bluuebunny/crossref_metadata_2025 huggingface dataset:
In situ Polar observation of transverse cold‐ion acceleration: Evidence that electric field generation is a hot‐ion finite gyroradii effect
Authors: Lennartsson O. W.
Published: May 2003

Recent results with the Polar TIMAS instrument (ion mass spectrometer) have revealed that earthward magnetic field‐aligned and velocity‐dispersed flows of 1‐ to 33‐keV protons (and other ions) from the outer magnetosphere are inherently “blast‐like” and exhibit a complex filamentary structure where transverse scale sizes may often be a few proton gyroradii. These velocity‐dispersed protons (and accompanying hot electrons) have been found to have a close association with enhanced large pitch angle outflow of accelerated (sometimes to more than 10 keV) ionospheric ions (H+, O+, and He+) at Polar. Theoretical considerations suggest a mechanism by which the difference in gyroradii between hot ions and electrons in earthward plasma bursts generates charge imbalance and strong transverse electric fields at density gradients, as the bursts move into an increasingly strong magnetic field, which in turn accelerate the ambient cold ions. Model electric fields greater than 1 V m−1 at Polar are readily generated with density gradient scale lengths of a few earth radii in an equatorial source region of the bursts, assuming that the magnetic moments of the burst protons are preserved. The nonadiabatic acceleration of cold ions may act to reduce these fields to the ca 0.1‐V m−1 amplitude and few‐second period fluctuations actually observed. Sample TIMAS data are shown to be consistent with the transverse acceleration of cold O+ ions to keV energy on a timescale of less than a single gyroperiod.
Energetic particle acceleration and transport at coronal mass ejection–driven shocks
Authors: Li Gang , Zank G. P. , Rice W. K. M.
Published: May 2003

Evidence now exists which suggests that in large solar energetic particle (SEP) events, particles are often accelerated to ∼ MeV energies (and perhaps up to GeV energies) at shock waves driven by coronal mass ejections (CMEs). These energetic particles are of considerable importance to space weather studies since they serve as a precursor signal for possible disruptive events at the Earth. As a CME‐driven shock propagates, expands and weakens, particles accelerated diffusively at the shock can escape upstream and downstream into the interplanetary medium. The escaping energized particles propagate along the interplanetary magnetic field, experiencing only weak scattering from fluctuations in the interplanetary magnetic field (IMF). In this work, we study the time‐dependent transport of energetic particles accelerated at a propagating shock using a Monte‐Carlo approach. This treatment, together with our previous work on particle acceleration at shocks, allows us to investigate the characteristics (intensity profiles, angular distribution, particle anisotropies) of high‐energy particles arriving at various distances from the sun. Such an approach is both easy to implement and allows us to study the affect of interplanetary turbulence on particle transport in a systematic manner. These theoretical models form an excellent basis on which to interpret observations of high‐energy particles made in situ at 1 AU by spacecraft such as ACE and WIND.
Interhemispheric asymmetry of the high‐latitude ionospheric convection on 11–12 May 1999
Authors: Nishitani Nozomu , Papitashvili Vladimir O. , Ogawa Tadahiko , Sato Natsuo , Yamagishi Hisao , Yukimatu Akira Sessai , Rich Frederick J.
Published: May 2003

Ionospheric convection over the southern polar cap on 11–12 May 1999 has been studied by using the Syowa East and South HF radar data and the DMSP ion driftmeter data, when the solar wind density was very low and geomagnetic activity was low. The overall convection pattern is consistent with the previous results by Ohtani et al. [2000]. However, the Syowa radars and the DMSP satellites observed very high (>1500 m/s) westward plasma flows at dusk directed from the nightside toward the dayside only in the Southern (dark) Hemisphere. The high‐speed flow was observed continuously across the fields of view of both radars from 1530 UT on 11 May to 0200 UT on 12 May, when the solar wind density was close to minimum. Comparison with the DMSP particle and auroral image data shows that the westward flow regions were located in the middle of the auroral precipitation area. The strong asymmetry of the convection between the two hemispheres indicates the importance of the presence (absence) of solar illumination for the absence (presence) of the strong and localized ionospheric flows.
Isotope composition of secondary hydrogen and helium above the atmosphere measured by the instruments NINA and NINA‐2
Authors: Bidoli V. , Casolino M. , De Pascale M. , Furano G. , Iannucci A. , Morselli A. , Picozza P. , Sparvoli R. , Bakaldin A. , Galper A. , Koldashov S. , Korotkov M. , Leonov A. , Mikhailov V. , Voronov S. , Boezio M. , Bonvicini V. , Vacchi A. , Zampa G. , Zampa N. , Ambriola M. , Cafagna F. , Circella M. , De Marzo C. , Adriani O. , Papini P. , Spillantini P. , Straulino S. , Vannuccini E. , Ricci M. , Castellini G.
Published: May 2003

