bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerodynamic roughness of fixed sandy beds
Authors: Dong Zhibao , Wang Xunming , Zhao Aiguo , Liu Lianyou , Liu Xianwan
Published: February 2004

The nature of interactions between surface winds and sand surfaces that can be characterized by aerodynamic roughness length has important implications for aeolian sediment transport. Here we report the wind tunnel results of systematic tests of the aerodynamic roughness lengths of different sized fixed sand beds at different free‐stream wind velocities. The results suggest that there are complex variations of aerodynamic roughness length. Wind is an active factor in determining the aerodynamic roughness length of fixed sand beds. The so‐called 1/30 law in aeolian research proposed by Bagnold more than a half century ago for sand surfaces is found to suffer some limitations. Some multivariate models relating aerodynamic roughness length and other factors are provided. The differences between the aerodynamic roughness of fixed and mobile beds are discussed, and it is implied that mobile surfaces adapt to the wind by changing roughness. The similarities between aerodynamic roughness and hydraulic roughness are compared. Aerodynamic roughness is divided into aerodynamically smooth, transitional, and completely rough regimes.
A global wave hindcast over the period 1958–1997: Validation and climate assessment
Authors: Cox Andrew T. , Swail Val R.
Published: February 2004

This study describes the first 40 year global wave simulation derived from the National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis (NRA) surface wind fields. The NRA 10 m wind fields were input into a deep water version of a proven spectral ocean wave model adapted onto a global grid of spacing 1.25° in latitude by 2.5° in longitude. In situ and satellite wind and wave data sets were used to evaluate the hindcast skill. The validation showed excellent agreement not only in terms of bias and scatter but over the entire frequency distribution out to 99th percentiles of both winds and waves. A global trend analysis showed statistically significant areas of both increasing and decreasing winds and waves. The increasing trend in the northeast Atlantic and decreasing trend in the central North Atlantic are particularly well defined and consistent with changes reported in previous studies, which were linked to changes in the North Atlantic Oscillation. The trend analysis highlighted the difficulty in separating creeping inhomogeneities in the NRA winds from real climate change, illustrating the need to use homogeneous in situ measured data to confirm trends derived from model output. The trends derived from the hindcast seem reasonable in the Northern Hemisphere and may provide a good upper bound to true trends in the wind and wave climate.
Observations and modeling of coastal internal waves driven by a diurnal sea breeze
Authors: Lerczak J. A. , Hendershott M. C. , Winant C. D.
Published: February 2004

During the Internal Waves on the Continental Margin (IWAVES) field experiments of 1996 and 1997 off of Mission Beach, California (32.75°N), we observed energetic, diurnal‐band motions across the entire study site in water depths ranging from 15 to 500 m and spanning a cross‐shore distance of 15 km. The spectral peak of the currents was at the diurnal frequency (σDi = 1 cpd) and was sufficiently well resolved to be clearly separated from the slightly higher local inertial frequency (ƒ = 1.08 cpd). These motions were surface enhanced and clockwise circularly polarized and had an upward phase propagation speed of ∼68 m d−1, suggesting that the motions were driven predominantly by the diurnal sea breeze. However, the downward energy (upward phase) propagation seems irreconcilable with the subinertial diurnal period, and moreover, the intermittent diurnal current events were not obviously associated with diurnal sea breeze events. We rationalize these features using a flat‐bottomed linear modal sum internal wave model that includes advection and refraction due to subtidal alongshore flow, V(x, t). Fluctuations in V at the observing site can change the “effective” local Coriolis parameter ƒ + Vx by as much as 50%, thus making the diurnal motions at different times effectively either subinertial or superinertial. The model is integrated numerically for 200 days at a latitude of 32.75°N under different wind and subtidal flow conditions: purely diurnal winds and no V, purely diurnal winds and a time‐independent V, narrow‐band diurnal winds and no V, and narrow‐band diurnal winds and subtidal, time‐dependent V. Model diurnal currents forced by narrow‐band diurnal winds and subtidal V show complex offshore structure with realistic intermittency and spectral broadening. This study suggests that continental margins in the vicinity of the 30° latitude (where σDi = ƒ) are regions that could potentially produce energetic, sea breeze‐driven baroclinic motions and that these motions could be regulated by the vorticity of the local subtidal currents.
Large‐eddy simulation of the diurnal cycle of oceanic boundary layer: Sensitivity to domain size and spatial resolution
Authors: Wang Dailin
Published: February 2004

