bluuebunny/crossref_metadata_2025 huggingface dataset:
Submillisecond resolution lightning currents and sprite development: Observations and implications
Authors: Cummer Steven A. , Stanley Mark
Published: September 2002

We analyze synchronized high speed video images and ELF‐VLF radio emissions from 11 sprite clusters observed on 6 October 1997. Quantitative analysis shows that vertical lightning charge moment changes of 150–1100 C · km occurred before the optical emissions reached their peak with delays of 2–11 ms from the lightning discharge. This threshold unexpectedly decreases with increasing delay from parent lightning to peak emissions. We also find that sprite charge moment change and minimum sprite altitude are not well correlated with the vertical charge moment change in the parent discharge. These observations do not agree well with present sprite generation models, and we suggest that streamer development and horizontal lightning charge motion can play a significant role in sprite generation.
Observed constraint on proton‐proton relative velocities in the solar wind
Authors: Goldstein Bruce E. , Neugebauer Marcia , Zhang L. Dennis , Gary S. Peter
Published: September 2002

From June 1994 to December 1995 the Ulysses spacecraft was within 3 AU of the Sun and traveled from high southern to high northern heliospheric latitudes. For this period, SWOOPS instrument data obtained when the magnetic field was approximately aligned with the radial direction from the Sun have been analyzed. In the fast wind at high latitudes, two resolvable proton components are typically present. Statistical studies of the relative densities and the field‐ aligned velocity difference, υo, of the two components are carried out, considering the dependence on distance from the Sun, the local Alfvén speed, and the type of solar wind flow. The observed values of υo/υA, where υA is the local Alfvén speed, are less than the upper bound determined from the linear theory for electromagnetic proton‐proton instabilities. This result is good evidence that these microinstabilities constrain the relative streaming of the two components in the solar wind.
Direct evidence of the interstellar gas flow velocity in the pickup ion cut‐off as observed with SOHO CELIAS CTOF
Authors: Möbius E. , Litvinenko Y. , Grüwaldt H. , Aellig M. R. , Bogdanov A. , Ipavich F. M. , Bochsler P. , Hilchenbach M. , Judge D. , Klecker B. , Lee M. A. , Ogawa H.
Published: September 2002

He+ pickup ions as observed with SOHO CELIAS CTOF have been analyzed for the time period DOY 160–190, 1996. During this time of the year the Earth is on the upwind side of the interstellar gas flow with respect to the sun. The high‐speed cut‐off in the frame of the sun is significantly higher v/Vsw = 2, predicted for pickup ions. The difference increases with lower solar wind speeds. This behavior is interpreted as an effect of the local interstellar gas flow velocity (inflow at large distances including gravitational acceleration by the sun) on the pickup ion distribution. The neutral velocity is added to the solar wind velocity in the determination of the pickup ion cut‐off on the upwind side and subtracted on the downwind side of the gas flow. This new observation will provide a valuable tool to determine the interstellar gas flow and will thus complement direct neutral gas measurements.
On the generation of a broad downshifted spectrum of HF wave enhanced plasma lines in the ionospheric heating experiments
Authors: Kuo S. P. , Lee M. C.
Published: September 2002

Generation of a broad downshifted spectrum of HF wave enhanced plasma Lines (HFPLs) in ionospheric heating experiments is explored. Langmuir waves are first excited within a cone around the geomagnetic field by the HF wave, in the region near its reflection height, through the oscillating two stream instability (OTSI). These Langmuir waves then cascade through a secondary parametric instability, whereby an obliquely propagating Langmuir pump wave decays into an obliquely propagating Langmuir sideband and a lower hybrid decay mode, which propagates in a direction perpendicular to the Langmuir pump wave. The excited Langmuir sidebands have a broad downshifted frequency spectrum and large propagation angles. Their propagation angles are further widened via the filamentation instability or scattering off short‐scale field‐aligned density irregularities. Thus they become detectable by backscatter radars as HFPLs with a broad downshifted frequency spectrum. The results of our analysis show that it requires HF field amplitude of 3.6 V/m to cascade the OTSI‐excited Langmuir waves, for example, 8 times to produce a downshifted spectral width of 50 KHz in the Arecibo heating experiments.
HF chirps: Eigenmode trapping in density depletions
Authors: McAdams K. L. , Ergun R. E. , LaBelle J.
Published: September 2002

Short‐duration, narrow‐bandwidth, HF chirps occur at slowly drifting frequencies above fpe when fpe > fce [McAdams and LaBelle, 1999]. We put forth a model of the HF chirp emissions as quasi‐trapped eigenmodes in a density depletion perpendicular to the ambient magnetic field. The escaping waves retain the frequency structure of the eigenmodes and are observed as HF chirps. We show that this model is quantitatively consistent with observed characteristics of HF chirps, most notably the frequency spacing of the chirp emissions (0.1–4 kHz), the fact that they are equally spaced independent of mode number, and the number of modes predicted for a given density cavity. Several mechanisms for the escape of the waves from the cavity are suggested.
Simultaneous triggered VLF emissions and energetic electron distributions observed on POLAR with PWI and HYDRA
Authors: Bell T. F. , Inan U. S. , Helliwell R. A. , Scudder J. D.
Published: September 2002

