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Session 46 - Interstellar Scattering and Scintillation as Tools in Radio Astronomy.
Topical, Oral session, Tuesday, June 09
Sierra/Padre,
Interstellar scintillations (ISS) of small angular diameter radio sources have been studied for 30 years. They arise from fluctuations of the interstellar electron density. These exhibit an anisotropic Kolmogorov spectrum over scales ranging from about 10^9 cm to 10^15 cm, with the power in these fluctuations varying by orders of magnitude from place to place. The optics of this phenomenon is fairly well understood. The same cannot be said for the physical process responsible for producing the electron density fluctuations. In recent work, Goldreich and Sridhar argue that incompressible MHD turbulence will produce a power law velocity spectrum of shear Alfven waves that matches the ISS electron density spectrum. The velocity spectrum arises from a critical balance between the linear wave periods and nonlinear wave interactions. Its connection to the electron density spectrum remains a puzzle. One might speculate that the turbulent velocity field mixes specific entropy as a passive contaminant thereby giving rise to isobaric temperature and density fluctuations whose spectra mimic that of the velocity field. This is the manner in which the atmospheric scintillations of optical stars arise. However, as a consequence of its rapid of cooling, specific entropy is not conserved in large scale motions of ionized interstellar gas. This severely limits the outer scale of the electron density spectrum. Implications of this limitation to potential sources and sites of ISS will be explored in this lecture.
