bluuebunny/crossref_metadata_2025 huggingface dataset:
Walén and variance analyses of high‐speed flows observed by Wind in the midtail plasma sheet: Evidence for reconnection
Authors: Øieroset Marit , Phan Tai D. , Lin Robert P. , Sonnerup Bengt U. Ö.
Published: February 2004

We have analyzed 4 days of Wind observations of high‐speed convective flows of up to ∼800 km s−1 in the plasma sheet at XGSE ∼ − 60 RE during late March and early April 1999. Both earthward and tailward flows were observed. The high‐speed flows had a duration of several hours, unlike the shorter‐lasting (tens of minutes) bursty bulk flows, which are typically observed closer to the Earth. We have analyzed in detail a ∼10 hour interval of high‐speed flows detected during rather low geomagnetic activity and northward interplanetary magnetic field. Our analysis indicates that the fast flows are produced by magnetic reconnection and that the observed flow reversals are consistent with the passage of a reconnection X line. First, the results of the shear stress balance test (the Walén test) indicate that the flow in the deHoffmann‐Teller frame, which is aligned with the magnetic field, has an average flow speed that is ∼60% of the Alfvén speed, consistent with the presence of slow shocks in the magnetotail reconnection layer. Furthermore, the slope of the Walén regression line switches sign at the flow reversals, as expected from reconnection. Consistent with the passage of a reconnection X line, the magnetic field component normal to the neutral sheet also reverses sign at the flow reversals. For this event the tailward flowing plasma is hotter than the earthward flowing plasma, consistent with the two plasmas being magnetically disconnected. Our observations imply that quasi‐steady reconnection can occur in the midmagnetotail region during periods of persistent northward interplanetary magnetic field.
Flow transients in the postnoon ionosphere: The role of solar wind dynamic pressure
Authors: Thorolfsson A. , Cerisier J.‐C. , Pinnock M.
Published: February 2004

Super Dual Auroral Radar Network (SuperDARN) HF radar, magnetometer, and satellite data are used to study ionospheric convection flow bursts in the sunward return flow in the 1200–1500 MLT auroral region. A comparison between the latitudinal position of the flow bursts and the position of the open/closed field line boundary shows that the sunward bursts occur on closed field lines. The bursts occur simultaneously with poleward motions of the convection reversal boundary. Decreases in the solar wind dynamic pressure are shown to correlate well with the flow bursts. Comparison of radar arid magnetometer data shows that these observations can be explained in terms of the global current model proposed for sudden expansions of the rnagnetosphere [Araki and Nagano, 1988]. The sunward flow bursts are shown to be consistent with the high‐latitude dusk to dawn electric field predicted by the model during pressure decreases.
Three‐dimensional observations of gyrating ion distributions far upstream from the Earth's bow shock and their association with low‐frequency waves
Authors: Meziane K. , Mazelle C. , Lin R. P. , LeQuéau D. , Larson D. E. , Parks G. K. , Lepping R. P.
Published: February 2004

This report discusses the nature of gyrating ion distributions observed on board the Wind spacecraft by the three‐dimensional ion electrostatic analyzer with high geometrical factor (3DP PESA‐High). The gyrating ion distributions are observed near the inner ion beam foreshock boundary at distances between ∼9 and ∼83 RE. Our upstream measurements confirm several features previously reported using two‐dimensional measurements. These distributions are observed in association with low‐frequency waves with substantial amplitude (|δB|/B > 0.2). The analysis of the waves shows that they propagate in the right‐hand mode roughly along the background magnetic field. The ions are bunched in gyrophase angle when the associated waves are quasi‐monochromatic and high in amplitude. The peak of the ion distribution function rotates in the gyrophase plane. If the wave train is nonmonochromatic, the particle phase angle distribution is extended over a larger range, suggesting the occurrence of a phase mixing effect or a source at the shock. The phase angle distribution also seems to be energy‐dependent, and no gyrophase rotation is observed in this case. Furthermore, we have characterized the ion distributions by computing their densities as well as parallel and perpendicular velocities. The results clearly indicate that the waves are cyclotron‐resonant with the field‐aligned beams observed just upstream. The resonance condition strongly suggests the local production of these gyrating ions in a field‐aligned‐beam disruption. Such a resonant wave‐particle interaction may be a dominant characteristic of the back‐streaming ion population in the foreshock at large distances from the Earth's bow shock.
Substorm and convection bay compared: Auroral and magnetotail dynamics during convection bay
Authors: Sergeev V. A. , Kubyshkina M. V. , Liou K. , Newell P. T. , Parks G. , Nakamura R. , Mukai T.
Published: February 2004