In this paper we report on the energy spectra and abundance ratios of hydrogen and helium isotopes of albedo origin, measured by the instruments NINA and NINA‐2 in near‐equatorial regions. The instrument NINA flew on board the satellite Resurs‐01‐N4 between 1998 and 1999, at a 830 km average altitude. The NINA‐2 apparatus, on board the satellite MITA, was put into orbit in July 2000 at an altitude of about 450 km. NINA and NINA‐2 measurements revealed that 2H, 3H, 3He, and 4He are a significant portion of the secondary flux above the atmosphere. The energy spectra of hydrogen isotopes are practically flat across the energy range of 10–40 MeV/n, while the spectra of helium isotopes can be fitted by a power law of spectral indexes γ = 0.8 and γ = 1.5 for 3He and 4He, respectively.
Signatures of collisionless magnetic reconnection
Authors: Rogers B. N. , Denton R. E. , Drake J. F.
Published: May 2003

Simulations of magnetic reconnection in simple systems without a guide field predict the formation of a spatially antisymmetric out‐of‐plane magnetic field signature and a symmetric perturbation in the plasma pressure. Here we show, using two fluid simulations and analytic arguments, that these signatures are not in fact a generic feature of fast reconnection. Rather, in the strong guide field limit the pressure perturbation becomes antisymmetric, while the perturbation in the out‐of‐plane field can be either symmetric for βx ∼ 1 or antisymmetric for βx ≫ 1, where βx is the plasma β defined using the reconnection magnetic field just upstream of the reconnection region.
Shock aurora: FAST and DMSP observations
Authors: Zhou X.‐Y. , Strangeway R. J. , Anderson P. C. , Sibeck D. G. , Tsurutani B. T. , Haerendel G. , Frey H. U. , Arballo J. K.
Published: May 2003

Global signatures of the aurora caused by interplanetary shocks/pressure pulses have been studied in recent years using ultraviolet imager data from polar orbiting spacecraft. The signatures include the occurrence of the aurora first near local noon and then propagation antisunward along the auroral oval at very high speeds. To better understand the mechanisms of particle precipitation, in this paper we study shock auroras using near‐Earth observations of the FAST and DMSP satellites. We have studied the events that occurred during 1996–2000 where FAST and/or DMSP crossed the dawnside or duskside auroral zone within 10 min after shocks/pressure pulses arrived at the nose of the magnetopause. It is found that the electron precipitation increased significantly above the dawnside and duskside auroral oval zone after the shock/pressure pulse arrivals. The precipitation structure is low‐energy electrons (<∼1 keV) at higher latitudes (∼75°–83° ILAT within 0600–0900 MLT) and high‐energy electrons (∼1–10 keV) at lower latitudes (∼65°–79° ILAT) of the auroral zone. There are a few degrees (1°–4° ILAT) of overlap between these two categories of precipitated electrons. The precipitation of low‐energy electrons was along highly structured field‐aligned currents. The precipitation of the high‐energy electrons was highly isotropic filling the loss cone. Possible mechanisms of field‐aligned current generation are some dynamic processes occurring on the dayside magnetopause, such as magnetic shearing, magnetopause perturbation, magnetic reconnection, and Alfvén wave generation. Adiabatic compression might have caused the high‐energy electron precipitation. On the basis of observations of FAST and DMSP, shock auroras are speculated to be diffuse auroras at the lower latitudes of the dayside auroral oval and discrete auroras on the poleward boundary of the oval with a few latitude degree overlap of the two types of auroras.
Tail plasma sheet models derived from Geotail particle data
Authors: Tsyganenko N. A. , Mukai T.
Published: May 2003

Simple analytical models have been derived for the first time, describing the 2‐D distribution (along and across the Earth's magnetotail) of the central plasma sheet (CPS) ion temperature, density, and pressure, as functions of the incoming solar wind and interplanetary magnetic field (IMF) parameters, at distances between 10 and 50 RE. The models are based on a large set of data of the Low‐Energy Particle (LEP) and Magnetic Field (MGF) instruments, taken by Geotail spacecraft between 1994 and 1998, comprising 7234 1‐min average values of the CPS temperature and density. Concurrent solar wind and IMF data were provided by the Wind and IMP 8 spacecraft. The accuracy of the models was gauged by the correlation coefficient (c.c.) R between the observed and predicted values of a parameter. The CPS ion density N is controlled mostly by the solar wind proton density and by the northward component of the IMF. Being the least stable characteristic of the CPS, it yielded the lowest c.c. RN = 0.57. The CPS temperature T, controlled mainly by the solar wind speed V and the IMF Bz, gave a higher c.c. RT = 0.71. The CPS ion pressure P was best controlled by the solar wind ram pressure Psw and by an IMF‐related parameter F = B⟂, where B⟂ is the perpendicular component of the IMF and θ is its clock angle. In a striking contrast with N and T, the model pressure P revealed a very high c.c. with the data, RP = 0.95, an apparent consequence of the force balance between the CPS and the tail lobe magnetic field. No significant dawn‐dusk asymmetry of the CPS was found beyond the distance 10 RE, in line with the observed symmetry of the tail lobe magnetic field. The plasma density N is lowest at midnight and increases toward the tail's flanks. Larger (smaller) solar wind ion densities and northward (southward) IMF Bz result in larger (smaller) N in the CPS. In contrast to the density N, the temperature T peaks at the midnight meridian and falls off toward the dawn/dusk flanks. Faster (slower) solar wind flow and southward (northward) IMF Bz result in a hotter (cooler) CPS. The CPS ion pressure P is essentially a function of only XGSM in the midtail (20–50 RE); at closer distances the isobars gradually bend to approximately follow the contours of constant geomagnetic field strength. For northward IMF conditions combined with a slow solar wind, the isobars remain quasi‐circular up to larger distances, reflecting a weaker tail current and, hence, more dipole‐like magnetic field.
Ray trajectories of lower hybrid solitary structures
Authors: Schuck P. W. , Bonnell J. W.
Published: May 2003

