bluuebunny/crossref_metadata_2025 huggingface dataset:
Elasticity of marine sediments: Rock physics modeling
Authors: Dvorkin Jack , Prasad Manika , Sakai Akio , Lavoie Dawn
Published: September 2002

We offer an effective medium model for the elastic moduli of high‐porosity ocean‐bottom sediments. The elastic constants of the dry‐sediment frame depend on porosity, elastic moduli of the solid phase, and effective pressure. The model connects two end points in the elastic‐modulus‐porosity plane: the Hertz‐Mindlin modulus of a dense elastic sphere pack at critical porosity; and zero at 100% porosity. The elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. Unlike the suspension model, our model assigns non‐zero elastic constants to the dry‐sediment frame and can predict the shear‐wave velocity. Unlike various modifications of the travel‐time‐average equation, it is first‐principle‐based and contains only physical parameters. We justify this model by matching sonic data in shallow marine sediments and in an ODP well.
Observation of NOy uptake by particles in the Arctic tropopause region at low temperatures
Authors: Feigl C. , Schlager H. , Ziereis H. , Curtius J. , Arnold F. , Schiller C.
Published: September 2002

Measurements of gas‐ and condensed‐phase NOy, HNO3, O3, and particle concentration and size were performed on 24 January 1997 during POLSTAR on a flight through an ice cloud. In the ice cloud below the tropopause only 2 to 4 pptv particle NOy were detected. The observed average mixing ratio of total HNO3 is significantly larger than the detected particle NOy. Considering the combined error limits of the NOy and HNO3 measurements, however, it cannot be ruled out that the observed uptake of NOy was limited by the available gas‐phase HNO3. The ratio of total HNO3 to gaseous NOy measured during the cirrus cloud penetration amounts only to 0.2 in contrast to 0.9 in the stratospheric part of the flight.
Solar wind density as a driver for the ring current in mild storms
Authors: Smith J. P. , Thomsen M. F. , Borovsky J. E. , Collier M.
Published: September 2002

Using the methods of multi‐ and partial‐linear‐correlation, we demonstrate that both the solar wind density and the solar wind electric field make significant and independent contributions to the strength of the ring current, as measured by the Dst* index, during mild storms. The solar wind data were obtained from WIND, and the Dst data from OmniWeb. After appropriate cleaning of the data, 55 events from the Wind‐era with Dst* excursions below −50 were obtained. Correlations with Dst* were performed with independent, variable time lags for the density and for the electric field, and the optimal time lags are found to be approximately 5 hours for density and less than one hour for the electric field (in addition to the time shift to Earth).
Modeling the temperature response to forced climate change over the last six centuries
Authors: Crowley Thomas J. , Kim Kwang‐Yul
Published: September 2002

Development of improved proxy estimates of climate forcing and temperature change over the past six centuries provides a new opportunity to examine the role of forced variability in the climate system. We utilize an energy balance model to estimate that as much as 18–34% of low frequency temperature change over the preanthropogenic interval could be forced by volcanism and solar variability. Comparison of residuals with estimates of unforced variability from control runs of coupled models indicates that the spectra of the two time series agree at the 90% significance level.
Current sheet control of recurrent particle increases at 4–5 AU
Authors: Sanderson T. R. , Lario D. , Maksimovic M. , Marsden R. G. , Tranquille C. , Balogh A. , Forsyth R. J. , Goldstein B. E.
Published: September 2002

We examine the relation between the current sheet as derived from observations of the Sun's neutral line, and recurrent particle increases associated with co‐rotating interactions regions (CIR) at 4 to 5 AU radial distance as observed by the Ulysses spacecraft during its first out‐of‐ecliptic orbit around the Sun. We compare observations during the ascent to high southern latitudes, close to solar maximum, with observations during the return to the ecliptic from high northern latitudes, close to solar minimum, when the spacecraft was passing from the streamer belt to high‐speed solar wind flow or vice‐versa every solar rotation, and when regular recurrent particle increases were observed. Although conditions are very different, we show that the position of the Sun's neutral line, propagated out to the position of the spacecraft, can be used to explain the recurrence of the particle increases observed at 4–5 AU at Ulysses.
Sensitivity of the European LGM climate to North Atlantic sea‐surface temperature
Authors: Pinot Sophie , Ramstein Gilles , Marsiat Isabelle , de Vernal Anne , Peyron Odile , Duplessy Jean‐Claude , Weinelt Maria
Published: September 2002

Recent reconstructions of Sea‐Surface Temperatures (SSTs) for the Last Glacial Maximum (LGM, 21kyr BP) based on foraminifera and dinoflagellate proxies suggest that the north Atlantic may have been warmer than estimated by CLIMAP [1981]. To better understand the impact of such a warm north Atlantic on the global LGM climate, we used two different AGCMs to perform sensitivity studies. With the new, warmer SSTs, both models simulate a hydrological cycle and temperatures very different from those obtained with the CLIMAP boundary conditions. The most noticeable differences occur in winter over North America and Siberia whereas southern Europe is only weakly affected at all seasons. Whichever the conditions prescribed over the north Atlantic, both models underestimate the large cooling recorded by continental proxy data over the Mediterranean Basin.
Multidecadal global and regional trends in 1000 mb and 500 mb cyclone frequencies
Authors: Key Jeffrey R. , Chan Alan C. K.
Published: September 2002

