bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure of the dayside magnetopause for low magnetic shear
Authors: Paschmann G. , Baumjohann W. , Sckopke N. , Phan T. ‐D. , Lühr H.
Published: February 2008

We have analyzed 22 AMPTE/IRM satellite passes through the low‐latitude magnetopause region for which the magnetic shear, i.e., the field rotation angle on transit from the magnetosheath to the magnetosphere, was less than 30°. We found that on all passes a key time could be identified where the proton temperature, and usually also in the electron temperature and the temperature anisotropies, show net and often discontinuous changes. A change in plasma flow direction also occurs at this time. The small field rotations that occur have no fixed relationship to the key time. Earthward of the key time, the plasma has the characteristics of the magnetopause boundary layer, i.e., reduced density and bulk velocity, and in particular an electron temperature anisotropy with Te∥ > Te⊥, until further and usually more dramatic changes in plasma thermal properties mark the entry into the magnetosphere proper. Boundary layer durations varied widely, from 4s to 14 min. On the magnetosheath side we observed a layer of plasma density depletion (by a factor of two or more) and concurrent magnetic field pile‐up in less than half the crossings. This depletion layer was 3 min wide on average and often characterized by a drop in Tp∥, as predicted. From the observational evidence we conclude that the changes in plasma thermal and flow properties at the key time mark the crossing of the magnetopause under conditions of low magnetic shear. It is tempting to attribute these changes to the crossing of a topological boundary, e.g., a transition from open interplanetary to closed geomagnetic field lines. There is evidence, however, that in some cases the field lines immediately earthward of the key time are not closed. The consistent presence of a boundary layer inside the low‐shear magnetopause confirms earlier inferences that solar wind plasma can enter the magnetosphere regardless of field orientation. Inspection of the velocity distribution functions indicates that the plasma is heated upon entry. Taking measured plasma velocities along the magnetopause normal direction, we have inferred an average magnetopause speed of 11 km/s, an indication that the low‐shear magnetopause moves more slowly. The duration of the temperature transition translates into a thickness of often less than 50 km, comparable with the typical proton gyroradius. For the plasma depletion layer and the boundary layer we obtained average thicknesses of 0.4 and 0.3 RE, respectively.
Observational test of hot flow anomaly formation by the interaction of a magnetic discontinuity with the bow shock
Authors: Thomsen M. F. , Thomas V. A. , Winske D. , Gosling J. T. , Farris M. H. , Russell C. T.
Published: February 2008

The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma. Recent single‐particle and two‐dimensional hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock‐reflected ions into it. We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature. The principal difficulty with the test is the determination of the normals to the discontinuities. Application of a minimum variance analysis produces discontinuity normals which suggest that the majority of the discontinuities were probably tangential, even though it is doubtful that the conditions for validity are very well satisfied for these events. Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events). Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field: The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements. Two‐dimensional hybrid simulations appear to confirm the association between the magnetic field signature and the electric field orientations, and they show further that rotational discontinuities appear to be less effective at producing distinct HFA‐like signatures than tangential discontinuities.
Implications of the altitude of transient 630‐nm dayside auroral emissions
Authors: Lockwood M. , Carlson H. C. , Sandholt P. E.
Published: February 2008

The altitude from which transient 630‐nm (“red line”) light is emitted in transient dayside auroral breakup events is discussed. Theoretically, the emissions should normally originate from approximately 250 to 550 km. Because the luminosity in dayside breakup events moves in a way that is consistent with newly opened field lines, they have been interpreted as the ionospheric signatures of transient reconnection at the dayside magnetopause. For this model the importance of these events for convection can be assessed from the rate of change of their area. The area derived from analysis of images from an all‐sky camera and meridian scans from a photometer, however, depends on the square of the assumed emission altitude. From field line mapping, it is shown for both a westward and an eastward moving event, that the main 557.7‐nm emission comes from the edge of the 630 nm transient, where a flux transfer event model would place the upward field‐aligned current (on the poleward and equatorward edge, respectively). The observing geometry for the two cases presented is such that this is true, irrespective of the 630‐nm emission altitude. From comparisons with the European incoherent scatter radar data for the westward (interplanetary magnetic field By > 0) event on January 12, 1988, the 630‐nm emission appears to emanate from an altitude of 250 km, and to be accompanied by some 557.7‐nm “green‐line” emission. However, for a large, eastward moving event observed on January 9, 1989, there is evidence that the emission altitude is considerably greater and, in this case, the only 557.7‐nm emission is that on the equatorward edge of the event, consistent with a higher altitude 630‐nm excitation source. Assuming an emission altitude of 250 km for this event yields a reconnection voltage of >50 kV during the reconnection burst but a contribution to the convection voltage of >15 kV. However, from the motion of the event we infer that the luminosity peaks at an altitude in the range of 400 and 500 km, and for the top of this range the reconnection and average convection voltages would be increased to >200 kV and >60 kV, respectively. (These are all minimum estimates because the event extends in longitude beyond the field‐of‐view of the camera). Hence the higher‐emission altitude has a highly significant implication, namely that the reconnection bursts which cause the dayside breakup events could explain most of the voltage placed across the magnetosphere and polar cap by the solar wind flow. Analysis of the plasma density and temperatures during the event on January 9, 1989, predicts the required thermal excitation of significant 630‐nm intensities at altitudes of 400‐500 km.
F region plasma drifts over Arecibo: Solar cycle, seasonal, and magnetic activity effects
Authors: Fejer B. G.
Published: February 2008

