bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear stability of the geomagnetic field
Authors: Hutcheson Kenneth A. , Fearn David R.
Published: February 2004

By subjecting a model axisymmetric magnetic field to a nonaxisymmetric perturbation, or instability, linear stability analyses can provide an indirect measure of the toroidal field strength in the Earth's core. It is thought that such field strengths have an upper bound of the order of 50 Gauss. We have constructed a model of the Earth's core to investigate how magnetic field instabilities evolve nonlinearly and find that at reasonably low viscosities, the instability evolves to a finite amplitude and rotates rigidly, as predicted from the theory of bifurcations in rotating systems. As the field strength is increased the solution adopts a completely different spatial configuration and has a different characteristic frequency to the branch found at the lower field strength. The primary Hopf bifurcation is thought to be of a subcritical nature which accords with a previous weakly nonlinear analysis. We conjecture that such an instability could provide the geomagnetic field with a mechanism for evolution to a different state at field strengths lower than the strength at the point of marginal stability.
Alleviation of the Backus Effect in geomagnetic field modelling
Authors: Holme Richard , Bloxham Jeremy
Published: February 2004

Satellite magnetometry is plagued by the Backus effect which renders maps of the magnetic field derived from measurements of the total field intensity unreliable, especially near the geomagnetic equator. The only remedy is to use oriented, vector component data in addition to scalar intensity data, but this requires either accurate determination of the satellite attitude or the use of data from permanent magnetic observatories. The former adds considerably to the cost and complexity of satellite magnetometry; the latter suffers in practice from the poor geographical distribution of magnetic observatories. Here we re‐examine the problem of attitude determination, and ask how well the attitude needs to be determined to alleviate the Backus effect. We show that the use of a more complete model of the errors in satellite vector component data allows the requirements on attitude determination to be relaxed significantly, so that lower‐cost means of obtaining attitude determination, such as GPS, can be employed.
Fluid velocity and path length in fractured media
Authors: Parney Robert , Smith Leslie
Published: February 2004

Field studies and numerical simulations suggest that solute transport in fractured rock occurs primarily along preferred pathways or channels that involve only a small percentage of the available fracture network. By examining relationships between path length and fluid velocity in a series of discrete network simulations, we conclude that mass tends to travel longer distances in fractures in which the fluid velocity is higher, although the longest paths traveled do not correspond to the highest fluid velocities within the network. These observations suggest that a velocity optimization process, operating at fracture intersections, causes mass channeling at the network scale. In networks composed of fractures of varying aperture, this mechanism explains the increasingly tortuose paths traveled as mass moves selectively through the larger apertures.
Assessment of mechanisms for Jovian synchrotron variability associated with comet SL‐9
Authors: Bolton Scott J. , Thorne Richard M.
Published: February 2004

The impact of comet SL9 with Jupiter induced a number of variations in Jupiter's synchrotron radiation; including a 20–30% increase in emission intensity, spectral changes, and a possible broadening in the latitudinal distribution of the emission. Here we consider the consequences of three potential mechanisms for inducing such effects; namely electron acceleration, radial diffusion and pitch‐angle scattering. While none of the processes can be ruled out as insignificant, we show that pitch‐angle scattering is consistent with all of the available radio frequency data and demonstrate that this could be due to realistic enhanced amplitudes of cyclotron resonant whistler‐mode waves associated with the comet impacts. We suggest that the waves could result from electrical storm activity or be excited by natural instabilities of the electron distribution in Jupiter's radiation belts.
Observations of magnetic flux ropes and associated currents in Earth's magnetotail with the Galileo spacecraft
Authors: Khurana K. K. , Kivelson M. G. , Frank L. A. , Paterson W. R.
Published: February 2004

On December 8, 1990, during its first flyby of the Earth, Galileo recorded signatures of several flux rope type structures in the Earth's magnetotail. One of the prominent structures had a negative/positive polarity in the Bz component and was moving earthward, whereas another of the prominent structures had a positive/negative polarity in Bz and was observed to be traveling away from the Earth. Both of these flux ropes had strong core fields (≥10 nT) aligned in the positive YGSM direction. We have performed an analysis of combined field and particle data obtained within these flux ropes. We show that the length scale of a flux rope can be determined quite accurately from magnetic data if an independent measure of either the current density or the convection velocity is available. A surprising result that emerges from this analysis is that even though temporal durations (5–10 min) and magnetic field perturbations of the two flux ropes are similar, the scale sizes of the two structures are very different. The earthward moving flux rope with a radius ≤1000 km was traveling very slowly (υx < 10 km/s). The tailward moving flux rope with a radius of ∼8 RE had a speed of ∼150 km/s. Even though the earthward moving flux rope was small in size, the current density in its core was well above that of the tail current sheet and was detected directly by the plasma instrument. We believe that this is the first time that measurements of field aligned current in a flux rope core have been made directly by a plasma instrument. We discuss the details of the distribution functions of the current‐carrying particles. We also speculate on the conditions that led to the vast differences in the scale sizes of the two flux ropes.
Slow inflation of the Furnas Volcano, São Miguel, Azores, suggested from initial leveling and global positioning system measurements
Authors: Sigmundsson Freysteinn , Tryggvason Eysteinn , Alves Maria Madalena , Alves Jose Luiz , Pálsson Karl , Ólafsson Halldór
Published: February 2004

