Showing papers in "Reviews of Geophysics in 1974"

The impulse response of a Maxwell Earth

Abstract: An extended form of the correspondence principle is employed to determine directly the quasi-static deformation of viscoelastic earth models by mass loads applied to the surface. The stress-strain relation employed is that appropriate to a Maxwell medium. Most emphasis is placed on the discussion of spherically stratified self-gravitating earth models, although some consideration is given to the uniform elastic half space and to the uniform viscous sphere, since they determine certain limiting behaviors that are useful for interpretation and proper normalization of the general problem. Laplace transform domain solutions are obtained in the form of ‘s spectra’ of a set of viscoelastic Love numbers. These Love numbers are defined in analogy with the equivalent elastic problem. An efficient technique is described for the inversion of these s spectra, and this technique is employed to produce sets of time dependent Love numbers for a series of illustrative earth models. These sets of time dependent Love numbers are combined to produce Green functions for the surface mass load boundary value problem. Through these impulse response functions, which are obtained for radial displacement, gravity anomaly, and tilt, a brief discussion is given of the approach to isostatic equilibrium. The response of the earth to an arbitrary quasi-static surface loading may be determined by evaluating a space-time convolution integral over the loaded region using these response functions.

Early chemical history of the solar system

Abstract: By using equilibrium thermodynamics, the sequence of condensation of mineral phases from a cooling nebula of solar composition has been calculated. The theoretical models suggest that the chemistry and mineralogy of Ca-Al-rich inclusions in C2 and C3 chondrites were established during condensation at temperatures above 1300 K. Fractionation of such inclusions is necessary to account for the refractory element depletions of ordinary and enstatite chondrites relative to the carbonaceous chondrites. The metal-silicate fractionation in ordinary chondrites took place in the nebula at a temperature below 1000 K and at .00001 atm total pressure. The volatile element depletion of C2 and C3 chondrites relative to C1 chondrites took place during chondrule formation: the most volatile elements are depleted in ordinary chondrites because they accreted before these elements were totally condensed.

Acoustic‐gravity waves in the upper atmosphere

Abstract: In this paper we review the theory of acoustic-gravity waves, the interaction of such waves with the ionosphere, the experimental support for the existence of such waves in the upper atmosphere, and the role played by acoustic-gravity waves in thermospheric dynamics. After a thorough discussion on the properties of acoustic-gravity waves in an ideal isothermal atmosphere, the effects produced by horizontal winds, sharp boundary discontinuities, and dissipative processes are discussed. The generation of these waves by stationary or moving sources is then treated. It is shown that the atmospheric response to a stationary impulse source can be described by the emission of three waves: acoustic, buoyancy, and gravity. These discussions are then followed by reviewing propagation effects in a realistic atmosphere for both free waves and guided waves. Recent numerical results are given. When acoustic-gravity waves propagate through the ionosphere, interaction between the wave and the ionosphere will take place. The physical processes involved in such an interaction are examined.

Structure of the Moon

Abstract: Seismic data fron the four stations of the Apollo passive seismic network have been analyzed to obtain the velocity structure of the moon. Analysis of body wave phases from artificial impacts of known impact time and position yields a crustal section. In the Mare Cognitum region the crust is about 60 km thick and is layered. In the 20-km-thick upper layer, velocity gradients are high and microcracks may play an important role. The 40-km-thick lower layer has a nearly constant 6.8-km/sec velocity. There may be a thin high-velocity layer present beneath the crust. The determination of seismic velocities in the lunar mantle is attempted by using natural impacts and deep moonquakes. The simplest model that can be proposed for the mantle consists of a 'lithosphere' overlying an 'asthenosphere'.

