Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields.

磁场对 M 矮星的内部和大气特性起着重要作用，并极大地影响在这些低质量恒星的宜居带中运行的类地行星。确定恒星表面和整个恒星磁层的磁场强度和拓扑结构是推进恒星和行星科学的关键因素。在这里，回顾了应用于 M 矮星的现代磁场测量方法，重点是基于高分辨率强度和偏振光谱中塞曼效应特征的解释的直接诊断。总结并严格评估了从塞曼展宽分析得出的平均场强测量结果以及通过将层析成像测绘方法应用于分光偏振观测而推断出的全球磁几何信息。在水磁发电机和磁场改变的恒星内部结构的理论模型的背景下，讨论了对 M 矮星磁场复杂、多尺度性质的新认识。