bluuebunny/crossref_metadata_2025 huggingface dataset:
Coseismic deformation of the Wrights tunnel during the 1906 San Francisco earthquake: A key to understanding 1906 fault slip and 1989 surface ruptures in the southern Santa Cruz Mountains, California
Authors: Prentice Carol S. , Ponti Daniel J.
Published: February 2004

The Wrights tunnel is an abandoned railroad tunnel that crosses the San Andreas fault in the southern Santa Cruz Mountains in the vicinity of the 1989 Loma Prieta earthquake. The tunnel was damaged and deformed during the 1906 San Francisco earthquake and a plot showing postearthquake measurements made in the tunnel is given by Lawson [1908]. The amount of offset shown on this plot (1.5 m) has been used in several studies as being representative of the amount of fault offset along this segment of the San Andreas fault in 1906. Our historical research shows that different observers reported different amounts of fault offset in the tunnel and that the 1.5 m given on the plot is not a surveyed measurement. In addition, the plot of the tunnel has been interpreted in several previous studies as evidence of a broad (1.5 km) zone of faulting beneath Summit Ridge. Our analysis shows that this plot need not indicate a broad zone of deformation. Our historical research and modeling of the tunnel measurements indicate that faulting was confined to a zone less than 400 m wide and that 60–85% of the coseismic slip occurred across a single fault plane. There is no evidence for offset across a second shear zone beneath Summit Ridge in 1906. This implies that surface fractures reported on Summit Ridge in 1906 were not associated with significant deformation of the tunnel, implying that they were shallow, surficial features. By analogy, the very similar fractures that occurred on Summit Ridge in 1989 were also probably the result of shallow gravitational, rather than deep‐seated tectonic, processes. Our modeling also indicates that total coseismic, near‐surface slip across the San Andreas fault zone in the Wrights tunnel in 1906 was at least 1.7–1.8 m.
Tectonics of the Umbria‐Marche‐Romagna Arc (central northern Apennines, Italy): New paleomagnetic constraints
Authors: Speranza Fabio , Sagnotti Leonardo , Mattei Massimo
Published: February 2004

We present the results of a paleomagnetic study carried out on 32 sites from mainly Messinian clayey sediments distributed throughout the external Umbria‐Marche‐Romagna Arc (UMRA). These data, together with published results from coeval sediments, demonstrate that this arc is an orocline in its central northern sector. Bending, not well constrained in time, was due to about 15° clockwise rotations of the central part of this arc and to counterclockwise rotations farther north. In this latter area, post‐Messinian counterclockwise rotations are of the same amplitude as those calculated for some classic Mesozoic paleomagnetic sections in northern Umbria, suggesting a Plio‐Pleistocene age for the rotations reported from the older sequences.
Early sediment diagenesis on the Blake/Bahama Outer Ridge, North Atlantic Ocean, and its effects on sediment magnetism
Authors: Schwartz Martha , Lund Steve P. , Hammond Douglas E. , Schwartz Richard , Wong Kathy
Published: February 2004

Sediment magnetic and geochemical studies of a suite of deep‐sea sediment cores from the Blake/Bahama Outer Ridge (BBOR), North Atlantic Ocean, have identified two current redox boundaries in surficial (Holocene), carbonate‐rich sediments over much of the BBOR. The upper Mn+4/Mn+2 redox boundary is associated with a spike in the concentration of solid‐phase Mn (as MnO2); the lower Fe+3/Fe+2 redox boundary is associated with a spike in the concentration of solid‐phase Fe (as goethite, αFeOOH). Over much of the BBOR, high sediment magnetic intensities occur in surficial, carbonate‐rich sediments associated with these redox boundaries and lower intensities occur in deeper (late Pleistocene) carbonate‐poor sediments. This relationship is opposite to that expected if sediment magnetism simply reflects the clastic (noncarbonate) sediment fraction. The surficial, high sediment magnetic intensities are due primarily to the following two factors: (1) magnetic mineral authigenesis associated with early diagenesis and (2) the presence of abundant <0.1 μm magnetite crystals interpreted to be bacterial magnetosomes. Magnetosomes are almost absent in the late Pleistocene low‐carbonate sediments owing, most likely, to local Pleistocene environmental conditions (high clastic flux, low organic flux) which did not favor their growth. The sediment natural remanent magnetization is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and the presence of abundant bacterial magnetite. If this is typical, bacterial magnetite may be more abundant in nature but less important to sediment paleomagnetic records, than previously thought.
Paleomagnetic data and U‐Pb isotopic age determinations from Coats Land, Antarctica: Implications for late Proterozoic plate reconstructions
Authors: Gose Wulf A. , Helper Mark A. , Connelly James N. , Hutson Fred E. , Dalziel Ian W. D.
Published: February 2004