Using observations from eight spacecraft and a ground network, we study two subsequent bay‐like disturbances on December 10, 1996, initiated by southward interplanetary magnetic field intervals, one being a classic substorm and another one a convection bay. Both events showed enhanced convection and Dst decreases as well as Pi2 pulsations in the auroral zone. Contrasting to the well‐defined substorm signatures of the first event (poleward auroral expansion, substorm current wedge, strong particle injection to 6.6 RE) resulting from energy loading/unloading and near‐Earth reconnection in the tail, these signatures were virtually absent during the convection bay (CB). Distinctive features of the CB event were the same as those during the Steady Magnetospheric Convection intervals: (1) wide double oval at the nightside; (2) thick plasma sheet, relaxed lobe field, and enhanced magnetic flux closure (large Bz) and multiple bursty earthward flows (BBFs) in the midtail; (3) sporadic narrow soft injections to 6.6 RE; (4) auroral streamers associated with both BBFs and narrow injections. We emphasize the development of multiple and sporadic auroral streamers which start at the poleward oval boundary, propagate equatorward (in 3–8 min) and end with a long‐duration bright spot in the equatorward oval. We conclude that the plasma sheet and auroral dynamics during the convection bay was formed by sporadic narrow (a few RE wide) plasma streams (plasma bubbles) which transported the plasma sheet material from the distant magnetotail reconnection regions to the inner magnetosphere and may significantly contribute to the magnetospheric circulation on the nightside. We modeled the nightside tail configuration using magnetotail magnetic observations and low‐altitude particle boundaries to show that at the beginning of the convection bay the increase of magnetic flux tube volume with distance was small in the midtail. Therefore the “pressure crisis” in the tail was significantly reduced during the convection bay, and the efficient earthward transport by sporadic narrow plasma streams was probably able to balance the magnetospheric circulation to avoid the large‐scale instability of the magnetotail.
Geotail observations of low‐frequency waves from 0.001 to 16 Hz during the November 24, 1996, Geospace Environment Modeling substorm challenge event
Authors: Sigsbee K. , Cattell C. A. , Mozer F. S. , Tsuruda K. , Kokubun S.
Published: February 2004

We present observations of low‐frequency waves from 0.001 Hz up to the lower hybrid frequency during the time periods corresponding to the growth, expansion, and recovery phases of a substorm that occurred from ∼ 2100 to 2310 UT on November 24, 1996, while Geotail was located in the magnetotail at XGSE ∼ 25 RE and 0100 LT. Large‐amplitude waves near the lower hybrid frequency were observed when large‐scale density and magnetic field gradients were present during thinning of the plasma sheet at substorm onset and the beginning of the recovery phase. Later in the recovery phase, waves near the lower hybrid frequency were observed close to the neutral sheet in localized areas with strong small‐scale density and magnetic field gradients. Fluctuations of the electric and magnetic fields near the ion gyrofrequency were also observed during this event; however, the amplitudes were smaller than those often observed in the near tail in association with substorms. Compressional fluctuations of the magnetic field in the Pi2 frequency range were observed close to substorm onset. Although Geotail was not in the region considered in recent studies of Pi2 pulsations, it may still be possible that the magnetic field fluctuations observed by Geotail during the November 24, 1996, substorm are related to this phenomenon since little is known about propagation of the compressional pulses and generation of the waves.
Global three‐dimensional MHD simulation of a space weather event: CME formation, interplanetary propagation, and interaction with the magnetosphere
Authors: Groth Clinton P. T. , De Zeeuw Darren L. , Gombosi Tamas I. , Powell Kenneth G.
Published: February 2004

A parallel adaptive mesh refinement (AMR) finite‐volume scheme for predicting ideal MHD flows is used to simulate the initiation, structure, and evolution of a coronal mass ejection (CME) and its interaction with the magnetosphere‐ionosphere system. The simulated CME is driven by a local plasma density enhancement on the solar surface with the background initial state of the corona and solar wind represented by a newly devised “steady state” solution. The initial solution has been constructed to provide a reasonable description of the time‐averaged solar wind for conditions near solar minimum: (1) the computed magnetic field near the Sun possesses high‐latitude polar coronal holes, closed magnetic field flux tubes at low latitudes, and a helmet streamer structure with a neutral line and current sheet; (2) the Archimedean spiral topology of the interplanetary magnetic field is reproduced; (3) the observed two‐state nature of the solar wind is also reproduced with the simulation yielding fast and slow solar wind streams at high and low latitudes, respectively; and (4) the predicted solar wind plasma properties at 1 AU are consistent with observations. Starting with the generation of a CME at the Sun, the simulation follows the evolution of the solar wind disturbance as it evolves into a magnetic cloud and travels through interplanetary space and subsequently interacts with the terrestrial magnetosphere‐ionosphere system. The density‐driven CME exhibits a two‐step release process, with the front of the CME rapidly accelerating following the disruption of the near‐Sun closed magnetic field line structure and then moving at a nearly constant speed of ∼560 km/s through interplanetary space. The CME also produces a large magnetic cloud (> 100 RS across) characterized by a magnetic field that smoothly rotates northward and then back again over a period of ∼2 days at 1 AU. The cloud does not contain a sustained period with a strong southward component of the magnetic field, and, as a consequence, the simulated CME is somewhat ineffective in generating strong geo‐magnetic activity at Earth. Nevertheless, the simulation results illustrate the potential, as well as current limitations, of the MHD‐based space weather model for enhancing the understanding of coronal physics, solar wind plasma processes, magnetospheric physics, and space weather phenomena. Such models will provide the foundation for future, more comprehensive space weather prediction tools.
The response of the HF radar spectral width boundary to a switch in the IMF By direction: Ionospheric consequences of transient dayside reconnection?
Authors: Chisham G. , Pinnock M. , Rodger A. S.
Published: February 2004

