bluuebunny/crossref_metadata_2025 huggingface dataset:
Aspect sensitivity of equatorial electrojet irregularities and theoretical implications
Authors: Kudeki Erhan , Farley Donald T.
Published: February 2008

We used a multi baseline 50‐MHz radar interferometer at the Jicamarca Radio Observatory to study the dependence of echo strength on the aspect angle, the angle of arrival relative to normal to the geomagnetic field. We observed rms values of this angle in the range 0.1°–0.4°, which implies an aspect sensitivity somewhat greater than the nominal 10 dB/deg often quoted for auroral echoes. The technique has sufficient resolution to allow deviations from normal of less than 0.1° to be measured fairly easily, and we were able to determine variations of the aspect sensitivity with altitude, Doppler shift, and electrojet conditions. During weakly driven periods, when only type 2 echoes were observed, the width, or rms angular deviation from normal, decreased monotonically with increasing altitude, as simple linear theory would suggest, but the variation was less rapid than the variation of νe/Ωe, especially on the bottomside of the electrojet. A particularly interesting observation was that, during strongly driven “type 1” conditions, there was a pronounced variation of width with Doppler shift. The width for small phase velocities was sometimes more than twice that at the ion‐acoustic velocity. We believe that this broadening is due to nonlinear three‐wave mode coupling which produces a reverse cascade of energy from short wavelengths to long and an increase in the mean aspect angle. Such a cascade represents an increase in the effective diffusion coefficient for the short wavelength waves and so could provide an explanation alternative to that of the anomalous collision process of Sudan (1983) for the limitation of the type 1 phase velocity at the ion‐acoustic velocity.
Satellite observation of atmospheric nuclear gamma radiation
Authors: Letaw John R. , Share G. H. , Kinzer R. L. , Silberberg R. , Chupp E. L. , Forrest D. J. , Rieger E.
Published: February 2008

We present a satellite observation of the spectrum of gamma radiation from the Earth's atmosphere in the energy interval from 300 keV to 8.5 MeV. The data were accumulated by the gamma ray spectrometer on the Solar Maximum Mission over 3½ years, from 1980 to 1983. The excellent statistical accuracy of the data allows 20 atmospheric line features to be identified. The features are superimposed on a continuum background which is modeled using a power law with index −1.16. Many of these features contain a blend of more than one nuclear line. All of these lines (with the exception of the 511‐keV annihilation line) are Doppler broadened. Line energies and intensities are consistent with production by secondary neutrons interacting with atmospheric 14N and 16O. Although we find no evidence for other production mechanisms, we cannot rule out significant contributions from direct excitation or spallation by primary cosmic ray protons. The relative intensities of the observed line features are in fair agreement with theoretical models; however, existing models are limited by the availability of neutron cross sections, especially at high energies. The intensity and spectrum of photons at energies below the 511‐keV line, in excess of a power law continuum, can be explained by Compton scattering of the annihilation line photons in traversing an average of ∼21g cm−2 of atmosphere.
Solar wind interaction with the ionosphere of Venus inferred from radio scintillation measurements
Authors: Woo Richard , Sjogren William L. , Luhmann Janet G. , Kliore Arvydas J. , Brace Larry H.
Published: February 2008

This paper presents the first S‐band (2.3 GHz) radio scintillations observed in the ionosphere of Venus and discovered when the Pioneer Venus Orbiter spacecraft traversed the ionosphere of Venus. In situ plasma measurements as well as propagation calculations confirm that the scintillations are caused by electron density irregularities in the topside ionosphere of Venus below the ionopause. While these topside plasma irregularities have not been studied before, simultaneous magnetic field measurements presented here reveal that they are associated with the penetration of large‐scale magnetic fields in the ionosphere. Previous studies based on extensive magnetic field measurements have shown that the presence of large‐scale magnetic fields occurs in the subsolar region when the solar wind dynamic pressure exceeds the ionospheric plasma pressure. As with the large‐scale magnetic fields, the disturbed plasma and resulting scintillations are therefore a manifestation of high‐dynamic solar wind interaction with the ionosphere. Since the scintillations only occur in the subsolar region of Venus, the global morphology of ionospheric scintillations at Venus is different from that of the terrestrial ionosphere, where scintillations are observed in both polar and equatorial regions, with peaks occurring during nighttime. This difference apparently stems from the fact that Venus is not a magnetic planet. We also demonstrate that the disturbed plasma produced by the high‐dynamic solar wind interaction can be remotely sensed by scintillations during radio occultation measurements, that is, when the spacecraft is outside the ionosphere. Such observations can therefore provide important information on the high‐dynamic solar wind interaction of nonmagnetic planets such as Venus and Mars when in situ measurements are not available. Venus is of particular interest at solar cycle minimum, when high‐dynamic conditions may be expected to be more prevalent.
Radio emission from the magnetic equator of Uranus
Authors: Kaiser M. L. , Desch M. D. , Connerney J. E. P.
Published: February 2008