Paleomagnetic results and isotopic age determinations for granophyre and rhyolite from small, isolated nunataks in southern Coats Land, Antarctica, are used to evaluate late Proterozoic plate reconstructions. U‐Pb zircon dates for the two rock types indicate coeval crystallization at 1112±4 Ma. A concordant 1106±3 Ma titanite date from the granophyre overlaps the crystallization age, implying rapid cooling, and is consistent with field and petrographic evidence of no subsequent penetrative deformation, metamorphism, or hydrothermal disturbance. The mean direction of magnetization of the rhyolite at Littlewood Nunataks is statistically indistinguishable from the mean directions of five sites in the granophyre and crosscutting rhyolite dikes at Bertrab Nunataks. The group mean virtual geomagnetic pole of 22.9°N, 80.3°E (N=6, A95=6.8°) compares favorably with the only other extant Precambrian paleomagnetic poles for the East Antarctic craton, two poles from western Dronning Maud Land. The East Antarctic and Laurentian poles of 1.1 Ga do not coincide after restoration of the continents to a position suggested by the SWEAT hypothesis juxtaposing the Pacific margins of East Antarctica‐Australia and Laurentia, indicating either that the hypothesis is incorrect or that Coats Land and parts of western Dronning Maud Land (herein the CMG province) were not part of the East Antarctic craton at 1.1 Ga. In support of the latter, there is reasonable agreement of the 1.1 Ga CMG poles and approximately coeval poles from the Kalahari craton of West Gondwana when the CMG is restored to a position adjacent the Kalahari craton. Such a reconstruction places the CMG in West Gondwana rather than East Gondwana, as originally implied in Rodinia reconstructions, and is consistent with previously recognized links between the geology of the Kalahari craton and western Dronning Maud Land. It further implies that the CMG did not become part of East Antarctica until latest Precambrian to Cambrian time. A new reconstruction places a partially assembled West Gondwana off the present southeastern margin of Laurentia at 1.1 Ga such that poles of the CMG and Kalahari fall on the Laurentian polar wander path for this time period.
Resistivity cross section through the southern central Andes as inferred from magnetotelluric and geomagnetic deep soundings
Authors: Schwarz Gerhard , Krüger Detlef
Published: February 2004

To investigate the upper lithospheric structure and subduction‐induced processes across an active continental margin, magnetotelluric (MT) and geomagnetic deep sounding (GDS) experiments were done in the southern central Andes (21°–25°S) of northern Chile, southern Bolivia and northwestern Argentina. Using two‐dimensional modeling, we have constructed an E‐W resistivity cross section at latitude 21°45′S that involves all the Andean units from the Pacific coast to the lowland plains. The crust of the Coastal Cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded. Increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted Nazca plate. The electrical resistivity of the ‘normal Andean’ crust seems to be low, having values of only 200 ohm m. A wide zone of extremely high electrical conductivity with an E‐W extent of more than 330 km was detected. This high‐ conductivity zone (HCZ) is found at a depth of about 25 km under the Western Cordillera and has a total conductance of more than 23,000 S. Here, it may be caused by partially melted crust, an interpretation which would correlate with those from gravity and seismic data. The HCZ can be followed in the lower crust eastward below the Altiplano where it has a total conductance of about 15,000 S. In the western part of the Eastern Cordillera, total conductance first increases and then drops abruptly at about longitude 65°10′W. The latter HCZs can be explained in terms of thrust tectonics, where detachment zones may involve fluids as well as increased mineralizations. The lowlands of the sub‐Andean and the Chaco are characterized by a cover of anisotropic low resistivity. The resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m, while the deeper mantle gets unexpectedly much more conductive.
A three‐dimensional gravity inversion applied to São Miguel Island (Azores)
Authors: Camacho A. G. , Montesinos F. G. , Vieira R.
Published: February 2004

Gravimetric studies are becoming more and more widely acknowledged as a useful tool for studying and modeling the distributions of subsurface masses that are associated with volcanic activity. In this paper, new gravimetric data for the volcanic island of São Miguel (Azores) were analyzed and interpreted by a stabilized linear inversion methodology. An inversion model of higher resolution was calculated for the Caldera of Furnas, which has a larger density of data. In order to filter out the noncorrelatable anomalies, least squares prediction was used, resulting in a correlated gravimetric signal model with an accuracy of the order of 0.9 mGal. The gravimetric inversion technique is based on the adjustment of a three‐dimensional (3‐D) model of cubes of unknown density that represents the island's subsurface. The problem of non‐uniqueness is solved by minimization with appropriate covariance matrices of the data (resulting from the least squares prediction) and of the unknowns. We also propose a criterion for choosing a balance between the data fit (which in this case corresponds to residues with rms of the order of 0.6 mGal) and the smoothness of the solution. The global model of the island includes a low‐density zone in a WNW‐ESE direction and a depth of the order of 20 km, associated with the Terceira rift spreading center. The minimums located at a depth of 4 km may be associated with shallow magmatic chambers beneath the main volcanoes of the island. The main high‐density area is related to the Nordeste basaltic shield. With regard to the Caldera Furnas, in addition to the minimum that can be associated with a magmatic chamber, there are other shallow minimums that correspond to eruptive processes.
Compression directions in southern California (from Santa Barbara to Los Angeles Basin) obtained from borehole breakouts
Authors: Wilde Melita , Stock Joann
Published: February 2004

Borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress SH for six areas west of the San Andreas fault in southern California: Santa Barbara, Ojai, Central Ventura Basin, East Ventura Basin, West Los Angeles Basin, and East Los Angeles Basin. Breakouts were determined from analysis of oriented four‐arm caliper data. The breakouts form at the position of the maximum compressive stress on the borehole wall; if the borehole is vertical and parallel to one of the principal stress directions, the breakouts will form parallel to the minimum horizontal principal stress Sh orthogonal to the maximum horizontal principal stress SH. Observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses. In most cases the data permit either a thrust faulting (Sv<Sh<SH) or strike‐slip faulting (Sh<Sv<SH) stress regime with NE to NW directions of SH. These results are broadly consistent with results from focal mechanism studies [Hauksson, 1990; Li, 1996] and with breakout and focal mechanism data present in the world stress map database [Zoback, 1992]. However, we find systematic variations in SH directions suggestive of strong heterogeneity in the stress field at shallow depths, similar to that present in the Cajon Pass borehole [Shamir and Zoback, 1992]. Anomalous NW directions of SH in the San Fernando Valley region and near the Whittier fault may be related to structural complexities and/or lateral ramps in nearby fault systems.
Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps
Authors: Zebker Howard A. , Rosen Paul A. , Hensley Scott
Published: February 2004

Interferogram images derived from repeat‐pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor. Other tropospheric variations, such as pressure and temperature, also induce distortions, but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term. Spatial and temporal changes of 20% in relative humidity lead to 10 cm errors in deformation products, and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries. In wet regions such as Hawaii, these are by far the dominant errors in the Spaceborne Imaging Radar‐C and X Band Synthetic Aperature Radar (SIR‐C/X‐SAR) interferometric products. The unknown time delay from tropospheric distortion is independent of frequency, and thus multiwavelength measurements, such as those commonly used to correct radar altimeter and Global Positioning System (GPS) ionospheric biases, cannot be used to rectify the error. In the topographic case, the errors may be mitigated by choosing interferometric pairs with relatively long baselines, as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline. For the SIR‐C/X‐SAR Hawaii data we found that the best (longest) baseline pair produced a map supporting 100 m contouring, whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval. In the case of deformation map errors the result is either independent of baseline parameters or else very nearly so. Here the only solution is averaging of independent interferograms, so in order to create accurate deformation products in wet regions many multiple passes may be required. Rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are (1) to use the longest radar wavelengths possible, within ionospheric scintillation and Faraday rotation limits, (2) for topography, maximize interferometer baseline within decorrelation limits* and (3) for surface deformation, use multiple observations and average the derived products. Following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions, such as Hawaii.
Mantle discontinuity structure from midpoint stacks of converted P to S waves across the Yellowstone hotspot track
Authors: Dueker Kenneth G. , Sheehan Anne F.
Published: February 2004

Analysis of a deployment of broadband sensors along a 500‐km‐long line crossing the Yellowstone hotspot track (YHT) has provided 423 in‐plane receiver functions with which to image lateral variations in mantle discontinuity structure. Imaging is accomplished by performing the converted wave equivalent of a common midpoint stack, which significantly improves resolution of mantle discontinuity structure with respect to single‐station stacks. Timing corrections are calculated from locally derived tomographic P and S wave velocity images and applied to the Pds (where d is the depth of the conversion) ray set in order to isolate true discontinuity topography. Using the one‐dimensional TNA velocity model and a Vp/Vs ratio of 1.82 to map our Pds times to depth, the average depths of the 410‐ and 660‐km discontinuities are 423 and 664 km, respectively, giving an average transition zone thickness of 241 km. Our most robust observation is provided by comparing the stack of all NW back‐azimuth arrivals versus all SE back‐azimuth arrivals. This shows that the transition zone thickness varies between 261 and 232 km, between the NW and SE portions of our line. More spatially resolved images show that this transition zone thickness variation results from the occurrence of 20–30 km of topography over 200–300 lateral scale lengths on the 410‐ and 660‐km discontinuities. The topography on the 410‐ and 660‐km discontinuities is not correlated either positively or negatively beneath the 600‐km‐long transect, albeit correlation could be present for wavelengths larger than the length of our transect. If this discontinuity topography is controlled exclusively by thermal effects, then uncorrelated 250° lateral temperature variations are required at the 410‐ and 660‐km discontinuities. However, other sources of discontinuity topography such as the effects of garnet‐pyroxene phase transformations, chemical layering, or variations in mantle hydration may contribute. The most obvious correlation between the discontinuity structure and the track of the Yellowstone hotspot is the downward dip of the 410‐km discontinuity from 415 km beneath the NW margin of the YHT to 435 km beneath the easternmost extent of Basin and Range faulting. Assuming this topography is thermally controlled, the warmest mantle resides not beneath the Yellowstone hotspot track, but 150 km to the SE along the easternmost edge of the active Basin and Range faulting.
Numerical simulation of high‐pressure rock tensile fracture experiments: Evidence of an increase in fracture energy with pressure?
Authors: Fialko Yuri A. , Rubin Allan M.
Published: February 2004