The Furnas volcano, São Miguel island, Azores is an active stratovolcano, with a 6‐km‐diameter caldera. In order to mitigate volcanic risk in the area, crustal deformation research based on optical leveling tilt measurements and lake leveling was initiated at the volcano in 1991. Progressive tilt as high as 4 µrad/yr has been observed, but the measurements do not all fit a simple model. The Global Positioning System (GPS) was used to measure a network of fourteen stations at the volcano in 1993, and again in 1994. Horizontal displacements in the one year period between the surveys are less than 1 cm, but significant with 95% confidence. Stations are being displaced away from the caldera and strain analysis indicates areal dilation of 0.84±0.40 µstrain/yr of the volcanic edifice. The areal dilation can be explained by slight inflation of the volcano. The location of the center of inflation is rather uncertain, but based on both the GPS and the leveling data the preferred location is in the northern part of the caldera. A central uplift rate of >7 mm/year is suggested.
Attenuation anisotropy beneath the subduction zones in Japan
Authors: Hiramatsu Yoshihiro , Ando Masataka
Published: February 2004

We analyze high quality digital waveform data at stations in Japan from the POSEIDON and IRIS broad‐band seismograph networks. The analysis of multiple ScS waves of the deep event beneath Sakhalin on 12 May 1990 shows that the orientation of the principal axis of horizontal ScSn particle motion is rotated clockwise from NWN‐SES to N‐S as the number of reflections increases. This suggests that the attenuation of the E‐W component is higher than that of the N‐S component of ScS waves. The spectral ratio ScS2/ScS1 differs between the two horizontal components. We estimate the quality factor QScS of the whole mantle for each horizontal component using the decay rate of spectral ratio of band‐passed ScS1 and ScS2 phases in the frequency range from 0.01 to 0.03 Hz. We obtain a higher average QScS for the N‐S component (141±8) than that for the E‐W component (91±5). That is, the E‐W component attenuates more than the N‐S component. For upcoming ScS waves the N‐S direction is nearly parallel to the strike of the subducting slab beneath the mantle wedge in the Japanese islands, and the E‐W is perpendicular to the strike. The up‐welling and downgoing flow in the mantle wedge due to drag caused by the subduction is likely to cause the difference in attenuation between the two directions, in other words, azimuthal attenuation anisotropy.
A CME‐driven solar wind disturbance observed at both low and high heliographic latitudes
Authors: Gosling J. T. , McComas D. J. , Phillips J. L. , Pizzo V. J. , Goldstein B. E. , Forsyth R. J. , Lepping R. P.
Published: February 2004

A solar wind disturbance produced by a fast coronal mass ejection, CME, that departed from the Sun on Feburary 20, 1994 was observed in the ecliptic plane at 1 AU by IMP 8 and at high heliographic latitudes at 3.53 AU by Ulysses. In the ecliptic the disturbance included a strong forward shock but no reverse shock, while at high latitudes the disturbance was bounded by a relatively weak forward‐reverse shock pair. It is clear that the disturbance in the ecliptic plane was driven primarily by the relative speed between the CME and a slower ambient solar wind ahead, whereas at higher latitudes the disturbance was driven by expansion of the CME. The combined IMP 8 and Ulysses observations thus provide a graphic illustration of how a single fast CME can produce very different types of solar wind disturbances at low and high heliographic latitudes. Simple numerical simulations help explain observed differences at the two spacecraft.
Geodetic measurements of convergence across the New Hebrides Subduction Zone
Authors: Calmant Stéphane , Lebellegard Pierre , Taylor Fred , Bevis Michael , Maillard Didier , Récy Jacques , Bonneau Jocelyne
Published: February 2004

Between 1990 and 1994, geodetic measurements (GPS observations) have been conducted across the New Hebrides subduction zone where the Australia plate subducts under the New Hebrides Arc. This paper establishes convergence rate variations along the trench. In the South, at Tanna, the relative motion is oriented N244 ±4 and has a uniform rate of 11.7±0.8 cm/yr. The rate at Efaté is 10.3±0.9 cm/yr, oriented N242 ±4. Both azimuths very well compare with slip vectors of the last major earthquakes. In the North, the rate at Santo is only 3.6 ±1.2 cm/yr, oriented N253 ± 26. The difference in the convergence rates between Santo on the one hand and Efaté and Tanna in the other hand points to a right lateral shear zone between Santo and Efaté. At Santo where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No significant variations are detected for the baselines within the Australia plate.
Application of the wavelet transform in detecting multiple events of microearthquake seismograms
Authors: Lou M. , Rial J. A.
Published: February 2004

The Wavelet Transform (WT) is employed here to discriminate multiple events contained in microearthquake seismograms. The WT decomposes the signal into well localized time‐frequency compartments (or “Heisenberg boxes”) that display the energy distribution over the time and frequency domains simultaneously, therefore providing a useful technique to characterize dispersion and attenuation of a seismic signal, as well as to identify multiple interfering events. This letter suggests that the WT could provide a useful tool to analyze complicated seismic signals, especially to detect the multiple events contained in microearthquake seismograms.
Simultaneous estimation of the masses of Mars, Phobos, and Deimos using spacecraft distant encounters
Authors: Smith David E. , Lemoine Frank G. , Zuber Maria T.
Published: February 2004

