bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy‐dependent ionization states of shock‐accelerated particles in the solar corona
Authors: Reames D. V. , Ng C. K. , Tylka A. J.
Published: September 2002

We examine the range of possible energy dependence of the ionization states of ions that are shock‐accelerated from the ambient plasma of the solar corona. If acceleration begins in a region of moderate density, sufficiently low in the corona, ions above ∼0.1 MeV/amu approach an equilibrium charge state that depends primarily upon their speed and only weakly on the plasma temperature. We suggest that the large variations of the charge states with energy for ions such as Si and Fe observed in the 1997 November 6 event are consistent with stripping in moderately dense coronal plasma during shock acceleration. In the large solar‐particle events studied previously, acceleration occurs sufficiently high in the corona that even Fe ions up to 600 MeV/amu are not stripped of electrons.
Acceleration of relativistic electrons via drift‐resonant interaction with toroidal‐mode Pc‐5 ULF oscillations
Authors: Elkington Scot R. , Hudson Mary K. , Chan Anthony A.
Published: September 2002

There has been increasing evidence that Pc‐5 ULF oscillations play a fundamental role in the dynamics of outer zone electrons. In this work we examine the adiabatic response of electrons to toroidal‐mode Pc‐5 field line resonances using a simplified magnetic field model. We find that electrons can be adiabatically accelerated through a drift‐resonant interaction with the waves, and present expressions describing the resonance condition and half‐width for resonant interaction. The presence of magnetospheric convection electric fields is seen to increase the rate of resonant energization, and allow bulk acceleration of radiation belt electrons. Conditions leading to the greatest rate of acceleration in the proposed mechanism, a nonaxisymmetric magnetic field, superimposed toroidal oscillations, and strong convection electric fields, are likely to prevail during storms associated with high solar wind speeds.
Radio‐rich solar eruptive events
Authors: Gopalswamy N. , Kaiser M. L. , Thompson B. J. , Burlaga L. F. , Szabo A. , Lara A. , Vourlidas A. , Yashiro S. , Bougeret J. ‐L.
Published: September 2002

We report on the analysis of a large number of solar eruptive events that produced radio emission in the dekameter‐hectometric (DH) radio window (1–14 MHz), newly opened by the Wind/WAVES experiment. The distinguishing characteristics of coronal mass ejections (CMEs) associated with the DH type II radio bursts are larger‐than‐average width and speed. Flares of all sizes (X‐ray importance B to X) occurring at all longitudes were associated with the DH type II bursts and CMEs. We found a global enhancement in EUV over an area much larger than the flaring active region in the beginning many events. A comparison between the ‘Shock Associated‧ events and microwave bursts shows that at least half of the events do not have temporal relation. A majority of the DH type II bursts were associated with IP shocks and kilometric type II bursts. In particular, we found a very close relationship between the kilometric type II bursts and the IP shocks.
Global multispectral auroral imaging of an isolated substorm
Authors: Cummer S. A. , Vondrak R. R. , Østgaard N. , Stadsnes J. , Bjordal J. , Chenette D. L. , Brittnacher M. J. , Parks G. K. , Sigwarth J. B. , Frank L. A.
Published: September 2002

We analyze simultaneous visible, ultraviolet (UV), and X‐ray auroral images of an isolated substorm on January 25, 1998. The total precipitating electron energy flux and mean energy extracted near substorm maximum from each of the imagers are in acceptable agreement. The expansion phase visible, UV, and X‐ray emissions are morphologically similar, with the brightest emissions coming from the auroral surge. However, during the recovery phase, there are regions of bright X‐ray emissions near local midnight and dawn that do not correspond to obvious features in the visible and UV images. X‐ray energy analysis in these regions reveals a high energy (∼30–100 keV) component of the precipitating electron population that contributes ∼30% of the total electron energy flux. The temporal development of the X rays indicates that the dawn emissions originate in a drifting electron population, while the newly revealed midnight emissions originate in a continuous injection of energetic electrons in a narrow local time region.
On the nature of 150‐km radar echoes over the magnetic dip equator
Authors: Tsunoda Ronald T. , Ecklund Warner L.
Published: September 2002

We present new experimental findings that provide insight into the nature of 150‐km echoes. A finding, using data obtained with a 49.92‐MHz radar on Pohnpei (6.96°N, 158.19°E, 0.5° magnetic dip angle), is an unusual east‐west asymmetry in which the altitude profile of echo occurrence depends on viewing direction. The other is the narrowness of Doppler spectral widths associated with these echoes. When considered with other known properties of 150‐km echoes, a scenario for 150‐km echoes emerges in which 3‐m‐scale field‐aligned irregularities (FAI) are imbedded in tilted, sheetlike structures in plasma density. These meter‐scale FAI are envisioned to have narrow Doppler spectral widths if excited directly by a linear plasma instability, and they would consist of a narrow angular spectrum of plane plasma waves if the instability is weak. The latter could produce the observed east‐west viewing asymmetry.
2D MHD simulation of the emergence and merging of coherent structures
Authors: Wu C. C. , Chang Tom
Published: September 2002

