bluuebunny/crossref_metadata_2025 huggingface dataset:
An improved algorithm for micromagnetics
Authors: Wright T. M. , Williams W. , Dunlop D. J.
Published: February 2004

We present an improved micromagnetic model for predicting the minimum energy magnetization states in fine magnetite grains. The resolution, or the number of elemental magnetization vectors, has been increased over previous models: first, by the use of a Fourier transform algorithm to reduce the number of calculations from O(N2) to O(N log N) (N being the number of elements into which the grain is subdivided); and second, by implementing the model on a parallel computer. Using a parallel computer reduces the computation time by a factor of approximately 1/(4Np), where Np represents the number of processors. The improved model enables equilibrium magnetization states to be predicted using a resolution of 64×64 ×64 subcubes to a grain in 16 hours of CPU time; this compares with a resolution of 12×12 ×12 in 24 hours of CPU time for previous models. High‐resolution models allow the examination of multidomain states in materials such as magnetite and different sized grains or assemblages of interacting grains.
Seismic images of crustal duplexing and continental subduction in the Brooks Range
Authors: Wissinger E. S. , Levander A. , Christensen Nikolas I.
Published: February 2004

We have interpreted an integrated vertical incidence to wide‐angle seismic data set to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. The common midpoint (CMP) reflection data image the principal structures comprising the Brooks Range: the Endicott Mountains allochthon (EMA), the crustal scale Doonerak duplex, the master detachment, a 1.0–1.5 s thick zone of lower crustal reflectivity just above the crust‐mantle boundary, and a complex crustal root. The master detachment separates the crust into units which have been uplifted and deformed in the fold and thrust belt from those which have not. Least squares inversion of both reflection and refraction travel time data produced a velocity model consistent with the CMP image of the Brooks Range as well as with the Bouguer gravity data. The different layers comprising the seismic velocity model correlate well with the principal structural elements identified in the seismic reflection data, and seismic velocities in the model compare favorably to petrophysical data from Brooks Range rock samples. Maximum crustal thickness in the Brooks Range is 49 km, at an asymmetric root located under the EMA. At the root we observe an offset in the lower crustal reflectivity and two deep zones of reflections north of the root. We interpret these as a Moho offset of some 5 km near the range front, which in our favored interpretation resulted from subduction of the Brooks Range lower crust northward beneath the North Slope. A mantle reflective zone which we interpret as the subducted lower crust can be traced to depths as great as 65 km. Above this zone at considerably shallower depth is the original North Slope Moho. Proximity of the continental subduction zone to the crustal scale Doonerak duplex suggests that the development of the fold and thrust belt has been at least partially controlled by the lower crust/mantle subduction.
Monte Carlo simulation of two‐dimensional domain structures in magnetite
Authors: Fukuma Koji , Dunlop David J.
Published: February 2004

A Monte Carlo method was applied for micromagnetic studies of two‐dimensional domain structures of a 1‐μm magnetite cube in zero field. By using this method we could incorporate the effect of thermal agitation into micromagnetic modeling. Starting from an initial single‐domain structure (a saturated state), we obtained a closure domain structure with three body and four closure domains. This structure is quite different from the checkerboard‐like structure obtained by a conjugate gradient method, and it gives a much lower energy and saturation remanence ratio. An initial lamellar two‐domain structure evolved into a vortex structure, which was also reached from an initial quasi‐vortex structure. Although such a vortex structure has a lower energy than the closure domain structure, it was not attainable from an initial single‐domain structure at room temperature. The Monte Carlo method is effective in finding a path to escape from unstable local energy minima and reach a stable local energy minimum, although not necessarily a global minimum, at a given temperature. The structure corresponding to such a stable local minimum should represent a realistic domain structure, comparable to what would be attained in nature with the aid of thermal fluctuations of spins.
An integrated model for the tectonic development of the frontal Brooks Range and Colville Basin 250 km west of the Trans‐Alaska Crustal Transect
Authors: Cole Frances , Bird Kenneth J. , Toro Jaime , Roure François , O'Sullivan Paul B. , Pawlewicz Mark , Howelll David G.
Published: February 2004

We present a kinematic model for the sequence of deformation and sedimentation in the frontal Brooks Range and adjacent Colville Basin in the Etivluk River region, 250 km west of the Trans‐Alaska Crustal Transect (TACT). The model is based on a tectonic subsidence analysis of the foreland basin, combined with structural, stratigraphic, and thermal studies of the northern edge of the Brooks Range thrust belt. We interpret six discrete tectonic events that led to the present‐day configuration of the thrust belt in this area: (1) emplacement of ophiolitic allochthons over the distal continental margin rocks in Valanginian time, hundreds of kilometers south of this study, (2) Hauterivian uplift of the Barrow Arch rift margin, affecting the northern part of the Colville Basin, (3) Barremian contraction involving emplacement of distal continental margin and ophiolitic allochthons onto the Endicott Mountains allochthon and creation of a southward dipping flexural basin on the North Slope autochthon, (4) mid‐Cretaceous exhumation of imbricated rocks in the Brooks Range during northward propagation of the thrust front into the foreland, (5) minor thrusting in Late Cretaceous‐Paleocene in the northern foreland to the northern limit of contractional structures, and (6) regional exhumation of the orogen and the foreland in Paleocene‐Eocene time. This sequence of deformation agrees well with a simple model of a forward propagating thrust system.
Measurements and global models of surface wave propagation
Authors: Ekström Göran , Tromp Jeroen , Larson Erik W. F.
Published: February 2004