The major observational characteristics of the smooth, narrow bandwidth component of Uranus' radio emissions are well described by sources radiating near the local electron gyrofrequency, confined to the magnetic equatorial plane and encircling the planet at radial distances of approximately 2 to 3 RU. The most intense emission appears to be generated in association with the ε ring at 2.0 RU radial distance. We infer a cold electron density of ≤4 cm−3 in this region.
Estimation of ionospheric electrodynamic parameters using ionospheric conductance deduced from Bremsstrahlung X ray image data
Authors: Ahn B.‐H. , Kroehl H. W. , Kamide Y. , Gorney D. J.
Published: February 2008

Various ionospheric electrodynamic parameters for the period July 23–24, 1983, are calculated by using ground magnetic records from a total of 88 stations in the northern hemisphere. For this purpose, an “instantaneous” conductance distribution deduced from the DMSP‐F6 bremsstrahlung X ray image data is utilized. Since the conductance distribution is, for the first time, completely independent of ground magnetic data, it is a unique opportunity to examine some of the inherent ambiguity in the magnetogram‐inversion technique based on a statistically derived conductance model. Several important conclusions of this study are as follows: (1) The poleward portion of the westward electrojet in the morning sector is dominated by the electric field, while its equatorward portion is dominated by the ionospheric conductance. Although less definite, a reverse trend seems to pervade the eastward electrojet region in the dusk sector. (2) During a quiet or moderately disturbed period, the major electric potential pattern is roughly circumscribed by the auroral zone conductance belt with the subauroral zone being a substantially lower electric field region. (3) The global pattern of the equivalent current system resembles the electrical potential distribution during summer conditions. It may thus be possible to use the equivalent current system as a reasonable approximation of the electric potential distribution in studying the global pattern of magnetospheric convection for prevailing sunlit conditions. (4) The electric potential distribution consists generally of a smooth and well‐defined two‐cell convection pattern without any significantly localized structure. (5) A sunward convection flow is clearly identified over the polar cap region during strongly northward IMF periods. The multicell nature of the convection pattern is still unclear. (6) During strongly northward IMF periods, significant currents and Joule dissipation are observed in the polar cap region, indicating that the magnetosphere is far from its ground state. (7) The regions of intense Joule heating are generally confined to relatively narrow belts along the auroral electrojets, with the major heating region in the westward electrojet region shifted poleward and the one in the eastward electrojet region shifted equatorward. The Joule dissipation rate is relatively low in the local midnight sector. (8) The presently available statistical conductance models can be used, as a first approximation, to study global‐scale polar ionospheric electrodynamics. However, the fact that the statistical models cannot simulate an instantaneous situation severely restricts their usefulness for studying the spatial and temporal variations of individual substorms.
Charge exchange cross sections for the Io plasma torus
Authors: McGrath M. A. , Johnson R. E.
Published: February 2008

An impact parameter method for calculating cross sections as a function of incident ion energy is used in conjunction with an improved exchange energy formulation to update several of the charge exchange cross sections currently used in Io plasma torus modeling. New cross sections for S+ + S2+ → S2+ + S+ and Na+ on neutral targets, useful in analyzing the fast Na jets observed at Io, are also calculated.
Ion distributions and thermalization at perpendicular and quasi‐perpendicular supercritical collisionless shocks
Authors: Burgess D. , Wilkinson W. P. , Schwartz S. J.
Published: February 2008

Computer simulations and observations in laboratory and space plasmas have revealed that some incident ions are reflected at perpendicular and quasi‐perpendicular high Mach number (i.e., supercritical) shocks. Moreover, these studies have established that the gyration and subsequent thermalization of these ions play a dominant role in the the shock dissipation process. We use a hybrid kinetic simulation in order to study the selection of an incoming ion as either reflected‐gyrating or transmitted. We find that the reflected ions come from a limited region in the upstream velocity space distribution. None of the reflected ions come from the core of the distribution. Whether a particle becomes reflected depends on its energy in the upstream frame and its gyrophase as it encounters the shock. In the simulations we have carefully identified and separated the two subpopulations: transmitted and reflected. The transmitted ions do not heat appreciably in passing through the shock, although there may be wave‐particle effects neglected in the hybrid simulation. There is a contribution to the total downstream pressure due to the gyration of the two subpopulations about their common center of mass, in addition to the pressure associated with the reflected component relative to its own center of mass.
The equilibrium dayside magnetosphere
Authors: Zavriyev Anton , Hasegawa Akira
Published: February 2008

We present a method of computing the dayside global Earth magnetic field which is in equilibrium with the plasma pressure based on satellite observations at a local region of the magnetosphere. The method, which utilizes a perturbation around a dipole magnetic field, involves computation of the global plasma pressure profile, P⊥(r, θ) and P∥(r, θ) based on the equatorial (anisotropic) pressure data P⊥(r, π/2) and P∥(r, π/2), derivation of the current profile which satisfies the equilibrium condition, J × B − ▽ · P = 0, and computation of the magnetic field using the current profile and the boundary current produced by the solar wind. The method is applied for the Active Magnetospheric Particle Tracer Explorers data, and the result of the computation is found to compare reasonably well with the observed magnetic field profile near the geomagnetic equator.
Ground‐satellite study of a Pc 1 ion cyclotron wave event
Authors: Fraser B. J. , Kemp W. J. , Webster D. J.
Published: February 2008

