the relative wave height saturates in natural surf zones, equilibrium profiles converge to a relatively flat profile near the shoreline. Second, under some situations a seaward sloping equilibrium profile may not exist. Third, the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature.
Nearshore currents and turbulent exchange processes during upwelling and downwelling events in Lake Ontario
Authors: Rao Y. R. , Murthy C. R.
Published: February 2004

The nearshore circulation and exchange processes during summer stratification and certain episodic events like upwelling and downwelling have been examined using a time series data of horizontal velocity and temperature profiles in Lake Ontario. The primary peak of spectral energy of currents and temperature for the summer season is located at a period of 10–12 days corresponding to the large‐scale response of the lake due to meteorological forcing and the propagation of internal Kelvin waves. A secondary peak of spectral energy of coastal currents is situated near the inertial frequency band. Each upwelling and downwelling episode along the western Lake Ontario has lasted for nearly 4–6 days under the influence of prevailing winds. Currents associated with downwelling cycles are slightly stronger than currents associated with upwelling events. Although the kinetic energy associated with alongshore currents is generally higher, cross‐shore flow exhibited higher energy at middle and lower levels during these episodes. The alongshore horizontal exchange coefficients are generally higher than the cross‐shore component. However, during upwelling episodes, horizontal exchange coefficients are reduced in the surface layers and increased in the bottom layers. Full upwelling events are characterized with weak static stability and reduced vertical current shear in surface layers. Near bottom layers are affected by intense turbulence associated with increased vertical current shear. During downwelling events with migration and intersection of the thermocline with the bottom, vertical exchange coefficients are relatively small due to weak turbulent activity.
Optics, particles, stratification, and storms on the New England continental shelf
Authors: Gardner W. D. , Blakey J. C. , Walsh I. D. , Richardson M. J. , Pegau S. , Zaneveld J. R. V. , Roesler C. , Gregg M. C. , MacKinnon J. A. , Sosik H. M. , Williams A. J.
Published: February 2004

In situ beam attenuation and chlorophyll fluorescence were correlated with concentration and bulk composition of particles in shelf waters during summer and spring under different physical forcing conditions to determine if optical parameters could be used as an additional tracer in examining the process of mixing in shelf waters. Time series measurements were made for two 18 day periods during high stratification (late summer 1996, Δσt = ∼3.0 kg m−3 surface to bottom) and low but rapidly developing stratification (spring 1997, Δσt = 0.05 to 0.5 kg m−3) in 70 m of water in a midshelf environment south of Martha's Vineyard, Massachusetts. When defined by hydrography and optical profiles, four layers were identified during the summer: the surface mixed layer, the particle/chlorophyll maximum, the midwater particle minimum, and the bottom nepheloid layer. Fast moving solitons perturbed the water column briefly, but no storms perturbed the system until large surface swells from Hurricane Edouard intensified and thickened the nepheloid layer. Bulk composition and optics of particles in and above the nepheloid layer were distinctly different after the passage of Hurricane Edouard. The hurricane passage demonstrated that intense atmospheric forcing greatly influences both hydrographic and optical properties in the entire water column, even when highly stratified (Δσt = ∼3.0 kg m−3, decreasing to 0.8 kg m−3 post hurricane), and causes massive resuspension, due initially to wave shear stress that was later dominated by current shear. Restratification progressed rapidly after the hurricane passed. During spring the water column started as a weakly stratified two‐layer system hydrographically and optically but evolved into three layers as stratification developed. Strong spring storms affected both surface and bottom layers but with decreasing impact as the water column stratified.
Evidence for internal hydraulic control in the northern Øresund
Authors: Nielsen Morten Holtegaard
Published: February 2004

New observations of mainly flow velocity, salinity, and temperature are used to show and discuss some of the physical conditions in Øresund, the strait between Denmark and Sweden, one of three connections between the brackish Baltic Sea and the saline North Sea. The main geometric features are a contraction in the northern Øresund and the shallow Drogden sill at the entrance to the Baltic. The observations show that the two‐layer flows through the contraction are often hydraulically controlled. The observations also reveal details of the transition from subcritical to supercritical flow. In terms of the composite Froude number, on the basis of local flow parameters these details are that the flow may be subcritical as well as supercritical in different areas of some cross section. Existing theories on rotating hydraulics are unable to account for these circumstances, which are due to the strong influence of the Earth's rotation and the curvature of the streamlines. In the present study it is not attempted to explain these conditions, but the probable effects of rotation and curvature on the controlled flow rate are discussed briefly. Also, the possible effects of hydraulic control on the exchange of the Baltic are discussed. It is argued that the stratification in the Kattegat, the sea to the north, is more important than the presence of the Drogden sill for the amount of high saline water to enter the Baltic through Øresund. This result is supported by observations of the stratification in Øresund and the flow at the sill. The observations show that the interface in Øresund rises significantly during flow to the Baltic and that the transport of saline water into the Baltic is closely connected to the shallowness of the interface in Øresund.
Hydrography and high‐temperature heat flux of the Rainbow hydrothermal site (36°14′N, Mid‐Atlantic Ridge)
Authors: Thurnherr A. M. , Richards K. J.
Published: February 2004

