bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic tomography, surface uplift, and the breakup of Gondwanaland: Integrating mantle convection backwards in time
Authors: Conrad Clinton P. , Gurnis Michael
Published: May 2003

Mantle density heterogeneities, imaged using seismic tomography, contain information about time‐dependent mantle flow and mantle structures that existed in the past. We model the history of mantle flow using a tomographic image of the mantle beneath southern Africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition. If the resulting (backwards integrated) model for structures is used as a starting point for a forwards convection model, today's mantle can be adequately reconstructed if we do not integrate backwards more than than about 50–75 Ma. Flow can also be reliably reversed through the Mesozoic, but only if instability of the lower boundary layer can be suppressed. Our model predicts that the large seismically‐slow and presumably hot structure beneath southern Africa produced 500–700 m of dynamic topography throughout the Cenozoic. Since ∼30 Ma, uplift has moved from eastern to southern Africa, where uplift rates are ∼10 m/Myr, consistent with observations. During the Mesozoic, the modeled topographic high is situated near Gondwanaland rifting, raising the possibility that this buoyant structure may have been involved with this breakup.
A statistically significant long term characteristic time‐scale of test size variation of Calcareous Trochospiral Benthic Foraminifera (CTBF) during the past 120 m.y.
Authors: Tiwari R. K. , Rao K. N. N.
Published: May 2003

Geo‐marine records are faithful recorders of the earth's evolutionary history and hence are most suitable for identifying long‐term cyclic patterns. Although a remarkable synchronism among Calcareous Trochospiral Benthic Foraminifera (CTBF) test size variation, ocean productivity, dissolved oxygen and global climate exists, global “geospectroscopy” is essential for understanding the wider ramification of significant physical linkages of the various terrestrial and extra‐terrestrial processes. Earlier claims of a quasi‐periodicity of 26–36 m.y. in various geo‐bio‐ocean‐atmospheric records sparked a major controversy. Here, we examine spectral characteristics of the latest available time series representing fluctuations in test size of CTBF for the past 120 m.y. using the Multi‐Taper Method (MTM) and Maximum Entropy Method (MEM) of spectral analyses. The analyses of test size variations of CTBF time series reveal, hitherto unidentified, a statistically significant and stable harmonic component of 35 ± 5 m.y. against a robust “red background noise”. Further, wavelet spectral analysis of this record exhibits a strong stationary power in the above frequency band suggesting temporal persistence of spectral peak through the entire evolutionary period. This finding renders credible support to the concept of cyclic geo‐marine processes and will provide stimulus for further quantitative research into the coupling of marine geo‐bio‐chemical cycles.
ULF and ELF magnetic activity from a terrestrial dust devil
Authors: Houser Jeffrey G. , Farrell William M. , Metzger S. M.
Published: May 2003

Terrestrial dust devils are known to be triboelectric generators: electric charge is generated and exchanged via the mixing and colliding of moving grains. Static electric fields from the structures have been previously reported, this in excess of 1/2 kV/m at a couple hundred meters from the devil. In this work, we report on AC magnetic measurements made during the passage of an intense ∼10‐m wide, couple hundred‐meter tall dust devil in the Nevada desert. The devil‐related magnetic activity appears in two forms: Impulsive extremely‐low‐frequency (ELF: 30–300 Hz) static discharges to the instrument as the sensor became immersed directly into the devil's electrified dust and ultra‐low‐frequency (ULF: 3–30 Hz), continuous emissions that were sensed remotely as the storm approached and receded from the mobile station. The latter is a new finding not previously reported. Such measurements not only prove that individual dust grains within the devil are charged, but that the grains are transported in bulk to form an extended, coherent radiation source.
Sediment core profiles of long‐chain n‐alkanes in the Sea of Okhotsk: Enhanced transport of terrestrial organic matter from the last deglaciation to the early Holocene
Authors: Seki O. , Kawamura K. , Nakatsuka T. , Ohnishi K. , Ikehara M. , Wakatsuchi M.
Published: May 2003

Terrestrial plant‐derived n‐alkanes (C25–C35) were measured in three piston cores (PC1, PC2 and PC4) in the Sea of Okhotsk covering the last 30 kyrs. Down core profiles of the n‐alkane concentrations and mass accumulation rates (MAR) were characterized by deglacial maxima. In particular, cores PC2 and PC4, which were collected from the central and western Sea of Okhotsk, respectively, show a two‐step increase around the Meltwater Pulse events (MWP) 1A (14.5–13.5 kyr BP) and 1B (about 10 kyr BP). This finding was interpreted by the outflow of terrestrial organic matter from the submerged land shelf to the Sea of Okhotsk through the East Sakhalin Current. This study demonstrated that the sea level rise forced by global warming in the deglaciation period may have caused the enhanced transport of terrestrial organic matter in marginal seas.
Evidence of gravity wave breaking in lidar data from the mesopause region
Authors: Franke Patricia Minthorn , Collins Richard L.
Published: May 2003

