bluuebunny/crossref_metadata_2025 huggingface dataset:
A review of anomalous resistivity for the ionosphere
Authors: Papadopoulos K.
Published: February 2008

This is a general review of anomalous resistivity with emphasis on its applicability in space and more specifically on ionospheric plasmas. It is addressed to the general ionospheric community rather than the specialist. Therefore a substantial amount of rigor has been sacrificed in favor of simplified physical pictures. However, several prescriptions are presented, on the basis of which one can compute the anomalous resistivity resulting from each specific mechanism. Following a conceptual discussion of resistivity a general formalism is presented for its computation on the basis of the spectrum of electric field fluctuations. On the basis of this it is shown that stable nonthermal plasmas can at most enhance resistivity by a few percent. The same is true for collisionally driven instabilities. From the current‐driven instabilities, only the ion acoustic instability can produce a steady state anomalous resistivity. The rest result in transient resistivity and the appearance of hot electron or ion spots. A more satisfying picture emerges when the low‐frequency turbulence that produces resistivity is excited parametrically by a high‐frequency instability. The case where such a driver arises from the interaction of precipitating electrons is discussed in detail. Finally, the relevance of the various resistivity mechanisms and their importance in ionospheric electron acceleration is discussed. Although a large number of physical notions are well understood, the efforts toward their incorporation into a gross modeling picture remain embarrassingly small.
Coronal holes and high‐speed wind streams
Authors: Zirker Jack B.
Published: February 2008

Coronal holes are regions of unusually low density and low temperature in the solar corona. During a 9‐month Skylab solar workshop, 50 participants established some of the basic properties of coronal holes and their associated high‐speed wind streams using a combination of Skylab, satellite, and ground‐based observations. The holes have been identified now as Bartel's M regions, i.e., sources of high‐speed wind streams that produce recurrent geomagnetic variations. Throughout the Skylab period the polar caps of the sun were coronal holes, and at lower latitudes the most persistent and recurrent holes were equatorial extensions of the polar caps. The holes rotated ‘rigidly’ at the equatorial synodic rate. They formed in regions of unipolar photospheric magnetic field, and their internal magnetic fields diverged rapidly with increasing distance from the sun. The geometry of the magnetic field in the inner corona seems to control both the physical properties of the holes and the global distribution of high‐speed wind streams in the heliosphere. The diverging field lines in well‐established holes act as Laval nozzle which produces supersonic flow and depressed densities at low altitudes. The latitude variation of the divergence of the coronal magnetic field lines produces corresponding variations in wind speed, in agreement with interplanetary scintillation measurements and multisatellite observations. During the years of declining solar activity the global field of the corona approximates a perturbed dipole. The divergence of field lines in each hemisphere produces a high‐speed wind near the poles and low‐speed wind in a narrow belt that coincides with the magnetic neutral sheet. The width and magnetic polarity of recurrent wind streams, as measured near earth, has been predicted successfully on the basis of the global coronal magnetic field geometry, at least during the Skylab period. The analysis of electron density measurements within a polar hole indicates that solar wind is accelerated principally in the region between 2 and 5 Rs and that mechanical wave pressure (possibly Alfvén wave) may be responsible for the acceleration of the wind. Phenomenological models for the birth and decay of coronal holes have been proposed. A global pattern of formation, with a systematic eastward drift of successive hole appearances, suggests that the diverging magnetic fields of the coronal holes arise through dynamo action in the deep convection zone. However, attempts to explain the birth and rigid rotation of holes through dynamo action have been only partially successful. More observational data exist on the solar cycle variation of wind streams than on coronal holes. The polar holes shrink, and the volume occupied by the neutral sheet may increase near solar maximum. The 11‐year variation of cosmic ray intensities at the earth may result from cyclic variation of open field regions associated with coronal holes.
Energetic ion composition in the Earth's magnetosphere
Authors: Johnson R. G.
Published: February 2008

In the previous quadrennium, 1971–1974, energetic ion composition measurements provided the first compelling evidence that the ionosphere was contributing large quantities of ions to the energetic ion fluxes in the earth's magnetosphere. With an energetic ion mass spectrometer aboard a low altitude polar orbiting satellite (1971‐089A) relatively large fluxes of energetic (0.7–12 keV) O+ and He+ ions were discovered precipitating from the magnetosphere (Shelley et al., 1972; Sharp et al., 1974; and Johnson et al., 1974). Because of the instrumentation orientation and the low altitude of the satellite, it was difficult to obtain specific information on the important basic questions relating to these ions: What were the relative contributions of the solar wind and the ionosphere to the energetic ion populations? Where was the ionospheric source region located in latitude, altitude, and local time? What physical processes were responsible for the acceleration, transport, and loss of the ionospheric ions? The presence of large fluxes of heavy (O+ and He+) energetic ionospheric ions also made it appear likely that some of the energetic H+ ions in the magnetosphere were also of ionospheric origin. This further emphasized the need for ion mass and charge identification for investigations of the entry of solar wind ions into the magnetosphere and of their subsequent acceleration, transport, and loss processes since the solar wind H+ ions could no longer always be assumed to be dominant over other sources of H+ ions in the magnetosphere.
Magnetospheric plasma wave research 1975–1978
Authors: Shawhan Stanley D.
Published: February 2008

