bluuebunny/crossref_metadata_2025 huggingface dataset:
The composition of galactic cosmic rays and solar energetic particles
Authors: Mason G. M.
Published: February 2008

Galactic cosmic rays and solar energetic particles constitute two high energy particle populations which originate from different astrophysical reservoirs, and yet which we may study directly at earth. Since galactic cosmic rays stay in the galactic disk for only about 10 million years, they may carry information about recent nucleosynthetic activity in the galaxy. On the other hand, solar energetic particles originate in the corona, whose composition has probably remained essentially unchanged for several billion years, and thus they carry information concerning the origin of the solar system. Details of the composition of these high energy particles carry information of vital importance to other diverse topics as well, such as dynamical processes in the solar corona and in supernovae shocks, and the propagation and transport of particles in the interplanetary and interstellar space.
Future earthquakes
Authors: Bakun William H.
Published: February 2008

While significant progress toward earthquake prediction has occurred in the past four years, the current guarded optimism for continued progress reflects in part revised goals and new definitions. First, recognition [e.g., Evernden, 1982] of the failures of the search during the 1970s for a reliable readily‐observable earthquake precursor has reemphasized the need to understand the fundamental physics of the earthquake generation process [e.g., Stuart, 1984/85] and brought acceptance of a probabilistic rather than a deterministic approach to earthquake prediction. That is, we now accept the notion that earthquake predictions must be couched in terms of probability gain and likelihood estimates. Second, a prediction terminology [Wallace et al., 1984] has been largely adopted that separates and expands the tasks into long‐term earthquake potential (no specific time window) and long‐term prediction (time windows of a few years to a few decades), where progress has been significant, and intermediate‐term prediction (time windows of a few weeks to a few years) and short‐term prediction (time windows up to a few weeks), where progress has lagged. In the 1970s, earthquake prediction usually was taken to mean a deterministic short‐term or intermediate‐term warning; now, probabilistic estimates of earthquake potential [e.g., Lindh, 1983; Sykes and Nirhenko, 1984] are accepted as the most reasonable and valid form for expressing the likelihood of a future earthquake and its hazards, as well as the uncertainties.
Structure of the Earth: Mantle and core
Authors: Lay Thorne
Published: February 2008

The deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years. Most studies have emphasized quantitative three‐dimensional mapping of the lateral velocity heterogeneity of the mantle and core. These aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes. A first generation of global models for the lateral velocity variations in the deep earth has been produced, providing tantalizing images of large scale structures suggestive of a non‐steady state thermal convection system.

The upper 400 km of the mantle has the strongest lateral velocity variations, of up to ±10% for shear velocity. Surface‐wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations. Thus, the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics. The upper mantle models support the contention that continents have deep roots, with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km. Very long‐wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km, as well as throughout the lower mantle. These deep‐seated variations have little correspondence to surface tectonics, and efforts to interpret their nature are just beginning. The lowermost 200 km of the mantle (D″ region) has lateral velocity fluctuations comparable to those in the upper mantle, and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the D″ layer. A combined thermal and compositional boundary layer, roughly mirroring the lithosphere, is a likely explanation for this anomalous zone. The core‐mantle boundary appears to have significant (10 km) long‐wavelength topography, presumably sustained by dynamic stresses from deep mantle convection. The inner core may have strong lateral heterogeneity or axially symmetric anisotropy, suggesting a complex thermal and compositional state.

Significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short‐period body‐wave band. Models for teleseismic P‐wave attenuation operators have converged, with t* values of 0.7–1.0 s appropriate at 1 Hz, and t* values of 0.4–0.6 s appropriate at 4 Hz. Regional variations of attenuation are slowly being mapped out as well.
Lithospheric stress and deformation
Authors: McNutt Marcia
Published: February 2008

The study of the mechanisms, magnitudes, and modes of lithospheric stress and deformation occupies a central position in the discipline of Tectonophysics. It is therefore difficult in assembling a comprehensive review to place limits on what should be included. For example, one cannot discuss stress and deformation apart from considerations of the rheological models of the lithosphere which link the two and ultimately control the behavior of plates at their margins and in their interiors. Once admitting to the importance of rheological considerations in discussing stress and deformation, one is then compelled to include information relating to the thermal state of the lithosphere, since temperature is one of the foremost environmental variables controlling rheology. Furthermore, lithospheric processes cannot be considered in isolation from those affecting the asthenosphere, since surely there exists some coupling between the motion of the lithosphere and convection in the Earth's interior. In fact, an understanding of lithospheric stress and deformation can be considered a prerequisite to solving problems in geodynamics because it is through the lithospheric filter that one detects the consequences of thermal and compositional anomalies at greater depth. The topic of this report cannot even be cleanly separated from reviews in other disciplines such as Seismology and Geodesy. The magnitude and orientation of lithospheric stress is often indicated by earthquakes, while lithospheric strain can be observed geodetically. The philosophy I have adopted here is that some overlap with other review papers in this series is unavoidable, and probably desirable from the standpoint of emphasizing how recent work n other areas of geophysics has contributed to the advancement in our understanding of the lithosphere's state‐of‐stress and mechanisms of deformation. I have, however, resisted the temptation to include extraterrestrial studies, presumably covered in the Planetary section, despite the similar causes for deviatoric stress and deformational mechanisms operating on other terrestrial planets. Certainly ideas of how silicate materials behave and how lithosphere responds to internal forces benefit from the perspective of omparative planetology. Finally, in defining the scope of this portion of the U.S. National Report to the IUGG, I did not consciously exclude papers by non‐U.S. citizens, work done at non‐U.S. institutions, projects not funded by the U.S. Government, or however else “foreign” might be defined. Topics in geophysical research cannot be divided along political boundaries. However, I did not attempt to seek out articles written in languages other than English or published in journals not readily available in the United States, and I apologise for this obvious shortcoming.
Mineralogy of chondritic interplanetary dust particles
Authors: Mackinnon Ian D. R. , Rietmeijer Frans J. M.
Published: February 2008

