bluuebunny/crossref_metadata_2025 huggingface dataset:
Mantle discontinuity structure beneath the Colorado Rocky Mountains and High Plains
Authors: Dueker Kenneth G. , Sheehan Anne F.
Published: February 2004

Analysis of mantle discontinuity structure using converted P to S (PaS) phases beneath Colorado from the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) Rocky Mountain Front (RMF) experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations. A stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km, respectively. Imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the Pds conversion points and then stacking the receiver functions in each bin to form spatial images, analogous to common depth point stacking. Corrections for lateral velocity heterogeneity are calculated using the local S wave tomographic model of Lee and Grand [1996] and a constant ∂lnVs/∂lnVP scaling of 1.3. This scaling value is determined from the relative scaling between teleseismic P and S wave travel time residuals measured from the Rocky Mountain Front deployment. Mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography, 30 km of 660 km discontinuity topography, and up to 40 km of transition zone thickness variation. Features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western Colorado and a gradual 10 km dip of the 410 km discontinuity beneath the High Plains. The thickening of the transition zone beneath southwest Colorado is consistent with the presence of the subducted Farallon slab in this region as imaged by Van der Lee and Nolet [1997]. In general, our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies.
Transition from oblique subduction to collision: Earthquakes in the southernmost Ryukyu arc‐Taiwan region
Authors: Kao Honn , Shen Sern‐su Jack , Ma Kuo‐Fong
Published: February 2004

Tectonic characteristics of the region between Taiwan and the southernmost Ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5.5≤mb≤6.6. Five major seismogenic structures can be delineated: the Collision Seismic Zone (CSZ), the Interface Seismic Zone (ISZ), the Wadati‐Benioff Seismic Zone (WBSZ), the Lateral Compression Seismic Zone (LCSZ), and the Okinawa Seismic Zone (OSZ). In the CSZ, located along the east coast of Taiwan and offshore, earthquake focal mechanisms show horizontal P axes distributed in two directions, 287°±10° and 333°±16°, possibly reflecting a strain partition associated with the relative plate convergence between the Eurasia plate and the Philippine Sea plate. The corresponding seismic strain tensor indicates a maximum compressive strain rate of 1.2×10−7 yr−1 along 293° and a comparable extension in vertical direction, presumably resulted from plate collision in the region. The geometry of the ISZ, which is distorted significantly at its westernmost end, can be approximated by a north dipping plane that is gradually pushed northward with increasing dip. The seismogenic portion of the interface spans a short depth range from ∼10 km to ∼35 km. A clear pattern of earthquake slip partition is observed; the average slip vector residual is as large as 35°. Seismic strain patterns within the subducted Philippine Sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at ∼270 km, different from the pattern of strain segmentation observed for the rest of the Ryukyu arc where the northern and southern portions are dominated by downdip extension and compression, respectively. Owing to the large convergence obliquity, the slab is descending at a rate significantly slower near Taiwan than in the southern Ryukyu. Thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab's negative buoyancy. A number of thrust or oblique strike‐slip earthquakes between ISZ and WBSZ show a consistent pattern of lateral compression with P axes oriented roughly parallel to the local strike of the trench‐arc system. They are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere. Shallow normal‐faulting earthquakes show successive rotation of T axes from approximately N‐S in the Okinawa trough to approximately E‐W in northeast Taiwan, possibly as a result of interaction between the extension from the opening of the Okinawa trough and the compression from collision. One normal event (January 18, 1991, mb=5.9) occurred in the Central Range with T axis roughly parallel to the structural trend of Taiwan, implying that the nature of the orogeny in Taiwan has changed from “thin‐skinned” deformation to lithospheric collision involving the whole crust and uppermost mantle.
Estimating shear velocities in the oceanic crust from compliance measurements by two‐dimensional finite difference modeling
Authors: Crawford Wayne C. , Webb Spahr C. , Hildebrand John A.
Published: February 2004

The deep seafloor deforms under the pressure loading of linear ocean surface gravity (water) waves at low frequencies (0.003 to 0.04 Hz). The ratio of seafloor displacement to pressure loading as a function of frequency, known as the seafloor compliance function, depends on the shear velocity structure of the oceanic crust and upper mantle. Compliance measurements are used to estimate oceanic crustal structure, particularly within low shear velocity regions such as sediments, fractured rock, and partial melt. Compliance calculated from laterally homogeneous (one‐dimensional, 1‐D) crustal models shows that a buried low‐velocity zone (LVZ) causes a peak in the compliance function at wavelengths 4 to 6 times longer than the LVZ depth, and that the compliance amplitude depends primarily on the LVZ shear velocity. A new numerical code allows forward modeling of compliance for two‐dimensional oceanic crustal models. The new code demonstrates that the peak in the compliance function directly over a finite width LVZ reaches a maximum value at higher frequency, and is of smaller amplitude, than predicted from 1‐D modeling. The compliance maximum persists outside of the region underlain by the LVZ but diminishes in amplitude and shifts to lower frequencies with increasing distance from the LVZ. The numerical models indicate that significant peaks in the compliance function indicate crustal LVZs, but that multiple compliance measurements are necessary to independently constrain the depth, location, and shear velocity of these features.
Estimating horizontal gradients of tropospheric path delay with a single GPS receiver
Authors: Bar‐Sever Yoaz E. , Kroger Peter M. , Borjesson Jorgen A.
Published: February 2004

