Chromospheric rotation I: Dependance on the lifetime of chromospheric features

In this paper the dependance of the rotation rate of chromospheric features on their is discussed. The angular velocity of short lived emission features at chromospheric level has been determined by analyzing daily filtergrams in the Ca 11K3 line, obtained at the Anacapri Observatory, in the period 8 May-14August 1972. An accurate method of analysis has been developed in order to evaluate the average daily displacement of chromospheric tracers, by computing an 'average cross-correlation' of chromospheric brightness features for consecutive days at constant latitude. This method yields the average rotation rate for short-lived K3 chromospheric features (lifetime>1day). These results are compared with the determinations of the rotation rate of long-lived chromospheric features (lifetime>27days) obtained by using the same kind of data. Long-lived tracers are rotating rigidly in years of declining activity, while short-lived ones rotate differentially. Short-lived chromospheric features rotate at the same rate as the chromospheric plasma, i.e. faster than the photospheric plasma.