From a mineralogical survey of approximately 30 chondritic micrometeorites collected from the lower stratosphere and studied in detail using current electron microscopy techniques, it is concluded that these particles represent a unique group of extraterrestrial materials. These micrometeorites differ significantly in form and texture from components of carbonaceous chondrites and contain some mineral assemblages which do not occur in any meteorite class. Electron microscope investigations of chondritic micrometeorites have established that these materials (1) are extraterrestrial in origin, (2) existed in space as small objects, (3) endured minimal alteration by planetary processes since formation, and (4) can suffer minimal pulse heating (<600°C) on entering earth's atmosphere. The probable sources for chondritic interplanetary dust particles (IDPs) are cometary and asteroidal debris and, perhaps to a lesser extent, interstellar regions. These sources have not been conclusively linked to any specific mineralogical subset of IDP, although the chondritic porous (CP) aggregate is considered of likely cometary origin. Chondritic IDPs occur in two predominant mineral assemblages: (1) carbonaceous phases and phyllosilicates and (2) carbonaceous phases and nesosilicates or inosilicates, although particles with both types of silicate assemblages are observed. Olivines, pyroxenes, layer silicates, and carbon‐rich phases are the most commonly occurring minerals in many chondritic IDPs. Other phases often observed in variable proportions include sulphides, spinels, metals, metal carbides, carbonates, and minor amounts of sulphates and phosphates. Individual mineral grain sizes range from micrometers (primarily pyroxenes and olivines) to nanometers, with the predominant size for all phases less than 100 nm. Specific mineral characteristics for particular chondritic IDPs provide an indication of processes which may have occurred prior to collection in the earth's stratosphere. For example, pyroxene mineralogy in some chondritic aggregates is consistent with condensation from a vapor phase and, we consider, with condensation in a turbulent solar nebula at relatively low temperatures (<1000°C). Carbonaceous phases present in other CP aggregates have been used to imply low‐temperature formation processes such as Fischer‐Tropsch synthesis (∼530°C) or carbonization and graphitization (∼315°C). Alteration processes have been implicated in the formation of some layer silicates in CP aggregates and may have involved hydrocryogenic alteration at <0°C. In general, interpretations of transformation processes on submicrometer‐size minerals in chondritic IDPs are consistent with formation at a radius equivalent to the asteroid belt or greater during the later stages of solar nebula evolution using currently available models.
Long‐term air quality monitoring at the South Pole by the NOAA Program Geophysical Monitoring for Climatic Change
Authors: Robinson E. , Bodhaine B. A. , Komhyr W. D. , Oltmans S. J. , Steele L. P. , Tans P. , Thompson T. M.
Published: February 2008

The National Oceanic and Atmospheric Administration program of Geophysical Monitoring for Climatic Change (GMCC) has operated a background atmospheric monitoring observatory in Antarctica at the United States Amundsen‐Scott South Pole Station since 1972. The program objectives at South Pole, as at the other three GMCC observatories, include the determination of concentrations, variations with time, and other properties of atmospheric trace gases and aerosol particles which can potentially impact climate. In addition, GMCC monitors solar radiation and meteorological factors to determine long‐term means and any trends that could be associated with climatic changes as well as to examine correlations between meteorological and air chemistry parameters. This discussion emphasizes the long‐term GMCC South Pole air chemistry record for carbon dioxide, total ozone, surface ozone, methane, halocarbons, nitrous oxide, and aerosol concentrations. Comparisons of South Pole findings with other global GMCC data are also given. The total ozone discussion includes the results of recent GMCC ozonesonde operations and an assessment of Dobson ozone spectrophotometer data taken at South Pole by NOAA since 1964. These data sets are directly applicable to Antarctic “ozone hole” investigations, and current findings related to this phenomenon are discussed.
Balloon‐borne measurements of middle atmosphere aerosols and trace gases in Antarctica
Authors: Hofmann D. J.
Published: February 2008