The magnetospheric generation and propagation characteristics of ion cyclotron waves associated with a Pc 1 emission observed by a network of four middle and low‐latitude ground stations are determined using ground source location techniques and ISEE 1 plasma data. The source region at L = 4.7 ± 0.7 is determined from propagation in the F2 region ionospheric duct using wave polarization characteristics at ground stations. This source is just inside the steep plasmapause seen by ISEE 1 at L = 4.9 ± 0.1. The ion cyclotron wave packet interhemispheric bounce period measured from the ground spectra increases with time from 140 to 155 s during the event but is in agreement with dispersion calculations undertaken using ISEE 1 electron density and cool to cold ion composition data in a H+ plasma with 8–10% He+ and <1% 0+ ions. The frequency of the Pc 1 emission band seen on the ground (0.5–0.8 Hz) corresponds to the propagation region between the equatorial 0+ cutoff frequency and the He+ cyclotron frequency. Linear convective growth rate calculations inside the plasmapause show significant wave amplitudes in this band. With these results it is possible to completely describe the wave generation mechanism and the magnetosphere ionosphere propagation characteristics for the Pc 1 event.
Continuum radiation at Uranus
Authors: Kurth W. S. , Gurnett D. A. , Desch M. D.
Published: February 2008

Uranus has proven to be a radio source of remarkable complexity with as many as six distinctly different types of emission. One Uranian radio emission which has thus far escaped attention is an analog of continuum radiation at Earth, Jupiter, and Saturn. The emission is found to be propagating in the ordinary mode in the range of one to a few kHz on the inbound leg of the Voyager 2 encounter, shortly after the magnetopause crossing. The continuum radiation spectrum at Uranus also includes bands with frequencies as high as 12 kHz or greater on both the inbound and outbound legs. The Uranian continuum radiation is notably weak, making it more like that detected at Saturn than the extremely intense Jovian continuum radiation. The Uranian emission shows some evidence for narrow‐band components lying in the same frequency regime as the continuum, completing the analogy with the other planets, which also show narrow‐band components superimposed on the continuum spectrum. We argue that the low intensity of the Uranian continuum is most likely related to the lack of a density cavity within the Uranian magnetosphere that is deep relative to the solar wind plasma density.
Trapped electrons as a free energy source for the auroral kilometric radiation
Authors: Louarn P. , Roux A. , de Féraudy H. , Le Quéau D. , André M. , Matson L.
Published: February 2008

Simultaneous measurements of electromagnetic fields and particle distributions, measured during the crossing by the Swedish spacecraft Viking of an auroral kilometric radiation (AKR) source, are presented. It is shown that AKR is generated within an acceleration region characterized by an upward directed parallel electric field, as evidenced by its signature on the proton and electron distributions. From particle observations inside the AKR source it is clear that the potential drop below the spacecraft produces upward moving field‐aligned ion beams and a depletion in the density of low energy electrons. The potential drop above the spacecraft produces downward accelerated electrons. A large fraction of these electrons have small parallel velocities; they mirror above the ionosphere. These trapped electrons lie in a region of velocity space which should be empty in a simple adiabatic theory. We suggest that these electrons get trapped when they experience a time‐varying (or space‐varying) parallel electric field. This conclusion is supported by the comparison between the observed electron distribution function and a model distribution function built by applying Liouville theorem. Since trapped electrons can cause positive gradients (∂fe/∂V⊥ > 0) over a broad range of parallel velocities, around v⫽ ≈ 0, it is suggested that they are the free energy source for the AKR. This conclusion is substantiated by an evaluation of the convective growth rate, where the various input parameters have been determined by fitting particle data.
The Venus nitric oxide night airglow: Model calculations based on the Venus thermospheric general circulation model
Authors: Bougher S. W. , Gérard J. C. , Stewart A. I. F. , Fesen C. G.
Published: February 2008

Pioneer Venus (PV) orbiter ultraviolet spectrometer (OUVS) images of the nightside airglow in the (0, 1) δ band of nitric oxide showed a maximum whose average location was at 0200 local solar time just south of the equator. The average airglow brightness calculated over a portion of the nightside for 35 early orbits during the Pioneer Venus mission was a factor of 4 lower than this maximum. Recent recalibration of the PV OUVS instrument and reanalysis of the data yield new values for this statistical maximum (1.9±0.6 kR) and the nightside average (400–460± 20 R) nightglow. This emission is produced by radiative recombination of N and O atoms transported from their source on the dayside to the nightside by the Venus thermospheric circulation. The Venus Thermospheric General Circulation Model (VTGCM) has been extended to incorporate odd nitrogen chemistry in order to examine the dynamical and chemical processes required to give rise to this emission. Its predictions of dayside N atom densities are also compared with empirical models based on Pioneer Venus measurements. Calculations are presented corresponding to OUVS data taken during solar maximum (F10.7 = 180–200). The average production of nitrogen atoms on the dayside is about 9.0×109 atoms cm−2 s−1. Approximately 30% of this dayside source is required for transport to the nightside to yield the observed dark‐disk nightglow features. The statistical location and intensity of the bright spot are well reproduced, as well as the altitude of the airglow layer. The importance of the large‐scale transport and eddy diffusion on the global N(4S) distribution is also evaluated. Uncertainties in measured rate coefficients and branching ratios and their effects on the calculated airglow are discussed. It is found that model parameters can be adjusted to reproduce either the OUVS nightglow or the observed dayside N densities; a best match to both implies a VTGCM global wind system that is a factor of 1.5–2.0 slower than at present. Predictions are also made for comparison with OUVS solar minimum data.
Ion thermalization in quasi‐perpendicular shocks involving reflected ions
Authors: Sckopke N. , Paschmann G. , Brinca A. L. , Carlson C. W. , Lühr H.
Published: February 2008