Trends in seasonal and annual frequencies of 1000 mb and 500 mb closed low pressure centers, or “cyclones”, over the 40 year period 1958–1997 are determined for latitudinal zones and regions of cyclogenesis. Statistically significant trends are found throughout the northern hemisphere and in the southern hemisphere tropics, though the trends vary by season. Opposite trends were observed at the two levels in some regions. No significant differences in cyclone frequencies were found when El Niño and La Niña years were compared. Cyclone frequencies in North America and Europe are poorly correlated with the North Atlantic Oscillation.
Dynamic 3D simulations of earthquakes on En Echelon Faults
Authors: Harris Ruth A. , Day Steven M.
Published: September 2002

One of the mysteries of earthquake mechanics is why earthquakes stop. This process determines the difference between small and devastating ruptures. One possibility is that fault geometry controls earthquake size. We test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three‐dimensional medium and apply our knowledge to two California fault zones. We find that the size difference between the 1934 and 1966 Parkfield, California, earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 Landers, California, earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event. If there are no linking structures, such as transfer faults, then strike‐slip earthquakes are unlikely to propagate through stepover s >5 km wide.
Swiss glacier recession since the Little Ice Age: Reconciliation with climate records
Authors: Greene Arthur M. , Broecker Wallace S. , Rind David
Published: September 2002

Since the culmination of the Little Ice Age, Alpine glaciers have been in a state of general retreat. The present study, focusing on the Swiss Alps between 1850 and 1973, seeks to relate the concomitant rise in equilibrium‐line altitudes (ELAs) to recorded climate shifts. The approach taken involves the development of a regression model, but differs from most other studies in that the relationship of (midlatitude) ELAs to climate is examined at a resolution such that regional values are treated as individual data‐points. Although such a treatment implies loss of resolution at small scales, a coherent relationship between climate and ELA is nonetheless obtained, due in large part to the primary control exercised by temperature, as well as the relatively wide range of ELAs included in the regression dataset. The derived relationship is applied to observed Swiss climate shifts, and is found to predict a secular rise in ELA somewhat greater than that observed. It is hypothesized that the difference represents climate change that had not yet been expressed by changes in glacier morphology.
Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes
Authors: Ito Takamori , Nagamine Koichiro , Yamamoto Koshi , Adachi Mamoru , Kawabe Iwao
Published: September 2002

Simultaneous monitoring of He, H2, Ar, N2, CH4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H2, as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H2. We suggest that the selective and preseimic increases of H2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress‐corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H2 production by mechanochemical reaction of fractured rock surface with water in pre‐existing fault planes as groundwater conduits.
Elastic‐wave velocity in marine sediments with gas hydrates: Effective medium modeling
Authors: Helgerud M. B. , Dvorkin J. , Nur A. , Sakai A. , Collett T.
Published: September 2002

We offer a first‐principle‐based effective medium model for elastic‐wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame; (b) hydrate becomes a component of the solid phase, modifying the elasticity of the frame. The goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) consistent with estimates obtained from resistivity, chlorinity and evolved gas data.
HNO3 partitioning in cirrus clouds
Authors: Meilinger S. K. , Tsias A. , Dreiling V. , Kuhn M. , Feigl Ch. , Ziereis H. , Schlager H. , Curtius J. , Sierau B. , Arnold F. , Zöger M. , Schiller C. , Peter Th.
Published: September 2002

During the 1997 POLSTAR‐1 winter campaign in northern Sweden a flight was performed across a cold trough of air (≃ 196 K) in the tropopause region. Measurements of total water vapour, nitric acid, particles and reactive nitrogen (NOy) were taken. The particle measurements indicate that about 3% of the particles in the moist tropospheric air were ice particles. Forward and backward facing NOy inlets were used simultaneously to determine condensed phase HNO3. The combined NOy and particle measurements reveal that less than 1% of a monolayer of NOy could have resided on the ice particles. This casts doubt on the hypothesis that sedimenting cirrus particles generally lead to a strong downward flux of NOy. In addition to the NOy measurements, independent HNO3 measurements were used to determine total HNO3. Although quantitative uncertainties do not allow to completely rule out that the NOy uptake on ice was limited by total HNO3, the combined NOy and HNO3 data suggest that there was low uptake of NOy on ice despite abundant HNO3 in the gas phase. Model studies indicate, that the most likely explanation of the measured nitric acid partitioning is given by HNO3 in ternary solution droplets coexisting with almost HNO3 free ice in the same air mass.
The influence of boundary heterogeneity in experimental models of mantle convection
Authors: Namiki A. , Kurita K.
Published: September 2002

