chromospheric activity index

The chromospheric activity index (or S-index) is a measure of the chromospheric magnetic field activity of a star. It is derivable from the star's spectroscopy, and is of interest regarding the Sun for comparison with other stars. It essentially compares the strength of the H and K (calcium) Fraunhofer lines, a ratio which on the Sun has been found to shift with the amount of sunspots, which are associated with increased Solar activity. It is presumed this is the case for Sun-like stars, allowing tracking of their cycles and yielding some sense of their activity.