New space value of the solar oblateness obtained with PICARD

Description

The PICARD spacecraft was launched on June 15, 2010 with one of its sci- entific objectives the study of the geometry of the Sun. The measure of the solar oblateness remains difficult because images are affected by optical distortions. Rolling the satellite as performed in previous space missions, allows determi- nation of the telescope contribution by assuming the geometry of the Sun is constant during the observations. This supposes that the telescope optical re- sponse is time-invariant during the roll operations. It is not the case for PICARD since an orbital signature is clearly observed in the solar radius computed from its images. We take into account this effect and provide the new space value of the solar oblateness obtained from PICARD images recorded in the solar continuum at 535.7 nm in July 4-5, 2011. We obtain 8.4 ± 0.5 mas for the equator- pole radius difference, which corresponds to an absolute radius difference of 6.1 kilometers. It is in good agreement with the mean value of all solar oblate- ness measurements obtained during the two last decades from ground, balloons and space. It is also consistent with values determined from models using helioseismology data.

Abstract

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