bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanism and rate of denitrification in an agricultural watershed: Electron and mass balance along groundwater flow paths
Authors: Tesoriero Anthony J. , Liebscher Hugh , Cox Stephen E.
Published: September 2002

The rate and mechanism of nitrate removal along and between groundwater flow paths were investigated using a series of well nests screened in an unconfined sand and gravel aquifer. Intensive agricultural activity in this area has resulted in nitrate concentrations in groundwater often exceeding drinking water standards. Both the extent and rate of denitrification varied depending on the groundwater flow path. While little or no denitrification occurred in much of the upland portions of the aquifer, a gradual redox gradient is observed as aerobic upland groundwater moves deeper in the aquifer. In contrast, a sharp shallow redox gradient is observed adjacent to a third‐order stream as aerobic groundwater enters reduced sediments. An essentially complete loss of nitrate concurrent with increases in excess N2provide evidence that denitrification occurs as groundwater enters this zone. Electron and mass balance calculations suggest that iron sulfide (e.g., pyrite) oxidation is the primary source of electrons for denitrification. Denitrification rate estimates were based on mass balance calculations using nitrate and excess N2coupled with groundwater travel times. Travel times were determined using a groundwater flow model and were constrained by chlorofluorocarbon‐based age dates. Denitrification rates were found to vary considerably between the two areas where denitrification occurs. Denitrification rates in the deep, upland portions of the aquifer were found to range from <0.01 to 0.14 mMof N per year; rates at the redoxcline along the shallow flow path range from 1.0 to 2.7 mMof N per year. Potential denitrification rates in groundwater adjacent to the stream may be much faster, with rates up to 140 mMper year based on an in situ experiment conducted in this zone.
On the calibration and verification of two‐dimensional, distributed, Hortonian, continuous watershed models
Authors: Senarath Sharika U. S. , Ogden Fred L. , Downer Charles W. , Sharif Hatim O.
Published: September 2002

Physically based, two‐dimensional, distributed parameter Hortonian hydrologic models are sensitive to a number of spatially varied parameters and inputs and are particularly sensitive to the initial soil moisture field. However, soil moisture data are generally unavailable for most catchments. Given an erroneous initial soil moisture field, single‐event calibrations are easily achieved using different combinations of model parameters, including physically unrealistic values. Verification of single‐event calibrations is very difficult for models of this type because of parameter estimation errors that arise from initial soil moisture field uncertainty. The purpose of this study is to determine if the likelihood of obtaining a verifiable calibration increases when a continuous flow record, consisting of multiple runoff producing events is used for model calibration. The physically based, two‐dimensional, distributed, Hortonian hydrologic model CASC2D [Julien et al., 1995] is converted to a continuous formulation that simulates the temporal evolution of soil moisture between rainfall events. Calibration is performed using 6 weeks of record from the 21.3 km 2 Goodwin Creek Experimental Watershed, located in northern Mississippi. Model parameters are assigned based on soil textures, land use/land cover maps, and a combination of both. The sensitivity of the new model formulation to parameter variation is evaluated. Calibration is performed using the shuffled complex evolution method [Duan et al., 1991]. Three different tests are conducted to evaluate model performance based on continuous calibration. Results show that calibration on a continuous basis significantly improves model performance for periods, or subcatchments, not used in calibration and the likelihood of obtaining realistic simulations of spatially varied catchment dynamics. The automated calibration reveals that the parameter assignment methodology used in this study results in overparameterization. Additional research is needed in spatially distributed hydrologic model parameter assignment methodologies for hydrologic forecasting.
A simple extension of two‐phase characteristic curves to include the dry region
Authors: Webb Stephen W.
Published: September 2002

Two‐phase characteristic curves are necessary for the simulation of water and vapor flow in porous media. Existing functions such as van Genuchten [1980], Brooks and Corey [1966], and Luckner et al. [1989] have significant limitations in the dry region as the liquid saturation goes to zero. This region, which is important in a number of applications, including liquid and vapor flow and vapor‐solid sorption, has been the subject of a number of previous investigations. Most previous studies extended standard capillary pressure curves into the adsorption region to zero water content and required a refitting of the revised curves to the data. In contrast, the present method provides for a simple extension of existing capillary pressure curves without the need to refit the experimental data. Therefore previous curve fits can be used, and the transition between the existing fit and the relationship in the adsorption region is easily calculated. The data‐model comparison shows good agreement. This extension is a simple and convenient way to extend existing curves to the dry region.
The transport of plasma sheet material from the distant tail to geosynchronous orbit
Authors: Borovsky Joseph E. , Thomsen Michelle F. , Elphic Richard C. , Cayton Thomas E. , McComas David J.
Published: September 2002