We present evidence that modeling troposphere delay gradients in precise Global Positioning System (GPS) geodesy improves the accuracy and precision of the estimated quantities, and that the estimated gradients resemble real atmospheric moisture gradients observed with a water vapor radiometer (WVR). Using a low elevation angle cutoff, combined with a model of the atmospheric delay gradient as a random walk process leads to 19.5% and 15% average improvement in radial and horizontal site position repeatabilities, respectively, relative to a current state‐of‐the‐art estimation strategy that does not model horizontal gradients and imposes high elevation angle cutoff. The agreement between estimated values of zenith wet delay from collocated GPS receivers and WVRs was improved by at least 25%. Merely lowering the elevation angle cutoff improves the repeatability of the radial component of the site's position vector but tends to degrade the repeatability of the horizontal components of the position vector if troposphere gradients are not properly modeled. The estimates of wet delay gradients from a collocated GPS receiver and a WVR at Onsala, Sweden, seem to be correlated over timescales as short as 15 min. The agreement in azimuth between the GPS‐based and the WVR‐based gradients was at the 10° level, for significant gradients. The GPS was found to under‐estimate the magnitude of the gradients by about 60% relative to the WVR‐based gradients. The ability to sense atmospheric moisture gradients from a single GPS receiver increases the useful information content from networks of GPS receivers by providing additional spatial information for weather forecasting applications.
Seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern Sierra Nevada and vicinity, California
Authors: Jones Craig H. , Phinney Robert A.
Published: February 2004

21 well‐distributed teleseisms (30° to 100° distance) were recorded by mixed broad‐band (BB) and short‐period (SP) seismic arrays at Mineral King (MK) and Horseshoe Meadow (HM) in the southern Sierra Nevada and at Darwin Plateau (DP) between the Inyo and Argus ranges. These arrays permit identification and separation of direct P and S arrivals, reflections from topography, scattered energy, and arrivals from different back azimuths (multipath arrivals). P‐to‐S conversions can be identified from beamed BB or SP seismograms. Least squares time domain processing recovered single‐event receiver functions from these beamed seismograms. Converted phases attributable to the Moho are clear and uncomplicated at DP; Ps‐P times of 3.9–4.2 s correspond to crustal thicknesses of 31–33 km, assuming a mean P crustal velocity of 6.2 km/s and a Poisson's ratio (ν) of 0.255. Ps‐P times at HM (eastern Sierra) are 3.9–4.6 s (32–37 km) excepting some unusual seismograms from events at back azimuths of 225°–239°. MK (western Sierra) times for Ps‐P show a strong E‐W asymmetry: 3.9–4.1 s (31–33 km) from the east, and 4.8–5.3 s (39–42 km) from the west. The MK arrivals from the east are multiple and substantially weaker by comparison with HM and DP. These results confirm the absence of a thick crust under the southern High Sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies. At DP the Moho event follows an intracrustal negative polarity event defining the top of an S wave low‐velocity zone. This feature dips west under the Sierran crest at HM but is absent farther west at MK. This feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed Sierra overlies a tectonized lower crust. Sub‐Moho energy is absent under the Basin and Range (DP) but is conspicuous under the High Sierra at positive arrivals ∼7.3 s (MK) and ∼9 s (HM) after the P. These arrivals might be from the base of a low‐velocity, low‐density upper mantle body supporting the Sierra.
Episodic imbricate thrusting and underthrusting: Analog experiments and mechanical analysis applied to the Alaskan Accretionary Wedge
Authors: Gutscher Marc‐André , Kukowski Nina , Malavieille Jacques , Lallemand Serge
Published: February 2004

Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated sequences of long, underthrust sheets. Rapid landward increases in wedge thickness, backthrusting, and uplift of the forearc are observed, suggesting underthrusting beneath the wedge. These features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations. Episodic wedge growth is observed in high basal friction experiments using sand as an analog material. Two phases of an accretionary cycle can be distinguished: frontal accretion of short imbricate thrust slices, alternating with underthrusting of long, undeformed sheets. The phase is shown experimentally to depend upon the surface slope of the wedge. Mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry. Maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations. For a steep frontal slope (at the upper limit of the Mohr‐Coulomb taper stability field) the overburden is too great to permit underthrusting, and failure occurs repeatedly at the wedge front producing short imbricate slices. The wedge grows forward, lowering the surface angle to the minimum critical taper. For a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting, causing frontal erosion and backthrusting, steepening the wedge and thus completing the cycle.
Contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world's oceans
Authors: Dyment J. , Arkani‐Hamed J.
Published: February 2004

