bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy dependence of the ionic charge state distribution during the November 1997 solar energetic particle event
Authors: Möbius E. , Popecki M. , Klecker B. , Kistler L. M. , Bogdanov A. , Galvin A. B. , Heirtzler D. , Hovestadt D. , Lund E. J. , Morris D. , Schmidt W. K. H.
Published: September 2002

Ionic charge state distributions for a variety of species, such as C, O, Ne, Mg, Si and Fe were obtained with the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) on ACE for the strongest of a series of energetic particle events after the November 4 and 7, 1997, flares. The capabilities of SEPICA allow a much more detailed analysis of the charge distributions than previous instrumentation. Over the energy range from ≈ 0.2 to 1 MeV/Nuc a trend is observed that shows charge states increasing with energy, in particular for Mg, Si and Fe. In addition, for Fe a mixed charge state distribution with a distinct peak at lower charge states (10–14) is observed simultaneously with a tail reaching to charge states up to ≈ 20. This may be an indication of a mixture of different energetic particle populations.
The free core nutation period stays between 431 and 434 sidereal days
Authors: Roosbeek F. , Defraigne P. , Feissel M. , Dehant V.
Published: September 2002

We determine the parameters of the Free Core Nutation (FCN) from the resonance induced in the forced nutations by a least‐squares analysis of VLBI observations. By applying the procedure to consecutive observation series, we deduce the time evolution of the FCN period, and show that it varies only between 431 and 434 sidereal days. These bounds are within the limits of uncertainty arising from noise in the data. We therefore find no evidence for time variation of the FCN period.
Seismic reflections from depths of less than two meters
Authors: Baker Gregory S. , Schmeissner Chris , Steeples Don W. , Plumb Richard G.
Published: September 2002

Three distinct seismic reflections were obtained from within the upper 2.1 m of flood‐plain alluvium in the Arkansas River valley near Great Bend, Kansas. Reflections were observed at depths of 0.63, 1.46, and 2.10 m and confirmed by finite‐difference wave‐equation modeling. The wavefield was densely sampled by placing geophones at 5‐cm intervals, and near‐source nonelastic deformation was minimized by using a very small seismic impulse source. For the reflections to be visible within this shallow range, low seismic P‐wave velocities (<300 m/s) and high dominant‐frequency content of the data (∼450 Hz) were essential. The practical implementation of high‐resolution seismic imaging at these depths has the potential to complement ground‐penetrating radar (GPR), chiefly in areas where materials exhibiting high electrical conductivity, such as clays, prevent the effective use of GPR. Potential applications of these results exist in hydrogeology and environmental, Quaternary, and neotectonic geology.
Formation and gas/particle partitioning of monoterpenes photo‐oxidation products over forests
Authors: Kavouras I. G. , Mihalopoulos N. , Stephanou E. G.
Published: September 2002

By using adequate sampling techniques we studied the chemical pathways, the gas‐to‐particle partition and the conversion processes leading to the formation of organic aerosols via the photo‐oxidation of biogenic hydrocarbons in the atmosphere over a conifer forest. Photo‐oxidation products of monoterpenes such as pinonic acid, nor‐pinonic acid, pinic acid, pinonaldehyde and nopinone were detected in the gas and particulate phases of all forest aerosol samples. Considering the diurnal concentration pattern of the photo‐oxidation products of α‐ and β‐pinene and Aitken nuclei concentration measured during the same periods, we observed that the acidic photo‐oxidation products of monoterpenes play a more important role in the formation of new particles than the corresponding carbonyl compounds.
Permeability‐porosity relationship in vesicular basalts
Authors: Saar Martin O. , Manga Michael
Published: September 2002

The permeability κ and porosity ϕ of vesicular basalts are measured. The relationship between κ and ϕ reflects the formation and emplacement of the basalts and can be related to the crystal and vesicle microstructure obtained by image analysis. Standard theoretical models relating κ and ϕ that work well for granular materials are unsuccessful for vesicular rocks due to the fundamental difference in pore structure. Specifically, κ in vesicular rocks is governed by apertures between bubbles. The difference between calculated and measured κ reflects the small size of these apertures with aperture radii typically O(10) times smaller than the mean bubble radii.
On the frequency distribution of net annual snow accumulation at the South Pole
Authors: van der Veen C. J. , Whillans I. M. , Gow A. J.
Published: September 2002

The frequency distribution of stratigraphic layer thickness in cores and a snowmine at South Pole is not compatible with a significant number (>1%) of missing layers associated with zero‐accumulation years inferred from pole‐height measurements. A reconciliation of these data sets is needed if observed stratigraphic records are to be used as reliable paleoclimate indicators. Three explanations for the discrepancy are offered, namely (i) during a significant number of years, a visible stratigraphic horizon does not form or is not identified, (ii) the true distribution is characterized by two maxima, with a secondary maximum centered around zero layer thickness, or (iii) the pole‐height measurements are misinterpreted and there are very few zero‐accumulation years at South Pole. With the currently available data, it is not possible to discriminate among these three possibilities.
Intercomparison of ATMOS, SAGE II, and ER‐2 Observations in Arctic Vortex and Extra‐Vortex Air Masses during Spring 1993
Authors: Michelsen H. A. , Manney G. L. , Webster C. R. , May R. D. , Gunson M. R. , Baumgardner D. , Kelly K. K. , Loewenstein M. , Podolske J. R. , Proffitt M. H. , Wofsy S. C. , Yue G. K.
Published: September 2002

