摘要:

One of the most fascinating and challenging problems of modern interdisciplinary research is to understand the causes of global climatic changes. This is an area of research which has been extensively covered in the past at the various IUGG meetings by more than two associations, and the same holds for this assembly. And the thrust of research continues with all force from many directions.Presently there seems to be a general consensus that the influence of orbital variations on climate is real enough and that all efforts should be made to test the astronomical theory with climate models. The progress made in understanding and modeling physical mechanisms by which the climate system responds to the astronomically forced changes in the pattern of incoming solar radiation was recently evaluated at a climate symposium organized by Professor Andre Berger, who will deliver today'S talk.

ISSN:8755-1209    

DOI:10.1029/RG026i004p00623

年代:1988

数据来源: WILEY

摘要:

Among the longest astrophysical and astronomical cycles that might influence climate (and even among all forcing mechanisms external to the climatic system itself), only those involving variations in the elements of the Earth's orbit have been found to be significantly related to the long‐term climatic data deduced from the geological record. The aim of the astronomical theory of paleoclimates, a particular version of which being due to Milankovitch, is to study this relationship between insolation and climate at the global scale. It comprises four different parts: the orbital elements, the insolation, the climate model, and the geological data. In the nineteenth century, Croll and Pilgrim stressed the importance of severe winters as a cause of ice ages. Later, mainly during the first half of the twentieth century, Köppen, Spitaler, and Milankovitch regarded mild winters and cool summers as favoring glaciation. After Köppen and Wegener related the Milankovitch new radiation curve to Penck and Brückner's subdivision of the Quaternary, there was a long‐lasting debate on whether or not such changes in the insolation can explain the Quaternary glacial‐interglacial cycles. In the 1970s, with the improvements in dating, in acquiring, and in interpreting the geological data, with the advent of computers, and with the development of astronomical and climate models, the Milankovitch theory revived. Over the last 5 years it overcame most of the geological, astronomical, and climatological difficulties. The accuracy of the long‐term variations of the astronomical elements and of the insolation values and the stability of their spectra have been analyzed by comparing seven different astronomical solutions and four different time spans (0–0.8 million years before present (Myr B.P.), 0.8–1.6 Myr B.P., 1.6–2.4 Myr B.P., and 2.4–3.2 Myr B.P.). For accuracy in the time domain, improvements are necessary for periods earlier than 2 Myr B.P. As for the stability of the frequencies, the fundamental periods (around 40, 23, and 19 kyr) do not deteriorate with time over the last 5 Myr, but their relative importance for each insolation and each astronomical parameter is a function of the period considered. Spectral analysis of paleoclimatic records has provided substantial evidence that, at least near the obliquity and precession frequencies, a considerable fraction of the climatic variance is driven in some way by insolation changes forced by changes in the Earth's orbit. Not only are the fundamental astronomical and climatic frequencies alike, but also the climatic series are phase‐locked and strongly coherent with orbital variations. Provided that monthly insolation (i.e., a detailed seasonal cycle) is considered for the different latitudes, their long‐term deviations can be as large as 13% of the long‐term average, and sometimes considerable changes between extreme values can occur in less than 10,000 years. Models of different categories of complexity, from conceptual ones to three‐dimensional atmospheric general circulation models and two‐dimensional time‐dependent models of the whole climate system, have now been astronomically forced in order to test the physical reality of the astronomical theory. The output of most recent modeling efforts compares favorably with data of the past 400,000 years. Accordingly, the model predictions for the next 100,000 years are used as a basis for forecasting how climate would evolve when forced by orbital variations in the absence of anthropogenic disturbance. The long‐term cooling trend which began some 6,000 years ago will continue for the next 5,000 years; this first temperature minimum will be followed by an amelioration at around 15 kyr A.P. (after present), by a cold interval centered at 23 kyr A.P., and by a major

ISSN:8755-1209    

DOI:10.1029/RG026i004p00624

年代:1988

数据来源: WILEY

摘要:

Professor Berger has explained to us in a very clear manner the premises of our belief in the control of orbital variations over the terrestrial climate. His lecture has been a textbook example of how cross‐disciplinary ties are forged in a field by experimentalists and theorists working from so many different directions, and how these ideas converge to understanding of a problem. Researchers in laboratories all over the globe, finally culminating in the researchers meeting in the campus of a university: that is how challenging problems such as the climate are solve

ISSN:8755-1209    

DOI:10.1029/RG026i004p00658

年代:1988

数据来源: WILEY

摘要:

