Chromospheric network properties derived from one year of PSPT images

Description of mechanisms responsible of network variations on a long time scale deserves large interest in solar variability studies, due to the well known network contributtion to long term variations of the solar bolometric and spectral irradiance. In fact, to understand this variability and develop consistent models of the solar activity cycle it is important to know in detail how the solar magnetic field varies and the physical mechanisms responsible for its variation. We applied an automated procedure to derive the chromospheric network pattern and to identify the internetwork cells on a sequence of Ca II K images obtained in a period of quiescent solar activity (from July 1996 to May 1997) with the PSPT telescope at the Rome Observatory. Skeletons of the identified network patterns have been used to study variations of the network contrast and of the internetwork cell geometry. By analyzing one year of images we found variations of the chromospheric network contrast correlated with variations of the He 1083 nm and the X-ray emissions. In particular, we found a weakening of the network to internetwork intensity ratio that coincides with an increase of the soft X-ray flux and of the He 1083 nm absorption. The fractal dimension of the internetwork cells and their area distribution did not seem to show time variations during the period analyzed, while the value of the cell characteristic scale, as obtained by the wavelet analysis, shows a slight decrease.