A new technique for making single‐station phase velocity measurements is developed and applied to a large number of globally recorded Rayleigh and Love waves in the period range 35–150 s. The method is based on phase‐matched filter theory and iteratively suppresses the effect of interfering overtones by minimizing residual dispersion. The model surface wave signal is described by its amplitude and apparent phase velocity, both of which are parameterized in terms of smooth B‐spline functions of frequency. A misfit function is constructed which represents the difference between the model and observed waveforms, and the optimal spline coefficients are estimated in an iterative misfit minimization algorithm. In order to eliminate cycle skips in the measurements of phase at short periods, the waveforms are first matched at long periods, and the frequency range is gradually extended to include higher frequencies. The application of the algorithm to records from the Global Seismographic Network, using earthquakes in the Harvard centroid‐moment tensor catalog, results in the determination of more than 50,000 high‐quality dispersion curves. The observed variations in measured dispersion for pairwise similar paths are used to estimate realistic uncertainties in the data. Phase delays at discrete periods are inverted for global maps of variations in phase velocity expanded in spherical harmonics up to degree 40. A realistic resolution test indicates that structures are well recovered up to at least degree 20. The new phase velocity maps explain 70–96% of the observed variance in phase residuals, reflecting the high internal consistency of the dispersion measurements.
Tectonic evolution of the central Brooks Range mountain front: Evidence from the Atigun Gorge region
Authors: Mull Charles G. , Glenn Richard K. , Adams Karen E.
Published: February 2004

Atigun Gorge, at the northern front of the eastern Endicott Mountains, contains well‐exposed rocks of the upper part of the Endicott Mountains allochthon and rocks of the structurally higher Picnic Creek or Ipnavik River allochthon. These allochthons contain rocks as young as Early Cretaceous (Valanginian) and are separated by a nearly vertical fault zone that contains exotic blocks of Triassic and Jurassic chert and silicified mudstone. Siliceous rocks of this type are not present in the Endicott Mountains allochthon but are characteristic of the Picnic Creek, Ipnavik River, and some of the other allochthons that structurally overlie the Endicott Mountains allochthon in the central and western Brooks Range. These exotic blocks, therefore indicate that structurally higher rocks of either the Picnic Creek or Ipnavik River allochthon were emplaced during the Early Cretaceous and are preserved along the northern flank of the eastern Endicott Mountains. The deformed thickness of this higher allochthon in the subsurface north of the mountains is unknown but probably exceeds 2 kilometers. Similar relations are mapped east of Atigun Gorge in an area of structural transition from the eastern Endicott Mountains into the northern Philip Smith Mountains, which are formed by the parautochthonous North Slope stratigraphic assemblage. The allochthonous rocks at the mountain front are regionally unconformably overlain by proximal Lower Cretaceous (Albian) foredeep conglomerate at the southern flank of the Colville basin, but at Atigun Gorge, the base of these deposits is interpreted as a possible back thrust at a triangle zone. Conglomerate clasts in the foredeep deposits are dominantly chert, mafic igneous rock, and other lithologies characteristic of the Picnic Creek and Ipnavik River allochthons and scattered clasts from the Endicott Mountains allochthon. The conglomerates show that the chert‐rich allochthonous rocks and the Endicott Mountains allochthon were emplaced in the north‐central Brooks Range by large‐scale crustal shortening (>300 km) between the Valanginian and Albian (∼135 to ∼112 Ma). This orogenic event significantly postdates early stages of Brooks Range orogeny but predates later stages of orogeny documented by stratigraphic and apatite fission‐track data. These relations reduce the magnitude of shortening inferred at the triangle zone at the Brooks Range mountain front. The outcrop data suggest that some of the strata preserved at a structurally low level north of the mountain front and visible in the seismic data of the Trans‐Alaska Crustal Transect (TACT) may consist of clastic sedimentary rocks of the structurally higher Picnic Creek or Ipnavik River allochthon.
A method for determining significant structures in a cloud of earthquakes
Authors: Jones R. H. , Stewart R. C.
Published: February 2004

The interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task. Locations are subject to uncertainties, which will not be the same for every earthquake. The data should therefore not be interpreted simply by inspection, which is difficult in the case of three‐dimensional data anyway. Instead, we propose using the location uncertainties as a guide in processing the data. Earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained, which can then be interpreted in terms of some simplified structure such as faults. The aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data. The method is applied to three synthetic sets of data. These illustrate the potential and limitations of the method. Application to a real earthquake data set from Rabaul Caldera in Papua New Guinea gives an image of the caldera ring fault that suggests departures from the simple ring‐fault structure previously assumed. Sensitivity analysis on the Rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it.
Quantitative implementation of Preisach‐Mayergoyz space to find static and dynamic elastic moduli in rock
Authors: Guyer R. A. , McCall K. R. , Boitnott G. N. , Hilbert L. B. , Plona T. J.
Published: February 2004

In this paper we describe the analysis of quasi‐static stress‐strain data using a Preisach‐Mayergoyz (PM) [after Preisach, 1935; Mayergoyz, 1985] space picture for the elastic behavior of rock. In contrast to the traditional analytic approach to stress strain (an energy density as a function of the strain invariants), the PM space picture reproduces hysteresis and discrete memory seen in the data. In addition, the PM space picture establishes a relationship between experimental data and a number density ρ of microscopic mechanical units within the rock. The density ρ allows us to make quantitative predictions of dynamic elastic properties. Determining ρ from quasi‐static stress‐strain data requires us to solve a highly underdetermined inverse problem. We explore the following three methods of solving the inverse problem: simulated annealing, normal modes, and exponential decay. All three methods are tested on a Berea sandstone data set and found to give an excellent description of stress versus strain. Choosing one method, the normal mode method, we analyze quasi‐static stress‐strain curves on two additional sandstones, namely, another sample of Berea and a sample of Castlegate sandstone. From the density ρ for each sample we predict the dynamic modulus as a function of pressure and the nonlinear elastic constants. For each of these cases the agreement between the predictions based on ρ and experiment is quite good. We establish that PM space provides a quantitative description of the elastic response of a rock and that PM space may be found by a variety of inversion methods.
Lateral variations in lowermost mantle shear wave anisotropy beneath the north Pacific and Alaska
Authors: Garnero Edward J. , Lay Thorne
Published: February 2004

S waves recorded by long‐period World‐Wide Standardized Seismograph Network and broadband stations in North America for deep northwest Pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north Pacific and Alaska. Systematic delays of up to 4 s are observed between longitudinal components (SV) and transverse components (SH) of motion for core‐reflected ScS waves as well as for core‐grazing and diffracted S waves. The absence of significant splitting for S waves that have turning points more than a few hundred kilometers above the core‐mantle boundary indicates that anisotropy is localized within the D′′ region (the lowermost portion of the mantle). SV‐SH differential arrival times for both ScS and Sdiff, along with path length estimates assuming a 250 km thick D′′ region, indicate spatial variations in the strength of shear wave anisotropy. The strongest anisotropy (1–1.5%) is found in the eastern part of the study area, with systematic reduction in magnitude toward the west. A transverse isotropy model can explain the data, with the velocity structure for horizontally polarized waves (VSH) having a 2–3% discontinuous shear velocity increase at the top of D′′ (as proposed in earlier studies of the region) and a similar structure for S wave particle motion in the direction normal to the core‐mantle boundary (VSV) but with the velocity jump at the top of D′′ and the velocity within D′′ being reduced from that for VSH by 0.5–1.5 km/s. Large uncertainties exist for velocity gradients above and below the velocity jump, but the requirement of a reduced VSV relative to VSH in D′′ is clear. Synthetic waveforms calculated by using separate isotropic structures for SH and SV match the observations well and constrain the basic anisotropic structure, because the shear waves all traverse the region with near‐grazing geometries. The study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the D′′ layer. Topography of the D′′ layer is not well resolved (because of trade‐off with volumetric heterogeneity).
Lateral variations of Pn wave velocity in northwestern Italy
Authors: Parolai S. , Spallarossa D. , Eva C.
Published: February 2004

The Pn arrival times recorded from seismic networks operating throughout northwestern Italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical outliers, poorly recorded events, and scantily sampled stations. Moreover, synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique. The agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete. A low‐velocity zone was found beneath the western and northwestern side of the Alps; it may be related to an increase in depth of the Moho, as supported by other geophysical data. The high Pn velocities found in the eastern side of the western Alps indicate the presence of high velocity and density, lower crust rocks of the so‐called “Ivrea Body.” The high Pn velocity underlying the Ligurian Sea could be related to a high‐velocity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the Bouguer gravity anomalies of the area.
Grain‐size‐induced weakening of H2O ices I and II and associated anisotropic recrystallization
Authors: Stern Laura A. , Durham William B. , Kirby Stephen H.
Published: February 2004