The correlation between large‐amplitude lower hybrid waves and density depletions, termed “spikelets,” “lower hybrid cavities,” or “lower hybrid solitary structures” has been attributed to the change in the refractive index computed from the homogeneous plasma dispersion relation. However, the appropriate refractive index includes the electron Hall term involving the density gradient. We demonstrate that the Hall term produces ray trajectories which are profoundly different from the homogeneous plasma dispersion relation.
Validation of the STORM response in IRI2000
Authors: Araujo‐Pradere E. A. , Fuller‐Rowell T. J. , Bilitza D.
Published: May 2003

The latest version of the International reference ionosphere, IRI2000 [Bilitza, 2001], contains a dependence on geomagnetic activity based on an empirical storm‐time ionospheric correction model (STORM) [Araujo‐Pradere et al., 2002]. The new storm correction in IRI is driven by the previous time history (33 hours) ofapand is designed to scale the normal quiet‐timeFlayer critical frequency (foF2) to account for storm‐time changes in the ionosphere. An extensive validation of IRI2000 has been performed during geomagnetic storm conditions to determine the validity of the new algorithms. The quality of the storm‐time correction has been evaluated by comparing the model with the observed ionospheric response during all the geomagnetic storms withap> 150 in 2000 and 2001, a total of 14 intervals. The model output was compared with the actual ionospheric response for all available ionosonde stations for each storm. The comparisons show that the model captures the decreases in electron density particularly well in summer and equinox conditions. To quantify the improvement in IRI2000, the root‐mean‐square error has been evaluated and compared with the previous version of IRI, which had no geomagnetic dependence. The results indicate that IRI2000 has almost a 30% improvement over IRI95 during the storm days and is able to capture more than 50% of the increase in variability, above quiet times, due to the storms.
Interplanetary magnetic field control of polar patch velocity
Authors: Zhang Y. , McEwen D. J. , Cogger L. L.
Published: May 2003

Polar patch drift speed and direction have been investigated using oxygen 630 nm images recorded by an all‐sky imager at Eureka (89° CGM), Canada over an extended period, January and December 1998. Statistical studies showed that (1) the interplanetary magnetic field (IMF, from the WIND satellite) Bz component linearly controls the patch speed when Bz is between −7.5 nT and 0 nT. The speed tends to saturate when Bz is less than −7.5nT due to nonlinear coupling between the solar wind and the magnetosphere. The average patch speed of 600 m/s is in agreement with results from earlier studies; (2) The IMF By or the IMF clock angle has a clear control of the patch drift direction as determined by the drift azimuth angle. When ∣By∣ is less than 7.5nT, the drift azimuth angle is linearly and positively correlated with By. For a large ∣By∣ (>7.5 nT), the positive correlation is replaced by a negative correlated linear relation and the azimuth angle tends to turn towards 180 degrees; that is, the patches drift in an antisunward direction. These IMF By effects can be qualitatively explained by the northern winter polar ionospheric convection models developed by Weimer [1995] and Hairston and Heelis [1990]. Results from our quantitative study on the IMF control of polar patch speed and drift direction provide constraints for the development of future polar ionospheric convection models.
Can solar variability explain global warming since 1970?
Authors: Solanki S. K. , Krivova N. A.
Published: May 2003

The magnitude of the Sun's influence on climate has been a subject of intense debate. Estimates of this magnitude are generally based on assumptions regarding the forcing due to solar irradiance variations and climate modeling. This approach suffers from uncertainties that are difficult to estimate. Such uncertainties are introduced because the employed models may not include important but complex processes or mechanisms or may treat these in too simplified a manner. Here we take a more empirical approach. We employ time series of the most relevant solar quantities, the total and UV irradiance between 1856 and 1999 and the cosmic rays flux between 1868 and 1999. The time series are constructed using direct measurements wherever possible and reconstructions based on models and proxies at earlier times. These time series are compared with the climate record for the period 1856 to 1970. The solar records are scaled such that statistically the solar contribution to climate is as large as possible in this period. Under this assumption we repeat the comparison but now including the period 1970–1999. This comparison shows without requiring any recourse to modeling that since roughly 1970 the solar influence on climate (through the channels considered here) cannot have been dominant. In particular, the Sun cannot have contributed more than 30% to the steep temperature increase that has taken place since then, irrespective of which of the three considered channels is the dominant one determining Sun‐climate interactions: tropospheric heating caused by changes in total solar irradiance, stratospheric chemistry influenced by changes in the solar UV spectrum, or cloud coverage affected by the cosmic ray flux.
Dependence of the equatorial anomaly and of equatorial spread F on the maximum prereversal E × B drift velocity measured at solar maximum
Authors: Whalen J. A.
Published: May 2003