In the high‐latitude dayside ionosphere, the movement of the HF radar spectral width boundary (SWB) provides a good proxy for the movement of the open‐closed field line boundary around magnetic local noon. By studying the dynamics of the spectral width boundary we can investigate features of the dayside ionospheric response to changes in the interplanetary magnetic field (IMF). The high temporal and spatial resolution of the Super Dual Auroral Radar Network (SuperDARN) HF radars make them good tools to study these features. In this paper, we use the Halley HF radar in Antarctica to study the equatorward motion of the SWB which appears to occur in response to a large change in the direction of IMF By. The spectral width boundary initially moves equatorward in the form of a U‐shaped bulge close to magnetic local noon. This bulge then expands longitudinally to earlier and later magnetic local times. Merged velocity vectors from two Antarctic HF radars describe the flow velocity variation in the boundary region. The flow equatorward of the boundary follows the contours of the boundary as it expands. The flow poleward of the boundary is directed at more oblique angles to the boundary. This study represents the first clear two‐dimensional observation of the formation of an equatorward bulge on the polar cap boundary which may be associated with changes in dayside reconnection and presents a unique observation of the variation of the ionospheric flow in the locality of the boundary. We discuss the possible interpretations of this event and its consequences to our present understanding of the ionospheric response to changes in the IMF.
Model for the partition of temperature between electrons and ions across collisionless, fast mode shocks
Authors: Hull A. J. , Scudder J. D.
Published: February 2004

The partition of temperature between electrons and ions across Earth's bow shock is a long‐standing problem in modeling particle entry into the magnetosphere. Observations have shown that the ion temperature increase across collisionless, fast mode shocks (as in Earth's bow shock) is substantially larger than that of the electrons. A model that can quantitatively explain this result will provide a better understanding of the relationships between collisionless shock parameters and particle behavior across the shock layer, finding applications to multifluid models of particle entry to the Earth's magnetosphere, as in the polar rain. In this paper we present a model that quantitatively determines the partition of temperature between the electrons and ions across the shock. The model couples a simplified Vlasov guiding center ordered electron fluid problem with the Rankine‐Hugoniot conservation equations to determine the downstream partition of electron and ion temperature given a few observational constraints. We work in the deHoffmann‐Teller reference Frame (HTF) because in HTF the electrons are only coupled to the ions through the electrostatic potential. This approach does not preclude the subsequent secondary thermalization of the ions by means of wave‐particle interactions. We demonstrate under this approximation that the model recovers the electron temperature jumps of a new ISEE 1 database of 129 Earth bow shock crossings developed and analyzed by Hull et al. [2000]. The model also recovers trends in the downstream ion temperature at shocks observed by ISEE 1 previously cataloged by Thomsen et al. [1987].
Evolution of the outer plasmasphere during low geomagnetic activity observed by the EUV scanner onboard Planet‐B
Authors: Yoshikawa I. , Yamazaki A. , Shiomi K. , Yamashita K. , Takizawa Y. , Nakamura M.
Published: February 2004

The extreme ultraviolet scanner onboard the Planet‐B spacecraft has taken EUV images (He II 304 Å) of both the duskside plasmasphere and nearby outer magnetosphere on September 9–10, 1998. Even during a relatively quiet period the images have revealed that there is a significant amount of escape of plasmaspheric ions from the outer plasmasphere toward the magnetosphere. Such a phenomenon is not expected from the conventional teardrop model of the plasmasphere. Plasmaspheric materials of at least 2.9×1028 He+ ions were peeled off the main body of the plasmasphere within 12 hours. This quantity corresponds to an ion flux of 6.7 × 1024 ions/s under the assumption that the He+/H+ ratio is 10%. If geomagnetic storms occur twice a month, this flux is comparable to the predictions for a stormtime transport. This suggests that the plasmasphere may provide the magnetosphere with significant amount of plasmas even when the geomagnetic activity is low.
Magnetopause motion driven by interplanetary magnetic field variations
Authors: Sibeck D. G. , Kudela K. , Lepping R. P. , Lin R. , Nemecek Z. , Nozdrachev M. N. , Phan T.‐D. , Prech L. , Safrankova J. , Singer H. , Yermolaev Y.
Published: February 2004

