bluuebunny/crossref_metadata_2025 huggingface dataset:
Paleomagnetism of the Duke Island, Alaska, ultramafic complex revisited
Authors: Butler Robert F. , Gehrels George E. , Saleeby Jason B.
Published: February 2004

The Duke Island ultramafic intrusion was emplaced into the Alexander terrane immediately preceding development of a regional mid‐Cretaceous thrust belt. Paleomagnetic samples were collected from exposures of ultramafic rock with cumulate layering northwest of Judd Harbor and northwest of Hall Cove. Thermal demagnetization results were analyzed using principal component analysis to isolate the characteristic remanent magnetization. Site‐mean characteristic directions determined from 16 sites fail the fold test at 95% confidence, indicating that cumulate layering attitudes were highly contorted at the time of magnetization, at least on a scale of tens of meters. Variations in cumulate layering attitudes probably resulted from the combined effects of thermal convection phenomena during crystallization and deformation following crystallization but prior to magnetization. Analysis of cumulate layering over larger structural domains indicates that kilometer‐scale deformation produced southwest plunging folds within the Hall Cove and Judd Harbor bodies. Bogue et al. [1995] proposed that a compound structural correction involving unplunging of fold axes followed by unfolding of average cumulate layering could restore cumulate layering to horizontal. However, using the full set of 21 site‐mean paleomagnetic directions from Duke Island (16 from the current study and 5 from Bogue et al. [1995]), the compound structural correction yields mean paleomagnetic directions from the Judd Harbor and Hall Cove areas that are statistically distinguishable at 99% confidence. This result indicates that even on the kilometer‐scale, cumulate layering within the Duke Island ultramafic intrusion was neither coplanar nor horizontal at the time of magnetization. Observations of cumulate layering in other ultramafic intrusive rocks indicate that this layering can significantly depart from horizontal by 10°–20° even on the kilometer scale. Therefore use of cumulate layering of ultramafic rocks as a proxy for paleohorizontal is not justified, and paleomagnetic directions from the Duke Island ultramafic intrusion cannot be used to infer the Cretaceous paleolatitude of the Insular superterrane.
Orientation of the stress field surrounding the creeping section of the San Andreas Fault: Evidence for a narrow mechanically weak fault zone
Authors: Provost Ann‐Sophie , Houston Heidi
Published: February 2004

We determine how the orientations of the principal stresses change with distance from the creeping section of the San Andreas fault (SAF). Our data are 2392 earthquake focal mechanisms, which are grouped according to distance from the SAF into 33 bins parallel to fault strike; the width of the bins are dictated by the spatial organization and density of the seismicity. Only events with M ≥ 1.5, depth ≥ 2 km, and well‐constrained focal mechanisms are used. The focal mechanisms are inverted simultaneously for the orientations and relative amplitudes of the principal stresses using two different inversion programs. A bootstrap resampling method provides confidence limits on the stress orientations. Our general result is that the maximum horizontal compression lies mostly at high angles to the SAF except in a narrow zone on the fault (no wider than 1–3 km), in which it appears to lie at a smaller angle (∼45°) from the fault strike. In particular, the maximum horizontal compression lies at a high angle to the SAF trend immediately adjacent to the fault. This stress state is most readily interpreted as due to a narrow mechanically weak SAF in this region. The narrowness of this zone contrasts with the region wider than 20–30 km of rotated stresses found surrounding the SAF in southern California [Hardebeck and Hauksson, 1999], suggestive of different mechanical behaviors of the SAF in these two regions. This approach may provide a way to define the width of the mechanically weak part of faults.
Compacting and dilating shear bands in porous rock: Theoretical and experimental conditions
Authors: Bésuelle Pierre
Published: February 2004

The failure of rocks in the brittle regime is generally associated with the appearance of strain localization bands. For very porous rocks, three types of strain localization can be distinguished: extension bands, shear bands, and compaction bands. The first is associated with an extensional normal strain concentration inside the band; the second, with a shear strain concentration; and the third, with a compressive normal strain concentration. This paper shows the continuous transition between pure extension bands and pure compaction bands, via shear bands that evolve from dilating shear bands to compacting shear bands. By an extension to the analysis of Rudnicki and Rice [1975] (RR) on strain localization in pressure sensitive rocks, the prediction of the strain type inside bands at the onset of localization shows that inside shear bands, the shear strain can be associated with a volumetric dilatancy or compaction depending on the constitutive parameters of the material. The theoretical determination of the strain type is in accordance with recent observations of dilating and compacting shear bands in laboratory tests on porous sandstone specimens. A limit for the existence of a localized reduction of porosity within the band is expressed. A physical limit to the RR model is also proposed to insure continuity of the strain mechanism of localization with respect to the constitutive parameters.
Vertical axis rotation in the Bolivian orocline, South America: 1. Paleomagnetic analysis of Cretaceous and Cenozoic rocks
Authors: Lamb Simon
Published: February 2004