A model of sporadic localized merging of coherent structures has recently been proposed by Chang to describe the dynamics of the Earth's magnetotail. Here we report the results of MHD simulations regarding the development and merging of 2D coherent structures. With a magnetic shear, such coherent structures are generated in alignment with the imposed current sheet. The calculated fluctuation spectra suggest long‐ranged correlations with power‐law characteristics.
The latitudinal distribution of solar wind magnetic holes
Authors: Winterhalter Daniel , Smith Edward J. , Neugebauer Marcia , Goldstein Bruce E. , Tsurutani Bruce T.
Published: September 2002

Using the fast latitude scan period of the Ulysses spacecraft, the occurrence of magnetic holes in the solar wind is investigated from −80° to +80° helio‐latitude. Near the ecliptic, the large average value and the large variations of the hole count rate appear to be correlated with interaction regions. It is found, however, that at 30°north and south latitude, approximately, the hole count rate drops to about 10% of that in the low latitude region and stays constant at this level to near the poles. The small but non‐zero hole count rate at high helio‐latitudes indicates that these holes were formed by processes other than the large scale dynamic solar wind features operating in the ecliptic region.
Signatures of equatorial electrojet in the mesospheric partial reflection drifts over magnetic equator
Authors: Gurubaran S. , Rajaram R.
Published: September 2002

The partial reflection (PR) drifts at 98 km measured by the medium frequency (MF) radar operating at Tirunelveli, India, situated close to the magnetic equator, are shown to have the combined effects of mesospheric winds and the equatorial electrojet. The evidences for this behavior of the PR drifts are brought out from a detailed examination of the temporal variation of the dynamical and the geometrical parameters associated with the ground diffraction pattern.
Seismic tomographic inversion of Russian PNE data along profile Kraton
Authors: Nielsen Lars , Thybo Hans , Solodilov Leonid
Published: September 2002

Tomographic inversion of Russian peaceful nuclear explosion (PNE) data collected along the 3500 km east‐west striking profile Kraton has provided new information about the two‐dimensional P‐wave velocity structure of the upper mantle beneath the Siberian platform. The 8° discontinuity is clearly identified as the top of a zone in the 100–200 km depth range. This zone appears as a low‐velocity zone in large parts of the profile and has zero velocity gradient along the profile. The 400 km discontinuity is imaged as an increase in velocity of at least 0.4 km/s at ∼380–420 km depth.
Postseismic deformation in Chile: Constraints on the asthenospheric viscosity
Authors: Piersanti Antonio
Published: September 2002

By means of a spherical Earth model with Maxwell rheology, an analysis of the postseismic deformation field due to the viscoelastic relaxation of the mantle and asthenosphere following the great 1960 Chile earthquake is performed. Due to the exceptional energy release of that earthquake, geomorphological evidences of postseismic deformation are available in a broad area surrounding the rupture zone. The comparison of the synthetic results with the available data allows to put some constraints on the viscosity of the asthenosphere beneath the Chilean region. Values between 8 · 1019 and 1020Pa s are compatible with the available postseismic deformation field data.
Martian soil component in impact glasses in a Martian meteorite
Authors: Rao M. N. , Borg L. E. , McKay D. S. , Wentworth S. J.
Published: September 2002

Chemical compositions of impact melt glass veins, called Lithology C (Lith C) in Martian meteorite EET79001 were determined by electron microprobe analysis. A large enrichment of S, and significant enrichments of Al, Ca, and Na were observed in Lith C glass compared to Lithology A (Lith A). The S enrichment is due to mixing of plagioclase‐enriched Lith A material with Martian soil, either prior to or during impact on Mars. A mixture of 87% Lath A, 7% plagioclase, and 6% Martian soil reproduces the average elemental abundances observed in Lith C. Shock melting of such a mixture of plagioclase‐enriched, fine‐grained Lith A host rock and Martian soil could yield large excesses of S (observed in this study) and Martian atmospheric noble gases (found by Bogard et al., 1983) in Lith C. These mixing proportions can be used to constrain the elemental abundance of phosphorus in Martian soil.
Very long‐period signals associated with Vulcanian Explosions at Popocatepetl Volcano, Mexico
Authors: Arciniega‐Ceballos Alejandra , Chouet Bernard A. , Dawson Phillipe
Published: September 2002

Very long period (VLP) seismic signals associated with large summit eruptions of Popocatepetl volcano in the last four years are investigated using data from a broadband seismometer (0.04–120 s) deployed on the north flank of the volcano at a radial distance of 5 km from the active crater. The VLP signals associated with individual eruptions share similar waveforms. Discrete VLP signals accompanying long‐period (LP) events also share similar signatures and have dominant periods that are nearly identical to those observed in the VLP waveforms of explosions. The VLP particle motions for eruption onsets consistently point to a source located a few km beneath the crater. The VLP ground displacement response to each explosion is marked by a compression, followed by a dilatation and terminating with another compression, suggesting a sequence of pressurization‐depressurization‐repressurization of the conduit. The repetitive nature of the waveforms points to a non‐destructive source process which has remained active in the magmatic system of Popocatepetl at least since April 1997.
Slip tendency analysis of the Vesuvius Faults: Implications for the seismotectonic and volcanic hazard assessment
Authors: Ventura Guido , Vilardo Giuseppe
Published: September 2002

