bluuebunny/crossref_metadata_2025 huggingface dataset:
Radial evolution of corotating merged interaction regions and flows between ≈14 AU and ≈43 AU
Authors: Burlaga L. F. , Ness N. F. , Belcher J. W.
Published: February 2004

During 1993 and 1994 the solar coronal holes and the heliospheric current sheet were relatively stationary, and recurrent streams and interaction regions with periods of the order of the solar rotation period were present within 5 AU. One expects that during 1994 Voyager 2 (V2) (located at ≈43 AU and at ≈12°S latitude, in the sector zone) would have observed some evolutionary form of corotating streams and interaction regions. The “sector zone” is the latitude band in which a spacecraft observes both positive and negative sectors [Burlaga and Ness, 1996]. We present the observations of the magnetic field strength (B), the speed (V), density (N), and proton temperature (T) made by V2‐94, and for reference we also discuss the observations made by V2 at ≈14 AU during 1983 (V2‐83) a solar cycle earlier. Correlated, quasiperiodic variations inBandNwith a period of ≈26 days (corotating merged interaction regions) were observed at ≈14 AU but not at ≈43 AU. The speed and temperature profiles were irregular at ≈14 AU but quasiperiodic at ≈43 AU. An ƒ−2spectrum of the magnetic field strengthB(indicating the dominance of shocks) was observed at ≈14 AU, but an ƒ−5/3spectrum (indicating the dominance of Kolmogorov turbulence) was observed at ≈43 AU in the range (2.7 × 10−6to 2.3 × 10−5)Hz. An ƒ−2.5spectrum of the speed fluctuations was observed at ≈14 AU, but an ƒ−2spectrum was observed at ≈43 AU in the range (8.8 × 10−7to 2.3 × 10−5)Hz. We suggest the hypothesis that the qualitative differences between the observations at ≈14 AU and ≈43 AU represent a change in the state of the solar wind as it moves between these two positions. This change involves a transition from a quasiperiodic (ordered) state inBandNat ≈14 AU to a disordered state at ≈43 AU and from an aperiodic state inVandTat ≈14 AU to a quasiperiodic state at ≈43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions have not predicted such a transition. Our results suggest that it would be fruitful to develop a new MHD model of such flows, which should include (1) three‐dimensional effects, (2) the intermediate‐scale fluctuations, and (3) the interstellar pickup ion pressure.
Study of propagation characteristics of very low latitude whistlers by means of three‐dimensional ray‐tracing computations
Authors: Ohta Kenji , Nishimura Yasuhiro , Kitagawa Tomomi , Hayakawa Masashi
Published: February 2004

The propagation mechanism of very low latitude (geomagnetic latitudes of less than 10–15°) whistlers is poorly understood. There is a controversy on their propagation; some workers using the observational facts have suggested field‐aligned propagation, but some theoretical (ray tracing) works have all indicated nonducted propagation. This paper reexamines the propagation characteristics of nonducted propagation, but we use three‐dimensional ray tracing (different from previous works) for realistic ionosphere/magnetosphere models (the electron density profile with latitudinal and longitudinal gradients and the International Geomagnetic Reference Field (IGRF) magnetic field model instead of the conventional dipole model). By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input southern hemisphere, we have found that it is possible for us to detect simultaneously, at a very low latitude position in the northern ionosphere, one‐hop whistler rays started from slightly spaced locations in the south with different initial wave normal angles and that some of them can penetrate through the ionosphere, but some others cannot. On the basis of systematic analysis of important parameters, we come to the general conclusion that it is possible for us to find a closely spaced set of paths to reproduce the one‐hop and three‐hop whistlers in the north and to have the dispersion ratio of 1:3. The echo train whistlers, as were often observed by Hayakawa et al. [1990], are realized also by this nonducted propagation without any serious requirements.
Undriven magnetic reconnection in magnetohydrodynamics and Hall magnetohydrodynamics
Authors: Lottermoser R.‐F. , Scholar M.
Published: February 2004