We have used Arecibo incoherent scatter measurements from 1981 to 1990 to determine the characteristics of low‐latitude F region plasma drifts. The measurements show large day‐to‐day variability even during magnetically quiet periods. The average poleward/perpendicular plasma drifts do not change significantly with season and solar cycle except in the midnight‐morning sector. The zonal drifts show clear solar cycle and seasonal effects. The afternoon‐nighttime eastward drifts increase with solar flux; the westward drifts in the early morning‐afternoon sector show a large increase from summer to winter but are independent of solar activity. The two perpendicular velocity components also respond differently to magnetic activity. The average northward/perpendicular drifts decrease with magnetic activity during the day but do not exhibit a systematic response at night. The zonal component shows increased westward drifts occurring predominantly at night. The plasma drifts along the magnetic field lines exhibit large altitudinal and seasonal variations, particularly near solar minimum, and are generally anticorrelated with the perpendicular/north drifts. The drift patterns observed by the Arecibo and the middle and upper atmosphere radars have significantly different seasonal dependence. This can be explained by electrodynamic effects in the corresponding local and conjugate ionospheres. The large longitudinal variation of the quiet time F region plasma drifts results from the displacement between the geographic and dip equators and from magnetic field declination effects. In general, the longitudinal variation should also depend on magnetic activity.
A global magnetohydrodynamic simulation of the magnetosphere when the interplanetary magnetic field is southward: The onset of magnetotail reconnection
Authors: Walker Raymond J. , Ogino Tatsuki , Raeder Joachim , Ashour‐Abdalla Maha
Published: February 2008

We have used a new high‐resolution global magnetohydrodynamic simulation model to investigate the onset of reconnection in the magnetotail during intervals with southward interplanetary magnetic field (IMF). After the southward IMF reaches the dayside magnetopause reconnection begins and magnetic flux is convected into the tail lobes. After about 35 min, reconnection begins within the plasma sheet near midnight at x = −14RE. Later the x line moves toward dawn and dusk. The reconnection occurs just tailward of the region where the tail attaches onto the dipole‐dominated inner magnetosphere. The simulation shows that prior to the onset of reconnection, the Poynting flux is concentrated in this region. The time required for the start of reconnection depends on the component of the magnetic field normal to the equator (BZ). Reconnection occurs only after the BZ component has been reduced sufficiently for the tearing mode to grow. Later, when all the plasma sheet field lines have reconnected, a plasmoid moves down the tail.
Simultaneous observations of solar MeV particles in a magnetic cloud and in the Earth's northern tail lobe: Implications for the global field line topology of magnetic clouds and for the entry of solar particles into the magnetosphere during cloud passage
Authors: Farrugia C. J. , Richardson I. G. , Burlaga L. F. , Lepping R. P. , Osherovich V. A.
Published: February 2008

We present simultaneous observations of magnetic fields and flow anisotropies of solar energetic (several MeV) protons and electrons made by the ISEE 3 and IMP 8 spacecraft during the passage of an interplanetary magnetic cloud on February 16 ‐ 17, 1980. ISEE 3 was situated ∼250 RE upstream of Earth at the Ll libration point, and IMP 8 was crossing the Earth's northern tail lobe and magnetosheath in an dusk‐dawn direction at a mean downtail distance of ∼27 RE. Several solar particle onsets were seen at ISEE 3 during cloud passage, with corresponding energetic particle signatures being observed in the geomagnetic tail by IMP 8. Various differences in the particle signatures at the two spacecraft occur, however, and these may be interpreted in terms of the magnetic field line topology of the cloud, the connectivity of the cloud field lines to the solar surface, and the interconnection between the magnetic fields of the magnetic cloud and of the Earth. The observations are consistent with a model of magnetic clouds according to which these mesoscale configurations are curved magnetic flux ropes attached at both "ends" to the Sun's surface and extending to 1 AU. The energetic particles stream out of the Sun along the cloud's helical field lines, arrive with appropriate time delays at an observation site inside the cloud, and become counterstreaming after mirroring within the cloud. When the field in the magnetic cloud had a persistent southward component, we can directly confirm the interconnection between the terrestrial and the cloud magnetic fields at the location of IMP 8 in the magnetotail. While at ISEE 3 inside the magnetic cloud the energetic particles have a strong east‐west bidirectional flow, IMP 8 observes a tail‐aligned bidirectional flow (i.e., roughly orthogonal to the flow direction at ISEE 3) as the energetic particles travel earthward along lobe field lines closer to Earth. Thus we have a case of solar particles gaining access into the magnetosphere at low altitudes first through magnetic connection of the magnetic cloud to the Sun and, subsequently, via the interconnection of the cloud's field lines to those of the Earth.
Termination shock: Solar cycle variations of location and speed
Authors: Whang Y. C. , Burlaga L. F.
Published: February 2008

