1999A&A...344..163O

We provide a diagnosis of photometric starspot models through modelling active areas on the Sun using softwares originally written for starspot modelling. The data we used were one-dimensional measurements of the Sun in radio (10.7cm, DRAO, Canada) and in soft X-rays (GOES satellites). In these wavelengths the response to magnetic activity results in similar amplitude variability on the Sun to those we attribute to starspots in visual wavelengths. The modelling results were compared with contemporaneous direct images (obtained at Nobeyama, Japan and with the Yohkoh and SOHO spacecraft). We found that: a) knowing well the basic physical parameters of a star, the resulting total spotted area is a fairly good approximation of the reality, thus making sense of photometric starspot modelling; b) long-term variability coupled with the rotational modulation may result in artificially high latitude spots; c) in two- or multi-spot models a resulting small spot can account for short living spots; d) systematic change in spot size could be partially due to flux ratio changes. The contrast between the studied bright active region and the undisturbed area on the Sun decreased in time following roughly a power law. At the same time, the emission measure of the active region's core similarly decreased.