The masses of Mars and its satellites, Phobos and Deimos, have been estimated from the Mariner 9 and Viking 1 and 2 Orbiter tracking data. These spacecraft were sensitive to the gravitational force of Mars as well as to its satellites. Although the satellite masses are eight orders of magnitude smaller than Mars, their regular effect on the orbits of the spacecraft is evident in the tracking data and has enabled us to derive their masses simultaneously with that of Mars. Our method for estimating the satellite masses uses the many “distant encounters” of the spacecraft with these small bodies rather than the few “close encounters” used in previous studies. The mass estimate for Phobos leads to a mean density of 1530±100 kg m−3 based on a volume of 5748±190 km³ (Thomas, 1993), while the mass estimate of Deimos leads to a poorly constrained mean density of 1340±828 kg m−3 based on a volume of 1017±130 km³ (Thomas, 1993). Our analysis confirms, within the bounds of error, the anomalously low density of Phobos using an independent method and data set. If the result is valid within several times the estimated error (1σ), then factors other than composition, i.e., porosity, a thick regolith and/or a significant interior ice content, are required to explain the observed mass of this body.
Episodic ages of Greenstones: A key to mantle dynamics?
Authors: Condie Kent C.
Published: February 2004

Two greenstone age minima at 2.5–2.2 Ga and 1.65–1.35 Ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction. From the age distributions of greenstones and juvenile continental crust, earth history can be divided into three stages: I) >2.8 Ga, when greenstones and microcontinents formed and collided continuously, although probably not forming a supercontinent until about 3 Ga; II) 2.8–1.3 Ga, where a clear episodisity is apparent in greenstone eruption and collision ages, and where two greenstone age minima coincide in age with supercontinents; and III;) <1.3 Ga, when greenstones and continents appear to have formed and collided continuously, perhaps in response to significant overlap in the times of dispersal and assembly of supercontinents. Greenstone age peaks 2.7, 1.9 and 1.3 Ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs through the 660‐km discontinuity to the D″ layer initiating an episode of major mantle plume activity. Continuous production and preservation of greenstones in the last 1.3 Gy is consistent with seismic tomography data, which suggest a steady sinking of slabs into the lower mantle.
High, room and low temperature photoabsorption cross sections of CS2 in the 1800–2300 Å region
Authors: Chen F. Z. , Robert Wu C. Y.
Published: February 2004

Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of CS2 with a spectral bandwidth (FWHM) of 0.8 Å. The absorption cross sections of CS2 have been measured from 1800 to 2300 Å at temperatures of 383 and 295 K and from 1800 to 2150 Å at 203 K. Significant temperature effects and hot bands are observed, particularly in the long wavelength region. The apparent continuum cross section underlying the vibrational features in the 2200–2300 Å region increases by about an order of magnitude when the gaseous CS2 temperature increases from 295 K to 383 K. The new temperature‐dependent laboratory cross section data should be useful in the determination of the CS2 abundance in Jupiter's atmosphere after the impact of Comet Shoemaker‐Levy 9.
Rocket FUV observations of the Io plasma torus during the Shoemaker‐Levy/9 impacts
Authors: Stern S. A. , Slater D. , Cash W. , Wilkinson E. , Green J. , Gladstone R.
Published: February 2004

We observed the Io torus from 820–1140 Å on UT 20.25 July 1994 from a sounding rocket telescope/spectrograph. These observations serve as only the fourth published spectrum of the torus in this wavelength range, and the only FUV data documenting the state of the torus during the Shoemaker‐Levy 9 impacts.
On the importance of E‐field variability for Joule heating in the high‐latitude thermosphere
Authors: Codrescu M. V. , Fuller‐Rowell T. J. , Foster J. C.
Published: February 2004

Joule heating is known to be one of the major energy sources of the upper atmosphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the region's physics. However, Joule heating is currently one of the largest sources of uncertainty in the thermosphere's energy budget. In numerical models the distribution of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E‐field. That method ignores the important component of Joule heating due to rapid or small‐scale fluctuations in E‐field or ion drifts. However, E‐field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heating in the thermosphere is proportional to the average of the square of the E‐field. The computation of the average of the square of the E‐field requires knowledge of the statistical characteristics of E‐field variability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E‐field variability as an upper‐atmosphere energy source. We show that the inclusion of E‐field variability in the high‐latitude convection model can significantly increase the amount of Joule heating for a given pattern.
Ion heating in a dusty plasma due to the dust/ion acoustic instability
Authors: Winske D. , Gary S. Peter , Jones Michael E. , Rosenberg M. , Chow V. W. , Mendis D. A.
Published: February 2004

The drift of plasma ions relative to charged grains in a dusty plasma can give rise to a dust/ion acoustic instability. We investigate the linear properties of the instability by numerically solving an appropriate linear dispersion equation and examine the nonlinear behavior through one‐dimensional electrostatic particle simulations, in which the plasma and dust ions are treated as discrete particles and the electrons are modeled as a Boltzmann fluid. The instability saturates by trapping some of the plasma ions. The instability is slightly weaker when the dust particles have a range of sizes, and corresponding range of charges and masses. It is argued that due to dust particles that comprise planetary rings, this process can contribute to ion heating and diffusion observed in the inner magnetosphere of Saturn.
Formation, propagation, and decay of coherent pulses of solar cosmic rays
Authors: Ruffolo D. , Khumlumlert T.
Published: February 2004

