bluuebunny/crossref_metadata_2025 huggingface dataset:
Unusual composition of the solar wind in the 2–3 May 1998 CME observed with SWICS on ACE
Authors: Gloeckler G. , Fisk L. A. , Hefti S. , Schwadron N. A. , Zurbuchen T. H. , Ipavich F. M. , Geiss J. , Bochsler P. , Wimmer‐Schweingruber R. F.
Published: September 2002

Elemental, isotopic and charge state abundances provide valuable information about the source and acceleration mechanism of Coronal Mass Ejections (CMEs). Even though the kinetic properties of the plasma might be subject to changes because of dynamic effects occurring during the expansion of the CME, the composition of the solar wind remains unchanged after it leaves the low corona. Data from the Solar Wind Ion Composition Spectrometer (SWICS) on ACE are used to study the elemental and charge state composition of He, O, C, N, and Fe as well as the isotopic ratio of He during the very large CME of May 2–3, 1998. We find in this CME anomalously large enrichment of ³He++/4He++, He/O and Fe/O. During the 28 hour long cloud portion of the CME unusually cold material (4He+ and very low charge state heavy ions) was observed together with hot (high charge state ions) and normal solar wind plasma.
On the bulk isotopic composition of magnesium and silicon during the May 1998 CME: ACE/SWIMS
Authors: Wimmer‐Schweingruber Robert F. , Bochsler Peter , Gloeckler George , Ipavich Fred M. , Geiss Johannes , Kallenbach Reinald , Fisk Len A. , Hefti Simon , Zurbuchen Thomas H.
Published: September 2002

The coronal mass ejection (CME) observed at the Advanced Composition Explorer (ACE) spacecraft on May 2 and 3, 1998, exhibited very high as well as exceptionally low charge states of all ions. In addition, ³He was considerably enhanced by in the bulk material of the CME. High ³He/4He ratios in solar energetic particles have recently been observed in some events to coincide with substantial enrichments in the heavy isotopes of heavy elements. We use data from the Solar Wind Ion Mass Spectrometer (SWIMS) on ACE to investigate whether the enrichment of the heavy isotopes in solar energetic particles is mirrored in the isotopic composition of solar wind Mg and Si. We concentrate on the time period where the unusual mixture of charge states and unusually high ³He/4He was observed to test for isotopic fractionation in such extreme solar wind conditions. We find very little or no enrichment of the neutron‐rich isotopes with respect to the main isotopes. Incidentally, this is also the first report on the isotopic composition of ions with Z > 2 in the bulk material in a CME.
Estimation of global vehicular methyl bromide emissions: Extrapolation from a case study in Santiago, Chile
Authors: Chen Tai‐Yih , Blake Donald R. , Lopez Jimena P. , Rowland F. Sherwood
Published: September 2002

Between June 1 and June 8, 1996, 144 whole air samples were collected in Santiago, Chile. The temporal and geographical enhancement of CH3Br correlated with incomplete combustion tracers emitted from vehicles during the morning commute. From these, a city‐wide CH3Br/CO volume emission ratio of 2.2 × 10−6 was measured in ambient air. Without using the CO measurements, we estimate an annual release of 8.9 tons of CH3Br in Santiago based solely upon enhanced concentrations observed throughout the study area during the morning traffic period. This enhancement corresponds to 8.0 × 10−6 kg CH3Br emitted for each liter of gasoline used (leaded and unleaded). By scaling the annual gasoline usage in Santiago to countries still using leaded gasoline, and assuming the above 8.0 × 10−6 kg/L value holds true, a global vehicular CH3Br emission of 4 ± 3 Gg/year is calculated. This small vehicular CH3Br emission source strength will not improve the current CH3Br budget imbalance.
Blob instability in rotating compositional convection
Authors: Claßen Sabine , Heimpel Moritz , Christensen Ulrich
Published: September 2002

The onset and structure of compositional convection in a rotating system are investigated experimentally. A vertically oriented cylindrical annulus filled with NH4Cl‐H2O solution is cooled from the bottom and can be rotated about its axis at rates ranging up to 10.5 rad s−1, corresponding to Ekman numbers down to 7.5 × 10−6. The Coriolis force has a strong effect on the structure of plumes above the mush‐liquid interface. Helical motion of the conduit, which is weakly developed in the non‐rotating case, is amplified by Coriolis forces that twist the plume conduits to lie nearly horizontally. This results in secondary plumes (or blobs) that rise from the sub‐horizontal primary plume conduits. This new instability could be an efficient mechanism for producing small‐scale flow in the form of buoyant blobs that ascend through the polar regions of the outer core.
Kinetic requirements for the measurement of mesospheric water vapor at 6.8 µm under non‐LTE conditions
Authors: Mlynczak Martin G. , Zhou Daniel K. , Lopez‐Puertas Manuel , Zaragoza Guillermo , Russell James M.
Published: September 2002