Grain‐size‐dependent flow mechanisms tend to be favored over dislocation creep at low differential stresses and can potentially influence the rheology of low‐stress, low‐strain rate environments such as those of planetary interiors. We experimentally investigated the effect of reduced grain size on the solid‐state flow of water ice I, a principal component of the asthenospheres of many icy moons of the outer solar system, using techniques new to studies of this deformation regime. We fabricated fully dense ice samples of approximate grain size 2±1 μm by transforming “standard” ice I samples of 250±50 μm grain size to the higher‐pressure phase ice II, deforming them in the ice II field, and then rapidly releasing the pressure deep into the ice I stability field. At T≤200 K, slow growth and rapid nucleation of ice I combine to produce a fine grain size. Constant‐strain rate deformation tests conducted on these samples show that deformation rates are less stress sensitive than for standard ice and that the fine‐grained material is markedly weaker than standard ice, particularly during the transient approach to steady state deformation. Scanning electron microscope examination of the deformed fine‐grained ice samples revealed an unusual microstructure dominated by platelike grains that grew normal to the compression direction, with c axes preferentially oriented parallel to compression. In samples tested at T≥220 K the elongation of the grains is so pronounced that the samples appear finely banded, with aspect ratios of grains approaching 50:1. The anisotropic growth of these crystallographically oriented neoblasts likely contributes to progressive work hardening observed during the transient stage of deformation. We have also documented remarkably similar microstructural development and weak mechanical behavior in fine‐grained ice samples partially transformed and deformed in the ice II field.
Continental signature of a ridge‐trench‐triple junction: Northern Vancouver Island
Authors: Lewis T. J. , Lowe C. , Hamilton T. S.
Published: February 2004

South of the Juan de Fuca‐Pacific‐North America triple junction located off northern Vancouver Island there is subduction, and to the north there is oblique convergence along the Queen Charlotte transform fault. In the mid‐Tertiary, periods of extension, including crustal thinning and volcanism, formed Queen Charlotte Basin. Heat flow on northern Vancouver Island (67 mW/m2) is the same as in Queen Charlotte Basin to the north, but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mW/m2. Compared to the rest of the island, the northern island has more subdued topography, a lower mean elevation, slightly higher Bouguer gravity values, and lower seismicity. Late Tertiary volcanics, which occur only on the northern part of the island, geochemically resemble the Tertiary Masset Formation which formed during periods of extension in Queen Charlotte Basin. These data are consistent with a Tertiary history including periods of extension for northern Vancouver Island, similar to Queen Charlotte Basin, and distinguish it from the rest of Vancouver Island. The southern boundary of this northern block extends northeastward from Brooks Peninsula, approximately above the inferred northern edge of the subducted Juan de Fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone. The abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate, which produces large temperature differences at depths of 30–40 km. The time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years, and therefore the location of the subducting plate's northern edge must have been stationary or moving very slowly over this period. Consequently it is likely that the triple junction has been located on average for the last 40 Myr off Brooks peninsula.
Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17°10′–17°40′S, from a study of layer 2A geometry
Authors: Carbotte Suzanne M. , Mutter John C. , Xu Liqing
Published: February 2004

Multichannel seismic images from the East Pacific Rise 17°10′–17°40′S are used to study the geometry of seismic layer 2A in the near‐axis region. Wave‐equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2A event. Processed stacks are compared with seafloor morphology and tectonic fabric imaged in side‐scan sonar data. Assuming layer 2A corresponds with the extrusive crust, these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge‐crest and ridge‐flank morphology. We find variations in axial thickening of layer 2A which imply twofold variation in the width of the extrusive layer accumulation zone, as well as systematic changes in the pattern of accumulation of this layer. On the ridge flanks the layer 2A horizon mimics abyssal hill relief, consistent with a horst and graben origin for this topography. Short‐wavelength variations in the thickness of layer 2A are superimposed on this relief, which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone. The integrated off‐axis thickness of layer 2A is correlated with ridge cross‐sectional area and indicates persistent (100,000 years) spatial gradients in extrusive layer thickness. Systematic changes in the cross‐axis shape of the ridge, observed over distances of 5–10 km, cannot be attributed to the extrusive layer, and it appears that axial structure beneath the volcanics governs the cross‐axis morphology of the ridge.
A gravity and magnetic anomaly study of the extinct Aegir Ridge, Norwegian Sea
Authors: Jung Woo‐Yeol , Vogt Peter R.
Published: February 2004

