bluuebunny/crossref_metadata_2025 huggingface dataset:
The oceanic zone of convergence on the eastern edge of the Pacific warm pool: A synthesis of results and implications for El Niño‐Southern Oscillation and biogeochemical phenomena
Authors: Picaut Joel , Ioualalen Mansour , Delcroix Thierry , Masia François , Murtugudde Ragu , Vialard Jérome
Published: February 2004

The eastern edge of the western Pacific warm pool corresponds to the separation between the warm, rainfall‐induced low‐salinity waters of the warm pool and the cold, high‐salinity upwelled waters of the cold tongue in the central‐eastern equatorial Pacific. Although not well defined in sea surface temperature (SST), this eastern edge is characterized by a sharp salinity front that is trapped to the equator. Several studies, using numerous in situ and satellite data and three classes of ocean models, indicate that this front is the result of the zonal convergence of the western and central Pacific water masses into the eastern edge of the warm pool. This occurs through the frequent encounter of the eastward jets in the warm pool and the westward South Equatorial Current in the cold tongue. The notable and alternate variations of these wind‐driven zonal currents are trapped to the equator and are chiefly interannual in the vicinity of the edge. Consequently, the Eastern Warm Pool Convergence Zone (EWPCZ) is subject to eastward or westward displacements over several thousands of kilometers along the equatorial band, in synchrony with the warm phase (El Niño) and the cold phase (La Niña) of the El Niño‐Southern Oscillation (ENSO) phenomenon. Zonal advection appears to be the predominant mechanism for the ENSO displacements of the eastern edge of the warm and fresh pool. The existence of the EWPCZ and its ENSO displacements have significant effects on the physics of the tropical Pacific and on related biogeochemical phenomena. The EWPCZ is important for the formation of the barrier layer in the isothermal layer of the warm pool. Its zonal displacements control SST in the central equatorial Pacific, which in turn drives the surface winds and atmospheric convection (and vice versa). Hence the central equatorial Pacific is a key region for ENSO coupled interactions. All these findings from several studies and additional analyses lead to a revision of the delayed action oscillator theory of ENSO. The existence of the EWPCZ and its zonal displacements are also reasons for the ENSO variations in production and exchange of CO2 with the atmosphere over the equatorial Pacific. The zone of one‐dimensional convergence seems to congregate the world's most important tuna fishery in the western equatorial Pacific, and its displacements are likely the reason for this fishery to move zonally over thousands of kilometers in phase with ENSO.
Flow structure in depth‐limited, vegetated flow
Authors: Nepf H. M. , Vivoni E. R.
Published: February 2004

Aquatic vegetation controls the mean and turbulent flow structure in channels and coastal regions and thus impacts the fate and transport of sediment and contaminants. Experiments in an open‐channel flume with model vegetation were used to better understand how vegetation impacts flow. In particular, this study describes the transition between submerged and emergent regimes based on three aspects of canopy flow: mean momentum, turbulence, and exchange dynamics. The observations suggest that flow within an aquatic canopy may be divided into two regions. In the upper canopy, called the “vertical exchange zone”, vertical turbulent exchange with the overlying water is dynamically significant to the momentum balance and turbulence; and turbulence produced by mean shear at the top of the canopy is important. The lower canopy is called the “longitudinal exchange zone” because it communicates with surrounding water predominantly through longitudinal advection. In this region turbulence is generated locally by the canopy elements, and the momentum budget is a simple balance of vegetative drag and pressure gradient. In emergent canopies, only a longitudinal exchange zone is present. When the canopy becomes submerged, a vertical exchange zone appears at the top of the canopy and deepens into the canopy as the depth of submergence increases.
Resuspension by saline and particle‐driven gravity currents
Authors: Eames I. , Hogg A. J. , Gething S. , Dalziel S. B.
Published: February 2004

Saline and particle‐driven gravity currents play an important role in resuspension by environmental and industrial flows. We develop a model of resuspension by unsteady buoyancy‐driven flows by bringing together results from the large body of literature dealing with the dynamics of gravity current and resuspension in channel flows. The criterion for resuspending material of density ρp and diameter b is expressed in terms of a modified Shields parameter, Θ = ρfu2/(ρp − ρf)gb, where u is the depth‐averaged velocity within the current and ρf is the bulk fluid density. The critical modified Shields parameter, Θc, which corresponds to the condition when erosion is initiated, is a function of the particle Reynolds number, Rep = bυT/ν, expressed in terms of the terminal fall velocity of the sediment particles υT. Particles characterized by Rep ≫ 1 are resuspended when Θ > Θc ≈ 0.04/cD where cD is the bed drag coefficient. According to this resuspension criterion, a two‐dimensional saline gravity current generated by the release of a volume V per unit width of fluid density ρc in an ambient fluid ρa resuspends material over an erosive distance which scales as
. These results are extended to describe resuspension by particle‐driven gravity currents, an analysis complicated by the change of bulk density of the suspension with time as particles sediment from the current. Here the erosive length scale is bounded by the maximum extent of the particle‐driven gravity current. A general form for the vertical mass flux qd is assumed, and the total mass resuspended is calculated in terms of the initial characteristics of the gravity current and shown to be a function of the volume V of dense fluid released, independent of the geometry of release, and independent of gravitational acceleration. Complementary laboratory experiments of resuspension by two‐dimensional saline and particle‐driven gravity currents are presented. These experiments consisted of a lock release of dense fluid running over a layer of particles. The critical conditions for resuspension were determined for different materials, and the variation of the erosive distance with gravity current characteristics was studied. These observations are discussed with reference to the theoretical model.
An approach to parameterization of the oceanic turbulent boundary layer in the western Pacific warm pool
Authors: Soloviev Alexander , Lukas Roger , Hacker Peter
Published: February 2004