In this review the term “plasma wave” is used to denote all waves which are generated in the magnetospheric plasma or which have their wave characteristics significantly modified on propagation through the magnetosphere. These plasma waves are both electromagnetic and electrostatic and are generally generated by the conversion of free energy within the plasma into wave energy through a variety of plasma‐wave processes. Plasma waves may in turn interact with plasma particles locally or at remote locations in the magnetosphere to modify the particle populations leading to such phenomena as particle precipitation.

Plasma wave research has led to the identification of numerous plasma wave phenomena within the terrestrial magnetosphere. Many of these plasma wave types are associated with the different regions of the magnetosphere as depicted in Figure 1. A listing of the location of these plasma wave types along with the observed frequency range and a description of the wave properties is given in Table 1. Within the magnetosphere, detected plasma waves range in frequency from millihertz—well below the ion gyrofrequency at the bow shock—to 10 megahertz—approximately the electron plasma frequency in the ionosphere. Within the Jovian magnetosphere plasma wave phenomena extend up to 40 MHz which is the maximum electron gyrofrequency in the Jovian ionosphere.
Polar motion and Earth rotation
Authors: McCarthy Dennis D.
Published: February 2008

During the reporting period 1975–1978, the activity in the field of polar motion and Earth rotation in the United States has been concerned with the continuation of regular observations of polar motion and the rotational speed of the Earth, improvement in the present observational techniques, development of new observational techniques and the analysis of the observations in relation to geophysical phenomena. Significant work has been accomplished in each of these areas, and these efforts are summarized separately in this report.

Observations

Classical observations of the variation of astronomic latitude at the International Latitude Observatories at Gaithersburg, Maryland and Ukiah, California were continued by the National Geodetic Survey of the National Ocean Survey. These observations were made with the same visual zenith telescopes that have been used in the past observations. The data were transmitted to the Central Bureau of the International Polar Motion Service in Mizusawa, Japan, and to the Bureau International de l'Heure in Paris, France. Results are listed in the annual publications of the IPMS and the BIH.
Dynamic satellite geodesy
Authors: Smith David E.
Published: February 2008

Since the last report (Gaposchkin, 1975) dynamic satellite geodesy has continued to develop and refine its capability to determine locations on the earth's surface and to improve our knowledge of the global gravity field (Anderle 1975, 1978, 1978a; Gaposchkin 1977, 1979; Lerch, et al., 1977, 1978a;Smith, et al., 1976, 1978;Wagner, et al., 1977). The launching of two spacecraft, GEOS‐3 and Lageos (Laser Geodynamics Satellite) in 1975 and 1976 respectively, are having a profound impact on satellite geodesy. GEOS‐3 provided the first real opportunity to test two new techniques, satellite altimetry and satellite‐to‐ satellite tracking, Altimetry is the measure of the distance between the satellite and the ocean surface and the technique requires a comparable knowledge of the spacecraft motion to the precision of the altimeter. For GEOS‐3 the altimeter had a capability of a few decimeters and a similar accuracy in orbit determination required a significant improvement in all areas, tracking, computation, gravity field modeling, etc.
Numerical models of ocean circulation and climate interaction
Authors: Haney Robert L.
Published: February 2008

This article reviews recent advances in ocean circulation models and climate interactions. Related reviews of one‐dimensional models and models of equatorial and polar regions are given in this Quadrennial Report by Garwood [1979], O'Brien [1979] and Baker [1979], respectively. Related reviews of climate are given in the Meteorology section of the Quadrennial Report by Gates [1979], Kutzbach [1979], Perry [1979], and Rotty and Marland [1979].

Several detailed review articles on ocean circulation models and climate were written in the last four years. National Academy of Sciences [1975], U. S. Committee for GARP [1975] and WMO/ICSU [1975] give the status at the beginning of this review period while Pond and Bryan [1976] and Holland [1977a] represent more recent reviews. Kraus [1977] contains review articles on modeling the upper ocean, while Gribbin [1978] contains review articles on the ocean's role in climatic change. The latest detailed reviews of ocean circulation models and their relation to climate were presented at a JOC/SCOR Joint Study Conference held in Helsinki in May, 1977 [WMO/ICSU, 1977]. Many of these reviews will be published in a forthcoming issue of Dynamics of Atmospheres and Oceans. The present review begins with studies of the possible effect of ocean temperature variabilities on the atmosphere, and then proceeds to studies using idealized basin models, world ocean models, and finally coupled ocean‐atmosphere models.
Planetary seismology and interiors
Authors: Toksöz M. Nafi
Published: February 2008