Although trace stratospheric gases such as ozone have been measured in Antarctica from the surface since 1957 and from balloons since 1966 and stratospheric aerosols have been measured routinely by balloon‐borne particle counters since 1972, only recently has any real interest in these measurements developed. This new awareness is directly related to the realization, in 1985, that springtime Antarctic total ozone had declined about 50% since about 1977. Such an unprecedented, unpredicted change understandably created a great deal of interest in past Antarctic ozone‐related measurements. Early theories of this phenomenon included both chemical and dynamical ozone depletion mechanisms. Chemical theories generally utilized stratospheric particles (created in the low‐temperature environment of the Antarctic stratosphere) to enhance chemical conversion of reservoir chlorine compounds to active ozone‐destroying chlorine heterogeneously. Dynamic models utilized particle heating to lift the polar vortex air mass in spring and cause reductions in ozone. Thus these theories generated an interest not only in ozone measurements but also in stratospheric particulates (aerosols) and in measurements of trace gases which could be used as tracers of atmospheric motions, for example, nitrous oxide. In 1986 the first major field studies, in the National Ozone Expedition, were directed at this problem. In the following review of in situ balloon‐borne measurements of aerosols and trace gases the limited amounts of data prior to 1986 are summarized. The new measurements of 1986 have revealed much concerning the nature of the ozone depletion mechanism. While the steady decline in ozone in the 12‐ to 20‐km region in September can probably only be explained by fast chemistry, the phenomenon appears to be shaped spatially and temporally by dynamical phenomena. The next several years will see expanded research in this area, not only in Antarctica but also in other regions in an attempt to detect reductions in ozone which may occur at these locations.
Geologic nozzles
Authors: Kieffer Susan Werner
Published: February 2008

Sonic velocities of geologic fluids, such as volcanic magmas and geothermal fluids, can be as low as 1 m/s. Critical velocities in large rivers can be of the order of 1–10 m/s. Because velocities of fluids moving in these settings can exceed these characteristic velocities, sonic and supersonic gas flow and critical and supercritical shallow‐water flow can occur. The importance of the low characteristic velocities of geologic fluids has not been widely recognized, and as a result, the importance of supercritical and supersonic flow in geological processes has generally been underestimated. The lateral blast at Mount St. Helens, Washington, propelled a gas heavily laden with dust into the atmosphere. Because of the low sound speed in this gas (about 100 m/s), the flow was internally supersonic. Old Faithful Geyser, Wyoming, is a converging‐diverging nozzle in which liquid water refilling the conduit during the recharge cycle changes during eruption into a two‐phase liquid‐vapor mixture with a very low sound velocity. The high sound speed of liquid water determines the characteristics of harmonic tremor observed at the gyeser during the recharge interval, whereas the low sound speed of the liquid‐vapor mixture influences the fluid flow characteristics of the eruption. At the rapids of the Colorado River in the Grand Canyon, Arizona, the channel is constricted into the shape of a converging‐diverging nozzle by debris flows that enter from tributary canyons. Both subcritical and supercritical flow occur within the rapids. The transport capacity in the rapids can be so great that the river contours the channel to a characteristic shape. This shape can be used to interpret the flood history of the Colorado River over the past 10³–105 years. The unity of fluid mechanics in these three natural phenomena is provided by the well‐known analogy between gas flow and shallow‐water flow in converging‐diverging nozzles.
Coordination chemistry of weathering: Kinetics of the surface‐controlled dissolution of oxide minerals
Authors: Stumm Werner , Wollast Roland
Published: February 2008

Chemical weathering processes, essentially caused by the interaction of water and the atmosphere with the Earth's crust, transform primary minerals into solutes and clays and, eventually, into sedimentary rocks; these processes participate in controlling the global hydrogeochemical cycles of many elements. Many mineral dissolution processes are controlled by a chemical mechanism at the solid‐water interface. The reaction‐controlling steps can be interpreted in terms of a surface coordination model. The tendency of a mineral to dissolve is influenced by the interaction of solutes—H+, OH−, ligands, and metal ions—with its surface. The surface reactivity is shown to depend on the surface species and their structural identity; specifically, the dependence of dissolution rates on pH and on dissolved ligand concentrations can be explained in terms of surface protonation (and deprotonation) and of ligand surface complexes. A general rate law for the dissolution of minerals is derived by considering, in addition to the surface coordination chemistry, established models of lattice statistics and activated complex theory.
The time‐averaged paleomagnetic field
Authors: Schneider David A. , Kent Dennis V.
Published: February 2008

