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Astron. Astrophys. 318, 970-974 (1997)

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1. Introduction

The solar irradiance background signal is defined as the contribution from the time and spatial variation of small scale solar surface structures integrated over the solar disk. From low to high frequencies, surface inhomogeneities such as faculae, sunspots, etc. coupled with the sun's rotation, convective structures (super, meso and granulation) and their evolution produce the "noise" signal that we call the irradiance background signal. Knowledge of this signal, and of the relative contribution at different frequencies from each of these structures, is of great importance to understand these processes and look for other weak signals which have disappeared in the background. In particular, this information is highly relevant for Asteroseismology, where photometric data is easier to obtain than velocity data and, in most cases, is the only data available. It is also useful when searching for earth-like planets in other stars by measuring the occultation of the starlight by the planet.

For Helioseismology in particular, it is of paramount importance for detecting long period solar oscillations, the so-called g-modes, to gain information on the solar core. Global solar oscillations are concentrated on a frequency band below about 6000 µHz. Particularly, the frequency band below 200 µHz is extremely interesting since it contains the majority of g-modes. All searches for g-modes have yielded negative results, either in velocity or in irradiance measurements (Pallé 1991; Hill et al. 1991). These results are the consequence of a combination of several factors: the low surface amplitude of g-modes, instrumental noise, bad window functions, the terrestrial atmospheric noise and the solar background signal. Both in order to increase the duty cycle and to avoid the noise introduced by the earth's atmosphere g-mode detection requires the use of space missions. But even in the case of measurements taken under ideal observing conditions, the observations would ultimately be contaminated by the unavoidable solar background "noise" produced by these non-periodic, surface phenomena.

The VIRGO experiment proposal to ESA (Fröhlich et al. 1987) contains an estimate of the solar irradiance background variations spectrum. Further, a direct numerical model simulation of the convective structures (Andersen 1991a, b) was made which has been compared to observational data (Andersen et al. 1994) in limited frequency ranges.

Unfortunately, intensity or irradiance measurements are less abundant than velocity measurements which allowed a good estimation of the background velocity solar spectrum (Pallé et al. 1995). However, sufficient data exist to estimate the upper limit of the solar background spectrum in a wide frequency range. In this work the best available Earth-based and space data sets have been used to generate a power spectrum which has been compared with the one obtained from a theoretical numerical simulation of the involved phenomena.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998