Magnetism and the interior of the Moon

Abstract: The application of lunar magnetic field measurements to the study of properties of the lunar crust and deep interior is reviewed. Following a brief description of lunar magnetometers and the lunar magnetic environment, measurements of lunar remanent fields and their interaction with the solar plasma are discussed. The magnetization induction mode is considered with reference to lunar magnetic permeability and iron abundance calculations. Finally, electrical conductivity and temperature calculations from analyses of poloidal induction, for data taken in both the solar wind and in the geomagnetic tail, are reviewed.

Lunar near‐surface structure

Abstract: Seismic refraction data obtained at the Apollo 14, 16, and 17 landing sites permit a compressional wave velocity profile of the lunar near surface to be derived. Beneath the regolith at the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is material with a seismic velocity of about 300 m/sec, believed to be brecciated material or impact-derived debris. Considerable detail is known about the velocity structure at the Apollo 17 Taurus-Littrow site. Seismic velocities of 100, 327, 495, 960, and 4700 m/sec are observed. The depth to the top of the 4700-m/sec material is 1385 m, compatible with gravity estimates for the thickness of mare basaltic flows, which fill the Taurus-Littrow valley. The observed magnitude of the velocity change with depth and the implied steep velocity-depth gradient of more than 2 km/sec/km are much larger than have been observed on compaction experiments on granular materials and preclude simple cold compaction of a fine-grained rock powder to thicknesses of the order of kilometers.

Auroral particle precipitation and Birkeland currents

Abstract: Correlated measurements that provide information on Birkeland (field aligned) currents are reviewed. Because of the obvious importance of field-aligned electric fields with regard to Birkeland currents, the last section of the paper is devoted to a presentation of recent data on the observations of field-aligned auroral electron fluxes and monoenergetic peaks in the spectrum. Experimental data cannot yet define the overall magnetospheric pattern of the Birkeland current system. The largest body of data regarding these currents comes from polar-orbiting satellites. These data indicate that current sheets are associated with auroral displays and that field-aligned electron fluxes on the order of 1-keV energy are significant charge carriers for the conventional current out of the atmosphere. Rocket observations verify that electrons responsible for the production of auroral forms carry a significant fraction of the current necessary to produce local magnetic effects that require Birkeland currents for their explanations.

Lunar Seismicity, Structure, and Tectonics

Abstract: Natural seismic events have been detected by the long-period seismometers at Apollo stations 16, 14, 15, and 12 at annual rates of 3300, 1700, 800, and 700, respectively, with peak activity at 13- to 14-day intervals. The data are used to describe magnitudes, source characteristics, and periodic features of lunar seismicity. In a present model, the rigid lithosphere overlies an asthenosphere of reduced rigidity in which present-day partial melting is probable. Tidal deformation presumably leads to critical stress concentrations at the base of the lithosphere, where moonquakes are found to occur. The striking tidal periodicities in the pattern of moonquake occurrence and energy release suggest that tidal energy is the dominant source of energy released as moonquakes. Thus, tidal energy is dissipated by moonquakes in the lithosphere and probably by inelastic processes in the asthenosphere.

Origin of the plasma sheet

Abstract: The plasma sheet is a region of hot plasma (∼107 °K) that surrounds the earth, filling the distant magnetosphere and separating the northern and southern lobes of the magnetospheric tail; its origin is one of the basic outstanding problems of magnetospheric physics. A critical review of the available observations indicates that the plasma sheet source must provide roughly 1025–1026 particles/s to balance known loss mechanisms operating in the quiet time magnetosphere and that an adiabatic or quasi-adiabatic (ΔE ∝ E) acceleration mechanism must provide most of the observed particle energy. These constraints together with auroral helium ion measurements indicate that the plasma sheet particles must originate in the solar wind rather than in the terrestrial ionosphere. The available hypotheses for a solar wind source are examined, and it is concluded that a two-step process of magnetopause diffusion plus electrostatic acceleration is the simplest source mechanism that is consistent with the available observations.

