bluuebunny/crossref_metadata_2025 huggingface dataset:
Healing and sealing of a simulated fault gouge under hydrothermal conditions: Implications for fault healing
Authors: Olsen Martin P. , Scholz Christopher H. , Léger Albert
Published: February 2004

We present results from an experimental program designed to simulate interseismic healing of fault gouge at midcrustal depths. Simulated fault gouge specimens consisting of a quartzo‐feldspathic sand were deformed in triaxial shear at temperatures up to 250°C. Specimens were loaded to steady state sliding and then subjected to a hold period of up to 2 days at a reduced level of axial load. During the hold periods, the aqueous pore fluid was cycled through the gouge layer, allowing for continuous permeability measurements. It was found that the specimens would progressively “seal” during hold periods. Sealing rates were found to be faster at higher temperature and also at earlier stages of the experiments. Subsequent reloading showed that strengthening, or “healing,” had occurred and that the amount of this healing was closely related to the net amount of sealing, rather than to holding time. During reloading, some recovery of permeability occurred, although every hold period resulted in a net loss of permeability. Based on postexperimental pore fluid analysis, observation of the indurated and deformed gouge, and associated work from a concurrent study, we conclude that the sealing was a result of secondary mineral precipitation and that the healing resulted from the cementation of grains by the precipitating minerals. This “precipitation sealing” mechanism is distinct from mechanisms that have been observed to cause healing or “aging” in dry friction experiments. Thus, healing from precipitation sealing is a likely mechanism for explaining the underprediction of laboratory estimates of healing based on dry friction laws relative to healing observed in natural faults. Precipitation sealing also may contribute to maintenance of abnormally high pore fluid pressures within fault gouge zones.
Predictions of Antarctic crustal motions driven by present‐day ice sheet evolution and by isostatic memory of the Last Glacial Maximum
Authors: James Thomas S. , Ivins Erik R.
Published: February 2004

Detectable crustal motion and secular rate of change of solid‐surface gravity may be produced by the Earth's response to present‐day and past ice mass changes in Antarctica. Scenarios of present‐day ice mass balance, previously utilized to explore the global geodetic signatures of the Antarctic ice sheet, produce elastic crustal responses that are typically bounded by uplift rates ≤5 mm/yr, horizontal motion ≤1 mm/yr, and solid‐surface gravity change rates ≤1 μGal/yr. In a restricted locality, one scenario produces uplift rates slightly in excess of 10 mm/yr and correspondingly enhanced horizontal and gravity rates. In contrast, the viscoelastic response to ice mass changes occurring since Last Glacial Maximum (LGM) exceeds 5 mm/yr (uplift) over substantial portions of West Antarctica for a wide range of plausible choices of timing and magnitude of deglaciation and mantle viscosity. Similarly, viscoelastic gravity rate predictions exceed 1 μGal/yr (decrease) over large regions, confirming suggestions that a Global Positioning System (GPS) and absolute gravity‐based program of crustal monitoring in Antarctica could potentially detect postglacial rebound. A published revision to the CLIMAP model of the Antarctic ice sheet at LGM, herein called the D91 model, features a substantially altered West Antarctic ice sheet reconstruction. This revision predicts a spatial pattern of present‐day crustal motion and surface gravity change that diverges strikingly from CLIMAP‐based models. Peak D91 crustal rates, assuming deglaciation begins at 12 kyr and ends at 5 kyr, are around 16 mm/yr (uplift), 2 mm/yr (horizontal), and −2.5 μGal/yr (gravity). Tabulated crustal response predictions for selected Antarctic bedrock sites indicate critical localities in the interior of West Antarctica where expected responses are large and D91 predictions differ from CLIMAP‐based models by a factor of 2 or more. Observations of the postglacial rebound signal in Antarctica might help constrain Antarctic mass balance and contribution to sea level rise over the past 20,000 years.
Remagnetization in lava flows recording pretransitional directions
Authors: Valet Jean‐Pierre , Kidane Tesfaye , Soler Vicente , Brassart Jacques , Courtillot Vincent , Meynadier Laure
Published: February 2004

Basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of La Palma are characterized by overlapping components with very high unblocking temperatures. A medium high‐temperature (MHT) component with normal polarity is resistant up to 500–550°C, while in most cases the high‐temperature (HT) primary reverse component cannot be isolated below 520°C. The same situation (with opposite polarities) is observed also in a lava flow associated with the onset of the upper Réunion reversal in the Gamarri sequence in Ethiopia. The presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling. The direction of MHT is either close or similar to the direction of the HT component of the overlying flows, which indicates that remagnetization occurred shortly after emplacement. Rock magnetic studies, microscopic observations, microprobe analyses and remagnetization experiments have been carried out to investigate the origin of MHT. The rock magnetic parameters do not exhibit specific anomalies. The unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling. The most plausible scenario is that baking by the overlying flows was accompanied by low‐temperature oxidation of titanomagnetite to cation‐deficient titanomagnetite. This description also includes the presence of cracks, fractures and other heterogeneities of the lava which induce variability between parallel profiles. Similarities with characteristics observed at Steens Mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution.
Midcrustal shear zones in postorogenic extension: Example from the northern Tyrrhenian Sea
Authors: Jolivet Laurent , Faccenna Claudio , Goffé Bruno , Mattei Massimo , Rossetti Federico , Brunet Christophe , Storti Fabrizio , Funiciello Renato , Cadet Jean Paul , d'Agostino Nicola , Parra Teddy
Published: February 2004

