bluuebunny/crossref_metadata_2025 huggingface dataset:
The ionopause current layer at Venus
Authors: Cravens T. E. , Shinagawa H.
Published: February 2008

Magnetic fields are induced by the solar wind in the ionospheres of planets such as Venus and Mars, which possess only weak, or nonexistent, intrinsic magnetic fields. The magnetometer on the Pioneer Venus Orbiter (PVO) observed large‐scale magnetic fields in the ionosphere when the solar wind dynamic pressure was high. During high solar wind dynamic pressure conditions the ionopause was observed to be broad and at low altitudes, whereas during low solar wind dynamic pressure conditions the ionopause was located at high altitudes (z > 300 km) and was narrow (Δz ≈ 20 km). We review the results of recent theoretical models of the magnetic field in the ionosphere of Venus, and, in particular, the one‐dimensional MHD model of Shinagawa and Cravens (1988) and the two‐dimensional kinematic dynamo model of Cravens et al. (1990). The results of these models are used as a starting point for an explanation of the behavior of the ionopause. It is well known that during low solar wind dynamic pressure conditions the location of the ionopause is determined by vertical pressure balance between the ionospheric thermal pressure and the magnetic pressure of the overlying magnetic barrier. The structure of the ionopause is not well understood; we suggest that the sensitivity of horizontal plasma transport to the presence of a magnetic field and the convergence of the vertical flow provide the explanation for the narrowness of the Venus ionopause. However, during high solar wind dynamic pressure conditions, ion‐neutral friction also plays an important role in determining the thickness of the ionopause.
Evidence of Saturn's magnetic field anomaly from Saturnian kilometric radiation high‐frequency limit
Authors: Galopeau Patrick , Ortega ‐Molina Arturo , Zarka Philippe
Published: February 2008

The detailed analysis of Voyager observations of Saturn kilometric radiation (SKR), taking into account the polarization response of the planetary radio astronomy experiment, has allowed us to study separately the high‐frequency limit of SKR emitted from the northern and southern auroral regions of the planet. Interpretation of the variations of these high‐frequency limits, in the frame of a model inspired by the cyclotron maser mechanism, has allowed us to demonstrate unambiguously the existence of a magnetic “anomaly”, postulated for years, in the near‐surface Saturnian field. Its location and amplitude are estimated, and it is shown that, although it accounts for the observed SKR modulation, it could not be detected by the spacecraft‐borne magnetometers which passed too far from the planetary surface. This demonstrates that the current degree of comprehension of planetary radio emissions makes them a unique and very pertinent tool for the fine study of planetary magnetic fields.
Turbulent acceleration of auroral electrons
Authors: Bryant D. A. , Cook A. C. , Wang Z.‐S. , de Angelis U. , Perry C. H.
Published: February 2008

It is shown that the characteristic peak in the auroral electron velocity distribution can be generated stochastically through resonant interactions with lower hybrid electrostatic turbulence. The peak itself is shown to be a direct consequence of restrictions imposed on reflection of electron velocities in the frame of reference of individual wave packets by the limitation in group velocity. A Monte‐Carlo model demonstrates how the various properties of the acceleration region are reflected in the resultant electron distribution. It is shown, in particular, that the width of the peak is governed by the amplitude of the turbulence, while the amplitude of the peak reflects the column density of wave energy. Electron distributions encountered within three auroral arcs are interpreted to yield order of magnitude estimates of the amplitude and rms electric field of lower hybrid wave packets. The velocities and frequencies of the resonant waves, the net electric field, the column density of wave energy, and the electric‐field energy density are also estimated. The results are found to be consistent with available electric‐field measurements. A general broadening of the electron distribution caused by less systematic interactions between electrons and wave packets is shown to have a negligible effect on the peak resulting from the reflection process; it does, though, lead to the creation of a characteristic high‐energy tail.
A self‐consistent model for the particles and fields upstream of an outgassing comet: 1. Gyrotropic and isotropic ion distributions
Authors: Flammer K. R. , Mendis D. A. , Northrop T. G. , Whipple E. C.
Published: February 2008

The magnetic field and solar wind flow parameters in the vicinity of a weakly outgassing comet are determined using a self‐consistent model which treats the cometary ions kinetically. Two different assumptions are made concerning the cometary ion distribution function; the pickup cometary ions form either a velocity space gyrotropic ring distribution or a velocity space Isotropic shell distribution in the solar wind frame of reference. The individual currents due to the solar wind and cometary ions, which determine the magnetic field, are calculated in each case. Using theoretically determined parameters which reflect the conditions expected at comet Kopff (a typical short‐period comet and one possible target for the future CRAF/Cassini mission) for various heliocentric distances, we are able to determine how the global field and flow characteristics are influenced by the nature of the cometary ion distribution function.
Polar cusp topology and position as a function of interplanetary magnetic field and magnetic activity: Comparison of a model with Viking and other observations
Authors: Stasiewicz K.
Published: February 2008

