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Lee, L. C.

Publications and source records attributed to Lee, L. C..

132 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Strong scintillations in astrophysics. II - A theory of temporal broadening of pulses

A theory of temporal broadening of pulses propagating in a turbulent medium is developed on the basis of the Markov approximation. The theory may be applied to quite general turbulence spectra and to thin or thick turbulent regions. Since the basis of the theory is the wave equation, no reliance is placed on geometrical optics and no assumptions are made about the scattered angular spectrum. The observed smearing is found to be the combination of three effects: the dispersion effect, the pure refractive effect, and the diffraction effect. The last of these dominates for typical pulsar parameters. Pulse shapes are calculated for both Gaussian and Kolmogorov turbulence spectra and it is shown how these scale with the various turbulence parameters.

Lee, L. C.↗
NASA NTRS1975

Strong scintillations in astrophysics. I - The Markov approximation, its validity and application to angular broadening

The Markov approximation to the propagation of waves in an extended, irregular medium is discussed in an astrophysical context. A new derivation is presented which is simple and which shows that the assumption of Gaussian statistics used by previous authors is irrelevant. We discuss the relevance of the approximation and show that it may apply in many situations of interest, including interstellar scintillations of pulsar signals. The approximation does not require the assumption of weak scattering or Gaussian correlation functions. The Markov equation for the angular spectrum is particularly simple, and solutions are discussed for typical turbulence spectra. It is found that the equation for the angular spectrum is very nearly that used by previous authors, and the present discussion shows that these results are much more general than previously thought. A possible observational test for distinguishing between Gaussian and power-law interstellar density spectra is discussed.

Lee, L. C.↗
NASA NTRS1974

Wave propagation in a random medium - A complete set of the moment equations with different wavenumbers

The propagation of waves in a random medium is studied in the 'quasi-optics' and the 'Markov random process' approximations. Under these assumptions, a Fokker-Planck equation satisfied by the characteristic functional of the random wave field is derived. A complete set of moment equations with different transverse coordinates and different wave numbers is then obtained from the Fokker-Planck equation of the characteristic functional. The application of those results to the pulse smearing of the pulsar signal and the frequency correlation function of the wave intensity in interstellar scintillation is briefly discussed.

Lee, L. C.↗
NASA NTRS1974

Cross sections and band strengths for the N2O/+/ /A 2Sigma+ to X 2Pi/ system produced by vacuum ultraviolet radiation

Analysis of cross sections that have been obtained for the production of the N2O(+) (A 2Sigma+ to X 2Pi) fluorescence, using vacuum ultraviolet radiation between 462 and 755 A. The fluorescence spectra produced using incident photons of 715.6- and 754.9-A wavelengths are presented, as well as the relative fluorescence cross sections for the individual observed bands of the above-mentioned N2O(+) system. Finally, absolute cross sections for the production of the N2O(+) (A 2Sigma+ to X 2Pi) system are presented, as well as band strengths for the A 2Sigma+(0,0,0) to X 2Pi(n1,n2,0) fluorescence.

Lee, L. C.↗
NASA NTRS1972

Velocity of the solar wind as determined from interplanetary scintillations.

The method of smooth perturbations is used to derive the equations for interplanetary scintillation in a (statistically) spherically symmetric solar wind with constant wind velocity. The expressions, valid in the limit of small scintillation index, are used to discuss the relation between the radial wind velocity and the observed projected velocity of the scintillation pattern. It is found that the pattern velocity is systematically less than the wind velocity by a substantial fraction, depending on the radial variation of the density fluctuations. If the fluctuations fall off as 1/r-squared the correction is 18 percent, whereas if the fluctuations are proportional to 1/r the correction is 57 percent. The effects may be even more severe if the solar wind is not spherically symmetric.

Jokipii, J. R.↗