Regional studies have shown that remanent magnetization of the Cretaceous quiet zones (CQZs), created during the long period of geomagnetic normal polarity at 118–84 Ma, produce well‐defined magnetic anomalies at satellite altitudes. We investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale. We consider entire oceanic areas because oceanic lithosphere formed during periods other than CQZ, but with a predominant polarity, also have appreciable contributions to the satellite anomalies. The magnetic anomalies of the world's oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map, plate relative motions, and the apparent polar wander path of Africa. Three magnetization models are examined: thermoremanent magnetization confined to extrusive layer 2A, and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km. Although all models lead to rather similar anomaly distributions, the amount of magnetization required suggests the need for deeper sources. All models produce the satellite anomalies associated with the CQZ in the North Atlantic and parts of the Indian and Pacific Oceans, with slight differences in location, depending on the models. Low‐amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the Indian and Pacific Oceans are successfully modeled. A major difference between models and observation is the north‐south elongated model anomalies associated, for instance, with the CQZs in the South Atlantic and the fast spreading East Pacific Rise. These elongated anomalies are systematically absent on the observed map, probably because they were removed by along‐track filtering in the early stages of processing the satellite data. A careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4, 0, 1, and 0.6 A/m for the extrusive basalts, intrusive basalts, gabbros, and peridotites, respectively.
Seismic velocities in the uppermost oceanic crust: Age dependence and the fate of layer 2A
Authors: Carlson R. L.
Published: February 2004

It has been widely accepted that upper crustal seismic velocities gradually increase from low values in the youngest crust to ‘typical’ crustal velocities (−5.2 km s−1) over a period of 30–40 million years, with a corresponding decrease in the observed heat flow deficiency caused by hydrothermal circulation. Some recent seismic studies, however, suggest that the seismic velocities actually rise rapidly, nearly doubling in <10 million years and remain essentially unchanged thereafter. On the basis of a new compilation of published high‐quality seismic survey data, this study finds the following: In crust <1 million years old, seismic velocities are commonly <3 km s−1, while in crust between 1 and 5 million years old, velocities range from 2 to >5 km s−1, and velocities <3 km s−1 are not observed in crust older than 5 Ma. There is no detectable systematic change in the seismic properties of the uppermost oceanic crust after 10 million years. Median and average velocities increase from 2.3 to 4.4 km s−1 by ∼7.5 Ma and remain unchanged thereafter. If layer 2A is an interval capping the oceanic crust in which velocities are significantly lower than velocities that are typical of the underlying rocks (layer 2B, 5.2 km s−1), it follows that layer 2A does not disappear but is a persistent feature of the oceanic crust. These results are consistent with geochemical evidence that most hydrothermal alteration occurs within 3 million years after the formation of the oceanic crust and suggest that there is little correlation between seismic velocities and ongoing hydrothermal circulation.
Major element chemistry of volcanic glasses from the Easter Seamount Chain: Constraints on melting conditions in the plume channel
Authors: Pan Yucheng , Batiza Rodey
Published: February 2004

Lavas from the Easter Seamount Chain (ESC) between Salas y Gomez Island and the Ahu volcanic field are tholeiitic and alkalic basalts showing regular and systematic chemical variations with longitude. With progressive distance eastward from the east rift of the Easter microplate, the lavas become progressively richer in K2O, Na2O, FeO, TiO2, and P2O5, and have higher K2O/TiO2 and lower MgO and CaO. These changes reflect differences in the total extent of shallow fractionation and differences in the conditions under which it occurred. Below the Salas y Gomez ridge, where large isostatically compensated volcanoes lead to locally thicker crust, fractionation took place under higher pressure and/or conditions of higher H2O, compared with lavas of the Easter ridge and the east rift. Differences in K2O, P2O5, and K2O/TiO2 reflect differences in mantle source composition and binary mixing between an enriched plume component and a depleted mid‐ocean ridge basalt (MORB)‐like component, as indicated by Pb isotopic data. Mixing along the ESC apparently occurred in the solid state prior to melting, whereas mixing below the east rift involved fractionated liquids. We also see evidence for differences in the conditions of melting, using oxide abundances corrected for shallow fractionation and mantle heterogeneity. Melting below the Salas y Gomez region seems to be initially deeper and more extensive, with progressively shallower and less extensive melting toward the east rift. If this model is correct, it implies that some alkali basalts may form by larger extents of melting than previously thought on the basis of trace element modeling. Since independent evidence suggests the Easter plume has a modest to large excess temperature compared with ambient MORB mantle, we conclude the plume is under the Salas y Gomez region.
Structure and elasticity of CaO at high pressure
Authors: Karki Bijaya B. , Crain Jason
Published: February 2004