A family of quantitative magnetospheric models is used to determine the cusp position in the ionosphere and to analyze the field topology around the outer cusp. It is demonstrated that the “cusp proper” (∼1° × 2 hours), observed in particle precipitation patterns, can be defined as the footprint of the magnetopause current layer with field lines connected to the interplanetary medium via a normal magnetic field component. A wider magnetic local time extent of the cleft (∼8–16 hours MLT) can be associated with field lines connected to the low‐latitude boundary layer between XGSM = 0 and 20 RE downtail and extending 1–2 RE inward from the magnetopause layer. It is suggested that the finite‐sized physical cusp originates as a result of gyroviscous erosion at the magnetopause current layer that enlarges the topologically singular magnetic cusp. The model‐predicted magnetic latitude (MLAT) of the ionospheric footprint of the cusp is 79° during quiet conditions and 74° during active periods. The latitude of the cusp varies also with universal time and with season, due to variation of the dipole tilt angle. The corrected magnetic coordinates are found to exhibit the smallest amplitude of diurnal variation of the cusp position and are recommended for mapping of auroral phenomena. Other coordinate systems produce artificial diurnal variations of the cusp position. These artificial effects have an amplitude of ±2.5 (±1) hours MLT and ±4° (±2°) MLAT for dipole (eccentric dipole) coordinates, respectively. An interplanetary magnetic field (IMF) By component linearly superposed with the magnetospheric field can shift the magnetic local time of the cusp by a maximum of 0.07–0.15 hour/nT. A negative (positive) IMF Bz component can shift equatorward (poleward) the position of the cusp by a maximum of 0.1°–0.2° /nT. A similar effect is obtained with a positive (negative) Bz component. These results imply that the instantaneous effect of a negative Bz component on the equatorward shift of the cusp is smaller than an indirect effect due to increased magnetic activity. Some of the model predictions are compared with cusp observations made on board the Swedish Viking satellite. Both the Viking and other observations can be quantitatively explained by the model predictions.
The physics of cosmic ray modulation: Heliospheric propagation during the 1987 Minimum
Authors: Lopate C. , Simpson J. A.
Published: February 2008

The period in and near the 1987 solar minimum of modulation is a unique interval in the 11‐year solar cycle modulation for determining whether both the heliospheric current sheet and charged particle gradient drift contribute significantly to the modulation of galactic cosmic rays and the anomalous components. Our investigation of this period is based on measurements from neutron monitors and the IMP 8 satellite at 1 AU and the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft distributed throughout the heliosphere to ∼46 AU. Three observations essential for the analysis are as follows: (1) the brief time‐intensity profile for cosmic ray minimum could be traced relatively unchanged from 1 AU to ∼41 AU at the average solar wind velocity, (2) the onset of enhanced modulation in the new solar cycle at 1 AU began at the highest magnetic rigidity and progressed in time over a period of months to particles of low magnetic rigidity, and (3) the average tilt angle of the neutral current sheet was available from Hoeksema (1989) as a function of time for the period centered on the 1987 solar minimum at 1 AU. To account for the observations, we propose a model for the onset of the new modulation cycle which includes a change in the average tilt angle of the current sheet as it propagates outward through the heliosphere at the solar wind velocity. Calculations based on our model with an inwardly directed north solar polar magnetic field (which prevailed in the 1985–1988 period) include diffusion, convection, adiabatic deceleration, and drift for the inward propagation of cosmic ray protons in the energy range from 20 to 1500 MeV and reproduced the rigidity phase lag effect observed in the 1987 data. On the other hand, for the reversed solar magnetic field polarity (e.g., 1954, 1977) the model calculations show that an enhanced onset for these solar cycles has a negligible rigidity phase lag at 1 AU, in agreement with the observed 1977 onset of modulation. We point out that transient phenomena (e.g., Forbush decreases and interaction regions) appear to dominate the higher levels of solar modulation following the 1987 period. A consequence of our analysis is that at the time of solar minimum there is a minimum in the amplitude of the 11‐year variation of the radial gradient of integral cosmic ray intensity which propagates outward at the solar wind velocity. Therefore the time dependence of the amplitude of the radial gradient is not the result of the inward‐outward motion of a modulation boundary.
Whistler doublets and hyperfine structure recorded digitally by the signal analyzer and sampler on the active satellite
Authors: Lichtenberger János , Tarcsai György , Pásztor Szilárd , Ferencz Csaba , Hamar Dániel , Molchanov Oleg A. , Golyavin Aleksander M.
Published: February 2008