Ion thermalization mechanisms downstream of the quasi‐perpendicular Earth's bow shock are examined by means of plasma and magnetic field data from the AMPTE/IRM spacecraft which include three‐dimensional ion distributions, plasma fluid parameters derived every ∼4.3 s, and spectra of transverse and parallel magnetic fluctuations up to 16 Hz. The objects studied in detail are low‐Mach number, low‐β shocks in which reflected‐gyrating ions are present and contribute to the downstream ion temperature but where processes beyond the ramp take place slowly, so that the basic phenomenology becomes apparent. In MHD terms, most of these shocks qualify as marginally critical. Downstream of the ramp, the initially separated core and ring ions slowly merge into a joint, less anisotropic distribution possessing a high‐energy tail. The ion temperature ratio, T⊥/T∥, is high not only in the shock foot and ramp but also within some distance downstream; its speed of decline rises and the residual level lessens with increasing β. The ions diffuse about equally fast in energy and in pitch angle. An asymmetry of the distributions with respect to the field direction is present when the shock is slightly oblique. It decays only slowly, which might indicate that the pitch angle diffusion rate near zero pitch angle is reduced. Low‐frequency electromagnetic waves are present below the proton gyrofrequency; they are characterized by strong left‐hand‐polarized emissions and a low level of parallel fluctuations except very close to the shock. The left‐hand emissions are often concentrated into a narrow frequency band but sometimes they exhibit a double‐humped structure. Waves and ion distributions approach a slowly varying equilibrium some distance downstream of the shock. After extending the analysis to one supercritical shock representing the majority of bow shock encounters, we conclude that our deductions are more generally valid, although the thermalization is faster, usually, and appears to involve nonlinear processes which tend to obscure most of the features noted.
Cyclotron resonance precipitation of energetic electrons from the inner magnetosphere
Authors: Datlowe D. W. , Imhof W. L.
Published: February 2008

Cyclotron resonance between trapped energetic electrons and VLF waves produces pitch angle scattering which leads to electron precipitation in the inner magnetosphere. Previous experiments have shown that in the drift loss cone at L values from 1.5 to 1.8 the energy spectrum of the electrons above 50 ReV is often dominated by a single narrow peak. The center energy of this peak varies with L in a manner characteristic of cyclotron resonance between the electrons and monochromatic VLF waves in the vicinity of the geomagnetic equator. The source of the waves is probably VLF communication or navigation transmitters. We report here the results of a study of 680 occurrences of these peaks detected by the low altitude polar orbiting satellite S81‐1. The present data, from altitudes between 170 and 270 km, show the resonance peaks only in two restricted longitude zones centered at 110°E and 300°E; this result contrasts with the previous measurements at higher altitudes, which detected peaks over a wide range of longitudes. The majority of nighttime events were near transmitter locations where the cyclotron resonance condition was met by electrons with pitch angles near the edge of the loss cone. Detailed study of the events occurring near 110°E reveals a 27‐day periodicity in the frequency of occurrence; peaks are most often observed during minima in solar/geomagnetic activity. The peaks observed at nighttime in both longitude zones have a possible seasonal variation, being most frequent when it is wintertime at the site of the VLF transmitters responsible for the precipitation.
The motion of magnetic flux tube at the dayside magnetopause under the influence of solar wind flow
Authors: Liu Z. X. , Hu Y. D. , Li F. , Pu Z. Y.
Published: February 2008

We propose that flux transfer events (FTEs) at the dayside magnetopause are formed by fluid vortices in the flow field. According to the view of vortex‐induced reconnection a FTE tube is a magnetic fluid vortex tube (MF vortex tube). The motion of a FTE tube can be represented by that of a MF vortex in the formation region located in the dayside magnetopause region. This study deals with the internal and external influences governing the motion of MF vortex tubes. The equations of motion of a vortex tube are established and solved. It is found that a FTE tube moves from low latitude to high latitude with a certain speed. However, the motional path is not a straight line but oscillates about the northward direction for the northern hemisphere. The motional velocity, amplitude and period of the oscillation depend on the flow field and magnetic field in the magnetosheath and magnetosphere as well as the size of the FTE tube.
On dust charges and plasma potentials in a dusty plasma with dust size distribution
Authors: Havnes Ove , Aanesen Torsten K. , Melandsø Frank
Published: February 2008