High confining pressure fracture tests of Indiana limestone [Abou‐Sayed, 1977] and Iidate granite [Hashida et al., 1993] were simulated using boundary element techniques and a Dugdale‐Barenblatt (tension‐softening) model of the fracture process zone. Our results suggest a substantial (more than a factor of 2) increase in the fracture energy of Indiana limestone when the confining pressure was increased from zero to only 6–7 MPa. While Hashida et al. [1993] concluded that there was no change in the fracture energy of Iidate granite at confining pressures up to 26.5 MPa, we find that data from one series of experiments (“compact‐tension” tests in their terminology) are also consistent with a significant (more than a factor of 2) increase in fracture energy. Data from another set of their experiments (thick‐walled cylinder tests) seem to indicate a decrease in the fracture energy of Iidate granite at confining pressures of 6–8 MPa, but these may be biased due to the very small specimen size. To our knowledge these results are the first reliable indication from laboratory experiments that rock tensile fracture energy varies with confining pressure. Based on these results, some possible mechanisms of pressure sensitive fracture are discussed. We suggest that the inferred increase in fracture energy results from more extensive inelastic deformation near the crack tip that increases the effective critical crack opening displacement. Such deformation might have occurred due to the large deviatoric stress in the vicinity of the crack tip in the Abou‐Sayed experiments, and due to the enlarged region of significant tensile stress near the crack tip in the Hashida et al. compact tension tests. These results also highlight the fact that at confining pressures that exceed the tensile strength of the material, tensile fracture energy will in general depend upon the crack size and the distribution of loads within it, as well as the ambient stress.
Cretaceous and Cenozoic episodic denudation of the Transantarctic Mountains, Antarctica: New constraints from apatite fission track thermochronology in the Scott Glacier region
Authors: Fitzgerald Paul G. , Stump Edmund
Published: February 2004

Apatite fission track thermochronology utilizing vertical sampling profiles, with results interpreted using the concept of exhumed partial annealing zones, is applied in the Scott Glacier area (86°S) of the Transantarctic Mountains (TAM). Patterns in age profiles indicate that episodes of denudation in the Early Cretaceous, Late Cretaceous, and Cenozoic were separated by periods of relative tectonic stability. Thermal modeling of time‐temperature histories compared to observed data indicates that denudation episodes commenced at ∼125 Ma, ∼95 Ma, and 50–45 Ma. Magnitude of denudation is constrained only as >700 m for the Early Cretaceous and from barely detectable to 1.5 km for the Late Cretaceous. Since the early Cenozoic, denudation within the TAM Front was similar in magnitude to other localities along the TAM (∼4–6 km), decreasing inland. Rock uplift was also a maximum at the coast, decreasing inland. Patterns of rock uplift and denudation are complicated by Cenozoic faulting, mostly by faults oriented ∼45° to the TAM Front. Along the length of the TAM there is an apparent systematic variation in the angle of these Cenozoic faults to the TAM Front, possibly reflecting greater components of dextral transtension southward along the TAM. The three denudation episodes correspond to regional tectonic events: Early Cretaceous southward translation of the Ellsworth‐Whitmore Mountains block of West Antarctica relative to East Antarctica; Late Cretaceous extension in the Ross Embayment between East and West Antarctica; and Cenozoic rejuvenated faulting, magmatism, and deformation within the Victoria Land Basin and its presumed southward extension under the Ross Ice Sheet.
Using borehole breakouts to constrain the complete stress tensor: Results from the Sijan Deep Drilling Project and offshore Santa Maria Basin, California
Authors: Zajac Blair J. , Stock Joann M.
Published: February 2004

We use borehole breakouts in nonvertical drill holes to constrain the directions of the three principal stresses and their relative magnitudes. In this paper we start by modifying previously published breakout selection criteria to work with highly deviated borehole data. We present a forward modeling technique using genetic algorithms and a nongradient N‐dimensional optimizer to find the best fitting stress state for a set of breakout data. The stress state is parameterized by three Euler angles and the stress state ratio ϕ. A technique is developed to determine the 95% confidence weighted misfit between a model and the data. We then map out the 95% misfit confidence limits on the best fitting stress state. This technique is applied to data published by Qian and Pedersen [1991]. Removing their constraint of a vertical principal stress direction reduces the misfit between the stress state and the breakout data. We find that the best fitting stress state they report with a vertical principal stress direction lies outside our 95% confidence limits. We also invert breakouts in the offshore Santa Maria Basin, California. These data show a “thrust faulting” stress state with the maximum principal stress, S1, at N148.5°E plunging 31.5°. The 95% confidence range for the azimuth of S1 ranges from N143.0°E to N198.1°E. The stress ratio ϕ was found to be 0.8210.5841.
ARIA II neutral flywheel‐driven field‐aligned currents in the postmidnight sector of the auroral oval: A case study
Authors: Odom C. D. , Larsen M. F. , Christensen A. B. , Anderson P. C. , Hecht J. H. , Brinkman D. G. , Walterscheid R. L. , Lyons L. R. , Pfaff R. , Emery B. A.
Published: February 2004