We review indications of persistent deviations from the geocentric axial dipole model of the time‐averaged geomagnetic field and present a zonal harmonic model derived from 185 deep‐sea sediment piston cores taken from low to middle latitudes (to approximately ±45°). Analysis of the paleomagnetic inclination recorded in these cores for the Brunhes (normal polarity; 0–73 Ma) and Matuyama (reverse polarity; 0.73–2.47 Ma) chrons, after plate motion correction, gives well‐constrained estimates of the dominant long‐term nondipole contributions (the axial quadrupole and axial octupole) and shows no significant deviation from axial symmetry. The amplitude of the axial quadrupole is found to vary with polarity (2.6% of the geocentric axial dipole for normal; 4.6% for reverse), while the axial octupole does not show appreciable change (−2.9% for normal; −2.1% for reverse). These estimates of the quadrupole contribution agree well with prior determinations for the Plio‐Pleistocene (0–5 Ma); however, the octupole contribution we find is opposite in sign to previous estimates. We suggest that a negative octupole is representative of the actual time‐averaged paleomagnetic field, while prior positive octupole estimates probably reflect spurious inclination shallowing. The lack of polarity asymmetry in the octupole suggests that this nondipole component may be more closely linked to the main dipole field than is the quadrupole and so supports models of the geodynamo in which dipole and quadrupole families do not interact.
Ray Description for Point Source Excited Surface Waves in Stratified Anisotropic Media
Authors: Bertoni H. L. , Hessel A.
Published: February 2008

A ray‐optical description for the surface wave contribution to the far fields radiated by an electromagnetic point source in the presence of a planar interface between two lossless, homogeneous, anisotropic media is formulated. The surface wave rays are viewed as extending radially along the interface from the normal projection of the source onto the interface. Locally, the ray fields are those of the modal surface waves carrying energy in the ray direction. The effect of anisotropy on the surface wave radiation pattern is also considered.
Differential Absorption Measurements by a Phase Difference Technique
Authors: Boys J. T.
Published: February 2008

Measurements of the phase difference between the signals from ionospherically propagated radio waves received on a vertical whip antenna and on a horizontal dipole at the same location are shown to lead to an estimate of the relative powers in the ordinary and extraordinary components when the propagation path is such that quasi‐longitudinal approximations may be used. The technique is valid for incoherent signals, where the incoming waves are considered to be made up of many individual rays. When spaced antennas are used, an estimate of the mean difference in bearing angle between the ordinary and extraordinary components is possible.
The Location of the Dip Equator at E‐layer altitude
Authors: Cain Joseph C.
Published: February 2008

The position of the magnetic dip equator at 100 km and the gradient of position with time and altitude are tabulated using the POGO (10/68), GSFC (12/66), and IGRF (10/68) geomagnetic field models. The positions agree within a few tenths of a degree with each other but differ up to 2° from older models and charts. The maximum gradients with time (0.2°/yr) and altitude (−0.3°/100 km) occur to the west of Africa.
Relation Between the Equatorial Anomaly and Trans‐Equatorial VHF Radio Propagation
Authors: Gibson‐Wilde B. C.
Published: February 2008

Comparison of the occurrence of VHF radio propagation over a 6620‐km transequatorial circuit between Korea and northern Australia with the development of the equatorial anomaly in the F layer shows that the transequatorial propagation mechanism is closely related to changes in the form of the equatorial anomaly. Signal occurrences are most common when the anomaly in foF2 exhibits sharp well‐defined crests located fairly symmetrically on either side of the dip equator at dip angles between 20° and 30°. These crests correspond to large horizontal gradients in the F layer ionization density, which are known to be capable of refracting radio waves at frequencies well above the normal for a horizontally homogeneous F layer. It is suggested that all transequatorial VHF signal occurrences observed over this circuit can be explained in terms of an F2F2 propagation mode, with the ionospheric refractions occurring at the crests of the equatorial anomaly.
Influence of Electromagnetic Drifts and Neutral Air Winds on Some Features of the F2 Region
Authors: Sterling D. L. , Hanson W. B. , Moffett R. J. , Baxter R. G.
Published: February 2008

Solutions of the continuity equation for electrons in the F2 region of the earth's ionosphere are obtained by including parameters for the various physical processes experienced by the plasma moving in the electromagnetic‐drift frame of reference. The electric field responsible for such a drift is derived from an electric potential, so the E X B/B2 drift velocity has an east‐west component in addition to the usual vertical one. The Jacchia model atmosphere is used, and the description of the photoionization process uses the Chapman function for absorption of radiation by the neutral atmosphere. The ratio of electron temperature to ion temperature can be varied with time. Account is taken of recombination with both molecular oxygen and molecular nitrogen. Other processes included are diffusion and neutral air winds. The velocity of the neutral air winds is assumed to be zero at the equator for equinox conditions and at 15° latitude reaches a maximum of 35 m sec−1 directed away from the equator at 1300 LT and a maximum of 55 m sec−1 directed toward the equator at 0100 LT. It is shown that the east‐west component of the electromagnetic drift has little effect on the solutions. For sunspot‐minimum atmospheric conditions, better agreement with observations in the equatorial region is obtained when the vertical component of the electromagnetic drift velocity peaks at noon rather than at 1500 LT. The sunspot‐maximum results show that there is a phase difference in the electromagnetic drift between sunspot‐maximum and sunspot‐minimum conditions. The over‐all magnitudes of equatorial NmF2 agree fairly well with observations for both sunspot‐minimum and sunspot‐maximum conditions. The equatorward wind at night is a necessary feature to maintain NmF2 at its observed levels at latitudes higher than about 10°. Within 10° of the equator winds do not play an essential role in the maintenance of the nighttime ionosphere. Preliminary results for a moderate latitude field line with a model variation of Te/Ti show several features that are in accordance with Evans' data for the period when the evening increase in NmF2 occurs.
Atmospheric Structure and Radar Backscattering in Clear Air
Authors: Ottersten Hans
Published: February 2008