We have reconsidered the question of charges on dust particles and local potential of a collection of dust embedded in a plasma. Distribution of dust sizes and photoelectric effects are included. We show that the existence of small undetected dust can have a considerable effect on the charges in, for example, a planetary ring. We give simple rational function approximations to facilitate the calculations of dust charges and cloud potentials. The results are given for different ions and ratios between photoelectron and plasma electron fluxes.
Goose Bay radar observations of Earth‐reflected, atmospheric gravity waves in the high‐latitude ionosphere
Authors: Samson J. C. , Greenwald R. A. , Ruohoniemi J. M. , Frey A. , Baker K. B.
Published: February 2008

In the late fall and early winter, The Johns Hopkins University HF radar at Goose Bay, Labrador, observes the effects of atmospheric gravity waves on radar transmissions that are obliquely reflected from the ionosphere and subsequently backscattered from the Earth's surface. The waves exist under a wide variety of geomagnetic conditions; how ever, they are particularly noticeable under quiet conditions (0 ≤ Kp ≤ 1 +). The clearest signatures of the waves are spatially localized enhancements in the backscattered power and quasi‐periodic fluctuations in the backscattered powers, Doppler velocities, and reflection heights. The waves are generally observed during daylight hours and propagate equatorward from regions of high‐latitude ionospheric backscatter that are located near the ionospheric convection reversal boundary. The gravity waves appear to be generated just equatorward of the dayside flow‐reversal boundary in the vicinity of the auroral electrojet at altitudes of 115 to 135 km and propagate approximately perpendicular to the boundary along azimuths ranging from 156° to 180°. The waves propagate obliquely downward through the lower atmosphere until they are reflected by the Earth's surface back into the upper atmosphere. The frequencies associated with these gravity waves cover the range of 0.3 to 0.6 mHz, with wavelengths of 300 to 500 km, and with average phase velocities of 110 to 180 m/s. The maximum phase speeds are 270 to 300 m/s, which is slightly less than the speed of sound in the lower atmosphere. Poleward‐propagating gravity waves are sometimes observed under disturbed conditions when the polar cap and convection reversal boundary have expanded equatorward.
High‐resolution sounding rocket observations of large‐amplitude Alfvén waves
Authors: Boehm M. H. , Carlson C. W. , McFadden J. P. , Clemmons J. H. , Mozer F. S.
Published: February 2008

Shear Alfvén waves with amplitudes >100 mV/m were observed on two recent sounding rocket flights: one flown from Poker Flat, Alaska, to an altitude of 1040 km in the evening auroral zone, and another flown from Sondre Stromfjord, Greenland, to an altitude of 770 km in the morning cusp region. The largest waveforms are best described as a series of step functions, rather than as broadband noise or as single frequency waves. Complete two‐dimensional E and B measurements at 4‐ms time resolution were made, showing a downward propagation direction and implying insignificant reflection from the ionosphere at frequencies greater than 1 Hz. Intense, field‐aligned, low‐energy electron fluxes accompany the waves. Acceleration of these electrons by the Alfvén waves, either through the resonant acceleration mechanism described by Temerin et al. (1986), or through the nonresonant mechanism described by Goertz and Boswell (1979), is shown to be feasible. The waves in at least one case have a sufficiently large ponderomotive potential to generate the observed density fluctuations of order 1.
Dependence of stimulated electromagnetic emission on the ionosphere and pump wave
Authors: Leyser T. B. , Thidé B. , Derblom H. , Hedberg Å. , Lundborg B. , Stubbe P. , Kopka H.
Published: February 2008

Electromagnetic emission, stimulated by a powerful high frequency radio wave injected into the ionospheric F region, exhibits a rich spectral structure in the sidebands of the reflected pump wave. Results from such experiments at the Heating facility near Tromsø, Norway, are presented. Spectral features in a steady state, a few seconds after the onset of the pump, are shown to depend on ionospheric conditions, the pump frequency, and power. Different types of stimulated radiation spectra may be obtained when the ionospheric critical frequency is well above or near the pump frequency. Further, the appearance of the spectral features is very sensitive to the pump frequency near harmonics of the F region electron cyclotron frequency. Our interpretation is that the considered stimulated electromagnetic emissions are generated primarily in two interaction regions, the plasma resonance region at the pump reflection height and the upper hybrid resonance region, typically a few kilometers below the reflection height.
Theory of electromagnetic cyclotron wave growth in a time‐varying magnetoplasma
Authors: Gail William B.
Published: February 2008

