New data analysis technique developed for the determination of the solar limb position in measurements of the solar diameter and oblateness, and application to observations obtained with the balloon-borne Solar Disk Sextant (SDS) experiment.

Abstract

Solar diameter measurements performed from ground by several instruments during these last decades show variations which are not in agreement. In relation with solar activity, these measurements do not reveal consistent results. These results can be either attributed to Earth atmosphere effects or to instrumental ones especially in presence of noise. Noise affects directly the determination of the solar diameter defined as the zero crossing of the second derivative of the solar limb. Furthermore, presence of noise in data causes additional problems requiring appropriate data filtering without changing the solar limb slope. Several methods have been developed and used for a correct inflexion point position determination, among them, is the Fast Fourier Transform Definition (FFTD). We first present a complete description of the FFTD tool and in particular a new method to choose the filtering parameter (a) to be determined for applying FFDT. An alternative method by filtering using the wavelet analysis is also shown. The Solar Disk Sextant (SDS) is an instrument which has been flown on stratospheric balloons from 1992 to 1998 at 37 km altitude preventing all atmospheric effects. SDS uses a prism as angular reference. We present and discuss results obtained from SDS data analysis and compare them using others methods of inflexion point position detection. Finally, we discuss all other SDS experimental parameters able to cause solar diameter measurement variations.We show the relationship between the diameter variation and solar variability