We have compared measured abundances of NOy, O3, H2O, CH4, HCl, and N2O from ATMOS and aerosol from SAGE II from inside and outside the Arctic vortex with in situ observations from the SPADE aircraft campaign in April/May 1993. When the distinction between vortex and extra‐vortex air is taken into account, SPADE measurements of NOy, O3, H2O, CH4, HCl, and aerosol abundances, correlated with [N2O], demonstrate excellent agreement with ATMOS and SAGE II data. Observations of low [HCl] and high [ClNO3] inside the vortex by ATMOS and low [HCl] in vortex fragments by ALIAS suggest that vortex‐influenced air masses maintained unusually high levels of [ClNO3] throughout spring 1993.
Compressive creep of ice containing a liquid intergranular phase: Rate‐controlling processes in the dislocation creep regime
Authors: De La Chapelle S. , Milsch H. , Castelnau O. , Duval P.
Published: September 2002

Experiments have been conducted to investigate the effect of melt on the creep behavior of polycrystalline ice deformed in the dislocation creep regime. The transition between a mode with a stress exponent n = 3 and a mode with n < 2 is observed for both melt‐free and melt‐added ice samples. The large influence of the melt phase is not related to the wetting characteristics of the liquid. Owing to the large plastic anisotropy of the ice crystal, the liquid phase would attenuate the internal stress field which develops during the primary creep. The contribution to the deformation of the basal slip (the weaker slip system) would increase with the melt content.
Comparisons of Polar satellite observations of solitary wave velocities in the plasma sheet boundary and the high altitude cusp to those in the auroral zone
Authors: Cattell C. A. , Dombeck J. , Wygant J. R. , Hudson M. K. , Mozer F. S. , Temerin M. A. , Peterson W. K. , Kletzing C. A. , Russell C. T. , Pfaff R. F.
Published: September 2002

Characteristics of solitary waves observed by Polar in the high altitude cusp, polar cap and plasma sheet boundary are reported and compared to observations in the auroral zone. The study presented herein shows that, at high altitudes, the solitary waves are positive potential structures (electron holes), with scale sizes of the order of 10's of Debye lengths, which usually propagate with velocities of a few thousand km/s. At the plasma sheet boundary, the direction of propagation can be either upward or downward; whereas at the leading edge of high altitude cusp energetic particle injections, it is downward. For these high altitude events, explanations based on ion modes and on electron modes are both examined, and the electron mode interpretation is shown to be more consistent with observations.
New Particle Formation in the Remote Troposphere: A Comparison of Observations at Various Sites
Authors: Weber R. J. , McMurry P. H. , Mauldin R. L. , Tanner D. J. , Eisele F. L. , Clarke A. D. , Kapustin V. N.
Published: September 2002

Measurements show that new particles are formed by homogenous nucleation over a wide range of conditions in the remote troposphere. In our studies, large nucleation events are found exclusively in regions of enhanced sulfuric acid vapor (H2SO4g) concentrations, with maximum concentrations never exceeding 5×107 molecules cm−3. Although these data suggest that H2SO4g participated, comparisons between ambient conditions in regions of nucleation to conditions necessary for binary H2SO4 water (H2O) nucleation indicate that the mechanism may vary with elevation. In remote marine regions, at altitudes greater than ∼4 km above sea level, observations of nucleation in clear air along cloud perimeters are in fair agreement with current classical binary nucleation models. In these regions, the low temperatures associated with high altitudes may produce sufficiently saturated H2SO4 for the production of new H2SO4/H2O particles. However, uncertainties with current binary nucleation models limit decisive comparisons. In warmer regions, closer to the earth's surface, measured H2SO4 concentrations are clearly insufficient for binary nucleation. Conditions at these sites are similar to those observed in an earlier study where there was circumstantial evidence for a ternary mechanism involving H2SO4, H2O, and ammonia (NH3) [Weber et al., 1998], suggesting that this may be a significant route for particle production at lower altitudes where surface‐derived species, like NH3, are more apt to participate.
Development of auroral streamers in association with localized impulsive injections to the inner magnetotail
Authors: Sergeev V. A. , Liou K. , Meng C. ‐I. , Newell P. T. , Brittnacher M. , Parks G. , Reeves G. D.
Published: September 2002

