Showing papers in "Reviews of Geophysics in 1977"

Coronal holes and high-speed wind streams

Abstract: Coronal holes, regions of unusually low density and low temperature in the solar corona, are identified 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. Phenomenological models for the birth and decay of coronal holes have been proposed.

Stress corrosion theory of crack propagation with applications to geophysics

Abstract: The theory of stress corrosion for slow crack propagation is reviewed in the light of classical Griffith theory of fracture. Experimental data for stress corrosion cracking for glasses, ceramics, and metals are reviewed. We suggest that stress corrosion cracking plays an important role in the intrusion of magmas and in the transport of magmas upward through the lithosphere. It is shown that the effect of decreasing temperature (at progressively shallower levels along the geotherm) would be to decrease the crack velocity by several orders of magnitude if other factors were equal. We also propose that stress corrosion may be an important process in time-dependent earthquake phenomena such as premonitory behavior and earthquake aftershocks. We suggest that slow cracking in the earth is not seismically detectable but may nevertheless precede the terminal (catastrophic) phase of the fracture that is discerned as an earthquake. The seismically quiet periods before some earthquakes and the seismically quiet regions beneath some volcanoes may in fact be regimes of slow crack propagation. Slow crack propagation in a lithospheric plate may provide access routes for magmas which give rise to prominent linear volcanic chains.

Mathematical structure of transport equations for multispecies flows

Abstract: In this review we attempt to present a unified picture of transport in multispecies gas mixtures. To accomplish this task, it is necessary to outline the mathematical structure of the transport equations. Starting from Boltzmann's equation, we derive a general system of transport equations using an approach that is valid for flow situations in which there are large temperature and drift velocity differences between the interacting species. However, this system of equations, which is obtained by taking velocity moments of the Boltzmann equation, does not constitute a closed set, since the equation governing the velocity moment of order r contains the velocity moment of order r + 1. To close the system of transport equations, it is therefore necessary to adopt an approximate expression for the species velocity distribution function. For near-equilibrium flows, various levels of approximation are considered, including the 5-, 8-, 10-, 13-, and 20-moment approximations. The procedure for obtaining closed sets of transport equations for far-from-equilibrium flows is also discussed. When the transport equations are ordered with respect to the collisional mean free path, the result is the Euler, Navier-Stokes, or extended Navier-Stokes equations depending upon whether terms proportional to the zeroth, first, or second power of the mean free path are retained. For a collisionless plasma the analogous expansion using the Larmor radius yields the Chew-Goldberger-Low (CGL) equations to zeroth order and the extended CGL equations to first order.

Depositional and postdepositional processes in the magnetization of sediments

Abstract: This paper surveys 40 years of research on the processes by which a sediment acquires a detrital remanent magnetization (DRM). These processes can be divided into two types: depositional processes, which arise from interactions between the magnetic carriers and the substrate at the sediment/water interface, and postdepositional processes, which arise from the mobility of magnetic carriers within fluid-filled voids in the sediment. Depositional DRM is well understood theoretically and experimentally, but its applicability to natural sediments may be quite limited. Postdepositional DRM, which has not been studied in sufficient detail, may well be the dominant process by which sediments acquire a magnetization. The nature and composition of both the magnetic carriers and the matrix as well as the pore water content of the sediment influence the balance between depositional and postdepositional DRM. The identification of the method of magnetization in a given sedimentary environment, such as glacial lakes, the deep sea, or lakes and marginal seas, can be a difficult problem. Since postdepositional DRM accurately records the ambient geomagnetic field, whereas depositional DRM does not, such an identification is necessary in order to use the detrital remanent magnetism of sediments in studies of the fine scale behavior of the geomagnetic field, the paleomagnetic correlation between sedimentary environments, and the possible relation between paleomagnetic parameters and paleoclimatic indicators.

A review of anomalous resistivity for the ionosphere

Abstract: 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.