We investigate the structural and elastic properties of CaO up to 140 GPa using the plane wave pseudopotential method within the local density approximation. Free energy comparison suggests that the transition from the low‐pressure sodium chloride (B1) to the high‐pressure cesium chloride (B2) structure in CaO occurs at 58 GPa, which falls within the experimental range of 53–70 GPa. The zero‐pressure values and initial pressure dependences of the calculated athermal elastic moduli compare favorably with the experiments. We find that the elastic properties of the two phases vary differently with pressure. The degree of the anisotropy strongly increases with pressure in the B1 phase (due to softening of c44), whereas it strongly decreases in the B2 phase. However, the sign of anisotropy and the propagation directions of the extremum wave velocities remain unchanged with pressure. Both compressional and shear wave velocities of CaO are substantially lower than the seismic velocities throughout the Earth's lower mantle, suggesting that CaO is unlikely to be present in the mantle in large amounts.
Lithospheric mantle structure beneath the Trans‐Hudson Orogen and the origin of diamondiferous kimberlites
Authors: Bank Carl‐Georg , Bostock Michael G. , Ellis Robert M. , Hajnal Zoltan , VanDecar John C.
Published: February 2004

An array of seismographs was deployed over the central Trans‐Hudson Orogen from July 1991 to January 1992 and from October 1994 to July 1996 with the objective of characterizing subcrustal lithospheric structure in a region of diamondiferous kimberlite occurrence using tomographic imaging techniques. The two‐dimensional array was located in south central Saskatchewan and consisted of 17 stations with an average spacing of 100 km. We obtained relative travel time residuals for 321 teleseismic events and inverted them for subcrustal velocity variations. The ray coverage affords resolution from 60 to 400 km depth. Our results reveal heterogeneities in mantle velocity that deviate by up to ±1.5% from the iasp91 Earth model. The most pronounced low‐velocity anomaly is quasi‐cylindrical, 120 km in diameter and extends to ∼220 km depth. This feature is partly surrounded by a region of high velocity which penetrates to slightly greater depths. Cretaceous diamondiferous kimberlites and high concentrations of kimberlitic minerals in glacial tills occur above or near the rims of the low velocity anomalies. In addition, correlations exist between a long‐wavelength gravity low and the high‐velocity region, as well as between high heat flow and low mantle velocities in the southern portions of the study area. Taken together, these observations are consistent with the interpretation of the imaged anomalies as due to thermomechanical erosion of the lithospheric keel of the Sask craton during the Cretaceous by plume activity or Rayleigh‐Taylor‐like instability within the asthenosphere. The diamondiferous kimberlites are viewed as a direct consequence of this process. Low levels of heterogeneity below 250 km depth are interpreted to be indicative of effective homogenization in a convecting asthenosphere.
Earthward expansion of tail current disruption: Dual‐satellite study
Authors: Ohtani Shin‐ichi
Published: February 2004

The present paper addresses the spatial development of tail current disruption in the near‐Earth tail by making use of magnetic field measurements made by two satellites located close to each other. The paper consists of two parts: a statistical study and a case study of a substorm event that took place on August 30, 1986. In the first part the relative timing of the onsets of tail magnetic field reconfiguration (dipolarization) at the AMPTE/CCE (r < 8.8 RE) and GOES geosynchronous satellites is statistically examined. The result shows that the tail current disruption region expands azimuthally outside geosynchronous orbit. Another, and most unexpected, finding is that the tail current disruption region often expands earthward. The substorm event of August 30, 1986, observed by the AMPTE/CCE and SCATHA satellites provides a good opportunity to examine the earthward expansion of the tail current disruption region. In this event, AMPTE/CCE was located on the earthward side of SCATHA by 0.4 RE and observed the onset of magnetic fluctuations 15 s after the SCATHA onset. The earthward expansion velocity is estimated at 180 km/s, much slower than the propagation velocity of the fast magnetosonic mode, indicating that the observed earthward expansion is not an effect of the wave propagation. The very initial signature observed by AMPTE/CCE in this event is a decrease in the north‐south magnetic component accompanied by the enhancement of the flux of energetic ions having their guiding centers tailward. This result suggests that the tail current is intensified by injected particles at the front of the earthward expanding region of the tail current disruption. This enhanced current may trigger the local disruption of the tail current, making the disruption region expand further earthward.
The Geospace Environment Modeling Grand Challenge: Results from a Global Geospace Circulation Model
Authors: Raeder J. , Berchem J. , Ashour‐Abdalla M.
Published: February 2004

