Long-term project observations for project P1321 semester 2025APRS_20

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.

中性原子星际介质（atomic interstellar medium）在0.1~10000天文单位（astronomical unit, AU）尺度上存在微小尺度光学深度涨落，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）的存在具有深远意义，甚至可能对我们关于星际介质中加热、冷却与动力学过程的基本认知构成挑战。然而当前相关观测数据仍较为稀缺。本长期研究计划通过搜寻背景脉冲星背景下的中性氢（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TSAS进行定量表征。本次观测是迄今为止单次实验中覆盖视线方向最多、时间采样最密集的观测项目，将验证“TSAS是湍流级联末端产物”的理论预测，限定其最小可分辨尺度（低至~0.05天文单位），并有望首次直接测得尺度大于1000天文单位的“大型”TSAS结构内的压强。本研究利用我们近期在帕克斯（Parkes）望远镜上实现的相位分辨谱线观测模式，相较于过往研究，该模式将数据速率与处理时间压缩了约1000倍。本项目是对先导项目P1321的长期拓展。