bluuebunny/crossref_metadata_2025 huggingface dataset:
Kilometer‐sized waves in electron density in the Venusian nightside ionosphere
Authors: Brace Larry H.
Published: February 2008

As periapsis of the Pioneer Venus Orbiter (PVO) descended into the lower nightside ionosphere of Venus in the Fall of 1992, wave‐like ionospheric density structures began to appear on some of the volt‐ampere characteristics of the Orbiter Electron Temperature Probe. The number of such events is insufficient to fully define their morphology but enough to provide an indication of the wave amplitudes, scale sizes, occurrence altitudes, and local time variation. The density variations were quasi‐sinudoidal, with wavelengths of the order of 1 km along the nearly horizontal trajectory near periapsis. Nearly all of the wave events were encountered within an altitude band lying between 140 and 160 km, a region containing the steep negative Ne gradient just above the ionospheric peak. The waves generally did not fill the occurrence band but were seen primarily as isolated events on curves taken intermittently as PVO crossed through the band. Peak‐to‐trough amplitudes (ΔN/N) were in the range of 5% to 50%. The waves exhibited little local time variation within the available viewing period (01–04 hrs), with perhaps a tendency for the waves to rise to slightly higher altitudes toward dawn. The latitudinal extent of the waves could not be resolved because volt‐ampere curves were obtained only intermittent, however, their occurrence on both inbound and outbound passages through the wave band suggests that the waves sometimes exist in layers that extend over at least 15° of latitude. The generation mechanism for these waves is unknown, but we suspect that it involves the steep density gradient that separates the main nightside ionosphere from the tenuous, and probably rapidly flowing plasma above.
Southern‐hemisphere PMSE: Where are they?
Authors: Balsley B. B. , Woodman R. F. , Sarango M. , Urbina J. , Rodriguez R. , Ragaini E. , Carey J.
Published: February 2008

We report a surprising absence of PMSE (Polar Mesospheric Summer Echoes) in VHF radar observations of the southern summer high‐latitude mesosphere. Our observations cover about twenty days during January‐February, 1993, from King George Island, Antarctica.
Semiannual variation of the geomagnetic activity and solar wind parameters
Authors: Orlando M. , Moreno G. , Parisi M. , Storini M.
Published: February 2008

The semiannual variation of the geomagnetic activity is investigated in connection with a large set of solar wind and interplanetary magnetic field data (4494 daily averages from 1965 to 1987). Our analysis confirms that the geomagnetic activity (described by the aa index), is mainly modulated by the southward component of the magnetic field (Bs), as suggested by Russell and McPherron. On the other hand, it is also found that the solar wind velocity (V) has a relevant role in this phenomenon. In fact, the amplitude of the aa modulation is best correlated with the function BsV². We also explore the linkage between the annual trend of aa and the sunspot activity (1868–1989), showing that the modulation of the geomagnetic activity follows a more regular pattern during the descending phase of the solar cycle than during the rising and maximum parts.
Estimation of methane discharge from a plume: A case of landfill
Authors: Tohjima Yasunori , Wakita Hiroshi
Published: February 2008

The distribution of atmospheric methane concentrations downwind of a landfill was measured by a continuous measuring system equipped with a flame ionization detector. The observed data show dispersion of the methane plume emitted from the landfill. On the basis of an atmospheric diffusion model, the methane emission rate of 3‐5×108 g/day was estimated for this 2×106 m² landfill.
3D plasma observations near Mars
Authors: Kallio E. , Koskinen H. , Barabash S. , Lundin R. , Norberg O. , Luhmann J. G.
Published: February 2008

3‐dimensional (3D) proton velocity data measured by the ASPERA mass spectrometer onboard the Phobos‐2 spacecraft are presented. The mass spectrometer was capable of providing 3D plasma distribution data by scanning through the 4π space, but most of the time its own scanner was switched off, and only 2D data have been presented so far. Because the spacecraft was often spinning around its Sun‐pointing axis, the specific data treatment presented in this paper gives a possibility to restore the full angular coverage and obtain 3D vectors of the proton velocity near Mars for the first time. For the circular orbits analysed in this work the flow was deflected at the bow shock and almost stopped at a boundary termed the magnetopause. The main features of the flow and the location of the boundary are compared with a gas‐dynamic model of the flow. In general, the model and data are in good agreement. Other observed effects, possibly associated with reflected and pick‐up ions, non‐thermal proton population, and/or mass loading, which are not taken into account in the gas dynamic model, are discussed.
The importance of the reactions between OH and ClO for stratospheric ozone
Authors: Toumi R. , Bekki S.
Published: February 2008

Recent analysis of ozone trends in the upper stratosphere suggest that models overestimate the decrease of ozone in the upper stratosphere over the last decade (WMO, 1992). We have included the reaction OH + ClO → HCl + O2 in a two‐dimensional radiative‐chemical‐transport model and find that this reaction reduces the ClO/HCl ratio to agree better with observations. The calculated trend in the upper stratosphere is lower and agrees well with some observations. The effect of this reaction on the lower stratosphere is small. The reaction OH + ClO → Cl + HO2 is the main sink for OH in regions of elevated ClO and low ozone such as during the Antarctic Spring.
Origin and maintenance of the oxygen torus in Saturn's magnetosphere
Authors: Morfill G. E. , Havnes O. , Goertz C. K.
Published: February 2008

