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Solar Variability and the Relation of Facular to Sunspot Areas during Solar Cycle 22
The total irradiance of the Sun has been found to vary mostly because of changes in the areas of dark sunspots and bright faculae. Improved observations, such as those discussed in this paper, are needed to understand better the interplay between these two competing features. In this paper, faculae are determined by observations using a filter centered at the Ca II K line (393.4 nm) with a bandpass of 0.9 nm. This filter allows the detection of faculae across the entire solar disk rather than just at the limb, as is the case for white-light faculae. Sunspots are detected with a filter at 672.3 nm with a bandpass of 9.7 nm. The mean ratio of facular to sunspot area was found to be 16.7 ± 0.51 for a 7½ year period during solar cycle 22 but showed a significant increase as the solar cycle progressed. This ratio suggests that the irradiance excess associated with faculae outweighs the irradiance deficit associated with sunspots by about 50%. The facular area also exhibited a quadratic dependence on sunspot area, as suggested by Foukal, but there is no clear evidence of a turnover in facular area at large sunspot areas. Lagged cross-correlations between facular and sunspot areas showed a clear rotational modulation extending to lags of five to six rotations when spots led faculae. Lags in the opposite direction, however, showed the rotational modulation falling abruptly after about two rotations.