Published online by Cambridge University Press: 04 August 2017
It is now firmly established that lower main sequence (LMS) stars show a qualitative correlation between rotation rate and chromospheric and coronal emission. By analogy with the Sun, the emission is believed to be intimately associated with surface magnetic fields. This association is especially close on the Sun for the Ca II H and K lines, for which the spatial correlation between chromospheric emission and photospheric fields is essentially one-to-one down to scales at least as fine as a few arcseconds and for which the emission flux from an area on the Sun is approximately proportional to the total magnetic flux passing through the same area in the underlying photosphere (Leighton 1959; Skumanich, Smythe, and Frazier 1975; Frazier 1971). The extension of the association to other LMS stars, while based on appeal to analogy, has been strengthened by recent detections of strong magnetic fields covering large fraction of the surface area of chromospherically active stars (see review by Marcy in this symposium).