On the relation between total irradiance and radius variations

¹ Department of Physics and Astronomy, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095, USA
² OCA/CERGA, Avenue Copernic, 06130 Grasse, France
³ UCLA, Department of Physics, 90095-1562, Los Angeles, USA

Corresponding author: J. P. Rozelot, rozelot@obs-azur.fr

Received: 29 August 2000
Accepted: 18 January 2001

Abstract

We use Singular Spectrum Analysis (SSA) to analyze total solar irradiance variations and CERGA radius measurements. Total solar irradiance has been monitored from space for more than two decades, whilst ground-based radius measurements are available as a coherent time series from 1975. We compare these 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 observed variations of the Sun's radius by distinguishing the signal from the noise. Two approaches were used: one using SSA on ground-based data averaged over 90 days, in order to smooth the signal (especially over periods when no data were obtained, mainly in winter time); the second repeats the analysis on individual measurements corrected by reporting data to the zenith. As expected, the level of noise is higher in the first case and the reconstructed noise level, which is large, indicates the difficulty in ascertaining the solar origin in the apparent variability of the solar radius. It is shown from the reconstructed components that the main variation in amplitude (over 930 days) is pronounced during the first part of the measurements and seems 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 both cases, these variations disappear during the rising portion of cycle 23. The first reconstructed component shows that total irradiance varies in parallel 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 activity of cycle 23 is less obvious. The observed value of the solar radius increased by about 0.11 arcsec from the maximum of cycle 21 to the minimum between cycles 21 and 22. Most importantly, we report a long-term radius variation which increased from the maximum of cycle 21 to minimum by about 0.015% , while a smaller decrease (around 0.01% ) is seen from the minimum of cycle 21 to the maximum of cycle 22. This study indicates need for measurements of the degree of the radius changes taken from space, together with total irradiance measurements to establish the phase relation between these two quantities.