On the relation between total irradiance and radius variations

We use Singular Spectrum Analysis (SSA) to analyze total solar irradiance v ariations and CERGA radius measurements. Total solar irradiance has been mo nitored from space for more than two decades, whilst ground-based radius me asurements are available as a coherent time series from 1975. We compare th ese indicators to try to understand the origin of energy production inside the Sun. One of the main objectives was to assess the reality of the observ ed variations of the Sun's radius by distinguishing the signal from the noi se. Two approaches were used: one using SSA on ground-based data averaged o ver 90 days, in order to smooth the signal (especially over periods when no data were obtained, mainly in winter time); the second repeats the analysi s on individual measurements corrected by reporting data to the zenith. As expected, the level of noise is higher in the first case and the reconstruc ted noise level, which is large, indicates the difficulty in ascertaining t he solar origin in the apparent variability of the solar radius. It is show n from the reconstructed components that the main variation in amplitude (o ver 930 days) is pronounced during the first part of the measurements and s eems to disappear after 1988. There is also a variation with a periodicity of 1380 days, of lower amplitude than that of the shorter component. In bot h cases, these variations disappear during the rising portion of cycle 23. The first reconstructed component shows that total irradiance varies in par allel with the solar cycle, being higher during maximum activity conditions . The reconstructed radius trend indicates that the solar radius was higher during the minimum of solar cycle 21, but its decrease with the rising act ivity of cycle 23 is less obvious. The observed value of the solar radius i ncreased by about 0.11 arcsec from the maximum of cycle 21 to the minimum b etween cycles 21 and 22. Most importantly, we report a long-term radius var iation which increased from the maximum of cycle 21 to minimum by about 0.0 15%, while a smaller decrease (around 0.01%) is seen from the minimum of cy cle 21 to the maximum of cycle 22. This study indicates need for measuremen ts of the degree of the radius changes taken from space, together with tota l irradiance measurements to establish the phase relation between these two quantities.