Showing papers in "Reviews of Geophysics in 1971"

Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes

Abstract: A region-by-region analysis of 204 reliable focal-mechanism solutions for deep and intermediate-depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake-generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World-Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre-1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double-couple or shear dislocation model of the source mechanism is adequate for all the data.

Propagation of cosmic rays in the solar wind

Abstract: This paper presents a coherent exposition of the modern statistical theory of the transport of fast charged particles (cosmic rays) in the solar wind. Observations are discussed only as they illustrate the phenomena under discussion. A brief introductory section surveys the historical development of the theory. The dominant effect on the motion of cosmic rays in the solar wind is the interplanetary magnetic field, which is irregular and which is therefore best treated statistically, using random functions. The magnetic irregularities scatter the cosmic rays in pitch angle, so that to a good approximation the cosmic rays diffuse through the irregular magnetic field. Using a statistical analysis of the equations of motion, one may relate the diffusion tensor to the power spectrum of the magnetic field, which is in principle measurable. The resulting general transport theory relates the motion of cosmic rays, statistically, to the solar-wind velocity and magnetic field. Application of the theory both to the modulation of galactic cosmic rays by the solar wind and to the propagation of solar cosmic rays is discussed in detail. It is concluded that the present theory explains the principal phenomena quite well. Future theoretical work will probably be devoted to obtaining better solutions of the equations, to obtaining better values of the parameters, and to studying higher-order or more subtle effects.

Critical review of ultraviolet photoabsorption cross sections for molecules of astrophysical and aeronomic interest.

Abstract: This paper is devoted to a critical review of photoabsorption cross sections for molecules of aeronomic and astrophysical interest at wavelengths less than 3000 A. A discussion of the relative merits of various experimental techniques is given along with possible systematic and random errors that may be associated with them. The problems in data analysis associated with finite spectral bandwidths are reviewed, with special emphasis on the interpretation of published absorption cross sections. This review does not contain a complete set of cross-section-versus-wavelength values for each molecule; the prepared figures are used to compare the results of several determinations or to point out where difficulties of interpretation might arise. However, references to all papers believed to contain the more reliable data are given.

Seismological evidence for anomalous structure of island arcs with special reference to the Japanese region

Abstract: Evidence indicating lateral heterogeneity in the deep structure of island arcs has been collected from seismological literature. Abundant evidence from seismological studies in Japan during the last fifty years generally supports the large-scale anomalous structure beneath this typical island-arc region, i.e., the existence of a high-Q, high-velocity zone about 100 km thick that dips from the vicinity of the trench beneath the arc to a depth of several hundred kilometers, and two low-Q, low-velocity zones in the upper mantle on the inner (continental) and outer (oceanic) sides of this dipping zone. Most of the seismic activity is confined to this dipping zone and to the crust near and on the inner side of the volcanic belt. In other island-arc regions, such as Kurile-Kamchatka, Indonesia, Tonga-Kermadec, and New Zealand, a number of seismological studies have indicated the existence of similar anomalous structure. At present, more detailed, more accurate determination of the configuration and physical parameters of these anomalous zones by seismological techniques is in progress, providing important information for understanding the global tectonic process and the origins of regional geophysical and geological features found in island-arc regions.

Spectral studies of tropospheric wave disturbances in the tropical western Pacific