Recent global seismological observations have revealed lateral variations in the thickness of the D″ layer. We have performed laboratory experiments to explore how undulations of various sizes in the D″ layer affect convection patterns. We find that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns. Observed undulations in the D″ layer exceed this critical height, suggesting that they may control mantle convection patterns.
A double‐layered seismic zone in Arica, northern Chile
Authors: Comte Diana , Dorbath Louis , Pardo Mario , Monfret Tony , Haessler Henri , Rivera Luis , Frogneux Michel , Glass Bianca , Meneses Carlos
Published: September 2002

A double layered seismic zone is determined in Arica, northern Chile using locally recorded events. At depths >100 km two planes of seismicity can be observed: one dipping at ∼30°E with ∼10 km of thickness and a second parallel plane 20–25 km deeper, with the same average thickness. Fault plane solutions for both layers show a wide variability, even between nearby events. The genesis of the Arica double seismic zone seems to be independent of the age, the relative convergence rate and direction of the Nazca plate, because all of these parameters are almost the same along the whole northern Chile and a clear separation of the seismicity into two layers is only observed around the Arica elbow. Moreover, it cannot be responsible of the double layered seismic zone because no similar significant changes are observed in the other well studied double seismic zones.
First passive radar observations of auroral E‐region irregularities
Authors: Lind Frank D. , Sahr John D. , Gidner Dawn M.
Published: September 2002

On August 27, 1998 we observed auroral E‐region irregularities using the Manastash Ridge Radar system. These are the first radio science results from passive radar observations of commercial FM broadcasts near 100 MHz. These are also the first reported observations of 1.5 m irregularities in 15 years. The data show scatter most consistent with type‐1 and type‐3 ion‐acoustic turbulence as observed by 50 MHz and 140 MHz radars. The instrument is extraordinarily low in cost and provides data free of range and doppler velocity aliasing problems. The range and velocity resolution achieved by this radar is substantially superior to that of other comparable coherent scatter radars, and the time resolution is similar.
Sea level changes from Topex‐Poseidon altimetry and tide gauges, and vertical crustal motions from DORIS
Authors: Cazenave A. , Dominh K. , Ponchaut F. , Soudarin L. , Cretaux J. F. , Le Provost C.
Published: September 2002

Sea level difference (Topex‐Poseidon minus tide gauge) time series have been computed over 1993–1997 at 53 selected gauges sites. Comparison of these sea level differences with vertical crustal motions derived from the DORIS space geodesy system at 6 colocated sites shows good consistency. At one site (the Socorro volcanic island), a striking correlation is reported between sea level differences and DORIS height time series. The observed trend likely reflects a post eruptive deformation associated with a volcanic eruption that occurred in early 1993. At several other gauge sites, the sea level differences present large linear trends possibly bearing evidence of land motion. This may be the case at Rabaul (Papua New Guinea) where a volcanic eruption took place in autumn of 1994. The sea level differences at Rabaul show a negative trend from this date, likely related to the volcanic event.
The scaling of collisionless, magnetic reconnection for large systems
Authors: Shay M. A. , Drake J. F. , Rogers B. N. , Denton R. E.
Published: September 2002

Hybrid simulations with electron inertia, along with analytic scaling arguments, are presented which demonstrate that magnetic reconnection remains Alfvénic in a collisionless system even as the macroscopic scale length of the system becomes very large. This fast reconnection is facilitated by the whistler physics present near the x‐line. The reconnection rate is found to be a universal constant corresponding to an inflow velocity towards the x‐line of around 0.1 cA.
Measuring near field coseismic displacements from SAR images: Application to the Landers Earthquake
Authors: Michel Rémi , Avouac Jean‐Philippe , Taboury Jean
Published: September 2002

We describe a procedure to compute SAR interferograms based on the compensation of the images' mis‐registrations due to the ground deformation and on a specific adaptive filtering to improve the SNR in the measurement where displacement gradients are large. Applied to the Landers earthquake case, the registration reduces the phase noise by 20° (rms) near the fault and the interferogram reveals fringes that would not show out otherwise. The interferogram is validated by comparison, in the far field, with a GPS/EDM‐based model. The inferred displacement field shows discontinuities that follow the fault within few hundred meters. Close to the fault the measured displacements differ significantly from that predicted by elastic modeling of the far‐field data. Interferometry and measurements derived from the images' amplitude (offsets) appear to be complementary techniques providing original measurements that may help to reconcile seismological and geodetic models of coseismic deformation with measurements of fault‐slip at the surface.
Conditions for existence of orthogonal coordinate systems oriented by magnetic field lines
Authors: Salat A. , Tataronis J. A.
Published: September 2002