We report simultaneous observations of energetic 1–20 keV electrons and VLF emissions triggered within the plasmasphere by pulses from ground based VLF transmitters, using the PWI and HYDRA instruments on the POLAR spacecraft. The 1–20 keV electrons have the correct energy to interact with the input pulses through gyoresonance. Emissions are generated by the pulses only when the particle flux is enhanced well above background and the particle pitch angle distribution is very highly anisotropic, with the average equatorial pitch angle exceeding ∼75°. Because of these high pitch angles, the particles are trapped typically within 7° of the magnetic equator. Only pulses which propagate within whistler mode ducts are observed to trigger emissions. The observed pitch angle anisoptropies are much larger than those assumed in present models of the VLF emission triggering phenomenon, and thus our observations provide a new starting point for understanding the emission process.
First measurements of tidal modes in the lower thermosphere by the EISCAT Svalbard radar
Authors: van Eyken A. P. , Williams P. J. S. , Buchert S. C. , Kunitake M.
Published: September 2002

First measurements by the EISCAT Svalbard Radar of ion velocities in the lower thermosphere show the presence of strong tidal or quasi‐tidal modes with periods of 24, 12 and 8 hours. The diurnal mode shows no variation of phase with height above 100 km. In contrast, the semidiurnal and terdiurnal modes show a steady variation of phase with height corresponding to vertical wavelengths of about 40 km and 30 km respectively. In addition there is evidence at 93 km, the lowest height measured, of a planetary wave with a period of 2–3 days.
Lessons from the ring current injection during the September 24, 25, 1998 storm
Authors: Russell C. T. , Lu Gang , Luhmann J. G.
Published: September 2002

Because the conditions in the solar wind that lead to the injection of energetic particles into the ring current simultaneously affect a variety of geomagnetic phenomena, it is often difficult to determine whether these magnetospheric processes are causally related or are simply contemporaneous. The sequence of interplanetary conditions leading up to the September 24–25, 1998 geomagnetic storm are particularly suited to illustrating what conditions lead to significant activity in the auroral zone as distinct from the injection into the ring current. This interval illustrates that the magnetosphere requires a period of strong, steady, not time‐varying, convection for the buildup of the ring current, and that the AE index cannot be used to forecast the ring current buildup. Furthermore, when the ring current reached its peak strength, it contained 6 PJ and was dissipating energy at a rate of about 0.4 TW. The northern auroral regions and southern added another 1.2 TW of dissipation. This rate of dissipation could be maintained readily by the mechanical energy flux of the solar wind of about 40 TW but would require coupling to much of the incident solar‐wind Poynting flux if that were the sole energy source.
Investigating the 1996 Pyrenean Earthquake (France) with SAR interferograms heavily distorted by atmosphere
Authors: Rigo Alexis , Massonnet Didier
Published: September 2002

We form interferograms by pairwise combination of four synthetic aperture radar (SAR) images acquired by the ERS‐1/2 satellites before and after the ML 5.2, earthquake of February 18, 1996 near Saint‐Paul de Fenouillet in the eastern Pyrenees, France. Of the four interferograms we interpret, two span the seismic event. The interferometric fringes, which correspond to contours of equal satellite‐to‐ground line length change (28.4 mm), are distorted by atmosphere in such a way that the amplitude of the coseismic signature is much smaller than that of the atmospheric disturbances. These noisy conditions contrast with previous earthquake studies with radar interferometry. Nevertheless, we are able to obtain a realistic elastic dislocation model fitting the observations. By comparisons with forward modeling, our results can discriminate between competing models that are not resolved by previous seismological studies. The data derived from radar are especially useful for a more accurate characterization of the vertical motion.
New GPS constraints on the motion of the Yakutat Block
Authors: Fletcher Hilary J. , Freymueller Jeffrey T.
Published: September 2002

Global Positioning System (GPS) measurements were made at Yakutat, on the Yakutat terrane of southern Alaska, to investigate the motion of the Yakutat block with respect to the North American plate and to help constrain motion along the Fairweather fault. The velocity of Yakutat derived from the GPS data is 44.1 +/− 1.9 mm/yr toward N37° +/− 4°W relative to stable North America. The magnitude of this velocity is similar to that of the Pacific plate predicted by NUVEL1A, although there is a significant difference in the azimuth of these two vectors. The motion of Yakutat relative to North America is almost exactly parallel to the strike of the Fairweather fault, suggesting that most deformation inboard of Yakutat is right‐lateral strike slip on the Fairweather fault or faults parallel to it, and that significant motion normal to the Fairweather fault occurs offshore of Yakutat. The GPS velocity at Yakutat is also used to help constrain the slip rate and locking depth of the Fairweather fault.
Preseismic changes in groundwater level and volumetric strain associated with earthquake swarms off the east coast of the Izu Peninsula, Japan
Authors: Koizumi Naoji , Tsukuda Eikichi , Kamigaichi Osamu , Matsumoto Norio , Takahashi Makoto , Sato Tsutomu
Published: September 2002