An outstanding question about the dynamics of the mesosphere is the temporal and spatial distribution of nonlinear events such as wave‐breaking, wave saturation, and wave‐critical layer interactions. A climatology of these events will help us understand how the mesoscale dynamical features, such as gravity waves, interact with the background mean wind and temperature structure. New lidar systems have the resolution to show us a height versus time “picture” of the dynamics so that identifying individual events within the observation window of the instrument is now possible. At the Starfire Optical Range (SOR) a sodium resonance lidar provides simulataneous sodium density, temperature, and three components of the winds. In this paper we present “pictures” of individual wave events apparent in the lidar data using the temperature (plotted as potential temperature and spectra) to show the time evolution of the wave structure.
Responses of the open–closed field line boundary in the evening sector to IMF changes: A source mechanism for Sun‐aligned arcs
Authors: Maynard N. C. , Burke W. J. , Moen J. , Ober D. M. , Scudder J. D. , Sigwarth J. B. , Siscoe G. L. , Sonnerup B. U. Ö. , White W. W. , Siebert K. D. , Weimer D. R. , Erickson G. M. , Frank L. A. , Lester M. , Peterson W. K. , Russell C. T. , Wilson G. R. , Egeland A.
Published: May 2003

Simultaneous measurements from the Polar satellite and by ground‐based optical sensors suggest that brief variations of the poleward auroral boundary on the nightside correlate with changes in the interplanetary magnetic field (IMF) about an hour after a structure has propagated in the solar wind past the Earth. Short‐lived Sun‐aligned arcs may emerge along the open–closed magnetic field line boundary (OCB) and then disappear after ∼10 min. The arcs are fueled by energetic particles whose spectral characteristics are similar to those of the of boundary plasma sheet (BPS). Polar measurements confirm that these auroral protrusions into the polar cap occur on nearly isolated closed magnetic flux. Optical emissions from these arcs appear strongest at their intersection with the poleward boundary of the auroral oval. Detailed magnetohydrodynamic (MHD) simulations of dayside interactions, when the dominant IMF component BY changes sign, indicate that near the polarity reversal merging can occur between interplanetary field line segments within the magnetosheath [Maynard et al., 2001b]. Newly formed loops of interplanetary flux sweep past the Earth without interacting with the magnetosphere. Here, we consider some consequences within the distant magnetotail as the loops of disconnected flux propagate to XGSM ≈ −200 RE. The MHD simulations indicate that about an hour after intra‐IMF merging events fingers of closed field lines protrude from the OCB into the polar cap. Like the observed Sun‐aligned arc, these simulated auroral features grow and decay on scales of ∼10 min and have ionospheric footprints that are nearly surrounded by open magnetic flux. The simulated auroral fingers are conjugate to high‐pressure channels in the distant plasma sheet. We suggest that the short‐lived Sun‐aligned arcs are created via an interchange process similar to that proposed by Kan and Burke [1985] to explain one class of theta auroral forms. The continuity of magnetotail currents across a high‐pressure channel requires the development of field‐aligned currents carried by obliquely propagating Alfvén waves. Plasma drifts associated with the dusk‐to‐dawn electric fields of the Alfvén waves are away from the Earth in the magnetotail and poleward in the nightside ionosphere. The correlation of phenomena at the nightside OCB with variations in the IMF indicate that processes other than substorms can influence boundary dynamics. Effects of self‐interactions of the IMF within the dayside magnetosheath may be felt along the OCB as much as 1 hour later.
Electrostatic solitary waves as collective charges in a magnetospheric plasma: Physical structure and properties of Bernstein–Greene–Kruskal (BGK) solitons
Authors: Krasovsky V. L. , Matsumoto H. , Omura Y.
Published: May 2003

The electrostatic pulses recorded by the Geotail spacecraft and labeled electrostatic solitary waves (ESW) are considered within the framework of Bernstein–Greene–Kruskal (BGK) solitons. The main goal of this paper is to develop sufficiently general and simple theoretical models of the nonlinear wave perturbations under study. Such models are necessary for understanding the physical structure and properties of the observed waves as well as for the treatment of the extensive experimental material, which has been accumulated on the ESW waveforms and for meaningful data processing. Examples of localized BGK modes described previously in literature are difficult to compare with the experimental data due to their particular nature. In contrast to these particular models, the general approach developed in the article applies to arbitrary particle distributions of the background plasma, velocities of the BGK solitons and wide variety of the recorded ESW waveforms. Relying on the theory of BGK solitons, with an emphasis on their general properties, the work contains a consistent analytical description and physical interpretation of ESW as effective spatial charges shielded by a collisionless plasma. The general analysis of localized BGK waves leads to a number of new results, such as unified physical models of ESW based on the additivity of the distribution function, as well as energetic relations and universal restrictions on the parameters and waveforms of the solitons. The new models and physical interrelations reveal universal features of the BGK soliton structure and allow a direct juxtaposition with the observations. The established interconnections between the physical characteristics of the waves agree well with the Geotail data on ESW waveforms. We also discuss less studied nonstationary phenomena, among which are wave emission by the BGK solitons, mechanisms of electron phase density hole interaction and certain questions of stability. Some of the most typical examples of the various BGK solutions described in the article are useful for a correlation between the theory and measurements as well as for an identification of the ESW waveforms observed in the magnetosphere.
Fast Auroral Snapshot observations of the dependence of dayside auroral field‐aligned currents on solar wind parameters and solar illumination
Authors: Cattell C. , Dombeck J. , Peria W. , Strangeway R. , Elphic R. , Carlson C.
Published: May 2003