We have performed numerical simulations of the interplanetary transport of solar cosmic rays. The particles form a coherent pulse within ∼0.01 AU after their injection. The gradual decrease of a pulse's speed and anisotropy can be understood in terms of an equilibrium between pitch‐angle scattering and focusing. The results should be useful for estimating times of particle injection.
Very low oxygen concentration in basal ice from Summit, central Greenland
Authors: Souchez R. , Janssens L. , Lemmens M. , Stauffer B.
Published: February 2004

Oxygen concentration as low as 3% has been detected in the basal silty ice of the GRIP core. Such values were never observed in ice from ice sheets. They are most probably the consequence of organic matter oxidation in ice developed in a peaty deposit when the Greenland Ice Sheet was not present at the site. Flow‐induced mixing has further incorporated this ice into glacier ice during the ice sheet build up. The part of the local ice component in the mixing process diminishes with the distance from the bed. This is the process which explains the oxygen profile.
Chemistry of deep (3500–5600 m) Pacific MORB—Why is the Pacific anomalous?
Authors: Batiza R. , Hekinian R. , Bideau D. , Francheteau J.
Published: February 2004

In contrast with the Atlantic and Indian Oceans, the Pacific shows the global correlations of axial depth and basalt chemistry only weakly or not at all for unaveraged data. In the past, this observation has been partly explained by the limited depth window of available Pacific samples. We report on new, very young basalt samples collected by submersible from Pacific ridges at ∼3500–5600 m, nearly doubling the depth interval of Pacific samples. These samples, from the Pito Deep, Hess Deep, and the Garrett transform, do not conform to the global correlation of depth vs. chemistry and show the local trend of chemical variation. The Pacific as a whole is thus unusual compared to the Atlantic and Indian Oceans. We propose that the anomalous behavior of the Pacific is due mostly to its relatively rapid spreading rate and tectonic shifts leading to an absence of a dynamic equilibrium between mantle temperature and seafloor depth observed elsewhere.
Mantle convection experiments with evolving plates
Authors: Puster Peter , Hager Bradford H. , Jordan Thomas H.
Published: February 2004

We have performed numerical mantle convection experiments in an annulus with plates whose geometries evolve with time. A thermally activated rheology results in a high‐viscosity surface boundary layer, which is mobilized by weak zones (small regions of low viscosity). These weak zones are advected with the flow, leading to evolving plates. Surface plates organize the mantle flow system, resulting in large‐scale structures, and a thermal angular power spectrum dominated by low angular degrees. Plate size and plate velocity distributions are used to characterize the system of surface plates. These statistics are sensitive to the radial flow structure and provide a new way of evaluating convection models using reconstructions of Earth's plate‐tectonic record. A direct comparison of the results from our two‐dimensional (2D) experiments to the plate‐tectonic record for the past 120 Ma indicates that models with a 30‐fold viscosity increase in the lower mantle are most consistent with these observational constraints, while calculations with a strong endothermic phase change that leads to intermittently layered convection are least consistent with the plate‐tectonic record.
Microcracking during stress‐relief of polycrystalline ice formed at high pressure
Authors: Meglis Irene L. , Gagnon R. E. , Young R. P.
Published: February 2004

To study microcracking in a brittle material in response to stress‐relief, samples of polycrystalline ice were frozen under hydrostatic pressures up to 19 MPa and subsequently decompressed. Eight cylindrical samples (approximately 56 mm in diameter and 120 mm long) were made by freezing a slurry of crushed ice and degassed water inside a clear Teflon jacket. Four additional tests were done by reloading samples which were previously unloaded, allowing them to re‐equilibrate, and then unloading again. Tests were filmed through windows in the confining cell. Acoustic emissions were monitored in some samples. When the stress was released, intense cracking occurred for approximately 0.4 seconds and then tapered off. Thin sections showed that the stress‐relief cracks were approximately equal to the facet size (<5 mm), were primarily along grain boundaries, and were distributed throughout the samples with no obvious preferred orientation. Within a few hours of unloading, grain boundaries became cloudy, possibly a result of air trapped at the grain surfaces. Preliminary results from further tests indicate that this air plays a significant role in crack nucleation.
Observing tropospheric water vapor by radio occultation using the Global Positioning System
Authors: Kursinski E. R. , Hajj G. A. , Hardy K. R. , Romans L. J. , Schofield J. T.
Published: February 2004

Given the importance of water vapor to weather, climate and hydrology, global humidity observations from satellites are critical. At low latitudes, radio occultation observations of Earth's atmosphere using the Global Positioning System (GPS) satellites allow water vapor profiles to be retrieved with accuracies of 10 to 20% below 6 to 7 km altitude and ∼5% or better within the boundary layer. GPS observations provide a unique combination of accuracy, vertical resolution (≤ 1 km) and insensitivity to cloud and aerosol particles that is well suited to observations of the lower troposphere. These characteristics combined with the inherent stability of radio occultation observations make it an excellent candidate for the measurement of long term trends.
Interpretation of an unusual high latitude density decrease in terms of thermospheric density cells
Authors: Schoendorf J. , Crowley G.
Published: February 2004