We have observed the groundwater level at Omuroyama‐kita (OMR) well on the Izu Peninsula, Japan since October 1994. During the observation period, there were four earthquake swarms. We detected one or two preseismic groundwater level changes together with preseismic crustal deformation recorded at the other nearby stations. Therefore those preseismic groundwater level changes are considered to be caused by crustal deformation or volumetric strain changes. The crustal deformations are inferred to be caused by the magmatic dike intrusion, which is a possible source of the earthquake swarms off the east coast of the Izu Peninsula.
Evidences for cessation of Indian Plate Subduction in the Burmese Arc Region
Authors: Rao N. Purnachandra , Kumar M. Ravi
Published: September 2002

The issue of whether subduction is still active in the India‐Burma plate boundary zone has been rather controversial. While the presence of an eastward dipping Indian lithospheric slab is undisputed, different opinions have been voiced regarding the continuance of subduction at present. Analysis of the Harvard CMT data in comparison with major subduction zones of the world demonstrates that the Burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down‐dip direction. We propose a major right‐lateral shearing of the Indian plate along with its subducted slab past the Burmese plate in the NNE direction.
Seismic risk at the rift‐transform junction in North Iceland
Authors: Jouanne F. , Villemin T. , Ferber V. , Maveyraud C. , Ammann J. , Henriot O. , Got J.‐L.
Published: September 2002

The current tectonics of north Iceland are characterised by max M 6–7 earthquakes in the transform zone between the northern rift zone and the Kobleinsey Ridge. The last rifting period (1975–1984) was associated with an opening of up to 9 m along the Krafla fissure swarm. This period was followed by the disappearance of microseismicity along the eastern part of Husavik‐Flatey Fault, the main structure of the transform zone. A GPS network was occupied in 1995 and again in 1997 to study present‐day deformation on both sides of the on‐land part of HFF and around its junction with the northern rift zone. The main deformational features observed are: (1) in Tjörnes, the HFF is locked above the ductile/brittle boundary below which ductile creep is occurring, and (2) aseismic opening along the Theystareykir and Krafla fissure swarms and along a N 120°E superficial fault that marks the southern boundary of Tjörnes.
Crack density and saturation rate in the 1995 Kobe Earthquake Region
Authors: Zhao Dapeng , Mizuno Takashi
Published: September 2002

We have estimated the 3‐D distribution of crack density parameter (ϵ) and saturation rate (ξ) in the 1995 Kobe earthquake region by applying a crack theory to values of seismic velocity and Poisson's ratio obtained from seismic tomography. Our results show that ϵ and ξ exhibit considerable heterogeneity. Higher ϵ and ξ exist in the Kobe fault zone, in particular, at the mainshock hypocenter. High ξ areas are generally consistent with high ϵ areas, but high ξ areas have a much wider distribution, indicating that microcracks exist in more localized areas, while fluids are permeating through wider areas in the crust. A significant discrepancy between ϵ and ξ exists beneath Osaka Bay where ϵ is low while ξ is high. ξ is generally high beneath seas but low beneath land areas. Osaka Bay and the present sea/land distribution in southwest Japan have existed for 2 millions years. This period is long enough for the sea water to permeate down to the deep crust through the active faults that would have been ruptured during many earthquakes cycles.
The effect of bathymetry on tsunami characteristics at Sisano Lagoon, Papua New Guinea
Authors: Matsuyama Masafumi , Walsh J. P. , Yeh Harry
Published: September 2002

The 1998 Papua New Guinea tsunami was greater than expected from its earthquake magnitude. The area of significant impact was small, approximately a 30 km stretch near the mouth of Sissano Lagoon, Papua New Guinea. To explain the localized nature of the event, a submarine landslide has been conjectured to be responsible. Our study indicates that offshore bathymetry is critical to predicting tsunami coastal behavior. Model runs with newly obtained bathymetric data indicate that an earthquake fault source combined with the existing seafloor geometry may also explain the concentrated tsunami. Although the definitive cause of the Papua New Guinea tsunami remains uncertain, local bathymetry had a notable effect on the wave behavior.
Non‐double‐couple moment tensor of the March 25, 1998, Antarctic Earthquake: Composite rupture of strike‐slip and normal faults
Authors: Kuge Keiko , Kikuchi Masayuki , Yamanaka Yoshiko
Published: September 2002

The 1998 Antarctic intraplate earthquake (Mw 8.1) has a significant non‐double‐couple component in the moment tensor solution as determined from long‐period surface waves. In the teleseismic P and SH wave‐forms, there are two distinctive wave packets that can be explained by multiple strike‐slip subevents. We propose a composite rupture model comprising two different episodes: two predominant clusters of en echelon strike‐slip segments (∼16 × 1020 Nm) and normal faulting with a long duration of approximately 100 s (∼4 × 1020 Nm). This model agrees with observations of both teleseismic body waves and long‐period surface waves, including the large non‐double‐couple component. The normal faulting with as long a duration as the main strike‐slip rupture might be secondarily induced by the en echelon segments of strike‐slip faulting.
Fluid‐magma decoupling in a hot‐spot volcano
Authors: Bureau H. , Métrich N. , Semet M. P. , Staudacher T.
Published: September 2002

