Apparent variations of the sun's radius observed at the Côte d'Azur observatory (Solar astrolabe at Calern site, 1975-1998)

The results here discussed were produced by a program of visual observations carried out at the Calern Observatory from 1975 to 1998. The sun radius data collected display some variability, in particular an oscillation of 0.2″amplitude and 11.8 years period which seems anticorrelated with the sunspot cycle. These apparent variations might also be put together with other structural solar parameters: frequency shifts of low-degree p-modes or neutrino flux. Moreover, the series shows a deviation from the mean diameter as a function of heliographic latitude. The origin of these apparent variations is currently unclear and will be better understood by ground measurements over a long period of time (e.g. the DORAYSOL Instrument and Program at Calern) and by the results expected from the future PICARD mission of the French Space Agency CNES.