In 1990 the U.S. Naval Research Laboratory mapped the extinct Aegir Ridge in the Norwegian Sea with SeaMARC II side‐scan sonar, Hydrosweep multibeam, seismics, gravity, and magnetics onboard R/V Maurice Ewing. We analyzed the 92 center beam bathymetry, gravity, and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 Ma. In bathymetric expression the rift valley deepens and narrows southward, while the rift mountain summits become higher. South of ∼67°N, the western rift mountains and flanks stand higher than the eastern mountains and flanks. Free‐air anomalies along the ridge axis show undulating intermediate wavelength (∼100 km) anomalies superimposed on a regional decrease of the rift valley negative from ∼0 mGal in the north to −55 mGal in the south. After removal of sediment effects, results from three‐dimensional gravity modeling imply a variation of crustal thickness. A new rotation pole for the movement of the Jan Mayen microplate relative to the Eurasia plate from ∼50 Ma to 25 Ma was found at 64°34′N, 13°W, based on magnetic lineations and inferred discontinuity traces. A plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of ∼2.0°/m.y. (>36 Ma) followed by gradual deceleration to 0 at 25 Ma. Full spreading rate between chrons C13n and C21n ranged from ∼13 mm/yr in the north to 8 mm/yr in the south.
A note on the use of the two‐stream delta‐scaling approximation for calculating atmospheric photolysis rate coefficients
Authors: Zeng J. , Madronich S. , Stamnes K.
Published: February 2004

The delta‐scaling approximation is widely used in radiative transfer models to improve the accuracy of calculated irradiances under strongly forward scattering conditions. At completion of the scaled radiative transfer calculations, the inverse transformation (or descaling) is applied to partition the total downward irradiance, which is assumed invariant with respect to the scaling/descaling transformation, into its diffuse and direct components. However, this invariance does not apply to the total downward actinic flux, and ambiguities arise when the actinic flux components are calculated from either scaled or descaled components of the irradiance. The problem is particularly severe in two‐stream models because of the lack of angular resolution. We use a realistic example (the calculation of the photodissociation rate coefficient, or J value, for the photolysis reaction NO2 → NO + O(3P)) to show in particular that, for high Sun and low surface albedo, the use of descaled J values leads to large overestimation of rather constant or modest increases of J values below a thin cloud or aerosol layer. Even larger overestimations of J values are found near the top of a thick cloud layer at high Sun when descaling is used. Use of scaled values appears preferable under most practical conditions, although they, too, suffer from inaccuracies generally associated with two‐stream methods.
Measuring regional economic impacts of streamflow depletions
Authors: Leones Julie , Colby Bonnie , Cory Dennis , Ryan Liz
Published: February 2004

Because of large upstream diversions for agriculture and an absence of policies to protect in‐stream flows, flows in the Rio Grande near Taos, New Mexico, routinely are low by midsummer. The reach is a popular Whitewater run in the southwestern United States when flows are adequate for river running. This article estimates the regional economic impacts attributable to summer streamflow depletions. Economic analysis indicates that while lower water levels affect the number of people coming to the region to raft on one river reach, low water levels had no effect on another nearby rafting area. Total expenditures and economic impacts were simulated for streamflows maintained at levels adequate for Whitewater recreation throughout the summer season. These simulations indicate a 24% ($0.74 million) increase in rafting‐linked expenditures and a 25% ($0.94 million) increase in value added from rafting, compared to actual 1992 expenditures and value added.
A plasma temperature anomaly in the equatorial topside ionosphere
Authors: Balan N. , Oyama K.‐I. , Bailey G. J. , Fukao S. , Watanabe S. , Abdu M. A.
Published: February 2004

A study of the thermal structure of the low‐latitude (30°N to 30°S) ionosphere under equinoctial conditions at low, medium, and high solar activity has been carried out using the Sheffield University plasmasphere‐ionosphere model (SUPIM) and Hinotori satellite observations. The study reveals the existence of an anomaly in the plasma (electron and ion) temperature in the topside ionosphere during the evening‐midnight period. The anomaly, called the equatorial plasma temperature anomaly (EPTA), is characterized by a trough around the magnetic equator with crests on either side. The trough develops before the crests. The model results show that the anomaly occurs between 1900 and 0100 LT at altitudes between 450 and 1250 km; the strongest anomaly occurs around 2130 LT at 950 km altitude during high solar activity. The temperature trough of the anomaly arises from the adiabatic expansion of the plasma and an increase in plasma density caused by the prereversal strengthening of the upward vertical E × B drift. The temperature crests arise from the combined effect of the reverse plasma fountain and nighttime plasma cooling. The electron temperature measured by the Hinotori satellite near 600 km altitude during medium and high solar activity periods shows the existence of the EPTA with characteristics in close agreement with those obtained by the model. The model also reproduces the occurrence of a daytime temperature bulge in the electron temperature in the bottomside ionosphere; the ion temperature shows no bulge.
Geotail observations of energetic ion species and magnetic field in plasmoid‐like structures in the course of an isolated substorm event
Authors: Zong Q.‐G. , Wilken B. , Reeves G. D. , Daglis I. A. , Doke T. , Iyemori T. , Livi S. , Maezawa K. , Mukai T. , Kokubun S. , Pu Z.‐Y. , Ullaland S. , Woch J. , Lepping R. , Yamamoto T.
Published: February 2004