Fault reactivation processes may be precursors of volcanic eruptions. Reactivation is generally due to an increase of fluid pressure, which induces a decrease of the effective normal stress on the fault surface. Historical information from the Vesuvius volcano indicates that faults reactivation occurred during pre‐eruptive and eruptive phases. A slip tendency analysis of the Vesuvius faults has been performed with the aim to investigate the potential fault activity associated to an increase of fluid pressure. A detailed mapping of the Vesuvius faults has been performed combining DTM and geological data. Slip tendency of the Vesuvius faults has been calculated using available information about the stress field acting on the volcano. The stress field has been perturbed by increasing the fluid pressure and slip tendency fault maps have been derived for different values of fluid pressure. Results indicate that both surface and seismic monitoring of the Vesuvius faults could provide useful insights into the volcano dynamics evolution allowing to refine assessment of volcanic hazard.
GPS geodetic constraints on Caribbean‐North America Plate Motion
Authors: DeMets Charles , Jansma Pamela E. , Mattioli Glen S. , Dixon Timothy H. , Farina Fred , Bilham Roger , Calais Eric , Mann Paul
Published: September 2002

We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr−1 velocity uncertainties. The new model predicts Caribbean‐North America motion ∼65% faster than predicted by NUVEL‐1A, averaging 18–20±3 mm yr−1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike‐slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr−1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.
Bedrock features and ice flow near the EPICA Ice Core Site (Dome C, Antarctica)
Authors: Rémy Frédérique , Tabacco Ignazio E.
Published: September 2002

The precise bedrock topography near the EPICA ice core site, at Dome C in Antarctica exhibits impressive bedrock mountains, bowls and a clear network of elongated valleys. Some of these features are probably tectonic and are likely to be buried pre‐ice‐sheet landforms. The elongated bedrock valleys are clearly detectable in the surface topography suggesting a large‐scale network. The analysis of the relationship between basal and surface elongated structures indicates a damping factor decreasing from 120 near the dome to 80 at few tens of kilometer. There is evidence of presence of subglacial water and of erosed bed, that can play a role of the ice flow. A complete flow modeling, taking into account longitudinal stresses, should be performed around EPICA site in order to improve dating of the ice.
Winter‐to‐spring transition in Europe 48–54°N: From temperature control by advection to control by insolation
Authors: Otterman J. , Ardizzone J. , Atlas R. , Hu H. , Jusem J. C. , Starr D.
Published: September 2002

As established in previous studies, and analyzed further herein for the years 1988–1998, warm advection from the North Atlantic is the predominant control of the surface‐air temperature in northern‐latitude Europe in late winter. This thesis is supported by the substantial correlation Cti between the speed of the southwesterly surface winds over the eastern North Atlantic, as quantified by a specific Index Ina, and the 2‐meter level temperature Ts over central Europe ( 48–54°N; 5–25°E), for January, February and early March. In mid‐March and subsequently, the correlation Cti drops drastically (quite often it is negative). The change in the relationship between Ts and Ina marks a transition in the control of the surface‐air temperature. As (a) the sun rises higher in the sky, (b) the snows melt (the surface absorptivity can increase by a factor of 3.0), (c) the ocean‐surface winds weaken, and (d) the temperature difference between land and ocean (which we analyze) becomes small, absorption of insolation replaces the warm advection as the dominant control of the continental temperature. We define the onset of spring by this transition, which evaluated for the period of our study occurs at pentad 16 (Julian Date 76, that is, March 16). The control by insolation means that the surface is cooler under cloudy conditions than under clear skies. This control produces a much smaller interannual variability of the surface temperature and of the lapse rate than prevailing in winter, when the control is by advection.

Regional climatic data would be of greatest value for agriculture and forestry if compiled for well‐defined seasons. For continental northern latitudes, analysis presented here of factors controlling the surface temperature appears an appropriate tool for this task.
Projected impact of climate change on the freshwater and salt budgets of the Arctic Ocean by a global climate model
Authors: Miller James R. , Russell Gary L.
Published: September 2002

The annual flux of freshwater into the Arctic Ocean by the atmosphere and rivers is balanced by the export of sea ice and oceanic freshwater. Two 150‐year simulations of a global climate model are used to examine how this balance might change if atmospheric greenhouse gases (GHGs) increase. Relative to the control, the last 50‐year period of the GHG experiment indicates that the total inflow of water from the atmosphere and rivers increases by 10% primarily due to an increase in river discharge, the annual sea‐ice export decreases by about half, the oceanic liquid water export increases, salinity decreases, sea‐ice cover decreases, and the total mass and sea‐surface height of the Arctic Ocean increase. The closed, compact, and multi‐phased nature of the hydrologic cycle in the Arctic Ocean makes it an ideal test of water budgets that could be included in model intercomparisons.
Demonstration of using crossed dipole GPR antennae for site characterization
Authors: Guy Erich D. , Daniels Jeffrey J. , Radzevicius Stanley J. , Vendl Mark A.
Published: September 2002

Crossed dipole (cross‐pole) and parallel dipole (co‐pole) GPR data were acquired at an industrial site that formerly operated as a creosote wood treating facility in order to locate buried pipes and tanks or other possible contaminant‐filled subsurface structures. Cross‐pole data are not typically considered during GPR field studies, but proved essential for accurate site characterization at this location, as images produced using co‐pole data had a poor signal to noise ratio. Data interpretations were confirmed through exploratory trenching conducted subsequent to this study. The GPR data proved successful in locating back‐filled trenches that contained creosote‐filled drainage tile, as well as vaults and a pit filled with pure creosote product at the site.
Experimental study of homogeneous bubble nucleation in rhyolitic magmas
Authors: Mourtada‐Bonnefoi Catherine C. , Laporte Didier
Published: September 2002