Vertical profiles of zonal velocity and the dissipation rate ε of the turbulent kinetic energy obtained during the Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (COARE) are analyzed in the context of planetary boundary layer relationships previously derived from atmospheric measurements. The presence of a barrier layer and the striking effect of increased dimensionless shear and ε at the bottom of the surface mixed layer of the ocean, features often observed in the western Pacific warm pool area, are consistent with the boundary layer laws. The gradient Richardson number Ri is found to be a convenient parameter for scaling the nonstationary and horizontally heterogeneous mixed layer in the warm pool area. The approach to parameterization of the turbulent eddy coefficient within the mixed layer as a function of Ri is tested as part of a one‐dimensional model. A comparison between the observed and modeled upper ocean temperatures for two COARE examples shows a reasonable agreement over a wide range of wind speed conditions.
Interdisciplinary ocean process studies on the New England shelf
Authors: Dickey T. D. , Williams A. J.
Published: February 2004

A coordinated study of physical, bio‐optical, and sedimentary processes and acoustic transmission was conducted on the New England shelf about 110 km south of Cape Cod, Massachusetts, from September 1995 to August 1997. The experiment utilized several different observing platforms enabling measurements over space scales from centimeters in the vertical to tens of kilometers in the horizontal and timescales from minutes to the annual cycle. The present report provides information concerning previous work in the region and objectives, sampling methods, locations, and timelines for the experiment. Many of the results focus on the understanding of interrelationships and couplings among physical, bio‐optical, sedimentary, and acoustical properties and processes. Two hurricanes passed near the study site, enabling novel research concerning the effects of intense atmospheric forcing.
Structure and modeling of surf zone turbulence due to wave breaking
Authors: Mocke Gary P.
Published: February 2004

The turbulence structure under breaking waves in the surf zone is investigated through reference to experimental measurements and turbulence closure modeling. This includes a unique set of recent turbulence measurements that have extended into the roller area through the application of digital particle image velocimetry. Measurements highlight the surface roller as the primary agent for turbulence production, from where it is transported downward principally because of turbulent diffusion. Reflecting the breaker transition process, turbulence intensities are largest some distance shoreward of the breaker point where the roller is fully developed. The temporal variation of turbulence at a fixed point below the wave trough level is generally found to be minimal, supporting the present use of a time‐independent turbulence closure model. Production of turbulent kinetic energy in the two‐equation (k‐ε) model is presumed to occur in the vicinity of the surface roller, from where it is transported downward through the water column by diffusion. This production, which is estimated from the rate of energy dissipation in the wave roller, is the principal input parameter for the k‐ε model. Froude scaled turbulence measurements from laboratory and field experiments are found to be satisfactorily predicted within the limits of the developed surf zone, with improved methods of incorporating wave breaker transition zone effects leading to further model refinement. The turbulence closure model is further shown to be effective in predicting time mean suspended sediment concentration and undertow velocity profiles in the surf zone.
Seasonal variation of the North Atlantic Current
Authors: Yaremchuk Max I. , Nechaev Dmitri A. , Thompson Keith R.
Published: February 2004

The seasonal circulation of the upper 1000 m of the North Atlantic between 40°–55°N and 20°–40°W is calculated using the traditional dynamic method and a circulation model with a density field that evolves with the flow. The model is of finite difference form and is based on dynamics that describe the nonlinear evolution of the ocean at low Rossby number. The model is controlled by initial and boundary conditions that include air‐sea buoyancy and momentum fluxes. The model is run in two ways: with controls specified directly from observations and with controls inferred by the assimilation of all available data. These data include surface drifter trajectories, sea levels from the TOPEX/Poseidon altimeter, Bunker air‐sea fluxes, and the Levitus climatological monthly means of temperature and salinity. We conclude that the North Atlantic Current transport is 40±18 Sv with seasonal variations of the order of 2 Sv. The mean vertical transport out of the region is 2±9 Sv and is subject to seasonal variations of 2 Sv. Overall, these estimates are in good agreement with integral North Atlantic Current features derived from independent long‐term measurements made in the region over the past decade. The optimal ocean state has a volume transport across the western boundary of 51±3 Sv with a maximum transport of 61±5 Sv in April‐May and a minimum of 42±3 Sv in October‐November. This western inflow is compensated by mean outflows of 28±2 (east), 16±2 (north), 5±2 (south), and 1.8±0.4 Sv out of the domain at 1000 m. Sensitivity studies show that nonlinear mixing and seasonality are important in determining the overall circulation. Specifically, steady boundary forcing leads to annual mean transports that are 15–25% smaller than transports obtained with seasonal forcing. Winter convection is also shown to play a significant role in determining the overall circulation pattern.
Absolute measurement of the abundance of atmospheric carbon monoxide
Authors: Brenninkmeijer C. A. M. , Koeppel C. , Röckmann T. , Scharffe D. S. , Bräunlich Maya , Gros Valerie
Published: February 2004

