Parkes observations for project P1014 semester 2021OCTS_02

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The detections of overdense and overpressured Tiny Scale Atomic Structures (TSAS) challenge the standard picture of cooling and heating mechanisms in the interstellar medium (ISM). Multi-epoch observations of HI absorption against pulsars are a tested, practical observational method to probe the TSAS. Based on our observations in the 2019APR, 2019OCT, and 2020APR, we successfully detected a TSAS towards PSR B1557-50 with a spatial scale of 17 AU and significant dispersion measurements (DM) variations simultaneously. If TSAS is dust-free and molecule-free with a large fraction of carbon gas and not highly shielded from starlight, most of the carbon is ionized. Such electron column density in the TSAS can be observed in DM variations at a level of ~10^-2 pc cm^-3. Therefore, the ratio of HI column density variations and electron density variations measures the ionization fractions of the TSAS directly. The ionization fractions estimation in such cold atomic gas is very difficult usually through low-frequency carbon radio recombination lines with interferometers. In order to constrain the ionization fractions in TSAS, we propose two epoch observations towards PSR B1557-50 and two epoch observations towards PSR B1749-28. This will help us to better understand the cooling and heating mechanisms in the ISM.