The results of a paleomagnetic study are reported for 45 new sites or localities in the central Andes of Bolivia and northern Chile (17°S‐23°S), South America, in intrusive, volcanic and sedimentary rocks, ranging in age between ∼7 and ∼80 Ma. These outcrop across the width of the northern and southern limbs of the Bolivian orocline, from the sub‐Andean zone in the east to the volcanic arc in the west. For most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field. A comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable South America reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks (hundreds of meters to tens of kilometers across), relative to stable South America. These data, when combined with previous paleomagnetic studies of Upper Cretaceous and Cenozoic rocks, show a pattern of vertical axis rotations which defines three regional domains, extending across the width of the Bolivian Andes. The differences between the various domains are best seen in rocks in the age range ∼20 to 80 Ma; younger rocks acquired a primary magnetization at later stages in the rotation history. For rocks older than 20 Ma, crustal blocks in domain 1, on the northern flank of the Bolivian orocline, have undergone an anticlockwise rotation about a vertical axis in the range ∼8 to ∼33°, with a geometric mean of ∼22; blocks from domain 2 immediately south of the main oroclinal bend have rotated between ∼19° anticlockwise and ∼40° clockwise, with a geometric mean of ∼8° clockwise. Even farther south, in domain 3b, blocks have rotated between ∼3° and ∼57° clockwise, with a geometric mean of 37°. A localised area (domain 3b), in which blocks have undergone ∼32° clockwise rotation in the last ∼65 Myr, lies within the southeastern part of domain 1. The northern boundary of domain 1 and the southern boundary of domain 3a are as yet not well defined. The amount and timing of Cenozoic vertical axis rotation vary across the width of the Andes, and are interpreted to record both regional bending of both limbs of the Bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the Cenozoic: (1) ∼45 to ∼25 Ma, (2) ∼25 to ∼10 Ma, (3) ∼10 Ma to 0. Bending of the Bolivian Andes during the last ∼25 Myr was accommodated, relative to stable South America, by gradients of shortening along the eastern margin of the Andes, particularly in the sub‐Andean zone.
Studies of Antarctic sea ice concentrations from satellite data and their applications
Authors: Comiso Josefino C. , Steffen Konrad
Published: February 2004

Large regional and temporal changes in the global sea ice cover have been observed recently. Because of the relevance of such changes to climate change studies it is important that key ice concentration data sets currently used for evaluating such a phenomenon are assessed for accuracy and interpreted properly. Sea ice concentrations derived from passive microwave data using the Bootstrap and NASA Team algorithms are shown to be generally consistent but are also observed to differ by 10–35% in large areas within the ice pack, especially in the Weddell Sea, Amundsen Sea, and Ross Sea regions. Comparative analyses of such passive microwave data with coregistered visible and infrared (i.e., Landsat, AVHRR, and OLS) data show a predominance of thick consolidated ice in these areas and good agreement with the Bootstrap algorithm results. The relatively low values from the NASA Team algorithm results are likely due to layering within the ice and snow and/or surface flooding, which are known to affect the polarization ratio. Large seasonality in the physical characteristics and emissivity of the ice cover is also observed, and in predominantly new ice regions, the ice concentrations from passive microwave data are usually lower than retrievals from Landsat and OLS data in which the thresholding technique is used. The passive microwave results are biased because of relatively low emissivity of new ice, but they may be more useful since the bias allows for the identification of areas of significant divergence and polynya activities. Such areas need to be identified since heat and salinity fluxes are proportionately increased in these areas compared to those from the thicker ice areas. Time series data from 1978 through 2000 also show a slight but insignificant positive trend of 0.17±0.33%/decade in ice extent which is consistent with slight continental cooling during the period. This is a big contrast to the observed negative trend of about −3%/decade in the Arctic sea ice cover. It should be noted, however, that because the overlap period for key instruments is just 1 month, the error due to changes in sensor characteristics, calibration, and threshold for the 15% ice edge may not be negligible.
Three‐dimensional nature of flow near a sill
Authors: Klymak Jody M. , Gregg Michael C.
Published: February 2004

Velocity and density sections across Knight Inlet, British Columbia, demonstrate that lateral recirculations are a first‐order feature of the flow in the lee of the sill. The flow over this sill has been used as a typical example of two‐dimensional hydraulics, with a lower layer that thins and accelerates as it moves downstream below an almost stagnant layer that widens downstream, creating a distinctive wedge shape. However, we find that rather than being stagnant, the velocity in this wedge‐shaped layer is actually quite large, consisting of a swiftly recirculating dipole vortex during flood tide and a monopole vortex during ebb tide, though we may have missed the matching half of a dipole with our sampling during ebb. The recirculations during flood tide carry an amount of water equal to 25% of the tidal flux, while the monopole during ebb tide carries 20% of the tidal flux. These recirculations bias along‐channel estimates of volume flux, especially in the middle wedge‐shaped layer, and demonstrate that accurate volume fluxes in the lee wave are only possible if three‐dimensional surveys are made. Our three‐dimensional survey shows that there is a net isopycnal convergence of water in the middle layer at a rate adequate to close the volume budget of the flow without recourse to diapycnal fluxes. We also calculate the strength of the vorticity in the recirculations observed during flood tide and attribute their formation to boundary layer separation.
A model for sand ridges on the shelf: Effect of tidal and steady currents
Authors: Calvete D. , Walgreen M. , de Swart H. E. , Falqués A.
Published: February 2004