Importance of physical dispersion in surface wave and free oscillation problems: Review

Abstract: Physical dispersion resulting from anelasticity is investigated from the point of view of linear viscoelastic models and causality relations. It is concluded that inasmuch as Q in the earth's mantle is nearly independent of frequency, at least in the seismic frequency band, a dispersion relation in the form of C(ω) = C(ω_r)[1 + (1/πQ_m) In (ω/ω_r)] must be used for correcting the effect of physical dispersion arising from anelasticity. (Here C(ω) is the phase velocity of either body waves, surface waves, or free oscillations, ω is the angular frequency, ωr is the reference angular frequency, and Q_m is the path average Q for body waves or Q of a surface wave or a mode of angular frequency ω; for surface waves and free oscillations, C(ω_r) should be understood as the phase velocity at ω computed by using the elastic moduli at ω = ω_r.) The values of Q outside the seismic frequency band affect mainly the absolute value of the phase velocity but do not affect significantly the relative dispersion within the seismic frequency band. Even if the microscopic mechanism of attenuation is nonlinear, this dispersion relation can be used if departure from elasticity is relatively small, so that the signal can be approximated by a superposition of propagating harmonic waves. Since surface wave and free oscillation Q is 100–500 for fundamental modes, a correction of 0.5–1.5% must be made for joint interpretation of body wave and surface wave data. This correction is nearly 1 order of magnitude larger than the uncertainties associated with these data and are therefore very significant. When this correction is made, the discrepancy between the observed surface wave phase velocities and free oscillation periods and those predicted by the Jeffreys or Gutenberg model becomes much smaller than has previously been considered.

Electric currents in cosmic plasmas

Abstract: It is suggested that dualism is essential for the physics of cosmic plasmas, that is, that some phenomena should be described by a magnetic field formalism, and others by an electric current formalism. While in earlier work the magnetic field aspect has dominated, at present there is a systematic exploration of the particle (or current) aspect. A number of phenomena which can be understood only from the particle aspect are surveyed. Topics include the formation of electric double layers, the origin of 'explosive' events like magnetic substorms and solar flares, and the transfer of energy from one region to another. A method for exploring many of these phenomena is to draw the electric circuit in which the current flows and then study its properties. A number of simple circuits are analyzed in this way.

West Antarctic Ice Streams

Abstract: Solar heat is the acknowledged driving force for climatic change. However, ice sheets are also capable of causing climatic change. This property of ice sheets derives from the facts that ice and rock are crystalline whereas the oceans and atmosphere are fluids and that ice sheets are massive enough to depress the earth's crust well below sea level. These features allow time constants for glacial flow and isostatic compensation to be much larger than those for ocean and atmospheric circulation and therefore somewhat independent of the solar variations that control this circulation. This review examines the nature of dynamic processes in ice streams that give ice sheets their degree of independent behavior and emphasizes the consequences of viscoplastic instability inherent in anisotropic polycrystalline solids such as glacial ice. Viscoplastic instability and subglacial topography are responsible for the formation of ice streams near ice sheet margins grounded below sea level. As a result the West Antarctic marine ice sheet is inherently unstable and can be rapidly carved away by calving bays which migrate up surging ice streams. Analyses of tidal flexure along floating ice stream margins, stress and velocity fields in ice streams, and ice stream boundary conditions are presented and used to interpret ERTS 1 photomosaics for West Antarctica in terms of characteristic ice sheet crevasse patterns that can be used to monitor ice stream surges and to study calving bay dynamics.

Implications of Pacific Island and seamount ages for the origin of volcanic chains

Abstract: Available age data from the Hawaiian, Marquesas, Kodiak-Bowie, Society, Caroline, and Guadalupe chains, all WNW trending Pacific chains, follow patterns of generally increasing ages to the WNW. The inferred rates of volcanic propagation for these chains are not significantly different and apparently offer strong support for the hypothesis that volcanic chains are formed by ‘hot spots’ which do not move with respect to each other. However, age data from the Austral-Cook chain follow no simple pattern: ages are both younger and older than would be expected from the ‘fixed hot spot’ hypothesis. Eocene and Cretaceous ages from along the Hawaiian ridge are inconsistent with the otherwise systematic age progression along this chain. The limited age data from the older NNW trending chains show little evidence of age progression. The best dated of these older chains, the Line chain, could have formed synchronously along most of its length.