Undriven magnetic reconnection in a one‐dimensional current sheet is investigated by means of a two‐dimensional compressible and resistive Hall MHD (HMHD) code. Reconnection is set up by applying a spatially limited resistivity in the center of an isothermal Harris sheet. Although the magnetic fields on the two sides of the initial current sheet are equal and exactly antiparallel, both magnetic field and flow velocity components out of the simulation plane are self‐consistently generated in the course of the reconnection process. The quasi steady state exhibits all features of Petschek‐like reconnection, such as two pairs of shock waves attached to the diffusion region, a strongly converging flow toward the field line reversal region, and a weak fast mode expansion of the inflowing plasma. The reconnection rate measured in terms of the Alfvén Mach number well upstream of the diffusion region is within the analytic limits for compressible Petschek reconnection and slightly higher than in the corresponding MHD case. In contrast to MHD, the shock in HMHD is a subfast, strong (2 → 4), intermediate shock; the thickness is ≃10 ion inertial lengths, and the shocks exhibit an internal structure. The trailing left‐handed slow mode wave train predicted from the integration of the stationary one‐dimensional Hall MHD equations is not found. This is probably due to the fact that waves downstream of the shock are smeared out along the shock layer by the fast outflow jet. The shock waves generated during the decay of a current sheet are studied by a one‐dimensional (1‐D) Hall MHD simulation. In the 1‐D case, the shocks are of slow mode type, and thus a left‐handed intermediate mode wave train can form upstream of the shocks. The shock thickness is estimated to be at least a factor of 3 thicker than in 2‐D HMHD fast reconnection. The intermediate shock in the 2‐D HMHD simulation as well as the slow shock in the 1‐D HMHD simulation exhibit an S‐shaped hodogram. When starting the simulation from a quasi‐steady Petschek state and subsequently allowing the resistivity to be spatially constant, a long and thin current sheet develops. Such a current sheet becomes unstable against the tearing mode.
Anomalous sudden commencement on March 24, 1991
Authors: Araki T. , Fujitani S. , Emoto M. , Yumoto K. , Shiokawa K. , Ichinose T. , Luehr H. , Orr D. , Milling D. K. , Singer H. , Rostoker G. , Tsunomura S. , Yamada Y. , Liu C. F.
Published: February 2004

An anomalous geomagnetic sudden commencement (SC) occurred on March 24, 1991. It is characterized by an exceptionally large and sharp impulse observed in its initial part along the noon meridian in middle and low latitudes. The analysis of the SC was made by using high time resolution digital data from the 210° Meridian Magnetometer Chain in the west Pacific, Sub‐Auroral Magnetometer Network (SAMNET) in the United Kingdom and southern Scandinavia, the EISCAT Magnetometer Cross in northern Scandinavia and Svalbard, and Canopus in Canada together with other ground and satellite (GOES 6, GOES 7, CRRES, and GMS) data. The results of the analysis suggest that the pulse observed at lower‐latitude ground stations was caused by the propagation of a strong magnetospheric compression of short duration (less than l min) which has never been observed before this event. The HF Doppler observation in Kyoto near local noon seems to be consistent with existence of the bipolar electric field associated with the propagating compressional magnetic pulse. The SAMNET stations and CRRES in the early morning also detected positive pulses which delays 30–50 s from the pulses in noon sector. Although the delay in the peak time of the pulse observed on the ground is consistent with ionospheric hydromagnetic wave propagation from the dayside to the nightside with finite speed, the initial onset time of the pulse on the ground was almost simultaneous everywhere suggesting the existence of an “almost instantaneous” propagation mode below the ionosphere.
Dynamics of charged space debris in the Earth's plasma environment
Authors: Juhász Antal , Horányi Mihály
Published: February 2004

Degrading objects in orbit around the Earth as well as solid rocket motors generate micron‐ and submicron‐sized space debris. The motion of these particles is dictated by gravity, solar radiation pressure, and electromagnetic forces, since these grains collect electrostatic charges and become vulnerable to the electric and magnetic fields in the Earth's magnetosphere. We show that magnetospheric effects tend to reduce the lifetime of these grains either by forcing them onto elliptical orbits to collide with the Earth or by swiftly ejecting them into the interplanetary space.
Laboratory experiments on shear Alfvén waves and their relationship to space plasmas
Authors: Gekelman Walter , Vincena Stephen , Leneman David , Maggs James
Published: February 2004

Alfvén waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfvén waves radiated from sources with cross‐field scale size of the order of the electron inertial length, δ = c/ωpe, have properties which differ considerably from planar magnetohydrodynamic waves. Currents of cross‐field size, δ, are common in space plasmas. A series of experiments in the Large Plasma Device at the University of California, Los Angeles is presented which illustrates that waves generated by small‐scale fluctuating currents have a parallel electric field and radiate across magnetic field lines. Data and theory are presented for varying plasma collisionality and ratio of Alfvén speed to electron thermal velocity. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. The shear Alfvén wave is important in the Earth's auroral regions and may play a role in auroral dynamics.
Auroral and geomagnetic events at cusp/mantle latitudes in the prenoon sector during positive IMF By conditions: Signatures of pulsed magnetopause reconnection
Authors: Øieroset M. , Sandholt P. E. , Lühr H. , Denig W. F. , Moretto T.
Published: February 2004