Melt and fluid inclusions in olivines from the last 1998 eruption of Piton de la Fournaise (PdF), have recorded a range of volatile partial pressures (350–420 MPa), the highest, so far for this volcano, are found in Fo‐rich olivines from a vent remote from the main eruptive activity. Such pressures indicate that fractionation of olivine (and other crystals) occur below the crust‐mantle boundary in a CO2‐rich volatile saturated environment. Together with previously published data, the model that emerges for PdF is one where olivines which have crystallized from multiple past magma injection events [see Albarède et al., 1997], may be picked up by newly intruding magma, anywhere from upper‐mantle depths to the surface. This model is likely to apply to other shield volcanoes. Magma production and transport to the surface is accompanied by continuous open‐system degassing through the permeable volcanic pile. The calculated H2O content of primary basalts (MgO ≈ 12–14 wt.%) from PdF may reach 0.7–1 wt.% implying a rather H2O‐rich hot spot mantle source.
Borehole versus isotope temperatures on Greenland: Seasonality does matter
Authors: Werner Martin , Mikolajewicz Uwe , Heimann Martin , Hoffmann Georg
Published: September 2002

New simulation results obtained with the Hamburg Atmosphere General Circulation Model ECHAM‐4 under maximum glacial boundary (LGM) conditions confirm the paleotemperatures on Greenland determined by borehole thermometry. The disagreement between δ18O isotope based temperatures and the borehole temperatures of the LGM is not only reproduced by the model, but the simulation results provide a plausible explanation: Paleotemperatures inferred from δ18O measurements in ice cores are biased by a substantially increased seasonality of precipitation over Greenland during the LGM. During the glacial winter a much more zonal circulation prevents the effective transport of moisture to the Greenland ice sheet, and therefore reduces the contribution of isotopically strongly depleted winter snow to the annual mean isotope signal.
Absence of strain accumulation in the Western Shumagin Segment of the Alaska Subduction Zone
Authors: Freymueller Jeffrey T. , Beavan John
Published: September 2002

GPS measurements in the western Shumagin segment of the Alaska subduction zone show no significant strain. The elastic dislocation model that best fits the data has the entire plate interface slipping freely. The simplest explanation is that steady slip at the plate convergence rate is occurring on the entire plate interface, although we cannot rule out the existence of a shallow locked patch close to the trench large enough to generate a M∼7 earthquake. The 99% confidence upper bound on the down‐dip width of the locked patch is 55 km assuming the trench is the the updip limit of the locked zone. Unless there is significant temporal variability in the locking of the plate interface or viscoelastic effects from a past earthquake, the Shumagin segment cannot by itself release enough moment to generate a great (M∼8) earthquake.
DC trains and Pc3s: Source effects in mid‐latitude geomagnetic transfer functions
Authors: Egbert Gary D. , Eisel Markus , Boyd O. Sierra , Morrison H. Frank
Published: September 2002

Magnetotelluric (MT) data from two sites 150 and 300 km southeast of San Francisco, California (geomagnetic dipole latitude: 43 degrees, L approximately 1.9) show that the usual MT assumption of spatially uniform external magnetic fields is violated to a significant degree in the period range 10–30 s. Inter‐station transfer functions exhibit large systematic temporal variations which are consistent with a combination of two distinct sources: electromagnetic noise due to the San Francisco Bay Area Rapid Transit (BART) DC electric railway, and Pc3 geomagnetic pulsations. There is a suggestion in the data that some of the Pc activity may actually be excited by BART.
First testing of a volcano Doppler radar (Voldorad) at Mount Etna, Italy
Authors: Dubosclard G. , Cordesses R. , Allard P. , Hervier C. , Coltelli M. , Kornprobst J.
Published: September 2002

A medium‐power (1 kW) Doppler radar of limited weight (110 kg) and working in the UHF band has been developed for active remote sensing of volcanic eruptions. Named Voldorad, this radar is designed to monitor eruptive activities of variable magnitude at medium distance range (∼0.5–5 km), in any weather condition, and fills the gap between low‐ and high‐power radar or sodar systems previously tested on volcanoes. Voldorad was operated for the first time during an eruption of Mt. Etna on October 11–12, 1998. Doppler spectra recorded at 1 km distance from the vent reveal powerful echoes during the eruption paroxysm (lava fountains), which gradually decreased along with declining activity. An average vertical velocity of 80–87 m s−1 is inferred for the eruptive jets.
Regional wave propagation in Turkey and surrounding regions
Authors: Gök Rengin , Türkelli Niyazi , Sandvol Eric , Seber Dogan , Barazangi Muawia
Published: September 2002

Digital and analog seismic waveform data collected by 34 stations in and around Turkey provided excellent ray coverage for a detailed attenuation study of regional shear waves (Sn and Lg). Over 2000 seismograms within a distance range of 15° were visually inspected and the quality of Sn and Lg phases categorized into three different classes: efficient, inefficient or not present. Our results show that Sn and Lg propagation is mostly inefficient in western Turkey and the Aegean Sea. Sn is efficient in parts of southwestern Turkey, the western Pontides and western Greece. Sn is not observed in eastern Turkey and along the Aegean volcanic arc. Lg propagates efficiently in the Arabian plate including paths that cross the Dead Sea fault zone and in northwestern Turkey. Lg does not propagate in northeast Anatolia, across the Lesser Caucasus and north of the Hellenic arc (Sea of Crete). These results are a major improvement on prior attenuation studies in this region and provide new constraints for proposed tectonic models.
CO2 degassing along the San Andreas Fault, Parkfield, California
Authors: Lewicki Jennifer L. , Brantley Susan L.
Published: September 2002