Large-scale electric fields in the Earth's magnetosphere

Abstract: Studies of the earth's magnetosphere have indicated that a large-scale electric field E plays a central role in its electrodynamics and in the flow and acceleration of charged particles there; while many observations relevant to E have accumulated, quite a few basic problems involving the origin and structure of this field remain unsolved. The ultimate source of E is presumably the flow of the solar wind past the earth, but the mechanism by which E arises is still unclear, and several independent sources may contribute to it, some of them being of a rather transient nature. This review attempts to sum up the main observed facts and theoretical concepts related to E.

Neutral hydrogen in interplanetary space

Abstract: The theory and observations relevant to the problem of neutral hydrogen in interplanetary space are reviewed. Emphasis is placed on those theoretical problems whose treatment in the existing literature is not entirely satisfactory, but discussion of all significant observational and theoretical aspects of the interplanetary H problem is provided. Attention is also given to other neutral constituents (particularly He) that are relatively abundant in interplanetary space and in the local interstellar medium, which is the primary source of the interplanetary neutral gas. Some consequences of the passage of the solar system through an interstellar cloud are also briefly considered.

Strain heating and creep instability in glaciers and ice sheets

Abstract: Creep instability, the runaway increase of internal temperature and deformation rate, may affect the boundary condition at the base of glaciers and ice sheets and thus influence their flow and dimensions. We consider a simple slab model of heat transport in which three dimensionless parameters determine the relative importance of strain heating, ice advection normal to the surface, and boundary conditions. We find that an ice mass will be unstable if the strain-heating parameter exceeds a critical value which depends strongly on advection and boundary conditions. Critical values over the range of parameters appropriate to natural ice masses are presented. Accumulation (downward advection) or ablation (upward advection) affects the critical value by up to 5 orders of magnitude: ablation tends to reduce stability, and accumulation increases it. For an ice mass frozen to its bed, instability eventually raises the basal ice to melting point. This can restore thermal stability, but the ice mass will start to slide over its bed. If the strain-heating parameter exceeds a second, higher critical value, a layer of basal ice at melting point will form. We find that the conditions for instability are likely to exist in the accumulation and ablation zones of certain glaciers and ice sheets. However, times calculated for instability to develop are in the range 102–103 yr for glaciers and 103–104 yr for ice sheets. As these times exceed the normal residence time for ice in the ablation zone, it appears that instability is most likely to develop in the accumulation zone. This conclusion is reinforced by the fact that ablation increases the growth time for instability, whereas accumulation decreases it. The growth times quoted above are longer than the periods of most glacier surges, and thus creep instability is an unlikely surge mechanism. Unstable conditions may, however, obtain in East Antarctica and may have existed in the central part of ice age ice sheets. Surges of ice sheets triggered by creep instability may be possible.

Anomalies in the time‐averaged paleomagnetic field and their implications for the lower mantle