The solar illumination dependence of solar wind‐magnetosphere coupling has been studied by examining the dependence of auroral zone field‐aligned currents on solar wind parameters. A database of Region I currents from ∼900 crossings of the auroral zone by Fast Auroral Snapshot (FAST) was parameterized by magnetic local time, invariant latitude, and whether the foot point was illuminated. The magnitudes of dayside currents (∼200 events) were correlated with solar wind parameters using the technique of rank correlation. Very significant, strong correlation between dayside sunlit currents and solar wind parameters such as Btsin(θ/2)v2 and Btsin(θ/2)Pd1/2 was observed, consistent with a reconnection source. There was no correlation with upstream parameters for dayside Region I currents when the ionospheric foot point was in darkness. This is the first experimental evidence showing that the correlation of dayside Region I currents to upstream parameters depends on ionospheric illumination. Both the strong dependence of the correlation on solar illumination (ionospheric conductivity) and the weakness of observed correlations, compared to those obtained in previous studies of the cross polar cap potential, may be interpreted to imply that the reconnection process is a voltage, rather than a current, source. However, several theoretical studies have suggested that reconnection does not act as either a pure voltage source or a current source. In addition, because the mapping of the field‐aligned currents was not examined, the observed difference between the sunlit and dark events could be explained by current preferentially flowing in the hemisphere with the higher conductivity if the source were on closed field lines.
Solar wind disturbances responsible for geomagnetic storms
Authors: Lyatsky W. , Tan A.
Published: May 2003

The superposed epoch method is used to construct a typical behavior of solar wind parameters before and during strong isolated geomagnetic storms. A total of 130 geomagnetic storms during the period of 1966–2000 were analyzed. The results obtained show that the averaged disturbance in the solar wind responsible for geomagnetic storms is associated with the compression of ambient solar wind plasma and interplanetary magnetic field (IMF) ahead of a high‐speed plasma flow. The magnetic field strength and plasma density start to increase several hours before geomagnetic storm onset. However, the negative IMF Bz (which is responsible for the onset of strong negative Dst) starts to increase approximately 4–5 hours after the maximum variations in plasma density and IMF By. The time delays between peaks in negative IMF Bz and plasma density as well as IMF ∣By∣ (in the solar‐magnetospheric coordinate system) may be a result of the compression and three‐dimensional draping of the ambient magnetic field about high‐speed flows as discussed earlier by some authors. The combined effect of the compression and draping of the ambient magnetic field may lead to an increase in plasma density and IMF ∣By∣ ahead of a high‐speed flow followed by an increase in IMF ∣Bz∣ in its central portion. The motion of such structure leads to a time delay between the peaks in the IMF ∣By∣ and ∣Bz∣ observed at satellites.
Reanalysis of Saturn's magnetospheric field data view of spin‐periodic perturbations
Authors: Espinosa Stéphane A. , Southwood David J. , Dougherty Michèle K.
Published: May 2003

Periodic perturbations with their period close to that of planetary rotation are observed in most of the magnetic field data from Saturn's magnetosphere. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). The long‐held view that no planetary spin‐periodic modulation was present in the magnetic field observations is thus not true. Here we present several new pieces of information obtained from a careful analysis of the magnetic field data in view of this peculiar periodic feature. First, by simple considerations of the magnetic field morphology, we argue that these perturbations cannot be directly due to the planetary intrinsic field. Also, we analyze by means of two‐dimensional (2‐D) hodographs the rotation of the magnetic field vector, expressed in an inertial planetocentric spherical polar coordinate system, and obtain a definitive argument against the possibility of a dipole tilt signature. In addition, we find that the inbound and outbound Pioneer 11 observations of the perturbations are nearly in phase (once the spatial distance is accounted for), indicating that this periodic feature is of a global nature. Finally, we discuss the fact that in the same data set, the magnetopause position in the dawn sector seems to be modulated in phase with the radial component of the perturbation magnetic field.
Southern Hemisphere poleward moving auroral forms
Authors: Drury E. E. , Mende S. B. , Frey H. U. , Doolittle J. H.
Published: May 2003

This paper presents a statistical study of Southern Hemisphere poleward moving auroral forms (PMAFs) using optical data from the US Automatic Geophysical Observatory network in Antarctica. These Southern Hemisphere events are compared with Northern Hemisphere PMAFs, which have previously been observed during varied interplanetary magnetic field (IMF) configurations. The frequency of PMAF occurrence is studied as a function of IMF orientation and magnetic local time. Southern Hemisphere PMAFs are biased to Bz < 0 constituting 62% of events, but are frequently observed during conditions of Bz > 0 constituting 38% of events. Southern Hemisphere PMAFs share a similar IMF Bz dependence with Northern Hemisphere events [e.g., Fasel, 1995]. The primary modulator of PMAFs was found to be IMF By, which was negative for 81% of events. Although the average background IMF was biased to By < 0, we find significant enhancement during By < 0 after this bias was removed. In a statistical study of Northern Hemisphere events, Fasel [1995] found 77% of events occur during positive By. PMAFs appear to have an antisymmetric By dependence between hemispheres. We find By < 0 (By > 0) enhances the observation of PMAFs in the prenoon (postnoon) region, consistent with the release of magnetic stress. A morning bias of Southern Hemisphere PMAFs is observed, consistent with Northern Hemisphere events.
Width and brightness of auroral arcs driven by inertial Alfven waves
Authors: Chaston C. C. , Peticolas L. M. , Bonnell J. W. , Carlson C. W. , Ergun R. E. , McFadden J. P. , Strangeway R. J.
Published: May 2003