The main aspects of an absolute method for measurement of the mixing ratio of atmospheric carbon monoxide (CO) are presented. The method is based on cryogenic extraction of CO from air after oxidation to CO2 followed by accurate volumetric determination. Gravimetric measurement is used to determine the quantity of sample air processed. In routine operation the overall error can be kept below 1%. Furthermore, the results of a laboratory intercomparison are analyzed. It is shown how offsets in the commonly applied analytical methods can occur and how these can seriously affect results obtained at the low concentration end (<100 nmole/mole).
Thermospheric gravity waves in the southern polar cap from 5 years of photometric observations at Davis, Antarctica
Authors: Innis J. L. , Greet P. A. , Dyson P. L.
Published: February 2004

Five years of photometric data of the λ630 nm oxygen auroral/airglow emission (∼240‐km altitude) over Davis, Antarctica (Λ=−74.6°), are searched for evidence of periodic oscillations due to atmospheric gravity waves. Data were selected for intervals when the auroral oval was equatorward of Davis and hence are from the thermospheric polar cap. A total of 106 cloud‐free nights, each with a minimum of 2 hours in the cap, were subjected to a full spectral analysis. Wave‐like features were seen on 45 nights (42%), with some nights showing evidence for two intervals of waves: A total of 53 waves were identified. Periods range from ∼13 to 40 min. We identify the oscillations as the signatures of gravity waves. Derived wave properties include horizontal wavelengths from 220 to 1600 km and phase speeds ranging from ∼200 to ∼850 ms−1. Around half of the waves are directed within ∼ ±40° of magnetic south, with most of the remainder being directed either approximately magnetic east or magnetic west. We interpret this as indicating the wave source regions are of significant linear extent and are aligned either parallel or perpendicular to the auroral oval. Source locations are estimated using the known dependence on frequency of the gravity wave angle of ascent, assuming the source is near the electrojet height (120 km) and ignoring the effects of background winds. The source locations are found to be near the magnetic latitude of the poleward edge of the auroral oval. Phase speeds and travel directions are similar to the speeds and direction of substorm surges, which is suggestive of the source of these waves.
Which magnetic storms produce relativistic electrons at geosynchronous orbit?
Authors: O'Brien T. P. , McPherron R. L. , Sornette D. , Reeves G. D. , Friedel R. , Singer H. J.
Published: February 2004

Relativistic electrons appear in the geosynchronous environment following some, but not all, geomagnetic storms. The ability to identify which storms produce these electrons would bring us much closer to explaining the mechanism responsible for their appearance, and it would provide the space weather community with a means to anticipate the electron hazard to geosynchronous spacecraft. We apply a recently developed statistical technique to produce an hourly time series of relativistic electron conditions at local noon along geosynchronous orbit using several geosynchronous monitors. We use a cross‐correlation analysis to determine what parameters in the solar wind and magnetosphere might influence the flux of relativistic electrons. We then perform a superposed epoch analysis to compare storms with and storms without the appearance of these electrons. We investigate a number of solar wind and magnetospheric parameters for these two sets of storms at 1‐hour resolution. In particular, sustained solar wind velocity in excess of 450 km s−1 is a strong external indicator of the subsequent appearance of relativistic electrons. In the magnetosphere, long‐duration elevated Pc 5 ULF wave power during the recovery phase of magnetic storms appears to discriminate best between those storms that do and do not produce relativistic electrons.
One‐gas models with height‐dependent mean molecular weight: Effects on gravity wave propagation
Authors: Walterscheid R. L. , Hickey M. P.
Published: February 2004