Plasma and magnetic field data from Voyagers 1 and 2 over an 11‐year period (1978‐1988) together with a one‐fluid MHD model are used to study the motion of the termination shock. The observed large fluctuations in solar wind parameters cause variations in the speed, in the jump conditions, and in the location of the termination shock. If the interstellar stagnation pressure is equivalent to a magnetic field of 0.5 nT plus draping enhancements, the termination shock moves between 88 AU and 102 AU. The location of the termination shock is anticorrelated with the sunspot number. The solar cycle variation appears to be the basic variation for the position of the termination shock. In the declining phase of the solar cycle, the solar wind has relatively large momentum and the shock moves outward. Near solar minimum the shock reaches its maximum distance. In the rising phase of the solar cycle, the shock moves inward as the shock speed is negative. Due to uncertainty of the interstellar conditions it is difficult to predict accurately the position of the termination shock. However, the anticorrelation between the shock distance and the sunspot number remains unchanged independent of the average distance of the termination shock. The termination shock is very strong with pressure ratios of a few thousands, the fast Mach number is ∼12, and the plasma β ratio jumps from ∼0.1 to ∼14. During the declining phase of the solar cycle, the shock speed is often greater than or comparable to the spacecraft speed. If the crossing of the termination shock by a spacecraft occurs during the declining phase, the spacecraft will cross the termination shock 3 or more times over a period of about 4 years.
Large‐scale distant heliospheric magnetic field: Voyager 1 and 2 observations from 1986 through 1989
Authors: Burlaga L. F. , Ness N. F.
Published: February 2008

We analyze the magnetic field observed by Voyagers 1 and 2 (V1 and V2) as solar activity increased from near minimum in 1986 to near maximum at the end of 1989. The radial distance of V1 and V2 increased from 25.4 to 39.9 AU and from 18.9 to 30.2 AU, respectively. The elevation angle of the magnetic field observed by both V1 and V2 for each of the years was δ = 0° ± 4.5°. The azimuthal angle λ was consistent with Parker's spiral angle when the solar activity was low but not during 1988 and 1989 when there were many transient ejecta. V1 observed predominantly a single negative polarity from 1986 through 1988, when its latitude was in the range 27.8°‐30.8°N, because the heliospheric current sheet (HCS) was below V1. V1 observed both polarities during 1989, when the maximum latitude of the HCS was far above V1. During 1988 and 1989, V2 observed sectors but no sector pattern, even though an extrapolation of the neutral line near the Sun would imply a two‐sector pattern. This indicates a significant radial evolution of the sector structure and the HCS. We suggest that the numerous transient ejecta perturb the HCS and the radial positions of the sector boundaries randomly when the Sun is active. There was no relation among the sector boundaries, sectors, and interaction regions at either V1 or V2. This implies a considerable evolution of the relation between interaction regions and sector boundaries with increasing distance from the Sun.
Configurations of the solar wind flow and magnetic field around the planets with no magnetic field: Calculation by a new MHD simulation scheme
Authors: Tanaka T.
Published: February 2008

A new MHD simulation scheme is developed on an unstructured grid system using the finite volume total variation diminishing scheme and applied to the problem of solar wind‐planet interaction, assuming a perfect‐conducting ionosphere around the planet. It is shown that the scheme presented here enables one to calculate effectively the configuration of three‐dimensional MHD flow near planets, together with the draping process of magnetic field. Results of the calculation give a general confirmation for the formations of the bow shock, the magnetic barrier, the magnetosheath, the magnetopause, the wake region, the plasma sheet, and the magnetotail. Because of the J × B forces, flow near the planet is accelerated in the polar regions and decelerated in the equatorial plane in the solar wind velocity magnetic field (VB) coordinate system. In the tail region, the J × B forces act to accelerate the flow in the midnight meridian plane and suppress the formation of the trailing shock. However, the present calculation gives a weaker magnitude for the tail magnetic field than that given by the observations and suggests the role of some additional mechanisms for the pileup of the magnetic field in the tail.
The spin of cosmic dust: rotational bursting of circumsolar dust in the F corona
Authors: Misconi Nebil Y.
Published: February 2008

