摘要:

I have been asked to give two talks [Walker‐Ames Lecture at the University of Washington, June 18, 1979] on recent developments in branches of my main interest. Unfortunately, I have not been able to follow the most recent literature, but I believe it is worthwhile to recognize the essential features of modern computation techniques by confronting them with older ones; this might be achieved by a historical survey, to be given in particular in my first talk, while the second one will emphasize the common aspects of procedures which in the literature have been worked out only for one special branch of physics or technology but are just as well applicable to other related fields. This division requires, moreover, that my first talk almost exclusively concern phenomena the essential properties of which already follow from the effects produced by a single infinitesimal source, while the second lecture considers the consequences of the detailed structure of the source in view of its finite dimensions. Thus let me begin with a discussion of the propagation of terrestrial radio waves, a problem the development of which I have followed with great interest during my scientific career. Represented in its simplest idealized form, we here want to know the electromagnetic field of a point source transmitter situated near the earth's surface and situated there in the presence of a homogeneous, spherical earth surrounded by an also homogeneous medium, the atmosphere. The interest in this diffraction problem arose when, in 1902, Marconi succeeded for the first time in transmitting damped radio waves across the Atlantic Ocean. Although this success was in the beginning mainly viewed from the economical side (possible future competition with telecommunication using sea cables), a bit later scientists wondered about its physical aspect, since here the curvature of the earth excluded rectilinear propagation; indeed the latter should take place along a chord through the earth, which, however, would involve an extremely high attenuation. The question then became whether the atmosphere really behaves as an almost homogeneous medium or whether just its small inhomogeneity might make possible the transmission over large distances along curved rays, in the sense of geometrical optics. We should not forget that the ionosphere had yet to be discovered at the time, although as early as 1878 the American BaIfour Stewart had introduced a model of electric currents at high altitudes in the atmosphere in order to explain the part of the earth's magnetic field that had to be ascribed (according to a theory of Gauss) to a source outside the earth. Kennelly, not conscious of Stewart's model, then put forward the hypothesis of some reflecting layer, while Heaviside believed that oceans might guide waves, in view of their electric conductivity. According to Heaviside, however, the high conductivity of a rarefied gas (large mean free paths) at high altitudes might also play a role, though only a secondary one. In honor of these two scientists the name Kennelly‐Heaviside layer became customary a long time before the present term ionosphere. Since all these assumptions could not be verified at the time, pure mathematicians preferred to consider first the simplest model, that of pure diffraction around a completely homogeneous body of the earth surrounded by an also completely homogeneous atmosphere; this then constitutes the idealized mode I mentioned at the beginn

ISSN:0048-6604    

DOI:10.1029/RS015i003p00459

年代:1980

数据来源: WILEY

摘要:

In my preceding talk I treated phenomena the main characteristics of which were already apparent from effects produced by an infinitesimal point source. Let me now [Walker‐Ames Lecture at the University of Washington, June 25, 1979] consider cases in which, on the contrary, the structure of an extended source is essential for the problem in question. Having some experience in problems dealing with rather different applications, such as radio waves, optical and electron‐optical imagings, and plasma physics problems, it struck me that fundamental phenomena occurring in any of them are often discussed while overlooking results that have been obtained in one of the related fields. Today I would like to emphasize the existence of some common features in all of these problems, all having to do with sources of finite dimensions. This may be illustrated by considering a special parameter with the dimension of a length and connected with the size of the source, which plays a role in all of them. As a matter of fact, it is just the finiteness of the source which implies that some length (however, not necessarily just the size itself of the source) becomes most essential. This is at once clear in the case of the three‐dimensional source representing a transmitting antenna system. In the immediate vicinity of the latter the field depends mainly on the nearest parts of the system, but at larger distances the effects of all parts interfere, and we arrive in the so‐called ‘wave zone.’ The transition occurs at distancesDat which the various parts of the source, with separationsLperpendicular to the propagation direction, can reach the point of observation along path lengths differing by not more than about the operating wavelength. According to the relation we find the expression for this critical distance. Of course, this transition distance is rather vague, since the dominating scale lengthLof the antenna, here introduced, is not well defined. However, it is well known that the approximations needed in order to determine radiation diagrams are only applicable in the wave zone which is just fixed, apart from the almost trivial conditionsD≫ λ andD≫Lby the pr