Many models of the thermosphere employ the one‐gas approximation where the governing equations apply only to the total gas and the physical properties of the gas that depend on composition (mean molecular weight and specific heats) are height‐dependent. It is further assumed that the physical properties of the gas are locally constant; thus motion‐induced perturbations are nil. However, motion in a diffusively separated atmosphere perturbs local values of mean molecular weight and specific heats. These motion‐induced changes are opposed by mutual diffusion of the constituent gases, which attempts to restore diffusive equilibrium. Assuming that composition is locally constant is equivalent to assuming that diffusion instantaneously damps the changes that winds attempt to produce. This is the limit of fast diffusion. In the limit of slow diffusion, gas properties are constant (conserved) following the motion but are perturbed locally by advection. An analysis of the static stability shows that composition effects significantly change the static stability, with greater changes for the slow‐diffusion limit than for the fast‐diffusion limit. We have used a one‐gas full‐wave model to examine the effects of wave‐perturbed composition on gravity waves propagating through the lower thermosphere. We have augmented the conventional system (fixed gas properties) with predictive equations for composition‐dependent gas properties. These equations include vertical advection and mutual diffusion. The latter is included in parameterized form as second‐order scale‐dependent diffusion. We have found that the fast diffusion implied by locally fixed properties has a significant effect on the dynamics. Predicted temperatures are larger for locally fixed composition than for conserved composition. The simulations with parameterized mutual diffusion gave results that are much closer to the results for conserved gas properties than for fixed properties. We found that the divergence between the fast and slow limits was greatest for fast waves and for colder thermospheres. This is because the propagation characteristics of fast waves are sensitive to changes in the static stability and because compositional gradients are stronger for colder thermospheres. We conclude that future models that use the one‐gas approximation for fast waves in the lower thermosphere should include, at minimum, the simplification of conserved rather than fixed properties, especially for colder thermospheres.
Predicting the 1‐AU arrival times of coronal mass ejections
Authors: Gopalswamy Nat , Lara Alejandro , Yashiro Seiji , Kaiser Mike L. , Howard Russell A.
Published: February 2004

We describe an empirical model to predict the 1‐AU arrival of coronal mass ejections (CMEs). This model is based on an effective interplanetary (IP) acceleration described by Gopalswamy et al. [2000b] that the CMEs are subject to, as they propagate from the Sun to 1 AU. We have improved this model (1) by minimizing the projection effects (using data from spacecraft in quadrature) in determining the initial speed of CMEs, and (2) by allowing for the cessation of the interplanetary acceleration before 1 AU. The resulting effective IP acceleration was higher in magnitude than what was obtained from CME measurements from spacecraft along the Sun‐Earth line. We evaluated the predictive capability of the CME arrival model using recent two‐point measurements from the Solar and Heliospheric Observatory (SOHO), Wind, and ACE spacecraft. We found that an acceleration cessation distance of 0.76 AU is in reasonable agreement with the observations. The new prediction model reduces the average prediction error from 15.4 to 10.7 hours. The model is in good agreement with the observations for high‐speed CMEs. For slow CMEs the model as well as observations show a flat arrival time of ∼4.3 days. Use of quadrature observations minimized the projection effects naturally without the need to assume the width of the CMEs. However, there is no simple way of estimating the projection effects based on the surface location of the Earth‐directed CMEs observed by a spacecraft (such as SOHO) located along the Sun‐Earth line because it is impossible to measure the width of these CMEs. The standard assumption that the CME is a rigid cone may not be correct. In fact, the predicted arrival times have a better agreement with the observed arrival times when no projection correction is applied to the SOHO CME measurements. The results presented in this work suggest that CMEs expand and accelerate near the Sun (inside 0.7 AU) more than our model supposes; these aspects will have to be included in future models.
Determining the mass density along magnetic field lines from toroidal eigenfrequencies: Polynomial expansion applied to CRRES data
Authors: Denton R. E. , Lessard M. R. , Anderson R. , Miftakhova E. G. , Hughes J. W.
Published: February 2004

Because the harmonics of toroidal Alfvén wave eigenmodes have a different response to mass density at different points along a magnetic field line, the frequencies of these harmonics can, in principal, be used to infer the distribution of mass density along the field line. Here we present several improvements to the methodology and test our method using magnetic and plasma wave data from the CRRES spacecraft. Our method includes the calculation of toroidal frequencies in a Tsyganenko magnetic field assuming a polynomial expansion for the logarithm of the mass density with respect to a coordinate related to distance along the field line. We use a Monte Carlo distribution of frequencies about the observed peaks in order to infer an uncertainty for the mass density. The method only works well if the frequencies of the fundamental and second harmonic are known. We compare the inferred mass density at the spacecraft location with the electron density determined from the plasma wave experiment onboard CRRES. The observed electron density is about a factor of 2 lower than the mean of the inferred mass density for an ensemble of frequency combinations based on the uncertainties of the measured spectral peaks but is close to or within the error bars of the inferred mass density. In one of the cases examined, the inferred mass density had a local maximum at the equator, while in the other case the inferred mass density increased monotonically away from the equator.
Characteristics of coronal mass ejections associated with long‐wavelength type II radio bursts
Authors: Gopalswamy N. , Yashiro S. , Kaiser M. L. , Howard R. A. , Bougeret J.‐L.
Published: February 2004