The placing of the first seismometer on the Moon by the Apollo 11 astronauts in 1969 started the field of planetary seismology. In the following years five other seismic stations were established on the Moon. The seismic network was operated continuously until all of the ALSEP stations were turned off on 1 October 1977. The Viking missions placed two three‐component seismometers on Mars in 1976.
Interplanetary dust
Authors: Brownlee D. E.
Published: February 2008

Small sizes of individual particles and exceedingly low spatial density are formidable problems which greatly impeded progress in the dust field during the previous decade. The major experimental problems which prevented reliable measurements appear finally to have been solved and the entire dust field has seen dramatic progress over the past four years. Major accomplishments were reliable impact and zodiacal light measurements over the range of solar distance from 0.3 AU to 5 AU and successful collection of micrometeorites from the stratosphere which has provided hundreds of proven interplanetary particles for laboratory studies. Interest in dust has grown because of the increasing realization that the solar system dust cloud is an astrophysical site where grain processes such as radiation pressure, rotational bursting, thermal alteration, ion implantation, sputtering, and magnetic effects can be studied in‐situ.
The ionospheres of the major planets
Authors: Strobel Darrell F.
Published: February 2008

A review of the ionospheres of the major planets is given from the pre‐Voyager 1 encounter perspective of Jupiter. Ion chemistry, recombination, and sources are individually discussed. It is concluded that material sputtered off the surfaces of the Galilean satellites, ionized by electron impact, and accelerated by magnetic field capture and inward radial diffusion by flux tube interchange may dissipate sufficient energy in Jupiter's upper atmosphere to completely dominate the solar column production of ionization and Lyman α radiation and heating by dissipation of upward propagating inertia‐gravity waves. These precipitating ions may also be a source of the pronounced layers observed by the Pioneer 10 and 11 radio occultation experiments in Jupiter's lower ionosphere. If Saturn has a strong magnetic field, similar conclusions also apply to hydrogen escaping Titan's atmosphere. Cosmic rays are probably the most important ionization source on Neptune.
Organic chemistry on Titan
Authors: Chang Sherwood , Scattergood Thomas , Aronowitz Sheldon , Flores Jose
Published: February 2008

Observations of nonequilibrium phenomena on the Saturn satellite Titan indicate the occurrence of organic chemical evolution. Features taken from various models of Titan's atmosphere are combined in a working composite model that provides environmental constraints within which different pathways for organic chemical synthesis are assessed. Experimental results and theoretical modeling studies suggest that the organic chemistry of the satellite is dominated by two atmospheric processes: photochemistry and energetic particle bombardment. Photochemical reactions of CH4 in the upper atmosphere can account for the presence of C2 hydrocarbons. Reactions initiated in various levels of the atmosphere by cosmic ray, Saturn ‘wind,’ and solar wind particle bombardment of a CH4‐N2 atmospheric mixture can account for the ultraviolet‐visible absorbing stratospheric haze, the reddish appearance of the satellite, and some of the C2 hydrocarbons. In the lower atmosphere, photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. Hot H atom reactions initiated by photodissociation of NH3 can couple the chemical reactions of NH3 and CH4; if 0.1% of the incident ultraviolet light from 1600 to 2270 Å reaches the lower atmosphere, these reactions will be capable of producing organic matter at a rate comparable to or higher than that resulting from particle‐initiated reactions. Electric discharges are highly improbable on Titan; if they occurred at all, they would be restricted to the lower atmosphere and clouds. Their yield of organic matter might approach that of hot H atom reactions if the conversion of solar to electrical discharge energy on Titan was as efficient as that on earth. These assessments indicate that future missions to Titan should include organic chemical analyses of its atmosphere and surface among the prime science objectives.
Solar activity and the solar wind
Authors: Hundhausen A. J.
Published: February 2008

The availability of in situ solar wind observations from the complete sunspot cycle 20 (1964–1976) suggests an examination of the changes in the character of the solar wind related to the solar cycle and comparison of these observed changes with those expected from earlier studies of solar‐terrestrial physics. In many ways the observations correspond to these expectations; flare‐associated shock waves were probably most important near sunspot maximum, while long‐lived streams of high‐speed wind, now known to originate in coronal holes, were clearly most important in the declining years of the cycle. However, one important pattern of deviation from the expected behavior must be emphasized. The largest elevations of solar wind speed and the highest level of geomagnetic activity, associated with coronal holes and long‐lived streams, occurred very late in the cycle rather than near sunspot maximum. The implications of this ‘anomalous’ behavior are not well understood.
The application of artificial electron beams to magnetospheric research
Authors: Winckler J. R.
Published: February 2008

