摘要:

Coronal holes are regions of unusually low density and low temperature in the solar corona. During a 9‐month Skylab solar workshop, 50 participants established some of the basic properties of coronal holes and their associated high‐speed wind streams using a combination of Skylab, satellite, and ground‐based observations. The holes have been identified now 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. The diverging field lines in well‐established holes act as Laval nozzle which produces supersonic flow and depressed densities at low altitudes. The latitude variation of the divergence of the coronal magnetic field lines produces corresponding variations in wind speed, in agreement with interplanetary scintillation measurements and multisatellite observations. During the years of declining solar activity the global field of the corona approximates a perturbed dipole. The divergence of field lines in each hemisphere produces a high‐speed wind near the poles and low‐speed wind in a narrow belt that coincides with the magnetic neutral sheet. The width and magnetic polarity of recurrent wind streams, as measured near earth, has been predicted successfully on the basis of the global coronal magnetic field geometry, at least during the Skylab period. The analysis of electron density measurements within a polar hole indicates that solar wind is accelerated principally in the region between 2 and 5Rsand that mechanical wave pressure (possibly Alfvén wave) may be responsible for the acceleration of the wind. Phenomenological models for the birth and decay of coronal holes have been proposed. A global pattern of formation, with a systematic eastward drift of successive hole appearances, suggests that the diverging magnetic fields of the coronal holes arise through dynamo action in the deep convection zone. However, attempts to explain the birth and rigid rotation of holes through dynamo action have been only partially successful. More observational data exist on the solar cycle variation of wind streams than on coronal holes. The polar holes shrink, and the volume occupied by the neutral sheet may increase near solar maximum. The 11‐year variation of cosmic ray intensities at the earth may result from cyclic variation of open field regions associa

ISSN:8755-1209    

DOI:10.1029/RG015i003p00257

年代:1977

数据来源: WILEY

摘要:

Since the beginning of the century, physics has been dualistic in the sense that some phenomena are described by a field concept and others by a particle concept. This dualism is essential also in the physics of cosmic plasmas: some phenomena should be described by a magnetic field formalism, and others by an electric current formalism. During the first period of evolution of cosmic plasma physics the magnetic field aspect has dominated, and a fairly exhaustive description has been given of those phenomena, like the propagation of waves, which can be described in this way. We have now entered a second period, which is dominated by a systematic exploration of the particle (or current) aspect. A survey is given of a number of phenomena which can be understood only from the particle aspect. These include the formation of electric double layers, the origin of ‘explosive’ events like magnetic substorms and solar flares, and further, the transfer of energy from one region to another. A useful method of exploring many of these phenomena is to draw the electric circuit in which the current flows and to study its properties. A number of simple circuits are analyzed in this

ISSN:8755-1209    

DOI:10.1029/RG015i003p00271

年代:1977

数据来源: WILEY

摘要:

The configuration of high‐latitude electric and magnetic fields is reviewed. Various results indicate that high‐latitude magnetic field lines from the outermost regions of the dayside magnetosphere converge toward a point near the noon meridian. This configuration funnels particles toward noon, where they can flow down field lines to form the midday cusp. Plasma convects away from this entry region and is associated with a dawn‐dusk electric field across the polar cap. Plasma flow is enhanced on the side of the polar cap where the components of interplanetary and magnetospheric magnetic fields are in the same direction. The electric fields associated with these flows produce Hall currents on the polar cap which vary with sector structure. Some evidence suggests that polar cap convection may reverse during intervals of strong northward interplanetary field. The flow reversal at the polar cap boundary is probably associated with ‘inverted V’ particle events, auroral arcs, parallel electric fields, and field‐aligned currents. It is concluded that most observations are consistent with an open field magnetos

ISSN:8755-1209    

DOI:10.1029/RG015i003p00285

年代:1977

数据来源: WILEY

摘要:

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

ISSN:8755-1209    

DOI:10.1029/RG015i003p00299

年代:1977

数据来源: WILEY

摘要:

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 ΔIaround latitude strips to determine the zonal harmonics and the declination anomaly ΔDaround 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 propose

ISSN:8755-1209    

DOI:10.1029/RG015i003p00309

年代:1977

数据来源: WILEY

摘要:

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 theFlayer 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 diff

ISSN:8755-1209    

DOI:10.1029/RG015i003p00325

年代:1977

数据来源: WILEY

摘要:

Available data on the solubility of H2O and CO2in silicate melts at high pressures and temperatures reveal that (1) the solubility of H2O is several times greater than that of CO2and (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 CO2and 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. AtP≲ 20 at the CO2may 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 com

ISSN:8755-1209    

DOI:10.1029/RG015i003p00351

年代:1977

数据来源: WILEY

摘要:

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 documente

ISSN:8755-1209    

DOI:10.1029/RG015i003p00363

年代:1977

数据来源: WILEY

摘要:

The 1976 U.S. Standard Atmosphere, representing a mid‐latitude atmosphere for moderate solar activity, has recently been adopted by the Committee for the Extension of the Standard Atmosphere (COESA). For heights of 51 km and below, this standard is identical with its immediate predecessor, the 1962 U.S. Standard Atmosphere; above 50 km the 1976 standard completely replaces its predecessor. The new standard represents the latest in a century‐long history of model atmospheres, beginning in 1867 with a 10°C isothermal model. A comparison of the temperature‐height profileT(Z) of the 1976 standard withT(/2) of each of 11 important standards and models used during the preceding century traces the development of our knowledge of the earth’s atmosphere. Comparisons of the density‐height profileρ(Z) of each of five earlier model atmospheres with that of the 1976 standard indicate an oscillation ofρ(Z) around the currently accepted average value. This oscillation is partly the result of true density changes related to the 11‐year cycle of solar activity and partly the result of uncertainty in earlier models. Number densities of each of six atmospheric species computed for the 1976 U.S. standard are compared over the height regio

ISSN:8755-1209    

DOI:10.1029/RG015i003p00375

年代:1977

数据来源: WILEY