Alfven waves with scale sizes perpendicular to the geomagnetic field of the order of an electron skin depth (λe), or electron inertial length, and larger are often observed by polar orbiting spacecraft above the aurora. Using observations from the FAST spacecraft, predictions from an MHD simulation, and theoretically derived results for the efficiency of light production from auroral electron spectra characteristic of those found in Alfven waves, the width and brightness distribution of auroral arcs driven by linear inertial Alfven waves are determined. It is shown that inertial Alfven waves can drive aurora to optical intensities as high as 100 kR over widths in the ionosphere of the order of 1 km. It is also shown that while linear inertial Alfven waves can cause auroral arcs of widths less than 100 m, it is unlikely that these waves can account for the optically observed distribution of narrow arc widths reported in the literature.
Statistical investigation of the saturation effect in the ionospheric foF2 versus sunspot, solar radio noise, and solar EUV radiation
Authors: Liu J. Y. , Chen Y. I. , Lin J. S.
Published: May 2003

This study explores the saturation effects in the ionospheric foF2 due to sunspot number R, solar radio noise (10.7 cm) flux F10.7, and solar EUV fluxes. To locate the R, F10.7, or EUV value at which the foF2 values are saturated, a two‐segmented regression model is built based on the data of the strictly rise period of the 21st solar cycle recorded by eight ionosonde stations scattering roughly between 40°N and 40°S geomagnetic latitude. Results show that clear saturation features appear around the equatorial anomaly crest regions. The regression model is then fitted into the foF2 data observed at Chung‐Li station (13.8°N, geomagnetic) to investigate the hourly variation of the saturation effect. The saturated foF2 slope and the value of the R or EUV at which the foF2 values are saturated in the fitted models are closely related to the overall mean and standard deviation of foF2. The hourly results demonstrate that the daily ionospheric equatorial fountain and prereversal enhancement are important for the saturation features.
Empirical modeling of global ionospheric foF2 response to geomagnetic activity
Authors: Kutiev Ivan , Muhtarov Plamen
Published: May 2003

The authors expand the previously developed midlatitude model, providing the relative deviation of foF2 from its monthly median value as a function of local time and Kp, to the global scale. To achieve this, 55 ionosonde stations, having at least 11 years of continuous data, have been selected, and the model was applied to the data from each station separately. Data from each station were grouped into 12‐month bins, every bin containing all the available hourly data within the respective month of the year. The model considers the distribution of the relative deviation along the local time at any fixed moment as composed of a diurnal and a semidiurnal waves, expressed by five parameters: daily mean (average offset), diurnal and semidiurnal amplitudes and phases. The model expression is scaled by a modified function of Kp, which reflects the delayed reaction of foF2 to Kp changes. The model parameters are determined by fitting the model expression to the data in each bin. Their distribution along the geomagnetic latitude is obtained in three longitude sectors: North America‐South America, Europe‐Africa, and East Asia‐Australia. The seasonal symmetry of model parameters in the Northern and Southern Hemispheres, which is found to be acceptable, allows the use of parameter values from both hemispheres in obtaining their latitudinal profiles. In order to produce global distribution of each of the model parameters, the respective latitudinal profiles from the three sectors were averaged and approximated by analytical expressions.
Evolution of energetic electron pitch angle distributions during storm time electron acceleration to megaelectronvolt energies
Authors: Horne Richard B. , Meredith Nigel P. , Thorne Richard M. , Heynderickx Daniel , Iles Roger H. A. , Anderson Roger R.
Published: May 2003

We investigate the pitch angle distributions of 0.15–1.58 MeV electrons observed during the 9–15 October 1990 storm measured by the Combined Release and Radiation Effects Satellite (CRRES) spacecraft. This storm period is characterized by an enhancement in the electron flux at L ≈ 4 by more than an order of magnitude over the prestorm level. The overall change in flux at L ≈ 6.6 is small in comparison. Previous work shows that radial diffusion underestimates the flux enhancement by up to a factor of 5 for L ≤ 4.5 [Brautigam and Albert, 2000], indicating the need for an additional acceleration process. The pitch angle distributions presented here are examined for evidence of the acceleration mechanism. The distributions at L ≈ 2 are rounded and are dominated by Coulomb collisions. They show little variation during the storm. The distributions at L ≈ 3 are pancake‐shaped before the storm, characteristic of pitch angle scattering by plasmaspheric hiss. During the main phase, they become broad and flat, and they evolve back into pancake distributions during the recovery phase. At L ≈ 4–6, the pitch angle distributions are characterized as butterfly distributions at storm onset, and they become broad flat top distributions during the recovery phase. The flat top distributions persist throughout the ∼3‐day recovery phase and are observed in the region of highest flux enhancement. The flat top distributions are energy dependent and are broader at lower energies (30°–150°) than at higher energies (50°–130°). The higher energies exhibit a much faster fall off toward the loss cone than at lower energies. Inward radial diffusion should result in anisotropic distributions peaked near 90° and does not explain the observed energy dependence. Furthermore, the direction of diffusion is outward at higher energies. Model calculations of the pitch angles resonant with whistler mode waves show that flat top distributions are consistent with pitch angle and energy scattering in regions where fpe/fce ∼ 1. Although radial diffusion may be very important for particle energization, the observed pitch angle distributions provide strong evidence that wave particle interactions play an important role in the energization process.
Relationship between magnetotail variations and auroral activities during substorms
Authors: Miyashita Y. , Machida S. , Liou K. , Mukai T. , Saito Y. , Meng C.‐I. , Parks G. K.
Published: May 2003

