Long-term project observations for project P1321 semester 2025APRS_04

The atomic interstellar medium shows tiny-scale optical depth fluctuations on the scale of 0.1~10,000 AU, whose origin and nature are poorly understood. The existence of this Tiny-Scale Atomic Structure (TSAS) has significant implications, potentially calling into question our fundamental understanding of heating, cooling and dynamical processes in the interstellar medium. Yet observations remain sparse. This long-term project plans to search for temporal variations in HI absorption spectra seen against background pulsars to characterise TSAS in the Milky Way interstellar medium (ISM). These observations constitute the largest number of sightlines and densest temporal sampling ever performed in a single experiment, and will test predictions that TSAS is the tail end of a turbulent cascade, constrain its minimum size scale (down to resolutions of ~0.05 AU) and potentially provide the first direct measurements of pressures in "large" TSAS features of > 1000 AU. We make use of the phase-resolved spectral line mode that we have recently implemented on Parkes, which has cut data rates and processing times by factors of ~1000 compared to past studies. This is an expansion of our pilot P1321 to a long term study.

星际中性原子介质（atomic interstellar medium）存在尺度介于0.1~10000天文单位（Astronomical Unit, AU）的微观光深涨落，其起源与本质目前仍未得到充分阐释。这类小尺度原子结构（Tiny-Scale Atomic Structure, TSAS）的存在具有重要学术意义，甚至可能动摇我们对星际介质中加热、冷却与动力学过程的基本认知，然而当前相关观测数据仍较为匮乏。
本长期研究计划通过搜寻以背景脉冲星为参照的中性氢（HI）吸收光谱的时间变化特征，对银河系星际介质（interstellar medium, ISM）中的小尺度原子结构进行表征。
本次观测将是单实验中开展的观测视线数目最多、时间采样密度最高的研究，将对“小尺度原子结构是湍流级联末端产物”这一理论预测开展检验，约束其最小可分辨尺度（低至约0.05天文单位），并有望首次直接测量尺度大于1000天文单位的“大型”小尺度原子结构的压强。
本研究利用我们近期在帕克斯（Parkes）射电望远镜上实现的相位分辨谱线观测模式，相较于过往相关研究，该模式可将数据采集速率与处理时长缩减至原有水平的约1/1000。
本项目是对先导项目P1321的长期拓展。