In the study of Alfvén waves and resonances in the magnetospheres of the Earth and other celestial bodies, orthogonal, magnetic‐field‐aligned coordinates (OF coordinates) are widely used. Two of the coordinates are constant on field lines, and the direction of the third is parallel to the direction of the ambient magnetic field. In dipolar fields or in nondipolar but still purely meridional axisymmetric fields the existence of such coordinates is evident. OF coordinates, however, are also used occasionally in magnetospheric configurations of more general “nontrivial” geometry where their existence is far from obvious. In the present paper it is proved that magnetic field configurations cannot support OF coordinates wherever local shear of the magnetic field lines is present. The previously known condition that no field‐aligned currents must exist further restricts the admissible geometry of the magnetic field. If the geometry is axisymmetric, a purely meridional or a purely azimuthal magnetic field is the only remaining OF compatible configuration. Therefore publications in which OF coordinates are used together with “nontrivial” magnetic field configurations need a critical reassessment.
Magnetic field strength distribution in interplanetary turbulence
Authors: Hartlep T. , Matthaeus W. H. , Padhye N. S. , Smith C. W.
Published: September 2002

The distribution of magnetic field magnitudes is derived for the special case of a constant mean field and uncorrelated components having Gaussian distributions. Of the three cases considered (isotropic, transverse, and axisymmetric) the latter most closely resembles a lognormal distribution when the parallel variance is less than the perpendicular variance. Thus a normally distributed, nonintermittent vector field can produce a magnitude distribution that closely resembles a functional form often associated with models of intermittency. This result is illustrated by comparison with a magnetic field magnitude distribution computed from the 1 AU Omni dataset. The parameters of the Gaussian component magnitude distribution can be chosen to compare with the data which is also well fit as a lognormal type. We conclude that the magnitude distribution may not be a sensitive indicator of intermittency and that further examination of the sensitivity of such indicators is warranted.
Global Positioning System measurements of the ionospheric zonal apparent velocity at Cachoeira Paulista in Brazil
Authors: Kil Hyosub , Kintner Paul M. , de Paula Eurico R. , Kantor Ivan J.
Published: September 2002

Ionospheric irregularities and their zonal apparent drift were studied using Global Positioning System (GPS) measurements at Cachoeira Paulista (22.41°S, 45.00°W, −26° dip angle) in Brazil during November 6–19, 1998. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using four GPS receivers spaced geomagnetically east–west and north–south. Total electron content (TEC) was measured through the ionospheric advance of the GPS L1 and L2 (1.227 GHz) phases. Strong amplitude scintillations coincided with TEC fluctuations associated with spread F bubbles elongated along the magnetic field. Movement of the Presnel‐scale (400 m) ionospheric irregularity layers caused the scintillation to drift, and their zonal apparent drift velocities were measured using a cross‐correlation technique. Our measurements show that the apparent eastward velocity varies from 200 m/s to 150 m/s at 2000 LT, and then it decreases to 100–50 m/s at midnight. On a magnetically disturbed day, reversal of the zonal apparent drift was observed just after midnight, and the apparent westward velocities observed at early in the morning showed large variations with location in the sky. From the receivers spaced in the geomagnetic north–south direction we measured near‐zero time shifts, from which we conclude that the correlation length of several‐hundred‐meter‐scale irregularities is much larger than 70‐m separation between the north and south receivers.
Excitation of electron acoustic waves near the electron plasma frequency and at twice the plasma frequency
Authors: Schriver D. , Ashour‐Abdalla M. , Sotnikov V. , Hellinger P. , Fiala V. , Bingham R. , Mangeney A.
Published: September 2002

In this paper we investigate the nonlinear development of the electron acoustic instability that can lead to the transfer of wave energy to frequencies just above the electron plasma frequency (ωpe) and to waves with approximately twice the electron plasma frequency (2ωpe). Using plasma conditions in the upstream electron foreshock region based on data from the AMPTE‐UKS spacecraft, an electron beam is considered in plasma containing a background of hot and cold electrons. This leads to the linear excitation of large‐amplitude electron acoustic waves at frequencies between about 0.8 and 1.0 ωpe. A modified decay instability then excites waves in the spectrum just above ωpe. This is followed by a second nonlinear coalescence process that causes the excitation of waves at frequencies just below 2ωpe. The linear and nonlinear properties of the electron acoustic instability are examined for observed conditions using analytical theory, particle‐in‐cell simulations, and Vlasov simulations. These results have application to observations made inside the electron foreshock region, as well as the polar cap and auroral zone, where plasma oscillations and waves at 2ωpe are observed.
Shock propagation in the outer heliosphere: 3. Pickup ions, MHD, cosmic rays, and energetic particles
Authors: Rice W. K. M. , Zank G. P.
Published: September 2002