Observations of thermal ions in Saturn's inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by “self‐sputtering” of E ring dust. Sputtered particles are “picked up” by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is ∼5, which implies that an avalanche of self‐sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planet's ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.
Energetic (>0.2 MeV) electron bursts observed by ISEE 3 in the deep (<240 RE) geomagnetic tail
Authors: Richardson I. G. , Owen C. J. , Slavin J. A. , von Rosenvinge T. T.
Published: February 2008

The Goddard Space Flight Center medium‐energy cosmic ray experiment on ISEE 3 made as yet unreported observations of 0.2‐2 MeV electrons and >1 MeV/amu ions in the deep geomagnetic tail out to ∼240 RE from the Earth during the geotail mission in 1982‐1983. The most notable feature of these data is the presence of brief (<1‐hour duration) electron bursts which exceed the instrument background of ∼0.02 (s cm2; sr MeV)−1. These occur most frequently and have the highest intensities, within ∼80 RE of the Earth and are relatively absent at greater distances downtail. Approximately 60% of these bursts are observed in the plasma sheet. These occur most frequently on closed field lines connected to the Earth, based on their association with slow earthward or mixed plasma sheet flows and northward directed magnetic fields. The near‐absence of such events beyond ∼80 RE downtail provides evidence for predominantly open rather than closed magnetic field structures in the plasma sheet beyond this distance downtail. The observations are consistent with the presence of a distant neutral line in the tail plasma sheet at an average distance of ∼80 RE from the Earth. Approximately 30% of plasma sheet electron bursts are associated with north then south turning magnetic field deviations and fast tailward plasma flows, which may be related to the expulsion of plasmoids following substorm onsets. Approximately 20% of all the electron bursts are seen in the tail lobes, probably associated with encounters with the plasma sheet boundary (separatrix) layer. The lobe events show no clear trend in intensity with downtail distance. We show examples of plasma sheet and lobe electron bursts that are clearly associated with substorms, indicating that temporal as well as spatial variations in the energetic electron intensity do indeed occur in the geomagnetic tail. Finally, ∼20% of the electron bursts are observed in the magnetosheath. The intensity of magnetosheath bursts also falls off with downtail distance such that they are nearly absent beyond ∼100 RE from the Earth.
Well‐resolved observations by ISEE 2 of ion dispersion in the magnetospheric cusp
Authors: Phillips J. L. , Bame S. J. , Elphic R. C. , Gosling J. T. , Thomsen M. F. , Onsager T. G.
Published: February 2008

During a prolonged period of southward interplanetary magnetic field on October 30, 1978, the ISEE 2 spacecraft observed an unusually prolonged and distinct region of ion energy‐time dispersion in the dayside, dawn magnetosphere. Observed plasma features included the (1) presence of magnetosheath electrons and initial absence of magnetosheath ions and (2) subsequent arrival and energy‐time dispersion of magnetosheath ions as the spacecraft transited outbound from 6.03 to 7.66 Earth radii. We use ISEE 2 ion measurements to illustrate these dispersive time‐of‐flight effects and to support our interpretation of a location in the North magnetospheric cusp. Ion energy dispersion, well documented at lower altitudes but generally only observable for a few minutes due to rapid cusp crossings by the spacecraft, in this case persists for ∼35 min and includes well‐resolved sequences of magnetosheath and magnetospheric ion distributions. We focus primarily on observations and time‐of‐flight analysis of ions from the magnetosheath, which are observed simultaneously entering and exiting the cusp at different energies. We discuss the observational support and shortcomings of various ion injection locations and durations and conclude that quasi‐steady merging at the dayside magnetopause, with ion dispersion caused by spacecraft motion away from the last closed field line, is the most likely scenario.
Plasma expansion and evolution of density perturbations in the polar wind: Comparison of semikinetic and transport models
Authors: Ho C. W. , Horwitz J. L. , Singh N. , Wilson G. R.
Published: February 2008

Comparisons are made between transport and semikinetic models in a study of the time evolution of plasma density perturbations in the polar wind. The situations modeled include plasma expansion into a low‐density region and time evolution of localized density enhancements and cavities. The results show that the semikinetic model generally yields smoother profiles in density, drift velocity, and ion temperature than the transport model, principally because of ion velocity dispersion. While shocks frequently develop in the results of the transport model, they do not occur in the semikinetic results. In addition, in the semikinetic results, two ion streams, or double‐humped distributions, frequently develop. In the transport model results the bulk parameters, at a given time, often have a one‐to‐one correspondence in the locations of their local minima or maxima. This is a consequence of the coupling of the fluid equations. There is, however, no such relationship among the moments produced by the semikinetic model where the local moment maxima and minima are often shifted in altitude. In general, incorporation of enhanced heat fluxes in the transport model leads to somewhat improved agreement with the semikinetic results.
Evolution of corotating stream structures in the heliospheric equatorial plane
Authors: Hu Y. Q.
Published: February 2008