Several aspects of mass transport in the Earth's plasma sheet are examined. The evolution of plasma sheet material as it moves earthward is examined by statistically comparing plasma sheet properties at three different downtail distances: near‐Earth plasma sheet properties obtained from measurements by 1989‐046 near the geomagnetic equator near midnight at 6.6 RE, midtail plasma sheet properties obtained from ISEE 2 measurements during 333 encounters with the neutral sheet, and distant‐plasma sheet properties obtained from ISEE 2 measurements during 53 encounters with the interface between the plasma sheet and the plasma sheet boundary layer. Examination of the evolution of the plasma sheet through pressure‐density space shows that the transport is nearly adiabatic (γ = 1.52), with a loss of entropy observed in the near‐Earth region. The estimated pressure loss from the plasma‐sheet associated with the aurora is able to account for the observed decrease in entropy. The near‐Earth plasma sheet plasma is also found to be compressed much less than would be expected from magnetic field models. Examination of the evolution of the plasma sheet through density‐flux tube‐volume space (with the aid of the T89c magnetic field model) indicates that there is a substantial loss of mass from plasma sheet flux tubes. Global magnetic reconnection during substorms and patchy reconnection at other times is invoked to account (1) for the required mass loss, (2) for the related lack of compression, and (3) for an observed disconnection between ionospheric convection and plasma sheet convection. This reconnection must occur closer than 20 RE downtail. Selective transport is examined by statistically analyzing the ISEE 2 neutral sheet crossing data set: strong transport is found to be associated with low densities, with weak Bz, and with large flux tube volume. A correlation between the direction of the flow in the plasma sheet and the solar wind velocity indicates that earth‐ward transport is stronger when the solar wind velocity is lower. An examination of near‐Earth and of midtail plasma sheet densities, temperatures, and entropies shows that the plasma sheet is usually spatially homogeneous, contrary to a “bubbles and blobs” picture of transport. Several new points of view about plasma sheet transport are discussed, including the dominant role of near‐Earth reconnection, the importance of auroral zone pressure loss, the control of the plasma sheet properties by the density and speed of the solar wind, and the disconnection of the ionospheric and plasma sheet flow patterns.
The sub‐Alfvénic interaction of the Galilean satellites with the Jovian magnetosphere
Authors: Neubauer Fritz M.
Published: September 2002

Recent observations by the Galileo spacecraft and Earth‐based techniques have motivated us to reconsider the sub‐Alfvénic interaction between the Galilean satellites of Jupiter and the magnetosphere. (1) We show that the atomic processes causing the interaction between the magnetoplasma and a neutral atmosphere can be described by generalized collision frequencies with contributions from elastic collisions, ion pickup, etc. Thus there is no fundamental difference in the effect of these processes on the plasma dynamics claimed in the recent literature. For a magnetic field configuration including possible internal fields, we show that the sub‐Alfvénic, low‐beta interaction can be described by an anisotropically conducting atmosphere joined to an Alfvén wing as one extreme case and the Jovian ionosphere as the other extreme case. (2) The addition of a small magnetic field of internal origin does not modify the general Alfvén wing model qualitatively but only quantitatively. All magnetic moments discussed in the literature for Io are small in this sense. For an aligned internal dipole and ambient Jovian magnetic field the interaction will be enhanced by focusing of the electric field. (3) A qualitative change occurs by the additional occurrence of closed magnetic field lines for larger internal magnetic fields as in the case of Ganymede. Here the focusing is even enhanced. (4) The first discussion of nonstationary plasma flows at the satellites shows that electromagnetically induced magnetic fields may play an important role if the satellite interiors are highly conducting. From the point of view of the external excitation, induction effects may be strong for Callisto, Io, Europa, and Ganymede in order of decreasing importance. The magnetic field observations at the first Callisto encounter can be explained by these effects.
Magma production and growth of the lava dome of the Soufriere Hills Volcano, Montserrat, West Indies: November 1995 to December 1997
Authors: Sparks R. S. J. , Young S. R. , Barclay J. , Calder E. S. , Cole P. , Darroux B. , Davies M. A. , Druitt T. H. , Harford C. , Herd R. , James M. , Lejeune A. M. , Loughlin S. , Norton G. , Skerrit G. , Stasiuk M. V. , Stevens N. S. , Toothill J. , Wadge G. , Watts R.
Published: September 2002