We present accuracy requirements for specific kinetic parameters used to calculate the populations and vibrational temperatures of the H2O(010) and H2O(020) states in the terrestrial mesosphere. The requirements are based on rigorous simulations of the retrieval of mesospheric water vapor profiles from measurements of water vapor infrared emission made by limb scanning instruments on orbiting satellites. Major improvements in the rate constants that describe vibration‐to‐vibration exchange between the H2O(010) and O2(1) states are required in addition to improved specification of the rate of quenching of O2(1) by atomic oxygen (O). It is also necessary to more accurately determine the yield of vibrationally excited O2(1) resulting from ozone photolysis. A contemporary measurement of the rate of quenching of H2O(010) by N2 and O2 is also desirable. These rates are either highly uncertain or have never before been measured at atmospheric temperatures. The suggested improvements are necessary for the interpretation of water vapor emission measurements at 6.8 µm to be made from a new spaceflight experiment in less than 2 years. The approach to retrieving water vapor under non‐LTE conditions is also presented.
Estimation of tropospheric zenith delay and gradients over the Madrid area using GPS and WVR data
Authors: Ruffini G. , Kruse L. P. , Rius A. , Bürki B. , Cucurull L. , Flores A.
Published: September 2002

We have analyzed data from an experiment over the Madrid (Spain) area obtained from 5 GPS receivers and 3 Water Vapor Radiometers (WVR), in order to compare their retrievals of Tropospheric Slant Delays. For this purpose we have fitted a simple gradient model to both types of data, using a Kalman filter to account for the temporal variability of the zenith and gradient parameters. We show that the retrieved gradients with the two instruments are compatible, thus suggesting that the derived slant delays can be useful for tomographic analysis. We compare the estimates of ZWD obtained with the GIPSY and the Bernese software packages. Finally, we compare the estimated gradients with those obtained with HIRLAM, a Numerical Weather Prediction model.
Observation of short‐term variation of Jupiter's synchrotron radiation
Authors: Miyoshi Yoshizumi , Misawa Hiroaki , Morioka Akira , Kondo Tetsuro , Koyama Yasuhiro , Nakajima Junichi
Published: September 2002

Detailed observations of Jovian decimetric radiation (DIM) from relativistic electrons in the inner Jovian radiation belt was carried out to seek the existence of a short‐term variation which has information on the electromagnetic disturbances in the inner Jovian magnetosphere. Results demonstrated the DIM flux enhancement continuing for a few days for the case of November 1996. It was shown that the relativistic electrons were highly anisotropic during the DIM flux enhancement. The correlation between the DIM flux enhancement and the solar F10.7 flux enhancement was also detected. A model simulation suggested that the observed DIM flux variation was caused by the enhanced radial diffusion due to the intense solar UV/EUV event which heated the Jupiter's upper atmosphere.
The total ozone anomaly at Lauder, NZ in 1997
Authors: Connor B. J. , Bodeker G. E. , McKenzie R. L. , Boyd I. S.
Published: September 2002

A record low annual mean value of total ozone was observed at Lauder, New Zealand in 1997, namely 294.5 DU, compared to a 1985–96 mean of 307.9±3.7. Ozone values were low through most of the year, especially in winter and spring. These observations are similar to TOMS results for Southern mid‐latitudes, although the long‐term comparison is clouded by the change in TOMS instruments. A regression analysis of the Lauder record through 1996 allows a generally accurate prediction of the 1997 values, shows that the QBO dominates the 1996–97 difference, and that the 1997 values are consistent with a linear long‐term trend in ozone. The analysis does not completely explain low monthly values in austral spring. Enhancements in biologically active UV radiation were also observed, with monthly mean values increased by as much as 20–40% from recent years. The unusual ozone values were the dominant cause of the UV enhancement.
Thermoelastic stresses with reference to scale and morphology of the Earth's surface
Authors: Zimmer F. , Hergarten St. , Neugebauer H. J.
Published: September 2002

The variability of thermoelastic stresses at the Earth's surface is investigated on different scales with a three‐dimensional numerical model. Energy transfer by heat conduction and radiation of sun, atmosphere and Earth's surface is regarded. It is shown that thermoelastic stresses are considerably high, so that they are able to assist faulting and thus force weathering of rocks. Their morphological distribution strongly depends on the spatial scale of the relief; on large scales it is governed by the temperature distribution, on small scales (below a few meters) the surface morphology plays the dominant part.
Viscous heating in heterogeneous media as applied to the thermal interaction between the crust and mantle
Authors: Schott Bertram , Yuen David A. , Schmeling Harro
Published: September 2002

We have studied the effects of viscous heating involving heterogeneous media which occurs in the interaction between the mantle‐lithosphere—crust system with the mantle‐lithosphere in delamination. High rates of viscous heating are found in corner flow regions, where the deformation rate of the flow is highest. The weak lower crust is sheared between the upper crust and the mantle‐lithosphere, giving rise to a second type of viscous heating with high dissipation values several ten to hundred times the chondritic value. Greater dissipation rates are found in the shear interaction between heterogeneous media with different non‐Newtonian rheologies than in the shear heating inside a single non‐Newtonian or Newtonian homogeneous medium. High spatial resolution of at least 2.5 km or finer is needed to resolve the sharp variations of viscous heating near the interface of the two media.
An upper limit on the spontaneous fission decay constant of232Th derived from xenon in monazites with extremely high Th/U ratios
Authors: Wieler Rainer , Eikenberg Jost
Published: September 2002