Martian dust storms

Abstract: A brief survey is presented of observational evidence and theoretical speculations concerning Martian dust storms. The observations fall into two major classes: earth-based photographic records and Mariner 9 infrared spectra. Among the conclusions reached are that: the mechanism for loss of particles from the atmosphere is not Stokes-Cunningham fallout, more experiments are necessary in order to reach an understanding of injection processes, observations of storm genesis from Mars orbit will probably be necessary to distinguish between different theories of storm dynamics.

Interpenetrating solar wind streams.

Abstract: Observations of solar wind interpenetrating ion streams are reviewed. They occur preferentially during periods of low but increasing plasma number density and most likely result from spatial and/or temporal inhomogeneities in the solar wind bulk velocity. The exact nature of these inhomogeneities is not known, but three conceptual models that account for the observations are summarized. Double ion streams are shown to occur during extended time periods during which the proton anisotropy and dimensionless heat flux undergo large coherent variations. This fact provides strong evidence that a major source for solar wind internal ion energy is its bulk convection energy. The observed continuum of proton velocity configurations further suggests that the ion internal energy state at 1 AU is often perturbed by bulk flow spatial and/or temporal inhomogeneities. Self-consistent interactions with the interplanetary wave mix that results from these inhomogeneities as well as other aspects of the solar wind expansion establish preferred proton velocity configurations. These configurations can be summarized by simple statistical relations that couple the two independent third velocity moments to the second moments.

Observations of interactions between interplanetary and geomagnetic fields

Abstract: Magnetospheric effects associated with variations of the north-south component of the interplanetary magnetic field are examined in light of recent recent experimental and theoretical results. Although the occurrence of magnetospheric substorms is statistically related to periods of southward interplanetary magnetic field, the details of the interaction are not understood. In particular, attempts to separate effects resulting directly from the interaction between the interplanetary and geomagnetic fields from those associated with substorms have produced conflicting results. The transfer of magnetic flux from the dayside to the nightside magnetosphere is evidenced by equatorward motion of the polar cusp and increases of the magnetic energy density in the lobes of the geomagnetic tail. The formation of a macroscopic X-type neutral line at tail distances less than 35 R sub E appears to be a substorm phenomenon.

Theories of the geomagnetic and solar dynamos

Abstract: A review of recent advances in dynamo theory is presented in relation to the problems of the generation of the earth's and the sun's magnetic fields. Some relevant modern observations and estimates of physical quantities are discussed, and the lack of knowledge about the dynamical state of the earth's core is emphasized. The dynamo problem is conveniently separated into the kinematic approach, the area in which most work has been done so far, and the dynamical problem, which is exciting more current interest. Most attention is given to the low-conductivity limit because this allows many of the theories to be exhibited in their simplest form. Dynamical dynamo models, driven either by an applied body force or by thermal convection, are described. Emphasis throughout the paper is on simple examples, at least as far as Cowling's theorem permits.

Lunar basin formation and highland stratigraphy

Abstract: Multiring impact basins, formed after solidification of the lunar crust, account for most or all premare regional deposits and structures expressed in the lunar landscape and for major topographic and gravity variations. A fresh basin has two or more concentric mountain rings, a lineated ejecta blanket, and secondary impact craters. Crackled material on the floor may be impact melt. The ejecta blanket was emplaced at least partly as a ground-hugging flow and was probably hot. A suggested model of basin formation is that the center lifts up and the rings form by inward collapse during evisceration. The resulting basin is shallow and has a central uplift of the mantle. This results in a central gravity high and a ring low. Later flooding by mare basalt has since modified most near side basins. Highland deposits of plains, furrowed and pitted terrain, and various hills, domes, and craters that were interpreted before the Apollo missions as being volcanic can now be interpreted as being basin related.