On the basis of an extensive set of conductivity‐temperature‐depth, lowered acoustic Doppler current profiler (LADCP), and nephelometry profiles and tow‐yos the hydrography, flow field, and particle plume of the Rainbow hydrothermal site on the Mid‐Atlantic Ridge are analyzed. In the rift valley the water column is less dense and stratified than both eastern and western off‐ridge water, with T/S characteristics consistent with inflow across a sill from the east. The buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m. The bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m. Large‐amplitude internal waves consistent with tidal forcing are observed near the sill, but LADCP measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth. Above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream. Density‐averaged light‐scattering profiles show the hydrothermal particle plume to be Gaussian in depth, even in the near field, where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest. The temperature anomalies associated with the mean near‐source particle plume are of order −5×10−3°C; that is, the plume is cold/fresh as expected from the background hydrography of the deep Atlantic. Using the flow field, light‐scattering, and hydrographic anomaly observations, the heat flux associated with the hydrothermal particle plume at Rainbow is estimated to lie between 1 and 5 GW.
Glacio‐chemical study spanning the past 2 kyr on three ice cores from Dronning Maud Land, Antarctica: 1. Annually resolved accumulation rates
Authors: Sommer S. , Appenzeller C. , Röthlisberger R. , Hutterli M. A. , Stauffer B. , Wagenbach D. , Oerter H. , Wilhelms F. , Miller H. , Mulvaney R.
Published: February 2004

For the first time, annually resolved accumulation rates have been determined in central Antarctica by means of counting seasonal signals of ammonium, calcium, and sodium. All records, obtained from three intermediate depth ice cores from Dronning Maud Land, East Antarctica, show rather constant accumulation rates throughout the last 9 centuries with mean values of 63, 61, and 44 mm H2O yr−1 and a typical year‐to‐year variation of about 30%. For the last few decades, no trend was detected accounting for the high natural variability of all records. A significant weak intersite correlation is apparent only between two cores when the high‐frequency part with periods less than 30 years is removed. By analyzing the records in the frequency domain, no persistent periods were found. This suggests that the snow accumulation in this area is mainly influenced by local deposition patterns and may be additionally masked by redistribution of snow due to wind. By comparing accumulation rates over the last 2 millennia a distinct change in the layer thickness in one of the three cores was found, which might be attributed either to an area upstream of the drilling site with lower accumulation rates, or to deposition processes influenced by surface undulations. The missing of a clear correlation between the accumulation rate histories at the three locations is also important for the interpretation of small, short time variations of past precipitation records obtained from deep ice cores.
Photochemical escape of atomic carbon from Mars
Authors: Fox J. L. , Bakalian F. M.
Published: February 2004

We have modeled the escape fluxes of atomic carbon from the Martian atmosphere for low and high solar activities due to various photochemical escape mechanisms, including photodissociation of CO, dissociative recombination of CO+, electron impact dissociation and dissociative ionization of CO, photodissociative ionization of CO, and dissociative charge transfer of O++ to CO. Only photodissociation of CO and dissociative recombination of CO+ are found to be important, and the time‐averaged escape flux is predicted to be controlled by the high solar activity values. The computed global average escape fluxes of C due to photodissociation of CO at low and high solar activities are 1.65 × 105 and 1.8 × 106 cm−2 s−1, respectively, and those for dissociative recombination are 2.9 × 104 and 6.2 × 105 cm−2 s−1, respectively. The other sources contribute <10% to the total. The photochemical escape rates are slightly larger than those estimated for sputtering by O+ pickup ions in the present era. If current estimates are correct, however, sputtering will dominate the escape in earlier epochs when the solar wind is predicted to have been much stronger. Ion outflow may also constitute an important loss mechanism if escape rates are on the order of their predicted upper limits.
Origin and development of transient coronal holes
Authors: Kahler S. W. , Hudson H. S.
Published: February 2004