During continuous magnetospheric activity it is not uncommon to observe narrow (in MLT) transient particle injections (duration about 1–2 minute at E=100 keV and local time extent ≤ 1 hour MLT) in the nightside part of geosynchronous orbit. Using global UV images from POLAR spacecraft we analyze the development of auroral activity on December 22, 1996 during a sequence of such injections observed by two LANL spacecraft. We found that narrow transient injections are associated with specific localized auroral form, the auroral streamer, which develops in this local time sector. The streamer first appear as a bright spot in the poleward part of the double oval ≈2–5 minutes before the geosynchronous plasma injection, and then develops equatorward, reaching in many cases the equatorward boundary of the UV aurora. We interprete the observations as evidence that some high speed flow bursts (BBFs) of small cross‐tail extent (less than 1 h MLT), formed in the distant tail or midtail, can intrude as close to the Earth as the geosynchronous distance before being stopped.
The polar cusp location and its dependence on dipole tilt
Authors: Zhou X.‐W. , Russell C. T. , Le G. , Fuselier S. A. , Scudder J. D.
Published: September 2002

Polar cusp crossings at high altitudes are seen in the POLAR data as decreases in the magnetic field and increases in magnetosheath‐like plasma. Close to 500 polar cusp crossings identified from the magnetic field, low‐energy electron and ion data observed by POLAR, are used to determine the statistical location of the polar cusp. When compared with Tsyganenko's 1989 vacuum model with an ellipsoidal magnetopause [Tsyganenko, 1989], the medians of the cusp crossings are located between the magnetic field lines with invariant latitudes of 80° and 82°. Statistically the shape of the polar cusp in this region is consistent with this model although there is much scatter around the median value. The position of the cusp is significantly dependent on the dipole tilt angle. When dipole tilts more toward the Sun, the cusp moves more poleward to higher invariant latitude from 77.2° at −30° tilt, to 80.0° at 0° tilt, to 81.8° at 30° or roughly 1° for every 14° of tilt.
A neural network approach for modeling the Heat Island phenomenon in urban areas during the summer period
Authors: Santamouris M. , Mihalakakou G. , Papanikolaou N. , Asimakopoulos D. N.
Published: September 2002

The distribution of ambient air temperature in a city and the urban heat island intensity are investigated during the summer period in the major Athens region where ambient air temperature data are recorded at twenty stations. A neural network approach, based on predicted or recorded hourly values, is designed for modeling, predicting and estimating the air temperature at each station. Various feedforward, multiple layered, neural network architectures based on backpropagation algorithm are designed and trained for the stations' temperature prediction and estimation. The results were tested using extensive sets of measurements and it was found that they correspond well with the actual values. Furthermore, each one of the estimated stations is used as one input for the estimation of the next station's temperatures. The results were compared with the measured data and the neural network method was found able to simulate with sufficient accuracy the urban temperature field at several locations in a large urban region during the summer.
ScP; A probe of ultralow velocity zones at the base of the mantle
Authors: Garnero Edward J. , Vidale John E.
Published: September 2002

The core‐reflected phase ScP is a sensitive probe of the properties of an ultralow velocity zone (ULVZ) at the base of the mantle. In synthetic experiments, a ULVZ with large depressions in P and S velocities results in ScP having two diagnostic precursors and one later arrival, whose timing depends on ULVZ VP, VS, and thickness. Amplitudes of the additional arrivals can be large if ULVZ shear velocity has extreme reductions, such as 30% and greater. Some data examples are presented for southwest Pacific events recorded at short period and broadband arrays in Hawaii, which show evidence for a ULVZ. Resolving all three additional arrivals will help in addressing the strength and ratio of VS and VP reductions in ULVZ, and thus the issue of partial melt.
On the excitation of Io's atmosphere by the photoelectrons: Application of the analytical yield spectral model of SO2
Authors: Bhardwaj Anil , Michael Marykutty
Published: September 2002

The analytical yield spectral model is applied to the calculation of photoelectron fluxes, excitation rates, and photoelectron‐excited intensities of several neutral and ionized S and O UV emissions, and SO (240–265 nm) and SO2 (264–430) band emissions in the atmosphere of Io. Most of the OI emission intensity is produced in the e‐O direct collisional‐excitation rather than in the e‐SO2 dissociative excitation process. Using the existing models of Io's atmosphere the calculated brightness of neutral O and S emissions on Io is found to be 2 to 3 orders of magnitude smaller compared to the HST‐observed intensity. We have shown that larger intensities can be produced on increasing the densities of the atmospheric species, particularly the atomic species: a model where O dominates over SO2 above a few tens of km altitude can explain 10–30% of the HST brightness. Our model calculations have demonstrated that it is possible to explain the HST‐observed brightness provided that (a) the Io's atmospheric densities are much larger (>factor of 10) and more extended in altitude than those predicted by the existing models, and (b) there is a mechanism to accelerate the ionospheric photoelectrons to energies ∼100 eV. Otherwise, there should be some other source of the production of these emissions on Io.
Atlantic Ocean baroclinic heat flux at 24 to 26° N
Authors: O'Neil Baringer Molly , Molinari Robert
Published: September 2002