A characteristic sequence of poleward moving auroral events and the associated ground magnetic deflections in the vicinity of cusp‐type precipitation in the convection throat are studied. The occurrence, structure, and motion pattern of these events are found to be strongly regulated by the IMF Bz and By components. The present cases are observed in the prenoon sector during positive IMF By and negative Bz conditions. New, improved results on the dynamical structure of the magnetosphere‐ionosphere current system of these events are obtained. The current system consists of a central filament or sheet‐like field‐aligned current (FAC), located on newly opened field lines with oppositely directed return currents on the poleward and equatorward sides. The ground magnetic deflections can be explained by ionospheric Hall currents generated by this FAC system and the associated pattern of ionospheric electric fields. The propagation speed of the events (auroral and magnetic signatures) are found to be comparable with the simultaneously observed antisunward convection speeds (1–2 km/s); that is, they are related to the tailward convection of newly opened magnetic flux. A qualitatively similar current system with opposite current polarity has recently been proposed for the corresponding auroral and magnetic signatures observed during negative IMF By conditions. The events are found to be consistent with an interpretation in terms of pulsed magnetopause reconnection, that is, flux transfer events.
Super storms
Authors: Bell J. T. , Gussenhoven M. S. , Mullen E. G.
Published: February 2004

“Super storms” are defined to be the largest 2% of geomagnetic storms from 1932 through 1995, selected using ground‐based magnetic indices. These storms are significant not only because they are prolonged periods of extremely high magnetic activity but also because data taken during super storms in the space era show other anomalous features, such as abnormally high energy input to the auroral regions from precipitating particles, and/or the creation of additional, trapped radiation belts in the inner magnetosphere. Super storms are most likely to occur on the downslope from solar maximum and near the equinoxes. One half of the super storms have multiple SSCs that reactivate magnetospheric currents and prolong magnetic activity. AE “spikes” occur during some of the super storms. The relationship between super storms and the locations of trapped radiation populations is briefly examined. The auroral energy input to the inner magnetosphere during recent super storms is calculated and presented.
Coordinated EISCAT/DMSP measurements of electron density and energetic electron precipitation
Authors: Anderson P. C. , McCrea I. W. , Strickland D. J. , Blake J. B. , Looper M. D.
Published: February 2004

A campaign was recently undertaken to obtain simultaneous radar and satellite measurements that would provide insight into the temporal and spatial morphology of the energetic particle precipitation. It included coordinated field‐aligned measurements of the altitudinal profile of the E region electron density from the European Incoherent Scatter (EISCAT) radar, and measurements of the energetic particle precipitation from the SAMPEX and DMSP F10 and F12 satellites. The Strickland auroral electron transport code, with the recent addition of auroral proton transport, was employed to produce density profiles from the energetic particle precipitation measurements by the SSJ/4 sensors on the DMSP F10 and F12 spacecraft for comparison with the EISCAT measurements. We found good agreement between the EISCAT measurements and the model results for three of the four conjunctions including a very close conjunction (< 12 km) when the integral ion energy flux was greater than the integral electron energy flux by a factor of 3 to 4. All of the conjunctions for which the results of the comparison were good occurred within a stable diffuse aurora. Conductivity calculations using the model results varied less than 15% from calculations using the radar measurements. The comparison was good even when the separation between the satellite and the radar was greater than 200 km. The fourth conjunction, for which the agreement was poor, occurred within the discrete aurora during a very active period when the ionospheric conditions were dynamic both spatially and temporally.
Electrodynamics of midlatitude spread F 2. A new theory of gravity wave electric fields
Authors: Miller Clark A.
Published: February 2004

In this paper, we investigate the response of the nighttime, midlatitude ionosphere to atmospheric gravity waves in the presence of the global dynamo electric field. Previous theories of gravity wave‐ionosphere interaction have neglected the zero‐order electric field generated by the global dynamo. We find, however, that gravity wave winds carrying the ionosphere upward and downward along the tilted midlatitude magnetic field can cause spatial variations in the total conductive load experienced by the global circuit. In turn, these variations in ionospheric conductivity create divergences in the global current which produce local electric fields. Since ionospheric conductivity depends on altitude, any nighttime gravity wave which perturbs the height of the ionosphere may, through this mechanism, produce such fields. If true, the consequences of this interaction are potentially quite important. Gravity wave‐induced electric fields may be the source of much of the day‐to‐day variation of the total ionospheric electric field; they may substantially alter the ionospheric electron density response to gravity waves; and they may seed ionospheric plasma instabilities. Further experimental investigations are necessary to test these predictions.
Solar origin of the 26‐day periodicity observed by Ulysses
Authors: Bai T. , Hoeksema J. T. , Weber M. , Acton L. W.
Published: February 2004