This paper develops a numerical method and applies it to the study of corotating high‐speed streams in the equatorial plane of the heliosphere from 0.3 AU up to 10 AU, where corotating shocks undergo processes of formation, development and interaction. The numerical method is designed specially for solving two‐dimensional steady supersonic magnetohydrodynamic flows, and it has two distinguishing features: (1) The conservation laws are maintained with high precision along the streamlines, and (2) a shock or a tangential discontinuity is treated as a surface with zero thickness, using the Rankine‐Hugoniot relations to directly match the flow conditions upstream and downstream. By using this method, the whole physical picture of the corotating high‐speed streams is constructed throughout the domain of 0.3‐10 AU for several specific theoretical examples. Our simulation unifies previous simulations which were limited to either the near‐Sun region within 1 AU or the far region outside several AU. In addition, several new physical features and processes are revealed by this study, especially those concerning the global geometry and the detailed evolution of the corotating shocks.
Electromagnetic ion cyclotron waves in the plasma depletion layer
Authors: Denton Richard E. , Hudson Mary K. , Fuselier Stephen A. , Anderson Brian J.
Published: February 2008

We have studied the theoretical properties of electromagnetic ion cyclotron (EMIC) waves which occur in the plasma depletion layer. Our analysis assumes a homogeneous plasma with the characteristics which were measured by the AMPTE/CCE satellite at 1450‐1501 UT on October 5, 1984. During this time, waves were observed in the Pc 1 frequency range (0.2‐5 Hz) below the hydrogen gyrofrequency, and we identify these waves as EMIC waves. In agreement with observations, the theory predicts two frequency bands of waves, above and below about 0.4 ΩH, where ΩH is the hydrogen gyrofrequency. The higher‐frequency instability is driven by the temperature anisotropy of the H+ ions, while the lower‐frequency instability is driven by the temperature anisotropy of the He2+ ions. These two frequency bands occur on the same dispersion surface. The EMIC waves tend to quasi‐linearly control the value of the proton temperature ratio T⊥/T∥ by an increase in T∥ due to scattering so that the bulk plasma stays near marginal stability. Here, T∥ (T⊥) is the temperature parallel (perpendicular) to B0. Without convective effects, the nonlinear wave amplitude tends to be large over the entire angular range 0 ≤ θkB ≤ 75°, where θkB is the angle between the wave vector and the background magnetic field (B0). However, waves for which the perpendicular (to B0) group velocity υg⊥ is large, are likely to be convectively limited in growth. Within the higher‐frequency band, υg⊥ is small only for wave vectors k which are close to parallel to B0. Within the lower‐frequency band, υg⊥ is small only for oblique k. Therefore, it is likely that the higher‐frequency waves will have k roughly parallel to B0 and will be left‐hand polarized, while the lower‐frequency wave band will have k oblique to B0 and will be linearly polarized, in agreement with observations.
Observations of the temporal and spatial development of induced and natural plasma lines during HF modification experiments at Arecibo using chirped incoherent scatter radar
Authors: Isham B. , Hagfors T.
Published: February 2008

Observations were made of the HF‐modified ionosphere over Arecibo during the daytime on March 4, 1987 and April 6‐8, 1988, and during the nighttime on April 13 and 14 and 14 and 15, 1988, using chirped incoherent scatter radar (ISR). The daytime observations show a difference in the frequencies of the HF‐induced plasma line (HFPL) and the photoelectron‐enhanced (natural) plasma line (PEPL) at the same height of the order of 100 kHz. In addition, the daytime HFPL originates within about 200 m of the reflection height (that is, the O mode cutoff), often within 20 ms of HF turn‐on, and may remain at that location for many seconds. A second HFPL source region was sometimes observed to form between approximately 1 and 4 km below the usual source region after 5 to 10 s of heating. During the nighttime, rapidly changing regions of HF‐induced plasma lines spread over more than 3 km in height were observed; the existence of these regions appears to be strongly dependent on HF heating power. At times, frequency shifting and broadening of the PEPL during heating was noted; the shift in frequency is due to increasing electron temperature and the frequency broadening to the appearance of short‐scale density striations. These effects continued even during periods when the heater remained on and the HF‐induced line was not observed by the radar.
Magnetospheric response to solar wind dynamic pressure variations: Interaction of interplanetary tangential discontinuities with the bow shock
Authors: Wu B. H. , Mandt M. E. , Lee L. C. , Chao J. K.
Published: February 2008