The inversion of local earthquake data (LED) for three‐dimensional velocity structure requires the simultaneous solution of the coupled hypocenter‐model problem. The Aki‐Christoffersson‐Husebye method (ACH) involves the inversion of large matrices, a task that is often performed by approximative solutions when the matrices become too big, as is the case for most LED, considering the coupled inverse problem. Such an approximate method (herein referred to as approximate geotomographic method) is used to perform tests with LED to obtain the best suited inversion parameters, such as velocity damping and number of iteration steps. The ACH method has been proposed for use of teleseismic data. Several adjustments to the original ACH method, which are necessary for use of LED, have been developed and are discussed. Such adjustments are the separation of the unknown hypocentral from the velocity model parameters for the inversion, the use of geometric weighting and step length weighting, the calculation of a minimum one‐dimensional (1D) model as the starting three‐dimensional (3D) model for the model inversion, and the display of an approximate resolution matrix (ray density tensors) before the inversion is performed. The ray density tensors allow the block cutting, e.g., the definition of the 3D velocity grid, to better correspond with the resolution capability of the specific data set. The adjustments to the method are tested by inversion of realistic LED of known variance. Synthetic LED are also used to demonstrate the effects of systematic errors, such as mislocations of seismic stations, on the resulting velocity field. Using the data sets from Long Valley, California, Yellowstone National Park, Wyoming, and Borah Peak, Idaho, the effects of improvements to the ACH method and of the data filtering process are shown. The use of the minimum 1D models for routine earthquake location improves this location procedure, as shown with the relocation of shots for the Long Valley and Yellowstone areas. The three‐dimensional velocity fields obtained by the ACH method for the Long Valley and Yellowstone areas show local anomalies in the p velocity that can be correlated with tectonic and volcanic features. A pronounced anomaly of low p velocity below the Yellowstone caldera can be interpreted as a large magma chamber. However, the bulk of the paper addresses problems of the inversion method. The LED from the areas mentioned above are used to numerically and theoretically tune the inversion method for the defects that all real data contain. It is shown that one of the most important steps for any inversion of LED is the selection of the data for quality and for geometrical

ISSN:8755-1209    

DOI:10.1029/RG026i004p00659

年代:1988

数据来源: WILEY

摘要:

During the past decade, significant progress has been made in defining the nature of midcontinent tectonism as a result of an improved geophysical/geologic data base which permits an integration of seismicity data, crustal structures, and current stress directions. Critically placed microseismic networks have shown that many earthquakes with common focal mechanisms occur in specific spatial patterns. The direction of the maximum horizontal compressive stress is generally directed east‐northeast, which supports an origin associated with ridge‐push forces related to movement of the North American plate. Crustal structures have been mapped locally by seismic studies and regionally by extensive gravity and magnetic anomaly data. These data have been used to relate current seismicity and associated tectonism to one of two models. Increasing evidence from across the midcontinent supports reactivation of preexisting zones of crustal weakness appropriately oriented with respect to the prevailing stress field, the ‘zone of weakness model’, as the dominant control on contemporary tectonism and the ‘local basement inhomogeneity model’ as a mechanism for minor, low‐energy release earthq

ISSN:8755-1209    

DOI:10.1029/RG026i004p00699

年代:1988

数据来源: WILEY

摘要:

The most intense,Fregion irregularities in the high‐latitude ionosphere appear to be produced by convective plasma processes and in particular, by the fluid(gradient drift) interchange instability. Such irregularities are produced by convectively mixing plasma across a mean plasma density gradient with the transport of higher‐density plasma into regions of lower‐density plasma (and vice versa) leading to the development of an irregularity spectrum that extends in scale from about 10 km down to the ion gyroradius. The mean plasma density gradient that must be present to allow irregularity production by this interchange process appears to be associated with larger‐scale (>10 km) plasma structure produced by other means. Because much of the recent progress on this research topic stems from this recognition, a significant portion of this review is dedicated to a description of the characteristics and processes of 10‐km plasma structure and their relationships to those of smaller‐scale irregularities. From this review, we synthesize a descriptive model of plasma structures in the high‐latitudeFlayer that unifies most of the diverse and independent observations. For the large‐scale plasma processes, the model includes (1) the formation of 1000‐km‐scale “patches” in the polar cap from solar‐produced plasma that is transported poleward from lower latitudes; (2) the reconfiguration of patches as they convect into the auroral region and become the latitudinally confined, but longitudinally extended, plasma density enhancements near the equatorward auroral boundary; and (3) the production of localized enhancements and depletions along the poleward auroral boundary by soft‐particle precipitation and large but localized electric fields. In the model, the most intense, smaller‐scale irregularities are in spatial proximity to these large‐scale plasma features, the implication being that the presence of the latter allows formation of the former. The irregularity characteristics are consistent with production by theinstability and a morphology controlled by (1) a “slip” velocity (i.e., plasma drift relative to the neutral gas) that is moderately small except in regions of nonuniform plasma convection or under time‐varying conditions (e.g., substorms, pulsation events) and (2) a highly conducting auroralElayer that damps irregularity growth and enhances decay. The final irregularity spectrum appears to be produced by (1) global convective processes acting on solar‐produced plasma at the largest scales (>50 km), (2) particle precipitation at scales greater than 10 km, (3) perhaps some form of wave activity around 10 km, and (4) theins

