Title: Irradiance and solar Radius variations
Author: Rozelot J.P.
Affil: OCA/CERGA
Email: rozelot@obs-azur.fr
Authors: Lefebvre, S., Godier, S.
Abstract: Study of the Sun's variability is of interest for both astrophysics and solar-terrestrial physics. Observations of the total irradiance, obtained by space-based experiments over the last two decades, have demonstrated that irradiance varies on time scales of minutes to the 11-year solar cycle. If the central energy source remains constant while the rate of energy emission from the surface varies, there must be an intermediate layer where the energy can be stored or released depending on the variable rate of energy transport. The gravitational field of the Sun is one such energy reservoir. If the energy is stored in this layer, it will result in a change in the solar radius. Thus, a careful determination of the time dependence of the solar radius can provide a constraint on models of total irradiance variations. Therefore, it is particularly important to define the relation between total irradiance and radius variations. Historical observations of the solar radius over the last 300 years may provide an estimate of the past luminosity changes and their possible climate effects. In this paper we compare the long-term composite total irradiance, (compiled by Fröhlich and Lean (1998)), with the radius measurements which have been carried out at the Calern Observatory in South-East France since 1975 and in other places (Brazil and Chili). Recent studies indicate that small but persistent variations in solar energy flux may play an important role in climate changes. One of the most important implication concerns the changes in the irradiance due to the changes in the radius. Sofia (1979), calculated the so-called w parameter which measures such changes which is strongly model dependent. Observational measurements are still very poor. We will give here some new estimates, from space and ground measurements. Our ability to make more precise measurements may be expected to improve in an era of dedicated space missions, such as PICARD which will be briefly presented. A direct application of a precise determination of w would help in understanding how small perturbations in the solar irradiance may induce changes in the terrestrial surface temperature of a fraction of a degree centigrade.