Some magnetic impulse events observed in the polar region are related to vortices associated with plasma convection in the ionosphere. Recent analyses of satellite and ground data suggest that the interaction of solar wind dynamic pressure pulses and the magnetosphere may lead to the formation of velocity vortices in the magnetopause boundary layer region. This can in turn lead to the presence of vortices in the polar ionosphere. However, before reaching the Earth's magnetopause, these interplanetary pressure pulses must interact with and pass through the bow shock. A variation of the solar wind dynamic pressure (ΔρV²) may be associated with shocks, magnetic holes, or tangential discontinuities (TDs) in the interplanetary medium. We study the interaction of interplanetary TDs with the Earth's bow shock (BS) using both theoretical analysis and MHD computer simulations. It is found that as a result of the collision between a TD and the BS, the jump in the solar wind dynamic pressure associated with the TD is significantly modified, the bow shock moves, and a new fast shock or fast rarefaction wave, which propagates in the downstream direction, is excited. Our theoretical analysis shows that the change in the plasma density across the interplanetary TD plays the most important role in the collision process. In the case with an enhanced dynamic pressure behind the interplanetary TD, the bow shock is intensified in strength and moves in the earthward direction. The dynamic pressure jump associated with the transmitted TD is generally reduced from the value before the interaction. A fast compressional shock is excited ahead of the transmitted TD and propagates toward the Earth's magnetosphere. For the case in which the dynamic pressure is reduced behind the interplanetary TD, the pressure jump across the transmitted TD is substantially weakened, the bow shock moves in the sunward direction, and a rarefaction wave which propagates downstream is excited. We also simulate and discuss the interaction of a pair of tangential discontinuities, which may correspond to a magnetic hole, with the BS.
On the momentum balance along the Comet Halley — Sun line during the Giotto flyby
Authors: Israelevich P. L. , Neubauer F. M. , Ershkovich A. I.
Published: February 2008

The plasma velocity along the Sun ‐ comet Halley line is derived by using in situ measurements of the magnetic field along the Giotto trajectory. Including this velocity profile in the momentum balance turns out to be insufficient in order to support the magnetic barrier with conventional parameters of ion‐neutral interaction. The observed magnetic barrier of comet Halley is shown to be consistent with the following parameters of interaction: kin = 5.8 × 10−9 cm3 · s−1 (with Q = 6.9 × 1029 s−1) or Q = 19 × 1029 s−1 (with kin = 1.1 × 10−9 cm3 · s−1). An estimated electric field of convection enables one to find a shielding factor of 0.03 ‐ 0.07 in the maximum of the magnetic barrier of comet Halley.
Quasi‐drift effects of high‐energy solar cosmic rays in the magnetosphere
Authors: Shumilov O. I. , Vashenyuk E. V. , Henriksen K.
Published: February 2008

The structure of high‐energy solar proton penetration zones in the polar and subpolar regions using data from riometers, transpolar satellite DMSP‐F6, and neutron monitors during an extremely anisotropic solar proton event on February 16, 1984, has been studied. Particular emphasis was placed upon interpretation of the solar proton intensity maximum located at the auroral and nearby latitudes. Precipitation of quasi‐trapped particles drifting along the latitudinal direction from the entry site of the anisotropic flux at the dawnside of the magnetosphere is discussed as a cause of this maximum.
A generalization of the Rayleigh criterion for atmospheres with both energy and momentum sources: Implications for atmospheric wave propagation through convecting plasma
Authors: Robinson T. R.
Published: February 2008

A method is presented for calculating the effects of steady energy and momentum sources on atmospheric wave propagation. This involves evaluating the weak feedback produced by the modulation of the source field by small‐amplitude waves. Although the sources may, in practice, depend on a number of the wave field parameters it is only necessary to formally treat the fluid density modulation. This constitutes a generalization of Rayleigh's instability criterion for the generation of sound in heated air. The approach is applied to the propagation of internal gravity waves in a Boussinesq approximation and to high‐frequency acoustic waves. In particular, a number of effects associated with the propagation of atmospheric waves through regions of convecting plasma are treated. These cases are relevant to the propagation of internal gravity waves in the F region of the high‐latitude ionosphere. Some new theoretical results concerning the damping and stability of internal gravity waves in rapidly flowing plasma and under conditions of critical coupling between plasma flow and phase propagation are presented.
Normal modes of an expanding plasma shell
Authors: Gladd N. T. , Brecht Stephen H.
Published: February 2008

We present a cold plasma, two‐fluid electromagnetic theory of the normal modes of a plasma shell expanding, at sub‐Alfvenic velocities, into a magnetic field. A two‐fluid approach is used in order that the specific effects of plasma and magnetic field geometry can be considered. Three main results are obtained. First, a general normal mode equation for lower‐hybrid frequency range oscillations is derived which is more accurate in its treatment of the plasma and magnetic field geometry than previous published normal mode equations. The higher accuracy accrues because no a priori assumptions are made about the ratio of perturbation and equilibrium scale lengths. The second set of results is the establishment of specific criteria for the existence of localized solutions of the lower‐hybrid drift normal mode equation and the development of analytical formulae which describe the dispersive properties of this instability. These are useful because results concerning lower‐hybrid drift instability growth rates and maximal growing wave numbers are usually given in the context of the local approximation without specifying where the normal modes are localized within the highly inhomogeneous plasma profile. The third result is a cautionary one. Two equilibrium models are constructed with identical magnetic field profiles, similar density profiles, but distinctive electron cross‐field velocity profiles. The localization criteria and analytical formulae for lower‐hybrid drift dispersive properties are found to be quite accurate for one equilibrium model but to be inappropriate for the other.
Riometer observations at Sanae (L = 4.0) related to solar proton events
Authors: Stoker P. H. , van Wyk J. P.
Published: February 2008