Metamorphic core complexes of the Aegean region have revealed midcrustal, shallow‐dipping extensional shear zones. These shear zones display constant kinematic indicators over large regions (100–200 km). We analyze the example of the northern Tyrrhenian Sea and then compare it to the Aegean region. We first summarize our observations on ductile extension and metamorphic evolution in the northern Tyrrhenian Sea from Alpine Corsica to Tuscany. (1) Extension migrated from west to east from the early Miocene in Corsica to the Recent in the Apennines; (2) Extension is accommodated by shallow east dipping extensional shear zones at the depth of the brittle‐ductile transition, from the early Miocene to the Pliocene. (3) West dipping normal faults accommodate extension on the eastern side of the volcanic arc. (4) Extension is preceded along the convergence front by the formation of a thrust wedge, where high‐pressure and low‐temperature conditions are recorded; maximum PT conditions decrease toward the east, and PT paths are systematically very cold, suggesting that a large part of the exhumation occurred during synorogenic extension. We discuss the possible mechanisms that account for constant shear sense over large domains. The model involves retreat of the slab and migration of the volcanic arc. Partially molten lower crust acts as a low strength zone where extensional strain is localized. Eastward motion of the upper mantle as a consequence of the migration of the slab induced a component of shear toward the volcanic arc at the base of the stronger upper crust. In the weak upper mantle and lower crust, to the west of the volcanic arc, extensional stresses are not transmitted; this produces a top‐to‐the‐east sense of shear at the base of the upper crust that migrates eastward, following arc migration.
The 1995 Kobe earthquake: Seismic image of the source zone and its implications for the rupture nucleation
Authors: Zhao Dapeng , Negishi Hiroaki
Published: February 2004

In order to understand what may have triggered the 1995 Kobe, Japan, earthquake and how the rupture proceeded after initiation, we determined high‐resolution three‐dimensional (3‐D) P and S wave velocity and Poisson's ratio structures in the Kobe epicentral area and relocated the aftershocks with the obtained 3‐D velocity model. We used 64,337 P and 49,200 S wave high‐quality arrival times from 3634 Kobe aftershocks and local microearthquakes recorded by both permanent seismic networks and portable stations that were set up following the Kobe mainshock. Significant velocity variations of up to 6% are revealed in the aftershock area. We found that areas with high aftershock activity are generally associated with low Poisson's ratio, which may be the strong and competent parts of the fault zone and were apt to generate aftershocks. The Kobe mainshock hypocenter is located in a distinctive zone characterized by low P and S wave velocities and a high Poisson's ratio. This anomaly exists in the depth range of 16 to 21 km and extends 15 to 20 km laterally. This anomaly may be due to a fluid‐filled, fractured rock matrix that contributed to the initiation of the Kobe earthquake. Our interpretation has been supported by many pieces of evidence from hydrological, geochemical, seismological, and geophysical investigations conducted at the Kobe earthquake region.
Equation of state of MgSiO3 perovskite and its thermoelastic properties under lower mantle conditions
Authors: Hama Juichiro , Suito Kaichi
Published: February 2004

A simple four‐parameter model for calculating thermoelastic properties of MgSiO3 perovskite is presented based on the Vinet model for static lattice and the Debye approximation for lattice vibration. The input parameters are the volume of the unit cell, V0, the bulk modulus, K0, its pressure derivative, K0′, and the Debye temperature, Θ0, in the static lattice at zero pressure. For V0, K0 and K0′ the theoretical values by Stixrude and Cohen [1993] are used and Θ0 is determined to reproduce the experimental value at ambient conditions, 980 K, by Akaogi and Ito [1993]. The resulting isobars are in good agreement with experimental data to 1300 K and 11 GPa by Wang et al. [1994], with those to 1200 K at 20 GPa by Utsumi et al. [1995], and with those to 1500 K and to 2000 K, respectively, by Kato et al. [1995] and Funawori et al. [1996] both at 25 GPa. Using the present equation of state together with the method for calculating adiabatic Lamé constants λS and μS for isotropic medium given in the present paper, density ρ, and sound velocities νp and νs of MgSiO3 perovskite under lower mantle conditions have been calculated where the constant‐entropy model is assumed with the temperature at the core‐mantle boundary being taken to be 3000 K. The results for ρ, νp, and νs are in agreement with the preliminary reference Earth model (PREM) within −2.4%∼ −3.7%, +3.3%∼+1.1%, and +0.8%∼−6.8%, respectively, over the lower mantle from 670 to 2891 km in depth. The calculated thermal expansivity under lower mantle conditions is in good agreement with that of the lower mantle estimated by Anderson [1982]. Using the present model with the parameters determined from experimental data at room temperature by Knittle and Jeanloz [1987], assuming Θ0 to be the same as that of MgSiO3 perovskite, thermoelastic properties of (Mg0.9, Fe0.1)SiO3 perovskite under lower mantle conditions have been calculated. The density becomes in much better agreement (+0.4%∼−0.8 %) with PREM and νp and νs remain almost unchanged from those of MgSiO3 perovskite.
New statistics for estimating the bulk rheology of active lava flows: Puu Oo examples
Authors: Baloga S. M. , Glaze L. S. , Crisp J. A. , Stockman S. A.
Published: February 2004