In the last decade, more than 25 large research rockets have injected electron beams into the ionosphere and magnetosphere to probe the magnetosphere or to investigate various basic problems in space plasma physics. The problem of vehicle neutralization has been studied in the altitude region 100–300 km, and it is now known that the beam injection produces a hot plasma extending possibly as much as 100 m from the vehicle. Vehicle neutralization is enhanced by a discharge associated with the beam or with the return current. The neutralization current may be locally enhanced by neutral gas. Beams have been projected for long distances and appear to be stable in many cases. However, certain beam‐plasma discharges similar to those observed in laboratory experiments may have also occurred in space, tending to thermalize high‐energy beams. Several types of experiments search for beam echoes from field‐aligned potential differences thought to cause auroras. Others have made observations at conjugate regions in the opposite hemisphere for magnetic geometry studies. In the Minnesota ‘Echo’ technique, conjugate hemisphere beam echoes are detected on board the source rocket. From the Echo experiments it was found that ionospheric electric fields map along auroral zone field lines as equipotentials through the equatorial plane. Echo bounce times can be fitted in some cases to model fields, but in other cases the field may be distorted or extended in the night region. Examples from the Echo program at Minnesota are used to illustrate electron beams as magnetospheric ‘probes.’ The injection of such beams creates rich sources of plasma and electromagnetic waves, which have been detected by ejected subsystems near the mother rocket or at ground level. The frequency range extends from a few kilohertz to at least 50 MHz and includes lower hybrid, whistler mode, electron cyclotron harmonics, upper hybrid, and plasma frequency emission. Electron beams have been used to produce artificial auroras that were observed by sensitive television techniques, by other optical methods, and by radar reflections. The scattering, energy loss, and magnetic reflection of beams in the 100‐km atmosphere region have also received detailed study. Future programs including Space Shuttle are discussed. Summaries of the current experimental programs and complete literature references are included.
Geomagnetic models and the solar cycle effect
Authors: Alldredge Leroy R.
Published: February 2008

An examination of first differences in annual means of the northward component at widely separated observatories over an 80‐year period shows many large spikes. These spikes occur simultaneously at many observatories and coincide with rapid changes in sunspot numbers, which shows that they are real and that they come from external sources. A smoothed time derivative curve of the same component made by plotting, for each year, the slope of a straight line determined by least squares from five consecutive years still shows a very strong solar cycle effect. It is important to know how this solar cycle effect modifies the spherical harmonic coefficients of geomagnetic models. A simple example illustrates that a zonal external field makes no contribution to internal spherical harmonic coefficients when a least squares solution is made using a uniform distribution of all component data. This effect is an example of the general theorem that any harmonic field of external origin is orthogonal to any harmonic field of internal origin. Totally different results are obtained if a single component or the total field intensity is used in the least squares solution.
Asteroids and comets
Authors: Chapman Clark R.
Published: February 2008

Asteroids and comets are populations of countless bodies ranging downwards in size from Ceres, which is about 1000 km in diameter. Although they constitute a virtually negligible fraction of the mass of the solar system, their physical/chemical properties and locations in the system are of particular interest, since they provide most of our observational and experimental clues about the earliest epochs of solar system history. This is particularly due to the fundamental role of meteorites in planetary science; meteorites are fragments of asteroids and comets, although they are treated in this review only tangentialIy. In addition, the ongoing behavior of asteroids and comets (primarily their response to the interplanetary radiation and impact environment) provides interesting insight to unusual physical and geological processes.

Both asteroids and comets have been the subject of an important international meeting during the period covered by this report. A comprehensive book and a special issue of a journal emanated from each conference (Asteroids. Gehreis, ed., 1979, and Icarus 40. 1979; Comets. Wilkening, ed., 1982, and Icarus 47. 1981). Particularly in the case of comets, for which the book was published in mid‐1982, it would be superfluous to treat the pre‐1982 literature in great detail. More has happened since the 1979 publication of the Asteroids book, so I rely more on subsequent journal publications about asteroids.
Ionospheric electrodynamics and irregularities: A review of contributions by U.S. scientists from 1979 to 1982
Authors: Richmond A. D.
Published: February 2008

The past four years have seen important advances in our understanding of global ionospheric electrodynamics and of ionospheric irregularities. The accumulation of ionization drift measurements by radar and satellite have led to the development of quantitative global electric field models. Extensive ground‐ and satellite‐based magnetic field measurements have given improved information about electric currents. Quantitative models of high‐latitude ionospheric electrical conductivities have been produced. Theoretical modeling efforts have given us the capability of inferring unknown electrodynamic parameters from measured quantities on a global basis, and allow us to assess quantitatively the influence of ionospheric electric fields and currents on the ionospheric plasma and on the neutral atmosphere. Ionospheric irregularities have also been extensively studied by both experimental and theoretical means in recent years, but are treated only very briefly in this report, with the reader referred to more comprehensive review articles for further information.
The atmospheric aerosol system: An overview
Authors: Prospero J. M. , Charlson R. J. , Mohnen V. , Jaenicke R. , Delany A. C. , Moyers J. , Zoller W. , Rahn K.
Published: February 2008

