Low-energy cosmic rays (up to the GeV energy domain) play a crucial role in the physics and chemistry of the densest phase of the interstellar medium. Unlike interstellar ionising radiation, they can penetrate large column densities of gas, and reach molecular cloud cores. By maintaining there a small but not negligible gas ionisation fraction, they dictate the coupling between the plasma and the magnetic field, which in turn affects the dynamical evolution of clouds and impacts on the process of star and planet formation. The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also drives the rich interstellar chemistry revealed by observations of spectral lines in a broad region of the electromagnetic spectrum, spanning from the submillimetre to the visual band. Some recent developments in various branches of astrophysics provide us with an unprecedented view on low-energy cosmic rays. Accurate measurements and constraints on the intensity of such particles are now available both for the very local interstellar medium and for distant interstellar clouds. The interpretation of these recent data is currently debated, and the emerging picture calls for a reassessment of the scenario invoked to describe the origin and/or the transport of low-energy cosmic rays in the Galaxy.

低能宇宙射线（高达GeV能域）在星际介质最致密相的物理和化学中发挥着至关重要的作用。与星际电离辐射不同，它们可以穿透大柱密度的气体，并到达分子云核心。通过维持较小但不可忽略的气体电离分数，它们决定了等离子体和磁场之间的耦合，这反过来又影响云的动态演化以及对恒星和行星形成过程的影响。星际云中分子氢的宇宙射线电离也驱动了丰富的星际化学，这是通过对电磁波谱的广泛区域（从亚毫米到可见光波段）的谱线观测所揭示的。天体物理学各个分支的一些最新进展为我们提供了关于低能宇宙射线的前所未有的观点。现在，对于非常局部的星际介质和遥远的星际云，都可以对此类粒子的强度进行精确测量和限制。目前对这些最新数据的解释存在争议，新出现的情况需要重新评估用于描述银河系低能宇宙射线的起源和/或传输的场景。