We have compared magnetotail variations with auroral activities during 8 substorms using GEOTAIL and Polar UVI data. In the 1737 UT substorm event on 15 December 1996, auroral breakups and intensifications were highly correlated with fast plasma flows with the variations in the north‐south magnetic field and the total pressure in the magnetotail. GEOTAIL was located around X ∼ −21 RE, and several fast tailward flows were observed in the early expansion phase with the southward magnetic field and the total pressure enhancement, associated with plasmoids. These flows were observed simultaneously with or within about 1 min of auroral breakups or pseudobreakups. In the late expansion or recovery phase, some fast Earthward flows were observed with the northward magnetic field as well as the total pressure enhancement slightly earlier than small auroral intensifications. These observations imply that the activation and tailward retreat of the near‐Earth neutral line is intermittent. Furthermore, including the other events, we found that the total pressure decrease in the magnetotail can be correlated with auroral activity better than the fast plasma flow. The total pressure in the magnetotail significantly decreases during auroral breakups or poleward expansions of the auroral bulge but slightly decreases during pseudobreakups. The duration of the expansion and the maximum size of the auroral bulge are closely correlated with the duration and amount of total pressure decrease in the magnetotail, respectively. These results imply that the magnitude of auroral activities associated with substorms depends on that of energy dissipation in the magnetotail.
Whistler‐mediated magnetic reconnection in large systems: Magnetic flux pileup and the formation of thin current sheets
Authors: Dorelli John C. , Birn Joachim
Published: May 2003

We compute numerical solutions of the resistive Hall MHD equations corresponding to pairwise magnetic island coalescence. The simulation results can be organized according to the relative sizes of three length scales: the electron dissipation length, ℓe; the ion inertial length, di; and the island wavelength, λ. We identify three qualitatively distinct regimes of magnetic island coalescence: (1) the resistive MHD limit, di ≲ ℓe ≪ λ; (2) the “whistler‐mediated” limit, ℓe ≪ di ≪ λ; and (3) the “whistler‐driven” limit, ℓe ≪ λ ≲ di. In the resistive MHD limit, magnetic flux piles up outside thin current sheets between the islands. The upstream Alfvén speed increases with increasing Lundquist number, and the reconnection rate is insensitive to the Lundquist number. In the whistler‐driven limit, the electron and ion bulk flows decouple on the island wavelength scale. Magnetic flux pileup does not occur, and the coalescence proceeds on a whistler timescale that is much shorter than the Alfvén time. In the whistler‐mediated limit, electron and ion bulk flows decouple in spatially localized “ion inertial sheets” around the island separatrices. Flux pileup is reduced, and the upstream Alfvén speed approaches a nearly constant value as the Lundquist number is increased. The maximum reconnection rate in the whistler‐mediated limit is comparable to that observed in the resitive MHD limit over the Lundquist number range 500 < Sλ < 10000.
A numerical study on the acceleration and transit time of coronal mass ejections in the interplanetary medium
Authors: González‐Esparza J. Américo , Lara Alejandro , Pérez‐Tijerina Eduardo , Santillán Alfredo , Gopalswamy Nat
Published: May 2003

Recently, an empirical model of the acceleration/deceleration of coronal mass ejections (CMEs) as they propagate through the solar wind was developed using near‐Sun (coronagraphic) and near‐Earth (in situ) observations [Gopalswamy et al., 2000, 2001a]. This model states and quantifies the fact that slow CMEs are accelerated and fast CMEs are decelerated toward the ambient solar wind speed (∼400 km/s). In this work we study the propagation of CMEs from near the Sun (0.083 AU) to 1 AU using numerical simulations and compare the results with those of the empirical model. This is a parametric study of CME‐like disturbances in the solar wind using a one‐dimensional, hydrodynamic single‐fluid model. Simulated CMEs are propagated through a variable ambient solar wind and their 1 AU characteristics are derived to compare with observations and the empirical CME arrival model. We were able to reproduce the general characteristics of the prediction model and to obtain reasonable agreement with two‐point measurements from spacecraft. Our results also show that the dynamical evolution of fast CMEs has three phases: (1) an abrupt and strong deceleration just after their injection against the ambient wind, which ceases before 0.1 AU, followed by (2) a constant speed propagation until about 0.45 AU, and, finally, (3) a gradual and small deceleration that continues beyond 1 AU. The results show that it is somewhat difficult to predict the arrival time of slow CMEs (Vcme < 400 km/s) probably because the travel time depends not only on the CME initial speed but also on the characteristics of the ambient solar wind and CMEs. However, the simulations show that the arrival time of very fast CMEs (Vcme > 1000 km/s) has a smaller dispersion so the prediction can be more accurate.
Akebono/Suprathermal Mass Spectrometer observations of low‐energy ion outflow: Dependence on magnetic activity and solar wind conditions
Authors: Cully C. M. , Donovan E. F. , Yau A. W. , Arkos G. G.
Published: May 2003