We present a comprehensive experimental study of homogeneous bubble nucleation in rhyolitic magmas. It was undertaken by performing a series of isothermal (800°C) rapid decompression experiments using crystal‐free rhyolitic liquids containing from 4 to 7 wt% H2O and various CO2 contents of up to 820 ppm. No homogeneous nucleation was observed in samples containing 4.5 wt% H2O and <120 ppm CO2 even at pressures of a few MPa. On the contrary, homogeneous nucleation occurred between 101 and 63 MPa in samples containing 4.5 wt% H2O plus 800 ppm CO2 and between 211 and 198 MPa in samples containing 7 wt% H2O plus 650 ppm CO2. Homogeneous nucleation produced a high number density (1013 to 1015 m−3) of uniformly distributed, small and nearly equal‐sized bubbles. Our experiments demonstrate that an event of homogeneous nucleation is possible in natural crystal‐poor and volatile‐rich rhyolitic magmas, that it may require supersaturation pressures much larger than previously anticipated (>150–200 MPa), and that the pressure of homogeneous nucleation will be strongly sensitive to the multicomponent volatile composition of the liquid.
Steady‐state flow of solid CO2: Preliminary results
Authors: Durham William B. , Kirby Stephen H. , Stern Laura A.
Published: September 2002

To help answer the question of how much solid CO2 exists in the Martian south polar cap, we performed a series of laboratory triaxial deformation experiments at constant displacement rate in compression on jacketed cylinders of pure, polycrystalline CO2. Test conditions were temperatures 150 < T < 190 K, hydrostatic confining pressures 5≤ P ≤40 MPa, and strain rates 4.5×10−8 ≤ ⋵ ≤4.3×10−4 s−1. Most of the measurements follow a constitutive law of the form ⋵ = Aσnexp(−Q/RT), where σ is the applied differential stress, R is the gas constant, and the other constants have values as follows: A = 103.86 MPa−ns−1, n = 5.6, and Q = 33 kJ/mol. Solid CO2 is markedly weaker than water ice. Our results suggest that the south polar cap on Mars is unlikely to be predominately solid CO2, because the elevation and estimated age of the cap is difficult to reconcile with the very weak rheology of the material.
Overpressure detection from compressional‐ and shear‐wave data
Authors: Dvorkin Jack , Mavko Gary , Nur Amos
Published: September 2002

By analyzing experimental data we show that in many room‐dry rocks, the Poisson's ratio (PR) decreases with decreasing differential pressure (confining minus pore pressure). This means that in gas‐saturated rocks, PR decreases with increasing pore pressure. We confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling. This effect can be used as a new tool for overpressure detection from surface seismic, cross‐well, sonic logs and measurements ahead of the drill bit.
Instability and collapse of hazardous gas‐pressurized lava domes
Authors: Voight Barry , Elsworth Derek
Published: September 2002

An important type of gravitational lava‐dome failure involves significant internal gas overpressure that influences the failure process. For the first time, this paper presents a rigorous analysis of the mechanics of gas‐pressurized dome failure. We develop diffusion models to calculate gas overpressures in a lava dome, and embed these pressure data into stability analyses to demonstrate how gas pressurization can initiate instability. The results bear on the mechanistic understanding of hazardous dome failures. Gas pressurization can promote deep‐seated failure, and the timing of conduit pressurization can control the timing of dome collapse. Theory explains why dome collapse can be delayed for cases of oscillating effusion. Dome collapses and explosive events linked to collapse of lava domes can be hazardous. Monitoring criteria are considered to anticipate whether or not such collapses are likely to be explosive.
Multifractality and punctuated equilibrium in the Earth's magnetic field polarity reversals
Authors: Consolini Giuseppe , De Michelis Paola , Meloni Antonio
Published: September 2002

The Earth's magnetic field has reversed its polarity hundreds of times in the Earth's history. This phenomenon is generally viewed as due to a Poisson nonstationary process. Here, the geomagnetic chron time length sequence, for the past 166 Ma, is analyzed using a multifractal approach, based on ‘singularity analysis’. The existence of a multifractal structure is the signature of the occurrence of ‘punctuated equilibrium‘, which can be read as an indication of a critical configuration of the geodynamo. The results obtained are compared with those from a prototypical model of a dynamical system, introduced by Seki & Ito [1993], and are discussed in the context of fluid motions within the Earth's outer liquid core.
The effect of mineral bond strength and adsorbed water on fault gouge frictional strength
Authors: Morrow C. A. , Moore D. E. , Lockner D. A.
Published: September 2002

