摘要:

A comprehensive review and extension of the theoretical bases for the measurement of the characteristics of rain and snow with vertically pointing Doppler radar are presented. The drop size distribution in rain can be computed from the Doppler spectrum, provided that the updraft can be estimated, but difficulties are involved in the case of snow. Doppler spectra and their moments are computed for rain by using various power law relations of fall speed υ versus particle diameterDand an exponential fit to the actual fall speed data. In the former case, there is no sharp upper bound to the spectra and all the spectral moments increase with rainfall rateRwithout limit; in the latter case, there is a sharp upper bound of the spectra corresponding to the limiting terminal velocity of raindrops, and the spectral moments approach an asymptote. Accordingly, the power laws are useful approximations over only limited ranges of precipitation rate. A comparison of theoretical and experimental mean Doppler velocity 〈υ〉 as a function of radar reflectivity factorZshows that the empirical relation 〈υ〉 = 2.6Z0.107of J. Joss and A. Waldvogel seems to be the only practical relation; even so, the scatter in 〈υ〉 is about ±1 m sec−1. This is also the kind of error to be expected in measuring updraft speeds by present methods. Such updraft errors result in unacceptably large errors in the drop number concentration estimated from Doppler spectra. In the absence of updrafts the mean Doppler velocity 〈υ〉 is uniquely related to Λ, the slope of the exponential drop size distribution. Simultaneous measurements ofZand 〈υ〉 can then be used to estimateN0, Λ,D0,M, andR, whereN0is the intercept of the exponential drop size distribution atD= 0,D0is the median volume diameter, a

ISSN:8755-1209    

DOI:10.1029/RG011i001p00001

年代:1973

数据来源: WILEY

摘要:

Controversy over the use of rectilinear plots for the interpretation of isotopic rock lead studies has prompted the following analysis of discrete two‐ and three‐stage episodic models, which is partly in the nature of a review, but which also introduces some new results. An investigation is made of the restrictive conditions under which these models lead to rectilinear plots in the isochron, lead‐lead, and modified concordia diagrams; the question of the solubility of these models and the possibility of discriminating between them are discussed. It is shown that there are two special three‐stage models that lead to rectilinear plots in all diagrams and that one of these models cannot (from uranium‐lead data alone) be distinguished from the two‐stage model. The alternative special rectilinear three‐stage model, which can be distinguished both from the two‐stage and from other three‐stage models on the basis of uranium‐lead data alone, cannot be fully solved on the basis of these data alone. The necessity of using data other than the uranium‐lead data (such as the ‘age of the earth’ or the duration of the last stage of lead evolution) either to solve or to distinguish the models makes it clear that a fully independent age of the earth cannot be derived from uranium‐lead data alone. A discussion is also given of the solubility of general three‐ and four‐stage models by use of ‘age‐corrected’ isotope ratios to correct back one stage. It is shown that careless application of a particular multistage model without proper investigation of its consistency with the data can lead to false interpretations. Recent controversy over the use of multistage models is discussed; much of the controversy is shown to arise out of a confusion of the models with the particular diagram used to plot the data. In general no one diagram is to be preferred to another, with the exceptions that ifµ1is variable the modified concordia diagram must be used, whereas the discussion of age‐corrected isotope ratios necessitates use of the lead‐lead diagram. A brief discussion is given of the interpretation of oceanic lead data, for which the analytical precision at present may often be insufficient for the proper application of consistency tests; individual sets of oceanic lead therefore are not susceptible of unambiguous interpretation at this time. It is emphasized that this real lack of information cannot be overcome (as has been implied in some papers) by the use of alternative presentations of the data. The new Tera‐Wasserburg diagram is examined, as is the recently introduced idea that the initial207Pb/206Pb ratio may serve as an important chronometer for early planetary system evolution. Both ideas are shown to contribute no information that cannot be derived from the modified concordia diagram. The application of the Tera‐Wasserburg diagram in the attempt to derive from lunar basalt U‐Pb data an independent age of the moon is shown to rest on precisely the same principles and to be subject to the same restrictions and uncertainties as earlier attempts to derive an age of the earth from terrestrial oceanic basalts. A discussion is also given of the question of homogenization and of the application o