The dynamics of sand ridges, as observed on the innershelves and midshelves of various coastal seas, are investigated within the framework of an idealized model. These seabed undulations include shoreface‐connected sand ridges and tidal sand banks. The new aspects in this study are the explicit incorporation of both steady and tidal currents, a shelf with a sloping bottom, and a statistical approach to describe the sediment transport during storms (linear in the current) and fair weather conditions (cubic in the current). The present model is used to gain understanding of the different ridge characteristics in different geographical regions. This is done by analyzing the initial growth of small bottom perturbations, which evolve on a basic state describing a longshore uniform flow over a reference topography. It is found that basically five different bottom modes can be excited: shoreface‐connected sand ridges, Coriolis ridges, friction‐induced bars, trapped tidal ridges, and tidal sand banks. Their growth, which is due to different physical mechanisms, strongly depends on the weather climate and the geometrical characteristics of the shelf.
Comparison between predicted and observed sand waves and sand banks in the North Sea
Authors: Hulscher Suzanne J. M. H. , van den Brink G. Matthijs
Published: February 2004

For the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire North Sea. The model, which originates from first physical principles, predicts this occurrence via two dimensionless parameters on the basis of the water depth, the tidal velocity amplitude, the level of zero intercept above the seabed (z0), and a viscosity variation parameter ε. The latter two quantities were varied in a number of predictions for the entire North Sea, whereas for the first two, local values were used. The range of realistic values of ε and z0 was large enough to let these two parameters distinguish between the possible (combinations of) bed forms, as is shown in the shallower southern bight of the North Sea. The results were more sensitive to variations in z0 than in ε. A slightly more detailed approach focused on sand waves only in the southern North Sea and estimated local values for z0 using depth information. Quantification of the results showed that the model was able to predict the contours of the sand wave patches, but it could not account for the absence of the bed features within this area. The type of bed deposit partly explains the smaller‐scale variation. The work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large‐scale seabed features are formed as free instabilities of tide‐topography interactions.
Thin electron‐scale layers at the magnetopause
Authors: André M. , Vaivads A. , Buchert S. C. , Fazakerley A. N. , Lahiff A.
Published: February 2004

We use data from the four Cluster satellites to examine the microphysics of a thin electron‐scale layer discovered on the magnetospheric side of the magnetopause. Here the ion and electron motions are decoupled in a layer about 20 km (a few electron scales) wide, including currents and strong electric fields. In this layer the electrons are E × B drifting with the ions as a background, and the region can be described by Hall MHD physics. A unique identification of the source of the thin layer is not possible, but our observations are consistent with recent simulations showing thin layers associated with the separatrix extending far away from a reconnection diffusion region.
Slip tendency analysis as a tool to constrain fault reactivation: A numerical approach applied to three‐dimensional fault models in the Roer Valley rift system (southeast Netherlands)
Authors: Worum Geza , van Wees Jan‐Diederik , Bada Gabor , van Balen Ronald T. , Cloetingh Sierd , Pagnier Henk
Published: February 2004

We describe a new numerical approach to constrain the three‐dimensional (3‐D) pattern of fault reactivation. Taking advantage of the knowledge of the tectonic stress field, the ratio of the resolved shear and normal stresses (slip tendency) as well as the direction of the shear stress is calculated at every location on the faults modelled by triangulated surfaces. Although the calculated contact stresses represent only a first order approximation of the real stresses, comparison of the 3‐D pattern of slip tendency with the frictional resistance of the fault can provide useful constraints on the probability of fault reactivation. The method was applied to 3‐D geometrical fault models in the Roer Valley Rift System (southeast Netherlands) which is presently characterized by pronounced tectonic activity. The input stress tensors were constrained by published stress indicators. The analysis demonstrated that the observed fault activity could be explained within a reasonable range of frictional parameters and input stress magnitudes. In addition a fairly good correlation was found between the predicted slip directions and the focal mechanisms of local earthquakes. This suggests that in the study area, fault models being valid in the uppermost part of the crust are suitable to constrain fault reactivation even in the deeper part of the seismogenic layer. The analysis further demonstrated that fault hierarchy and the regional tectonic contexts of the fault system are important factors in fault reactivation. Therefore they always should be taken into account during evaluation of the calculated slip tendency and slip direction patterns.
First measurements of ClOOCl in the stratosphere: The coupling of ClOOCl and ClO in the Arctic polar vortex
Authors: Stimpfle R. M. , Wilmouth D. M. , Salawitch R. J. , Anderson J. G.
Published: February 2004