Solar transient coronal holes (TCHs) are short‐lived (≤2 days) regions of dimmed X‐ray intensity sometimes observed in association with coronal mass ejections. These features, first discovered from Skylab observations, can occur in magnetic unipolar regions on either side of the X‐ray posteruptive arcades. They have been suggested as the magnetically open footpoints of associated transient flux ropes observed at 1 AU. We have used images from the Yohkoh Soft X‐ray Telescope (SXT) to study the development of 19 TCH events obtained in a survey of 9 years of Yohkoh observations. We find that the boundaries of the TCHs are never static. The boundaries closer to the magnetic neutral line generally move away from it as the closed‐loop X‐ray arcades expand. In addition, previously closed coronal loops at the ends of the arcades often continue to expand and open on the outer boundaries of the TCHs. These processes typically last for hours. The arcade brightenings do not extend into the full areas of the TCHs. The TCHs tend to disappear only by a net contraction of the boundaries, rather than by brightening within their boundaries. The location of a TCH appears to coincide with a large‐scale curvature of the magnetic neutral line or the occurrence of a nearby active region at one end of the coronal eruption. This distinguishes the formation of TCHs from the arcade development, suggesting that there is no requirement for a pair of TCHs or even any TCH to be formed in an eruptive event. The moving magnetic boundaries, uniformly dark interiors, and short lifetimes of TCHs pose significant problems for the interpretation that TCHs are footpoints of interplanetary magnetic flux ropes.
Nature of fluctuations on directional discontinuities inside a solar ejection: Wind and IMP 8 observations
Authors: Vasquez Bernard J. , Farrugia Charles J. , Markovskii Sergei A. , Hollweg Joseph V. , Richardson Ian G. , Ogilvie Keith W. , Lepping Ronald P. , Lin Robert P. , Larson Davin
Published: February 2004

A solar ejection passed the Wind spacecraft between December 23 and 26, 1996. On closer examination, we find a sequence of ejecta material, as identified by abnormally low proton temperatures, separated by plasmas with typical solar wind temperatures at 1 AU. Large and abrupt changes in field and plasma properties occurred near the separation boundaries of these regions. At the one boundary we examine here, a series of directional discontinuities was observed. We argue that Alfvénic fluctuations in the immediate vicinity of these discontinuities distort minimum variance normals, introducing uncertainty into the identification of the discontinuities as either rotational or tangential. Carrying out a series of tests on plasma and field data including minimum variance, velocity and magnetic field correlations, and jump conditions, we conclude that the discontinuities are tangential. Furthermore, we find waves superposed on these tangential discontinuities (TDs). The presence of discontinuities allows the existence of both surface waves and ducted body waves. Both probably form in the solar atmosphere where many transverse nonuniformities exist and where theoretically they have been expected. We add to prior speculation that waves on discontinuities may in fact be a common occurrence. In the solar wind, these waves can attain large amplitudes and low frequencies. We argue that such waves can generate dynamical changes at TDs through advection or forced reconnection. The dynamics might so extensively alter the internal structure that the discontinuity would no longer be identified as tangential. Such processes could help explain why the occurrence frequency of TDs observed throughout the solar wind falls off with increasing heliocentric distance. The presence of waves may also alter the nature of the interactions of TDs with the Earth's bow shock in so‐called hot flow anomalies.
The kinetic shell model of coronal heating and acceleration by ion cyclotron waves: 2. Inward and outward propagating waves
Authors: Isenberg Philip A.
Published: February 2004

We extend the kinetic shell model of the cyclotron resonant interaction between coronal hole protons and outward propagating ion cyclotron waves presented in the first paper of this series [Isenberg et al., 2001]. That work showed that the resonant dissipation of outward propagating waves produced proton distributions that were unstable to the generation of inward propagating waves. Here we include the kinetic shell interaction with the inward waves, assuming that both the wave generation and wave dissipation proceed much faster than all other processes in the collisionless coronal hole. In this case, the entire proton distribution will be resonant with one set of waves or the other and is thus composed of constant‐energy shells in both the sunward and antisunward regions of velocity space. The evolution of the distribution as the plasma flows away from the Sun is then described by following the motion of these shells under the action of the nonresonant forces in the coronal hole. We find that the distribution consists of a core population which circulates through velocity space and a halo which continually expands to higher energy. The halo population is shown to be essential to obtain acceleration of the bulk proton plasma through its response to the mirror force. We present an illustrative calculation of this system which assumes the waves to be dispersionless. We find for this case that the halo particles soon reach extremely high energies, leading to a continuous, rather than declining, acceleration of the plasma. We suggest that these properties are due to the dispersionless assumption and that an improved model incorporating wave dispersion may give more reasonable quantitative results.
Foreword [to special section on Remote Sensing of the Low‐ and Middle‐Latitude Thermosphere and Ionosphere From the Ground and From Space]
Authors: Dymond Kenneth F.
Published: February 2004