The ARIA II experiment carried out on February 12, 1994, provided the neutral wind and electron density measurements needed to calculate the neutral‐driven field‐aligned current densities. In situ rocket measurements were made approximately 60 min after the onset of an auroral substorm in a moderately disturbed (Kp=5−) postmidnight auroral oval over Alaska. Two chemical release rockets deployed four widely separated TMA trails, and E region neutral wind profiles were obtained from the motion of those trails. An instrumented rocket launched near‐simultaneously measured the electric fields, electron densities, and neutral composition. The divergence and the vertical component of the vorticity in the neutral flow were obtained from the neutral wind profiles, and those values, together with the measured electron densities, were used to calculate the neutral‐driven field‐aligned current densities. Our best estimate is that the current density below 114 km was effectively zero. Above 114 km the field‐aligned current was downward with a peak value 0.23 μA m−2. Our results indicate that, although the wind‐driven current was a significant contributor to the total field‐aligned current in the recovery phase of the substorm, the dominant sources of the field‐aligned current were due to conductivity gradients and the divergence in the ionospheric electric field. The measurements also show that the current density profile has vertical structure with a scale of ∼ 10 km due to height variations in the vorticity, divergence, and conductivity profiles.
Statistical nature of the magnetotail current in the near‐Earth region
Authors: Nakai H. , Kamide Y. , Russell C. T.
Published: February 2004

ISEE 1 magnetic field data for 1978‐1987 are examined to obtain the average distribution of the magnetotail current in the region . The whole data set is divided into three subsets according to different geomagnetic conditions, that is, AL −50 nT, −200AL−50 nT, and AL−200 nT. Magnetic field values are averaged in each cell with a volume of 1 × 1 × 1 RE3, and the corresponding current densities are calculated from the average tail field. The radial distribution of the neutral sheet current for disturbed periods is characterized by intense currents near geostationary altitudes. In the midnight sector, it reaches as large as 140 mA/m, twice that for quiet periods. It is found that the neutral sheet current is maximized in the region −3Y6 RE, exhibiting clear dawn‐dusk asymmetry. There is no evident indication for the current disruption in the near‐Earth magnetotail, except for a 20 mA/m depression in the current intensity for disturbed periods near the midnight meridian within X−7 RE. Currents parallel to the average magnetic field are also derived. They are regarded as the average field‐aligned currents (FACs) in this paper. It is shown that irrespective of variable geomagnetic activity, large‐scale FACs are distributed in the conventional pattern, that is, region 1 and region 2 currents. The total intensity of FACs in the 2100‐0300 MLT sector is found to increase rapidly with increasing X in the region X−15 RE. This implies that the main source of FACs in the midnight sector is located in the near‐Earth magnetotail. A new model of the magnetotail current system for disturbed periods is proposed on the basis of our findings.
Intensities of anomalous and galactic cosmic rays in the outer heliosphere near the heliospheric equator in 1987 and during the period from 1994 to 1996
Authors: Webber W. R. , Lockwood J. A.
Published: February 2004

Using galactic and anomalous cosmic ray data from the Pioneer 10 and Voyager 1 and 2 spacecraft in the outer heliosphere between ∼42 and 64 AU near the heliospheric equator, we find the following comparisons between the previous intensity maximum in 1987 and the current period in 1994–1996: (1) the radial and latitudinal gradients are much smaller in 1994–1996; (2) the intensity recovery in 1994–1996 is very slow, typically only a few percent per year; and (3) the intensities in 1994–1996 are much lower than those in 1987 by a factor ∼2 for galactic cosmic rays and ∼6 for anomalous cosmic rays. These features all combine to suggest that in 1997 at the probable time of the next sunspot minimum the intensities of both galactic and anomalous cosmic rays will be much lower than they were in the previous cycle in the outer heliosphere at 42 AU and beyond, including the location of a possible termination shock in the range 70–85 AU. We attribute these intensity differences between 11‐year cycles to drift effects near the termination shock. These effects reduce the galactic cosmic ray spectrum near the equator at low energies in 1996 relative to 1987, producing a barrier‐like reduction of intensity across the shock region at this time, while at the same time the intensity just inside the shock near the poles may be much higher in 1996 than in 1987. This effect gives rise to a 22‐year modulation wave that is strongest in the outer heliosphere near the shock but has important effects near the Earth as well.
Plasma depletion layer model for low Alfvén Mach number: Comparison with ISEE observations
Authors: Farrugia C. J. , Erkaev N. V. , Biernat H. K. , Lawrence G. R. , Elphic R. C.
Published: February 2004