We have used our Global Geospace Circulation Model (GGCM) to simulate two time intervals that were proposed as the Geospace Environment Modeling (GEM) Grand Challenge for modelers to investigate to what extent and accuracy models can predict the ionosphere's response to the solar wind and interplanetary magnetic field. In this paper we present comparisons of our GGCM with the comprehensive experimental study by Lyons [this issue] (which provided synoptic maps of the polar cap electrodynamics and particle precipitation) for the two time intervals, January 27, 1992, 1325–1715 UT and 1730–1930 UT. We find a very good agreement between the potential patterns predicted by our model and those obtained by the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. We also find that the separatrix and cusp locations predicted by our model generally compare well with those obtained from particle precipitation data. The soft electron zone of ionospheric precipitation, as defined by Lyons, lies almost entirely in the region for which our model predicts open field lines. However, the model predicts cross polar cap potential drops that are roughly a factor of 2 larger than those predicted by AMIE.
Pc5 field line resonance frequencies and structure observed by SuperDARN and CANOPUS
Authors: Ziesolleck C. W. S. , Fenrich F. R. , Samson J. C. , McDiarmid D. R.
Published: February 2004

We have analyzed a number of low‐m Pc5 field line resonance events observed simultaneously by one or more of the Super Dual Auroral Radar Network (SuperDARN) radars and the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) magnetometer array. At times there was excellent agreement in the Pc5 signal frequencies observed by radar and magnetometers. For events with good agreement in the measured signal frequencies, there was also reasonable agreement in the observed resonance positions and m numbers. Sometimes, however, the ordering of frequencies by their spectral power did not coincide between the radar and magnetometer data and even the frequencies of the spectral peaks did not always coincide. In these cases, there was also some apparent discrepancy in the observed resonance positions and m numbers. The resonance half power widths observed by the magnetometers were found to be a factor of 2 – 6 wider than those measured with the radar, owing to the spatial integration by ground‐based magnetometers of ionospheric current structures having scale size less than ionospheric height. Although there was general agreement in the direction of the longitudinal propagation, the m numbers obtained from the magnetometer data were found to be somewhat smaller that those calculated from the radar data. We consider and discuss possible reasons for the observed discrepancies.
Magnetic variance analysis for small‐amplitude waves and flux transfer events on a current sheet
Authors: Khrabrov A. V. , Sonnerup B. U. Ö.
Published: February 2004

The properties of the magnetic variance matrix M in the context of remote sensing of two‐dimensional (2‐D) coherent flux transfer events and waves on the magnetopause or other current sheets are investigated. It is known from previous work that, in theory, the smallest eigenvalue of M should be zero and that the corresponding eigenvector should lie in the plane tangent to the unperturbed current layer, forming a certain, theoretically predictable angle, θN, with the invariant direction (or axis) of the event. For a certain class of events, we now demonstrate the following additional properties: (1) The direction of the vector normal to the unperturbed current layer should coincide with either the maximum or the intermediate variance eigenvector, depending on the Mach numbers of the plasma; and (2) the ratio of maximum to intermediate eigenvalue of M is related in a simple manner to the so‐called stretching factor R and to the angle θN, permitting both to be determined without use of the Hubert‐transform fitting technique and the trial‐and‐error approach to finding θN previously employed [Walthour et al., 1993]. Magnetopause events from the spacecraft AMPTE/CCE, AMPTE/IRM, and ISEE are used to illustrate the new results.
Whistler instability enhanced by suprathermal electrons within the Earth's foreshock
Authors: Mace R. L.
Published: February 2004

The electron temperature‐anisotropy driven whistler instability in the presence of a suprathermal power law tail on the electron population, such as is observed in the Earth's electron foreshock, is investigated. The enhancement in energetic electrons significantly accentuates whistler growth at wavenumbers smaller than those expected for a bi‐Maxwellian electron distribution of similar temperature anisotropy and brings the unstable frequencies and wavelengths closer to those expected for the 1‐Hz (in the spacecraft frame) whistler waves observed in the electron foreshock. The presence of a power law tail, furthermore, causes a rapid increase in growth rate with increasing electron temperature anisotropy. This means that even very modest temperature anisotropies, like those that are frequently present in the magnetically connected foreshock, can strongly destabilize parallel whistlers. This instability could be responsible for the nearly field aligned so‐called “1 Hz” whistlers previously observed in the Earth's foreshock, which are more difficult to account for in models of the oblique whistler instability.
Ground‐satellite coherence analysis of Pc3 pulsations
Authors: Kim Khan‐Hyuk , Takahashi Kazue , Anderson Brian J.
Published: February 2004

Although Pc3 magnetic pulsations are commonly observed in the dayside magnetosphere and are believed to propagate into the inner magnetosphere and to the ground, how and where they establish themselves as a regular oscillation are not completely understood. In particular, it is not clear whether the cavity mode plays a significant role in determining the spectral properties of Pc3 pulsations. The mode, in principle, can be identified by multipoint observations of the spatial variation of the amplitude and phase of magnetic field perturbations, with at least one satellite providing evidence of compressional magnetic field oscillation in the magnetosphere. Motivated by these requirements for detection of the cavity mode, we surveyed combined magnetic field data from the Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE CCE) satellite and the Kakioka ground station. We identified an interval on November 4–5, 1984, during which both the satellite (dipole L = 3–6) and ground station (L = 1.25) were on the morningside (near 8‐hour local time) with a small local time separation (within 0.9 hour) and observed Pc3 magnetic pulsations exhibiting a compressional component in space. Unlike previous studies, which used satellites located at L > 6, we observed high coherence in the Pc3 band between the compressional component (bz)in space and the horizontal component (H) on the ground. A cross‐phase analysis of the merged bz and H time series indicates that these components oscillated in antiphase over a range of satellite radial distance. We discuss whether the observations can be taken as evidence of a cavity mode oscillation in the magnetosphere.
SuperDARN‐detected plasma convection vortices and the global plasma convection
Authors: Kustov A. V. , Weimer D. R.
Published: February 2004