The fluxes, concentrations, and carbon isotopic compositions of soil CO2 were measured along the Parkfield segment of the San Andreas fault (SAF). Single or double‐peak CO2 flux anomalies (> 18 g m−2 d−1) were observed along 12 of 16 fault‐crossing transects at five sites. Flux anomalies did not coincide with concentration anomalies. Flux anomalies paralleled the fault trace, suggesting zones of high diffusivity/permeability. One flux anomaly may have been accentuated by a 0.3 mm creep event. However, values of δ13C (‐23.7 to ‐21.6 ‰) and 14C (110.5 to 111.9 pmC) for soil gas CO2 are characteristic of CO2 of biogenic origin. The CO2 flux anomalies are therefore consistent with fault‐related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2.
geophysical and geochemical constraints on crustal accretion at the very‐slow spreading mohns ridge
Authors: Klingelhöfer Frauke , Géli Louis , White Robert S.
Published: September 2002

The composition of upper mantle and lower crustal material at very‐slow spreading centers cannot be reliably determined by seismic studies alone. Since the range of P‐wave velocities for serpentinized peridotites and gabbros overlap, additional information provided by the major and rare earth element (REE) content of the basalts is useful to constrain interpretations of seismic data. Refraction seismic data from the very‐slow spreading (16 mm/a, full rate) Mohns Ridge in the Norwegian‐Greenland Sea yields a highly variable thin crust of 4.0 ± 0.5 km thickness. Analysis of S‐waves suggests that Layer 3 is composed primarily of gabbro containing at most a small percentage (< 20%) of mantle material. The Na8 content of Mohns Ridge basalts suggests a magmatic crustal thickness of 4–5 km. Inversion of the REE concentrations yields a melt thickness of ∼5 km. This agreement between seismic and geochemical data suggests that neither large quantities of mantle material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle.
Box modeling the chemical evolution of geophysical systems: Case study of the Earth's mantle
Authors: Coltice N. , Ferrachat S. , Ricard Y.
Published: September 2002

Geochemical measurements have been widely used for understanding geophysical dynamic systems. Fluid mechanics and box models are quantitative tools for testing the reliability of these interpretations. We present here the connection between these two methods, especially in the case of the Earth's mantle. We show that box models implicitly assume a chemical diffusivity inversely proportional to the number of boxes. From fluid dynamics considerations we suggest that at least 15 boxes should be used to model the mantle. Then, we compare the results of a simple convective geochemical model with box models to illustrate a way to incorporate dynamical constraints in them.
Mantle convection and the dynamic geoid of Mars
Authors: Harder Helmut
Published: September 2002

This study compares different 3D spherical models of thermal convection in the Martian mantle. The best fit to the Tharsis bulge is obtained if the endothermic phase transition to perovskite occurs closely above the core‐mantle boundary. In this case, a large‐scale flow develops and the associated geoid anomaly explains approximately half of the observed long‐wavelength geoid. The shallower exothermic phase transitions have little influence on the planform of the flow and on the shape of the geoid. Previous studies that included exothermic transitions were biased by effects of a soft lithosphere, which introduces a large‐scale mantle flow even without a deep endothermic phase transition. However, such models predict a continous resurfacing and a dominant geoid low, which is in conflict with observations.
Summer monsoon variations over the last 1.2 Ma from the weathering of loess‐soil sequences in China
Authors: Guo Zhengtang , Biscaye Pierre , Wei Lanying , Chen Xihui , Peng Shuzhen , Liu Tungsheng
Published: September 2002

The loess‐soil sequence in northern China contains a near continuous record of Quaternary paleoclimate. Magnetic susceptibility and grain size have so far been the only proxies available to address the long‐term changes of the East‐Asian paleomonsoon extending back to more than one million years. In this study, the ratio of CBD (citrate‐bicarbonate‐dithionite)‐extractable free Fe2O3 (FeD), a measure of iron liberated by chemical weathering, versus the total Fe2O3 available (FeT) was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1.2 Ma. Variations of this index are highly consistent with other pedological indicators, but in addition provide a quantitative measurement of the degree of pedogenesis in the Loess Plateau. Since chemical weathering in the region mainly depends upon summer precipitation and temperature, weathering intensity primarily reflects changes in the East‐Asian summer monsoon. The new proxy has been used to document a series of summer monsoon changes of global significance, which are not necessarily recorded by magnetic susceptibility.
Integration of volcanic and sedimentary records of paleointensity: Constraints imposed by irregular eruption rates
Authors: Guyodo Yohan , Valet Jean‐Pierre
Published: September 2002

The reading of absolute paleointensity by lava flows has been simulated by sampling the field intensities from dipole (extracted from sedimentary paleointensities) and non dipole (similar to the present field) components. Results show that specific features in the geomagnetic intensity cannot be reproduced unless a minimum of 10 flows per 100 ky have been emitted. However, this criteria does not guaranty the high quality of the record, which depends also on the temporal control. For cases where the volcanic activity is episodic, a minimum of 4 dated flows per 100 ky is suggested. The construction of a composite curve from volcanic paleointensity, intended as a type‐curve of field intensity changes, requires a very large number of data in order to overcome the effects of non‐dipole components, errors in ages, and variable sampling. The present volcanic database is far from having this resolution to perform this analysis for the last 800 ky.
Density measurements of liquid Fe‐S alloys at high‐pressure
Authors: Sanloup C. , Guyot F. , Gillet P. , Fiquet G. , Mezouar M. , Martinez I.
Published: September 2002