The time‐dependent growth rate for parallel propagating electromagnetic cyclotron waves is derived for a magnetoplasma which is characterized by a time dependent compressional perturbation superimposed on an equilibrium configuration. Such perturbations are commonly observed in the Earth's magnetosphere as a consequence of resonant field line oscillations, solar‐wind disturbances, and other phenomena. The time dependencies of the magnetic field, thermal plasma density, energetic particle distribution function, and resonance condition are first related through a single dimensionless lime parameter b(t) using the ideal MHD assumption. For cases in which the particle distribution can be described by F(α, E) = f(E)sina(E)α, the time dependent wave growth rate is then given by γ ≃ γ0(1+Γ) where γ0 is the equilibrium growth rate and Γ(b) is a function of the equilibrium parameters and the time parameter b. The term |Γ| is generally small compared to 1, and the effect is a small modulation of the equilibrium growth rate by Γ. If the particle distribution is locally near marginal stability, however, |Γ| is large compared to 1, and the growth rate modulation can be much larger than for a distribution which is not near marginal stability. The results suggest that particle populations which are near marginal stability may be strongly influenced by perturbations in the magnetic field and plasma. Marginally stable distributions may thus play an important role in magnetospheric dynamics as well as determination of radiation belt characteristics.
Vertical seismic noise at very low frequencies
Authors: Agnew Duncan Carr , Berger Jon
Published: February 2008

We use records from the Project IDA modified LaCoste gravimeters to investigate ground noise at. frequencies from 1 to 10 mHz. At most sites the level between 2 and 10 mHz is nearly flat and close to 2 × 10−18 m2 s−3 Much higher values which are observed at island and coastal stations are due to loading by waves trapped along the shore. Data from the superconducting gravimeter at Piñon Flat Geophysical Observatory show that the noise power increases as ƒ−2.7 for frequencies between 1 and 0.001 mHz.
Mars atmospheric phenomena during major dust storms, as measured at surface
Authors: Ryan J. A. , Henry R. M.
Published: February 2008

Meteorological instrumentation aboard the Viking Mars Landers measures wind, temperature, and pressure. Two global dust storms occurred during northern autumn and winter, observed both by the orbiters and by the landers. The meteorological data from the landers has been analyzed for the period just before first storm arrival to just after second storm arrival, with the objectives being definition of meteorological phenomena during the storm period, determination of those associated with storm and dust arrival, and evaluation of effects on synoptic conditions and the general circulation. Times of dust arrival over the sites could be defined fairly closely from optical and pressure (solar tide) data, and dust arrival was also accompanied by changes in diurnal temperature range, temperature maxima, and temperature minima. The arrivals of the storms at VL‐1 were accompanied by significant increase in wind speed and pressure. No such changes were observed at VL‐2. It is possible that surface material could have been raised locally at VL‐1. Throughout the period except following the second dust storm the synoptic picture at VL‐2 was one of eastward moving cyclonic and anticyclonic systems. These disappeared following the second storm, a phenomenon which may be related to the storm.
The Martian polar vortex: Theory of seasonal variation and observations of eolian features
Authors: French Richard G. , Gierasch Peter J.
Published: February 2008

Observations of eolian features in the south polar region of Mars show strikingly axisymmetric patterns. Dark streaks and splotches show a pattern spiraling away from the pole in a retrograde direction. Frost streaks appearing in the lee of craters within the polar cap show a spiral away from the pole in a prograde direction. We present an explanation for the patterns in terms of the simplest possible dynamical model. We assume axisymmetric steady flow and adopt the Boussinesq approximation. Atmospheric temperatures are assumed to be known. The vertical exchange of momentum is parameterized in terms of a constant eddy viscosity. The swirl component of the surface stress is controlled by the transfer of angular momentum by the Hadley circulation and by the spin‐up or spin‐down produced by net mass transfer associated with condensation or sublimation of the seasonal CO2 cap. The results show excellent agreement with the observations, suggesting that the important momentum exchange processes have been included in the model. Surface stresses are largest during rapid condensation or sublimation of the cap. Between Ls = 40° and 140° a strong prograde stress field matches the observed frost streak directions. During recession of the CO2 cap the swirl is reversed, and large retrograde surface stresses are predicted between Ls = 200° and 260° consistent with dark streak directions. When the CO2 cap is absent, stresses are greatly reduced.
Is the Martian lithosphere sulfur rich?
Authors: Clark Benton C. , Baird A. K.
Published: February 2008

It is proposed that the high sulfur measured in Martian fines may represent more than enrichment by upward migration of soluble sulfate salts. It may in fact reflect intrinsic surface abundances of S much higher than for the earth or moon. High S in the regolith is permitted by several alternative models including remnant primitive lithosphere, a primitive component in the regolith, or trapping of volcanic gases. The alternative models encompass various features of the accretion chemistry, thermal history, and core size‐mass relationships that have been proposed for Mars. Possible consequences of ubiquitous sulfur include mantle mineral assemblages with unusual properties, significant sulfide content of unweathered rocks, high initial weathering rates and susceptibility to erosion of the surface, very high level of incorporation of H2O and O2 by surface fines, recycling of carbon and nitrogen, paleoclimatic effects of extinct atmospheric sulfur‐containing gases, and a previous, if not contemporary, basis for biological activity without photosynthesis. The Martian regolith may also contain heavy volatile elements in relatively high abundance and a significant component of late accretionary material.
Shock deformation of brittle solids
Authors: Grady D. E.
Published: February 2008

