Apparent solar radius variations: The influence of magnetic network and plage

Solar radius measurements, a by-product of the magnetograms recorded several times daily at Mt. Wilson Observatory over a period of a few decades, have revealed apparent variations of about 0.4" that are correlated with the solar cycle. We note that the radius definition used for the analysis of those magnetograms automatically converts intensity variations near the limb into apparent radius variations. A change in the average temperature structure of the quiet Sun can be ruled out as the source of these variations, since such a change would need to be very significant and would lead to other easily measurable consequences that are not observed. We show that plage emission near the solar limb associated with the magnetic activity variation during a solar cycle produces apparent radius changes of the correct sign. The use of plane-parallel or spherically-symmetric models to describe the faculae gives apparent radius variations that are a factor of 4-10 too small in magnitude. If the Mt. Wilson results are correct, then this implies that the small-scale structure of faculae produces limb extensions that are considerably larger than those returned by a plane-parallel or spherically-symmetric model.