This paper will list, investigate, and evaluate the various spin mechanisms for cosmic dust particles that have been suggested in the literature. This evaluation will concentrate on the possibility of particle fragmentation (rotational bursting) of cosmic dust. The importance of particle fragmentation is manifested by the fact that dust particle size distribution will be changed. More importantly, repeated fragmentation of interplanetary dust particles will give rise to some of the observed “β meteoroids.” Some of these particles are driven out of the solar system and into interstellar space by solar radiation pressure. If this mass loss mechanism is taking place in stellar systems, then it becomes a source for interstellar dust. This paper also addresses the possibility that massive rotational bursting by circumsolar dust particles is taking place in the F corona region. This rotational bursting of F coronal dust could be taking place because of the classical “Paddack effect,” along with a similar spin mechanism suggested by the author, and caused by coronal mass ejections.
The solar flare myth
Authors: Gosling J. T.
Published: February 2008

Many years of research have demonstrated that large, nonrecurrent geomagnetic storms, shock wave disturbances in the solar wind, and energetic particle events in interplanetary space often occur in close association with large solar flares. This result has led to a paradigm of cause and effect ‐ that large solar flares are the fundamental cause of these events in the near‐Earth space environment. This paradigm, which I call “the solar flare myth,” dominates the popular perception of the relationship between solar activity and interplanetary and geomagnetic events and has provided much of the pragmatic rationale for the study of the solar flare phenomenon. Yet there is good evidence that this paradigm is wrong and that flares do not generally play a central role in producing major transient disturbances in the near‐Earth space environment. In this paper I outline a different paradigm of cause and effect that removes solar flares from their central position in the chain of events leading from the Sun to near‐Earth space. Instead, this central role is given to events known as coronal mass ejections.
In situ diagnostics of ionospheric plasma with the resonance cone technique
Authors: Rohde V. , Piel A. , Thiemann H. , Oyama K. I.
Published: February 2008

Electron density and temperature profiles in the mid latitude ionosphere are derived from the ”resonance cone” in the radiation pattern of high‐frequency point antennas aboard a sounding rocket. By comparing the shapes for reversed wave propagation direction it is possible to study electron drift motion and field‐aligned beams. The data from the COREX‐I (Cooperative Resonance Cone Experiment) experiment show close agreement of electron density and temperature profiles from resonance cones with results from independent instruments. There was no indication of a substantial temperature anomaly atEregion altitude during this flight. An unexpected electron drift can be interpreted as electron Hall current in the presheath of the negatively charged payload. Field‐aligned beams, which have been suggested in connection with the temperature anomaly, were not detected.
Recent solar extreme ultraviolet irradiance observations and modeling: A review
Authors: Tobiska W. Kent
Published: February 2008

For more than 90 years, solar extreme ultraviolet (EUV) irradiance modeling has progressed from empirical blackbody radiation formulations, through fudge factors, to typically measured irradiances and reference spectra as well as time‐dependent empirical models representing continua and line emissions. A summary of recent EUV measurements by five rockets and three satellites during the 1980s is presented along with the major modeling efforts. The most significant reference spectra are reviewed and three independently derived empirical models are described. These include Hinteregger's 1981 SERF1, Nusinov's 1984 two‐component, and Tobiska's 1990/1991 SERF2/EUV91 flux models. They each provide daily full‐disk broad spectrum flux values from 2 to 105 nm at 1 AU. All the models depend to one degree or another on the long time series of the Atmosphere Explorer E (AE‐E) EUV database. Each model uses ground‐ and/or space‐based proxies to create emissions from solar atmospheric regions. Future challenges in EUV modeling are summarized including the basic requirements of models, the task of incorporating new observations and theory into the models, the task of comparing models with solar‐terrestrial data sets, and long‐term goals and modeling objectives. By the late 1990s, empirical models will potentially be improved through the use of proposed solar EUV irradiance measurements and images at selected wavelengths that will greatly enhance modeling and predictive capabilities.
A discussion of plausible solar irradiance variations, 1700‐1992
Authors: Hoyt Douglas V. , Schatten Kenneth H.
Published: February 2008

From satellite observations the solar total irradiance is known to vary. Sunspot blocking, facular emission, and network emission are three identified causes for the variations. In this paper we examine several different solar indices measured over the past century that are potential proxy measures for the Sun's irradiance. These indices are (1) the equatorial solar rotation rate, (2) the sunspot structure, the decay rate of individual sunspots, and the number of sunspots without umbrae, and (3) the length and decay rate of the sunspot cycle. Each index can be used to develop a model for the Sun's total irradiance as seen at the Earth. Three solar indices allow the irradiance to be modeled back to the mid‐1700s. The indices are (1) the length of the solar cycle, (2) the normalized decay rate of the solar cycle, and (3) the mean level of solar activity. All the indices are well correlated, and one possible explanation for their nearly simultaneous variations is changes in the Sun's convective energy transport. Although changes in the Sun's convective energy transport are outside the realm of normal stellar structure theory (e.g., mixing length theory), one can imagine variations arising from even the simplest view of sunspots as vertical tubes of magnetic flux, which would serve as rigid pillars affecting the energy flow patterns by ensuring larger‐scale eddies. A composite solar irradiance model, based upon these proxies, is compared to the northern hemisphere temperature departures for 1700‐1992. Approximately 71% of the decadal variance in the last century can be modeled with these solar indices, although this analysis does not include anthropogenic or other variations which would affect the results. Over the entire three centuries, ∼50% of the variance is modeled. Both this analysis and previous similar analyses have correlations of model solar irradiances and measured Earth surface temperatures that are significant at better than the 95% confidence level. To understand our present climate variations, we must place the anthropogenic variations in the context of natural variability from solar, volcanic, oceanic, and other sources.
Steady state and dynamical structure of a cosmic‐ray‐modified termination shock
Authors: Donohue D. J. , Zank G. P.
Published: February 2008