We present the first static measurements of the density of metallic liquids in the Fe‐S system in the pressure and temperature range 1.5 GPa–6.2 GPa and 1500 K–1780 K. Density is inferred from X‐ray absorption experiments carried out with a large volume press at the European Synchrotron Radiation Facility. It is shown that increasing the amount of sulfur in liquid iron decreases the bulk incompressibility by −2.5 GPa per 1 weight% of S. These data are important for constraining the presence and amount of sulfur in the cores of small planetary bodies.
Out‐of‐sequence thrust faults developed in the coseismic slip zone of the 1946 Nankai Earthquake (Mw=8.2) off Shikoku, southwest Japan
Authors: Park Jin‐Oh , Tsuru Tetsuro , Kodaira Shuichi , Nakanishi Ayako , Miura Seiichi , Kaneda Yoshiyuki , Kono Yoshiteru , Takahashi Narumi
Published: September 2002

A multi‐channel seismic (MCS) reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 Nankai earthquake (Mw=8.2) off Shikoku Island in 1997. We could identify a splay fault system consisting of several sigmoid out‐of‐sequence thrust (OST) faults dipping landward with slope 10∼25° on the poststack depth migrated MCS profiles. Most of the OSTs are apparently developed from the subducting oceanic basement to the seafloor in the forearc region, cutting both underthrust sediments and the overriding accretionary prism. In addition, most of the OSTs are within the coseismic zone of the 1946 Nankai earthquake and the interseismic locked zone. The OSTs are considered to be related to large interplate earthquakes including the 1946 Nankai earthquake. These OSTs may be responsible for tsunami generation following deformation of forearc accretionary wedge.
Multivariate projection of ocean surface data onto subsurface sections
Authors: Fischer Martin
Published: September 2002

A multivariate projection of ocean surface data onto subsurface fields is presented. The method is applied to the projection of sea surface height and sea surface temperature anomalies onto the three dimensional temperature field. The results are compared to the conventional univariate regression between sea surface height and the underlying temperature structure.

The multivariate method clearly outperforms the univariate approach and succeeds in capturing the equatorial anomaly structures even during the early nineties, a period in which the univariate projection encounters great difficulties. A multivariate scheme projects onto linear combinations of different vertical modes, depending on the contribution of each input variable. Thus, vertical patterns, different not only in amplitude but also in shape can be affected, as opposed to the univariate case. During the ENSO cycle two different vertical temperature modes become dominant. To address both modes, two linearly independent variables are required. Therefore, the combination of SSH and SST contains additional information about the vertical temperature structure that cannot be extracted from either sea surface height measurements, or sea surface temperature only. This is of relevance in the context of ENSO predictions, and it may help to improve the initialization of ENSO forecast models.
Tectonic deformation in western Washington from continuous GPS measurements
Authors: Khazaradze Giorgi , Qamar Anthony , Dragert Herb
Published: September 2002

The analysis of 3 years of continuous data from 7 permanent GPS stations along the western Washington section of the Cascadia Subduction Zone indicates that the direction of the observed horizontal velocities (with respect to station DRAO, nominally representing the stable North American continent) is roughly parallel to the relative plate convergence direction of the Juan de Fuca and North America plates and that their magnitude decreases away from the trench. Most of this deformation can be attributed to inter‐seismic strain accumulation due to the locking of the thrust interface. When the dislocation model predictions are subtracted from the observed velocities, there is evidence for an additional N‐S oriented contraction at a rate of ∼4 mm/yr over a distance of 250 km. This signal presumably represents a more long‐term deformation pattern than the periodic accumulation and release of elastic strain connected with subduction earthquakes and is most likely related to the occurrence of shallow earthquakes in western Washington that are characterized by predominantly N‐S oriented maximum principal stress.
Oceanic mixed layer feedback and tropical Atlantic variability
Authors: Saravanan R. , Chang Ping
Published: September 2002

Models of atmospheric circulation are usually forced by prescribing the sea surface temperature (SST), which is equivalent to assuming an infinite heat capacity for the ocean. This assumption breaks down on timescales of a month or longer, when the finite heat capacity of the oceanic mixed layer allows the SST to respond to atmospheric forcing. Experiments using an atmospheric general circulation model coupled to a slab ocean model demonstrate that this SST feedback can play an important role in tropical Atlantic variability. The amplitude of surface heat flux variability is attenuated by this feedback and the persistence of atmospheric flow anomalies is enhanced. The latter effect could have important implications for atmospheric predictability on monthly to seasonal timescales. The results also show that modeling the spatial variations in mixed layer depth is important for an accurate simulation of SST variability.
Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling
Authors: de F. Forster Piers M. , Shine Keith P.
Published: September 2002