Aerosols could play a critical role in many processes which impact on our lives either indirectly (e.g., climate) or directly (e.g., health). However, our ability to assess these possible impacts is constrained by our limited knowledge of the physical and chemical properties of aerosols, both anthropogenic and natural. This deficiency is attributable in part to the fact that aerosols are the end product of a vast array of chemical and physical processes. Consequently, the properties of the aerosol can exhibit a great deal of variability in both time and space. Furthermore, most aerosol studies have focused on measurements of a single aerosol characteristic such as composition or size distribution. Such information is generally not useful for the assessment of impacts because the degree of impact may depend on the integral properties of the aerosol, for example, the aerosol composition as a function of particle size. In this overview we discuss recent work on atmospheric aerosols that illustrates the complex nature of the aerosol chemical and physical system, and we suggest strategies for future research. A major conclusion is that man has had a great impact on the global budgets of certain species, especially sulfur and nitrogen, that play a dominant role in the atmospheric aerosol system. These changes could conceivably affect climate. Large‐scale impacts are implied because it has recently been demonstrated that natural and pollutant aerosol episodes can be propagated over great distances. However, at present there is no evidence linking anthropogenic activities with a persistent increase in aerosol concentrations on a global scale. A major problem in assessing man's impact on the atmospheric aerosol system and on global budgets is the absence of aerosol measurements in remote marine and continental areas. This dearth is especially acute for the southern hemisphere, where we could expect natural sources to predominate because of the relatively low level of industrial development and energy utilization.
Solar wind controlled pulsations: A review
Authors: Odera T. J.
Published: February 2008

Studies of the solar wind controlled Pc 3, 4 pulsations by early and recent researchers are highlighted. The review focuses on the recent observations, which cover the time during the International Magnetospheric Study (IMS). Results from early and recent observations agree on one point, that is, that the Pc 3, 4 pulsations are influenced by three main solar wind parameters, namely, the solar wind velocity Vsw, the IMF orientation θxB, and magnitude B. The results can be interpreted, preferably, in terms of an external origin for Pc 3, 4 pulsations. This implies, essentially, the signal model, which means that the pulsations originate in the upstream waves (in the interplanetary medium) and are transported by convection to the magnetopause, where they couple to oscillations of the magnetospheric field lines.
Correction to “SNC meteorites: Clues to Martian petrologic evolution?”
Authors: McSween Harry Y.
Published: February 2008

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The Martian surface
Authors: Carr Michael H.
Published: February 2008

The past four years have been particularly fruitful for Martian research as the enormous volumes of data collected during the Viking mission became readily available to the general science community, and as reformatting of the remote sensing data into cartographic products made the data more useable. The 1:5,000,000‐scale map series is complete, and 1:2,000,000‐scale controlled mosaics of the entire planet are nearing completion. In addition 1:500,000‐scale mosaics have been prepared of about thirty local areas. A uniform scale, digital imaging data base is in preparation (Batson, 1986), and the infrared data have been reformatted to make it more accessible to the general user (Martin and Kieffer, 1985).
Core plasma in the magnetosphere
Authors: Horwitz J. L.
Published: February 2008

The “core” of magnetospheric plasma distribution functions is of prime importance in the magnetospheric plasma behavior. For the purposes of this review and because of recent instrumental characteristics, we shall regard the term “core plasma” to include ions and electrons with energies less than 50 eV. In the period 1983–1986, several new and important core plasma phenomena and regions have been identified, including: the cleft ion fountain, the plasmaspheric heavy ion torus, core molecular ions, high core plasma densities in the plasma sheet boundary layer, intense transverse heating of core plasmas in the equatorial regions, the low‐energy ion transition (LEIT) delineating the plasmaspheric and plasma sheet regions, the supersonic polar wind, toroidal or ring heavy ion distribution functions in the auroral region, conic‐to‐field‐aligned ion signatures of inverted V auroral events, and large‐scale electron density structure in the magnetosphere. On the theoretical/modeling side, significant progress has been made on: the effect of hot electrons on the polar wind; simulation of thermal helium heating at the equator; kinetic models of the plasma transport from localized ionospheric regions; time‐dependent hydrodynamic models of the polar wind and plasmasphere‐ionosphere coupling; studies of the behavior of the plasmaspheric He+, O+ + and counterstreaming He+‐H+; and kinetic models of plasmasphere refilling. In this report, we shall briefly review the progress during the past quadrennium in these and other topics under the general headings: (1) core plasma in the plasmasphere and plasmapause region, (2) core plasma in the polar cleft and polar cap magnetosphere, and (3) core plasma in the auroral and plasma sheet magnetosphere.
Overview of cosmic ray, solar, and interplanetary research (1983–1986)
Authors: Shea M. A.
Published: February 2008

This report briefly summarizes the research results in the fields of cosmic radiation, solar, and interplanetary physics (including the interaction of the solar wind with comets) as obtained by scientists in the United States during the quadrennium 1983–1986.
Overview of geomagnetism and paleomagnetism, 1983–1986
Authors: Blakely Richard J.
Published: February 2008

