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Paper: Extreme Scattering Events: Insights into the Interstellar Medium on AU Scales
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 299
Authors: Walker, M.A.
Abstract: Several radio-wave scintillation phenomena exhibit properties which are difficult to accommodate within the standard propagation model based on distributed Kolmogorov turbulence in the ionised ISM; here we discuss one such phenomenon, namely Extreme Scattering Events. By analysis of the data we demonstrate that these events are caused by ionised gas associated with selfgravitating, AU-sized gas clouds. The data also show that the ionised gas is confined by ram pressure, with the clouds moving at hundreds of km/s relative to the diffuse ISM and causing strong shocks. These conclusions are supported by a quantitative model in which heat from the shocked ISM evaporates gas from the surface of a cold cloud; this model readily explains the physical conditions which are required for Extreme Scattering and yields passable reproductions of the light-curves. The magnetotail of the cloud provides a site in which two other “anomalous” radio-wave propagation phenomena – Intra-Day Variability of quasars, and pulsar parabolic arcs – can plausibly arise, thus linking three anomalous propagation phenomena in a single physical model. Locally there must be thousands of these neutral clouds per cubic parsec and by mass they are the primary constituent of interstellar matter.
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