The Ulysses spacecraft discovered that the interplanetary magnetic sector structure went through a major restructuring in mid‐1992. The observed recurrence period changed from about 25.4 days to about 26.2 days. Another interesting discovery is that the solar wind speed, energetic particle fluxes, and interplanetary magnetic field all varied quasiperiodically with a similar 26.2‐day period during Ulysses' midlatitude passage south of the ecliptic in 1992–1993. In order to find the solar origins of these interplanetary phenomena, we compared Ulysses observations with relevant solar data. According to our study the global pattern of the open magnetic field lines originating in the photosphere changed drastically in June 1992, and this resulted in a major restructuring of the interplanetary sector structure. After that time the magnetic field pattern in the midlatitude and high‐latitude zones of the southern hemisphere was dominated by two large unipolar regions (covering the entire longitude interval) that rotated with a synodic period of about 28.5 days until mid‐1993. Because the heliographic longitude of the spacecraft remained the same while it approached the Sun, the 26.2‐day period seen by Ulysses is equivalent to the terrestrial synodic period of 28.5 days. By analyzing soft X ray data observed by the Yohkoh satellite we confirm the existence of a stable lobe protruding from the polar coronal hole. This protrusion persisted from 1992 until the end of the study in mid‐1995.
Atmospheric O/N2 ratios from photoelectron spectra
Authors: Goembel L. , Doering J. P. , Morrison D. , Paxton L. J.
Published: February 2004

In this work we compare the O/N2 number density ratios determined from mass spectrometer measurements to O/N2 ratios determined from photoelectron spectra. The photoelectron spectra analyzed are from the Photoelectron Spectrometer (PES) experiment on the Atmosphere Explorer E (AE‐E) satellite. Relative production rates from the photoionization of O and N2 (as calculated from PES data) were used to derive O/N2 ratios for altitudes from 150 to 250 km for a number of orbits of AE‐E. In order to demonstrate the validity of the method we have used to analyze the spectra, we include a comparison of a modeled spectrum to a spectrum from PES. The ambient O/N2 is derived for each 15‐s interval of the orbits studied. The O/N2 ratio derived from the photoelectron spectrum is compared to the O/N2 ratio from each of two mass spectrometers (Neutral Atmosphere Composition Experiment and open source neutral mass spectrometer) that flew with PES on AE‐E. We conclude that atmospheric O/N2 ratios from photoelectron spectra agree (within the error of our measurement) with those from the mass spectrometers. We include in this work suggestions for improvements to the PES experiment that would reduce the uncertainty of any future determinations of atmospheric O/N2 ratios from photoelectron spectra.
Global structure of mirror modes in the magnetosheath
Authors: Johnson Jay R. , Cheng C. Z.
Published: February 2004

A global stability analysis of mirror modes in the magnetosheath is presented. The analysis is based upon the kinetic‐MHD formulation which includes relevant kinetic effects such as Landau resonance and gradient drift effects related to inhomogeneities in the background density, temperature, pressure and its anisotropy, magnetic field, and plasma flow velocity. Pressure anisotropy provides the free energy for the global mirror mode. The local theory of mirror modes predicts purely growing modes in the frame of the moving plasma confined to the unstable magnetosheath region; however, the nonlocal theory that includes the effects of gradients and plasma flow predicts modes with real frequency which propagate with the flow from the magnetosheath toward the magnetopause boundary. The real frequency is of the order of a combination of the diamagnetic drift frequency and the Doppler shift frequency associated with plasma flow. The real frequency is not simply a Doppler‐shifted frequency obtained from local calculation. Rather, it is determined from a boundary value constraint on an eigenvalue problem and can differ significantly from the local Doppler‐shifted frequency. Without plasma flow, the diamagnetic drift frequency provides a wave phase velocity in the direction of the magnetopause so that wave energy accumulates against the magnetopause boundary, and the amplitude is skewed in that direction. Such a wave structure might be observed under conditions of small magnetosheath flow. For larger plasma flow (Alfvén Mach number ≥ 0.1), the flow also causes a real phase velocity in the direction of the flow, but the phase velocity increases from a small value near the bow shock to nearly the flow velocity as the wave propagates toward the magnetopause. As a result, wave amplitude is large where the phase velocity is small and small where the phase velocity is large resulting in a skew in wave amplitude toward the bow shock.
A quantitative assessment of energy storage and release in the Earth's magnetotail
Authors: Baker D. N. , Pulkkinen T. I. , Hesse M. , McPherron R. L.
Published: February 2004

Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated in an explosive instability that occurs in the near‐Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate the amount of energy transferred from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the plasma sheet and magnetotail lobes. Possible energy sources to drive substorm evolution are evaluated in one particularly well‐observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, etc. is assessed during the substorm. It is found that stored tail‐lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On the other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.
Gravity wave seeding of equatorial plasma bubbles
Authors: Singh Sardul , Johnson F. S. , Power R. A.
Published: February 2004

