摘要:

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 permit

ISSN:8755-1209    

DOI:10.1029/RG012i002p00137

年代:1974

数据来源: WILEY

摘要:

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. Specifically, we summarize the characteristics and use of UV radiometry, magnetometers, and ion probes for determining solar wind properties near planets, as well as the application of the occultation experiment. Some of the significant results were the very small, perhaps zero, magnetic moment of Venus and the possibly small but nonzero magnetic moment of Mars (Mars 2 and Mars 3 spacecraft); observation of daytime ionospheres on both Mars and Venus with peak number densities of 105and 6 × 105cm−3, respectively; observation of the top side scale heights for both planets; observation of a ledge of ionization with a large scale height and the anemopause above Venus; revelation by the two‐frequency radar occultation experiment carried by Mariner 5 of the existence of a nighttime Venusian ionosphere with a peak number density of 10³ cm−3and a very large plasma scale height above the peak; and measurement by Mariner 6, 7, and 9 and Mars 2 and 3 of EUV radiation intensities (scattered solar radiation and dayglow emission), which yielded data for reasonably precise estimates of the concentration profiles of CO2, CO, O, C, and H as well as the thermospheric temperature (350°K for Mariner 6 and 7) on Mars. Corresponding observations by Mariner 5 and Venera spacecraft have not yielded as much information about Venus; Mariner 5 did measure the atomic hydrogen profile, whereas Venera 4 established an upper limit on atomic oxygen on the night side. The structure of the neutral atmosphere at high altitudes is of key significance to ionospheric structure. Although CO2is the major constituent up to and including the main layer of the ionosphere, lighter gases such as O, He, and H probably play important roles at heights between the main layer and the anemopause. The occurrence of these gases, their high‐altitude profiles, and the closely related problem of the photolytic stability of the Martian and Venusian atmospheres are discussed in considerable detail. The anemopause also has an important bearing on the structure of the upper ionosphere. Unlike the earth the solar wind impinges directly on the ionospheres of Mars and Venus, thus acting as a possible heat source. However, no adequate theory has yet been developed to predict details of the interaction. The crude method of computing the day side boundary is discussed: it requires the balancing of the solar wind dynamic pressure by the ionospheric static pressure. The ‘downwind’ side of the boundary is also discussed, but mainly from the standpoint of our ignorance of the wake structure. 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 general principles in varying degrees of simplification have been employed by several investigators to model the upper ionospheres of Mars and Venus. The models discussed are the model of Cloutier and co‐workers, who suggested that direct hydrodynamic pressure of the solar wind on the top side Martian ionosphere depresses the ionospheric scale height to a value equal to the neutral atmosphere scale height; the models of Banks and Axford, of Whitten, and of Herman and co‐workers, who attempted to account for the ionospheric ledge above the main layer of the Venusian ionosphere by proposing the ionization of light constituents; 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. In presenting the essentials of each model, their strengths and weaknesses are also discussed. The paper concludes with a summary of current knowledge of the upper ionospheres of Mars and Venus and the information needed to understand full

ISSN:8755-1209    

DOI:10.1029/RG012i002p00155

年代:1974

数据来源: WILEY

摘要:

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. The response of the ionosphere to acoustic‐gravity waves can be fairly complex, but its understanding is necessary to interpret various experimental data. The existing experimental data on traveling disturbances are then reviewed. The existence of acoustic‐gravity waves throughout the atmosphere implies coupling between the lower atmosphere and the upper atmosphere. Transport of both momentum and energy are accompanied by the wave process. The implication of momentum and energy transport on thermospheric dynamics i

ISSN:8755-1209    

DOI:10.1029/RG012i002p00193

年代:1974

数据来源: WILEY

摘要:

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. There are a few measurements that suggest that the return current might be carried by low‐energy electrons (less than a few hundred electron volts) streaming out of the

ISSN:8755-1209    

DOI:10.1029/RG012i002p00217

年代:1974

数据来源: WILEY

摘要:

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 neede

ISSN:8755-1209    

DOI:10.1029/RG012i002p00233

年代:1974

数据来源: WILEY

摘要:

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 theFregion of the ionosph

ISSN:8755-1209    

DOI:10.1029/RG012i002p00259

年代:1974

数据来源: WILEY

摘要:

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 discus

ISSN:8755-1209    

DOI:10.1029/RG012i002p00273

年代:1974

数据来源: WILEY

摘要:

A concise review is given of the role of eddy processes in the dissipation of kinetic energy in large‐scale atmospheric circulations above the planetary boundary layer. A distillation of the more prominent works relevant to this problem is accomplished with the following two goals in mind: (1) to consolidate existing evidence for the presence of a dissipation maximum above the boundary layer in the so‐called ‘free’ atmosphere and (2) to provide new insight into the concept of dissipation as applied to large‐scale open atmospheric cir

ISSN:8755-1209    

DOI:10.1029/RG012i002p00281

年代:1974

数据来源: WILEY

摘要:

The main results of the symposium, which dealt with bothEandFregion irregularities, are summarized and discussed. The basic processes generating theEregion (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. TheFregion 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 theE × Bdrift patt

ISSN:8755-1209    

DOI:10.1029/RG012i002p00285

年代:1974

数据来源: WILEY