The signal analyzer and sampler (SAS) experiment was placed on the Active spacecraft as a collaborative effort between the Eötvös University (Budapest), the Technical University of Budapest, and IZMIRAN (Moscow). The scientific objective of the experiment was to study whistler/VLF ducted propagation, VLF duct structure, and the hyperfine structure of whistlers. Digitally sampled waveforms of several field components were transmitted in real time by the SAS telemetry system at 460.4 MHz. For the transmission 900‐Hz wide bands were selected between 0.5 and 21.5 kHz or a single 5 kHz wide band was transmitted. Data were received in Budapest, Hungary and at Wallops Island, Virginia. Although the obtained data were processed for various purposes, here some results concerning the hyperfine structure of whistlers are presented. The studied whistlers, recorded as several pairs of closely spaced traces (doublets), were interpreted as ducted whistlers escaping at a high altitude from a single or two closely spaced narrow ducts and reaching the satellite directly from above or after reflection from below. The whistler traces were processed by a sophisticated matched filter technique which enabled us to obtain very high resolution dynamic (frequency‐time‐amplitude) spectra. The hyperfine structure of traces revealed by this technique demonstrates the complexity of whistler propagation. The observed splitting of traces may be explained, for example, in terms of a number of guided modes (waveguide mode splitting) or by the superposition of closely spaced ducting structures (duct splitting).
Mapping of auroral arcs during substorm growth phase
Authors: Pulkkinen T. I. , Koskinen H. E. J. , Pellinen R. J.
Published: February 2008

The substorm growth phase is manifested in the auroral oval by brightening and equatorward motion of auroral arcs, prior to the formation of the westward traveling surge at substorm onset. In the magnetosphere the cross‐tail current enhances and the current sheet becomes very thin, and at substorm onset the tail current system disrupts, connecting the magnetosphere and the ionosphere through field‐aligned currents. The relation between the auroral and tail processes is studied here by magnetic‐field‐aligned mapping from the ionosphere to the current sheet using the Tsyganenko 1989 field model. The dynamical changes in the field and current distributions are modeled by stepwise modification of the field model to include the current sheet thinning and enhancement of the tail current. In addition, the field signatures of a current meander suggested to form in the tail are demonstrated. These signatures are compared with a model where line currents are added to represent the effects of the current wedge forming at substorm onset. Using the Tsyganenko model, the mapping of a growth phase arc recorded by Finnish all‐sky cameras moves earthward from 12 RE at the beginning of the growth phase to about 6 RE by the time of the substorm onset, corresponding to the equatorward motion of the arc. If the observed motion of the arc is interpreted to be entirely due to the stretching of the tail field during the growth phase, the enhancement of the cross‐tail current and the changes in the field configuration can be estimated. In this particular case, the current sheet has to be thinned to 10% of the original value at X = −8 RE, and the cross‐tail current enhanced by a factor of 1.3 to correspond to the equatorward motion of the arc.
Model of electron and ion distributions in the plasma sheet boundary layer
Authors: Onsager T. G. , Thomsen M. F. , Elphic R. C. , Gosling J. T.
Published: February 2008

Electron and ion velocity space distributions in the plasma sheet boundary layer have distinct features and exhibit a characteristic evolution with depth in the boundary layer. Near the lobeward edge of the layer, enhanced earthward and tailward directed electron flux is observed. Somewhat deeper in the boundary layer, earthward and tailward directed ion beams are observed. The electron and ion beams have low‐speed cutoffs, and the earthward directed beams are consistently observed at lower speeds than the simultaneously observed tailward directed beams. The ion distributions evolve from magnetic‐field‐aligned beams, to “kidney bean” shaped distributions, to isotropic shells with increasing equatorward penetration into the boundary layer. A two‐dimensional model based on quasi‐steady reconnection occurring in the distant magnetotail is able to reproduce all of these observed features in the electron and ion distribution functions. The essential features of the model are the finite time‐of‐flight effect (velocity filter effect), conservation of energy, and conservation of magnetic moment as the particles stream from the low magnetic field region near the central plasma sheet to the higher field region in the boundary layer. The model can be used to estimate the central plasma sheet density, temperature, and bulk flow speed as functions of position earthward of a reconnection site from observed plasma sheet boundary layer distributions. These plasma distributions obtained from the model may be useful in determining the stability of the boundary layer plasma to various electrostatic and electromagnetic modes.
Observations of Pi 2 pulsations at a very low latitude (L = 1.06) station and magnetospheric cavity resonances
Authors: Lin C. A. , Lee L. C. , Sun Y. J.
Published: February 2008

Observations of Pi 2 pulsations at the Chung‐Li (CNL) station are presented and interpreted. The geomagnetic latitude of this station is 13.8°N (L = 1.06). The power spectra of the observed Pi pulsations show both single and multiple peaks during daytime and nighttime. Typical periods of Pi pulsations observed are 30–100 s. The observed frequency ratios of the first three harmonics are found to be about 1∶1.5∶2. Large‐amplitude pulsations are more frequently observed during local noon and midnight. The observed characteristics are explained on the basis of two cavity resonance models: (1) a magnetosphere cavity model (model A) that includes the plasmasphere and the plasma outside the plasmapause and (2) a plasmasphere cavity model (model B) that ignores the plasma outside the plasmapause. It is found that the presence of the plasma outside the plasmapause can significantly modify the amplitude and eigenfrequencies of the waves excited in the resonance cavity. The eigenfrequencies of the cavity waves also depend on the position and the density drop at the plasmapause. The eigenfrequencies as a function of the azimuthal wave number ky are also studied. It is found that for small ky, the ratios of the eigenfrequencies of the first three harmonics in the radial direction calculated from model A are consistent with the observed values.
A three‐dimensional MHD simulation of plasma flow past Io
Authors: Linker J. A. , Kivelson M. G. , Walker R. J.
Published: February 2008