We investigated the characteristics of coronal mass ejections (CMEs) associated with long‐wavelength type II radio bursts in the near‐Sun interplanetary medium. Type II radio bursts in the decameter‐hectometric (DH) wavelengths indicate powerful MHD shocks leaving the inner solar corona and entering the interplanetary medium. Almost all of these bursts are associated with wider and faster than average CMEs. A large fraction of these radio‐rich CMEs were found to decelerate in the coronagraph field of view, in contrast to the prevailing view that most CMEs display either constant acceleration or constant speed. We found a similar deceleration for the fast CMEs (speed > 900 km s−1) in general. We suggest that the coronal drag could be responsible for the deceleration, based on the result that the deceleration has a quadratic dependence on the CME speed. About 60% of the fast CMEs were not associated with DH type II bursts, suggesting that some additional condition needs to be satisfied to be radio‐rich. The average width (66°) of the radio‐poor, fast CMEs is much smaller than that (102°) of the radio‐rich CMEs, suggesting that the CME width plays an important role. The special characteristics of the radio‐rich CMEs suggest that the detection of DH radio bursts may provide a useful tool in identifying the population of geoeffective CMEs.
Far ultraviolet equatorial aurora during geomagnetic storms as observed by the Low‐Resolution Airglow and Aurora Spectrograph
Authors: Stephan Andrew W. , Chakrabarti Supriya , Dymond Kenneth F. , Budzien Scott A. , Thonnard Stefan E. , McCoy Robert P.
Published: February 2004

We report the detection of storm time enhancements in the low‐latitude far ultraviolet airglow as observed by the Low‐Resolution Airglow and Aurora Spectrograph on the Advanced Research and Global Observation Satellite. The enhancements are present in several of the dayside and nightside emission lines, including the prominent 1304‐ and 1356‐Å lines of atomic oxygen as well as the N2 Lyman‐Birge‐Hopfield bands near 1465 and 1495 Å. Time histories of the average low‐latitude intensities of all emissions show a correlation with geomagnetic activity, as measured by the Dst index. Comparisons between the prestorm and storm time latitude profiles indicate that the emission increases are confined to magnetic latitudes < 20°. We have used the ratio of 1356 Å/1495 Å as a measure of O/N2 composition changes at these low latitudes. Although this ratio shows composition changes during the storm, no change in the ratio is observed during the peak in the emission. On the basis of the emission morphology, we conclude that these emission enhancements are most likely the result of energetic neutral atoms, which are created in the ring current and collisionally excite ambient atomic oxygen and molecular nitrogen in the low‐altitude, low‐latitude ionosphere and thermosphere.
Theoretical modeling of ƒoF2 and hmF2 ionospheric parameters during a strong magnetic disturbance
Authors: Kilifarska N. A. , Ouzounov D. P.
Published: February 2004

The theoretical data assimilative model of the ionosphere (TDAMI) was created to answer the most challenging problem in ionospheric physics: modeling the ionospheric disturbances (in particular, parameters related to the peak of the F layer ƒoF2 and hmF2) during geomagnetic storms. It is an analytical model that calculates maximum electron density (Nm) distribution and the height of this maximum (hm) as a function of time, latitude, longitude, season and solar activity. Tested as a “climatological”, i.e., monthly average, model over the worldwide midlatitude ionospheric data set for different seasons and solar activity conditions, TDAMI yields results comparable with the well‐known empirical models of the International Radio Consultative Committee (CCIR) and the Union Radio Scientifique Internationale (URSI). The mean annual relative error ƒoF2 is <8% for all levels of solar activity, while the mean seasonal errors do not exceed 12%. A comparison between the model simulations and all existing vertical incidence (VI) data for the geomagnetically disturbed period October 10–11, 1988, shows that TDAMI adequately simulates the UT‐latitudinal‐longitudinal dependence of the ionospheric reaction to the geomagnetic disturbance. During the hours of maximum depletion in the electron density, the improvement achieved by the model is more than 70–80% compared to international reference ionosphere (IRI) global distributions (by CCIR and URSI coefficients sets). The maximum TDAMI model error is 20–25%.
Space‐based ultraviolet optical signatures of simulated equatorial ionospheric bubbles
Authors: Keskinen M. J. , Dymond K. F. , Ossakow S. L.
Published: February 2004

Space‐based ultraviolet (UV) optical signatures of equatorial ionospheric bubbles have been computed from a first‐principles time‐dependent physical mesoscale ionospheric simulation model. We find that for nadir‐viewing geometry, extreme UV signatures at 911 Å of equatorial ionospheric bubble structures contain large bite‐outs in intensity, are structured over tens of kilometers scales, and are distinctly different from the signatures of the quiet ionosphere.
Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study
Authors: Hamrin Maria , André Mats , Ganguli Gurudas , Gavrishchaka Valeri V. , Koepke Mark E. , Zintl Michael W. , Ivchenko Nickolay , Karlsson Tomas , Clemmons James H.
Published: February 2004

We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma flow perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current‐driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.
Effects of charge exchange and photoionization on the interaction between the solar wind and unmagnetized planets
Authors: Shimazu Hironori
Published: February 2004