From November 1995 to December 1997 a total volume of 246 × 106 (DRE) m³ of andesite magma erupted, partitioned into 93 × 106 m³ of the dome, 125 × 106 m³ of pyroclastic flow deposits and 28 × 106 m³ of explosive ejecta. In the first 11 weeks magma discharge rate was low (0.5 m³/s). From February 1996 to May 1997 discharge rates have averaged 2.1 m³/s, but have fluctuated significantly and have increased with time. Three pulses lasting a few months can be recognised with discharge rates reaching 3 to 8 m³/s. Short term pulsations in growth lasting a few days reach discharge rates of over 10 m³/s and there are periods of days to a few weeks when dome growth is < 0.5 m³/s. Discharge rate increased from May 1997 with an average rate of 7.5 m³/s to December 1997. The observations indicate an open magmatic system.
Experimental phase equilibria constraints on pre‐eruptive storage conditions of the Soufriere Hills magma
Authors: Barclay J. , Rutherford M. J. , Carroll M. R. , Murphy M. D. , Devine J. D. , Gardner J. , Sparks R. S. J.
Published: September 2002

New experimental results are used to constrain the P, T, X(H2O) conditions of the Soufriere Hills magma prior to ascent and eruption. The experiments were performed on a powdered andesite erupted in January, 1996, at an fO2 corresponding to ∼NNO+1 with PH2O and temperatures in the range 50 to 200 MPa and 800 to 940°C. Amphibole is stable at PH2O >115 MPa and temperatures <875°C. Quartz only becomes stable at low temperatures and after high degrees of crystallization (T <840°C, >72 wt% SiO2 in residual melt) at PH2O >115 MPa. Analyses of rhyolitic glass inclusions in quartz and plagioclase from recently erupted samples indicate melt water contents of 4.27±0.54 wt% H2O and CO2 contents <60 ppm. The evolved Soufriere Hills magma would therefore be H2O‐saturated at pressures <130 MPa.

These results suggest that the Soufriere Hills magma containing the stable assemblage amphibole, quartz, plagioclase, orthopyroxene, magnetite and ilmenite was stored at PH2O of 115‐130 MPa, equivalent to a minimum depth for a water‐saturated magma chamber of 5–6 km depth. Magma temperatures were initially low (820–840°C). Quartz is believed to have been destabilised by a heating event involving injection of new basaltic magma. The stability field of hornblende provides a useful upper limit (∼880°C) for the extent of this reheating.
Soufrière Hills Eruption, Montserrat, 1995–1997: Volcanic earthquake locations and fault plane solutions
Authors: Aspinall W. P. , Miller A. D. , Lynch L. L. , Latchman J. L. , Stewart R. C. , White R. A. , Power J. A.
Published: September 2002

A total of 9242 seismic events, recorded since the start of the eruption on Montserrat in July 1995, have been uniformly relocated with station travel‐time corrections. Early seismicity was generally diffuse under southern Montserrat, and mostly restricted to depths less than 7 km. However, a NE‐SW alignment of epicentres beneath the NE flank of the volcano emerged in one swarm of volcano‐tectonic earthquakes (VTs) and later nests of VT hypocentres developed beneath the volcano and at a separated location, under St. George's Hill. The overall spatial distribution of hypocentres suggests a minimum depth of about 5 km for any substantial magma body. Activity associated with the opening of a conduit to the surface became increasingly shallow, with foci concentrated below the crater and, after dome building started in Fall 1995, VTs diminished and repetitive swarms of ‘hybrid’ seismic events became predominant. By late‐1996, as magma effusion rates escalated, most seismic events were originating within a volume about 2 km diameter which extended up to the surface from only about 3 km depth ‐ the diminution of shear failure earthquakes suggests the pathway for magma discharge had become effectively unconstricted. Individual and composite fault plane solutions have been determined for a few larger earthquakes. We postulate that localised extensional stress conditions near the linear VT activity, due to interaction with stresses in the overriding lithospheric plate, may encourage normal fault growth and promote sector weaknesses in the volcano.
Remarkable cyclic ground deformation monitored in real‐time on Montserrat, and its use in eruption forecasting
Authors: Voight B. , Hoblitt R. P. , Clarke A. B. , Lockhart A. B. , Miller A. D. , Lynch L. , McMahon J.
Published: September 2002

Telemetered high‐resolution tiltmeters were installed in Montserrat in summer of 1995, in December 1996, and in May 1997. The 1995 installations, several km from the Soufriere Hills vent, were too distant to yield useful data. However, the 1996 and 1997 installations on the crater rim revealed 6–14 h inflation cycles caused by magma pressurization at shallow depths (< 0.6 km below the base of dome). The tilt data correlated with seismicity, explosions, and pyroclastic flow activity, and were used to forecast times of increased volcanic hazard to protect scientific field workers and the general public.
On the sodium tail of comet Hale‐Bopp (C/1995 O1)
Authors: Kupperman D. G. , Paxton L. J. , Carbary J. , Romick G. J. , Anderson D. E. , Meng C.‐I. , Feldman P. D.
Published: September 2002

