Observations of changes in sunspot cooling

The larger and more rapid changes in the total solar irradiance have been identified with the passage of large sunspot groups across the solar disk. These dips have been modeled by use of the PSI function which assumes a constant value of thermal contrast, [alpha], from one spot to another. We present results from digital images of sunspot groups obtained at 672.3 nm using a photometric telescope with an aperture of 1.5 cm. In addition to a digital PSI, these data can also be used to determine the pixel-by-pixel photometric deficit, DEF. The ratio of these two quantities, PSI/DEF, is proportional to [alpha]. We define [alpha] to be 02. * (PSI/DEF). This definition will differ by - 15% from that used in the PSI because our deficit measurement is monochromatic. For 18 spot groups, we find a mean [alpha] of 0.34 [+-] 0//014, although extreme values range from 0.24 to 0.46. We find that [alpha] for a particular sunspot group can vary by over 100% for a small group (corrected area of the hemisphere, A[sub H], of about 100-200 millionths) to, typically, 35% for large groups (A[sub H] of about 3000 millionths). Some groups show little or not change. Theremore » is no consistent correlation between changes in [alpha] with changes in the corrected area of the spot group. In summary, we believe that we are able to determine the thermal structure of sunspots which will allow us to determine the change in solar luminosity caused by individual sunspot groups.« less