VARIATION OF THE LOW-DEGREE SOLAR ACOUSTIC MODE PARAMETERS OVER THE SOLAR CYCLE A. JIMÉNEZ 1 , T. ROCA CORTÉS 2 and S. J. JIMÉNEZ-REYES 3 1 Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain 2 Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain and Departamento de Astrofísica, U. de La Laguna, La Laguna, Tenerife, Spain. 3 Themis, Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain (Received 25 February 2002; accepted 18 June 2002) Abstract. VIRGO/SPM is a helioseismic sunphotometer on board SOHO that observes the disk- integrated sunlight irradiance at three different colors (red, green, and blue). The data obtained for SPM since the beginning of the SOHO mission, April 1996, to March 2001 have been used to study the differences of the p-mode parameters during the solar activity cycle. These time series have been divided in sub-series of 100 days, transformed to power spectra and averaged in sets of three to yield a total number of six averaged power spectra (around one per year). A new way of analyzing the power spectrum has been applied to the six power spectra of each color; it consists of fitting the whole p-mode spectrum at once with a unique background. The results for the frequencies, line widths, power, mode energy, energy rate fed in the mode and splittings along the activity cycle are found, compared and discussed. 1. Introduction Since the discovery of the solar low-ℓ acoustic spectrum by Claverie et al. (1979), helioseismology has produced a wealth of information about the internal structure of our Sun. The very-low-degree modes are the only ones that penetrate into the core of the Sun and, most probably, are the only ones to be obtained for other main-sequence stars. Disk-integrated sunlight observations as the ones obtained with VIRGO/SPM, contain information of the modes spatially averaged over the solar disk. Such av- eraging allows the detection of modes (n,ℓ,m) of the lowest degree (0 ≤ ℓ ≤ 3) grouped in multiplets (n,ℓ) with azimuthal degrees -ℓ ≤ m ≤+ℓ; and from these, only the modes with (ℓ + m) even will be observable since the observations are taken nearly perpendicularly to the rotation axis of the Sun. It is also known that p-mode parameters change with the solar activity. Us- ing ACRIM (Activity Cavity Radiometer on-board SMM) data of 1980 and 1984, Woodard and Noyes (1985) found a frequency shift for ℓ = 0 and ℓ = 1 around 420 nHz for the first time. A similar shift was found by Fossat et al. (1987) and Gelly et al. (1988). Further, Pallé, Régulo, and Roca Cortés (1989) using the best Doppler velocity data from 1977 to 1988 obtained at Observatorio del Teide, found a good correlation between the frequency shifts of these low degree p-modes and Solar Physics 209: 247–263, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.