Observations of the sodium tail of comet Hale‐Bopp (C/ 1995 O1) were made with the Midcourse Space Experiment (MSX) satellite near perihelion. Observations of the dust continuum, H2O+ emissions, and sodium tail are reported. The measurements show the Na tail to be diffuse and located between the dust and ion tails. The midrange spatial scale of the MSX measurements (104 to 106 km) is complementary to the high spatial resolution measurements of Brown et al. (submitted to Icarus, 1997) and large‐scale measurements of Cremonese et al. [1997], less than 104 km and greater than 3.5×106 km, respectively. The primary Na source appears to be from the inner coma and nucleus region. The observations are fit with a sodium point source model affected only by radiation pressure to obtain excellent agreement.
17O/16O and 18O/16O isotope measurements of atmospheric carbon monoxide and its sources
Authors: Huff Amy K. , Thiemens Mark H.
Published: September 2002

Simultaneous 17O/16O and 18O/16O measurements of atmospheric carbon monoxide (CO) and two of its major sources are presented. CO was collected in situ at ground level in La Jolla, CA, Pasadena, CA and White Mountain, CA, altitude 3.8 km. Samples were also taken directly from engine exhaust and during a local wild brushfire. Results indicate that atmospheric carbon monoxide has a significant mass independent component, which increases as carbon monoxide concentrations decrease. The origin of the observed mass independent fractionation is discussed. CO samples from engine exhaust and brushfires are clearly mass dependent, but with a wide variation in 18O values from different engines.
On the effect of the mean shear wind in the predicted “universal” spectral shape of the atmospheric variables
Authors: Giraldez Alberto , de la Torre Alejandro
Published: September 2002

The Doppler shift undergone by a single quasi‐sinusoidal wave in a constant, stratified, mean shear background atmosphere is analytically and numerically considered. This wave does not encounter critical levels for the elementary cases here considered. The idealized picture intends to illustrate the “zeroth order” effect of a mean shear wind profile on the resulting shape of temperature and wind spectra. The trial of different vertical shear structures suggests that only the linear or quasilinear wind profiles are able to reproduce adequately the experimental spectral slopes. For a linear shear and different incident wavelengths, slope values near to −3 are found and the calculated spectra close resemble those observed from atmospheric data.
Variability of the ring current source population
Authors: Thomsen M. F. , Borovsky J. E. , McComas D. J. , Collier M. R.
Published: September 2002

The relationship between the strength of the storm‐time ring current and the available density in the presumed source region, the plasma sheet, is examined for 23 storms with Dst* (minimum Dst corrected for magnetopause currents) ranging from ‐50 to ‐164 nT. A good correlation is found between the plasma‐sheet density at geosynchronous orbit and the minimum Dst*. The minimum Dst* is also well correlated with the eastward interplanetary electric field measured upstream by the Wind spacecraft, in agreement with previous studies, but the dependence of Dst* on plasma‐sheet density is unrelated to its dependence on the electric field. The best correlation is between the minimum Dst* and the product of the plasma‐sheet density and the eastward interplanetary electric field. These results are consistent with a scenario in which the intensity of the storm‐time ring current is determined by a combination of source strength (plasma‐sheet density) and injection strength (interplanetary electric field).
Rapid subsidence over oil fields measured by SAR interferometry
Authors: Fielding Eric J. , Blom Ronald G. , Goldstein Richard M.
Published: September 2002

The Lost Hills and Belridge oilfields are in the San Joaquin Valley, California. The major oil reservoir is high porosity and low permeability diatomite. Extraction of large volumes from shallow depths causes reduction in pore pressure and subsequent compaction, forming a surface subsidence bowl. We measure this subsidence from space using interferometric analysis of SAR (Synthetic Aperture Radar) data collected by the European Space Agency Remote Sensing Satellites (ERS‐1 and ERS‐2). Maximum subsidence rates are as high as 40 mm in 35 days or >400 mm/yr, measured from interferograms with time separations ranging from one day to 26 months. The 8‐ and 26‐month interferograms contain areas where the subsidence gradient exceeds the measurement possible with ERS SAR, but shows increased detail in areas of less rapid subsidence. Synoptic mapping of subsidence distribution from satellite data powerfully complements ground‐based techniques, permits measurements where access is difficult, and aids identification of underlying causes.
Intense low latitude VLF emissions observed aboard Ariel 4
Authors: Singh U. P. , Singh D. P.
Published: September 2002