Together with the magnetic shear across the magnetopause, the solar wind Alfvén Mach number, MA∞ plays a central role in determining the structure of the magnetosheath. Recent theoretical modeling has shown, in particular, that as MA∞ decreases, the region adjacent to the sunward side of the magnetopause where the interplanetary magnetic field (IMF) exerts a strong influence on the flow (i.e., the so‐called “plasma depletion layer”), is no longer confined to a thin layer ∼ 0.3 Earth radii (RE) thick but occupies an increasingly larger fraction of the magnetosheath. Furthermore, the model predicts the possibility of a plasma depletion layer for low MA∞, irrespective of the size of the magnetic shear at the magnetopause. In this paper we study three examples of low latitude ISEE 2 passes through the dayside magnetosheath near noon: December 3, 1979; October 5, 1979; and November 11, 1979. In all three examples, MA∞ was lower than normal. During the December 3 pass (which we treat, qualitatively), we find evidence of a plasma depletion layer when the IMF was pointing south. On the other two passes (which we study quantitatively), the interplanetary magnetic field was strongly northward pointing, leading to low magnetic shear at the respective magnetopause crossings. The October 5 pass was under steady interplanetary conditions and we find good agreement between theory and data. Temporal variations of the interplanetary medium during the November 11 pass necessitated an extension of the steady state theory to encompass piecewise steady (on average) interplanetary conditions. Better agreement with the data results when the theory is extended further to correct the total pressure at the sunward side of the magnetopause by integrating the magnetic tension term across the layer. For wide plasma depletion layers, this correction can be substantial.
Onset of nonadiabatic particle motion in the near‐Earth magnetotail
Authors: Anderson B. J. , Decker R. B. , Paschalidis N. P. , Sarris T.
Published: February 2004

The onset of nonadiabatic proton motion is studied using direct integration of the Lorentz force equation of motion in the T89c magnetic field model with no electric field. Irreversible changes in the magnetic moment μ occur on traversals of the equator and give the gyrophase dependence predicted by Birmingham [1984]. Birmingham's expression δB and the semiemipirical centrifugal impulse model of Delcourt et al. [1996] δCIM2 nave linear regression coefficients with Δμ/μ of 0.99 and 0.95, respectively, for Δμ/μ ≤ 1. By contrast, ε = 1/κ2, where κ is the kappa parameter, has a linear regression coefficient with Δμ/μ of only 0.5. To reliably estimate the onset of nonadiabatic behavior, one must therefore use δB or δCIM2 rather than κ. Using isocontours of constant δB we map the regions of nonadiabatic ion motion. For a given energy the transition to nonadiabatic motion occurs over a radial distance of ∼2 RE on the nightside and is closest to the Earth at midnight. At midnight the nonadiabatic regime for protons extends inward to ∼11 RE (∼7.5 RE) for 1 keV and to ∼6 RE (∼4.5 RE) for 1 MeV with the Kp = 0 (Kp = 6) model. For O+ the nonadiabatic regime is 1.5 to 2 RE closer to the Earth than for protons. Drift trajectory calculations and analytical estimates show that particles drifting through regions with δB > 0.01 suffer net Δμ ∼ μ. The net Δμ is extremely sensitive to initial gyrophase and it is shown that for δB < 0.01 differences in gyrophase diverge exponentially with repeated equatorial crossings. Because the equatorial gyrophase determines the μ scattering, this implies that the μ scattering is chaotic so that no gyrophase‐averaged invariant exists for the nonadiabatic drift motion. Despite this, the average nonadiabatic drift paths are fairly well defined. The resulting hybrid drift consists of dayside adiabatic and nightside nonadiabatic drift. A single nonadiabatic nightside drift path is associated with a family of adiabatic dayside drift paths. If some of the adiabatic drift paths are open to the magnetopause, all of the particles on the family of hybrid drift trajectories will be subject to loss on a timescale comparable to the drift period. Because the nonadiabatic behavior is due solely to field line curvature, the same behavior will be present with a nonzero convection electric field with the important difference that the lower‐energy particles will be on open convection drift paths. The hybrid drift path‐induced loss effects are therefore most important for higher‐energy particles, > 50 keV, whose adiabatic drift paths are closed in the presence of a convection electric field. The implications of nonadiabatic effects for ring current modeling based on Liouville's theorem apply equally well in the zero and finite electric field cases.
Ion energization mechanisms at 1700 km in the auroral region
Authors: André Mats , Norqvist Patrik , Andersson Laila , Eliasson Lars , Eriksson Anders I. , Blomberg Lars , Erlandson Robert E. , Waldemark Joakim
Published: February 2004