We attempted to analyse xenon produced by spontaneous fission of232Th in monazites with Th/U ratios on the order of several thousand and Th concentrations of ∼4000–6000 ppm. The samples are from the Bayan Obo ore deposit in China and previously yielded precise Th‐Pb internal isochron ages of ∼400–500 Ma (Wang et al., 1994, Geochim. Cosmochim. Acta 58, 3155). The monazites retained between 20–50% of their radiogenic4He, and we can assume that fission Xe has essentially been completely retained. The minute amounts of fission Xe detected are compatible with production entirely from the traces of238U. We derive an upper limit of ∼2*10−21y−1for the spontaneous fission decay constant of232Th. This shows that the Th‐derived contribution can be neglected when dating U‐bearing minerals by fission Xe.
Long term ionospheric trends over Ahmedabad
Authors: Sharma Som , Chandra H. , Vyas G. D.
Published: September 2002

Ionospheric data over Ahmedabad (23°N, 73°E) during 1955–96 have been examined for the long term changes arising due to the predicted cooling of the mesosphere and thermosphere because of the increased concentration of trace gases resulting from anthropogenic causes. The long term changes in the critical frequencies of the ionospheric layers and the height of the maximum ionization as characterized by hPF2, the virtual height at 0.834 of foF2, have been obtained after removing the seasonal and solar cycle variations. While the trend of a small decrease in foE (0.15 MHz in 40 years) may not be considered significant, there is a clear indication of a decrease in foF2 (1.6 MHz for midday, 1.0 MHz for midnight). A small increase is noted in foF1 (0.3 MHz). There is a decrease in hPF2 (16 km for midday and 10 km for midnight). The lowering of F2 layer peak will affect radio propagation.
CF4 and the Age of Mesospheric and Polar Vortex Air
Authors: Harnisch Jochen , Borchers Reinhard , Fabian Peter , Maiss Manfred
Published: September 2002

A chronology of tropospheric CF4 mixing ratios is reported for the northern hemisphere. A decline of global emissions of CF4 from 16,000 to 11,000 metric tons yr−1 is found for the periods 1978–1990 and 1992–1998, respectively. The atmospheric chronology of CF4 is applied to determine the age of air of cryogenic air samples collected on rocket and balloon platforms. Age values of 7 years are found inside the polar vortex between 25–44 km altitude and of 11 years between 56 and 61 km altitude, which is significantly higher than predicted by most models currently used to assess the effects of high‐flying aircraft. The results suggest that the mesosphere is more isolated from below than previously believed. A comparison of age values derived from CF4 and SF6 relative to predicted deviations constrains the atmospheric lifetime of SF6 to >5,000 years.
The magnetic and plasma structure of flux transfer events
Authors: Le G. , Gosling J. T. , Russell C. T. , Elphic R. C. , Thomsen M. F. , Newbury J. A.
Published: September 2002

Flux transfer events have been interpreted to be manifestations of transient reconnection of the magnetospheric and magnetosheath magnetic fields at the Earth's magnetopause. In this study we determine the interior structure of flux transfer events by examining high‐resolution magnetic field and plasma distribution functions from the ISEE 2 spacecraft. The sampling time and cadence of these data are more than adequate to resolve the rapidly changing plasma regimes and to avoid spatial aliasing. From these data we have confirmed the existence of two distinct regions within a flux transfer event (FTE), a central core and a field‐draping region, and these two regions have been found within FTEs observed both on the magnetosheath side and the magnetospheric side of the magnetopause. The boundaries between the two regions are apparent in both the field and the plasma data. In the field‐draping region the plasma distribution functions show little difference from those in the neighboring magnetosheath or magnetosphere, and the magnetic field consistently exhibits signatures expected from field‐line draping around a flux tube. Within the central core region, plasma appears to be a mixture of the magnetosheath and magnetospheric components. In FTEs observed in the magnetosphere, we find transmitted magnetosheath plasma and a strong depletion of hot magnetospheric plasma. In FTEs observed in the magnetosheath, we find an outflow of magnetospheric plasma mixed with the magnetosheath plasma. These signatures unambiguously show the reconnection of the interplanetary magnetic field and the Earth's magnetic field. Thus our observations indicate that the magnetic field lines within the central core region are open allowing the inflow of cold magnetosheath plasma and the outflow of hot magnetospheric plasma through the open flux tube. The reconnection picture is further supported by the observation of separate electron and ion edges at the trailing boundary of a northward moving flux tube, expected as time‐of‐flight effects on newly reconnected field lines. Thus our observations are consistent with the reconnected open flux tube interpretation for FTEs.
Three‐dimensional propagation of coronal mass ejections (CMEs) in a structured solar wind flow: 2. CME launched adjacent to the streamer belt
Authors: Odstrčil D. , Pizzo V. J.
Published: September 2002