Analysis of the peak intensity of low‐latitude VLF emissions observed aboard the Ariel satellite reveal that some north zone 3.2 kHz VLF emissions are unexpectedly more intense than south zone VLF emissions. In fact, the north zone intensity peaks are 5–10 dB higher than those in the south zone. This discrepancy in intensity peaks can be explained in terms of wave growth during transverse resonant interaction between the wave and counterstreaming energetic electrons. It is argued that VLF signals observed on board the Ariel satellite were generated in the south zone during lightning discharges and propagated to equatorial heights in the whistler mode. At the equator, the emissions are amplified by taking energy from energetic electrons, and then these amplified emissions, again propagating in the whistler mode, arrive at the Ariel satellite in the north zone. Thus the intense signals in the south zone are preamplified, and in the north zone, these are observed after amplification. This explains the discrepancy in the observations of intensity. Analysis of VLF satellite data reveals that VLF emissions have high spectral densities in the longitudinal range of 100°–140°E. These phenomena of VLF emissions are explained through the effect of the South Pacific Magnetic Anomaly (SPMA). It is found that atmospheric scattering, enhancing precipitated fluxes of energetic electrons in the South Atlantic Magnetic Anomaly (SAMA) region, has no role in VLF wave amplification.
Numerical simulations and simplified models of nonlinear electron inertial Alfvén waves
Authors: Rankin R. , Tikhonchuk V. T.
Published: September 2002

We describe nonlinear resonance absorption of compressional Alfvén waves in a model magnetosphere. It is shown that the ponderomotive force of excited standing shear Alfvén waves can lead to nonlinear saturation and spatial structuring of field line resonances and that in low‐beta plasmas ponderomotive saturation may occur before other nonlinear saturation processes such as the Kelvin‐Helmholtz instability. The effects of finite electron inertia are also considered, and it is shown that spatial structuring of field line resonances may occur with scale sizes compatible with those found in discrete auroral arcs. Results are discussed in the context of three‐dimensional numerical solutions to the resistive magnetohydrodynamic equations and based on a simplified analytical model of nonlinear resonance absorption.
Large‐scale imaging of high‐latitude convection with Super Dual Auroral Radar Network HF radar observations
Authors: Ruohoniemi J. M. , Baker K. B.
Published: September 2002

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide measurements of the E × B drift of ionospheric plasma over extended regions of the high‐latitude ionosphere. With the recent augmentation of the northern hemisphere component to six radars, a sizable fraction of the entire convection zone (approximately one‐third) can be imaged nearly instantaneously (∼2 min). To date, the two‐dimensional convection velocity has been mapped by combining line‐of‐sight velocity measurements obtained from pairs of radars within common‐volume areas. We describe a new method of deriving large‐scale convection maps based on all the available velocity data. The measurements are used to determine a solution for the distribution of electrostatic potential, Φ, expressed as a series expansion in spherical harmonics. The addition of data from a statistical model constrains the solution in regions of no data coverage. For low‐order expansions the results provide a gross characterization of the global convection. We discuss the processing of the radar velocity data, the factors that condition the fitting, and the reliability of the results. We present examples of imaging that demonstrate the response of the global convection to variations in the interplanetary magnetic field (IMF). In the case of a sudden polarity change from northward to southward IMF, the convection is seen to reconfigure globally on very short (<6 min) timescales.
Dstindex prediction using data‐derived analogues of the magnetospheric dynamics
Authors: Klimas A. J. , Vassiliadis D. , Baker D. N.
Published: September 2002

The method ofKlimas et al.[1997] for constructing dynamical analogues of physical input‐output systems is generalized. Higher‐order analogues with sensitivity to more features of the input data are derived. Solar windVBsparameter data are used for input andDstindex data are used for output to construct analogues of the magnetospheric dynamics responsible forDststorms. A detailed study of the dynamics involved in a single storm is presented. It is shown that the relationship betweenVBsinput andDstoutput for this storm can be described in the context of the model ofBurton et al.[1975] but with variable decay time and strength of coupling to the solar windVBsparameter. During the storm recovery it is found that the decay time varies from ≈ 4 hours at the storm maximum to ≈ 20 hours midway in the recovery and then back to ≈ 10 hours. There appears to be nothing in the simultaneous solar wind data to explain this reversal in the evolution of the decay time. It is shown that the strength of coupling to the solar windVBsparameter varies considerably. The coupling strength peaks strongly at the time of the storm maximum and decays to low values during the storm recovery. Solar windVBsinput during the storm recovery does not affect the recovery rate. Using this storm data, empirical nonlinear analogues are constructed. These analogues are tested out of sample for their prediction effectiveness. Comparisons with the predictions of the model due to Burton et al. are given. It is shown that these analogues are promising prediction tools, but their lack of sensitivity to solar wind dynamic pressure must be corrected.
Relativistic theory of wave‐particle resonant diffusion with application to electron acceleration in the magnetosphere
Authors: Summers Danny , Thorne Richard M. , Xiao Fuliang
Published: September 2002

Resonant diffusion curves for electron cyclotron resonance with field‐aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single‐ion plasma, and a numerical scheme is developed for the more realistic case of a multi‐ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth's magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler (R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere.
Ionospheric storm simulations driven by magnetospheric MHD and by empirical models with data comparisons
Authors: Sojka J. J. , Schunk R. W. , Bowline M. D. , Chen J. , Slinker S. , Fedder J. , Sultan P. J.
Published: September 2002