Routine Super Dual Auroral Radar Network (SuperDARN) ionospheric plasma convection maps were used to assess the performance of the Weimer 96 model, which predicts a convection pattern for the entire high‐latitude ionosphere for any value of the interplanetary magnetic field (IMF) By and Bz components, the solar wind speed, and the seasonal dipole tilt angle. Reasonable agreement between model predictions and SuperDARN measurements are shown for the most typical IMF orientations on the clock diagram and moderate IMF intensities between 1 nT and 5 nT. Global convection patterns were then derived for the periods when the SuperDARN HF radars detected mesoscale quasi‐stable convection vortices. The earlier finding that convection vortices with clockwise (CW) plasma circulation occur at the dayside ends of large‐scale dusk convection cells (for Bz < 0) was confirmed, but only for 9 events out of 25 events that were studied. We found that vortices can occur at the dayside ends of dawn convection cells. The sense of plasma circulation in these vortices is counterclockwise (CCW). We also found that CW and CCW convection vortices can occur, though rarely, at the night ends of large‐scale convection cells, dusk and dawn, respectively. We found that in 11 vortex events out of all events studied the SuperDARN maps were simply showing plasma circulation around the foci of the global convection cells (Bz > 0 and Bz < 0). For strongly northward IMF, there were located two vortex events for which vortices represented the entire cell in a multicell global plasma convection pattern. These results indicate that different mechanisms for vortex formation should be considered, depending on what kind of vortex has formed.
A collisional kinetic model of the polar wind
Authors: Pierrard V. , Lemaire J.
Published: February 2004

The effects of Coulomb collisions in the transition region between the collision‐dominated regime at low altitudes and the collisionless regime at high altitudes have been taken into account to study the escape of H+ ions in the polar wind. A specialized spectral method is used to solve the steady state Fokker‐Planck equation describing the diffusion and upward bulk motion of H+ ions through a background of O+ ions. The H+ ion velocity distribution function is determined as a function of altitude for realistic initial conditions in the whole velocity space. We emphasize the importance for a correct choice of the boundary conditions at v = 0 to obtain regular mathematical solutions. The results of this kinetic model are compared with results of earlier polar wind models based on different approximations of the plasma transport equations.
Transport of anisotropic interstellar pickup ions on bent flux tubes
Authors: Isenberg Philip A. , Lee Martin A.
Published: February 2004

We present a simple model of anisotropic pickup ion distributions on a flux tube containing both a radial segment and segments oriented perpendicular to the radial direction. The flux tube is carried away from the Sun by a constant speed solar wind. We model the pitch‐angle scattering of these ions by a hemispherical approximation, which assumes that the scattering is instantaneous except through the pitch angle of 90°, where the scattering is finite. The model includes a radially dependent ionization of pickup ions, with the newly created ions concentrated in the sunward hemisphere within the radial segment but distributed equally between the hemispheres in the perpendicular segments. The pickup ions stream along the magnetic field and decelerate in the expanding solar wind, but we assume that the radial segment is short enough that the effects of adiabatic focusing may be neglected. For weak scattering through 90°, the distributions exhibit nonlocal properties due to ion streaming along the field from regions where conditions are different than observed locally. We find that this model can yield a depletion of antisunward ions in the radial segment of the flux tube, such as has been observed, but the streaming of ions into the radial region limits the size of this effect unless the radial segment is very long.
An overview of the early November 1993 geomagnetic storm
Authors: Knipp D. J. , Emery B. A. , Engebretson M. , Li X. , McAllister A. H. , Mukai T. , Kokubun S. , Reeves G. D. , Evans D. , Obara T. , Pi X. , Rosenberg T. , Weatherwax A. , McHarg M. G. , Chun F. , Mosely K. , Codrescu M. , Lanzerotti L. , Rich F. J. , Sharber J. , Wilkinson P.
Published: February 2004

