Variations of the chromospheric network with the solar cycle

Abstract

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 identified and masked by applying an algorithm based on the geometrical connection between active pixels. Contrast histograms have been derived for the full disk and for 'quiet' regions. Both of them show a neat Gaussian profile on the dark side and a significant tail on the bright side, due to the brightness enhancements associated with magnetic fields. A solar cycle dependence of the histogram asymmetry is clearly evident in 'quiet' region data, thus confirming that the network component might provide a significant contribution to long-term variations of the total irradiance.