ISSN:0048-6604    

DOI:10.1029/RS015i003p00465

年代:1980

数据来源: WILEY

摘要:

A method of calculating the current distribution and input impedance of thin horizontal cylindrical antennas is presented and the impedance computed for cases in which the antennas are located at heights of 0.05 λ and 0.02 λ above a semi‐infinite plane earth. Results indicate that it would be practical to determine the electrical parameters εrand σ of the earth directly from measurements of the input impedance of a half‐wave dipole provided these parameters were essentially constant to a depth equal to the penetration depth of radiation at the operating fr

ISSN:0048-6604    

DOI:10.1029/RS015i003p00471

年代:1980

数据来源: WILEY

摘要:

The time structure of a representative set of weakly and strongly scintillating transionospheric beacon signals is analyzed. Under conditions of weak scatter, the coherence time of the signal intensity is a monotonic function of the Fresnel radius divided by the effective scan velocity. The shape of the function, however, is controlled by the power law index. Data from a Peruvian station show evidence of a slightly steeper spectral distribution than do data from Kwajalein in the Marshall Islands. Under conditions of strong scattering, the intensity coherence time depends only on the perturbation strength. The strong scatter data show remarkably little dispersion when they are plotted against the perturbation strength. The data are all consistent with a phase spectral slope somewhat less than 3, which is independently verified by using phase scintillation data.

ISSN:0048-6604    

DOI:10.1029/RS015i003p00479

年代:1980

数据来源: WILEY

摘要:

Two rocket payloads carrying plasma density probes with high spatial resolution have been flown in the auroral zone during active conditions. Simultaneous Wideband satellite scintillation and Chatanika incoherent scatter radar observations were made in order to study the properties of high‐latitude irregularities and their effects on radio wave transmission. Unlike barium cloud striations and bottomside equatorial spreadF, the observed power law dependence of the irregularities does not seem to be due to steepening of kilometer‐scale structures, rather, a turbulent process seems to occur. In addition the power law indexes determined both from the probe and from the scintillation measurements indicates an in situ one‐dimensional spectrum less steep than thek−2value often reported. Both the probe and the scintillation data indicate absolute electron density fluctuations (Δn2e)½of several times 109m−3during the expansion phase of an auroral substorm, with a layer thickness of several hundred kilometers. The observedS4levels at VHF were in the range of 0.1–0.4. This level of scintillation, as well as the absolute density fluctuation levels and the power spectral density at the kilometer scale, are shown to be comparable with bottomside equatorial spreadF. It is suggested that differences between the power spectral index in the present data set and the other ionospheric experiments mentioned above may be due to a highly conductiveElayer and its effects upon the nonlinear evolution of irregularities. During another flight with lower magnetic activity but several bright auroral areas in the trajectory, much lower levels of absolute and relative density fluctuations were observed with a corresponding lower value forS4. Two very sharp changes in electron density were observed (e‐folding scales of 1.45 and 0.7 km) near the field line projected position of the auroral arcs. The associated density spectra were peaked at short wavelengths. The detection of very structured plasma within minutes of the poleward expansion phase of a substorm suggests that theFlayer irregularities were formed in the precipitation event. On the other hand, evidence is also presented for production or enhancement of irregularities in the presence of horizontal density gradients which suggests that plasma instabilities also play a role in the production of auroral zone

ISSN:0048-6604    

DOI:10.1029/RS015i003p00491

年代:1980

数据来源: WILEY

摘要:

