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Paper: The Small-Scale Structures in Interstellar HI: A Puzzle Resolved?
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 105
Authors: Deshpande, A.A.
Abstract: During the past decade or so, measurements of Galactic HI absorption using VLBI against extragalactic sources, as well as multi-epoch observations in pulsar directions, have detected small-scale transverse variations corresponding to tens of AU at the distance of the absorbing matter. Hitherto these measurements have been interpreted as small-scale structure in the HI distribution with densities nHI ~ 104 − 105 cm−3, orders of magnitude greater than those of the parsec-scale structure. Naturally it is difficult to imagine how such structures could exist in equilibrium with other components of the ISM.

In this talk we show that structure on all scales contributes to the differences on neighbouring lines of sight, and that the observed differences can be accounted for by a natural extension of the distribution of irregularities in the distribution of HI opacities at larger scales, using a single power law. Appreciating certain similarity of the contexts, we also examine if a suitable single power-law description of the electron density distribution in the ISM is consistent with the observations of the Extreme Scattering Events. In either case, we find a) no compelling evidence for over-dense, over-pressured structures, and b) no surprise in these apparent density enhancements on small transverse scales.

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