Abstract: Tropospheric wave disturbances in the tropical western Pacific have been subjected to intensive analysis during three separate seasons. These all show evidence of spectral peaks in the 4- to 5-day range and at periods longer than 10 days. In this review I attempt to synthesize the results for these three seasons, together with the analysis of a fourth season, into a tentative description of the various types of wave disturbances in this region. The conclusions are outlined below. It appears that there are two types of tropical disturbances that contribute to the 4- to 5-day peaks: synoptic-scale, westward-propagating waves with phase speeds on the order of 6-7 degrees per day, and planetary-scale equatorial waves that extend into the lower stratosphere. There are also one or more wave types associated with the low-frequency spectral peaks. All the tropical waves exhibit a similar vertical structure, with a phase reversal in the wind field between upper and lower troposphere. There are warm temperature anomalies at 300 mb, above the low-level troughs, in qualitative agreement with the thermal-wind equation. These warm anomalies coincide with the regions of maximum rising motion. As distinguished from tropical waves, the subtropical disturbances do not exhibit a phase reversal in the wind field between upper and lower tropospheric levels. The troughs of these waves appear to be colder than their surroundings at 300 mb. The available potential energy the tropical waves derive from condensation heating is immediately converted into kinetic energy by the synoptic-scale divergent wind field. This appears to be the dominant energy source for these waves. The leakage of wave energy upward through the tropopause appears to be at least an order of magnitude smaller than the rate of energy generation. The spectral results indicate that most of the synoptic-scale mass convergence into regions of disturbed weather takes place above the subcloud layer. Although the vertical distribution of latent heat release displays the same gross features in different regions, a simple linear proportionality between boundary-layer convergence and condensation heating (as is usually assumed in the CISK hypothesis) does not appear to exist.

The tectonics of the eastern Mediterranean: A geophysical review

Abstract: A summary is given of present geophysical data available for the eastern Mediterranean, and the structural and tectonic history of the area is interpreted. The western Mediterranean is described in sufficient detail only for comparison. The region is delineated into plates on the basis of seismic activity at their margins. The African plate is moving northward relative to Europe with crustal destruction and underthrusting occurring along the Crete and Calabrian island arc systems. The intervening Aegean, Apulian, and Turkish plates are small and rapidly moving; they show intense seismicity at the boundaries. All observations (sediment deformation, gravity, magnetics and heat flow) suggest the existence of a zone of compression in the eastern Mediterranean. Anticlockwise rotation relative to Eurasia of parts of southern Europe occurred during an extensional phase in the west, permitting the development of new oceanic crust and accompanied by compression in the east. What little evidence there is suggests that the crust in the eastern Mediterranean is oceanic, but there are no magnetic stripes to indicate the orientation of the mid-oceanic ridge from which it developed.

Structure of the magnetopause

Abstract: This paper reviews present understanding of the internal structure of the thin boundary layer, termed the magnetopause, that separates the distorted geomagnetic field in the magnetosphere from the flow of solar plasma in the magnetosheath. The fundamental theoretical concepts of the subject are introduced by considering the structure of the boundary layer that exists when a cold, unmagnetized stream of ions and electrons impinges normally on a vacuum magnetic field. This idealized model indicates how the earth's magnetic field is confined by the impact pressure of the solar wind, the geomagnetic field being terminated by induced shielding currents flowing in the magnetopause. The various idealizations and approximations in this model, such as the assumption of a cold solar plasma and the neglect of the interplanetary magnetic field, are examined critically. The presence of thermal plasma in the magnetosphere modifies the structure of the boundary layer so that the ions penetrate substantially deeper (∼100 km) than the electrons into the geomagnetic field. Complications then arise if the solar plasma has a component of velocity parallel to the confined field, as occurs in the downstream magnetopause that bounds the geomagnetic tail. This additional component of velocity generates electric currents parallel to the magnetic field that may destroy the small-scale equilibrium of the magnetopause and result in a tangential drag on the geomagnetic cavity. The available experimental information on the magnetopause structure is summarized and related to the theoretical investigations. Although the observations confirm some of the theoretical predictions, much of the detailed theory remains speculative at the present time.

Solar wind interaction with planetary atmospheres

Abstract: Several quite dissimilar models have been proposed for the direct interaction of the solar wind with planetary ionospheres, based, respectively, on the assumption of (1) restricted flow into the ionosphere, (2) an interface between the solar wind and the ionospheric plasma, or (3) a magnetic field barrier (‘induced magnetopause’). All three require the existence of a standing bow shock in the solar wind. We review these models, discuss their individual limitations, and consider what observational data would distinguish among them.

New polar magnetic disturbances: Sq p , SP, DPC, and DP 2

Abstract: The analyses that have led to the finding of a new polar magnetic disturbance are reviewed, and a unified picture is presented. Although the new disturbance has been reported under various names, an essential common feature is that the disturbance does not originate from the auroral electrojet. The high correlation observed between the disturbance and the orientation of the interplanetary magnetic field suggests that the disturbance reflects a basic interaction process between the solar wind and the magnetosphere.