We describe a time‐dependent three‐dimensional magnetohydrodynamic (MHD) simulation of plasma flow past Jupiter's satellite Io. The simulation starts with a uniform, magnetized flow interacting with a finite conducting sphere (representing Io), and the integration is carried forward in time until a quasi‐steady state develops. Previously we have reported on the presence of standing slow mode perturbations in the flow, attached to Io. In this paper we present details of the simulation model and describe the overall features of the interaction. Field‐aligned currents are found in regions extending downstream from the obstacle at an angle to the background field; these currents form the Alfvén wings, an expected result. The plasma flow diverts both around Io and the Alfvén wings. An “Alfvén ellipse,” the intersection of a cylindrical Alfvén wing with an arbitrary plane above Io, describes the obstacle that an Alfvén wing presents to the flow. In addition to the Alfvén mode, which carries the field‐aligned currents, perturbations carried by the other MHD modes are also present. Standing slow mode perturbations are the most significant of these, but a fast mode signature also appears. Near Io the interaction is clearly nonlinear; all three MHD modes interact, and the local Alfvén and sound speeds are altered. However, estimates of the spatial location of the standing MHD mode disturbances based on the background characteristic velocities are qualitatively accurate.
Probe measurements of low plasma densities with applications to the auroral acceleration region and auroral kilometric radiation sources
Authors: Hilgers A. , Holback B. , Holmgren G. , Boström R.
Published: February 2008

When the Viking satellite crossed the auroral acceleration region, where the electron density is supposed to be less than a few particles per cubic centimeter, a strong spin modulation of the signal of a positively biased Langmuir probe was observed. This spin modulation is better correlated with the angle of the antenna to the geomagnetic field than with the angle to the Sun. A simple model based on the photoelectron orbits in a spherically symmetric electric field, in the presence of a uniform magnetic field, gives the right phase and amplitude of the extrema. In addition, it shows that the perturbation could be negligible when the antenna is perpendicular to the magnetic field, provided that the photoelectron sources are far enough from the magnetic field lines connected to the probe (about 2 Larmor radii). As a consequence, we deduce important information for the design of experiments devoted to very low density measurements, and we show that the magnetic field may be a crucial parameter for the study of the exchange of photoelectrons between parts of a satellite. The most important result of this study is to facilitate the probe density measurements inside the acceleration structures. It is shown that in these regions the cold plasma density is depleted, with a density of thermal electrons below 5 cm−3 at altitudes between 6500 km and 8500 km. When auroral kilometric radiation (AKR) is generated inside such structures, the density of thermal electrons can be below 1 cm−3. The edges of the structures are sharply defined, and the gradient scale can be as low as 1.4 km. This is the first time that density measurements of thermal electrons have been realized inside clearly identified AKR sources, with appropriate resolution.
Association of electron conical distributions with upper hybrid waves
Authors: Menietti J. D. , Lin C. S. , Wong H. K. , Bahnsen A. , Gurnett D. A.
Published: February 2008

The particle and plasma wave data of the DE 1 and Swedish Viking satellites shows that incense (> 1 mV/m) upper hybrid emissions are sometimes present in the mid‐altitude polar magnetosphere on both the dayside cusp/cleft and the nightside auroral regions and that waves near the upper hybrid frequency are often associated with electron conical distributions. These observations are consistent with the production of at least some electron conical distributions by oblique heating of the electrons by upper hybrid waves. Examination of the wave data to establish the role of parallel heating remains to be performed.
The Nimbus 7 solar total irradiance: A new algorithm for its derivation
Authors: Hoyt Douglas V. , Kyle H. Lee , Hickey John R. , Maschhoff Robert H.
Published: February 2008