Abstract: The proposal that the time-averaged paleomagnetic field is not strictly that of a geocentric axial dipole but that of an axial dipole displaced slightly northward from the geocenter is examined in terms of spherical harmonic expansions. Standard procedures for spherical harmonic expansions such as are applied to the present instantaneous geomagnetic field are not necessarily applicable to paleomagnetic data. A technique is proposed that enhances the time-averaging process that is a necessary part of determining the paleomagnetic field. This involves analyzing the inclination anomaly ΔI around latitude strips to determine the zonal harmonics and the declination anomaly ΔD around longitude sectors to determine the first nonzonal harmonics. The technique is applied to a carefully selected data set covering the last 5 m.y. All data are divided into normal or reversed sets providing 266 land-based points (171 normal, 95 reversed) and 100 deep-sea core results with inclination data only (50 normal, 50 reversed). These data demonstrate clearly that the time-averaged field is not simply that of a geocentric axial dipole and also that it is different for the normal and reversed fields. With respect to the present field the zonal harmonics of the time-averaged field are reduced less than the nonzonal harmonics, the indication being that westward and/or eastward drift of the nondipole field is dominant over 5 m.y. The magnitude of the second zonal harmonic suggests that on the average, paleomagnetic poles could be in error by about 3° when calculated by the usual geocentric axial dipole assumption. The data show clear asymmetries between the northern and southern hemispheres and possibly between the oceanic (Pacific) and continental hemispheres. Possible explanations of these asymmetries include variations in topography and/or temperature at the core-mantle interface. Large-scale asymmetries in the boundary conditions at this interface could affect the magnetic field through alteration of the convection pattern in the core. The asymmetries correlate with other global asymmetries such as the distribution of continents and oceans and the locations of subduction zones. They also apparently correlate with seismic velocity anomalies observed in the lower mantle. All these may reflect large-scale inhomogeneities related to dynamic processes that have occurred in the lower mantle. Although all the Gaussian coefficients determined change sign when the axial dipole field reverses, there are significant differences between the normal and reversed fields, suggesting that the distribution of sources for these fields is not identical. This is consistent with the cyclonic convection models for reversal of the geomagnetic field proposed by Parker and Levy.

Geomagnetic excursions and their paleomagnetic record

Abstract: The paleomagnetic record of recent lake and marine sediments contains anomalous magnetic directions which have been interpreted as large-scale fluctuations of the geomagnetic field (excursions). In view of the many ways in which isolated distortions of the paleomagnetic recording process can arise, proof of the existence of an excursion must depend on consistent results from within a given lake or marine environment as well as from adjacent sedimentary environments. A critical review of the proposed excursions reveals that although some excursions probably did occur, there is not yet sufficient evidence to confirm the existence of many of the claimed excursions. When the existence of regional or global excursions is established, additional information will become available on the nature of the geomagnetic field, on the use of excursions as time-stratigraphic horizons, and on possible correlations between geomagnetic activity and climatic change.

Space and time scales of mesoscale motion in the western North Atlantic

Abstract: From moored data, primarily temperature, of the Mid-Ocean Dynamics Experiment (Mode I) and its successor experiments we find a statistical description of the mesoscale variability. In the Mode I area itself the spectral characteristics of the thermocline and the deep water are different. The thermocline is conveniently described as being made up of three spectral bands: a ‘low-frequency’ band dominated by zonal velocity fluctuations, an ‘eddy-containing’ band in which the velocity field is nearly isotropic, and a ‘high-frequency’ band consistent with models of geostrophic turbulence. In the deep water the zonal dominance at low frequencies is not apparent, and there is enhanced energy at periods of 20–50 days. Vertical structure scales with WKBJ approximation in the high-frequency band but not in the lower frequencies, where low vertical modes dominate the motion. Linear models do not adequately describe the data in the Mode I region. Differences between rough and smooth topography regions are clearly seen only at 1500 m, where there is a loss of energy consistent with a reduced barotropic motion. Other differences, while apparently real, are small. It is found, consistent with the results of Schmitz (1976a), that the Mode I region is atypical of the midocean in that large changes of energy level are found elsewhere. A region due east of Mode I has slightly reduced kinetic energy levels in the main thermocline, but deep energy levels are much lower. Potential energy is less variable than kinetic; in the eastern region the frequency spectra change structure slightly. Linear models may be more adequate there. With more than 2 years of data, no statistically significant heat flux was found in the Mode I area, except for a weak zonal flux in the deep water. There is no direct evidence for baroclinic instability as a significant mechanism of eddy generation; the Gulf Stream is a possible, if unconfirmed, source.

