Geometrical information on the solar shape: high precision results with SDO/HMI

TL;DR

This paper presents high-precision measurements of the solar diameter and oblateness using SDO/HMI satellite data and ground observations, addressing calibration challenges and instrument effects.

Contribution

It provides refined measurements of solar shape parameters by analyzing satellite and ground data, highlighting calibration and instrument effects.

Findings

High-precision solar diameter values obtained

Solar oblateness measured with milli-arcsecond accuracy

Instrument effects significantly impact shape measurements

Abstract

The uncertainty of measurement of solar diameter is depending on the observational time scale. Full-disc images of SDO/HMI and the images from ground observations in Huairou Solar Observing Station have been analyzed to get the values of solar diameter. The satellite observations reach a very high precision, but the absolute image scale still need to be calibrated. The solar oblateness is a more challenging measurement than the diameter, since the signal amplitude is a few milli-arcseconds. It is a relative measurement, then not affected by the pixel scale calibration required by the diameter measurement. But the results are strongly dependent on the state of instrument such as focus plane deformation and on the calculation process.