The results of two ionospheric simulations are compared with each other and with ionospheric observations of the southern hemisphere for the magnetic cloud passage event of January 14, 1988. For most of the event one simulation agrees with observations, while the other does not. Electric fields and electron precipitation patterns generated by a magnetospheric MHD model are used as inputs to a physical model of the ionosphere in the successful simulation, while empirical electric fields and electron precipitation are used as the inputs for the second simulation. In spite of ionospheric summer conditions a large and deep polar hole is developed. This is seen in the in situ plasma observations made by the DMSP‐F8 satellite. The hole is surprisingly present during both northward and southward IMF conditions. It is deepest for the storm phase of the southward IMF period. A well‐defined tongue of ionization is formed during this period. These features have been reproduced by the TDIM‐MHD simulation and to a lesser extent by the TDIM‐empirical simulation. However, the model simulations have not been able to generate a storm enhanced density where one was observed by DMSP‐F8 during the initial phase of the storm. The differences between the two F region ionospheric simulations are attributed to differences in the magnetospheric electric fields and precipitation patterns used as inputs. This study provides a unique first simulation of the ionosphere's response to self‐consistent electric field and auroral precipitation patterns over a 24‐hour period that leads into a major geomagnetic storm.
Cyclotron amplification of whistler waves by electron beams in an inhomogeneous magnetic field
Authors: Hobara Y. , Trakhtengerts V. Y. , Demekhov A. G. , Hayakawa M.
Published: September 2002

Cyclotron wave‐particle interactions in the case of well‐organized distributions of energetic electrons under an inhomogeneous magnetic field are studied. Step‐like and δ function distributions over the field‐aligned velocity are considered. The one‐hop amplification of whistler waves is calculated by simple analytical solution and numerical computation based on strict approach. The strict consideration, taking into account third‐order expansion of the spatial dependence of the electron phase with respect to the wave, reveals sonic new important features of the one‐hop amplification Γ as a function of frequency and electron beam parameters. The main result is that Γ exhibits a quasi‐periodic structure as a function of wave frequency or characteristic electron parallel velocity, remaining always positive in the case of the step‐like distribution but being sign alternative for δ‐function. Dependence of Γ on the parameters of energetic electrons such as their total energy, characteristic parallel velocity, position of the injection point in relation to the equator, and dispersion in parallel velocity is discussed.
Predicted electron precipitation during inward convection driven by the cross‐tail electric field in the near‐Earth plasma sheet
Authors: Calvert W.
Published: September 2002

The electron precipitation that is caused by inward convection driven by the cross‐tail electric field is found to be insufficient to account for the aurora during a substorm, thereby requiring scattering or diffusion into the loss cone in order to account for the aurora that occurs during substorm expansion. Since it has also been found that discrete arcs can be attributed to scattering into the loss cone inside the auroral electron acceleration region, an auroral substorm can therefore be attributed to inward convection driven by the cross‐tail electric field, followed by scattering into the loss cone inside the auroral electron acceleration region.
Effect of heavy ions on ponderomotive forces due to ion cyclotron waves
Authors: Feygin F. Z. , Pokhotelov O. A. , Pokhotelov D. O. , Mursula K. , Kangas J. , Bräysy T. , Kerttula R.
Published: September 2002

We study ponderomotive effects induced by the electromagnetic ion cyclotron (EMIC) waves in the Pel frequency band (0.2 to 5.0 Hz) in a two‐ion (H+ and one heavy ion, e.g., He+) plasma. Near the dayside boundary of the magnetosphere, the ponderomotive forces lead to a noticeable accumulation of cold plasma along the field line in two regions of minimum magnetic field intensity located symmetrically around the equator. In the inner magnetosphere, one maximum of cold plasma at the equator is found. At frequencies less than the heavy ion gyrofrequency, the accumulation of cold plasma increases with increasing heavy ion concentration. At frequencies above the heavy ion gyrofrequency, the ponderomotive forces due to EMIC waves are enhanced because of a resonance at the stop band frequencies. We investigate the stop band structure of EMIC waves in a nondipolar magnetosphere and discuss the properties of wave propagation along the field line for different heavy ion plasma concentrations.
Magnetotail structures in a laboratory magnetosphere
Authors: Yur G. , Chang T.‐F. , Rahman H. U. , Birn J. , Chao C. K.
Published: September 2002