The first measurements of ClOOCl in the stratosphere have been acquired from a NASA ER‐2 aircraft, deployed from Kiruna, Sweden (68°N, 21°E), during the joint SOLVE/THESEO‐2000 mission of the winter of 1999/2000. ClOOCl is detected by thermal dissociation into two ClO fragments that are measured by the well‐known technique of chemical conversion, vacuum ultraviolet resonance fluorescence. Ambient ClO is detected simultaneously. Observations of the ratio [ClOOCl]/[ClO]2 (estimated uncertainty of ±25%, 1 σ) are used with a time‐dependent photochemical model, to test the model representation of the ratios of kinetic parameters J/kProd and kLoss/kProd for day and nighttime observations, respectively. Here, kProd and kLoss are the rate constants for ClOOCl production and loss, respectively, and J is the photolysis rate of ClOOCl. The observations are in good agreement with J based upon the 2002 JPL recommended cross sections for ClOOCl [Sander et al., 2003], if the true value of kProd is given by either the 2000 JPL recommendation [Sander et al., 2000] or the work of Trolier et al. [1990]. The larger values of kProd given by Bloss et al. [2001] and the 2002 JPL recommendation are consistent with the observations only if J is increased by a significant amount. This is accomplished if J is calculated with the larger ClOOCl cross sections measured by Burkholder et al. [1990]. The J values for ClOOCl based on the Huder and DeMore [1995] cross sections are too small, by factors of ∼1.6 to 2.5 for all values of kProd, based on the observations. Nighttime results suggest that, for 190 < T < 200 K, the values for KEq (the equilibrium constant, equal to the ratio of kProd/kLoss) of Cox and Hayman [1988] and Avallone and Toohey [2001] are in best agreement with the observations.
Carbonate crash and biogenic bloom in the late Miocene: Evidence from ODP Sites 1085, 1086, and 1087 in the Cape Basin, southeast Atlantic Ocean
Authors: Diester‐Haass L. , Meyers P. A. , Bickert T.
Published: February 2004

Middle/late Miocene to early Pliocene sedimentary sequences along the continental margin of southwest Africa have changes that correspond to the carbonate crash (12–9 Ma) and biogenic bloom events (∼7–4 Ma) described in the equatorial Pacific by Farrell et al. [1995]. To explore the origins of these changes, we analyzed the carbon and coarse fraction contents of sediments from ODP Sites 1085, 1086, and 1087 at a time resolution of 5 to 30 kyr. Several major drops in CaCO3 concentration between 12 and 9 Ma are caused by dilution from major increases in clastic input from the Oranje River during global sea level regressions. Abundant pyrite crystals and good preservation of fish debris reflect low oxygenation of bottom/pore waters. Regional productivity was enhanced during the time equivalent to the carbonate crash period. Higher benthic/planktic foraminiferal ratios indicate that CaCO3 dissolution at Site 1085 peaked between 9 to 7 Ma, which was after the global carbonate crash. This period of enhanced dissolution suggests that Site 1085 was located within a low‐oxygen water mass that dissolved CaCO3 more easily than North Atlantic Deep Water, which began to bathe this site at 7 Ma. At 7 to 6 Ma, the onset of the biogenic bloom, increases and variations in total organic carbon and benthic foraminiferal accumulation rates show that paleoproductivity increased significantly above values observed during the carbonate crash period and fluctuated widely. We attribute the late Miocene paleoproductivity increase off southwest Africa to ocean‐wide increases in nutrient supply and delivery1.
Modified lower hybrid fan instability excited by precipitating auroral electrons
Authors: Omelchenko Yu. A. , Shapiro V. D. , Shevchenko V. I. , Ashour‐Abdalla M. , Schriver D.
Published: February 2004

This paper analyses a possible mechanism for ion conic formation in the auroral zone that result from the wave‐particle interactions between ions and VLF waves excited due to an instability triggered by precipitating electrons in magnetospheric plasma. This so‐called “fan” instability does not require the presence of an electron beam or other type of positive slope in the electron distribution function, but only an asymmetric high‐energy tail on one side of the distribution function. Analytical solutions of the quasi‐linear equations describing the instability are calculated, and the wave spectra and amplitudes are shown to be in reasonable good agreement with observations. Also, the calculated energy distribution function for the accelerated ions agrees with experimental data. The analytical results are supported by full numerical solutions of the quasilinear equations. The application of the fan instability to the formation of ion conics is discussed.
Recovery phase of magnetospheric substorms and its association with morning‐sector aurora
Authors: Opgenoorth H. J. , Persson M. A. L. , Pulkkinen T. I. , Pellinen R. J.
Published: February 2004