Atmospheric ionization by protons of integral intensity above a cutoff of 0.86 GV at Sanae is too small to record a riometer absorption above noise even during the large ground‐level enhancements (GLEs) of September 29 and October 19 and 24, 1989. However, linear increases may be depicted in the absorptions of 20‐, 30‐, and 51.4‐MHz cosmic radio noise that ranged from 0.25 dB per hour at 30 MHz for a GLE with a rising phase of 26% per hour to 0.81 dB per hour for a GLE of 360% per hour. Lowering of cutoff rigidity by magnetospheric ring currents to give direct access to solar flare protons of rigidity below 0.86 GV or precipitation of energetic magnetospheric electrons due to magnetic activity may be ruled out. Mutual consistency appears between the rates in ground‐level solar proton enhancements and the rates of increases in absorptions at all three of the frequencies for all three events. This suggests that nonrelativistic solar protons should have traversed the geomagnetic field lines to be precipitated at Sanae. This traversal may be due to equatorward gradient and curvature drifts from the polar cap region, a westward longitudinal drift along L = 4.0, and subsequent precipitation into the atmosphere at Sanae due to the South Atlantic Magnetic Anomaly. Preceding these linear increases, solar flare effects with absorption maxima coincident with the maxima of solar flare X rays and the 2.2‐MeV gamma ray line have been observed. The rates of recovery in absorption that are faster than that of the 0.5‐ to 4‐Å X rays suggest that these absorptions are due to ionization caused by still shorter wavelength electromagnetic radiation.
Contrast of faculae near the disk center and solar variability
Authors: Lawrence J. K. , Topka K. P. , Jones H. P.
Published: February 2008

We analyze simultaneous, or near‐simultaneous, coregistered, digital, photometric images of solar photospheric intensity and line‐of‐sight magnetic field. Images were made with the Lockheed tunable filter instrument at the Swedish Solar Observatory, La Palma, with the video spectra‐spectroheliograph system at the San Fernando Observatory and with the new NASA spectromagnetograph at the National Solar Observatory at Kitt Peak. We study the disk center contrasts of small magnetic elements. While active region faculae are dark at disk center quiet Sun network features are bright. The populations of magnetic field elements that make up these two kinds of features are quite different. Different contrast center‐limb functions must be used when estimating their irradiance or luminosity contributions. The disk center contrasts of active region faculae are color dependent and indicate a depth effect related to the H− opacity of the facular atmosphere. This result is important for calibration of monochromatic observations of faculae to bolometric irradiance fluctuations. We emphasize the value of cooperative observations among installations whose differing strengths are complementary.
Large cosmic ray transient decreases observed in the heliosphere at the Earth, Voyagers 1 and 2 and Pioneer 10 from 1987 to 1991
Authors: Webber W. R. , Lockwood J. A.
Published: February 2008

Using the > 70‐MeV cosmic ray rate data from the IMP, Voyager, and Pioneer spacecraft, we have examined the role of large transient decreases in the overall cosmic ray decrease from 1987 to 1990. At least 12 separate transient decreases with magnitudes > 5% were observed at Voyager 2, which moved from a heliocentric radius of ∼ 23 to 35 AU during the time period. Many, but not all, of these decreases were observed at IMP 8 near the Earth and at Pioneer 10 between 42 and 54 AU near the ecliptic plane. However, only five of these decreases were observed at Voyager 1 at north heliographic latitude ∼ 30° and radial distance between 32 and 46 AU. The period of decreasing cosmic ray intensity from 1987 to 1990 can be divided into two phases. During phase 1, between mid‐1987 and early 1989, the average heliospheric current sheet tilt rapidly increased from ∼ 8° to 65° as the solar activity was also increasing rapidly and the intensity at the Earth decreased by about 35%. Four large transient decreases were observed at the Earth during this time; however, transient decreases beyond the Earth were observed only near the ecliptic plane. In phase 2, from early 1989 to mid‐1990 when the intensity at the Earth decreased by an additional ∼ 60%, five large transient decreases were observed at all latitudes. During this time the current sheet tilt was always greater than 60°. Using a procedure to separate these large transient decreases from any possible slower long‐term variations, we argue that these decreases appear to be responsible for much of the overall intensity decrease of 20‐35% observed at all spacecraft near the equatorial plane in phase 1 as well as most of the larger intensity decrease of about 45‐60% observed at all spacecraft during phase 2. The recovery of the cosmic ray intensity that started at the Earth in early 1990 moved outward in the heliosphere reaching the Voyager and Pioneer spacecraft about 6 months later. This is in contrast to what happened as the intensity began to recover in the previous cycle in 1981, when the recovery was observed to begin almost simultaneously at all radii < 25 AU. This difference suggests that during the current cycle, the mechanism responsible for the recovery must have propagated outward from the Sun. This could have been related to the reversal of the solar polar magnetic field which occurred in early 1990. The effects of large transients propagating outward in the heliosphere and becoming ineffective when they reach the heliospheric boundary could not have played a major role in the onset of the recovery in 1990, since in this case the recovery should have been observed first in the outer heliosphere.
Optically thick cascade to the O I 3s3S state in the Earth's thermosphere
Authors: Cotton D. M. , Chakrabarti S. , Gladstone G. R.
Published: February 2008

