Parkes observations for project P916 semester 2016APRS

We propose to search for temporal variations in HI absorption spectra seen against background pulsars to characterise Tiny Scale Atomic Structure (TSAS) in the Milky Way interstellar medium (ISM). We will re-observe four previously studied bright pulsars, extending the experimental baseline to 16 years and providing sensitivity to poorly probed 1000 AU scales. These observations will test predictions that there is a minimum size scale set by the thermal properties of the ISM, below which TSAS is only sporadically seen, and above which it becomes ubiquitous. Due to their unique on-off state, and proper motions of 100 AU per year, our sample of bright pulsars provide the perfect tool to carry out this experiment. Furthermore, our project provides an excellent demonstration of recent developments in the spectral line capability of the CASPSR backend, made possible by modern advances in GPUs. In total we request 50 hours of Parkes time to achieve our science goal of detecting significant opacity variation in the atomic ISM on 1000 AU scales.