The discovery of a cellular structure in the high latitude thermospheric (120–350 km) neutral density provides a new framework in which to interpret high latitude density data. During geomagnetically active times, above about 170 km, the cells consist of density enhancements near noon and midnight and depletions near dawn and dusk. In this paper, a previously unexplained observation is presented and interpreted in terms of the cells. In general the high latitude neutral density is expected to increase during magnetically active conditions. On the contrary, the SETA‐1 satellite measured a 40% density decrease near 200 km during the geomagnetic storm of April 3, 1979. In the past, such ‘anomalous’ observations have often been attributed to the inseparability of density and winds in accelerometer measurements. We first show that the depletion cannot be caused by winds, and then show that the depletion is a natural consequence of the cellular structure inherent in the mass density at 200 km.
Electrostatic charging properties of simulated lunar dust
Authors: Horányi Mihály , Robertson Scott , Walch Bob
Published: February 2004

The charge on grains of simulated lunar regoliths (MLS‐1 and JSC‐1) has been measured in a plasma containing thermal electrons with a few electron volts temperature and a second population of nearly monoenergetic fast electrons with energy variable from 15 to 100 eV. The measured charges are similar to that on glass microspheres of the same diameter. Both the glass and the simulated lunar samples showed multiple charge states in a finite electron energy range, that might help the theoretical models suggesting active electrostatic dust transport on the Moon. The measurements can be well described by a charging model which includes currents due the background thermal plasma, monoenergetic fast electrons and secondary electrons.
4He/³He dispersion and mantle convection
Authors: Allègre Claude J. , Moreira Manuel , Staudacher Thomas
Published: February 2004

Histograms of the 4He/³He ratios analysed in MORB glasses from individual ridge segments show a linar correlation between the ridge spreading rate and the standard deviation of the corresponding averaged 4He/³He ratio. We interpret the form of these histograms in terms of stirring time (τstir) of the upper mantle. This stirring time corresponds to the time that perturbations brought in from outside into the upper mantle, in the form of oceanic island source material or slabs, were attenuated by a factor of 1/e through the effect of convection.

Using 4He/³He ratios of oceanic basalts, we calculate a stirring time close to 250 Ma, distinct from the residence time of ∼1 Ga. This may indicate the existence of two scale upper mantle convection, rapid convection being responsible for the homogeneisation of helium in the upper mantle, and the consequent uniformity of the 4He/³He ratio, and slower convection being responsible for mantle outgassing and plate tectonic motion.
Coronal heating and plasma parameters at 1 AU
Authors: Esser Ruth , Habbal Shadia Rifai
Published: February 2004

A three‐fluid description of the solar wind is used to investigate the mass flux, helium abundance and flow speeds at 1 AU as a function of heat input in the inner corona. The solar wind model includes continuity, momentum and energy equations for each of the three species, electrons, protons, and alpha particles. The energy equations contain parameterized heat sources for any of the three species. The energy equation for the electrons also includes radiation losses in the transition region. We study the effect of heating different particle species. We show that for a given set of plasma parameters at the coronal base, heating the electrons, protons and alpha particles, or all three species, can result in the same mass flux, helium abundance and proton flow speed at 1 AU, yet very different plasma properties in the inner corona. The plasma parameters we consider are characteristic of the high speed solar wind.
The adiabatic transport of superthermal distributions modelled by Kappa Functions
Authors: Collier Michael R.
Published: February 2004

The Boltzmann entropy of a superthermal distribution modelled by a kappa function is calculated. It is shown that, for adiabatic transport, the quantity (Tcore3/2/n)ξ(κ) remains constant where Tcore is the core Maxwellian temperature, n is the density, and ξ(κ) is a function of the high energy spectral index, κ. For large values of kappa commonly found in space plasmas (κ > 4), the quantity T3/2/n where T = {κ/(κ ‐ 3/2)} Tcore is approximately constant for adiabatic transport.
Observation of electromagnetic oxygen cyclotron waves in a flickering aurora
Authors: Lund E. J. , LaBelle J. , Torbert R. B. , Liou K. , Peria W. , Kletzing C. A. , Kelley M. C. , Baker S. D. , Primdahl F. , Stenbaek‐Nielsen H. C. , Ranta A. , Haerendel G. , Frey H. U.
Published: February 2004

Instruments on the Auroral Turbulence rocket detected several intervals of weak electromagnetic oscillations at frequencies of 6–13 Hz in a strongly flickering auroral arc. These oscillations have amplitudes of up to δB ∼ 3 nT and δE ∼ 4 mV/m and have downward field‐aligned Poynting fluxes of up to ∼10−5 W/m². Fluctuations in the parallel electron flux at about 9 Hz were observed in association with the strongest of these oscillations. Simultaneous ground‐based optical data show that the arc was flickering at frequencies of 8–15 Hz. The observed frequencies would match the oxygen cyclotron frequency at ∼4500 km altitude. In one wave/particle event the apparent lag of the waves behind the modulated electrons implies a modulation source altitude of 2500–5000 km. We interpret these waves as electromagnetic ion cyclotron waves originating in the auroral acceleration region.
The latitude gradients of galactic cosmic ray and anomalous helium fluxes measured on Ulysses from the Sun's south polar region to the equator
Authors: Simpson J. A. , Connell J. J. , Lopate C. , McKibben R. B. , Zhang M.
Published: February 2004