Recent studies suggest that the tendency of many fault gouge minerals to take on adsorbed or interlayer water may strongly influence their frictional strength. To test this hypothesis, triaxial sliding experiments were conducted on 15 different single‐mineral gouges with various water‐adsorbing affinities. Vacuum dried samples were sheared at 100 MPa, then saturated with water and sheared farther to compare dry and wet strengths. The coefficients of friction, μ, for the dry sheet‐structure minerals (0.2–0.8), were related to mineral bond strength, and dropped 20–60% with the addition of water. For non‐adsorbing minerals (μ = 0.6–0.8), the strength remained unchanged after saturation. These results confirm that the ability of minerals to adsorb various amounts of water is related to their relative frictional strengths, and may explain the anomalously low strength of certain natural fault gouges.
Seismic velocity decrement ratios for regions of partial melt in the lower mantle
Authors: Berryman James G.
Published: September 2002

For regions of partial melt in the lower mantle, both compressional and shear wave velocities decrease monotonically with increasing melt volume fraction. Those regions close to the core‐mantle boundary thought to contain partial melt have a velocity decrement ratio (relative change in shear velocity over relative change of compressional velocity) of about 3. Arguments based on effective medium theories have been given to show that such high values of this ratio are predicted for partial melt systems. The present work confirms the value of 3 near the core‐mantle boundary. It is shown, furthermore, that this velocity decrement ratio can be estimated without detailed knowledge of, or assumptions about, the microstructure of the partial melt system by using Gassmann‧s equation of poroelasticity together with some reasonable assumptions about the change (or lack of change) of the density and bulk modulus for lower mantle pressure and temperature conditions.
Effect of pore and confining pressures on VP in thermally pre‐cracked granites
Authors: Darot Michel , Reuschlé Thierry
Published: September 2002

The acoustic P‐waves velocity VP is measured in a water saturated fine‐grained granite as a function of pore pressure for a series of confining pressures ranging from 10 to 75 MPa. Prior to VP measurements, different crack contents were generated in the granite specimens by means of controlled thermal pre‐treatments at 450, 510, and 555°C. VP variations are interpreted as resulting from crack behaviour evolution in response to imposed pressures. An effective pressure law with an effective pressure coefficient close to 0.9 is derived for VP from a delimited pressure domain. This behaviour is interpreted in terms of crack deformation under pressure.
Transition from arc to intraplate magmatism associated with backarc rifting: Evolution of the Sikhote Alin Volcanism
Authors: Tatsumi Yoshiyuki , Sato Keiko , Sano Takashi , Arai Rieko , Prikhodko Vladimir S.
Published: September 2002

K‐Ar dates and chemical compositions of lavas in Sikhote Alin, Far East Russia, demonstrate marked secular variation in lava chemistry and suggest a linkage between magmatism and the major tectonic opening of the Japan Sea backarc basin. Lavas having typical arc magma chemistry were produced at 40–25 Ma in the eastern part of Sikhote Alin. A conspicuous volcanic hiatus is revealed in the region at 25–20 Ma, which is contemporaneous with the period of the major rifting of the backarc basin. Subduction‐related magmatism may have been switched off by an oceanward retreat of the arc‐trench system. In contrast, intraplate‐type lavas with typical hotspot magma compositions have dominated magmatism in both the eastern and western Sikhote Alin since 20 Ma, and occupy the eastern margin of the Cenozoic intraplate magmatism in the NE Asian continent.
Continuous monitoring of fumarolic gas flux at a bore hole in an active volcanic island
Authors: Igarashi George , Fujii Tetsuya , Mori Toshiya , Notsu Kenji , Watanabe Sei‐ichiro
Published: September 2002

A new method for continuous monitoring of fumarolic gas flux at a bore hole is here described. We measured the vertical temperature distribution continuously using a DTS (Distributed Temperature Sensor) system in a volcanic island, Izu‐Oshima, central Japan. The vertical temperature profile is expressed as a function of gas flux. We then obtain continuous data on relative gas flux with a time interval of one hour. The gas flux appears to fluctuate in response to atmospheric pressure changes, and we support that the characteristic response of the gas flux can well be simulated using a pore pressure diffusion model. This method will be useful to detect changes, even small, in the fumarolic gas flux related to volcanic activity.
Effect of grain size and domain state on thermal demagnetization tails
Authors: Dunlop David J. , Özdemir Özden
Published: September 2002

Thermal demagnetization of viscous remanence (VRM) and partial thermoremanence (pTRM) of 20‐ and 135‐µm natural magnetites reveals a broad spectrum f(TUB) of unblocking temperatures TUB, both > and < TB, the blocking temperature. In contrast, 0.04‐µm single‐domain (SD) grains demagnetize sharply at TUB ≈ TB. High‐ and low‐TUB tails of f(TUB) were wider for 135‐µm multidomain (MD) grains than for 20‐µm PSD grains. 10‐mT alternating‐field demagnetization rendered the VRM of the 20‐µm magnetite more SD‐like in subsequent thermal cleaning, selectively erasing both low‐ and high‐TUB tails of f(TUB). Primary and secondary remanences are then cleanly separated, and non‐linear Thellier paleointensity determination becomes linear. Our results agree well with (TB, TUB) data for 105‐year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because (TUB)av ≈ TB for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (TUB)av values are used instead of (TUB)max values.
Simultaneous EISCAT Svalbard and VHF radar observations of ion upflows at different aspect angles
Authors: Ogawa Y. , Fujii R. , Buchert S. C. , Nozawa S. , Watanabe S. , van Eyken A. P.
Published: September 2002