D‐region ion chemistry

Abstract: The status of D-region positive- and negative-ion chemistry is reviewed. In the case of the positive-ion chemistry, the presently accepted ion production rates and ion-neutral reaction scheme are incompatible with the observed qualitative ion composition measurements. The observations indicate that water-ion clusters, H3O+(H2O)n, particularly H5O2+, are dominant below 80 km, whereas calculations predict that NO+ and its hydrates would be dominant. It seems rather likely that an essential element of the physics involved has so far escaped attention. In the case of the negative ions, the few observations only recently obtained are insufficent to critically test the detailed reaction schemes available from laboratory studies. Uncertainties in the concentrations of minor neutral constituents severely limit the ability to predict theoretical negative-ion profiles at present. The first rocket negative-ion observations do support the laboratory-derived prediction of a dominant role for NO3− and its hydrates in the D-region.

Plasma instabilities in the magnetosphere

Abstract: An introductory review of theories of plasma instabilities in the magnetosphere is presented. Part A is a review of theories of plasma instabilities that are relevant to magnetospheric plasmas. Instabilities arising from velocity-distribution anisotropies, such as a pitch-angle anisotropy or the presence of beams, as well as instabilities from nonuniform distributions of plasmas and magnetic fields, are discussed. Particular emphasis is placed on the effect of a mixture of a cold plasma in a high-β plasma. Part B is a summary of works related to actual plasma instabilities in the magnetosphere. In view of the observed plasma parameters, it is shown that the magnetosphere is rather stable against most macroscopic instabilities, and its dynamics are predominantly governed by microscopic instabilities.

A stochastic model for the occurrence of main-sequence earthquakes

Abstract: If a long earthquake sequence is considered to be a stationary stochastic process, the stored elastic energy of deformation can be shown to be an independent variable in the usual ‘backward’ equation. Three unknown probability functions are introduced: the probability that the stored energy of deformation is at a certain level; the probability that, if this energy is at a given level, an earthquake will occur; and the transition probability that, if the earthquake occurs, the final energy state will be at a certain level. It is assumed that the frequency-energy distribution is known. The equations can be solved, if the transition probability is assumed to be known; and they have been solved for the model in which the transition probability is a function of the energy released in the shock but is not otherwise dependent on the final energy state. In this case, the results can be used to describe the earthquake history for some time after a great shock, and possibly for times just before a great shock. The results have some features of inconsistency with observations.

Large‐scale properties of the interplanetary magnetic field

Abstract: Our knowledge of the large-scale properties of the interplanetary magnetic field is reviewed. The early theoretical work of Parker is presented, along with the observational evidence supporting his Archimedes spiral model. The variations present in the interplanetary magnetic field from the spiral angle are related to structures in the solar wind. The causes of these structures are found to be either nonuniform radial solar wind flow or the time evolution of the photospheric field. The coronal magnetic models are related to the connection between the solar magnetic field and the interplanetary magnetic field. The direct extension of solar field-magnetic nozzle controversy is discussed, along with the coronal magnetic models. The effect of active regions on the interplanetary magnetic field is discussed with particular reference to the evolution of interplanetary sectors. The variation of the interplanetary magnetic field magnitude is shown throughout the solar cycle. The percentage of time the field magnitude is greater than 10 γ is shown to closely parallel sunspot number. The suggested influence of the sun's polar field on the interplanetary field and alternative views of the magnetic field structure out of the ecliptic plane are presented. In addition, a variety of significantly different interplanetary field structures are discussed.