In contrast to the extensively studied growth and expansion phases of magnetospheric substorms, the substorm recovery phase has not received much attention in the published literature. It has generally been considered as a period in which all disturbances caused by the previous two phases decay, the magnetosphere “recovers” to reach a quiet state again. Using mainly ground‐based data, we show that the “recovery phase” contains a number of features which are qualitatively different from the expansion phase, that is, not just a decay of previously excited forms of energy release. Typical recovery phase phenomena include intense electrojet activity in the morning sector, high‐energy particle precipitation, the development of large‐scale auroral vortex streets (so‐called Ω bands and associated magnetic Ps 6 pulsations), and very often new eastward expanding active auroral phenomena, not unlike evening‐sector expansion phase features but concentrated to the morning sector of the auroral oval. Such features must be associated with the substorm mechanism itself. We suggest that our observations during the substorm recovery phase are explained by the magnetospheric reorganization after the ejection of a plasmoid. We show that a more detailed investigation of the late substorm features can increase our understanding of the physical processes leading to the complete cycle of magnetospheric energy release.
Electron collection by a highly positive satellite in the ionosphere: Test particle simulation
Authors: Singh Nagendra , Chaganti V. S.
Published: February 2004

Collection of electrons by a satellite at a relatively high potential in low Earth orbit, including the effects of the satellite's orbital motion, remains a poorly understood phenomenon. Using a test particle simulation in which charged particle motion is tracked in prescribed electric fields, we calculate here the current collection and the nature of the energy distribution function of the electrons collected by the satellite, including the effects of the satellite orbital motion. Calculations of the collected current without the orbital motion show an excellent agreement with the current from the Parker‐Murphy model, but with the orbital motion of the satellite the current collection is enhanced, the degree of enhancement depending on the size of the satellite sheath extending along the magnetic field line. In the latter case, the flow of electrons around the satellite shows some interesting behaviors including azimuthal E × B drift around the satellite, axial trapping along the magnetic field, and formation of field‐aligned flow of electrons in the wake region. The total energy of the collected electrons is ≃eϕo, where ϕo is the satellite potential, but the partition of the energy into components parallel (W∥) and perpendicular (W⊥) to the magnetic field shows interesting features. For the magnetic field along the polar axis Z, the energy distributions near the poles are perfectly field aligned, that is, the parallel energy W∥ = eϕo. The perpendicular component (W⊥) progressively increases toward the “equator” of the spherical satellite. For a sufficiently large bias potential ϕo, the equatorial energy distributions f(W∥) and f(W⊥) show fine structures having multiple peaks. These features of the distribution functions result from the chaotic motion of electrons and the associated coupling between the electron motions parallel and perpendicular to the magnetic field in strongly nonuniform electric fields near the satellite.
Magnitude of BZ in the neutral sheet of the magnetotail
Authors: Huang C. Y. , Frank L. A.
Published: February 2004

Statistical estimates of the average value of BZ in the magnetotail neutral sheet between 10 and 22 RE in X, and −10 to 10 RE in Y (GSM coordinates) are given for different phases of geomagnetic activity. With few exceptions the average value of BZ is found to be between 5 and 8 nT, but generally near 7 nT, irrespective of the type or phase of activity. This is considerably higher than is predicted in current magnetic field models of the magnetotail. The magnetic field within 2 RE of the neutral sheet is examined to determine whether BZ remains approximately constant. We do not find this to be true, BZ decreasing significantly over this distance from ΔZ = 0. The structure of the cross‐tail current and variations in current density with substorrn phase appear to be more complicated than is commonly accepted.
Ground‐based observations of MF/HF radio noise in the auroral zone
Authors: Weatherwax A. T. , LaBelle J. , Trimpi M. L. , Brittain R. , Treumann R. A.
Published: February 2004

Broadband noise enhancements in the frequency range of ∼1.4‐4.8 MHz have been observed with a ground‐based receiver located at Two Rivers, Alaska (near Fairbanks). During the 5‐month period from November 1991 to March 1992, eight broadband noise enhancements were recorded. A correlation is observed between the radio noise enhancements and magnetic activity recorded with the magnetometer in College, Alaska (45 km away). Initial examination of college ionosonde data also suggests that sporadic E is associated with the termination of some of the events. The enhancements are characterized by a low‐frequency cutoff at ∼1.4 MHz and a decrease in intensity near ∼2.8 MHz, which roughly correspond to 1 and 2 times the ionospheric electron cyclotron frequency, respectively. Although some discrete man‐made signals are observed to be enhanced during the events, the bulk of the broadband enhancements may be composed of natural signals. If natural, the observed spectral shape is consistent with calculations of synchrotron radiation combined with cyclotron absorption; in this case, these signals would be diagnostic of the hardness of the auroral electron energy spectrum.
Effects of vertical winds and electric fields in the nonlinear evolution of equatorial spread F
Authors: Sekar R. , Suhasini R. , Raghavarao R.
Published: February 2004

In view of the present understanding of Equatorial Spread F (ESF) through the generalized Rayleigh‐Taylor instability mechanism, the importance of dynamical parameters is examined by means of a nonlinear numerical simulation technique. The investigation reveals that agencies like downward wind and eastward electric field of measured magnitudes accelerate the evolutionary process of ESF even beyond 350 km height, where according to the linear theories they are considered to be less significant. It is also shown that the acceleration must be due to the presence of in‐situ winds or electric fields. The variabilities in the occurrence characterstic of ESF are discussed in connection with the possible day‐to‐day variabilities of vertical winds.
A statistical survey of the central plasma sheet
Authors: Huang C. Y. , Frank L. A.
Published: February 2004