The Nimbus 7 satellite has measured the solar total irradiance from November 1978 to July 1991 (153 months). These measurements are important both in solar physics and for climate change. To insure that the Nimbus 7 measurements are capturing the true behavior of the Sun, it is essential that the properties of the radiometer and its changes over time be understood. The calibration of the radiometer can be viewed as a process of removing instrumental influences from the raw measurements, leaving the experimenter with an estimate of solar variability. In this paper the changing radiometer pointing, the zero offsets, the stability of the gain, the temperature sensitivity, and the influences of other platform instruments are all examined and their effects on the measurements considered. Only the question of relative accuracy (not absolute) is examined. The resulting derived solar irradiances are compared to previous analyses of the Nimbus 7 radiometer and to the Solar Maximum Mission (SMM) measurements. Compared to previous analyses, the newly derived values are higher and somewhat less variable than the older values. Removal of the off‐axis pointing errors and a new treatment of the zero offsets are the major reasons for the changes. Compared to the SMM measurements, both agree quite well so long as any solar activity is present. When the Sun becomes quiet, so its irradiance variability is less than the Nimbus 7 radiometer resolution, the comparison to the SMM results breaks down. Between 1980 and 1988 the correlation of the daily values is 0.83, compared to 0.62 using previously published values from both satellites. The monthly means have a correlation of 0.90, indicating that about 80% of the longer‐term variance is in common. In 1980 when both satellites were operating without problems, Nimbus 7 was 0.3155% higher on average. For May 1984 to December 1988, Nimbus 7 was 0.3255% higher. Therefore if the Nimbus 7 satellite continues to operate properly until a period of about 1 year after the launch of UARS with its active cavity radiometer irradiance monitor (ACRIM), it appears that the differences between Nimbus 7, SMM ACRIM, and UARS ACRIM can be measured to within a few hundredths of a percent. A self‐consistent set of solar irradiance measurements from several satellites over nearly two solar cycles appears feasible.
Effects of gravity waves on complex airglow chemistries: 2. OH emission
Authors: Tarasick David W. , Shepherd Gordon G.
Published: February 2008

The theory of Hines and Tarasick (1987) for the effects of gravity waves on airglow emissions is extended to consider more complex airglow chemistries, including multiple and mutiple‐step production mechanisms, quenching, and other loss processes. Attention is given to nonsteady state chemistry, since the time constants of some species involved in OH* production are comparable with gravity wave periods. Relations for the dependence of η, the ratio of brightness to temperature fluctuations, on emission chemistry are presented in a generalized form which is readily applicable to other emissions. The specific case of OH airglow is examined in detail for the quenching rates of Llewellyn et al. (1978) and for those of Lowe (1991). Quenching is found to be important to predictions of η The results obtained are compared with the limited set of published observations of gravity waves in OH airglow. Although the theory predicts significant differences in the behavior of η for the two sets of rates, the existing data are not adequate to draw firm conclusions.
A case study of lightning, whistlers, and associated ionospheric effects during a substorm particle injection event
Authors: Rodriguez J. V. , Inan U. S. , Li Y. Q. , Holzworth R. H. , Smith A. J. , Orville R. E. , Rosenberg T. J.
Published: February 2008

Simultaneous ground‐based observations of narrowband and broadband VLF radio waves and of cloud‐to‐ground lightning were made at widely spaced locations during the 1987 Wave‐Induced Particle Precipitation (WIPP) campaign, conducted from Wallops Island, Virginia. Based on these observations, the first case study has been made of the relationships among located cloud‐to‐ground (CG) lightning flashes, whistlers, and associated ionospheric effects during a substorm particle injection event. This event took place 2 days after the strongest geomagnetic storm of 1987, during a reintensification in geomagnetic activity (Kp = 5) that did not affect the high rate of whistlers observed at Faraday Station, Antarctica (L = 2.46). At the time of the injection event, several intense nighttime thunderstorms were located over Long Island and the coast of New England, between 400 km northwest and 600 km north of the region geomagnetically conjugate to Faraday. About two thirds of the CG flashes that were detected in these thunderstorms during the hour following the injection event onset were found to be causatively associated with whistlers received at Faraday. During the same period the amplitude of the 24.0‐kHz signal from the NAA transmitter in Cutler, Maine, propagating over the thunderstorm centers toward Wallops Island was repeatedly perturbed in a manner characteristic of previously reported VLF signatures of transient and localized ionization enhancements at D region altitudes. Though such enhancements may have been caused by whistler‐induced burst electron precipitation from the magnetosphere, the data in this case are insufficient to establish a clear connection between the NAA amplitude perturbations and the Faraday Station whistlers. In view of the proximity of the NAA great circle path to the storm center, heating of the lower ionosphere by intense radiation from lightning may also have played a role in the observed VLF perturbations. The onset of each of the NAA signal perturbation events coincided with an intense cluster of radio atmospherics. Detailed temporal variations in the ELF (0.3–3 kHz) and VLF (3–30 kHz) power of similar “sferic clusters” correlated well with variations in the power of simultaneous “anomalous” optical events (AOEs) observed by a down‐looking photodiode detector on a rocket at altitudes between 150 and 412 km.
Soliton propagation on multiple‐interface magnetic structures
Authors: Ruderman Michael S.
Published: February 2008

The magnetospheres of Earth and other planets, the solar wind, and the solar and stellar atmospheres are magnetically structured. The structuring can include tangential discontinuities. Wave propagation in magnetic structures consisting of an arbitrary number of parallel tangential discontinuities is considered. The governing equation for long nonlinear small‐amplitude waves is derived. The solutions of this equation in the form of algebraic solitary waves and nonlinear periodic waves are found.
Observations of a new class of upstream waves with periods near 3 seconds
Authors: Le G. , Russell C. T. , Thomsen M. F. , Gosling J. T.
Published: February 2008