Cosmic-ray-produced electrons and gamma rays in the atmosphere

Abstract: The propagation of the primary cosmic radiation through the earth's atmosphere is a phenomenon of considerable intrinsic complexity stemming from the large number and variety of parameters involved and their dependence on energy, latitude, and atmospheric depth. Although in the past the study of the hadronic component in the atmosphere has received appreciable attention, not much interest has been focused on the electron-photon component until very recent years. As a consequence, the observational data now available on the latter are meager and fragmentary; such information as is available is reviewed here, and the existing gaps in our knowledge are highlighted. Because of the very complexity of the problem, it has been recognized that extensive and refined calculations are needed to understand the observations and to provide new motivation to experimenters. In the present article the limited theoretical calculations so far attempted are first reviewed; a summary of detailed calculations recently carried out by the present authors follows. Finally, the results from the calculations are compared with those available from experiments; wherever it is necessary, attempts are made to identify the areas in which further work is needed.

Laboratory measurements of ionospheric ion‐molecule reaction rates

Abstract: The present status of laboratory measurements of ion-molecule reactions of ionospheric importance is reviewed. Rate constants for most of the normal terrestrial E and F region reactions have been reliably measured at 300°K. Recently, some data have become available on the energy dependences of the most important of these binary positive ion reactions. In the case of D region reactions a great deal of new data on both positive and negative ion reaction rate constants has appeared in recent years. Both binary reactions and three-body reactions are important in the D region, and important reactions still remain to be measured.

The Sun and cosmic rays

Abstract: Solar phenomena produce cosmic ray intensity variations over a wide range of time scales. The observed flux is modulated and rendered anisotropic as the particles propagate in the solar wind, and it is occasionally enhanced by the sporadic emission of solar cosmic rays. Both quasi-static phenomena (the long-term omnidirectional intensity variation and the steady state diurnal anisotropy) and transient fluctuations (disturbed daily variation and Forbush decrease) as well as the spatial distribution of solar flare particles are represented by a theoretical model that prescribes the role of the several solar-controlled parameters that characterize the electromagnetic properties of the interplanetary medium. Considerable information concerning the ambient conditions has been obtained with spacecraft. However, the in situ measurements are confined to a limited region near the ecliptic plane. Consequently, in some cases, theoretical predictions based upon them are not in accord with observations of cosmic ray intensity variations. Thus the modulations and anisotropies must be treated in a three-dimensional framework. It is therefore reasonable to attempt to deduce the properties of the relevant inaccessible regions of the heliosphere from observations of the sun itself. These properties can be determined by relating the various cosmic ray phenomena to changes during a solar cycle and from one cycle to another. To this end, data covering at least two solar cycles are studied to determine the solar cycle dependence of the following effects: long-term variations in the omnidirectional intensity, Forbush decreases, solar diurnal variations, and solar cosmic ray events (ground level enhancement and polar cap absorption).

Observations of moon-plasma interactions by orbital and surface experiments

Abstract: Extensive magnetic field observations together with crucial plasma measurements by the Explorer 35 lunar orbiter and Apollo surface and orbital experiments have established the basic nature of the moon's interaction with the solar wind and interplanetary magnetic field. The effective absorption of the incident solar wind by the moon creates a plasma void or cavity behind the moon. The cavity-associated magnetic signature is characterized by an enhancement in magnetic field magnitude B within the cavity as compared with the mean level of B in the surrounding interplanetary plasma and dips or decreases in B near the cavity boundaries with the solar wind. Apollo particle and field measurements on the lunar surface have provided evidence of a regional interaction of the highly conducting solar wind with lunar remanent magnetic fields. Simultaneous plasma and magnetic field data, from the spectrometer and the lunar surface magnetometer at the Apollo 12 location, show the compression of the local remanent field by large solar wind and magnetosheath plasma dynamic pressures.

