VARIATIONS OF THE CHROMOSPHERIC NETWORK WITH THE SOLAR-CYCLE

We have digitized a small sample of Ca II K spectroheliograms from the photographic archive of Arcetri Observatory for the period 1950-1970. For each digitized plate, active regions have been automatically iden tified and masked by applying an algorithm based on the geometrical co nnection between active pixels. Contrast histograms have been derived for the full disk and for 'quiet' regions. Both of them show a neat Ga ussian profile on the dark side and a significant tail on the bright s ide, due to the brightness enhancements associated with magnetic field s. A solar cycle dependence of the histogram asymmetry is clearly evid ent in 'quiet' region data, thus confirming that the network component might provide a significant contribution to long-term variations of t he total irradiance.