We carried out synthetic observations of interstellar atomic hydrogen at 21cm wavelength by utilizing the magneto-hydrodynamical numerical simulations of the inhomogeneous turbulent interstellar medium (ISM) Inoue and Inutsuka (2012). The cold neutral medium (CNM) shows significantly clumpy distribution with a small volume filling factor of 3.5%, whereas the warm neutral medium (WNM) distinctly different smooth distribution with a large filling factor of 96.5%. In projection on the sky, the CNM exhibits highly filamentary distribution with a sub-pc width, whereas the WNM shows smooth extended distribution. In the HI optical depth the CNM is dominant and the contribution of the WNM is negligibly small. The CNM has an area covering factor of 30% in projection, while the WNM has a covering factor of 70%. This causes that the emission-absorption measurements toward radio continuum compact sources tend to sample the WNM with a probability of 70%, yielding smaller HI optical depth and smaller HI column density than those of the bulk HI gas. The emission-absorption measurements, which are significantly affected by the small-scale large fluctuations of the CNM properties, are not suitable to characterize the bulk HI gas. Larger-beam emission measurements which are able to fully sample the HI gas will provide a better tool for that purpose, if a reliable proxy for hydrogen column density, possibly dust optical depth and gamma rays, is available.
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