The relation of equatorial bubbles to the equatorial anomaly is important because scintillation that is most disruptive to transionospheric RF propagation occurs when it passes through the intersection of the two. However, measurement of the relation between the two and of the electric field from which both arise is difficult because of large separations in space and time. This first attempt to perform these measurements employs a latitudinal array of ionospheric sounders spanning 0° to 40° dip latitude (DLAT) in the Western American sector. Measured on each day of a solar maximum year are the following: (1) the maximum electron density of the postsunset equatorial anomaly, Ne, at 16° and at 20.3° DLAT at 2100 LT, the time when the anomaly crest is at its maximum latitude; (2) equatorial spread F (ESF), detected by the occurrence of macroscopic bubbles and of bottomside spread F (BSSF), the latter recorded at levels of none, weak and strong; (3) Kp averaged over the 6 hours before sunset. Ne and ESF are considered functions of the maximum prereversal F layer drift E × B drift velocity measured by the Jicamarca incoherent scatter radar also during solar maximum and at the same longitude. Parameters are averaged over two levels of Kp for the three seasons, the E months (March, April, September, and October), D months (November–February), and J months (May–August) to yield the following results: (1) Ne measured at 16°, at 20.3° DLAT or at the anomaly crest are linearly dependent on maximum E × B drift velocity. (2) Occurrence of each level of ESF increases with Ne approximately linearly during the E and J months but not during the D months. (3) ESF occurrence is dependent on and increases approximately linearly with maximum E × B drift velocity during the E and J months. During the D months this dependence is absent. Except for the D months, these results indicate that scintillation increases with maximum prereversal E × B drift velocity: at L‐band at the bubble‐anomaly intersection because bubble occurrence increases, Ne increases, and the latitudinal extent of the anomaly increases; and at VHF/UHF near the equator because the occurrence of strong BSSF increases.
Evolution of the thin current sheet in a substorm observed by Geotail
Authors: Asano Y. , Mukai T. , Hoshino M. , Saito Y. , Hayakawa H. , Nagai T.
Published: May 2003

Time evolution of a thin current sheet during a substorm is studied in the near‐Earth magnetotail of X = −15 RE. Specifically, we study the thickness of the current sheet and its dynamical structure. Using the Geotail plasma data, we obtain the electric current density directly from both ion and electron velocity moments, and the thickness of the current sheet is evaluated from the Ampère's law with the pressure balance of the plasma sheet. It is found that the averaged thickness of the highly fluctuating current sheet becomes as thin as the ion inertial scale during the late growth phase and the early expansion phase. The current sheet temporally forms a double‐peaked current sheet in which the current density becomes the largest away from the neutral sheet. We also found that the intense current is mainly carried by electrons, contrary to the ion‐dominated diamagnetic current caused by the higher ion temperature than the electron one. We discuss that the Hall electric field plays an important role on the formation of the double‐peaked current sheet. The Hall electric field toward the neutral sheet causes the electron dominated current sheet by enhancing the dawnward drift for both ion and electron.
Evidence of a role for modulated atmospheric tides in the dependence of sporadic E layers on planetary waves
Authors: Pancheva D. , Haldoupis C. , Meek C. E. , Manson A. H. , Mitchell N. J.
Published: May 2003

A large amplitude, 7‐day period westward propagating S = 1 planetary wave (PW) of global response has been reported from ground radar and satellite wind measurements in the mesosphere‐lower thermosphere (MLT) during the second half of August and well into September 1993. Following recent suggestions that PW might play a role in the formation of midlatitude sporadic E layers (Es), Haldoupis and Pancheva [2002] found a strong 7‐day periodicity present in all stations concurrently with the 7‐day planetary wave reported elsewhere, by analyzing sporadic E critical frequency (foEs) time series from eight midlatitude ionosonde stations covering a longitudinal zone from about 58°E to 157°W. This study provided the first direct proof in favor of a PW role on Es formation. In the present paper we further investigate this role by considering the same PW event and correlating the 7‐day periodicity in foEs directly with concurrent variations in the mesospheric neutral wind measured with atmospheric radars in Saskatoon, Canada, and in Sheffield, United Kingdom. Although our analysis cannot exclude a direct PW role on Es formation, it shows clearly that Es is affected indirectly by the PW through the action of the diurnal and semidiurnal tides which are strongly modulated by the same PW, apparently through a nonlinear interaction process at altitudes below 100 km. This 7‐day PW modulation was found to be clearly present simultaneously in the amplitude of the zonal 12‐hour tidal wind, the meridional 24‐hour tidal wind, and in both, the 12‐hour and 24‐hour periodicities which existed in the foEs time series. The results here provide a new physical explanation for the observed relation between sporadic E layers and planetary waves.
On radial heliospheric magnetic fields: Voyager 2 observation and model
Authors: Wang C. , Richardson J. D. , Burlaga L. F. , Ness N. F.
Published: May 2003

