Solar cycle evolution of the contrast of small photospheric magnetic elements Original Research Article

Solar irradiance variations produced on the solar rotation time-scale are known to be driven by the passage of active regions while, during the last years, the origin of variations on the solar cycle time-scale has been under debate. Nowadays, there is an agreement that the magnetic network has an important contribution to these long-term variations, although it has not been fully quantified. This important role motivated us to study its physical properties along the solar cycle, such as contrast and population. We combine magnetograms and intensity images from the MDI instrument on board the SOHO spacecraft to analyze the radiative properties of small magnetic elements. We determine the contrast of faculae and network elements as a function of position over the disk, magnetic flux and time, finding that these elements exhibit a very different center-to-limb variation of the contrast. This implies that their contribution to irradiance variability is distinct. By extending this analysis through the rising phase of solar cycle 23, we conclude that the functional dependence of the contrast of small elements results to be time independent, implying that the physical properties of the underlying flux tubes may not vary with time. We decompose magnetograms into two structures identifying both faculae and network features and we examine their populations along the solar cycle.