ISSN:8755-1209    

DOI:10.1029/RG011i001p00037

年代:1973

数据来源: WILEY

摘要:

The Herglotz‐Wiechert integral for the direct inversion of ray parameter versus distance curves can be manipulated to find the envelope of all possible models consistent with geometrical body wave observations (travel time and ray parameter versus distance). Such an extremal inversion approach has been used to find the uncertainty bounds for the velocity structure in the mantle and core. We find, for example, that there is an uncertainty of ±40 km in the radius of the inner core boundary, ±18 km at the core‐mantle boundary, and ±35 km at the 435‐km transition zone. The velocity uncertainty is about ±0.08 km/sec forPandSwaves in the lower mantle and about ±0.20 km/sec in the core. Experiments with various combinations of ray types in the core indicate that rather crude observations ofSKKS‐SKStravel times confine the range of possible models far more dramatically than do the most precise estimates ofPmKPtravel times. Comparisons of results from extremal inversion and linearized perturbation inversions indicate that body wave behavior is too strongly nonlinear for linearized schemes to be effective for predicting

ISSN:8755-1209    

DOI:10.1029/RG011i001p00087

年代:1973

数据来源: WILEY

摘要:

The solar abundance of helium (more specifically the ratio of solar helium to hydrogen) is a basic quantity in understanding many astrophysical and space physical problems. However, in spite of a great deal of work on the subject, it has proved to be very difficult to accurately determine the helium abundance of the sun. We here review critically the four methods that have been used to estimate the ratio of helium to hydrogen; the solar neutrino flux, spectral intensity of helium lines in prominences and the chromosphere, elemental abundance of solar cosmic rays, and variations of solar wind He/H. At present the measured value for the neutrino flux cannot be explained by any of the theoretical models of the sun, and thus the neutrino measurements cannot yield an estimate of He/H. Helium line intensity measurements give the most accurate values for He/H now available, i.e., 5–8% helium by number. However, these results refer to the chromosphere and prominences only, and questions of spatial and temporal variations of the observed quantities and of the inferred helium abundances have not yet been dealt with adequately. The cosmic ray observations are usually interpreted as indicating 6% helium at the flare site. However, this interpretation is here called into question because of an apparent energy dependence of the observations. The variations of He/H in the solar wind yield information on the coronal distribution of helium, rather than the solar abundance. The solar wind observations are most easily interpreted in terms of a moderate helium enhancement (He/H ∼ 10–15%) in the deeper levels of the solar atmosphere, but a uniformly mixed corona cannot at present be completely ruled out. It is concluded that the solar abundance of helium remains uncertain to within a factor of 2

ISSN:8755-1209    

DOI:10.1029/RG011i001p00115

年代:1973

数据来源: WILEY

摘要:

Recent research on the structure and dynamics of the magnetospheric thermal plasma indicates that the vast region above an altitude of ∼1000 km rivals the underlying ionosphere in complexity and that it is coupled to the lower region in complicated, physically important ways. An example involves the relation of the electron content of magnetospheric tubes of ionization to the electron content of the regular ionosphere. Tube volume between ∼1000 km and the magnetic equator varies rapidly over a relatively small range of tube end point latitudes, which gives rise to correspondingly rapid variations with latitude in coupled effects that involve interchange of ionization between the upper and lower regions. In the past, some correlative studies involving the plasmapause have been hindered by lack of information concerning (1) the unsteady nature of the process by which the disturbed‐time plasmapause profile is established and (2) the fact that at most times and at most locations the plasmasphere‐plasmapause system is in a state of recovery. A series of equatorial density profiles is shown to illustrate the reduction of plasmapause radius during brief periods of increased disturbance and the recovery of the plasmasphere by various processes, particularly by filling from the underlying ionosphere. A number of research results are presented as part of a ‘quick‐reference guide’ to the plasmasphere. To the ionospheric observer, the plasmapause should appear to have a complex but generally predictable geometry as well as characteristic motions. A crude predictor of plasmapauseLvalue (Lpp) in the post‐midnight period as a function of magnetic disturbance is the formulaLpp= 5.7‐0.47Kp, whereKpis the maximum 3‐hourKpvalue in the preceding 12 hours. A ground station atL∼ 3.7 is recommended as optimum for observation of plasmapause‐associated effects directly overhead. The plasmasphere is regularly perturbed by substorm‐associated convection electric fields, and these apparently have important effects on the nightside ionosphere at middle latitudes. Other known departures of the plasmasphere from corotation are expected to have their count