We use previously reported observations of hot flow anomalies (HFAs) and foreshock cavities to predict the characteristics of corresponding features in the dayside magnetosheath, at the magnetopause, and in the outer dayside magnetosphere. We compare these predictions with Interball 1, Magion 4, and GOES 8/GOES 9 observations of magneto‐pause motion on the dusk flank of the magnetosphere from 1800 UT on January 17 to 0200 UT on January 18, 1996. As the model predicts, strong (factor of 2 or more) density enhancements bound regions of depressed magnetosheath densities and/or outward magnetopause displacements. During the most prominent event, the geosynchronous spacecraft observe an interval of depressed magnetospheric magnetic field strength bounded by two enhancements. Simultaneous Wind observations indicate that the intervals of depressed magnetosheath densities and outward magnetopause displacements correspond to periods in which the east/west (By) component of the interplanetary magnetic field (IMF) decreases to values near zero rather than to variations in the solar wind dynamic pressure, the north/south component of the IMF, or the IMF cone angle.
Auroral polar cap boundary ion conic outflow observed on FAST
Authors: Tung Y.‐K. , Carlson C. W. , McFadden J. P. , Klumpar D. M. , Parks G. K. , Peria W. J. , Liou K.
Published: February 2004

We observe large ion outflow fluxes (> 108 cm−2s−1 mapped to 100 km) flowing from the auroral polar cap boundary on many FAST auroral passes near midnight. The outflow is in the form of ion conies composed mostly of light ions with energies of hundreds of eV. A statistical study of 606 FAST orbits during January and February 1997 was done to determine the MLT distribution and ion outflow properties relative to substorms. We find that the ion conic events occur most frequently near midnight, and therefore their direct magnetic connection to the nightside central plasma sheet makes this ion outflow a candidate for the ionospheric source of the nightside plasma sheet. To study the relationship between the ion conic outflow events and auroral substorm conditions, we use global auroral images acquired from the ultraviolet imager (UVI) on Polar. We find that the outflow events are well correlated with substorm expansion phase, but ion conic outflow events also occur during generally active aurora, when onset and expansion phases were not clearly identifiable. By combining FAST data to determine ion outflow fluxes and ILAT extent and Polar UVI images to determine MLT extent, we estimate the total outflow due to these ion conic events to be 1022 to 1024 ions/s. By comparing with earlier ion outflow studies, we conclude that these polar cap boundary ion conic events are not the dominant or only ionospheric source of ion outflow over the entire auroral oval but are the dominant nightside auroral ion outflow.
Origin and geometry of upward parallel electric fields in the auroral acceleration region
Authors: Mozer F. S. , Hull A.
Published: February 2004

Electric fields, magnetic fields, and plasmas measured on the Polar satellite are studied to determine the altitude extent of the upward field‐aligned current portion of the auroral acceleration region and the physical processes that populate it with parallel electric fields. This region extends upward to a geocentric altitude of 3 or 3.5 Earth radii at premidnight local times during the spring and fall. In a model for this region a necessary but not sufficient condition for the appearance of parallel electric fields is an upward field‐aligned current. Parallel electric fields will exist where the plasma density is insufficient to carry this imposed field‐aligned current through the converging magnetic field. These parallel fields are associated with (1) high‐altitude electron acceleration required to carry the imposed current through the low‐density region; (2) a low‐altitude sheath required to inject compensating positive charge into the region and to restrict entry of secondary electrons from below; and (3) the midaltitude maintenance of local charge neutrality. The parallel electric field in the low‐altitude sheath is directly measured to be as large as several hundred mV m−1, while the higher‐altitude fields appear to have magnitudes of a few mV m−1. The low‐altitude sheath is corrugated in a manner that does not depend on striations of the field‐aligned current but that may depend on density variations.
Asymmetric mass loading effect at Titan's ionosphere
Authors: Kopp Andreas , Ip Wing‐Huen
Published: February 2004

Because of the fact that Titan does not possess an intrinsic magnetic field, the corotating plasma in the Saturnian magnetosphere can interact directly with the nitrogen‐rich atmosphere of Titan. In this work we perform three‐dimensional resistive MHD simulations of the interaction between Titan and the Saturnian magnetosphere. The magnetic field of Saturn is modeled as a dipole field located at the origin of a cylindrical coordinate system. Of particular interest is the mass loading effect in the vicinity of Titan. The pickup of atmospheric ions is strongly influenced by the ionization rate and scale lengths of the system. The fact that the ion gyroradius of the system is of the order of the radius of Titan has important consequences. Ions emerging from Titan's ionosphere at the hemisphere facing toward Saturn get lost back into the ionosphere, whereas ions at the opposite hemisphere may contribute to the satellite's mass loading by enhanced atmospheric sputtering. This results in an asymmetry of the mass loading patterns between the planet‐facing hemisphere and the antiplanet hemisphere. For different mass loading ratios we investigate the effect of this asymmetry. By comparing our results with Voyager 1 measurements we conclude that a mass loading asymmetry could in principle provide an explanation of the observed deflection in the magnetic fields as well as the asymmetry of the plasma flow in the wake region.
A global MHD simulation of the Jovian magnetosphere interacting with/without the interplanetary magnetic field
Authors: Miyoshi T. , Kusano K.
Published: February 2004

