A measure of the network radiative properties over the solar activity cycle

Magnetic activity contributes to solar irradiance variations, both on short and long time scales. While sunspots and active region faculae are the dominant contributors to irradiance changes on time scales of days to weeks, the origin of the long term increase of the irradiance between activity minimum and maximum (≈0.1%) is still debated. It has been proposed that the small-scale magnetic elements composing the enhanced and quiet network take part substantially in this increase. To contribute to this debate, we present the results obtained measuring changes in the radiative properties of the quiet network pattern along the current solar cycle. We use daily near simultaneous full-disk images provided by the Rome PSPT in three pass bands centered on CaII K line, blue and red Continua. The period analyzed ranges from July 1996 to September 2002, during the whole ascending phase of the current solar cycle. We measured a network contrast change of about 0.05% in the two continuum PSPT bands and a network disk coverage change of the order of 6%. Under the simple hypothesis that network and quiet sun radiate as black bodies with the network at a slightly elevated temperature, the network contribution to TSI cycle variation, estimated over the period analyzed, is ≈3−4 × 10 −4 .