The solubility of H2O and CO2 under predicted magma genesis conditions and some petrological and geophysical implications

Abstract: Available data on the solubility of H2O and CO2 in silicate melts at high pressures and temperatures reveal that (1) the solubility of H2O is several times greater than that of CO2 and (2) the solubility of H2O depends strongly on pressure and, compared to that of CO2, depends only to a small extent on temperature. It has been suggested that the species in silicate melts can be chosen so that the molar solubility of H2O may not depend on the bulk composition of the melt. The solubility of CO2, on the other hand, varies significantly with pressure, temperature, and bulk composition of the melt. Solution of volatiles at high pressure affects the structure of the silicate melts. Water depolymerizes the melt, the result being lowered viscosity. The same depolymerization is manifested in the enhanced stability of silicate minerals on the liquidus, which are less polymerized than the minerals precipitating from the same melt composition at the same pressure under volatile-free conditions. Carbon dioxide, on the other hand, enhances polymerization of the melt, the result being increased viscosity and increased stability of liquidus minerals which are more polymerized than those that would precipitate under volatile-free conditions. Because of the large difference in the solubilities of CO2 and H2O in silicate melts, partial melting of an (H2O + CO2)-bearing mantle results in enrichment of H2O in the liquid, whereas the residual mantle becomes, enriched in CO2. At P ≲ 20 at the CO2 may be retained in a vapor phase. At higher pressures, carbonate is likely to be the stable phase. Therefore it would be expected that as the result of partial melting throughout geological history the upper mantle would be heterogeneous with respect to vapor components.

The homogeneous chemistry of atmospheric sulfur

Abstract: Sulfur is present in the atmosphere as a constituent of more than 15 compounds and in valence states ranging from −2 to +6. Nearly all of the compounds have both natural and anthropogenic sources. This paper reviews the present information on the reactions of these compounds and their products with typical atmospheric species. It is shown that atmospheric sulfur chemistry proceeds monotonically in the direction of increasing sulfur oxidation and that the gas phase transformation of reduced sulfur compounds to sulfate aerosol in all ambient atmospheres can be described by a common chain consisting of five chemical steps followed by a heterogeneous loss process.

Secondary and tertiary creep of glacier ice as measured by borehole closure rates

Abstract: Published and previously unpublished measurements of closure rates of five boreholes in polar ice caps are reviewed. The data cover effective shear stresses between 0.15 and 1.0 MN m−2, temperatures between −16° and −28°C, and strains up to 2.2. Curves of strain at the borehole wall (logarithm of the ratio of hole diameter to its initial diameter) against time show a stage of constant closure rate corresponding to secondary (steady state) creep of the ice followed by accelerating closure rate attributed to recrystallization of the ice (tertiary creep). Curves for low stresses also show an initial transient stage of decreasing closure rate. The onset of tertiary creep is largely determined by the strain; critical values range from 0.03 to 0.10, and the lower the temperature, the higher the critical value. Secondary creep rates in the different boreholes are consistent with each other; the data yield a creep activation energy of 54 kJ/mol and a flow law index close to 3. The borehole data reduced to a common temperature of −22°C are compared with the results of two laboratory experiments at this temperature. For a given stress the strain rates measured by Steinemann (1958a, b) are 2–3 times those in the boreholes, and for the experiments of Barnes et al. (1971) the factor is about 8. Differences between laboratory and glacier ice, probably in grain size, may explain the differences between the borehole data and the results of Steinemann. Some evidence is presented that the creep rates measured by Barnes et al. at this temperature may contain a significant component of transient creep; this might account for the large difference between their results and those of Steinemann. The ratio of tertiary to secondary creep rate increases approximately linearly with the strain. No steady state tertiary creep rate is observed even at a strain of 1.5, at which point the ratio of tertiary to secondary creep rate is about 10. However, the ice is not strained uniformly during borehole closure. Even if recrystallization has been completed in the ice near the borehole wall, the ice further away, having been strained less, may still be recrystallizing. This may account for the failure to observe steady state tertiary creep. Near the bottom of one borehole, creep rates (tertiary) are about 4 times those in the ice immediately above. The boundary between the two deformation regions corresponds closely to the boundary between ice deposited during the Wisconsin glaciation and ice deposited since that time. The crystals in the Wisconsin ice are smaller, much less variable in size, and more nearly equidimensional than those elsewhere. Moreover, the Wisconsin ice has a much higher microparticle content and a much lower content of salts of marine origin. It is suggested that one or more of these differences make the Wisconsin ice ‘softer’ than the remainder of the ice. The decrease in grain size is considered to be the most likely factor.