The sensitivity of ocean large‐eddy simulations (LES) to model domain size and spatial resolution is systematically investigated in the context of a diurnal cycling oceanic boundary layer. The model domain size varies by a factor of 64, horizontal resolution varies by a factor of 12, and vertical resolution varies by a factor of 8. A control experiment with a high resolution serves as the “truth” by which other experiments with smaller domains and/or coarser resolutions are judged. During nighttime the primary balance in the turbulent kinetic energy budget is between buoyancy production, turbulent transport, pressure transport, and dissipation. During daytime the primary balance is between shear production, buoyancy destruction, and dissipation. It is found that mean fields and turbulent fluxes are insensitive to domain size as long as it is comparable to or greater than the mixed layer depth. The mean fields and turbulent fluxes are also insensitive to resolution, provided there are several vertical levels in the entrainment layer. Turbulent kinetic energy (TKE), however, shows varying degrees of sensitivity. Simulations with larger domains or with coarser resolutions tend to have larger values of TKE. Furthermore, TKE is more sensitive to domain size and resolution during the day (shear turbulence) than during the night (convection). Practically, it is better to use a larger domain with an anisotropic resolution than a small domain with an isotropic resolution, if computer resources are severely limited. With the same grid geometry, the former is not only cheaper to conduct but also can be more accurate as far as turbulent kinetic energy and turbulent fluxes are concerned. This underlines the importance of resolving the largest eddies in the oceanic boundary layer. We also conclude that validations of simple one‐dimensional mixed layer models using LES solutions with a resolution of order 32×32×32 are not without merit in the sense that mean fields and turbulent fluxes are not much different when much higher resolution is used. However, if TKE is used as a variable of validation, higher resolution is needed.
Turbulent flow over a wavy surface: Stratified case
Authors: Calhoun Ronald J. , Street Robert L. , Koseff Jeffrey R.
Published: February 2004

Stably stratified turbulent flow over wavy terrain is investigated using large‐eddy simulation for laboratory‐scale domains. Basic near‐wall turbulence structure and production mechanisms appear similar to the neutral case, though dampened. The state of turbulence in the stratified fluid is analyzed by graphing the relationships between key nondimensional parameters. A shifting and smearing of the curve of flux Richardson number Rif versus turbulent Froude number Frt caused by the wavy boundary (compared with a flat boundary) demonstrates the higher level of turbulence near the wavy wall and shows strong mixing in the detached shear layer over the trough. The relationship describing the state of turbulence in a stratifed fluid given by Ivey and Imberger [1991] is approximately satisfied. Conditional sampling is used to analyze the nature of Reynolds stress events.
Estuarine salt flux through an isohaline surface
Authors: MacCready Parker , Geyer W. Rockwell
Published: February 2004

The salinity budget in an estuary is analyzed using an isohaline as the seaward bounding surface of the volume of integration. It is found that the rate of change of volume integrated salinity is governed by two processes: (1) the “drift” of the isohaline through the fluid, and (2) turbulent salt flux through the isohaline. This analysis highlights the role of turbulent salt flux in maintaining the salinity intrusion. In contrast, more standard budgets using a stationary, vertical cross section as the seaward end of the volume of integration attribute almost all along‐estuary salt flux to advective processes such as the gravitational circulation. The isohaline budget is explored using an analytical model and using results from a three‐dimensional numerical model. The numerical results highlight the complex spatial and temporal interplay between turbulent mixing and the shape of the isohaline. Averaging over a time period during which the freshwater volume behind the isohaline is constant reduces the isohaline budget to a balance of two terms: the time‐mean, area‐integrated turbulent salinity flux across the isohaline being equal to the mean river flow times the salinity of the isohaline.
Cluster observations of lower hybrid turbulence within thin layers at the magnetopause
Authors: Vaivads A. , André M. , Buchert S. C. , Wahlund J.‐E. , Fazakerley A. N. , Cornilleau‐Wehrlin N.
Published: February 2004

We use data from the Cluster satellites to study waves in the frequency range of ∼2–150 Hz at the magnetopause. In addition to broad band wave turbulence we identify lower hybrid drift waves and whistler waves. Wave amplitudes maximize within narrow current sheets that are associated with density gradients and electron beams and that can be the separatrices of the reconnection. The dispersion properties of lower hybrid drift waves are studied using interferometric measurements of electric field. A direct estimate of the wave role in the particle diffusion across the magnetopause shows that their importance is limited only to the narrow current sheet.
Paleo‐stress estimates on ancient seismogenic faults based on frictional heating of coal
Authors: O'Hara Kieran
Published: February 2004

Faulted coals from eastern Kentucky and Montana (USA), and the south Wales coalfield (UK) show fault‐related vitrinite reflectance anomalies interpreted as due to frictional heating during seismic slip. The vitrinite reflectance anomalies are converted to temperature using kinetic‐based software (Easy%R) for post‐seismic cooling rates. Shear stresses are calculated using Lachenbruch's [1986] piezometer, which depends only on the area under the thermal anomaly and fault displacement. For maximum displacements of 1–3 meters, the minimum shear stresses are 13–64 MPa, and overlap with seismological estimates of stress drops during modern earthquakes. The calculated local mean normal stress is ∼100 MPa, resulting in coefficients of friction of 0.1–0.6 (thrust) and 0.2–0.4 (normal faults), which are lower than laboratory‐based values.
A comparative study of Markov chain Monte Carlo methods for conceptual rainfall‐runoff modeling
Authors: Marshall Lucy , Nott David , Sharma Ashish
Published: February 2004