Measurements from the Ulysses COSPIN (Cosmic Ray and Solar Particle INvestigations) High Energy Telescope between 80.2°S solar latitude and the sun's equator from September 1994 to March 1995 confirm that the modulated fluxes of galactic cosmic rays and anomalous components depend only weakly on heliographic latitude in the inner heliosphere at this phase of the solar cycle. The new observations were made over a radial range of only ∼1 AU during a period of nearly constant modulation and thus require less correction for radial and temporal variations in modulation than measurments made during Ulysses climb to maximum latitude.
Determination of dispersion relations in quasi‐stationary plasma turbulence using dual satellite data
Authors: Dudok de Wit T. , Krasnosel'skikh V. V. , Bale S. D. , Dunlop M. W. , Lühr H. , Schwartz S. J. , Woolliscroft L. J. C.
Published: February 2004

The joint frequency‐wavenumber spectrum is one of the basic quantities for analyzing plasma turbulence. It is shown how the full spectrum can be recovered from wavefields measured by two or more satellites via spectral methods based on wavelet transforms. Compared to standard cross‐correlation techniques, different branches in the dispersion relation can be resolved and quasi‐stationary wavefields can be accessed. Using this new approach, low frequency magnetic field data from the AMPTE‐UKS and AMPTE‐IRM spacecraft are investigated and the impact of nonlinear processes on wave propagation at the Earth's foreshock is revealed.
Observations of strong wind shears and temperature enhancements during several sporadic Na layer events above Haleakala
Authors: Gardner Chester S. , Tao Xin , Papen George C.
Published: February 2004

We describe two sporadic Na (Nas) layer events observed on 26 September and 6 October 1993 by the Na Wind/Temperature (W/T) lidar above Haleakala, Maui (20.8°N, 156.2°W) during ALOHA‐93. Both events occurred between 90 and 95 km altitude and appeared to be associated with large amplitude gravity waves. Both were accompanied by temperature enhancements of more than 40 K. The temperature and Na density variations at the peak of the Nas, as well as the vertical motions of the Nas layers and regions of elevated temperature were highly correlated. The vertical wind structure exhibited no unusual characteristics in the vicinities of the Nas. The zonal wind profile for the 6 October event exhibited a very strong vertical shear approximately 1 km below the Nas layer that approached the dynamic instability limit of 42 m/s/km. However the polarity of the shear was opposite that predicted by the classical wind shear theory of sporadic E layers (Es) formation.
Validation of sub‐grid‐scale mixing schemes using CFCs in a global ocean model
Authors: Robitaille Daniel Y. , Weaver Andrew J.
Published: February 2004

Three sub‐grid‐scale mixing parameterizations (lateral/vertical; isopycnal; Gent and McWilliams, 1990) are used in a global ocean model in an attempt to determine which yields the best ocean climate. Observed CFC‐11 distributions, in both the North and South Atlantic, are used in evaluating the model results. While the isopycnal mixing scheme does improve the deep ocean potential temperature and salinity distributions, when compared to results from the traditional lateral/vertical mixing scheme, the CFC‐11 distribution is worse in the upper ocean due to too much mixing. The Gent and McWilliams (1990) parameterization significantly improves the CFC‐11 distributions when compared to both of the other schemes. The main improvement comes from a reduction of CFC uptake in the southern ocean where the ‘bolus’ transport cancels the mean advection of tracers and hence causes the Deacon Cell to disappear. These results suggest that the asymmetric response found in CO2‐increase experiments, whereby the climate over the southern ocean does not warm as much as in the northern hemisphere, may be due to the particular mixing schemes used.
High precision 230Th and 232Th in the Norwegian Sea and Denmark by thermal ionization mass spectrometry
Authors: Moran S. B. , Hoff J. A. , Buesseler K. O. , Edwards R. L.
Published: February 2004

Seawater samples (1–2 liters) were collected from the Norwegian Sea and Denmark Strait and analyzed for 230Th and 232Th using highly sensitive thermal ionization mass spectrometry (TIMS). Depth profiles of dissolved 230Th and 232Th are characterized by surface water minima (<1 fg/kg, <5 pg/kg), subsurface maxima (12 fg/kg, 134 pg/kg), and intermediate concentrations that progressively decrease toward the bottom (∼5 fg/kg, ∼17 pg/kg), respectively. The lack of an increase in 230Th with depth is suggested to result from the short ventilation age of Norwegian Sea Deep Water combined with enhanced scavenging near the basin margins. The 230Th maximum is attributed to advection of high 230Th in the Arctic Intermediate Water, whereas the 232Th maximum may be related to a particulate source. The low dissolved 230Th and 232Th concentrations observed in the NADW formation regions implies a minor advective export of these long‐lived Th tracers to the North Atlantic.
The 410‐km‐depth discontinuity: A sharpness estimate from near‐critical reflections
Authors: Vidale John E. , Ding Xiao‐Yang , Grand Stephen P.
Published: February 2004