Michael Fuller, author of a similar overview four years ago, concluded that “...the next quadrennium promises to be a most interesting one, with plenty to keep us busy.” The 11 review papers that follow support his prediction; research in geomagnetism and paleomagnetism indeed has flourished over the last four years. In this brief overview, I will attempt to summarize what I perceive as important advances during the last quadrennium and to predict where research might lead during the next four years. These observations are seen through the eyes of a very specialized geophysicist; another author undoubtedly would see the past and future much differently.
The role of pore fluids in tectonic processes
Authors: Mase Charles W. , Smith Leslie
Published: February 2008

Most rocks within the upper crust are porous and are likely to contain water that is mechanically free and is not trapped in isolated pores. Because the pressure of the pore water influences the mechanical strength of rock, stresses and deformation are coupled to fluid flow within a porous medium. Furthermore, local heating during tectonic processes can lead to thermal expansion, pressurization and flow of pore fluids, coupling the thermal field to the stress and fluid fields. Fluid migration can also be an important transport mechanism for large‐scale redistribution of solutes and heat within the upper crust. Consequently, the behavior of pore fluids is of considerable importance in the physics and evolution of tectonic processes. During the past four years, there has been a general recognition that pore fluids are not a passive component of tectonic environments, but are rather best viewed as an integral part of the system dynamics.
Geotomography with local earthquake data
Authors: Kissling Edi
Published: February 2008

The inversion of local earthquake data (LED) for three‐dimensional velocity structure requires the simultaneous solution of the coupled hypocenter‐model problem. The Aki‐Christoffersson‐Husebye method (ACH) involves the inversion of large matrices, a task that is often performed by approximative solutions when the matrices become too big, as is the case for most LED, considering the coupled inverse problem. Such an approximate method (herein referred to as approximate geotomographic method) is used to perform tests with LED to obtain the best suited inversion parameters, such as velocity damping and number of iteration steps. The ACH method has been proposed for use of teleseismic data. Several adjustments to the original ACH method, which are necessary for use of LED, have been developed and are discussed. Such adjustments are the separation of the unknown hypocentral from the velocity model parameters for the inversion, the use of geometric weighting and step length weighting, the calculation of a minimum one‐dimensional (1D) model as the starting three‐dimensional (3D) model for the model inversion, and the display of an approximate resolution matrix (ray density tensors) before the inversion is performed. The ray density tensors allow the block cutting, e.g., the definition of the 3D velocity grid, to better correspond with the resolution capability of the specific data set. The adjustments to the method are tested by inversion of realistic LED of known variance. Synthetic LED are also used to demonstrate the effects of systematic errors, such as mislocations of seismic stations, on the resulting velocity field. Using the data sets from Long Valley, California, Yellowstone National Park, Wyoming, and Borah Peak, Idaho, the effects of improvements to the ACH method and of the data filtering process are shown. The use of the minimum 1D models for routine earthquake location improves this location procedure, as shown with the relocation of shots for the Long Valley and Yellowstone areas. The three‐dimensional velocity fields obtained by the ACH method for the Long Valley and Yellowstone areas show local anomalies in the p velocity that can be correlated with tectonic and volcanic features. A pronounced anomaly of low p velocity below the Yellowstone caldera can be interpreted as a large magma chamber. However, the bulk of the paper addresses problems of the inversion method. The LED from the areas mentioned above are used to numerically and theoretically tune the inversion method for the defects that all real data contain. It is shown that one of the most important steps for any inversion of LED is the selection of the data for quality and for geometrical distribution.
Geophysiology, the science of Gaia
Authors: Lovelock James E.
Published: February 2008

The Gaia hypothesis postulates that the climate and chemical composition of the Earth's surface environment is, and has been, regulated at a state tolerable for the biota. This notion was introduced in 1972 and 1973 (Lovelock, 1972; Margulis and Lovelock, 1974; Lovelock and Margulis, 1973). The wording of these early papers was sometimes poetic, rather than scientific, but Gaia has matured and might be better stated as a theory that views the evolution of the biota and of their material environment as a single, tightly coupled process, with the self‐regulation of climate and chemistry as an emergent property. It is a theory that makes “risky” predictions, for example, that oxygen is and has been regulated during the existence of land plants, within ±5% of its present level; it is therefore falsifiable. Numerical models are used to illustrate the potential for stable self‐regulation of tightly coupled systems of organisms and their environments.
Dusty plasmas in the solar system
Authors: Goertz C. K.
Published: February 2008