Observations obtained by the Freja satellite at altitudes around 1700 km in the high‐latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations of ions, electrons, plasma densities, electric and magnetic wave fields, and field‐aligned currents are used to study O+ heating mechanisms. Three ion heating events are studied in detail, and 20 events are used in a detailed statistical study. More than 200 events are classified as belonging to one of four major types of ion heating and are ordered as a function of magnetic local time. The most common types of ion heating are associated with broadband low‐frequency electric wave fields occurring at all local times. These waves cover frequencies from below one up to several hundred hertz and correspond to the most intense O+ energization. Heating by these waves at frequencies of the order of the O+ gyrofrequency at 25 Hz seems to be the important energization mechanism, causing O+ ion mean energies up to hundreds of eV. The broadband waves are associated with Alfvén waves with frequencies up to at least a few hertz and with field‐aligned currents. Other types of O+ energization events are less common. During these events the ions are heated by waves near the lower hybrid frequency or near half the proton gyrofrequency. These waves are generated by auroral electrons or in a few cases by precipitating ions.
Auroral activity associated with unsteady magnetospheric erosion: Observations on December 18, 1990
Authors: Sandholt P. E. , Farrugia C. J. , Øieroset M. , Stauning P. , Denig W. F.
Published: February 2004

We report auroral observations made on December 18, 1990, when interplanetary conditions should lead to large‐scale erosion of the dayside magnetosphere during a substorm growth phase. A long interval of strongly northward pointing interplanetary magnetic field (IMF) was succeeded by several hours of strongly southward pointing IMF. The interval of southward pointing IMF was punctuated by a number of IMF directional discontinuities during which the IMF north‐south component, Bz changed polarity abruptly. The auroral responses, monitored at Ny Ålesund (75° magnetic latitude) by meridian scanning photometers and all‐sky cameras, were as follows: The interval of negative IMF Bz was characterized by a net equatorward migration of the equatorward boundary of the dayside cusp/cleft aurora, as expected from previous studies. On this occasion, however, we find that the latitudinal shift occurred in steps which consisted of an initial brightening of individual auroral events at ∼0.5° MLAT equatorward of the preexisting luminosity, followed by a steady poleward retreat lasting typically 4–5 min. The net effect over the first hour of IMF Bz < 0 conditions was to move the equatorward boundary toward the geomagnetic equator by ∼2.7° MLAT. The auroral data suggest that in this instance dayside magnetosphere erosion took place intermittently: bursts of reconnection (initial brightenings) are followed by a switch‐off of the reconnection electric field (subsequent poleward retreat). The bursts of reconnection may be identified with flux transfer events or, equivalently, flux erosion events.
Observation and analysis of selected IMP 8 and WIND bow shock crossings in late 1994
Authors: Huterer D. , Paularena K. I. , Szabo A.
Published: February 2004

Nine bow shock crossings from late 1994 are examined using data from the IMP 8 spacecraft as input to the Rankine‐Hugoniot (RH) conservation equations. In addition, two crossings from the same time period are examined using WIND spacecraft data. A nonlinear chi‐square minimization method is used to obtain the best fit bow shock (BS) normal and speed for each crossing. The shock normals and speeds are also calculated neglecting temperature, as well as using only the magnetic coplanarity and velocity coplanarity techniques. All of the methods generally give mutually consistent results. The orientations of the shock normals are consistent with the nominal bow shock shape. The shocks were expected to move according to a simple “breathing model in which the bow shock moves inward and outward from Earth according to changes in external solar wind conditions. However, the results show outward motion of the bow shock for all but one of all the crossings considered. Moreover, the direction of motion of the BS depends on the exact choice of the shock normal so that a slightly different shock normal can give a different sense of motion of the BS. We conclude that the RH equations that we use may be missing some terms which would provide an accurate description of the balance on both sides of the BS. In particular, the BS acceleration at its flanks is not taken into account by these equations.
Formation of current layers in three‐dimensional, inhomogeneous coronal magnetic fields by photospheric motions
Authors: Inverarity G. W. , Titov V. S.
Published: February 2004

We present a method for calculating analytical expressions for the velocity, magnetic field, electric field, and current density arising from the quasistatic stressing of an initially potential, inhomogeneous coronal magnetic field by photospheric flows. Four exact solutions that are produced by a wide variety of driving flows are examined. We have found that large gradients in the vorticity can lead to the generation of strong current layers, across which the magnetic field is continuous. In two of the exact solutions these appear as logarithmic singularities in the current density and first spatial derivatives of the vorticity. Two other solutions, on the other hand, driven by continuous and differentiable photospheric flows lacking large gradients in the vorticity, exhibit little intensification of the current density. When current layers appear, they are located at the quasi‐separatrix layers (QSL) of the magnetic configuration, where small displacements of the magnetic field footpoints at one part of the photosphere lead to large displacements of the footpoints at the other end of the field line. This extends previous QSL work by providing an additional constraint on the driving flow in order that strong currents are generated at such layers in the course of a quasi‐static evolution of the magnetic field.
Interaction of a nonuniform solar wind with the local interstellar medium: 2. A two‐fluid model
Authors: Pauls H. L. , Zank G. P.
Published: February 2004