This study summarizes advances in radar backscattering from refractive index irregularities in the clear air and its relationship to atmospheric structure and turbulence. This recent discipline of radar meteorology originated from radar angel studies and the efforts to explain these mysterious echoes. The significance of specular atmospheric radar reflections now appears limited, and consequently this study is devoted entirely to radar backscattering from turbulent fluctuations in the clear‐air refractive index. Two regimes of atmospheric turbulence are distinguished: (1) in the convective domain, backscattering from water vapor fluctuations outlines regular, three‐dimensional clear‐air structures associated with buoyant, moist air parcels; (2) in the stable regime, horizontally stratified clear‐air echoes originate from water vapor and/or temperature perturbations due to wind‐shear‐generated turbulent mixing in zones of enhanced static stability. Theoretical relationships are given between the radar reflectivity and the refractive index microstructure for the general case, for the isotropic case, and for the inertial subrange. The connection between the microstructure and the atmospheric mean fields is illustrated for a simplified situation in the stable regime; this relationship provides insight to the radar backscattering from clear‐air turbulence (CAT). It is concluded that strong CAT will be associated with zones of increased refractive index variability and enhanced radar returns. A brief résumé of experimental work illustrates the application of radar methods in clear‐air research and summarizes characteristic features of clear‐air structures.
Amplitude Modulation of Microwaves Propagated Through a Periodically Varying Plasma
Authors: John P. I. , Sarkar D. C.
Published: February 2008

A theory has been developed for the amplitude modulation of microwave signals propagated through a plasma created by an amplitude modulated RF field by taking into account the variations in the electron density and the electron molecule collision frequency of the plasma due to the modulation of the electron temperature. Experimental studies have verified the predicted dependence of the microwave modulation index on the modulation frequency, the exciting field strength, and the carrier modulation index. Slight deviations have been found between the experimental and theoretical values at both very low and very high modulation frequencies. The dependence of the RF conductivity on the exciting field has been suggested as the probable cause for the departures of the experimental values from the theoretical ones.
Chemical Analysis of the Lunar Surface in the Surveyor Program—Summary*
Authors: Patterson James H. , Turkevich Anthony L. , Franzgrote Ernest J.
Published: February 2008

Chemical analyses of lunar surface materials were obtained in situ by Surveyors 5, 6, and 7. Mare samples were analyzed by Surveyors 5 and 6, which landed in Mare Tranquillitatis and Sinus Medii, respectively [Turkevich et al., 1967b, 1968]. The Surveyor‐7 analyses were made in a terra region near the crater Tycho, which is thought to be covered with éjecta from that crater [Patterson et al., 1969]. The two adjacent samples in Mare Tranquillitatis were at least partially covered with material thrown out by the footpads of the spacecraft as it slid down the side of a small crater.
Properties of Lunar Surface Material From Direct Observation by Spacecraft—Summary*
Authors: Jaffe Leonard D.
Published: February 2008

The seven unmanned spacecraft that have landed on the moon, together with the lunar orbiting spacecraft, provided considerable information on the chemical composition, mechanical properties, and small‐scale structure of the lunar surface. This abstract, prepared prior to manned landing on the moon, summarizes some of the results; a more complete account is available elsewhere [Jaffe, 1969a, c]. Space‐craft results pertaining to the lunar interior and to the surroundings of the moon are omitted from this paper, as are results based on data from electromagnetic detectors alone, which other papers at this symposium discuss and interpret.

SURFACE OVER

The disturbances of the lunar surface produced by Surveyor landing gear, surface sampler devices, and rocket engines showed that both maria and high‐lands are covered with a layer of fine particles.
Bistatic Radar Measurements of Topographic Variations in Lunar Surface Slopes With Explorer 35
Authors: Tyler G. L. , Simpson R. A.
Published: February 2008

Bistatic radar observations of the lunar surface conducted with Explorer 35 at 2.2 meters have been used to measure the average large‐scale (tens to hundreds of meters) lunar slopes. Data obtained for the equatorial band between 70° east and west longitude show significant (3:1) regional variations. Unidirectional rms slopes of 2°, 3°, and 6° were obtained for Mare Fecunditatis and Oceanus Procellarum, the central highlands and terra surrounding the crater Alfraganus, and the Censorinus highlands, respectively. A comparison of the slope variations and visible surface structure suggests that the meter wavelength radar slopes are those of the visible surface measured on the set of scales given above.
The Atmosphere of The Planet Venus
Authors: Kuzmin A. D.
Published: February 2008