The abrupt increase in seismic velocity and density about 410 km below the surface of the Earth occurs at the depth where a phase change in olivine is expected [Akaogi et al., 1989; Katsura and Ito, 1989; Ringwood, 1969]. We have assembled recordings from 22 short‐period seismometers in Washington and British Columbia of 35 nuclear explosions in Nevada. These data reveal relatively noise‐free reflections from the “410” discontinuity. Most of the transition is completed within a depth interval of less than 10 km under the western United States. This result adds to the accumulating seismic evidence of a fairly sharp “410” transition.
Simultaneous lidar observations of vertical wind, temperature, and density profiles in the upper mesophere: Evidence for nonseparability of atmospheric perturbation spectra
Authors: Gardner Chester S. , Tao Xin , Papen George C.
Published: February 2004

A Na Wind/Temperature lidar was used to measure Na density, atmospheric temperature, and vertical wind profiles between 84 and 100 km altitude on September 27, 1993 at Haleakala, Maui during the 1993 Airborne Lidar and Observations of the Hawaiian Airglow (ALOHA‐93) Campaign. The data were used to compute the vertical wave number (m) and observed temporal frequency spectra (ω) of relative atmospheric density, relative temperature, and vertical winds perturbations. The shapes and magnitudes of the relative density and temperature spectra are consistent with the large body of previously published observations. The spectral indices are near −3 for the m‐spectra and near −2 for the ω‐spectra. The spectral index of the vertical wind ω‐spectrum is near 0. The spectral index of the vertical wind m‐spectrum is very small (−1.4). This result is inconsistent with the concept of separable (m, ω) joint spectra for wave induced perturbations. The measured rms perturbations during the observation period are, respectively, 4.9%, 4.5%, and 3.9 m/s for relative atmospheric density, relative temperature, and vertical winds.
Consistency between variations of ozone and temperature in the stratosphere
Authors: Finger F. G. , Nagatani R. M. , Gelman M. E. , Long C. S. , Miller A. J.
Published: February 2004

We compare time series of monthly mean total ozone values, provided by the Solar Backscatter Ultraviolet Radiometer (SBUV), with temperatures derived from the operational stratospheric analyses produced by the National Centers for Environmental Prediction, Climate Prediction Center (NCEP/CPC). Results generally substantiate the expectancy of positive correlations between variations of ozone and temperature in the lower stratosphere, and negative correlations between the two in the upper stratosphere. It is found that the correlations did not behave in the expected manner during the periods immediately after the eruptions of El Chichon (April 1982) and Mt.Pinatubo (June 1991). Possible problems are also pointed out for other time periods, when the anticipated correlations are not indicated. In some of the cases, the evaluations lead to recommendations for continued investigation, in order to determine whether some values might be erroneous and should be removed to improve consistency within the datasets. In a few cases it is found that real variations can be mistakenly identified as errors.
Temperature anisotropy effects and the generation of anomalous slow shocks
Authors: Karimabadi H. , Krauss‐Varban D. , Omidi N.
Published: February 2004

Temperature anisotropy is a persistent feature of many space plasmas (e.g., the magnetosheath) and can result in significant modifications to the jump conditions at various discontinuities. The effect of anisotropy on the Rankine‐Hugoniot (RH) relations is reexamined and the topological changes in the solutions are illustrated by means of a graphical technique. A class of slow shocks is found with quite unusual properties. In particular, there exists a class of nonswitchoff slow shocks which rotate the tranverse component of the magnetic field. To distinguish these new solutions from the usual solutions, they are termed anomalous slow shocks.
Mass balance change as a control on the frequency and occurrence of glacier surges in Svalbard, Norwegian High Arctic
Authors: Dowdeswell J. A. , Hodgkins R. , Nuttall A.‐M. , Hagen J. O. , Hamilton G. S.
Published: February 2004

The end of the Little Ice Age (LIA) in Svalbard (76–81°N), a climate‐sensitive region at the northern extreme of strong poleward heat transfer, was marked by an abrupt increase in mean annual air temperature of up to 5°C around 1920. Glacier mass balance has been consistently negative since this time, and large cumulative net losses of mass have occurred at most glaciers. Energy‐balance modelling confirms the sensitivity of Svalbard glaciers to climate change, predicting a negative shift in net mass balance of up to 0.8 m a−1 (water equivalent) per degree temperature rise. This climate‐related shift in glacier mass balance has reduced the intensity of glacier surge activity in Svalbard. One glacier, known to have surged since the end of the LIA, has since failed to accumulate the mass required to re‐initiate the surge cycle, and is also now cold at its base and incapable of rapid flow by basal sliding. Three overviews of the total number of actively‐surging glaciers in Svalbard between 1936–90 show a decrease from 18 to 5. This is significant compared with the expected numbers of surges based on LIA conditions. Post‐LIA climate change in Svalbard has therefore affected not only glacier extent, but also ice dynamics. This is trend will probably continue given CO2‐induced climate‐warming.
Results of ultra‐low‐frequency magnetic field measurements during the Guam Earthquake of 8 August 1993
Authors: Hayakawa Masashi , Kawate Ryusuke , Molchanov Oleg A. , Yumoto Kiyohumi
Published: February 2004