Downstream changes in lava rheology due to cooling, crystallization, and vesiculation have a strong influence on the final length and morphology of a lava flow. Three statistics are proposed to estimate the change in lava rheology with distance along the path of an active flow. These statistics correspond to three separate models of the volumetric flow rate dependence on the thickness of the flow. Each statistic is based on flow dimensions and topographic data that are often available from field measurements or remote sensing. One model assumes an elementary laminar Newtonian flow. A second empirical model often used to describe the flow of complex geologic materials, such as lahars, sediment‐laden floods, and debris flows, is also investigated for comparison. A new volume‐loss model is also proposed to account for stationary components such as levees and stagnant areas. The three statistics derived from these flow rate models are applied and interpreted for two well‐defined lobes from episodes 2 and 18 of the 1983–1984 Puu Oo eruption. The power law dependence of the first two models results only in modest differences in the estimates of rheologic change along the flow path. However, the removal of lava from the active flow to construct stationary components produces significant differences in both the magnitude of computed rheologic changes and the ability to discern trends in rheologic changes along the path of the flow.
Paleomagnetic volcanic data and geometric regularity of reversals and excursions
Authors: Love J. J.
Published: February 2004

Mostly on the basis of paleomagnetic sedimentary data, it has been suggested that maps of virtual geomagnetic poles (VGPs), corresponding to directions of the magnetic field at each site, tend to fall along American and Asian longitudes during reversals and excursions. Such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled. However, studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields. In this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 Myr. Volcanic eruptions occur sporadically, thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions, but we prefer not to use averaging methods of previous investigators, who discarded or combined directions when they appeared to be similar. We find that volcanic data give intermediate VGPs which tend to fall along American and Asian longitudes, roughly consistent with the sedimentary data. This result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites. Provided the appropriate polarity is assigned to intermediate VGPs, we find that Icelandic VGPs tend to fall along Asian longitudes. Other patterns in the data, for example, latitudinal clustering of VGPs or distinguishing longitudinal preferences of excursions from reversals, are not resolved. However, it appears that in general, transitional fields are nondipolar.
Magma chambers modeled as cavities explain the formation of rift zone central volcanoes and their eruption and intrusion statistics
Authors: Gudmundsson Agust
Published: February 2004

The volcanic rift zone in Iceland is characterized by elongate volcanic systems, consisting of tension fractures, normal faults and volcanic fissures, where most of the volcanotectonic activity takes place. Most volcanic systems develop central volcanoes, the formation of which is still poorly understood. There is great difference in the eruption and intrusion statistics of the volcanic systems inside and outside the central volcanoes. In the central volcanoes, eruptions are frequent (typically one every several hundred years), of small volume (normally less than 0.1 km3) and fed mainly by thin (average thickness of 0.5 m), inclined sheets. Outside central volcanoes the eruptions are rare (typically one every several thousand years) but of large volume (typically more than 1 km3) and fed mainly by thick (average thickness of 4–5 m), subvertical dikes. Using the results of a boundary element study of magma chambers modeled as a cavities or holes in a semi‐infinite plate, these empirical relations, as well as the formation of specific central volcanoes in the volcanic systems, can be explained. Once a cavity‐like magma chamber has formed, its existence in the rift zone concentrates tensile stress. This stress concentration causes the segment containing the magma chamber to rupture much more frequently than the other parts of the volcanic system, which partly explains the formation of central volcanoes and their high eruption frequencies. This concentration also gives rise to a local stress field that encourages injection of small‐volume sheets in all directions and at various dips from their source magma chamber. The magma chamber acts as a trap for upward propagating dikes from mantle reservoirs and channels magma, through inclined sheets, toward a limited area at the surface where the central volcano gradually forms. The inverse relationship between eruption frequency and eruption volume, when the central volcano is compared with other parts of the volcanic system, is also partly due to the trap‐like nature of the magma chamber. The chamber is normally much smaller than its source mantle reservoir, so that a single magma flow (through a dike) from the reservoir (lasting perhaps many years) may trigger tens of magma flows (through sheets) from the chamber, many of which would reach the surface in the central volcano.
Transmission of solar radiation in boreal conifer forests: Measurements and models
Authors: Ni Wenge , Li Xiaowen , Woodcock Curtis E. , Roujean Jean‐Louis , Davis Robert E.
Published: February 2004