It has been shown previously that to study the propagation charcteristics of shock waves accurately in the outer helioshpere, the heliospheric model should contain pickup ions (PIs). The inclusion of a magnetic field was also shown to affect shock wave propagation but was found to be far less important than the effect of the PIs. To further this study, we consider here the effect of galactic cosmic rays and energetic particles on interplanetary traveling shock waves in the outer heliosphere. The energetic particles are taken to be protons and are assumed to be produced by injection and acceleration at the shock fronts. The galactic cosmic rays are found to have almost no effect on the shock waves. Depending on their injection efficiency and diffusion coefficient, the energetic protons, on the other hand, can significantly modify the shock structure.
Solar wind observations over Ulysses' first full polar orbit
Authors: McComas D. J. , Barraclough B. L. , Funsten H. O. , Gosling J. T. , Santiago‐Muñoz E. , Skoug R. M. , Goldstein B. E. , Neugebauer M. , Riley P. , Balogh A.
Published: September 2002

This study examines solar wind plasma and magnetic field observations from Ulysses' first full polar orbit in order to characterize the high‐latitude solar wind under conditions of decreasing and low solar activity. By comparing observations taken over nearly all heliolatitudes and two different intervals covering the same radial distances, we are able to separate the radial and latitudinal variations in the solar wind. We find that once the radial gradients are removed, none of the high‐latitude solar wind parameters show much latitudinal variation, indicating that the solar wind emanating from the polar coronal holes is extremely uniform. In addition, by examining nearly 6 years of data starting in the declining phase of the last solar cycle and extending through the most recent solar minimum, we are able to address hemispheric asymmetries in the observations. We find that these asymmetries are most likely driven by differences in the solar wind source over the solar cycle and indicate that more energy goes into the polar solar wind during the declining phase of the solar cycle than around minimum. Because the mass flux is larger in the declining phase while the speeds are very similar, we conclude that this energy is introduced at an altitude below the solar wind acceleration critical point. Finally, we provide details of the statistics of over 20 solar wind parameters so that upcoming observations from Ulysses' second polar orbit, during much more active times on the Sun, can be readily compared to the quieter first orbit results.
Electromagnetic fluctuations near the electron plasma frequency from an electron/electron instability
Authors: Kazimura Yoshihiro , Gary S. Peter , Li Hui , Sakai Jun‐Ichi
Published: September 2002

Enhanced electromagnetic fluctuations near the electron plasma frequency ωe are observed in the auroral and polar regions of the terrestrial magnetosphere; until recently, their origin has not been understood. Linear Vlasov theory and a particle‐in‐cell simulation with a two‐dimensional fully relativistic electromagnetic code have been carried out for an electron/electron instability in a homogeneous, collisionless, magnetized plasma model under the condition ωe/|Ωe| < 1 where Ωe is the electron cyclotron frequency. At propagation parallel to the background magnetic field the instability is strictly electrostatic, but at oblique propagation significant electromagnetic fluctuations are also excited. The simulation reproduces the following essential features of the observed plasma frequency fluctuations: (1) narrowband frequencies near ωe, (2) relatively large electric fields (δE/δB ≫ 1), and (3) episodes of both right‐hand and left‐hand polarization. This polarization result is new and has not been published before. Furthermore, the present theory and simulation show that the right‐hand polarized fluctuations are on a short‐wavelength beam mode and are excited by linear instability, growth; the left‐hand fluctuations are at relatively long wavelengths and are apparently driven by nonlinear processes.
Reduced dynamical equations for the high‐latitude thermosphere: Ion drag balance
Authors: Kinney R. M. , McWilliams J. C.
Published: September 2002

Although highly variable on small scales, the middle thermosphere shows evidence of slow‐timescale mesoscale dynamics. A reduced set of equations is derived by asymptotic scale analysis. The drag‐balance equations describe the nonlinear evolution of the thermosphere during magnetic quiet times under an approximate balance of pressure, Coriolis, and ion‐drag forces. The equations explicitly remove gravity and sound oscillations, making them well suited for theoretical study of slow‐timescale evolution and efficient for numerical modeling. Linear analysis is used to establish the limits of their validity in terms of timescale separation. The slow‐timescale component is only weakly damped for horizontally broad motions, even when the ion‐neutral collision frequency is large.
Double‐probe measurements in field‐aligned irregularities produced by intense electromagnetic radiation
Authors: Peria W. J. , Kelley M. C. , Franz T.
Published: September 2002

We present the signature of filamentary field‐aligned irregularities (FAI) in the artificially modified ionosphere using data from a rocket‐borne floating double‐probe gathered near the critical layer of the Arecibo HF heater beam. We model the double‐probe signature as proportional to each filament's density gradient, in the plane perpendicular to the geomagnetic field. We find that this signature is consistent with the presence of a radially directed effective electric field
within each of the over 180 filaments. The direction of is consistent with that of an ambipolar electric field associated with the rapid perpendicular diffusion of ions out of each filament, or with an apparent electric field due to an inward electron temperature gradient associated with the presence of hotter plasma inside each filament. Our model also gives an estimate of the impact parameter at which each filament is encountered. The mean square impact parameter shows the correct relation to the mean square filament transit time, assuming cylindrical field‐aligned filaments. The consistency of these results confirms that the previously reported density depletions are quasi‐steady, cylindrically symmetric, spatial structures. A small shift in the apparent angle between the double‐probe boom and the filament‐rocket velocity places a lower limit on the Earth‐frame drift velocity of the filaments, away from the heater beam. Since the filaments are observed near the westward edge of the heater beam, well away from the bulk of the heater Poynting flux, we expect that the filaments are observed in the process of decaying, that is, that ions are diffusing inward rather than outward. Thus we identify as a temperature gradient and use it to estimate a minimum filament temperature enhancement of ∼ 100 K.
Ion and electron characteristics in auroral density cavities associated with ion beams: No evidence for cold ionospheric plasma
Authors: McFadden J. P. , Carlson C. W. , Ergun R. E. , Klumpar D. M. , Moebius E.
Published: September 2002

