The radio frequency 1.4 GHz transition of the atomic hydrogen is one of the
important tracers of the diffuse neutral interstellar medium. Radio
astronomical observations of this transition, using either a single dish
telescope or an array interferometer, reveal different properties of the
interstellar medium. Such observations are particularly useful to study the
multiphase nature and turbulence in the interstellar gas. Observations with
multiple radio telescopes have recently been used to study these two closely
related aspects in greater detail. Using various observational techniques, the
density and the velocity fluctuations in the Galactic interstellar medium was
found to have a Kolmogorov-like power law power spectra. The observed power law
scaling of the turbulent velocity dispersion with the length scale can be used
to derive the true temperature distribution of the medium. Observations from a
large ongoing atomic hydrogen absorption line survey have also been used to
study the distribution of gas at different temperature. The thermal steady
state model predicts that the multiphase neutral gas will exist in cold and
warm phase with temperature below 200 K and above 5000 K respectively. However,
these observations clearly show the presence of a large fraction of gas in the
intermediate unstable phase. These results raise serious doubt about the
validity of the standard model, and highlight the necessity of alternative
theoretical models. Interestingly, numerical simulations suggest that some of
the observational results can be explained consistently by including the
effects of turbulence in the models of the multiphase medium. This review
article presents a brief outline of some of the basic ideas of radio
astronomical observations and data analysis, summarizes the results from recent
observations, and discusses possible implications of the results.Comment: 20 pages, 10 figures. Invited review accepted for publication in the
Proceedings of the Indian National Science Academy. The definitive version
will be available at http://insaindia.org/journals/proceedings.ph