The patterns and sources of high‐latitude particle precipitation

Abstract: We survey, in this paper, the properties of the zones of particle precipitation at high latitudes. We consider the average properties of particle influx from the outer magnetosphere as determined by satellite and rocket observations, as well as ground-based techniques. The point of view of this paper is to try to associate source regions, and, if possible, precipitation mechanisms with each of the families of precipitating particles that collectively form the auroral zone. The precipitation patterns for both electrons and protons exhibit a diurnal variation. The several zones of particle precipitation can be placed in relationship to each other. The zone of electron precipitation is best described as consisting of a region at low latitudes (60° ⪝ Λ ⪝ 70° near local midnight) where electrons with characteristic energies of tens of kev precipitate; this region merges and overlaps at the higher latitudes with another region (70° ⪝ Λ ⪝ 80° near local midnight) characterized by precipitating electrons of ≈0.5 kev. The limited data available indicate that the zone of proton precipitation spatially overlaps the zone of electron precipitation. For example, the peak of precipitation of > 4-kev protons near local midnight is found between Λ = 65° and Λ = 70°. Near local noon a separate region of very soft proton precipitation is found in the vicinity of Λ ≈ 80°, in addition to a region of proton influx at lower latitudes. We consider the plasma sheet, the extraterrestrial ring current, the polar cusp, and the outer radiation belt as source regions of precipitating particles. Brief surveys of the precipitation of helium ions and of possible means of artificially inducing particle precipitation are also included.

Dynamical aspects of lunar origin

Abstract: According to tidal friction calculations in which estimates of the current dissipation factor are used, the moon was close to the earth less than two billion years ago. The most plausible solution to this ‘time scale’ problem appears to be a much lesser extent of shallow seas in the past; a coupling of the moon's orbit with Venus is also a possibility. The same calculations indicate that the moon had an inclination of at least 10° to the equator, and that the earth's rotation period was five hours. Fission hypotheses, of which the most carefully developed is by O'Keefe, have not yet dealt successfully with these constraints. Fission theories are also unsatisfyingly episodic: matter must first lose angular momentum to fall into the earth, and then regain it to separate. All capture hypotheses are inherently improbable, particularly if the moon came from a different enough part of the solar system to account for the moon's density difference. The least implausible capture hypothesis, by Urey and MacDonald, entails collision with a pre-existing earth satellite at 30–40 earth radii, but the reasoning behind it is not compelling. Most compatible with the idea that formation of the moon is closely connected to the formation of the earth are hypotheses that the moon formed out of a geocentric swarm of matter to which matter from heliocentric orbit was continually added. The theory of Ruskol seems to incorporate most of the dynamical essentials. However, the model needs to be developed further to include gas drag and other effects of the severe heating associated with chemical fractionation between the earth and moon, as well as to account for the inclination of the lunar orbit. A possibly severe constraint on the distance from the earth at which the moon's formation was completed is a systematic ellipticity of lunar craters pointed out by Opik; it should be re-examined after oriented metric photographs from Apollo 15–17 flights are obtained.

Radial diffusion of trapped particles and some of its consequences

Abstract: The nonadiabatic motion of geomagnetically trapped particles across L shells has been inferred from the measured distributions of trapped particles and has also been observed directly in a number of cases. This type of motion has important influences on both the steady-state structure and the dynamics of the radiation belts. The characteristics of the radial motion have been evaluated and are generally consistent (within an order of magnitude) with the radial diffusion expected from fluctuations in the magnetospheric electric and/or magnetic fields. However, the complexity and variability of the observed phenomena suggest that several basic processes are acting simultaneously, and further efforts are needed to identify specific mechanisms.

Geomagnetic models from satellite surveys.

Abstract: The use of satellite scalar magnetic field data to produce geomagnetic field models is reviewed. There have been nine separate spacecraft that have acquired observations of the geomagnetic field from low (<1500 km) satellite altitudes since 1958. The magnetic field models produced from such data have not been sufficiently compared with surface vector data to firmly, establish their validity. One comparison has indicated that satellite-derived models are as valid over-all as those produced from surface data. Variations in the field components are shown to be larger than those in scalar field by factors of 4–10. Recent satellite models indicate an increase in dipole decay to −27 γ/year, a value that may be plausible, judging from extrapolation of earlier surface-derived models. Also noted is a recent slowing of eccentric dipole westward drift.