We present a statistical analysis of the ion plasma properties and magnetic field of the central plasma sheet at distances from 10 to 23 RE from the Earth. This study incorporates ISEE 1 plasma sheet samples accumulated with the University of Iowa Lepedea plasma analyzer during 1979. Our study of the bulk properties of the ion plasma shows that there is relatively little change as a result of increasing geomagnetic activity. The main macroscopic effect is an increase in plasma temperature. When the transverse profile of the plasma sheet is examined, it is clear that this increase in temperature occurs preferentially at high latitudes. The magnetic field shows an increase in magnitude in the vicinity of the neutral sheet during periods of high geomagnetic activity. The radial profiles of the density, temperature, plasma, and magnetic pressures show a gradient in total energy density in the plasma sheet. The convective velocities are systematically directed earthward, and there is a bias toward positive Vy on the duskside of the magnetotail. We infer that particle motion can be attributed to a pressure gradient, combined with steady state convective electric fields. We compute the convective electric fields from our measured bulk velocities, arriving at a pattern similar to that observed at low altitudes. The cross‐tail electric field Ey is dominant near the neutral sheet region, but at high latitudes Ez is a major component. Our results suggest that the plasma sheet boundary layer, rather than the central plasma sheet, is a region of diverging electric fields. The picture of the plasma sheet that emerges from our study is that of a stable reservoir of hot plasma in which thermodynamic, rather than dynamic, processes is important.
Simulation of magnetic field line stochasticity at the magnetopause
Authors: Wang Zhi , Ashour‐Abdalla Maha
Published: February 2004

We have conducted a three‐dimensional particle simulation to study the magnetic field line stochasticity at the magnetopause current layer. Our results show that the magnetic field lines become stochastic due to the overlap of the destabilized multiple tearing mode islands, which agrees with the percolation model suggested by Galeev et al. (1986). After the field lines become stochastic, these tearing modes grow even 2‐3 times faster than in the linear stage and saturate at an amplitude level 3‐4 times bigger than the single tearing mode without mode‐mode coupling. The field line stochasticity also causes a strong particle diffusion across the current layer. The diffusion coefficient reaches to 109 m2/s for typical magnetopause parameters. Associated with the particle diffusion, the current layer becomes broader in width. As a result, the magnetic energy is dissipated into particle energy by heating parallel to the local magnetic field. The particle energy increases by 60%, while the magnetic helicity, which has always been regarded as a good invariant, changes by 20%.
A mechanism to produce a dawn‐dusk component of plasma flow during magnetic reconnection in the magnetotail
Authors: Hawkins J. G. , Lee L. C. , Yan M. , Lin Y. , Perkins F. W. , Yamada M.
Published: February 2004

Magnetic reconnection between antiparallel field lines in the magnetotail is generally thought to produce plasma acceleration in the earthward‐tailward direction. However, measurements of the plasma velocity in the magnetotail during substorm activity sometimes reveal a dawn‐dusk component of plasma flow. In this paper, we show that a dawn‐dusk component of plasma acceleration may be produced during reconnection if the neutral line is not perpendicular to the magnetic field. In this case, MHD simulations show that reconnection between antiparallel field lines will initially produce plasma acceleration that is nearly parallel to the neutral line because the magnetic tension force is not opposed by a pressure gradient force in this direction. As the magnetic field topology evolves to a steady state, the plasma flow direction rotates until it is nearly parallel to the plane that initially contained the antiparallel magnetic field lines before reconnection (hereafter referred to as the initial field plane). However, the time required to reach a steady state (typically several hundred seconds in the magnetotail region) may be greater than the time during which the reconnection process is active. Consequently, bursts of plasma flow with a dawn‐dusk component may occur in the magnetotail. The initial acceleration along the neutral line depends on the angle (θB;) between the neutral line and the initial field plane, with the largest burst of plasma flow along the neutral line occurring when θB = 45°.
The energetics of Titan's ionosphere
Authors: Roboz A. , Nagy A. F.
Published: February 2004

We have developed a comprehensive model to study the dynamics and energetics of the ionosphere of Titan. We solved the one‐dimensional, time‐dependent, coupled continuity and momentum equations for several ion species, together with single ion and electron energy equations, in order to calculate density, velocity, and temperature profiles. Calculations were carried out for several cases corresponding to different local times and configurations of the Titan‐Saturn system. In our model the effects of horizontal magnetic fields were assumed to be negligible, except for their effect on reducing the electron and ion thermal conductivities and inhibiting vertical transport in the subram region. The ionospheric density peak was found to be at an altitude of about 1100 km, in accordance with earlier model calculations. The ionosphere is chemically controlled below an altitude of about 1500 km. Above this level, ion densities differ significantly from their chemical equilibrium values due to strong upward ion velocities. Heat is deposited in a narrow region around the ionospheric peak, resulting in temperature profiles increasing sharply and reaching nearly constant values of 800‐1000°K for electrons and 300°K for ions in the topside, assuming conditions appropriate for the wake region. In the subram region magnetic correction factors make the electron heat conductivities negligible, resulting in electron temperatures increasing strongly with altitude and reaching values in the order of 5000°K at our upper boundary located at 2200 km. Ion chemical heating is found to play an important role in shaping the ion energy balance in Titan's ionosphere.
Altitude characteristics of plasma turbulence excited with the Tromsø Superheater
Authors: Djuth F. T. , Stubbe P. , Sulzer M. P. , Kohl H. , Rietveld M. T. , Elder J. H.
Published: February 2004