A three‐dimensional (3‐D) numerical hydrodynamic model is used to investigate the evolution of coronal mass ejections (CMEs) launched at several heliographic positions into a tilted‐dipole ambient solar wind (SW) flow, which is appropriate around solar activity minimum and declining phase. The CME is injected as an overpressured plasma cloud. Results show that the motion and local appearance of a CME in interplanetary space is strongly affected by its interaction with the background SW velocity and density structures. The most complicated scenario occurs when a CME is injected directly into the slow stream‐belt flow. Temporal profiles corresponding to the interaction of fast CME and slow SW flows, to the merging of CME‐driven and corotating disturbances at the fast stream leading edge, and to expanding structures at the trailing edge of the preceding fast stream are presented. The simplest configuration is when a CME is injected just to the west of the streamer belt. In that case the CME‐driven disturbance evolves and propagates freely into the preceding fast stream, untouched by the streamer belt slow flow near which it originates, except for a small segment at low latitudes. When a CME is injected just to the east of the streamer belt, the CME expansion and shock evolution are constrained by the slow, dense flow ahead of the fast stream in which the CME propagates initially. Low‐latitude portions of the CME are trapped between the slow streamer belt flow and the leading edge of the oncoming fast stream. In this case, CME‐driven and corotating shock structures can merge. These results are related to a previous study, where the CME is launched into the midst of the slow streamer belt flow. Numerical results confirm recent Ulysses findings about the overexpanding CME structure, the extreme latitudinal distortions in the CME shape, and the inclination of shock fronts. These results also provide a basis for interpreting variations in the radial width of the CME and the shock stand‐off distance. Simulations of spatial structures resulting from 3‐D dynamic interactions help illuminate differing temporal onsets and profiles of observed CMEs.
A test particle simulation of the motion of oxygen ions and solar wind protons near Mars
Authors: Kallio E. , Koskinen H.
Published: September 2002

Outflowing oxygen ions (O+) have been observed to be a persistent feature in the Martian tail. However, not much is known of the spatial distribution of oxygen ions, of the correlation between oxygen ions and solar wind protons (H+), and, especially, of the acceleration of oxygen ions in the tail. We present a test particle simulation study of the motion of O+ and H+ ions and compare the results to Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) Phobos 2 particle measurements. We have studied the spatial distribution, velocity, density, and flux of oxygen ions in the nightside. In the simulation the oxygen ions were ionized from the Martian hot oxygen corona and the H+ ions were solar wind protons. We used an empirical flow model to derive the magnetic and electric field everywhere around the planet. The work is the first test particle simulation for Mars where a fully three‐dimensional magnetotail configuration has been used. The model reproduces many observed plasma features. The solar wind protons flow fluid‐like near the terminator despite their finite Larmor radii. The oxygen ions produce a plasmasheet‐like layer near the cross‐tail current sheet and empty magnetic lobes, and have north‐to‐south asymmetry in the magnetosheath and in the tail much as observed. The energy of oxygen ions in the tail is close to, but slightly less than, observed. The particle density and the particle flux of oxygen ions also agree quite well with the observations, suggesting that the total O+ outflow rate is ∼ 2 × 1025 s−1. Overall, the study suggest that most of the observed O+ outflow features can be understood by assuming that the ions are accelerated by the convective electric field associated with the flow of the solar wind protons.
Size of the auroral oval: UV ovals and precipitation boundaries compared
Authors: Kauristie K. , Weygand J. , Pulkkinen T. I. , Murphree J. S. , Newell P. T.
Published: September 2002

The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval (often associated with the earthward edge of the main plasma sheet), the boundary between smooth and structured precipitation, and the poleward boundary of the particle oval (close to the open‐closed field line separatrix). The UV oval is characterized by the latitude of maximum UV intensity, equatorward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundary of the particle oval is often at ≥2° higher latitudes than the most intense UV luminosity. Large differences are typical especially in the midnight and morning sectors. The present results suggest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to compute changes in the amount of open flux under different states of substorm activity.
Pronounced enhancements of pickup hydrogen and helium in high‐latitude compressional regions
Authors: Schwadron N. A. , Zurbuchen T. H. , Fisk L. A. , Gloeckler G.
Published: September 2002

Pickup hydrogen and helium distributions are measured in compressive corotating regions at high heliographic latitudes. The enhancements can be explained using a pickup ion transport model which includes compression but requires a long scattering mean free path. The long mean free path is generally observed for pickup ions at high latitudes, but it is not explained by current theories of wave‐particle interaction. The feet that the mean free path is not reduced in the high‐latitude compressed events suggests that the long mean free path is not isolated to highly static solar wind conditions. It also has important implications for wave‐particle interaction theories since the turbulence within the compressed events is expected to be qualitatively different from the turbulence in static noncompressed solar wind.
Observations of high‐latitude lower thermospheric winds from Thule Air Base and Søndre Strømfjord, Greenland
Authors: Won Y.‐I. , Niciejewski R. J. , Killeen T. L. , Johnson R. M. , Lee B. Y.
Published: September 2002