The structure of the magnetotail is investigated in a laboratory simulated magnetosphere. This study builds upon our previous investigation of the magnetotail [Birn et al., 1992], which was explored magnetotail through models and magnetic field measurements in the simulated magnetosphere. In this paper the magnetic field measurements are complemented by measurements of ion and electron fluxes, plasma temperature and high‐resolution optical images. Plasma fluxes are measured with a double‐sided Faraday cup that measures the ion and electron fluxes of counterstreaming plasma simultaneously in both directions. The overall magnetotail structure is examined with high‐resolution imaging using a gated optical imager and applying digital enhancement to the magnetotail region. These images confirm the structures inferred earlier from the magnetic field measurements, a Y‐type magnetic neutral line for northward interplanetary magnetic field, and the expected X‐type structure for southward IMF.
Magnetopause shape determinations from measured position and estimated flaring angle
Authors: Kawano H. , Petrinec S. M. , Russell C. T. , Higuchi T.
Published: September 2002

In this paper we unify two approaches to determining magnetopause shape and location from observations by fitting one model simultaneously to both the crossing data and the calculated data of magnetopause flaring angle, to achieve more accurate magnetopause shapes. We prepare two magnetopause models, each of which is an ellipsoid from the subsolar point to the position where the distance of the ellipsoid from the X axis is maximum. The model magnetopause beyond this position is a cylinder attached to the edge of the ellipsoid at this location. This choice of shapes makes it easier to compare our results with earlier work which fit ellipsoids to (only) crossing data. In one of our two models, the focus of the ellipsoid lies at the center of the Earth, whereas in the other it lies at a position determined from fits to the data. We use a statistical criterion (called AIC) that lets us objectively determine which model better fits the data. As a result, we find that a model with an off‐center focus better expresses the nature of the observed data. Despite differing mathematical formulations, our model results generally resemble those of Petrinec and Russell [1996]. Consequently, the mathematical expression is not the major reason why these models flare more (less) than the model of Roelof and Sibeck [1993] for very small (large) solar wind dynamic pressure.
A study of tomographically reconstructed ionospheric images during a solar eclipse
Authors: Huang C. R. , Liu C. H. , Yeh K. C. , Lin K. H. , Tsai W. H. , Yeh H. C. , Liu J. Y.
Published: September 2002

The low‐latitude ionospheric tomography network (LITN) consists of a chain of six Naval Navigation Satellite System (NNSS) receiving stations established along 121°E longitude from a geographic latitude of 14.6°N to 31°N. It is specifically designed to observe large‐scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques. Recently, the network LITN was applied to observations of the October 24, 1995, solar eclipse. Two‐dimensional images of ionospheric electron density during the eclipse period were reconstructed. These images and the corresponding results from a nearby ionosonde were compared with those for a reference day. It is shown that during the eclipse day the ionosphere experienced some large‐scale changes. In particular, four episodes of electron density enhancement or depression have been identified. (1) The maximum enhancement occurred before the maximum phase of the solar eclipse at approximately 7°–10°N geomagnetic latitude at the 275–300 km ionospheric height. (2) The second enhancement appeared roughly 3 1/2 hours after the maximum obscuration at 15°–22°N geomagnetic latitude and 300–325 km ionospheric height. (3) The largest electron density depression occurred roughly 2 hours after the maximum obscuration at approximately 9°–15°N geomagnetic latitude and on both the bottom and topside ionosphere. (4) The second depression occurred about 4 hours after the maximum obscuration at approximately 5.5°N geomagnetic latitude and mainly on the topside ionosphere. More detailed study suggests that the two enhancements have their origins in the ionospheric day‐to‐day variations, the first depression is related to the combined photochemical and the equatorial fountain effects, and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres. These observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region.
Regional, scale size, and interplanetary magnetic field variability of magnetic field and ion drift structures in the high‐latitude ionosphere
Authors: Keady John P. , Heelis R. A.
Published: September 2002

Using data from the Dynamics Explorer 2 satellite's ion drift meter and magnetometer, we have examined fluctuations in the high‐latitude velocity and magnetic field structure to better understand the coupling between the magnetosphere and ionosphere. Our study examines perturbations in the frequency range from 0.25 to 8 Hz, equivalent to static scale sizes between about 30 and 1 km. Seasonal variations in the perturbations at large‐scale sizes indicate that magnetosphere‐ionosphere coupling is dominated by static currents. These static currents display behavior consistent with a current driver in the auroral zone and a voltage driver in the cusp. In the frequency range 0.6 to 1.5 Hz the magnetosphere‐ionosphere coupling is significantly influenced by Alfven waves with the high‐frequency range extending up to 8 Hz in the cusp.
Emission of a bounded nonmonoenergetic electron beam injected in the ionosphere
Authors: Lundin B. , Kraft C.
Published: September 2002