One challenge that faces hydrologists in water resources planning is to predict the catchment's response to a given rainfall. Estimation of parameter uncertainty (and model uncertainty) allows assessment of the risk in likely applications of hydrological models. Bayesian statistical inference provides an ideal means of assessing parameter uncertainty, whereby prior knowledge about the parameter is combined with information from the available data to produce a probability distribution (the posterior distribution) that describes uncertainty about the parameter and serves as a basis for selecting appropriate values for use in modeling applications. Widespread use of Bayesian techniques in hydrology has been hindered by difficulties in summarizing and exploring the posterior distribution. These difficulties have been largely overcome by recent advances in Markov chain Monte Carlo (MCMC) methods that involve Monte Carlo sampling of the posterior distribution. This study compares four MCMC sampling algorithms in the context of rainfall‐runoff modeling. The algorithms compared include a conventional Metropolis‐Hastings algorithm used previously in hydrological applications which uses a combination of block and single‐site updating and an adaptive Metropolis algorithm that has characteristics that are well suited to model parameters with a high degree of correlation and interdependence, as is often evident in hydrological models. In addition to these, two other algorithms are evaluated to clarify the relative importance of updating all model parameters as a block versus updating each parameter one at a time. The MCMC techniques are compared for simplicity, ease of use, statistical efficiency in exploration of the parameter space, and speed of implementation, using 11 years of daily rainfall‐runoff data from the Bass river catchment in Australia. The results show that the adaptive Metropolis algorithm is superior in many respects and can offer a relatively simple basis for assessing parameter uncertainty in hydrological modeling studies and that the efficiency of the adaptive algorithm is not solely attributed to the block‐updating element of the algorithm.
The occurrence of organic peroxides in air at a mountain site
Authors: Fels Michael , Junkermann Wolfgang
Published: February 2004

Mixing ratios of individual organic hydroperoxides were measured during summer 1990 on a mountain summit (1780 m a.s.l.). The dominating compound during the early summer months was hydroxymethyl hydroperoxide (HMHP) while later in the summer methyl hydroperoxide (MHP) prevailed. HMHP and MHP together made up more than 90% of total organic hydroperoxides (ROOH) which constituted about 10–40% of hydrogen peroxide measured concurrently at the same site. The ROOH mixing ratios at an altitude of 1175 m a.s.l., representing the planetary boundary layer (PBL), were about twice as large as those at the summit. Higher ROOH values in the PBL point to ROOH formation from anthropogenic and biogenic hydrocarbon precursors. The tendency to higher HMHP values in early summer points to the possible HMHP formation from biogenically emitted unsaturated hydrocarbon precursors.
A statistical seasonal analysis of winter decreases in ozone at Macquarie Island
Authors: Lehmann P.
Published: February 2004

The change in total ozone between the seasonal periods April to June and July to September at Macquarie Island (Australia, 55°S, 159°E) are shown to be highly statistically significant during the latter half of the 1980s when compared to the seasonal changes observed over the period 1963–79. These results support a previous analysis by the author which indicated significant winter ozone decreases using a less rigorous method and a shorter time series.
Rapid variations of the plasma bulk flow direction observed in the plasma sheet at XGSE∼ −60Re
Authors: Saito Yoshifumi , Mukai Toshifumi , Machida Shinobu , Hirahara Masafumi , Nishida Atsuhiro , Terasawa Toshio
Published: February 2004

Three‐dimensional variations of the plasma bulk flow direction in the plasma sheet at XGSE ∼ −60 Re were observed by the Low Energy Particle (LEP) instrument on board the GEOTAIL satellite. These variations have two components, namely a rapidly varying component of which the period is about 8 minutes and a more slowly varying component. Separating these two components, we have found a period when the rapidly varying component showed clear rotations while the direction of the slowly varying component nearly corresponded with the direction of the magnetic field. We have calculated the rotation axis of the rapidly varying component during this period and have found that the direction of the rotation axis changed by nearly 90° in a step‐like motion while the vortex motion remained coherent in the plane perpendicular to the moving rotation axis. We discuss several possible mechanisms for these variations, MHD waves, vortex structures, and tail flapping motion with velocity shears.
There is no “cometopause” at comet Halley
Authors: Rème H. , Mazelle C. , d'Uston C. , Korth A. , Lin R. P. , Chaizy P.
Published: February 2004

Immediately after the flybys at comet Halley by a fleet of spacecraft in 1986, Gringauz et al. (1986a) reported the detection by the Vega‐ 2 spacecraft of a chemical and sharp plasma boundary, which they named the “cometopause,” at a distance of ∼1.6 × 105 km from the nucleus. Gringauz and Verigin (1991) presented the “cometopause” as a permanent feature of the solar wind ‐ Halley type comet interaction at ∼1 UA from the Sun. This permanent boundary presumably separates an upstream region dominated by the solar wind from the downstream region where heavy cometary ions dominate. We present here the analysis of the results of the Giotto positive ion cluster composition analyzer ‐ Rème plasma analyzer (PICCA‐RPA2) ion mass spectrometer and electron electrostatic analyzer ‐ Rème plasma analyzer (EESA‐RPA1) electron spectrometer data, which clearly show that there is no such boundary at comet Halley.
Growing “Alfvenic” modes in the upstream region of Saturn
Authors: Orlowski D. S. , Russell C. T. , Krauss‐Varban D. , Omidi N.
Published: February 2004

