Physics of Stellar Coronae

Stellar coronae, defined by the ensemble of magnetic field structures above stellar photospheres and chromospheres together with their thermal or non-thermal plasma content, play an important role in our understanding of stellar magnetic fields, the stellar magnetic dynamo, and interactions between a star and its environment. The most important wavelength regions from which we have learned diagnostically on stellar coronae include the radio range and the X-ray domain. The former is sensitive to accelerated electrons in magnetic fields, and that has provided the only direct means of imaging stellar coronal structure, through very long baseline interferometry. The soft X-ray diagnostic power has been instrumental for our understanding of physical processes in the hot, magnetically trapped coronal plasma, and the recent advent of high-resolution X-ray spectroscopy is now accessing physical parameters of coronal plasma directly. The adjacent extreme ultraviolet range contains diagnostics relevant for the same temperature range as X-rays. The present chapter provides a ``stellar astronomer's view'' of magnetic coronae, summarizing basic physical processes relevant for our understanding of radio/X-ray observations of coronal stars.