This paper describes the development of a major space storm during November 2–11, 1993. We discuss the history of the contributing high‐speed stream, the powerful combination of solar wind transients and a corotating interaction region which initiated the storm, the high‐speed flow which prolonged the storm and the near‐Earth manifestations of the storm. The 8‐day storm period was unusually long; the result of a high‐speed stream (maximum speed 800 km/s) emanating from a distended coronal hole. Storm onset was accompanied by a compression of the entire dayside magnetopause to within geosynchronous Earth orbit (GEO). For nearly 12 hours the near‐Earth environment was in a state of tumult. A super‐dense plasma sheet was observed at GEO, and severe spacecraft charging was reported. The effects of electrons precipitating into the atmosphere penetrated into the stratosphere. Subauroral electron content varied by 100% and F layer heights oscillated by 200 km. Equatorial plasma irregularities extended in plumes to heights of 1400 km. Later, energetic particle fluxes at GEO recovered and rose by more than an order of magnitude. A satellite anomaly was reported during the interval of high energetic electron flux. Model results indicate an upper atmospheric temperature increase of 200°K within 24 hours of storm onset. Joule heating for the first 24 hours of the storm was more than 3 times that for typical active geomagnetic conditions. We estimate that total global ionospheric heating for the full storm interval was ∼190 PJ, with 30% ofthat generated within 24 hours of storm onset.
Very low frequency waves in the heliosphere: Ulysses observations
Authors: Lin Naiguo , Kellogg P. J. , MacDowall R. J. , Scime E. E. , Balogh A. , Forsyth R. J. , McComas D. J. , Phillips J. L.
Published: February 2004

An overall profile of plasma wave activity in a frequency range of 0.2 to 448 Hz in the solar wind is presented using 6 years of Ulysses data which cover a large range of heliographic latitudes (0° to ±80°) at distances from 1 to 5 AU from the Sun. The spacecraft has continuously observed wave activity with peak power below the local electron cyclotron frequency ƒce. Four distinct types of fluctuation phenomena have been observed: (1) enhanced electromagnetic fluctuations associated with the interplanetary shocks and heliospheric current sheet crossings and other solar wind turbulence; (2) enhanced electromagnetic fluctuations associated with compression regions of high‐speed stream interfaces, which were observed in periods of increasing solar wind velocity; (3) electric fluctuations associated with the expanding solar wind, which were observed in periods of decreasing solar wind velocity; and (4) enhanced electric fluctuations in the high‐latitude fast solar wind plasma. The fourth type of wave activity was observed nearly continuously with the relative power observed peaked near the local ƒce. The first three types of waves were observed in the heliomagnetic streamer belt flows, where the spacecraft frequently encountered enhanced solar wind turbulence, interplanetary shocks, and current sheet crossings. The electromagnetic wave bursts (types 1 and 2) are likely to be whistler mode. The occurrences of apparently electrostatic waves during periods of expanding solar wind are coincident with significant reductions in the electron heat flux intensity. The generation mechanism of these electrostatic waves is still under investigation, but the observations may imply that these waves reduce the intensity of the heat flux through enhanced wave particle scattering associated with a heat flux instability.
Model of the 5.3 μm radiance from NO during the sunlit terrestrial thermosphere
Authors: Sharma R. D. , Dothe H. , Duff J. W.
Published: February 2004

This paper models the fundamental vibration‐rotation band emission from NO around 5.3 μm observed by the interferometer aboard the cryogenic infrared radiance instrumentation for shuttle (CIRRIS 1A) during the sunlit terrestrial thermosphere. The four dominant contributions to the 5.3 μm emission are solar pumping, the inelastic collisions with O of NO(v=0), the reactions of N(2D) with O2, and the reactions of N(4S) with O2. The contribution to the chemiluminescence due to the reaction of N(4S) with O2 is calculated using the energy distribution function (EDF) of these atoms obtained by solving the time dependent Boltzmann equation. The calculated radiance is derived using two model atmospheres: (1) the model atmosphere obtained from the atmospheric ultraviolet radiance integrated code (AURIC) [Strickland et al., 1998] and (2) the model atmosphere obtained from the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) [Roble and Ridley, 1994]. The calculated results reproduce gross features of the CIRRIS 1A observations, and disagreement by a factor of ∼2 in the total band radiance calls for a fine tuning of the model atmospheres and/or the underlying phenomenology. The cooling of the atmosphere at high altitudes due to chemiluminescence from the reaction of N(4S) with O2 is found to be comparable to that due to collisions of NO with O.
The predicted size of cycle 23 based on the inferred three‐cycle quasi‐periodicity of the planetary index Ap
Authors: Ahluwalia H. S.
Published: February 2004

A review of the solar and the geophysical data for the last 6 decades indicates the existence of a three‐solar‐activity‐cycle quasi‐periodicity in them. It is also present in other data sets extending over several centuries. A new cycle may have started after 1996. If true, cycle 23 will be more modest (à la cycle 17) with an annual mean sunspot number count of 119.3 ± 30 at the maximum. This inference is in contrast to the predictions of some of the solar astronomers who are predicting a very active cycle 23 (à la cycle 19).
Cold torus whistlers: An indirect probe of the inner Jovian plasmasphere
Authors: Wang K. , Thorne R. M. , Home R. B. , Kurth W. S.
Published: February 2004