The spatially varying, interior geostrophic baroclinic heat flux component of the total meridional oceanic heat flux near 24°N in the Atlantic Ocean is examined using four transatlantic hydrographic sections including the October 1957 DISCOVERY II IGY section, the September 1981 ATLANTIS section, the August 1992 HESPERIDES section, the February 1998 RONALD H. BROWN section and the 1982 Levitus and the Lozier, Owens, Curry climatologies. The 1992 section is complemented by shorter western boundary sections obtained concurrently during the Trident cruise. We find an average southward baroclinic heat flux of 0.9±0.3 PW with an annual cycle amplitude of ±0.3 PW. More than 90% of the annual cycle is captured within 30° of the western boundary.
Polarization pattern of low‐frequency geomagnetic field fluctuations (0.8–3.6 mHz) at high and low latitude
Authors: Lepidi S. , Francia P. , Villante U. , Lanzerotti L. J. , Meloni A.
Published: September 2002

A statistical analysis of the polarization pattern of low‐frequency geomagnetic field fluctuations (0.8–3.6 mHz) covering the entire 24‐hour interval has been performed at an Antarctic station (Terra Nova Bay) and a low‐latitude station (L'Aquila, Italy) during the entire 1995. The results show a complex pattern in which, in agreement with predictions, four polarization reversals occur at high latitude during the local day. A comparison with another Antarctic station, McMurdo, during a shorter interval in 1994 confirms these results. At low latitude the polarization sense in the afternoon reverses with respect to the morning, but the reversal is delayed by a few hours after the expected local noon. In Antarctica the polarization pattern does not show any dependence on frequency and season, while at L'Aquila it is better defined for frequencies below ≈2 mHz and during local summer.
Effect of ionospheric self‐conditioning and preconditioning on the broad upshifted maximum component of stimulated electromagnetic emission
Authors: Wagner L. S. , Bernhardt P. A. , Goldstein J. A. , Selcher C. A. , Frolov V. L. , Sergeev E. N.
Published: September 2002

Stimulated electromagnetic emissions (SEE) are high‐frequency radio emissions that are generated during high‐power, high‐frequency, ordinary mode (O‐mode), radiowave heating of the ionosphere. These emissions are particularly useful in ionospheric heating experiments because they provide a way of monitoring space plasma processes remotely and passively. In order to utilize these emissions for diagnostic purposes, it is necessary to understand the space plasma processes involved in their generation. The purpose of these experiments was to observe the responses of a particular component of the SEE, the broad upshifted maximum (BUM), to a variety of heating stimuli in an effort to understand the factors involved in its development. Heating experiments were conducted at the Radiophysical Research Institute SURA Ionospheric Modification Facility in Russia. Experiments consisted of single‐pump, two‐pump, and single‐pump power‐stepping experiments. The single‐pump and two‐pump transmissions were organized into groups of pulses of varying widths and spacings to facilitate the investigation of self‐conditioning, preconditioning, and two‐pump‐interaction conditioning effects. The major findings of these experiments are that the action of a pump can have a conditioning effect on the medium that affects the time development of the BUM. The result of the conditioning process is the formation of an overshoot in the temporal development of the BUM. A residual conditioning effect is sustained after the end of a pump pulse for a period of time (∼30 s). The residual conditioning acts as preconditioning for the BUM of a subsequent pump pulse. A second O‐mode pump (pump2), at a frequency a few hundred kilohertz above that of the first pump (pump1), is observed to cause additional suppression of the pump1 BUM, implying an enhanced conditioning effect. Time constants for the buildup and decay of the conditioning effects are estimated. During power‐stepping experiments, the BUM spectrum was observed to evolve from a weak, narrow spectrum at an effective radiated power (ERP) of ≈5 MW, to a strong, broad spectrum with a ramp‐like spectral tail at an ERP of ≈150 MW. Other features noted during power stepping include (1) strong BUM transients at pump power level transitions, (2) BUM amplitude asymmetry between power‐up and power‐down sides of a power stepping cycle, (3) reduction of the BUM spectral peak offset from the pump frequency with increasing pump power, and (4) power law dependence of BUM power on pump power (exponent ∼2). Results of these experiments are used in an attempt to assess the relevance of small‐scale irregularity generation and electron heating mechanisms to the observed conditioning effects.
Traveling convection vortices as seen by the SuperDARN HF radars
Authors: Lyatsky W. B. , Sofko G. J. , Kustov A. V. , André D. , Hughes W. J. , Murr D.
Published: September 2002