Recent studies of low‐frequency electromagnetic waves upstream of the Saturn bow shock have shown that these waves, in contrast to those at Earth, are observed not in one, but in at least two, distinct frequency bands. The results of wave mode identification based on the Hall‐MHD model of plasma and observed wave polarization suggested that these waves propagate in the high beta intermediate mode. However, the underlying instability was not unambiguously determined. In the present paper we use the full electromagnetic dispersion relation derived from linear Vlasov theory in order to examine which of the plasma modes, with observed properties, are unstable in an isotropic Maxwellian plasma in the presence of backstreaming proton beams consistent with Voyager 2 observations at Saturn. As a result we find that the unstable “Alfvenic” beam mode, as well as resonant and non‐resonant fast magnetosonic modes have properties consistent with the data. Moreover, we find that in contrast to the Earth' upstream waves, at Saturn no “kinetic” normal mode can account for the observed magnetic polarization.
A new mechanism for polar patch formation
Authors: Rodger A. S. , Pinnock M. , Dudeney J. R. , Baker K. B. , Greenwald R. A.
Published: February 2004

Polar patches are regions within the polar cap where the F‐region electron concentration and airglow emission at 630 nm are enhanced above a background level. Previous observations have demonstrated that polar patches can be readily identified in Polar Anglo‐American Conjugate Experiment (PACE) data. Here PACE data and those from complementary instruments are used to show that some polar patches form in the dayside cusp within a few minutes of the simultaneous occurrence of a flow channel event (short‐lived plasma jets ∼2 km s−1) and azimuthal flow changes in the ionospheric convection pattern. The latter are caused by variations of the y‐component of the interplanetary magnetic field. The physical processes by which these phenomena cause plasma enhancements and depletions in the vicinity of the dayside cusp and cleft are discussed. Subsequently, these features are transported into the polar cap where they continue to evolve. The spatial scale of patches when formed is usually 200‐1000 km in longitude and 2°‐3° wide in latitude. Their motion after formation and the velocity of the plasma within the patches are the same, indicating that they are drifting under the action of an electric field. Occasionally, patches are observed to occur simultaneously in geomagnetic conjugate regions. Since some of these observations are incompatible with the presently‐accepted model for patch formation involving the expansion of the high latitude convection pattern entraining solar‐produced plasma, further modeling of the effects of energetic particle precipitation in the cusp, the consequences of flow channel events on the plasma concentrations, and the time dependence of plasma convection as a result of interplanetary magnetic field By changes is strongly recommended. Such studies could be used to determine the relative importance of this new mechanism compared with the existing theory for patch formation as a function of universal time and season.
Electron time dispersion
Authors: Kletzing C. A. , Torbert R. B.
Published: February 2004

Observations of electrons during a sounding rocket flight from Sondrestromfjord, Greenland, in 1985 showed numerous instances of electron time‐of‐flight dispersion, that is, a coherent structure in which fast electrons arrived before slow electrons. The instrumentation measured many pitch angles simultaneously with high time resolution, providing a detailed view of the temporal evolution of the electron signature at rocket altitudes which was indicative of an impulsive injection of electrons over a range of energies. A model utilizing an extended, quasi‐static (static on a timescale of 1 s), parallel electric field was developed to investigate a possible mechanism for the injection of these electrons. The model is found to fit the data well only when a background is added which cannot be self‐consistently generated by the model, although other parameters used in the model are in general agreement with previous reports. This leads to the conclusion that electron dispersion is more probably generated by a wave acceleration mechanism.
Comment on “Solar wind from a corona with a large helium abundance” by Egil Leer, Thomas E. Holzer, and Edward C. Shoub
Authors: McKenzie J. F.
Published: February 2004

This comment provides the correct expression for the Alfvén wave force/unit volume acting on an ion species flowing along the background magnetic field direction. In very sub‐Alfvénic flow the exact expression is well approximated by the one used by Leer et al. (1992), and it therefore provides an adequate description of the action of Alfvén wave forces in the initial acceleration of the wind. It is pointed out that the “regulation effect” can be interpreted within the concept of the generalized “sonic points” of the coupled bi‐ion flow equations.
Elsässer variable analysis of fluctuations in the ion foreshock and undisturbed solar wind
Authors: LaBelle James , Treumann Rudolf A. , Marsch Eckart
Published: February 2004

MHD fluctuations in the solar wind have been investigated previously by use of Elsässer variables. In this paper, we present a comparison of the spectra of Elsässer variables in the undisturbed solar wind at 1 AU and in the ion foreshock in front of the Earth. Both observations take place under relatively strong solar wind flow speed conditions (≈ 600 km s−1). In the undisturbed solar wind we find that outward propagating Alfvén waves dominate, as reported by other observers. In the ion foreshock the situation is more complex, with neither outward nor inward propagation dominating over the entire range investigated (1‐10 mHz). Measurements of the Poynting vectors associated with the fluctuations are consistent with the Elsässer variable analysis. These results generally support interpretations of the Elsässer variables which have been made based strictly on solar wind data and provide additional insight into the nature of the ion foreshock turbulence.
Electromagnetic ion cyclotron wave amplification and source regions in the magnetosphere
Authors: Hu Y. D. , Fraser B. J.
Published: February 2004