A simultaneous EISCAT Svalbard and VHF radar experiment has shown that field‐aligned (FA) ion upflows observed at an altitude of 665 km in the dayside cusp are associated with significant anisotropy of ion temperature, isotropic increases of electron temperature and enhancements of electron density. There is no clear correspondence between the enhancements of the electric field strength and the occurrence of the ion upflows. This suggests that the upflow is driven primarily by precipitation. The data support that in addition to “direct” precipitation effects, namely enhanced ambipolar diffusion and heat flux, also wave‐particle interaction, like wave‐induced transverse ion heating, which causes a hydrodynamic mirror force, may play a role.
Electron heat flow carried by Kappa Distributions in the solar corona
Authors: Dorelli John C. , Scudder Jack D.
Published: September 2002

We generalize the Spitzer and Härm [1953] description of electron heat flow to model the effects of the electron velocity distribution power law tails which are frequently observed [Vasyliunas, 1968; Christon et al., 1988; Maksimovic et al., 1997] in astrophysical plasmas. We apply our generalized heat flow model to the problem of electron heat flow in the solar corona, demonstrating that a weak power law tail in the electron velocity distribution can allow heat to flow up a radially directed temperature gradient.
Evolution of coronal streamer structure during the rising phase of solar cycle 23
Authors: Wang Y. ‐M. , Sheeley N. R. , Rich N. B.
Published: September 2002

White‐light images of the outer corona recorded with the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) show a rapid widening of the streamer belt during 1998 and early 1999. The observed streamer structure and its evolution from rotation to rotation are reproduced with a model in which the Thomson‐scattering electrons are concentrated within a narrow layer centered around the heliospheric current sheet. The latitudinal spreading of the streamer belt is shown to be a consequence of the increased rate of magnetic flux emergence in the sunspot latitudes, which led to a weakening of the Sun's axisymmetric dipole moment, to a rapid growth in the nonaxisymmetric components of the coronal field, and hence to a strong tilting and warping of the plasma/current sheet.
The tilt angle control of the outer cusp position
Authors: Němeček Zdeněk , Měrka Jan , Šafránková Jana
Published: September 2002

For two years, the MAGION‐4 satellite collected a unique data set of cusp‐like plasma observations at middle to high altitudes as a part of the INTERBALL project. To compare with low‐altitude cusp studies, we map our observations of cusp‐like plasma along magnetic field lines to the 835km altitude of the DMSP orbit. We use the Tsyganenko 1996 model of the magnetospheric magnetic field for the mapping, taking into account actual solar wind and IMF parameters from the WIND observations. The footprint positions show a substantial latitudinal dependence on the dipole tilt angle. The dependence can be fitted by a line with a slope of 0.15° MLAT per 1° of tilt. This slope increases for observations made in the vicinity of the magnetopause (0.16° MLAT per 1° of tilt) and during periods of southward interplanetary magnetic field (0.17° MLAT per 1° of tilt). Very similar results were obtained for mapping using the Tsyganenko 1989 model.
Distribution of X‐type magnetic neutral lines in the magnetotail with Geotail observations
Authors: Ueno G. , Machida S. , Mukai T. , Saito Y. , Nishida A.
Published: September 2002

We obtained the distribution of the X‐type magnetic neutral lines in the magnetotail from XGSM = 0 to −210 RE with Geotail observations. For the detection of the neutral lines, we adopted simultaneous reversals of magnetic field from north to south (south to north) and ion bulk velocity from earthward to tailward (tailward to earthward). In the near‐tail region (0 ≥ XGSM > −30 RE), the occurrence rate of the reversal events had a peak at YGSM ≃ +9 RE and was characterized by a very asymmetric form in the YGSM direction. The region of the reversal events appeared to expand in the YGSM direction up to a 50 RE width at XGSM ≃ −50 RE. This width was maintained in the region further downtail (XGSM < −50 RE). The occurrence rate integrated in the YGSM direction was found to have a maximum at XGSM ≃ −60 RE.
Acceleration of charged particles by large amplitude MHD waves: Effect of wave spatial correlation
Authors: Kuramitsu Yasuhiro , Hada Tohru
Published: September 2002

We discuss energy diffusion and acceleration of charged particles by large amplitude magnetohydrodynamic (MHD) waves by test particle simulations, paying particular attention to the effects of phase correlation of the waves. In quasi‐linear theory, the wave phases are assumed to be random, but in reality, interaction between different wave modes may produce phase correlation. When the wave phases are strongly correlated, the waveform appears as spatially localized traveling wave packet. These wave packets can efficiently accelerate charged particles, as they are repeatedly mirror reflected by oppositely propagating wave packets: a process analogous to the Fermi acceleration.
Recurrence in MeV proton fluxes and anisotropies at 5 AU from the Sun
Authors: Dalla S. , Balogh A.
Published: September 2002

We report the observation of a recurrent pattern in proton fluxes and anisotropies at 5.2 AU from the Sun, made by the Anisotropy Telescopes on board the Ulysses spacecraft between April 1998 and May 1999, during the rising phase of solar cycle 23. The time delay between recurrent features of the pattern is not constant and increases from 122 to 155 days over the three recurrences observed. We discuss a possible connection between these observations and the well documented existence of a 154‐day periodicity in the rate of occurrence of solar flares.
Terrestrial plasmaspheric imaging by an Extreme Ultraviolet Scanner on planet‐B
Authors: Nakamura M. , Yoshikawa I. , Yamazaki A. , Shiomi K. , Takizawa Y. , Hirahara M. , Yamashita K. , Saito Y. , Miyake W.
Published: September 2002