A hydrodynamic model is developed for the structure of a cosmic‐ray‐modified termination shock. The model is based on the two‐fluid equations of diffusive shock acceleration (Drury and Völk, 1981). Both the steady state structure of the shock and its interaction with outer heliospheric disturbances are considered. Under the assumption that the solar wind is decelerated by diffusing interstellar cosmic rays, it is shown that the natural state of the termination shock is a gradual deceleration and compression, followed by a discontinuous jump to a downstream state which is dominated by the pressure contribution of the cosmic rays. A representative model is calculated for the steady state which incorporates both interstellar cosmic ray mediation and diffusively accelerated anomalous ions through a proposed thermal leakage mechanism. The interaction of large‐scale disturbances with the equilibrium termination shock model is shown to result in some unusual downstream structure, including transmitted shocks and cosmic‐ray‐modified contact discontinuities. The structure observed may be connected to the 2‐kHz outer heliospheric radio emission (Cairns et al., 1992a, b). The time‐dependent simulations also demonstrate that interaction with solar wind compressible turbulence (e.g., traveling interplanetary shocks, etc.) could induce the termination shock to continually fluctuate between cosmic‐ray‐dominated and gas‐dynamic states. This fluctuation may represent a partial explanation of the galactic cosmic ray modulation effect and illustrates that the Pioneer and Voyager satellites will encounter an evolving shock whose structure and dynamic properties are strongly influenced by the mediation of interstellar and anomalous cosmic rays.
Mass loading and velocity diffusion models for heavy pickup ions at comet Grigg‐Skjellerup
Authors: Huddleston D. E. , Coates A. J. , Johnstone A. D. , Neubauer F. M.
Published: February 2008

We compare model predictions of cometary water group ion densities and the solar wind slow down with measurements made by the Giotto Johnstone plasma analyzer implanted ion sensor at the encounter with comet Grigg‐Skjellerup (G‐S) on July 10, 1992. The observed slope of the ion density profile on approach to the comet is unexpectedly steep. Possible explanations for this are discussed. We present also a preliminary investigation of the quasilinear velocity‐space diffusion of the implanted heavy ion population at G‐S using a transport equation including source, convection, adiabatic compression, and velocity diffusion terms. Resulting distributions are anisotropic, in agreement with observations. We consider theoretically the waves that may be generated by the diffusion process for the observed solar wind conditions. At initial ion injection, waves are generated at ω ∼ Ωi the ion gyrofrequency, and lower frequencies are predicted for diffusion toward a bispherical shell.
Nongyrotropic distribution of pickup ions at comet P/Grigg‐Skjellerup: A possible source of wave activity
Authors: Motschmann Uwe , Glassmeier Karl‐Heinz
Published: February 2008

The pickup of newborn cometary ions in the solar wind excites strong wave activity. At comet P/Grigg‐Skjellerup the interplanetary magnetic field is nearly perpendicular to the solar wind velocity; thus the pickup ions should form a ring distribution. In opposition to P/Halley, comet P/Grigg‐Skjellerup is rather small, and its inhomogeneity scale is even smaller than the pickup scale. Therefore the ring will be filled incompletely, and the distribution becomes nongyrotropic. The dispersion of a nongyrotropic cometary pickup distribution in a cold proton‐electron background plasma is studied. Nongyrotropy couples the gyrotropic eigenmodes and drives an instability. Left‐hand polarized waves close to the cometary ion cyclotron frequency are excited.
Determination of source parameters at regional distances with three‐component sparse network data
Authors: Dreger Douglas S. , Helmberger Donald V.
Published: February 2008

We find remarkable similarities between regional body waves recorded by the TERRAscope network of broadband stations and synthetics constructed from a standard southern California velocity model. This model is shown to be effective for a variety of azimuths and ranges throughout southern California. At short periods some of the relative timing of the body waves are discordant, but at longer periods this becomes less of a factor. Thus we have developed a waveform inversion technique to rapidly determine source parameters using stored Green's functions for events out to 500 km, well outside the TERRAscope network. Often, only the three‐component records of a single station are required because the ratio of SV to SH energy is dependent upon source orientation. Sensitivity analyses examining the effects of source mislocations and velocity model on the inversion results show that the long‐period body waves appear relatively insensitive to lateral mislocations but are sensitive to source depth. However, the choice of velocity model can be a factor in obtaining reliable estimates of source depth. In this study the October 24, 1990, (Mw = 5.2) Lee Vining and the December 3, 1991, (Mw = 5.1) Baja California events are used to demonstrate the effectiveness of the inversion method. For the Baja event, we obtained unique results using a single station. For the Lee Vining event, inversions using a single station were not as stable. However, we found that using two stations with only a 24° aperture provided enough constraint to obtain unique results.
Global positioning system reoccupation of early triangulation sites: Tectonic deformation of the Southern Coast Ranges
Authors: Shen Zheng‐Kang , Jackson David D.
Published: February 2008