Langmuir/ion turbulence excited with the upgraded high‐power (1.2‐GW effective radiated power) HF heating facility at Tromsø, Norway, has been recently studied with the European Incoherent Scatter VHF and UHF incoherent scatter radars. In this report we focus on the altitudinal development of the turbulence observed at the highest HF power levels available. Quite remarkably, the observed plasma turbulence plunges downward in altitude over timescales of tens of seconds following HF beam turn‐on; the bottom altitude is generally reached after ∼30 s. This phenomenon has a well‐defined HF power threshold. It is most likely caused by changes in the electron density profile brought about by HF heating of the electron gas. If this is the case, then the heat source must be nonlinearly dependent on HF power. Overall, the characteristics of the Tromsø turbulence are quite distinctive when compared to similar high‐resolution measurements made at Arecibo Observatory, Puerto Rico. After HF transmissions have been made for tens of seconds at Tromsø, billowing altitude structures are often seen, in sharp contrast to layers of turbulence observed at Arecibo.
A multi‐ion generalized transport model of the polar wind
Authors: Demars H. G. , Schunk R. W.
Published: February 2004

The higher‐order generalizations of the equations of standard hydrodynamics, known collectively as generalized transport theories, have been used since the early 1980s to describe the terrestrial polar wind. Inherent in the structure of generalized transport theories is the ability to describe not only interparticle collisions but also certain non‐Maxwellian processes, such as heat flow and viscous stress, that are characteristic of any plasma flow that is not collision dominated. Because the polar wind exhibits a transition from collision‐dominated to collisionless flow, generalized transport theories possess advantages for polar wind modeling not shared by either collision‐dominated models (such as standard hydrodynamics) or collisionless models (such as those based on solving the collisionless Boltzmann equation). In general, previous polar wind models have used generalized transport equations to describe electrons and only one species of ion (H+). If other ion species were included in the models at all, it was in a simplified or semiempirical manner. The model described in this paper is the first polar wind model that uses a generalized transport theory (bi‐Maxwellian‐based 16‐moment theory) to describe all of the species, both major and minor, in the polar wind plasma. In the model, electrons and three ion species (H+, He+, O+) are assumed to be major and several ion species are assumed to be minor (NO+, Fe+, O++). For all species, a complete 16‐moment transport formulation is used, so that profiles of density, drift velocity, parallel and perpendicular temperatures, and the field‐aligned parallel and perpendicular energy flows are obtained. In the results presented here, emphasis is placed on describing those constituents of the polar wind that have received little attention in past studies. In particular, characteristic solutions are presented for supersonic H+ outflow and for both supersonic and subsonic outflows of the major ion He+. Solutions are also presented for various minor ions, both atomic and molecular and both singly and multiply charged.
Effect of the relative flow velocity on the structure and stability of the magnetopause current layer
Authors: Kuznetsova M. M. , Roth M. , Wang Z. , Ashour‐Abdalla M.
Published: February 2004

The Vlasov kinetic approach is used to study the stability of the magnetopause current layer (MCL) when a sheared flow velocity and a sheared magnetic field both exist simultaneously within it. A modified Harris‐Sestero equilibrium where the magnetic field and bulk velocity are changing direction on the same spatial scale is suggested to illustrate the generation of a y component of the magnetic field in the center of the MCL. With this equilibrium it is shown that By(0) can be of the order of Bz(∞) when the value of the shear flow (U) tends to the ion drift velocity (Ud). The modifications of the initial symmetrical Harris configuration, introduced by the presence of a shear flow, strongly influence the adiabatic interaction of the plasma with low‐frequency tearing‐type electromagnetic perturbations as well as the nonadiabatic response of the particles near the center of the MCL. This results in a reduction of the growth rate of the tearing mode.
Generation of region 1 current by magnetospheric pressure gradients
Authors: Yang Y. S. , Spiro R. W. , Wolf R. A.
Published: February 2004