The low and midlatitude thermosphere and ionosphere constitute a tightly coupled dynamic region of the near‐Earth environment. This region exhibits strong and complex coupling between charged and neutral species, between different altitudes, and between equatorial and nonequatorial latitudes. The ion and neutral composition and temperature are affected by dynamics, electric and magnetic fields, photochemistry, and geomagnetic and solar forcing. This special section focuses on this region and the remote sensing techniques that are providing the measurements needed to better understand this complex region of the near‐Earth space environment.
Westward plasma drift in the midlatitude ionospheric F region in the midnight‐dawn sector
Authors: Huang Chao‐Song , Foster J. C. , Holt J. M.
Published: February 2004

In this paper we report the observations of a sharp ionospheric plasma velocity reversal boundary at subauroral latitudes during magnetic storms in the midnight‐dawn sector. The zonal plasma drift was eastward poleward of the velocity boundary and became westward equatorward of the boundary. The latitudinal extent of the westward drifts was ∼20°. A local maximum of the drift occurred only 2°–5° below the boundary; the magnitude of the maximum velocity was 200–400 m s−1 between 2100 and 0300 local time (LT) and as high as 780 m s−1 near midnight. The large westward plasma drift immediately equatorward of the velocity boundary first occurred at 2100 LT and continuously extended across midnight to the dawn sector. The location of the maximum velocity coincided with the minimum density within the midlatitude ionospheric trough. The latitude of the velocity boundary decreased from geographic latitude ∼53° to ∼43° (corresponding to geomagnetic latitude ∼65° to ∼55°) between 2100 and 0800 LT. We propose that the large westward drift at subauroral latitudes is driven by the polarization jet electric fields in the ring current and that the lower‐latitude westward drift with moderate magnitude is caused by the disturbance wind dynamo.
Observations of equatorial F region plasma bubbles using simultaneous OI 777.4 nm and OI 630.0 nm imaging: New results
Authors: Abalde J. R. , Fagundes P. R. , Bittencourt J. A. , Sahai Y.
Published: February 2004

Simultaneous observations of the OI 630.0 nm and OI 777.4 nm nightglow emissions using all‐sky imaging systems and ionospheric radio sounding using a Canadian Advanced Digital Ionosonde (CADI) digisonde have been recently carried out at São José dos Campos (23.21°S, 45.86°W), Brazil. The all‐sky imaging systems use novel CCD devices, with high quantum efficiency and which provide an exceptional capacity for quantitative measurement of faint‐ and low‐contrast emissions. On October 23–24, 2000 (high solar activity), the presence of large‐scale F region plasma irregularities (plasma bubbles) was observed using both techniques (i.e., optical and radio). The high‐resolution images, recorded using the OI 777.4 nm nightglow emission, show a new striated or ray‐like pattern, which has not been detected before. These OI 777.4 nm optical observations show for the first time, in great detail, the field‐aligned ionospheric plasma bubble structures, in contrast with the OI 630.0 nm images, which show a diffuse image of the bubbles. The optical signatures of the OI 777.4 nm emission are more closely related to the actual ionospheric bubble structure, owing to its prompt emission and dependence only on the electron density, with no F layer height dependence. On the other hand, the OI 630.0 nm emission comes from the bottomside of the F layer with a strong F layer height dependence and shows blurred images due to its 110‐s lifetime. An additional advantage of using the OI 777.4 nm emission for ionospheric irregularity studies is that the plasma bubbles can be observed earlier on the OI 777.4 nm images than on the OI 630.0 nm images (by ∼ 15 min).
Electron heating and phase space signatures at supercritical, fast mode shocks
Authors: Hull A. J. , Scudder J. D. , Larson D. E. , Lin R.
Published: February 2004