Two impulsive traveling convection vortex (TCV) events observed simultaneously by ground based magnetometers and the SuperDARN HF radars in the prenoon sector were studied. In both cases, disturbances traveled westward at speeds of 4–6 km/s. Convection patterns derived from magnetometer measurements and radar observations were overall in reasonable agreement; observed differences at some points might be caused by both the nonuniform ionospheric conductivity distribution and difference in the integration time of the radar and magnetometer data. For one event, the convection patterns obtained from magnetometer data and SuperDARN radar measurements were relatively simple; they can be interpreted as a result of the westward motion of a convection vortex system associated with a pair of field‐aligned currents separated in azimuthal direction. This TCV event was associated with relatively low Pc5 pulsation activity, contrary to the second TCV event that was accompannied by a train of Pc5 magnetic pulsations of large amplitude. Convection patterns for the second event were complicated. A simple scenario for the interpretation of the generation of TCVs and Pc5 pulsations is suggested. A sudden impulse in the solar wind dynamic pressure produces disturbances on several boundaries of magnetospheric plasma: on the magnetopause, the LLBL inner edge, and the plasma sheet inner edge. These boundaries are elastic so that surface waves can propagate along them. The high‐latitude wave is responsible mainly for TCVs, whereas the low‐latitude waves may be responsible for excitation of Pc5 field line resonance pulsations. The scenario explains important features of both TCV events and Pc5 pulsations: both phenomena appear simultaneously and show westward (eastward) propagation, but the TCVs are observed at latitudes close to the LLBL inner edge, whereas the Pc5 pulsations occur at lower latitudes, close to the inner boundary of the plasma sheet.
Atmospheric holes: Instrumental and geophysical effects
Authors: Frank L. A. , Sigwarth J. B.
Published: September 2002

Global images of far‐ultraviolet dayglow with the Earth Camera on board the Polar spacecraft were used for a comprehensive study of transient, localized decreases of intensities, also known as atmospheric holes. An automated determination for detection of an atmospheric hole was developed and employed in this survey of the characteristics of this phenomenon, which avoided the possibility of aliasing by visual inspection of the images. An extensive investigation of possible instrumental effects included (1) evaluation of random rates for a period in mid‐January for which there were no atmospheric holes, thus providing an excellent inflight calibration of the instrument's noise, (2) the effects in the images due to energetic electrons in the outer radiation zone, (3) the nonuniform sensitivities for the pixels of the sensor, or “hot spots,” and (4) the contributions of longer wavelength radiation from the atmosphere to the camera's responses. For the straightforward selection criteria for the identification of atmospheric holes the instrumental effects were not major contributors to the occurrence frequencies of this phenomenon. The subsequent search for geophysical effects revealed (1) a strong altitude dependence of the frequency of atmospheric holes, (2) a substantial local‐time variation of rates which favored locations in the local‐morning sectors of the atmosphere relative to those in the evening, (3) increasing sizes of the atmospheric holes as the spacecraft altitude decreases, and (4) large seasonal variations in the hole rates during the period November 1997 through late January 1998 which were remarkably similar to those observed during the same months but 16 years earlier with Dynamics Explorer 1.
Dynamics of double‐theta aurora: Polar UVI study of January 10–11, 1997
Authors: Newell Patrick T. , Liou Kan , Meng Ching‐I. , Brittnacher Mitchell J. , Parks George
Published: September 2002

During the much studied magnetic cloud encounter with Earth on January 10–11, 1997, pairs of polar cap arcs twice became widely separated from the auroral oval, that is, an extremely rare double θ aurora configuration arose. These events exhibit many dynamic features hitherto rarely if ever reported in the literature of θ aurora. The two independent bars proved capable of merging into a single bar, crossing the polar cap, then splitting again. Moreover, in both events, after completely crossing the polar cap, the transpolar arcs reversed direction, and crossed again. The dynamics of these double θ‐aurora events appear to constrain any models of their origin. Interestingly, in the January 10 event, a double θ‐aurora which had twice crossed the polar cap ended up in a configuration previously termed a double oval. Particle data from the DMSP F13 satellite demonstrates that an isolated plasma sheet fragment formed also in the southern hemisphere, at least for the January 11 event for which correlative data is available.
Morphology of large‐scale traveling atmospheric disturbances in the polar thermosphere
Authors: Balthazor Richard L. , Moffett R. J.
Published: September 2002

Using a fully coupled thermosphere‐ionosphere‐plasmasphere model, we investigate the morphology of modelled Large‐Scale Traveling Atmospheric Disturbances (LS‐TADs, hereinafter TADs) propagating both poleward and equatorward from auroral sources. We confirm observational evidence of daytime ion‐drag, reducing equatorward far‐field TAD activity. We further suggest an additional source‐dependent mechanism that associates TADs propagating preferentially poleward with an auroral electric field burst that contains an eastward rather than a westward component. These poleward TADs are superimposed on the high‐speed neutral air winds in the polar cap (originating from the day‐night plasma convection) and exhibit speed and spatial resonance effects with the neutral wind. The direction of propagation is affected by Coriolis forcing due to the Earth's rotation.
A kinetic‐fluid model
Authors: Cheng C. Z. , Johnson Jay R.
Published: September 2002

