Chromospheric rotation : dependence on the size of cromospheric features

The dependence of solar rotation on the size of the chromospheric tracers is considered. On the basis of an analysis of Ca n K3 daily filtergrams taken in the period 8 May-14 August, 1972, chromo-spheric features can be divided into two classes according to their size. Features with size falling into the range 24 000-110 000 km can be identified with network elements, while those falling into~the range 120 000-300 000 km with active regions, or brightness features of comparable size present at high latitudes. The rotation rate is determined separately for the two families of chromospheric features by means of a cross-correlation technique which directly yields the average daily displacement of tracers due to rotation. Before computing the cross-correlation functions, chromospheric brightness data have been filtered with appropriate bandpass and highpass filters for separating spatial periodicities whose wavelengths fall into the two ranges of size, characteristic of the network pattern and of the activity centers. A difference less than 1% of the rotation rate of the two families of chromospheric features has been found. This is an indication for a substantial corotation at chromospheric levels of different short-lived features, both related to solar activity and controlled by the convective supergranular motions.