Problems related to macroscopic electric fields in the magnetosphere

Abstract: The macroscopic electric fields in the magnetosphere originate from internal as well as external sources. The fields are intimately coupled with the dynamics of magnetospheric plasma convection. Th ...

Electric and magnetic fields in the high-latitude magnetosphere

Abstract: The configuration of high-latitude electric and magnetic fields is reviewed. Various results suggest that high-latitude magnetic field lines from the outermost regions of the dayside magnetosphere converge toward a point near the noon meridian. Plasma flows, the midday cusp, and a dawn-dusk electric field across the polar cap are characterized. The electric fields associated with plasma flows produce Hall currents on the polar cap which vary with sector structure. Some evidence indicates that polar cap convection may reverse during intervals of strong northward interplanetary field. It is concluded that most observations are consistent with an open field magnetosphere model.

Interaction of the surfaces of the moon and Mercury with their exospheric atmospheres

Abstract: A model of atmosphere-surface interactions on the moon and on Mercury is presented. This model rejects the traditional assumption that the atmospheric particle source is a 'saturated' absorbed surface layer of gas and instead assumes that interaction is with solid atoms bound in a lattice structure so that most collisions with the light atmospheric particles are free-free rather than free-bound. The energy accommodation per collision for He and H is dominated by first-order interaction terms and is generally less than 0.1 per collision. It follows that estimated thermal escape is drastically reduced for the light atoms on both bodies. Thus the model requires much reduced solar wind collection efficiencies for protons and alpha particles. Some peculiarities of the Mercury He and H atmospheres might possibly be explained by the nature of the gas-surface coupling.

Theoretical aspects of tidal and planetary wave propagation at thermospheric heights

Abstract: A simple semiquantitative model is presented which allows analytic solutions of tidal and planetary wave propagation at thermospheric heights. This model is based on perturbation approximation and mode separation. The effects of viscosity and heat conduction are parameterized by Rayleigh friction and Newtonian cooling. Because of this simplicity, one gains a clear physical insight into basic features of atmospheric wave propagation. In particular, we discuss the meridional structures of pressure and horizontal wind (the solutions of Laplace's equation) and their modification due to dissipative effects at thermospheric heights. Furthermore, we solve the equations governing the height structure of the wave modes and arrive at a very simple asymptotic solution valid in the upper part of the thermosphere. That 'system transfer function' of the thermosphere allows one to estimate immediately the reaction of the thermospheric wave mode parameters such as pressure, temperature, and winds to an external heat source of arbitrary temporal and spatial distribution. Finally, the diffusion effects of the minor constituents due to the global wind circulation are discussed, and some results of numerical calculations are presented.

The 1976 Standard Atmosphere and its relationship to earlier standards

Abstract: The 1976 U.S. Standard Atmosphere, representing a mid-latitude atmosphere for moderate solar activity, is compared to earlier standards. For heights of 51 km and below, this standard is identical with its immediate predecessor, the 1962 U.S. Standard Atmosphere. When the density-height profile of each of five earlier model atmospheres is compared with that of the 1976 standard, an oscillation of this parameter around the currently accepted average value is observed, which is partly the result of true density changes related to the 11-year cycle of solar activity and partly the result of earlier uncertainties. The development of knowledge is also elucidated by comparing the temperature-height profiles of each of the important standards used during the preceding century. Number densities of each of six atmospheric species computed for the 1976 U.S. standard are compared over the height region of 0 to 1000 km.