Low‐density cavities associated with upgoing ion beams were identified in the auroral regions over two decades ago. In order to understand the waves, double layers, and solitary structures observed within these cavities, accurate measurements of the plasma distribution function are required. Although measurements by DE 1 indicated that these cavities were composed primarily of hot plasma in the form of ion beams, plasma sheet ions, and inverted V electrons, later reports from Viking showed these cavities contained a cold plasma component whose density was an order of magnitude larger than the hot component. Recent measurements by the FAST satellite contrast sharply with the Viking results and support the earlier DE 1 observations. Regions of upgoing ion beams observed by FAST are shown to contain little or no cold plasma. The hot electron densities (>100 eV) and the combined plasma sheet ion and upgoing ion beam densities (>30 eV) agree remarkably well. Furthermore, no cold ions (0–30 eV) are measured at low energies by the mass spectrometer, which precludes the presence of significant (>20%) cold electrons to preserve charge neutrality. Characteristics of the plasma for 11 ion beam events are tabulated.
Configuration of the magnetosphere‐ionosphere convection system under northward IMF conditions with nonzero IMF By
Authors: Tanaka T.
Published: September 2002

The configuration of the magnetosphere‐ionosphere convection system is investigated by analyzing outputs of a global magnetohydrodynamic (MHD) model for the solar wind‐magnetosphere‐ionosphere (S‐M‐I) coupling process. The input interplanetary magnetic field (IMF) to the model is inclined 45° from the due‐northward direction with downward IMF. The results clearly show that the round cell and crescent cell are formed in the polar ionosphere and that the lobe cell and merging cell in the magnetosphere are connected to the core of the round cell and to the outer round and crescent cells, respectively. The open field lines that construct the lobe‐cell circulation are generated through the open‐field‐line reconnection on the poleward side of the cusp, together with unconnected field lines. The open magnetic field lines that extend from the outer round cell in the Northern (Southern) Hemisphere and construct the merging‐cell convection are generated through the closed‐field‐line reconnection in the Northern (Southern) Hemisphere, while the open magnetic field lines that extend from the crescent cell in the Northern (Southern) Hemisphere are generated in the Southern (Northern) Hemisphere. Convecting through the lobe, the open field lines from the northern outer round cell reconnect with the open field lines from the southern crescent cell in the northeast edge of the twisted plasma sheet. The features of the merging‐cell convection can become very complicated in the plasma sheet.
Observations and interpretation of gravity wave induced fluctuations in the O I (557.7 nm) airglow
Authors: Schubert G. , Walterscheid R. L. , Hickey M. P. , Tepley C. A.
Published: September 2002

Observations of fluctuations in the intensity and temperature of the O I (557.7 nm) airglow taken at Arecibo in 1989 are reported and interpreted on the assumption that they are caused by gravity waves propagating through the emission layer. The data give the magnitude of Krassovsky's ratio as 3.5 ± 2.2, at periods between about 5 and 10 hours. Comparison with theory shows that the gravity waves responsible for the measured airglow variations must have long wavelengths of several thousand kilometers. The observed phases of Krassovsky's ratio are in good agreement with theoretically predicted values at the long wavelengths and large periods for about half the cases. In the other cases, observed phases are near −180°, suggesting that the waves responsible for the airglow fluctuations have experienced strong reflections in the emission layer. The observations emphasize the importance of knowing the full altitude profiles of temperature and winds for extraction of wave information from the airglow fluctuations.
MHD description of the dynamical relationships between a flux rope, streamer, coronal mass ejection, and magnetic cloud: An analysis of the January 1997 Sun‐Earth connection event
Authors: Wu S. T. , Guo W. P. , Michels D. J. , Burlaga L. F.
Published: September 2002