We study tectonic deformation in the Southern Coast Ranges, California. We use triangulation and astronomic azimuth data collected since 1875, trilateration since 1970, and global positioning system data collected from 1986 to 1987. Two modeling techniques have been used. An elastic block‐fault model is applied to study the tectonic motion of the San Andreas Fault and the San Gregorio‐Hosgri Fault. Station velocities are modeled to study regional deformations. Results show that the regional deformation is predominantly controlled by deep strike‐slip motion along the San Andreas Fault, at a rate of 33±2 mm/yr. Deep slip along the San Gregorio‐Hosgri Fault is about 0–4 mm/yr, assuming a locked fault to a depth of 20 km. Convergence normal to the San Andreas Fault in the Southern Coast Ranges is not greater than 0.02 μrad/yr.
Estimation of optimum average surficial density from gravity data: An objective Bayesian Approach
Authors: Murata Yasuaki
Published: February 2008

A new objective method is proposed for estimating an average surficial density, Bouguer density, from gravity data, which then may be correlated with the surface and/or near‐surface geology in analyzed area. The method assumes that a suitable Bouguer density will lead to a smooth Bouguer anomaly. With this assumption, the method fits a two‐dimensional curved function to the observed Bouguer anomaly by finding a Bouguer density that gives the optimum trade‐off between smoothness and goodness of fit to the Bouguer anomalies. This trade‐off is interpreted by an objective Bayesian procedure and determined by minimization of Akaike's Bayesian Information Criterion (ABIC). Further, the distribution of Bouguer density is assumed to be smooth enough to apply our method to any of the spatial windows with an area of 20 × 20 km2 in the district, and therefore be obtainable by moving successively the windows. The proposed method is tested against the gravitational data in Kyushu district, southwest Japan. The distribution of Bouguer density which is obtained by this method agrees remarkably well with the geology of the relevant area.
Separation of scattering attenuation and intrinsic absorption in Japan using the multiple lapse time window analysis of full seismogram envelope
Authors: Hoshiba Mitsuyuki
Published: February 2008

Two seismic wave attenuation factors, scattering attenuation (Qs−1) and intrinsic absorption (Qi−1), can be determined from estimates of two parameters, the reciprocal of extinction length Le−1 ≡ (Qs−1 + Qi−1) ·ω/v and seismic albedo B0≡ Qs−1 / (Qs−1 + Qi−1), where ω and v are angular frequency and seismic wave velocity, respectively. These parameters are measured using the multiple lapse time window analysis method over frequency bands of 1–2, 2–4, and 4–8 Hz from 16 stations located throughout Japan. In the analysis, time‐integrated seismic wave energy from earthquakes having hypocentral distances less than 120 km and focal depths less than 40 km are compared with simulations of multiple isotropic scattering based on the assumption of spatial uniformity of Qs−1 and Qi−1. The modified coda normalization method is used to correct for sources and site effects, where the spatially non uniform distribution of the coda wave energy is considered. The Le−1 estimates do not have a distinct frequency dependence, but the Be estimates do: 0.2–0.7 for 1–2 Hz and 0.1–0.3 for 4–8 Hz. The results mean that Qs−1 is similar to Qi−1 for 1–2 Hz but is 3–5 times smaller for 4–8 Hz and that the frequency dependence of Qs−1 is larger than Qi−1. Analyses using different components of motion and different window lengths give nearly the same results. Coda attenuation (Qc−1) estimates based on the single scattering approximation are nearly equal to intrinsic absorption Qi−1 rather than to total attenuation (Qs−1 + Qi−1) for Japan.
Magmatic processes under arcs and formation of the volcanic front
Authors: Furukawa Yoshitsugu
Published: February 2008

Temperature structure and stress under arcs are simulated in a two‐dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge. Results of the calculations show three important features with respect to magmatic processes under arcs: (1) Temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction, the temperature structure of the slab, and that of the back‐arc region. This indicates that physical conditions such as temperature and pressure are similar under various arcs. It is thus inferred that primary magmas generated under various arcs should have similar chemical compositions, if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same. (2) Calculated deviatoric stress magnitude is relatively large (more than a few tens of megapascals) in the partially molten mantle. Cracks may open under high differential stress, and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds. (3) The deviatoric stress values in the region over the partially molten mantle are relatively large, and the direction of the principal stress changes horizontally; the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore‐arc region. It is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism. The propagation direction of magma‐filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress. The calculated stress is highly compressive horizontally on the trench side, while it becomes tensile on the back‐arc side. The location of this stress transition coincides with that of the volcanic front. The location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma‐filled cracks under relatively high differential stress field.
Spectral analysis of volcanic glass chemistry along the East Pacific Rise, 13°–23°S
Authors: Butler Rhett , Sinton John M. , Mahoney John J. , Smaglik Suzanne M.
Published: February 2008