The heliospheric magnetic field (HMF) direction, on average, conforms well to the Parker spiral. However, numerous examples of events where the HMF is oriented in near‐radial directions for many hours have been reported on the basis of observations inside 5 AU from spacecraft such as ISEE‐3 and Ulysses. The magnetic field data observed by Voyager 2 from launch in 1977 through the end of 1982 (i.e., between 1 and ∼10 AU) were searched for all instances of radial fields with durations of 6 hours or more. Radial fields of significant durations at large distances are unusual as the Parker spiral is very tightly wound. The radial HMF events in the inner heliosphere typically occur at times when the solar wind speed is declining gradually, while they tend to be associated with steady wind speeds at distances beyond ∼6 AU. The durations of these events appear to be independent of distance and solar cycle, with an average duration of ∼11 hours. They generally are not associated with interplanetary coronal mass ejections (ICMEs). Possible generation mechanisms of the radial field events related to speed variations near the Sun are investigated by use of a MHD model. We find that a noticeable low‐speed plateau of limited duration in solar wind speed near the Sun can produce radial field events having durations of the order of 10 hours in the heliosphere as observed by Voyager 2.
Particle‐in‐cell simulations of Alfvén‐cyclotron wave scattering: Proton velocity distributions
Authors: Gary S. Peter , Saito Shinji
Published: May 2003

Alfvén‐cyclotron fluctuations propagating parallel or antiparallel to the background magnetic field Bo help shape solar wind ion velocity distributions fi(v). Alfvén waves may be generated at low, nonresonant frequencies and, by propagation through the inhomogeneous plasma, attain ion cyclotron resonances and thereby scatter the fi(v) to anisotropy. Ion anisotropies of sufficient magnitude lead to the growth of ion cyclotron instabilities; the resulting enhanced Alfvén‐cyclotron fluctuations scatter ions so as to reduce the anisotropy. Here particle‐in‐cell simulations were carried out in a magnetized, homogeneous, collisionless plasma of electrons and one ion species to study the evolution of the fi(v) in response to both of these scattering processes. A simulation with a spectrum of right‐traveling Alfvén‐cyclotron fluctuations imposed at t = 0 leads to non‐Maxwellian ion distributions. The computations show that the pitch angle scattering of left‐traveling (v∥ < 0) ions becomes weaker as v∥ becomes less negative but also that this scattering can transport ions across the condition v∥ = 0, where the subscript denotes the direction parallel to Bo.
Magma chamber behavior beneath a volcanic edifice
Authors: Pinel Virginie , Jaupart Claude
Published: May 2003

The construction of a large volcanic edifice at Earth's surface generates stresses in the upper crust whose magnitude is comparable to those of tectonic stresses and overpressures within a magma chamber. We study how this affects eruption behavior. Analytical calculations are carried out in two dimensions for a cylindrical reservoir with an internal overpressure in an elastic half‐space with an edifice at the surface. Different edifice shapes are considered, from shield volcanoes with gentle slopes to stratovolcanoes with steeper flanks. Without an edifice at the top, the hoop stress at the cavity walls reaches a maximum at two symmetrical points at some distance from the axis, away from the top of the chamber. With an edifice at the top, the maximum is reached at the top of the chamber, just beneath the edifice summit. This implies preferential failure of chamber walls at the axis and hence the focussing of volcanic activity through a central vent system. Tensile failure of the cavity walls occurs for a critical value of magma overpressure which depends on the dimensions of the edifice and on the depth and size of the cavity. For a small magma chamber beneath a large stratovolcano, the magmatic overpressure at the onset of eruption increases as the edifice grows and decreases following edifice destruction. These effects may explain why pressures recorded in phenocryst assemblages at Mount St. Helens, have varied over the past 4000 years as the edifice went through successive phases of growth and destruction.
P wave tomography of the mantle under the Alpine‐Mediterranean area
Authors: Piromallo Claudia , Morelli Andrea
Published: May 2003

We study the upper mantle P wave velocity structure below the Euro‐Mediterranean area, down to 1000 km depth, by seismic travel time tomography. We invert summary residuals constructed with both regional and teleseismic first arrival data reported by the International Seismological Centre (ISC) (1964–1995), introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure. Our high‐resolution model PM0.5 is parameterized with three‐dimensional (3‐D) linear splines on a grid of nodes with 0.5° spacing in both horizontal directions and 50 km vertical spacing. We obtain about 26% root‐mean‐square (RMS) reduction of residuals by inversion in addition to roughly 31% reduction after summary rays formation and selection. Sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales. The distribution of large‐scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions. The mantle dynamics of western central Europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone. Contrarily, in the eastern Mediterranean, fast velocity material sinks into the lower mantle, suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity. On a smaller scale, the existence of tears in the subducted slab (lithospheric detachment) all along both margins of the Adriatic plate, as proposed by some authors, is not supported by our tomographic images.
Activation of diffuse discontinuities and folding of sedimentary layers
Authors: Guiton Martin L. E. , Leroy Yves M. , Sassi William
Published: May 2003