We have found a new class of ULF waves with periods near 3 s in the Earth's upstream region by examining the high time resolution magnetic field data from the ISEE spacecraft. These waves are observed in the part of the upstream region which is magnetically connected to the bow shock, but only when the solar wind plasma beta is high (greater than 1). The waves are always right‐handed, nearly circularly polarized in the spacecraft frame. The directions of the wave vectors are in the general direction of the average magnetic field, and the waves are convected downstream in the spacecraft frame. Our study of these waves has shown that they appear to be intrinsically left‐handed ion cyclotron waves in the plasma rest frame.
Theory of field line resonances of standing shear Alfvén waves in three‐dimensional inhomogeneous plasmas
Authors: Schulze‐Berge Sibylle , Cowley Steven , Chen Liu
Published: February 2008

We have analyzed field line resonances of Alfvén waves in a rectangular box model with a straight uniform magnetic field but three‐dimensionally varying density. Field line resonances are shown to exist even with this three‐dimensional nonuniformity. For a given wave frequency we can construct the surface on which the resonance occurs and derive the local form of the singular solution. Magnetic perturbations are found to lie predominantly in the resonant surface. In the presence of azimuthal inhomogeneities the present theory could explain why some satellite measurements show geomagnetic pulsations of comparable magnitude in radial and azimuthal components.
Thermal ion observations of depletion and refilling in the plasmaspheric trough
Authors: Watanabe S. , Whalen B. A. , Yau A. W.
Published: February 2008

Thermal (0–25 eV) ion observations in the altitude range 5000 km to 10,000 km and at invariant latitudes greater than ∼60°, made by the Suprathermal Ion Mass Spectrometer (SMS) on the EXOS‐D satellite, are used to estimate temperatures, densities, composition, and drift velocities of the local major thermal ion population during a 22‐day period in February 1990. This preliminary study indicates that equatorward of a high‐latitude boundary, higher‐density cold ions corotate with Earth and field‐aligned drift velocities are low (<1 km/s). These observations are consistent with a plasmaspheric origin for these ions; however, densities measured near the boundary (10‐100 cm−3) suggest that this boundary lies in or near the trough region. Poleward of this boundary, convection patterns deviate from corotation, and large upward directed field‐aligned flows of ionospheric plasma, consistent with a “polar wind like” source mechanism, are detected. Typically, parallel drift velocities of 12, 7, and 3 km/s for H+, He+, and O+ are observed. Correlations of these plasma parameters, which were obtained approximately once per day, with magnetic activity (Kp) suggest response times of 2–3 days between the onset of Kp changes and the establishment of new, large‐scale, steady state conditions. The implications of these results with regard to ionospheric plasma dynamics associated with depletion and replenishment of high‐latitude flux tubes are discussed.
Proton and alpha particle fluxes in the solar wind: Results of a three‐fluid model
Authors: Bürgi Alfred
Published: February 2008

A three‐fluid model for the solar wind, with continuity, momentum, and energy equations for the three species protons, alpha particles, and electrons, including electron heat conduction and a parameterized coronal heat source, is used to study the behavior of the resulting proton and alpha fluxes as a function of coronal energy input and the proton‐to‐alpha density ratio at the coronal base. While in model solutions without alpha particles the proton flux is a strongly increasing function of coronal temperature, quite opposite to the observed near constancy of the proton flux in the solar wind, solutions with alpha particle abundances of the order of a few percent at the coronal base have a reduced proton flux and much less variation of the proton flux. The mechanism by which the proton flux is reduced is a pileup of the alpha particle abundance in the region of the coronal temperature maximum and the increased frictional drag on the protons resulting therefrom. We find a range of model solutions in which the resulting proton flux is almost independent of coronal energy input or coronal temperature, and we conclude that proton‐alpha friction and dynamic accumulation of alphas in the corona can provide an explanation for the near constancy of the proton flux which is observed in the solar wind.
Propagation characteristics of dayside low‐altitude hiss: Case studies
Authors: Lefeuvre F. , Rauch J. L. , LaGoutte D. , Berthelier J. J. , Cerisier J. C.
Published: February 2008