Modes of magnetohydrodynamic waves in the magnetosphere

Abstract: Magnetohydrodynamic plasma waves (waves with frequencies less than the local ion gyrofrequency) constitute an important part of the wave frequency spectrum in the earth's magnetosphere and in the solar wind. Such plasma waves also probably play an important role in astrophysical plasmas. The nature and modes of the plasma waves excited in the earth’s magnetosphere are of interest because they are generated and propagate in a plasma environment imbedded in a dipole magnetic field configuration. As such, analyses of the nature of these waves may prove useful in studies of some planetary and stellar plasma environments. Measurements made at conjugate points on the earth's surface and in the magnetosphere between spacecraft altitudes and the ground provide information on the nature of the modes of the magnetohydrodynamic waves excited in the magnetosphere. This paper reviews the available data on wave modes and concludes that near L=4 (corresponding to geomagnetic latitudes ∼60°) the mode for waves in the period range ∼20 to ∼200 s is predominantly odd mode. This may be a fundamental aspect of the physical process that couples the driving source to the resonant field line. For other geomagnetic and frequency ranges there is presently a disappointing dearth of experimental observations.

Tracing of high‐latitude magnetic field lines by solar particles

Abstract: ABS>Recent measurements of solar particles in the energy interval between hundreds of keV and a few MeV have shown that a direct connection exists between a portion of the high-latitude geomagnetic field and the interplanetary magnetic field. The access window for 300-keV solar protons that reach the center of the polar cap may be as near as 150 R/sub E/ of the downstream magnetotail. Solar protons that precipitate into the atmosphere at latitudes near the geomagnetic cutoff enter through the flanks of the magnetosphere and the nearby neutral sheet, possibly within 30 RE of the Earth. Comparison of the patterns of auroral particle precipitation with the zones of access of energetic solar electrons and protons indicates that a substantial fraction of the aurora originates on field lines connected to the interplanetary field. (auth)

Solid Earth and ocean tides estimated from satellite orbit analyses

Abstract: The earth's tidal deformations cause perturbations in the motions of close earth satellites, observations of which give estimates of the Love number k2 and phase lag δ. The contribution of the ocean tides has generally been considered unimportant, but this is not so. These ocean tides cause the same spectrum of orbital perturbations as the solid tide, and a complete separation, by analyzing satellite orbits of different elements, is not possible. Neglect of the ocean tide will introduce errors in k2 of as much as 15% and in phase angle of as much as 10° depending on the orbit. This explains the low values for k2 near 0.25 that have been obtained recently from orbital analyses. Comparison of several models for the principal lunar tide (M2) of the ocean indicates that these models are insufficient for making precise orbital corrections. For the other tidal frequencies that often have important effects on satellite orbits, no satisfactory tide models exist. The equilibrium theory does not suffice for these corrections, and improved values for k2 and particularly for the phase lag δ can only come from a concomitant improvement in our knowledge of the ocean tides. It is possible to extract some information on the ocean tides from the orbital analyses, and this could be used as a constraint in numerical ocean tide computations. We have analyzed the orbit of the satellite Geos 2 for the lunar M2 tide. After correcting for the ocean tides the value found for k2 varies between 0.27 and 0.30, depending on which ocean tide model is used. The solutions for the phase angles are quite unsatisfactory, and this condition stems from insufficiently precise tracking data and inadequate coverage as well as from inadequate tide models. We have applied approximate ocean tide corrections to the tide parameters determined by several authors to give a mean value of k2=0.31 and δ2=0.5°. This phase lag corresponds to a mantle Q of about 60.