A global model of the Jovian magnetosphere is presented based on magnetohydrodynamic (MHD) simulation that takes into account the interaction between a rapidly rotating magnetosphere and external plasma flow. In this study, a steady solar wind with, as well as without, the interplanetary magnetic field (IMF) is applied to the inflow boundary. The results clearly indicate that the global structure of the magnetic field lines of force is strongly affected by the rotation of Jupiter as well as by the solar wind. It is also found that magnetic reconnection at the tail region generates the plasma outflow and greatly modifies the corotational structure of the plasmas in the magnetosphere. Especially, when a relatively high‐speed solar wind or a solar wind with a northward IMF collides with the magnetosphere, strong magnetic reconnection at the tail induces the tail‐dusk flow, which is against the corotation. On the other hand, the evident tail‐dusk flow does not appear in the case of a low‐speed solar wind or in the case with a southward IMF. Thus the internal structure of the rotating magnetosphere could be altered qualitatively in response to changes in the solar wind parameters.
Spiky electric fields in the magnetotail
Authors: Streed T. , Cattell C. , Mozer F. , Kokubun S. , Tsuruda K.
Published: February 2004

Large‐amplitude spiky electric fields which have been proposed to provide energization and heating of particles have been observed in many regions of the magnetosphere, including the bow shock, auroral zone, and plasma sheet boundary. We extend previous statistical studies of such fields in the magnetotail (which were limited to ≲ 22 RE) to cover the region explored by the Geotail satellite. Results are presented concerning the occurrence frequency, location, and association with density gradients of spiky electric fields in the magnetotail at radial distances of ∼ 10–200 RE. Spiky fields do not occur beyond ∼ 100 RE, and they predominantly occur at or near the plasma sheet boundary (as indicated by changes in the spacecraft potential) and are always associated with waves near the lower hybrid frequency. Spiky fields occur preferentially during periods of higher magnetic activity (Kp > 3). Statistical analysis of the direction of the largest field amplitudes provides information on the polarization of the waves and the spikes and therefore provides constraints on the origin of the fields. Waves which accompany the spikes are primarily polarized perpendicular to the magnetic field and are consistent with lower hybrid waves, while the spikes are polarized obliquely to the magnetic field. The results are described and compared with various possible generation mechanisms, including lower hybrid wave collapse, a leading candidate for explaining these spikes.
The dawn‐dusk plasma sheet asymmetry of energetic particles: An Interball perspective
Authors: Sarafopoulos D. V. , Sidiropoulos N. F. , Sarris E. T. , Lutsenko V. , Kudela K.
Published: February 2004

We systematically study the energetic particle (∼25 to 850 keV) dawn‐dusk plasma sheet asymmetry using both ion and electron spectra composed of 56 energy channels. These spectra, corresponding to an average duration of ∼8 hours, have been provided by the Interball tail probe and cover the extent between 15 and 28 RE away from the Earth. The events are classified in four Y coordinate categories: the “dawnside” with YGSM<−13 RE, the “center dawn” with 0>Y>−13 RE, the “center dusk” with 0<Y<13 RE, and the “duskside” with Y>13 RE. The asymmetry is profound between the two extreme dawn‐dusk flanks and weaker between the two central plasma sheet regions. In all the energy channels the dusk flank ion fluxes exceed those of electrons. At the dawnside the energetic electron fluxes frequently outnumber those of ions above a bottom energy threshold. During quite times (Kp<2) the ion (electron) fluxes in the duskside (dawnside) plasma sheet exceed by far those in the dawn (dusk) flank. In general, the ion and electron fluxes significantly increase as the Kp index steps upward. However, owing to the dawn‐dusk asymmetry, one can observe higher electron fluxes at dawn with low Kp than at dusk with high Kp. On the basis of two case studies, it is shown that ongoing substorm processes and plasma temperature transitions temporarily affect the spatial dawn‐dusk asymmetry and the associated spectra. In one case study, via successive spectra it was concluded that the betatron acceleration mechanism is probably the dominant energization process, although the observed spectra did not preserve their spectral shape. This is interpreted as due to the action of energy‐dependent large‐scale drifts across the tail that strongly modify the initial source spectra. The dawn‐dusk asymmetry and all the spectral characteristics from inside the plasma sheet are transmitted out into the magnetosheath, and this may help us (in the future) to discriminate between merging and leakage processes taking place over the magnetopause.
Aerodynamic influences on atmospheric in situ measurements from sounding rockets
Authors: Gumbel Jörg
Published: February 2004