We present observations by the Suprathermal Mass Spectrometer (SMS) on Akebono (EXOS‐D) of ion outflow in the energy range from <1 to ∼70 eV. These observations cover a unique region of phase space and present an opportunity to “tie together” observations from disparate satellites. Variation of the total hemispheric O+ and H+ outflow rates with solar radio flux (monitored by the Penticton F10.7 index), with geomagnetic activity (monitored by the Kp index), and with solar wind parameters is discussed. Comparisons of F10.7 and Kp trends to results from Polar and Dynamics Explorer‐1 (DE‐1) lead us to conclude that flows of H+ in this low energy range are entirely sufficient to account for higher‐energy flows at higher altitudes. On the other hand, we infer a substantial amount of O+ at energies above 70 eV. Both H+ and O+ outflow rates in this range exhibit a strong correlation with the solar wind kinetic pressure, the solar wind electric field, and the variability in the interplanetary magnetic field (IMF) in the hour preceding. While these factors are also associated with increased geomagnetic activity (Kp), a separate, Kp‐independent effect is also found, showing a correlation of ion outflow with solar wind density and an anticorrelation with solar wind velocity.
Trimpi occurrence and geomagnetic activity: Analysis of events detected at Comandante Ferraz Brazilian Antarctic Station (L = 2.25)
Authors: Fernandez J. H. , Rizzo Piazza L. , Kaufmann P.
Published: May 2003

We present an analysis of the occurrence of Trimpi events observed at Comandante Ferraz Brazilian Antarctic Station (EACF), at L = 2.25, as observed by the amplitude of very low frequency (VLF) signals transmitted from Hawaii (NPM 21.4 kHz) from April 1996 to August 1999. The event parameters (total duration, amplitude variation, time incidence, and type (negative or positive)) were analyzed for 4394 events detected in the first year (solar minimum and relatively low Trimpi activity). The Trimpi occurrence was compared with geomagnetic activity for the whole period. A higher incidence of Trimpi events was observed near the equinoxes, with minima at the solstices following the same behavior as the Ap indices annual variation. During the day, their incidence peaked at ∼0700 and ∼1045 UT in accordance with the occurrence of lightning flashes in the geomagnetic conjugate regions but apparently not in agreement to the daily mean geomagnetic variation (Kp index) found to be peaked (in entire years sampling average) at 0300 UT and weaker at 0900 UT for the whole 4‐year period. The relationship between Trimpi occurrence and rise in solar activity for cycle 23 is discussed. A sudden strong incidence of Trimpi events around 5 May 1998 and 27 August 1998 was attributed to pronounced magnetic storms (Dst index).
On nonideal MHD properties of the partially ionized interstellar gas
Authors: Baranov Vladimir B. , Fahr Hans J.
Published: May 2003

The counterflow configuration resulting from the motion of the heliosphere through the ambient interstellar medium meanwhile has been described by several hydrodynamic and magnetohydrodynamic (MHD) model approaches. The self‐consistent inclusion of interstellar and solar magnetic fields in presently existing MHD approaches has been achieved assuming the validity of ideal magnetohydrodynamics where frozen‐in magnetic field condition is fulfilled. This assumption is valid only, however, if the electrical conductivity of plasma is very high (i.e., magnetic Reynolds number Rem ≫ 1). As we are going to show here, the presence of neutral H atoms in the interstellar medium strongly questions this requirement and leads to a nonideal Ohm's law invalidating the frozen‐in field condition. The MHD modeling of the heliospheric interface thus has to be carried out on a new basis presented in this paper.
Lower crustal structure in northern California: Implications from strain rate variations following the 1906 San Francisco earthquake
Authors: Kenner Shelley J. , Segall Paul
Published: May 2003

It is well known that geodetic data from a single instant in time cannot uniquely characterize structure or rheology beneath active seismogenic zones. Nevertheless, comparison of spatial and temporal variations in deformation rate with time‐dependent mechanical models can place valuable constraints on fault zone geometry and rheology. We consider postseismic strain rate transients by comparing geodetic data from north of San Francisco Bay obtained between 1906 and 1995 to predictions from viscoelastic finite element models. Models include (1) an elastic plate over a viscoelastic half‐space, (2) distributed shear within a viscoelastic layer, (3) discrete shear zones within an otherwise elastic layer, (4) discrete shear zones in combination with distributed viscoelastic shear, and (5) midcrustal detachment surfaces. We vary, as applicable, locking depth, elastic thickness, depth to the top and bottom of the distributed shear layer, distributed shear relaxation time, discrete shear zone relaxation time, and discrete shear zone width. The best fitting, physically reasonable elastic plate over viscoelastic half‐space models (1) do a poor job simultaneously predicting spatial and temporal variations in the data. The best fitting distributed shear models (2) do a poor job predicting spatial variations in the deformation rate. Although they fit the geodetic data, recent findings from seismic reflection‐refraction studies in northern California argue against models with shallow subhorizontal detachments (5). Models incorporating discrete shear zones (3, 4) provide the best fit to the geodetic data and are consistent with seismic studies that argue for discrete fault zones extending through the entire crust.
Hydrothermal microearthquake swarms beneath active vents at Middle Valley, northern Juan de Fuca Ridge
Authors: Golden Charles E. , Webb Spahr C. , Sohn Robert A.
Published: May 2003