The electron impact excitation cross section for the O I (1304 Å) emission is enhanced in the terrestrial thermosphere owing to an optically thick cascade mechanism. Values for the contribution of the cascade have been calculated and deduced from measured 1304‐Å airglow emissions to between 0 and 200%. We have developed a model to calculate a large portion of the optically thick cascade contribution, including contributions from the radiative entrapment of the O I (1040, 1027, and 989 Å) lines. High‐resolution spectroscopic data from a sounding rocket experiment are analyzed using this model. The optically thick cascade contribution for the three lower lying states was evaluated to be 24.5% and compares well with recent calculations. The data, however, support a total contribution of 40%. The 15.5% difference is likely due to the contribution from higher Rydberg states.
Resonant electromagnetic ion‐ion beam turbulence at comet P/Grigg‐Skjellerup
Authors: Verheest Frank , Lakhina G. S.
Published: February 2008

The electromagnetic turbulence and associated frequencies observed far upstream of comet P/Grigg‐Skjellerup during the recent Giotto spacecraft fly by can be attributed to relative streaming between the cometary ions and the solar wind, and the hypothesis that unstable parallel modes are responsible is used to deduce the wavelengths involved. Such a self‐consistent approach yields resonant instabilities with wavelengths of 7000 ∼ 8000 km, phase velocities which are sub‐Alfvénic and polarizations which are left‐handed in the spacecraft frame.
Velocity space diffusion and nongyrotropy of pickup water group ions at comet Grigg‐Skjellerup
Authors: Coates A. J. , Johnstone A. D. , Wilken B. , Neubauer F. M.
Published: February 2008

The diffusion of water group cometary ions in velocity space at comet Grigg‐Skjellerup was measured during the Giotto spacecraft encounter. The evolution of the collapsed pitch angle and energy distributions during the inbound and outbound passes shows that the timescale for energy diffusion may be similar to that for pitch angle diffusion. Fully isotropic pitch angle distributions were never seen. Also the bulk parameters of the three‐dimensional distributions are examined. Transformation of these parameters into a field‐aligned solar wind frame allows us to test the gyrotropy of the distributions. The observations imply that there were deviations from gyrotropy throughout the encounter becoming most important near to closest approach.
Effects of hydrothermal alteration on the magnetization of the Oligocene Carpenter Ridge Tuff, Bachelor Caldera, San Juan Mountains, Colorado
Authors: Sweetkind Donald S. , Reynolds Richard L. , Sawyer David A. , Rosenbaum Joseph G.
Published: February 2008

Intracaldera Oligocene Carpenter Ridge Tuff fills the Bachelor caldera in the central San Juan caldera complex and hosts mineral deposits of the Creede mineral district. The Carpenter Ridge Tuff and unaltered portions of its intracaldera Bachelor Mountain Member, have strong, high‐coercivity, reverse magnetizations with average magnetic susceptibility (MS) and natural remanent magnetization (NRM) of 6 × 10−3 volume SI and 8 A/m, respectively. Oxide phenocrysts in these rocks are titanomagnetite and ilmenite; however, magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and Curie temperatures between 580°C and 620°C. Much of the intracaldera tuff was affected by potassic metasomatism, a type of hydrothermal alteration characterized by addition of K and loss of Ca and Na, between 27.3 Ma and 25.1 Ma. Potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz, oxidation of the original oxide phenocrysts to hematite and rutile, and consequent suppression of MS and NRM by a factor of 5. Also present, however, are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed; values of MS and NRM are similar to those of outflow tuff. Such rocks are suspected of containing secondary magnetite on the basis of (1) Curie and thermal unblocking temperatures at and below 580°C, indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed, and (2) observation in one sample of magnetite rimming cores of hematite+rutile. The presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry, although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition. These altered rocks may record a change from a pervasive alteration by alkaline, oxidizing fluids to a vein‐controlled alteration dominated by more reduced fluids in which magnetite was stable.
Toward tubular tomography
Authors: Stark Philip B. , Nikolayev Dmitry I.
Published: February 2008