The multiple‐scattering effects in rain are negligibly small in comparison with the absorption effects when raindrop sizes are much smaller than a wavelength. This is often true for light to moderate rain below about 30 GHz, and the total atmospheric attenuation along the path may be estimated by the emission method using the usual radiometric formula. At higher frequencies and heavy rain the multiple‐scattering effects cannot be ignored. This paper describes a theoretical study of the correction due to the multiple‐scattering effect for the determination of the total atmospheric attenuation by the emission method. The observed temperatures at the ground both for vertical and horizontal polarizations and for various raindrop temperatures, precipitation rates, ground temperatures, ground albedos, and frequencies ranging from 30 GHz to 120 GHz are calculated on the basis of the equation of transfer, taking into account the polarizations and the Stokes parameters. The cross sections of rain droplets are calculated using the Mie solution with the Laws and Parsons drop size distribution and Saxton's formula for refractive index of water. These calculations show the differences between the total rain attenuation and the attenuation calculated from the sky temperature measurements and the assumed rain temperatures. The results are compared to some experimental data, showing a good agre

ISSN:0048-6604    

DOI:10.1029/RS015i003p00507

年代:1980

数据来源: WILEY

摘要:

Full wave methods and wave guide concepts are used to calculate the nighttime 17.8‐kHz radio fields along the 40°W parallel of longitude at 500‐km altitude. The fields are generated by the ground‐based transmitter at Cutler, Maine. The results are compared with data obtained by the Ogo 4 satellite and with results of Souza and Scarabucci, which were calculated using other methods. Wave guide leakage accounts for general features of measurements made in the northern geomagnetic hemisphere. Measurements made in the southern geomagnetic hemisphere suggest that those fields can be estimated by assuming lossless whistler mode propagation of wave guide leakage fields at the geomagnetic con

ISSN:0048-6604    

DOI:10.1029/RS015i003p00517

年代:1980

数据来源: WILEY

摘要:

The scanning multichannel microwave radiometer is a five‐frequency (6.6, 10.7, 18, 21, and 37 GHz), dual‐polarized microwave radiometer, which was launched in two separate satellites, Nimbus 7 and Seasat, in 1978. A formalism is developed which can be used to interpret the data in terms of sea surface temperature, sea surface wind speed, and the atmospheric content of water vapor and liquid water. It is shown with reasonable instrumental performance assumptions that these parameters can be derived to useful accuracies. Although the algorithms are not derived for use in rain, it is shown that at least token rain rates can be tolerated without invalidating the retrieved geophysical paramet

ISSN:0048-6604    

DOI:10.1029/RS015i003p00525

年代:1980

数据来源: WILEY

摘要:

The wave spectrum of the fluctuations in a plasma can be determined by first measuring the cross‐power density spectrum and then performing a spatial Fourier transform. The feasibility of using this method to measure the fluctuations in the ionosphere by placing a cross‐power spectrum analyzer on an orbiting satellite is assessed by determining the integration time required to make the measurement. The theory considers in particular spherical probes biased into the electron saturation region. The method consists in assuming an equilibrium plasma wave spectrum, from which the power spectrum directed to each spherical probe and the available power at the amplifier input are determined. Using statistical theory, this information is used to determine the integration time needed to measure the cross‐power spectral density at the output terminals of the cross‐power spectrum analyzer to a specified accuracy. For typical experimental conditions the integration times are found to increase from 0.1 to 1000 s as the probe spacing increases from 2 to 10 Debye wavelengths for ion wave frequencies. For electron plasma wave frequencies the corresponding increase is from 10

ISSN:0048-6604    

DOI:10.1029/RS015i003p00545

年代:1980

数据来源: WILEY

摘要:

In situ data measured on board AE satellites and rockets reveal spiky and wedgelike electron density structures inside the equatorial ionospheric bubbles. Two models are constructed to simulate the initial stage and fully developed stage of a bubble. Effects of radio propagation through such bubbles are simulated by solving the parabolic equation numerically. The results show that even though the amplitude scintillation at 136 MHz appears to be stationary, such is not the case at gigahertz frequencies. Instead, the amplitude at gigahertz frequencies shows outbursts with large excursions whenever the direct ray intersects the spicky ionization structure. Both the peak‐to‐peak excursion and the amplitude distribution cannot be predicted by the scintillation theory that assumes the medium to be ran

ISSN:0048-6604    

DOI:10.1029/RS015i003p00559

年代:1980

数据来源: WILEY