Over 3000 local and regional earthquakes were recorded by a compact network of eight ocean bottom seismographs (500 m instrument spacing) from August 1996 to January 1997 in Middle Valley, a sediment‐covered rift valley on the northern Juan de Fuca Ridge. Thirteen swarms of small‐magnitude microearthquakes (−1.2 < Mw < 0.2) were detected beneath Dead Dog vent field, a major hydrothermal area in Middle Valley with exit fluid temperatures near 270°C. High precision relative positions for 304 events within swarms were determined using waveform cross‐correlation techniques. The events were relocated into small, spatially distinct clusters. The intensity of the swarms is correlated with high heat flow with the largest swarm positioned 1.3 km beneath the Dead Dog vents. Smaller clusters of earthquakes are located up to hundreds of meters outside the vent field. The results suggest that the observed seismicity in the Dead Dog region is triggered by thermal strain (contraction) in the hydrothermal reaction zone as fluids extract heat from hot basement rock. Microearthquake swarms appear to be concentrated in regions where faulting has promoted seawater penetration through the sediment layer, cooling the crust, and yielding larger strain rates than those produced by seafloor spreading.
Evaluation of paleomagnetic and finite strain relationships due to the Alleghanian Orogeny in the Mississippian Mauch Chunk Formation, Pennsylvania
Authors: Cioppa M. T. , Kodama K. P.
Published: May 2003

Three localities of the Mississippian Mauch Chunk Formation around the Pennsylvania Salient (Broad Top Synclinorium, Minersville Syncline, and a sublocality in the Minersville Syncline) were sampled for detailed paleomagnetic and strain analysis. Paleomagnetic analysis showed three magnetization components: an intermediate‐temperature B component (Tb < 650°C), a high‐temperature C component (Tb > 620°C), and an SW component, unblocked over a wide range of temperatures and observed only on the northern segment of the salient. Normalized Fry strain and Rf/ϕ strain were measured in three dimensions and correlated to the paleomagnetic results. At one site (JTSF locality), a clear correlation between the amount of strain (εs) and remanence direction indicates that axial planar cleavage formation has caused remanence rotation. The correlations at the Minersville Syncline and Broad Top Synclinorium are not definitive, but suggest that different mechanisms are operating. The observed heterogeneity in paleomagnetic response to strain indicates that the effects will be strongly dependent on the strain history at a particular site.
Searching for seismic scattering off mantle interfaces between 800 km and 2000 km depth
Authors: Castle John C. , van der Hilst Rob D.
Published: May 2003

We systematically examined seismic data from deep earthquakes in the western and northwestern Pacific, Tonga/Fiji, and South America for signs of sharp gradients in seismic properties that could represent either a mineralogical phase change or the boundary between geochemical reservoirs in the Earth's lower mantle. This involved the stacking of short‐period, vertical component teleseismic waveforms from the dense Pacific Northwest Seismic Network and searching for evidence of S‐to‐P conversions at sharp mantle structures between 800 and 2000 km depth. With these data, we estimate we should be able to detect seismic discontinuities with ΔVs > 2%, transition width <20 km, and limited local topography. Using data from over 45 earthquakes, we found no evidence of a horizontal global discontinuity, neither near 920 km nor near 1700 km, beneath the four convergent margins studied here. Consistent with previous results, we detected a deep structure with topography near 1600 km beneath the Marianas region. Because we observed the signal over a large geographical region beneath the Marianas (10°N to 20°N), it is likely a coherent local structural and not a point source. The absence of such a structure beneath the northwest Pacific, the Tonga/Fiji region, or South America, argues against a global feature near 1600 km depth, however. The lack of convincing and consistent scattering suggests that no composition boundary or thermal boundary layer exists between 800 to 2000 km depth beneath the convergent zones studied that has ΔVs < 2%, locally limited topography, and a transition interval <20 km.
Resolution, uncertainty, and whole Earth tomography
Authors: Vasco D. W. , Johnson Lane R. , Marques Osni
Published: May 2003