The observed cooling of the lower stratosphere over the last two decades has been attributed, in previous studies, largely to a combination of stratospheric ozone loss and carbon dioxide increase, and as such it is meant to provide one of the best pieces of evidence for an anthropogenic cause to climate change. This study shows how increases in stratospheric water vapour, inferred from available observations, may be capable of causing as much of the observed cooling as ozone loss does; as the reasons for the stratospheric water vapour increase are neither fully understood nor well characterized, it shows that it remains uncertain whether the cooling of the lower stratosphere can yet be fully attributable to human influences. In addition, the changes in stratospheric water vapour may have contributed, since 1980, a radiative forcing which enhances that due to carbon dioxide alone by 40%.
Stress loading from viscous flow in the lower crust and triggering of aftershocks following the 1994 Northridge, California, Earthquake
Authors: Deng Jishu , Hudnut Kenneth , Gurnis Michael , Hauksson Egill
Published: September 2002

Following the Mw 6.7 Northridge earthquake, significant postseismic displacements were resolved with GPS. Using a three‐dimensional viscoelastic model, we suggest that this deformation is mainly driven by viscous flow in the lower crust. Such flow can transfer stress to the upper crust and load the rupture zone of the main shock at a decaying rate. Most aftershocks within the rupture zone, especially those that occurred after the first several weeks of the main shock, may have been triggered by continuous stress loading from viscous flow. The long‐term decay time of aftershocks (about 2 years) approximately matches the decay of viscoelastic loading, and thus is controlled by the viscosity of the lower crust. Our model provides a physical interpretation of the observed correlation between after‐shock decay rate and surface heat flow.
A possible explanation for rapid, large‐scale ionospheric responses to southward turnings of the IMF
Authors: Shepherd S. G. , Greenwald R. A. , Ruohoniemi J. M.
Published: September 2002

We have examined the dayside high‐latitude convection response to a sudden southward turning of the Interplanetary Magnetic Field (IMF) and found that the response is nearly instantaneous (<2 min) over a spatial region extending from ∼75° to 85° and from ∼9 to 16 MLT. Observations of the magnetic field were made with the WIND spacecraft in the solar wind and the GEOTAIL and IMP8 spacecraft in the magnetosheath. In the high‐latitude ionosphere, the HF radars of the Super Dual Auroral Radar Network (SuperDARN) were used to monitor the convection. Based on the magnetosheath flow of the gas dynamic approximation, the field lines of the new IMF state were draped over a large portion of the dayside magnetopause when the first significant indication of convection response was measured in the ionosphere. Significant magnetic field line draping accompanied by extended reconnection on the dayside magnetopause may help explain the rapid, large‐scale response of ionospheric convection.
Ionospheric effects of relativistic electron enhancement events
Authors: Demirkol Mehmet K. , Inan Umran S. , Bell Timothy F. , Kanekal S. G. , Wilkinson D. C.
Published: September 2002

The relativistic electron population as measured both at geosynchronous orbit and at low altitudes at subauroral latitudes exhibits pronounced fluctuations in association with magnetospheric substorm and solar activity. A ground‐satellite correlative study based on amplitude and phase measurements of VLF signals propagating in the earth‐ionosphere waveguide indicates that the relativistic electron enhancements are accompanied by similar enhancements in nighttime ionospheric conductivity produced by associated enhanced precipitation. VLF signal amplitudes are found to exhibit >10 dB changes, showing the same 27 day cycle and 2–3 day rise and fall time pattern as relativistic electron enhancement events recorded by GOES 7 and SAMPEX, and indicating that the nighttime lower ionospheric electron density at subauroral latitudes is detect ably affected by 27‐day periodicity in solar rotation.
Response of the ionosphere to a density pulse in the solar wind: Simulation of traveling convection vortices
Authors: Slinker S. P. , Fedder J. A. , Hughes W. J. , Lyon J. G.
Published: September 2002

We show the response of the Earth's magnetosphere and ionosphere to a density pulse in the solar wind using a global 3D MHD simulation model. Flow vortices are generated in the ionosphere and they exhibit many properties similar to those observed during impulsive traveling convection vortices. Two oppositely rotating flow vortices are formed at about 70° magnetic latitude near noon. They separate and move down the morning and evening flanks, greatly weakening as they pass the terminator. Meanwhile a second pair of flow vortices appears at noon at a slightly higher latitude and with opposite flow directions than the first pair. The second pair follows the first and also fades as it reaches the nightside. The results are interpreted as a hydromagnetic wave propagating in the inhomogeneous magnetosphere plasma.
Ionization of the lower ionosphere by γ‐rays from a Magnetar: Detection of a low energy (3–10 keV) component
Authors: Inan U. S. , Lehtinen N. G. , Lev‐Tov S. J. , Johnson M. P. , Bell T. F. , Hurley K.
Published: September 2002

A gigantic periodic flare from the soft γ repeater SGR 1900+14 produced enhanced ionization at ionospheric altitudes of 30 to 90 km, which was observed as unusually large amplitude and phase changes of very low frequency (VLF) signals propagating in the Earth‐ionosphere waveguide. The VLF signals remained perturbed for ∼5 min and exhibited the 5.16 s periodicity of the giant flare detected on the Ulysses spacecraft [Hurley et al., 1999]. Quantitative analysis indicates the presence of an intense initial low energy (3–10 keV) photon component that was not detectable by the Ulysses instrument.
Sprites triggered by negative lightning discharges
Authors: Barrington‐Leigh C. P. , Inan U. S. , Stanley M. , Cummer S. A.
Published: September 2002

