On the origins of chromospheric activity in late-type stars

The correlation between stellar rotation rate and overall chromospheric activity levels in late-type stars has been known for some time. As such stars age they slow down, and their chromospheric emission decreases with their surface rotation rate. Similar relations even more strongly dependent on rotation have been found for coronal X-ray emission. The casual nature of these relationships - the details of the physical mechanisms responsible - have been more difficult to elucidate. However, the last few years have seen a tremendous increase in the available data on chromospheric emission and rotation rates of late-type stars, as well as much theoretical work on dynamos. The purpose in this short review is the discussion of the new data and the constraints they place on theories of the origin of chromospheric activity, and the suggestion of further observations which would be especially useful