Lower thermospheric winds have been determined from Fabry‐Perot interferometer (FPI) measurements of the Doppler shift of the 5577‐Å O(1S) emission over Thule Air Base (76.5°N, 69.0°W) and Søndre Strømfjord (67.0°N, 50.9°W), Greenland. These winds normally correspond to the altitude of the peak of the airglow O(1S) emission layer, near 97 km. The altitude ambiguity due to auroral contamination has been reduced by eliminating data when the intensity of the emission increases significantly. Contamination by airglow emission of 5577‐Å O(1S) originating from higher altitudes has been investigated by an FPI simulation code. The simulation results indicate that this latter emission may contribute an anomalous diurnal oscillation to ground‐based 5577‐Å O(1S) FPI measurements of lower thermospheric wind. The agreement of diurnal phases between that deduced from the green‐line measurements and that determined from simultaneous red‐line observation supports this conclusion. The same simulation applied to observations from Søndre Strømfjord shows that the upper layer contamination is much weaker and is not serious. Significant day‐to‐day variation is evident in the lower thermospheric wind field. Average neutral winds are calculated, and a harmonic analysis is carried out to examine the major low‐frequency wind components. The seasonal variations of these wind components are compared with radar data and model predictions. The observations are generally in good agreement with model results. The comparison between FPI and radar results also shows reasonable agreement. The semidiurnal amplitudes observed with the Søndre Strømfjord radar during the Lower Thermospheric Coupling Study (LTCS‐I) and LTCS‐2 periods are always greater than the climatological values obtained from averaging FPI and Chatanika radar observations. This result shows the variability that can be expected when comparing “instantaneous” estimates of tidal parameters with climatological results.
Relevance of plasma and neutral wind velocities to the topology and the excitation of modes for the onset of the equatorial spread F
Authors: Basu Bamandas , Coppi Bruno
Published: September 2002

A theoretical model for the onset of the equatorial spread F (ESF) is introduced that considers all the effects of the electric fields that are present together with the effects of the neutral wind velocity, of gravity, and of the density gradient. These impose well‐defined topological constraints on the modes that can be excited. The horizontal (east‐west) ion velocity is shown to be one of the key factors for the intrinsic vertical localization of modes that are driven unstable by the effective vertical gravity (including the E field contribution). The thickness of the layer of localization is related to the vertical gradient of the ion horizontal velocity, of the neutral wind velocity, and of the ion‐neutral collision frequency. The “horizontal gravity” resulting from the difference between the horizontal ion and neutral velocities is shown to not be a driving factor for the instability, under realistic conditions when the eastward electric field is included in the analysis, but to have an important influence on the mode topology (vertical profile). An explicit analytical solution is found for the relevant dispersion equation, which includes, in particular, the contribution of a vertical ion velocity. The mode vertical profile under conditions that are shown to be realistic is found. It is suggested that the necessary conditions for the intrinsic localization of the modes determine the onset of the ESF. The mode ballooning structure, along the magnetic field, including the effect of the plasma finite resistivity is also described.
Finite Larmor radius convection instability in the near‐Earth plasma sheet
Authors: Kozlovsky A. , Lyatsky W.
Published: September 2002

Violation of the frozen‐in condition for the plasma sheet ions when the spatial scale of disturbance is of the order of the Larmor radius leads to the magnetospheric convection instability. This instability results in the separation of the convection plasma flow into jets with the spatial scale of the order of 1km (from a few hundreds meters up to a few kilometers) at the ionosphere level. The instability develops in these regions of the plasma sheet where the contents of the hot ions in the unit magnetic flux tube vary with the distance from the Earth. This takes place at the boundaries of the plasma sheet. The instability has a large growth rate (up to 0.01 s−1). In the nighside plasma sheet region the developing structures are stretched approximately along the auroral oval.
A prediction of geomagnetic activity for solar cycle 23
Authors: Cliver E. W. , Ling A. G. , Wise J. E. , Lanzerotti L. J.
Published: September 2002

Using a database of 13 solar cycles of geomagnetic aa data, we obtained correlations between cycle averages of geomagnetic activity (and sunspot number) and the numbers of days with disturbance levels above certain aa thresholds. We then used a precursor‐type relation to predict an average aa index of 23.1 nT for cycle 23 and inserted this average aa value into the above correlations to forecast the integral size distribution of geomagnetic activity for the new cycle. The predicted size distribution is similar to that observed for cycles 21 and 22 but most closely resembles that of solar cycle 18 (1944–1954), which was slightly smaller than cycles 21 and 22. Our prediction agrees reasonably well with the “climatology‐based” forecast made by the intergovernmental panel tasked to predict geomagnetic activity for the coming solar cycle and is significantly different from their “precursor‐based” prediction.
Impulsive reconnection dynamics in collisionless laboratory and space plasmas
Authors: Bhattacharjee A. , Ma Z. W. , Wang Xiaogang
Published: September 2002