A comprehensive set of wave analysis techniques is applied to dayside ELF hiss observed over three typical orbits of the low‐altitude AUREOL 3 satellite. Validity domains are established for each technique, whose results indicate the following propagation characteristics. Within the plasmasphere the waves are narrow‐band, have a lower cutoff frequency close to the local proton gyrofrequency ƒH+, and propagate downward with very oblique k vectors directed toward lower L values. Left‐hand mode waves are detected just below ƒH+. Within the plasmapause gradient the waves are broad‐band, have a lower cutoff frequency at the vicinity of ƒH+, and propagate mainly upward with oblique k vectors directed toward lower L values. Within the light‐ion trough region and the auroral zones the waves are broad‐band, have lower cutoff frequencies that can be below the local ƒH+, and propagate downward with k vectors along B0. Narrowband emissions detected in the vicinity of the cusp propagate upward. Exceptions are found at frequencies just above ƒH+ where, at nearly all invariant latitudes, waves are commonly upgoing. We conclude that (1) the reflection at the two‐ion cutoff frequency is a very efficient process for returning waves to higher altitudes and (2) the large density gradients that characterize the plasmapause and the cusp seem to act as traps for waves reflected below the satellite.
The great solar energetic particle events of 1989 observed from geosynchronous orbit
Authors: Reeves G. D. , Cayton T. E. , Gary S. P. , Belian R. D.
Published: February 2008

Los Alamos energetic proton instruments at geosynchronous orbit observed more major solar energetic particle events during 1989 than any other year since this series of detectors began observations in 1976. The temporal flux profiles of four intervals, which contain six distinct events, are compared illustrating the uniqueness of each event. Characteristic risetime and decay time are computed for each event. During two of these events, brief order‐of‐magnitude increases of the proton flux are observed. They are associated with sudden commencement events and dramatic changes in the solar wind. We conclude that these two brief events are likely the result of shock acceleration in the solar wind. We have fit the measured count rates to a spectral form which is exponential in rigidity, and we have examined the changes in spectral slope with time for each of the four intervals. In general, harder spectra are measured near the onset of an event followed by a softening of the spectrum as the fluxes decay. We have also investigated the effects of these events on geomagnetic activity by comparing the fluxes of >30‐keV electrons at geosynchronous orbit and Kp geomagnetic index during the early part of two of the solar energetic particle events.
Prediction probabilities from foreshocks
Authors: Agnew Duncan Carr , Jones Lucile M.
Published: February 2008

When any earthquake occurs, the possibility that it might be a foreshock increases the probability that a larger earthquake will occur nearby within the next few days. Clearly, the probability of a very large earthquake ought to be higher if the candidate foreshock were on or near a fault capable of producing that very large mainshock, especially if the fault is towards the end of its seismic cycle. We derive an expression for the probability of a major earthquake characteristic to a particular fault segment, given the occurrence of a potential foreshock near the fault. To evaluate this expression, we need: (1) the rate of background seismic activity in the area, (2) the long‐term probability of a large earthquake on the fault, and (3) the rate at which foreshocks precede large earthquakes, as a function of time, magnitude, and spatial location. For this last function we assume the average properties of foreshocks to moderate earthquakes in California: (1) the rate of mainshock occurrence after foreshocks decays roughly as t−1, so that most foreshocks are within three days of their mainshock, (2) foreshocks and mainshocks occur within 10 km of each other, and (3) the fraction of mainshocks with foreshocks increases linearly as the magnitude threshold for foreshocks decreases, with 50% of the mainshocks having foreshocks with magnitudes within three units of the mainshock magnitude (within three days). We apply our results to the San Andreas, Hayward, San Jacinto, and Imperial faults, using the probabilities of large earthquakes from the report of the Working Group on California Earthquake Probabilities (1988). The magnitude of candidate event required to produce a 1% probability of a large earthquake on the San Andreas fault within three days ranges from a high of 5.3 for the segment in San Gorgonio Pass to a low of 3.6 for the Carrizo Plain.
Seismic fluids and percolation theory
Authors: Stark Colin P. , Stark J. Alex
Published: February 2008

We present a first attempt at a quantitative assessment of seismic fluid flux. It is assumed that during an earthquake, slipped regions experience transient high permeability, high porosity, and high pressure gradients, which rapidly redistribute fluid. We use percolation theory to model the size distribution of such slip planes. Interseismic fluid migration along background pressure gradients is ignored. To render the problem tractable, we make a number of assumptions regarding fluid movement: fluid is transported from a well supplied reservoir across the fault plane to an efficient sink; the system reaches steady state with a linear fluid concentration function across the fault. Using probability methods, we obtain a simple relation between fluid flux, slip rate, and fracture porosity. It is then a straightforward matter to calculate the total seismic fluid throughput for a region. Such estimates agree well with water/rock ratio and other data.
Imaging global body wave phases by stacking long‐period seismograms
Authors: Shearer Peter M.
Published: February 2008

Stacked record sections combining over 5 years of long‐period Global Digital Seismograph Network (GDSN) seismograms from shallow events show dozens of seismic body wave phases. Separate images are obtained for the vertical, radial, and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking. These images provide an indication of the relative signal‐to‐noise characteristics of different parts of the GDSN data set. Core‐mantle boundary diffracted phases are particularly prominent in these stacks, which show Pdiff to 150°, PKPdiff to 200°, PPdiff to 230°, Sdiff to 160°, and SSdiff to 220°. High‐order S surface multiples can be seen extending through the surface wave train up to ranges of 540°. Two examples of possible upper mantle discontinuity reflected phases are visible, one following PPP between about 95° and 125°, the other following ScS3 between about 210° and 240°. An apparent offset between SH and SV arrivals is seen which becomes larger for the higher‐order S multiples (SSS, SSSS, etc.), and appears consistent with models containing transverse isotropy in the upper mantle.
High‐frequency paleovariability in climate and CO2 levels from Vostok Ice Core Records
Authors: Yiou P. , Genthon C. , Ghil M. , Jouzel J. , Le Treut H. , Barnola J. M. , Lorius C. , Korotkevitch Y. N.
Published: February 2008