The amplification of electromagnetic ion cyclotron (EMIC) waves in the magnetosphere is studied by integrating local convective growth rates along the wave path. The integrated wave amplification is used to investigate the radial extent of wave source regions. Typical heavy ion concentrations (He+ ∼10‐20%, O+ ∼1‐10%) are considered in the calculation. In contrast to previous studies, the results presented indicate that the radial structure of the integrated wave amplification is controlled by the ion composition in the magnetosphere, as well as by the cold (or thermal) plasma density profile. It is found that the source region of the lowest‐frequency wave branch (i.e., the O+ wave branch here) is confined to the plasmapause and inside the plasmasphere, whereas the other wave branches (i.e., the He+ and H+ wave branches here) can be amplified over most of the magnetosphere. It is suggested that the structured Pc 1‐2 waves observed mainly on the ground are due to the lowest‐frequency wave branch, and the unstructured Pc 1‐2 waves observed both on the ground and in space result from the other wave branches.
Quiet‐time intensifications along the poleward auroral boundary near midnight
Authors: de la Beaujardière O. , Lyons L. R. , Ruohoniemi J. M. , Friis‐Christensen E. , Danielsen C. , Rich F. J. , Newell P. T.
Published: February 2004

Radar and optical measurements from Sondrestrom are combined with satellite and Goose Bay data in a study of the poleward edge of the nightside auroral oval during a quiet period. The By and Bz components of the interplanetary magnetic field were close to zero, and the Bx component was ∼8 nT for more than 24 hours. On a large scale, the convection and precipitation patterns remained almost constant during this period; on a small scale, however, the conditions were quite dynamic. At 10‐ to 20‐min intervals the arc that marked the poleward auroral boundary intensified, and a new arc appeared poleward of it. About once per hour, stronger intensifications were observed. One such event is examined in detail. The auroral arcs first appeared to dim, and then they brightened, with a factor of 10 increase in E region electron density. At the time of the brightening a new arc formed poleward of all the arcs. The arcs then drifted southward at velocities of ∼270 m/s. A plasma drift disturbance, characterized by a doubling of the southward velocity and a reversal in the east‐west component, propagated westward at 900 m/s through the fields of view of the Sondrestrom and Goose Bay radars. A simultaneous satellite overpass close to the radars revealed the presence of an energetic ion event similar to the “velocity dispersed ion structures” observed on the Aureol satellite and presumed to be the signature of fast ion beams within the plasma sheet boundary layer. The stronger arc intensification events observed by the Sondrestrom radar are associated with an increase in plasma flow across the boundary between open and closed magnetic field lines. We interpret this increased flow as the ionospheric signature of abrupt, localized increases in the reconnection rate in the midnight sector.
Four cells or two? Are four convection cells really necessary?
Authors: Reiff P. H. , Heelis R. A.
Published: February 2004

This paper addresses the question whether a four‐cell convection pattern in the polar cap ionosphere is required by observations, or whether the data are fully explainable by a (perhaps highly distorted) two‐cell convection pattern. We present convection data from Atmosphere Explorer C, which, if only the flow component in the sunward‐antisunward direction were measured, could be explained either as one of two possible distorted two‐cell patterns or as a full four‐cell pattern. However, neither of the distorted two‐cell patterns that are consistent with the sunward‐antisunward flow component can be made consistent with the dawn‐dusk flow component over the entire spacecraft trajectory, without postulating a severe flow kink and extra field‐aligned currents sunward of the spacecraft track. In addition, the zero potential point (which in a four‐cell model would mark the division between the two reverse convection cells) also exactly corresponded to the location of the reversal of the east‐west component in the flow, a feature predicted from the four‐cell model but more difficult to explain in a distorted two‐cell model. Because the pattern was repeated on two consecutive passes, time variations can probably be ruled out as a cause of the sunward flow. Between the two northern hemisphere dayside passes, a southern hemisphere nightside pass also showed a region of sunward flow in the polar cap. The fact that in this case the sunward flow was not confined to the dayside also favors a four‐cell explanation.
Response of the thermosphere and ionosphere to geomagnetic storms
Authors: Fuller‐Rowell T. J. , Codrescu M. V. , Moffett R. J. , Quegan S.
Published: February 2004