On January 15, 1994, the ion spectrometer high energy particle ‐ low energy particle detector (HEP‐LD) on the Japanese spacecraft Geotail observed five quasi‐periodic energetic ion bursts in the deep tail (X=−96 RE). These bursts were associated with plasmoid‐like structures in the magnetic field components. In. addition, three multiple TCR groups were identified in the interval. The observations in the distant tail occurred during a time interval of substorm activity which also produced multiple injections in the geosynchronous orbit region. The HEP‐LD observations show that Bz bipolar plasmoid‐like structures are associated with tailward flowing particle bursts. However, earthward flowing particle bursts are predominantly associated with bipolar signatures in By. In addition, an oxygen burst was seen in the back of a plasmoid (postplasmoid) which showed both By and Bz bipolar magnetic field signatures. The oxygen burst lasted for 23 min, and the density ratio (O/H) reached 15% for the HEP‐LD energy range (in the same plasmoid, this ratio was approximately 1% before the oxygen burst). The oxygen burst exhibited a strong beam‐like structure which occupied only 6 ∼ 7% of the full solid angle (4π). We suggest that energized oxygen ions of ionospheric origin travel downtail in the narrow postplasmoid‐plasma sheet which trails the plasmoid. Furthermore, we suggest that the magnetosphere dissipated larger quantities of energy during this very intense substorm event by ejecting multiple relatively small plasmoids rather than through the formation and ejection of a single large plasmoid.
Ion temperature measurements from the Akebono suprathermal mass spectrometer: Application to the polar wind
Authors: Drakou E. , Yau A. W. , Abe T.
Published: February 2004

We present a method for estimating the thermal ion drift velocity, ion temperature, and spacecraft potential in the polar ionosphere from data acquired with the suprathermal mass spectrometer (SMS) on the Akebono satellite. The method is based on fitting the spin angle distributions of the observed ion flux for a number of retarding potential analyzer (RPA) settings to a drifting Maxwellian distribution. A nonlinear fitting procedure is used to relate the observed fluxes to the plasma parameters. The spacecraft potential is taken into account by means of the thin‐sheath approximation. The analysis is applicable to the “thermal” (total energy few eV) polar wind ion population. A study of a number of representative passes at various altitudes, latitudes, and local times indicates that all major ions (H+, He+, and O+) have low temperatures, in the range of 0.05–0.35 eV, with little temperature dependence on altitude, longitude, or latitude. The velocity estimates confirm the previous analysis using the moment method: all ions have a significant upward velocity component (antiparallel to the magnetic field, in the northern hemisphere) which increases with altitude. The velocity estimates from data at the higher RPA settings are sometimes higher than those at low RPA settings. This indicates that the actual ion distributions are not Maxwellian, perhaps due to a higher‐energy tail component drifting at higher velocity. All ions are in general supersonic at high altitudes.
A new functional form to study the solar wind control of the magnetopause size and shape
Authors: Shue J.‐H. , Chao J. K. , Fu H. C. , Russell C. T. , Song P. , Khurana K. K. , Singer H. J.
Published: February 2004

In this study a new functional form, r = r0[2/(1 + cos θ)]α, is used to fit the size and shape of the magnetopause using crossings from ISEE 1 and 2, Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM), and IMP 8 satellites. This functional form has two parameters, r0 and α, representing the standoff distance and the level of tail flaring. The value r is the radial distance at an angle (θ) between the Sun‐Earth line and the direction of r. It is found that r0 varies with the interplanetary magnetic field (IMF) Bz component and has a break in the slope at Bz = 0 nT. The best‐fit value of r0 decreases with increasing southward IMF Bz. For northward IMF Bz, the best‐fit value of r0 increases slightly with increasing Bz. The best‐fit value of α increases monotonically with decreasing IMF Bz. The dynamic pressure (Dp) also changes r0 and α. The parameters Dp and r0 are related by a power law of −1/(6.6±0.8). The best‐fit value of α is slightly larger for larger dynamic pressure, which implies that Dp also has a role in flux transfer from the dayside to the nightside, but the size of this effect is small. An explicit function for the size and shape of the magnetopause, in terms of Dp and Bz, is obtained by using multiple parameter fitting in a form that is useful for operational space applications such as predicting when satellites at geosynchronous orbit will be found in the magnetosheath.
Response of the termination shock to interplanetary disturbances: 2. MHD
Authors: Story Todd R. , Zank Gary P.
Published: February 2004