We investigate the dynamical relationships between a coronal flux rope, a streamer, a coronal mass ejection (CME), and a magnetic cloud by using observations from the satellites of the International Solar‐Terrestrial Physics observatories together with a streamer and flux rope interaction model [Wu and Guo, 1997a]. This is the first physical description of the evolution of a CME related to a flux rope in a streamer near the Sun to a magnetic cloud at 1 AU. The distinctive physical configuration of the model is based on a theoretical suggestion [Low, 1994] and observations [Hundhausen, 1993] that the magnetic structure of a streamer with an embedded cavity provides favorable condition for launch of a CME. We explore this physical scenario by identifying a flux rope as the cavity and using a fully self‐consistent numerical simulation to illustrate the dynamical process of evolution of the flux rope/CME into a magnetic cloud. The simulation results are then compared to solar and interplanetary data from the well‐observed Sun‐Earth connection event of January 6–12, 1997. The data used for this analysis were collected chiefly by the Solar and Heliospheric Observatory (SOHO) Large‐Angle and Spectrometric Coronagraph Experiment coronagraph and the solar wind particle and field sensors on the Wind spacecraft, but ground‐based solar data were used as well. Because we have detailed observations of the same disturbance both at the Sun (SOHO) and at 1 AU (Wind), this event gives us an unusual opportunity to test the magnetohydrodynamic methodology and to learn about the physical processes of the Sun‐Earth connection. In this study we show that when the flux rope rises (owing to increasing axial current, as assumed here, or to some other mechanism), it disrupts the streamer‐flux rope system, thus launching a coronal mass ejection. The flux rope then escapes from the streamer and evolves to become a magnetic cloud, as expected, in interplanetary space. The CME is a visible feature moving ahead of the flux rope. The model also predicts a fast‐mode shock in front of the magnetic cloud, as observed.
Formation and evolution of subauroral ion drifts in the course of a substorm
Authors: De Keyser J.
Published: September 2002

We propose a physical mechanism that explains how “polarization jets” (PJ) or “subauroral ion drifts” (SAID) are formed in the course of a substorm and how they evolve. A PJ/SAID is considered to be the ionospheric signature of an inward moving injected plasma front. The flow shear that exists across this interface when it arrives in the vicinity of the plasmapause is responsible for the generation of intense electric fields in the premidnight sector, where PJ/SAID are observed. Quantitative simulation of this mechanism accounts for PJ/SAID width and peak drift velocity. The mechanism explains why PJ/SAID are observed poleward of or in the vicinity of the plasmapause. The inward traveling time of the injected plasma agrees with the delay between substorm onset and the apparition of PJ/SAID; the evolution of the ionospheric signature is consistent with observations as well.
Modeling of time‐evolving magnetic fields during substorms
Authors: Lu G. , Tsyganenko N. A. , Lui A. T. Y. , Singer H. J. , Nagai T. , Kokubun S.
Published: September 2002

An attempt has been made to model the dynamics of the magnetospheric magnetic field during substorms by modifying the 1996 version of the data‐based model by Tsyganenko [1996]. The modified model incorporates an adjustment to the intensity and thickness of the near‐tail current sheet and a contribution from the substorm current wedge. These improvements make it possible to use the model to represent the evolution of the magnetic field during the entire substorm sequence of growth, expansion, and recovery. The modeled magnetic fields have been compared to satellite observations during three isolated substorms. According to the model results, during the substorm growth phase the thickness of the tail current sheet was gradually decreased while the intensity of the tail current was gradually increased; by the end of the growth phase a thin current sheet of 190–1340 km in half thickness was formed in a narrow region around X ∼ −7.5 RE, with a maximum westward current density of 9–23 nA/m2. During the substorm expansion phase an eastward current associated with the substorm current wedge started to develop around X ∼ −12 RE, resulting in a collapse of the previously stretched field configuration. At the peak of an intense substorm the net tail current flow became eastward between X = −11 and X = −13 RE, accompanied by a negative (southward) Bz tailward of −13 RE.
Substorms: A global instability of the magnetosphere‐ionosphere system
Authors: Baker D. N. , Pulkkinen T. I. , Büchner J. , Klimas A. J.
Published: September 2002

Observational and numerical modeling evidence demonstrates that substorms are a global, coherent set of processes within the magnetosphere and ionosphere. This supports the view that substorms are a configurational instability of the coupled system since the entire magnetosphere changes during the expansion phase onset. It is shown that the magnetosphere progresses through a specific sequence of energy‐loading and stress‐developing states until the entire system collapses. This energy loading‐unloading sequence is the essential basis of the Faraday Loop non‐linear dynamics model which has been quite successful in describing the fundamental behavior of substorms without invoking detailed treatments of the internal substorm instability mechanism. Present‐day MHD models also are seen to produce substorm‐like global instabilities despite the fact that they do not treat realistically the extremely thin current sheets that play such an essential role in the near‐tail dynamics prior to substorm onset. This paper discusses three‐dimensional kinetic simulations that have recently shown a variety of initial plasma kinetic instability modes which all evolve quickly to a similar, globally unstable reconnection configuration. Continuing research concerning the substorm onset location and mechanisms addresses important questions of when and exactly how the substorm expansion develops. However, the loaded magnetosphere almost always progresses rapidly to the same basic reconnection configuration irrespective of the detailed localized initiation mechanism. This is likened to the catastrophic collapse of a sand dune that has reached a highly unstable configuration: Any small local perturbation can cause a complete and large‐scale collapse irrespective of the perturbation details. It is concluded that the global magnetospheric substorm problem has now largely been solved and that future work should be directed toward understanding the detailed plasma physical processes that occur during substorms.
Observation of nonlinear interactions in large‐scale density enhancements of the high‐latitude ionosphere
Authors: Masson Arnaud , Lefeuvre François , Zhao Zheng Y. , Lagoutte Dominique , Rauch Jean‐Louis
Published: September 2002