Folding of sedimentary layers is often accommodated by the opening or sliding of inherited and new discontinuities which are assumed here to be diffuse so that a continuum description applies at the fold scale. The rock rheology is then described with an elastoplasticity model for which the permanent deformation is of simple shear (sliding) or dilation (opening) with respect to specific orientations of the new or inherited diffuse discontinuities. To illustrate the relation between folding and activation of diffuse discontinuities, a three‐dimensional layer under compression in the two horizontal directions and sustaining the overburden lithostatic pressure is studied. Cylindrical buckling occurs either before (elastic) or after the diffuse discontinuities have been activated. If buckling is elastic, inherited vertical discontinuities, striking obliquely to the fold geometrical axes, are activated in a sliding mode in the outer arc, leading to a rotation of the principal stress directions. Opening is then detected across new vertical planes striking obliquely to the fold axis. The activation of inherited or new vertical discontinuities can be suppressed if sliding takes place along weak bedding interfaces. Alternatively, early and homogeneous layer‐parallel shortening, marked by a reverse fault mode, drastically reduces the critical buckling load compared to the Euler load and modifies the final geometry of buckling which is then more of a circular dome shape. The switching in buckling mode results in the fold limbs in a change from the early reverse fault to a strike‐slip fault sliding and to opening across diffuse planes oriented consistently with the final circular structure.
Probabilistic earthquake location in complex three‐dimensional velocity models: Application to Switzerland
Authors: Husen S. , Kissling E. , Deichmann N. , Wiemer S. , Giardini D. , Baer M.
Published: May 2003

We present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of Switzerland. A three‐dimensional (3‐D) P wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3‐D P wave velocity structure. A 3‐D P wave velocity model derived from controlled source seismology (CSS) is used as an initial reference model. The final 3‐D model thus combines all available information from both CSS and local earthquake data. The probabilistic, nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model. In particular, the combination of nonlinear, global search algorithms, such as the Oct‐Tree Importance Sampling, with probabilistic earthquake location provides a fast and reliable tool for earthquake location. The comparison of hypocenter locations obtained routinely by the Swiss Seismological Service (SED) to those relocated in the new 3‐D velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths. We can attribute these large shifts in part to large uncertainties in the hypocenter location. Events with a low number of observations (<8) and no observation within the critical focal depth distance typically show large location uncertainties. Improved hypocentral locations, particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable, confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for Switzerland.
Earthquake triggering by static and dynamic stress changes
Authors: Belardinelli M. E. , Bizzarri A. , Cocco M.
Published: May 2003

In this study we aim to understand the effect of static and dynamic stress changes in promoting earthquake failures on secondary faults. Toward this goal we solve the equation of motion of a spring‐slider dynamic system including inertia and using rate‐ and state‐dependent constitutive laws. We separately investigate the dynamic response of this fault analog system to a sudden stress change represented either as a stress step or as a stress pulse, which are used to model permanent (static) and transient (dynamic) stress perturbations. The induced earthquake failure does not occur immediately at the application of the coseismic stress change, but it is delayed in time: we define this time interval as the triggering delay. For a given stress perturbation, we analyze the dependence of triggering delays on different system conditions and constitutive parameters. Our results clearly show that the effects of static and dynamic stress changes are quite different. While a static stress change is able to advance as well as to delay an induced instability depending on its sign, a dynamic stress pulse is only able to promote a nearly instantaneous failure, provided its amplitude is positive and large enough with respect to the direct effect of friction. In other words, dynamic stress changes can only cause nearly instantaneous failures, without any relevant triggering delay. These results should be considered in interpreting the seismicity rate changes caused by large earthquakes at least as long as seismic events are interpreted as sliding instabilities obeying rate‐ and state‐dependent friction laws.
Laboratory experiments simulating the geomorphic response to tectonic uplift
Authors: Lague Dimitri , Crave Alain , Davy Philippe
Published: May 2003

We present the results of an experimental study of topography dynamics under conditions of constant precipitation and uplift rate. The experiment is designed to develop a complete drainage network by the growth and propagation of erosion instabilities in response to tectonic perturbations. The quantitative analysis of topographic evolution is made possible by using telemetric lasers that perform elevation measurements at an excellent level of precision. We focus our study on the effect of initial surface organization and of uplift rate on both the transient dynamics and the steady state forms of topography. We show that the transient phase is strongly dependent on the initial internally drained area, which is found to decrease exponentially with time. The topography always reaches a steady state whose mean elevation depends linearly on uplift rate with a strictly positive value when uplift is zero. Steady state surfaces are characterized by a well‐defined slope–area power law with a constant exponent of −0.12 and an amplitude that depends linearly on uplift rate with a strictly positive value when uplift is zero. These results are consistent with a stream power law erosion model that includes a nonnegligible threshold for particle detachment. Uncertainty regarding the sediment transport length is resolved by calibrating the transient dynamics with a surface process model. Reappraising published results on the linear dependency between mean elevation, or relief, and denudation rate, we suggest that an erosion threshold is worth considering for large‐scale systems.
Lower mantle conductivity anomalies estimated from geomagnetic jerks
Authors: Nagao Hiromichi , Iyemori Toshihiko , Higuchi Tomoyuki , Araki Tohru
Published: May 2003