Satellite beacon contributions to studies of the structure of the ionosphere

Abstract: The signals transmitted by radio beacons on board artificial earth satellites have been widely used for studies of the earth's ionosphere. Following the launch of Sputnik 1, two lines of investigation quickly emerged, the study of the total columnar content of the ionosphere (up to the height of the satellite) and the study of the fine-scale irregularities within the layer responsible for causing rapid fading of the received signals. Early studies of total content employed either the Faraday rotation or the differential Doppler method and suffered because of an ambiguity in the results. This was overcome when satellites were launched that carried beacons transmitting on closely spaced frequencies. The measurements then provided information on the diurnal, seasonal, and sunspot cycle variations of the total content. In addition, by comparing the measurements with true height analyses of ionosonde records, useful results were obtained concerning the ratio of the number of electrons lying above the peak of the F layer to the number below and the layer thickness. Presently, the most useful product of the low-altitude satellite measurements of ionospheric total content is in revealing latitudinal irregularities produced, for example, by traveling ionospheric disturbances or by auroral zone processes. The advent of geostationary satellites has made possible long continuous records of total content for many fixed locations on the earth, and by using multiple frequencies it has also been possible to study the exchange of plasma between the ionosphere and the magnetosphere. An extension of these observations to higher latitudes could resolve the question of whether the shrinkage of the plasmasphere during magnetically disturbed periods is accomplished by ‘peeling away’ the outer shells or by an inward compression of the plasma into the ionosphere. The morphology of ionospheric scintillation has been studied extensively by using beacon signals. In addition, these studies have shown that the spectrum of irregularities is power law (rather than Gaussian) and at the equator can extend to very small spatial scales. It is generally agreed that the irregularities responsibile for scintillation are created by some form of plasma instability; there seems to be a good candidate in the case of the equatorial irregularities but not in the case of irregularities at mid-and auroral latitudes. It may be that different instability mechanisms operate in the auroral zone and at mid-latitudes and/or at different times.

Plasma waves and instabilities in the polar cusp: a review.

Abstract: A review is presented of the experimental and theoretical work performed in recent years on plasma waves and instabilities in the polar cusp (cleft).

A survey of Waterdrop Interaction Experiments

Abstract: The growth of waterdrops by inertial capture and coalescence induced by the size-dependent terminal velocities in the earth's gravitational field is an important process in the formation of rain. The extensive experimental data on this fundamental process are reviewed and organized into a coherent summary of the collection efficiencies applicable to natural clouds, in the following circumstances: negligible effects of electric charges and fields, normal laboratory temperatures and pressures, and relative humidities between 50 and 100%. The judgments required to reduce the experimental data to the brief summary form are documented.

Linear inference and underparameterized models

Abstract: A version of Backus's theory of linear inference is developed by using a new finite-dimensional space. This approach affords a clear geometric interpretation of the essential role played by a priori model smoothing assumptions and also facilitates the construction of a theory for the treatment of random data errors that is quite different from the treatment of Backus. When the unknown parameters form a (necessarily incomplete) description of the model, it is possible to formulate a special smoothing assumption that is particularly appropriate; in practical examples this strategy often leads to tighter bounds on the model uncertainty than those obtained with previous assumptions. An analysis of the numerical aspects of the problem forces one to the conclusion that the theory is not competitive numerically with conventional least squares parameter estimation, unless one of the large submatrices in the problem possesses a simple inverse. An example of this kind is discussed briefly.

On probing the outer planets with the Raman effect

Abstract: The paper examines the feasibility of using the H2 rotational Raman spectrum to study the physical structure of the atmospheres of the outer planets. Computations of the strengths of the S(0) and S(1) lines are made for a wide range of physical conditions on the basis of a semi-infinite homogeneous pure H2 atmospheric model. Initial applications of the Raman probe technique to Jupiter and Uranus are reported.

Stratospheric Aitken particles

Abstract: The nature and sources of stratospheric Aitken particles have remained highly uncertain in spite of considerable research by various groups. Most of the field studies of these particles have been performed by using condensation nucleus counters and by impactors. Numerous theoretical studies have been made of their formation, transport, and fate. This article reviews and analyzes the results. This analysis strongly indicates that the Aitken particles in the stratosphere below 20 km in altitude are usually of tropospheric origin and like the larger particles consist to a major extent of impure sulfuric acid. Above 20 km they may contain a considerable fraction of Aitken particles of meteoric origin.

Comments on “The distribution of water vapor in the stratosphere” by J. E. Harries

Abstract: The author responds to a letter by Harries who reports that insufficient data do not exist in his studies to support the claim for a stratospheric sink specific for water vapor. (JMT)