Using the arrival times of 10 distinct phases from the International Seismological Center catalogue (P, S, PP, PcP, SS, ScS, PKPab, PKPbc, PKPdf, SKSac) and two sets of differential times (SS − S410S and SS − S660S), we estimate the P and S velocity structure of the whole Earth (mantle, outer core, and inner core), boundary topography of four interfaces within the Earth, hypocentral relocation parameters, and station corrections. The entire catalogue is relocated and reassociated using the radial velocity model ak135. Both P and S phases are used in the relocation of over 41,000 events. In depth, isotropic velocity variations within the Earth are parameterized by 22 layers. Lateral variations in shear and compressional velocity within the layers are represented by one of two grids. In the mantle, each layer is subdivided into 4560 equal‐volume cells, of dimension 3° × 3° at the equator. However, in the outer and inner core, the cell size is increased to 6° × 6°. This change in grid spacing accounts for decreasing cell volumes with depth and variations in sampling between the mantle and core. Previous attempts using a fine grid throughout the Earth resulted in poor resolution of core structure. With the current grid, the resolution of velocity heterogeneity in the mantle and outer core is comparable. In the upper and midmantle, the well‐resolved cells lie beneath Eurasia and a narrow zone surrounding the mantle beneath the Pacific basin. In the lowest mantle, the resolution decreases significantly. An examination of the averaging kernels associated with six locations near the base of the mantle reveals the significant lateral and vertical averaging inherent in estimates of compressional velocity. In well‐sampled regions of the outer core, we are able to resolve compressional velocity variations in our 6° × 6° cells. However, velocity variations in the inner core are poorly constrained at the scale of our blocks. In well‐sampled regions of the mantle and outer core model, parameter standard deviations remain below 0.25%. For much of the inner core, the standard deviations are quite large, exceeding 0.5%. Narrow, high‐velocity anomalies are imaged in the upper mantle, associated with subduction zones. In a number of locations, such as beneath the Americas, these features extend well into the lower mantle. In the outer core, heterogeneity is generally lower in magnitude, of the order of 0.5%. The most notable feature in the outer core is a systematic shift to slightly higher velocity. In addition, we find a roughly symmetric distribution of heterogeneity at the base of the outer core. Compressional velocities are generally higher in the equatorial regions and lower at the poles.
Slip rate and earthquake recurrence along the central Septentrional fault, North American–Caribbean plate boundary, Dominican Republic
Authors: Prentice Carol S. , Mann Paul , Peña Luis R. , Burr G.
Published: May 2003

The Septentrional fault zone (SFZ) is the major North American‐Caribbean, strike‐slip, plate boundary fault at the longitude of eastern Hispaniola. The SFZ traverses the densely populated Cibao Valley of the Dominican Republic, forming a prominent scarp in alluvium. Our studies at four sites along the central SFZ are aimed at quantifying the late Quaternary behavior of this structure to better understand the seismic hazard it represents for the northeastern Caribbean. Our investigations of excavations at sites near Rio Cenovi show that the most recent ground‐rupturing earthquake along this fault in the north central Dominican Republic occurred between A.D. 1040 and A.D. 1230, and involved a minimum of ∼4 m of left‐lateral slip and 2.3 m of normal dip slip at that site. Our studies of offset stream terraces at two locations, Rio Juan Lopez and Rio Licey, provide late Holocene slip rate estimates of 6–9 mm/yr and a maximum of 11–12 mm/yr, respectively, across the Septentrional fault. Combining these results gives a best estimate of 6–12 mm/yr for the slip rate across the SFZ. Three excavations, two near Tenares and one at the Rio Licey site, yielded evidence for the occurrence of earlier prehistoric earthquakes. Dates of strata associated with the penultimate event suggest that it occurred post‐A.D. 30, giving a recurrence interval of 800–1200 years. These studies indicate that the SFZ has likely accumulated elastic strain sufficient to generate a major earthquake during the more than 800 years since it last slipped and should be considered likely to produce a destructive future earthquake.
Crustal structure of northern and southern Tibet from surface wave dispersion analysis
Authors: Rapine Richard , Tilmann Frederik , West Michael , Ni James , Rodgers Arthur
Published: May 2003

Group and phase velocities of fundamental mode Rayleigh waves, in the period range of 10 to 70 s, are obtained for southern and northern Tibet. Significant variations in crustal velocity structure are found. The group velocity minimum for Tibet occurs at ∼33 s and the minimum is ∼0.12 km/s lower for southern Tibet than for northern Tibet. At periods greater than 50 s, however, group velocities are up to 0.2 km/s faster in southern Tibet. The group and phase velocities are inverted for layered S wave models. The dispersion observations in southern Tibet can only be fit with a low‐velocity layer in the middle crust. In contrast, the velocity models for northern Tibet do not require any low‐velocity zone in the crust. The S wave velocity of the lower crust of southern Tibet is ∼0.2 km/s faster than the lower crust of northern Tibet. In southern Tibet the sub‐Moho velocity increases with a positive gradient that is similar to a shield, while there is no velocity gradient beneath northern Tibet. The high‐velocity lower crust of southern Tibet is consistent with the underthrusting of Indian continental lithosphere. The most plausible explanation of the mid‐crustal low velocity zone is the presence of crustal melt resulting from H2O‐saturated melting of the interplate shear zone between the underthrusting Indian crust and overflowing Asian crust. The lack of a pronounced crustal low‐velocity zone in northern Tibet is an indication of a relatively dry crust. The low S wave velocity in the lower crust of northern Tibet is interpreted to be due to a combination of compositional differences, high temperatures, presumably caused by a high mantle heat flux, and possibly small amounts of partial melt. Combined with all available observations in Tibet, the new surface wave results are consistent with a hot and weak upper mantle beneath northern Tibet.