Authors: Gerard Thuillier and the PICARD Team
Affiliation: Service d'Aeronomie du CNRS, France
Ancient and modern observations of the solar diameter do not reveal consistent
variations which may be explained by atmosphere, instruments and solar effects
or a combination of them. Furthermore, diameter determined by helioseismologic
measurements also show some inconsistencies. However, solar modeling as made
by Li et al. (2003), shows the importance of knowing how behaves the solar diameter
as a function of time for parametrization of the role of the turbulence. In
particular it is of prime importance to determine the diameter/luminosity relationship.
The PICARD mission objectives are the study of the physics of the Sun and the
study of the solar forcing on the Earth's climate by modeling.
This mission consists in a microsatellite and a ground-based station. In space,
three instruments will carry out simultaneous measurements of the absolute total
and spectral solar irradiance, diameter, solar limb shape and asphericity, and
solar oscillations to probe the Sun's interior. On the ground, several instruments
will observe the solar diameter and measure the local turbulence to understand
differences space/ground. Diameter variation will be referred to an internal
angular scale, which will be periodically scaled to star angular distances.
It is expected to provide by this method a means to measure the solar secular
variation. Solar diameter and luminosity being related, a contribution to the
total solar irradiance historical reconstruction is expected. The launch is
foreseen in early 2008. At that time, several missions will be operation, in
particular the Solar Dynamics Observatory. A specific synergy is anticipated
by its observations related to magnetic fields and time variation in irradiance.