Four numerical simulations have been performed, at equinox, using a coupled thermosphere‐ionosphere model, to illustrate the response of the upper atmosphere to geomagnetic storms. The storms are characterized by an increase in magnetospheric energy input at high latitude for a 12‐hour period; each storm commences at a different universal time (UT). The initial response at high latitude is that Joule heating raises the temperature of the upper thermosphere and ion drag drives high‐velocity neutral winds. The heat source drives a global wind surge, from both polar regions, which propagates to low latitudes and into the opposite hemisphere. The surge has the character of a large‐scale gravity wave with a phase speed of about 600 m s−1. Behind the surge a global circulation of magnitude 100 m s −1 is established at middle latitudes, indicating that the wave and the onset of global circulation are manifestations of the same phenomena. A dominant feature of the response is the penetration of the surge into the opposite hemisphere where it drives poleward winds for a few hours. The global wind surge has a preference for the night sector and for the longitude of the magnetic pole and therefore depends on the UT start time of the storm. A second phase of the meridional circulation develops after the wave interaction but is also restricted, in this case by the buildup of zonal winds via the Coriolis interaction. Conservation of angular momentum may limit the buildup of zonal wind in extreme cases. The divergent wind field drives upwelling and composition change on both height and pressure surfaces. The composition bulge responds to both the background and the storm‐induced horizontal winds; it does not simply rotate with Earth. During the storm the disturbance wind modulates the location of the bulge; during the recovery the background winds induce a diurnal variation in its position. Equatorward winds in sunlight produce positive ionospheric changes during the main driving phase of the storm. Negative ionospheric phases are caused by increases of molecular nitrogen in regions of sunlight, the strength of which depends on longitude and the local time of the sector during the storm input. Regions of positive phase in the ionosphere persist in the recovery period due to decreases in mean molecular mass in regions of previous downwelling. Ion density changes, expressed as a ratio of disturbed to quiet values, exhibit a diurnal variation that is driven by the location of the composition bulge; this variation explains the ac component of the local time variation of the observed negative storm phase.
Nonlinear current sheet formation in ideal plasmas
Authors: Voge A. , Otto A. , Schindler K.
Published: February 2004

We present a numerical study of the formation of current sheets in ideal plasmas. First we confirm the development of singular current sheets in a one‐dimensional model. In a second step we extend the analysis to two‐dimensional equilibria. Here it is found that the resulting structures are quite insensitive to the boundary conditions. For the special case of a magnetotaillike equilibrium it will be shown that the resulting current distribution provides a possibility to understand the onset of a localized anomalous resistivity from a macroscopic point of view. Furthermore, the resulting structures provide an explanation for the dramatic decrease of the thickness of the current sheet in the magnetotail prior to the onset of geomagnetic substorms.
Planetary wave and solar emission signatures in the equatorial electrojet
Authors: Parish H. F. , Forbes J. M. , Kamalabadi F.
Published: February 2004

Recent analyses of observational data reveal the presence of perturbations in the E and F regions of the equatorial ionosphere with periods ranging from 2 to 45 days. The characteristic periods of many of these perturbations suggest an association with free Rossby (resonant mode) oscillations, perhaps excited either in the lower atmosphere or in situ. In the present work we analyze hourly magnetic observations from Huancayo Observatory, Peru (12.00°S, 75.30°W geographic; 0.72°S, 4.78°W geomagnetic), for the presence and persistence of these oscillations during the whole year of 1979. The measured variations can be interpreted in terms of oscillations of the wind field in the E region (approximately 100‐160 km), which in turn cause perturbations in the electric fields generated by the wind‐driven atmospheric dynamo and in the magnetic field intensity measured at the ground. The observations suggest that the effects of planetary wave oscillations with periods close to 2.5, 3, 6, 7, 9, 10.5, and 16 days may regularly propagate into the thermosphere and ionosphere, causing oscillations which are significant in magnitude. On the basis of an averaged periodogram analysis, we estimate that planetary wave effects may account for up to 75% of the total energy in ΔH values in the 2 to 35 day period range, suggesting that planetary waves may provide an important contribution to the dynamics and electrodynamics of the lower ionosphere and thermosphere. EUV fluxes during 1979 are noted to have a predominant 13.5‐day periodicity during the first half of the year and the more typical 27‐day oscillation during the latter half of 1979. These features can in principle affect the ΔH variations through their influence on the E region conductivity. We examine such influences here, especially those that affect the interpretation of the quasi 16‐day oscillation.
Midlatitude detection of ELF whistlers
Authors: Sentman D. D. , Ehring D. A.
Published: February 2004

Narrow‐band, whistlerlike magnetic events distinguished by nearly monochromatic signals decreasing in frequency with time have been observed for the first time at midlatitudes in the ELF band. Measurements performed during September 3 to October 5, 1985 at Table Mountain, California (34.4°N, 117.7°W), show that the frequency and dispersion characteristics of these events are similar to events detected at auroral latitudes (Heacock, 1974), including a narrow‐band magnetic signal monotonically decreasing in frequency from 120 to 60 Hz over a 40 s interval with a mean center frequency of approximately 90 Hz. No echoes were observed. Maximum amplitudes of the magnetic signals ranged from just above the approximately 1 pT Hz−1/2 floor of the ambient background to roughly 20 pT Hz−1/2. The polarization was predominantly linear in the geographic east‐west direction. The midlatitude ELF whistlers reported here have a significantly lower average daily rate of occurrence than those reported for auroral latitudes. However, as with the high‐latitude events, they displayed an occurrence rate that is maximum during local daytime. Following Heacock (1974), it is suggested that a possible source for these events is whistler mode lion roars occurring in field‐aligned ducts of enhanced cold plasma densities in the distant magnetosheath. For favorable configurations of these ducts extending from the magnetosheath into the polar cusp, the waves may propagate to the Earth through the cusp acting as a waveguide. Although lightning is usually considered to be the dominant source of ELF noise in the Earth ionosphere cavity, magnetosheath ELF noise coupled into the cavity at high latitudes may represent an additional source. The fractional intensities of the natural ELF noise power within the cavity that are generated by this mechanism are presently unknown. However, a determination of these fractional intensities would be useful in the interpretation of the Schumann resonances as a measure of the totality of global lightning, particularly in the case of measurements made at high latitudes in the vicinity of the cusp‐ionosphere coupling region. Similarly, high‐latitude detection of very weak, artificially generated ELF signals, such as by HF heating of the polar electrojet, may be affected by the proximity of such a local ionospheric ELF noise source.
Spacecraft potential effects on the Dynamics Explorer 2 satellite
Authors: Anderson P. C. , Hanson W. B. , Coley W. R. , Hoegy W. R.
Published: February 2004