The dynamics and energetics of mature tropical cyclones

Abstract: Rapid progress toward the understanding of tropical cyclones has been made during the past 10 years, largely as a result of the development of numerical models. The dynamics and energetics of the mature tropical cyclone are reviewed in this article. First, the pressure, wind, temperature, and moisture structures of the hurricane are summarized. Then a scale analysis is applied over four separate regions of the hurricane domain to emphasize the most important dynamic processes in each region. The energetics of the tropical cyclone is examined in detail. The storm is shown to be a self-sustaining quasi-steady thermodynamic heat engine that is driven primarily by latent heat release. The relationship between latent heating and production of kinetic energy is discussed from the available potential energy viewpoint. The possibility of a steady state axisymmetric hurricane in a closed domain is considered. Although such a model appears possible from an energetic viewpoint, angular momentum considerations reveal that a steady axisymmetric closed system is impossible on any scale. For small domains (radius ∼500 km), in which axisymmetric circulations are possible, the system must be open in order that angular momentum be imported from the environment. For larger domains, axisymmetric circulations are unrealistic. The water vapor budget of the hurricane is examined to indicate the relative importance of evaporation and horizontal transport of water vapor in maintaining the moisture supply. Evaporation is shown to be an important percentage of the precipitation in the mature storm. Finally, the numerical modeling of tropical cyclones is summarized. An important problem in the development of hurricane models is the treatment or parameterization of the cumulus cloud scale and the hurricane scale interactions. The implications of the concept of conditional instability of the second kind on hurricane modeling are discussed. Results from several axisymmetric (two-dimensional) hurricane models, including attempts to simulate hurricane modification experiments, are summarized. Some recent results from a time-dependent three-dimensional hurricane model are presented.

Aeronomy of the lower atmosphere of Mars

Abstract: Recent models and observations relevant to Martian aeronomy are discussed. The stability of the CO2 atmosphere against photodissociation into CO and O2 has finally been accounted for. The basic mechanism is catalysis by odd hydrogen, aided by very rapid downward mixing, photolysis of H2O2, or some combination. The low abundance of ozone at low latitudes is readily explained, and its higher abundance in cold regions is accounted for by the freezing out of H2O, H2O2, and possibly HO2. The system seems likely to be unstable, and major changes of the CO and O2 abundances may be expected on a time scale of 1 year.

The ionospheres of Mars and Venus

Abstract: Observations made by spacecraft in the Mariner and Venera series have provided considerable knowledge of the structure of the ionospheres and atmospheres of Mars and Venus. This paper begins with brief but complete discussions of these measurements and their interpretations. The principles for constructing models of ionospheres are presented from the standpoint of mass, momentum, and energy conservation. The relevant ion chemistry is presented, and the meaning and use of plasma scale heights are discussed. The models discussed are the model of Cloutier and co-workers; the models of Banks and Axford, of Whitten, and of Herman and co-workers; the model developed by McElroy and Strobel to account for the nighttime ionosphere of Venus; and the models developed by a number of investigators for the thermal structure of the Martian and Venusian ionospheres.

Photoelectrons in the ionosphere: Radar measurements and theoretical computations

Abstract: Ionospheric photoelectrons may be studied by measuring the power returned in the plasma lines of the radar incoherent scatter spectrum. This paper reviews the type and quality of information about photoelectrons available from radar measurements. Distinctive capabilities and limitations of this ground-based technique are discussed, and results are presented for a number of plasma line studies, mostly at Arecibo, Puerto Rico, and Millstone Hill, Massachusetts. These results are compared with measurements by satellite-borne instruments and certain findings of theoretical studies of photoelectrons. It is noted that measured photoelectron escape fluxes generally exceed those predicted by current theories and that inadequate input data for theoretical computations are the likely cause of the discrepancy. Other outstanding problems involving photoelectrons require further research and are described here. One of these is the quantitative evaluation of photoelectron transport and energy degradation in the plasma contained in the magnetospheric field tubes. Experimental and theoretical work is needed on this problem to achieve quantitative understanding of protonospheric thermal structure and of predawn temperature and airglow enhancements in the F region of the ionosphere.

Waves and irregularities in the solar wind

Abstract: This review is meant for the nonspecialist who wants a simple physical picture. Some simple calculations are given to show why waves and irregularities in the solar wind are of importance in obtaining a physical understanding of the dynamical processes occurring in the interplanetary medium. The basic physics of various phenomena observed in the heliosphere, resulting because of the presence of waves in the solar wind, is spelled out in some detail. The most fundamental point made in this review is that a considerable body of phenomena occurring in the solar wind are either caused directly or modified significantly by the turbulent waves that are always present in the solar wind.