Some examples from the Atmosphere Explorer E data showing plasma bubble development from wavy ion density structures in the bottomside F layer are described. The wavy structures mostly had east–west wavelengths of 150–800 km; in one example it was about 3000 km. The ionization troughs in the wavy structures later broke up into either a multiple‐bubble patch or a single bubble, depending upon whether, in the precursor wavy structure, shorter wavelengths were superimposed on the larger‐scale wavelengths. In the multiple‐bubble patches, intrabubble spacings varied from 55 km to 140 km. In a fully developed equatorial spread F case, east–west wavelengths from 690 km down to about 0.5 km were present simultaneously. The spacings between bubble patches or between bubbles in a patch appear to be determined by the wavelengths present in the precursor wave structure. In some cases, deeper bubbles developed on the western edge of a bubble patch, suggesting an east–west asymmetry. Simultaneous horizontal neutral wind measurements showed wavelike perturbations that were closely associated with perturbations in the plasma horizontal drift velocity. We argue that the wave structures observed here that served as the initial seed ion density perturbations were caused by gravity waves, strengthening the view that gravity waves seed equatorial spread F irregularities.
On the mapping of ionospheric convection into the magnetosphere
Authors: Hesse Michael , Birn Joachim , Hoffman Robert A.
Published: February 2004

Under steady state conditions and in the absence of parallel electric fields, ionospheric convection is a direct map of plasma and magnetic flux convection in the magnetosphere, and quantitative estimates can be obtained from the mapping along magnetic field lines of electrostatic ionospheric electric fields. The resulting magnetospheric electrostatic potential distribution then provides the convection electric field in various magnetospheric regions. We present a quantitative framework for the investigation of the applicability and limitations of this approach based on an analytical theory derived from first principles. Particular emphasis is on the role of parallel electric field regions and on inductive effects, such as expected during the growth and expansive phases of magnetospheric substorms. We derive quantitative estimates for the limits in which either effect leads to a significant decoupling between ionospheric and magnetospheric convection and provide an interpretation of ionospheric convection which is independent of the presence of inductive electric fields elsewhere in the magnetosphere. Finally, we present a study of the relation between average and instantaneous convection, using two periodic dynamical models. The models demonstrate and quantify the potential mismatch between the average electric fields in the ionosphere and the magnetosphere in strongly time‐dependent cases that may exist even when they are governed entirely by ideal MHD.
A statistical study of the behavior of the electron temperature in ejecta
Authors: Richardson I. G. , Farrugia C. J. , Cane H. V.
Published: February 2004

Ejecta, which are the interplanetary manifestations of coronal mass ejections, are identified by various signatures. It was first reported over 20 years ago that ejecta are generally associated with low plasma electron temperatures (Te). However, further case studies, in particular, recent studies of magnetic clouds (a subset of ejecta characterized by large rotations in the magnetic field direction), have shown that in some ejecta, Te is enhanced, not depressed, after the forward boundary is crossed. To clarify the general behavior of Te in ejecta, we have examined, in a statistical study, the behavior of Te in 95 ejecta in the near‐Earth solar wind. Of these ejecta, 13 were magnetic clouds. We find that the behavior of Te in ejecta is variable. Most typically, Te fluctuates and may be enhanced (relative to the temperature in the ambient (unshocked) solar wind) in one region of the ejecta and depressed in another. This structure in Te is often reflected in the electron density (ne), with variations in Te being significantly anticorrelated with variations in ne. The behavior of Te contrasts with that of the proton temperature (Tp) in that Tp is depressed in most ejecta so that Te/Tp ≫ 1 in virtually all events. In addition, Tp shows no correlation or anticorrelation with the plasma density. Our study suggests that a criterion according to which Te/Tp > 2 is a more appropriate, though still not unique, indicator of an ejecta than one based on depressions in Te.
Global postseismic rebound of a viscoelastic Earth: Theory for finite faults and application to the 1964 Alaska earthquake
Authors: Piersanti A. , Spada G. , Sabadini R.
Published: February 2004

A spherically symmetric Earth model with viscoelastic rheology is used to study the postseismic rebound associated with finite lithospheric dislocations. We perform a systematic study of surface deformations due to sources characterized by two‐ and three‐dimensional faults, modeled by a linear and planar distribution of point sources. Our approach is based on the normal mode technique for a layered Earth with linear viscoelastic rheology and allows for a self‐consistent description of the time evolution of postseismic displacements due to strike‐ and dip‐slip faults. As a case study, we compare the predicted horizontal displacements due to the 1964 Alaska earthquake with very long baseline interferometry (VLBI) baselines changes observed in this region in the period 1984–1989. Although subduction and postglacial isostatic adjustment are expected to contribute significantly to present‐day velocities, our results show that the postseismic rebound due to the 1964 Alaska earthquake plays a relevant role in the changes of several VLBI baselines. The last section is devoted to the analysis of the evolution of other North American baselines. As suggested by recent investigations based on forward approaches, isostatic readjustment of the Earth's crust in response to the melting of the last Pleistocene ice sheets is a major contributor to baseline variations in this area. Our results indicate that a correct interpretation of VLBI baseline changes in North America should account also for the effects of postseismic deformation.
Uniaxial compaction creep of wet gypsum aggregates
Authors: de Meer Siese , Spiers Christopher J.
Published: February 2004