There are some phenomena involving magnetic reconnection in collisionless laboratory and space plasmas that are characterized not only by fast growth but also by an impulsiveness, that is, a sudden increase in the time derivative of the growth rate. Three examples are the sawtooth collapse in tokamaks, the sudden enhancement of the cross‐tail current density just prior to substorm onset in the Earth's magnetotail, and the impulsive phase of a solar flare. Not only is the peak growth rate of these phenomena large (near‐Alfvénic), they are triggered suddenly, with the growth rate increasing from small to large values very rapidly. Traditional steady state reconnection models, such as those due to Sweet‐Parker or Petschek, do not include time dependency, and hence cannot account for the impulsiveness. It is demonstrated that in collisionless plasmas, the effects of electron pressure gradients and Hall currents (referred to collectively as Hall MHD effects) can provide a plausible explanation not only of the fast growth but also of the impulsiveness. The geometrical structure of thin current sheets realized during the dynamical evolution of such collisionless systems changes spontaneously from a Y point in the early nonlinear regime (characteristic of the models of Sweet‐Parker and Syrovatsky) to an X point (characteristic of the Petschek model). However, the underlying physics of reconnection is qualitatively different from that in resistive MHD models. In Hall MHD, the electron and ion dynamics are decoupled from each other in the reconnection layer. Such decoupling is forbidden in resistive MHD. In Hall MHD, while the spatial scale of the parallel electric field (that is, the component of the electric field parallel to the background magnetic field) is the ion skin depth, the spatial scale of the parallel current density is narrower, determined by the Lundquist number (or the electron skin depth), if resistivity (or electron inertia) is the dominant mechanism that breaks field lines. Analytical results as well as high‐resolution numerical simulations are presented, and the predictions of the model are compared with data from laboratory experiments as well as space.
Interplanetary Lyman α line profiles derived from SWAN/SOHO hydrogen cell measurements: Full‐sky Velocity Field
Authors: Quémerais Eric , Bertaux Jean‐Loup , Lallement Rosine , Berthé Michel , Kyrölä Errki , Schmidt Walter
Published: September 2002

We present here an analysis of 1 year of data obtained by the solar wind anisotropies (SWAN) instrument on board the SOHO spacecraft orbiting around the Ll Lagrange point at 1.5 × 106 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman a line profile reflects the velocity distribution of H atoms projected onto the line of sight (LOS). Here we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. For a LOS in a fixed celestial direction, the Doppler shift between the interplanetary emission profile and the H cell absorption profile varies by up to ±0.12 Å during 1 year, owing to the Earth's orbital velocity around the Sun, equal to 30 km s−1. Such a Doppler spectral scan across the emission line allows us to derive Lyman α line profiles, and hence the velocity distribution, in and out of the ecliptic independent of any modeling of the neutral hydrogen atom distribution in the heliosphere or of the multiple scattering of solar photons. The spatial distribution of the apparent velocity relative to the Sun as observed from the orbit of SOHO is derived for all directions, except within 5° of the ecliptic poles. This determination strongly constrains models of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° in J2000 ecliptic coordinates) show a small discrepancy by 3° – 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in the upwind direction is −25.4 ± 1 km/s, whereas it is equal to 21.6 ± 1.3 km s−1 in the downwind direction. A preliminary analysis shows that the Zero Doppler shift cone and the difference between the upwind and downwind velocities correspond to a ratio μ of Lyman α radiation pressure to solar gravity of 0.9–1.0. It follows that the observed upwind apparent velocity is compatible with a velocity at infinity of H atoms of the order of 21–22 km s−1. However, extensive modeling is required in order to get more definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan method and the H cell data.
Postsunset vortex in equatorial F‐region plasma drifts and implications for bottomside spread‐F
Authors: Kudeki Erhan , Bhattacharyya Santanu
Published: September 2002

Recent high‐resolution incoherent scatter drift measurements conducted at Jicamarca during solar minimum conditions indicate that a vortical plasma flow pattern accompanies the well known prereversal enhancement events observed in the equatorial F region. The vortex is centered at ∼250 km altitude and about 2000–3000 km east of the evening terminator and is characterized by upward and downward flows to the west and to the east and eastward and westward flows on the top and the bottom, respectively. Postsunset bottomside spread‐F events are observed to commence in the interior of the vortex in regions of westward plasma drifts. The formation of the vortex implies localized charging of the postsunset bottomside F region in response to eastward neutral wind driven F region dynamo action.
Pc5 ULF pulsations associated with waveguide modes observed with the IMAGE magnetometer array
Authors: Mathie R. A. , Mann I. R. , Menk F. W. , Orr D.
Published: September 2002