Simultaneous measurements of electron density and ELF electric field by the AUREOL‐3 satellite have allowed Cerisier et al. [1985] to point out the existence of large‐scale density irregularities in the high‐latitude topside F region and to establish the role of the gradient drift instability in the plasma structuring. High‐resolution spectra of the density and field fluctuations show the presence of low‐frequency peaks linked by relationships such that ƒm + ƒn = ƒm+n. Bispectral analyses point out nonlinear interactions between waves at 7.8 and 15.6 Hz and density irregularities at 5.85 Hz. The consistency of the results with regard to instability models is discussed. The existence of an interaction process involving Schumann resonances is suggested.
Energy and pitch angle‐dispersed auroral electrons suggesting a time‐variable, inverted‐V potential structure
Authors: Arnoldy R. L. , Lynch K. A. , Austin J. B. , Kintner P. M.
Published: September 2002

High temporal resolution electron detectors aboard the PHAZE II rocket flight have shown that the energy‐dispersed, field‐aligned bursts (FABs) are time coincident with pitch angle‐dispersed electrons having energies at the maximum voltage of the inverted‐V potential. This modulation of the energetic inverted‐V electrons is superimposed upon an energy‐diffused background resulting in a peak‐to‐valley ratio of ∼2 for the pitch angle‐dispersed electrons. Since the characteristic energy of the FABs, the order of an eV, is considerably less than that of the plasma sheet electrons (the order of a keV) presumably falling through the inverted‐V potential to create the discrete aurora, the modulation mechanism has to be independent of the electron temperature. The mechanism must accelerate the cold electrons over a range of energies from the inverted‐V energy down to a few tens of eV. It must do this at the same time it is creating a population of hot, pitch angle‐dispersed electrons at the inverted‐V energy. Both the energy dispersion of the FABs and the pitch angle dispersion of the inverted‐V electrons can be used to determine a source height assuming both populations start from the same source region at the same time. These calculations give source heights between 3500 and 5300 km for various events and disagreement between the two methods the order of 20%, which is within the rather substantial error limits of both calculations. A simple mechanism of providing a common start time for both populations of electrons would be a turning on/off of a spatially limited (vertically), inverted‐V potential. The energy‐dispersed FABs can be reconstructed at rocket altitudes if one assumes that cold electrons are accelerated to an energy determined by how much of the inverted‐V potential they fall through when it is turned on. Similarly, the pitch angle‐dispersed, inverted‐V electrons can be modeled at rocket altitudes if one assumes that the plasma sheet electrons falling through the entire potential drop all start to do so at the same time when the potential is turned on. The FABs seem to fluctuate at either ∼10 Hz or near 100 Hz. An important constraint of the on/off mechanism is whether cold electrons (1 eV) can fill the inverted‐V volume during the off cycle. The maximum vertical height of the 10 kV potential region for the 10 Hz events would be the order of 100 and 10 km for the 100 Hz events. To get 10 kV, these heights require parallel electric fields of 0.1 and 1 V/m respectively for the 10 and 100 Hz events assuming that the filling is along B from below the inverted‐V potential. Alternative mechanisms are also discussed in the light of the data presented.
Hybrid Input Algorithm: An event‐oriented magnetospheric model
Authors: Kubyshkina Marina V. , Sergeev Victor A. , Pulkkinen Tuija I.
Published: September 2002

We introduce and test a new approach suitable to model the magnetotail configuration during individual events. This Hybrid Input Algorithm (HIA) uses, in addition to spacecraft magnetic measurements in the tail, other complementary information (in this version, the isotropic boundaries of energetic particles observed at low altitudes), therefore increasing the amount of input data for the modeling and making models more accurate for mapping purposes as well as for evaluation of the current sheet thickness and current density. We test the HIA on two previously well‐studied and modeled events, compare different models as well as the model‐based and observation‐based estimates for the plasma pressure and current density, and discuss the uncertainty in the resulting mapping. We apply the HIA‐based models to evaluate the location of substorm onset in the tail during Coordinated Data Analysis Workshop 6A (CDAW‐6A) substorm event and the characteristics of the current sheet at 7–15 RE just before the substorm onset obtained for several other events. We found moderate values for the maximal current densities in the thin current sheet region (10–35 nA/m2) and the minimal current sheet half thickness between 0.1 and 0.7 RE. The X scale of the thinning region was sim 5 RE. By assembling the modeling results for many events we found that the maximum current densities in the tail current sheet could be effectively predicted by taking into account the observed positions of isotropic boundaries and dipole tilt angle. We also briefly discuss possible future extension of the Hybrid Input Algorithm.