High altitude air breakdown, manifested as “red sprites,” is reported in close association with negative cloud‐to‐ground lightning (−CG) on at least two occasions above an unusual storm on August 29, 1998. Data from high speed photometry, low‐light‐level video, and receivers of lightning electromagnetic signatures in the frequency range 10 Hz to 20 kHz are used to establish the association and indicate that the causative −CG discharges effected unusually large vertical charge moment changes (ΔMQv) of up to 1550 C · km in 5 ms. The existence of sprites caused by −CG's, rather than the regularly associated +CG's, has immediate implications for sprite models and observations.
The north‐south asymmetry of the solar and heliospheric magnetic field during activity minima
Authors: Bravo S. , González‐Esparza J. A.
Published: September 2002

Recent in situ spacecraft observations and the analysis of the galactic cosmic ray intensity at Earth have shown that the heliospheric magnetic field exhibited a north‐south asymmetry and that the heliospheric current sheet was inclined with respect to the solar equator during the years around the last sunspot minimum (1995–1996). In this paper we show that both heliospheric characteristics are due to the particular way the solar magnetic field evolves. The north‐south asymmetry can be explained with the evolution of the quadrupolar component of the field in the solar magnetosphere and the inclination of the heliospheric current sheet could be a result of the presence of an inclined relic dipolar field in the Sun, as has been suggested by Bravo and Stewart [1995].
Plasmaspheric depletion and refilling associated with the September 25, 1998 magnetic storm observed by ground magnetometers at L = 2
Authors: Chi P. J. , Russell C. T. , Musman S. , Peterson W. K. , Le G. , Angelopoulos V. , Reeves G. D. , Moldwin M. B. , Chun F. K.
Published: September 2002

The plasmaspheric mass density at L ≃ 2 was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to ≃ 1/4 of the pre‐storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as L = 2, which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern.
On the perpendicular scale of electron phase‐space holes
Authors: Franz J. R. , Kintner P. M. , Seyler C. E. , Pickett J. S. , Scudder J. D.
Published: September 2002

The perpendicular scale of electron phase‐space holes is investigated using electric field data from the Polar Plasma Wave Instrument. We show that the electron phase‐space holes are roughly spherical for Ωe/ωp > 1, and become more oblate (with the perpendicular scale larger than the parallel scale) with decreasing Ωe/ωp. A scaling argument based upon electron gyrokinetic theory is proposed as a possible explanation for the observed scaling. The data indicate that the ratio of the parallel dimension (L∥) to the perpendicular dimension (L⟂) is such that L∥/L⟂ ≃ (1 + ρe²/λD²)−1/2. Our results provide a connection between the Geotail measurements in the deep magnetotail, where Ωe/ωp ≪ 1, and the FAST measurements in the low altitude auroral zone, where Ωe/ωp ≫ 1.
Observations of QP radar echo structure consistent with neutral wind shear control of the initiation mechanism
Authors: Pan C. J. , Larsen M. F.
Published: September 2002

The semidiumal variations in quasi‐periodic echo structure parameters associated with midlatitude sporadic E layers observed with the Chung‐Li VHF radar located in Taiwan (24.9 N, 121.2 E; 52 MHz) are presented. The results show that the periodicity decreases with increasing range rate and that the spread of Doppler velocities increases as the altitude coverage of the echoes increases. These characteristics are shown to be consistent with the variations to be expected if the shears in the neutral wind and the variations in the background mean flow are the dominant mechanisms controlling the initiation of the echo structures.
Fractal structure of sprites
Authors: Pasko Victor P. , Inan Umran S. , Bell Timothy F.
Published: September 2002

A large scale model of sprites based on a phenomenological probabilistic approach to modeling of streamer corona discharges is developed. The model utilizes the experimentally documented macroscopic properties of positive and negative streamer corona in air and allows a realistic determination of the propagation of multiple breakdown branches in a self‐consistent electric field. The model results reproduce the large scale volumetric shapes of sprites, agree with the experimentally documented thundercloud charge moment changes in sprite producing cloud to ground lightning discharges (CGs), and demonstrate fundamental asymmetries between sprites generated by CGs of positive and negative polarity.
Some comments on transient and steady‐state reconnection at the dayside magnetopause
Authors: Rodger Alan S. , Coleman Iain J. , Pinnock Mike
Published: September 2002

Reconnection at the dayside magnetopause is the principal method by which energy is transferred from the solar wind into the magnetosphere‐ionosphere system. There is still considerable uncertainty as to whether reconnection is transient or quasi‐steady. Here we discuss some of the implications of assuming that reconnection occurring in regions where the magnetosheath flow is super‐Alfvénic must be transient. We use a simple magnetospheric model to illustrate where on the magnetopause transient reconnection is most likely to occur, and show that the location of these regions is dependent critically upon the dipole tilt angle and the interplanetary magnetic field orientation. Although our idealised examples do not take into account temporal variations of the solar wind conditions, or the influence of the bow shock or magnetosheath, we believe that they demonstrate interesting features. For example, the results suggest that reconnection for northward IMF is almost always likely to be transient.