We investigate the effect of the nonmonotonic features of the macroscopic magnetic field B(chi;) and the deHoffmann‐Teller frame electrostatic potential ΦHT(chi;) on the electron distribution functions within collisionless, fast mode shocks. The signatures of electron distribution functions are explored by using Liouville's theorem in the adiabatic approximation to map model upstream and downstream boundary electron velocity distribution functions to regions inside model shocks with monotonic and nonmonotonic magnetic fields under the empirically motivated approximation that δΦHT ∝ δB. In the case of shocks with monotonically increasing magnetic fields, we show that there are no “exclusion” regions and that the electron distribution function at all pitch angles and hence the electron temperature increase can be explained by the reversible behavior of magnetized electrons in the shock macroscopic electric and magnetic fields. However, at shocks with nonmonotonic magnetic fields, there exist regions of inaccessibility which are outside the domain of the one‐Dimensional (1‐D), steady state Vlasov‐Liouville (V‐L) approach as defined by the upstream and downstream boundaries. Such regions, if occupied, may be filled by electrons scattered into these regions by waves, or perhaps by reversible processes such as the adiabatic convection of electrons into these regions of a curved bow shock, or by coherent nonadiabatic access. As a further test, the V‐L method is employed to study, for the first time, the detailed signatures of full 3‐D electron velocity distribution functions observed by the Wind spacecraft through the resolved layer of a supercritical, fast mode Earth bow shock crossing. We demonstrate that much of the complex structure of the observed electron distribution function within the shock layer can be explained by the motion of adiabatic electrons in the non‐monotonic shock macroscopic magnetic and electric fields. However, a significant portion of electron phase space appears to be the remnants of electron phase space holes. The mechanisms responsible for allowing electrons to gain access to the exclusion regions are not well understood and may have important implications on the thermodynamic properties of collisionless shocks.
Ionospheric HF pump wave triggering of local auroral activation
Authors: Blagoveshchenskaya N. F. , Kornienko V. A. , Borisova T. D. , Thidé B. , Kosch M. J. , Rietveld M. T. , Mishin E. V. , Luk'yanova R. Y. , Troshichev O. A.
Published: February 2004

Experimental results from ionospheric HF pumping experiments in the nightside auroral E region above Tromsø are reported. We found intriguing evidence that a modification of the ionosphere‐magnetosphere coupling, due to the effects of powerful HF waves beamed into an auroral sporadic E layer, can lead to a local intensification of the auroral activity. Summarizing multi‐instrument observations during two consecutive nights, one can distinguish the following peculiarities related to this auroral activation: a modification of the auroral arc and its breakup above Tromsø, local changes in the horizontal currents above Tromsø, a burst‐like increase of the electron density and temperature, a large increase in the ion temperature in a wide altitude range and in the north‐south component of the electric field, distinctive features in dynamic HF radio scatter Doppler spectra, pump‐induced electron precipitation, and substorm activation exactly above Tromsø. We discuss the modification of the ionosphere‐magnetosphere coupling in terms of the excitation of a turbulent Alfvén boundary layer between the base of the ionosphere (∼100 km) and the level of sharp increase of the Alfvén velocity (at heights up to one Earth radius), and the formation of a local magnetospheric current system. The results suggest that a possible triggering of local auroral activation requires specific geophysical conditions.
Two‐dimensional simulations of a curved shock: Self‐consistent formation of the electron foreshock
Authors: Savoini Philippe , Lembège Bertrand
Published: February 2004

A collisionless curved shock is analyzed in a supercritical regime with the help of a two‐dimensional electromagnetic full particle code. Curvature effects are included self‐consistently and allow one to follow continuously the transition from a narrow and step‐like strictly perpendicular shock to a wider and more turbulent oblique shock within the quasi‐perpendicular range 65° < θBn < 90°. Present results reproduce the formation of the electron foreshock without any simplifying assumptions. In agreement with experimental data, local bump‐on‐tail parallel distribution functions are well recovered in the foreshock region and correspond to electrons backstreaming along the magnetic field lines. Present detailed analysis shows that local back‐streaming distributions have two components: (i) a high parallel energy component corresponding to back‐streaming electrons characterized by a field‐aligned bump‐in‐tail or beam signature, and (ii) a low‐energy parallel component characterized by a loss cone signature (mirrored electron). Two types of bump‐in‐tail patterns, broad and narrow, are identified at short and large distances from the curved shock, respectively, and are due to different contributions of these two components according to the local impact of the time‐of‐flight effects. Present results allow one to identify more clearly the nature of the bump‐in‐tail pattern evidenced experimentally (narrow type). These also confirm that mirroring electrons make the dominant contribution to the bump‐in‐tail pattern in the total distribution in agreement with previous studies. Results suggest that low and high parallel energy populations are intimately related and may contribute together to the upstream wave turbulence.
Periodic variations of magnetosheath energetic electron flux associated with global Pc5 pulsations
Authors: Sarafopoulos D. V. , Takahashi K. , McEntire R. W.
Published: February 2004