Results of a fully three‐dimensional model of the interaction of the solar wind with the local interstellar medium are presented. The effects of charge‐exchange with interstellar neutral hydrogen are taken into account self‐consistently, while the effects of hot solar wind neutral hydrogen, as well as cosmic rays and magnetic fields, are ignored in this study. In accord with solar medium observations by Ulysses, the solar wind is assumed to depend on heliolatitude. Two large, long‐lived polar coronal holes, one in the northern hemisphere and the other in the southern hemisphere, are assumed to produce a hot, low‐density, high‐speed wind which bounds a cooler, higher‐density, low‐speed ecliptic wind. The solar wind boundary conditions, which allow for a 1.5 increase in solar wind ram pressure over the poles of the Sun compared with the ecliptic plane, are drawn from published Ulysses data [Phillips et al., 1995, 1996]. The results of this simulation are compared with the no‐charge‐exchange asymmetric solar wind simulation, described by Pauls and Zank [1996]. It is found that the elongation of the heliosphere along the solar poles, resulting from the ram pressure increase with heliolatitude, induces a greater influx of interstellar hydrogen over the poles of the Sun than in the ecliptic plane. This, in turn, reduces the extent of elongation of the heliosphere along the poles of the Sun. The vorticity, found in the no‐charge‐exchange simulation, is absent in the presence of charge‐exchange. Once again, as found by Baranov and Malama [1993] and Pauls et al. [1995], the interaction of the solar wind with the local interstellar medium is influenced strongly by charge‐exchange processes.
Ion dynamics and resultant velocity space distributions in the course of magnetotail reconnection
Authors: Nakamura M. S. , Fujimoto M. , Maezawa K.
Published: February 2004

We study a large‐scale magnetic reconnection using 2‐D hybrid simulation (ion particle, charge neutralizing massless electron fluid). After formation of fast plasma jets from the diffusion region, a quasi‐steady state is achieved. The outstanding difference in field configuration between the present result and the MHD is an appearance of global cross‐tail magnetic field component with a halflobe field strength. Inside the fast plasma jet, ion distribution functions show various features that can be classified into three types according to the distances from the reconnection line: (1) the modified Speiser distribution observed near the reconnection line, (2) the counterstreaming ion distribution observed around the jet front where the plasma density is compressed and the field lines are piled up due to the collision of the plasma jet with the preexisting plasma sheet, and (3) the partial‐shell distribution observed in the region between the above two. The ion dynamics leading to the formation of these characteristic distributions are fairly well understood with the aid of test particle studies. The results are expected to serve as a guide to identify the reconnection dynamics on the basis of the ion distribution functions.
The October 22, 1989, solar cosmic ray enhancement: An analysis of the anisotropy and spectral characteristics
Authors: Cramp J. L. , Duldig M. L. , Flückiger E. O. , Humble J. E. , Shea M. A. , Smart D. F.
Published: February 2004

A solar cosmic ray ground‐level enhancement was observed at Earth on October 22, 1989, with an extremely anisotropic onset. On the basis of neutron monitor data we have derived the arrival directions, spectra, and pitch angle distributions of the ≥1 GV solar protons. The results indicate an extended time period during which high‐energy particles propagated past Earth with minimal scattering in the local interplanetary medium. The deduced pitch angle distributions of the particles propagating from the Sun show a relatively slow evolution from a focused particle flux, even though there are dramatic changes in the observed intensity‐time profiles. Strong evidence is found that reverse scattering of particles beyond Earth resulted in bidirectional flow along the local interplanetary magnetic field during the second phase of the event.
Entry process of low‐energy electrons into the magnetosphere along open field lines: Polar rain electrons as field line tracers
Authors: Shirai H. , Maezawa K. , Fujimoto M. , Mukai T. , Yamamoto T. , Saito Y. , Kokubun S.
Published: February 2004

The strahl component of solar wind electrons, which constitutes field‐aligned electron heat flux running away from the Sun, is a strong candidate for the origin of the polar rain. We investigate the entry process of the strahl electrons into the distant‐tail magnetosphere and discuss topologies of Earth's field lines in this paper. The Geotail satellite has often observed either gradual or abrupt transitions from the magnetosheath electrons to the bidirectional lobe electrons at the magnetopause. In some cases, the strahl flux of the sheath electrons flowing tailward gradually turned near the magnetopause and finally became the earthward flux of the bidirectional lobe electrons. This transition is accompanied by the rotation of the magnetic field direction and indicates the direct entry of the strahl electrons along open field lines. On the basis of the data of 38 magnetopause crossings by Geotail, we investigate variation of density of the strahl (polar rain) electrons and that of ion density near the magnetopause. It is shown that the strahl electrons decrease upon crossing the magnetopause, and the decrease is correlated with that of ions, although, quantitatively, the strahl electrons do not decrease so much as ions. It is suggested that the strahl electrons enter the magnetosphere along open field lines more freely than ions, but their entry is under the influence of charge neutrality with ions. It remains as a problem why the Geotail data do not show the presence of electrons escaping from the magnetosphere along open field lines.