The occurrence epochs and duration periods of the 1969, 1978, and 1991 geomagnetic jerks are estimated for 65 geomagnetic observatories by applying a statistical time series model to monthly means of the geomagnetic eastward component. It is found for every jerk that the occurrence epochs around South Africa and South Pacific Ocean are delayed several years from those in other regions. Moreover, it is found for the 1969 and 1978 jerks that the duration times around South Africa, which are defined as the lengths of the duration periods, are longer than those in other regions. The distribution of these anomalous regions of the jerks (i.e., South Africa and South Pacific Ocean) may imply the existence of higher conductivities in the lower mantle beneath these regions, which possibly relates to the fact that the velocity of a seismic S wave in the D″ layer beneath these regions is slower than that in other regions. The mantle conductivities beneath South Africa, South Pacific Ocean, and other regions are estimated through a numerical simulation; the magnetic diffusion equation in the mantle is solved for an abrupt magnetic change generated at the core‐mantle boundary. Comparing the results obtained by the simulation with those obtained by the data analysis, it is plausible that the mantle conductivities beneath South Africa and South Pacific Ocean are higher than that beneath other regions, although there is an ambiguity in the estimated magnitudes of the mantle conductivity.
Pore pressure development and progressive dewatering in underthrust sediments at the Costa Rican subduction margin: Comparison with northern Barbados and Nankai
Authors: Saffer Demian M.
Published: May 2003

At subduction zones, pore pressure affects fault strength, deformation style, structural development, and potentially the updip limit of seismogenic faulting behavior through its control on effective stress and consolidation state. Despite its importance for a wide range of subduction zone processes, few detailed measurements or estimates of pore pressure at subduction zones exist. In this paper, I combine logging‐while‐drilling (LWD) data, downhole physical properties data, and laboratory consolidation tests from the Costa Rican, Nankai, and Barbados subduction zones, to document the development and downsection variability of effective stress and pore pressure within underthrust sediments as they are progressively loaded by subduction. At Costa Rica, my results suggest that the lower portion of the underthrust section remains nearly undrained, whereas the upper portion is partially drained. An inferred minimum in effective stress developed within the section ∼1.5 km landward of the trench is consistent with core and seismic observations of faulting, and illustrates the important effects of heterogeneous drainage on structural development. Inferred pore pressures at the Nankai and northern Barbados subduction zones indicate nearly undrained conditions throughout the studied intervals, and are consistent with existing direct measurements and consolidation test results. Slower dewatering at Nankai and Barbados than at Costa Rica can be attributed to higher permeability and larger compressibility of near‐surface sediments underthrust at Costa Rica. Results for the three margins indicate that the pore pressure ratio (λ) in poorly drained underthrust sediments should increase systematically with distance landward of the trench, and may vary with depth.
Magnetic susceptibility anisotropy: A new petrofabric tool in migmatites
Authors: Ferré Eric C. , Teyssier Christian , Jackson Mike , Thill James W. , Rainey Emma S. G.
Published: May 2003

The rheological behavior of the lower to middle continental crust during orogenic deformation is poorly understood. Migmatite, common at this crustal level, remains difficult to analyze from a structural viewpoint because of pervasive recrystallization. We present new anisotropy of magnetic susceptibility (AMS) data from 217 specimens of migmatites of the Superior Province in Minnesota with the aim of investigating its potential as a petrofabric tool. Quarry exposures provide the ideal sampling playground for such a study. The migmatites are coarse grained, are granitic to tonalitic in composition, and display a centimeter‐scale compositional layering. The layering and the regional foliation are subhorizontal to shallowly dipping to the NE. Mineral lineations are difficult to observe due to recrystallization but trend around 70°N with gentle plunges. Magnetic properties are carried mainly by multidomain (MD) primary magnetite grains ranging in size from 20 to 200 μm. The magnetic susceptibilities are generally high, around (5–10) × 10−3 SI. The degree of magnetic anisotropy is large, between 1.2 and 1.3. The principal axes of the AMS ellipsoids coincide with those of the finite strain, and the degree of anisotropy broadly correlates with the mineral fabric intensity. This study is the first detailed investigation of magnetic fabrics in migmatites. These preliminary results suggest that the AMS technique is an easy and powerful tool for the petrofabric analysis of migmatites.
Single‐chamber silicic magma system inferred from shear wave discontinuities of the crust and uppermost mantle, Coso geothermal area, California
Authors: Wilson Charles K. , Jones Craig H. , Gilbert Hersh J.
Published: May 2003