A combined geometric‐optical and radiative transfer (GORT) model allows incorporation of multiple scales of clustering in conifer canopies on the estimation of radiation transmission. Consideration of clustering of branches into whorls is the latest addition to this model. Modification of the GORT model to include whorl orientation improves the ability to model the observed patterns of solar radiation transmission as a function of solar zenith angle and height in the canopy. Whorl orientation distributions are derived from multidirectional measurements using a geometric optical mutual shadowing model. For BOREAS test stands, model estimates and vertical measurements of photosynthetically active radiation transmittance within the canopy show (1) general decreases in transmission as solar zenith angles increase in the range of solar zenith angles dominated by beam irradiance, (2) increases in PAR transmission at very high solar zenith angles where diffuse skylight is dominant, (3) maximum scattering and absorption occur in the middle of the canopy. Model estimates match measurements from the forest floor, indicating the value of the model for providing radiation inputs to snowmelt models in forested landscapes.
Soil moisture estimation using special satellite microwave/imager satellite data over a grassland region
Authors: Jackson Thomas J.
Published: February 2004

The special sensor microwave/imager (SSM/I) is an instrument that has been a component of several satellite platforms since 1987. Although not designed for soil moisture sensing, it is possible based on theory to extract soil moisture information under some conditions. The limiting feature of the SSM/I for soil moisture–related studies is that the frequencies are quite high and are significantly affected by vegetation. However, other features of the data, such as the frequency of measurements, are very good for observing time‐varying hydrologic variables such as soil moisture. There have been no quantitative evaluations of the SSM/I using observed soil moisture data. In this study, data collected in two large‐scale experiments conducted over the Little Washita watershed, in Oklahoma were available for evaluating the capabilities of SSM/I data for soil moisture mapping. Physically based models were used to relate the satellite data to the ground observations. The results indicated that for this grass‐dominated subhumid area a soil moisture–emissivity relationship with an error of estimate of 5.3% could be developed that incorporated the range of temperature and vegetation conditions encountered. An approach to adapting this approach for other vegetation regimes is still needed for wider application.
Linearity of basin response as a function of scale in a semiarid watershed
Authors: Goodrich David C. , Lane Leonard J. , Shillito Rose M. , Miller Scott N. , Syed Kamran H. , Woolhiser David A.
Published: February 2004

Linearity of basin runoff and peak response as a function of watershed scale was examined for a set of 29 nested semiarid watersheds within the U.S. Department of Agriculture–Agricultural Research Service Walnut Gulch Experimental Watershed, located in southeastern Arizona. Watershed drainage areas range from 1.83 × 103 to 1.48 × 108 m2 (0.183–14800 ha), and all stream channels are ephemeral. Observations of mean annual runoff, database‐derived 2‐ and 100‐year peak runoff rates, ephemeral channel area, and areal rainfall characteristics derived from 304 events were examined to assess the nature of runoff response behavior over this range of watershed scales. Two types of distributed rainfall‐runoff models of differing complexity were applied to a subset of the watersheds to further investigate the scale‐dependent nature of the collected data. Contrary to the conclusions of numerous studies in more humid regions, it was found that watershed runoff response becomes more nonlinear with increasing watershed scale, with a critical transition threshold area occurring roughly around the range of 3.7 × 105 to 6.0 × 105 m2 (37–60 ha). The primary causes of increasingly nonlinear response are the increasing importance of ephemeral channel losses and partial storm area coverage. The modeling results indicate that significant error will result in model estimates of peak runoff rates when rainfall inputs from depth area‐frequency relationships are applied beyond the area of typical storm coverage. For runoff modeling in Walnut Gulch and similar semiarid environments, explicit treatment of channel routing and transmission losses from channel infiltration will be required for watersheds larger than the critical drainage area.
The Geospace Modeling Program Grand Challenge
Authors: Lyons L. R.
Published: February 2004

The first Geospace Modeling Program (GEM) campaign, the boundary layer campaign, focused on the cusp, the dayside magnetopause, and dayside magnetospheric boundary layers. One of the projects pursued during this campaign was a construction of synoptic maps of convective flows and particle regions within the polar ionosphere for different orientations of the interplanetary magnetic field. In 1996, the GEM steering committee recommended that these synoptic maps be used for data‐model comparisons to provide tests of model convection patterns, convection strengths, separatrix locations, boundary layers, and currents. The papers that follow this introductory paper describe results of the data‐model comparisons. This paper summarizes the data analysis approach and the results that were obtained that are important for the comparisons.
Midlatitude particle and electric field effects at the onset of the November 1993 geomagnetic storm
Authors: Foster J. C. , Cummer S. , Inan U. S.
Published: February 2004