Within ray theory, travel times are sensitive to the medium only along the ray path. If the arrival time is defined to be the point where the observed seismogram crosses a nonzero threshold, time domain computations using Born scattering in a homogeneous unbounded medium show that the sensitivity is large within a considerable volume, and most intense near the source and receiver. Details of this volume or “tube” depend on the threshold value, the source time function and the instrument response. Generally, as the threshold value gets larger, the volume of sensitivity and magnitude of sensitivity get larger. If the source time function is oscillatory, the region of highest sensitivity can be quite complex. Due to the high sensitivity near the endpoints, ray‐theoretical tomography is likely to alias near‐source and near‐receiver structure to produce spurious structure along the ray path.
Plate tectonics and crustal deformation around the Japanese Islands
Authors: Hashimoto Manabu , Jackson David D.
Published: February 2008

We analyze over a century of geodetic data to study crustal deformation and plate motion around the Japanese Islands, using the block‐fault model for crustal deformation developed by Matsu'ura et al. (1986). In this model, crustal deformation corresponds to the sum of relative movement among designated blocks and displacement due to dislocations along the locked parts of designated faults. We model the area including the Japanese Islands with 19 crustal blocks and 104 faults based on the distribution of active faults and seismicity. Three kinds of data are used; rates of change of angles and lengths, and average velocities of very long baseline interferometry stations. With the inversion of these geodetic data we obtain block motions and average slip rates of faults. This geodetic model predicts that the Pacific plate moves N69° ± 2°W at about 80 ± 3 mm/yr relative to the Eurasian plate which is much lower than that predicted in geologic models. Substantial aseismic slip occurs on the subduction boundaries. The block containing the Izu Peninsula may be separated from the rigid part of the Philippine Sea plate. The faults on the coast of Japan Sea and the western part of the Median Tectonic Line have slip rates exceeding 4 mm/yr, while the Fossa Magna does not play an important role in the tectonics of the central Japan. The geodetic model requires the division of northeastern Japan, contrary to the hypothesis that northeastern Japan is a part of the North American plate. Owing to rapid convergence, the seismic risk in the Nankai trough may be larger than that of the Tokai gap.
Thermal effects of sediment thickening and fluid expulsion in accretionary prisms: Model and parameter analysis
Authors: Wang K. , Hyndman R. D. , Davis E. E.
Published: February 2008

We investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically. Variations landward across the wedge in the porosity‐depth function are included in the model. The most important contribution to the thermal regime arises from the thickening of the wedge, which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent. This effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments. The total thermal perturbation is sensitive to the prism taper angle, the incoming sediment thickness, and the convergence velocity. Anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth. The thermal consequences of retarded fluid expulsion are significant, and result in locally depressed seafloor heat flow in this region. For the full observed range of accretion parameters, the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km.
Aseismic growth of Durmid Hill, southeasternmost San Andreas Fault, California
Authors: Sylvester Arthur Gibbs , Bilham Roger , Jackson Michael , Barrientos Sergio
Published: February 2008

Our 2.27‐km leveling line across and normal to the San Andreas fault at Durmid Hill domed upward by 9 mm between September 1985 and December 1991. The flanks of the line tilted more than 1 μrad/yr, implying that the uplift extends beyond the ends of the line and that absolute uplift of the crest of the hill may greatly exceed the measured rate of 1.5 mm/yr. Trilateration measurements sampling 5 km of the fault zone within the hill from 1978 to 1992 yield a mean dextral slip rate of 2.4 ± 0.3 mm/yr and fault normal extension of ≈1 ± 0.3 mm/yr. The inferred uplift rate during the 5.5‐year leveling period (as well as from geomorphic samples at 330 years and 35,000 years and from erosion of sedimentary strata younger than 738,000 years in monoclines abutting the fault at Durmid Hill) is remarkably consistent at 1 to 4 mm/yr and implies that the current processes may have persisted for millennia. We investigate two mechanisms for the deformation that are defined by the symmetry of the observed doming: transpression on a vertical or near‐vertical subsurface San Andreas fault, or slip on a shallow flat fault or detachment that terminates beneath Durmid Hill. The resultant models suggest that uplift rates of 1 mm/yr will result from 6 mm/yr dextral shear along a vertical fault together with 1 mm/yr horizontal convergence across that fault, and this strain field is consistent with the paleoseismic record and our leveling data. We conclude that interseismic processes account for most of the growth of Durmid Hill, because the interseismic vertical deformation rates equal or exceed the long‐term vertical slip rate measured across the fault. Because dextral shear of 6 mm/yr at Durmid Hill represents a small fraction of the 25 mm/yr measured across the entire San Andreas fault zone in Coachella Valley and because the San Andreas fault at Durmid Hill may represent an earthquake nucleation zone, the observed deformation processes may be of significant importance for understanding future earthquake rupture initiation or termination.
Anisotropic convection with implications for the upper mantle
Authors: Chastel Yvan B. , Dawson Paul R. , Wenk Hans‐Rudolf , Bennett Kristin
Published: February 2008