Several recent publications have reported high‐latitude radar observations of field line resonances (FLRs) displaying discrete and very stable frequencies in the Pc5 range. It is believed that these quantized frequency FLRs may be associated with cavity/waveguide harmonics of the outer magnetosphere but the prevalence and reported stability of the frequencies remains controversial. We examine the statistics of 137 Pc5 pulsation events identified from 3 months magnetometer data from the International Monitor for Auroral Geomagnetic Effects (IMAGE) and show that these discrete frequencies are also prominent in ground‐based data but find no evidence to support their reported day‐to‐day stability. However, our observations are consistent with FLRs in the outer magnetosphere being driven by waveguide mode harmonics. Further, we examine amplitude, phase, polarization, and azimuthal wave number characteristics and show that these FLRs have features which are strongly local time dependent. Our analysis indicates that the local time dependence of the waveguide modes' magnetopause boundary condition may be critical in determining where Pc5 pulsation resonances can develop.
Geomagnetic and solar data
Authors: Coffey Helen E.
Published: September 2002

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Detailed structures of the subducted Philippine Sea Plate beneath northeast Taiwan: A new type of double seismic zone
Authors: Kao Honn , Rau Ruey‐Juin
Published: September 2002

We studied the detailed structure of the subducted Philippine Sea plate beneath northeast Taiwan where oblique subduction, regional collision, and back arc opening are all actively occurring. Simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast, high‐quality data recorded by the Taiwan Seismic Network. We further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic P and SH waveforms, a critical step to define the position of plate interface and the state of strain within the subducted slab. The most interesting feature is that relocated hypocenters tend to occur along a two‐layered structure. The upper layer is located immediately below the plate interface and extends down to 70–80 km at a dip of 40°–50°. Below approximately 100 km, the dip increases dramatically to 70°–80°. The lower layer commences at 45–50 km and stays approximately parallel to the upper layer with a separation of 15±5 km in between down to 70–80 km. Below that the separation decreases and the two layers seem to gradually merge into one Wadati‐Benioff Zone. We propose to term the classic double seismic zones observed beneath Japan and Kuril as “type I” and that we observed as “type II,” respectively. A global survey indicates that type II double seismic zones are also observed in New Zealand near the southernmost North Island, Cascadia, just north of the Mendocino triple junction, and the Cook Inlet area of Alaska. All of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction. We interpret the seismogenesis of type II double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere (originating from oblique subduction) and the downdip extension (from slab pulling force). The upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and/or along the plate interface, whereas the lower layer is associated with events in the uppermost mantle.
Lithologic tomography: From plural geophysical data to lithology estimation
Authors: Bosch Miguel
Published: September 2002

The information provided by different geophysical data sets (gravimetric, magnetic, seismic, etc.) can be used, together with petrophysical and geostatistical information, to estimate the major lithological properties of the rocks within the studied volume. The formalization of this inverse problem requires a joint representation and parameterization of the different media properties in the model. The information relating rock properties together couples the inversion of the plural geophysical data sets and allows one to relate the observations with the lithological parameters of the model. The representation by probability density functions (pdfs) of the different types of information entering the problem is also required and provides the mathematical framework to formulate their combination. The resulting joint posteriorpdfis composed of two factors: the joint likelihood function, which is the product of independent likelihood functions associated with each geophysical data set, and the joint priorpdf. The latter decomposes, following a partition of the model parameter space in a primary (lithological) subspace and secondary (physical) subspace, as a marginalpdfover the primary model parameter space and a conditionalpdfover the secondary model parameter space. A Markov‐chain Monte Carlo method was adapted to sample joint models from the posteriorpdf: (1) the method starts with a Markov‐chain sampling primary models from the marginal priorpdf, (2) the chain is extended to the joint model space, by sampling from the conditionalpdfof the secondary parameters with respect to the primary parameters, and (3) it is modified to sample from the posteriorpdf, by applying the Metropolis rule, which uses the evaluation of the joint likelihood function to accept or reject model transitions in the sampling chain. Finally, posterior marginal or posterior conditionalpdfs for the model parameters or the model properties can be straightforwardly calculated from the set of joint models sampled by the chain.
Dynamic geometrical effects on strong ground motion in a normal fault model
Authors: Brune James N. , Anooshehpoor Abdolrasool
Published: September 2002

We report results of measurement of particle motion in a foam‐rubber model of normal faulting and compare the results with similar results for a strike‐slip geometry. Standard computer modeling of strong ground motion from normal fault earthquakes has used dislocation theory, in which slip along the shallow part of the fault is prescribed by assuming particular time functions for fault slip. Unfortunately, in the case of normal faults, there are essentially no near‐fault data from large earthquakes to constrain the modeling. In an extensional faulting regime the static normal and shear stresses along the fault must approach zero at the surface, and thus the upper few kilometers of the fault have inherently less stored strain energy than the maximum possible for strike‐slip faults. In addition there are dynamic effects from geometry and drop in fault‐normal stress which affect the fault motion. Physical models of faulting, such as foam‐rubber modeling, are guaranteed to obey static and dynamic mechanical laws and thus can be used to gain insight into the physical processes involved. In this study we compare surface accelerations from normal fault and strike‐slip geometries. The data show surface accelerations near the normal fault trace that are systematically lower, by an average factor of about 0.1, compared to the accelerations at the side sensors, which represent strike‐slip motion. These results suggest that kinematic modeling of ground motion using classical dislocation techniques is inappropriate on the shallow part of the fault. The results of this study are qualitatively similar to those obtained for numerical models, such as a dynamic lattice model [Shi et al., 1997] and a finite element model [Oglesby et al., 1998], lending support to the probability that similar effects take place in the real Earth.
Collection and three‐dimensional modeling of GPS and tilt data at Merapi volcano, Java
Authors: Beauducel François , Cornet François H.
Published: September 2002