The relationship between the plasma environment and spacecraft potential is examined for the Dynamics Explorer 2 (DE 2) spacecraft in an attempt to improve the accuracy of ion drift measurements by the retarding potential analyzer (RPA). Because of the DE 2 orbit characteristics (apogee near 1000 km and perigee near 300 km) and the configuration of conducting surfaces on the spacecraft, thermal electrons and ions constituted the only significant contributions to the charging currents to the spacecraft surface for the majority of geophysical conditions encountered. The geomagnetic field had considerable effect on the spacecraft potential due to magnetic field confinement of the electrons as well as to the V × B electric field resulting from the movement of the spacecraft across magnetic field lines. Using a database of inferred spacecraft potentials from the RPA, measured electron temperatures from the Langmuir probe (LANG), and calculated V × B electric fields, we derive an algorithm for determining the spacecraft potential (at the location of the RPA on the spacecraft) for any point of the DE 2 orbit. Knowledge of the spacecraft potential subsequently allows us to retrieve relatively accurate ion drifts from the RPA data.
Compressional ULF waves in the outer magnetosphere:, 2. A case study of Pc 5 type wave activity
Authors: Zhu Xiaoming , Kivelson Margaret G.
Published: February 2004

In previously published work (Zhu and Kivelson, 1991) the spatial distribution of compressional magnetic pulsations of period 2 ‐ 20 min in the outer magnetosphere was described. In this companion paper, we study some specific compressional events within our data set, seeking to determine the structure of the waves and identifying the wave generation mechanism. We use both the magnetic field and three‐dimensional plasma data observed by the ISEE 1 and/or 2 spacecraft to characterize eight compressional ULF wave events with frequencies below 8 mHz in the outer magnetosphere. High time resolution plasma data for the event of July 24, 1978, made possible a detailed analysis of the waves. Wave properties specific to the event of July 24, 1978, can be summarized as follows: (1) Partial plasma pressures in the different energy ranges responded to the magnetic field pressure differently. In the low‐energy range they oscillated in phase with the magnetic pressure, while oscillations in higher‐energy ranges were out‐of‐phase; (2) Perpendicular wavelengths for the event were determined to be 60,000 and 30,000 km in the radial and azimuthal directions, respectively. Wave properties common to all events can be summarized as follows: (1) Compressional Pc 5 wave activity is correlated with β, the ratio of the plasma pressure to the magnetic pressure; the absolute magnitude of the plasma pressure plays aminor role for the wave activity; (2) The magnetic equator is a node of the compressional perturbation of the magnetic field; (3) The criterion for the mirror mode instability is often satisfied near the equator in the outer magnetosphere when the compressional waves are present. We believe these waves are generated by internal MHD instabilities.
Comparison of low‐latitude ion and neutral zonal drifts using DE 2 data
Authors: Coley W. R. , Heelis R. A. , Spencer N. W.
Published: February 2004

We have used data from the ion drift meter and the wind and temperature spectrometer on the DE 2 spacecraft to make statistical comparisons of the zonal ion and neutral drifts at dip latitudes (DLAT) in the ±35° range over all local times. Fourier analysis indicates that the superrotation and the diurnal components of both flows are strongly peaked at the dip equator, with the superrotation term becoming negative for |DLAT| ≥20°. One interesting feature is the presence of a period (2200‐0500 solar local time) in the 300‐400 km altitude region near the dip equator where the ion drift is more strongly eastward than the neutral flow. This would seem to indicate the presence of an electric field source of greater strength than the F region dynamo elsewhere along the geomagnetic field line. Model calculations indicate that a possible mechanism for this source lies in the vertical shear in the zonal neutral wind in the 100‐200 km altitude region.
Electrodynamic parameters in the nighttime sector during auroral substorms
Authors: Fujii R. , Hoffman R. A. , Anderson P. C. , Craven J. D. , Sugiura M. , Frank L. A. , Maynard N. C.
Published: February 2004