We report the results of measurements of ultra‐low‐frequency magnetic noise during a large earthquake (Ms=7.1) at Guam of 8 August, 1993 (depth ∼60 km). The ULF observing system is located in the Guam Island, about 65 km from the epicenter. Several distinct features of this analysis are summarized. (1) We have proposed rather sophisticated statistical analyses (monthly mean, standard deviation) in order to estimate the wave intensity and polarization (i.e. ratio Z/H). (2) A comparison between the ULF wave activity and ΣKp, is useful in distinguishing between the space geomagnetic pulsations and non‐space emissions. (3) Then, the use of the ratio (Z/H) is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves. (4) The statistical analysis of the temporal evolution of this ratio, has yielded that it shows a broad maximum only about one month before the earthquake, and this suggests that the emissions during this period are very likely to be magnetic precursors. (5) The temporal variation of Z component is similar to that for the Loma Prieta earthquake such that it shows a broad maximum ten days ∼ two weeks before the earthquake and another increase a few days before the earthquake. (6) The emissions presumably associated with the earthquake are of noise‐like nature, and their main frequency is 0.02 ∼ 0.05 Hz (with maximum intensity ∼0.1 nT).
Concurrent observations of auroral activity and a large sporadic sodium layer event during ANLC‐93
Authors: Gu Y. Y. , Qian J. , Papen G. C. , Swenson G. R. , Espy P. J.
Published: February 2004

Prominent sporadic sodium (Nas) layers, apparently associated with intense auroral activity, were observed during the 1993 Arctic Noctilucent Cloud Campaign (ANLC‐93) with an airborne sodium lidar on the August 6 flight near 51°N latitude. The peak density of the Nas layer reached 74,500 cm−3. The zonal extent of the Nas layers exceeded 1600 km. Strong auroral emissions were recorded with an all‐sky imager on the aircraft. A Michelson Interferometer was used to measure the intensity of OH nightglow between 85–90 km. OH rotational temperature exhibited no obvious correlation with the Nas layer peak density near 94 km. The data suggest a connection between the Nas layers and auroral electron precipitations. The data also support the conjecture that dust and smoke particles may play a crucial role in the Nas layer formation.
On the stability of rotational discontinuities: One‐ and two‐dimensional hybrid simulations
Authors: Karimabadi H. , Krauss‐Varban D. , Omidi N.
Published: February 2004

Studies of rotational discontinuities (RDs) and their role in affecting the magnetic field rotations at the magnetopause have been generally conducted using one‐dimensional simulations and for RDs in isolation from external perturbations. The conditions at the magnetopause are, however, often turbulent with large Alfvénic and mirror mode fluctuations. Here, previous calculations are extended to two‐dimensions using the hybrid code and the effect of external perturbations on the stability of both isotropic and anisotropic RDs is examined for the first time. As in the one‐dimensional simulations and in agreement with observations at the magnetopause, RDs are found to satisfy the minimum shear condition (rotation angles less than or equal to 180°) and have gradient scale half‐widths of the order of 1–4 ion inertial lengths. In addition, RDs are found to be quite stable even when large amplitude mirror and Alfvén waves impinge upon them. No evidence for two‐dimensional (e.g., surface or shear) instabilities were found. Although simulations show the presence of a coherent wavetrain for some of the RDs, the observations of RDs at the magnetopause have not identified any wavetrain. It is shown that under the turbulent conditions prevalent at the magnetopause, the detection of wavetrain in the data would most likely require multispacecraft measurements.
Independent marine and atmospheric model estimates of the sea‐air flux of dimethylsulfide in the Southern Ocean
Authors: Gabric Albert J. , Ayers Gregory P. , Sander Graham C.
Published: February 2004

Marine and atmospheric models have been combined with data collected at the Cape Grim atmospheric baseline station, to estimate the flux of dimethylsulfide to the atmosphere during the spring‐summer period in the Subantarctic Southern Ocean in the vicinity of the Cape Grim baseline atmospheric monitoring station. The marine model predicts that production of phytoplankton and of dissolved DMS will increase during spring to reach a maximum in summer consistent with the data on atmospheric DMS. Local wind and sea temperature data have been used to calculate the DMS transfer velocity which was used to compute the sea‐to‐air flux of DMS. Independent predictions of the DMS flux using an atmospheric model and Cape Grim data are in excellent agreement with the marine based prediction.
Electron acceleration from localized lower‐hybrid waves
Authors: Dubouloz N. , Treumann R. A. , Pottelette R. , Lynch K. A.
Published: February 2004

The stochastic acceleration of electrons in lower hybrid (LH) wave packets is investigated in the context of the Earth's auroral ionosphere and magnetosphere. As suggested by rocket and satellite observations, the packets are assumed to contain a broad spectrum of short wavelength LH waves. The interaction of the strongly magnetized electrons with a LH packet occurs mainly in the direction parallel to the background magnetic field, and can in a first approximation be described by a quasi‐linear velocity diffusion equation. An analytical expression is derived for the diffusion coefficient and used in Monte‐Carlo test‐particle calculations. These simulations exhibit the formation of high‐energy tails on both sides of the electron parallel velocity distribution. A model is proposed in order to explain the downgoing field‐aligned bursts of about 10 eV to 1 keV electrons which are observed by auroral rockets within inverted‐V structures at altitudes of several hundreds of kilometers.