The uniaxial compaction creep behavior of wet, granular gypsum is investigated under both chemically closed (i.e., drained) and open (i.e., flow through) conditions known to favor pressure solution. The experiments were performed using applied stresses of 0.5 to 2.5 MPa and grain sizes of 32–280 μm, at room temperature, using pore fluids saturated with respect to unstressed sample material. All wet‐tested samples crept rapidly. In contrast, control experiments using dry and oil‐saturated samples showed no measurable creep. The microstructures developed in the wet tests provide classical evidence for the operation of grain boundary diffusional pressure solution. To enable detailed comparison with theory, it is demonstrated that despite minor effects of the water of crystallization, conventional pressure solution creep models, for closed systems, are applicable to gypsum. However, the mechanical behavior observed in the closed‐system experiments does not fully match these models for either dissolution, diffusion, or precipitation control. Nonetheless, independent crystal growth data suggest that precipitation is most likely to be rate controlling. Additional evidence for this was provided by the experiments with through‐flowing solution. In such experiments, precipitation can no longer control the rate of deformation, so that dissolution or diffusion are expected to take over as the rate‐limiting process, thus enhancing the creep rate. Indeed, a 10 to 30 times increase in compaction creep rate was observed in the flow‐through tests, confirming that creep in the closed‐system case probably occurred by precipitation‐controlled grain boundary diffusional pressure solution.
Generation and maintenance of pore pressure excess in a dehydrating system 1. Experimental and microstructural observations
Authors: Ko Suz‐Chung , Olgaard David L. , Wong Teng‐Fong
Published: February 2004

The generation and maintenance of excess pore pressures in dehydrating gypsum aggregates were investigated using experiments and microstructural analyses. A triaxial deformation apparatus, was equipped with a pore fluid system connected directly to the dehydrating sample. This system was operated in constant fluid volume mode to monitor pore pressure increase under undrained conditions, and in constant pore pressure mode to monitor fluid expulsion under drained conditions. X ray diffraction and backscatter scanning electron microscopy were used to characterize the spatial relationship among gypsum, the product phase bassanite, and the pores. In addition, we measured the permeability and pore compressibility of the starting material and explored the influence of effective and pore pressures, temperature, and axial load on fluid expulsion. Three stages of fluid expulsion and microstructural evolution during dehydration of an initially low‐porosity, low‐permeability gypsum aggregate are defined: (1) Initially, fluid released by the reaction is trapped in isolated or discontinuous pore networks and high pore pressures are possible. (2) An interconnected pore network eventually develops and fluid readily escapes. (3) Fluid expulsion slows down drastically as the reaction nears completion. As a result of coupling between dehydration and porosity production, both the cumulative volume of fluid expelled and the expulsion rate increase with increasing temperature, effective pressure, and axial load and with decreasing pore pressure. Our hydrological and microstructural data, combined with previous mechanical data, provide a better understanding of the relationships among changes in fluid volume, porosity, and pore pressure excess, and the deformation behavior of a dehydrating system where drainage evolves with time.
Vertical movements following intracontinental magmatism: An example from southern Israel
Authors: Gvirtzman Zohar , Garfunkel Zvi
Published: February 2004

We present a quantitative thermal model for vertical movements following continental magmatism, focusing on how the associated elevation changes depend on the depth of intrusion. When an intrusion is emplaced within the lithosphere, its buoyancy causes a quick initial movement which is followed by long‐term movements caused by thermal relaxation. Intrusions emplaced within the gabbro stability field produce initial uplifting which is about 12% of their thickness. Subsequent thermal relaxation reduces the uplift to a residual value of 9–10% of the intrusion thickness. In contrast, intrusions emplaced within the eclogite stability field produce a small subsidence from the very beginning which is slowly increased by thermal relaxation and may reach a residual value of some 4% of the intrusion thickness. In both cases the rates of the thermal subsidence depend on the depth of intrusion: it is relatively fast when the intrusions are shallow but considerably slower when the intrusions are deep. The model enables us to infer volumes and depths of intrusions from amplitudes and rates of vertical movements. As an example we apply the model to analyze the geodynamic evolution of the central Negev, southern Israel, during the Early Cretaceous. Two distinct magmatic pulses that were recognized there represent the two basic situations envisaged by the model, i.e., shallow magma emplacement in the gabbro field associated with uplifting, and deep intrusion in the eclogite field associated with subsidence. In a wider context we think that this model may help in understanding intracratonic basins in nonextensional settings. In particular, deep and thick eclogite intrusions can explain subsidence of regions which were not extended nor uplifted and in regions where crustal magmatism and heating were not observed.
Stress distribution and failure in anisotropic rock near a bend on a weak fault
Authors: Chester Judith S. , Fletcher Raymond C.
Published: February 2004