The high resolution of the Vostok records provides a unique look at the causes of paleoclimatic variability during the last complete glacial cycle. The records present strong evidence for the interaction between orbital forcing and internal, physico‐chemical mechanisms of variability. This interaction appears to account for the great wealth of spectral features found in the records.
Global Earth dynamics and induced gravity changes
Authors: Hinderer Jacques , Legros Hilaire , Crossley David
Published: February 2008

We consider the elastogravitational deformation of an Earth model with a liquid outer core and a surficial fluid layer modeling the oceans or the atmosphere. The surface gravity changes caused by planetary‐scale dynamical phenomena can be expressed in terms of generalized gravimetric factors. Each of these factors is related to a specific boundary condition (in normal or transverse traction, in potential and derivative) which has to be considered at the different interfaces (inner core‐outer core, outer core‐mantle, surface) according to the problem. We apply this formalism to express in a self‐consistent and uniform manner the gravity disturbances due to various sources: tides, Earth rotation, loading process, inner and outer core dynamics, with special attention paid to possible resonance effects.
Eocene potassic magmatism in the Highwood Mountains, Montana: Petrology, geochemistry, and tectonic implications
Authors: O'Brien Hugh E. , Irving Anthony J. , McCallum I. Stewart
Published: February 2008

Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north‐central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite‐diopside‐phyric minettes to yield heteromorphic leucite‐salite‐phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba‐LREE‐enriched, U‐Th‐HFSE‐depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite‐veined harzburgite and phlogopite dunite xenoliths (one with ‐εNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnöitic diatremes) and a young subduction‐related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr‐87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low‐angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite‐rich, metasome veins upon ascent through the Wyoming Craton mantle keel.
Monitoring resistivity changes prior to earthquakes in Parkfield, California, with telluric arrays
Authors: Park Stephen K.
Published: February 2008

A telluric array has been designed to monitor long‐period changes of resistivity in Parkfield prior to the next characteristic earthquake there. The array straddles the San Andreas fault and utilizes dipoles which range from 5 to 18 km long. A method of data analysis using robust processing in the time domain and singular value decomposition yields daily estimates of telluric coefficients which are stable to within 1 %. The array has detected several fluctuations above the noise level of the measurements. Most of these fluctuations are uncorrelated with bursts of seismic activity, changes in water levels, anomalous strain and creep, or magnetic storms. However, the largest fluctuation in 2 years coincided with a tensor strain anomaly, and both of these preceded a M=3.7 earthquake by approximately 1 month. This is the largest earthquake to occur in Parkfield since the array was installed.
Aftershock sequences of intermediate‐depth earthquakes in the Pamir‐Hindu Kush Seismic Zone
Authors: Pavlis Gary L. , Hamburger Michael W.
Published: February 2008

We examine the space‐time seismicity patterns associated with the 40 largest (M≥5.6), intermediate‐depth earthquakes that occurred in the Pamir‐Hindu Kush zone between 1962 and 1984 to determine if any of these events were followed by aftershock sequences. To address this issue we examined data from the Soviet Central Asia regional earthquake catalog with two complementary techniques: (1) a subjective technique based on inspection of epicentral maps, and (2) a quantitative method which assumed that the background seismicity was a Poisson process and which detected aftershocks as significant departures, as quantified by the Poisson model, from background rates. Of the 40 events we examined, only three were identified by both methods as having clearly defined aftershock sequences. These three events are also remarkably similar in several other ways: (1) they occurred at almost the same point in space, (2) they have indistinguishable focal mechanisms, (3) they have similar magnitudes, and (4) they define two nearly equal time intervals (9.37 and 9.41 years). No other events in the Pamir‐Hindu Kush zone exhibit comparable aftershock behavior. Six other events show rate anomalies, but the subjective method rated them negative or at least ambiguous. Four of these six we argue are caused by coincidental or man‐made rate increases at the time of the mainshock. The remaining two are noteworthy because they are the two largest events to have occurred beneath the central Pamir. Each of these events shows a small, short burst of seismicity around the time period of the largest event in the sequence. We suggest two alternative hypotheses to explain our observations: (1) more events may have aftershock sequences, but the aftershocks are too small to be detected; or (2) the three events that had clear aftershocks are unique, and trigger a burst of aftershocks only because they have a special relationship to the the most active nest of activity in the Hindu Kush zone.