Speaker
Description
Among all the fundamental solar parameters, mass, surface gravity, temperature, luminosity..., all well inventoried for several years in reference books, solar diameter is still a controversial issue, both its true value and any possible temporal variations. The most exact value of the solar diameter is of importance, as it serves as an astronomical standard. Changing its absolute value can, for example, significantly alter the absolute diameter of stars, since the diameter of all stars is defined relative to that of the Sun (it may result an impact on the inferred stellar structures - though the density for instance). Considerable efforts to measure the solar diameter(s) have been made during the second half of the XXth century, involving dedicated space instruments, as well as eclipse observations of high precision. Modern 3-D solar theories show that the leptocline (inside the near sub-solar surface -NSSL-, a shallow and sharp rotational shear layer in the top around 8 Mm), can be modelized and is in rather good agreement with helioseismic observations deduced, for instance, from the HMI experiment on board the Solar Dynamics Observatory (SDO) NASA satellite. The role of the leptocline in causing solar diameter(s) variations is now well documented. In this presentation, we attempt to shed further light on the matter. After defining what a solar diameter refers to -photospheric, seismic, acoustic…-, we will give an overview of what we know today about changes in the Sun’s diameter over time. A quick aside will be made on diameter variations with heliographic latitude. We will address the role played by the leptocline in shaping the solar sub-surfacing layers, hence enables us to interpret the different solar diameters. We propose a fully updated glossary with meaning of solar diameters in use today in an attempt to reach a consensus on this still somewhat disputed subject. To conclude, this new look on such modern measurements of the Sun’s global changes gives a new way for peering into the solar interior, mainly to better understand the subsurface fields which play an important role in the implementation of solar cycles.