Facular influences on the apparent solar shape

Abstract

The apparent shape of the Sun is controversial¹: Dicke and Goldenberg² measured a Sun with an oblateness (equatorial minus polar radius divided by polar radius) of 5 × 10⁻⁵, whereas Hill and Stebbins measured a value of 0.9 × 10⁻⁵, consistent with a slowly rotating core. More recently, Claverie et al.³ have found the Sun's interior may be rotating faster than the surface after all. Dicke and Goldenberg's observations were questioned by Chapman and Ingersoll⁴ on the basis that facular influences could introduce a signal which appeared similar to the solar oblateness signal both in shape and magnitude. Dicke⁵ analysed the data in many different ways (such as dividing the data on the basis of facular brightness, and different window size) and concluded that the faculae did not contribute significantly. We reexamine this question by means of more general facular contrast models. We find that the faculae can contribute a signal which has a time dependence similar to the Dicke and Goldenberg oblateness signal, and which for a facular contrast within the range of acceptable values allows an acceptable fit to the oblateness measurements.