The amplitude of the electromagnetic field radiated (as an imposed antenna current) by a semi‐infinite rarefied nonmonoenergetic electron beam artificially injected from a satellite into the Earth ionosphere is calculated far from the injection point (near the remote receiver) by the use of a two‐dimensional Stationary Phase Point method. The Stationary Phase Point, which gives the main contribution to the radiated field near the remote receiver, is defined by two variables: the parallel beam velocity u and the frequency ω of the radiated signal. The size of its vicinity, {δu, δω}2D, governing the wave packet amplitude far from the beam, is inversely proportional to the transverse distance of the receiver to the beam axis. A qualitative scheme of formation of the wave field near the remote receiver is discussed for different limiting cases on the level of beam monoenergeticity and spectral characteristics of the remote receiver.
Simulation of off‐equatorial ring current ion spectra measured by Polar for a moderate storm at solar minimum
Authors: Jordanova V. K. , Farrugia C. J. , Quinn J. M. , Torbert R. B. , Borovsky J. E. , Sheldon R. B. , Peterson W. K.
Published: September 2002

We use our kinetic drift‐loss model to study ring current dynamics during moderate geomagnetic activity at the minimum phase of the current solar cycle, on March 20–22, 1996. Interplanetary conditions monitored by Wind during this period show fluctuations about zero in the (GSM) Bz component, which are due to Alfvén waves in a stream‐stream interaction region. The corresponding Dst index shows general activity punctuated by 2 moderate storms (min Dst ≈−50 nT and ≈−70 nT, respectively) of ∼15 hours duration each. We calculate the off‐equatorial ring current proton spectra in the energy range 100 eV to 300 keV and compare them with measurements provided by the HYDRA, TIMAS, and CEPPAD instruments on the Polar spacecraft at MLT≈2 and MLT≈14. We find reasonable overall agreement. An eastward offset of 3 hours in the Kp‐dependent Volland‐Stern convection electric field with γ=2 is necessary in order to match modeled with observed dips in the energy spectra. Collisional losses have larger effect on the postnoon spectra and reduce significantly the low‐energy proton distribution at low L shells and at higher magnetic latitudes. These losses reach maximum values for slowly drifting (∼2–10 keV) particles. Our study suggests that a more realistic magnetospheric electric field and additional diffusion processes should be considered to reduce the overestimated width and depth of the dip in the modeled dayside spectra.
Observation of neutron bursts associated with atmospheric lightning discharge
Authors: Shyam A. , Kaushik T. C.
Published: September 2002

Neutron bursts with magnitude up to 109 were observed to be associated with atmospheric lightning discharge.
Topography on the 410‐km seismic velocity discontinuity near subduction zones from stacking of sS, sP, and pP precursors
Authors: Flanagan Megan P. , Shearer Peter M.
Published: September 2002

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near‐source, underside reflections off the 410‐km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high‐quality depth phase arrivals recorded by several long‐period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel‐time curves reveals clear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross‐correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25‐s period, corrected to an oceanic crustal thickness. The lateral resolution of our long‐period stacks for 410 topography is limited compared to that sometimes achieved in short‐period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410‐km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410‐km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long‐period data. Coverage for the Tonga and Peru‐Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the slab orientation as large as 13 km. The cross‐slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.
GPS observations of fault afterslip and upper crustal deformation following the Northridge earthquake
Authors: Donnellan Andrea , Lyzenga Gregory A.
Published: September 2002

Global Positioning System (GPS) observations indicate that significant aseismic deformation occurred in the year following the January 17, 1994, Northridge earthquake. The postseismic observations show the same sense of motion as the coseismic. Compared with the coseismic displacements, the far‐field postseismic motions (1–2 fault dimensions away) are proportionally larger than those seen in the near field. The postseismic data are best modeled with two faults: one on the rupture plane and one located in the shallow crust. The upper crustal fault may represent an actual fault or may be indicative of viscous relaxation occurring in the upper crust. The inferred afterslip and/or relaxation moment is approximately 2.9×1018 N m or 22% of the mainshock moment release. We expect that the moment release due to the afterslip and relaxation effectively reduce the earthquake hazard locally. It is not clear from this study how the postseismic deformation loads the surrounding faults or alters the state of stress on those faults.
Structure of an island‐arc: Wide‐angle seismic studies in the eastern Aleutian Islands, Alaska
Authors: Fliedner Moritz M. , Klemperer Simon L.
Published: September 2002

New seismic wide‐angle data from the eastern Aleutian Islands show a mafic composition and a 30‐km‐thick island‐arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three‐dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities of 7.2–7.4 km/s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the arc, composed mainly of metabasalts, diorite and diabase in the upper crust, and garnet‐granulite or amphibolite‐to‐hornblendite in the lower crust. Reflected arrivals from the subducting Pacific plate at depths of 45–55 km beneath the fore‐arc, together with Pn, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 km/s with an average of about 7.7 km/s. A mantle composition that grades from mainly pyroxenite (probably ultramafic cumulates) near the Moho to dunite at greater depths best explains these observed velocities.