We study here the deformations associated with the November 1996 to March 1997 eruption period at Mount Merapi (Central Java), one of the most active volcanoes in Indonesia. This activity period includes a vertical explosion on January 17 and an increase of the lava dome volume by about 3×106m3. Two Global Positioning System (GPS) campaigns have been carried out on a six‐benchmark network at the beginning and at the end of the period. Relative displacements with respect to the reference point show an average subsidence of 6.5 cm. A multicomponent tilt station installed on the southeast flank, 3 km from the summit, recorded a tilt of 11.1 ± 0.7 μrad in the tangential direction and 0.9 ± 0.4 μrad in the radial direction. These data are interpreted using a three‐dimensional (3‐D) elastic model based on the mixed boundary element method and a near‐neighbor Monte Carlo inversion. Interpretation of tilt data requires an accurate mesh for discretizing the 3‐D topography. The final result supports a horizontal elliptic magma source located 8.5 ± 0.4 km below the summit and 2 ± 0.4 km to the east of it. In particular, the data cannot be consistent with the location of a magma chamber determined from seismic activity analysis (i.e., 2 km below the summit). The computed depth depends strongly on the source shape and cannot be constrained properly because of the small amount of data. The computed deflation of 11 ± 2×106m3is about 3 times larger than the observed increase in the lava dome volume. This difference is attributed to rock avalanches and pyroclastic flows on the flanks of the volcano.
Recent crustal deformation in southern California deduced from the restoration of folded and faulted strata
Authors: Gratier J. P. , Hopps T. , Sorlien C. , Wright T.
Published: September 2002

A rigid element method of restoration (UNFOLD) is used to restore competent folded and faulted layers of the Ventura and Los Angeles basins to their initial horizontal state. Comparison of initial (undeformed) state with present (deformed state) allows one to estimate both the finite crustal deformation and its associated horizontal displacement field (relative to an arbitrary fixed line). Including data from the Santa Barbara Channel basin, the total finite displacement field for the western Transverse Ranges and vicinity (within the Pacific plate) is inferred from this map restoration and is modeled as a double fan closure. This model implies a 10° clockwise rotation of the northern boundary of the western Transverse Ranges and a 5° counterclockwise rotation of its northeast boundary. Lateral variation of the deformation reveals the heterogeneity of the subsurface deformation. Most of the major thrusts appear to initiate as en echelon structures along the left‐lateral northern margin and the right‐lateral northeastern margin of the studied area. The resulting deformation and displacement values closely match those derived by other geological methods (section balancing techniques or fault slip measurements) and by geophysical methods (geodetic, paleomagnetic, and focal mechanism data). Map restoration thus is a method that can independently quantify both local and regional deformation including folds and faults. This method also reveals the zones where problems of compatibility appear with the available geological and geophysical data and thus where the next studies might be focused.
Network modeling of the evolution of permeability and dilatancy in compact rock
Authors: Zhu Wenlu , Wong Teng‐fong
Published: September 2002

Rock deformation and fluid transport are coupled together in many crustal settings. Nonhydrostatic stress can greatly affect pore structure and transport properties of a rock. When a compact rock is stressed to failure, dilatancy and permeability enhancement are generally observed whether the failure mode is brittle faulting or cataclastic flow. Laboratory data for the brittle faulting regime (in Westerly granite) and for the cataclastic flow regime (in Carrara marble and synthetic halite) are modeled. Dilatancy is simulated by incorporating an increasing number of stress‐induced microcracks with similar geometric attributes in a random network model, thus enhancing permeability. The microcracks are represented by sheet‐like conduits, with crack length and aspect ratio distributions constrained by microstructural data. Before the onset of dilatancy, a rock under overall compression has very low density of open cracks that occur in isolated clusters with relatively low connectivity. Nonhydrostatic loading induces damage in the form of extensile microcracks that gradually form a fully connected percolation network. Significant permeability increase of up to several orders of magnitude may occur in this percolative regime. Once the crack network is fully connected, the accumulation of additional cracks is not as effective in enhancing permeability, and our model predicts permeability and porosity changes to be linearly related in this fully connected regime. After accounting for the existence of fine cracks that are below the microscope resolution, our simulation results agree reasonably well with laboratory data.