Measurements from the Wind spacecraft in the solar wind, the Geotail spacecraft in the magnetosheath, and multiple magnetometers on the ground are combined to study a unique electron flux modulation event in the magnetosheath that occurred during an interval of global Pc5 waves. The electron event is characterized by a Pc5‐band oscillation occurring in the magnetic‐field‐aligned component of the electron flux (energy > 38 keV) while the local magnetic field and plasma density remained unperturbed. During this event the whole magnetosphere oscillated as inferred from magnetic field recorded at the CANOPUS, IMAGE, and 210° MM chains. At the ground stations that are mapped to the dayside and duskside magnetosphere, we find a one‐to‐one correspondence of the peaks in the horizontal component to those in the electron flux. At the stations mapped to the dawnside magnetosphere the observed frequency is twice that of the electron flux oscillation. The time‐lagged solar wind density and velocity at Wind were constant, which leads us to exclude the possibility that the waves were driven by ram pressure variations upstream of the bow shock. The Kelvin‐Helmholtz (K‐H) instability on the magnetopause is the more probable source of the Pc5 waves because (1) the solar wind velocity was ∼730 km s−1, well above the average value; (2) the ground‐based observations show that the Pc5 amplitude decreases with distance from the magnetopause; and (3) the dawnside and duskside magnetospheric flanks show oppositely polarized waves. The electron flux oscillation in the magnetosheath can be explained by intermittent magnetic connection between the spacecraft and the dawnside pulsating magnetopause. The ∼60 keV protons are continuously streaming throughout the whole interval, providing evidence for a continuously active source and permanent leakage through a tangential magnetopause discontinuity. It is worth noticing that at progressively higher energies the energetic particle angular distributions are gradually more isotropic because at higher energies the particles are strongly scattered in the magnetosheath.
Loss of plasmaspheric ions during a storm observed by the EUV scanner onboard Planet‐B
Authors: Yoshikawa I. , Yamazaki A. , Shiomi K. , Yamashita K. , Takizawa Y. , Nakamura M.
Published: February 2004

The first measurement of He II (304 Å) emission in the dayside magnetosphere has been done by the EUV scanner onboard Planet‐B. A magnetic storm with the minimum Dst of −70nT occurred during the observation. Before the initial phase of the magnetic storm, the average He+ density of ∼1 cm−3 in the dayside magnetosphere was observed. During the main phase, the average He+ density rapidly increased up to 4–20 cm−3. During the recovery phase, the average density smoothly decreased to the lowest density, which is due to the daily ionospheric filling. The total loss of the ions to the outer magnetosphere is estimated by 3.6×1030 ions. This is sufficiently large to populate the plasma sheet.
Modeling of midlatitude F region response to geomagnetic activity
Authors: Kutiev Ivan , Muhtarov Plamen
Published: February 2004

An empirical model is developed to describe the variations of midlatitude F region ionization along all longitudes within the dip latitude band (30°–55°N), induced by geomagnetic activity, by using the relative deviations (Φ) of the F region critical frequency f0F2 from its monthly median. The geomagnetic activity is represented by the Kp index. The main statistical relationship between Φ and Kp is obtained by using 11 years of data from 26 midlatitude ionosondes. The statistical analysis reveals that the average dependence of Φ on Kp is quadratic, the average response of the ionosphere to geomagnetic forcing is delayed with a time constant T of about 18 hours, and the instantaneous distribution of Φ along local times can be assumed sinusoidal. A continuity equation is written for Φ with the “production term” being a function of Kp modulated by a sinusoidal function of local time and the “loss” term proportional to Φ with a loss coefficient β=1/T. A new, modified function of geomagnetic activity (Kf) is introduced, being proportional to Φ averaged over all longitudes. The model Φ is defined by two standing sinusoidal waves with periods of 24 and 12 hours, rotating synchronously with the Sun, modulated by the modified function Kf. The wave amplitudes and phases, as well as their average offset, are obtained by fitting to the data. A new error estimate called “prediction efficiency” (Peff) is used, which assigns equal weights to the model errors at all deviations of data from medians. The prediction efficiency estimate gives a gain of accuracy of 29%.