Astrophysics (index)about

chromospheric activity index

(measure of magnetic activity of a star's chromosphere)

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, e.g., for comparison with other stars. A star's S-index changes over time, i.e., it is not a constant value.