Millstone Hill incoherent scatter radar elevation scans across midlatitudes captured the ionospheric response to storm‐induced electric field and precipitation‐induced changes near the equatorward extent of the expanding auroral region during the early phases of the November 3–4, 1993, magnetic storm. Solar wind‐induced magnetospheric compression was observed at ∼2307 UT [Borovsky et al., this issue] and a prompt, short‐duration (10 min) increase in the upward plasma velocity to > 100 m/s at ∼2319 UT on November 3, 1993 signaled the onset of the storm‐induced enhancement of the eastward electric field over Millstone Hill in the premidnight sector at 54° invariant latitude (Λ). This resulted in an uplifting of the F region ionosphere above the site by ∼80 km by 2330 UT. Formation of a narrow ionospheric trough poleward of the Millstone site accompanied the auroral convection enhancement at somewhat later times while plasmasheet precipitation produced the ionization at altitudes between 200 and 300 km at Λ < 60° observed by the radar. Strong precipitation of energetic particles from the outer radiation belt was observed by SAMPEX at 58.5°Λ, near the equatorward limit of the plasmasheet precipitation observed by DMSP and Millstone Hill. Amplitude perturbations of VLF signals propagating in the Earth‐ionosphere waveguide serve to localize the energetic radiation belt precipitation to latitudes between 56°Λ and 58.5°Λ and provide accurate timing of storm‐induced energetic precipitation, whose onset was at ∼2332 UT in the premidnight sector. A later enhancement of the eastward electric field at latitudes equatorward of Millstone Hill and the storm‐induced trough led to a perturbation of the midlatitude ionosphere to A < 40° and is the subject of a companion paper [Foster and Rich, this issue].
Electron temperature in the ambient solar wind: Typical properties and a lower bound at 1 AU
Authors: Newbury J. A. , Russell C. T. , Phillips J. L. , Gary S. P.
Published: February 2004

Our understanding of what controls the solar wind electron temperature is far from complete. Previous studies from the Vela and IMP spacecraft have suggested that twice the proton temperature or an assumed average of ∼ 150,000 K are reasonable estimations of total electron temperature at 1 AU. Eighteen months of continuous ISEE 3 solar wind data are analyzed in this paper and are found to have a mean electron temperature of 141,000 ± 38,000 K, in good agreement with past measurements. No correlation is found between electron temperature and other solar wind parameters, including proton temperature. However, a very distinct lower bound on the electron temperature is found; this bound increases with proton temperature and is observed by both ISEE 3 and Ulysses spacecraft. The bound is also found to vary with bulk solar speed of the solar wind and with distance from the Sun. Solar wind plasma observed following stream interactions are often associated with temperatures near this bound, and enhanced electromagnetic wave activity in the 18–100 Hz range is coincident with intervals where this apparent temperature coupling is observed, suggesting the possible presence of wave‐particle interactions. Possible explanations for the existence of this electron temperature bound are explored, but no definitive answer has been found at this time.
Io's plasma environment during the Galileo flyby: Global three‐dimensional MHD modeling with adaptive mesh refinement
Authors: Combi M. R. , Kabin K. , Gombosi T. I. , DeZeeuw D. L. , Powell K. G.
Published: February 2004

The first results for applying a three‐dimensional multiscale ideal MHD model for the mass‐loaded flow of Jupiter's corotating magnetospheric plasma past Io are presented. The model is able to consider simultaneously physically realistic conditions for ion mass loading, ion‐neutral drag, and intrinsic magnetic field in a full global calculation without imposing artificial dissipation. Io is modeled with an extended neutral atmosphere which loads the corotating plasma torus flow with mass, momentum, and energy. The governing equations are solved using adaptive mesh refinement on an unstructured Cartesian grid using an upwind scheme for MHD. For the work described in this paper we explored a range of models without an intrinsic magnetic field for Io. We compare our results with particle and field measurements made during the December 7, 1995, flyby of Io, as published by the Galileo Orbiter experiment teams. For two extreme cases of lower boundary conditions at Io, our model can quantitatively explain the variation of density along the spacecraft trajectory and can reproduce the general appearance of the variations of magnetic field and ion pressure and temperature. The net fresh ion mass‐loading rates are in the range of ∼300–650 kg s−1, and equivalent charge exchange mass‐loading rates are in the range ∼540–1150 kg s−1 in the vicinity of Io.
The ionosphere and thermosphere at southern midlatitudes during the November 1993 ionospheric storm: A comparison of measurement and modeling
Authors: Richards P. G. , Wilkinson P. J.
Published: February 2004