Sounding rockets are essential tools for studies of the mesosphere and lower thermosphere. However, in situ measurements from rockets are potentially subject to a number of perturbations related to the gas flow around the vehicle. This paper reviews the aerodynamic principles behind these perturbations. With respect to both data analysis and experiment design, there is a substantial need for improved understanding of aerodynamic effects. Any such analysis is complicated by the different flow regimes experienced during a rocket flight through the rarefied environment of the mesosphere and thermosphere. Numerical studies are presented using the Direct Simulation Monte Carlo (DSMC) approach, which is based on a tracing of individual molecules. Complementary experiments have been performed in a low‐density wind tunnel. These experiments are crucial for the development of appropriate model parameterization. However, direct similarity between scaled wind tunnel results and arbitrary atmospheric flight conditions is usually difficult to achieve. Density, velocity, and temperature results are presented for different payload geometries and flow conditions. These illustrate a wide range of aerodynamic effects representative for rocket flights in the mesosphere and lower thermosphere.
Dynamics of a toroidal magnetic cloud in the solar wind
Authors: Romashets E. P. , Vandas M.
Published: February 2004

Knowledge about the behavior of compact toroidal magnetic force‐free objects in ambient magnetic field may help to better understand dynamical processes in association with coronal mass ejections and their interplanetary counterparts. The problem to find the diamagnetic force acting on a toroidal object in an inhomogeneous magnetic field is solved analytically. At first the drapery of the inhomogeneous magnetic field caused by an insertion of a toroid was found, and then the force acting on the toroid by this disturbed magnetic field was obtained. The problem is considered in purely toroidal coordinates. The obtained solution can be used for calculations of the repulsing diamagnetic force acting on isolated objects in the solar corona such as magnetic clouds of a toroidal shape and for the determination of their velocity profiles. Deformations of toroidal transients due to this melon seed force are not investigated here. Only the force acting on the toroid as a whole is taken in consideration.
The failure of earthquake failure models
Authors: Gomberg Joan
Published: February 2004

In this study I show that simple heuristic models and numerical calculations suggest that an entire class of commonly invoked models of earthquake failure processes cannot explain triggering of seismicity by transient or “dynamic” stress changes, such as stress changes associated with passing seismic waves. The models of this class have the common feature that the physical property characterizing failure increases at an accelerating rate when a fault is loaded (stressed) at a constant rate. Examples include models that invoke rate state friction or subcritical crack growth, in which the properties characterizing failure are slip or crack length, respectively. Failure occurs when the rate at which these grow accelerates to values exceeding some critical threshold. These accelerating failure models do not predict the finite durations of dynamically triggered earthquake sequences (e.g., at aftershock or remote distances). Some of the failure models belonging to this class have been used to explain static stress triggering of aftershocks. This may imply that the physical processes underlying dynamic triggering differs or that currently applied models of static triggering require modification. If the former is the case, we might appeal to physical mechanisms relying on oscillatory deformations such as compaction of saturated fault gouge leading to pore pressure increase, or cyclic fatigue. However, if dynamic and static triggering mechanisms differ, one still needs to ask why static triggering models that neglect these dynamic mechanisms appear to explain many observations. If the static and dynamic triggering mechanisms are the same, perhaps assumptions about accelerating failure and/or that triggering advances the failure times of a population of inevitable earthquakes are incorrect.
Crustal structure and thermal anomalies of the Dunedin Region, South Island, New Zealand
Authors: Godfrey Nicola J. , Davey Fred , Stern Tim A. , Okaya David
Published: February 2004

Seven geophysical data sets are used to investigate a transect along the southeast coast of South Island, New Zealand. The specific focus of this study is the Dunedin volcanic center, which last produced volcanics at the surface 13–10 Myr ago. Wide‐angle reflection/refraction seismic data along a two‐dimensional profile reveal a low‐velocity lower crust and mantle beneath the Dunedin volcanic center. The low‐velocity lower crust coincides with a highly reflective region on a nearby multichannel seismic line and may represent a hot, fluid‐rich region of the crust. In addition, high mantle helium ratios measured in the Dunedin region suggest a current or recent mantle‐melting event. High heat flow recorded in the Dunedin region is consistent with a hot body emplaced in the midcrust ∼10 Myr ago (Miocene) whose heat is just reaching the surface today. Uplift of an Oligocene limestone horizon in the Canterbury basin can be explained by a buoyant load beneath the Dunedin volcanic center and low flexural rigidity of the lithosphere beneath the volcanic center during the Miocene. We interpret the data as revealing two separate thermal events beneath the Dunedin volcanic center, one during the Miocene, when volcanism was last occurring at the surface, and the other occurring currently. Active volcanism associated with the current mantle‐melting event has yet to reach the surface.
Three‐dimensional structure of Vp, Vs, and Vp/Vs beneath northeastern Japan: Implications for arc magmatism and fluids
Authors: Nakajima Junichi , Matsuzawa Toru , Hasegawa Akira , Zhao Dapeng
Published: February 2004

