Cool stars like the Sun harbor convection zones capable of producing substantial surface magnetic fields leading to stellar magnetic activity. The influence of stellar parameters like rotation, radius, and age on cool-star magnetism, and the importance of the shear layer between a radiative core and the convective envelope for the generation of magnetic fields are keys for our understanding of low-mass stellar dynamos, the solar dynamo, and also for other large-scale and planetary dynamos. Our observational picture of cool-star magnetic fields has improved tremendously over the last years. Sophisticated methods were developed to search for the subtle effects of magnetism, which are difficult to detect particularly in cool stars. With an emphasis on the assumptions and capabilities of modern methods used to measure magnetism in cool stars, I review the different techniques available for magnetic field measurements. I collect the analyses on cool-star magnetic fields and try to compare results from different methods, and I review empirical evidence that led to our current picture of magnetic fields and their generation in cool stars and brown dwarfs.

中文翻译：

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冷星磁场的观测

太阳之类的凉爽恒星具有对流区，它们能够产生大量的表面磁场，从而导致恒星的磁场活动。恒星参数（如旋转，半径和年龄）对冷星磁场的影响，以及辐射芯和对流包壳之间的剪切层对于产生磁场的重要性，是我们了解低质量恒星动力的关键，太阳能发电机，以及其他大型和行星发电机。在过去的几年中，我们对冷星磁场的观察图像有了很大的改善。已开发出复杂的方法来寻找磁力的微妙影响，这在冷恒星中尤其难以发现。重点介绍了用于测量凉爽恒星中磁性的现代方法的假设和功能，我回顾了可用于磁场测量的不同技术。我收集了有关冷星磁场的分析，并试图比较不同方法的结果，并回顾了经验证据，这些证据导致了我们对冷星和褐矮星的磁场及其产生情况的了解。