In terms of shock‐compression characteristics, minerals of geophysical interest are placed in a class of materials which, because of some rather unique Hugoniot and shock‐compression properties, are classified as brittle solids. The physical processes responsible for these shock‐wave properties are not yet well established. However, recent experimental observations and theories based on localized, nonhomogeneous shear deformation and a transient nonuniform thermal state appear to provide a reasonable qualitative picture. In the present work, some of the terminal and transient shock‐compression features observed in brittle solids are reviewed with particular emphasis on Hugoniot release‐wave measurements. The possibility of instabilities in the laws governing shear deformation leading to observed heterogeneous deformation is considered, and although an elementary model is treated, the method shows promise of predicting the material and kinematic properties governing occurrence, growth, and frequency of localized deformation features. Transient stress wave calculations in crystalline quartz demonstrate how the kinematic environment governing instability growth is established under shock‐wave compression. In addition, the transient nonuniform thermal state resulting from heterogeneous deformation is shown to provide a possible explanation for the observation of both ‘fluidlike’ and ‘solidlike’ shock release waves depending on the competing properties of thermal diffusion, melting temperature, and degree of thermal localization. The analysis shows a striking difference between those minerals which do, and do not, undergo a shock‐induced phase transition and leads to speculated similarities between the kinetics of shock‐induced phase transformation in brittle solids and the kinetics of thermal detonation in explosives.
A predictive macroscopic integral radiation efficiency model
Authors: ReVelle Douglas O.
Published: February 2008

A procedure is developed which unites the photometric and dynamic methods of calculating large meteoroid properties from their observed behavior in the atmosphere. By utilizing a linear separation of variables solution for the kinetic energy depletion factor in the classical single‐body entry model the relationship between this factor and the various efficiencies of heat, radiation, ionization, compressional waves, etc. is obtained. By using this equivalency the radiation efficiency (integral value of the fractional ‘light’ production) can be calculated immediately without recourse to the complex quantum mechanical considerations first introduced by Öpik if only the ratios of the various efficiencies to one another are known. By using the free molecular flow value of these ratios the radiation efficiency is predicted via this macroscopic approach without using further meteor data. If other nonintegral values of the radiation efficiency, such as those of ReVelle and Rajan (1979), are converted to the present definition, good agreement between the methods is obtained. Such agreement is also found for integral radiation efficiency values computed recently by Ceplecha for a number of the Prairie Network fireballs. These results suggest that the above ratios do not vary greatly between the extremes of free molecular and continuum flow. This conclusion is also consistent with the results of earlier workers.
Seismicity and tectonics of the Philippine Islands
Authors: Acharya H. K. , Aggarwal Y. P.
Published: February 2008

Seismic and volcanic activity in the Philippine Islands was examined in an attempt to decipher the tectonics of this region. Several new fault plane solutions for shallow, intermediate, and deep focus earthquakes were determined. This study has revealed the presence of a zone of eastward underthrusting in the western Philippines which is well developed near Negros Island. Fault plane solutions of several shallow earthquakes in the western Philippines show thrust faulting with slip vectors toward east or northeast. As active eastward subduction of the Eurasian plate is also taking place along the Manila trench near west central Luzon, it suggests that the underthrusting of the Eurasian plate may have occurred at one time along the western Philippines from Taiwan to Sulawesi in the Molucca Sea. Subduction has ceased along sections where continental crust is present. This interpretation is consistent with the geology and gravity anomalies in the area. Near the eastern Philippines the westward subduction of the Philippine Sea plate occurs (1) along the Philippine trench and (2) in a localized zone near the western edge of the Benham rise. The Philippine Islands are therefore flanked in the east and west by active but disjointed subduction systems. Left lateral strike slip faulting has been deduced near one end of several of these active trench systems and suggests movement on transverse features. Seismic activity on the Philippine fault is concentrated in the zone between 10°N and 15°N and appears to be due to stresses generated by opposing movements of the Philippine and Eurasian plates which are not released in underthrusting. Fault plane solutions of shallow earthquakes associated with the Philppine fault show left lateral strike slip motion consistent with field observations. Our study suggests that the extent and magnitude of earthquake activity on the Philippine fault forms one component of movement between the Eurasian plate and the Philippine Sea plate.
Paleomagnetic evidence for tectonic rotation of Jurassic plutons in Blue Mountains, eastern Oregon
Authors: Wilson Douglas , Cox Allan
Published: February 2008