An analysis is presented for the variation in chemistry of 162 normal mid‐ocean ridge basalt glass groups, collected with an average spacing of about 10 km along a −1100 km section of the East Pacific Rise between 13° and 23°S. Long‐wavelength periodicities have been determined for oxides Al2O3, CaO, Cr2O3, FeO*, K2O, MgO, MnO, Na2O, NiO, P2O5, SiO2, TiO2; for ratios FeO*/MgO, K/P, and K/Ti; for FeO*, Na2O, and SiO2 corrected for fractionation to 8.0 wt % MgO; and for bathymetry. The data were modeled as the sum of a long‐wavelength periodic component and a residual random component. The power spectrum of the periodic component was determined using the maximum entropy method. The spectral phase of significant spectral peaks was determined by cross correlation with a cosine wave with the wavelength of the respective peaks. In most cases the characteristics of the random component were found to be consistent with a normal distribution. Spatial aliasing considerations and the variance of the random component place confidence limits upon the spectral features. The results of the spectral analyses indicate substantial correlation in the power spectra of the oxides. The oxide data have spectral peaks with wavelengths in the vicinity of 575 and 225 km and a local minimum in spectral power at wavelengths between 300 and 500 km. In general, oxide components compatible in the crystallizing minerals olivine and plagioclase are approximately 180° out of phase with the incompatible oxides, indicating that fractionation is an important process in controlling compositional variations along this portion of the East Pacific Rise. Unlike the fractionation‐sensitive oxide data, periodic variations are not pronounced for K/Ti, indicating that the length scales of mantle composition being melted are uncorrelated with those of magmatic temperature variation. The analysis of fractionation‐corrected values of Na8.0 and Fe8.0 indicates that degree and depth of partial melting show a strong spectral peak near a wavelength about 430 km. There is also significant power in the spectrum of Na8.0 near 260 km and of Fe8.0 near 200 km, bounding the average spectral peak for the oxides at 225 km. There appears to be strong coupling between the degree and depth of melting, and magmatic temperature at two length scales, around 225 km and 400–600 km, corresponding to wavelengths of geoid undulations observed in the vicinity of the East Pacific Rise, and suggesting a relationship with convection.
Triple junction and ridge hotspots: Earthquakes, faults, and volcanism in Afar, the Azores, and Iceland
Authors: Hirn Alfred , Lépine Jean‐Claude , Sapin Martine
Published: February 2008

The Afar depression of the Horn of Africa, the Azores Archipelago, and Iceland are zones of plate divergence and mantle plume discharge. Strike‐slip focal mechanisms dominate in major earthquakes. Among nodal planes a particular fault plane can be identified by precise epicentral location, from the strike of the aftershock zone, or by relation to field evidence of active faulting. These fault planes are orthogonal to the directions expected in previous models for transform faults along a continuous divergent plate boundary. Recent volcanic features are consistently at small angles with the fault planes. Rotation of blocks within a deforming zone is implied by the direction of fault planes, the sense of motion along them, and their location on a series of parallel features transverse to the deformed zone. The drift of oceanic or continental large lithospheric plates deforms a zone of lithosphere, deriving from both the plume discharge and ridge accretion while the zone is created.
An episode of reinflation of the Long Valley Caldera, eastern California: 1989–1991
Authors: Langbein John , Hill David P. , Parker Timothy N. , Wilkinson Stuart K.
Published: February 2008