The interaction of the solar wind termination shock with disturbances incident from the upstream solar wind is considered using a one‐dimensional MHD (both transverse and oblique) model. This paper extends the work of Story and Zank [1995] (Paper 1) to include the effects of the interplanetary magnetic field. A decay law is derived to describe the damping of compound rarefaction/shock structures. The inclusion of a parallel magnetic field component leads to the production of slow‐mode rarefactions and shocks. Therefore, depending on parameters, the interaction of the termination shock with interplanetary disturbances may serve to generate both slow‐ and fast‐mode magnetosonic waves that propagate through the heliosheath. Furthermore, it follows from the simulations presented here that slow‐mode waves and rotational discontinuities can be expected to occur, at least to some extent, at any region of the solar system where collisions between oblique MHD shocks occur. Some discussion regarding the relative local orientation of the heliospheric magnetic field, with respect to the interstellar magnetic field, is presented to address the possibility of a global rotational discontinuity(s) which may be necessary to connect the interplanetary magnetic field to the interstellar field. Some further results concerning the interaction of waves and shocks reflected from heliosheath structure with the termination shock are also presented.
Lower thermospheric neutral winds at Søndre Strømfjord: A seasonal analysis
Authors: Azeem S. M. I. , Johnson R. M.
Published: February 2004

Data obtained by Søndre Strømfjord incoherent scatter radar during recent Lower Thermospheric Coupling Study (LTCS) campaigns have been analyzed to examine seasonal changes in high‐latitude lower thermosphere neutral winds. Data included in this study were obtained with a 2 × 160 μs multipulse as well as with higher‐resolution multipulse and alternating code techniques and were analyzed with different auto correlation function (ACF) fitting routines. We report neutral wind results obtained during LTCS experiments with the new acquisition and analysis system and compare these results with those acquired using earlier versions of the software. We investigate the effects of different assumptions in the ACF fitting program on the data return and quality. Wind results are intercompared to examine average seasonal structure and variability. Strong semidiurnal oscillations characterize high‐latitude lower thermospheric winds at 105 km irrespective of season; however, summer amplitudes are nearly a factor of 2 greater than those observed during winter. Average winter winds appear to have more variability than those deduced for summer. Equinox winds at fall and spring show similar oscillations but are nearly reversed in phase. At 115 km, diurnal oscillations grow and their amplitudes often exceeds typical semidiurnal amplitudes. Tidal amplitudes observed in radar data are compared with the predictions of several models, including National Center for Atmospheric Research thermosphere‐ionosphere‐general‐circulation model (TIGCM) [Roble et al., 1988], global scale wave model (GSWM) [Hagan et al., 1994], and Forbes‐Vial model (FV91) [Forbes and Vial, 1991]. These comparisons suggest that theoretical predictions significantly underestimate tidal amplitudes calculated from Sondrestrom incoherent scatter radar measurements.
Large fluxes of auroral electrons in filaments of 100 m width
Authors: Lanchester B. S. , Rees M. H. , Lummerzheim D. , Otto A. , Frey H. U. , Kaila K. U.
Published: February 2004

The combination of high time and space resolution measurements in the magnetic zenith (optical and radar) and detailed modeling of the ionospheric response to auroral particle precipitation has produced the new result that most of the energy density in a bright arc resides in an extremely narrow filament, embedded in a broader feature. We are able to show that the narrow filaments, of the order of 100 m width, are produced by monoenergetic beams of electrons, whereas the surrounding emissions of lower energy density are produced by broad spectral distributions. The new result is achieved by combining different energy spectra as input to an auroral model and comparing the resulting electron density profiles with those measured with the European incoherent scatter radar at 0.2 s time resolution. The result is dependent on the optical evidence that the narrow energetic beams do not fill the fields of view of the detectors, i.e. both photometer and radar. Agreement between the observed structure, scale lengths, and evolution of the aurora and modeled predictions suggests that transient parallel electric fields are important acceleration mechanisms above the ionosphere.
Effect of Sun‐aligned arcs on the polar thermosphere
Authors: Ma T.‐Z. , Schunk R. W.
Published: February 2004

Sun‐aligned polar cap arcs cause regions of enhanced ionization that often appear when the interplanetary magnetic field is near zero or northward. They are typically associated with a shear in the convection electric field, which leads to electron precipitation and plasma density enhancements that can be up to 10 times background plasma densities. The Sun‐aligned arcs are 100–300 km in width and 1000 km in length. Because the arcs offer resistance to the basically antisunward thermospheric flow, they could cause an appreciable thermospheric perturbation. To determine their effect, a time‐dependent, three‐dimensional, high‐resolution model of the global thermosphere was used to calculate the thermospheric response to “representative” Sun‐aligned arcs. The model predicts that the largest perturbations occur when the arcs are associated with a sheared electric field and when multiple arcs are present. In this case, there are both density and temperature enhancements on the upstream side of the arcs and depressions on the downstream side. There is about a 20% neutral density variation and about a 100°K neutral temperature variation across the arcs.
Direct experimental measurement of electron impact ionization‐excitation branching ratios: 3. Branching ratios and cross sections for the N2+X2Σg+, A2Πu, and B2Σu+ states at 100 eV
Authors: Doering J. P. , Yang J.
Published: February 2004