The effects of photoionization and charge exchange were compared by using three‐dimensional hybrid code simulations of the interaction between the solar wind and Venus. The direction of the shock altitude asymmetry in the direction of the convection electric field when photoionization was included was contrary to observations near Venus. However, when charge exchange was included, the direction of the asymmetry did agree with the observations. An analysis of the simulation results showed that the velocities of pickup ions and electrons differed near locations where charge exchange occurred. This velocity difference caused an electric current, and it generated a strong magnetic barrier on the side of the planet to which the convection electric field was pointing. This magnetic barrier acted as an obstacle to the solar wind, and the shock was inflated on this side.
Possible control of plasma transport in the near‐Earth plasma sheet via current‐driven Alfvén waves (ƒ ≃ ƒH+)
Authors: Le Contel O. , Roux A. , Perraut S. , Pellat R. , Holter Ø. , Pedersen A. , Korth A.
Published: February 2004

Two time periods, each covering both quiet and disturbed conditions (growth phase, breakup, and post breakup phase), are studied. Electric and magnetic field measurements, carried out in the near‐Earth plasma sheet (NEPS), are used to calculate the two components (radial and azimuthal) of the electric E × B/B2 drift. These calculations are compared with independent estimates of the ion flow direction deduced from ion flux measurements. During active periods, the two flow directions coincide to a large degree. Evidence is given for two regimes of transport: (1) During the growth phase, and after the active phase, the electric field (radial and azimuthal) and hence the azimuthal and radial flow velocities are small in the near‐equatorial region. This is interpreted as the consequence of an electrostatic field that tends to shield the induced electric field associated with time‐varying external conditions. (2) During active phases (breakup and pseudobreakup), however, large‐amplitude bursts in E × B/B2 radial and azimuthal components (interpreted as flow bursts), with typical velocities of the order of 100 km s−1, are observed. The direction of these flow bursts is somewhat arbitrary, and in particular, for the two substorm events described here, sudden reversals in the flow direction are observed. These fast flow bursts coincide with intense low‐frequency electromagnetic fluctuations: current‐driven Alfvén waves (CDA waves) with frequency ƒ ≃ ƒH+, the proton gyrofrequency. CDA waves produce “anomalous” collisions on timescales shorter than the electron bounce period, thus violating the second adiabatic invariant for electrons. As a consequence, the electrostatic shielding is destroyed, which leads to enhanced radial transport. Thus the transport in the NEPS seems to be controlled by a microscopic current‐driven instability.
Particle acceleration at an X‐type reconnection site with a parallel magnetic field
Authors: Browning P. K. , Vekstein G. E.
Published: February 2004

The acceleration of charged particles at a two‐dimensional magnetic reconnection site is investigated. The magnetic field has an X‐type neutral point, while reconnection is driven by a uniform transverse electric field; the effect of including a uniform magnetic field component parallel to the driving electric field and transverse to the plane of the X point is studied. We focus on the adiabatic motion of strongly magnetized particles, a valid assumption everywhere for sufficiently strong parallel magnetic fields but one which excludes a region around the neutral point for weaker fields. The regime of interest is fast driven reconnection, in which the electric drift is strong. The trajectories of particles and their dependence on the magnitude of the parallel magnetic field component are investigated. Particles can be accelerated along the magnetic field lines both because of the coupling of the perpendicular electric drift with the parallel motion, which occurs in an inhomogeneous magnetic field, and the direct acceleration by the electric field. The energy spectra of particles leaving the reconnection site are also calculated.
Model of ionic charge states of impulsive solar energetic particles in solar flares
Authors: Rodríguez‐Frías M. Dolores , del Peral Luis , Pérez‐Peraza Jorge
Published: February 2004

We have fully developed a computational model (ESCAPE) to follow the behavior of the mean charge state of ions in solar energetic particle events while the ions are accelerated. Our model combines acceleration with energy loss and charge stripping low in the corona. Therefore we have taken into account explicitly the second‐order Fermi‐type stochastic acceleration under a magnetohydrodynamic turbulence. We have found that the mean ionic charge states depend sensitively on plasma parameters as source temperature or density and on acceleration parameters as efficiency or the timescales for acceleration. Our model finds a systematic increase of the ionic charge states with energy for all the ions studied. This energy dependence differs between ions, and in the energy range of observations this dependence is stronger for heavy ions.
Detection of S (IV) 10.51 μm emission from the Io plasma torus
Authors: Lichtenberg G. , Thomas N. , Fouchet T.
Published: February 2004

We report the first detection of the forbidden 10.51 μm line of S3+ in the Io plasma torus (IPT). The observations were performed with European Space Agency's Infrared Space Observatory. Seven measurements were acquired, four at the dusk ansa and three at the dawn ansa. Variations with λIII have been investigated, and an upper limit for variation of 20% at the dusk ansa was determined. Using this value, a maximum possible change of 8 – 10% from the nominal value of electron temperature with λIII has been calculated. Alternatively, the density of hot electrons can vary by 20%. The absolute intensity of S (IV) was found to be 1.5–3.5 times higher than that predicted by a model of the IPT for a Voyager‐like torus. Possible reasons for this difference have been investigated. It was found that an increased electron temperature, a higher concentration of suprathermal electrons, or a higher plasma density can lead to an intensity increase of a factor of 1.5. To increase the intensity by a factor of 3.5 it is necessary to reduce the electron temperature and at the same time increase the plasma density. Additionally, we report a detection of an S (III) line at 33.48 μm.
An overview of observations by the Cassini radio and plasma wave investigation at Earth
Authors: Kurth W. S. , Hospodarsky G. B. , Gurnett D. A. , Kaiser M. L. , Wahlund J.‐E. , Roux A. , Canu P. , Zarka P. , Tokarev Y.
Published: February 2004