A network of ionosondes in the Australian‐Japanese region recorded the ionospheric response to a major magnetic storm that occurred near 2400 UT on November 3, 1993, when Kp reached 7. Most stations recorded a positive phase (increase) in NmF2 on November 4 which was well produced by the field line interhemispheric plasma (FLIP) model despite a large relative increase in the model molecular neutral densities. The positive phase appears to be related to a storm‐induced large abrupt uplift of the ionospheric peak height (hmF2) that may have been caused by a neutral wind surge or by an electric field. The storm caused large winds to blow from the pole to the equator, raising the measured hmF2 by 50–100 km over quiet time values. There was a negative phase (decrease) on November 5 at the higher midlatitude stations but not at the low‐latitude stations. The negative phase was not well reproduced by the model. At low latitudes, a short‐period wave‐like structure was seen in hmF2, NmF2, and equivalent winds. The quiet time equivalent winds in the eastern Australian sector show a pronounced semidiurnal tide signature at all latitudes with a phase shift of about 1 hour for every 15 deg increase in latitude. An algorithm which adjusts the atomic to molecular neutral density ratio by changing the neutral temperature and atomic oxygen density was used to help reproduce the observed electron density. The changes to the standard neutral density ratio were small at Hobart except during the stormy period. At Townsville, the needed changes to the neutral density ratio were generally small at day‐time, but large increases were needed at night during quiet periods. The algorithm is unable to uniquely determine whether the changes in neutral density ratio come from changes in temperature or changes in atomic oxygen density. When averaged over the full 14‐day campaign, there was good agreement between the modified and standard model neutral temperatures except at the low‐latitude stations at night. In this study the ionospheric model reproduced the nighttime ionosphere well at the higher latitudes, but the lower the latitude, the more the model underestimated the nighttime density. Because the low‐latitude stations do not have a significant plasmasphere to support the ionosphere, this study provides some evidence that problems in modeling the night‐time ionosphere may not be due to uncertainties in the plasmaspheric ion flux.
Quiet time densities of hot ions at geosynchronous orbit
Authors: Maurice S. , Thomsen M. F. , McComas D. J. , Elphic R. C.
Published: February 2004

Densities of hot (100eV–40keV) ions vary with local time at geosynchronous orbit. Typically, the density is maximum near midnight, falls off toward both terminators, and is minimum near noon. This local time dependence is routinely observed in the Los Alamos geosynchronous plasma data. We use a month of these data (June 1996) and a simple model of particle drifts within the magnetosphere to explore its origins. We first show that the nightside density profile is shaped according to the drift path topology: trapped orbits are depleted, and only particles drifting on open trajectories contribute to this population, particularly for observations taken at times of low Kp. Then, we highlight spectral absorption in dayside fluxes, which occurs when the particles pass through the geocorona. A model of charge exchange accounts for the shape and energy range of the absorption signatures observed at low Kp for varying pitch angles. The density absorption computed versus local time matches the observations. Hence these basic principles of particle motion in the terrestrial magnetosphere readily explain the local time dependence of the hot ion density at geosynchronous orbit.
Conjugacy of isolated auroral arcs and nonconjugate auroral breakups
Authors: Sato Natsuo , Nagaoka Takayuki , Hashimoto Kumiko , Saemundsson Thorsteinn
Published: February 2004

Fine examples of both conjugate and nonconjugate isolated auroral arcs were observed at two geomagnetically conjugate stations near L = 6, Syowa Station in Antarctica and Husafell in Iceland on September 12, 1988. These events exhibited some interesting characteristics. An auroral loop structure that appeared in both hemispheres was ∼2.0 times larger in the north‐south direction at Syowa than at Husafell. This scale difference is greater than expected from the difference in geographic and geomagnetic (IGRF) coordinates between the two points of observation. However, temporal and spatial variations in the loop structures were almost identical in both hemispheres. After the disappearance of the loop structure, closely conjugate auroras were formed. Nonconjugate auroral features appeared again at Syowa on the poleward side, while the equatorward auroras maintained conjugacy. The nonconjugate aurora at Syowa then began to break up, showing fast moving vortex‐like structures (auroral spirals). At this time, all auroral features at Husafell seemed to have their conjugate counterparts in equatorward auroras at Syowa and none exhibited rapid motions. These conjugate auroras at Husafell were gradually extending poleward, while the corresponding features at Syowa were compressed toward the equator and shrinking in size. The onset of auroral breakup was about one minute earlier at Syowa than at Husafell. The nonconjugate auroral features were reflected in corresponding magnetic field variations on the ground. The events summarized above give interesting clues to the development and decay of auroral conjugacy and the question why the beginning of auroral breakup is not simultaneous at conjugate stations. The time lag and nonconjugacy of auroral breakup in conjugate areas suggests that the triggering source of auroral breakup was not located near the equatorial plane in the magnetosphere but most likely in a localized region near the ionosphere in the southern hemisphere. The nonconjugate auroral spirals also suggest the existence of asymmetrical field‐aligned currents.
A survey of small‐scale spatially periodic distortions of auroral forms
Authors: Trondsen T. S. , Cogger L. L.
Published: February 2004

