The small-scale structure in interstellar HI: a resolvable puzzle

During the past decade or so, measurements of Galactic H I absorption using VLBI against extragalactic sources, as well as multi-epoch observations in pulsar directions, have detected small-scale transverse variations corresp onding to tens of au at the distance of the absorbing matter. Hitherto thes e measurements have been interpreted as small-scale structure in the H I di stribution with densities n(H I)similar to 10(4)-10(5) cm(-3), orders of ma gnitude greater than those of the pc-scale structure. Naturally, it is diff icult to imagine how such structures could exist in equilibrium with other components of the ISM. In this paper we show that structure on all scales contributes to the diffe rences on neighbouring lines of sight, and that the observed differences ca n be accounted for by a natural extension of the distribution of irregulari ties in the distribution of H I opacities at larger scales, using a single power law. This, in our opinion, should put an end to the decades-long puzz le of the so-called small-scale structure in H I and other species in the G alaxy.