The Rice Convection Model (RCM) is used to illustrate theoretical possibilities for generating region 1 Birkeland currents by pressure gradients on closed field lines in the Earth's magnetosphere. Inertial effects and viscous forces are neglected. The RCM is applied to idealized cases, to emphasize the basic physical ideas rather than realistic representation of the actual magnetosphere. Ionospheric conductance is taken to be uniform, and the simplest possible representations of the magnetospheric plasma are used. Three basic cases are considered: (1) the case of pure northward IMF, with cusp merging assumed to create new closed field lines near the nose of the magnetosphere, following the suggestion by Song and Russell [1992]; (2) the case where Dungey‐type reconnection occurs at the nose, but magnetosheath plasma somehow enters closed field lines on the dawnside and duskside of the merging region, causing a pressure‐driven low‐latitude boundary layer; and (3) the case where Dungey‐type reconnection occurs at the nose, but region 1 currents flow on sunward drifting plasma sheet field lines. In case 1, currents of region 1 sense are generated by pressure gradients, but those currents do not supply the power for ionospheric convection. Results for case 2 suggest that pressure gradients at the inner edge of the low‐latitude boundary layer might generate a large fraction of the region 1 Birkeland currents that drive magnetospheric convection. Results for case 3 indicate that pressure gradients in the plasma sheet could provide part of the region 1 current.
Momentum transport near a magnetic X line in collisionless reconnection
Authors: Cai H. J. , Ding D. Q. , Lee L. C.
Published: February 2004

Plasma dynamics and momentum transport near an X line during time‐dependent magnetic reconnection in a collisionless plasma are investigated based on two‐dimensional particle simulations. We find that a weakly skewed velocity distribution is formed near the magnetic X line, leading to the presence of off‐diagonal elements of the plasma pressure tensor. Let the reconnection electric field be in the y direction. The gradients of the off‐diagonal elements of the pressure tensor can provide a transport of the y momentum. During the normal magnetic reconnection, the momentum transport associated with the off‐diagonal terms of the pressure tensor mediates a transfer of the y momentum from the region near the X line to regions outside the X line. A period of “reverse magnetic reconnection,” during which the plasma kinetic energy is converted into magnetic energy, is also observed in the simulation. When reverse reconnection occurs, the gradients of the off‐diagonal pressure tensor elements can mediate a transfer of y momentum into the X line. It is found that the inertial term also plays a significant role in the force balance near the magnetic X line. An explanation for the origin of the off‐diagonal pressure terms is also given in this paper.
Landau damping of auroral hiss
Authors: Morgan D. D. , Gurnett D. A. , Menietti J. D. , Winningham J. D. , Burch J. L.
Published: February 2004

Auroral hiss is observed to propagate over distances comparable to an Earth radius from its source in the auroral oval. The role of Landau damping is investigated for upward propagating auroral hiss. By using a ray tracing code and a simplified model of the distribution function, the effect of Landau damping is calculated for auroral hiss propagation through the environment around the auroral oval. Landau damping is found to be the likely mechanism for explaining some of the one‐sided auroral hiss funnels observed by Dynamics Explorer 1. It is also found that Landau damping puts a lower limit on the wavelength of auroral hiss. Poleward of the auroral oval, Landau damping is found in a typical case to limit ω/k∥ to values of 3.4 × 104 km/s or greater, corresponding to resonance energies of 3.2 keV or greater and wavelengths of 2 km or greater. For equatorward propagation, ω/k∥ is limited to values greater than 6.8 × 104 km/s, corresponding to resonance energies greater than 13 keV and wavelengths greater than 3 km. Independent estimates based on measured ratios of the magnetic to electric field intensity also show that ω/k∥ corresponds to resonance energies greater than 1 keV and wavelengths greater than 1 km. These results lead to the difficulty that upgoing electron beams sufficiently energetic to directly generate auroral hiss of the inferred wavelength are not usually observed. A partial transmission mechanism utilizing density discontinuities oblique to the magnetic field is proposed for converting auroral hiss to wavelengths long enough to avoid damping of the wave over long distances. Numerous reflections of the wave in an upwardly flared density cavity could convert waves to significantly increased wavelengths and resonance velocities.
A note on two‐dimensional asymptotic magnetotail equilibria
Authors: Voigt Gerd‐Hannes , Moore Brian D.
Published: February 2004

In order to understand, on the fluid level, the structure, the time evolution, and the stability of current sheets, such as the magnetotail plasma sheet in Earth's magnetosphere, one has to consider magnetic field configurations that are in magnetohydrodynamic (MHD) force equilibrium. Any reasonable MHD current sheet model has to be two‐dimensional, at least in an asymptotic sense (Bz/Bx) = ε « 1. The necessary two‐dimensionality is described by a rather arbitrary function fnof;(x). We utilize the free function fnof'(x) to construct two‐dimensional magnetotail equilibria that are “equivalent” to current sheets in empirical three‐dimensional models. We obtain a class of asymptotic magnetotail equilibria ordered with respect to the magnetic disturbance index Kp. For low Kp values the two‐dimensional MHD equilibria reflect some of the realistic, observation based, aspects of three‐dimensional models. For high Kp values the three‐dimensional models do not fit the asymptotic MHD equilibria, which is indicative of their inconsistency with the assumed pressure function. This, in turn, implies that high magnetic activity levels of the real magnetosphere might be ruled by thermodynamic conditions different from local thermodynamic equilibrium.