Oceanic volcanism: A review

Abstract: Recognition of the magnitude and tectonic significance of oceanic volcanism has recently stimulated intensive investigation of the volcanic rocks and structures of the sea floor. An adequate interpretation of these features depends on a thorough understanding of submarine volcanic mechanisms. The pressure of deep water restricts the amount of vesiculation of eruptive magmas so that explosive activity is suppressed. Pillow lavas are probably shorter but thicker than subaerial flows of the same composition. Fragmental hyaloclastites are also important, but seem to be most common at relatively shallow depths. Most lavas and hyaloclastites are basaltic, but more siliceous pyroclastic deposits may be common in shallow seas of the continental shelves. Circumstantial evidence indicates that most abyssal basalts of the ocean floor were erupted at spreading centers and have been transported laterally. The rate of eruption of lavas is probably related to the mechanism of dike intrusion into a spreading center. Slow spreading rates result in less vertical transfer of heat, so that only relatively thick dikes reach the surface. Thinner dikes are chilled before erupting and cause a differential dilation of the crust that is reflected in shallow normal faulting. With increasing spreading rates, wall rocks are maintained at higher temperatures, and almost all dikes reach the surface to produce lava flows and smoother topography. Seamounts may begin to grow at ridge crests and continue as the volcano moves outward, but many must begin their activity far from the spreading center. Most of the submarine structure of seamounts must consist of pillow lavas with increasing amounts of hyaloclastites in the upper portion. Injection of dikes and sills adds substantially to the volume of the structure. Most abyssal basalts erupted at ridge crests are tholeiites, commonly with relatively high alumina contents. Silica-deficient alkaline basalts become more common outward, especially at the crests of seamounts. The ultramafic rocks dredged from the mid-Atlantic ridge fall into two classes. Some are clearly cumulate rocks related to associated gabbro and differentiated basalt. Others are strongly depleted in low-melting components and appear to have been emplaced along fault zones; they may be derived from a residual layer from which basalt was extracted during an earlier episode of partial melting. It is difficult to find conclusive evidence that oceanic volcanism was as intense in pre-Mesozoic time as it is postulated to be today.

Crustal resistivity anomalies from geomagnetic deep-sounding studies

Abstract: A number of magnetic variation anomalies observed in geomagnetic deep-sounding studies can be attributed to inhomogeneities of electrical resistivity in the upper crust. The electromagnetic induction in isolated conductors is usually too small to explain anomalies associated with crustal inhomogeneities, and anomalous variation fields mainly arise from concentration and channeling of currents induced elsewhere. These systems of currents are induced over a large region of the earth and have dimensions comparable to those of the source field. Most of the crustal anomalies are associated with deep conducting sedimentary basins or with regions of rapid change of sedimentary conductivity. Examples of anomalies of this origin are the North German basin, the Anadarko basin in Oklahoma and a region in north-central Texas where more resistive paleozoic sediments are adjacent to conductive sediments in the Gulf coast plains. Other anomalies are related to currents concentrated in channels of conducting sea water located between two resistive continental blocks. Anomalous fields arising in this way have been observed on both sides of the Bonifacio Straits between Corsica and Sardinia. It is also thought that the Alert anomaly in the Canadian Arctic is closely related to the channeling of current through the Robeson Channel. An additional inland anomaly seems to be due to leakage of currents induced in the oceans into conductive structures in the upper crust. A magnetic variation anomaly with an amplitude larger than any other reported in the literature has been found in the northern Great Plains province of the United States. It extends along the eastern edge of the Black Hills north to the Williston basin. Higher conductivities than those characteristic of sediments are needed to explain this anomaly, which has been tentatively attributed to a zone of conducting graphite schists in the basement. Examples of magnetotelluric observations near current concentrations compared with two-dimensional model calculations show that plane-layered solutions are rarely adequate as representations of magnetotelluric resistivity curves in the vicinity of these features. In spite of these difficulties, the geomagnetic deep-sounding technique can yield information about the lateral variations of resistivities of the upper mantle, providing that the conductivities at the surface are not too high or are relatively uniform, or both.