The dominant features of Jovian whistlers observed by Voyager 1 in the inner cold torus around RJ ≈ 5.3 are studied using the technique of ray tracing with an offset tilted dipole. These whistlers are unducted, and the electron density in the inner plasmasphere plays a controlling role in the access of whistlers to the cold torus. The upper frequency cutoff of inner zone whistlers allows a firm limit to be placed on the minimum electron density 3–5 cm−3 at higher latitudes in the inner plasmasphere around L ≈ 5.3, a region where no spacecraft have ever been. Backward ray tracing from the observation location is used to determine the source location at the top of the Jovian atmosphere, and it is shown to be consistent with the latitude of lightning events observed by Voyager 1.
Nature, properties, and origin of low‐frequency waves from an oblique shock to the inner magnetosheath
Authors: Hubert D. , Lacombe C. , Harvey C. C. , Moncuquet M. , Russell C. T. , Thomsen M. F.
Published: February 2004

We analyze the high time‐resolution profiles of the electron density and of the magnetic field and the plasma parameters recorded by ISEE 1 and 2 during a crossing of the Earth's magnetosheath at 1430 LT. Compressive and Alfvén ion cyclotron modes (AIC modes) are identified by comparing the measured magnetic polarization and electron parallel compressibility with the results of calculations in an unstable kinetic linear model. A criterion to discuss the accuracy of the wave vector direction of mirror modes is established; an efficient method to disentangle mirror and AIC modes is presented and applied. From the bow shock to the inner sheath we identify successively (1) compressive modes and AIC modes in the oblique shock, (2) a pure AIC mode region of circularly and elliptically polarized waves in a layer 0.3 RE thick adjacent to the undershoot, (3) a mixed region 2 RE thick where both mirror modes and AIC modes are observed, (4) a pure mirror mode region. The nature of the dominant mode appears to be controlled by the depth in the magnetosheath, more than by the local values of βp and the proton temperature anisotropy Tp⊥/Tp‖. In the outer sheath the unusual identification of a pure Alfvénic region for a large average proton beta βp = 13 and a moderate proton temperature anisotropy could be explained by a relatively low density of α particles. The mirror modes are three‐dimensional structures with their major axis along the magnetic field and with their minor axis nearly perpendicular to the magnetopause surface. We estimate the dimensions of ordered structures observed in the middle of the magnetosheath for a βp around 7 ± 1 and Tp⊥/Tp‖ around 1.5; the minor axis of regular mirror modes is typically between 1300 and 1900 km long; the intermediate dimension is larger than either 2200 or 2700 km, while the major axis is larger than either 2700 or 3400 km. For the first time the measured parallel compressibility of the pure mirror modes is shown to be in relatively good agreement with the linear model predictions for 4 < βp < 11. The absence of AIC modes in the inner sheath suggests that these modes cannot grow or propagate in regions where mirror modes are well developed and that AIC wave energy is not transferred across a large‐amplitude mirror mode region.
Reaction rate of O+ with O2, N2, and NO under highly disturbed auroral conditions
Authors: St.‐Maurice J.‐P. , Laneville P. J.
Published: February 2004

There are no published reaction rates of O+ ions with O2, N2, and NO for ion temperatures in excess of 6000 K in the ionosphere. Yet, there are now documented cases of very strong perpendicular high‐latitude ionospheric electric fields, with resulting ion temperatures well in excess of 10,000 K. We have therefore extended the calculations of O+ reaction rates to fields as high as 250 mV/m, that is, to ion temperatures as high as 30,000 K. In the process of presenting new formulae for the revised rates, we stress that at temperatures less than 6000 K, our new rates are entirely consistent with previously published values. The new rates are presented in order to allow ionospheric modelers to apply their composition calculations to highly disturbed conditions.
An improved magnetic field model for Jupiter's inner magnetosphere using a microsignature of Amalthea
Authors: Randall Bruce A.
Published: February 2004

Observation of a particle absorption microsignature of the Jovian satellite Amalthea during the Pioneer 11 close flyby of Jupiter on December 3, 1974, has been described by McKibben et al. [1983]. The microsignature was also observed by the University of Iowa/Pioneer 11 instrument in the distribution of protons in the kinetic energy range 0.61 < Tp < 3.41 MeV but has not been previously reported. The finer time resolution and superior data quality of the latter observations provide a fresh basis for assessing the accuracy with which various published magnetic field models of Jupiter's magnetic field models describe Jupiter's magnetic field for radial distances of the order of or less than several planetary radii. The expected time of occurrence of the minimum of Amalthea's microsignature was calculated for each model and compared to the time of the observed minimum. The discrepancy between these two times was significant for each of the published models, but it could be reduced to zero by interpolating between two of the closely related models. The spherical harmonic coefficients of this combination model are tabulated. Also included is a novel method for calculating the intensity‐time profile of the microsignature. In a later paper, the improved model is utilized to analyze the energetic particle measurements in Jupiter's innermost magnetosphere by the Galileo entry probe in December 1995 [Fischer et al., 1996].