We estimated both P (Vp) and S wave velocity (Vs) structures beneath northeastern Japan by applying a tomographic method to 169,712 P and 103,993 S wave arrival time data from 4338 local events. The average value of Vp/Vs ratio is ∼1.69 in the upper crust, ∼1.75 in the lower crust, and ∼1.77 in the uppermost mantle. These differences in Vp/Vs ratio may mainly reflect the lithological variations with depth. Low‐Vp, low‐Vs, and high Vp/Vs zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge. Here the low‐Vp, low‐Vs, and high Vp/Vs values are interpreted because of partial melting materials, which imply the presence of a vast amount of melt in the uppermost mantle. In the lower crust, low‐Vp, low‐Vs, and high Vp/Vs zones are not continuously visible along the volcanic front but are confined to individual volcanic areas, which suggests that melting of lower crustal materials occurs just beneath each active volcano. In contrast, the upper crust beneath active volcanoes exhibits low Vp, low Vs, and low Vp/Vs. These results suggest that the partial melting zones under northeastern Japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes. The low‐Vp, low‐Vs, and low Vp/Vs features in the upper crust suggest the existence of H2O (rather than melt) right beneath active volcanoes.
Permeability reduction in granite under hydrothermal conditions
Authors: Morrow C. A. , Moore D. E. , Lockner D. A.
Published: February 2004

The formation of impermeable fault seals between earthquake events is a feature of many models of earthquake generation, suggesting that earthquake recurrence may depend in part on the rate of permeability reduction of fault zone materials under hydrothermal conditions. In this study, permeability measurements were conducted on intact, fractured, and gouge‐bearing Westerly granite at an effective pressure of 50 MPa and at temperatures from 150° to 500°C, simulating conditions in the earthquake‐generating portions of fault zones. Pore fluids were cycled back and forth under a 2 MPa pressure differential for periods of up to 40 days. Permeability of the granite decreased with time t, following the exponential relation k=c(10−rt). For intact samples run between 250° and 500°C the time constant for permeability decrease r was proportional to temperature and ranged between 0.001 and 0.1 days−1 (i.e., between 0.4 and 40 decades year−1 loss of permeability). Values of r for the lower‐temperature experiments differed little from the 250°C runs. In contrast, prefractured samples showed higher rates of permeability decrease at a given temperature. The surfaces of the fractured samples showed evidence of dissolution and mineral growth that increased in abundance with both temperature and time. The experimentally grown mineral assemblages varied with temperature and were consistent with a rock‐dominated hydrothermal system. As such mineral deposits progressively seal the fractured samples, their rates of permeability decrease approach the rates for intact rocks at the same temperature. These results place constraints on models of precipitation sealing and suggest that fault rocks may seal at a rate consistent with earthquake recurrence intervals of typical fault zones.
Topographic effects of the Eastern California Shear Zone in the Mojave Desert
Authors: Dokka Roy K. , Macaluso Kristine Y.
Published: February 2004

Digital elevation data were used to evaluate the topographic implications of the late Neogene Eastern California Shear Zone (ECSZ). Analysis shows that the ECSZ has directly affected >18% of the surface of the Mojave Desert block (MDB); uncertainties regarding the distribution of strain in basins suggest that the area of effect may be larger by perhaps a factor of 2. Remaining areas are likely due to Quaternary erosion or are inherited from pre‐ECSZ times. Major drainages and basins of the MDB are developed along transtensional depressions within the ECSZ. These depressions established the regional and local base levels that have governed erosion by running water. Local topography is closely tied to the style of strain. Highlands of the MDB are spatially associated with (1) areas that have or are currently undergoing contractional strain associated with transpression within the ECSZ and (2) areas that lie adjacent to uplifted transpressional belts that bound the Mojave Desert (San Bernardino and San Gabriel Mountains). In contrast, lowland areas are spatially associated only with transtensional areas within the ECSZ. Examination of the topography in the transtensional areas also reveals two, distinct populations of elevation. Spatial analysis indicates that areas that have undergone the greatest local transtension associated with dextral shear are also the lowest topographically. These areas formed during the earliest phase of movement of the ECSZ (late Miocene to early Pleistocene). The second population is coextensive with recently formed transtensional basins that continue to be active.
Viscous forces acting on subducting lithosphere
Authors: Winder R. O. , Peacock Simon M.
Published: February 2004