Clear‐air turbulence, aviation, and atmospheric science

Abstract: Clear-air turbulence (CAT) presents a scientific problem of considerable importance to aviation and to attempts to understand atmospheric processes that must be incorporated in a long-range numerical prediction model. Present knowledge about CAT is reviewed briefly, and the fact that the requirements of both aviation and atmospheric science for new information necessitate a measurement program whose objective is increased understanding of the physical mechanisms of CAT is emphasized. Specific investigations that should be a part of such a measurement program are presented, and the importance of the energy budget of CAT is stressed both for its scientific value and for its potential as a method of verifying accuracy of measured data. A new approach to determining the required accuracy of turbulence instrumentation, utilizing root-mean-square error techniques, is presented. Some aspects of the management of a large measurement program are also considered.

Ionospheric effects of Birkeland currents.

Abstract: Since Birkeland's observations of high-latitude magnetic perturbations led him to postulate the existence of geomagnetically aligned electric currents, theoretical studies have led to a general understanding of the mechanisms that are capable of driving such currents, and of the processes associated with formation and control of current configurations Recent advances in experimental techniques have yielded information on the spatial and temporal behavior of such field-aligned Birkeland currents and have established their association to an individual auroral arc and energetic precipitating particles This paper gives a brief summary of ionospheric effects related to production, maintenance, and control of Birkeland current systems Available experimental data are discussed in relation to some of these effects

The new Moon: A view

Abstract: A review of Apollo 11, 12, and 14 and Luna 16 data shows that a genuine step-function increase has occurred in our understanding of the moon. This results largely from the ability to conduct detailed analyses, most of which cannot be done by remote means, on returned sample. Geophysical data from Apollo-emplaced science stations is a valuable complement to the new sample knowledge. It is evident that the great store of data previously gathered from earth-based telescopic observations and unmanned spacecraft and the interpretations of it are enabling us to construct much better lunar models than otherwise possible. Without this pre-existing framework, the lunar samples acquired would be of diminished value. The lunar ‘sum total experience’ will reach beyond the moon and should be particularly valuable in interpreting remotely sensed data from Mars and other planets. It appears that the moon originated about 4.6 b.y. ago, as did the earth and meteorites, thus at the beginning of the solar system. It seems to have suffered most of its internal thermal spasms in the first 1 or 2 b.y. of life and has been slowly dying since, in contrast to the earth, which today may be as active as ever. The lunar surface was exposed to a large but rapidly decreasing flux of infalling objects in its first 1½ b.y., some of which might have been part of the accretionary population that evidently formed the moon. That activity has been at a greatly diminished level for the past 3 b.y. and may now be comparable to the action of solar and galactic atomic particles in effecting surface modifications. The igneous processes that resulted in flooding of the mare basins with basaltic lavas are now reasonably well understood, as are the effects of meteoroids in generating the shallow (meter scale) surficial soil or regolith. Processes responsible for modification of the near-surface layer (micron scale), which gives rise to the remotely sensed spectra, are not wholly clear. Evidence exists that the lunar near-surface highlands are compositionally heterogeneous, but neither the mare basalts nor Imbrium ejecta (Fra Mauro) can be representative of the lunar deep-interior composition. However, a good model of the ‘primordial’ or existing interior composition does not yet exist, although geochemical and geophysical evidence indicates that the deep interior may represent accreted material that has never been above the melting point. On the other hand, the outer regions have undergone severe chemical modification. These outer regions are, and probably always have been, depleted in volatile elements and enriched in refractories, an observation constraining models of lunar origin. All basic models of lunar origin (capture, dual-planet, earth-fission) are alive, although the idea of direct fission from earth is quite sick. Sicker yet, and essentially dead, are tektites- and meteorites-from-the-moon hypotheses. The meteorite evidence does indicate, though, that processes similar to those forming lunar-mare basalts occurred at other places in the early solar system. Questions relating to the origin of life must await future planetary exploration, for no life forms have been found, nor have organic molecules been unambiguously identified as being indigenous to the moon. The virtually complete lack of water and the 4+ b.y. of exposure to a harsh space environment make eventual detection of lunar life unlikely.

