Photospheric Faculae and the Solar Oblateness

Abstract

Photospheric faculae near the equatorial solar limb may provide the excess brightness which Ingersoll and Spiegel showed would explain Dicke and Goldenberg's oblateness measurement. Three lines of evidence support this statement: (1) the excess emission of faculae may arise in optically thin regions, as required by the Ingersoll-Spiegel hypothesis; (2) faculae are sufficiently widespread on the solar surface to account quantitatively for the observed signal; and (3) temporal fluctuations in the expected signal due to faculae in 1966 are correlated with fluctuations in the observed signal at the 1 percent level. (The probability of the correlation coefficient for uncorrelated data exceeding the observed value is less than 1 percent.) Although this evidence clearly demonstrates that faculae make a sizable contribution to the observed oblateness signal, it does not preclude an equally sizable contribution due to true gravitational oblateness. Evidence that faculae may not be the only source of oblateness signal comes from the apparent fact that the ratio of fluctuation amplitude to mean signal amplitude is greater for the facular signal than for the observed oblateness signal. However, this difference may be due to errors in reading the photographs from which the facular signal was derived, or to differences in processing the two sets of data. A better test of our hypothesis cannot be made until the daily oblateness signals and their standard deviations are available. In any case, it appears that further data analysis will be necessary before a reliable value of the solar oblateness can be inferred.

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