An extreme ultraviolet (XUV) scanner on board the Mars orbiter, Planet‐B, observed the terrestrial plasmasphere while it was in a parking orbit around the earth. The image was available only for the dusk side of the plasmasphere. Nonetheless, this was the first image from the outside of the plasmasphere. Future missions, such as the NASA IMAGE mission in 2000 as well as the Japanese plasmaspheric imaging telescope on SELENE in 2003 will provide further information on this region.
The formation of an electron hole in the topside equatorial ionosphere
Authors: Huba J. D. , Joyce G. , Fedder J. A.
Published: September 2002

In this letter we report new modeling results that show the formation of an electron hole (depletion) in the topside equatorial ionosphere. The reduction in the electron density occurs in the altitude range 1500–2500 km at geomagnetic equatorial latitudes. The hole is produced by transhemispheric O+ flows that collisionally couple to H+ and transport it to lower altitudes, thereby reducing the electron density at high altitudes. The transhemispheric O+ flows are caused by an interhemispheric pressure anisotropy that can be generated by the neutral wind, primarily during solstice conditions. The formation of the electron hole has a seasonal and longitudinal dependence. This result has been found with a new low‐latitude ionospheric model that has been developed at the Naval Research Laboratory: SAMI2 (Sami2 is Another Model of the Ionosphere).
Observations of centrifugal acceleration during compression of magnetosphere
Authors: Cladis J. B. , Collin H. L. , Lennartsson O. W. , Moore T. E. , Peterson W. K. , Russell C. T.
Published: September 2002

Centrifugal acceleration of escaping ionospheric plasma is one of the important and fundamental processes responsible for energizing and transporting ionospheric plasma to all regions of the magnetosphere. Normally this mechanism operates over extremely large distances. Because of this feature and limitations in particle instrumentation, it has not been possible to directly confirm from in‐situ plasma measurements the very specific predictions made by basic plasma theory about centrifugal acceleration. We report here data obtained near Polar satellite apogee over the northern polar cap during a magnetospheric compression event that occurred on September 24, 1998. Between 2345:18 and 2347:18 UT the magnetic field rotated by about 20° and increased in intensity. The Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) instrument on Polar obtained bulk velocity moments of preexisting upflowing H+, O+, He+, and He++ ions with enough precision and resolution to directly demonstrate, together with computer simulation, that the magnitude and direction of changes in plasma motion during this event follow directly from the changes in the magnetic field configuration as predicted by the equations describing centrifugal acceleration.
Possible effects of the Martian crustal magnetization on its magnetosphere
Authors: Szegő Károly
Published: September 2002

The magnetometer carried aboard the Mars Global Surveyor (MGS) has recently found evidence for strong crustal magnetization in certain regions of Mars, exceeding 1600 nT field strength at about 100 km altitude. In this paper we show that the plasma instruments carried on board of the Phobos 2 spacecraft detected a plasma population between 18:48 and 18:56 UT on 1 February 1989, in the 1000 to 2000 km altitude region, that was very likely strongly influenced by crustal magnetism. It is a plausible interpretation that local magnetic features trapped the observed charged particles.
A continuous view of the dawn‐dusk polar cap
Authors: McEwen D. J. , Zhang Y.
Published: September 2002

Continuous monitoring of airglow and auroral emissions over Eureka (89° magnetic latitude) through each winter has shown that the dusk and dawn boundaries of the polar cap can be routinely seen when the IMF Bz is northward. When this condition prevails the auroral oval shrinks poleward, and the polar boundaries are clearly and continuously seen in photometric dusk‐dawn meridian scans of OI 630 nm emission brightness. This fact is illustrated with two 24‐hr days of records on December 19, 1996 and January 19, 1998. These examples show, and others similarly, that polar arcs emerge from the dusk or dawn flanks of the auroral oval (depending on the sign of By) and thus occur on closed field lines. In a few cases polar arcs emerge from the midnight or noon sector of the oval and extend along the sun‐earth line to cross the Eureka meridian, bifurcating the polar cap. This new technique for monitoring the polar cap dusk‐dawn extent and all auroral activity within promises to be valuable for diagnostic studies of both polar auroral activity and magnetospheric topology during solar wind changes and major sun‐earth events.
A new class of oxygen isotopic fractionation in photodissociation of carbon dioxide: Potential Implications for atmospheres of Mars and Earth
Authors: Bhattacharya Sourendra K. , Savarino Joël , Thiemens Mark H.
Published: September 2002

Photodissociation of CO2 by ultraviolet light (λ = 185 nm) generates CO and O2, which are unusually enriched (more than 100‰) in 17O. The dissociation takes place through a spin forbidden process during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The 17O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin‐orbit coupling of the low energy vibrational levels of the 16O12C17O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (λ < 160 nm) involves no spin violation and produces CO and O2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13C enriched CO2; in this case the products are enriched in both heavy isotopes but about 100‰ more in 18O. The 17O enrichment in CO and O2 generated by CO2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.
Characteristics of tropospheric biennial oscillation and its possible association with the stratospheric QBO
Authors: Sathiyamoorthy V. , Mohanakumar K.
Published: September 2002