Extraterrestrial Lyman alpha

Abstract: Theoretical work and observations on extraterrestrial Lyman α are reviewed. A source of extraterrestrial Lyman α near the solar apex is probably scattering of solar Lyman α on cold interplanetary hydrogen that has penetrated to the inner solar system. A minor source of interplanetary hydrogen comes from the breakup of comets. Galactic sources of Lyman α are probably also present, and such emission would tend to be rendered isotropic, depending on the amount of interstellar hydrogen in the near vicinity of the solar system.

Recent developments in theory of solar wind.

Abstract: Current problems and developments in the theory of the large-scale expansion of the solar corona are reviewed. The outstanding question is whether the energy supply to the quiet corona is mainly thermal conduction outward from a region of active heating at its base or mainly wage propagation outward from the base. It is suggested that the question can be settled only when the properties of the wind can be sampled over a wide range of radial distance from the sun, from far inside the orbit of earth to well beyond. It has been suggested that hydromagnetic waves may drive the expansion of the active corona by direct transfer of momentum as well as energy.

Polar ion flow: Wind or breeze?

Abstract: A review of the theories of light ion flow from the polar cap in their hydrodynamic and evaporative forms is offered. Both types of theories should be able to provide correct treatments of the phenomenon. Some difficulties with the hydrodynamic theory are mainly interpretative and should disappear if the so-called pressure gradient force term in the equations of motion is recognized as really inertial in nature. An important new insight has been provided recently in an evaporative theory by the realization that the electric field in the exosphere required to balance electron and ion fluxes is quite different from the electric field of gravitational separation usually used in ionospheric theory. However, an evaporative theory that is based on realistic boundary conditions as well as an acceptable electric field remains to be worked out. The situation in the polar ionosphere is sufficiently complex that experimental studies will undoubtedly be needed to establish the actual conditions that exist there.

Energy-Momentum Tensor for Linearized Waves in Material Media

Abstract: Within the WKB (Wentzel-Kremers-Brillouin) or geometrical optics approximation, a wave packet can be taken to be made up of phonons transporting energy, momentum, and angular momentum. The energy-momentum tensor formalism provides single generalized expressions for the conservation of these quantities and for the exchange of energy and momentum between the wave packet and the background medium. The formalism also leads to a clearer interpretation of wave energy and wave momentum; in many problems of geophysical interest, it is necessary to introduce modified concepts of the momentum density of a packet.

The terrestrial influx of small meteoric particles

Abstract: The available data on the terrestrial influx of small meteoric particles (mass < 1 gram) are surveyed. Whereas, in the past, flux determinations differed by factors of as much as 100 million, it is shown that recent experimental data are far more consistent, variations being limited to a factor of 10 for particles of mass greater than 10−10 gram. The sensors from which the data are obtained are discussed, with particular emphasis on those used in situ, and inherent uncertainties in the results are pointed out. A best estimate of the cumulative mass distribution influx is given on the basis of the most recent data available. This estimate uses the NASA Meteoroid Environment Model, 1969. Uncertainty limits due to differing experimental results and interpretations are placed upon this flux model. For masses larger than about 10−10 gram, the uncertainty is less than 1 order of magnitude; whereas for smaller particles the uncertainty may still be as large as 4 or 5 orders of magnitude. The present best estimate of the mean density of the small meteoric particles is 0.5 g/cm³, but there are indications that several classes of meteoroids exist with densities ranging from 0.3 to 1.2 g/cm³.

Relativity experiments in space vehicles

Abstract: This paper reviews experiments under way or contemplated that make use of space vehicles for the quantitative study of gravitation. Several experiments are discussed briefly, some which can be performed in earth orbit, and others which require deep space probes. It is anticipated that the decade of the 1970's will place the relativistic theory of gravitation on a firm experimental basis.

Planetary exploration: Accomplishments and goals

Abstract: Observations from space vehicles have vastly altered our conceptions of Mars and Venus. Mars has changed from an earth-like planet with water ice caps into a heavily cratered, coreless object whose polar caps are dry ice and whose tenuous atmosphere is CO2. Venus has been found to be blanketed by clouds and a dense CO2 atmosphere that has driven the surface temperature to 700–800°K. Attention is drawn to the practical returns we can expect from comparative studies of these three planets: for understanding of the origins of life, the potential development of a runaway greenhouse effect on earth, and atmospheric circulation.