A nonlinear kinetic‐fluid model for high β plasmas with multiple ion species which can be applied to multiscale phenomena is presented. The model embeds important kinetic effects due to finite ion Larmor radius (FLR), wave‐particle resonances, magnetic particle trapping, etc., in the framework of simple fluid descriptions. When further restricted to low‐frequency phenomena, with frequencies less than the ion cyclotron frequency, the kinetic‐fluid model takes a simpler form in which the fluid equations of multiple ion species collapse into one‐fluid density and momentum equations and a low‐frequency generalized Ohm's law. The kinetic effects are introduced via plasma pressure tensors for ions and electrons which are computed from particle distribution functions that are governed by the Vlasov equation or simplified plasma dynamics equations such as the gyrokinetic equation. The ion FLR effects provide a finite parallel electric field, a perpendicular velocity that modifies the E × B drift, and a gyroviscosity tensor, all of which are neglected in the usual one‐fluid MHD description. Eigenmode equations are derived, which include magnetosphere‐ionosphere coupling effects for low‐frequency waves (e.g., kinetic/inertial Alfvén waves and ballooning‐mirror instabilities).
High‐voltage interactions in plasma wakes: Simulation and flight measurements from the Charge Hazards and Wake Studies (CHAWS) experiment
Authors: Davis V. A. , Mandell M. J. , Cooke D. L. , Enloe C. L.
Published: September 2002

The Charge Hazards and Wake Studies (CHAWS) flight experiment flew on the Wake Shield Facility (WSF) aboard STS‐60 and STS‐69. The experiment studied high‐voltage current collection within the spacecraft wake. The wake‐side sensor was a 45‐cm‐long, biasable cylindrical probe mounted on the 3.66‐m‐diameter WSF. Operations were performed in free flight and at various attitudes while on the shuttle orbiter remote manipulator system (RMS) arm. Preflight and postflight simulations were performed using the programs Potentials of Large Objects in the Auroral Region (POLAR) and Dynamic Plasma Analysis Code (DynaPAC) and are compared here with the flight results. Both programs perform three‐dimensional, self‐consistent, steady state plasma simulations. During high‐voltage operations the wake‐side probe collected current consistent with preflight predictions. In both the flight data and the steady state simulations the current collected has a power law dependence on the potential and has a less than linear dependence on density. Growth of the sheath beyond the WSF edge controls the high‐voltage current collection, and kinematic and space charge effects both play important roles in attracting ions into the wake region. Measurements made at low voltage differ from the calculations. The preflight calculations for a pure oxygen plasma predict a collection threshold at −100 V bias. The flight data show little or no threshold, implying a source of ions not accounted for in the simulations. Possible sources for these ions are discussed.
“Pancake” electron distributions in the outer radiation belts
Authors: Meredith Nigel P. , Johnstone Alan D. , Szita Sarah , Horne Richard B. , Anderson Roger R.
Published: September 2002

Electron pitch angle distributions sharply peaked at 90° pitch angle were first recorded in the energy range 50 eV < E < 500 eV by the GEOSl and GEOS2 spacecraft in 1977/1978, from the plasmapause out to geostationary orbit. At the time they were explained as the remnants of pitch angle diffusion driven solely by electron cyclotron harmonic (ECH) waves. Here we report new observations by the Low Energy Plasma Analyser on board the Combined Release and Radiation Effects Satellite, which measured the complete pitch angle distribution over the energy range 100 eV < E < 30 keV. The pancake distributions are seen to develop from injected distributions that are nearly isotropic in velocity space, on a timescale that is greater than 2 hours. The freshly injected distributions are associated with strong ECH and whistler mode waves suggesting that the pancake distributions are likely to be caused by a combination of both wave types. Outside L = 6.0 the fitting analysis at energies in the range 100 eV < E < 1 keV shows that in the marginally stable state the phase space density contours lie approximately along the characteristic curves for diffusion by whistler mode waves determined independently from the plasma wave data. However, inside L = 6.0, significant departures are observed. Our results suggest that whistler mode waves play a dominant role in the formation of pancake distributions outside L = 6.0, whereas inside L = 6.0 and, in particular, in the vicinity of the plasmapause, the ECH waves also play a significant role. Consequently, both types of waves should be considered in any attempt to explain the diffuse aurora and the variation with L taken into account.
Whistler turbulence at the magnetopause: 1. Reduced equations and linear theory
Authors: Vetoulis G. , Drake J. F.
Published: September 2002

A set of reduced nonlinear equations is presented, which describes the dynamics of a current layer at scale sizes below the ion skin depth. At these scales the ion motion can be ignored. These equations contain the effects of electron inertia and the Hall term with the intent of describing small‐scale turbulence in the environment of the magnetopause. Magnetic shear effects are also included. It is found that the entire spectrum is linearly unstable either to the current gradient instability or to a kink‐type mode. The kink‐like instability, in contrast to the current gradient instability, also exists in the limit of massless electrons. The growth rate increases with decreasing current layer width and the jump of the north‐south component of the magnetic field across the magnetopause.
Analysis of quasi‐linear diffusion coefficients
Authors: Albert J. M.
Published: September 2002

This paper considers the quasi‐linear treatment of either electrons or protons cyclotron‐resonant with either whistler or ion cyclotron waves. The dispersion relation used does not yield a simple, explicit expression for the resonant frequency ω. Detailed analysis is made of the conditions determining the resonant frequencies ω for a given wave normal angle θ; there may be up to three values of ω(θ). Criteria are derived which identify the frequencies and indicate when they are outside specified frequency cutoffs. From this analysis, much unnecessary computational effort to evaluate the diffusion coefficients may be avoided. For electron‐whistler interactions in the radiation belts, the sensitivity of the local results to the parameters of the wave distribution is considered. It is shown how parameter‐independent calculations may be used to analyze the dependence on the parameter values. This approach is extended to the bounce‐averaged coefficients, which determine estimates of the pitch angle distribution and corresponding time to pitch angle scatter into the loss cone. Numerical results are obtained using recently reported parameters for whistler waves due to hiss, lightning, and VLF transmitters, as well as perturbations to these values.
Whistler waves in space and laboratory plasmas
Authors: Stenzel R. L.
Published: September 2002