ISSN:8755-1209    

DOI:10.1029/RG026i004p00719

年代:1988

数据来源: WILEY

摘要:

Multilevel parameterizations of the atmospheric boundary layer using first‐order and turbulent kinetic energy (TKE) closure schemes are reviewed. Eleven schemes, chosen as representative of both first‐order and TKE closure, are then used for comparison in a one‐dimensional barotropic planetary boundary layer model. TKE closure schemes evaluated are the E‐ε schemes in which eddy viscosityKmis determined from turbulent kinetic energy and energy dissipation ε and thelmodel schemes in whichKmis determined from TKE and mixing lengthl. Comparison of model simulations of mean and turbulence structure for first‐order closure and TKE closure schemes to observational data (MONEX79) is given. The two main conclusions drawn from this comparison are that (1) the mean structure of the boundary layer is fairly insensitive to the type of closure scheme, given that the scheme properly accounts for turbulent boundary layer mixing, and (2) TKE closure is preferable to first‐order closure in predicting the overall turbulence structure of the boundary layer. Among the TKE schemes compared in this paper, the modified Detering and Etling (1985) scheme

ISSN:8755-1209    

DOI:10.1029/RG026i004p00761

年代:1988

数据来源: WILEY

摘要:

Global properties of the Earth's magnetosphere are mainly studied in one of two ways : by focusing on specific events, e.g., substorms, or by analyzing average behavior. This collection of reviews deals with the second aspect.All four articles began as invited reviews at the International Association of Geomagnetism and Aeronomy symposium on the quiet magnetosphere, held in Vancouver, Canada, August 17, 1987, as part of the nineteenth International Union of Geodesy and Geophysics General Assembly. One invited speaker, Igor I. Alexeev, was unable to attend, and one talk, by G.‐H. Voigt, was transferred to the symposium from the general contributions sessio

ISSN:8755-1209    

DOI:10.1029/RG026i004p00781

年代:1988

数据来源: WILEY

摘要:

The state of our knowledge is reviewed concerning the global pattern of geomagnetic field lines. Sources of information on that pattern include (1) magnetic field models, derived directly from magnetic data or indirectly from general observed properties and from physics, (2) the tracing of magnetospheric features, e.g., polar cusps or the inner edge of the plasma sheet, (3) matching of magnetic flux, and (4) analysis of magnetic fields. Field line structure inside about 8 Earth radii is known fairly well, but beyond that, especially in the tail, the situation becomes rather uncertain and variable. Two particularly difficult problems are (1) the linkage between open field lines and the interplanetary field and (2) the field line structure of the “quiescent” magnetosphere following periods of prolonged northwar

ISSN:8755-1209    

DOI:10.1029/RG026i004p00782

年代:1988

数据来源: WILEY

摘要:

Three states of the magnetosphere may be identified from polar auroral and magnetic activity: the active auroral oval state, the active polar cap state, and the quiet state. The quiet time magnetosphere occurs when the energy transfer from the solar wind has been maintained at a minimum level for several hours. The conditions for minimal energy transfer occur for low solar wind velocity and for small IMFBandBz. The quiet magnetosphere is characterized by minimal, but nonzero, cross‐cap electric field potentials (6–20 kV) and, therefore, low earthward convection velocities in the plasma sheet. The auroral oval shrinks to a circle of radius 20° geomagnetic latitude (MLAT) offset, as in active times, toward midnight. Diffuse auroral precipitation is continuous throughout these periods but has low average energy and number flux. Discrete arcs are weak (subvisual) and multiple, often extending to extremely high latitudes, particularly on the dayside of the cap. The cusp lies between 77° and 84° MLAT at noon. The total particle energy flux into both hemispheres is ∼ 10 GW. Region 1 and region 2 field‐aligned current systems frequently are not observed during extended periods of magnetic quiet. However, small‐scale current systems, which are associated with the weak discrete arcs, are still seen. Cusp currents in the sunlit hemisphere remain strong. Of particular interest in modeling the quiet magnetosphere are the following topics: the entry and transport of cusp particle populations, stable convection in the plasma sheet, and the closure of tail lobe magnetic

ISSN:8755-1209    

DOI:10.1029/RG026i004p00792

年代:1988

数据来源: WILEY