The fact that Venus has an atmosphere was discovered by M. V. Lomonosov in 1761. However, attempts to investigate its properties during the following two centuries were not sufficiently successful for construction of an adequate model. This situation has only recently been changed. The successful flights of the Soviet automatic interplanetary stations Venera 4, 5, and 6, the flight of the American space probe Mariner 5, and recent radio astronomical and radar investigations of Venus provide a great deal of important new information about this planet.
Approximate Dispersion Relations for Hydromagnetic Waves
Authors: Abbas M. , Poeverlein H.
Published: February 2008

An approximate dispersion relation for hydromagnetic waves is derived from the macroscopic equations of the plasma. Collision terms corresponding to collisions between electrons, ions, and neutrals are included in the macroscopic equations, and the three collision frequencies involved are retained in the approximation. Some simplification results from omission of certain terms in the macroscopic equations on the basis of an estimate of their relative magnitudes. An arbitrary direction of propagation is assumed in the derivation of the dispersion relation. The propagation constants for propagation parallel and normal to the magnetic field are represented by fairly simple expressions. There is good agreement between the numerical values of real and imaginary parts of the refractive index for these two directions of propagation and the various ionospheric heights computed from the approximative fomulas and from a more rigorous dispersion relation.
Propagation of Coherent Waves in a Warm Turbulent Magnetoplasma
Authors: Youakim Maurice , Liu C. H.
Published: February 2008

The problem of wave propagation in a warm magnetoplasma in the presence of random density fluctuations is studied. Using a perturbation technique similar to the renormalization procedure in field theory, a dispersion relation for plane coherent wave propagation is derived. For propagation along the magnetic field, the coherent longitudinal and transverse modes are found to be independent. But there is coupling among the modes for the random waves scattered by the irregularities. The coupling is strong for short correlation length of the turbulence. Attenuation constants for the coherent waves are obtained, and the effect of random scattering on the Faraday rotation phenomenon is discussed.
VLF Phase Disturbances, HF Absorption, and Solar Protons in the PCA Events of 1967
Authors: Potemra T. A. , Zmuda A. J. , Shaw B. W. , Haave C. R.
Published: February 2008

The ionization model developed in conjunction with VLF and HF propagation parameters from our analysis of the PCA's of August 28 and September 2, 1966, is applied in unmodified form to the 1967 events of January 28, February 2, May 23, and May 28. As for the 1966 events, proton intensities, energies, and cutoffs measured on the polar orbiting satellite 1963–38C are successfully connected to VLF phase disturbances and multifrequency riometer absorption increases. Considering all these events, the model provides the following rms fits between observed and computed values: 4.0 μ sec for 141 daytime and nighttime VLF phase values for seven paths at five frequencies during six proton events; 0.45 db for 181 riometer absorption values at 30 MHz during the six events; and 0.20 db for 36 riometer absorption values at 50 MHz during the two events that produced this absorption. The variability of the proton intensities and spectrums provides a rigorous test of the validity and internal consistency of the ionospheric model deduced through the complementary use of riometer absorption and VLF propagation changes occurring during PCA's. In the present analysis, as in the preceding one, the temporal variations in the proton cutoffs play an important role in the VLF analysis as a result of the lengths of the VLF transmission paths in relation to the polar‐cap area disturbed by the solar protons.
Fitting Distributions of Telecommunication Variables with Chi‐Square Distributions
Authors: Zacharisen D. H. , Crow Edwin L.
Published: February 2008

A method is presented for fitting chi‐square (or gamma) distributions to observed distributions, either positively or negatively skewed, of telecommunication variables such as critical frequencies and maximum usable frequencies. These observed distributions have been recorded for years only in terms of the 10, 50, and 90% points, but in applications it is necessary to estimate the entire distribution. It is assumed in the present study that the observed variable can be approximated by a linear function of a chi‐square variable with unknown degrees of freedom. The three unknown constants are determined from the three per cent points and a convenient table. Several examples of complete observed distributions are compared graphically with normal, chi‐square, Gram‐Charlier, and Edgeworth fitted distributions.
Utilization of Image Theory Techniques in Determining the Mutual Coupling Between Elevated Long Horizontal Line Sources
Authors: Bannister Peter R.
Published: February 2008

Suitable engineering expressions for the external impedance per unit length of a long elevated horizontal line source and the mutual impedance between elements of an array of parallel line sources are derived by employing finitely conducting earth image theory techniques. It is shown that the image theory and previously derived analytical results, which are rather complicated, are in excellent agreement.
On the Reflection of Electromagnetic Pulses From the Ionosphere
Authors: Wait James R.
Published: February 2008

The reflection of a transient plane wave, from an ionospheric model with a linear electron‐density profile, is considered. After a discussion of causality and physical realizability, the impulse response is calculated for the special case of no collisions. For this special case, it is shown that the wave form of the reflected wave has a smoothly rising initial portion that is followed by a slowly damped oscillatory portion which comprises the main signal.
An Exact Solution for Wave‐Form Distortion of Arbitrary Signals in Ideal Wave Guides
Authors: Vogler L. E.
Published: February 2008

