Solar irradiance variation during rapid sunspot growth

Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. Observations show a strong correlation between spot area and TSI. During the growth phase of a sunspot, though, other physics might conceivably affect this correlation. In this study we analyze the growth phase of NOAA active region 8179, in which large sunspots suddenly appeared near disk center at a time when few competing sunspots or plage regions were present on the visible hemisphere. The spot area of AR 8179 increased by a factor of two in less than 12 hours on March 13, 1998. The study makes use of the continuum images from SOHO/MDI and total-irradiance data from SOHO/VIRGO. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with high thermal conductivity even at small depths below the photosphere. We have also searched for localized irradiance variations (transient bright rings) associated with the period of rapid growth. We discuss the significance of this lack of irradiance signatures of spot formation.