An overview of whistler wave phenomena in space and laboratory plasmas is given. Common features and different approaches between laboratory and space plasma research are pointed out. Both research activities have discovered a rich variety of whistler wave effects. Many useful applications have emerged. Open research topics and interactions between lab and space research on whistlers are pointed out.
Kinetic Alfvén wave revisited
Authors: Hollweg Joseph V.
Published: September 2002

We develop a series of new analytical expressions describing the physical properties of the kinetic Alfvén wave. The wave becomes strongly compressive when
is of the order of the ion inertial length. Thus, in a low‐β plasma, the kinetic Alfvén wave can be compressive at values of k⊥ for which the dispersion relation departs only slightly from that of the usual MHD Alfvén wave. The compression is accompanied by a magnetic field fluctuation δB‖ such that the total pressure perturbation δptot ≈ 0. Thus the wave undergoes transit‐time damping as well as Landau damping; the two effects are comparable if the ion thermal speed is of the order of the Alfvén speed. We find that the transverse electric field is elliptically polarized but rotating in the electron sense; this surprising behavior of the polarization of the Alfvén branch was discovered numerically by Gary [1986]. We derive a new dispersion relation which explicitly shows how the kinetic Alfvén wave takes on some properties of the large‐k⊥ limit of the slow mode. We also derive approximate dispersion relations valid for a multi‐ion plasma with differential streaming. We suggest that the kinetic Alfvén wave may be responsible for the flattening of density fluctuation spectra observed at large wavenumbers in the corona and in the solar wind. We also find that our derived properties of the kinetic Alfvén wave are consistent with its presence in the dissipation range of MHD turbulence [Leamon et al., 1998a, b].
Review of controlled laboratory experiments on physics of magnetic reconnection
Authors: Yamada Masaaki
Published: September 2002

We review results from the most recent experiments in the past 2 decades in which magnetic reconnection has been generated and studied in controlled laboratory settings. As a whole, research on the fundamental physics of the reconnection process and its hydromagnetic consequences has been largely theoretical. Laboratory experiments are crucial for understanding the fundamental physics of magnetic reconnection since they can provide well‐correlated plasma parameters at multiple plasma locations simultaneously, while satellites can only provide information from a single location at a given time in a space plasma. Thanks to the significant progress of data acquisition technology, the detailed magnetic field structures of the reconnection regions were measured and plasma acceleration and strong ion heating were identified. Extensive data have been accumulated in the electron MHD plasma regimes [in which electrons are magnetized] with relatively low Lundquist number of S = 1 – 10 as well as in MHD plasmas with S = 100 – 1000. This article puts a special focus on the most recent plasma merging experiments in the MHD regime since they are new and have been published in the past several years. Two distinctly different shapes of diffusion regions were identified both in the electron MHD plasma regime [Stenzel and Gekelman, 1983] and in the MHD plasmas in MRX [Yamada et al., 1997b]. The familiar two‐dimensional (2‐D) feature, a double‐Y‐shaped diffusion region, was identified when there is no or very little axial magnetic field (third vector component). In cohelicity merging with a sizable third field component, an O‐shaped diffusion region appears and the reconnection rate decreases substantially; this is attributed to the axial field pressure and the incompressibility of the plasma. Another important achievement is the experimental verification of a generalized Sweet‐Parker model [Ji et al., 1998]. In this recent work it is found that the observed reconnection rate can be explained by a generalized Sweet‐Parker model, which incorporates compressibility, downstream pressure and the effective resistivity. The latter is often enhanced over its classical values in the collisionless limit. A significant implication of this result is that the generalized Sweet‐Parker model is valid in certain 2‐D reconnection cases with axisymmetric geometry. In MHD plasmas, the thickness of this thin current layer is found to be of the order of the ion gyroradius and the ion skin depth as well.
Cross‐field current instability as a catalyst of the explosive reconnection in the geomagnetotail
Authors: Sitnov M. I. , Lui A. T. Y.
Published: September 2002

Consequences of the turbulence excited by the cross‐field current instability (CCI) for explosive reconnection and the tapping of lobe energy into the current disruption region are considered. It is shown that choking of the electron compressibility effect to restore the appropriate reconnection (tearing) mode may be considerably facilitated by the combined action of the pitch angle diffusion induced by CCI and the shielding effect of transient electrons. Another promising way of exciting the tearing mode is by coordinating it with the particle exchange between trapped and transient electron populations induced by the CCI turbulence, which is modulated in turn by the tearing mode because of local thinning of the current sheet.
Bathymetry of Shatsky Rise, northwest Pacific Ocean: Implications for ocean plateau development at a triple junction
Authors: Sager William W. , Kim Jinho , Klaus Adam , Nakanishi Masao , Khankishieva Larisa M.
Published: September 2002

