Vol. 7, Issue 2, Part C (2025)

This chapter presents a comprehensive examination of magnetic activity in the Sun and other stars, with particular emphasis on its physical origin, observational manifestations, and implications for exoplanetary studies. The discussion begins with an overview of the Sun’s magnetic activity, tracing its evolution from the photosphere through the outer atmospheric layers. Key features, including sunspots, faculae, and chromospheric structures, are described in the context of their temporal variability, commonly referred to as the solar activity cycle. The persistence of these phenomena throughout the solar lifetime is attributed to the operation of a magnetic dynamo, maintained by differential rotation and convective flows. Extending these concepts to stellar contexts, the chapter explores the diagnostics employed to monitor and quantify stellar magnetic activity, with particular attention to spectropolarimetry as a primary observational technique for characterising stellar magnetic fields. The synthesis of empirical results and theoretical modelling is then presented across a range of spectral types, providing insights into the diversity of stellar activity patterns. Finally, the chapter reviews current methodologies for detecting stellar activity cycles and highlights the multi-wavelength approaches that underpin contemporary investigations in this domain.