To test the hypothesis that pre‐Tertiary rocks of eastern Oregon have undergone clockwise rotation, a paleomagnetic investigation was made of granitic and high‐temperature metamorphic rocks of the upper Jurassic‐Lower Cretaceous intrusive suite of the Blue Mountains province, northeastern Oregon. At nine sampling sites in three widely separated regions the paleomagnetic directions showed a high level of intrasite and intersite consistency, with a mean direction of I = 63°, D = 30°, a95 = 10°. The observed inclination is consistent with that expected on the basis of paleomagnetic results for stable North America, indicating that the intrusives have not undergone northward translation since cooling. The paleomagnetic declination, on the other hand, indicates that the region sampled has undergone a clockwise rotation of 60°±29° relative to the stable craton. Two of the bodies sampled are intrusive into the exotic Seven Devils terrane, and the remaining body is intrusive into the tectonic melange of a separate arc‐trench terrane, which may well have been the North American margin. The consistency in magnetic directions between the three regions (k = 151) indicates that suturing of the exotic terrane took place prior to the emplacement of the intrusives. The early Tertiary Clarno Formation situated to the west of the region sampled has rotated substantially less than the Blue Mountains [Beck et al., 1978], indicating that much of the rotation of the Blue Mountains occurred prior to the Eocene. To account for the observed rotations, we discuss two alternative models. In the first, rotation accompanies back arc rifting associated with the Early Cretaceous westward rifting of the Klamath Mountains. In the second, rotation reflects Late Cretaceous regional dextral shearing in the Blue Mountain area, which is located several hundred kilometers south of major right lateral faults known to extend from Alaska down at least to northern Washington.
The elevation of volcanoes and their edifice heights at subduction zones
Authors: Ben‐Avraham Zvi , Nur Amos
Published: February 2008

The elevation above sea level of circum‐Pacific volcanoes situated on continental crust varies greatly, not only between various chains but also within chains. Their edifice heights, however, are essentially constant with each chain. This pattern is reversed for oceanic volcanoes: The elevation of circum‐Pacific volcanoes situated on oceanic crust is constant within arcs, while edifice heights are greatly variable. In continents the depth to the root zones of volcanoes may be within the elastic part of the lithosphere, whereas in the oceans it may be well below the elastic part of the lithosphere. We suggest that melting, or the onset of the volcanic uprising, may be controlled in both cases primarily by pressure: in the continental lithosphere by the overburden pressure determined by depth below the local surface and in the oceanic lithosphere by the isostatically compensated pressure zone controlled by depth below sea level. The pattern seems to hold even in complex geological regions and may be used to identify the nature of the crust in such regions.
Isotope geochemistry of water in Gulf Coast salt domes
Authors: Knauth L. Paul , Kumar M. B. , Martinez J. D.
Published: February 2008

Water found as active leaks and isolated pools in the Weeks Island, Jefferson Island, and Belle Isle salt mines of south Louisiana has δ18O values ranging from −4 to +11.5‰ and δD values from −2.3 to −53‰. One sample from Weeks Island and one from Jefferson Island are isotopically similar to local surface waters and are clearly of meteoric origin. All other samples are too enriched in 18O to be meteoric waters. In the Weeks Island mine the isotopic data define a linear array given by δD = 3.0δ18O ‐ 40.1. Active leaks define the positive end of this array. Isolated pools are interpreted as inactive leaks with initial δ18O and δD values of +9.1±0.5‰ and −11‰±7‰, which have subsequently exchanged with water vapor in the mine air to produce the linear array of δ values. The water derived from active leaks in these three mines is too enriched in 18O and too depleted in D to be connate ocean water or evaporite connate water trapped in the salt. The isotopic composition of water derived from the dehydration of gypsum is probably dissimilar to that of the active leaks. It is unlikely that the water has originated from the dehydration of gypsum. It is also unlikely that isotopic exchange with anhydrite is responsible for the observed 18O enrichments. Nonmeteoric water from the active leaks displays the type of 18O enrichments characteristic of saline formation waters, where water exchanges isotopically with calcite and clay minerals. It is concluded that the nonmeteoric waters are formation waters which have become incorporated in the salt. From the observed 18O enrichment it is calculated that the formation waters were incorporated during diapiric rise of the salt at a depth of 3–4 km and have been trapped within the salt for 10–13 m.y. Large volumes of salt within salt domes are not naturally penetrated by meteoric groundwaters but can contain limited amounts of trapped formation water which will have to be contended with in any attempt to store radioactive wastes in salt domes.
Lunar near‐surface shear wave velocities at the Apollo Landing Sites as inferred from spectral amplitude ratios
Authors: Horvath Peter , Latham Gary V. , Nakamura Yosio , Dorman H. James
Published: February 2008

We reexamined the horizontal‐to‐vertical amplitude ratios of the long‐period seismograms to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity‐depth functions are well represented by two piecewise linear segments, although the presence of first‐order discontinuities cannot be ruled out.
The systems albite‐orthoclase‐water and albite‐orthoclase‐quartz‐water: Chemographic phase relationships
Authors: Boettcher A. L.
Published: February 2008

Phase relationships in the systems NaAlSi3O8‐KAlSi3O8‐H2O and NaAlSi3O8‐KAlSi3O8‐SiO2‐H2O to 10 kbars pressure are discussed using pressure‐temperature projections and isobaric, polythermal projections. Complexities in these systems result at about 4.2 and 2.5 kbar, respectively, as the result of critical endpoints generated by the intersections of the vapor‐saturated solidi and the feldspar solvi, previously investigated experimentally by Seek and by Morse. A little‐known method derived by Hillert is used to distinguish eutectic and peritectic reactions involving alkali feldspars and water‐undersaturated liquids.