摘要:

Radio waves traveling through the irregular plasma density in the interstellar medium are distorted in various ways. The differing observable quantities are introduced and the major conclusions that can be drawn from each about the wave‐number spectrum of plasma density are discussed. Distinction is drawn between diffractive effects on scales 106–108m and refractive effects on scales of 1010–1012m.

ISSN:0094-243X    

DOI:10.1063/1.37597

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The inverse problem consists of two parts: (i) To establish theformof the electron density fluctuation spectrum. (ii) To determine thestrengthof fluctuations in different regions of the ISM.(i) Based on a variety of observations of scintillation‐related phenomena in radio pulsars and compact galactic and extragalactic radio sources, it appears that the density fluctuation spectrum for lengthscales shorter than ∼1014cm is close to that predicted by Kolmogorov turbulence theory. Refractive scintillation is found to be unusually strong in several sources, suggesting either that there is enhanced fluctuation power at ∼1013–1014cm or that the inner scale is larger than thought earlier, possibly as large as ∼1011–1012cm.(ii) Electron density fluctuations in the Galaxy consist of a strong clumpy component with a scale height ≤100 pc and a weaker nearly homogeneous component with a scale height ≥500 pc. The former component is particularly strong in the regions of the Galaxy interior to the sun and may be associated with HII regions and supernova remnants.

ISSN:0094-243X    

DOI:10.1063/1.37588

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Recent observations of radio source angular broadening and pulsar dynamic spectra are reviewed, with the goal of determining the spectrum of the interstellar density irregularities and the sources which generate the turbulence. Measurements of angular broadening and certain aspects of pulsar dynamic spectra favor ‘‘quasi‐Kolmogoroff’’ density spectra, which are power law with an index less than four. However, pulsar dynamic spectra frequently display phenomena such as drifting bands or quasi‐periodic structures which seem to require spectra with more power at long wavenumbers than can be supplied by quasi‐Kolmogoroff spectra. The resolution of this seemingly contradictory situation is not obvious at the present time. Observations of angular broadening of extragalactic sources and spectral corrugation of pulsars show that the intensity of turbulence in the interstellar medium must be highly spatially inhomogeneous. The paper concludes with suggestions for future observations.

ISSN:0094-243X    

DOI:10.1063/1.37598

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The transport of cosmic rays in the interstellar medium is reviewed with emphasis on interactions with the turbulent interstellar magnetic field. It is shown that the standard picture of cosmic‐ray transport suggests strongly the existence of a smooth turbulence spectrum of the range of scales between 1012and 1019cm. This, coupled with observations of radio wave scattering and other direct measurements suggests a smooth, power‐law turbulence spectrum over the range of scales from 109to 1019cm, with the index of the power law being close to that of the Kolomogorov equilibrium subrange. Attributing this to a turbulent cascade leads to severe difficulties. Suggestions for avoiding these difficulties are discussed.

ISSN:0094-243X    

DOI:10.1063/1.37599

出版商:AIP    

年代:1988

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.37600

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We have performed computer simulations to investigate whether sizeable electron density fluctuations might be produced in the process of cosmic‐ray acceleration at supernova remnant shock waves. The hypothesis is the following: Cosmic‐ray acceleration via a Fermi I mechanism leads to large‐amplitude Alfven waves upstream of a supernova remnant shock wave. If the Alfven waves reach a large enough amplitude, they can drive sound waves in the interstellar medium. The fluctuations in the electron density due to these sound waves will then contribute to the observed interstellar scintillation. Our simulations suggest that this mechanism may be a plausible one. Issues remaining to be addressed include the resulting filling factor, and the integrated strength C2nto be expected for the turbulence.

ISSN:0094-243X    

DOI:10.1063/1.37601

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We discuss the possibility that the density fluctuations responsible for radio scintillations could be due to ion‐beam‐generated magnetohydrodynamic waves near interstellar shock waves. This suggestion is inspired by spacecraft observations which reveal these phenomena near shocks in the solar system. The model quite naturally accounts for the scale on which these fluctuations occur; it is dictated by the wavelength of the unstable waves. A feature of the model is that there is no cascasde from very large (of order parsec) scales to small scales. The fluctuations are directly produced on the small scales where they have a pronounced effect on radio waves. An embarrassment to the model is the apparently small spatial extent of region of intense MHD waves and associated density fluctuations. Simple estimates based on quasilinear theory yield regions too thin to produce the observed scattering. A number of ways in which this conclusion can be circumvented are discussed.

ISSN:0094-243X    

DOI:10.1063/1.37578

出版商:AIP    

年代:1988

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.37579

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Turbulence in the stratified ocean is inhibited at large scales by gravity, leaving internal waves and partially mixed fluctuations in the density field called fossil turbulence. Information about previous turbulence activity is preserved by the fossil remnants of 3‐D and possibly 2‐D turbulence. Turbulence in the interstellar medium may be inhibited by gravity at large scales, and by self gravity and electromagnetic forces at small scales, also producing fossils of previous active turbulence.

ISSN:0094-243X    

DOI:10.1063/1.37580

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Ground‐based observations of radio wave scattering caused by density irregularities in the solar wind provide a useful means of studying the innter heliosphere. This review outlines the different methods that have been used and highlights some of the important results obtained.

ISSN:0094-243X    

DOI:10.1063/1.37581

出版商:AIP    

年代:1988

数据来源: AIP