The processes that lead to charging of dust grains in a plasma are briefly reviewed. Whereas for single grains the results have been long known, the reduction of the average charge on a grain by “Debye screening” has only recently been discovered. This reduction can be important in the Jovian ring and in the rings of Uranus. The emerging field of gravitoelectrodynamics which deals with the motion of charged grains in a planetary magnetosphere is then reviewed. Important mechanisms for distributing grains in radial distance are due to stochastic fluctuations of the grain charge and a systematic variation due to motion through plasma gradients. The electrostatic levitation model for the formation of spokes is discussed, and it is shown that the radial transport of dust contained in the spokes may be responsible for the rich radial structure in Saturn's rings. Finally, collective effects in dusty plasmas are discussed which affect various waves, such as density waves in planetary rings and low‐frequency plasma waves. The possibility of charged grains forming a Coulomb lattice is briefly described.
Spacecraft observations of the interaction of active comets with the solar wind
Authors: Neugebauer M.
Published: February 2008

In 1985 and 1986, six spacecraft encountered two comets and obtained a wealth of new data about the plasma processes at work in the interaction of an active comet with the solar wind. This paper summarizes those data and their interpretations. The interaction process starts millions of kilometers from the nucleus where the solar wind first picks up cometary ions. The cometary ions not only slow down the wind but also create disturbances which are ultimately responsible for the surprisingly high fluxes of energetic particles detected at comets. After passing through a structure that is arguably called a bow shock, the plasma continues to slow down as it accretes more cometary ions. The deceleration of the wind causes the interplanetary magnetic field to pile up and drape around the comet's ionosphere, as predicted by Alfvén in 1957. It is this draped field that defines the geometry of a comet's plasma tail. At Halley the Giotto spacecraft detected a well‐defined boundary separating the mixture of solar wind and cometary plasmas and the interplanetary magnetic field from the field‐free, nearly pure cometary plasma in the inner coma. Other unexpected features in the inner coma were a flux of fast “granddaughter” ions and high densities of negative ions. For all the comet encounters, the region between the bow shock and the field‐free cavity was distinguished by several sudden changes in the properties of the plasma; the natures and causes of these features are subjects of intense debate.
Single‐Polynomial Analysis of Ionograms
Authors: Titheridge J. E.
Published: February 2008

A new formulation of the single‐polynomial analysis is presented, in which the electron density profile is assumed to begin with a flat base at a height equal to the lowest measured virtual height. This formulation gives a more accurately defined profile with only a small number of points and agrees with the assumptions normally made. Near the critical frequency he electron density profile is required to approach a parabolic distribution. Coefficients are given for the rapid analysis of ionograms taken anywhere in the world. The height of the peak, the scale height at the peak, and the subpeak electron content, along with five or six points on the electron density profile, are obtained directly from a small number of measured virtual heights.
Polarization of Ionospherically Propagated HF Radio Waves with Applications to Radio Communication
Authors: Epstein M. R.
Published: February 2008

Sweep‐frequency CW radio signals transmitted over 1300‐km north‐south and 1900‐km east‐west temperate‐zone paths were received 24 hours per day for 21½ ‐day periods. Measurements of the incoming one‐hop F‐layer polarization, as a function of time‐of‐day and radio frequency, indicate that daytime rates of change of polarization rotation with frequency (at an instant of time) average 1 turn/MHz over the north‐south path and 0.25 turn/MHz over the east‐west path. These values are in good agreement with computer raytracing predictions of the rates of change of polarization rotation with frequency over similarly disposed paths. It was observed that nighttime polarization rotation rates are about 1 to 4 times as high as those measured during the day. It was also experimentally found that the rate of change of the incoming polarization with frequency first decreased and then increased with incoming radio frequency, the minimum rate occurring near 0.8 MOF0. Daytime rates of polarization rotation with time (at a given frequency) average 0.25 turn/min. The polarization rotation rates with time do not appear to vary either with path azimuth or transmitted radio frequency. Near‐zero rates of polarization rotation with time occur for much of the nighttime period. Large fluctuations in the polarization variations occur with both time and frequency throughout the day.

Polarization measurements made with signals that were reflected from a nonblanketing, nighttime sporadic E layer of about two hours duration indicate that, for this layer, circumstances other than polarization effects, probably multipath, determine much of the observed signal strength variations with frequency and time.

The results suggest that for round‐the‐clock temperate zone propagation, when linearly polarized antennas are employed, a qualitative threshold for envelope distortion of broadband signals (due to variation in the incoming polarization with frequency) occurs when signal bandwidths exceed approximately 100 kHz for north‐south paths and 400 kHz for east‐west paths. The effects of such distortion may be reduced either by operating near 0.8 MOF0 or by the use of circularly polarized antennas.
Gross Estimates of the Conductivity, Dielectric Constant, and Magnetic Permeability Distributions in the Moon
Authors: Ward Stanley H.
Published: February 2008