On August 18, 1999, the Cassini spacecraft flew by Earth at an altitude of 1186 km on its way to Saturn. Although the flyby was performed exclusively to provide the spacecraft with sufficient velocity to get to Saturn, the radio and plasma wave science (RPWS) instrument, along with several others, was operated to gain valuable calibration data and to validate the operation of a number of capabilities. In addition, an opportunity to study the terrestrial radio and plasma wave environment with a highly capable instrument on a swift fly‐through of the magnetosphere was afforded by the encounter. This paper provides an overview of the RPWS observations at Earth, including the identification of a number of magnetospheric plasma wave modes, an accurate measurement of the plasma density over a significant portion of the trajectory using the natural wave spectrum in addition to a relaxation sounder and Langmuir probe, the detection of natural and human‐produced radio emissions, and the validation of the capability to measure the wave normal angle and Poynting flux of whistler‐mode chorus emissions. The results include the observation of a double‐banded structure at closest approach including a band of Cerenkov emission bounded by electron plasma and upper hybrid frequencies and an electron cyclotron harmonic band just above the second harmonic of the electron cyclotron frequency. In the near‐Earth plasma sheet, evidence for electron phase space holes is observed, similar to those first reported by Geotail in the magnetotail. The wave normal analysis confirms the Polar result that chorus is generated very close to the magnetic equator and propagates to higher latitudes. The integrated power flux of auroral kilometric radiation is also used to identify a series of substorms observed during the outbound passage through the magnetotail.
A 2.5 dimensional MHD simulation of multiple‐plasmoid‐like structures in the course of a substorm
Authors: Jin S.‐P. , Hu X.‐P. , Zong Q.‐G. , Fu S.‐Y. , Wilken B. , Büchner J.
Published: February 2004

As well known, the magnetic cross‐tail component By in the magnetotail is well correlated with the interplanetary magnetic field (IMF) By component, and its variation is significant during substorms. A 2.5‐dimensional MHD simulation has been carried out on the basis of equilibrium solutions of the quiet magnetotail. Two types of distributions of the By component and the plasma density (Type I and Type II) are used as the initial states of the simulation studies. The results of all cases with different initial conditions illustrate that the formation of plasmoids occurs intermittently and repeatedly. Also, all the plasmoids are high‐density and high‐temperature regions in comparison with the ambient environment; that means the plasma with high‐temperature is embedded in plasmoids formed repeatedly. These results are in line with features of multiple‐plasmoid‐like structures observed on January 15, 1994, with Geotail at 96 RE in the tail. Thus it can be concluded that a large amount of the energy stored in the magnetotail is gradually dissipated by ejecting multiple plasmoids in the course of substorms. In this simulation it is shown that the lasting inflow caused by the electric field E imposed on the boundary of the simulation box controls the recurrent formation of plasmoids. Further, as an additional evidence, the multiple‐plasmoid‐like structures have been detected by Geotail in conditions of high‐speed solar wind streams and southward IMF. Therefore our simulation suggests that the solar wind and the IMF have close control over the magnetotail dynamic process. The features of a classic plasmoid (bipolar Bz) and a travelling compression region (TCR) in the (X, Z) plane can be seen in all cases. Taking the time evolution of the By component into account, two kinds of plasmoid‐like structures with a flux rope core and with both By and Bz bipolar signatures can be reproduced for the Type I and Type II conditions, respectively. Therefore the occurrence of various magnetic structures in the magnetotail might be related to nonsteady driven reconnection with different initial distributions of the By component.
Energetic particle measurements during the Earth swing‐by of the Cassini spacecraft in August 1999
Authors: Lagg A. , Krupp N. , Livi S. , Woch J. , Krimigis S. M. , Dougherty M. K.
Published: February 2004