An exact expression is derived for the transient response of an arbitrarily modulated signal propagating through an ideal wave guide. The solution is in the form of an infinite series of Bessel functions, the coefficients of which depend in a simple manner on the time derivatives of the input signal. It is shown that for the specific example of a step‐modulated carrier signal (the only earlier case for which an exact solution is known) the general expression reduces to that derived in earlier works. A particular case of a ‘double exponential’ pulse is evaluated and briefly discussed.
Geomagnetic Crochets and Associated Ionospheric Current Systems
Authors: Richmond A. D. , Venkateswaran S. V.
Published: February 2008

It is shown that time‐structure analysis of geomagnetic crochets can be useful in the ground‐based detection of XUV radiation from solar flares. The crochets often manifest a composite structure that consists of a ‘fast’ component presumably produced by EUV radiation (100–1000Å) and a ‘slow’ component produced by soft X rays (1–100Å). This composite structure is taken into account in the proposed method of analysis for crochet current systems. Some preliminary results are given from the simulation studies of Sq and crochet current systems. These studies are intended to probe into the nature of the atmospheric winds that drive the ionospheric dynamo and that generate both the Sq and crochet currents.
Simultaneous Electric Field Measurements Made in the Auroral Ionosphere by Using three Independent Techniques
Authors: Fahleson U. , Fälthammar C.‐G. , Pedersen A. , Knott K. , Brommundt G. , Schumann G. , Haerendel G. , Rieger E.
Published: February 2008

Two Skylark rockets were launched from the range of the European Space Research Organization (ESRO) near Kiruna on February 14, 1970, at 17.15 CET and on April 2 at 02.46 for investigation of electric fields. Each rocket carried an electric double‐probe experiment, a Langmuir probe, an experiment to detect the influence of the plasma drift on the wake of the rocket, a Geiger‐Müller counter, and finally a barium cloud experiment. Three of these (double‐probe, wake experiment, and barium cloud) provided independent information on the ambient electric field. The present paper deals with the initial results of these three electric field experiments. In the first flight, which took place under slightly disturbed conditions (+40 γ), all methods indicated a roughly northerly field of about 22 mv/m. In the second flight (under quiet conditions), a southerly field between 3 and 6 mv/m was observed by the different experiments. The direction of the plasma drift derived from the measured orientation of the wake of the rocket agrees within 10° with that derived from the double‐probe results and the barium cloud observations. Estimates of electron density and temperature were obtained from the operation of the double‐probe system in different modes. These electron density values were in good agreement with those derived from the ionosonde data and from the Langmuir‐probe data.
A Method for Predicting the F1 Layer Critical Frequency
Authors: DuCharme E. D. , Petrie L. E. , Eyfrig R.
Published: February 2008

The development of a method for predicting monthly median values of the F1 layer penetration frequency (ƒ0F1) at any location and time is described. The expressions derived for predicting the monthly median ƒ0F1 are functions of the zenith angle of the sun, the ionospheric index IF2, and the geomagnetic latitude.
The Waveguide Mode Theory of Whispering‐Gallery Propagation in the F Region of the Ionosphere
Authors: Chang Hsi‐Tien
Published: February 2008

At frequencies above the maximum usable frequency for ground‐to‐ground ionospheric communication, propagation is possible between terminals located in the ionosphere. This phenomenon has been studied in the past by use of the ray theory and is treated in this paper by use of the mode theory. At a given HF frequency there are a number of modes that are well trapped and a number that leak profusely to the topside of the ionosphere. As the frequency is increased, the number of well trapped modes decreases. Ultimately, only one mode is reasonably well trapped and leakage even of this mode finally sets in. The frequency at which this happens determines the maximum usable frequency of the ionosphere for whispering‐gallery propagation. By use of a Chapman model of ionospheric density, this frequency is calculated to be about six times the vertical penetration frequency for the F region. Mode theory shows that the whispering gallery associated with the F region can be used to explain both round‐the‐world echoes and long distance satellite observations.
Computing the Effects of Tropospheric Refraction at Frequencies Below 10 Megahertz
Authors: Gerks Irvin H.
Published: February 2008

The extended WKB method is employed to obtain a solution of the differential equation governing the dependence of field strength on height. The excess permittivity is represented as an exponential function of height. The method yields asymptotic solutions which become less accurate as the frequency decreases. It is found that the first‐order solution can be used down to 300 kHz. For lower frequencies, the more complicated second‐order solution is necessary, which can then be used down to 10 kHz. The results are compared with those based on an airless earth and an equivalent earth of 4/3 times the true radius.
Subsurface Electromagnetic Fields of a Line Source on a Conducting Half‐Space
Authors: Wait James R. , Spies Kenneth P.
Published: February 2008