Following the episodes of inflation of the resurgent dome associated with the May 1980 earthquake sequence (fourM6 earthquakes) and the January 1983 earthquake swarm (twoM5.2 events), 7 years of frequently repeated two‐color geodimeter measurements spanning the Long Valley caldera document gradually decreasing extensional strain rates from 5 ppm/yr in mid‐1983, when the measurements began, to near zero in mid‐1989. The corresponding seismic activity within the caldera persisted at a low rate of fewer than 10M≥ 1.2 earthquakes per week from 1985 through November 1989 with no events exceedingM3.0. Early October 1989 marked a change in activity when measurements of the two‐color geodimeter network showed a significant increase in extensional strain rate (9 ppm/yr) across the caldera. The seismic activity began exceeding 10M≥ 1.2 per week in early December 1989 and rapidly increased to a sustained level of tens ofM≥ 1.2 per week with bursts having hundreds of events per day. Many events exceededM3.0 and the largest event wasM≈ 4. The 1989–1991 inflation episode is the first time that we have sufficient geodetic measurements in Long Valley to define the temporal relation between onset of an inflation episode and onset of brittle failure (earthquake swarm within the caldera). Here, the onset of deformation preceded the onset of increased earthquake activity by more than 2 months. The seismicity rate began to decrease in mid‐July 1990, consistent with a gradually slowing of extension across the caldera as measured by the two‐color geodimeter. The recent episode of inflation can be modeled by a single Mogi point source located about 7 km beneath the center of the resurgent dome. In contrast, the deformation pattern observed between mid‐1983 and mid‐1989 is best reproduced by fault slip in the south moat, inflation at 6.5 km depth near Casa Diablo Hot Springs and inflation beneath the resurgent dome. It appears that the 7‐km source beneath the resurgent dome that was active for the earlier episodes is the primary source for the more recent episode. The model used to satisfy the line length observations predicts 7.5 cm of uplift along leveling route along highway 395 from mid‐1983 to mid‐1989 and an additional 11 cm through the end of 1991. To comp are with the energy release from seismicity, the modeled inflation from late 1989 through the end of 1991 has a moment that is a factor of 40 more than the cumulative seismic moment from earthquakes located within the caldera from the same period. Thus the recent inflation episode represents a significant portion of the observed geodetic deformation with only little seismic energy release.
Evolution of the caldera‐forming eruption at Crater Lake, Oregon, indicated by component analysis of lithic fragments
Authors: Suzuki‐Kamata Keiko , Kamata Hiroki , Bacon Charles R.
Published: February 2008

Crater Lake caldera (8 × 10 km), formed 6845 years B. P. (14C age) during the climactic eruption of the volcanic edifice known as Mount Mazama, is intermediate in size between small calderas associated with central vent eruptions and large calderas that have ring fracture vent systems. Our quantitative study of lithic fragments in the ejecta confirms the existing model of changes in vent configuration during the climactic eruption of Mount Mazama. Initial activity was from a single vent that produced a rhyodacite pumice fall from a Plinian column. Altered preexisting volcanic rocks are the predominant lithic type in the Plinian deposit, and their extensive hydrothermal alteration is considered as evidence of their relatively deep origin. The Wineglass Welded Tuff lies atop the Plinian deposit and contains a higher proportion of fresh volcanic rocks, suggesting enlargement of the single vent by slumping of its walls. This same vent enlargement caused the Plinian eruption column to collapse and feed valley‐hugging pyroclastic flows that deposited the Wineglass Welded Tuff. When enough material was erupted from the shallow magma chamber that its roof was no longer adequately supported, Mount Mazama collapsed to form the caldera, while highly energetic pyroclastic flows produced the climactic ignimbrite. A lag breccia that represents the proximal facies of the compositionally zoned climactic ignimbrite lies atop the Wineglass Welded Tuff and contains predominantly altered volcanic rocks of deeper origin, accompanied by minor granitoids from the magma chamber walls. Azimuthal differences in lithic component proportions in the lag breccia correlate well with the geology of the caldera walls, indicating that the climactic ignimbrite was ejected by multiple vents along a ring fracture system. Systematic lithic component changes within the lag breccia suggest different quarrying levels that reflect waxing and waning of the discharge rate during the caldera collapse phase of the climactic eruption. Our lithic component analysis demonstrates that calderas that may be too small to experience structural resurgence, such as Crater Lake, nevertheless may form by syneruptive subsidence along ring fractures.
Anisotropic reflection and transmission calculations with application to a crustal seismic survey from the East Greenland Shelf
Authors: Guest W. S. , Thomson C. J. , Spencer C. P.
Published: February 2008

A three‐component refraction data set recorded over the East Greenland Shelf contained two anomalous shear wave arrivals. The direct‐S phase emerged at the receiver as an apparent SH wave even though the airgun source was purely compressional. We have concluded that this arrival is the result of anisotropy in the upper crust. By employing a number of fairly simple techniques we were able to highlight a second, delayed quasi‐shear arrival and interpret shear wave splitting. Modeling of the polarization, amplitude, and delay time of direct‐S suggested that the anisotropy is due to aligned subvertical fractures perpendicular to the local seafloor spreading direction. The second shear arrival of interest is a P to S conversion from the Moho (PmS). The relative strength of this arrival suggests that the Moho is a sharp transition in this region. PmS also contained a significant transverse component but attempts to identify shear wave splitting were inconclusive. However, modeling revealed that the transverse PmS displacement may be due to anisotropy in the upper mantle (alignment of olivine crystals with the spreading direction), which causes the converted S reflection to have a significant SH (transverse) component. An understanding of, and ability to model, anisotropic reflection and transmission effects are essential to these interpretations. Therefore we also outline an efficient method to obtain all of the reflection and transmission coefficients at boundaries between anisotropic media. This has been incorporated into a new ray tracing package, A‐TRAK, based on asymptotic ray theory, capable of modeling three‐dimensional inhomogeneous media separated by curved interfaces.