Sources of direct information on the electrical parameters of the moon include earth‐based astatic radar, earth‐satellite bistatic radar, lunar‐lander astatic radar, earth‐based radiothermic measurements, lunar orbital magnetic field measurements, and lunar‐lander magnetic attraction experiments. These pieces of information, when fitted to a reasonable geological model of the moon, permit estimation of the depth profiles of electrical conductivity, dielectric constant, and magnetic permeability. At lunar surface dc conductivities as low as 10‐12 mho/m may occur, and conductivities as high as 10 mhos/m appear possible for the deep lunar interior. It is demonstrated that interpretation of the Explorer 35 lunar satellite magnetometer data is at present based on such inadequate theory that the above range of values of conductivity could be quite consistent with the orbital magnetic field data. Anomalous ‘dc’ dielectric constants as high as 1010 could exist in the lunar interior if pore water and metallic minerals are present in juxtaposition. Magnetic permeabilities as large as 1.7 can be present in the lunar surface rocks. Typical depth profiles of conductivity and dielectic constant that fall within the extremes noted above are presented.
Faraday Rotation Measurements Using Incoherent Scatter
Authors: Farley D. T.
Published: February 2008

A complete description is given of the method used at the Jicamarca Radar Observatory to derive Faraday rotation information (and hence absolute electron density profiles that do not require subsequent normalization or correction for the effects of Te/Ti) from incoherent scatter data. Similar techniques which could be used at other observatories with radars not having the unusual properties of the Jicamarca radar are discussed. At temperate and polar latitudes the optimum frequencies for incoherent scatter Faraday measurements in most of the ionosphere are roughly 100–200 MHz. Somewhat higher frequencies should give acceptable daytime results, however. A slight modification of the Faraday measurement, used at Jicamarca to obtain the autocorrelation function of the scattered signal, is also briefly described.
Amplitude Distributions Resulting From Curvature Distributions of a Reflector
Authors: Austin G. L. , Thorpe M. R.
Published: February 2008

A simple theory of focusing is discussed, and amplitude probability distributions are related to distributions of curvature at the reflector. The results are applicable to amplitude distributions of radio signals reflected from the ionosphere.
Fluctuations of a Beam Wave Propagating Through a Locally Homogeneous Medium
Authors: Ishimaru Akira
Published: February 2008

General formulations for the fluctuations of a beam wave propagating through a homogeneous or locally homogeneous medium are given in terms of the spectral density of the index of refraction. The amplitude and phase correlation functions and the mean‐square fluctuations are derived for a homogeneous medium showing the dependence on the radial distance in the transverse plane of the beam. The amplitude and phase structure functions are derived for a locally homogeneous medium. The correlation functions and the structure functions do not depend only on the difference coordinate; they are functions of the radial coordinates in the beam cross section. This particular inhomogeneity, however, is shown to be an analytic continuation of the homogeneous or locally homogeneous case. The meansquare amplitude fluctuation for the Kolmogorov's locally homogeneus medium is shown to behave as a plane wave for a short distance and then to become less than that of a spherical wave, and its spectrum is shown to behave as K−1 for large K in contrast with the plane and spherical waves. The spread of the beam radius is shown to be approximately the 8/3 powers of the distance L for small distance, and its, increase depends on the magnitude of the index of refraction fluctuation.
Propagation in a Superrefractive Troposphere with a Trapping Surface Layer
Authors: Gerks Irvin H.
Published: February 2008

The problem of UHF propagation is examined for the case of a spherically stratified troposphere having a profile of refractive modulus with a minimum near the surface of the earth. The methods of R. E. Langer are used to develop an asymptotic solution in terms of Whittaker functions, and eigenvalues are determined numerically with a fast computer. The results, which agree with those obtained earlier by other methods, are used to show how the distant field strength varies as the profile changes from a ‘normal’ case to a highly superrefractive case.
The Simple Thin‐Film Optical Method in Electromagnetic Wave Propagation
Authors: Altman C. , Cory H.
Published: February 2008

Ionospheric transmission and reflection coefficients for vertical incidence are determined by means of a simple and rapid numerical method based on thin‐film optics. By this method the ionosphere is divided into a large number of thin layers, all partial reflections within a given layer are summed, and the process is then extended layer by layer to include the entire ionospheric region. The iterative procedure in its simplest form is started high in the ionosphere and extended downward to terminate at the ionospheric base, but by suitable modification it may be started low in the ionosphere and extended upward to yield in a single ‘run’ the contribution of each portion of the electron density profile to the over‐all reflection and transmission coefficients. The method is free of the tendency to numerical swamping and is valid as long as the wave polarization remains constant, which is true in the D region in most of the VLF range. Comparison with full‐wave analysis shows that the method breaks down at higher frequencies when weak coupling processes between the magnetoionic modes become relatively important owing to varying wave polarizations.
Determination of D‐Region Electron Densities From Multifrequency Absorption Measurements
Authors: Musser Jeffrey M.
Published: February 2008

The use of a cubic approximation to the electron density determined by means of multifrequency absorption measurements does not always yield satisfactory results. An alternate method of determining the electron density is presented. A family of proton spectra is used, and the electron density profile resulting from each spectrum is determined. It is found that each profile produces absorption which varies as ƒ−n with the exponent n characteristic of the particular profile. These profiles are then varied in magnitude to produce the measured absorption. The result is an electron density profile that gives a more realistic representation of the D region during a proton event than does a cubic approximation.