The characteristics of the large‐scale electrodynamic parameters, field‐aligned currents (FACs), electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, have been obtained through a unique analysis which places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. A generic bulge‐type auroral emission region has been deduced from auroral images taken by the Dynamics Explorer 1 (DE 1) satellite during a number of isolated substorms, and the form has been divided into six sectors, based on the peculiar emission characteristics in each sector: west of bulge, surge horn, surge, middle surge, eastern bulge, and east of bulge. By comparing the location of passes of the Dynamics Explorer 2 (DE 2) satellite to the simultaneously obtained auroral images, each pass is placed onto the generic aurora. The organization of DE 2 data in this way has systematically clarified peculiar characteristics in the electrodynamic parameters. An upward net current mainly appears in the surge, with little net current in the surge horn and the west of bulge. The downward net current is distributed over wide longitudinal regions from the eastern bulge to the east of bulge. Near the poleward boundary of the expanding auroral bulge, a pair of oppositely directed FAC sheets is observed, with the downward FAC on the poleward side. This downward FAC and most of the upward FAC in the surge and the middle surge are associated with narrow, intense antisunward convection, corresponding to an equatorward directed spikelike electric field. This pair of currents decreases in amplitude and latitudinal width toward dusk in the surge and the west of bulge, and the region 1 and 2 FACs become embedded in the sunward convection region. The upward FAC region associated with the spikelike field on the poleward edge of the bulge coincides well with intense electron precipitation and aurora appearing in this western and poleward portion of the bulge. The convection reversal is sharp in the west of bulge and surge horn sectors, and near the high‐latitude boundary of the upward region 1 FAC. In the surge, the convection reversal is near the low‐latitude boundary of the upward region 1, with a near stagnation region often extending over a large interval of latitude. In the eastern bulge and east of bulge sectors, the region 1 and 2 FACs are located in the sunward convection region, while a spikelike electric field occasionally appears poleward of the aurora but usually not associated with a pair of FAC sheets. In the eastern bulge, magnetic field data show complicated FAC distributions which correspond to current segments and filamentary currents.
A simulation study of radial expansion of an electron beam injected into an ionospheric plasma
Authors: Koga J. , Lin C. S.
Published: February 2004

Injections of nonrelativistic electron beams from a finite equipotential conductor into an ionospheric plasma have been simulated using a two‐dimensional electrostatic particle code. The purpose of the study is to survey the simulation parameters for understanding the dependence of beam radius on physical variables. The conductor is charged to a high potential when the background plasma density is less than the beam density. Beam electrons attracted by the charged conductor are decelerated to zero velocity near the stagnation point, which is at a few Debye lengths from the conductor. The simulations suggest that the beam electrons at the stagnation point receive a large transverse kick and the beam expands radially thereafter. The buildup of beam electrons at the stagnation point produces a large electrostatic force responsible for the transverse kick. However, for the weak charging cases where the background plasma density is larger than the beam density, the radial expansion mechanism is different; the beam plasma instability is found to be responsible for the radial expansion. The simulations show that the electron beam radius for high spacecraft charging cases is of the order of the beam gyroradius, defined as the beam velocity divided by the gyrofrequency. In the weak charging cases, the beam radius is only a fraction of the beam gyroradius. The parameter survey indicates that the beam radius increases with beam density and decreases with magnetic field and beam velocity. The beam radius normalized by the beam gyroradius is found to scale according to the ratio of the beam electron Debye length to the ambient electron Debye length. The parameter dependence deduced would be useful for interpreting the beam radius and beam density of electron beam injection experiments conducted from rockets and the space shuttle.
Multipoint observations of a dayside transient event
Authors: Vo H. B. , Murphree J. S. , Elphinstone R. D. , Elphic R. C. , Friis‐Christensen E. , Luhr H. , Sibeck D. G.
Published: February 2004

Many different processes may take place at the magnetopause. To distinguish the various mechanisms requires simultaneous multipoint observations in the dayside magnetosphere. Such coincident observations were made by a number of spacecraft located in the solar wind, at the magnetopause, at geosynchronous orbit, and at polar orbit on September 23, 1986. Beginning at 1110 UT, the Bz component of the IMF underwent three periodic cycles, each with a duration of about 7 min, and there were 4 solar wind pressure pulses with a period of 10 min, as recorded by the IMP 8 satellite. The ISEE satellite pair crossed the magnetopause near local noon at 1125 UT and recorded two bipolar events in the normal component of the magnetic field at 1115 and 1120 UT. In addition, ISEE 1 observed a bipolar event at 1118 UT that ISEE 2 did not. Periodic oscillations and compressions of the magnetic field were then subsequently seen at geosynchronous orbit in the dawn sector. Greenland magnetometer stations also located in the morning sector recorded transient responses to the solar wind pressure pulses. The East Coast stations (near local noon) apparently observed a different vortex at 1122 UT. Viking auroral images show an activation of a localized feature poleward of the dayside auroral distribution at 1122 UT. An auroral enhancement, seen at 1128 UT in the afternoon sector, moved eastward at a speed of 5 km s−1 and decayed in intensity by 1141 UT. Clear Pc 5 pulsations were seen equatorward of this activation. Differences in both in situ measurements and ionospheric responses to the two bipolar events lead to the conclusion that the 1118 UT event can be interpreted as being associated with reconnection at the same time that the magnetopause was in motion.