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C.R. Gwinn (UC Santa Barbara)
I suggest that radio-wave scattering in the interstellar medium is responsible for the observed ultra-small-scale structure of HI absorption. Absorption in the 21-cm transition of HI shows changes of about 10 percent, between lines of sight separated by tens of AU. This observation is commonly interpreted as indicating that much of the ISM resides in small clumps with relatively high column density. However, velocity gradients in the absorbing gas can produce closely similar effect, by the interaction of their resonant absorption and refraction with the well-known scattering of radio waves in the interstellar plasma. The superthermal linewidths of the absorbing gas indicate that such velocity gradients exist. I suggest observational tests of this hypothesis. This work was supported by the National Science Foundation.
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