High‐resolution television observations by the University of Calgary portable auroral imager of the small‐scale spatially periodic distortions of auroral forms known as “curls” are presented. During winter 1995 and 1997 field trips to Rabbit Lake, Saskatchewan, Canada, a large amount of video data containing auroral curls was acquired. Analysis of this data set has led to much improved insight into the properties of these features. Observed spatial and temporal characteristics are presented, with results compared to those of earlier surveys wherever applicable. Striking parallels between the characteristics of these ∼1 km scale size curls and the characteristics of the ∼ 100 km scale size auroral “spirals” observed by the Freja UV imager are pointed out and discussed in terms of shear‐driven instabilities. In addition, observations of the interesting new phenomena of “auroral kinks” and “spinning auroral patches” are briefly described.
On the differential ion velocity in the inner solar corona and the observed solar wind ionic charge states
Authors: Ko Yuan‐Kuen , Geiss Johannes , Gloeckler George
Published: February 2004

Theoretical calculations on multifluid solar wind have indicated that the velocities of ions of the same element are not the same in the inner coronal region where the freeze‐in process of these solar wind heavy ions occurs. This may have nonnegligible effect on the inference of the electron temperature in this region from the observed ionic charge states. In this paper, we investigate the effect of differential ion velocities on the inferred freeze‐in temperatures and the coronal electron temperature diagnostics. We then investigate if the ionic charge state data are able to provide constraints on the differential velocities of these ions in their freezing‐in region. We use the ionic charge composition data observed by the SWICS instrument on Ulysses during the south polar pass as an example.
A new reference spectrum for the EUV irradiance of the quiet Sun: 2. Comparisons with observations and previous models
Authors: Warren H. P. , Mariska J. T. , Lean J.
Published: February 2004

In a companion paper we presented a new reference spectrum for the EUV irradiance of the quiet Sun based primarily on intensities calculated from a quiet Sun emission measure distribution and recent compilations of atomic data. The contributions of optically thick emission lines and continua were included empirically. In this paper we present comparisons between this reference spectrum and independently measured irradiance observations, previous quiet Sun reference spectra, and the predictions of empirical EUV irradiance models. These comparisons indicate relatively good agreement among fluxes of emission lines formed in the solar chromosphere and transition region. The fluxes for coronal emission lines in previous quiet Sun reference spectra, however, do not agree with our calculated fluxes or with a recent irradiance observation taken at a low level of solar activity. Coronal emission lines in the Atmospheric Explorer E (AE‐E) quiet Sun reference spectrum SC21REFW have fluxes that are typically smaller than our calculated fluxes by factors of 2 or more. We also identify inconsistencies in the earlier reference spectrum of Heroux and Hinteregger [1978] (F74113), where the fluxes of many coronal emission lines with wavelengths below 250 Å are inconsistent with the fluxes of coronal emission lines at longer wavelengths. The fluxes of EUV continua in the various reference spectra and irradiance observations also differ significantly.
Global energy deposition during the January 1997 magnetic cloud event
Authors: Lu G. , Baker D. N. , McPherron R. L. , Farrugia C. J. , Lummerzheim D. , Ruohoniemi J. M. , Rich F. J. , Evans D. S. , Lepping R. P. , Brittnacher M. , Li X. , Greenwald R. , Sofko G. , Villain J. , Lester M. , Thayer J. , Moretto T. , Milling D. , Troshichev O. , Zaitzev A. , Odintzov V. , Makarov G. , Hayashi K.
Published: February 2004

The passage of an interplanetary magnetic cloud at Earth on January 10–11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about −85 nT. We use a comprehensive set of data collected from space‐borne instruments and from ground‐based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2‐day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the ε parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross‐polar‐cap potential drop, hemisphere‐integrated Joule heating rate, and hemisphere‐integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross‐polar‐cap potential drop for large AE (>1000 nT), but further studies are needed to confirm this.
Ion energy equation for the high‐speed solar wind: Ulysses observations
Authors: Feldman W. C. , Barraclough B. L. , Gosling J. T. , McComas D. J. , Riley P. , Goldstein B. E. , Balogh A.
Published: February 2004

Ulysses data in the high‐speed solar wind that cover a wide range of latitudes centered on the solar poles were studied to test whether a polytrope law can be used to close the ion energy equation. Three approaches were taken. We determined the correlation between proton temperature and density (1) in the free expansion of the high‐speed solar wind between 1.5 and 4.8 AU, (2) in steepened microstreams at high latitudes, and (3) at the edges of the equatorial band of solar wind variability. Strong correlations were observed in all data subsets that are consistent with a single polytrope relation, where our best estimate for γ* is between 1.5 and 1.7. The best fitting relation is
Constraints on Jovian plasma properties from a dispersion analysis of unducted whistlers in the warm Io torus
Authors: Wang K. , Thorne R. M. , Home R. B. , Kurth W. S.
Published: February 2004