Oceanic plateaus are large igneous edifices thought to have been created by nascent mantle plumes, but owing to sparse data, their origins remain uncertain. Understanding plateau evolution is important because they are significant ocean features and may provide clues about mantle plume dynamics. We constructed a bathymetry map of Shatsky Rise, a large Pacific plateau, combining multibeam and wide‐beam echosounder data from 87 cruises and U.S. Navy multibeam contours. The rise consists of three large, isolated volcanic edifices (massifs), surrounded by nearly normal lithosphere, a linear volcanic ridge, and a group of about 80 scattered seamounts. Massif flank slopes are typically gentle (∼1.5°) and often parallel magnetic lineations or fracture zones. The slope angles imply effusive volcanism, similar to flood basalts, whereas the rise shape suggests formation near the Pacific‐Izanagi‐Farallon triple junction with modification of volcano flanks by spreading‐ridge tectonics. Edifice sizes and inferred ages imply a trend of decreasing volume and age from southwest to northeast. Furthermore, gaps between massifs suggest episodic volcanism. Existing data are broadly consistent with the “plume head” hypothesis wherein the largest edifice formed by massive plume head eruptions, the ridge formed from the plume tail, and the two massifs in between represent a transition. Seamounts east of the rise are morphologically distinct, which suggests that they may have been formed by a different source or mechanism than Shatsky Rise proper.
Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40Ar/39Ar timescale of reversals and events
Authors: Singer Bradley S. , Hoffman Kenneth A. , Chauvin Annick , Coe Robert S. , Pringle Malcolm S.
Published: September 2002

The K‐Ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed, but rarely dated, the transitions between polarity intervals. Subsequent 40Ar/39Ar dating indicated that the ages of some polarity transitions had been underestimated by about 6%. Although the accepted ages of the polarity chron boundaries have increased, their precise temporal definition remained uncertain. We have taken a different approach and used incremental‐heating techniques to obtain 18 new 40Ar/39Ar ages from basaltic lavas within flow sequences at Punaruu Valley, Tahiti, and Haleakala volcano, Hawaii. These lavas record transitional paleomagnetic directions corresponding to four mid‐Pleistocene polarity reversals or events. Three lavas from Punaruu Valley previously thought to record the Cobb Mountain Normal Polarity Subchron (CMNS) gave a mean age of 1.105 ± 0.005 Ma, indicating that they were erupted about 76 kyr after the CMNS; this period of transitional field behavior is designated the Punaruu event. In addition, seven new 40Ar/39Ar ages from the Punaruu Valley indicate that the Jaramillo Normal Polarity Subchron (JNS) lasted about 67 kyr, starting at 1.053 ± 0.006 Ma and ending 0.986 ± 0.005 Ma. This agrees with astronomical estimates but conflicts with JNS ages proposed by Spell and McDougall [1992] and Izett and Obradovich [1994] on the basis of 40Ar/39Ar dating of rhyolite domes in the Valles Caldera. Indistinguishable 40Ar/39Ar ages of seven lavas, including one from Punaruu Valley and six from Haleakala that record broadly similar intermediate paleodirections, suggest that the Kamikatsura event occurred at 0.886 ± 0.003 Ma. Moreover, these data indicate that the Kamikatsura event occurred 20–40 kyr after another geomagnetic event, most probably taking place at 0.92 Ma. We designate this earlier field behavior the Santa Rosa event, adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the JNS in the 1968 study of Doell et al. [1968]. The discovery of these new short‐lived polarity events during the Matuyama reversed chron suggests that the 400 kyr period between 1.18 and 0.78 Ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse. This newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks.
On the initiation of shear faults during brittle compressive failure: A new mechanism
Authors: Schulson Erland M. , Iliescu Daniel , Renshaw Carl E.
Published: September 2002

Brittle materials loaded under compression generally fail by shear faulting. This paper addresses the initiation of the fault. It presents direct observational evidence from ice, which is used as a model material for rock, and shows that wing cracking and “splay cracking” are important processes in the localization of deformation, both prior to and during fault initiation. Wing cracks develop at the tips of sliding intergranular cracks and tend to align with the maximum principal stress. Splay cracks emanate from one side of the sliding parent crack. The theme of the paper is that the splay cracks play the dominant role in triggering the fault. The central idea is that the slender columns between the splay cracks are more likely to buckle and fail than are the columns between adjacent wing cracks because they do not have two fixed ends; instead, the end stemming from the inclined parent crack is free. A moment is then applied by frictional sliding of the parent inclined crack, and this causes the fixed‐free columns to break at a much lower stress than the fixed‐fixed columns. Columns created near a free surface are more likely to fail than those created elsewhere, and this explains the observation that shear localization tends to initiate near free surfaces. A first‐order calculation shows that the failure stress of the splay‐created columns is of the same order of magnitude as the terminal failure stress.