Signals of oscillation with periods ranging from 20 to 32 months are found in the tropospheric temperature over the near‐equatorial Indian station, Thumba (8° 23′ N, 76° 52′ E). The phase of this Tropospheric Biennial Oscillation (TBO) in temperature does not vary with height from surface to the level of tropopause and is associated with the intensity of the Indian monsoon rainfall. A quasi‐biennial oscillation (QBO) signal is seen in the lower stratosphere. Marked differences in amplitude of QBO and TBO are noticed between the two decades 1971–81 and 1982–92. During the first decade the downward propagating phase of QBO is disturbed in the 18–23 km altitudes. The QBO in zonal wind, however, has neither inter decadal variability nor disturbances.
Ion chemistry for the detection of isoprene and other volatile organic compounds in ambient air
Authors: Leibrock Edeltraud , Huey L. Gregory
Published: September 2002

A chemical ionization mass spectrometer (CIMS) and a flowing afterglow apparatus were used to study reactions of benzene cations (C6H6+ and (C6H6)2+) with a series of volatile organic compounds (VOCs). Both cations react at the collision rate with compounds of lower ionization potential than benzene, such as isoprene (C5H8), other conjugated dienes, and aromatics. These ions are generally unreactive with substances of higher ionization potential such as alkanes, simple alcohols, simple carbonyls, etc. The results demonstrate that C6H6+ and (C6H6)2+ are excellent reagent ions for the sensitive detection of isoprene in air with a CIMS. However, 2‐methyl‐3‐buten‐2‐ol (MBO) and C5H8 conjugated dienes were identified as potential interferences to this technique. This indicates that the selectivity of the CIMS isoprene measurement must be tested by intercomparison with well‐established methods, e.g. gas chromatography techniques.
The 1997–98 El Nino event and related wintertime lightning variations in the southeastern United States
Authors: Goodman S. J. , Buechler D. E. , Knupp K. , Driscoll K. , McCaul E. W.
Published: September 2002

The El Nino Southern Oscillation (ENSO) is a climate anomaly responsible for worldwide weather impacts ranging from droughts to floods. In the United States, warm episode years are known to produce above normal rainfall along the Southeast U.S. Gulf Coast and into the Gulf of Mexico, with the greatest response observed in the October–March period of the warm episode year. The 1997–98 warm episode is notable for being the strongest event since 1982–83. With the recent launch of a lightning sensor on NASA's Tropical Rainfall Measuring Mission (TRMM) in November 1997 and the detailed coverage of the U.S. National Lightning Detection Network (NLDN), such interannual changes in lightning activity can be examined with far greater detail than ever before. For the 1997–98 ENSO event the most significant year‐to‐year changes in lightning frequency worldwide occurred along the Gulf Coast and within the Gulf of Mexico basin during the Northern Hemisphere winter. Within a broad swath across the northern Gulf of Mexico basin there is a 100–150% increase in lightning days year‐to‐year (a peak of 33 days in the winter of 1997–98 vs. only 15 days or fewer in both the 1996–97 and 1998–99 winter). In addition, there is a nearly 200% increase in lightning hours (a peak of 138 hours in 1996–97 vs. 50 hours in both 1996–97 and 1998–99). The increase in lightning activity during ENSO occurs in association with a 100% increase in the number of synoptic scale cyclones that developed within or moved through the Gulf basin. The primary variables controlling these enhancements in thunderstorm activity are the position and strength of the jet stream.
Plume transformation index (PTI) of the subsonic aircraft exhausts and their dependence on the external conditions
Authors: Karol Igor L. , Ozolin Yury E. , Kiselev Andrey A. , Rozanov Eugene V.
Published: September 2002

A concept of the PTI is proposed to characterize the photochemical interactions of aircraft exhausts with the ambient air. The PTI is the ratio of NOx, CO,, ozone, and NOy transformed in the plume during its lifetime period tp to NOx and CO released into the plume initially. An estimate of tp is calculated by prescribing the 10% excess of emitted (NOx)p in the plume over the (NOx)a of the ambient air at the end of plume life. Simple analytical formulas for PTI are obtained from the known plume box model. The tp, PTI of NOx, HNO3 and O3 are calculated and analyzed for different moments of releases during the daytime in and out of flight corridor. The in‐plume NOx conversion to HNO3 and O3 enhancement are found to be the largest in daytime, in July and in the lower stratosphere.
Interannual variations of global UV radiation in Santiago, Chile (33.5°S)
Authors: Cabrera Sergio , Fuenzalida Humberto A.
Published: September 2002

Observations with a four‐channel UV radiometer in Santiago, Chile (33.5°S, 70.6°W) from January 1992 to December 1998 are presented. Channels are centered at 305, 320, 340 and 380 nm with a 10 nm bandwidth. Measurements were made at one‐minute intervals. Hourly mean values at noon for 305 and 340 nm are presented as well as instantaneous irradiances for 60° of solar zenith angle. Their aperiodic variations on a seasonal scale are discussed with respect to the quasi‐biennial oscillation (QBO) and ENSO phenomena. A significant positive trend that must be caused by decreasing total ozone is found in 305 nm irradiance. On a seasonal basis, this negative trend appears strong and significant during winter.