The dispersion of unducted lightning‐generated whistlers observed by Voyager 1 in the warm torus around density peaks at L = 5.7 and L = 5.9 are analyzed using the HOTRAY code, which incorporates a newly developed diffusive equilibrium density model for the Io torus. Since the wave propagation characteristics are primarily controlled by electron density, a simplified two‐ion (H+ and O+) model has been used to simulate wave dispersion. The properties of O+ are adjusted to simulate the electron density variation at low latitudes (≤20°), where heavy ions dominate, and a variable H+ component is added to model the electron density at higher latitudes. Both the offset and tilt of the Jovian magnetic dipole are taken into account to determine the electron distribution as a function of System III longitude. The results confirm earlier suggestions that modest therml anisotropies (T⊥ > T‖) of heavy ions are required to match the observed whistler dispersion. Proton concentrations typically lie in the range 5–10%, with larger values in the outer torus. On the basis of these optimum plasma parameters, the observed upper cutoff frequencies (∼ 6 kHz) imply a minimum electron density of about 8 cm−3 at high latitudes along field lines that map into the warm torus. This analysis of unducted whistlers indicates that all observed waves originate in the northern hemisphere rather than the southern hemisphere, as assumed in earlier studies of ducted waves. This new result is consistent with optical lightning events, which were only observed in the northern hemisphere by Voyager 1.
Localized activation of the distant tail neutral line just prior to substorm onsets
Authors: Watanabe Masakazu , Pinnock Michael , Rodger Alan S. , Sato Natsuo , Yamagishi Hisao , Yukimatu A. Sessai , Greenwald Raymond A. , Villain Jean‐Paul , Hairston Marc R.
Published: February 2004

We have found flow burst features in the nightside ionosphere that are thought to be the ionospheric signature of distant tail reconnection. These are observed to form just prior to substorm onsets. Simultaneous observations by the Goose Bay‐Stokkseyri dual HF radars and DMSP satellites provide the data. Our conclusions are based on equatorward flow bursts on the nightside during two isolated substorms that followed a long period of magnetospheric inactivity associated with a northward interplanetary magnetic field. Both flow bursts start ∼60 min after the growth phase onset and last ∼10–20 min until the expansion phase onset, migrating equatorward with time. Simultaneous DMSP observations of precipitating particles show that the flow burst occurs at the polar cap boundary, suggesting that the equatorward migration corresponds to the expansion of the polar cap during the growth phase. For one event, the reconnection electric field at 400 km altitude was 14 mV/m and its longitudinal scale was 290 km, which is equivalent to a reconnection voltage of 4.1 kV. For the other event, these values were 11 mV/m (reconnection electric field), 380 km (longitudinal scale), and 4.0 kV (reconnection voltage). In addition to the reconnection signatures, we discuss implications for substorm dynamics during the final stage of the substorm growth phase. The morphology indicates that the distant tail neutral line is activated ∼1 hour after the growth phase onset and at the same time the nightside separatrix starts to move equatorward much faster than during the preceding early and middle growth phases. The 1‐hour time lag would correspond to the timescale on which slow rarefaction waves from both northern and southern tail lobes converge in the equatorial magnetotail. The fast‐moving separatrix on the nightside implies a rapid change of magnetotail configuration resulting from nonlinear enhancement and/or earthward movement of the cross‐tail current for the last 10–20 min prior to the expansion phase onset.
Magnetopause location under extreme solar wind conditions
Authors: Shue J.‐H. , Song P. , Russell C. T. , Steinberg J. T. , Chao J. K. , Zastenker G. , Vaisberg O. L. , Kokubun S. , Singer H. J. , Detman T. R. , Kawano H.
Published: February 2004

During the solar wind dynamic pressure enhancement, around 0200 UT on January 11, 1997, at the end of the January 6–11 magnetic cloud event, the magnetopause was pushed inside geosynchronous orbit. The LANL 1994–084 and GMS 4 geosynchronous satellites crossed the magnetopause and moved into the magnetosheath. Also, the Geotail satellite was in the magnetosheath while the Interball 1 satellite observed magnetopause crossings. This event provides an excellent opportunity to test and validate the prediction capabilities and accuracy of existing models of the magnetopause location for producing space weather forecasts. In this paper, we compare predictions of two models: the Petrinec and Russell [1996] model and the Shue et al. [1997] model. These two models correctly predict the magnetopause crossings on the dayside; however, there are some differences in the predictions along the flank. The Shue et al. [1997] model correctly predicts the Geotail magnetopause crossings and partially predicts the Interball 1 crossings. The Petrinec and Russell [1996] model correctly predicts the Interball 1 crossings and is partially consistent with the Geotail observations. We further found that some of the inaccuracy in Shue et al.'s predictions is due to the inappropriate linear extrapolation from the parameter range for average solar wind conditions to that for extreme conditions. To improve predictions under extreme solar wind conditions, we introduce a nonlinear dependence of the parameters on the solar wind conditions to represent the saturation effects of the solar wind dynamic pressure on the flaring of the magnetopause and saturation effects of the interplanetary magnetic field Bz on the subsolar standoff distance. These changes lead to a better agreement with the Interball 1 observations for this event.