We report on energetic particle measurements from the Low‐Energy Magnetospheric Measurement System (LEMMS) aboard the Cassini spacecraft during the Earth swing‐by maneuver in August 1999. LEMMS is capable of identifying the energy and incidence direction of energetic ions and electrons with energies of a few tens of keV to several tens of MeV. Cassini flew by Earth with a velocity of 16 kms−1 or 9 Earth radii (RE) per hour, which means that the entire day side magnetosphere was passed within 1 hour. This fast flyby trajectory of the spacecraft provided a snapshot of the Earth's magnetosphere where key regions were passed within a few hours instead of typically tens of hours or days for orbiting spacecraft. The measurements provide a means to show the performance of the instrument's capabilities as well as to test models of energetic particle distributions within the Earth's magnetosphere [e.g. Sibeck et al., 1987]. Particle pitch angle distributions (PADs) on the inbound pass along the 1300 magnetic local time (MLT) meridian gradually changed from normal distributions (maximum at 90° pitch angle) to butterfly pitch angle distributions (maximum at intermediate pitch angles). Close to the plasmapause, LEMMS observed field‐aligned bidirectional distributions. During the outbound pass through the inner magnetosphere at 0130 MLT, pitch angle distributions are generally close to isotropic. Shortly after a substorm onset, identified by ground‐based observations and aboard the Polar spacecraft [Khan et al., this issue], LEMMS observed an energy‐time dispersed enhancement in the differential flux of low‐energy electrons. We further address the question of whether signatures of the Earth magnetotail were observed at distances beyond 5000 RE when Cassini passed through the Earth's downstream region. LEMMS measured a series of particle increases during that time. The angular distributions during these enhancements show that most of these particles did indeed arrive from the Earth's direction. Nevertheless, the lack of singly charged heavy ions does not allow for a definite identification of this event as a deep‐tail encounter.
Antenna‐plasma and antenna‐spacecraft resistance on the Wind spacecraft
Authors: Kellogg Paul J. , Bale Stuart D.
Published: February 2004

Measurements have been made of the resistance of three different electric field sensing antennas on the Wind spacecraft in the solar wind. The measurements have been separated into base resistance and plasma resistance by assuming that the plasma resistance is inversely proportional to the antenna length. Simple formulas for the resistances, as functions of the plasma electron flux, are given and fitted to the data. The results imply that the division of the signal by the base resistance is important and needs to be taken into account in low‐frequency measurements of electric fields when the antennas are shorter than a few tens of meters.
Nonlinear response of the polar ionosphere to large values of the interplanetary electric field
Authors: Russell C. T. , Luhmann J. G. , Lu G.
Published: February 2004

During large geomagnetic storms the polar cap potential drop fluctuates, but its average value seldom reaches 150 kV and its peak value seldom reaches 200 kV. Since the quiet time potential drop is about 40 kV and since the interplanetary electric field (IEF) is greatly enhanced during storms, this relative constancy appears to be due to a saturation effect. This apparent saturation phenomenon has been previously noted but has not received much attention in the recent literature, in spite of its importance during magnetic storms and in spite of the fact that it seems not to be understood. The IEF and the polar cap potential drop appear to have a very linear relationship up to about 80 kV. At greater IEF the potential drop increases only slightly. We use the assimilative mapping of ionospheric electrodynamics procedure to illustrate this saturation on a case‐by‐case basis during a number of recent storms. We confirm using DE 2 data that the saturation of the potential drop is a real effect and is not an artifact of the measurement process. Observations of the plasmapause and the buildup of the ring current indicate a linear response to the IEF over a wide range of solar wind conditions. Thus the convection of the magnetospheric plasma and the injection of energy into the ring current seem not to be affected by this saturation.
A multi‐instrument study of a Jovian magnetospheric disturbance
Authors: Louarn P. , Mauk B. H. , Kivelson M. G. , Kurth W. S. , Roux A. , Zimmer C. , Gurnett D. A. , Williams D. J.
Published: February 2004

Using observations from different Galileo experiments (plasma wave system, magnetometer and energetic particle detector), we analyze a strong magnetospheric disturbance that occurs on day 311 of 1996 as Galileo was close to Jupiter (less than 15 Jovian radii). This perturbation is characterized by multiple injections of energetic particles in the inner magnetosphere and has been described as a possible analog of the terrestrial magnetic storm by Mauk et al. [1999]. We show here that it also corresponds to a large‐scale magnetospheric perturbation similar to the “energetic events” described by Louarn et al., [1998, 2000]. It is associated with the development of a particular magnetic activity in the outermost part of the Io torus, over periods of 2–4 hours and in sectors of longitude with a typical 30°–80° longitudinal extension. At distances ranging from 10 to 13 Rj, the activity itself is characterized by the generation of low‐frequency magnetic oscillations (18 min periodicity in the present case) that correlate with dispersionless energetic electron injections and modulations of the auroral radio flux. When they are observed a few hours after their formation, these injections present a weak energy‐time dispersion and are still periodic. They then progressively mix and finally define a region of limited longitudinal extension where the density of energetic particles is particularly large. We show that this region corresponds to the source of the narrowband kilometric radiation (n‐KOM). By combining remote sensing radio observations, in situ particle, and magnetic field measurements, we show that the active zone where the large scale disturbance initially develops most probably does not corotate and would even be almost fixed in local time. In the present case, the magnetospheric event is the consequence of two activations separated by a few hours. They occur in two separated longitude sectors and give rise to two different n‐KOM sources. During the event, some 1012 W are transferred to the electron population. It is proposed that this set of phenomena is the manifestation of a sporadic dissipation of a part of the Io torus rotational energy and would be thus associated with the development of a large‐scale instability in the external part of the Io torus.