Long-term project observations for project P1321 semester 2025APRS_19

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.星际原子介质存在尺度介于0.1~10000天文单位（Astronomical Unit, AU）的微观光深涨落，其起源与本质至今仍未得到充分阐释。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。本长期项目计划通过搜寻以背景脉冲星为观测背景的中性氢（H I）吸收光谱的时间变化特征，对银河系星际介质（ISM）中的TSAS进行表征。本次观测是迄今单次实验中覆盖视线方向最多、时间采样密度最高的一次，将验证“TSAS是湍流级联末端产物”的理论预测，约束其最小尺度（分辨率可达约0.05 AU），并有望首次直接测量尺度大于1000 AU的“大尺度”TSAS结构中的压强。本项目利用我们近期在帕克斯望远镜（Parkes）上搭建的相位分辨谱线观测模式，与过往研究相比，该模式将数据速率与处理时间压缩了约三个数量级。本项目是对先导项目P1321的长期扩展研究。