The three electronic states of N2+ which are important to aeronomy are the X 2Σg+, A 2Πu, and B 2Σu+ states. The transitions X 2Σg+ ← A 2Πu, and X 2Σg+ ← B 2Σu+ are responsible for the prominent Meinel bands and first negative bands which occur in the aurora and airglow. In this paper, we use new electron‐electron coincidence (e,2e) measurements to extend our previous work on the B 2Σu+ state cross section to give a complete set of branching ratios and cross sections for N2+ production in the three electronic states given above by 100‐eV electron impact on N2. An improved electron‐electron coincidence apparatus has allowed the direct measurement of the cross section and branching ratio for the X 2Σg+ state for the first time. The cross sections for the X and A states agree within experimental error with the existing optical excitation results in the literature, but the error bars on the present measurements are smaller than those for the optical experiments. The branching ratios for the N2+ X and A states at 100 eV are 0.448±0.033 and 0.453±0.033, respectively. The corresponding 100‐eV excitation cross sections for the X and A states deduced from the total N2+ 100‐eV electron impact ionization cross section which is known from other measurements are 86.9±7.0 × 10−18 cm2 and 87.9±7.0 × 10−18 cm2, respectively. The B state results, which have been discussed in detail in a previous paper, are branching ratio of 0.099±0.017 and excitation cross section of 19.2±3.3 × 10−18 cm2. These values are significantly lower than the result deduced from the widely accepted optical emission cross section of Borst and Zipf [1970] but in satisfactory agreement with the “best” value for the cross section of 23.6 × 10−18 cm2 previously deduced from a combination of optical and (e,2e) results.
Subauroral electric fields observed by the Freja satellite: A statistical study
Authors: Karlsson T. , Marklund G. T. , Blomberg L. G. , Mälkki A.
Published: February 2004

Over 12 months of Freja electric field data have been scanned for subauroral electric fields (SAEF) to enable a comprehensive study of the ionospheric signatures of such electric fields. SAEF are encountered from 1800 to 0200 MLT in agreement with an earlier study. However, a large majority of the SAEF are encountered at a time slightly premidnight (2200 – 2300 MLT), with rather few occurrences before 2000 MLT and after 2400 MLT. Furthermore, the strength of the subauroral electric field is generally much larger for events close to 2200 MLT than for other events. The data confirm that SAEF occur during the substorm recovery phase but also show that SAEF occur earlier during recovery when located close to 2200 MLT than at other local times. The dependence on season and geomagnetic activity is studied, and it is found that the SAEF are more commonly observed at times of high activity when the subauroral electric fields are also generally stronger, except close to winter solstice, when strong electric fields are observed during low activity. The potentials associated with the SAEF and the relation to interplanetary magnetic field By are also studied. The observations are discussed in context with substorm‐related field‐aligned currents and the midlatitude trough, and we present evidence that support and refine one of the proposed production mechanisms.
On low‐altitude particle acceleration and intense electric fields and their relationship to black aurora
Authors: Marklund Göran , Karlsson Tomas , Clemmons James
Published: February 2004

Recent findings by the Freja satellite have shown the existence of extremely intense (1–2 V/m) and small‐scale (1 km) diverging electric fields which are interpreted to be associated with east–west aligned dark striations or black auroral curls. Precipitating or transversely energized ions, downward field‐aligned currents carried by upward fluxes of ionospheric electrons and dropouts of energetic electron precipitation, are found to be characteristic features of such events. A comparison of these characteristics to those of the aurora point at a symmetry between the aurora and the black aurora, the aurora being associated with negative divergence of the electric field and the black aurora with positive divergence. The diverging field events typically occur during winter conditions within the midnight to early morning sector of the auroral oval. Estimates of the ambient conductivity due to solar EUV radiation for each of these events show a clear anticorrelation with the electric field magnitude. The black auroral structures are likely to be associated with localized ionospheric density depletions below that of the ambient density and caused by the upward flow of ionospheric electrons. The efficiency by which such density holes are created in regions of downward field‐aligned current flow have recently been demonstrated in model studies. The electric field magnitudes are found to decrease with the scale size, not inversely as suggested in recent theoretical work but with a power law exponent of 0.6–0.8. At lower altitudes (around 800 km) the maximum intensities for a majority of the events are in the range of values that have been reported from rocket and radar measurements in the ionosphere, i.e., around 150–200 mV/m. However, close to magnetic midnight and during winter conditions small‐scale diverging electric fields of 1 V/m are occasionally found to exist down to at least 800 km. We suggest that the diverging electric fields observed by Freja are associated with low‐altitude and narrow (≈1–2 km) potential structures similar to the auroral potential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of narrow upward beams of 2 keV electrons in good agreement with a 2 kV positive peak in the electrostatic potential for a black aurora event. The existence of a downward parallel electric field at low altitudes is also supported by low‐altitude observations by the S3–3 and Viking satellites. If such low‐altitude potential structures do exist as our results suggest, an outstanding problem for future investigation is how they may be formed and maintained.