ISSN:8755-1209    

DOI:10.1029/RG011i001p00133

年代:1973

数据来源: WILEY

摘要:

The auroral oval represents a region of auroral occurrence where particular spectral features have sufficient integrated intensity to exceed all‐sky camera thresholds. It has been argued that this is not a good representation of the characteristics of particle precipitation or of energy deposition. Consequently, the auroral oval concept may not be a good one to use to order diverse auroral data on a 24‐hour basis. Rather, the precipitation patterns for specific energy classes of protons and electrons should form the basis for the interpretation of high‐latitude geophysical measure

ISSN:8755-1209    

DOI:10.1029/RG011i001p00155

年代:1973

数据来源: WILEY

摘要:

Several measurements of the flux and charge state of auroral helium precipitation have been made and reported between 1967 and 1972. The primary purpose of these experiments has been to deduce the source location of auroral ions by comparison of auroral helium populations with those measured in the ionosphere and the solar wind. Accordingly, results of these experiments are reviewed and summarized, along with measurements of solar wind and ionospheric helium abundances. It is concluded that the available data suggest a solar wind origin of auroral ions and in addition place certain constraints on the acceleration mechanism. The problem of modification of the incident charge state of a helium beam by atmospheric charge exchange is discussed, and it is shown that auroral helium measurements should be made at altitudes above 400 km to insure that the measurements are a true indication of the original beam charge state composition. Attempts to measure optical emissions of auroral helium are also reviewed, with the conclusion that definitive reproducible detection of such emissions has not been accomplished to date.

ISSN:8755-1209    

DOI:10.1029/RG011i001p00169

年代:1973

数据来源: WILEY

摘要:

This description of the present status of work on magnetospheric substorms is based on the talks and discussions that occurred at the Rice University‐AGU Conference on Magnetospheric Substorms. Various types of new data were presented, including, for example, Isis 2 scanning photometer observations of the aurora from above and lunar shadowing observations made from the Apollo 15 subsatellite. The subject of greatest controversy at the conference was the substorm growth phase: gaps in the network of ground magnetometer stations can result in the expansion phase of one substorm being mistaken for the growth phase of another, and there was considerable debate over whether this observational ambiguity could invalidate popular ideas about the growth phase and its signatures. Correlations between magnetospheric phenomena and the direction of the interplanetary magnetic field constituted another area of considerable discussion: the picture appears complicated, with different magnetospheric phenomena correlating with different components of the interplanetary magnetic field. Two areas in which previous controversies have been closed for some time now are the question of whether the magnetosphere is open or closed and the general pattern of the average magnetospheric electric field: it now seems generally agreed that there is direct connection between interplanetary and magnetospheric magnetic field lines, although the details of the connection process remain uncertain, and electric‐field measurements from balloons and satellites have convincingly verified the existence of systematic magnetospheric convection, with dawn‐to‐dusk electric fields over the polar caps. A variety of theoretical approaches to the substorm problem are being pursued, with, as yet, no commonly accepted unifying

ISSN:8755-1209    

DOI:10.1029/RG011i001p00181

年代:1973

数据来源: WILEY