Large transform faults, thin‐skinned thrust faults, and listric normal faults often contain restraining and releasing bends that can alter the state of stress in the adjacent crust during faulting episodes. Crustal rock commonly has anisotropic mechanical properties due to the presence of sedimentary layering, subsidiary fractures and faults, schistosity, or other foliation. We present an analytical solution for the stress distribution in anisotropic rock produced by sliding on a wavy, frictionless surface. The frictionless surface represents the limiting case of a weak fault. The fault shape treated is either a sinusoid, or a periodic array of isolated restraining and releasing bends. The rheological behavior of the rock is that of an incompressible linear viscous fluid with an orthotropic anisotropy characterized by a greater viscosity for shortening and extension than for shear in the principal directions of anisotropy. Our results illustrate that stress and flow associated with a bend in a fault will extend to much greater distances from the fault when the medium is anisotropic. The magnitude of the stress perturbation increases with degree of anisotropy and decreases with radius of curvature of the fault surface. Principal stress directions tend to align parallel to the principal directions of anisotropy except in the immediate region of the fault bend. Mean stress and maximum shear stress magnitudes vary along the fault in a cellular manner, with multiple maxima near a bend. Stress concentrations emanate from the bend and are elongate in directions parallel to the principal directions of anisotropy. For isotropic rock, locations and orientations of shear failure near restraining bends are distinctly different from those near releasing bends. In contrast, with application of an anisotropic failure criterion, as the magnitude of anisotropy increases, the patterns of shear failure at a restraining and a releasing bend become increasingly similar. The model may help explain field observations from dip‐slip and strike‐slip regimes that indicate a complex stress history and local stress reorientation adjacent to a bend in a fault.
Laboratory observations of the hydraulic behavior of a permeable fracture from 3800 m depth in the KTB pilot hole
Authors: Durham William B.
Published: February 2004

A thorough search through core recovered from the German Continental Deep Drilling Program (KTB) pilot hole in the vicinity of the high‐permeability zone near 3800 m depth revealed three intervals containing undisturbed fractures that had the appearance of high‐permeability fluid conduits. We report here a detailed study in the laboratory of one of those fractures, an epidote‐filled fracture in amphibolite, aimed at determining if the fracture was fluid conducting at in situ stresses and, if so, what characteristics of the fracture allowed it to remain open over geologic times. We found that the presence of the fracture made the core sample 4–7 orders of magnitude more permeable than the host rock at confining pressures near 100 MPa. Fine scale measurements of the shape of the fracture aperture, made with a specially designed profilometer, suggest that water flows through a distributed network of very small (order of 1 mm) high‐aspect‐ratio (order of 10) channels that cover 30–70% of the fracture surface. In contrast, fresh, well‐mated fractures in granite at comparable conditions and for a comparable amount of fracture closure are slightly more compliant normal to the fracture plane and more prone to concentrate flow into channels than the mineralized fracture, suggesting fundamentally different hydraulic conduction mechanisms in fresh versus mineralized fluid‐conducting fractures.
On the heat flow variation from Archean cratons to Proterozoic mobile belts
Authors: Lenardic A.
Published: February 2004

Heat flow data from a number of continental shield regions show a trend of relatively low to high values from Archean cratons to bordering Proterozoic mobile belts. Of the two end‐member explanations for this trend, low heat production in Archean crust or a relatively thick cratonic lithosphere, the latter has come to be generally preferred. Such an explanation assumes a strong, one‐to‐one correspondence between the mantle component of surface heat flow and local lithospheric thickness. This assumption has been well validated below oceans and its applicability to continental lithosphere has gone largely unchallenged. It is, however, not fully valid. This is demonstrated through numerical models that allow continents to form over a convecting mantle. Model continents consist of a core region of thickened crust and mantle residuum and a peripheral region of thick crust, analogs to a craton and a mobile belt, respectively. Despite a thicker thermal lithosphere in the core relative to the periphery, the equilibrium surface heat flux across a continent shows little variation. The finite thermal conductivity of buoyant continental material is at the heart of this behavior as it allows continents to enforce a spatially near‐constant heat flux condition on the mantle below. Such a condition is associated with a weak correspondence between mantle heat flow and lithospheric thickness defined thermally or mechanically. This general result, together with specific modeling results applied to heat flow data, suggests that variable lithospheric thickness is most likely not the primary cause of heat flow variations near Archean cratons, leaving differing degrees of crustal heat production as the more likely candidate.
LAGEOS I once‐per‐revolution force due to solar heating
Authors: Rubincam David Parry , Currie Douglas G. , Robbins John W.
Published: February 2004