On the basis of polycrystalline theory describing the plasticity in olivine and enstatite, the flow in a convection cell has been simulated using a finite element formulation. The spatial variations in anisotropic properties are computed from the textures that evolve with the flow. A kinematically constrained equilibrium‐based assumption is used to partition the macroscopic deformation among crystals within an aggregate. We model the convection for one specific cell geometry and two sets of boundary conditions. A complete map of textures throughout the cell is obtained. The textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain. The results of the simulations indicate that during up welling a strong texture develops rapidly. It convects during spreading and is attenuated during subduction. Results are compared with features of the upper mantle. In our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear. This is contrary to Hess' [1964] intuition that (001) slip planes of olivine orient themselves parallel to the flow planes, yet the pattern is consistent with natural fabric data. Significant differences are observed as a function of depth within the cell. The variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data.
Gravity and hydrothermal modeling of the Roosevelt Hot Springs Area, southwestern Utah
Authors: Becker David J. , Blackwell David
Published: February 2008

The geothermal field at Roosevelt Hot Springs, Utah, produces water at temperatures exceeding 230°C. The source of the heat for the geothermal field and the nature of the hydrothermal system have been investigated by analyses of gravity, thermal, and hydrogeologic data. A deep, cylindrically shaped, anomalous mass approximately 10–15 km in diameter is inferred to exist approximately 5 km beneath the geothermal field based on inverse modeling of a −17 mGal gravity anomaly isolated by strike filtering. This body is assumed to be a young intrusion and the heat source for the geothermal field. The configuration of the anomalous body was used in a vertical cross‐sectional model of the hydrothermal system. The results of the modeling indicate (1) the source of the fluid recharge for the field is the Mineral Mountains; (2) the age of the source of the geothermal field may be younger than 500,000 years, the age of nearby rhyolites; (3) water at high temperature may be found beneath the sedimentary basin west of the geothermal field if sufficient permeability can be found; (4) the deep body beneath the field has significant permeability, in excess of 5 × 10−18 m2; and (5) the fault zone which acts as a conduit for upwelling thermal waters and provides the production at the field is probably bounded by low‐permeability material.
Flow of ices in the ammonia‐water system
Authors: Durham W. B. , Kirby S. H. , Stern L. A.
Published: February 2008

We have fabricated in the laboratory and subsequently deformed crystalline hydrates and partial melts of the water‐rich end of the NH3‐H2O system, with the aim of improving our understanding of physical processes occurring in icy moons of the outer solar system. Deformation experiments were carried out at constant strain rate. The range of experimental variables was strain rate 3.5 × 10−7 < ϵé < 3.5 × 10−4 s−1, temperature 132 < T < 220 K, pressure 50 ≤ P ≤ 100 MPa, and mole fraction NH3 0 ≤ xNH3 ≤ 0.295. Phase relationships in the NH3‐H2O system indicate that water ice and ammonia dihydrate, NH3 · 2H2O, are the stable phases under our experimental conditions. X ray diffraction of our samples usually revealed these as the dominant phases, but we have also observed an amorphous phase (in unpressurized samples only) and occasionally significant ammonia monohydrate, NH3 · H2O. The onset of partial melting at the peritectic temperature at about 176 K appeared as a sharp transition in strength observed in samples of xNH3 = 0.15 and 0.295. In samples of xNH3 = 0.05 and 0.01, the effect of melt was less pronounced. For any given water ice + dihydrate alloy in the subsolidus region, we observed one rheological law over the entire temperature range from 176 K to about 140 K. Below 140 K, a shear instability similar to that occurring in pure water ice under the same conditions limited our ability to measure ductile flow. The rheological laws for the several alloys vary systematically from that of pure ice to that of dihydrate. Pure dihydrate is about 4 orders of magnitude less viscous than water ice just below the peritectic temperature, but because of a very pronounced temperature dependence in dihydrate (100 kJ/mol versus 43 kJ/mol for water ice) the viscosity of dihydrate equals or exceeds that of water ice at T < 140 K. The large variation in viscosity of dihydrate with relatively small changes in temperature may be helpful in explaining the rich variety of tectonic and volcanic features seen on the surfaces of icy moons in the outer solar system.
The paradox of the Loma Prieta Earthquake: Why did rupture terminate at depth?
Authors: Wallace Michelle Hall , Wallace Terry C.
Published: February 2008

The 1989 Loma Prieta earthquake was an oblique thrusting event that raptured a moderately dipping fault that is subparallel to the San Andreas fault. Rupture initiated at 18 km and propagated both updip and bilaterally. Seismic and geodetic studies indicate that rupture terminated between 8 and 5 km depth. We examine the role of the elastic properties of the crust as a mechanism for reducing stress in the upper crust and terminating rupture. We use linear elastic fracture mechanics and the finite element method to model the Loma Prieta fault rupture as a mode II and mode III shear crack in a layered elastic crust. Our analysis shows that a low‐stress condition, generated in our models by a change in material properties in the upper crust, is capable of stopping rapture. We also examine the stress condition at the intersection of the San Andreas and Loma Prieta faults to determine the present state of stress and seismic hazard in the region. We find that the Loma Prieta fault in the upper crust is still closer to the point of failure than the San Andreas fault over the same depth. Rupture of the Loma Prieta fault did not increase the seismic hazard of the San Andreas fault within the crustal volume above the Loma Prieta rupture.