Outer-atmospheric imaging

The nearest cool star confronted us with the reality that cool stars have extremely inhomogeneous outer atmospheres. This was first confirmed for stars other than the Sun by the modulation of broadband signals, caused by starspots, and later by the discovery of the quasi-periodic variation in the Ca II H+K signal of some cool stars by Vaughan et al. (1981) caused by the rotation of an inhomogeneously covered stellar surface. Insight in stellar dynamos requires observational data on the properties of stellar active regions and their emergence patterns. For instance, we would like to know the sizes of stellar active regions and their lifetimes, the details of the structure of starspots, the emission scale height at different temperatures, and so on. In fact, we would like to know the entire three-dimensional geometrical structure of the outer atmospheres of cool stars. For that knowledge to be obtained, stellar surfaces should somehow be imaged by sounding the atmosphere from the photosphere on out. We would like to learn all this not merely for stars with activity levels similar to that of the Sun, but also for other stars, from the extremely active, tidally interacting binary systems for which much of the surface seems to be covered by areas as bright as solar active regions with a small fraction being even brighter, down to the very slowly rotating giant stars whose average coronal brightness is well below that of a solar coronal hole.