Subducting lithosphere is driven by gravitational forces and resisted primarily by viscous forces. Three independent types of mantle flow combine to create viscous tractions on the surface of a subducting slab which terminates in the upper mantle: (1) flow induced by plate convergence and slab subduction, (2) flow caused by trench migration, and (3) “regional” upper mantle flow unrelated to convergence or trench migration. The equilibrium dip of a slab reflects a balance of viscous tractions with gravitational forces and slab stiffness. We present results from a two‐dimensional finite difference model of upper mantle flow for different slab geometries where mantle viscosity increases exponentially with depth and the slab, terminating in the upper mantle, is assumed to be infinitely viscous. In the overlying mantle wedge the subducting slab creates a strong eddy, which is largely unaffected by trench migration or regional flow. Trench migration and regional mantle flow produce tractions which tend to lift the slab and which are independent of slab dip for a slab terminating in the upper mantle. For dips >22.5° the upward torquing force on the slab due to convergence is negligible compared to the pressures created by trench migration and regional mantle flow. Slabs dipping <87.5° which are in torquing equilibrium require regional flow in the same direction as subduction (or trench migration in the opposite direction), which simultaneously tends to eliminate most of the horizontal pressure difference across the subduction zone. The force calculations based on this model suggest that viscosity at the base of the lithosphere is <2×1020 Pa s, assuming viscosity increases exponentially with depth to <2.8×1021 Pa s just above the lower mantle. For force‐balanced slabs which penetrate halfway into the upper mantle with dip in the range of 30° to 75° slab dip increases linearly with decreasing trench migration rate and decreasing regional mantle flow.
Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep
Authors: Gillis Kathryn M. , Muehlenbachs Karlis , Stewart Michael , Gleeson Thomas , Karson Jeffrey
Published: February 2004

Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4‐km‐wide region at Hess Deep (equatorial Pacific) reveal significant spatial heterogeneity (10–103 m) in the extent and nature of hydrothermal alteration in young, fast spreading East Pacific Rise crust. The volcanic sequence is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60°. Their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole, clinopyroxene replacement increases (<20% to >40%), whole rock δ18O values decreases (4.4–5.5‰ to 3.5–4.5‰), and calculated peak metamorphic temperatures increase (∼250°C to 450°–700°C). Within the deepest dikes, localized zones up to 400‐m‐wide are chlorite‐rich and have low‐δ18O (2.9–4.1‰) and low peak metamorphic temperatures (∼345°C). These alteration patterns likely formed within broad recharge zones whereby the low‐δ18O zones developed in the regions with the highest fluid flux. In the west, massive, slightly rotated sheeted dikes near the volcanic‐sheeted dike transition are δ18O and Cu depleted and display higher peak temperatures (≥345°C) than elsewhere in the shallow dikes. These characteristics are consistent with formation within a high temperature, hydrothermal discharge zone. We propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes, similar to those preserved in many ophiolites.
Surface heat flux histories from inversion of geothermal data: Energy balance at the Earth's surface
Authors: Beltrami Hugo
Published: February 2004

Past changes in the Earth's surface energy balance propagate into the subsurface and appear as perturbations of the subsurface thermal regime. This paper presents a singular value decomposition inversion method used to reconstruct surface heat flux histories (SHFH) from the heat flux anomalies detected in the shallow subsurface. Synthetic tests were used to assess the robustness of the inversion procedure. It was found that data noise can have a significant effect on the stability of the SHFH inferred from inversion. This translates in SHFHs having lower temporal resolution than ground surface temperature histories (GSTHs) obtained from the same data, but the long‐term trends are robust. Results are encouraging for temperature data noise levels typically encountered in field measurements. Synthetic data tests yield results in agreement with analytical expressions derived from GSTHs for the same parameterization. Temperature‐depth profiles from Canada's geothermal database were used to illustrate the inversion procedure. Individual temperature profile inversions are shown as examples. All 112 temperature logs in the database were used to obtain a mean heat flux history for the region. Results indicate that the ground heat flux has increased an average of 24 mW m−2 over the last 200 years in Canada. Application of this method to the existing global geothermal data base should allow for a quantification of the global energy balance at the Earth's surface for the past few centuries and may be useful for land surface models.
Causes for axial high topography at mid‐ocean ridges and the role of crustal thermal structure
Authors: Shah Anjana K. , Buck W. Roger
Published: February 2004

Mid‐ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow, but not require, a region of rapid cooling near the axis. Loads on the lithosphere arise from the presence of low‐density melt, densification due to cooling with distance from the ridge axis, and thermal contraction stresses. We find two end‐member classes of temperature and melt structure that can produce axial high topography and gravity observed at the East Pacific Rise (EPR). One class is very similar to previous models, requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis. The other is a new class, predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis, with melt which is contained primarily within the crust. The latter solution is consistent with tomography and compliance studies at the EPR which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation. This solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle. Model fits predict 0–30% melt in the lower crust, depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography. The model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis. This is consistent with previous correlations between large cross‐sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime. For a slightly slower rate of lithospheric cooling at distances more than ∼5 km from the axis the model predicts a trough at the base of the axial high. Such troughs have been previously observed at the base of the high on the western flank of the southern EPR, where subsidence rates are anomalously low. Finally, thick axial lithosphere reduces the amplitude of the high, making it sometimes difficult to distinguish from long‐wavelength subsidence. This morphology is comparable to that of some intermediate spreading ridges, where topography is relatively flat, suggesting a transition from fast to intermediate style morphology.