Analysis of seismograms from teleseismic rays traversing the Coso geothermal area near Ridgecrest, California, suggests the geothermal system lies over a single shallow magma reservoir (∼5 km below the surface) that also plays a crucial role in the local change in deformation style from areas to the north and west. The character of the magma reservoir and the absence of a lower crustal magma reservoir is inferred from three crustal P‐to‐S conversions observed using receiver function analysis: (1) A high‐amplitude, shallow, negative arrival, Ps‐P time of 0.7–0.9 s (3–5 km below sea level (bsl)), (2) a moderate amplitude, positive conversion, Ps‐P time of 2.1–2.5 s (14–17 km bsl), and (3) the Moho conversion, Ps‐P time of 4.0–4.2 s (30–32 km bsl). Observations of Moho converted arrivals indicate that the interface is mostly flat and uncomplicated throughout the study area, while the midcrustal conversion is laterally variable in amplitude and depth. The absence of the large negative amplitude conversion on waveforms recorded at stations outside the geothermal area strongly suggests that the feature lies only underneath the modern geothermal area. In addition, rays sampling the shallow converter also contain later arrivals with retrograde moveout consistent with an origin as reverberations above the conversion. Receiver functions calculated from synthetic data using a single isotropic layer over a half‐space indicates that the shear velocity decreases by 30% across the interface (VS1 = 2.6 km/s; VS2 = 1.8 km/s; layer one thickness 4.9 km), further supporting the presence of shallow magma.
Geometrical effects and low‐attenuation resonance of volcanic fluid inclusions for the source mechanism of long‐period earthquakes
Authors: Fujita Eisuke , Ida Yoshiaki
Published: May 2003

We formulate the resonances of three simple geometrical inclusions (a plane, a cylinder and a sphere) and find several clues to distinguishing the shape of volcanic fluid systems and their relevance to the observation of long‐period earthquakes. The series of eigenfrequencies for a cylindrical and a spherical inclusion have shifts of π/4 and π/2, respectively, in their higher modes from the eigenfrequencies of a planar inclusion. The ratios between the higher and fundamental characteristic frequencies can be indicators of the geometry of volcanic fluid inclusions. Furthermore, the eigenattenuation factor of a planar inclusion is a constant value that is determined uniquely by the impedance contrast between the fluid and the surrounding elastic medium, and is independent of mode. In marked contrast, cylindrical and spherical inclusions have smaller or greater eigenattenuation factors, particularly in lower modes. Under certain conditions of P wave velocity contrast, and Poisson's ratio of the surrounding elastic medium, lower modes have less than 0.1 times the attenuation factor of higher modes. We name these “low‐attenuation modes” (LAMs). LAMs are physically related to the conversion of volumetric changes to shear deformation of the surrounding elastic medium and the radiation of shear waves from the curved boundaries. Some monotonic waveforms of long‐period earthquakes can be interpreted by the excitation of these LAMs. Crack modes are also simply formulated by the resonance of a rectangular inclusion and also become LAMs whose attenuation factors are related to the aspect ratio of the crack.
The 2000 Nemuro‐Hanto‐Oki earthquake, off eastern Hokkaido, Japan, and the high intraslab seismic activity in the southwestern Kuril Trench
Authors: Takahashi Hiroaki , Hirata Kenji
Published: May 2003

The 2000 Nemuro‐Hanto‐Oki earthquake (Mw6.8) occurred in the southwestern part of the Kuril Trench. The hypocenter was located close to the aftershock region of the great 1994 Kuril earthquake (Mw8.3), named “the 1994 Hokkaido‐Toho‐Oki earthquake” by the Japan Meteorological Agency, for which the fault plane is still in debate. Analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event. The hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage. They clearly show that the 2000 event was an intraslab event occurring on a shallow‐dipping fault plane between 55 and 65 km in depth. The well‐focused aftershock distribution of the 2000 event, the relative location of the 1994 event with respect to the 2000 event, and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow‐dipping fault plane in the subducting slab. The recent hypocenter distribution around the 1994 aftershock region also supports this result. Large intraslab earthquakes occurring to the southeast of Hokkaido may occur due to a strong coupling on the plate boundary, which generates relatively large stress field within the subducting Pacific plate.
Creep of polycrystalline anorthite and diopside
Authors: Dimanov A. , Lavie M. P. , Dresen G. , Ingrin J. , Jaoul O.
Published: May 2003

We investigated the creep strength of fine‐grained aggregates of single‐phase anorthite and diopside at temperatures ranging from 1253 to 1553 K at flow stress between 5 and 250 MPa and at 0.1 MPa pressure. Powders of crushed natural, iron‐bearing diopside and of pure anorthite and diopside glasses were hot pressed at temperatures of 1373–1423 K and 300 MPa confining pressure in a gas‐medium apparatus and at 1423–1473 K and 2200 MPa in a piston‐cylinder device. Grain size of the synthetic materials was 3.4 μm for anorthite and between 1.5 and 7.5 μm for diopside. Creep tests were performed using two uniaxial creep machines in Toulouse and Potsdam. Synthetic anorthite and diopside aggregates showed a stress exponent n ≈ 1 and a grain size exponent m ≈ −3, suggesting grain boundary diffusion‐controlled creep. Creep rates were independent of oxygen fugacity. Coarser‐grained (43 μm) iron‐bearing samples showed a transition from diffusion‐controlled creep to dislocation creep with n ≈ 5. Activation energies for diffusion and dislocation creep of iron‐bearing diopside polycrystals were Q = 364–468 and 719 kJ/mol, respectively. The activation energies for diffusion‐controlled creep of synthetic fine‐grained anorthite and diopside aggregates were Q = 362–383 and 558 kJ/mol, respectively. At similar thermodynamic conditions, diffusion creep rates of diopside are nearly 10 times lower than for anorthite. Irrespective of processing route, the data for anorthite and diopside aggregates from both laboratories are in good agreement.