An infinite line source or current‐carrying cable on the surface of a flat homogeneous earth is considered. The integral formula, for the case of negligible displacement currents, is reduced either to tabulated functions or to a form suitable for numerical integration. A table of numerical values of these basic integrals is presented. We also show that, in principle, the relative location of the line source can be deduced from measurements of the subsurface field The results have possible application to mine rescue operations.
Cross‐Polarization Distortion in Microwave Radio Transmission Due to Rain
Authors: Thomas David T.
Published: February 2008

Cross‐polarization distortion (XPD) due to nonspherical raindrops may be a serious problem for 18.5‐ and 30‐GHz microwave radio communications, but no measured data have yet been published. This paper presents a simple approach for estimating XPD due to rain from the effects of differential attenuation on canted spheroidal raindrops. Values of average raindrop canting angle, θA, are predicted from R. A. Semplak's measurements of differential attenuation taken over a one‐year period, published in 1970. Independent observations of canting angle from two rainstorms, published in 1971 by M. J. Saunders, agree with the predicted values and also indicate values for the senses (positive versus negative) of raindrop canting angles. Estimated values of XPD due to rain are graphically presented for various frequencies, rain rates, and typical path lengths. The paper concludes that the operational limits placed on communications by XPD due to rain are comparable to the limits due to rain attenuation or fading.
Energy Balance and Radiation Forces for Arbitrary Moving Objects
Authors: Censor Dan
Published: February 2008

In scattering processes involving moving objects an exchange of electromagnetic and mechanical power takes place. The problem is considered for arbitrary objects in one, two, and three dimensions. It is shown that the energy balance depends on the scattering amplitude, evaluated at the frequency of excitation, on this frequency itself, and on the velocity in question. Three cases are considered: objects moving in free space, objects moving together with the surrounding medium, and objects moving through a medium, at rest with respect to the observer. An interesting result is the fact that objects moving in the direction of the incident wave cause a loss of electromagnetic energy smaller than the gain occurring when the objects move in the opposite direction. This is a first‐order effect in the velocity.
Ray‐Optical Calculation of Modes Excited by Sources and Scatterers in a Weakly Inhomogeneous Duct
Authors: Batorsky D. V. , Felsen L. B.
Published: February 2008

Recently a ray‐optical procedure has been developed to determine the modes scattered by an obstacle in a homogeneously filled, perfectly conducting waveguide from knowledge of the free‐space diffraction pattern ƒ(θ) of the obstacle. The present paper extends the procedure to two‐dimensional waveguides or ducts filled with a weakly inhomogeneous medium and bounded by impedance walls. As in previous studies for simpler cases, the analysis deals initially with radiation from a nonisotropic line source with free‐space radiation pattern ƒ(θ); it involves the ray‐optical construction of the solution, ray‐modal conversion, comparison with the asymptotic form of the rigorous solution for an isotropic source (ƒ(θ) = 1), and a semiheuristic modification to make the result applicable even to lower‐order modes. While the method applies most directly to real rays and propagating modes, the result is extended subsequently to complex modes descriptive of evanescent, lossy, or leaky wave processes. Moreover, special attention is given to commonly ignored ray‐optical phenomena associated with impedance boundaries. Application of the results is illustrated briefly by considering scattering due to a perfectly conducting strip obstacle located inside an inhomogeneous, lossless waveguide. Detailed calculations for this and other examples are to be presented in a future publication.
Reflection and Transmission of Electromagnetic Waves by a Semi‐Infinite Moving Anisotropic Plasma
Authors: Ohkubo M.
Published: February 2008

The reflection and transmission of electromagnetic waves by a semi‐infinite moving plasma with a static magnetic field parallel to the moving direction are investigated for the incident H‐plane wave. Numerical results for the reflection coefficient as functions of incident angle, velocity, frequency, and cyclotron frequency are given. It is shown that the moving anisotropic plasma exhibits several phenomena different from manifestations of the stationary case.
Scattering of a Transient Plane Wave by a Periodic Grating
Authors: Hill D. A. , Wait J. R.
Published: February 2008

The reflection of a transient plane wave from a grating of thin conducting cylinders is considered. The inverse Fourier transform of the known steady‐state solution is performed both analytically and numerically. Both impulse and step function excitations are treated explicitly. Expressions for the scattered fields are obtained that are useful for either large or small time. It was found that a simple decaying exponential function of time is a fairly good approximation for the waveform of the induced current in the grating.
Input Resistance of Horizontal Loops Above a Conducting Ground Plane
Authors: Galejs Janis
Published: February 2008

Analytical expressions are derived for the resistance of a finite loop with a uniform current distribution located above a lossy half‐space. For loops of radii p0 large relative to the skin depth δ of the lossy medium, the resistance remains the same as for a long, straight, horizontal wire. For p0 much less than δ, the loop resistance is proportional to the square of (p0/δ). Numerical solutions show that the above formulas are not valid under conditions of nonuniform loop current at loop elevations h that are a small fraction of δ. The resistance expressions of a magnetic dipole are obtained only in the limit of h ≫ δ.