Photon thrust from the solar heating of the LAGEOS I satellite appears to explain much of the eccentricity variations seen in the satellite's orbital elements. We invoke a thermal model of LAGEOS I in which the photon thrust from solar heating is directed along the satellite's spin axis and functionally depends only on the cosine of the angle between the Sun's position and the spin axis. We calibrated the amplitude of the force from the 1980–1983 equivalent along‐track acceleration derived from the observed orbital perturbations; during this time the spin axis position is assumed to be known and to be that at orbit injection. The photon thrust from this simple thermal model, plus later spin axis positions obtained from Sun glint data (which show LAGEOS I to be precessing), give reasonable agreement with the observed along‐track acceleration in the time period 1988–1995. Thus much of the eccentricity variations seem to be due to thermal thrust and do not have a geophysical origin (atmospheric tides) as has been proposed. However, our solar heating model does not appear to explain the highest peaks and deepest troughs seen in the along‐track acceleration, indicating the need for a better thermal model and consideration of other forces, such as that due to anisotropic reflection.
Investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the Southern East Pacific Rise
Authors: Shen Yang , Forsyth Donald W. , Conder James , Dorman LeRoy M.
Published: February 2004

Between February 1991 and May 1992, 33 intraplate earthquakes having body wave magnitudes between 4.3 and 6.0 were located on the west flank of the Southern East Pacific Rise by the International Seismological Center. Seven months after the last teleseismic event, we deployed four ocean bottom seismometers at the site of the teleseismic swarm. One hundred and ninety‐two microearthquakes were located using P and S travel times of events recorded by three or more instruments during the 16‐day deployment. Most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains. In addition, several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis. The focal depths of the microearthquakes range from 1 to 15 km, and most are between 5 and 12 km, similar to the depth range of the teleseismic events [Hung and Forsyth, 1996]. The composite P wave polarities indicate that the microearthquakes had a variety of focal mechanisms. We developed a new grid‐search, inversion technique that utilizes the P wave polarities and the empirically corrected ratios of P and S wave amplitudes to find the focal mechanisms of individual events. Within the acceptable travel time and amplitude misfits, focal solutions are fairly stable. Normal faulting is found in the ridge‐parallel seismicity line. The thrust and strike‐slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events. There is no apparent depth dependence of fault types. None of the existing models of the sources of stress (ridge push, thermoelastic stresses, loading by local topographic features, caldera collapse, and north‐south extension of the Pacific Plate) provides a satisfactory explanation for both the teleseismic swarm and microearthquakes. We propose a new tectonic scenario. In this scenario, the lithosphere is prestressed by the cooling of the plate. Magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere, which is already under extensional, thermal stress, producing the larger, teleseismically detected events. Once the dikes propagate into the lithosphere, the region surrounding the dikes behind the tips is compressed by the overpressure of magma. Depending on the geometry of the dikes, the local orientations of the minimum principal stress, and the local weaknesses in the lithosphere, thrust or strike‐slip faulting (the microearthquakes) may occur.
Long‐term seismic behavior of the focal and adjacent regions of great earthquakes during the time between two successive shocks
Authors: Pérez Omar J. , Scholz Christopher H.
Published: February 2004

We analyze the spatial‐temporal‐energy distribution of shallow (depth, h<70 km) seismicity (surface wave magnitude, Ms≥7) occurring in the focal and adjacent regions of very great shallow earthquakes (rupture lengths of the order of hundreds of kilometers) in the circum‐Pacific seismic belt for as many decades as possible before and after the occurrence of the main shock. For this purpose we have revised the catalog of data for the world instrumental seismicity (1900–1995) by removing a series of heterogeneities in the earthquake reporting and magnitude determination. This revised catalog is fairly complete for earthquakes with M≥∼7 since early in the century. Our findings indicate that once a main shock‐aftershock sequence occurs the focal region enters a period of relative quiescence for large (Ms≥7) earthquakes. Significant seismic activity (Ms≥7) occurs during many decades prior to the next main shock. This activity is fairly continuous or random in time and lasts until the onset of the main event but clusters in space around the ends of the forthcoming rupture and/or in the vicinity of the principal epicenter (except in the case of rare bilateral breaks the main epicenter is located at one of these ends). Thus major portions of the rupture zone experience no events with Ms≥7 during the seismic cycle. Larger aftershocks also cluster near the edges of the rupture and near the principal epicenter. Preshock activity, minutes to weeks before the main shock (if any), occurs, in general, near the main epicenter. We also find that the nucleation and ends of the rupture often coincide in space with recognizable geometrical and/or geological inhomogeneities, “barriers” existing in the lithosphere. It appears that while strain energy accumulates, high stress concentrations often occur at the ends of the future rupture zone, which results in the long‐term foreshock activity and in the subsequent initiation of the main rupture from within those regions. We suggest a simple mechanical model that explains the observed pattern of earthquake occurrence for both unilateral and bilateral ruptures. In all cases of great earthquakes that have occurred this century for which a long and complete (to a level of 2 to 3 orders of magnitude smaller than the moment of the main shock) catalog of prior seismicity exists, we can not recognize any statistically significant “quiescent” or “doughnut” patterns in the preceding seismicity at the Ms≥7 level.