Instabilities and nonlinear processes in geophysics and astrophysics

Abstract: A review of some nonlinear effects related to plasma instabilities is presented. Discussions are included on (1) quasilinear diffusion, in which a comparison between the effects of electromagnetic and electrostatic turbulence is given; (2) anomalous resistivity, in which a general derivation of the anomalous resistivity is presented and compared with some concrete examples; (3) wave-wave and wave-particle interactions, in which a general theory of parametric instabilities and quenching is presented together with the effects of the nonlinear Landau damping on stabilization of linear instabilities; and (4) modulational instability, in which a general derivation of the instability conditions and the consequence of the instability are discussed.

Comparison of activity in solar cycles 18, 19, and 20

Abstract: Although solar activity cycles are in general remarkably similar, cycles 18, 19, and 20, with maximums in 1947.5, 1958.2, and 1968.9, respectively, differed in the heights of their maximums, in the patterns of their development, and to a certain extent in the character of their manifestations of solar activity. Of the 20 solar cycles since 1755, the maximums in sunspot numbers for cycles 18, 19, and 20 rank as 3, 1, and 10, respectively. Cycle 18, with a maximum smoothed monthly sunspot number of 151.8, was the cycle of 'giant' spots. Cycle 19, with the highest recorded maximum, 201.3, included many large spots but failed to produce spots with areas equal to those of cycle 18. The highest known daily 2800-MHz flux occurred in cycle 18, not in cycle 19. Cycle 20, with a sunspot maximum of only 110.6, appears to have been close to the average cycle of the last 220 years. An attempt is made to compare the general levels of flare production in the three cycles. Cycle 19 is estimated to have been more flare-rich than cycle 18 by a much smaller factor than the original data implied. In cycle 20, general flare production drops conspicuously.

Geomagnetic Deep Sounding With Arrays of Magnetometers

Abstract: Arrays of about 25 portable magnetic variometers have recently been used to record the total varying geomagnetic field over an area 1000–2000 km square The total varying field consists of a portion generated outside the solid earth (in the ionosphere and the magnetosphere) and a portion induced by this external field (in the material of varying conductivity in the crust and upper mantle of the earth) By plotting sets of magnetograms of the observed varying fields, Fourier spectra over the full range of periods available, or contours of Fourier amplitudes at specified periods, localized anomalies in the total field are identified Careful examination of these various plots yields considerable qualitative information about the subsurface variations in conductivity that produce these anomalies Formal methods of separating the observed field into components of internal and external origin exist and can be applied to this problem, but with questionable accuracy Semiquantitative estimates of the separate components may be just as accurate Quantitative interpretation of the anomalous internal fields in terms of inhomogeneities in conductivity is at present almost entirely based on numerical modeling techniques Three such methods have been used so far, the one using finite differences being the most successful Results of such modeling are neither unique nor very close to the observed field patterns Another method of analysis is based on transfer functions and cross-correlation spectra, and this method may turn out to be more useful

Irregularities in the equatorial ionosphere: The Berkner Symposium

Abstract: The main results of the symposium, which dealt with both E and F region irregularities, are summarized and discussed. The basic processes generating the E region (electrojet) irregularities now seem to be understood, but the details are not. Linear plasma instability theory appears to account for the presence or absence of irregularities. However, the irregularity amplitudes, the wavelength distribution, the propagation velocities, etc., all of which depend on nonlinear effects, are not well explained. The question of nonlinear mechanisms is controversial and is being attacked from a number of directions. The F region irregularities are still poorly understood; the basic driving mechanisms are not yet established. Two new ideas that seem promising involve (1) a gravitational interchange instability involving entire magnetic tubes of force and (2) irregular ‘fingers’ of metallic ions extending into the lower ionosphere that alter the E × B drift patterns.